Energy storage element and method for manufacturing energy storage element

文档序号：723271 发布日期：2021-04-16 浏览：31次中文

阅读说明：本技术 蓄电元件和蓄电元件的制造方法 (Energy storage element and method for manufacturing energy storage element ) 是由尾木谦太中野史也下川亮介伊藤祥太增田真规山福太郎熊林慧宫崎明彦于 2019-09-10 设计创作，主要内容包括：本发明的一个方式为一种蓄电元件,其具备具有负极和正极的电极体,上述负极具有负极基材和含有负极活性物质且沿着该负极基材的至少一侧的面以未压制的状态配置的负极活性物质层,其中,上述负极活性物质含有实心石墨粒子作为主成分,上述实心石墨粒子的长宽比为1～5。(One embodiment of the present invention is an electricity storage device including an electrode body having a negative electrode and a positive electrode, the negative electrode including a negative electrode substrate and a negative electrode active material layer containing a negative electrode active material and disposed in an uncompressed state along at least one surface of the negative electrode substrate, wherein the negative electrode active material contains solid graphite particles as a main component, and the solid graphite particles have an aspect ratio of 1 to 5.)

1. An electric storage element comprises an electrode body having a negative electrode and a positive electrode,

the negative electrode comprises a negative electrode base material and a negative electrode active material layer containing a negative electrode active material and arranged in an unpressed state along at least one surface of the negative electrode base material,

the negative electrode active material contains solid graphite particles as a main component,

the aspect ratio of the solid graphite particles is 1 to 5.

2. An electric storage element comprises an electrode body having a negative electrode and a positive electrode,

the negative electrode comprises a negative electrode base material and a negative electrode active material layer containing a negative electrode active material and arranged along at least one surface of the negative electrode base material,

the negative electrode active material contains solid graphite particles as a main component,

the aspect ratio of the solid graphite particles is 1 to 5,

the density of the negative electrode active material layer was 1.20g/cm³～1.55g/cm³。

3. An electric storage element comprises an electrode body having a negative electrode and a positive electrode,

the negative electrode comprises a negative electrode base material and a negative electrode active material layer containing a negative electrode active material and arranged along at least one surface of the negative electrode base material,

the negative electrode active material contains solid graphite particles as a main component,

the aspect ratio of the solid graphite particles is 1 to 5,

wherein a ratio R2/R1 of a surface roughness R2 of the negative electrode substrate in a region where the negative electrode active material layer is not arranged to a surface roughness R1 of the negative electrode substrate in a region where the negative electrode active material layer is arranged is 0.90 or more.

4. The power storage element according to any one of claims 1 to 3, wherein the negative electrode active material further contains non-graphitizable carbon.

5. The power storage element according to any one of claims 1 to 4, comprising an electrode body in which the negative electrode and the positive electrode are wound in a laminated state,

the electrode body has a hollow region in a central portion.

6. The power storage element according to any one of claims 1 to 5, comprising: and a pressure-sensitive cutting mechanism for cutting off the electrical connection between the negative electrode and the positive electrode when the internal pressure rises to a predetermined pressure, or a pressure-sensitive short-circuiting mechanism for electrically short-circuiting the negative electrode and the positive electrode outside the electrode body.

7. The power storage element according to any one of claims 1 to 6, comprising:

a case that houses the electrode body and has an inner surface in direct or indirect contact with an outer surface of the electrode body, and

and a pressurizing member that pressurizes the housing from the outside.

8. A method for manufacturing an electric storage element includes the steps of:

preparing a negative electrode in which a negative electrode active material layer containing a negative electrode active material is disposed along at least one surface of a negative electrode base material,

preparing a positive electrode in which a positive electrode active material layer containing a positive electrode active material is disposed along one surface of a positive electrode substrate, and

laminating the negative electrode and the positive electrode;

wherein the negative electrode active material contains solid graphite particles,

the aspect ratio of the solid graphite particles is 1 to 5,

the negative electrode is not pressed with the negative electrode active material layer before the negative electrode is laminated with the positive electrode.

Technical Field

The present invention relates to an electric storage device and a method for manufacturing an electric storage device.

Background

A nonaqueous electrolyte secondary battery represented by a lithium ion nonaqueous electrolyte secondary battery is often used for electronic devices such as personal computers and communication terminals, automobiles, and the like because of its high energy density. The nonaqueous electrolyte secondary battery generally includes an electrode body having a pair of electrodes electrically separated by a separator, and a nonaqueous electrolyte interposed between the electrodes, and is charged and discharged by exchanging ions between the electrodes. Further, as an electric storage element other than the nonaqueous electrolyte secondary battery, a capacitor such as a lithium ion capacitor or an electric double layer capacitor has been widely used.

In order to achieve such an increase in energy density and an improvement in charge/discharge efficiency of the power storage device, a carbon material such as graphite has been used as a negative electrode active material of the power storage device (see patent document 1).

Documents of the prior art

Patent document

Patent document 1: japanese patent application laid-open No. 2005-222933

Disclosure of Invention

However, graphite expands and contracts largely during charge and discharge. Therefore, as the electrode expands due to charge and discharge, a load is applied to the electrode itself or the separator stacked adjacent to the electrode, and there is a possibility that the performance of the energy storage element is degraded.

The present invention has been made in view of the above circumstances, and an object thereof is to provide an energy storage element having a high effect of suppressing expansion of a negative electrode occurring at the time of initial charging when graphite is used as a negative electrode active material, and a method for manufacturing the energy storage element.

One embodiment of the present invention, which has been made to solve the above problems, is an electric storage device including an electrode body having a negative electrode and a positive electrode, the negative electrode including a negative electrode substrate and a negative electrode active material layer containing a negative electrode active material and disposed in an uncompressed state along at least one surface of the negative electrode substrate, the negative electrode active material containing solid graphite particles as a main component, the solid graphite particles having an aspect ratio of 1 to 5.

Another embodiment of the present invention is an electricity storage device including an electrode body including a negative electrode and a positive electrode, the negative electrode including a negative electrode substrate and a negative electrode active material layer containing a negative electrode active material and disposed along at least one surface of the negative electrode substrate, the negative electrode active material containing solid graphite particles as a main component, the solid graphite particles having an aspect ratio of 1 to 5, and the negative electrode active material layer having a density of 1.20g/cm³～1.55g/cm³。

Another embodiment of the present invention is an electricity storage device including an electrode body including a negative electrode and a positive electrode, the negative electrode including a negative electrode substrate and a negative electrode active material layer containing a negative electrode active material and disposed along at least one surface of the negative electrode substrate, the negative electrode active material containing solid graphite particles as a main component, the solid graphite particles having an aspect ratio of 1 to 5, and a ratio R2/R1 of a surface roughness R2 of the negative electrode substrate in a region where the negative electrode active material layer is not disposed to a surface roughness R1 of the negative electrode substrate in a region where the negative electrode active material layer is disposed being 0.90 or more.

Another aspect of the present invention is a method for manufacturing an electric storage device, including: preparing a negative electrode in which a negative electrode active material layer containing a negative electrode active material is disposed along at least one surface of a negative electrode base material, a positive electrode in which a positive electrode active material layer containing a positive electrode active material is disposed along one surface of a positive electrode base material, and laminating the negative electrode and the positive electrode; wherein the negative electrode active material contains solid graphite particles, the aspect ratio of the solid graphite particles is 1 to 5, and the negative electrode active material layer is not compressed before the negative electrode and the positive electrode are laminated.

According to the present invention, it is possible to provide an energy storage device having a high effect of suppressing expansion of a negative electrode occurring at the time of initial charging when graphite is used as a negative electrode active material, and a method for manufacturing the energy storage device.

Drawings

Fig. 1 is a schematic exploded perspective view illustrating an electric storage device according to an embodiment of the present invention.

Fig. 2 is a schematic cross-sectional view of an electricity storage element according to an embodiment of the present invention.

Fig. 3 is a schematic diagram showing a power storage device configured by grouping a plurality of power storage elements according to an embodiment of the present invention.

Detailed Description

This energy storage device has a high effect of suppressing the expansion of the negative electrode during initial charging when graphite is used as the negative electrode active material. The reason is not clear, but is considered as follows.

The energy storage device includes a negative electrode in which a negative electrode active material layer containing solid graphite particles as a main component is disposed in an uncompressed state, and stress is hardly applied to the negative electrode active material until an electrode body is formed. Therefore, the graphite particles themselves have a small residual stress, and uneven expansion of the negative electrode due to release of the residual stress can be suppressed. Further, since the graphite particles are solid, the density in the graphite particles is uniform, and the graphite particles are made to be nearly spherical by the aspect ratio of 1 to 5, so that current concentration is less likely to occur, and therefore, uneven expansion of the negative electrode can be suppressed. Further, since the graphite particles are nearly spherical as described above, the graphite particles incorporated in the active material layer have low orientation and are likely to be randomly oriented, and thus uneven expansion of the negative electrode can be suppressed. Further, since the graphite particles are close to spherical, adjacent graphite particles are less likely to be caught, and the graphite particles slide relative to each other appropriately, so that even if the graphite particles expand, the graphite particles are easily maintained in a state close to the closest packing. As described above, in the present embodiment, even if the graphite particles expand, the graphite particles expand relatively uniformly, and the negative electrode active material layer having a high filling ratio of the graphite particles is maintained by appropriate mutual sliding, and as a result, it is estimated that expansion of the negative electrode occurring at the initial charging can be suppressed.

Note that the term "not pressed" means that the following steps are not performed during production: a pressure (line pressure) of 10kgf/mm or more (for example, 5kgf/mm or more) is applied to the negative electrode active material layer by an apparatus such as a roll press machine for applying pressure to a work. That is, an operation of applying a slight pressure to the negative electrode active material layer in another step such as winding the negative electrode is also included in "not pressing". Further, "not pressed" includes a step of applying a pressure (line pressure) of less than 10kgf/mm (for example, less than 5 kgf/mm). "solid" refers to the condition where the interior of the particle is filled and there are substantially no voids. More specifically, in the present specification, the solid state means that the area ratio of voids excluding the voids in the particles with respect to the entire area of the particles in the cross section of the particles observed in an SEM image obtained using a Scanning Electron Microscope (SEM) is 95% or more (for example, 96% or more, typically 98% or more). The "main component" refers to a component having the largest content, and is, for example, a component contained in an amount of 50 mass% or more with respect to the total mass of the negative electrode active material. The "aspect ratio" refers to a value of a ratio a/B of a longest diameter a of a particle to a longest diameter B in a direction perpendicular to the diameter a in a cross section of the particle observed in an SEM image obtained using a scanning electron microscope.

One embodiment of the present invention is an electricity storage device including an electrode body including a negative electrode and a positive electrode, the negative electrode including a negative electrode substrate and a negative electrode active material layer containing a negative electrode active material and disposed along at least one surface of the negative electrode substrate, the negative electrode active material containing solid graphite particles as a main component, the solid graphite particles having an aspect ratio of 1 to 5, and the negative electrode active material layer having a density of 1.20g/cm³～1.55g/cm³。

The more the negative electrode active material layer is pressed by a roll press or the like, the higher the density of the negative electrode active material layer. In other words, when the density of the anode active material layer is small, the pressure applied to the anode active material layer is small. In the storage element, the density of the negative electrode active material layer containing solid graphite particles as a main component was 1.20g/cm³～1.55g/cm³The negative electrode active material layer is in a state of no pressure or less applied thereto. Therefore, the graphite particles themselves have a small residual stress, and uneven expansion of the negative electrode due to release of the residual stress can be suppressed. Further, since the graphite particles are solid, the density in the graphite particles is uniform, and the graphite particles are made to be nearly spherical by the aspect ratio of 1 to 5, so that current concentration is less likely to occur, and therefore, uneven expansion of the negative electrode can be suppressed. Further, since the graphite particles are nearly spherical as described above, the graphite particles incorporated in the active material layer have low orientation and are likely to be randomly oriented, and thus uneven expansion of the negative electrode can be suppressed. In addition, adjacent graphite particles are less likely to be locked to each other due to the proximity of the spherical shape,the graphite particles slide relative to each other appropriately, and even if the graphite particles expand, they are easily maintained in a state close to the closest packing. As described above, in the present embodiment, even if the graphite particles expand, the graphite particles expand relatively uniformly, and the negative electrode active material layer having a high filling ratio of the graphite particles is maintained by appropriate mutual sliding, and as a result, it is estimated that expansion of the negative electrode occurring at the initial charging can be suppressed. The negative electrode active material layer of the energy storage device contains solid graphite particles having an aspect ratio of 1 to 5 as a main component. Since such graphite particles have few voids in the particle itself, the particle shape is not easily deformed and is close to a spherical shape, and therefore adjacent graphite particles are not easily caught by each other, and the graphite particles are easily packed densely. Therefore, the density of the negative electrode active material layer of the storage element containing the graphite particles can be set within the above range even when the pressure applied to the negative electrode active material layer is not high or low.

One embodiment of the present invention is an electricity storage device including an electrode body including a negative electrode and a positive electrode, the negative electrode including a negative electrode substrate and a negative electrode active material layer containing a negative electrode active material and disposed along at least one surface of the negative electrode substrate, the negative electrode active material containing solid graphite particles as a main component, the solid graphite particles having an aspect ratio of 1 to 5, and a ratio R2/R1 of a surface roughness R2 of the negative electrode substrate in a region where the negative electrode active material layer is not disposed to a surface roughness R1 of the negative electrode substrate in a region where the negative electrode active material layer is disposed is 0.90 or more.

The region where the anode active material layer is formed becomes coarser as the anode substrate is subjected to pressure, and therefore R2/R1 becomes smaller. In other words, when the negative electrode substrate is not pressurized, the surface roughness is almost equal between the region where the negative electrode active material layer is disposed and the region where the negative electrode active material layer is not disposed (so-called exposed region of the negative electrode substrate). That is, R2/R1 becomes close to 1. In this energy storage element, the above-mentioned R2/R1 is 0.90 or more, and the pressure applied to the negative electrode active material layer is not applied or is small. Therefore, the graphite particles themselves have a small residual stress, and uneven expansion of the negative electrode due to release of the residual stress can be suppressed. Further, since the graphite particles are solid, the density in the graphite particles is uniform, and the graphite particles are made to be nearly spherical by the aspect ratio of 1 to 5, so that current concentration is less likely to occur, and therefore, uneven expansion of the negative electrode can be suppressed. Further, since the graphite particles are nearly spherical as described above, the graphite particles incorporated in the active material layer have low orientation and are likely to be randomly oriented, and thus uneven expansion of the negative electrode can be suppressed. Further, since the graphite particles are close to spherical, adjacent graphite particles are less likely to be caught, and the graphite particles slide relative to each other appropriately, so that even if the graphite particles expand, the graphite particles are easily maintained in a state close to the closest packing. As described above, in the present embodiment, even if the graphite particles expand, the graphite particles expand relatively uniformly, and the negative electrode active material layer having a high filling ratio of the graphite particles is maintained by appropriate mutual sliding, and as a result, it is estimated that expansion of the negative electrode occurring at the initial charging can be suppressed.

Preferably, the negative electrode active material further contains non-graphitizable carbon. By further containing the non-graphitizable carbon as the negative electrode active material, an energy storage device having a high effect of suppressing expansion of the negative electrode occurring at the initial charge can be obtained.

Preferably, the electricity storage device includes an electrode body in which the negative electrode and the positive electrode are wound in a stacked state, and the electrode body has a hollow region in a central portion. By providing the electrode body with the hollow region in the central portion, it is possible to suppress peeling of the active material layer due to bending of the negative electrode or the positive electrode present at a position close to the central portion, and by the energy storage element, it is possible to obtain an energy storage element which has a high effect of suppressing expansion of the negative electrode occurring at the initial charging, and which can suppress uneven charging and discharging due to an increase in the inter-electrode distance which has conventionally occurred in the electrode body having the hollow region.

The electric storage element preferably includes: and a pressure-sensitive cutting mechanism for cutting off the electrical connection between the negative electrode and the positive electrode when the internal pressure rises to a predetermined pressure, or a pressure-sensitive short-circuiting mechanism for electrically short-circuiting the negative electrode and the positive electrode outside the electrode body. If the power storage element is overcharged or the electrolyte is decomposed, the internal pressure or temperature may be greatly increased to such an extent that the power storage element cannot exhibit the required charge/discharge performance. Therefore, in the electric storage device, in order to further improve safety, the following means have been provided: the mechanism is a pressure-sensitive cutting mechanism for cutting the electrical connection between the negative electrode and the positive electrode by, for example, reversing the separator when the internal pressure rises due to overcharge or the like, or a pressure-sensitive short-circuiting mechanism for electrically short-circuiting the negative electrode and the positive electrode. However, in these mechanisms, if the amount of expansion of the electrode plate increases, the internal pressure of the power storage element increases, and the mechanism may be activated early. The electric storage device has a mechanism for cutting off the electrical connection between the negative electrode and the positive electrode or a mechanism for electrically short-circuiting the negative electrode and the positive electrode outside the electrode body, thereby further improving safety, and the electric storage device has a high effect of suppressing the expansion of the negative electrode occurring at the time of initial charging, and therefore, the mechanism can be suppressed from being activated too early.

The electric storage element preferably includes a case that houses the electrode assembly and has an inner surface that is in direct or indirect contact with an outer surface of the electrode assembly, and a pressure member that presses the case from outside. Since the effect of the electric storage element on suppressing expansion of the negative electrode occurring during initial charging is high, there is a possibility that the electrode body moves in the case by reducing the frictional force against the inner surface of the case due to expansion of the electrode body. The electric storage element is provided with a pressurizing member for pressurizing the case from the outside, so that the frictional force between the case and the electrode body is increased, and the holding capability of the electrode body can be improved.

Another embodiment of the present invention is a method for manufacturing an electric storage device, including the steps of: preparing a negative electrode in which a negative electrode active material layer containing a negative electrode active material is disposed along at least one surface of a negative electrode base material, a positive electrode in which a positive electrode active material layer containing a positive electrode active material is disposed along one surface of a positive electrode base material, and laminating the negative electrode and the positive electrode; wherein the negative electrode active material contains solid graphite particles, the aspect ratio of the solid graphite particles is 1 to 5, and the negative electrode active material layer is not compressed before the negative electrode and the positive electrode are laminated. According to this method for producing an energy storage element, the negative electrode active material layer is not compressed before the negative electrode and the positive electrode are laminated, and thus an energy storage element having a high effect of suppressing expansion of the negative electrode occurring during initial charging can be produced.

Hereinafter, the power storage element according to the present embodiment will be described in detail with reference to the drawings.

< storage element >

[ embodiment 1 ]

Hereinafter, a nonaqueous electrolyte electricity storage element as a secondary battery will be described as an example of the electricity storage element. A nonaqueous electrolyte electricity storage element is provided with an electrode body, a nonaqueous electrolyte, and a case that houses the electrode body and the nonaqueous electrolyte. The electrode body has a negative electrode and a positive electrode. The electrode body is generally formed as a wound electrode body in which a positive electrode and a negative electrode laminated with a separator interposed therebetween are wound, or as a laminated electrode in which a positive electrode and a negative electrode are alternately laminated with a separator interposed therebetween. The nonaqueous electrolyte is interposed between the positive electrode and the negative electrode in a state of being impregnated in the separator.

[ negative electrode ]

The negative electrode has a negative electrode substrate and a negative electrode active material layer. The negative electrode active material layer contains a negative electrode active material and is disposed along at least one surface of the negative electrode substrate. The anode active material layer according to embodiment 1 of the present invention is disposed in an uncompressed state.

(negative electrode substrate)

The negative electrode substrate is a substrate having conductivity. As a material of the negative electrode substrate, a metal such as copper, nickel, stainless steel, nickel-plated steel, or an alloy thereof can be used, and copper or a copper alloy is preferable. The form of the negative electrode substrate includes foil, vapor-deposited film, and the like, and foil is preferable from the viewpoint of cost. That is, copper foil is preferable as the negative electrode base material. Examples of the copper foil include rolled copper foil and electrolytic copper foil. The term "electrically conductive" means that the volume resistivity measured in accordance with JIS-H0505 (1975) is 1X 10⁷The term "non-conductive" means that the volume resistivity is more than 1X 10⁷Ω·cm。

The upper limit of the average thickness of the negative electrode base material may be, for example, 30 μm, preferably 20 μm, and more preferably 10 μm. When the average thickness of the negative electrode base material is not more than the upper limit, the energy density can be further improved. On the other hand, the lower limit of the average thickness may be, for example, 1 μm or 5 μm. The average thickness is an average value of thicknesses measured at 10 arbitrarily selected positions.

[ negative electrode active material layer ]

The negative electrode active material layer is disposed along at least one surface of the negative electrode substrate directly or with an intermediate layer interposed therebetween. The negative electrode active material layer is formed of a so-called negative electrode mixture containing a negative electrode active material. The negative electrode active material contains solid graphite particles as a main component. The negative electrode mixture contains optional components such as a conductive agent, a binder (binder), a thickener, and a filler, as required.

As the negative electrode active material, a material capable of occluding and releasing lithium ions is generally used. In the power storage element according to embodiment 1 of the present invention, the negative electrode active material contains solid graphite particles as a main component.

(solid graphite particles)

The solid graphite particles are graphite particles having substantially no voids filled in the particles. As described above, in the present specification, the solid graphite particles mean graphite particles in which the area ratio R of voids in the particles excluding the area of the entire particles in the cross section of the particles observed in an SEM image obtained using a scanning electron microscope is 95% or more. The area ratio R can be determined as follows.

(1) Preparation of measurement sample

The powder of the negative electrode active material particles to be measured is fixed with a thermosetting resin. The negative electrode active material particles fixed with the resin were exposed in cross section by using a cross-section polisher, to prepare a sample for measurement.

(2) Acquisition of SEM images

In obtaining the SEM image, JSM-7001F (manufactured by Nippon electronics Co., Ltd.) was used as a scanning electron microscope. The SEM image is an observed secondary electron image. The acceleration voltage was 15 kV. The observation magnification is set to a magnification of 3 to 15 negative electrode active material particles appearing in one visual field. The obtained SEM image was saved as an image file. Various conditions such as the beam spot diameter, the working distance, the irradiation current, the brightness, and the focal point are appropriately set so that the contour of the negative electrode active material particles becomes clear.

(3) Cutting of Rougo of negative active material particles

The outline of the negative active material particles was cut from the obtained SEM image using the image cutting function of the image editing software Adobe Photoshop Elements 11. The cutting of the contour is performed by selecting the outer side of the contour of the active material particles using a quick selection tool and editing the region other than the negative electrode active material particles to a black background. At this time, when the number of the negative electrode active material particles subjected to the contour cutting is less than 3, the SEM image is obtained again until the number of the negative electrode active material particles subjected to the contour cutting is 3 or more.

(4) Binarization processing

The image of the 1 st negative electrode active material particle out of the cut negative electrode active material particles was binarized by setting a density 20% lower than the density at which the intensity was maximized as a threshold value using image analysis software PopImaging 6.00. The area on the side where the density is low is calculated by the binarization processing, and this is taken as "the area excluding the voids in the particles S1".

Next, the same image of the 1 st negative electrode active material particle as above was subjected to binarization processing using the density 10 as a threshold value. The outer edge of the negative electrode active material particle was identified by binarization processing, and the area inside the outer edge was calculated to obtain "the entire particle area S0".

By calculating the ratio of S1 to S0 (i.e., S1/S0) using the above-calculated S1 and S0, the "area ratio R1 excluding voids in the particles with respect to the area of the entire particles" of the 1 st negative electrode active material particle was calculated.

The images of the 2 nd and subsequent negative electrode active material particles out of the cut negative electrode active material particles are also subjected to the binarization processing described above, and the areas S1 and S0 are calculated. Based on these calculated areas S1, S0, area ratios R2, R3, · of the respective negative electrode active material particles were calculated.

(5) Determination of the area ratio R

The average value of all the area ratios R1, R2, R3, · · · · calculated by the binarization processing was calculated, thereby specifying "the area ratio R of the negative electrode active material particles excluding the voids in the particles with respect to the area of the entire particles".

Graphite is a carbon substance having an average lattice spacing d (002) of (002) crystal planes of less than 0.340nm as measured by X-ray diffraction in a discharge state. The solid graphite particles preferably have a d (002) of less than 0.338 nm. The average lattice spacing d (002) of the solid graphite particles is preferably 0.335nm or more. The spherical solid graphite particles are preferably in the shape close to a true sphere, may be in the shape of an ellipse, an oval or the like, and may have irregularities on the surface. The solid graphite particles may comprise particles in which a plurality of solid graphite particles are aggregated.

The lower limit of the aspect ratio of the solid graphite particles is 1.0 (e.g., 1.5), preferably 2.0. In some embodiments, the aspect ratio of the solid graphite particles may be 2.2 or more (e.g., 2.5 or more). On the other hand, the upper limit of the aspect ratio of the solid graphite particles is 5.0 (e.g., 4.5), preferably 4.0. In some embodiments, the solid graphite particles may have an aspect ratio of 3.5 or less (e.g., 3.0 or less). When the aspect ratio of the solid graphite particles is in the above range, the graphite particles are made to be nearly spherical, and current concentration is less likely to occur, so that uneven expansion of the negative electrode can be suppressed.

As described above, "aspect ratio" refers to a value of a ratio a/B of the longest diameter a of a particle to the longest diameter B in a direction perpendicular to the diameter a in a cross section of the particle observed in an SEM image obtained using a scanning electron microscope. The aspect ratio may be determined as follows.

(1) Preparation of measurement sample

A measurement sample in which the cross section used for determining the area ratio R is exposed is used.

(2) Acquisition of SEM images

In obtaining the SEM image, JSM-7001F (manufactured by Nippon electronics Co., Ltd.) was used as a scanning electron microscope. The SEM image is an observed secondary electron image. The acceleration voltage was 15 kV. The observation magnification is set to a magnification of 100 to 1000 negative electrode active material particles appearing in one visual field. The obtained SEM image was saved as an image file. Various conditions such as the beam spot diameter, the working distance, the irradiation current, the brightness, and the focal point are appropriately set so that the contour of the negative electrode active material particles becomes clear.

(3) Determination of aspect ratio

From the obtained SEM images, 100 negative electrode active material particles were randomly selected, and the longest diameter a of the negative electrode active material particles and the longest diameter B in the direction perpendicular to the diameter a were measured, respectively, to calculate the a/B value. The aspect ratio of the negative electrode active material particles was determined by calculating the average of all the calculated a/B values.

The median diameter of the solid graphite particles is not particularly limited, and the upper limit is preferably 10 μm (for example, 8 μm), and more preferably 5 μm, from the viewpoint of improving the output of the energy storage device. For example, the median diameter of the solid graphite particles is preferably less than 5 μm, and more preferably 4.5 μm or less. In some embodiments, the median diameter of the solid graphite particles may be 4 μm or less, or may be 3.5 μm or less (e.g., 3 μm or less). The lower limit is preferably 1 μm, and more preferably 2 μm, from the viewpoint of ease of handling during production and production cost. The technique disclosed herein can be preferably implemented so that the median particle diameter of the solid graphite particles is 1 μm or more and less than 5 μm (further 1.5 to 4.5 μm, particularly 2 to 4 μm).

Preferred examples of the solid graphite particles disclosed herein include those having an aspect ratio of 1 to 5 and a median diameter of 10 μm or less; the length-width ratio is 11.2-4.5, and the median particle size is less than 5 mu m; an aspect ratio of 1.3 to 4 and a median particle diameter of 4.5 μm or less; solid graphite particles having an aspect ratio of 1.5 to 3.5 and a median diameter of 4 μm or less. By using such solid graphite particles having a small diameter and a nearly spherical shape, the above-described effects can be more effectively exhibited.

The "median particle diameter" is a value (D50) of 50% in a volume-based cumulative distribution calculated according to JIS-Z8819-2 (2001). Specifically, the measurement value can be obtained by the following method. The measurement was carried out using a laser diffraction particle size distribution measuring apparatus ("SALD-2200" by Shimadzu corporation) as the measuring apparatus and using Wing SALD-2200 as the measurement control software. In the scattering measurement mode, a dispersion liquid in which a measurement sample is dispersed in a dispersion solvent is circulated, and a laser beam is irradiated to the wet cell, thereby obtaining a scattered light distribution from the measurement sample. Then, the scattered light distribution was approximated by a lognormal distribution, and the particle diameter at which the cumulative degree of 50% was reached was regarded as the median particle diameter (D50).

The solid graphite particles can be suitably selected from various known graphite particles and used as graphite particles having a suitable aspect ratio and shape. Examples of such known graphite particles include artificial graphite particles and natural graphite particles. Here, the artificial graphite is a generic term for artificially produced graphite, and the natural graphite is a generic term for graphite collected from natural minerals. Specific examples of the natural graphite particles include flake graphite (scale graphite), block graphite, and soil graphite. The solid graphite particles may be flat flake-shaped natural graphite particles or spheroidized natural graphite particles obtained by spheroidizing the flake-shaped graphite. In a preferred embodiment, the solid graphite particles are artificial graphite particles. The above-described effects are more effectively exhibited by using solid artificial graphite particles. The solid graphite particles may be graphite particles having a coating layer (for example, an amorphous carbon coating layer) applied to the surface thereof.

The R value of the solid graphite particles may be substantially 0.25 or more (for example, 0.25 to 0.8). Here, the "R value" means the peak intensity (I) of the D band in the Raman spectrum_D1) Peak intensity with G band (I)_G1) Ratio of (I)_D1/I_G1). The solid graphite particles have an R value of, for example, 0.28 or more (e.g., 0.28 to 0.7), and typically 0.3 or more (e.g., 0.3 to 0.6). In some embodiments, the R value of the solid graphite particles may be 0.5 or less, or may be 0.4 or less.

Here, the "Raman spectrum" is obtained by using "HRRelease" from horiba Ltd under the conditions of 532nm wavelength (YAG laser), 600g/mm grating, and 100 times measurement magnification of 200cm^-1～4000cm^-1The range of (b) is obtained by raman spectroscopy. In addition, "peak intensity ratio of G band (I)_G1) Peak intensity ratio of "and" D band (I)_D1) "can be obtained by the following method. First, 4000cm of the obtained Raman spectrum was measured^-1The intensity at (b) is normalized by the maximum intensity (e.g., intensity of G band) in the above measurement range as a base intensity. Then, the obtained spectrum was fitted with a Lorentz function, and 1580cm was calculated^-1Nearby G-band and 1350cm^-1The intensity of each of the nearby D bands was defined as "peak intensity of G band (I)_G1) Peak intensity of the "and" D band (I)_D1)”。

The solid graphite particles preferably have a true density of 2.1g/cm³The above. By using such solid graphite particles having a high true density, the energy density can be further increased. On the other hand, the upper limit of the true density of the solid graphite particles is, for example, 2.5g/cm³. The true density is determined by gas-volumetric methods based on pycnometer using helium.

The lower limit of the content of the solid graphite particles with respect to the total mass of the negative electrode active material is preferably 60 mass%. In some embodiments, the content of the solid graphite particles with respect to the total mass of the negative electrode active material may be, for example, 70 mass% or more, or may be 80 mass%. When the negative electrode active material does not contain a negative electrode active material other than the solid graphite particles, the lower limit of the content of the solid graphite particles is preferably 90 mass%. By setting the content of the solid graphite particles to the lower limit or more, the charge and discharge efficiency can be further improved. On the other hand, the upper limit of the content of the solid graphite particles with respect to the total mass of the negative electrode active material may be, for example, 100 mass%.

The negative electrode active material disclosed herein may contain carbon particles other than the solid graphite particles. Examples of the carbon particles other than the solid graphite particles include hollow graphite particles and non-graphitic carbon particles. Examples of the non-graphitic carbon particles include hard-to-graphitize carbon particles and easy-to-graphitize carbon particles. Here, the "hard-to-graphitize carbon" refers to a carbon material having an average lattice spacing d (002) of (002) crystal planes of 0.36nm to 0.42nm as measured by X-ray diffraction method before charge and discharge or in a discharge state. The "graphitizable carbon" refers to a carbon material having an average lattice spacing d (002) of 0.34nm or more and less than 0.36 nm.

(hard-to-graphitize carbon)

When the negative electrode active material contains carbon particles other than the solid graphite particles, the carbon particles are preferably non-graphitizable carbon particles. The non-graphitizable carbon is generally a structure in which fine graphite crystals are arranged in random directions and a nano-order void is formed between a crystal layer and a crystal layer. The average particle size of the non-graphitizable carbon may be, for example, 1 to 10 μm, and is preferably 2 to 5 μm from the viewpoint of improving the filling property of the negative electrode active material in the negative electrode. The non-graphitizable carbon may be used alone in 1 kind, or may be used in combination of plural kinds.

When the negative electrode active material contains hard-to-graphitize carbon, the lower limit of the content of the hard-to-graphitize carbon with respect to the total mass of the negative electrode active material is preferably 5 mass%, and more preferably 10 mass%. On the other hand, the upper limit of the content of the non-graphitizable carbon with respect to the total mass of the negative electrode active material is preferably 40 mass%, and more preferably 30 mass%. When the content of the non-graphitizable carbon is in the above range, the porosity of the negative electrode can be reduced, and an energy storage device having a negative electrode with a high packing density of the active material can be obtained.

(other negative electrode active Material)

The negative electrode active material disclosed herein may contain a negative electrode active material made of a material other than the above-described carbon particles (i.e., solid graphite particles and carbon particles other than solid graphite particles). Examples of the negative electrode active material (hereinafter, also referred to as "non-carbonaceous active material") that may be contained in addition to the carbon particles include semimetals such as Si, metals such as Sn, oxides of these metals and semimetals, and composites of these metals, semimetals, and carbon materials. The content of the non-carbonaceous active material is, for example, preferably 30% by mass or less, more preferably 20% by mass or less, and still more preferably 10% by mass or less, of the total mass of the negative electrode active material. In some embodiments, the content of the non-carbonaceous active material in the total mass of the negative electrode active material may be 5 mass% or less (for example, 1 mass% or less, typically 0 mass%).

(other optional ingredients)

The solid graphite particles and the non-graphitizable carbon also have conductivity, and examples of the conductive agent include graphite such as metal, conductive ceramics, acetylene black, and carbon materials other than the non-graphitizable carbon.

Examples of the binder include elastomers such as ethylene-propylene-diene rubber (EPDM), sulfonated EPDM, Styrene Butadiene Rubber (SBR), and fluororubber; thermoplastic resins other than elastomers such as fluororesins (polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), etc.), polyethylene, polypropylene, polyimide, etc.; polysaccharide polymers, and the like.

Examples of the thickener include polysaccharide polymers such as carboxymethyl cellulose (CMC) and methyl cellulose. When the thickener has a functional group reactive with lithium, the functional group is preferably inactivated by methylation or the like.

The filler is not particularly limited. Examples of the main component of the filler include polyolefins such as polypropylene and polyethylene, silica, alumina, zeolite, and glass.

(intermediate layer)

The intermediate layer is a coating layer on the surface of the negative electrode base material, and contains conductive particles such as carbon particles, thereby reducing the contact resistance between the negative electrode base material and the negative electrode mixture layer. The intermediate layer may cover a part of the negative electrode substrate or the entire surface. The negative electrode substrate may have a region in which the intermediate layer is laminated and the negative electrode active material layer is not laminated. The intermediate layer is not particularly limited in its structure, and may be formed of, for example, a composition containing a resin binder and conductive particles. The term "conductive" means that the term "conductive" is based on JIS-H0505 (1975) and a volume resistivity of 1X 10⁷Omega cm or less.

The porosity of the negative electrode is preferably 40% or less. By setting the porosity of the negative electrode to 40% or less, the energy density of the energy storage device can be further improved. The porosity of the negative electrode is preferably 25% or more. The "porosity" of the negative electrode is a value based on volume, and is a calculated value calculated from the mass, the true density, and the thickness of the active material layer of the constituent components contained in the active material layer.

[ Positive electrode ]

The positive electrode has a positive electrode substrate and a positive electrode active material layer. The positive electrode active material layer contains a positive electrode active material and is disposed along at least one surface of the positive electrode substrate directly or with an intermediate layer interposed therebetween.

The positive electrode substrate has conductivity. As a material of the substrate, a metal such as aluminum, titanium, tantalum, stainless steel, or an alloy thereof can be used. Among them, aluminum and aluminum alloys are preferable in terms of balance between high potential resistance and conductivity and cost. The form of the positive electrode base material includes foil, vapor-deposited film, and the like, and foil is preferable from the viewpoint of cost. That is, as the positive electrode substrate, aluminum foil is preferable. Examples of the aluminum or aluminum alloy include a1085P and a3003P defined in JIS-H4000 (2014).

The positive electrode active material layer is formed of a so-called positive electrode mixture containing a positive electrode active material. The positive electrode mixture forming the positive electrode active material layer contains optional components such as a conductive agent, a binder (binder), a thickener, and a filler, as necessary.

Examples of the positive electrode active material include a lithium metal composite oxide and a polyanion compound. Examples of the lithium metal composite oxide include Li_xMO_y(M represents at least one transition metal), specifically, there may be mentioned α -NaFeO having a layered structure₂Li of type crystal structure_xCoO₂、Li_xNiO₂、Li_xMnO₃、Li_xNiαCO_(1-α)O₂、Li_xNi_αMn_βCO_(1-α-β)O₂Etc. Li having a spinel-type crystal structure_xMn₂O₄、Li_xNi_αMn_(2-α)O₄And the like. The polyanion compound includes, for example, Li_wMe_x(XO_y)_z(Me represents at least one transition metal, X represents, for example, P, Si, B, V, etc.), and specific examples thereof include LiFePO₄、LiMnPO₄、LiNiPO₄、LiCoPO₄、Li₃V₂(PO₄)₃、Li₂MnSiO₄、Li₂CoPO₄F, and the like. The elements or polyanions in these compounds may have a portion replaced with other elements or anionic species. In the positive electrode active material layer, 1 of these compounds may be used alone, or 2 or more of these compounds may be mixed and used.

The conductive agent is not particularly limited as long as it is a conductive material. Examples of such a conductive agent include carbon black such as graphite, furnace black, acetylene black, and ketjen black, metal, and conductive ceramics. Examples of the shape of the conductive agent include a powder shape and a fiber shape.

Examples of the binder (binder) include thermoplastic resins such as fluororesins (polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), etc.), polyethylene, polypropylene, polyimide, etc.; elastomers such as ethylene-propylene-diene rubber (EPDM), sulfonated EPDM, Styrene Butadiene Rubber (SBR), and fluororubber; polysaccharide polymers, and the like.

The filler is not particularly limited. Examples of the main component of the filler include polyolefins such as polypropylene and polyethylene, silica, alumina, zeolite, glass, and carbon.

The intermediate layer is a coating layer on the surface of the positive electrode base material, and contains conductive particles such as carbon particles to reduce the contact resistance between the positive electrode base material and the positive electrode active material layer. The intermediate layer may cover a part of the positive electrode substrate or the entire surface. The intermediate layer is not particularly limited in its structure, and may be formed of a composition containing a resin binder and conductive particles, for example, as in the negative electrode.

[ spacers ]

Examples of the material of the separator include woven fabric, nonwoven fabric, and porous resin film. Among these, a porous resin film is preferable from the viewpoint of strength, and a nonwoven fabric is preferable from the viewpoint of liquid retention of the nonaqueous electrolyte. As the main component of the separator, for example, polyolefin such as polyethylene and polypropylene is preferable from the viewpoint of strength, and for example, polyimide, aramid and the like are preferable from the viewpoint of oxidative decomposition resistance. These resins may be compounded.

An inorganic layer may be provided between the separator and the electrode (usually, the positive electrode). The inorganic layer is a porous layer also called a heat-resistant layer or the like. In addition, a separator in which an inorganic layer is formed on one surface of a porous resin film may be used. The inorganic layer is generally composed of inorganic particles and a binder, and may contain other components.

[ non-aqueous electrolyte ]

As the nonaqueous electrolyte, a known nonaqueous electrolyte generally used for a general nonaqueous electrolyte secondary battery (power storage element) can be used. The nonaqueous electrolyte includes a nonaqueous solvent and an electrolyte salt dissolved in the nonaqueous solvent. The nonaqueous electrolyte may be a solid electrolyte or the like.

As the nonaqueous solvent, a known nonaqueous solvent generally used as a nonaqueous solvent for a nonaqueous electrolyte for an electric storage element can be used. Examples of the nonaqueous solvent include cyclic carbonates, chain carbonates, esters, ethers, amides, sulfones, lactones, nitriles, and the like. Among them, at least a cyclic carbonate or a chain carbonate is preferably used, and a cyclic carbonate and a chain carbonate are more preferably used in combination. When the cyclic carbonate and the chain carbonate are used in combination, the volume ratio of the cyclic carbonate to the chain carbonate (cyclic carbonate: chain carbonate) is not particularly limited, and is, for example, preferably 5: 95-50: 50.

examples of the cyclic carbonate include Ethylene Carbonate (EC), Propylene Carbonate (PC), Butylene Carbonate (BC), Vinylene Carbonate (VC), Vinyl Ethylene Carbonate (VEC), chloroethylene carbonate, fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), styrene carbonate, catechol carbonate, 1-phenylenevinylene carbonate, 1, 2-diphenylvinylene carbonate, and the like, and EC is preferable.

Examples of the chain carbonate include diethyl carbonate (DEC), dimethyl carbonate (DMC), Ethyl Methyl Carbonate (EMC), diphenyl carbonate, and the like, and EMC is preferable.

As the electrolyte salt, a known electrolyte salt generally used as an electrolyte salt of a nonaqueous electrolyte for an electric storage element can be used. Examples of the electrolyte salt include a lithium salt, a sodium salt, a potassium salt, a magnesium salt, and an onium salt, and a lithium salt is preferable.

The lithium salt may be LiPF₆、LiPO₂F₂、LiBF₄、LiClO₄、LiN(SO₂F)₂Iso inorganic lithium salt, LiSO₃CF₃、LiN(SO₂CF₃)₂、LiN(SO₂C₂F₅)₂、LiN(SO₂CF₃)(SO₂C₄F₉)、LiC(SO₂CF₃)₃、LiC(SO₂C₂F₅)₃And lithium salts having a hydrocarbon group whose hydrogen is substituted with fluorine. Among them, inorganic lithium salt is preferable, and LiPF is more preferable₆。

The lower limit of the content of the electrolyte salt in the nonaqueous electrolyte is preferably 0.1M, more preferably 0.3M, still more preferably 0.5M, and particularly preferably 0.7M. On the other hand, the upper limit is not particularly limited, but is preferably 2.5M, more preferably 2M, and still more preferably 1.5M.

[ 2 nd embodiment ]

In the electric storage device according to embodiment 2 of the present invention, the negative electrode active material contains solid graphite particles as a main componentThe aspect ratio of the solid graphite particles is 1 to 5, and the density of the negative electrode active material layer is 1.20g/cm³～1.55g/cm³. The more the negative electrode active material layer is pressed by a roll press or the like, the higher the density of the negative electrode active material layer. In other words, when the density of the anode active material layer is small, the pressure applied to the anode active material layer is small. In the storage element, the density of the negative electrode active material layer containing solid graphite particles as a main component was 1.20g/cm³～1.55g/cm³The negative electrode active material layer is in a state of no pressure or less applied thereto. Therefore, the graphite particles themselves have a small residual stress, and uneven expansion of the negative electrode due to release of the residual stress can be suppressed. Further, since the graphite particles are solid, the density in the graphite particles is uniform, and the graphite particles are made to be nearly spherical by the aspect ratio of 1 to 5, so that current concentration is less likely to occur, and therefore, uneven expansion of the negative electrode can be suppressed. Further, since the graphite particles are nearly spherical as described above, the graphite particles incorporated in the active material layer have low orientation and are likely to be randomly oriented, and thus uneven expansion of the negative electrode can be suppressed. Further, since the graphite particles are close to spherical, adjacent graphite particles are less likely to be caught, and the graphite particles slide relative to each other appropriately, so that even if the graphite particles expand, the graphite particles are easily maintained in a state close to the closest packing. As described above, in the present embodiment, even if the graphite particles expand, the graphite particles expand relatively uniformly, and the negative electrode active material layer having a high filling ratio of the graphite particles is maintained by appropriate mutual sliding, and as a result, it is estimated that expansion of the negative electrode occurring at the initial charging can be suppressed. The negative electrode active material layer of the energy storage device contains solid graphite particles having an aspect ratio of 1 to 5 as a main component. Since such graphite particles have few voids in the particle itself, the particle shape is not easily deformed and is close to a spherical shape, and therefore adjacent graphite particles are not easily caught by each other, and the graphite particles are easily packed densely. Therefore, the density of the negative electrode active material layer of the storage element containing the graphite particles can be set within the above range even when the pressure applied to the negative electrode active material layer is not high or low.

The negative electrode active material contains a solid stoneWhen the ink particles are used as a main component, the lower limit of the density of the negative electrode active material layer is 1.20g/cm³Preferably 1.30g/cm³More preferably 1.40g/cm³. On the other hand, the upper limit of the density of the negative electrode active material layer is 1.55g/cm³Preferably 1.50g/cm³. For example, the density of the anode active material layer may be less than 1.50g/cm³(e.g., 1.49 g/cm)³Below). In some embodiments, the density of the anode active material layer may be 1.45g/cm³The following. When the negative electrode active material contains hard-to-graphite carbon in addition to the solid graphite particles as the main component, the lower limit of the density of the negative electrode active material layer is 1.20g/cm³Preferably 1.25g/cm³. On the other hand, the upper limit of the density of the negative electrode active material layer is 1.55g/cm³Preferably 1.45g/cm³More preferably 1.40g/cm³. When the density of the negative electrode active material layer is in the above range, a power storage element having a high effect of suppressing the expansion of the negative electrode occurring at the initial charge can be obtained.

[ embodiment 3]

In the electric storage device according to embodiment 3 of the present invention, the negative electrode active material contains solid graphite particles as a main component, the solid graphite particles have an aspect ratio of 1 to 5, and a ratio of a surface roughness R2 of the negative electrode substrate in a region where the negative electrode active material layer is not disposed (laminated) (exposed region) to a surface roughness R1 of the negative electrode substrate in a region where the negative electrode active material layer is disposed (laminated), that is, R2/R1, is 0.90 or more. The more the negative electrode substrate is laminated on the negative electrode substrate, the more the surface roughness R1 of the region where the negative electrode active material layer is formed becomes rough, and therefore the ratio R2/R1 to the surface roughness R2 of the region where the negative electrode active material layer is not disposed becomes smaller. In other words, when the negative electrode substrate is not pressurized, the surface roughness is substantially equal between the region where the negative electrode active material layer is disposed and the region where the negative electrode active material layer is not disposed (for example, when the negative electrode substrate has a portion where the negative electrode substrate is exposed, the exposed region of the negative electrode substrate). That is, R2/R1 are close to 1. In this energy storage device, the above-mentioned R2/R1 being 0.90 or more means that the pressure applied to the negative electrode active material layer in a state of being laminated on the negative electrode substrate is not high or low. Therefore, the graphite particles themselves have a small residual stress, and uneven expansion of the negative electrode due to release of the residual stress can be suppressed. Further, since the graphite particles are solid, the density in the graphite particles is uniform, and the graphite particles are made to be nearly spherical by the aspect ratio of 1 to 5, so that current concentration is less likely to occur, and therefore, uneven expansion of the negative electrode can be suppressed. Further, since the graphite particles are nearly spherical as described above, the graphite particles incorporated in the active material layer have low orientation and are likely to be randomly oriented, and thus uneven expansion of the negative electrode can be suppressed. Further, since the graphite particles are close to spherical, adjacent graphite particles are less likely to be caught, and the graphite particles slide relative to each other appropriately, so that even if the graphite particles expand, the graphite particles are easily maintained in a state close to the closest packing. As described above, in the present embodiment, even if the graphite particles expand, the graphite particles expand relatively uniformly, and the negative electrode active material layer having a high filling ratio of the graphite particles is maintained by appropriate mutual sliding, and as a result, it is estimated that expansion of the negative electrode occurring at the initial charging can be suppressed.

The "surface roughness" is a value measured by a laser microscope in accordance with JIS-B0601 (2013) for the center line roughness Ra of the surface of the substrate (the surface of the region where the active material layer is formed, from which the active material layer is removed). Specifically, the measurement value can be obtained by the following method.

First, when there is a portion where the negative electrode substrate is exposed in the negative electrode, the surface roughness of the portion is measured according to JIS-B0601 (2013) using a commercially available laser microscope (instrument name "VK-8510" manufactured by KEYENCE) as the surface roughness R2 of the region where the negative electrode active material layer is not disposed. In this case, as the measurement conditions, the measurement region (area) was 149. mu. m.times.112. mu.m (16688. mu.m)²) The measurement pitch was set to 0.1. mu.m. Next, the negative electrode was shaken by an ultrasonic cleaning machine to remove the negative electrode active material layer and other layers from the negative electrode base material. Use and upperThe surface roughness R1 of the region where the negative electrode active material layer was formed was measured by the same method as described above for the surface roughness of the exposed portion of the negative electrode base material. When the negative electrode has no exposed portion of the negative electrode substrate (for example, when the entire surface of the negative electrode substrate is covered with the intermediate layer), the surface roughness R2 of a region where the negative electrode active material layer is not disposed (for example, a region where the negative electrode active material layer is not disposed and covered with the intermediate layer) is also measured by the same method. The ultrasonic cleaning machine was used for shaking, and a benchtop ultrasonic cleaning machine "2510J-DTH" manufactured by brasson corporation was used, and the shaking was performed by dipping in water and shaking for 3 minutes, followed by dipping in ethanol and shaking for 1 minute.

The lower limit of the ratio of the surface roughness (R2/R1) is preferably 0.92, and more preferably 0.94, because the pressure applied to the anode active material layer is not high or low. On the other hand, the upper limit of the ratio of the surface roughness (R2/R1) is preferably 1.10, and more preferably 1.05.

[ concrete constitution of the Electricity storage element ]

Next, a specific configuration example of the power storage element according to one embodiment of the present invention will be described. Fig. 1 is a schematic exploded perspective view showing an electrode body and a case of a nonaqueous electrolyte electricity storage element as an electricity storage element according to an embodiment of the present invention. Fig. 2 is a schematic cross-sectional view of the nonaqueous electrolyte storage element in fig. 1. The nonaqueous electrolyte storage element 1 includes: the battery includes an electrode body 2, a positive electrode collector 4 'and a negative electrode collector 5' connected to both ends of the electrode body 2, respectively, and a case 3 housing these. The nonaqueous electrolyte electricity storage element 1 houses the electrode assembly 2 in the case 3, and the nonaqueous electrolyte is disposed in the case 3. The electrode assembly 2 is formed by winding a positive electrode 10 provided with a positive electrode active material and a negative electrode 12 provided with a negative electrode active material in a flat shape with a separator 11 interposed therebetween. In the present embodiment, the winding axial direction of the electrode body 2 is defined as the Z-axis direction, and the long axis direction of a cross section perpendicular to the Z-axis of the electrode body 2 is defined as the X-axis direction. The direction orthogonal to the Z axis and the X axis is the Y axis.

An exposed region of the positive electrode substrate on which the positive electrode active material layer is not formed is formed at one end of the positive electrode 10 in one direction. In addition, an exposed region of the negative electrode substrate on which the negative electrode active material layer is not formed is formed at one end of the negative electrode 12 in one direction. The positive electrode current collector 4 'is electrically connected to the exposed region of the positive electrode base material by clamping with a clip, welding, or the like, and the negative electrode current collector 5' is electrically connected to the exposed region of the negative electrode base material in the same manner. The positive electrode 10 is electrically connected to the positive electrode terminal 4 via a positive electrode current collector 4 ', and the negative electrode 12 is electrically connected to the negative electrode terminal 5 via a negative electrode current collector 5'.

(case)

The case 3 is a rectangular parallelepiped case that accommodates the electrode body 2, the positive electrode collector 4 ', and the negative electrode collector 5' and has a surface (upper surface) on one side perpendicular to the second direction (X direction) opened. Specifically, the housing 3 has a bottom surface, a pair of long side surfaces facing the third direction (Y direction), and a pair of short side surfaces facing the first direction (Z direction). Also, the inner surface of the case 3 is in direct contact with the outer surface (typically, separator) of the electrode body 2. The case 3 may be provided with a separator, a sheet, or the like interposed between the electrode body 2 and the case. The material of the separator, sheet, or the like is not particularly limited as long as it has insulation properties. When the case 3 is provided with a separator, a sheet, or the like, the inner surface of the case 3 indirectly contacts the outer surface of the electrode body 2 via the separator, the sheet, or the like.

The upper surface of the housing 3 is covered with a cover 6. The case 3 and the cover 6 are made of a metal plate. As a material of the metal plate, for example, aluminum can be used.

The lid 6 is provided with a positive electrode terminal 4 and a negative electrode terminal 5 to which external power is supplied. The positive electrode terminal 4 is connected to a positive electrode current collector 4 ', and the negative electrode terminal 5 is connected to a negative electrode current collector 5'. When the power storage element is a nonaqueous electrolyte power storage element, a nonaqueous electrolyte (electrolytic solution) is injected into the case 3 through an injection hole (not shown) provided in the lid 6.

(electrode body)

The electrode body 2 includes a positive electrode 10, a negative electrode 12, and a separator 11 for insulating them, and is formed by alternately stacking the positive electrode 10 and the negative electrode with the separator 11 interposed therebetween. The electrode body 2 is a wound electrode body in which a sheet-like body including a positive electrode 10, a negative electrode 12, and a separator is wound in a flat shape.

The electrode body 2 preferably has a hollow region in the central portion 8. In the case where the electrode body 2 is formed by winding the positive electrode 10 and the negative electrode 12 around a winding core with the separator 11 interposed therebetween, the winding core preferably has a hollow internal structure or a hollow region in the central portion 8 of the electrode body by having a gap in a portion thereof not in close contact with the outer surface of the winding core. When a wound electrode body wound in a state in which a negative electrode and a positive electrode are stacked has a hollow portion in which an electrode plate or a separator, which is the innermost circumference of the electrode body, is not present, when the negative electrode expands, a part of the negative electrode may move into the hollow portion. In particular, the negative electrode near the inner peripheral portion moves toward the hollow portion because of the proximity to the hollow portion, and as a result, a portion in which the inter-electrode distance between the positive electrode and the negative electrode is increased may be generated. Such behavior on the inner peripheral side of the wound electrode assembly is likely to occur because the electrode assembly is less likely to move toward the outer peripheral side when the outer peripheral surface of the electrode assembly contacts the inner surface of the case. In this way, when a portion where the inter-electrode distance between the positive electrode and the negative electrode increases occurs, the resistance of the portion where the inter-electrode distance is increased becomes large, and the charge and discharge reaction is less likely to occur. It is predicted that charge/discharge unevenness occurs in the vicinity of the inner peripheral portion due to such concentration of charge/discharge reactions. Whether or not the above-described charge/discharge unevenness occurs can be observed by examining whether or not there is unevenness in the color of the negative electrode active material layer in the inner peripheral portion of the negative electrode plate obtained by disassembling the electric storage element. In this case, it is considered that the discolored portion of the negative electrode active material layer causes charge/discharge unevenness due to an increase in the inter-electrode distance when the entire width direction (short side direction) of the negative electrode plate extends.

By providing the electrode body 2 with a hollow region in the central portion 8, it is possible to suppress peeling of the active material layer due to bending of the negative electrode or the positive electrode present at a position close to the central portion 8, and by providing the energy storage element with a high effect of suppressing expansion of the negative electrode occurring at the time of initial charging, it is possible to obtain an energy storage element capable of suppressing charge/discharge unevenness due to an increase in the inter-electrode distance, which has conventionally occurred in an electrode body having a hollow region.

The material of the winding core is not particularly limited as long as it has insulation properties and is stable in an electrolyte solution. Examples of the material of the winding core include polyethylene and polypropylene.

(pressure-sensitive electric connection disconnection mechanism and pressure-sensitive electric short-circuit mechanism)

The energy storage device preferably includes a pressure-sensitive cutting mechanism for cutting off electrical connection between the negative electrode and the positive electrode when the internal pressure rises to a predetermined pressure (preferably, a pressure of 0.2MPa to 1.0MPa or less) or a pressure-sensitive short-circuiting mechanism for electrically short-circuiting the negative electrode and the positive electrode outside the electrode body. When the power storage element is overcharged or the electrolyte is decomposed, the internal pressure or temperature may be greatly increased to such an extent that the power storage element cannot exhibit the charge/discharge performance required of the power storage element. Therefore, in the conventional electric storage device, when the internal pressure rises due to overcharge or the like, for example, a mechanism for electrically disconnecting the negative electrode and the positive electrode by reversing the separator or electrically short-circuiting the negative electrode and the positive electrode outside the electrode body has been provided, thereby further improving safety. However, in these mechanisms, when the amount of expansion of the electrode plate increases, the internal pressure of the power storage element increases, and the mechanisms may be activated early. The electric storage device has a mechanism for cutting off the electric connection between the negative electrode and the positive electrode or a mechanism for electrically short-circuiting the negative electrode and the positive electrode outside the electrode body, thereby further improving the safety, and the electric storage device has a high effect of suppressing the expansion of the negative electrode generated at the time of initial charging, thereby suppressing the premature activation of the mechanism.

These mechanisms operate by putting a compound that promotes gas generation at the time of temperature rise or voltage rise into the electrolyte solution in advance, and increasing the internal pressure of the battery when overcharge or the like occurs.

The pressure-sensitive electrical connection/disconnection mechanism includes, for example, an electrically conductive path provided between the positive electrode and the positive electrode terminal, an electrically conductive path provided between the negative electrode and the negative electrode terminal, and the like. When the pressure-sensitive electrical connection interrupting mechanism is operated, the charging current does not flow, and therefore, an increase in the voltage of the power storage element can be suppressed, and safety during overcharging can be further improved. In the case of the pressure-sensitive electrical connection interrupting mechanism, for example, when the internal pressure of the power storage element rises due to overcharge of the power storage element, the current is interrupted by the floating of the central portion of the separator to interrupt the conductive path. This prevents further charging of the storage element during overcharge.

The pressure-sensitive electrical short-circuiting means is provided, for example, outside the electrode body (for example, the negative electrode collector 5'). In the pressure-sensitive electrical short-circuiting mechanism, when the electrical storage device is in an overcharged state and the pressure inside the electrical storage device is equal to or greater than a predetermined value, the central portion of the metal separator floats, and the positive electrode and the negative electrode are short-circuited due to the contact of the separator with the conductive member. This can prevent the charging current from flowing into the electrode body. Since the short circuit occurs outside the electrode body, it is possible to suppress a temperature increase in the storage element due to a heat generation reaction of the active material layer, as in the case where a short circuit occurs inside the electrode body. This further improves the safety of the battery when the battery reaches an overcharged state.

(pressing means)

The power storage element preferably includes a pressurizing member that pressurizes the case 3 from the outside. Since this electricity storage device has a high effect of suppressing the expansion of the negative electrode occurring during initial charging, the frictional force against the inner surface of the case due to the expansion of the silver electrode body is reduced, and the electrode body may move in the case. By providing the power storage element with the pressure member that presses the case from the outside, the frictional force between the case and the electrode assembly is increased, and the holding capability of the electrode assembly can be improved.

Examples of the pressing member include a restraint band attached to the outer periphery of the housing, and a metal frame.

When graphite is used as the negative electrode active material, the storage element has a high effect of suppressing the expansion of the negative electrode during initial charging.

< method for manufacturing electric storage device >

A method for manufacturing an electric storage device according to an embodiment of the present invention includes: the method includes preparing a negative electrode in which a negative electrode active material layer containing a negative electrode active material is disposed along at least one surface of a negative electrode base material, preparing a positive electrode in which a positive electrode active material layer containing a positive electrode active material is disposed along one surface of a positive electrode base material, and laminating the negative electrode and the positive electrode.

In the step of preparing the negative electrode, the negative electrode mixture may be applied to the negative electrode substrate to dispose the negative electrode active material layer containing the negative electrode active material along at least one surface of the negative electrode substrate. Specifically, the negative electrode active material layer is disposed by applying a negative electrode mixture to a negative electrode substrate and drying the negative electrode mixture. As described above, the negative electrode active material contains solid graphite particles, and the aspect ratio of the solid graphite particles is 1 to 5.

The negative electrode mixture may be a negative electrode mixture paste containing a dispersion medium in addition to the above-mentioned optional components. As the dispersion medium, for example, an aqueous solvent such as water or a mixed solvent mainly containing water; organic solvents such as N-methylpyrrolidone and toluene.

In the step of preparing the positive electrode, the positive electrode mixture may be applied to the positive electrode substrate so that the positive electrode active material layer containing the positive electrode active material is disposed along one surface of the positive electrode substrate. Specifically, the positive electrode active material layer is disposed by applying a positive electrode mixture to a positive electrode substrate and drying the same. The conditions for drying may be the same as in the above-described negative electrode active material layer forming step. The positive electrode mixture may be a positive electrode mixture paste containing a dispersion medium in addition to the above-mentioned optional components. The dispersion medium may be arbitrarily selected from the dispersion media exemplified for the negative electrode mixture.

The negative electrode and the positive electrode are stacked with a separator interposed therebetween to form an electrode body. The negative electrode is not pressed with the negative electrode active material layer before the negative electrode and the positive electrode are laminated. The positive electrode may be pressed by using a roll press or the like.

In addition to the above steps, for example, the method includes a step of housing an electrode body in a case and a step of injecting the nonaqueous electrolyte into the case. The implantation can be performed by a known method. After the injection, the nonaqueous electrolyte storage element can be obtained by sealing the injection port. The details of each element constituting the nonaqueous electrolyte storage element obtained by the manufacturing method are as described above.

According to the method for producing an energy storage element, the negative electrode active material layer is not compressed before the negative electrode and the positive electrode are laminated, whereby an energy storage element having a high effect of suppressing expansion of the negative electrode occurring at the time of initial charging can be produced.

[ other embodiments ]

The power storage element of the present invention is not limited to the above-described embodiments.

In the above embodiment, the description has been mainly given of the form in which the power storage element is a nonaqueous electrolyte secondary battery, but other power storage elements may be used. Examples of the other electric storage element include a capacitor (an electric double layer capacitor, a lithium ion capacitor), and the like. The nonaqueous electrolyte secondary battery includes a lithium ion nonaqueous electrolyte secondary battery.

In the above embodiment, a wound electrode body is used, but a laminated electrode body formed by a laminate in which a plurality of sheet bodies including a positive electrode, a negative electrode, and a separator are stacked may be provided.

The present invention can also be realized as a power storage device including a plurality of the power storage elements. Further, a single or a plurality of the electric storage elements (battery cells) of the present invention may be used to form an electric storage unit, and the electric storage unit may be further used to form an electric storage device. In this case, the technique of the present invention may be applied to at least one power storage element included in the power storage unit or the power storage device. The power storage device can be used as a power source for automobiles such as Electric Vehicles (EV), Hybrid Electric Vehicles (HEV), plug-in hybrid electric vehicles (PHEV), and the like. The power storage device may be used in various power supply devices such as an engine operation power supply device, an auxiliary power supply device, and an Uninterruptible Power Supply (UPS).

Fig. 3 shows an example of a power storage device 30 in which power storage cells 20, which are a group of two or more electrically connected power storage elements 1, are further grouped together. Power storage device 30 may include a bus bar (not shown) that electrically connects two or more power storage elements 1, and a bus bar (not shown) that electrically connects two or more power storage cells 20. Power storage unit 20 or power storage device 30 may include a state monitoring device (not shown) that monitors the state of one or more power storage elements.

Examples

The present invention will be described more specifically with reference to the following examples, but the present invention is not limited to the following examples.

Negative electrodes of examples 1 to 2 and comparative examples 1 to 6

A coating liquid (negative electrode mixture paste) containing a negative electrode active material having a composition shown in table 1, styrene-butadiene rubber as a binder, and carboxymethyl cellulose as a thickener and water as a dispersion medium was prepared. The ratio of the negative electrode active material, the binder, and the thickener was 97.4: 2.0: 0.6. the coating liquid was applied to both sides of a copper foil substrate (surface roughness 0.74 μm) having a thickness of 8 μm, and dried to form a negative electrode active material layer, thereby obtaining negative electrodes of examples 1 to 2 and comparative examples 1 to 6. The physical properties of the negative electrode active material and the presence or absence of the pressurizing step are shown in table 1. In examples 1 and 2, solid graphite having an R value of 0.30 was used. In comparative examples 1 to 3, hollow graphite having an R value of 0.21 was used. The amount of the negative electrode mixture (negative electrode mixture obtained by evaporating the dispersion medium from the negative electrode mixture paste) per unit area of the dried one surface was 1.55g/100cm². Further, the press was performed by using a roll press machine so that the pressure (line pressure) of example 2 was less than 10kgf/mm and the pressure (line pressure) of comparative examples 1,2, 4 and 6 was 40kgf/mm or more.

(calculation of the area ratio R of the negative electrode active material particles excluding the voids in the particles)

(1) Preparation of measurement sample

(2) Acquisition of SEM images

In obtaining the SEM image, JSM-7001F (manufactured by Nippon electronics Co., Ltd.) was used as a scanning electron microscope. The SEM image was obtained under the condition of observing a secondary electron image. The acceleration voltage was 15 kV. The observation magnification is set to a magnification of 3 to 15 negative electrode active material particles appearing in one visual field. The obtained SEM image was saved as an image file. Various conditions such as the beam spot diameter, the working distance, the irradiation current, the brightness, and the focal point are appropriately set so that the contour of the negative electrode active material particles becomes clear.

(3) Cutting of Rougo of negative active material particles

The contour of the negative active material particles was cut from the obtained SEM image using the image cutting function of the image editing software Adobe Photoshop Elements 11. The cutting of the contour is performed by selecting the outer side of the contour of the active material particles using a quick selection tool and editing the region other than the negative electrode active material particles to a black background. Next, the images of all the negative electrode active material particles subjected to the contour cutting are subjected to binarization processing. At this time, when the number of the negative electrode active material particles subjected to the contour cutting is less than 3, SEM images are again obtained, and the contour cutting of the negative electrode active material particles is performed until the number of the negative electrode active material particles subjected to the contour cutting is 3 or more.

(4) Binarization processing

The image of the 1 st negative electrode active material particle out of the cut negative electrode active material particles was binarized by setting a density 20% lower than the maximum intensity as a threshold value using image analysis software PopImaging 6.00. The area on the side where the density is low is calculated by the binarization processing, and this is taken as "the area excluding the voids in the particles S1".

By calculating the ratio of S1 to S0 (S1/S0) using the calculated S1 and S0, the "area ratio R1 of voids in the particles excluding the area of the entire particles" in the 1 st anode active material particle was calculated.

The images of the 2 nd and subsequent negative electrode active material particles out of the cut negative electrode active material particles are also subjected to the binarization processing described above, and the areas S1 and S0 are calculated. Based on the calculated areas S1, S0, the area ratios R2, R3, · of the respective negative electrode active material particles were calculated.

(5) Determination of the Crystal area Rate R

(determination of aspect ratio)

(1) Preparation of measurement sample

The measurement sample with the exposed cross section used for determining the area ratio R was used.

(2) Acquisition of SEM images

In obtaining the SEM image, JSM-7001F (manufactured by Nippon electronics Co., Ltd.) was used as a scanning electron microscope. The SEM image was obtained under the condition of observing a secondary electron image. The acceleration voltage was 15 kV. The observation magnification is set to a magnification of 100 to 1000 negative electrode active material particles appearing in one visual field. The obtained SEM image was saved as an image file. Various conditions such as the beam spot diameter, the working distance, the irradiation current, the brightness, and the focal point are appropriately set so that the contour of the negative electrode active material particles becomes clear.

(3) Determination of aspect ratio

100 negative electrode active material particles were randomly selected from the obtained SEM images, and the longest diameter a of the negative electrode active material particles and the longest diameter B in the direction perpendicular to the diameter a were measured, respectively, to calculate the a/B value. The aspect ratio of the negative electrode active material particles was determined by calculating the average of all the calculated a/B values.

(Density of negative electrode active material layer)

Density of negative electrode active material layer when coating amount of negative electrode mixture (g/100 cm)²) When W is taken as the value and T is taken as the thickness (cm) of the negative electrode active material layer before charge and discharge, which will be described later, the value can be calculated by the following formula.

Density (g/cm) of negative electrode active material layer³)＝W/(T×100)

(ratio of surface roughness of negative electrode base Material)

As described above, the surface roughness R1 of the region where the negative electrode active material layer was formed and the surface roughness R2 of the exposed portion of the negative electrode substrate in the negative electrode were measured using a laser microscope. Then, using the measured R1 and R2, the ratio of the surface roughness of the negative electrode base material (R2/R1) was calculated. Here, when the surface roughness R1 of the region where the negative electrode active material layer was formed was measured, the negative electrode active material layer was removed by ultrasonic cleaning in water for 3 minutes and ultrasonic cleaning in ethanol for 1 minute using a desktop ultrasonic cleaning machine 2510J-DTH manufactured by brasson corporation.

Negative electrodes of examples 3 to 6

Negative electrodes of examples 2 to 6 were obtained in the same manner as in example 1, except that the compositions of the negative electrode active materials were set as shown in tables 1 and 2. In each of examples 3 to 6, the same graphite as that used in example 1 (area ratio 99.1%, aspect ratio 2.7) was used. Table 2 shows the presence or absence of the pressing step, the density of the negative electrode active material layer, and the ratio (R2/R1) of the surface roughness of the negative electrode substrate.

[ production of energy storage devices according to examples 7 to 8 and comparative examples 7 to 8 ]

The negative electrodes shown in table 3, positive electrodes described later, and polyethylene separators having a thickness of 20 μm were wound in a laminated state, thereby producing energy storage devices of examples 7 to 8 and comparative examples 7 to 8. For the positive electrode, a positive electrode containing LiNI as a positive electrode active material was prepared_1/3CO_1/3Mn_1/3O₂Coating material comprising polyvinylidene fluoride (PVDF) as binder and acetylene black as conductive agent and N-methyl-2-pyrrolidone (NMP) as dispersion mediumLiquid (positive electrode mixture paste). The ratio of the positive electrode active material, the binder and the conductive agent is 94: 3: 3. the coating liquid was applied to both sides of an aluminum foil substrate having a thickness of 12 μm, and dried and pressed to form a positive electrode active material layer. The amount of the positive electrode mixture (the positive electrode mixture obtained by evaporating the dispersion medium from the positive electrode mixture paste) per unit area of the dried one surface was 2.1g/100cm²。

In example 7, comparative example 7, and comparative example 8, the wound electrode body was produced by arranging a core formed by welding a polypropylene resin sheet having a thickness of 0.3mm in a state of being wound in a racetrack shape at the center. In example 8, in the wound element, instead of disposing no hollow core, the winding start portion was loosened to form an electrode body in which a hollow region having a thickness of 0.5mm was formed in the center of the electrode body. In all of the power storage elements of examples 7 to 8 and comparative examples 7 to 8, the outer peripheral surface of the wound element was in contact with the inner surface of the battery case via the insulating sheet. In the thickness (mm) of the hollow region in the central portion of the electrode body in the electric storage elements of examples 7 to 8 and comparative examples 7 to 8, the thickness of the hollow portion when a core is present substantially corresponds to the thickness of the inner side of the core excluding the thickness of the resin sheet. The electrode assembly is wound so that its cross section becomes an elongated circular shape (see fig. 2). The thickness of the hollow region refers to the length of the hollow region in the thickness direction (Y-axis direction in fig. 2) of the electrode body.

In table 1, the evaluation results of the negative electrodes of examples 1 to 2 and comparative examples 1 to 6 are shown in table 1, and the evaluation results of the negative electrodes of example 1 and examples 3 to 6 are shown in table 2. Table 3 shows evaluation results of the energy storage devices of examples 7 to 8 and comparative examples 7 to 8.

[ evaluation ]

(measurement of thickness of negative electrode active material layer before Charge/discharge)

Samples having an area of 2cm × 1cm of the negative electrode before 10 electric storage device fabrication were prepared as measurement samples, and the thickness of the negative electrode was measured using a high-precision micrometer manufactured by Mitutoyo corporation. The thickness of the negative electrode active material layer before charging and discharging of one negative electrode was measured by measuring the thickness of the negative electrode at 5 positions, respectively, and subtracting the thickness of the copper foil base material by 8 μm from the average value. The average value of the thicknesses of the negative electrode active material layers before charge and discharge measured for 10 negative electrodes was calculated as the thickness of the negative electrode active material layer before charge and discharge.

(measurement of porosity of negative electrode active material layer)

As described above, the "porosity" is a value based on volume and is a calculated value calculated from the mass, the true density, and the thickness of the active material layer of the constituent components contained in the active material layer. Specifically, it is calculated by the following equation.

Porosity (%) { 1- (density of anode active material layer/true density of anode active material layer) } × 100

Here, the "density of the negative electrode active material layer" (g/cm)³) As described above, the coating amount W of the negative electrode mixture and the thickness T of the negative electrode active material layer before charge and discharge are calculated.

"true density of negative electrode active material layer" (g/cm)³) The actual density of each constituent component contained in the negative electrode active material layer and the mass of each constituent component were calculated. Specifically, the true density of the negative electrode active material was D1 (g/cm)³) The true density of the adhesive was D2 (g/cm)³) The true density of the tackifier was D3 (g/cm)³) When the mass of the negative electrode active material contained in 1g of the negative electrode mixture is W1(g), the mass of the binder contained in 1g of the negative electrode mixture is W2(g), and the mass of the thickener contained in 1g of the negative electrode mixture is W3(g), the following equation is calculated.

True density (g/cm) of negative electrode active material layer³)＝1/{(W1/D1)+(W2/D2)+(W3/D3)}

(measurement of thickness of negative electrode active material layer at full charge)

For the measurement of the thickness of the negative electrode active material layer at the time of full charge, the storage element at the time of full charge was disassembled in a glove box filled with argon having a dew point value of-60 ℃ or lower, and the negative electrode after DMC cleaning was used as a sample for measurement, and this was dividedThe measurement was performed in the same manner as the measurement of the thickness of the negative electrode active material layer before charge and discharge. In the case of full charge, the current density of the electric storage element before charge and discharge in examples and comparative examples was 2mA/cm²The charge termination current density was 0.04mA/cm²And a state in which constant-current constant-voltage charging is performed under the condition that the upper limit voltage is 4.25V.

(measurement of swelling amount of negative electrode active Material at initial Charge)

The amount of swelling of the negative electrode active material during initial charging is calculated by subtracting "the thickness of the negative electrode active material layer before charge and discharge" from "the thickness of the negative electrode active material layer during full charge" calculated by the above method.

(uneven charging and discharging after charging and discharging test)

The prepared electric storage device was subjected to a charge/discharge test under conditions of an atmosphere having an upper limit voltage of 4.15V and a lower limit voltage of 2.75V at 60 ℃, and then constant current discharge was performed until the voltage reached 2.75V. The electric storage element was disassembled, and the negative electrode active material layer on the inner peripheral portion (portion adjacent to the hollow region or the winding core in the state of the electrode body) of the negative electrode plate taken out was visually confirmed, and as a result, a whitened region was observed. When the region where the discoloration is observed extends over the entire width of the negative electrode plate, it is evaluated that charge/discharge unevenness is observed.

From table 1, it can be seen that: the negative electrode active material layer was disposed in an unpressed state and had a density of 1.20g/cm³～1.55g/cm³And the solid graphite particles as the negative electrode active material have an aspect ratio of 1 to 5 and a surface roughness ratio R2/R1 of 0.90 or more, and examples 1 to 2 are excellent in the effect of suppressing the swelling amount of the negative electrode active material layer at the initial charging.

On the other hand, comparative examples 1 and 1 in which the negative electrode active material layer was disposed in a compressed state and the ratio of the surface roughness of the negative electrode base material R2/R1 was less than 0.902. In comparative example 4 and comparative example 6, the swelling amount of the negative electrode active material at the initial charge was significantly increased as compared with examples 1 to 2. The negative electrode active material layer is disposed in an uncompressed state, the ratio of the surface roughness of the negative electrode substrate R2/R1 is 0.90 or more, and the density of the negative electrode active material layer is less than 1.20g/cm³In comparative examples 3 and 5, the negative electrode active material swelling amount at the initial charging of the negative electrode active material layer was also increased as compared with examples 1 to 2.

In addition, as for the porosity of the anode active material layer, when comparing example 1, comparative example 3, and comparative example 5 in which the anode active material layer was arranged in an uncompressed state, it can be seen that: in example 1, the porosity was small although the negative electrode active material was disposed in an uncompressed state, and the filling ratio of the negative electrode active material could be increased.

As is clear from table 2, examples 3 to 6 in which the negative electrode active material contains solid graphite particles and non-graphitizable carbon have an effect of suppressing the amount of swelling of the negative electrode active material layer at the initial charging, and also have an effect of improving the filling factor of the negative electrode active material by reducing the porosity even if the negative electrode active material layer is not compressed, as in example 1 in which the negative electrode active material contains solid graphite particles. In view of the effect of suppressing the amount of swelling of the negative electrode active material layer during initial charging and the effect of improving the filling rate of the negative electrode active material, the mass ratio of the non-graphitizable carbon to the total mass of the negative electrode active material is preferably 15 to 35 mass%, and more preferably 20 to 30 mass%.

[ Table 3]

From table 3, it can be seen that: the energy storage devices of examples 7 to 8, in which the electrode body in which the negative electrode and the positive electrode having the same composition as in example 1 were wound in a laminated state had a hollow region in the central portion, were different from the energy storage devices of comparative examples 7 to 8, in which the same form of the negative electrode having the same composition as in comparative examples 2 and 6 was provided, and no charge/discharge unevenness was observed after the charge/discharge test. From these results, it is estimated that since expansion of the negative electrode of the storage elements according to examples 7 to 8 is relatively suppressed, the increase in the distance between the positive electrode and the negative electrode due to the movement of the negative electrode plate toward the hollow portion as described above is less likely to occur, and therefore, uneven charging and discharging is not observed.

As described above, it was found that the storage device has a high effect of suppressing the expansion of the negative electrode during initial charging when graphite is used as the negative electrode active material.

Industrial applicability

The present invention is preferably used as an electric storage element represented by a nonaqueous electrolyte secondary battery used as a power source for electronic devices such as personal computers and communication terminals, automobiles, and the like.

Description of the symbols

1 electric storage element

2 electrode body

3 case

4 positive terminal

4' positive electrode current collector

5 negative electrode terminal

5' negative electrode current collector

6 cover

8 center part

10 positive electrode

11 spacer

12 negative electrode

20 electric storage unit

30 electric storage device

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Energy storage element and method for manufacturing energy storage element

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