阅读说明：本技术 负极和包含所述负极的二次电池 (Negative electrode and secondary battery comprising same ) 是由 金睿邻 柳正宇 金泰坤 金荣宰 于 2020-06-19 设计创作，主要内容包括：本发明涉及一种负极,所述负极包含：负极集电器；和形成在所述负极集电器上的负极活性材料层,其中所述负极活性材料层包含硅类负极活性材料、粘合剂、导电材料和单壁碳纳米管,并且所述单壁碳纳米管在所述负极活性材料层中的含量为0.001重量％～1重量％。(The present invention relates to an anode comprising: a negative electrode current collector; and an anode active material layer formed on the anode current collector, wherein the anode active material layer includes a silicon-based anode active material, a binder, a conductive material, and single-walled carbon nanotubes, and the content of the single-walled carbon nanotubes in the anode active material layer is 0.001 wt% to 1 wt%.)

1. An anode, comprising:

a negative electrode current collector; and

an anode active material layer formed on the anode current collector,

wherein the negative electrode active material layer contains a silicon-based negative electrode active material, a binder, a conductive material, and single-walled carbon nanotubes, and

the content of the single-walled carbon nanotube in the negative electrode active material layer is 0.001-1 wt%.

2. The negative electrode according to claim 1, wherein the single-walled carbon nanotube has a D/G value represented by the following formula 1 of 0.15 or less in Raman spectrum,

[ formula 1]

D/G ═ D band peak intensity/G band peak intensity.

3. The negative electrode of claim 1, wherein the single-walled carbon nanotubes have an average length of 3 μ ι η or more.

4. The anode according to claim 1, wherein a content of the silicon-based anode active material in the anode active material layer is 50 to 90 wt%.

5. The anode of claim 1, wherein the silicon-based anode active material is Si.

6. The anode according to claim 1, wherein the silicon-based anode active material has an average particle diameter (D) of 0.5 to 10 μm₅₀)。

7. The anode of claim 1, wherein the silicon-based anode active material and the single-walled carbon nanotubes are contained in the anode active material layer at a weight ratio of 50,000:1 to 90: 1.

8. The negative electrode according to claim 1, wherein the conductive material and the single-walled carbon nanotubes are contained in the negative electrode active material layer at a weight ratio of 500:1 to 5:1.

9. The anode according to claim 1, wherein the conductive material is at least one selected from the group consisting of: natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, conductive fiber, fluorocarbon, aluminum powder, nickel powder, zinc oxide, potassium titanate, titanium oxide, and polyphenylene derivative.

10. The anode according to claim 1, wherein the anode active material layer has a thickness of 5 μm to 40 μm.

11. A secondary battery, comprising:

the negative electrode as set forth in claim 1,

a positive electrode disposed to face the negative electrode;

a separator disposed between the negative electrode and the positive electrode; and

an electrolyte.

Technical Field

Cross Reference to Related Applications

The present application claims priority and benefit from korean patent application No. 10-2019-0078182, filed on 28.6.2019, the disclosure of which is incorporated herein by reference in its entirety.

The present invention relates to an anode and a secondary battery including the anode.

Background

Recently, with the rapid spread of electronic devices using batteries, such as mobile phones, notebook computers, and electric vehicles, the demand for secondary batteries having a small size, a light weight, and a relatively high capacity is rapidly increasing. In particular, lithium secondary batteries are in the spotlight as driving power sources for portable devices because they are lightweight and have high energy density. Accordingly, research and development efforts have been continuously made to improve the performance of lithium secondary batteries.

A lithium secondary battery generally includes a cathode, an anode, a separator provided between the cathode and the anode, an electrolyte, an organic solvent, and the like. In addition, in the cathode and the anode, an active material layer containing a cathode active material or an anode active material may be formed on the current collector. Typically, a lithium-containing metal oxide such as LiCoO₂、LiMn₂O₄Etc. are used as a cathode active material in a cathode, and a carbon-based material or a silicon-based material containing no lithium is used as an anode active material in an anode.

Among the anode active materials, particularly, the silicon-based anode active material receives much attention because its capacity is about 10 times that of the carbon-based anode active material, and has an advantage of being able to achieve high energy density even with a thin electrode because its capacity is high. However, the silicon-based anode active material has not been generally used due to the following problems: volume expansion occurs due to charging and discharging, and active material particles are broken/damaged due to the volume expansion, and thus, life characteristics are deteriorated.

In particular, in the case of a silicon-based active material, electrical disconnection occurs between the active materials due to volume expansion/contraction occurring upon charge and discharge, and thus lithium may not be smoothly inserted/extracted into/from the silicon-based active material, resulting in rapid deterioration of the life span of the silicon-based active material.

Therefore, it is required to develop a secondary battery having improved life characteristics while achieving high capacity and high energy density of a silicon-based anode active material.

Korean unexamined patent publication No. 10-2017-0074030 relates to an anode active material for a lithium secondary battery, a method of preparing the same, and a lithium secondary battery comprising the same, and discloses an anode active material comprising a porous silicon-carbon composite, but there is still a limitation in solving the above problems.

Documents of the prior art

[ patent document ]

Korean unexamined patent publication No. 10-2017-

Disclosure of Invention

[ problem ] to

The present invention aims to provide an anode using a silicon-based anode active material, whereby an electrical short between the active materials due to charge and discharge can be effectively prevented.

The present invention is also directed to a secondary battery comprising the above negative electrode.

[ solution ]

One aspect of the present invention provides an anode, including: a negative electrode current collector; and an anode active material layer formed on the anode current collector, wherein the anode active material layer includes a silicon-based anode active material, a binder, a conductive material, and single-walled carbon nanotubes, and the content of the single-walled carbon nanotubes in the anode active material layer is 0.001 wt% to 1 wt%.

Another aspect of the present invention provides a secondary battery including: the above negative electrode; a positive electrode disposed to face the negative electrode; a separator disposed between the negative electrode and the positive electrode; and an electrolyte.

[ advantageous effects ]

The anode according to the present invention includes a specific amount of single-walled carbon nanotubes in an anode active material layer when a silicon-based anode active material is used, and thus even when the silicon-based anode active material undergoes volume expansion due to charge and discharge, the single-walled carbon nanotubes can improve electrical connection between the active materials, thereby improving the life characteristics of the anode. In addition, since the electrical connection between the active materials can be easily maintained by the single-walled carbon nanotube, the anode according to the present invention is preferable from the viewpoint of initial efficiency and reduction in electrical resistance.

Drawings

Fig. 1 is a graph in which initial efficiencies according to examples and comparative examples are evaluated.

Fig. 2 is a graph evaluating the cycle capacity retention rates according to examples and comparative examples.

Fig. 3 is a graph in which the resistance increase rates according to the examples and comparative examples were evaluated.

Detailed Description

The terms and words used in the present specification and claims should not be construed as being limited to general meanings or dictionary meanings, but interpreted as having meanings and concepts consistent with the technical scope of the present invention on the basis of the principle that the inventor can appropriately define the concept of the term to describe the invention in the best way.

The terminology provided herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise.

It will be understood that the terms "comprises," "comprising," "includes" and/or "including," when used herein, specify the presence of stated features, integers, steps, operations, elements, components, and/or combinations thereof, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

In the present invention, the average particle diameter (D)₅₀) Can be defined as corresponding to the particle size at 50% cumulative volume in the particle size distribution curve. The average particle diameter (D) can be measured using, for example, a laser diffraction method₅₀). Laser diffraction methods are generally capable of measuring particle sizes ranging from submicron to several millimeters and can produce results with high reproducibility and high resolution.

Hereinafter, the present invention will be described in detail.

< negative electrode >

The invention provides a negative electrode, specifically a negative electrode for a lithium secondary battery.

The anode according to the present invention includes: a negative electrode current collector; and an anode active material layer formed on the anode current collector, wherein the anode active material layer includes a silicon-based anode active material, a binder, a conductive material, and single-walled carbon nanotubes, and the content of the single-walled carbon nanotubes in the anode active material layer is 0.001 wt% to 1 wt%.

Generally, silicon-based anode active materials are known to have a high capacity of about 10 times as high as that of carbon-based anode active materials. Therefore, when applied to an anode, even a low-thickness silicon-based anode active material is expected to realize a thin-film electrode having a high level of energy density. However, the silicon-based anode active material has the following problems: the lifetime deterioration is caused due to volume expansion/contraction occurring with lithium intercalation/deintercalation during charge and discharge. In particular, when the silicon-based anode active material undergoes volume expansion/contraction due to charge and discharge, electrical connection between the active materials deteriorates, and an electrical short occurs, resulting in rapid deterioration of the life of the anode.

In order to solve the problems, when a silicon-based anode active material is used, the anode according to the present invention includes 0.001 wt% to 1 wt% of single-walled carbon nanotubes (hereinafter, referred to as "SWCNTs") in an anode active material layer. Since the fiber length of the SWCNT is long, even when the silicon-based anode active material undergoes volume expansion due to charge and discharge, electrical connection between the active materials can be maintained, and thus effective improvement of the life characteristics of the anode, reduction in resistance, and improvement in initial efficiency can be achieved.

The anode current collector is not particularly limited as long as it does not cause chemical changes in the battery and has high conductivity. Specifically, as the anode current collector, there may be used: copper, stainless steel, aluminum, nickel, titanium, and calcined carbon; copper or stainless steel with surface treated with carbon, nickel, titanium, silver, etc.; aluminum-cadmium alloys, and the like.

The anode current collector may generally have a thickness of 3 to 100 μm, preferably 4 to 40 μm, to realize an anode having a low thickness.

The anode current collector may have fine protrusions and recesses formed on the surface thereof to improve adhesion of the anode active material. In addition, the anode current collector may be used in any of various forms such as a film, a sheet, a foil, a mesh, a porous material, a foam, a non-woven fabric, and the like.

The anode active material layer is formed on an anode current collector.

The negative active material layer includes a silicon-based negative active material, a binder, a conductive material, and SWCNTs.

The silicon-based negative active material may include a silicon oxide film composed of SiO_x(0≤x<2) The compound shown in the specification. Because of SiO₂Does not react with lithium ions, and thus cannot store lithium. Therefore, x is preferably within the above range.

Specifically, the silicon-based anode active material may be Si. Conventionally, Si has an advantage in that its capacity is silicon oxide (e.g., SiO)_x(0<x<2) About 2.5 to 3 times of Si), but there is a problem in that commercialization of Si is not easy because the degree of volume expansion/contraction of Si due to charge and discharge is very large compared to silicon oxide. On the other hand, according to the present invention, since a specific amount of SWCNTs is contained in the anode active material layer, the electrical connection and the conductive network between the active materials can be maintained even when the volume of Si expands, whereby the solution can be effectively solvedThe high capacity and the high energy density of the silicon-based anode active material are more preferably achieved due to the problem of the deterioration of the life characteristics of the silicon-based anode active material caused by the volume expansion.

Average particle diameter (D) of silicon-based negative electrode active material in view of ensuring structural stability of the active material during charge and discharge, reducing side reactions by reducing reaction area with electrolyte, and reducing manufacturing cost₅₀) It may be 0.5 to 10 μm, preferably 2 to 6 μm. Particularly, when the average particle diameter (D) is set₅₀) When the silicon-based negative electrode active material in the above range is used together with SWCNTs described later, the electrical connection between the negative electrode active materials can be stably maintained.

The content of the silicon-based anode active material in the anode active material layer may be 50 to 90 wt%, preferably 60 to 80 wt%, in view of sufficiently achieving a high capacity of the silicon-based anode active material in a secondary battery.

The binder may include at least one selected from the group consisting of: styrene Butadiene Rubber (SBR), acrylonitrile butadiene rubber, acrylic rubber, butyl rubber, fluororubber, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl alcohol (PVA), polyacrylic acid (PAA), polyethylene glycol (PEG), Polyacrylonitrile (PAN), and Polyacrylamide (PAM).

The binder may include at least one selected from the group consisting of: polyvinyl alcohol, polyacrylic acid, polyacrylonitrile and polyacrylamide, preferably comprising polyvinyl alcohol and polyacrylic acid. When the binder includes polyvinyl alcohol and polyacrylic acid, the polyvinyl alcohol and polyacrylic acid may be included in the binder in a weight ratio of 50:50 to 90:10, preferably 55:45 to 80:20, from the viewpoint of further enhancing the above effects.

The content of the binder in the anode active material layer may be 5 to 30 wt%, preferably 10 to 25 wt%. In view of more effectively controlling the volume expansion of the active material, it is preferable that the content of the binder is within the above range.

The conductive material may be used to improve the conductivity of the negative electrode, and it is preferable to use any conductive material that does not cause chemical changes and has conductivity. Specifically, the conductive material may include at least one selected from the group consisting of: natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, conductive fiber, fluorocarbon, aluminum powder, nickel powder, zinc oxide, potassium titanate, titanium oxide, and polyphenylene derivative, and carbon black is preferably contained in view of achieving high conductivity and excellent dispersibility.

Average particle diameter (D) of the conductive material₅₀) It may be 20nm to 60nm, preferably 25nm to 55 nm. From the viewpoint of promoting dispersion of the conductive material, improving the conductivity of the negative electrode, compensating for the low conductivity of the silicon-based negative electrode active material to improve the battery capacity, it is preferable that the average particle diameter of the conductive material is within the above range.

The content of the conductive material in the anode active material layer may be 3 to 20 wt%, preferably 5 to 15 wt%. When the content of the conductive material is within the above range, excellent conductivity can be exhibited, and the conductive material can complement the conductive network formed of the SWCNTs to improve the electrical connection between the active materials.

The SWCNT is a carbon nanotube having a single cylindrical wall and has a fiber shape. Compared to multi-walled carbon nanotubes (hereinafter, referred to as "MWCNTs"), SWCNTs have a long fiber length because they do not break during the growth of the tube, and also have a high graphitization degree and a high crystallinity degree.

Therefore, when the SWCNTs are included in the anode active material layer, the active material is effectively wrapped due to its long fiber length and high crystallinity, and thus, even when the active material undergoes volume expansion, electrical connection between the active materials can be stably maintained. Therefore, according to the anode of the present invention, an electrical short circuit caused by volume expansion of the active material and rapid deterioration of the life of the active material caused by the electrical short circuit can be effectively prevented, and the life characteristics of the anode can be improved. In addition, SWCNTs are preferable in view of reduction in resistance and improvement in efficiency because they easily maintain electrical connection between active materials even if the active materials undergo volume expansion/contraction.

The content of the SWCNT in the negative electrode active material layer is 0.001 wt% to 1 wt%. When the content of the SWCNTs is less than 0.001 wt%, it is difficult to effectively wrap the active materials or maintain electrical connection between the active materials. On the other hand, when the content of the SWCNTs is more than 1 wt%, an excessive amount of the SWCNTs causes an increase in side reactions with the electrolyte, and since the used amount of the dispersant for dispersing the SWCNTs is increased, the viscosity and elasticity of the anode slurry excessively increase, resulting in deterioration of workability in the manufacture of the anode.

The content of the SWCNT in the negative electrode active material layer is preferably 0.1 wt% to 0.5 wt%, and more preferably 0.2 wt% to 0.4 wt%. Within the above range, the electrical connection between the active materials can be improved, side reactions with the electrolyte can be reduced, the resistance of the negative electrode can be reduced, and the amount of the dispersant for dispersing the SWCNTs can be appropriately adjusted, whereby the negative electrode slurry can have a viscosity suitable for realizing a thin film negative electrode.

The SWCNTs may have an average length of 3 μm or more, preferably 4 μm or more, and more preferably 4.5 to 10 μm. From the viewpoint of maintaining a conductive network between the active materials and preventing aggregation and dispersion degradation due to excessively elongated SWCNTs, it is preferable that the average length of the SWCNTs is within the above range.

In the present specification, the average length of SWCNTs was measured as follows. A solution (containing 1 wt% solids based on the total weight of the solution) obtained by adding SWCNT and carboxymethylcellulose (CMC) to water at a weight ratio of 40:60 was diluted 1,000-fold in water. Thereafter, 20mL of the diluted solution was filtered through a filter, and the filter containing the SWCNTs filtered thereon was dried. One hundred Scanning Electron Microscope (SEM) images of the dried filters were taken, the lengths of the SWCNTs were measured using ImageJ program, and the average of the measured lengths was defined as the average length of the SWCNTs.

The SWCNTs may have an average diameter of 0.3nm to 5nm, preferably 0.5nm to 3.5 nm. From the viewpoint of reducing the resistance and improving the conductivity, the average diameter of the SWCNTs is preferably within the above range.

In the present specification, the average diameter of SWCNTs is measured as follows. A solution (containing 1 wt% solids based on the total weight of the solution) obtained by adding SWCNT and carboxymethylcellulose (CMC) to water at a weight ratio of 40:60 was diluted 1,000-fold in water. One drop of the dilution solution was dropped on a grid (grid) of a Transmission Electron Microscope (TEM), and the TEM grid was dried. The dried TEM grid was observed by a TEM device (H-7650 manufactured by Hitachi High-Tech Corporation) and the average diameter of the SWCNTs was measured.

The ratio of the average length of the SWCNTs to the average diameter thereof may be 1,000:1 or more, preferably 1,000:1 to 5,000: 1. The ratio is preferably within the above range from the viewpoint of improving the electrical conductivity of the SWCNTs and maintaining electrical connection even when the active material undergoes volume expansion/contraction.

The SWCNT may have a D/G value represented by the following formula 1 in a Raman spectrum of 0.15 or less, preferably 0.09 or less, more preferably 0.005 to 0.05, and still more preferably 0.01 to 0.03.

[ formula 1]

D/G ═ D band peak intensity/G band peak intensity

The D/G value can be used as an index indicating the crystallinity of the SWCNT. For example, as the D/G value is smaller (i.e., the G band peak intensity is higher), more similar properties to graphite are exhibited, and thus it can be determined that the SWCNT has higher crystallinity.

In the negative electrode according to the present invention, when the D/G value of the SWCNT is adjusted within the above range, crystallinity and conductivity can be improved, and charge transfer resistance (charge transfer resistance) of the negative electrode can be reduced. In particular, when the SWCNTs having the above D/G value are used together with a silicon-based anode active material (e.g., Si), a conductive network may be formed to enable smooth transfer of charges between the silicon-based anode active materials, the efficiency of the silicon-based anode active material may be improved through excellent conductivity and resistance reduction, and breakage of the CNTs, shortening of fiber lengths, insufficient formation of the conductive network, and the like, which may occur when the SWCNTs have a high D/G value, may be prevented.

In the present invention, the silicon-based negative electrode active material and the SWCNTs may be included in the negative electrode active material layer at a weight ratio of 50,000:1 to 90:1, preferably 5,000:1 to 150:1, more preferably 450:1 to 200: 1. Within the above range, the conductive network of SWCNTs can sufficiently wrap the anode active material, increase of side reaction with the electrolyte caused by an excessive amount of SWCNTs can be prevented, and by using the above content ratio, anode slurry having a desired level of viscosity and solid content can be prepared, thereby preferably realizing a thin film anode.

In the present invention, the conductive material and the SWCNTs may be included in the anode active material layer at a weight ratio of 500:1 to 5:1, preferably 300:1 to 10:1, more preferably 40:1 to 20: 1. Within the above range, the conductive material can complement the conductive network formed of the SWCNTs, thereby improving the electrical connection between the active materials, and can more preferably achieve the resistance reduction effect of the SWCNTs.

The negative active material layer may further include a thickener. When included in the anode active material layer, the thickener may improve dispersibility of the components. Also, when included in the anode slurry for preparing the anode active material layer, the thickener may improve the dispersibility of the components and enable the anode slurry to have a viscosity suitable for coating.

The thickener may be carboxymethyl cellulose (CMC).

The content of the thickener in the negative electrode active material layer may be 0.1 to 1.5 wt%, preferably 0.3 to 0.5 wt%.

The SWCNT described above can increase electrical connection of the silicon-based negative active material according to the negative active material layer, and can realize a thin film negative electrode having high energy density. Specifically, the thickness of the anode active material layer may be 5 to 40 μm, preferably 15 to 30 μm.

The anode may be manufactured by dispersing a silicon-based anode active material, a binder, a conductive material, SWCNTs, and an optional thickener in a solvent for forming an anode slurry to prepare an anode slurry and applying the anode slurry to an anode current collector, followed by drying and roll-pressing.

Specifically, the anode paste may be prepared by preparing a conductive material solution in which SWCNTs and a thickener are added to a solvent (e.g., distilled water) and adding a silicon-based anode active material, a binder, a conductive material, and the conductive material solution to the solvent for forming the anode paste. Since the negative electrode paste is prepared after preparing the conductive material solution in which the SWCNTs and the thickener are pre-dispersed, the dispersibility of the SWCNTs can be improved.

The conductive material solution may include SWCNTs and a thickener at a weight ratio of 20:80 to 50:50, preferably 35:65 to 45: 55. In this case, the SWCNTs can be smoothly dispersed.

The solvent for forming the anode slurry may include at least one selected from the group consisting of: distilled water, ethanol, methanol, and isopropanol, and preferably contains distilled water from the viewpoint of promoting dispersion of the components.

< Secondary Battery >

The present invention provides a secondary battery, specifically a lithium secondary battery, comprising the above-described anode.

Specifically, the secondary battery according to the present invention includes: the above negative electrode; a positive electrode disposed to face the negative electrode; a separator disposed between the negative electrode and the positive electrode; and an electrolyte.

The positive electrode may include a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector.

The positive electrode current collector is not particularly limited as long as it does not cause chemical changes in the battery and has high conductivity. Specifically, as the positive electrode current collector, there may be used: copper, stainless steel, aluminum, nickel, titanium, calcined carbon; copper or stainless steel whose surface is treated with carbon, nickel, titanium, silver, etc.; aluminum-cadmium alloys, and the like.

The positive electrode current collector may generally have a thickness of 3 to 500 μm.

The positive electrode current collector may have fine protrusions and recesses formed on the surface thereof to improve adhesion of the positive electrode active material. In addition, the cathode current collector may be used in any of various forms such as a film, a sheet, a foil, a mesh, a porous material, a foam, a non-woven fabric, and the like.

The positive electrode active material layer may include a positive electrode active material.

The positive active material may include a compound capable of reversibly intercalating and deintercalating lithium, specifically, a lithium-transition metal composite oxide including lithium and at least one transition metal selected from the group consisting of nickel, cobalt, manganese and aluminum, preferably a lithium-transition metal composite oxide including lithium and a transition metal including nickel, cobalt and manganese.

More specifically, the lithium-transition metal composite oxide may be a lithium-manganese-based oxide (e.g., LiMnO)₂、LiMn₂O₄Etc.), lithium-cobalt-based oxides (e.g., LiCoO)₂Etc.), lithium-nickel based oxides (e.g., LiNiO)₂Etc.), lithium-nickel-manganese-based oxides (e.g., LiNi)_1-YMn_YO₂(wherein 0)<Y<1)、LiMn_2-ZNi_ZO₄(wherein 0)<Z<2) Etc.), lithium-nickel-cobalt-based oxides (e.g., LiNi)_1-Y1Co_Y1O₂(wherein 0)<Y1<1) Etc.), lithium-manganese-cobalt-based oxides (e.g., LiCo)_1-Y2Mn_Y2O₂(wherein 0)<Y2<1)、LiMn_2-Z1Co_Z1O₄(wherein 0)<Z1<2) Etc.), lithium-nickel-manganese-cobalt-based oxides (e.g., Li (Ni)_pCo_qMn_r1)O₂(wherein 0)<p<1，0<q<1，0<r1<1，p+q+r1＝1)、Li(Ni_p1Co_q1Mn_r2)O₄(wherein 0)<p1<2，0<q1<2，0<r2<2, p1+ q1+ r2 ═ 2), or a lithium-nickel-cobalt-transition metal (M) oxide (e.g., Li (Ni)_p2Co_q2Mn_r3M_s2)O₂(wherein M is selected from the group consisting of Al, Fe, V, Cr, Ti, Ta, Mg and Mo, and p2, q2, r3 and s2 are each an atomic fraction of an element independent of each other, and 0<p2<1，0<q2<1，0<r3<1，0<s2<1, p2+ q2+ r3+ s2 ═ 1), etc.), which may be used alone or in combination of two or more thereof. Among the above, the lithium-transition metal composite oxide may be LiCoO from the viewpoint of improving the capacity characteristics and stability of the battery₂、LiMnO₂、LiNiO₂Lithium-nickel-manganese-cobalt oxides (e.g. Li (Ni)_0.6Mn_0.2Co_0.2)O₂、Li(Ni_0.5Mn_0.3Co_0.2)O₂、Li(Ni_0.7Mn_0.15Co_0.15)O₂、Li(Ni_0.8Mn_0.1Co_0.1)O₂Etc.) or lithium-nickel-cobalt-aluminum oxides (e.g., Li (Ni)_0.8Co_0.15Al_0.05)O₂Etc.). In addition, the lithium-transition metal composite oxide may be Li (Ni) in consideration of controlling the type and content ratio of elements constituting the lithium-transition metal composite oxide to achieve a significant improvement effect_0.6Mn_0.2Co_0.2)O₂、Li(Ni_0.5Mn_0.3Co_0.2)O₂、Li(Ni_0.7Mn_0.15Co_0.15)O₂、Li(Ni_0.8Mn_0.1Co_0.1)O₂Etc., which may be used alone or in combination of two or more thereof.

The content of the cathode active material in the cathode active material layer may be 80 to 99 wt%, preferably 92 to 98.5 wt%, in view of sufficiently exhibiting the capacity of the cathode active material.

The positive electrode active material layer may further include a binder and/or a conductive material, in addition to the above-described positive electrode active material.

The binder is used to assist in the bonding between the active material and the conductive material and the bonding to the current collector. Specifically, the binder may include at least one selected from the group consisting of: polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, Ethylene Propylene Diene Monomer (EPDM), sulfonated EPDM, styrene butadiene rubber, and fluororubber, preferably containing polyvinylidene fluoride.

The content of the binder in the positive electrode active material layer may be 1 to 20% by weight, preferably 1.2 to 10% by weight, from the viewpoint of sufficiently ensuring the bonding between components such as the positive electrode active material.

The conductive material may be used to impart conductivity to the secondary battery and improve conductivity, and is not particularly limited as long as it does not cause chemical changes and has conductivity. Specifically, the conductive material may include at least one selected from the group consisting of: graphite such as natural graphite, artificial graphite, etc.; carbon black-based materials such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, and the like; conductive fibers such as carbon fibers, metal fibers, and the like; conductive tubes such as carbon nanotubes and the like; a fluorocarbon compound; metal powders such as aluminum powder, nickel powder, etc.; conductive whiskers composed of zinc oxide, potassium titanate, or the like; conductive metal oxides such as titanium oxide and the like; and a polyphenylene derivative, and preferably contains carbon black from the viewpoint of improving conductivity.

The content of the conductive material in the positive electrode active material layer may be 1 to 20% by weight, preferably 1.2 to 10% by weight, from the viewpoint of sufficiently ensuring conductivity.

The thickness of the positive electrode active material layer may be 30 to 400 μm, preferably 50 to 110 μm.

The positive electrode may be manufactured by coating a positive electrode slurry including a positive electrode active material and optionally a binder, a conductive material, and a solvent for forming the positive electrode slurry onto a positive electrode current collector, followed by drying and roll-pressing.

When the cathode active material and optional binder and conductive material are included, the solvent for forming the cathode slurry may include an organic solvent such as N-methyl-2-pyrrolidone (NMP) or the like and may be used in an amount suitable for achieving a preferred viscosity. For example, the solvent used to form the cathode slurry may be included in the cathode slurry such that the amount of solid matter including the cathode active material and optional binder and conductive material is in the range of 50 to 95 wt%, preferably 70 to 90 wt%.

The separator serves to separate the anode and the cathode and provide a path for lithium ion migration, and any separator used as a separator in a typical lithium secondary battery may be used without limitation. In particular, a separator having low resistance to migration of electrolyte ions and excellent electrolyte impregnation ability is preferable. Specifically, as the separator, there can be used: porous polymer films such as those formed from polyolefin-based polymers such as ethylene homopolymers, propylene homopolymers, ethylene/butene copolymers, ethylene/hexene copolymers, ethylene/methacrylate copolymers, and the like; or have a stacked structure of two or more layers thereof. Further, as the separator, a common porous nonwoven fabric, for example, a nonwoven fabric made of high-melting glass fibers, polyethylene terephthalate fibers, or the like, may be used. In addition, in order to secure heat resistance or mechanical strength, a coated separator including a ceramic component or a polymer material and optionally having a single-layer or multi-layer structure may be used as the separator.

Examples of the electrolyte used in the present invention include organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, inorganic solid electrolytes, melt-type inorganic electrolytes, and the like, which can be used to manufacture secondary batteries, but the present invention is not limited thereto.

Specifically, the electrolyte may include an organic solvent and a lithium salt.

As the organic solvent, any organic solvent may be used without particular limitation so long as it can serve as a medium through which ions participating in the electrochemical reaction of the battery can migrate. Specifically, the organic solvent may be: ester solvents such as methyl acetate, ethyl acetate, gamma-butyrolactone, epsilon-caprolactone and the like; ether solvents such as dibutyl ether, tetrahydrofuran, etc.; ketone solvents such as cyclohexanone, etc.; aromatic hydrocarbon solvents such as benzene, fluorobenzene and the like; carbonate-based solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), Ethyl Methyl Carbonate (EMC), Ethylene Carbonate (EC), Propylene Carbonate (PC), and the like; alcohol solvents such as ethanol, isopropanol, and the like; nitriles such as R-CN (R is a C2-C20 hydrocarbon group having a linear, branched or cyclic structure and may contain a double bond, an aromatic ring or an ether bond), etc.; amides such as dimethylformamide and the like; dioxolanes such as 1, 3-dioxolane, and the like; or sulfolane. Among the above, preferred are carbonate-based solvents, and more preferred are mixtures of cyclic carbonate-based compounds (e.g., EC, PC, etc.) having high ion conductivity and high dielectric constant and linear carbonate-based compounds (e.g., EMC, DMC, DEC, etc.) having low viscosity, which can improve charge/discharge performance of a battery. In this case, when a mixture obtained by mixing the cyclic carbonate-based compound and the linear carbonate-based compound at a volume ratio of about 1:1 to about 1:9 is used, excellent electrolyte performance can be exhibited.

As the lithium salt, any compound may be used without particular limitation as long as it can provide lithium ions used in a lithium secondary battery. Specifically, the lithium salt may be LiPF₆、LiClO₄、LiAsF₆、LiBF₄、LiSbF₆、LiAlO₄、LiAlCl₄、LiCF₃SO₃、LiC₄F₉SO₃、LiN(C₂F₅SO₃)₂、LiN(C₂F₅SO₂)₂、LiN(CF₃SO₂)₂、LiCl、LiI、LiB(C₂O₄)₂And the like. The lithium salt is preferably used at a concentration of 0.1M to 2.0M. When the concentration of the lithium salt is within the above range, the electrolyte has appropriate levels of conductivity and viscosity, whereby excellent electrolyte performance can be exhibited and lithium ions can be efficiently transferred.

The secondary battery can be manufactured by a conventional method of manufacturing a secondary battery, that is, by disposing a separator between the above-described anode and cathode and injecting an electrolytic solution.

The secondary battery according to the present invention may be used in the following fields: portable devices such as mobile phones, notebook computers, digital cameras, and the like; and electric vehicles such as Hybrid Electric Vehicles (HEVs), etc., and are particularly preferably used as batteries constituting medium-and large-sized battery modules. Therefore, the present invention also provides a middle or large-sized battery module including the above-described secondary battery as a unit cell.

Of these, large-sized battery modules are preferably used as power sources for devices requiring high output and high capacity, such as electric vehicles, hybrid electric vehicles, power storage systems, and the like.

Hereinafter, the present invention will be described in detail with reference to examples so that those skilled in the art can easily practice the present invention. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein.

Examples

Example 1: manufacture of negative electrode

By mixing SWCNT with carboxymethyl cellulose (CMC, weight average molecular weight (M) as a thickener_w): 150,000) was dispersed in water at a weight ratio of 40:60 to prepare a conductive material solution.

A silicon-based negative electrode active material (Si, average particle diameter (D)) as a negative electrode active material₅₀): 3 μm), carbon black (average particle diameter (D) as a conductive material₅₀): 35nm, Super C65 manufactured by Imerys corporation, a binder, and a conductive material solution containing SWCNT and CMC were added to a solvent (distilled water) for forming a negative electrode paste to prepare a negative electrode paste (containing 30 wt% of solids based on the total weight of the negative electrode paste). In this case, the anode active material, the conductive material, the binder, the SWCNT and the CMC were mixed in a weight ratio of 70:7:22.25:0.30:0.45 in the anode paste.

As the binder, a polyvinyl alcohol (PVA)/Na-substituted polyacrylic acid (PAA) copolymer (hereinafter referred to as "PVA/PAA", Aquacharge manufactured by SUMITOMO SEIKA) was used.

The SWCNTs had an average length of 5 μm, an average diameter of 1.5nm and a D/G value (measured by Raman spectroscopy) of 0.02.

Mixing the negative electrode slurryAt 68mg/cm²(7.4mAh/cm²) The amount of the supporting substance was applied to a copper current collector (thickness: 15 μm), rolled, and dried in a vacuum oven at 130 ℃ for 10 hours to form a negative electrode active material layer (thickness: 21.5 μm), and the resultant was used as an anode according to example 1 (thickness: 36.5 μm).

Examples 2 to 4 and comparative examples 1 to 4

Negative electrodes according to examples 2 to 4 and comparative examples 1 to 4 were manufactured in the same manner as example 1, except that the kinds and contents of a negative electrode active material, CNT, a conductive material, a binder, and a dispersant were used as shown in table 1 below.

[ Table 1]

In table 1, the average length and average diameter of SWCNTs were measured by the following methods.

1) Average length

Each of the conductive material solutions prepared in examples 1 to 4 and comparative examples 2 to 4 was diluted 1,000 times in water. Thereafter, 20mL of the diluted solution was filtered through a filter, and the filter containing the SWCNTs filtered thereon was dried. One hundred Scanning Electron Microscope (SEM) images of the dried filters were taken, the lengths of the SWCNTs were measured using ImageJ program, and the average of the measured lengths was defined as the average length of the SWCNTs.

2) Average diameter

Each of the conductive material solutions prepared in examples 1 to 4 and comparative examples 2 to 4 was diluted 1,000 times in water. A drop of the dilution solution was dropped onto a grid of a TEM, which was then dried. The dried TEM grid was observed by a TEM device (H-7650 manufactured by Hitachi high tech Co., Ltd.), and the average diameter of CNTs was measured.

3) D/G value

The D/G values of CNTs used in examples 1 to 4 and comparative examples 2 to 4 were measured using a Raman spectrometer (FEX manufactured by NOST Co.).

Examples of the experiments

Experimental example 1: evaluation of initial Capacity and efficiency

< production of Secondary Battery >

As the positive electrode, lithium metal was used.

A polyethylene separator was interposed between each of the negative and positive electrodes manufactured in examples 1 to 4 and comparative examples 1 to 4, and an electrolyte was injected to manufacture coin-type half-cell secondary batteries. By adding vinylene carbonate in an amount of 3 wt% with respect to the total weight of the electrolyte to an organic solvent in which fluoroethylene carbonate (FEC) and dimethyl carbonate (DMC) were mixed at a volume ratio of 30:70 and adding LiPF as a lithium salt at a concentration of 1M₆An electrolyte was prepared.

< evaluation of initial Charge Capacity, initial discharge Capacity, and initial efficiency >

The initial charge capacity, initial discharge capacity and initial efficiency (initial discharge capacity/initial charge capacity) of the secondary batteries according to examples 1 to 4 and comparative examples 1 to 4 were evaluated using an electrochemical charge/discharge apparatus.

Initial charge capacity, initial discharge capacity and initial efficiency were measured by charging and discharging the secondary batteries according to examples 1 to 4 and comparative examples 1 to 4 under the following charge and discharge conditions. The results are shown in fig. 1 and table 2.

Charging conditions are as follows: 0.1C, CC/CV (1.5V, 0.05C cutoff)

Discharge conditions: 0.1C, CC (0.05V cut-off)

[ Table 2]

Referring to fig. 1 and table 2, it can be seen that the secondary batteries using the negative electrodes according to examples 1 to 4 are significantly more excellent in initial discharge capacity, initial charge capacity, and initial efficiency than the secondary batteries according to comparative examples 1 to 4.

Experimental example 2: evaluation of Life characteristics

< production of Secondary Battery >

LiNi to be used as a positive electrode active material_0.8Co_0.1Mn_0.1O₂And LiNiO₂The mixture of (1) in a weight ratio of 96:4, carbon black as a conductive material, and PVdF as a binder were added to an N-methyl-2-pyrrolidone (NMP) solvent in a weight ratio of 97:1.5:1.5 to prepare a positive electrode slurry. The positive electrode slurry was added at 458mg/cm²(3.7mAh/cm²) The amount of the supporting substance was applied to an aluminum current collector (thickness: 12 μm), rolled, and dried in a vacuum oven at 130 ℃ for 10 hours to form a positive electrode active material layer (thickness: 20.1 μm), and the resultant was used as a positive electrode (thickness: 32.1 μm).

A polyethylene separator was interposed between each of the negative and positive electrodes manufactured in examples 1 to 4 and comparative examples 1 to 4, and an electrolyte was injected to manufacture coin-type all-cell (full-cell) secondary batteries. By adding vinylene carbonate in an amount of 3 wt% with respect to the total weight of the electrolyte to an organic solvent in which FEC and DMC were mixed at a volume ratio of 30:70 and adding LiPF as a lithium salt at a concentration of 1M₆An electrolyte was prepared.

< evaluation of Capacity Retention >

The cycle capacity retention rates of the secondary batteries according to examples 1 to 4 and comparative examples 1 to 4 were evaluated using an electrochemical charge/discharge device.

The cycle capacity retention ratio was measured by charging and discharging the secondary battery under the charge/discharge condition of 0.5C/0.5C, 4.2V to 2.5V, 0.05C end, and was calculated by the following formula 2. The results are shown in fig. 2, and the capacity retention rate for 100 cycles is shown in table 3.

[ formula 2]

Cycle capacity retention rate (%) { (discharge capacity at the nth cycle)/(discharge capacity at the 1 st cycle) } × 100

(in the formula 2, N is an integer of 1 to 100.)

[ Table 3]

Referring to fig. 2 and table 3, it can be seen that the secondary batteries using the negative electrodes according to examples 1 to 4 exhibited significantly improved life characteristics as compared to the secondary batteries according to comparative examples 1 to 4.

Experimental example 3: evaluation of resistance increase rate

< production of Secondary Battery >

LiNi to be used as a positive electrode active material_0.8Co_0.1Mn_0.1O₂And LiNiO₂The mixture of (1) in a weight ratio of 96:4, carbon black as a conductive material, and PVdF as a binder were added to an NMP solvent in a weight ratio of 97:1.5:1.5 to prepare a positive electrode slurry. The positive electrode slurry was added at 458mg/cm²(3.7mAh/cm²) The amount of the supporting substance was applied to an aluminum current collector (thickness: 12 μm), rolled, and dried in a vacuum oven at 130 ℃ for 10 hours to form a positive electrode active material layer (thickness: 20.1 μm), and the resultant was used as a positive electrode (thickness: 32.1 μm).

A polyethylene separator was interposed between each of the negative and positive electrodes manufactured in examples 1 to 4 and comparative examples 1 to 4, and an electrolyte was injected to manufacture a pouch-type all-cell secondary battery. By adding vinylene carbonate in an amount of 3 wt% with respect to the total weight of the electrolyte to an organic solvent in which FEC and DMC were mixed at a volume ratio of 30:70 and adding LiPF as a lithium salt in a concentration of 1M₆An electrolyte was prepared.

< evaluation of the rate of increase in resistance >

The resistance increase rates of the secondary batteries according to examples 1 to 4 and comparative examples 1 to 4 were evaluated using an electrochemical charge/discharge device.

The secondary battery was charged and discharged for 100 cycles at the end of 0.5C/0.5C, 4.2V-2.5V, 0.05C, but an HPPC test was performed at 50% SOC every 20 cycles (C rate: 3C), whereby the resistance increase rate was measured. The resistance increase rate was calculated by the following formula 3, and the result thereof is shown in fig. 3. The resistance increase rate for 100 cycles is shown in table 4.

[ formula 3]

Cyclic resistance increase rate (%) { (resistance at N-th cycle)/(resistance at 1-th cycle) } × 100

(in formula 3, N is an integer of 1 to 100.)

[ Table 4]

Referring to fig. 3 and table 4, it can be seen that the secondary batteries using the negative electrodes according to examples 1 to 4 exhibited more excellent resistance reduction effects than the secondary batteries according to comparative examples 1 to 4.