阅读说明：本技术 钛酸锂烧结体板 (Lithium titanate sintered body plate ) 是由 由良幸信 冈田茂树 小林伸行 于 2018-05-09 设计创作，主要内容包括：本发明提供钛酸锂烧结体板,其锂离子传导性及电子传导性优异,并且,在作为负极组装于锂二次电池的情况下能够呈现出优异的高速充放电性能及高温低温动作性。本发明的钛酸锂烧结体板用于锂二次电池的负极。该钛酸锂烧结体板具有多个一次粒子结合得到的结构,且厚度为10～290μm,多个一次粒子的平均粒径、即一次粒径为0.70μm以下,气孔率为21～45％,开口气孔比率为60％以上,平均气孔纵横尺寸比为1.15以上,纵横尺寸比为1.30以上的气孔在全部气孔中所占的比例为30％以上,平均气孔径为0.70μm以下,体积基准D10及D90气孔径满足4.0≤D90/D10≤50的关系。(Provided is a lithium titanate sintered plate which has excellent lithium ion conductivity and electron conductivity, and which, when assembled as a negative electrode in a lithium secondary battery, can exhibit excellent high-speed charge/discharge performance and high-temperature/low-temperature operability. The lithium titanate sintered plate of the present invention is used for a negative electrode of a lithium secondary battery. The lithium titanate sintered plate has a structure in which a plurality of primary particles are bonded together, has a thickness of 10 to 290 [ mu ] m, has an average primary particle diameter of 0.70 [ mu ] m or less, has a porosity of 21 to 45%, has an open pore ratio of 60% or more, has an average pore aspect ratio of 1.15 or more, has a proportion of pores having an aspect ratio of 1.30 or more in all pores of 30% or more, has an average pore diameter of 0.70 [ mu ] m or less, and has a volume basis D10 and a volume basis D90 pore diameter satisfying a relationship of 4.0. ltoreq. D90/D10. ltoreq.50.)

1. A lithium titanate sintered body plate which is a lithium titanate sintered body plate used for a negative electrode of a lithium secondary battery,

the lithium titanate sintered body plate has a structure in which a plurality of primary particles are bonded together,

the thickness is 10 to 290 μm,

the average particle diameter of the plurality of primary particles, namely the primary particle diameter, is less than or equal to 1.2 mu m,

the porosity is 21 to 45%,

the ratio of open pores is 60% or more,

the average aspect ratio of pores is 1.15 or more,

the proportion of pores having an aspect ratio of 1.30 or more in all pores is 30% or more,

the average pore diameter is less than 0.70 μm,

the volume references D10 and D90 air hole diameters satisfy the relation that D90/D10 is less than or equal to 4.0 and less than or equal to 50.

2. The lithium titanate sintered body plate according to claim 1, wherein,

the average particle diameter of the plurality of primary particles, that is, the primary particle diameter, is 0.02 to 1.2,

the ratio of the open pores is 65-90%,

the average pore aspect ratio is 1.15 to 3.50,

the proportion of the pores having an aspect ratio of 1.30 or more in all pores is 30 to 90%,

the average pore diameter is 0.02-0.70 μm,

the volume references D10 and D90 air hole diameters satisfy the relation that D90/D10 is less than or equal to 4.5 and less than or equal to 50.

3. The lithium titanate sintered plate according to claim 1 or 2, wherein,

the thickness is 40-200 μm.

4. A lithium secondary battery is provided with:

a positive electrode, a negative electrode comprising the lithium titanate sintered plate according to any one of claims 1 to 3, and an electrolyte solution.

Technical Field

The present invention relates to a lithium titanate sintered plate used for a negative electrode of a lithium secondary battery.

Background

In recent years, lithium titanate Li has been used as a negative electrode material for lithium secondary batteries (also referred to as lithium ion secondary batteries)₄Ti₅O₁₂(LTO hereinafter) is attracting attention. LTO has the following advantages: when used as a negative electrode material for a lithium secondary battery, the negative electrode material is less susceptible to volume change associated with insertion/desorption of lithium ions, has a cycle life and safety superior to those of a carbon negative electrode, and has excellent low-temperature operability.

In addition, sintering LTO has been proposed to improve energy density and the like. That is, it has been proposed to use the LTO sintered compact as a positive electrode or a negative electrode of a lithium secondary battery. For example, patent document 1 (Japanese patent No. 5174283) discloses an LTO sintered body having an average pore diameter of 0.10 to 0.20 μm and an average pore diameter of 1.0 to 3.0m²A specific surface area per gram and a relative density of 80 to 90%, and contains titanium oxide crystal particles. Patent document 2 (jp 2002-42785 a) discloses an LTO sintered body having a filling rate of an active material of 50 to 80% and a thickness of more than 20 μm and 200 μm or less. Patent document 3 (jp 2015-185337 a) discloses an LTO sintered body having a relative density of 90% or more and a particle diameter of 50nm or more.

Disclosure of Invention

Generally, Lithium Titanate (LTO) has significantly lower electron conductivity and also lower ion conductivity than widely used lithium cobaltate. Therefore, when an electrode is manufactured by mixing LTO powder with a general binder and a conductive additive, powder having a small particle size is used. However, the negative electrode having such a configuration cannot obtain sufficient characteristics for increasing the energy density and realizing high-speed charge and discharge and high-temperature operation, which are required for IoT applications. In this regard, the LTO sintered bodies disclosed in patent documents 1 to 3 have good electron conductivity due to the increased densification by sintering, and are also suitable for high-temperature operation, but have poor lithium ion conductivity due to lack of pores that allow penetration of an electrolyte solution, and the rate characteristics may become insufficient.

The inventors of the present invention have recently found that: the LTO sintered plate satisfying the predetermined particle size and pore conditions is excellent in lithium ion conductivity and electron conductivity, and exhibits excellent high-rate charge/discharge performance and high-temperature operability when assembled as a negative electrode in a lithium secondary battery.

Accordingly, an object of the present invention is to provide an LTO sintered plate which is excellent in lithium ion conductivity and electron conductivity and which can exhibit excellent high-speed charge/discharge performance and high-temperature/low-temperature operability when assembled as a negative electrode in a lithium secondary battery.

According to an aspect of the present invention, there is provided a sintered LTO plate, which is a sintered Lithium Titanate (LTO) plate for a negative electrode of a lithium secondary battery, wherein,

the LTO sintered body plate has a structure in which a plurality of primary particles are bonded to each other,

the thickness is 10 to 290 μm,

the average particle diameter of the plurality of primary particles, namely the primary particle diameter, is less than or equal to 1.2 mu m,

the porosity is 21 to 45%,

the ratio of open pores is 60% or more,

the average aspect ratio of pores is 1.15 or more,

the proportion of pores having an aspect ratio of 1.30 or more in all pores is 30% or more, the average pore diameter is 0.70 μm or less,

the volume references D10 and D90 air hole diameters satisfy the relation that D90/D10 is less than or equal to 4.0 and less than or equal to 50.

According to one aspect of the present invention, there is provided a lithium secondary battery comprising: a positive electrode, a negative electrode comprising the LTO sintered body plate, and an electrolyte solution.

Detailed Description

Definition of

In the following, definitions for determining the parameters of the present invention are given.

In the present specification, "primary particle diameter" is: the average particle diameter of the primary particles constituting the LTO sintered body plate. The primary particle size can be measured by image analysis of a cross-sectional SEM image of the sintered body plate. For example, the sintered body plate is processed by a section polishing machine (CP) to expose the polished section. The polished cross section is observed with an SEM (scanning electron microscope) at a predetermined magnification (for example, 1000 times) and a predetermined field of view (for example, 125. mu. m.times.125. mu.m). At this time, the field of view is set so that 20 or more primary particles are present in the field of view. Circumscribed circles were drawn for all primary particles in the obtained SEM images, and the diameters of these circumscribed circles at that time were obtained, and the average value of these was taken as the primary particle diameter.

In the present specification, "porosity" is: volume fraction of pores (including open pores and closed pores) in the LTO sintered body plate. This porosity can be measured by image analysis of a cross-sectional SEM image of the sintered body plate. For example, the sintered body plate is processed by a section polishing machine (CP) to expose the polished section. The polished cross section is observed with an SEM (scanning electron microscope) at a predetermined magnification (for example, 1000 times) and a predetermined field of view (for example, 125. mu. m.times.125. mu.m). The obtained SEM image was subjected to image analysis, and the area of all pores in the field of view was divided by the area (cross-sectional area) of the sintered body plate in the field of view to obtain a quotient, and the quotient was multiplied by 100 to obtain a porosity (%).

The "open pore ratio" in the present specification is: the volume ratio (vol%) of open pores to all pores (including open pores and closed pores) included in the LTO sintered body plate. By "open pore" is meant: and air holes communicating with the outside of the sintered body plate among the air holes contained in the sintered body plate. "closed pore" means: among the pores contained in the sintered body plate, there are no pores communicating with the outside of the sintered body plate. The open pore ratio can be calculated from the total porosity corresponding to the total of open pores and closed pores obtained from the bulk density and the closed porosity corresponding to closed pores obtained from the apparent density. The parameters for calculating the open pore ratio can be measured using archimedes' method or the like. For example, the closed porosity (vol%) can be determined from the apparent density measured by the archimedes method, and the total porosity (vol%) can be determined from the bulk density measured by the archimedes method. Then, the open pore ratio can be determined from the closed pore ratio and the total pore ratio by the following calculation.

(open porosity ratio) ═ open porosity)/(total porosity)

(open porosity)/[ (open porosity) + (closed porosity) ]

[ (total porosity) - (closed porosity) ]/(total porosity)

In the present specification, the "average pore aspect ratio" is: average value of aspect ratio of pores contained in the LTO sintered body plate. The aspect ratio of the pores is: a ratio of a length of the air hole in a longitudinal direction to a length of the air hole in a width direction. The average pore aspect ratio can be measured by image analysis of a cross-sectional SEM image of the sintered body plate. For example, the sintered body plate is processed by a section polishing machine (CP) to expose the polished section. The polished cross section is observed with an SEM (scanning electron microscope) at a predetermined magnification (for example, 1000 times) and a predetermined field of view (for example, 125. mu. m.times.125. mu.m). And binarizing the obtained SEM image by using image analysis software, and judging the pores according to the obtained binarized image. For the identified pores, the aspect ratio is calculated by dividing the length in the longitudinal direction by the length in the width direction. And the aspect ratios of all the pores in the binarized image were calculated, and the average value of these aspect ratios was taken as the average aspect ratio. The "proportion of pores having an aspect ratio of 1.30 or more in all pores" referred to in the present specification may be determined in accordance with the image analysis step.

In this specification, "average pore diameter" is: the volume standard D50 gas pore diameter in the pore diameter distribution (integrated distribution) obtained by measurement of the LTO sintered plate, where the horizontal axis represents the gas pore diameter and the vertical axis represents the cumulative volume% (100% of the total pore volume). The meaning of the volume-based D50 gas pore size is the same as the well-known volume-based D50 diameter in the particle size distribution of the powder. Therefore, the volume reference D50 pore size is: the cumulative pore volume is a pore diameter of 50% of the total pore volume. The pore size distribution can be measured by mercury intrusion using a mercury porosimeter.

In the specification, "the volume reference D10 and D90 air hole diameters" are: volume references D10 and D90 gas pore diameters in a pore diameter distribution (integrated distribution) obtained by measurement of an LTO sintered plate, in which the horizontal axis represents a gas pore diameter and the vertical axis represents a cumulative volume% (relative to 100% of the total pore volume). The meaning of the volume basis D10 and D90 gas pore diameters is the same as the well known meaning of the volume basis D10 and D90 diameters in the particle size distribution of powders. Therefore, the volume references D10 and D90 pore diameters refer to: the cumulative pore volume is the pore diameter of 10% and 90% of the total pore volume, respectively. The pore size distribution can be measured by mercury intrusion using a mercury porosimeter.

LTO sintered body plate

The LTO sintered plate of the present invention is used for a negative electrode of a lithium secondary battery. The LTO sintered body plate has a structure in which a plurality of primary particles are bonded. The LTO sintered body has a thickness of 10 to 290 μm, an average primary particle diameter of a plurality of primary particles, that is, a primary particle diameter of 1.2 μm or less, and a porosity of 21 to 45%. The LTO sintered plate has an open pore ratio of 60% or more, an average pore aspect ratio of 1.15 or more, a proportion of pores having an aspect ratio of 1.30 or more in all pores of 30% or more, an average pore diameter of 0.70 μm or less, and volume-based D10 and D90 pore diameters satisfying a relationship of 4.0. ltoreq. D90/D10. ltoreq.50. The LTO sintered plate satisfying the predetermined particle size and pore conditions as described above is excellent in lithium ion conductivity and electron conductivity, and can exhibit excellent high-speed charge/discharge performance and high-temperature/low-temperature operability when assembled as a negative electrode in a lithium secondary battery. The low-temperature operability is a generally known performance of the LTO negative electrode, but the high-temperature operability is a performance exhibited by the LTO sintered plate without containing an auxiliary component such as a binder that is easily reacted with the electrolyte solution for high temperature.

In particular, the LTO sintered plate of the present invention has a porosity of 21 to 45%, in other words, a density of 55 to 79%. In this regard, although LTO sintered plates of various densities are known (see, for example, patent documents 1 to 3), the LTO sintered plate of the present invention has a specific porosity or density, and in addition, has specific ranges for the size, shape, structure, and distribution of pores. The porosity or the density can be studied mainly from the viewpoint of energy density and the viewpoint of electron conductivity. For example, high densification (i.e., low porosity) exhibited by sintering improves electron conductivity and also improves energy density. On the other hand, high densification (i.e., low porosity) may reduce lithium ion conductivity. This is because: in the case of a sintered body plate having a small void, the electrolyte solution cannot sufficiently flow to and from the sintered body plate, and therefore, lithium ion conduction cannot be promoted. In this sense, it can be said that there is a trade-off relationship between the electron conductivity and the lithium ion conductivity. In contrast, in the present invention, by controlling the size, shape, structure, and distribution of pores, electron conductivity and lithium ion conductivity can be improved at the same time, and excellent high-speed charge and discharge performance and high-temperature and low-temperature operability can be achieved.

The LTO sintered body plate has a structure in which a plurality of (i.e., a plurality of) primary particles are bonded. Thus, these oneThe secondary particles being formed from lithium titanate Li₄Ti₅O₁₂(LTO). A typical embodiment of LTO is known to have a spinel structure, however, other structures may be used during charging and discharging. For example, LTO is charged and discharged with Li₄Ti₅O₁₂(spinel structure) and Li₇Ti₅O₁₂The reaction proceeds in a form in which two phases (rock salt structure) coexist. Therefore, LTO is not limited to the spinel structure.

The thickness of the LTO sintered body plate is 10 to 290 μm, preferably 10 to 200 μm, more preferably 40 to 200 μm, further preferably 40 to 175 μm, and particularly preferably 50 to 160 μm. The thicker the LTO sintered plate, the easier it is to realize a battery with high capacity and high energy density. The thickness of the LTO sintered plate can be obtained by observing the cross section of the LTO sintered plate with an SEM (scanning electron microscope), for example, and measuring the distance between the plate surfaces observed substantially in parallel at this time.

The average primary particle diameter, i.e., the primary particle diameter, of the plurality of primary particles constituting the LTO sintered plate is 1.2 μm or less, preferably 0.02 to 1.2 μm, and more preferably 0.05 to 0.7. mu.m. When the amount is within the above range, lithium ion conductivity and electron conductivity are easily achieved at the same time, which contributes to improvement of rate characteristics.

The LTO sintered body plate contains pores. When the sintered plate includes pores, particularly open pores, the electrolyte solution can permeate into the sintered plate when the sintered plate is incorporated in a battery as a negative electrode plate, and as a result, the lithium ion conductivity can be improved. This is because: in the case where the lithium ion conduction in the sintered body is 2 types of conduction via the constituent particles of the sintered body and conduction via the electrolyte in the pores, the speed of conduction via the electrolyte in the pores has an overwhelming advantage.

The porosity of the LTO sintered plate is 21 to 45%, more preferably 22 to 40%, and still more preferably 25 to 35%. When the amount is within the above range, lithium ion conductivity and electron conductivity are easily achieved at the same time, which contributes to improvement of rate characteristics.

The LTO sintered plate has an open pore ratio of 60% or more, more preferably 65% or more, still more preferably 70% or more, and particularly preferably 80% or more. The open pore ratio may be 100%, typically 98% or less, more typically 95% or less, and still more typically 90% or less. If the number of open pores is large, the electrolyte solution easily permeates into the sintered body plate, and the lithium ion conductivity is improved. Therefore, if the amount is within the above range, lithium ion conductivity and electron conductivity are easily achieved at the same time, which contributes to improvement of rate characteristics.

The LTO sintered plate has an average pore aspect ratio of 1.15 or more, preferably 1.15 to 3.50, and more preferably 1.3 to 3.5. And, it is considered that: the anisotropic pore shape defined by the aspect ratio is present in the sintered body, and thus an interface with the electrolyte can be efficiently established, contributing to improvement of rate characteristics. For the same reason, the proportion of pores having an aspect ratio of 1.30 or more in the sintered LTO plate is preferably 30% or more, more preferably 30 to 90%, and still more preferably 50 to 90% of the total pores.

The average pore diameter of the LTO sintered plate is 0.70 μm or less, preferably 0.02 to 0.70 μm, and more preferably 0.15 to 0.60. mu.m. When the amount is within the above range, lithium ion conductivity and electron conductivity are easily achieved at the same time, which contributes to improvement of rate characteristics.

The pore diameters of D10 and D90 in the LTO sintered body plates based on the volume are set to satisfy 4.0. ltoreq. D90/D10. ltoreq.50, preferably 4.5. ltoreq. D90/D10. ltoreq.50, more preferably 5.0. ltoreq. D90/D10. ltoreq.40, and particularly preferably 5.0. ltoreq. D90/D10. ltoreq.20. The above relationship means: the pore size distribution is a gentle (broad) distribution. For example, the relationship D90/D10 ≧ 4.0 means: the D10 pore size and D90 pore size are clearly separated. And, it is considered that: the above-described characteristic pore size distribution contributes to permeation of the electrolyte into the sintered body, thereby contributing to improvement of rate characteristics.

The LTO sintered plate of the present invention is used for a negative electrode of a lithium secondary battery. Therefore, according to a preferred aspect of the present invention, there is provided a lithium secondary battery comprising: a positive electrode, a negative electrode including an LTO sintered body plate, and an electrolytic solution. The positive electrode preferably comprises a lithium compositeAn oxide. Examples of the lithium composite oxide include: lithium cobaltate, lithium nickelate, lithium manganate, lithium nickel cobaltate, lithium cobalt nickel manganate, lithium cobalt manganate, and the like. The lithium composite oxide may contain one or more elements selected from Mg, Al, Si, Ca, Ti, V, Cr, Fe, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, Ag, Sn, Sb, Te, Ba, Bi, W, and the like. The most preferred lithium composite oxide is lithium cobaltate (LiCoO)₂). Therefore, the positive electrode is particularly preferably a lithium composite oxide sintered plate, and most preferably a lithium cobaltate sintered plate. The electrolyte solution may be any known electrolyte solution generally used in lithium secondary batteries. The electrolyte solution may contain 96% by volume or more of 1 or 2 or more selected from γ -butyrolactone, propylene carbonate, and ethylene carbonate. By using the electrolyte, the battery can be stably manufactured without deteriorating the battery when the battery is manufactured through high-temperature operation and high-temperature process of the battery.

The lithium secondary battery manufactured by using the LTO sintered plate of the present invention exhibits: the battery has high reliability such as good cycle performance and good storage performance (less self-discharge), and can be connected in series by simple control.

In addition, since a lithium secondary battery using the LTO sintered plate of the present invention as a negative electrode does not generate dendrite, constant voltage charging (CV charging) can be performed. For the charging, constant current charging (CC charging), constant current and constant voltage charging (CC-CV charging), and CV charging may be performed. When only CV charging is performed, the charging IC is not required, and therefore, there are advantages in that the battery can be operated by simple control, and the battery can be made thin and small.

In the case where both the positive electrode and the negative electrode are made of ceramic, the separator may also be made of ceramic, thereby integrating the 3 electrode members. For example, the ceramic positive electrode, the ceramic negative electrode, and the ceramic separator may be produced and then joined to be integrated. Alternatively, before firing the ceramic member, 3 green sheets each provided with the positive electrode, the negative electrode, and the separator may be pressed to prepare a laminate, and the laminate may be fired to obtain an integrated ceramic member. AsPreferable examples of the constituent material of the ceramic separator include: al (Al)₂O₃、ZrO₂、MgO、SiC、Si₃N₄And the like.

When a battery in which both the positive electrode and the negative electrode are ceramic plates is manufactured, since the energy density of both the electrode members is high, a thin battery can be manufactured. In particular, since the thin battery can be subjected to CV charging as described above, it is suitable for use as a battery for smart cards and IoT.

Manufacturing method

The LTO sintered plate of the present invention may be produced by any method, but is preferably produced by (a) producing a LTO-containing green sheet and (b) firing the LTO-containing green sheet.

(a) Production of Green sheet containing LTO

First, lithium titanate Li is prepared₄Ti₅O₁₂The constituted raw material powder (LTO powder). The starting powder may be commercially available LTO powder or may be newly synthesized. For example, a powder obtained by hydrolyzing a mixture of titanium tetraisopropoxide and lithium isopropoxide may be used, or a mixture containing lithium carbonate, titanium dioxide, or the like may be fired. The volume-based D50 particle size of the raw material powder is preferably 0.05 to 5.0 μm, and more preferably 0.1 to 2.0. mu.m. If the particle diameter of the raw material powder is large, pores tend to increase. When the particle size of the raw material is large, a pulverization treatment (for example, pot ball mill pulverization, bead mill pulverization, jet mill pulverization, or the like) may be performed to obtain a desired particle size. Then, the raw material powder is mixed with a dispersion medium and various additives (a binder, a plasticizer, a dispersant, etc.) to form a slurry. In the slurry, for the purpose of promoting the grain growth in the firing step described later or compensating for volatile components, LiMO may be added in excess to the extent of 0.5 to 30 mol% except for the purpose of₂Other lithium compounds (e.g., lithium carbonate). Preferably, no pore-forming material is added to the slurry. The slurry is preferably stirred and defoamed under reduced pressure, and the viscosity is preferably adjusted to 4000 to 10000 cP. The obtained slurry was formed into a sheet to obtain a green sheet containing LTO. The green sheet thus obtained was an independent sheet-like molded body. The independent sheets (sometimes also referred to as "free-standing films") refer to: a sheet (including a sheet having an aspect ratio of 5 or more) which can be handled as a single body independently from another support. That is, the independent sheet does not include a sheet fixedly bonded to another support (such as a substrate) and integrated with the support (which cannot be separated or is difficult to separate). The sheet molding can be carried out by various known methods, but is preferably carried out by a doctor blade method. The thickness of the LTO-containing green sheet may be appropriately set so that the thickness after firing becomes a desired thickness as described above.

(b) Firing of LTO-containing Green sheet

The green sheet containing LTO was placed on a setter plate. The setter plates are made of ceramic, preferably zirconia or magnesia. The setter is preferably subjected to press-texturing. The green sheet thus placed on the setter was placed in a sagger. The sagger is also made of ceramic, preferably alumina. Then, in this state, degreasing is performed as necessary, and then firing is performed, thereby obtaining an LTO sintered plate. The firing is preferably carried out at 600 to 900 ℃ for 1 to 50 hours, and more preferably at 700 to 800 ℃ for 3 to 20 hours. The sintered body plate thus obtained is also in the form of a separate independent sheet. The temperature rise rate during firing is preferably 100 to 1000 ℃/h, more preferably 100 to 600 ℃/h. In particular, the rate of temperature rise is preferably employed in the temperature rise process of 300 to 800 ℃ and more preferably 400 to 800 ℃.

(c) Small knot

As described above, the LTO sintered body plate of the present invention can be preferably manufactured. In this preferred production method, it is effective to 1) adjust the particle size distribution of the LTO powder and/or 2) change the temperature increase rate during firing, and these operations are considered to contribute to the realization of various characteristics of the lithium composite oxide sintered plate of the present invention. For example, in patent document 2, a lithium material and a titanium material are fired at 850 ℃, and then cast and fired without being crushed, pulverized, or the like, but in the method for producing an LTO sintered body plate of the present invention, it is preferable to adjust the particle size distribution of the LTO material as described above.