Non-aqueous electrolyte secondary battery

Abstract

In a non-aqueous electrolyte secondary battery according to one embodiment of the present invention, a positive electrode (11) has: a positive electrode core material (30); and a positive electrode mixture layer (31) formed on the surface of the positive electrode core material (30). The electrode mixture layer (31) contains, as a positive electrode active material, first lithium metal composite oxide particles that are non-aggregated particles having a volume-based median diameter of 2-10 μm, and second lithium metal composite oxide particles that are secondary particles that have a volume-based median diameter of 10-30 μm and that are each obtained by aggregation of primary particles having an average particle diameter from 50 nm to 2 μm. The first lithium metal composite oxide particles are contained more in a first region (31a) than in a second region (31b). A non-aqueous electrolyte contains a diisocyanate compound.

Description

[Technical Field]

[0001] This disclosure relates to a non-aqueous electrolyte secondary battery. [Background technology]

[0002] In recent years, with the increasing adoption of non-aqueous electrolyte secondary batteries in automotive and energy storage applications, there has been a growing demand for non-aqueous electrolyte secondary batteries with high capacity and excellent cycle characteristics. For example, Patent Document 1 discloses a non-aqueous electrolyte secondary battery that uses single-crystal particles, which are non-aggregated particles, and secondary particles formed by the aggregation of multiple primary particles as the positive electrode active material in order to improve cycle characteristics. Patent Document 2 also discloses a non-aqueous electrolyte secondary battery comprising an electrolyte containing a lithium salt and a diisocyanate compound. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2020-53386 [Patent Document 2] Japanese Patent Publication No. 2007-242411 [Overview of the Initiative]

[0004] However, our investigations revealed that while using non-aggregated particles as the positive electrode active material improves cycle performance, it reduces battery capacity due to decreased packing efficiency of the positive electrode active material. On the other hand, using secondary particles as the positive electrode active material resulted in worse cycle performance compared to using non-aggregated particles. Furthermore, it was found that using a mixture of non-aggregated and secondary particles did not allow for both high capacity and good cycle performance.

[0005] Furthermore, while the addition of diisocyanate compounds contributes to improving cycle characteristics, further improvements in these characteristics are needed.

[0006] The purpose of this disclosure is to provide a non-aqueous electrolyte secondary battery with high capacity and excellent cycle characteristics.

[0007] A non-aqueous electrolyte secondary battery according to one aspect of the present disclosure is a non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the positive electrode has a positive electrode core material and a positive electrode mixture layer formed on the surface of the positive electrode core material, and the positive electrode mixture layer contains, as a positive electrode active material, first lithium metal composite oxide particles which are non-aggregated particles with a volume-based median diameter of 2 to 10 μm, and second lithium metal composite oxide particles which are secondary particles which are aggregated primary particles with an average particle size of 50 nm to 2 μm and have a volume-based median diameter of 10 to 30 μm, and when the positive electrode mixture layer is divided into two equal parts in the thickness direction and defined as a first region and a second region in order from the surface side of the positive electrode, the first lithium metal composite oxide particles are contained in greater quantities in the first region than in the second region, and the non-aqueous electrolyte contains a diisocyanate compound.

[0008] According to one aspect of this disclosure, a non-aqueous electrolyte secondary battery with high capacity and excellent cycle characteristics can be provided. [Brief explanation of the drawing]

[0009] [Figure 1] This is a cross-sectional view of a non-aqueous electrolyte secondary battery, which is an example of an embodiment. [Figure 2] This is a cross-sectional view of a positive electrode, which is an example of an embodiment. [Modes for carrying out the invention]

[0010] As described above, achieving both high capacity and excellent cycle characteristics in non-aqueous electrolyte secondary batteries is a crucial challenge. The inventors of this invention have diligently studied to solve this problem and have found that by using a combination of specific non-aggregated particles and specific secondary particles as the positive electrode active material, and by having more non-aggregated particles on the surface side than on the core side of the positive electrode mixture layer, as well as by adding a diisocyanate compound to the non-aqueous electrolyte, a non-aqueous electrolyte secondary battery with high capacity and excellent cycle characteristics can be obtained.

[0011] The addition of diisocyanate compounds contributes to improving cycle characteristics, but the improvement effect is small with diisocyanate compounds alone. Similarly, while the presence of a large number of non-aggregated particles on the surface of the cathode mixture layer contributes to improving cycle characteristics, the improvement effect is small with non-aggregated particles alone. As a result of our investigations, we found that the combination of non-aggregated particles and diisocyanate compounds yields a synergistic effect far exceeding the expected effect, resulting in a specific improvement in cycle characteristics.

[0012] Diisocyanate compounds are thought to improve cycle characteristics by forming a protective film on the negative electrode surface. However, the addition of diisocyanate compounds can cause side reactions at the positive electrode, easily increasing the positive electrode resistance. However, by having a large number of non-aggregated particles on the surface side of the positive electrode mixture layer, the increase in resistance due to such side reactions can be suppressed. In other words, the negative electrode can be modified while suppressing the problems caused by the addition of diisocyanate compounds. For this reason, the non-aqueous electrolyte secondary battery according to this disclosure exhibits a significant improvement in cycle characteristics.

[0013] When non-aggregated particles are used alone as the positive electrode active material, cycle characteristics improve, but the battery capacity decreases due to a reduction in the packing density of the positive electrode active material. On the other hand, when secondary particles are used, particle cracking is likely to occur during the positive electrode compression process, and the gaps between particles that form the electrolyte flow path become blocked by the cracked particles. As a result, although the packing density of the active material increases, cycle characteristics deteriorate. Furthermore, it is not possible to achieve both of these characteristics when using a mixture of non-aggregated and secondary particles.

[0014] In other words, by setting the content of non-aggregated particles in the positive electrode mixture layer such that the first region on the surface side of the positive electrode mixture layer is greater than the second region on the core material side, and by adding a diisocyanate compound to the non-electrolyte, a unique combination of high capacity and excellent cycle characteristics can be achieved.

[0015] Hereinafter, an example of an embodiment of a non-aqueous electrolyte secondary battery according to the present disclosure will be described in detail with reference to the drawings. Note that selectively combining a plurality of embodiments and modification examples described below is included in the present disclosure.

[0016] Hereinafter, a cylindrical battery in which a wound electrode body 14 is housed in a bottomed cylindrical outer can 16 will be exemplified. However, the outer package of the battery is not limited to a cylindrical outer can. For example, it may be a rectangular outer can (rectangular battery) or an outer package (laminated battery) composed of a laminated sheet including a metal layer and a resin layer. Further, the electrode body may be a laminated electrode body in which a plurality of positive electrodes and a plurality of negative electrodes are alternately laminated via a separator.

[0017] FIG. 1 is a diagram schematically showing a cross section of a non-aqueous electrolyte secondary battery 10 which is an example of an embodiment. As shown in FIG. 1, the non-aqueous electrolyte secondary battery 10 includes a wound electrode body 14, a non-aqueous electrolyte, and an outer can 16 that houses the electrode body 14 and the non-aqueous electrolyte. The electrode body 14 has a positive electrode 11, a negative electrode 12, and a separator 13, and has a wound structure in which the positive electrode 11 and the negative electrode 12 are wound in a spiral shape via the separator 13. The outer can 16 is a metal container having a bottomed cylindrical shape with one side in the axial direction open, and the opening of the outer can 16 is closed by a sealing body 17. Hereinafter, for convenience of explanation, the side of the battery where the sealing body 17 is located is referred to as the upper side, and the bottom side of the outer can 16 is referred to as the lower side.

[0018] The positive electrode 11, the negative electrode 12, and the separator 13 that constitute the electrode body 14 are all strip-shaped elongated bodies, and are alternately laminated in the radial direction of the electrode body 14 by being wound in a spiral shape. The negative electrode 12 is formed with dimensions slightly larger than those of the positive electrode 11 in order to prevent precipitation of lithium. That is, the negative electrode 12 is formed longer in the longitudinal direction and the width direction (short side direction) than the positive electrode 11. The separator 13 is formed with dimensions at least slightly larger than those of the positive electrode 11, and two sheets are arranged so as to sandwich the positive electrode 11. The electrode body 14 has a positive electrode lead 20 connected to the positive electrode 11 by welding or the like and a negative electrode lead 21 connected to the negative electrode 12 by welding or the like.

[0019] Insulating plates 18 and 19 are positioned above and below the electrode body 14, respectively. In the example shown in Figure 1, the positive electrode lead 20 extends through a through-hole in the insulating plate 18 towards the sealing body 17, and the negative electrode lead 21 extends outside the insulating plate 19 towards the bottom of the outer can 16. The positive electrode lead 20 is connected to the lower surface of the internal terminal plate 23 of the sealing body 17 by welding or the like, and the cap 27, which is the top plate of the sealing body 17 and is electrically connected to the internal terminal plate 23, becomes the positive electrode terminal. The negative electrode lead 21 is connected to the inner bottom surface of the outer can 16 by welding or the like, and the outer can 16 becomes the negative electrode terminal.

[0020] As described above, the outer casing 16 is a bottomed cylindrical metal container with one side open in the axial direction. A gasket 28 is provided between the outer casing 16 and the sealing body 17 to ensure airtightness inside the battery and insulation between the outer casing 16 and the sealing body 17. The outer casing 16 has a grooved portion 22 formed on its side surface, which protrudes inward to support the sealing body 17. The grooved portion 22 is preferably formed in an annular shape along the circumferential direction of the outer casing 16, and its upper surface supports the sealing body 17. The sealing body 17 is fixed to the upper part of the outer casing 16 by the grooved portion 22 and the open end of the outer casing 16 which is crimped to the sealing body 17.

[0021] The sealing body 17 has a structure in which an internal terminal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are stacked in order from the electrode body 14 side. Each component constituting the sealing body 17 has, for example, a disc shape or a ring shape, and each component except the insulating member 25 is electrically connected to one another. The lower valve body 24 and the upper valve body 26 are connected at their respective centers, with the insulating member 25 interposed between their respective peripheries. When a malfunction occurs in the battery and the internal pressure rises, the lower valve body 24 deforms and ruptures, pushing the upper valve body 26 towards the cap 27, thereby interrupting the current path between the lower valve body 24 and the upper valve body 26. If the internal pressure rises further, the upper valve body 26 ruptures, and gas is discharged from the opening of the cap 27.

[0022] The following describes in detail the positive electrode 11, negative electrode 12, separator 13, and non-aqueous electrolyte that constitute the non-aqueous electrolyte secondary battery 10, with particular attention paid to the positive electrode 11 and the non-aqueous electrolyte.

[0023] [Positive electrode] The positive electrode 11 comprises a positive electrode core material 30 and a positive electrode mixture layer 31 formed on the surface of the positive electrode core material 30. The positive electrode core material 30 can be a foil of a metal that is stable in the potential range of the positive electrode 11, such as aluminum or an aluminum alloy, or a film on which the metal is arranged on the surface. An example of the positive electrode core material 30 is an aluminum or aluminum alloy foil with a thickness of 10 to 20 μm. The positive electrode mixture layer 31 contains a positive electrode active material, a conductive agent, and a binder, and is preferably formed on both sides of the positive electrode core material 30. The thickness of the positive electrode mixture layer 31 is, for example, 30 to 100 μm on one side of the positive electrode core material 30. The positive electrode 11 can be manufactured, for example, by applying a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, and a binder onto the positive electrode core material 30, drying the coating, and then compressing it to form the positive electrode mixture layer 31 on both sides of the positive electrode core material 30.

[0024] Examples of conductive agents included in the positive electrode mixture layer 31 include carbon materials such as carbon black, acetylene black, Ketjenblack, graphite, carbon nanotubes, carbon nanofibers, and graphene. The content of the conductive agent is, for example, 0.01 to 10 parts by mass, preferably 0.05 to 5 parts by mass, per 100 parts by mass of positive electrode active material.

[0025] Examples of binders included in the positive electrode mixture layer 31 include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide, acrylic resin, and polyolefin. These resins may also be used in combination with carboxymethylcellulose (CMC) or its salts, polyethylene oxide (PEO), etc. The binder content is, for example, 0.1 to 10 parts by mass, preferably 0.5 to 5 parts by mass, per 100 parts by mass of positive electrode active material.

[0026] The positive electrode composite layer 31 contains particulate lithium metal composite oxide as the positive electrode active material. The lithium metal composite oxide is a composite oxide containing metal elements such as Co, Mn, Ni, and Al in addition to Li. The metal elements constituting the lithium metal composite oxide are, for example, at least one selected from Mg, Al, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Y, Zr, Sn, Sb, W, Pb, and Bi. In particular, it is preferable to contain at least one selected from Co, Ni, and Mn. Examples of suitable composite oxides include lithium metal composite oxides containing Ni, Co, and Mn, and lithium metal composite oxides containing Ni, Co, and Al.

[0027] The positive electrode mixture layer 31 contains two types of lithium metal composite oxide particles. Furthermore, when the positive electrode mixture layer 31 is divided into two equal parts in the thickness direction and defined as the first region 31a and the second region 31b, respectively from the surface side of the positive electrode 11, the materials contained in the first region 31a and the second region 31b are different from each other. In this embodiment, the types of lithium metal composite oxide particles contained in the first region 31a and the second region 31b are different, or if two types of lithium metal composite oxide particles are contained in each region, their mass ratio is different. On the other hand, the types and contents of the conductive agent and binder may be the same or different in the first region 31a and the second region 31b.

[0028] The positive electrode mixture layer 31 contains first lithium metal composite oxide particles, which are non-aggregated particles, and second lithium metal composite oxide particles, which are secondary particles formed by the aggregation of primary particles with an average particle size of 50 nm to 2 μm. The positive electrode mixture layer 31 may contain only the first and second lithium metal composite oxide particles as the positive electrode active material, or it may contain third lithium metal composite oxide particles to the extent that it does not impair the purpose of this disclosure. An example of the third lithium metal composite oxide particles is composite oxide particles that do not satisfy the particle size conditions described later.

[0029] The volume-based median diameter (hereinafter sometimes referred to as "D50") of the first lithium metal composite oxide particles is 2 to 10 μm, preferably 3 to 8 μm. The D50 of the second lithium metal composite oxide particles is 10 to 30 μm, preferably 12 to 20 μm. D50 refers to the particle size at which the cumulative frequency of the smallest particle size accounts for 50% in the volume-based particle size distribution. The particle size distribution of lithium metal composite oxide particles can be measured using a laser diffraction particle size distribution analyzer (e.g., Microtrac-Bell Co., Ltd., MT3000II) with water as the dispersion medium.

[0030] The first lithium metal composite oxide particles are particles that do not have grain boundaries inside, and are, for example, primary particles of a single crystal. The crystallinity of the lithium metal composite oxide particles can be confirmed using a scanning ion microscope. The first composite oxide particles, which are non-aggregated particles, may contain five or fewer primary particles. In this specification, non-aggregated particles mean particles composed of one primary particle that does not have grain boundaries inside, and particles composed of five or fewer primary particles.

[0031] The second lithium metal composite oxide particles are secondary particles formed by the aggregation of primary particles with an average particle size of 50 nm to 2 μm, preferably 500 nm to 2 μm. Grain boundaries of the primary particles exist within the second lithium metal composite oxide particles. The primary particles can be confirmed by observing the second lithium metal composite oxide particles with a scanning electron microscope (SEM). The multiple primary particles are firmly bonded to each other, so as not to break apart even when strong forces are applied, such as during the pulverization of the second lithium metal composite oxide particles after synthesis or during the preparation of the cathode mixture slurry.

[0032] The average particle size of the primary particles constituting the second lithium metal composite oxide particle can be determined by analyzing SEM images of the particle cross-section. For example, the positive electrode 11 is embedded in resin, a cross-section is prepared by cross-section polishing (CP), and this cross-section is photographed with an SEM. From the SEM image, 30 primary particles are randomly selected and their grain boundaries are observed. The diameter of the circumscribed circle of each of the 30 primary particles is determined, and the average value of these diameters is taken as the average particle size.

[0033] Each lithium metal composite oxide particle can be synthesized by the method described in the examples to be described later. For example, the first composite oxide particle can be synthesized by increasing the pH of the alkaline aqueous solution used when synthesizing a precursor (metal composite hydroxide) containing Ni, Co, Mn, Al, etc. compared to the case of synthesizing the second composite oxide particle. Alternatively, instead of increasing the pH of the alkaline aqueous solution, or in addition to this, it can be synthesized by increasing the firing temperature of the precursor.

[0034] An example of a suitable pH of the alkaline aqueous solution when synthesizing the first composite oxide particle is 10 to 11, and an example of a suitable firing temperature is 950 to 1100 °C. When synthesizing the second composite oxide particle, for example, an alkaline aqueous solution with a pH of 9 to 10 is used and the firing temperature is set to 950 °C or lower.

[0035] Each lithium metal composite oxide particle is composed of, for example, a lithium metal composite oxide having a hexagonal crystal structure belonging to the space group R-3m, preferably LiNi x Co y Mn z O2 (0.3 < x < 0.6, x + y + z = 1), or LiNi x Co y Al z [ O2 (0.8 < x < 0.95, x + y + z = 1) as the main component. Here, the main component means the component with the largest mass among the components constituting the lithium metal composite oxide particle. Note that the compositions of the respective lithium metal composite oxide particles may be the same as or different from each other.

[0036] As described above, the positive electrode mixture layer 31 contains first and second lithium metal composite oxide particles as positive electrode active materials, but the positive electrode active materials contained in the first region 31a and the second region 31b are different. The content of the first lithium metal composite oxide particles in the positive electrode mixture layer 31 is not uniform, and the first lithium metal composite oxide particles are contained in greater quantities in the first region 31a than in the second region 31b. By adding a large amount of the first lithium metal composite oxide particles, which are non-aggregated particles, to the first region 31a that forms the surface of the positive electrode mixture layer 31, the contact area with the electrolyte can be reduced, and it is thought that side reactions between the diisocyanate compound and the electrolyte in the positive electrode 11 can be suppressed.

[0037] Furthermore, since the first lithium metal composite oxide particles are less prone to particle cracking during the manufacturing process of the positive electrode 11 compared to the second lithium metal composite oxide particles, by having a large amount of the first lithium metal composite oxide particles in the first region 31a of the positive electrode mixture layer 31, a flow path for the electrolyte is secured in the first region 31a. In this case, the penetration of the electrolyte into the second region 31b is improved, and it is thought that the cycle characteristics are improved.

[0038] The content of the positive electrode active material may differ between the first region 31a and the second region 31b, but is preferably substantially the same. The content of the positive electrode active material is, for example, 90 to 99.9% by mass, preferably 95 to 99% by mass, relative to the mass of the positive electrode mixture layer 31. The content of the first lithium metal composite oxide particles in the first region 31a is preferably 50% by mass or more relative to the mass of the positive electrode active material contained in the first region 31a. In this case, the improvement effect on cycle characteristics becomes more pronounced.

[0039] The first region 31a may substantially contain only the first lithium metal composite oxide particles as the positive electrode active material. The first lithium metal composite oxide particles may, for example, be contained only in the first region 31a and not be present in the second region 31b. If the first region 31a contains the second lithium metal composite oxide particles, it is preferable that the mass ratio of the first lithium metal composite oxide particles is higher in the first region 31a. That is, it is preferable that the content of the first lithium metal composite oxide particles in the first region 31a is greater than the content of the second lithium metal composite oxide particles in the first region 31a. The content of the first lithium metal composite oxide particles in the first region 31a is more preferably 70% by mass or more, and particularly preferably 80% by mass or more, relative to the mass of the positive electrode active material contained in the first region 31a.

[0040] The second lithium metal composite oxide particles are present in greater quantities in the second region 31b than in the first region 31a. Since the second lithium metal composite oxide particles can be packed in greater quantities per unit volume compared to the first lithium metal composite oxide particles, the cathode composite layer 31 can be made more densely packed by using the second lithium metal composite oxide particles. By having a large amount of the second lithium metal composite oxide particles in the second region 31b on the cathode core material 30 side, which has less influence on the permeability of the electrolyte, it is possible to increase capacity while maintaining good cycle characteristics.

[0041] The second region 31b may substantially contain only the second lithium metal composite oxide particles as the positive electrode active material. The second lithium metal composite oxide particles may, for example, be contained only in the second region 31b and not be present in the first region 31a. If the second region 31a contains the first lithium metal composite oxide particles, the content of the second lithium metal composite oxide particles in the second region 31b is preferably greater than the content of the first lithium metal composite oxide particles in the second region 31b. The content of the second lithium metal composite oxide particles in the second region 31b is preferably 50% by mass or more, more preferably 70% by mass or more, and particularly preferably 80% by mass or more, based on the mass of the positive electrode active material contained in the second region 31b.

[0042] [Negative electrode] The negative electrode 12 comprises a negative electrode core material and a negative electrode mixture layer formed on the surface of the negative electrode core material. The negative electrode core material can be a foil of a metal that is stable in the potential range of the negative electrode 12, such as copper or a copper alloy, or a film on which the metal is arranged on the surface. An example of a negative electrode core material is a copper or copper alloy foil with a thickness of 5 to 15 μm. The negative electrode mixture layer contains a negative electrode active material and a binder, and is preferably formed on both sides of the negative electrode core material. The thickness of the negative electrode mixture layer is, for example, 30 to 150 μm on one side of the negative electrode core material. The negative electrode 12 can be manufactured by applying a negative electrode mixture slurry containing a negative electrode active material, a binder, etc., onto the negative electrode core material, drying the coating film, and then compressing it to form the negative electrode mixture layer on both sides of the negative electrode core material.

[0043] The negative electrode mixture layer contains, for example, a carbon-based active material that reversibly intercepts and releases lithium ions as the negative electrode active material. Suitable carbon-based active materials include natural graphite such as flake graphite, lumpy graphite, and clay-like graphite, as well as artificial graphite such as lumpy artificial graphite (MAG) and graphitized mesophase carbon microbeads (MCMB). In addition, a Si-based active material composed of at least one of Si and a Si-containing compound may be used as the negative electrode active material, and carbon-based active materials and Si-based active materials may be used in combination.

[0044] The binder included in the negative electrode mixture layer can be a fluororesin such as PTFE or PVdF, PAN, polyimide, acrylic resin, polyolefin, or styrene-butadiene rubber (SBR). The negative electrode mixture layer may also contain CMC or its salt, polyacrylic acid (PAA) or its salt, or polyvinyl alcohol (PVA). The binder content is, for example, 0.1 to 10 parts by mass, preferably 0.5 to 5 parts by mass, per 100 parts by mass of the negative electrode active material. Conductive agents such as carbon black, acetylene black, or Ketjenblack may also be added to the negative electrode mixture layer.

[0045] [Separator] A porous sheet having ion permeability and insulating properties is used for the separator 13. Specific examples of porous sheets include microporous thin films, woven fabrics, and nonwoven fabrics. Suitable materials for the separator include polyethylene, polyolefins such as polypropylene, and cellulose. The separator 13 may have a single-layer structure or a multi-layer structure. Furthermore, a highly heat-resistant resin layer, such as aramid resin, may be formed on the surface of the separator 13.

[0046] A filler layer containing an inorganic filler may be formed at the interface between the separator 13 and at least one of the positive electrode 11 and the negative electrode 12. Examples of inorganic fillers include oxides containing metal elements such as Ti, Al, Si, and Mg, and phosphoric acid compounds. The filler layer can be formed by applying a slurry containing the filler to the surface of the positive electrode 11, the negative electrode 12, or the separator 13.

[0047] [Non-aqueous electrolytes] Non-aqueous electrolytes comprise a non-aqueous solvent and an electrolyte salt. Examples of non-aqueous solvents include esters, ethers, nitriles such as acetonitrile, amides such as dimethylformamide, and mixtures of two or more of these. The non-aqueous solvent may also contain halogen-substituted solvents in which at least some of the hydrogen atoms in the solvent are replaced with halogen atoms such as fluorine. Examples of halogen-substituted solvents include fluorinated cyclic carbonate esters such as fluoroethylene carbonate (FEC), fluorinated linear carbonate esters, and fluorinated linear carboxylic acid esters such as methyl fluoropropionate (FMP).

[0048] Examples of the above esters include cyclic carbonate esters such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate; linear carbonate esters such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl propyl carbonate, ethyl propyl carbonate, and methyl isopropyl carbonate; cyclic carboxylic acid esters such as γ-butyrolactone (GBL) and γ-valerolactone (GVL); and linear carboxylic acid esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate. Among these, it is preferable to use at least one selected from EC, EMC, and DMC, and it is particularly preferable to use a mixed solvent of EC, EMC, and DMC.

[0049] Examples of the above ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineole, cyclic ethers such as crown ethers, 1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, and methylphenyl ether. Examples of chain ethers include ethylphenyl ether, butylphenyl ether, pentylphenyl ether, methoxytoluene, benzyl ethyl ether, diphenyl ether, dibenzyl ether, o-dimethoxybenzene, 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, 1,1-dimethoxymethane, 1,1-diethoxyethane, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether.

[0050] The non-aqueous electrolyte further contains a diisocyanate compound. The diisocyanate compound is dissolved in the non-aqueous solvent. The diisocyanate compound is thought to polymerize on the negative electrode surface to form a protective film, thereby improving the battery's cycle characteristics. The diisocyanate compound may cause a side reaction at the positive electrode 11, potentially increasing the positive electrode resistance. However, according to this embodiment, the synergistic effect with the non-aggregated particles contained in the first region 31a of the positive electrode mixture layer 31 specifically improves the cycle characteristics. The non-aqueous electrolyte may also contain additives such as vinylene carbonate (VC).

[0051] Specific examples of diisocyanate compounds include diisocyanatomethane, 1,3-diisocyanatopropane, 1,4-diisocyanatobutane, 1,5-diisocyanatopentane, 1,6-diisocyanatohexane (HDI), dicyclohexylmethane diisocyanate, 1,3-bisisocyanatomethylcyclohexane, 1,7-diisocyanatheptane, 1,8-diisocyanatooctane, and 1,9-diisocyanatooctane. Cyanatononane, 1,10-diisocyanatodecane, 1,3-bis(isocyanatomethyl)cyclohexane, dicyclohexylmethane-1,1'-diisocyanate, dicyclohexylmethane-2,2'-diisocyanate, dicyclohexylmethane-3,3'-diisocyanate, dicyclohexylmethane-4,4'-diisocyanate, isophorone diisocyanate (IPDI), 1,6,11- Diisocyanatoundecane, 4-isocyanatomethyl-1,8-octamethylenediisocyanate, diisocyanatobenzene, toluene diisocyanate (TDI), diphenylmethane diisocyanate (MDI), xylene diisocyanate (XDI), ethyl diisocyanatobenzene, trimethyldiisocyanatobenzene, diisocyanatonaphthalene, diisocyanatobiphenyl, 1,4-diisocyana Examples include anato-2-butene, 1,5-diisocyanato-2-pentene, 1,5-diisocyanato-2-methylpentane, 1,6-diisocyanato-2-hexene, 1,6-diisocyanato-3-hexene, 1,3-bis(isocyanatomethyl)cyclohexane, carbonyl diisocyanate, 1,4-diisocyanatobutane-1,4-dione, and 1,5-diisocyanatopentane-1,5-dione.

[0052] The diisocyanate compound is preferably a compound represented by the general formula: O=C=N-R-N=C=O. In the formula, R is an aliphatic hydrocarbon group having 1 to 12 carbon atoms or an aromatic hydrocarbon group having 6 to 20 carbon atoms. The aliphatic hydrocarbon group and the aromatic hydrocarbon group may contain hetero elements. The number of carbon atoms in the aliphatic hydrocarbon group is more preferably 3 to 10, and particularly preferably 4 to 8. Specific examples of suitable diisocyanate compounds include 1,6-diisocyanatohexane (HDI). Note that one type of diisocyanate compound may be used, or two or more types may be used in combination.

[0053] Even a small amount of the diisocyanate compound contributes to the improvement of cycle characteristics. However, its content is preferably 0.01 to 3% by mass based on the mass of the non-aqueous electrolyte. The content of the diisocyanate compound may be more than 3% by mass, but from the perspective of suppressing the anode resistance, it is preferably limited to 3% by mass. The content of the diisocyanate compound is more preferably 0.02 to 2% by mass, and particularly preferably 0.03 to 1% by mass based on the mass of the non-aqueous electrolyte.

[0054] The electrolyte salt is preferably a lithium salt. Examples of lithium salts include LiBF4, LiClO4, LiPF6, LiAsF6, LiSbF6, LiAlCl4, LiSCN, LiCF3SO3, LiCF3CO2, Li(P(C2O4)F4), LiPF 6-x (C n F 2n+1 ) x (1 < x < 6, n is 1 or 2), LiB 10 Cl 10 , LiCl, LiBr, LiI, lithium chloroborane, lithium lower aliphatic carboxylate, borate salts such as Li2B4O7, Li(B(C2O4)F2), LiN(SO2CF3)2, LiN(C1F 2l+1 SO2)(C m F 2m+1Examples include imide salts such as SO2){l and m are integers greater than or equal to 0}. Lithium salts may be used individually or in mixtures of multiple types. Among these, LiPF6 is preferred. The concentration of the lithium salt is, for example, 0.8 to 4 moles per liter of non-aqueous solvent. [Examples]

[0055] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0056] <Example 1> [Synthesis of the first lithium metal composite oxide particles] Nickel sulfate, cobalt sulfate, and manganese sulfate were mixed in predetermined proportions and uniformly mixed in an alkaline aqueous solution with a pH of 10-11 to prepare a precursor. Next, this precursor was mixed with lithium carbonate, calcined at a temperature of 1000°C for 15 hours, and then pulverized to obtain first lithium metal composite oxide particles, which are non-aggregated particles. The composition and D50 of these particles are as follows. Composition: LiNi 0.5 Co 0.2 Mn 0.3 O2 D50: 4.5 μm

[0057] [Synthesis of the second lithium metal composite oxide particle] Except for changing the pH of the alkaline aqueous solution to 9-10 and the calcination temperature to 900°C, a second set of lithium metal composite oxide particles, which are secondary particles formed by the aggregation of primary particles, was obtained in the same manner as for the first set of lithium metal composite oxide particles. The composition of these particles, the average particle size of the primary particles, and the D50 of the secondary particles are as follows. Composition: LiNi 0.5 Co 0.2 Mn 0.3 O2 Average particle size of primary particles: 1.6 μm Secondary particles (second lithium metal composite oxide particles) D50: 14.1 μm

[0058] [Fabrication of the positive electrode] A first positive electrode slurry with a solid content of 70% by mass was prepared by mixing first lithium metal composite oxide particles, acetylene black (AB), and polyvinylidene fluoride (PVdF) in a mass ratio of 98:1:1, and adding an appropriate amount of N-methyl-2-pyrrolidone (NMP) as a dispersion medium. A second positive electrode slurry was prepared in the same manner as the first positive electrode slurry, except that second lithium metal composite oxide particles were used instead of the first lithium metal composite oxide particles. Next, the second positive electrode slurry was applied to both sides of a positive electrode core made of aluminum foil, and then the first positive electrode slurry was applied onto the coating of the second positive electrode slurry. After drying and compressing the coating (linear pressure 3000 N / m), it was cut to a predetermined electrode size to produce a positive electrode with positive electrode slurry layers formed on both sides of the positive electrode core. The application amounts of the first cathode mixture slurry and the second cathode mixture slurry were the same.

[0059] [Fabrication of the negative electrode] The negative electrode active material consists of 95 parts by mass of graphite powder and 5 parts by mass of SiO2. x A mixture of a Si-containing compound represented by [formula] was used. 100 parts by mass of negative electrode active material, 1 part by mass of carboxymethylcellulose sodium (CMC-Na), and water were mixed, and then 1.2 parts by mass of styrene-butadiene rubber (SBR) dispersion was added to prepare a negative electrode mixture slurry. Next, the negative electrode mixture slurry was applied to both sides of a negative electrode core material made of copper foil, the coating was dried and compressed, and then cut to a predetermined electrode size to produce a negative electrode in which negative electrode mixture layers were formed on both sides of the negative electrode core material.

[0060] [Preparation of non-aqueous electrolyte solution] Ethylene carbonate (EC), methyl ethyl carbonate (MEC), and dimethyl carbonate (DMC) were mixed in a volume ratio of 20:5:75 (at 25°C). LiPF6 was dissolved in this mixed solvent to a concentration of 1.4 mol / L, and vinylene carbonate (VC) and 1,6-diisocyanatohexane (HDI) were added. VC and HDI were added to concentrations of 3% by mass and 0.5% by mass, respectively.

[0061] [Battery construction] Lead terminals were attached to the positive and negative electrodes, respectively, and a wound electrode body was fabricated by winding the positive and negative electrodes in a spiral shape via a separator. The electrode body was housed in a bottomed cylindrical outer casing, the negative electrode lead was welded to the inner surface of the bottom of the outer casing, and the positive electrode lead was welded to the internal terminal plate of the sealing body. Then, the non-aqueous electrolyte was poured into the outer casing, and the opening edge of the outer casing was crimped and fixed to the sealing body to fabricate a cylindrical secondary battery A1 with a battery capacity of 3400mAh.

[0062] <Comparative Example 1> In the preparation of the positive electrode, the second positive electrode mixture slurry was used instead of the first positive electrode mixture slurry (forming the positive electrode mixture layer using only the second positive electrode mixture slurry), and HDI was not added in the preparation of the non-aqueous electrolyte. In addition, a non-aqueous electrolyte secondary battery B1 was prepared in the same manner as in Example 1.

[0063] <Comparative Example 2> A non-aqueous electrolyte secondary battery B2 was prepared in the same manner as in Example 1, except that a second positive electrode mixture slurry was used instead of the first positive electrode mixture slurry in the preparation of the positive electrode.

[0064] <Comparative Example 3> Non-aqueous electrolyte secondary battery B3 was prepared in the same manner as in Example 1, except that HDI was not added during the preparation of the non-aqueous electrolyte.

[0065] [Evaluation of cycle characteristics (capacity retention rate)] For the batteries in the examples and comparative examples, constant current charging was performed at a current of 990mA (0.3-hour rate) at a temperature of 45°C until the battery voltage reached 4.2V, followed by constant voltage charging with a termination current of 66mA at 4.2V. Subsequently, constant current discharge was performed at a current of 990mA until the battery voltage reached 3V. This charge-discharge cycle was performed 400 times, and the capacity retention rate was calculated using the following formula. The evaluation results are shown in Table 1. Capacity retention rate = (Discharge capacity at 400 cycles / Discharge capacity at 1 cycle) × 100

[0066] [Table 1]

[0067] As shown in Table 1, Battery A1 of Example 1 has a high capacity retention rate and excellent cycle characteristics. On the other hand, Battery B1 of Comparative Example 1, which does not contain non-aggregated particles and diisocyanate compounds, has a significantly lower capacity retention rate compared to Battery A1. Battery B2 of Comparative Example 2, which has a diisocyanate compound added to Battery B1, also showed little improvement in cycle characteristics and a low capacity retention rate. Battery B3 of Comparative Example 3, which contains non-aggregated particles but does not contain diisocyanate compounds, has a significantly improved capacity retention rate compared to B1 and B2, but it does not reach the capacity retention rate of Battery A1. Furthermore, according to Battery A1 of Example 1, the packing density of the positive electrode active material can be increased by using non-aggregated particles and secondary particles in combination, thereby increasing the capacity of the battery. [Explanation of Symbols]

[0068] 10 Non-aqueous electrolyte secondary battery, 11 Positive electrode, 12 Negative electrode, 13 Separator, 14 Electrode body, 16 Outer casing, 17 Sealing body, 18,19 Insulating plate, 20 Positive electrode lead, 21 Negative electrode lead, 22 Grooved section, 23 Internal terminal plate, 24 Lower valve body, 25 Insulating material, 26 Upper valve body, 27 Cap, 28 Gasket, 30 Positive electrode core material, 31 Positive electrode mixture layer, 31a First region, 31b Second region

Claims

1. A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, The positive electrode comprises a positive electrode core material and a positive electrode mixture layer formed on the surface of the positive electrode core material. The positive electrode mixture layer includes, as a positive electrode active material, first lithium metal composite oxide particles which are non-aggregated particles with a volume-based median diameter of 2 to 10 μm, and second lithium metal composite oxide particles which are secondary particles formed by aggregation of primary particles with an average particle size of 50 nm to 2 μm and a volume-based median diameter of 10 to 30 μm. When the positive electrode composite layer is divided into two equal parts in the thickness direction, and these are defined as the first region and the second region in order from the surface side of the positive electrode, the first lithium metal composite oxide particles are present in greater quantities in the first region than in the second region. The aforementioned non-aqueous electrolyte is a non-aqueous electrolyte secondary battery containing a diisocyanate compound.

2. The diisocyanate compound is a compound represented by the general formula: O=C=N-R-N=C=O, where R is an aliphatic hydrocarbon group having 1 to 12 carbon atoms, or an aromatic hydrocarbon group having 6 to 20 carbon atoms, as described in claim 1, for the non-aqueous electrolyte secondary battery.

3. The non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the content of the diisocyanate compound is 0.01 to 3% by mass relative to the mass of the non-aqueous electrolyte.

4. The non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the content of the first lithium metal composite oxide particles in the first region is 50% by mass or more relative to the mass of the positive electrode active material contained in the first region.

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