Nonaqueous electrolyte secondary battery and method for manufacturing nonaqueous electrolyte secondary battery

Abstract

Provided is a non-aqueous electrolyte secondary battery having an excellent capacity retention rate. According to the present invention, a non-aqueous electrolyte secondary battery (100) is provided with: an electrode body having a positive electrode (22) and a negative electrode (24); and a non-aqueous electrolyte. The positive electrode (22) contains a lithium transition metal composite oxide, the negative electrode (24) contains graphite particles and a Si-containing material, and the non-aqueous electrolyte solution contains fluoroethylene carbonate and a difluorophosphate. The ratio of the mass of the fluoroethylene carbonate to the mass of the Si-containing material is 0.2 or more.

Description

Technical Field

[0001] The present invention relates to a non-aqueous electrolyte secondary battery and a method for manufacturing the non-aqueous electrolyte secondary battery. Background Art

[0002] In the past, non-aqueous electrolyte secondary batteries with positive electrodes, negative electrodes and non-aqueous electrolytes were known. In such non-aqueous electrolyte secondary batteries, various combinations of positive electrode active materials, negative electrode active materials and non-aqueous electrolytes have been studied. As related prior art, Japanese Patent Application Publication No. 2020-64717 and 2 can be cited. Japanese Patent Application Publication No. 2020-64717 describes a non-aqueous electrolyte secondary battery, which contains a positive electrode active material represented by a prescribed general formula, and an additive selected from cyclohexylbenzene and biphenyl and a non-aqueous electrolyte of lithium difluorophosphate. Japanese Patent Application Publication No. 2019-185992 describes a non-aqueous electrolyte secondary battery, which contains a negative electrode active material including silicon oxide particles and a silicon layer, and a non-aqueous electrolyte containing at least one selected from fluoroethylene carbonate and vinylene carbonate.

[0003] Prior art literature

[0004] Patent Literature

[0005] Patent Document 1: Japanese Patent Application Publication No. 2020-64717

[0006] Patent Document 2: Japanese Patent Application Publication No. 2019-185992 Summary of the invention

[0007] In recent years, non-aqueous electrolyte secondary batteries containing Si-containing materials have been studied for the purpose of high capacity, etc. Fluorinated ethylene carbonate is adsorbed on the surface of the negative electrode to protect the surface of the Si-containing material, suppressing the side reaction with the electrolyte, thereby improving the capacity retention rate after the cycle. In addition, lithium difluorophosphate is adsorbed on the surface of the positive electrode to protect the surface of the positive electrode active material, which can suppress the increase in internal resistance. However, according to the results of the present inventors' research, it is found that when the negative electrode contains Si-containing material and uses a non-aqueous electrolyte containing only any one of fluoroethylene carbonate and lithium difluorophosphate, the cycle characteristics of the non-aqueous electrolyte secondary battery (for example, the capacity retention rate after the cycle) are not sufficient.

[0008] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a non-aqueous electrolyte secondary battery having a negative electrode including a Si-containing material and having an excellent capacity retention rate, and a method for producing the same.

[0009] According to the present invention, a non-aqueous electrolyte secondary battery is provided, which comprises an electrode body having a positive electrode and a negative electrode, a non-aqueous electrolyte, and a battery case. The positive electrode of the non-aqueous electrolyte secondary battery comprises a lithium transition metal composite oxide containing at least lithium and manganese as a positive electrode active material, the negative electrode comprises graphite particles and a Si-containing material as a negative electrode active material, and the non-aqueous electrolyte comprises fluoroethylene carbonate and difluorophosphate. Moreover, the ratio of the mass of the fluoroethylene carbonate to the mass of the Si-containing material is greater than 0.2.

[0010] According to the above-mentioned constitution, the protective effect of fluoroethylene carbonate on the negative electrode surface (especially the surface of the Si-containing material) and the protective effect of difluorophosphate on the positive electrode surface (especially the surface of the positive electrode active material) are appropriately exerted. Thus, cracks of the Si-containing material accompanying charge and discharge are suppressed, and a non-aqueous electrolyte secondary battery with excellent capacity retention can be realized.

[0011] In addition, from another perspective, a method for manufacturing a non-aqueous electrolyte secondary battery having an electrode body with a positive electrode and a negative electrode, a non-aqueous electrolyte, and a battery case is provided. The manufacturing method includes: a construction step of accommodating the above-mentioned electrode body in the above-mentioned battery case to construct an assembly, a preparation step of preparing a non-aqueous electrolyte containing fluoroethylene carbonate and difluorophosphate, and a liquid injection step of injecting the above-mentioned non-aqueous electrolyte into the above-mentioned battery case. The above-mentioned positive electrode contains a positive electrode active material, and the above-mentioned negative electrode contains graphite particles and Si-containing materials as negative electrode active materials. In the above-mentioned preparation step, the above-mentioned non-aqueous electrolyte is prepared in a manner such that the ratio of the mass of the above-mentioned fluoroethylene carbonate to the mass of the above-mentioned Si-containing material is 0.2 or more. According to the above-mentioned composition, a non-aqueous electrolyte secondary battery with excellent capacity retention rate can be appropriately manufactured. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 It is a perspective view schematically showing a nonaqueous electrolyte secondary battery according to one embodiment.

[0013] Figure 2 It is along Figure 1 Schematic longitudinal cross-sectional view along line II-II.

[0014] Figure 3 It is a schematic diagram showing the structure of an electrode body.

[0015] Figure 4 The present invention is a flowchart of a manufacturing method according to an embodiment.

[0016] Explanation of symbols

[0017] 10 Battery housing

[0018] 20 Electrode body

[0019] 22 Positive electrode

[0020] 22a Positive electrode active material layer

[0021] 24 negative electrode

[0022] 24a Negative electrode active material layer

[0023] 100 battery (non-aqueous electrolyte secondary battery)

[0024] S1 Construction Process

[0025] S2 Preparation Process

[0026] S3 injection process DETAILED DESCRIPTION

[0027] Hereinafter, some preferred embodiments of the technology disclosed herein are described with reference to the accompanying drawings. It should be noted that matters other than matters not specifically mentioned in this specification and necessary for the implementation of the present invention (for example, general structures and manufacturing processes that do not characterize the non-aqueous electrolyte secondary battery of the present invention) can be grasped as design matters for those skilled in the art based on the prior art in the field. The present invention can be implemented based on the contents disclosed in this specification and the technical common sense in the field.

[0028] It should be noted that in this specification, "non-aqueous electrolyte secondary battery" refers to all batteries that can be repeatedly charged and discharged by charge carriers moving between the positive electrode and the negative electrode via a non-aqueous electrolyte. Non-aqueous electrolyte secondary batteries are a concept that includes so-called storage batteries such as lithium-ion secondary batteries and capacitors such as lithium-ion capacitors and double-layer capacitors. In addition, in this specification, the description of "A to B" indicating a range includes the meaning of more than A and less than B, as well as the meaning of "greater than A" and "less than B".

[0029] <Battery 100>

[0030] Figure 1 It is a perspective view of the battery 100 . Figure 2 It is along Figure 1 Schematic longitudinal cross-sectional view of line II-II of FIG. In the following description, the symbols L, R, F, Rr, U, and D in the drawings represent left, right, front, back, top, and bottom, and the symbols X, Y, and Z in the drawings represent the short side direction of the battery 100, the long side direction orthogonal to the short side direction, and the up and down direction orthogonal to the short side direction and the long side direction, respectively. These are directions for convenience of explanation only, and there is no limitation on the setting form of the battery 100.

[0031] like Figure 2As shown, the battery 100 includes a battery case 10, an electrode body 20, a positive electrode terminal 30, a negative electrode terminal 40, a positive electrode current collecting portion 50, a negative electrode current collecting portion 60, and a non-aqueous electrolyte (not shown). The battery 100 is a lithium ion secondary battery. The battery 100 is preferably a lithium ion secondary battery.

[0032] The battery case 10 is a case that contains the electrode body 20 and the non-aqueous electrolyte. Figure 1 As shown, the battery case 10 has a flat and bottomed cubic (square) shape. The material of the battery case 10 can be the same as that used in the past, and there is no particular limitation. The battery case 10 is preferably made of metal, and more preferably is made of, for example, aluminum, aluminum alloy, iron, iron alloy, etc. However, in another embodiment, the battery case 10 may also be in the shape of a bag made of, for example, a laminated film. Figure 2 As shown, the battery case 10 includes: an outer body 12 having an opening 12h, and a sealing plate (lid) 14 that seals the opening 12h. The sealing plate 14 is attached to the outer body 12 in a manner that seals the opening 12h of the outer body 12. The battery case 10 is integrated by joining (for example, welding) the sealing plate 14 to the periphery of the opening 12h of the outer body 12. The battery case 10 is hermetically sealed (sealed).

[0033] like Figure 2 As shown, the sealing plate 14 is provided with a liquid injection hole 15 and two terminal lead holes 18 and 19. The liquid injection hole 15 is used to inject non-aqueous electrolyte into the battery case 10 after the sealing plate 14 is assembled to the outer body 12. It is preferred to provide the liquid injection hole 15 on the sealing plate 14. The liquid injection hole 15 is sealed by a sealing member 16. The terminal lead holes 18 and 19 are respectively formed at both ends of the long side direction Y of the sealing plate 14 ( Figure 2 The terminal lead holes 18 and 19 penetrate the sealing plate 14 in the thickness direction (vertical direction Z).

[0034] like Figure 2 As shown, the positive terminal 30 and the negative terminal 40 are inserted through the terminal lead-out holes 18 and 19 and extend from the inside of the sealing plate 14 to the outside. The positive terminal 30 and the negative terminal 40 are riveted to the peripheral portion of the sealing plate 14 surrounding the terminal lead-out holes 18 and 19 by riveting. The end portion ( Figure 2 A rivet portion 30c, 40c is formed at the lower end portion of the rivet portion.

[0035] The positive terminal 30 is electrically connected to the positive electrode tab group 23 of the electrode body 20 via the positive electrode collector 50 inside the battery case 10. The positive terminal 30 is insulated from the sealing plate 14 by the positive electrode insulating component 70 and the gasket 90. The positive terminal 30 is preferably made of metal, and more preferably, it is composed of, for example, aluminum or an aluminum alloy. The negative terminal 40 is electrically connected to the negative electrode tab group 25 of the electrode body 20 via the negative electrode collector 60 inside the battery case 10. The negative terminal 40 is insulated from the sealing plate 14 by the negative electrode insulating component 80 and the gasket 90. The negative terminal 40 is preferably made of metal, and more preferably, it is composed of, for example, copper or a copper alloy.

[0036] As described above, the electrode body 20 and the non-aqueous electrolyte are housed inside the battery case 10. The non-aqueous electrolyte of the battery 100 disclosed herein contains fluoroethylene carbonate (hereinafter also referred to as "FEC") and difluorophosphate. The non-aqueous electrolyte may contain a non-aqueous solvent other than fluoroethylene carbonate and a salt (auxiliary salt) other than difluorophosphate within the scope of not damaging the technology disclosed herein.

[0037] The above-mentioned FEC is attached to the surface of the Si-containing material contained in the negative electrode 24, and the effect of protecting the Si-containing material is particularly high. On the other hand, FEC has a carbonate skeleton, so there is a tendency to be easily adsorbed on the surface of the positive electrode 22 (specifically, the surface of the positive electrode active material). In addition, difluorophosphate easily interacts with the surface of the positive electrode active material and is a material adsorbed on the surface of the positive electrode active material to protect the active material. According to the results of the research of the present inventors, it was found that when a non-aqueous electrolyte containing only one of FEC and difluorophosphate is used, the effect of improving the capacity retention rate of the non-aqueous electrolyte secondary battery cannot be fully exerted. It is speculated that this is because the Si-containing material is not protected in the absence of FEC, and a film formation on the surface of the Si-containing material will occur. In addition, it is speculated that since FEC is also attached to the positive electrode surface in the absence of difluorophosphate, the Si-containing material cannot be fully protected, and a film formation on the surface of the Si-containing material will occur. Therefore, the battery 100 disclosed herein contains FEC and difluorophosphate. Since the difluorophosphate is preferentially attached to the surface of the positive electrode 22 over FEC, the protective effect of FEC on the Si-containing material is appropriately exerted. As a result, the cycle characteristics of the nonaqueous electrolyte secondary battery (for example, the capacity retention rate after cycling) are improved.

[0038] Fluorinated ethylene carbonate (FEC) is a type of fluorinated cyclic carbonate. Carbonates are divided into non-fluorinated carbonates and fluorinated carbonates according to whether the constituent elements contain fluorine (F). In addition, carbonates are divided into chain carbonates and cyclic carbonates according to the chemical structure. Chain carbonates are non-cyclic (chain) carbonate compounds having a carbonate skeleton (O-CO-O). Cyclic carbonates are carbonate compounds having a carbonate skeleton closed in a ring with a C-C bond. It should be noted that the non-aqueous solvent may contain fluorinated carbonates other than FEC, such as fluorinated chain carbonates and fluorinated cyclic carbonates other than FEC.

[0039] As a non-aqueous solvent other than FEC, one or more non-aqueous solvents known in the past that can be used for non-aqueous electrolyte secondary batteries can be used. As an example of a non-aqueous solvent, organic solvents such as carbonates, ethers, esters, nitriles, sulfones, and lactones can be cited. The non-aqueous solvent preferably contains carbonates. Specifically, for example, ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), etc. can be cited.

[0040] When the content of FEC in the non-aqueous electrolyte is too small, it is difficult to properly exert the protective effect of the above-mentioned Si-containing material. Here, from the viewpoint of properly exerting the technical effect disclosed herein, the content of FEC in the non-aqueous electrolyte is preferably determined by the ratio to the Si-containing material contained in the negative electrode 24. The ratio of the mass of FEC to the mass of the Si-containing material is preferably greater than 0.2, more preferably greater than 0.28, and may also be greater than 0.45. As a result, the capacity retention rate after cycling is better improved. The upper limit of the ratio of the mass of FEC to the mass of the Si-containing material is not particularly limited. If the content of FEC in the non-aqueous solvent increases, there is a tendency for the amount of gas generated during charging and discharging to increase. From the viewpoint of reducing the amount of gas generated, the ratio of the mass of FEC to the mass of the Si-containing material is, for example, preferably less than 1.6, may be less than 1.2, or may be less than 1.

[0041] Although not particularly limited, the ratio of the mass of FEC to the total amount of negative electrode active material contained in the negative electrode 24 (i.e., the total mass of graphite particles, Si-containing materials and other negative electrode active materials) is preferably 0.04 or less. As a result, the amount of gas generated after the cycle is appropriately reduced. The ratio of the mass of FEC to the mass of the negative electrode active material is preferably 0.005 to 0.04, more preferably 0.01 to 0.04, and can also be 0.01 to 0.035. Although not particularly limited, when the ratio of the mass of FEC to the mass of the negative electrode active material is 0.007 to 0.048, and the ratio of the mass of FEC to the mass of the Si-containing material is 0.2 or more, the capacity retention rate after the cycle can be appropriately improved and the gas generation can be reduced.

[0042] Although not particularly limited, when the non-aqueous solvent as a whole is set to 100% by volume, the volume ratio of FEC is preferably 10% by volume or less. Thus, the protective effect of the Si-containing material is properly exerted, and the amount of gas generated during charge and discharge can be properly suppressed. For example, when the non-aqueous solvent as a whole is set to 100% by volume, FEC is preferably 0.5% by volume to 10% by volume, more preferably 1% by volume to 10% by volume, and further preferably 1% by volume to 7% by volume.

[0043] Difluorophosphate is PO2F2 － The salt of the anion represented by and the cation. As the cation, alkali metal ions such as Li, Na, K, ammonium ions, etc. can be cited, among which Li is preferred. As a specific example of a preferred difluorophosphate, lithium difluorophosphate (lithium difluorophosphate (LiDFP), LiPO2F2) can be cited.

[0044] When the content of difluorophosphate in the non-aqueous electrolyte is too little, since FEC can preferentially adhere to the positive electrode surface as described above, it is difficult to properly exert the protective effect of the Si-containing material based on FEC. Here, from the perspective of properly exerting the technical effects disclosed herein, the content of difluorophosphate in the non-aqueous electrolyte is preferably determined by the ratio with the positive electrode active material contained in the positive electrode 22. The ratio of the mass of difluorophosphate to the mass of the positive electrode active material is preferably 0.0008 or more, more preferably 0.001 or more, and further preferably 0.0015 or more. Thus, the protective effect on the positive electrode surface and the negative electrode surface is more appropriately exerted, and the capacity retention rate after cycling is appropriately improved. The upper limit of the ratio of the mass of difluorophosphate to the mass of the positive electrode active material is not particularly limited, for example, preferably less than 0.01, more preferably less than 0.005, and can also be less than 0.0045.

[0045] Although not particularly limited, the proportion of the difluorophosphate in the entire nonaqueous electrolyte is preferably 0.1% by mass to 2% by mass, and more preferably 0.2% by mass to 1.2% by mass.

[0046] As salts other than difluorophosphate, one or more salts known to be usable in non-aqueous electrolyte secondary batteries can be used. In the case of lithium ion secondary batteries, for example, fluorine-containing lithium salts such as LiPF6 and LiBF4 can be used.

[0047] The non-aqueous electrolyte may further contain additives. As additives, one or more additives that are known to be added to non-aqueous electrolytes may be used. For example, various additives such as biphenyl (BP) and cyclohexylbenzene (CHB), gas generators, dispersants, and thickeners may be contained.

[0048] like Figure 2 As shown, the electrode body 20 is housed inside the battery case 10 (specifically, inside the outer casing 12). The number of electrode bodies 20 disposed inside one battery case 10 is not particularly limited, and may be one or more than two (plural).

[0049] Figure 3 Schematic diagram showing the structure of the electrode body 20. Figure 3 As shown, the electrode body 20 includes a positive electrode 22, a negative electrode 24, and a separator 26. The electrode body 20 is a flat wound electrode body. The electrode body 20 is formed by stacking a strip-shaped positive electrode 22 and a strip-shaped negative electrode 24 with a strip-shaped separator 26 interposed therebetween and winding them around a winding axis WL. The electrode body 20 may also be a stacked electrode body in which a square (typically rectangular) positive electrode and a square (typically rectangular) negative electrode are stacked in an insulated state.

[0050] The positive electrode 22 has a positive electrode collector 22c and a positive electrode active material layer 22a fixed on at least one surface of the positive electrode collector 22c. The positive electrode collector 22c is in a strip shape. The positive electrode collector 22c is composed of a conductive metal such as aluminum, aluminum alloy, nickel, stainless steel, etc. The positive electrode collector 22c is a metal foil, specifically an aluminum foil. The positive electrode 22 may have a positive electrode protective layer 22p at one end in the long side direction. When the positive electrode 22 has a positive electrode protective layer 22p, the positive electrode protective layer 22p is arranged between the positive electrode collector 22c and the positive electrode active material layer 22a in the long side direction Y. The positive electrode protective layer 22p contains, for example, an inorganic filler (for example, aluminum oxide).

[0051] One end portion of the positive electrode current collector 22c in the longitudinal direction Y ( Figure 3 A plurality of positive electrode tabs 22t are provided at the left end of the positive electrode collector 22c. The positive electrode tab 22t is a part of the positive electrode collector 22c and is made of metal foil (aluminum foil). The plurality of positive electrode tabs 22t are stacked at one end in the longitudinal direction Y ( Figure 3 ), such as Figure 2 As shown, a positive electrode tab group 23 is formed. A positive electrode current collecting portion 50 is attached to (specifically, joined to) the positive electrode tab group 23. The positive electrode tab group 23 is electrically connected to the positive electrode terminal 30 via the positive electrode current collecting portion 50.

[0052] The positive electrode active material layer 22a is arranged in a strip shape along the long side direction of the strip-shaped positive electrode collector 22c. The positive electrode active material layer 22a contains a positive electrode active material that can reversibly absorb and release charge carriers. The positive electrode active material contains a lithium transition metal composite oxide. The positive electrode active material is preferably a lithium transition metal composite oxide containing at least lithium and manganese. The crystal structure of the lithium transition metal composite oxide is not particularly limited, and may be a layered structure, a spinel structure, an olivine structure, or the like. In some embodiments, from the viewpoint of achieving high energy density, the lithium transition metal composite oxide preferably has a layered rock salt type crystal structure (so-called lithium excess type crystal structure) of space group C2 / m. As such a compound, a lithium excess transition metal composite oxide represented by the following formula (I) can be cited. As a specific example of such a lithium excess transition metal composite oxide, for example, Li 1.1 Ni 0.35 Mn 0.55 O2、Li 1.14 Ni 0.29 Mn 0.57 O2、Li 1.17 Ni 0.25 Mn 0.58 O2、Li 1.2 Ni 0.2 Mn 0.6 O2、Li 1.33 Mn 0.67 O2, etc.

[0053] Li 1+a Ni x Mn y M z O2 Formula (I)

[0054] In formula (I), when a, x, y, and z satisfy 0.1≤a≤0.33, 0≤x≤0.5, 0.5≤y≤0.7, 0≤z≤0.2, a+x+y+z=1, and 0<z, M is one or more elements selected from Co, Al, Mg, Ca, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W.

[0055] Although not particularly limited, the ratio of the amount of Li in the positive electrode active material to the amount of metal elements (Me) other than Li (Li / Me) is preferably 1.1 or more, more preferably 1.2 or more, further preferably 1.26 or more, and particularly preferably 1.3 or more. The upper limit of the value of Li / Me is 2.0 or less, and can be 1.5 or less, or 1.4 or less. It should be noted that the unit of amount of substance is mole (mol). The ratio of the amount of Li in the positive electrode active material to the amount of metal elements (Me) other than Li (Li / Me) is, for example, preferably 1.2 to 2, more preferably 1.2 to 1.5, and further preferably 1.26 to 1.45. As a positive electrode active material, by using a positive electrode active material such that Li / Me satisfies the above numerical range, the cycle characteristics (for example, the capacity retention rate after cycling) are better improved.

[0056] It is not intended to limit the technology disclosed herein, but the reasons for obtaining the above-mentioned effects are presumed to be as follows. As described above, by containing FEC and difluorophosphate in the non-aqueous electrolyte, the positive electrode surface and the negative electrode surface are properly protected, and the capacity retention rate after the cycle is improved. Here, in the lithium transition metal composite oxide of the lithium excess system as described above, there is a tendency to more easily adsorb difluorophosphate. In particular, in the Li excess positive electrode active material with a Li / Me value of 1.2 or more (preferably 1.26 or more, more preferably 1.3 or more), difluorophosphate is more easily adsorbed, and as a result, the protective effect of the negative electrode surface based on FEC is particularly appropriately exerted. As a result, the protective effect of the negative electrode 24 of FEC and the protective effect of the positive electrode 22 of the difluorophosphate are properly exerted, and the cycle characteristics (for example, the capacity retention rate after the cycle) of the battery 100 are improved.

[0057] The content of the positive electrode active material in the positive electrode active material layer 22a is not particularly limited. The content of the positive electrode active material in the positive electrode active material layer 22a (i.e., relative to the total mass of the positive electrode active material layer) is preferably 70% by mass or more, more preferably 80% by mass to 99% by mass, and further preferably 85% by mass to 98% by mass.

[0058] The average particle size (D50 particle size) of the positive electrode active material is not particularly limited. The average particle size (D50 particle size) of the positive electrode active material is, for example, 0.5 μm to 25 μm, preferably 10 μm to 25 μm. It should be noted that in this specification, "D50 particle size" refers to the particle size equivalent to 50% cumulative from the microparticle side in the volume-based particle size distribution measured by particle size distribution measurement based on laser diffraction scattering method.

[0059] The positive electrode active material layer 22a may contain any component other than the positive electrode active material, such as a binder, a conductive material, various additives, etc. As a binder, for example, polyvinylidene fluoride (PVdF) or the like may be appropriately used. The content of the binder in the positive electrode active material layer 22a is not particularly limited, and is, for example, 0.5% to 15% by mass, preferably 1% to 10% by mass, and more preferably 1.5% to 8% by mass. As a conductive material, carbon black such as acetylene black (AB), and other carbon materials (for example, graphite, etc.) may be appropriately used. The content of the conductive material in the positive electrode active material layer 22a is not particularly limited, and is, for example, 0.1% to 20% by mass, preferably 1% to 15% by mass, and more preferably 2% to 10% by mass.

[0060] The negative electrode 24 has a negative electrode current collector 24c and a negative electrode active material layer 24a fixed on at least one surface of the negative electrode current collector 24c. The negative electrode current collector 24c is in a strip shape. The negative electrode current collector 24c is made of a conductive metal such as copper, a copper alloy, nickel, stainless steel, etc. The negative electrode current collector 24c is a metal foil, specifically a copper foil.

[0061] One end portion of the negative electrode current collector 24c in the longitudinal direction Y ( Figure 3 A plurality of negative electrode tabs 24t are provided at the right end of the negative electrode collector 24c. The negative electrode tab 24t is a part of the negative electrode collector 24c and is made of metal foil (copper foil). The plurality of negative electrode tabs 24t are stacked at one end in the longitudinal direction Y ( Figure 3 ), such as Figure 2 As shown, the negative electrode tab group 25 is formed. A negative electrode current collecting portion 60 is attached to (specifically, joined to) the negative electrode tab group 25. The negative electrode tab group 25 is electrically connected to the negative electrode terminal 40 via the negative electrode current collecting portion 60.

[0062] The negative electrode active material layer 24a is arranged in a strip shape along the long side direction of the strip-shaped negative electrode collector 24c. The negative electrode active material layer 24a contains a negative electrode active material that can reversibly absorb and release charge carriers. The negative electrode active material contains at least graphite particles and Si-containing materials. In the battery 100 disclosed herein, the protective effect of the non-aqueous electrolyte on the Si-containing material is properly exerted, so that cracks in the Si-containing material are suppressed. As a result, the cycle characteristics (especially the capacity retention rate after the cycle) are improved. In addition, by using the above-mentioned lithium-excess lithium transition metal composite oxide as the positive electrode active material, cracks in the Si-containing material are further suppressed. The technology disclosed herein is not intended to be limited, but the reason for obtaining the above-mentioned effect is presumed to be as follows. By containing FEC and difluorophosphate in the non-aqueous electrolyte, the surface of the Si-containing material is protected, the decomposition of the electrolyte is suppressed, and the formation of the film is suppressed. As a result, the cycle characteristics (especially the capacity retention rate after the cycle) are improved. In addition, the lithium-excess lithium transition metal composite oxide is different from the previous lithium transition metal composite oxide (for example, LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 Compared with non-lithium-excess lithium transition metal composite oxides such as lithium-ion batteries (such as lithium transition metal composite oxides) in the lithium-ion battery 100, there is a tendency for the range of working potential to be slightly narrower. Therefore, the use range of the Si-containing material during charge and discharge also becomes narrower. As a result, the volume change of the Si-containing material during charge and discharge is suppressed, and the cracks of the Si-containing material are appropriately suppressed. By suppressing the cracks of the Si-containing material, the new surfaces generated by the cracks are also reduced, and there is a tendency for the need for FEC to further protect the new surfaces to decrease. As a result, the protective effect of FEC is fully exerted. As a result, the capacity retention rate of the battery 100 after cycling is appropriately improved.

[0063] Si-containing materials have a larger specific capacity than carbon materials, and therefore are appropriately used for purposes such as increasing the capacity of the battery 100. As long as the Si-containing material contains Si, it may also contain components other than Si. As a preferred example of a Si-containing material, there are Si (silicon), SiO (silicon oxide), a composite of silicon and carbon (including silicon carbide, SiC composites in which carbon is dispersed inside silicon particles), a material containing SiN (silicon nitride), and a material in which nano-Si particles are dispersed in porous particles. The Si-containing material may use one or more of the above materials. Among them, as the Si-containing material, silicon, silicon oxide, and a composite of silicon and carbon can be preferably used. Although not particularly limited, the D50 particle size of the Si-containing particles is preferably, for example, 1 μm to 15 μm, and more preferably 2 μm to 10 μm.

[0064] The mass ratio of the Si-containing material in the negative electrode active material is not particularly limited. For example, from the viewpoint of achieving both high capacity and improved cycle characteristics of the battery 100, when the total amount of the negative electrode active material is set to 100 mass%, the content ratio of the Si-containing material is preferably 1 mass % to 15 mass %, and more preferably 1 mass % to 10 mass %.

[0065] As graphite particles, for example, artificial graphite, natural graphite, etc. can be appropriately used. The graphite particles can have a coating layer of amorphous carbon on their surface. Without particular limitation, the graphite particles can be roughly spherical. It should be noted that in this specification, "roughly spherical" refers to terms including spherical, rugby-shaped, etc., for example, the average aspect ratio (the ratio of the length in the major axis direction to the length in the minor axis direction in the smallest rectangle circumscribed by the particle) is, for example, 1 to 2 (preferably 1 to 1.5). The D50 particle size of the graphite particles is not particularly limited, for example, preferably 5μm to 30μm, more preferably 10μm to 25μm.

[0066] Although not particularly limited, the content of the graphite particles is preferably 85 to 99 mass %, more preferably 90 to 99 mass %, when the total amount of the negative electrode active material is 100 mass %. This appropriately achieves both increased capacity and improved cycle characteristics of the battery 100 .

[0067] The negative electrode active material may include negative electrode active materials other than Si-containing materials and graphite. Specific examples of negative electrode active materials other than Si-containing materials and graphite include carbon materials such as hard carbon, soft carbon, and amorphous carbon.

[0068] The content of the negative electrode active material in the negative electrode active material layer 24 a is not particularly limited. The content of the negative electrode active material in the negative electrode active material layer 24 a is preferably 90% by mass or more, and more preferably 95% by mass to 99% by mass.

[0069] The negative electrode active material layer 24a may contain any component other than the negative electrode active material, such as a binder, a dispersant, a conductive material, various additives, etc. As a binder, rubbers such as styrene butadiene rubber (SBR) and acrylic resins such as polyacrylic acid (PAA) can be appropriately used. The content of the binder in the negative electrode active material layer 24a is not particularly limited, and is, for example, 0.1% to 10% by mass, preferably 0.5% to 5% by mass. As a thickener, celluloses such as carboxymethyl cellulose (CMC) can be appropriately used. The content of the dispersant in the negative electrode active material layer 24a is not particularly limited, and is, for example, 0.1% to 10% by mass, and is preferably 0.5% to 5% by mass.

[0070] The separator 26 is a component that insulates the positive electrode active material layer 22a of the positive electrode 22 from the negative electrode active material layer 24a of the negative electrode 24. The separator 26 can be the same as before, and there is no particular limitation. As the separator 26, a resin porous sheet composed of a polyolefin resin such as polyethylene (PE) and polypropylene (PP) is preferred. The separator 26 may have a heat resistance layer (Heat Resistance Layer: HRL) and an adhesive layer on the surface of the base material portion composed of the resin porous sheet. The composition of the heat resistance layer and the adhesive layer can be the same as before.

[0071] <Method for producing non-aqueous electrolyte secondary battery>

[0072] Figure 4 FIG. 1 is a flow chart of a manufacturing method according to an embodiment. Figure 4 As shown, the battery 100 can be manufactured by, for example, a manufacturing method including a construction step (step S1), a preparation step (step S2), and a liquid injection step (step S3). The manufacturing method disclosed herein can further include other steps at any stage.

[0073] The construction process (step S1) is a process of accommodating the electrode body 20 in the battery case 10 to construct an assembly (a combination of the battery case 10 and the electrode body 20). In a preferred embodiment, this process includes an electrode production process (step S1-1), an electrode body production process (step S1-2), a configuration process (step S1-3) and a welding and joining process (step S1-4). In addition, other processes may be further included at any stage.

[0074] First, in the electrode manufacturing process (step S1-1), the positive electrode 22 and the negative electrode 24 are respectively manufactured. The positive electrode 22 can be manufactured by, for example, mixing a positive electrode active material, a conductive material, a binder and a dispersion solvent to prepare a positive electrode composite slurry, applying the prepared positive electrode composite slurry on the positive electrode collector 22c according to a conventionally known method, drying, and appropriately pressurizing. As the positive electrode active material, one material can be used alone or two or more materials can be appropriately mixed and used from the materials described above, among which the lithium-excess transition metal composite oxide represented by the above formula (I) can be appropriately used.

[0075] The negative electrode 24 can be prepared by, for example, mixing a negative electrode active material, a binder, a dispersant and a dispersing solvent to prepare a negative electrode composite slurry, applying the prepared negative electrode composite slurry on the negative electrode collector 24c according to a conventionally known method, drying, and appropriately pressurizing. As the negative electrode active material, at least graphite particles and Si-containing materials (preferably at least one of silicon, silicon oxide, and a composite of silicon and carbon) are used. From the viewpoint of both high capacity and improved cycle characteristics of the battery 100, when the total amount of the negative electrode active material is set to 100 mass%, it is preferably prepared in a manner that the content ratio of the Si-containing material is 1 mass% to 15 mass%, and more preferably prepared in a manner that the content ratio of the Si-containing material is 1 mass% to 10 mass%. It should be noted that as the negative electrode active material, one or more of the above-mentioned materials can be further used.

[0076] Next, in the electrode body manufacturing process (step S1-2), the positive electrode 22 and the negative electrode 24 manufactured above are placed opposite to each other via the separator 26 to manufacture the electrode body 20. Next, in the configuration process (step S1-3), the electrode body 20 is configured inside the outer body 12. For example, the electrode body 20 is received inside the outer body 12 from the opening 12h of the outer body 12. Next, in the welding and joining process (step S1-4), the sealing plate 14 is fitted into the opening 12h of the outer body 12, and the sealing plate 14 is welded to the periphery of the opening 12h to integrate the outer body 12 and the sealing plate 14.

[0077] The preparation process (step S2) is a process for preparing a non-aqueous electrolyte. The non-aqueous electrolyte of the battery 100 disclosed herein contains fluoroethylene carbonate and difluorophosphate. As described above, the surface of the positive electrode active material and the surface of the negative electrode active material (especially the Si-containing material) are properly protected by the non-aqueous electrolyte, and the capacity retention rate of the battery 100 is improved. From the viewpoint of exerting the technical effects disclosed herein at a high level, in the preparation process, it is preferred to prepare the non-aqueous electrolyte in a manner such that the ratio of the mass of FEC to the mass of the Si-containing material is 0.2 or more. In the preparation process, it is preferred to prepare the non-aqueous electrolyte in a manner such that the ratio of the mass of FEC to the mass of the Si-containing material is 0.2 to 1.6, and it is more preferred to prepare the non-aqueous electrolyte in a manner such that the ratio of the mass of FEC to the mass of the Si-containing material is 0.28 to 1.2.

[0078] Although not particularly limited, in the preparation process, it is preferred to prepare the nonaqueous electrolyte in a manner where the ratio of the mass of FEC to the total amount of the negative electrode active material (i.e., the total mass of the graphite particles, the Si-containing material, and other negative electrode active materials) is 0.04 or less. In the preparation process, it is preferred to prepare the nonaqueous electrolyte in a manner where the ratio of the mass of FEC to the mass of the negative electrode active material is 0.005 to 0.04, more preferably in a manner where the ratio of the mass of FEC to the mass of the negative electrode active material is 0.01 to 0.04, and it is also possible to prepare the nonaqueous electrolyte in a manner where the ratio of the mass of FEC to the mass of the negative electrode active material is 0.01 to 0.035.

[0079] Although not particularly limited, in the preparation process, when the non-aqueous solvent is set to 100% by volume, it is preferably prepared in a manner that the volume ratio of FEC is 10% by volume or less. Thus, the protective effect of the Si-containing material is properly exerted, and the amount of gas generated during charge and discharge can be properly suppressed. The preparation process is preferably implemented in a manner that the volume ratio of FEC is 0.5% by volume to 10% by volume when the non-aqueous solvent is set to 100% by volume, more preferably implemented in a manner that the volume ratio of FEC is 1% by volume to 10% by volume, and further preferably implemented in a manner that the volume ratio of FEC is 1% by volume to 7% by volume.

[0080] In a preferred embodiment, the non-aqueous electrolyte contains carbonates other than FEC in addition to FEC. Carbonates can be used alone or in combination of two or more of the above materials. In the preparation process, the non-aqueous electrolyte is preferably prepared in a manner that includes at least one of DMC, DEC, EMC, EC and VC.

[0081] As a difluorophosphate, LiPO2F2 can be appropriately used. Although not particularly limited, in the preparation process, it is preferred to prepare a non-aqueous electrolyte in a manner where the ratio of the mass of the difluorophosphate to the mass of the positive electrode active material is 0.001 or more. In the preparation process, it is preferred to prepare a non-aqueous electrolyte in a manner where the ratio of the mass of the difluorophosphate to the mass of the positive electrode active material is 0.001 to 0.01, more preferably in a manner where the ratio of the mass of the difluorophosphate to the mass of the positive electrode active material is 0.001 to 0.005, and further preferably in a manner where the ratio of the mass of the difluorophosphate to the mass of the positive electrode active material is 0.0015 to 0.0045. As a result, the protective effect on the positive electrode surface and the Si-containing material is more appropriately exerted, and the capacity retention rate after cycling is appropriately improved.

[0082] Although not particularly limited, in the preparation process, it is preferred to prepare the non-aqueous electrolyte in a manner such that the proportion of difluorophosphate in the entire non-aqueous electrolyte is 0.1% by mass to 2% by mass, and it is more preferred to prepare the non-aqueous electrolyte in a manner such that the proportion of difluorophosphate in the entire non-aqueous electrolyte is 0.2% by mass to 1.2% by mass.

[0083] The non-aqueous electrolyte preferably contains an auxiliary salt (a lithium salt in the case of a lithium ion secondary battery) in addition to FEC and difluorophosphate. As the auxiliary salt, one of the above materials can be used alone or two or more can be appropriately mixed and used, among which LiPF6 can be appropriately used.

[0084] In the liquid injection process (step S3), the prepared non-aqueous electrolyte is injected into the battery case 10 from the liquid injection hole 15 of the sealing plate 14. The liquid injection can be performed under atmospheric pressure, and for the purpose of, for example, increasing the impregnation of the non-aqueous electrolyte in the electrode body 20, the battery case 10 can be decompressed. The non-aqueous electrolyte injected from the liquid injection hole 15 is impregnated in the electrode body 20. In other words, the non-aqueous electrolyte is distributed in the electrode body 20 without omission.

[0085] Although not particularly limited, the assembly can be placed (kept) for a predetermined time after the injection process. Thus, for example, when the length of the long side direction Y is long, the non-aqueous electrolyte is well-balanced and distributed in the long side direction Y, so that the non-aqueous electrolyte can be fully impregnated in the electrode body 20. The placement temperature can be either room temperature (e.g., 25°C ± 10°C, about 25°C ± 5°C), or a high temperature environment of about 35 to 45°C for the purpose of, for example, improving the impregnation of the non-aqueous electrolyte in the electrode body 20. The placement time also depends on, for example, the size of the electrode body 20 (especially the length of the long side direction Y), the viscosity of the first non-aqueous electrolyte, the presence or absence of decompression, the placement temperature, etc., and is therefore not particularly limited, for example, preferably more than 5 minutes, more preferably more than 10 minutes, and particularly preferably more than 1 hour. In addition, from the viewpoint of manufacturing efficiency, it is preferably within 240 hours (10 days), more preferably within 48 hours (2 days), and particularly preferably within 24 hours (1 day).

[0086] In addition, the manufacturing method disclosed herein may include a charging step. The charging step is a step of charging the assembly injected with a non-aqueous electrolyte at least once. The charging of the assembly can be performed in the same manner as before. Typically, an external power source is connected between the positive terminal and the negative terminal of the assembly, and charging is performed until the assembly reaches a specified voltage. Although not particularly limited, it is preferably charged to a voltage of approximately 4V or more, preferably 4.1V or more, 4.2V or more, for example, 4.2V. In the case of using a lithium-excess transition metal composite oxide as a positive electrode active material, it is more preferably charged to a voltage of approximately 4.6V or more, preferably 4.7V or more, for example, 4.7V or more, from the viewpoint of showing a high capacity in an electrochemically active state. The charge and discharge rate can be, for example, about 0.1 to 2C. Charging can be once, for example, it can also be repeated twice or more with discharging. In addition, after charging, discharging can also be performed. The charging process can be performed at room temperature (for example, 25°C ± 10°C, about 25°C ± 5°C) or at a high temperature of about 35 to 45°C. In summary, the battery 100 can be appropriately manufactured.

[0087] <Application of battery 100>

[0088] The battery 100 can be used for various purposes. For example, due to its excellent cycle characteristics (e.g., capacity retention rate after cycling), it can be suitably used as a motor power source (driving power source) mounted on vehicles such as cars and trucks. The type of vehicle is not particularly limited, and examples thereof include plug-in hybrid electric vehicles (PHEV), hybrid electric vehicles (HEV), and battery electric vehicles (BEV).

[0089] Hereinafter, some embodiments related to the present invention will be described, but the present invention is not intended to be limited to these embodiments.

[0090] Experiment 1

[0091] <Production of secondary battery for evaluation>

[0092] (Example 1)

[0093] First, a positive electrode and a negative electrode were prepared. The positive electrode was prepared as follows. Li 1.14 Ni 0.29 Mn 0.57O2, acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVdF) as a binder. These materials are mixed in a mass ratio of positive electrode active material: AB: PVdF = 100:1:1, and the fluidity is adjusted with N-methylpyrrolidone (NMP) as a dispersion solvent to prepare a positive electrode composite slurry. Next, the prepared positive electrode composite slurry is applied to the Al foil as the positive electrode collector, dried, and pressed to a specified thickness. Then, it is cut into a specified size to make a positive electrode.

[0094] The negative electrode is made as follows. First, graphite and SiO (Si-containing material) are prepared as negative electrode active materials. These materials are mixed in a mass ratio of graphite:SiO=95:5 to obtain a mixture. Next, SBR as a binder and CMC as a dispersant are added to the obtained mixture (100 parts by mass) at a ratio of 1 part by mass, respectively (negative electrode active material: SBR:CMC=100:1:1), and the fluidity is adjusted with a dispersion solvent (water) to prepare a negative electrode composite slurry. Next, the prepared negative electrode composite slurry is applied to a Cu foil serving as a negative electrode collector, dried, and pressed to a specified thickness. Furthermore, it is cut into a specified size to make a negative electrode.

[0095] Next, a non-aqueous electrolyte is prepared as follows. As solvents, fluoroethylene carbonate (FEC), ethylene carbonate (EC) and ethyl methyl carbonate (EMC) are prepared. They are mixed in a volume ratio of FEC:EC:EMC=0.08:0.92:3 to prepare a non-aqueous solvent. LiPF6 is dissolved in the prepared non-aqueous solvent at a concentration of 1 mol / L. In addition, LiPO2F2 is dissolved at 0.7 mass%. Thus, a non-aqueous electrolyte is prepared.

[0096] As a separator, a porous sheet consisting of three layers of PP / PE / PP is prepared. The positive electrode and negative electrode prepared above are placed opposite to each other through a separator to form an electrode body. Next, the positive terminal and the negative terminal are installed on the prepared electrode body, and are contained in a bag-shaped battery case made of aluminum laminate film together with a non-aqueous electrolyte and sealed. The above-prepared secondary battery is charged to 4.2V at a constant current of a charge rate of 0.1C, and then discharged to 3.0V at a constant current of a discharge rate of 0.1C. The secondary battery for evaluation of Example 1 is thus prepared.

[0097] (Example 2)

[0098] As a non-aqueous electrolyte, a non-aqueous electrolyte that does not contain FEC and LiPO2F2 is prepared. Specifically, a non-aqueous solvent is prepared by mixing at a volume ratio of EC:EMC=1:3. LiPF6 is dissolved in the prepared non-aqueous solvent at a concentration of 1 mol / L. The non-aqueous electrolyte of Example 2 is thus prepared. In addition, a secondary battery for evaluation of Example 2 is prepared in the same manner as Example 1.

[0099] (Example 3)

[0100] As a non-aqueous electrolyte, a non-aqueous electrolyte that does not contain LiPO2F2 is prepared. Specifically, a non-aqueous solvent is prepared by mixing FEC:EC:EMC=0.08:0.92:3 by volume. LiPF6 is dissolved in the prepared non-aqueous solvent at a concentration of 1 mol / L. Thus, the non-aqueous electrolyte of Example 3 is prepared. In addition, a secondary battery for evaluation of Example 3 is prepared in the same manner as Example 1.

[0101] (Example 4)

[0102] As a non-aqueous electrolyte, a non-aqueous electrolyte that does not contain FEC is prepared. Specifically, a non-aqueous solvent is prepared by mixing at a volume ratio of EC:EMC=1:3. LiPF6 is dissolved in the prepared non-aqueous solvent at a concentration of 1 mol / L. In addition, LiPO2F2 is dissolved at 0.7 mass%. The non-aqueous electrolyte of Example 4 is thus prepared. In addition, a secondary battery for evaluation of Example 4 is prepared in the same manner as Example 1.

[0103] (Example 5~Example 8)

[0104] As the nonaqueous electrolyte, a nonaqueous electrolyte was prepared in which the concentration (mass %) of LiPO2F2 in the nonaqueous electrolyte of Example 1 was changed as shown in Table 1. Secondary batteries for evaluation of Examples 5 to 8 were prepared in the same manner as in Example 1 except for the above.

[0105] (Example 9~Example 11)

[0106] As the nonaqueous electrolyte, a nonaqueous electrolyte was prepared in which the concentration (volume %) of FEC in the nonaqueous electrolyte of Example 1 was changed as shown in Table 1. Secondary batteries for evaluation of Examples 9 to 11 were prepared in the same manner as in Example 1 except for the above.

[0107] (Example 12~Example 15)

[0108] As the negative electrode active material, a mixture was prepared in a mass ratio of graphite:SiO=97:3. In addition, as the non-aqueous electrolyte, a non-aqueous electrolyte was prepared in which the concentration (volume %) of FEC in the non-aqueous electrolyte of Example 1 was changed as shown in Table 1. In addition, secondary batteries for evaluation of Examples 12 to 15 were prepared in the same manner as Example 1.

[0109] (Example 16~Example 19)

[0110] As the negative electrode active material, a mixture was prepared in a mass ratio of graphite:SiO=90:10. In addition, as the non-aqueous electrolyte, a non-aqueous electrolyte was prepared in which the concentration (volume %) of FEC in the non-aqueous electrolyte of Example 1 was changed as shown in Table 1. Other than this, secondary batteries for evaluation of Examples 16 to 19 were prepared in the same manner as Example 1.

[0111] <Evaluation of capacity retention rate>

[0112] At 25°C, charge to 4.2V at a charge rate of 0.5C with a constant current, then discharge to 3.0V at a discharge rate of 0.5C with a constant current. The above charge and discharge is regarded as one cycle, and the charge and discharge are repeated for 300 cycles. The discharge capacity of the first cycle is referred to as the "initial capacity". Next, the evaluation secondary battery after 300 cycles is charged to 4.2V at a charge rate of 0.1C at 25°C with a constant current, and then discharged to 3.0V at a discharge rate of 0.1C with a constant current. The discharge capacity at this time is referred to as the "capacity after cycle". The capacity retention rate (%) is calculated by the following formula: Capacity retention rate = (capacity after cycle / initial capacity) × 100. The results are shown in Table 1.

[0113] <Evaluation of gas generation amount>

[0114] The evaluation secondary battery of each example before and after the cycle is measured by the Archimedes method using Freon as a solvent to determine the volume in the shell, thereby calculating the amount of gas generated. Specifically, first, the evaluation secondary battery of each example after the activation treatment uses the Archimedes method to calculate the volume in the shell according to the relationship between the weight in the air and the weight in the water. It is used as the "initial volume". Then, under an environment of 25°C, after constant current charging to 4.2V at a charge rate of 0.5C, constant current discharge is performed to 3.0V at a discharge rate of 0.5C, and the above charge and discharge is regarded as 1 cycle, and the charge and discharge of 300 cycles are repeated. The evaluation secondary battery after 300 cycles is used to calculate the volume in the shell in the same manner as above. It is used as the "volume after the cycle". Then, the difference (mL) between the volume after the cycle and the initial volume is obtained and used as the amount of gas generated. Furthermore, the amount of gas generated (mL) was divided by the rated capacity (Ah) of each secondary battery for evaluation to calculate the amount of gas generated per unit battery capacity (ml / Ah).

[0115] [Table 1]

[0116]

[0117] As shown in Table 1, it can be seen that the capacity retention rates of Examples 1, 5 to 8, 10 to 15, 18 and 19, in which the negative electrode active material includes a Si-containing material, the non-aqueous electrolyte contains LiPO2F2 and FEC, and the ratio of the mass of FEC to the mass of the Si-containing material is 0.2 or more, are 93% or more. That is, by including a lithium transition metal composite oxide containing at least lithium and manganese as a positive electrode active material, graphite particles and a Si-containing material as a negative electrode active material, and a non-aqueous electrolyte containing FEC and a difluorophosphate, and the ratio of the mass of FEC to the mass of the Si-containing material is 0.2 or more, a non-aqueous electrolyte secondary battery with excellent capacity retention after cycling is achieved.

[0118] On the other hand, it can be seen that the capacity retention rate is low in Examples 2 and 4 where FEC is not added. In addition, it can be seen that the capacity retention rate is also low in Examples 9, 16, and 17 where the ratio of the mass of FEC to the mass of the Si-containing material is less than 0.2. It is speculated that this is because the protective effect of FEC on the negative electrode (especially the Si-containing material contained in the negative electrode) is not fully exerted. In addition, it can be seen that the capacity retention rate of Example 3, which does not contain LiPO2F2, is also low. It is speculated that this is because FEC is also attached to the surface of the positive electrode due to the absence of LiPO2F2, and the protective effect of FEC on the surface of the negative electrode is reduced.

[0119] As shown in Table 1, when comparing Example 1 with Example 5, it can be seen that the capacity retention rate is slightly lower in Example 5 in which the ratio of the mass of LiPO2F2 to the mass of the positive electrode active material is less than 0.001. It is speculated that this is because the amount of FEC adsorbed on the positive electrode surface is increased by the small amount of LiPO2F2 relative to the positive electrode active material, and the protective effect of FEC on the negative electrode surface is slightly reduced.

[0120] As shown in Table 1, it can be seen that the amount of gas generated is large in Examples 11, 15, and 19 where the ratio of the mass of FEC to the mass of the negative electrode active material is higher than 0.04. This is presumably because the amount of gas when FEC is decomposed increases as the mass of FEC relative to the mass of the negative electrode active material increases.

[0121] Experiment 2

[0122] <Production of secondary battery for evaluation>

[0123] (Example 20~Example 24)

[0124] In the second test, the type of positive electrode active material was changed to prepare the evaluation secondary battery of each example, and the capacity retention rate and gas generation amount were evaluated. Specifically, the evaluation secondary battery of each example was prepared using the positive electrode active material shown in Table 2. Except for this, the evaluation secondary batteries of Examples 20 to 24 were prepared in the same manner as Example 1.

[0125] <Evaluation of capacity retention rate and gas generation amount>

[0126] The capacity retention rate and gas generation amount of the secondary battery for evaluation of each example were evaluated in the same manner as the above-mentioned evaluation method. The results are shown in Table 2. In addition, in Table 2, the results of Example 1 and Example 2 are also shown for comparison.

[0127] [Table 2]

[0128]

[0129] As shown in Table 2, it can be seen that the same effect can be obtained even when the type of positive electrode active material is different. In addition, as shown in Table 2, the capacity retention rate is particularly good in Examples 1 and 22 to 24 where the ratio of the amount of Li to the amount of metal other than Li (Li / Me) is higher than 1.2.

[0130] Although the specific examples of the present invention have been described in detail above, these are only examples and do not limit the scope of the claims. The technology described in the scope of the claims includes various modifications and changes to the specific examples described above.

[0131] In summary, the following items of description can be cited as specific aspects of the technology disclosed herein.

[0132] Item 1: A non-aqueous electrolyte secondary battery, comprising an electrode body having a positive electrode and a negative electrode, a non-aqueous electrolyte, and a battery case, wherein the positive electrode comprises a lithium transition metal composite oxide containing at least lithium and manganese as a positive electrode active material, the negative electrode comprises graphite particles and a Si-containing material as a negative electrode active material, the non-aqueous electrolyte comprises fluoroethylene carbonate and difluorophosphate, and the ratio of the mass of the fluoroethylene carbonate to the mass of the Si-containing material is greater than 0.2.

[0133] Item 2: The nonaqueous electrolyte secondary battery according to Item 1, wherein a ratio of the mass of the fluoroethylene carbonate to the mass of the negative electrode active material is 0.04 or less.

[0134] Item 3: The nonaqueous electrolyte secondary battery according to Item 1 or Item 2, wherein the Si-containing material accounts for 1 to 15% by mass when the total of the graphite particles and the Si-containing material is 100% by mass.

[0135] Item 4: The nonaqueous electrolyte secondary battery according to any one of Items 1 to 3, wherein the ratio of the mass of the difluorophosphate to the mass of the positive electrode active material is 0.001 or more.

[0136] Item 5: The nonaqueous electrolyte secondary battery according to any one of Items 1 to 4, wherein the positive electrode active material is a lithium transition metal composite oxide represented by the following general formula.

[0137] Li 1+a Ni x Mn y M z O2(I)

[0138] (Wherein, 0.1≤a≤0.33, 0≤x≤0.5, 0.5≤y≤0.7, 0≤z≤0.2, a+x+y+z=1, and M is at least one element selected from Co, Al, Mg, Ca, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W.)

[0139] Item 6: The nonaqueous electrolyte secondary battery according to any one of Items 1 to 5, wherein when the metal element other than Li contained in the positive electrode active material is Me, the ratio of the amount of Li to the amount of Me (Li / Me) is 1.2 to 2.

[0140] Item 7: The nonaqueous electrolyte secondary battery according to any one of Items 1 to 6, wherein the Si-containing material is at least one selected from the group consisting of silicon, silicon oxide, and a composite of silicon and carbon.

[0141] Item 8: The nonaqueous electrolyte secondary battery according to any one of Items 1 to 7, wherein the difluorophosphate salt comprises lithium difluorophosphate.

[0142] Item 9: A method for manufacturing a non-aqueous electrolyte secondary battery, which is a method for manufacturing a non-aqueous electrolyte secondary battery having an electrode body having a positive electrode and a negative electrode, a non-aqueous electrolyte, and a battery case, comprising: a construction step of accommodating the above-mentioned electrode body in the above-mentioned battery case to construct an assembly, a preparation step of preparing a non-aqueous electrolyte containing fluoroethylene carbonate and difluorophosphate, and a liquid injection step of injecting the above-mentioned non-aqueous electrolyte into the above-mentioned battery case, wherein the above-mentioned positive electrode contains a positive electrode active material, and the above-mentioned negative electrode contains graphite particles and Si-containing material as negative electrode active materials. In the above-mentioned preparation step, the above-mentioned non-aqueous electrolyte is prepared in a manner such that the ratio of the mass of the above-mentioned fluoroethylene carbonate to the mass of the above-mentioned Si-containing material is 0.2 or more.

[0143] Item 10: The production method according to Item 9, wherein in the preparation step, the nonaqueous electrolyte is prepared so that the ratio of the mass of the fluoroethylene carbonate to the mass of the negative electrode active material is 0.04 or less.

[0144] Item 11: The production method according to Item 9 or Item 10, wherein in the preparation step, the nonaqueous electrolyte is prepared so that the ratio of the mass of the difluorophosphate to the mass of the positive electrode active material is 0.001 or more.

[0145] Item 12: The production method according to any one of Items 9 to 11, wherein the positive electrode active material is a lithium transition metal composite oxide represented by the following general formula.

[0146] Li 1+a Ni x Mn y M z O2(I)

[0147] (Wherein, 0.1≤a≤0.33, 0≤x≤0.5, 0.5≤y≤0.7, 0≤z≤0.2, a+x+y+z=1, and M is at least one element selected from Co, Al, Mg, Ca, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W.)

[0148] Item 13: The production method according to any one of Items 9 to 12, wherein when the metal element other than Li contained in the positive electrode active material is Me, the ratio of the amount of Li to the amount of Me (Li / Me) is 1.2 to 2.

Claims

1. A non-aqueous electrolyte secondary battery, comprising an electrode body, a non-aqueous electrolyte and a battery case, wherein the electrode body has a positive electrode and a negative electrode, The positive electrode comprises a lithium transition metal composite oxide containing at least lithium and manganese as a positive electrode active material. The negative electrode comprises graphite particles and Si-containing materials as negative electrode active materials, The non-aqueous electrolyte contains fluoroethylene carbonate and difluorophosphate, The ratio of the mass of the fluoroethylene carbonate to the mass of the Si-containing material is greater than 0.

2.

2. The nonaqueous electrolyte secondary battery according to claim 1, wherein: The ratio of the mass of the fluoroethylene carbonate to the mass of the negative electrode active material is 0.04 or less.

3. The non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein: When the total of the graphite particles and the Si-containing material is 100 mass %, the Si-containing material is 1 mass % to 15 mass %.

4. The nonaqueous electrolyte secondary battery according to claim 1 or 2, wherein: The ratio of the mass of the difluorophosphate to the mass of the positive electrode active material is greater than 0.

001.

5. The non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein: The positive electrode active material is a lithium transition metal composite oxide represented by the following general formula: Li 1+a Ni x Mr y M z O2(I) Among them, 0.1≤a≤0.33, 0≤x≤0.5, 0.5≤y≤0.7, 0≤z≤0.2, a+x+y+z=1, and M is at least one element selected from Co, Al, Mg, Ca, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W.

6. The nonaqueous electrolyte secondary battery according to claim 5, wherein: When the metal element other than Li contained in the positive electrode active material is Me, Li / Me, which is a ratio of the amount of Li to the amount of Me, is 1.2 to 2.

7. The nonaqueous electrolyte secondary battery according to claim 1 or 2, wherein: The Si-containing material is at least one selected from the group consisting of silicon, silicon oxide, and a composite of silicon and carbon.

8. The nonaqueous electrolyte secondary battery according to claim 1 or 2, wherein: The difluorophosphate comprises lithium difluorophosphate.

9. A method for manufacturing a non-aqueous electrolyte secondary battery, the non-aqueous electrolyte secondary battery comprising an electrode body, a non-aqueous electrolyte, and a battery case, the electrode body having a positive electrode and a negative electrode, The manufacturing method comprises the following steps: a construction step of housing the electrode body in the battery case to construct an assembly, Preparation process, preparing a non-aqueous electrolyte containing fluoroethylene carbonate and difluorophosphate, and A liquid injection step of injecting the non-aqueous electrolyte into the battery housing; Furthermore, the positive electrode contains a positive electrode active material, and the negative electrode contains graphite particles and a Si-containing material as a negative electrode active material. In the preparation step, the non-aqueous electrolyte is prepared such that the ratio of the mass of the fluoroethylene carbonate to the mass of the Si-containing material is 0.2 or more. 10 . The production method according to claim 9 , wherein in the preparation step, the nonaqueous electrolyte is prepared so that the ratio of the mass of the fluoroethylene carbonate to the mass of the negative electrode active material is 0.04 or less. 11 . The production method according to claim 9 , wherein in the preparation step, the nonaqueous electrolyte is prepared so that the ratio of the mass of the difluorophosphate to the mass of the positive electrode active material is 0.001 or more.

12. The manufacturing method according to claim 9 or 10, wherein: The positive electrode active material is a lithium transition metal composite oxide represented by the following general formula: Li 1+a Ni x Mr y M z O2(I) Among them, 0.1≤a≤0.33, 0≤x≤0.5, 0.5≤y≤0.7, 0≤z≤0.2, a+x+y+z=1, and M is at least one element selected from Co, Al, Mg, Ca, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W.

13. The manufacturing method according to claim 12, wherein: When the metal element other than Li contained in the positive electrode active material is Me, Li / Me, which is a ratio of the amount of Li to the amount of Me, is 1.2 to 2.

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