Nonaqueous electrolyte secondary battery and method for manufacturing the same

Abstract

To provide a non-aqueous electrolyte secondary battery having a negative electrode containing a Si-containing material and having an excellent capacity retention rate.SOLUTION: A battery 100 disclosed herein includes an electrode body having a positive electrode and a negative electrode, a non-aqueous electrolyte, and a battery case 10. The positive electrode includes a lithium transition metal composite oxide containing at least lithium and manganese as a positive electrode active material, the negative electrode includes graphite particles and a Si-containing material as negative electrode active materials, and the non-aqueous electrolyte includes fluoroethylene carbonate and difluorophosphate. The ratio of the mass of the fluoroethylene carbonate to the mass of the Si-containing material is 0.2 or more.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a nonaqueous electrolyte secondary battery and a method for manufacturing a nonaqueous electrolyte secondary battery. [Background technology]

[0002] Conventionally, nonaqueous electrolyte secondary batteries having a positive electrode, a negative electrode, and a nonaqueous electrolyte have been known. For such nonaqueous electrolyte secondary batteries, various combinations of positive electrode active materials, negative electrode active materials, and nonaqueous electrolytes have been studied. Related prior art includes references 1 and 2. Reference 1 describes a nonaqueous electrolyte secondary battery containing a positive electrode active material represented by a specific general formula, an additive selected from the group consisting of cyclohexylbenzene and biphenyl, and a nonaqueous electrolyte containing lithium difluorophosphate. Reference 2 describes a nonaqueous electrolyte secondary battery containing a negative electrode active material containing silicon oxide particles and a silicon layer, and a nonaqueous electrolyte containing at least one selected from the group consisting of fluoroethylene carbonate and vinylene carbonate. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2020-64717 A [Patent Document 2] JP 2019-185992 A Summary of the Invention [Problem to be solved by the invention]

[0004] In recent years, non-aqueous electrolyte secondary batteries containing a Si-containing material in the negative electrode have been studied for the purpose of increasing capacity. Fluoroethylene carbonate is adsorbed to the surface of the negative electrode to protect the surface of the Si-containing material, and can improve the capacity retention rate after cycling by suppressing side reactions with the electrolyte. Lithium difluorophosphate is adsorbed to the surface of the positive electrode to protect the surface of the positive electrode active material, and can suppress an increase in internal resistance. However, according to the results of the study by the present inventors, it was found that when the negative electrode contains a Si-containing material and a non-aqueous electrolyte containing only one of fluoroethylene carbonate and lithium difluorophosphate is used, the effect of improving the cycle characteristics (e.g., capacity retention rate after cycling) of the non-aqueous electrolyte secondary battery is insufficient.

[0005] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a nonaqueous electrolyte secondary battery in which a negative electrode contains a Si-containing material and which has an excellent capacity retention rate, and a method for producing the same. [Means for solving the problem]

[0006] The present invention provides a nonaqueous electrolyte secondary battery comprising an electrode assembly having a positive electrode and a negative electrode, a nonaqueous electrolyte, and a battery case. The positive electrode of this nonaqueous electrolyte secondary battery contains a lithium transition metal composite oxide containing at least lithium and manganese as a positive electrode active material, the negative electrode contains graphite particles and a Si-containing material as negative electrode active materials, and the nonaqueous electrolyte contains fluoroethylene carbonate and difluorophosphate. The ratio of the mass of the fluoroethylene carbonate to the mass of the Si-containing material is 0.2 or more.

[0007] According to this configuration, the protective effect of the fluoroethylene carbonate on the negative electrode surface (particularly the surface of the Si-containing material) and the protective effect of the difluorophosphate on the positive electrode surface (particularly the surface of the positive electrode active material) are effectively exerted, thereby suppressing cracking of the Si-containing material due to charging and discharging, and realizing a nonaqueous electrolyte secondary battery with excellent capacity retention rate.

[0008] From another aspect, there is provided a method for producing a nonaqueous electrolyte secondary battery including an electrode assembly having a positive electrode and a negative electrode, a nonaqueous electrolyte, and a battery case. The method includes a construction step of housing the electrode assembly in the battery case to construct an assembly, a preparation step of preparing a nonaqueous electrolyte containing fluoroethylene carbonate and difluorophosphate, and a liquid injection step of injecting the nonaqueous electrolyte into the battery case. Here, the positive electrode of the electrode assembly includes a positive electrode active material, and the negative electrode includes graphite particles and a Si-containing material as negative electrode active materials, and 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 Si-containing material is 0.2 or more. According to this configuration, a nonaqueous electrolyte secondary battery having an excellent capacity retention rate can be suitably manufactured. [Brief description of the drawings]

[0009] [Figure 1] FIG. 1 is a perspective view that illustrates a nonaqueous electrolyte secondary battery according to one embodiment. [Diagram 2] FIG. 2 is a schematic vertical cross-sectional view taken along line II-II in FIG. [Diagram 3] FIG. 3 is a schematic diagram showing the configuration of the electrode body. [Figure 4] FIG. 4 is a flow chart of a manufacturing method according to one embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Hereinafter, some preferred embodiments of the technology disclosed herein will be described with reference to the drawings. Note that matters other than those specifically mentioned in this specification and necessary for carrying out the present invention (for example, the general configuration and manufacturing process of a non-aqueous electrolyte secondary battery that does not characterize the present invention) can be understood as design matters of a person skilled in the art based on the prior art in the field. The present invention can be carried out based on the contents disclosed in this specification and the technical common sense in the field.

[0011] In this specification, the term "non-aqueous electrolyte secondary battery" refers to any battery that can be repeatedly charged and discharged by the movement of charge carriers between the positive electrode and the negative electrode via the non-aqueous electrolyte. The concept of non-aqueous electrolyte secondary battery includes so-called storage batteries such as lithium ion secondary batteries, and capacitors such as lithium ion capacitors and electric double layer capacitors. In this specification, the expression "A to B" indicating a range includes the meaning of "greater than A" and "smaller than B" as well as the meaning of "A or more and B or less."

[0012] <Battery 100> Fig. 1 is a perspective view of the battery 100. Fig. 2 is a schematic longitudinal sectional view taken along line II-II in Fig. 1. In the following description, the symbols L, R, F, Rr, U, and D in the drawings represent left, right, front, rear, 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 perpendicular to the short side direction, and the up-down direction perpendicular to the short side direction and the long side direction, respectively. However, these directions are merely for the convenience of description, and do not limit the installation form of the battery 100 in any way.

[0013] 2, the battery 100 includes a battery case 10, an electrode assembly 20, a positive electrode terminal 30, a negative electrode terminal 40, a positive electrode current collector 50, a negative electrode current collector 60, and a non-aqueous electrolyte (not shown). The battery 100 here is a lithium ion secondary battery. The battery 100 is preferably a lithium ion secondary battery.

[0014] The battery case 10 is a housing that contains the electrode assembly 20 and the nonaqueous electrolyte. As shown in FIG. 1, the battery case 10 has a flat, bottomed rectangular parallelepiped (rectangular) outer shape. The material of the battery case 10 may be the same as that used conventionally, and is not particularly limited. The battery case 10 is preferably made of a metal, and more preferably made of, for example, aluminum, an aluminum alloy, iron, an iron alloy, or the like. However, in other embodiments, the battery case 10 may be in the shape of a bag made of, for example, a laminate film. As shown in FIG. 2, the battery case 10 includes an exterior body 12 having an opening 12h, and a sealing plate (lid body) 14 that closes the opening 12h. The sealing plate 14 is attached to the exterior body 12 so as to close the opening 12h of the exterior 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 exterior body 12. The battery case 10 is hermetically sealed (sealed).

[0015] As shown in Fig. 2, the sealing plate 14 is provided with a liquid injection hole 15 and two terminal extraction holes 18, 19. The liquid injection hole 15 is for injecting a non-aqueous electrolyte into the battery case 10 after the sealing plate 14 is assembled to the exterior body 12. The sealing plate 14 is preferably provided with the liquid injection hole 15. The liquid injection hole 15 is sealed with a sealing member 16. The terminal extraction holes 18, 19 are formed at both ends of the sealing plate 14 in the long side direction Y (the left end and the right end in Fig. 2). The terminal extraction holes 18, 19 penetrate the sealing plate 14 in the thickness direction (the up-down direction Z).

[0016] As shown in Fig. 2, the positive electrode terminal 30 and the negative electrode terminal 40 extend from the inside to the outside of the sealing plate 14 through the terminal pull-out holes 18, 19. Here, the positive electrode terminal 30 and the negative electrode terminal 40 are crimped to the peripheral portion surrounding the terminal pull-out holes 18, 19 of the sealing plate 14 by crimping. Crimped portions 30c, 40c are formed on the ends of the positive electrode terminal 30 and the negative electrode terminal 40 on the side of the exterior body 12 (the lower ends in Fig. 2).

[0017] The positive electrode terminal 30 is electrically connected to the positive electrode tab group 23 of the electrode body 20 through the positive electrode current collector 50 inside the battery case 10. The positive electrode terminal 30 is insulated from the sealing plate 14 by a positive electrode insulating member 70 and a gasket 90. The positive electrode terminal 30 is preferably made of metal, more preferably made of aluminum or an aluminum alloy, for example. The negative electrode terminal 40 is electrically connected to the negative electrode tab group 25 of the electrode body 20 through the negative electrode current collector 60 inside the battery case 10. The negative electrode terminal 40 is insulated from the sealing plate 14 by a negative electrode insulating member 80 and a gasket 90. The negative electrode terminal 40 is preferably made of metal, more preferably made of copper or a copper alloy, for example.

[0018] As described above, a nonaqueous electrolyte is housed inside the battery case 10 together with the electrode assembly 20. The nonaqueous electrolyte of the battery 100 disclosed herein contains fluoroethylene carbonate (hereinafter also referred to as "FEC") and difluorophosphate. The nonaqueous electrolyte may contain a nonaqueous solvent other than fluoroethylene carbonate and a salt (supporting salt) other than difluorophosphate, as long as it does not impair the technology disclosed herein.

[0019] The above-mentioned FEC adheres to the surface of the Si-containing material contained in the negative electrode 24, and is particularly effective in protecting the Si-containing material. On the other hand, since FEC has a carbonate skeleton, it tends to be easily adsorbed to the surface of the positive electrode 22 (specifically, the surface of the positive electrode active material). Difluorophosphate is a material that easily interacts with the surface of the positive electrode active material and is adsorbed to the surface of the positive electrode active material to protect the active material. According to the results of the study by 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 a non-aqueous electrolyte secondary battery is not fully exhibited. This is presumably because, when FEC is not contained, the Si-containing material is not protected in the first place, and a coating is formed on the surface of the Si-containing material. Also, when difluorophosphate is not contained, FEC adheres to the surface of the positive electrode, and the Si-containing material is not sufficiently protected, and a coating is formed on the surface of the Si-containing material. Therefore, the battery 100 disclosed herein contains FEC and difluorophosphate. The difluorophosphate is preferentially attached to the surface of the positive electrode 22 over the FEC, so that the FEC can adequately exert its protective effect against the Si-containing material, thereby favorably improving the cycle characteristics (e.g., capacity retention rate after cycling) of the nonaqueous electrolyte secondary battery.

[0020] Fluoroethylene carbonate (FEC) is a type of fluorinated cyclic carbonate. Carbonates are divided into non-fluorinated carbonates and fluorinated carbonates depending on whether or not they contain fluorine (F) as a constituent element. Carbonates are also divided into chain carbonates and cyclic carbonates depending on their chemical structure. Chain carbonates are acyclic (chain) carbonate compounds having a carbonate skeleton (O-CO-O). Cyclic carbonates are carbonate compounds having a carbonate skeleton closed in a ring by a C-C bond. The non-aqueous solvent may contain a fluorinated carbonate other than FEC, for example, a fluorinated chain carbonate or a fluorinated cyclic carbonate other than FEC.

[0021] As the non-aqueous solvent other than FEC, one or more of those that are known to be usable in non-aqueous electrolyte secondary batteries can be used. Examples of the non-aqueous solvent include organic solvents such as carbonates, ethers, esters, nitriles, sulfones, and lactones. The non-aqueous solvent preferably contains a carbonate. Specific examples include ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC).

[0022] If the content of FEC in the non-aqueous electrolyte is too small, the protective effect of the Si-containing material described above is difficult to be properly exerted. Here, 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 from the viewpoint of properly exerting the effects of the technology disclosed herein. The ratio of the mass of FEC to the mass of the Si-containing material is preferably 0.2 or more, more preferably 0.28 or more, and may be 0.45 or more. This appropriately improves the capacity retention rate after cycling. There is no particular upper limit to the ratio of the mass of FEC to the mass of the Si-containing material. When the content of FEC in the non-aqueous solvent increases, the amount of gas generated during charging and discharging tends 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 preferably 1.6 or less, and may be 1.2 or less, or may be 1 or less.

[0023] Although not particularly limited, the ratio of the mass of FEC to the total amount of the negative electrode active material contained in the negative electrode 24 (i.e., the total mass of the graphite particles, the Si-containing material, and other negative electrode active material) is preferably 0.04 or less. This allows the amount of gas generated after cycling to be suitably 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 may 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, an improvement in the capacity retention rate after cycling and a reduction in the amount of gas generated are suitably realized.

[0024] Although not particularly limited, when the entire non-aqueous solvent is taken as 100 volume %, the volume ratio of FEC is preferably 10 volume % or less. This allows the protective effect of the Si-containing material to be adequately exhibited, while the amount of gas generated during charging and discharging can be suitably suppressed. For example, when the entire non-aqueous solvent is taken as 100 volume %, the volume ratio of FEC is preferably 0.5 volume % to 10 volume %, more preferably 1 volume % to 10 volume %, and even more preferably 1 volume % to 7 volume %.

[0025] Difluorophosphate is PO2F2 - The difluorophosphate salt is a salt of an anion represented by the formula: and a cation. Examples of the cation include alkali metal ions such as Li, Na, and K, and ammonium ions, among which Li is preferred. A specific example of a preferred difluorophosphate salt is lithium difluorophosphate (lithium difluorophosphate (LiDFP), LiPO2F2).

[0026] When the content of difluorophosphate in the non-aqueous electrolyte is too small, FEC may preferentially adhere to the positive electrode surface as described above, so that the protective effect of FEC on the Si-containing material is not adequately exhibited. Here, the content of difluorophosphate in the non-aqueous electrolyte is preferably determined by the ratio to the positive electrode active material contained in the positive electrode 22 from the viewpoint of adequately exhibiting the effects of the technology disclosed herein. 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 even more preferably 0.0015 or more. This allows the protective effect on the positive electrode surface and the negative electrode surface to be more adequately exhibited, and the capacity retention rate after cycling is suitably 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, but may be, for example, preferably 0.01 or less, more preferably 0.005 or less, and may be 0.0045 or less.

[0027] Although not particularly limited, the proportion of the difluorophosphate in the entire nonaqueous electrolyte is preferably 0.1% by mass or more and 2% by mass or less, and more preferably 0.2% by mass or more and 1.2% by mass or less.

[0028] As the salt other than the difluorophosphate, one or more of those known to be usable in non-aqueous electrolyte secondary batteries can be used. In the case of a lithium ion secondary battery, for example, fluorine-containing lithium salts such as LiPF6 and LiBF4 can be used.

[0029] The non-aqueous electrolyte may further contain additional components (additives). As the additives, one or more of those that are conventionally known to be added to non-aqueous electrolytes may be used. For example, various additives such as gas generating agents such as biphenyl (BP) and cyclohexylbenzene (CHB), dispersants, and thickeners may be included.

[0030] 2, the electrode body 20 is housed inside the battery case 10 (more specifically, inside the exterior body 12). The number of electrode bodies 20 arranged inside one battery case 10 is not particularly limited, and may be one or two or more (plural).

[0031] FIG. 3 is a schematic diagram showing the configuration of the electrode body 20. As shown in FIG. 3, the electrode body 20 includes a positive electrode 22, a negative electrode 24, and a separator 26. Here, the electrode body 20 is a flat-shaped wound electrode body. The electrode body 20 is configured 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 the electrodes around a winding axis WL. However, the electrode body 20 may be a laminated electrode body in which a square-shaped (typically rectangular) positive electrode and a square-shaped (typically rectangular) negative electrode are stacked in an insulated state.

[0032] 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 strip-shaped. The positive electrode collector 22c is made of a conductive metal such as aluminum, an aluminum alloy, nickel, or stainless steel. The positive electrode collector 22c is a metal foil, specifically an aluminum foil, here. 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 the positive electrode protective layer 22p, the positive electrode protective layer 22p is provided 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, alumina).

[0033] A plurality of positive electrode tabs 22t are provided at one end (left end in FIG. 3) in the long side direction Y 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 (left end in FIG. 3) in the long side direction Y to form a positive electrode tab group 23 as shown in FIG. 2. A positive electrode current collector 50 is attached (more 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 collector 50.

[0034] The positive electrode active material layer 22a is provided in a strip shape along the longitudinal direction of the strip-shaped positive electrode current collector 22c. The positive electrode active material layer 22a contains a positive electrode active material capable of reversibly occluding and releasing 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, 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 from the viewpoint of realizing a high energy density. Examples of such a compound include lithium-excess transition metal composite oxides represented by the following formula (I). Specific examples of such lithium-excess transition metal composite oxides include, 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 and the like.

[0035] Li 1+a Ni x Mn y M z O2 Formula (I) In formula (I), a, x, y, z satisfy 0.1 ≦ a ≦ 0.33, 0 ≦ x ≦ 0.5, 0.5 ≦ y ≦ 0.7, 0 ≦ z ≦ 0.2, and a + x + y + z = 1. When 0 < z, M is one or more elements selected from Co, Al, Mg, Ca, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W.

[0036] Although not particularly limited, the positive electrode active material preferably has a ratio (Li / Me) of the amount of substance of Li to the amount of substance of metal elements other than Li (Me) of 1.1 or more, more preferably 1.2 or more, even more 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, may be 1.5 or less, or may be 1.4 or less. The ratio (Li / Me) of the amount of substance of Li to the amount of substance of metal elements other than Li (Me) in the positive electrode active material is, for example, preferably 1.2 or more and 2 or less, more preferably 1.2 or more and 1.5 or less, and even more preferably 1.26 or more and 1.45 or less. By using a positive electrode active material in which Li / Me satisfies the above-mentioned numerical range as the positive electrode active material, the cycle characteristics (for example, capacity retention rate after cycles) are suitably improved.

[0037] Although there is no intention to limit the technology disclosed herein, the reason why such an effect is obtained is presumed to be as follows. As described above, the nonaqueous electrolyte contains FEC and difluorophosphate, so that the positive electrode surface and the negative electrode surface are suitably protected, and the capacity retention rate after cycling is improved. Here, difluorophosphate tends to adhere more easily to the lithium-excess lithium transition metal composite oxide as described above. In particular, difluorophosphate is more easily adsorbed in a Li-excess positive electrode active material having a Li / Me value of 1.2 or more (preferably 1.26 or more, more preferably 1.3 or more), and as a result, the protective effect of FEC on the negative electrode surface is particularly suitably exhibited. As a result, the protective effect of FEC on the negative electrode 24 and the protective effect of difluorophosphate on the positive electrode 22 are suitably exhibited, and the cycle characteristics (e.g., capacity retention rate after cycling) of the battery 100 are improved.

[0038] 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., with respect to the total mass of the positive electrode active material layer) is preferably 70 mass% or more, more preferably 80 mass% or more and 99 mass% or less, and even more preferably 85 mass% or more and 98 mass% or less.

[0039] The average particle diameter (D50 particle diameter) of the positive electrode active material is not particularly limited. The average particle diameter (D50 particle diameter) of the positive electrode active material is, for example, 0.5 μm or more and 25 μm or less, and preferably 10 μm or more and 25 μm or less. In this specification, the "D50 particle diameter" refers to a particle diameter corresponding to a cumulative 50% from the fine particle side in a volume-based particle size distribution measured by a particle size distribution measurement based on a laser diffraction / light scattering method.

[0040] 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 additive components, etc. As the binder, for example, polyvinylidene fluoride (PVdF) or the like can be suitably used. The content of the binder in the positive electrode active material layer 22a is not particularly limited, but is, for example, 0.5 mass% or more and 15 mass% or less, preferably 1 mass% or more and 10 mass% or less, and more preferably 1.5 mass% or more and 8 mass% or less. As the conductive material, for example, carbon black such as acetylene black (AB) or other carbon materials (for example, graphite, etc.) can be suitably used. The content of the conductive material in the positive electrode active material layer 22a is not particularly limited, but is, for example, 0.1 mass% or more and 20 mass% or less, preferably 1 mass% or more and 15 mass% or less, and more preferably 2 mass% or more and 10 mass% or less.

[0041] 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 strip-shaped. The negative electrode current collector 24c is made of a conductive metal such as copper, a copper alloy, nickel, or stainless steel. Here, the negative electrode current collector 24c is a metal foil, specifically a copper foil.

[0042] A plurality of negative electrode tabs 24t are provided at one end (right end in FIG. 3) in the long side direction Y of the negative electrode current collector 24c. The negative electrode tab 24t is a part of the negative electrode current collector 24c here, and is made of metal foil (copper foil). The plurality of negative electrode tabs 24t are stacked at one end (right end in FIG. 3) in the long side direction Y to form a negative electrode tab group 25 as shown in FIG. 2. A negative electrode current collector 60 is attached (more 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 collector 60.

[0043] The negative electrode active material layer 24a is provided in a strip shape along the longitudinal direction of the strip-shaped negative electrode current collector 24c. The negative electrode active material layer 24a contains a negative electrode active material capable of reversibly absorbing and releasing charge carriers. The negative electrode active material contains at least graphite particles and a Si-containing material. In the battery 100 disclosed herein, the above-mentioned nonaqueous electrolyte provides a suitable protective effect on the Si-containing material, thereby suppressing cracking of the Si-containing material. This improves cycle characteristics (particularly, capacity retention rate after cycling). In addition, by using the above-mentioned lithium-excess lithium transition metal composite oxide as the positive electrode active material, cracking of the Si-containing material is further suppressed. Although there is no intention to limit the technology disclosed herein, the reason for obtaining such an effect is presumed to be as follows. Since the nonaqueous electrolyte contains FEC and difluorophosphate, the surface of the Si-containing material is protected, and decomposition of the electrolyte is suppressed, thereby suppressing film formation. This improves cycle characteristics (particularly, capacity retention rate after cycling). Furthermore, the lithium-rich lithium transition metal composite oxide is different from the conventional lithium transition metal composite oxide (e.g., LiNi 1 / 3 Co 1 / 3 Mn 1 / 3Compared to non-lithium-rich lithium transition metal composite oxides such as O2), the range of operating potential tends to be somewhat narrower. Therefore, the range of use of the Si-containing material during charging and discharging can be made relatively narrow. This suppresses the volume change of the Si-containing material during charging and discharging, and favorably suppresses cracking of the Si-containing material. By suppressing cracking of the Si-containing material, the number of newly generated surfaces caused by cracking is also reduced, and the need for further protection of the newly generated surfaces by FEC tends to decrease. As a result, the protective effect of the FEC is fully exerted. This favorably improves the capacity retention rate after cycling of the battery 100.

[0044] Since the Si-containing material has a larger specific capacity than the carbon material, it is preferably used for the purpose of increasing the capacity of the battery 100. The Si-containing material may contain components other than Si as long as it contains Si. Suitable examples of the Si-containing material include Si (silicon), SiO (silicon oxide), a composite of silicon and carbon (including silicon carbide and SiC composites in which carbon is dispersed inside silicon particles), SiN-containing materials (silicon nitride), nano-Si particles dispersed in porous particles, and the like. The Si-containing material may be one or more of the above-mentioned materials. Among them, silicon, silicon oxide, and a composite of silicon and carbon are preferably used as the Si-containing material. Although not particularly limited, the D50 particle size of the Si-containing particles is preferably, for example, 1 μm or more and 15 μm or less, and more preferably 2 μm or more and 10 μm or less.

[0045] 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 taken as 100 mass%, the content ratio of the Si-containing material is preferably 1 mass% or more and 15 mass% or less, and more preferably 1 mass% or more and 10 mass% or less.

[0046] As the graphite particles, for example, artificial graphite, natural graphite, etc. are preferably used. The graphite particles may have a coating layer of amorphous carbon on the surface. Although not particularly limited, the graphite particles may be approximately spherical. In this specification, the term "approximately spherical" includes a spherical shape, a rugby ball shape, etc., and refers to a shape having an average aspect ratio (the ratio of the length in the superaxial direction to the length in the short axis direction in the smallest rectangle circumscribing the particle) of, for example, 1 to 2 (preferably, 1 to 1.5). The D50 particle size of the graphite particles is not particularly limited, but is preferably, for example, 5 μm or more and 30 μm or less, and more preferably, 10 μm or more and 25 μm or less.

[0047] Although not particularly limited, when the total amount of the negative electrode active material is taken as 100 mass%, the content of the graphite particles is preferably 85 mass% or more and 99 mass% or less, and more preferably 90 mass% or more and 99 mass% or less, which satisfies both the high capacity and the improved cycle characteristics of the battery 100.

[0048] The negative electrode active material may contain a negative electrode active material other than the Si-containing material and graphite. Specific examples of the negative electrode active material other than the Si-containing material and graphite include carbon materials such as hard carbon, soft carbon, and amorphous carbon.

[0049] The content of the negative electrode active material in the negative electrode active material layer 24a is not particularly limited. The content of the negative electrode active material in the negative electrode active material layer 24a is preferably 90 mass % or more, and more preferably 95 mass % or more and 99 mass % or less.

[0050] 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 additive components, and the like. As the binder, for example, rubbers such as styrene butadiene rubber (SBR) and acrylic resins such as polyacrylic acid (PAA) can be suitably used. The content of the binder in the negative electrode active material layer 24a is not particularly limited, but is, for example, 0.1 mass% to 10 mass%, and preferably 0.5 mass% to 5 mass%. As the thickener, for example, celluloses such as carboxymethylcellulose (CMC) can be suitably used. The content of the dispersant in the negative electrode active material layer 24a is not particularly limited, but is, for example, 0.1 mass% to 10 mass%, and preferably 0.5 mass% to 5 mass%.

[0051] The separator 26 is a member 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 may be the same as a conventional one and is not particularly limited. The separator 26 is preferably a resin porous sheet made of a polyolefin resin such as polyethylene (PE) or polypropylene (PP). The separator 26 may have a heat resistance layer (HRL) or an adhesive layer on the surface of the base material portion made of a resin porous sheet. The configuration of the heat resistance layer and adhesive layer may be the same as a conventional one.

[0052] <Manufacturing method of non-aqueous electrolyte secondary battery> Fig. 4 is a flow chart of a manufacturing method according to an embodiment. As shown in Fig. 4, the battery 100 can be manufactured by a manufacturing method including, for example, a construction step (step S1), a preparation step (step S2), and a liquid injection step (step S3). However, the manufacturing method disclosed herein may further include other steps at any stage.

[0053] The construction step (step S1) is a step of housing 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 step includes an electrode fabrication step (step S1-1), an electrode body fabrication step (step S1-2), an arrangement step (step S1-3), and a welding and joining step (step S1-4). In addition, other steps may be included at any stage.

[0054] First, in the electrode preparation step (step S1-1), the positive electrode 22 and the negative electrode 24 are prepared. The positive electrode 22 can be prepared, for example, by 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 onto the positive electrode current collector 22c by a conventionally known method, drying, and appropriately pressing. As the positive electrode active material, one type of material as described above can be used alone or two or more types can be mixed and appropriately used, and among them, the lithium-excess transition metal complex oxide represented by the above formula (I) can be preferably used.

[0055] The negative electrode 24 can be produced, for example, by mixing a negative electrode active material, a binder, a dispersant, and a dispersion solvent to prepare a negative electrode composite slurry, applying the prepared negative electrode composite slurry to the negative electrode current collector 24c by a conventionally known method, drying, and appropriately pressing. At least graphite particles and a Si-containing material (preferably at least one of silicon, silicon oxide, and a composite of silicon and carbon) are used as the negative electrode active material. From the viewpoint of achieving both high capacity and improved cycle characteristics of the battery 100, it is preferable to prepare the content ratio of the Si-containing material to be 1% by mass or more and 15% by mass or less, and more preferably 1% by mass or more and 10% by mass or less, when the total amount of the negative electrode active material is taken as 100% by mass. Note that one or more of the above-mentioned materials may be further used as the negative electrode active material.

[0056] Next, in the electrode body preparation step (step S1-2), the positive electrode 22 and the negative electrode 24 prepared above are opposed to each other with the separator 26 interposed therebetween to prepare the electrode body 20. Next, in the arrangement step (step S1-3), the electrode body 20 is arranged inside the exterior body 12. For example, the electrode body 20 is accommodated inside the exterior body 12 through the opening 12h of the exterior body 12. Next, in the welding and joining step (step S1-4), the sealing plate 14 is fitted into the opening 12h of the exterior body 12 and welded to the periphery of the opening 12h to integrate the exterior body 12 and the sealing plate 14.

[0057] The preparation step (step S2) is a step of preparing a nonaqueous electrolyte. The nonaqueous electrolyte of the battery 100 disclosed herein contains fluoroethylene carbonate and difluorophosphate. As described above, the nonaqueous electrolyte contains FEC and difluorophosphate, so that the surface of the positive electrode active material and the surface of the negative electrode active material (particularly the Si-containing material) are both suitably protected, and the capacity retention rate of the battery 100 is improved. From the viewpoint of exerting the effects of the technology disclosed herein at a high level, in the preparation step, it is preferable to prepare the nonaqueous electrolyte so that the ratio of the mass of FEC to the mass of the Si-containing material is 0.2 or more. In the preparation step, it is preferable to prepare the nonaqueous electrolyte so that the ratio of the mass of FEC to the mass of the Si-containing material is 0.2 or more and 1.6 or less, and more preferably 0.28 or more and 1.2 or less.

[0058] Although not particularly limited, in the preparation step, the nonaqueous electrolyte is preferably prepared so that 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 material materials) is 0.04 or less. In the preparation step, the nonaqueous electrolyte is preferably prepared so that the ratio of the mass of FEC to the mass of the negative electrode active material is 0.005 to 0.04, more preferably 0.01 to 0.04, and may be prepared so that the ratio is 0.01 to 0.035.

[0059] Although not particularly limited, in the preparation step, it is preferable to prepare the volume ratio of FEC to be 10 volume % or less when the non-aqueous solvent is 100 volume %. This allows the protective effect of the Si-containing material to be adequately exhibited, while suitably suppressing the amount of gas generated during charging and discharging. The preparation step is preferably carried out so that the volume ratio of FEC is 0.5 volume % to 10 volume %, more preferably 1 volume % to 10 volume %, and even more preferably 1 volume % to 7 volume %, when the entire non-aqueous solvent is 100 volume %.

[0060] In a preferred embodiment, the non-aqueous electrolyte contains carbonates other than FEC in addition to FEC. The carbonates may be used alone or in combination of two or more of the above-mentioned materials. In particular, in the preparation step, the non-aqueous electrolyte is preferably prepared to contain at least one of DMC, DEC, EMC, EC, and VC.

[0061] As the difluorophosphate, LiPO2F2 can be suitably used. Although not particularly limited, in the preparation step, it is preferable to prepare a nonaqueous electrolyte so that 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 step, it is preferable to prepare a nonaqueous electrolyte so that the ratio of the mass of the difluorophosphate to the mass of the positive electrode active material is 0.001 or more and 0.01 or less, more preferably 0.001 or more and 0.005 or less, and even more preferably 0.0015 or more and 0.0045 or less. This allows the protective effect on the positive electrode surface and the Si-containing material to be more appropriately exhibited, and the capacity retention rate after cycling is suitably improved.

[0062] Although not particularly limited, in the preparation step, the nonaqueous electrolyte solution is preferably prepared so that the proportion of difluorophosphate in the entire nonaqueous electrolyte solution is 0.1% by mass or more and 2% by mass or less, and more preferably 0.2% by mass or more and 1.2% by mass or less.

[0063] Preferably, the non-aqueous electrolyte contains a supporting salt (lithium salt in the case of a lithium ion secondary battery) in addition to FEC and difluorophosphate. As the supporting salt, one of the above-mentioned materials can be used alone or two or more of them can be mixed appropriately, and LiPF6 is particularly suitable.

[0064] In the liquid injection step (step S3), the nonaqueous electrolyte prepared as above is injected into the battery case 10 through the liquid injection hole 15 of the sealing plate 14. The liquid injection may be performed at atmospheric pressure, or may be performed under a reduced pressure inside the battery case 10, for example, for the purpose of improving the impregnation of the nonaqueous electrolyte into the electrode body 20. The nonaqueous electrolyte injected through the liquid injection hole 15 is impregnated into the electrode body 20. In other words, the nonaqueous electrolyte is evenly distributed throughout the electrode body 20.

[0065] Although not particularly limited, after the injection step, the assembly may be left (held) for a predetermined time. As a result, even if the length of the long side direction Y is relatively long, the nonaqueous electrolyte spreads evenly in the long side direction Y, and the electrode body 20 can be impregnated with the nonaqueous electrolyte. The temperature at which the electrode body 20 is left may be room temperature (for example, about 25°C ± 10°C, about 25°C ± 5°C), or may be a high temperature environment of about 35 to 45°C for the purpose of improving the impregnation of the nonaqueous electrolyte into the electrode body 20. The time at which the electrode body 20 is left is not particularly limited because it depends on, for example, the size of the electrode body 20 (particularly the length of the long side direction Y), the viscosity of the first nonaqueous electrolyte, the presence or absence of pressure reduction, the leaving temperature, etc., but is preferably 5 minutes or more, more preferably 10 minutes or more, and particularly preferably 1 hour or more. In addition, from the viewpoint of production efficiency, etc., the time is preferably within 240 hours (10 days), more preferably within 48 hours (2 days), and particularly preferably within 24 hours (1 day).

[0066] The manufacturing method disclosed herein may also include a charging step. The charging step is a step of charging the assembly into which the non-aqueous electrolyte solution has been injected at least once. The charging of the assembly can be performed in the same manner as in the past. Typically, an external power source is connected between the positive electrode terminal and the negative electrode terminal of the assembly, and charging is performed until the assembly reaches a predetermined voltage. Although not particularly limited, it is preferable to charge until the voltage reaches about 4V or more, preferably 4.1V or more, 4.2V or more, for example, 4.2V. When a lithium-excess transition metal composite oxide is used as the positive electrode active material, it is more preferable to charge until the voltage reaches about 4.6V or more, preferably 4.7V or more, for example, 4.7V, from the viewpoint of expressing a high capacity as an electrochemically active state. The charge and discharge rate can be, for example, about 0.1 to 2C. Charging may be performed once, or may be repeated two or more times, for example, with a discharge in between. Discharging may also be performed after charging. The charging step may be performed in a room temperature environment (for example, about 25° C.±10° C., about 25° C.±5° C.) or in a high temperature environment of about 35 to 45° C. In this manner, the battery 100 can be suitably manufactured.

[0067] <Uses of Battery 100> The battery 100 can be used for various purposes, but for example, because of its excellent cycle characteristics (e.g., capacity retention rate after cycling), it can be suitably used as a power source (driving power source) for motors mounted on vehicles such as passenger cars and trucks. The type of vehicle is not particularly limited, but examples include plug-in hybrid electric vehicles (PHEVs), hybrid electric vehicles (HEVs), battery electric vehicles (BEVs), and the like.

[0068] Several examples of the present invention will be described below, but it is not intended that the present invention be limited to these examples.

[0069] <First Test> <Preparation of secondary battery for evaluation> (Example 1) First, the positive and negative electrodes were prepared. The positive electrode was prepared as follows. Li was used as the positive electrode active material. 1.14 Ni 0.29 Mn 0.57 O2, acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVdF) as a binder were prepared. These materials were mixed in a mass ratio of positive electrode active material:AB:PVdF=100:1:1, and the fluidity was adjusted with N-methylpyrrolidone (NMP) as a dispersion solvent to prepare a positive electrode composite slurry. Next, the prepared positive electrode composite slurry was applied onto an Al foil as a positive electrode current collector, dried, and pressed to a predetermined thickness. Then, the mixture was cut to a predetermined size to prepare a positive electrode.

[0070] The negative electrode was prepared as follows. First, graphite and SiO (Si-containing material) were prepared as the negative electrode active material. These materials were mixed in a mass ratio of graphite:SiO = 95:5 to obtain a mixture. Next, SBR as a binder and CMC as a dispersant were added to the obtained mixture (100 parts by mass) in a ratio of 1 part by mass each (negative electrode active material: SBR: CMC = 100: 1: 1), and the fluidity was adjusted with a dispersion solvent (water) to prepare a negative electrode composite slurry. Next, the prepared negative electrode composite slurry was applied onto Cu foil as a negative electrode current collector, dried, and pressed to a predetermined thickness. Then, it was cut into a predetermined size to prepare a negative electrode.

[0071] Next, the non-aqueous electrolyte was prepared as follows. As a solvent, fluoroethylene carbonate (FEC), ethylene carbonate (EC), and ethyl methyl carbonate (EMC) were prepared. These were mixed in a volume ratio of FEC:EC:EMC=0.08:0.92:3 to prepare a non-aqueous solvent. LiPF6 was dissolved in the prepared non-aqueous solvent at a concentration of 1 mol / L. Also, LiPO2F2 was dissolved at 0.7 mass%. In this way, a non-aqueous electrolyte was prepared.

[0072] As the separator, a porous sheet consisting of three layers of PP / PE / PP was prepared. The positive electrode and negative electrode prepared above were opposed to each other through a separator to prepare an electrode body. Next, a positive electrode terminal and a negative electrode terminal were attached to the prepared electrode body, and the electrode body was housed in a bag-shaped battery case made of an aluminum laminate film together with a non-aqueous electrolyte and sealed. The secondary battery prepared above was charged at a constant current of 0.1C to 4.2V, and then discharged at a constant current of 0.1C to 3.0V to perform an activation treatment. In this way, the evaluation secondary battery of Example 1 was prepared.

[0073] (Example 2) A non-aqueous electrolyte solution not containing FEC and LiPO2F2 was prepared. Specifically, EC:EMC were mixed at a volume ratio of 1:3 to prepare a non-aqueous solvent. LiPF6 was dissolved in the prepared non-aqueous solvent at a concentration of 1 mol / L. In this way, the non-aqueous electrolyte solution of Example 2 was prepared. A secondary battery for evaluation of Example 2 was prepared in the same manner as Example 1 except for this.

[0074] (Example 3) A non-aqueous electrolyte that does not contain LiPO2F2 was prepared. Specifically, FEC:EC:EMC were mixed in a volume ratio of 0.08:0.92:3 to prepare a non-aqueous solvent. LiPF6 was dissolved in the prepared non-aqueous solvent at a concentration of 1 mol / L. In this way, the non-aqueous electrolyte of Example 3 was prepared. A secondary battery for evaluation of Example 3 was prepared in the same manner as Example 1 except for this.

[0075] (Example 4) A non-aqueous electrolyte solution not containing FEC was prepared. Specifically, EC:EMC were mixed at a volume ratio of 1:3 to prepare a non-aqueous solvent. LiPF6 was dissolved in the prepared non-aqueous solvent at a concentration of 1 mol / L. LiPO2F2 was also dissolved at 0.7 mass%. In this manner, a non-aqueous electrolyte solution of Example 4 was prepared. A secondary battery for evaluation of Example 4 was prepared in the same manner as Example 1 except for this.

[0076] (Examples 5 to 8) As the non-aqueous electrolyte, the concentration (mass%) of LiPO2F2 in the non-aqueous electrolyte of Example 1 was changed as shown in Table 1. Except for this, the evaluation secondary batteries of Examples 5 to 8 were prepared in the same manner as Example 1.

[0077] (Example 9 to Example 11) As the nonaqueous electrolyte, the concentration (vol %) of FEC in the nonaqueous electrolyte of Example 1 was changed as shown in Table 1. Except for this, the evaluation secondary batteries of Examples 9 to 11 were prepared in the same manner as in Example 1.

[0078] (Examples 12 to 15) As the negative electrode active material, a mixture of graphite and SiO in a mass ratio of 97:3 was prepared. In addition, as the nonaqueous electrolyte, the concentration (volume %) of FEC in the nonaqueous electrolyte of Example 1 was changed as shown in Table 1. Except for this, the evaluation secondary batteries of Examples 12 to 15 were prepared in the same manner as Example 1.

[0079] (Example 16 to Example 19) A mixture of graphite and SiO in a mass ratio of 90:10 was prepared as the negative electrode active material. In addition, 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. Except for this, the evaluation secondary batteries of Examples 16 to 19 were prepared in the same manner as Example 1.

[0080] <Evaluation of capacity retention rate> In an environment of 25°C, constant current charging was performed at a charge rate of 0.5C up to 4.2V, and then constant current discharging was performed at a discharge rate of 0.5C up to 3.0V, with 300 cycles of charging and discharging being repeated. The discharge capacity in the first cycle was taken as the "initial capacity". Next, in an environment of 25°C, the evaluation secondary battery after 300 cycles was charged at a constant current of 0.1C up to 4.2V, and then constant current discharging was performed at a discharge rate of 0.1C up to 3.0V. The discharge capacity at this time was taken as the "post-cycle capacity". The capacity retention rate (%) was calculated using the following formula: capacity retention rate = (post-cycle capacity / initial capacity) x 100; The results are shown in Table 1.

[0081] <Evaluation of gas generation amount> The amount of gas generated was calculated by measuring the volume inside the case of each evaluation secondary battery before and after the cycles using the Archimedes method with Fluorinert as a solvent. Specifically, the volume inside the case was calculated from the relationship between the weight in air and the weight in water using the Archimedes method for each evaluation secondary battery after the activation treatment. This was defined as the "initial volume". Next, in an environment of 25°C, constant current charging was performed to 4.2V at a charge rate of 0.5C, and then constant current discharging was performed to 3.0V at a discharge rate of 0.5C, and 300 cycles of charging and discharging were repeated. The volume inside the case of the evaluation secondary battery after 300 cycles was calculated in the same manner as above using the Archimedes method. This was defined as the "volume after cycling". Then, the difference (mL) between the volume after cycling and the initial volume was calculated, and this was defined as the amount of gas generated. Furthermore, the amount of gas generated (mL) was divided by the rated capacity (Ah) of each evaluation secondary battery to calculate the amount of gas generated per battery capacity (ml / Ah). The results are shown in Table 1.

[0082] [Table 1]

[0083] As shown in Table 1, in Examples 1, 5 to 8, 10 to 15, 18 and 19 in which the negative electrode active material contains a Si-containing material, the nonaqueous 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, the capacity retention rate is 93% or more. That is, a nonaqueous electrolyte secondary battery having an excellent capacity retention rate after cycling is realized by including a lithium transition metal composite oxide containing at least lithium and manganese as the positive electrode active material, graphite particles and a Si-containing material as the negative electrode active material, and a nonaqueous electrolyte containing FEC and difluorophosphate, and the ratio of the mass of FEC to the mass of the Si-containing material is 0.2 or more.

[0084] On the other hand, it is seen that the capacity retention rate is low in Examples 2 and 4 where FEC is not added. It is also seen that the capacity retention rate is 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. This is presumed to be because the FEC does not fully exert the protective effect on the negative electrode (especially the Si-containing material contained in the negative electrode). Furthermore, it is seen that Example 3 which does not contain LiPO2F2 also has a low capacity retention rate. This is presumed to be because the absence of LiPO2F2 causes FEC to adhere to the positive electrode surface, reducing the protective effect of FEC on the negative electrode surface.

[0085] As shown in Table 1, comparing Example 1 and Example 5, it can be seen that the capacity retention rate is somewhat low 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. This is presumably because the amount of LiPO2F2 is small relative to the positive electrode active material, so that the amount of FEC adsorbed on the positive electrode surface increases, and the protective effect of FEC on the negative electrode surface becomes somewhat smaller.

[0086] As shown in Table 1, in Examples 11, 15, and 19, in which the ratio of the mass of FEC to the mass of the negative electrode active material exceeds 0.04, the amount of gas generated is relatively large. This is presumably because the amount of gas generated during decomposition of FEC increases when the mass of FEC increases relative to the mass of the negative electrode active material.

[0087] <<Second Test>> <Preparation of secondary battery for evaluation> (Example 20 to Example 24) In the second test, the type of positive electrode active material was changed to prepare secondary batteries for evaluation in each example, and the capacity retention rate and the amount of gas generated were evaluated. Specifically, the secondary batteries for evaluation in each example were prepared using the positive electrode active material shown in Table 2. Except for this, the secondary batteries for evaluation in Examples 20 to 24 were prepared in the same manner as in Example 1.

[0088] <Evaluation of capacity retention rate and gas generation rate> The capacity retention rate and the amount of gas generated of the secondary batteries for evaluation of each example were evaluated in the same manner as described above. The results are shown in Table 2. For comparison, Table 2 also shows the results of Examples 1 and 2.

[0089] [Table 2]

[0090] As shown in Table 2, it can be seen that similar effects can be obtained even when the type of positive electrode active material is different. Also, as shown in Table 2, it can be seen that the capacity retention rate is particularly good in Examples 1 and 22 to 24, in which the ratio of the substance amount of Li to the substance amount of metals other than Li (Li / Me) exceeds 1.2.

[0091] Although the specific examples of the present invention have been described in detail above, these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and changes of the specific examples exemplified above.

[0092] As described above, specific aspects of the technology disclosed herein include those described in the following sections. Item 1: A nonaqueous electrolyte secondary battery comprising an electrode assembly having a positive electrode and a negative electrode, a nonaqueous electrolyte, and a battery case, wherein the positive electrode contains a lithium transition metal composite oxide containing at least lithium and manganese as a positive electrode active material, the negative electrode contains graphite particles and a Si-containing material as negative electrode active materials, the nonaqueous electrolyte contains fluoroethylene carbonate and difluorophosphate, and a ratio of the mass of the fluoroethylene carbonate to the mass of the Si-containing material is 0.2 or more. 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. Item 3: The nonaqueous electrolyte secondary battery according to item 1 or 2, wherein the Si-containing material is 1 mass % or more and 15 mass % or less when the total of the graphite particles and the Si-containing material is 100 mass %. Item 4: The nonaqueous electrolyte secondary battery according to any one of Items 1 to 3, wherein a ratio of the mass of the difluorophosphate to the mass of the positive electrode active material is 0.001 or more. Item 5: The positive electrode active material has the following general formula: Li 1+a Ni x Mn y M z O2(I) (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 the group consisting of Co, Al, Mg, Ca, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W); 5. The nonaqueous electrolyte secondary battery according to any one of items 1 to 4, wherein the lithium transition metal composite oxide is represented by the following formula: Item 6: The nonaqueous electrolyte secondary battery according to any one of items 1 to 5, wherein, when a metal element other than Li contained in the positive electrode active material is Me, a ratio of the amount of substance of Li to the amount of substance of Me (Li / Me) is 1.2 or more and 2 or less. 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 complex of silicon and carbon. Item 8: The nonaqueous electrolyte secondary battery according to any one of Items 1 to 7, wherein the difluorophosphate includes lithium difluorophosphate. Item 9: A method for producing a nonaqueous electrolyte secondary battery including an electrode assembly having a positive electrode and a negative electrode, a nonaqueous electrolyte, and a battery case, the method comprising: an assembly step of accommodating the electrode assembly in the battery case to construct an assembly; a preparation step of preparing a nonaqueous electrolyte containing fluoroethylene carbonate and a difluorophosphate; and a liquid injection step of injecting the nonaqueous electrolyte into the battery case, wherein the positive electrode of the electrode assembly contains a positive electrode active material, and the negative electrode contains graphite particles and a Si-containing material as negative electrode active materials, and in the preparation step, the nonaqueous electrolyte is prepared so that a ratio of the mass of the fluoroethylene carbonate to the mass of the Si-containing material is 0.2 or more. Item 10: The method according to item 9, wherein in the preparation step, the nonaqueous electrolyte solution is prepared so that a ratio of a mass of the fluoroethylene carbonate to a mass of the negative electrode active material is 0.04 or less. Item 11: The method according to item 9 or 10, wherein in the preparation step, a ratio of a mass of the difluorophosphate to a mass of the positive electrode active material is 0.001 or more. Item 12: The positive electrode active material has the following general formula: Li 1+a Ni x Mn y M z O2(I) (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 the group consisting of Co, Al, Mg, Ca, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W); Item 12. The method according to any one of items 9 to 11, wherein the lithium transition metal composite oxide is represented by the following formula: Item 13: The method according to any one of items 9 to 12, wherein, when a metal element other than Li contained in the positive electrode active material is Me, a ratio of the amount of substance of Li to the amount of substance of Me (Li / Me) is 1.2 or more and 2 or less. [Explanation of symbols]

[0093] 10 Battery case 20 Electrode body 22 Positive electrode 22a Cathode active material layer 24 Negative electrode 24a Negative electrode active material layer 100 batteries S1 construction process S2 preparation process S3 Liquid injection process

Claims

1. A non-aqueous electrolyte secondary battery comprising an electrode assembly having a positive electrode and a negative electrode, a non-aqueous electrolyte, and a battery case, the positive electrode contains a lithium transition metal composite oxide containing at least lithium and manganese as a positive electrode active material, The negative electrode includes graphite particles and a Si-containing material as a negative electrode active material, The non-aqueous electrolyte contains fluoroethylene carbonate and difluorophosphate, a ratio of the mass of the fluoroethylene carbonate to the mass of the Si-containing material is 0.2 or more.

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

3. 3. The nonaqueous electrolyte secondary battery according to claim 1, wherein the Si-containing material is present in an amount of 1 mass % to 15 mass % when the total amount of the graphite particles and the Si-containing material is taken as 100 mass %.

4. 3. The nonaqueous electrolyte secondary battery according to claim 1, wherein a ratio of a mass of the difluorophosphate to a mass of the positive electrode active material is 0.001 or more.

5. The positive electrode active material has the following general formula: Li １＋ａ Ni ｘ Mn ｙ M ｚ O ２ (I) (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 the group consisting of Co, Al, Mg, Ca, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W); 3. The nonaqueous electrolyte secondary battery according to claim 1, wherein the lithium transition metal composite oxide is represented by the formula:

6. 6. The nonaqueous electrolyte secondary battery according to claim 5, wherein when a metal element other than Li contained in the positive electrode active material is Me, a ratio of the amount of substance of Li to the amount of substance of Me (Li / Me) is 1.2 or more and 2 or less.

7. 3. The nonaqueous electrolyte secondary battery according to claim 1, 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. 3. The nonaqueous electrolyte secondary battery according to claim 1, wherein the difluorophosphate salt includes lithium difluorophosphate.

9. A method for manufacturing a non-aqueous electrolyte secondary battery including an electrode assembly having a positive electrode and a negative electrode, a non-aqueous electrolyte, and a battery case, comprising the steps of: a construction step of housing the electrode body in the battery case to construct an assembly; A preparation step of preparing a non-aqueous electrolyte solution containing fluoroethylene carbonate and a difluorophosphate; a liquid injection step of injecting the nonaqueous electrolyte into the battery case; Including, wherein the positive electrode of the electrode body includes a positive electrode active material, and the negative electrode includes graphite particles and a Si-containing material as negative electrode active materials, the preparing step comprises preparing the nonaqueous electrolyte so that a ratio of a mass of the fluoroethylene carbonate to a mass of the Si-containing material is 0.2 or more.

10. The method according to claim 9 , wherein in the preparing step, the nonaqueous electrolyte solution is prepared so that a ratio of a mass of the fluoroethylene carbonate to a mass of the negative electrode active material is 0.04 or less.

11. The method according to claim 9 or 10, wherein in the preparing step, a ratio of a mass of the difluorophosphate to a mass of the positive electrode active material is 0.001 or more.

12. The positive electrode active material has the following general formula: Li １＋ａ Ni ｘ Mn ｙ M ｚ O ２ (I) (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 the group consisting of Co, Al, Mg, Ca, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W); The method according to claim 9 or 10, wherein the lithium transition metal composite oxide is represented by the formula:

13. 13. The method according to claim 12, wherein a ratio of the amount of substance of Li to the amount of substance of Me (Li / Me) is 1.2 or more and 2 or less, where Me is a metal element other than Li contained in the positive electrode active material.

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