Nonaqueous electrolyte secondary battery and method of 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 investigated. Related prior art includes Patent Documents 1 and 2. Patent Document 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. Patent Document 2 describes a nonaqueous electrolyte secondary battery containing a negative electrode active material including 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] Japanese Patent Publication No. 2020-64717 [Patent Document 2] Japanese Patent Application Publication No. 2019-185992 Summary of the Invention [Problem to be solved by the invention]

[0004] In recent years, nonaqueous electrolyte secondary batteries containing a Si-containing material in the negative electrode have been studied for the purpose of increasing capacity, etc. Fluoroethylene carbonate is adsorbed to the surface of the negative electrode to protect the surface of the Si-containing material and suppress side reactions with the electrolyte, thereby improving the capacity retention rate after cycling. Lithium difluorophosphate is adsorbed to the surface of the positive electrode to protect the surface of the positive electrode active material and suppress an increase in internal resistance. However, according to the results of studies by the present inventors, it was found that when a negative electrode contains a Si-containing material and a nonaqueous 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 nonaqueous 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 having a negative electrode containing a Si-containing material and having 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 a difluorophosphate. The mass ratio of the fluoroethylene carbonate to the Si-containing material is 0.2 or more.

[0007] This configuration effectively protects the negative electrode surface (particularly the surface of the Si-containing material) by the fluoroethylene carbonate and the positive electrode surface (particularly the surface of the positive electrode active material) by the difluorophosphate, thereby suppressing cracking of the Si-containing material during charge and discharge, and realizing a nonaqueous electrolyte secondary battery with excellent capacity retention.

[0008] Another aspect provides a method for manufacturing 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 an assembly 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. 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. 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 non-aqueous electrolyte secondary battery having an excellent capacity retention rate can be suitably manufactured. [Brief explanation of the drawings]

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

[0010] Hereinafter, some preferred embodiments of the technology disclosed herein will be described with reference to the drawings. It should be noted that matters necessary for carrying out the present invention other than those specifically mentioned in this specification (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 for those skilled in the art based on prior art in the relevant field. The present invention can be carried out based on the contents disclosed in this specification and common general technical knowledge in the relevant field.

[0011] In this specification, the term "nonaqueous electrolyte secondary battery" refers to any battery that can be repeatedly charged and discharged by the transfer of charge carriers between a positive electrode and a negative electrode via a nonaqueous electrolyte. The concept of nonaqueous electrolyte secondary batteries encompasses 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 not only the meaning of A or more but also the meaning of "greater than A" and "smaller than B."

[0012] <Battery 100> FIG. 1 is a perspective view of battery 100. FIG. 2 is a schematic longitudinal cross-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 battery 100, the long side direction perpendicular to the short side direction, and the up-down direction perpendicular to the short side and long side directions, respectively. However, these directions are merely used for convenience of explanation and do not limit the installation form of 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 nonaqueous 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 houses 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, more preferably made of, for example, aluminum, an aluminum alloy, iron, or an iron alloy. 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) 14 that closes the opening 12h. The sealing plate 14 is attached to the exterior body 12 so as to close the opening 12h. The battery case 10 is integrated by joining (e.g., 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 inlet 15 and two terminal outlet holes 18, 19. The liquid inlet 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 liquid inlet 15 is preferably provided in the sealing plate 14. The liquid inlet 15 is sealed with a sealing member 16. The terminal outlet 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 outlet 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 lead-out holes 18, 19. Here, the positive electrode terminal 30 and the negative electrode terminal 40 are crimped to the peripheral portion of the sealing plate 14 surrounding the terminal lead-out holes 18, 19 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 assembly 20 via a 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, and more preferably made of, for example, aluminum or an aluminum alloy. The negative electrode terminal 40 is electrically connected to the negative electrode tab group 25 of the electrode assembly 20 via a 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, and more preferably made of, for example, copper or a copper alloy.

[0018] As described above, the battery case 10 contains a nonaqueous electrolyte 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 the technology disclosed herein is not impaired.

[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, because FEC has a carbonate skeleton, it tends to easily adsorb 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 adsorbs to the surface of the positive electrode active material to protect the active material. The inventors have found that using a nonaqueous electrolyte containing only either FEC or difluorophosphate does not fully improve the capacity retention rate of nonaqueous electrolyte secondary batteries. This is presumably because, without FEC, the Si-containing material is not protected in the first place, resulting in the formation of a coating on the surface of the Si-containing material. Furthermore, without difluorophosphate, FEC also adheres to the surface of the positive electrode, resulting in insufficient protection of the Si-containing material and the formation of a coating on the surface of the Si-containing material. Therefore, the battery 100 disclosed herein contains FEC and difluorophosphate. The difluorophosphate adheres preferentially to the surface of the positive electrode 22 over the FEC, thereby allowing the FEC to adequately exert its protective effect on 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 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, such as 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 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 nonaqueous electrolyte is too low, the protective effect of the Si-containing material described above is difficult to adequately exhibit. Here, the content of FEC in the nonaqueous electrolyte is preferably determined by the ratio to the Si-containing material contained in the negative electrode 24, from the viewpoint of adequately exhibiting the effects of the technology disclosed herein. The mass ratio of FEC to the Si-containing material is preferably 0.2 or more, more preferably 0.28 or more, and may be 0.45 or more. This favorably improves the capacity retention rate after cycling. There is no particular upper limit to the mass ratio of FEC to the Si-containing material. As the content of FEC in the nonaqueous 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 mass ratio of FEC to the Si-containing material is preferably 1.6 or less, and may be 1.2 or less, or 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, Si-containing material, and other negative electrode active materials) is preferably 0.04 or less. This suitably reduces the amount of gas generated after cycling. 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 achieved.

[0024] Although not particularly limited, when the total volume of the nonaqueous solvent is taken as 100%, the volume ratio of FEC is preferably 10% by volume or less. This allows the Si-containing material to be adequately protected while suitably suppressing the amount of gas generated during charge and discharge. For example, when the total volume of the nonaqueous solvent is taken as 100%, the volume ratio of FEC is preferably 0.5% by volume to 10% by volume, more preferably 1% by volume to 10% by volume, and even more preferably 1% by volume to 7% by volume.

[0025] Difluorophosphate is PO2F2 - The difluorophosphate salt is a salt of a cation and an anion represented by the formula:

[0026] . Examples of the cation include alkali metal ions such as Li, Na, and K, and ammonium ions, with Li being preferred. A specific example of a preferred difluorophosphate salt is lithium difluorophosphate (lithium difluorophosphate (LiDFP), LiPO2F2).

[0026] If the difluorophosphate content in the non-aqueous electrolyte is too low, the FEC may preferentially adhere to the positive electrode surface, as described above, making it difficult to adequately protect the Si-containing material through the FEC. Here, the content of the 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 achieving the effects of the technology disclosed herein. The mass ratio of the difluorophosphate to 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 for more adequate protection of the positive electrode surface and the negative electrode surface, and favorably improves the capacity retention rate after cycling. The upper limit of the mass ratio of the difluorophosphate to the positive electrode active material is not particularly limited, but is preferably 0.01 or less, more preferably 0.005 or less, and may be, for example, 0.0045 or less.

[0027] Although not particularly limited, the proportion of difluorophosphate in the entire non-aqueous 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 salts known to be usable in non-aqueous electrolyte secondary batteries can be used. In the case of lithium ion secondary batteries, examples of such salts include fluorine-containing lithium salts such as LiPF6 and LiBF4.

[0029] The non-aqueous electrolyte may further contain additional components (additives). The additives may be one or more of those known to be additives to non-aqueous electrolytes. 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 assembly 20 is housed inside the battery case 10 (more specifically, inside the exterior body 12). The number of electrode assemblies 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 assembly 20. As shown in FIG. 3, the electrode assembly 20 includes a positive electrode 22, a negative electrode 24, and a separator 26. Here, the electrode assembly 20 is a flat wound electrode assembly. The electrode assembly 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 between them and winding them around a winding axis WL. However, the electrode assembly 20 may also be a laminated electrode assembly 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 includes a positive electrode current collector 22c and a positive electrode active material layer 22a fixed to at least one surface of the positive electrode current collector 22c. The positive electrode current collector 22c is strip-shaped. The positive electrode current collector 22c is made of a conductive metal such as aluminum, an aluminum alloy, nickel, or stainless steel. Here, the positive electrode current collector 22c is a metal foil, specifically, an aluminum foil. The positive electrode 22 may include a positive electrode protective layer 22p at one end in the long side direction. When the positive electrode 22 includes the positive electrode protective layer 22p, the positive electrode protective layer 22p is disposed between the positive electrode current collector 22c and the positive electrode active material layer 22a in the long side direction Y. The positive electrode protective layer 22p includes, for example, an inorganic filler (e.g., alumina).

[0033] A plurality of positive electrode tabs 22t are provided at one end of the positive electrode current collector 22c in the long side direction Y (the left end in FIG. 3). The positive electrode tab 22t is part of the positive electrode current collector 22c and is made of metal foil (aluminum foil). The plurality of positive electrode tabs 22t are stacked at one end of the positive electrode current collector 22c in the long side direction Y (the left end in FIG. 3) to form a positive electrode tab group 23 as shown in FIG. 2. A positive electrode current collector 50 is attached to (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, etc. In some embodiments, the lithium transition metal composite oxide preferably has a layered rock salt-type crystal structure of space group C2 / m (so-called, lithium-excess-type crystal structure) from the viewpoint of realizing a high energy density. Examples of such a compound include a lithium-excess transition metal composite oxide represented by the following formula (I). Specific examples of such a lithium-excess transition metal composite oxide 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, etc.

[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, a + x + y + z = 1, and 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 ratio (Li / Me) of the amount of Li to the amount of metal elements other than Li (Me) in the positive electrode active material is preferably 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 Li / Me value is 2.0 or less, and may be 1.5 or less, or may be 1.4 or less. The ratio (Li / Me) of the amount of Li to the amount 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 whose Li / Me satisfies the above-mentioned numerical range as the positive electrode active material, the cycle characteristics (e.g., capacity retention rate after cycling) are suitably improved.

[0037] While not intending to limit the technology disclosed herein, the reason for such an effect is presumed to be as follows. As described above, when the nonaqueous electrolyte contains FEC and difluorophosphate, the positive electrode surface and the negative electrode surface are suitably protected, improving the capacity retention rate after cycling. Here, difluorophosphate tends to adhere more easily to the lithium-excess lithium transition metal composite oxides described above. In particular, difluorophosphate is more easily adsorbed to Li-excess positive electrode active materials having a Li / Me ratio 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 well exhibited. This allows the protective effect of FEC on the negative electrode 24 and the protective effect of difluorophosphate on the positive electrode 22 to be properly exhibited, improving the cycle characteristics (e.g., the capacity retention rate after cycling) of the battery 100.

[0038] The content of the positive electrode active material in the positive electrode active material layer 22a is not particularly limited, and is preferably 70 mass % or more, more preferably 80 mass % to 99 mass % or less, and even more preferably 85 mass % to 98 mass % or less, in the positive electrode active material layer 22a (i.e., relative to the total mass of the positive electrode active material layer).

[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 the particle diameter corresponding to the cumulative 50% from the fine particle side in the volume-based particle size distribution measured by particle size distribution measurement based on a laser diffraction / light scattering method.

[0040] The positive electrode active material layer 22a may contain optional components other than the positive electrode active material, such as a binder, a conductive material, and various additives. Examples of suitable binders include polyvinylidene fluoride (PVdF). The binder content in the positive electrode active material layer 22a is not particularly limited, but is, for example, 0.5% by mass to 15% by mass, preferably 1% by mass to 10% by mass, and more preferably 1.5% by mass to 8% by mass. Examples of suitable conductive materials include carbon black such as acetylene black (AB) and other carbon materials (e.g., graphite). The conductive material content in the positive electrode active material layer 22a is not particularly limited, but is, for example, 0.1% by mass to 20% by mass, preferably 1% by mass to 15% by mass, and more preferably 2% by mass to 10% by mass.

[0041] The negative electrode 24 includes a negative electrode current collector 24c and a negative electrode active material layer 24a fixed to 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. In this example, 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 of the negative electrode current collector 24c in the long side direction Y (the right end in FIG. 3). The negative electrode tab 24t is part of the negative electrode current collector 24c and is made of metal foil (copper foil). The plurality of negative electrode tabs 24t are stacked at one end of the negative electrode current collector 24c in the long side direction Y (the right end in FIG. 3) 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-like 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 nonaqueous electrolyte adequately protects the Si-containing material, thereby suppressing cracking and other problems in the Si-containing material. This improves cycle characteristics (particularly, the capacity retention rate after cycling). Furthermore, the use of the lithium-excess lithium transition metal composite oxide described above as the positive electrode active material further suppresses cracking and other problems in the Si-containing material. While not intending to limit the technology disclosed herein, the reason for this effect is presumed to be as follows: The nonaqueous electrolyte containing FEC and difluorophosphate protects the surface of the Si-containing material, suppressing decomposition of the electrolyte and film formation. This improves cycle characteristics (particularly, the capacity retention rate after cycling). Furthermore, lithium-excess lithium transition metal composite oxides are not as good as conventional lithium transition metal composite oxides (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 also be narrowed. 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 formed surfaces caused by cracking is reduced, and the need for additional protection of the newly formed surfaces by FEC tends to be reduced. 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] Because Si-containing materials have a larger specific capacity than carbon materials, they are preferably used for purposes such as 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 Si-containing materials include Si (silicon), SiO (silicon oxide), silicon-carbon composites (including silicon carbide and SiC composites in which carbon is dispersed inside silicon particles), SiN-containing materials (silicon nitride), and porous particles in which nano-Si particles are dispersed. The Si-containing material may be one or more of the above materials. Among these, silicon, silicon oxide, and silicon-carbon composites are particularly preferred. While 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.

[0045] The mass proportion of the Si-containing material in the negative electrode active material is not particularly limited. For example, from the viewpoint of achieving both a high capacity and improved cycle characteristics of the battery 100, the content 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, when the total amount of the negative electrode active material is taken as 100 mass %.

[0046] Suitable graphite particles include, for example, artificial graphite and natural graphite. The graphite particles may have a coating layer of amorphous carbon on their surfaces. While not particularly limited, the graphite particles preferably have a substantially spherical shape. In this specification, the term "substantially spherical" encompasses spherical shapes, rugby ball shapes, and the like, and refers to particles having an average aspect ratio (the ratio of the length in the superaxial direction to the length in the minor 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 to 30 μm, and more preferably, 10 μm to 25 μm.

[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% to 99 mass%, and more preferably 90 mass% to 99 mass%, which allows the battery 100 to have both high capacity and improved cycle characteristics.

[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, and is preferably 90% by mass or more, and more preferably 95% by mass or more and 99% by mass or less, in the negative electrode active material layer 24a.

[0050] The negative electrode active material layer 24a may contain optional components other than the negative electrode active material, such as a binder, a dispersant, a conductive material, and various additives. Examples of suitable binders include rubbers such as styrene butadiene rubber (SBR) and acrylic resins such as polyacrylic acid (PAA). The binder content in the negative electrode active material layer 24a is not particularly limited, but is, for example, 0.1% by mass to 10% by mass, and preferably 0.5% by mass to 5% by mass. Examples of suitable thickeners include celluloses such as carboxymethyl cellulose (CMC). The dispersant content in the negative electrode active material layer 24a is not particularly limited, but is, for example, 0.1% by mass to 10% by mass, and preferably 0.5% by mass to 5% by 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 separator and is not particularly limited. The separator 26 is preferably a porous resin 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 a substrate portion made of a porous resin sheet. The configuration of the heat resistance layer or adhesive layer may be the same as a conventional one.

[0052] <Manufacturing method of non-aqueous electrolyte secondary battery> 4 is a flowchart of a manufacturing method according to one 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). Furthermore, 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 each 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 to the positive electrode current collector 22c by a conventionally known method, drying, and appropriately pressing. As the positive electrode active material, one type from the above-mentioned materials can be used alone, or two or more types can be mixed and used as appropriate. 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 fabricated, 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 conventional method, drying, and appropriately pressing. The negative electrode active material includes at least graphite particles and a Si-containing material (preferably, at least one of silicon, silicon oxide, and a silicon-carbon composite). From the viewpoint of achieving both high capacity and improved cycle characteristics of the battery 100, the content of the Si-containing material is preferably adjusted to 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, based on the total amount of the negative electrode active material taken as 100% by mass. Note that the negative electrode active material may further include one or more of the above-described materials.

[0056] Next, in the electrode assembly fabrication step (step S1-2), the positive electrode 22 and negative electrode 24 fabricated above are opposed to each other with the separator 26 interposed therebetween to fabricate the electrode assembly 20. Next, in the placement step (step S1-3), the electrode assembly 20 is placed inside the exterior housing 12. For example, the electrode assembly 20 is accommodated inside the exterior housing 12 through the opening 12h of the exterior housing 12. Next, in the welding and joining step (step S1-4), the sealing plate 14 is fitted into the opening 12h of the exterior housing 12 and welded to the periphery of the opening 12h to integrate the exterior housing 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, when the nonaqueous electrolyte contains FEC and difluorophosphate, the surfaces of both the positive electrode active material and the negative electrode active material (particularly the Si-containing material) are suitably protected, improving the capacity retention rate of the battery 100. From the viewpoint of achieving a high level of the effects of the technology disclosed herein, it is preferable that the nonaqueous electrolyte be prepared in the preparation step so that the mass ratio of FEC to the Si-containing material is 0.2 or more. It is preferable that the nonaqueous electrolyte be prepared in the preparation step so that the mass ratio of FEC to 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 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 or more and 0.04 or less, more preferably 0.01 or more and 0.04 or less, and the nonaqueous electrolyte may be prepared so that the ratio is 0.01 or more and 0.035 or less.

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

[0060] In a preferred embodiment, the non-aqueous electrolyte contains, in addition to FEC, a carbonate other than FEC. The carbonate may be selected from the above-mentioned materials and used alone or in combination of two or more. 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 solution 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 solution 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 more suitably exhibits the protective effect on the positive electrode surface and the Si-containing material, and suitably improves the capacity retention rate after cycling.

[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 in combination of two or more, and LiPF6 is particularly suitable.

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

[0065] Although not particularly limited, after the liquid injection step, the assembly may be left (held) for a predetermined time. This allows the nonaqueous electrolyte to be evenly distributed in the long side direction Y, even when the length in the long side direction Y is relatively long, thereby thoroughly impregnating the electrode assembly 20 with the nonaqueous electrolyte. The leaving temperature may be room temperature (e.g., 25°C ± 10°C, approximately 25°C ± 5°C), or may be a high-temperature environment of approximately 35 to 45°C, for example, for the purpose of improving the impregnation of the nonaqueous electrolyte into the electrode assembly 20. The leaving time is not particularly limited because it depends on, for example, the size of the electrode assembly 20 (particularly the length in the long side direction Y), the viscosity of the first nonaqueous electrolyte, whether or not pressure is applied, the leaving temperature, etc. However, for example, 5 minutes or more is preferable, 10 minutes or more is more preferable, and 1 hour or more is particularly preferable. Furthermore, from the viewpoint of production efficiency, etc., the leaving 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 a nonaqueous electrolyte solution has been injected, at least once. Charging of the assembly can be carried out in a conventional manner. Typically, an external power source is connected between the positive and negative electrode terminals of the assembly, and charging is carried out until the assembly reaches a predetermined voltage. While not particularly limited, charging is preferably carried out until the voltage reaches approximately 4 V or higher, preferably 4.1 V or higher, 4.2 V or higher, for example, 4.2 V. 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 approximately 4.6 V or higher, preferably 4.7 V or higher, for example, 4.7 V, from the viewpoint of achieving high capacity in an electrochemically active state. The charge / discharge rate can be, for example, about 0.1 to 2 C. Charging may be carried out once, or may be repeated two or more times, for example, with a discharge in between. Discharging may also be carried out after charging. The charging step may be carried out in a room temperature environment (for example, about 25° C.±10° C., 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> Battery 100 can be used for a variety of purposes, but 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, and examples include plug-in hybrid electric vehicles (PHEVs), hybrid electric vehicles (HEVs), and battery electric vehicles (BEVs).

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

[0069] <First Exam> <Preparation of secondary battery for evaluation> (Example 1) First, a positive electrode and a negative electrode 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 to an Al foil as a positive electrode current collector, dried, and pressed to a predetermined thickness. The mixture was then cut to a predetermined size to prepare a positive electrode.

[0070] The negative electrode was fabricated as follows. First, graphite and SiO (a 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, 1 part by mass of SBR as a binder and 1 part by mass of CMC as a dispersant were added to the resulting mixture (100 parts by mass) (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 to a Cu foil as a negative electrode current collector, dried, and pressed to a predetermined thickness. The negative electrode was then fabricated by cutting to a predetermined size.

[0071] Next, a non-aqueous electrolyte solution was prepared as follows. Fluoroethylene carbonate (FEC), ethylene carbonate (EC), and ethyl methyl carbonate (EMC) were prepared as solvents. 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. LiPO2F2 was also dissolved at 0.7 mass%. In this way, a non-aqueous electrolyte solution was prepared.

[0072] A porous sheet consisting of three layers of PP / PE / PP was prepared as a separator. The positive electrode and negative electrode prepared above were opposed to each other with the separator interposed therebetween to prepare an electrode assembly. Next, a positive electrode terminal and a negative electrode terminal were attached to the prepared electrode assembly, and the electrode assembly was housed together with a non-aqueous electrolyte in a bag-shaped battery case made of aluminum laminate film and sealed. The prepared secondary battery was subjected to activation treatment by constant current charging at a charge rate of 0.1 C to 4.2 V, followed by constant current discharging at a discharge rate of 0.1 C to 3.0 V. In this way, the evaluation secondary battery of Example 1 was prepared.

[0073] (Example 2) A non-aqueous electrolyte solution containing neither FEC nor LiPO2F2 was prepared. Specifically, EC and 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. Except for this, a secondary battery for evaluation of Example 2 was prepared in the same manner as in Example 1.

[0074] (Example 3) A non-aqueous electrolyte solution not containing 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 solution of Example 3 was prepared. Except for this, a secondary battery for evaluation of Example 3 was prepared in the same manner as in Example 1.

[0075] (Example 4) A non-aqueous electrolyte solution containing no FEC was prepared. Specifically, EC and 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% by mass. In this manner, the non-aqueous electrolyte solution of Example 4 was prepared. Except for this, a secondary battery for evaluation of Example 4 was prepared in the same manner as in Example 1.

[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, secondary batteries for evaluation of Examples 5 to 8 were prepared in the same manner as in Example 1.

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

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

[0079] (Examples 16 to 19) A mixture of graphite and SiO in a mass ratio of 90:10 was prepared as the negative electrode active material. Furthermore, a nonaqueous electrolyte solution was prepared in which the concentration (vol %) of FEC in the nonaqueous electrolyte solution of Example 1 was changed as shown in Table 1. Except for this, secondary batteries for evaluation of Examples 16 to 19 were prepared in the same manner as in Example 1.

[0080] <Evaluation of capacity retention rate> The test battery was subjected to 300 cycles of constant current charging at a charge rate of 0.5C up to 4.2V at 25°C, followed by constant current discharging at a discharge rate of 0.5C down to 3.0V. The discharge capacity at the first cycle was designated the "initial capacity." After 300 cycles, the test secondary battery was subjected to constant current charging at a charge rate of 0.1C up to 4.2V at 25°C, followed by constant current discharging at a discharge rate of 0.1C down to 3.0V. The discharge capacity at this time was designated the "post-cycle capacity." The capacity retention rate (%) was calculated using the following formula: capacity retention rate = (post-cycle capacity / initial capacity) × 100. The results are shown in Table 1.

[0081] <Evaluation of gas generation amount> The volume inside the case of each test secondary battery before and after cycling was measured using the Archimedes method with Fluorinert as a solvent, and the amount of gas generated was calculated. Specifically, the volume inside the case of each test secondary battery after activation was calculated using the Archimedes method based on the relationship between the weight in air and the weight in water. This volume was designated the "initial volume." Next, 300 charge / discharge cycles were performed at 25°C, with one cycle consisting of constant current charging to 4.2 V at a charge rate of 0.5 C and constant current discharging to 3.0 V at a discharge rate of 0.5 C. The volume inside the case of the test secondary battery after 300 cycles was calculated using the Archimedes method in the same manner as above. This volume was designated the "post-cycle volume." The difference (mL) between the post-cycle volume and the initial volume was then calculated and used as the amount of gas generated. The amount of gas generated (mL) was then divided by the rated capacity (Ah) of each test 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, 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, have a capacity retention rate of 93% or more. That is, a nonaqueous electrolyte secondary battery having an excellent post-cycle capacity retention rate 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 having a ratio of the mass of FEC to the mass of the Si-containing material of 0.2 or more.

[0084] On the other hand, it can be seen that Examples 2 and 4, in which FEC is not added, have low capacity retention rates. It can also be seen that Examples 9, 16, and 17, in which the ratio of the mass of FEC to the mass of the Si-containing material is less than 0.2, also have low capacity retention rates. This is presumably because FEC does not sufficiently protect the negative electrode (particularly the Si-containing material contained in the negative electrode). Furthermore, it can be seen that Example 3, which does not contain LiPO2F2, also has a low capacity retention rate. This is presumably because, due to the absence of LiPO2F2, FEC also adheres 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 slightly low in Example 5, where 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 mass of the positive electrode active material, which increases the amount of FEC adsorbed on the positive electrode surface and slightly reduces the protective effect of FEC on the negative electrode surface.

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

[0087] <<Second Exam>> <Preparation of secondary battery for evaluation> (Examples 20 to 24) In the second test, the type of positive electrode active material was changed to fabricate evaluation secondary batteries for each example, and the capacity retention rate and gas generation amount were evaluated. Specifically, the evaluation secondary batteries for each example were fabricated using the positive electrode active materials shown in Table 2. Except for this, the evaluation secondary batteries of 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 gas generation rate of the test secondary batteries of each example were evaluated using the same evaluation methods as described above. The results are shown in Table 2. For comparison, Table 2 also lists 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. Furthermore, as shown in Table 2, it can be seen that the capacity retention rate is particularly good in Examples 1 and 22 to 24, where the ratio of the substance amount of Li to the substance amount of metals other than Li (Li / Me) exceeds 1.2.

[0091] Although specific examples of the present invention have been described above in detail, these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and alterations 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 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, 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 non-aqueous electrolyte contains fluoroethylene carbonate and difluorophosphate, and the 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 the 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 accounts for 1% by mass or more and 15% by mass or less when the total of the graphite particles and the Si-containing material is 100% by mass. 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. 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 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 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, the ratio of the amount of Li to the amount 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 composite of silicon and carbon. Item 8: The nonaqueous electrolyte secondary battery according to any one of Items 1 to 7, wherein the difluorophosphate salt includes lithium difluorophosphate. Item 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, 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 non-aqueous electrolyte containing fluoroethylene carbonate and difluorophosphate; and a liquid injection step of injecting the non-aqueous 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 non-aqueous 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. Item 10: The manufacturing method according to Item 9, wherein in the preparation step, the nonaqueous electrolyte solution 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. Item 11: The method according to Item 9 or 10, wherein in the preparation step, the ratio of the mass of the difluorophosphate to the 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 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 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, the ratio of the amount of Li to the amount 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 negative electrode active materials, the non-aqueous electrolyte solution 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; a ratio of the amount of Li to the amount of Me (Li / Me) of 1.3 to 2.0, where Me is a metal element other than lithium contained in the positive electrode active material;

2. 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the ratio of the mass of said fluoroethylene carbonate to the 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 accounts for 1% by mass or more and 15% by mass or less when the total of the graphite particles and the Si-containing material is 100% by mass.

4. 3. The nonaqueous electrolyte secondary battery according to claim 1, wherein the ratio of the mass of said difluorophosphate to the mass of said 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 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. 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.

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

8. 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: 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 difluorophosphate; a liquid injection step of injecting the nonaqueous electrolyte into the battery case; Including, wherein the positive electrode of the electrode assembly contains a lithium transition metal composite oxide containing at least lithium and manganese as a positive electrode active material, and the negative electrode contains graphite particles and a Si-containing material as negative electrode active materials; When a metal element other than lithium contained in the positive electrode active material is defined as Me, the ratio of the amount of substance of Li to the amount of substance of Me (Li / Me) is 1.3 or more and 2 or less, 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.

9. The method according to claim 8 , wherein in the preparing step, the nonaqueous electrolyte solution 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.

10. The method according to claim 8 or 9, 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.

11. 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 Co, Al, Mg, Ca, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W); The method according to claim 8 or 9, wherein the lithium transition metal composite oxide is represented by the formula:

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