Non-aqueous electrolyte secondary battery

Abstract

To provide a non-aqueous electrolyte secondary battery having improved storage characteristics.SOLUTION: A nonaqueous electrolyte secondary battery 100 includes a positive electrode 50, a negative electrode 60, and a nonaqueous electrolyte 80. The negative electrode 60 includes a negative electrode active material layer 64 containing a negative electrode active material, and the negative electrode active material includes a carbon material and an amorphous carbon coating layer that covers the carbon material. The negative electrode active material has an intensity ratio (D / G) of 0.3 or more and 0.5 or less between a peak intensity G at 1580 cm-1 and a peak intensity D at 1360 cm-1 in a Raman spectrum analysis measured by Raman spectroscopy. The nonaqueous electrolyte 80 includes at least a sulfur-based electrolyte salt and a nonaqueous solvent, and the nonaqueous solvent includes a carbonate-based solvent and methyl acetate. Here, the negative electrode active material layer 64 has a SOx concentration of 0.3 atomic% or more calculated based on an XPS spectrum measured by X-ray photoelectron spectroscopy.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The technology disclosed herein relates to a non-aqueous electrolyte secondary battery. [Background technology]

[0002] 2. Description of the Related Art In recent years, non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries have been suitably used as portable power sources for personal computers, mobile terminals, and the like, and as power sources for driving vehicles such as electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs), among others.

[0003] A non-aqueous electrolyte secondary battery is generally constructed by housing an electrode assembly having a positive electrode and a negative electrode, and a non-aqueous electrolyte in a battery case. For example, Patent Documents 1 to 3 are listed as related techniques. Patent Document 1 discloses a non-aqueous electrolyte containing an oxalate salt, a compound having two or more cyclic carboxylic anhydride skeletons in the molecule, and a chain carboxylic ester having 5 or less carbon atoms, for the purpose of improving the rate characteristics, the storage capacity retention rate, and the amount of gas generated during high-temperature storage. Patent Document 2 discloses a non-aqueous electrolyte containing an ester-based solvent and a carbonate-based solvent, but not containing a sulfur compound. Patent Document 3 discloses a secondary battery having a negative electrode containing at least a carbon fiber aggregate and lithium metal. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2022-129682 A [Patent Document 2] JP 2017-103240 A [Patent Document 3] JP 2019-160730 A Summary of the Invention [Problem to be solved by the invention]

[0005] In non-aqueous electrolyte secondary batteries, it is difficult to simultaneously reduce the rate of increase in resistance (low resistance) after storage and ensure a high capacity retention rate (high capacity retention rate). On the other hand, in recent years, there has been a demand for improving the storage characteristics of non-aqueous electrolyte secondary batteries (for example, suppressing the rate of increase in resistance after storage and suppressing the decrease in capacity retention rate).

[0006] The present invention has been made in view of the above circumstances, and a main object of the present invention is to provide a nonaqueous electrolyte secondary battery with improved storage characteristics. [Means for solving the problem]

[0007] The non-aqueous electrolyte of the battery member disclosed herein comprises a positive electrode, a negative electrode, and a non-aqueous electrolyte. The negative electrode comprises a negative electrode active material layer containing a negative electrode active material, and the negative electrode active material comprises a carbon material and an amorphous carbon coating layer that coats the carbon material. The negative electrode active material has a Raman spectrum analysis measured by Raman spectroscopy that shows a Raman scattering peak at 1580 cm -1 The peak intensity G at 1360 cm -1 The intensity ratio (D / G) of the peak intensity D of the negative electrode active material layer is 0.3 or more and 0.5 or less. The nonaqueous electrolyte solution includes at least a sulfur-based electrolyte salt and a nonaqueous solvent, and the nonaqueous solvent includes a carbonate-based solvent and methyl acetate. Here, the negative electrode active material layer has a SOx concentration of 0.3 atomic% or more calculated based on an XPS spectrum measured by X-ray photoelectron spectroscopy.

[0008] In the nonaqueous electrolyte secondary battery having such a configuration, a suitable coating can be formed on the negative electrode by incorporating sulfur (S) element in the form of SOx into the coating formed on the negative electrode after activation treatment. Such a coating has low resistance and reduces Li loss, which is an irreversible reaction. This realizes a nonaqueous electrolyte secondary battery with excellent storage characteristics (particularly, suppression of resistance increase and suppression of capacity retention rate decrease). [Brief description of the drawings]

[0009] [Figure 1]FIG. 1 is a cross-sectional view that illustrates a schematic internal structure of a lithium-ion secondary battery according to one embodiment. [Diagram 2] FIG. 2 is a schematic exploded view showing the configuration of a wound electrode body of a lithium ion secondary battery according to one embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Matters not mentioned in this specification but necessary for carrying out the present invention can be understood as design matters for a person skilled in the art based on the prior art in the relevant field. The present invention can be carried out based on the contents disclosed in this specification and the technical common sense in the relevant field. In addition, in the following drawings, members and parts having the same function are described with the same reference numerals. Furthermore, the dimensional relationships (length, width, thickness, etc.) in each drawing do not reflect the actual dimensional relationships. In addition, the numerical range expressed as "A to B" in this specification includes A and B.

[0011] In this specification, the term "secondary battery" refers to an electricity storage device that can be repeatedly charged and discharged. In addition, in this specification, the term "non-aqueous electrolyte secondary battery" refers to a battery in general that can be repeatedly charged and discharged by the movement of charge carriers between the positive electrode and the negative electrode via a non-aqueous electrolyte. In addition, the term "lithium ion secondary battery" refers to a secondary battery that uses lithium ions as charge carriers and realizes charging and discharging by the movement of charges associated with the lithium ions between the positive electrode and the negative electrode.

[0012] Hereinafter, the present invention will be described in detail using as an example a flat rectangular lithium ion secondary battery having a flat wound electrode body and a flat battery case, but it is not intended that the present invention be limited to the embodiment described therein.

[0013] FIG. 1 is a cross-sectional view showing a schematic internal structure of a nonaqueous electrolyte secondary battery 100 disclosed herein. The nonaqueous electrolyte secondary battery 100 shown in FIG. 1 is a sealed battery constructed by housing a flat wound electrode assembly 20 and a nonaqueous electrolyte 80 in a flat rectangular battery case (i.e., an outer container) 30. The battery case 30 is provided with a positive electrode terminal 42 and a negative electrode terminal 44 for external connection, and a thin-walled safety valve 36 set to release the internal pressure when the internal pressure of the battery case 30 rises to a predetermined level or higher. The battery case 30 is also provided with an injection port (not shown) for injecting the nonaqueous electrolyte 80. The positive electrode terminal 42 is electrically connected to the positive electrode current collector plate 42a. The negative electrode terminal 44 is electrically connected to the negative electrode current collector plate 44a. The material of the battery case 30 is, for example, a lightweight metal material with good thermal conductivity, such as aluminum. Note that FIG. 1 does not accurately represent the amount of the nonaqueous electrolyte 80.

[0014] FIG. 2 is a schematic exploded view showing the configuration of the wound electrode body 20 of the nonaqueous electrolyte secondary battery 100 disclosed herein. As shown in FIG. 1 and FIG. 2, the wound electrode body 20 has a configuration in which a positive electrode sheet 50 and a negative electrode sheet 60 are overlapped with two long separator sheets 70 interposed therebetween and wound in the longitudinal direction. The positive electrode sheet 50 has a configuration in which a positive electrode active material layer 54 is formed along the longitudinal direction on one or both sides (both sides here) of a long positive electrode current collector 52. The negative electrode sheet 60 has a configuration in which a negative electrode active material layer 64 is formed along the longitudinal direction on one or both sides (both sides here) of a long negative electrode current collector 62. The positive electrode active material layer-free portion 52a (i.e., a portion where the positive electrode active material layer 54 is not formed and the positive electrode current collector 52 is exposed) and the negative electrode active material layer-free portion 62a (i.e., a portion where the negative electrode active material layer 64 is not formed and the negative electrode current collector 62 is exposed) are formed so as to protrude outward from both ends in the winding axis direction (i.e., the sheet width direction perpendicular to the longitudinal direction) of the wound electrode body 20. The positive electrode active material layer-free portion 52a and the negative electrode active material layer-free portion 62a are joined to the positive electrode current collector 42a and the negative electrode current collector 44a, respectively.

[0015] The positive electrode 50 (positive electrode sheet 50) has a positive electrode collector 52 and a positive electrode active material layer 54 disposed on at least one surface of the positive electrode collector 52. The positive electrode collector 52 may be a known positive electrode collector used in lithium ion secondary batteries, and examples thereof include a sheet or foil made of a metal with good electrical conductivity (e.g., aluminum, nickel, titanium, stainless steel, etc.). The positive electrode collector 52 is preferably an aluminum foil. The dimensions of the positive electrode collector 52 are not particularly limited and may be appropriately determined according to the battery design. When an aluminum foil is used as the positive electrode collector 52, its thickness is not particularly limited, but is, for example, 5 μm to 35 μm, and preferably 7 μm to 20 μm.

[0016] The positive electrode active material layer 54 contains a positive electrode active material. As the positive electrode active material, a known positive electrode active material used in a lithium ion secondary battery may be used. Specifically, for example, as the positive electrode active material, a lithium composite oxide, a lithium transition metal phosphate compound, or the like may be used. The crystal structure of the positive electrode active material is not particularly limited, and may be a layered structure, a spinel structure, an olivine structure, or the like.

[0017] The lithium composite oxide is preferably a lithium transition metal composite oxide containing at least one of Ni, Co, and Mn as a transition metal element, and specific examples thereof include lithium nickel composite oxide, lithium cobalt composite oxide, lithium manganese composite oxide, lithium nickel manganese composite oxide, lithium nickel cobalt manganese composite oxide, lithium nickel cobalt aluminum composite oxide, lithium iron nickel manganese composite oxide, etc. These positive electrode active materials may be used alone or in combination of two or more.

[0018] In this specification, the term "lithium nickel cobalt manganese composite oxide" includes oxides containing Li, Ni, Co, Mn, and O as constituent elements, as well as oxides containing one or more additional elements other than those. Examples of such additional elements include transition metal elements and typical metal elements such as Mg, Ca, Al, Ti, V, Cr, Y, Zr, Nb, Mo, Hf, Ta, W, Na, Fe, Zn, and Sn. The additional element may be a semimetal element such as B, C, Si, or P, or a nonmetal element such as S, F, Cl, Br, or I. This also applies to the above-mentioned lithium nickel composite oxide, lithium cobalt composite oxide, lithium manganese composite oxide, lithium nickel manganese composite oxide, lithium nickel cobalt aluminum composite oxide, and lithium iron nickel manganese composite oxide.

[0019] Examples of the lithium transition metal phosphate compound include lithium iron phosphate (LiFePO4), lithium manganese phosphate (LiMnPO4), and lithium manganese iron phosphate.

[0020] As the positive electrode active material, a lithium nickel cobalt manganese based composite oxide can be particularly preferably used.

[0021] The average particle diameter of the positive electrode active material (the cumulative 50% particle diameter in the volume-based particle size distribution obtained by particle size distribution measurement based on a laser diffraction / light scattering method) is not particularly limited, but is, for example, 0.05 μm or more and 25 μm or less, preferably 1 μm or more and 20 μm or less, and more preferably 3 μm or more and 15 μm or less.

[0022] The positive electrode active material layer 54 may contain components other than the positive electrode active material, such as a conductive material, a binder, etc. As the conductive material, for example, a carbon material such as carbon black (e.g., acetylene black (AB)), carbon nanotubes, graphite, etc. can be suitably used. As the binder, for example, polyvinylidene fluoride (PVdF) can be used.

[0023] The content of the positive electrode active material in the positive electrode active material layer 54 (i.e., the content of the positive electrode active material relative to the total mass of the positive electrode active material layer 54) is not particularly limited, but is preferably 70 mass% or more, more preferably 80 mass% to 97 mass% or less, and even more preferably 85 mass% to 96 mass% or less. The content of the conductive material in the positive electrode active material layer 54 is not particularly limited, but is preferably 1 mass% to 15 mass% or less, and more preferably 3 mass% to 13 mass% or less. The content of the binder in the positive electrode active material layer 54 is not particularly limited, but is preferably 1 mass% to 15 mass% or less, and more preferably 1.5 mass% to 10 mass% or less.

[0024] The thickness of the positive electrode active material layer 54 is not particularly limited, but is, for example, not less than 10 μm and not more than 400 μm, and preferably not less than 20 μm and not more than 300 μm.

[0025] The positive electrode sheet 50 may have an insulating layer (not shown) at the boundary between the positive electrode active material layer non-forming portion 52a and the positive electrode active material layer 54. The insulating layer contains, for example, ceramic particles.

[0026] The negative electrode 60 has a negative electrode current collector 62 and a negative electrode active material layer 64 fixed on at least one surface of the negative electrode current collector 62. The negative electrode current collector 62 may be a known negative electrode current collector used in lithium ion secondary batteries, and examples thereof include a sheet or foil made of a metal with good conductivity (e.g., copper, nickel, titanium, stainless steel, etc.). Copper foil is preferable as the negative electrode current collector 62. The dimensions of the negative electrode current collector 62 are not particularly limited and may be appropriately determined according to the battery design. When copper foil is used as the negative electrode current collector 62, its thickness is not particularly limited, but is, for example, 5 μm to 35 μm, and preferably 6 μm to 20 μm.

[0027] It was found by X-ray photoelectron spectroscopy (XPS) that the negative electrode active material layer 64 of the nonaqueous electrolyte secondary battery 100 disclosed herein has sulfur (S) element incorporated in the film formed on the negative electrode in the form of SOx. It was also found that when the SOx concentration is 0.3 atomic% or more, the resistance increase rate of the nonaqueous electrolyte secondary battery 100 after storage is suitably suppressed, and the capacity retention rate after storage is high. Although it is not intended to limit the technology disclosed herein, the reason why such an effect is obtained is presumed to be as follows. In the nonaqueous electrolyte secondary battery 100 after the activation treatment, a part of the nonaqueous electrolyte 80 is reduced and decomposed, and a film called an SEI film (Solid Electrolyte Interface) is formed on the surface of the negative electrode. When the SEI film is suitably formed, the negative electrode is stabilized, and a nonaqueous electrolyte secondary battery 100 with good storage characteristics can be provided. On the other hand, when the SEI film is excessively formed, the storage characteristics of the secondary battery tend to deteriorate (for example, the resistivity increases after storage and the capacity retention rate decreases). According to the results of the studies by the present inventors, sulfur (S) element is incorporated in the form of SOx into the coating formed on the negative electrode, and the high concentration of SOx (specifically, 0.3 atomic % or more) suppresses excessive formation (growth) of the SEI film, and a suitable SEI film can be formed on the negative electrode. Such an SEI film has low resistance and reduces Li loss, which is an irreversible reaction. This makes it possible to realize a nonaqueous electrolyte secondary battery 100 with excellent storage characteristics.

[0028] The presence of SOx on the negative electrode can be confirmed, for example, by X-ray photoelectron spectroscopy (XPS) analysis. Specifically, the XPS spectrum measured by XPS has a peak Ps attributed to SOx at a position where the binding energy is 165 to 175 eV (for example, around 169 eV). Such peak Ps means the presence of SOx. The SOx state ratio (SOx concentration) in the negative electrode active material layer can be measured by XPS analysis. Specifically, the SOx state ratio can be obtained by correcting the peak area of ​​the acquired peak Ps with a sensitivity coefficient that takes into account the ease of generation of photoelectrons. Note that this SOx state ratio means the state ratio (concentration) of SOx on the outermost surface of the negative electrode active material layer 64 when the peak area obtained in the range of 0 to 1100 eV is expressed in atomic percentage (100 atomic%).

[0029] The SOx concentration (SOx state ratio) measured by the above method may be 0.3 atomic% or more, 0.4 atomic% or more, or 0.5 atomic% or more. The upper limit of the SOx concentration in the negative electrode active material layer is not particularly limited, but may be 2 atomic% or less, 1 atomic% or less, or 0.7 atomic% or less.

[0030] The negative electrode active material layer 64 contains a negative electrode active material. In the nonaqueous electrolyte secondary battery 100 disclosed herein, the negative electrode active material includes a carbon material and an amorphous carbon coating layer that coats the carbon material. The type of carbon material is not particularly limited, and examples thereof include natural graphite and artificial graphite. The amorphous carbon coating layer may be composed of, for example, non-graphitizable carbon, easily graphitizable carbon, activated carbon, and the like. In particular, the negative electrode active material is preferably amorphous carbon-coated graphite in which the surface of graphite is coated with a carbon material with low crystallinity.

[0031] As described above, the negative electrode active material includes a carbon material and an amorphous carbon coating layer that coats the carbon material. In other words, the negative electrode active material can be a mixture or composite of carbon materials with different crystallinity. Here, the crystallinity of the negative electrode active material can be evaluated by Raman spectroscopy. A conventionally known method can be appropriately adopted for measuring the Raman spectrum. For example, the Raman spectrum can be measured by laser Raman spectroscopy using an appropriate laser light (e.g., an argon ion laser) as a light source. The negative electrode active material of the nonaqueous electrolyte secondary battery 100 disclosed herein has a Raman spectrum of 1580 cm in Raman spectroscopy. -1 The peak intensity G at 1360 cm -1 The intensity ratio (D / G) between the peak intensity D of the graphite layer 64 and the intensity of the G peak reflecting the regular graphite structure is 0.3 or more and 0.5 or less. The D / G ratio is the ratio of the intensity of the D peak reflecting the irregular structure to the intensity of the G peak reflecting the regular graphite structure, and is also called the R value. By adjusting the intensity ratio (D / G) to 0.3 or more and 0.5 or less, it can be said that the negative electrode active material has appropriate crystallinity. By combining the negative electrode active material having the intensity ratio (D / G) adjusted to 0.3 or more and 0.5 or less with a nonaqueous electrolyte solution described later, the SOx concentration in the negative electrode active material layer 64 can be suitably improved. In this specification, the above-mentioned "1580 cm -1 The peak intensity is 1580 cm -1 Near (for example, 1580 cm -1 From 1620cm -1 In this specification, the term "1360 cm" refers to the peak intensity in the range of 1360 cm -1 The peak intensity is 1360 cm -1 Near (for example, 1300 cm -1 From 1400cm -1 This refers to the peak intensity in the range (D band) of

[0032] The intensity ratio (D / G) of the negative electrode active material may be 0.3 or more, 0.32 or more, 0.34 or more, or 0.35 or more. The intensity ratio (D / G) of the negative electrode active material may be 0.5 or less, 0.48 or less, 0.45 or less, 0.42 or less, 0.4 or less, or 0.38 or less. This makes it possible to realize a negative electrode active material with a crystallinity that is neither too high nor too low.

[0033] The negative electrode active material is preferably particulate. Although not particularly limited, the average particle size of the negative electrode active material (the cumulative 50% particle size in the volume-based particle size distribution obtained by particle size distribution measurement based on the laser diffraction / light scattering method) is, for example, 0.5 μm to 50 μm, preferably 1 μm to 20 μm, and more preferably 5 μm to 10 μm.

[0034] The negative electrode active material having the above-mentioned intensity ratio (D / G) can be manufactured by a conventionally known method. For example, first, a graphite material and an amorphous carbon material (e.g., easily graphitized carbon) are prepared as raw materials. Next, the amorphous carbon material is attached to the surface of the graphite material by a gas phase method such as a CVD method (Chemical Vapor Deposition), or a liquid phase method, a solid phase method, or the like. Then, the graphite material to which the amorphous carbon material is attached is fired at, for example, about 800 to 1500°C, and carbonized to produce a negative electrode active material (amorphous carbon-coated graphite). The D / G value of the negative electrode active material can be adjusted, for example, by the D / G value of the raw materials (graphite material and / or amorphous carbon material) used, the mixing ratio of the raw materials, the firing temperature during firing, and the like.

[0035] The negative electrode active material layer 64 may contain components other than the active material, such as a binder, a thickener, etc. As the binder, for example, styrene butadiene rubber (SBR), polyvinylidene fluoride (PVdF), etc. may be used. As the thickener, for example, carboxymethyl cellulose (CMC), etc. may be used.

[0036] The content of the negative electrode active material in the negative electrode active material layer 64 is preferably 90% by mass or more, and more preferably 95% by mass or more and 99% by mass or less. The content of the binder in the negative electrode active material layer 64 is preferably 0.1% by mass or more and 8% by mass or less, and more preferably 0.5% by mass or more and 3% by mass or less. The content of the thickener in the negative electrode active material layer 64 is preferably 0.3% by mass or more and 3% by mass or less, and more preferably 0.5% by mass or more and 2% by mass or less.

[0037] The thickness of the negative electrode active material layer 64 is not particularly limited, but is, for example, not less than 10 μm and not more than 400 μm, and preferably not less than 20 μm and not more than 300 μm.

[0038] The separator 70 (separator sheet 70) may be a porous sheet (film) made of a resin such as polyethylene (PE), polypropylene (PP), polyester, cellulose, or polyamide. Such a porous sheet may have a single-layer structure or a laminated structure of two or more layers (for example, a three-layer structure in which a PP layer is laminated on both sides of a PE layer). A heat-resistant layer (HRL) containing ceramic particles or the like may be provided on the surface of the separator 70. The thickness of the separator 70 is not particularly limited, but is, for example, 5 μm to 50 μm, and preferably 10 μm to 30 μm.

[0039] The nonaqueous electrolyte 80 contains a nonaqueous solvent and an electrolyte salt (also called a supporting salt). The nonaqueous electrolyte 80 contains at least a sulfur-based electrolyte salt, a carbonate-based solvent, and methyl acetate. By combining such a nonaqueous electrolyte with the above-mentioned negative electrode active material having a suitably adjusted D / G ratio, the SOx derived from the sulfur-based electrolyte salt is suitably incorporated into the surface of the negative electrode active material layer 64, and the SOx concentration can be improved. This realizes suppression of an increase in resistance of the nonaqueous electrolyte secondary battery 100 after storage and suppression of a decrease in the capacity retention rate.

[0040] The sulfur-based electrolyte salt is not particularly limited as long as it is an electrolyte salt containing a sulfur atom. Examples of the sulfur-based electrolyte salt include lithium bis(fluorosulfonyl)imide (LiFSI) and lithium bis(trifluoromethane)sulfonimide (LiTFSI). Among them, the sulfur-based electrolyte salt preferably contains LiFSI. The concentration of the sulfur-based electrolyte salt in the non-aqueous electrolyte solution is not particularly limited, but is preferably 0.5% by mass or more and 3% by mass or less, and more preferably 0.5% by mass or more and 1.5% by mass or less.

[0041] The non-aqueous electrolyte solution 80 may contain an electrolyte salt other than the sulfur-based electrolyte salt. Examples of such electrolyte salts include LiPF6 and LiBF4. The concentration of such electrolyte salts other than the sulfur-based electrolyte salt in the non-aqueous electrolyte solution is not particularly limited, but is preferably 0.7 mol / L or more and 1.3 mol / L or less.

[0042] In the nonaqueous electrolyte secondary battery 100 disclosed herein, the nonaqueous solvent contains a carbonate-based solvent and methyl acetate. Examples of the carbonate-based solvent include ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), monofluoroethylene carbonate (MFEC), difluoroethylene carbonate (DFEC), monofluoromethyl difluoromethyl carbonate (F-DMC), trifluorodimethyl carbonate (TFDMC), and the like. Such carbonate-based solvents can be used alone or in appropriate combination of two or more. Among them, it is preferable that the carbonate-based solvent contains at least ethylene carbonate.

[0043] From the viewpoint of suitably increasing the SOx concentration on the surface of the negative electrode active material layer, it is preferable that the non-aqueous solvent contains at least 10 vol% or more of methyl acetate when the total volume of the non-aqueous solvent is 100 vol%. The content of methyl acetate is, for example, preferably 20 vol% or more, more preferably 30 vol% or more. The upper limit of the content of methyl acetate is not particularly limited, but is, for example, preferably 80 vol% or less, more preferably 70 vol% or less. The content (vol%) of such methyl acetate can be determined, for example, by NMR analysis or the like.

[0044] Although not particularly limited, the content ratio of the carbonate solvent and the methyl acetate in the nonaqueous solvent is, for example, preferably 90:10 to 20:80, and more preferably 90:10 to 30:70, whereby the above-mentioned effects are exhibited more stably, and the storage characteristics of the nonaqueous electrolyte secondary battery 100 can be suitably improved.

[0045] The nonaqueous electrolyte 80 may contain components other than the nonaqueous solvent and the electrolyte salt (hereinafter, also referred to as optional components). Suitable examples of the optional components include various additives such as film-forming agents such as vinylene carbonate and oxalate complexes, thickeners, and gas generating agents such as biphenyl (BP) and cyclohexylbenzene (CHB).

[0046] The above describes, as an example, a rectangular nonaqueous electrolyte secondary battery 100 equipped with a flat wound electrode assembly 20. However, the lithium ion secondary battery can also be configured as a nonaqueous electrolyte secondary battery equipped with a laminated electrode assembly (i.e., an electrode assembly in which multiple positive electrodes and multiple negative electrodes are alternately laminated). The lithium ion secondary battery can also be configured as a cylindrical nonaqueous electrolyte secondary battery, a laminated case type nonaqueous electrolyte secondary battery, etc.

[0047] The nonaqueous electrolyte secondary battery 100 can be used for various purposes, but since it can simultaneously suppress the resistance increase rate after storage and the decrease in the capacity maintenance rate, it can be particularly suitably used as a power source (driving power source) for a motor mounted on a moving body, for example, a vehicle such as a passenger car or a truck. The type of vehicle is not particularly limited, and examples thereof include a plug-in hybrid electric vehicle (PHEV), a hybrid electric vehicle (HEV), and an electric vehicle (BEV). The nonaqueous electrolyte secondary battery 100 can also be used in the form of a battery pack in which a plurality of batteries are connected in series and / or parallel.

[0048] Examples of the present invention will be described below, but it is not intended that the present invention be limited to those shown in these examples.

[0049] Example 1 Graphite coated with amorphous carbon was prepared as the negative electrode active material. Specifically, first, commercially available graphite and coal tar pitch as a raw material of amorphous carbon were prepared. Next, coal tar pitch was attached to the surface of the graphite using a CVD method. Then, the graphite with the coal tar pitch attached was fired at 800 to 1500 ° C. in a non-oxidizing atmosphere to produce graphite coated with amorphous carbon. This was used as the negative electrode active material of Example 1.

[0050] Next, styrene butadiene rubber (SBR) as a binder and carboxymethyl cellulose (CMC) as a thickener were prepared. The negative electrode active material of Example 1 prepared above, SBR and CMC were mixed with ion-exchanged water in a mass ratio of negative electrode active material:SBR:CMC=98:1:1 to prepare a negative electrode slurry. This slurry was applied in a strip shape to both sides of a long copper foil, dried, and then pressed to prepare a negative electrode sheet.

[0051] Next, LiNi as the positive electrode active material 1 / 3 Co 1 / 3 Mn1 / 3 O2 (NCM), acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVdF) as a binder were prepared. These materials were mixed with N-methylpyrrolidone (NMP) in a mass ratio of NCM:AB:PVdF = 92:5:3 to prepare a slurry for forming a positive electrode active material layer. This slurry was applied in strips on both sides of a long aluminum foil, dried, and then pressed to produce a positive electrode sheet.

[0052] A separator was prepared by providing a HRL on a porous polyolefin sheet having a three-layer structure of PP / PE / PP. The positive electrode sheet and the negative electrode sheet prepared above and two of the separator sheets prepared above were laminated and wound to prepare a wound electrode body.

[0053] Next, a non-aqueous electrolyte was prepared. As the carbonate-based solvent, ethylene carbonate and dimethyl carbonate were prepared. A mixed solvent containing ethylene carbonate (EC), dimethyl carbonate (DMC), and methyl acetate (MA) in a volume ratio of 30:70-X:X was prepared. The value of X (volume ratio; also volume %) was set to the value shown in Table 1. Lithium bis(fluorosulfonyl)imide (LiFSI) was dissolved in this mixed solvent at a concentration of 0.5 mass%, and LiPF6 as an electrolyte salt was dissolved at a concentration of 1.0 mol / L. This prepared a non-aqueous electrolyte.

[0054] Terminals were attached to the electrode body produced above, and the electrode body was housed in a battery case together with the nonaqueous electrolyte solution prepared above and sealed airtight. Thereafter, activation treatment was performed to obtain a lithium ion secondary battery for evaluation of Example 1.

[0055] (Examples 2 and 3) In Examples 2 and 3, the content (vol%) of methyl acetate (MA) was changed as shown in Table 1. Except for this, lithium ion secondary batteries for evaluation in Examples 2 and 3 were produced in the same manner as in Example 1.

[0056] Example 4 In Example 4, the baking temperature was changed to vary the crystallinity of the outermost surface of the negative electrode active material. Also, the content (vol%) of methyl acetate (MA) was changed as shown in Table 1. Except for these, the lithium ion secondary battery for evaluation of Example 4 was produced in the same manner as in Example 1.

[0057] Comparative Example 1 In Comparative Example 1, the content (vol%) of methyl acetate (MA) was changed as shown in Table 1. Except for these, the lithium ion secondary battery for evaluation in Comparative Example 1 was produced in the same manner as in Example 1.

[0058] (Comparative Examples 2 to 4) In Comparative Examples 2 to 4, the baking temperature was changed to vary the crystallinity of the outermost surface of the negative electrode active material. In addition, the content (vol%) of methyl acetate (MA) was changed as shown in Table 1. Except for these, the lithium ion secondary batteries for evaluation of Comparative Examples 2 to 4 were fabricated in the same manner as in Example 1.

[0059] (Comparative Examples 5 to 7) In Comparative Examples 5 and 7, the baking temperature was changed to make the crystallinity of the outermost surface of the negative electrode active material different. In Comparative Examples 5 to 7, methyl propionate (MP) was used instead of methyl acetate. The content (vol%) of the methyl propionate was changed as shown in Table 1. That is, in Comparative Examples 5 to 7, a mixed solvent containing ethylene carbonate (EC), dimethyl carbonate (DMC), and methyl propionate (MP) in a volume ratio of 30:70-X:X was prepared. Except for these, the evaluation lithium ion secondary batteries of Comparative Examples 5 to 7 were fabricated in the same manner as in Example 1.

[0060] <Initial capacity measurement> Each of the activated lithium ion secondary batteries for evaluation was charged at a constant current of 0.2 C to 4.10 V, and then charged at a constant voltage until the current value became 1 / 50 C, thereby fully charging the battery. Then, the battery was discharged at a constant current of 0.2 C to 3.00 V, and then discharged at a constant voltage until the current value became 1 / 50 C, thereby performing a constant current-constant voltage discharge. The discharge capacity at this time was measured, and this value was designated as the initial capacity of each lithium ion secondary battery for evaluation.

[0061] <Evaluation of resistance increase rate> The initial capacity was taken as SOC 100%, and each evaluation lithium ion secondary battery was charged at a current value of 1C in a thermostatic bath at -10°C until the SOC reached 50%. Next, the battery was charged for 10 seconds at current values ​​of 1C, 3C, 5C, and 10C in a thermostatic bath at -10°C, and the battery voltage after charging at each current value was measured. Each current value and each battery voltage were plotted to determine the IV characteristics during charging, and the IV resistance (Ω) during discharging was calculated from the slope of the obtained straight line. This value was taken as the initial resistance value of each evaluation lithium ion secondary battery. Next, the above-mentioned initial capacity was taken as SOC100%, and each evaluation lithium ion secondary battery was charged in a thermostatic chamber at -10°C at a current value of 15C until SOC reached 50%. Each evaluation lithium ion secondary battery was placed in a thermostatic chamber set at 60°C and stored for 150 days. Thereafter, in a thermostatic chamber at -10°C, charging was performed at current values ​​of 1C, 3C, 5C, and 10C for 10 seconds, and the battery voltage after charging at each current value was measured. Each current value and each battery voltage were plotted to determine the IV characteristics during charging, and the IV resistance (Ω) during discharging was determined from the slope of the obtained straight line. This value was taken as the resistance value after storage of each evaluation lithium ion secondary battery. The resistance increase rate (%) was calculated from the formula: (IV resistance value after storage / initial IV resistance value)×100. The ratios of the resistance increase rates of the other evaluation lithium ion secondary batteries to the evaluation lithium ion secondary battery of Comparative Example 6 were calculated, assuming that the resistance increase rate of the evaluation lithium ion secondary battery of Comparative Example 6 was 100. The results are shown in Table 1.

[0062] <Evaluation of capacity retention rate> After the above-mentioned lithium-ion secondary batteries for evaluation after storage were charged at a constant current to 4.10 V at a current value of 0.2C, they were charged at a constant voltage until the current value reached 1 / 50C by constant current-constant voltage charging to bring them to a fully charged state. After discharging at a constant current to 3.00 V at a current value of 0.2C, constant current-constant voltage discharging was performed in which discharging at a constant voltage was carried out until the current value reached 1 / 50C. The discharge capacity at this time was measured and taken as the discharge capacity after storage. Using the above-measured initial capacity and discharge capacity, the capacity retention rate (%) was determined from the formula: (discharge capacity after storage / initial capacity)×100. When the capacity retention rate of the lithium-ion secondary battery for evaluation in Comparative Example 6 was taken as 100, the ratio of the initial resistance of other lithium-ion secondary batteries for evaluation to the lithium-ion secondary battery for evaluation in Comparative Example 6 was determined. The results are shown in Table 1.

[0063] <Calculation of D / G ratio> After disassembling the lithium-ion secondary batteries for evaluation in each example after the storage test, the negative electrode active material was analyzed. The negative electrode active material in each example was analyzed with a Raman spectrometer (a microscopic Raman spectrometer manufactured by Renishaw, inVia Reflex 785S, excitation wavelength 532 nm, Ar laser) to obtain a Raman spectrum. The peak intensity G at 1580 cm -1 and the peak intensity D at 1360 cm -1 of the Raman spectrum were measured, and the ratio (D / G ratio) of the peak intensity D to the peak intensity G was determined. The calculated values are shown in Table 1.

[0064] <XPS measurement of negative electrode active material layer> The lithium ion secondary batteries for evaluation of each example after the storage test were disassembled, and the negative electrode active material layer was analyzed using an XPS device. Specifically, these negative electrode active material layers were measured using an XPS device (ULVAC-PHI's "PHI 5000 VersaProbe2") under the following conditions: X-ray source: AlKα ray (monochromatic light), irradiation range: φ100μm, current and voltage: 25kW 15kV. From the obtained XPS spectrum, curve fitting was performed for the SOx peak Ps, which appears at a position where the binding energy is around 169eV, to obtain the peak area. Note that the peak shift correction was performed after correcting the C1s peak to 284.80eV. The SOx concentration (atomic%) was calculated from this. The results are shown in Table 1.

[0065] [Table 1]

[0066] As shown in Table 1, Examples 1 to 4, in which the D / G ratio of the negative electrode active material is 0.3 or more and 0.5 or less, the nonaqueous electrolyte contains a sulfur-based electrolyte salt, a carbonate-based solvent, and methyl acetate, and the SOx concentration based on XPS is 0.3 atomic% or more, have a small resistance increase rate and a high capacity retention rate. From these results, the nonaqueous electrolyte secondary battery disclosed herein realizes a nonaqueous electrolyte secondary battery in which the increase in resistance value is suitably suppressed even after storage and which can ensure a high capacity retention rate.

[0067] Although the technology disclosed herein has 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 to the specific examples exemplified above.

[0068] 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 a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the negative electrode comprises a negative electrode active material layer containing a negative electrode active material, the negative electrode active material comprises a carbon material and an amorphous carbon coating layer coating the carbon material, and the negative electrode active material has a Raman spectrum analysis measured by Raman spectroscopy that has a Raman peak at 1580 cm -1 The peak intensity G at 1360 cm -1 the intensity ratio (D / G) of a peak intensity D of the negative electrode active material layer to a peak intensity D of the negative electrode active material layer is 0.3 or more and 0.5 or less, the nonaqueous electrolyte contains at least a sulfur-based electrolyte salt and a nonaqueous solvent, the nonaqueous solvent contains a carbonate-based solvent and methyl acetate, and the negative electrode active material layer has an SOx concentration of 0.3 atomic% or more calculated based on an XPS spectrum measured by X-ray photoelectron spectroscopy. Item 2: The nonaqueous electrolyte secondary battery according to item 1, wherein the nonaqueous solvent of the nonaqueous electrolyte has a methyl acetate content of 10 vol % or more and 70 vol % or less, when the entire nonaqueous solvent is taken as 100 vol %. Item 3: The nonaqueous electrolyte secondary battery according to item 1 or 2, wherein the carbonate-based solvent contains at least ethylene carbonate. Item 4: The nonaqueous electrolyte secondary battery according to any one of Items 1 to 3, wherein the sulfur-based electrolyte salt contains lithium bis(fluorosulfonyl)imide. [Explanation of symbols]

[0069] 20 Wound electrode body 30 Battery case (outer container) 36 Safety valve 42 Positive terminal 42a Positive current collector 44 Negative terminal 44a Negative current collector plate 50 Positive electrode (positive electrode sheet) 52 Positive electrode current collector 52a Portion where positive electrode active material layer is not formed 54 Cathode active material layer 60 Negative electrode (negative electrode sheet) 62 Negative electrode current collector 62a Part where negative electrode active material layer is not formed 64 Negative electrode active material layer 70 Separator (separator sheet) 80 Nonaqueous electrolyte 100 Nonaqueous electrolyte secondary battery

Claims

1. A positive electrode and A negative electrode; A non-aqueous electrolyte; A non-aqueous electrolyte secondary battery comprising: the negative electrode includes a negative electrode active material layer containing a negative electrode active material, The negative electrode active material includes a carbon material and an amorphous carbon coating layer that coats the carbon material, The negative electrode active material has a Raman spectrum measured by Raman spectroscopy at 1580 cm －１ Peak intensity G at 1360 cm －１ The intensity ratio (D / G) of the peak intensity D to the peak intensity D is 0.3 or more and 0.5 or less, The non-aqueous electrolyte solution contains at least a sulfur-based electrolyte salt and a non-aqueous solvent, The non-aqueous solvent includes a carbonate-based solvent and methyl acetate, Here, in the negative electrode active material layer, a SOx concentration calculated based on an XPS spectrum measured by X-ray photoelectron spectroscopy is 0.3 atomic % or more.

2. 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the nonaqueous solvent of the nonaqueous electrolyte has a methyl acetate content of 10 vol % or more and 70 vol % or less when the entire nonaqueous solvent is taken as 100 vol %.

3. 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the carbonate-based solvent contains at least ethylene carbonate.

4. 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the sulfur-based electrolyte salt contains lithium bis(fluorosulfonyl)imide.

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