Non-aqueous electrolyte secondary battery

Abstract

To provide a non-aqueous electrolyte secondary battery capable of suppressing an increase in resistance when exposed to high temperatures for a long period of time.SOLUTION: A non-aqueous electrolyte secondary battery includes a positive electrode, a negative electrode, and a non-aqueous electrolyte. The positive electrode includes a positive electrode current collector and a positive electrode active material layer supported on the positive electrode current collector. The positive electrode active material layer contains a positive electrode active material and at least one positive electrode additive selected from the group consisting of thiophosphates and dithiophosphates.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] 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, etc., and as power sources for driving vehicles such as electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs).

[0003] The positive electrode of a non-aqueous electrolyte secondary battery typically includes a positive electrode active material layer containing a positive electrode active material. A known technique for improving the performance of a non-aqueous electrolyte secondary battery involves incorporating an additive into the positive electrode active material layer, which additive forms a coating on the surface of the positive electrode active material. For example, Patent Document 1 describes that incorporating a thiophosphate ester or a thiophosphate ester salt into the positive electrode active material layer improves the cycle characteristics of a non-aqueous electrolyte secondary battery. For example, Patent Document 2 describes that incorporating lithium phosphate into the positive electrode active material layer improves the output characteristics and cycle characteristics of a non-aqueous electrolyte secondary battery when a high-potential positive electrode active material is used. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-352804 [Patent Document 2] Japanese Patent Application Laid-Open No. 2016-062644 Summary of the Invention [Problem to be solved by the invention]

[0005] However, as a result of intensive research, the present inventors have found that, in the above-mentioned conventional technology, there is room for improvement in the high-temperature storage characteristics of nonaqueous electrolyte secondary batteries, specifically, there is room for improvement in suppressing an increase in resistance when a nonaqueous electrolyte secondary battery is placed under high temperatures for a long period of time.

[0006] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a non-aqueous electrolyte secondary battery capable of suppressing an increase in resistance when placed at high temperatures for a long period of time. [Means for solving the problem]

[0007] The nonaqueous electrolyte secondary battery disclosed herein includes a positive electrode, a negative electrode, and a nonaqueous electrolyte. The positive electrode includes a positive electrode current collector and a positive electrode active material layer supported on the positive electrode current collector. The positive electrode active material layer contains a positive electrode active material and at least one positive electrode additive selected from the group consisting of thiophosphates and dithiophosphates.

[0008] With this configuration, it is possible to provide a nonaqueous electrolyte secondary battery that can suppress an increase in resistance when placed under high temperatures for a long period of time. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a cross-sectional view schematically showing the internal structure of a lithium-ion secondary battery according to one embodiment of the present invention. [Figure 2] 1 is a schematic exploded view showing the configuration of a wound electrode body of a lithium ion secondary battery according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments of the present invention will be described with reference to the drawings. Matters not mentioned in this specification but necessary for implementing 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 implemented based on the contents disclosed in this specification and the technical common sense in the relevant field. Furthermore, in the following drawings, components and parts that perform the same function are denoted by the same reference numerals. Furthermore, the dimensional relationships (length, width, thickness, etc.) in each drawing do not reflect the actual dimensional relationships. In this specification, a numerical range expressed as "A to B" 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, and is a term that encompasses so-called storage batteries and electricity storage elements such as electric double layer capacitors. In addition, in this specification, the term "lithium ion secondary battery" refers to a secondary battery that uses lithium ions as charge carriers and achieves charging and discharging by the transfer of charge associated with the lithium ions between the positive and negative electrodes.

[0012] The present invention will be described in detail below using as an example a flat prismatic 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 above.

[0013] The lithium-ion 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., outer container) 30. The battery case 30 is provided with a positive electrode terminal 42 and a negative electrode terminal 44 for external connection, as well as a thin-walled safety valve 36 that is designed to release internal pressure when the internal pressure of the battery case 30 rises above a predetermined level. The battery case 30 also has an injection port (not shown) for injecting the nonaqueous electrolyte 80. The positive electrode terminal 42 is electrically connected to a positive electrode current collector plate 42a. The negative electrode terminal 44 is electrically connected to a negative electrode current collector plate 44a. The battery case 30 is made of a lightweight metal material with good thermal conductivity, such as aluminum. Note that FIG. 1 does not accurately represent the amount of nonaqueous electrolyte 80.

[0014] As shown in Figures 1 and 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 in this case) 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 in this case) 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 current collector 52 constituting the positive electrode sheet 50 may be a known positive 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., aluminum, nickel, titanium, stainless steel, etc.). Aluminum foil is preferred as the positive electrode current collector 52.

[0016] The dimensions of the positive electrode current collector 52 are not particularly limited and may be determined appropriately depending on the battery design. When an aluminum foil is used as the positive electrode current collector 52, the thickness thereof is not particularly limited, but is, for example, 5 μm to 35 μm, and preferably 7 μm to 20 μm.

[0017] The positive electrode active material layer 54 contains a positive electrode active material and at least one positive electrode additive selected from the group consisting of thiophosphates and dithiophosphates. The positive electrode active material may be a known positive electrode active material used in lithium-ion secondary batteries. Specific examples of the positive electrode active material include lithium composite oxides and lithium transition metal phosphate compounds. 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.

[0018] 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, and lithium iron nickel manganese composite oxide.

[0019] In this specification, the term "lithium nickel cobalt manganese composite oxide" refers to oxides containing Li, Ni, Co, Mn, and O as constituent elements, as well as oxides containing one or more additional elements. 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 also be a metalloid 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 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.

[0020] Examples of lithium transition metal phosphate compounds include lithium iron phosphate (LiFePO4), lithium manganese phosphate (LiMnPO4), and lithium manganese iron phosphate.

[0021] These positive electrode active materials may be used alone or in combination of two or more.

[0022] By subjecting the lithium-ion secondary battery 100 to an initial charge, the positive electrode additive decomposes, and a coating derived from the positive electrode additive is formed on the surface of the positive electrode active material. This initial charge is generally performed at a voltage equal to or higher than the decomposition onset potential of the positive electrode additive. However, since it is sufficient to apply a voltage equal to or higher than the decomposition onset potential of the positive electrode additive to the lithium-ion secondary battery only during the initial charge, the upper limit potential of the positive electrode active material in a typical usage mode of the lithium-ion secondary battery may be lower than the decomposition onset potential of the positive electrode additive.

[0023] From the viewpoints of battery characteristics and stability of the crystal structure during initial charging, a lithium composite oxide having a layered structure is preferred as the positive electrode active material. The lithium composite oxide preferably contains Ni. Therefore, lithium nickel cobalt manganese composite oxide and lithium nickel cobalt aluminum composite oxide are preferred as the positive electrode active material, and lithium nickel cobalt manganese composite oxide is more preferred.

[0024] The proportion of Ni to the total of metal elements other than Li in the lithium composite oxide is preferably 20 mol % to 60 mol %, and more preferably 30 mol % to 50 mol %.

[0025] The average particle diameter (median diameter: D50) of the positive electrode active material is not particularly limited, but is, for example, 0.05 μm to 25 μm, preferably 1 μm to 20 μm, and more preferably 3 μm to 15 μm. The average particle diameter (D50) of the positive electrode active material can be determined, for example, by a laser diffraction scattering method.

[0026] 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, for example, 80 mass % or more, preferably 85 mass % or more, and more preferably 87 mass % or more.

[0027] The positive electrode additive is a component that forms a coating on the surface of the positive electrode active material when initial charging is performed on the lithium ion secondary battery 100. In this embodiment, at least one selected from the group consisting of thiophosphates and dithiophosphates is used as the positive electrode additive.

[0028] Thiophosphates are salts of a thiophosphate anion represented by the following formula (1) and a cation. Dithiophosphates are salts of a dithiophosphate anion represented by the following formula (2) and a cation. Thiophosphates and dithiophosphates are typically inorganic salts and therefore typically do not contain an organic group. The thiophosphate anion and dithiophosphate anion are each trivalent anions, and the cation is a monovalent, divalent, or trivalent cation (particularly a metal cation).

[0029] [ka]

[0030] Since thiophosphate anions and dithiophosphate anions contribute significantly to the formation of the coating, the type of cation is not particularly limited. Examples of cations include Li + , Na + , K. + Alkali metal cations such as Mg 2+ , Ca 2+ , Sr 2+ , Ba 2+ alkaline earth metal cations such as Zn 2+ Group 12 element cations such as Al 3+ Among these, alkali metal cations are preferred.

[0031] From the viewpoint of easy availability, sodium thiophosphate is particularly preferred as the thiophosphate. Similarly, from the viewpoint of easy availability, sodium dithiophosphate is particularly preferred as the dithiophosphate. From the viewpoint of battery performance, lithium thiophosphate is particularly preferred as the thiophosphate. Similarly, from the viewpoint of battery performance, lithium dithiophosphate is particularly preferred as the dithiophosphate.

[0032] From the viewpoint of smaller initial resistance and better high-temperature storage characteristics, dithiophosphates are preferred as the positive electrode additive, and alkali metal salts of dithiophosphates are more preferred.

[0033] Thiophosphates and dithiophosphates have a relatively low decomposition potential as positive electrode additives. For example, the decomposition potential of Li3PO3 is 4.50 V (vs Li + / Li), whereas the decomposition potential of sodium thiophosphate (Na3PSO3) is 4.35V (vsLi + / Li), and the decomposition potential of sodium dithiophosphate (Na3PS2O2) is 4.25V (vsLi + / Li). Therefore, when the lithium ion secondary battery 100 is initially charged, these can be efficiently decomposed, and a high-quality coating can be formed on the surface of the positive electrode active material layer. As a result, an increase in resistance when the lithium ion secondary battery 100 is left at high temperatures for a long period of time can be suppressed.

[0034] The higher the content of the positive electrode additive in the positive electrode active material layer 54, the greater the effect of suppressing an increase in resistance when the lithium-ion secondary battery 100 is placed under high temperatures for a long period of time. Therefore, the content of the positive electrode additive in the positive electrode active material layer 54 (i.e., the mass ratio of the positive electrode additive to the total mass of all components of the positive electrode active material layer) is preferably 0.1 mass% or more, more preferably 0.5 mass% or more, and even more preferably 1 mass% or more. On the other hand, a high content of the positive electrode additive tends to increase the initial resistance. Therefore, the content of the positive electrode additive in the positive electrode active material layer 54 is preferably 15 mass% or less, more preferably 10 mass% or less, and even more preferably 5 mass% or less.

[0035] The positive electrode active material layer 54 may contain only at least one selected from the group consisting of thiophosphates and dithiophosphates as a positive electrode additive, or may further contain other positive electrode additives within a range that does not significantly impair the effects of the present invention.

[0036] The positive electrode active material layer 54 may contain components (i.e., optional components) other than the above-mentioned positive electrode active material and positive electrode additive. Examples of the optional components include a conductive material, a binder, etc. Suitable conductive materials include carbon materials such as carbon black (e.g., acetylene black), carbon nanotubes (CNTs), and graphite. Suitable binders include polyvinylidene fluoride (PVDF), etc. When CNTs are used as the conductive material, the positive electrode active material layer 54 may further contain a dispersant for the CNTs.

[0037] The content of the conductive material in the positive electrode active material layer 54 is not particularly limited, but is preferably 0.1% by mass to 15% by mass, and more preferably 0.5% by mass to 13% by mass. The content of the binder in the positive electrode active material layer 54 is not particularly limited, but is preferably 1% by mass to 15% by mass, and more preferably 1.5% by mass to 10% by mass.

[0038] The thickness of the positive electrode active material layer 54 is not particularly limited, but is, for example, 10 μm or more and 300 μm or less, and preferably 20 μm or more and 200 μm or less.

[0039] The positive electrode sheet 50 may contain 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.

[0040] The negative electrode current collector 62 constituting the negative electrode sheet 60 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 preferred as the negative electrode current collector 62.

[0041] The dimensions of the negative electrode current collector 62 are not particularly limited and may be determined appropriately depending on the battery design. When a copper foil is used as the negative electrode current collector 62, the thickness thereof is not particularly limited, but is, for example, 5 μm to 35 μm, and preferably 7 μm to 20 μm.

[0042] The negative electrode active material layer 64 contains a negative electrode active material. As the negative electrode active material, for example, a carbon material such as graphite, hard carbon, or soft carbon can be used. The graphite may be natural graphite or artificial graphite, or may be amorphous carbon-coated graphite in which graphite is coated with an amorphous carbon material.

[0043] The average particle diameter (median diameter: D50) of the negative electrode active material is not particularly limited, but is, for example, 0.1 μm to 50 μm, preferably 1 μm to 25 μm, and more preferably 5 μm to 20 μm. The average particle diameter (D50) of the negative electrode active material can be determined, for example, by a laser diffraction scattering method.

[0044] The negative electrode active material layer 64 may contain components other than the active material, such as a binder or a thickener. Examples of binders that may be used include styrene butadiene rubber (SBR) and polyvinylidene fluoride (PVDF). Examples of thickeners that may be used include carboxymethyl cellulose (CMC).

[0045] 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.

[0046] The thickness of the negative electrode active material layer 64 is not particularly limited, but is, for example, 10 μm or more and 300 μm or less, and preferably 20 μm or more and 200 μm or less.

[0047] The separator 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 PP layers are laminated on both sides of a PE layer). A heat-resistant layer (HRL) may be provided on the surface of the separator 70.

[0048] 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. The air permeability of the separator 70 measured by the Gurley test method is not particularly limited, but is preferably 350 seconds / 100 cc or less.

[0049] The nonaqueous electrolyte 80 typically contains a nonaqueous solvent and a supporting salt (electrolyte salt). As the nonaqueous solvent, various organic solvents such as carbonates, ethers, esters, nitriles, sulfones, and lactones commonly used in electrolytes for lithium-ion secondary batteries can be used without any particular limitation. Of these, carbonates and esters are preferred, and specific examples thereof 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), methyl acetate, and methyl propionate. These nonaqueous solvents can be used singly or in appropriate combinations of two or more.

[0050] As the supporting salt, for example, lithium salts such as LiPF6, LiBF4, and lithium bis(fluorosulfonyl)imide (LiFSI) (preferably LiPF6) can be suitably used. The concentration of the supporting salt is preferably 0.7 mol / L or more and 1.3 mol / L or less.

[0051] The nonaqueous electrolyte 80 may contain various additives other than the above-mentioned components, such as film-forming agents such as vinylene carbonate (VC) and oxalate complexes; gas generating agents such as biphenyl (BP) and cyclohexylbenzene (CHB); and thickeners, as long as the effects of the present invention are not significantly impaired.

[0052] By initially charging the lithium ion secondary battery 100 configured as described above to a voltage equal to or higher than the decomposition starting potential of the positive electrode additive, it is possible to suppress an increase in resistance when the battery is left at high temperatures for a long period of time. Therefore, with the above configuration, it is possible to provide a lithium ion secondary battery 100 with excellent durability.

[0053] The lithium ion secondary battery 100 can be used for a variety of purposes. Specific applications include portable power sources for personal computers, portable electronic devices, portable terminals, etc.; power sources for driving vehicles such as electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs); and storage batteries for small-sized power storage devices, with the battery being preferred as a power source for driving vehicles. The lithium ion secondary battery 100 can also be used in the form of a battery pack, typically consisting of a plurality of batteries connected in series and / or parallel.

[0054] The above describes, as an example, a rectangular lithium ion secondary battery 100 equipped with a flat wound electrode assembly 20. However, the lithium ion secondary battery can also be configured as a lithium ion secondary battery equipped with a stacked electrode assembly (i.e., an electrode assembly in which multiple positive electrodes and multiple negative electrodes are stacked alternately). The lithium ion secondary battery can also be configured as a cylindrical lithium ion secondary battery, a laminated case lithium ion secondary battery, etc.

[0055] The secondary battery according to this embodiment can be constructed as a non-aqueous electrolyte secondary battery other than a lithium ion secondary battery according to a known method.

[0056] Examples of the present invention will be described in detail below, but it is not intended that the present invention be limited to those shown in these examples.

[0057] [Examples 1 to 9 and Comparative Examples 1 to 4] LiNi as a positive electrode active material powder 0.5 Co 0.2 Mn 0.3 O2 (NCM), acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVdF) as a binder were mixed in a mass ratio of NCM:AB:PVdF = 90:5:5. The positive electrode additives shown in Table 1 were added thereto so that the content in the positive electrode active material layer was the amount shown in Table 1, and then mixed with N-methylpyrrolidone (NMP). This prepared a slurry for forming a positive electrode active material layer. This slurry was applied to both sides of aluminum foil and then dried to form a positive electrode active material layer. The total basis weight of both sides at this time was 15 mg / cm. 2 Next, the positive electrode active material layer was 3 A positive electrode sheet was obtained by rolling and pressing the sheet so that the thickness became

[0058] A negative electrode active material layer was prepared by mixing natural graphite (C) as a negative electrode active material, styrene butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener with ion-exchanged water in a mass ratio of C:SBR:CMC = 97:2:1. This slurry was applied to both sides of copper foil and then dried to form a negative electrode active material layer. The total basis weight of both sides was 9 mg / cm. 2 Next, the negative electrode active material layer was coated with a 1.2 g / cm 3 A negative electrode sheet was obtained by rolling and pressing the sheet so that the thickness of the negative electrode sheet became 1 / 3.

[0059] A polyolefin porous membrane was prepared as a separator. The prepared positive electrode sheet and negative electrode sheet were laminated with the separator interposed therebetween to prepare a laminated electrode assembly. Terminals were attached to this electrode assembly, which was then housed in an aluminum battery case.

[0060] A mixed solvent containing ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 25:40:35 was prepared. A nonaqueous electrolyte was prepared by dissolving LiPF6 in this mixed solvent at a concentration of 1.1 mol / L. The prepared nonaqueous electrolyte was poured into a battery case, which was then sealed to produce a lithium-ion secondary battery for evaluation in each example and comparative example.

[0061] <Activation process> Each of the lithium ion secondary batteries for evaluation prepared above was placed in a thermostatic chamber at 25°C. Each of the lithium ion secondary batteries for evaluation was charged at a constant current of 0.1 C up to a predetermined upper limit voltage, and then discharged at a constant current down to 3.0 V. This predetermined upper limit voltage was determined by the positive electrode potential (vsLi + / Li). This charge / discharge cycle was repeated twice. Next, each evaluation lithium ion secondary battery was adjusted to a voltage of 3.7 V, and then placed in a thermostatic chamber at 60°C for 12 hours to undergo aging treatment. In this manner, the evaluation lithium ion secondary batteries of each Example and Comparative Example were activated.

[0062] <Initial characteristic evaluation> Each activated lithium ion secondary battery for evaluation was charged at a constant current of 0.1 C up to the predetermined upper limit voltage, and then discharged at a constant current to 3.0 V. The discharge capacity at this time was measured and used as the initial capacity.

[0063] This initial capacity was taken as SOC 100%, and each evaluation lithium-ion secondary battery was adjusted to SOC 50% at 25°C. Each evaluation lithium-ion secondary battery was then placed in a thermostatic chamber at -10°C and discharged at a current of 10 C for 10 seconds. The voltage change ΔV at this time was measured, and the output resistance of each evaluation lithium-ion secondary battery was calculated as the initial resistance using this voltage change ΔV and the current value. The results are shown in Table 1.

[0064] <High-temperature storage characteristic evaluation> Each activated lithium-ion secondary battery for evaluation was adjusted to an SOC of 80% in a temperature environment of 25°C. Each lithium-ion secondary battery for evaluation was placed in a thermostatic chamber at 60°C and stored for 60 days. Thereafter, the output resistance after storage was measured using the same method as for the initial resistance. The resistance increase rate (%) was calculated using the formula: (output resistance after high-temperature storage / initial resistance) x 100. The results are shown in Table 1.

[0065] [Table 1]

[0066] As shown in Table 1, in Examples 1 to 9, thiophosphate and dithiophosphate were used as the positive electrode additive. In Comparative Example 1, no positive electrode additive was used. In Comparative Example 2, Li3PO4, a conventional positive electrode additive, was used. In Comparative Example 3, Li2CO3 was used as the positive electrode additive. In Comparative Example 4, trimethylthiophosphate, a conventional thiophosphate ester, was used as the positive electrode additive.

[0067] As shown in the results in Table 1, the resistance increase after high-temperature storage was significantly smaller in Examples 1 to 9 than in Comparative Examples 1 to 4. This demonstrates that the nonaqueous electrolyte secondary battery disclosed herein can suppress an increase in resistance when placed under high temperatures for a long period of time.

[0068] 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.

[0069] That is, the nonaqueous electrolyte secondary battery disclosed herein has the following features [1] to [5]. [1] A positive electrode, a negative electrode; a non-aqueous electrolyte; A non-aqueous electrolyte secondary battery comprising: the positive electrode includes a positive electrode current collector and a positive electrode active material layer supported on the positive electrode current collector; the positive electrode active material layer contains a positive electrode active material and at least one positive electrode additive selected from the group consisting of thiophosphates and dithiophosphates; Nonaqueous electrolyte secondary battery. [2] The nonaqueous electrolyte secondary battery according to item [1], wherein the content of the positive electrode additive in the positive electrode active material layer is 1% by mass to 10% by mass. [3] The nonaqueous electrolyte secondary battery according to item [1] or [2], wherein the content of the positive electrode additive in the positive electrode active material layer is 1% by mass to 5% by mass. [4] The positive electrode active material is a layered lithium composite oxide, The nonaqueous electrolyte secondary battery according to any one of items [1] to [3], wherein the lithium composite oxide contains Ni. [5] The nonaqueous electrolyte secondary battery according to any one of items [1] to [4], wherein the positive electrode additive is an alkali metal salt of dithiophosphoric acid. [Explanation of symbols]

[0070] 20 Wound electrode body 30 Battery case 36 Safety valve 42 Positive terminal 42a Positive current collector plate 44 Negative terminal 44a Negative current collector plate 50 Positive electrode sheet (positive electrode) 52 Positive electrode current collector 52a Portion where positive electrode active material layer is not formed 54 Cathode active material layer 60 Negative electrode sheet (negative electrode) 62 Negative electrode current collector 62a Part where negative electrode active material layer is not formed 64 Negative electrode active material layer 70 Separator sheet (separator) 80 Non-aqueous electrolyte 100 Lithium-ion secondary battery

Claims

1. A positive electrode and a negative electrode; a non-aqueous electrolyte; A non-aqueous electrolyte secondary battery comprising: the positive electrode includes a positive electrode current collector and a positive electrode active material layer supported on the positive electrode current collector, the positive electrode active material layer contains a positive electrode active material and at least one positive electrode additive selected from the group consisting of thiophosphates and dithiophosphates, the non-aqueous electrolyte contains a non-aqueous solvent and a supporting salt, The thiophosphate is a salt of a thiophosphate anion represented by formula (1) with Li + or Na + , and the dithiophosphate is a salt of a dithiophosphate anion represented by formula (2) with Li + or Na + . Nonaqueous electrolyte secondary battery. 【Chemistry 1】

2. 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the content of said positive electrode additive in said positive electrode active material layer is 1% by mass to 10% by mass.

3. 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the content of said positive electrode additive in said positive electrode active material layer is 1% by mass to 5% by mass.

4. the positive electrode active material is a lithium composite oxide having a layered structure, 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the lithium composite oxide contains Ni.

5. 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the positive electrode additive is a salt of the dithiophosphate anion represented by formula (2) with Li + or Na + .

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