Non-aqueous electrolyte secondary battery

Abstract

This nonaqueous electrolyte secondary battery is provided with a positive electrode, a negative electrode and a nonaqueous electrolyte; and during the charging thereof, lithium metal is precipitated at the negative electrode, and during the discharging thereof, the lithium metal is dissolved in the nonaqueous electrolyte from the negative electrode. The positive electrode contains a positive electrode active material; the positive electrode active material contains a composite oxide that contains lithium and a transition metal; and the nonaqueous electrolyte contains an oxalate salt. The composite oxide contains Ni and at least one element that is selected from the group consisting of Fe, V, Ti and Nb, while having a structure that is based on a crystal structure belonging to the space group R-3m.

Description

[Technical Field]

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

[0002] Non-aqueous electrolyte secondary batteries, typified by lithium ion secondary batteries, have high energy density and high output, and are considered promising as power sources for mobile devices such as smartphones, power sources for vehicles such as electric vehicles, storage devices for natural energy such as solar energy, etc. A composite oxide containing lithium and a transition metal is used as the positive electrode active material of non-aqueous electrolyte secondary batteries.

[0003] Meanwhile, with the aim of further increasing the capacity of batteries, non-aqueous electrolyte secondary batteries in which lithium metal is deposited on the negative electrode current collector during charging and the lithium metal dissolves during discharging are being studied (for example, Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 9-259929 Summary of the Invention

[0005] When a non-aqueous electrolyte secondary battery is exposed to high temperatures due to an internal short circuit or the like, oxygen is generated from the positive electrode containing a composite oxide. In the battery described in Patent Document 1, oxygen generated from the positive electrode easily reacts with lithium metal deposited on the negative electrode current collector, and the reaction generates heat in the battery, accelerating oxygen generation from the positive electrode and resulting in an increase in battery internal pressure. Furthermore, the oxygen generated from the positive electrode oxidizes and decomposes the non-aqueous electrolyte, which generates gas and increases battery internal pressure. The increase in battery internal pressure can damage the battery case, so there is a demand for improved battery safety.

[0006] In view of the above, one aspect of the present disclosure relates to a non-aqueous electrolyte secondary battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte, in which lithium metal is deposited on the negative electrode during charging and the lithium metal dissolves from the negative electrode into the non-aqueous electrolyte during discharging, wherein the positive electrode includes a positive electrode active material, and the positive electrode active material includes a composite oxide containing lithium and a transition metal, the non-aqueous electrolyte includes an oxalate salt, and the composite oxide includes Ni and at least one selected from the group consisting of Fe, V, Ti, and Nb, and has a structure based on a crystal structure belonging to the space group R-3m.

[0007] According to the present disclosure, the safety of non-aqueous electrolyte secondary batteries can be improved. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a schematic perspective view of a non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure, with a portion cut away. DETAILED DESCRIPTION OF THE INVENTION

[0009] A nonaqueous electrolyte secondary battery according to an embodiment of the present disclosure includes a positive electrode, a negative electrode, and a nonaqueous electrolyte. During charging, lithium metal precipitates on the negative electrode, and during discharging, the lithium metal dissolves from the negative electrode into the nonaqueous electrolyte. The positive electrode includes a positive electrode active material, which includes a composite oxide containing lithium and a transition metal. The nonaqueous electrolyte includes an oxalate salt. The composite oxide includes, as the transition metal, Ni and at least one selected from the group consisting of Fe, V, Ti, and Nb, and has a structure based on a layered rock-salt crystal structure belonging to the space group R-3m. This configuration reduces the rate of gas generation during battery heat generation.

[0010] When a battery is exposed to high temperatures due to an internal short circuit or other reason, the composite oxide, which is the positive electrode active material, thermally decomposes and generates oxygen. In particular, Ni-containing composite oxides can achieve high capacity by increasing the Ni content, but they are also susceptible to thermal decomposition. Thermal decomposition causes the crystal structure to change from a layered rock-salt structure belonging to the space group R-3m to a non-layered rock-salt structure belonging to the space group Fm-3m, releasing oxygen. However, adding at least one element selected from the group consisting of Fe, V, Ti, and Nb to the composite oxide can slow the oxygen release rate. This is thought to be because adding at least one element selected from the group consisting of Fe, V, Ti, and Nb causes the crystal structure to transition to a structure based on a spinel structure belonging to the space group Fd-3m during thermal decomposition. In terms of slowing the oxygen release rate, it is preferable to add Fe, among the elements of the group consisting of Fe, V, Ti, and Nb, to the composite oxide.

[0011] Therefore, by reducing the rate of oxygen generation at the positive electrode, the rate of reaction between oxygen generated from the positive electrode and lithium metal deposited on the surface of the negative electrode is also reduced. As a result, heat generation in the battery associated with this reaction is also suppressed, and further oxygen generation from the positive electrode can also be suppressed. Therefore, an increase in internal battery pressure due to oxygen generation from the positive electrode is suppressed. Furthermore, oxidative decomposition of the nonaqueous electrolyte by oxygen generated from the positive electrode is suppressed, and an increase in internal battery pressure due to gas generation associated with this oxidative decomposition is also suppressed.

[0012] The non-aqueous electrolyte contains an oxalate salt. The oxalate salt is a salt containing a cation (e.g., a lithium ion) and an anion of an oxalate complex. The inclusion of the oxalate salt in the non-aqueous electrolyte facilitates uniform precipitation of lithium metal in the form of fine particles due to the interaction between the anion of the oxalate complex and lithium, thereby suppressing localized dendritic precipitation of lithium metal. This reduces the surface area of ​​the lithium metal, further suppressing the reaction between oxygen generated from the positive electrode and the lithium metal, and further suppressing heat generation in the battery associated with this reaction. As a result, the increase in internal battery pressure due to oxygen generation from the positive electrode is further suppressed.

[0013] Therefore, the combination of a non-aqueous electrolyte containing an oxalate salt and a composite oxide containing at least one element selected from the group consisting of Fe, V, Ti, and Nb synergistically suppresses the reaction between oxygen generated from the positive electrode and lithium metal, significantly suppressing heat generation in the battery. Furthermore, the increase in internal battery pressure due to oxygen generation from the positive electrode is significantly suppressed, suppressing damage to the battery case due to the increase in internal battery pressure, and improving battery safety.

[0014] The oxalate salt is preferably lithium difluorooxalatoborate (LiFOB). The concentration of the oxalate salt in the non-aqueous electrolyte (the concentration of the anion of the oxalate complex) may be, for example, 0.05 mol / L or more and 1 mol / L or less. In order to suppress the reaction between oxygen and lithium metal, the concentration of the oxalate salt in the non-aqueous electrolyte is preferably 0.2 mol / L or more and 0.6 mol / L or less.

[0015] The composite oxide may further contain Al, which stabilizes the crystal structure of the composite oxide and improves thermal stability.

[0016] More specifically, for example, the composite oxide may be represented by the general formula LiNi 1-x-y M 1 x M 2 y O2 (where 0.03≦x≦0.15, 0.02≦y≦0.6). 1 contains at least one element selected from the group consisting of Fe, Ti, V and Nb. 2 contains at least one selected from the group consisting of Al, Mn and Co.

[0017] From the viewpoint of facilitating high capacity, Ni and M are used in composite oxides. 1 and M 2The atomic ratio of Ni to the sum of x and y is preferably 0.55 or more and less than 1, more preferably 0.7 or more and less than 1, and even more preferably 0.8 or more and less than 1. That is, in the above general formula, x + y ≦ 0.45 is preferable, x + y ≦ 0.3 is more preferable, and x + y ≦ 0.2 is even more preferable. When the Ni ratio is 0.7 or more (when x + y is 0.3 or less), the amount of oxygen generated when the battery is exposed to high temperatures is large, and therefore, the configuration of the present disclosure can significantly suppress an increase in the internal pressure of the battery due to oxygen generation from the positive electrode.

[0018] element M 1 The element M may be 50 atomic % or more of iron (Fe), or 90 atomic % or more, or 95 atomic % or more of iron (Fe). 1 The composite oxide may be substantially composed of iron (Fe). 1-x-y Fe x M 2 y O2 (where 0.03≦x≦0.15, 0.02≦y≦0.6).

[0019] element M 2 The element M is added to the composite oxide from the viewpoint of improving the output characteristics. 2 It is preferable that M contains at least aluminum (Al). 2 may contain Al and manganese (Mn) and / or cobalt (Co). 2 By including Co, the phase transition of the composite oxide containing Li and Ni is suppressed during charge and discharge, the stability of the crystal structure is improved, and the cycle characteristics are likely to be improved. 2 When contains Mn and / or Al, the thermal stability is improved.

[0020] Ni and M 1 and M 2 M for the sum of 1 The atomic ratio x is preferably 0.06≦x≦0.15, more preferably 0.06≦x≦0.12, and even more preferably 0.09≦x≦0.12.

[0021] Ni and M1 and M 2 M for the sum of 2 The atomic ratio y of element M 2 From the viewpoint of realizing a high capacity by increasing the Ni ratio while obtaining the effect of improving the stability of the composite oxide by the addition of y, 0.02≦y≦0.27 is preferable, and 0.02≦y≦0.1 is more preferable.

[0022] element M 2 If contains Co, Ni and M 1 and M 2 The atomic ratio of Co to the total of element M may be more than 0 and 0.2 or less. In this case, it is easy to maintain high capacity and high output, and it is easy to improve the stability of the crystal structure during charge and discharge. 2 When contains Al, Ni and M 1 and M 2 The atomic ratio of Al to the sum of may be more than 0 and not more than 0.05. In this case, high capacity and high output are easily maintained, and thermal stability is easily improved.

[0023] Ni and M 1 and M 2 M for the sum of 1 and M 2 When the total atomic ratio x+y is 0.3 or less, the proportion of Ni in the metals other than Li is large, the thermal decomposition rate is fast when the battery is exposed to high temperatures, and the amount of oxygen generated per unit time is large. In particular, when x+y is 0.2 or less, the oxygen generation rate is likely to increase significantly. However, the configuration of the present disclosure can significantly suppress the increase in battery internal pressure due to oxygen generation from the positive electrode. In addition, in this case, it is easy to increase the capacity, and the effect of Ni and the effect of element M are also obtained. 1 and M 2 The effects of this are well balanced.

[0024] The composite oxide is composed of the above Ni and element M. 1 and M 2In addition to lithium, the composite oxide may contain elements other than lithium. Specifically, the composite oxide may contain at least one element selected from the group consisting of copper (Cu), chromium (Cr), zirconium (Zr), vanadium (V), tantalum (Ta), molybdenum (Mo), magnesium (Mg), calcium (Ca), strontium (Sr), zinc (Zn), silicon (Si), and boron (B).

[0025] In the nonaqueous electrolyte secondary battery of this embodiment, for example, 70% or more of the rated capacity is achieved by the deposition and dissolution of lithium metal. The movement of electrons in the negative electrode during charge and discharge is mainly due to the deposition and dissolution of lithium metal in the negative electrode. Specifically, 70 to 100% (e.g., 80 to 100% or 90 to 100%) of the movement of electrons (current from another perspective) in the negative electrode during charge and discharge is due to the deposition and dissolution of lithium metal. In other words, the negative electrode in the nonaqueous electrolyte secondary battery of this embodiment differs from a negative electrode in which the movement of electrons in the negative electrode during charge and discharge is mainly due to the absorption and release of lithium ions by the negative electrode active material (such as graphite).

[0026] In a battery in which lithium metal is deposited on the negative electrode during charging, the open circuit potential (OCV) of the negative electrode when fully charged is, for example, 70 mV or less relative to lithium metal. When the rated capacity of the battery is C, the fully charged state refers to a state in which the battery is charged until the state of charge (SOC) reaches, for example, 0.98×C or more. The OCV of the negative electrode when fully charged can be measured by disassembling a fully charged battery under an argon atmosphere, removing the negative electrode, and assembling a cell using lithium metal as the counter electrode. The nonaqueous electrolyte of the cell may have the same composition as the nonaqueous electrolyte in the disassembled battery. For example, the nonaqueous electrolyte used in Example 1 described below may be used as a model nonaqueous electrolyte.

[0027] The configuration of the nonaqueous electrolyte secondary battery will be described in detail below.

[0028] (positive electrode) The positive electrode includes, for example, a positive electrode current collector and a positive electrode mixture layer supported on the surface of the positive electrode current collector. The positive electrode mixture layer can be formed by applying a positive electrode slurry, in which the positive electrode mixture is dispersed in a dispersion medium, to the surface of the positive electrode current collector and drying the applied layer. The dried coating may be rolled as necessary. The positive electrode mixture layer may be formed on one surface or both surfaces of the positive electrode current collector. The positive electrode mixture contains a positive electrode active material and an additive as essential components, and may contain a binder, a conductive agent, etc. as optional components. N-methyl-2-pyrrolidone (NMP) or the like is used as the dispersion medium.

[0029] Examples of binders include resin materials such as fluororesins, polyolefin resins, polyamide resins, polyimide resins, acrylic resins, vinyl resins, polyvinylpyrrolidone, polyethersulfone, and rubber materials. Examples of fluororesins include polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF). Examples of rubber materials include styrene-butadiene copolymer rubber (SBR). One type of binder may be used alone, or two or more types may be used in combination.

[0030] Examples of the conductive agent include carbon blacks such as acetylene black, conductive fibers such as carbon fibers and metal fibers, and carbon fluoride. One type of conductive agent may be used alone, or two or more types may be used in combination.

[0031] The positive electrode current collector may be, for example, a metal foil. Examples of metals constituting the positive electrode current collector include aluminum (Al), titanium (Ti), alloys containing these metal elements, and stainless steel. The thickness of the positive electrode current collector is not particularly limited, but is, for example, 3 to 50 μm.

[0032] (Negative electrode) The negative electrode may include a negative electrode current collector. In this case, lithium metal is deposited on the surface of the negative electrode current collector during charging, and the lithium metal deposited on the surface of the negative electrode current collector dissolves in the non-aqueous electrolyte during discharging.

[0033] The negative electrode current collector may be, for example, a metal foil. The metal constituting the negative electrode current collector is preferably a metal that does not react with lithium metal, such as copper (Cu), nickel (Ni), iron (Fe), or an alloy containing these metal elements. The thickness of the negative electrode current collector is not particularly limited, and is, for example, 5 μm or more and 300 μm or less.

[0034] The negative electrode may include a negative electrode current collector and a negative electrode mixture layer supported on the surface of the negative electrode current collector. However, from the viewpoint of achieving a high-capacity nonaqueous electrolyte secondary battery, the thickness of the negative electrode mixture layer is set to be sufficiently thin so that lithium metal can be deposited on the negative electrode during charge. In this case, the design capacity Cn of the negative electrode active material in the negative electrode mixture layer relative to the design capacity Cp of the positive electrode satisfies Cn / Cp<1, and may also satisfy Cn / Cp<0.8. In this case, lithium metal is deposited on the surface of the negative electrode mixture layer during charge, and the lithium metal deposited on the surface of the negative electrode mixture layer dissolves in the nonaqueous electrolyte during discharge.

[0035] The negative electrode mixture layer can be formed, for example, by applying a negative electrode slurry, in which the negative electrode mixture is dispersed in a dispersion medium, to the surface of the negative electrode current collector and drying it. The dried coating film may be rolled as necessary. The negative electrode mixture layer may be formed on one surface or both surfaces of the negative electrode current collector. Examples of the dispersion medium include water and NMP.

[0036] The negative electrode mixture contains a negative electrode active material as an essential component, and may contain optional components such as a binder, a conductive agent, and a thickener. The binder and conductive agent may be the same as those exemplified for the positive electrode. Examples of thickeners include carboxymethyl cellulose (CMC) and its modified form (such as the Na salt).

[0037] The negative electrode active material may contain a carbon material that absorbs and releases lithium ions. Examples of carbon materials that absorb and release lithium ions include graphite (natural graphite and artificial graphite), easily graphitizable carbon (soft carbon), and hardly graphitizable carbon (hard carbon). Among these, graphite is preferred because of its excellent charge / discharge stability and low irreversible capacity.

[0038] The negative electrode active material may include an alloy-based material. The alloy-based material is a material containing at least one metal capable of forming an alloy with lithium, such as silicon, tin, a silicon alloy, a tin alloy, or a silicon compound. The alloy-based material may be a composite material having a lithium ion conductive phase and silicon particles dispersed in the phase. The lithium ion conductive phase may be a silicate phase, a silicon oxide phase containing 95% or more by mass of silicon dioxide, a carbon phase, or the like.

[0039] The negative electrode active material may be a combination of an alloy material and a carbon material. In this case, the mass ratio of the carbon material to the total of the alloy material and the carbon material is, for example, preferably 80 mass % or more, and more preferably 90 mass % or more.

[0040] (non-aqueous electrolyte) The non-aqueous electrolyte contains lithium ions and anions and has lithium ion conductivity. The non-aqueous electrolyte contains at least an oxalate salt. The non-aqueous electrolyte may be liquid. The liquid non-aqueous electrolyte contains, for example, lithium ions, anions, and a non-aqueous solvent. The liquid non-aqueous electrolyte is prepared by dissolving a lithium salt in a non-aqueous solvent. The dissolution of the lithium salt in the non-aqueous solvent generates lithium ions and anions.

[0041] The non-aqueous electrolyte may be in a gel state. The gel-state non-aqueous electrolyte may contain, for example, lithium ions, anions, and a matrix polymer, and may further contain a non-aqueous solvent. As the matrix polymer, for example, a polymer material that absorbs the non-aqueous solvent and gels is used. Examples of the polymer material include fluororesin, acrylic resin, and polyether resin.

[0042] As the anion, in addition to the anion of an oxalate complex, known anions used in non-aqueous electrolytes of lithium secondary batteries can be used. Specifically, BF4 - , ClO4 - , PF6 - , CF3SO3 - , CF3CO2 -, anions of imides, etc. Examples of anions of imides include N(SO2CF3)2 - , N(C m F 2m+1 SO2) x (C n F 2n+1 SO2)y - (m and n are each independently an integer of 0 or 1 or greater, and x and y are each independently 0, 1, or 2, satisfying the relationship x+y=2.) The anion of the oxalate complex may contain boron and / or phosphorus. Examples of the anion of the oxalate complex include B(C2O4)2 - , difluorooxalate borate anion: BF2(C2O4) - , PF4(C2O4) - , PF2(C2O4)2 - The anions may be used singly or in combination of two or more.

[0043] From the viewpoint of suppressing the deposition of lithium metal in a dendritic form, it is preferable that the nonaqueous electrolyte contains at least an anion of an oxalate complex. The interaction between the anion of the oxalate complex and lithium facilitates the uniform deposition of lithium metal in the form of fine particles. This facilitates the suppression of localized deposition of lithium metal. The anion of the oxalate complex may be combined with other anions. The other anions may be PF6 - and / or an anion of an imide.

[0044] The concentration of the anion in the non-aqueous electrolyte may be 0.5 mol / L or more and 3.5 mol / L or less, and the concentration of the anion of the oxalate complex in the non-aqueous electrolyte may be 0.05 mol / L or more and 1 mol / L or less.

[0045] Examples of non-aqueous solvents include esters, ethers, nitriles, amides, and halogen-substituted derivatives thereof. The non-aqueous solvents may be used singly or in combination of two or more. Examples of halogen-substituted derivatives include fluorides.

[0046] Examples of esters include cyclic carbonates, chain carbonates, cyclic carboxylic acid esters, and chain carboxylic acid esters. Examples of cyclic carbonates include ethylene carbonate (EC) and propylene carbonate (PC). Examples of chain carbonates include dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC). Examples of cyclic carboxylic acid esters include γ-butyrolactone (GBL) and γ-valerolactone (GVL). Examples of chain carboxylic acid esters include ethyl acetate, propyl acetate, and methyl propionate (PM).

[0047] Examples of ethers include cyclic ethers and chain ethers. Examples of cyclic ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, and 2-methyltetrahydrofuran. Examples of chain ethers include 1,2-dimethoxyethane, diethyl ether, ethyl vinyl ether, and 1,2-diethoxyethane.

[0048] The non-aqueous electrolyte may contain at least one component selected from the group consisting of vinylene carbonate (VC), fluoroethylene carbonate (FEC), and vinyl ethylene carbonate (VEC). When the non-aqueous electrolyte contains the above component, a good coating is formed on the surface of the negative electrode, and the formation of lithium metal dendrites is suppressed.

[0049] (separator) It is usually desirable to interpose a separator between the positive electrode and the negative electrode. The separator has high ion permeability and adequate mechanical strength and insulating properties. The separator may be made of a microporous thin film, woven fabric, nonwoven fabric, or the like. The separator is preferably made of polyolefin such as polypropylene or polyethylene.

[0050] An example of the structure of a nonaqueous electrolyte secondary battery is a structure in which an electrode group formed by winding a positive electrode and a negative electrode with a separator interposed therebetween and a nonaqueous electrolyte are housed in an outer casing. Alternatively, instead of a wound electrode group, other types of electrode groups may be used, such as a stacked electrode group formed by stacking a positive electrode and a negative electrode with a separator interposed therebetween. The nonaqueous electrolyte secondary battery may be in any shape, such as a cylindrical shape, a prismatic shape, a coin shape, a button shape, or a laminate shape.

[0051] FIG. 1 is a schematic perspective view, with a portion cut away, of a prismatic nonaqueous electrolyte secondary battery according to one embodiment of the present disclosure.

[0052] The battery includes a bottomed prismatic battery case 4, an electrode group 1, and a non-aqueous electrolyte housed within the battery case 4. The electrode group 1 includes a long strip-shaped negative electrode, a long strip-shaped positive electrode, and a separator interposed between them to prevent direct contact. The electrode group 1 is formed by winding the negative electrode, positive electrode, and separator around a flat plate-shaped winding core and then removing the winding core.

[0053] One end of a negative electrode lead 3 is attached to the negative electrode current collector of the negative electrode by welding or the like. The other end of the negative electrode lead 3 is electrically connected to a negative electrode terminal 6 provided on the sealing plate 5 via a resin insulating plate. The negative electrode terminal 6 is insulated from the sealing plate 5 by a resin gasket 7. One end of a positive electrode lead 2 is attached to the positive electrode current collector of the positive electrode by welding or the like. The other end of the positive electrode lead 2 is connected to the back surface of the sealing plate 5 via an insulating plate. In other words, the positive electrode lead 2 is electrically connected to the battery case 4, which also serves as the positive electrode terminal. The insulating plate separates the electrode group 1 from the sealing plate 5 and also separates the negative electrode lead 3 from the battery case 4. The periphery of the sealing plate 5 fits into the open edge of the battery case 4, and the fitting portion is laser-welded. In this way, the opening of the battery case 4 is sealed with the sealing plate 5. The injection hole for the non-aqueous electrolyte provided in the sealing plate 5 is closed by a sealing plug 8 .

[0054] <Example> The present disclosure will be specifically described below based on examples and comparative examples, but the present invention is not limited to the following examples.

[0055] Examples 1 to 4, Comparative Example 1 (1) Preparation of the positive electrode Hydroxides containing Ni, Co, Al, Mn, and / or Fe in the proportions shown in Table 1 were synthesized by coprecipitation. Lithium hydroxide monohydrate (LiOH·HO) was then mixed with the resulting hydroxide so that the molar ratio of the total amount of metal elements other than Li to Li was 1:1.02. The mixture was then calcined under a 95% oxygen stream (flow rate of 5 L / min per kg of mixture) at a heating rate of 2.0°C / min from room temperature to 650°C, and then at a heating rate of 1°C / min from 650°C to 780°C to obtain a composite oxide. The calcined composite oxide was washed with water and subjected to solid-liquid separation to obtain a composite oxide powder with a water content of 3–8% by mass.

[0056] The obtained positive electrode active material, acetylene black (AB), and polyvinylidene fluoride (PVDF) were mixed to prepare a positive electrode mixture. N-methyl-2-pyrrolidone (NMP) was added to the positive electrode mixture and stirred to prepare a positive electrode slurry. In the positive electrode mixture, the mass ratio of the composite oxide, AB, and PVDF was 100:2:2.

[0057] The positive electrode slurry was applied to the surface of an aluminum foil (thickness 15 μm) serving as a positive electrode current collector, and the coating was dried and then rolled. In this way, a positive electrode mixture layer was formed on both sides of the aluminum foil, and a laminate was obtained. The laminate was cut to a predetermined size to obtain a positive electrode. Note that an exposed portion of the positive electrode current collector not having a positive electrode mixture layer was formed in a partial region of the positive electrode. One end of an aluminum positive electrode lead was attached to the exposed portion of the positive electrode current collector by welding.

[0058] (2) Preparation of the negative electrode An electrolytic copper foil (thickness: 10 μm) serving as a negative electrode current collector was cut to a predetermined size to form a negative electrode. One end of a nickel negative electrode lead was attached to the negative electrode current collector by welding.

[0059] (3) Preparation of non-aqueous electrolyte A non-aqueous electrolyte was prepared by dissolving LiPF6 and LiBF2(C2O4) in a non-aqueous solvent. The non-aqueous solvent used was a mixed solvent containing fluoroethylene carbonate (FEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of 20:5:75. The concentration of LiPF6 in the non-aqueous electrolyte was 1.0 mol / L. The concentration of LiBF2(C2O4) in the non-aqueous electrolyte was 0.5 mol / L.

[0060] (4) Fabrication of non-aqueous electrolyte secondary battery A wound electrode assembly was prepared by winding the positive and negative electrodes with a polyethylene separator interposed therebetween. The electrode assembly was vacuum-dried and then housed in a battery case that also served as the negative electrode terminal. Resin upper and lower insulating plates were placed above and below the electrode assembly, respectively. A cylindrical iron case with a bottom (outer diameter 21 mm, height 70 mm) was used as the battery case. A nonaqueous electrolyte was then poured into the battery case, and the opening of the battery case was closed using a metal sealing member that also served as the positive electrode terminal. A resin gasket was interposed between the sealing member and the open end of the battery case. The other end of the positive electrode lead was connected to the sealing member, and the other end of the negative electrode lead was connected to the inner bottom surface of the battery case. In this manner, a 21700-type cylindrical nonaqueous electrolyte secondary battery was prepared.

[0061] Non-aqueous electrolyte secondary batteries A1 to A4 and B1 with different composite oxide compositions were fabricated by changing the composition of Ni, Co, Al, Mn, and Fe in the composite oxide as shown in Table 1. Batteries A1 to A4 correspond to Examples 1 to 4, and battery B1 corresponds to Comparative Example 1.

[0062] Comparative Examples 2 and 3 In preparing the non-aqueous electrolyte, only LiPF6 was dissolved in the non-aqueous solvent without adding LiBF2(C2O4). The concentration of LiPF6 in the non-aqueous electrolyte was 1.5 mol / L.

[0063] Other than this, nonaqueous electrolyte secondary batteries B2 and B3 were fabricated in the same manner as in Example 1, and the compositions of Ni, Co, Al, Mn, and Fe in the composite oxide were set to the values ​​shown in Table 1. Batteries B2 and B3 correspond to Comparative Examples 2 and 3.

[0064] 《Reference example 1》 (1) Preparation of the negative electrode Water was added to the negative electrode mixture and stirred to prepare a negative electrode slurry. The negative electrode mixture was a mixture of artificial graphite (average particle size 25 μm), styrene-butadiene rubber (SBR), and carboxymethyl cellulose sodium (CMC-Na). The mass ratio of the artificial graphite to the SBR and CMC-Na in the negative electrode mixture was 100:1:1.

[0065] The negative electrode slurry was applied to the surface of a copper foil, the coating was dried, and then rolled to obtain a laminate in which a negative electrode mixture layer was formed on both sides of the copper foil. The laminate was cut to a predetermined size to prepare a negative electrode.

[0066] (2) Preparation of non-aqueous electrolyte A non-aqueous electrolyte was prepared by dissolving LiPF6 at a concentration of 1.2 mol / L in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) (volume ratio 1:1).

[0067] The negative electrode and non-aqueous electrolyte obtained above were used.

[0068] Except for the above, a battery C1 was fabricated in the same manner as in Example 1. The thickness of the negative electrode mixture layer was adjusted so that Li metal would not precipitate on the surface of the negative electrode during charging. That is, the design capacity Cn of the negative electrode, which is carried by the negative electrode active material in the negative electrode mixture layer, was set to be larger than the design capacity Cp of the positive electrode.

[0069] The following evaluations were carried out on the batteries A1 to A4, B1 to B3, and C1.

[0070] [Evaluation: Measurement of gas generation rate] The batteries obtained above were charged at a constant current of 0.1 C at 25°C until the voltage reached 4.3 V. After the constant current charging, they were charged at a constant voltage of 4.3 V until the current reached 0.01 C. In this manner, fully charged batteries were obtained. In batteries A1 to A4 and B1 to B4, Li metal was deposited on the surface of the negative electrode current collector by charging. In battery C1, lithium ions were absorbed into the graphite in the negative electrode mixture by charging.

[0071] A fully charged battery was placed in a sealed container, and an internal short circuit was caused in the battery by piercing it with a nail, causing the battery to heat up. The change over time in the amount of gas generated from the battery was measured. The amount of gas generated from the battery was calculated by measuring the pressure P inside the sealed container with a pressure sensor, measuring the temperature T inside the sealed container with a thermocouple, and using the gas equation: PV=nRT (V is the volume inside the sealed container, n is the amount of gas, and R is the gas constant). Based on the measurement results, the maximum amount of gas generated per unit time was determined and used as the gas generation rate. The gas generation rate was expressed as an index, with the gas generation rate for Battery B1 of Comparative Example 1 set to 100.

[0072] The evaluation results are shown in Table 1. Table 1 shows the composition of the composite oxide used in each battery and whether or not LiFOB was added to the electrolyte, along with the evaluation results of the gas generation rate.

[0073] [Table 1]

[0074] In the batteries A1 to A4 of Examples 1 to 4, the gas generation rate was slower and the increase in the internal pressure of the battery was suppressed compared to the batteries B1 to B3 of Comparative Examples 1 to 3.

[0075] Comparing Batteries B1 and B2, which have the same composite oxide composition, the gas generation rate of Battery B1 was reduced to approximately two-thirds of that of Battery B2 by adding LiFOB to the electrolyte. On the other hand, comparing Batteries A1 and B3, which have the same composite oxide composition, the gas generation rate of Battery A1, which contains Fe, was reduced to approximately one-thirtieth of that of Battery B3 by adding LiFOB to the electrolyte, demonstrating a significant improvement in the gas generation rate.

[0076] In Battery A1, the gas generation rate was significantly reduced and the increase in the internal battery pressure was suppressed, even compared to Battery C1 of Reference Example 1, in which Li ions were absorbed into the graphite in the negative electrode mixture during charging. [Industrial Applicability]

[0077] The nonaqueous electrolyte secondary battery according to the present disclosure is suitable for use, for example, as a power source for mobile devices such as smartphones, a power source for vehicles such as electric vehicles, and a storage device for natural energy such as solar energy. [Explanation of symbols]

[0078] 1 electrode group 2 positive leads 3 Negative lead 4 Battery case 5 Sealing plate 6 Negative terminal 7 Gasket 8. Seal

Claims

1. A positive electrode, a negative electrode, and a non-aqueous electrolyte, a non-aqueous electrolyte secondary battery in which lithium metal is deposited on the negative electrode during charging, and the lithium metal is dissolved from the negative electrode into the non-aqueous electrolyte during discharging, the positive electrode includes a positive electrode active material, the positive electrode active material includes a composite oxide containing lithium and a transition metal, the non-aqueous electrolyte contains an oxalate salt, The composite oxide is LiNi １－ｘ－ｙ Fe ｘ M ２ ｙ O ２ (where 0.09≦x≦0.12, 0.02≦y≦0.6, x+y≦0.20), and M ２ A non-aqueous electrolyte secondary battery containing Al and having a structure based on a layered rock salt type crystal structure.

2. M ２ The non-aqueous electrolyte secondary battery according to claim 1 , further comprising Mn and Co.

3. 3. The nonaqueous electrolyte secondary battery according to claim 1, wherein 0.02≦y≦0.

1.

4. 4. The nonaqueous electrolyte secondary battery according to claim 1, wherein the oxalate salt includes lithium difluorooxalatoborate.

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