Secondary battery and nonaqueous electrolyte for secondary battery

Abstract

The present disclosure provides a secondary battery having high reliability. A secondary battery according to the present disclosure comprises a positive electrode, a negative electrode, and a nonaqueous electrolyte. The negative electrode contains titanium oxide as a negative electrode active material, and the nonaqueous electrolyte contains a solvent and a lithium salt. The solvent includes a cyclic ester, and the lithium salt includes LiTFSI and LiBF4. The total content of LiTFSI and LiBF4 in the lithium salt is 50 mol% or more with respect to the total amount of the lithium salt, and the concentration of lithium ions in the nonaqueous electrolyte is 0.4-2.3 mol / L. C1 (mol / L), which is the concentration of LiTFSI in the nonaqueous electrolyte, and C2 (mol / L), which is the concentration of LiBF4 in the nonaqueous electrolyte, satisfy 0.25≤C1≤1.85, 0.15≤C2≤0.55, and 0.46≤C1 / C2≤9.50.

Description

Secondary battery and non-aqueous electrolyte for secondary battery

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

[0002] 2. Description of the Related Art Various studies have been conducted on secondary batteries that include a positive electrode, a negative electrode, and a non-aqueous electrolyte.

[0003] For example, Patent Document 1 describes a negative electrode using a lithium-titanium oxide having a spinel structure as an active material, and a ２ MnO ３ or LiMnO ２ A non-aqueous electrolyte lithium secondary battery comprising a positive electrode using the active material as an active material and a non-aqueous electrolyte has been proposed.

[0004] Patent Document 2 proposes "a nonaqueous electrolyte secondary battery including a nonaqueous electrolyte solution, a separator, and a positive electrode and a negative electrode capable of absorbing and releasing lithium, the nonaqueous electrolyte solution being prepared by dissolving a supporting electrolyte having lithium as a cation and a surfactant in a solvent made of a plurality of cyclic compounds."

[0005] Japanese Patent Laid-Open No. 7-320784 Japanese Patent Laid-Open No. 2002-33119

[0006] In recent years, there has been a demand for further improvement in the reliability of secondary batteries.

[0007] One aspect of the present disclosure provides a battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the negative electrode contains titanium oxide as a negative electrode active material, the non-aqueous electrolyte contains a solvent and a lithium salt, the solvent contains a cyclic ester, and the lithium salt is LiTFSI and LiBF ４ and wherein the LiTFSI and the LiBF in the lithium salt ４ and a total content of the lithium salt is 50 mol% or more with respect to the total amount of the lithium salt, a concentration of lithium ions in the non-aqueous electrolyte is 0.4 mol / L or more and 2.3 mol / L or less, a concentration of the LiTFSI in the non-aqueous electrolyte is 1 mol / L, and a concentration of the LiBF in the non-aqueous electrolyte is 1 mol / L. ４The concentration C2 mol / L relates to a secondary battery that satisfies 0.25≦C1≦1.85, 0.15≦C2≦0.55, and 0.46≦C1 / C2≦9.50.

[0008] Another aspect of the present disclosure is a non-aqueous electrolyte for use in a secondary battery, the secondary battery including a positive electrode, a negative electrode, and the non-aqueous electrolyte, the negative electrode including a titanium oxide as a negative electrode active material, the non-aqueous electrolyte including a solvent and a lithium salt, the solvent including a cyclic ester, and the lithium salt including LiTFSI and LiBF ４ and wherein the LiTFSI and the LiBF in the lithium salt ４ and a total content of the lithium salt is 50 mol% or more with respect to the total amount of the lithium salt, a concentration of lithium ions in the non-aqueous electrolyte is 0.4 mol / L or more and 2.3 mol / L or less, a concentration of the LiTFSI in the non-aqueous electrolyte is 1 mol / L, and a concentration of the LiBF in the non-aqueous electrolyte is 1 mol / L. ４ The concentration C2 mol / L relates to a non-aqueous electrolyte for a secondary battery that satisfies 0.25≦C1≦1.85, 0.15≦C2≦0.55, and 0.46≦C1 / C2≦9.50.

[0009] According to the present disclosure, the reliability of secondary batteries can be improved.

[0010] FIG. 1 is a cross-sectional view schematically illustrating a secondary battery according to an embodiment of the present disclosure.

[0011] The following describes embodiments of the present disclosure using examples, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and materials may be exemplified, but other numerical values ​​and materials may be applied as long as the effects of the present disclosure are obtained. In this specification, the term "numerical value A to numerical value B" includes numerical value A and numerical value B and can be read as "numerical value A or more and numerical value B or less." In the following description, when lower and upper limits are exemplified for numerical values ​​of specific physical properties or conditions, any of the exemplified lower limits and any of the exemplified upper limits can be arbitrarily combined, as long as the lower limit is not equal to or greater than the upper limit. When multiple materials are exemplified, one of the materials may be selected and used alone, or two or more materials may be used in combination.

[0012] A secondary battery according to an embodiment of the present disclosure includes a positive electrode, a negative electrode, and a non-aqueous electrolyte. The negative electrode contains titanium oxide as a negative electrode active material. The non-aqueous electrolyte contains a solvent and a lithium salt. The solvent contains a cyclic ester. The lithium salt contains LiTFSI and LiBF as main components. ４ That is, LiTFSI and LiBF in the lithium salt ４ The total content of the lithium salts is 50 mol % or more. LiTFSI is lithium bis(trifluoromethanesulfonyl)imide: LiN(CF ３ SO ２ ) ２ The concentration of lithium ions in the non-aqueous electrolyte is 0.4 mol / L or more and 2.3 mol / L or less. The concentration of LiTFSI in the non-aqueous electrolyte is C1 mol / L, and the concentration of LiBF in the non-aqueous electrolyte is C1 mol / L. ４ The concentration C2 mol / L satisfies the following relationships (i) to (iii).

[0013] (i) 0.25≦C1≦1.85 (ii) 0.15≦C2≦0.55 (iii) 0.46≦C1 / C2≦9.50 When the above conditions are satisfied, the reliability of the secondary battery can be improved. For example, the deterioration of the output characteristics of the secondary battery after high-temperature storage can be significantly suppressed. Although the detailed reason is not clear, it is believed that the lithium salt contains LiTFSI and LiBF as its main components. ４ It is believed that when these compounds are used in combination at specific concentrations and in specific concentration ratios, a special synergistic effect is significantly obtained.

[0014] The secondary battery according to the embodiment of the present disclosure can be used in various environments, but is particularly suitable for use at high temperatures. The secondary battery may be used in an environment of 85°C or higher, or in an environment of 100°C or higher. The temperature of the usage environment may be 150°C or lower. Use at high temperatures includes storage of a secondary battery in a fully charged state or a nearly fully charged state (e.g., a state of charge (SOC) of 80% or higher) at high temperatures for a certain period of time.

[0015] LiTFSI is advantageous in terms of improving the heat resistance and ionic conductivity of the non-aqueous electrolyte. ４When LiBF is used in combination with LiTFSI, the LiTFSI remains in the non-aqueous electrolyte without being decomposed at high temperatures, thereby suppressing a decrease in the ionic conductivity of the non-aqueous electrolyte. The secondary battery includes a metal case that houses a positive electrode, a negative electrode, and a non-aqueous electrolyte. ４ This forms a protective film on the inner surface of the case, which suppresses corrosion of the inner surface of the case due to LiTFSI at high temperatures and the resulting increase in internal resistance, and is therefore thought to suppress the deterioration of output characteristics after high-temperature storage.

[0016] When the concentration C1 of LiTFSI is 0.25 mol / L or more, LiTFSI is present in sufficient amount, and LiBF ４ This suppresses the decomposition of LiTFSI at high temperatures, thereby suppressing a decrease in the ionic conductivity of the non-aqueous electrolyte due to a decrease in the amount of LiTFSI at high temperatures. When the LiTFSI concentration C1 is 1.85 mol / L or less, a non-aqueous electrolyte with an appropriate viscosity is obtained, and a decrease in the ionic conductivity of the non-aqueous electrolyte due to high viscosity is suppressed.

[0017] LiBF ４ When the concentration C2 of LiBF is 0.15 mol / L or more, the decomposition of LiTFSI at high temperatures and the accompanying deterioration of the non-aqueous electrolyte are suppressed. ４ When the concentration C2 of LiBF is 0.55 mol / L or less, ４ The generation of by-products resulting from the reaction is suppressed, and an increase in internal resistance due to the generation of the by-products is suppressed.

[0018] When C1 / C2 is 0.46 or more, LiBF ４ When the C1 / C2 ratio is 9.50 or less, the ratio of LiBF to LiTFSI is 1.0. ４ The presence of sufficient ４ This effectively prevents corrosion of the inner surface of the case, and suppresses an increase in internal resistance due to the corrosion.

[0019] LiTFSI and LiBF in lithium salts ４When the total content of LiTFSI and LiBF is 50 mol % or more based on the total lithium salt, ４ The combined effect of these two drugs can be fully achieved.

[0020] When the lithium ion concentration in the nonaqueous electrolyte is 0.4 mol / L or more, a nonaqueous electrolyte with good ionic conductivity is obtained. When the lithium ion concentration in the nonaqueous electrolyte is 2.3 mol / L or less, a nonaqueous electrolyte with appropriate viscosity is obtained, and a decrease in the ionic conductivity of the nonaqueous electrolyte due to high viscosity is suppressed.

[0021] The lithium ion concentration in the non-aqueous electrolyte is the lithium ion concentration when it is assumed that all of the lithium salt contained in the non-aqueous electrolyte dissociates to form lithium ions and anions, and is determined from the amount of lithium contained in the non-aqueous electrolyte. The lithium ion concentration in the non-aqueous electrolyte is 0.4 mol / L or more and 2.3 mol / L or less, and may be 1.20 mol / L or more and 2.18 mol / L or less.

[0022] The concentration C1 of LiTFSI can be determined by measuring the amount of S element by an analytical method such as ICP-MS (inductively coupled plasma mass spectrometry). ４ The concentration C2 of the lithium salt is determined by measuring the amount of B element by an analytical method such as ICP-MS. ４ When the non-aqueous electrolyte is composed of only the concentration C1 and the concentration C2, the sum of the concentrations C1 and C2 is calculated as the lithium ion concentration in the non-aqueous electrolyte.

[0023] C1 / C2 is 0.46 to 9.50, and may be 1.8 to 7.4. C1 is 0.25 to 1.85 mol / L, and may be 0.50 to 1.85 mol / L, or may be 0.75 to 1.85 mol / L. C2 is 0.15 to 0.55 mol / L, and may be 0.15 to 0.50 mol / L, or may be 0.25 to 0.50 mol / L.

[0024] The concentration of lithium ions in the nonaqueous electrolyte is preferably 1.20 mol / L or more and 2.18 mol / L or less, and satisfies 0.15≦C2≦0.50 and 1.8≦C1 / C2≦7.4. In this case, the capacity decrease after high-temperature storage is easily suppressed.

[0025] The lithium salts are LiTFSI and LiBF ４ In this case, the relationship 0.4≦C1+C2≦2.3 is satisfied.

[0026] The nonaqueous electrolyte (solvent and lithium salt) can be analyzed using a known analytical method used in nonaqueous electrolyte secondary batteries. For example, an analytical method such as ICP-MS can be used. The analysis may be performed using a nonaqueous electrolyte prepared (prepared) during battery production, or may be performed using a nonaqueous electrolyte collected by disassembling an initial battery (e.g., an unused battery immediately after production or less than six months after production).

[0027] Titanium oxide particles usually contain secondary particles (aggregates of primary particles). The average particle diameter of the primary particles contained in the secondary particles is preferably 0.5 μm or more and 10 μm or less. The average particle diameter of the secondary particles is preferably 5 μm or more and 60 μm or less. The secondary particles, which are aggregated particles, have pores. From the viewpoint of improving output characteristics, the pore mode diameter (most frequent pore diameter) of the secondary particles is preferably 0.5 μm or more. When the average particle diameters of the secondary particles and primary particles are within the above range, the pore mode diameter within the above range is easily obtained. When the pore mode diameter is 0.5 μm or more, a coating is formed on the surface of the secondary particles, and the pores of the secondary particles are impregnated with the electrolyte, which easily ensures a large effective surface area of ​​the active material particles after high-temperature storage, and easily improves output characteristics after high-temperature storage. The average particle diameter of the primary particles may be 0.5 μm or more and 5 μm or less.

[0028] The average particle diameter of the secondary particles is determined by photographing a cross section of the negative electrode with a scanning electron microscope (SEM), determining the equivalent circle diameters of 15 or more (e.g., 20 to 50) randomly selected secondary particles using the SEM image of the negative electrode cross section, and calculating the average of these. The equivalent circle diameter is the diameter of a circle having the same area as a secondary particle in the SEM image.

[0029] The average particle size of the primary particles can be determined by measuring the equivalent circle diameters of 15 or more (e.g., 20 to 50) arbitrarily selected primary particles using an SEM image of the cross section of the negative electrode, and then calculating the average of these diameters. The equivalent circle diameter is the diameter of a circle having the same area as the primary particle in the SEM image.

[0030] The pore mode diameter (modal pore diameter) of secondary particles is determined by mercury intrusion porosimetry using titanium oxide particles containing secondary particles. The titanium oxide particles may contain aggregated particles (secondary particles) and a small amount of non-aggregated particles (particles in which primary particles are separated without agglomeration). Even when a small amount of non-aggregated particles is contained, the determined pore mode diameter is based almost entirely on the pores of the secondary particles and may be considered to be the pore mode diameter of the secondary particles.

[0031] (Negative electrode) The negative electrode includes a negative electrode active material capable of reversibly absorbing and releasing lithium ions. The negative electrode may include other additives (e.g., a conductive material, a binder, etc.) as needed. That is, the negative electrode may include a negative electrode mixture including the negative electrode active material and other additives. The additives are not particularly limited, and known additives may be used.

[0032] Titanium oxide is used as the negative electrode active material. Titanium oxide has excellent high temperature resistance and is also advantageous in terms of output characteristics. As the titanium oxide, a lithium titanium composite oxide can be used. The titanium oxide may be lithium titanate. Examples of lithium titanate include lithium titanate having a spinel structure. An example of lithium titanate having a spinel structure is represented by the composition formula Li ４ Ti ５ O １２ A part of Ti (for example, 10 mol % or less) may be substituted with another metal element.

[0033] The lithium-titanium composite oxide is produced, for example, by firing a mixture of Li and Ti raw materials. The average particle size and pore mode diameter can be adjusted by the firing conditions (firing temperature, firing time), the particle size of the raw materials, etc.

[0034] The conductive material for the negative electrode is not particularly limited, and may be any known conductive material used as a conductive material for the negative electrode of a nonaqueous electrolyte secondary battery. Examples of the conductive material include conductive carbon materials, such as carbon black (e.g., acetylene black) and carbon fibers (e.g., carbon nanotubes).

[0035] The binder used in the negative electrode is not particularly limited, and known binders may be used. Examples of binders include acrylic resins, polyolefin resins, polyamide resins, polyimide resins, fluororesins, rubber, etc. Examples of acrylic resins include polyacrylic acid, polymethacrylic acid, sodium polyacrylate, sodium polymethacrylate, and acrylic acid-ethylene copolymers. Examples of fluororesins include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), and vinylidene fluoride-hexafluoropropylene copolymer. Examples of rubbers include styrene-butadiene copolymer (SBR), ethylene-propylene-diene copolymer, etc. Furthermore, thickeners such as carboxymethyl cellulose may be used as other additives.

[0036] The method for producing the negative electrode is not particularly limited, and the negative electrode may be produced by a known method. In one example of the production method, first, the negative electrode materials (and a dispersion medium, if necessary) are mixed to prepare a negative electrode mixture. Next, the negative electrode mixture is dried as necessary, and then molded into coin-shaped pellets to obtain the negative electrode.

[0037] Alternatively, a negative electrode may be fabricated by applying a negative electrode slurry containing a negative electrode composite and a dispersion medium to a negative electrode current collector, drying the coating, and compressing it as necessary to form a negative electrode composite layer. Such a negative electrode includes a negative electrode current collector and a negative electrode composite layer supported on the surface of the negative electrode current collector. The negative electrode composite layer is formed on both sides or one side of the negative electrode current collector.

[0038] (Positive Electrode) The positive electrode includes a positive electrode active material capable of reversibly absorbing and releasing lithium ions. The positive electrode may include other additives (e.g., a conductive material, a binder, etc.) as needed. That is, the positive electrode may include a positive electrode mixture including a positive electrode active material and other additives. The additives are not particularly limited, and known additives may be used. The conductive material and binder may be the same as those exemplified as the materials for the negative electrode. Graphite may be used as the conductive material.

[0039] Examples of positive electrode active materials include lithium cobalt oxide (LiCoO ２ ), lithium nickel oxide (LiNiO ２ ), lithium iron phosphate (LiFePO ４ ), general formula: Li ｘ MnO ｙ The positive electrode active material includes lithium-containing manganese oxides and lithium-containing transition metal oxides represented by the formula: ２ and LiCoO ２ From the viewpoint of increasing capacity, the lithium-containing transition metal oxide may be a composite oxide containing lithium and nickel and having a layered rock salt type crystal structure.

[0040] The composite oxide has the general formula: Li α Ni ｘ１ M1 ｘ２ M2 （１－ｘ１－ｘ２） O ２＋β In this formula, the following conditions are satisfied: 0.9≦α≦1.1, -0.05≦β≦0.05, 0.5≦x1<1, 0≦x2≦0.5, and 0≦1-x1-x2≦0.5. The element M1 is at least one element selected from the group consisting of V, Co, and Mn. The element M2 is at least one element selected from the group consisting of Mg, Al, Ca, Ti, Cu, Zn, and Nb. α increases or decreases with charge and discharge. For example, the oxide may be a composite oxide containing lithium, nickel, cobalt, and manganese.

[0041] The positive electrode active material is LiCoO ２ and LiCoO ２ LiCoO ２ is preferable in terms of excellent output characteristics.

[0042] The manufacturing method of the positive electrode is not particularly limited, and the positive electrode may be manufactured by a known method. In one example of the manufacturing method, first, the positive electrode materials (and a dispersion medium, if necessary) are mixed to prepare a positive electrode mixture. Next, the positive electrode mixture is dried as necessary, and then formed into a coin-shaped pellet to obtain the positive electrode.

[0043] Alternatively, a positive electrode may be fabricated by applying a positive electrode slurry containing a positive electrode mixture and a dispersion medium to the surface of a positive electrode current collector, drying the coating, and compressing it as necessary to form a positive electrode mixture layer. Such a positive electrode includes 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 is formed on both sides or one side of the positive electrode current collector.

[0044] (Non-aqueous electrolyte) The non-aqueous electrolyte contains a solvent (nonaqueous solvent) and a lithium salt (solute). The non-aqueous electrolyte may be liquid or gel-like. The gel-like non-aqueous electrolyte may contain a polymer material. Examples of the polymer material include fluororesin, acrylic resin, polyether resin, polyethylene oxide, etc. The lithium salt contains a cation component and an anion component. The lithium salt dissolves in the solvent to form lithium ions (cations) and anions. The concentration of the lithium salt in the non-aqueous electrolyte may be 0.4 mol / L or more and 2.3 mol / L or less. The non-aqueous electrolyte may contain known additives.

[0045] The lithium salts include at least LiTSFI and LiBF ４ As mentioned above, the lithium salts LiTSFI and LiBF ４ The total content of LiTFSI and LiBF is 50 mol% or more, or may be 60 mol% or more, 75 mol% or more, or 90 mol% or more. ４ Only may be used.

[0046] The solvent contains at least a cyclic ester. By including a cyclic ester with excellent thermal stability in the solvent, degradation of the non-aqueous electrolyte at high temperatures and the resulting decrease in output characteristics are suppressed. Examples of cyclic esters include cyclic carbonates and cyclic carboxylic acid esters. Examples of cyclic carbonates include propylene carbonate. Examples of cyclic carboxylic acid esters include γ-butyrolactone and γ-valerolactone. The cyclic ester preferably contains at least one selected from the group consisting of γ-butyrolactone and propylene carbonate. The proportion of the cyclic ester in the solvent may be 70% by volume or more, or may be 90% by volume or more. The solvent may contain only at least one selected from the group consisting of γ-butyrolactone and propylene carbonate.

[0047] The non-aqueous electrolyte may contain a solvent other than the cyclic ester. The solvent other than the cyclic ester is not particularly limited, and a solvent used in a known non-aqueous electrolyte secondary battery may be used. Examples of the solvent include a chain ester. Examples of the chain ester include a chain carbonate. Examples of the chain carbonate include diethyl carbonate, ethyl methyl carbonate, and dimethyl carbonate.

[0048] The non-aqueous electrolyte is LiTFSI and LiBF ４ Lithium salts other than LiTFSI and LiBF may also be contained. ４ The lithium salt other than the above is not particularly limited, and may be a salt used in known non-aqueous electrolyte secondary batteries. For example, a lithium salt of a chlorine-containing acid (LiClO ４ , LiAlCl ４ , LiB １０ Cl １０ etc.), LiBF ４ Lithium salts of fluorine-containing acids other than LiPF ６ , LiSbF ６ , LiAsF ６ , LiCF ３ SO ３ , LiCF ３ CO ２ etc.), lithium salts of fluorine-containing acid imides other than LiTFSI (LiN(SO２ F) ２ , LiN(CF ３ SO ２ ) ２ , LiN(CF ３ SO ２ ) (C ４ F ９ SO ２ ), LiN(C ２ F ５ SO ２ ) ２ etc.), lithium halides (LiCl, LiBr, LiI etc.), and the like.

[0049] (Separator) A separator is preferably disposed between the positive electrode and the negative electrode. The separator is not particularly limited, and separators used in known non-aqueous electrolyte secondary batteries may be used. An insulating porous film can be used as the separator. Examples of porous films include microporous films, woven fabrics, and nonwoven fabrics. Examples of separator materials include polyolefins (polyethylene, polypropylene, etc.), high heat-resistant resins (polyphenylene sulfide (PPS), polyether ether ketone (PEEK), etc.), and other insulating resin materials.

[0050] An example of a secondary battery will be described below with reference to Fig. 1. Fig. 1 is a cross-sectional view schematically showing a secondary battery according to an embodiment of the present disclosure. Note that the secondary battery of the present disclosure is not limited to this.

[0051] The coin-type secondary battery 10 includes a positive electrode 11, a negative electrode 12, and a separator 13 disposed between the positive electrode 11 and the negative electrode 12. The positive electrode 11, the negative electrode 12, and the separator 13 are in contact with a non-aqueous electrolyte (not shown).

[0052] One surface of the positive electrode 11 is electrically connected to the positive electrode case 14. The positive electrode case 14 is a member that houses the positive electrode 11 and a separator 13 (described later), and also serves as a positive electrode current collector and a positive electrode terminal. The positive electrode case 14 can be formed from various materials known in the field of non-aqueous electrolyte secondary batteries. Specific examples include stainless steel.

[0053] One surface of the negative electrode 12 is electrically connected to the negative electrode case 15. The negative electrode case 15 is a member that contacts the negative electrode 12 and acts as a negative electrode current collector and a negative electrode terminal. The negative electrode case 15 also serves as a sealing plate for the coin-shaped battery. Materials for forming the negative electrode case 15 include various materials known in the field of non-aqueous electrolyte secondary batteries. Specific examples include iron, titanium, and stainless steel. A conductive layer may be formed on the inner surface of the negative electrode case 15. In this case, the negative electrode is electrically connected to the negative electrode case via the conductive layer. The conductive layer is, for example, a layer containing a conductive carbon material.

[0054] The positive electrode case 14 and the negative electrode case 15 are insulated from each other by a gasket 16. Examples of materials constituting the gasket 16 include synthetic resins such as polypropylene, polyphenylene sulfide (PPS), and polyether ether ketone. Among these, polypropylene is preferable.

[0055] The shape of the nonaqueous electrolyte secondary battery shown in FIG. 1 is a coin shape, but the shape of the nonaqueous electrolyte secondary battery can be appropriately selected from various shapes other than the coin shape, such as a cylindrical shape, a prismatic shape, a sheet shape, a flat shape, and a laminated shape.

[0056] <<Additional Notes>> The above description of the embodiment discloses the following techniques.

[0057] (Technology 1) A battery includes a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the negative electrode contains titanium oxide as a negative electrode active material, the non-aqueous electrolyte contains a solvent and a lithium salt, the solvent contains a cyclic ester, and the lithium salt is LiTFSI and LiBF ４ and wherein the LiTFSI and the LiBF in the lithium salt ４ the total content of the lithium salt is 50 mol % or more, the concentration of lithium ions in the non-aqueous electrolyte is 0.4 mol / L or more and 2.3 mol / L or less, the concentration of LiTFSI in the non-aqueous electrolyte is 1 mol / L, and the concentration of LiBF in the non-aqueous electrolyte is 1 mol / L. ４The concentration C2 mol / L of the secondary battery satisfies the following conditions: 0.25≦C1≦1.85, 0.15≦C2≦0.55, and 0.46≦C1 / C2≦9.50.

[0058] (Technology 2) The secondary battery according to Technology 1, wherein the concentration of lithium ions in the non-aqueous electrolyte is 1.20 mol / L or more and 2.18 mol / L or less, and 0.15≦C2≦0.50 and 1.8≦C1 / C2≦7.4 are satisfied.

[0059] (Technology 3) The lithium salt is LiTFSI and LiBF ４ The secondary battery according to claim 1 or 2, which is composed of only C1 and C2 and satisfies 0.4≦C1+C2≦2.3.

[0060] (Technology 4) The secondary battery according to any one of Technologies 1 to 3, wherein the cyclic ester includes at least one selected from the group consisting of γ-butyrolactone and propylene carbonate.

[0061] (Technology 5) The secondary battery according to any one of Technologies 1 to 4, wherein the titanium oxide is lithium titanate.

[0062] (Technology 6) The secondary battery according to any one of Technologies 1 to 5, wherein the titanium oxide particles include secondary particles, the primary particles included in the secondary particles have an average particle diameter of 0.5 μm or more and 10 μm or less, the secondary particles have an average particle diameter of 5 μm or more and 60 μm or less, and the secondary particles have a pore mode diameter of 0.5 μm or more.

[0063] (Technology 7) The secondary battery according to any one of Technologies 1 to 6, wherein the positive electrode active material of the positive electrode is lithium cobalt oxide.

[0064] (Technology 8) The secondary battery according to any one of Technologies 1 to 7, which is used in an environment of 85°C or higher.

[0065] (Technology 9) A non-aqueous electrolyte for use in a secondary battery, the secondary battery comprising: a positive electrode; a negative electrode; and the non-aqueous electrolyte; the negative electrode contains titanium oxide as a negative electrode active material; the non-aqueous electrolyte contains a solvent and a lithium salt; the solvent contains a cyclic ester; and the lithium salt is LiTFSI and LiBF ４ and wherein the LiTFSI and the LiBF in the lithium salt ４ the total content of the lithium salt is 50 mol % or more, the concentration of lithium ions in the non-aqueous electrolyte is 0.4 mol / L or more and 2.3 mol / L or less, the concentration of LiTFSI in the non-aqueous electrolyte is 1 mol / L, and the concentration of LiBF in the non-aqueous electrolyte is 1 mol / L. ４ The concentration C2 mol / L satisfies the following conditions: 0.25≦C1≦1.85, 0.15≦C2≦0.55, and 0.46≦C1 / C2≦9.50.

[0066] (Technology 10) The nonaqueous electrolyte for a secondary battery according to Technology 9, wherein a concentration of lithium ions in the nonaqueous electrolyte is 1.20 mol / L or more and 2.18 mol / L or less, and 0.15≦C2≦0.50 and 1.8≦C1 / C2≦7.4 are satisfied.

[0067] (Technology 11) The lithium salt is the LiTFSI and the LiBF ４ 11. The nonaqueous electrolyte for a secondary battery according to claim 9 or 10, which is composed of only C1 and C2, and satisfies 0.4≦C1+C2≦2.3.

[0068] (Technology 12) The nonaqueous electrolyte for a secondary battery according to any one of Techniques 9 to 11, wherein the cyclic ester includes at least one selected from the group consisting of γ-butyrolactone and propylene carbonate.

[0069] [Examples] Hereinafter, the present disclosure will be specifically described based on examples, but the present disclosure is not limited to the following examples.

[0070] <Batteries A1 to A21, B1 to B14> Coin-type secondary batteries were fabricated according to the following procedure.

[0071] (Preparation of Negative Electrode) 95 parts by mass of the negative electrode active material, 5 parts by mass of acetylene black as a conductive material, 5 parts by mass of styrene-butadiene rubber as a binder, and an appropriate amount of water were mixed to obtain a negative electrode composite paste. The negative electrode composite paste was dried to obtain a negative electrode composite. The obtained negative electrode composite was subjected to a 20 kN / cm ２ The mixture was pressed at a pressure of 1000 kJ / cm2 to obtain pellets (diameter: 10 mm). The obtained pellets were dried at 150° C. to obtain pellet-shaped negative electrodes.

[0072] The negative electrode active material is lithium titanate (Li ４ Ti ５ O １２ As the lithium titanate powder, powder of LTO1 or LTO2 shown in Table 1 was used.

[0073]

[0074] (Preparation of Positive Electrode) 95 parts by mass of the positive electrode active material, 5 parts by mass of acetylene black as a conductive material, 5 parts by mass of polytetrafluoroethylene as a binder, and an appropriate amount of water were mixed to obtain a positive electrode composite paste. The positive electrode composite paste was dried to obtain a positive electrode composite. Approximately 100 mg of the obtained positive electrode composite was subjected to a 10 kN / cm ２ The mixture was pressed at a pressure of 1000 kJ / cm2 to obtain a pellet (diameter: 10 mm). The obtained pellet was dried at 200° C. to obtain a pellet-shaped positive electrode.

[0075] The positive electrode active material used was a composite oxide containing lithium and a transition metal, as shown in Tables 2 and 3. In the tables, LCO stands for lithium cobalt oxide (LiCoO ２ ), and NCM is a composite oxide containing lithium, nickel, cobalt, and manganese.

[0076] (Preparation of Nonaqueous Electrolytes) Nonaqueous electrolytes were prepared with the compositions shown in Tables 2 and 3. In the tables, with regard to the solvents, GBL stands for γ-butyrolactone, PC stands for propylene carbonate, and EC stands for ethylene carbonate.

[0077] In A1 to A16, A19 to A20, and B4 to B12, the lithium salts are LiTFSI and LiBF ４In A17 to A18 and B3, LiTFSI and LiBF were used as lithium salts. ４ , and LiBETI(LiN(SO ２ C ２ F ５ ) ２ In A19, PS (1,3-propane sultone) was added as an additive to the non-aqueous electrolyte at a molar ratio of PS:lithium salt=2:98.

[0078] (Fabrication of Battery) Using the above-described positive electrode, negative electrode, non-aqueous electrolyte, and other components, a coin-type secondary battery (outer diameter 16 mm) having the same configuration as the battery shown in Fig. 1 was fabricated. A separator (thickness 50 µm) made of polyphenylene sulfide resin was placed between the positive electrode and the negative electrode.

[0079] The fabricated batteries were evaluated according to the following procedures.

[0080] (Pulse Discharge Voltage Vd After High-Temperature Storage) The battery was stored at a high temperature. Specifically, the battery was stored in a thermostatic chamber at 100° C. for 500 hours. The storage at a high temperature was performed in a state where the battery was charged at a constant voltage of 2.6 V in order to maintain a fully charged state.

[0081] Thereafter, the battery after high-temperature storage was subjected to constant current discharge at 10 mA in an environment of 20° C. The discharge voltage 1 second after the start of discharge was determined as the pulse discharge voltage Vd. This was used to evaluate the output characteristics after high-temperature storage.

[0082] For the batteries A1 to A21, the capacity retention rate R after high-temperature storage was also determined.

[0083] (Capacity Retention Rate R After High-Temperature Storage) The battery in an initial fully charged state was discharged at a constant current of 0.01 mA in an environment of 20° C. until the voltage reached 1.5 V, and the initial discharge capacity D1 was determined.

[0084] A battery in an initial fully charged state was separately prepared and stored at high temperature in the same manner as above. After high temperature storage, the discharge capacity D2 of the battery was determined in the same manner as above.

[0085] The capacity retention rate R after high-temperature storage was calculated as D2 / D1×100.

[0086] The evaluation results are shown in Tables 2 and 3. Note that batteries A1 to A21 in Table 2 are examples, and batteries B1 to B14 in Table 3 are comparative examples.

[0087]

[0088]

[0089] The batteries A1 to A21 in Table 2 had a higher pulse discharge voltage Vd and better output characteristics than the batteries B1 to B14 in Table 3. Furthermore, the batteries A1 to A21 had a better capacity retention rate R.

[0090] In batteries A1 to A5 and A9 to A16, LiTFSI and LiBF ４ The batteries were fabricated using a lithium salt, GBL (solvent), LCO (positive electrode active material), and LTO1 (negative electrode active material) under the same conditions except for the concentrations C1, C2, and lithium ion concentration. Comparing these batteries A1 to A5 and A9 to A16, all batteries exhibited good capacity retention rates R, with batteries A5 and A13 to A16 exhibiting particularly high capacity retention rates R of 90% or more.

[0091] Both battery A1, which used LCO as the positive electrode active material, and battery A5, which used NCM as the positive electrode active material, exhibited good output characteristics, but battery A1 exhibited a higher pulse discharge voltage Vd.

[0092] Both battery A1, which used LTO1 as the negative electrode active material, and battery A21, which used LTO2 as the negative electrode active material, exhibited good output characteristics, but battery A1 exhibited a higher pulse discharge voltage Vd.

[0093] In batteries B2 and B4, where the LiTFSI concentration C1 was less than 0.25 mol / L, the ionic conductivity of the non-aqueous electrolyte decreased at high temperatures, resulting in a decrease in output characteristics.In battery B5, where the LiTFSI concentration C1 was greater than 1.85 mol / L, the viscosity of the non-aqueous electrolyte increased, resulting in a decrease in output characteristics.

[0094] LiBF ４ In the batteries B1, B6, B13, and B14 in which the concentration C2 of LiBF was less than 0.15 mol / L, LiTFSI was oxidized at high temperatures, and the output characteristics were reduced. ４In the battery B7 in which the concentration C2 is greater than 0.55 mol / L, LiBF ４ The generation of by-products from the reaction increased the internal resistance and reduced the output characteristics.

[0095] In Battery B8, in which the lithium ion concentration in the nonaqueous electrolyte was less than 0.4 mol / L, the conductivity of the nonaqueous electrolyte decreased, resulting in a decrease in output characteristics. In Battery B9, in which the lithium ion concentration in the nonaqueous electrolyte was greater than 2.3 mol / L, the viscosity of the nonaqueous electrolyte increased, resulting in a decrease in output characteristics.

[0096] In battery B10 where C1 / C2 is less than 0.46, LiBF ４ In Battery B11, where C1 / C2 was greater than 9.50, the inner surface of the case was corroded by LiTFSI, increasing the internal resistance and deteriorating the output characteristics.

[0097] LiTFSI and LiBF in lithium salts ４ In battery B3, the total content of LiTFSI and LiBF is less than 50 mol% based on the total lithium salt. ４ In battery B12, which did not contain a cyclic ester in the solvent, the output characteristics were reduced due to the deterioration of the non-aqueous electrolyte at high temperatures caused by the reduced thermal stability of the solvent.

[0098] The secondary battery according to the present disclosure is suitable for use as a power source for portable devices and electronic devices.

[0099] 10: Battery 11: Positive electrode 12: Negative electrode 13: Separator 14: Positive electrode case 15: Negative electrode case 16: Gasket

Claims

1. A battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the negative electrode contains titanium oxide as a negative electrode active material, the non-aqueous electrolyte contains a solvent and a lithium salt, the solvent contains a cyclic ester, and the lithium salt is LiTFSI and LiBF ４ and wherein the LiTFSI and the LiBF in the lithium salt ４ the total content of the lithium salt is 50 mol % or more, the concentration of lithium ions in the non-aqueous electrolyte is 0.4 mol / L or more and 2.3 mol / L or less, the concentration of LiTFSI in the non-aqueous electrolyte is 1 mol / L, and the concentration of LiBF in the non-aqueous electrolyte is 1 mol / L. ４ The concentration C2 mol / L of the secondary battery satisfies the following conditions: 0.25≦C1≦1.85, 0.15≦C2≦0.55, and 0.46≦C1 / C2≦9.

50.

2. The secondary battery according to claim 1, wherein the concentration of lithium ions in the non-aqueous electrolyte is 1.20 mol / L or more and 2.18 mol / L or less, and the following conditions are satisfied: 0.15≦C2≦0.50, and 1.8≦C1 / C2≦7.

4.

3. The lithium salt is LiTFSI and LiBF ４ The secondary battery according to claim 1 , wherein the secondary battery is composed of only C1 and C2, and satisfies 0.4≦C1+C2≦2.

3.

4. The secondary battery according to claim 1, wherein the cyclic ester includes at least one selected from the group consisting of γ-butyrolactone and propylene carbonate.

5. The secondary battery according to claim 1, wherein the titanium oxide is lithium titanate.

6. The secondary battery according to claim 1, wherein the titanium oxide particles include secondary particles, the primary particles included in the secondary particles have an average particle diameter of 0.5 μm or more and 10 μm or less, the secondary particles have an average particle diameter of 5 μm or more and 60 μm or less, and the secondary particles have a pore mode diameter of 0.5 μm or more.

7. The secondary battery according to claim 1, wherein the positive electrode active material of the positive electrode is lithium cobalt oxide.

8. The secondary battery according to claim 1, which is used in an environment of 85°C or higher.

9. A non-aqueous electrolyte for use in a secondary battery, the secondary battery comprising a positive electrode, a negative electrode, and the non-aqueous electrolyte, the negative electrode containing titanium oxide as a negative electrode active material, the non-aqueous electrolyte containing a solvent and a lithium salt, the solvent containing a cyclic ester, and the lithium salt containing LiTFSI and LiBF ４ and wherein the LiTFSI and the LiBF in the lithium salt ４ the total content of the lithium salt is 50 mol % or more, the concentration of lithium ions in the non-aqueous electrolyte is 0.4 mol / L or more and 2.3 mol / L or less, the concentration of LiTFSI in the non-aqueous electrolyte is 1 mol / L, and the concentration of LiBF in the non-aqueous electrolyte is 1 mol / L. ４ The concentration C2 mol / L satisfies the following conditions: 0.25≦C1≦1.85, 0.15≦C2≦0.55, and 0.46≦C1 / C2≦9.

50.

10. The non-aqueous electrolyte for a secondary battery according to claim 9, wherein the concentration of lithium ions in the non-aqueous electrolyte is 1.20 mol / L or more and 2.18 mol / L or less, and the following relationships are satisfied: 0.15≦C2≦0.50, and 1.8≦C1 / C2≦7.

4.

11. The lithium salt is LiTFSI and LiBF ４ The nonaqueous electrolyte for a secondary battery according to claim 9 , which is composed only of C1 and C2, and satisfies 0.4≦C1+C2≦2.

3.

12. The nonaqueous electrolyte for a secondary battery according to claim 9, wherein the cyclic ester includes at least one selected from the group consisting of γ-butyrolactone and propylene carbonate.

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