Non-aqueous electrolytic solution and non-aqueous electrolyte secondary battery

Abstract

A non-aqueous electrolytic solution according to the present disclosure includes a non-aqueous solvent, a phosphorus-containing compound, and two or more electrolyte salts dissolved in the non-aqueous solvent. The two or more electrolyte salts include a second lithium salt and a first lithium salt having an oxalate structure. The first lithium salt may include at least one selected from the group consisting of LiFOB, LiBOB, LiTFOP, and LiDOBFP.

Description

Nonaqueous electrolyte and nonaqueous electrolyte secondary battery

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

[0002] As known to those skilled in the art, conventional battery electrolytes contain various components. For example, Patent Document 1 discloses that a non-aqueous electrolyte secondary battery containing vinylene carbonate as a non-aqueous solvent has good cycle characteristics.

[0003] Japanese Patent Application Laid-Open No. 2005-268230

[0004] The present disclosure provides a nonaqueous electrolyte suitable for improving the cycle characteristics of a battery.

[0005] The present disclosure provides a non-aqueous electrolyte solution comprising: a non-aqueous solvent; a phosphorus-containing compound; and two or more types of electrolyte salts dissolved in the non-aqueous solvent, wherein the two or more types of electrolyte salts include a first lithium salt having an oxalate structure and a second lithium salt.

[0006] According to the technology of the present disclosure, the cycle characteristics of the battery can be improved.

[0007] Fig. 1 is a schematic cross-sectional view showing an example of a nonaqueous electrolyte secondary battery according to embodiment 2. Fig. 2 is a diagram showing the structural formulas of various first lithium salts.

[0008] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. The present disclosure is not limited to the following embodiments.

[0009] (Embodiment 1) The nonaqueous electrolyte solution of embodiment 1 includes a nonaqueous solvent, a phosphorus-containing compound, and two or more electrolyte salts dissolved in the nonaqueous solvent. The two or more electrolyte salts include a first lithium salt and a second lithium salt. The first lithium salt has an oxalate structure. When a battery using the nonaqueous electrolyte solution of embodiment 1 is operated, the phosphorus-containing compound contained in the nonaqueous electrolyte solution forms a coating on the surface of the positive electrode. This can suppress oxidative decomposition of the solvent at the interface between the nonaqueous electrolyte solution and the positive electrode. Furthermore, even if the coating is damaged over time or due to repeated charge and discharge, the phosphorus-containing compound contained in the nonaqueous electrolyte solution can repair the coating. As a result, the nonaqueous electrolyte solution of the present disclosure can suppress oxidative decomposition of the solvent for a longer period of time than, for example, a battery assembled with a coating formed directly on the positive electrode. As a result, the nonaqueous electrolyte solution of embodiment 1 can improve the cycle characteristics of the battery.

[0010] Here, the phosphorus-containing compound tends to increase the viscosity of the non-aqueous electrolyte. From the viewpoint of lithium ion conductivity and low-temperature characteristics, the viscosity of the non-aqueous electrolyte is generally desired to be low. Therefore, it is desirable to reduce the viscosity of the non-aqueous electrolyte while maintaining the effect of improving the cycle characteristics by the phosphorus-containing compound.

[0011] The non-aqueous electrolyte solution in embodiment 1 contains two or more electrolyte salts. The two or more electrolyte salts include a first lithium salt having an oxalate structure. The first lithium salt reduces the viscosity of the non-aqueous electrolyte solution while maintaining the effect of improving cycle characteristics due to the phosphorus-containing compound. The effect of reducing the viscosity of the non-aqueous electrolyte solution is due to the large surface area of ​​the anion contained in the oxalate structure. The surface area of ​​the anion can be confirmed by theoretical calculation.

[0012] In this specification, the term "oxalate structure" refers to a structure formed by coordinating an oxalate ion to an atom such as boron (B) or phosphorus (P).

[0013] The first lithium salt can exert an effect of reducing the amount of gas generated depending on its concentration in the nonaqueous electrolyte solution.

[0014] 2 shows structural formulas of various first lithium salts. The first lithium salt may include at least one selected from the group consisting of lithium difluorooxalatoborate (LiFOB) shown in FIG. 2( a), lithium bis(oxalato)borate (LiBOB) shown in FIG. 2( b), lithium tetrafluoro(oxalato)phosphate (LiTFOP) shown in FIG. 2( c), and lithium bis(oxalatodifluorophosphate) (LiDOBFP) shown in FIG. 2( d). These first lithium salts make it easy to reduce the viscosity of the nonaqueous electrolyte while maintaining the effects of the phosphorus-containing compound.

[0015] The first lithium salt is dissolved in the non-aqueous solvent at a concentration of, for example, 0.005 mol / L or more and 0.70 mol / L or less. In other words, the concentration of the first lithium salt in the portion of the non-aqueous electrolyte excluding the phosphorus-containing compound is, for example, 0.005 mol / L or more and 0.70 mol / L or less. By appropriately adjusting the concentration of the first lithium salt, it is possible to obtain the effects based on the first lithium salt while maintaining the performance required of the non-aqueous electrolyte. The first lithium salt may be dissolved in the non-aqueous solvent at a concentration of preferably more than 0.03 mol / L and less than 0.25 mol / L, more preferably 0.04 mol / L or more and 0.15 mol / L or less. The "portion of the non-aqueous electrolyte excluding the phosphorus-containing compound" may be the liquid phase portion of the non-aqueous electrolyte.

[0016] Examples of the second lithium salt include lithium hexafluorophosphate (LiPF), lithium tetrafluoroborate (LiBF), lithium perchlorate (LiClO), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bisperfluoroethylsulfonylimide (LiN(SO2C2F5)2), LiAsF6, and LiCF3SO3. The lithium salts selectable as the first lithium salt can also be used as the second lithium salt. At least one selected from the above-described substances can be used as the second lithium salt. The second lithium salt may contain fluorine (F).

[0017] The second lithium salt may include LiPF. LiPF is a lithium salt with excellent properties and is therefore recommended as the second lithium salt. On the other hand, non-aqueous electrolytes containing a phosphorus-containing compound and LiPF tend to have high viscosity. Therefore, it is desirable to use LiPF in combination with a first lithium salt having an oxalate structure.

[0018] The concentration of the electrolyte salt in the nonaqueous electrolyte solution may be, for example, 0.5 mol / L or more and 2 mol / L or less. By controlling the concentration of the electrolyte salt within the above range, a nonaqueous electrolyte solution having excellent ionic conductivity and appropriate viscosity can be obtained. However, the concentration of the electrolyte salt is not limited to the above.

[0019] The proportion of the first lithium salt in the electrolyte salt may be equal to or smaller than the proportion of the second lithium salt in the electrolyte salt, on a mass of substance basis. With this configuration, the characteristics of the second lithium salt dominate the nonaqueous electrolyte solution, making it easy to select the type of second lithium salt so as to obtain a nonaqueous electrolyte solution with desired performance.

[0020] However, the ratio of the first lithium salt to the second lithium salt is not particularly limited. In one example, the ratio of the first lithium salt to the second lithium salt is, by mass, in the range of 0.005:0.995 to 0.5:0.5, or may be in the range of 0.01:0.99 to 0.5:0.5, or may be in the range of 0.04:0.96 to 0.15:0.85. The ratio of the first lithium salt to the total amount of lithium salts may be, by mass, 0.005 to 0.5, 0.01 to 0.5, or 0.04 to 0.15.

[0021] The non-aqueous electrolyte solution in the first embodiment is, for example, liquid at 25°C. The liquid state includes a sol. The non-aqueous electrolyte solution in the first embodiment can have fluidity at 25°C.

[0022] In the present disclosure, "having fluidity at 25°C" means having a viscosity of 20,000 mPa·s or less at 25°C.

[0023] The viscosity of the nonaqueous electrolyte solution in embodiment 1 at 25° C. may be 5000 mPa·s or less, 3000 mPa·s or less, or 1000 mPa·s or less.

[0024] The phosphorus-containing compound may be insoluble in the non-aqueous solvent. The non-aqueous electrolyte in embodiment 1 may contain particles of the phosphorus-containing compound that are insoluble in the non-aqueous solvent. The non-aqueous electrolyte in embodiment 1 may be a non-aqueous colloidal solution in which particles of the phosphorus-containing compound are dispersed.

[0025] In the present disclosure, "particles of a phosphorus-containing compound insoluble in a non-aqueous solvent" refers to particles of a phosphorus-containing compound that require 100 mL or more of non-aqueous solvent to dissolve 1 g of the compound at 25°C. That is, the solubility of the particles of the phosphorus-containing compound in 100 mL of non-aqueous solvent is 1 g or less. Here, "dissolution" means that the permeability of the solution obtained when the particles of the phosphorus-containing compound are dissolved in the non-aqueous solvent in a container does not change from the permeability of the solvent, that is, the solution is not cloudy, and no precipitate is observed on the bottom of the container after standing for 24 hours.

[0026] The particles of the phosphorus-containing compound may be particles of a phosphorus-containing compound having a solubility of 0.1 g or less in 100 mL.

[0027] The phosphorus-containing compound is, for example, an inorganic phosphorus-based compound. The phosphorus-containing compound may include phosphoric acid or a phosphoric acid compound. The phosphoric acid compound is, for example, an inorganic phosphate. The phosphoric acid may include phosphorous acid, and the phosphoric acid compound may include phosphite. The phosphorus-containing compound preferably includes a PO4 skeleton. The phosphorus-containing compound may include an organic phosphorus compound. The organic phosphorus compound is, for example, a phosphoric acid ester including a PO4 skeleton.

[0028] The phosphorus-containing compound may include at least one selected from the group consisting of Li3PO4, Na3PO4, K3PO4, LiPO3, LiPO3F, metaphosphoric acid, PO5, and calcium phosphate. Calcium phosphate is, for example, hydroxyapatite. The phosphorus-containing compound may include at least one selected from the group consisting of Li3PO4, Na3PO4, K3PO4, LiPO3, LiPO3F, metaphosphoric acid, and PO5. The phosphorus-containing compound may include at least one selected from the group consisting of Li3PO4, Na3PO4, LiPO3, LiPO3F, metaphosphoric acid, and PO5.

[0029] The phosphorus-containing compound may contain Li3PO4. That is, the nonaqueous electrolyte solution in embodiment 1 may contain Li3PO4 particles. According to the above configuration, the cycle characteristics of the battery can be improved.

[0030] The phosphorus-containing compound may contain at least one element selected from the group consisting of magnesium, strontium, and barium. Magnesium, strontium, and barium are classified as alkaline earth metals and tend to have a divalent oxidation state in phosphorus-containing compounds. Therefore, when magnesium, strontium, or barium is contained in a phosphorus-containing compound, the Coulomb force between these elements and phosphorus is strengthened, stabilizing the phosphorus-containing compound.

[0031] The phosphorus-containing compound containing at least one element selected from the group consisting of magnesium, strontium, and barium may be an inorganic phosphorus-based compound. The phosphorus-containing compound may contain only one element selected from the group consisting of magnesium, strontium, and barium as an alkaline earth metal element. Such phosphorus-containing compounds can be synthesized relatively easily or are available as commercially available reagents.

[0032] The phosphorus-containing compound is M x P2O y(1≦x≦3, 5≦y≦10). M contains at least one selected from the group consisting of magnesium, strontium, and barium. Such a compound has the effect of improving the output characteristics of the battery. M may be magnesium, strontium, or barium.

[0033] The phosphorus-containing compound may contain at least one selected from the group consisting of phosphates and pyrophosphates. Phosphates are compounds containing alkaline earth metal ions and phosphate ions (PO 2- Pyrophosphate is a salt of an alkaline earth metal ion and a pyrophosphate ion (P2O7 4- ) and salt.

[0034] The phosphorus-containing compound may contain a transition metal element in addition to an alkaline earth metal element.

[0035] The phosphorus-containing compound may include at least one selected from the group consisting of Sr(PO), Ba(PO), and BaP0, which is effective in improving the output characteristics of the battery.

[0036] The phosphorus-containing compound containing at least one selected from the group consisting of magnesium, strontium, and barium may be used in combination with the other phosphorus-containing compounds described above.

[0037] The phosphorus-containing compound may be a compound that does not contain lithium, and may be a material different from the electrolyte normally contained in the non-aqueous electrolyte.

[0038] The phosphorus-containing compound may be a compound that does not contain water of crystallization, which can prevent the battery performance from being deteriorated by moisture.

[0039] In the non-aqueous electrolyte solution of embodiment 1, the content of the phosphorus-containing compound particles may be 0.1 vol% or more and 10 vol% or less. The content of the phosphorus-containing compound particles may be 0.1 vol% or more and 8 vol% or less, 0.5 vol% or more and 6 vol% or less, 1 vol% or more and 5 vol% or less, or 1 vol% or more and 4 vol% or less. The above configuration can improve the dispersibility of the phosphorus-containing compound particles and the fluidity of the non-aqueous electrolyte solution.

[0040] When the nonaqueous electrolyte solution of embodiment 1 contains particles of a phosphorus-containing compound, the content of the phosphorus-containing compound particles can be determined, for example, by the following method. After measuring the volume of the nonaqueous electrolyte solution, the nonaqueous electrolyte solution is filtered to separate the particles. The separated particles are washed with a solvent such as dimethyl carbonate, and the washing solvent is evaporated and dried, after which the mass of the particles is measured. The volume of the particles is calculated from the specific gravity determined from the mass of the particles and the particle components. The particle components can be identified by various analytical methods such as inductively coupled plasma analysis (ICP), X-ray diffraction (XRD), infrared absorption spectroscopy (IR), and nuclear magnetic resonance analysis (NMR). In this way, the content of the phosphorus-containing compound particles in the nonaqueous electrolyte solution can be calculated. The volume of the nonaqueous electrolyte solution can also be calculated from the composition and mass. The composition of the nonaqueous electrolyte solution can be measured using a liquid chromatograph, gas chromatograph, or the like.

[0041] The particles of the phosphorus-containing compound may be nanoparticles.

[0042] The average particle diameter of the phosphorus-containing compound particles may be 1 nm or more and 500 nm or less. This configuration improves the dispersibility of the phosphorus-containing compound particles in the non-aqueous electrolyte, thereby enabling increased industrial productivity of the non-aqueous electrolyte. The average particle diameter of the phosphorus-containing compound particles may be 5 nm or more and 400 nm or less, or 10 nm or more and 300 nm or less. When the average particle diameter of the phosphorus-containing compound particles is 500 nm or less, in a battery using the non-aqueous electrolyte of the present disclosure, when the non-aqueous electrolyte permeates the positive electrode active material layer, the phosphorus-containing compound particles may permeate between the positive electrode active material particles arranged inside the positive electrode active material layer. Therefore, a coating of the phosphorus-containing compound is also formed on the surface of the positive electrode active material arranged inside the positive electrode active material layer, thereby suppressing oxidative decomposition of the solvent inside the positive electrode active material layer.

[0043] The average particle size of the particles of the phosphorus-containing compound may be equal to or smaller than the pore size of the separator of a battery using a nonaqueous electrolyte. According to the above configuration, the particles of the phosphorus-containing compound do not clog the pores of the separator, so that circulation of the electrolyte inside the electrode group is not hindered even during charge and discharge.

[0044] In the present disclosure, the average particle size refers to the median diameter (d50). The median diameter is the particle size at which the cumulative volume in the volume-based particle size distribution is 50%. The volume-based particle size distribution can be determined by a laser diffraction scattering method using a commercially available laser diffraction measuring device.

[0045] The non-aqueous solvent is not particularly limited, and for example, a cyclic carbonate, a chain carbonate, a cyclic carboxylic acid ester, or the like may be used.

[0046] Examples of the cyclic carbonate include propylene carbonate (PC) and ethylene carbonate (EC).

[0047] Examples of the chain carbonate ester include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC).

[0048] Examples of the cyclic carboxylic acid ester include γ-butyrolactone (GBL) and γ-valerolactone (GVL).

[0049] The nonaqueous solvent may be used alone or in combination of two or more. The nonaqueous solvent may contain ethylene carbonate. This can increase the solubility of an electrolyte such as a lithium salt in the nonaqueous solvent.

[0050] The nonaqueous electrolyte solution in embodiment 1 may further contain other substances than those described above. For example, the nonaqueous electrolyte solution in embodiment 1 may further contain an additive to improve the dispersibility of the phosphorus-containing compound particles. The additive is, for example, a fluorine-containing solvent. That is, the nonaqueous electrolyte solution in embodiment 1 may further contain a fluorine-containing solvent. With the above configuration, it is possible to reduce aggregation of the phosphorus-containing compound particles over time and the resulting sedimentation of the particles.

[0051] Examples of fluorine-containing solvents include fluorinated cyclic esters and fluorinated ethers. The fluorinated cyclic esters may include fluoroethylene carbonate. The fluorinated ethers may include 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether.

[0052] The nonaqueous electrolyte solution in the first embodiment can be produced, for example, by the following method.

[0053] Two or more electrolyte salts are dissolved in a non-aqueous solvent. The resulting solution is mixed with the phosphorus-containing compound and ZrO balls as a mixing medium in a ball mill. The mixing medium is removed from the resulting mixture to obtain the non-aqueous electrolyte solution of embodiment 1.

[0054] The method for producing the non-aqueous electrolyte is not limited to the above, and may be, for example, by dispersing particles of a phosphorus-containing compound in a non-aqueous solvent in which an electrolyte salt has been dissolved, using an ultrasonic homogenizer.

[0055] (Embodiment 2) A nonaqueous electrolyte secondary battery according to Embodiment 2 includes a positive electrode, a negative electrode, and the nonaqueous electrolyte solution according to Embodiment 1. By using the nonaqueous electrolyte solution according to Embodiment 1, the cycle characteristics of the secondary battery can be improved.

[0056] FIG. 1 is a schematic cross-sectional view showing an example of a nonaqueous electrolyte secondary battery according to Embodiment 2. The secondary battery 100 includes a container 1, an electrode group 4, and an electrolyte solution (not shown). The electrolyte solution is the nonaqueous electrolyte solution of Embodiment 1. The electrode group 4 has a wound structure. The electrode group 4 is housed in the container 1. The electrode group 4 includes a positive electrode 5, a negative electrode 6, and a pair of separators 7. The electrode group 4 is impregnated with the electrolyte solution. The opening of the container 1 is closed with a sealing plate 2. The positive electrode 5 includes a positive electrode current collector 5a and a positive electrode active material layer 5b. One end of a positive electrode lead 5c is connected to the positive electrode 5. The other end of the positive electrode lead 5c is connected to the back surface of the sealing plate 2. An insulating packing 3 is disposed around the sealing plate 2. The negative electrode 6 includes a negative electrode current collector 6a and a negative electrode active material layer 6b. One end of the negative electrode lead 6c is connected to the negative electrode 6. The other end of the negative electrode lead 6c is connected to the bottom surface of the container 1. An insulating ring 8 is disposed on each of the upper and lower surfaces of the electrode group 4.

[0057] Each component of the secondary battery 100 will be specifically described below.

[0058] The positive electrode current collector 5a can be a sheet or film made of a metal material such as aluminum, stainless steel, titanium, or an alloy thereof. Aluminum and its alloys are suitable materials for the positive electrode current collector 5a because they are inexpensive and easy to form into thin films. The sheet or film may be porous or non-porous. Examples of the sheet or film include metal foil and metal mesh. A carbon material such as carbon may be applied to the surface of the positive electrode current collector 5a as a conductive auxiliary material.

[0059] The positive electrode active material layer 5b includes a positive electrode active material. The positive electrode active material may be a material capable of absorbing and releasing lithium ions. Examples of the positive electrode active material include lithium-containing transition metal oxides, lithium-containing transition metal phosphates, transition metal fluorides, polyanionic materials, fluorinated polyanionic materials, transition metal sulfides, transition metal oxysulfides, and transition metal oxynitrides. In particular, using a lithium-containing transition metal oxide or a lithium-containing transition metal phosphate as the positive electrode active material can reduce the manufacturing cost of the battery and increase the average discharge voltage. Examples of lithium-containing transition metal oxides include lithium cobalt oxide, lithium nickel cobalt aluminum oxide, lithium nickel cobalt manganese oxide, and lithium nickel manganese oxide. Examples of lithium-containing transition metal phosphates include lithium iron phosphate, lithium vanadium phosphate, lithium cobalt phosphate, and lithium nickel phosphate.

[0060] The positive electrode active material may contain lithium nickel oxide having a layered rock salt crystal structure. The proportion of Ni among metal elements other than Li contained in the lithium nickel oxide may be 50 atomic % or more. The lithium nickel oxide may also contain other transition metals. The lithium nickel oxide is useful for achieving a high operating voltage.

[0061] The lithium nickel oxide may be represented by the following composition formula (I): Element M1 is at least one selected from the group consisting of V, Co, and Mn. Element M2 is at least one selected from the group consisting of Mg, Al, Ca, Ti, Cu, Zn, and Nb. Composition formula (I) satisfies 0.9≦α≦1.10, −0.05≦β≦0.05, 0.5≦x1<1, 0≦x2≦0.5, and 0<1−x1−x2≦0.5.

[0062] Li α Ni x1 M1 x2 M2 (1-x1-x2) O 2+β ...(I)

[0063] The positive electrode active material layer 5b may contain other materials such as a conductive additive and a binder.

[0064] The conductive additive is used to reduce the resistance of the positive electrode 5. Examples of the conductive additive include a carbon material and a conductive polymer compound. Examples of the carbon material include carbon black, graphite, acetylene black, carbon nanotubes, carbon nanofibers, graphene, fullerene, and graphite oxide. Examples of the conductive polymer compound include polyaniline, polypyrrole, and polythiophene.

[0065] The binder is used to improve the binding property of the material constituting the positive electrode 5. As the binder, polymer materials such as polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, polytetrafluoroethylene, carboxymethyl cellulose, polyacrylic acid, styrene-butadiene copolymer rubber, polypropylene, polyethylene, and polyimide can be used.

[0066] The negative electrode current collector 6a may be a sheet or film made of a metal material such as stainless steel, nickel, copper, or an alloy thereof. The sheet or film may be porous or non-porous. Examples of the sheet or film include metal foil and metal mesh. A carbon material such as carbon may be applied to the surface of the negative electrode current collector 6a as a conductive auxiliary material.

[0067] The negative electrode active material layer 6b includes a negative electrode active material. The negative electrode active material can be a material capable of absorbing and releasing lithium ions. The negative electrode active material includes, for example, at least one selected from the group consisting of metallic lithium, a carbon material, and a material capable of forming an alloy with lithium. Examples of the carbon material include graphite. Examples of materials capable of forming an alloy with lithium include silicon, silicon-containing oxides, tin, zinc alloys, bismuth, and germanium. One type selected from these negative electrode active materials may be used, or two or more types may be used in combination.

[0068] The negative electrode active material layer 6b may contain at least one selected from the group consisting of graphite and silicon as the negative electrode active material. The negative electrode active material layer 6b may contain only graphite as the negative electrode active material. Graphite is recommended because it is resistant to deterioration even when repeatedly charged and discharged at a deep depth. Carbon materials other than graphite may also be used as the negative electrode active material. Silicon has a larger capacity than graphite and is therefore advantageous for increasing the capacity of the secondary battery 100.

[0069] The negative electrode 6 (negative electrode active material layer 6 b) may contain metallic lithium as the negative electrode active material. The nonaqueous electrolyte solution of embodiment 1 contains a first lithium salt having an oxalate structure. The first lithium salt having an oxalate structure can form a strong solid electrolyte interface that can withstand the expansion and contraction of metallic lithium.

[0070] The negative electrode active material layer 6b may contain other materials such as a conductive additive, a binder, etc. Materials that can be used as the conductive additive and binder for the positive electrode active material layer 5b can also be used for the negative electrode active material layer 6b.

[0071] The electrolyte is the nonaqueous electrolyte in embodiment 1. The electrolyte is impregnated into the positive electrode 5, the negative electrode 6, and the separator 7. The electrolyte may fill the internal space of the container 1. The electrolyte allows lithium ions to move between the positive electrode 5 and the negative electrode 6.

[0072] The average particle size of the particles of the phosphorus-containing compound contained in the electrolyte may be equal to or smaller than the pore size of the separator 7 .

[0073] The separator 7 has lithium ion conductivity. The material of the separator 7 is not particularly limited as long as it allows the passage of lithium ions. The material of the separator 7 can be at least one selected from the group consisting of a gel electrolyte, an ion exchange resin membrane, a semipermeable membrane, and a porous membrane. Using these materials for the separator 7 can sufficiently ensure the safety of the secondary battery 100. Examples of gel electrolytes include gel electrolytes containing fluororesins such as PVdF. Examples of ion exchange resin membranes include cation exchange membranes and anion exchange membranes. Examples of porous membranes include porous membranes made of polyolefin resins and porous membranes containing glass paper obtained by weaving glass fibers into a nonwoven fabric. Using the nonaqueous electrolyte of embodiment 1 in the secondary battery 100 suppresses oxidation of the separator 7 and reduces deterioration in the strength of the separator 7.

[0074] The container 1 is made of a metal such as aluminum or stainless steel, and may have a cylindrical shape or a rectangular tube shape.

[0075] The electrode group 4 may be wound into a cylindrical shape or an oval shape.

[0076] The shape of the secondary battery 100 is not particularly limited. In the present disclosure, as an example of the structure of the nonaqueous electrolyte secondary battery according to embodiment 2, the configuration example shown in FIG. 1 is described, i.e., a secondary battery in which an electrode group formed by winding a positive electrode and a negative electrode with a separator interposed therebetween and an electrolyte solution are housed in an exterior body. However, the secondary battery according to the present disclosure is not limited to this configuration example. The secondary battery according to the present disclosure may have any shape, such as a cylindrical shape, a prismatic shape, a coin shape, a button shape, a laminate shape, or the like. Furthermore, as the electrode group in the secondary battery according to the present disclosure, instead of a wound-type electrode group, an electrode group of another shape, such as a stacked-type electrode group formed by stacking a positive electrode and a negative electrode with a separator interposed therebetween, may be used.

[0077] (Other Embodiments) (Additional Notes) The above description of the embodiments discloses the following techniques.

[0078] (Technology 1) A non-aqueous electrolyte solution comprising: a non-aqueous solvent; a phosphorus-containing compound; and two or more types of electrolyte salts dissolved in the non-aqueous solvent, wherein the two or more types of electrolyte salts include a first lithium salt having an oxalate structure and a second lithium salt.

[0079] According to the first technique, the cycle characteristics of the battery can be improved.

[0080] (Technology 2) The nonaqueous electrolyte solution according to Technology 1, wherein the first lithium salt includes at least one selected from the group consisting of LiFOB, LiBOB, LiTFOP, and LiDOBFP. These first lithium salts make it easy to reduce the viscosity of the nonaqueous electrolyte solution while maintaining the effect of the phosphorus-containing compound.

[0081] (Technology 3) The nonaqueous electrolyte solution according to Technology 1 or 2, wherein the first lithium salt is dissolved in the nonaqueous solvent at a concentration of 0.005 mol / L or more and 0.70 mol / L or less. By appropriately adjusting the concentration of the first lithium salt, it is possible to obtain the effects based on the first lithium salt while maintaining the performance required of the nonaqueous electrolyte solution.

[0082] (Technology 4) The nonaqueous electrolyte solution according to any one of Technologies 1 to 3, wherein the second lithium salt includes at least one selected from the group consisting of LiPF, LiBF, LiClO, LiFSI, LiTFSI, LiN(SOCF), LiAsF, and LiCFSO. These lithium salts have excellent properties and are therefore recommended as the second lithium salt.

[0083] (Technology 5) The nonaqueous electrolyte solution according to any one of Technologies 1 to 4, wherein the proportion of the first lithium salt in the electrolyte salt is equal to or smaller than the proportion of the second lithium salt in the electrolyte salt, on a substance amount basis. With this configuration, the characteristics of the second lithium salt dominate the nonaqueous electrolyte solution, making it easy to select the type of second lithium salt so as to obtain a nonaqueous electrolyte solution having desired performance.

[0084] (Technology 6) The nonaqueous electrolyte solution according to any one of Technologies 1 to 5, wherein the phosphorus-containing compound includes at least one selected from the group consisting of LiPO, NaPO, LiPO, LiPOF, metaphosphoric acid, and PO. According to the above configuration, the cycle characteristics of the battery can be improved.

[0085] (Technology 7) The nonaqueous electrolyte according to any one of Technologies 1 to 6, wherein the phosphorus-containing compound includes Li3PO4. According to the above configuration, the cycle characteristics of the battery can be improved.

[0086] (Technology 8) The nonaqueous electrolyte solution according to any one of Technologies 1 to 7, wherein the content of the phosphorus-containing compound in the nonaqueous electrolyte solution is 0.1% by volume or more and 10% by volume or less.

[0087] (Technology 9) The nonaqueous electrolyte solution according to any one of Technologies 1 to 8, wherein the phosphorus-containing compound is insoluble in the nonaqueous solvent, particles of the phosphorus-containing compound are contained in the nonaqueous electrolyte solution, and the particles have an average particle size of 1 nm or more and 500 nm or less. This configuration can improve the dispersibility of the phosphorus-containing compound particles and the fluidity of the nonaqueous electrolyte solution.

[0088] (Technology 10) A non-aqueous electrolyte secondary battery comprising: a positive electrode; a negative electrode; and the non-aqueous electrolyte solution according to any one of Technologies 1 to 9.

[0089] According to the tenth technique, a nonaqueous electrolyte secondary battery with improved cycle characteristics can be provided.

[0090] (Technology 11) The nonaqueous electrolyte secondary battery according to Technology 10, wherein the negative electrode contains metallic lithium as a negative electrode active material. The technology of the present disclosure is particularly useful for nonaqueous electrolyte secondary batteries containing metallic lithium as a negative electrode active material.

[0091] (Reference Example 1) (Preparation of Nonaqueous Electrolyte) Ethylene carbonate (EC), fluoroethylene carbonate (FEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) were mixed in a volume ratio of EC:FEC:EMC:DMC = 10:10:5:75 to prepare a nonaqueous solvent. LiPF6 was dissolved in the obtained nonaqueous solvent to a concentration of 1.0 mol / L to obtain a solution. 15 g of the obtained solution, an appropriate amount of vinylene carbonate (VC), 1.2 g of Li3PO4 particles (average particle diameter 10 μm) as particles of a phosphorus-containing compound, and 70 g of ZrO2 balls (average particle diameter 0.5 mm) as mixing media were placed in a ball mill and mixed at 300 rpm for 2 hours. The average particle diameter of the Li3PO4 particles after mixing was 50 nm. The supernatant of the resulting mixture was sucked up with a dropper to remove the ZrO2 balls, yielding a nonaqueous electrolyte solution of Reference Example 1. The nonaqueous electrolyte solution of Reference Example 1 contained 4 volume % of Li3PO4 particles dispersed therein. The vinylene carbonate content in the nonaqueous electrolyte solution of Reference Example 1 was 1.5 mass %. The nonaqueous electrolyte solution of Reference Example 1 had fluidity at 25°C.

[0092] Example 1 A non-aqueous electrolyte solution of Example 1 was obtained in the same manner as in Reference Example 1, except that LiPF and LiFOB were used in combination as the electrolyte salts. In Example 1, LiPF was dissolved in a non-aqueous solvent at a concentration of 0.99 mol / L, and LiFOB was dissolved in a non-aqueous solvent at a concentration of 0.01 mol / L.

[0093] Examples 2 to 6 Non-aqueous electrolyte solutions of Examples 2 to 6 were obtained in the same manner as in Example 1, except that the concentrations of LiPF 6 and LiFOB were changed to the values ​​shown in Table 1.

[0094] Comparative Examples 1 to 7 Non-aqueous electrolyte solutions of Comparative Examples 1 to 7 were obtained in the same manner as in Reference Example 1 and Examples 1 to 6, except that they did not contain Li3PO4 particles.

[0095] The viscosity of the nonaqueous electrolytes of the examples and comparative examples was measured at 25°C. An Anton Paar "MCR92" was used for viscosity measurement. A cone-type probe was used to obtain viscoelasticity data while varying the angular velocity under conditions of a measurement gap of 0.21 mm and 25°C (stage temperature). The viscosity values ​​were measured at an angular velocity of 1000 rad / sec. The results are shown in Table 1.

[0096] [Preparation of Test Cells] Test cells using the nonaqueous electrolyte solutions of Comparative Examples 1 to 7, Reference Example 1, and Examples 1 to 6 were prepared according to the following procedure.

[0097] LiNi 0.6 Co 0.2 Mn 0.2 A positive electrode slurry was prepared by adding N-methyl-2-pyrrolidone (NMP) to a positive electrode active material having a composition of O, acetylene black (AB), and polyvinylidene fluoride (PVDF). The mass ratio of these materials in the positive electrode active material layer was positive electrode active material:AB:PVDF=96:2:2.

[0098] The positive electrode slurry was applied to the surface of an aluminum foil (1.45 cm x 1.45 cm), the coating was dried, and then rolled to form a positive electrode active material layer. In this way, a positive electrode was obtained.

[0099] An appropriate amount of water was added to the negative electrode mixture and mixed to obtain a negative electrode slurry. The negative electrode mixture was a mixture of a negative electrode active material, a binder, and a conductive agent. Graphite (average particle size (D50) 25 μm) was used as the negative electrode active material. Sodium polyacrylate (PAA-Na), a sodium salt of CMC (CMC-Na), and styrene butadiene rubber (SBR) were used as the binder. The contents of PAA-Na, CMC-Na, and SBR in the negative electrode mixture were each 1% by mass. Next, the negative electrode slurry was applied to the surface of copper foil, the coating film was dried, and then rolled to form a negative electrode mixture layer (thickness 80 μm, density 1.6 g / cm 3 ) was formed to obtain a negative electrode.

[0100] A cell for evaluation was fabricated using the positive electrode, the negative electrode, the separator, and the non-aqueous electrolyte. A polyethylene separator (#2320, manufactured by Celgard) was used as the separator.

[0101] As a result of the above, evaluation cells of Comparative Examples 1 to 7, Reference Example 1, and Examples 1 to 6 were obtained.

[0102] [Cycle Test] A cycle test was performed on each evaluation cell according to the following procedure. Each evaluation cell was charged at a constant current of 0.2 C at an ambient temperature of 45°C until the voltage reached 4.3 V. Thereafter, a constant current discharge was performed at a current of 0.2 C until the voltage reached 2.5 V. The above charge / discharge cycle was counted as one cycle, and this cycle was repeated 50 times to evaluate the capacity retention rate. The results are shown in Table 1. In Table 1, the capacity retention rate represents the ratio of the discharge capacity at the 50th cycle to the discharge capacity at the first cycle.

[0103] In the cycle test, the amount of gas generated from the evaluation cell was also measured using volume measurement by Archimedes' method. Specifically, the weight of the evaluation cell in air and the weight of the evaluation cell in water at 25°C were measured, and the volume of the evaluation cell was calculated from the difference in weight. The same operation was performed before and after the cycle test, and the change in the volume of the evaluation cell was considered to be the gas amount. Using the following equations 1 and 2, the amount of gas (unit: mL) derived from LiFOB in Example 1 was calculated from the measured gas amount. Similarly, the amount of gas (unit: mL) derived from LiFOB in Examples 2 to 6 was calculated. The results are shown in Table 1.

[0104] <Formula 1> (Gas amount in Reference Example 1) - (Gas amount in Comparative Example 1) = Amount of gas G derived from Li3PO4 <Formula 2> (Gas amount in Example 1) - (Gas amount in Comparative Example 2) - G = Amount of gas derived from LiFOB in Example 1

[0105]

[0106] In Table 1, Example 1 is compared to Comparative Example 2, which has the same LiFOB ratio as Example 1. Similarly, Examples 2 to 6 are compared to Comparative Examples 3 to 7, respectively. As shown in Table 1, the capacity retention rates at 50 cycles of the evaluation cells of Examples 1 to 6 exceeded those of the corresponding Comparative Examples 2 to 7.

[0107] As shown in Table 1, the viscosity of the nonaqueous electrolyte solution of Reference Example 1 was higher than the viscosities of the nonaqueous electrolyte solutions of Comparative Examples 1 to 7. On the other hand, the evaluation cell of Reference Example 1 exhibited a higher capacity retention rate than the capacity retention rates of the evaluation cells of Comparative Examples 1 to 7.

[0108] The viscosities of the nonaqueous electrolyte solutions of Examples 1 to 6 were lower than that of the nonaqueous electrolyte solution of Reference Example 1. In other words, LiFOB exerted the effect of reducing the viscosity of the nonaqueous electrolyte solution.

[0109] In the evaluation cell of Example 3, the amount of gas derived from LiFOB was very small. In the evaluation cells of Examples 4 to 6, the amount of gas derived from LiFOB was a negative value. The negative value of the amount of gas derived from LiFOB indicates that the amount of gas generated was reduced by the addition of LiFOB. In other words, when an appropriate amount of the first lithium salt having an oxalate structure was contained, not only was the effect of improving the capacity retention rate of the evaluation cell achieved, but also the effect of reducing the amount of gas generated.

[0110] According to Examples 1 to 6, the amount of the first lithium salt suitable for obtaining the effects of improving cycle characteristics and suppressing gas generation while achieving the object of appropriately adjusting the viscosity of the nonaqueous electrolyte solution is considered to be as follows: That is, the first lithium salt may be dissolved in the nonaqueous solvent at a concentration of more than 0.03 mol / L and less than 0.25 mol / L, more preferably 0.04 mol / L or more and 0.15 mol / L or less.

[0111] The technology of the present disclosure is useful for, for example, lithium ion secondary batteries.

Claims

1. A non-aqueous electrolyte solution comprising: a non-aqueous solvent; a phosphorus-containing compound; and two or more electrolyte salts dissolved in the non-aqueous solvent, wherein the two or more electrolyte salts include a first lithium salt having an oxalate structure and a second lithium salt.

2. The nonaqueous electrolyte according to claim 1, wherein the first lithium salt comprises at least one selected from the group consisting of LiFOB, LiBOB, LiTFOP, and LiDOBFP.

3. The non-aqueous electrolyte according to claim 1, wherein the first lithium salt is dissolved in the non-aqueous solvent at a concentration of 0.005 mol / liter or more and 0.70 mol / liter or less.

4. The nonaqueous electrolyte according to claim 1, wherein the second lithium salt comprises at least one selected from the group consisting of LiPF6, LiBF4, LiClO4, LiFSI, LiTFSI, LiN(SO2C2F5)2, LiAsF6, and LiCF3SO3.

5. The nonaqueous electrolyte solution according to claim 1, wherein the proportion of the first lithium salt in the electrolyte salt is equal to or smaller than the proportion of the second lithium salt in the electrolyte salt, on a mass of substance basis.

6. The nonaqueous electrolyte according to claim 1, wherein the phosphorus-containing compound comprises at least one selected from the group consisting of Li3PO4, Na3PO4, LiPO3, LiPO3F, metaphosphate, calcium phosphate, and P2O5.

7. The nonaqueous electrolyte of claim 1, wherein the phosphorus-containing compound comprises Li3PO4.

8. The non-aqueous electrolyte according to claim 1, wherein the content of the phosphorus-containing compound in the non-aqueous electrolyte is 0.1% by volume or more and 10% by volume or less.

9. The non-aqueous electrolyte according to claim 1, wherein the phosphorus-containing compound is insoluble in the non-aqueous solvent, particles of the phosphorus-containing compound are contained in the non-aqueous electrolyte, and the average particle size of the particles is 1 nm or more and 500 nm or less.

10. A non-aqueous electrolyte secondary battery comprising: a positive electrode; a negative electrode; and the non-aqueous electrolyte solution according to claim 1.

11. The nonaqueous electrolyte secondary battery according to claim 10, wherein the negative electrode contains metallic lithium as a negative electrode active material.

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