Nonaqueous electrolytic solution and nonaqueous electrolyte secondary battery

Abstract

A nonaqueous electrolytic solution according to the present disclosure comprises a nonaqueous solvent, an electrolyte dissolved in the nonaqueous solvent, and particles that are insoluble in the nonaqueous solvent, the particles containing an alkali metal salt represented by formula (1), wherein M is an alkali metal and n is an integer of 1 or more. A secondary battery 100 according to the present disclosure comprises a positive electrode 5, a negative electrode 6, and the nonaqueous electrolytic solution according to the present disclosure.　(1) CnH2n+1COOM

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] Patent Document 1 discloses that a non-aqueous electrolyte secondary battery containing vinylene carbonate as a non-aqueous solvent has good cycle characteristics. Patent Document 2 discloses an ion secondary battery that is composed of a positive electrode made of a composite lithium oxide or composite sodium oxide, a negative electrode made of a material capable of retaining lithium or sodium, a separator, and an electrolyte solution made of a non-aqueous solvent, the electrolyte solution containing fluoroethylene carbonate and phosphorus compound particles.

[0003] JP 2005-268230 A JP 2012-018801 A

[0004] Non-aqueous electrolyte secondary batteries are required to be safe even under severe conditions such as high temperatures. The present disclosure provides a non-aqueous electrolyte solution that is advantageous in terms of battery safety.

[0005] The non-aqueous electrolyte solution of the present disclosure includes a non-aqueous solvent, an electrolyte dissolved in the non-aqueous solvent, and particles insoluble in the non-aqueous solvent, wherein the particles include an alkali metal salt represented by the following formula (1): n H 2n+1 COOM (1) In formula (1), M is an alkali metal, and n is an integer of 1 or more.

[0006] According to the technology of the present disclosure, it is possible to provide a nonaqueous electrolyte solution that is advantageous in terms of battery safety.

[0007] Fig. 1 is a schematic cross-sectional view showing an example of a secondary battery according to the present embodiment. Fig. 2 is a graph showing the temperature transition of the batteries of Example 1 and Comparative Example 1 in Heating Test II.

[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 in embodiment 1 includes a nonaqueous solvent, an electrolyte dissolved in the nonaqueous solvent, and particles insoluble in the nonaqueous solvent, the particles including an alkali metal salt represented by the following formula (1): C n H 2n+1 COOM (1) In formula (1), M is an alkali metal, and n is an integer of 1 or more.

[0010] By using the nonaqueous electrolyte of embodiment 1 in a battery, thermal runaway can be suppressed even at high temperatures, for example, above 100°C. The mechanism by which the nonaqueous electrolyte of embodiment 1 achieves the above-mentioned effects is not necessarily clear, but the following mechanism is presumed. That is, particles containing the alkali metal salt represented by formula (1) form flame-retardant coatings at various locations within the battery, such as the positive electrode surface and the negative electrode surface, at high temperatures. This suppresses the exothermic reaction between the negative electrode active material and the nonaqueous electrolyte, and oxygen generation due to thermal decomposition of the positive electrode active material. As a result, thermal runaway can be suppressed. Furthermore, the battery can be made flame-retardant. As a result, the nonaqueous electrolyte of embodiment 1 can improve the safety of the battery.

[0011] In the present disclosure, "particles insoluble in non-aqueous solvents" refers to particles that require 100 mL or more of non-aqueous solvent to dissolve 1 g of particles at 25°C. That is, the solubility of the particles 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 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.

[0012] The non-aqueous electrolyte is, for example, liquid at 25°C. Liquids also include sols. The non-aqueous electrolyte may have fluidity at 25°C. In the present disclosure, "having fluidity at 25°C" means having a viscosity of 20,000 mPa·s or less at 25°C. The non-aqueous electrolyte in embodiment 1 does not always have fluidity at 25°C, and may have, for example, thixotropy. For example, "having fluidity at 25°C" also includes a case where the non-aqueous electrolyte exhibits a viscosity of more than 20,000 mPa·s at 25°C when left standing, but exhibits a viscosity of 20,000 mPa·s or less at 25°C when a force is applied, such as shearing due to stirring.

[0013] The viscosity of the non-aqueous electrolyte at 25° C. may be 5000 mPa·s or less, 3000 mPa·s or less, or 1000 mPa·s or less. The viscosity can be measured using a rheometer.

[0014] The particles may be dispersed in a non-aqueous solvent, and the non-aqueous electrolyte may be a non-aqueous colloidal solution in which the particles are dispersed. That is, the non-aqueous electrolyte in embodiment 1 is a suspension.

[0015] The particles include an alkali metal salt represented by formula (1). The alkali metal salt represented by formula (1) is an alkali metal carboxylate. That is, the nonaqueous electrolyte of embodiment 1 includes particles of an alkali metal carboxylate. In formula (1), M may be one selected from the group consisting of K, Na, Rb, and Cs, or one selected from the group consisting of K, Na, and Rb, or may be K or Na, or may be K.

[0016] Hereinafter, in this disclosure, the term "alkali metal salt" refers to the alkali metal salt represented by formula (1).

[0017] In the above formula (1), 1≦n≦18 may be satisfied, 1≦n≦15 may be satisfied, 1≦n≦10 may be satisfied, 1≦n≦6 may be satisfied, or 1≦n≦3 may be satisfied. With such a configuration, the dispersibility of the particles can be improved, and the above-mentioned effects can be sufficiently obtained. n=1 may also be satisfied.

[0018] The alkali metal salt may be an alkali metal acetate. That is, the nonaqueous electrolyte of embodiment 1 may contain particles of an alkali metal acetate. The alkali metal salt may be at least one selected from the group consisting of CH3COONa, CH3COOK, and CH3COORb, or at least one selected from the group consisting of CH3COONa and CH3COOK, or may be CH3COOK.

[0019] The particles may contain one type of alkali metal salt or two or more types of alkali metal salts.

[0020] The particles may contain an alkali metal salt as a main component. Here, the term "main component" refers to the component that is contained in the largest amount by mass. The content of the alkali metal salt in the particles may be 50% by mass or more, 60% by mass or more, 70% by mass or more, 80% by mass or more, 90% by mass or more, 95% by mass or more, or even 99% by mass or more. The particles may not intentionally contain any substance other than the alkali metal salt represented by formula (1) above. The particles may consist only of an alkali metal salt. Note that when the particles contain two or more types of alkali metal salts, the content of the alkali metal salts refers to the total content of the alkali metal salts.

[0021] The particle content in the non-aqueous electrolyte may be 0.1% by volume or more and 50% by volume or less, 0.1% by volume or more and 20% by volume or less, 0.1% by volume or more and 10% by volume or less, 0.5% by volume or more and 10% by volume or less, 1% by volume or more and 10% by volume or less, 2% by volume or more and 10% by volume or less, or 4% by volume or more and 10% by volume or less. The above configuration can improve the dispersibility of the particles in the non-aqueous electrolyte and the fluidity of the non-aqueous electrolyte.

[0022] The particle content in the nonaqueous electrolyte can be determined, for example, by the following method. After measuring the volume of the nonaqueous electrolyte in the secondary battery, the nonaqueous electrolyte 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 particle volume is calculated from the specific gravity determined from the particle mass and particle components. The particle components can be determined 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 particle content in the nonaqueous electrolyte can be calculated. The volume of the nonaqueous electrolyte can also be calculated from the composition and mass. The composition of the nonaqueous electrolyte can be measured using liquid chromatography, gas chromatography, etc.

[0023] The particles may be nanoparticles.

[0024] The average particle diameter of the particles may be 1 nm or more and 500 nm or less, 1 nm or more and 300 nm or less, 1 nm or more and 150 nm or less, or 1 nm or more and 90 nm or less. This configuration increases the surface area of ​​the particles in the nonaqueous electrolyte, thereby fully achieving the above-mentioned effects of the particles. Furthermore, the dispersibility of the particles in the nonaqueous electrolyte is improved, thereby enabling increased industrial productivity of the nonaqueous electrolyte and batteries including the same. Furthermore, in a battery using the nonaqueous electrolyte of embodiment 1, when the nonaqueous electrolyte permeates the active material layer, the particles may permeate between the active material particles arranged inside the active material layer. As a result, a coating is formed on the surface of the active material arranged inside the active material layer, thereby suppressing exothermic reactions inside the active material layer. The average particle diameter of the particles may be 5 nm or more and 90 nm or less, or 10 nm or more and 90 nm or less.

[0025] The average particle size of the particles may be equal to or smaller than the pore size of the separator. With this configuration, the particles do not clog the pores of the separator, and therefore circulation of the nonaqueous electrolyte inside the electrode group is not hindered even during charge and discharge.

[0026] 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.

[0027] The shape of the particles is not particularly limited, and may be needle-like, scale-like, spherical, or oval-spherical.

[0028] The electrolyte may include, for example, a lithium salt. Examples of the 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, LiCF3SO3, and lithium difluoro(oxalato)borate. The lithium salt may be at least one selected from the above-mentioned substances. The lithium salt may contain fluorine (F). The lithium salt may be LiPF6.

[0029] The concentration of the lithium salt in the non-aqueous electrolyte may be, for example, 0.5 mol / L or more and 2 mol / L or less. By controlling the lithium salt concentration within the above range, a non-aqueous electrolyte having excellent ionic conductivity and appropriate viscosity can be obtained. However, the lithium salt concentration is not limited to the above.

[0030] 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.

[0031] Examples of the cyclic carbonate include propylene carbonate (PC) and ethylene carbonate (EC).

[0032] Examples of the chain carbonate ester include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC).

[0033] Examples of the cyclic carboxylic acid ester include γ-butyrolactone (GBL) and γ-valerolactone (GVL).

[0034] 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.

[0035] 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 particle dispersibility. 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, particle aggregation over time and the resulting particle sedimentation can be reduced.

[0036] 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.

[0037] The nonaqueous electrolyte solution in the first embodiment can be produced, for example, by the following method.

[0038] A lithium salt is dissolved in a non-aqueous solvent. The resulting solution, the alkali metal salt particles, and ZrO balls as a mixing medium are mixed in a ball mill. The mixing medium is removed from the resulting mixture to obtain the non-aqueous electrolyte solution of embodiment 1.

[0039] The method for producing the non-aqueous electrolyte is not limited to the above, and for example, particles may be dispersed in a non-aqueous solvent in which a lithium salt has been dissolved, using an ultrasonic homogenizer.

[0040] (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, reactions of the negative electrode active material and the positive electrode active material are inhibited, particularly at high temperatures, and heat generation and oxygen generation are suppressed. This makes it possible to suppress thermal runaway even when the battery is exposed to high temperatures, for example, above 100°C. Furthermore, the battery can be made flame-retardant. Furthermore, the spread of fire can be prevented. As described above, the nonaqueous electrolyte secondary battery according to Embodiment 2 is excellent in safety.

[0041] 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.

[0042] Each component of the secondary battery 100 will be specifically described below.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] The lithium nickel oxide may be represented by the following composition formula (2): Element M2 is at least one selected from the group consisting of V, Co, and Mn. Element M3 is at least one selected from the group consisting of Mg, Al, Ca, Ti, Cu, Zn, and Nb. Composition formula (2) 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.

[0047] Li α Ni x1 M2 x2 M3 (1-x1-x2) O 2+β ... (2)

[0048] The positive electrode active material layer 5b may contain other materials such as a conductive additive and a binder.

[0049] 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.

[0050] 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.

[0051] 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.

[0052] 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 carbon materials and materials capable of forming an alloy with lithium. Examples of carbon materials 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.

[0053] 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.

[0054] 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.

[0055] 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.

[0056] The average particle size of the particles contained in the electrolyte may be equal to or smaller than the pore size of the separator 7 .

[0057] 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. Making the separator 7 from these materials 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. In the secondary battery disclosed herein, combustion of the separator 7 is suppressed even at high temperatures, thereby improving safety.

[0058] 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.

[0059] The electrode group 4 may be wound into a cylindrical shape or an oval shape.

[0060] 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.

[0061] The application of the nonaqueous electrolyte solution of the present disclosure is not limited to the secondary battery 100. In addition to lithium secondary batteries, the nonaqueous electrolyte solution of the present disclosure can be applied to various secondary batteries such as sodium secondary batteries and magnesium secondary batteries.

[0062] (Other Embodiments) (Additional Notes) The above description of the embodiments discloses the following techniques.

[0063] (Technology 1) A non-aqueous solvent, an electrolyte dissolved in the non-aqueous solvent, and particles insoluble in the non-aqueous solvent, wherein the particles contain an alkali metal salt represented by the following formula (1): C n H 2n+1 COOM (1) In formula (1), M is an alkali metal, and n is an integer of 1 or more.

[0064] The nonaqueous electrolyte solution of the present disclosure can suppress thermal runaway of the battery at high temperatures, thereby improving the safety of the battery.

[0065] (Technology 2) The nonaqueous electrolyte according to Technology 1, wherein the content of the particles in the nonaqueous electrolyte is 0.1% by volume or more and 10% by volume or less. With this configuration, the dispersibility of the particles and the fluidity of the nonaqueous electrolyte can be improved.

[0066] (Technology 3) The nonaqueous electrolyte according to Technology 1 or 2, wherein the alkali metal salt is at least one selected from the group consisting of CH3COONa, CH3COOK, and CH3COORb. With this configuration, the above-described effects of the particles can be sufficiently obtained.

[0067] (Technology 4) The nonaqueous electrolyte solution according to any one of Technologies 1 to 3, wherein the particles have an average particle size of 1 nm or more and 90 nm or less. This configuration allows the above-described effects of the particles to be fully achieved. Furthermore, the dispersibility of the particles in the nonaqueous electrolyte solution is improved, enabling industrial productivity to be increased.

[0068] (Technology 5) 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 4.

[0069] The nonaqueous electrolyte solution of the present disclosure is suitable for nonaqueous electrolyte secondary batteries, and this structure makes it possible to obtain nonaqueous electrolyte secondary batteries with excellent safety.

[0070] The present disclosure will be described in more detail below using examples. The following examples are merely examples of embodiments, and are not intended to limit the scope of the present disclosure.

[0071] Example 1 Ethylene carbonate (EC), fluoroethylene carbonate (FEC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) were used as non-aqueous solvents. EC, FEC, DMC, and EMC were mixed in a volume ratio of EC:FEC:DMC:EMC = 10:10:75:5 to prepare a non-aqueous solvent.

[0072] LiPF6 was dissolved in the resulting non-aqueous solvent to a concentration of 1.35 mol / L to obtain a solution. The resulting solution, potassium acetate particles (average particle diameter 90 μm), and ZrO2 balls (particle diameter 5 mm) as mixing media were placed in a planetary ball mill container. The amount of potassium acetate particles was adjusted so that the content in the non-aqueous electrolyte was 6% by volume. First, as a pretreatment, the mixture was mixed at 300 rpm for 30 minutes. Next, the mixing media was changed to ZrO2 balls (particle diameter 0.5 mm), and the container was mixed at 300 rpm for 3 hours. The supernatant of the resulting mixture was removed with a dropper to obtain the non-aqueous electrolyte of Example 1. Potassium acetate particles with an average particle diameter of 45 nm were dispersed in the non-aqueous electrolyte of Example 1.

[0073] Example 2 A nonaqueous electrolyte solution of Example 2 was obtained in the same manner as in Example 1, except that sodium acetate particles (average particle size: 140 μm) were used instead of the potassium acetate particles. The average particle size of the sodium acetate particles in the nonaqueous electrolyte solution of Example 2 was 60 nm.

[0074] Example 3 A non-aqueous electrolyte solution of Example 3 was obtained in the same manner as in Example 1, except that rubidium acetate particles (average particle size: 100 μm) were used instead of potassium acetate particles. The average particle size of the rubidium acetate particles in the non-aqueous electrolyte solution of Example 3 was 40 nm.

[0075] Comparative Example 1 A nonaqueous electrolyte solution of Comparative Example 1 was prepared in the same manner as in Example 1, except that particles were not dispersed in the nonaqueous solvent. That is, the nonaqueous electrolyte solution of Comparative Example 1 was a solution obtained by dissolving LiPF at a concentration of 1.35 mol / L in a nonaqueous solvent in which EC, FEC, DMC, and EMC were mixed in a volume ratio of EC:FEC:DMC:EMC=10:10:75:5.

[0076] [Heating Test I] The safety of the non-aqueous electrolyte solutions was evaluated by carrying out Heating Test I as follows: Cellulose nonwoven fabrics onto which the non-aqueous electrolyte solutions of Examples 1 to 3 and Comparative Example 1 had been dropped were heated on a hot plate at 180°C for 30 seconds.

[0077] The extent of combustion of the portion of the cellulose nonwoven fabric onto which the nonaqueous electrolyte solution had been dropped was confirmed. The results are shown in Table 1. Regarding the safety evaluation in Table 1, a mark of ◎ indicates that almost no carbonization was observed, a mark of ○ indicates that there were some blackened carbonized areas but the degree of carbonization was reduced compared to Comparative Example 1, and a mark of × indicates that the entire portion onto which the nonaqueous electrolyte solution had been dropped was blackened. Furthermore, for the cellulose nonwoven fabrics onto which the nonaqueous electrolyte solutions of Examples 1 to 3 had been dropped, a coating was observed on the surface of the cellulose nonwoven fabric after Heating Test I. No coating was observed on the cellulose nonwoven fabric onto which the nonaqueous electrolyte solution of Comparative Example 1 had been dropped.

[0078]

[0079] In the cellulose nonwoven fabric to which the nonaqueous electrolyte solution of Comparative Example 1 was dropped, the entire area where the nonaqueous electrolyte solution was dropped was blackened and carbonized due to combustion. In the cellulose nonwoven fabric to which the nonaqueous electrolyte solution of Example 1 was dropped, almost no blackened and carbonized areas were observed. In Examples 2 and 3, although there were some blackened and carbonized areas compared to Example 1, most of the areas of the cellulose nonwoven fabric to which the nonaqueous electrolyte solution was dropped were brown. In other words, the degree of carbonization in Examples 2 and 3 was less than that of Comparative Example 1.

[0080] It is presumed that the nonaqueous electrolyte solutions of Examples 1 to 3 formed a flame-retardant film on the cellulose nonwoven fabric when heated to a high temperature, thereby preventing the supply of oxygen and preventing combustion. From the above, the nonaqueous electrolyte solution of the present disclosure can suppress combustion reactions at high temperatures in materials in contact with the nonaqueous electrolyte solution, and is therefore excellent in safety.

[0081] <Fabrication of Batteries> Batteries using the nonaqueous electrolyte solutions of Example 1 and Comparative Example 1 were fabricated according to the following procedure.

[0082] LiNi 0.5 Co 0.2 Mn 0.3 A positive electrode slurry was prepared by mixing a positive electrode active material (NCM) having a composition of O, acetylene black (AB), and polyvinylidene fluoride (PVDF) with N-methyl-2-pyrrolidone (NMP). The mass ratio of these materials was NCM:AB:PVDF = 96:2:2.

[0083] The positive electrode slurry was applied to both sides of an aluminum foil (2.5 cm x 15 cm), the coating film was dried, and then rolled to form a positive electrode active material layer. In this way, a positive electrode was obtained.

[0084] Cylindrical batteries were fabricated using a positive electrode, a graphite electrode as a negative electrode, a separator, and the nonaqueous electrolyte solutions of Example 1 and Comparative Example 1. A polyethylene microporous film (manufactured by Asahi Kasei, film thickness 16 μm) was used as the separator.

[0085] In this manner, the batteries of Example 1 and Comparative Example 1 were fabricated.

[0086] [Battery Characteristics] The initial characteristics of the batteries of Example 1 and Comparative Example 1 were evaluated according to the following procedure: The evaluation of the initial characteristics was carried out at an ambient temperature of 25°C.

[0087] The battery was charged at a constant current of 0.1 C until the voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the current reached 0.02 C. The battery was then discharged at a constant current of 0.1 C until the voltage reached 2.5 V. The initial battery characteristics were measured in this manner. The results are shown in Table 2.

[0088] [Heating Test II] Heating test II was carried out on the batteries of Example 1 and Comparative Example 1 according to the following procedure.

[0089] The battery, charged to 4.2 V, was placed on a hot plate set at a plate temperature of 290°C and heated. The battery temperature was observed using a thermocouple attached to the top surface of the battery (the part not in contact with the hot plate). Figure 2 is a graph showing the temperature transitions of the batteries of Example 1 and Comparative Example 1 during Heating Test II. Table 2 shows the maximum temperature reached by the batteries during Heating Test II. Under these severe conditions, the battery of Comparative Example 1 experienced a rapid temperature rise five minutes after the start of the test, resulting in thermal runaway. On the other hand, the battery of Example 1 did not experience thermal runaway even after being heated for one hour.

[0090]

[0091] (Discussion) As can be seen from Table 2, the battery of Example 1 had initial characteristics equivalent to those of the battery of Comparative Example 1. Furthermore, the battery of Example 1 did not experience thermal runaway even under the harsh conditions of being heated to extremely high temperatures, and the maximum temperature reached was limited to 175°C. It is presumed that thermal runaway did not occur because the flame-retardant coating formed on the surface of the positive electrode by potassium acetate in the non-aqueous electrolyte prevented contact between the high-temperature non-aqueous electrolyte and the negative electrode active material, thereby suppressing the exothermic reaction of the negative electrode active material and the gas generation associated with this reaction, and furthermore, the coating formed on the surface of the negative electrode suppressed the reaction between the gas and the positive electrode active material and the oxygen generation associated with this reaction.

[0092] As described above, the nonaqueous electrolyte solution of the present disclosure can improve the safety of the battery at high temperatures while maintaining the battery characteristics.

[0093] The technology of the present disclosure is useful for, for example, lithium ion secondary batteries.

Claims

1. A method for producing a liquid electrolyte comprising: a non-aqueous solvent; an electrolyte dissolved in the non-aqueous solvent; and particles insoluble in the non-aqueous solvent, wherein the particles contain an alkali metal salt represented by the following formula (1): n H 2n+1 COOM (1) In formula (1), M is an alkali metal, and n is an integer of 1 or more.

2. The non-aqueous electrolyte according to claim 1, wherein the content of said particles in said non-aqueous electrolyte is 0.1% by volume or more and 10% by volume or less.

3. The non-aqueous electrolyte according to claim 1, wherein the alkali metal salt is at least one selected from the group consisting of CH3COONa, CH3COOK, and CH3COORb.

4. The nonaqueous electrolyte according to claim 1, wherein the particles have an average particle size of 1 nm or more and 90 nm or less.

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

Images