Nonaqueous electrolyte solution and nonaqueous electrolyte secondary battery

Abstract

A nonaqueous electrolyte solution according to the present disclosure comprises: a nonaqueous solvent; an electrolyte dissolved in the nonaqueous solvent; and an organic boronic acid. The concentration of the organic boronic acid in the nonaqueous electrolyte solution is 0.02 mass % or more and 0.4 mass % or less. The organic boronic acid may contain a cyclic, linear or branched alkyl group in which at least one hydrogen atom may be substituted with a fluorine atom.

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] One of the technical issues with non-aqueous electrolyte secondary batteries is the generation of gas during charge and discharge. The present disclosure provides a non-aqueous electrolyte solution that can suppress the generation of gas during charge and discharge of the battery.

[0005] The present disclosure provides a non-aqueous electrolyte solution comprising: a non-aqueous solvent; an electrolyte dissolved in the non-aqueous solvent; and an organoboronic acid, wherein a concentration of the organoboronic acid in the non-aqueous electrolyte solution is 0.02 mass % or more and 0.4 mass % or less.

[0006] The nonaqueous electrolyte solution of the present disclosure can suppress gas generation that occurs during charging and discharging of a battery.

[0007] FIG. 1 is a schematic cross-sectional view showing an example of a nonaqueous electrolyte secondary battery according to the second embodiment.

[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 an organic boronic acid. The concentration of the organic boronic acid in the nonaqueous electrolyte solution is 0.02 mass% or more and 0.4 mass% or less. Use of the nonaqueous electrolyte solution in Embodiment 1 in a battery can suppress gas generation associated with charging and discharging of the battery.

[0010] The mechanism by which the nonaqueous electrolyte in embodiment 1 exhibits the effect of suppressing gas generation is not entirely clear, but the following mechanism is presumed. When a battery using the nonaqueous electrolyte in embodiment 1 is operated, the organic boronic acid contained in the nonaqueous electrolyte forms a coating containing boron on the surface of the positive electrode. This can suppress decomposition of the nonaqueous solvent at the interface between the nonaqueous electrolyte and the positive electrode. The presence of the coating containing boron on the surface of the positive electrode can be confirmed by analyzing the components of the surface of the positive electrode after charging and discharging the battery. A method for analyzing the components of the surface of the positive electrode is, for example, ICP (Inductively Coupled Plasma) analysis.

[0011] Here, the concentration of the organic boronic acid in the non-aqueous electrolyte, particularly the upper limit of the concentration, is important. If the concentration of the organic boronic acid in the non-aqueous electrolyte is less than 0.02% by mass, the effect of suppressing gas generation cannot be sufficiently achieved. If the concentration of the organic boronic acid in the non-aqueous electrolyte exceeds 0.4% by mass, discoloration of the non-aqueous electrolyte occurs, impairing the functionality of the non-aqueous electrolyte. This discoloration is presumably due to a significant reaction between the organic boronic acid and the electrolyte.

[0012] The organic boronic acid is preferably dissolved in a non-aqueous solvent. In this case, the organic boronic acid penetrates into the inside of the positive electrode, and the effect of suppressing gas generation is also obtained inside the positive electrode. Here, "dissolution" means that the permeability of the solution obtained when the organic boronic acid is dissolved in the non-aqueous solvent does not change from the permeability of the solvent, i.e., the solution is not cloudy and no precipitate is observed on the bottom of the container after standing for 24 hours.

[0013] 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.

[0014] In the present disclosure, "having fluidity at 25°C" means having a viscosity of 20,000 mPa·s or less at 25°C.

[0015] 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. The viscosity can be measured using a rheometer.

[0016] Organoboronic acid refers to a boronic acid that contains a boron-carbon covalent bond.

[0017] The organic boronic acid may contain a saturated hydrocarbon group in which at least one hydrogen atom may be substituted with a fluorine atom. In other words, the organic boronic acid may be an alkylboronic acid. The number of carbon atoms in the saturated hydrocarbon group is not particularly limited and may be, for example, 1 to 6. This configuration sufficiently suppresses gas generation. A saturated hydrocarbon group with a relatively small number of carbon atoms allows the organic boronic acid to be sufficiently dissolved in a non-aqueous solvent.

[0018] The saturated hydrocarbon group may contain a cyclic, linear or branched alkyl group. This configuration provides a sufficient effect of suppressing gas generation.

[0019] The organic boronic acid may contain a compound represented by the following formula (1): In formula (1), X is a methyl group in which at least one hydrogen atom may be substituted with a fluorine atom, and n is an integer from 0 to 6. This configuration satisfactorily suppresses gas generation.

[0020]

[0021] In formula (1), n ​​is preferably an integer of 0 to 4, more preferably 1 to 4, and even more preferably 1 or 2. With this configuration, the effect of suppressing gas generation can be sufficiently obtained.

[0022] The organic boronic acid may contain a compound represented by the following formula (2): In formula (2), X is a methyl group in which at least one hydrogen atom may be substituted with a fluorine atom; m and n are each independently an integer from 0 to 6, and 0≦m+n≦6 is satisfied. Such a configuration satisfies the effect of suppressing gas generation.

[0023]

[0024] The organic boronic acid may contain at least one selected from the group consisting of a compound represented by formula (1) and a compound represented by formula (2).

[0025] The compound represented by formula (1) is an organic boronic acid having a linear alkyl group, such as methylboronic acid, ethylboronic acid, propylboronic acid, butylboronic acid, pentylboronic acid, and hexylboronic acid.

[0026] The compound represented by formula (2) is an organic boronic acid having a branched alkyl group. Examples of such organic boronic acids include isobutylboronic acid and isopropylboronic acid. Organic boronic acids having a branched alkyl group are also called isoalkylboronic acids.

[0027] Examples of boronic acids having a cycloalkyl group, which is a cyclic alkyl group, include cyclopentylboronic acid and cyclohexylboronic acid.

[0028] The organic boronic acids may be used alone or in combination of two or more thereof, which can satisfactorily suppress gas generation.

[0029] 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.

[0030] The concentration of the lithium salt in the nonaqueous electrolyte may be, for example, 0.6 mol / L or more and 2.0 mol / L or less. By controlling the lithium salt concentration within the above range, an electrolyte having excellent ionic conductivity and appropriate viscosity can be obtained. However, the lithium salt concentration is not limited to the above.

[0031] The electrolyte may contain LiPF. The concentration of LiPF in the non-aqueous electrolyte may be 0.6 mol / L or more and 1.5 mol / L or less. LiPF has advantages such as being easily soluble in non-aqueous solvents, having high electrochemical stability, and exhibiting high lithium ion conductivity. Although there is concern about a side reaction between the organoboronic acid used in the non-aqueous electrolyte of the present disclosure and LiPF, the side reaction can be suppressed by limiting the concentration of the organoboronic acid.

[0032] 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.

[0033] Examples of the cyclic carbonate include propylene carbonate (PC) and ethylene carbonate (EC).

[0034] Examples of the chain carbonate ester include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC).

[0035] Examples of the cyclic carboxylic acid ester include γ-butyrolactone (GBL) and γ-valerolactone (GVL).

[0036] 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.

[0037] 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.

[0038] The non-aqueous electrolyte solution in embodiment 1 is obtained by mixing a non-aqueous solvent, an electrolyte, and an organic boronic acid, which may be prepared using a mixing device such as a ball mill or an ultrasonic homogenizer.

[0039] (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.

[0040] 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.

[0041] Each component of the secondary battery 100 will be specifically described below.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] Li α Ni x1 M1 x2 M2 (1-x1-x2) O 2+β ...(I)

[0047] The positive electrode active material layer 5b may contain other materials such as a conductive additive and a binder.

[0048] 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.

[0049] 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.

[0050] 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.

[0051] 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.

[0052] 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.

[0053] 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.

[0054] 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.

[0055] 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.

[0056] 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.

[0057] The electrode group 4 may be wound into a cylindrical shape or an oval shape.

[0058] 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.

[0059] 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.

[0060] (Other Embodiments) (Additional Notes) The above description of the embodiments discloses the following techniques.

[0061] (Technology 1) A non-aqueous electrolyte solution comprising: a non-aqueous solvent; an electrolyte dissolved in the non-aqueous solvent; and an organic boronic acid, wherein the concentration of the organic boronic acid in the non-aqueous electrolyte solution is 0.02% by mass or more and 0.4% by mass or less.

[0062] The nonaqueous electrolyte solution of the present disclosure can suppress gas generation that occurs during charging and discharging of a battery.

[0063] (Technology 2) The nonaqueous electrolyte solution according to Technology 1, wherein the organic boronic acid contains a saturated hydrocarbon group in which at least one hydrogen atom may be substituted with a fluorine atom. With this configuration, the effect of suppressing gas generation can be sufficiently obtained.

[0064] (Technology 3) The nonaqueous electrolyte solution according to Technology 2, wherein the saturated hydrocarbon group includes a cyclic, linear, or branched alkyl group. With this configuration, the effect of suppressing gas generation can be sufficiently obtained.

[0065] (Technology 4) The nonaqueous electrolyte solution according to Technology 1, wherein the organic boronic acid contains a compound represented by the following formula (1), wherein in formula (1), X is a methyl group in which at least one hydrogen atom may be substituted with a fluorine atom, and n is an integer of 0 to 6. With this configuration, the effect of suppressing gas generation can be sufficiently obtained.

[0066] (Technology 5) The nonaqueous electrolyte solution according to Technology 4, wherein in the formula (1), n ​​is 1 to 4. With this configuration, the effect of suppressing gas generation can be sufficiently obtained.

[0067] (Technology 6) The nonaqueous electrolyte solution according to Technology 1, wherein the organic boronic acid contains a compound represented by the following formula (2), wherein in formula (2), X is a methyl group in which at least one hydrogen atom may be substituted with a fluorine atom, R is a methyl group or an ethyl group, and m and n are each independently an integer of 0 to 6, and 0≦m+n≦6 is satisfied. With this configuration, the effect of suppressing gas generation can be sufficiently obtained.

[0068] (Technology 7) The nonaqueous electrolyte solution according to Technology 1 or 2, wherein the organic boronic acid includes at least one selected from the group consisting of methylboronic acid, ethylboronic acid, propylboronic acid, butylboronic acid, pentylboronic acid, hexylboronic acid, isobutylboronic acid, and isopropylboronic acid. With this configuration, the effect of suppressing gas generation can be sufficiently obtained.

[0069] (Technology 8) The nonaqueous electrolyte solution according to any one of Techniques 1 to 7, wherein the electrolyte contains LiPF6, and the concentration of LiPF6 in the nonaqueous electrolyte solution is 0.6 mol / L or more and 1.5 mol / L or less. Although there is a concern about a side reaction between the organoboronic acid used in the nonaqueous electrolyte solution of the present disclosure and LiPF6, the side reaction can be suppressed by limiting the concentration of the organoboronic acid.

[0070] (Technology 9) A nonaqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and the nonaqueous electrolyte solution according to any one of Technologies 1 to 8. The nonaqueous electrolyte solution of the present disclosure can suppress gas generation during charging and discharging of the battery.

[0071] Example 1 (Preparation of Non-Aqueous Electrolyte) Ethylene carbonate (EC), fluoroethylene carbonate (FEC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) were mixed in a volume ratio of EC:FEC:DMC:EMC = 10:10:75:5 to prepare a non-aqueous solvent. LiPF was dissolved in the obtained non-aqueous solvent to a concentration of 1.3 mol / L to obtain a solution. Propylboronic acid as an additive was dissolved in the solution to obtain a concentration of 0.03 mass% of propylboronic acid in the non-aqueous electrolyte, thereby obtaining the non-aqueous electrolyte of Example 1.

[0072] Example 2 A non-aqueous electrolyte solution of Example 2 was obtained in the same manner as in Example 1, except that the concentration of propylboronic acid in the non-aqueous electrolyte solution was changed to 0.05% by mass.

[0073] Example 3 A non-aqueous electrolyte solution of Example 3 was obtained in the same manner as in Example 1, except that the concentration of propylboronic acid in the non-aqueous electrolyte solution was changed to 0.1 mass %.

[0074] Example 4 A non-aqueous electrolyte solution of Example 4 was obtained in the same manner as in Example 1, except that the concentration of propylboronic acid in the non-aqueous electrolyte solution was changed to 0.2 mass %.

[0075] Comparative Example 1 A non-aqueous electrolyte solution of Comparative Example 1 was obtained in the same manner as in Example 1, except that propylboronic acid was not used.

[0076] Comparative Example 2 A nonaqueous electrolyte solution of Comparative Example 2 was obtained in the same manner as in Example 1, except that the concentration of propylboronic acid in the nonaqueous electrolyte solution was changed to 1 mass %.

[0077] Comparative Example 3 A nonaqueous electrolyte solution of Comparative Example 3 was obtained in the same manner as in Example 1, except that the concentration of propylboronic acid in the nonaqueous electrolyte solution was changed to 2 mass %.

[0078] [Preparation of Evaluation Cells] Evaluation cells using the nonaqueous electrolyte solutions of Examples 1 to 4 and Comparative Examples 1 to 3 were prepared according to the following procedure.

[0079] LiNi 0.8 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.

[0080] The positive electrode slurry was applied to the surface of an aluminum foil (1.5 cm x 1.5 cm), the applied film was dried, and then rolled to form a positive electrode active material layer. In this way, a positive electrode was obtained.

[0081] Laminate half cells were fabricated using a positive electrode, a Li metal foil (2 cm × 2 cm, 200 μm thick) as a counter electrode, a separator, and the nonaqueous electrolyte solutions of Examples 1 to 4 and Comparative Examples 1 to 3. A polyethylene separator (#2320, manufactured by Celgard) was used as the separator.

[0082] As a result of the above, evaluation cells of Examples 1 to 4 and Comparative Examples 1 to 3 were obtained.

[0083] [Measurement of the amount of gas generated during charge and discharge] The evaluation cells of Examples 1 to 4 and Comparative Examples 1 to 3 were charged and discharged according to the following procedure, and the amount of gas generated during charge and discharge was measured. Charging and discharging were performed in an ambient atmosphere at 55°C. The evaluation cells were charged to 4.3 V (fully charged state) and then discharged at a constant current to 2.5 V (fully discharged state). The current value at which the discharge time was 1 hour was defined as "1 C."

[0084] (First cycle) Constant current charging was performed at a current value of 0.2 C until the voltage reached 4.3 V, and constant voltage charging was performed at a voltage of 4.3 V until the current value reached 0.02 C. After a rest period of 20 minutes, constant current discharging was performed at a current value of 0.2 C until the voltage reached 2.5 V. After the rest period of 20 minutes, the initial volume of the evaluation cell was measured by the Archimedes method in an ambient atmosphere at 20°C.

[0085] (2nd to 25th cycles) Constant current charging was performed at a current value of 0.3 C until the voltage reached 4.3 V, followed by constant voltage charging at a voltage of 4.3 V until the current value reached 0.02 C. Subsequently, constant current discharging was performed at a current value of 0.5 C until the voltage reached 2.5 V. A 20-minute rest period was provided between charging and discharging. This charge / discharge process was repeated 24 times, for a total of 25 charge / discharge cycles. The volume of the evaluation cell was then measured using the Archimedes method under an ambient atmosphere at 20°C. The amount of gas generated at the 25th cycle was calculated by subtracting the initial volume from the volume of the evaluation cell at the 25th cycle. The results are shown in Table 1.

[0086] (26th to 50th cycles) Next, the charge-discharge cycle was repeated 25 more times using the same method as the charge-discharge cycles from the 2nd to 25th cycles. The volume of the evaluation cell was then measured by Archimedes' method in an ambient atmosphere at 20°C. The amount of gas generated at the 50th cycle was calculated by subtracting the initial volume from the volume of the evaluation cell at the 50th cycle. The results are shown in Table 1.

[0087]

[0088] In Table 1, the items "Gas generation amount (%) at 25 cycles" and "Gas generation amount (%) at 50 cycles" represent values ​​relative to those of Comparative Example 1.

[0089] The "unmeasurable" result for Comparative Example 2 was due to a significant increase in the resistance of the evaluation cell, which resulted in the charge-discharge treatment being discontinued before 50 charge-discharge cycles were completed. For the same reason, the charge-discharge treatment for the evaluation cell of Comparative Example 3 was terminated before 25 charge-discharge cycles were completed. The nonaqueous electrolytes of Comparative Examples 2 and 3 were discolored at the time of preparation. The discoloration of the nonaqueous electrolyte is believed to be due to a significant reaction between LiPF and propylboronic acid. As a result, the batteries using the nonaqueous electrolytes of Comparative Examples 2 and 3 exhibited high resistance values ​​and were unable to continue charging and discharging.

[0090] As shown in Table 1, the gas generation amounts of the evaluation cells of Examples 1 to 4 at both the 25th and 50th cycles were significantly lower than the gas generation amount of the evaluation cell of Comparative Example 1. As shown by the results of Example 1, the effect of suppressing gas generation was obtained even when the concentration of propylboronic acid in the nonaqueous electrolyte was relatively low.

[0091] As can be seen from a comparison of the amount of gas generated at 25 cycles and the amount of gas generated at 50 cycles, there is a tendency for more gas to be generated with fewer cycles. According to each example, the amount of gas generated with fewer cycles was significantly reduced.

[0092] On the other hand, as shown by the results of Comparative Example 2 at the 25th cycle, when the concentration of propylboronic acid in the non-aqueous electrolyte reached 1 mass %, the amount of gas generated increased.

[0093] From these results, it is desirable to adjust the concentration of the organic boronic acid in the non-aqueous electrolyte solution to a range of 0.02 mass % or more and 0.4 mass % or less.

[0094] The effect of suppressing gas generation was significant in Examples 3 and 4. From these results, it can be seen that the concentration of the organoboronic acid is preferably 0.075% by mass or more and 0.30% by mass or less, and more preferably 0.075% by mass or more and 0.25% by mass or less.

[0095] In parallel with the measurement of the amount of gas generated, the discharge capacity of each evaluation cell was also measured. The discharge capacities of the evaluation cells of Examples 1 to 4 and Comparative Example 1 at the 25th cycle were roughly equal, with no significant difference observed. On the other hand, the discharge capacity of the evaluation cell of Comparative Example 2 was significantly lower than that of the evaluation cell of Comparative Example 1. The discharge capacity of the evaluation cell of Comparative Example 2 at the end of the 25-cycle charge-discharge treatment was 52% of the value of Comparative Example 1.

[0096] (Examples 11 to 18) (Preparation of Nonaqueous Electrolyte) The nonaqueous electrolytes of Examples 11 to 17 were obtained in the same manner as Example 1, except that an organic boronic acid listed in Table 2 was used and the concentration of the organic boronic acid in the nonaqueous electrolyte was changed to 0.2 mass%. The nonaqueous electrolyte of Example 13 was the same as the nonaqueous electrolyte of Example 4. The nonaqueous electrolyte of Example 18 was obtained in the same manner as Example 1, except that isopropylboronic acid was used as the organic boronic acid and the concentration of isopropylboronic acid in the nonaqueous electrolyte was changed to 0.1 mass%.

[0097] [Preparation of Evaluation Cells] Using the nonaqueous electrolyte solutions of Examples 11 to 18 and Comparative Example 1, evaluation cells of the Examples and Comparative Examples were prepared by the method described above.

[0098] [Measurement of the amount of gas generated during charge and discharge] The amount of gas generated during charge and discharge was measured in the same manner as described above, except that 35 charge and discharge cycles were performed. The results are shown in Table 2.

[0099]

[0100] In Table 2, the item "Gas generation amount (%) at 35 cycles" indicates a relative value to that of Comparative Example 1.

[0101] As shown in Table 2, at the 35th cycle, the gas generation rates of the evaluation cells of Examples 11 to 18 were significantly lower than the gas generation rate of the evaluation cell of Comparative Example 1. In other words, the effect of suppressing gas generation was obtained regardless of the type of alkylboronic acid.

[0102] Among Examples 11 to 18, the results of Examples 12 to 15 were particularly excellent. From these results, it is preferable to use at least one selected from the group consisting of ethylboronic acid, propylboronic acid, butylboronic acid, and pentylboronic acid among organic boronic acids. In the above-described formula (1), n ​​is preferably 1 to 4.

[0103] In parallel with the measurement of the amount of gas generated, the discharge capacity of each evaluation cell was also measured. The discharge capacities of the evaluation cells of Examples 11 to 18 and Comparative Example 1 at the time of 35 cycles were roughly equal, and no significant difference was observed.

[0104] (ICP Analysis) After two charge / discharge cycles, the evaluation cells of Comparative Example 1 and Example 4 were disassembled, and the positive electrodes after charge / discharge were removed. The positive electrodes after charge / discharge were immersed in a container filled with dimethyl carbonate for 30 seconds or more and stirred to remove the nonaqueous electrolyte adhering to the surface of the positive electrode. The boron atomic concentration in the positive electrode was then measured using an ICP analyzer (iCAP7400 Duo, manufactured by Thermo Fisher Scientific).

[0105] The boron atom concentration of the positive electrode taken from the evaluation cell of Comparative Example 1 was below the detection limit (0.002% by mass) of the ICP analyzer. In contrast, the boron atom concentration of the positive electrode taken from the evaluation cell of Example 4 was 0.005% by mass. This result indicates that a coating containing boron was formed on the positive electrode due to the organic boronic acid.

[0106] The technology of the present disclosure is useful for, for example, lithium ion secondary batteries.

Claims

1. A non-aqueous electrolyte comprising: a non-aqueous solvent; an electrolyte dissolved in the non-aqueous solvent; and an organic boronic acid, wherein the concentration of the organic boronic acid in the non-aqueous electrolyte is 0.02 mass % or more and 0.4 mass % or less.

2. The non-aqueous electrolyte solution according to claim 1, wherein the organic boronic acid contains a saturated hydrocarbon group in which at least one hydrogen atom may be substituted with a fluorine atom.

3. The nonaqueous electrolyte according to claim 2, wherein the saturated hydrocarbon group includes a cyclic, linear, or branched alkyl group.

4. The organic boronic acid includes a compound represented by the following formula (1):

2. The nonaqueous electrolyte solution according to claim 1, wherein in formula (1), X is a methyl group in which at least one hydrogen atom may be substituted with a fluorine atom, and n is an integer of 0 to 6.

5. The nonaqueous electrolyte according to claim 4, wherein in formula (1), n ​​is 1 to 4.

6. The organic boronic acid includes a compound represented by the following formula (2): wherein, in formula (2), X is a methyl group in which at least one hydrogen atom may be substituted with a fluorine atom; R is a methyl group or an ethyl group; m and n are each independently an integer of 0 to 6, and 0≦m+n≦6 is satisfied.

7. The nonaqueous electrolyte solution according to claim 1, wherein the organic boronic acid comprises at least one selected from the group consisting of methylboronic acid, ethylboronic acid, propylboronic acid, butylboronic acid, pentylboronic acid, hexylboronic acid, isobutylboronic acid, and isopropylboronic acid.

8. The non-aqueous electrolyte according to claim 1, wherein the electrolyte contains LiPF6, and the concentration of LiPF6 in the non-aqueous electrolyte is 0.6 mol / L or more and 1.5 mol / L or less.

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

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