Nonaqueous electrolytic solution and nonaqueous electrolyte secondary battery

Abstract

A nonaqueous electrolytic solution according to the present disclosure contains a nonaqueous solvent, a phosphorus-containing compound, and two or more types of electrolyte salts dissolved in the nonaqueous solvent. The two or more types of electrolyte salts include LiBF4 as a first lithium salt and a second lithium salt having a composition different from LiBF4. A nonaqueous electrolyte secondary battery 100 according to the present disclosure comprises a positive electrode 5, a negative electrode 6, and the nonaqueous electrolytic solution.

Description

Non-aqueous electrolytes and non-aqueous electrolyte secondary batteries

[0001] This disclosure relates to a non-aqueous electrolyte and a non-aqueous electrolyte secondary battery.

[0002] As is known to those skilled in the art, the electrolytes of conventional non-aqueous electrolyte secondary batteries contain various components depending on the purpose. For example, Patent Document 1 discloses an electrolyte containing vinylene carbonate as a non-aqueous solvent for the purpose of obtaining good cycle characteristics of a non-aqueous electrolyte secondary battery.

[0003] Japanese Patent Publication No. 2005-268230

[0004] Since various components are added to the electrolyte depending on the purpose, an increase in viscosity can sometimes be a problem.

[0005] This disclosure provides a non-aqueous electrolyte capable of reducing the increase in viscosity.

[0006] The non-aqueous electrolyte of this disclosure comprises 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 LiBF4 as a first lithium salt and a second lithium salt having a different composition from LiBF4.

[0007] According to this disclosure, it is possible to provide a non-aqueous electrolyte that can reduce the increase in viscosity.

[0008] Figure 1 is a schematic cross-sectional view showing an example of a non-aqueous electrolyte secondary battery in Embodiment 2.

[0009] The embodiments of this disclosure will be described below with reference to the drawings. This disclosure is not limited to the embodiments described below.

[0010] (Embodiment 1) The non-aqueous electrolyte in Embodiment 1 comprises 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 LiBF4 as a first lithium salt and a second lithium salt having a different composition from LiBF4.

[0011] Phosphorus-containing compounds are added to non-aqueous electrolytes, for example, to improve the battery's cycle characteristics. Specifically, when a battery using a non-aqueous electrolyte containing phosphorus-containing compounds is operated, the phosphorus-containing compounds in the non-aqueous electrolyte form a film on the surface of the positive electrode. This can suppress the oxidative decomposition of the solvent at the interface between the non-aqueous electrolyte and the positive electrode. Furthermore, even if this film is damaged over time and through repeated charging and discharging, the phosphorus-containing compounds in the non-aqueous electrolyte can repair the film. As a result, a non-aqueous electrolyte containing phosphorus-containing compounds can provide a longer-lasting effect of suppressing solvent oxidative decomposition than, for example, a battery assembled with a film directly formed on the positive electrode.

[0012] However, phosphorus-containing compounds tend to increase the viscosity of non-aqueous electrolytes. From the viewpoint of lithium-ion conductivity and low-temperature properties, a low viscosity of the non-aqueous electrolyte is generally desirable.

[0013] The non-aqueous electrolyte in Embodiment 1 contains two or more electrolyte salts. The two or more electrolyte salts include lithium tetrafluoroborate (LiBF4) as a first lithium salt and a second lithium salt having a different composition from LiBF4. LiBF4 can reduce the increase in viscosity of the non-aqueous electrolyte caused by phosphorus-containing compounds. Furthermore, by simultaneously including a second lithium salt having a different composition from LiBF4 as an electrolyte in addition to LiBF4, the non-aqueous electrolyte in Embodiment 1 can maintain the effect of improving cycle characteristics due to phosphorus-containing compounds. Therefore, the non-aqueous electrolyte in Embodiment 1 can reduce the increase in viscosity of the non-aqueous electrolyte while maintaining the effect of improving cycle characteristics due to phosphorus-containing compounds. In other words, the non-aqueous electrolyte in Embodiment 1 can reduce the increase in viscosity of the non-aqueous electrolyte without impairing the desired properties due to the added components.

[0014] Furthermore, phosphorus-containing compounds tend to increase the amount of gas generated in non-aqueous electrolytes. In Embodiment 1, the non-aqueous electrolyte can reduce the amount of gas generated by including LiBF4 as the electrolyte.

[0015] The first lithium salt, LiBF4, is dissolved in a non-aqueous solvent at a concentration of, for example, 0.005 mol / liter or more and 0.70 mol / liter or less. In other words, the concentration of the first lithium salt in the portion of the non-aqueous electrolyte from which the phosphorus-containing compound has been removed is, for example, 0.005 mol / liter or more and 0.75 mol / liter or less. By appropriately adjusting the concentration of LiBF4, it is possible to obtain the effects based on LiBF4 while maintaining the performance required for the non-aqueous electrolyte. LiBF4 may also be dissolved in a non-aqueous solvent at a concentration preferably greater than 0.03 mol / liter and less than 0.25 mol / liter, and more preferably 0.04 mol / liter or more and 0.15 mol / liter or less. The "portion of the non-aqueous electrolyte from which the phosphorus-containing compound has been removed" may be the liquid phase portion of the non-aqueous electrolyte.

[0016] Examples of lithium dihydrotestosterone salts include lithium hexafluoride phosphate (LiPF6), lithium perchlorate (LiClO4), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), bisperfluoroethylsulfonylimide lithium (LiN(SO2C2F5)2), LiAsF6, and LiCF3SO3. At least one of the above-mentioned substances can be used as the lithium dihydrotestosterone salt. The lithium dihydrotestosterone salt may also contain fluorine (F).

[0017] The second lithium salt may contain LiPF6. LiPF6 is recommended as the second lithium salt because it has excellent properties. On the other hand, non-aqueous electrolytes using a combination of phosphorus-containing compounds and LiPF6 tend to have high viscosity. However, the non-aqueous electrolyte in Embodiment 1 can reduce viscosity by suppressing the increase in viscosity by containing LiBF4 as the first lithium salt.

[0018] The concentration of the electrolyte salt in the non-aqueous electrolyte may be, for example, 0.5 mol / liter or more and 2 mol / liter or less. By controlling the concentration of the electrolyte salt within the above range, a non-aqueous electrolyte with 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 lithium triphosphate (i.e., LiBF4) in the electrolyte salt may be 1 mol% or more and 20 mol% or less. With this configuration, it is possible to reduce the increase in viscosity of the non-aqueous electrolyte while maintaining or enhancing the effect of improving the cycle characteristics by the phosphorus-containing compound.

[0020] The non-aqueous electrolyte in Embodiment 1 is, for example, liquid at 25°C. This liquid state may also include a sol. The non-aqueous electrolyte in Embodiment 1 may be fluid at 25°C.

[0021] In this disclosure, "having fluidity at 25°C" means having a viscosity of 20,000 mPa·s or less at 25°C.

[0022] The viscosity of the non-aqueous electrolyte in Embodiment 1 at 25°C may be 5000 mPa·s or less, 3000 mPa·s or less, or 1000 mPa·s or less.

[0023] The phosphorus-containing compound may be insoluble in a non-aqueous solvent. The non-aqueous electrolyte in Embodiment 1 may contain particles of the phosphorus-containing compound that are insoluble in a 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.

[0024] In this disclosure, "phosphorus-containing compound particles insoluble in non-aqueous solvents" means phosphorus-containing compound particles that require 100 mL or more of non-aqueous solvent to dissolve 1 g at 25°C. That is, the solubility of phosphorus-containing compound particles in 100 mL of non-aqueous solvent is 1 g or less. Here, dissolution means that when phosphorus-containing compound particles are dissolved in a non-aqueous solvent in a container, the permeability of the solution obtained does not change from the permeability of the solvent, i.e., the solution is not cloudy, and no precipitate is observed at the bottom of the container after standing for 24 hours.

[0025] The phosphorus-containing compound particles may be phosphorus-containing compound particles having a solubility of 0.1 g or less per 100 mL.

[0026] The phosphorus-containing compound may be, for example, a compound containing a phosphorus atom and an oxygen atom.

[0027] Phosphorus-containing compounds are, for example, inorganic phosphorus compounds. Phosphorus-containing compounds may contain phosphoric acid or phosphorus compounds. Phosphorus compounds are, for example, inorganic phosphates. Phosphoric acid may contain phosphorous acid, and phosphorus compounds may contain phosphates. It is desirable that phosphorus-containing compounds contain a PO4 skeleton. Phosphorus-containing compounds may also contain organophosphorus compounds. Organophosphorus compounds are, for example, phosphoric acid esters containing a PO4 skeleton.

[0028] The phosphorus-containing compound may contain at least one selected from the group consisting of Li3PO4, Na3PO4, K3PO4, LiPO3, LiPO3F, metaphosphoric acid, P2O5, and calcium phosphate. Calcium phosphate is, for example, hydroxyapatite. The phosphorus-containing compound may contain at least one selected from the group consisting of Li3PO4, Na3PO4, K3PO4, LiPO3, LiPO3F, metaphosphoric acid, and P2O5. The phosphorus-containing compound may contain at least one selected from the group consisting of Li3PO4, Na3PO4, LiPO3, LiPO3F, metaphosphoric acid, and P2O5.

[0029] The phosphorus-containing compound may also contain Li3PO4. That is, the non-aqueous electrolyte in Embodiment 1 may contain Li3PO4 particles. With the above configuration, the battery's cycle characteristics 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 positive divalent oxidation state in phosphorus-containing compounds. Therefore, when magnesium, strontium, and barium are included in the phosphorus-containing compound, the Coulomb forces between these elements and phosphorus are strengthened, stabilizing the phosphorus-containing compound.

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

[0032] The phosphorus-containing compound is M x P2O y (1 ≦ x ≦ 3, 5 ≦ y ≦ 10) may be included. M contains at least one selected from the group consisting of magnesium, strontium, and barium. According to such a compound, an effect of improving the output characteristics of the battery can be obtained. 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. The phosphate is a salt of an alkaline earth metal ion and a phosphate ion (PO4 2- ). The pyrophosphate is a salt of an alkaline earth metal ion and a pyrophosphate ion (P2O7 4- ).

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

[0035] The phosphorus-containing compound may contain at least one selected from the group consisting of Sr3(PO4)2, Ba3(PO4)2, and Ba2P2O7. According to such a compound, an effect of improving the output characteristics of the battery can be sufficiently obtained.

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

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

[0038] The phosphorus-containing compound may be a compound that does not contain water of crystallization. By not containing water of crystallization, it is possible to avoid deterioration of the battery performance due to moisture.

[0039] In the non-aqueous electrolyte in Embodiment 1, the content ratio of the particles of the phosphorus-containing compound may be 0.1% by volume or more and 10% by volume or less. The content ratio of the particles of the phosphorus-containing compound may be 0.1% by volume or more and 8% by volume or less, may be 0.5% by volume or more and 6% by volume or less, may be 1% by volume or more and 5% by volume or less, or may be 1% by volume or more and 4% by volume or less. With the above configuration, the dispersibility of the particles of the phosphorus-containing compound and the fluidity of the non-aqueous electrolyte can be improved.

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

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

[0042] The average particle size of the phosphorus-containing compound particles may be 1 nm or more and 500 nm or less. With the above configuration, the dispersibility of the phosphorus-containing compound particles in the non-aqueous electrolyte is improved, and the industrial productivity of the non-aqueous electrolyte can be increased. The average particle size 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 size of the phosphorus-containing compound particles is 500 nm or less, in a battery using the non-aqueous electrolyte of this disclosure, when the non-aqueous electrolyte penetrates the positive electrode active material layer, the phosphorus-containing compound particles can penetrate between the positive electrode active material particles arranged inside the positive electrode active material layer. Therefore, a film of phosphorus-containing compound is also formed on the surface of the positive electrode active material arranged inside the positive electrode active material layer, so that the oxidative decomposition of the solvent inside the positive electrode active material layer can be suppressed.

[0043] The average particle size of the phosphorus-containing compound may be less than or equal to the pore size of the separator in a battery using a non-aqueous electrolyte. With this configuration, the phosphorus-containing compound particles do not clog the pores of the separator, and therefore the circulation of the electrolyte within the electrode group is not obstructed even during charging and discharging.

[0044] In this disclosure, the average particle size of the phosphorus-containing compound particles can be determined by dynamic light scattering (DLS) spectroscopy.

[0045] The non-aqueous solvent is not particularly limited, and examples include cyclic carbonate esters, linear carbonate esters, and cyclic carboxylic acid esters.

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

[0047] Examples of linear carbonate esters include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC).

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

[0049] The non-aqueous solvent may be used alone or in combination of two or more types. The non-aqueous solvent may also contain ethylene carbonate. This can increase the solubility of electrolytes such as lithium salts in the non-aqueous solvent.

[0050] The non-aqueous electrolyte in Embodiment 1 may further contain other substances not mentioned above. For example, the non-aqueous electrolyte in Embodiment 1 may further contain additives to improve the dispersibility of phosphorus-containing compound particles. The additive is, for example, a fluorine-containing solvent. That is, the non-aqueous electrolyte in Embodiment 1 may further contain a fluorine-containing solvent. With the above configuration, aggregation of phosphorus-containing compound particles over time and the resulting sedimentation of particles can be reduced.

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

[0052] The non-aqueous electrolyte in Embodiment 1 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, the phosphorus-containing compound mentioned above, and ZrO2 balls as a mixing medium are mixed using a ball mill. By removing the mixing medium from the resulting mixture, the non-aqueous electrolyte of Embodiment 1 is obtained.

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

[0055] (Embodiment 2) The non-aqueous electrolyte secondary battery in Embodiment 2 comprises a positive electrode, a negative electrode, and a non-aqueous electrolyte according to Embodiment 1. By using the non-aqueous electrolyte in Embodiment 1, a secondary battery with reduced viscosity of the non-aqueous electrolyte can be obtained. When the viscosity of the non-aqueous electrolyte is high, various problems arise in the battery manufacturing process, such as the need for shearing during the injection process of the non-aqueous electrolyte. However, the non-aqueous electrolyte secondary battery in Embodiment 2 can solve these manufacturing problems.

[0056] The non-aqueous electrolyte secondary battery in Embodiment 2, by using the non-aqueous electrolyte in Embodiment 1, can also achieve effects such as improved cycle characteristics and reduced gas generation.

[0057] Figure 1 is a schematic cross-sectional view showing an example of a non-aqueous electrolyte secondary battery in Embodiment 2. The secondary battery 100 comprises a container 1, an electrode group 4, and an electrolyte (not shown). The electrolyte is the non-aqueous electrolyte in Embodiment 1. The electrode group 4 has a wound structure. The electrode group 4 is housed in the container 1. The electrode group 4 has a positive electrode 5, a negative electrode 6, and a pair of separators 7. The electrode group 4 is impregnated with the electrolyte. The opening of the container 1 is sealed with a sealing plate 2. The positive electrode 5 has 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 arranged around the sealing plate 2. The negative electrode 6 has a negative electrode current collector 6a and a negative electrode active material layer 6b. One end of a 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. Insulating rings 8 are positioned on the upper and lower surfaces of the electrode group 4, respectively.

[0058] The components of the secondary battery 100 will be described in detail below.

[0059] As the positive electrode current collector 5a, a sheet or film made of a metallic material such as aluminum, stainless steel, titanium, or their alloys can be used. Aluminum and its alloys are suitable materials for the positive electrode current collector 5a because they are inexpensive and easy to make into thin films. The sheet or film may be porous or non-porous. Metal foil, metal mesh, etc., can be used as the sheet or film. A carbon material such as carbon may be coated on the surface of the positive electrode current collector 5a as a conductive auxiliary material.

[0060] The positive electrode active material layer 5b contains a positive electrode active material. The positive electrode active material may be a material that has the ability to intercept and release lithium ions. As the positive electrode active material, lithium-containing transition metal oxides, lithium-containing transition metal phosphates, transition metal fluorides, polyanionic materials, fluorinated polyanionic materials, transition metal sulfides, transition metal oxysulfides, transition metal oxynitrides, etc., can be used. In particular, when lithium-containing transition metal oxides or lithium-containing transition metal phosphates are used as the positive electrode active material, the manufacturing cost of the battery can be reduced and the average discharge voltage can be increased. 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.

[0061] The positive electrode active material may include lithium nickel oxide having a layered rock salt-type crystalline structure. The proportion of Ni among the metal elements other than Li in the lithium nickel oxide may be 50 atomic percent or more. Other transition metals may be included in the lithium nickel oxide. Lithium nickel oxide is useful for achieving a high operating voltage.

[0062] Lithium nickel oxide may be represented by the following compositional 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. The compositional 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.

[0063] Li α Ni x1 M1 x2 M2 (1-x1-x2) O 2+β ... (I)

[0064] The positive electrode active material layer 5b may contain other materials such as a conductive assistant and a binder.

[0065] The conductive assistant is used to reduce the resistance of the positive electrode 5. Examples of the conductive assistant include carbon materials and conductive polymer compounds. Examples of the carbon materials include carbon black, graphite, acetylene black, carbon nanotubes, carbon nanofibers, graphene, fullerenes, and oxidized graphite. Examples of the conductive polymer compounds include polyaniline, polypyrrole, and polythiophene.

[0066] The binder is used to improve the binding property of the materials constituting the positive electrode 5. Examples of the binder include 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.

[0067] As the negative electrode current collector 6a, a sheet or film made of a metallic material such as stainless steel, nickel, copper, or alloys thereof may be used. The sheet or film may be porous or non-porous. As the sheet or film, metal foil, metal mesh, etc., may be used. A carbon material such as carbon may be coated on the surface of the negative electrode current collector 6a as a conductive auxiliary material.

[0068] The negative electrode active material layer 6b contains a negative electrode active material. The negative electrode active material may be a material having the ability to intercept and release lithium ions. The negative electrode active material includes, for example, at least one selected from the group consisting of metallic lithium, carbon materials, and materials capable of forming alloys with lithium. Examples of carbon materials include graphite. Examples of materials capable of forming alloys with lithium include silicon, silicon-containing oxides, tin, zinc alloys, bismuth, and germanium. One of these negative electrode active materials may be used, or two or more may be used in combination.

[0069] 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. Alternatively, only graphite may be included in the negative electrode active material layer 6b. Graphite is recommended because it does not degrade easily even when repeatedly charged and discharged at great depths. Other carbon materials may be used as the negative electrode active material. Silicon exhibits a larger capacity than graphite, which is advantageous for increasing the capacity of the secondary battery 100.

[0070] The negative electrode 6 (negative electrode active material layer 6b) may contain metallic lithium as the negative electrode active material.

[0071] The negative electrode active material layer 6b may contain other materials such as conductive additives and binders. Materials that can be used in the positive electrode active material layer 5b can also be used in the negative electrode active material layer 6b as conductive additives and binders.

[0072] The electrolyte is the non-aqueous electrolyte in Embodiment 1. The electrolyte is impregnated into the positive electrode 5, the negative electrode 6, and the separator 7. The electrolyte may also fill the internal space of the container 1. Due to the action of the electrolyte, lithium ions can move between the positive electrode 5 and the negative electrode 6.

[0073] The average particle size of the phosphorus-containing compound particles in the electrolyte may be less than or equal to the pore size of the separator 7.

[0074] The separator 7 is lithium ion conductive. The material of the separator 7 is not particularly limited as long as the passage of lithium ions is permitted. The material of the separator 7 may be at least one selected from the group consisting of gel electrolytes, ion exchange resin membranes, semipermeable membranes, and porous membranes. If the separator 7 is made of these materials, the safety of the secondary battery 100 can be sufficiently ensured. 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 resin and porous membranes containing glass paper obtained by weaving glass fibers into a nonwoven fabric. By using the non-aqueous electrolyte in Embodiment 1 in the secondary battery 100, oxidation of the separator 7 is suppressed and the decrease in the strength of the separator 7 can be reduced.

[0075] Container 1 is, for example, a metal container such as aluminum or stainless steel. Container 1 may have a cylindrical shape or a rectangular tube shape.

[0076] The electrode group 4 may be wound in a cylindrical shape or in an elliptical shape.

[0077] The shape of the secondary battery 100 is not particularly limited. In this disclosure, as an example of the structure of a non-aqueous electrolyte secondary battery according to Embodiment 2, the configuration example shown in Figure 1, namely a secondary battery in which an electrode group in which a positive electrode and a negative electrode are wound around a separator, and an electrolyte are housed in an outer casing, is described. However, the secondary battery according to this disclosure is not limited to this configuration example. The secondary battery according to this disclosure may take any form, such as cylindrical, prismatic, coin-shaped, button-shaped, laminated, etc. Furthermore, as the electrode group in the secondary battery according to this disclosure, other forms of electrode groups may be used instead of wound electrode groups, such as laminated electrode groups in which a positive electrode and a negative electrode are stacked around a separator.

[0078] (Other Embodiments) (Note) The above description of embodiments discloses the following technologies.

[0079] (Technology 1) A non-aqueous electrolyte 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 LiBF4 as a first lithium salt and a second lithium salt having a different composition from LiBF4.

[0080] According to Technology 1, it is possible to provide a non-aqueous electrolyte that can reduce the increase in viscosity.

[0081] (Technical 2) The non-aqueous electrolyte according to Technical 1, wherein the first lithium salt is dissolved in the non-aqueous solvent at a concentration of 0.005 mol / liter or more and 0.75 mol / liter or less.

[0082] According to Technology 2, by appropriately adjusting the concentration of the first lithium salt within the above range, it is possible to obtain the effects based on the first lithium salt while maintaining the performance required for a non-aqueous electrolyte.

[0083] (Technical 3) The non-aqueous electrolyte according to Technical 1 or 2, wherein the second lithium salt comprises at least one selected from the group consisting of LiPF6, LiClO4, LiFSI, LiTFSI, LiN(SO2C2F5)2, LiAsF6, and LiCF3SO3.

[0084] The lithium salt described above is recommended as a secondary lithium salt because it possesses excellent properties. Therefore, according to Technology 3, for example, the cycle characteristics of the battery can be improved.

[0085] (Technical 4) The non-aqueous electrolyte according to any one of Technical 1 to 3, wherein the proportion of the first lithium salt in the electrolyte salt is 1 mol% or more and 20 mol% or less.

[0086] According to Technology 4, by appropriately adjusting the proportion of the first lithium salt in the electrolyte salt within the above range, it is possible to reduce the increase in viscosity of the non-aqueous electrolyte while maintaining or enhancing the effect of the phosphorus-containing compound on improving cycle characteristics.

[0087] (Technical 5) The non-aqueous electrolyte according to any one of Technical 1 to 4, wherein the phosphorus-containing compound comprises at least one selected from the group consisting of Li3PO4, Na3PO4, K3PO4, LiPO3, LiPO3F, metaphosphoric acid, P2O5, and calcium phosphate.

[0088] With the above configuration, the battery's cycle characteristics can be improved.

[0089] (Technical 6) The phosphorus-containing compound is M x P2O y A non-aqueous electrolyte according to any one of the Art 1 to 5, wherein M comprises at least one selected from the group consisting of magnesium, strontium, and barium, x satisfies 1 ≤ x ≤ 3, and y satisfies 5 ≤ ​​y ≤ 10.

[0090] With the above configuration, the battery's cycle characteristics can be improved.

[0091] (Technical 7) 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. The non-aqueous electrolyte according to any one of Technical 1 to 6.

[0092] With the above configuration, the battery's cycle characteristics can be improved.

[0093] (Technical 8) The non-aqueous electrolyte according to any one of Technical 1 to 7, 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.

[0094] With the above configuration, the dispersibility of phosphorus-containing compound particles and the fluidity of the non-aqueous electrolyte can be improved.

[0095] (Technical 9) A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte according to any one of Technical 1 to 8.

[0096] According to Technology 9, the viscosity of the non-aqueous electrolyte can be reduced, thus solving manufacturing problems that arise when the viscosity of the non-aqueous electrolyte is high, such as the need for shear during the injection process of the non-aqueous electrolyte.

[0097] The present disclosure will be described in more detail below with reference to examples, comparative examples, and reference examples. The following examples are merely illustrative and not limited to any one aspect.

[0098] [Preparation of Non-Aqueous Electrolyte] (Reference Example 1) 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 non-aqueous solvent. LiPF6 was dissolved in the obtained non-aqueous 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 size 10 μm) as phosphorus-containing compound particles, and 70 g of ZrO2 balls (average particle size 0.5 mm) as a mixing medium were placed in a ball mill and mixed at 300 rpm for 2 hours. The average particle size of the Li3PO4 particles after mixing was 50 nm. The supernatant of the resulting mixture was collected with a dropper to remove the ZrO2 balls, and the non-aqueous electrolyte of Reference Example 1 was obtained. In the non-aqueous electrolyte of Reference Example 1, 4% by volume of Li3PO4 particles were dispersed. The vinylene carbonate content in the non-aqueous electrolyte of Reference Example 1 was 1.5% by mass. The non-aqueous electrolyte of Reference Example 1 was fluid at 25°C.

[0099] The average particle size of Li3PO4 particles, which are phosphorus-containing compounds, was determined by the DLS method. The specific measurement method was as follows.

[0100] Measurements were performed using the HORIBA nanoparticle analyzer "nano Partica SZ-100V2". Prior to measuring Li3PO4 particles, 100 nm size polystyrene latex particles were measured at various concentrations. The detector angle was 173°, the peak fitting shape used for calculations was narrow, and it was assumed that the distribution had one peak, and the analysis was performed using monodisperse. The analysis was performed automatically by the instrument's dedicated software. For samples with a polystyrene concentration of 1 ppm or less, the result was 120 nm, and the autocorrelation function value with a single-digit delay time was approximately 0.7. The closest result to 100 nm was 103 nm at 1000 ppm; at higher concentrations, sufficient transmitted light could not be obtained for measurement, making analysis impossible. At 1000 ppm, the autocorrelation function showed a value of 0.9 or higher with a single-digit delay time. Based on the above, we established a criterion for determining that measurements are being taken appropriately in this device if the autocorrelation function is 0.9 or higher. In the measurement of Li3PO4 particles used as the phosphorus-containing compound, we also confirmed the autocorrelation function at various concentrations as described above, and confirmed that there were no problems with the sample in which 0.03 g of isolated particles were added to 10 g of water. The detector angle was 173°, the peak fitting shape used in the calculation was standard, and furthermore, the analysis was performed using polydispersity assuming that there were two or more peaks in the distribution. Through such measurements, we obtained the result that the average particle size of the Li3PO4 particles used as the phosphorus-containing compound was 50 nm.

[0101] (Example 1) The non-aqueous electrolyte of Example 1 was obtained in the same manner as in Reference Example 1, except that LiPF6 and LiBF4 were used in combination as the electrolyte salt. In Example 1, LiPF6 was dissolved in a non-aqueous solvent at a concentration of 0.9 mol / L, and LiBF4 was dissolved in a non-aqueous solvent at a concentration of 0.1 mol / L. That is, the proportion of LiBF4 in the electrolyte salt was 10 mol%.

[0102] (Example 2) The non-aqueous electrolyte of Example 1 was obtained in the same manner as in Reference Example 1, except that LiPF6 and LiBF4 were used in combination as the electrolyte salt. In Example 1, LiPF6 was dissolved in a non-aqueous solvent at a concentration of 0.75 mol / L, and LiBF4 was dissolved in a non-aqueous solvent at a concentration of 0.25 mol / L. That is, the proportion of LiBF4 in the electrolyte salt was 25 mol%.

[0103] (Example 3) The non-aqueous electrolyte of Example 1 was obtained in the same manner as in Reference Example 1, except that LiPF6 and LiBF4 were used in combination as the electrolyte salt. In Example 1, LiPF6 was dissolved in a non-aqueous solvent at a concentration of 0.5 mol / L, and LiBF4 was dissolved in a non-aqueous solvent at a concentration of 0.5 mol / L. That is, the proportion of LiBF4 in the electrolyte salt was 50 mol%.

[0104] (Reference Example 2) The non-aqueous electrolyte of Example 1 was obtained in the same manner as in Reference Example 1, except that LiBF4 was used instead of LiPF6 as the electrolyte salt.

[0105] (Comparative Examples 1 to 5) Non-aqueous electrolytes of Comparative Examples 1 to 5 were obtained in the same manner as in Reference Example 1, Examples 1 to 3, and Reference Example 2, except that they did not contain Li3PO4 particles.

[0106] The viscosity of the non-aqueous electrolytes of Examples 1 to 3, Reference Examples 1 to 2, and Comparative Examples 1 to 5 at 25°C was measured. Anton Paar's "MCR92" was used for viscosity measurement. Using a cone-type probe, viscoelastic data was acquired while varying the angular velocity under the conditions of a measurement gap of 0.21 mm and 25°C (stage temperature). For viscosity, the value at an angular velocity of 1000 rad / sec was used. The results are shown in Table 1.

[0107] [Preparation of test cells] Test cells were prepared using the non-aqueous electrolytes of Examples 1 to 3, Reference Examples 1 to 2, and Comparative Examples 1 to 5, respectively, by following the procedure below.

[0108] LiNi 0.6 Co 0.2 Mn 0.2A positive electrode slurry was prepared by stirring a positive electrode active material having an O2 composition, acetylene black (AB), and polyvinylidene fluoride (PVDF) with N-methyl-2-pyrrolidone (NMP). The mass ratio of these materials in the positive electrode active material layer was positive electrode active material:AB:PVDF = 96:2:2.

[0109] A positive electrode slurry was applied to the surface of an aluminum foil (1.45 cm x 1.45 cm), the coating film was dried, and then the foil was rolled to form a positive electrode active material layer. In this way, a positive electrode was obtained.

[0110] Cells were fabricated using graphite electrodes (1.5 cm x 1.5 cm) as the positive and negative electrodes, a separator, and a non-aqueous electrolyte. A polyethylene separator (Celgard, #2320) was used as the separator.

[0111] Based on the above, evaluation cells for Examples 1 to 3, Reference Examples 1 to 2, and Comparative Examples 1 to 5 were obtained.

[0112] [Cycle Test] A cycle test was performed on each evaluation cell according to the following procedure. Each evaluation cell was charged with a constant current of 0.2C at an ambient temperature of 45°C until the voltage reached 4.3V. Then, it was discharged with a constant current of 0.2C until the voltage reached 2.5V. The above charge and discharge was considered one cycle, and this was repeated 100 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 cycle 100 to the discharge capacity at cycle 1.

[0113] In the cycle test, the Archimedes method was used to measure volume, and the amount of gas generated from the evaluation cell after 100 cycles was also measured. 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 procedure was performed before and after the cycle test, and the change in the volume of the evaluation cell was considered as the amount of gas generated. The results are shown in Table 1.

[0114]

[0115] As shown in Table 1, when comparing the non-aqueous electrolytes of Reference Example 1, Examples 1 to 3, and Reference Example 2 with the non-aqueous electrolytes of Comparative Examples 1 to 5, which have the same electrolyte salt components, it is clear that the viscosity of the non-aqueous electrolytes containing the phosphorus-containing compound Li3PO4 increased compared to the non-aqueous electrolytes without the phosphorus-containing compound. However, the non-aqueous electrolytes of Examples 1 to 3, which contain LiBF4 and a second lithium salt (here, Li3PO4) having a different composition from LiBF4 as the electrolyte salt, showed a reduced increase in viscosity compared to the non-aqueous electrolyte without the phosphorus-containing compound, compared to the non-aqueous electrolyte of Reference Example 1, which contains only the second lithium salt and no LiBF4 as the electrolyte salt. Thus, the non-aqueous electrolytes of Examples 1 to 3, which contain a non-aqueous solvent, a phosphorus-containing compound, and an electrolyte salt, and which contain LiBF4 and a second lithium salt having a different composition from LiBF4 as the electrolyte salt, were able to reduce the increase in viscosity.

[0116] As shown in Table 1, the non-aqueous electrolytes of Examples 1 to 3 exhibited higher volume retention rates after 100 cycles compared to the corresponding non-aqueous electrolytes of Comparative Examples 2 to 4, which differed only in the presence or absence of phosphorus-containing compounds. Thus, it was confirmed that the non-aqueous electrolytes of Examples 1 to 3 can reduce the increase in viscosity of the non-aqueous electrolyte while maintaining or enhancing the effect of improving cycle characteristics due to phosphorus-containing compounds.

[0117] Furthermore, the non-aqueous electrolyte of Reference Example 2, which contains a phosphorus-containing compound and contains only LiBF4 as the electrolyte salt, was able to suppress the increase in viscosity to a level comparable to that of the non-aqueous electrolyte of Comparative Example 5, which does not contain a phosphorus-containing compound. However, the non-aqueous electrolyte of Reference Example 2 had a lower volume retention rate at 100 cycles than the non-aqueous electrolyte of Comparative Example 5, which does not contain a phosphorus-containing compound, thus actually diminishing the effect of the phosphorus-containing compound on improving cycle characteristics.

[0118] As shown in Table 1, the non-aqueous electrolytes of Examples 1 to 3 contained a phosphorus-containing compound and, compared to the non-aqueous electrolyte of Reference Example 1, contained only a second lithium salt and no LiBF4 as the electrolyte salt, the amount of gas generated was reduced. Thus, the non-aqueous electrolytes of Examples 1 to 3 also demonstrated the effect of reducing the amount of gas generated, which increases due to the phosphorus-containing compound.

[0119] The technology disclosed herein is useful, for example, in lithium-ion secondary batteries.

Claims

1. A non-aqueous electrolyte 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 LiBF4 as a first lithium salt and a second lithium salt having a different composition from LiBF4.

2. 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 0.75 mol / liter or less.

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

4. The non-aqueous electrolyte according to claim 1, wherein the proportion of the first lithium salt in the electrolyte salt is 1 mol% or more and 20 mol% or less.

5. The non-aqueous electrolyte according to claim 1, wherein the phosphorus-containing compound comprises at least one selected from the group consisting of Li3PO4, Na3PO4, K3PO4, LiPO3, LiPO3F, metaphosphoric acid, P2O5, and calcium phosphate.

6. The phosphorus-containing compound is M x P2O y The non-aqueous electrolyte according to claim 1, wherein M comprises at least one selected from the group consisting of magnesium, strontium, and barium, x satisfies 1 ≤ x ≤ 3, and y satisfies 5 ≤ ​​y ≤ 10.

7. 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.

8. 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.

9. A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte according to any one of claims 1 to 8.

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