JP7267823B2 - Non-volatile electrolyte, secondary battery - Google Patents

Description

TECHNICAL FIELD The present invention relates to a non-volatile electrolyte containing a non-aqueous electrolyte using an ester solvent and a carbonate solvent, and a secondary battery.

BACKGROUND ART Lithium ion secondary batteries are used in various applications, including power sources for mobile devices such as mobile phones and portable personal computers, drive power sources for electric vehicles and hybrid vehicles, and stationary power sources for power storage. In recent years, the use of lithium ion secondary batteries has expanded to include large-sized products, etc., and there is a demand for higher energy density, higher capacity, and longer life.

Non-aqueous electrolytes used in lithium-ion secondary batteries, etc., have general characteristics such as stability under electrochemical and temperature conditions during use, flame retardancy, and the width of the potential window.・High ionic conductivity and appropriate viscosity are required. Conventionally, carbonates have been widely used as solvents for electrolytes in lithium-ion secondary batteries, and non-aqueous electrolytes are also being improved in order to improve battery performance.

Patent Document 1 discloses the following content as a technology related to the non-aqueous electrolyte. "Contains a solvated electrolyte salt, an ester solvent that constitutes the solvated electrolyte salt and the solvated ionic liquid, and a low-viscosity solvent, and the mixing ratio of the ester solvent to the solvated electrolyte salt is 0.00 in terms of moles. 5 or more and 1.5 or less, and the mixing ratio of the low-viscosity solvent to the solvated electrolyte salt is 4 or more and 16 or less in terms of moles, semi-solid electrolyte layer, electrode, secondary battery. 1)

WO2018/179990

In Patent Document 1, a carbon-based material or the like is used as a negative electrode active material, an ether-based solvent or a low-viscosity solvent is used as a solvent for an electrolytic solution, and vinylene carbonate, fluoroethylene carbonate, or the like is used as a film-forming agent. When a film-forming agent such as vinylene carbonate is added to the electrolytic solution and electrochemically reacted, an SEI (Solid Electrolyte Interphase) film is formed on the surface of the negative electrode active material, so reductive decomposition of the electrolytic solution can be suppressed to some extent. can.

However, when a carbonate-based solvent or ether-based solvent, which is known as a general solvent, is used, there is still a possibility that reductive decomposition of the solvent will occur on the surface of the negative electrode active material. Further, when graphite or silicon is used as the negative electrode active material, carrier ions and the solvent may co-insert between the layers of the negative electrode active material. The effect of adding a film-forming agent reaches a ceiling at a certain amount, and excessive addition lowers the battery performance.

Accordingly, an object of the present invention is to provide a non-volatile electrolyte containing a non-aqueous electrolyte that improves the discharge capacity of a secondary battery, and a secondary battery using the same.

In order to solve the above problems, the present invention has, for example, the following configurations.
A non-aqueous electrolyte containing a non-aqueous solvent containing an electrolyte salt, an ester-based solvent and a carbonate-based solvent, an additive for forming a film on the surface of the negative electrode, and particles carrying the non-aqueous electrolyte A non-volatile electrolyte containing
The donor number of the ester solvent is larger than that of the carbonate solvent,
A nonvolatile electrolyte in which the molar ratio of a carbonate-based solvent to an ester-based solvent is 0.06 or more and 0.65 or less .

ADVANTAGE OF THE INVENTION According to this invention, the secondary battery using the non-volatile electrolyte containing the non-aqueous electrolyte which improves the discharge capacity of a secondary battery can be provided. Problems, configurations, and effects other than those described above will be clarified by the following description of the embodiments.

1 is a schematic cross-sectional view illustrating the configuration of a secondary battery according to an embodiment of the invention; FIG. FIG. 2 is a diagram showing the relationship between the molar ratio of a carbonate-based solvent to an ester-based solvent and the discharge capacity of a secondary battery; FIG. 3 is a diagram showing the relationship between the molar ratio of an electrolyte salt to an ester-based solvent and the discharge capacity of a secondary battery;

A nonaqueous electrolyte, a nonvolatile electrolyte, and a secondary battery using the same according to one embodiment of the present invention will be described below with reference to the drawings.

The following description provides specific examples of the subject matter of the present invention. The present invention is not limited to the following description, and various changes and modifications can be made by those skilled in the art within the scope of the technical ideas disclosed in this specification. The present invention includes various modifications different from the embodiment. The embodiments are described in detail for easy understanding of the present invention, and are not necessarily limited to those having all the described configurations. It is possible to replace part of the configuration of one embodiment with the configuration of another embodiment. Moreover, it is possible to add the configuration of another embodiment to the configuration of one embodiment. Moreover, it is possible to add, delete, or replace a part of the configuration of the embodiment with another configuration.

In this specification, "-" is used to mean that the numbers before and after it are the lower limit and the upper limit. However, when the numerical value described before and after is "0", "0" shall not be included as an upper limit value or a lower limit value. In the numerical ranges described step by step in this specification, the upper and lower limits described in one numerical range may be replaced with other upper and lower limits described step by step. good. The upper and lower limits of the numerical ranges described herein may be replaced with the numerical values shown in the examples.

In this specification, a lithium ion secondary battery will be described as an example of a secondary battery. A lithium-ion secondary battery means an electrochemical device in which lithium ions are absorbed into and released from electrodes to generate a potential difference between electrodes, thereby storing or utilizing electrical energy. Lithium-ion secondary batteries are also called by other names such as lithium-ion batteries, non-aqueous electrolyte secondary batteries, and non-aqueous electrolyte secondary batteries. be. The technical idea of the present invention can also be applied to sodium ion secondary batteries, magnesium ion secondary batteries, calcium ion secondary batteries, zinc secondary batteries, aluminum ion secondary batteries and the like.

When materials are selected and used from the group of materials exemplified below, the materials may be used singly or in combination within a range that does not contradict the content disclosed in the present specification. Materials other than those exemplified below may also be used as long as they do not contradict the content disclosed in this specification.

FIG. 1 is a schematic cross-sectional view illustrating the configuration of a secondary battery according to one embodiment of the present invention. FIG. 1 shows a laminated secondary battery as an example of a secondary battery. As shown in FIG. 1, the secondary battery 1000 includes a positive electrode 100, a negative electrode 200, a separator 300, and an outer package 500.

The outer package 500 accommodates the positive electrode 100 , the negative electrode 200 and the separator 300 . As the material of the exterior body 500, a material exhibiting corrosion resistance to the non-aqueous electrolyte, such as aluminum, stainless steel, nickel-plated steel, is used.

The positive electrode 100 , the separator 300 and the negative electrode 200 are laminated in this order to form an electrode assembly 400 . A plurality of positive electrodes 100 , separators 300 , and negative electrodes 200 are stacked and housed inside the exterior body 500 . The positive electrode 100 and the negative electrode 200 are arranged with the separator 300 interposed therebetween.

Note that FIG. 1 shows a laminated secondary battery, but the secondary battery 1000 may be a wound type. Moreover, the shape of the secondary battery 1000 may be any shape such as a cylindrical shape, a square shape, and a button shape. The exterior body 500 may be provided as a bag-like laminated container instead of the metal can.

The positive electrode 100 has a positive electrode mixture layer 110 , a positive electrode current collector 120 and a positive electrode tab 130 . In FIG. 1, positive electrode mixture layers 110 are formed on both sides of a flat plate-shaped positive electrode current collector 120 . At the end of the positive electrode current collector 120, a flat positive electrode tab 130 is provided for extracting current. The positive electrode tab 130 is pulled out from each positive electrode 100 to the outside of the outer package 500 .

The negative electrode 200 has a negative electrode mixture layer 210 , a negative electrode current collector 220 and a negative electrode tab 230 . In FIG. 1 , negative electrode mixture layers 210 are formed on both sides of a flat plate-shaped negative electrode current collector 220 . At the end of the negative electrode current collector 220, a flat plate-shaped negative electrode tab 230 for taking out current is provided. The negative electrode tab 230 is pulled out from each negative electrode 200 to the outside of the outer package 500 .

The positive electrode tab 130 and the negative electrode tab 230 are electrically connected to leads, external terminals, and the like (not shown) outside the package 500 . As a method for joining the tab, an appropriate method such as spot welding or ultrasonic joining is used. The electrode assembly 400 can be connected in parallel by joining the positive electrode tabs 130 and the negative electrode tabs 230 together. Alternatively, the electrode body 400 can form a bipolar secondary battery that is connected in series.

The positive electrode mixture layer 110 includes a positive electrode active material capable of absorbing and releasing lithium ions, a conductive agent for improving the conductivity of the positive electrode mixture layer 110, and a binder for binding the positive electrode active material and the conductive agent. and The positive electrode active material has an electrochemical activity of intercalating and deintercalating lithium ions at a potential nobler than that of the negative electrode 200, desorbing lithium ions during charging, and intercalating lithium ions during discharging.

The negative electrode mixture layer 210 contains a negative electrode active material capable of intercalating and deintercalating lithium ions. The negative electrode active material has the electrochemical activity of intercalating and deintercalating lithium ions at a potential lower than that of the positive electrode 100, and the lithium ions are desorbed during the discharging process and the lithium ions are inserted during the charging process. The negative electrode mixture layer 210 contains a conductive agent for improving the conductivity of the negative electrode mixture layer 210 and a binder for binding the negative electrode active material and the conductive agent, depending on the type of the negative electrode active material. good too.

<Positive electrode active material>
The positive electrode active material is selected from a group of materials such as, for example, LiCo-based composite oxides, LiNi-based composite oxides, LiMn-based composite oxides, LiCoNiMn-based composite oxides, LiFeP-based composite oxides, and LiMnP-based composite oxides. . Specific examples of the positive electrode active material include LiCoO ₂ , Li(Co, Mn)O ₂ , Li(Ni, Mn)O ₂ , LiMn ₂ O ₄ , Li ₄ Mn ₅ O ₁₂ , Li(Co, Ni, Mn). O ₂ , LiFePO ₄ , LiCoPO ₄ , LiNiPO ₄ , LiMnPO ₄ , LiMnVO ₄ , LiFeBO ₃ , LiMnBO ₃ , Li ₂ FeSiO ₄ , Li ₂ CoSiO ₄ , Li ₂ MnSiO ₄ and the like. As the positive electrode active material, oxides obtained by substituting these transition metals with dissimilar elements, and oxides having different stoichiometric ratios can also be used. Examples of different elements include Co, Ni, Mn, Fe, Cr, Zn, Ta, Al, Mg, Cu, Cd, Mo, Nb, W, and Ru.

Examples of the positive electrode active material include sulfur, chalcogenides such as TiS ₂ , MoS ₂ , Mo ₆ S ₈ and TiSe ₂ , vanadium oxides such as V ₂ O ₅ , halides such as FeF ₃ , Fe(MoO ₄ ) ₃ , Fe ₂ (SO ₄ ) ₃ , Li ₃ Fe ₂ (PO ₄ ) ₃ and other polyanionic materials, and organic materials such as quinone organic crystals can also be used.

<Negative electrode active material>
The negative electrode active material is selected, for example, from a group of materials such as carbon-based materials, oxide-based materials, and metal-based materials. Specific examples of carbonaceous materials include natural graphite, artificial graphite, easily graphitizable carbon materials, non-graphitizable carbon materials, amorphous carbon materials, and organic crystalline carbon materials. Specific examples of oxide-based materials include lithium titanates such as Li ₄ Ti ₅ O ₁₂ and oxides containing lithium and tin, silicon, iron, germanium, or the like. Specific examples of metallic materials include metal lithium, alloys of lithium with tin, silicon, aluminum, and the like.

<Conductive agent>
The conductive agent is selected, for example, from a group of materials such as ketjen black, acetylene black, furnace black, thermal black, graphite, and carbon fiber.

<Binder>
The binder of the positive electrode mixture layer 110 is selected from a group of materials such as polyvinylidene fluoride (PVDF), for example. The binder of the negative electrode mixture layer 210 is selected from a group of materials such as styrene-butadiene rubber, carboxymethylcellulose, and polyvinylidene fluoride, for example.

<Positive current collector, positive electrode tab>
A metal foil, a perforated foil, an expanded metal, a foamed metal plate, or the like can be used as the positive electrode current collector 120 . The positive electrode current collector 120 is selected from a group of materials such as aluminum and aluminum alloys. Stainless steel, titanium, or the like can also be used depending on the oxidation-reduction potential of the positive electrode 100 and the like. From the viewpoint of achieving both mechanical strength and energy density, the thickness of the positive electrode current collector 120 is preferably 10 nm to 1 mm, more preferably 1 to 100 μm. The positive electrode tab 130 can be made of the same material as the positive electrode current collector 120 .

<Negative electrode current collector, negative electrode tab>
A metal foil, a perforated foil, an expanded metal, a foam metal plate, or the like can be used as the negative electrode current collector 220 . The negative electrode current collector 220 is selected from a group of materials such as copper and copper alloys. Stainless steel, titanium, nickel, or the like can also be used depending on the oxidation-reduction potential of the negative electrode 200 and the like. From the viewpoint of achieving both mechanical strength and energy density, the thickness of the negative electrode current collector 220 is preferably 10 nm to 1 mm, more preferably 1 to 100 μm. The negative electrode tab 230 can be made of the same material as the negative electrode current collector 220 .

<Electrode mixture layer forming method>
The positive electrode mixture layer 110 and the negative electrode mixture layer 210 are formed by kneading an active material, a conductive agent, and a binder in a solvent to prepare an electrode mixture, coating the prepared electrode mixture on a current collector, and applying the coating. It can be formed by drying the electrode mixture. The electrode mixture layer formed on the current collector is pressure-molded by a roll press or the like so that the active material has a predetermined density. The electrode mixture layer can also be laminated on the current collector by repeating the steps from coating to drying. The electrode on which the electrode mixture layer is formed can be subjected to punching, cutting, or the like.

Kneading of the electrode mixture can be performed by various devices such as a planetary mixer, a disper mixer, a butterfly mixer, a twin-screw kneader, a ball mill, and a bead mill. Various solvents such as 1-methyl-2-pyrrolidone (NMP) and water can be used as the solvent in which the active material is dispersed, depending on the electrode. Various methods such as a roll coating method, a doctor blade method, a dipping method, and a spray method can be used as methods for applying the electrode mixture.

<Separator>
The separator 300 electrically insulates between the positive electrode 100 and the negative electrode 200 to prevent a short circuit between the positive electrode 100 and the negative electrode 200 , while acting as a medium that conducts ions between the positive electrode 100 and the negative electrode 200 . The separator 300 is an insulating microporous film having fine pores, a non-volatile electrolyte obtained by allowing particles to support a non-aqueous electrolyte, and a solid electrolyte. can be formed by The thickness of the separator 300 is preferably several nanometers to several millimeters from the viewpoint of achieving both electronic insulation and energy density.

<Microporous membrane>
Microporous membranes are made of cellulose-based resins such as cellulose, carboxymethylcellulose, and hydroxypropylcellulose, polyolefin-based resins such as polypropylene and polyethylene-polypropylene copolymer, and polyester-based resins such as polyethylene terephthalate, polyethylene naphthalate, and polybutylene terephthalate. or from a group of materials such as aramid, polyamide-imide, polyimide, and glass. As the microporous membrane, in addition to a porous membrane obtained by film-forming a resin or the like, a porous sheet, a non-woven fabric, or the like can also be used.

<Nonvolatile electrolyte>
As the nonvolatile electrolyte, a semi-solid electrolyte composed of a non-aqueous electrolytic solution in which an electrolyte salt is dissolved and supporting particles (particles) for supporting the non-aqueous electrolytic solution can be used. The non-volatile electrolyte may contain a binder for binding the support particles. According to the non-volatile electrolyte, a non-aqueous electrolyte is retained in the pores between the particles of the support particles to mediate ionic conduction. Since volatilization and flow of the non-aqueous electrolyte are suppressed, the secondary battery 1000 is obtained in which leakage of the non-aqueous electrolyte and change in composition are less likely to occur.

<Carried particles>
Various particles that are highly insulating and insoluble in a non-aqueous electrolytic solution can be used as the carrier particles. Specific examples of the support particles include inorganic particles of metal oxides such as γ-alumina (Al ₂ O ₃ ), silica (SiO ₂ ), zirconia (ZrO ₂ ), ceria (CeO ₂ ). A particulate solid electrolyte can also be used as the support particles.

The average particle size of the primary particles of the support particles is preferably 1 nm to 10 μm, more preferably 1 to 50 nm, still more preferably 1 to 10 nm. When the average particle size is 1 nm or more, the support particles are less likely to agglomerate due to the force between the surfaces, and thus the separator 300 can be easily formed. Further, when the average particle size is 10 μm or less, the surface area of the support particles is large, so that a large amount of electrolytic solution can be retained. The particle size of the carrier particles can be measured, for example, using a transmission electron microscope (TEM).

<Solid electrolyte>
Solid electrolytes have high ionic conductivity but low conductivity, and solid electrolytes can be used within operating temperatures. Specific examples _of solid electrolytes include oxide solid electrolytes _such as _LiLaZrO such as _Li7La3Zr2O12 , and _sulfides such _{as Li10Ge2PS12 and Li2SP2S5 .} system solid electrolytes, and the like.

Separator 300 is preferably formed from a nonvolatile electrolyte alone or from a combination of a nonvolatile electrolyte and a microporous membrane. When such a separator 300 is formed, a non-volatile electrolyte layer composed of a non-volatile electrolyte containing a non-aqueous electrolyte and supporting particles is formed between the positive electrode 100 and the negative electrode 200 . Since the non-volatile electrolyte layer suppresses volatilization and flow of the non-aqueous electrolyte, leakage of the non-aqueous electrolyte and changes in composition are less likely to occur, and a safe and long-life secondary battery can be obtained.

When a non-volatile electrolyte is used for the separator 300, the content of the non-aqueous electrolyte is preferably 40 to 90 vol%, more preferably 50 to 80 vol%, and even more preferably 60 to 80 vol%. . When the content of the non-aqueous electrolyte is 40 vol % or more, a sufficiently high ion conductivity can be obtained. Moreover, when the content of the non-aqueous electrolyte is 90 vol % or less, the non-aqueous electrolyte is less likely to leak out of the semi-solid electrolyte.

<Separator forming method>
When a non-volatile electrolyte is used for the separator 300, a method of compression-molding the carrier particles into a pellet shape, a sheet shape, or the like, a method of mixing binder powder with the carrier particles and molding them into a highly flexible sheet shape, etc. It can be formed by a method of kneading the carrier particles and the binder in a solvent to form a sheet or the like, followed by drying to remove the solvent. The separator 300 may be formed integrally with the electrode by kneading carrier particles and a binder in a solvent, coating the surface of the electrode mixture layer, and drying.

Further, when the separator 300 uses a combination of a nonvolatile electrolyte and a microporous membrane, support particles and a binder are kneaded in a solvent, the resulting mixture is applied to the microporous membrane, and the coated mixture is It can be formed by a method of drying to remove the solvent. Alternatively, the separator 300 may be formed separately, or may be formed integrally with the electrode, and the electrode body may be formed by sandwiching the separator 300 between the electrode and the microporous membrane.

The non-aqueous electrolyte may be filled between the support particles after forming the support particles into a sheet or the like, or may be mixed with the support particles before forming the support particles. Mixing the non-aqueous electrolyte and the support particles yields a semi-solid non-volatile electrolyte. For example, an organic solvent such as methanol is added to the supported particles and the non-aqueous electrolytic solution, mixed, and the resulting slurry is spread on a petri dish or the like and the organic solvent is distilled off to obtain a powdery non-volatile electrolyte. .

Moreover, the non-aqueous electrolyte may be retained not only in the separator 300 but also in the positive electrode mixture layer 110 and the negative electrode mixture layer 210 . When the non-aqueous electrolyte is retained between the particles of the active material and the conductive agent in the electrode mixture layer, the ionic conductivity in the electrode increases and a high discharge capacity can be obtained. As a method for holding the non-aqueous electrolyte in the electrode mixture layer, there is a method of injecting the non-aqueous electrolyte into the exterior body 500, or a method of kneading the non-aqueous electrolyte, the active material, the conductive agent, and the binder to form an electrode mixture. and applying the prepared electrode mixture to a current collector to produce an electrode. When the positive electrode mixture layer 110 , the negative electrode mixture layer 210 and the separator 300 hold the non-aqueous electrolyte, it is not necessary to inject the non-aqueous electrolyte into the package 500 .

Binders used for forming the nonvolatile electrolyte layer include, for example, polyethylene, polypropylene, polytetrafluoroethylene (PTFE), polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer (P(VdF-HFP)), and styrene. - butadiene rubber, polyalginic acid, polyacrylic acid and the like. As the microporous membrane used for forming the nonvolatile electrolyte layer, a fluororesin porous sheet is preferable. The average particle size of the primary particles of the support particles is preferably 1/100 to 1/2 of the thickness of the microporous membrane.

<Non-aqueous electrolyte>
The non-aqueous electrolyte contains a non-aqueous solvent containing an electrolyte salt, an ester solvent and a carbonate solvent, and a film-forming agent (additive) for forming a film on the surface of the negative electrode. As a solvent for the non-aqueous electrolyte, a mixed solvent of an ester solvent having a larger number of donors than the carbonate solvent and a carbonate solvent is used.

In general, carbonate-based solvents are often used as solvents for non-aqueous electrolytes in lithium-ion secondary batteries. Solvents for general non-aqueous electrolytes include cyclic carbonates such as ethylene carbonate (EC) and propylene carbonate (PC), which have a high relative dielectric constant, and dimethyl carbonate (DMC), which has a low viscosity, and ethyl methyl carbonate (EMC). It is a mixed solvent with chain carbonate.

However, in a secondary battery using graphite or silicon as a negative electrode active material, when a carbonate-based solvent is used as a solvent, the solvent is reductively decomposed on the surface of the negative electrode active material, or carrier ions are formed between the layers of the negative electrode active material. Co-insertion with the solvent causes the problem that the discharge capacity becomes lower than the theoretical capacity and the cycle characteristics deteriorate.

In order to solve this problem, when a film-forming agent is added to the non-aqueous electrolyte, an SEI film is formed on the surface of the negative electrode active material during the initial charging and discharging, so that reductive decomposition and co-insertion of the solvent are prevented. can be done. However, if the amount of the film-forming agent added is too large, an excessive SEI film will be formed, resulting in an increase in ionic conduction resistance and, on the contrary, a decrease in battery performance. be.

Another method for improving battery performance is to use an ester solvent as a solvent for the non-aqueous electrolyte. Ester-based solvents have relatively good oxidation-reduction resistance, so deterioration of the non-aqueous electrolyte can be avoided to some extent. However, the ester solvent has a large number of donors and tends to increase the viscosity of the non-aqueous electrolyte, so it cannot necessarily be said to be the optimum solvent. If only an ester-based solvent is used, the non-aqueous electrolyte solution becomes high in ionic conduction resistance due to solvation to lithium ions and high viscosity, and the discharge capacity of the secondary battery becomes low.

On the other hand, as in the present embodiment, when a mixed solvent of an ester solvent having a larger number of donors than a carbonate solvent and a carbonate solvent is used at an appropriate mixing ratio, the interaction between lithium ions and the ester solvent can be alleviated and the viscosity of the non-aqueous electrolyte can be reduced. This is because a carbonate-based solvent has a smaller donor number than an ester-based solvent and generally has a lower viscosity than a mixed solution of an ester-based solvent and an electrolyte salt. Relaxing the interaction and lowering the viscosity increases the ionic conductivity, resulting in a higher discharge capacity.

The donor number (DN) of a solvent is a measure of the electron pair donating ability of a solvent molecule proposed by Gutmann et al. The donor number is defined as the heat of reaction when the solvent of interest forms a 1:1 adduct with _SbCl5 in an inert medium (dichloromethane), with hexamethylphosphoric triamide as the reference material. dimensionally normalized. The number of donors in the solvent can be obtained from the chemical shift of nuclear magnetic resonance (NMR).

The non-aqueous electrolyte may contain an electrolyte salt, a non-aqueous solvent, a film-forming agent, a corrosion inhibitor, and other additives. When a non-volatile electrolyte is used for the separator 300, the non-aqueous electrolyte is used in the pores between the supporting particles, the interface between the separator 300 and the electrode mixture layer, and the pores between the active material and conductive agent particles. retained.

<Electrolyte salt>
Electrolyte salts include _LiPF6 , _LiBF4 , LiClO4, _{LiCF3SO3 , LiCF3CO2} , _{LiAsF6 , LiSbF6} , lithium bis( _{fluorosulfonyl} )imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI ), lithium bis(pentafluoroethanesulfonyl)imide (LiBETI), lithium tetrafluoroborate (LiBF ₄ ), lithium bisoxalateborate (LiBOB), lithium triflate, and various other lithium salt materials. As the electrolyte salt, LiFSI, LiTFSI, or LiBETI is preferable from the viewpoint of high degree of dissociation and high chemical stability.

<Ester solvent>
As the ester solvent, γ-butyrolactone (GBL, DN=18), trimethyl phosphate (TMP, DN=23), triethyl phosphate (TEP, DN=26), etc. can be used. As the ester solvent, γ-butyrolactone is preferable because of its excellent oxidation-reduction resistance.

The content of the ester-based solvent is preferably 30 to 80 mol % based on the non-aqueous solvent in the non-aqueous electrolyte, from the viewpoint of optimizing the viscosity of the non-aqueous electrolyte. The content of the ester solvent can be quantified using NMR or the like.

<Carbonate solvent>
Ethylene carbonate (EC, DN=16.4), propylene carbonate (PC, DN=15.1), etc. can be used as the carbonate-based solvent. As the carbonate-based solvent, propylene carbonate is preferable from the viewpoint of increasing the ion conductivity of the non-aqueous electrolyte.

From the viewpoint of improving the discharge capacity of the secondary battery, the content of the carbonate-based solvent is preferably 0 to 60 mol % based on the total amount of the non-aqueous solvent used in the non-aqueous electrolyte. The content of the carbonate-based solvent can be quantified using NMR or the like.

The molar ratio of the carbonate solvent to the ester solvent is preferably 0 to 0.65, more preferably 0.06 to 0.59, even more preferably 0.12 to 0.52. With such a molar ratio, the discharge capacity of the secondary battery is higher than when only the ester solvent is used. The molar ratio of the carbonate-based solvent to the ester-based solvent can be determined using NMR.

The discharge capacity of the secondary battery is, for example, in a system of a ternary positive electrode and a graphite negative electrode, when the molar ratio of the carbonate solvent to the ester solvent is in the range of 0 to 0.65, it is 357 mAh or more, and the molar ratio is 0.06 to 0.06. In the range of 0.59, it can be 380 mAh or more, and in the range of 0.12 to 0.52, it can be 400 mAh or more.

The molar ratio of the electrolyte salt to the ester solvent is preferably 0 to 0.24, more preferably 0 to 0.20, even more preferably 0 to 0.16. Within such a molar ratio range, the higher the ratio of the electrolyte salt, the higher the concentration of lithium ions and the lower the ionic conduction resistance of the non-aqueous electrolyte. Also, with such a molar ratio, the molar ratio of the electrolyte salt is suppressed, so the interaction between the ester-based solvent having a large number of donors and lithium ions is weakened. As a result, lithium ions move more easily, and the ionic conductivity of the non-aqueous electrolytic solution increases, so that a higher discharge capacity can be obtained. The molar ratio of the electrolyte salt to the ester solvent can be determined using NMR.

The discharge capacity of the secondary battery is, for example, 350 mAh or more when the molar ratio of the electrolyte salt to the ester solvent is in the range of 0 to 0.24 in a system of a ternary positive electrode and a graphite negative electrode, and the molar ratio is 0 to 0.21. 375 mAh or higher in the range of , and 400 mAh or higher in the molar ratio range of 0 to 0.16.

<Film forming agent>
Film formers include vinylene carbonate (VC), fluoroethylene carbonate (FEC), succinic anhydride, vinylethylene carbonate, tris(trimethylsilyl)borate, tris(trimethylsilyl)phosphate, lithium 4,5-dicyano-2-(trifluoro It is selected from a group of materials such as methyl)imidazolide (LiTDI), lithium bis(2-methyl-2-fluoromalonate)borate (LiBMFMB), lithium difluorooxalateborate (LiDFOB).

From the viewpoint of improving the discharge capacity and cycle characteristics of the secondary battery, the content of the film-forming agent is preferably 0 to 30 wt%, more preferably 0 to 10 wt%, relative to the total of the electrolyte salt and the solvent. The content of the film-forming agent can be quantified using NMR or the like.

<Corrosion inhibitor>
As the corrosion inhibitor, various substances capable of preventing corrosion of metals such as current collectors at high potential can be used. As the corrosion inhibitor, a film-forming type or a surface-adsorbing type is preferable, and a substance with high stability in the presence of moisture is more preferable.

Indicators of stability in the presence of water include water solubility and hydrolyzability. If the corrosion inhibitor is a solid, it preferably has a water solubility of less than 1% at room temperature. The hydrolyzability of the corrosion inhibitor can be evaluated by conducting molecular structural analysis after the corrosion inhibitor has been absorbed moisture or mixed with water. When the moisture-absorbed corrosion inhibitor or the corrosion inhibitor mixed with water is heated at 100°C or higher to remove the moisture, if 95 wt% or more of the residue has the same molecular structure as the original corrosion inhibitor, the corrosion The inhibitor is non-hydrolyzable and can be said to be stable in the presence of moisture.

As a corrosion inhibitor, an organic salt represented by (MR) ⁺ An ^- is particularly preferred. However, in the formula, (M−R) ⁺ is a cation and An ⁻ is an anion. M is a central atom selected from nitrogen (N), boron (B), phosphorus (P) and sulfur (S); R is a substituent that is fully substituted on the central atom; is the base. Such organic cations are highly polar and thus provide highly stable corrosion inhibitors.

The anion of the corrosion inhibitor is preferably tetrafluoroborate anion (BF ₄ ⁻ ), hexafluorophosphate anion (PF ₆ ⁻ ), or the like. Since these anions are thought to react with metals to form a passive film, they are effective in preventing elution of metals constituting current collectors and the like. As the anion, a hexafluorophosphate anion is particularly preferable because it can further suppress metal elution. As the organic salt, an ionic liquid that exists as a liquid at room temperature is particularly preferable in terms of high non-volatility, good heat resistance, chemical stability and electrical stability.

Specific examples of corrosion inhibitors include quaternary ammonium salts such as tetrabutylammonium hexafluorophosphate (TBA-PF6) and tetrabutylammonium tetrafluoroborate (TBA-BF4), and 1-ethyl-3-methylimidazolium tetra Fluoroborate (EMI-BF4), 1-ethyl-3-methylimidazolium hexafluorophosphate (EMI-PF6), 1-butyl-3-methylimidazolium tetrafluoroborate (BMI-BF4), 1-butyl-3- and imidazolium salts such as methylimidazolium hexafluorophosphate (BMI-PF6).

The content of the corrosion inhibitor is preferably 0.5 to 20 wt%, more preferably 1 to 10 wt%, relative to the total of the electrolyte salt and the solvent. When the content of the corrosion inhibitor is 0.5 wt % or more, a sufficient effect of preventing corrosion of the current collector and the like can be obtained, so a high discharge capacity and good cycle characteristics can be obtained. Further, when the content of the corrosion inhibitor is 20 wt % or less, the conduction of lithium ions is less likely to be hindered, so the ionic conductivity of the non-aqueous electrolyte can be ensured. In addition, since the consumption of electric energy due to self-discharge that dissociates and decomposes the corrosion inhibitor is reduced, the possibility of obtaining a high discharge capacity increases.

According to the above non-aqueous electrolyte, non-volatile electrolyte, and secondary battery using the same, the non-aqueous electrolyte is appropriately a mixed solvent of an ester solvent having a larger number of donors than a carbonate solvent and a carbonate solvent. Therefore, the discharge capacity of the secondary battery can be improved compared to the case where the carbonate-based solvent is not used in combination, despite the use of an ester-based solvent having a large number of donors. In addition, according to a secondary battery using a non-volatile electrolyte in which a non-aqueous electrolyte is supported on particles, or a separator formed between a positive electrode and a negative electrode, the non-aqueous electrolyte leaks or changes in composition. It is possible to realize a safe and long-life secondary battery.

As a secondary battery, a secondary battery having a negative electrode containing graphite as a negative electrode active material is particularly preferable. A relatively high discharge capacity can be obtained by using a negative electrode containing graphite as the negative electrode active material. Since the non-aqueous electrolyte contains an ester solvent, reductive decomposition of the solvent on the surface of the negative electrode active material is suppressed more than before, even when the reduction potential is low as in the case of a graphite negative electrode. become. In addition, co-insertion between the layers of the negative electrode active material is reduced despite the presence of the carbonate-based solvent. Therefore, according to the secondary battery described above, high discharge capacity and good cycle characteristics can be obtained.

EXAMPLES The present invention will be specifically described below with reference to Examples, but the present invention is not limited to these Examples.

<Example 1>
<Preparation of non-aqueous electrolyte>
Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) as electrolyte salt, γ-butyrolactone (GBL) as ester solvent, propylene carbonate (PC) as carbonate solvent, vinylene carbonate (VC) as additive, tetra Butyl ammonium hexafluorophosphate (TBA-PF6) was weighed and mixed to prepare a non-aqueous electrolyte.

Components contained in the prepared non-aqueous electrolyte were quantified by NMR. As a result, the molar ratio of PC to GBL was 0.65, and the molar ratio of LiTFSI to GBL was 0.22. Also, the content of VC was 3 wt % with respect to the total of electrolyte salt and solvent, and the content of TBA-PF6 was 2.5 wt % with respect to the total of electrolyte salt and solvent.

<Production of secondary battery>
<Preparation of positive electrode>
LiNi _1/3 Co _1/3 Mn _1/3 O ₂ -based oxide as a positive electrode active material, acetylene black as a conductive agent, and PVDF dissolved in N-methylpyrrolidone as a binder at a solid weight ratio of 94:4. : 2, and uniformly mixed in a kneader. The resulting mixture was slurried with NMP and adjusted to the desired solids concentration. Then, the slurry with the adjusted concentration was applied to both sides of an aluminum foil, which was a positive electrode current collecting foil, with a desktop coater, and passed through a drying oven at 120° C. to obtain a positive electrode. The total coating amount of the positive electrode mixture on both surfaces was 30.1 mg/cm ² . The obtained positive electrode was adjusted to have an electrode density of 3.15 g/cm ³ by a roll press.

<Production of negative electrode>
Graphite as a negative electrode active material, and styrene-butadiene rubber and carboxymethyl cellulose as binders were weighed so that the weight ratio of the solid content was 98:1:1, and these were uniformly mixed in a kneader. The resulting mixture was slurried with water and adjusted to the desired solids concentration. Then, the slurry with the adjusted concentration was applied to both sides of the copper foil, which was the negative electrode current collecting foil, with a desktop coater, and passed through a drying furnace at 100° C. to obtain a negative electrode. The total coating amount of the negative electrode mixture on both surfaces was 18.1 mg/cm ² . The obtained negative electrode was adjusted to have an electrode density of 1.55 g/cm ³ by a roll press.

<Formation of Separator>
A separator was formed by applying a nonvolatile electrolyte to the surface of the electrode mixture layer. First, SiO ₂ with an average particle size of 1 μm as support particles and vinylidene fluoride-hexafluoropropylene copolymer (P(VdF-HFP)) as a binder were mixed in a weight ratio of 89.3:10.7. and uniformly mixed in a kneader. The resulting mixture was slurried with NMP and adjusted to the desired solids concentration. Then, the slurry with the adjusted concentration was applied to both surfaces of the positive electrode and the negative electrode using a desktop coater, and passed through a drying oven at 100° C. to obtain a positive electrode and a negative electrode on which a nonvolatile electrolyte layer was formed.

<Assembly>
The prepared positive electrode and negative electrode were punched out with an air punch so that the positive electrode mixture layer was 45 mm×70 mm and the negative electrode mixture layer was 47 mm×74 mm, and tab portions were formed on the positive electrode and the negative electrode. Then, the positive electrode and the negative electrode were dried at 100° C. for 2 hours to remove NMP in the electrodes. The dried positive electrode was sandwiched between resin-made microporous membranes having a three-layer structure of PP/PE/PP with a thickness of 30 μm, and three sides other than the side where the tab portion was formed were thermally welded.

After stacking the positive electrode covered with the microporous film and the punched negative electrode in the order of negative electrode/positive electrode/negative electrode, a PTFE sheet having a thickness of 50 μm was placed on the negative electrode. The tab portions provided on the positive electrode and the negative electrode, the aluminum positive electrode terminal, and the nickel negative electrode terminal were welded by ultrasonic welding. The obtained electrode assembly was sandwiched between laminate films, and three sides including the side where the tab portion was formed, except one side for liquid injection, were heat-sealed at 200°C by a lamination sealing device, and then at 50°C for 20 hours. Vacuum dried. After injecting the non-aqueous electrolyte from one side for liquid injection, the one side for liquid injection was vacuum-sealed to obtain a secondary battery.

<Measurement of discharge capacity>
The produced secondary battery was held at 25° C., and was charged at a constant current of 0.05C to an upper limit voltage of 4.2V, and then charged at a constant voltage of 4.2V until the voltage decreased to 0.005C. After that, the secondary battery was discharged at a constant current of 0.05 C to a lower limit voltage of 2.7 V, and the discharge capacity of the secondary battery was measured.

<Examples 2 to 7, Comparative Examples 1 to 2>
In the same manner as in Example 1 except that the composition of the non-aqueous electrolyte was changed as shown in Table 1, the components contained in the non-aqueous electrolyte were quantified and the discharge capacity was measured.

Table 1 shows the types of electrolyte salts (Li salt (1), Li salt (2)) used in Examples 1 to 7 and Comparative Examples 1 and 2, the types of ester solvents and carbonate solvents (solvent (1) , solvent (2)), molar ratio of carbonate solvent to ester solvent (solvent (2)/solvent (1)), molar ratio of electrolyte salt to ester solvent (Li salt (1)/solvent (1)) and measurement results of the discharge capacity of the secondary battery.

<Results and discussion>
FIG. 2 is a diagram showing the relationship between the molar ratio of the carbonate-based solvent to the ester-based solvent and the discharge capacity of the secondary battery. As shown in FIG. 2, in compositions where the molar ratio of the carbonate-based solvent to the ester-based solvent is less than about 0.32, the higher the molar ratio, the greater the discharge capacity. This result is considered to be due to the fact that the addition of PC having a low viscosity lowers the viscosity of the non-aqueous electrolyte and increases the ionic conductivity of the non-aqueous electrolyte. In Comparative Example 1, the concentration of the electrolyte salt was lower than in Comparative Example 2, so the viscosity of the non-aqueous electrolyte was reduced to some extent without adding PC, but the ionic conduction resistance of the non-aqueous electrolyte was large. This is presumed to be the cause of the decrease in discharge capacity.

On the other hand, in compositions where the molar ratio of the carbonate-based solvent to the ester-based solvent was greater than about 0.32, the smaller the molar ratio, the larger the discharge capacity. A possible reason for this is that the excessive addition of PC increased the coinsertion between the layers of the negative electrode active material and the reductive decomposition of PC on the surface of the negative electrode active material. The optimum mixing ratio that balances the ionic conduction resistance of the non-aqueous electrolyte and the stability of the solvent in the negative electrode and increases the discharge capacity of the secondary battery is the ester solvent (GBL) and the carbonate solvent (PC). shown to exist.

It was confirmed that the discharge capacity Z of the secondary battery is represented by the following formula (1) when the molar ratio X of the carbonate-based solvent to the ester-based solvent is in the range of 0 to 0.65.

Z=−660.34X ² +427.01X+357 Expression (1)

It was also confirmed that the discharge capacity Z of the secondary battery monotonically decreases as X increases in the range where the molar ratio X exceeds 0.65, and is expressed by the following formula (2).

Z=−97.787X+418.11 Expression (2)

According to formula (1), the range of X in which the discharge capacity of the secondary battery exceeds 357 mAh was 0 to 0.65, as in the case of X=0. Also, the range of X at which the discharge capacity was 380 mAh or more was 0.06 to 0.59. Also, the range of X at which the discharge capacity was 400 mAh or more was 0.12 to 0.52.

FIG. 3 is a diagram showing the relationship between the molar ratio of the electrolyte salt to the ester solvent and the discharge capacity of the secondary battery. As shown in FIG. 3, in compositions where the molar ratio of the electrolyte salt to the ester solvent is less than about 0.06, the higher the molar ratio, the greater the discharge capacity. On the other hand, in compositions where the molar ratio of the electrolyte salt to the ester solvent was greater than about 0.06, the smaller the molar ratio, the larger the discharge capacity. If the concentration of the ester-based solvent, which affects the dissociation of the electrolyte salt, is too high relative to the concentration of the electrolyte salt, the interaction between the electrolyte salt and the ester-based solvent becomes stronger, and the ionic conductivity of the non-aqueous electrolyte decreases. Therefore, it is thought that the discharge capacity becomes smaller.

It has been confirmed that the discharge capacity Z of the secondary battery is represented by the following formula (3) by the molar ratio Y of the electrolyte salt to the ester solvent.

Z=−2388.8Y ² +298.23Y+412.71 Expression (3)

According to formula (3), it was found that the discharge capacity of the secondary battery was 350 mAh or more when Y was in the range of 0 to 0.24. Further, it was found that the discharge capacity of the secondary battery was 375 mAh or more when Y was in the range of 0 to 0.20. Moreover, it was found that the discharge capacity of the secondary battery was 400 mAh or more when Y was in the range of 0 to 0.16.

100 Positive electrode 110 Positive electrode mixture layer 120 Positive electrode current collector 130 Positive electrode tab 200 Negative electrode 210 Negative electrode mixture layer 220 Negative electrode current collector 230 Negative electrode tab 300 Separator 400 Electrode body 500 Armor body 1000 Secondary battery

Claims (8)

A non-aqueous electrolyte containing a non-aqueous solvent containing an electrolyte salt, an ester-based solvent and a carbonate-based solvent, an additive for forming a film on the surface of the negative electrode, and particles supporting the non-aqueous electrolyte A non-volatile electrolyte containing
The donor number of the ester solvent is greater than the donor number of the carbonate solvent,
A non-volatile electrolyte, wherein the molar ratio of the carbonate-based solvent to the ester-based solvent is 0.06 or more and 0.65 or less . The non-volatile electrolyte of claim 1,
The non-aqueous electrolyte is a non-volatile electrolyte in which the molar ratio of the carbonate-based solvent to the ester-based solvent is 0.06 or more and 0.59 or less . The non-volatile electrolyte of claim 1,
The non-aqueous electrolyte is a nonvolatile electrolyte in which the molar ratio of the electrolyte salt to the ester solvent is more than 0 and 0.24 or less. The non-volatile electrolyte of claim 1,
The non-aqueous electrolyte is a nonvolatile electrolyte in which the molar ratio of the electrolyte salt to the ester solvent is more than 0 and 0.20 or less. The non-volatile electrolyte of claim 1,
The non-volatile electrolyte, wherein the ester solvent is γ-butyrolactone. The non-volatile electrolyte of claim 1,
The non-volatile electrolyte, wherein the carbonate-based solvent is propylene carbonate. The non-volatile electrolyte of claim 1,
A non-volatile electrolyte wherein said particles are silica. a positive electrode;
a negative electrode;
a separator formed between the positive electrode and the negative electrode;
The separator is a non-volatile electrolyte containing a non-aqueous electrolyte and particles supporting the non-aqueous electrolyte,
The non-aqueous electrolyte is a non-aqueous electrolyte containing a non-aqueous solvent containing an electrolyte salt, an ester solvent and a carbonate solvent, and an additive for forming a film on the surface of the negative electrode,
The donor number of the ester solvent is greater than the donor number of the carbonate solvent,
A secondary battery, wherein the molar ratio of the carbonate-based solvent to the ester-based solvent is 0.06 or more and 0.65 or less .

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