WO2020213268A1

WO2020213268A1 - Nonaqueous electrolytic solution, nonvolatile electrolyte, and secondary battery

Info

Publication number: WO2020213268A1
Application number: PCT/JP2020/008682
Authority: WO
Inventors: 篤宇根本; 栄二關
Original assignee: 株式会社日立製作所
Priority date: 2019-04-17
Filing date: 2020-03-02
Publication date: 2020-10-22
Also published as: JP2020177779A; JP7267823B2

Abstract

The present invention provides a nonaqueous electrolytic solution and a nonvolatile electrolyte which improve the discharge capacity of a secondary battery, and a secondary battery using the same. The nonaqueous electrolytic solution comprises: a nonaqueous solvent containing an electrolyte salt, an ester-based solvent, and a carbonate-based solvent; and an additive for forming a film on a surface of a negative electrode. The donor number of the ester-based solvent is greater than the donor number of the carbonate-based solvent, and the molar ratio of the carbonate-based solvent to the ester-based solvent is 0 to 0.65.

Description

Non-aqueous electrolyte, non-volatile electrolyte, secondary battery

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

Lithium-ion secondary batteries are used in various applications such as mobile power sources for 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 been expanded to large-scale products and the like, and further high energy density, high capacity, and long life are required.

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, wide potential window, and relative permittivity. -High ionic conductivity and appropriate viscosity are required. Conventionally, carbonates have been widely used as a solvent for an electrolytic solution of a lithium ion secondary battery, but improvements are being made for non-aqueous electrolytic solutions to improve battery performance.

Patent Document 1 discloses the following contents as a technique relating to a non-aqueous electrolyte solution. "It has a solvent-containing electrolyte salt, an ester-based solvent constituting the solvent-containing electrolyte salt and a solvent-harmonious ionic liquid, and a low-viscosity solvent, and the mixing ratio of the ester-based solvent to the solvent-containing electrolyte salt is 0 in terms of molars. A semi-solid electrolyte, a semi-solid electrolyte layer, an electrode, and a secondary battery in which the mixing ratio of the low-viscosity solvent to the solvent-containing electrolyte salt is 4 or more and 16 or less in terms of molars. 1)

International Publication No. 2018/179990

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

However, when a carbonate-based solvent or an ether-based solvent 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 a solvent may be co-inserted between the layers of the negative electrode active material. The effect of adding the film-forming agent reaches a plateau at a certain amount of addition, and if it is added excessively, the battery performance is deteriorated. Therefore, a non-aqueous electrolytic solution capable of obtaining a higher discharge capacity is required.

Therefore, an object of the present invention is to provide a non-aqueous electrolyte solution, a non-volatile electrolyte, and a secondary battery using the non-aqueous electrolyte solution, which improves the discharge capacity of the secondary battery.

In order to solve the above problems, the present invention has, for example, the following configuration.
A non-aqueous electrolyte solution 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 number of donors for ester solvents is larger than the number of donors for carbonate solvents,
A non-aqueous electrolyte solution in which the molar ratio of the carbonate solvent to the ester solvent is 0 to 0.65.

According to the present invention, it is possible to provide a non-aqueous electrolyte solution, a non-volatile electrolyte, and a secondary battery using the non-aqueous electrolyte solution, which improves the discharge capacity of the secondary battery. Issues, configurations and effects other than those described above will be clarified by the description of the following embodiments.

It is a schematic cross-sectional view explaining the structure of the secondary battery which concerns on one Embodiment of this invention. It is a figure which shows the relationship between the molar ratio of a carbonate solvent with respect to an ester solvent, and the discharge capacity of a secondary battery. It is a figure which shows the relationship between the molar ratio of the electrolyte salt with respect to an ester solvent, and the discharge capacity of a secondary battery.

Hereinafter, the non-aqueous electrolyte solution, the non-volatile electrolyte, and the secondary battery using the non-aqueous electrolyte solution according to the embodiment of the present invention will be described with reference to the drawings and the like.

The following description shows a specific example of the contents of the present invention. The present invention is not limited to the following description, and various modifications and modifications by those skilled in the art can be made within the scope of the technical ideas disclosed in the present specification. The present invention includes various modifications different from the embodiments. The embodiments have been described in detail in order to explain the present invention in an easy-to-understand manner, 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. It is also possible to add the configuration of another embodiment to the configuration of one embodiment. Further, it is possible to add / delete / replace a part of the configuration of the embodiment with another configuration.

"-" Described in this specification is used to mean that the numerical values described before and after it are used as the lower limit value and the upper limit value. However, when the numerical values described before and after are "0", "0" is not included as the upper limit value or the lower limit value. In the numerical range described stepwise in the present specification, the upper limit value and the lower limit value described in one numerical range may be replaced with other upper limit values and other lower limit values described stepwise. Good. The upper and lower limits of the numerical range described in the present specification 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 as a secondary battery. A lithium ion secondary battery means an electrochemical device that creates a potential difference between electrodes by occluding lithium ions into and releasing them from the electrodes, thereby storing or making available electrical energy. The lithium ion secondary battery is also called by another name such as a lithium ion battery, a non-aqueous electrolyte secondary battery, a non-aqueous electrolyte secondary battery, etc., and any battery having any name is the subject of the present invention. is there. The technical idea of the present invention can also be applied to a sodium ion secondary battery, a magnesium ion secondary battery, a calcium ion secondary battery, a zinc secondary battery, an aluminum ion secondary battery and the like.

When a material is selected from the material group illustrated below and used, the material may be used alone or in combination of a plurality as long as it does not contradict the contents disclosed in the present specification. Further, materials other than the material group exemplified below may be used as long as they do not contradict the contents disclosed in the present specification.

FIG. 1 is a schematic cross-sectional view illustrating the configuration of a secondary battery according to an embodiment of the present invention. FIG. 1 shows a laminated secondary battery as an example of the 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 exterior body 500.

The exterior body 500 houses the positive electrode 100, the negative electrode 200, and the separator 300. As the material of the exterior body 500, a material that exhibits corrosion resistance to a non-aqueous electrolytic solution, such as aluminum, stainless steel, and nickel-plated steel, is used.

The positive electrode 100, the separator 300, and the negative electrode 200 are laminated in this order to form the electrode body 400. Inside the exterior body 500, a plurality of positive electrodes 100, separators 300, and negative electrodes 200 are laminated and housed. The positive electrode 100 and the negative electrode 200 are arranged so as to sandwich the separator 300.

Although FIG. 1 shows a laminated type secondary battery, the secondary battery 1000 may be a wound type. The shape of the secondary battery 1000 may be any of a cylindrical shape, a square shape, a button shape, and the like. The exterior body 500 may be provided as a bag-shaped 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, the positive electrode mixture layer 110 is formed on both sides of the flat plate-shaped positive electrode current collector 120. At the end of the positive electrode current collector 120, a flat plate-shaped positive electrode tab 130 for extracting an electric current is provided. The positive electrode tab 130 is pulled out from each positive electrode 100 to the outside of the exterior body 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, the negative electrode mixture layer 210 is formed on both sides of the flat negative electrode current collector 220. At the end of the negative electrode current collector 220, a flat plate-shaped negative electrode tab 230 for extracting an electric current is provided. The negative electrode tab 230 is pulled out from each negative electrode 200 to the outside of the exterior body 500.

The positive electrode tab 130 and the negative electrode tab 230 are electrically connected to leads, external terminals, etc. (not shown) on the outside of the exterior body 500. As a method of joining the tabs, an appropriate method such as spot welding or ultrasonic bonding is used. The electrode body 400 can be connected in parallel by joining the positive electrode tabs 130 to each other and the negative electrode tabs 230 to each other. Alternatively, the electrode body 400 can also form a bipolar type secondary battery that is connected in series.

The positive electrode mixture layer 110 is a binder for binding a positive electrode active material capable of storing and releasing lithium ions, a conductive agent for improving the conductivity of the positive electrode mixture layer 110, and the positive electrode active material and the conductive agent. And contains. The positive electrode active material has an electrochemical activity of occluding and releasing lithium ions at a potential nobler than that of the negative electrode 200, lithium ions are desorbed in the charging process, and lithium ions are inserted in the discharging process.

The negative electrode mixture layer 210 contains a negative electrode active material capable of storing and releasing lithium ions. The negative electrode active material has an electrochemical activity of occluding and releasing lithium ions at a potential lower than that of the positive electrode 100, the lithium ions are desorbed in the discharging process, and the lithium ions are inserted in 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. May be good.

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

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 , and Fe (MoO). ₄ ) Polyanionic materials such as ₃ , Fe ₂ (SO ₄ ) ₃ , Li ₃ Fe ₂ (PO ₄ ) ₃ and organic materials such as quinone organic crystals can also be used.

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

<Conducting agent>
The conductive agent is selected 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, carboxymethyl cellulose, and polyvinylidene fluoride.

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

<Negative electrode current collector, negative electrode tab>
As the negative electrode current collector 220, a metal foil, a perforated foil, an expanded metal, a foamed metal plate, or the like can be used. The negative electrode current collector 220 is selected from a group of materials such as copper and copper alloy. Further, stainless steel, titanium, nickel and the like can also be used depending on the redox potential of the negative electrode 200 and the like. The thickness of the negative electrode current collector 220 is preferably 10 nm to 1 mm, more preferably 1 to 100 μm, from the viewpoint of achieving both mechanical strength and energy density. The negative electrode tab 230 can be formed of the same material as the negative electrode current collector 220.

<Electrode mixture layer formation method>
In the positive electrode mixture layer 110 and the negative electrode mixture layer 210, an active material and a conductive agent or a binder are kneaded in a solvent to prepare an electrode mixture, and the prepared electrode mixture is applied to a current collector and coated. It can be formed by drying the prepared 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 punched, cut, or the like.

The electrode mixture can be kneaded with various devices such as a planetary mixer, a disposable mixer, a butterfly mixer, a twin-screw kneader, a ball mill, and a bead mill. As the solvent for dispersing the active material or the like, various solvents such as 1-methyl-2-pyrrolidone (NMP) and water can be used depending on the electrode. As a method of applying the electrode mixture, various methods such as a roll coating method, a doctor blade method, a dipping method, and a spray method can be used.

<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, and acts as a medium for conducting ions between the positive electrode 100 and the negative electrode 200. The separator 300 uses any one or a combination of an insulating microporous membrane having minute pores, a non-volatile electrolyte obtained by supporting a non-aqueous electrolyte solution on particles, and a solid electrolyte. Can be formed. The thickness of the separator 300 is preferably several nm to several mm from the viewpoint of achieving both electron insulation and energy density.

<Microporous membrane>
The microporous film is a cellulose resin such as cellulose, carboxymethyl cellulose or hydroxypropyl cellulose, a polyolefin resin such as polypropylene or a polyethylene-polypropylene copolymer, or a polyester resin such as polyethylene terephthalate, polyethylene naphthalate or polybutylene terephthalate. , Aramid, Polyethyleneimide, Polyethylene, Glass and the like. As the microporous film, in addition to the porous film obtained by forming a resin or the like, a porous sheet, a non-woven fabric, or the like can also be used.

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

<Supported particles>
As the supported particles, various particles having high insulating properties and insoluble in a non-aqueous electrolytic solution can be used. Specific examples of the supported particles include inorganic particles of metal oxides such as γ-alumina (Al ₂ O ₃ ), silica (SiO ₂ ), zirconia (ZrO ₂ ), and ceria (CeO ₂ ). As the supported particles, a particulate solid electrolyte can also be used.

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

<Solid electrolyte>
As the solid electrolyte, the ionic conductivity is high but the conductivity is low, and a solid electrolyte can be used within the operating temperature. Specific examples of the solid electrolyte include oxide-based solid electrolytes such as Li LaZrO based on Li ₇ La ₃ Zr ₂ O ₁₂ , and sulfides such as Li ₁₀ Ge ₂ PS ₁₂ and Li ₂ SP ₂ S ₅ . Examples include system solid electrolytes.

The separator 300 is preferably formed only of the non-volatile electrolyte or a combination of the non-volatile electrolyte and the microporous membrane. When such a separator 300 is formed, a non-volatile electrolyte layer made of a non-volatile electrolyte containing a non-aqueous electrolyte solution and supported particles is formed between the positive electrode 100 and the negative electrode 200. In the non-volatile electrolyte layer, volatilization and flow of the non-aqueous electrolyte solution are suppressed, so that leakage and composition change of the non-aqueous electrolyte solution are less likely to occur, and a safe and long-life secondary battery can be obtained.

When a non-volatile electrolyte is used in the separator 300, the content of the non-aqueous electrolyte solution is preferably 40 to 90 vol%, more preferably 50 to 80 vol%, still more preferably 60 to 80 vol%. .. When the content of the non-aqueous electrolyte solution is 40 vol% or more, a sufficiently high ionic conductivity can be obtained. Further, when the content of the non-aqueous electrolyte solution is 90 vol% or less, the possibility that the non-aqueous electrolyte solution leaks from the semi-solid electrolyte is reduced.

<Separator forming method>
When a non-volatile electrolyte is used for the separator 300, a method of compression-molding the supported particles into pellets, sheets, etc., a method of mixing the supported particles with binder powder, and forming into a highly flexible sheet, etc. The supported particles and the binder can be kneaded in a solvent, formed into a sheet or the like, and dried to remove the solvent. The separator 300 may be integrally formed with the electrode by kneading the supported particles and the binder in a solvent, applying the mixture to the surface of the electrode mixture layer, and drying the separator 300.

When the separator 300 uses a combination of a non-volatile electrolyte and a microporous membrane, the supporting particles and the binder are kneaded in a solvent, and the obtained mixture is applied to the microporous membrane, and the coated mixture is used. It can be formed by a method of drying to remove the solvent. Alternatively, the separator 300 may be formed individually, or may be integrally formed with the electrode, and the separator 300 may be sandwiched between the electrode and the microporous film to form an electrode body.

The non-aqueous electrolytic solution may be filled between the particles of the supported particles after the supported particles are formed into a sheet or the like, or may be mixed with the supported particles before the supported particles are formed. Mixing the non-aqueous electrolyte solution with the supported particles gives a semi-solid non-volatile electrolyte. For example, when the supported particles and the non-aqueous electrolyte solution are mixed by adding an organic solvent such as methanol and the obtained slurry is spread on a petri dish or the like to distill off the organic solvent, a powdery non-volatile electrolyte can be obtained. ..

Further, the non-aqueous electrolytic solution may be held 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 electrolytic solution is held between the particles of the active material and the conductive agent in the electrode mixture layer, the ionic conductivity in the electrode is increased and a high discharge capacity can be obtained. As a method of holding the non-aqueous electrolyte solution in the electrode mixture layer, a method of injecting the non-aqueous electrolyte solution into the exterior body 500 or a method of kneading the non-aqueous electrolyte solution, the active material, the conductive agent or the binder is used to prepare the electrode mixture. , And the prepared electrode mixture is applied to the current collector to prepare an electrode. When the positive electrode mixture layer 110, the negative electrode mixture layer 210, and the separator 300 hold the non-aqueous electrolytic solution, it is not necessary to inject the non-aqueous electrolytic solution into the exterior body 500.

Examples of the binder used for forming the non-volatile electrolyte layer include polyethylene, polypropylene, polytetrafluoroethylene (PTFE), polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer (P (VdF-HFP)), and styrene. -Cancel rubber, polyalginic acid, polyacrylic acid and the like can be mentioned. As the microporous membrane used for forming the non-volatile electrolyte layer, a porous sheet made of fluororesin is preferable. The average particle size of the primary particles of the supported particles is preferably 1/100 to 1/2 of the thickness of the microporous membrane.

<Non-aqueous electrolyte>
The non-aqueous electrolyte solution 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 the solvent of the non-aqueous electrolyte solution, a mixed solvent of an ester solvent having a larger number of donors than the carbonate solvent and a carbonate solvent is used.

Generally, a carbonate solvent is often used as a solvent for a non-aqueous electrolyte solution of a lithium ion secondary battery. Solvents for general non-aqueous electrolyte solutions include cyclic carbonates such as ethylene carbonate (EC) and propylene carbonate (PC) having a high relative permittivity, and dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) having a low viscosity. It is a mixed solvent with chain carbonate.

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

To solve this problem, when a film-forming agent is added to the non-aqueous electrolytic solution, a SEI film is formed on the surface of the negative electrode active material by initial charging and discharging, which makes it difficult for the solvent to undergo reductive decomposition or co-insertion. Can be done. However, if the amount of the film forming agent added is too large, the ion conduction resistance becomes high due to the formation of an excessive SEI film and the battery performance is deteriorated, so that the effect of adding the film forming agent is limited. is there.

As another method for improving the battery performance, there is a method of using an ester solvent as a solvent for the non-aqueous electrolyte solution. Since the ester solvent has relatively good redox resistance, deterioration of the non-aqueous electrolytic solution can be avoided to some extent. However, the ester solvent is not always the optimum solvent because it has a large number of donors and tends to increase the viscosity of the non-aqueous electrolyte solution. When only the ester solvent is used, the solvation with lithium ions and the increase in viscosity increase the ion conduction resistance of the non-aqueous electrolyte solution and decrease the discharge capacity of the secondary battery.

On the other hand, when a mixed solvent of an ester solvent and a carbonate solvent having a larger number of donors than the carbonate solvent is used as in the present embodiment at an appropriate mixing ratio, the interaction between the lithium ion and the ester solvent It is possible to reduce the viscosity of the non-aqueous electrolyte solution. This is because the carbonate solvent has a smaller number of donors than the ester solvent and generally has a lower viscosity than the mixed solution of the ester solvent and the electrolyte salt. When the interaction is relaxed and the viscosity is lowered, the ionic conductivity becomes high, so that a high discharge capacity can be obtained.

The number of donors of a solvent (DN) is a measure of the electron pair donation of a molecule of a solvent proposed by Gutmann et al. The number of donors is defined as the heat of reaction when the solvent of interest forms a 1: 1 adduct with SbCl ₅ in an inert medium (dichloromethane), with hexamethylphosphoric acid triamide as the reference material. It is dimensionally standardized. The number of solvent donors can be determined from the chemical shift of Nuclear Magnetic Resonance (NMR).

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

<Electrolyte salt>
Electrolyte _{_{_{salt, LiPF 6, LiBF 4, LiClO}}} 4, LiCF 3 SO 3, LiCF 3 CO 2, LiAsF 6, LiSbF 6, lithium bis (fluorosulfonyl) imide (LiFSI), lithium bis (trifluoromethanesulfonyl) imide (LiTFSI ), Lithium bis (pentafluoroethanesulfonyl) imide (LiBETI), lithium tetrafluoroborate (LiBF ₄ ), lithium bisoxalate borate (LiBOB), lithium trifurate and other materials of various lithium salts. As the electrolyte salt, LiFSI, LiTFSI or LiBETI are preferable from the viewpoint of high dissociation degree 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) and the like can be used. As the ester solvent, γ-butyrolactone is preferable because it has excellent redox resistance.

The content of the ester solvent is preferably 30 to 80 mol% per non-aqueous solvent in the non-aqueous electrolyte solution from the viewpoint of making the viscosity of the non-aqueous electrolyte solution appropriate. The content of the ester solvent can be quantified by using NMR or the like.

<Carbonate solvent>
As the carbonate solvent, ethylene carbonate (EC, DN = 16.4), propylene carbonate (PC, DN = 15.1) and the like can be used. As the carbonate solvent, propylene carbonate is preferable from the viewpoint of increasing the ionic conductivity of the non-aqueous electrolyte solution.

The content of the carbonate solvent is preferably 0 to 60 mol% per total of the non-aqueous solvent used in the non-aqueous electrolyte solution from the viewpoint of improving the discharge capacity of the secondary battery. The content of the carbonate solvent can be quantified by 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, and even more preferably 0.12 to 0.52. With such a molar ratio, the discharge capacity of the secondary battery becomes higher than when only the ester solvent is used. The molar ratio of the carbonate solvent to the ester solvent can be determined by 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, the molar ratio 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 the molar ratio 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, and 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 smaller the ion conduction resistance of the non-aqueous electrolyte solution. Further, with such a molar ratio, since the molar ratio of the electrolyte salt is suppressed, the interaction between the ester solvent having a large number of donors and the lithium ion is weakened. As a result, lithium ions become easy to move, and the ionic conductivity of the non-aqueous electrolyte solution becomes high, 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 and a molar ratio of 0 to 0.21 when the molar ratio of the electrolyte salt to the ester solvent is 0 to 0.24 in the system of the ternary positive electrode and the graphite negative electrode. In the range of, it can be 375 mAh or more, and in the range of the molar ratio of 0 to 0.16, it can be 400 mAh or more.

<Film forming agent>
The film-forming agents are 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 difluorooxalate borate (LiDFOB).

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

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

As an index of stability in the presence of water, solubility in water and hydrolyzability can be mentioned. When the corrosion inhibitor is a solid, its solubility in water is preferably less than 1% at room temperature. The hydrolyzability of the corrosion inhibitor can be evaluated by performing a molecular structure analysis after absorbing the corrosion inhibitor or mixing it with water. When a moisture-absorbed corrosion inhibitor or a corrosion inhibitor mixed with water is heated at 100 ° C or higher to remove water, if 95 wt% or more of the residue has the same molecular structure as the original corrosion inhibitor, the corrosion. It can be said that the inhibitor is non-corrosive and stable in the presence of moisture.

As the corrosion inhibitor, an organic salt represented by (MR) ⁺ An ⁻ is particularly preferable. However, in the formula, (MR) ⁺ is a cation and An ⁻ is an anion. M is a central atom selected from nitrogen (N), boron (B), phosphorus (P) and sulfur (S), and R is a substituent that is totally substituted with respect to the central atom and is a hydrocarbon. It is the basis. Since such organic cations have high polarity, a highly stable corrosion inhibitor can be obtained.

Examples of the anion of the corrosion inhibitor, tetrafluoroborate anion (BF ₄ ^-), hexafluorophosphate anion (PF ₆ ^-) and the like are preferable. Since these anions are considered to react with metals to form a passivation 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 the elution of metal. As the organic salt, an ionic liquid that exists as a liquid at room temperature is particularly preferable in that it is highly non-volatile and has good heat resistance, chemical stability, and electrical stability.

Specific examples of the corrosion inhibitor include quaternary ammonium salts such as tetrabutylammonium hexafluorophosphate (TBA-PF6) and tetrabutylammonium tetrafluoroborate (TBA-BF4), and 1-ethyl-3-methylimidazolium tetra. Fluorobolate (EMI-BF4), 1-Ethyl-3-methylimidazolium hexafluorophosphate (EMI-PF6), 1-Butyl-3-methylimidazolium tetrafluoroborate (BMI-BF4), 1-Butyl-3- Examples thereof include imidazolium salts such as methyl imidazolium hexafluorophosphate (BMI-PF6).

The content of the corrosion inhibitor is preferably 0.5 to 20 wt% and more preferably 1 to 10 wt% with respect 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 or the like can be obtained, so that 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 not easily hindered, so that the ionic conductivity of the non-aqueous electrolytic solution can be secured. In addition, since the consumption of electrical energy due to self-discharge that dissociates and decomposes the corrosion inhibitor is reduced, there is a high possibility that a high discharge capacity can be obtained.

According to the above non-aqueous electrolyte solution, non-volatile electrolyte, and secondary battery using the above, the non-aqueous electrolyte solution is suitable as a mixed solvent of an ester solvent having a larger number of donors than the carbonate solvent and a carbonate solvent. Since the mixture is contained in an appropriate mixing ratio, the discharge capacity of the secondary battery can be improved as compared with the case where the carbonate solvent is not used in combination, even though the ester solvent having a large number of donors is used. Further, according to a non-aqueous electrolyte in which a non-aqueous electrolyte is supported on particles and a secondary battery in which a non-aqueous electrolyte is used as a separator formed between a positive electrode and a negative electrode, the non-aqueous electrolyte leaks or changes in composition. Is unlikely to occur, and a safe and long-life secondary battery can be realized.

As the secondary battery, a secondary battery including a negative electrode containing graphite as a negative electrode active material is particularly preferable. When a negative electrode containing graphite is used as the negative electrode active material, a relatively high discharge capacity can be obtained. Since the non-aqueous electrolyte solution contains an ester solvent, the reduction 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 graphite negative electrode. become. Further, despite the inclusion of the carbonate solvent, the co-insertion of the negative electrode active material between the layers is reduced. Therefore, according to the above secondary battery, high discharge capacity and good cycle characteristics can be obtained.

Hereinafter, the present invention will be specifically described with reference to examples, but the present invention is not limited to these examples.

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

The components contained in the prepared non-aqueous electrolytic solution 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. The VC content was 3 wt% with respect to the total of the electrolyte salt and the solvent, and the content of TBA-PF6 was 2.5 wt% with respect to the total of the electrolyte salt and the solvent.

<Making secondary batteries>
<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 the binder, the weight ratio of the solids 94: 4 Each was weighed to a ratio of: 2, and these were uniformly mixed in a kneader. The obtained mixture was slurried by adding NMP to adjust the solid content concentration. Then, the slurry whose concentration was adjusted was applied to both sides of the aluminum foil which is the positive electrode current collecting foil with a tabletop coater, and passed through a drying furnace at 120 ° C. to obtain a positive electrode. The total amount of the positive electrode mixture applied on both sides was 30.1 mg / cm ² . The electrode density of the obtained positive electrode was adjusted to 3.15 g / cm ³ by a roll press.

<Manufacturing of negative electrode>
Graphite as the negative electrode active material and styrene-butadiene rubber and carboxylmethyl cellulose as the binder were weighed so that the weight ratio of the solid content was 98: 1: 1, and these were uniformly mixed by a kneader. The obtained mixture was slurried by adding water to adjust the solid content concentration. Then, the slurry whose concentration was adjusted was applied to both sides of the copper foil, which is the negative electrode current collector foil, with a tabletop coater, and passed through a drying furnace at 100 ° C. to obtain a negative electrode. The total amount of the negative electrode mixture applied on both sides was 18.1 mg / cm ² . The electrode density of the obtained negative electrode was adjusted to 1.55 g / cm ³ by a roll press.

<Formation of separator>
The separator was formed by applying a non-volatile electrolyte to the surface of the electrode mixture layer. First, SiO ₂ having an average particle size of 1 μm as the supporting particles and vinylidene fluoride-hexafluoropropylene copolymer (P (VdF-HFP)) as the binder have a weight ratio of 89.3: 10.7. Each was weighed so as to be, and these were uniformly mixed in a kneader. The obtained mixture was slurried by adding NMP to adjust the solid content concentration. Then, the slurry having the adjusted concentration was applied to both the positive electrode and the negative electrode with a tabletop coater and passed through a drying oven at 100 ° C. to obtain a positive electrode and a negative electrode having a non-volatile electrolyte layer formed therein.

<Assembly>
The prepared positive electrode and negative electrode were punched with an air punching machine so that the positive electrode mixture layer had a size of 45 mm × 70 mm and the negative electrode mixture layer had a size of 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 microporous films made of resin having a thickness of 30 μm and a three-layer structure of PP / PE / PP, and three sides other than the side on which the tab portion was formed were heat-welded.

A positive electrode covered with a microporous film and a punched negative electrode were laminated in the order of negative electrode / positive electrode / negative electrode, and then a 50 μm-thick PTFE sheet was placed on the negative electrode. Each tab portion provided on the positive electrode and the negative electrode, an aluminum positive electrode terminal, and a nickel negative electrode terminal were welded by ultrasonic welding, respectively. The obtained electrode body is sandwiched between laminate films, one side for injecting liquid is left, and three sides including the side on which the tab portion is formed are heat-sealed at 200 ° C. by a laminate sealing device for 20 hours at 50 ° C. It was vacuum dried. Then, after injecting the non-aqueous electrolytic solution from one side for liquid injection, one side for liquid injection was vacuum-sealed to obtain a secondary battery.

<Measurement of discharge capacity>
The prepared secondary battery was held at 25 ° C., charged at a constant current with a charging rate of 0.05 C up to an upper limit voltage of 4.2 V, and then charged at a constant voltage at a voltage of 4.2 V until it decreased to 0.005 C. Then, a constant current was discharged at 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>
The components contained in the non-aqueous electrolyte solution were quantified and the discharge capacity was measured in the same manner as in Example 1 except that the composition of the non-aqueous electrolyte solution was changed as shown in Table 1.

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, and the types of ester-based solvent and carbonate-based solvent (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 the measurement result of the discharge capacity of the secondary battery is shown.

<Results and discussion>
FIG. 2 is a diagram showing the relationship between the molar ratio of the carbonate solvent to the ester solvent and the discharge capacity of the secondary battery. As shown in FIG. 2, in the composition in which the molar ratio of the carbonate solvent to the ester solvent was smaller than about 0.32, the larger the molar ratio, the larger the discharge capacity. This result is considered to be due to the fact that when PC having a low viscosity was added, the viscosity of the non-aqueous electrolyte solution decreased and the ionic conductivity of the non-aqueous electrolyte solution increased. In Comparative Example 1, since the concentration of the electrolyte salt was lower than that in Comparative Example 2, the viscosity of the non-aqueous electrolyte solution was reduced to some extent without adding PC, but the ion conduction resistance of the non-aqueous electrolyte solution was large. It is presumed that this is the cause of the reduced discharge capacity.

On the other hand, in a composition in which the molar ratio of the carbonate solvent to the ester solvent was larger than about 0.32, the smaller the molar ratio, the larger the discharge capacity. The reason for this is considered to be that as a result of the excessive addition of PC, the co-insertion of the negative electrode active material between the layers and the reductive decomposition of PC on the surface of the negative electrode active material increased. The optimum mixing ratio that can balance the ion conduction resistance of the non-aqueous electrolyte solution and the stability of the solvent at the negative electrode and increase the discharge capacity of the secondary battery is the ester solvent (GBL) and the carbonate solvent (PC). It was shown to exist.

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

Z = -660.34X ² + 427.01X + 357 ... Equation (1)

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

Z = -97.787X + 418.11 ... Equation (2)

According to the formula (1), the range of X that is larger than 357 mAh is 0 to 0.65 as in the case where the discharge capacity of the secondary battery is X = 0. The range of X having a discharge capacity of 380 mAh or more was 0.06 to 0.59. The range of X having a discharge capacity of 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 the composition in which the molar ratio of the electrolyte salt to the ester solvent was smaller than about 0.06, the larger the molar ratio, the larger the discharge capacity. On the other hand, in the composition in which the molar ratio of the electrolyte salt to the ester solvent was larger than about 0.06, the smaller the molar ratio, the larger the discharge capacity. If the concentration of the ester solvent that affects the dissociation of the electrolyte salt is too high with respect to the concentration of the electrolyte salt, the interaction between the electrolyte salt and the ester solvent becomes strong, and the ionic conductivity of the non-aqueous electrolyte solution decreases. Therefore, it is considered that the discharge capacity becomes smaller.

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

Z = -2388.8Y ² + 298.23Y + 412.71 ... Equation (3)

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

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 Exterior body 1000 Secondary battery

Claims

A non-aqueous electrolyte solution 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 number of donors of the ester solvent is larger than the number of donors of the carbonate solvent.
A non-aqueous electrolytic solution in which the molar ratio of the carbonate solvent to the ester solvent is 0 to 0.65.
The non-aqueous electrolyte solution according to claim 1.
A non-aqueous electrolytic solution in which the molar ratio of the carbonate solvent to the ester solvent is 0.06 to 0.59.
The non-aqueous electrolyte solution according to claim 1.
A non-aqueous electrolyte solution in which the molar ratio of the electrolyte salt to the ester solvent is 0 to 0.24.
The non-aqueous electrolyte solution according to claim 1.
A non-aqueous electrolyte solution in which the molar ratio of the electrolyte salt to the ester solvent is 0 to 0.20.
The non-aqueous electrolyte solution according to claim 1.
The ester solvent is a non-aqueous electrolytic solution which is γ-butyrolactone.
The non-aqueous electrolyte solution according to claim 1.
The carbonate solvent is a non-aqueous electrolyte solution which is propylene carbonate.
The non-aqueous electrolyte solution of claim 1 and
A non-volatile electrolyte containing particles supporting the non-aqueous electrolyte solution.
The non-volatile electrolyte according to claim 7.
The particles are non-volatile electrolytes that are silica.
With the positive electrode
With the negative electrode
A separator formed between the positive electrode and the negative electrode is provided.
The separator contains a non-aqueous electrolytic solution and particles supporting the non-aqueous electrolytic solution.
The non-aqueous electrolyte solution is a non-aqueous electrolyte solution 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 number of donors of the ester solvent is larger than the number of donors of the carbonate solvent.
A secondary battery in which the molar ratio of the carbonate solvent to the ester solvent is 0 to 0.65.