CN115461907A

CN115461907A - Nonaqueous electrolyte solution, semisolid electrolyte layer, sheet for secondary battery, and secondary battery

Info

Publication number: CN115461907A
Application number: CN202180028662.8A
Authority: CN
Inventors: 宇根本笃; 熊代祥晃; 关荣二; 上野和英; 独古薰; 渡边正义
Original assignee: Hitachi Ltd; Yokohama National University NUC
Current assignee: Hitachi Ltd; Yokohama National University NUC
Priority date: 2020-05-08
Filing date: 2021-04-15
Publication date: 2022-12-09
Also published as: JP2021177470A; WO2021225065A1

Abstract

The purpose of the present invention is to provide a nonaqueous electrolyte solution that can maintain the discharge capacity maintenance rate of a secondary battery at a high level when the secondary battery is operated at a high temperature. The nonaqueous electrolytic solution is characterized by containing an electrolyte salt and an organic solvent, wherein the organic solvent contains a main solvent selected from at least one of sulfolane and derivatives thereof and a low-viscosity organic solvent, the concentration of the electrolyte salt relative to the main solvent is 1.06 mol/L-3.46 mol/L, and the relative dielectric constant of the organic solvent is 63 or less.

Description

Nonaqueous electrolyte solution, semisolid electrolyte layer, sheet for secondary battery, and secondary battery

Technical Field

The invention relates to a nonaqueous electrolytic solution, a semisolid electrolyte layer, a sheet for a secondary battery, and a secondary battery.

Background

As a prior art relating to nonaqueous electrolytic solutions used for various secondary batteries, patent document 1 discloses a liquid or gel-state nonaqueous electrolyte containing a nonaqueous solvent, an electrolyte salt, an overcharge control agent that causes a redox reaction at a predetermined potential, and at least one selected from a heat-stable salt that is highly thermally stable and stably remains in the nonaqueous electrolytic solution, a protective film-forming material that forms a protective film on a positive electrode and a negative electrode to suppress decomposition of the overcharge control agent, and a complex-forming agent that forms a complex with a transition metal.

Documents of the prior art

Patent document

Patent document 1: japanese patent laid-open No. 2012-256502

Disclosure of Invention

Technical problems to be solved by the invention

With the electrolyte solution in the lithium-ion secondary battery of patent document 1, there is room for improvement in that the discharge capacity retention rate of the secondary battery when the secondary battery is operated at a relatively high temperature cannot be sufficiently maintained at a relatively high level.

The purpose of the present invention is to provide a nonaqueous electrolyte solution that can maintain the discharge capacity maintenance rate of a secondary battery at a high level when the secondary battery is operated at a high temperature. It is also an object of the present invention to provide a semi-solid electrolyte layer, a sheet for a secondary battery, and a secondary battery using the non-aqueous electrolyte.

Technical solution for solving technical problem

In order to solve the above technical problem, the nonaqueous electrolytic solution of the present invention is characterized by containing an electrolyte salt and an organic solvent, wherein the organic solvent contains a main solvent selected from at least one of sulfolane and derivatives thereof and a low-viscosity organic solvent, the concentration of the electrolyte salt relative to the main solvent is 1.06mol/L to 3.46mol/L, and the relative dielectric constant of the organic solvent is 63 or less.

This specification contains the disclosure of Japanese patent application No. 2020-082631, which is the basis of priority of the present application.

Effects of the invention

According to the nonaqueous electrolytic solution of the present invention, a nonaqueous electrolytic solution is provided which can maintain the discharge capacity maintenance rate of a secondary battery at a high level when the secondary battery is operated at a high temperature. Problems, structures, and effects other than those described above will become more apparent from the following description of the embodiments.

Drawings

Fig. 1 is a sectional view showing one embodiment of a secondary battery of the present invention.

FIG. 2 is a graph showing the relationship between the lithium salt concentration (mol/L) relative to the main solvent and the discharge capacity maintaining rate (%) at 45 ℃ in examples and comparative examples.

FIG. 3 is a graph showing the relationship between the relative dielectric constant of the organic solvent (mixed solvent) and the discharge capacity maintaining rate (%) at 45 ℃ in examples and comparative examples.

Detailed Description

Hereinafter, embodiments of the present invention will be described with reference to the drawings and the like. The following description is a specific example showing the content of the present invention, and the present invention is not limited to the description, and various changes and modifications can be made by those skilled in the art within the scope of the technical idea disclosed in the present specification. In all the drawings for describing the present invention, the same reference numerals are given to the components having the same functions, and the redundant description thereof may be omitted.

In the present specification, "to" means to have the meaning that the numerical values described before and after the "to" are the lower limit value and the upper limit value. When the upper limit value or the lower limit value is 0, the upper limit value or the lower limit value is not included. In the numerical ranges recited in the present specification, the upper limit or the lower limit recited in one numerical range may be replaced with another upper limit or a lower limit recited in another numerical range. The upper limit or the lower limit of the numerical range described in the present specification may be replaced with values shown in the examples.

As one embodiment of the secondary battery of the present invention, a lithium ion secondary battery will be explained below as an example. A lithium ion secondary battery is an electrochemical device capable of storing or utilizing electric energy by occlusion and release of lithium ions in an electrolyte to electrodes. Lithium ion secondary batteries are also called lithium ion batteries, nonaqueous electrolyte secondary batteries and other names, and these batteries are the objects of the present invention. 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.

In the case where a material is selected from the material group exemplified below, one material may be selected alone or a plurality of materials may be selected in combination within a range not inconsistent with the disclosure in the present specification. In addition, materials other than the material group exemplified below may be selected within a range not inconsistent with the disclosure in the present specification.

Fig. 1 is a sectional view of a lithium-ion secondary battery according to an embodiment of the present invention. Fig. 1 shows a stacked lithium ion secondary battery, and a lithium ion secondary battery 1000 includes a positive electrode 100, a negative electrode 200, an outer package 500, and an insulating layer 300. The exterior 500 houses the insulating layer 300, the positive electrode 100, and the negative electrode 200. The exterior body 500 is selected from a group of materials having corrosion resistance to the nonaqueous electrolytic solution, such as aluminum, stainless steel, and nickel-plated steel. The lithium ion secondary battery may have a wound structure.

In the lithium ion secondary battery 1000, an electrode assembly 400 including a positive electrode 100, an insulating layer 300, and a negative electrode 200 is stacked to form an electrode group. Hereinafter, the positive electrode 100 or the negative electrode 200 may be referred to as an electrode. The member obtained by laminating the positive electrode 100, the negative electrode 200, or both of them and the insulating layer 300 may be referred to as a secondary battery sheet. When the insulating layer 300 and the electrodes are formed as an integral structure, the electrode group can be produced by merely stacking the secondary battery sheets.

The positive electrode 100 includes a positive electrode current collector 120 and a positive electrode mixture layer 110. The positive electrode mixture layer 110 is formed on both surfaces of the positive electrode current collector 120. Negative electrode 200 includes negative electrode current collector 220 and negative electrode mixture layer 210. Negative electrode mixture layers 210 are formed on both surfaces of the negative electrode current collector 220. The positive electrode mixture layer 110 or the negative electrode mixture layer 210 may be referred to as an electrode mixture layer, and the positive electrode collector 120 or the negative electrode collector 220 may be referred to as an electrode collector.

The positive electrode collector 120 has a positive electrode sheet 130. The negative electrode collector 220 has a negative electrode tab 230. The positive electrode tab 130 or the negative electrode tab 230 is sometimes referred to as an electrode tab. The electrode mixture layer is not formed on the electrode sheet. However, the electrode mixture layer may be formed on the electrode sheet within a range that does not adversely affect the performance of the lithium-ion secondary battery 1000. The positive electrode sheets 130 and the negative electrode sheets 230 protrude outside the exterior body 500, and the protruding positive electrode sheets 130 and the protruding negative electrode sheets 230 are joined to each other by, for example, ultrasonic welding, thereby forming parallel connections in the lithium ion secondary battery 1000. The lithium ion secondary battery of the present invention may have a bipolar structure in which the secondary batteries are electrically connected in series.

The positive electrode mixture layer 110 contains a positive electrode active material, a positive electrode conductive agent, and a positive electrode binder. Negative electrode mixture layer 210 contains a negative electrode active material, a negative electrode conductive agent, and a negative electrode binder. The positive electrode active material or the negative electrode active material is sometimes referred to as an electrode active material, the positive electrode conductive agent or the negative electrode conductive agent is sometimes referred to as an electrode conductive agent, and the positive electrode binder or the negative electrode binder is sometimes referred to as an electrode binder.

< electrode conductive agent >

The electrode conductive agent is used to improve the conductivity of the electrode mixture layer. The electrode conductive agent may be appropriately selected from the group consisting of ketjen black, acetylene black, and graphite.

< electrode Binder >

The electrode binder is used to bind an electrode active material in an electrode to an electrode conductive agent and the like. The electrode binder may be appropriately selected from the group consisting of Styrene Butadiene Rubber (SBR), carboxymethylcellulose (CMC), polyvinylidene fluoride (PVDF), and a copolymer of vinylidene fluoride (VDF) and Hexafluoropropylene (HFP) (P (VDF-HFP)).

< Positive electrode active Material >

The positive electrode active material exhibiting a high potential is released with lithium ions during charging, and the lithium ions released from the negative electrode active material are inserted during discharging. As the positive electrode active material, a lithium composite oxide having a transition metal is preferable. Specific examples thereof include LiMO ₂ Li [ LiM ] with Li excess]O ₂ 、LiM ₂ O ₄ 、LiMPO ₄ 、LiMVO ₄ 、LiMBO ₃ 、Li ₂ MSiO ₄ (wherein M is at least one member selected from the group consisting of Co, ni, mn, fe, cr, zn, ta, al, mg, cu, cd, mo, nb, W, ru, etc.). In addition, part of oxygen in these materials may be replaced with another element such as fluorine. Furthermore, the positive electrode active material may also contain a material selected from TiS ₂ 、MoS ₂ 、Mo ₆ S ₈ 、TiSe ₂ Isochalcogenides, or V ₂ O ₅ Vanadium-containing oxide, feF ₃ Isohalides, fe (MoO) constituting polyanions ₄ ) ₃ 、Fe ₂ (SO ₄ ) ₃ 、Li ₃ Fe ₂ (PO ₄ ) ₃ And at least one material selected from the group consisting of quinone organic crystals, oxygen, and the like.

< Positive electrode Current collector 120>

The positive electrode current collector 120 may be suitably selected from the group consisting of aluminum foil having a thickness of 1 to 100 μm, perforated aluminum foil having a thickness of 10 to 100 μm and having pores with a diameter of 0.1 to 10mm, metal mesh, metal foam, stainless steel, titanium, and other materials.

< negative electrode active Material >

The negative electrode active material exhibiting a low potential is released with lithium ions during discharge, and lithium ions released from the positive electrode active material in the positive electrode mixture layer 110 are inserted during charge. The negative electrode active material may be selected from carbon-based materials (graphite, easily graphitizable carbon materials, amorphous carbon materials, organic crystals, activated carbon, etc.), conductive polymer materials (polyacene, polyparaphenylene, polyaniline, polyacetylene, etc.), lithium composite oxides (lithium titanate: li) ₄ Ti ₅ O ₁₂ Or Li ₂ TiO ₄ Etc.), metallic lithium, a metal alloyed with lithium (having at least one or more of aluminum, silicon, tin, etc.), or an oxide thereof.

< negative electrode Current collector 220>

The negative electrode current collector 220 may be suitably selected from the group consisting of copper foil having a thickness of 1 to 100 μm, copper perforated foil having a thickness of 1 to 100 μm and a pore diameter of 0.1 to 10mm, metal mesh, metal foam, stainless steel, titanium, nickel, and the like.

< electrode >

An electrode slurry obtained by mixing an electrode active material, an electrode conductive agent, an electrode binder, and a solvent is attached to an electrode current collector by a coating method such as a doctor blade method, a dipping method, or a spray method, thereby producing an electrode mixture layer. The solvent is selected from the group of materials such as N-methylpyrrolidone (NMP) and water. Thereafter, the electrode mixture layer is dried to remove the solvent, and the electrode mixture layer is pressure-molded by roll pressing, thereby producing an electrode.

When the nonaqueous electrolyte solution is contained in the electrode mixture layer, the content of the nonaqueous electrolyte solution in the electrode mixture layer is preferably 20 to 40 vol%. When the content of the nonaqueous electrolytic solution is small, an ion conduction path inside the electrode mixture layer may not be sufficiently formed, and the rate characteristics may be degraded. In addition, when the content of the nonaqueous electrolytic solution is large, the nonaqueous electrolytic solution may leak from the electrode mixture layer, and the relative amount of the electrode active material may be insufficient, which may reduce the energy density.

In order to contain the nonaqueous electrolyte solution in the electrode mixture layer, the nonaqueous electrolyte solution may be injected into the lithium ion secondary battery 1000 from the open side or the injection hole of the exterior body 500, and the nonaqueous electrolyte solution may be filled into the pores of the electrode mixture layer. Alternatively, a slurry obtained by mixing the nonaqueous electrolytic solution, the electrode active material, the electrode conductive agent, and the electrode binder may be prepared, and the prepared slurry may be applied to the electrode current collector together to fill the nonaqueous electrolytic solution into the pores of the electrode mixture layer. Thus, the particles such as the electrode active material and the electrode conductive agent in the electrode mixture layer can function as the carrier particles without the carrier particles contained in the semi-solid electrolyte, and the nonaqueous electrolytic solution is held by these particles.

The thickness of the electrode mixture layer is preferably equal to or greater than the average particle diameter of the electrode active material. If the thickness of the electrode mixture layer is small, the electron conductivity between adjacent electrode active materials may be deteriorated. When coarse particles having an average particle size of not less than the thickness of the electrode material mixture layer are present in the electrode active material powder, the coarse particles are preferably removed in advance by sieving, air classification, or the like to produce particles having a thickness of not more than the thickness of the electrode material mixture layer.

< insulating layer 300>

Insulating layer 300 is a medium for transferring ions between positive electrode 100 and negative electrode 200. Insulating layer 300 also functions as an electron insulator to prevent short circuit between positive electrode 100 and negative electrode 200. The insulating layer 300 may have a semi-solid electrolyte layer. As the insulating layer 300, a separator and a semisolid electrolyte layer may be used in combination.

< separator >

As the separator, a porous sheet may be used. The porous sheet may be selected from the group consisting of cellulose, modified cellulose (carboxymethyl cellulose (CMC), hydroxypropyl cellulose (HPC), and the like), polyolefin (polypropylene (PP), copolymer of propylene, and the like), polyester (polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polybutylene terephthalate (PBT), and the like), polyacrylonitrile (PAN), resin such as aramid, polyamideimide, and polyimide, and glass. By making the area of the separator larger than that of the positive electrode 100 or the negative electrode 200, short circuit between the positive electrode 100 and the negative electrode 200 can be prevented.

The separator may be formed by coating a separator-forming mixture having separator particles, a separator binder, and a solvent on the electrode mixture layer. Alternatively, the porous sheet may be coated with a mixture for forming a separator.

The separator particles may be made from gamma-alumina (Al) ₂ O ₃ ) Silicon dioxide (SiO) ₂ ) Zirconium oxide (ZrO) ₂ ) Etc. are selected from the group of materials. The average particle diameter of the separator particles is preferably 1/100 to 1/2 of the thickness of the separator. The separator binder can be appropriately selected from Polyethylene (PE), PP, polytetrafluoroethylene (PTFE), PVDF, P (VdF-HFP), styrene Butadiene Rubber (SBR), alginic acid, polyacrylic acid, and the like.

When the insulating layer 300 includes a separator, the nonaqueous electrolytic solution may be injected into the lithium ion secondary battery 1000 from the open side or the injection hole of the exterior body 500, and the separator may be filled with the nonaqueous electrolytic solution.

< semi-solid electrolyte layer >

The semi-solid electrolyte layer has a semi-solid electrolyte binder and a semi-solid electrolyte. The semi-solid electrolyte has supported particles and a non-aqueous electrolyte. The semi-solid electrolyte has pores formed of an aggregate of the supported particles, and retains a nonaqueous electrolytic solution therein. By holding the nonaqueous electrolytic solution in the semisolid electrolyte, the semisolid electrolyte permeates lithium ions. In the case where a semi-solid electrolyte layer is used as the insulating layer 300 and a nonaqueous electrolytic solution is filled in the electrode mixture layer, it is not necessary to inject a nonaqueous electrolytic solution into the lithium ion secondary battery 1000.

The semi-solid electrolyte layer can be handled as a solid, although it contains a liquid component such as a nonaqueous electrolyte solution, and may be a translucent self-supporting film. Locally, a liquid component such as a nonaqueous electrolytic solution is responsible for lithium ion conduction, and therefore exhibits high ion conductivity. That is, the semisolid electrolyte layer has both advantages of high safety possessed by a solid and high ion conduction characteristics possessed by a liquid.

As a method for producing the semisolid electrolyte layer, there are a method of compression-molding a powder of a semisolid electrolyte into a pellet form using a molding die or the like, a method of adding a semisolid electrolyte binder to a powder of a semisolid electrolyte and mixing them to prepare a sheet, and the like. By adding and mixing the semisolid electrolyte binder to the semisolid electrolyte powder, a sheet-like semisolid electrolyte layer having high flexibility can be produced. The semisolid electrolyte layer may be produced by adding a solution of a binder in which a semisolid electrolyte binder is dissolved in a dispersion solvent to a semisolid electrolyte, mixing the solution, applying the mixture to a substrate such as an electrode, and drying the mixture to remove the dispersion solvent.

< Supported particles >

The carrier particles are preferably insulating particles and insoluble in the nonaqueous electrolytic solution from the viewpoint of electrochemical stability. The support particles may be selected from SiO ₂ Particles of Al ₂ O ₃ Particles, cerium oxide (CeO) ₂ ) Particles, zrO ₂ The material is suitably selected from the group consisting of inorganic oxide particles such as particles, and solid electrolytes. By using the oxide inorganic particles as the carrier particles, the nonaqueous electrolytic solution can be held at a high concentration in the semisolid electrolyte layer. Further, since no gas is generated from the oxide inorganic particles, the semisolid electrolyte layer can be produced by a roll-to-roll process in the atmosphere. The solid electrolyte may be selected from oxide-based solid electrolytes such as Li-La-Zr-O, and Li ₁₀ Ge ₂ PS ₁₂ And the sulfide-based solid electrolyte and the like are appropriately selected and used.

The amount of the nonaqueous electrolytic solution to be held is considered to be proportional to the specific surface area of the supporting particles, and therefore, the average particle diameter of the primary particles of the supporting particles is preferably 1nm to 10 μm. If the average particle diameter of the primary particles of the carrier particles is large, the carrier particles cannot properly hold a sufficient amount of the nonaqueous electrolytic solution, possibly causing difficulty in the formation of a semisolid electrolyte. Further, if the average particle diameter of the primary particles of the supported particles is small, the interfacial force between the supported particles increases, the supported particles easily aggregate with each other, and the formation of a semisolid electrolyte may become difficult. The average particle diameter of the primary particles of the carrier particles is more preferably in the range of 1nm to 50nm, and still more preferably in the range of 1nm to 10 nm. The average particle diameter of the primary particles supporting the particles can be measured by TEM.

< nonaqueous electrolyte solution >

The nonaqueous electrolytic solution contains a main solvent, and a low-viscosity organic solvent having a relative dielectric constant different from that of the main solvent, an electrolyte salt, and optionally a negative electrode interface stabilizing material. The main solvent has a high relative dielectric constant, and has an effect of increasing the lithium ion concentration by dissociation of the lithium salt. The main solvent is at least one selected from the group consisting of sulfolane and derivatives thereof (also referred to as "sulfolane and/or derivatives thereof"). Thus, the main solvent may contain two or more solvents selected from sulfolane and its derivatives, for example, two, three or four solvents. Sulfolane and/or its derivatives constitute a solvated ionic liquid together with an electrolyte salt. In the following description, sulfolane and/or its derivative may be referred to as a main solvent. The components contained in the nonaqueous electrolytic solution can be measured by NMR or the like. The relative dielectric constants of the main solvent and the low-viscosity organic solvent can be measured using a relative dielectric constant measuring apparatus.

The content of the nonaqueous electrolytic solution in the semisolid electrolyte layer is not particularly limited, and is preferably 40 to 90 vol%. When the content of the nonaqueous electrolytic solution is small, the interface resistance between the electrode and the semisolid electrolyte layer may increase. In addition, when the content of the nonaqueous electrolytic solution is large, there is a risk that the nonaqueous electrolytic solution leaks from the semisolid electrolyte layer. In the case where the semi-solid electrolyte layer is in the form of a sheet, the content of the nonaqueous electrolyte solution in the semi-solid electrolyte layer is preferably 50 to 80 vol%, more preferably 60 to 80 vol%. In the case where the semi-solid electrolyte layer is formed by coating a mixture of the semi-solid electrolyte and a solution in which a semi-solid electrolyte binder is dissolved in a dispersion solvent on the electrode, the content of the nonaqueous electrolytic solution in the semi-solid electrolyte layer is preferably 40 to 60 vol%.

The weight ratio of the main solvent in the nonaqueous electrolytic solution is not particularly limited, but from the viewpoint of stability of the lithium ion secondary battery, high-speed charge and discharge are possible, and therefore, the weight ratio of the main solvent in the nonaqueous electrolytic solution is preferably 30% by weight (wt%) to 70 wt%, particularly preferably 40% by weight to 60% by weight, and more preferably 45% by weight to 55% by weight.

< solvating Ionic liquids >

The solvated ionic liquid has sulfolane and/or a derivative thereof, and an electrolyte salt. When a solvated ionic liquid containing sulfolane and/or its derivative is used, since sulfolane and/or its derivative forms an inherent coordination structure with lithium ions, the transport speed of lithium ions in the semisolid electrolyte layer is increased. Therefore, unlike a solvated ionic liquid including an ether solvent and an electrolyte salt, which have input/output characteristics of a secondary battery that decrease with increasing viscosity, even if the viscosity of the solvated ionic liquid is increased, it is possible to suppress a decrease in input/output characteristics of a secondary battery including the solvated ionic liquid.

Examples of the sulfolane derivative include those in which a hydrogen atom bonded to a carbon atom constituting a sulfolane ring is substituted with a fluorine atom, an alkyl group, or the like. Specifically, it can be appropriately selected from the group of materials such as fluorosulfolane, difluorosulfolane, and methylsulfolane.

< electrolyte salt >

As the electrolyte salt, a salt capable of being uniformly dispersed in a low-viscosity organic solvent is preferable, and in the case where the cation is lithium, various lithium salts can be used. As the electrolyte salt, lithium tetrafluoroborate (LiBF) can be selected ₄ ) Lithium bis (trifluoromethanesulfonylimide) (LiTFSI), lithium bis (fluorosulfonylimide) (LiFSI), liClO ₄ Lithium bifluorosulfonamide (LiFSA), lithium bistrifluoromethanesulfonamide (LiTFSA), a mixture of two or more of these, and the likeThe material satisfying the above conditions is appropriately selected and used. LiBF is preferably used ₄ As an electrolyte salt. LiBF ₄ The negative electrode active material such as graphite is stable, and the capacity of the secondary battery can be improved.

Solvated ionic liquids having sulfolane and/or derivatives thereof, and an electrolyte salt can be represented in an apparent compositional entity. For example, a solvated ionic liquid composed of sulfolane and LiTFSI can be expressed as Li (SL) as the apparent composition _x TFSI (x =2 to 6), the number of moles calculated as a single substance having this composition.

The concentration of the electrolyte salt with respect to the main solvent is preferably 1.06mol/L to 3.46mol/L. If the concentration is too high, the viscosity tends to increase, the resistance tends to increase, and the capacity tends to decrease. If the concentration is too low, the solvent tends to be unstable and the life characteristics tend to be lowered.

< Low viscosity organic solvent >

The low-viscosity organic solvent is an organic solvent having a relative dielectric constant different from that of the main solvent and having a viscosity lower than that of the solvated ionic liquid. Therefore, by mixing a low-viscosity organic solvent with the solvated ionic liquid, the viscosity of the solvated ionic liquid can be reduced, and the ionic conductivity can be improved. In addition, when the internal resistance of the nonaqueous electrolytic solution is large, the ionic conductivity of the nonaqueous electrolytic solution can be increased by adding a low-viscosity organic solvent, and the internal resistance of the nonaqueous electrolytic solution can be reduced.

The low-viscosity solvent may be suitably selected from the group consisting of cyclic carbonates such as Ethylene Carbonate (EC), propylene Carbonate (PC), 1,2-Butylene Carbonate (BC), and fluoroethylene carbonate (FEC), acyclic esters such as Ethyl Methyl Carbonate (EMC), dimethyl carbonate (DMC), and diethyl carbonate (DEC), phosphoric esters such as trimethyl phosphate (TMP), triethyl phosphate (TEP), tris (2,2,2-trifluoroethyl) phosphite (TFP), and dimethyl methyl phosphonate (DMMP), lactones such as γ -butyrolactone (GBL), and mixtures of two or more of these.

When the nonaqueous electrolytic solution contains one or more main solvents and one or more low-viscosity organic solvents, the relative dielectric constant of the i-th main solvent is p _i Q represents a molar ratio of the i-th main solvent in the mixed solvent _i And the relative dielectric constant of the i-th low-viscosity organic solvent is p' _i And the molar ratio of the i-th low-viscosity organic solvent in the mixed solvent is q' _i In this case, the relative dielectric constant Z of the mixed solvent (main solvent + low-viscosity organic solvent) can be represented by the following formula (1).

(wherein n is the number of types of main solvents contained in the mixed solvent, and m is the number of types of low-viscosity organic solvents contained in the mixed solvent.)

Preferably, Z is 63 or less.

For example, when the nonaqueous electrolytic solution contains one kind of main solvent and one kind of low-viscosity organic solvent, the relative dielectric constant Z of the mixed solvent (main solvent + low-viscosity organic solvent) can be represented by the following formula (2) when the relative dielectric constant of the main solvent is p, the molar ratio of the main solvent in the mixed solvent is q, the relative dielectric constant of the low-viscosity organic solvent is p ', and the molar ratio of the low-viscosity organic solvent in the mixed solvent is q'.

Z = p × q + p '× q' · formula (2)

Preferably, Z is 63 or less.

The addition of a low-viscosity organic solvent having a low relative dielectric constant to the solvated ionic liquid can reduce the viscosity of the solvated ionic liquid. On the other hand, if a low-viscosity organic solvent is excessively added to the solvated ionic liquid, the dissociation degree of the lithium salt decreases, and a good SEI (Solid Electrolyte phase interface film) cannot be formed, and the lifetime characteristics at high temperatures (45 ℃) decrease. Therefore, when Z is 63 or less, the viscosity of the solvated ionic liquid can be reduced, and the decrease in the dissociation degree of the lithium salt can be suppressed, thereby improving the lifetime characteristics at high temperatures (45 ℃). The method for measuring the molar ratio of the main solvent in the mixed solvent and the molar ratio of the low-viscosity organic solvent in the mixed solvent can be measured by NMR.

< Material for stabilizing interface of negative electrode >

By containing the negative electrode interface stabilizing material in the nonaqueous electrolytic solution, the rate characteristics of the secondary battery can be improved and the battery life can be improved. The negative electrode interface stabilizing material may be appropriately selected from the group consisting of Vinylene Carbonate (VC), fluoroethylene carbonate (FEC), and the like.

< anticorrosive agent >

The nonaqueous electrolytic solution may contain an anticorrosive agent as necessary. The anticorrosive agent can form a coating film in which a metal is not easily eluted even when the positive electrode current collector 120 is exposed to a high electrochemical potential. As the anticorrosive agent, PF is preferably contained ₆ Or BF ₄ Such anionic species and include materials having cationic species with strong chemical bonds for forming stable compounds in an aqueous atmosphere.

As an index of a compound which is stable in the atmosphere, solubility in water or presence or absence of hydrolysis can be cited. In the case where the anticorrosive agent is a solid, the solubility to water is preferably less than 1%. The presence or absence of hydrolysis can be evaluated by analyzing the molecular structure of the sample mixed with water. Here, the term "hydrolysis-free" means that the anticorrosive absorbs moisture or is mixed with water, and then the water is removed by heating at a temperature of 100 ℃ or higher, and 95% of the residue has the same molecular structure as the original anticorrosive.

The anticorrosive agent is represented by (M-R) ⁺ An ^－。(M－R) ⁺ An ^－ The cation of (A) is (M-R) ⁺ . M is selected from nitrogen (N), boron (B), phosphorus (P) or sulfur (S). R is composed of a hydrocarbon group.

(M－R) ⁺ An ^－ The anion of (A) is An ^－ . As An ^－ Preferably, BF is used ₄ ^－ Or PF ₆ ^－ . The anion passing through the anticorrosive being BF ₄ ^－ Or PF ₆ ^－ The elution of the positive electrode current collector 120 can be effectively suppressed. It is considered that this is due to BF ₄ ^－ Or PF ₆ ^－ The F anion (B) has an influence on a passivation film formed by reacting with SUS or aluminum of the electrode current collector。

The corrosion inhibitor may be selected from tetrabutylammonium hexafluorophosphate (TBAPF) ₆ ) Tetrabutylammonium Tetrafluoroborate (TBABF) ₄ ) Isoquaternary ammonium salt, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI-BF) ₄ ) 1-ethyl-3-methylimidazolium hexafluorophosphate (EMI-PF) ₆ ) 1-butyl-3-methylimidazolium tetrafluoroborate (BMI-BF) ₄ ) 1-butyl-3-methylimidazolium hexafluorophosphate (BMI-PF) ₆ ) And imidazolium salts and the like are appropriately selected and used. In particular the anion being PF ₆ ^－ In this case, elution of the positive electrode current collector 120 can be appropriately suppressed.

The content of the anticorrosive is preferably 0.5 to 20 wt%, more preferably 1 to 10 wt%, based on the total weight of the nonaqueous electrolytic solution. If the content of the anticorrosive agent is small, the effect of suppressing elution of the electrode current collector is reduced, and the battery capacity may be reduced with charge and discharge. In addition, if the content of the anticorrosive agent is large, lithium ion conductivity decreases, and a large amount of stored energy is consumed for decomposition of the anticorrosive agent, with the result that battery capacity may decrease.

< semi-solid electrolyte Binder >

As the semisolid electrolyte binder, a fluorine-based resin is preferably used. The fluorine-based resin can be selected and used as appropriate from the group consisting of PTFE, PVDF, and P (VdF-HFP). By using PVDF or P (VdF-HFP), the adhesion between the insulating layer 300 and the electrode current collector is improved, and thus the battery performance is improved.

< semi-solid electrolyte >

The nonaqueous electrolytic solution is supported or held by the supporting particles, whereby a semisolid electrolyte is formed. Examples of the method for producing the semi-solid electrolyte include the following methods: the method for producing a semisolid electrolyte includes mixing a nonaqueous electrolytic solution and carrier particles at a specific volume ratio, adding an organic solvent such as methanol, mixing the mixture to prepare a slurry of a semisolid electrolyte, spreading the slurry on a petri dish or the like, and removing the organic solvent to obtain a powder of a semisolid electrolyte.

Examples

The present invention will be described more specifically with reference to examples and comparative examples, but the present invention is not limited to these examples.

< example 1>

< preparation of nonaqueous electrolyte solution >

Lithium salt (Li salt) lithium bistrifluoromethanesulfonamide (LiTFSA) as an electrolyte salt, sulfolane (SL) as a main solvent, propylene Carbonate (PC) as a low-viscosity solvent, vinylene Carbonate (VC) as a graphite film-forming material for stabilizing the interface of a negative electrode, and tetrabutylammonium hexafluorophosphate (TBA-PF) as an Al collector foil inhibitor as an anticorrosive were weighed out separately ₆ ) And mixing them to prepare a nonaqueous electrolyte. The components contained in the obtained nonaqueous electrolytic solution were quantified by NMR. As a result, the molar ratio of the main Solvent (SL) to the low-viscosity solvent (PC) was 65.4: 34.6, and the lithium salt concentration was 2.37mol/L.

< preparation of Positive electrode >

LiNi as a positive electrode active material was weighed so that the weight ratio of the solid components was 94: 4: 2, respectively _1/3 Co _1/ ₃ Mn _1/3 O ₂ The base oxide, acetylene black as a positive electrode conductive agent, and PVDF dissolved in N-methylpyrrolidone as a positive electrode binder were uniformly mixed by a mixer. NMP was added to the obtained mixture to form a slurry, and the slurry was adjusted to a predetermined solid content concentration. Subsequently, the slurry with the adjusted concentration was applied to both surfaces of an aluminum foil as a positive electrode current collector foil by a bench coater, and passed through a drying furnace at 120 ℃. The amount of the positive electrode mixture (positive electrode active material + positive electrode conductive agent + positive electrode binder) applied was 30.1mg/cm in total of both surfaces ² . The obtained positive electrode was adjusted to an electrode density of 3.15g/cm by roll pressing ³ 。

< preparation of negative electrode >

Graphite as a negative electrode active material, styrene-butadiene rubber as a negative electrode binder, and carboxymethyl cellulose were weighed in a weight ratio of the solid components of 98: 1, respectively, and mixed uniformly by a mixer. Water was added to the obtained mixture to form a slurry, and the solid content concentration was adjusted to a predetermined value. Is connected withThen, the slurry having the adjusted concentration was applied to both sides of a copper foil as a negative electrode current collector by a desk coater, and the coated copper foil was passed through a drying furnace at 100 ℃. The coating amount of the negative electrode mixture (negative electrode active material + negative electrode binder) was 18.1mg/cm in total of both surfaces ² . The obtained negative electrode was adjusted to an electrode density of 1.55g/cm by roll pressing ³ 。

< formation of separator >

The separator is formed by applying a nonvolatile electrolyte to the surface of the electrode material mixture layer. First, siO having an average particle diameter of 1 μm was weighed as carrier particles at a weight ratio of 89.3: 10.7 ₂ And a vinylidene fluoride-hexafluoropropylene copolymer (P (VdF-HFP)) as a binder, and these were uniformly mixed by a mixer. NMP was added to the obtained mixture to form a slurry, and the solid content was adjusted to a predetermined concentration. Subsequently, the slurry having the adjusted concentration was applied to both surfaces of the positive electrode and the negative electrode by a desk coater, and the resultant was introduced into a drying furnace at 100 ℃.

< production of Secondary Battery >

< Assembly >

The obtained positive electrode and negative electrode were punched out by a pneumatic punch to have a positive electrode mixture layer of 45mm × 70mm and a negative electrode mixture layer of 47mm × 74mm, and electrode piece portions were formed on the positive electrode and negative electrode. Subsequently, the positive electrode and the negative electrode were dried at 100 ℃ for 2 hours, and NMP in the electrodes was removed. The dried positive electrode was sandwiched between a microporous resin film having a three-layer structure of PP/PE/PP with a thickness of 30 μm, and three sides except the side on which the electrode piece was formed were heat-welded.

The positive electrode covered with the microporous film and the punched negative electrode were stacked in the order of negative electrode/positive electrode/negative electrode, and a PTFE sheet having a thickness of 50 μm was disposed on the negative electrode. The positive electrode terminal and the negative electrode terminal made of aluminum were welded to the respective electrode piece portions provided on the positive electrode and the negative electrode by ultrasonic welding. The obtained electrode body was sandwiched between laminated films, leaving one side for injection, three sides including the side on which the pole piece portions were formed were heat-sealed at 200 ℃ by a lamination sealing apparatus, and vacuum-dried at 50 ℃ for 20 hours. Next, a nonaqueous electrolytic solution was injected from the side for injection, and then the side for injection was vacuum-sealed to obtain a secondary battery.

< measurement of discharge Capacity >

(1) Measurement of initial discharge Capacity

The obtained secondary battery was maintained at 25 ℃, and after being charged to an upper limit voltage of 4.2V at a Constant Current (CC) at a charge rate of 0.05C, the voltage was maintained at 4.2V, and then charged to a current value reduced to 0.005C at a Constant Voltage (CV). Thereafter, constant current discharge was performed at a discharge rate of 0.05C to a lower limit voltage of 2.7V, and the initial discharge capacity of the secondary battery was measured.

(2) Cycle test at 45 deg.C

After the initial discharge capacity was measured, the secondary battery was heated to 45 ℃ and charge and discharge at 0.3 ℃ were repeated for 100 cycles.

(3) Measurement of discharge Capacity after cycle test

After the temperature of the secondary battery after the cycle test was lowered to 25 ℃, the temperature was maintained, and after charging to an upper limit voltage of 4.2V at a charging rate CC of 0.05C, the voltage was maintained at 4.2V, and CV charging was performed until the current value was reduced to 0.005C. Thereafter, the discharge capacity of the secondary battery after the cycle test was measured by discharging the battery to a lower limit voltage of 2.7V at a discharge rate CC of 0.05C.

(4) Calculation of discharge Capacity maintenance Rate at 45 deg.C

The discharge capacity maintaining rate at 45 ℃ obtained from the ratio of the discharge capacity after the cycle test measured in (3) to the initial discharge capacity measured in (1) was evaluated as 100 cycle lives.

A discharge capacity maintenance rate (%) at 45 ℃ (100 cycles life) = (discharge capacity after cycle test/initial discharge capacity) × 100 · equation (3)

< examples 2 to 12 and comparative examples 1 to 2>

A secondary battery was produced in the same manner as in example 1, except that the composition of the nonaqueous electrolytic solution was changed to the composition shown in table 1, and the quantitative determination of the components contained in the nonaqueous electrolytic solution and the measurement of the discharge capacity were performed.

Table 1 shows the compositions and the discharge capacity maintaining rates at 45 ℃ of the secondary batteries of examples 1 to 12 and comparative examples 1 to 2. Fig. 2 shows the relationship between the lithium salt concentration (mol/L) relative to the main solvent and the discharge capacity maintaining rate (%) at 45 ℃ in examples and comparative examples, and fig. 3 shows the relationship between the relative dielectric constant of the mixed solvent of examples and comparative examples and the discharge capacity maintaining rate (%) at 45 ℃.

< results and examination >

In fig. 2, the discharge capacity maintaining rate at 45 ℃ of the secondary battery is plotted with respect to the lithium salt concentration. As shown in FIG. 2, the rate of maintenance of the discharge capacity at 45 ℃ was small in the composition in which the lithium salt concentration was less than 2.21mol/L, and the rate of maintenance of the discharge capacity at 45 ℃ was large in the composition in which the lithium salt concentration was 2.21mol/L or more. As a result, it is considered that mixing a low-viscosity organic solvent having a low viscosity with a solvated ionic liquid lowers the viscosity of the nonaqueous electrolytic solution and increases the ionic conductivity of the nonaqueous electrolytic solution. In addition, in the composition having a lithium salt concentration of more than 2.21mol/L, the discharge capacity maintaining rate tends to be larger at 45 ℃ as the concentration is lower. This is presumably because the solvent is coordinated with lithium ions and stabilized at a low concentration.

From fig. 2, it was confirmed that the discharge capacity maintaining rate Y at 45 ℃ of the secondary battery is represented by the following formula (4) using the lithium salt concentration X in the range of the lithium salt concentration of 0.8mol/L to 3.8 mol/L.

Y＝－4.0167X ² +18.13X +. 73.074. Formula (4)

According to the formula (4), the range of the lithium salt concentration X at which the maintenance rate of the discharge capacity at 45 ℃ is larger than the maintenance rate of the discharge capacity at 45 ℃ of comparative example 1 (lithium salt concentration X = 1) (87.7%) is 1.06mol/L to 3.46mol/L. The lithium salt concentration X at which the retention rate of the discharge capacity at 45 ℃ of the secondary battery is 89.5% or more is in the range of 1.25 to 3.26mol/L. The lithium salt concentration X at which the 45 ℃ discharge capacity retention rate of the secondary battery is 90% or more is in the range of 1.32 to 3.18mol/L. Further, the lithium salt concentration X at which the retention rate of discharge capacity at 45 ℃ of the secondary battery becomes 92% or more is in the range of 1.64 to 2.86mol/L. Therefore, the lithium salt concentration X is in the range of 1.06 to 3.46mol/L, preferably 1.25 to 3.26mol/L, and more preferably 1.32 to 3.18mol/L.

In fig. 3, the discharge capacity maintenance rate at 45 ℃ of the secondary battery is plotted against the relative dielectric constant of the mixed solvent in the nonaqueous electrolytic solution. As shown in FIG. 3, in the composition having a relative permittivity of less than about 31.5, the larger the relative permittivity, the larger the discharge capacity maintenance rate at 45 ℃. On the other hand, in the composition having a relative dielectric constant of more than about 31.5, the smaller the relative dielectric constant, the larger the discharge capacity maintenance rate at 45 ℃. It is considered that the higher the relative dielectric constant affecting the dissociation of the electrolyte salt, the more significant the reductive decomposition of the solvent on the graphite surface when the electrolyte salt is added at a high concentration, and therefore the smaller the discharge capacity maintenance rate at 45 ℃.

When the relative dielectric constant of the main solvent is a, the molar ratio of the main solvent to the mixed solvent is M, the relative dielectric constant of the low-viscosity organic solvent is B, and the molar ratio of the low-viscosity organic solvent to the mixed solvent is N, the relative dielectric constant T of the mixed solvent can be defined by the following formula (5).

T = A × M + B × N · formula (5)

Therefore, it was confirmed from fig. 3 that the discharge capacity maintaining rate Y at 45 ℃ of the secondary battery is represented by the following formula (6) using the relative dielectric constant T of the mixed solvent.

Y＝－0.0065T ² +0.41T + 88. Formula (6)

According to the formula (6), the range of the relative permittivity T of the mixed solvent in which the 45 ℃ discharge capacity maintaining rate is larger than the 45 ℃ discharge capacity maintaining rate (87.7%) of the secondary battery of comparative example 1 is 63 or less. The range of the relative dielectric constant T of the mixed solvent in which the 45 ℃ discharge capacity maintenance rate Z of the secondary battery is 89.5% or more is 3.9 to 59. The mixed solvent has a relative permittivity T within a range of 5.5 to 58, wherein the maintaining rate of the discharge capacity at 45 ℃ of the secondary battery is 90% or more. Further, the mixed solvent has a relative dielectric constant T in the range of 12.5 to 51.0, wherein the retention rate of discharge capacity at 45 ℃ of the secondary battery is 92% or more. Therefore, the relative dielectric constant T of the mixed solvent is 63 or less, preferably 3.9 to 59, and more preferably 5.5 to 58.

Description of the reference numerals

100: a positive electrode; 110: a positive electrode mixture layer; 120: a positive electrode current collector; 130: a positive plate; 200: a negative electrode; 210: a negative electrode mixture layer; 220: a negative electrode current collector; 230: a negative plate; 300: an insulating layer; 400: an electrode body; 500: an exterior body; 1000: a lithium ion secondary battery.

All publications, patents and patent applications cited in this specification are herein incorporated by reference as if fully set forth.

Claims

1. A non-aqueous electrolyte solution characterized in that,

contains an electrolyte salt and an organic solvent, and the electrolyte salt,

the organic solvent comprises a main solvent and a low-viscosity organic solvent, the main solvent is at least one selected from sulfolane and derivatives thereof,

the concentration of the electrolyte salt to the main solvent is 1.06mol/L to 3.46mol/L, and the relative dielectric constant of the organic solvent is 63 or less.

2. The nonaqueous electrolytic solution of claim 1,

the low viscosity organic solvent comprises at least one selected from the group consisting of ethylene carbonate, propylene carbonate, 1,2-butylene carbonate, fluoroethylene carbonate, methylethyl carbonate, dimethyl carbonate, diethyl carbonate, trimethyl phosphate, triethyl phosphate, tris (2,2,2-trifluoroethyl) phosphite, dimethyl methylphosphonate, and γ -butyrolactone.

3. The nonaqueous electrolytic solution of claim 1,

the electrolyte salt contains at least one selected from lithium tetrafluoroborate and lithium bistrifluoromethanesulfonamide.

4. A semi-solid electrolyte layer characterized in that,

comprising the nonaqueous electrolytic solution of claim 1, a carrier particle, and a semisolid electrolyte binder.

5. A sheet for a secondary battery, which comprises a positive electrode and/or a negative electrode and the semisolid electrolyte layer according to claim 4 laminated thereon.

6. A secondary battery, characterized by comprising:

a positive electrode;

a negative electrode; and

the semi-solid electrolyte layer of claim 4 disposed between the positive electrode and the negative electrode.