Electrolyte, secondary battery and electronic device

Abstract

The invention provides an electrolyte, a secondary battery and an electronic device. The electrolyte comprises a compound as shown in a formula I and a compound as shown in a formula II, in the compound represented by the formula I, R is selected from any one of an unsubstituted or Ra substituted C2-C6 alkyl group, an unsubstituted or Ra substituted C2-C6 alkenyl group, an unsubstituted or Ra substituted C2-C6 alkynyl group, an unsubstituted or Ra substituted C5-C12 nitrogen-containing heteroaryl group, an unsubstituted or Ra substituted C5-C12 nitrogen-containing heteroaryl group, and an unsubstituted or Ra substituted C6-C12 aryl group; wherein Ra is independently selected from any one of fluorine or C1-C6 fluoroalkyl groups, and Ra is selected from any one of fluorine and C1-C6 fluoroalkyl groups; and in the compound shown in the formula II, R1 and R2 are independently selected from any one of fluorine or C1-C3 fluoroalkyl groups. The compound shown in the formula I and the compound shown in the formula II in the electrolyte can form a stable sulfur-based interface layer on a negative electrode interface of the secondary battery, so that the impedance increase of the secondary battery in the cycle process is obviously improved.

Description

Technical Field

[0001] The present application relates to the field of energy storage, and more specifically, to an electrolyte, a secondary battery and an electronic device. Background Art

[0002] Secondary batteries such as lithium-ion batteries are widely used in portable electronic products, electric vehicles, aerospace, energy storage and other fields due to their high energy density, good cycle performance, safety, environmental protection and no memory effect. In existing secondary batteries, as the number of secondary battery cycles increases, the impedance during the cycle will increase significantly, resulting in a decrease in energy density. Summary of the invention

[0003] The present application provides an electrolyte, a secondary battery and an electronic device. When the electrolyte in the present application is used in a secondary battery, the phenomenon of impedance growth of the secondary battery during the cycle process can be significantly improved.

[0004] In a first aspect, the present application provides an electrolyte, which comprises a compound of formula I and a compound of formula II, wherein the general structural formula of the compound of formula I is shown in formula I below:

[0005]

[0006] wherein R is selected from any one of C2-C6 alkyl which is unsubstituted or substituted by Ra, C2-C6 alkenyl which is unsubstituted or substituted by Ra, C2-C6 alkynyl which is unsubstituted or substituted by Ra, C5-C12 nitrogen-containing heteroaryl which is unsubstituted or substituted by Ra, and C6-C12 aryl which is unsubstituted or substituted by Ra; wherein Ra is independently selected from any one of fluorine or C1-C6 fluoroalkyl;

[0007] The general structural formula of the compound of formula II is shown in the following formula II:

[0008]

[0009] Wherein R1 and R2 are each independently selected from fluorine or any one of C1-C3 fluoroalkyl groups; based on the mass of the electrolyte, the mass content of the compound of formula I is A%, the mass content of the compound of formula II is B%, 8≤A≤52, 0.4≤A / B≤3.47.

[0010] In the above electrolyte, the inventors found that since both the compound of formula I and the compound of formula II contain sulfone groups, the compound of formula I and the compound of formula II have strong molecular coordination ability for lithium ions, and the compound of formula II can also provide lithium ions. Therefore, when the above electrolyte is used in a secondary battery, this strong coordination ability can make the molecules of the compound of formula I and the anions of the compound of formula II preferentially decompose at the negative electrode interface of the secondary battery, thereby forming a stable sulfur-based interface layer. This interface layer can maintain high stability during the cycle process and is not easy to be decomposed or ruptured, so it can significantly improve the impedance growth during the cycle process. However, the values ​​of A and A / B cannot be too large, otherwise it is easy to over-coordinate with lithium ions, affecting the ion transmission capacity of the electrolyte, and will increase the impedance growth of the secondary battery during the cycle process; similarly, the values ​​of A and A / B cannot be too small, otherwise the sulfur-based interface layer formed is unstable, easy to decompose or rupture, and cannot well improve the impedance growth during the cycle process.

[0011] In a possible implementation, the electrolyte satisfies at least one of the following conditions: (1) 20≤A≤45; (2) 10≤B≤25; (3) 0.8≤A / B≤2.

[0012] In one possible implementation, the compound of formula I includes at least one of the following compounds:

[0013]

[0014] In one possible implementation, the compound of formula II includes at least one of the following compounds:

[0015]

[0016] In a possible implementation, the electrolyte further includes a compound of formula III, and the general structural formula of the compound of formula III is shown in the following formula III:

[0017]

[0018] Wherein R3 to R8 are each independently selected from at least one of fluorine, C1-C3 alkyl or C1-C3 fluoroalkyl; based on the mass of the electrolyte, the mass content of the compound of formula III is C%, 3≤C≤38.

[0019] In the above technical solution, the compound of formula III with a mass content of 3% to 38% can form a SEI film (SEI, i.e., Solid Electrolyte Interface) of suitable thickness at the interface of the negative electrode plate. The formed SEI film contains benzene organic matter, which can cooperate with the sulfur-based interface layer to further improve the impedance growth of the secondary battery during the cycle process. The content of the compound of formula III cannot be too high or too low. If it is too high, it is easy to have an adverse effect on the transmission of lithium ions, and if it is too low, it is not easy to form a stable SEI film.

[0020] In one possible implementation, the compound of formula III includes at least one of the following compounds:

[0021]

[0022] In one possible implementation, 24≤C≤35.

[0023] In one possible implementation, the electrolyte also includes a first component, which includes at least one of fluoroethylene carbonate, vinylene carbonate, 1,2-bis(1,1,2,2-tetrafluoroethoxy)ethane, 1H,1H,5H-octafluoropentyl-1,1,2,2-tetrafluoroethyl ether, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether or ethoxypentafluorocyclotriphosphazene; based on the mass of the electrolyte, the mass content of the first component is D%, 0.1≤D≤22.

[0024] In the above technical solution, 0.1% to 22% of the first component can form an SEI film at the interface of the negative electrode, which can cooperate with the sulfur-based interface layer to further improve the impedance growth of the secondary battery during the cycle process. Similarly, the content of the first component cannot be too high or too low; if it is too high, it is easy to have side reactions with the positive and negative electrodes of the secondary battery, causing the secondary battery to easily produce gas, and if it is too low, it is not easy to form a stable SEI film.

[0025] In one possible implementation, 1≤D≤12.

[0026] In a second aspect, the present application provides a secondary battery, which includes the above-mentioned electrolyte. Therefore, the impedance growth of the secondary battery provided by the present application during the cycle process can be well suppressed.

[0027] In a third aspect, the present application provides an electronic device, which includes the above-mentioned secondary battery. Therefore, the electronic device provided by the present application has good performance.

[0028] Beneficial effects of this application:

[0029] The present application provides an electrolyte, a secondary battery and an electronic device, wherein the electrolyte comprises a compound of formula I and a compound of formula II; in the compound of formula I, R is selected from any one of an alkyl group of C2 to C6 which is unsubstituted or substituted by Ra, an alkenyl group of C2 to C6 which is unsubstituted or substituted by Ra, an alkynyl group of C2 to C6 which is unsubstituted or substituted by Ra, a nitrogen-containing heteroaryl group of C5 to C12 which is unsubstituted or substituted by Ra, and an aryl group of C6-C12 which is unsubstituted or substituted by Ra; wherein Ra is independently selected from any one of fluorine or a fluoroalkyl group of C1 to C6; in the compound of formula II, R1 and R2 are independently selected from any one of fluorine or a fluoroalkyl group of C1 to C3. By setting the compound of formula I and the compound of formula II, a stable sulfur-based interface layer can be formed at the negative electrode interface of the secondary battery, which can significantly improve the impedance growth of the secondary battery during the cycle process. DETAILED DESCRIPTION

[0030] In order to make the purpose, technical scheme and advantages of the embodiments of the present application clearer, the technical scheme in the embodiments of the present application will be described clearly and completely below. If the specific conditions are not specified in the embodiments, they are carried out according to the conventional conditions or the conditions recommended by the manufacturer. If the manufacturer is not specified for the reagents or instruments used, they are all conventional products that can be purchased commercially.

[0031] The electrolyte, secondary battery and electronic device of the embodiments of the present application are described in detail below.

[0032] In a first aspect, the present application provides an electrolyte, which comprises a compound of formula I and a compound of formula II, wherein the general structural formula of the compound of formula I is shown in formula I below:

[0033]

[0034] Wherein R is selected from any one of C2-C6 alkyl which is unsubstituted or substituted by Ra, C2-C6 alkenyl which is unsubstituted or substituted by Ra, C2-C6 alkynyl which is unsubstituted or substituted by Ra, C5-C12 nitrogen-containing heteroaryl which is unsubstituted or substituted by Ra, and C6-C12 aryl which is unsubstituted or substituted by Ra; wherein Ra is independently selected from any one of fluorine or C1-C6 fluoroalkyl.

[0035] The general structural formula of the compound of formula II is shown in the following formula II:

[0036]

[0037] Wherein R1 and R2 are each independently selected from fluorine or any one of C1-C3 fluoroalkyl groups; based on the mass of the electrolyte, the mass content of the compound of formula I is A%, the mass content of the compound of formula II is B%, 8≤A≤52, 0.4≤A / B≤3.47.

[0038] The inventors found that the above-mentioned compounds of formula I and formula II both contain sulfone groups, so the compounds of formula I and formula II have strong coordination ability for metal cations such as lithium ions, and the compounds of formula II can also provide lithium ions. Therefore, when the electrolyte containing the compounds of formula I and formula II is used in a secondary battery, due to the strong coordination ability, the anions of the compounds of formula I and formula II will preferentially decompose at the negative electrode interface of the secondary battery, thereby forming a stable sulfur-based interface layer, which can maintain a high stability during the cycle process and is not easy to be decomposed or ruptured, so it can significantly improve the impedance growth of the secondary battery during the cycle process. However, the values ​​of A and A / B cannot be too large, otherwise it is easy to transitionally coordinate with lithium ions, affecting the ion transmission capacity of the electrolyte, but will increase the impedance growth of the secondary battery during the cycle process; similarly, the values ​​of A and A / B cannot be too small, otherwise the sulfur-based interface layer formed is unstable, easy to decompose or rupture, and cannot well improve the impedance growth during the cycle process.

[0039] Preferably, in order to further suppress the impedance growth of the secondary battery during the cycle process, based on the mass of the electrolyte, the mass content of the compound of formula I is A%, 20≤A≤45, for example, A can be 8, 10, 20, 25, 30, 35, 40, 45, 50, 52, etc. or within the range consisting of any two of the above values; based on the mass of the electrolyte, the mass content of the compound of formula II is B%, 10≤B≤25, for example, B can be 10, 20, 25, 30, 35, 40, 45, etc. or within the range consisting of any two of the above values; 0.8≤A / B≤2.

[0040] In addition, as an example, the present application also provides specific structures of the compound of formula I and the compound of formula II, wherein the compound of formula I can be at least one of the following compounds:

[0041]

[0042] The compound of formula II may be at least one of the following compounds:

[0043]

[0044] In some embodiments of the present application, in order to further suppress the impedance growth of the secondary battery during the cycle process, the electrolyte may further include a compound of formula III, and the general structural formula of the compound of formula III is shown in the following formula III:

[0045]

[0046] Wherein R3 to R8 are each independently selected from at least one of fluorine, C1-C3 alkyl or C1-C3 fluoroalkyl; for example, the compound of formula III can be at least one of the following compounds:

[0047]

[0048] In the present application, based on the mass of the electrolyte, the mass content of the compound of formula III is C%, 3≤C≤38, so that the compound of formula III has a sufficient amount to form an SEI film of suitable thickness and high stability at the interface of the negative electrode plate, and is not easy to have an adverse effect on the transmission of lithium ions. The SEI film formed by the compound of formula III contains benzene organic matter, which can cooperate with the sulfur-based interface layer to further improve the impedance growth of the secondary battery during the cycle process. Preferably, in order to further suppress the impedance growth of the secondary battery during the cycle process, in some embodiments of the present application, based on the mass of the electrolyte, the mass content of the compound of formula III is C%, 24≤C≤35; for example, C can be 5, 10, 18, 24, 29, 32, 35, etc. or within the range composed of any two of the above values.

[0049] In some embodiments of the present application, in order to further suppress the impedance growth of the secondary battery during the cycle, the electrolyte also includes a first component, and the first component includes at least one of fluoroethylene carbonate, vinylene carbonate, 1,2-bis(1,1,2,2-tetrafluoroethoxy)ethane, 1H,1H,5H-octafluoropentyl-1,1,2,2-tetrafluoroethyl ether, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether or ethoxy pentafluorocyclotriphosphazene. Since the content of the first component is too high and it is easy to react with the positive electrode of the secondary battery, causing the secondary battery to easily produce gas, and it is not easy to form a stable SEI film if it is too low, therefore, in some embodiments of the present application, based on the mass of the electrolyte, the mass content of the first component is D%, 0.2≤D≤20, preferably 1≤D≤12. For example, D can be 0.2, 0.8, 1, 4, 8, 12, 16, 20, etc. or within the range composed of any two of the above values.

[0050] Of course, in some embodiments of the present application, other substances such as lithium salts, solvents, additives, etc. may be added to the electrolyte according to actual conditions. The lithium salt may be dissolved in the solvent to form an ion conductor, used as a conductive medium and a lithium ion transmission medium; the lithium salt includes but is not limited to at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium perchlorate, lithium bis(oxalatoborate), lithium bis(trifluoromethylsulfonylimide), lithium bis(fluorooxalatoborate), lithium bis(fluorosulfonylimide), etc.

[0051] The solvent can dissolve the lithium salt and the additive, and the solvent can be at least one of carbonate, carboxylic acid ester, ether, alcohol, etc., wherein the carbonate can be divided into cyclic carbonate, linear carbonate, etc. Cyclic carbonate specifically includes but is not limited to at least one of ethylene carbonate, propylene carbonate, etc.; linear carbonate specifically includes but is not limited to at least one of dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, etc.; carboxylic acid ester includes but is not limited to at least one of methyl formate, methyl acetate, methyl butyrate, ethyl propionate, propyl propionate, propyl acetate, etc.; ethers include but are not limited to at least one of tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, 4-methyl-1,3-dioxolane, etc.; alcohols include but are not limited to at least one of ethanol, ethylene glycol, glycerol, etc.

[0052] The additives include, but are not limited to, nitriles, sulfones, sulfoxides, fluoronitriles, fluoroesters, and the like.

[0053] On the second aspect, the present application also provides a secondary battery, which includes a positive electrode, a negative electrode and an electrolyte according to the present application. Therefore, the impedance growth of the secondary battery of the present application can be well suppressed during the cycle. It should be noted that in the subsequent specific implementation of the present application, the lithium ion secondary battery (i.e., lithium ion battery) is used as an example of a secondary battery to explain the present application, but the secondary battery of the present application is not limited to lithium ion batteries. For example, the secondary battery of the present application may include but is not limited to: lithium metal secondary battery, lithium polymer secondary battery or lithium ion polymer secondary battery (lithium ion polymer battery), etc.

[0054] Except for the electrolyte, the structures of the various parts of the secondary battery of the present application are specifically as follows:

[0055] Positive electrode

[0056] The positive electrode sheet includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector. That is, in the present application, the positive active material layer can be disposed on one surface in the thickness direction of the positive current collector, or on two surfaces in the thickness direction of the positive current collector. Moreover, in the present application, the "surface of the positive current collector" can be the entire area of ​​the positive current collector, or a partial area of ​​the positive current collector. The present application has no special restrictions, as long as the purpose of the present application can be achieved.

[0057] The components of the positive electrode active material layer include a positive electrode active material, which can be any material that can reversibly embed and de-embed Li + 、Na +For example, the positive electrode active material includes but is not limited to at least one of lithium iron phosphate (LiFePO4), lithium cobalt oxide (LiCoO2), lithium manganese oxide, lithium nickel oxide, ternary materials, etc., and the ternary materials include but are not limited to LiNi x Co y Mn z O2、LiNi x Co y Al z O2, etc., and the content of Ni, Co, Mn, Al, etc. can be adjusted to ensure that x+y+z=1. For example, the ternary material can be LiNi 0.6 Co 0.2 Mn 0.2 O2、LiNi 0.88 Co 0.08 Mn 0.04 O2、LiNi 0.8 Co 0.15 Mn 0.05 O2、LiNi 0.8 Co 0.1 Mn 0.1 O2、LiNi 0.88 Co 0.1 Mn 0.02 O2、LiNi 0.8 Co 0.15 Al 0.05 O2、LiNi 0.88 Co 0.1 Al 0.02 O2, etc.

[0058] In some embodiments, the components of the positive electrode active material layer also include a positive electrode conductive agent; the present application has no limitation on the type of the positive electrode conductive agent, and any known conductive material can be used. Specifically, the positive electrode conductive agent includes, but is not limited to, at least one of acetylene black, carbon black such as Super-P, or amorphous carbon such as needle coke, or carbon nanotubes, or graphene.

[0059] In some embodiments, the components of the positive electrode active material layer generally also contain a positive electrode binder. There is no particular limitation on the type of positive electrode binder used in the manufacture of the positive electrode active material layer. In the case of a coating method, any material that can be dissolved or dispersed in the liquid medium used in electrode manufacturing can be used. The positive electrode binder includes, but is not limited to, any one or at least two of the following: resin polymers such as polyethylene, polypropylene, polyethylene terephthalate, polymethyl methacrylate, polyimide, aromatic polyamide, cellulose, and nitrocellulose; rubber polymers such as styrene-butadiene rubber (SBR), nitrile rubber (NBR), fluororubber, isoprene rubber, polybutadiene rubber, and ethylene-propylene rubber; thermoplastic elastomer polymers such as styrene-butadiene-styrene block copolymers or their hydrides, ethylene-propylene-diene terpolymers (EPDM), styrene-ethylene-butadiene-ethylene copolymers, styrene-isoprene-styrene block copolymers or their hydrides; soft resin polymers such as isotactic-1,2-polybutadiene, polyvinyl acetate, ethylene-vinyl acetate copolymers, and propylene-α-olefin copolymers; fluorine-based polymers such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene, fluorinated polyvinylidene fluoride, and polytetrafluoroethylene-ethylene copolymers; polymer compositions having ion conductivity of alkali metal ions (especially lithium ions), etc.

[0060] In the positive electrode sheet, the type of positive current collector is not particularly limited, and it can be any material known to be suitable for use as a positive current collector. The material of the positive current collector includes, but is not limited to, metal materials such as aluminum, stainless steel, nickel plating, titanium, and tantalum. In addition, in order to reduce the electronic contact resistance between the positive current collector and the positive active material layer, a conductive additive or a conductive coating may be provided on the surface of the positive current collector. Conductive additives include, but are not limited to, carbon and precious metals such as gold, platinum, and silver. The conductive coating may be a mixture layer of an inorganic oxide, a conductive agent, and a positive electrode binder.

[0061] When preparing the positive electrode sheet, the components in the above-mentioned positive electrode active material layer can be dissolved or dispersed in a liquid solvent to prepare a positive electrode slurry, and then the positive electrode slurry is coated on the positive electrode collector and dried to form a positive electrode active material layer on the positive electrode collector, thereby obtaining a positive electrode sheet. When using this method to prepare the positive electrode sheet, there is no special restriction on the solvent in the positive electrode slurry, as long as it can dissolve or disperse the above-mentioned components. Specifically, the solvent in the positive electrode slurry includes but is not limited to N-methylpyrrolidone (NMP), ethylene carbonate (EC), etc. In addition, when preparing the positive electrode sheet, the various components in the positive electrode active material layer can also be dry mixed to form a sheet, and then the obtained sheet is pressed onto the positive electrode collector.

[0062] Negative electrode

[0063] The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector, and the components of the negative electrode active material layer include a negative electrode sheet active material. That is, in the present application, the negative electrode active material layer can be disposed on one surface in the thickness direction of the negative electrode current collector, or on two surfaces in the thickness direction of the negative electrode current collector. Moreover, in the present application, the "surface of the negative electrode current collector" can be the entire area of ​​the negative electrode current collector, or it can be a partial area of ​​the negative electrode current collector. There is no special limitation in the present application, as long as the purpose of the present application can be achieved.

[0064] The negative electrode active material layer generally contains negative electrode sheet active materials, and the present application does not have any particular restrictions on the negative electrode sheet active materials. Specifically, the negative electrode sheet active material may include at least one of a carbon material or a silicon-based material. More specifically, the carbon material includes but is not limited to at least one of natural graphite, artificial graphite, mesophase microcarbon beads, hard carbon or soft carbon; the silicon-based material includes but is not limited to at least one of silicon, silicon-oxygen composite materials or silicon-carbon composite materials.

[0065] In some embodiments, the negative electrode active material layer generally further contains a negative electrode sheet conductive agent. The present application has no particular restrictions on the type of the negative electrode sheet conductive agent, as long as the purpose of the present application can be achieved. For example, the negative electrode sheet conductive agent includes but is not limited to at least one of acetylene black, Ketjen black, carbon nanotubes, carbon fibers, carbon dots or graphene.

[0066] In some embodiments, the negative electrode active material layer may also contain a negative electrode binder and a thickener. The present application has no particular restrictions on the types of the negative electrode binder and the thickener, as long as the purpose of the present application can be achieved. For example, the negative electrode binder may include but is not limited to at least one of polyvinyl alcohol, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, styrene-butadiene rubber or acrylic (ester) styrene-butadiene rubber; the thickener in the negative electrode slurry may include but is not limited to at least one of sodium carboxymethyl cellulose or lithium carboxymethyl cellulose.

[0067] In the negative electrode sheet, the material of the negative electrode current collector includes but is not limited to copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam or a polymer substrate covered with a conductive metal, etc., and there is no special limitation in this application. Among them, the conductive metal includes but is not limited to copper, nickel or titanium, and the material of the polymer substrate includes but is not limited to at least one of polyethylene, polypropylene, ethylene propylene copolymer, polyethylene terephthalate, polyethylene naphthalate or poly(p-phenylene terephthalamide).

[0068] In addition, in the present application, there is no particular limitation on the thickness of the negative electrode current collector and the negative electrode active material layer, as long as the purpose of the present application can be achieved. For example, the thickness of the negative electrode current collector is 4 μm to 12 μm, and the thickness of the single-sided negative electrode active material layer is 30 μm to 160 μm.

[0069] In addition, similar to the preparation of the positive electrode sheet, when preparing the negative electrode sheet, it can be configured into a negative electrode slurry, and then the negative electrode slurry is coated on the negative electrode collector and dried to form a negative electrode active material layer on the negative electrode collector to obtain a negative electrode sheet; or the components in the negative electrode active material layer can be dry mixed to form a sheet, and then the obtained sheet is pressed onto the negative electrode collector to form a negative electrode active material layer to obtain a negative electrode sheet. The solvent in the negative electrode slurry includes any one of an aqueous solvent and an organic solvent. The aqueous solvent includes but is not limited to a mixed solvent of alcohol and water or water. Organic solvents include, but are not limited to, aliphatic hydrocarbons such as hexane; aromatic hydrocarbons such as benzene, toluene, xylene, methylnaphthalene; heterocyclic compounds such as quinoline and pyridine; ketones such as acetone, methyl ethyl ketone, and cyclohexanone; esters such as methyl acetate and methyl acrylate; amines such as diethylenetriamine and N,N-dimethylaminopropylamine; ethers such as diethyl ether, propylene oxide, and tetrahydrofuran (THF); amides such as N-methylpyrrolidone (NMP), dimethylformamide, and dimethylacetamide; aprotic polar solvents such as hexamethylphosphoramide and dimethyl sulfoxide. In addition, in some other embodiments, when using an aqueous solvent, the components of the negative electrode slurry will also include a thickener and styrene-butadiene rubber (SBR) emulsion to slurry the negative electrode slurry, thereby adjusting the viscosity of the negative electrode slurry. The types of thickeners in the positive electrode slurry include, but are not limited to, at least one of carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol, oxidized starch, phosphorylated starch, casein, and their salts.

[0070] Isolation film

[0071] In order to prevent short circuit, a separator is usually provided between the positive electrode sheet and the negative electrode sheet. In this case, the electrolyte of the present application is usually used by infiltrating the separator.

[0072] There is no particular restriction on the material and shape of the isolation membrane, as long as the effect of the present application is not significantly impaired. The material of the isolation membrane may be a resin, glass fiber, inorganic substance, etc. formed by a material that is stable to the electrolyte of the present application. In some embodiments, the isolation membrane includes a porous sheet or a non-woven fabric-like material with excellent liquid retention. Examples of materials for resin or glass fiber isolation membranes include but are not limited to polyolefins, aromatic polyamides, polyimides (PI), polyamides (PA), polytetrafluoroethylene, polyether sulfone, spandex or aramid, etc. In some embodiments, the polyolefin is polyethylene or polypropylene. In some embodiments, the polyolefin is polypropylene. The materials of the above isolation membranes can be used alone or in any combination.

[0073] The separator may also be a material formed by laminating the above materials, examples of which include, but are not limited to, a three-layer separator formed by laminating polypropylene, polyethylene, and polypropylene in this order.

[0074] The materials of the inorganic substance include, but are not limited to, oxides such as aluminum oxide and silicon dioxide, nitrides such as aluminum nitride and silicon nitride, and sulfates (eg, barium sulfate, calcium sulfate, etc.). The form of the inorganic substance includes, but is not limited to, granular or fibrous forms.

[0075] The separator may be in the form of a thin film, including but not limited to non-woven fabrics, woven fabrics, microporous films, etc. In the thin film form, the pore size of the separator is 0.01 μm to 1 μm, and the thickness is 5 μm to 50 μm. In addition to the above-mentioned independent thin film separators, the following separators can also be used: a separator formed by forming a composite porous layer containing the above-mentioned inorganic particles on the surface of the positive electrode and / or the negative electrode using a resin-based adhesive, for example, a separator formed by using fluororesin as an adhesive to form a porous layer on both sides of the positive electrode with 90% of the aluminum oxide particles having a particle size less than 1 μm.

[0076] The thickness of the separator is arbitrary. In some embodiments, the thickness of the separator is greater than 1 μm, greater than 5 μm, or greater than 8 μm. In some embodiments, the thickness of the separator is less than 50 μm, less than 40 μm, or less than 30 μm. When the thickness of the separator is within the above range, insulation and mechanical strength can be ensured, and the rate characteristics and energy density of the secondary battery can be ensured.

[0077] In the present application, the diaphragm may include a substrate and a surface treatment layer. The substrate may be a non-woven fabric or a composite film having a porous structure, and the material of the substrate may include at least one of polyethylene, polypropylene, polyethylene terephthalate or polyimide. Optionally, a polypropylene porous film, a polyethylene porous film, a polypropylene non-woven fabric, a polyethylene non-woven fabric or a polypropylene-polyethylene-polypropylene porous composite film may be used. Optionally, a surface treatment layer is provided on at least one surface of the substrate, and the surface treatment layer may be a polymer layer or an inorganic layer, or a layer formed by a mixed polymer and an inorganic substance. For example, the inorganic layer includes inorganic particles and a binder, and the present application has no particular restrictions on the above-mentioned inorganic particles, for example, it may include at least one of aluminum oxide, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide or barium sulfate. The present application has no particular restrictions on the above-mentioned binder, for example, it may be at least one of the aforementioned binders. The polymer layer contains polymers, and the polymer material includes at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyvinyl pyrrolidone, polyvinyl ether, polyvinylidene fluoride or poly(vinylidene fluoride-hexafluoropropylene).

[0078] The secondary battery of the present application also includes a packaging bag for containing a positive electrode sheet, a separator, a negative electrode sheet and an electrolyte, as well as other components known in the art in the secondary battery, and the present application does not limit the above other components. The present application does not specifically limit the packaging bag, and it can be a packaging bag known in the art, as long as it can achieve the purpose of the present application.

[0079] In a third aspect, the present application further provides an electronic device, which includes the secondary battery according to the present application.

[0080] The use of the secondary battery of the present application is not particularly limited, and it can be used for any electronic device known in the prior art. In some embodiments, the secondary battery of the present application can be used for, but not limited to, laptop computers, pen-input computers, mobile computers, e-book players, portable phones, portable fax machines, portable copiers, portable printers, head-mounted stereo headphones, video recorders, LCD TVs, portable cleaners, portable CD players, mini-discs, transceivers, electronic notepads, calculators, memory cards, portable recorders, radios, backup power supplies, motors, cars, motorcycles, power-assisted bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household batteries and lithium-ion capacitors, etc.

[0081] Example

[0082] The following uses a lithium ion secondary battery as an example to provide a more specific description of the implementation of the secondary battery of the present application by way of examples and comparative examples. Those skilled in the art will appreciate that the preparation method described in the present application is merely an example, and any other suitable preparation method is within the scope of the present application.

[0083] Test methods and equipment:

[0084] Impedance growth test during cycling

[0085] The secondary battery was charged to 3.95V at a constant current of 0.5C at 45°C, and the initial battery internal resistance of the positive and negative electrodes was tested with an internal resistance tester. The battery was left for 30 minutes and discharged to 3.0V at 0.5C. After 200 cycles of charge and discharge, the internal resistance of the fully charged secondary battery in the cooled state was tested again; Impedance growth rate (%) = (internal resistance after cycle - initial internal resistance) / initial internal resistance × 100%.

[0086] Example 1-1

[0087] <Preparation of Electrolyte>

[0088] In an argon atmosphere glove box with a water content of less than 10 ppm, ethyl methyl carbonate and ethyl acetate were mixed in a mass percentage of 1:1 to prepare a base solvent, and then lithium salt lithium hexafluorophosphate (LiPF6), a compound of formula I and a compound of formula II were added. Based on the total mass of the electrolyte, the mass percentage of LiPF6 was 12.5%, the mass content of the compound of formula I and the compound of formula II is shown in Table 1, and the remainder is the base solvent.

[0089] The codes in the table are detailed in the above content of this article.

[0090] <Preparation of positive electrode sheet>

[0091] Lithium cobalt oxide, conductive carbon black, and polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 95:2:3, and N-methylpyrrolidone (NMP) was added and stirred evenly under the action of a vacuum mixer to obtain a positive electrode slurry with a solid content of 70wt%. The positive electrode slurry was evenly coated on the upper and lower surfaces of a positive electrode current collector aluminum foil with a thickness of 9μm, and then dried and pressurized and cut into a specified size to obtain a positive electrode sheet.

[0092] <Preparation of negative electrode sheet>

[0093] Artificial graphite, styrene-butadiene rubber (SBR), polyacrylic acid (PAA), carbon nanotubes (CNT), and carboxymethyl cellulose (CMC) were mixed in a mass ratio of 95.8:2.4:0.5:0.5:0.8, and then deionized water was added as a solvent and stirred evenly to prepare a negative electrode slurry with a solid content of 45wt%. The negative electrode slurry was evenly coated on the upper and lower surfaces of a negative electrode current collector copper foil with a thickness of 6μm, and then dried and pressurized and cut into a specified size to obtain a negative electrode sheet.

[0094] <Isolation film>

[0095] A porous polyethylene film with a thickness of 15 μm was used as the separator.

[0096] <Preparation of Secondary Battery>

[0097] The prepared positive electrode sheet, separator, negative electrode sheet and separator are stacked in order, so that the separator is between the positive electrode sheet and the negative electrode sheet to play a role of isolation, and then wound to obtain an electrode assembly. After welding the pole ear, the electrode assembly is placed in an aluminum-plastic film packaging bag, placed in a vacuum oven at 85°C for 12 hours to remove moisture, and the prepared electrolyte is injected. After vacuum packaging, standing, formation, shaping, and capacity testing, a lithium-ion battery is obtained.

[0098] Example 1-2 to Example 1-30

[0099] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as Example 1-1.

[0100] Comparative Example 1 to Comparative Example 2

[0101] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as Example 1-4.

[0102] Table 1

[0103]

[0104]

[0105] In Table 1, it can be seen from the examples and comparative examples 1, 3, and 5 that when the electrolyte does not contain the compound of formula I, or the content of the compound of formula I is less than 8%, or the value of A / B is less than 0.4, the formed sulfur-based interface layer is unstable, easily decomposed or ruptured, and the impedance growth of the secondary battery during the cycle cannot be well improved. It can be seen from the examples and comparative examples 2 and 4 that when the electrolyte does not contain the compound of formula II, or the value of A / B is greater than 3.47, the compound of formula I in the electrolyte is prone to excessive coordination with lithium ions, affecting the ion transport capacity of the electrolyte, and instead increasing the impedance growth of the secondary battery during the cycle. In particular, it can be seen from Examples 1-4 to 1-9 that when 20≤A≤45, the formed sulfur-based interface layer is more stable, and the impedance growth of the secondary battery during the cycle process can be further improved; it can be seen from Examples 1-18 to 1-20 that when 10≤B≤25, a more stable sulfur-based interface layer can also be formed, and the impedance growth of the secondary battery during the cycle process can also be further improved; in addition, Table 1 shows that when 0.8≤A / B≤2, the impedance growth of the secondary battery during the cycle process can also be further improved; this is because the ratio of the compound of formula I and the compound of formula II is more suitable, and the lithium ions in the compound of formula II under this ratio can also better participate in ion transport, and the formed sulfur-based interface layer is better stabilized.

[0106] Example 2-1 to Example 2-17

[0107] Except for adjusting the relevant preparation parameters of the electrolyte according to Table 2, the rest is the same as Example 1-4.

[0108] Table 2

[0109]

[0110] In Table 2, it can be seen from Examples 1-4 and 2-1 to 2-17 that when the mass content of the compound of formula III is 3% to 38%, the compound of formula III can form an SEI film containing benzene organic matter and having a suitable thickness at the interface of the negative electrode plate, and it is not easy to have an adverse effect on the transmission of lithium ions. The formed SEI film can cooperate with the sulfur-based interface layer to further improve the impedance growth of the secondary battery during the cycle process. In particular, in Examples 2-5 to 2-8, when 24≤C≤35, the impedance growth of the secondary battery during the cycle process can be further suppressed.

[0111] Example 3-1 to Example 3-15

[0112] Except for adjusting the relevant preparation parameters of the electrolyte according to Table 3, the rest is the same as Example 1-4.

[0113] Table 3

[0114]

[0115]

[0116] As shown in Table 3, in Examples 3-1 to 3-15, when the mass content D% of the first component in the electrolyte satisfies 0.2≤D≤20, the first component can form a stable SEI film at the interface of the negative electrode plate, and is not easy to react with the positive electrode plate to cause the secondary battery to easily produce gas, so the compound of formula I and the compound of formula II can be combined to further improve the impedance growth of the secondary battery during the cycle. In particular, in Examples 3-4 to 3-7, when 1≤D≤12, the impedance growth of the secondary battery during the cycle can be further suppressed.

[0117] It should be noted that, in this article, the terms "comprises", "includes" or any other variations thereof are intended to cover non-exclusive inclusion, so that a process, method or article that includes a series of elements includes not only those elements, but also includes other elements that are not explicitly listed, or also includes elements that are inherent to such process, method or article.

[0118] Each embodiment in this specification is described in a related manner, and the same or similar parts between the embodiments can be referenced to each other, and each embodiment focuses on the differences from other embodiments.

[0119] The above are only embodiments of the present application and are not intended to limit the scope of protection of the present application. For those skilled in the art, the present application may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the scope of protection of the present application.

Claims

1. An electrolyte, characterized in that: It includes a compound of formula I and a compound of formula II, wherein the general structural formula of the compound of formula I is shown in the following formula I: wherein R is selected from any one of C2-C6 alkyl which is unsubstituted or substituted by Ra, C2-C6 alkenyl which is unsubstituted or substituted by Ra, C2-C6 alkynyl which is unsubstituted or substituted by Ra, C5-C12 nitrogen-containing heteroaryl which is unsubstituted or substituted by Ra, and C6-C12 aryl which is unsubstituted or substituted by Ra; wherein Ra is independently selected from any one of fluorine or C1-C6 fluoroalkyl; The general structural formula of the compound of formula II is shown in the following formula II: Wherein R1 and R2 are each independently selected from any one of fluorine or C1-C3 fluoroalkyl; Based on the mass of the electrolyte, the mass content of the compound of formula I is A%, the mass content of the compound of formula II is B%, 8≤A≤52, 0.4≤A / B≤3.

47.

2. The electrolyte according to claim 1, characterized in that It meets at least one of the following conditions: (1)20≤A≤45； (2)10≤B≤25； (3)0.8≤A / B≤2.

3. The electrolyte according to claim 1, characterized in that The compound of formula I includes at least one of the following compounds:

4. The electrolyte according to claim 1, characterized in that The compound of formula II includes at least one of the following compounds:

5. The electrolyte according to claim 1, characterized in that It also includes a compound of formula III, the general structural formula of the compound of formula III is shown in the following formula III: wherein R3 to R8 are each independently selected from at least one of fluorine, C1-C3 alkyl or C1-C3 fluoroalkyl; Based on the mass of the electrolyte, the mass content of the compound of formula III is C%, 3≤C≤38.

6. The electrolyte according to claim 5, characterized in that The compound of formula III includes at least one of the following compounds:

7. The electrolyte according to claim 5 or 6, characterized in that Based on the mass of the electrolyte, the mass content of the compound of formula III is C%, 24≤C≤35.

8. The electrolyte according to claim 1, characterized in that The electrolyte further includes a first component, the first component including at least one of fluoroethylene carbonate, vinylene carbonate, 1,2-bis(1,1,2,2-tetrafluoroethoxy)ethane, 1H,1H,5H-octafluoropentyl-1,1,2,2-tetrafluoroethyl ether, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether or ethoxypentafluorocyclotriphosphazene; Based on the mass of the electrolyte, the mass content of the first component is D%, and 0.1≤D≤22.

9. The electrolyte according to claim 8, characterized in that Based on the mass of the electrolyte, the mass content of the first component is D%, 1≤D≤12.

10. A secondary battery, characterized in that: It comprises the electrolyte according to any one of claims 1 to 9.

11. An electronic device, characterized in that: It comprises the secondary battery according to claim 10.

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