Non-aqueous electrolyte solution and lithium secondary battery

Abstract

The invention discloses a non-aqueous electrolyte and a lithium secondary battery. The non-aqueous electrolyte comprises vinylene carbonate and a compound shown as a formula I, based on the total mass of the non-aqueous electrolyte, the mass percentage content of the vinylene carbonate is A%, 0.01 < = A < = 3, the mass percentage content of the compound shown as the formula I is B%, P = B / A, and 0.5 < = P < = 50. The lithium secondary battery prepared from the non-aqueous electrolyte provided by the invention shows relatively excellent electrochemical performance, especially relatively excellent low-temperature discharge performance and cycle performance. # imgabs0 #

Description

Technical Field

[0001] The present application relates to the field of energy storage technology, and in particular to a non-aqueous electrolyte and a lithium secondary battery. Background Art

[0002] With the continuous development of secondary batteries, people have put forward higher requirements for their performance. The existing technology often leads to a decrease in cycle performance while improving the low-temperature discharge performance of secondary batteries. Therefore, the problem of how to improve the low-temperature discharge performance of secondary batteries while taking into account the cycle performance has attracted widespread attention. Summary of the invention

[0003] The embodiments of the present application provide a non-aqueous electrolyte and a lithium secondary battery, which can achieve good low-temperature discharge performance while improving cycle performance.

[0004] In a first aspect, an embodiment of the present application provides a non-aqueous electrolyte, the non-aqueous electrolyte comprising vinylene carbonate and a compound of formula I:

[0005]

[0006] wherein R is selected from C2 to C6 alkyl which is unsubstituted or substituted by Ra, C2 to C6 alkenyl which is unsubstituted or substituted by Ra, C2 to C6 alkynyl which is unsubstituted or substituted by Ra, C5 to C6 alkyl which is unsubstituted or substituted by Ra, 12 A nitrogen-containing heteroaryl group, a C6 to C 12 The substituent Ra of each group is independently selected from fluorine or a fluoroalkyl group of C1 to C6; based on the total mass of the non-aqueous electrolyte, the mass percentage of vinylene carbonate is A%, 0.01≤A≤3, and the mass percentage of the compound of formula I is B%, P=B / A, 0.5≤P≤50.

[0007] The inventor of the non-aqueous electrolyte based on the embodiment of the present application found that when the non-aqueous electrolyte further includes a compound of formula I, and controls the A value of the mass percentage of vinylene carbonate in the non-aqueous electrolyte, and the ratio P of the mass percentage of the B value of the A value and the compound of formula I satisfies the above range, a strong SEI / CEI layer for improving ion conduction can be formed in coordination, and vinylene carbonate can form a dense SEI / CEI film layer at the electrode interface during the battery formation process, thereby greatly improving the cycle performance of the secondary battery, but due to the compactness of the film layer, ion transmission is hindered to a certain extent, resulting in an increase in the initial impedance of the secondary battery, especially under low temperature conditions, the discharge capacity is reduced, and the compound of formula I can form a lithium-containing inorganic compound rich in S and F elements at the electrode interface, introduce more grain boundaries for ion conduction, and therefore modify the SEI / CEI film formed by vinylene carbonate, while maintaining the compactness of the passivation film, improve the conduction of ions. Therefore, the low-temperature discharge performance of lithium secondary batteries can be improved, while taking into account the improvement of cycle performance.

[0008] In some of the embodiments, the mass percentage A% of vinylene carbonate in the non-aqueous electrolyte, the mass percentage B% of the compound of formula I, and the ratio P of the two satisfy at least one of the following conditions: (1) 0.1≤A≤2; (2) 0.5≤B≤5; (3) 1≤P≤5. Based on the above embodiments, by adjusting the mass percentage A% of vinylene carbonate and the mass percentage B% of the compound of formula I, and the ratio of the two to satisfy any of the above conditions, the vinylene carbonate in the non-aqueous electrolyte can better cooperate with the compound of formula I, thereby further improving the low-temperature discharge performance of the lithium secondary battery and improving the cycle performance.

[0009] In some embodiments, the compound of formula I is selected from at least one of the following compounds:

[0010]

[0011] Based on the above embodiments, by selecting the above types of compounds of formula I, the low-temperature discharge performance of the lithium secondary battery can be further improved while also improving the cycle performance.

[0012] In some embodiments, the non-aqueous electrolyte further comprises a cyclic carbonate, and the cyclic carbonate is selected from any two of ethylene carbonate, propylene carbonate or fluoroethylene carbonate; based on the total mass of the non-aqueous electrolyte, the mass percentage of the cyclic carbonate is M%, 5≤M≤30. Based on the above embodiments, the above-mentioned content of cyclic carbonate is further added to the non-aqueous electrolyte of the present application, and the mass percentage of the cyclic carbonate is controlled within a specific range, so as to further improve the low-temperature discharge performance of the lithium secondary battery, while taking into account the improvement of the cycle performance.

[0013] In some embodiments, the mass percentage of the cyclic carbonate in the non-aqueous electrolyte is M%, and the mass percentage of the compound of formula I is B%, 10≤M≤20; and / or, 0.1≤B / M≤3. Based on the above embodiments, by adjusting the mass percentage M value of the cyclic carbonate in the non-aqueous electrolyte and the ratio of the mass percentage B% of the compound of formula I to the mass percentage M% of the cyclic carbonate within the above range, the low-temperature discharge performance of the lithium secondary battery can be further improved while taking into account the improvement of the cycle performance.

[0014] In some embodiments, the non-aqueous electrolyte further comprises a linear ester, wherein the linear ester comprises a fluorinated linear ester and a non-fluorinated linear ester; the linear ester satisfies at least one of the following conditions: (1) the fluorinated linear ester is at least selected from methyl difluoroethyl carbonate, methyl trifluoroethyl carbonate, ethyl trifluoroethyl carbonate, methyl hexafluoroisopropyl carbonate, di(2,2,2-trifluoroethyl) carbonate, 2,2-difluoroethyl acetate, 2,2,2-trifluoroethyl acetate, hexafluoroisopropyl acetate, 2,2-difluoroethyl propionate, 2,2,2-trifluoroethyl propionate or hexafluoroisopropyl propionate; (2) the non-fluorinated linear ester is at least selected from dimethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate or propyl propionate; (3) based on the total mass of the non-aqueous electrolyte, the mass percentage of the linear ester is N0%, 10≤N0≤70. Based on the above embodiments, further adding a certain amount of linear ester to the non-aqueous electrolyte of the present application and controlling the total mass percentage of the linear ester within a specific range can further enhance the low-temperature discharge performance of the lithium secondary battery while also improving the cycle performance.

[0015] In some of the embodiments, based on the total mass of the non-aqueous electrolyte, for the mass percentage of the fluorinated linear ester being N1%, the mass percentage of the non-fluorinated linear ester being N2%, and the mass percentage of the linear ester being N0%, at least one of the following conditions is satisfied: (1) 5≤N1≤50; (2) 5≤N2≤50; (3) 20≤N0≤60. Based on the above embodiments, by regulating the mass percentage of the fluorinated linear ester and the non-fluorinated linear ester in the linear ester and the sum of the mass percentages of the two within the above range, the low-temperature discharge performance of the lithium secondary battery can be further improved while also improving the cycle performance.

[0016] In some of the embodiments, the non-aqueous electrolyte further includes a polycyanide substance, and the polycyanide substance is at least selected from one of succinonitrile, glutaronitrile, methylglutaronitrile, adiponitrile, pimelonitrile, octanedinitrile, trans-butene dinitrile or 1,2-bis(cyanoethoxy)ethane; based on the total mass of the non-aqueous electrolyte, the mass percentage of the polycyanide substance is X%, and 0.1≤X≤10. Based on the above embodiments, a certain amount of polycyanide substances is further added to the non-aqueous electrolyte of the present application, and the mass percentage of the polycyanide substances X% is regulated within the above range. In the process of forming CEI from vinyl carbonate, a transition metal complex layer can be simultaneously formed on the surface of the positive electrode active material, thereby reducing the complete oxidation and decomposition of the passivation film formed by vinyl carbonate on the surface of the positive electrode material, inhibiting the occurrence of oxidation-generated gas, improving high-temperature storage, and taking into account the improvement of low-temperature discharge performance and cycle performance.

[0017] In some of the embodiments, for the mass percentage of the polycyanide substance X%, 0.5≤X≤5. Based on the above embodiments, by regulating the mass percentage of the polycyanide substance X% in the non-aqueous electrolyte within the above range, high-temperature storage can be further improved while improving low-temperature discharge performance and cycle performance.

[0018] In some embodiments, the non-aqueous electrolyte further comprises a compound of formula II and / or a compound of formula III:

[0019]

[0020] Wherein, R1 to R5 are each independently selected from one of hydrogen atoms, fluorine atoms, vinyl groups, acetylene groups or anhydride groups; based on the total mass of the non-aqueous electrolyte, the mass percentage of the compound of formula II is Y1%, and the mass percentage of the compound of formula III is Y2%, Y%=Y1%+Y2%, 0.1≤Y≤0.5; wherein, any two adjacent groups among R1 to R5 exist independently or are connected by covalent bonds to form a ring with the parent ring. Based on the above embodiment, a certain amount of the compound of formula II and the compound of formula III are further added to the non-aqueous electrolyte of the present application, and the sum of the mass percentage of the compound of formula II Y1% and the mass percentage of the compound of formula III Y2% is regulated within the above range, which can be enriched at the interface, and because of the presence of the nitrogen-containing heterocyclic ring, its frontier orbital energy level is narrow, has a lower oxidation potential and a higher reduction potential, and during the first formation process, a molecular skeleton rich in nitrogen heterocyclic rings can be formed to reduce the overreaction and consumption of vinylene carbonate and the molecule of formula I, thereby further improving high-temperature storage, while taking into account the improvement of low-temperature discharge performance and cycle performance.

[0021] In some embodiments, 0.05≤Y1≤0.5, such as 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, or a value within a range consisting of any two of these values.

[0022] In some embodiments, 0.05≤Y2≤0.5, such as 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, or a value within a range consisting of any two of these values.

[0023] In some embodiments, the compound of formula II is selected from at least one of the following compounds:

[0024]

[0025] In some embodiments, the compound of formula III is selected from at least one of the following compounds:

[0026]

[0027] Based on the above embodiments, by selecting the above-mentioned compounds of formula II and / or formula III, high-temperature storage can be further improved while improving low-temperature discharge performance and cycle performance.

[0028] In the second aspect, an embodiment of the present application provides a lithium secondary battery, comprising a positive electrode: the positive electrode comprises a positive electrode collector and a positive electrode active layer arranged on at least one surface of the positive electrode collector; a negative electrode: the negative electrode comprises a negative electrode collector and a negative electrode active layer arranged on at least one surface of the negative electrode collector; an isolation membrane; a non-aqueous electrolyte: the non-aqueous electrolyte is the above-mentioned non-aqueous electrolyte. DETAILED DESCRIPTION

[0029] In order to make the purpose, technical solution and advantages of the present application more clear, the present application is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described here are only used to explain the present application and are not used to limit the present application.

[0030] An embodiment of the present application provides a lithium secondary battery, including a non-aqueous electrolyte, a positive electrode, a negative electrode and a separator.

[0031] Non-aqueous electrolyte

[0032] The non-aqueous electrolyte used in the lithium secondary battery of the present application embodiment includes an electrolyte and a solvent for dissolving the electrolyte. In some embodiments, the non-aqueous electrolyte includes vinylene carbonate and a compound of formula I:

[0033]

[0034] wherein R is selected from C2 to C6 alkyl which is unsubstituted or substituted by Ra, C2 to C6 alkenyl which is unsubstituted or substituted by Ra, C2 to C6 alkynyl which is unsubstituted or substituted by Ra, C5 to C6 alkyl which is unsubstituted or substituted by Ra, 12 A nitrogen-containing heteroaryl group, a C6 to C 12 The substituents Ra of each group are independently selected from fluorine or C1 to C6 fluoroalkyl.

[0035] In some embodiments, based on the total mass of the non-aqueous electrolyte, the mass percentage of vinylene carbonate is A%, 0.01≤A≤3, preferably 0.1≤A≤2. For example, A is 0.01, 0.1, 1, 1.5, 2, 2.5, 3 or a value within the range of any two of these values. The mass percentage of the compound of formula I is B%, 0.5≤B≤5. For example, B is 0.5, 0.8, 1.0, 2.2, 3.8, 4.5, 5 or a value within the range of any two of these values. The ratio of the mass percentage B% of the compound of formula I to the mass percentage A% of vinylene carbonate is P=B / A, 0.5≤P≤50, preferably 1≤P≤5. For example, P is 0.5, 2.0, 6.9, 10.1, 22.5, 39.3, 50 or a value within the range of any two of these values. By adding vinylene carbonate and the compound of formula I to the non-aqueous electrolyte, and adjusting the mass percentage A% of vinylene carbonate and the mass percentage B% of the compound of formula I and the ratio P of the two to meet the above range, the low-temperature discharge performance can be improved while also improving the cycle performance.

[0036] In some embodiments, the compound of Formula I is selected from at least one of the following compounds:

[0037]

[0038] Adding the above-mentioned compound of formula I to the non-aqueous electrolyte can further enhance the low-temperature discharge performance of the lithium secondary battery while also improving the cycle performance.

[0039] In some embodiments, the non-aqueous electrolyte further includes a cyclic carbonate, and the cyclic carbonate is selected from any two of ethylene carbonate, propylene carbonate or fluoroethylene carbonate; based on the total mass of the non-aqueous electrolyte, the mass percentage of the cyclic carbonate is M%, 5≤M≤30. In some embodiments, 10≤M≤20. For example, M is 5, 6.5, 7.6, 15.0, 19.5, 23.8, 30 or a value within the range of any two of these values. Further adding a certain amount of cyclic carbonate to the non-aqueous electrolyte and controlling the mass percentage of the cyclic carbonate within the above-mentioned specific range can further improve the low-temperature discharge performance of the lithium secondary battery while taking into account the improvement of the cycle performance.

[0040] In some embodiments, the ratio of the mass percentage content B% of the compound of formula I to the mass percentage content M% of the cyclic carbonate in the above non-aqueous electrolyte is 0.1≤B / M≤3. In some embodiments, 0.1≤B / M≤2. In some embodiments, 1≤B / M≤3. In some embodiments, 0.3≤B / M≤1. In some embodiments, 2≤B / M≤3. In some embodiments, 1.5≤B / M≤3. By regulating the ratio of the mass percentage content B% of the compound of formula I to the mass percentage content M% of the cyclic carbonate within the above range, the low-temperature discharge performance of the lithium secondary battery can be further improved, while taking into account the improvement of the cycle performance.

[0041] In some embodiments, the non-aqueous electrolyte further comprises a linear ester, and the linear ester comprises a fluorinated linear ester and a non-fluorinated linear ester.

[0042] In some embodiments, the fluorinated linear ester is at least one selected from methyl difluoroethyl carbonate, methyl trifluoroethyl carbonate, ethyl trifluoroethyl carbonate, methyl hexafluoroisopropyl carbonate, di(2,2,2-trifluoroethyl) carbonate, 2,2-difluoroethyl acetate, 2,2,2-trifluoroethyl acetate, hexafluoroisopropyl acetate, 2,2-difluoroethyl propionate, 2,2,2-trifluoroethyl propionate or hexafluoroisopropyl propionate.

[0043] In some embodiments, the non-fluorinated linear ester is at least one selected from dimethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate or propyl propionate.

[0044] In some embodiments, based on the total mass of the non-aqueous electrolyte, the mass percentage of the linear ester is N0%, 10≤N0≤70. For example, N0 is 10, 20, 30, 40, 50, 60, 70, or a value in the range of any two of these values. Further adding a certain amount of linear ester to the non-aqueous electrolyte and controlling the total mass percentage of the linear ester within a specific range can further improve the low-temperature discharge performance of the lithium secondary battery while also improving the cycle performance.

[0045] In some embodiments, based on the total mass of the non-aqueous electrolyte, the mass percentage of the fluorinated linear ester is N1%, 5≤N1≤50. For example, N1 is 5, 10, 15, 20, 30, 40, 50 or a value in the range consisting of any two of these values.

[0046] In some embodiments, based on the total mass of the non-aqueous electrolyte, the mass percentage of the non-fluorinated linear ester is N2%, 5≤N2≤50. For example, N2 is 5, 10, 15, 20, 30, 40, 50, or a value in the range of any two of these values. By regulating the mass percentage of the fluorinated linear ester and the non-fluorinated linear ester in the linear ester within the above range, the low-temperature discharge performance of the lithium secondary battery can be further improved, while taking into account the improvement of the cycle performance.

[0047] In some embodiments, the non-aqueous electrolyte further includes a polycyanide substance, and the polycyanide substance is at least selected from one of succinonitrile, glutaronitrile, methylglutaronitrile, adiponitrile, pimelonitrile, octanonitrile, trans-butene dinitrile or 1,2-bis(cyanoethoxy)ethane; based on the total mass of the non-aqueous electrolyte, the mass percentage of the polycyanide substance is X%, and 0.1≤X≤10. For example, X is 0.1, 0.5, 1.5, 2, 5, 7, 10 or a value in the range of any two of these values. Further adding a polycyanide substance to the non-aqueous electrolyte and regulating the mass percentage of the polycyanide substance X% within the above range can improve high-temperature storage while taking into account the improvement of low-temperature discharge performance and cycle performance.

[0048] In some embodiments, the non-aqueous electrolyte comprises a compound of formula II and / or a compound of formula III:

[0049]

[0051] Wherein, R1 to R5 are each independently selected from one of hydrogen atoms, fluorine atoms, vinyl groups, acetylene groups or anhydride groups; based on the total mass of the non-aqueous electrolyte, the mass percentage of the compound of formula II is Y1%, the mass percentage of the compound of formula III is Y2%, Y%=Y1%+Y2%, 0.1≤Y≤0.5. For example, 0.1, 0.2, 0.3, 0.4, 0.5 or a value within the range of any two of these values. Wherein, any two adjacent groups among R1 to R5 exist independently or are connected by covalent bonds to form a ring with the parent ring. Further adding a certain amount of the compound of formula II and the compound of formula III to the non-aqueous electrolyte, and regulating the sum of the mass percentage of the compound of formula II Y1% and the mass percentage of the compound of formula III Y2% within the above range, can further improve high-temperature storage, while taking into account the improvement of low-temperature discharge performance and cycle performance.

[0052] In some embodiments, the compound of formula II is selected from at least one of the following compounds:

[0053]

[0054] In some embodiments, the compound of formula III is selected from at least one of the following compounds:

[0055]

[0056]

[0057] By selecting the above-mentioned compounds of formula II and / or formula III, high-temperature storage can be further improved while also improving low-temperature discharge performance and cycle performance.

[0058] The non-aqueous electrolyte may also include lithium salts and non-aqueous solvents. The present application does not particularly limit the types of lithium salts, as long as the purpose of the present application can be achieved. For example, the lithium salt may include but is not limited to at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium difluorophosphate, bistrifluoromethanesulfonyl imide lithium, bis(fluorosulfonyl)imide lithium, bisoxalate borate or difluorooxalate borate. Based on the mass of the non-aqueous electrolyte, the mass percentage of the lithium salt may be 8% to 15%, for example, the mass percentage of the lithium salt may be 8%, 9%, 10%, 11%, 12.5%, 13%, 15% or a range consisting of any two of the values. The types of the above-mentioned non-aqueous solvents may include, for example, but are not limited to at least one of ether compounds or other organic solvents.

[0059] The above-mentioned ether compound may include, but is not limited to, at least one of ethylene glycol dimethyl ether, dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1-ethoxy-1-methoxyethane, 2-methyltetrahydrofuran or tetrahydrofuran.

[0060] The above-mentioned other organic solvents may include but are not limited to at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, N-methyl-2-pyrrolidone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate or trioctyl phosphate.

[0061] positive electrode

[0062] The positive electrode includes a positive electrode collector and a positive electrode active layer disposed on the surface of the positive electrode collector.

[0063] The positive electrode active layer contains a positive electrode active material, and the positive electrode active layer can be one layer or multiple layers. Each layer of the multiple layers of positive electrode active material can contain the same or different positive electrode active materials. The positive electrode active material is any material that can reversibly embed and de-embed alkali metal ions.

[0064] The positive electrode active material includes a lithium transition metal oxide containing nickel and other transition metals. In the lithium transition metal oxide including nickel and other transition metals, the amount of nickel relative to the total molar number of the transition metal may be 60 mol% or more, for example, 75 mol% or more, for example, 80 mol% or more, for example, 85 mol% or more, or for example, 90 mol% or more.

[0065] In some embodiments, the positive electrode active material includes at least one active material selected from the group consisting of: Li-Ni-Co-Al (NCA), Li-Ni-Co-Mn (NCM), lithium cobalt oxide (LiCoO2), lithium manganese oxide (LiMnO2), lithium nickel oxide (LiNiO2) or lithium iron phosphate (LiFePO4).

[0066] In some embodiments, the positive electrode active layer includes a positive electrode conductive material; the type of the positive electrode conductive material is not limited, and any known conductive material can be used. Examples of positive electrode conductive materials may include, but are not limited to, carbon black such as acetylene black and Super-P; amorphous carbon materials such as needle coke; carbon nanotubes; graphene, etc. The above positive electrode conductive materials may be used alone or in any combination.

[0067] In some embodiments, the positive electrode material layer includes a positive electrode binder. The type of the positive electrode binder is not particularly limited, and in the case of a coating method, any material that can be dissolved or dispersed in a liquid medium used in electrode manufacturing can be used. Examples of positive electrode binders may include, but are not limited to, one or more of the following: resin-based polymers such as polyethylene, polypropylene, polyethylene terephthalate, polymethyl methacrylate, polyimide, aromatic polyamide, cellulose or nitrocellulose; rubber-like polymers such as styrene-butadiene rubber (SBR), nitrile rubber (NBR), fluororubber, isoprene rubber, polybutadiene rubber or ethylene-propylene rubber; thermoplastic elastomer-like polymers such as styrene-butadiene-styrene block copolymer or its hydride, ethylene-propylene-diene terpolymer (EPDM), styrene-ethylene-butadiene-ethylene copolymer, styrene-isoprene-styrene block copolymer or its hydride; soft resin-like polymers such as syndiotactic-1,2-polybutadiene, polyvinyl acetate, ethylene-vinyl acetate copolymer or propylene-α-olefin copolymer; fluorine-based polymers such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene, fluorinated polyvinylidene fluoride or polytetrafluoroethylene-ethylene copolymer; polymer compositions having ionic conductivity of alkali metal ions, etc. The above positive electrode binders may be used alone or in any combination.

[0068] There is no limitation on the type of solvent used to form the positive electrode slurry, as long as it is a solvent that can dissolve or disperse the positive electrode active material, the conductive material, the positive electrode binder, and the thickener used as needed. Examples of solvents used to form the positive electrode slurry may include any of aqueous solvents and organic solvents. Examples of aqueous media may include, but are not limited to, a mixed medium of alcohol and water or water, etc. Examples of organic media may 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; amides such as N-methylpyrrolidone, dimethylformamide, and dimethylacetamide; non-protonic polar solvents such as hexamethylphosphoramide or dimethyl sulfoxide, etc.

[0069] The thickener is usually used to adjust the viscosity of the slurry. In the case of using an aqueous medium, the thickener and styrene-butadiene rubber emulsion can be used for slurrying. The type of thickener is not particularly limited, and examples thereof may include, but are not limited to, carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol, oxidized starch, phosphorylated starch or casein and their salts, etc. The above thickeners may be used alone or in any combination.

[0070] There is no particular limitation on the type of positive electrode current collector, which can be any known material suitable for use as a positive electrode current collector. Examples of positive electrode current collectors may include, but are not limited to, metal materials such as aluminum, stainless steel, nickel plating, titanium or tantalum; materials such as carbon cloth and carbon paper. In some embodiments, the positive electrode current collector is a metal material. In some embodiments, the positive electrode current collector is aluminum.

[0071] In order to reduce the electronic contact resistance between the positive electrode current collector and the positive electrode active layer, the surface of the positive electrode current collector may include a conductive additive or a conductive coating. Examples of conductive additives may include, but are not limited to, noble metals such as carbon, gold, platinum or silver. Examples of conductive coatings may include a mixture layer containing an inorganic oxide, a conductive agent and a binder.

[0072] negative electrode

[0073] The negative electrode comprises a negative electrode current collector and a negative electrode active layer disposed on at least one surface of the negative electrode current collector, wherein the negative electrode active layer comprises a negative electrode active material. In some embodiments, the chargeable capacity of the negative electrode active material is greater than the discharge capacity of the positive electrode active material to prevent lithium metal from being accidentally precipitated on the negative electrode during charging.

[0074] The negative electrode active material may include natural graphite, artificial graphite, mesophase microcarbon beads (MCMB), silicon, silicon-carbon composites, SiO x(0.5<x<1.6), Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, SnO2, spinel structure lithium titanate Li4Ti5O 12 , Li-Al alloy or metallic lithium, etc. Optionally, the negative electrode active material may further include an amorphous carbon material, and the amorphous carbon may be soft carbon (low temperature calcined carbon), hard carbon, mesophase pitch carbide or calcined coke, etc.

[0075] The negative electrode material layer of the present application also includes a negative electrode binder. The negative electrode binder can improve the bonding between the negative electrode active material particles and the bonding between the negative electrode active material and the current collector. The type of negative electrode binder is not particularly limited, as long as it is a material that is stable to the solvent used in the manufacture of the electrolyte or electrode. In some embodiments, the negative electrode binder includes a resin binder. Examples of resin binders include, but are not limited to, fluororesins, polyacrylonitrile (PAN), polyimide resins, acrylic resins, polyolefin resins, and the like. When an aqueous solvent is used to prepare the negative electrode mixture slurry, the negative electrode binder includes, but is not limited to, carboxymethyl cellulose (CMC) or its salt, styrene-butadiene rubber (SBR), polyacrylic acid (PAA) or its salt, polyvinyl alcohol, and the like.

[0076] The negative electrode material layer of the present application also includes a conductive agent. The present application has no particular restrictions on the type of the negative electrode conductive agent, as long as the purpose of the present application can be achieved. For example, the negative electrode conductive agent can be at least one of acetylene black, Ketjen black, carbon nanotubes, carbon fibers, carbon dots or graphene, and the above-mentioned carbon nanotubes can include but are not limited to at least one of single-walled carbon nanotubes or multi-walled carbon nanotubes.

[0077] The present application has no particular restrictions on the negative electrode current collector, as long as the purpose of the present application can be achieved. For example, the negative electrode current collector may include copper foil, aluminum foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam or a polymer substrate covered with a conductive metal, etc. 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). In the present application, there is no particular restriction on the thickness of the negative electrode current collector and the negative electrode active layer, as long as the purpose of the present application can be achieved. For example, the thickness of the negative electrode current collector is 4μm to 12μm, and the thickness of the single-sided negative electrode active layer is 30μm to 160μm. In the present application, the negative electrode active layer can be arranged 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. It should be noted that the "surface" here can be the entire area of ​​the negative electrode current collector or a partial area of ​​the negative electrode current collector. This application has no special restrictions as long as the purpose of this application can be achieved.

[0078] The present application has no particular restrictions on the compaction density of the negative electrode sheet, as long as the purpose of the present application can be achieved. For example, the compaction density of the negative electrode sheet can be 1.0 g / cm 3 Up to 1.85g / cm 3 The present application has no particular limitation on the cold pressing pressure of the negative electrode sheet, as long as the purpose of the present application can be achieved. For example, the cold pressing pressure of the negative electrode sheet can be 3 tons to 30 tons.

[0079] Optionally, the negative electrode plate may further include a conductive layer, which is located between the negative electrode current collector and the negative electrode material layer. The present application has no particular restrictions on the composition of the conductive layer, which may be a conductive layer commonly used in the art. The conductive layer includes a conductive agent and a binder. The present application has no particular restrictions on the conductive agent and the binder in the conductive layer, which may be at least one of the above conductive agent and the above binder. The present application has no particular restrictions on the mass ratio of the conductive agent and the binder in the conductive layer, and those skilled in the art can choose according to actual needs, as long as the purpose of the present application can be achieved. The present application has no particular restrictions on the thickness of the conductive layer, as long as the purpose of the present application can be achieved, for example, the thickness of the conductive layer is 1μm to 10μm.

[0080] Diaphragm

[0081] In the present application, a diaphragm is usually provided between the positive electrode and the negative electrode. The diaphragm is used to separate the positive electrode plate and the negative electrode plate, prevent the internal short circuit of the secondary battery, allow the electrolyte ions to pass freely, and do not affect the electrochemical charge and discharge process.

[0082] The present application has no particular restrictions on the separator, as long as the purpose of the present application can be achieved. For example, the material of the separator may include but is not limited to polyethylene (PE), polypropylene (PP)-based polyolefins (PO), polyester (e.g., polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex or aramid; the type of the separator may include at least one of woven membrane, non-woven membrane, microporous membrane, composite membrane, rolled membrane or spun membrane.

[0083] 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 a polymer, and the material of the polymer includes at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyvinyl pyrrolidone, polyvinyl ether, polyvinylidene fluoride or poly(vinylidene fluoride-hexafluoropropylene).

[0084] In the present application, the pore size of the diaphragm is 0.01 μm to 1 μm, and the thickness is 5 μm to 50 μm.

[0085] In some embodiments, the thickness of the membrane is greater than 1 μm, greater than 5 μm, or greater than 8 μm.

[0086] 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 rate characteristics and energy density of the secondary battery can be ensured.

[0087] The electrochemical device 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 electrochemical device, 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.

[0088] Example

[0089] Hereinafter, taking lithium-ion batteries as an example, examples and comparative examples are given to more specifically describe the implementation of the secondary battery of the present application. Those skilled in the art will understand that the preparation method described in the present application is only an example, and any other suitable preparation method is within the scope of the present application. Various tests and evaluations are carried out according to the following methods. In addition, unless otherwise specified, "parts" and "%" are based on mass.

[0090] Example 1-1

[0091] <Preparation of lithium-ion batteries>

[0092] (1) Preparation of positive electrode: The positive electrode active material lithium cobalt oxide, the conductive agent conductive carbon black and polyvinylidene fluoride (PVDF) are mixed in a mass ratio of 95:2:3, N-methylpyrrolidone (NMP) is added, and the mixture is 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 is evenly coated on one surface of a positive electrode current collector aluminum foil with a thickness of 9μm, and dried to obtain a positive electrode sheet coated with a positive electrode mixture layer on one side. Repeat the above steps on the other surface of the positive electrode current collector aluminum foil to obtain a positive electrode sheet coated with a positive electrode mixture layer on both sides. After cold pressing, cutting, slitting and drying, a positive electrode sheet with a specification of 74mm×867mm is obtained.

[0093] (2) Preparation of non-aqueous electrolyte: In a dry argon atmosphere glove box, diethyl carbonate was used as a base solvent, and then vinylene carbonate, the compound of formula I and lithium hexafluorophosphate (LiPF6) were dissolved in the above base solvent to obtain an electrolyte. Wherein, based on the total mass of the electrolyte, the mass percentage of LiPF6 was 12.5%, the mass percentage of vinylene carbonate and the compound of formula I was as shown in Table 1, and the balance was diethyl carbonate.

[0094] (3) Preparation of negative electrode: artificial graphite is used as the negative electrode active material, and the negative electrode active material, styrene-butadiene rubber (SBR), polyacrylic acid (PAA), carbon nanotubes (CNT) and carboxymethyl cellulose (CMC) are mixed according to a mass ratio of 95.8:2.4:0.5:0.5:0.8, and then deionized water is added as a solvent and stirred evenly to prepare a negative electrode slurry with a solid content of 45wt%. The negative electrode slurry is evenly coated on one surface of a negative electrode current collector copper foil with a thickness of 6μm, and dried to obtain a negative electrode sheet coated with a negative electrode mixture layer on one side. Repeat the above steps on the other surface of the negative electrode current collector copper foil to obtain a negative electrode sheet coated with a negative electrode mixture layer on both sides. After cold pressing, cutting, slitting and drying, a negative electrode sheet with a specification of 76.6mm×875mm is obtained.

[0095] (4) Preparation of diaphragm: A porous polyethylene film with a thickness of 15 μm was used as the diaphragm.

[0096] (5) Preparation of lithium-ion batteries: stack the positive electrode sheet, separator, and negative electrode sheet in order, place the separator between the positive electrode sheet and the negative electrode sheet to play an isolating role, and wind them to obtain a bare cell. Place the bare cell in a packaging bag, inject electrolyte, and package. After the process of formation, degassing, trimming, and capacity testing, a lithium-ion battery is obtained.

[0097] <Test Method>

[0098] (1) Low temperature discharge performance test:

[0099] Place the lithium-ion battery in a high and low temperature box, adjust the temperature to 25°C, and let it stand for 30 minutes to allow the lithium-ion battery to reach a constant temperature. The lithium-ion battery that has reached a constant temperature is discharged to 3.0V at a current constant current of 0.5C, then charged to 4.5V at a current constant current of 0.5C, and then charged to a current of 0.05C at the 4.5V voltage constant voltage. Also at a temperature of 25°C, discharge to 3.0V at a current constant current of 0.5C, and record the discharge capacity as the initial discharge capacity. At a temperature of 25°C, charge to 4.5V at a current constant current of 0.5C, and then charge to a current of 0.05C at the 4.5V voltage constant voltage. Afterwards, place the lithium-ion battery at a temperature of 0°C and let it stand for 30 minutes to keep the temperature of the lithium-ion battery consistent with the outside temperature. At 0°C, discharge to 3.0V at a current constant current of 0.5C, and record the discharge capacity as the low-temperature discharge capacity.

[0100] Low-temperature discharge capacity retention rate = (low-temperature discharge capacity / initial discharge capacity) × 100%.

[0101] (2) Cyclic performance test:

[0102] Place the lithium-ion battery in a 45°C constant temperature test box and let it stand for 30 minutes to allow the lithium-ion battery to reach a constant temperature. Charge it to 4.5V at a constant current of 0.5C, charge it to a current of 0.025C at a constant voltage, let it stand for 5 minutes, and discharge it to 3.0V at a constant current of 0.5C, and record it as the initial discharge capacity C0. Repeat this step for 100 cycles, record the discharge capacity C1 after 100 cycles, and calculate the cycle capacity retention rate of the lithium-ion battery.

[0103] Cycle capacity retention rate = C1 / C0×100%.

[0104] (3) High temperature storage performance test:

[0105] Place the lithium-ion battery in a 25°C constant temperature environment and let it stand for 30 minutes to allow the lithium-ion battery to reach a constant temperature. Charge it to 4.5V at a constant current of 0.5C, and then charge it to a current of 0.025C at a constant voltage of 4.5V. Record the thickness of the lithium-ion battery as the initial thickness. Transfer the lithium-ion battery to a 60°C constant temperature box and store it for 30 days. During this period, test and record the thickness of the lithium-ion battery every 6 days. Record the test thickness after 30 days as the storage thickness. Calculate the thickness expansion rate of the lithium-ion battery and use it as an indicator to evaluate the high-temperature storage performance of the lithium-ion battery.

[0106] High temperature storage expansion ratio = (storage thickness - initial thickness) / initial thickness × 100%.

[0107] The lithium ion batteries of the following embodiments or comparative examples differ from those of embodiment 1-1 only in that the A value of the mass percentage of vinylene carbonate, the type of compound of formula I and the B value of the mass percentage of the compound of formula I are adjusted according to Table 1. The performance test results of the lithium ion batteries of the embodiments and comparative examples are shown in Table 1 below.

[0108] Table 1

[0109]

[0110]

[0111] In the above table, the mass percentage of each substance is based on the mass of the electrolyte, A is the mass percentage of vinylene carbonate, B is the mass percentage of the compound of formula I, and “ / ” means that the substance is not contained.

[0112] As can be seen from Table 1, the non-aqueous electrolyte of the lithium ion battery prepared in the embodiment of the present application includes vinylene carbonate and a compound of formula I, the mass percentage of vinylene carbonate is A%, the mass percentage of the compound of formula I is B%, and P=B / A. When 0.01≤A≤3 and 0.5≤P≤50 are regulated, the low-temperature discharge capacity retention rate of the lithium secondary battery can be improved, while taking into account the improvement of the cycle capacity retention rate. Among them, when at least one of 0.1≤A≤2, 0.5≤B≤5 or 1≤P≤5 is satisfied, the low-temperature discharge capacity retention rate of the lithium secondary battery can be further improved, while taking into account the improvement of the cycle capacity retention rate.

[0113] The lithium ion batteries of the following embodiments are different from those of embodiments 1-3 only in that specific types and contents of cyclic carbonates and linear esters are added to the base solvent during the preparation of the electrolyte, and their mass percentages in the electrolyte are shown in Table 2. The performance test results of the lithium ion batteries of each embodiment are shown in Table 2 below.

[0114] Table 2

[0115]

[0116]

[0117] In the above table, the mass percentage of each substance is based on the mass of the electrolyte, and " / " means that the substance is not contained. The corresponding compounds of each code are as follows: EC: ethylene carbonate, PC: propylene carbonate, FEC: fluoroethylene carbonate.

[0118] As can be seen from Table 2, the non-aqueous electrolyte of the lithium ion battery prepared in the embodiment of the present application further includes a cyclic carbonate, and when the mass percentage content M% of the cyclic carbonate satisfies 5≤M≤30, the low-temperature discharge capacity retention rate of the lithium secondary battery can be further improved, while taking into account the improvement of the cycle capacity retention rate. In particular, when at least one of 10≤M≤20 or 0.1≤B / M≤3 is satisfied, the effect of improving the low-temperature discharge capacity retention rate and the cycle capacity retention rate of the lithium secondary battery is more significant. The non-aqueous electrolyte of the lithium ion battery prepared in the embodiment of the present application further includes a linear ester, and when the mass percentage content N0% of the linear ester satisfies 10≤N0≤70, the low-temperature discharge capacity retention rate of the lithium secondary battery can be further improved, while taking into account the improvement of the cycle capacity retention rate. In particular, for the mass percentage of fluorinated linear ester being N1% and the mass percentage of non-fluorinated linear ester being N2%, when at least one of 5≤N1≤50, 5≤N2≤50 or 20≤N0≤60 is satisfied, the effect of improving the low-temperature discharge capacity retention rate and the cycle capacity retention rate of the lithium secondary battery is more significant.

[0119] The lithium ion batteries of the following Examples 3-1 to 3-19 are adjusted based on the parameters of Examples 1-3; the lithium ion batteries of Example 3-20 are adjusted based on the parameters of Examples 2-28. The specific adjustment is: when preparing the electrolyte, a specific type and content of a polycyano substance, a compound of Formula II and a compound of Formula III are added to the base solvent, and the mass percentage of each substance in the electrolyte is shown in Table 3. The performance test results of the lithium ion batteries of each embodiment are shown in Table 3 below.

[0120] Table 3

[0121]

[0122] In the above table, the mass percentage of each substance is based on the mass of the electrolyte, and “ / ” means that the substance is not contained.

[0123] As can be seen from Table 3, the non-aqueous electrolyte of the lithium ion battery prepared in the embodiment of the present application further includes a polycyanide substance, and when the mass percentage content X% of the polycyanide substance satisfies 0.5≤X≤5, the high-temperature storage expansion rate of the lithium secondary battery can be improved, while taking into account the improvement of the low-temperature discharge capacity retention rate and the cycle capacity retention rate. The non-aqueous electrolyte of the lithium ion battery prepared in the embodiment of the present application further includes an internal salt substance, and when the mass percentage content Y% of the internal salt substance satisfies 0.1≤Y≤0.5, the high-temperature storage expansion rate of the lithium secondary battery can be improved, while taking into account the improvement of the low-temperature discharge capacity retention rate and the cycle capacity retention rate.

[0124] The above description is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent substitutions and improvements made within the principles of the present application should be included in the protection scope of the present application.

Claims

1. A non-aqueous electrolyte, characterized in that: The non-aqueous electrolyte comprises vinylene carbonate and a compound of formula I: wherein R is selected from C2 to C6 alkyl which is unsubstituted or substituted by Ra, C2 to C6 alkenyl which is unsubstituted or substituted by Ra, C2 to C6 alkynyl which is unsubstituted or substituted by Ra, C5 to C6 alkyl which is unsubstituted or substituted by Ra, 12 A nitrogen-containing heteroaryl group, a C6 to C 12 The substituents Ra of each group are independently selected from fluorine or C1 to C6 fluoroalkyl; Based on the total mass of the non-aqueous electrolyte, the mass percentage of the vinylene carbonate is A%, 0.01≤A≤3, and the mass percentage of the compound of formula I is B%; P=B / A, 0.5≤P≤50.

2. The non-aqueous electrolyte according to claim 1, characterized in that The non-aqueous electrolyte satisfies at least one of the following conditions: (1)0.1≤A≤2； (2)0.5≤B≤5； (3)1≤P≤5。 3. The non-aqueous electrolyte according to claim 1, characterized in that The compound of formula I is selected from at least one of the following compounds:

4. The non-aqueous electrolyte according to any one of claims 1 to 3, characterized in that: The non-aqueous electrolyte further comprises a cyclic carbonate, wherein the cyclic carbonate is selected from any two of ethylene carbonate, propylene carbonate or fluoroethylene carbonate; Based on the total mass of the non-aqueous electrolyte, the mass percentage of the cyclic carbonate is M%, 5≤M≤30.

5. The non-aqueous electrolyte according to claim 4, characterized in that 10≤M≤20; and / or, 0.1≤B / M≤3.

6. The non-aqueous electrolyte according to any one of claims 1 to 3, characterized in that: The non-aqueous electrolyte further comprises a linear ester, wherein the linear ester comprises a fluorinated linear ester and a non-fluorinated linear ester; and the linear ester satisfies at least one of the following conditions: (1) The fluorinated linear ester is at least one selected from methyl difluoroethyl carbonate, methyl trifluoroethyl carbonate, ethyl trifluoroethyl carbonate, methyl hexafluoroisopropyl carbonate, di(2,2,2-trifluoroethyl) carbonate, 2,2-difluoroethyl acetate, 2,2,2-trifluoroethyl acetate, hexafluoroisopropyl acetate, 2,2-difluoroethyl propionate, 2,2,2-trifluoroethyl propionate or hexafluoroisopropyl propionate; (2) the non-fluorinated linear ester is at least one selected from dimethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate or propyl propionate; (3) Based on the total mass of the non-aqueous electrolyte, the mass percentage of the linear ester is N0%, 10≤N0≤70.

7. The non-aqueous electrolyte according to claim 6, characterized in that Based on the total mass of the nonaqueous electrolyte, the nonaqueous electrolyte satisfies at least one of the following conditions: (1) The mass percentage of the fluorinated linear ester is N1%, 5≤N1≤50; (2) The mass percentage of the non-fluorinated linear ester is N2%, 5≤N2≤50; (3)20≤N0≤60。 8. The non-aqueous electrolyte according to any one of claims 1 to 3, characterized in that: The non-aqueous electrolyte further comprises a polycyano substance, wherein the polycyano substance is selected from at least one of succinonitrile, glutaronitrile, methylglutaronitrile, adiponitrile, pimelonitrile, suberonitrile, fumaronitrile or 1,2-bis(cyanoethoxy)ethane; Based on the total mass of the non-aqueous electrolyte, the mass percentage of the polycyanide substance is X%, and 0.1≤X≤10.

9. The non-aqueous electrolyte according to claim 8, characterized in that 0.5≤X≤5。 10. The non-aqueous electrolyte according to any one of claims 1 to 3, characterized in that: The non-aqueous electrolyte further comprises a compound of formula II and / or a compound of formula III: wherein R1 to R5 are each independently selected from a hydrogen atom, a fluorine atom, a vinyl group, an acetylene group or an anhydride group; Based on the total mass of the non-aqueous electrolyte, the mass percentage of the compound of formula II is Y1%, the mass percentage of the compound of formula III is Y2%, Y%=Y1%+Y2%, 0.1≤Y≤0.5; Any two adjacent groups among R1 to R5 exist independently or are connected by covalent bonds to form a ring with the parent ring.

11. The non-aqueous electrolyte according to claim 10, characterized in that The compound of formula II is selected from at least one of the following compounds: And / or, the compound of formula III is selected from at least one of the following compounds:

12. A lithium secondary battery, characterized in that: include: Positive electrode: the positive electrode comprises a positive electrode current collector and a positive electrode active layer disposed on at least one surface of the positive electrode current collector; Negative electrode: the negative electrode comprises a negative electrode current collector and a negative electrode active layer disposed on at least one surface of the negative electrode current collector; Isolation film; Non-aqueous electrolyte: The non-aqueous electrolyte is the non-aqueous electrolyte according to any one of claims 1 to 11.

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