Electrochemical device and electronic device

Abstract

The invention relates to an electrochemical device and an electronic device. Specifically, the present application provides an electrochemical device including: a positive electrode including a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector, the positive electrode active material layer including a positive electrode active material, in which: the positive electrode active material has a first element; the first element comprises at least one of lanthanum, yttrium, cerium or tungsten, the content of the first element is a% based on the weight of the positive electrode active material, and a is less than or equal to 1; and the electrolyte solution includes a propionate and a compound having a cyano group. The electrochemical device has improved voltage drop and safety at high temperature and high pressure.

Description

[0001] This application is a divisional application of a patent application with an application date of December 29, 2021, application number 202180027072.3, and invention name “Electrochemical Device and Electronic Device”. Technical Field

[0002] The present application relates to the field of energy storage, and in particular to an electrochemical device and an electronic device, in particular a lithium-ion battery. Background Art

[0003] In recent years, with the rapid development of electronic products such as smartphones, tablets and smart wearables, the demand for electrochemical devices (e.g., lithium-ion batteries) has increased significantly. Considering the different usage time and working environment of electronic products, consumers have higher and higher requirements for the energy density and safety of lithium-ion batteries.

[0004] High-voltage lithium cobalt oxide (4.4V and above) is usually used as the positive electrode active material and graphite negative electrode material with high capacity and high compaction density to improve the energy density of lithium-ion batteries. However, as the temperature and voltage increase, the stability of lithium cobalt oxide deteriorates, and the commonly used electrolyte is prone to decomposition, which will deteriorate the performance of lithium-ion batteries.

[0005] In view of the above, there is a real need to provide electrochemical devices and electronic devices with improved performance at high temperature and pressure. Summary of the invention

[0006] The embodiments of the present application solve the problems existing in the prior art to some extent by providing an electrochemical device and an electronic device with improved voltage drop and safety under high temperature and high pressure.

[0007] In one aspect of the present application, the present application provides an electrochemical device comprising: a positive electrode, a negative electrode and an electrolyte, the positive electrode comprising a positive electrode collector and a positive electrode active material layer formed on the positive electrode collector, the positive electrode active material layer comprising a positive electrode active material, wherein: the positive electrode active material has a first element, the first element comprises at least one of lanthanum, yttrium, cerium or tungsten, and based on the weight of the positive electrode active material, the content of the first element is a%, a≤1; and the electrolyte comprises propionate and a compound having a cyano group.

[0008] According to an embodiment of the present application, the positive electrode active material further comprises a second element, and the second element comprises at least one of aluminum, magnesium, titanium or zirconium.

[0009] According to an embodiment of the present application, based on the weight of the electrolyte, the sum of the contents of the propionate and the compound having a cyano group is b%, and a and b satisfy: 0.1≤a×b≤20.

[0010] According to an embodiment of the present application, 0.2≤a×b≤15.

[0011] According to an embodiment of the present application, 0.01≤a≤1.

[0012] According to an embodiment of the present application, based on the weight of the electrolyte, the sum of the contents of the propionate and the compound having a cyano group is b%, and the value range of b is 5 to 65.

[0013] According to an embodiment of the present application, based on the weight of the electrolyte, the content of the propionate is x%, the content of the compound having a cyano group is y%, and x and y satisfy: 1≤x / y≤20.

[0014] According to an embodiment of the present application, based on the weight of the electrolyte, the content of the propionate is x%, and the value range of x is 5 to 50.

[0015] According to an embodiment of the present application, based on the weight of the electrolyte, the content of the compound having a cyano group is y%, and the value range of y is 0.1 to 15.

[0016] According to an embodiment of the present application, the weight fraction of the positive electrode active material in the positive electrode active material layer is M%, and the value range of M is 95 to 99; based on the weight of the electrolyte, the content of the propionate is x%; and M and x satisfy: 1.5≤M / x≤18.

[0017] According to an embodiment of the present application, the average particle size of the positive electrode active material is Dμm, and the value range of D is 5 to 30; based on the weight of the electrolyte, the content of the compound having a cyano group is y%; and D and y satisfy: 1≤D / y≤20.

[0018] According to an embodiment of the present application, the propionate ester comprises at least one of ethyl propionate or propyl propionate.

[0019] According to an embodiment of the present application, the cyano compound includes succinonitrile, glutaronitrile, adiponitrile, 1,5-dicyanopentane, 1,6-dicyanohexane, tetramethylsuccinonitrile, 2-methylglutaronitrile, 2,4-dimethylglutaronitrile, 2,2,4,4-tetramethylglutaronitrile, 1,4-dicyanopentane, 1,2-dicyanobenzene, 1,3-dicyanobenzene, 1,4-dicyanobenzene, ethylene glycol bis(propionitrile) ether, 3,5-dioxa- Heptanedinitrile, 1,4-bis(cyanoethoxy)butane, diethylene glycol di(2-cyanoethyl)ether, triethylene glycol di(2-cyanoethyl)ether, tetraethylene glycol di(2-cyanoethyl)ether, 1,3-bis(2-cyanoethoxy)propane, 1,4-bis(2-cyanoethoxy)butane, 1,5-bis(2-cyanoethoxy)pentane, ethylene glycol di(4-cyanobutyl)ether, 1,4-dicyano-2-butene, 1,4-dicyano-2 -methyl-2-butene, 1,4-dicyano-2-ethyl-2-butene, 1,4-dicyano-2,3-dimethyl-2-butene, 1,4-dicyano-2,3-diethyl-2-butene, 1,6-dicyano-3-hexene, 1,6-dicyano-2-methyl-3-hexene, 1,3,5-pentanetricarbonitrile, 1,2,3-propanetricarbonitrile, 1,3,6-hexatriconitrile, 1,2,6-hexatriconitrile, 1,2,3- At least one of tris(2-cyanoethoxy)propane, 1,2,4-tris(2-cyanoethoxy)butane, 1,1,1-tris(cyanoethoxymethylene)ethane, 1,1,1-tris(cyanoethoxymethylene)propane, 3-methyl-1,3,5-tris(cyanoethoxy)pentane, 1,2,7-tris(cyanoethoxy)heptane, 1,2,6-tris(cyanoethoxy)hexane or 1,2,5-tris(cyanoethoxy)pentane.

[0020] According to an embodiment of the present application, the electrolyte further comprises at least one of lithium difluorophosphate or a compound of formula I,

[0021]

[0022] Among them, M + represents a quaternary ammonium cation or an imidazolium cation, and R represents a C1-5 alkyl group.

[0023] According to an embodiment of the present application, the electrolyte further includes at least one of fluoroethylene carbonate, 1,3-propane sultone, vinyl sulfate, vinylene carbonate or 1-propylphosphoric acid cyclic anhydride.

[0024] According to an embodiment of the present application, the positive electrode active material includes at least one of lithium cobalt oxide containing at least one of lanthanum, yttrium or cerium or a ternary positive electrode active material containing tungsten.

[0025] In another aspect of the present application, the present application provides an electronic device, which includes the electrochemical device according to the present application.

[0026] The positive electrode active material layer used in the present application has improved interface stability under thermal runaway, and can fully suppress the voltage drop of the electrochemical device under high pressure and high temperature. At the same time, the use of an electrolyte including propionate and a compound having a cyanide group can significantly improve the high-temperature safety of the electrochemical device.

[0027] Additional aspects and advantages of the embodiments of the present application will be described, shown, or explained in part in the subsequent description through implementation of the embodiments of the present application. DETAILED DESCRIPTION

[0028] The embodiments of the present application will be described in detail below. The embodiments of the present application should not be interpreted as limiting the present application.

[0029] Unless expressly stated otherwise, the following terms used herein have the meanings indicated below.

[0030] In the detailed description and claims, a list of items connected by the term "at least one of" may mean any combination of the listed items. For example, if items A and B are listed, the phrase "at least one of A and B" means only A; only B; or A and B. In another example, if items A, B, and C are listed, the phrase "at least one of A, B, and C" means only A; or only B; only C; A and B (excluding C); A and C (excluding B); B and C (excluding A); or all of A, B, and C. Item A may include a single element or multiple elements. Item B may include a single element or multiple elements. Item C may include a single element or multiple elements. The term "at least one of" has the same meaning as the term "at least one of."

[0031] As used herein, the term "alkyl" is expected to be a straight chain saturated hydrocarbon structure with 1 to 20 carbon atoms. "Alkyl" is also expected to be a branched or cyclic hydrocarbon structure with 3 to 20 carbon atoms. When specifying an alkyl with a specific carbon number, it is expected to cover all geometric isomers with that carbon number; therefore, for example, "butyl" means including n-butyl, sec-butyl, isobutyl, tert-butyl and cyclobutyl; "propyl" includes n-propyl, isopropyl and cyclopropyl. Alkyl examples include, but are not limited to methyl, ethyl, n-propyl, isopropyl, cyclopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, cyclobutyl, n-pentyl, isopentyl, neopentyl, cyclopentyl, methylcyclopentyl, ethylcyclopentyl, n-hexyl, isohexyl, cyclohexyl, n-heptyl, octyl, cyclopropyl, cyclobutyl, norbornyl, etc.

[0032] Under high temperature and high pressure, the structural stability of the positive electrode active material is related to the safety and life of the electrochemical device. However, under high temperature and high pressure conditions, the structural stability of high-voltage lithium cobalt oxide deteriorates, metal ions are easily dissolved from the positive electrode, and reduced and deposited on the surface of the negative electrode, thereby destroying the structure of the negative electrode solid electrolyte interface (SEI) film, resulting in the continuous increase of the negative electrode impedance and the thickness of the lithium-ion battery. At the same time, the electrolyte is prone to oxidative decomposition on the surface of the positive electrode to produce a large amount of gas, which leads to the swelling of the lithium-ion battery and the destruction of the electrode interface. In addition, under high temperature and high voltage, the oxidation activity of lithium cobalt oxide is high, and the side reactions between it and the electrolyte are intensified, resulting in the continuous deposition of the decomposition products of the electrolyte on the surface of the positive electrode, and the internal resistance of the lithium-ion battery increases. These factors will have an adverse effect on the voltage drop and safety of lithium-ion batteries.

[0033] The industry usually uses rare earth elements to dope positive electrode active materials (such as lithium cobalt oxide) in order to improve electronic conductivity. However, there have been no reports of using rare earth doping to improve the safety of electrochemical devices. In addition, it is generally believed that propionate esters are used in electrolytes to adjust viscosity and improve low-temperature performance; while compounds with cyanide groups are mostly added to inhibit the dissolution of positive transition metal ions. Based on existing knowledge, it is impossible to predict the impact of the simultaneous use of rare earth element-doped positive electrode active materials and electrolytes including propionate esters and compounds with cyanide groups on the voltage drop and safety performance of electrochemical devices.

[0034] The present application solves the problems related to voltage drop and safety performance of electrochemical devices by using a positive electrode active material including at least one of lanthanum, yttrium, cerium or tungsten and an electrolyte including propionate and a compound having a cyano group. The inventor unexpectedly discovered that by doping rare earth elements such as lanthanum, yttrium, cerium or tungsten in the positive electrode active material, the crystal phase transformation of the positive electrode active material (e.g., lithium cobalt oxide) can be suppressed, but the internal voltage drop of the electrochemical device will be increased. The use of an electrolyte including propionate and a compound having a cyano group can effectively suppress the phase change or delamination of the positive electrode active material under high temperature and high pressure, passivate the positive electrode surface, and stabilize the bulk structure of the positive electrode active material. In addition, the higher the content of the doping element (lanthanum, yttrium, cerium and / or tungsten) in the positive electrode active material, the higher the stability of the crystal structure of the positive electrode active material under high temperature and high pressure. However, when the content of these doping elements is too high (e.g., more than 1%), it will have a serious impact on the voltage drop of the electrochemical device. Therefore, it is necessary to control the content of these doping elements in the positive electrode active material. The specific combination of the positive electrode active material and the electrolyte of the present application can not only effectively reduce the voltage drop of the electrochemical device under high temperature and high pressure, but also significantly improve the safety of the electrochemical device (for example, short circuit safety and thermal abuse safety, etc.).

[0035] I. Positive electrode

[0036] The positive electrode includes a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector. The positive electrode active material layer can be one layer or multiple layers. The positive electrode active material layer includes a positive electrode active material, and each layer of the multiple layers of positive electrode active material can contain the same or different positive electrode active materials.

[0037] A main feature of the electrochemical device of the present application is that the positive electrode active material has a first element, the first element includes at least one of lanthanum, yttrium, cerium or tungsten, and the content of the first element is a%, a≤1 based on the weight of the positive electrode active material.

[0038] In some embodiments, 0.01≤a≤1. In some embodiments, 0.05≤a≤0.5. In some embodiments, 0.1≤a≤0.3. In some embodiments, a is 0.01, 0.02, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, or in a range consisting of any two of the foregoing values.

[0039] In some embodiments, the positive electrode active material further comprises a second element, wherein the second element comprises at least one of aluminum, magnesium, titanium or zirconium. When the positive electrode active material comprises both the first element (at least one of lanthanum, yttrium, cerium or tungsten) and the second element (at least one of aluminum, magnesium, titanium and zirconium), the stability of the crystal structure under high temperature and high pressure is enhanced, and the interface stability can be improved under the joint action of the propionate and the compound having a cyano group, and the oxidative decomposition of the electrolyte is inhibited, thereby further improving the voltage drop and safety of the electrochemical device under high temperature and high pressure.

[0040] In some embodiments, the weight fraction of the positive electrode active material in the positive electrode active material layer is M%, and the value range of M is 95 to 99. In some embodiments, M is 95, 96, 97, 98, 99 or in a range consisting of any two of the above values. When the weight fraction of the positive electrode active material in the positive electrode active material layer is within the above range, the energy density of the electrochemical device can be significantly improved.

[0041] In some embodiments, the average particle size of the positive electrode active material is Dμm, and the value range of D is 5 to 30. In some embodiments, the value range of D is 10 to 25. In some embodiments, the value range of D is 12 to 20. In some embodiments, D is 5, 7, 9, 10, 12, 15, 18, 20, 25, 30 or in the range consisting of any two of the above values. When the average particle size of the positive electrode active material is within the above range, a positive electrode active material with a high tap density can be obtained, which helps to improve the performance of the electrochemical device. In addition, in the preparation process of the positive electrode of the electrochemical device (that is, when the positive electrode active material, the conductive material and the adhesive are slurried with a solvent and coated in a thin film), the use of a positive electrode active material with a high tap density can prevent problems such as streaks. By mixing two or more positive electrode active materials with different average particle sizes, the filling property during the preparation of the positive electrode can be further improved.

[0042] The average particle size of the positive electrode active material can be measured using a laser diffraction / scattering particle size distribution measuring device: when using LA-920 manufactured by HORIBA as a particle size distribution meter, a 0.1% sodium hexametaphosphate aqueous solution is used as the dispersion medium used in the measurement, and the refractive index is set to 1.24 after 5 minutes of ultrasonic dispersion. The average particle size of the positive electrode active material can also be measured by a laser diffraction particle size analyzer (Shimadzu SALD-2300) and a scanning electron microscope (ZEISS EVO18, the number of samples is not less than 100).

[0043] In some embodiments, the positive electrode active material includes at least one of lithium cobalt oxide containing at least one of lanthanum, yttrium or cerium or a ternary positive electrode active material containing tungsten. In some embodiments, the positive electrode active material includes lithium nickel cobalt manganese oxide containing tungsten.

[0044] A substance with a different composition may be attached to the surface of the above-mentioned positive electrode active material. Examples of surface-attached substances may include, but are not limited to: oxides such as aluminum oxide, silicon dioxide, titanium dioxide, zirconium oxide, magnesium oxide, calcium oxide, boron oxide, antimony oxide, and bismuth oxide; sulfates such as lithium sulfate, sodium sulfate, potassium sulfate, magnesium sulfate, calcium sulfate, and aluminum sulfate; carbonates such as lithium carbonate, calcium carbonate, and magnesium carbonate; carbon, etc. By attaching substances to the surface of the positive electrode active material, the oxidation reaction of the electrolyte on the surface of the positive electrode active material can be suppressed, and the life of the electrochemical device can be improved. When the amount of surface-attached substances is too small, the effect cannot be fully manifested; when the amount of surface-attached substances is too much, it will hinder the entry and exit of lithium ions, and thus the resistance will sometimes increase. In this application, a positive electrode active material having a substance with a different composition attached to the surface of the positive electrode active material is also referred to as a "positive electrode active material."

[0045] In some embodiments, the shape of the positive electrode active material particles includes, but is not limited to, block, polyhedron, sphere, ellipsoid, plate, needle and column, etc. In some embodiments, the positive electrode active material particles include primary particles, secondary particles or a combination thereof. In some embodiments, the primary particles can be agglomerated to form secondary particles.

[0046] There is no limitation on the type of positive electrode conductive material, and any known conductive material can be used. Examples of positive electrode conductive materials may include, but are not limited to, graphite such as natural graphite and artificial graphite; carbon black such as acetylene black; carbon materials such as amorphous carbon such as needle coke; carbon nanotubes; graphene, etc. The above positive electrode conductive materials may be used alone or in any combination.

[0047] There is no particular restriction 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 the manufacture of the electrode can be used. Examples of positive electrode binders may include, but are not limited to, one or more 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; styrene-butadiene-styrene block copolymers or their hydrogenates, ethylene-propylene-diene terpolymers (EPDM), Thermoplastic elastomer polymers such as styrene·ethylene·butadiene·ethylene copolymers, styrene·isoprene·styrene block copolymers or their hydrogenated products; soft resin polymers such as syndiotactic-1,2-polybutadiene, polyvinyl acetate, ethylene·vinyl acetate copolymers, propylene·α-olefin copolymers; fluorine-based polymers such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene, fluorinated polyvinylidene fluoride, polytetrafluoroethylene·ethylene copolymers; polymer compositions having ion conductivity of alkali metal ions (especially lithium ions), etc. The above positive electrode binders can be used alone or in any combination.

[0048] 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, water and mixed media of alcohol and water. 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 (THF); amides such as N-methylpyrrolidone (NMP), dimethylformamide, and dimethylacetamide; aprotic polar solvents such as hexamethylphosphoramide and dimethyl sulfoxide, etc.

[0049] Thickeners are generally used to adjust the viscosity of the slurry. In the case of using an aqueous medium, a thickener and a styrene-butadiene rubber (SBR) 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, casein, and salts thereof. The above thickeners may be used alone or in any combination.

[0050] The type of positive electrode current collector is not particularly limited, and it can be any material known to be 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, tantalum, and carbon 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.

[0051] In order to reduce the electronic contact resistance between the positive electrode current collector and the positive electrode active material layer, the surface of the positive electrode current collector may include a conductive additive. Examples of the conductive additive may include, but are not limited to, carbon and precious metals such as gold, platinum, and silver.

[0052] The positive electrode can be made by forming a positive electrode active material layer containing a positive electrode active material and a binder on a current collector. The manufacture of a positive electrode using a positive electrode active material can be carried out by a conventional method, that is, the positive electrode active material and the binder, as well as the conductive material and the thickener as required, are dry-mixed to form a sheet, and the obtained sheet is pressed onto the positive electrode current collector; or these materials are dissolved or dispersed in a liquid medium to form a slurry, and the slurry is applied to the positive electrode current collector and dried to form a positive electrode active material layer on the current collector, thereby obtaining a positive electrode.

[0053] When the positive electrode active material is a primary particle, the average particle size of the positive electrode active material refers to the primary particle size of the positive electrode active material particles. When the primary particles of the positive electrode active material particles aggregate to form secondary particles, the average particle size of the positive electrode active material particles refers to the secondary particle size of the positive electrode active material particles.

[0054] II. Electrolyte

[0055] The electrolyte used in the electrochemical device of the present application includes an electrolyte and a solvent for dissolving the electrolyte. Another main feature of the electrochemical device of the present application is that the electrolyte includes a propionate ester and a compound having a cyano group.

[0056] The inventors have found that when the first element in the positive electrode active material is used together with the propionate ester and the compound having a cyano group in the electrolyte, it can produce an unexpected improvement effect on the stability of the structure and electrode interface of the positive electrode active material under high voltage. A good voltage drop can also be further obtained by regulating the content of the first element in the positive electrode active material and the content of the propionate ester and the compound having a cyano group in the electrolyte. Adjustment of the content of the first element in the positive electrode active material or the content of the propionate ester and the compound having a cyano group in the electrolyte will cause changes in the structure of the positive electrode material, changes in other components and contents of the electrolyte, changes in elements on the surface of the positive electrode material particles, and the stability of the interface between the positive electrode and the electrolyte, thereby affecting the performance of the entire electrochemical device.

[0057] In some embodiments, based on the weight of the electrolyte, the sum of the contents of the propionate and the compound having a cyano group is b%, and a and b satisfy: 0.1≤a×b≤20. In some embodiments, a and b satisfy: 0.2≤a×b≤15. In some embodiments, a and b satisfy: 0.5≤a×b≤10. In some embodiments, a and b satisfy: 1≤a×b≤5. In some embodiments, a×b is 0.1, 0.2, 0.5, 1, 2, 2.5, 3, 3.5, 4, 4.5, 5, 8, 10, 15, 20 or in a range consisting of any two of the above values. When the content of the first element in the positive electrode active material and the sum of the contents of the propionate and the compound having a cyano group in the electrolyte satisfy the above relationship, the surface defects of the crystal structure of the positive electrode active material can be reduced, the continuous destruction of the positive electrode surface passivation layer during the charge and discharge cycle of the electrochemical device can be effectively inhibited, the number of repairs can be reduced, and the interfacial stability of the positive electrode active material layer can be fully improved, thereby further improving the voltage drop and safety of the electrochemical device under high temperature and high pressure.

[0058] In some embodiments, b is in the range of 5 to 65. In some embodiments, b is in the range of 10 to 60. In some embodiments, b is in the range of 20 to 50. In some embodiments, b is in the range of 30 to 40. In some embodiments, b is 5, 7, 8, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, or in the range consisting of any two of the above values.

[0059] The propionate ester as a solvent in the electrolyte and the cyanide compound as an additive can not only play their own functions, but the inventors also found that the propionate ester helps to enhance the passivation effect of the cyanide compound at the positive electrode, so that the protective film formed has a smaller impedance. This improvement is more obvious in the positive electrode active material containing the first element.

[0060] Based on the weight of the electrolyte, the content of the propionate is x%, the content of the compound having a cyano group is y%, and x and y satisfy: 1≤x / y≤20. In some embodiments, 5≤x / y≤15. In some embodiments, 10≤x / y≤12. In some embodiments, x / y is 1, 3, 5, 8, 10, 13, 15, 18, 20 or within the range consisting of any two of the above values. When the content of the propionate and the content of the compound having a cyano group in the electrolyte satisfy the above relationship, it helps to improve the structural stability of the positive electrode surface during the charge and discharge cycle, and further improves the voltage drop and safety of the electrochemical device under high temperature and high pressure.

[0061] In some embodiments, x is in the range of 5 to 70. In some embodiments, x is in the range of 10 to 60. In some embodiments, x is in the range of 20 to 50. In some embodiments, x is 5, 7, 8, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, or in the range consisting of any two of the above values.

[0062] In some embodiments, y ranges from 0.1 to 15. In some embodiments, y ranges from 0.5 to 10. In some embodiments, y ranges from 1 to 8. In some embodiments, y ranges from 3 to 5. In some embodiments, y is 0.1, 0.5, 1, 3, 5, 8, 10, 13, 15 or in the range consisting of any two of the above values. When the weight fraction of the positive electrode active material in the positive electrode active material is increased, the content of the conductive agent or the binder needs to be lowered accordingly, but this will bring about problems such as poor interface stability and decreased wettability of the electrolyte. The inventors found that by adjusting the weight fraction M% of the positive electrode active material and the content x% of the propionate in the electrolyte so that 1.5≤M / x≤18, better performance was unexpectedly obtained.

[0063] In some embodiments, M and x satisfy: 2≤M / x≤15. In some embodiments, M and x satisfy: 3≤M / x≤10. In some embodiments, M and x satisfy: 5≤M / x≤8. In some embodiments, M / x is 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 8, 10, 15, 18 or within the range consisting of any two of the above values. When the weight fraction of the positive electrode active material in the positive electrode active material layer and the content of propionate in the electrolyte satisfy the above relationship, it helps to improve the structural stability of the positive electrode surface during the charge and discharge cycle, and further improves the voltage drop and safety of the electrochemical device at high temperature and high pressure.

[0064] The appropriate particle size of the positive electrode active material helps to improve the particle breakage or structural collapse during the cycle under high voltage. The use of positive electrode film-forming additives with a specific chemical skeleton structure and / or content can further control the thickness and density of the film.

[0065] In some embodiments, the average particle size Dμm of the positive electrode active material and the weight y% of the compound having a cyano group in the electrolyte satisfy: 1≤D / y≤20. In some embodiments, D and y satisfy: 1.5≤D / y≤15. In some embodiments, D and y satisfy: 2≤D / y≤10. In some embodiments, D and y satisfy the following relationship: 3≤D / y≤5. In some embodiments, D / y is 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 8, 10, 15, 20 or within the range consisting of any two of the above values. When the average particle size of the positive electrode active material and the weight of the compound having a cyano group in the electrolyte satisfy the above relationship, it helps to improve the structural stability of the positive electrode surface during the charge and discharge cycle, and further improves the voltage drop and safety of the electrochemical device at high temperature and high pressure.

[0066] In some embodiments, the propionate ester comprises at least one of ethyl propionate or propyl propionate.

[0067] In some embodiments, the compound having a cyano group comprises succinonitrile, glutaronitrile, adiponitrile, 1,5-dicyanopentane, 1,6-dicyanohexane, tetramethylsuccinonitrile, 2-methylglutaronitrile, 2,4-dimethylglutaronitrile, 2,2,4,4-tetramethylglutaronitrile, 1,4-dicyanopentane, 1,2-dicyanobenzene, 1,3-dicyanobenzene, 1,4-dicyanobenzene, ethylene glycol bis(propionitrile) ether, 3,5-dioxa-heptane Nitrile, 1,4-bis(cyanoethoxy)butane, diethylene glycol di(2-cyanoethyl)ether, triethylene glycol di(2-cyanoethyl)ether, tetraethylene glycol di(2-cyanoethyl)ether, 1,3-bis(2-cyanoethoxy)propane, 1,4-bis(2-cyanoethoxy)butane, 1,5-bis(2-cyanoethoxy)pentane, ethylene glycol di(4-cyanobutyl)ether, 1,4-dicyano-2-butene, 1,4-dicyano-2- methyl-2-butene, 1,4-dicyano-2-ethyl-2-butene, 1,4-dicyano-2,3-dimethyl-2-butene, 1,4-dicyano-2,3-diethyl-2-butene, 1,6-dicyano-3-hexene, 1,6-dicyano-2-methyl-3-hexene, 1,3,5-pentanetricarbonitrile, 1,2,3-propanetricarbonitrile, 1,3,6-hexatriconitrile, 1,2,6-hexatriconitrile, 1,2,3-tricarbonitrile At least one of (2-cyanoethoxy)propane, 1,2,4-tris(2-cyanoethoxy)butane, 1,1,1-tris(cyanoethoxymethylene)ethane, 1,1,1-tris(cyanoethoxymethylene)propane, 3-methyl-1,3,5-tris(cyanoethoxy)pentane, 1,2,7-tris(cyanoethoxy)heptane, 1,2,6-tris(cyanoethoxy)hexane or 1,2,5-tris(cyanoethoxy)pentane. The above-mentioned compounds having a cyano group can be used alone or in any combination. If the electrolyte contains two or more compounds having a cyano group, the content of the compound having a cyano group refers to the total content of the two or more compounds having a cyano group.

[0068] In some embodiments, the electrolyte further comprises at least one of lithium difluorophosphate or a compound of formula I,

[0069]

[0070] Among them, M + represents a quaternary ammonium cation or an imidazolium cation, and R represents a C1-5 alkyl group.

[0071] In some embodiments, the compound of formula I includes at least one of 1-ethyl-3-methylimidazolium methyl sulfate, 1-ethyl-3-methylimidazolium ethyl sulfate, 1,3-dimethylimidazolium methyl sulfate, 1,3-dimethylimidazolium ethyl sulfate, 1-butyl-3-methylimidazolium methyl sulfate, 1-butyl-3-methylimidazolium ethyl sulfate, N-methyl-N-propylpyrrolidinium methyl sulfate or N-methyl-N-propylpyrrolidinium ethyl sulfate.

[0072] When the electrolyte includes lithium difluorophosphate and / or the compound of formula I, the surface defects caused by the decomposition and reconstruction of the positive electrode surface protective film during the cycle of the electrochemical device can be further suppressed, thereby further improving the voltage drop and safety of the electrochemical device under high temperature and high pressure.

[0073] In some embodiments, the electrolyte further comprises at least one of fluoroethylene carbonate, 1,3-propane sultone, vinyl sulfate, vinylene carbonate or 1-propylphosphoric acid cyclic anhydride. These compounds help stabilize the interface between the positive electrode and the electrolyte, thereby further improving the voltage drop and safety of the electrochemical device at high temperature and high pressure.

[0074] In some embodiments, the electrolyte further comprises any non-aqueous solvent known in the prior art that can be used as a solvent for the electrolyte.

[0075] In some embodiments, the non-aqueous solvent includes, but is not limited to, one or more of the following: cyclic carbonates, chain carbonates, cyclic carboxylates, chain carboxylates, cyclic ethers, chain ethers, phosphorus-containing organic solvents, sulfur-containing organic solvents, and aromatic fluorine-containing solvents.

[0076] In some embodiments, examples of the cyclic carbonate may include, but are not limited to, one or more of the following: ethylene carbonate (EC), propylene carbonate (PC) and butylene carbonate. In some embodiments, the cyclic carbonate has 3-6 carbon atoms.

[0077] In some embodiments, examples of the linear carbonates may include, but are not limited to, one or more of the following: dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate (DEC), methyl n-propyl carbonate, ethyl n-propyl carbonate, di-n-propyl carbonate, etc. Examples of fluorine-substituted linear carbonates may include, but are not limited to, one or more of the following: bis(fluoromethyl) carbonate, bis(difluoromethyl) carbonate, bis(trifluoromethyl) carbonate, bis(2-fluoroethyl) carbonate, bis(2,2-difluoroethyl) carbonate, bis(2,2,2-trifluoroethyl) carbonate, 2-fluoroethyl methyl carbonate, 2,2-difluoroethyl methyl carbonate, and 2,2,2-trifluoroethyl methyl carbonate, etc.

[0078] In some embodiments, examples of the cyclic carboxylic acid ester may include, but are not limited to, one or more of the following: one or more of γ-butyrolactone and γ-valerolactone. In some embodiments, part of the hydrogen atoms of the cyclic carboxylic acid ester may be substituted with fluorine.

[0079] In some embodiments, examples of the chain carboxylic acid esters may include, but are not limited to, one or more of the following: methyl acetate, ethyl acetate, propyl acetate, isopropyl acetate, butyl acetate, sec-butyl acetate, isobutyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, isopropyl propionate, methyl butyrate, ethyl butyrate, propyl butyrate, methyl isobutyrate, ethyl isobutyrate, methyl valerate, ethyl valerate, methyl pivalate and ethyl pivalate, etc. In some embodiments, some hydrogen atoms of the chain carboxylic acid ester may be substituted by fluorine. In some embodiments, examples of fluorine-substituted chain carboxylic acid esters may include, but are not limited to, methyl trifluoroacetate, ethyl trifluoroacetate, propyl trifluoroacetate, butyl trifluoroacetate and 2,2,2-trifluoroethyl trifluoroacetate, etc.

[0080] In some embodiments, examples of the cyclic ether may include, but are not limited to, one or more of the following: tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, 2-methyl1,3-dioxolane, 4-methyl1,3-dioxolane, 1,3-dioxane, 1,4-dioxane and dimethoxypropane.

[0081] In some embodiments, examples of the chain ether may include, but are not limited to, one or more of the following: dimethoxymethane, 1,1-dimethoxyethane, 1,2-dimethoxyethane, diethoxymethane, 1,1-diethoxyethane, 1,2-diethoxyethane, ethoxymethoxymethane, 1,1-ethoxymethoxyethane and 1,2-ethoxymethoxyethane, etc.

[0082] In some embodiments, examples of the phosphorus-containing organic solvent may include, but are not limited to, one or more of the following: trimethyl phosphate, triethyl phosphate, dimethyl ethyl phosphate, diethyl methyl phosphate, ethylidene methyl phosphate, ethylidene ethyl phosphate, triphenyl phosphate, trimethyl phosphite, triethyl phosphite, triphenyl phosphite, tris(2,2,2-trifluoroethyl) phosphate, and tris(2,2,3,3,3-pentafluoropropyl) phosphate, etc.

[0083] In some embodiments, examples of the sulfur-containing organic solvent may include, but are not limited to, one or more of the following: sulfolane, 2-methylsulfolane, 3-methylsulfolane, dimethyl sulfone, diethyl sulfone, ethyl methyl sulfone, methyl propyl sulfone, dimethyl sulfoxide, methyl methanesulfonate, ethyl methanesulfonate, methyl ethanesulfonate, ethyl ethanesulfonate, dimethyl sulfate, diethyl sulfate, and dibutyl sulfate. In some embodiments, some hydrogen atoms of the sulfur-containing organic solvent may be substituted by fluorine.

[0084] In some embodiments, the aromatic fluorine-containing solvent includes, but is not limited to, one or more of the following: fluorobenzene, difluorobenzene, trifluorobenzene, tetrafluorobenzene, pentafluorobenzene, hexafluorobenzene, and trifluoromethylbenzene.

[0085] In some embodiments, the solvent used in the electrolyte of the present application includes cyclic carbonates, chain carbonates, cyclic carboxylates, chain carboxylates, and combinations thereof. In some embodiments, the solvent used in the electrolyte of the present application includes an organic solvent selected from the group consisting of the following substances: ethylene carbonate, propylene carbonate, diethyl carbonate, ethyl propionate, propyl propionate, n-propyl acetate, ethyl acetate, and combinations thereof. In some embodiments, the solvent used in the electrolyte of the present application includes: ethylene carbonate, propylene carbonate, diethyl carbonate, ethyl propionate, propyl propionate, γ-butyrolactone, and combinations thereof.

[0086] In some embodiments, the electrolyte is not particularly limited, and any substance known as an electrolyte can be used arbitrarily. In the case of a lithium secondary battery, a lithium salt is generally used. Examples of electrolytes may include, but are not limited to, inorganic lithium salts such as LiPF6, LiBF4, LiClO4, LiAlF4, LiSbF6, LiWF7; lithium tungstates such as LiWOF5; lithium carboxylates such as HCO2Li, CH3CO2Li, CH2FCO2Li, CHF2CO2Li, CF3CO2Li, CF3CH2CO2Li, CF3CF2CO2Li, CF3CF2CF2CO2Li, CF3CF2CF2CF2CO2Li; lithium salts such as FSO3Li, CH3SO3Li 、CH2FSO3Li、CHF2SO3Li、CF3SO3Li、CF3CF2SO3Li、CF3CF2CF2SO3Li、CF3CF2CF2CF2SO3Li and other lithium sulfonate salts; LiN(FCO)2、LiN(FCO)(FSO2)、LiN(FSO2)2、LiN(FSO2)(CF3SO2)、LiN(CF3SO2)2、LiN(C2F5SO2)2、Cyclic 1,2-perfluoroethane bissulfonyl imide lithium、Cyclic 1,3-perfluoropropane bissulfonyl imide Lithium, LiN(CF3SO2)(C4F9SO2) and other imide lithium salts; LiC(FSO2)3, LiC(CF3SO2)3, LiC(C2F5SO2)3 and other methylated lithium salts; (malonate) borate lithium salts such as bis(malonate) borate and difluoro(malonate) borate lithium salts; (malonate) phosphate lithium salts such as tris(malonate) phosphate, difluorobis(malonate) phosphate, tetrafluoro(malonate) phosphate; and LiPF4(CF3)2, LiPF4(C2F5)2, Fluorine-containing organic lithium salts such as LiPF4(CF3SO2)2, LiPF4(C2F5SO2)2, LiBF3CF3, LiBF3C2F5, LiBF3C3F7, LiBF2(CF3)2, LiBF2(C2F5)2, LiBF2(CF3SO2)2, LiBF2(C2F5SO2)2; lithium oxalate borate salts such as lithium difluorooxalatoborate and lithium bis(oxalato)borate; lithium oxalate phosphate salts such as lithium tetrafluorooxalato phosphate, lithium difluorobis(oxalato) phosphate and lithium tris(oxalato) phosphate, etc.

[0087] In some embodiments, the electrolyte is selected from LiPF6, LiSbF6, FSO3Li, CF3SO3Li, LiN(FSO2)2, LiN(FSO2)(CF3SO2), LiN(CF3SO2)2, LiN(C2F5SO2)2, cyclic 1,2-perfluoroethane bissulfonyl imide lithium, cyclic 1,3-perfluoropropane bissulfonyl imide lithium, LiC(FSO2)3, LiC(CF3SO2)3, LiC(C2F5SO2)3, LiBF3CF3, LiBF3C2F5, LiPF3(CF3)3, LiPF3(C2F5)3, lithium difluorooxalatoborate, lithium bis(oxalate)borate or lithium difluorobis(oxalato)phosphate, which helps to improve the output power characteristics, high-rate charge and discharge characteristics, high-temperature storage characteristics and cycle characteristics of the electrochemical device.

[0088] The content of the electrolyte is not particularly limited as long as it does not impair the effect of the present application. In some embodiments, the total molar concentration of lithium in the electrolyte is greater than 0.3 mol / L, greater than 0.4 mol / L or greater than 0.5 mol / L. In some embodiments, the total molar concentration of lithium in the electrolyte is less than 3 mol / L, less than 2.5 mol / L or less than 2.0 mol / L. In some embodiments, the total molar concentration of lithium in the electrolyte is within the range formed by any two of the above values. When the electrolyte concentration is within the above range, there will not be too little lithium as a charged particle, and the viscosity can be kept within an appropriate range, thereby easily ensuring good conductivity.

[0089] When more than two electrolytes are used, the electrolyte includes at least one salt selected from the group consisting of monofluorophosphate, borate, oxalate and fluorosulfonate. In some embodiments, the electrolyte includes a salt selected from the group consisting of monofluorophosphate, oxalate and fluorosulfonate. In some embodiments, the electrolyte includes a lithium salt. In some embodiments, based on the weight of the electrolyte, the content of the salt selected from the group consisting of monofluorophosphate, borate, oxalate and fluorosulfonate is greater than 0.01% or greater than 0.1%. In some embodiments, based on the weight of the electrolyte, the content of the salt selected from the group consisting of monofluorophosphate, borate, oxalate and fluorosulfonate is less than 20% or less than 10%. In some embodiments, the content of the salt selected from the group consisting of monofluorophosphate, borate, oxalate and fluorosulfonate is within the range of any two of the above values.

[0090] In some embodiments, the electrolyte comprises one or more substances selected from the group consisting of monofluorophosphate, borate, oxalate and fluorosulfonate and one or more salts other than this. As salts other than this, the lithium salts exemplified above can be cited, and in some embodiments, they are LiPF6, LiN(FSO2)(CF3SO2), LiN(CF3SO2)2, LiN(C2F5SO2)2, cyclic 1,2-perfluoroethane bissulfonyl imide lithium, cyclic 1,3-perfluoropropane bissulfonyl imide lithium, LiC(FSO2)3, LiC(CF3SO2)3, LiC(C2F5SO2)3, LiBF3CF3, LiBF3C2F5, LiPF3(CF3)3, LiPF3(C2F5)3. In some embodiments, the salt other than this is LiPF6.

[0091] In some embodiments, the content of the other salt is greater than 0.01% or greater than 0.1% based on the weight of the electrolyte. In some embodiments, the content of the other salt is less than 20%, less than 15%, or less than 10% based on the weight of the electrolyte. In some embodiments, the content of the other salt is within the range formed by any two of the above values. The other salt having the above content helps to balance the conductivity and viscosity of the electrolyte.

[0092] III. Negative electrode

[0093] The negative electrode includes a negative electrode current collector and a positive electrode active material layer disposed on one or both surfaces of the negative electrode current collector, and the negative electrode active material layer contains a negative electrode active material. The negative electrode active material layer can be one or more layers, and each layer of the multi-layer negative electrode active material can contain the same or different negative electrode active materials. The negative electrode active material is any substance that can reversibly embed and deintercalate metal ions such as lithium ions. 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.

[0094] As a current collector for holding the negative electrode active material, any known current collector can be used. Examples of negative electrode current collectors include, but are not limited to, metal materials such as aluminum, copper, nickel, stainless steel, and nickel-plated steel. In some embodiments, the negative electrode current collector is copper.

[0095] In the case where the negative electrode current collector is a metal material, the negative electrode current collector may include, but is not limited to, metal foil, metal cylinder, metal strip, metal plate, metal film, metal mesh, stamped metal, foamed metal, etc. In some embodiments, the negative electrode current collector is a metal film. In some embodiments, the negative electrode current collector is a copper foil. In some embodiments, the negative electrode current collector is a rolled copper foil based on a rolling method or an electrolytic copper foil based on an electrolytic method.

[0096] In some embodiments, the thickness of the negative electrode current collector is greater than 1 μm or greater than 5 μm. In some embodiments, the thickness of the negative electrode current collector is less than 100 μm or less than 50 μm. In some embodiments, the thickness of the negative electrode current collector is within the range of any two of the above values.

[0097] There is no particular limitation on the negative electrode active material, as long as it can reversibly absorb and release lithium ions. Examples of negative electrode active materials may include, but are not limited to, carbon materials such as natural graphite and artificial graphite; metals such as silicon (Si) and tin (Sn); or oxides of metal elements such as Si and Sn. Negative electrode active materials may be used alone or in combination.

[0098] The negative electrode active material layer may further include 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 electrolyte or electrode manufacturing. 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.

[0099] The negative electrode can be prepared by applying a negative electrode mixture slurry containing a negative electrode active material, a resin binder, etc. on a negative electrode current collector, drying it, and then rolling it to form a negative electrode active material layer on both sides of the negative electrode current collector, thereby obtaining a negative electrode.

[0100] IV. Isolation membrane

[0101] In order to prevent short circuit, a separator is usually provided between the positive electrode and the negative electrode. In this case, the electrolyte of the present application is usually used by infiltrating the separator.

[0102] 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 isolation membrane may be a resin, glass fiber, inorganic substance, etc. formed of 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 may include, but are not limited to, polyolefins, aromatic polyamides, polytetrafluoroethylene, polyether sulfone, etc. In some embodiments, the polyolefin is polyethylene or polypropylene. In some embodiments, the polyolefin is polypropylene. The materials of the above isolation membranes may be used alone or in any combination.

[0103] The isolation film may also be a material formed by laminating the above materials, and examples thereof include, but are not limited to, a three-layer isolation film formed by laminating polypropylene, polyethylene, and polypropylene in this order.

[0104] Examples of inorganic materials may include, but are not limited to, oxides such as aluminum oxide and silicon dioxide, nitrides such as aluminum nitride and silicon nitride, and sulfates (e.g., barium sulfate, calcium sulfate, etc.) The form of the inorganic material may include, but is not limited to, granular or fibrous.

[0105] The separator may be in the form of a thin film, examples of which include, but are not limited to, nonwoven fabrics, woven fabrics, microporous films, etc. In the thin film form, the separator has a pore size of 0.01 μm to 1 μm and a thickness of 5 μm to 50 μm. In addition to the above-mentioned independent thin film separators, the following separators may 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 a fluororesin as an adhesive to form a porous layer on both sides of the positive electrode with aluminum oxide particles having a particle size of 90% less than 1 μm.

[0106] The thickness of the isolation membrane is arbitrary. In some embodiments, the thickness of the isolation membrane is greater than 1 μm, greater than 5 μm, or greater than 8 μm. In some embodiments, the thickness of the isolation membrane is less than 50 μm, less than 40 μm, or less than 30 μm. In some embodiments, the thickness of the isolation membrane is within the range formed by any two of the above values. When the thickness of the isolation membrane is within the above range, the insulation and mechanical strength can be ensured, and the rate characteristics and energy density of the electrochemical device can be ensured.

[0107] When a porous material such as a porous sheet or non-woven fabric is used as an isolation membrane, the porosity of the isolation membrane is arbitrary. In some embodiments, the porosity of the isolation membrane is greater than 10%, greater than 15%, or greater than 20%. In some embodiments, the porosity of the isolation membrane is less than 60%, less than 50%, or less than 45%. In some embodiments, the porosity of the isolation membrane is within the range formed by any two of the above values. When the porosity of the isolation membrane is within the above range, insulation and mechanical strength can be ensured, and membrane resistance can be suppressed, so that the electrochemical device has good safety characteristics.

[0108] The average pore size of the isolation membrane is also arbitrary. In some embodiments, the average pore size of the isolation membrane is less than 0.5 μm or less than 0.2 μm. In some embodiments, the average pore size of the isolation membrane is greater than 0.05 μm. In some embodiments, the average pore size of the isolation membrane is within the range formed by any two of the above values. If the average pore size of the isolation membrane exceeds the above range, short circuit is likely to occur. When the average pore size of the isolation membrane is within the above range, the electrochemical device has good safety characteristics.

[0109] V. Electrochemical Device Components

[0110] The electrochemical device assembly includes an electrode group, a current collecting structure, an outer casing and a protective element.

[0111] The electrode group may be any one of a stacked structure formed by stacking the positive electrode and the negative electrode with the separator interposed therebetween, and a structure formed by spirally winding the positive electrode and the negative electrode with the separator interposed therebetween. In some embodiments, the weight of the electrode group accounts for a proportion of the internal volume of the battery (electrode group occupancy) greater than 40% or greater than 50%. In some embodiments, the electrode group occupancy is less than 90% or less than 80%. In some embodiments, the electrode group occupancy is within the range formed by any two of the above values. When the electrode group occupancy is within the above range, the capacity of the electrochemical device can be ensured, and at the same time, the reduction in characteristics such as repeated charge and discharge performance and high temperature storage associated with the increase in internal pressure can be suppressed.

[0112] There is no particular limitation on the current collection structure. In some embodiments, the current collection structure is a structure that reduces the resistance of the wiring portion and the joint portion. When the electrode group is the above-mentioned stacked structure, it is suitable to use a structure formed by bundling the metal core portions of each electrode layer and welding them to the terminal. When the electrode area of ​​a sheet increases, the internal resistance increases, so it is also suitable to set more than two terminals in the electrode to reduce the resistance. When the electrode group is the above-mentioned winding structure, the internal resistance can be reduced by setting more than two lead structures at the positive electrode and the negative electrode respectively, and bundling them at the terminal.

[0113] The material of the outer casing is not particularly limited, as long as it is a material that is stable to the electrolyte used. The outer casing can be made of, but not limited to, nickel-plated steel, stainless steel, aluminum or aluminum alloy, magnesium alloy and other metals, or a laminated film of resin and aluminum foil. In some embodiments, the outer casing is a metal or laminated film of aluminum or aluminum alloy.

[0114] The metal outer casing includes, but is not limited to, a packaged closed structure formed by welding metals to each other by laser welding, resistance welding, and ultrasonic welding; or a riveted structure formed by using the above-mentioned metals through a resin gasket. The outer casing using the above-mentioned laminated film includes, but is not limited to, a packaged closed structure formed by thermally bonding resin layers to each other. In order to improve the sealing performance, a resin different from the resin used in the laminated film can also be sandwiched between the above-mentioned resin layers. When the resin layer is thermally bonded to form a closed structure through the collector terminal, due to the bonding between the metal and the resin, a resin with a polar group or a modified resin into which a polar group is introduced can be used as the sandwiched resin. In addition, the shape of the outer casing is also arbitrary, for example, it can be any of cylindrical, square, laminated, button-shaped, large, etc.

[0115] The protection element may be a positive temperature coefficient (PTC) whose resistance increases when abnormal heat is released or excessive current flows, a temperature fuse, a thermistor, or a valve (current cutoff valve) that cuts off the current flowing in the circuit by causing the internal pressure or internal temperature of the battery to rise sharply when abnormal heat is released. The above protection element may be selected to be an element that does not work in normal use of high current, or may be designed in a form that does not cause abnormal heat release or thermal runaway even if the protection element is not present.

[0116] The electrochemical device of the present application includes any device that generates an electrochemical reaction, and specific examples thereof include a lithium metal secondary battery or a lithium ion secondary battery.

[0117] The present application further provides an electronic device, which includes the electrochemical device according to the present application.

[0118] The use of the electrochemical device 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 electrochemical device 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 televisions, 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.

[0119] The preparation of lithium-ion batteries is described below by taking lithium-ion batteries as an example and combining specific embodiments. Those skilled in the art will understand that the preparation method described in this application is only an example, and any other suitable preparation method is within the scope of this application.

[0120] Example

[0121] 1. Preparation of lithium-ion batteries

[0122] 1. Preparation of negative electrode

[0123] Artificial graphite, styrene-butadiene rubber and sodium carboxymethyl cellulose were mixed with deionized water in a weight ratio of 96%:2%:2%, and stirred evenly to obtain negative electrode slurry. The negative electrode slurry was coated on a 9μm copper foil, dried, cold pressed, cut into pieces, and welded to the pole ears to obtain the negative electrode.

[0124] 2. Preparation of positive electrode

[0125] The positive electrode active material having the first element is prepared by the following method:

[0126] (1) Using lithium cobalt oxide as the positive electrode active material: Weigh a certain amount of Co3O4 and LiOH powder, mix and grind them thoroughly in an agate mortar, and then calcine them at 900°C for 10 hours. Add a specific amount of sulfate or nitrate containing the first element to the calcined mixture, use ethanol as a solvent, ball mill for 10 hours, and calcine at 800°C for 10 hours to obtain a positive electrode active material containing the first element.

[0127] (2) Using lithium nickel cobalt manganese oxide as the positive electrode active material: Anhydrous ethanol and deionized water are mixed in a volume ratio of 7:3, and 0.05 mol nickel nitrate hexahydrate, 0.02 mol cobalt nitrate hexahydrate, 0.03 mol manganese acetate tetrahydrate and a specific amount of sulfate or nitrate with the first element are added thereto, and ultrasonic stirring is performed to completely dissolve them. The above-mentioned mixed solution is transferred to a 500 mL high-pressure reactor, 0.6 mol ammonium bicarbonate is added thereto, and the mixture is heated to 120°C for 8 hours, cooled, centrifuged, and the precipitate is collected, washed with anhydrous ethanol for 3 times, and vacuum dried at 60°C for 8 hours to obtain a nickel cobalt manganese hydroxide precursor. Subsequently, lithium carbonate, tungsten oxide with the first element and the obtained cobalt manganese hydroxide precursor are fully mixed and ground, calcined at 900°C for 20 hours in a tube furnace, and ground again after cooling to obtain a positive electrode active material with the first element.

[0128] The positive electrode active material having the first element, carbon nanotubes and polyvinylidene fluoride are mixed with N-methylpyrrolidone (NMP) in a weight ratio of 97:1:2, and stirred evenly to obtain a positive electrode slurry. The positive electrode slurry is coated on a 12 μm aluminum foil, dried, cold pressed, cut into pieces, and welded to the tabs to obtain a positive electrode.

[0129] 3. Preparation of electrolyte

[0130] In a dry argon environment, EC, PC and DEC (weight ratio 1:1:1) were mixed, and LiPF6 was added and mixed evenly to form a basic electrolyte, wherein the concentration of LiPF6 was 12.5%. Different amounts of additives were added to the basic electrolyte to obtain electrolytes of different embodiments and comparative examples.

[0131] The abbreviations and names of the components in the electrolyte are shown in the following table:

[0132]

[0133]

[0134] 4. Preparation of isolation membrane

[0135] A polyethylene porous polymer film is used as the isolation membrane.

[0136] 5. Preparation of lithium-ion batteries

[0137] The obtained positive electrode, separator and negative electrode are wound in order and placed in an outer packaging foil, leaving a liquid injection port. The electrolyte is poured from the liquid injection port, packaged, and then subjected to formation, capacity and other processes to obtain a lithium-ion battery.

[0138] 2. Test Method

[0139] 1. Test method for voltage drop of lithium-ion battery

[0140] At 25°C, the lithium-ion battery is charged to 4.7V at 1C constant current, then charged to 0.05C at constant voltage, and then discharged to 3.2V at 1C constant current, left to stand for 5 minutes, and the voltage before storage is tested. The lithium-ion battery is then stored at 85°C for 24 hours, and the voltage after storage is tested. The voltage drop of the lithium-ion battery is calculated according to the following formula:

[0141] Voltage drop = voltage before storage - voltage after storage.

[0142] 2. Test method for high temperature short circuit deformation rate of lithium-ion batteries

[0143] At 25°C, the lithium-ion battery was left to stand for 30 minutes, then charged to 4.7V at a constant current rate of 0.5C, and then charged to 0.05C at a constant voltage rate of 4.7V, and left to stand for 60 minutes, and the thickness T1 of the lithium-ion battery was measured. The lithium-ion battery was then short-circuited for 10 seconds at 100mΩ, and then the thickness T2 of the lithium-ion battery was measured. The high-temperature short-circuit deformation rate of the lithium-ion battery was calculated by the following formula:

[0144] Short circuit deformation rate = [(T2-T1) / T1] x 100%.

[0145] 3. Test method for thickness expansion rate of lithium-ion battery due to thermal abuse

[0146] At 25°C, let the lithium-ion battery stand for 30 minutes, measure the thickness H1 of the lithium-ion battery, and then start heating at a heating rate of 5°C / min. When the temperature rises to 130°C, keep it for 30 minutes, and measure the thickness H2 of the lithium-ion battery. The thermal abuse thickness expansion rate of the lithium-ion battery is calculated by the following formula:

[0147] Thickness expansion ratio = [(H2-H1) / H1] x 100%.

[0148] 3. Test Results

[0149] Table 1 shows the effect of electrolyte on the voltage drop and safety of lithium-ion batteries under high temperature and high pressure, wherein the positive electrode active material of each embodiment and comparative example is lithium cobalt oxide containing 0.1% lanthanum.

[0150] Table 1

[0151]

[0152] As shown in Comparative Examples 1-1 to 1-3, although the lithium-ion battery uses lithium cobalt oxide containing 0.1% lanthanum as the positive electrode active material, the electrolyte does not contain propionate (PP) and a cyanide compound (ADN) or contains only one of them, and the lithium-ion battery has a high voltage drop, a high short-circuit deformation rate, and a high thickness expansion rate, and its performance is poor and cannot meet the use requirements.

[0153] As shown in Examples 1-1 to 1-21, when lithium cobalt oxide containing 0.1% lanthanum is used as the positive electrode active material and an electrolyte containing propionate (PP) and a cyanide compound (ADN, SN, EDN, HTCN and / or TCEP) is used, the voltage drop, short-circuit deformation rate and thickness expansion rate of the lithium-ion battery are significantly reduced.

[0154] Tables 2 and 3 show the effects of the type and content of doping elements in the positive electrode active material on the voltage drop and safety of lithium-ion batteries under high temperature and high pressure, wherein lithium cobalt oxide containing or not containing doping elements is used as the positive electrode active material in each embodiment and comparative example listed in Table 2, and lithium nickel cobalt manganese oxide containing or not containing doping elements is used as the positive electrode active material in each embodiment and comparative example listed in Table 3, and the electrolyte used in each embodiment and comparative example listed in Tables 2 and 3 is the same as that in Example 1-1.

[0155] Table 2

[0156]

[0157]

[0158] Table 3

[0159]

[0160]

[0161] The results show that using lithium nickel cobalt manganese oxide with or without doping elements and using lithium cobalt oxide with or without doping elements as the positive electrode active material can achieve basically the same effect.

[0162] As shown in Comparative Examples 2-1 to 2-5 in Table 2 and Comparative Examples 3-1 to 3-5 in Table 3, although the electrolyte contains propionate (PP) and a compound having a cyano group (ADN), the positive electrode active material does not contain the first element (at least one of lanthanum, yttrium, cerium or tungsten), and the lithium-ion battery has a high voltage drop, a high short-circuit deformation rate and a high thickness expansion rate, and its safety is poor and cannot meet the use requirements.

[0163] As shown in Examples 1-1 and 2-1 to 2-6 in Table 2 and Examples 3-1 to 3-6 in Table 3, when an electrolyte containing propionate (PP) and a compound having a cyano group (ADN) is used while a positive electrode active material containing a first element is used, the voltage drop, short-circuit deformation rate and thickness expansion rate of the lithium-ion battery are significantly reduced.

[0164] As shown in Examples 2-7 to 2-25 in Table 2 and Examples 3-7 to 3-25 in Table 3, on the basis of the electrolyte comprising propionate (PP) and a compound having a cyano group (ADN), when the positive electrode active material simultaneously comprises the first element and the second element, the voltage drop, short-circuit deformation rate and thickness expansion rate of the lithium-ion battery can be further reduced.

[0165] Table 4 shows the relationship between the content a% of the first element in the positive electrode active material and the sum of the contents b% of propionate and compounds having a cyano group in the electrolyte on the voltage drop and safety of lithium-ion batteries under high temperature and high pressure, wherein lithium nickel cobalt manganese oxide containing doped elements is used as the positive electrode active material in each embodiment and comparative example.

[0166] Table 4

[0167]

[0168] The results show that when the content a% of the first element in the positive electrode active material and the sum of the contents b% of propionate and cyanide compounds in the electrolyte satisfy 0.1≤a×b≤20, the voltage drop, short-circuit deformation rate and thickness expansion rate of the lithium-ion battery can be further reduced.

[0169] Table 5 shows the influence of the content relationship of electrolyte components on the voltage drop and safety of lithium-ion batteries under high temperature and high pressure, wherein lithium nickel cobalt manganese oxide containing 0.1% tungsten is used as the positive electrode active material in each embodiment and comparative example.

[0170] Table 5

[0171]

[0172] The results show that when the content of propionate ester in the electrolyte is x% and the content of the compound having a cyano group is y% which satisfies 1≤x / y≤20, the voltage drop, short-circuit deformation rate and thickness expansion rate of the lithium-ion battery can be further reduced.

[0173] In addition, when the content of propionate in the electrolyte is in the range of 5% to 50% and / or the content of the compound having a cyano group is in the range of 0.1% to 15%, the voltage drop, short circuit deformation rate and thickness expansion rate of the lithium ion battery can be further reduced.

[0174] Table 6 shows the influence of the weight fraction M% of the positive electrode active material in the positive electrode active material layer and its content x% of propionate in the electrolyte on the voltage drop and safety of the lithium-ion battery under high temperature and high pressure, wherein lithium nickel cobalt manganese oxide containing 0.1% tungsten is used as the positive electrode active material in each embodiment and comparative example, and the electrolyte contains different contents of PP and 3% ADN. When M is 97, the positive electrode active material, carbon nanotubes and polyvinylidene fluoride are mixed in a weight ratio of 97:1:2; when M is 95, the positive electrode active material, carbon nanotubes and polyvinylidene fluoride are mixed in a weight ratio of 95:2:3; when M is 96, the positive electrode active material, carbon nanotubes and polyvinylidene fluoride are mixed in a weight ratio of 96:2:2; when M is 98, the positive electrode active material, carbon nanotubes and polyvinylidene fluoride are mixed in a weight ratio of 98:1:1; when M is 99, the positive electrode active material, carbon nanotubes and polyvinylidene fluoride are mixed in a weight ratio of 99:0.5:0.5.

[0175] Table 6

[0176]

[0177] The results show that when the weight fraction M% of the positive electrode active material in the positive electrode active material layer and the content x% of the propionate in the electrolyte satisfy 1.5≤M / x≤18, the voltage drop, short-circuit deformation rate and thickness expansion rate of the lithium-ion battery can be further reduced.

[0178] Table 7 shows the effects of the particle size Dμm of the positive electrode active material in the positive electrode active material layer and the content y% of the compound having a cyano group in the electrolyte on the safety performance and voltage drop of the lithium-ion battery under high temperature and high pressure, wherein lithium nickel cobalt manganese oxide containing 0.1% tungsten is used as the positive electrode active material in each embodiment and comparative example, and the electrolyte contains 30% PP and different contents of compounds having a cyano group.

[0179] Table 7

[0180]

[0181]

[0182] The results show that when the particle size Dμm of the positive electrode active material in the positive electrode active material layer and the content y% of the cyanide compound in the electrolyte satisfy 1≤D / y≤20, the voltage drop, short circuit deformation rate and thickness expansion rate of the lithium ion battery can be further reduced.

[0183] Table 8 shows the effect of further adding additional compounds to the electrolyte on the safety performance and voltage drop of lithium-ion batteries under high temperature and high pressure, wherein lithium nickel cobalt manganese oxide containing 0.1% tungsten is used as the positive electrode active material in each embodiment and comparative example. Except for the electrolyte components and their contents listed in Table 8, the composition of the electrolyte in each embodiment and comparative example is the same as that in Example 3-1.

[0184] Table 8

[0185]

[0186] The results show that when at least one of lithium difluorophosphate, a compound of formula I, fluoroethylene carbonate, 1,3-propane sultone, vinyl sulfate, vinylene carbonate or 1-propylphosphoric acid cyclic anhydride is further added to the electrolyte on the basis of the electrolyte containing propionate and a compound having a cyano group, the voltage drop, short-circuit deformation rate and thickness expansion rate of the lithium-ion battery can be further reduced.

[0187] References to "embodiments", "partial embodiments", "one embodiment", "another example", "example", "specific example" or "partial example" throughout the specification mean that at least one embodiment or example in the present application includes the specific features, structures, materials or characteristics described in the embodiment or example. Therefore, descriptions appearing in various places throughout the specification, such as: "in some embodiments", "in an embodiment", "in one embodiment", "in another example", "in an example", "in a specific example" or "example", do not necessarily refer to the same embodiment or example in the present application. In addition, the specific features, structures, materials or characteristics herein may be combined in one or more embodiments or examples in any suitable manner.

[0188] Although illustrative embodiments have been demonstrated and described, those skilled in the art should understand that the above embodiments should not be construed as limitations on the present application, and that changes, substitutions and modifications may be made to the embodiments without departing from the spirit, principles and scope of the present application.

Claims

1. An electrochemical device comprising: A positive electrode, a negative electrode and an electrolyte, wherein the positive electrode comprises a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector, wherein the positive electrode active material layer comprises a positive electrode active material, wherein: The positive electrode active material has a first element, the first element includes at least one of lanthanum, yttrium, cerium or tungsten, and the content of the first element is a%, based on the weight of the positive electrode active material, and a≤1; The positive electrode active material further comprises a second element, wherein the second element comprises at least one of aluminum, magnesium, titanium or zirconium; The electrolyte includes propionate and a compound having a cyano group. 2 . The electrochemical device according to claim 1 , wherein the sum of the contents of the propionate ester and the compound having a cyano group is b % based on the weight of the electrolyte, and a and b satisfy: 0.1≤a×b≤20. The electrochemical device according to claim 2 , wherein 0.2≤a×b≤15. The electrochemical device according to claim 1 , wherein 0.01≤a≤1.

5. The electrochemical device according to claim 1, wherein the sum of the contents of the propionate ester and the compound having a cyano group is b% based on the weight of the electrolyte, and the value of b ranges from 5 to 65. 6 . The electrochemical device according to claim 1 , wherein the content of the propionate is x%, the content of the compound having a cyano group is y%, based on the weight of the electrolyte, and x and y satisfy: 1≤x / y≤20.

7. The electrochemical device according to claim 1, wherein the content of the propionate is x% based on the weight of the electrolyte, and the value of x ranges from 5 to 50.

8. The electrochemical device according to claim 1, wherein the content of the compound having a cyano group is y% based on the weight of the electrolyte, and the value of y ranges from 0.1 to 15.

9. The electrochemical device according to claim 1, wherein: The weight fraction of the positive electrode active material in the positive electrode active material layer is M%, and the value range of M is 95 to 99; The content of the propionate is x% based on the weight of the electrolyte; and M and x satisfy: 1.5≤M / x≤18.

10. The electrochemical device according to claim 1, wherein: The average particle size of the positive electrode active material is D μm, and the value of D ranges from 5 to 30; The content of the compound having a cyano group is y% based on the weight of the electrolyte; and D and y satisfy: 1≤D / y≤20.

11. The electrochemical device according to claim 1, wherein the propionate ester comprises at least one of ethyl propionate or propyl propionate.

12. The electrochemical device according to claim 1, wherein the compound having a cyano group comprises succinonitrile, glutaronitrile, adiponitrile, 1,5-dicyanopentane, 1,6-dicyanohexane, tetramethylsuccinonitrile, 2-methylglutaronitrile, 2,4-dimethylglutaronitrile, 2,2,4,4-tetramethylglutaronitrile, 1,4-dicyanopentane, 1,2-dicyanobenzene, 1,3-dicyanobenzene, 1,4-dicyanobenzene, ethylene glycol bis(propionitrile) ether, 3,5-Dioxa-heptanenitrile, 1,4-bis(cyanoethoxy)butane, diethylene glycol di(2-cyanoethyl)ether, triethylene glycol di(2-cyanoethyl)ether, tetraethylene glycol di(2-cyanoethyl)ether, 1,3-bis(2-cyanoethoxy)propane, 1,4-bis(2-cyanoethoxy)butane, 1,5-bis(2-cyanoethoxy)pentane, ethylene glycol di(4-cyanobutyl)ether, 1,4-dicyano-2-butene, 1,4 -Dicyano-2-methyl-2-butene, 1,4-dicyano-2-ethyl-2-butene, 1,4-dicyano-2,3-dimethyl-2-butene, 1,4-dicyano-2,3-diethyl-2-butene, 1,6-dicyano-3-hexene, 1,6-dicyano-2-methyl-3-hexene, 1,3,5-pentanetricarbonitrile, 1,2,3-propanetricarbonitrile, 1,3,6-hexatriconitrile, 1,2,6-hexatriconitrile, 1,2 , at least one of 3-tris(2-cyanoethoxy)propane, 1,2,4-tris(2-cyanoethoxy)butane, 1,1,1-tris(cyanoethoxymethylene)ethane, 1,1,1-tris(cyanoethoxymethylene)propane, 3-methyl-1,3,5-tris(cyanoethoxy)pentane, 1,2,7-tris(cyanoethoxy)heptane, 1,2,6-tris(cyanoethoxy)hexane or 1,2,5-tris(cyanoethoxy)pentane.

13. The electrochemical device according to claim 1, wherein the electrolyte further comprises at least one of lithium difluorophosphate or a compound of formula I, Among them, M + represents a quaternary ammonium cation or an imidazolium cation, and R represents a C1-5 alkyl group.

14. The electrochemical device according to claim 1, wherein the electrolyte further comprises at least one of fluoroethylene carbonate, 1,3-propane sultone, vinyl sulfate, vinylene carbonate, or 1-propylphosphoric acid cyclic anhydride. 15 . The electrochemical device according to claim 1 , wherein the positive electrode active material comprises at least one of lithium cobalt oxide containing at least one of lanthanum, yttrium, or cerium, or a ternary positive electrode active material containing tungsten.

16. An electronic device comprising the electrochemical device according to any one of claims 1 to 15.

Images