Electrochemical device and electronic device

Abstract

An electrochemical device and an electronic device. The electrochemical device comprises a positive electrode. The positive electrode comprises a current collector and positive electrode active material layers. The positive electrode active material layers are located on at least one surface of the current collector and comprise a first positive electrode active material layer and a second positive electrode active material layer. The first positive electrode active material layer is located between the current collector and the second positive electrode active material layer, wherein the adhesion between the current collector and the first positive electrode active material layer is F1 N / m, the cohesion of the second positive electrode active material layer is F2 N / m, and a positive electrode satisfies: f1 / f2 = 6. The positive electrode is applied to the electrochemical device, so that the safety performance of the electrochemical device can be enhanced, the direct-current internal resistance of the electrochemical device at high voltage and high temperature can be fully inhibited, and the cycle performance of the electrochemical device is improved.

Description

TECHNICAL FIELD

[0001] This application relates to the field of energy storage, and specifically, to anelectrochemical device and an electronic device.BACKGROUND

[0002] In recent years, people have increasingly high requirements for energydensity of electrochemical devices. However, while in pursuit of high energy density,the associated safety issues have become more prominent. For example, when theelectrochemical device is subjected to misuse such as compression, nail penetration, orovercharging, short circuit easily occurs inside the electrochemical device, and a largeamount of heat is accumulated, causing thermal runaway, and even the problems suchas fire and explosion.

[0003] The safety of electrochemical devices during use can be improved to someextent by changing the configuration of the electrode plate in electrochemical devices.For example, in the prior art, providing a safety coating on the surface of the electrodeplate can increase the contact internal resistance, reducing the risk of thermal runaway.However, the provision of the safety coating (for example, a ceramic layer) reduces theenergy density of the electrochemical device and deteriorates other electrochemicalperformance. Therefore, how the energy density and electrochemical performance ofelectrochemical devices are enhanced while guaranteeing the safety performance ofelectrochemical devices has gained an increasing attention.SUMMARY

[0004] This application is intended to solve the problems in the prior art to someextent by improving configuration of a positive electrode of an electrochemical deviceand matching between the positive electrode and an electrolyte.

[0005] According to an aspect of this application, this application provides anelectrochemical device, including a positive electrode, a negative electrode, and anelectrolyte, where the positive electrode includes: a current collector; and a positiveelectrode active material layer, where the positive electrode active material layer islocated on at least one surface of the current collector and includes a first positiveelectrode active material layer and a second positive electrode active material layer, thefirst positive electrode active material layer being located between the current collectorand the second positive electrode active material layer, where an adhesion forcebetween the current collector and the first positive electrode active material layer is F1N / m, and a cohesion force of the second positive electrode active material layer is F2N / m, the positive electrode satisfying F1 / F2 ≥ 6.

[0006] According to an embodiment of this application, in the electrochemicaldevice, F1 ≥ 200.

[0007] According to an embodiment of this application, in the electrochemicaldevice, the first positive electrode active material layer includes a first binder, and thefirst binder satisfies at least one of the following conditions: (1) the first binder is watersoluble; (2) the first binder has unsaturated acid functional groups; (3) the first binderincludes polyacrylate containing polar functional groups; or (4) based on a mass of thefirst positive electrode active material layer, a mass fraction of the first binder is b%,where 2 ≤ b ≤ 20.

[0008] According to an embodiment of this application, in the electrochemicaldevice, the first positive electrode active material layer includes a first binder, and basedon a mass of the first positive electrode active material layer, a mass fraction of the firstbinder is b%, where F1 and b satisfy F1 ≥ 200, 2 ≤ b ≤ 20, and F1 / b ≥ 10.

[0009] According to an embodiment of this application, in the electrochemicaldevice, the second positive electrode active material layer includes a second binder, andthe second binder satisfies at least one of the following conditions: (1) the second binderis water-insoluble; (2) the second binder includes a fluoropolymer; (3) the secondbinder includes polyvinylidene fluoride containing an α crystalline form; or (4) basedon a mass of the second positive electrode active material layer, a mass fraction of thesecond binder is a%, where 0.5 ≤ a ≤ 5.

[0010] According to an embodiment of this application, in the electrochemicaldevice, the second positive electrode active material layer includes a second binder, andbased on a mass of the second positive electrode active material layer, a mass fractionof the second binder is a%, where F2 and a satisfy 5 ≤ F2 ≤ 60, 0.5 ≤ a ≤ 5, and F2 / a ≥1.

[0011] According to an embodiment of this application, in the electrochemicaldevice, the first positive electrode active material layer includes a water-soluble binder,and the second positive electrode active material layer includes a water-insoluble binder.

[0012] According to an embodiment of this application, in the electrochemicaldevice, the first positive electrode active material layer includes a first binder, and basedon a mass of the first positive electrode active material layer, a mass fraction of the firstbinder is b%; and the second positive electrode active material layer includes a secondbinder, and based on a mass of the second positive electrode active material layer, amass fraction of the second binder is a%; where a and b satisfy 2.5 ≤ a + b ≤ 25 and 1≤ b / a ≤ 40.

[0013] According to an embodiment of this application, in the electrochemicaldevice, thickness of the first positive electrode active material layer is H1 μm, andthickness of the positive electrode active material layer is H μm, the positive electrodesatisfying H1 / H ≤ 0.1.

[0014] According to an embodiment of this application, in the electrochemicaldevice, 0.1 ≤ H1 ≤ 5.

[0015] According to an embodiment of this application, in the electrochemicaldevice, the electrolyte includes a compound containing a cyano group.

[0016] According to an embodiment of this application, in the electrochemicaldevice, based on a mass of the electrolyte, a percentage of the compound containing acyano group is x%, where 0.1 ≤ x ≤ 15.

[0017] According to an embodiment of this application, in the electrochemicaldevice, F1 ≥ 200 and F1 / x ≥ 13.33.

[0018] According to an embodiment of this application, in the electrochemicaldevice, the compound containing a cyano group includes at least one of 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-pimelonitrile, 1,4-bis(cyanoethoxy)butane, diethylene glycol bis(2-cyanoethyl)ether, triethylene glycol bis(2-cyanoethyl) ether, tetraethylene glycol bis(2-cyanoethyl)ether, 1,3-bis(2-cyanoethoxy)propane, 1,4-bis(2-cyanoethoxy)butane, 1,5-bis(2-cyanoethoxy)pentane, ethylene glycol bis(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-hexanetricarbonitrile, 1,2,6-hexanetricarbonitrile, 1,2,3-tris(2-20 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.

[0019] According to an embodiment of this application, in the electrochemicaldevice, the electrolyte includes propylene carbonate.

[0020] According to an embodiment of this application, in the electrochemicaldevice, based on a mass of the electrolyte, a percentage of the propylene carbonate isy%, where 2 ≤ y ≤ 25.

[0021] According to an embodiment of this application, in the electrochemicaldevice, based on a mass of the electrolyte, a percentage of the propylene carbonate isy%, where F2 / y ≥ 0.5.

[0022] According to an embodiment of this application, in the electrochemicaldevice, the electrolyte includes propyl propionate, and based on a mass of theelectrolyte, a percentage of the propyl propionate is z%, where 5 ≤ z ≤ 50.

[0023] According to an embodiment of this application, in the electrochemicaldevice, the electrolyte includes a compound containing a cyano group and propylpropionate, where based on a mass of the electrolyte, a percentage of the compoundcontaining a cyano group is x% and a percentage of the propyl propionate is z%, where12 ≤ x + z ≤ 65 and 0.5 ≤ z / x ≤ 50.

[0024] According to an embodiment of this application, in the electrochemicaldevice, the electrolyte includes propylene carbonate and propyl propionate, wherebased on a mass of the electrolyte, a percentage of the propylene carbonate is y% and apercentage of the propyl propionate is z%, where 15 ≤ y + z ≤ 70 and 1 ≤ z / y ≤ 5.

[0025] According to an embodiment of this application, in the electrochemicaldevice, the electrolyte includes at least one of fluoroethylene carbonate, 1,3-propanesultone, vinyl sulfate, vinylene carbonate, or 1-propylphosphoric acid cyclic anhydride.

[0026] According to an aspect of this application, this application provides anelectronic device, including the electrochemical device described in the foregoingembodiments.

[0027] According to at least one aspect, controlling the adhesion force F1 betweenthe first positive electrode active material layer and the current collector and thecohesion force F2 of the second positive electrode active material layer withinappropriate ranges can effectively reduce the probability of internal short circuit of theelectrochemical device in the case of misuse such as compression, nail penetration, orovercharging, thereby improving safety of the electrochemical device. In addition, thiscan also effectively improve cycling performance of the electrochemical device at highvoltage and high temperature, and reduce direct current internal resistance of theelectrochemical device at high voltage and high temperature.

[0028] Additional aspects and advantages of some embodiments of this applicationare partly described and presented in subsequent descriptions, or explained byimplementation of some embodiments of this application.DETAILED DESCRIPTION OF EMBODIMENTS

[0029] Embodiments of this application are described in detail below. Same orsimilar components and components with same or similar functions are denoted withsimilar reference signs throughout this specification of this application. Embodimentsin related accompanying drawings described herein are descriptive and illustrative, andare used to provide a basic understanding of this application. The embodiments of thisapplication should not be construed as any limitations on this application.

[0030] Quantities, ratios, and other values are presented in the format of range inthis specification. It should be understood that such format of range is used forconvenience and simplicity and should be flexibly understood as including not onlyvalues explicitly designated as falling within the range but also all individual values orsub-ranges covered by the range as if each value and sub-range are explicitly designated.

[0031] In specific embodiments 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 itemsA and B are listed, the phrase "at least one of A or B" means only A, only B, or A andB. In another example, if items A, B, and C are listed, the phrase "at least one of A, B,or C" means only A, only B, only C, A and B (exclusive of C), A and C (exclusive ofB), B and C (exclusive of A), or all of A, B, and C. The item A may include a singleelement or a plurality of elements. The item B may include a single element or aplurality of elements. The item C may include a single element or a plurality of elements.The term "at least one type of" has the same meaning as the term "at least one of".

[0032] I. Positive electrode

[0033] In this application, the positive electrode includes a positive electrodecurrent collector and a positive electrode active material layer disposed on at least onesurface of the positive electrode current collector, where the positive electrode activematerial layer includes a positive electrode active material and a binder. In thisapplication, the positive electrode active material may be any material capable ofreversibly intercalating and deintercalating metal ions such as lithium ions and sodiumions. In some embodiments, the positive electrode current collector may be a positiveelectrode current collector commonly used in the art, and includes but is not limited toaluminum foil or nickel foil.

[0034] According to at least one aspect, it has been found in this application thatapplying at least two positive electrode active material layers onto at least one surfaceof the positive electrode current collector and controlling a cohesion force and adhesionforce of the at least two positive electrode active material layers within appropriateranges can improve safety performance of the electrochemical device, reduce directcurrent internal resistance of the electrochemical device at high temperature and highvoltage, and improve cycling stability of the electrochemical device.

[0035] Specifically, in some embodiments, this application provides a positiveelectrode, including a current collector and a positive electrode active material layer.The positive electrode active material layer is located on at least one surface of thecurrent collector and includes a first positive electrode active material layer and asecond positive electrode active material layer, the first positive electrode activematerial layer being located between the current collector and the second positiveelectrode active material layer, where an adhesion force between the current collectorand the first positive electrode active material layer is F1 N / m, and a cohesion force ofthe second positive electrode active material layer is F2 N / m, the positive electrodesatisfying F1 / F2 ≥ 6.

[0036] There are forces between different layers of an electrode plate of theelectrochemical device and within each layer. For example, during charge / dischargecycles of the electrochemical device, deintercalation and intercalation of active metalions (for example, lithium ions) in active material particles may cause the activematerial particles to expand or crack, resulting in interaction forces between the currentcollector, first active material layer, and second active material layer of the electrodeplate. The value of F1 / F2 can reflect mechanical stability of the positive electrode of theelectrochemical device in the case of misuse such as compression, nail penetration, orovercharging or during charge / discharge cycles. When the value of F1 / F2 satisfies therelation F1 / F2 ≥ 6, stress is evenly distributed on the electrode plate and the currentcollector, resulting in lower probability of misalignment or short circuit in the case ofcompression or nail penetration or during charge / discharge cycles at high temperatureand high voltage, thereby effectively improving the safety of the electrochemical deviceand effectively improving cycling performance of the electrochemical device at hightemperature and high voltage. In addition, unexpectedly, the positive electrode can alsoreduce the direct current internal resistance of the electrochemical device at hightemperature and high voltage.

[0037] In some embodiments, F1 and F2 satisfy F1 / F2 ≥ 10. In some embodiments,F1 and F2 satisfy F1 / F2 ≥ 20. In some embodiments, F1 and F2 satisfy F1 / F2 ≥ 30. In someembodiments, F1 and F2 satisfy F1 / F2 ≥ 40. In some embodiments, F1 and F2 satisfyF1 / F2 ≥ 50. The electrochemical device satisfying the foregoing relation can havefurther improved performance, especially in safety, stability, cycling performance, andimpedance reduction.

[0038] As the adhesion force F1 between the positive electrode current collector andthe first positive electrode active material layer increases, the probability of film fallingbetween the positive electrode active material layer and the current collector decreases,leading to lower risk of internal short circuit and higher thermal safety performance ofthe electrochemical device at high temperature and high voltage or in the case of misuse.In some embodiments, F1 ≥ 200. In some embodiments, F1 ≥ 220. In some embodiments,F1 is 250, 300, 350, 400, 450, 500, 600, 700, or 800, or falls within a range defined byany two of these values.

[0039] In some embodiments, 5 ≤ F2 ≤ 100. In some embodiments, 15 ≤ F2 ≤ 80. Insome embodiments, F2 is 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, or 100, or falls withina range defined by any two of these values.

[0040] It has been found in this application that selecting at least type or percentageof a binder in the first positive electrode active material layer and / or the second positiveelectrode active material layer helps to further implement F1 / F2 ≥ 6. In someembodiments, the first positive electrode active material layer includes a first binder,and the first binder satisfies at least one of the following conditions:(1) the first binder is water-soluble;(2) the first binder has unsaturated acid functional groups;(3) the first binder includes polyacrylate containing polar functional groups;or(4) based on a mass of the first positive electrode active material layer, amass fraction of the first binder is b%, where 2 ≤ b ≤ 20.

[0041] The water-soluble binder being used in the first positive electrode activematerial layer can more effectively improve the performance of the electrochemicaldevice. A possible reason is that hydrogen bonding or other intermolecular forcesbetween the water-soluble binder and the polar functional groups such as hydroxylgroups on the surface of the positive electrode current collector (for example, aluminumfoil) increase the adhesion force between the positive electrode active material layerand the current collector and greatly reduce the risk of the active material layer fallingoff the current collector, and in turn significantly reduce the probability of misalignmentor short circuit, effectively improving the safety of the electrochemical device andimproving the cycling stability of the electrochemical device at high temperature andhigh voltage.

[0042] In some embodiments, when the first positive electrode active material layersatisfies F1 / b ≥ 10, a positive electrode with better stability can be obtained, therebyfurther improving the electrochemical performance of the electrochemical device (forexample, thermal safety performance, cycling performance, and direct current internalresistance reduction at high temperature and high voltage). In particular, when F1 and b simultaneously satisfy F1 ≥ 200, 2 ≤ b ≤ 20, and F1 / b ≥ 10, the stability of the positiveelectrode can be further improved.

[0043] In some embodiments, the second positive electrode active material layerincludes a second binder, and the second binder satisfies at least one of the followingconditions:(1) the second binder is water-insoluble;(2) the second binder includes a fluoropolymer;(3) the second binder includes polyvinylidene fluoride containing an αcrystalline form; or(4) based on a mass of the second positive electrode active material layer, amass fraction of the second binder is a%, where 0.5 ≤ a ≤ 5.

[0044] In some embodiments, when the second positive electrode active materiallayer satisfies F2 / a ≥ 1, a positive electrode with better stability can be obtained, therebyfurther improving the electrochemical performance of the electrochemical device (forexample, thermal safety performance, cycling performance, and direct current internalresistance reduction at high temperature and high voltage). In particular, when F2 and asimultaneously satisfy 5 ≤ F2 ≤ 60, 0.5 ≤ a ≤ 5, and F2 / a ≥ 1, the stability of the positiveelectrode can be further improved.

[0045] In some embodiments, the positive electrode simultaneously satisfies F1 / b ≥and F2 / a ≥ 1. In this configuration, the positive electrode has better stability, and theobtained electrochemical device also shows better electrochemical performance. Insome embodiments, the positive electrode simultaneously satisfies F1 ≥ 200, 2 ≤ b ≤ 20,F1 / b ≥ 10, 5 ≤ F2 ≤ 60, 0.5 ≤ a ≤ 5, and F2 / a ≥ 1.

[0046] When the first positive electrode active material layer and the secondpositive electrode active material layer use binders with different solubilities, theobtained electrochemical device shows better thermal safety performance, cyclingperformance, and impedance reduction at high temperature and high voltage. Inparticular, using the water-soluble binder in the first positive electrode active materiallayer and the water-insoluble binder in the second positive electrode active materiallayer improve the electrochemical performance of the electrochemical device moresignificantly.

[0047] Because the binder cannot implement deintercalation and intercalation ofmetal ions, excessive addition compromises energy density of the electrochemicaldevice. However, when the percentage of the binder is excessively low, the activematerial layer cannot be firmly adhered to the positive electrode current collector,thereby increasing the risk of film falling. Therefore, adjusting the percentages of thefirst binder and the second binder in the first positive electrode active material layer andthe second positive electrode active material layer respectively can balance the energydensity and other electrochemical performance (for example, safety performance,cycling stability, and impedance reduction) of the electrochemical device. In someembodiments, the mass fractions b% and a% of the first binder and the second bindersatisfy 2.5 ≤ a + b ≤ 25 and 1 ≤ b / a ≤ 40. In some embodiments, a and b satisfy 3 ≤ a +b ≤ 20 and 2 ≤ b / a ≤ 30. In some embodiments, a and b satisfy 3.5 ≤ a + b ≤ 15 and 5 ≤b / a ≤ 20. In some embodiments, a and b satisfy 6 ≤ a + b ≤ 12 and 5 ≤ b / a ≤ 10.

[0048] In some embodiments, a satisfies 0.5 ≤ a ≤ 5. In some embodiments, a10 satisfies 1 ≤ a ≤ 4. In some embodiments, a is 0.8, 1, 1.2, 1.5, 1.8, 2, 2.5, 3, 3.5, 4, 4.5,or 5, or falls within a range defined by any two of these values.

[0049] In some embodiments, b satisfies 2 ≤ b ≤ 20. In some embodiments, bsatisfies 3 ≤ b ≤ 18. In some embodiments, b is 3, 4, 5, 6, 7, 8, 9, 10, 12, 15, 18, or 20,or falls within a range defined by any two of these values.

[0050] Cohesion force and adhesion force tests are performed with reference to thedetailed description in the examples of this application.

[0051] Adjusting thickness of the first positive electrode active material layer canalso further improve the electrochemical performance of the electrochemical device.The thickness mentioned herein is thickness of the positive electrode active materiallayer in a direction perpendicular to the current collector. In some embodiments,thickness of the first positive electrode active material layer is H1 μm, and thickness ofthe positive electrode active material layer is H μm, the positive electrode satisfyingH1 / H ≤ 0.1. In some embodiments, the thickness H1 μm of the first positive electrodeactive material layer satisfies 0.1 ≤ H1 ≤ 5. In some embodiments, the thickness H1 μmof the first positive electrode active material layer satisfies 0.1 ≤ H1 ≤ 3. In particular,when the water-soluble binder is used in the first positive electrode active material layer,controlling the thickness of the first positive electrode active material layer within theforegoing range can further optimize the performance of the electrochemical device.

[0052] The positive electrode active material is not limited to a particular type inthis application, provided that it is capable of electrochemically absorbing and releasingmetal ions (for example, lithium ions and sodium ions). In addition, the positiveelectrode active materials applicable to the first positive electrode active material layerand the second positive electrode active material layer in this application may beindependently lithium-containing oxides which may be the same or different. A lithium5 containing transition metal oxide may be used as the lithium-containing oxide.

[0053] In some embodiments, the positive electrode active material containinglithium and at least one transition metal. Examples of the positive electrode activematerial may include but are not limited to a lithium-containing transition metal oxideand a lithium-containing transition metal phosphate compound.

[0054] In some embodiments, a transition metal in the lithium-containing transitionmetal oxide includes V, Ti, Cr, Mn, Fe, Co, Ni, Cu, or the like. In some embodiments,the lithium-containing transition metal oxide includes lithium cobalt composite oxidesuch as LiCoO2, lithium nickel composite oxide such as LiNiO2, lithium manganesecomposite oxide such as LiMnO2, LiMn2O4, or Li2MnO4, or lithium nickel manganese cobalt composite oxide such as LiNi1 / 3Mn1 / 3Co1 / 3O2 or LiNi0.5Mn0.3Co0.2O2, wheresome of transition metal atoms constituting the main body of these lithium-containingtransition metal oxides are replaced with other elements such as Na, K, B, F, Al, Ti, V,Cr, Mn, Fe, Co, Li, Ni, Cu, Zn, Mg, Ga, Zr, Si, Nb, Mo, Sn, or W. Examples of thelithium-containing transition metal oxide may include but are not limited to LiNi0.5Mn0.5O2, LiNi0.85Co0.10Al0.05O2, LiNi0.33Co0.33Mn0.33O2, LiNi0.45Co0.10Al0.45O2,LiMn1.8Al0.2O4, and LiMn1.5Ni0.5O4. Examples of a combination of the lithiumcontaining transition metal oxides include but are not limited to a combination ofLiCoO2 and LiMn2O4, where some of Mn in LiMn2O4 may be replaced with a transitionmetal (for example, LiNi0.33Co0.33Mn0.33O2), and some of Co in LiCoO2 may bereplaced with a transition metal.

[0055] In some embodiments, a transition metal in the lithium-containing transitionmetal phosphate compound includes V, Ti, Cr, Mn, Fe, Co, Ni, Cu, or the like. In someembodiments, the lithium-containing transition metal phosphate compound includesiron phosphate such as LiFePO4, Li3Fe2(PO4)3, or LiFeP2O7, or cobalt phosphate suchas LiCoPO4, where some of transition metal atoms constituting the main body of theselithium transition metal phosphate compounds are replaced with other elements such asAl, Ti, V, Cr, Mn, Fe, Co, Li, Ni, Cu, Zn, Mg, Ga, Zr, Nb, or Si.

[0056] In some embodiments, the positive electrode active material containslithium phosphate, which can improve continuous charging performance of theelectrochemical device. The use of the lithium phosphate is not limited. In someembodiments, the positive electrode active material and the lithium phosphate are usedin a mixed manner. In some embodiments, as compared with a mass of the positiveelectrode active material and lithium phosphate, a percentage of the lithium phosphateis greater than 0.1%, greater than 0.3%, or greater than 0.5%. In some embodiments, ascompared with the mass of the positive electrode active material and lithium phosphate,the percentage of the lithium phosphate is less than 10%, less than 8%, or less than 5%.In some embodiments, the percentage of the lithium phosphate falls within a rangedefined by any two of these values.

[0057] Material with a composition different from that of the positive electrodeactive material may be attached to a surface of the positive electrode active material.Examples of the surface-attached material may include but are not limited to oxide suchas aluminum oxide, silicon dioxide, titanium dioxide, zirconium oxide, magnesiumoxide, calcium oxide, boron oxide, antimony oxide, and bismuth oxide; sulfate such aslithium sulfate, sodium sulfate, potassium sulfate, magnesium sulfate, calcium sulfate,and aluminum sulfate; carbonate such as lithium carbonate, calcium carbonate, andmagnesium carbonate; carbon, and the like.

[0058] These surface-attached materials may be attached to the surface of thepositive electrode active material in the following methods: a method for dissolving orsuspending the surface-attached material in a solvent to infiltrate into the positiveelectrode active material, and then performing drying; a method for dissolving orsuspending a precursor of the surface-attached material in a solvent to infiltrate into thepositive electrode active material, and then performing heating or the like to implementreaction of the surface-attached material; a method for adding the surface-attachedmaterial to a precursor of the positive electrode active material while performingsintering; and the like. In the case of attaching carbon, a method of mechanicalattachment by using a carbon material (for example, activated carbon) may be used.

[0059] In some embodiments, based on a mass of the positive electrode activematerial layer, a percentage of the surface-attached material is greater than 0.1 ppm,greater than 1 ppm, or greater than 10 ppm. In some embodiments, based on the massof the positive electrode active material layer, the percentage of the surface-attachedmaterial is less than 10%, less than 5%, or less than 2%. In some embodiments, basedon the mass of the positive electrode active material layer, the percentage of the surfaceattached material falls within a range defined by any two of these values.

[0060] Attaching material to the surface of the positive electrode active materialcan suppress oxidation reaction of the electrolyte on the surface of the positive electrodeactive material, prolonging service life of the electrochemical device. An excessivelysmall amount of surface-attached material cannot fully exhibit the effects, while anexcessively large amount of surface-attached material hinders deintercalation andintercalation of lithium ions, causing an increase in resistance sometimes.

[0061] In this application, materials with different compositions attached to thesurface of the positive electrode active material are also referred to as "positiveelectrode active materials"..

[0062] In some embodiments, shapes of positive electrode active material particlesinclude but are not limited to block, polyhedron, spherality, ellipsoid, plate, needle, andcolumn. In some embodiments, the positive electrode active material particles includeprimary particles, secondary particles, or a combination thereof. In some embodiments,the primary particles may agglomerate to form the secondary particles.

[0063] In some embodiments, a tap density of the positive electrode active materialis greater than 0.5 g / cm3, greater than 0.8 g / cm3, or greater than 1.0 g / cm3. The tapdensity of the positive electrode active material falling within the foregoing range cansuppress amounts of dispersion medium, conductive material, and positive electrodebinder required for forming the positive electrode active material layer, therebyguaranteeing filling rate of the positive electrode active material and capacity of theelectrochemical device. Composite oxide powder with high tap density being used canform a high-density positive electrode active material layer. Generally, a larger tapdensity is preferable, and there is no particular upper limit. In some embodiments, thetap density of the positive electrode active material is less than 4.0 g / cm3, less than 3.7g / cm3, or less than 3.5 g / cm3. The tap density of the positive electrode active materialhaving the foregoing upper limit can suppress a decrease in load characteristics.

[0064] The tap density of the positive electrode active material can be calculated inthe following manner: placing 5 g to 10 g of positive electrode active material powderinto a 10 mL glass measuring cylinder and tapping 200 times at a stroke of 20 mm toobtain a powder filling density (the tap density).

[0065] When the positive electrode active material particles are primary particles,a median particle size (D50) of the positive electrode active material particles is aprimary particle size of the positive electrode active material particles. When theprimary particles of the positive electrode active material particles agglomerate to formsecondary particles, a median particle size (D50) of the positive electrode activematerial particles is a secondary particle size of the positive electrode active materialparticles.

[0066] In some embodiments, the median particle size (D50) of the positiveelectrode active material particles is greater than 0.3 μm, greater than 0.5 μm, greaterthan 0.8 μm, or greater than 1.0 μm. In some embodiments, the median particle size(D50) of the positive electrode active material particles is less than 30 μm, less than 27μm, less than 25 μm, or less than 22 μm. In some embodiments, the median particlesize (D50) of the positive electrode active material particles falls within a range definedby any two of these values. When the median particle size (D50) of the positiveelectrode active material particles falls within the foregoing range, a positive electrode25 active material with a high tap density can be obtained, suppressing degradation in theperformance of the electrochemical device. In addition, problems such as stripes can beprevented during preparation of the positive electrode of the electrochemical device(that is, when the positive electrode active material, the conductive material, the binder,and the like are made into a slurry with a solvent and the slurry is applied in a thin-filmform). Herein, more than two positive electrode active materials with different medianparticle sizes are mixed, further improving the filling property during preparation of thepositive electrode.

[0067] The median particle size (D50) of the positive electrode active materialparticles can be measured by using a laser diffraction / scattering particle sizedistribution tester: when LA-920 manufactured by HORIBA is used as a particle sizedistribution tester, using a 0.1% sodium hexametaphosphate aqueous solution as adispersion medium for testing, and measuring a result at an refractive index of 1.24after ultrasonic dispersion for 5 minutes.

[0068] According to at least one aspect, this application further provides a methodfor preparing the positive electrode. The method includes:dispersing a first positive electrode active material, a first conductivematerial, and a first binder in a solvent to prepare a slurry for a first positive electrodeactive material layer, and dispersing a second positive electrode active material, asecond conductive material, and a second binder in a solvent to prepare a slurry for asecond positive electrode active material layer; andapplying the slurry for the first positive electrode active material layer ontoat least one surface of a positive electrode current collector, and then applying the slurryfor the second positive electrode active material layer onto the slurry for the firstpositive electrode active material layer before or after drying the first positive electrodeactive material layer.

[0069] In the foregoing method, if the slurries for the first and second positiveelectrode active material layers are applied onto both surfaces of the positive electrodecurrent collector, thickness and load of the first and second positive electrode activematerial layers applied onto one side of the positive electrode current collector may bethe same as or different from those of the first and second positive electrode activematerial layers applied onto the other side of the positive electrode current collector.

[0070] II. Electrolyte

[0071] The electrolyte used in the electrochemical device of this applicationincludes an electrolytic salt and a solvent for dissolving the electrolytic salt. In someembodiments, the electrolyte used in the electrochemical device of this applicationfurther includes an additive.

[0072] In some embodiments, the electrolyte in this application includes acompound containing a cyano group. The compound containing a cyano group can forma stable protective film on the surface of the positive electrode active material, therebyimproving the thermal safety, cycling performance, and impedance reduction of theelectrochemical device at high temperature and high voltage.

[0073] In some embodiments, the compound containing a cyano group includes butis not limited to at least one of 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 glycolbis(propionitrile) ether, 3,5-dioxa-pimelonitrile, 1,4-bis(cyanoethoxy)butane,diethylene glycol bis(2-cyanoethyl) ether, triethylene glycol bis(2-cyanoethyl) ether,tetraethylene glycol bis(2-cyanoethyl) ether, 1,3-bis(2-cyanoethoxy)propane, 1,4-bis(2-cyanoethoxy)butane, 1,5-bis(2-cyanoethoxy)pentane, ethylene glycol bis(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-hexanetricarbonitrile, 1,2,6-hexanetricarbonitrile, 1,2,3-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.

[0074] These compounds containing a cyano group may be used alone or in anycombination. When the electrolyte includes two or more compounds containing a cyanogroup, a percentage of the compounds containing a cyano group is a total percentage ofthe two or more compounds containing a cyano group.

[0075] The protective effect of the compound containing a cyano group is relatedto its amount used. In some embodiments, based on a mass of the electrolyte, apercentage of the compound containing a cyano group is x%, where 0.1 ≤ x ≤ 15. Insome embodiments, x satisfies 0.5 ≤ x ≤ 10, 1 ≤ x ≤ 8, or 3 ≤ x ≤ 5. In some embodiments,x may be but is not limited to 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, or 15, or falls within a rangedefined by any two of these values.

[0076] active materialIt is particularly important for repairing broken sites ofparticles that the compound containing a cyano group can form a stable protective filmon the surface of the positive electrode active material. It has been further found in thisapplication that controlling the adhesion force F1 between the current collector and thefirst positive electrode active material layer and the percentage x% of the compoundcontaining a cyano group to satisfy relations in the following embodiments can furthercompensate for broken particles caused by the electrochemical device duringcharge / discharge cycles, thereby further improving the thermal safety performance andcycling performance of the electrochemical device at high temperature and high voltageand reducing the direct current internal resistance of the electrochemical device. Insome embodiments, F1 and x satisfy F1 / x ≥ 13.33. In some embodiments, F1 and xsatisfy F1 / x ≥ 25. In some embodiments, F1 and x satisfy F1 / x ≥ 33.33. In someembodiments, F1 and x satisfy F1 / x ≥ 50. In some embodiments, F1 and x satisfy F1 / x ≥100. In particular, in the foregoing embodiments, controlling F1 to be above 200 N / mcan achieve better electrochemical performance.

[0077] In some embodiments, the electrolyte further includes any non-aqueoussolvent known in the prior art that can be used as a solvent of the electrolyte.

[0078] In some embodiments, the non-aqueous solvent includes but is not limitedto one or more of cyclic carbonate, linear carbonate, cyclic carboxylate, linearcarboxylate, cyclic ether, linear ether, a phosphorus-containing organic solvent, asulfur-containing organic solvent, or an aromatic fluorine-containing solvent.

[0079] In some embodiments, examples of the cyclic carbonate may include but arenot limited to one or more of ethylene carbonate (EC), propylene carbonate (PC), andbutylene carbonate. In some embodiments, the cyclic carbonate has 3 to 6 carbon atoms.

[0080] In some embodiments, the electrolyte includes propylene carbonate.Controlling a percentage of the propylene carbonate in the electrolyte can obtain anelectrochemical device with excellent thermal safety performance and cyclingperformance and low direct current resistance at high temperature and high voltage. Insome embodiments, based on a mass of the electrolyte, a percentage of the propylenecarbonate is y%, where 2 ≤ y ≤ 25. In some embodiments, y may be but is not limited5 to 6, 7, 8, 10, 12, 14, 18, 20, 22, or 25, or falls within a range defined by any two ofthese values.

[0081] In addition, it has been found in this application that controlling the relationbetween the cohesion force F2 of the second positive electrode active material layer andthe percentage y% of the propylene carbonate can further optimize the thermal safetyperformance, cycling performance, and direct current resistance of the electrochemicaldevice. At high temperature and high voltage, the propylene carbonate easily causesswelling of the positive electrode and may destroy an interface between the positiveelectrode and the electrolyte, resulting in side reactions. When the cohesion force of thesecond positive electrode active material layer increases, although the swellingdecreases, the side reactions of the electrolyte being oxidized by the positive electrodeincrease. However, when the cohesion force of the second positive electrode activematerial layer decreases, although the side reactions weaken, the swelling increases.Therefore, controlling F2 and y to satisfy a certain relation can better balance theswelling and the side reactions, thereby further improving the performance of theelectrochemical device. In some embodiments, F2 and y satisfy F2 / y ≥ 0.5. In someembodiments, F2 and y satisfy F2 / y ≥ 1. In some embodiments, F2 and y satisfy F2 / y ≥2. In some embodiments, F2 and y satisfy F2 / y ≥ 5. In some embodiments, F2 and ysatisfy F2 / y ≥ 6.

[0082] In some embodiments, the electrolyte includes propyl propionate.Controlling a percentage of the propyl propionate in the electrolyte can obtain anelectrochemical device with excellent thermal safety performance and cyclingperformance at high temperature and high voltage. In addition, unexpectedly, lowtemperature rate performance of the obtained electrochemical device can also be greatlyimproved. In some embodiments, based on a mass of the electrolyte, a percentage ofthe propyl propionate is z%, where 5 ≤ z ≤ 50. In some embodiments, z may be but isnot limited to 6, 8, 10, 20, 25, 30, 35, or 40, or falls within a range defined by any twoof these values.

[0083] When the electrolyte includes both the compound containing a cyano groupand the propyl propionate, controlling the relation between the percentages of thecompound containing a cyano group and propyl propionate in the electrolyte can alsoobtain an electrochemical device with excellent thermal safety performance and cyclingperformance and low direct current resistance at high temperature and high voltage. Insome embodiments, x and z satisfy 12 ≤ x + z ≤ 65 and 0.5 ≤ z / x ≤ 50.

[0084] When the electrolyte includes both the propylene carbonate and the propylpropionate, controlling the relation between the percentages of the propylene carbonateand propyl propionate in the electrolyte can also obtain an electrochemical device withexcellent thermal safety performance, cycling performance, and low-temperature rateperformance at high temperature and high voltage. In some embodiments, y and zsatisfy 15 ≤ y + z ≤ 70 and 1 ≤ z / y ≤ 5.

[0085] In some embodiments, the electrolyte further includes at least one offluoroethylene carbonate, 1,3-propane sultone, vinyl sulfate, vinylene carbonate, or 1-propylphosphoric acid cyclic anhydride. When the compound containing a cyano group,the propylene carbonate, the propyl propionate, the fluoroethylene carbonate, the 1,3-propane sultone, the vinyl sulfate, the vinylene carbonate, and the 1-propylphosphoricacid cyclic anhydride are used in combination, the interface between the electrode andthe electrolyte can be further stabilized, thereby improving the thermal safetyperformance and cycling performance of the electrochemical device at hightemperature and high voltage, especially greatly improving high-temperature storageperformance.

[0086] In some embodiments, the electrolytic salt is not particularly limited, andmay be any well-known material that can be used as an electrolytic salt. In the case ofa lithium secondary battery, a lithium salt is typically used as the electrolytic salt.Examples of the electrolytic salt may include but are not limited to inorganic lithiumsalt such as LiPF6, LiBF4, LiClO4, LiAlF4, LiSbF6, and LiWF7; lithium tungstate suchas LiWOF5; lithium carboxylate salt such as HCO2Li, CH3CO2Li, CH2FCO2Li,CHF2CO2Li, CF3CO2Li, CF3CH2CO2Li, CF3CF2CO2Li, CF3CF2CF2CO2Li, andCF3CF2CF2CF2CO2Li; lithium sulfonate salt such as FSO3Li, CH3SO3Li, CH2FSO3Li,CHF2SO3Li, CF3SO3Li, CF3CF2SO3Li, CF3CF2CF2SO3Li, and CF3CF2CF2CF2SO3Li;lithium imide salt such as LiN(FCO)2, LiN(FCO)(FSO2), LiN(FSO2)2,LiN(FSO2)(CF3SO2), LiN(CF3SO2)2, LiN(C2F5SO2)2, lithium cyclic 1,2-perfluoroethane disulfonylimide, lithium cyclic 1,3-perfluoropropane disulfonylimide,and LiN(CF3SO2)(C4F9SO2); lithium methide salt such as LiC(FSO2)3, LiC(CF3SO2)3,and LiC(C2F5SO2)3; lithium (malonato)borate salt such as lithium bis(malonato)boratesalt and lithium difluoro(malonato)borate salt; lithium (malonato)phosphate salt suchas lithium tris(malonato)phosphate, lithium difluorobis(malonato)phosphate, andlithium tetrafluoro(malonato)phosphate; fluorine-containing organic lithium salt suchas LiPF4(CF3)2, LiPF4(C2F5)2, LiPF4(CF3SO2)2, LiPF4(C2F5SO2)2, LiBF3CF3,LiBF3C2F5, LiBF3C3F7, LiBF2(CF3)2, LiBF2(C2F5)2, LiBF2(CF3SO2)2, andLiBF2(C2F5SO2)2; lithium oxalatoborate salt such as lithium difluoro(oxalato)borateand lithium bis(oxalato)borate; and lithium oxalatophosphate salt such as lithiumtetrafluoro(oxalato)phosphate, lithium difluorobis(oxalato)phosphate, and lithiumtris(oxalato)phosphate.

[0087] In some embodiments, the electrolytic salt is selected from LiPF6, LiSbF6,FSO3Li, CF3SO3Li, LiN(FSO2)2, LiN(FSO2)(CF3SO2), LiN(CF3SO2)2, LiN(C2F5SO2)2,lithium cyclic 1,2-perfluoroethane disulfonylimide, lithium cyclic 1,3-perfluoropropane disulfonylimide, LiC(FSO2)3, LiC(CF3SO2)3, LiC(C2F5SO2)3,LiBF3CF3, LiBF3C2F5, LiPF3(CF3)3, LiPF3(C2F5)3, lithium difluoro(oxalato)borate,lithium bis(oxalato)borate, or lithium difluorobis(oxalato)phosphate. This helps toimprove the performance of the electrochemical device such as output power, high-ratecharging and discharging, high-temperature storage, and cycling.

[0088] A percentage of the electrolytic salt is not particularly limited in thisapplication, provided that the effects of this application are not impaired. In someembodiments, a total molar concentration of lithium in the electrolyte is above 0.3mol / L, above 0.4 mol / L, or above 0.5 mol / L. In some embodiments, the total molarconcentration of lithium in the electrolyte is below 3.0 mol / L, below 2.5 mol / L, orbelow 2.0 mol / L. In some embodiments, the total molar concentration of lithium in theelectrolyte falls within a range defined by any two of these values. When theconcentration of the electrolytic salt falls within the foregoing range, the amount oflithium as charged particles is not excessively small, and the viscosity can be controlledwithin an appropriate range, so as to ensure good conductivity.

[0089] When two or more electrolytic salts are used, the electrolytic salts includeat least one salt selected from a group consisting of monofluorophosphate, borate,oxalate, and fluorosulfonate. In some embodiments, the electrolytic salt includes a saltselected from a group consisting of monofluorophosphate, oxalate, and fluorosulfonate.In some embodiments, the electrolytic salt includes a lithium salt. In some embodiments,based on a mass of the electrolytic salt, a percentage of the salt selected from the groupconsisting of monofluorophosphate, borate, oxalate, and fluorosulfonate is greater than0.01% or greater than 0.1%. In some embodiments, based on the mass of the electrolyticsalt, the percentage of the salt selected from the group consisting ofmonofluorophosphate, borate, oxalate, and fluorosulfonate is less than 20% or less than10%. In some embodiments, the percentage of the salt selected from the groupconsisting of monofluorophosphate, borate, oxalate, and fluorosulfonate falls within arange defined by any two of these values.

[0090] In some embodiments, the electrolytic salt includes more than one saltselected from the group consisting of monofluorophosphate, borate, oxalate, andfluorosulfonate, and more than one other salt. Examples of the other salt may includethe lithium salts exemplified above, and in some embodiments, are LiPF6,LiN(FSO2)(CF3SO2), LiN(CF3SO2)2, LiN(C2F5SO2)2, lithium cyclic 1,2-perfluoroethane disulfonylimide, lithium cyclic 1,3-perfluoropropane disulfonylimide,LiC(FSO2)3, LiC(CF3SO2)3, LiC(C2F5SO2)3, LiBF3CF3, LiBF3C2F5, LiPF3(CF3)3, andLiPF3(C2F5)3. In some embodiments, the other salt than the more than one material isLiPF6.

[0091] In some embodiments, based on a mass of the electrolytic salt, a percentageof the other salt than the more than one material is greater than 0.01% or greater than30 0.1%. In some embodiments, based on the mass of the electrolytic salt, the percentageof the other salt than the more than one material is less than 20%, less than 15%, or lessthan 10%. In some embodiments, the percentage of the other salt than the more thanone material falls within a range defined by any two of these values. The other salt thanthe more than one material with the foregoing percentage helps to balance theconductivity and viscosity of the electrolyte.

[0092] III. Negative electrode

[0093] The negative electrode includes a negative electrode current collector and anegative electrode active material layer disposed on at least one surface of the negativeelectrode current collector, and the negative electrode active material layer includes anegative electrode active material. One or more negative electrode active materiallayers may be provided, and the negative electrode active material layers may containthe same or different negative electrode active materials. The negative electrode activematerial is any material capable of reversibly intercalating and deintercalating metalions such as lithium ions and sodium ions. In some embodiments, a charge capacity ofthe negative electrode active material is larger than a discharge capacity of the positiveelectrode active material, so as to prevent lithium metal from unexpectedly precipitatingonto the negative electrode during charging.

[0094] In some embodiments, the negative electrode current collector may be anegative electrode current collector commonly used in the art, and includes but is notlimited to a metal material such as aluminum, copper, nickel, stainless steel, or nickelplated steel.

[0095] In a case that the negative electrode current collector is a metal material, thenegative electrode current collector may take forms including but not limited to a metalfoil, a metal cylinder, a metal coil, a metal plate, a metal film, a sheet metal mesh, apunched metal, and a foamed metal. In some embodiments, the negative electrodecurrent collector is a metal film. In some embodiments, the negative electrode currentcollector is a copper foil. In some embodiments, the negative electrode current collectoris a rolled copper foil based on a rolling method or an electrolytic copper foil based onan electrolytic method.

[0096] In some embodiments, thickness of the negative electrode current collectoris greater than 1 μm or greater than 5 μm. In some embodiments, the thickness of thenegative electrode current collector is less than 100 μm or less than 50 μm. In someembodiments, the thickness of the negative electrode current collector falls within arange defined by any two of these values.

[0097] The negative electrode active material is not particularly limited, providedthat it can reversibly absorb and release lithium ions or sodium ions. Examples of thenegative electrode active material may include but are not limited to carbon materialsuch as natural graphite and artificial graphite; metal such as silicon (Si) and tin (Sn);and oxide of metal elements such as Si and Sn. These negative electrode active materialsmay be used alone or in combination.

[0098] The negative electrode active material layer further includes a negativeelectrode binder. The negative electrode binder can improve bonding between negativeelectrode active material particles and bonding between the negative electrode activematerial and the current collector. The negative electrode binder is not limited to aparticular type, provided that its material is stable to the electrolyte or a solvent usedduring preparation of the electrode. In some embodiments, the negative electrode binderincludes a resin binder. Examples of the resin binder include but are not limited tofluororesin, polyacrylonitrile (PAN), polyimide resin, acrylic resin, and polyolefin resin.When an aqueous solvent is used for preparing a negative electrode mixture slurry, thenegative electrode binder includes but is not limited to carboxymethyl cellulose (CMC)or its salt, styrene-butadiene rubber (SBR), polyacrylic acid (PAA) or its salt, orpolyvinyl alcohol.

[0099] IV. Electrochemical device

[00100] According to at least one aspect, this application provides anelectrochemical device, including a positive electrode, a negative electrode, anelectrolyte, and a separator located between the positive electrode and the negativeelectrode. In some embodiments, the positive electrode includes the positive electrodedescribed in the foregoing embodiments of this application. In some embodiments, theelectrolyte includes the electrolyte described in the foregoing embodiments of thisapplication.

[00101] The separator is not limited to any particular material or shape in thisapplication, provided that the effects of this application are not significantly impaired.The separator may be a resin, glass fiber, inorganic material, or the like that is formedof materials stable to the electrolyte of this application. In some embodiments, theseparator includes a porous sheet or non-woven fabric-like material having excellentelectrolyte retention, or the like. Examples of the material of the resin or glass fiberseparator may include but are not limited to polyolefin, aromatic polyamide,polytetrafluoroethylene, and polyethersulfone. In some embodiments, the polyolefin ispolyethylene or polypropylene. In some embodiments, the polyolefin is polypropylene.These materials of the separator may be used alone or in any combination.

[00102] The separator may alternatively be formed by stacking the foregoingmaterials, and examples thereof include but are not limited to a three-layer separatorformed by stacking polypropylene, polyethylene, and polypropylene in order.

[00103] Examples of the material of the inorganic material may include but are notlimited to oxide such as aluminum oxide and silicon dioxide, nitride such as aluminumnitride and silicon nitride, and sulfate (for example, barium sulfate and calcium sulfate).The form of the inorganic material may include but is not limited to a granular or fibrousform.

[00104] The form of the separator may be a thin-film form, and examples thereofinclude but are not limited to a non-woven fabric, a woven fabric, and a microporousfilm. In the thin-film form, the separator has a pore size of 0.01 μm to 1 μm and athickness of 5 μm to 50 μm. In addition to the standalone thin-film-like separator, thefollowing separator may alternatively be used: a separator that is obtained by using aresin-based binder to form a composite porous layer containing inorganic particles onthe surface of the positive electrode and / or the negative electrode, for example, aseparator that is obtained by using fluororesin as a binder to form a porous layer on twosurfaces of the positive electrode with aluminum oxide particles of which 90% have aparticle size less than 1 μm.

[00105] The thickness of the separator is random. In some embodiments, thethickness of the separator is greater than 1 μm, greater than 5 μm, or greater than 8 μm.In some embodiments, the thickness of the separator is less than 50 μm, less than 40μm, or less than 30 μm. In some embodiments, the thickness of the separator falls withina range defined by any two of these values. The thickness of the separator falling withinthe foregoing range can guarantee insulation performance and mechanical strength, andcan guarantee the rate performance and energy density of the electrochemical device.

[00106] When a porous material such as a porous sheet or a non-woven fabric is usedas the separator, porosity of the separator is random. In some embodiments, the porosityof the separator is greater than 10%, greater than 15%, or greater than 20%. In someembodiments, the porosity of the separator is less than 60%, less than 50%, or less than45%. In some embodiments, the porosity of the separator falls within a range definedby any two of these values. The porosity of the separator falling within the foregoingrange can guarantee insulation performance and mechanical strength, and can suppresssheet resistance, allowing the electrochemical device to have good safety performance.

[00107] An average pore size of the separator is also random. In some embodiments,the average pore size of the separator is less than 0.5 μm or less than 0.2 μm. In someembodiments, the average pore size of the separator is greater than 0.05 μm. In someembodiments, the average pore size of the separator falls within a range defined by anytwo of these values. If the average pore size of the separator exceeds the foregoingrange, a short circuit is likely to occur. When the average pore size of the separator fallswithin the foregoing range, the electrochemical device has good safety performance.

[00108] V. Application

[00109] The electrochemical device of this application includes any device in whichelectrochemical reactions take place. Specific examples of the device include all typesof primary batteries, secondary batteries, fuel batteries, solar batteries, or capacitors.Especially, the electrochemical device is a lithium secondary battery, including alithium metal secondary battery or a lithium-ion secondary battery.

[00110] This application further provides an electronic device, including theelectrochemical device according to this application.

[00111] The electrochemical device of this application is not particularly limited toany purpose, and may be used in any known electronic device in the prior art. In someembodiments, the electrochemical device of this application may be used withoutlimitation in a notebook computer, a pen-input computer, a mobile computer, anelectronic book player, a portable telephone, a portable fax machine, a portable copier,a portable printer, a stereo headset, a video recorder, a liquid crystal television, aportable cleaner, a portable CD player, a mini-disc, a transceiver, an electronic notebook,a calculator, a storage card, a portable recorder, a radio, a standby power source, a motor,an automobile, a motorcycle, a motor bicycle, a bicycle, a lighting appliance, a toy, agame console, a clock, an electric tool, a flash lamp, a camera, a large household battery,or a lithium-ion capacitor.

[00112] The following describes preparation of lithium-ion battery by using alithium-ion battery as an example and with reference to specific examples. Personsskilled in the art understand that the preparation method described in this application isonly an example, and that all other suitable preparation methods fall within the scopeof this application.

[00113] Examples

[00114] I. Preparation of lithium-ion battery

[00115] (1) Preparation of negative electrode: Artificial graphite, styrene-butadienerubber, and sodium carboxymethyl cellulose were mixed at a mass ratio of 96%:2%:2%in deionized water, and the mixture was stirred to produce a uniform slurry. The slurrywas applied onto a 9 μm copper foil, followed by drying, cold pressing, cutting, and tabwelding, to obtain a negative electrode.

[00116] (2) Preparation of positive electrode: Lithium iron phosphate, conductivecarbon black Super-P, and a first binder were mixed at a mass ratio of 96.5%:1%:b% indeionized water, and the mixture was stirred to produce a uniform first positiveelectrode active material layer slurry; and lithium cobalt oxide, conductive carbon blackSuper-P, and a second binder were mixed at a mass ratio of 97.5%:1%:a% in Nmethylpyrrolidone (NMP), and the mixture was stirred to produce a uniform secondpositive electrode active material layer slurry. The first positive electrode activematerial layer slurry was applied onto the front and back sides of a positive electrodecurrent collector made of aluminum foil, with a thickness of 5 μm; and then the secondpositive electrode active material layer slurry was applied onto the positive electrodecurrent collector, with a thickness of 50 μm, such that a total coating thickness on onesurface was 55 μm. Then drying, cold pressing, cutting, and tab welding wereperformed to obtain a positive electrode.

[00117] (3) Preparation of electrolyte: In a dry argon environment, EC and DECwere mixed at a mass ratio of 1:1, and LiPF6 was added. The mixture was evenly mixedto form a base electrolyte, where a concentration of the LiPF6 was 1.15 mol / L. Additivesof different percentages were added to the base electrolyte, to obtain electrolytes in theexamples and comparative examples in this application.

[00118] Abbreviations and full names of components in the electrolyte are listed inthe following table.

[00119] (4) Preparation of separator: An 8 μm polyethylene film was used as aseparator.

[00120] (5) Preparation of lithium-ion battery: The obtained positive electrode,separator, and negative electrode were stacked in sequence and wound, and then placedin an outer package foil, with an electrolyte injection opening left. The electrolyte wasinjected from the electrolyte injection opening which was then sealed, followed byprocesses such as formation and capacity, to obtain a lithium-ion battery.

[00121] (II) Test methods of positive electrode and lithium-ion battery

[00122] (1) Cohesion force test: Electrode plates disassembled from the battery weretaken, from which a single-sided coated electrode plate was selected (or a double-sidedcoated electrode plate was made into a single-sided electrode plate with a scraper), andthe electrode plate was cut into a sample under test with a length 100 mm and a widthof 10 mm. A 25 mm wide stainless steel plate was taken, a 3M double-sided adhesive(with a width of 11 mm) was applied onto the stainless steel plate, and the sample undertest was attached to the 3M double-sided adhesive on the stainless steel plate, where thecurrent collector was adhered to the double-sided adhesive. A 2000 g roller was used toroll back and forth on the surface of the sample for three times (300 mm / min). Then, anadhesive tape with a width of 10 mm and a thickness of 50 μm (model:NITTO.NO5000NS) was attached to the surface of the active material layer, and the2000 g roller was used to roll back and forth on the surface of the adhesive tape forthree times (300 mm / min). The adhesive tape was bent at 180 degrees, and the adhesivetape was manually peeled off the active material layer by 25 mm. The sample was fixedon an Instron 336 tensile testing machine, with a peeling surface aligned with a forceline of the testing machine (that is, peeling was performed at 180°). Continuous peelingwas performed at 300 mm / min to obtain a cohesion force curve, and an average valueof a stable section was taken as a peeling force F0. In this case, the cohesion force ofthe electrode plate under test was: F2 = F0 / width of sample under test, where F2 wasmeasured in N / m.

[00123] (2) Adhesion force test: An electrode plate disassembled from the batterywas taken and cut into a 20 mm x 10 cm test sample strip, and the strip was attached toa clean stainless steel plate with a double-sided adhesive with a width of 20 mm (model:NITTO.NO5000NS). The tensile testing machine was used to perform a 180° peelingtest on the strip, where a tensile speed of the tensile testing machine was 50 mm / min.An average value of peeling forces collected when the first positive electrode activematerial layer was entirely peeled off the positive electrode current collector was anadhesion force F between the positive electrode current collector and the first positiveelectrode active material layer. The adhesion force of the electrode plate under test was:F1 = F / width of sample under test, where F1 was measured in N / m.

[00124] (3) Thermal safety temperature rise test

[00125] At 25°C, the lithium-ion battery was left standing for 30 minutes, and athickness T1 was measured. Then the temperature was increased to 130°C at atemperature rise velocity of 5°C / min, the battery was maintained at that temperature forminutes, and then a thickness T2 at that point was measured. Thickness swelling rateof the lithium-ion battery was calculated according to the following formula:Thickness swelling rate at temperature rise =[(T2 - T1) / T1] x 100%.

[00126] (4) Direct current internal resistance test at high temperature and highvoltage

[00127] At 65°C, the lithium-ion battery was charged to 4.7 V at a constant currentof 1.5C, then charged to 0.05C at a constant voltage of 4.7 V, and left standing for 30minutes. The battery was discharged at 0.1C for 10 seconds, and a voltage value wasrecorded as U1. The battery was discharged at 1C for 360 seconds, and a voltage valuewas recorded as U2. Such charging and discharging steps were repeated for 5 times."1C" refers to a current value at which a lithium-ion battery is fully discharged in onehour.Direct current resistance R = (U2 - U1) / (1C - 0.1C).

[00128] (5) Capacity retention rate test at high temperature and high voltage

[00129] At 45°C, the lithium-ion battery was charged to 4.7 V at a constant currentof 1C, then charged to a current of 0.05C at a constant voltage, and discharged to 3.0 Vat a constant current of 1C. This was the first cycle, and a discharge capacity C1 afterthe first cycle was recorded. 800 charge / discharge cycles were performed on thelithium-ion battery according to the foregoing conditions, and a discharge capacity C800after 800 cycles was recorded. The capacity retention rate after cycles was calculatedaccording to the following formula:Capacity retention rate = (C800 / C1) x 100%.

[00130] (6) Low-temperature rate performance test

[00131] At 25°C, the lithium-ion battery was charged to 4.7 V at a constant currentof 0.5C, then charged to 0.05C at a constant voltage, and discharged to 3.0 V at aconstant current of 0.5C. A discharge capacity at 25°C was recorded as C(25°C). At25°C, the lithium-ion battery was charged to 4.7 V at a constant current of 0.5C, thencharged to 0.05C at a constant voltage, placed in a -20°C thermostat, left standing for2 hours, and discharged to 3.0 V at a constant current of 0.5C. A discharge capacity at-20°C was recorded as C(-20°C). The capacity retention rate of the lithium-ion batteryat low temperature compared with room temperature was calculated according to thefollowing formula:Capacity retention rate = [C(-20°C) / C(25°C)] x 100%.

[00132] (7) High-temperature storage performance test

[00133] At 25°C, the lithium-ion battery was left standing for 30 minutes, thencharged to 4.7 V at a constant current of 0.5C, charged to 0.05C at a constant voltageof 4.7 V, and left standing for 5 minutes. A battery thickness was measured and recordedas T3. After storage at 60°C for 21 days, a battery thickness was measured and recordedas T4. The thickness swelling rate of the lithium-ion battery after high-temperaturestorage was calculated according to the following formula:Thickness swelling rate after high-temperature storage = [(T4 - T3) / T3] x100%.

[00134] III. Test results

[00135] The difference between the lithium-ion batteries prepared in Examples 1-1to 1-8 and Comparative Examples 1-1 to 1-4 according to the foregoing preparationmethod lies in the percentage of the binder, where the first binder is water-solublepolyacrylate and the second binder is oil-soluble PVDF. Table 1 shows influences ofthe adhesion force F1 between the positive electrode current collector and the firstpositive electrode active material layer and the cohesion force F2 of the second positiveelectrode active material layer on the thermal safety performance, direct current internalresistance, and cycling performance of the lithium-ion battery at high temperature andhigh voltage.Table 1

[00136] It can be learned from the electrochemical test results in Table 1 that ascompared with Comparative Examples 1-1 to 1-4, in Examples 1-1 to 1-8 of thisapplication, the positive electrode satisfies F1 / F2 ≥ 6, and the obtained correspondingelectrochemical device has lower thickness swelling at high temperature and has lowerdirect current resistance and higher capacity retention rate during charging anddischarging at high temperature and high voltage. In addition, it can be learned fromthe data of Examples 1-5 to 1-8 that as the adhesion force F1 between the positiveelectrode current collector and the first positive electrode active material layer increases,the obtained corresponding electrochemical device has lower risk of internal shortcircuit and higher thermal safety performance at high temperature and high voltage orin the case of misuse.

[00137] Table 2 shows influences of the first binder and the second binder on thethermal safety performance, direct current internal resistance, and cycling performanceof the lithium-ion battery at high temperature and high voltage, where the differencebetween Examples 2-1 to 2-14 and Example 1-1 lies only in the parameters listed inTable 2.Table 2

[00138] It can be learned from the comparison of the data of Examples 2-9 and 2-12in Table 2 that when the water-soluble binder is used in the first positive electrode activematerial layer, the obtained corresponding electrochemical device has betterelectrochemical performance. The comparison between Example 2-8 and Example 2-11 leads to a same conclusion. A possible reason is that hydrogen bonding or otherintermolecular forces between the water-soluble binder and the polar functional groupssuch as hydroxyl groups on the surface of the positive electrode current collectoraluminum foil increase the adhesion force. In addition, it can be learned from thecomparison between Examples 2-13 and 2-14 and Example 2-9 that in a case that thesame binders are used, when the percentages of the first binder and the second bindersatisfy 2.5 ≤ a + b ≤ 25 and 1 ≤ b / a ≤ 40, the positive electrode has a more stablestructure, further improving the performance of the electrochemical device. In addition,it can be learned from the data of Examples 1-1 and 2-1 to 2-12 that when other bindersare used and the percentages thereof satisfy 2.5 ≤ a + b ≤ 25 and 1 ≤ b / a ≤ 40, a morestable positive electrode can also be obtained.

[00139] Table 3 shows influences of F1 / b and F2 / a on the thermal safety performance,direct current internal resistance, and cycling performance of the electrochemicaldevice at high temperature and high voltage, where the difference between Examples3-1 to 3-6 and Example 1-1 lies only in the parameters listed in Table 3.Table 3

[00140] It can be learned from the electrochemical test results in Table 3 that whenthe positive electrode further satisfies F1 / b ≥ 10 and F2 / a ≥ 1, the obtained correspondingelectrochemical device has lower thickness swelling, lower direct current internalresistance, and higher capacity retention rate at high temperature and high voltage.

[00141] Table 4 shows influences of the total thickness H of the positive electrodeactive material layer and the thickness H1 of the first positive electrode active materiallayer on the thermal safety performance, direct current internal resistance, and cyclingperformance of the electrochemical device at high temperature and high voltage, wherethe difference between Examples 4-1 to 4-5 and Example 1-1 lies only in the parameterslisted in Table 4.Table 4

[00142] It can be learned from the electrochemical test results in Table 4 that whenthe positive electrode further satisfies H1 / H ≤ 0.1, the obtained correspondingelectrochemical device has lower thickness swelling, lower direct current internalresistance, and higher capacity retention rate at high temperature and high voltage.

[00143] Table 5 shows influences of the compound containing a cyano groupincluded in the electrolyte on the thermal safety performance, direct current internalresistance, and cycling performance of the electrochemical device at high temperatureand high voltage, where the difference between Examples 5-1 to 5-25 and Example 1-1 lies only in the parameters listed in Table 5.Table 5

[00146] It can be learned from the electrochemical test results in Table 6 that whenthe propylene carbonate is added to the electrolyte, especially when F2 and y furthersatisfy F2 / y ≥ 0.5, the obtained corresponding electrochemical device shows not onlyexcellent thermal safety performance and cycling performance but also unexpectedlygreatly reduced direct current resistance at high temperature and high voltage.

[00147] Table 7 shows influences of the percentages of the compound containing acyano group and the propyl propionate on the thermal safety performance, direct currentinternal resistance, and cycling performance of the electrochemical device at hightemperature and high voltage, where the difference between Examples 7-1 to 7-16 andExample 1-1 lies only in the parameters listed in Table 7.Table 7

[00148] It can be learned from the electrochemical test results in Table 7 that whenboth the compound containing a cyano group and the propyl propionate are added tothe electrolyte, especially when the percentages of the two in the electrolyte satisfy 12≤ x + z ≤ 65 and 0.5 ≤ z / x ≤ 50, the thermal safety performance, cycling performance,and impedance reduction of the electrochemical device at high temperature and highvoltage can be further optimized.

[00149] Table 8 shows influences of the percentages of the propylene carbonate andthe propyl propionate on the thermal safety performance, low-temperature rateperformance, and cycling performance of the electrochemical device at hightemperature and high voltage, where the difference between Examples 8-1 to 8-8 andExample 1-1 lies only in the parameters listed in Table 8.Table 8

[00150] It can be learned from the electrochemical test results in Table 8 that ascompared with Examples 8-7 and 8-8, in Examples 1-1 and 8-1 to 8-6, both thepropylene carbonate and the propyl propionate are added to the electrolyte, and theobtained corresponding electrochemical device shows better thermal safetyperformance, low-temperature rate performance, and cycling performance at hightemperature and high voltage. As compared with Examples 8-4 to 8-6, in Examples 8-1 to 8-3, the percentages of the propylene carbonate and the propyl propionate satisfy15 ≤ y + z ≤ 70 and 1 ≤ z / y ≤ 5, and the electrochemical performance of the obtainedcorresponding electrochemical device at high temperature and high voltage is furtherimproved, especially the low-temperature rate performance is greatly improved.

[00151] Table 9 shows influences of the solvent and additive in the electrolyte on thethermal safety performance and cycling performance of the electrochemical device athigh temperature and high voltage, where the difference between Examples 9-1 to 9-17and Example 1-1 lies only in the parameters listed in Table 9.Table 9

[00152] It can be learned from the electrochemical test results in Table 9 that whenthe compound containing a cyano group, the propylene carbonate, the propyl propionate,the fluoroethylene carbonate, the 1,3-propane sultone, the vinyl sulfate, the vinylenecarbonate, and the 1-propylphosphoric acid cyclic anhydride are used in combinationin the electrolyte, the obtained electrochemical device shows excellent thermal safetyperformance, high-temperature storage performance, and cycling performance at hightemperature and high voltage.

[00153] References to "an embodiment", "some embodiments", "one embodiment","another example", "an example", "a specific example", or "some examples" in thisspecification mean the inclusion of specific features, structures, materials, orcharacteristics described in the embodiment or example in at least one embodiment orexample of this application. Therefore, descriptions in various places throughout thisspecification, such as "in some embodiments", "in an embodiment", "in oneembodiment", "in another example", "in one example", "in a specific example", or "anexample", do not necessarily refer to the same embodiment or example of thisapplication. In addition, specific features, structures, materials, or characteristics hereinmay be combined in any suitable manner in one or more embodiments or examples.

[00154] Although illustrative embodiments have been demonstrated and described,persons skilled in the art should understand that the foregoing embodiments should notbe construed as any limitation on this application, and that some embodiments may bechanged, replaced, and modified without departing from the spirit, principle, and scopeof this application.

Claims

1. An electrochemical device, comprising a positive electrode, a negative electrode, and an electrolyte; wherein the positive electrode comprises: a current collector, and a positive electrode active material layer; characterized in that, the positive electrode active material layer is located on at least one surface of the current collector, and comprises a first positive electrode active material layer and a second positive electrode active material layer, the first positive electrode active material layer being located between the current collector and the second positive electrode active material layer; wherein an adhesion force between the current collector and the first positive electrode active material layer is F1 N / m, and a cohesion force of the second positive electrode active material layer is F2 N / m, and F1 / F2 ≥ 6.

2. The electrochemical device as claimed in claim 1, wherein F1 ≥ 200.

3. The electrochemical device as claimed in claim 1 or 2, wherein the first positive electrode active material layer comprises a first binder, and the first binder satisfies at least one of the following conditions: (1) the first binder is water-soluble; (2) the first binder has unsaturated acid functional groups; (3) the first binder comprises polyacrylate containing polar functional groups; or (4) based on a mass of the first positive electrode active material layer, a mass fraction of the first binder is b%, wherein 2 ≤ b ≤ 20.

4. The electrochemical device as claimed in any one of claims 1 to 3, wherein the first positive electrode active material layer comprises a first binder; based on a mass of the first positive electrode active material layer, a mass fraction of the first binder is b%, F1 ≥ 200, 2 ≤ b ≤ 20, and F1 / b ≥ 10.

5. The electrochemical device as claimed in any one of claims 1 to 4, wherein the second positive electrode active material layer comprises a second binder, and the second binder satisfies at least one of the following conditions: (1) the second binder is water-insoluble; (2) the second binder comprises a fluoropolymer; (3) the second binder comprises polyvinylidene fluoride containing an α crystalline form; or (4) based on a mass of the second positive electrode active material layer, a mass fraction of the second binder is a%, 0.5 ≤ a ≤ 5.

6. The electrochemical device as claimed in any one of claims 1 to 5, wherein the second positive electrode active material layer comprises a second binder; based on a mass of the second positive electrode active material layer, a mass fraction of the second binder is a%, 5 ≤ F2 ≤ 60, 0.5 ≤ a ≤ 5, and F2 / a ≥ 1.

7. The electrochemical device as claimed in any one of claims 1 to 6, wherein the first positive electrode active material layer comprises a water-soluble binder, and the second positive electrode active material layer comprises a water-insoluble binder.

8. The electrochemical device as claimed in any one of claims 1 to 7, wherein the first positive electrode active material layer comprises a first binder; and based on a mass of the first positive electrode active material layer, a mass fraction of the first binder is b%; and the second positive electrode active material layer comprises a second binder, based on a mass of the second positive electrode active material layer, a mass fraction of the second binder is a%; wherein 2.5 ≤ a + b ≤ 25 and 1 ≤ b / a ≤ 40.

9. The electrochemical device as claimed in any one of claims 1 to 8, wherein a thickness of the first positive electrode active material layer is H1 μm, and the positive electrode satisfies at least one of the following conditions: (1) a thickness of the positive electrode active material layer is H μm, H1 / H ≤ 0.1; or (2) 0.1 ≤ H1 ≤ 5.

10. The electrochemical device as claimed in any one of claims 1 to 9, wherein the electrolyte comprises a compound containing a cyano group.

11. The electrochemical device as claimed in claim 10, wherein the electrochemical device satisfies at least one of the following conditions: (1) based on a mass of the electrolyte, a percentage of the compound containing a cyano group is x%, 0.1 ≤ x ≤ 15; (2) based on a mass of the electrolyte, a percentage of the compound containing a cyano group is x%, F1 ≥ 200 and F1 / x ≥ 13.33; or (3) the compound containing a cyano group comprises at least one selected from the group consisting of 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-pimelonitrile, 1,4-bis(cyanoethoxy)butane, diethylene glycol bis(2-cyanoethyl) ether, triethylene glycol bis(2-cyanoethyl) ether, tetraethylene glycol bis(2-cyanoethyl) ether, 1,3-bis(2-cyanoethoxy)propane, 1,4-bis(2- cyanoethoxy)butane, 1,5-bis(2-cyanoethoxy)pentane, ethylene glycol bis(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-hexanetricarbonitrile, 1,2,6-hexanetricarbonitrile, 1,2,3- 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, and 1,2,5-tris(cyanoethoxy)pentane.

12. The electrochemical device as claimed in any one of claims 1 to 11, wherein the electrochemical device satisfies at least one of the following conditions: (1) the electrolyte comprises propylene carbonate; (2) the electrolyte comprises propylene carbonate, based on a mass of the electrolyte, a percentage of the propylene carbonate is y%, wherein 2 ≤ y ≤ 25; or (3) the electrolyte comprises propylene carbonate, based on a mass of the electrolyte, a percentage of the propylene carbonate is y%, wherein F2 / y ≥ 0.

5.

13. The electrochemical device as claimed in any one of claims 1 to 12, wherein the electrochemical device satisfies at least one of the following conditions: (1) the electrolyte comprises propyl propionate, based on a mass of the electrolyte, a percentage of the propyl propionate is z%, wherein 5 ≤ z ≤ 50; (2) the electrolyte comprises a compound containing a cyano group and propyl propionate; wherein based on a mass of the electrolyte, a percentage of the compound containing a cyano group is x% and a percentage of the propyl propionate is z%, wherein 12 ≤ x + z ≤ 65 and 0.5 ≤ z / x ≤ 50; or (3) the electrolyte comprises propylene carbonate and propyl propionate; wherein based on a mass of the electrolyte, a percentage of the propylene carbonate is y% and a percentage of the propyl propionate is z%, wherein 15 ≤ y + z ≤ 70 and 1 ≤ z / y ≤ 5.

14. The electrochemical device as claimed in any one of claims 1 to 13, wherein the electrolyte comprises at least one selected from the group consisting of fluoroethylene carbonate, 1,3-propane sultone, vinyl sulfate, vinylene carbonate, and 1- propylphosphoric acid cyclic anhydride.