Electrolyte, secondary battery comprising same, and electronic device

Abstract

The present application provides an electrolyte, a secondary battery comprising same, and an electronic device. The electrolyte comprises lithium difluorophosphate and a first lithium salt, wherein the first lithium salt comprises at least one of lithium hexafluorophosphate or lithium bisfluorosulfonylimide. Based on the mass of the electrolyte, the mass percentage content of lithium difluorophosphate is A%, and the mass percentage content of the first lithium salt is B%, wherein 2≤A≤8.8, and 0.1≤A / B≤1. The electrolyte further comprises a first solvent, wherein the donor number of the first solvent is 10 or above, or the dielectric constant of the first solvent is 5 or above. Based on the mass of the electrolyte, the mass percentage content of the first solvent is C%, wherein 5≤C≤60. The electrolyte comprises lithium difluorophosphate, and the first lithium salt and the first solvent, and the values of A, A / B and C are regulated to be within the ranges, such that the cycle performance and high-temperature storage performance of a secondary battery can be improved.

Description

Electrolyte, secondary battery containing the same, and electronic device Technical Field

[0001] The present application relates to the field of electrochemical technology, and in particular to an electrolyte, a secondary battery containing the electrolyte, and an electronic device. Background Art

[0002] Secondary batteries, such as lithium-ion batteries, have attracted widespread attention due to their high energy density, low maintenance, relatively low self-discharge, long cycle life, lack of memory effect, stable operating voltage, and environmental friendliness. They are widely used in portable electronic devices, power tools, and electric vehicles. However, with the rapid development of technology and the diversification of market demands, higher requirements are being placed on secondary batteries.

[0003] During the charge and discharge process, lithium salts in the electrolyte participate in the formation of the negative electrode solid electrolyte interface film (SEI film) and the positive electrode solid electrolyte interface film (CEI film). The SEI film and CEI film have a significant impact on the performance of lithium-ion batteries. Electrolytes containing lithium difluorophosphate are conducive to the formation of high-performance SEI films and CEI films, improving the cycle performance of lithium-ion batteries. However, in commonly used electrolytes (such as carbonate-based electrolytes), the solubility of lithium difluorophosphate is very low, only about 1%, which severely limits its role and is not conducive to improving the cycle performance of secondary batteries.

[0004] Summary of the Invention

[0005] The purpose of this application is to provide an electrolyte, a secondary battery containing the electrolyte, and an electronic device to improve the cycle performance of the secondary battery. The specific technical solution is as follows:

[0006] In a first aspect, the present application provides an electrolyte comprising lithium difluorophosphate and a first lithium salt, the first lithium salt comprising at least one of lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide; the lithium difluorophosphate having a mass percentage of A% and a mass percentage of B% of the first lithium salt based on the mass of the electrolyte, 2≤A≤8.8, 0.1≤A / B≤1; and a first solvent having a donor number of 10 or greater or a dielectric constant of 5 or greater, and a mass percentage of C% and 5≤C≤60 based on the mass of the electrolyte. In the electrolyte provided herein, lithium difluorophosphate has a high solubility. By rationally combining lithium difluorophosphate with the first lithium salt, a synergistic effect between the two can be exerted, facilitating the formation of stable SEI and CEI films. Furthermore, lithium difluorophosphate has a high solubility in the first solvent, enabling better performance of the lithium difluorophosphate and improving the cycling performance and high-temperature storage performance of the secondary battery. Therefore, the electrolyte includes lithium difluorophosphate and the first lithium salt and the first solvent, and the values ​​of A, A / B and C are adjusted within the above ranges, which can improve the cycle performance and high-temperature storage performance of the secondary battery.

[0007] In some embodiments of the present application, 6.25≤B≤17. By regulating the value of B within the above range, the electrolyte can have a suitable viscosity and a high conductivity, thereby increasing the migration rate of lithium ions and improving the cycle performance of the secondary battery.

[0008] In some embodiments of the present application, 2.2≤A≤7, 0.2≤A / B≤0.8. By regulating the values ​​of A and A / B within the above ranges, the synergistic effect between lithium difluorophosphate and the first lithium salt is more effectively exerted, which facilitates the formation of more stable SEI and CEI films, thereby further improving the cycling performance and high-temperature storage performance of the secondary battery.

[0009] In some embodiments of the present application, 3≤C / A≤25. Lithium difluorophosphate has a high solubility in the first solvent, which can better exert the function of lithium difluorophosphate. On the other hand, since the first solvent itself has a narrow electrochemical window, by properly matching the content of the first solvent and lithium difluorophosphate, that is, regulating the C / A value within the above range, the decomposition of the first solvent can be reduced, which is conducive to further improving the cycle performance and high-temperature storage performance of the secondary battery.

[0010] In some embodiments of the present application, the first solvent includes at least one of ethylene carbonate, ethyl acetate, ethylene glycol dimethyl ether, trimethyl phosphate, triethyl phosphate, tripropyl phosphate, tributyl phosphate, sulfolane, dimethyl sulfoxide, N,N-dimethylformamide, N,N-dimethylacetamide, N-methylacetamide, tetramethylurea, or gamma-butyrolactone. Selecting a first solvent within the above range is conducive to better improving the cycle performance and high-temperature storage performance of the secondary battery.

[0011] In some embodiments of the present application, the electrolyte further includes a first additive, the first additive including at least one of 1,3-propane sultone, 1,3-propylene sultone, vinyl sulfate, 1,3-propylene glycol cyclic sulfate, 2,4-butane sultone, 1,4-butane sultone, vinylene carbonate, or fluoroethylene carbonate, and the mass percentage of the first additive based on the mass of the electrolyte is X1, and 0.05≤X1≤12. In addition to the electrolyte including lithium difluorophosphate, further introducing the first additive and regulating the value of X1 within the above range can achieve a synergistic effect between the lithium difluorophosphate and the first additive, which is beneficial for increasing the organic-inorganic hybrid interface in the SEI film and the CEI film, further enhancing the stability of the SEI film and the CEI film, and thus further improving the cycle performance and high-temperature storage performance of the secondary battery.

[0012] In some embodiments of the present application, 0.3≤A / X1≤120. By regulating A / X1 within the above range, the synergistic effect of lithium difluorophosphate and the above-mentioned first additive can be better exerted, further enhancing the stability of the SEI film and CEI film, thereby enabling the secondary battery to have good cycle performance and high-temperature storage performance.

[0013] In some embodiments of the present application, the electrolyte further includes a second additive, the second additive including at least one of lithium bis(oxalatoborate), lithium tetrafluoroborate, lithium difluorooxalatoborate, lithium trifluoromethanesulfonate, 4,5-dicyano-2-trifluoromethyl-imidazole lithium, or lithium tetraborate, and the mass percentage of the second additive based on the mass of the electrolyte is X2%, 0.1≤X2≤2. In addition to the electrolyte including lithium difluorophosphate, the introduction of the second additive and the regulation of the value of X2 within the above range can exert a synergistic effect between the lithium difluorophosphate and the second additive, further enhancing the stability of the SEI film and the CEI film, and reducing interfacial side reactions on the surfaces of the positive and negative electrode sheets, thereby further improving the cycle performance and high-temperature storage performance of the secondary battery.

[0014] In some embodiments of the present application, 2≤A / X2≤44. By regulating A / X2 within the above range, the synergistic effect of lithium difluorophosphate and the second additive can be better exerted, further enhancing the stability of the SEI and CEI films, thereby enabling the secondary battery to have good cycle performance and high-temperature storage performance.

[0015] The second aspect of the present application provides a secondary battery, which includes the electrolyte provided by the first aspect of the present application. The secondary battery provided by the present application has good cycle performance and high-temperature storage performance.

[0016] The third aspect of the present application provides an electronic device, which includes the secondary battery provided in the second aspect of the present application. The secondary battery provided in the present application has good cycle performance and high-temperature storage performance, so that the electronic device of the present application has a long service life.

[0017] Beneficial effects of this application:

[0018] The present application provides an electrolyte, a secondary battery containing the electrolyte, and an electronic device. The electrolyte includes lithium difluorophosphate and a first lithium salt, the first lithium salt including at least one of lithium hexafluorophosphate or lithium bis(fluorosulfonyl)imide; based on the mass of the electrolyte, the mass percentage of lithium difluorophosphate is A%, the mass percentage of the first lithium salt is B%, 2≤A≤8.8, 0.1≤A / B≤1; the electrolyte also includes a first solvent, the number of donors of the first solvent is greater than 10, or the dielectric constant of the first solvent is greater than 5, and the mass percentage of the first solvent is C%, 5≤C≤60 based on the mass of the electrolyte. In the electrolyte provided by the present application, lithium difluorophosphate has a high solubility. By rationally matching lithium difluorophosphate and the first lithium salt, the synergistic effect of the two can be brought into play, which is conducive to the formation of a stable SEI film and CEI film, thereby improving the cycle performance and high-temperature storage performance of the secondary battery. DETAILED DESCRIPTION

[0019] To make the purpose, technical solutions, and advantages of this application more clearly understood, the following examples are given to further describe this application in detail. Obviously, the described examples are only some examples of this application, rather than all examples. All other examples obtained by those skilled in the art based on this application are within the scope of protection of this application.

[0020] It should be noted that in the following description, the present application is explained using a lithium-ion battery as an example of a secondary battery, but the secondary battery of the present application is not limited to a lithium-ion battery.

[0021] A first aspect of the present application provides an electrolyte comprising lithium difluorophosphate (LiPO2F2) and a first lithium salt, the first lithium salt comprising at least one of lithium hexafluorophosphate (LiPF6) or lithium bis(fluorosulfonyl)imide (LiFSI); based on the mass of the electrolyte, the mass percentage of lithium difluorophosphate is A%, the mass percentage of the first lithium salt is B%, 2≤A≤8.8, 0.1≤A / B≤1, preferably 2.2≤A≤7, 0.2≤A / B≤0.8, for example, the value of A can be 2, 2.2, 2.8, 3, 3.5, 4, 4.6, 5, 5.7, 6, 6.4, 7, 7.5, 8, 8.8 or a range consisting of any two values ​​therein, and the value of A / B can be 0.1, 0.15, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1 or a range consisting of any two values ​​therein. The electrolyte further includes a first solvent, wherein the donor number (DN) of the first solvent is greater than 10, or the dielectric constant (ε) of the first solvent is greater than 5, and the mass percentage of the first solvent based on the mass of the electrolyte is C%, 5≤C≤60. In some embodiments, the donor number DN of the first solvent may be 10 to 50, and the dielectric constant ε of the first solvent may be 5 to 100. 7, 8, 9, 10, 12, 15, 17, 20, 30, 40, 50, 60, 70, 75, 80, 85, 90, 91, 95, 100, or a range consisting of any two values ​​therein; and the value of C can be 5, 8, 10, 14, 17, 18, 20, 23, 25, 26, 30, 33, 35, 37, 40, 44, 45, 47, 50, 53, 55, 58, 60, or a range consisting of any two values ​​therein.

[0022] The inventors found that when the value of A is too small, for example, less than 2, the content of lithium difluorophosphate in the electrolyte is too low, which is not conducive to the formation of stable SEI film and CEI film, and thus cannot improve the cycle performance and high-temperature storage performance of the secondary battery; when the value of A is too large, for example, greater than 8.8, the content of lithium difluorophosphate in the electrolyte is too high, the requirements for the solvent are too high or lithium difluorophosphate cannot be dissolved, and the dissociation degree of lithium difluorophosphate in the solution is very low. A higher concentration of lithium difluorophosphate will significantly increase the viscosity of the electrolyte and affect the transport of ions in the electrolyte, thereby affecting the cycle performance of the secondary battery. When the value of A / B is too small, for example, less than 0.1, the combination of lithium difluorophosphate and the first lithium salt is unreasonable, which is not conducive to the synergistic effect between the two. The stability of the SEI film and CEI film formed at the interface of the positive electrode and the negative electrode is poor. At the same time, the lithium ions dissociated from the first lithium salt will be solvated with the solvent in the electrolyte. In this way, during the migration of lithium ions between the positive electrode and the negative electrode, more solvent molecules will reach the reaction interface, and the interface side reactions between the positive electrode and the negative electrode will increase, thereby affecting the cycle performance and high-temperature storage performance of the secondary battery; when the value of A / B is too large, for example, greater than 1, the combination of lithium difluorophosphate and the first lithium salt is unreasonable, which is not conducive to the synergistic effect between the two. Excessively high concentrations of lithium difluorophosphate itself cannot provide conductive lithium ions, and because its concentration is too high, the viscosity of the electrolyte will be too high, the migration of lithium ions will be hindered, and the conductivity of the electrolyte will be further reduced, which is not conducive to improving the cycle performance of the secondary battery. When the value of C is too small, for example, less than 5, it is not conducive to the dissolution of lithium difluorophosphate in the electrolyte, that is, the solubility of lithium difluorophosphate in the electrolyte is low, and a stable SEI film and CEI film cannot be formed, thereby failing to improve the cycle performance and high-temperature storage performance of the secondary battery. For an electrolyte with a higher content of lithium difluorophosphate, when the content of the first solvent is too low, lithium difluorophosphate may not even be completely dissolved, and the resulting electrolyte cannot be used in a secondary battery. When the value of C is too large, for example, greater than 60, due to the narrow electrochemical window of the first solvent itself, when its content is too high, it will lead to an increase in the interface side reaction between the positive and negative electrodes, which is not conducive to improving the cycle performance and high-temperature storage performance of the secondary battery. When the donor number and dielectric constant of the first solvent are both small, for example, DN < 10 and ε < 5, the solubility of lithium difluorophosphate in the first solvent is low, which is not conducive to the role of lithium difluorophosphate and the synergistic effect of lithium difluorophosphate and the first lithium salt, thereby failing to improve the cycle performance and high-temperature storage performance of the secondary battery. Therefore, in the electrolyte provided by the present application, lithium difluorophosphate has a high solubility. By reasonably matching lithium difluorophosphate with the first lithium salt, the synergistic effect of the two can be exerted, which is conducive to the formation of stable SEI film and CEI film. At the same time, lithium difluorophosphate has a high solubility in the above-mentioned first solvent, which can better exert the role of lithium difluorophosphate and improve the cycle performance and high-temperature storage performance of the secondary battery.Therefore, the electrolyte includes lithium difluorophosphate and the first lithium salt and the first solvent, and the values ​​of A, A / B and C are adjusted within the above ranges, which can improve the cycle performance and high-temperature storage performance of the secondary battery.

[0023] In some embodiments of the present application, 6.25≤B≤17. For example, the value of B can be 6.25, 8, 9, 10, 12, 14, 15, 16, 17, or a range consisting of any two of these values. By regulating the value of B within the above range, the electrolyte can have a suitable viscosity and a high conductivity, thereby increasing the ion migration rate, thereby further improving the cycle performance of the secondary battery.

[0024] In some embodiments of the present application, 3≤C / A≤25, for example, the value of C / A can be 3, 4, 5, 6, 8, 9, 10, 12, 14, 15, 17, 20, 21, 23, 25, or a range consisting of any two of these values. Lithium difluorophosphate has a high solubility in the first solvent, which can better play the role of lithium difluorophosphate. On the other hand, since the electrochemical window of the first solvent itself is relatively narrow, by reasonably matching the content of the first solvent and lithium difluorophosphate, that is, regulating the C / A value within the above range, the decomposition of the first solvent during the cycle or at high temperature can be reduced, which is conducive to further improving the cycle performance and high-temperature storage performance of the secondary battery.

[0025] In some embodiments of the present application, the first solvent includes at least one of ethylene carbonate, ethyl acetate, ethylene glycol dimethyl ether, trimethyl phosphate, triethyl phosphate, tripropyl phosphate, tributyl phosphate, sulfolane, dimethyl sulfoxide, N,N-dimethylformamide, N,N-dimethylacetamide, N-methylacetamide, tetramethylurea, or γ-butyrolactone. Lithium difluorophosphate has a high solubility in the above-mentioned first solvent, which can better exert the function of lithium difluorophosphate. Therefore, selecting the first solvent within the above-mentioned range is conducive to better improving the cycle performance and high-temperature storage performance of the secondary battery.

[0026] In some embodiments of the present application, the electrolyte further includes a first additive, the first additive including at least one of 1,3-propane sultone (1,3-PS), 1,3-propylene sultone, vinyl sulfate (VC), 1,3-propylene glycol cyclic sulfate, 2,4-butane sultone (2,4-BS), 1,4-butane sultone (1,4-BS), vinylene carbonate or fluoroethylene carbonate (FEC), based on the mass of the electrolyte, the mass percentage of the first additive is X1%, 0.05≤X1≤12, for example, the value of X1 can be 0.05, 0.1, 0.3, 0.8, 1, 3, 5, 6, 8, 10, 12 or a range consisting of any two values ​​thereof. On the basis that the electrolyte includes lithium difluorophosphate, the above-mentioned first additive is further introduced, and the value of X1 is regulated within the above-mentioned range, so that the synergistic effect of lithium difluorophosphate and the above-mentioned first additive can be exerted, and organic fluorine-containing phosphates (such as Li) can be generated on the surfaces of the positive and negative electrodes. x PFy, Li x PF y O z ) and inorganic LiF and other components are beneficial to increase the organic-inorganic hybrid interface in the SEI film and the CEI film, further enhancing the stability of the SEI film and the CEI film, thereby further improving the cycle performance and high-temperature storage performance of the secondary battery.

[0027] In some embodiments of the present application, 0.3≤A / X1≤120, for example, the value of A / X1 can be 0.3, 0.5, 0.8, 1, 1.25, 1.5, 3, 5, 8, 10, 12, 20, 40, 50, 75, 90, 100, 105, 120, or a range consisting of any two of these values. By regulating A / X1 within the above range, the synergistic effect of lithium difluorophosphate and the above-mentioned first additive can be better exerted, further enhancing the stability of the SEI film and the CEI film, thereby enabling the secondary battery to have good cycle performance and high-temperature storage performance.

[0028] In some embodiments of the present application, the electrolyte further includes a second additive, the second additive including at least one of lithium bis(oxalatoborate) (LiBOB), lithium tetrafluoroborate (LiBF4), lithium difluorooxalatoborate (LiDFOB), lithium trifluoromethanesulfonate (LiCF3SO3), 4,5-dicyano-2-trifluoromethyl-imidazole lithium (LITDI) or lithium tetraborate (Li2B4O7). Based on the mass of the electrolyte, the mass percentage of the second additive is X2%, 0.1≤X2≤2, for example, the value of X2 can be 0.1, 0.3, 0.5, 0.8, 1, 1.2, 1.4, 1.5, 1.6, 1.8, 2 or a range consisting of any two values ​​therein. On the basis of the electrolyte including lithium difluorophosphate, the above-mentioned second additive is further introduced, and the value of X2 is controlled within the above-mentioned range, so that the synergistic effect of lithium difluorophosphate and the above-mentioned second additive can be brought into play, the stability of the SEI film and the CEI film can be further enhanced, and the interfacial side reactions on the surfaces of the positive electrode and the negative electrode can be reduced, thereby further improving the cycle performance and high-temperature storage performance of the secondary battery.

[0029] In some embodiments of the present application, 2≤A / X2≤44, for example, the value of A / X2 can be 2, 3, 5, 8, 10, 12, 18, 20, 24, 25, 28, 30, 33, 35, 37, 40, 44, or a range consisting of any two of these values. By regulating A / X2 within the above range, the synergistic effect of lithium difluorophosphate and the above-mentioned second additive can be better exerted, further enhancing the stability of the SEI film and the CEI film, thereby enabling the secondary battery to have good cycle performance and high-temperature storage performance.

[0030] The electrolyte of the present application may further include a second solvent. The present application has no particular restrictions on the type of the above-mentioned second solvent, as long as the purpose of the present application can be achieved. For example, it may include but is not limited to at least one of dimethyl carbonate, diethyl carbonate (DEC), dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate or methyl ethyl carbonate. The present application has no particular restrictions on the mass percentage of the second solvent, as long as the purpose of the present application can be achieved. For example, based on the mass of the electrolyte, the mass percentage of the second solvent can be 0.2% to 86.75%. For example, the mass percentage of the second solvent can be 0.2%, 2.2%, 5%, 8%, 10%, 12.2%, 14.2%, 17%, 20%, 23%, 26%, 30%, 33%, 35%, 37%, 40%, 43%, 45%, 47%, 50%, 53%, 55%, 58%, 60%, 62%, 66%, 69%, 70%, 72%, 75%, 78%, 80%, 82%, 83%, 85%, 86.6%, 86.75% or a range consisting of any two of the values.

[0031] In some embodiments, the electrolyte includes lithium difluorophosphate, a first lithium salt, a first solvent, and a second solvent, wherein the weight percentages of lithium difluorophosphate, the first lithium salt, and the first solvent are as described above, and the weight percentage of the second solvent is 14.2% to 86.75%. Applying the electrolyte having the above characteristics to a secondary battery can improve the cycling performance and high-temperature storage performance of the secondary battery.

[0032] In some embodiments, the electrolyte includes lithium difluorophosphate, a first lithium salt, a first solvent, a first additive, and a second solvent. The weight percentages of lithium difluorophosphate, the first lithium salt, the first solvent, and the first additive are as described above, and the weight percentage of the second solvent is 2.2% to 86.7%. Application of the electrolyte having the above characteristics to a secondary battery is beneficial for further improving the cycling performance and high-temperature storage performance of the secondary battery.

[0033] In some embodiments, the electrolyte includes lithium difluorophosphate, a first lithium salt, a first solvent, a second additive, and a second solvent. The weight percentages of lithium difluorophosphate, the first lithium salt, the first solvent, and the second additive are as described above, and the weight percentage of the second solvent is 12.2% to 86.65%. Applying the electrolyte having the above characteristics to a secondary battery can further improve the cycling performance and high-temperature storage performance of the secondary battery.

[0034] In some embodiments, the electrolyte includes lithium difluorophosphate, a first lithium salt, a first solvent, a first additive, a second additive, and a second solvent, wherein the weight percentages of lithium difluorophosphate, the first lithium salt, the first solvent, the first additive, and the second additive are as described above, and the weight percentage of the second solvent is 0.2% to 86.6%. Application of the electrolyte having the above characteristics to a secondary battery is beneficial for further improving the cycling performance and high-temperature storage performance of the secondary battery.

[0035] The second aspect of the present application provides a secondary battery, which includes the electrolyte provided by the first aspect of the present application. The secondary battery provided by the present application has good cycle performance and high-temperature storage performance.

[0036] The secondary battery of the present application also includes a positive electrode sheet. The present application has no special restrictions on the positive electrode sheet, as long as the purpose of the present application can be achieved. For example, the positive electrode sheet includes a positive electrode collector and a positive electrode material layer provided on at least one surface of the positive electrode collector. In the present application, the positive electrode material layer can be provided on one surface in the thickness direction of the positive electrode collector, or on two surfaces in the thickness direction of the positive electrode collector. It should be noted that the "surface" here can be the entire area of ​​the surface of the positive electrode collector, or it can be a partial area of ​​the surface of the positive electrode collector. The present application has no special restrictions, as long as the purpose of the present application can be achieved.

[0037] The secondary battery of the present application also includes a negative electrode plate. The present application has no special restrictions on the negative electrode plate, as long as the purpose of the present application can be achieved. For example, the negative electrode plate includes a negative electrode collector and a negative electrode material layer provided on at least one surface of the negative electrode collector. In the present application, the negative electrode material layer can be provided on one surface in the thickness direction of the negative electrode collector, or on two surfaces in the thickness direction of the negative electrode collector. It should be noted that the "surface" here can be the entire area of ​​the negative electrode collector or a partial area of ​​the negative electrode collector. The present application has no special restrictions, as long as the purpose of the present application can be achieved. The negative electrode material layer of the present application includes a negative electrode active material. The present application has no special restrictions on the type of negative electrode active material, as long as the purpose of the present application can be achieved. For example, the negative electrode active material can include natural graphite, artificial graphite, mesophase microcarbon beads (MCMB), hard carbon, soft carbon, silicon, silicon-carbon composites, SiO x (0<x≤2), Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, SnO2, spinel structure lithium titanate Li4Ti5O 12 , Li-Al alloy, or metallic lithium. In the present application, there is no particular restriction on the thickness of the negative electrode current collector and the negative electrode material layer, as long as the purpose of the present application can be achieved. For example, the thickness of the negative electrode current collector is 4μm to 12μm, and the thickness of the single-sided negative electrode material layer is 30μm to 160μm. In the present application, there is no particular restriction on the mass ratio of the negative electrode active material, negative electrode conductive agent, and negative electrode binder in the negative electrode material layer, as long as the purpose of the present application can be achieved.

[0038] The negative electrode material layer of the present application may also include a negative electrode conductive agent, a negative electrode binder and a negative electrode dispersant. The present application has no particular restrictions on the negative electrode conductive agent and the negative electrode binder, as long as the purpose of the present application can be achieved. For example, the negative electrode conductive agent may include but is not limited to at least one of conductive carbon black (Super P), carbon nanotubes (CNTs), graphite, carbon fiber, carbon nanowire, graphene, a metal material or a conductive polymer, and the above-mentioned carbon nanotubes may include but are not limited to single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The above-mentioned carbon fiber may include but is not limited to vapor-grown carbon fiber (VGCF) and / or nano-carbon fiber. The above-mentioned metal material may include but is not limited to metal powder and / or metal fiber, specifically, the metal may include but is not limited to at least one of copper, nickel, aluminum or silver. The above-mentioned conductive polymer may include but is not limited to at least one of polyphenylene derivatives, polyaniline, polythiophene, polyacetylene or polypyrrole. The negative electrode binder may include, but is not limited to, at least one of polyacrylic acid, sodium polyacrylate, potassium polyacrylate, lithium polyacrylate, polyimide, polyvinyl alcohol, carboxymethyl cellulose, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, polyamide-imide, styrene-butadiene rubber, or polyvinylidene fluoride (PVDF). The negative electrode dispersant may include sodium carboxymethyl cellulose.

[0039] In the present application, there is no particular limitation on the method for preparing the negative electrode sheet, as long as the purpose of the present application can be achieved. For example, it can be prepared by the following method: a negative electrode active material, a negative electrode binder, and a negative electrode dispersant are mixed, deionized water is added and stirred evenly, and a negative electrode slurry with a solid content of 50wt% to 75wt% is obtained. The negative electrode slurry is evenly coated on one surface of the negative electrode current collector, and after drying, a negative electrode sheet coated with a negative electrode material layer on one side is obtained. Then, the above coating steps are repeated on the other surface of the negative electrode current collector, and after drying, a negative electrode sheet coated with a negative electrode material layer on both sides is obtained. After coating is completed, the negative electrode sheet is obtained by cold pressing and cutting.

[0040] The present application has no particular restrictions on the positive electrode current collector, as long as the purpose of the present application can be achieved. For example, the positive electrode current collector may include aluminum foil, aluminum alloy foil or a composite current collector (such as an aluminum-carbon composite current collector), etc. The present application has no particular restrictions on the thickness of the positive electrode current collector and the positive electrode material layer, as long as the purpose of the present application can be achieved. For example, the thickness of the positive electrode current collector is 5μm to 20μm, and the thickness of the single-sided positive electrode material layer is 30μm to 120μm. In the present application, the positive electrode material layer includes a positive electrode active material, and the present application has no particular restrictions on the type of positive electrode active material, as long as the purpose of the present application can be achieved. The positive electrode active material includes a compound that reversibly embeds and deintercalates lithium ions. In some embodiments, the positive electrode material layer includes a positive electrode active material with a working potential of 4.5V or more relative to metallic lithium. That is, the positive electrode active material of the present application can work under high voltage. In some embodiments, the positive electrode active material may include at least one of lithium nickel cobalt manganese oxide (NCM), lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium-rich manganese-based materials, lithium cobalt oxide, lithium manganese oxide, or lithium iron manganese phosphate. Lithium nickel cobalt manganese oxide may include but is not limited to LiNi 0.8 Co 0.1 Mn 0.1 O2(NCM811), LiNi 0.6 Co 0.2 Mn 0.2 O2(NCM622), LiNi 0.5 Co 0.2 Mn 0.3 O2(NCM523), LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2(NCM333) or LiNi 0.9 Co 0.05 Mn 0.05At least one of O2 (NCM955). The above-mentioned positive electrode active material may be subjected to a doping treatment. In some embodiments, the element used for doping may include at least one of K, Na, Ca, Mg, B, Al, Co, Si, V, Ga, Sn or Zr. The positive electrode material layer of the present application may also include a positive electrode conductor and a positive electrode binder. The present application has no particular restrictions on the positive electrode conductor and the positive electrode binder, as long as the purpose of the present application can be achieved. For example, the positive electrode conductor may be the same as the above-mentioned negative electrode conductor, and the positive electrode binder may be the same as the above-mentioned negative electrode binder. The present application has no particular restrictions on the mass ratio of the positive electrode active material, the positive electrode conductor and the positive electrode binder in the positive electrode material layer, as long as the purpose of the present application can be achieved.

[0041] In the present application, there is no particular limitation on the preparation method of the positive electrode sheet, as long as the purpose of the present application can be achieved. For example, it can be prepared by the following method: the positive electrode active material, the positive electrode conductive agent, and the positive electrode binder are mixed, N-methylpyrrolidone (NMP) is added and stirred evenly to obtain a positive electrode slurry with a solid content of 65wt% to 85wt%. The positive electrode slurry is evenly coated on one surface of the positive electrode collector, and after drying, a positive electrode sheet coated with a positive electrode material layer on one side is obtained. Then, the above coating steps are repeated on the other surface of the positive electrode collector, and after drying, a positive electrode sheet coated with a positive electrode material layer on both sides is obtained. After coating is completed, the positive electrode sheet is obtained by cold pressing and cutting.

[0042] The secondary battery of the present application also includes a separator. The present application has no particular restrictions on the separator, as long as it can achieve the purpose of the present application. For example, the material of the separator may include but is not limited to polyethylene (PE), polypropylene (PP), polytetrafluoroethylene-based polyolefin (PO) separators, polyester films (such as polyethylene terephthalate (PET) films), cellulose films, polyimide films (PI), polyamide films (PA), spandex or aramid films, etc. At least one of the types of separators may include but is not limited to woven membranes, non-woven membranes (non-woven fabrics), microporous membranes, composite membranes, rolled membranes or spun membranes, etc. The separator of the present application may have a porous structure, and the porous layer is provided on at least one surface of the separator, and the porous layer includes inorganic particles and a binder. The inorganic particles may include at least one of aluminum oxide, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide or barium sulfate. The binder may include at least one of polyvinylidene fluoride, a copolymer of vinylidene fluoride and hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, sodium carboxymethyl cellulose, polyvinyl pyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, or polyhexafluoropropylene. The present application does not particularly limit the size of the pores of the porous structure, as long as the purpose of the present application can be achieved. For example, the pore size can be 0.01 μm to 1 μm. In the present application, the thickness of the isolation membrane is not particularly limited, as long as the purpose of the present application can be achieved. For example, the thickness of the isolation membrane can be 5 μm to 50 μm.

[0043] This application does not specifically limit the type of secondary battery, which may include any device that generates an electrochemical reaction. For example, secondary batteries may include, but are not limited to, lithium metal secondary batteries, lithium ion batteries, sodium ion batteries, lithium polymer secondary batteries, and lithium ion polymer secondary batteries. This application does not specifically limit the shape of the secondary battery, as long as it can achieve the purpose of this application. The secondary battery of this application also includes a packaging bag. This application does not specifically limit the packaging bag, as long as it can achieve the purpose of this application. For example, an aluminum-plastic film packaging bag can be used.

[0044] The preparation process of a secondary battery is well known to those skilled in the art and is not particularly limited in this application. For example, it may include but is not limited to the following steps: stacking the positive electrode sheet, the separator and the negative electrode sheet in order, welding the tabs, and then winding, folding and other operations as needed to obtain an electrode assembly with a wound structure, placing the electrode assembly in a packaging bag, injecting the electrolyte into the packaging bag and sealing it to obtain a secondary battery; or stacking the positive electrode sheet, the separator and the negative electrode sheet in order, welding the tabs, and then fixing the four corners of the entire stacked structure with tape to obtain an electrode assembly with a stacked structure, placing the electrode assembly in a packaging bag, injecting the electrolyte into the packaging bag and sealing it to obtain a secondary battery. In addition, as needed, overcurrent protection elements, guide plates, etc. may be placed in the packaging bag to prevent pressure rise and overcharge and discharge inside the secondary battery.

[0045] The third aspect of the present application provides an electronic device, which includes the secondary battery provided in the second aspect of the present application. The secondary battery provided in the present application has good cycle performance and high-temperature storage performance, so that the electronic device of the present application has a long service life.

[0046] The present application does not particularly limit the type of electronic device, and the electronic device may be any electronic device known in the art. In some embodiments, the electronic device may include, but is not limited to, a laptop computer, a pen-type computer, a mobile computer, an electronic book player, a portable phone, a portable fax machine, a portable copier, a portable printer, a headset, a video recorder, an LCD television, a portable cleaner, a portable CD player, a mini-disc, a transceiver, an electronic notepad, a calculator, a memory card, a portable recorder, a radio, a backup power supply, a motor, an automobile, a motorcycle, a power-assisted bicycle, a bicycle, a lighting fixture, a toy, a game console, a clock, a power tool, a flashlight, a camera, a large household battery, and a lithium-ion capacitor.

[0047] Example

[0048] The following examples and comparative examples are provided to more specifically illustrate the embodiments of the present invention. Various tests and evaluations were performed according to the following methods. In addition, unless otherwise specified, "parts" and "%" are based on mass.

[0049] Test methods and equipment:

[0050] Test of the content of each component in the electrolyte:

[0051] The lithium-ion battery was discharged at a constant current of 1C to 2.5V and then disassembled to collect the electrolyte. The positive electrode sheet, negative electrode sheet, and separator were centrifuged, and the liquid obtained after centrifugation was mixed with the above electrolyte. Then, the components in the electrolyte were obtained and their contents were tested using a gas chromatography-mass spectrometer (instrument model: Agilent 8890) and an ion chromatography (instrument model: AQUION ion chromatography).

[0052] Cyclic performance test:

[0053] The cycle performance of lithium-ion batteries under high voltage conditions is evaluated by the capacity retention rate of lithium-ion batteries. The higher the capacity retention rate, the better the cycle performance of the lithium-ion battery. The test process is: place the lithium-ion battery in a 25°C constant temperature box, let it stand for 5 minutes, then charge the lithium-ion battery at a constant current of 1C to a voltage of 4.28V, then charge at a constant voltage of 4.28V to a current of 0.05C, and then discharge at a constant current of 1C to 2.5V. This is a charge and discharge cycle. Record the first discharge capacity as Q1, repeat the charge and discharge cycle 800 times, stop the test, and record the 800th discharge capacity as Q2. The 25°C cycle capacity retention rate (%) of the lithium-ion battery = Q2 / Q1×100%.

[0054] High temperature storage performance test:

[0055] The high-temperature storage performance of lithium-ion batteries is evaluated by measuring the thickness expansion rate of lithium-ion batteries when stored at 60°C. The smaller the thickness expansion rate, the better the high-temperature storage performance of the lithium-ion battery. The test process is as follows: In a 25°C environment, the lithium-ion battery is charged to 4.28V at a constant current of 0.5C, and then charged to a current of 0.05C at a constant voltage of 4.28V. The thickness of the lithium-ion battery is tested to be D1. The lithium-ion battery is then placed in a 60°C oven and taken out after storage for 180 days. The thickness of the lithium-ion battery at this time is tested to be D2. The thickness expansion rate of the lithium-ion battery stored at 60°C = (D2-D1) / D1×100%.

[0056] 50% State of Charge (SOC) DC Resistance (DCR) Test:

[0057] At 25°C, discharge the lithium-ion battery to 2.5V at a current of 0.5C, let it stand for 5 minutes, then charge it to 4.28V at a current of 0.5C, and then maintain a constant voltage of 0.025C at 4.28V. Let it stand for 5 minutes, discharge it to 2.5V using a current of 0.1C, and mark the discharge capacity at this time as C1. Charge it to 4.28V using a capacity of 0.5C1, maintain a constant voltage of 0.025C1 at a voltage of 4.28V, let it stand for 5 minutes, and discharge it for 5 hours using a current of 0.1C1. At this time, the voltage of the lithium-ion battery is recorded as V1. Then, discharge it for 1 second using a current of 1C. At this time, the voltage of the lithium-ion battery is recorded as V2. 50% SOC DCR = (V1-V2) / (1C-0.1C1).

[0058] Example 1-1

[0059] <Preparation of Electrolyte>

[0060] In an argon atmosphere glove box with a water content of <10 ppm, a first solvent, ethyl acetate, and a second solvent, diethyl carbonate, were uniformly mixed to obtain an organic solvent. Lithium difluorophosphate and a first lithium salt, lithium hexafluorophosphate (LiPF6), were then sequentially added to the organic solvent, dissolved, and uniformly mixed to obtain an electrolyte. The weight percentages of lithium difluorophosphate (A%), the first lithium salt (B%), and the first solvent (C%), based on the mass of the electrolyte, are as shown in Table 1, with the remainder being the second solvent, diethyl carbonate.

[0061] <Preparation of positive electrode sheet>

[0062] The positive electrode active material LiCoO2, the positive electrode conductive agent conductive carbon black, and the positive electrode binder polyvinylidene fluoride are mixed in a mass ratio of 97:1.4:1.6, and N-methylpyrrolidone is added and stirred evenly to obtain a positive electrode slurry with a solid content of 72wt%. The positive electrode slurry is evenly coated on one surface of a positive electrode current collector aluminum foil with a thickness of 10μm, and the aluminum foil is dried at 85°C for 4h to obtain a positive electrode sheet coated with a positive electrode material layer on one side. Repeat the above steps on the other surface of the aluminum foil to obtain a positive electrode sheet coated with a positive electrode material layer on both sides. Then, after drying under vacuum conditions at 85°C for 4h, a positive electrode sheet with a specification of 74mm×867mm is obtained by cold pressing, cutting, and slitting. Among them, the compaction density of the positive electrode material layer is 4.15g / cm 3 The thickness of the single-sided positive electrode material layer is 60μm.

[0063] <Preparation of negative electrode sheet>

[0064] The negative electrode active material graphite, the negative electrode binder styrene-butadiene rubber, and the negative electrode thickener sodium carboxymethyl cellulose are mixed in a mass ratio of 97.4:1.4:1.2, and deionized water is added and stirred evenly to obtain a negative electrode slurry with a solid content of 54wt%. The negative electrode slurry is evenly coated on one surface of a negative electrode current collector copper foil with a thickness of 6μm, and the copper foil is dried at 85°C for 4h to obtain a negative electrode sheet coated with a negative electrode material layer on one side. Repeat the above steps on the other surface of the copper foil to obtain a negative electrode sheet coated with a negative electrode material layer on both sides. Then, after drying at 85°C under vacuum conditions for 4h, a negative electrode sheet with a specification of 78mm×875mm is obtained by cold pressing, cutting, and slitting. Among them, the compaction density of the positive electrode material layer is 1.75g / cm 3 The thickness of the single-sided positive electrode material layer is 70μm.

[0065] <Preparation of Separator>

[0066] PVDF and alumina ceramic were mixed in a 9:1 mass ratio, and deionized water was added as a solvent to prepare a ceramic layer slurry with a solid content of 25wt%. The mixture was stirred evenly and then evenly coated on one surface of a 16μm thick polyethylene porous film substrate. After drying, a separator membrane with a 2μm alumina ceramic layer coated on one side was obtained. The above coating steps were then repeated on the other surface of the substrate to obtain a separator membrane with a 2μm alumina ceramic layer coated on both sides.

[0067] <Preparation of lithium-ion batteries>

[0068] The prepared positive electrode sheet, separator, and negative electrode sheet are stacked in order, with the separator placed between the positive and negative electrode sheets to act as an insulator, and then wound to form an electrode assembly. After welding the tabs, the electrode assembly is placed in an aluminum-plastic film packaging bag, placed in an 85°C vacuum oven to dry for 12 hours to remove moisture, and then injected with the prepared electrolyte. After vacuum packaging, standing, formation (0.02C constant current charging to 3.5V, then 0.1C constant current charging to 3.9V), shaping, capacity testing, and secondary packaging, a lithium-ion battery is obtained.

[0069] Example 1-2 to Example 1-33

[0070] The same procedures as in Example 1-1 were followed except for adjusting the parameters according to Table 1. When the mass percentage A% of lithium difluorophosphate, the mass percentage B% of the first lithium salt, or the mass percentage C% of the first solvent were changed according to Table 1, the mass percentage of the second solvent was also changed accordingly.

[0071] Example 2-1 to Example 2-15

[0072] The same procedures as in Example 1-22 were employed, except that the first additive was introduced into the electrolyte and the relevant parameters were adjusted according to Table 2. When the mass percentage of lithium difluorophosphate (A%), the mass percentage of the first lithium salt (B%), the mass percentage of the first solvent (C%), or the mass percentage of the first additive (×1%) was varied according to Table 2, the mass percentage of the second solvent was also varied accordingly.

[0073] Example 3-1 to Example 3-3

[0074] The process was identical to Example 1-22, except that the second additive was introduced into the electrolyte and the relevant parameters were adjusted according to Table 3. When the weight percentage of the second additive was changed by ×2% according to Table 3, the weight percentage A% of lithium difluorophosphate, the weight percentage C% of the first solvent, and the weight percentage B% of the first lithium salt remained unchanged, while the weight percentage of the second solvent changed accordingly.

[0075] Example 3-4 to Example 3-14

[0076] The process was identical to Example 2-11, except for introducing the second additive into the electrolyte and adjusting the relevant parameters according to Table 3. When the mass percentage of lithium difluorophosphate (A%) or the mass percentage of the second additive (X2%) was varied according to Table 3, the mass percentage of the first solvent (C%) and the mass percentage of the first lithium salt (B%) remained unchanged, while the mass percentage of the second solvent varied accordingly.

[0077] Comparative Examples 1 to 8

[0078] The same procedures as in Example 1-1 were used except that the parameters were adjusted according to Table 1. When the mass percentage A% of lithium difluorophosphate, the mass percentage B% of the first lithium salt, or the mass percentage C% of the first solvent were changed according to Table 1, the mass percentage of the second solvent was changed accordingly.

[0079] The preparation parameters and performance parameters of each embodiment and comparative example are shown in Tables 1 to 3.

[0080] Table 1

[0081] Note: “-” in Table 1 indicates that the performance data of lithium-ion batteries cannot be measured.

[0082] It can be seen from Examples 1-1 to 1-33 and Comparative Examples 1 to 8 that the electrolyte includes lithium difluorophosphate and the first lithium salt and the first solvent of the present application, and regulating the values ​​of A, A / B and C within the scope of the present application, the lithium ion battery can have a lower impedance, a higher cycle capacity retention rate and a lower storage thickness expansion rate, indicating that the lithium ion battery has a lower impedance, better cycle performance and high temperature storage performance. In the lithium ion batteries of Comparative Examples 1 to 7, at least one of A, A / B or C is not within the scope of the present application. In the lithium ion battery of Comparative Example 8, phenyl ether is used as a solvent to replace the first solvent ethyl acetate. The lithium ion batteries of Comparative Examples 1 to 6 have a lower cycle capacity retention rate and a higher storage thickness expansion rate, indicating that the cycle performance and high temperature storage performance of the lithium ion battery are poor. In Comparative Examples 7 and 8, lithium difluorophosphate is not completely dissolved, and the performance data of the lithium ion battery cannot be measured.

[0083] From Examples 1-1 to 1-7, Comparative Examples 1 and 2, it can be seen that when the value of A is too small, such as in Comparative Example 1, the lithium-ion battery's cycle capacity retention rate is lower and the storage thickness expansion rate is higher; when the value of A is too large, such as in Comparative Example 2, the lithium-ion battery's cycle capacity retention rate is lower and the storage thickness expansion rate is higher, indicating that the lithium-ion battery's cycle performance and high-temperature storage performance are poor. Therefore, by adjusting the value of A within the range of this application, the lithium-ion battery can have a higher cycle capacity retention rate and a lower storage thickness expansion rate, indicating that the lithium-ion battery has better cycle performance and high-temperature storage performance.

[0084] From Examples 1-1 to 1-15 and Comparative Examples 3 to 5, it can be seen that when the value of A / B is too small, such as in Comparative Example 3, the lithium-ion battery's cycle capacity retention rate is lower and the storage thickness expansion rate is higher; when the value of A / B is too large, such as in Comparative Examples 4 and 5, the lithium-ion battery's cycle capacity retention rate is lower and the storage thickness expansion rate is higher, indicating that the lithium-ion battery's cycle performance and high-temperature storage performance are poor. Therefore, by adjusting the value of A / B within the range of this application, the lithium-ion battery can have a higher cycle capacity retention rate and a lower storage thickness expansion rate, indicating that the lithium-ion battery has better cycle performance and high-temperature storage performance.

[0085] From Examples 1-3, 1-20 to 1-23, and Comparative Examples 6 to 7, it can be seen that when the value of C is too small, such as in Comparative Example 7, the lithium difluorophosphate is not completely dissolved, and the performance data of the lithium-ion battery cannot be measured; when the value of C is too large, such as in Comparative Example 6, the cycle capacity retention rate of the lithium-ion battery is lower and the storage thickness expansion rate is higher, indicating that the cycle performance and high-temperature storage performance of the lithium-ion battery are poor. Therefore, by adjusting the value of C within the range of this application, the lithium-ion battery can have a higher cycle capacity retention rate and a lower storage thickness expansion rate, indicating that the lithium-ion battery has better cycle performance and high-temperature storage performance.

[0086] As can be seen from Examples 1-1, 1-27, 1-30, 1-31 to 1-33, and Comparative Example 8, when the number of donors and the dielectric constant of the first solvent are outside the ranges of this application, the lithium difluorophosphate is not completely dissolved, and the performance data of the lithium-ion battery cannot be measured. However, using a first solvent with a number of donors or a dielectric constant within the ranges of this application can result in a lithium-ion battery with a higher cycle capacity retention rate and a lower storage thickness expansion rate, indicating that the lithium-ion battery has better cycle performance and high-temperature storage performance.

[0087] The value of B generally affects the cycling performance and high-temperature storage performance of lithium-ion batteries. As can be seen from Examples 1-4, 1-13, and 1-15, regulating the value of B within the scope of this application can result in lithium-ion batteries with lower impedance, higher cycling capacity retention, and lower storage thickness expansion, indicating that lithium-ion batteries have lower impedance, good cycling performance, and high-temperature storage performance.

[0088] The C / A value generally affects the cycling performance and high-temperature storage performance of lithium-ion batteries. As can be seen from Examples 1-1 to 1-33, regulating the C / A value within the scope of this application can result in lithium-ion batteries with lower impedance, higher cycle capacity retention, and lower storage thickness expansion, indicating that lithium-ion batteries have lower impedance, good cycling performance, and high-temperature storage performance.

[0089] The type of first solvent generally affects the cycling performance and high-temperature storage performance of lithium-ion batteries. As can be seen from Examples 1-1, 1-27, 1-30, 1-31, and 1-33, selecting a first solvent within the scope of this application can result in lithium-ion batteries having lower impedance, higher cycling capacity retention, and lower storage thickness expansion, indicating that lithium-ion batteries have lower impedance, better cycling performance, and higher-temperature storage performance.

[0090] Table 2

[0091] Note: In Table 2, “ / ” indicates that there is no relevant parameter or the corresponding substance does not exist. The mass percentage of “FEC+1,3-PS” is “2.5+0.5”, which means that the mass percentage of FEC is 2.5%, the mass percentage of 1,3-PS is 0.5%, and the sum of the two is X1%. Other similar expressions can be deduced by analogy.

[0092] The electrolyte includes a first additive and the value of X1 generally affects the cycling performance and high-temperature storage performance of the lithium-ion battery. As can be seen from Examples 1-22 and 2-1 to 2-9, the electrolyte includes lithium difluorophosphate and the first lithium salt and first solvent of the present application, and further introduces the first additive, and regulates the value of X1 within the range of the present application. This can result in the lithium-ion battery having lower impedance, higher cycle capacity retention, and lower storage thickness expansion rate, indicating that the lithium-ion battery has lower impedance, good cycling performance, and high-temperature storage performance.

[0093] The value of A / X1 generally affects the cycling performance and high-temperature storage performance of lithium-ion batteries. As can be seen from Examples 1-22 and 2-1 to 2-15, regulating the value of A / X1 within the scope of this application can result in lithium-ion batteries with lower impedance, higher cycling capacity retention, and lower storage thickness expansion, indicating that lithium-ion batteries have lower impedance, good cycling performance, and high-temperature storage performance.

[0094] The type of first additive typically affects the cycling performance and high-temperature storage performance of lithium-ion batteries. As can be seen from Examples 2-1, 2-3, and 2-4, selecting a first additive within the scope of this application can result in lithium-ion batteries with lower impedance, higher cycling capacity retention, and lower storage thickness expansion, demonstrating that lithium-ion batteries have lower impedance, good cycling performance, and high-temperature storage performance.

[0095] Table 3

[0096] Note: In Table 3, “ / ” indicates that there is no relevant parameter or the corresponding substance does not exist. The mass percentage of “LiBF4+LiDFOB+LiBOB” is “0.2+0.2+0.2”, which means that the mass percentage of LiBF4 is 0.2%, the mass percentage of LiDFOB is 0.2%, and the mass percentage of LiBOB is 0.2%. The sum of the three is X2%. Other similar expressions can be deduced by analogy.

[0097] The electrolyte includes a second additive and the value of X2 usually affects the cycle performance and high temperature storage performance of the lithium-ion battery. From Examples 1-22, 3-1 to 3-3, it can be seen that the electrolyte includes lithium difluorophosphate and the first lithium salt and the first solvent of the present application, and further introduces a second additive, and regulates the value of X2 within the scope of the present application, which can make the lithium-ion battery have a lower impedance, a higher cycle capacity retention rate and a lower storage thickness expansion rate, indicating that the lithium-ion battery has a lower impedance, good cycle performance and high temperature storage performance. From Examples 2-11, 3-4 to 3-11, it can be seen that the electrolyte includes lithium difluorophosphate and the first lithium salt, the first solvent and the first additive of the present application, and further introduces a second additive, and regulates the value of X2 within the scope of the present application, which can make the lithium-ion battery have a lower impedance, a higher cycle capacity retention rate and a lower storage thickness expansion rate, indicating that the lithium-ion battery has a lower impedance, good cycle performance and high temperature storage performance.

[0098] The value of A / X2 generally affects the cycling performance and high-temperature storage performance of lithium-ion batteries. As can be seen from Examples 2-11, 3-4, and 3-14, regulating the value of A / X2 within the scope of this application can result in lithium-ion batteries with lower impedance, higher cycling capacity retention, and lower storage thickness expansion, indicating that lithium-ion batteries have lower impedance, good cycling performance, and high-temperature storage performance.

[0099] The type of second additive typically affects the cycling performance and high-temperature storage performance of lithium-ion batteries. As can be seen from Examples 3-4, 3-9, and 3-10, selecting a second additive within the scope of this application can result in lithium-ion batteries with lower impedance, higher cycle capacity retention, and lower storage thickness expansion, demonstrating that lithium-ion batteries have lower impedance, good cycling performance, and high-temperature storage performance.

[0100] The above description is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present application shall be included in the scope of protection of the present application.

Claims

1. An electrolyte comprising lithium difluorophosphate and a first lithium salt, wherein the first lithium salt comprises at least one of lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide; based on the mass of the electrolyte, the mass percentage of the lithium difluorophosphate is A%, the mass percentage of the first lithium salt is B%, 2≤A≤8.8, and 0.1≤A / B≤1; The electrolyte further includes a first solvent, the number of donors of the first solvent is greater than 10, or the dielectric constant of the first solvent is greater than 5, and based on the mass of the electrolyte, the mass percentage of the first solvent is C%, 5≤C≤60.

2. The electrolyte according to claim 1, wherein 6.25≤B≤17。 3. The electrolyte according to claim 1, wherein 2.2≤A≤7, 0.2≤A / B≤0.

8.

4. The electrolyte according to claim 1, wherein 3≤C / A≤25.

5. The electrolyte according to claim 1, wherein The first solvent includes at least one of ethylene carbonate, ethyl acetate, ethylene glycol dimethyl ether, trimethyl phosphate, triethyl phosphate, tripropyl phosphate, tributyl phosphate, sulfolane, dimethyl sulfoxide, N,N-dimethylformamide, N,N-dimethylacetamide, N-methylacetamide, tetramethylurea or γ-butyrolactone.

6. The electrolyte according to any one of claims 1 to 5, wherein The electrolyte further includes a first additive, the first additive including at least one of 1,3-propane sultone, 1,3-propylene sultone, vinyl sulfate, 1,3-propylene glycol cyclic sulfate, 2,4-butane sultone, 1,4-butane sultone, vinylene carbonate, or fluoroethylene carbonate. Based on the mass of the electrolyte, the mass percentage of the first additive is X1%, 0.05≤X1≤12.

7. The electrolyte according to claim 6, wherein 0.3≤A / X1≤120.

8. The electrolyte according to any one of claims 1 to 5, wherein The electrolyte further includes a second additive, the second additive including at least one of lithium bis(oxalatoborate), lithium tetrafluoroborate, lithium difluorooxalatoborate, lithium trifluoromethanesulfonate, 4,5-dicyano-2-trifluoromethyl-imidazole lithium or lithium tetraborate, Based on the mass of the electrolyte, the mass percentage of the second additive is X2%, 0.1≤X2≤2.

9. The electrolyte according to claim 8, wherein 2≤A / X2≤44. 10 . A secondary battery comprising the electrolyte according to claim 1 . 11 . An electronic device comprising the secondary battery according to claim 10 .

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