Electrolyte of secondary battery, secondary battery, and electronic device

Abstract

The present application provides an electrolyte of a secondary battery, a secondary battery, and an electronic device. The electrolyte comprises ethyl fluoroacetate, fluoroethylene carbonate and lithium difluorophosphate, wherein ethyl fluoroacetate comprises at least one of difluoroethyl acetate or trifluoroethyl acetate. On the basis of the mass of the electrolyte, the mass percentage content of ethyl fluoroacetate is A%, wherein 12≤A≤65; the mass percentage content of fluoroethylene carbonate is B%, wherein 2.7≤B≤10.3; and the mass percentage content of lithium difluorophosphate is C%, wherein 0.05≤C≤0.5. The electrolyte comprises ethyl fluoroacetate, fluoroethylene carbonate and lithium difluorophosphate, and the values of A, B and C are regulated to be within the described ranges. The synergistic effect of the three substances can mitigate the capacity attenuation problem during secondary battery cycling, reduce the impedance increase during cycling, and improve the cycle stability of secondary batteries.

Description

Electrolyte of secondary battery, secondary battery and electronic device

[0001] This application claims priority to the Chinese patent application No. 202410840207.3, filed on June 26, 2024, and entitled "Electrolyte of secondary battery, secondary battery and electronic device", the whole content of which is incorporated herein by reference. TECHNICAL FIELD

[0002] The present application relates to the technical field of electrochemistry, in particular to an electrolyte of secondary battery, a secondary battery and an electronic device. BACKGROUND

[0003] Secondary batteries, such as lithium ion batteries, are widely used in portable electronic products, electric vehicles, aerospace, energy storage and other fields due to their high energy density, good cycle performance, safety, environmental protection and no memory effect.

[0004] In order to meet the needs of social development, it is urgent to seek lithium ion batteries with higher energy density, which leads to the design of lithium ion batteries with higher voltage, which intensifies the electrode interface reaction, leading to the capacity attenuation of lithium ion batteries during the cycle process, the rapid growth of impedance, and the deterioration of the cycle stability of lithium ion batteries at high voltage. SUMMARY

[0005] The purpose of the present application is to provide an electrolyte of secondary battery, a secondary battery and an electronic device to improve the cycle stability of the secondary battery at high voltage. The specific technical solutions are as follows:

[0006] The first aspect of the present application provides an electrolyte of a secondary battery, which comprises fluoroacetic acid ethyl ester, fluoroethylene carbonate and lithium difluorophosphate, the fluoroacetic acid ethyl ester comprising at least one of difluoroacetic acid ethyl ester or trifluoroacetic acid ethyl ester. The mass percentage of the fluoroacetic acid ethyl ester is A%, 12≤A≤65, the mass percentage of the fluoroethylene carbonate is B%, 2.7≤B≤10.3, and the mass percentage of the lithium difluorophosphate is C%, 0.05≤C≤0.5, based on the mass of the electrolyte. The electrolyte comprises the fluoroacetic acid ethyl ester, the fluoroethylene carbonate and the lithium difluorophosphate, and the values of A, B and C are regulated within the above ranges. Since the fluoroacetic acid ethyl ester has a high oxidation potential, the positive electrode reactivity is low, which is beneficial to reduce the interface reaction on the positive electrode side and reduce the increase of the positive electrode interface impedance during the cycle process. At the same time, the fluoroethylene carbonate and / or the lithium difluorophosphate preferentially undergoes a reduction reaction at the negative electrode than the fluoroacetic acid ethyl ester, which can alleviate the reduction reaction of the fluoroacetic acid ethyl ester at the negative electrode, generate a negative electrode electrolyte interface film (SEI film) with low impedance, and reduce the negative electrode interface impedance, thereby improving the capacity decay problem of the secondary battery during the cycle process as a whole, reducing the impedance growth during the cycle process, and improving the cycle stability of the secondary battery at high voltage.

[0007] In some embodiments of the present application, 15≤A+B≤73.8. By regulating the value of A+B within the above range, it is beneficial to form a stable interface film with low impedance and rich in inorganic components (such as LiF) at both the positive electrode and the negative electrode, thereby better improving the capacity decay problem of the secondary battery during the cycle process, further reducing the impedance growth during the cycle process, and improving the cycle stability of the secondary battery at high voltage.

[0008] In some embodiments of the present application, 1.2≤A / B≤21.5. By regulating the value of A / B within the above range, the fluoroacetic acid ethyl ester can be used to reduce the increase of the positive electrode interface impedance during the cycle process, while better exerting the synergistic effect between the fluoroethylene carbonate and the fluoroacetic acid ethyl ester, optimizing the components of the SEI film, generating a SEI film with low impedance, reducing the negative electrode interface impedance, thereby further improving the capacity decay problem of the secondary battery during the cycle process and reducing the impedance growth during the cycle process, and further more beneficial to improve the cycle stability of the secondary battery at high voltage.

[0009] In some embodiments of the present application, the electrolyte satisfies at least one of the following characteristics: (1) 20≤A≤50; (2) 2.7≤B≤8. By regulating the values of A, B and C within the above ranges, the capacity decay problem of the secondary battery during the cycle process and the impedance growth during the cycle process can be further improved, and the cycle stability of the secondary battery at high voltage can be more beneficially improved.

[0010] In some embodiments of the present application, the electrolyte further comprises a compound of Formula I, the compound of Formula I comprising at least one of the following compounds:

[0011] Based on the mass of the electrolyte, the mass percentage content of the compound of Formula I is D%, and 0.2≤D≤6. On the basis of the electrolyte comprising ethyl fluoroacetate, fluoroethylene carbonate and lithium difluorophosphate, the compound of Formula I is further introduced and its mass percentage content is regulated within the above range. The compound of Formula I and the above electrolyte components (ethyl fluoroacetate, fluoroethylene carbonate and lithium difluorophosphate) jointly participate in the interface film formation reaction of the positive electrode and the negative electrode, which is conducive to the formation of low-impedance interface components at the positive electrode and the negative electrode, and the interface components are stable during the cycle process, thereby further improving the capacity decay problem of the secondary battery during the cycle process, reducing the impedance increase during the cycle process, and improving the cycle stability of the secondary battery at high voltage.

[0012] In some embodiments of the present application, 0.06≤D / B≤1.4. Regulating the value of D / B within the above range can better exert the synergistic effect between the compound of Formula I and fluoroethylene carbonate, optimize the composition of the SEI film, and more favorably form interface components with high ion conductivity, low impedance and stability during the cycle process, thereby further improving the capacity decay problem of the secondary battery during the cycle process, reducing the impedance increase during the cycle process, and improving the cycle stability of the secondary battery at high voltage.

[0013] In some embodiments of the present application, the electrolyte further comprises a compound of Formula II, the compound of Formula II comprising at least one of the following compounds:

[0014] Based on the mass of the electrolyte, the mass percentage content of the compound of Formula II is E%, and 0.2≤E≤3. On the basis of the electrolyte comprising ethyl fluoroacetate, fluoroethylene carbonate and lithium difluorophosphate, the compound of Formula II is further introduced and its mass percentage content is regulated within the above range. The compound of Formula II and the above electrolyte components (ethyl fluoroacetate, fluoroethylene carbonate and lithium difluorophosphate) jointly participate in the interface film formation reaction of the positive electrode and the negative electrode, which is conducive to the formation of low-impedance interface components at the positive electrode and the negative electrode, and the interface components are stable during the cycle process, thereby further improving the capacity decay problem of the secondary battery during the cycle process, reducing the impedance increase during the cycle process, and improving the cycle stability of the secondary battery at high voltage.

[0015] In some embodiments of the present application, the electrolyte further comprises the compound of formula I and the compound of formula II, the mass percentage of the compound of formula I is D%, the mass percentage of the compound of formula II is E%, and 1≤D+E≤6.5, based on the mass of the electrolyte. On the basis of the electrolyte comprising ethyl fluoroacetate, fluoroethylene carbonate and lithium difluorophosphate, the compound of formula I and the compound of formula II are further introduced simultaneously, the compound of formula I and the compound of formula II participate in the interface film formation reaction of the positive electrode and the negative electrode together with the above-mentioned electrolyte components, and the interface of the positive electrode and the negative electrode is improved, so that the capacity decay problem in the cycle process of the secondary battery can be further improved, the impedance increase in the cycle process is reduced, and the cycle stability of the secondary battery at high voltage is improved.

[0016] The second aspect of the present application provides a secondary battery comprising a positive electrode sheet, a negative electrode sheet and the electrolyte provided by the first aspect of the present application. The electrolyte provided by the first aspect of the present application is applied to the secondary battery, which is beneficial to improve the capacity decay problem in the cycle process of the secondary battery, reduce the impedance increase in the cycle process, and improve the cycle stability of the secondary battery at high voltage.

[0017] In some embodiments of the present application, the positive electrode sheet comprises an aluminum current collector, and the thickness of the aluminum current collector is 4-10 μm. The positive electrode sheet comprises an aluminum current collector, and the thickness thereof is regulated within the above-mentioned range, so that the positive electrode sheet has a suitable strength, the risk of fracture is reduced, and the impedance of the secondary battery in the cycle process is reduced, thereby reducing the impedance increase in the cycle process and improving the cycle stability of the secondary battery at high voltage.

[0018] In some embodiments of the present application, the negative electrode sheet comprises a copper current collector, and the thickness of the copper current collector is 4-10 μm. The negative electrode sheet comprises a copper current collector, and the thickness thereof is regulated within the above-mentioned range, so that the negative electrode sheet has a suitable strength, the risk of fracture is reduced, and the impedance of the secondary battery in the cycle process is reduced, thereby reducing the impedance increase in the cycle process and improving the cycle stability of the secondary battery at high voltage.

[0019] The third aspect of the present application provides an electronic device comprising the secondary battery provided by the second aspect of the present application. The secondary battery provided by the present application has good cycle stability at high voltage, so that the electronic device provided by the present application has good use performance and a longer service life.

[0020] The application provides a kind of secondary battery electrolyte, secondary battery and electronic device.The electrolyte includes fluoroacetic acid ethyl ester, fluorinated carbonate and lithium difluorophosphate, and the fluoroacetic acid ethyl ester includes at least one of difluoroacetic acid ethyl ester or trifluoroacetic acid ethyl ester;The mass percentage of fluoroacetic acid ethyl ester is A%, 12≤A≤65, the mass percentage of fluorinated carbonate is B%, 2.7≤B≤10.3, and the mass percentage of lithium difluorophosphate is C%, 0.05≤C≤0.5 based on the mass of the electrolyte.The electrolyte includes fluoroacetic acid ethyl ester, fluorinated carbonate and lithium difluorophosphate, and the values of A, B and C are regulated within the above range, and since the above-mentioned fluoroacetic acid ethyl ester has a high oxidation potential, the positive electrode is low in reactivity, which is beneficial to reduce the interfacial reaction on the positive electrode side and reduce the increase of the positive electrode interfacial impedance during the cycle process, and at the same time, the fluorinated carbonate and lithium difluorophosphate synergistically reduce the reduction reaction of the above-mentioned fluoroacetic acid ethyl ester on the negative electrode, which can alleviate the continuous reduction reaction of the fluoroacetic acid ethyl ester on the negative electrode, generate SEI film with low impedance, reduce the negative electrode interfacial impedance, thereby the capacity decay problem during the cycle process of the secondary battery can be improved as a whole, the impedance increase during the cycle process is reduced, and the cycle stability of the secondary battery under high voltage is improved.

[0021] Of course, implementing any product or method of the present application does not necessarily require achieving all the advantages described above at the same time. DETAILED DESCRIPTION

[0022] The technical solutions in the embodiments of the present application will be described clearly and completely below, obviously, the described embodiments are only part of the embodiments of the present application, not all the embodiments. All other embodiments obtained by those skilled in the art based on the present application belong to the scope of protection of the present application.

[0023] It should be noted that in the following content, lithium ion battery is taken as an example to explain the present application, but the secondary battery of the present application is not limited to lithium ion battery. The specific technical solutions are as follows:

[0024] The first aspect of the present application provides an electrolyte for a secondary battery, comprising fluoroacetic acid ethyl ester, fluoroethylene carbonate and lithium difluorophosphate, the fluoroacetic acid ethyl ester comprising at least one of di-fluoroacetic acid ethyl ester (CAS No. 1550-44-3) or tri-fluoroacetic acid ethyl ester (CAS No. 406-95-1). The mass percentage of fluoroacetic acid ethyl ester is A%, 12≤A≤65, preferably 20≤A≤50, the mass percentage of fluoroethylene carbonate is B%, 2.7≤B≤10.3, preferably 2.7≤B≤8, and the mass percentage of lithium difluorophosphate is C%, 0.05≤C≤0.5, based on the mass of the electrolyte. For example, the value of A can be 12, 15, 20, 23, 26, 30, 35, 37, 40, 43, 45, 50, 55, 58, 60, 65 or a range between any two of these values, the value of B can be 2.7, 3, 4, 5, 6, 8, 9, 10, 10.3 or a range between any two of these values, and the value of C can be 0.05, 0.08, 0.1, 0.15, 0.2, 0.24, 0.3, 0.36, 0.4, 0.45, 0.5 or a range between any two of these values.

[0025] The electrolyte comprises ethyl fluoroacetate, fluoroethylene carbonate and lithium difluorophosphate. The ethyl fluoroacetate has a high oxidation potential, and is less reactive at the positive electrode, which is conducive to reducing the interface reaction at the positive electrode side and reducing the increase in the positive electrode interface impedance during the cycle process. At the same time, the fluoroethylene carbonate and / or lithium difluorophosphate is preferentially reduced at the negative electrode than the ethyl fluoroacetate, which can alleviate the reduction reaction of the ethyl fluoroacetate at the negative electrode, generate a negative electrolyte interface film (SEI film) with low impedance, reduce the negative electrode interface impedance, and is conducive to overall improving the capacity decay problem of the secondary battery during the cycle process and reducing the impedance growth during the cycle process, thereby improving the cycle stability of the secondary battery at high voltage. When the value of A is too small, for example, less than 12, the ethyl fluoroacetate cannot play a role, there is more interface reaction at the positive electrode side, the consumption of active lithium increases during the cycle process, and the positive electrode interface impedance increases, thereby failing to improve the capacity decay and impedance growth problems of the secondary battery during the cycle process. When the value of A is too large, for example, greater than 65, the continuous reduction reaction of the ethyl fluoroacetate at the negative electrode is more, which increases the negative electrode interface impedance, thereby causing the impedance of the secondary battery to grow too much during the cycle process. When the value of B is too small, for example, less than 2.7, it is not conducive to the role of the fluoroethylene carbonate, the reduction reaction of the ethyl fluoroacetate at the negative electrode cannot be alleviated, the consumption of active lithium increases during the negative electrode reaction in the cycle process, the impedance of the SEI film and the negative electrode interface impedance increase too much, causing the capacity to decay quickly and the impedance to grow large during the cycle process of the secondary battery. When the value of B is too large, for example, greater than 10.3, the adverse reaction of the electrolyte at the positive electrode side increases, HF is easily produced to promote the dissolution of transition metals in the positive electrode material layer, which accelerates the capacity decay of the positive electrode material, thereby being not conducive to overall improving the capacity decay problem of the secondary battery during the cycle process and reducing the impedance growth during the cycle process. When the value of C is too small, for example, less than 0.05, the role of lithium difluorophosphate cannot be played, and the reduction reaction of the ethyl fluoroacetate at the negative electrode cannot be alleviated, and the impedance of the SEI film and the negative electrode interface impedance cannot be reduced, causing the capacity to decay quickly and the impedance to grow large during the cycle process of the secondary battery. When the value of C is too large, for example, greater than 0.5, the content of lithium difluorophosphate is too high, which is difficult to completely dissolve in the electrolyte, and a stable and uniform liquid electrolyte cannot be formed, and cannot be used for a secondary battery. Therefore, by controlling the values of A, B and C within the above range, the capacity decay problem of the secondary battery during the cycle process can be overall improved, the impedance growth during the cycle process can be reduced, and the cycle stability of the secondary battery at high voltage can be improved.

[0026] In the present application, the high voltage refers to a voltage ≥ 4.4 V, for example, the charging voltage can be 4.4 V, 4.5 V, 4.53 V, 4.6 V, etc. The above-mentioned cycle process is not particularly limited in the present application and can be designed according to actual needs, as long as the voltage meets the above-mentioned range. For example, one charge-discharge cycle of the above-mentioned cycle process can be: at 45°C, the secondary battery is charged to 4.53 V at a constant current of 0.5 C, and charged to 0.05 C at 4.53 V under constant voltage; stand for 5 min, then discharge to 3.0 V at a constant current of 0.5 C, stand for 5 min.

[0027] In some embodiments of the present application, 15≤A+B≤73.8, for example, the value of A+B can be 15, 20, 23, 26, 30, 35, 37, 40, 43, 45, 50, 55, 58, 60, 65, 70, 73.8 or a range composed of any two of the above values. By adjusting the value of A+B within the above range, a stable interface layer with low impedance and rich in inorganic components (such as LiF) can be formed on both the positive electrode and the negative electrode, thereby better improving the capacity decay problem in the cycle process of the secondary battery, while the electrolyte can have a suitable viscosity, a lower ion transport impedance, and a smaller degree of polarization in the cycle process, thereby further reducing the impedance increase in the cycle process, and further improving the cycle stability of the secondary battery at high voltage.

[0028] In some embodiments of the present application, 1.2≤A / B≤21.5, for example, the value of A / B can be 1.2, 2, 3, 5, 8, 10, 12, 15, 18, 20, 21.5 or a range composed of any two of the above values. By adjusting the value of A / B within the above range, the increase of the positive electrode interface impedance in the cycle process can be reduced by using fluoroethyl acetate, while the synergistic effect between fluoroethylene carbonate and fluoroethyl acetate is better played, the components of the SEI film are optimized, the SEI film with low impedance is generated, and the negative electrode interface impedance is reduced, thereby further improving the capacity decay problem in the cycle process of the secondary battery; while the electrolyte can have a suitable viscosity, a lower ion transport impedance, and a smaller degree of polarization in the cycle process, thereby more favorably reducing the impedance increase in the cycle process, and further improving the cycle stability of the secondary battery at high voltage.

[0029] In some embodiments of the present application, the electrolyte further comprises a compound of formula I, the compound of formula I comprises at least one of the following compounds:

[0030] Based on the mass of the electrolyte, the mass percentage of the compound of Formula I is D%, 0.2≤D≤6, for example, the value of D can be 0.2, 0.3, 0.5, 0.8, 1, 1.5, 1.8, 2, 2.4, 3, 3.5, 4, 4.4, 5, 5.5, 6, or a range between any two of them. Based on the electrolyte including ethyl fluoroacetate, fluoroethylene carbonate and lithium difluorophosphate, further introducing the compound of Formula I and adjusting its mass percentage within the above range, the compound of Formula I and the above electrolyte components (ethyl fluoroacetate, fluoroethylene carbonate and lithium difluorophosphate) participate in the interface film reaction of the positive electrode and the negative electrode, which is beneficial to the formation of low impedance interface components on the positive electrode and the negative electrode, and stable in the cycle process, thereby further improving the capacity decay problem in the cycle process of the secondary battery, reducing the impedance increase in the cycle process, and improving the cycle stability of the secondary battery at high voltage.

[0031] In some embodiments of the present application, 0.06≤D / B≤1.4, for example, the value of D / B can be 0.06, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.8, 1, 1.2, 1.4, or a range between any two of them. Since the compound of Formula I and fluoroethylene carbonate have a competitive reaction at the negative electrode, the compound of Formula I tends to form a Li x SO y F z component at the negative electrode, and fluoroethylene carbonate tends to form a polycarbonate and LiF component with better ion transport capacity but slightly worse stability at the negative electrode. By adjusting the value of D / B within the above range, the synergistic effect between the compound of Formula I and fluoroethylene carbonate can be better exerted, the composition of the SEI film is optimized, and the SEI film combined with the above Li x SO y F z , polycarbonate and LiF components is formed, forming an interface component with high ion conductivity, low impedance and stability in the cycle process, thereby further improving the capacity decay problem in the cycle process of the secondary battery, reducing the impedance increase in the cycle process, and improving the cycle stability of the secondary battery at high voltage.

[0032] In some embodiments of the present application, the electrolyte further includes a compound of Formula II, the compound of Formula II includes at least one of the following compounds:

[0033] The mass percentage content of the compound of formula II is E%, 0.2≤E≤3, for example, the value of E can be 0.2, 0.3, 0.5, 0.6, 0.8, 1, 1.2, 1.6, 1.8, 2, 2.3, 2.5, 3, or a range between any two of them. Based on the electrolyte including ethyl fluoroacetate, fluoroethylene carbonate and lithium difluorophosphate, the compound of formula II is further introduced and its mass percentage content is controlled within the above range, the compound of formula II and the above electrolyte components (ethyl fluoroacetate, fluoroethylene carbonate and lithium difluorophosphate) participate in the interface film formation reaction of the positive electrode and the negative electrode, which is conducive to the formation of an interface component with high ion conductivity and low impedance at the positive electrode and the negative electrode, and the interface component is stable during the cycle, thereby further improving the capacity decay problem of the secondary battery during the cycle, reducing the impedance increase during the cycle, and improving the cycle stability of the secondary battery at high voltage.

[0034] In some embodiments of the present application, the electrolyte further includes the above-mentioned compound of formula I and the compound of formula II, the mass percentage content of the compound of formula I is D%, and the mass percentage content of the compound of formula II is E%, based on the mass of the electrolyte, 1≤D+E≤6.5, for example, the value of D+E can be 1, 1.5, 1.8, 2, 2.4, 3, 3.5, 4, 4.4, 4.8, 5, 5.3, 5.5, 6, 6.5, or a range between any two of them. Based on the electrolyte including ethyl fluoroacetate, fluoroethylene carbonate and lithium difluorophosphate, the above-mentioned compound of formula I and the compound of formula II are further introduced at the same time, the compound of formula I and the compound of formula II and the above electrolyte components participate in the interface film formation reaction of the positive electrode and the negative electrode, further optimizing the interface component of the positive electrode and the negative electrode, on the one hand, the protective layer can be formed in advance at the positive electrode, further reducing the oxidation of the electrolyte at the positive electrode interface, on the other hand, it can also coordinate with other components at the negative electrode to form an interface layer with high ion conductivity, low impedance and better stability during the cycle at the negative electrode. The above electrolyte components synergistically improve the positive electrode and negative electrode interface, which can further improve the capacity decay problem of the secondary battery during the cycle, reduce the impedance increase during the cycle, and improve the cycle stability of the secondary battery at high voltage.

[0035] In the present application, a lithium salt can also be contained in the electrolyte. The kind of the lithium salt is not particularly limited in the present application as long as the object of the present application can be achieved. The lithium salt can include, but is not limited to, at least one of lithium hexafluorophosphate (LiPF6), lithium bis-trifluoromethanesulfonimide (LiN(CF3SO2)2), lithium bis(fluorosulfonyl)imide (Li(N(SO2F)2)), or lithium hexafluorocerate (LiCsF6), lithium perchlorate (LiClO4), or lithium trifluoromethanesulfonate (LiCF3SO3). The mass percentage of the lithium salt is not particularly limited in the present application as long as the object of the present application can be achieved. The mass percentage of the lithium salt is 8% to 20% based on the mass of the electrolyte. For example, the mass percentage of the lithium salt can be 8%, 10%, 12%, 13%, 15%, 16%, 18%, 19%, 20%, or a range between any two of them.

[0036] In the present application, the electrolyte can further include a non-aqueous organic solvent. The non-aqueous organic solvent can include, but is not limited to, at least one of a carbonate compound, a carboxylate compound, an ether compound, or other organic solvents. The carbonate compound can include, but is not limited to, at least one of a chain carbonate compound or a cyclic carbonate compound. The chain carbonate compound can include, but is not limited to, at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), or methyl ethyl carbonate (EMC). The cyclic carbonate compound can include, but is not limited to, at least one of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), or vinyl ethylene carbonate (VEC). The carboxylate compound can include, but is not limited to, at least one of methyl formate, ethyl formate, propyl formate, n-butyl formate, isobutyl formate, methyl acetate, ethyl acetate, n-propyl acetate, t-butyl acetate, methyl propionate, ethyl propionate, or propyl propionate. The ether compound can include, but is not limited to, at least one of dibutyl ether, tetraglyme, diglyme, 1,2-dimethoxyethane, 1,2-diethoxyethane, ethoxymethoxyethane, 2-methyltetrahydrofuran, or tetrahydrofuran. The other organic solvents can include, but are not limited to, at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, N-methyl-2-pyrrolidinone, formamide, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, trioctyl phosphate, or phosphates. The present application does not particularly limit the mass percentage of the non-aqueous organic solvent, as long as the purpose of the present application is achieved. The mass percentage of the non-aqueous organic solvent is 15.2% to 85.25% based on the mass of the electrolyte. The mass percentage of the non-aqueous organic solvent can be 15.2%, 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%, 85.25%, or a range defined by any two of them.

[0037] In an embodiment of the present application, the electrolyte includes ethyl fluoroacetate, fluoroethylene carbonate, lithium difluorophosphate, a lithium salt, and a non-aqueous organic solvent, the mass percentages of the ethyl fluoroacetate, the fluoroethylene carbonate, the lithium difluorophosphate, and the lithium salt are as described above, and the mass percentage of the non-aqueous organic solvent is 24.2% to 85.25%. The secondary battery includes the electrolyte composed of the above components, which is advantageous to improve the capacity fading problem during the cycle of the secondary battery, and to reduce the impedance increase during the cycle, thereby improving the cycle stability of the secondary battery at a high voltage.

[0038] In an embodiment of the present application, the electrolyte comprises ethyl fluoroacetate, fluoroethylene carbonate, lithium difluorophosphate, the compound of formula I, a lithium salt and a non-aqueous organic solvent, the mass percentage of ethyl fluoroacetate, fluoroethylene carbonate, lithium difluorophosphate, the compound of formula I and the lithium salt is as described above, and the mass percentage of the non-aqueous organic solvent is 18.2% to 85.15%. The secondary battery comprises the electrolyte with the above composition, which is conducive to further improving the cycle stability of the secondary battery at high voltage.

[0039] In an embodiment of the present application, the electrolyte comprises ethyl fluoroacetate, fluoroethylene carbonate, lithium difluorophosphate, the compound of formula II, a lithium salt and a non-aqueous organic solvent, the mass percentage of ethyl fluoroacetate, fluoroethylene carbonate, lithium difluorophosphate, the compound of formula II and the lithium salt is as described above, and the mass percentage of the non-aqueous organic solvent is 21.2% to 85.15%. The secondary battery comprises the electrolyte with the above composition, which is conducive to further improving the cycle stability of the secondary battery at high voltage.

[0040] In an embodiment of the present application, the electrolyte comprises ethyl fluoroacetate, fluoroethylene carbonate, lithium difluorophosphate, the compound of formula I, the compound of formula II, a lithium salt and a non-aqueous organic solvent, the mass percentage of ethyl fluoroacetate, fluoroethylene carbonate, lithium difluorophosphate, the compound of formula I, the compound of formula II and the lithium salt is as described above, and the mass percentage of the non-aqueous organic solvent is 15.2% to 84.85%. The secondary battery comprises the electrolyte with the above composition, which is conducive to further improving the cycle stability of the secondary battery at high voltage.

[0041] The second aspect of the present application provides a secondary battery, which comprises a positive electrode sheet, a negative electrode sheet and the electrolyte provided by the first aspect of the present application. The application of the electrolyte provided by the first aspect of the present application to the secondary battery is conducive to improving the capacity decay problem of the secondary battery during the cycle process, reducing the impedance growth during the cycle process and improving the cycle stability of the secondary battery at high voltage.

[0042] In some embodiments of the present application, the positive electrode sheet comprises an aluminum current collector, and the thickness of the aluminum current collector is 4 μm to 10 μm, for example, the thickness of the aluminum current collector can be 4 μm, 5 μm, 5.5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm or a range between any two of the above values. In the case of the secondary battery comprising the above electrolyte, the positive electrode sheet comprises an aluminum current collector, and the thickness of the aluminum current collector is regulated within the above range. The aluminum current collector has low cost, is thin and has appropriate potential, reactivity and strength, which is conducive to reducing the risk of fracture, reducing the impedance of the secondary battery during the cycle process and thus reducing the impedance growth during the cycle process, and further improving the cycle stability of the secondary battery at high voltage.

[0043] In some embodiments of the present application, the negative electrode tab comprises a copper current collector, and the thickness of the copper current collector is 4-10 μm, for example, the thickness of the aluminum current collector can be 4 μm, 5 μm, 5.5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm or a range between any two of them. Based on the above-mentioned electrolyte, the negative electrode tab comprises a copper current collector, and the thickness thereof is regulated within the above-mentioned range. The negative electrode current collector is low in cost, thin, has a suitable potential, reactivity and strength, is beneficial to reduce the risk of fracture, and can reduce the impedance of the secondary battery during the cycle, thereby being beneficial to reduce the impedance increase during the cycle and further improve the cycle stability of the secondary battery at high voltage.

[0044] In the present application, the positive electrode tab comprises an aluminum current collector and a positive electrode material layer disposed on at least one surface of the aluminum current collector. In the present application, the positive electrode material layer can be disposed on one surface in the thickness direction of the aluminum current collector, or on both surfaces in the thickness direction of the aluminum current collector. It should be noted that the "surface" here can be the entire area of the surface of the aluminum current collector, or part of the area of the surface of the aluminum current collector, which is not particularly limited in the present application as long as the purpose of the present application can be achieved.

[0045] The thickness of the positive electrode material layer is not particularly limited in the present application as long as the purpose of the present application can be achieved. For example, the thickness of the single-sided positive electrode material layer is 30-120 μm. In the present application, the positive electrode material layer comprises a positive electrode active material, and the type of the positive electrode active material is not particularly limited in the present application as long as the purpose of the present application can be achieved. The positive electrode active material can comprise at least one of lithium nickel cobalt manganese oxide (for example, NCM811, NCM622, NCM523, NCM111), lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium-rich manganese-based material, lithium cobalt oxide, lithium manganese oxide or lithium manganese iron phosphate, etc.

[0046] The positive electrode material layer of the present application can further comprise a positive electrode conductive agent and a positive electrode binder. The present application does not have a particular limitation on the positive electrode conductive agent as long as the purpose of the present application can be achieved, for example, the positive electrode conductive agent can include, but is not limited to, at least one of conductive carbon black (Super P), carbon nanotubes (CNTs), carbon fibers, flake graphite, graphene, metal materials, or conductive polymers, and the conductive carbon black can include, but is not limited to, at least one of acetylene black or Ketjen black. The above-mentioned carbon nanotubes can include, but are not limited to, single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The above-mentioned carbon fibers can include, but are not limited to, vapor-grown carbon fibers (VGCF) and / or nanocarbon fibers. The above-mentioned metal materials can include, but are not limited to, metal powder and / or metal fibers, and in particular, the metal can include, but is not limited to, at least one of copper, nickel, aluminum, or silver. The above-mentioned conductive polymers can include, but are not limited to, at least one of polyphenylene derivatives, polyaniline, polythiophene, polyacetylene, or polypyrrole. The present application does not have a particular limitation on the binder as long as the purpose of the present application can be achieved, for example, the positive electrode binder can 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, butadiene-styrene rubber, or polyvinylidene fluoride.

[0047] In the present application, the method for preparing the positive electrode tab is not particularly limited as long as the purpose of the present application can be achieved, for example, it can be prepared by the following method: mixing the positive electrode active material, the positive electrode conductive agent, and the positive electrode binder, adding N-methyl pyrrolidone (NMP) and stirring uniformly to obtain a positive electrode slurry with a solid content of 65wt% to 85wt%. The positive electrode slurry is uniformly coated on one surface of the aluminum current collector, and after drying, a positive electrode tab with a single-sided coated positive electrode material layer is obtained. Then the above coating step is repeated on the other surface of the aluminum current collector, and after drying, a positive electrode tab with a double-sided coated positive electrode material layer is obtained. After the coating is completed, the positive electrode tab is obtained by cold pressing and cutting. The present application does not have a particular limitation on the mass ratio of the positive electrode active material, the conductive agent, and the binder in the positive electrode material layer as long as the purpose of the present application can be achieved.

[0048] In the present application, the negative electrode tab comprises a copper current collector and a negative electrode material layer disposed on at least one surface of the copper current collector. In the present application, the negative electrode material layer can be disposed on one surface in the thickness direction of the copper current collector, or it can be disposed on both surfaces in the thickness direction of the copper current collector. It should be noted that the "surface" here can be the entire area of the copper current collector, or it can be a partial area of the copper current collector, and the present application does not have a particular limitation as long as the purpose of the present application can be achieved.

[0049] In the present application, the thickness of the negative electrode material layer is not particularly limited, as long as the purpose of the present application can be achieved. For example, the thickness of the single-sided negative electrode material layer is 30 μm to 160 μm. The negative electrode material layer of the present application contains a negative electrode active material. The present application does not particularly limit the type of the negative electrode active material, as long as the purpose of the present application can be achieved. For example, the negative electrode active material can contain at least one of natural graphite, artificial graphite, meso-carbon microbeads (MCMB), hard carbon, soft carbon, silicon, silicon-carbon composite, SiO x (0 < x < 2), Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, SnO2, lithium titanate Li4Ti5O 12 , Li-Al alloy, or metallic lithium.

[0050] The negative electrode material layer of the present application can also contain a negative electrode conductive agent, a negative electrode binder, and a negative electrode dispersing agent. The present application does not particularly limit 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 can include, but is not limited to, at least one of conductive carbon black (Super P), carbon nanotubes (CNTs), graphite, carbon fibers, carbon nanowires, graphene, a metal material, or a conductive polymer, and the above-mentioned carbon nanotubes can include, but are not limited to, single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The above-mentioned carbon fibers can include, but are not limited to, vapor-grown carbon fibers (VGCF) and / or nanocarbon fibers. The above-mentioned metal material can include, but is not limited to, metal powder and / or metal fibers, and specifically, the metal can include, but is not limited to, at least one of copper, nickel, aluminum, or silver. The above-mentioned conductive polymer can include, but is not limited to, at least one of polyphenylene derivative, polyaniline, polythiophene, polyacetylene, or polypyrrole. The negative electrode dispersing agent can include sodium carboxymethyl cellulose.

[0051] In the present application, the method for preparing the negative electrode tab is not particularly limited, as long as the purpose of the present application can be achieved, for example, it can be prepared by the following method: mixing the negative electrode active material, the negative electrode conductive agent, the negative electrode binder, and the negative electrode dispersing agent, adding deionized water and stirring uniformly to obtain a negative electrode slurry with a solid content of 50 wt% to 75 wt%. The negative electrode slurry is uniformly coated on one surface of the copper current collector, and after drying, a negative electrode tab with a single-sided negative electrode material layer is obtained. Then the above coating step is repeated on the other surface of the copper current collector, and after drying, a negative electrode tab with a double-sided negative electrode material layer is obtained. After the coating is completed, the negative electrode tab is obtained by cold pressing and cutting. The present application does not particularly limit the mass ratio of the negative electrode active material, the negative electrode conductive agent, and the negative electrode binder in the negative electrode material layer, as long as the purpose of the present application can be achieved.

[0052] The secondary battery of the present application further includes a separator. The separator of the present application is not particularly limited as long as the object of the present application can be achieved. For example, the material of the separator can include, but is not limited to, at least one of polyethylene (PE), polyolefin (PO) based on polypropylene (PP), polyester (e.g., polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex, or aramid. The type of the separator can include at least one of a woven film, a non-woven film, a microporous film, a composite film, a calendered film, or a spun film. In some embodiments, the separator can include a base layer and a surface treatment layer. The base layer can be a non-woven fabric, a film, or a composite film having a porous structure, and the material of the base layer can include at least one of polyethylene, polypropylene, polyethylene terephthalate, or polyimide. Optionally, a polypropylene porous film, a polyethylene porous film, a polypropylene non-woven fabric, a polyethylene non-woven fabric, or a polypropylene-polyethylene-polypropylene porous composite film can be used. Optionally, a surface treatment layer is provided on at least one surface of the base layer, and the surface treatment layer can be a polymer layer or an inorganic layer, or a layer formed by mixing a polymer and an inorganic substance. In some embodiments, the inorganic layer includes inorganic particles and a binder. The inorganic particles of the present application are not particularly limited, and for example, the inorganic particles can 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 of the present application is not particularly limited, and for example, the binder can be at least one of the above-described binders. In some embodiments of the present application, the polymer layer includes a polymer, and the material of the polymer includes at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylic acid salt, polyvinylpyrrolidone, polyvinyl ether, or polyvinylidene fluoride or poly(vinylidene fluoride-hexafluoropropylene). In the present application, the thickness of the separator is not particularly limited as long as the object of the present application can be achieved, and for example, the thickness of the separator can be 3 μm to 30 μm.

[0053] The secondary battery further includes a case for accommodating the positive electrode sheet, the separator, the negative electrode sheet, and the electrolyte, and other components known in the art of secondary batteries, and the present application does not limit the above-described other components. The case of the present application is not particularly limited and can be a case known in the art as long as the object of the present application can be achieved. For example, the case can be a hard case or a flexible case. The material of the hard case can be a metal, and the type of the metal is not particularly limited, and a metal hard case known in the art can be used as long as the object of the present application can be achieved. The flexible case can be a metal plastic film, for example, an aluminum plastic film, a steel plastic film, or the like.

[0054] The kind of the secondary battery of the present application is not particularly limited, and it can include any device in which an electrochemical reaction occurs. For example, the secondary battery can include, but is not limited to, a lithium ion battery, a sodium ion battery, a lithium polymer secondary battery, a lithium ion polymer secondary battery.

[0055] The preparation process of the secondary battery of the present application is well known to those skilled in the art, and the present application is not particularly limited. For example, the preparation process of the secondary battery can include, but is not limited to, the following steps: stacking the positive electrode sheet, the separator, and the negative electrode sheet in order, and winding, folding, or the like as needed to obtain an electrode assembly in a wound structure, placing the electrode assembly in a case, injecting an electrolyte into the case and sealing it, and obtaining a secondary battery. Alternatively, the positive electrode sheet, the separator, and the negative electrode sheet are stacked in order, and then the four corners of the entire stack structure are fixed with adhesive tape to obtain an electrode assembly in a stack structure, the electrode assembly is placed in a case, an electrolyte is injected into the case and sealed, and a secondary battery is obtained. In addition, a current overprotection element, a guide plate, or the like can also be placed in the case as needed to prevent the pressure inside the secondary battery from rising and overcharging and discharging.

[0056] The third aspect of the present application provides an electronic device comprising the secondary battery provided by the second aspect of the present application. The secondary battery provided by the present application has good cycle stability, so that the electronic device provided by the present application has good use performance and a long service life.

[0057] The kind of the electronic device of the present application is not particularly limited, and it can be any electronic device known in the art. In some embodiments, the electronic device can include, but is not limited to, a notebook computer, a pen input computer, a mobile computer, an electronic book player, a portable telephone, a portable facsimile machine, a portable copying machine, a portable printer, a head-mounted stereo headphone, a video recorder, a liquid crystal television, a portable cleaner, a portable CD player, a mini disc, a transceiver, an electronic organizer, a calculator, a memory card, a portable recorder, a radio, a backup power supply, an electric motor, an automobile, a motorcycle, a power-assisted bicycle, a bicycle, a lighting appliance, a toy, a game machine, a clock, a power tool, a flashlight, a camera, a household large storage battery, and a lithium ion capacitor, etc.

[0058] Embodiments

[0059] Hereinafter, embodiments and comparative examples are given to more specifically describe the embodiments of the present application. Various tests and evaluations were carried out according to the following methods. In addition, unless otherwise specified, "parts" and "%" are on a mass basis.

[0060] Test methods and apparatus:

[0061] Current collector thickness test:

[0062] The lithium ion battery was disassembled after being discharged at 0.5C to 3.0V, and the positive electrode sheet and the negative electrode sheet were taken out. The positive electrode material layer and the negative electrode material layer were scraped off to obtain aluminum current collectors and copper current collectors. The aluminum current collectors and the copper current collectors were cleaned with dimethyl carbonate (DMC), dried at 60°C, and then the thicknesses of the aluminum current collectors and the copper current collectors were measured by a screw micrometer, in units of μm. When the positive electrode sheet and the negative electrode sheet taken out had an empty foil area (an area not coated with a positive electrode material layer or a negative electrode material layer), the thickness of the empty foil area could be directly tested to obtain the thicknesses of the aluminum current collectors and the copper current collectors.

[0063] Cycle stability test:

[0064] The capacity retention rate and the impedance growth rate of the lithium ion battery cycled 500 times at 45°C were used to evaluate the capacity decay and impedance growth of the lithium ion battery during the cycling process, respectively. The higher the capacity retention rate of the lithium ion battery cycled 500 times at 45°C, and the smaller the impedance growth rate, the better the cycle stability of the lithium ion battery.

[0065] The lithium ion battery was placed in a 45°C constant temperature oven, and charged at a constant current of 0.5C to 4.53V, and then charged at a constant voltage of 4.53V to 0.05C. After standing for 5 min, it was discharged at a constant current of 0.5C to 3.0V, and then stood for 5 min. This was one charge-discharge cycle process. 500 charge-discharge cycles were carried out in the above manner, and the initial discharge capacity D0 and the initial internal resistance R0 of the lithium ion battery after the first cycle were recorded. After 500 cycles, the residual discharge capacity Dz of the lithium ion battery after the 500th cycle and the internal resistance R1 after the cycle were recorded. The internal resistance of the lithium ion battery was measured by a multimeter. The capacity retention rate and the impedance growth rate of the lithium ion battery cycled 500 times at 45°C were calculated by the following formula: capacity retention rate (%) = Dz / D0 x 100%, impedance growth rate (%) = (R1-R0) / R0 x 100%.

[0066] Example 1-1

[0067] Preparation of the positive electrode sheet

[0068] The positive electrode active material lithium cobaltate (LiCoO2), the positive electrode conductive agent Super P, and the positive electrode binder polyvinylidene fluoride (PVDF) are mixed in a mass ratio of 97:1.4:1.6, N-methyl pyrrolidone (NMP) is added as a solvent, and a slurry with a solid content of 72 wt% is prepared. After uniform stirring in a vacuum stirrer, a positive electrode slurry is obtained. The positive electrode slurry is uniformly coated on one surface of an aluminum current collector aluminum foil with a thickness of 10 μm, and is dried at 85°C to obtain a positive electrode tab with a single-coated positive electrode material layer with a coating thickness of 110 μm. Then, the above steps are repeated on the other surface of the aluminum foil to obtain a positive electrode tab with a double-coated positive electrode material layer. After coating, the positive electrode tab is cold-pressed and cut into a specification of 74 mm x 867 mm for use. The compacted density of the positive electrode material layer after cold-pressing is 4.15 g / cm3. 3 .

[0069] <Preparation of a negative electrode tab>

[0070] The negative electrode active material artificial graphite, the negative electrode conductive agent Super P, the negative electrode thickening agent sodium carboxymethyl cellulose, and the negative electrode binder styrene-butadiene rubber are mixed in a mass ratio of 96.4:1.5:0.5:1.6, deionized water is added as a solvent, and a slurry with a solid content of 54 wt% is prepared. After uniform stirring in a vacuum stirrer, a negative electrode slurry is obtained. The negative electrode slurry is uniformly coated on one surface of a copper current collector copper foil with a thickness of 10 μm, and is dried at 90°C to obtain a negative electrode tab with a single-coated negative electrode material layer with a coating thickness of 130 μm. Then, the above steps are repeated on the other surface of the copper foil to obtain a negative electrode tab with a double-coated negative electrode material layer. After coating, the negative electrode tab is cold-pressed and cut into a specification of 76 mm x 851 mm for use. The compacted density of the negative electrode material layer after cold-pressing is 1.70 g / cm3. 3 .

[0071] <Separator>

[0072] The alumina and PVDF are mixed in a mass ratio of 95:5, NMP is added as a solvent, a slurry with a solid content of 12wt% is prepared and stirred uniformly to obtain an inorganic layer slurry; the inorganic layer slurry is uniformly coated on one surface of a polyethylene substrate with a thickness of 9μm, and after drying, a single-sided inorganic layer coated separator film with a coating thickness of 2μm is obtained. Then PVDF is added to NMP solvent and stirred uniformly to prepare a polymer layer slurry with a solid content of 25wt%, and then the polymer layer slurry is uniformly coated on the surface of the inorganic layer away from the substrate, and after drying, a single-sided inorganic layer and polymer layer coated separator film is obtained; then the polymer layer slurry is uniformly coated on the other surface of the polyethylene substrate, and after drying, a separator film with one side coated with an inorganic layer and a polymer layer and the other side coated only with a polymer layer is obtained. The areal density of the polymer layer slurry coated side is 0.15mg / cm 2 .

[0073] <Preparation of electrolyte>

[0074] In an argon atmosphere glove box with a water content of less than 10ppm, EC, PC, DEC are mixed in a mass ratio of 20:20:60 to obtain a non-aqueous organic solvent, then lithium salt LiPF6 is added to the non-aqueous organic solvent, dissolved and mixed uniformly, then ethyl fluoroacetate, difluoroacetic ethyl acetate, fluoroethylene carbonate and lithium difluorophosphate are added, dissolved and mixed uniformly to obtain an electrolyte. The mass percentage of lithium salt is 12.5% based on the mass of the electrolyte, the mass percentage of ethyl fluoroacetate, fluoroethylene carbonate and lithium difluorophosphate is shown in Table 1, and the balance is the non-aqueous organic solvent with a mass ratio of EC:PC:DEC of 20:20:60.

[0075] <Preparation of lithium ion battery>

[0076] The positive electrode sheet, separator film, negative electrode sheet and separator film prepared above are stacked in order, with the side of the separator film coated with an inorganic layer and a polymer layer facing the positive electrode sheet, and the side coated only with a polymer layer facing the negative electrode sheet, and the separator film is between the positive electrode sheet and the negative electrode sheet to act as a separator, then the electrode assembly is obtained by winding; the electrode assembly is placed in an outer packaging aluminum plastic film, after removing water at 80℃, the prepared non-aqueous electrolyte is injected and packaged, and after standing, formation, degassing, edge cutting, shaping and capacity testing processes, a lithium ion battery is obtained.

[0077] Examples 1-2 to 1-23

[0078] Except for adjusting the relevant parameters according to Table 1, the rest is the same as Example 1-1. When the mass percentage of ethyl fluoroacetate, fluoroethylene carbonate and lithium difluorophosphate changes, the mass percentage of lithium salt and the mass ratio of EC, PC, DEC remain unchanged, and the mass percentage of non-aqueous organic solvent changes accordingly.

[0079] Examples 2-1 to 2-21

[0080] The rest is the same as Example 1-1 except that the relevant parameters are adjusted according to Table 2 and the compound of Formula I and / or the compound of Formula II are further introduced into the electrolyte. When the mass percentage of the compound of Formula I and / or the compound of Formula II changes, the mass percentage of the lithium salt and the mass ratio of EC, PC, DEC remain unchanged, and the mass percentage of the non-aqueous organic solvent changes accordingly.

[0081] Examples 3-1 to 3-2

[0082] The rest is the same as Example 1-1 except that the relevant parameters are adjusted according to Table 3.

[0083] Comparative Examples 1 to 7

[0084] The rest is the same as Example 1-1 except that at least one of ethyl fluoroacetate, fluoroethylene carbonate or lithium difluorophosphate is not added in the preparation of the electrolyte according to Table 1, the mass percentage of the lithium salt and the mass ratio of EC, PC, DEC remain unchanged, the mass percentage of the non-aqueous organic solvent changes accordingly, and the content of ethyl fluoroacetate, fluoroethylene carbonate and / or lithium difluorophosphate is shown in Table 1.

[0085] Comparative Examples 8 to 12

[0086] The rest is the same as Example 1-1 except that the relevant parameters are adjusted according to Table 1. When the mass percentage of ethyl fluoroacetate, fluoroethylene carbonate and lithium difluorophosphate changes, the mass percentage of the lithium salt and the mass ratio of EC, PC, DEC remain unchanged, and the mass percentage of the non-aqueous organic solvent changes accordingly.

[0087] The preparation parameters and performance parameters of each example and comparative example are shown in Tables 1 to 3.

[0088] Table 1

[0089] Note: “ / ” in Table 1 indicates that the corresponding substance does not exist or there is no relevant parameter, and “-” indicates that the performance data of the lithium ion battery cannot be measured.

[0090] As can be seen from Examples 1-1 to 1-23, Comparative Examples 1 to 12, when the electrolyte comprises fluoroethyl acetate, fluoroethylene carbonate and lithium difluorophosphate within the scope of the present application and the values of A, B and C are controlled within the scope of the present application, the lithium ion battery has a higher capacity retention rate and a lower impedance growth rate, indicating that the lithium ion battery has better cycle stability. The electrolyte of Comparative Example 1 does not comprise fluoroethyl acetate, fluoroethylene carbonate or lithium difluorophosphate, the electrolyte of Comparative Examples 2 to 4 comprises any one of fluoroethyl acetate, fluoroethylene carbonate and lithium difluorophosphate, the electrolyte of Comparative Examples 5 to 7 comprises any two of fluoroethyl acetate, fluoroethylene carbonate and lithium difluorophosphate, and in Comparative Examples 8 to 12, at least one of the values of A, B and C is not within the scope of the present application. The lithium ion batteries of Comparative Examples 1 to 3 and Comparative Examples 5 to 11 have lower capacity retention rates and higher impedance growth rates, indicating that the lithium ion batteries have poorer cycle stability. Since the lithium difluorophosphate in the electrolytes of Comparative Example 4 and Comparative Example 12 is difficult to completely dissolve, a stable and uniform liquid electrolyte cannot be formed, and the performance data of the lithium ion battery cannot be measured.

[0091] The values of A+B and A / B generally affect the cycle stability of the lithium ion battery. As can be seen from Examples 1-1 to 1-9 and Examples 1-14 to 1-21, when the values of A+B and A / B are controlled within the scope of the present application, the lithium ion battery has a higher capacity retention rate and a lower impedance growth rate, indicating that the lithium ion battery has good cycle stability.

[0092] The type of fluoroethyl acetate generally affects the cycle stability of the lithium ion battery. As can be seen from Examples 1-1 to 1-2 and Examples 1-22 to 1-23, when the fluoroethyl acetate within the scope of the present application is selected, the lithium ion battery has a higher capacity retention rate and a lower impedance growth rate, indicating that the lithium ion battery has good cycle stability.

[0093] Table 2

[0094] Note: In Table 2, " / " indicates the absence of the corresponding substance or the absence of the relevant parameter. The mass percentage content of "I-3+I-4" is "1.5+1.5", indicating that the mass percentage content of I-3 is 1.5%, the mass percentage content of I-4 is 1.5%, and the sum of the two is D%, and the like is similarly indicated.

[0095] The electrolyte comprising the compound of formula I and the mass percentage content D% of the compound of formula I generally affects the cycle stability of the lithium ion battery. As can be seen from Example 1-1, Example 2-1 to Example 2-4, the introduction of the compound of formula I in the electrolyte and the regulation of the value of D within the scope of the present application can make the lithium ion battery have a higher capacity retention rate and a lower impedance growth rate, indicating that the lithium ion battery has good cycle stability.

[0096] The value of D / B generally affects the cycle stability of the lithium ion battery. As can be seen from Example 2-1 to Example 2-6, the regulation of the value of D / B within the scope of the present application can make the lithium ion battery have a higher capacity retention rate and a lower impedance growth rate, indicating that the lithium ion battery has good cycle stability.

[0097] The electrolyte comprising the compound of formula II and the mass percentage content E% of the compound of formula II generally affects the cycle stability of the lithium ion battery. As can be seen from Example 1-1, Example 2-7 to Example 2-9, the introduction of the compound of formula II in the electrolyte and the regulation of the value of E within the scope of the present application can make the lithium ion battery have a higher capacity retention rate and a lower impedance growth rate, indicating that the lithium ion battery has good cycle stability.

[0098] The electrolyte comprising the compound of formula I and the compound of formula II and the value of D+E generally affects the cycle stability of the lithium ion battery. As can be seen from Example 2-1, Example 2-7, Example 2-10 to Example 2-14, the simultaneous introduction of the compound of formula I and the compound of formula II in the electrolyte and the regulation of the value of D+E within the scope of the present application can make the lithium ion battery have a higher capacity retention rate and a lower impedance growth rate, indicating that the lithium ion battery has good cycle stability.

[0099] The type of compound of formula I generally affects the cycle stability of the lithium ion battery. As can be seen from Example 2-10, Example 2-15 to Example 2-18, the selection of the compound of formula I within the scope of the present application can make the lithium ion battery have a higher capacity retention rate and a lower impedance growth rate, indicating that the lithium ion battery has good cycle stability.

[0100] The type of compound of formula II generally affects the cycle stability of the lithium ion battery. As can be seen from Example 2-10, Example 2-19 to Example 2-21, the selection of the compound of formula II within the scope of the present application can make the lithium ion battery have a higher capacity retention rate and a lower impedance growth rate, indicating that the lithium ion battery has good cycle stability.

[0101] Table 3

[0102] The thickness of the aluminum current collector and the copper current collector usually affects the cycle stability of the lithium ion battery. As can be seen from Example 1-1, Example 3-1 to Example 3-2, regulating the thickness of the aluminum current collector and the copper current collector is within the scope of the present application, which can make the lithium ion battery have a higher capacity retention rate and a lower impedance growth rate, indicating that the lithium ion battery has good cycle stability.

[0103] It should be noted that the terms "comprising", "containing", or any other variant thereof are intended to cover non-exclusive inclusions, so that a process, method or article that includes a series of elements not only includes those elements, but also includes other elements not explicitly listed, or inherent to such a process, method or article.

[0104] Each of the embodiments in the specification is described in a related manner, and the same or similar parts between the embodiments can be referred to each other. Each embodiment focuses on the difference from other embodiments.

[0105] The above only describes the preferred embodiments of the present application and is not intended to limit the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. An electrolyte for a secondary battery, comprising ethyl fluorocarbonate, ethylene fluorocarbonate and lithium difluorophosphate, wherein the ethyl fluorocarbonate comprises at least one of difluoroethyl acetate or trifluoroethyl acetate; Based on the mass of the electrolyte, the mass percentage of the ethyl fluorophosphate is A%, 12≤A≤65%, the mass percentage of the ethylene fluorocarbonate is B%, 2.7≤B≤10.3%, and the mass percentage of the lithium difluorophosphate is C%, 0.05≤C≤0.5%.

2. The electrolyte according to claim 1, wherein, 15≤A+B≤73.

8.

3. The electrolyte according to claim 1, wherein, 1.2≤A / B≤21.

5.

4. The electrolyte according to claim 1, wherein it satisfies at least one of the following characteristics: (1)20≤A≤50； (2)2.7≤B≤8。 5. The electrolyte according to any one of claims 1 to 4, wherein, The electrolyte further includes a compound of formula I, which includes at least one of the following compounds: Based on the mass of the electrolyte, the mass percentage of the compound of formula I is D%, 0.2≤D≤6.

6. The electrolyte according to claim 5, wherein, 0.06≤D / B≤1.

4.

7. The electrolyte according to any one of claims 1 to 6, wherein, The electrolyte further includes a compound of formula II, which includes at least one of the following compounds: Based on the mass of the electrolyte, the mass percentage of the compound of formula II is E%, 0.2≤E≤3.

8. The electrolyte according to any one of claims 5 to 7, wherein, The electrolyte also includes a compound of formula I and a compound of formula II. Based on the mass of the electrolyte, the mass percentage of compound of formula I is D%, the mass percentage of compound of formula II is E%, and 1 ≤ D + E ≤ 6.

5.

9. A secondary battery comprising a positive electrode, a negative electrode, and an electrolyte according to any one of claims 1 to 8.

10. The secondary battery according to claim 9, wherein, The positive electrode includes an aluminum current collector with a thickness of 4 μm to 10 μm.

11. The secondary battery according to claim 9, wherein, The negative electrode includes a copper current collector with a thickness of 4 μm to 10 μm.

12. An electronic device comprising a secondary battery as described in any one of claims 9 to 11.

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