Electrolyte solution and secondary battery

Abstract

An electrolyte solution and a secondary battery. The electrolyte solution comprises a solvent, an electrolyte, and an additive. The additive comprises a compound represented by formula 1, fluoroethylene carbonate, and lithium difluoro(bisoxalato)phosphate, which act synergistically to form, on a negative electrode interface, a solid electrolyte interface film having high mechanical strength, good stability, high ion conductively, and high toughness, thereby significantly improving the normal-temperature cycle performance, high-temperature cycle performance, and high-temperature storage performance of secondary batteries, and reducing the impedance growth rate.

Description

Electrolyte and secondary battery

[0001] The present application claims priority to the Chinese patent application No. 202411091343.3 filed on August 9, 2024, and entitled "Electrolyte and secondary battery", 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 and a secondary battery. BACKGROUND

[0003] Secondary batteries, such as lithium ion batteries, are widely used in the fields of smart phones, wearable devices, consumer drones and electric vehicles, etc. due to their high energy density, long cycle life and no memory effect. With the wide application of lithium ion batteries in the above fields, the market requires lithium ion batteries to have higher energy density, and at the same time, good room temperature cycle performance, high temperature storage performance, high temperature cycle performance and lower impedance growth rate.

[0004] The electrolyte is the ion conduction carrier between the positive and negative electrodes of the lithium ion battery, and it is very crucial to the performance of the lithium ion battery. Therefore, it is urgent to develop a more comprehensive electrolyte to improve the room temperature cycle performance, high temperature cycle performance, high temperature storage performance and reduce the impedance growth rate of the lithium ion battery. SUMMARY

[0005] The purpose of the present application is to provide an electrolyte and a secondary battery to improve the room temperature cycle performance, high temperature cycle performance, high temperature storage performance and reduce the impedance growth rate of the secondary battery. The specific technical solutions are as follows:

[0006] The first aspect of the present application provides an electrolyte, which comprises a solvent, an electrolyte and an additive, the additive comprising a compound represented by Formula 1, fluoroethylene carbonate and lithium difluorophosphate;

[0007] wherein n is 0 or 1, A is selected from methylene or O, X is selected from sulfonyl or carbonyl, R1, R2 are each independently selected from H, one of X, R1and R2is selected from H, and at least one of X, R1and R2contains a sulfur atom. The electrolyte includes the compound of Formula 1 within the scope of the present application, fluoroethylene carbonate and lithium difluoro(oxalato)phosphate, which synergistically act together to facilitate the formation of a solid electrolyte interface film (SEI film) at the negative electrode interface, which has high mechanical strength, good stability, high ionic conductivity and toughness, effectively reduces the risk of SEI film damage caused by volume change of the negative electrode sheet during the cycle process of the secondary battery, reduces the side reactions caused by direct contact between the electrode material and the electrolyte, reduces the risk of lithium precipitation on the negative electrode surface, and improves the room temperature cycle performance, high temperature cycle performance, high temperature storage performance and impedance growth rate of the secondary battery.

[0008] In some embodiments of the present application, the mass percentage content of the compound of Formula 1 is M A , the mass percentage content of fluoroethylene carbonate is M B , and the mass percentage content of lithium difluoro(oxalato)phosphate is M C , which satisfy: 0.5%≤M A ≤7%, 2%≤M B ≤20%, 0.1%≤M C ≤1%. The electrolyte includes the compound of Formula 1 within the scope of the present application, fluoroethylene carbonate and lithium difluoro(oxalato)phosphate, and regulates the content of the compound of Formula 1, fluoroethylene carbonate and lithium difluoro(oxalato)phosphate within the scope of the present application, which synergistically act together to further facilitate the formation of a solid electrolyte interface film (SEI film) at the negative electrode interface, which has high mechanical strength, good stability, high ionic conductivity and toughness, effectively reduces the risk of SEI film damage caused by volume change of the negative electrode sheet during the cycle process of the secondary battery, reduces the side reactions caused by direct contact between the electrode material and the electrolyte, reduces the risk of lithium precipitation on the negative electrode surface, and further improves the room temperature cycle performance, high temperature cycle performance, high temperature storage performance and impedance growth rate of the secondary battery.

[0009] In some embodiments of the present application, the compound of Formula 1 is selected from the compound of Formula 2:

[0010] wherein X, R1and R2contain 1 or 2 sulfur atoms.

[0011] In some embodiments of the present application, the compound of Formula 1 is selected from at least one of the following compounds:

[0012] In some embodiments of the present application, the electrolyte satisfies at least one of the following conditions: (1) 1%≤M A ≤5%; (2) 10%≤M B ≤15%; (3) 0.5%≤M C≤1%. The electrolyte satisfies at least one of (1) to (3), which can further improve the performance of the SEI film, thereby further improving the normal-temperature cycle performance, high-temperature cycle performance, high-temperature storage performance of the secondary battery and reducing the impedance growth rate.

[0013] In some embodiments of the present application, the additive further comprises other additives, the other additives being selected from at least one of vinylene carbonate and lithium bisoxalato borate; the mass percentage of the other additives is M D , 0.3%≤M D ≤5%. The electrolyte comprises the other additives within the above range and regulates the mass percentage of the other additives within the range of the present application, which is conducive to the formation of stable interface films on the positive electrode and the negative electrode, and further improves the high-temperature cycle performance of the secondary battery.

[0014] In some embodiments of the present application, the electrolyte is selected from at least one of lithium hexafluorophosphate, lithium difluorophosphate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium perchlorate, lithium bisfluorosulfonimide, lithium bis(trifluoromethanesulfonimide), lithium bisoxalato borate and lithium difluoro oxalato borate; the mass percentage of the electrolyte is M E , 12%≤M E ≤18%. The electrolyte comprises the electrolyte within the above range and regulates the mass percentage of the electrolyte within the range of the present application, which can make the electrolyte have higher ionic conductivity and good electrochemical stability, and can further improve the normal-temperature cycle performance, high-temperature cycle performance and high-temperature storage performance of the secondary battery.

[0015] In some embodiments of the present application, the solvent is selected from at least two of propylene carbonate, ethyl methyl carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, γ-butyrolactone, sulfolane, methyl formate, ethyl formate, propyl formate, butyl formate, methyl acetate, ethyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, propyl propionate, butyl propionate, methyl butyrate, ethyl butyrate, propyl butyrate and butyl butyrate; the mass percentage of the solvent is M F , 55%≤M F ≤80%. The electrolyte comprises the solvent within the above range and regulates the mass percentage of the solvent within the range of the present application, which can make the electrolyte have suitable viscosity, higher ionic conductivity and good electrochemical stability, and can further improve the normal-temperature cycle performance, high-temperature cycle performance and high-temperature storage performance of the secondary battery.

[0016] The second aspect of the present application provides a secondary battery, which comprises a negative electrode sheet, a positive electrode sheet, a separator and the electrolyte provided by the first aspect of the present application.

[0017] In some embodiments of the present application, the negative electrode tab comprises a negative electrode material layer, the negative electrode material layer comprises a negative electrode active material, the mass percentage of silicon element is M Si ; after formation of the secondary battery, the mass percentage of the compound represented by Formula 1 is M a , the mass percentage of fluoroethylene carbonate is M b , and the mass percentage of lithium difluorophosphate is M c ; the formation condition is that the secondary battery is placed in an environment of 40-50°C, charged at a constant current, and charged to a state of charge of 70-100%, the constant current being 0.05-0.15C; the secondary battery satisfies the relationship formula I: 15≤[15×(M A +M B ) / (M a +M b )+10×M C ×(M C -M c )] / (1.5+M Si )≤234, wherein the injection coefficient of the secondary battery is 1.5+M Si , unit: g / Ah. When the secondary battery satisfies the above relationship formula I, the energy density of the secondary battery can be improved, and the high-temperature cycle performance, high-temperature storage performance, and impedance growth rate of the secondary battery are also considered.

[0018] In some embodiments of the present application, 10%≤M Si ≤30%. By adjusting the value of M si within the range of the present application, the energy density of the secondary battery can be improved, and the normal-temperature cycle performance, high-temperature cycle performance, high-temperature storage performance, and impedance growth rate of the secondary battery are also considered.

[0019] In some embodiments of the present application, the secondary battery satisfies at least one of the following conditions: (1) 0.01%≤M a ≤1.81%; (2) 0.24%≤M b ≤7.23%; (3) 0%≤M c≤0.33%. After the formation of the SEI film by the compound of formula 1, FEC or LiODFP in the electrolyte after the formation of the secondary battery, the remaining compound of formula 1 is beneficial to weaken the interaction between lithium ions and solvent molecules, so that the lithium ions are easy to desolvate, and the reversible deintercalation of lithium ions in the negative electrode is improved; the remaining FEC is beneficial to repair the damaged SEI film; the remaining LiODFP can further improve the ionic conductivity of the electrolyte. Therefore, when the secondary battery meets at least one of the above conditions, the room temperature cycle performance, high temperature cycle performance, high temperature storage performance of the secondary battery can be further improved, and the impedance growth rate is reduced.

[0020] In some embodiments of the present application, the secondary battery satisfies relationship formula II: 16≤[15×(M A +M B ) / (M a +M b )+10×M C ×(M C -M c )] / (1.5+M Si )≤83.3. When the secondary battery meets the above relationship formula II, the energy density of the secondary battery can be improved, and the room temperature cycle performance, high temperature cycle performance, high temperature storage performance of the secondary battery are taken into account and the impedance growth rate is reduced.

[0021] In some embodiments of the present application, the secondary battery satisfies relationship formula III: 16≤[15×(M A +M B ) / (M a +M b )+10×M C ×(M C -M c )] / (1.5+M Si )≤33. When the secondary battery meets the above relationship formula III, the energy density of the secondary battery can be further improved, and the room temperature cycle performance, high temperature cycle performance, high temperature storage performance of the secondary battery are taken into account and the impedance growth rate is reduced.

[0022] Advantages of the present application:

[0023] The present application provides an electrolyte and a secondary battery, the electrolyte comprising a solvent, an electrolyte and an additive, the additive comprising a compound of formula 1, fluoroethylene carbonate and lithium difluorophosphate, which synergistically act together to form a solid electrolyte interface film with high mechanical strength, good stability, high ionic conductivity and high toughness at the negative electrode interface, which can significantly improve the room temperature cycle performance, high temperature cycle performance and high temperature storage performance of the secondary battery, and reduce the impedance growth rate.

[0024] Of course, practicing any of the products or methods of the present application does not necessarily require achieving all of the above advantages simultaneously. DETAILED DESCRIPTION

[0025] The technical solutions in the embodiments of the present application will be clearly and completely described below. Obviously, the described embodiments are only some 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 are within the scope of protection of the present application.

[0026] It should be noted that in the specific embodiments of the present application, lithium ion batteries are used as examples of secondary batteries to explain the present application, but the secondary batteries of the present application are not limited to lithium ion batteries. The specific technical solutions are as follows:

[0027] The first aspect of the present application provides an electrolyte, which comprises a solvent, an electrolyte and an additive, the additive comprising a compound represented by Formula 1, fluoroethylene carbonate and lithium difluoro(oxalato)phosphate;

[0028] wherein n is 0 or 1, A is selected from methylene or O, X is selected from sulfonyl or carbonyl, R1, R2 are each independently selected from H, one of H, and R1 and R2 are not simultaneously selected from H, and at least one of X, R1 and R2 contains a sulfur atom. The electrolyte comprises the compound represented by Formula 1, fluoroethylene carbonate and lithium difluoro(oxalato)phosphate within the scope of the present application, which synergistically act together to form a solid electrolyte interface film (SEI film) with high mechanical strength, good stability, high ionic conductivity and toughness at the negative electrode interface, effectively reducing the risk of SEI film damage caused by volume change of the negative electrode sheet during the cycle process of the secondary battery, reducing the side reactions caused by direct contact of electrode materials and electrolyte, reducing the risk of lithium precipitation on the negative electrode surface, and improving the room temperature cycle performance, high temperature cycle performance, high temperature storage performance and impedance growth rate of the secondary battery.

[0029] The inventors have found that during the first charging process of the secondary battery, the compound represented by Formula 1 in the present application will preferentially decompose at low potential, resulting in a solid electrolyte interface film (SEI film) rich in inorganic components such as Li2SO4, Li2CO3, etc., and active metal ions (such as Li +) diffusion sites, which is conducive to reducing the film impedance of the negative electrode interface. Moreover, the compound represented by Formula 1 rich in double bonds is prone to reduction reaction at the negative electrode to form an insoluble organic film, which can act as an elastic component in the SEI film to make the SEI film have toughness to cope with the volume expansion of the negative electrode. However, when the volume expansion of the negative electrode is large, the toughness of the SEI film is not enough, and the SEI film will still break, causing the negative active material and the electrolyte to directly contact and react, affecting the high-temperature cycle performance and high-temperature storage performance of the secondary battery and increasing the impedance of the secondary battery. To solve this problem, the present application further introduces fluoroethylene carbonate (FEC) and lithium difluorophosphate (LiODFP) into the electrolyte. The FEC decomposes at the negative electrode interface to provide a large number of double bond and triple bond monomers, which polymerize to form a polymer film with low Young's modulus and excellent toughness, further improving the toughness of the SEI film. This can effectively inhibit the damage of the SEI film caused by the volume change of the negative electrode and reduce the side reactions caused by the direct contact of the negative active material and the electrolyte, further improving the room-temperature cycle performance, high-temperature cycle performance, and high-temperature storage performance of the secondary battery and reducing the impedance growth rate. The introduction of LiODFP provides a large amount of Li + vacancies for the SEI film, and the space charge layer formed between the defects of the negative electrode interface can further improve the ionic conductivity of the negative electrode interface, which is conducive to the transmission of lithium ions and reduces the risk of lithium precipitation on the negative electrode surface, further improving the high-temperature cycle performance and safety performance of the secondary battery.

[0030] In some embodiments of the present application, based on the mass of the electrolyte, the mass percentage of the compound represented by Formula 1 is M A , the mass percentage of fluoroethylene carbonate is M B , and the mass percentage of lithium difluorophosphate is M C , which satisfies: 0.5%≤M A ≤7%, 2%≤M B ≤20%, 0.1%≤M C ≤1%, optionally, 1%≤M A ≤3%, 10%≤M B ≤15%, 0.5%≤M C ≤1%. For example, the value of M A may be 0.5%, 0.8%, 1%, 1.2%, 1.5%, 1.8%, 2%, 2.2%, 2.5%, 2.8%, 3%, 3.2%, 3.5%, 3.8%, 4%, 4.2%, 4.5%, 4.8%, 5%, 5.2%, 5.5%, 5.8%, 6%, 6.2%, 6.5%, 6.8%, 7%, or a range formed by any two of the above values, M Bmay be 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, or a range between any two of them, M C may be 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, or a range between any two of them. When M A is too small, for example, M A is less than 0.5%, the generated SEI film has less Li2SO4, Li2CO3 inorganic components, and less active metal ion (e.g., Li + ) diffusion sites, which is not conducive to reducing the impedance growth rate; when M A is too large, for example, greater than 7%, it will cause the formed SEI film to be too thick, and the capacity loss of the secondary battery is obvious. When M B is too small, for example, M B is less than 2%, the formed SEI film has insufficient toughness, and when the negative electrode volume expands greatly, the SEI film is easy to break, and the negative active material and the electrolyte directly contact to cause side reactions, which affects the normal temperature cycle performance, high temperature cycle performance and high temperature storage performance of the secondary battery and increases the impedance of the secondary battery; when M B is too large, for example, greater than 20%, it causes the electrolyte to have a large viscosity, which is not conducive to the transmission of active metal ions (e.g., Li + ), and the residual FEC in the electrolyte is easy to decompose and produce gas at high temperature, causing the secondary battery to expand seriously, which seriously affects the normal temperature cycle performance, high temperature cycle performance and high temperature storage performance of the secondary battery and increases the impedance of the secondary battery. M B Within the scope of the present application, for example, when M B is 2% to 5%, the secondary battery can have better normal temperature performance, because under normal temperature working conditions, the negative electrode is less deteriorated, the demand for FEC is reduced, and the addition of FEC in small amounts can avoid the influence of excessive FEC on normal temperature performance; when M B is 5% to 20%, the secondary battery can have better high temperature performance, because under high temperature working conditions, the negative electrode deteriorates rapidly, and the increase in the amount of FEC can better improve the toughness of the SEI film. The FEC improves the toughness by forming a VC-like polymer, and the formation of LiF with high Young's modulus better isolates the electrolyte, improves the quality of the SEI film, and reduces the side reactions caused by the direct contact of the negative active material and the electrolyte. When M C is too small, for example, M Cless than 0.1%, the SEI film has less inorganic components, the strength of the SEI film is reduced, the SEI film is easy to break when the negative electrode expands, the negative active material and the electrolyte directly contact to cause side reactions, which affects the high-temperature cycle performance and high-temperature storage performance of the secondary battery and increases the impedance of the secondary battery; when the value of M C is too large, for example, greater than 1%, too much LiODFP is decomposed in the electrolyte, the -F in LiODFP is broken, and HF is generated by reacting with trace water in the electrolyte, which can damage the interface film of the positive electrode and the negative electrode, affecting the high-temperature cycle performance and high-temperature storage performance of the secondary battery. The electrolyte of the present application simultaneously includes the compound represented by formula 1, FEC and LiODFP, and the mass percentage contents of the compound represented by formula 1, FEC and LiODFP are controlled within the range of the present application, and the three components synergistically act to form a solid electrolyte interface film with high mechanical strength, good stability, high ionic conductivity and high toughness at the negative electrode interface, which can significantly improve the room-temperature cycle performance, high-temperature cycle performance and high-temperature storage performance of the secondary battery and reduce the impedance growth rate.

[0031] In some embodiments of the present application, the compound represented by formula 1 is selected from the compound represented by formula 2:

[0032] wherein X, R1 and R2 contain 1 or 2 sulfur atoms. The compound represented by formula 1 is selected from the compound represented by formula 2, which is beneficial to improve the mechanical strength, stability and toughness of the SEI film, and can further improve the room-temperature cycle performance, high-temperature cycle performance and high-temperature storage performance of the secondary battery and reduce the impedance growth rate.

[0033] In some embodiments of the present application, the compound represented by formula 1 is selected from at least one of the following compounds:

[0034] The electrolyte includes the compound represented by formula 1 within the above range, which is beneficial to improve the mechanical strength, stability and toughness of the SEI film, and can further improve the room-temperature cycle performance, high-temperature cycle performance and high-temperature storage performance of the secondary battery and reduce the impedance growth rate.

[0035] In some embodiments of the present application, the additive further includes other additives, and the other additives are selected from at least one of vinyl carbonate (VC) and lithium bis(oxalato)borate (LiBOB); the mass percentage content of the other additives is M D , 0.3%≤M D ≤5% based on the mass of the electrolyte. For example, M DThe value of M can be 0.3%, 0.5%, 0.8%, 1%, 1.2%, 1.5%, 1.8%, 2%, 2.2%, 2.5%, 2.8%, 3%, 3.2%, 3.5%, 3.8%, 4%, 4.2%, 4.5%, 4.8%, 5%, or a range between any two of the values. The electrolyte includes other additives within the above range and regulates the mass percentage of the other additives within the scope of the present application, which is conducive to forming a stable interface film between the positive electrode and the negative electrode and further improving the high-temperature cycle performance of the secondary battery.

[0036] In some embodiments of the present application, the electrolyte is selected from at least one of lithium hexafluorophosphate, lithium difluorophosphate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium perchlorate, lithium bisfluorosulfonimide, lithium bis-trifluoromethanesulfonimide, lithium bisoxalate borate, and lithium difluoro oxalate borate, and the mass percentage of the electrolyte is M E , 12%≤M E ≤18%; for example, M E The value of M can be 12%, 13%, 14%, 15%, 16%, 17%, 18%, or a range between any two of the values. The electrolyte includes the electrolyte within the above range and regulates the mass percentage of the electrolyte within the scope of the present application, which can make the electrolyte have higher ionic conductivity and good electrochemical stability, and can further improve the normal-temperature cycle performance, high-temperature cycle performance, and high-temperature storage performance of the secondary battery.

[0037] In some embodiments of the present application, the solvent is selected from at least two of propylene carbonate, ethyl methyl carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, γ-butyrolactone, sulfolane, methyl formate, ethyl formate, propyl formate, butyl formate, methyl acetate, ethyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, propyl propionate, butyl propionate, methyl butyrate, ethyl butyrate, propyl butyrate, and butyl butyrate, and the mass percentage of the solvent is M F , 55%≤M F ≤80%; for example, M F The value of M can be 55%, 58%, 60%, 62%, 65%, 68%, 70%, 72%, 75%, 78%, 80%, or a range between any two of the values. The electrolyte includes the solvent within the above range and regulates the mass percentage of the solvent within the scope of the present application, which can make the electrolyte have suitable viscosity, higher ionic conductivity, and good electrochemical stability, and can further improve the normal-temperature cycle performance, high-temperature cycle performance, and high-temperature storage performance of the secondary battery.

[0038] A second aspect of the present application provides a secondary battery, which includes a negative electrode sheet, a positive electrode sheet, a separator, and the electrolyte provided by the first aspect of the present application.

[0039] In some embodiments of the present application, the negative electrode tab comprises a negative electrode material layer, the negative electrode material layer comprises a negative electrode active material, the mass percentage of silicon element is M Si ; after formation of the secondary battery, the mass percentage of the compound represented by Formula 1 is M a , the mass percentage of fluoroethylene carbonate is M b , and the mass percentage of lithium difluorophosphate bisoxalate is M c ; the formation condition is that the secondary battery is charged at a constant current in an environment of 40-50°C, the secondary battery is charged to a state of charge of 70-100%, and the constant current is 0.05-0.15C; the secondary battery satisfies the relationship formula I: 15≤[15×(M A +M B ) / (M a +M b )+10×M C ×(M C -M c )] / (1.5+M Si )≤234, wherein the injection coefficient of the secondary battery is 1.5+M Si , and the unit is g / Ah; optionally, the secondary battery satisfies the relationship formula II: 16≤[15×(M A +M B ) / (M a +M b )+10×M C ×(M C -M c )] / (1.5+M Si )≤83.3; further optionally, the secondary battery satisfies the relationship formula III: 16≤[15×(M A +M B ) / (M a +M b )+10×M C ×(M C -M c )] / (1.5+M Si )≤33. For example, the value of the relationship formula I can be 15, 16, 17, 18, 19, 20, 21, 22, 24, 25, 26, 27, 30, 33, 35, 40, 50, 60, 70, 80, 83.3, 90, 100, 120, 140, 160, 180, 200, 220, 230, 234, or a range formed by any two of the above values. It should be noted that the unit of the injection coefficient can also be mg / mAh.

[0040] In the present application, the silicon element in the negative active material is derived from a silicon-based material, which can include but is not limited to at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, or silicon alloys. The negative active material also includes a carbon-based material, which can include but is not limited to at least one of artificial graphite, natural graphite, soft carbon, hard carbon, or mesophase carbon microbeads.

[0041] The silicon-based material has a high theoretical capacity, which is conducive to improving the energy density of the secondary battery. However, the silicon-based material will undergo a volume expansion of 120% to 300% during the lithium ion insertion and extraction in the charging and discharging process of the secondary battery. The use of the electrolyte provided by the present application is conducive to forming a solid electrolyte interface film with high mechanical strength, good stability, high ionic conductivity and high toughness at the negative electrode interface, which can effectively inhibit the rupture of the SEI film caused by the expansion of the silicon-based material, reduce the side reactions caused by the direct contact of the negative active material with the electrolyte; and after the formation of the SEI film by the compound represented by formula 1, FEC or LiODFP in the electrolyte during the formation and capacity of the secondary battery, the remaining compound represented by formula 1 is conducive to weakening the interaction between lithium ions and solvent molecules, making it easy for lithium ions to desolvate and improve the reversible deintercalation of lithium ions at the negative electrode; the remaining FEC is conducive to repairing the damaged SEI film; and the remaining LiODFP can further improve the ionic conductivity of the electrolyte. When the secondary battery satisfies the above relationship formula I, the energy density of the secondary battery can be improved, and the normal temperature cycle performance, high temperature cycle performance, high temperature storage performance and impedance growth rate of the secondary battery can be considered.

[0042] In some embodiments of the present application, 10%≤M Si ≤30%, for example, M si The value of M si can be 10%, 12%, 15%, 18%, 20%, 22%, 25%, 28%, 30%, or a range composed of any two of the above values. By adjusting the value of M si within the range of the present application, the energy density of the secondary battery can be improved, and the normal temperature cycle performance, high temperature cycle performance, high temperature storage performance and impedance growth rate of the secondary battery can be considered.

[0043] In some embodiments of the present application, 0.01%≤M a ≤1.81%, for example, M a The value of M aThe value of M within the scope of the present application is conducive to weakening the interaction between lithium ions and solvent molecules, making the lithium ions easy to desolvate, improving the reversible deintercalation of lithium ions in the negative electrode, and further improving the room temperature cycle performance, high temperature cycle performance, high temperature storage performance of the secondary battery and reducing the impedance growth rate.

[0044] In some embodiments of the present application, 0.24%≤M b ≤7.23%, for example, M b The value of M can be 0.24%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.23%, or a range formed by any two of them. M b The value of M within the scope of the present application is conducive to repairing the damaged SEI film, further improving the room temperature cycle performance, high temperature cycle performance, high temperature storage performance of the secondary battery and reducing the impedance growth rate.

[0045] In some embodiments of the present application, 0%≤M c ≤0.33%, for example, M c The value of M can be 0%, 0.05%, 0.1%, 0.12%, 0.15%, 0.18%, 0.2%, 0.22%, 0.25%, 0.28%, 0.3%, 0.33%, or a range formed by any two of them. M c The value of M within the scope of the present application is conducive to further improving the ionic conductivity of the electrolyte, further improving the room temperature cycle performance, high temperature cycle performance, high temperature storage performance of the secondary battery and reducing the impedance growth rate.

[0046] Generally, when other conditions are unchanged, the value of M a , the value of M b , the value of M c can be adjusted by changing the value of M A , the value of M B , the value of M C . For example, increasing the value of M A , M a increases; reducing the value of M A , M a decreases. Increasing the value of M B , M b increases; reducing the value of M B , M b decreases. Increasing the value of M C , M c increases; reducing the value of M C , M c decreases. When other conditions are unchanged, the value of M a , the value of M bThe value of M c may also be regulated by changing the value of M Si . For example, increasing the value of M Si , the value of M a , M b , M c decreases; decreasing the value of M Si , the value of M a , M b , M c increases. In combination with the "test of M a , M b , M c provided in the present application, the corresponding values of M a , M b , M c are measured.

[0047] In the present application, the negative electrode tab includes a negative electrode current collector and a negative electrode material layer disposed on at least one surface of the negative electrode current collector. The above-mentioned "negative electrode material layer disposed on at least one surface of the negative electrode current collector" means that the negative electrode material layer can be disposed on one surface of the negative electrode current collector along the thickness direction of the negative electrode current collector, or can be disposed on two surfaces of the negative electrode current collector along the thickness direction of the negative electrode current collector. It should be noted that the "surface" here can be the entire area of the surface of the negative electrode current collector, or can be part of the area of the surface of the negative electrode current collector, which is not particularly limited in the present application as long as the purpose of the present application can be achieved. The negative electrode current collector is not particularly limited in the present application as long as the purpose of the present application can be achieved, for example, the negative electrode current collector can be a copper foil, a copper alloy foil, a nickel foil, a stainless steel foil, a titanium foil, a foamed nickel or foamed copper, an aluminum foil or a composite negative electrode current collector. The above-mentioned composite negative electrode current collector can be a high polymer material base layer and a metal layer formed on at least one surface of the high polymer material base material, and the material of the above-mentioned high polymer material base layer can include but is not limited to at least one of polypropylene (PP), polyethylene terephthalate (PET) or polybutylene terephthalate (PBT), and the material of the above-mentioned metal layer can include but is not limited to at least one of copper, copper alloy, nickel or nickel alloy. The thickness of the negative electrode material layer and the negative electrode current collector 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 negative electrode material layer is 50 μm to 180 μm, and the thickness of the negative electrode current collector is 3 μm to 10 μm.

[0048] The application does not have special restrictions on the types of negative electrode conductive agent and negative electrode binder, as long as the purpose of the application can be achieved. For example, the negative electrode conductive agent can include, but is not limited to, at least one of Super P, acetylene black, Ketjen black, carbon nanotubes, graphene, or carbon fibers. 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. For example, the negative electrode binder can include, but is not limited to, at least one of styrene butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), sodium carboxymethyl cellulose (CMC), polymethacrylic acid (PMAA), or carboxymethyl chitosan (CMCS).

[0049] In the present application, the preparation method of the negative electrode tab is not particularly limited, as long as the purpose of the application can be achieved, for example, it can be prepared by the following method: mixing carbon-based materials and silicon-based materials in a certain proportion, uniformly mixing to obtain a negative electrode active material, then adding deionized water to the negative electrode active material, negative electrode conductive agent, negative electrode binder, and stirring uniformly to obtain a negative electrode slurry with a solid content of 45wt% to 70wt%. The negative electrode slurry is uniformly coated on one surface of the negative electrode current collector, and after drying, a negative electrode tab with a single-sided coated negative electrode material layer is obtained. Then repeat the above coating step on the other surface of the negative electrode current collector, and after drying, a negative electrode tab with a double-sided coated negative electrode material layer is obtained. After coating is completed, cold pressing and cutting are performed to obtain a negative electrode tab.

[0050] Generally, under other conditions unchanged, the mass percentage content of silicon element in the negative electrode active material can be adjusted by adjusting the ratio of carbon-based materials and silicon-based materials, for example, increasing the ratio of carbon-based materials and silicon-based materials, the mass percentage content of silicon element in the negative electrode active material decreases, and decreasing the ratio of carbon-based materials and silicon-based materials, the mass percentage content of silicon element in the negative electrode active material increases.

[0051] In the present application, the secondary battery includes a positive electrode sheet, and the positive electrode sheet includes a positive electrode current collector and a positive electrode material layer disposed on at least one surface of the positive electrode current collector. The "positive electrode material layer disposed on at least one surface of the positive electrode current collector" means that the positive electrode material layer can be disposed on one surface of the positive electrode current collector in the thickness direction of the positive electrode current collector, or can be disposed on both surfaces of the positive electrode current collector in the thickness direction of the positive electrode current collector. It should be noted that the "surface" herein can be the entire area of the surface of the positive electrode current collector, or can be part of the area of the surface of the positive electrode current collector, and the present application does not have a particular limitation, as long as the purpose of the present application can be achieved. The present application does not have a particular limitation 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 can be an aluminum foil, an aluminum alloy foil or a composite positive electrode current collector. The composite positive electrode current collector can be a high polymer material base layer and a metal layer formed on at least one surface of the high polymer material base layer, and the material of the high polymer material base layer can include but is not limited to at least one of polypropylene (PP), polyethylene terephthalate (PET) or polybutylene terephthalate (PBT), and the material of the metal layer can include but is not limited to at least one of aluminum, aluminum alloy, nickel or nickel alloy. The present application does not have a particular limitation on the thickness of the positive electrode material layer and the positive electrode current collector, 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 50 μm to 250 μm, and the thickness of the positive electrode current collector is 7 μm to 15 μm.

[0052] The positive electrode material layer includes a positive electrode active material, and the present application does not have a particular limitation on the positive electrode active material, as long as the purpose of the present application can be achieved, for example, the positive electrode active material can include but is not limited to at least one of lithium nickel cobalt manganese oxide (such as NCM811, NCM622, NCM523, NCM111), lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium-rich manganese-based material, lithium cobaltate (LiCoO2), lithium manganate, lithium manganese iron phosphate or lithium titanate.

[0053] The positive electrode material layer can further include a positive electrode conductive agent and a positive electrode binder, and the present application does not have a particular limitation on the types of the positive electrode conductive agent and the positive electrode binder, 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 Super P, acetylene black, Ketjen black, carbon nanotube, graphene or carbon fiber. For example, the positive electrode binder can include but is not limited to at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer or fluorine-containing acrylic ester resin. 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, and a person skilled in the art can select according to the actual needs, as long as the purpose of the present application can be achieved.

[0054] In the present application, the preparation method of 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 50wt% to 85wt%. The positive electrode slurry is uniformly coated on one surface of the positive electrode current collector, and after drying, a positive electrode tab with a single-sided coated positive electrode material layer is obtained. Then repeat the above coating step on the other surface of the positive electrode current collector, and after drying, a positive electrode tab with a double-sided coated positive electrode material layer is obtained. After coating is completed, cold pressing and cutting are performed to obtain the positive electrode tab.

[0055] In the present application, the secondary battery also includes a separator. The present application does not have a particular limitation on the separator, as long as the purpose 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), polypropylene (PP), glass fiber, polyester (e.g., polyethylene terephthalate (PET) film), cellulose, polyimide (PI), and polyamide (PA). The type of the separator can include at least one of woven film, non-woven fabric, microporous film, composite film, calendered film, or spunlaced film. In the present application, the thickness of the separator is not particularly limited, as long as the purpose of the present application can be achieved, for example, the thickness of the separator can be 4μm to 20μm.

[0056] In the present application, the secondary battery also includes a housing for accommodating the positive electrode tab, the separator, the negative electrode tab, and the electrolyte, as well as other components known in the art of secondary batteries, which are not limited in the present application. The housing is not particularly limited in the present application, and can be a housing known in the art, as long as the purpose of the present application can be achieved. For example, the housing can be a hard shell housing or a flexible housing. The material of the hard shell housing can be metal, and the type of metal is not limited in the present application, and a metal hard shell housing known in the art can be used, as long as the purpose of the present application can be achieved. The flexible housing can be a metal plastic film, such as an aluminum plastic film, a steel plastic film, etc.

[0057] The secondary battery of the present application is not particularly limited, and it can include any device that undergoes an electrochemical reaction. In an embodiment of the present application, the secondary battery can include, but is not limited to: a lithium ion secondary battery (lithium ion battery), a sodium ion secondary battery (sodium ion battery), etc.

[0058] 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, etc. as needed to obtain an electrode assembly of a wound structure, placing the electrode assembly into the shell, injecting the electrolyte into the shell and sealing, to obtain 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 of a stack structure, the electrode assembly is placed into the shell, the electrolyte is injected into the shell and sealed, to obtain a secondary battery. In addition, the overcurrent prevention element, the guide plate, etc. can also be placed in the shell as needed, so as to prevent the pressure rise in the secondary battery.

[0059] Examples

[0060] Hereinafter, examples and comparative examples are given to more specifically describe the embodiments of the present application. Various tests and evaluations are carried out according to the following methods. In addition, unless otherwise specified, "parts" and "%" are mass-based.

[0061] Test methods and equipment:

[0062] M a , M b , M c Test

[0063] The lithium ion batteries after formation of the examples and comparative examples were disassembled, the electrolyte was collected, and the disassembled positive electrode sheet, negative electrode sheet, and separator were centrifuged. The liquid obtained after centrifugation and the above-mentioned electrolyte were mixed uniformly to obtain a liquid sample, which was subjected to ion chromatography (IC) test to measure the types and contents of salt compounds such as lithium hexafluorophosphate and lithium difluorophosphate dioxalate in the electrolyte. The above-mentioned liquid sample was tested by gas chromatography-mass spectrometry (GC-MS) to measure the mass ratio of each component in the electrolyte, and then the mass percentage of each component in the electrolyte was calculated by combining the above- measured contents of lithium hexafluorophosphate and lithium difluorophosphate dioxalate, so as to obtain the values of M a , M b , M c .

[0064] Normal temperature cycle performance test

[0065] The lithium ion battery was placed in a 25°C constant temperature box and rested for 4 h, then the lithium ion battery was charged at 1C constant current to a voltage of 4.25 V, then charged at 4.25 V constant voltage to a current of 0.05C, then discharged at 1C constant current to a voltage of 2.75 V, and the initial discharge capacity was recorded as C1. This was one cycle of charging and discharging, and the above charging and discharging cycle was repeated 1200 times, and the discharge capacity after 1200 cycles was recorded as C2. The charging and discharging cycle tester was a new Wei BTS.

[0066] 25°C capacity retention rate (%) = C2 / C1 x 100%; the 25°C capacity retention rate was used to evaluate the normal temperature cycle performance of the lithium ion battery, and the greater the capacity retention rate indicated the better the normal temperature cycle performance of the lithium ion battery.

[0067] High temperature cycle performance test

[0068] The lithium ion battery was placed in a 45°C constant temperature box and rested for 4 h, then the lithium ion battery was charged at 1C constant current to a voltage of 4.25 V, then charged at 4.25 V constant voltage to a current of 0.05C, then discharged at 1C constant current to a voltage of 2.75 V, and the initial discharge capacity was recorded as C3. This was one cycle of charging and discharging, and the above charging and discharging cycle was repeated 1200 times, and the discharge capacity after 1200 cycles was recorded as C4. The charging and discharging cycle tester was a new Wei BTS.

[0069] 45°C capacity retention rate (%) = C4 / C3 x 100%; the 45°C capacity retention rate was used to evaluate the high temperature cycle performance of the lithium ion battery, and the greater the capacity retention rate indicated the better the high temperature cycle performance of the lithium ion battery.

[0070] High temperature storage performance test

[0071] The lithium ion battery was placed in a 25℃ constant temperature box, and rested for 4h, then the lithium ion battery was charged at 1C constant current to 4.25V, then charged at 4.25V constant voltage to 0.05C, then discharged at 1C constant current to 2.75V, and the initial discharge capacity Q1 was recorded, the initial thickness D1 of the lithium ion battery was tested, then the lithium ion battery was charged at 1C constant current to 4.25V, then charged at 4.25V constant voltage to 0.05C, then the lithium ion battery was placed in a 60℃ explosion-proof oven, and the battery thickness D2 was tested in the oven after 30 days of storage, then the lithium ion battery was taken out and cooled to room temperature, and the lithium ion battery was discharged at 1C constant current to 2.75V at 25℃, and the discharge capacity Q2 was recorded, then the lithium ion battery was charged at 1C constant current to 4.25V, then charged at 4.25V constant voltage to 0.05C, and the charge capacity Q3 was recorded. Storage thickness expansion rate (%) = (D2-D1) / D1x100%; storage capacity retention rate (%) = Q2 / Q1x100%; storage capacity recovery rate (%) = Q3 / Q1x100%;

[0072] The high-temperature storage performance of the lithium ion battery was evaluated by the storage thickness expansion rate, the storage capacity retention rate and the storage capacity recovery rate, and the smaller the value of the storage thickness expansion rate, the larger the values of the storage capacity retention rate and the storage capacity recovery rate, the better the high-temperature storage performance of the lithium ion battery.

[0073] Test of direct current resistance (DCR)

[0074] Test of DCR1 of lithium ion battery before storage: the lithium ion battery was placed in a 25℃ environment, charged at 1C constant current to 4.25V, then charged at 4.25V constant voltage to 0.05C, and rested for 30min, then discharged at 1C constant current for 30min (adjusted to 50% SOC, SOC refers to the state of charge of the battery), and the end voltage V1 was recorded, then discharged at 2C constant current for 10s, and the end voltage V2 was recorded, and the DCR1 before storage was calculated as (V1-V2) / (2C-1C);

[0075] Test of DCR2 after storage of lithium ion battery: the above lithium ion battery was placed in a 60℃ explosion-proof oven, taken out after 30 days of storage, and placed in a 25℃ environment, 1C constant current charged to a voltage of 4.25V, then charged to a current of 0.05C at a constant voltage of 4.25V, and then left for 30min, then discharged at a constant current of 1C for 30min (adjusted to 50% SOC, SOC refers to the state of charge of the battery), and the end voltage was recorded as V3, left for 1h, then discharged at a constant current of 2C for 10s, and the end voltage was recorded as V4, and the DCR2 before storage was calculated as (V3-V4) / (2C-1C); impedance growth rate (%) = (DCR2-DCR1) / DCR1x100%.

[0076] Example 1-1

[0077] Preparation of electrolyte

[0078] Under the inert atmosphere environment with water content less than 0.1ppm and oxygen content less than 1ppm, ethylene carbonate (EC), diethyl carbonate (DEC) and methyl ethyl carbonate (EMC) were mixed in a mass ratio of 1:1:1 to obtain a base solvent, and then the base solvent was added with electrolyte lithium hexafluorophosphate (LiPF6), the compound shown in formula 1-1, fluoroethylene carbonate (FEC) and lithium difluorophosphate bisoxalate (LiODFP) to obtain the electrolyte. Among them, the mass percentage of LiPF6 is 15%, the mass percentage of the compound shown in formula 1-1 is 2%, the mass percentage of FEC is 12%, the mass percentage of LiODFP is 0.7%, and the remaining base solvent is 70.3% based on the mass of the electrolyte.

[0079] Preparation of negative electrode sheet

[0080] The negative active material artificial graphite, the negative active material elemental silicon, the conductive agent acetylene black, the binder carboxymethyl cellulose sodium (CMC) and the butadiene rubber (SBR) were mixed in a mass ratio of 87.3:9.7:1:1:1, deionized water was added as a solvent, and a slurry with a solid content of 49wt% was prepared. After uniform stirring by a vacuum stirrer, a negative electrode slurry was obtained. The negative electrode slurry was uniformly coated on one surface of a negative current collector copper foil with a thickness of 9μm, and dried at 90℃ to obtain a negative electrode sheet with a single-side coated negative electrode material layer, and the coating weight of the negative electrode material layer was 8mg / mm 2 . Then the above steps were repeated on the other surface of the copper foil to obtain a negative electrode sheet with a double-side coated negative electrode material layer. After drying at 85℃, rolling, slitting and welding of the tabs, a negative electrode sheet with a size of 74mmx59mm was obtained for use. The thickness of the single-side negative electrode material layer was 111μm; the mass percentage of silicon element M Si10%.

[0081] <Manufacture of the positive electrode sheet>

[0082] The positive electrode active material LiNi 0.8 Co 0.1 Mn 0.1 O2(NCM811), a positive electrode conductive agent acetylene black, and a positive electrode binder polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 96:2:2, N-methyl pyrrolidone (NMP) was added as a solvent, and a slurry with a solid content of 55 wt% was prepared. After uniform stirring in a vacuum, a positive electrode slurry was obtained. The positive electrode slurry was uniformly coated on one surface of a positive electrode current collector aluminum foil with a thickness of 12 μm, and was dried at 90°C to obtain a positive electrode sheet with a single-coated positive electrode material layer. The coating weight of the positive electrode material layer was 17 mg / mm 2 . Then, the above steps were repeated on the other surface of the aluminum foil to obtain a positive electrode sheet with a double-coated positive electrode material layer. After drying at 85°C, the positive electrode sheet was rolled, and then was cut and the tabs were welded to obtain a positive electrode sheet with a size of 70 mm x 55 mm. The thickness of the single-coated positive electrode material layer was 158 μm.

[0083] <Manufacture of the separator>

[0084] A polyethylene (PE) porous film (provided by Shenzhen Xingyuan Material Technology Co., Ltd.) with a thickness of 10 μm was used as the separator.

[0085] <Manufacture of the lithium ion battery>

[0086] The positive electrode sheet, the separator, and the negative electrode sheet prepared above were stacked in order, with the separator between the positive electrode sheet and the negative electrode sheet to play a role of isolation, and were wound to obtain an electrode assembly. The electrode assembly was placed in an aluminum-plastic film packaging bag, and was dehydrated at 85°C. The electrolyte prepared above was injected into the bag at an injection coefficient of 1.6 g / Ah. After vacuum packaging, standing, formation, shaping, and other processes, a lithium ion battery was obtained. The standing time was 48 hours, the formation current was 0.1C, the upper limit of the formation voltage was 4.25V, and the formation temperature was 45°C.

[0087] Examples 1-2 to 1-15

[0088] Except that in the <Manufacture of the electrolyte>, the relevant parameters were adjusted according to Table 1, the rest was the same as in Example 1-1. When the values of M A , M B , and M C in the electrolyte changed, the mass percentage of the solvent M F changed accordingly, and the mass percentage of the electrolyte M E remained unchanged.

[0089] Example 1-16 to Example 1-30

[0090] Except that in <Preparation of negative electrode sheet>, the negative active material artificial graphite, the negative active material elemental silicon, the conductive agent acetylene black, the binder sodium carboxymethyl cellulose (CMC) and the butadiene rubber (SBR) are mixed in a mass ratio of 77.6:19.4:1:1:1, in <Preparation of electrolyte>, the related parameters are adjusted according to Table 1, and in <Preparation of lithium ion battery>, the injection coefficient is adjusted to 1.7 g / Ah except for the rest, which is the same as Example 1-1. Among them, when the values of M A , M B , M C in the electrolyte change, the mass percentage content of the solvent M F changes accordingly, the mass percentage content of the electrolyte M E does not change; the mass percentage content of silicon element M Si is 20% based on the mass of the negative active material.

[0091] Example 1-31 to Example 1-45

[0092] Except that in <Preparation of negative electrode sheet>, the negative active material artificial graphite, the negative active material elemental silicon, the conductive agent acetylene black, the binder sodium carboxymethyl cellulose (CMC) and the butadiene rubber (SBR) are mixed in a mass ratio of 67.9:29.1:1:1:1, in <Preparation of electrolyte>, the related parameters are adjusted according to Table 1, and in <Preparation of lithium ion battery>, the injection coefficient is adjusted to 1.8 g / Ah except for the rest, which is the same as Example 1-1. Among them, when the values of M A , M B , M C in the electrolyte change, the mass percentage content of the solvent M F changes accordingly, the mass percentage content of the electrolyte M E does not change; the mass percentage content of silicon element M Si is 30% based on the mass of the negative active material.

[0093] Example 1-46

[0094] Except that in <Preparation of electrolyte>, the related parameters are adjusted according to Table 1, and the rest is the same as Example 1-1. Among them, when the values of M A , M B , M C in the electrolyte change, the mass percentage content of the solvent M F changes accordingly, the mass percentage content of the electrolyte M E does not change.

[0095] Example 2-1 to Example 2-3

[0096] The rest is the same as Example 1-1 except that in the preparation of electrolyte, the type of compound shown in Formula 1 is adjusted according to Table 2.

[0097] Examples 2-4 to 2-8

[0098] The rest is the same as Example 1-1 except that in the preparation of electrolyte, other additives are introduced into the electrolyte according to Table 2, and the content and type of other additives are adjusted. Among them, when the mass percentage content M D of other additives changes, the mass percentage content M F of the solvent changes accordingly, and the mass percentage content of other components in the electrolyte remains unchanged. Among them, in Table 2, vinylene carbonate is abbreviated as VC, and lithium bisoxalate borate is abbreviated as LiBOB.

[0099] Example 2-9

[0100] The rest is the same as Example 1-1 except that in the preparation of electrolyte, the electrolyte is adjusted to lithium hexafluorophosphate (LiPF6) and lithium bisfluorosulfonylimide (LiFSI), and the mass percentage content of LiPF6 is 10% and the mass percentage content of LiFSI is 5% based on the mass of the electrolyte.

[0101] Comparative Examples 1 to 6

[0102] The rest is the same as Example 1-1 except that in the preparation of electrolyte, the relevant parameters are adjusted according to Table 1. Among them, when the values of M A , M B , and M C in the electrolyte change, the mass percentage content M F of the solvent changes accordingly, and the mass percentage content M E of the electrolyte remains unchanged.

[0103] Comparative Examples 7 to 12

[0104] The rest is the same as Example 1-16 except that in the preparation of electrolyte, the relevant parameters are adjusted according to Table 1. Among them, when the values of M A , M B , and M C in the electrolyte change, the mass percentage content M F of the solvent changes accordingly, and the mass percentage content M E of the electrolyte remains unchanged.

[0105] Comparative Examples 13 to 18

[0106] The rest is the same as Example 1-31 except that in the preparation of electrolyte, the relevant parameters are adjusted according to Table 1. Among them, when the values of M A , M BM C When the value changes, the mass percentage of the solvent M F Consequently, the mass percentage of electrolytes, M, changes. E constant.

[0107] Comparative Example 19

[0108] Except for adjusting the relevant parameters according to Table 1 in the section on <Preparation of Electrolyte>, the rest is the same as in Examples 1-1. Specifically, M in the electrolyte... A M B M C When the value changes, the mass percentage of the solvent M F Consequently, the mass percentage of electrolytes, M, changes. E constant.

[0109] The preparation parameters and performance parameters of each embodiment and comparative example are shown in Tables 1 and 2. Note: " / " in Table 1 indicates that there are no relevant preparation parameters.

[0110] Examples 1-1 to 1-46 and Comparative Examples 1 to 19 show that the electrolyte, comprising the compound of Formula 1 within the scope of this application, fluoroethylene carbonate, and lithium difluorodioxarate phosphate, enables the secondary battery to exhibit high capacity retention at 25°C, high capacity retention at 45°C, high capacity recovery rate, high storage capacity retention rate, low thickness expansion rate, and low impedance growth rate. This indicates that the secondary battery has good room temperature cycling performance, high temperature cycling performance, high temperature storage performance, and a low impedance growth rate. However, Comparative Examples 1 to 19 do not simultaneously meet the above characteristics; their secondary batteries exhibit low capacity retention at 25°C, low capacity retention at 45°C, low capacity recovery rate, low storage capacity retention rate, high thickness expansion rate, and high impedance growth rate. This indicates that the secondary batteries have poor room temperature cycling performance, high temperature cycling performance, and high temperature storage performance, and a large impedance growth rate.

[0111] M A M B M C M a M b M c M Si The relationship between these factors affects the room temperature cycling performance, high temperature cycling performance, high temperature storage performance, and impedance growth rate of the secondary battery. As can be seen from Examples 1-1 to 1-46, by adjusting M... A M B M C M a M b Mc The relationship between M Si The obtained secondary battery has higher 25℃ capacity retention rate, 45℃ capacity retention rate, higher capacity recovery rate, higher storage capacity retention rate, lower thickness expansion rate and lower impedance growth rate, which indicates that the secondary battery has better normal temperature cycle performance, high temperature cycle performance, high temperature storage performance and lower impedance growth rate.

[0112] The value of M Si The value of M Si The obtained secondary battery has higher 25℃ capacity retention rate, 45℃ capacity retention rate, higher capacity recovery rate, higher storage capacity retention rate, lower thickness expansion rate and lower impedance growth rate, which indicates that the secondary battery has better normal temperature cycle performance, high temperature cycle performance, high temperature storage performance and lower impedance growth rate.

[0113] The value of M a The value of M b The value of M c The value of M a The value of M b The value of M c The obtained secondary battery has higher 25℃ capacity retention rate, 45℃ capacity retention rate, higher capacity recovery rate, higher storage capacity retention rate, lower thickness expansion rate and lower impedance growth rate, which indicates that the secondary battery has better normal temperature cycle performance, high temperature cycle performance, high temperature storage performance and lower impedance growth rate.

[0114] The type of the compound represented by Formula 1 usually affects the high temperature cycle performance, high temperature storage performance and impedance growth rate of the secondary battery. As can be seen from Example 1-1, Example 2-1 to Example 2-3, and Comparative Example 1, when the electrolyte includes the compound represented by Formula 1 within the scope of the present application, the obtained secondary battery has higher 45℃ capacity retention rate, higher capacity recovery rate, higher storage capacity retention rate, lower thickness expansion rate and lower impedance growth rate, which indicates that the secondary battery has better high temperature cycle performance, high temperature storage performance and lower impedance growth rate.

[0115] The type and mass percentage of other additives M DGenerally, the high-temperature cycle performance, the high-temperature storage performance and the impedance growth rate of the secondary battery are affected. As can be seen from Example 1-1, Example 2-4 to Example 2-8, when the electrolyte comprises other additives within the scope of the present application and the content of the other additives is regulated within the scope of the present application, the secondary battery obtained has a higher 45℃ capacity retention rate, a higher capacity recovery rate, a higher storage capacity retention rate, a lower thickness expansion rate and a lower impedance growth rate, indicating that the secondary battery has better high-temperature cycle performance, high-temperature storage performance and lower impedance growth rate.

[0116] The type and mass percentage content M of the electrolyte E Generally, the high-temperature cycle performance, the high-temperature storage performance and the impedance growth rate of the secondary battery are affected. As can be seen from Example 1-1, Example 2-9, when the electrolyte comprises other additives within the scope of the present application and the content of the other additives is regulated within the scope of the present application, the secondary battery obtained has a higher 45℃ capacity retention rate, a higher capacity recovery rate, a higher storage capacity retention rate, a lower thickness expansion rate and a lower impedance growth rate, indicating that the secondary battery has better high-temperature cycle performance, high-temperature storage performance and lower impedance growth rate.

[0117] The type and mass percentage content M of the solvent F Generally, the high-temperature cycle performance, the high-temperature storage performance and the impedance growth rate of the secondary battery are affected. As can be seen from Example 1-1, Example 2-1 to Example 2-9, when the electrolyte comprises other additives within the scope of the present application and the content of the other additives is regulated within the scope of the present application, the secondary battery obtained has a higher 45℃ capacity retention rate, a higher capacity recovery rate, a higher storage capacity retention rate, a lower thickness expansion rate and a lower impedance growth rate, indicating that the secondary battery has better high-temperature cycle performance, high-temperature storage performance and lower impedance growth rate.

[0118] 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 scope of protection of the present application.

Claims

1. An electrolyte comprising a solvent, an electrolyte, and an additive, said additive comprising a compound of Formula 1, fluoroethylene carbonate, and lithium difluorodioxazophosphate; in, n is 0 or 1, A is selected from methylene or O, X is selected from sulfonyl or carbonyl, and R1 and R2 are each independently selected from H, One of them, and R1 and R2 are not both selected from H, and at least one sulfur atom is contained in X, R1 and R2.

2. The electrolyte according to claim 1, wherein the mass percentage of the compound represented by formula 1 is M based on the mass of the electrolyte. A The mass percentage of the fluoroethylene carbonate is M. B The mass percentage of lithium difluorodioxanol phosphate is M. C Satisfying: 0.5% ≤ M A ≤7%, 2%≤M B ≤20%, 0.1%≤M C ≤1%.

3. The electrolyte according to claim 1, wherein, The compound shown in Formula 1 is selected from the compound shown in Formula 2: X, R1, and R2 contain one or two sulfur atoms.

4. The electrolyte according to claim 1, wherein, The compound represented by Formula 1 is selected from at least one of the following compounds:

5. The electrolyte according to claim 2, wherein it satisfies at least one of the following conditions: (1)1％≤M A ≤5％； (2)10％≤M B ≤15％； (3)0.5％≤M C ≤1％。 6. The electrolyte according to any one of claims 1 to 5, wherein, The additives also include other additives selected from at least one of vinylene carbonate and lithium bis(oxalato)borate; the mass percentage of the other additives is M based on the mass of the electrolyte. D 0.3% ≤ M D ≤5%.

7. The electrolyte according to any one of claims 1 to 5, wherein the electrolyte is selected from at least one of lithium hexafluorophosphate, lithium difluorophosphate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium perchlorate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium bis(oxalateborate), and lithium di(fluorooxalateborate), and the mass percentage of the electrolyte is M based on the mass of the electrolyte. E 12% ≤ M E ≤18%; The solvent is selected from at least two of propylene carbonate, ethyl methyl carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, γ-butyrolactone, sulfolane, methyl formate, ethyl formate, propyl formate, butyl formate, methyl acetate, ethyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, propyl propionate, butyl propionate, methyl butyrate, ethyl butyrate, propyl butyrate, and butyl butyrate. Based on the mass of the electrolyte, the mass percentage of the solvent is M. F 55% ≤ M F ≤80%.

8. A secondary battery comprising a negative electrode, a positive electrode, a separator, and an electrolyte according to any one of claims 1 to 7.

9. The secondary battery according to claim 8, wherein, The negative electrode sheet includes a negative electrode material layer, which includes a negative electrode active material. Based on the mass of the negative electrode active material, the mass percentage of silicon is M. Si ; After the secondary battery is formed, based on the mass of the electrolyte, the mass percentage of the compound shown in Formula 1 is M. a The mass percentage of the fluoroethylene carbonate is M. b The mass percentage of lithium difluorodioxanol phosphate is M. c The formation conditions are as follows: the secondary battery is placed in an environment of 40°C to 50°C and charged with a constant current until it reaches a charge state of 70% to 100%, and the constant current is 0.05C to 0.15C. The secondary battery satisfies equation I: 15 ≤ [15 × (M)] A +M B ) / (M a +M b )+10×M C ×(M C -M c )] / (1.5+M Si )≤234, wherein the electrolyte injection coefficient of the secondary battery is 1.5+M Si The unit is g / Ah.

10. The secondary battery according to claim 9, wherein, 10％≤M Si ≤30％。 11. The secondary battery according to claim 9 or 10, wherein it satisfies at least one of the following conditions: (1)0.01％≤M a ≤1.81％； (2)0.24％≤M b ≤7.23％； (3)0％≤M c ≤0.33％。 12. The secondary battery according to claim 9 or 10, wherein the secondary battery satisfies relation II: 16 ≤ [15 × (M A +M B ) / (M a +M b )+10×M C ×(M C -M c )] / (1.5+M Si )≤83.

3.

13. The secondary battery according to claim 12, wherein the secondary battery satisfies relation III: 16 ≤ [15 × (M A +M B ) / (M a +M b )+10×M C ×(M C -M c )] / (1.5+M Si )≤33.

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