Non-aqueous electrolyte, secondary battery, and method of manufacturing

Abstract

This application relates to a non-aqueous electrolyte, a secondary battery, and a preparation method thereof. The non-aqueous electrolyte includes a first electrolyte and a second electrolyte. The first electrolyte includes a first electrolyte salt, a first organic solvent, and a first additive. The first additive includes vinylene carbonate and a thioester compound. The thioester compound contains at least two sulfur-containing and oxygen-containing ester segments. The sulfur-containing and oxygen-containing ester segments are selected from at least one of sulfate ester structural units, sulfite diester structural units, and substituted sulfonyloxy groups. The sulfur-containing and oxygen-containing ester segments are connected by linker segments. The second electrolyte includes a second electrolyte salt, a second organic solvent, and vinylene carbonate. The content of vinylene carbonate in the first additive can be 0. This application can simultaneously improve high-temperature cycling performance, high-temperature storage performance, low battery impedance, high discharge capacity, and high initial coulombic efficiency.

Description

[0001] This application claims priority to Chinese Patent Application No. 202611192445.3, filed on August 6, 2026, entitled “Non-aqueous electrolyte, secondary battery and preparation method”, the entire contents of which are incorporated herein by reference. Technical Field

[0002] This application relates to the field of battery technology, and in particular, to a non-aqueous electrolyte, a secondary battery, and a method for preparing it. Background Technology

[0003] With the widespread application of lithium-ion batteries in electric vehicles and energy storage systems, the market has placed higher demands on their energy density, cycle life, and safety performance. For example, in the energy storage field, a cycle life of 10,000 cycles and a calendar life of 20 years are required. Electrolyte additives are one of the key strategies for regulating electrode interface properties and improving the overall performance of batteries.

[0004] It is widely believed in the industry that the electrolyte interphase (SEI) film at the negative electrode-electrolyte interface of lithium-ion batteries has a layered structure, with the inner layer dominated by inorganic phases and the outer layer enriched with organic phases. Vinylene carbonate (VC), as a mature film-forming additive, preferentially undergoes a carbon-carbon double bond ring-opening reaction on the negative electrode surface to form an electrolyte interphase (SEI) film mainly composed of inorganic lithium carbonate (Li₂CO₃) and organic polymer phases. This effectively prevents the electrolyte from continuously undergoing side decomposition reactions with the negative electrode active material, significantly improving the cycle life of lithium iron phosphate (LFP) batteries. However, the organic phase of the VC-formed SEI film has poor thermal stability and is prone to decomposition and swelling at high temperatures, leading to performance degradation during high-temperature cycling and storage. Furthermore, there is an optimal range for VC addition; within this range, battery cycle life improves with increasing VC content, but it significantly increases interfacial impedance, reduces battery energy efficiency and rate performance, and increases the burden on thermal management. Additionally, the price of VC remains relatively high due to the increasing demand for energy storage. Other commonly used film-forming additives, such as 1,3-propanesulfonate lactone (PS), 1,3-propenesulfonate lactone (PST), and tetravinylsilane (TVSi), can mitigate electrolyte side reactions to some extent and improve storage performance at high temperatures, but they still cause the battery's DC internal resistance (DCR) to continue to increase.

[0005] Therefore, traditional negative electrode film-forming additives, while protecting the electrode, often exhibit high impedance characteristics, making it difficult to simultaneously achieve high-temperature performance and low resistance. For lithium-ion batteries, this hinders the insertion and extraction of lithium ions at the negative electrode, resulting in the loss of active lithium and battery capacity, directly affecting the battery's calendar life and cycle life. Summary of the Invention

[0006] The technical problem to be solved by this application is to provide a non-aqueous electrolyte, a secondary battery and a preparation method, so as to solve the defect in the prior art that the DC internal resistance DCR increases when the additives improve the high temperature cycling and storage performance.

[0007] To address the aforementioned technical problems, according to one aspect of this application, a non-aqueous electrolyte is provided, comprising: a first electrolyte, including: a first electrolyte salt, a first organic solvent, and a first additive, wherein the first additive comprises: vinylene carbonate and a thioester compound, the thioester compound comprising at least two sulfur-containing oxygen-containing ester segments, the sulfur-containing oxygen-containing ester segments being selected from at least one of sulfate ester structural units, sulfite diester structural units, and substituted sulfonyloxy groups, and the sulfur-containing oxygen-containing ester segments are connected by a linking segment, the linking segment being selected from substituted or unsubstituted C1-C8 alkyl segments or -(R 1 ) n -X,R 1 Selected from C1-C4 alkyl segments, X is selected from diester carbonate structural units, chain-like diester sulfate structural units, chain-like diester sulfite structural units, R 2 R is a substituent of any one of the following structural units: dialkoxyphosphonate diester, phosphate triester, oxalate diester, or quaternary ammonium cation group. 2 The first electrolyte is selected from any one of halogen, unsubstituted or fluorinated C1-C6 alkyl, unsubstituted or fluorinated C1-C6 saturated alkoxy, unsubstituted or fluorinated C2-C6 unsaturated alkoxy, unsubstituted or fluorinated C2-C6 unsaturated hydrocarbon, substituted or unsubstituted silyl, and cyano, where n is a positive integer; and the second electrolyte comprises: a second electrolyte salt, a second organic solvent, and a second additive, wherein the second additive comprises vinylene carbonate, and the mass fraction of the thioester compound in the first additive is greater than 0% and less than or equal to 100% based on the total mass of the thioester compound and the vinylene carbonate in the first additive is greater than or equal to 0% and less than 100%.

[0008] According to embodiments of this application, the sum of the mass fractions of thioester compounds, vinylene carbonate in the first electrolyte, and vinylene carbonate in the second electrolyte, based on the total mass of the non-aqueous electrolyte, is 0.01%-8%.

[0009] According to an embodiment of this application, the mass ratio of the first electrolyte to the second electrolyte is 7:3-9:1.

[0010] According to embodiments of this application, the thioester compounds have a symmetrical structure, and the sulfur-containing oxygen-containing ester fragments are selected from at least one of cyclic sulfate structural units and cyclic sulfite groups.

[0011] According to embodiments of this application, the substituents in the substituted C1-C8 alkyl segments are selected from fluorine or carboxylic acid ester groups.

[0012] The structural formula of the substituted sulfonyloxy group is: -O-SO2-R 3 R 3 It is selected from any one of the following: fluorinated or unsubstituted C1-C6 alkyl, unsubstituted or fluorinated C1-C6 alkoxy, unsubstituted or fluorinated C2-C6 unsaturated hydrocarbon, substituted or unsubstituted silyl, and cyano.

[0013] According to embodiments of this application, the thioester compounds are selected from any of the following structures: , , , , , , , , , , , , , , .

[0014] According to embodiments of this application, the first additive further includes at least one of vinyl sulfate, fluorovinyl carbonate, methylene disulfonate, 1,3-propanesulfonate lactone, lithium difluorooxalate borate, lithium difluorophosphate, and tris(trimethylsilane) phosphate.

[0015] According to embodiments of this application, based on the total mass of the non-aqueous electrolyte, the total mass percentage of the first electrolyte salt and the second electrolyte salt is 8%-20%, and the total mass percentage of the first organic solvent and the second organic solvent is 70%-90%. Both the first electrolyte salt and the second electrolyte salt are selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium difluorooxalate borate, lithium bis(oxalate borate), lithium difluorooxalate phosphate, lithium bis(fluorosulfonyl)imide, and lithium bis(trifluoromethanesulfonyl)imide. Both the first organic solvent and the second organic solvent are selected from at least one of ethylene carbonate, propylene carbonate, butene carbonate, dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, 1,4-butyrolactone, methyl propionate, methyl butyrate, ethyl acetate, ethyl propionate, or ethyl butyrate.

[0016] According to another aspect of this application, a method for preparing a secondary battery is provided, comprising the following steps: Step S1, fabricating a bare cell by stacking a positive electrode, a negative electrode, and a separator, encapsulating the bare cell to obtain a cell to be injected with electrolyte, and injecting the cell to be injected with electrolyte into a first non-aqueous electrolyte as described above; Step S2, performing formation and aging on the cell after electrolyte injection; Step S3, injecting a second non-aqueous electrolyte into the aged cell and sealing it to obtain a non-aqueous electrolyte secondary battery.

[0017] According to an embodiment of this application, in step S2, the formation is carried out using an incremental stepped current formation method. The incremental stepped current formation method is as follows: first, the battery is charged with a constant current at a first charging rate to a first voltage, and then charged with a constant current at a second charging rate to a second voltage. The first charging rate is greater than or equal to 0.05C and less than the second charging rate, the first voltage is less than the second voltage, and the second voltage is 2.8V-3.4V. The aging method is as follows: the formed battery cell is left to stand at a temperature of 35℃-50℃ for 24h-48h.

[0018] According to another aspect of this application, a secondary battery is provided, which is prepared by the method described above.

[0019] Compared with the prior art, the beneficial effects of this application are as follows: The non-aqueous electrolyte of this application uses a combination of thioester compounds and vinylene carbonate (VC) and is integrated with the battery refilling process to effectively resolve the contradiction between high-temperature storage, cycle performance, and high impedance. The non-aqueous electrolyte is divided into a first electrolyte and a second electrolyte. The first electrolyte contains thioester compounds and vinylene carbonate (VC), while the second electrolyte contains VC. The thioester compounds in the first electrolyte preferentially reduce and construct a thin, dense, inorganic sulfur-phase SEI inner layer on the negative electrode surface. This layer exhibits excellent ionic conductivity, thermal stability, and mechanical strength, avoiding the drawbacks of traditional additives like VC and PS, which tend to cause high initial impedance and continuously increasing DCR during cycling. This effectively improves the battery's initial coulombic efficiency and reduces irreversible lithium loss. Furthermore, the VC in the first electrolyte synergistically with the thioester compounds results in an SEI film on the negative electrode surface containing an organic cross-linked polymer phase formed by VC bond breaking and ring opening. This enhances the SEI film's flexibility, allowing it to better withstand the deformation caused by repeated lithium-ion insertion and extraction during cycling, thus improving the SEI film's mechanical properties and ultimately enhancing the battery's cycle performance. The ratio of VC to thioester compounds can be adjusted to control the organic-inorganic phase ratio of the SEI film. This allows the battery to leverage the highly flexible organic phase to enhance SEI cycle deformation tolerance and slow capacity decay, while also utilizing the highly thermally stable inorganic sulfur phase to suppress continuous side reactions between the electrolyte and electrode, thereby adapting to different battery performance requirements. Furthermore, during battery fabrication, a sequential two-stage electrolyte injection method is employed. First, the first electrolyte is injected and aged before the second electrolyte is injected. This effectively solves the competitive film-forming problem between thioester compounds and vinylene carbonate, achieving a synergistic film-forming effect where 1+1>2. Moreover, the thioester compounds are completely reacted and consumed during the first injection stage, eliminating the problem of electrolyte acidification caused by residual thioester compounds, which could degrade subsequent battery performance. Therefore, the battery fabricated using the non-aqueous electrolyte of this application can simultaneously improve high-temperature cycling and high-temperature storage performance, reduce battery impedance, and achieve high initial coulombic efficiency. It also reduces the unstable impact of VC production capacity and price fluctuations on the industry, is compatible with existing mass production processes, and possesses advantages in industrialization, economy, and safety. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without inventive effort are within the scope of protection of this application. Unless otherwise defined, the technical or scientific terms used herein should have the ordinary meaning understood by those skilled in the art to which this application pertains. The terms "comprising" and similar expressions used herein mean that the element or object preceding the word covers the element or object listed following the word and its equivalents, but does not exclude other elements or objects. Unless otherwise specified, the term "connection" as used herein can refer to a direct connection or an indirect connection, i.e., a connection through an intermediate object.

[0021] The terms “first,” “second,” and similar terms used in this patent application specification and claims do not indicate any order, quantity, or importance, but are merely used to distinguish different components.

[0022] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0023] According to one embodiment of this application, a non-aqueous electrolyte includes: a first electrolyte and a second electrolyte. The first electrolyte includes: a first electrolyte salt, a first organic solvent, and a first additive. The first additive includes: vinylene carbonate and a thioester compound. The thioester compound contains at least two sulfur-containing and oxygen-containing ester segments, which are selected from sulfate ester structural units (-O-SO2-O-), sulfite diester structural units (-OS(=O)-O-), and substituted sulfonyloxy groups (-O-SO2-R). 3 At least one of the following, and the sulfur-containing and oxygen-containing ester segments are linked by a linking segment selected from substituted or unsubstituted C1-C8 alkyl segments or -(R 1 ) n -X,R 1 Selected from C1-C4 alkyl segments, X is selected from diester carbonate structural units, chain-like diester sulfate structural units, chain-like diester sulfite structural units, R 2 R substituted with any one of the following structural units: dialkoxyphosphonate diester, phosphate triester, oxalate diester, or quaternary ammonium cation group. 2 The first electrolyte is selected from any one of halogen, unsubstituted or fluorinated C1-C6 alkyl groups, unsubstituted or fluorinated C1-C6 saturated alkoxy groups, unsubstituted or fluorinated C2-C6 unsaturated alkoxy groups, unsubstituted or fluorinated C2-C6 unsaturated hydrocarbon groups, substituted or unsubstituted silyl groups, and cyano groups, where n is a positive integer. The second electrolyte comprises: a second electrolyte salt, a second organic solvent, and a second additive, wherein the second additive includes vinylene carbonate. Specifically, based on the total mass of the thioester compounds and vinylene carbonate in the first additive, the mass fraction of the thioester compounds is greater than 0% and less than or equal to 100%, and the mass fraction of vinylene carbonate in the first additive is greater than or equal to 0% and less than 100%. The structural formula of the diester unit is shown below.

[0024]

[0025] The structural formula of the chain-like sulfate diester unit is shown below.

[0026]

[0027] The structural formula of the chain-like diester unit is shown below.

[0028]

[0029] R 2 The structural formula of the substituted diekoxyphosphonate diester is shown below.

[0030]

[0031] The structural formula of the phosphate triester structural unit is shown below.

[0032]

[0033] The structural formula of the oxalate diester unit is shown below.

[0034]

[0035] The structural formula of the quaternary ammonium cation group is shown below.

[0036]

[0037] It is understandable that the first additive only needs to contain thioester compounds and may not contain vinylene carbonate (VC). As an example, the mass ratio of thioester compounds to vinylene carbonate in the first additive can be 1:0, 2:8, 3:7, 4:6, 1:1, 6:4, 8:2, 9:1, or 9.9:1. Specifically, a mass ratio of 1:0 for thioester compounds to vinylene carbonate in the first additive means that, based on the total mass of the two compounds in the first additive, the mass fraction of thioester compounds is 100%, and the mass fraction of vinylene carbonate is 0%. When the proportion of thioester compounds in the first additive is high, an SEI film composed of multiple inorganic sulfur components will preferentially form, resulting in better thermal stability and superior high-temperature storage performance. When the proportion of vinylene carbonate in the first additive is high, the SEI membrane composition contains more organic cross-linked polymer phases formed by VC bond breaking and ring opening. This results in better flexibility, allowing it to better withstand the deformation caused by repeated lithium-ion insertion and extraction during cycling, leading to better mechanical properties and cycle performance of the SEI membrane. In practical applications, the dynamic equilibrium relationship between the organic and inorganic phases of the SEI membrane can be adjusted by changing the ratio of vinylene carbonate and thioester compounds in the first additive, according to battery performance requirements, thereby achieving the desired optimal performance.

[0038] This application does not specifically limit the source of thioester compounds; commercially available products well known to those skilled in the art, or products that can be prepared according to the methods described in this application or conventional preparation methods, may be used.

[0039] The non-aqueous electrolyte provided in the above embodiments of this disclosure uses a combination of thioester compounds and vinylene carbonate (VC) and is combined with the battery secondary electrolyte injection process to effectively solve the contradiction between high-temperature storage, cycle performance, and high impedance. The non-aqueous electrolyte is divided into a first electrolyte and a second electrolyte. The first electrolyte contains thioester compounds and vinylene carbonate (VC), and the second electrolyte contains VC. The thioester compounds in the first electrolyte preferentially reduce and construct a thin, dense, inorganic sulfur-phase SEI inner layer on the negative electrode surface. This layer exhibits excellent ionic conductivity, thermal stability, and mechanical strength, avoiding the drawbacks of traditional additives like VC and PS, which tend to cause high initial impedance and continuously increasing DCR during cycling. This effectively improves the battery's initial coulombic efficiency and reduces irreversible lithium loss. Furthermore, the VC in the first electrolyte synergistically with the thioester compounds results in an SEI film on the negative electrode surface containing an organic cross-linked polymer phase formed by VC bond breaking and ring opening. This enhances the SEI film's flexibility, allowing it to better withstand the deformation caused by repeated lithium-ion insertion and extraction during cycling, thus improving the SEI film's mechanical properties and ultimately enhancing the battery's cycle performance. The ratio of VC to thioester compounds can be adjusted to control the organic-inorganic phase ratio of the SEI film. This allows for both enhanced SEI cycle deformation tolerance and capacity decay due to the highly flexible organic phase, and suppression of continuous side reactions between the electrolyte and electrode by the highly thermally stable inorganic sulfur phase, adapting to different battery performance requirements. Furthermore, during battery fabrication, a sequential two-stage electrolyte injection method is employed. First, the first electrolyte is injected and aged before the second electrolyte is injected. This effectively solves the competitive film-forming problem between thioester compounds and vinylene carbonate, achieving a synergistic film-forming effect where 1+1>2. Moreover, the thioester compounds are completely reacted and consumed during the first injection stage, eliminating the problem of electrolyte acidification caused by residual thioester compounds, which could degrade subsequent battery performance. Therefore, the battery fabricated using the non-aqueous electrolyte of this application can simultaneously improve high-temperature cycling and high-temperature storage performance, reduce battery impedance, and achieve high initial coulombic efficiency. It also reduces the unstable impact of VC production capacity and price fluctuations on the industry, is compatible with existing mass production processes, and possesses advantages in industrialization, economy, and safety.

[0040] Specifically, both the high-temperature cycle performance and high-temperature storage performance of the battery initially improve and then deteriorate as the proportion of thioester compounds decreases. Furthermore, the high-temperature cycle performance is better when the ratio of thioester compounds to VC is 3:7, and the high-temperature storage performance is better when the ratio is 6:4. This indicates that thioester compounds and VC have both synergistic and competitive relationships. They synergistically form an inorganic-organic combined SEI film on the negative electrode surface. High-temperature storage focuses more on the static thermal stability of the SEI film, while high-temperature cycle performance focuses more on the dynamic mechanical properties of the SEI film. When the proportion of thioester compounds is higher than that of VC, an SEI film composed of multiple inorganic sulfur components is preferentially formed, resulting in better thermal stability and thus better high-temperature storage performance. Conversely, when the proportion of VC is higher than that of thioester compounds, the SEI film composition contains more organic cross-linked polymer phases formed by the breaking and ring-opening of VC bonds, resulting in better flexibility and the ability to better withstand the deformation caused by repeated lithium-ion insertion and extraction during cycling, leading to better SEI film mechanical properties and thus better cycle performance. In practice, the dynamic equilibrium between the organic and inorganic phases of the SEI membrane should be achieved by adjusting the ratio of the two components according to the specific battery performance requirements. For example, when focusing on high-temperature cycling performance, the proportion of thioester compounds in the first electrolyte should be greater than that of vitamin C. Preferably, the ratio of thioester compounds to vitamin C in the first electrolyte should be around 3:7. When focusing on high-temperature storage performance, the proportion of thioester compounds in the first electrolyte should be less than that of vitamin C. Preferably, the ratio of thioester compounds to vitamin C in the first electrolyte should be around 6:4.

[0041] In some embodiments, the sum of the mass fractions of thioester compounds, vinylene carbonate in the first electrolyte, and vinylene carbonate in the second electrolyte, based on the total mass of the non-aqueous electrolyte, is 0.01%-8%. That is, the sum of the mass fractions of thioester compounds and total vinylene carbonate in the non-aqueous electrolyte is 0.01%-8%. Within this range, the prepared battery exhibits superior performance. As an example, the sum of the mass fractions of thioester compounds and total vinylene carbonate in the non-aqueous electrolyte can be 0.01%, 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, or 8%. As an example, the range of the sum of the mass fractions of thioester compounds and total vinylene carbonate in the non-aqueous electrolyte can be 0.01%-0.1%, 0.01%-1%, 0.01%-3%, 0.01%-5%, 0.5%-2%, 4%-6%, 6%-7%, or 7%-8%.

[0042] In some embodiments, the mass ratio of the first electrolyte to the second electrolyte is 7:3-9:1. Within this range, the prepared battery exhibits superior performance. As examples, the mass ratio of the first electrolyte to the second electrolyte can be 7:3, 7.5:2.5, 8:2, 8.5:1.5, or 9:1. As further examples, the range of the mass ratio of the first electrolyte to the second electrolyte can be 7:3-7.5:2.5, 7:3-8:2, or 8:2-9:1.

[0043] Preferably, the thioester compound has a symmetrical structure, and the sulfur-containing and oxygen-containing ester fragment is selected from at least one of cyclic sulfate structural units and cyclic sulfite groups. The thioester compound with this structure forms a more uniform, thinner, and denser SEI film at the negative electrode interface, with better ion conductivity and fewer defects. At the same time, the symmetrical molecule has greater steric hindrance, and the molecular thermodynamic stability is significantly improved.

[0044] Specifically, thioester compounds can have a globally symmetrical structure or a locally symmetrical structure containing only sulfur-containing oxygen-containing ester segments. Preferably, thioester compounds have a globally symmetrical structure.

[0045] Specifically, the cyclic sulfate ester structural unit is a cyclic sulfate ester group, and the structural formula of the cyclic sulfate ester group is shown below.

[0046]

[0047] Specifically, the structural formula of the cyclic sulfite group is shown below.

[0048]

[0049] As an example, the structural formula of thioester compounds can be shown below.

[0050]

[0051] More preferably, the thioester compounds have a symmetrical structure, and the two cyclic sulfate groups or the two cyclic diester groups are not directly connected. The SEI film formed at the negative electrode interface by thioester compounds with this structure is more uniform, thinner, and denser than that formed by thioester compounds with directly connected cyclic sulfate groups or cyclic diester groups, exhibiting better ion conductivity, fewer defects, and greater steric hindrance due to the symmetrical molecular structure, resulting in significantly improved molecular thermodynamic stability.

[0052] Preferably, the substituents in the substituted C1-C8 alkyl segments are selected from fluorine or carboxylic acid ester groups.

[0053] The structural formula of the substituted sulfonyloxy group is: -O-SO2-R 3 R 3It is selected from any one of the following: fluorinated or unsubstituted C1-C6 alkyl, unsubstituted or fluorinated C1-C6 alkoxy, unsubstituted or fluorinated C2-C6 unsaturated alkyl, substituted or unsubstituted silyl, and cyano.

[0054] In some embodiments, the thioester compound is selected from any of the following structures.

[0055]

[0056] S1

[0057] S2

[0058] S3

[0059] S4

[0060] S5

[0061] S6

[0062] S7

[0063] S8

[0064] S9

[0065] S10

[0066] S11

[0067] S12

[0068] S13

[0069] S14

[0070] S15 In some embodiments, the first additive further includes at least one of vinyl sulfate (DTD), fluorovinyl carbonate (FEC), methylene disulfonate (MMDS), 1,3-propanesulfonate lactone (PS), lithium difluorooxalate borate (LiDFOB), lithium difluorophosphate (LiDFP), and tris(trimethylsilane) phosphate (TMSP).

[0071] Specifically, based on the total mass of the first electrolyte, the total mass percentage of vinyl sulfate, fluorovinyl carbonate, methylene disulfonate, 1,3-propanesulfonate lactone, lithium difluorooxalate borate, lithium difluorophosphate, and tris(trimethylsilane) phosphate is 0.01%-5%.

[0072] In some embodiments, the total mass percentage of the first electrolyte salt and the second electrolyte salt, based on the total mass of the non-aqueous electrolyte, can be 8%-20%. For example, the total mass percentage of the first electrolyte salt and the second electrolyte salt can be 8%, 10%, 12%, 14%, 16%, 18%, or 20%. For example, the range of the total mass percentage of the first electrolyte salt and the second electrolyte salt can be 8%-10%, 8%-12%, 10%-12%, 10%-14%, 16%-18%, or 18%-20%.

[0073] Both the first electrolyte salt and the second electrolyte salt are selected from at least one of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium difluorooxalate borate (LiDFOB), lithium bis(oxalate borate) (LiBOB), lithium difluorooxalate phosphate (LiODFP), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI).

[0074] In some embodiments, the total mass percentage of the first organic solvent and the second organic solvent, based on the total mass of the non-aqueous electrolyte, can be 70%-90%. For example, the total mass percentage of the first organic solvent and the second organic solvent can be 70%, 75%, 80%, 85%, or 90%. For example, the range of the total mass percentage of the first organic solvent and the second organic solvent can be 70%-75%, 70%-80%, 75%-80%, 80%-85%, or 85%-90%.

[0075] The organic solvent is selected from at least one of ethylene carbonate (EC), propylene carbonate (PC), butene carbonate, dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, methyl ethyl carbonate (EMC), methyl propyl carbonate, ethyl propyl carbonate, 1,4-butyrolactone, methyl propionate, methyl butyrate, ethyl acetate, ethyl propionate, or ethyl butyrate.

[0076] As an example, the organic solvent can be prepared by mixing EC, EMC and DMC in a mass ratio of 1:1:1.

[0077] This application also provides a method for preparing a secondary battery. The method for preparing a secondary battery includes the following steps: Step S1: The positive electrode, negative electrode and separator are stacked to form a bare cell. The bare cell is then packaged to obtain a cell to be injected with electrolyte. The first electrolyte, which is a non-aqueous electrolyte, is then injected into the cell to be injected with electrolyte.

[0078] Step S2 involves forming and aging the battery cells after electrolyte injection.

[0079] Step S3: Inject a second electrolyte (non-aqueous electrolyte) into the aged cell and seal it to obtain a non-aqueous electrolyte secondary battery.

[0080] In some embodiments, step S2 involves incremental step-current formation. The incremental step-current formation method involves first charging with a first charging rate to a constant current to a first voltage, and then charging with a second charging rate to a constant current to a second voltage. The first charging rate is greater than or equal to 0.05C and less than the second charging rate, the first voltage is less than the second voltage, and the second voltage is 2.8V-3.4V. Incremental step-current formation gradually increases the current, allowing the SEI film to grow appropriately while maintaining both density and ion conductivity. Forming with a very small current for a long time will result in an excessively thick SEI film, high internal resistance, and poor rate capability; while directly starting formation with a large current will result in a porous SEI film with many defects, leading to continuous damage and regeneration, and constant consumption of the lithium source.

[0081] As an example, the second voltage can be 2.8V, 2.9V, 3.0V, 3.1V, 3.2V, 3.3V, or 3.4V. As an example, the range of the second voltage can be 2.8V-3.0V, 2.8V-3.1V, 2.8V-3.2V, 3.1V-3.3V, or 3.3V-3.4V.

[0082] Specifically, the first voltage is greater than or equal to 2.4V. The second charging rate ranges from 0.1C to 0.2C.

[0083] The aging method involves placing the formed battery cells at a temperature of 35℃-50℃ for 24-48 hours. For example, the aging temperatures can be 35℃, 37℃, 40℃, 43℃, 47℃, or 50℃, and the resting times can be 24h, 30h, 34h, 38h, 42h, 46h, or 48h. Alternatively, the aging temperature range can be 35℃-37℃, 35℃-40℃, 35℃-43℃, 43℃-47℃, or 47℃-50℃, and the resting time range can be 24h-30h, 24h-34h, 34h-38h, 34h-42h, or 46h-48h.

[0084] Specifically, the mass ratio of the first electrolyte to the second electrolyte injected is 7:3-9:1. As an example, the mass ratio of the first electrolyte to the second electrolyte injected can be 7:3, 7.5:2.5, 8:2, 8.5:1.5, or 9:1. As an example, the range of the mass ratio of the first electrolyte to the second electrolyte can be 7:3-7.5:2.5, 7:3-8:2, or 8:2-9:1.

[0085] Specifically, the positive electrode (positive electrode sheet) includes: a positive current collector and a positive electrode coating disposed on at least one surface of the positive current collector. The positive electrode coating includes: a positive electrode active material, a conductive agent, and a binder.

[0086] Specifically, there are no particular limitations on the positive electrode active material, and any positive electrode active material commonly used by those skilled in the art can be used. As an example, the positive electrode active material LiNi... (1-x-y) Co x Mn y (where 0≤x≤1, 0≤y≤1, and x+y≤1) or LiMn x Fe （1 x） At least one of PO4 (where 0≤x≤1), preferably, the positive electrode active material is LiFePO4. These positive electrode active materials can be used alone or in any combination and proportion.

[0087] There are no particular limitations on the conductive agent, and conductive agents commonly used by those skilled in the art can be used. For example, the conductive agent may be at least one of conductive carbon black, conductive carbon spheres, graphene, conductive carbon fibers, and carbon nanotubes. These conductive agents can be used alone or in any combination and proportion.

[0088] There are no particular limitations on the adhesive, and any adhesive commonly used by those skilled in the art can be used. As an example, the adhesive may be at least one selected from polyvinylidene fluoride (PVDF), copolymers of PVDF, polytetrafluoroethylene (PTFE), copolymers of tetrafluoroethylene and hexafluoropropylene, copolymers of PVDF and hexafluoropropylene, polyacrylic acid resin, styrene-butadiene rubber (SBR), polyimide, polypropylene resin, and carboxymethyl cellulose. These adhesives may be used alone or in combination or proportion of two or more. Preferably, the adhesive is PVDF.

[0089] There are no particular limitations on the positive electrode current collector, and any current collector commonly used by those skilled in the art can be used. As an example, the positive electrode current collector can be a metal foil such as aluminum foil, nickel foil, or a composite current collector. A composite current collector can be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.). Preferably, aluminum foil is used as the positive electrode current collector.

[0090] The positive electrode sheet can be prepared according to methods commonly used in the art. For example, the positive electrode sheet can be formed by uniformly dispersing the positive electrode active material, conductive agent, and binder in a solvent (e.g., N-methylpyrrolidone (NMP)) at a predetermined mass ratio to obtain a positive electrode slurry; coating the positive electrode slurry onto a positive electrode current collector, drying it, and then rolling and cutting it to obtain the positive electrode sheet.

[0091] Specifically, the negative electrode (negative electrode sheet) includes: a negative electrode current collector and a negative electrode coating disposed on at least one surface of the negative electrode current collector. The negative electrode coating includes: a negative electrode active material, a conductive agent, and a binder.

[0092] Specifically, the negative electrode active material is selected from at least one of carbon-based negative electrode materials, silicon-based negative electrode materials, tin-based negative electrode materials, silicon-containing alloy negative electrode materials, and tin-containing alloy negative electrode materials. Preferably, the negative electrode active material is graphite and / or silicon. These negative electrode active materials can be used alone or in any combination and proportion.

[0093] There are no particular limitations on the conductive agent, and conductive agents commonly used by those skilled in the art can be used. For example, the conductive agent may be at least one of conductive carbon black, conductive carbon spheres, graphene, conductive carbon fibers, and carbon nanotubes. These conductive agents can be used alone or in any combination and proportion.

[0094] There are no particular limitations on the adhesive, and any adhesive commonly used by those skilled in the art can be used. As an example, the adhesive may be at least one selected from polyvinylidene fluoride, copolymers of polyvinylidene fluoride, polytetrafluoroethylene, copolymers of tetrafluoroethylene and hexafluoropropylene, copolymers of polyvinylidene fluoride and hexafluoropropylene, polyacrylic acid resin, styrene-butadiene rubber, polyimide, polypropylene resin, and carboxymethyl cellulose. These adhesives may be used alone or in combination or proportion of two or more.

[0095] There are no particular limitations on the negative electrode current collector, and any negative electrode current collector commonly used by those skilled in the art can be used. As an example, the negative electrode current collector can be a metal foil such as copper foil or a composite current collector. Composite current collectors typically employ a "sandwich" structure, using an insulating polymer film (such as PET, PP, PE) as the intermediate substrate, and depositing a certain thickness of copper layer on both its upper and lower surfaces using a combination of magnetron sputtering and electroplating processes. Preferably, the negative electrode current collector is copper foil.

[0096] The negative electrode sheet can be prepared according to methods commonly used in the art. For example, the negative electrode sheet can be formed by uniformly dispersing the negative electrode active material, conductive agent, and binder in a solvent (e.g., water) at a predetermined mass ratio to obtain a negative electrode slurry; coating the negative electrode slurry onto a negative electrode current collector, drying it, and then rolling and cutting it to obtain the negative electrode sheet.

[0097] Specifically, there are no particular limitations on the membrane, and commonly used porous membranes with electrochemical and chemical stability can be used, such as polymer membranes, ceramic membranes, non-woven fabrics, inorganic-organic composite membranes, etc. Preferably, the membrane is selected from PE membranes.

[0098] Specifically, the secondary battery can be a pouch battery, a square aluminum-cased battery, or a cylindrical battery.

[0099] The present application will be further described in detail below with reference to the embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. Where no specific technology or conditions are specified in the embodiments, the technology or conditions described in the literature in the art or the product instructions shall apply. Instruments used without a specified manufacturer are all commercially available conventional products, and reagents used without a specified manufacturer are all commercially available conventional products, or products that can be prepared according to the methods described in this application or using conventional preparation methods.

[0100] Example 1 Preparation of thioester compounds S1 The structural formula of thioester compound S1 is shown below:

[0101] The preparation method of thioester compound S1 is as follows: A multi-necked flask is purged with nitrogen, and 122.12 g (1 mol) of erythritol and 611 g of dichloromethane are added. While stirring at 25°C, 297.5 g (2.5 mol) of thionyl chloride is added dropwise to the reaction system, and the reaction is maintained at 25°C for 28 h. After gas production, the reaction solution is purged with nitrogen until the pH paper is neutral, indicating the end of the reaction. The reaction solution is concentrated to obtain sulfite intermediate 1. 21.4 g (0.1 mol) of intermediate 1 is taken and 110 g of dichloromethane and 44.92 g (0.21 mol) of sodium periodate are added for oxidation. After the reaction, the product S1 is obtained by recrystallization from acetonitrile / dichloromethane. The yield of the target product S1 is 37%, and the purity is 98.9%.

[0102] The NMR spectrum of the target product S1 is as follows: ¹H NMR (400 MHz, DMSO-d6) δ: 4.70–5.30 (m, CH and CH2 oxygen-containing aliphatic hydrogens on the two 1,3,2-dioxothionecyclopentane rings); 2.10 (m, bridging -CH2 hydrogen connecting the bicyclic rings); 2.50 (DMSO-d6 solvent peak); 3.33 (H2O water peak). The five-membered ring oxygen-containing aliphatic hydrogens form a dense cluster of multiple peaks. The bridging methylene signal is clear, and the characteristic peak positions match the target molecule structure. Impurity signals are relatively few.

[0103] Preparation of non-aqueous electrolyte In a glove box filled with argon (water content <0.02ppm, oxygen content <0.02ppm), ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) were mixed thoroughly at a mass ratio of 1:1:1 to obtain an organic solvent. LiPF6 was added to the organic solvent and stirred until the lithium salt was completely dissolved, thus preparing a blank electrolyte. The prepared blank electrolyte was then divided into two parts: a first blank electrolyte and a second blank electrolyte.

[0104] Ethylene carbonate (VC) and thioester compound S1 were added to a first blank electrolyte and mixed thoroughly to obtain a first electrolyte. The mass fraction of ethylene carbonate in the first electrolyte was 0.6%, and the mass fraction of thioester compound S1 was 1.4%. The mass ratio of compound S1 to ethylene carbonate was 7:3.

[0105] Ethylene carbonate (VC) was added to the second blank electrolyte and mixed evenly to obtain the second electrolyte, wherein the mass fraction of ethylene carbonate in the second electrolyte was 12%.

[0106] The mass fraction of lithium hexafluorophosphate (LiPF6) is 12.5% ​​based on the total mass of the first and second electrolytes.

[0107] Preparation of secondary batteries Preparation of positive electrode sheet Lithium iron phosphate (LiFePO4), conductive agent Super P (conductive carbon black), and binder PVDF (polyvinylidene fluoride) were mixed at a mass ratio of 96.5:1.5:2, and then added to N-methylpyrrolidone (NMP) solvent for high-speed shear dispersion to obtain a homogenized positive electrode slurry. The positive electrode slurry was uniformly coated onto both sides of an aluminum foil current collector using an automatic coating machine. After drying at 100°C, it was cold-pressed, then slit and sliced, and finally baked in a vacuum oven at 85°C for 4 hours to produce a lithium-ion battery positive electrode sheet.

[0108] Preparation of negative electrode sheet Artificial graphite, conductive agent Super P (conductive carbon black), thickener CMC (sodium carboxymethyl cellulose), and binder SBR (styrene-butadiene rubber) are mixed in a mass ratio of 95:1.5:1:2.5. Then, deionized water is added and the mixture is stirred until homogeneous to obtain the negative electrode slurry. The negative electrode slurry is uniformly coated on both sides of a copper foil current collector, dried, and then rolled and die-cut to obtain the lithium-ion battery negative electrode sheet.

[0109] Battery assembly The battery separator is a PE film. The prepared positive electrode, negative electrode, and separator are stacked to form a bare cell. The bare cell is then packaged to obtain a 2Ah capacity cell ready for electrolyte injection. After vacuum baking at 95℃ for 48 hours, and confirming that the moisture content of both the positive and negative electrodes is <100ppm, the first electrolyte is injected. After sealing, the cell is immersed at high temperature for 24 hours, charged to 2.4V at 45℃ using a 0.05C constant current, followed by charging to 3.4V using a 0.1C constant current, and then left to stand at 40℃ for 36 hours. After this, the battery is injected a second time using a second electrolyte. After sealing, the battery is capacity-tested to obtain a lithium-ion battery. The mass ratio of the first electrolyte to the second electrolyte is 8:2.

[0110] Example 2 The identical descriptions in Example 2 and Example 1 are omitted. The difference between Example 2 and Example 1 is that the thioester compound S1 added to the first blank electrolyte is replaced with thioester compound S4. The structural formula of thioester compound S4 is shown below:

[0111] The preparation method of thioester compound S4 is as follows: A multi-necked flask was purged with nitrogen, and 152.12 g (1 mol) of xylitol, 760.6 g of dichloromethane, and 1.52 g of pyridine were added. During stirring at 25°C, 237.94 g (2 mol) of thionyl chloride was added dropwise to the reaction system, and the reaction was maintained at 25°C for 48 h. After gas production, the reaction solution was purged with nitrogen until the pH paper was neutral, indicating the end of the reaction. The reaction solution was concentrated to obtain sulfite intermediate 2. 24.4 g (0.1 mol) of intermediate 2 was taken and 124.5 g of dichloromethane was added and stirred until dissolved. 86.35 g (0.11 mol) of acetyl chloride was added dropwise to the reaction system at 0°C, and the reaction was maintained at 0°C for 12 h. After the reaction was completed, the target product was purified by column chromatography to obtain intermediate 3. 28.6 g of intermediate 3 (0.1 mol) was dissolved and clarified in 148 g of acetonitrile, and then 44.92 g (0.21 mol) of sodium periodate was added for oxidation. After the reaction was completed, the target product was extracted with dichloromethane, and purified by recrystallization of acetonitrile / dichloromethane to obtain the target product S4. The yield of the target product S4 was 41%, and the purity was 99.1%.

[0112] The NMR spectrum of the target product S4 is as follows: ¹H NMR (400 MHz, DMSO-d6) δ: 4.40–5.80 (m, CH, CH2 on two 1,3,2-dioxothionyl pentane rings and skeletal-linked methine aliphatic hydrogens); 2.05 (s, -OCOCH3 acetylmethyl hydrogen); 2.50 (DMSO-d6 solvent peak); 3.33 (H2O water peak). The five-membered ring oxygen-containing aliphatic hydrogens form a dense cluster of multiple peaks. The acetylmethyl signal is clear, and the characteristic peak positions match the target molecule structure. Impurity signals are relatively few.

[0113] Example 3 The identical descriptions in Example 3 and Example 1 are omitted. The difference between Example 3 and Example 1 is that the thioester compound S1 added to the first blank electrolyte is replaced with thioester compound S6. The structural formula of thioester compound S6 is shown below:

[0114] The preparation method of thioester compound S6 is as follows: A multi-necked flask is purged with nitrogen, and 92.09 g (1 mol) of glycerol and 460 g of dichloromethane are added. While stirring at 25°C, 130.86 g (1.1 mol) of thionyl chloride is added dropwise to the reaction system, and the reaction is maintained at 25°C for 24 h. After gas production, the reaction solution is purged with nitrogen until a moist pH paper test is neutral, indicating the end of the reaction. The reaction solution is concentrated to obtain sulfite intermediate 4. 27.6 g (0.2 mol) of intermediate 4 is taken, and 13.1 g (0.11 mol) of thionyl chloride is added dropwise at 25°C, and the reaction is stirred for 12 h. The reaction solution is purged with nitrogen until a moist pH paper test is neutral, indicating the end of the reaction. The resulting reaction solution is concentrated under reduced pressure at -0.1 MPa and 45°C, and dried to obtain the target product S6. The yield of the target product S6 is 35%, and the purity is 99.3%.

[0115] The NMR spectrum of the target product S6 is as follows: ¹H NMR (400 MHz, DMSO-d6) δ: 4.12–5.38 (m, CH on the ring, CH2 on the ring, and bridging -CH2-O-SO2- aliphatic hydrogen); 2.50 (quint, DMSO-d6 solvent peak); 3.33 (H2O water peak); 0.00 (TMS). The oxygen-containing five-membered ring and the methylene hydrogen linked to the sulfate ester are concentrated in the 4.1–5.4 ppm range. The multiple peak clusters match the molecular structure characteristics, and there are no obvious impurity characteristic peaks.

[0116] Example 4 The identical descriptions in Example 4 and Example 1 are omitted. The difference between Example 4 and Example 1 is that the thioester compound S1 added to the first blank electrolyte is replaced with thioester compound S7. The structural formula of thioester compound S7 is shown below:

[0117] The CAS number for thioester compound S7 is 2793408-99-6, and the supplier is Jiangxi Fufu Polyfluorine New Material Technology Co., Ltd.

[0118] Example 5 The identical descriptions to those in Example 1 are omitted in Example 5. The difference between Example 5 and Example 1 is that the thioester compound S1 added to the first blank electrolyte is replaced with thioester compound S10. The structural formula of thioester compound S10 is shown below:

[0119] The CAS number for thioester compound S10 is 1373610-02-6, and the supplier is Jiangxi Fufu Polyfluorine New Material Technology Co., Ltd.

[0120] Example 6 The identical descriptions to those in Example 1 are omitted in Example 6. The difference between Example 6 and Example 1 is that the thioester compound S1 added to the first blank electrolyte is replaced with thioester compound S12. The structural formula of thioester compound S12 is shown below:

[0121] The preparation method of thioester compound S12 is as follows: A multi-necked flask is purged with nitrogen, and 92.09 g (1 mol) of glycerol, 460 g of dichloromethane, and 0.9 g of triethylamine are added. During stirring at 25°C, 130.87 g (1.1 mol) of thionyl chloride is added dropwise to the reaction system, and the reaction is maintained at 25°C for 36 h. After gas production, the reaction solution is purged with nitrogen until the moist pH paper is neutral, indicating the end of the reaction. The reaction solution is concentrated to obtain sulfite intermediate 6. 25.4 g (0.1 mol) of intermediate 6 is taken, and 130.6 g of dichloromethane is added and stirred until dissolved. 20.22 g (0.12 mol) of trifluoromethylsulfinyl chloride is added dropwise to the reaction system at 0°C, and the reaction is maintained at 10°C for 12 h. After the reaction is completed, the product is purified by vacuum distillation at -0.1 MPa and 110°C to obtain intermediate 7. 28.6 g of intermediate 7 (0.1 mol) was dissolved and clarified in 150 g of ethyl acetate, and then 0.12 mol of sodium hypochlorite was added for oxidation. After the reaction was completed, the product was purified by recrystallization from ethyl acetate / methyl tert-butyl ether to obtain the target product S12. The yield of the target product S12 was 39%, and the purity was 98.9%.

[0122] The NMR spectrum of the target product S12 is as follows: ¹H NMR (400 MHz, DMSO-d6) δ: 4.40–5.60 (m, 1,3,2-dioxothionyl pentane ring CH, CH2 and side chain CH2-OSO2CF3 oxygen-containing aliphatic hydrogens); 2.50 (DMSO-d6 solvent peak); 3.33 (H2O water peak). The five-membered ring and the side chain oxygen-containing aliphatic hydrogens form a dense cluster of multiple peaks, and the signal range matches the structure of the target molecule.

[0123] The NMR F-spectrum of the target product S12 is as follows: ¹ 9 F NMR (376 MHz, DMSO-d6) δ: -78.0 (s, -OSO2CF3 trifluoromethyl fluorine signal), which is a characteristic peak of trifluoromethanesulfonate.

[0124] Example 7 The same parts of Example 7 and Example 3 are omitted. The difference between Example 7 and Example 3 is that no vitamin C was added to the first blank electrolyte.

[0125] Example 8 The identical descriptions in Example 8 and Example 3 are omitted. The difference between Example 8 and Example 3 is that the mass fraction of VC added to the first blank electrolyte is 0.8%, and the mass fraction of thioester compound S6 is 1.2%. The mass ratio of thioester compound S6 to VC is 6:4.

[0126] Example 9 The identical descriptions in Example 9 and Example 3 are omitted. The difference between Example 9 and Example 3 is that the mass fraction of VC and the mass fraction of thioester compound S6 added to the first blank electrolyte are 1.0%. The mass ratio of VC to thioester compound S6 is 1:1.

[0127] Example 10 The parts that are the same as those in Example 10 and Example 3 are omitted. The differences between Example 10 and Example 3 are:

[0128] The first blank electrolyte contained 1.2% VC by mass and 0.8% thioester compound S6 by mass. The mass ratio of thioester compound S6 to VC was 4:6.

[0129] Example 11 The identical descriptions in Example 11 and Example 3 are omitted. The difference between Example 11 and Example 3 is that the mass fraction of VC added to the first blank electrolyte is 1.4%, and the mass fraction of thioester compound S6 is 0.6%. The mass ratio of thioester compound S6 to VC is 3:7.

[0130] Example Twelve The same descriptions as in Example 11 are omitted. The difference between Example 12 and Example 1 is that the mass fraction of VC added to the first blank electrolyte is 1.6%, and the mass fraction of thioester compound S6 is 0.4%. The mass ratio of thioester compound S6 to VC is 2:8.

[0131] Example Thirteen The identical descriptions in Example 13 and Example 1 are omitted. The differences between Example 13 and Example 1 are as follows: the mass ratio of the first electrolyte to the second electrolyte is 7:3; vinyl sulfate (DTD) is added to the first blank electrolyte, and the mass fraction of DTD in the first electrolyte is 1%; the mass fraction of vinylene carbonate in the second electrolyte is 22%.

[0132] Example Fourteen The same descriptions as in Example 14 are omitted. The difference between Example 14 and Example 1 is that the mass ratio of the first electrolyte to the second electrolyte is 9:1. Based on the total mass of the first electrolyte, the first blank electrolyte also contains 1% fluoroethylene carbonate (FEC) and 0.5% methane disulfonate (MMDS).

[0133] The mass fraction of vinylene carbonate in the second electrolyte is 5%.

[0134] Example 15 The same descriptions as in Example 1 are omitted in Example 15. The difference between Example 15 and Example 1 is that, based on the total mass of the first electrolyte, the first blank electrolyte also contains 0.5% methylene disulfonate (MMDS), 0.5% lithium difluorophosphate (LiDFP), and 0.3% tris(trimethylsilane) phosphate (TMSP).

[0135] Comparative Example 1 The same descriptions as those in Comparative Example 1 and Example 3 are omitted. The difference between Comparative Example 1 and Example 3 is that no thioester compound S6 was added to the first blank electrolyte.

[0136] Comparative Example 2 The same descriptions as those in Comparative Example 1 are omitted in Comparative Example 2. The difference between Comparative Example 2 and Example 1 is that a second electrolyte is not used to inject the battery a second time during battery assembly.

[0137] Comparative Example 3 The identical descriptions in Comparative Example 3 and Example 2 are omitted. The difference between Comparative Example 3 and Example 2 is that, during battery assembly, a second electrolyte is not used for a second electrolyte injection.

[0138] Comparative Example 4 The identical descriptions in Comparative Example 4 and Example 3 are omitted. The difference between Comparative Example 4 and Example 3 is that, during battery assembly, a second electrolyte is not used for a second electrolyte injection.

[0139] Comparative Example 5 The same descriptions as those in Comparative Example 5 and Example 4 are omitted. The difference between Comparative Example 5 and Example 4 is that a second electrolyte is not used to inject the battery a second time during battery assembly.

[0140] The amounts of each substance added in Examples 1 to 15 and Comparative Examples 1 to 5 are shown in Table 1.

[0141] Table 1.

[0142] Electrochemical performance testing Electrochemical performance tests were conducted on the batteries of Examples 1 to 15 and Comparative Examples 1 to 5. The test results of Examples 1 to 15 and Comparative Examples 1 to 5 are shown in Table 2.

[0143] 1. First Coulomb efficiency test Under 45°C conditions, the soft-pack lithium-ion batteries prepared in Examples 1 to 15 and Comparative Examples 1 to 5 were charged at a constant current of 0.05C to a cutoff voltage of 3.2V, then charged at a constant current of 0.2C to 3.4V, and then charged at a constant voltage of 3.4V until the current dropped to 0.05C and the charging capacity was recorded as C0. They were then discharged at a constant current of 1 / 3C to 2.5V, and the discharge capacity was recorded as C1. The initial coulombic efficiency (first-time efficiency) is calculated using the following formula.

[0144] First effect = C1 / C0 100% 2. DC Internal Resistance (DCR) Test At 25°C, the soft-pack lithium-ion batteries prepared in Examples 1 to 15 and Comparative Examples 1 to 5 were discharged at a constant current of 1C to 50% SOC. Then, a 2C discharge pulse current was applied and held for 10s. The DC internal resistance (DCR) was calculated by dividing the voltage difference before and after the pulse by the current value.

[0145] 3. 45℃ Cyclic Test Under 45°C conditions, the soft-pack lithium-ion batteries prepared in Examples 1 to 15 and Comparative Examples 1 to 5 above were charged to 3.65V at a constant current of 1C. Then, they were charged at a constant voltage of 3.65V until the current dropped to 0.05C and then discharged to 2.5V at a constant current of 1C. This charging / discharging process was repeated, and the capacity retention rate of the battery after 800 cycles was calculated.

[0146] 4. Battery capacity retention test at 60℃ The soft-pack lithium-ion batteries prepared in Examples 1 to 15 and Comparative Examples 1 to 5 were charged at a constant current of 1C to 3.65V at 25℃±2℃. Then, they were charged at a constant voltage of 3.65V until the current dropped to 0.05C. After resting for 10 minutes, they were discharged at a constant current of 1C to 2.5V and rested for 10 minutes. This process was repeated three times, and the average discharge capacity of the three cycles was taken as C1. The soft-pack lithium-ion batteries were then stored at 60℃±2℃ for 30 days. After that, the batteries were removed and stored at 25℃±2℃ for 5 hours. They were then discharged at a constant current of 1C to 2.5V, and the discharge capacity C2 was recorded. The formula for calculating the capacity retention rate after 30 days of storage at 60℃ is as follows: Capacity retention rate after 30 days of storage at 60℃ = C2 / C1 100% Table 2.

[0147] According to the comparisons in Table 2 between Example 1 and Comparative Example 2, Example 2 and Comparative Example 3, Example 3 and Comparative Example 4, and Example 4 and Comparative Example 5, the initial coulombic efficiency, capacity retention after 800 cycles at 45℃, and capacity retention after 30 days of storage at 60℃ for Comparative Examples 2, 3, 4, and 5 are significantly lower than those of the corresponding Examples 1-4, while their DC internal resistance (DCR) is significantly higher. This indicates that when the electrolyte contains both thioester compounds and vitamin C, and the battery is prepared using a traditional one-time electrolyte injection process, there is an uncoordinated competitive and antagonistic effect between the thioester compounds and vitamin C. Specifically, if the traditional one-time electrolyte injection process is used, during the formation and film formation stage, thioester compounds and VC compete for film formation, resulting in a loose, thick, and uneven SEI film, producing a 1+1<2 effect. Even if thioester compounds can reduce impedance and form an SEI film with more inorganic sulfur components at a higher proportion, the thioester compounds that are not completely consumed during the formation process remain in the electrolyte, causing the electrolyte acidity to increase, which in turn degrades the subsequent battery performance. The two-stage electrolyte injection method involves first adding a first electrolyte to the battery, in which thioester compounds partially or completely replace vitamin C, and the ratio of the two is adjusted. This allows sulfur compounds to preferentially react completely on the negative electrode surface to form a thin, dense, highly stable, and low-resistance SEI film rich in various inorganic sulfur components. After formation, a second electrolyte is added, in which vitamin C forms an organic flexible polymer layer on the outer layer. This fully leverages the advantages of vitamin C in subsequent cycle repair and film formation while effectively suppressing the internal resistance growth caused by vitamin C, further improving the battery's high-temperature, room-temperature cycle, and high-temperature storage performance, thus achieving a 1+1>2 effect.

[0148] A comparison of the data from Comparative Example 1 and Examples 1-15 shows that the first-round coulombic efficiency and capacity retention rate after 30 days of storage at 60℃ in Examples 1-15 are significantly higher than those in Comparative Example 1, while the DC internal resistance (DCR) is significantly lower. Furthermore, when the first additive contains both thioester compounds and vitamin C, the capacity retention rate after 800 cycles at 45℃ is also significantly higher than that in Comparative Example 1. This indicates that after adding thioester compounds, vitamin C and thioester compounds can synergistically undergo a polymerization reaction at the negative electrode interface, generating a thin, uniform SEI film with better ion conductivity and more stable thermodynamics and kinetics. This is more conducive to lithium-ion transport and charge transfer at the interface, overcoming the adverse effect of increased impedance caused by adding VC alone. This allows the additive to simultaneously improve high-temperature cycling and high-temperature storage performance while reducing battery impedance.

[0149] Based on the data from Examples 3, 7-12, and Comparative Example 1, it can be seen that as the proportion of thioester compounds in the first additive gradually decreases, the DC internal resistance (DCR) first gradually decreases to a certain value and then gradually increases; the capacity retention rate after 800 cycles at 45°C gradually increases, reaching its maximum value when the ratio of thioester compounds to vinylene carbonate (VC) in the first additive is 3:7. As the proportion of thioester compounds continues to decrease, the capacity retention rate after 800 cycles at 45°C gradually decreases; the capacity retention rate after 30 days of storage at 60°C gradually increases, reaching its maximum value when the ratio of thioester compounds to vinylene carbonate (VC) in the first additive is 6:4. As the proportion of thioester compounds continues to decrease, the capacity retention rate after 30 days of storage at 60°C gradually decreases. Both the high-temperature cycle performance and high-temperature storage performance of the battery initially improve and then deteriorate as the proportion of thioester compounds decreases. Furthermore, the high-temperature cycle performance is better when the ratio of thioester compounds to VC is 3:7, and the high-temperature storage performance is better when the ratio is 6:4. This indicates that thioester compounds and VC have both synergistic and competitive relationships. They synergistically form an inorganic-organic combined SEI film on the negative electrode surface. High-temperature storage focuses more on the static thermal stability of the SEI film, while high-temperature cycle performance focuses more on the dynamic mechanical properties of the SEI film. When the proportion of thioester compounds is higher than that of VC, an SEI film composed of multiple inorganic sulfur components is preferentially formed, resulting in better thermal stability and thus better high-temperature storage performance. Conversely, when the proportion of VC is higher than that of thioester compounds, the SEI film composition contains more organic cross-linked polymer phases formed by VC bond breaking and ring opening, resulting in better flexibility and the ability to better withstand the deformation caused by repeated lithium-ion insertion and extraction during cycling, leading to better SEI film mechanical properties and thus better cycle performance. The dynamic equilibrium relationship between the organic and inorganic phases of the SEI film should be achieved by adjusting the ratio of the two according to the specific battery performance requirements.

[0150] Based on the data from Examples 1-6, it can be seen that the batteries prepared in Examples 1-5 have higher DC internal resistance (DCR), higher capacity retention after 800 cycles at 45°C, and higher capacity retention after 30 days of storage at 60°C than the battery prepared in Example 6. This indicates that when the sulfur-containing oxygen-containing ester fragment in the thioester compound is symmetrical, and the sulfur-containing oxygen-containing ester fragment is selected from at least one of cyclic sulfate groups and cyclic sulfite diester groups, the thioester compound with this structure forms a more uniform, thinner, and denser SEI film at the negative electrode interface, with better ion conductivity and fewer defects. At the same time, the symmetrical molecular steric hindrance is greater, and the molecular thermodynamic stability is significantly improved. The batteries prepared in Examples 2, 4, and 5 have better capacity retention after the first efficiency test, 800 cycles at 45°C, and 30 days of storage at 60°C than the battery prepared in Example 3, indicating that when the thioester compound has an overall symmetrical structure, its high-temperature cycling performance and high-temperature storage performance are superior. The batteries prepared in Examples 2, 4, and 5 showed significantly higher DC internal resistance (DCR), capacity retention after 800 cycles at 45°C, and capacity retention after 30 days of storage at 60°C compared to the battery prepared in Example 1. This indicates that when thioester compounds have a symmetrical structure and the cyclic sulfate groups or cyclic diester groups are not directly connected, the SEI film formed at the negative electrode interface is more uniform, thinner, and denser than that of thioester compounds with two directly connected cyclic sulfate groups or cyclic diester groups. It also exhibits better ion conductivity, fewer defects, greater steric hindrance due to the symmetrical molecular structure, and significantly improved molecular thermodynamic stability.

[0151] The above description is merely a specific implementation of the embodiments of this application, but the protection scope of the embodiments of this application is not limited thereto. Any changes or substitutions within the technical scope disclosed in the embodiments of this application should be covered within the protection scope of the embodiments of this application. Therefore, the protection scope of the embodiments of this application should be determined by the protection scope of the appended claims.

Claims

1. A non-aqueous electrolyte, characterized in that, include: The first electrolyte comprises: a first electrolyte salt, a first organic solvent, and a first additive. The first additive comprises: vinylene carbonate and a thioester compound. The thioester compound contains at least two sulfur-containing oxygen-containing ester segments. The sulfur-containing oxygen-containing ester segments are selected from at least one of sulfate ester structural units, sulfite diester structural units, and substituted sulfonyloxy groups. The sulfur-containing oxygen-containing ester segments are connected by a linking segment, which is selected from substituted or unsubstituted C1-C8 alkyl segments or -(R... 1 ) n -X,R 1 Selected from C1-C4 alkyl segments, X is selected from diester carbonate structural units, chain-like diester sulfate structural units, chain-like diester sulfite structural units, R 2 R is a substituent of any one of the following structural units: dialkoxyphosphonate diester, phosphate triester, oxalate diester, or quaternary ammonium cation group. 2 Selected from any one of halogen, unsubstituted or fluorinated C1-C6 alkyl, unsubstituted or fluorinated C1-C6 saturated alkoxy, unsubstituted or fluorinated C2-C6 unsaturated alkoxy, unsubstituted or fluorinated C2-C6 unsaturated hydrocarbon, substituted or unsubstituted silyl, and cyano, where n is a positive integer; and The second electrolyte comprises: a second electrolyte salt, a second organic solvent, and a second additive, wherein the second additive comprises vinylene carbonate. Specifically, based on the total mass of the thioester compounds and vinylene carbonate in the first additive, the mass fraction of the thioester compounds is greater than 0% and less than or equal to 100%, and the mass fraction of vinylene carbonate in the first additive is greater than or equal to 0% and less than 100%.

2. The non-aqueous electrolyte according to claim 1, characterized in that, Based on the total mass of the non-aqueous electrolyte, the sum of the mass fractions of the thioester compounds, the vinylene carbonate in the first electrolyte, and the vinylene carbonate in the second electrolyte is 0.01%-8%.

3. The non-aqueous electrolyte according to claim 1, characterized in that, The mass ratio of the first electrolyte to the second electrolyte is 7:3-9:

1.

4. The non-aqueous electrolyte according to claim 1, characterized in that, The thioester compounds have a symmetrical structure, and the sulfur-containing oxygen-containing ester fragments are selected from at least one of cyclic sulfate structural units and cyclic sulfite groups.

5. The non-aqueous electrolyte according to claim 1, characterized in that, The substituents in the substituted C1-C8 alkyl segments are selected from fluorine or carboxylic acid ester groups. The structural formula of the substituted sulfonyloxy group is: -O-SO2-R 3 R 3 It is selected from any one of the following: fluorinated or unsubstituted C1-C6 alkyl, unsubstituted or fluorinated C1-C6 alkoxy, unsubstituted or fluorinated C2-C6 unsaturated hydrocarbon, substituted or unsubstituted silyl, and cyano.

6. The non-aqueous electrolyte according to claim 1, characterized in that, The thioester compounds are selected from any one of the following structures: 、 、 、 、 、 、 、 、 、 、 、 、 、 、 。 7. The non-aqueous electrolyte according to claim 1, characterized in that, The first additive also includes at least one of vinyl sulfate, fluorovinyl carbonate, methylene disulfonate, 1,3-propanesulfonate lactone, lithium difluorooxalate borate, lithium difluorophosphate, and tris(trimethylsilane) phosphate.

8. The non-aqueous electrolyte according to claim 1, characterized in that, Based on the total mass of the non-aqueous electrolyte, the total mass percentage of the first electrolyte salt and the second electrolyte salt is 8%-20%, and the total mass percentage of the first organic solvent and the second organic solvent is 70%-90%. Both the first electrolyte salt and the second electrolyte salt are selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium difluorooxalate borate, lithium bis(oxalate borate), lithium difluorooxalate phosphate, lithium bis(fluorosulfonyl)imide, and lithium bis(trifluoromethanesulfonyl)imide. Both the first organic solvent and the second organic solvent are selected from at least one of ethylene carbonate, propylene carbonate, butene carbonate, dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, 1,4-butyrolactone, methyl propionate, methyl butyrate, ethyl acetate, ethyl propionate, or ethyl butyrate.

9. A method for preparing a secondary battery, characterized in that, Includes the following steps: Step S1: The positive electrode, negative electrode and separator are stacked to form a bare cell, the bare cell is packaged to obtain a cell to be injected with electrolyte, and the first electrolyte of any one of claims 1-7 is injected into the cell to be injected with electrolyte. Step S2 involves forming and aging the battery cells after electrolyte injection. Step S3: Inject the second electrolyte of the non-aqueous electrolyte into the aged cell and seal it to obtain a non-aqueous electrolyte secondary battery.

10. The method for preparing a secondary battery according to claim 9, characterized in that, In step S2, the transformation is carried out by incremental step current transformation. The incremental step current transformation method is as follows: first, a constant current is charged to a first voltage using a first charging rate, and then a constant current is charged to a second voltage using a second charging rate. The first charging rate is greater than or equal to 0.05C and less than the second charging rate, the first voltage is less than the second voltage, and the second voltage is 2.8V-3.4V. The aging method is to leave the formed battery cells at a temperature of 35℃-50℃ for 24h-48h.

11. A secondary battery, characterized in that, include: It is prepared by the method described in claim 9 or 10.

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