Electrolyte and secondary battery

Abstract

The application provides an electrolyte and a secondary battery. The electrolyte provided by the application comprises an electrolyte, an additive and a solvent, the additive comprises a first additive and a second additive, the first additive is selected from a compound shown in formula I, and the second additive is a compound containing an unsaturated carbon-carbon bond. The first additive and the second additive synergistically act, are beneficial to the formation of a reticular solid electrolyte interface film, effectively alleviate the volume expansion of the battery during high-temperature storage, and significantly improve the cycle performance of the secondary battery.

Description

Technical Field

[0001] This application relates to the field of electrochemical technology, and in particular to an electrolyte and a secondary battery. Background Technology

[0002] In recent years, rechargeable batteries have achieved great success in the field of high-energy batteries. However, consumers still expect batteries with even higher overall performance, which depends on the research and development of new electrode materials and electrolyte systems. However, factors such as the dissolution of cathode metal ions under high-temperature conditions, the occurrence of electrolyte side reactions, and the degradation of the solid electrolyte interface (SEI) film at the electrode-electrolyte interface lead to severe gas generation in rechargeable batteries, making it difficult to improve their cycle performance. Therefore, it is urgent to develop electrolytes to alleviate battery volume expansion during high-temperature storage and improve the cycle performance of rechargeable batteries. Summary of the Invention

[0003] The purpose of this application is to provide an electrolyte and a secondary battery to alleviate battery volume expansion during high-temperature storage and improve the cycle performance of the secondary battery. The specific technical solution is as follows:

[0004] A first aspect of this application provides an electrolyte comprising an electrolyte, an additive, and a solvent, wherein the additive comprises a first additive and a second additive;

[0005] The first additive is selected from the compounds shown in Formula I;

[0006]

[0007] The second additive is a compound containing unsaturated carbon-carbon bonds, wherein the compound containing unsaturated carbon-carbon bonds is selected from at least one of formula II-a to II-d;

[0008]

[0009] Where A is selected from B is selected from

[0010] R1 to R 19 Each compound is independently selected from C1-C4 alkyl, C2-C4 alkenyl, or C2-C4 ynyl groups, and the compound containing an unsaturated carbon-carbon bond contains at least one C2-C4 alkenyl or C2-C4 ynyl group;

[0011] Based on the mass of the electrolyte, the mass percentage of the first additive is w1, where w1 is 0.05% to 3%, preferably 0.1% to 2%; the mass percentage of the second additive is w2, where w2 is 0.05% to 2%, preferably 0.5% to 2%.

[0012] In one embodiment of this application, the mass ratio w1 / w2 of the first additive and the second additive is 1:(0.05-20), preferably 1:(0.1-10).

[0013] In one embodiment of this application, the compound containing unsaturated carbon-carbon bonds is selected from at least one of the following compounds:

[0014]

[0015] In one embodiment of this application, the electrolyte is selected from at least one of lithium hexafluorophosphate, lithium difluorophosphate, lithium bis(trifluoromethyl)sulfonylimide, lithium bisfluorosulfonylimide, lithium monofluorosulfonate, and lithium trifluoromethylsulfonate; the mass percentage of the electrolyte is 10% to 20% based on the mass of the electrolyte solution.

[0016] In one embodiment of this application, the solvent is selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, dimethyl carbonate, and diethyl carbonate; the mass percentage of the solvent is 75% to 89% based on the mass of the electrolyte.

[0017] The second aspect of this application provides a secondary battery comprising a positive electrode, a negative electrode, a separator, and the electrolyte described in the first aspect of this application.

[0018] In one embodiment of this application, the positive electrode includes a positive current collector and a positive electrode material layer disposed on at least one surface of the positive current collector, the positive electrode material layer including a positive electrode active material, the positive electrode active material including LiNi. a Co b M c O2, M includes at least one of Mn, Al, Ti, V, Fe, Zn, V, Zr, Ce, Cr and Cu, a+b+c=1, 0.5≤a≤0.8, 0.1≤b≤0.2, 0.1≤c≤0.3.

[0019] The beneficial effects of this application are:

[0020] This application provides an electrolyte and a secondary battery. The electrolyte includes an electrolyte, additives, and a solvent. The additives include a first additive and a second additive, which are used in combination. The mass ratio of the first additive and the second additive is limited within the range specified in this application. The first additive can form a thin SEI film that facilitates ion conduction during the formation and capacity testing stages, thereby improving the cycle performance at room temperature. At the same time, the introduction of the second additive makes the SEI film more resilient, which can effectively suppress the SEI film rupture caused by high temperature conditions and reduce the side reactions caused by direct contact between the electrode and the electrolyte. The synergistic effect of the first additive and the second additive effectively alleviates the volume expansion of the battery during high-temperature storage and improves the cycle performance of the secondary battery.

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

[0022] The technical solutions in this application will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. All other embodiments obtained by those skilled in the art based on this application are within the scope of protection of this application.

[0023] It should be noted that, in the specific embodiments of this application, lithium-ion batteries are used as an example of secondary batteries to explain this application, but the secondary batteries in this application are not limited to lithium-ion batteries.

[0024] This application provides an electrolyte comprising an electrolyte, an additive, and a solvent, wherein the additive comprises a first additive and a second additive;

[0025] The first additive is selected from the compounds shown in Formula I;

[0026]

[0027] The second additive is a compound containing unsaturated carbon-carbon bonds, wherein the compound containing unsaturated carbon-carbon bonds is selected from at least one of formula II-a to II-d;

[0028]

[0029] Where A is selected from B is selected from R1 to R 19 Each compound is independently selected from C1-C4 alkyl, C2-C4 alkenyl, or C2-C4 ynyl groups, and the compound containing an unsaturated carbon-carbon bond contains at least one C2-C4 alkenyl or C2-C4 ynyl group; for example, R1 to R 19Each is independently selected from methyl, ethyl, propyl, butyl, vinyl, propenyl, butenyl, ethynyl, propynyl, or butynyl;

[0030] Based on the mass of the electrolyte, the mass percentage of the first additive is w1, which is 0.05% to 3%, preferably 0.1% to 2%; the mass percentage of the second additive is w2, which is 0.05% to 2%, preferably 0.5% to 2%. For example, based on the mass of the electrolyte, the mass percentage of the first additive w1 can be 0.05%, 0.1%, 1%, 1.5%, 2%, 2.5%, 3%, or a range of any two values ​​therein; the mass percentage of the second additive w2 can be 0.05%, 0.1%, 0.5%, 1%, 1.5%, 2%, or a range of any two values ​​therein. When the contents of the first and second additives are within the above ranges, the synergistic effect of the first and second additives can be achieved, mitigating the volume expansion of the battery during high-temperature storage and improving the cycle performance of the secondary battery.

[0031] In one embodiment of this application, the mass ratio w1 / w2 of the first additive and the second additive is 1:(0.05-20), preferably 1:(0.1-10). For example, the mass ratio w1 / w2 of the first additive and the second additive can be 1:0.05, 1:0.1, 1:1, 1:5, 1:10, 1:15, 1:20, or a range consisting of any two of these values. When the mass ratio of the first additive and the second additive is within the above range, the synergistic effect of the first additive and the second additive can be better utilized, further mitigating the volume expansion of the battery during high-temperature storage and improving the cycle performance of the secondary battery.

[0032] Lithium-ion battery electrolytes often exhibit increased initial impedance due to the reduction and decomposition of solvents during the formation stage, forming membrane components such as alkyl lithium carbonate and lithium carbonate, which are detrimental to ion conduction. Furthermore, as charge-discharge cycles continue, alkyl lithium carbonate and LiF membrane components continuously deposit at the electrode-electrolyte interface, making it difficult to meet the cycle performance requirements of practical applications. The inventors discovered that the compound shown in Formula I can form a thin SEI film that promotes ion conduction during the formation and capacity testing stages. The membrane component rich in alkyl lithium sulfonate exhibits excellent performance in improving room temperature cycling due to the flexibility and stability of alkyl lithium sulfonate and the thinness of the SEI film. However, under high-temperature conditions, the SEI film lacks sufficient protection and is prone to rupture, leading to side reactions between the electrode and electrolyte, resulting in battery volume expansion and increased impedance. The second additive, a compound containing unsaturated carbon-carbon bonds, opens double bonds during the formation and capacity testing stages, forming a membrane component rich in long carbon chains. Simultaneously, it can combine with the reduction decomposition product (compound shown in Formula I-1) or oxidation decomposition product (compound shown in Formula I-2) of the first additive (compound shown in Formula I-2) to further form a network SEI film rich in S groups. The combined action of the compound of Formula I and the compound containing unsaturated carbon-carbon bonds solves the problem of insufficient protection of the SEI film of the compound of Formula I. At the same time, the introduction of a network SEI film component rich in S groups into the long carbon chain-rich film component formed by the compound containing unsaturated carbon-carbon bonds also solves the problem of high impedance of the long carbon chain-rich film component formed by the compound containing unsaturated carbon-carbon bonds.

[0033] In summary, the synergistic effect of Formula 1 and compounds containing unsaturated carbon-carbon bonds can achieve the effect of not affecting ion conduction and further improving stability.

[0034]

[0035] In one embodiment of this application, the compound containing unsaturated carbon-carbon bonds is selected from at least one of the following compounds:

[0036]

[0037] In one embodiment of this application, the electrolyte is selected from at least one of lithium hexafluorophosphate, lithium difluorophosphate, lithium bis(trifluoromethyl)sulfonylimide, lithium bisfluorosulfonylimide, lithium monofluorosulfonate, and lithium trifluoromethylsulfonate; based on the mass of the electrolyte, the mass percentage content of the electrolyte is 10% to 20%. For example, based on the mass of the electrolyte, the mass percentage content of the electrolyte can be 10%, 12%, 14%, 16%, 18%, 20%, or a range of any two of these values. By including the above-mentioned electrolytes in the electrolyte and controlling the mass percentage content of the electrolyte within the scope of this application, the electrolyte can have high ionic conductivity and good electrochemical stability, which can further improve the cycle performance of the secondary battery.

[0038] In one embodiment of this application, the solvent is selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, dimethyl carbonate, and diethyl carbonate; the mass percentage of the solvent is 75% to 89% based on the mass of the electrolyte. For example, the mass percentage of the solvent can be 75%, 77%, 80%, 85%, 89%, or a range of any two of these values, based on the mass of the electrolyte. By including the above-mentioned solvents in the electrolyte and controlling the mass percentage of the solvent within the range of this application, the electrolyte can have suitable viscosity, high ionic conductivity, and good electrochemical stability, which can further improve the cycle performance of the secondary battery.

[0039] This application does not impose any particular restrictions on the preparation method of the electrolyte, as long as it can achieve the purpose of this application. For example, various solvents in the electrolyte can be mixed, and then electrolytes and additives can be added and mixed evenly.

[0040] The second aspect of this application provides a secondary battery comprising a positive electrode, a negative electrode, a separator, and the electrolyte described in the first aspect of this application.

[0041] In this application, the negative electrode sheet 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 phrase "a 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 its thickness direction, or on two surfaces of the negative electrode current collector along its thickness direction. It should be noted that the term "surface" here can refer to the entire surface area of ​​the negative electrode current collector, or only a portion thereof; this application does not impose any particular limitation, as long as the purpose of this application is achieved.

[0042] This application does not impose any particular limitation on the negative electrode current collector, as long as it can achieve the purpose of this application. For example, the negative electrode current collector can be a metal foil or a composite current collector. For example, as a metal foil, copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, foamed nickel, or foamed copper can be used. The composite current collector may include a polymer material substrate and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector can be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), etc.).

[0043] This application does not impose any particular limitation on the thickness of the negative electrode material layer and the negative electrode current collector, as long as the purpose of this application can be achieved. For example, the thickness of the single-sided negative electrode material layer is 60 μm to 80 μm, and the thickness of the negative electrode current collector is 5 μm to 18 μm.

[0044] The negative electrode material layer includes a negative electrode active material. This application does not impose any particular limitation on the negative electrode active material, as long as it can achieve the purpose of this application. For example, the negative electrode active material can be selected from, but is not limited to, at least one of artificial graphite, natural graphite, soft carbon, hard carbon, mesophase carbon microspheres, silicon-based materials, tin-based materials, and lithium titanate. The silicon-based material can be selected from at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material can be selected from at least one of elemental tin, tin oxide compounds, and tin alloys.

[0045] The negative electrode material layer may further include a conductive agent and a binder. This application does not impose any particular limitation on the types of conductive agents and binders, as long as they achieve the purpose of this application. For example, the conductive agent may include, but is not limited to, at least one of superconducting carbon black (SuperP), acetylene black, Ketjen black, carbon nanotubes, graphene, and carbon fibers. The aforementioned carbon nanotubes may include, but are not limited to, single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The aforementioned carbon fibers may include, but are not limited to, vapor-grown carbon fibers (VGCF) and / or carbon nanofibers. The binder may include, but is not limited to, at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), lithium polyacrylate (PAALi), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS). This application does not impose any particular limitation on the mass ratio of the negative electrode active material, conductive agent, and binder in the negative electrode material layer. Those skilled in the art can select according to actual needs, as long as the purpose of this application is achieved.

[0046] The negative electrode material layer may also include a thickener, which may include, but is not limited to, at least one of sodium carboxymethyl cellulose and lithium carboxymethyl cellulose. This application does not impose any particular limitation on the mass ratio of the negative electrode active material, conductive agent, binder, and thickener in the negative electrode material layer; those skilled in the art can select them according to actual needs, as long as the purpose of this application can be achieved.

[0047] Optionally, the negative electrode sheet may further include a conductive layer located between the negative electrode current collector and the negative electrode material layer. This application does not impose any particular limitation on the composition of the conductive layer, which can be a conductive layer commonly used in the art. For example, the conductive layer may include a conductive agent and a binder. This application does not impose any particular limitation on the conductive agent and binder in the conductive layer; for example, it can be at least one of the aforementioned conductive agents and binders.

[0048] This application does not impose any particular limitation on the preparation method of the negative electrode sheet, as long as it achieves the purpose of this application. For example, the negative electrode active material, conductive agent, thickener, and binder can be mixed in a certain proportion, and deionized water can be added and stirred evenly to obtain a negative electrode slurry with a solid content of 40wt% to 60wt%. The negative electrode slurry is uniformly coated on one surface of the negative electrode current collector, and after drying, a negative electrode sheet with a single-sided negative electrode material layer is obtained. Then, the above coating steps are repeated on the other surface of the negative electrode current collector, and after drying, a negative electrode sheet with a double-sided negative electrode material layer is obtained. After cold pressing, edge trimming, cutting, slitting, drying, and welding of electrode tabs, the negative electrode sheet is obtained.

[0049] In one embodiment of this application, the positive electrode includes a positive current collector and a positive electrode material layer disposed on at least one surface of the positive current collector, the positive electrode material layer including a positive electrode active material, the positive electrode active material including LiNi. a Co b M c O2, M includes at least one of Mn, Al, Ti, V, Fe, Zn, V, Zr, Ce, Cr and Cu, a+b+c=1, 0.5≤a≤0.8, 0.1≤b≤0.2, 0.1≤c≤0.3.

[0050] The aforementioned "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 along its own thickness direction, or it can be disposed on two surfaces of the positive electrode current collector along its own thickness direction. It should be noted that the "surface" here can be the entire area of ​​the surface of the positive electrode current collector, or it can be a part of the surface of the positive electrode current collector. This application has no particular limitation, as long as the purpose of this application can be achieved.

[0051] This application does not impose any particular limitation on the positive electrode current collector, as long as it can achieve the purpose of this application. For example, the positive electrode current collector can be a metal foil or a composite current collector. For example, as a metal foil, aluminum foil, aluminum alloy foil, nickel foil, or nickel alloy can be used. The composite current collector can include a polymer material substrate and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector can be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene, polyethylene terephthalate, polybutylene terephthalate, polystyrene, polyethylene, etc.).

[0052] This application does not impose any particular limitation on the thickness of the positive electrode material layer and the positive electrode current collector, as long as the purpose of this application can be achieved. For example, the thickness of the single-sided positive electrode material layer is 50 μm to 70 μm, and the thickness of the positive electrode current collector is 10 μm to 20 μm.

[0053] In some embodiments of this application, the positive electrode material layer may further include a conductive agent and a binder. This application does not impose any particular limitation on the types of conductive agents and binders, as long as they achieve the purpose of this application. For example, the conductive agent may include, but is not limited to, at least one of superconducting carbon black (Super P), acetylene black, Ketjen black, carbon nanotubes, graphene, and carbon fibers. The aforementioned carbon nanotubes may include, but are not limited to, single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The aforementioned carbon fibers may include, but are not limited to, vapor-grown carbon fibers (VGCF) and / or carbon nanofibers. The binder may include, but is not limited to, at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), ethylene-tetrafluoroethylene-propylene terpolymer, ethylene-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resins. This application does not impose any particular limitation on the mass ratio of the positive electrode active material, conductive agent, and binder in the positive electrode material layer. Those skilled in the art can select according to actual needs, as long as the purpose of this application is achieved.

[0054] Optionally, the positive electrode may further include a conductive layer located between the positive current collector and the positive electrode material layer. The composition of the conductive layer is not particularly limited and can be any conductive layer commonly used in the art. The conductive layer includes a conductive agent and a binder. This application does not impose any particular limitation on the conductive agent and binder in the conductive layer; for example, it can be at least one of the aforementioned conductive agents and binders.

[0055] This application does not impose any particular limitation on the preparation method of the positive electrode sheet, as long as it achieves the purpose of this application. For example, the positive electrode active material, conductive agent, and binder are mixed in a certain proportion, and N-methylpyrrolidone (NMP) is added and stirred evenly to obtain a positive electrode slurry with a solid content of 60wt% to 80wt%. The positive electrode slurry is uniformly coated on one surface of the positive electrode current collector, and after drying, a positive electrode sheet with a single-sided coating of positive electrode material is obtained. Then, the above coating steps are repeated on the other surface of the positive electrode current collector, and after drying, a positive electrode sheet with a double-sided coating of positive electrode material is obtained. After cold pressing, edge trimming, cutting, slitting, drying, and welding of electrode tabs, the positive electrode sheet is obtained.

[0056] This application does not impose any particular limitation on the diaphragm, as long as it can achieve the purpose of this application. For example, the diaphragm material can be selected from, 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 diaphragm can include at least one of woven membrane, ceramic diaphragm, nonwoven fabric, microporous membrane, composite membrane, rolled membrane, and spun membrane.

[0057] In this application, there is no particular limitation on the thickness of the diaphragm, as long as it can achieve the purpose of this application. For example, the thickness of the diaphragm can be from 4 μm to 20 μm.

[0058] In this application, the lithium-ion battery also includes a casing for housing the positive electrode, separator, negative electrode, and electrolyte, as well as other components known in the field of lithium-ion batteries. This application does not limit the scope of these other components. This application does not impose any particular limitation on the casing; it can be a casing known in the art, as long as it achieves the purpose of this application. For example, the casing can be a rigid casing or a flexible casing. The material of the rigid casing can be metal; this application does not limit the type of metal and can use known metal rigid casings, as long as they achieve the purpose of this application. The flexible casing can be a metal plastic film, such as aluminum-plastic film, steel-plastic film, etc.

[0059] The preparation process of the lithium-ion battery in this application is well known to those skilled in the art, and this application does not have any particular limitations. For example, the preparation process of the lithium-ion battery may include, but is not limited to, the following steps: stacking the positive electrode sheet, separator and negative electrode sheet in sequence, and winding, folding and other operations as needed to obtain a wound electrode assembly, placing the electrode assembly into the housing, injecting the electrolyte into the housing and undergoing vacuum sealing, formation, aging, capacity testing and other processes to obtain the lithium-ion battery.

[0060] Example

[0061] The embodiments and comparative examples provided below illustrate the implementation of this application in more detail. Various tests and evaluations were conducted according to the methods described below. Furthermore, unless otherwise specified, "parts" and "%" are quality standards.

[0062] Test methods and equipment:

[0063] ambient temperature cycling test

[0064] The lithium-ion battery was placed in a 25°C constant temperature chamber and left to stand for 0.5 hours to allow it to reach a constant temperature. It was then charged at a constant current of 1.0C until the voltage reached 4.2V, followed by constant voltage charging at 4.2V until the cutoff current reached 0.05C. Finally, it was discharged at a constant current of 1.0C until the voltage reached 3.0V. The initial discharge capacity was recorded as C0. This constituted one charge-discharge cycle. This cycle was repeated until the capacity decreased to 80% of C0. The number of cycles was recorded, which is the number of cycles at room temperature.

[0065] High-temperature storage performance test

[0066] The lithium-ion battery was placed in a 25°C constant temperature chamber and left to stand for 0.5 hours to allow it to reach a constant temperature. It was then charged at a constant current of 1.0C to a voltage of 4.2V, followed by constant voltage charging at 4.2V until a cutoff current of 0.05C was reached. Finally, it was discharged at a constant current of 0.5C until a voltage of 3V was reached. The initial discharge capacity was recorded as C2. The lithium-ion battery was then removed, and its initial thickness was measured using a thickness gauge as T1. The battery was then charged again at a constant current of 1.0C to a voltage of 4.2V, followed by constant voltage charging at 4.2V until a cutoff current of 0.05C was reached. The battery was then placed in a 60°C explosion-proof oven and stored for 30 days. Its thickness was measured again in the oven as T2. The battery was then removed and cooled to room temperature. At 25°C, it was discharged at a constant current of 1.0C until a voltage of 3V was reached. The discharge capacity at this point was recorded as C3.

[0067] High-temperature storage capacity retention rate (%) = C3 / C2 × 100%;

[0068] High-temperature storage thickness expansion rate (%) = (T2-T1) / T1×100%;

[0069] The high-temperature storage performance of lithium-ion batteries is evaluated by the high-temperature storage thickness expansion rate and the high-temperature storage capacity retention rate. The smaller the value of the storage thickness expansion rate and the larger the value of the storage capacity retention rate, the better the high-temperature storage performance of the lithium-ion battery.

[0070] DC resistance (DCR) test

[0071] The lithium-ion battery was placed in a 25°C constant temperature chamber and left to stand for 0.5 hours. Then, it was charged at a constant current of 1C to a voltage of 4.2V, left to stand for 5 minutes, and then discharged at a constant current of 1.0C for 30 minutes, left to stand for 1 hour, and finally discharged at a current I1 corresponding to a 2C rate for 10 seconds. The corresponding voltage was recorded as V1. The DCR of the lithium-ion battery at 50% state of charge (SOC) is calculated as DCR = V1 / I1, with units of mΩ.

[0072] Example 1-1

[0073] <Preparation of Electrolyte>

[0074] In an argon-atmospheric glove box (water content <10ppm, oxygen content <1ppm), ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) are mixed uniformly in a mass ratio of 3:5:2 to obtain a base solvent. Then, a first additive (the compound shown in Formula I) and a second additive (the compound shown in Formula II-1) are added, followed by lithium hexafluorophosphate (LiPF6). The mixture is dissolved and mixed uniformly to obtain an electrolyte. Based on the mass of the electrolyte, the mass percentage of LiPF6 is 12.5%, the mass percentage of the compound shown in Formula I (w1) is 0.05%, the mass percentage of the compound shown in Formula II-1 (w2) is 2%, and the remainder is the base solvent.

[0075] <Preparation of the positive electrode>

[0076] LiNi, the positive electrode active material 0.8 Co 0.1 Mn 0.1 O2, polyvinylidene fluoride (PVDF) binder, and superconducting carbon black (Super P) conductive agent were mixed in a mass ratio of 96.8:1.2:2. N-methylpyrrolidone (NMP) was added, and the mixture was stirred evenly under vacuum to obtain a positive electrode slurry with a solid content of 70 wt%. The positive electrode slurry was then uniformly coated onto one surface of a 15 μm thick aluminum foil for the positive electrode current collector, with a coating weight of 33 g / cm². 2 After drying at 85℃, a positive electrode sheet with a single-sided coating of positive electrode material layer with a coating thickness of 57μm is obtained. The above steps are repeated on the other surface of the positive current collector aluminum foil to obtain a positive electrode sheet with a double-sided coating of positive electrode material layer. After drying at 85℃, it is cold-pressed, trimmed, cut, and slit. After slitting, it is dried at 85℃ under vacuum for 4 hours and the tabs are welded to obtain a positive electrode sheet with a specification of 540mm×314mm.

[0077] <Preparation of Negative Electrode Sheets>

[0078] Artificial graphite (anode active material), superconducting carbon black (Super P) (conductive agent), sodium carboxymethyl cellulose (CMC) (thickener), and styrene-butadiene rubber (SBR) (binder) (mass ratio 95:1.5:2:1.5) were mixed, deionized water was added, and the mixture was stirred evenly under vacuum to obtain a cathode slurry with a solid content of 49 wt%. The cathode slurry was then uniformly coated onto one surface of a 10 μm thick copper foil cathode current collector, with a coating weight of 20.1 g / cm³. 2 After drying at 85℃, a negative electrode sheet with a single-sided coating of negative electrode material layer with a coating thickness of 68μm is obtained. The above steps are repeated on the other surface of the negative electrode current collector copper foil to obtain a negative electrode sheet with a double-sided coating of negative electrode material layer. After drying at 85℃, it is cold-pressed, trimmed, cut, and slit. After slitting, it is dried at 85℃ under vacuum for 4 hours and the tabs are welded to obtain a negative electrode sheet with a specification of 660mm×301mm.

[0079] <Preparation of the diaphragm>

[0080] A double-layer PP ceramic diaphragm with a thickness of 16μm is used.

[0081] <Preparation of Lithium-ion Batteries>

[0082] The prepared positive electrode, separator, and negative electrode are stacked sequentially, with the separator positioned between the positive and negative electrodes to act as a separator. They are then wound, with the positive tab connected to the positive electrode and the negative tab connected to the negative electrode, resulting in an electrode assembly. The electrode assembly is placed in an aluminum foil packaging bag, with the positive and negative tabs extended from the inside to the outside of the bag. Moisture is removed at 80°C, and the prepared electrolyte is injected at an injection rate of 3.0 g / Ah. After vacuum sealing, formation, aging, and capacity testing, a lithium-ion battery is obtained. The formation upper limit voltage is 4.2V, the formation temperature is 45°C, and the formation resting time is 24 hours. The aging temperature is 45°C, and the aging resting time is 24 hours. The capacity testing involves charging to 4.2V at 0.2C, resting for 5 minutes, then discharging to 3.0V at 0.2C, followed by repeating the steps at 0.5C and 1C.

[0083] Examples 1-2 to Examples 1-9

[0084] Except for adjusting the mass percentages of the first and second additives according to Table 1 in the <Preparation of Electrolyte>, changing the mass percentage of the base solvent accordingly, and keeping the percentages of other components in the electrolyte unchanged, the rest is the same as in Example 1-1.

[0085] Examples 1-10 to Examples 1-17

[0086] Except for adjusting the type of the second additive according to Table 1 in <Preparation of Electrolyte>, the rest is the same as in Examples 1-3.

[0087] Examples 1-18 to Examples 1-19

[0088] Except for adjusting the type and mass percentage of the second additive according to Table 1 in the <Preparation of Electrolyte>, changing the mass percentage of the base solvent accordingly, and keeping the percentage of other components in the electrolyte unchanged, the rest is the same as in Examples 1-3.

[0089] Comparative Example 1-1

[0090] Except that the first and second additives are not added in the <Preparation of Electrolyte>, the mass percentage of the base solvent is changed accordingly, and the percentage of other components in the electrolyte remains unchanged, the rest is the same as in Example 1-1.

[0091] Comparative Examples 1-2

[0092] Except that no second additive is added in the <Preparation of Electrolyte>, the mass percentage of the base solvent is changed accordingly, and the percentage of other components in the electrolyte remains unchanged, the rest is the same as in Examples 1-3.

[0093] Comparative Examples 1-3

[0094] Except that the first additive is not added in the <Preparation of Electrolyte>, the mass percentage of the base solvent is changed accordingly, and the percentage of other components in the electrolyte remains unchanged, the rest is the same as in Examples 1-3.

[0095] Comparative Examples 1-4

[0096] Except that the second additive is not added in the <Preparation of Electrolyte>, the mass percentage of the first additive is adjusted according to Table 1, and the percentage of other components in the electrolyte remains unchanged, the rest is the same as in Example 1-1.

[0097] Comparative Examples 1-5

[0098] Except that the first additive is not added in the <Preparation of Electrolyte>, the mass percentage of the second additive is adjusted according to Table 1, and the percentage of other components in the electrolyte remains unchanged, the rest is the same as in Example 1-1.

[0099] Comparative Examples 1-6 to 1-7

[0100] Except for adjusting the mass percentages of the first and second additives according to Table 1 in the <Preparation of Electrolyte>, changing the mass percentage of the base solvent accordingly, and keeping the percentages of other components in the electrolyte unchanged, the rest is the same as in Example 1-1.

[0101] The preparation parameters and performance parameters of each embodiment and comparative example are shown in Table 1.

[0102]

[0103] As can be seen from Examples 1-1 to 1-19 and Comparative Examples 1-1 to 1-7 above, when an electrolyte with neither the first nor the second additive is added simultaneously is applied to a lithium-ion battery, or when the content of the first and second additives is not within the scope of this application, the lithium-ion battery exhibits lower room-temperature cycle count and high-temperature storage capacity retention, as well as higher high-temperature storage thickness expansion rate and DCR. When an electrolyte with both the first and second additives is applied to a lithium-ion battery, and the content of the first and second additives is controlled within the scope of this application, the first additive can form a thin SEI film that facilitates ion conduction during the formation and capacity testing stages. Simultaneously, the introduction of the second additive makes the SEI film more resilient, effectively suppressing SEI film rupture under high-temperature conditions and reducing side reactions caused by direct contact between the electrode and the electrolyte. The synergistic effect of the first and second additives facilitates the formation of a network-like solid electrolyte interface film, resulting in a lithium-ion battery with higher room-temperature cycle count and high-temperature storage capacity retention, as well as lower high-temperature storage thickness expansion rate and DCR. The above results demonstrate that when the first and second additives are used in combination in the electrolyte, and the contents of the first and second additives are controlled within the range of this application, the two components work synergistically to significantly improve the cycle performance of the secondary battery.

[0104] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, or article that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, or article.

[0105] The above description is only a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. An electrolyte comprising an electrolyte, an additive, and a solvent, wherein the additive comprises a first additive and a second additive; The first additive is selected from the compounds shown in Formula I; The second additive is a compound containing unsaturated carbon-carbon bonds, wherein the compound containing unsaturated carbon-carbon bonds is selected from at least one of formula II-a to II-d; in, A is selected from B is selected from R1 to R 19 Each compound is independently selected from C1-C4 alkyl, C2-C4 alkenyl, or C2-C4 ynyl groups, and the compound containing an unsaturated carbon-carbon bond contains at least one C2-C4 alkenyl or C2-C4 ynyl group; Based on the mass of the electrolyte, the mass percentage of the first additive is w1, where w1 is 0.05% to 3%; the mass percentage of the second additive is w2, where w2 is 0.05% to 2%.

2. The electrolyte according to claim 1, wherein, w1 ranges from 0.1% to 2%.

3. The electrolyte according to claim 1, wherein, w2 ranges from 0.5% to 2%.

4. The electrolyte according to claim 1, wherein, The mass ratio of the first additive to the second additive, w1 / w2, is 1:(0.05~20).

5. The electrolyte according to claim 1, wherein, w1 / w2 is 1:(0.1~10).

6. The electrolyte according to claim 1, wherein, The compound containing unsaturated carbon-carbon bonds is selected from at least one of the following compounds:

7. The electrolyte according to claim 1, wherein, The electrolyte is selected from at least one of lithium hexafluorophosphate, lithium difluorophosphate, lithium bis(trifluoromethyl)sulfonylimide, lithium bisfluorosulfonylimide, lithium monofluorosulfonate, and lithium trifluoromethylsulfonate; the mass percentage of the electrolyte is 10% to 20% based on the mass of the electrolyte solution.

8. The electrolyte according to claim 1, wherein, The solvent is selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, dimethyl carbonate, and diethyl carbonate; the solvent has a mass percentage content of 75% to 89% based on the mass of the electrolyte.

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

10. The secondary battery according to claim 9, wherein, The positive electrode includes a positive current collector and a positive electrode material layer disposed on at least one surface of the positive current collector. The positive electrode material layer includes a positive electrode active material, which includes LiNi. a Co b M c O2, M includes at least one of Mn, Al, Ti, V, Fe, Zn, V, Zr, Ce, Cr and Cu, a+b+c=1, 0.5≤a≤0.8, 0.1≤b≤0.2, 0.1≤c≤0.3.

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