Electrolyte and secondary battery

Abstract

The present application relates to the field of secondary battery technology and provides an electrolyte and a secondary battery. The electrolyte includes a lithium salt, an organic solvent, and an additive. The additive includes a first additive, vinylene carbonate, and lithium tetrafluoroborate. The first additive is selected from the compound represented by Formula 1: wherein R1 and R2 are each independently selected from H, , , , , and R1 and R2 are not both H. Based on the total mass of the electrolyte, the mass percentage of the first additive is A, the mass percentage of the vinylene carbonate is B, and the mass percentage of the lithium tetrafluoroborate is C, with 1.2% ≤ A + B + C ≤ 13%. The present application combines the compound of Formula 1 with vinylene carbonate and lithium tetrafluoroborate. Through the synergistic effect of the three, the rate capability, high-temperature storage performance, and fast-charge cycle performance of the secondary battery can be improved.

Description

Technical Field

[0001] The present application relates to the technical field of secondary batteries, and in particular to an electrolyte and a secondary battery. Background Art

[0002] The rapid development and widespread application of various portable electronic devices, new energy electric vehicles, and energy storage systems in recent years have led to an increasingly urgent demand for secondary batteries with high energy density, good rate performance, long cycle life, and safe use. Increasing the charging voltage or increasing the capacity of active materials are the main methods for improving the energy density of secondary batteries. However, in actual use, increasing the charging voltage or increasing the capacity of active materials can bring about a series of problems, such as accelerating the decomposition and consumption of the electrolyte, exacerbating the interfacial side reactions of the battery cell, leading to gas production and increased impedance, which in turn affects the cycle performance and storage performance of the secondary battery.

[0003] In secondary batteries, the electrolyte is a crucial factor influencing their performance. Existing secondary batteries typically incorporate various additives into the electrolyte to form an organic passivation film on the surface of the active material. This passivation film prevents further decomposition of the electrolyte, inhibits interfacial side reactions, and further enhances the interfacial stability of the positive and negative electrodes, ensuring sufficient electrolyte retention and improving the battery's rate, cycling, and high-temperature storage performance. Therefore, it is crucial to develop an electrolyte for secondary batteries with excellent fast-charging, cycling, and high-temperature storage properties. Summary of the Invention

[0004] The purpose of this application is to provide an electrolyte and a secondary battery to improve the fast charge cycle performance and high temperature storage performance of the secondary battery. The specific technical solution is as follows:

[0005] The first aspect of the present application provides an electrolyte comprising a lithium salt, an organic solvent and an additive; the additive comprises a first additive, vinylene carbonate and lithium tetrafluoroborate;

[0006] The first additive is selected from the compound represented by Formula 1:

[0007] ;

[0008] Wherein, R1 and R2 are each independently selected from H, 、 、 、 , R1 and R2 are not H at the same time; based on the total mass of the electrolyte, the mass percentage of the first additive is A, the mass percentage of the vinylene carbonate is B, the mass percentage of the lithium tetrafluoroborate is C, 1.2%≤A+B+C≤13%.

[0009] In some embodiments of the present application, 2.2%≤A+B+C≤8%.

[0010] In some embodiments of the present application, B≥(A+C) / 8.

[0011] In some embodiments of the present application, 0.025≤A / B≤4, 0.025≤A / C≤40.

[0012] In some embodiments of the present application, 0.033≤A / B≤2, 0.05≤A / C≤20.

[0013] In some embodiments of the present application, 0.1%≤A≤4%, 1%≤B≤5%, and 0.1%≤C≤4%.

[0014] In some embodiments of the present application, 0.1%≤A≤2%, and / or, 2%≤B≤4%, and / or, 0.1%≤C≤2%.

[0015] In some embodiments of the present application, the first additive is selected from at least one of the following compounds:

[0016] .

[0017] In some embodiments of the present application, the organic solvent is selected from at least one of ethylene carbonate, propylene carbonate, γ-butyrolactone, phenyl acetate, 1,4-butanesultone, 3,3,3-trifluoropropylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methylpropyl carbonate, methyl formate, ethyl acetate, methyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, propyl propionate, butyl propionate, methyl butyrate, and ethyl butyrate.

[0018] A second aspect of the present application provides a secondary battery comprising a positive electrode sheet, a negative electrode sheet, a separator, and the electrolyte described in the first aspect of the present application.

[0019] In some embodiments of the present application, the positive electrode sheet includes a positive electrode active material, and the positive electrode active material is selected from LiMn2O4, Li 1+a Mn 1-x M x O2、LiCo 1-x M x O2、LiFe 1-x M x PO4 and Li2Mn 1-x At least one of O4, M is selected from at least one of Ni, Co, Mn, Al, Cr, Mg, Zr, Mo, V, Ti, B and F, 0≤a<0.2, 0≤x<1.

[0020] Beneficial effects of this application:

[0021] The present application provides an electrolyte and a secondary battery, wherein the electrolyte includes a lithium salt, an organic solvent, and an additive; the additive includes a first additive, vinylene carbonate, and lithium tetrafluoroborate; and the first additive is selected from the compound represented by Formula 1. The synergistic effect of the first additive, vinylene carbonate, and lithium tetrafluoroborate can improve the fast charge cycle performance and high-temperature storage performance of the secondary battery.

[0022] Of course, it is not necessary to achieve all the advantages described above at the same time when implementing any product or method of the present application. DETAILED DESCRIPTION

[0023] The following will be combined with the embodiments of the present application to clearly and completely describe the technical solutions in this application. Obviously, the embodiments described are only part of the embodiments of the present application, rather than all the embodiments. All other embodiments obtained by those skilled in the art based on this application are within the scope of protection of this application.

[0024] The present application provides an electrolyte comprising a lithium salt, an organic solvent and an additive; the additive comprises a first additive, vinylene carbonate and lithium tetrafluoroborate;

[0025] The first additive is selected from the compound represented by Formula 1:

[0026] ;

[0027] Wherein, R1 and R2 are each independently selected from H, 、 、 、 , R1 and R2 are not both H; based on the total mass of the electrolyte, the mass percentage of the first additive is A, the mass percentage of the vinylene carbonate is B, and the mass percentage of the lithium tetrafluoroborate is C, and 1.2%≤A+B+C≤13%. For example, the value of A+B+C can be 1.2%, 2%, 2.2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, or a range consisting of any two of these values.

[0028] Adding vinylene carbonate to the electrolyte can form a solid electrolyte interface (SEI) film containing unsaturated organic polymers on the surface of the negative electrode, thereby improving the stability of the SEI film. However, the unsaturated organic polymers in the SEI film can decompose during the middle and late stages of cycling and storage, increasing the impedance of the secondary battery and degrading its performance. The boron atoms in lithium tetrafluoroborate can complex with the unsaturated bonds in the organic polymer formed by vinylene carbonate, thereby reducing the polymerization of the organic polymer and forming a more stable oligomeric SEI film with the vinylene carbonate molecules, improving the long-cycle and high-temperature storage performance of the secondary battery. However, while lithium tetrafluoroborate can interrupt the copolymerization of vinylene carbonate and form a more stable oligomeric SEI film, it also increases the gaps in the SEI film, making the oligomeric SEI film formed at the electrode interface looser and significantly increasing its impedance. This has a negative impact on the fast-charge rate performance of the battery cell, exacerbates interfacial side reactions during cycling, and leads to rapid electrolyte consumption.

[0029] The compound of Formula 1 is rich in sulfone functional groups, which form a SEI film containing lithium alkylsulfonate (RSO3Li) on the negative electrode surface, thereby improving the lithium ion conductivity of the SEI film. However, when used alone, the sulfate groups of the compound of Formula 1 promote the hydrolysis of lithium salts, increasing the acidity of the electrolyte. Furthermore, due to the excessive inorganic components during film formation, the resulting SEI film is brittle and prone to rupture, increasing the resistance to lithium ion migration and reducing the battery cycle life. After in-depth research, the inventors discovered that combining the compound of Formula 1 with vinylene carbonate and lithium tetrafluoroborate, the boron atoms of the lithium tetrafluoroborate stabilize the sulfate groups. The double bonds of the vinylene carbonate further coordinate with the sulfate groups to form a film, increasing the flexibility of the SEI film, reducing the resistance to lithium ion migration, and improving the battery cycle life. Furthermore, the compound of Formula 1 can modify the unsaturated bond-containing oligomeric SEI film formed by vinylene carbonate and lithium tetrafluoroborate. Alkyl sulfonate lithium is rich in sulfone functional groups and is easily coordinated with the unsaturated double bonds of the oligomer SEI film. This coordination further reduces the unsaturation of the oligomer SEI film, enhances its stability, and inhibits the further decomposition of the oligomer SEI film during the cycle storage process, thereby modifying the SEI and inhibiting the loose rupture of the SEI. The present application combines the compound of formula 1 with vinylene carbonate and lithium tetrafluoroborate. Through the synergistic effect of the three, the high temperature storage performance and long cycle performance of the secondary battery can be improved. The value of A+B+C is controlled within the scope of this application, which is conducive to promoting the synergistic effect of the compound of formula 1 with vinylene carbonate and lithium tetrafluoroborate, and improving the high temperature storage performance, fast charge and long cycle performance of the secondary battery.

[0030] In some embodiments of the present application, 2.2% ≤ A + B + C ≤ 8%. For example, the value of A + B + C can be 2.2%, 3%, 4%, 5%, 6%, 7%, 8%, or a range consisting of any two of these values. Controlling the value of A + B + C within the range of this application can further promote the synergistic effect of the compound of Formula 1 with vinylene carbonate and lithium tetrafluoroborate, thereby improving the high-temperature storage performance, fast charging, and long-cycle performance of the secondary battery.

[0031] In some embodiments of the present application, based on the total mass of the electrolyte, the mass percentage of the first additive is A, the mass percentage of the vinylene carbonate is B, and the mass percentage of the lithium tetrafluoroborate is C, where B ≥ (A + C) / 8. By controlling the values ​​of A, B, and C to satisfy B ≥ (A + C) / 8, the synergistic effect of the compound of Formula 1 with vinylene carbonate and lithium tetrafluoroborate can be further enhanced, the unsaturation of the oligomer SEI film can be further reduced, and further decomposition of the oligomer SEI film during cyclic storage can be inhibited, thereby improving the high-temperature storage performance, fast charging, and long-cycle performance of the secondary battery.

[0032] In some embodiments of the present application, 0.025≤A / B≤4, 0.025≤A / C≤40; preferably, 0.033≤A / B≤2, 0.05≤A / C≤20. For example, the value of A / B can be 0.025, 0.033, 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, or a range consisting of any two thereof, and the value of A / C can be 0.025, 0.05, 0.5, 1, 5, 10, 15, 20, 25, 30, 35, 40, or a range consisting of any two thereof. By controlling the value range of A / B and A / C within the scope of this application, the boron atoms of lithium tetrafluoroborate can stabilize the sulfate groups, and the double bonds of vinylene carbonate further coordinate with the sulfate groups to form a film, which can increase the flexibility of the SEI film and reduce the migration resistance of lithium ions. The synergistic effect of the compound of formula 1 with vinylene carbonate and lithium tetrafluoroborate can be further enhanced, thereby improving the high-temperature storage performance, fast charging and long cycle performance of the secondary battery.

[0033] In some embodiments of the present application, 0.1%≤A≤4%, 1%≤B≤5%, and 0.1%≤C≤4%. For example, the value of A can be 0.1%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, or a range consisting of any two of these values; the value of B can be 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, or a range consisting of any two of these values; the value of C can be 0.1%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, or a range consisting of any two of these values. The mass percentage of the first additive, vinylene carbonate, and lithium tetrafluoroborate is controlled within the range of the present application. Through the synergistic effect of the compound of formula 1, vinylene carbonate, and lithium tetrafluoroborate, a low-impedance and dense SEI film can be formed on the surface of the negative electrode plate, which is beneficial to improving the structural stability of the SEI film and further improving the high-temperature storage performance, fast charging, and long-cycle performance of the secondary battery.

[0034] In some embodiments of the present application, 0.1%≤A≤2%, and / or, 2%≤B≤4%, and / or, 0.1%≤C≤2%. For example, the value of A can be 0.1%, 0.5%, 0.8%, 1%, 1.3%, 1.5%, 1.8%, 2%, or a range consisting of any two values ​​therein, the value of B can be 2%, 2.3%, 2.5%, 2.8%, 3%, 3.2%, 3.5%, 3.8%, 4%, or a range consisting of any two values ​​therein, and the value of C can be 0.1%, 0.5%, 0.8%, 1%, 1.3%, 1.5%, 1.8%, 2%, or a range consisting of any two values ​​therein.

[0035] In some embodiments of the present application, the first additive is selected from at least one of the following compounds:

[0036] .

[0037] Among them, the CAS number of the 1-1 is 1431298-10-0, the CAS number of the 1-2 is 2507955-35-1, the CAS number of the 1-3 is 2125520-24-1, and the CAS number of the 1-4 is 2943046-28-2.

[0038] In some embodiments of the present application, the organic solvent is selected from at least one of ethylene carbonate, propylene carbonate, γ-butyrolactone, phenyl acetate, 1,4-butanesultone, 3,3,3-trifluoropropylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methylpropyl carbonate, methyl formate, ethyl acetate, methyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, propyl propionate, butyl propionate, methyl butyrate, and ethyl butyrate.

[0039] The second aspect of the present application provides a secondary battery, which includes a positive electrode sheet, a negative electrode sheet, a separator and the electrolyte provided by the first aspect of the present application.

[0040] In some embodiments of the present application, the positive electrode sheet includes a positive electrode active material, and the positive electrode active material is selected from LiMn2O4, Li 1+a Mn 1-x M x O2、LiCo 1-x M x O2、LiFe 1-x M x PO4 and Li2Mn 1-x At least one of O4, M is selected from at least one of Ni, Co, Mn, Al, Cr, Mg, Zr, Mo, V, Ti, B, and F, 0 ≤ a < 0.2, 0 ≤ x < 1. In the present application, the electrolyte includes an electrolyte. Upon dissolution in the electrolyte, the electrolyte can release lithium ions or sodium ions. The lithium ions or sodium ions form a solvation structure with the solvent, facilitating rapid ion migration. The present application does not particularly limit the electrolyte, as long as it can achieve the objectives of the present application. For example, the electrolyte can include, but is not limited to, at least one of LiPF6, LiBF4, LiAsF6, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, Li2SiF6, LiFSI, lithium bis(oxalatoborate) (LiBOB), lithium bis(trifluoromethanesulfonyl imide) (LiTFSI), lithium difluoroborate, sodium hexafluorophosphate, and sodium bis(fluorosulfonyl imide). The present application does not particularly limit the content of the electrolyte in the electrolyte, as long as the purpose of the present application can be achieved. For example, based on the total mass of the electrolyte, the mass percentage of the electrolyte is 8% to 18%. For example, based on the total mass of the electrolyte, the mass percentage of the electrolyte can be 8%, 10%, 12%, 14%, 16%, 18% or a range consisting of any two of these values. By controlling the mass percentage of the electrolyte within the scope of the present application, the electrolyte can be fully dissolved in the non-aqueous organic solvent, and the electrolyte can have both high ionic conductivity and low manufacturing cost.

[0041] The secondary battery of the present application also includes a positive electrode sheet, which includes a positive electrode current collector and a positive electrode material layer disposed on at least one surface of the positive electrode current collector. In the present application, the positive electrode material layer can be disposed on one surface of the positive electrode current collector in the thickness direction, or on both surfaces in the thickness direction of the positive electrode current collector. The present application does not particularly limit the positive electrode current collector, as long as the objectives of the present application can be achieved. For example, the positive electrode current collector can include a metal foil or a composite current collector. For example, the metal foil is aluminum foil. The composite current collector can include a polymer material base layer and a metal material layer disposed on at least one surface of the polymer material base layer. For example, the material of the metal material layer can include at least one of aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, or silver alloy. The polymer material base layer can include at least one of polypropylene, polyethylene terephthalate, polybutylene terephthalate, polystyrene, or polyethylene. The present application does not particularly limit the thickness of the positive electrode current collector and the positive electrode material layer, as long as the objectives of the present application can be achieved. For example, the thickness of the positive electrode current collector is 160μm to 216μm, preferably 176μm to 196μm. The thickness of the single-sided positive electrode material layer is 72μm to 90μm. The positive electrode material layer of the present application may also include a conductive agent and a binder. There is no particular limitation on the conductive agent and binder in this application, as long as they can achieve the purpose of this application. For example, the conductive agent may include, but is not limited to, at least one of superconducting carbon (Super P), acetylene black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. The binder may include, but is not limited to, at least one of polyvinyl chloride, polypropylene alcohol, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, or a fluorine-containing acrylate resin.

[0042] The secondary battery of the present application includes a negative electrode plate, which comprises a negative electrode current collector and a negative electrode material layer disposed on at least one surface of the negative electrode current collector. In the present application, the negative electrode material layer can be disposed on one surface of the negative electrode current collector in the thickness direction, or on both surfaces in the thickness direction of the negative electrode current collector. The present application does not particularly limit the negative electrode current collector; any negative electrode current collector known in the art can be used, as long as it can achieve the objectives of the present application. The negative electrode current collector can be a metal foil or a composite current collector. For example, the metal foil can include at least one of aluminum foil, copper foil, nickel foil, and titanium foil. The composite current collector can include a polymer base layer and a metal layer formed on at least one surface of the polymer base layer. The composite current collector can be formed by laminating a metal material (copper, copper alloy, nickel, nickel alloy, etc.) onto a polymer base layer (such as polypropylene (PP), polyethylene terephthalate (PET), or polybutylene terephthalate (PBT)).

[0043] In this application, there are no specific restrictions on the thickness of the negative electrode current collector and the negative electrode material layer, as long as they can achieve the objectives of this application. For example, the thickness of the negative electrode current collector is 219 to 259 μm, and the thickness of the single-sided negative electrode material layer is 105 to 120 μm. Optionally, the negative electrode active material includes a thickener, which may include, but is not limited to, sodium carboxymethylcellulose (CMC-Na). The negative electrode material layer of this application may also include a conductive agent and a binder. There are no specific restrictions on the conductive agent and binder, as long as they can achieve the objectives of this application. For example, the binder may include at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS). The conductive agent may include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0044] The negative electrode material layer of the present application contains a negative electrode active material. The present application does not particularly limit the type of negative electrode active material, and any negative electrode active material known in the art can be used as long as it can achieve the purpose of the present application. For example, the negative electrode active material may include at least one of artificial graphite, natural graphite, soft carbon, hard carbon, mesophase carbon microbeads, silicon-based materials, tin-based materials and lithium titanate. The above-mentioned silicon-based materials may include but are not limited to at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites or silicon alloys; the above-mentioned tin-based materials may include at least one of elemental tin, tin oxide compounds or tin alloys.

[0045] The secondary battery of the present application also includes a diaphragm to separate the positive electrode plate and the negative electrode plate, prevent internal short circuits in the battery, allow ions to pass freely, and do not affect the electrochemical charge and discharge process. The present application has no particular restrictions on the type of diaphragm, and any porous structure diaphragm with good chemical stability and mechanical stability can be selected. For example, the material of the diaphragm can include at least one of glass fiber, non-woven fabric, polyethylene, polypropylene and polyvinylidene fluoride. The type of diaphragm can include but is not limited to at least one of woven membrane, non-woven membrane (non-woven fabric), microporous membrane, composite membrane, rolled membrane or spun membrane. The diaphragm can be a single-layer film or a multi-layer composite film. In the present application, the thickness of the diaphragm is not particularly limited, as long as it can achieve the purpose of the present application. For example, the thickness can be 10μm to 20μm.

[0046] The secondary battery of this application also includes a packaging bag for containing the positive electrode sheet, separator, negative electrode sheet, and electrolyte, as well as other components known in the art. This application does not limit these other components. This application does not specifically limit the packaging bag and can be any packaging bag known in the art, as long as it can achieve the purpose of this application. For example, an aluminum-plastic film packaging bag can be used.

[0047] The preparation process of the secondary battery of the present application is well known to those skilled in the art and is not particularly limited in the present application. For example, it may include but is not limited to the following steps: stacking the positive electrode sheet, the separator and the negative electrode sheet in order, and winding and folding them as needed to obtain an electrode assembly with a wound structure, placing the electrode assembly in a packaging bag, injecting the electrolyte into the packaging bag and sealing it to obtain a secondary battery; or stacking the positive electrode sheet, the separator and the negative electrode sheet in order, and then fixing the four corners of the entire stacked structure with tape to obtain an electrode assembly with a stacked structure, placing the electrode assembly in a packaging bag, injecting the electrolyte into the packaging bag and sealing it to obtain a secondary battery. In addition, as needed, overcurrent protection elements, guide plates, etc. may be placed in the packaging bag to prevent pressure rise and overcharge and discharge inside the secondary battery.

[0048] The secondary batteries of the present application may include battery cells, battery modules, and battery packs. Battery cells may be assembled into battery modules, which may contain one or more battery cells. A person skilled in the art may select the specific number based on the application and capacity of the battery module. The battery modules of the present application may also be assembled into battery packs, which may contain one or more battery modules. A person skilled in the art may select the specific number based on the application and capacity of the battery pack.

[0049] Example

[0050] Hereinafter, the embodiments of the present application will be described in more detail with reference to Examples and Comparative Examples. Various tests and evaluations were performed according to the following methods. In addition, unless otherwise specified, "parts" and "%" are by mass.

[0051] Test methods and equipment:

[0052] Room temperature capacity retention rate: At 25°C, charge the battery to 3.65V at 2C constant current and constant voltage. After standing for 5 minutes, discharge the battery to 2.5V at 1C constant current. Calculate the capacity retention rate after 1000 cycles. Calculation method: Room temperature capacity retention rate (%) = (1000th discharge capacity / 1st discharge capacity) × 100%.

[0053] High temperature storage performance test:

[0054] High-temperature storage expansion rate test: At 25°C, charge the battery to 3.65V with a 1C constant current and constant voltage test. Use a micrometer with an accuracy of ±1μm to measure the initial thickness of the lithium-ion battery. Then store the battery at 60°C for 30 days and measure the thickness again. Calculate the battery expansion rate using the formula: High-temperature storage expansion rate (%) = (thickness after storage - initial thickness) / initial thickness × 100%.

[0055] High-temperature storage capacity retention rate: At 25°C, charge to 3.65V with 1C constant current and constant voltage. The lithium-ion battery must be fully charged before storage. Then, the lithium-ion battery that has been stored at 60°C for 30 days is discharged to 2.5V with 1C. The discharge capacity at this time is recorded as the retention capacity. The discharge capacity measured by discharging the fully charged lithium-ion battery to 2.5V at 1C before high-temperature storage is recorded as the initial capacity, and the battery capacity retention rate is calculated. The calculation formula is as follows: High-temperature storage capacity retention rate (%) = retention capacity / initial capacity × 100%.

[0056] Rate charge test (rate performance): The battery is placed in a 25°C environment and discharged to 2.5V, then charged to 3.65V at a constant current of 1C, and then charged at a constant voltage to a cutoff current of 0.05C. The charge capacity is recorded as C 充 , leave it for 5 minutes, then discharge it at a constant current of 1C to 2.5V. Then charge it at a constant rate to 3.65V, charge it at a constant voltage until the current drops to 0.05C, leave it for 5 minutes, then discharge it at a constant current of 1C to 2.5V, leave it for 5 minutes. This is one charge and discharge cycle. Repeat the charge and discharge steps for 3 weeks. The battery is charged at rates of 0.5C, 1C, 2C, 3C, and 4C in sequence. The constant current charging capacity of the last cycle of the 4C rate is recorded as C4. The 4C constant current charge ratio = (C4 / C 充 )×100%.

[0057] Example 1

[0058] <Preparation of Electrolyte>

[0059] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and ethyl acetate (EA) are mixed in a mass ratio of 3:3:4 to obtain a base solvent, LiPF6 and LiFSI are added to the base solvent as electrolytes, and the compound of formula 1-1, vinylene carbonate, and lithium tetrafluoroborate are added as additives, and the mixture is uniformly mixed to obtain an electrolyte. Based on the total mass of the electrolyte, the mass percentage of LiPF6 is 10%, the mass percentage of LiFSI is 4%, the mass percentage of the compound of formula 1-1 is 0.5%, the mass percentage of vinylene carbonate is 3%, the mass percentage of lithium tetrafluoroborate is 0.5%, and the remainder is the base solvent.

[0060] <Preparation of positive electrode sheet>

[0061] The positive electrode active material, lithium iron phosphate (LiFePO4, purchased from Shenzhen Defang Nanotechnology Co., Ltd.), the conductive agent, conductive carbon black (Super P), and the binder, polyvinylidene fluoride (PVDF), were mixed in a mass ratio of 96:2:2. N-methylpyrrolidone (NMP) was added and stirred in a vacuum mixer until the mixture formed a uniform and fluid positive electrode slurry, resulting in a positive electrode slurry with a solid content of 68 wt%. The positive electrode slurry was evenly coated on one surface of a 16 μm thick positive electrode current collector aluminum foil and dried at 80°C to obtain a single-sided positive electrode material coating with a thickness of 198 μm. The above steps were repeated on the other surface of the positive electrode current collector aluminum foil to obtain a double-sided positive electrode material coating. After drying under vacuum at 80°C, the mixture was cold pressed, trimmed, cut, slit, sheeted, and welded with tabs and glued to produce a 70 mm × 54 mm positive electrode sheet. Among them, the compaction density of the positive electrode material layer is 2.5g / cm 3 .

[0062] <Preparation of negative electrode sheet>

[0063] The negative electrode active material, graphite (purchased from Jiangxi Zichen Technology Co., Ltd.), the conductive agent, conductive carbon black (Super P), the thickener, sodium carboxymethyl cellulose (CMC), and the binder, styrene-butadiene rubber (SBR), were mixed in a mass ratio of 95:2:1:2. Deionized water was added and the mixture was stirred evenly in a vacuum mixer to produce a negative electrode slurry with a solid content of 49 wt%. The negative electrode slurry was evenly coated on one surface of a 9 μm thick negative electrode current collector copper foil. After drying at 80°C, a negative electrode sheet with a single-sided negative electrode material layer and a coating thickness of 131 μm was obtained. The above steps were repeated on the other surface of the negative electrode current collector copper foil to obtain a negative electrode sheet with a double-sided negative electrode material layer. After drying under vacuum conditions at 80°C, the negative electrode sheet was cold pressed, trimmed, cut, slit, formed, and welded with tabs and glued. The resulting negative electrode sheet had a size of 74 mm × 58 mm. The compacted density of the negative electrode material layer was 1.6 g / cm 3 .

[0064] <Preparation of Separator>

[0065] The diaphragm was a polyethylene diaphragm purchased from Shenzhen Xingyuan Material Technology Co., Ltd.

[0066] <Preparation of Secondary Battery>

[0067] The separator, positive electrode sheet, separator, and negative electrode sheet are stacked in sequence, with the separator positioned between the positive and negative electrode sheets, isolating the positive and negative electrode sheets. The assembly is then wound, with the positive tabs connected to the positive and negative tabs connected to the negative electrode sheets, to obtain an electrode assembly. The electrode assembly is placed in an aluminum foil bag, and the positive and negative tabs are led out of the bag's interior to the exterior. After drying at 85°C for 36 hours to remove moisture, the bag is heat-pressed and sealed to obtain a battery cell ready for injection. The prepared electrolyte is then injected into the dried battery cell. After vacuum packaging, standing, formation, shaping, and constant capacity, a secondary battery is obtained.

[0068] Example 2

[0069] Except that the compound of formula 1-1 is changed to the compound of formula 1-2, the rest is the same as Example 1.

[0070] Example 3

[0071] Except that the compound of formula 1-1 is replaced by the compound of formula 1-3, the rest is the same as Example 1.

[0072] Example 4

[0073] Except that the compound of formula 1-1 is replaced by the compound of formula 1-4, the rest is the same as Example 1.

[0074] Example 5 to Example 9

[0075] Except for changing the mass percentage of the first additive and the mass percentage of the base solvent to satisfy the total amount of 100%, the rest is the same as Example 1.

[0076] Example 10 to Example 14

[0077] Except for changing the mass percentage of vinylene carbonate and the mass percentage of the base solvent to satisfy the total amount of 100%, the rest is the same as Example 1.

[0078] Example 15 to Example 19

[0079] Except for changing the mass percentage of lithium tetrafluoroborate and the mass percentage of the base solvent to satisfy the total amount of 100%, the rest is the same as Example 1.

[0080] Example 20 to Example 29

[0081] The process is the same as that of Example 1 except that the mass percentages of the first additive, vinylene carbonate, lithium tetrafluoroborate, and the mass percentage of the base solvent are changed so as to satisfy the total amount of 100%.

[0082] Example 30

[0083] The process is the same as that of Example 1 except that the type of the positive electrode active material is replaced with lithium cobalt oxide (LiCoO 2 ).

[0084] Example 31

[0085] In addition to replacing the type of positive electrode active material with NCM523 (LiNi 0.5 Co 0.2 Mn 0.3 Except for O2), the rest is the same as Example 1.

[0086] Comparative Example 1

[0087] The process is the same as that of Example 1 except that the first additive, vinylene carbonate and lithium tetrafluoroborate are not added and the mass percentage of the base solvent is changed to ensure that the total amount is 100%.

[0088] Comparative Example 2

[0089] Except that the first additive is not added and the mass percentage of the base solvent is changed to satisfy the total amount of 100%, the rest is the same as Example 1.

[0090] Comparative Example 3

[0091] The process is the same as in Example 1 except that vinylene carbonate is not added and the mass percentage of the base solvent is changed to satisfy the total amount of 100%.

[0092] Comparative Example 4

[0093] The process is the same as in Example 1 except that lithium tetrafluoroborate is not added and the mass percentage of the base solvent is changed to ensure that the total amount is 100%.

[0094] Comparative Example 5

[0095] The process is the same as that of Example 1 except that the first additive and lithium tetrafluoroborate are not added and the mass percentage of the base solvent is changed to ensure that the total amount is 100%.

[0096] Comparative Example 6 to Comparative Example 7

[0097] The process is the same as that of Example 1 except that the mass percentages of the first additive, vinylene carbonate, lithium tetrafluoroborate, and the mass percentage of the base solvent are changed so as to satisfy the total amount of 100%.

[0098] The preparation parameters and performance parameters of each embodiment and comparative example are shown in Table 1 and Table 2.

[0099] Table 1

[0100]

[0101] In Table 1, “ / ” indicates that the corresponding preparation parameters or substances do not exist.

[0102] Table 2

[0103]

[0104] In Table 2, “ / ” indicates that there is no corresponding preparation parameter or substance.

[0105] It can be seen from Examples 1 to 31 of the present application and Comparative Examples 1 to 5 that the electrolyte includes the first additive of the present application, vinylene carbonate, and lithium tetrafluoroborate. The synergistic effect of the first additive, vinylene carbonate, and lithium tetrafluoroborate can improve the fast charge cycle performance and high-temperature storage performance of the secondary battery. The electrolyte of Comparative Example 1 does not contain the first additive, vinylene carbonate, and lithium tetrafluoroborate. The electrolytes of Comparative Examples 2 to 5 do not contain at least one of the first additive, vinylene carbonate, or lithium tetrafluoroborate. The values ​​of A+B+C in the electrolytes of Comparative Examples 6 to 7 are too large or too small. The secondary batteries all have a low room temperature cycle capacity retention rate, a 4C constant current charge ratio, a high temperature cycle capacity retention rate, and a high high temperature storage expansion rate.

[0106] The type of the first additive affects the fast-charge cycle performance and high-temperature storage performance of the secondary battery. As can be seen from Examples 1 to 4, using the first additive within the scope of this application results in a secondary battery with a higher room-temperature cycle capacity retention rate, a 4C constant-current charge ratio, a high-temperature cycle capacity retention rate, and a lower high-temperature storage expansion rate.

[0107] The addition amount of the first additive, vinylene carbonate and lithium tetrafluoroborate will affect the fast charge cycle performance and high temperature storage performance of the secondary battery. From Examples 5 to 19, it can be seen that by controlling the value ranges of A, B and C within the scope of this application, the secondary battery has a higher room temperature cycle capacity retention rate, 4C constant current injection ratio, high temperature cycle capacity retention rate and a lower high temperature storage expansion rate. From Examples 20 to 23, it can be seen that by controlling the value of A+B+C within the scope of this application, the secondary battery has a higher room temperature cycle capacity retention rate, 4C constant current injection ratio, high temperature cycle capacity retention rate and a lower high temperature storage expansion rate. From Examples 27 to 29, it can be seen that by regulating the values ​​of A, B, and C to meet B≥(A+C) / 8, and regulating the values ​​of A / B and A / C within the scope of this application, the secondary battery has a higher room temperature cycle capacity retention rate, 4C constant current injection ratio, high temperature cycle capacity retention rate and a lower high temperature storage expansion rate.

[0108] It can be seen from Examples 30 to 31 that by using the electrolyte of the present application in combination with the positive electrode active material within the scope of the present application, the secondary battery has a higher room temperature cycle capacity retention rate, 4C constant current charge ratio, high temperature cycle capacity retention rate and a lower high temperature storage expansion rate.

[0109] The above description is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present application shall be included in the scope of protection of the present application.

Claims

1. An electrolyte comprising an electrolyte, an organic solvent and an additive; characterized in that: The additives include a first additive, vinylene carbonate and lithium tetrafluoroborate; The first additive is selected from the compound represented by Formula 1: ; wherein R1 and R2 are each independently selected from H, , R1 and R2 are not H at the same time; based on the total mass of the electrolyte, the mass percentage of the first additive is A, the mass percentage of the vinylene carbonate is B, and the mass percentage of the lithium tetrafluoroborate is C, 1.2%≤A+B+C≤13%; 0.025≤A / B≤4, 0.025≤A / C≤40.

2. The electrolyte according to claim 1, characterized in that 2.2%≤A+B+C≤8%.

3. The electrolyte according to claim 1, characterized in that B≥(A+C) / 8.

4. The electrolyte according to claim 1, characterized in that 0.033≤A / B≤2, 0.05≤A / C≤20.

5. The electrolyte according to claim 1, characterized in that 0.1%≤A≤4%, 1%≤B≤5%, 0.1%≤C≤4%.

6. The electrolyte according to claim 5, characterized in that 0.1%≤A≤2%, and / or, 2%≤B≤4%, and / or, 0.1%≤C≤2%.

7. The electrolyte according to claim 1, characterized in that The first additive is selected from at least one of the following compounds: .

8. The electrolyte according to claim 1, characterized in that The organic solvent is selected from at least one of ethylene carbonate, propylene carbonate, γ-butyrolactone, phenyl acetate, 1,4-butanesultone, 3,3,3-trifluoropropylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methylpropyl carbonate, methyl formate, ethyl acetate, methyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, propyl propionate, butyl propionate, methyl butyrate, and ethyl butyrate.

9. A secondary battery, characterized in that: The invention comprises a positive electrode sheet, a negative electrode sheet, a separator and the electrolyte according to any one of claims 1 to 8.

10. The secondary battery according to claim 9, wherein The positive electrode sheet includes a positive electrode active material, wherein the positive electrode active material is selected from LiMn2O4, Li 1+a Mn 1-x M x O2、LiCo 1-x M x O2、LiFe 1-x M x PO4 and Li2Mn 1-x At least one of O4, M is selected from at least one of Ni, Co, Mn, Al, Cr, Mg, Zr, Mo, V, Ti, B and F, 0≤a<0.2, 0≤x<1.

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