Electrolyte and secondary battery

Abstract

The invention relates to the technical field of secondary batteries, and provides an electrolyte and a secondary battery, the electrolyte comprises a lithium salt, an organic solvent and an additive; the additive comprises a first additive, vinylene carbonate and lithium tetrafluoroborate; the first additive is selected from a compound shown in a formula 1: # imgabs0 #; wherein R1 and R2 are respectively and independently selected from H, # imgabs 1 #, # imgabs 2 #, # imgabs 3 # and # imgabs 4 #, and R1 and R2 are not H at the same time; based on the total mass of the electrolyte, the mass percentage content of the first additive is A, the mass percentage content of the vinylene carbonate is B, the mass percentage content of the lithium tetrafluoroborate is C, and 1.2% < = A + B + C < = 13%. According to the present invention, the compound represented by the formula 1 is combined with the vinylene carbonate and the lithium tetrafluoroborate, and through the synergistic effect of the three, the rate, the high temperature storage performance and the rapid 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 particularly to an electrolyte and a secondary battery. Background Art

[0002] In recent years, with the rapid development and wide application of various portable electronic devices, new energy electric vehicles, and energy storage systems, the demand for secondary batteries with high energy density, good rate performance, long cycle life, and safe use has become increasingly urgent. Increasing the charging voltage or increasing the capacity of the active material is the main method to improve the energy density of secondary batteries. However, in actual use, increasing the charging voltage or increasing the capacity of the active material will bring a series of problems, such as accelerating the decomposition and consumption of the electrolyte, intensifying the interfacial side reactions of the battery core, resulting in problems such as gas generation and increased impedance, thus affecting the cycle performance and storage performance of secondary batteries.

[0003] In secondary batteries, the electrolyte is an important factor affecting the performance of secondary batteries. Existing secondary batteries usually add various additives to the electrolyte to form an organic passivation film on the surface of the active material. The passivation film can prevent the further decomposition of the electrolyte, inhibit interfacial side reactions, further enhance the interfacial stability of the positive and negative electrodes, ensure that the electrolyte has sufficient retention, and improve the rate, cycle, and high-temperature storage performance of secondary batteries. Based on the above, it is very necessary to develop an electrolyte for secondary batteries with good fast-charging cycle performance and high-temperature storage performance. Summary of the Invention

[0004] The purpose of the present application is to provide an electrolyte and a secondary battery to improve the fast-charging cycle performance and high-temperature storage performance of secondary batteries. The specific technical solutions are as follows:

[0005] In a first aspect of the present application, there is provided an electrolyte, which includes a lithium salt, an organic solvent, and an additive; the additive includes a first additive, vinylene carbonate, and lithium tetrafluoroborate;

[0006] The first additive is selected from the compound shown in Formula 1: ;

[0007] wherein, R1 and R2 are each independently selected from H, , , , , and R1 and R2 are not both H at the same time; based on the total mass of the electrolyte, the mass percentage content of the first additive is A, the mass percentage content of vinylene carbonate is B, and the mass percentage content of lithium tetrafluoroborate is C, and 1.2% ≤ A + B + C ≤ 13%.

[0008] In some embodiments of the present application, 2.2% ≤ A + B + C ≤ 8%.

[0009] In some embodiments of the present application, B ≥ (A + C) / 8.

[0010] In some embodiments of the present application, 0.025 ≤ A / B ≤ 4, 0.025 ≤ A / C ≤ 40.

[0011] In some embodiments of the present application, 0.033 ≤ A / B ≤ 2, 0.05 ≤ A / C ≤ 20.

[0012] In some embodiments of the present application, 0.1% ≤ A ≤ 4%, 1% ≤ B ≤ 5%, 0.1% ≤ C ≤ 4%.

[0013] In some embodiments of the present application, 0.1% ≤ A ≤ 2%, and / or, 2% ≤ B ≤ 4%, and / or, 0.1% ≤ C ≤ 2%.

[0014] In some embodiments of the present application, the first additive is selected from at least one of the following compounds: .

[0015] 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, methyl propyl carbonate, methyl formate, ethyl acetate, methyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, propyl propionate, butyl propionate, methyl butyrate, and ethyl butyrate.

[0016] 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 solution described in the first aspect of the present application.

[0017] 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 at least one of 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 O4, where 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.

[0018] Advantages of the present application:

[0019] The present application 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 compounds shown in Formula 1. Through the synergistic effect of the first additive, vinylene carbonate, and lithium tetrafluoroborate, the fast charge cycle performance and high-temperature storage performance of the secondary battery can be improved.

[0020] Of course, it is not necessary for any product or method implementing the present application to simultaneously achieve all the above-mentioned advantages. Detailed implementation manners

[0021] The technical solutions in the present application will be clearly and completely described below in conjunction with the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all of the embodiments. All other embodiments obtained by those skilled in the art based on the present application belong to the scope of protection of the present application.

[0022] The present application provides an electrolyte, which includes a lithium salt, an organic solvent, and an additive. The additive includes a first additive, vinylene carbonate, and lithium tetrafluoroborate.

[0023] The first additive is selected from the compounds shown in Formula 1: ;

[0024] Wherein, R1 and R2 are each independently selected from H, , , , , and R1 and R2 are not both H at the same time. Based on the total mass of the electrolyte, the mass percentage content of the first additive is A, the mass percentage content of vinylene carbonate is B, and the mass percentage content of 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 composed of any two of these values.

[0025] Adding vinylene carbonate to the electrolyte can form a SEI film (Solid electrolyte Interface) containing an organic polymer with unsaturated bonds on the surface of the negative electrode plate, thereby improving the stability of the SEI film. However, the organic polymer containing unsaturated bonds in the SEI film will decompose in the middle and late stages of cycling and storage, increasing the impedance of the secondary battery and deteriorating its performance. The boron atom 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. At the same time, it forms a more stable oligomer SEI film with vinylene carbonate molecules, improving the long-cycle and high-temperature storage performance of the secondary battery. However, although lithium tetrafluoroborate can interrupt the copolymerization of vinylene carbonate and form a more stable oligomer SEI film, it also increases the gaps in the SEI film, making the oligomer SEI film formed at the electrode interface relatively loose and significantly increasing the impedance, which has an adverse effect on the fast charging rate performance of the battery cell and exacerbates the interfacial side reactions during cycling, resulting in severe electrolyte consumption.

[0026] The compound of formula 1 is rich in sulfone functional groups and can generate a SEI film containing lithium alkyl sulfonate (RSO3Li) on the surface of the negative electrode, thereby improving the lithium ion conductivity of the SEI film. However, when the compound of formula 1 is used alone, its sulfate ester group will promote the hydrolysis of lithium salt, resulting in an increase in the acidity of the electrolyte. Moreover, due to the excessive inorganic components during film formation, the formed SEI film is brittle and easy to rupture, thus increasing the lithium ion migration resistance and reducing the battery cycle life. The inventors have found through in-depth research that when the compound of formula 1 is used in combination with vinylene carbonate and lithium tetrafluoroborate, the boron atom of lithium tetrafluoroborate can stabilize the sulfate ester group, and the double bond of vinylene carbonate can further coordinate with the sulfate ester group to form a film, which can increase the flexibility of the SEI film, reduce the lithium ion migration resistance, and improve the battery cycle life. At the same time, the compound of formula 1 can modify the oligomer SEI film containing unsaturated bonds formed by vinylene carbonate and lithium tetrafluoroborate. Lithium alkyl sulfonate is rich in sulfone functional groups and is easy to coordinate with the unsaturated double bonds of the oligomer SEI film. This coordination further reduces the unsaturation of the oligomer SEI film, enhances its stability, inhibits the further decomposition of the oligomer SEI film during cycling and storage, plays a role in modifying the SEI, and inhibits the loosening and rupture of the SEI. In this application, the compound of formula 1 is used in combination 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. Controlling the value of A + B + C within the scope of this application is beneficial to promoting the synergistic effect of the compound of formula 1, vinylene carbonate and lithium tetrafluoroborate, and improving the high-temperature storage performance, fast charging and long-cycle performance of the secondary battery.

[0027] 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 composed of any two of these values. Controlling the value of A + B + C within the scope of the present application can further promote the synergistic effect of the compound of Formula 1 with vinylene carbonate and lithium tetrafluoroborate, and improve the high-temperature storage performance, fast charging, and long-cycle performance of the secondary battery.

[0028] In some embodiments of the present application, based on the total mass of the electrolyte, the mass percentage content of the first additive is A, the mass percentage content of vinylene carbonate is B, and the mass percentage content of lithium tetrafluoroborate is C, and 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 degree of the oligomeric SEI film can be further reduced, the further decomposition of the oligomeric SEI film during the cycle storage process can be inhibited, and the high-temperature storage performance, fast charging, and long-cycle performance of the secondary battery can be improved.

[0029] 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 composed of any two of these values, 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 composed of any two of these values. Controlling the value ranges of A / B and A / C within the scope of the present application, the boron atom of lithium tetrafluoroborate can stabilize the sulfate group, and the double bond of vinylene carbonate can further coordinate with the sulfate group to form a film, which can increase the flexibility of the SEI film and reduce the migration resistance of lithium ions, further enhancing the synergistic effect of the compound of Formula 1 with vinylene carbonate and lithium tetrafluoroborate, and improving the high-temperature storage performance, fast charging, and long-cycle performance of the secondary battery.

[0030] In some embodiments of the present application, 0.1% ≤ A ≤ 4%, 1% ≤ B ≤ 5%, 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 composed 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 composed 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 composed of any two of these values. By controlling the mass percentages of the first additive, vinylene carbonate, and lithium tetrafluoroborate within the scope 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 sheet, which is beneficial to improving the structural stability of the SEI film and further enhancing the high-temperature storage performance, fast charging, and long-cycle performance of the secondary battery.

[0031] 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 composed of any two of these values, the value of B can be 2%, 2.3%, 2.5%, 2.8%, 3%, 3.2%, 3.5%, 3.8%, 4% or a range composed of any two of these values, and the value of C can be 0.1%, 0.5%, 0.8%, 1%, 1.3%, 1.5%, 1.8%, 2% or a range composed of any two of these values.

[0032] In some embodiments of the present application, the first additive is selected from at least one of the following compounds: .

[0033] Among them, the CAS number of 1-1 is 1431298-10-0, the CAS number of 1-2 is 2507955-35-1, the CAS number of 1-3 is 2125520-24-1, and the CAS number of 1-4 is 2943046-28-2.

[0034] 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, methyl propyl carbonate, methyl formate, ethyl acetate, methyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, propyl propionate, butyl propionate, methyl butyrate, and ethyl butyrate.

[0035] A 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.

[0036] 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 at least one of 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 O4, where 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. After the electrolyte is dissolved in the electrolyte solution, lithium ions or sodium ions can be released, and the lithium ions or sodium ions form a solvation structure with the solvent, which is beneficial to the rapid migration of ions. The present application places no particular limitation on the electrolyte, as long as the object of the present application can be achieved. For example, the electrolyte may 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(oxalato)borate (LiBOB), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium difluoroborate, sodium hexafluorophosphate, and sodium bis(fluorosulfonyl)imide. The present application places no particular limitation on the content of the electrolyte in the electrolyte solution, as long as the object of the present application can be achieved. For example, based on the total mass of the electrolyte solution, the mass percentage content of the electrolyte is 8% - 18%. For example, based on the total mass of the electrolyte solution, the mass percentage content of the electrolyte can be 8%, 10%, 12%, 14%, 16%, 18%, or any range composed of any two of these values. Controlling the mass percentage content of the electrolyte within the scope of the present application can enable the electrolyte to be fully dissolved in the non-aqueous organic solvent, and at the same time, the electrolyte solution has both high ionic conductivity and low manufacturing cost.

[0037] The secondary battery of the present application further includes a positive electrode plate, 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 in the thickness direction of the positive electrode current collector, or can be disposed on both surfaces in the thickness direction of the positive electrode current collector. The present application places no particular limitation on the positive electrode current collector, as long as the object of the present application can be achieved. For example, the positive electrode current collector can include a metal foil or a composite current collector, etc. For example, the metal foil is an aluminum foil. The composite current collector can include a polymer material base layer and a metal material layer located 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 places no particular limitation on the thicknesses of the positive electrode current collector and the positive electrode material layer, as long as the object of the present application can be achieved. For example, the thickness of the positive electrode current collector is 160 μm to 216 μm, preferably 176 μm to 196 μm. The thickness of the single-sided positive electrode material layer is 72 μm to 90 μm. The positive electrode material layer of the present application can further include a conductive agent and a binder. The present application places no particular limitation on the conductive agent and the binder, as long as the object of the present application can be achieved. For example, the conductive agent can include, but is not limited to, at least one of Super P, acetylene black, Ketjen black, carbon dots, carbon nanotubes, graphene and carbon nanofibers. The binder can include, but is not limited to, at least one of polyvinyl chloride, polyvinyl alcohol, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer or fluorinated acrylate resin.

[0038] The secondary battery of the present application includes a negative electrode plate, which includes 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 in the thickness direction of the negative electrode current collector, or can be disposed on both surfaces in the thickness direction of the negative electrode current collector. The present application places no particular limitation on the negative electrode current collector, and any negative electrode current collector well-known in the art can be used as long as the object of the present application can be achieved. The negative electrode current collector can be a metal foil or a composite current collector. For example, as the metal foil, it can include at least one of aluminum foil, copper foil, nickel foil, titanium foil. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector can be formed by laminating a metal material (such as 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.).

[0039] In the present application, there is no particular limitation on the thickness of the negative electrode current collector and the negative electrode material layer, as long as the object of the present application can be achieved. For example, the thickness of the negative electrode current collector is 219 μm to 259 μm, and the thickness of the single-sided negative electrode material layer is 105 μm to 120 μm. Optionally, the negative electrode active material includes a thickening agent, and the thickening agent may include, but is not limited to, sodium carboxymethyl cellulose (CMC-Na). The negative electrode material layer of the present application may also contain a conductive agent and a binder. There is no particular limitation on the conductive agent and the binder in the present application, as long as the object of the present application can be achieved. 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.

[0040] The negative electrode material layer of the present application contains a negative electrode active material. There is no particular limitation on the type of the negative electrode active material in the present application, and any negative electrode active material well known in the art can be used as long as the object of the present application can be achieved. For example, the negative electrode active material may include 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 above-mentioned silicon-based materials may include, but are not limited to, at least one of elemental silicon, silicon oxides, silicon-carbon composites, silicon-nitrogen composites, or silicon alloys; the above-mentioned tin-based materials may include at least one of elemental tin, tin oxides, or tin alloys.

[0041] The secondary battery of the present application further includes a separator for separating the positive electrode plate and the negative electrode plate, preventing internal short circuit of the battery, allowing ions to pass freely, and not affecting the progress of the electrochemical charge and discharge process. There is no particular limitation on the type of the separator in the present application, and any porous structure separator with good chemical stability and mechanical stability can be selected. For example, the material of the separator may include at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The type of the separator may include, but is not limited to, at least one of woven film, non-woven film (non-woven fabric), microporous film, composite film, rolled film, or spun film. The separator may be a single-layer thin film or a multi-layer composite thin film. In the present application, there is no particular limitation on the thickness of the separator, as long as the object of the present application can be achieved. For example, the thickness may be 10 μm to 20 μm.

[0042] The secondary battery of the present application further includes a packaging bag for accommodating the positive electrode plate, the separator, the negative electrode plate, and the electrolyte, as well as other components known in the art. The present application does not limit the above-mentioned other components. The present application has no special limitation on the packaging bag, and it can be a packaging bag well-known in the art as long as it can achieve the purpose of the present application. For example, an aluminum-plastic film packaging bag can be used.

[0043] The preparation process of the secondary battery of the present application is well-known to those skilled in the art, and the present application has no special limitation. For example, it may include but is not limited to the following steps: stacking the positive electrode plate, the separator, and the negative electrode plate in sequence, and winding or folding them as needed to obtain a wound electrode assembly, placing the electrode assembly into the packaging bag, injecting the electrolyte into the packaging bag and sealing it to obtain a secondary battery; or, stacking the positive electrode plate, the separator, and the negative electrode plate in sequence, and then fixing the four corners of the entire laminated structure with tape to obtain a laminated electrode assembly, placing the electrode assembly into the packaging bag, injecting the electrolyte into the packaging bag and sealing it to obtain a secondary battery. In addition, an overcurrent protection element, a guide plate, etc. can be placed in the packaging bag as needed to prevent the pressure inside the secondary battery from rising and overcharging and discharging.

[0044] The secondary battery of the present application can include the form of a battery cell, a battery module, and a battery pack. The battery cells can be assembled into a battery module, and the number of battery cells contained in the battery module can be one or more. Those skilled in the art can select the specific number according to the application and capacity of the battery module. The battery module of the present application can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more. Those skilled in the art can select the specific number according to the application and capacity of the battery pack.

[0045] Examples

[0046] Hereinafter, examples and comparative examples are given to more specifically illustrate the implementation manners of the present application. Various tests and evaluations are carried out according to the following methods. In addition, unless otherwise specified, "parts" and "%" are based on mass.

[0047] Test methods and equipment:

[0048] Normal temperature capacity retention rate: At 25°C, charge at a constant current of 2C and constant voltage to 3.65V, let it stand for 5 minutes, then discharge at a constant current of 1C to 2.5V. After 1000 cycles, calculate the capacity retention rate. Calculation method: Normal temperature capacity retention rate (%) = (discharge capacity at the 1000th time / discharge capacity at the 1st time) × 100%.

[0049] High-temperature storage performance test:

[0050] High-temperature storage expansion rate test: At 25°C, charge at a constant current of 1C and a constant voltage of 3.65V. Use a micrometer with an accuracy of ±1μm to measure the initial thickness of the lithium-ion battery at this time. Then store it at 60°C for 30 days, and measure the thickness of the lithium-ion battery again. Calculate the expansion rate of the battery according to the formula: High-temperature storage expansion rate (%) = (thickness after storage - initial thickness) / initial thickness × 100%.

[0051] High-temperature storage capacity retention rate: At 25°C, charge at a constant current of 1C and a constant voltage of 3.65V. Before storage, the lithium-ion battery needs to be kept fully charged. Then discharge the lithium-ion battery that has been stored at 60°C for 30 days at a constant current of 1C to 2.5V. Record the discharge capacity at this time as the retention capacity. The discharge capacity measured by discharging the fully charged lithium-ion battery at 1C to 2.5V before high-temperature storage is recorded as the initial capacity, and calculate the capacity retention rate of the battery. The calculation formula is as follows: High-temperature storage capacity retention rate (%) = retention capacity / initial capacity × 100%.

[0052] Rate charge test (rate performance): Place the battery in an environment of 25°C and discharge it to 2.5V, charge it at a constant current of 1C to 3.65V, and charge it at a constant voltage until the cut-off current is 0.05C. Record the charge capacity as C 充 , let it stand for 5 minutes, and 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, let it stand for 5 minutes, and then discharge it at a constant current of 1C to 2.5V, and let it stand for 5 minutes. This is one charge-discharge cycle. Repeat the charge-discharge steps for 3 weeks. Conduct rate charge tests on the battery at 0.5C, 1C, 2C, 3C, and 4C in sequence. The charge capacity of the constant current section of the last cycle at 4C is recorded as C4, and the 4C constant current charge ratio = (C4 / C 充 ) × 100%.

[0053] Example 1

[0054] <Preparation of electrolyte solution>

[0055] Mix ethylene carbonate (EC), ethyl methyl carbonate (EMC), and ethyl acetate (EA) in a mass ratio of 3:3:4 to obtain a basic solvent. Add LiPF6 and LiFSI as electrolytes, the compound of formula 1-1, vinylene carbonate, and lithium tetrafluoroborate as additives to the basic solvent, and mix evenly to obtain the electrolyte solution. Among them, based on the total mass of the electrolyte solution, the mass percentage content of LiPF6 is 10%, the mass percentage content of LiFSI is 4%, the mass percentage content of the compound of formula 1-1 is 0.5%, the mass percentage content of vinylene carbonate is 3%, the mass percentage content of lithium tetrafluoroborate is 0.5%, and the rest is the basic solvent.

[0056] <Preparation of positive electrode sheet>

[0057] The cathode active material lithium iron phosphate (LiFePO4, purchased from Shenzhen Defang Nano Technology Co., Ltd.), conductive agent conductive carbon black (Super P), and binder polyvinylidene fluoride (PVDF) were mixed at a mass ratio of 96:2:2. N-methylpyrrolidone (NMP) was added and stirred evenly under the action of a vacuum mixer until the mixed system became a homogeneous and flowing cathode slurry, obtaining a cathode slurry with a solid content of 68 wt%. The cathode slurry was evenly coated on one surface of a cathode current collector aluminum foil with a thickness of 16 μm. After drying at 80 °C, a cathode electrode sheet with a single-sided coated cathode material layer with a coating thickness of 198 μm was obtained. The above steps were repeated on the other surface of the cathode current collector aluminum foil, and a cathode electrode sheet with a double-sided coated cathode material layer was obtained. After drying under vacuum conditions at 80 °C, through processes such as cold pressing, edge trimming, slicing, slitting, sheet making, and welding tab sticking, a cathode electrode sheet with a specification of 70 mm × 54 mm was obtained. Among them, the tap density of the cathode material layer was 2.5 g / cm 3 。

[0058] <Preparation of the negative electrode sheet>

[0059] The negative electrode active material graphite (purchased from Jiangxi Zichen Technology Co., Ltd.), conductive agent conductive carbon black (Super P), thickening agent sodium carboxymethyl cellulose (CMC), and binder styrene-butadiene rubber (SBR) were mixed at a mass ratio of 95:2:1:2. Deionized water was added and stirred evenly under the action of a vacuum mixer to obtain a negative electrode slurry with a solid content of 49 wt%. The negative electrode slurry was evenly coated on one surface of a negative electrode current collector copper foil with a thickness of 9 μm. After drying at 80 °C, a negative electrode sheet with a single-sided coated negative electrode material layer with 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, and a negative electrode sheet with a double-sided coated negative electrode material layer was obtained. After drying under vacuum conditions at 80 °C, through processes such as cold pressing, edge trimming, slicing, slitting, sheet making, and welding tab sticking, a negative electrode sheet with a specification of 74 mm × 58 mm was obtained. Among them, the tap density of the negative electrode material layer was 1.6 g / cm 3 。

[0060] <Preparation of the separator>

[0061] The separator is a polyethylene separator purchased from Shenzhen Xingyuan Materials Technology Co., Ltd.

[0062] <Preparation of the secondary battery>

[0063] Stack the separator, the positive electrode sheet, the separator, and the negative electrode sheet in sequence, with the separator positioned between the positive electrode sheet and the negative electrode sheet to isolate the positive electrode sheet and the negative electrode sheet, and then wind them. Connect the positive electrode tab to the positive electrode sheet and the negative electrode tab to the negative electrode sheet to obtain the electrode assembly. Place the electrode assembly into an aluminum foil packaging bag, and lead the positive electrode tab and the negative electrode tab from the internal space of the packaging bag to the external space of the packaging bag. After drying at 85 °C for 36 hours to remove moisture, perform heat sealing to obtain the cell to be filled with electrolyte. Then inject the above-prepared electrolyte into the dried cell, and through processes such as vacuum packaging, standing, forming, shaping, and volume determination, the secondary battery is fabricated.

[0064] Example 2

[0065] Except for changing the compound of Formula 1-1 to the compound of Formula 1-2, the rest is the same as in Example 1.

[0066] Example 3

[0067] Except for changing the compound of Formula 1-1 to the compound of Formula 1-3, the rest is the same as in Example 1.

[0068] Example 4

[0069] Except for changing the compound of Formula 1-1 to the compound of Formula 1-4, the rest is the same as in Example 1.

[0070] Examples 5 to 9

[0071] Except for changing the mass percentage content of the first additive and the mass percentage content of the base solvent to meet a total of 100%, the rest is the same as in Example 1.

[0072] Examples 10 to 14

[0073] Except for changing the mass percentage content of vinylene carbonate and the mass percentage content of the base solvent to meet a total of 100%, the rest is the same as in Example 1.

[0074] Examples 15 to 19

[0075] Except for changing the mass percentage content of lithium tetrafluoroborate and the mass percentage content of the base solvent to meet a total of 100%, the rest is the same as in Example 1.

[0076] Examples 20 to 29

[0077] Except for changing the mass percentage content of the first additive, vinylene carbonate, lithium tetrafluoroborate, and the mass percentage content of the base solvent to meet a total of 100%, the rest is the same as in Example 1.

[0078] Example 30

[0079] Except that the type of the positive electrode active material is replaced with lithium cobalt oxide (LiCoO2), the rest is the same as in Example 1.

[0080] Example 31

[0081] Except that the type of the positive electrode active material is replaced with NCM523 (LiNi 0.5 Co 0.2 Mn 0.3 O2), the rest is the same as in Example 1.

[0082] Comparative Example 1

[0083] Except that the first additive, vinylene carbonate and lithium tetrafluoroborate are not added and the mass percentage content of the base solvent is changed to meet the total amount of 100%, the rest is the same as in Example 1.

[0084] Comparative Example 2

[0085] Except that the first additive is not added and the mass percentage content of the base solvent is changed to meet the total amount of 100%, the rest is the same as in Example 1.

[0086] Comparative Example 3

[0087] Except that vinylene carbonate is not added and the mass percentage content of the base solvent is changed to meet the total amount of 100%, the rest is the same as in Example 1.

[0088] Comparative Example 4

[0089] Except that lithium tetrafluoroborate is not added and the mass percentage content of the base solvent is changed to meet the total amount of 100%, the rest is the same as in Example 1.

[0090] Comparative Example 5

[0091] Except that the first additive and lithium tetrafluoroborate are not added, and the mass percentage content of the base solvent is changed to meet the total amount of 100%, the rest is the same as in Example 1.

[0092] Comparative Examples 6 to 7

[0093] Except that the mass percentage contents of the first additive, vinylene carbonate, lithium tetrafluoroborate and the base solvent are changed to meet the total amount of 100%, the rest is the same as in Example 1.

[0094] The preparation parameters and performance parameters of each example and comparative example are shown in Table 1 and Table 2.

[0095] Table 1

[0096] In Table 1, " / " indicates the absence of corresponding preparation parameters or substances.

[0097] Table 2

[0098] In Table 2, " / " indicates the absence of corresponding preparation parameters or substances.

[0099] As can be seen from Examples 1 to 31 and Comparative Examples 1 to 5 of the present application, the electrolyte includes the first additive, vinylene carbonate, and lithium tetrafluoroborate of the present application. Through the synergistic effect of the first additive, vinylene carbonate, and lithium tetrafluoroborate, the fast charge cycle performance and high-temperature storage performance of the secondary battery can be improved. 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 value of A + B + C in the electrolytes of Comparative Examples 6 to 7 is too large or too small, and the secondary battery has a low room-temperature cycle capacity retention rate, 4C constant current charge ratio, high-temperature cycle capacity retention rate, and a high high-temperature storage expansion rate.

[0100] 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, by selecting the first additive within the scope of the present application, the secondary battery has a high room-temperature cycle capacity retention rate, 4C constant current charge ratio, high-temperature cycle capacity retention rate, and a low high-temperature storage expansion rate.

[0101] The addition amounts of the first additive, vinylene carbonate, and lithium tetrafluoroborate affect the fast charge cycle performance and high-temperature storage performance of the secondary battery. As can be seen from Examples 5 to 19, by controlling the value ranges of A, B, and C within the scope of the present application, the secondary battery has a high room-temperature cycle capacity retention rate, 4C constant current charge ratio, high-temperature cycle capacity retention rate, and a low high-temperature storage expansion rate. As can be seen from Examples 20 to 23, by controlling the value of A + B + C within the scope of the present application, the secondary battery has a high room-temperature cycle capacity retention rate, 4C constant current charge ratio, high-temperature cycle capacity retention rate, and a low high-temperature storage expansion rate. As can be seen from Examples 27 to 29, by regulating the values of A, B, and C to satisfy B ≥ (A + C) / 8 and regulating the values of A / B and A / C within the scope of the present application, the secondary battery has a high room-temperature cycle capacity retention rate, 4C constant current charge ratio, high-temperature cycle capacity retention rate, and a low high-temperature storage expansion rate.

[0102] As can be seen from Examples 30 to 31, 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 high room-temperature cycle capacity retention rate, 4C constant current charge ratio, high-temperature cycle capacity retention rate, and a low high-temperature storage expansion rate.

[0103] The above are only the preferred embodiments of the present application, and are 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 within 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 additive includes a first additive, vinylene carbonate and lithium tetrafluoroborate; The first additive is selected from the compounds shown in Formula 1: ； wherein, R1 and R2 are each independently selected from H, , , , , and R1 and R2 are not simultaneously H; Based on the total mass of the electrolyte, the mass percentage of the first additive is A, the mass percentage of vinylene carbonate is B, and the mass percentage of lithium tetrafluoroborate is C, and 1.2% ≤ A + B + C ≤ 13%.

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, wherein 0.025 ≤ A / B ≤ 4, 0.025 ≤ A / C ≤ 40.

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

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

7. The electrolyte according to claim 6, wherein 0.1% ≤ A ≤ 2%, and / or, 2% ≤ B ≤ 4%, and / or, 0.1% ≤ C ≤ 2%.

8. The electrolyte according to claim 1, wherein The first additive is selected from at least one of the following compounds: 。 9. The electrolyte according to claim 1, wherein, 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, methyl propyl carbonate, methyl formate, ethyl acetate, methyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, propyl propionate, butyl propionate, methyl butyrate, ethyl butyrate.

10. A secondary battery, characterized in that, It includes a positive electrode sheet, a negative electrode sheet, a separator and the electrolyte according to any one of claims 1 to 9.

11. The secondary battery according to claim 10, wherein The positive electrode plate includes a positive electrode active material, and the positive electrode active material is selected from at least one of 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 O4, where 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, and 0 ≤ x < 1.

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