Electrolyte and secondary battery

Abstract

The invention provides an electrolyte and a secondary battery, the electrolyte comprises a solvent, an electrolyte salt and an additive, the additive comprises a first additive and a second additive, the first additive is selected from at least one of compounds represented by a formula I, and the second additive is selected from at least one of unsaturated carbonate compounds and cyclic anhydride compounds. The first additive and the second additive in the application range are added into the electrolyte and have a synergistic effect, so that an interfacial film which has relatively high thermal stability and is beneficial to lithium ion transmission is formed on a positive and negative electrode interface, the cycle performance and the high-temperature storage performance of the secondary battery are improved, and the impedance of the secondary battery is reduced.

Description

Technical Field

[0001] The present application relates to the field of electrochemistry technology, and particularly to an electrolyte and a secondary battery. Background Art

[0002] Secondary batteries (such as lithium-ion batteries) are widely used in electric vehicles and consumer electronic products due to their advantages of high energy density, high output power, long cycle life, and environmental friendliness. With the continuous expansion of the application scope of secondary batteries, their usage scenarios are more diverse, and the market has put forward higher requirements for the electrochemical performance of secondary batteries. For example, with the diversification of usage scenarios, secondary batteries must adapt to various climate conditions. However, under high-temperature conditions, factors such as the dissolution of positive metal ions, the occurrence of side reactions in the electrolyte, and the deterioration at the electrode-electrolyte interface lead to serious gas generation in secondary batteries, affecting the cycle performance and high-temperature storage performance of secondary batteries. Therefore, how to improve the cycle performance and high-temperature storage performance of secondary batteries has become an urgent problem to be solved. Summary of the Invention

[0003] The purpose of the present application is to provide an electrolyte and a secondary battery to improve the high-temperature storage performance and cycle performance of the secondary battery and reduce the impedance. The specific technical solutions are as follows:

[0004] The first aspect of the present application provides an electrolyte, which includes a solvent, an electrolyte salt, and an additive. The additive includes a first additive and a second additive. The first additive is selected from at least one of the compounds shown in Formula I, and the second additive is selected from at least one of unsaturated carbonate compounds and cyclic anhydride compounds;

[0005]

[0006] Wherein, R 1 and R 2 each independently selected from C1-C3 alkylene, C1-C3 alkoxy or oxygen atom, n and m are each independently 0 or 1, and n and m are not both 0 at the same time.

[0007] In some embodiments of the present application, based on the mass of the electrolyte, the mass percentage content of the first additive is W1, 0.01% ≤ W1 ≤ 3%, preferably, 0.1% ≤ W1 ≤ 2%.

[0008] In some embodiments of the present application, the mass percentage content of the second additive is W2, 0.01% ≤ W2 ≤ 5%, preferably, 0.05% ≤ W2 ≤ 3%.

[0009] In some embodiments of the present application, the compound shown in Formula I is selected from at least one of the following compounds:

[0010]

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

[0012]

[0013] In some embodiments of the present application, the cyclic anhydride compound is selected from at least one of the following compounds:

[0014]

[0015] In some embodiments of the present application, the electrolyte salt is selected from at least one of lithium hexafluorophosphate, lithium difluorophosphate, lithium bis(trifluoromethyl)sulfonylimide, lithium bis(fluorosulfonyl)imide, lithium monofluoromethanesulfonate, and lithium trifluoromethanesulfonate. Based on the mass of the electrolyte solution, the mass percentage content of the electrolyte salt is W3, and 10% ≤ W3 ≤ 20%.

[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 provided by the first aspect of the present application.

[0017] In some embodiments of the present application, the positive electrode sheet includes a positive electrode current collector and a positive electrode material layer provided on at least one surface of the positive electrode current collector. The positive electrode material layer includes a positive electrode active material, and the positive electrode active material is selected from lithium iron phosphate, lithium manganese iron phosphate, lithium cobaltate, lithium manganate, and Li a Co b M c O 2 , where M includes at least one of Mn, Al, Ti, V, Fe, Zn, V, Zr, Ce, Cr, and Cu, a + b + c = 1, 0.33 ≤ a ≤ 0.95, 0.02 ≤ b ≤ 0.33, and 0.03 ≤ c ≤ 0.33; preferably, the positive electrode active material is selected from LiNi a Co b M c O 2 .

[0018] In some embodiments of the present application, the separator includes a porous substrate and a ceramic coating provided on at least one surface of the porous substrate. The ceramic coating includes inorganic particles, and the inorganic particles are selected from at least one of boehmite, aluminum oxide, and Li 1.3 Al 0.3 Ti 1.7 (PO 4 ) 3 .

[0019] Advantages of the present application:

[0020] The present application provides an electrolyte and a secondary battery. The electrolyte includes a solvent, an electrolyte salt, and an additive. The additive includes a first additive and a second additive. The first additive is selected from at least one of the compounds represented by Formula I. By adding the first additive and the second additive within the scope of the present application to the electrolyte, the two act synergistically, which is conducive to forming an interfacial film with both high thermal stability and favorable lithium-ion transport at the positive and negative electrode interfaces, thereby improving the cycling performance and high-temperature storage performance of the secondary battery and reducing the impedance of the secondary battery.

[0021] 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

[0022] The technical solutions in the present application will be described clearly and completely below. Apparently, the described embodiments are only a 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 the present application fall within the scope of protection of the present application.

[0023] The first aspect of the present application provides an electrolyte, which includes a solvent, an electrolyte salt, and an additive. The additive includes a first additive and a second additive. The first additive is selected from at least one of the compounds represented by Formula I, and the second additive is selected from at least one of unsaturated carbonate compounds and cyclic anhydride compounds;

[0024]

[0025] wherein, R 1 and R 2 each independently selected from C1-C3 alkylene, C1-C3 alkoxy or oxygen atom, n and m are each independently 0 or 1, and n and m are not both 0 at the same time.

[0026] The inventors' research found that by adding the compound shown in Formula I of the present application to the electrolyte, due to the presence of the benzene ring in the compound shown in Formula I, it is beneficial for the ring-opening reaction of the compound shown in Formula I to occur during the formation stage of the secondary battery, and an interfacial film rich in inorganic sulfates, inorganic sulfites, lithium alkyl sulfonates, and lithium alkyl sulfates is formed at the positive and negative electrode interfaces, which is beneficial to enhancing the stability at the electrode-electrolyte interface and improving the cycle performance and high-temperature storage performance of the secondary battery. However, the free radical product with a benzene ring structure formed after the reduction and decomposition of the compound shown in Formula I during the formation stage is prone to inducing the decomposition of carbonate solvents and combining with the decomposition products of carbonate solvents to form an interfacial film component with low conductivity, which has a great hindrance to the conduction of lithium ions, resulting in an increase in the impedance of the secondary battery and a decline in the fast charging performance. Therefore, in the electrolyte, a second additive is further introduced. The second additive is selected from unsaturated carbonate compounds or cyclic anhydride compounds. The second additive has higher electrochemical activity than solvent molecules (such as carbonate solvents) and is more easily reduced compared to solvent molecules. The second additive can preferentially combine with the free radical product with a benzene ring structure formed by the reduction and decomposition of the compound shown in Formula I, and undergo an addition or free radical reaction to construct an interfacial film of organic polymers and inorganic lithium compounds rich in S elements at the electrode-electrolyte interface, making the interfacial film have both thermal stability and promote the conduction of lithium ions, avoiding the formation of an interfacial film component with low conductivity induced by the free radical product with a benzene ring structure. Through the synergistic effect of the first additive and the second additive, it is beneficial to improve the cycle performance and high-temperature storage performance of the secondary battery and reduce the impedance of the secondary battery.

[0027] In some embodiments of the present application, based on the mass of the electrolyte, the mass percentage content of the first additive is W1, 0.01% ≤ W1 ≤ 3%, preferably, 0.1% ≤ W1 ≤ 2%; for example, the mass percentage content of the first additive can be 0.01%, 0.05%, 0.1%, 0.3%, 0.5%, 0.8%, 1%, 1.3%, 1.5%, 1.8%, 2%, 2.2%, 2.5%, 2.8%, 3% or a range composed of any two of these values.

[0028] In some embodiments of the present application, the mass percentage content of the second additive is W2, 0.01% ≤ W2 ≤ 5%, preferably, 0.05% ≤ W2 ≤ 3%. For example, the mass percentage content of the second additive can be 0.01%, 0.03%, 0.05%, 0.08%, 0.1%, 0.2%, 0.3%, 0.4%, 0.7%, 0.8%, 0.9%, 1%, 1.2%, 1.5%, 1.8%, 2%, 2.2%, 2.5%, 2.8%, 3%, 3.2%, 3.5%, 3.8%, 4%, 4.2%, 4.5%, 4.8%, 5% or a range composed of any two of these values.

[0029] In some embodiments of the present application, the compound shown in Formula I is selected from at least one of the following compounds:

[0030]

[0031] By selecting the compound shown in Formula I within the scope of the present application, it is beneficial to form a stable interfacial film at the positive and negative interfaces, which is conducive to further improving the cycling performance and high-temperature storage performance of the secondary battery. Preferably, the first additive is selected from the compound shown in Formula I-3. The intermediate product formed after the bond breaking of the compound shown in Formula I-3 has more sites, which is more conducive to forming a stable interfacial film at the electrode and electrolyte interface, thereby further improving the cycling performance and high-temperature storage performance of the secondary battery.

[0032] In some embodiments of the present application, the unsaturated carbonate compound is selected from at least one of the following compounds:

[0033]

[0034] By selecting the unsaturated carbonate compound within the scope of the present application, which has higher electrochemical activity than solvent molecules (such as carbonate solvents) and is more easily reduced, it can preferentially combine with the radical product with a benzene ring structure formed by the reduction decomposition of the compound shown in Formula I, and undergo addition or radical reactions to construct an interfacial film of organic polymers and inorganic lithium compounds rich in S elements at the electrode-electrolyte interface, making the interface have both thermal stability and promoting lithium ion conduction. The compound shown in Formula I and the unsaturated carbonate compound within the scope of the present application act synergistically to further improve the cycling performance and high-temperature storage performance of the secondary battery and reduce the impedance of the secondary battery.

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

[0036]

[0037] By selecting the cyclic anhydride compound within the scope of the present application, which has higher electrochemical activity than solvent molecules (such as carbonate solvents) and is more easily reduced, it can preferentially combine with the radical product with a benzene ring structure formed by the reduction decomposition of the compound shown in Formula I, and undergo addition or radical reactions to construct an interfacial film of organic polymers and inorganic lithium compounds rich in S elements at the electrode-electrolyte interface, making the interface have both thermal stability and promoting lithium ion conduction. The compound shown in Formula I and the cyclic anhydride compound within the scope of the present application act synergistically to further improve the cycling performance and high-temperature storage performance of the secondary battery and reduce the impedance of the secondary battery.

[0038] In some embodiments of the present application, the electrolyte is selected from at least one of lithium hexafluorophosphate, lithium difluorophosphate, lithium bis(trifluoromethyl)sulfonylimide, lithium bis(fluorosulfonyl)imide, lithium monofluoromethanesulfonate, and lithium trifluoromethanesulfonate. Based on the mass of the electrolyte solution, the mass percentage content of the electrolyte is W3, and 10% ≤ W3 ≤ 20%. For example, the mass percentage content of the electrolyte can be 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, or a range composed of any two of these values. The electrolyte solution includes the electrolyte within the above range, and by regulating the mass percentage content of the electrolyte within the range of the present application, the electrolyte solution can have a high ionic conductivity and good electrochemical stability, further improving the cycle performance and high-temperature storage performance of the secondary battery and reducing the impedance.

[0039] The present application places no particular restrictions on the solvent, as long as the objectives of the present application can be achieved. For example, the solvent can include, but is not limited to, at least one of propylene carbonate, ethyl methyl carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, fluoroethylene carbonate, sulfolane, γ-butyrolactone, methyl formate, ethyl formate, propyl formate, butyl formate, methyl acetate, ethyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, propyl propionate, butyl propionate, methyl butyrate, ethyl butyrate, propyl butyrate, or butyl butyrate. The present application places no particular restrictions on the content of the solvent in the electrolyte solution, as long as the objectives of the present application can be achieved. For example, based on the mass of the electrolyte solution, the mass percentage content of the solvent can be from 76% to 89%.

[0040] The second aspect of the present application provides a secondary battery, which includes a positive electrode plate, a negative electrode plate, a separator, and the electrolyte solution provided in the first aspect of the present application.

[0041] In some embodiments of the present application, the positive electrode plate includes a positive electrode current collector and a positive electrode material layer provided on at least one surface of the positive electrode current collector. The positive electrode material layer includes a positive electrode active material, and the positive electrode active material is selected from lithium iron phosphate, lithium manganese iron phosphate, lithium cobaltate, lithium manganate, and LiNi a Co b M c O 2 , where M includes at least one of Mn, Al, Ti, V, Fe, Zn, V, Zr, Ce, Cr, and Cu, a + b + c = 1, 0.33 ≤ a ≤ 0.95, 0.02 ≤ b ≤ 0.33, 0.03 ≤ c ≤ 0.33; preferably, the positive electrode active material is selected from LiNi a Co b M c O 2 , for example, the positive electrode active material can be selected from LiNi 0.8 Co 0.1 Mn0.1 O 2 (NCM811), LiNi 0.7 Co 0.1 Mn 0.2 O 2 (NCM712), LiNi 0.6 Co 0.1 Mn 0.3 O 2 (NCM613), LiNi 0.6 Co 0.2 Mn 0.2 O 2 (NCM622), LiNi 0.5 Co 0.2 Mn 0.3 O 2 (NCM523), LiNi 0.8 Co 0.1 Al 0.1 O 2 (NCA811), LiNi 0.6 Co 0.2 Al 0.2 O 2 (NCA622), LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O 2 (NCM111), LiNi 0.95 Co 0.02 Mn 0.03 O 2 (Ni95), etc. Selecting the cathode active material within the scope of this application is beneficial to further improving the cycling performance and high-temperature storage performance of the secondary battery.

[0042] In the present application, the positive electrode sheet includes a positive electrode current collector and a positive electrode material layer disposed on at least one surface of the positive electrode current collector. The above-mentioned "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 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 a partial area of the surface of the positive electrode current collector. There is no special limitation in the present application, as long as the purpose of the present application can be achieved. There is no special limitation on the positive electrode current collector in the present application, as long as the purpose of the present application can be achieved. For example, the positive electrode current collector can be aluminum foil, aluminum alloy foil or a composite positive electrode current collector. The above-mentioned composite positive electrode current collector can be a polymer material base layer and a metal layer formed on at least one surface of the polymer material substrate. The material of the above-mentioned polymer material base layer can include, but is not limited to, at least one of polypropylene (PP), polyethylene terephthalate (PET) or polybutylene terephthalate (PBT). The material of the above-mentioned metal layer can include, but is not limited to, at least one of aluminum, aluminum alloy, nickel or nickel alloy. There is no special limitation on the thickness of the positive electrode material layer and the positive electrode current collector in the present application, as long as the purpose of the present application can be achieved. For example, the thickness of the single-sided positive electrode material layer is 50 μm to 250 μm, and the thickness of the positive electrode current collector is 7 μm to 15 μm.

[0043] The positive electrode material layer further includes a positive electrode conductive agent and a positive electrode binder. There is no special limitation on the types of the positive electrode conductive agent and the positive electrode binder in the present application, as long as the purpose of the present application can be achieved. For example, the positive electrode conductive agent can include, but is not limited to, at least one of Super P, acetylene black, Ketjen black, carbon nanotubes, graphene or carbon fiber. For example, the positive electrode binder can include, but is not limited to, at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer or fluorinated acrylate resin. There is no special limitation on the mass ratio of the positive electrode active material, the conductive agent and the binder in the positive electrode material layer in the present application, and those skilled in the art can select according to actual needs, as long as the purpose of the present application can be achieved.

[0044] In the present application, there is no special limitation on the preparation method of the positive electrode sheet, as long as the purpose of the present application can be achieved. For example, it can be prepared by the following method: Mix the positive electrode active material, the positive electrode conductive agent and the positive electrode binder, add N-methylpyrrolidone (NMP) and stir evenly to obtain a positive electrode slurry with a solid content of 50 wt% to 85 wt%. Coat the positive electrode slurry evenly on two surfaces of the positive electrode current collector, and after drying, obtain a positive electrode sheet with a double-sided coated positive electrode material layer. Then, through cold pressing and cutting, the positive electrode sheet is obtained.

[0045] In the present application, the negative electrode tab includes a negative electrode current collector and a negative electrode material layer disposed on at least one surface of the negative electrode current collector. The statement that "the negative electrode material layer is 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 can be disposed on both surfaces of the negative electrode current collector along its thickness direction. It should be noted that the "surface" here can be the entire area of the surface of the negative electrode current collector, or can be a partial area of the surface of the negative electrode current collector. There is no particular limitation in the present application, as long as the object of the present application can be achieved. There is no particular limitation on the negative electrode current collector in the present application, as long as the object of the present application can be achieved. For example, the negative electrode current collector can be copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, nickel foam or copper foam, aluminum foil or a composite negative electrode current collector. The above composite negative electrode current collector can be a polymer material base layer and a metal layer formed on at least one surface of the polymer material substrate. The material of the above polymer material base layer can include, but is not limited to, at least one of polypropylene (PP), polyethylene terephthalate (PET) or polybutylene terephthalate (PBT). The material of the above metal layer can include, but is not limited to, at least one of copper, copper alloy, nickel or nickel alloy. There is no particular limitation on the thickness of the negative electrode material layer and the negative electrode current collector in the present application, as long as the object of the present application can be achieved. For example, the thickness of the single-sided negative electrode material layer is 50 μm to 180 μm, and the thickness of the negative electrode current collector is 3 μm to 10 μm.

[0046] The negative electrode material layer includes a negative electrode active material. There is no particular limitation on the type of the negative electrode active material in the present application, as long as the object of the present application can be achieved. For example, the negative electrode active material can include, but is not limited to, graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials and lithium titanate, etc. Graphite can include, but is not limited to, at least one of natural graphite or artificial graphite; the above silicon-based materials can include, but is not limited to, at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites or silicon alloys; the above tin-based materials can include at least one of elemental tin, tin oxide compounds or tin alloys.

[0047] The negative electrode material layer further includes a negative electrode conductive agent and a negative electrode binder. There are no particular limitations on the types of the negative electrode conductive agent and the negative electrode binder in this application, as long as the objectives of this application can be achieved. For example, the negative electrode conductive agent may include, but is not limited to, at least one of super conductive carbon black (Super P), acetylene black, Ketjen black, carbon nanotubes, graphene, or carbon fiber. The above-mentioned carbon nanotubes may include, but are not limited to, single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The above-mentioned carbon fiber may include, but is not limited to, vapor-grown carbon fiber (VGCF) and / or nanofiber. For example, the negative electrode binder may include, but is not limited to, at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), or carboxymethyl chitosan (CMCS). There are no particular limitations on the mass ratio of the negative electrode active material, conductive agent, and binder in the negative electrode material layer in this application, and those skilled in the art can select according to actual needs as long as the objectives of this application can be achieved. In some embodiments of this application, the negative electrode material layer may further include a thickening agent. There are no particular limitations on the type of the thickening agent in this application, as long as the objectives of this application can be achieved. For example, the thickening agent may include, but is not limited to, at least one of sodium carboxymethyl cellulose or lithium carboxymethyl cellulose. There are no particular limitations on the mass ratio of the negative electrode active material, conductive agent, binder, and thickening agent in the negative electrode material layer in this application, and those skilled in the art can select according to actual needs as long as the objectives of this application can be achieved.

[0048] In this application, there are no particular limitations on the method for preparing the negative electrode plate, as long as the objectives of this application can be achieved. For example, it can be prepared by the following method: adding the negative electrode active material, negative electrode conductive agent, and negative electrode binder to deionized water and stirring evenly to obtain a negative electrode slurry with a solid content of 45 wt% to 70 wt%. The negative electrode slurry is evenly coated on both surfaces of the negative electrode current collector, and after drying, a negative electrode plate with a double-sided coated negative electrode material layer is obtained. Then, it is cold-pressed and cut to obtain the negative electrode plate.

[0049] In some embodiments of this application, the separator includes a porous substrate and a ceramic coating provided on at least one surface of the porous substrate. The ceramic coating includes inorganic particles, and the inorganic particles are selected from at least one of boehmite, aluminum oxide, and Li 1.3 Al 0.3 Ti 1.7 (PO 4 ) 3 By providing a ceramic coating on the separator, and the ceramic coating includes inorganic particles within the scope of this application, it is beneficial to improve the mechanical properties of the separator, reduce the occurrence of internal short circuits caused by metal dendrites piercing the separator, and is beneficial to the conduction of lithium ions on the separator, thereby further improving the cycle performance and high-temperature storage performance of the secondary battery.

[0050] The ceramic coating of the present application further includes a binder, and there is no particular limitation on the binder in the present application. For example, the binder may include but is not limited to at least one of polyacrylic acid, sodium polyacrylate, potassium polyacrylate, lithium polyacrylate, polyimide, polyvinyl alcohol, carboxymethyl cellulose, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, polyimide, polyamideimide, styrene-butadiene rubber, or polyvinylidene fluoride.

[0051] There is no particular limitation on the type of the porous substrate of the separator in the present application, and any porous substrate with good chemical stability and mechanical stability can be selected. For example, the material of the porous substrate may include at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The type of the porous substrate may include but is not limited to at least one of woven film, non-woven film (non-woven fabric), microporous film, composite film, calendared film, or spun film, etc. 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 5 μm to 20 μm.

[0052] There is no particular limitation on the preparation method of the separator in the present application, as long as the object of the present application can be achieved. For example, the preparation method of the separator may include but is not limited to the following steps: adding substances used in the ceramic coating, such as ceramic particles, binder, etc., into a solvent and mixing evenly to obtain a ceramic coating slurry, and then coating the ceramic coating slurry on the surface of the porous substrate and drying to obtain a separator including the ceramic coating. The separator of the present application can also be obtained by purchasing in the market.

[0053] In the present application, the secondary battery further includes a housing for accommodating the positive electrode plate, the separator, the negative electrode plate, and the electrolyte, as well as other components known in the field of secondary batteries, and the present application does not limit the above other components. There is no particular limitation on the housing in the present application, and it can be a housing well-known in the art, as long as the object of the present application can be achieved. For example, the housing can be a hard shell housing or a flexible housing. The material of the hard shell housing can be metal, and the present application does not limit the type of the metal, and a metal hard shell housing known in the art can be used, as long as the object of the present application can be achieved. The flexible housing can be a metal plastic film, such as an aluminum plastic film, a steel plastic film, etc.

[0054] There is no particular limitation on the secondary battery of the present application, and it may include any device that undergoes an electrochemical reaction. In one embodiment of the present application, the secondary battery may include but is not limited to: lithium ion secondary battery (lithium ion battery), sodium ion secondary battery (sodium ion battery), etc.

[0055] The preparation process of the secondary battery of the present application is well-known to those skilled in the art, and there is no special limitation in the present application. For example, the preparation process of the secondary battery may include, but is not limited to, the following steps: stacking the positive electrode sheet, the separator, and the negative electrode sheet in sequence, and winding, folding, etc. as needed to obtain a wound electrode assembly, placing the electrode assembly into a housing, injecting an electrolyte into the housing and sealing it to obtain a secondary battery. Alternatively, stack the positive electrode sheet, the separator, and the negative electrode sheet in sequence, and then fix the four corners of the entire laminated structure with tape to obtain a laminated electrode assembly, place the electrode assembly into a housing, inject an electrolyte into the housing and seal it to obtain a secondary battery. In addition, an overcurrent protection element, a guide plate, etc. can be placed in the housing as needed to prevent the pressure inside the secondary battery from rising and overcharging / discharging.

[0056] Examples

[0057] Hereinafter, examples and comparative examples are given to illustrate the embodiments of the present application more specifically. Various tests and evaluations are carried out according to the following methods. In addition, unless otherwise specified, "parts" and "%" are based on mass.

[0058] Test methods and equipment:

[0059] 25 °C cycling performance test

[0060] Place the lithium-ion battery in a constant temperature test chamber at 25 °C and let it stand for 30 minutes to make the lithium-ion battery reach a constant temperature. Charge it at a constant current of 3C to 4.4V, then charge it at a constant voltage of 4.4V until the cut-off current is 0.05C, and discharge it at a constant current of 1C to 3V. Record the initial discharge capacity as C0, and this is one charge-discharge cycle. Repeat the above charge-discharge cycle, and record the number of cycles when the capacity decays to 80% of C0.

[0061] When performing the 25 °C cycling performance test on the lithium-ion batteries of Examples 1-15 to 1-19 and Comparative Examples 4 to 6, adjust the upper limit voltage of 4.4V in the above steps to 3.65V and the lower limit voltage of 3V to 2V; when performing the 25 °C cycling performance test on the lithium-ion batteries of Examples 2-2 and 2-3, adjust the upper limit voltage of 4.4V in the above steps to 4.25V; the remaining examples and comparative examples are all tested with an upper limit voltage of 4.4V and a lower limit voltage of 3V.

[0062] 45 °C cycling performance test

[0063] Place the lithium-ion battery in a constant-temperature test chamber at 45°C and let it stand for 30 minutes to reach a constant temperature. Charge it at a constant current of 1C to 4.4V, then charge it at a constant voltage of 4.4V until the cut-off current is 0.05C, and discharge it at a constant current of 1C to 3V. Record the initial discharge capacity as D0, and this is one charge-discharge cycle. Repeat the above charge-discharge cycle and record the number of cycles when the capacity decays to 80% of D0.

[0064] When performing the 45°C cycle performance test on the lithium-ion batteries of Examples 1-15 to 1-19 and Comparative Examples 4 to 6, adjust the upper limit voltage of 4.4V in the above steps to 3.65V and the lower limit voltage of 3V to 2V; when performing the 45°C cycle performance test on the lithium-ion batteries of Examples 2-2 and 2-3, adjust the upper limit voltage of 4.4V in the above steps to 4.25V; for the rest of the examples and comparative examples, the tests are carried out according to the upper limit voltage of 4.4V and the lower limit voltage of 3V.

[0065] 60°C Storage Performance Test

[0066] Place the lithium-ion battery in a constant-temperature test chamber at 25°C and let it stand for 30 minutes to reach a constant temperature. Charge it at a constant current of 1C to 4.4V, charge it at a constant voltage of 4.4V until the cut-off current is 0.05C, and then discharge it at a constant current of 0.5C to 3V. Record the discharge capacity as Q1. Remove the battery and use a thickness tester to measure its initial thickness as T1. At 25°C, charge it at a constant current of 1C to 4.4V, charge it at a constant voltage of 4.4V until the cut-off current is 0.05C, and then transfer the lithium-ion battery to a constant-temperature oven at 60°C and leave it for 30 days. Use a thickness tester to measure its thickness after 15 days of storage as T2. Then, under the condition of 25°C, discharge the battery at a constant current, and the discharge cut-off voltage is 3V. Record the discharge capacity as Q2.

[0067] The 60°C storage capacity retention rate (%) = Q2 / Q1×100%, and the 60°C storage expansion rate (%) = [(T2 - T1) / T1]×100%.

[0068] When performing the 60°C storage performance test on the lithium-ion batteries of Examples 1-15 to 1-19 and Comparative Examples 4 to 6, adjust the upper limit voltage of 4.4V in the above steps to 3.65V and the lower limit voltage of 3V to 2V; when performing the 60°C storage performance test on the lithium-ion battery of Example 2-2, adjust the upper limit voltage of 4.4V in the above steps to 4.25V; when performing the 60°C storage performance test on the lithium-ion battery of Example 2-3, adjust the upper limit voltage of 4.4V in the above steps to 4.25V; for the rest of the examples and comparative examples, the tests are carried out according to the upper limit voltage of 4.4V and the lower limit voltage of 3V.

[0069] DC Impedance Test

[0070] The lithium-ion battery was placed in an incubator at 25 °C and left standing for 30 minutes. Then, the lithium-ion battery was charged at a constant current of 1.0C until the voltage reached 4.4V, and left standing for 5 min. Then, it was discharged at a constant current of 1.0C for 30 minutes, left standing for 1 h, and then discharged at a current I corresponding to a rate of 2.0C 1 for 10 s, and the corresponding voltage V1 was recorded. The direct current resistance (DCR) of the lithium-ion battery at a 50% state of charge (SOC), i.e., the initial DCR = V 1 / I 1 , with the unit of mΩ.

[0071] When performing the direct current impedance test on the lithium-ion batteries of Examples 1-15 to Examples 1-19 and Comparative Examples 4 to Comparative Examples 6, the upper limit voltage of 4.4V in the above steps was adjusted to 3.65V; when performing the direct current impedance test on the lithium-ion batteries of Examples 2-2 and Examples 2-3, the upper limit voltage of 4.4V in the above steps was adjusted to 4.25V. The remaining examples and comparative examples were all tested with an upper limit voltage of 4.4V.

[0072] Example 1-1

[0073] <Preparation of electrolyte>

[0074] In an argon atmosphere glove box with a water content < 10 ppm and an oxygen content < 1 ppm, ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate were mixed evenly in a mass ratio of 3:5:2 to obtain a basic solvent. Then, the first additive, Compound I-1, and the additive, Compound II-1, were added and mixed evenly. Then, the lithium salt, lithium hexafluorophosphate, was added, dissolved, and mixed evenly to obtain the electrolyte; wherein, based on the mass of the electrolyte, the mass percentage content W3 of lithium hexafluorophosphate was 12.5%, the mass percentage content W1 of the first additive, Compound I-1, was 0.005%, the mass percentage content W2 of the second additive, Compound II-1, was 0.5%, and the balance was the basic solvent, 86.995%.

[0075] <Preparation of positive electrode sheet>

[0076] The positive electrode active material LiNi 0.6 Co 0.1 Mn 0.3 O 2 , the binder polyvinylidene fluoride (PVDF), and the conductive agent Super P were mixed evenly in a mass ratio of 97:1.2:1.8, and N-methylpyrrolidone (NMP) was added as a solvent and stirred under a vacuum mixer until the mixed system became a homogeneous and fluid positive electrode slurry with a solid content of 65%; the positive electrode slurry was evenly coated on one surface of a positive electrode current collector aluminum foil with a thickness of 18 μm, and the coating amount was 2.5 g / m 2, dried at 85 °C, the positive electrode sheet with a positive electrode material layer coated on one side; then repeat the above steps on the other surface of the aluminum foil to obtain the positive electrode sheet with a positive electrode material layer coated on both sides; after drying at 85 °C, perform cold pressing, edge trimming, slitting, and strip cutting. After strip cutting, dry at 85 °C for 4 hours under vacuum conditions, and weld the tab to obtain a positive electrode sheet with a specification of 258 mm × 200 mm for standby; among them, the thickness of the single-sided positive electrode material layer is 45 μm.

[0077] <Preparation of the negative electrode sheet>

[0078] Mix the negative active material artificial graphite, conductive agent acetylene black, thickening agent sodium carboxymethyl cellulose (CMC), and binder styrene-butadiene rubber (SBR) according to a mass ratio of 95.8:1.2:1.5:1.5, add deionized water, and obtain a negative electrode slurry with a solid content of 50 wt% under the action of a vacuum mixer; evenly coat the negative electrode slurry on one surface of a negative electrode current collector copper foil with a thickness of 8 μm, and the coating amount is 1.5 g / m 2 , dried at 85 °C to obtain a negative electrode sheet with a negative electrode material layer coated on one side; then repeat the above steps on the other surface of the copper foil to obtain the negative electrode sheet with a negative electrode material layer coated on both sides; after drying at 85 °C, perform cold pressing, edge trimming, slitting, and strip cutting. After strip cutting, dry at 85 °C for 4 hours under vacuum conditions, and weld the tab to obtain a negative electrode sheet with a specification of 260 mm × 198 mm for standby; among them, the thickness of the single-sided negative electrode material layer is 50 μm.

[0079] <Preparation of the separator>

[0080] Adopt a double-layer PP ceramic separator with a thickness of 16 μm, the substrate thickness is 10 μm, the single-sided ceramic coating thickness is 3 μm, and the types of inorganic particles in the ceramic coating are Al 2 O 3 .

[0081] <Preparation of the lithium-ion battery>

[0082] The prepared positive electrode sheet, separator, and negative electrode sheet are stacked in sequence, with the separator placed in the middle between the positive electrode sheet and the negative electrode sheet to play an isolation role, and then wound. The positive electrode tab is connected to the positive electrode sheet, and the negative electrode tab is connected to the negative electrode sheet to obtain an electrode assembly. The electrode assembly is placed in an aluminum foil packaging bag, and the positive electrode tab and the negative electrode tab are led out from the internal space of the aluminum foil packaging bag to the external space, dehydrated at 80°C, and the prepared electrolyte is injected with a liquid injection coefficient of 3.0 g / Ah. After processes such as vacuum packaging, formation, aging, and grading, a lithium-ion battery is obtained. Among them, the upper limit voltage for formation is 4.4 V, the formation temperature is 45°C, and the formation standing time is 24 h; the aging temperature is 45°C, and the aging standing time is 24 h; for grading, it is charged at 0.2 C to 4.4 V, left standing for 5 minutes, then discharged at 0.2 C to 3.0 V, and then the above steps are repeated at 0.5 C and 1 C.

[0083] When forming and grading the lithium-ion batteries of Examples 1-15 to 1-19 and Comparative Examples 4 to 6, the upper limit voltage of 4.4 V in the above steps is adjusted to 3.65 V, and the lower limit voltage of 3 V is adjusted to 2 V; when forming and grading the lithium-ion batteries of Examples 2-2 and 2-3, the upper limit voltage of 4.4 V in the above steps is adjusted to 4.25 V; the remaining examples and comparative examples are all tested with an upper limit voltage of 4.4 V and a lower limit voltage of 3 V.

[0084] Examples 1-2 to 1-10

[0085] Except that in <Preparation of Electrolyte>, according to Table 1, the mass percentage content and type of the compound shown in Formula I of the first additive are adjusted, and the rest is the same as Example 1-1. Among them, when the mass percentage content of the first additive changes, the mass percentage content of the base solvent changes accordingly.

[0086] Examples 1-11 to 1-14

[0087] Except that in <Preparation of Electrolyte>, according to Table 1, the mass percentage content of the second additive is adjusted, and the rest is the same as Example 1-4. Among them, when the mass percentage content of the second additive changes, the mass percentage content of the base solvent changes accordingly.

[0088] Examples 1-15 to 1-19

[0089] Except that in <Preparation of Electrolyte>, according to Table 1, the mass percentage content of the second additive is adjusted, and in <Preparation of Positive Electrode Sheet>, according to Table 1, the type of the positive electrode active material is adjusted, and the rest is the same as Example 1-4. Among them, when the mass percentage content of the second additive changes, the mass percentage content of the base solvent changes accordingly.

[0090] Examples 1-20 to Examples 1-26

[0091] Except that in <Preparation of electrolyte solution>, the type of the second additive was adjusted according to Table 1, the rest was the same as in Example 1-4.

[0092] Examples 1-27 to Examples 1-29

[0093] Except that in <Preparation of electrolyte solution>, the mass percentage content and type of the electrolyte salt were adjusted according to Table 1, the rest was the same as in Example 1-4. Among them, when the mass percentage content of the electrolyte salt changed, the mass percentage content of the base solvent changed accordingly. Lithium bis(fluorosulfonyl)imide is abbreviated as LiFSI.

[0094] Examples 2-1 to Examples 2-4

[0095] Except that in <Preparation of positive electrode sheet>, the type of the positive electrode active material was adjusted according to Table 2, the rest was the same as in Example 1-4.

[0096] Example 2-5

[0097] Except that in <Preparation of separator>, the separator did not include a ceramic coating and a PP separator with a thickness of 16 μm was used, the rest was the same as in Example 1-4.

[0098] Examples 2-6, Examples 2-7

[0099] Except that in <Preparation of separator>, the type of the inorganic particles in the ceramic separator was adjusted according to Table 2, the rest was the same as in Example 1-4. Li 1.3 Al 0.3 Ti 1.7 (PO 4 ) 3 is abbreviated as LATP.

[0100] Comparative Example 1

[0101] Except that in <Preparation of electrolyte solution>, the first additive was not added and the mass percentage content of the base solvent changed accordingly, the rest was the same as in Example 1-4.

[0102] Comparative Examples 2, Comparative Example 3

[0103] Except that in <Preparation of electrolyte solution>, the mass percentage content of the first additive was adjusted according to Table 1 and the second additive was not added, and the mass percentage content of the base solvent changed accordingly, the rest was the same as in Example 1-4.

[0104] Comparative Example 4

[0105] Except that in <the preparation of the electrolyte>, the first additive is not added and the mass percentage content of the base solvent changes accordingly, the rest is the same as in Examples 1-16.

[0106] Comparative Example 5, Comparative Example 6

[0107] Except that in <the preparation of the electrolyte>, the mass percentage content of the first additive is adjusted according to Table 1 and the second additive is not added, and the mass percentage content of the base solvent changes accordingly, the rest is the same as in Examples 1-15.

[0108] The preparation parameters and performance parameters of each example and comparative example are shown in Tables 1 to 2.

[0109] Table 1

[0110]

[0111]

[0112] Note: " / " in Table 1 indicates the absence of the corresponding preparation parameter or substance.

[0113] It can be seen from Examples 1-1 to 1-29 and Comparative Examples 1 to 6 that for the lithium-ion batteries of each example of the present application, the electrolyte includes the first additive and the second additive within the scope of the present application, while the lithium-ion batteries in the comparative examples do not satisfy the above characteristics at the same time. The lithium-ion batteries obtained in the examples have a higher number of 25°C cycle times, a higher number of 45°C cycle times, a higher 60°C storage capacity retention rate, a lower 60°C storage expansion rate, and a lower initial impedance, indicating that the lithium-ion batteries of the examples of the present application have good cycle performance, high-temperature storage performance, and a lower initial impedance. The lithium-ion batteries in the comparative examples do not include the first additive and the second additive at the same time, and their lithium-ion batteries have a lower number of 25°C cycle times, a lower number of 45°C cycle times, a lower 60°C storage capacity retention rate, a higher 60°C storage expansion rate, and a higher initial impedance, indicating that the cycle performance and high-temperature storage performance of the lithium-ion batteries in the comparative examples are poor, and the initial impedance is high.

[0114] The type and mass percentage content of the first additive will affect the cycle performance, high-temperature storage performance, and initial impedance of the lithium-ion battery. It can be seen from Examples 1-1 to 1-10, Comparative Example 1, Example 1-16, and Comparative Example 4 that when the electrolyte includes the first additive within the scope of the present application and the mass percentage content of the first additive is regulated within the scope of the present application, the obtained lithium-ion battery has a higher number of 25°C cycle times, a higher number of 45°C cycle times, a higher 60°C storage capacity retention rate, a lower 60°C storage expansion rate, and a lower initial impedance, indicating that the lithium-ion battery has good cycle performance, high-temperature storage performance, and a lower initial impedance.

[0115] The type and mass percentage content of the second additive can affect the cycle performance, high-temperature storage performance, and initial impedance of the lithium-ion battery. It can be seen from Examples 1-11 to 1-26, Comparative Example 2, Comparative Example 3, Comparative Example 5, and Comparative Example 6 that when the electrolyte includes the second additive within the scope of the present application and the mass percentage content of the second additive is adjusted within the scope of the present application, the obtained lithium-ion battery has a higher number of 25°C cycle times, a higher number of 45°C cycle times, a higher 60°C storage capacity retention rate, a lower 60°C storage expansion rate, and a lower initial impedance, indicating that the lithium-ion battery has good cycle performance, high-temperature storage performance, and a lower initial impedance.

[0116] The type and mass percentage content of the electrolyte salt can affect the cycle performance, high-temperature storage performance, and initial impedance of the lithium-ion battery. It can be seen from Examples 1-27 to 1-29 that when the electrolyte includes the electrolyte salt within the scope of the present application and the mass percentage content of the electrolyte salt is adjusted within the scope of the present application, the obtained lithium-ion battery has a higher number of 25°C cycle times, a higher number of 45°C cycle times, a higher 60°C storage capacity retention rate, a lower 60°C storage expansion rate, and a lower initial impedance, indicating that the lithium-ion battery has good cycle performance, high-temperature storage performance, and a lower initial impedance.

[0117] Table 2

[0118]

[0119]

[0120] Note: " / " in Table 2 indicates the absence of the corresponding preparation parameter or substance.

[0121] The type of the positive electrode active material can affect the cycle performance, high-temperature storage performance, and initial impedance of the lithium-ion battery. It can be seen from Examples 1-4, Examples 2-1 to 2-4 that when the positive electrode active material is selected within the scope of the present application, the obtained lithium-ion battery has a higher number of 25°C cycle times, a higher number of 45°C cycle times, a higher 60°C storage capacity retention rate, a lower 60°C storage expansion rate, and a lower initial impedance, indicating that the lithium-ion battery has good cycle performance, high-temperature storage performance, and a lower initial impedance.

[0122] The types of inorganic particles in the separator can affect the cycle performance, high-temperature storage performance, and initial impedance of lithium-ion batteries. It can be seen from Examples 1-4, Examples 2-5 to Examples 2-7 that when the separator includes a ceramic coating and the ceramic coating includes inorganic particles within the scope of the present application, the obtained lithium-ion batteries have a higher number of cycles at 25 °C, a higher number of cycles at 45 °C, a higher storage capacity retention rate at 60 °C, a lower storage expansion rate at 60 °C, and a lower initial impedance, indicating that the cycle performance and high-temperature storage performance of the lithium-ion batteries are further improved and the initial impedance is further reduced.

[0123] 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 principle of the present application shall be included within the scope of protection of the present application.

Claims

1. An electrolyte comprising a solvent, an electrolyte salt and an additive, wherein the additive comprises a first additive and a second additive, wherein the first additive is selected from at least one of the compounds represented by formula I, and the second additive is selected from at least one of an unsaturated carbonate compound and a cyclic anhydride compound; in, R1 and R2 are each independently selected from a C1-C3 alkylene group, a C1-C3 alkyleneoxy group, an oxygen atom or a single bond, n and m are each independently 0 or 1, and n and m are not 0 at the same time.

2. The electrolyte according to claim 1, which satisfies at least one of the following conditions: (1) Based on the mass of the electrolyte, the mass percentage of the first additive is W1, 0.01%≤W1≤3%; (2) Based on the mass of the electrolyte, the mass percentage of the second additive is W2, 0.01%≤W2≤5%.

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

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

5. The electrolyte according to claim 1, wherein The cyclic anhydride compound is selected from at least one of the following compounds:

6. The electrolyte according to any one of claims 1 to 5, wherein The electrolyte satisfies at least one of the following conditions: (1)0.1％≤W1≤2％； (2)0.05％≤W2≤3％。 7. The electrolyte according to any one of claims 1 to 5, wherein The electrolyte salt is selected from at least one of lithium hexafluorophosphate, lithium difluorophosphate, lithium bis(trifluoromethyl)sulfonyl imide, lithium bis(fluorosulfonyl imide), lithium monofluorosulfonate and lithium trifluoromethylsulfonate. Based on the mass of the electrolyte, the mass percentage of the electrolyte salt is W3, 10%≤W3≤20%. 8 . A secondary battery comprising a positive electrode sheet, a negative electrode sheet, a separator and the electrolyte according to claim 1 .

9. The secondary battery according to claim 8, wherein The positive electrode sheet comprises a positive electrode current collector and a positive electrode material layer disposed on at least one surface of the positive electrode current collector, wherein the positive electrode material layer comprises a positive electrode active material, and the positive electrode active material is selected from lithium iron phosphate, lithium manganese iron phosphate, lithium cobalt oxide, lithium manganese oxide and LiNi a Co b M c O2, wherein M is selected from at least one of Mn, Al, Ti, V, Fe, Zn, V, Zr, Ce, Cr and Cu, a+b+c=1, 0.33≤a≤0.95, 0.02≤b≤0.33, 0.03≤c≤0.33; preferably, the positive electrode active material is selected from LiNi a Co b M c O2.

10. The secondary battery according to claim 8 or 9, wherein: The separator comprises a porous substrate and a ceramic coating disposed on at least one surface of the porous substrate, wherein the ceramic coating comprises inorganic particles selected from boehmite, aluminum oxide and Li 1.3 Al 0.3 Ti 1.7 At least one of (PO4)3.

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