Electrolyte and secondary battery

Abstract

The application 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 the compounds shown in formula I. The second additive is selected from at least one of unsaturated carbonate compounds and cyclic anhydride compounds. By adding the first additive and the second additive within the scope of the application to the electrolyte, the two additives synergistically form an interface film with high thermal stability and favorable lithium ion transmission at the interface between the positive and negative electrodes, thereby improving the cycle performance and high-temperature storage performance of the secondary battery and reducing the impedance of the secondary battery.

Description

TECHNICAL FIELD

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

[0002] Secondary batteries (for example, lithium ion batteries) are widely used in electric vehicles and consumer electronic products due to their advantages such as high energy density, high output power, long cycle life, and green environmental protection. As the use of secondary batteries continues to expand, their use scenarios are becoming more diverse, and the market has higher requirements for the electrochemical performance of secondary batteries. For example, as the use scenarios of secondary batteries become more diverse, they must adapt to various climate conditions. However, under high temperature conditions, factors such as the dissolution of positive metal ions, the occurrence of electrolyte side reactions, and the deterioration of the electrode-electrolyte interface cause the secondary battery to produce serious gas, which affects the cycle performance and high-temperature storage performance of the secondary battery. Therefore, how to improve the cycle performance and high-temperature storage performance of the secondary battery has become a problem to be solved. SUMMARY

[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 comprises a solvent, an electrolyte salt, and an additive, the additive comprising a first additive and a second additive, the first additive being selected from at least one of the compounds shown in Formula I, and the second additive being selected from at least one of unsaturated carbonate compounds and cyclic anhydride compounds;

[0005]

[0006] wherein R1 and R2 are each independently selected from C1-C3 alkylene, C1-C3 alkylenoxy, or an oxygen atom, n and m are each independently 0 or 1, and n and m are not 0 at the same time.

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

[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] Formula I-1, Formula I-2, Formula I-3.

[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] Formula II-7, Formula II-8, Formula II-9, Formula II-10.

[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 bisfluorosulfonylimide, lithium monofluorosulfonate and lithium trifluoromethylsulfonate, and the mass percentage of the electrolyte salt in the electrolyte is W3, 10%≤W3≤20%.

[0016] The second aspect of the present application provides a secondary battery, which comprises a positive electrode sheet, a negative electrode sheet, a separator and the electrolyte provided by the first aspect of the present application.

[0017] In some embodiments of the present application, 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, and the positive electrode material layer comprises a positive electrode active material selected from lithium iron phosphate, lithium manganese iron phosphate, lithium cobaltate, lithium manganate and LiNi a Co b M c O2, wherein M comprises 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.

[0018] In some embodiments of the present application, the separator comprises a porous substrate and a ceramic coating layer disposed on at least one surface of the porous substrate, and the ceramic coating layer comprises inorganic particles selected from at least one of boehmite, diatomic aluminum oxide and Li 1.3 Al 0.3 Ti 1.7 (PO4)3.

[0019] Advantages of the application:

[0020] The application 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 the compounds shown in formula I. By adding the first additive and the second additive within the scope of the application in the electrolyte, the two additives work synergistically, which is conducive to forming an interface film with high thermal stability and conducive to lithium ion transmission at the interface of the positive and negative electrodes, thereby improving the cycle performance and high-temperature storage performance of the secondary battery and reducing the impedance of the secondary battery.

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

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

[0023] The first aspect of the application provides an electrolyte, which 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 the compounds shown in formula I, and the second additive is selected from at least one of unsaturated carbonate compounds and cyclic anhydride compounds;

[0024]

[0025] wherein R1 and R2 are each independently selected from C1-C3 alkylene, C1-C3 alkylenoxy or an oxygen atom, n and m are each independently 0 or 1, and n and m are not 0 at the same time.

[0026] The inventors have found that by adding the compound of formula I to the electrolyte, the compound of formula I is prone to ring-opening reaction in the formation stage of the secondary battery due to the presence of the benzene ring in the compound of formula I, and an interface film rich in inorganic sulfate, inorganic sulfite, lithium alkyl sulfonate and lithium alkyl sulfate is formed at the interface of the positive and negative electrodes, which is conducive to enhancing the stability at the interface of the electrode and the electrolyte 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 by the reduction and decomposition of the compound of formula I in the formation stage is prone to inducing decomposition of the carbonate solvent and combining with the decomposition product of the carbonate solvent to form an interface film component with low conductivity, which hinders the conduction of lithium ions, resulting in increased impedance and decreased fast-charging performance of the secondary battery. Therefore, a second additive is further introduced into the electrolyte, the second additive is selected from an unsaturated carbonate compound or a cyclic anhydride compound, the second additive has higher electrochemical activity than the solvent molecules (e.g., the carbonate solvent) and is more prone to reduction than the 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 of formula I to occur addition or free radical reaction, and an interface film rich in S-containing organic polymers and inorganic lithium compounds is constructed at the electrode-electrolyte interface, so that the interface film has both thermal stability and the ability to promote the conduction of lithium ions, and the free radical product with a benzene ring structure is prevented from inducing decomposition of the carbonate solvent to form an interface film component with low conductivity. Through the synergistic effect of the first additive and the second additive, the cycle performance and high-temperature storage performance of the secondary battery are improved, and the impedance of the secondary battery is reduced.

[0027] In some embodiments of the present application, 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 formed by any two of them.

[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 formed by any two of them.

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

[0030] Formula I-1, Formula I-2, Formula I-3.

[0031] By selecting the compound of formula I within the scope of the present application, it is beneficial to form a stable interface film at the positive-negative interface, and it is beneficial to further improve the cycle performance and high-temperature storage performance of the secondary battery. Preferably, the first additive is selected from the compound of formula I-3, and the intermediate product site formed after the breaking of the compound of formula I-3 is more beneficial to form a stable interface film at the electrode-electrolyte liquid interface, thereby further improving the cycle 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, it has higher electrochemical activity than the solvent molecules (for example, carbonate solvents), is more easily reduced, can preferentially combine with the free radical product with benzene ring structure formed by the reduction and decomposition of the compound of formula I, and occurs addition or free radical reaction, constructs an interface film rich in S element organic polymer and inorganic lithium compound type at the electrode-electrolyte interface, and makes the interface have both thermal stability and promote lithium ion conduction. The compound of formula I and the unsaturated carbonate compound within the scope of the present application synergistically act to further improve the cycle 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] Formula II-7, Formula II-8, Formula II-9, Formula II-10.

[0037] By selecting the cyclic anhydride compound within the scope of the present application, it has higher electrochemical activity than the solvent molecules (for example, carbonate solvents), is easier to be reduced, can preferentially combine with the free radical product with benzene ring structure formed by the reduction and decomposition of the compound represented by Formula I, and occurs addition or free radical reaction to construct an interface film rich in S element organic polymer and inorganic lithium compound type at the electrode-electrolyte interface, so that the interface has both thermal stability and promotes lithium ion conduction. The compound represented by Formula I and the cyclic anhydride compound within the scope of the present application synergistically act to further improve the cycle 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 bisfluorosulfonylimide, lithium monofluorosulfonate and lithium trifluoromethylsulfonate, and the mass percentage of the electrolyte in the electrolyte is W3, 10%≤W3≤20%. For example, the mass percentage of the electrolyte can be 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20% or a range formed by any two of them. The electrolyte includes the electrolyte within the above range, and regulates the mass percentage of the electrolyte within the scope of the present application, so that the electrolyte has higher ion conductivity and good electrochemical stability, and can further improve the cycle performance and high-temperature storage performance of the secondary battery and reduce the impedance.

[0039] The solvent in the present application is not particularly limited as long as it can achieve the purpose of the present application, for example, the solvent can include but is not limited to at least one of propylene carbonate, methyl ethyl 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 content of the solvent in the electrolyte is not particularly limited as long as it can achieve the purpose of the present application. For example, the mass percentage of the solvent can be 76% to 89% based on the mass of the electrolyte.

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

[0041] 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, and the positive electrode material layer includes a positive electrode active material selected from lithium iron phosphate, lithium manganese iron phosphate, lithium cobaltate, lithium manganate and LiNi a Co bM c O2, wherein M comprises 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, for example, the positive electrode active material can be selected from LiNi 0.8 Co 0.1 Mn 0.1 O2 (NCM811), LiNi 0.7 Co 0.1 Mn 0.2 O2 (NCM712), LiNi 0.6 Co 0.1 Mn 0.3 O2 (NCM613), LiNi 0.6 Co 0.2 Mn 0.2 O2 (NCM622), LiNi 0.5 Co 0.2 Mn 0.3 O2 (NCM523), LiNi 0.8 Co 0.1 Al 0.1 O2 (NCA811), LiNi 0.6 Co 0.2 Al 0.2 O2 (NCA622), LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (NCM111), LiNi 0.95 Co 0.02 Mn 0.03 O2 (Ni95), etc. Selecting the positive electrode active material within the scope of the present application is conducive to further improving the cycle performance and high-temperature storage performance of the secondary battery.

[0042] In the present application, the positive electrode tab includes a positive current collector and a positive material layer disposed on at least one surface of the positive current collector. The "positive material layer disposed on at least one surface of the positive current collector" means that the positive material layer can be disposed on one surface of the positive current collector in the thickness direction of the positive current collector, or can be disposed on both surfaces of the positive current collector in the thickness direction of the positive current collector. It should be noted that the "surface" herein can be the entire area of the surface of the positive current collector, or can be part of the area of the surface of the positive current collector, and the present application does not have a particular limitation, as long as the purpose of the present application can be achieved. The present application does not have a particular limitation on the positive current collector, as long as the purpose of the present application can be achieved, for example, the positive current collector can be an aluminum foil, an aluminum alloy foil or a composite positive current collector. The composite positive current collector can be a polymer material base layer and a metal layer formed on at least one surface of the polymer material base material, and the material of the polymer material base layer can include but is not limited to at least one of polypropylene (PP), polyethylene terephthalate (PET) or polybutylene terephthalate (PBT), and the material of the metal layer can include but is not limited to at least one of aluminum, aluminum alloy, nickel or nickel alloy. The present application does not have a particular limitation on the thickness of the positive material layer and the positive current collector, as long as the purpose of the present application can be achieved, for example, the thickness of the single-sided positive material layer is 50 μm to 250 μm, and the thickness of the positive current collector is 7 μm to 15 μm.

[0043] The positive material layer further includes a positive conductive agent and a positive binder, and the present application does not have a particular limitation on the types of positive conductive agent and positive binder, as long as the purpose of the present application can be achieved, for example, the positive conductive agent can include but is not limited to at least one of Super P, acetylene black, Ketjen black, carbon nanotube, graphene or carbon fiber. For example, the positive 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 fluorine-containing acrylic ester resin. The present application does not have a particular limitation on the mass ratio of the positive active material, the conductive agent and the binder in the positive material layer, and those skilled in the art can select according to the actual needs, as long as the purpose of the present application can be achieved.

[0044] In the present application, the method for preparing the positive electrode tab is not particularly limited, as long as the purpose of the present application can be achieved, for example, it can be prepared by the following method: mixing the positive active material, the positive conductive agent and the positive binder, adding N-methyl pyrrolidone (NMP) and stirring uniformly to obtain a positive slurry with a solid content of 50wt% to 85wt%. The positive slurry is uniformly coated on both surfaces of the positive current collector, and after drying, a positive electrode tab with a double-sided positive material layer is obtained. Then, cold pressing and cutting are performed to obtain the positive electrode tab.

[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 above-mentioned "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 the thickness direction of the negative electrode current collector, or can be disposed on two surfaces of the negative electrode current collector along the thickness direction of the negative electrode current collector. It should be noted that the "surface" herein can be the entire area of the surface of the negative electrode current collector, or can be part of the area of the surface of the negative electrode current collector, which is not particularly limited in the present application as long as the purpose of the present application can be achieved. The negative electrode current collector is not particularly limited in the present application as long as the purpose of the present application can be achieved, for example, the negative electrode current collector can be a copper foil, a copper alloy foil, a nickel foil, a stainless steel foil, a titanium foil, a nickel foam or a copper foam, an aluminum foil or a composite negative electrode current collector. The above-mentioned 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 base material, and 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), and the material of the above-mentioned metal layer can include but is not limited to at least one of copper, copper alloy, nickel or nickel alloy. The thickness of the negative electrode material layer and the negative electrode current collector is not particularly limited in the present application as long as the purpose 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, and the type of the negative electrode active material is not particularly limited in the present application as long as the purpose 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 material, tin-based material and lithium titanate, etc. The graphite can include but is not limited to at least one of natural graphite or artificial graphite; the above-mentioned silicon-based material can include but is not limited to at least one of elemental silicon, silicon oxide compound, silicon-carbon composite, silicon-nitrogen composite or silicon alloy; and the above-mentioned tin-based material can include at least one of elemental tin, tin oxide compound or tin alloy.

[0047] The negative electrode material layer further comprises a negative electrode conductive agent and a negative electrode binder. The present application does not have a particular limitation on the types of the negative electrode conductive agent and the negative electrode binder, as long as the purpose of the present application can be achieved. For example, the negative electrode conductive agent can include, but is not limited to, at least one of Super P, acetylene black, Ketjen black, carbon nanotube, graphene, or carbon fiber. The above-mentioned carbon nanotube can include, but is not limited to, single-walled carbon nanotube and / or multi-walled carbon nanotube. The above-mentioned carbon fiber can include, but is not limited to, vapor grown carbon fiber (VGCF) and / or nanometer carbon fiber. For example, the negative electrode binder can 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). The present application does not have a particular limitation on the mass ratio of the negative electrode active material, the conductive agent, and the binder in the negative electrode material layer, and a person skilled in the art can select according to actual needs, as long as the purpose of the present application can be achieved. In some embodiments of the present application, the negative electrode material layer can further comprise a thickening agent, and the present application does not have a particular limitation on the type of the thickening agent, as long as the purpose of the present application can be achieved. For example, the thickening agent can include, but is not limited to, at least one of sodium carboxymethyl cellulose or lithium carboxymethyl cellulose. The present application does not have a particular limitation on the mass ratio of the negative electrode active material, the conductive agent, the binder, and the thickening agent in the negative electrode material layer, and a person skilled in the art can select according to actual needs, as long as the purpose of the present application can be achieved.

[0048] In the present application, the preparation method of the negative electrode tab is not particularly limited, as long as the purpose of the present application can be achieved. For example, it can be prepared by the following method: the negative electrode active material, the negative electrode conductive agent, and the negative electrode binder are added to deionized water and stirred uniformly to obtain a negative electrode slurry with a solid content of 45wt% to 70wt%. The negative electrode slurry is uniformly coated on both surfaces of the negative electrode current collector, and after drying, a negative electrode tab with a double-sided coated negative electrode material layer is obtained. Then, cold pressing and cutting are performed to obtain the negative electrode tab.

[0049] In some embodiments of the present application, the separator comprises a porous substrate and a ceramic coating layer provided on at least one surface of the porous substrate, and the ceramic coating layer comprises inorganic particles selected from at least one of boehmite, aluminum trioxide, and Li 1.3 Al 0.3 Ti 1.7 (PO4)3. By providing a ceramic coating layer on the separator, the ceramic coating layer comprising inorganic particles within the scope of the present application is beneficial to improve the mechanical properties of the separator, reduce the occurrence of internal short circuit caused by metal dendrite piercing the separator, and facilitate 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 also includes a binder, which is not particularly limited in the present application, for example, the binder can 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, polyamide-imide, styrene butadiene rubber, or polyvinylidene fluoride.

[0051] The type of the porous substrate of the separator is not particularly limited in the present application, and any porous structure having good chemical stability and mechanical stability can be selected. For example, the material of the porous substrate can include at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The type of the porous substrate can include, but is not limited to, at least one of woven membranes, non-woven membranes (non-woven fabrics), microporous membranes, composite membranes, calendered membranes, or spunlaced membranes. In the present application, the thickness of the separator is not particularly limited, as long as the purpose of the present application can be achieved, for example, the thickness can be 5 μm to 20 μm.

[0052] The preparation method of the separator is not particularly limited in the present application, as long as the purpose of the present application can be achieved, for example, the preparation method of the separator can include, but is not limited to, the following steps: adding the substances used in the ceramic coating, such as ceramic particles, binders, etc., into a solvent to mix uniformly to obtain a ceramic coating slurry, then coating the ceramic coating slurry on the surface of the porous substrate, and drying to obtain a separator including a ceramic coating. The separator of the present application can also be obtained by purchasing on the market.

[0053] In the present application, the secondary battery also includes a housing for accommodating the positive electrode sheet, the separator, the negative electrode sheet, and the electrolyte, as well as other components known in the art of secondary batteries, which are not limited in the present application. The housing is not particularly limited in the present application, and can be a housing known in the art, as long as the purpose 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 type of metal is not limited in the present application, and a metal hard shell housing known in the art can be used, as long as the purpose 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] The secondary battery of the present application is not particularly limited, and it can include any device that undergoes an electrochemical reaction. In an embodiment of the present application, the secondary battery can include, but is not limited to, a lithium ion secondary battery (lithium ion battery), a 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 the present application is not particularly limited, for example, the preparation process of the secondary battery can include but is not limited to the following steps: stacking the positive electrode sheet, the separator and the negative electrode sheet in order, and winding, folding and the like as needed to obtain an electrode assembly of a wound structure, placing the electrode assembly into the case, injecting the electrolyte into the case and sealing to obtain the secondary battery. Alternatively, the positive electrode sheet, the separator and the negative electrode sheet are stacked in order, and then the four corners of the entire stack structure are fixed with adhesive tape to obtain an electrode assembly of a stack structure, the electrode assembly is placed into the case, the electrolyte is injected into the case and sealed to obtain the secondary battery. In addition, the overcurrent prevention element, the guide plate and the like can also be placed in the case as needed, so as to prevent the pressure rise in the secondary battery.

[0056] Examples

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

[0058] Test methods and apparatus:

[0059] 25℃ cycle performance test

[0060] The lithium ion battery is placed in a 25℃ constant temperature test box, and allowed to stand for 30 minutes to reach constant temperature. It is charged at 3C constant current to 4.4V, then charged at constant voltage 4.4V to the cut-off current of 0.05C, and discharged at 1C constant current to 3V. The first discharge capacity is recorded as C0, which is one cycle of charge and discharge. The above charge and discharge cycle is repeated, and the cycle number when the capacity decays to 80% C0 is recorded.

[0061] When the lithium ion batteries of Examples 1-15 to 1-19 and Comparative Examples 4 to 6 are subjected to the 25℃ cycle performance test, the upper limit voltage 4.4V in the above step is adjusted to 3.65V, and the lower limit voltage 3V is adjusted to 2V; when the lithium ion batteries of Examples 2-2 and 2-3 are subjected to the 25℃ cycle performance test, the upper limit voltage 4.4V in the above step is adjusted to 4.25V; the rest of the examples and comparative examples are tested according to the upper limit voltage of 4.4V and the lower limit voltage of 3V.

[0062] 45℃ cycle performance test

[0063] The lithium ion battery was placed in a 45℃ constant temperature test box and rested for 30 minutes to allow the lithium ion battery to reach a constant temperature. Charged to 4.4V at 1C constant current, then charged to 0.05C cutoff current at 4.4V constant voltage, then discharged to 3V at 1C constant current, and the initial discharge capacity was recorded as D0. The above charging and discharging cycle was repeated, and the cycle number when the capacity decayed to 80% D0 was recorded.

[0064] When the lithium ion batteries of Example 1-15 to Example 1-19 and Comparative Example 4 to Comparative Example 6 were subjected to 45℃ cycle performance testing, the upper limit voltage 4.4V in the above steps was adjusted to 3.65V, and the lower limit voltage 3V was adjusted to 2V; when the lithium ion batteries of Example 2-2 and Example 2-3 were subjected to 45℃ cycle performance testing, the upper limit voltage 4.4V in the above steps was adjusted to 4.25V; the remaining examples and comparative examples were tested according to an upper limit voltage of 4.4V and a lower limit voltage of 3V.

[0065] 60℃ storage performance test

[0066] The lithium ion battery was placed in a 25℃ constant temperature test box and rested for 30 minutes to allow the lithium ion battery to reach a constant temperature. Charged to 4.4V at 1C constant current, then charged to 0.05C cutoff current at 4.4V constant voltage, then discharged to 3V at 0.5C constant current, and the discharge capacity was recorded as Q1. The battery was removed and its initial thickness was measured as T1. Charged to 4.4V at 1C constant current, then charged to 0.05C cutoff current at 4.4V constant voltage, then the lithium ion battery was transferred to a 60℃ constant temperature oven and rested for 30 days, and the thickness after 15 days of rest was measured as T2. Then the battery was discharged at 1C constant current at 25℃, and the discharge cutoff voltage was 3V, and the discharge capacity was recorded as Q2.

[0067] 60℃ storage capacity retention rate (%) = Q2 / Q1 x 100%, 60℃ storage expansion rate (%) = [(T2-T1) / T1] x 100%.

[0068] When the lithium ion batteries of Example 1-15 to Example 1-19 and Comparative Example 4 to Comparative Example 6 were subjected to 60℃ storage performance testing, the upper limit voltage 4.4V in the above steps was adjusted to 3.65V, and the lower limit voltage 3V was adjusted to 2V; when the lithium ion battery of Example 2-2 was subjected to 60℃ storage performance testing, the upper limit voltage 4.4V in the above steps was adjusted to 4.25V; when the lithium ion battery of Example 2-3 was subjected to 60℃ storage performance testing, the upper limit voltage 4.4V in the above steps was adjusted to 4.25V; the remaining examples and comparative examples were tested according to an upper limit voltage of 4.4V and a lower limit voltage of 3V.

[0069] DC impedance test

[0070] The lithium ion battery was placed in a 25℃ constant temperature box, and rested for 30 minutes. Then the lithium ion battery was charged at 1.0C constant current to a voltage of 4.4V, rested for 5min, then discharged at 1.0C constant current for 30 minutes, rested for 1h, then discharged at a current I1 corresponding to a rate of 2.0C for 10s, and the corresponding voltage V1 was recorded. The direct current resistance (DCR) of the lithium ion battery at 50% state of charge (SOC), i.e. the initial DCR = V1 / I1, was recorded in mΩ.

[0071] When the lithium ion batteries of Example 1-15 to Example 1-19 and Comparative Example 4 to Comparative Example 6 were subjected to direct current impedance testing, the upper limit voltage 4.4V in the above steps was adjusted to 3.65V; when the lithium ion batteries of Example 2-2 and Example 2-3 were subjected to direct current impedance testing, the upper limit voltage 4.4V in the above steps was adjusted to 4.25V; the rest of the examples and comparative examples were tested according to the upper limit voltage of 4.4V.

[0072] Example 1-1

[0073] Preparation of electrolyte

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

[0075] Preparation of positive electrode sheet

[0076] The positive electrode active material LiNi 0.6 Co 0.1 Mn 0.3 O2, a binder polyvinylidene fluoride (PVDF), and a conductive agent Super P were mixed uniformly according to a mass ratio of 97:1.2:1.8, N-methyl pyrrolidone (NMP) was added as a solvent, and stirring was performed under the action of a vacuum stirrer until the mixed system became a positive electrode slurry with uniform fluidity, and the solid content of the positive electrode slurry was 65%; the positive electrode slurry was uniformly coated on one surface of a positive electrode current collector aluminum foil with a thickness of 18μm, and the coating amount was 2.5 g / m 2The positive electrode tab with single-side coated positive electrode material layer is dried at 85°C, and then the above steps are repeated on the other surface of the aluminum foil to obtain the positive electrode tab with double-side coated positive electrode material layer; after drying at 85°C, cold pressing, edge cutting, tab cutting, and striping, the tab is dried at 85°C under vacuum for 4 hours, and the tab ears are welded to obtain the positive electrode tab with a size of 258 mm x 200 mm for use; wherein the thickness of the single-side positive electrode material layer is 45 μm.

[0077] <Preparation of negative electrode tab>

[0078] The negative electrode active material artificial graphite, conductive agent acetylene black, thickening agent sodium carboxymethyl cellulose (CMC), and binder styrene-butadiene rubber (SBR) are mixed in a mass ratio of 95.8:1.2:1.5:1.5, deionized water is added, and a negative electrode slurry with a solid content of 50wt% is obtained under the action of a vacuum stirrer; the negative electrode slurry is uniformly coated 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 The positive electrode tab with single-side coated positive electrode material layer is dried at 85°C, and then the above steps are repeated on the other surface of the aluminum foil to obtain the positive electrode tab with double-side coated positive electrode material layer; after drying at 85°C, cold pressing, edge cutting, tab cutting, and striping, the tab is dried at 85°C under vacuum for 4 hours, and the tab ears are welded to obtain the positive electrode tab with a size of 258 mm x 200 mm for use; wherein the thickness of the single-side positive electrode material layer is 45 μm.

[0079] <Preparation of separator>

[0080] A double-layer PP ceramic separator with a thickness of 16 μm is used, the substrate has a thickness of 10 μm, the single-side ceramic coating has a thickness of 3 μm, and the inorganic particles in the ceramic coating are Al2O3.

[0081] <Preparation of lithium ion battery>

[0082] The positive electrode tab, the separator, and the negative electrode tab prepared above are stacked in sequence, with the separator between the positive electrode tab and the negative electrode tab to play a role of isolation, and then the positive electrode tab ears and the negative electrode tab ears are connected to the positive electrode tab and the negative electrode tab, respectively, to obtain an electrode assembly. The electrode assembly is placed in an aluminum foil packaging bag, and the positive electrode tab ears and the negative electrode tab ears are led out from the internal space of the aluminum foil packaging bag to the external space of the aluminum foil packaging bag, water is removed at 80°C, the electrolyte prepared above is injected, the injection coefficient is 3.0 g / Ah, and a lithium ion battery is obtained through processes such as vacuum packaging, formation, aging, and capacity grading. The upper limit voltage of formation is 4.4 V, the formation temperature is 45°C, the standing time of formation is 24 h; the aging temperature is 45°C, the standing time of aging is 24 h; the capacity grading is 0.2C charging to 4.4 V, standing for 5 minutes, and then discharging to 3.0 V at 0.2C, followed by repeating the above steps at 0.5C and 1C.

[0083] The upper limit voltage 4.4V in the above step was adjusted to 3.65V and the lower limit voltage 3V was adjusted to 2V when the lithium ion batteries of Example 1-15 to Example 1-19 and Comparative Example 4 to Comparative Example 6 were subjected to formation and capacity distribution; the upper limit voltage 4.4V in the above step was adjusted to 4.25V when the lithium ion batteries of Example 2-2 and Example 2-3 were subjected to formation and capacity distribution; the remaining examples and comparative examples were tested according to the upper limit voltage of 4.4V and the lower limit voltage of 3V.

[0084] Example 1-2 to Example 1-10

[0085] Except that the mass percentage content and the type of the first additive compound represented by Formula I in the <Preparation of electrolyte> were adjusted according to Table 1, the rest was 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] Example 1-11 to Example 1-14

[0087] Except that the mass percentage content of the second additive in the <Preparation of electrolyte> was adjusted according to Table 1, the rest was 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] Example 1-15 to Example 1-19

[0089] Except that the mass percentage content of the second additive in the <Preparation of electrolyte> was adjusted according to Table 1, and the type of the positive active material in the <Preparation of positive electrode sheet> was adjusted according to Table 1, the rest was 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] Example 1-20 to Example 1-26

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

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

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

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

[0095] Except for adjusting the kind of positive active material in <Preparation of positive electrode sheet> according to Table 2, the rest is the same as Example 1-4.

[0096] Example 2-5

[0097] Except for <Preparation of separator> that the separator does not include ceramic coating and using PP separator with thickness of 16 μm, the rest is the same as Example 1-4.

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

[0099] Except for <Preparation of separator> that adjusting the kind of inorganic particles in ceramic separator according to Table 2, the rest is the same as Example 1-4. 1.3 Al 0.3 Ti 1.7 (PO4)3is abbreviated as LATP.

[0100] Comparative Example 1

[0101] Except for <Preparation of electrolyte> that not adding the first additive and the mass percentage of base solvent is changed accordingly, the rest is the same as Example 1-4.

[0102] Comparative Example 2, Comparative Example 3

[0103] Except for <Preparation of electrolyte> that adjusting the mass percentage of the first additive according to Table 1 and not adding the second additive and the mass percentage of base solvent is changed accordingly, the rest is the same as Example 1-4.

[0104] Comparative Example 4

[0105] Except for <Preparation of electrolyte> that not adding the first additive and the mass percentage of base solvent is changed accordingly, the rest is the same as Example 1-16.

[0106] Comparative Example 5, Comparative Example 6

[0107] Except for <Preparation of electrolyte> that adjusting the mass percentage of the first additive according to Table 1 and not adding the second additive and the mass percentage of base solvent is changed accordingly, the rest is the same as Example 1-15.

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

[0109] Table 1

[0110]

[0111]

[0112] Note: " / " in Table 1 means that the corresponding preparation parameter or substance does not exist.

[0113] As can be seen from Examples 1-1 to 1-29, Comparative Examples 1 to 6, the lithium ion battery of each embodiment of the present application has an electrolyte comprising 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 simultaneously satisfy the above characteristics, the lithium ion battery obtained by the embodiments has a higher number of 25°C cycles, a higher number of 45°C cycles, 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 of the embodiments of the present application has good cycle performance, high temperature storage performance and lower initial impedance. The lithium ion battery of the comparative examples does not simultaneously include the first additive and the second additive, and has a lower number of 25°C cycles, a lower number of 45°C cycles, 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 battery of the comparative examples are poor, and the initial impedance is high.

[0114] The type and mass percentage of the first additive will affect the cycle performance, high temperature storage performance and initial impedance of the lithium ion battery. As can be seen from Examples 1-1 to 1-10, Comparative Example 1, Example 1-16, Comparative Example 4, when the electrolyte comprises the first additive within the scope of the present application and the mass percentage of the first additive is controlled within the scope of the present application, the lithium ion battery obtained has a higher number of 25°C cycles, a higher number of 45°C cycles, 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 lower initial impedance.

[0115] The type and mass percentage of the second additive will affect the cycle performance, high temperature storage performance and initial impedance of the lithium ion battery. As can be seen from Examples 1-11 to 1-26, Comparative Example 2, Comparative Example 3, Comparative Example 5 and Comparative Example 6, when the electrolyte comprises the second additive within the scope of the present application and the mass percentage of the second additive is controlled within the scope of the present application, the lithium ion battery obtained has a higher number of 25°C cycles, a higher number of 45°C cycles, 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 lower initial impedance.

[0116] The type and mass percentage of the electrolyte salt can affect the cycle performance, high-temperature storage performance and initial impedance of the lithium ion battery. As can be seen from Examples 1-27 to 1-29, when the electrolyte includes the electrolyte salt within the scope of the present application and the mass percentage of the electrolyte salt is regulated within the scope of the present application, the obtained lithium ion battery has 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 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 that the corresponding preparation parameter or substance does not exist.

[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. As can be seen from Examples 1-4, 2-1 to 2-4, when the positive electrode active material is selected from within the scope of the present application, the obtained lithium ion battery has 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 lithium ion battery has good cycle performance, high-temperature storage performance and a lower initial impedance.

[0122] The type of the inorganic particles in the separator can affect the cycle performance, high-temperature storage performance and initial impedance of the lithium ion battery. As can be seen from Examples 1-4, 2-5 to 2-7, when the separator includes a ceramic coating and the ceramic coating includes the inorganic particles within the scope of the present application, the obtained lithium ion battery has 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 battery are further improved and the initial impedance is further reduced.

[0123] The above only describes the preferred embodiments of the present application and is not intended to limit the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in 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, the first additive being selected from at least one compound of formula I, and the second additive being selected from at least one unsaturated carbonate compound and cyclic anhydride compound; ； in, R1 and R2 are each independently selected from C1-C3 alkylene groups, C1-C3 alkoxy groups, oxygen atoms or single bonds, and n and m are each independently 0 or 1, and n and m are not simultaneously 0; The unsaturated carbonate compound is selected from at least one of the following compounds: 、 ； The cyclic anhydride compound is selected from at least one of the following compounds: ； Based on the mass of the electrolyte, the mass percentage of the first additive is W1, where 0.01% ≤ W1 ≤ 3%; Based on the mass of the electrolyte, the mass percentage of the second additive is W2, where 0.01% ≤ W2 ≤ 5%.

2. The electrolyte according to claim 1, wherein, The compound represented by Formula I is selected from at least one of the following compounds: 。 3. The electrolyte according to claim 1 or 2, wherein, The electrolyte satisfies at least one of the following conditions: （1）0.1%≤W1≤2%； （2）0.05%≤W2≤3%。 4. The electrolyte according to claim 1 or 2, wherein, The electrolyte salt is selected from at least one of lithium hexafluorophosphate, lithium difluorophosphate, lithium bis(trifluoromethyl)sulfonylimide, lithium bisfluorosulfonylimide, lithium monofluorosulfonate, and lithium trifluoromethylsulfonate. Based on the mass of the electrolyte, the mass percentage of the electrolyte salt is W3, where 10% ≤ W3 ≤ 20%.

5. A secondary battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte according to any one of claims 1 to 4.

6. The secondary battery according to claim 5, wherein, The positive electrode includes a positive current collector and a positive electrode material layer disposed on at least one surface of the positive current collector. The positive electrode material layer includes a positive electrode active material 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, 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.

7. The secondary battery according to claim 6, wherein, The positive electrode active material is selected from LiNi a Co b M c O2.

8. The secondary battery according to any one of claims 5 to 7, wherein, The membrane comprises a porous substrate and a ceramic coating disposed on at least one surface of the porous substrate, the ceramic coating comprising inorganic particles selected from boehmite, alumina, and Li. 1.3 Al 0.3 Ti 1.7 At least one of (PO4)3.

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