Secondary batteries and their preparation methods, electrical equipment

Abstract

This application relates to the field of battery technology, and specifically discloses a secondary battery and its preparation method, as well as an electrical device. The secondary battery includes a positive electrode, a negative electrode, a separator, and an electrolyte. The electrolyte includes a free electrolyte and a non-free electrolyte. Both the non-free electrolyte and the free electrolyte include cyclic carbonate compounds, including vinylene carbonate. The mass percentage of vinylene carbonate in the free electrolyte is m1%, and the total mass percentage of all other cyclic carbonate compounds is m2%. The mass percentage of vinylene carbonate in the non-free electrolyte is n1%, and the total mass percentage of all other cyclic carbonate compounds is n2%. m1, m2, n1, and n2 satisfy 0 < (m1 / m2) - (n1 / n2) ≤ 0.2. The positive electrode includes a positive current collector and an intermediate coating and a positive active material layer sequentially disposed on at least one side of the positive current collector. The peel force between the positive active material layer and the intermediate coating is S, and S satisfies 10 N / m ≤ S ≤ 30 N / m.

Description

Technical Field

[0001] This application relates to the field of battery technology, and in particular to a secondary battery and its preparation method, and an electrical device. Background Technology

[0002] Lithium-ion batteries using lithium iron phosphate (LiFePO4) cathodes and graphite anodes offer advantages such as lower cost and better safety, making them suitable for electrochemical energy storage. To further optimize resource utilization and promote sustainable development, it is necessary to continue improving the lifespan of lithium iron phosphate batteries.

[0003] For LiFePO4 cathodes, the microscopic manifestation of the lithium insertion / extraction process is a two-phase transformation between orthorhombic LiFePO4 and hexagonal FePO4. Upon completion of charging, the cell volume of FePO4 decreases by approximately 6.8% relative to LiFePO4. However, during long-term battery cycling, the repeated expansion and contraction of the cells accumulate stress within the particles, leading to material dislocation. The resulting stress release directly causes large-scale cracking of the particles, macroscopically manifesting as decreased electrode conductivity, active material stripping, battery capacity loss, and a sharp increase in impedance.

[0004] Similarly, after complete lithium intercalation to form LiC6 in the graphite anode, approximately 10% volume expansion occurs, along with the generation and propagation of microcracks, material layering, electrolyte co-intercalation, solvent reduction and gas generation, and a vicious cycle of these phenomena. Therefore, suppressing the cracking of the positive and negative electrode active materials is a key technical challenge for improving the cycle life of lithium iron phosphate batteries. Common methods to improve cycle life by suppressing cracking of the positive and negative electrode active materials include: increasing the amount of binder used in the positive and negative electrodes, optimizing the type or degree of polymerization of the binder; reducing the coating weight or compaction of the positive and negative electrode active materials, and strengthening the coating of the active materials. However, these measures all sacrifice electrode capacity, making it difficult to match the pursuit of high energy density by optimizing the binder and active materials themselves. Summary of the Invention

[0005] This application aims to at least partially address one of the technical problems in the related art. Therefore, the purpose of this application is to provide a secondary battery and its preparation method, as well as an electrical device, wherein the secondary battery has a durable positive electrode active material that can maintain a long cycle life without sacrificing battery capacity.

[0006] In one aspect of this application, a secondary battery is provided. According to an embodiment of this application, the secondary battery includes a positive electrode, a negative electrode, a separator, and an electrolyte;

[0007] The electrolyte includes a free electrolyte and a non-free electrolyte, and the non-free electrolyte includes electrolyte in the pores of the positive electrode, electrolyte in the pores of the negative electrode, and electrolyte in the pores of the diaphragm.

[0008] The non-ionized electrolyte and the ionized electrolyte include cyclic carbonate compounds, including vinylene carbonate;

[0009] The free electrolyte contains vinylene carbonate at a mass percentage of m1%, and other cyclic carbonate compounds besides vinylene carbonate at a total mass percentage of m2%.

[0010] The non-ionized electrolyte contains vinylene carbonate at a mass percentage of n1%, and other cyclic carbonate compounds besides vinylene carbonate at a total mass percentage of n2%. The values ​​of m1, m2, n1, and n2 satisfy 0 < (m1 / m2) - (n1 / n2) ≤ 0.2.

[0011] The positive electrode sheet includes a positive current collector and an intermediate coating and a positive active material layer sequentially disposed on at least one side surface of the positive current collector. The peel force between the positive active material layer and the intermediate coating is S, and S satisfies 10N / m≤S≤30N / m.

[0012] This application optimizes the charge-discharge performance, improves stability, and extends cycle life of the battery by designing the localized distribution characteristics of the free and non-free electrolytes. When the difference between (m1 / m2 - n1 / n2) between the ratio m1 / m2 of vinylene carbonate (VC) to the total mass of all cyclic carbonate compounds other than VC in the non-free electrolyte and n1 / n2 of the ratio n1 / n2 of VC to the total mass of all cyclic carbonate compounds other than VC in the free electrolyte is greater than 0, the proportion of VC in the free electrolyte is consistently higher than that in the non-free electrolyte. This design maintains sufficient VC in the free electrolyte, allowing VC to leverage its performance advantages. Simultaneously, (m1 / m2) - (n1 / n2) is less than or equal to 0.2, ensuring that VC effectively participates in the SEI film repair process, which is beneficial for achieving a long cycle life of the battery. Furthermore, the positive electrode sheet is limited to include an intermediate coating, and the peel force S between the coating and the positive active material layer is limited to 10 N / m ≤ S ≤ 30 N / m. This ensures that the positive current collector and the positive active material have sufficient adhesion, suppressing the expansion, contraction, and delamination of the electrode sheet during cycling, thereby improving the cycle stability of the battery. At the same time, it also ensures electron transport between the positive current collector and the positive active material layer, avoiding high resistance caused by the introduction of the intermediate coating.

[0013] In some embodiments of this application, m1 and n1 satisfy 0.5≤m1≤5 and 0.2≤n1≤3.

[0014] In some embodiments of this application, the cyclic carbonate compound further includes at least one of propylene carbonate, ethylene carbonate, butyl carbonate, methyl vinylene carbonate, 1,2-dimethyl vinylene carbonate, ethyl vinylene carbonate, 1,2-diethyl vinylene carbonate, propyl vinylene carbonate, 1,2-dipropyl vinylene carbonate, phenyl vinylene carbonate, 1,2-diphenyl vinylene carbonate, vinyl ethylene carbonate, divinyl ethylene carbonate, fluoroethylene carbonate, difluoroethylene carbonate, and 3,3,3-trifluoropropylene carbonate.

[0015] In some embodiments of this application, the intermediate coating comprises inorganic particles, a conductive agent, and a binder;

[0016] And / or, the inorganic particles include at least one of ceramics, boehmite, aluminum oxide, titanium dioxide, magnesium oxide, zirconium oxide, and zinc oxide;

[0017] And / or, the conductive agent includes at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0018] And / or, the adhesive comprises polyacrylate;

[0019] And / or, the polyacrylate includes at least one of lithium polyacrylate, sodium polyacrylate, and magnesium polyacrylate.

[0020] In some embodiments of this application, the intermediate coating satisfies at least one of the following conditions:

[0021] The thickness of the intermediate coating is 1μm-20μm;

[0022] The adhesion between the intermediate coating and the positive electrode current collector is 260 N / m-400 N / m.

[0023] In a second aspect of this application, a method for preparing the secondary battery described in the first aspect is proposed, comprising the following steps of injecting an electrolyte:

[0024] The electrolyte used in the first injection is the first electrolyte, and the electrolyte used in the second injection is the second electrolyte. The first electrolyte and the second electrolyte independently include cyclic carbonate compounds, and the cyclic carbonate compounds include vinylene carbonate.

[0025] In the first electrolyte, the mass percentage of vinylene carbonate is M1%, and the total mass percentage of cyclic carbonate compounds is M2%; in the second electrolyte, the mass percentage of vinylene carbonate is N1%, and the total mass percentage of cyclic carbonate compounds is N2%.

[0026] M1, N1, M2, N2, and S satisfy the following:

[0027] 15N / m≤M1×S≤50N / m, 100N / m≤N1×S≤200N / m;

[0028] 0.1≤n1<M1<m1<N1≤14.

[0029] Secondary electrolyte injection flexibly controls the local distribution characteristics of the non-ionized and ionized electrolytes in the initial state of the battery by separating the electrolyte components required during battery formation and those required during cycling. Localized electrolyte distribution is achieved by controlling n1 < M1 < m1 < N1, while limiting the relationship between M1 and M2 and S ensures the stability of the intermediate coating on the positive electrode after cycling, avoiding negative electrode impedance degradation and significant capacity decay during battery cycles.

[0030] In some embodiments of this application, M1, N1, M2, N2, n1, and m1 satisfy at least one of the following conditions:

[0031] M1 / M2-N1 / N2<0;

[0032] 0.01≤M1 / M2≤0.2;

[0033] 0.1≤N1 / N2≤0.6.

[0034] In some embodiments of this application, the mass ratio of the first electrolyte to the second electrolyte is (6:4) to (8:2).

[0035] In some embodiments of this application, the sum of the injection coefficients of the first electrolyte and the second electrolyte is 2.5 g / Ah to 5 g / Ah.

[0036] In a third aspect, this application proposes an electrical device. According to an embodiment of this application, the electrical device includes the secondary battery described in the first aspect.

[0037] Therefore, the electrical equipment has all the advantages of the secondary battery, which will not be elaborated here.

[0038] Additional aspects and advantages of this application will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of this application. Detailed Implementation

[0039] The embodiments of this application are described in detail below, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout.

[0040] In one aspect of this application, a secondary battery is provided. According to an embodiment of this application, the secondary battery includes a positive electrode, a negative electrode, a separator, and an electrolyte;

[0041] The electrolyte includes a free electrolyte and a non-ionized electrolyte. The non-ionized electrolyte includes electrolyte in the pores of the positive electrode, electrolyte in the pores of the negative electrode, and electrolyte in the pores of the diaphragm. The free electrolyte includes electrolyte outside the pores of the positive electrode, electrolyte outside the pores of the negative electrode, and electrolyte outside the pores of the diaphragm.

[0042] The non-ionized electrolyte and the ionized electrolyte include cyclic carbonate compounds, including vinylene carbonate;

[0043] The free electrolyte contains vinylene carbonate at a mass percentage of m1%, and other cyclic carbonate compounds besides vinylene carbonate at a total mass percentage of m2%.

[0044] The non-ionized electrolyte contains vinylene carbonate at a mass percentage of n1%, and other cyclic carbonate compounds besides vinylene carbonate at a total mass percentage of n2%. The values ​​of m1, m2, n1, and n2 satisfy 0 < (m1 / m2) - (n1 / n2) ≤ 0.2.

[0045] The positive electrode sheet includes a positive current collector and an intermediate coating and a positive active material layer sequentially disposed on at least one side surface of the positive current collector. The peel force between the positive active material layer and the intermediate coating is S, and S satisfies 10N / m≤S≤30N / m.

[0046] This application employs localized electrolyte technology, which distinguishes between the non-ionized and ionized electrolytes within the battery cell. The non-ionized electrolyte exists within the material pores, directly contacting the electrodes and separator, and plays a dominant role in the formation and repair of the SEI film. It participates in the desolvation effect of active metal ions (such as lithium ions) and directly determines the SEI film impedance R in the battery. SEI and charge transfer impedance R ct The free electrolyte exists in the cell cavity outside the material pores, and there is no absolute physical barrier between it and the non-ionized electrolyte. When the non-ionized electrolyte is insufficient, it is automatically absorbed by the capillary action of the electrodes and separator to replenish it. When the content of a certain component in the non-ionized electrolyte is higher than that in the free electrolyte, the content of that component decreases after the non-ionized electrolyte absorbs the free electrolyte; conversely, when the content of a certain component in the non-ionized electrolyte is lower than that in the free electrolyte, the content of that component increases after the non-ionized electrolyte absorbs the free electrolyte. The electrolyte composition in the two regions exhibits dynamic differences during cycle consumption; therefore, the component content of the battery also changes dynamically during use.

[0047] This application specifies that the difference between the ratio of VC to the total mass of all cyclic carbonate compounds other than VC in the non-ionized electrolyte (m1 / m2) and the ratio of VC to the total mass of all cyclic carbonate compounds other than VC in the free electrolyte (n1 / n2), where (m1 / m2) - (n1 / n2) is greater than 0, ensures that the VC proportion advantage in the free electrolyte is consistently superior to that in the non-ionized electrolyte. This design maintains sufficient VC in the free electrolyte, allowing VC to exert its performance advantages. Simultaneously, an upper limit needs to be set for (m1 / m2) - (n1 / n2) because excessively large m1 / m2 or excessively small n1 / n2 means that VC cannot effectively participate in the SEI repair process, which is detrimental to achieving a long cycle life. Specifically, (m1 / m2) - (n1 / n2) can be 0.01, 0.05, 0.1, 0.15, 0.2, etc.

[0048] Furthermore, this application specifies that both non-free electrolytes and free electrolytes include cyclic carbonate compounds because they can undergo cross-linking copolymerization reactions with the polymers that function as binders in the positive electrode, repairing broken chains. Commonly used cyclic carbonate compounds include VC, ethylene carbonate (EC), fluoroethylene carbonate (FEC), and vinyl ethylene carbonate (VEC). Among them, EC and FEC are linearly linked to other monomers or polymer branches after ring opening; VEC is linked by external double bonds; and VC is directly linked by internal double bonds or linearly polymerized after ring opening. Analysis of the differences in polymer grafting structures shows that EC and FEC can significantly improve polymer flexibility and increase the density of the binder layer, but their stability and regrafting ability after chain breakage are poor; VEC can significantly improve polymer rigidity, which is beneficial for the construction of network molecules, but it is prone to brittle fracture with material expansion; while VC can balance the improvement of flexibility and rigidity, maintaining the mechanical strength of the binder layer while giving the polymer molecules good self-healing ability. Therefore, this application selects VC, which has the most balanced flexible and rigid structure of the cross-linked product, as the main additive.

[0049] This application further defines the positive electrode sheet as including a positive current collector and an intermediate coating and a positive active material sequentially disposed on at least one side surface of the positive current collector. Since the binder in the positive electrode sheet ages more rapidly with battery cycling, its binding effect on the internal stress of the particles decreases, leading to cycle degradation. Therefore, an intermediate coating is added to the positive electrode sheet, and its peel force S from the positive active material layer is limited to 10 N / m ≤ S ≤ 30 N / m. This improves the adhesion between the active material layer and the current collector, suppresses the expansion, contraction, and delamination of the electrode sheet during cycling, thereby improving the cycle stability of the battery. Simultaneously, it does not affect the electron transport of the current collector and the active material layer, avoiding excessively high electrode resistance. Specifically, S can be 10 N / m, 11 N / m, 12 N / m, 13 N / m, 14 N / m, 15 N / m, 16 N / m, 17 N / m, 18 N / m, 19 N / m, 20 N / m, 21 N / m, 22 N / m, 23 N / m, 24 N / m, 25 N / m, 26 N / m, 27 N / m, 28 N / m, 29 N / m, 30 N / m, etc.

[0050] According to embodiments of this application, m1 and n1 satisfy 0.5≤m1≤5 and 0.2≤n1≤3. If m1 and n1 are too high, the VC content in the free or non-free electrolyte is excessive, and VC continuously and extensively participates in the reduction reaction during cycling, causing R... SEI Rapid increases in concentration worsen cycle capacity. If m1 and n1 are too low, the VC content in the free or non-ionized electrolyte is insufficient. During cycling, VC cannot effectively repair the positive electrode binder layer, leading to the stripping of the positive electrode active material after long cycles, thus affecting cycle performance. Specifically, m1 can be 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, etc., and n1 can be 0.2, 0.5, 1, 1.5, 2, 2.5, 3, etc.

[0051] According to embodiments of this application, the type of cyclic carbonate compound is not particularly limited. In addition to the VC that is always included, it may also be selected from propylene carbonate (PC), ethylene carbonate (EC), butyl carbonate (BC), methyl vinylene carbonate, 1,2-dimethyl vinylene carbonate, ethyl vinylene carbonate, 1,2-diethyl vinylene carbonate, propyl vinylene carbonate, 1,2-dipropyl vinylene carbonate, phenyl vinylene carbonate, 1,2-diphenyl vinylene carbonate, vinyl ethylene carbonate (VEC), and divinyl ethylene carbonate (DVEC), as well as fluorinated derivatives of the above compounds, such as fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), 3,3,3-trifluoropropylene carbonate, etc.

[0052] According to embodiments of this application, the composition of the intermediate coating is not particularly limited, as long as its peel strength from the positive electrode active material layer is within a defined range. As some specific examples, the intermediate coating includes inorganic particles, a conductive agent, and a binder to achieve a strong peel strength between the intermediate coating and the positive electrode active material layer.

[0053] According to some specific embodiments of this application, the inorganic particles include, but are not limited to, ceramics, boehmite, aluminum oxide, titanium dioxide, magnesium oxide, zirconium oxide, and zinc oxide, thereby improving the tensile strength and mechanical strength of the electrode.

[0054] According to some specific embodiments of this application, the conductive agent includes, but is not limited to, superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers, thereby ensuring a good electronic channel between the current collector, intermediate coating, and active material layer, achieving a low impedance effect.

[0055] According to some specific embodiments of this application, the binder can be selected from polyacrylates, including but not limited to lithium polyacrylate, sodium polyacrylate, and magnesium polyacrylate. These binders provide abundant carboxyl groups, which bond tightly with the active material, conductive agent, and hydroxyl groups on the current collector surface through hydrogen bonding. Polyacrylates are almost non-swellable in carbonate electrolyte solvents and have good chemical stability; polyacrylates have excellent deformation flexibility, but insufficient electronic conductivity and mechanical strength. When used alone, they can hinder electron transport from the current collector foil to the active material layer, and are prone to chain breakage and decomposition due to deformation of the positive electrode active material layer during charging and discharging. Therefore, mixing with conductive agents provides a good electronic channel between the current collector, the undercoat layer, and the active material layer, achieving a low impedance effect; and mixing with inorganic particles gives the overall membrane high tensile strength, improving mechanical strength and avoiding the risk of current collector deformation and breakage due to phase transition of the positive electrode active material during battery cycling.

[0056] According to some specific embodiments of this application, the thickness of the intermediate coating is not particularly limited, and those skilled in the art can select it as appropriate. As some specific examples, the thickness of the intermediate coating is 1μm-20μm, such as 1μm, 2μm, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, 11μm, 12μm, 13μm, 14μm, 15μm, 16μm, 17μm, 18μm, 19μm, 20μm, etc.

[0057] According to some specific embodiments of this application, the adhesion force between the intermediate coating and the positive current collector is 260 N / m-400 N / m. As some specific examples, the adhesion force between the intermediate coating and the positive current collector can be 260 N / m, 270 N / m, 280 N / m, 290 N / m, 300 N / m, 310 N / m, 320 N / m, 330 N / m, 340 N / m, 350 N / m, 360 N / m, 370 N / m, 380 N / m, 390 N / m, 400 N / m, etc.

[0058] According to some specific embodiments of this application, during battery charging and discharging, active ions repeatedly insert and extract between the positive and negative electrode plates. The electrolyte conducts ions between the positive and negative electrode plates. A separator is disposed between the positive and negative electrode plates to prevent short circuits while allowing ions to pass through.

[0059] According to some specific embodiments of this application, the secondary battery is a lithium-ion secondary battery.

[0060] According to some specific embodiments of this application, the positive electrode sheet includes a positive current collector and an intermediate coating and a positive active material layer sequentially disposed on at least one side surface of the positive current collector. The positive active material layer includes a positive active material and a lithium supplement agent. Preferably, the positive active material is lithium iron phosphate.

[0061] According to some specific embodiments of this application, the positive electrode current collector may include a metal foil or a composite positive electrode current collector. For example, the metal foil may be aluminum foil. The composite positive electrode current collector may include a polymer material substrate and a metal layer formed on at least one surface of the polymer material substrate. For example, the composite negative electrode current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, etc.) on a polymer material substrate (such as polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), etc.).

[0062] According to some specific embodiments of this application, the positive electrode active material layer may optionally include a conductive agent. As an example, the conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0063] According to some specific embodiments of this application, the positive electrode active material layer may optionally include a binder. As an example, the binder may include at least one selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0064] According to some specific embodiments of this application, the positive electrode sheet can be prepared in the following manner: the components used to prepare the positive electrode sheet, such as inorganic particles, conductive agents and binders, are dispersed in a solvent (e.g., N-methylpyrrolidone, NMP) to form an intermediate coating slurry; the positive electrode active material, lithium supplementer, conductive agent and binder are dispersed in a solvent (e.g. NMP) to form a positive electrode active material slurry; the intermediate coating slurry and the positive electrode active material slurry are sequentially coated on the positive electrode current collector, and after drying, cold pressing and other processes, the positive electrode sheet can be obtained.

[0065] According to some specific embodiments of this application, the negative electrode sheet includes a negative current collector and a negative active material layer disposed on at least one side surface of the negative current collector, the negative active material layer including a negative active material.

[0066] According to some specific embodiments of this application, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer material substrate and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), etc.).

[0067] According to some specific embodiments of this application, the negative electrode active material may be a negative electrode active material known in the art for use in batteries. As an example, the negative electrode active material may include at least one of the following materials: graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate, etc. Silicon-based materials may include at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. Tin-based materials may include at least one of elemental tin, tin oxide compounds, and tin alloys.

[0068] According to some specific embodiments of this application, the negative electrode active material layer may optionally include a binder. The binder may include at least one selected from styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0069] According to some specific embodiments of this application, the negative electrode active material layer may optionally include a conductive agent. The conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0070] According to some specific embodiments of this application, the negative electrode active material layer may also optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0071] According to some specific embodiments of this application, the negative electrode sheet can be prepared by dispersing the above-mentioned components for preparing the negative electrode sheet, such as negative electrode active material, conductive agent, binder and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto the negative electrode current collector, and after drying, cold pressing and other processes, the negative electrode sheet can be obtained.

[0072] According to some specific embodiments of this application, there are no particular limitations on the type of separator membrane; any known porous separator membrane with good chemical and mechanical stability can be selected. In some embodiments of this application, the material of the separator membrane may include at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, or polyvinylidene fluoride.

[0073] According to some specific embodiments of this application, the positive electrode sheet, the negative electrode sheet, and the separator can be fabricated into an electrode assembly by a winding process or a stacking process.

[0074] According to some specific embodiments of this application, the aforementioned secondary battery may include an outer packaging. This outer packaging can be used to encapsulate the aforementioned electrode assembly and electrolyte.

[0075] According to some specific embodiments of this application, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer packaging of the secondary battery can also be a soft pack, such as a pouch-type soft pack. The material of the soft pack can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0076] In a second aspect, this application proposes a method for preparing the secondary battery described in the first aspect. According to an embodiment of this application, the preparation method includes the following step of injecting an electrolyte:

[0077] A second injection is performed after the first injection has been completed;

[0078] The electrolyte used in the first injection is the first electrolyte, and the electrolyte used in the second injection is the second electrolyte. The first electrolyte and the second electrolyte independently include cyclic carbonate compounds, and the cyclic carbonate compounds include vinylene carbonate.

[0079] In the first electrolyte, the mass percentage of vinylene carbonate is M1%, and the total mass percentage of cyclic carbonate compounds is M2%; in the second electrolyte, the mass percentage of vinylene carbonate is N1%, and the total mass percentage of cyclic carbonate compounds is N2%.

[0080] The terms M1, N1, M2, N2, n1, m1, and S satisfy:

[0081] 15N / m≤M1×S≤50N / m, 100N / m≤N1×S≤200N / m;

[0082] 0.1≤n1<M1<m1<N1≤14.

[0083] This application employs a secondary electrolyte injection method to prepare the aforementioned secondary battery. By separating the electrolyte components required for the battery formation stage and the electrolyte components required for the cycling stage, the local distribution characteristics of the non-ionized and ionized electrolyte in the initial state of the battery can be flexibly controlled. Research shows that the local distribution characteristics of the electrolyte in the initial state determine the local distribution characteristics of the electrolyte in the subsequent operating state: assuming sufficient total electrolyte volume and smooth transport, the local distribution ratio of additives in the electrolyte in the initial state can qualitatively determine the local distribution ratio of additives in the electrolyte in the subsequent operating state.

[0084] This application employs a two-stage electrolyte injection process and limits n1 < M1 < m1 < N1 in the second stage of electrolyte injection, thereby achieving localized distribution of the electrolyte. Simultaneously, controlling the upper limit of N1 ensures good fluidity of the second electrolyte, preventing it from failing to penetrate the cell cavity.

[0085] Furthermore, since the intermediate coating and active material layer contain binders, their aging will accelerate with the cycle use of the battery, causing cycle deterioration. The abundant cyclic carbonate compounds in the electrolyte can undergo cross-linking copolymerization with the polymer that plays a binding role in the positive electrode, thereby repairing the broken chains. Among the cyclic carbonate compounds studied, the cross-linking product of VC has the most balanced flexible and rigid structure. Therefore, combined with the peeling force provided by the intermediate coating of the positive electrode, the localization content of the main additive VC is designed. The method is to control the VC content M1 and M2 in the first and second electrolytes with the relationship with S, that is, 15N / m≤M1×S≤50N / m, 100N / m≤N1×S≤200N / m. When S is low, M1 and M2 tend to have high content, and conversely, when S is high, M1 and M2 tend to have low content. When M1×S and N1×S are too low, the stability of the positive electrode undercoating cannot be maintained after cycling, and the technical effect cannot be achieved. When M1×S and N1×S are too high, the positive electrode impedance is severely degraded, resulting in an increase in the positive electrode potential and a decrease in the negative electrode potential during charging and discharging. At this time, the decomposition of the electrolyte at the positive and negative electrodes is intensified, and the ion channel is broken during cycling, causing a sharp decline in cycle capacity. M1×S can be 15N / m, 20N / m, 25N / m, 30N / m, 35N / m, 40N / m, 45N / m, 50N / m, etc., and N1×S can be 100N / m, 110N / m, 120N / m, 130N / m, 140N / m, 150N / m, 160N / m, 170N / m, 180N / m, 190N / m, 200N / m, etc.

[0086] According to embodiments of this application, the first electrolyte and the second electrolyte independently include cyclic carbonate compounds, and the cyclic carbonate compounds include vinylene carbonate. That is, the types of cyclic carbonate compounds in the first electrolyte and the second electrolyte may be the same or different, but both should contain vinylene carbonate.

[0087] According to an embodiment of the present application, M1, N1, M2, and N2 satisfy at least one of the following conditions: M1 / M2 - N1 / N2 < 0; 0.01 ≤ M1 / M2 ≤ 0.2; 0.1 ≤ N1 / N2 ≤ 0.6. Compared with the used battery cell, the intermediate coating in the positive electrode of the dry battery cell is not affected by cycling and the bonding force does not decrease. Therefore, the first electrolyte (i.e., the first injection electrolyte) is mainly used for forming SEI on the negative electrode and filling pores. The ratio of VC to cyclic carbonates M1 / M2 does not need to be particularly high. Cyclic carbonate compounds such as EC, VC, VEC, and FEC can meet the requirement of stabilizing the initial SEI. Therefore, the ratio M1 / M2 in the first injection is set within a specified range in combination with the stripping force of the positive electrode. The second electrolyte (i.e., the second injection electrolyte) has the ability to repair the aged positive electrode binder and the intermediate coating, which determines whether the battery can work stably for a long time. Therefore, a high content of VC, N1, is necessary. At the same time, since the second electrolyte also needs to play a role in maintaining long cycling, the unconsumed part of the first electrolyte after formation and the second electrolyte are fused together to form the free electrolyte in the initial state. Only when there is sufficient cyclic carbonate in the free electrolyte can the functions of SEI repair and ion channels be ensured. Therefore, the ratio N1 / N2 is set within the above range and the size relationship of M1 / M2 < N1 / N2 is determined.

[0088] Specifically, M1 / M2 can be 0.01, 0.05, 0.1, 0.15, 0.2, etc. N1 / N2 can be 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, etc.

[0089] According to an embodiment of the present application, the mass ratio of the first electrolyte to the second electrolyte is (6:4) to (8:2). As some specific examples, the mass ratio of the first electrolyte to the second electrolyte can be 6:4, 7:3, 8:2, etc. This can improve the cycling performance of the battery. The first injection (the first liquid injection) is mainly used for formation to form the initial SEI layer, and the second injection (the second liquid injection) is mainly used to supplement the electrolyte amount required for cycling. Therefore, the mass of the first electrolyte needs to satisfy the requirement of fully wetting the positive and negative electrode plates, and the mass of the second electrolyte needs to be able to meet the consumption required for cycling. When the mass ratio of the first electrolyte to the second electrolyte is lower than 6:4, the liquid injection amount during the first injection is not sufficient to fully wet, and the obtained SEI protection layer during formation is uneven. After the second injection, the area without SEI formation and the electrolyte are electronically conductive, resulting in the decomposition and accumulation of by-products of the electrolyte. If the mass ratio of the first electrolyte to the second electrolyte is too high, that is, greater than 8:2, the proportion of the second injection is low, and the composition of the free electrolyte mainly comes from the unconsumed electrolyte component of the first injection, which is basically the same as the non-free electrolyte component. At such a ratio, the second injection and the traditional first injection are basically no different, and it is difficult to achieve the design of localized electrolyte.

[0090] According to embodiments of this application, the sum of the injection coefficients of the first electrolyte and the second electrolyte is 2.5 g / Ah to 5 g / Ah. As some specific examples, the sum of the injection coefficients of the first electrolyte and the second electrolyte can be 2.5 g / Ah, 3 g / Ah, 3.5 g / Ah, 4 g / Ah, 4.5 g / Ah, 5 g / Ah, etc., preferably 3.5 g / Ah to 4.5 g / Ah. Specifically, the injection coefficient (injection volume / cell capacity) represents the electrolyte requirement. For energy storage batteries, a higher injection coefficient can maintain a long cycle life. This application limits the sum of the injection coefficients of the two injections to a suitable range, which can achieve sufficient wetting of the separator and electrode, and avoid electrolyte swelling due to excessive electrolyte volume, or local lithium plating caused by accumulation at the bottom of the battery.

[0091] According to a specific embodiment of this application, the preparation method can be carried out through the following steps: first electrolyte injection, static formation, second electrolyte injection, high-temperature aging, and capacity testing to obtain the secondary battery. The static aging and formation processes are not particularly limited, as long as sufficient wetting and uniform film formation are ensured. The battery can be fully charged and disassembled before the second electrolyte injection and before capacity testing. Visible electrolyte distribution at all positions of the positive and negative electrodes and the separator, and no abnormal spots or dark marks on the negative electrode, meet the requirements. The full charge process can be a constant current charge at 0.2C to 3.65V.

[0092] In a third aspect, this application proposes an electrical device. According to an embodiment of this application, the electrical device includes the secondary battery described in the first aspect.

[0093] According to some specific embodiments of this application, the secondary battery can be used as a power source for the electrical device, or as an energy storage unit for the electrical device. The aforementioned electrical device can be, but is not limited to, mobile phones, tablets, laptops, electric toys, power tools, electric vehicles, electric cars, ships, spacecraft, etc. Among them, electric toys can include stationary or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys, etc., and spacecraft can include airplanes, rockets, space shuttles, and spacecraft, etc.

[0094] The embodiments of this application are described in detail below. It should be noted that the embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. In addition, unless otherwise specified, all reagents used in the following embodiments are commercially available or can be synthesized according to the methods described herein or known methods. For reaction conditions not listed, they are also readily available to those skilled in the art.

[0095] The batteries in each embodiment and comparative example were prepared according to the following method:

[0096] (1) Preparation of the positive electrode sheet:

[0097] Alumina, conductive carbon black (SP), and polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 90:5:5, and an appropriate amount of solvent NMP was added. The mixture was stirred for different times to obtain a coating slurry with a solid content of approximately 65±5%. This slurry was then applied to the surface of the positive electrode current collector aluminum foil and dried to form a 3μm thick coating. The adhesion between the coating and the current collector was 300±50 N / m. This step was omitted for experiments without a coating design. Positive electrode sheets with different peel strengths were prepared using coating slurries stirred for different times, and the peel strength of the positive electrode sheets was tested to obtain the design parameter S (N / m) in Table 1. In the experiments in Tables 2 and 3, the same coating design as in Examples 1-2 of Table 1 was used.

[0098] Lithium iron phosphate (LiFePO4), SP, and PVDF binder were dispersed in NMP solvent at a mass ratio of 97:2.5:0.5 and mixed evenly to obtain a positive electrode slurry. The positive electrode slurry was then coated onto the aforementioned current collector aluminum foil, with a coating weight of 33 mg / cm² per unit area. 2 After drying, cold pressing, slitting, and cutting, positive electrode sheets are obtained.

[0099] Among them, Comparative Examples 1-1 and 1-2 in Table 1 have no positive electrode intermediate coating, while the Examples in Table 1, the Examples and Comparative Examples in Table 2, and all Examples in Table 3 have a positive electrode intermediate coating.

[0100] (2) Preparation of negative electrode sheet:

[0101] The negative electrode active material, artificial graphite, conductive carbon black (SP), thickener (CMC), and binder (SBR) were dispersed in deionized water at a mass ratio of 96.5:0.5::2 and mixed evenly to obtain a negative electrode slurry. The negative electrode slurry was then coated onto the negative electrode current collector copper foil, with a coating weight of 16 mg / cm² per unit area. 2 After drying, cold pressing, slitting, and cutting, negative electrode sheets are obtained.

[0102] (3) Preparation of electrolyte:

[0103] In an argon atmosphere glove box with moisture and oxygen content ≤0.1ppm, cyclic carbonate compounds and 12.5% ​​LiPF6 by mass were mixed according to the designed proportions in Table 1-3. Ethyl methyl carbonate (EMC) was added to supplement the remaining solvent components, and the mixture was stirred until completely dissolved. For test groups requiring secondary electrolyte injection, the first and second injection electrolytes were prepared using the above method.

[0104] (4) Preparation of the diaphragm:

[0105] A 16μm polyethylene film was used as the diaphragm;

[0106] (5) Battery assembly:

[0107] The prepared positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to separate them. The electrodes are then wound into a bare cell, and after welding the tabs, the cell is assembled into the outer packaging. After hot pressing, it undergoes vacuum drying. Then, the electrolyte injection process is carried out: the first injection of electrolyte is followed by encapsulation, settling, and formation of the cell; the second injection of electrolyte is followed by high-temperature settling and capacity testing of the cell. Finally, a soft-pack lithium-ion battery with a capacity of 3.2Ah is prepared.

[0108] Among them, Comparative Example 2-1 in Table 2 adopts a single electrolyte injection process: injecting electrolyte, encapsulating the cell, allowing it to stand, forming, allowing it to stand at high temperature, and testing its capacity, and finally preparing a soft-pack lithium-ion battery with a capacity of 3.2Ah. Except for the second electrolyte injection process, the other processes are consistent.

[0109] The formation of both the first and second injections involves the following steps:

[0110] (1) The battery cell was left to stand in a 45°C formation cabinet for 10 minutes and then charged at a 0.1C rate for 7 minutes to reach 1.17% SOC;

[0111] (2) Let stand for 3 minutes, then charge at 0.2C rate to 30% SOC.

[0112] In all test groups in Table 1, the examples in Table 2, Comparative Examples 2-2 and 2-3, the fixed injection coefficient was 4.0 g / Ah; the injection ratio for the first and second injections was 7:3; the standing conditions after the first injection were 45±5℃ for 24 hours, and the formation temperature was 45℃; the standing conditions (high-temperature aging) after the second injection were 45±5℃ for 24 hours.

[0113] Table 3 lists the injection coefficient and the injection ratio for the first and second injections. The temperature and time parameters for the second injection are consistent with those in Tables 1 and 2. The electrolyte formulations and positive electrode coating designs for Examples 3-1 to 3-7 and Comparative Examples 3-1 to 3-3 are consistent with those for Examples 1-2. The electrolyte formulations and positive electrode coating designs for Examples 3-8 are consistent with those for Examples 2-15.

[0114] The batteries prepared in the examples and comparative examples were subjected to the following tests:

[0115] (1) Positive electrode peeling force test:

[0116] In accordance with the requirements of GB-T2790-1995 "Test Method for 180° Peel Strength of Adhesives - Flexible Materials vs. Rigid Materials", the fully dried electrode is fixed in the clamp of the tensile testing machine. One end of the adhesive tape is attached to the electrode, and the other end is folded 180° and fixed in the lower clamp of the tensile testing machine. The electrode is pulled apart at the test rate specified in the above standard, and the peel force (N / m) is measured by the force required to continuously peel the tape from the electrode.

[0117] (2) Analysis of the composition of the free electrolyte:

[0118] Discharge the battery with its outer packaging to 0% SOC (2.5V) using a 0.05C current. Cut a notch of about 0.5cm on the right side of the battery facing the positive terminal and cut the same notch on the right side facing the negative terminal, ensuring that the two notches are diagonally opposite. Pour the free electrolyte into the fluoride bottle through the small opening on the positive terminal side until intermittent droplets flow down, and time for 30 seconds.

[0119] Gas chromatography-mass spectrometry (GC-MS) was used to test and collect the types and relative mass contents of vitamin C and other organic components in the electrolyte.

[0120] The relative mass content of lithium salts in the collected electrolyte was determined using ion chromatography (IC).

[0121] (3) Testing of non-ionized (porous) electrolyte:

[0122] Remove the outer packaging, take out the individual bare cells, quickly peel off the electrode sheets, put them into an aluminum-plastic bag, add a sufficient amount of acetonitrile solvent to ensure that the electrode sheets are completely immersed, seal the aluminum-plastic bag containing the acetonitrile solvent and electrode sheets, and let it stand at 35±10℃ for 3 days.

[0123] GC-MS was used to collect the types and relative mass contents of vitamin C and other organic components in the electrolyte.

[0124] (4) Cyclic performance test:

[0125] Charge the battery at 25°C to 3.65V with a constant power of 0.5P, then discharge it to 2.5V with a constant power of 0.5P. Record the capacity of the Xth charge and discharge cycle as CCx and DCx, respectively, until X = 1000. Record the capacity value of each cycle.

[0126] Battery cycle life is evaluated by the cycle capacity retention rate λ, where λ = DC. 1000 / DC3.

[0127] Table 1

[0128]

[0129] Continued from Table 1

[0130]

[0131]

[0132] Table 1 evaluates the impact of the localized distribution characteristics of VC and EC, as key components in the electrolyte, on battery cycle performance. Comparisons 1-1 and 1-2 show that secondary electrolyte injection is required to ensure the electrolyte meets the localized design parameter m1 / m2 > n1 / n2. When this parameter is not met, the cell cycle performance is poor. This is because (m1 / m2) - (n1 / n2) greater than 0 ensures that the proportion of VC in the ionized electrolyte is consistently higher than that in the non-ionized electrolyte. This design maintains sufficient VC in the ionized electrolyte; otherwise, the VC in the ionized electrolyte would be insufficient to primarily repair the adhesion between the positive electrode and the undercoat.

[0133] Comparison of Comparative Examples 1-2 and Example 1-1 shows that a positive electrode undercoat is needed to ensure that the peel strength of the active material layer on the electrode matches the electrolyte composition. Without a positive electrode undercoat, the S value is low (S is 4 N / m in Comparative Examples 1-2 and 15 N / m in Example 1-1), and M1×S and N1×S also exceed the lower limit of the specified range, resulting in poor cell cycle performance. This is because the binder in the positive electrode ages more rapidly with battery cycling, reducing its binding effect on the internal stress of the particles, leading to insufficient adhesion between the active material layer and the current collector, causing problems such as electrode expansion, contraction, and delamination.

[0134] As can be seen from Examples 1-1 to 1-4 and Comparative Examples 1-3 to 1-5, the localization design of VC and EC in the electrolyte (m1 / m2)-(n1 / n2) is greater than 0, which can effectively improve cycle performance. By adjusting the peeling force S of the electrode active material layer and the VC content M1 and M2 in the first and second injections, making M1×S and N1×S fall within the limited range, the value of the localization design (m1 / m2)-(n1 / n2) can be affected. Although M1, M2, and S individually take conventional values, they cannot satisfy the relationship in the formula, which means that the localization electrolyte design cannot be achieved, resulting in poor cycle performance. In Comparative Example 1-4, (m1 / m2)-(n1 / n2) is too large, which means that VC cannot effectively participate in the SEI repair process, and the cycle retention rate is not as good as in Examples 1-1 and 1-2. In Comparative Examples 1-5, excessively high S affects electron transport in the current collector and active material layer, which in turn leads to excessively high resistance of the positive electrode film, an increase in the positive electrode potential and a decrease in the negative electrode potential during charging and discharging. At this time, the decomposition of the electrolyte at the positive and negative electrodes is intensified, and the ion channel is broken during cycling, resulting in a sharp decline in cycling capacity.

[0135] Table 2

[0136]

[0137] Continued from Table 2

[0138]

[0139]

[0140] Among them, " / " indicates non-existence.

[0141] Table 2 tests the influence of the characteristics of the localized distribution of cyclic carbonate compounds on the battery cycle performance under the condition that the stripping force S of the active material layer of the positive electrode tab remains unchanged. The added amount of VC in Comparative Example 2-1 and Comparative Example 2-2 is 3.0% after being converted according to the injection coefficient and injection ratio. Comparative Example 2-1 shows that the primary injection process widens the gap between the free electrolyte and the non-free electrolyte in terms of VC. The electrolyte satisfies the localized design parameter m1 / m2 < n1 / n2, and the cycle performance of the battery cell is poor. Comparative Example 2-2 shows that in the secondary injection process, but all of the VC is placed in the first-injected electrolyte, and the proportion of VC in the free electrolyte is insufficient. Similarly, m1 / m2 < n1 / n2, and the cycle performance of the battery cell is poor. In Comparative Example 2-3, no VC is used in either the first-injected or second-injected electrolyte. Although the alternative additive FEC is introduced, there is an obvious gap in the cycle capacity retention rate compared with the VC system. It can be inferred that VC plays an irreplaceable key role in long-term cycling.

[0142] It can be found from Comparative Example 1-2, Comparative Examples 2-4 to 2-14, Examples 2-1 to 2-8, and Example 2-16 that by adjusting the contents of VC and EC in the electrolyte of the secondary injection, the localized differential distribution of VC and EC in the free state and the non-free state can be achieved. In Comparative Examples 2-4, 2-5, 2-12, and 2-14, m1 / m2 < n1 / n2. In Comparative Example 2-13, (m1 / m2) - (n1 / n2) > 0.2, and the cycle performance is poor. In Comparative Examples 2-6 and 2-8, there are problems of excessive or insufficient m1 or n1, and the cycle performance is poor. This is because if m1 and n1 are too high, the content of VC in the free or non-free electrolyte is too much, and VC continuously and massively participates in the reduction reaction during cycling, resulting in a sharp increase in R SEI which deteriorates the cycle capacity. Or if m1 and n1 are too low, the content of VC in the free or non-free electrolyte is insufficient, and VC cannot play the role of repairing the positive electrode bonding layer during cycling, resulting in the stripping of the positive electrode active material after long-term cycling, thereby affecting the cycle performance.

[0143] Comparative studies of Examples 2-9 to 2-14 reveal that adjusting the type and content of cyclic carbonates in the electrolyte of the secondary injection can also affect the localized distribution characteristics of the electrolyte. From the content of VC and other cyclic carbonates in the free and non-free states, it can be observed that there is no absolute physical barrier between the free and non-free electrolytes. The unconsumed portion of the first injection's electrolyte merges with the second injection's electrolyte to form the initial free electrolyte state. Similarly, when the non-free electrolyte is insufficient, it automatically absorbs free electrolyte as a supplement. Therefore, components present only in the first or second injection can coexist in both the free and non-free electrolytes, with only localized differences in content. Besides VC, using two or more different cyclic carbonates, such as PC, FEC, and VEC mixed with EC, does not affect the application of the principles of this application, thus having a universal effect on improving cycle life.

[0144] Examples 2-14 and 2-15 follow the technical requirements for localized electrolyte design and positive electrode undercoat to achieve the design M1×S and M2×S, with a maximum ideal effect of over 90%.

[0145] Table 3

[0146]

[0147]

[0148] Continued from Table 3

[0149]

[0150] Table 3 shows the effects of the secondary electrolyte injection coefficient and injection mass ratio on the localization distribution of cyclic carbonate compounds in the free and non-free electrolytes, as well as their impact on cycle performance.

[0151] In Comparative Example 3-1, the injection mass ratio was too low, and the separator and the electrode sheet could not be fully wetted, resulting in a lack of film formation protection in the local area of the electrode sheet that did not enter the electrolyte during the formation stage, manifested as m1 / m2 < n1 / n2; in Comparative Example 3-2, the battery swelled due to excessive electrolyte volume, and the redundant electrolyte would accumulate at the bottom of the battery, causing lithium deposition. At this time, (m1 / m2) - (n1 / n2) exceeded the upper limit requirement. Considering the influence on the actual cycle capacity retention rate, the injection coefficient of the sum of the first injection electrolyte and the second injection electrolyte was limited to 2.5 g / Ah to 5 g / Ah. In Comparative Example 3-3, since the injection volume ratio of the first and second injections was too low, that is, less than 6:4, the injection volume during the first injection was not sufficient to fully wet, and the SEI protection layer obtained during formation was uneven. After the second injection, the area without SEI formation was electronically conductive with the electrolyte, resulting in the decomposition and accumulation of by-products of the electrolyte; in Comparative Example 3-4, the injection volume ratio of the first and second injections was too high, that is, greater than 8:2, and the proportion of the second injection was low. The free electrolyte mainly came from the unconsumed VC and EC in the first injection, and there was no significant difference in the content of the non-free electrolyte. In such a ratio, the second injection was basically no different from the traditional first injection, and it was difficult to achieve the design of the localized electrolyte. Therefore, the mass ratio range of the electrolytes for the first injection and the second injection was limited to 6:4 to 8:2.

[0152] As can be seen from Examples 3-1 to 3-6, within the appropriate injection coefficient and injection mass ratio range, the influence degree of the lithium salt distribution in the non-free electrolyte and the free electrolyte is very small and can be ignored, and the solution provided by this application can still be realized. The injection coefficient is preferably 3.5 g / Ah to 4.5 g / Ah.

[0153] Based on Example 2-15, Example 3-7 with refined adjustment of the injection coefficient and the mass ratio of the second injection further improved the thermal runaway test safety performance and the cycle capacity retention rate, up to 93%. Since the test group was an electrolyte system without using additives of other functional group types, the long cycle life of the lithium-ion battery could be achieved at low cost; when additives of other functional group types were further introduced into the electrolyte of the test group, it was not mutually exclusive with the solution of this application, so it could be predicted that the cycle performance of the battery could be further improved.

[0154] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0155] Although embodiments of this application have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting this application. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of this application.

Claims

1. A secondary battery, characterized in that, The secondary battery includes a positive electrode, a negative electrode, a separator, and an electrolyte; The electrolyte includes a free electrolyte and a non-free electrolyte, and the non-free electrolyte includes electrolyte in the pores of the positive electrode, electrolyte in the pores of the negative electrode, and electrolyte in the pores of the diaphragm. The non-ionized electrolyte and the ionized electrolyte include cyclic carbonate compounds, including vinylene carbonate; In the free electrolyte, the mass percentage of vinylene carbonate is m1%, and the total mass percentage of other cyclic carbonate compounds other than vinylene carbonate is m2%. The non-ionized electrolyte contains vinylene carbonate at a mass percentage of n1%, and other cyclic carbonate compounds besides vinylene carbonate at a total mass percentage of n2%; m1, m2, n1, and n2 satisfy 0 < (m1 / m2) - (n1 / n2) ≤ 0.2; The positive electrode sheet includes a positive current collector and an intermediate coating and a positive active material layer sequentially disposed on at least one surface of the positive current collector. The peel force between the positive active material layer and the intermediate coating is S, wherein S satisfies 10 N / m ≤ S ≤ 30 N / m. The values ​​of m1 and n1 satisfy 0.5 ≤ m1 ≤ 5 and 0.2 ≤ n1 ≤ 3. The method for preparing the secondary battery includes the following steps: injecting electrolyte. A second injection is performed after the first injection has been completed; The electrolyte used in the first injection is the first electrolyte, and the electrolyte used in the second injection is the second electrolyte. The first electrolyte and the second electrolyte independently include cyclic carbonate compounds, and the cyclic carbonate compounds include vinylene carbonate. In the first electrolyte, the mass percentage of vinylene carbonate is M1%, and the total mass percentage of cyclic carbonate compounds is M2%; in the second electrolyte, the mass percentage of vinylene carbonate is N1%, and the total mass percentage of cyclic carbonate compounds is N2%. The terms M1, N1, M2, N2, n1, m1, and S satisfy: 15 N / m≤M1×S≤50 N / m, 100 N / m≤N1×S≤200 N / m; 0.1≤n1<M1<m1<N1≤14, M1 / M2-N1 / N2<0; 0.01≤M1 / M2≤0.2; 0.1≤N1 / N2≤0.6, The mass ratio of the first electrolyte to the second electrolyte is (6:4) to (8:2); The sum of the injection coefficients of the first electrolyte and the second electrolyte is 2.5 g / Ah to 5 g / Ah.

2. The secondary battery according to claim 1, characterized in that, The cyclic carbonate compounds further include at least one of the following: propylene carbonate, ethylene carbonate, butyl carbonate, methyl vinylene carbonate, 1,2-dimethyl vinylene carbonate, ethyl vinylene carbonate, 1,2-diethyl vinylene carbonate, propyl vinylene carbonate, 1,2-dipropyl vinylene carbonate, phenyl vinylene carbonate, 1,2-diphenyl vinylene carbonate, vinyl ethylene carbonate, divinyl ethylene carbonate, fluoroethylene carbonate, difluoroethylene carbonate, and 3,3,3-trifluoropropylene carbonate.

3. The secondary battery according to claim 1, characterized in that, The intermediate coating comprises inorganic particles, a conductive agent, and a binder; And / or, the inorganic particles include at least one of ceramics, boehmite, aluminum oxide, titanium dioxide, magnesium oxide, zirconium oxide, and zinc oxide; And / or, the conductive agent includes at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. And / or, the adhesive comprises polyacrylate; And / or, the polyacrylate includes at least one of lithium polyacrylate, sodium polyacrylate, and magnesium polyacrylate.

4. The secondary battery according to claim 1, characterized in that, The intermediate coating satisfies at least one of the following conditions: The thickness of the intermediate coating is 1 μm-20 μm; The adhesion between the intermediate coating and the positive electrode current collector is 260 N / m-400 N / m.

5. A method for preparing a secondary battery according to any one of claims 1-4, characterized in that, The following steps are included in the electrolyte injection process: A second injection is performed after the first injection has been completed; The electrolyte used in the first injection is the first electrolyte, and the electrolyte used in the second injection is the second electrolyte. The first electrolyte and the second electrolyte independently include cyclic carbonate compounds, and the cyclic carbonate compounds include vinylene carbonate. In the first electrolyte, the mass percentage of vinylene carbonate is M1%, and the total mass percentage of cyclic carbonate compounds is M2%; in the second electrolyte, the mass percentage of vinylene carbonate is N1%, and the total mass percentage of cyclic carbonate compounds is N2%. The terms M1, N1, M2, N2, n1, m1, and S satisfy: 15 N / m≤M1×S≤50 N / m, 100 N / m≤N1×S≤200 N / m; 0.1≤n1<M1<m1<N1≤14.

6. The preparation method according to claim 5, characterized in that, M1, N1, M2, and N2 satisfy at least one of the following conditions: M1 / M2-N1 / N2<0; 0.01≤M1 / M2≤0.2; 0.1≤N1 / N2≤0.

6.

7. The preparation method according to claim 5, characterized in that, The mass ratio of the first electrolyte to the second electrolyte is (6:4) to (8:2).

8. The preparation method according to claim 5, characterized in that, The sum of the injection coefficients of the first electrolyte and the second electrolyte is 2.5 g / Ah to 5 g / Ah.

9. An electrical appliance, characterized in that, The electrical equipment includes the secondary battery as described in any one of claims 1-4.

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