Battery cells, secondary batteries and electrical equipment

Abstract

The embodiment of the present invention discloses a battery cell, a secondary battery and an electrical device, wherein the battery cell comprises a positive electrode sheet, a negative electrode sheet and a separator located between the positive electrode sheet and the negative electrode sheet; the positive electrode sheet comprises a positive electrode collector and a positive electrode active layer arranged on the positive electrode collector, the positive electrode active layer comprises a positive electrode active material and a positive electrode conductive agent; the positive electrode active material, the positive electrode conductive agent, the positive electrode sheet, a surface of the positive electrode collector on one side close to the positive electrode active layer and a surface of at least one of the separators are coated with a film-forming additive, and the battery cell facilitates controllable and uniform distribution of the film-forming additive in the width and length directions of the positive electrode sheet by introducing the film-forming additive into the positive electrode or the separator, thereby facilitating improvement of uniformity and stability of the SEI film and improving the cycle performance of the battery.

Description

Technical Field

[0001] The present invention relates to the technical field of secondary batteries, and in particular to a battery cell, a secondary battery and an electrical device. Background Art

[0002] Secondary batteries have been widely used in energy storage base stations, consumer electronics, electric vehicles and other products due to their advantages such as high energy density, high operating voltage, long service life, low self-discharge rate and environmental friendliness. With the rapid development of the industry, higher requirements are placed on the performance of secondary batteries. One of the methods to improve the cycle performance and service life of the battery is to add film-forming additives to the electrolyte. For example, adding vinyl carbonate to the electrolyte of lithium iron phosphate / graphite batteries can promote the formation of a SEI (Solid Electrolyte Interphase) film containing polyvinyl carbonate (polyVC) on the surface of the negative electrode of the battery. The polyVC in the SEI film can effectively inhibit the diffusion of iron ions, thereby inhibiting the iron ions from penetrating the SEI film and obtaining electrons on the negative electrode surface for deposition, avoiding iron deposition from blocking the diffusion channel of lithium ions, and thus improving the cycle performance and service life of the battery. However, during the battery charging and discharging process, the film-forming additives in the electrolyte are continuously consumed. The content of the film-forming additives is high at the edge of the pole piece and low at the center. Due to the uneven distribution of the film-forming additives in the electrolyte, it is easy to cause the formation of an uneven SEI film with poor stability, which is not conducive to the improvement of the battery cycle performance. Summary of the invention

[0003] Based on this, an embodiment of the present invention provides a battery cell, a secondary battery and an electrical device, in which the positive electrode active material, the positive electrode conductive agent, the positive electrode sheet, the positive electrode current collector on one side close to the positive electrode active layer and the surface of at least one of the separators in the battery cell are coated with a film-forming additive. The battery cell introduces the film-forming additive into the positive electrode sheet or the separator, thereby facilitating the improvement of the uniformity and stability of the SEI film and the improvement of the cycle performance of the battery.

[0004] In the first aspect, an embodiment of the present invention provides a battery cell, comprising a positive electrode sheet, a negative electrode sheet, and a separator located between the positive electrode sheet and the negative electrode sheet; the positive electrode sheet comprises a positive electrode collector and a positive electrode active layer arranged on the positive electrode collector, and the positive electrode active layer comprises a positive electrode active material and a positive electrode conductive agent; the positive electrode active material, the positive electrode conductive agent, the positive electrode sheet, a surface of the positive electrode collector on one side close to the positive electrode active layer, and a surface of at least one of the separator are coated with a film-forming additive; the film-forming additive comprises one or more of vinylene carbonate, fluoroethylene carbonate, ethylene carbonate, lithium difluorophosphate, propylene sulfite, and vinyl sulfite.

[0005] In an embodiment of the present invention, the film-forming additive in the positive electrode sheet, and / or on the surface of the positive electrode sheet, and / or on the surface of the separator is uniformly distributed in the length direction and the width direction of the positive electrode sheet.

[0006] In some embodiments of the present invention, at least two of the positive electrode active material, the positive electrode conductor, the positive electrode sheet, a surface of the positive electrode current collector close to the positive electrode active layer, and the separator are coated with the film-forming additive.

[0007] In some embodiments of the present invention, the surface of the positive electrode sheet is coated with the film-forming additive, and the surface of at least one of the positive electrode conductor, the positive electrode active material, the surface of the positive electrode collector close to the positive electrode active layer, and the separator is coated with the film-forming additive.

[0008] In an embodiment of the present invention, in the positive electrode active layer, the total mass of the film-forming additive coated on the surface of the positive electrode active material is 0.1%-11% of the total mass of the positive electrode active material;

[0009] And / or, in the positive electrode active layer, the total mass of the film-forming additive coated on the surface of the positive electrode conductive agent is 0.1%-11% of the total mass of the positive electrode conductive agent;

[0010] And / or, the mass of the film-forming additive coated on the surface of the positive electrode sheet is 0.1%-11% of the mass of the positive electrode sheet;

[0011] and / or, the mass of the film-forming additive coated on the surface of the positive electrode current collector close to the positive electrode active layer is 0.1%-11% of the mass of the positive electrode current collector;

[0012] And / or, the mass of the film-forming additive coated on the surface of the diaphragm is 0.1%-11% of the mass of the diaphragm.

[0013] In an embodiment of the present invention, the coverage rate of the film-forming additive on the surface of the positive electrode sheet close to the separator is greater than 60%, and / or the coverage rate of the film-forming additive on the surface of the positive electrode sheet away from the separator is greater than 60%;

[0014] And / or, the coverage rate of the film-forming additive on the surface of the positive electrode current collector close to the positive electrode active layer is greater than 60%;

[0015] And / or, the coverage rate of the film-forming additive on the surface of the separator close to the positive electrode sheet is greater than 60%, and / or, the coverage rate of the film-forming additive on the surface of the separator close to the negative electrode sheet is greater than 60%.

[0016] In an embodiment of the present invention, the negative electrode sheet includes a negative electrode current collector and a negative electrode active layer disposed on the negative electrode current collector, the negative electrode active layer includes a negative electrode active material and a negative electrode conductive agent; the surface of at least one of the negative electrode active material, the negative electrode conductive agent, the negative electrode sheet and the negative electrode current collector on one side close to the negative electrode active layer is coated with the film-forming additive.

[0017] In an embodiment of the present invention, in the negative electrode active layer, the total mass of the film-forming additive coated on the surface of the negative electrode active material is 0.1%-11% of the total mass of the negative electrode active material;

[0018] And / or, in the negative electrode active layer, the total mass of the film-forming additive coated on the surface of the negative electrode conductive agent is 0.1%-11% of the total mass of the negative electrode conductive agent;

[0019] and / or, the mass of the film-forming additive coated on the surface of the negative electrode sheet is 0.1%-11% of the mass of the negative electrode sheet;

[0020] And / or, the mass of the film-forming additive coated on the surface of the negative electrode current collector on one side close to the negative electrode active layer is 0.1%-11% of the mass of the negative electrode current collector.

[0021] In an embodiment of the present invention, the coverage rate of the film-forming additive on the surface of the negative electrode sheet close to the separator is greater than 60%, and / or the coverage rate of the film-forming additive on the surface of the negative electrode sheet away from the separator is greater than 60%;

[0022] And / or, the coverage rate of the film-forming additive on the surface of the negative electrode current collector on the side close to the negative electrode active layer is greater than 60%.

[0023] In an embodiment of the present invention, the positive electrode active material includes an iron-based positive electrode material, and the iron-based positive electrode material includes at least one of lithium iron phosphate, lithium iron manganese phosphate, nickel cobalt manganese iron lithium, sodium iron phosphate, sodium iron pyrophosphate and sodium iron sulfate;

[0024] And / or, the positive electrode conductive agent includes at least one of conductive carbon black, conductive graphite, carbon nanotubes, carbon fibers and graphene;

[0025] And / or, the positive electrode current collector comprises at least one of aluminum foil and carbon-coated aluminum foil;

[0026] And / or, the separator includes at least one of polyethylene and polypropylene.

[0027] In an embodiment of the present invention, the negative electrode active material includes at least one of a carbon-based negative electrode active material, a silicon-based negative electrode active material and a tin-based negative electrode active material;

[0028] And / or, the negative electrode conductive agent includes at least one of conductive carbon black, conductive graphite, carbon nanotubes, carbon fibers and graphene;

[0029] And / or, the negative electrode current collector includes at least one of copper foil and carbon-coated copper foil.

[0030] The positive electrode active material, positive electrode conductive agent, positive electrode sheet, the surface of one side of the positive electrode collector close to the positive electrode active layer and the surface of at least one of the separators in the battery cell provided by the embodiment of the present invention are coated with a film-forming additive. By introducing the film-forming additive into the positive electrode or the separator, the battery cell facilitates the controllable and uniform distribution of the film-forming additive in the width and length directions of the positive electrode sheet, which is beneficial to improving the uniformity and stability of the SEI film, improving the cycle performance of the battery, and replenishing the consumption of the film-forming additive during the battery cycle, which is beneficial to extending the service life of the battery cell.

[0031] In a second aspect, an embodiment of the present invention provides a secondary battery, including a packaging shell, and a battery cell and an electrolyte located in the packaging shell, wherein the battery cell includes the battery cell described in the first aspect.

[0032] In an embodiment of the present invention, the electrolyte contains the film-forming additive.

[0033] The battery cell provided by the embodiment of the present invention contains a uniformly distributed film-forming additive, which can promote the formation of a stable and uniform SEI film on the surface of the battery negative electrode during the battery cycle, which is beneficial to inhibit iron deposition on the negative electrode surface and the self-discharge of the battery, thereby helping to improve the battery cycle performance.

[0034] In a third aspect, an embodiment of the present invention provides an electrical device, comprising the secondary battery described in the second aspect.

[0035] The electric device provided in the embodiment of the present invention comprises the secondary battery of the second aspect. The secondary battery has good cycle performance, which is beneficial to improving the battery life of the electric device and enhancing the market competitiveness of the electric device. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] In order to more clearly illustrate the technical solution of the embodiment of the present invention, the accompanying drawings required for use in the embodiment of the present invention will be described below.

[0037] Figure 1 is a schematic diagram of the side structure of a battery cell provided by an embodiment of the present invention;

[0038] Figure 2 is a scanning electron microscope image of vinylene carbonate-coated LFP@C prepared in Example 1 of the present invention;

[0039] Figure 3is a cycle performance diagram of the battery prepared in Example 1 of the present invention at 60°C;

[0040] Figure 4 is a cycle performance diagram of the battery prepared in Example 2 of the present invention at 60°C;

[0041] Figure 5 This is a cycle performance diagram of the battery prepared in Comparative Example 1 of the present invention at 60°C;

[0042] Figure 6 It is a partition diagram of the positive electrode sheets prepared in Examples 1-9 of the present invention and Comparative Example 1.

[0043] Description of Figure Numbers

[0044] 10-battery cell; 11-positive electrode sheet; 111-positive electrode current collector; 112-positive electrode active layer; 12-negative electrode sheet; 121-negative electrode current collector; 122-negative electrode active layer; 13-diaphragm. DETAILED DESCRIPTION

[0045] The embodiments of the present invention are described below in conjunction with the accompanying drawings in the embodiments of the present invention.

[0046] Iron-based cathode materials (such as lithium iron phosphate) have the advantages of high safety, long cycle life, good high temperature performance and stable discharge platform, and are widely used in secondary batteries. However, the residual Fe2P, Fe3O4 or other highly active iron compounds in the iron-based cathode materials will continuously undergo electrochemical or chemical reactions to dissolve iron ions during the battery cycle. Especially under high temperature conditions, the dissolution of iron ions will be aggravated, and the iron ions will migrate to the negative electrode, blocking the lithium ion diffusion channel, thereby causing a more serious self-discharge problem, resulting in a decrease in battery capacity and high temperature cycle performance. At present, film-forming additives (such as VC) are usually added to the battery electrolyte to promote the formation of an SEI film on the negative electrode surface that can inhibit the diffusion of iron ions, so as to reduce iron deposition on the negative electrode surface, thereby improving the battery cycle performance. However, the diffusion rate of film-forming additives in the electrolyte is slow and the consumption is uneven, which easily leads to an uneven SEI film and poor stability, which is not conducive to the improvement of battery cycle performance.

[0047] Based on this, an embodiment of the present invention provides a battery cell, which introduces a film-forming additive into the positive electrode sheet or the separator, thereby facilitating improvement of the uniformity and stability of the SEI film and improving the cycle performance of the battery.

[0048] like Figure 1As shown, an embodiment of the present invention provides a battery cell 10, which includes a positive electrode sheet 11, a negative electrode sheet 12, and a separator 13 located between the positive electrode sheet 11 and the negative electrode sheet 12. Among them, the positive electrode sheet 11 includes a positive electrode collector 111 and a positive electrode active layer 112 disposed on the positive electrode collector 111, and the positive electrode active layer 112 includes a positive electrode active material and a positive electrode conductive agent. In an embodiment of the present invention, the surface of at least one of the positive electrode active material, the positive electrode conductive agent, the positive electrode sheet 11, the positive electrode collector 111 close to the positive electrode active layer 112 and the separator 13 is coated with a film-forming additive. In an embodiment of the present invention, the film-forming additive includes one or more of vinylene carbonate (VC), fluoroethylene carbonate (FEC), vinyl ethylene carbonate (VEC), lithium difluorophosphate (LiPO2F2), propylene sulfite (PS) and vinyl sulfite (ES).

[0049] The surface of at least one of the positive active material, positive conductive agent, positive electrode sheet, positive current collector and separator in the battery provided by the embodiment of the present invention is coated with a film-forming additive. The battery introduces the film-forming additive into the positive electrode sheet or the separator, so as to realize the controllable uniform distribution of the film-forming additive in the width direction and the length direction of the positive electrode sheet, thereby facilitating the uniformity and stability of the SEI film, improving the cycle performance of the battery, and replenishing the consumption of the film-forming additive during the battery cycle, which is conducive to extending the service life of the battery. On the one hand, the active group of the film-forming additive used in the present application has a strong ability to absorb electrons, which can increase the reduction potential, and perform a reduction reaction before the lithium ions or sodium ions are embedded in the negative electrode to form a passivation film, inhibit the electrolyte from decomposing again, improve the performance of the SEI film, and improve the battery cycle performance; in addition, for iron-based positive electrode material batteries, in the SEI film formed by the film-forming additive used in the present application, the iron ions have a high diffusion barrier, and the diffusion of the iron ions is inhibited, so that it is difficult to obtain electrons on the negative electrode surface for deposition, thereby preventing the occurrence of self-discharge and slowing down the decay of the battery capacity. In summary, in the battery cell provided by the embodiments of the present invention, a film-forming additive is coated on the surface of the positive electrode active material, the positive electrode conductive agent, the positive electrode sheet, the positive electrode current collector on the side close to the positive electrode active layer and at least one of the diaphragms, which is conducive to forming a relatively stable SEI film, reducing the iron deposition rate, improving the high-temperature cycle performance of the battery and extending the battery life, and effectively preventing the battery performance from being degraded due to uneven distribution of the film-forming additive.

[0050] In the present invention, the surface of the positive electrode active material is coated with a film-forming additive, which means that at least part of the surface of the positive electrode active material in the active material layer is coated with the film-forming additive, and the surface of all the positive electrode active materials may be coated with the film-forming additive, or only part of the surface of the positive electrode active materials may be coated with the film-forming additive. The coating of the film-forming additive on the surface of the positive electrode active material may be partial coating or complete coating.

[0051] In the present invention, the surface of the positive electrode conductive agent is coated with a film-forming additive, which means that at least part of the surface of the positive electrode conductive agent in the active material layer is coated with the film-forming additive, and the surface of all the positive electrode conductive agents may be coated with the film-forming additive, or only part of the surface of the positive electrode conductive agent may be coated with the film-forming additive. The coating of the film-forming additive on the surface of the positive electrode conductive agent may be partial coating or complete coating.

[0052] In the embodiment of the present invention, the film-forming additive can be a compound or can include multiple compounds. In some embodiments, the film-forming additive can be vinylene carbonate (VC), the double bond of VC is connected to the electron-withdrawing ester group in the ring, and its charge density is low, and the reactivity between the double bonds is higher. The inorganic components of the formed SEI film are mainly Li2O, Li2CO3, LiF and LiOH, and the organic components are mainly lithium diethyl carbonate (LiEDC) and polyvinylene carbonate (polyVC). The influence of each component of the SEI film on the size of the Fe diffusion barrier is poly (VC)>>Li2O>LiF>Li2CO3, and poly (VC) has a better effect of inhibiting iron diffusion, so as to better inhibit iron ions from penetrating the SEI film and obtaining electrons on the surface of the graphite negative electrode. Deposition. In other embodiments, the film-forming additive includes vinylene carbonate (VC), and one or more of fluoroethylene carbonate (FEC), vinyl ethylene carbonate (VEC), lithium difluorophosphate (LiPO2F2), propylene sulfite (PS) and vinyl sulfite (ES). When the film-forming additives are two or more compounds compounded to form a film, the film-forming stability of the SEI film can be further improved, thereby further improving the electrochemical performance of the battery.

[0053] In some embodiments of the present invention, the film-forming additives in the positive electrode sheet, on the surface of the positive electrode sheet and / or on the surface of the separator are uniformly distributed in the length direction and width direction of the positive electrode sheet. The film-forming additives in the positive electrode sheet are uniformly distributed in the length direction and width direction of the positive electrode sheet, that is, the positive electrode active material coated with the film-forming additives is uniformly distributed in the length direction and width direction of the positive electrode sheet, and / or, the conductive agent coated with the film-forming additives is uniformly distributed in the length direction and width direction of the positive electrode sheet, and / or, the film-forming additives on the surface of the positive electrode current collector close to the positive electrode active layer are uniformly distributed in the length direction and width direction of the positive electrode sheet. In some embodiments of the present invention, the film-forming additives on the surface of the positive electrode sheet are uniformly distributed in the length direction and width direction of the positive electrode sheet, that is, the film-forming additives coated on the surface of the positive electrode sheet are uniformly distributed in the length direction and width direction of the positive electrode sheet. In other embodiments of the present invention, the film-forming additives on the surface of the separator are uniformly distributed in the length direction and width direction of the positive electrode sheet, that is, the film-forming additives coated on the surface of the separator are uniformly distributed in the length direction and width direction of the positive electrode sheet. The film-forming additives in the positive electrode sheet, on the surface of the positive electrode sheet and / or on the surface of the separator are uniformly distributed in the length direction and the width direction of the positive electrode sheet, which can further enhance the uniformity and stability of the SEI film.

[0054] In an embodiment of the present invention, at least two of the positive electrode active material, the positive electrode conductive agent, the positive electrode sheet, the positive electrode current collector on the side close to the positive electrode active layer, and the separator are coated with a film-forming additive. When two or more positions in the positive electrode or the separator are coated with a film-forming additive, the film formation of SEI can be further promoted and its stability can be improved. In some embodiments, the surface of the positive electrode sheet is coated with a film-forming additive, and the surface of at least one of the positive electrode conductive agent, the positive electrode active material, the positive electrode current collector on the side close to the positive electrode active layer, and the separator is coated with a film-forming additive.

[0055] In the embodiment of the present invention, the film-forming additive coated on the surface of the positive electrode active material, the surface of the positive electrode conductive agent, the surface of the positive electrode sheet, the surface of the positive electrode current collector on one side close to the positive electrode active layer and the surface of the separator can be the same compound or different compounds, one compound or multiple compounds. In some embodiments, the surfaces of the positive electrode active material, the positive electrode conductive agent, the positive electrode sheet, the surface of the positive electrode current collector on one side close to the positive electrode active layer and the separator (two or more) are coated with vinylene carbonate. In other embodiments, the surface of the positive electrode active material is coated with vinylene carbonate, and the surface of at least one of the positive electrode conductive agent, the positive electrode sheet, the surface of the positive electrode current collector on one side close to the positive electrode active layer and the separator is coated with fluoroethylene carbonate.

[0056] In an embodiment of the present invention, in the positive electrode active layer, the total mass of the film-forming additive coated on the surface of the positive electrode active material may be 0.1%-11% of the total mass of the positive electrode active material, which can improve the uniformity and stability of the SEI film without affecting the energy density of the battery. In some embodiments, the total mass of the film-forming additive coated on the surface of the positive electrode active material may be, for example, 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10% or 11% of the total mass of the positive electrode active material.

[0057] In the embodiment of the present invention, the positive electrode active material includes a positive electrode active material that can reversibly embed / de-embed metal ions (lithium ions, sodium ions, potassium ions, magnesium ions, zinc ions, aluminum ions, lead ions, etc.). The present application has no special restrictions on the selection of positive electrode active materials, and can be positive electrode active materials conventionally used in existing secondary batteries. In some embodiments, the positive electrode active material includes but is not limited to at least one of lithium iron phosphate, lithium iron manganese phosphate, nickel cobalt manganese iron lithium, sodium iron phosphate, sodium iron pyrophosphate and sodium iron sulfate.

[0058] In an embodiment of the present invention, in the positive electrode active layer, the total mass of the film-forming additive coated on the surface of the positive electrode conductive agent may be 0.1%-11% of the total mass of the positive electrode conductive agent, which can improve the uniformity and stability of the SEI film without affecting the energy density of the battery. In some embodiments, the total mass of the film-forming additive coated on the surface of the positive electrode conductive agent may be, for example, 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10% or 11% of the total mass of the positive electrode conductive agent.

[0059] In the embodiment of the present invention, the positive electrode conductive agent includes a conductive material that can efficiently conduct metal ions (lithium ions, sodium ions, potassium ions, magnesium ions, zinc ions, aluminum ions, lead ions, etc.) and electrons. The present application has no special restrictions on the selection of the positive electrode conductive agent, and it can be a positive electrode conductive agent conventionally used in existing secondary batteries. In some embodiments, the positive electrode conductive agent includes but is not limited to at least one of conductive carbon black (Super P, acetylene black, Ketjen black), conductive graphite, carbon nanotubes, carbon fibers, and graphene.

[0060] In an embodiment of the present invention, the mass of the film-forming additive coated on the surface of the positive electrode sheet may be 0.1%-11% of the mass of the positive electrode sheet, which can improve the uniformity and stability of the SEI film without affecting the energy density of the battery. In some embodiments, the mass of the film-forming additive coated on the surface of the positive electrode sheet may be, for example, 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10% or 11% of the mass of the positive electrode sheet.

[0061] In an embodiment of the present invention, the positive electrode sheet also includes a binder, which includes but is not limited to at least one of polyvinylidene fluoride, soluble polytetrafluoroethylene, styrene-butadiene rubber, hydroxypropyl methylcellulose, methyl cellulose, carboxymethyl cellulose, polyvinyl alcohol, acrylonitrile copolymer, sodium alginate, chitosan and chitosan derivatives.

[0062] In an embodiment of the present invention, the mass of the film-forming additive coated on the surface of one side of the positive current collector close to the positive active layer may be 0.1%-11% of the mass of the positive current collector, which can improve the uniformity and stability of the SEI film without affecting the energy density of the battery. In some embodiments, the mass of the film-forming additive coated on the surface of one side of the positive current collector close to the positive active layer may be, for example, 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10% or 11% of the mass of the positive current collector.

[0063] In the embodiment of the present invention, the positive electrode current collector may be an existing conventional positive electrode current collector, including but not limited to at least one of aluminum foil and carbon-coated aluminum foil.

[0064] In an embodiment of the present invention, the mass of the film-forming additive coated on the surface of the diaphragm may be 0.1%-11% of the mass of the diaphragm, which can improve the uniformity and stability of the SEI film without affecting the ion conductivity of the battery. In some embodiments, the mass of the film-forming additive coated on the surface of the diaphragm may be, for example, 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10% or 11% of the mass of the diaphragm.

[0065] In the embodiment of the present invention, the diaphragm can be an existing conventional diaphragm, including but not limited to at least one of polyethylene and polypropylene. Specifically, the diaphragm can be a base film such as single-layer PP (polypropylene), single-layer PE (polyethylene), double-layer PP / PE, double-layer PP / PP or three-layer PP / PE / PP, and a ceramic coating (aluminum oxide) and a glue layer (PVDF) can also be provided on the base film.

[0066] In an embodiment of the present invention, the coverage of the film-forming additive on the surface of the positive electrode sheet close to the separator is greater than 60%, where the coverage = (coverage area of ​​the film-forming additive on the surface of the positive electrode sheet close to the separator / surface area of ​​the positive electrode sheet close to the separator) × 100%. The film-forming additive has a high coverage on the surface of the positive electrode sheet close to the separator and is evenly dispersed, which can better improve the uniformity and stability of the SEI film. In some embodiments, the coverage of the film-forming additive on the surface of the positive electrode sheet close to the separator can be, for example, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% or 100%.

[0067] In an embodiment of the present invention, the coverage of the film-forming additive on the surface of the positive electrode sheet away from the separator is greater than 60%, where the coverage = (coverage area of ​​the film-forming additive on the surface of the positive electrode sheet away from the separator / surface area of ​​the positive electrode sheet away from the separator) × 100%. The coverage of the film-forming additive on the surface of the positive electrode sheet away from the separator is high and evenly dispersed, which can better improve the uniformity and stability of the SEI film. In some embodiments, the coverage of the film-forming additive on the surface of the positive electrode sheet away from the separator can be, for example, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% or 100%.

[0068] In an embodiment of the present invention, the coverage of the film-forming additive on the surface of the positive electrode current collector close to the positive electrode active layer is greater than 60%, where the coverage = (coverage area of ​​the film-forming additive on the surface of the positive electrode current collector close to the positive electrode active layer / surface area of ​​the positive electrode current collector close to the positive electrode active layer) × 100%, the coverage of the film-forming additive on the surface of the positive electrode current collector close to the positive electrode active layer is high, and the dispersion is uniform, which can better improve the uniformity and stability of the SEI film. In some embodiments, the coverage of the film-forming additive on the surface of the positive electrode current collector close to the positive electrode active layer can be, for example, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% or 100%.

[0069] In an embodiment of the present invention, the coverage of the film-forming additive on the surface of the side of the separator close to the positive electrode is greater than 60%, where the coverage = (coverage area of ​​the film-forming additive on the surface of the side of the separator close to the positive electrode / surface area of ​​the side of the separator close to the positive electrode) × 100%. The coverage of the film-forming additive on the surface of the side of the separator close to the positive electrode is high and evenly dispersed, which can better improve the uniformity and stability of the SEI film. In some embodiments, the coverage of the film-forming additive on the surface of the side of the separator close to the positive electrode can be, for example, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% or 100%.

[0070] In an embodiment of the present invention, the coverage of the film-forming additive on the surface of the side of the separator close to the negative electrode is greater than 60%, where the coverage = (coverage area of ​​the film-forming additive on the surface of the side of the separator close to the negative electrode / surface area of ​​the side of the separator close to the negative electrode) × 100%. The coverage of the film-forming additive on the surface of the side of the separator close to the negative electrode is high and evenly dispersed, which can better improve the uniformity and stability of the SEI film. In some embodiments, the coverage of the film-forming additive on the surface of the side of the separator close to the negative electrode can be, for example, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% or 100%.

[0071] In the embodiment of the present invention, Figure 1 As shown, the battery cell 10 also includes a negative electrode sheet 12, the negative electrode sheet 12 includes a negative electrode collector 121 and a negative electrode active layer 122 arranged on the negative electrode collector 121, and the negative electrode active layer 122 includes a negative electrode active material and a negative electrode conductive agent; the surface of at least one of the negative electrode active material, the negative electrode conductive agent, the negative electrode sheet 12 and the surface of the negative electrode collector 121 on one side close to the negative electrode active layer 122 is coated with a film-forming additive, so that the film-forming additive can be better and more evenly dispersed in the battery cell, thereby further improving the uniformity and stability of the formed SEI film.

[0072] In the present invention, the surface of the negative electrode active material is coated with a film-forming additive, which means that at least part of the surface of the negative electrode active material in the active material layer is coated with the film-forming additive. The surface of all negative electrode active materials may be coated with the film-forming additive, or only part of the surface of the negative electrode active material may be coated with the film-forming additive.

[0073] In the present invention, the surface of the negative electrode conductive agent is coated with a film-forming additive, which means that at least part of the surface of the negative electrode conductive agent in the active material layer is coated with the film-forming additive. The surface of all the negative electrode conductive agents may be coated with the film-forming additive, or only part of the surface of the negative electrode conductive agent may be coated with the film-forming additive.

[0074] In the embodiment of the present invention, the film-forming additives on the surface of the positive electrode active material, the surface of the positive electrode conductive agent, the surface of the positive electrode sheet, the surface of the positive electrode current collector close to the positive electrode active layer, the surface of the negative electrode active material, the surface of the negative electrode conductive agent, the surface of the negative electrode sheet, the surface of the negative electrode current collector close to the negative electrode active layer, and the surface of the separator may be the same compound or different compounds. In some embodiments, the surface of at least one of the positive electrode active material, the positive electrode conductive agent, the positive electrode sheet, the surface of the positive electrode current collector close to the positive electrode active layer, and the separator is coated with vinylene carbonate, and the surface of at least one of the negative electrode active material, the negative electrode conductive agent, the negative electrode sheet, and the surface of the negative electrode current collector close to the negative electrode active layer is also coated with vinylene carbonate. In other embodiments, at least one of the surfaces of the positive electrode active material, the positive electrode conductor, the positive electrode sheet, the positive electrode current collector on one side close to the positive electrode active layer and the separator is coated with vinylene carbonate, and at least one of the surfaces of the negative electrode active material, the negative electrode conductor, the negative electrode sheet and the negative electrode current collector on one side close to the negative electrode active layer is coated with fluoroethylene carbonate.

[0075] In an embodiment of the present invention, the film-forming additive can be coated on the surface of the positive electrode active material by any of the following methods: chemical vapor deposition (CVD), atomic layer deposition (ALD), solution immersion drying method, supercritical fluid method, sol-gel method or mechanical mixing method. Similarly, any of the above methods can be used to prepare negative electrode active materials coated with film-forming additives, positive electrode conductive agents coated with film-forming additives, negative electrode conductive agents coated with film-forming additives, positive electrode sheets coated with film-forming additives, negative electrode sheets coated with film-forming additives, positive electrode current collectors coated with film-forming additives, negative electrode current collectors coated with film-forming additives, and separators coated with film-forming additives.

[0076] In an embodiment of the present invention, in the negative electrode active layer, the total mass of the film-forming additive coated on the surface of the negative electrode active material may be 0.1%-11% of the total mass of the negative electrode active material, which can improve the uniformity and stability of the SEI film without affecting the energy density of the battery. In some embodiments, the total mass of the film-forming additive coated on the surface of the negative electrode active material may be, for example, 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10% or 11% of the total mass of the negative electrode active material.

[0077] In an embodiment of the present invention, the negative electrode active material includes but is not limited to at least one of a carbon-based negative electrode active material, a silicon-based negative electrode active material and a tin-based negative electrode active material. In some embodiments, the carbon-based negative electrode active material may be, for example, at least one of graphite, graphene, soft carbon and hard carbon. In other embodiments, the silicon-based negative electrode active material may be at least one of elemental silicon, a silicon-carbon compound, a silicon-oxygen compound and a silicon-metal compound. In still other embodiments, the tin-based negative electrode active material may be at least one of elemental tin, a tin-carbon compound, a tin-oxygen compound and a tin-metal compound.

[0078] In an embodiment of the present invention, the total mass of the film-forming additive coated on the surface of the negative electrode conductive agent may be 0.1%-11% of the total mass of the negative electrode conductive agent, which can improve the uniformity and stability of the SEI film without affecting the energy density of the battery. In some embodiments, the total mass of the film-forming additive coated on the surface of the negative electrode conductive agent may be, for example, 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10% or 11% of the total mass of the negative electrode conductive agent.

[0079] In the embodiment of the present invention, in the negative electrode active layer, the negative electrode conductive agent includes a conductive material that can efficiently conduct metal ions (lithium ions, sodium ions, potassium ions, magnesium ions, zinc ions, aluminum ions, lead ions, etc.) and electrons. The present application has no special restrictions on the selection of the negative electrode conductive agent, and it can be the negative electrode conductive agent conventionally used in existing secondary batteries. In some embodiments, the negative electrode conductive agent includes but is not limited to at least one of conductive carbon black (Super P, acetylene black, Ketjen black), conductive graphite, carbon nanotubes, carbon fibers and graphene.

[0080] In an embodiment of the present invention, the mass of the film-forming additive coated on the surface of the negative electrode sheet may be 0.1%-11% of the mass of the negative electrode sheet, which can improve the uniformity and stability of the SEI film without affecting the energy density of the battery. In some embodiments, the mass of the film-forming additive coated on the surface of the negative electrode sheet may be, for example, 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10% or 11% of the mass of the negative electrode sheet.

[0081] In an embodiment of the present invention, the mass of the film-forming additive coated on the surface of the negative electrode current collector on one side close to the negative electrode active layer may be 0.1%-11% of the mass of the negative electrode current collector, which can improve the uniformity and stability of the SEI film without affecting the energy density of the battery. In some embodiments, the mass of the film-forming additive coated on the surface of the negative electrode current collector on one side close to the negative electrode active layer may be, for example, 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10% or 11% of the mass of the negative electrode current collector.

[0082] In the embodiment of the present invention, the negative electrode current collector may be an existing conventional negative electrode current collector, including but not limited to at least one of copper foil and carbon-coated copper foil.

[0083] In an embodiment of the present invention, the coverage of the film-forming additive on the surface of the negative electrode sheet close to the separator is greater than 60%, where the coverage = (coverage area of ​​the film-forming additive on the surface of the negative electrode sheet close to the separator / surface area of ​​the negative electrode sheet close to the separator) × 100%. The film-forming additive has a high coverage on the surface of the negative electrode sheet close to the separator and is evenly dispersed, which can better improve the uniformity and stability of the SEI film. In some embodiments, the coverage of the film-forming additive on the surface of the negative electrode sheet close to the separator can be, for example, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% or 100%.

[0084] In an embodiment of the present invention, the coverage of the film-forming additive on the surface of the negative electrode sheet away from the separator is greater than 60%, where the coverage = (coverage area of ​​the film-forming additive on the surface of the negative electrode sheet away from the separator / surface area of ​​the negative electrode sheet away from the separator) × 100%. The coverage of the film-forming additive on the surface of the negative electrode sheet away from the separator is high and evenly dispersed, which can better improve the uniformity and stability of the SEI film. In some embodiments, the coverage of the film-forming additive on the surface of the negative electrode sheet away from the separator can be, for example, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% or 100%.

[0085] In an embodiment of the present invention, the coverage of the film-forming additive on the surface of the negative electrode current collector close to the negative electrode active layer is greater than 60%, where the coverage = (coverage area of ​​the film-forming additive on the surface of the negative electrode current collector close to the negative electrode active layer / surface area of ​​the negative electrode current collector close to the negative electrode active layer) × 100%, the coverage of the film-forming additive on the surface of the negative electrode current collector close to the negative electrode active layer is high, and the dispersion is uniform, which can better improve the uniformity and stability of the SEI film. In some embodiments, the coverage of the film-forming additive on the surface of the negative electrode current collector close to the negative electrode active layer can be, for example, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% or 100%.

[0086] An embodiment of the present invention further provides a secondary battery, including a packaging shell, and a battery cell and an electrolyte located in the packaging shell, wherein the battery cell includes the battery cell in any of the above embodiments.

[0087] The battery cell provided by the embodiment of the present invention contains a uniformly distributed film-forming additive, which can promote the formation of a stable and uniform SEI film on the surface of the battery negative electrode during the battery cycle, which is beneficial to inhibit iron deposition on the negative electrode surface and the self-discharge of the battery, thereby helping to improve the battery cycle performance.

[0088] In an embodiment of the present invention, the electrolyte contains a film-forming additive in any of the above embodiments, which can increase the concentration of the film-forming additive in the electrolyte, better improve the uniformity and stability of the SEI film, and further improve the cycle performance of the battery. The film-forming additive in the electrolyte and the positive electrode active material, the positive electrode conductive agent, the positive electrode sheet, the positive electrode current collector on the side of the surface close to the positive electrode active layer, the negative electrode active material, the negative electrode conductive agent, the negative electrode sheet, the negative electrode current collector on the side of the surface close to the negative electrode active layer and the film-forming additive on the surface of the separator can be the same compound or different compounds. In some embodiments, the surface of at least one of the positive electrode active material, the positive electrode conductive agent, the positive electrode sheet, the positive electrode current collector on the side of the surface close to the positive electrode active layer and the separator is coated with vinylene carbonate, and the electrolyte contains vinylene carbonate. In other embodiments, the surface of at least one of the positive electrode active material, the positive electrode conductive agent, the positive electrode sheet, the positive electrode current collector on the side of the surface close to the positive electrode active layer and the separator is coated with vinylene carbonate, and the electrolyte contains fluoroethylene carbonate.

[0089] In an embodiment of the present invention, the electrolyte further comprises an electrolyte salt and a solvent. The electrolyte salt may be an electrolyte salt commonly used in a secondary battery, for example, in a lithium secondary battery, the electrolyte salt includes but is not limited to at least one of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethylsulfonyl)imide, lithium trifluoromethylsulfonate, lithium bis(oxalatoborate), lithium difluorooxalatoborate, lithium tetrafluoroborate and lithium perchlorate.

[0090] In an embodiment of the present invention, the solvent may be at least one of a carbonate solvent, an ether solvent and a carboxylate solvent. In some embodiments, the carbonate solvent includes but is not limited to cyclic carbonate or chain carbonate, and the cyclic carbonate may be, for example, at least one of ethylene carbonate (EC), propylene carbonate (PC), γ-butyrolactone (GBL) and butylene carbonate (BC); the chain carbonate may be, for example, at least one of dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC) and dipropyl carbonate (DPC). In other embodiments, the ether solvent includes but is not limited to cyclic ether or chain ether, and the cyclic ether may be, for example, at least one of 1,3-dioxolane (DOL), 1,4-dioxane (DX), crown ether, tetrahydrofuran (THF), 2-methyltetrahydrofuran and 2-trifluoromethyltetrahydrofuran; the chain ether may be, for example, at least one of methylene dimethyl ether (DMM), ethylene glycol dimethyl ether (DME) and diethylene glycol dimethyl ether (DEDM). In some embodiments, the carboxylate solvent may be at least one of methyl acetate (MA), ethyl acetate (EA), propyl acetate (EP), butyl acetate, propyl propionate (PP) and butyl propionate.

[0091] The embodiment of the present invention further provides an electric device, comprising the secondary battery in any of the above embodiments. Specifically, the electric device can be an electric car, an electric motorcycle, an electric bicycle, a mobile power supply, an unmanned aerial vehicle, a mobile phone, a computer, a camera, an electric tool, a smart home or a wearable device.

[0092] The electric device provided in the embodiment of the present invention includes a secondary battery. The secondary battery has good cycle performance, which is beneficial to improving the battery life of the electric device and enhancing the market competitiveness of the electric device.

[0093] The technical solution of the present invention is further illustrated below through specific embodiments and comparative examples.

[0094] Example 1

[0095] (1) Preparation of lithium iron phosphate (LFP)

[0096] First, the ionic liquid was preheated to 50 °C to ensure that the precursor was well dissolved. Secondly, FeSO4·7H2O, LiOH, H3PO4 and polyvinylpyrrolidone (PVP) with a molar ratio of 1:1:1:1 were added to 10 mL of the ionic liquid as precursors and stirred vigorously for 30 min. Then, the obtained homogeneous solution was transferred to a 20 mL Teflon-lined stainless steel autoclave and heated at 200 °C for 24 h. After cooling to room temperature, the green precipitate was thoroughly washed with water and ethanol several times and dried in vacuum at 60 °C overnight to obtain lithium iron phosphate (LFP) material.

[0097] (2) Preparation of carbon-coated lithium iron phosphate composites (LFP@C)

[0098] LFP and sucrose were dispersed in distilled water and ethanol, further dispersed by ultrasonic treatment and concentrated to dryness. The mixture was ground in a mortar and then calcined at 300 °C for 0.5 h in an Ar atmosphere, then raised to 550 °C and kept for 2.5 h, with a heating rate of 5 °C min -1 The prepared LFP@C powder has a nanosheet structure and agglomerates into a spherical shape.

[0099] (3) Preparation of vinylene carbonate (VC)-coated LFP@C

[0100] Using a mechanical mixing method, VC was dissolved in a solvent, N-methylpyrrolidone (NMP), and stirred at a constant speed for 30 minutes. Then, LFP@C powder was added thereto and stirred at a constant speed for 60 minutes to obtain a suspension. The suspension was vacuum filtered, and the filter cake was dried at 60°C for 12 hours to obtain a VC-coated LFP@C powder material, in which the total mass of VC coated on the surface of LFP@C was 3% of the total mass of LFP@C. The morphology of the VC-coated LFP@C was characterized using a scanning electron microscope (SEM), and the obtained SEM image is shown in the figure. Figure 2 shown.

[0101] (4) Preparation of positive electrode

[0102] The VC-coated LFP@C, conductive agent carbon black, and binder polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 94:3:3, and NMP was used as a solvent. The positive electrode slurry was uniformly stirred for 12 hours to obtain the positive electrode slurry. The positive electrode slurry was coated on the carbon-coated aluminum foil and dried in an oven at 80°C for 12 hours to obtain the positive electrode sheet.

[0103] (5) Preparation of batteries

[0104] The positive electrode sheet prepared in step (4) is used as a graphite negative electrode and a PP / PE / PP composite separator to prepare a battery cell, the battery cell is packaged and injected with an electrolyte to obtain a battery, the electrolyte lithium salt is LiPF6, the electrolyte solvent is ethylene carbonate (EC), ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC), and the electrolyte contains VC, the mass of VC is 3% of the total mass of the electrolyte.

[0105] Example 2

[0106] (1) Preparation of lithium iron phosphate (LFP)

[0107] Same as Example 1.

[0108] (2) Preparation of LFP@C

[0109] Same as Example 1.

[0110] (3) Preparation of VC-coated carbon nanotubes

[0111] The carbon nanotubes coated with the conductive agent VC were prepared by mechanical mixing. VC was dissolved in a solvent N-methylpyrrolidone (NMP), stirred at a constant speed for 30 minutes, and then carbon nanotubes were added thereto, stirred at a constant speed for 60 minutes to obtain a suspension. The suspension was vacuum filtered, and the filter cake was dried at 60°C for 12 hours to obtain a VC-coated carbon nanotube material, wherein the total mass of VC coated on the surface of the carbon nanotubes was 3% of the total mass of the carbon nanotubes.

[0112] (4) Preparation of positive electrode

[0113] LFP@C, VC-coated carbon nanotubes, and PVDF were mixed in a mass ratio of 94:3:3, and NMP was used as a solvent, and stirred evenly for 12 hours to obtain a positive electrode slurry. The positive electrode slurry was coated on a carbon-coated aluminum foil and dried in an oven at 80°C for 12 hours to obtain a positive electrode sheet.

[0114] (5) Preparation of batteries

[0115] Same as Example 1.

[0116] Example 3

[0117] (1) Preparation of lithium iron phosphate (LFP)

[0118] Same as Example 1.

[0119] (2) Preparation of LFP@C

[0120] Same as Example 1.

[0121] (3) Preparation of VC-coated carbon-coated aluminum foil

[0122] The VC-coated carbon-coated aluminum foil was prepared by a mechanical mixing method. VC was dissolved in a solvent N-methylpyrrolidone (NMP), and the mixture was stirred at a uniform speed for 30 minutes. The obtained suspension was coated on the surface of the carbon-coated aluminum foil close to the negative electrode active layer to obtain the VC-coated carbon-coated aluminum foil, wherein the mass of VC coated on the surface of the carbon-coated aluminum foil close to the negative electrode active layer was 3% of the mass of the carbon-coated aluminum foil.

[0123] (4) Preparation of positive electrode

[0124] LFP@C, carbon nanotubes, and PVDF were mixed in a mass ratio of 94:3:3, and NMP was used as a solvent, and the mixture was stirred evenly for 12 hours to obtain a positive electrode slurry. The positive electrode slurry was coated on a VC-coated carbon-coated aluminum foil, and dried in an oven at 80°C for 12 hours to obtain a positive electrode sheet.

[0125] (5) Preparation of batteries

[0126] Same as Example 1.

[0127] Example 4

[0128] (1) Preparation of lithium iron phosphate (LFP)

[0129] Same as Example 1.

[0130] (2) Preparation of LFP@C

[0131] Same as Example 1.

[0132] (3) Preparation of VC-coated positive electrode sheets

[0133] LFP@C, carbon nanotubes, and PVDF were mixed in a mass ratio of 94:3:3, and NMP was used as a solvent. The positive electrode slurry was uniformly stirred for 12 hours to obtain a positive electrode slurry. The positive electrode slurry was coated on a carbon-coated aluminum foil and dried in an oven at 80°C for 12 hours to obtain a positive electrode sheet. The positive electrode sheet was further coated with VC on the surface of the positive electrode sheet by a mechanical mixing method to obtain a VC-coated positive electrode sheet, and VC was dissolved in a solvent N-methylpyrrolidone (NMP), uniformly stirred for 30 minutes, and the obtained suspension was coated on the surface of the positive electrode sheet to obtain a VC-coated positive electrode sheet, wherein the mass of VC coated on the surface of the positive electrode sheet was 3% of the mass of the positive electrode sheet.

[0134] (4) Preparation of batteries

[0135] Same as Example 1.

[0136] Example 5

[0137] (1) Preparation of lithium iron phosphate (LFP)

[0138] Same as Example 1.

[0139] (2) Preparation of LFP@C

[0140] Same as Example 1.

[0141] (3) Preparation of VC-coated diaphragm

[0142] A PP / PE / PP composite film was used as the base film, and VC was coated on the surface of the diaphragm by a mechanical mixing method to obtain a VC-coated diaphragm. VC was dissolved in a solvent, N-methylpyrrolidone (NMP), and stirred at a uniform speed for 30 minutes. The obtained suspension was coated on the surface of the diaphragm to obtain a VC-coated diaphragm, wherein the mass of VC coated on the surface of the diaphragm was 3% of the mass of the diaphragm.

[0143] (4) Preparation of positive electrode

[0144] LFP@C, carbon nanotubes, and PVDF were mixed in a mass ratio of 94:3:3, and NMP was used as a solvent, and the mixture was stirred evenly for 12 hours to obtain a positive electrode slurry. The positive electrode slurry was coated on a carbon-coated aluminum foil, and dried in an oven at 80°C for 12 hours to obtain a positive electrode sheet.

[0145] (5) Preparation of batteries

[0146] The positive electrode sheet prepared in step (4) is used as a graphite negative electrode, and the VC-coated diaphragm prepared in step (3) is used to prepare a battery cell, and the battery cell is packaged and injected with electrolyte to obtain a battery, the electrolyte lithium salt is LiPF6, the electrolyte solvent is ethylene carbonate (EC), ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC), and the electrolyte contains VC, and the mass of VC is 3% of the total mass of the electrolyte.

[0147] Example 6

[0148] (1) Preparation of lithium iron phosphate (LFP)

[0149] Same as Example 1.

[0150] (2) Preparation of LFP@C

[0151] Same as Example 1.

[0152] (3) Preparation of VC-coated LFP@C

[0153] The preparation method is the same as that of Example 1, wherein the total mass of VC coated on the surface of LFP@C is 1.5% of the total mass of LFP@C.

[0154] (4) Preparation of VC-coated carbon nanotubes

[0155] The preparation method is the same as that of Example 2, wherein the total mass of VC coated on the surface of the carbon nanotubes is 1.5% of the total mass of the carbon nanotubes.

[0156] (5) Preparation of positive electrode

[0157] VC-coated LFP@C, VC-coated carbon nanotubes, and PVDF were mixed in a mass ratio of 94:3:3, and NMP was used as a solvent, and stirred evenly for 12 hours to obtain a positive electrode slurry. The positive electrode slurry was coated on a carbon-coated aluminum foil and dried in an oven at 80°C for 12 hours to obtain a positive electrode sheet.

[0158] (6) Preparation of batteries

[0159] The positive electrode sheet prepared in step (5) is used to prepare a battery in the same manner as in Example 1.

[0160] Example 7

[0161] (1) Preparation of lithium iron phosphate (LFP)

[0162] Same as Example 1.

[0163] (2) Preparation of LFP@C

[0164] Same as Example 1.

[0165] (3) Preparation of VC-coated LFP@C

[0166] The preparation method is the same as that of Example 1, wherein the total mass of VC coated on the surface of LFP@C is 1.5% of the total mass of LFP@C.

[0167] (4) Preparation of VC-coated graphite

[0168] The negative electrode active material VC coated graphite was prepared by mechanical mixing. VC was dissolved in solvent N-methylpyrrolidone (NMP), stirred at a constant speed for 30 minutes, and then graphite was added thereto and stirred at a constant speed for 60 minutes to obtain a suspension. The above suspension was vacuum filtered, and the filter cake was dried at 60°C for 12 hours to obtain a VC coated graphite material, wherein the total mass of VC coated on the graphite surface was 1.5% of the total mass of graphite.

[0169] (5) Preparation of positive electrode

[0170] Same as Example 1.

[0171] (6) Preparation of negative electrode sheet

[0172] VC-coated graphite, carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR) and conductive carbon black (SP) were mixed in a mass ratio of 95.8:1.2:2:1, and water was used as a solvent. The mixture was stirred evenly for 12 hours to obtain a negative electrode slurry. The negative electrode slurry was coated on a carbon-coated copper foil and dried in an oven at 80°C for 12 hours to obtain a negative electrode sheet.

[0173] (7) Preparation of batteries

[0174] The positive electrode sheet prepared in step (5) and the negative electrode sheet prepared in step (6) are prepared into a battery cell using a PP / PE / PP composite separator, and the battery cell is packaged and injected with an electrolyte to obtain a battery. The electrolyte lithium salt is LiPF6, and the electrolyte solvent is ethylene carbonate (EC), ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC). The electrolyte contains VC, and the mass of VC is 3% of the total mass of the electrolyte.

[0175] Example 8

[0176] (1) Preparation of lithium iron phosphate (LFP)

[0177] Same as Example 1.

[0178] (2) Preparation of LFP@C

[0179] Same as Example 1.

[0180] (3) Preparation of VC-coated LFP@C

[0181] The only difference from Example 1 is that the total mass of VC coated on the surface of LFP@C is 0.5% of the total mass of LFP@C.

[0182] (4) Preparation of positive electrode

[0183] Same as Example 1.

[0184] (5) Preparation of batteries

[0185] Same as Example 1.

[0186] Example 9

[0187] (1) Preparation of lithium iron phosphate (LFP)

[0188] Same as Example 1.

[0189] (2) Preparation of LFP@C

[0190] Same as Example 1.

[0191] (3) Preparation of VC-coated LFP@C

[0192] The only difference from Example 1 is that the total mass of VC coated on the surface of LFP@C is 11% of the total mass of LFP@C.

[0193] (4) Preparation of positive electrode

[0194] Same as Example 1.

[0195] (5) Preparation of batteries

[0196] Same as Example 1.

[0197] Comparative Example 1

[0198] (1) Preparation of lithium iron phosphate (LFP)

[0199] Same as Example 1.

[0200] (2) Preparation of LFP@C

[0201] Same as Example 1.

[0202] (3) Preparation of positive electrode

[0203] LFP@C, carbon nanotubes, and PVDF were mixed in a mass ratio of 94:3:3, and NMP was used as a solvent, and the mixture was stirred evenly for 12 hours to obtain a positive electrode slurry. The positive electrode slurry was coated on a carbon-coated aluminum foil, and dried in an oven at 80°C for 12 hours to obtain a positive electrode sheet.

[0204] (4) Preparation of batteries

[0205] The positive electrode sheet prepared in step (3) is used to prepare a full battery, a graphite negative electrode and a PP / PE / PP composite separator are used to prepare a battery cell, the battery cell is packaged and injected with an electrolyte to obtain a battery, the electrolyte lithium salt is LiPF6, the electrolyte solvent is ethylene carbonate (EC), ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC), and the electrolyte contains VC, the mass of VC is 3% of the total mass of the electrolyte.

[0206] The batteries prepared in Examples 1-9 of the present invention and Comparative Example 1 were subjected to the following tests:

[0207] (1) Capacity retention test

[0208] Test method: 1) At room temperature, charge to 3.8V at 0.1C constant current and constant voltage, cut off at 0.02C, and leave for 10 minutes; then discharge to 2V at 0.1C, leave for 30 minutes, and record the first cycle discharge capacity as the initial capacity; 2) Place the battery in a 60°C environment, leave for 6 hours, charge to 3.8V at 1C constant current and constant voltage, cut off at 0.02C, and leave for 10 minutes; then discharge to 2V at 1C, and leave for 30 minutes; 3) Cycle step 2) 1000 times, record the 1000th cycle discharge capacity, and calculate the capacity retention rate. Using the above test method, the cycle performance diagram of the batteries prepared in Example 1, Example 2 and Comparative Example 1 at 60°C is as follows: Figures 3 to 5 As shown, the capacity retention rates of the batteries prepared in Examples 1-9 and Comparative Example 1 after 1000 cycles at 60° C. are shown in Table 1.

[0209] (2) VC distribution test on positive electrode

[0210] The batteries of Examples 1-9 and Comparative Example 1 were cycled for 1000 cycles at 60°C and then disassembled. The positive electrode sheets were taken out and immersed in ethyl acetate (EA). Figure 6 As shown, the surface of the positive electrode sheet in contact with the electrolyte is divided into 9 areas, and then the VC content of area 1, area 5 and area 9 is tested respectively by gas chromatography-mass spectrometry (GC-MS). The test results are shown in Table 1.

[0211] (3) Iron content test

[0212] The batteries of Examples 1-9 and Comparative Example 1 were cycled for 1000 cycles at 60° C. and then disassembled to remove the negative electrode sheets. The iron content of the negative electrode sheets was then tested using an inductively coupled plasma optical emission spectrometer (ICP-OES). The test results are shown in Table 1.

[0213] Table 1 Test results

[0214]

[0215]

[0216] It can be seen from Table 1 that, relative to Comparative Example 1, the battery of Examples 1-9 has a higher capacity retention rate after 1000 cycles at 60°C, a more uniform distribution of VC on the positive electrode sheet, and a higher VC content at the center of the positive electrode sheet (region 5), while the iron content in the negative electrode sheet is lower, indicating that the film-forming additives in the battery provided by the embodiment of the present invention are more evenly distributed, the content of the film-forming additives at the center of the battery is higher, and the negative electrode iron deposition can be well suppressed during the charge and discharge cycle, and the cycle performance is good. And by comparing Examples 1-5 and Examples 6-7, it can be found that when the addition amount of the film-forming additive VC is the same, the battery obtained by coating two positions at the same time has better cycle performance than that obtained by coating a single position.

[0217] The above-mentioned embodiments only express several implementation methods of the present application, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the present application. It should be pointed out that, for a person of ordinary skill in the art, several variations and improvements can be made without departing from the concept of the present application, and these all belong to the protection scope of the present application. Therefore, the protection scope of the present application shall be subject to the attached claims.

Claims

1. A battery cell, characterized in that: It includes a positive electrode sheet, a negative electrode sheet, and a separator located between the positive electrode sheet and the negative electrode sheet; the positive electrode sheet includes a positive electrode collector and a positive electrode active layer arranged on the positive electrode collector, and the positive electrode active layer includes a positive electrode active material and a positive electrode conductive agent; the surface of at least one of the positive electrode active material, the positive electrode conductive agent, the positive electrode sheet, the positive electrode collector on the side close to the positive electrode active layer and the separator is coated with a film-forming additive; the film-forming additive includes one or more of vinylene carbonate, fluoroethylene carbonate, vinyl ethylene carbonate, lithium difluorophosphate, propylene sulfite and vinyl sulfite.

2. The battery cell according to claim 1, characterized in that: The film-forming additive in the positive electrode sheet, and / or on the surface of the positive electrode sheet, and / or on the surface of the separator is uniformly distributed in the length direction and the width direction of the positive electrode sheet.

3. The battery cell according to claim 1, characterized in that: The film-forming additive is coated on at least two surfaces of the positive electrode active material, the positive electrode conductor, the positive electrode sheet, the positive electrode current collector on a side close to the positive electrode active layer, and the separator.

4. The battery cell according to claim 3, characterized in that: The surface of the positive electrode sheet is coated with the film-forming additive, and the surface of at least one of the positive electrode conductor, the positive electrode active material, the positive electrode current collector on the side close to the positive electrode active layer, and the separator is coated with the film-forming additive.

5. The battery cell according to claim 1, characterized in that: In the positive electrode active layer, the total mass of the film-forming additive coated on the surface of the positive electrode active material is 0.1%-11% of the total mass of the positive electrode active material; And / or, in the positive electrode active layer, the total mass of the film-forming additive coated on the surface of the positive electrode conductive agent is 0.1%-11% of the total mass of the positive electrode conductive agent; And / or, the mass of the film-forming additive coated on the surface of the positive electrode sheet is 0.1%-11% of the mass of the positive electrode sheet; and / or, the mass of the film-forming additive coated on the surface of the positive electrode current collector close to the positive electrode active layer is 0.1%-11% of the mass of the positive electrode current collector; And / or, the mass of the film-forming additive coated on the surface of the diaphragm is 0.1%-11% of the mass of the diaphragm.

6. The battery cell according to claim 1, characterized in that: The coverage rate of the film-forming additive on the surface of the positive electrode sheet close to the separator is greater than 60%, and / or the coverage rate of the film-forming additive on the surface of the positive electrode sheet away from the separator is greater than 60%; And / or, the coverage rate of the film-forming additive on the surface of the positive electrode current collector close to the positive electrode active layer is greater than 60%; And / or, the coverage rate of the film-forming additive on the surface of the separator close to the positive electrode sheet is greater than 60%, and / or, the coverage rate of the film-forming additive on the surface of the separator close to the negative electrode sheet is greater than 60%.

7. The battery cell according to any one of claims 1 to 6, characterized in that: The negative electrode sheet comprises a negative electrode current collector and a negative electrode active layer disposed on the negative electrode current collector, wherein the negative electrode active layer comprises a negative electrode active material and a negative electrode conductive agent; the surface of at least one of the negative electrode active material, the negative electrode conductive agent, the negative electrode sheet and the negative electrode current collector close to the negative electrode active layer is coated with the film-forming additive.

8. The battery cell according to claim 7, characterized in that: In the negative electrode active layer, the total mass of the film-forming additive coated on the surface of the negative electrode active material is 0.1%-11% of the total mass of the negative electrode active material; And / or, in the negative electrode active layer, the total mass of the film-forming additive coated on the surface of the negative electrode conductive agent is 0.1%-11% of the total mass of the negative electrode conductive agent; and / or, the mass of the film-forming additive coated on the surface of the negative electrode sheet is 0.1%-11% of the mass of the negative electrode sheet; And / or, the mass of the film-forming additive coated on the side of the negative electrode current collector close to the negative electrode active layer is 0.1%-11% of the mass of the negative electrode current collector.

9. The battery cell according to claim 7, characterized in that: The coverage rate of the film-forming additive on the surface of the negative electrode sheet close to the separator is greater than 60%, and / or the coverage rate of the film-forming additive on the surface of the negative electrode sheet away from the separator is greater than 60%; And / or, the coverage rate of the film-forming additive on the surface of the negative electrode current collector on a side close to the negative electrode active layer is greater than 60%.

10. The battery cell according to claim 1, characterized in that: The positive electrode active material includes an iron-based positive electrode material, and the iron-based positive electrode material includes at least one of lithium iron phosphate, lithium manganese iron phosphate, nickel cobalt manganese iron lithium, sodium iron phosphate, sodium iron pyrophosphate and sodium iron sulfate; And / or, the positive electrode conductive agent includes at least one of conductive carbon black, conductive graphite, carbon nanotubes, carbon fibers and graphene; And / or, the positive electrode current collector comprises at least one of aluminum foil and carbon-coated aluminum foil; And / or, the separator includes at least one of polyethylene and polypropylene.

11. The battery cell according to claim 7, characterized in that: The negative electrode active material comprises at least one of a carbon-based negative electrode active material, a silicon-based negative electrode active material and a tin-based negative electrode active material; And / or, the negative electrode conductive agent includes at least one of conductive carbon black, conductive graphite, carbon nanotubes, carbon fibers and graphene; And / or, the negative electrode current collector includes at least one of copper foil and carbon-coated copper foil.

12. A secondary battery, characterized in that: It comprises a packaging shell, and a battery cell and an electrolyte located in the packaging shell, wherein the battery cell comprises the battery cell according to any one of claims 1 to 11.

13. The secondary battery according to claim 12, characterized in that: The electrolyte contains the film-forming additive.

14. An electrical device, characterized in that: A secondary battery comprising the secondary battery according to claim 12 or 13.

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