A lithium / sodium ion battery negative electrode with a dual gradient structure and a preparation method thereof

Abstract

The present invention belongs to the technical field of alkali metal ion batteries, and discloses a negative electrode of a lithium / sodium ion battery with a dual gradient structure and a preparation method thereof. The dual gradient negative electrode is a double gradient sandwich structure constructed by a current collector modified with a strong lithium / sodium-philic conductive layer at the bottom of the negative electrode, an active layer in the middle of the negative electrode, and a lithium / sodium-phobic insulating passivation layer at the top of the negative electrode, wherein the bottom of the negative electrode is lithium / sodium-philic and conductive, the middle negative electrode active layer has moderate lithium / sodium-philicity and conductivity, and the top is lithium / sodium-phobic and insulating. By introducing a conductive and lithium / sodium-philic component at the bottom of the negative electrode with a low lithium / sodium nucleation barrier, after the negative electrode active layer accommodates lithium / sodium, it can induce excess lithium / sodium ions to nucleate and grow from the bottom of the negative electrode, avoid lithium / sodium precipitation on the surface of the negative electrode, effectively utilize the lower layer space of the negative electrode, realize dual-phase lithium / sodium storage, and greatly improve the specific capacity and cycle life of the negative electrode. At the same time, the insulating lithium-repellent passivation layer on the negative electrode surface is not only thin, but also well preserves the original pore structure of the negative electrode active layer, and has little effect on the wettability of the electrolyte and the diffusion process of lithium / sodium ions. Moreover, the insulating lithium-repellent property effectively hinders the reduction growth of lithium / sodium on the electrode surface. The synergistic effect of the dual-gradient structure not only greatly improves the specific capacity of the negative electrode through the dual-phase lithium / sodium storage characteristics, but also prevents the occurrence of lithium / sodium precipitation on the surface, significantly improving the cycle performance of the battery.

Description

Technical Field

[0001] The present invention belongs to the technical field of alkali metal ion batteries, and in particular relates to a lithium / sodium ion battery negative electrode with a double gradient structure and a preparation method thereof. Background Art

[0002] According to the global energy development strategy, vigorously developing new energy vehicles with lithium / sodium ion batteries as the main power core is undoubtedly the development trend of future automobiles. However, due to manufacturing process and design problems, or during use, there is an imbalance between the charging rate and the lithium / sodium ion embedding rate, the capacity of the positive and negative electrodes does not match, etc., which leads to the lithium / sodium ions tending to be reduced on the surface of the negative electrode, resulting in the notorious lithium / sodium precipitation problem. Lithium / sodium precipitation not only causes dendrite growth and easily evolves into "dead lithium / sodium", causing active material loss and increased interface impedance, but also lithium / sodium dendrite growth easily pierces the diaphragm, causing a series of safety problems such as thermal runaway of the battery. Therefore, improving the lithium / sodium precipitation phenomenon on the negative electrode interface, improving the utilization rate of lithium / sodium ions, and then improving the electrochemical and safety performance of the battery has become one of the technical difficulties in expanding the application market of new energy vehicles, which needs to be solved urgently.

[0003] Through the above analysis, the problems and defects of the prior art are as follows:

[0004] Due to the mismatch between the positive and negative electrode capacities caused by battery design, or the battery working under high-rate charging conditions, the charging rate and the rate at which lithium / sodium ions are embedded in the active material are unbalanced, which will cause lithium / sodium ions to tend to be reduced on the negative electrode surface, resulting in lithium / sodium precipitation on the electrode surface. Lithium / sodium precipitation not only easily leads to dendrite growth and evolution into "dead lithium / sodium", causing loss of active materials and increased interface impedance, but also lithium / sodium dendrite growth easily pierces the diaphragm, causing a series of safety problems such as thermal runaway of the battery. Summary of the invention

[0005] In view of the problems existing in the prior art, the present invention provides a lithium / sodium ion battery negative electrode with a dual gradient structure and a preparation method thereof.

[0006] The present invention is achieved in this way: a lithium / sodium ion battery negative electrode with a dual gradient structure comprises a negative electrode current collector modified by a strong lithium / sodium affinity conductive layer, a negative electrode active layer and a lithium / sodium phobic insulating passivation layer on the top of the negative electrode, forming a dual gradient sandwich structure with lithium / sodium affinity conductive at the bottom of the negative electrode active layer and lithium / sodium phobic insulation at the top; the negative electrode current collector modified by the strong lithium / sodium affinity conductive layer and the negative electrode active layer constitute a lithium / sodium ion battery negative electrode with a strong lithium / sodium affinity at the bottom; the negative electrode active layer comprises a negative electrode active material, a conductive additive, an adhesive, etc.; the strong lithium / sodium affinity modified layer has a thickness of 100-2000nm; the lithium phobic insulating passivation layer is highly conformal to the particle morphology on the surface of the active layer, and has a thickness of 10-1000nm.

[0007] A method for preparing a double-gradient modified lithium-ion battery negative electrode, comprising:

[0008] Step 1: prepare a certain concentration of AgNO3 aqueous solution, and quickly scrape the AgNO3 aqueous solution on the copper foil current collector by scraping with a scraper at room temperature, and let it stand for a few minutes to react; after washing and drying, a lithium-philic Ag particle modification layer with a thickness of about 1 μm is grown on the surface of the copper foil;

[0009] Step 2: Mix the negative electrode active material graphite, the conductive additive acetylene black, the binder PVDF, etc. in a solvent to prepare a slurry, and then coat it on the copper foil current collector modified with the above-mentioned lithium-philic Ag particle modification layer by a conventional slurry coating method; put the obtained electrode sheet into a vacuum drying oven at 120° C. for high-temperature drying to prepare a lithium-philic Ag-modified graphite negative electrode sheet;

[0010] Step 3, dissolving the lithium-phobic insulating polymer polyvinyl alcohol in a water solvent, heating and stirring on a magnetic stirrer at a heating temperature of 75° C. to fully dissolve it;

[0011] Step 4, the obtained polyvinyl alcohol solution is quickly scraped on the surface of the lithium-philic Ag-modified graphite negative electrode sheet by scraping with a scraper to obtain a graphite negative electrode sheet with a sandwich structure having an insulating lithium-phobic surface and a conductive lithium-philic bottom layer;

[0012] The dual gradient is a lithium-philic / sodium gradient and an electronic conductivity gradient, wherein the top lithium-phobic / sodium insulating passivation layer has the worst lithium-philicity and the lowest conductivity, the middle negative electrode active layer has moderate lithium-philicity and moderate conductivity, and the bottom strongly lithium-philic / sodium conductive modification layer has the strongest lithium-philicity and the highest conductivity.

[0013] Furthermore, the strong lithium / sodium affinity component is a mixture of one or more organic substances, polymers, inorganic substances, metal elements, etc. with high electronic conductivity and high lithium / sodium affinity, including at least one of Ag, Zn, Co, Sn, Al, In2O3, AgO, ZnS, CoP, Ni2P, TiN, Li-Zn, Li-Al, Li-Sn alloy, etc.

[0014] Furthermore, the current collector is a conventional current collector that has been mass-produced, including one of aluminum foil, nickel foil, stainless steel foil, copper foil, etc.

[0015] Furthermore, the negative electrode active material of the lithium / sodium ion battery includes one of graphite, silicon-carbon composite material, silicon oxide, hard carbon, etc.; the conductive agent includes at least one of conductive carbon black and carbon nanotubes; the binder includes at least one of sodium carboxymethyl cellulose, styrene-butadiene rubber, polyvinylidene fluoride, polyacrylic acid, polyvinyl alcohol, polyethylene oxide, and polyacrylonitrile; and the powder particles of the obtained negative electrode active layer have a certain porosity so that the electrolyte can infiltrate therein.

[0016] Furthermore, the introduction of a strong lithium / sodium-philic layer can be carried out by at least one of chemical vapor deposition, physical vapor deposition or wet chemical method; the chemical vapor deposition method includes plasma enhanced chemical deposition, atomic layer deposition, etc.; the physical vapor deposition method includes electron beam coating, magnetron sputtering, thermal evaporation coating, etc.; the wet chemical method includes hydrothermal method, electrochemical deposition method, spray pyrolysis, etc.

[0017] Another object of the present invention is to provide a method for preparing the negative electrode of a lithium / sodium ion battery with a strongly lithium-philic / sodium-philic conductive layer modified current collector, which is characterized in that the negative electrode active material, conductive additive, binder, etc. are first mixed in a solvent to form a slurry, which is then coated on a conventional current collector modified with a lithium / sodium-philic layer by a traditional slurry coating method, and dried to obtain the negative electrode sheet of a lithium / sodium ion battery with a strongly lithium-philic / sodium-philic layer modified current collector.

[0018] Another object of the present invention is to provide a method for preparing the above-mentioned dual gradient structure modified lithium / sodium ion battery negative electrode, characterized in that it comprises:

[0019] At least one of the lithium-phobic and insulating organic or polymer components is added to a solvent and fully dissolved to form a solution; preferably, in a preferred embodiment of the present invention, the solvent refers to an aqueous solution or an organic solvent, including at least one of ethylene glycol dimethyl ether (DME), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAC), dimethyl sulfoxide (DMSO), N-methylpyrrolidone (NMP), methanol, etc.;

[0020] The solution is coated on the surface of the finished product of the negative electrode sheet of the lithium / sodium ion battery with the strongly lithium-philic / sodium-philic layer modified current collector by spraying or rapid scraping, and is immediately dried at high temperature to evaporate the solvent quickly, thereby obtaining a double-gradient modified lithium / sodium ion battery negative electrode with a sandwich structure.

[0021] Furthermore, the top polymer component may be a lithium-phobic polymer with low electronic conductivity, including one or two of polyvinyl alcohol, melamine, etc.

[0022] Furthermore, the electrode sheet is directly coated by a simple spray coating method or a blade coating method; in particular, the morphology of the surface insulating passivation layer is highly conformal to the morphology of the powder particles on the surface of the negative electrode active layer, and the pore structure between the particles is retained to the greatest extent to allow electrolyte infiltration.

[0023] Furthermore, the working potential of the negative electrode can be lower than 0V vs.Na / Na + or 0V vs.Li / Li + ; In particular, when the amount of lithium / sodium ions is appropriate, it can induce the rapid diffusion of lithium / sodium ions, avoid concentration polarization caused by lithium / sodium ion enrichment, and achieve uniform lithium / sodium insertion; when lithium / sodium is excessive, it can induce lithium / sodium to precipitate uniformly on the current collector side, thereby improving the storage capacity and reversibility of lithium / sodium ions.

[0024] In combination with the above technical solutions and the technical problems solved, the advantages and positive effects of the technical solutions to be protected by the present invention are as follows:

[0025] First, in view of the technical problems existing in the above-mentioned prior art and the difficulty of solving the problems, the technical solutions to be protected by the present invention and the results and data during the research and development process are closely combined to analyze in detail and deeply how the technical solutions of the present invention solve the technical problems, and some creative technical effects brought about after solving the problems. The specific description is as follows:

[0026] The present invention provides a dual-gradient modified lithium / sodium ion battery negative electrode sheet capable of dual-phase lithium storage. The bottom is conductive and a lithium / sodium-philic component is introduced to construct a lithium / sodium-philic gradient, which can accelerate the diffusion of lithium / sodium ions inside the electrode, homogenize the lithium / sodium ion flux, and avoid the enrichment of lithium / sodium ions on the electrode surface to cause concentration polarization inside the battery, thereby precipitating lithium / sodium on the surface. The lithium / sodium-philic component at the bottom also has a low lithium / sodium nucleation barrier, and can induce excess lithium / sodium ions to nucleate and grow from the bottom of the negative electrode first after the negative electrode active layer accommodates lithium / sodium, effectively utilizing the lower space of the active layer, realizing dual-phase lithium / sodium storage, and improving the reversibility of lithium / sodium ions.

[0027] The present invention performs lithium / sodium-philic modification on the current collector. The lithium / sodium-philic layer material acts as a heterogeneous nucleation point while adsorbing lithium / sodium ions, inducing additional lithium / sodium ions that are not embedded in the active layer to preferentially precipitate at the bottom, so that the nucleation and precipitation of lithium / sodium has a bottom-up directionality. In coordination with the insulating lithium-phobic layer on the top, a dual-gradient modified negative electrode is constructed in which a lithium-philic gradient and an electronic conductive gradient coexist, which effectively improves the lithium / sodium precipitation behavior and dendrite growth at the interface and enhances the safety performance of the battery. In addition, lithium / sodium ions that exceed the normal lithium / sodium storage capacity of the negative electrode active material can also be uniformly deposited at the bottom and stored in the form of metallic lithium / sodium, achieving a negative electrode voltage below 0V vs. Li / Li + or 0V vs.Na / Na+ The normal working ability under the potential of widened the working voltage range and specific capacity of the battery. The upper, middle and lower layers of the sandwich configuration each have their own functions and cooperate with each other to construct a dual-gradient modified lithium / sodium ion battery negative electrode sheet capable of dual-phase lithium / sodium storage.

[0028] The present invention adopts the existing preparation process (plasma enhanced chemical deposition method, atomic layer deposition method, electron beam coating method, magnetron sputtering method, thermal evaporation coating method, spray pyrolysis method, solution replacement method, etc.) to achieve lithium / sodium-philic modification of the current collector, and the preparation process is mature, and the operation is simple and efficient. The current collector after lithium / sodium-philic modification has no effect on the subsequent slurry coating, and is compatible with the current electrode preparation process. In addition, when the slurry is coated with the negative electrode active layer, the binder in the slurry not only increases the bonding force between the active layer powder and the current collector, but also indirectly promotes the lithium / sodium-philic layer to be tightly bonded to the current collector, thereby enhancing the overall structural stability of the electrode sheet.

[0029] The insulating lithium-phobic passivation layer formed by the present invention uses a lithium-phobic polymer with low electronic conductivity and is relatively thin. The solution scraping method and the spray coating method used for the coating layer both well retain the original pore structure of the negative electrode active layer, which not only hinders the reduction of lithium on the electrode surface, but also basically does not affect the infiltration of the electrolyte and the diffusion process of lithium ions.

[0030] Second, considering the technical solution as a whole or from the perspective of the product, the technical effects and advantages of the technical solution to be protected by the present invention are described in detail as follows:

[0031] The present invention provides a dual-gradient modified lithium / sodium ion battery negative electrode sheet capable of dual-phase lithium storage. By introducing a conductive and lithium / sodium-philic component at the bottom to construct a lithium / sodium-philic gradient, the diffusion of lithium / sodium ions inside the electrode can be accelerated, the lithium / sodium ion flux can be homogenized, and the lithium / sodium ion enrichment on the electrode surface can be avoided to cause concentration polarization inside the battery, thereby preventing lithium / sodium from being precipitated on the surface. The lithium / sodium-philic component at the bottom also has a low lithium / sodium nucleation barrier, and can induce excess lithium / sodium ions to nucleate and grow from the bottom of the negative electrode first after the negative electrode active layer accommodates lithium / sodium, effectively utilizing the bottom space of the active layer, realizing dual-phase lithium / sodium storage, and improving the reversibility of charge and discharge of lithium / sodium ions.

[0032] The present invention performs lithium / sodium-philic modification on the current collector. The lithium / sodium-philic layer material acts as a heterogeneous nucleation point while adsorbing lithium / sodium ions, inducing additional lithium / sodium ions that are not embedded in the active layer to preferentially precipitate at the bottom, so that the nucleation and precipitation of lithium / sodium has a bottom-up directionality. In coordination with the insulating lithium-phobic layer on the top, a dual-gradient modified negative electrode is constructed in which a lithium-philic gradient and an electronic conductive gradient coexist, which effectively improves the lithium / sodium precipitation behavior and dendrite growth at the interface and enhances the safety performance of the battery. In addition, lithium / sodium ions that exceed the normal lithium / sodium storage capacity of the negative electrode active material can also be uniformly deposited at the bottom and stored in the form of metallic lithium / sodium, achieving a negative electrode voltage below 0V vs. Li / Li + or 0V vs.Na / Na + The normal working ability under the potential of wide voltage range of the battery is improved. The upper, middle and lower layers of the sandwich configuration each have their own functions and cooperate with each other to construct a dual-gradient modified lithium / sodium ion battery negative electrode sheet capable of dual-phase lithium / sodium storage.

[0033] The present invention adopts the existing preparation process (plasma enhanced chemical deposition method, atomic layer deposition method, electron beam coating method, magnetron sputtering method, thermal evaporation coating method, spray pyrolysis method, solution replacement method, etc.) to achieve lithium / sodium-philic modification of the current collector, and the preparation process is mature, and the operation is simple and efficient. The current collector after lithium / sodium-philic modification has no effect on the subsequent slurry coating, and is compatible with the current electrode preparation process. In addition, when the slurry is coated with the negative electrode active layer, the binder in the slurry not only increases the bonding force between the active layer powder and the current collector, but also indirectly promotes the lithium / sodium-philic layer to be tightly bonded to the current collector, thereby enhancing the overall structural stability of the electrode sheet.

[0034] The insulating lithium-phobic passivation layer formed by the present invention uses a lithium-phobic polymer with low electronic conductivity and is relatively thin. The solution scraping method and the spray coating method used for the coating layer both well retain the original pore structure of the negative electrode active layer, which not only hinders the reduction of lithium / sodium on the electrode surface, but also basically does not affect the infiltration of the electrolyte and the diffusion process of lithium / sodium ions. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 It is a flow chart of a method for preparing a double-gradient modified lithium-ion battery negative electrode provided by an embodiment of the present invention.

[0036] Figure 2 The present invention is a schematic diagram of a lithium-ion battery negative electrode with a sandwich configuration and double gradient modification prepared in Example 1 provided in an embodiment of the present invention. The negative electrode comprises a current collector (1) modified with a bottom strong lithium-philic conductive layer (2), a negative electrode active layer (3) and a top insulating lithium-phobic passivation layer (4), forming a double gradient (electronic conductivity gradient, lithium-philic gradient) modified negative electrode with a sandwich configuration.

[0037] Figure 3This is a cross-sectional elemental surface scanning diagram of a graphite negative electrode with a double gradient modification of silver particles and a surface coated with insulating lithium-phobic polyvinyl alcohol and a bottom layer prepared in Example 1 of the present invention. Figure 3 As shown, the copper element in the lower layer is attributed to the copper foil current collector. The Ag signal is clearly detected on the copper foil current collector, indicating that silver is successfully modified at the bottom of the graphite layer, realizing the construction of the bottom lithium-philic conductive structure. The carbon signal distribution above the silver layer is also clearly detected, which can be attributed to the carbon elements in the graphite and the insulating lithium-phobic polymer coated on the top. This shows that the double-gradient modified graphite negative electrode with the surface coated with insulating lithium-phobic polyvinyl alcohol and the bottom modified with silver particles is successfully prepared.

[0038] Figure 4 This is a cross-sectional scanning electron microscope image of the negative electrode of a lithium-ion battery prepared in Example 2 with 40% excess lithium provided in the embodiment of the present invention. The specific implementation method is to prepare a double gradient modified silicon-carbon negative electrode prepared in Example 2 with a lower layer modified with lithium-philic conductive Zn and an upper surface modified with insulating lithium-phobic melamine as the negative electrode, a metal lithium foil as the counter electrode, and an ester electrolyte, a diaphragm, etc. to assemble into a half-cell. The half-cell is tested by electrochemical deposition of lithium on a battery charging and discharging system. Figure 4 As shown in the scanning electron micrograph of the electrode cross section, even in the extreme case of 40% excess lithium deposition, there is no obvious lithium accumulation on the upper surface of the double-gradient modified negative electrode, and a large number of pore structures are still maintained between the negative electrode active layer particles on the top. Since the bottom is modified with the Zn layer, the lithium affinity is stronger and can induce lithium nucleation from bottom to top. Therefore, the pores between the negative electrode active particles are tightly filled with metallic lithium, indicating that the tight deposition of metallic lithium from bottom to top is achieved, and since the top is insulated and lithium-phobic, the synergistic effect avoids the precipitation of lithium on the top. In addition, this phenomenon also proves that the working potential of the modified negative electrode can be lower than 0Vvs.Li / Li + It can not only effectively improve the reversibility of lithium ions and realize dual-phase (active material / lithium element) lithium storage, but also improve the safety of the negative electrode.

[0039] Figure 5 This is a cycle stability diagram of a full battery assembled with a double-gradient modified hard carbon negative electrode and a lithium iron phosphate positive electrode prepared in Example 3 provided in an embodiment of the present invention. The preparation method of the lithium battery comprises: mixing the positive electrode active material (LiFePO4), the conductive agent (conductive carbon black), and the binder (PVDF) in a solvent NMP to obtain a positive electrode slurry, and then evenly coating the positive electrode slurry on the positive electrode collector aluminum foil, and obtaining the positive electrode plate after drying and rolling. The double-gradient modified hard carbon negative electrode plate, ester electrolyte, diaphragm, and positive electrode plate prepared in Example 3 are assembled to obtain a full battery. A charge and discharge test (1C) was carried out under a voltage range of 2.2-3.8V. The test results are as follows Figure 5As shown. Since it can induce excess lithium to deposit from the bottom, improve the utilization rate of lithium ions, and prevent metal lithium from precipitating on the negative electrode surface and evolving into lithium dendrites and dead lithium, compared with the unmodified negative electrode assembled LiFePO4 full battery, the double gradient modified graphite negative electrode assembled LiFePO4 full battery has higher reversibility, less capacity decay, and better cycle stability. DETAILED DESCRIPTION

[0040] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0041] In order to enable those skilled in the art to fully understand how to implement the present invention in detail, this section is an explanatory embodiment that expands and describes the technical solution of the claims.

[0042] like Figure 1 As shown, the present invention provides a method for preparing a double-gradient modified lithium-ion battery negative electrode, comprising:

[0043] S101, preparing a certain concentration of AgNO3 aqueous solution, and quickly scraping the AgNO3 aqueous solution on the copper foil current collector by scraping with a scraper at room temperature, and letting it stand for a few minutes to react; after washing and drying, a lithium-philic Ag particle modification layer with a thickness of about 1 μm is grown on the surface of the copper foil;

[0044] S102, mixing graphite, a negative electrode active material, acetylene black, a conductive additive, PVDF, etc. in a solvent to prepare a slurry, and then coating the slurry on the copper foil current collector modified with the lithium-philic Ag particle modification layer by a conventional slurry coating method; placing the obtained electrode sheet in a vacuum drying oven at 120° C. for high-temperature drying to obtain a lithium-philic Ag-modified graphite negative electrode sheet;

[0045] S103, dissolving the lithium-phobic insulating polymer polyvinyl alcohol in a water solvent, heating and stirring on a magnetic stirrer at a heating temperature of 75° C. to fully dissolve it;

[0046] S104, quickly coating the obtained polyvinyl alcohol solution on the surface of the lithium-philic Ag-modified graphite negative electrode sheet by scraping with a scraper, so as to obtain a graphite negative electrode sheet with a sandwich structure having an insulating lithium-phobic surface and a conductive lithium-philic bottom layer.

[0047] A lithium / sodium ion battery negative electrode with a dual gradient structure comprises a negative electrode current collector modified with a strong lithium / sodium affinity conductive layer, a negative electrode active layer and a lithium / sodium phobic insulating passivation layer on the top of the negative electrode, forming a dual gradient sandwich structure with lithium / sodium affinity conductive at the bottom of the negative electrode active layer and lithium / sodium phobic insulation at the top; the negative electrode current collector modified with a strong lithium / sodium affinity conductive layer and the negative electrode active layer constitute a lithium / sodium ion battery negative electrode with a strong lithium / sodium affinity at the bottom; the negative electrode active layer comprises a negative electrode active material, a conductive additive, an adhesive, etc.; the strong lithium / sodium affinity modified layer has a thickness of 100-2000nm; the lithium phobic insulating passivation layer is highly conformal to the surface particle morphology of the active layer, and has a thickness of 10-1000nm.

[0048] The dual gradient is a lithium-philic / sodium gradient and an electronic conductivity gradient, wherein the top lithium-phobic / sodium insulating passivation layer has the worst lithium-philicity and the lowest conductivity, the middle negative electrode active layer has moderate lithium-philicity and moderate conductivity, and the bottom strongly lithium-philic / sodium conductive modification layer has the strongest lithium-philicity and the highest conductivity.

[0049] The dual gradient provided by the present invention is a lithium-philic / sodium gradient and an electronic conductivity gradient, wherein the top lithium-phobic / sodium insulating passivation layer has the worst lithium-philicity and the lowest conductivity, the middle negative electrode active layer has moderate lithium-philicity and moderate conductivity, and the bottom strongly lithium-philic / sodium conductive modification layer has the strongest lithium-philicity and the highest conductivity.

[0050] The strong lithium / sodium affinity component provided by the present invention is a mixture of one or more organic substances, polymers, inorganic substances, metals, etc. with high electronic conductivity and high lithium / sodium affinity, including at least one of Ag, Zn, Co, Sn, Al, In2O3, AgO, ZnS, CoP, Ni2P, TiN, Li-Zn, Li-Al, Li-Sn alloy, etc.

[0051] The current collector provided by the present invention is a conventional current collector that has been mass-produced at present, including one of aluminum foil, nickel foil, stainless steel foil, copper foil and the like.

[0052] The lithium / sodium ion battery negative electrode active material provided by the present invention comprises one of graphite, silicon-carbon composite material, hard carbon, etc.; the conductive agent comprises at least one of conductive carbon black and carbon nanotubes; the binder comprises at least one of sodium carboxymethyl cellulose, styrene-butadiene rubber, polyvinylidene fluoride, polyacrylic acid, polyvinyl alcohol, polyethylene oxide, and polyacrylonitrile; and the powder particles of the obtained negative electrode active layer have a certain porosity so that the electrolyte can infiltrate therein.

[0053] The negative electrode current collector with a strong lithium / sodium-philic conductive modification layer provided by the present invention is characterized in that the introduction of the strong lithium / sodium-philic layer can be carried out by at least one of chemical vapor deposition, physical vapor deposition or wet chemical method, etc.; the chemical vapor deposition method includes plasma enhanced chemical deposition method, atomic layer deposition method, etc.; the physical vapor deposition method includes electron beam coating method, magnetron sputtering method, thermal evaporation coating method, etc.; the wet chemical method includes hydrothermal method, electrochemical deposition method, spray pyrolysis method, etc.

[0054] A method for preparing a negative electrode for a lithium / sodium ion battery with a strongly lithium-philic / sodium-philic conductive layer modified current collector, characterized in that a negative electrode active material, a conductive additive, a binder, etc. are first mixed in a solvent to form a slurry, which is then coated on a conventional current collector modified with a lithium / sodium-philic layer by a traditional slurry coating method, and dried to obtain the above-mentioned lithium / sodium ion battery negative electrode sheet with a strongly lithium-philic / sodium-philic layer modified current collector.

[0055] A method for preparing a lithium / sodium ion battery negative electrode modified with a dual gradient structure, characterized in that it comprises:

[0056] At least one of the lithium-phobic and insulating organic or polymer components is added to a solvent and fully dissolved to form a solution. Preferably, in a preferred embodiment of the present invention, the solvent refers to an aqueous solution or an organic solvent, including at least one of ethylene glycol dimethyl ether (DME), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAC), dimethyl sulfoxide (DMSO), N-methylpyrrolidone (NMP), methanol, etc.;

[0057] The solution is coated on the surface of the finished negative electrode sheet of the lithium / sodium ion battery with the strongly lithium-philic / sodium-philic layer modified current collector by spraying or rapid scraping, and is immediately placed in an oven for high-temperature drying to allow the solvent to evaporate rapidly, thereby obtaining a double-gradient modified lithium / sodium ion battery negative electrode with a sandwich structure.

[0058] The top polymer component provided by the present invention can be selected from lithium-phobic polymers with low electronic conductivity, including one or two of polyvinyl alcohol, melamine, etc.

[0059] The present invention provides a method for directly coating the electrode sheet using a simple spray coating method or a scraping coating method; in particular, the morphology of the surface insulating passivation layer is highly conformal to the morphology of the powder particles on the surface of the negative electrode active layer, and the pore structure between the particles is retained to the greatest extent to allow electrolyte infiltration.

[0060] The working potential of the negative electrode provided by the present invention can be lower than 0V vs.Na / Na + or 0V vs.Li / Li +; In particular, when the amount of lithium / sodium ions is appropriate, it can induce the rapid diffusion of lithium / sodium ions, avoid concentration polarization caused by lithium / sodium ion enrichment, and achieve uniform lithium / sodium insertion; when lithium / sodium is excessive, it can induce lithium / sodium to precipitate uniformly on the current collector side, thereby improving the storage capacity and reversibility of lithium / sodium ions.

[0061] In order to prove the creativity and technical value of the technical solution of the present invention, this section provides application examples of the claimed technical solution on specific products or related technologies.

[0062] Example 1

[0063] This embodiment provides a double gradient modified lithium ion battery negative electrode, the structure of which is as follows Figure 2 As shown. It includes a current collector (1) modified by a bottom strong lithium-philic conductive layer (2), a negative electrode active layer (3) and a top insulating lithium-phobic passivation layer (4), forming a sandwich negative electrode structure modified with a double gradient (electronic conductivity gradient, lithium-philic gradient). The modified current collector and the negative electrode active layer constitute a lithium-ion battery negative electrode modified with a conductive strong lithium-philic layer. The preparation method of the double gradient modified lithium-ion battery negative electrode comprises:

[0064] (1) Prepare a certain concentration of AgNO3 aqueous solution, and quickly scrape the AgNO3 aqueous solution on the copper foil current collector by scraping with a scraper at room temperature, and let it stand for a few minutes to react. After washing and drying, a lithium-philic Ag particle modification layer with a thickness of about 1 μm is grown on the copper foil surface.

[0065] (2) The negative electrode active material graphite, the conductive additive acetylene black, the binder PVDF, etc. are mixed in a solvent to prepare a slurry, and then the slurry is coated on the copper foil current collector modified with the above-mentioned lithium-philic Ag particle modification layer by a conventional slurry coating method. The obtained electrode sheet is placed in a vacuum drying oven at 120°C for high-temperature drying to prepare a lithium-philic Ag-modified graphite negative electrode sheet.

[0066] (3) Dissolve the lithium-phobic insulating polymer polyvinyl alcohol in a water solvent, and heat and stir it on a magnetic stirrer at a heating temperature of 75° C. to fully dissolve it.

[0067] (4) The obtained polyvinyl alcohol solution is quickly scraped onto the surface of the lithium-philic Ag-modified graphite negative electrode sheet by scraping with a scraper to obtain a graphite negative electrode sheet with a sandwich structure having an insulating lithium-phobic surface and a conductive lithium-philic bottom layer.

[0068] The elemental surface scanning image of the cross section of the graphite negative electrode with an insulating surface that is lithium-phobic and a conductive bottom layer that is lithium-philic is shown in the figure. Figure 3As shown, the copper element in the lower layer is attributed to the copper foil current collector. The Ag signal is clearly detected on the copper foil current collector, indicating that silver is successfully modified at the bottom of the graphite layer, realizing the construction of the bottom lithium-philic conductive structure. The carbon signal distribution above the silver layer is also clearly detected, which can be attributed to the carbon elements in the graphite and the insulating lithium-phobic polymer coated on the top. This shows that the double-gradient modified graphite negative electrode with the surface coated with insulating lithium-phobic polyvinyl alcohol and the bottom modified with silver particles is successfully prepared.

[0069] This embodiment also provides a lithium battery comprising the above-mentioned double-gradient modified graphite negative electrode, the lithium battery comprising a positive electrode plate, an electrolyte, and the above-mentioned negative electrode plate, and the preparation method of the lithium battery comprises:

[0070] The positive electrode active material (LiFePO4), conductive agent (conductive carbon black), and binder (PVDF) are mixed evenly in the solvent NMP to obtain a positive electrode slurry, and then the positive electrode slurry is evenly coated on the positive electrode current collector aluminum foil, and the positive electrode sheet is obtained after drying and rolling. The above negative electrode sheet, ester electrolyte, separator, and positive electrode sheet are assembled to obtain a lithium battery.

[0071] Example 2

[0072] This embodiment provides a double-gradient modified lithium-ion battery negative electrode, the structure of which is consistent with that of embodiment 1, and the preparation method includes:

[0073] (1) A Zn layer with a thickness of 500 nm was sputtered on the surface of the copper foil by magnetron sputtering to obtain a Zn-modified copper foil.

[0074] (2) The negative electrode active material silicon carbon composite, the conductive agent acetylene black, the binder CMC and SBR are mixed evenly in an aqueous solvent to obtain a negative electrode slurry, and then coated on the copper foil current collector modified with the above-mentioned lithium-philic conductive Zn layer by a conventional slurry coating method. The obtained electrode sheet is placed in a vacuum drying oven at 120° C. for high-temperature drying to obtain a lithium-philic Zn modified negative electrode sheet.

[0075] (3) Dissolve the lithium-phobic insulating polymer melamine in a methanol solvent and stir it on a magnetic stirrer to fully dissolve it.

[0076] (4) The obtained melamine solution is quickly sprayed on the surface of the lithium-philic Zn-modified silicon-carbon negative electrode sheet by spraying, and after drying, a sandwich structured negative electrode sheet with an insulating lithium-phobic surface and a conductive lithium-philic bottom layer is obtained.

[0077] In this embodiment, the double gradient modified silicon carbon negative electrode prepared above is used as the negative electrode, and the upper surface is modified with insulating lithium-phobic melamine. The metal lithium foil is used as the counter electrode, and an ester electrolyte, a separator, etc. are added to assemble into a half-cell. The half-cell is tested by electrochemical deposition of lithium on a battery charging and discharging system. Figure 4 As shown in the scanning electron micrograph of the electrode cross section, even in the extreme case of 40% excess lithium deposition, there is no obvious lithium accumulation on the upper surface of the double-gradient modified negative electrode, and a large number of pore structures are still maintained between the negative electrode active layer particles on the top. Since the bottom is modified with the Zn layer, the lithium affinity is stronger and can induce lithium nucleation from bottom to top. Therefore, the pores between the negative electrode active particles are tightly filled with metallic lithium, indicating that the tight deposition of metallic lithium from bottom to top is achieved, and since the top is insulated and lithium-phobic, the synergistic effect avoids the precipitation of lithium on the top. In addition, this phenomenon also proves that the working potential of the modified negative electrode can be lower than 0V vs.Li / Li + It can not only effectively improve the reversibility of lithium ions and realize dual-phase (active material / lithium element) lithium storage, but also improve the safety of the negative electrode.

[0078] This embodiment also provides a lithium battery comprising the above-mentioned dual-gradient modified silicon-carbon negative electrode, the lithium battery comprising a positive electrode plate, an electrolyte, and the above-mentioned negative electrode plate, and the preparation method of the lithium battery comprises:

[0079] The positive electrode active material (NCM811), conductive agent (conductive carbon black), and binder (PVDF) are mixed evenly in the solvent NMP to obtain a positive electrode slurry, and then the positive electrode slurry is evenly coated on the positive electrode current collector aluminum foil, and the positive electrode sheet is obtained after drying and rolling. The above negative electrode sheet, ester electrolyte, separator, and positive electrode sheet are assembled to obtain a lithium battery.

[0080] Example 3

[0081] This embodiment provides a double-gradient modified lithium-ion battery negative electrode, the structure of which is consistent with that of embodiment 1, and the preparation method includes:

[0082] (1) A TiN layer with a thickness of 200 nm was sputtered on the surface of the copper foil by magnetron sputtering to obtain a TiN-modified copper foil.

[0083] (2) The negative electrode active material hard carbon, the conductive agent acetylene black, carbon nanotubes, the binder CMC and SBR are mixed evenly in an aqueous solvent to obtain a negative electrode slurry, and then coated on the copper foil current collector modified with the above-mentioned lithium-philic conductive TiN modification layer by a conventional slurry coating method. The obtained electrode sheet is placed in a vacuum drying oven at 120°C for high-temperature drying to prepare a lithium-philic TiN modified hard carbon negative electrode sheet.

[0084] (3) The lithium-phobic insulating polymer polyvinyl alcohol is dissolved in an aqueous solvent and heated and stirred on a magnetic stirrer (75° C.) to fully dissolve the polymer.

[0085] (4) The obtained polyvinyl alcohol solution is quickly coated on the surface of the lithium-philic TiN-modified hard carbon negative electrode sheet by scraping with a scraper, and after drying, a hard carbon negative electrode sheet with a sandwich structure having an insulating lithium-phobic surface and a conductive lithium-philic bottom layer is obtained.

[0086] This embodiment also provides a lithium battery using the above double gradient modified hard carbon as a negative electrode, the lithium battery comprising a positive electrode plate, an electrolyte, and the above negative electrode plate, and the preparation method of the lithium battery comprises:

[0087] The positive electrode active material (LiFePO4), conductive agent (conductive carbon black), and binder (PVDF) are mixed evenly in the solvent NMP to obtain a positive electrode slurry, and then the positive electrode slurry is evenly coated on the positive electrode current collector aluminum foil, and the positive electrode sheet is obtained after drying and rolling. The above negative electrode sheet, ester electrolyte, separator, and positive electrode sheet are assembled to obtain a lithium battery.

[0088] The charge and discharge test (1C) was carried out under the voltage range of 2.2-3.8V. The test results are as follows Figure 5 As shown. Since it can induce excess lithium to deposit from the bottom, improve the utilization rate of lithium ions, and prevent metal lithium from precipitating on the negative electrode surface and evolving into lithium dendrites, compared with the unmodified negative electrode assembled LiFePO4 full battery, the double gradient modified graphite negative electrode assembled LiFePO4 full battery has less capacity decay and better cycle stability.

[0089] Example 4

[0090] This embodiment provides a double-gradient modified sodium ion battery negative electrode, the structure of which is consistent with that of embodiment 1, and the preparation method includes:

[0091] (1) A 150 nm thick Sn layer was sputtered on the surface of the copper foil by electrochemical deposition to obtain Zn-modified aluminum foil.

[0092] (2) The negative electrode active material hard carbon, the conductive agent acetylene black, and the binder polytetrafluoroethylene (PTFE) are dry mixed to form a film, and then pressed onto the aluminum foil current collector modified with the above-mentioned lithium-philic conductive Sn modification layer. The obtained electrode sheet is placed in a vacuum drying oven at 120°C for high-temperature drying to prepare a lithium-philic Sn modified negative electrode sheet.

[0093] (3) The lithium-phobic insulating polymer polyvinyl alcohol is dissolved in a water solvent, and heated and stirred at 75° C. on a magnetic stirrer to fully dissolve it.

[0094] (4) The obtained polyvinyl alcohol is quickly sprayed on the surface of the lithium-philic Sn-modified negative electrode sheet by scraping with a scraper, and after drying, a sandwich structured negative electrode sheet with an insulating sodium-repellent surface and a conductive sodium-philic bottom layer is obtained.

[0095] This embodiment also provides a sodium battery comprising the above-mentioned double-gradient modified negative electrode, the sodium battery comprising a positive electrode plate, an electrolyte, and the above-mentioned negative electrode plate, and the preparation method of the lithium battery comprises:

[0096] The positive electrode active material (Na 0.66 Cu 0.17 Mn 0.33 Ti 0.5 O2), conductive agent (conductive carbon black), and binder (PVDF) are mixed evenly in solvent NMP to obtain positive electrode slurry, and then the positive electrode slurry is evenly coated on the positive electrode current collector aluminum foil, and the positive electrode sheet is obtained after drying and rolling. The above negative electrode sheet, ester electrolyte, separator, and positive electrode sheet are assembled to obtain a sodium battery.

[0097] Comparative Example 1

[0098] This comparative example provides a lithium battery, which uses a common negative electrode plate, and the preparation method is as follows:

[0099] (1) A purchased commercial lithium-ion battery negative electrode active material graphite composite material, a conductive agent acetylene black, carbon nanotubes, a binder CMC and SBR are mixed evenly in an aqueous solvent to obtain a negative electrode slurry, and then the negative electrode slurry is evenly coated on the surface of an unmodified copper foil current collector, and a graphite negative electrode sheet is obtained after drying and rolling.

[0100] (2) The positive electrode active material (LiFePO4), the conductive agent (carbon nanotubes), and the binder (PVDF) are mixed evenly in a solvent to obtain a positive electrode slurry, and then the positive electrode slurry is evenly coated on the positive electrode current collector aluminum foil, and the positive electrode sheet is obtained after drying and rolling.

[0101] (3) Assemble the above-mentioned negative electrode plate, ester electrolyte, separator, and positive electrode plate to obtain a lithium battery.

[0102] Comparative Example 2

[0103] This comparative example provides a lithium battery, which uses a common negative electrode plate, and the preparation method is as follows:

[0104] (1) The purchased commercial lithium-ion battery negative electrode active material silicon carbon, conductive agent acetylene black, carbon nanotubes, binder CMC and SBR are mixed evenly in an aqueous solvent to obtain a negative electrode slurry, and then the negative electrode slurry is evenly coated on the surface of the copper foil current collector, and after drying and rolling, a hard carbon negative electrode sheet is obtained.

[0105] (2) The positive electrode active material (LiFePO4), the conductive agent (conductive carbon black), and the binder (PVDF) are mixed evenly in the solvent NMP to obtain a positive electrode slurry, and then the positive electrode slurry is evenly coated on the positive electrode current collector aluminum foil, and the positive electrode sheet is obtained after drying and rolling.

[0106] (3) Assemble the above-mentioned negative electrode plate, ester electrolyte, separator, and positive electrode plate to obtain a lithium battery.

[0107] Comparative Example 3

[0108] This comparative example provides a lithium battery, which uses a common negative electrode plate, and the preparation method is as follows:

[0109] (1) The purchased commercial lithium-ion battery negative electrode active material graphite, conductive agent acetylene black, carbon nanotubes, binder CMC and SBR are mixed evenly in an aqueous solvent to obtain a negative electrode slurry, and then the negative electrode slurry is evenly coated on the surface of the copper foil current collector, and after drying and rolling, a graphite hard carbon negative electrode sheet is obtained.

[0110] (2) The positive electrode active material (NCM811), the conductive agent (conductive carbon black), and the binder (PVDF) are mixed evenly in the solvent NMP to obtain a positive electrode slurry, and then the positive electrode slurry is evenly coated on the positive electrode current collector aluminum foil, and the positive electrode sheet is obtained after drying and rolling.

[0111] (3) Assemble the above-mentioned negative electrode plate, ester electrolyte, separator, and positive electrode plate to obtain a lithium battery.

[0112] Comparative Example 4

[0113] This comparative example provides a sodium battery, which uses a common negative electrode plate, and the preparation method is as follows:

[0114] (1) The purchased sodium ion battery negative electrode active material hard carbon, conductive agent acetylene black, and binder PTFE are dry-mixed to form a film, which is then pressed onto the surface of an aluminum foil current collector. After drying and rolling, a hard carbon negative electrode sheet is obtained.

[0115] (2) The positive electrode active material (Na 0.66 Cu 0.17 Mn 0.33 Ti 0.5 O2), a conductive agent (conductive carbon black), and a binder (PVDF) are uniformly mixed in a solvent NMP to obtain a positive electrode slurry, and then the positive electrode slurry is uniformly coated on the positive electrode current collector aluminum foil, and the positive electrode sheet is obtained after drying and rolling.

[0116] (3) Assemble the above-mentioned negative electrode plate, ester electrolyte, separator, and positive electrode plate to obtain a sodium battery.

[0117] The embodiments of the present invention have achieved some positive effects during the development or use process, and indeed have great advantages over the prior art. The following content is described in conjunction with data, charts, etc. of the test process.

[0118] The present invention provides a dual-gradient modified lithium / sodium ion battery negative electrode sheet capable of dual-phase lithium storage. The bottom is conductive and a lithium / sodium-philic component is introduced to construct a lithium / sodium-philic gradient, which can accelerate the diffusion of lithium / sodium ions inside the electrode, homogenize the lithium / sodium ion flux, and avoid the enrichment of lithium / sodium ions on the electrode surface to cause concentration polarization inside the battery, thereby precipitating lithium / sodium on the surface. The lithium / sodium-philic component at the bottom also has a low lithium / sodium nucleation barrier, and can induce excess lithium / sodium ions to nucleate and grow from the bottom of the negative electrode first after the negative electrode active layer accommodates lithium / sodium, effectively utilizing the lower space of the active layer, realizing dual-phase lithium / sodium storage, and improving the reversibility of lithium / sodium ions.

[0119] The present invention performs lithium / sodium-philic modification on the current collector. The lithium / sodium-philic layer material acts as a heterogeneous nucleation point while adsorbing lithium / sodium ions, inducing additional lithium / sodium ions that are not embedded in the active layer to preferentially precipitate at the bottom, so that the nucleation and precipitation of lithium / sodium has a bottom-up directionality. In coordination with the insulating lithium-phobic layer on the top, a dual-gradient modified negative electrode is constructed in which a lithium-philic gradient and an electronic conductive gradient coexist, which effectively improves the lithium / sodium precipitation behavior and dendrite growth at the interface and enhances the safety performance of the battery. In addition, lithium / sodium ions that exceed the normal lithium / sodium storage capacity of the negative electrode active material can also be uniformly deposited at the bottom and stored in the form of metallic lithium / sodium, achieving a negative electrode voltage below 0V vs. Li / Li + or 0V vs.Na / Na + The normal working ability under the potential of wide voltage range of the battery is improved. The upper, middle and lower layers of the sandwich configuration each have their own functions and cooperate with each other to construct a dual-gradient modified lithium / sodium ion battery negative electrode sheet capable of dual-phase lithium / sodium storage.

[0120] The present invention adopts the existing preparation process (plasma enhanced chemical deposition method, atomic layer deposition method, electron beam coating method, magnetron sputtering method, thermal evaporation coating method, spray pyrolysis method, solution replacement method, etc.) to achieve lithium / sodium-philic modification of the current collector, and the preparation process is mature, and the operation is simple and efficient. The current collector after lithium / sodium-philic modification has no effect on the subsequent slurry coating, and is compatible with the current electrode preparation process. In addition, when the slurry is coated with the negative electrode active layer, the binder in the slurry not only increases the bonding force between the active layer powder and the current collector, but also indirectly promotes the lithium / sodium-philic layer to be tightly bonded to the current collector, thereby enhancing the overall structural stability of the electrode sheet.

[0121] The insulating lithium-phobic passivation layer formed by the present invention uses a lithium-phobic polymer with low electronic conductivity and is relatively thin. The solution scraping method and the spray coating method used for the coating layer both well retain the original pore structure of the negative electrode active layer, which not only hinders the reduction of lithium on the electrode surface, but also basically does not affect the infiltration of the electrolyte and the diffusion process of lithium ions.

[0122] Figure 1It is a flow chart of a method for preparing a double-gradient modified lithium-ion battery negative electrode provided by an embodiment of the present invention.

[0123] Figure 2 The present invention is a schematic diagram of a lithium-ion battery negative electrode with a sandwich configuration and double gradient modification prepared in Example 1 provided in an embodiment of the present invention. The negative electrode comprises a current collector (1) modified with a bottom strong lithium-philic conductive layer (2), a negative electrode active layer (3) and a top insulating lithium-phobic passivation layer (4), forming a double gradient (electronic conductivity gradient, lithium-philic gradient) modified negative electrode with a sandwich configuration.

[0124] Figure 3 This is a cross-sectional elemental surface scanning diagram of a graphite negative electrode with a double gradient modification of silver particles and a surface coated with insulating lithium-phobic polyvinyl alcohol and a bottom layer prepared in Example 1 of the present invention. Figure 3 As shown, the copper element in the lower layer is attributed to the copper foil current collector. The Ag signal is clearly detected on the copper foil current collector, indicating that silver is successfully modified at the bottom of the graphite layer, realizing the construction of the bottom lithium-philic conductive structure. The carbon signal distribution above the silver layer is also clearly detected, which can be attributed to the carbon elements in the graphite and the insulating lithium-phobic polymer coated on the top. This shows that the double-gradient modified graphite negative electrode with the surface coated with insulating lithium-phobic polyvinyl alcohol and the bottom modified with silver particles is successfully prepared.

[0125] Figure 4 This is a cross-sectional scanning electron microscope image of the negative electrode of a lithium-ion battery prepared in Example 2 with 40% excess lithium provided in the embodiment of the present invention. The specific implementation method is to prepare a double gradient modified silicon-carbon negative electrode prepared in Example 2 with a lower layer modified with lithium-philic conductive Zn and an upper surface modified with insulating lithium-phobic melamine as the negative electrode, a metal lithium foil as the counter electrode, and an ester electrolyte, a diaphragm, etc. to assemble into a half-cell. The half-cell is tested by electrochemical deposition of lithium on a battery charging and discharging system. Figure 4 As shown in the scanning electron micrograph of the electrode cross section, even in the extreme case of 40% excess lithium deposition, there is no obvious lithium accumulation on the upper surface of the double-gradient modified negative electrode, and a large number of pore structures are still maintained between the negative electrode active layer particles on the top. Since the bottom is modified with the Zn layer, the lithium affinity is stronger and can induce lithium nucleation from bottom to top. Therefore, the pores between the negative electrode active particles are tightly filled with metallic lithium, indicating that the tight deposition of metallic lithium from bottom to top is achieved, and since the top is insulated and lithium-phobic, the synergistic effect avoids the precipitation of lithium on the top. In addition, this phenomenon also proves that the working potential of the modified negative electrode can be lower than 0Vvs.Li / Li + It can not only effectively improve the reversibility of lithium ions and realize dual-phase (active material / lithium element) lithium storage, but also improve the safety of the negative electrode.

[0126] Figure 5This is a cycle stability diagram of a full battery assembled with a double-gradient modified hard carbon negative electrode and a lithium iron phosphate positive electrode prepared in Example 3 provided in an embodiment of the present invention. The preparation method of the lithium battery comprises: mixing the positive electrode active material (LiFePO4), the conductive agent (conductive carbon black), and the binder (PVDF) in a solvent NMP to obtain a positive electrode slurry, and then evenly coating the positive electrode slurry on the positive electrode collector aluminum foil, and obtaining the positive electrode plate after drying and rolling. The double-gradient modified hard carbon negative electrode plate, ester electrolyte, diaphragm, and positive electrode plate prepared in Example 3 are assembled to obtain a full battery. A charge and discharge test (1C) was carried out under a voltage range of 2.2-3.8V. The test results are as follows Figure 5 As shown. Since it can induce excess lithium to deposit from the bottom, improve the utilization rate of lithium ions, and prevent metal lithium from precipitating on the negative electrode surface and evolving into lithium dendrites and dead lithium, compared with the unmodified negative electrode assembled LiFePO4 full battery, the double gradient modified graphite negative electrode assembled LiFePO4 full battery has higher reversibility, less capacity decay, and better cycle stability.

[0127] The above description is only a specific implementation mode of the present invention, but the protection scope of the present invention is not limited thereto. Any modifications, equivalent substitutions and improvements made by any technician familiar with the technical field within the technical scope disclosed by the present invention and within the spirit and principle of the present invention should be covered by the protection scope of the present invention.

Claims

1. A lithium / sodium ion battery negative electrode with a dual gradient structure, characterized in that: The lithium / sodium ion battery negative electrode with a double gradient structure is composed of a current collector modified with a strong lithium / sodium-philic conductive layer at the bottom of the negative electrode, an active layer in the middle of the negative electrode, and a lithium / sodium-phobic insulating passivation layer at the top of the negative electrode, with a double gradient sandwich structure of a negative electrode active layer with lithium / sodium-philic conductivity at the bottom and lithium / sodium-phobic insulation at the top; The negative electrode current collector modified by the strong lithium / sodium affinity conductive layer and the negative electrode active layer constitute a lithium / sodium ion battery negative electrode with strong lithium / sodium affinity at the bottom layer, wherein the thickness of the strong lithium / sodium affinity modification layer is 100-2000 nm; The negative electrode active layer comprises a negative electrode active material, a conductive additive, and a binder; The lithium-phobic insulating passivation layer on the top of the negative electrode is highly conformal to the particle morphology on the surface of the active layer, and its thickness is 10-1000nm; The current collector includes one of aluminum foil, nickel foil, stainless steel foil and copper foil; The dual gradient is a lithium-philic / sodium gradient and an electronic conductivity gradient, wherein the top lithium-phobic / sodium insulating passivation layer has the worst lithium-philicity and the lowest conductivity, the middle negative electrode active layer has moderate lithium-philicity and moderate conductivity, and the bottom strongly lithium-philic / sodium conductive modification layer has the strongest lithium-philicity and the highest conductivity.

2. The lithium / sodium ion battery negative electrode with a dual gradient structure as claimed in claim 1, characterized in that: The strong lithium / sodium affinity component is a mixture of one or more organic and inorganic substances with high electronic conductivity and high lithium / sodium affinity; Including at least one of Ag, Zn, Co, Sn, Al, In2O3, AgO, ZnS, CoP, Ni2P, TiN, Li-Zn, Li-Al, and Li-Sn alloy.

3. The lithium / sodium ion battery negative electrode with a dual gradient structure as claimed in claim 1, characterized in that: The negative electrode active material of the lithium / sodium ion battery as claimed in claim 1 comprises one of graphite, silicon-carbon composite material, silicon oxide, and hard carbon; the conductive agent comprises at least one of conductive carbon black and carbon nanotubes; the binder comprises at least one of sodium carboxymethyl cellulose, styrene-butadiene rubber, polyvinylidene fluoride, polyacrylic acid, polyvinyl alcohol, polyethylene oxide, and polyacrylonitrile; and the powder particles of the obtained negative electrode active layer have a certain porosity.

4. The lithium / sodium ion battery negative electrode with a dual gradient structure as claimed in claim 1, characterized in that: The introduction of a strong lithium / sodium-philic layer can be carried out by at least one of chemical vapor deposition, physical vapor deposition or wet chemical method; the chemical vapor deposition method includes plasma enhanced chemical deposition and atomic layer deposition; the physical vapor deposition method includes electron beam coating, magnetron sputtering and thermal evaporation coating; the wet chemical method includes hydrothermal method, electrochemical deposition and spray pyrolysis.

5. A method for preparing a negative electrode for a lithium / sodium ion battery according to any one of claims 1 to 4, characterized in that: The negative electrode active material, conductive additive and binder are first mixed in a solvent to prepare a slurry, which is then coated on a conventional current collector modified with a lithium / sodium-philic layer, and dried to obtain the lithium / sodium-ion battery negative electrode sheet with the above-mentioned strongly lithium / sodium-philic layer modified current collector.

6. The preparation method according to claim 5, characterized in that: Add at least one of the lithium-phobic and insulating polymer components into a solvent and fully dissolve it to form a solution; the solvent includes at least one of ethylene glycol dimethyl ether (DME), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAC), dimethyl sulfoxide (DMSO), N-methylpyrrolidone (NMP), and methanol; The solution is coated on the surface of the finished negative electrode sheet of the lithium / sodium ion battery with the strongly lithium-philic / sodium-philic layer modified current collector by spraying or rapid scraping, and is immediately placed in an oven for high-temperature drying to allow the solvent to evaporate rapidly, thereby obtaining a double-gradient modified lithium / sodium ion battery negative electrode with a sandwich structure.

7. The preparation method according to claim 6, characterized in that: The top polymer component is a lithium-phobic polymer with low electronic conductivity, including one or two of polyvinyl alcohol and melamine.

8. The preparation method according to claim 6, characterized in that: The preparation method of the top insulating lithium-phobic passivation layer is to directly coat the electrode sheet using a simple spray coating method or a scraping method; the morphology of the surface insulating passivation layer is highly conformal to the morphology of the powder particles on the surface of the negative electrode active layer, and the pore structure between the particles is retained to the greatest extent to allow electrolyte infiltration.

9. The method for preparing a negative electrode for a lithium / sodium ion battery having a dual gradient structure according to claim 1, characterized in that: The following steps are involved: Step 1: Prepare a certain concentration of AgNO3 aqueous solution, and quickly scrape the AgNO3 aqueous solution on the copper foil current collector by scraping with a scraper at room temperature, and let it stand for a few minutes to react; after washing and drying, a lithium-philic Ag particle modification layer with a thickness of about 1 μm is grown on the surface of the copper foil; Step 2: Mix the negative electrode active material graphite, the conductive additive acetylene black, and the binder PVDF in a solvent to prepare a slurry, and then coat it on the copper foil current collector modified with the above-mentioned lithium-philic Ag particle modification layer by a conventional slurry coating method; put the obtained electrode sheet into a vacuum drying oven at 120°C for high-temperature drying to prepare a lithium-philic Ag-modified graphite negative electrode sheet; Step 3, dissolving the lithium-phobic insulating polymer polyvinyl alcohol in a water solvent, heating and stirring on a magnetic stirrer at a heating temperature of 75° C. to fully dissolve it; Step 4: quickly apply the obtained polyvinyl alcohol solution on the surface of the lithium-philic Ag-modified graphite negative electrode sheet by scraping with a scraper to obtain a graphite negative electrode sheet with a sandwich structure that is insulating and lithium-phobic on the surface and conductive and lithium-philic on the bottom layer.

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