Composite pole piece, preparation method and sodium ion battery

Abstract

The invention discloses a composite pole piece, a preparation method and a sodium ion battery, the composite pole piece comprises a current collector and a conductive layer covering the surface of the current collector, and the conductive layer is a three-dimensional net structure comprising hollow carbon microspheres, graphene and carbon nanotubes. The surface of the current collector is covered with the conductive layer, the conductive layer is of a three-dimensional network structure comprising the hollow carbon microspheres, the graphene and the carbon nanotubes, and the hollow carbon microspheres are of a porous structure, large in specific surface area and capable of providing a large number of adsorption sites and reaction active sites and allowing liquid, ions and the like to be stored or adsorbed massively; the hollow carbon microspheres, the graphene and the carbon nanotubes cooperate with one another, so that the three-dimensional reticular conductive structure of the conductive layer can provide a large number of ion and electron transmission channels, the conductivity of the battery cell material and the composite pole piece is greatly improved, and the rate capability and the cycle performance of the sodium ion battery are improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of sodium ion batteries, and particularly relates to a composite electrode sheet, a preparation method and a sodium ion battery. BACKGROUND

[0002] In recent years, the industrialization process of sodium ion batteries has developed rapidly. Compared with widely used lithium ion batteries, sodium ion batteries are considered to be ideal secondary batteries suitable for future electric two-wheel vehicles and large-scale energy storage systems due to their abundant sodium salt reserves, low raw material costs, and wide working temperature range. However, the current sodium ion battery current collectors and electrode sheets have poor conductivity, especially the negative electrode current collector and negative electrode sheet, which leads to the rate performance and cycle performance of the sodium ion battery failing to meet the needs of various application scenarios. SUMMARY

[0003] In view of the problems in the prior art, the present application provides a composite electrode sheet, a preparation method and a sodium ion battery, and the technical solution is as follows: In one aspect, the present application provides a composite electrode sheet, comprising a current collector and a conductive layer covering the surface of the current collector, wherein the conductive layer is a three-dimensional network structure comprising hollow carbon microspheres, graphene and carbon nanotubes.

[0004] Further, the graphene sheet structure wraps the surface of the hollow carbon microspheres, and the carbon nanotubes are inserted and connected in the gaps between the hollow carbon microspheres to form the three-dimensional network structure.

[0005] Further, the conductive layer satisfies at least one of the following characteristics: The diameter of the hollow carbon microspheres is 5-100 microns; The shell of the hollow carbon microspheres is a porous structure, and the specific surface area of the hollow carbon microspheres is 1200-1500 m 2 / g; 2 / g; The pore size of the hollow carbon microspheres is 5-20 nm; The thickness of the graphene sheet is 3-10 nm; The diameter of the carbon nanotubes is 10-30 nm; The aspect ratio of the carbon nanotubes is greater than a preset aspect ratio, and the preset aspect ratio is 800-1200; The mass ratio of the hollow carbon microspheres to the graphene is 1:0.5-1:2; The mass ratio of the hollow carbon microspheres to the carbon nanotubes is 1:0.5-1:2; The porosity of the conductive layer is 60-80%; The conductivity of the conductive layer is greater than a preset conductivity, and the preset conductivity is 800 S / m-1200 S / m.

[0006] Further, the composite electrode plate further comprises an active layer, the active layer is located on the side of the conductive layer away from the current collector; The active layer comprises an active material and a first conductive agent, and the first conductive agent is a three-dimensional network structure comprising hollow carbon microspheres.

[0007] Further, the mass ratio between the active material and the first conductive agent in the active layer is 92:2-96:1.

[0008] Further, the active layer satisfies at least one of the following characteristics: The active layer further comprises a second conductive agent, and the second conductive agent comprises a mixture of one or more of conductive carbon black, acetylene black, conductive graphite, ketjen black, graphene and carbon nanotubes; The mass ratio between the first conductive agent and the second conductive agent in the active layer is 1:3-2:1. The active layer further comprises a first binder, and the first binder comprises a mixture of one or more of sodium carboxymethyl cellulose, butadiene rubber, polyacrylic acid and polyacrylonitrile; The mass ratio between the active material and the first binder is 92:3-96:1.

[0009] Further, the conductive layer further comprises a second binder, and the second binder satisfies at least one of the following characteristics: The second binder comprises one or more of polytetrafluoroethylene, polyvinylidene fluoride, polyolefin, polyvinyl alcohol, polyacrylic acid and polyurethane; The volume ratio between the hollow carbon microspheres, the graphene and the carbon nanotubes and the second binder is 6:7-8:2.

[0010] Further, the composite electrode plate satisfies at least one of the following characteristics: The areal density of the composite electrode plate is 20 g / m 2 -180 g / m 2 ; The compacted density of the composite electrode plate is 0.95 g / cm 3 -2.5 g / cm 3 .

[0011] In another aspect, the present application also provides a preparation method of a composite electrode plate, comprising: Forming a conductive layer on the surface of the current collector to obtain a composite electrode plate; the conductive layer is a three-dimensional network structure comprising hollow carbon microspheres, graphene and carbon nanotubes.

[0012] Further, the forming the conductive layer on the surface of the current collector to obtain the composite electrode sheet comprises: The hollow carbon microspheres, graphene and carbon nanotubes are mixed in a solvent according to a preset mass ratio to obtain a composite conductive slurry; The composite conductive slurry is coated on the current collector to form an initial conductive layer; The initial conductive layer is dried to form the conductive layer.

[0013] Further, the coating the composite conductive slurry on the current collector to form the initial conductive layer comprises: The composite conductive slurry is coated on the current collector by electrospinning to form the initial conductive layer; the voltage of the electrospinning is 10kV-30kV, the solution flow rate is 0.5ml / h-2.0ml / h, the diameter of the syringe needle is 0.01cm-0.05cm, the diameter of the needle cylinder is 1cm-5cm, and the distance between the nozzle and the current collector is 0.2cm-1.0cm.

[0014] Further, the hollow carbon microspheres are prepared by the following steps: Dopamine hydrochloride is added to a buffer solution of calcium carbonate microspheres to obtain a mixed solution; The mixed solution is filtered, washed and dried to obtain polydopamine-coated calcium carbonate microspheres; The polydopamine-coated calcium carbonate microspheres are high-temperature sintered and washed to obtain hollow carbon microspheres.

[0015] Further, the forming the conductive layer on the surface of the current collector to obtain the composite electrode sheet comprises: The conductive layer is formed on the surface of the current collector; An active layer is formed on the side of the conductive layer away from the current collector; the active layer comprises an active material and a first conductive agent, and the first conductive agent is a three-dimensional net-like structure comprising hollow carbon microspheres.

[0016] In another aspect, the application also provides a sodium ion battery comprising the composite electrode sheet according to any one of the above.

[0017] The application has the following beneficial effects: The present application covers the surface of the current collector with a conductive layer, which is a three-dimensional network structure including hollow carbon microspheres, graphene and carbon nanotubes, wherein the hollow carbon microspheres have a unique porous structure, a large specific surface area, and can provide a large number of adsorption sites and reactive sites, so that liquid and ions can be stored or adsorbed in the surface and interstices of the material in large quantities, while having good electrical conductivity and thermal stability. The hollow carbon microspheres, graphene and carbon nanotubes synergize with each other, so that the three-dimensional network conductive structure of the conductive layer can provide a large number of ion and electron transmission channels, greatly improving the electrical conductivity of the battery material and the composite electrode sheet. In addition, when the composite electrode sheet is a negative electrode sheet, the hollow carbon microspheres can also act as a special conductive agent, which can improve the liquid retention capacity of the battery, and is beneficial to improve the rate performance and cycle stability of the sodium ion battery. BRIEF DESCRIPTION OF DRAWINGS

[0018] In order to more clearly illustrate the technical solutions of the present application, the drawings used in the embodiments will be briefly introduced below, wherein the same parts are denoted by the same reference numerals. Obviously, the drawings described below are only some embodiments of the present application, and those skilled in the art can obtain other drawings according to these drawings without creative labor.

[0019] Figure 1 Discharge curve of the sodium ion battery cell at 5C rate in some embodiments of the present application; Figure 2 Capacity retention rate of the sodium ion battery at 25℃@0.5C / 0.5C in some embodiments of the present application. DETAILED DESCRIPTION

[0020] The technical solutions in the embodiments of the present application will be described clearly and completely below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only some embodiments of the present application, not all embodiments, and therefore cannot be understood as limiting the present application. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0021] It should be noted that in the description of the present invention, for the following defined terms, these definitions shall apply unless a different definition is given in the claims or elsewhere in this specification. All numerical values, whether or not explicitly indicated, are defined herein as being modified by the term "about". The term "about" generally refers to a numerical range that a person of ordinary skill in the art would consider to be equivalent to the stated value to produce substantially the same properties, functions, results, etc. A numerical range indicated by a lower value and a higher value is defined to include all numerical values ​​included in the numerical range and all subranges included in the numerical range.

[0022] It should be noted that the terms "first", "second", etc. in the specification, claims, and drawings of the present invention are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that the objects used in this way can be interchanged where appropriate so that the embodiments of the present invention can be implemented in an order other than the following diagrams or descriptions. In addition, the terms "including" and "having" and any variations thereof are intended to cover non-exclusive inclusions. For example, a process, method, system, or product that includes a series of steps or units is not necessarily limited to those steps or units clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, or products.

[0023] In response to the technical problem that the conductivity of the current collector and electrode in the existing sodium ion battery is insufficient, resulting in poor rate performance and cycle performance, the embodiments of the present invention provide a composite electrode, a preparation method and a sodium ion battery, wherein the preparation method of the composite electrode is used to prepare the composite electrode, the sodium ion battery includes the composite electrode, the composite electrode includes a current collector and a conductive layer covering the surface of the current collector, in some exemplary embodiments, the current collector includes any one of aluminum plain foil, aluminum composite plain foil, copper plain foil, and copper composite plain foil; the composite electrode can be at least one of a positive electrode and a negative electrode, and accordingly, the current collector 1 can include at least one of a positive electrode current collector and a negative electrode current collector, thereby expanding the scope of application of the composite electrode; in some exemplary embodiments, the composite electrode is a negative electrode, which is more efficient in improving the conductivity of the sodium ion battery.

[0024] Specifically, the conductive layer is a three-dimensional network structure including hollow carbon microspheres, graphene and carbon nanotubes, that is, the conductive layer is a carbon coating covering the surface of the current collector. The hollow carbon microspheres, graphene and carbon nanotubes together constitute a composite conductive agent, so that the conductive layer with the composite conductive agent forms a three-dimensional network conductive structure, which can provide a large number of ion and electron transmission channels, thereby improving the conductivity of the battery core material.

[0025] Specifically, the sheet structure of graphene wraps the surface of the hollow carbon microspheres, the hollow carbon microspheres have a porous structure and an extremely high specific surface area, can provide a large number of adsorption sites and reaction active sites, enable liquid and ions to be stored or adsorbed in large quantities on the surface and pores of the hollow carbon microspheres, and can provide good electrical conductivity and thermal stability, on the one hand, can serve as a special conductive agent to improve the electrical conductivity of the composite electrode sheet, and on the other hand, can significantly improve the liquid retention capacity of the battery cell, which is conducive to improving the rate performance and cycle stability of the battery cell; graphene also has good electrical conductivity, and has super-strong mechanical properties, good thermal conductivity and excellent adsorption, which can improve the electron transfer rate in the composite electrode sheet, and the wrapping on the surface of the hollow carbon microspheres can further improve the electrical conductivity; at the same time, the carbon nanotubes are connected in the gaps between the hollow carbon microspheres to form a three-dimensional network structure, the carbon nanotubes are connected in the gaps between the hollow carbon microspheres wrapped with the sheet structure of graphene to form a three-dimensional network conductive structure, the sheet structure of graphene enables itself to be in contact with the active material, which is convenient for building a larger conductive network in the composite electrode sheet, and the carbon nanotubes can form a continuous conductive network, reduce the internal resistance, improve the migration rate of electrons and ions, and significantly improve the ion transmission capacity of the conductive layer and the composite electrode sheet, and improve the electrical conductivity of the battery cell.

[0026] Specifically, the diameter of the hollow carbon microspheres is 5 μm to 100 μm; it can be understood that the diameter of the hollow carbon microspheres can be any point value in the range of 5 μm to 100 μm; for example, the diameter of the hollow carbon microspheres can be 5 μm, 10 μm, 15 μm, 25 μm, 50 μm, 75 μm, 100 μm, etc.; within this diameter range, the effective surface area of the hollow carbon microspheres can be improved, and the electron transfer path can be shortened to a certain extent, the resistance can be reduced, and the electrical conductivity of the conductive layer and the composite electrode sheet can be improved from two aspects.

[0027] Specifically, the shell of the hollow carbon microspheres is a porous structure, and the specific surface area of the hollow carbon microspheres is 1200 m 2 / g to 1500 m 2 / g; it can be understood that the specific surface area of the hollow carbon microspheres can be any point value in the range of 1200 m 2 / g to 1500 m 2 / g; for example, the specific surface area of the hollow carbon microspheres can be 1200 m 2 / g, 1250 m 2 / g, 1300 m 2 / g, 1350 m 2 / g, 1400 m 2 / g, 1500 m 2, etc. Thus, the specific surface area of the hollow carbon microspheres is extremely high, which can provide a large number of adsorption sites and reaction active sites, so that the liquid and ions can be stored or adsorbed in large quantities on the surface and pores of the hollow carbon microspheres, thereby improving the conductivity of the conductive layer and the composite electrode sheet, and also improving the liquid retention capacity of the battery cell, and improving the rate performance and cycle performance of the battery cell.

[0028] Specifically, the pore size of the hollow carbon microspheres is 5 nm to 20 nm. Understandably, the pore size of the hollow carbon microspheres can be any point value in the range of 5 nm to 20 nm. For example, the pore size of the hollow carbon microspheres can be 5 nm, 10 nm, 15 nm, 17 nm, 20 nm, etc. Within this pore size range, the liquid and ions can be adsorbed, the conductivity of the conductive layer and the composite electrode sheet having the conductive layer can be improved, and the liquid retention capacity of the battery cell can be improved.

[0029] Specifically, the sheet thickness of the graphene is 3 nm to 10 nm. Understandably, the sheet thickness of the graphene can be any point value in the range of 3 nm to 10 nm. For example, the sheet thickness of the graphene can be 3 nm, 5 nm, 7.5 nm, 9 nm, 10 nm, etc. The sheet thickness of the graphene is extremely thin, which can effectively shorten the diffusion path of the ions, improve the conductivity of the composite electrode sheet, and be beneficial to improve the rate performance of the battery cell. In addition, within this sheet thickness range, the graphene can also provide a large specific surface area, which can increase the contact area of the electrolyte and the composite electrode sheet, improve the performance of the sodium ion battery, and also be beneficial to reduce the polarization of the sodium ion battery and reduce energy loss.

[0030] Specifically, the diameter of the carbon nanotube is 10 nm to 30 nm. Understandably, the diameter of the carbon nanotube can be any point value in the range of 10 nm to 30 nm. For example, the diameter of the carbon nanotube can be 10 nm, 12 nm, 15 nm, 18 nm, 20 nm, 25 nm, 30 nm, etc. Within this diameter range, the electronic energy level distribution of the carbon nanotube is more dispersed, which is beneficial to improve the electron transport efficiency and thus improve the conductivity of the composite electrode sheet. In addition, it can also inhibit the growth of dendrites to a certain extent, which is beneficial to improve the safety and cycle stability of the composite electrode sheet.

[0031] Specifically, the aspect ratio of the carbon nanotube is greater than a preset aspect ratio, and the preset aspect ratio is 800-1200; it can be understood that the preset aspect ratio can be any point value in 800-1200; for example, the preset aspect ratio can be 800, 900, 1000, 1100, 1150, 1200, etc.; within the preset aspect ratio range, the carbon nanotube can provide excellent conductivity for the composite electrode piece, and can more effectively connect the active material in the composite electrode piece, further forming a high-efficiency three-dimensional conductive network, reducing the internal impedance of the composite electrode piece, and being conducive to improving the conductivity and energy density of the sodium ion battery; for example, in some specific embodiments, the preset aspect ratio is 1000, that is, the aspect ratio of the carbon nanotube is greater than 1000.

[0032] Specifically, the mass ratio of the hollow carbon microsphere to the graphene is 1:0.5-1:2; it can be understood that the mass ratio of the hollow carbon microsphere to the graphene can be any point value in 1:0.5-1:2; for example, the mass ratio of the hollow carbon microsphere to the graphene can be 1:0.5, 1:0.8, 1:1, 1:1.2, 1:1.5, 1:2, etc.

[0033] Specifically, the mass ratio of the hollow carbon microsphere to the carbon nanotube is 1:0.5-1:2; it can be understood that the mass ratio of the hollow carbon microsphere to the carbon nanotube can be any point value in 1:0.5-1:2; for example, the mass ratio of the hollow carbon microsphere to the carbon nanotube can be 1:0.5, 1:0.8, 1:1, 1:1.2, 1:1.5, 1:2, etc.

[0034] Within the mass ratio range of the hollow carbon microsphere, the graphene, and the carbon nanotube, the conductivity of the composite electrode piece can be effectively improved, wherein the hollow carbon microsphere acts as a "node", the graphene sheet structure wraps the surface of the hollow carbon microsphere to form a "conductive bridge", and the carbon nanotube is connected to form a three-dimensional network conductive structure to provide a large number of ion and electron transmission channels and improve the conductivity of the composite electrode piece and the sodium ion battery.

[0035] Specifically, the porosity of the conductive layer is 60%-80%; it can be understood that the porosity of the conductive layer can be any point value in 60%-80%; for example, the porosity of the conductive layer can be 60%, 62%, 66%, 70%, 75%, 80%, etc.; within the porosity range, it is convenient for ions to pass through, becomes a channel for ion transmission, effectively improves the transmission efficiency of ions, and further improves the conductivity, at the same time, it is also convenient for electrolyte to fill in, which is conducive to improving the rate performance and cycle performance of the sodium ion battery.

[0036] Specifically, the conductivity of the conductive layer is greater than a preset conductivity, and the preset conductivity is 800 S / m-1200 S / m. Understandably, the preset conductivity can be any point value in 800 S / m-1200 S / m. For example, the preset conductivity can be 800 S / m, 900 S / m, 1000 S / m, 1050 S / m, 1100 S / m, 1200 S / m, etc. The conductivity of the conductive layer is relatively large, indicating that the conductive layer has good conductivity, which can effectively improve the conductivity of the composite tab. For example, in some specific embodiments, the preset conductivity is 1000 S / m, that is, the conductivity of the conductive layer is greater than 1000 S / m.

[0037] Specifically, the conductive layer further comprises a second binder for improving the adhesion between various materials in the conductive layer, and for improving the adhesion between the conductive layer and the current collector, and the adhesion between the conductive layer and the subsequent active layer, preventing delamination, and being conducive to improving the performance stability and life of the composite tab. In some exemplary embodiments, the second binder comprises one or more of polytetrafluoroethylene, polyvinylidene fluoride, polyolefin, polyvinyl alcohol, polyacrylic acid, and polyurethane, and has good adhesion.

[0038] Specifically, the volume ratio of the total volume of the hollow carbon microspheres, graphene and carbon nanotubes to the second binder is 6:7-8:2. Understandably, the volume ratio of the total volume of the hollow carbon microspheres, graphene and carbon nanotubes to the second binder can be any point value in 6:7-8:2. For example, the volume ratio of the total volume of the hollow carbon microspheres, graphene and carbon nanotubes to the second binder can be 6:7, 1:1, 5:2, 3:1, 8:2, etc. Within this volume ratio range, the adhesion of the conductive layer can be improved, and the conductivity of the conductive layer and the capacity of the composite tab can be avoided, which is conducive to improving the rate performance and cycle performance of the sodium ion battery with the composite tab.

[0039] Specifically, the composite electrode sheet further comprises an active layer located on the side of the conductive layer away from the current collector; the active layer comprises an active material and a first conductive agent, wherein the active material, as a material directly involved in the electrochemical reaction, contributes to the energy density, cycle life and stability of the sodium-ion battery; in some exemplary embodiments, the composite electrode sheet is a positive electrode sheet, and the corresponding active material is a positive electrode active material, including any one of transition metal layered oxides, polyanion compounds and prussian blue analogues; in other exemplary embodiments, the composite electrode sheet is a negative electrode sheet, and the corresponding active material is a negative electrode active material, including alloy-based materials, metal oxides, and carbon materials such as hard carbon, soft carbon, and graphite; and the first conductive agent is a three-dimensional network structure including hollow carbon microspheres, which can on the one hand synergize with the conductive layer to further form ion transmission channels and improve the conductivity of the composite electrode sheet, and on the other hand improve the adhesion between the conductive layer and the active layer and further improve the liquid retention capacity of the battery cell, thereby improving the rate performance and cycle stability of the sodium-ion battery.

[0040] Specifically, the mass ratio of the active material to the first conductive agent in the active layer is 92:2-96:1; it can be understood that the mass ratio of the active material to the first conductive agent in the active layer can be any point value in the range of 92:2-96:1; for example, the mass ratio of the active material to the first conductive agent in the active layer can be 92:2, 52:1, 70:1, 85:1, 96:1, etc.; within this mass ratio range, the composite electrode sheet can have a higher capacity, and the ion transmission path can be increased to effectively improve the conductivity of the active layer and the composite electrode sheet.

[0041] Specifically, the active layer further comprises a second conductive agent, which comprises a mixture of one or more of conductive carbon black, acetylene black, conductive graphite, ketjen black, graphene and carbon nanotubes, has good conductivity, and can synergize with the first conductive agent to further improve the ion transmission capacity of the active layer material and improve the conductivity of the composite electrode sheet.

[0042] Specifically, the mass ratio of the first conductive agent to the second conductive agent in the active layer is 1:3-2:1; it can be understood that the mass ratio of the first conductive agent to the second conductive agent in the active layer can be any point value in the range of 1:3-2:1; for example, the mass ratio of the first conductive agent to the second conductive agent in the active layer can be 1:3, 2:3, 1:1, 1.5:1, 2:1, etc.; within this mass ratio range, the first conductive agent and the second conductive agent synergize to improve the conductivity of the composite electrode sheet, and provide a large number of adsorption sites and reaction active sites, significantly improving the liquid retention capacity of the battery cell, thereby improving the rate performance and cycle stability of the sodium-ion battery with the composite electrode sheet.

[0043] Specifically, the active layer further comprises a first binder for improving the adhesion between various materials in the active layer, and also improving the adhesion between the active layer and the conductive layer to prevent delamination; in some exemplary embodiments, the first binder comprises a mixture of one or more of sodium carboxymethyl cellulose, styrene butadiene rubber, polyacrylic acid and polyacrylonitrile, which can provide good adhesion.

[0044] Specifically, the mass ratio between the active material and the first binder is 92:3-96:1; it can be understood that the mass ratio between the active material and the first binder can be any point value in 92:3-96:1; for example, the mass ratio between the active material and the first binder can be 92:3, 92:2, 52:1, 70:1, 85:1, 96:1, etc.; within this mass ratio range, the adhesion of the active layer can be effectively improved, while avoiding affecting the capacity of the active layer, which is conducive to improving the rate performance and cycle performance of the sodium ion battery with the composite electrode sheet.

[0045] Specifically, the areal density of the composite electrode sheet is 20 g / m 2 -180 g / m 2 ; it can be understood that the areal density of the composite electrode sheet can be any point value in 20 g / m 2 -180 g / m 2 ; for example, the areal density of the composite electrode sheet can be 20 g / m 2 , 30 g / m 2 , 50 g / m 2 , 100 g / m 2 , 110 g / m 2 , 150 g / m 2 , 165 g / m 2 , 180 g / m 2 ; within this areal density range, ion migration is facilitated, which is conducive to improving the conductivity and cycle stability of the composite electrode sheet.

[0046] Specifically, the compacted density of the composite electrode sheet is 0.95 g / cm 3 -2.5 g / cm 3 ; it can be understood that the compacted density of the composite electrode sheet can be any point value in 0.95 g / cm 3 -2.5 g / cm 3 ; for example, the compacted density of the composite electrode sheet can be 0.95 g / cm 3 , 0.98 g / cm 3 , 1.00 g / cm 3 , 1.03 g / cm 3 , 1.05 g / cm 3 , 1.5 g / cm 31.95g / cm 3 2.15g / cm 3 2.5g / cm 3 and the like; in this range of the compacted density, it is beneficial to improve the energy density of the composite electrode sheet, accelerate the ion transmission speed and reduce the critical current density, thereby improving the discharge performance and cycle stability of the sodium ion battery.

[0047] In another aspect, the present application provides a method for preparing a composite electrode sheet, comprising: forming a conductive layer on the surface of the current collector to obtain the composite electrode sheet; the conductive layer is a three-dimensional network structure comprising hollow carbon microspheres, graphene and carbon nanotubes.

[0048] Specifically, before forming the conductive layer on the surface of the current collector, the hollow carbon microspheres in the conductive layer are prepared by the following steps: adding dopamine hydrochloride to the buffer solution of calcium carbonate microspheres to obtain a mixed solution; filtering, washing and drying the mixed solution to obtain polydopamine-coated calcium carbonate microspheres; sintering the polydopamine-coated calcium carbonate microspheres at high temperature, washing to obtain the hollow carbon microspheres.

[0049] The buffer solution of calcium carbonate microspheres can be prepared by the following steps: adjusting the pH of the initial buffer solution to obtain an alkaline buffer solution; dispersing the calcium carbonate microspheres in the alkaline buffer solution to obtain the buffer solution of the calcium carbonate microspheres.

[0050] The initial buffer solution is a Tris-HCl buffer solution with a concentration of 5mmol / L-20mmol / L, then the pH of the initial buffer solution is adjusted to 8-10 by using 0.1mol / L hydrochloric acid to obtain an alkaline buffer solution; then 1g-5g of calcium carbonate microspheres with a diameter of 5μm-100μm are added to the alkaline buffer solution, and the calcium carbonate microspheres are ultrasonically dispersed to uniformly distribute the calcium carbonate microspheres in the alkaline buffer solution to obtain the buffer solution of the calcium carbonate microspheres.

[0051] Afterwards, hydrochloric acid dopamine is added into the buffer solution of the calcium carbonate microspheres, and after uniform stirring at 20-35°C for 20-40h, a mixed solution is obtained; then the mixed solution is filtered, washed with deionized water for at least 3 times, and dried at 45°C under vacuum for 10-15h to obtain polydopamine-coated calcium carbonate microspheres; the polydopamine-coated calcium carbonate microspheres are placed in a tube furnace, heated at 800-900°C under nitrogen atmosphere for 2-8h with a heating rate of 5°C / min to realize high-temperature sintering, so that the outer layer of polydopamine is carbonized and the inner layer of calcium carbonate is decomposed into calcium oxide, and then the residual calcium oxide is removed by soaking and washing with dilute hydrochloric acid to obtain uniform-sized hollow carbon microspheres.

[0052] In the preparation process of the hollow carbon microspheres, polydopamine is the main active ingredient of the viscous substance secreted by the outer shell of shellfish, which has strong adhesion and can improve the interfacial adhesion performance; and the surface of hydrochloric acid dopamine contains many functional groups (hydroxyl, amino, catechol, etc.), which can be oxidized and self-polymerized to form polydopamine in an alkaline environment, and then adhere to the surface of the calcium carbonate microspheres to form a double-layer core-shell structure of polydopamine-coated calcium carbonate microspheres, and then high-temperature carbonization and acid washing are performed to remove the residual substances in the inner layer to obtain hollow carbon microspheres with hollow core and porous shell. The preparation method of the hollow carbon microspheres based on dopamine-coated calcium carbonate is simple to operate, and is convenient for preparing hollow carbon microspheres of different sizes, and is also convenient for improving the morphological uniformity, size consistency and property stability of the prepared hollow carbon microspheres. In addition, in some exemplary embodiments, the specific surface area of the hollow carbon microspheres can be adjusted by adjusting the particle size of the calcium carbonate microspheres and the sintering temperature of the high-temperature sintering process, so as to obtain hollow carbon microspheres with different specific surface areas, which is beneficial to the improvement of the conductivity of the subsequent composite electrode plate.

[0053] Specifically, in some exemplary embodiments, forming the conductive layer on the surface of the current collector to obtain the composite electrode plate comprises: forming the conductive layer on the surface of the current collector; forming an active layer on the side of the conductive layer away from the current collector; the active layer comprises an active material and a first conductive agent, and the first conductive agent is a three-dimensional network structure comprising hollow carbon microspheres.

[0054] Specifically, forming the conductive layer on the surface of the current collector comprises: mixing the hollow carbon microspheres, graphene and carbon nanotubes in a solvent according to a preset mass ratio to obtain a composite conductive slurry; coating the composite conductive slurry on the current collector to form an initial conductive layer; drying the initial conductive layer to form the conductive layer.

[0055] In some exemplary embodiments, the solvent is any one of N-methyl pyrrolidone (NMP), N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO), which has good solubility for the hollow carbon microspheres, graphene and carbon nanotubes. The hollow carbon microspheres, graphene and carbon nanotubes are mixed in the solvent according to a preset mass ratio, and a second binder is added to the solvent and uniformly mixed to obtain a composite conductive slurry. In the composite conductive slurry, the hollow carbon microspheres, graphene and carbon nanotubes constitute a composite conductive agent, the total volume of the three in the solvent accounts for 6% to 8% of the volume of the solvent, the volume percentage of the second binder in the solvent is 2% to 7%, the solid content in the composite conductive slurry is 4% to 35%, and the viscosity of the composite conductive slurry is 5 mPa·S to 400 mPa·S. The composite conductive slurry can effectively disperse the hollow carbon microspheres, graphene and carbon nanotubes, thereby facilitating the improvement of the film forming precision and uniformity of the subsequently prepared conductive layer.

[0056] Specifically, the composite conductive slurry is coated on the current collector to form an initial conductive layer, including: The composite conductive slurry is coated on the current collector by electrospinning to form an initial conductive layer. The voltage of electrospinning is 10 kV to 30 kV, the solution flow rate is 0.5 ml / h to 2.0 ml / h, the diameter of the syringe needle is 0.01 cm to 0.05 cm, the diameter of the needle cylinder is 1 cm to 5 cm, and the distance between the electrospinning nozzle and the current collector is 0.2 cm to 1.0 cm.

[0057] In some exemplary embodiments, the composite conductive slurry can be sprayed on the current collector by spraying. For example, in one exemplary embodiment, the composite conductive slurry is loaded into an electrospinning device, the electrostatic spraying voltage is 25 kV, the solution flow rate is 3 ml / h, the slurry viscosity is 5400 mPa·s, the diameter of the syringe needle is 0.03 cm, the diameter of the needle cylinder is 2 cm, the distance between the electrospinning nozzle and the aluminum foil is 10 cm, the conductive layer is sprayed on the aluminum foil current collector, and the negative electrode current collector with a surface covered with the conductive layer is obtained after drying. The current collector surface is covered with a carbon coating, in which the hollow carbon microspheres, graphene and carbon nanotubes jointly constitute a three-dimensional network conductive structure, which significantly improves the conductivity of the current collector and in turn improves the conductivity of the composite electrode plate.

[0058] Then, the active layer is formed on the side of the conductive layer away from the current collector. Specifically, the active material, the first conductive agent, the second conductive agent, and the first binder are dissolved in a solvent according to a predetermined mass ratio to obtain an active slurry. The active slurry is coated on the surface of the current collector covered with the conductive layer, and after drying, a composite electrode sheet is obtained. In the active slurry, hollow carbon microspheres are added as the first conductive agent, which can not only improve the conductivity of the active material, but also significantly improve the liquid retention capacity of the battery cell, which is beneficial to improve the rate performance and cycle stability of the battery cell. In addition, the hollow carbon microspheres in the conductive layer can synergize with the hollow carbon microspheres in the active slurry, further improving the conduction rate of ions and electrons, improving the conductivity of the composite electrode sheet, and improving the rate performance of the sodium-ion battery.

[0059] In addition, the hollow carbon microspheres of the first conductive agent and the hollow carbon microspheres of the conductive layer can be prepared by the above-mentioned preparation method of hollow carbon microspheres. Other raw material components of the composite electrode sheet, such as graphene, carbon nanotubes, the first binder, the second binder, the second conductive agent, the active material, and the solvent, and raw material components used for preparing the hollow carbon microspheres, such as dopamine hydrochloride, calcium carbonate microspheres, and Tris-HCl buffer, can be obtained from the market.

[0060] On the other hand, the present application provides a sodium-ion battery, which comprises the composite electrode sheet as described above. The composite electrode sheet can be a positive electrode sheet or a negative electrode sheet. Taking the case that the composite electrode sheet is a negative electrode sheet, the sodium-ion battery comprises a negative electrode sheet, a sodium-ion positive electrode sheet, an inorganic porous film, and a sodium-ion electrolyte. Each sheet is stacked and prepared into a sodium-ion battery through processes such as rolling, die cutting, sheet stacking, assembly, baking, liquid injection, standing, formation, and capacity distribution.

[0061] The following describes the embodiments of the present application in combination with the above technical solutions.

[0062] Embodiment 1 In the sodium-ion battery of the present embodiment, the negative electrode sheet is a composite electrode sheet, which is prepared by the following steps: 1. Preparation of hollow carbon microspheres: 0.1 mol / L hydrochloric acid is used to adjust the pH of 5 mmol / L Tris-HCl buffer solution to pH=9.0, 5 g of calcium carbonate microspheres with a diameter of 20 μm are added to the alkaline buffer solution, and the calcium carbonate microspheres are ultrasonically dispersed to uniformly distribute in the alkaline buffer solution to obtain a buffer solution of calcium carbonate microspheres; dopamine hydrochloride is added to the buffer solution of calcium carbonate microspheres, and after uniform stirring at 25℃ for 24 h, a mixed solution is obtained; the mixed solution is then filtered and washed with deionized water at least 3 times, and vacuum dried at 45℃ for 12 h to obtain polydopamine-coated calcium carbonate microspheres; the polydopamine-coated calcium carbonate microspheres are placed in a tube furnace for high-temperature sintering, heated at 800℃ under nitrogen atmosphere for 8 h with a heating rate of 5℃ / min, then soaked and washed with dilute hydrochloric acid to remove residual calcium oxide, and then uniform-sized hollow carbon microspheres are obtained, the specific surface area of the prepared hollow carbon microspheres is 1400 m² / g, and the pore size is 10 nm.

[0063] 2. Preparation of conductive layer on the surface of negative electrode current collector: hollow carbon microspheres, graphene and carbon nanotubes are dissolved in NMP solvent according to a predetermined mass ratio, a second binder polytetrafluoroethylene is added, and the mixture is uniformly mixed to obtain a composite conductive slurry, the volume percentage of the composite three-dimensional network structure composed of hollow carbon microspheres, graphene and carbon nanotubes in the solvent is 6%, the volume percentage of the second binder in the solvent is 5%, the solid content of the composite conductive slurry is 10%, and the viscosity of the composite conductive slurry is 200 mPa·S; then, the composite conductive slurry is loaded into an electrostatic spinning device, and the composite conductive slurry is sprayed on the aluminum foil current collector, the electrostatic spraying voltage is 20 kV, the solution flow rate is 1.5 ml / h, the diameter of the syringe needle is 0.03 cm, the diameter of the needle cylinder is 2 cm, the distance between the electrostatic spinning nozzle and the aluminum foil is 0.5 cm, and the conductive layer is obtained after drying.

[0064] 3. Preparation of negative electrode sheet: negative electrode active material, second conductive agent conductive carbon black, first conductive agent hollow carbon microspheres and first binder are dissolved in a solvent according to a predetermined mass ratio of 94.5%:1.5%:1.5%:2.5% to prepare negative electrode active slurry, the negative electrode active slurry is coated on the surface of the negative electrode current collector with the conductive layer, and after baking and drying, a negative electrode sheet is obtained; wherein the first binder is sodium carboxymethyl cellulose (CMC) + styrene-butadiene rubber (SBR); the areal density of the negative electrode sheet is 55 g / m 2 , and the compacted density is 0.95 g / cm 3 .

[0065] 4. Preparation of sodium ion battery: the negative electrode sheet, sodium ion positive electrode sheet and inorganic porous film are assembled, and sodium ion electrolyte is added, and the sodium ion battery is prepared through processes such as rolling, die cutting, lamination, assembly, baking, liquid injection, standing, formation and capacity distribution.

[0066] Example 2 In the sodium-ion battery of the present example, the negative electrode sheet is a composite electrode sheet, which is prepared by the following steps: 1. Preparation of hollow carbon microspheres: 5 mmol / L Tris-HCl buffer solution is adjusted to pH=8.5 by using 0.1 mol / L hydrochloric acid, 5 g of calcium carbonate microspheres with a diameter of 50 μm are added to the alkaline buffer solution, and the calcium carbonate microspheres are ultrasonically dispersed to uniformly distribute in the alkaline buffer solution to obtain a buffer solution of calcium carbonate microspheres; then, dopamine hydrochloride is added to the buffer solution of calcium carbonate microspheres, and after uniform stirring at 25°C for 20 h, a mixed solution is obtained; the mixed solution is then filtered and washed with deionized water for at least 3 times, and vacuum dried at 45°C for 12 h to obtain polydopamine-coated calcium carbonate microspheres; the polydopamine-coated calcium carbonate microspheres are placed in a tube furnace for high-temperature sintering, heated at 850°C for 6 h under nitrogen atmosphere at a heating rate of 5°C / min, and then soaked and washed with dilute hydrochloric acid to remove residual calcium oxide, thereby obtaining uniform-sized hollow carbon microspheres; the specific surface area of the prepared hollow carbon microspheres is 1300 m² / g, and the pore size is 15 nm.

[0067] 2. Preparation of the conductive layer on the surface of the negative electrode current collector: hollow carbon microspheres, graphene and carbon nanotubes are dissolved in NMP solvent according to a predetermined mass ratio, a second binder polytetrafluoroethylene is added, and the mixture is uniformly mixed to obtain a composite conductive slurry; the volume percentage of the composite three-dimensional network structure composed of hollow carbon microspheres, graphene and carbon nanotubes in the solvent is 6%, the volume percentage of the second binder in the solvent is 7%, the solid content of the composite conductive slurry is 20%, and the viscosity of the composite conductive slurry is 300 mPa·S; then, the composite conductive slurry is loaded into an electrostatic spinning device, and the composite conductive slurry is sprayed on the aluminum foil current collector; the electrostatic spraying voltage is 20 kV, the solution flow rate is 2 ml / h, the diameter of the syringe needle is 0.04 cm, the diameter of the needle cylinder is 2 cm, the distance between the electrostatic spinning nozzle and the aluminum foil is 1.5 cm, and the conductive layer is obtained after drying.

[0068] 3. Preparation of the negative electrode sheet: the negative electrode active material, the second conductive agent conductive carbon black, the first conductive agent hollow carbon microspheres and the first binder are dissolved in the solvent according to a predetermined mass ratio of 95.5%:1%:1%:2.5% to prepare a negative electrode active slurry; the negative electrode active slurry is coated on the surface of the negative electrode current collector with the conductive layer, and after baking and drying, the negative electrode sheet is obtained; wherein the first binder is sodium carboxymethyl cellulose (CMC) + styrene-butadiene rubber (SBR); the areal density of the negative electrode sheet is 55 g / m 2 , and the compacted density is 0.97 g / cm 3 .

[0069] 4. Preparing a sodium-ion battery: assembling the negative electrode sheet, the sodium-ion positive electrode sheet and the inorganic porous film, and adding a sodium-ion electrolyte, and preparing a sodium-ion battery through processes of rolling, die cutting, lamination, assembly, baking, liquid injection, standing, formation and capacity distribution.

[0070] Example 3 In the sodium-ion battery of the present example, the negative electrode sheet is a composite electrode sheet, which is prepared by the following steps: 1. Preparing hollow carbon microspheres: 5 g of calcium carbonate microspheres with a diameter of 100 μm are added to an alkaline buffer solution of 5 mmol / L Tris-HCl buffer solution adjusted to pH 8.0 with 0.1 mol / L hydrochloric acid, and the calcium carbonate microspheres are ultrasonically dispersed to uniformly distribute the calcium carbonate microspheres in the alkaline buffer solution to obtain a buffer solution of calcium carbonate microspheres; then, dopamine hydrochloride is added to the buffer solution of calcium carbonate microspheres, and after uniform stirring at 25°C for 16 h, a mixed solution is obtained; the mixed solution is then filtered and washed with deionized water at least 3 times, and vacuum dried at 45°C for 12 h to obtain polydopamine-coated calcium carbonate microspheres; the polydopamine-coated calcium carbonate microspheres are placed in a tube furnace for high-temperature sintering, heated at 900°C for 4 h under a nitrogen atmosphere at a heating rate of 5°C / min, and then soaked and washed with dilute hydrochloric acid to remove residual calcium oxide, thereby obtaining uniform-sized hollow carbon microspheres; the specific surface area of the prepared hollow carbon microspheres is 1250 m² / g, and the pore size is 17 nm.

[0071] 2. Preparing a conductive layer on the surface of the negative electrode current collector: hollow carbon microspheres, graphene and carbon nanotubes are dissolved in NMP solvent according to a predetermined mass ratio, and a second binder polytetrafluoroethylene is added and mixed uniformly to obtain a composite conductive slurry; the volume percentage of the composite three-dimensional network structure of hollow carbon microspheres, graphene and carbon nanotubes in the solvent is 8%, the volume percentage of the second binder in the solvent is 5%, the solid content of the composite conductive slurry is 30%, and the viscosity of the composite conductive slurry is 200 mPa·S; then, the composite conductive slurry is loaded into an electrospinning device, and the composite conductive slurry is sprayed on an aluminum foil current collector; the electrostatic spraying voltage is 20 kV, the solution flow rate is 1.5 ml / h, the diameter of the syringe needle is 0.05 cm, the diameter of the needle cylinder is 2 cm, the distance between the electrospinning nozzle and the aluminum foil is 2.0 cm, and the conductive layer is obtained after drying.

[0072] 3. Preparation of the negative electrode sheet: the negative electrode active material, the second conductive agent conductive carbon black, the first conductive agent hollow carbon microspheres, and the first binder are dissolved in a solvent according to a preset mass ratio of 95%:1.5%:1%:2.5% to prepare a negative electrode active paste, the negative electrode active paste is coated on the surface of the negative electrode current collector with the conductive layer, and the negative electrode sheet is obtained after baking and drying; wherein the first binder is sodium carboxymethyl cellulose (CMC) + styrene-butadiene rubber (SBR); the surface density of the negative electrode sheet is 55 g / m 2 , and the compacted density is 0.98 g / cm 3 .

[0073] 4. Preparation of the sodium ion battery: the negative electrode sheet, the sodium ion positive electrode sheet, and the inorganic porous film are assembled, and a sodium ion electrolyte is added, and the sodium ion battery is prepared through processes such as rolling, die cutting, lamination, assembly, baking, liquid injection, standing, formation, and capacity distribution.

[0074] Example 4 The difference between this embodiment and example 1 is that in the step of preparing the hollow carbon microspheres, 0.1 mol / L hydrochloric acid is used to adjust the pH of 5 mmol / L Tris-HCl buffer solution to pH=9.0, 5 g of calcium carbonate microspheres with a diameter of 10 μm are added to the alkaline buffer solution, the calcium carbonate microspheres are ultrasonically dispersed to uniformly distribute the calcium carbonate microspheres in the alkaline buffer solution, a buffer solution of calcium carbonate microspheres is obtained; dopamine hydrochloride is added to the buffer solution of calcium carbonate microspheres, and after uniform stirring at 25°C for 24 h, a mixed solution is obtained; the mixed solution is then filtered, washed with deionized water at least 3 times, and vacuum dried at 45°C for 12 h to obtain polydopamine-coated calcium carbonate microspheres; the polydopamine-coated calcium carbonate microspheres are placed in a tube furnace for high-temperature sintering, heated at 800°C for 8 h under a nitrogen atmosphere at a heating rate of 5°C / min, and then soaked and washed with dilute hydrochloric acid to remove residual calcium oxide, thereby obtaining uniform-sized hollow carbon microspheres; the specific surface area of the prepared hollow carbon microspheres is 1500 m² / g, and the pore size is 5 nm.

[0075] Example 5 The difference between this embodiment and example 1 is that in the step of preparing the negative electrode sheet, the second conductive agent is removed, and the negative electrode active material, the first conductive agent hollow carbon microspheres, and the first binder are dissolved in a solvent according to a preset mass ratio of 96%:2%:2%; the rest is the same as example 1.

[0076] Example 6 The difference between this embodiment and example 1 is that the composite electrode sheet is a positive electrode sheet, the positive electrode current collector aluminum foil and the positive electrode active material are used, and the positive electrode current collector surface is coated with a conductive layer containing 5% three-dimensional network conductive structure; the rest is the same as example 1.

[0077] In the sodium ion battery of the embodiment, the positive electrode sheet is a composite electrode sheet, which is prepared by the following steps: 1. Hollow carbon microspheres are prepared: 0.1 mol / L hydrochloric acid is used to adjust the pH of 5 mmol / L Tris-HCl buffer solution to pH=9.0, 5 g of calcium carbonate microspheres with a diameter of 20 μm are added to the alkaline buffer solution, and the calcium carbonate microspheres are ultrasonically dispersed to uniformly distribute the calcium carbonate microspheres in the alkaline buffer solution, to obtain a buffer solution of calcium carbonate microspheres; dopamine hydrochloride is added to the buffer solution of calcium carbonate microspheres, and after uniform stirring at 25°C for 24 h, a mixed solution is obtained; the mixed solution is then filtered, washed with deionized water at least 3 times, and vacuum dried at 45°C for 12 h, to obtain polydopamine-coated calcium carbonate microspheres; the polydopamine-coated calcium carbonate microspheres are placed in a tube furnace for high-temperature sintering, heated at 800°C for 8 h under a nitrogen atmosphere at a heating rate of 5°C / min, and then soaked and washed with dilute hydrochloric acid to remove residual calcium oxide, to obtain uniform-sized hollow carbon microspheres. The specific surface area of the prepared hollow carbon microspheres is 1400 m² / g, and the pore size is 10 nm.

[0078] 2. A conductive layer on the surface of the positive electrode current collector is prepared: hollow carbon microspheres, graphene, and carbon nanotubes are dissolved in NMP solvent according to a preset mass ratio, a second binder polytetrafluoroethylene is added, and the mixture is uniformly mixed to obtain a composite conductive slurry. The volume percentage of the composite three-dimensional network structure composed of hollow carbon microspheres, graphene, and carbon nanotubes in the solvent is 6%, the volume percentage of the second binder in the solvent is 5%, the solid content of the composite conductive slurry is 10%, and the viscosity of the composite conductive slurry is 200 mPa·S. Then, the composite conductive slurry is loaded into an electrostatic spinning device, the composite conductive slurry is sprayed on the aluminum foil current collector, the electrostatic spraying voltage is 20 kV, the solution flow rate is 1.5 ml / h, the diameter of the syringe needle is 0.03 cm, the diameter of the needle cylinder is 2 cm, the distance between the electrostatic spinning nozzle and the aluminum foil is 0.5 cm, and the conductive layer is obtained after drying.

[0079] 3. A positive electrode sheet is prepared: the positive electrode active material, the second conductive agent conductive carbon black, the first conductive agent hollow carbon microspheres, and the first binder are dissolved in a solvent according to a preset mass ratio of 94.0%:1.5%:1.0%:3.5% to prepare a positive electrode active slurry, the positive electrode active slurry is coated on the surface of the positive electrode current collector with the conductive layer, and after baking and drying, the positive electrode sheet is obtained. The first binder is polyvinylidene fluoride (PVDF). The areal density of the positive electrode sheet is 165 g / m 2 , and the compacted density is 2.1 g / cm 3 .

[0080] 4. Preparation of sodium-ion battery: the positive sheet, sodium-ion negative sheet and inorganic porous film are assembled, and sodium-ion electrolyte is added, and the sodium-ion battery is prepared through the processes of rolling, die cutting, lamination, assembly, baking, liquid injection, standing, formation, and capacity distribution.

[0081] Comparative Example 1 In the sodium-ion battery of the present comparative example, the conductive layer does not exist in the negative sheet, and the first conductive agent is also not included in the active layer, which is prepared by the following steps: 1. Preparation of negative sheet: the negative active material, the second conductive agent conductive carbon black and the first binder are dissolved in the solvent according to the mass ratio of 96%:2%:2% to prepare the negative active paste, and the negative active paste is coated on the surface of the negative current collector aluminum foil, and after baking and drying, the negative sheet is obtained; wherein the first binder is sodium carboxymethyl cellulose (CMC) + styrene-butadiene rubber (SBR); the areal density of the negative sheet is 55 g / m 2 , and the compacted density is 0.98 g / cm 3 .

[0082] 2. Preparation of sodium-ion battery: the negative sheet, sodium-ion positive sheet and inorganic porous film are assembled, and sodium-ion electrolyte is added, and the sodium-ion battery is prepared through the processes of rolling, die cutting, lamination, assembly, baking, liquid injection, standing, formation, and capacity distribution.

[0083] The sodium-ion batteries prepared in the above examples 1-6 and comparative examples are tested, and the test results are shown in Table 1.

[0084] Table 1 Performance test results of examples and comparative examples

[0085] As can be seen from Table 1, compared with Comparative Example 1, the composite sheet resistance and the cell AC resistance of Examples 1-5 are effectively reduced, which indicates that the conductivity of the composite sheet is significantly improved; and the 5C discharge capacity retention rate and the cycle 500 cycle capacity retention rate (%) of Examples 1-6 are also effectively improved relative to Comparative Example 1, combined with the discharge curves of Examples 1-3 and Comparative Example in Figure 1 , and Figure 2The capacity retention comparison chart of Examples 1-3 and Comparative Examples shows that the composite electrode sheet with the conductive layer is beneficial to improve the rate performance and cycle performance of the sodium ion battery; in addition, the liquid retention capacity test is performed on the sodium ion battery in Example 4, and the electrolyte absorption rate is measured to be 185%, which shows that the three-dimensional network conductive structure formed by the hollow carbon microspheres, graphene and carbon nanotubes and the three-dimensional network conductive structure formed by the first conductive agent in the active layer in the sodium ion battery with the composite electrode sheet can significantly improve the liquid retention capacity of the battery cell, and further improve the rate performance and cycle stability of the sodium ion battery; and the cycle attenuation rate test is performed on the sodium ion battery in Example 6, and the cycle attenuation rate is measured to be 0.002% / cycle, which shows that the composite electrode sheet is also suitable for the positive electrode sheet, and can greatly reduce the cycle attenuation and improve the cycle stability of the sodium ion battery.

[0086] It should be noted that each of the embodiments in the specification is described in a progressive manner, and each embodiment focuses on the difference from other embodiments. The same and similar parts of each embodiment can be referred to each other.

[0087] The above only describes some embodiments of the present application and is not used to limit the present application. It should be understood by those skilled in the art that the present application can have various changes and improvements. Any modification, equivalent replacement and improvement according to the present application fall within the scope of the present application.

Claims

1. A composite pole piece, characterized in that: The invention comprises a current collector and a conductive layer covering the surface of the current collector, wherein the conductive layer is a three-dimensional network structure comprising hollow carbon microspheres, graphene and carbon nanotubes.

2. The composite pole piece according to claim 1, characterized in that: The graphene sheet structure wraps the surface of the hollow carbon microspheres, and the carbon nanotubes are inserted and connected in the gaps between the hollow carbon microspheres to form the three-dimensional network structure.

3. The composite pole piece according to claim 1, characterized in that: The conductive layer satisfies at least one of the following characteristics: The diameter of the hollow carbon microspheres is 5 μm to 100 μm; The shell of the hollow carbon microspheres is a porous structure, and the specific surface area of ​​the hollow carbon microspheres is 1200m 2 / g~1500m 2 / g; The pore size of the hollow carbon microspheres is 5 nm to 20 nm; The graphene sheet has a thickness of 3 nm to 10 nm; The diameter of the carbon nanotubes is 10 nm to 30 nm; The aspect ratio of the carbon nanotubes is greater than a preset aspect ratio, and the preset aspect ratio is 800 to 1200; The mass ratio of the hollow carbon microspheres to the graphene is 1:0.5 to 1:2; The mass ratio of the hollow carbon microspheres to the carbon nanotubes is 1:0.5 to 1:2; The porosity of the conductive layer is 60% to 80%; The conductivity of the conductive layer is greater than a preset conductivity, and the preset conductivity is 800 S / m to 1200 S / m.

4. The composite pole piece according to claim 1, characterized in that: The composite electrode further comprises an active layer, and the active layer is located on a side of the conductive layer away from the current collector; The active layer includes an active material and a first conductive agent, wherein the first conductive agent is a three-dimensional network structure including hollow carbon microspheres.

5. The composite pole piece according to claim 4, characterized in that: The mass ratio between the active material and the first conductive agent in the active layer is 92:2 to 96:

1.

6. The composite pole piece according to claim 4, characterized in that: The active layer satisfies at least one of the following characteristics: The active layer further includes a second conductive agent, wherein the second conductive agent includes a mixture of one or more of conductive carbon black, acetylene black, conductive graphite, Ketjen black, graphene and carbon nanotubes; The mass ratio between the first conductive agent and the second conductive agent in the active layer is 1:3 to 2:1; The active layer further includes a first binder, wherein the first binder includes a mixture of one or more of sodium carboxymethyl cellulose, styrene-butadiene rubber, polyacrylic acid and polyacrylonitrile; The mass ratio of the active material to the first binder is 92:3 to 96:

1.

7. The composite pole piece according to any one of claims 1 to 6, characterized in that: The conductive layer further includes a second binder, wherein the second binder satisfies at least one of the following characteristics: The second binder includes one or more of polytetrafluoroethylene, polyvinylidene fluoride, polyolefins, polyvinyl alcohol, polyacrylic acid and polyurethane; The volume ratio of the total volume of the hollow carbon microspheres, the graphene and the carbon nanotubes to the second binder is 6:7 to 8:

2.

8. The composite pole piece according to any one of claims 1 to 6, characterized in that: The composite electrode meets at least one of the following characteristics: The surface density of the composite electrode is 20g / m 2 ~180g / m 2 ; The compaction density of the composite electrode is 0.95 g / cm 3 ~2.5g / cm 3 .

9. A method for preparing a composite electrode, characterized in that: include: forming a conductive layer on the surface of the current collector to obtain a composite electrode; The conductive layer is a three-dimensional network structure including hollow carbon microspheres, graphene and carbon nanotubes.

10. A sodium ion battery, characterized in that: Comprising the composite pole piece according to any one of claims 1 to 8.

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