Positive electrode active material, preparation method thereof, positive electrode slurry, positive electrode plate and battery

Abstract

The invention relates to the technical field of battery positive electrode slurry, in particular to a positive electrode active material, a preparation method thereof, positive electrode slurry, a positive plate and a battery, the positive electrode active material comprises an inner core and a coating layer coating the surface of the inner core, the inner core is a polyanionic compound, and the coating layer is a transition metal oxide. The positive electrode active material provided by the invention can improve the cycling stability of the sodium ion battery under high voltage, reduce the terminal voltage difference, and comprehensively improve the energy density and the structural stability of the battery.

Description

Technical Field

[0001] The present invention relates to the technical field of positive electrode slurries for batteries, in particular to positive electrode active materials, their preparation methods, positive electrode slurries, positive electrode sheets and batteries. Background Art

[0002] Positive electrode slurries play a crucial role in the core performance of batteries, such as energy density and cycle life. An ideal positive electrode slurry should possess characteristics such as high specific capacity, abundant resources, stable structure, and high working voltage. Currently, the research on positive electrode slurries for sodium-ion batteries mainly focuses on three categories: transition metal oxides, Prussian blue, and polyanion-type compounds. Layered oxides, with a crystal structure similar to that of ternary positive electrode slurries, have a high energy density but relatively weak safety and high costs. Prussian blue compounds have a high energy density and good rate performance, however, the crystal water in them is difficult to remove and will weaken the actual specific capacity and cycle performance. Polyanion-type compounds, with a strong and open three-dimensional network structure constructed by polyanion polyhedra and transition metal ion polyhedra through strong covalent bonds, exhibit excellent thermal stability and electrochemical stability and long cycle life, but have a low energy density and low platform voltage. When used in the high voltage range, due to the rapid change of the terminal voltage, it will cause too large a pressure difference within the Pack group, affecting the performance of the battery cells.

[0003] In the prior art, it is difficult for a single material to balance high energy density, long cycle life, and low-temperature performance. For example, uncoated polyanion materials, although having a stable structure, have a low ion diffusion rate, resulting in severe polarization; while transition metal oxides have a high energy density but significant volume expansion and many interfacial side reactions. For example, patent CN116435498A introduces a preparation method for a P2-phase layered oxide positive electrode slurry, which has relatively good cycle stability but is prone to gas swelling at high voltages.

[0004] Chinese Patent with application number 202310710690.9 discloses a sodium-ion battery positive electrode material, its preparation method and application, which discloses a positive electrode material that uses a layered oxide as the core and sodium iron sulfate as the shell layer. Through the synergistic effect of the core-shell heterostructure, it provides a positive electrode material with long life, high energy density, and low cost. However, this positive electrode material has poor low-temperature performance.

[0005] Therefore, there is an urgent need for a composite structure material to synergistically improve the comprehensive performance. Summary of the Invention

[0006] The first object of the present invention is to provide a sodium-ion composite positive electrode active material, which can improve the cycle stability of sodium-ion batteries at high voltages, reduce the terminal pressure difference, reduce the polarization of low-temperature charge and discharge, and comprehensively improve the energy density and structural stability of the battery.

[0007] To solve the above technical problems, the present invention adopts the following technical solutions:

[0008] A sodium-ion composite cathode active material includes a core and a coating layer coated on the surface of the core. The core is a polyanionic compound, and the coating layer is a transition metal oxide.

[0009] Furthermore, the mass percentage of the core and the coating layer is 70%-90%: 10%-30%.

[0010] Furthermore, the thickness of the coating layer is 10-200 nm.

[0011] Furthermore, the transition metal oxide is at least one of O3-phase layered oxide, P2-phase layered oxide, and tunnel oxide.

[0012] Furthermore, the chemical formula of the transition metal oxide is Na x MO2, where M is a transition metal element, x is the stoichiometric number of sodium, and 0 < x < 1.

[0013] The second object of the present invention is to provide a preparation method of a sodium-ion composite cathode active material, including the following preparation steps:

[0014] Step 1: Disperse the polyanionic compound in a solvent, then add sodium salt, soluble transition metal salt, and citric acid, and stir evenly to form a sol to obtain a precursor.

[0015] Step 2: Under an inert atmosphere, calcine the precursor at a calcination temperature of 200-500 °C for 2-8 h, and finally ball mill to obtain the cathode active material.

[0016] Furthermore, in Step 2, the temperature is gradually increased to the calcination temperature at a heating rate of 1-3 °C / min.

[0017] The third object of the present invention is to provide a cathode slurry including the above-mentioned cathode active material.

[0018] Furthermore, the cathode slurry includes the following components by mass percentage: 92.5%-94.7% of cathode active material, 0.3%-0.5% of dispersant, 2.5%-3.5% of conductive carbon, and 2.5%-3.5% of binder.

[0019] The fourth object of the present invention is to provide a cathode sheet including the above-mentioned cathode slurry.

[0020] The fifth object of the present invention is to provide a sodium-ion battery including the above-mentioned cathode sheet.

[0021] The beneficial effects of the present invention:

[0022] The present invention uses a polyanionic compound as the core and a transition metal oxide coating layer as the shell to form a coated composite cathode active material with a core-shell structure. The inner core provides structural stability, and the coating layer improves the energy density and inhibits sodium deposition. Through the synergistic effect of the inner core and the coating layer, the cycle stability of the sodium-ion battery at high voltages is improved, the terminal voltage difference is reduced, the low-temperature charge and discharge polarization is decreased, the sodium deposition risk is alleviated, and the battery energy density and structural stability are enhanced.

[0023] The present invention adopts the sol-gel method to prepare a coated composite cathode active material with a core-shell structure. By controlling the calcination temperature and time, the structural integrity of the generated core active material is controlled, so that the prepared active material has the effects of improving the battery energy density, having stable structure, and reducing the low-temperature charge and discharge polarization. When the calcination temperature is too high and the time is too long, the excessive temperature will cause excessive grain growth or phase transformation of the transition metal oxide coating layer, destroying the integrity of the core-shell structure, resulting in a decrease in the oxidation reaction activation energy of the inner core material and accelerating the structural degradation. The too long calcination time leads to excessive sintering and agglomeration of metal oxide particles, a decrease in specific surface area, and an extension of the ion migration path. When the calcination temperature is too low and the time is too short, the too low temperature cannot fully decompose the precursor, resulting in a loose and porous coating layer or even residual carbon source. It also cannot complete the crystallization of the metal oxide, resulting in the coating layer being mainly amorphous, and the undegraded organic matter will hinder ion transport. Therefore, too high or too low calcination temperature will affect the performance of the prepared active material. Description of the Drawings

[0024] The drawings described herein are used to provide a further understanding of the present invention and constitute a part of the present invention. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation to the present invention.

[0025] Figure 1 It is the charge and discharge curves of the batteries prepared in Example 1 and Comparative Example 1 of the present invention at room temperature.

[0026] Figure 2 It is the cycle life curves of the batteries prepared in Example 1 and Comparative Example 1 of the present invention at room temperature.

[0027] Figure 3 It is the -20°C low-temperature discharge curves of the batteries prepared in Example 1 and Comparative Example 1 of the present invention.

[0028] Figure 4 It is the -20°C low-temperature full charge curves of the batteries prepared in Example 1 and Comparative Example 1 of the present invention. Detailed Embodiments

[0029] To make the objectives, technical solutions, and technical effects of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention are clearly and completely described below. The embodiments described below are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art without creative efforts in combination with the embodiments of the present invention belong to the scope of protection of the present invention. For those conditions not specified in the embodiments, they are carried out according to conventional conditions or conditions recommended by the manufacturer; for those reagents or instruments without indicating the manufacturer, they are all conventional products that can be obtained through commercial purchase.

[0030] In the description of the present invention, the term "and / or" describes the association relationship of associated objects, indicating that three relationships may exist. For example, A and / or B may represent: A exists alone, A and B exist simultaneously, and B exists alone. Wherein A and B may be singular or plural. The character " / " generally indicates that the associated objects before and after are in an "or" relationship.

[0031] In the description of the present invention, "at least one" means one or more, and "a plurality" means two or more. "At least one (item)" or its similar expression below refers to any combination of these items, including any combination of single item (item) or plural items (items). For example, "at least one (item) of a, b, or c", or, "at least one (item) of a, b, and c" can all represent: a, b, c, a - b (that is, a and b), a - c, b - c, or a - b - c, where a, b, and c can be single or multiple respectively.

[0032] It should be understood that the weights of the relevant components mentioned in the embodiments of the present invention not only can refer to the specific contents of each component, but also can represent the proportional relationship of the weights between each component. Therefore, as long as the contents of the relevant components in the embodiments of the present invention are scaled up or down proportionally, they are within the scope disclosed by the present invention. Specifically, the weights described in the embodiments of the present invention can be mass units well-known in the chemical field such as μg, mg, g, kg, etc.

[0033] In addition, unless otherwise explicitly stated in the context, the expression of the singular form of a word should be understood to include the plural form of the word. The term "comprising" or "having" is intended to specify the existence of a feature, quantity, step, operation, element, part, or a combination thereof, but is not used to exclude the existence or possible addition of one or more other features, quantities, steps, operations, elements, parts, or a combination thereof.

[0034] Since it is difficult for existing single materials to balance high energy density, long cycle life, and low-temperature performance, Chinese Patent No. 202310710690.9 discloses a sodium-ion battery cathode material, its preparation method, and application. It discloses a cathode material that uses a layered oxide as the core and sodium iron sulfate as the shell layer. Through the synergy of the core-shell heterostructure, it provides a cathode material with long life, high energy density, and low cost. However, the cathode material prepared by this patent method has poor low-temperature performance. And although transition metal oxides have high energy density, due to their significant volume expansion, existing technologies (such as the above-mentioned Chinese patent) generally use transition metal oxides as the inner core of composite materials.

[0035] A sodium-ion composite cathode active material provided by an embodiment of the present invention, the cathode active material includes a core with a mass fraction of 70wt%-90wt% and a coating layer with a mass fraction of 10wt%-30wt%. The sum of the mass percentages of the core and the coating layer is 100%. The coating layer covers the surface of the core. The core is a polyanion-type compound, and the coating layer is a transition metal oxide.

[0036] The present invention uses a polyanion-type compound as the core and a transition metal oxide coating layer as the shell to form a coated composite cathode active material with a core-shell structure. The core provides structural stability, and the coating layer improves energy density and inhibits sodium deposition. Through the synergistic effect of the core and the coating layer, the cycle stability of the sodium-ion battery at high voltages is improved, the terminal voltage difference is reduced, the low-temperature charge and discharge polarization are reduced, the sodium deposition risk is alleviated, and the battery energy density and structural stability are improved.

[0037] The coating structure of the cathode active material of the present invention reduces the possibility of direct electrical contact between the core material and the coating layer material. On the one hand, the built-in electric field formed by the work function difference between the transition metal oxide coating layer and the polyanion-type compound core drives electrons to transport along the surface of the coating layer, rather than through the core-coating layer interface, thereby reducing the possibility of direct electrical contact between the core material and the coating layer material; on the other hand, the layered oxide in the coating layer has multi-dimensional ion channels, which preferentially guide Na + to diffuse along the coating layer structure rather than penetrate to the core interface, further reducing the electrical contact probability between the two. Reducing the direct electrical contact between the core material and the coating layer material can avoid interfacial side reactions, thereby improving the high-voltage cycle stability of the battery and extending the service life of the battery.

[0038] The present invention uses a transition metal oxide as the coating layer to enhance the surface ion diffusion ability and reduce the charge and discharge polarization under low-temperature conditions, so that the battery can maintain good charge and discharge performance even in a cold environment.

[0039] There is a synergistic effect between the polyanionic compound core and the transition metal oxide coating layer of the present invention, which improves the structural stability and reliability of the battery. The polyanionic compound as the core has an olivine structure. Its high structural stability enables the insertion and extraction of sodium ions during charge and discharge not to cause significant lattice distortion. Its low expansion property helps to maintain the structural integrity of the particles, reduce crack formation, and thus indirectly inhibit further expansion caused by particle rupture. At the same time, the gradient modulus structure formed between the polyanionic compound core and the coating layer limits the expansion deformation of the coating layer and absorbs the expansion stress through stress redistribution during charge and discharge, further ensuring the long-term stability and reliability of the battery.

[0040] In the present invention, a transition metal oxide is used as the coating layer to achieve the effect of inhibiting sodium deposition. The gradient conductive network formed by the coating layer can promote the uniform extraction of sodium ions, avoid local excessive current density, reduce the sodium ion accumulation at the negative electrode. At the same time, the coating layer can reduce the interfacial resistance, reduce the polarization during charge and discharge, make the insertion of sodium ions at the negative electrode more efficient, and reduce the probability of sodium deposition. The core-shell structure maintains the integrity of the electrode, prevents the active material from cracking or poor contact, and thus maintains a stable sodium ion transmission channel.

[0041] By strictly controlling the ratio between the core and the coating layer of the positive electrode active material of the present invention, the positive electrode active material of the present invention is more conducive to improving the battery energy density and structural stability. When the mass ratio of the coating layer is too small, it is difficult to form a continuous conductive network on the surface of the core material, the charge transfer efficiency becomes low, resulting in problems such as reduced battery energy density, increased battery internal resistance, shortened battery cycle life, and decreased rate performance. When the mass ratio of the coating layer is too large, it will cause the coating layer formed on the surface of the core to be too thick and extend the sodium ion diffusion path, thereby resulting in decreased rate performance.

[0042] In the examples, the mass percentages of the core and the coating layer can be, but are not limited to, 70wt%:30wt%, 75wt%:25wt%, 80wt%:20wt%, 85wt%:15wt%, 90wt%:10wt%, and the sum of the mass percentages of the core and the coating layer is 100%.

[0043] In the examples, the polyanionic compound is at least one of sodium iron phosphate and sodium iron sulfate. Sodium iron phosphate and sodium iron sulfate as core materials can provide good structural stability and longer cycle life. Preferably, it is sodium iron phosphate, which has a more stable olivine structure and is more conducive to improving the structural stability and reliability of the battery. In other examples, the polyanionic compound can also be at least one of sodium vanadium phosphate, sodium vanadium sulfate, sodium-containing silicate, and sodium fluorophosphate vanadate.

[0044] In an embodiment, the transition metal oxide is at least one of an O3-phase layered oxide, a P2-phase layered oxide, and a tunnel oxide. The O3-phase layered oxide has a high sodium content, can provide more reversible sodium ion insertion / extraction sites, has a high theoretical specific capacity, and exhibits a stable voltage plateau during charge and discharge. Its structure is stable, with excellent cycle stability and good low-temperature performance. The P2-phase layered oxide has high ionic conductivity, a high working voltage, and structural stability, and is suitable for high-rate charge and discharge. The tunnel-type oxide has ultra-high structural stability, an extremely long cycle life, excellent rate performance, and wide-temperature adaptability. In this embodiment, the O3-phase layered oxide is preferably used, which is beneficial to improving the energy density and cycle life of the battery.

[0045] In an embodiment, the chemical formula of the transition metal oxide is Na x MO2, where M is a transition metal element and x is the stoichiometric number of sodium, and 0 < x < 1. Specifically, M is at least one of scandium (Sc), titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), and zinc (Zn). In other embodiments, M can also be other types of transition metals.

[0046] In an embodiment, the thickness of the coating layer is 10 - 200 nm. By strictly controlling the thickness of the coating layer in the present invention, the cathode active material of the present invention is more conducive to improving the battery energy density and structural stability. An overly thick coating layer will extend the sodium ion diffusion path, resulting in a decline in rate performance; while an overly thin coating layer will cause the coating layer to fail to form a continuous conductive network on the surface of the core, resulting in low charge transfer efficiency. In other embodiments, the thickness of the coating layer can also be 20 - 180 nm, 30 - 170 nm, 40 - 160 nm, 50 - 150 nm, 70 - 130 nm, or 90 - 110 nm.

[0047] The embodiment of the present invention also provides a preparation method for the above sodium ion composite cathode active material, including the following preparation steps:

[0048] Step 1: Disperse the polyanionic compound in a solvent, then add a sodium salt, a soluble transition metal salt, and citric acid, and stir evenly to form a sol to obtain a precursor;

[0049] Step 2: Under an inert atmosphere, calcine the precursor at a calcination temperature of 200 - 500 °C for a calcination time of 2 - 8 h, and finally ball mill to obtain the cathode active material.

[0050] The present invention adopts the sol-gel method. By dispersing a polyanionic compound in a solvent, then adding a sodium salt, a soluble transition metal salt, and citric acid, and stirring evenly to dissolve the complexing agent citric acid to dissolve the transition metal ions and react with each substance to form a homogeneous and stable sol system. Finally, the sol is calcined, controlling the calcination temperature at 200-500 °C and the calcination time at 2-8 h. After grinding and dispersing, a coated composite cathode active material with a core-shell structure is obtained, which has a polyanionic compound as the core and a transition metal oxide as the coating layer. The core provides structural stability, and the coating layer improves the energy density and inhibits sodium deposition. Through the synergistic effect of the core and the coating layer, the cycle stability of the sodium-ion battery at high voltages can be improved, the terminal voltage difference can be reduced, the low-temperature charge and discharge polarization can be reduced, the sodium deposition risk can be alleviated, and the battery energy density and structural stability can be improved. When the calcination temperature is too high and the time is too long, the excessive temperature will cause excessive grain growth or phase transformation of the transition metal oxide coating layer, destroying the integrity of the core-shell structure, resulting in a decrease in the oxidation reaction activation energy of the core material and accelerating the structural degradation. The too long calcination time leads to excessive sintering of the metal oxide particles, causing agglomeration, a decrease in the specific surface area, and an extension of the ion migration path. When the calcination temperature is too low and the time is too short, the too low temperature cannot fully decompose the precursor, resulting in a loose and porous coating layer or even residual carbon source. The crystallization of the metal oxide cannot be completed either, resulting in the coating layer being mainly in an amorphous state, and the undegraded organic matter will hinder ion transport. Therefore, too high or too low calcination temperatures will affect the performance of the prepared active material.

[0051] In the embodiment, the soluble transition metal salt can be at least one of a soluble scandium salt, a soluble titanium salt, a soluble vanadium salt, a soluble chromium salt, a soluble manganese salt, a soluble iron salt, a soluble cobalt salt, a soluble nickel salt, a soluble copper salt, a soluble zinc salt, or a soluble salt of other transition metals.

[0052] In the embodiment, the sodium salt is sodium nitrate, and the soluble transition metal salt is a transition metal nitrate. The nitrate ions can be decomposed during the calcination process, which can maintain the purity of the active material and avoid impurities affecting its performance.

[0053] In the embodiment, in step two, the temperature is gradually increased to the calcination temperature at a heating rate of 1-3 °C / min, which is beneficial to the formation of crystals of the transition metal oxide and the uniform coating on the surface of the core.

[0054] In an embodiment, the ball milling speed is 200 - 400 rpm, and the ball milling time is 1 - 3 h. Controlling the ball milling time and speed is beneficial to improving the dispersibility of the prepared cathode active material. When the cathode active material is applied in a battery, it is more conducive to improving the high-voltage cycle life of the battery, reducing the polarization of charge and discharge at low temperatures, and reducing the risk of sodium precipitation. Specifically, in the embodiment, the ball milling speed can be, but is not limited to, 200 rpm, 220 rpm, 250 rpm, 270 rpm, 300 rpm, 320 rpm, 350 rpm, 380 rpm, 400 rpm; the ball milling time can be, but is not limited to, 1 h, 1.2 h, 1.5 h, 1.7 h, 2 h, 2.2 h, 2.5 h, 2.7 h, 3 h.

[0055] An embodiment of the present invention also provides a cathode slurry, which includes the above-mentioned cathode active material. The cathode slurry includes the following components in mass percentages: 92.5% - 94.7% of the cathode active material, 0.3% - 0.5% of the dispersant, 2.5% - 3.5% of the conductive carbon, and 2.5% - 3.5% of the binder. The sum of the mass percentages of the components of the cathode active material is 100%. The cathode slurry of the present invention uses the above-mentioned cathode active material and is compounded with the dispersant, conductive carbon, and binder strictly according to the ratio, which can improve the sodium ion transport ability of the electrode sheet.

[0056] An embodiment of the present invention also provides a cathode sheet, which includes the above-mentioned cathode slurry.

[0057] An embodiment of the present invention also provides a sodium ion battery, which includes the above-mentioned cathode sheet.

[0058] The cathode sheet of the present invention has a high sodium ion transport ability. The sodium ion battery prepared with the cathode sheet of the present invention has good cycle stability at high voltages, can reduce the terminal voltage difference, reduce the polarization of charge and discharge at low temperatures, reduce the risk of sodium precipitation, and improve the energy density and structural stability of the battery.

[0059] To enable those skilled in the art to clearly understand the above-mentioned implementation details and operations of the present invention, and to significantly reflect the progressive performance of the embodiments of the present invention, the following uses multiple embodiments to illustrate the above technical solutions.

[0060] Example 1

[0061] A sodium ion battery includes the following preparation steps:

[0062] Step 1: Prepare a sodium ion composite cathode active material:

[0063] S11. Disperse sodium iron phosphate in an ethanol solution by mass parts, then add sodium nitrate, soluble transition metal salts, and citric acid. The mass ratio of sodium iron phosphate, ethanol solution, sodium nitrate, soluble transition metal salts, and citric acid is 1:7:0.5:1.2:0.3. The soluble metal salts include iron nitrate, nickel nitrate, manganese nitrate, and zinc nitrate, and the mass ratio of iron nitrate, nickel nitrate, manganese nitrate, and zinc nitrate is 5:3:1.5:0.5. Mix and stir these substances at a stirring speed of 300 rpm for 0.5 h to form a sol and obtain a precursor.

[0064] S12. Under an inert atmosphere, gradually heat the precursor to 400 °C at a heating rate of 2 °C / min, calcine for 4 h, and finally perform ball milling at a ball milling speed of 300 rpm for 2 h to obtain a positive electrode active material.

[0065] The mass of the prepared positive electrode active material accounts for 80 wt% of the total mass of the active material, and the mass of the coating layer accounts for 20 wt% of the total mass of the active material. The coating layer is coated on the surface of the core. The core is sodium iron phosphate, and the coating layer is an O3-phase layered oxide with the chemical formula Na 0.91 Fe 0.32 Ni 0.23 Mn 0.37 Zn 0.08 O2, and the thickness of the coating layer is 100 nm.

[0066] Step Two: Prepare a positive electrode slurry:

[0067] Mix and stir 93.6% of the positive electrode active material, 0.4% of polyacrylic acid dispersant, 3% of conductive carbon, and 3% of polyvinylidene fluoride binder by mass percentage to obtain a positive electrode slurry.

[0068] Step Three: Prepare a sodium-ion battery: Coating, rolling, slitting, die-cutting, laminating, encapsulating, baking, injecting electrolyte, high-temperature fixture forming, degassing and second sealing, and forming the capacity of the above positive electrode slurry to obtain a finished battery cell, and then test its electrochemical performance. The preparation methods of the electrode sheets and sodium-ion batteries in this embodiment are the same as those in the prior art and will not be elaborated here.

[0069] Example 2

[0070] A sodium-ion battery, which includes the following preparation steps:

[0071] Step One: Prepare a sodium-ion composite positive electrode active material:

[0072] S11. Disperse sodium iron phosphate by mass parts in an ethanol solution, then add sodium nitrate, soluble transition metal salts and citric acid. The mass ratio of sodium iron phosphate, ethanol solution, sodium nitrate, soluble transition metal salts and citric acid is 1:7:0.5:1.2:0.3. The soluble metal salts include iron nitrate and nickel nitrate with a mass ratio of 1:1. Mix and stir all substances at a stirring speed of 300 rpm for 0.5 h. After stirring evenly, a sol is formed to obtain a precursor;

[0073] S12. Under an inert atmosphere, gradually heat the precursor to 200 °C at a heating rate of 1 °C / min, calcine for 8 h, and finally perform ball milling treatment at a ball milling speed of 200 rpm for 3 h to obtain a positive electrode active material;

[0074] The mass of the core of the prepared positive electrode active material accounts for 70 wt% of the total mass of the active material, and the mass of the coating layer accounts for 30 wt% of the total mass of the active material. The coating layer is coated on the surface of the core. The core is sodium iron phosphate, and the coating layer is an O3-phase layered oxide. The thickness of the coating layer is 10 nm.

[0075] Step Two. Prepare a positive electrode slurry:

[0076] Mix and stir evenly 92.5% of the positive electrode active material, 0.5% of polyacrylic acid dispersant, 3.5% of conductive carbon and 3.5% of polyvinylidene fluoride binder by mass percentage to obtain a positive electrode slurry.

[0077] Step Three. Prepare a sodium-ion battery: coat, roll, slit, die cut, stack, encapsulate, bake, inject electrolyte, perform high-temperature fixture formation, degas and second seal, and capacity grading on the above positive electrode slurry. After obtaining a finished battery cell, test its electrochemical performance. The preparation methods of the electrode and the sodium-ion battery in this embodiment are the same as those in the prior art and will not be elaborated here.

[0078] Example 3

[0079] A sodium-ion battery, which comprises the following preparation steps:

[0080] Step One. Prepare a sodium-ion composite positive electrode active material:

[0081] S11. Disperse sodium iron phosphate by mass parts in an ethanol solution, then add sodium nitrate, soluble transition metal salts and citric acid. The mass ratio of sodium iron phosphate, ethanol solution, sodium nitrate, soluble transition metal salts and citric acid is 1:7:0.5:1.2:0.3. The soluble metal salt is manganese nitrate. Mix and stir all substances at a stirring speed of 300 rpm for 0.5 h. After stirring evenly, a sol is formed to obtain a precursor;

[0082] S12. Under an inert atmosphere, the precursor is gradually heated to 500 °C at a heating rate of 3 °C / min for calcination for 2 h, and finally subjected to ball milling at a ball milling speed of 400 rpm for 1 h to obtain the positive electrode active material;

[0083] The mass of the core of the prepared positive electrode active material accounts for 90 wt% of the total mass of the active material, and the mass of the coating layer accounts for 10 wt% of the total mass of the active material. The coating layer is coated on the surface of the core. The core is sodium iron phosphate, and the coating layer is an O3-phase layered oxide with a thickness of 200 nm.

[0084] Step 2. Prepare the positive electrode slurry:

[0085] By mass percentage, 94.7% of the positive electrode active material, 0.3% of the polyacrylic acid dispersant, 2.5% of the conductive carbon, and 2.5% of the polyvinylidene fluoride binder are mixed and stirred evenly to obtain the positive electrode slurry.

[0086] Step 3. Prepare a sodium-ion battery: The above positive electrode slurry is coated, roll-pressed, slit, die-cut, laminated, encapsulated, baked, injected with electrolyte, subjected to high-temperature fixture formation, degassed and second-sealed, and capacitance-divided to obtain a finished battery cell, and then its electrochemical performance is tested. The preparation methods of the electrode sheets and sodium-ion batteries in this example are the same as those in the prior art and will not be elaborated here.

[0087] Comparative Example 1

[0088] A sodium-ion battery. The difference between Comparative Example 1 and Example 1 is that the positive electrode active material in Comparative Example 1 is sodium iron phosphate, and the other preparation steps in Comparative Example 1 are the same as those in Example 1 and will not be elaborated here.

[0089] Comparative Example 2

[0090] A sodium-ion battery. The difference between Comparative Example 2 and Example 1 is that the positive electrode active material in Comparative Example 2 is an O3-phase layered oxide with the chemical formula Na 0.91 Fe 0.32 Ni 0.23 Mn 0.37 Zn 0.08 O2. The other preparation steps in Comparative Example 2 are the same as those in Example 1 and will not be elaborated here.

[0091] Comparative Example 3

[0092] A sodium-ion battery. The difference between Comparative Example 3 and Example 1 is that the mass of the core of the sodium-ion composite positive electrode active material in Comparative Example 3 accounts for 95% of the total mass of the active material, the mass of the coating layer accounts for 5% of the total mass of the active material, the thickness of the coating layer is 5 nm, and the preparation of the positive electrode slurry in Step 2 and the preparation of the sodium-ion battery in Step 3 are the same as those in Example 1 and will not be elaborated here.

[0093] Comparative Example 4

[0094] A sodium-ion battery, the difference between Comparative Example 4 and Example 1 is that the mass of the core of the sodium-ion composite cathode active material in Comparative Example 4 accounts for 60% of the total mass of the active material, the mass of the coating layer accounts for 40% of the total mass of the active material, the thickness of the coating layer is 300 nm, and the preparation of the cathode slurry in Step 2 and the preparation of the sodium-ion battery in Step 3 are the same as those in Example 1, which will not be elaborated here.

[0095] Comparative Example 5

[0096] A sodium-ion battery, the difference between Comparative Example 5 and Example 1 lies in Step S12. The calcination temperature in Comparative Example 5 is 150 °C, and the other preparation steps of Comparative Example 5 are the same as those in Example 1, which will not be elaborated here.

[0097] Comparative Example 6

[0098] A sodium-ion battery, the difference between Comparative Example 6 and Example 1 lies in Step S12. The calcination temperature in Comparative Example 6 is 600 °C, and the other preparation steps of Comparative Example 6 are the same as those in Example 1, which will not be elaborated here.

[0099] Performance Test:

[0100] (1) Capacity Retention Rate Test

[0101] Test Method: In an environment of 25 °C, charge at a current of 1C to 3.8V, then maintain a voltage of 3.8V until the current drops to 0.05C, stop charging, and then discharge at a constant current of 1C to the cut-off voltage. Record the initial discharge capacity C0, repeat the above charge-discharge steps, record the discharge capacity (Cn) of the 100th cycle, divide C n by C0 and multiply by 100% to obtain the capacity retention rate of the 100th cycle, and record the test results in Table 1.

[0102] Table 1 Test Results of Capacity Retention Rate

[0103] Group Capacity retention rate after 100 cycles Example 1 98.6％ Example 2 98.3％ Example 3 98.0％ Comparative Example 1 71.4％ Comparative Example 2 73.2％ Comparative Example 3 78.6％ Comparative Example 4 80.1％ Comparative Example 5 72.8％ Comparative Example 6 75.1％

[0104] (2) Low-Temperature Discharge Capacity Residual Rate Test

[0105] Test method: In an environment of 25°C, charge at a constant current and constant voltage of 0.5C until 3.6V (cut-off current 0.05C), discharge at a constant current of 0.5C until 1.5V, and record the average discharge capacity C0 of 3 cycles. Then, in an environment of 25°C, charge at a constant current and constant voltage of 0.5C until 3.6V (cut-off current 0.05C), and then stand for 4h at different temperatures (-30°C / -20°C / -10°C / 0°C) and then discharge at a constant current of 0.5C until 1.5V. Record the discharge capacity (C1), divide C1 by C0 and multiply by 100% to obtain the residual rate of the discharge capacity, and record the test results in Table 2.

[0106] Table 2 Test Results Table of Residual Rate of Capacity

[0107]

[0108]

[0109] (3) Low-temperature Full Charge Test

[0110] Test method: In an environment of 25°C, charge at a constant current and constant voltage of 0.5C until 3.6V (cut-off current 0.05C), discharge at a constant current of 0.5C until 1.5V. In an environment of -20°C, stand for 6 hours, then charge at a constant current of 0.5C until 30% SOC, and then charge at a constant current of 1.75C until 3.6V (cut-off current 0.05C). The test results are recorded in Figure 4 in.

[0111] It can be seen from the test results in Table 1 that the battery prepared with the core-shell structure coated composite cathode active material of the present invention has higher capacity retention rate and longer cycle life. By comparing Comparative Example 1, Comparative Example 2 and Example 1, it can be seen that the core-shell coated composite cathode active material formed by the polyanion-type compound core and the transition metal oxide coating layer of the present invention can effectively improve the capacity retention rate and cycle life of the battery, and the two have a synergistic effect; by comparing Comparative Example 3 and Comparative Example 4 with Example 1, it can be seen that controlling the mass ratio of the core material to the coating material and the thickness of the coating layer is beneficial to improving the capacity retention rate and cycle life of the battery. When the proportion of the coating layer is too low or too high, or the coating layer is too thick or too thin, it will affect the capacity retention rate and cycle life of the battery; by comparing Example 1 with Comparative Example 5 and 6, it can be seen that the control of the calcination temperature of the present invention can effectively improve the cycle life of the battery.

[0112] From the test data in Table 2 and Figure 3 and Figure 4It can be seen that the positive electrode active material of the present invention can reduce the charge-discharge polarization of the battery at low temperature, enabling the battery to maintain good charge-discharge performance even under low temperature conditions. From the comparison of the test results of Example 1 with Comparative Example 1 and Comparative Example 2 in Table 2, it can be seen that the core-shell structure formed by using a polyanionic-type core and a layered oxide as the shell has a synergistic effect, and the combination of the two can better reduce the charge-discharge polarization at low temperature, enabling the battery to maintain good charge-discharge performance even under low temperature conditions. From the comparison of Example 1 with Comparative Example 3 and 4 in Table 2, it can be seen that the mass ratio of the core material to the coating material and the coating thickness are beneficial to better reducing the charge-discharge polarization at low temperature; from the comparison of Example 1 with Comparative Example 5 and 6, it can be seen that the preparation method of the present invention can improve the charge-discharge performance of the battery in a low temperature environment by controlling the calcination temperature.

[0113] Figure 1 are the charge-discharge curves of the batteries prepared in the examples and comparative examples of the present invention. It can be seen from Figure 1 that the charge-discharge curve of the battery prepared in the example of the present invention shows a more stable voltage plateau during the entire charge-discharge process. Especially in the high SOC region (90%-100%), the voltage remains at a relatively high level, indicating that it has better energy output ability and better voltage stability. The charge-discharge curve of the battery prepared in the comparative example shows an obvious downward trend in voltage in the high SOC region, indicating that its performance is less stable than that of the battery prepared in the example of the present invention under high state of charge. In the low SOC region (0%-20%), the voltage difference between the two is small, but as the SOC increases, the advantages of the battery prepared in the example of the present invention gradually emerge. That is to say, the battery prepared in the example of the present invention has better voltage stability and energy output ability in the high SOC region. This is because the present invention uses a core-shell structure-coated composite positive electrode active material to prepare a sodium ion battery, and the active material structure and preparation method of the present invention are beneficial to improving the overall performance of the battery.

[0114] Figure 2 are the cycle life curves of the batteries prepared in the examples and comparative examples of the present invention. It can be seen from Figure 2It can be seen that for the battery prepared in the embodiment of the present invention, at the beginning, the capacity retention rate of the battery is 100%, and the battery is in the best state. As the number of cycles increases, the capacity retention rate gradually decreases, but the decreasing trend is relatively gentle. When the number of cycles reaches 200, the capacity retention rate is still close to 98%, indicating that the battery prepared in the embodiment of the present invention has good cycle stability. For the battery prepared in the comparative example, the initial capacity retention rate is also 100%, but in the earlier cycle stage (about 50 cycles), the capacity retention rate rapidly drops to about 90%. Subsequently, the capacity retention rate continues to drop rapidly and drops to about 80% at 90 cycles. Therefore, the battery prepared in the embodiment of the present invention has better cycle stability and capacity retention ability. Even after multiple charge-discharge cycles, its capacity loss is relatively small. The battery prepared in the embodiment of the present invention has significantly better cycle performance at a voltage of 3.8V than the comparative example, with a higher capacity retention rate and a longer cycle life. This is because the present invention uses a core-shell structure coated composite cathode active material to prepare a sodium ion battery, and the active material structure and preparation method of the present invention are beneficial to improving the overall performance of the battery.

[0115] The above embodiments only represent several implementation manners of the present invention. The description thereof is relatively specific and detailed, but it should not be construed as a limitation on the scope of the patent of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the appended claims.

Claims

1. A sodium-ion composite cathode active material, characterized in that, It includes a core and a coating layer coated on the surface of the core. The core is a polyanionic compound, and the coating layer is a transition metal oxide.

2. The sodium ion composite cathode active material according to claim 1, wherein The mass percentage of the core and the coating layer is 70%-90%: 10%-30%.

3. The sodium ion composite cathode active material according to claim 1, wherein The thickness of the coating layer is 10-200 nm.

4. The sodium ion composite cathode active material according to claim 1, wherein, The transition metal oxide is at least one of O3-phase layered oxide, P2-phase layered oxide, and tunnel oxide.

5. The preparation method of a sodium-ion composite cathode active material according to any one of claims 1-4, characterized in that, It includes the following preparation steps: Step 1: Disperse the polyanionic compound in a solvent, then add sodium salt, soluble transition metal salt, and citric acid, and stir evenly to form a sol to obtain a precursor. Step 2: Under an inert atmosphere, calcine the precursor at a calcination temperature of 200-500 °C for 2-8 h, and finally ball mill to obtain the positive electrode active material.

6. The preparation method of a sodium-ion composite cathode active material according to claim 5, characterized in that, In Step 2, the temperature is gradually increased to the calcination temperature at a heating rate of 1-3 °C / min.

7. A positive electrode paste, characterized in that, It includes a positive electrode active material, which is the positive electrode active material described in any one of Claims 1-4 or the positive electrode active material prepared by the preparation method described in any one of Claims 5-6.

8. A positive electrode paste according to claim 7, wherein It includes components with the following mass percentages: 92.5%-94.7% of positive electrode active material, 0.3%-0.5% of dispersant, 2.5%-3.5% of conductive carbon, and 2.5%-3.5% of binder.

9. A positive electrode sheet, characterized in that, It includes the positive electrode paste described in Claim 7 or 8.

10. A sodium-ion battery, characterized in that, It includes the positive electrode sheet described in Claim 9.

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