Sodium-ion battery positive electrode material, positive electrode plate and sodium-ion battery

Abstract

The invention relates to a sodium-ion battery positive electrode material, a positive electrode plate and a sodium-ion battery, the positive electrode material combines a main body material and a blending material, optimizes related technical characteristics, improves the cycle stability of a polyanion material under high voltage, prolongs the service life of a battery cell, and meets the requirements of long-cycle application scenarios such as start-stop of the battery and energy storage. The average discharge or discharge platform voltage of the mixing material is greater than the average discharge or discharge platform voltage of the main body material, and the difference value is controlled to be greater than or equal to 0.1 V. Furthermore, a carbon coating layer is arranged on the surface of the main body material, the electron conduction capability is improved, the structural integrity is kept, the complementary advantage of electrochemical performance is fully considered in the selection of the mixing material, and the electrochemical performance is improved. The positive electrode material with excellent comprehensive performance is formed. In addition, key parameters such as median particle sizes of the main body material and the blending material, cut-off voltage of the main body material, compaction density of the positive electrode material and the like are regulated and controlled, and the electrochemical performance is optimized.

Description

Technical Field

[0001] The present application relates to the technical field of sodium-ion batteries, and in particular, to a cathode material, a cathode electrode sheet and a sodium-ion battery for sodium-ion batteries. Background Art

[0002] In the field of start-stop power supply applications, starting lead-acid batteries have long dominated the market. However, lead-acid batteries are not an ideal choice for future starting batteries. They contain sulfuric acid and some heavy metal substances, are corrosive, and cause serious environmental pollution. The development prospects of lead-acid batteries have faced an end. In addition, lead-acid batteries also have problems such as being bulky and having a low energy density. Especially in extremely cold conditions, their capacity and voltage will decrease significantly, making it difficult to meet the more frequent starting requirements of start-stop functions and the requirements of powering on-vehicle electrical appliances at idle speed. At the same time, the feedback charging ability of lead-acid batteries is weak, and they cannot replenish electricity in time, easily leading to the failure of the automatic start-stop function or even the inability to start. Although lithium-ion batteries, as another option, are also used in the field of start-stop power supplies, they also face performance challenges in low-temperature environments.

[0003] Compared with lead-acid batteries and lithium-ion batteries, sodium-ion batteries provide new possibilities for solving the problems in start-stop power supply applications with their excellent low-temperature characteristics and high-rate characteristics. In the composition of the battery cell, the material is the foundation, and the technical routes of sodium-ion batteries are mainly distinguished by the cathode material. Currently, the cathode materials of sodium-ion batteries are mainly divided into three technical routes: Prussian blue / white, polyanion, and layered oxide. Although sodium-ion batteries show great potential in the field of start-stop power supplies, the existing technology still has the problem of insufficient cycle stability, and it is necessary to develop materials with higher cycle performance to meet the requirements of long-life application scenarios such as start-stop batteries. Therefore, in the research and development of sodium-ion battery cathode materials, cathode electrode sheets and sodium-ion batteries, how to improve the cycle stability has become a key problem to be solved urgently. Summary of the Invention

[0004] The purpose of the present application is to provide a cathode electrode sheet and a sodium-ion battery using this electrode sheet to improve the cycle stability of polyanion materials under high voltage and increase the service life of the battery cell to meet the requirements of long-cycle application scenarios such as start-stop batteries and energy storage. The cathode material of the sodium-ion battery of the present application includes a main material and a blending material; The main material and the blending material are Na a M b (X c O d ) e Z f, where M is selected from one or more of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Ca, Mg, Al, and Nb, X is selected from one or more of C, Se, Si, S, P, As, B, Mo, W, and Ge, Z is selected from one or more of O, F, and OH, the values of a, b, c, d, and e are greater than 0, and the value of f is greater than or equal to 0.

[0005] Among them, the average discharge voltage of the blending material is greater than that of the host material, and the difference between the average discharge voltages of the blending material and the host material is ≥0.1V.

[0006] In one embodiment, the surface of the host material includes a carbon coating layer.

[0007] In one embodiment, the blending material includes one or more materials.

[0008] In one embodiment, the mass of the blending material is in the range of 1%-50% of the mass of the host material.

[0009] In one embodiment, the mass of the blending material is in the range of 5%-40% of the total mass of the host material and the blending material.

[0010] In one embodiment, the median particle size of the host material is in the range of 0.1-15μm.

[0011] In addition, the present application further provides a positive electrode sheet for a sodium-ion battery, which includes the aforementioned positive electrode material for a sodium-ion battery.

[0012] In one embodiment, the tap density of the positive electrode material for a sodium-ion battery is in the range of 1.45-2.65 g / cm 3 range.

[0013] The present application further provides a sodium-ion battery, which includes the aforementioned positive electrode sheet for a sodium-ion battery.

[0014] Compared with the prior art, the present application has the following beneficial effects: By selecting sodium iron pyrophosphate phosphate, etc. as the host material and blending a blending material with a higher cut-off voltage (such as sodium vanadium phosphate, sodium fluorovanadate phosphate, etc.), the positive electrode material can still maintain stable electrochemical performance under high-voltage conditions. The difference in the cut-off voltage between the blending material and the host material is controlled within the range of 0.1-1.6V, effectively balancing the energy output and cycle stability of the battery cell at high voltage, improving the service life of the battery cell, and making the sodium-ion battery of the present application particularly suitable for long-cycle application scenarios such as start-stop batteries and energy storage, meeting the high requirements for battery cycle stability and service life in these fields.

[0015] By coating the surface of the host material with a carbon layer, not only the conductivity of the material is improved, but also the structural stability of the material is enhanced, further improving the cycle stability of the battery cell under high voltage. At the same time, the selection and proportion control of the blending material (the mass ratio is in the range of 1% - 50%) optimize the overall performance of the cathode material, ensuring the health state of the battery to the greatest extent. During the long-term use of the sodium-ion battery of this application, it can maintain a high capacity retention rate and a small capacity attenuation rate.

[0016] In this application, the median particle size of the host material is controlled within the range of 0.1 - 15 μm, which not only ensures the active surface area of the material but also facilitates the close packing of the battery cell, thereby improving the energy density of the battery cell. By precisely controlling the composition and proportion of the blending material and the host material, as well as the tap density of the cathode material (in the range of 1.45 - 2.65 g / cm³), while maintaining a high energy density, the battery cell also has good safety and stability. In applications such as start-stop power supplies, the sodium-ion battery of this application can provide a longer cruising range and more reliable performance. Through the design of the cathode material and the optimization of technical features, this application improves the cycle stability, battery cell health, and energy density of the sodium-ion battery under high voltage, thereby extending the service life of the battery and meeting the urgent needs of long-cycle application scenarios such as start-stop batteries and energy storage. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 is the SEM image of the cathode material of the sodium-ion battery in the first embodiment of this application; Figure 2 is the SEM image of the cathode material of the sodium-ion battery in the second embodiment of this application; Figure 3 is the SEM image of the cathode material of the sodium-ion battery in the third embodiment of this application; Figure 4 is the charge-discharge curve of the full battery of the sodium-ion battery using the cathode material in one embodiment of this application; Figure 5 is the charge-discharge curve of the full battery of the sodium-ion battery using the cathode material in one comparative example of this application; Figure 6 is the cycle capacity retention rate of the sodium-ion battery using the cathode material in one embodiment of this application. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0018] To make the above objects, features, and advantages of the present application more apparent and understandable, the following will provide a detailed description of the specific embodiments of the present application in conjunction with the accompanying drawings. It can be understood that the specific embodiments described herein are only used to explain the present application, rather than limiting the present application. Additionally, it should be noted that for the convenience of description, only the parts related to the present application rather than all the structures are shown in the drawings. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present application.

[0019] The terms "comprising" and "having" in the present application and any variations thereof are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or device that includes a series of steps or units is not limited to the listed steps or units, but optionally further includes steps or units not listed, or optionally further includes other steps or units inherent to these processes, methods, products, or devices.

[0020] Referring to "embodiments" in this context means that the specific features, structures, or characteristics described in connection with the embodiments can be included in at least one embodiment of the present application. The phrase appears in various places in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment mutually exclusive with other embodiments. Those skilled in the art explicitly and implicitly understand that the embodiments described herein can be combined with other embodiments.

[0021] As a strong competitor to lithium-ion batteries, sodium-ion batteries show broad application prospects due to their rich sodium resource reserves, low cost, and similar working principles. The cathode material, as the core component of sodium-ion batteries, directly affects the overall performance of the battery. Currently, the cathode materials for sodium-ion batteries mainly fall into three technical routes: Prussian blue / white, polyanion, and layered oxide. Among them, polyanion materials have attracted much attention due to their high safety and good cycle stability. This application will focus on introducing a cathode material for sodium-ion batteries, the cathode electrode sheet prepared therefrom, and sodium-ion batteries. Through the combination of a host material and a blending material, as well as the optimization of specific technical features, this cathode material aims to improve the cycle stability of polyanion materials under high voltage and extend the service life of the battery cells to meet the requirements of long-cycle application scenarios such as start-stop batteries and energy storage. Next, the composition, structure, performance, and its performance in practical applications of this cathode material will be elaborated in detail. The cathode material for sodium-ion batteries in this application includes a host material and a blending material, and the chemical formulas of the host material and the blending material are Na a M b (X c O d ) e Z f, wherein M is selected from one or more of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Ca, Mg, Al, and Nb, X is selected from one or more of C, Se, Si, S, P, As, B, Mo, W, and Ge, Z is selected from one or more of O, F, and OH, the values of a, b, c, d, and e are greater than 0, the value of f is greater than or equal to 0, and when the value of f is 0, it is equivalent to F being blank. In a specific embodiment, the specific constituent materials of the host material are preferably sodium iron pyrophosphate phosphate and / or sodium iron fluoropyrophosphate phosphate, and the specific constituent materials of the blending material are preferably selected from one or more of sodium iron pyrophosphate phosphate, sodium vanadium phosphate, sodium fluorovanadium phosphate, sodium manganese vanadium phosphate, sodium iron sulfate, sodium manganese iron pyrophosphate phosphate, sodium cobalt pyrophosphate phosphate, sodium copper iron manganese, sodium nickel manganese, sodium iron manganese, sodium nickel iron manganese, and fluorinated derivatives of the above materials.

[0022] The cut-off voltage of the blending material is greater than that of the host material, and the difference in the cut-off voltages of the blending material and the host material is ≥0.1 V. The positive electrode material of the sodium-ion battery of the present application includes a host material and a blending material, and the electrochemical performance of the sodium-ion battery is improved through the synergistic effect between the materials. In a specific embodiment, the host material can be sodium iron pyrophosphate phosphate (Na a Fe b (PO4) c (P2O7) d ) or its fluorinated derivative sodium iron fluoropyrophosphate phosphate. Such materials can provide a stable sodium ion insertion / extraction path during charge and discharge due to their high energy density, good thermal stability, and structural stability, thereby ensuring the long cycle life and high safety of the battery. In addition, the present application introduces a blending material, the chemical formula composition of which is similar to that of the host material, and the chemical formulas of both the host material and the blending material can be represented by the following chemical formula: Na a M b (X c O d ) e Z f . In a specific embodiment, the constituent materials of the blending material can be selected from sodium iron pyrophosphate phosphate (Na a Fe b (PO4) c (P2O7) d , abbreviated as NFPP), sodium vanadium phosphate (Na3V2(PO4)3, abbreviated as NVP), sodium fluorovanadium phosphate (Na a V b (PO4) c F, abbreviated as NVPF), sodium fluorovanadium phosphate (Na a V b (PO4) cF, abbreviated as NVPF), sodium vanadium manganese phosphate (Na a V b Mn c (PO4) d , abbreviated as NVMP), sodium vanadium oxygen phosphate fluoride (Na a V b (PO4) c O d F, abbreviated as NVPOF), sodium iron sulfate (Na x Fe y SO4, abbreviated as NFS), sodium iron manganese pyrophosphate phosphate (Na a Fe b Mn c (PO4) c (P2O7) d , abbreviated as NMFPP), sodium cobalt pyrophosphate phosphate (Na a Co b (PO4) c (P2O7) d , abbreviated as NCPP), sodium manganese chromium phosphate (Na4MnCr(PO4)3, abbreviated as NMCP), sodium copper iron manganese (Na x Cu y Fe x / 3 Mn 2 / 3x-y O z , abbreviated as NCFMO), sodium nickel manganese, sodium iron manganese, sodium nickel iron manganese (Na x Ni x / 3Fe x / 3 Mn x / 3 O 2x, abbreviated as NNFMO), and its fluorinated derivatives, etc. These blending materials need to have a relatively high average discharge or discharge platform voltage, which is complementary to the host material. In this application, the average discharge or discharge platform voltage of the blending material is designed to be greater than that of the host material, and the voltage difference between the two is greater than or equal to 0.1V. In a further preferred technical solution, the voltage difference between the two is controlled within the range of 0.2 - 1.0V. By blending materials with a high average voltage, the overall voltage window of the cathode material can be effectively broadened, thereby improving the energy density of the sodium-ion battery. The synergistic effect between the blending material and the host material during charge and discharge helps to alleviate the volume change of the host material and reduce structural damage, thereby extending the cycle life of the battery. At the end of the constant voltage charging stage during the charging process of the battery cell, the voltage is maintained at the cut-off voltage for a period of time. The doping material can continue to provide Na extraction sites at a high potential, ensuring the structural integrity of the material and improving the rate performance of the battery, enabling the battery to charge and discharge quickly within a short time. By selecting the blending material, the thermal stability and chemical stability of the cathode material can be improved, and the safety risk of the battery under extreme conditions such as overcharging and over-discharging can be reduced. The SEM image of the formed composite material can be seen in Figures 1-3 , and it can be seen from the SEM image that the cathode material particles of this application are uniform. In addition, from the charge-discharge curve of the full battery ( Figure 4 , Figure 5 ), as well as the cycle capacity retention rate ( Figure 6 ), it can be seen that the cathode material of this application has extended the cycle life.

[0023] Specifically, the surface of the host material includes a carbon coating layer. A carbon coating layer is provided on the surface of the host material of the positive electrode material of the sodium-ion battery of the present application. In an electrochemical system, good electrical conductivity is the key to ensuring efficient charge transfer. By coating a layer of carbon on the surface of the host material, the electron conduction ability of the positive electrode material can be improved, thereby accelerating the electron transfer rate during charge and discharge, and improving the rate performance of the battery. Secondly, the carbon coating layer also plays a role in structural stabilization. During the charge and discharge cycle of the battery, the positive electrode material will experience repeated processes of ion insertion and extraction, which is often accompanied by changes in the volume of the material. If this volume change is not controlled, it may lead to the destruction of the initial material structure, thereby affecting the cycle life of the battery. The carbon coating layer can, to a certain extent, limit the relative movement of other materials in the host material, thereby alleviating the volume change of the host material during charge and discharge, maintaining the integrity of its structure, and thus improving the cycle stability of the battery. In addition, the carbon coating layer also has certain chemical stability. In the harsh working environment of the battery, such as high temperature, high pressure, or corrosive media, the positive electrode material is easily eroded, resulting in performance degradation. Due to the stable chemical properties of carbon materials, they can effectively protect the host material from these adverse factors, ensuring the performance stability of the battery during long-term use. Additionally, through experiments, it was found that for the composition of the host material and the blending material of the present application, it is only necessary to form a carbon coating layer on the surface of the host material, and there is no need to form a carbon coating layer on the surface of the blending material. Especially when the mass of the blending material is within the range of 10-25% of the total mass of the host material and the blending material, whether a carbon coating layer is formed on the blending material has no obvious change in the final performance of the battery positive electrode material.

[0024] Specifically, the blending material includes one or more materials. When including one material, the preparation method is relatively simple and has a high manufacturing efficiency. When choosing a blending system containing two or more materials, the complementary advantages of different materials in electrochemical performance can be fully utilized. For example, some materials may have a high average discharge voltage and can provide a higher energy density; while other materials may have excellent rate performance or cycle stability, which can ensure the performance of the battery during rapid charge and discharge or long-term cyclic use. By mixing these materials in an appropriate proportion, a positive electrode material with more excellent comprehensive performance can be formed.

[0025] The blending of multiple materials can also alleviate the possible performance bottlenecks of single materials to a certain extent. In the specific implementation, there are also differences in the average discharge or discharge platform voltage among multiple blending materials, and the voltage difference between each blending material is also greater than 0.1V. By further optimizing the blending materials, the stability can be improved. For example, some materials may experience large volume changes during charge and discharge, resulting in structural damage and performance degradation. Introducing other materials with better structural stability as blending components can effectively alleviate this volume change, thereby extending the service life of the battery. In addition, the blending of multiple materials can also provide more flexibility for the design of cathode materials. By adjusting the types and proportions of different materials, the performance of cathode materials can be customized and optimized for specific application scenarios and requirements. For example, for applications requiring high energy density, materials with a higher average discharge voltage can be selected as the main blending components, while for applications requiring long cycle life, more attention can be paid to the cycle stability of the blending materials.

[0026] Specifically, the mass of the blending material is in the range of 1% - 50% of the mass of the main material. Setting the mass ratio of the blending material in the range of 1% to 50% can ensure that the blending material plays an effective modification role in the cathode material. Within this ratio range, the blending material can be evenly dispersed in the main material, forming effective interfacial interactions, thereby improving the electrochemical performance of the cathode material, such as increasing the energy density, optimizing the rate performance, or enhancing the cycle stability. In a further preferred embodiment, the mass of the blending material is in the range of 5% - 40% of the total mass of the main material and the blending material, and the material performance improvement amplitude is greater within this range. Controlling the mass ratio of the blending material within the above range can not only ensure performance improvement but also maintain reasonable cost-effectiveness. In addition, the selection of this ratio range also provides more possibilities for the customized design of cathode materials. By adjusting the types and proportions of the blending materials, the performance of cathode materials can be optimized for different application scenarios and requirements.

[0027] Specifically, the ratio of the median particle size (D50) of the main material to that of the blending material is in the range of 0.03 - 30. By adjusting the ratio of the median particle sizes of the two materials, it can be ensured that during the mixing process, the blending material can be more effectively dispersed in the main material, reducing the agglomeration phenomenon and improving the uniformity of the overall mixture. When the ratio is relatively low (close to 0.03), the blending material, as fine particles, is easy to penetrate and fill the voids between the main material particles; while when the ratio is relatively high (close to 30), although the blending material particles are relatively large, an appropriate amount of large-sized particles can promote the fluidity and dispersibility of the main material to a certain extent. As long as the mixing process is appropriate, a good mixing effect can still be achieved. The combination of materials with different particle sizes can affect the physical properties of the final product, such as density, hardness, wear resistance, etc. By controlling this ratio, the performance of the product can be optimized specifically to meet the requirements of specific application scenarios. An appropriate particle size ratio helps to reduce energy consumption and wear during the processing.

[0028] Specifically, the median particle size of the main material is in the range of 0.1 - 15 μm. By adjusting the median particle size of the main material, the microscopic structure and phase distribution of the material can be finely controlled, thereby affecting its macroscopic properties. For example, a main material with a smaller particle size can provide more interfacial areas, promoting the interaction with the blending material and enhancing the interfacial bonding force; while a main material with a larger particle size may form a unique network structure, improving the overall stability and crack resistance of the material. The main material with a median particle size in this range usually has better dispersibility and fluidity. A smaller particle size helps to reduce the agglomeration phenomenon, improving the uniformity and processing accuracy of the material, while a larger particle size may reduce the viscosity of the material.

[0029] Specifically, the cut-off voltage of the main material is in the range of 3.25 - 4.45 V. The cut-off voltage of the main material directly determines the energy output capacity of the battery system. Within the set voltage range, the main material can efficiently embed and extract sodium ions, thereby maximizing the energy density of the battery while ensuring safety. By controlling the cut-off voltage of the main material, overcharging or over-discharging phenomena can be effectively avoided, reducing the damage to the material structure, thereby enhancing the cycle stability of the battery. Operating within an appropriate voltage window can ensure the integrity of the structure of the main material during charge and discharge, reducing the loss of active substances and extending the service life of the battery. In a battery system, too high a voltage may lead to safety hazards such as thermal runaway and electrolyte decomposition. Setting the cut-off voltage of the main material in the range of 3.25 to 4.45 V can effectively avoid the occurrence of these extreme situations and ensure the safety of the battery under normal operating conditions.

[0030] In addition, the present application also provides a positive electrode sheet for a sodium-ion battery, which includes the aforementioned positive electrode material for a sodium-ion battery. Due to the use of the optimized positive electrode material for a sodium-ion battery, during the charge and discharge process, this electrode sheet can exhibit higher energy density, more stable cycle performance, and more excellent rate performance, directly improving the overall performance of the sodium-ion battery and enabling it to meet the application requirements of high energy density and long cycle life in electric vehicles, energy storage systems, etc. The selection and optimization of the positive electrode material are crucial for the safety and reliability of the battery. In the positive electrode sheet of the present application, by using a positive electrode material with excellent thermal stability and chemical stability, the reaction activity of the battery under abnormal conditions such as overcharge, over-discharge, and short circuit is effectively reduced, thereby improving the safety and reliability of the battery.

[0031] Specifically, the tap density of the positive electrode material for a sodium-ion battery is in the range of 1.45 - 2.65 g / cm 3 range. Tap density is one of the important indicators for measuring the filling efficiency of battery materials. Within the above range, the positive electrode material can maintain a relatively high filling density, thereby effectively improving the energy density of the sodium-ion battery. Under the same volume, the battery can store more energy, providing longer battery life and higher energy output for applications such as electric vehicles and energy storage systems. Reasonable control of the tap density is crucial for the cycle stability of the positive electrode material. Excessively high tap density may lead to an increase in internal stress of the material, affecting the structural stability of the material; while too low tap density may reduce the conductivity and ion transport efficiency of the material. Within the above range, the positive electrode material can maintain an appropriate microstructure and porosity, which is conducive to the insertion and extraction of lithium ions, reducing the structural damage of the material, thereby improving the cycle stability of the battery. At an appropriate tap density, the positive electrode material can maintain stable thermal and chemical properties, reducing the reaction activity of the battery under abnormal conditions.

[0032] The present application further provides a sodium-ion battery, which includes the aforementioned positive electrode sheet for a sodium-ion battery. Due to the use of a high-performance positive electrode sheet for a sodium-ion battery, while maintaining a compact structure, this battery can store more energy, especially meeting the requirements of long cycle application scenarios such as start-stop batteries and energy storage. Specific embodiments The following will further introduce some specific implementation manners to further elaborate on the technical solutions of the present application and compare the specific materials and corresponding test results in each embodiment. The test items include the cycle capacity retention rate after 600 cycles and the cell capacity.

[0034] In the present application, the specific test method for voltage: The positive electrode formulation is based on Na a M b (X c O d ) e Z f: Prepare a slurry by mixing PVDF:CNT:SP at a ratio of 95:3:1:1, and set the coating loading to 15 mg / cm 2 , and use a sodium metal sheet for the negative electrode. At 25 °C and 0.1 C, set the charge-discharge cut-off voltage to 1.5 - 4.0 V to obtain the discharge voltage-capacity curve. The calculation formula for the average voltage Ü is Ü = ∫U(t)*I(t)dt / ∫I(t)dt.

[0035] The plateau voltage refers to the voltage value corresponding to the smallest voltage change and the largest capacity change. Calculation of the plateau voltage: During the discharge test, set the current to 0.2 C and the acquisition frequency to 5 s, then differentiate the discharge curve to obtain the dQ / dV vs. V curve, and determine the plateau voltage through the dQ / dV peak value. If there are multiple dQ / dV peaks in the positive electrode material, calculate the plateau voltage based on the highest voltage, and the upper limit of the plateau voltage does not exceed 4.0 V.

[0036] Cycling test conditions: Under normal temperature of 25 °C, perform the cycling life test on the cylindrical battery cell according to the following steps: 1) Constant current charge at 1 C to 3.85 V, then constant voltage charge to 0.05 C; 2) Stand for 10 min; 3) Constant current discharge at 1 C to 1.5 V; 4) Stand for 10 min. Record the capacity retention rate during cycling.

[0037] Preparation of blended materials, electrode sheets, and battery cells: 1. Material premixing (taking the main material: blended material = 70:30 as an example): Put the main material NFPP and the blended material NFS into the mixing tank according to the mass ratio, with a rotation speed of 30 rpm, and premix for 30 minutes.

[0038] 2. Preparation of the initial positive electrode sheet: Mix the premixed materials, binder PVDF (Solvay Solef 5130), conductive agent SP (Super P), and single-walled carbon nanotube SWCNT. Mix them evenly with the solvent N-methylpyrrolidone according to the mass ratio of 95:3:1:1 to form a slurry, then coat the slurry on both sides of the carbon-coated aluminum foil, and after drying, the single-sided coating amount of the positive electrode is 15 mg / cm 2 , and the water content of the electrode sheet is ≤ 100 ppm; then, after cold pressing and die cutting, it is ready for assembling the battery cell. Compaction density control method: Pass the electrode sheet through the roller, and control the compaction density of the electrode sheet by the roller pressure.

[0039] 3. Preparation of the initial negative electrode sheet a. Hard carbon electrode: The negative active material HC, binder SBR (Trinseo Voltabond 029), conductive agent SP (Super P), and sodium carboxymethyl cellulose CMC are mixed evenly with deionized water according to a mass ratio of 95:1.5:1.5:2 to form a slurry. Then, the slurry is coated on both the front and back sides of the aluminum foil. After drying, the coating amount on each side of the negative electrode is 7 mg / cm 2 , and the water content of the electrode is ≤100 ppm. Then, it is cold-pressed to obtain the initial electrode.

[0040] b. Carbon-coated aluminum foil electrode: The carbon-coated aluminum foil is dried until the water content is ≤100 ppm to obtain the initial electrode.

[0041] 4. Preparation of sodium-ion battery: 4.1 Preparation of sodium-ion battery: After drying the negative electrode, it is die-cut and assembled with the positive electrode and polypropylene separator mentioned above to form a battery cell. Then, the bare battery cell is dried and put into the shell and filled with electrolyte to obtain a sodium-ion battery (the electrolyte is a commercial product, and the same electrolyte is used in all examples and comparative examples).

[0042] In the present application, the blending process occurs before the electrode coating process. Without changing the original battery cell preparation process, the process is simple, the operation is convenient, and the cost is low, having extremely high practical value.

[0043] Table 1 Test results of the influence of materials on the average voltage in the examples It can be seen from the above specific examples and test results that when there is no potential difference between the positive electrode materials of Material 1 and Material 2 under the same cut-off voltage condition, when the cycle cut-off voltage is set at 1.5 - 3.85 V, due to the poor lattice stability of sodium iron pyrophosphate at high voltage, the cycle life is relatively low. When doping with other materials, such as in Examples 1 to 8, the capacity at high voltage is provided by other materials, thus stabilizing the lattice structure of sodium iron pyrophosphate and extending the cycle life. The voltage difference between the host material and the blending material needs to be discussed by classification: On the one hand, in principle, the higher the average voltage difference, the more beneficial it is to improve the stability of the material with a low average voltage. However, considering the reality and application significance, limited by the current technological development, under the same cut-off voltage condition of different positive electrode materials, the maximum achievable voltage difference is 1.6 V currently. On the other hand, when the voltage difference is too high, limited by the upper limit voltage of 3.85 V of the application scenario, the capacity of the blending material is restricted, and the capacity of the battery cell decreases, such as in Example 6. If the charging cut-off voltage is increased to above 4.3 V, side reactions will occur in the electrolyte at high voltage, reducing the performance of the battery cell. Therefore, it is preferred that the voltage difference is in the range of 0.2 - 1.0 V, and the balance between the capacity and cycle performance of the battery cell can be achieved.

[0044] Table 2 Test Results of the Influence of Material Blending Ratio in Examples It can be seen from Examples 9 - 14 and Comparative Examples 1 - 3 that the blending ratio is an important influencing factor for the capacity and life of the battery cell. Materials such as NVP, NVPF, or NVMP can increase the capacity, but they are not cost-effective. Considering the industrialization process, NFS is a material that takes into account both the high discharge platform voltage, industrial scale, and cost-effectiveness. Therefore, NFS is mainly selected as the blending material in the examples. In Comparative Example 1, due to the insufficient doping ratio of Material 2, the crystal lattice of the material is unstable at a high cut-off voltage of 3.85V. In Comparative Examples 2 and 3, due to the excessive blending ratio, although the cycle retention performance is increased by 15% because the proportion of the doping material is relatively large, it results in a loss of nearly 30% of the battery cell capacity, which cannot meet the actual application requirements. In Examples 9 - 14, the doping range of NFS is 1% - 50%, which significantly improves the cycle capacity retention rate. The preferred blending ratio range is about 5 - 40%, the loss of battery cell capacity is ≤12%, and the improvement of the cycle retention rate is close to 10%, achieving the best balance between cycle and capacity performance.

[0045] It can be seen from Example 15 that different types of materials can be compounded for Material 2. Due to the differences in the morphologies of different materials, a higher compaction density can be achieved, and thus a higher battery cell capacity can be obtained. The specific structure can be seen in Table 3.

[0046] Table 3 Test Results of Different Doping Materials in Examples From the perspective of processing performance, there is an upper limit to the compaction density of the electrode sheet. In Comparative Examples 4 - 5, although the rolling pressure is increased, the compaction density does not increase. In Examples 16 - 19, it is in a reasonable range, and the preferred compaction density is between 1.45 - 2.65 g / cm 3 , and the influence of specific materials and compaction density can be seen in Table 4.

[0047] Table 4 Test Results under the Rolling Pressure of Different Examples In addition, during the further optimization of the materials, it was found that the particle size could determine the charge transfer interface impedance, and the composite particle size between different materials had a significant impact on the cycle capacity retention rate. The present application further studied the influence of the particle size in the technical solution of the present application. In Comparative Example 7, the particle size of Material 1 was much larger than that of Material 2, and Material 2 was dispersed around Material 1. However, due to the too small particle size of Material 2, it could not be well embedded with sodium ions at high potentials, thus weakening the cycle performance. In Comparative Example 8, due to the too large particle size of Material 2, it was in an uneven state at high potentials, which was not conducive to fully exerting the stability of NFS at high voltages, so the cycle performance was weak. Examples 20 to 24 had better cycle performance in terms of particle size. Therefore, when the particle size ratio was within the range of 0.3 to 15, the sodium intercalation capacity of NFS at high voltages could be fully exerted. For the test results of specific particle size ratios, see Table 5.

[0048] Table 5 Test Results at Different Particle Size Ratios of Examples As can be seen from the foregoing, the present application proposes a cathode material for a sodium-ion battery. Through the combination of the host material and the blended material, the cycle stability of the polyanion material at high voltages is improved, and the service life of the battery cell is extended to meet the requirements of long-cycle application scenarios such as start-stop batteries and energy storage. By forming a cathode material with a host material and a blended material, the specific chemical formula is Na a M b (X c O d ) e Z f . The host material is preferably sodium pyrophosphate iron phosphate (Na4Fe3(PO4)2P2O7) or its fluorinated derivative sodium fluorophosphate pyrophosphate iron phosphate. Such materials have high energy density, good thermal stability and structural stability, and can provide a stable sodium ion insertion / extraction path for the battery, ensuring the long cycle life and high safety of the battery. The blended material is preferably selected from a variety of materials including sodium pyrophosphate iron phosphate, sodium vanadium phosphate, sodium fluorophosphate vanadate, sodium manganese vanadium phosphate, sodium iron sulfate, sodium manganese iron pyrophosphate phosphate, etc. These materials have a relatively high working average voltage, excellent rate performance or good cycle stability, and can form a complement with the host material. The cut-off voltage of the blended material is designed to be greater than the cut-off voltage of the host material, and the difference between the two is controlled to be at least greater than 0.1 V to widen the overall voltage window of the cathode material and improve the energy density of the battery.

[0049] In addition, a carbon coating layer is provided on the surface of the host material to enhance the electron conduction ability of the material, accelerate the electron transfer rate during charge and discharge, relieve the volume change of the material during charge and discharge, maintain the structural integrity, and improve the cycle stability and rate performance of the battery. The selection of the blended materials fully considers the complementary advantages of different materials in terms of electrochemical performance. Through a blending system containing two or more materials, a cathode material with more excellent comprehensive performance can be formed.

[0050] In the preparation process, the present application regulates key parameters such as the median particle size of the host material and the blended materials, the cut-off voltage of the host material, and the tap density of the cathode material. By adjusting these parameters, it can be ensured that the blended materials play an effective modification role in the cathode material, improve the uniformity of the overall mixture, and optimize the electrochemical performance of the cathode material, such as increasing the energy density, optimizing the rate performance, or enhancing the cycle stability.

[0051] The present application also provides a sodium-ion battery cathode electrode sheet and a sodium-ion battery comprising the above-mentioned cathode material. Due to the use of the optimized cathode material, the electrode sheet and the battery can exhibit higher energy density, more stable cycle performance, and more excellent rate performance during charge and discharge, directly improving the overall performance of the sodium-ion battery and enabling it to meet the application requirements of high energy density and long cycle life in electric vehicles, energy storage systems, etc. At the same time, by using a cathode material with excellent thermal stability and chemical stability, the reaction activity of the battery under abnormal conditions is effectively reduced, and the safety and reliability of the battery are improved.

[0052] The above is only a specific embodiment of the present application, and any improvement made on the premise of the concept of the present application is regarded as the protection scope of the present application.

Claims

1. A cathode material for a sodium-ion battery, characterized in that, It includes a main material and a blending material; The chemical formulas of the main material and the blending material are Na a M b (X c O d ) e Z f , where M is selected from one or more of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Ca, Mg, Al, and Nb, X is selected from one or more of C, Se, Si, S, P, As, B, Mo, W, and Ge, Z is selected from one or more of O, F, and OH, the values of a, b, c, d, and e are greater than 0, and the value of f is greater than or equal to 0; Among them, the average discharge or discharge platform voltage of the blending material is greater than that of the main material, and the difference between the average discharge or discharge platform voltage of the blending material and the main material is ≥ 0.1V.

2. The sodium-ion battery cathode material according to claim 1, wherein the difference range between the average discharge voltage or discharge platform of the blending material and the main material is 0.2 - 1.0V.

3. The cathode material for a sodium-ion battery according to claim 1, wherein The surface of the main material includes a carbon coating layer.

4. The cathode material for a sodium-ion battery according to claim 1, characterized in that, The blending material includes one or more materials.

5. The cathode material for a sodium-ion battery according to claim 1, wherein The mass of the blending material is within the range of 1% - 50% of the total mass of the main material and the blending material.

6. The sodium-ion battery cathode material according to claim 5, wherein the mass of the blending material is within the range of 5% - 40% of the total mass of the main material and the blending material.

7. The cathode material for a sodium-ion battery according to claim 1, wherein, The median particle size of the main material is within the range of 0.1 - 15μm.

8. A positive electrode sheet of a sodium-ion battery, characterized in that, It includes the sodium-ion battery cathode material according to any one of claims 1 - 7.

9. The positive electrode sheet of the sodium-ion battery according to claim 8, characterized in that, The tap density of the positive electrode material of the sodium ion battery is in the range of 1.45-2.65 g / cm 3 .

10. A sodium-ion battery, characterized in that, It includes the sodium-ion battery cathode plate according to any one of claims 8 - 9.

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