Sodium ion positive electrode material, preparation method thereof and sodium ion secondary battery

Abstract

The invention relates to a sodium ion positive electrode material, a preparation method thereof and a sodium ion secondary battery, the sodium ion positive electrode material comprises doping elements, the doping elements comprise M and N, M comprises one or more of Zn, Ca, Sr, Ba, Mg and A1, and N comprises one or more of Nb, Ta, MO, W, N, P, K, Ca, Rb, Sr, Cs and Ba; the orientation difference of adjacent crystal grains in the sodium ion positive electrode material crystal grains is detected through the backscattering electron diffraction technology, the crystal boundary with the orientation difference larger than or equal to 10 degrees is defined as the large-angle crystal boundary, and in the sodium ion positive electrode material, the proportion of the large-angle crystal boundary is smaller than or equal to 50%. By controlling the proportion of the large-angle grain boundary within the preset range, the stress action which is borne by grains or crystals and is generated by unit cell deformation is reduced, so that the structural stability of the sodium ion positive electrode material is improved, and the cracking risk of the sodium ion positive electrode material in the circulating process is reduced.

Description

Technical Field

[0001] This application relates to the field of electrochemical energy storage, specifically to a sodium-ion cathode material and its preparation method, and a sodium-ion secondary battery. Background Technology

[0002] Sodium-ion rechargeable batteries primarily use sodium salts as electrode materials. Compared to lithium resources, sodium resources are more abundant, more widely distributed, and cheaper on Earth, while also causing less environmental pollution. Sodium-ion batteries also offer advantages such as good power characteristics, wide temperature range adaptability, safety performance, and no over-discharge issues. However, due to the large radius of sodium ions, the extraction or insertion of sodium ions during charging and discharging causes significant deformation in the unit cells, resulting in substantial stress. This can lead to cracking or even failure of the electrode materials during cycling. Summary of the Invention

[0003] In view of this, this application provides a sodium-ion cathode material and a method for preparing the same, to solve at least one of the aforementioned technical problems. Furthermore, this application also provides a sodium-ion secondary battery.

[0004] In a first aspect, this application provides a sodium-ion cathode material, which includes doping elements, namely M and N, wherein M includes one or more of Zn, Ca, Sr, Ba, Mg and Al, and N includes Nb, Ta, and M. O One or more of W, N, P, K, Ca, Rb, Sr, Cs and Ba; the orientation difference between adjacent grains in sodium ion cathode material is detected by backscattered electron diffraction technology, and grain boundaries with an orientation difference greater than or equal to 10° are defined as large-angle grain boundaries. In sodium ion cathode material, the proportion of large-angle grain boundaries is less than or equal to 50%.

[0005] Based on the first aspect, in some possible implementations, the proportion of large-angle grain boundaries is 15% to 30%.

[0006] Based on the first aspect, in some possible implementations, M includes one or more of Zn, Ca, and Ba, and N includes one or more of Mo, W, and Cs.

[0007] Based on the first aspect, in some possible implementations, the particle size Dv10 of the sodium ion cathode material is 2 μm to 15 μm.

[0008] Based on the first aspect, in some possible implementations, the general chemical formula of the sodium ion cathode material is Na. x M a N b Me 1-a-bO2, 0.8≤x≤1.1, 0.002≤a≤0.05, 0.002≤b≤0.05, where Me includes one or more of Ni, Fe, Mn, Cr, Cu and V.

[0009] Secondly, this application provides a method for preparing the above-mentioned sodium-ion cathode material. The method includes: mixing a sodium-ion cathode material precursor, a sodium source, and a compound containing element M to obtain a mixture; stirring and heating the mixture to obtain a doped product; mixing the doped product with the sodium source to obtain a dry mixture; sintering the dry mixture to obtain a sintered product; mixing the sintered product, the compound containing element N, and the sodium source, and then spray drying the mixture to obtain a spray material; sintering the spray material to obtain the sodium-ion cathode material.

[0010] Based on the second aspect, in some possible implementations, an alkali is also added to the mixture, with the concentration of the alkali in the mixture being 0.1 mol / L to 1 mol / L.

[0011] Based on the second aspect, in some possible implementations, the heating temperature is 45 ℃ to 200 ℃, and the time is 10 min to 200 min.

[0012] Based on the second aspect, in some possible implementations, the dry mix is ​​sintered at a temperature of 650 °C to 1150 °C for a time of 5 h to 40 h.

[0013] Based on the second aspect, in some possible implementations, the temperature for sintering the sprayed material is 400 ℃ to 1000 ℃, and the time is 2 h to 30 h.

[0014] Based on the second aspect, in some possible implementations, the general chemical formula of the sodium ion cathode material precursor includes one or more of MeCO3, Me(OH)2 and MeC2O4, wherein Me includes one or more of Ni, Fe, Mn, Cr, Cu and V.

[0015] Thirdly, this application provides a sodium-ion secondary battery, including a positive electrode sheet comprising the aforementioned sodium-ion positive electrode material.

[0016] The sodium-ion cathode material of this application has specific doping elements. The M element induces the precursor crystal to preferentially grow along one plane, which helps reduce the possibility of grain boundaries forming between crystals with large phase differences. The N element can accumulate on the grain surface, which helps eliminate adhesion between crystal particles and reduces the formation of large-angle grain boundaries. Therefore, the aforementioned specific doping elements work synergistically to induce crystal plane growth and inhibit crystal adhesion, helping to control the proportion of large-angle grain boundaries. The large-angle grain boundaries obtained by defining angles in this application indicate that the adjacent crystals forming these large-angle grain boundaries experience significant stress from unit cell deformation. By controlling the proportion of these large-angle grain boundaries within a preset range, this application helps reduce the stress on the grains or crystals caused by unit cell deformation, thereby improving the structural stability of the sodium-ion cathode material and reducing the risk of cracking during cycling.

[0017] The preparation method described in this application controls the preparation process and involves stepwise and sequential doping of specific elements and stepwise replenishment of the sodium source, which helps to control the proportion of large-angle grain boundaries. Doping with element M first induces the precursor crystal to preferentially grow along one plane, reducing the likelihood of grain boundaries forming between crystals with large phase differences. Based on the effect of element M, the doped element N can accumulate on the grain surface, which helps eliminate adhesion between crystal particles and further reduces the formation of large-angle grain boundaries. Stepwise replenishment of the sodium source, compared to a one-time replenishment, can appropriately reduce the adhesion effect of sodium itself on crystal particles, also contributing to reducing the formation of large-angle grain boundaries. Attached Figure Description

[0018] Figure 1 This is a schematic diagram illustrating the identification of the grain orientation angle of the sodium-ion cathode material provided in Comparative Example 1 of this application using EBSD.

[0019] Figure 2 This is a schematic diagram of the sodium ion cathode material provided in Example 1 of this application, which uses EBSD to identify the grain orientation angle.

[0020] Figure 3 The image is a scanning electron microscope image of the sodium ion cathode material provided in Comparative Example 1 of this application after cyclic failure. Detailed Implementation

[0021] The embodiments of this application are described in detail below. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain this application, and should not be construed as limiting this application; it should be noted that, unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs; where there is no conflict, the implementation methods and features of the implementation methods of this application can be combined with each other; many specific details are set forth in the following description to provide a full understanding of this application, and the described implementation methods are only a part of the implementation methods of this application, and not all of the implementation methods.

[0022] In response to the problem that sodium-ion secondary battery electrode materials are prone to cracking during cycling, this application found that in the grains of sodium-ion cathode materials, adjacent crystals form crystal orientation angles at the grain boundaries. Different crystal orientation angles may affect the stress exerted on the corresponding crystals by different degrees from the deformation of the unit cell.

[0023] Based on this, this application also studies the crystal phase angle formed by adjacent crystals in grains that are significantly affected by stress generated by cell deformation. By improving the preparation method of sodium-ion cathode material, the proportion of crystal phase angles with this angle in sodium-ion cathode material is controlled, thereby reducing the stress generated by cell deformation on the grains or crystals, reducing the adverse effect of stress generated by cell deformation on the structural stability of sodium-ion cathode material, and achieving the goal of reducing the cracking risk of sodium-ion cathode material during cycling.

[0024] Based on this, one embodiment of this application provides a sodium-ion cathode material, which includes doping elements, namely M and N, wherein M includes one or more of Zn, Ca, Sr, Ba, Mg and Al, and N includes Nb, Ta, and M. O One or more of W, N, P, K, Ca, Rb, Sr, Cs, and Ba are used. The orientation difference between adjacent grains in the sodium-ion cathode material is detected using backscattered electron diffraction technology. Grain boundaries with an orientation difference greater than or equal to 10° are defined as large-angle grain boundaries. The proportion of large-angle grain boundaries in the sodium-ion cathode material is less than or equal to 50%. For example, the proportion of large-angle grain boundaries can be 50%, 45%, 40%, 35%, 30%, 25%, 20%, 15%, 10%, 5%, 3%, 1%, 0.5%, or any value within the range of any two of the above values.

[0025] The sodium-ion cathode material of this application has specific doping elements. The M element can induce the precursor crystal to preferentially grow along specific crystal planes ((003), (104), or (012) crystal planes). For example, Zn, Ca, Sr, Ba, Mg, and Al can induce the growth of the (003) crystal plane of the precursor crystal, thereby reducing the possibility of grain boundaries forming between crystals with large phase differences. The N element can be enriched on the grain surface, which is beneficial to eliminating the adhesion between crystal particles and reducing the formation of large-angle grain boundaries. Therefore, the above-mentioned specific doping elements work together to induce crystal plane growth and inhibit crystal adhesion, which helps to control the proportion of large-angle grain boundaries. The large-angle grain boundary obtained by defining the angle indicates that the adjacent crystals forming the large-angle grain boundary are subjected to more significant stress from the deformation of the unit cell. By controlling the proportion of the above-mentioned large-angle grain boundaries within a preset range, it is beneficial to reduce the stress from the deformation of the unit cell on the grains or crystals, thereby improving the structural stability of the sodium-ion cathode material and reducing the cracking risk of the sodium-ion cathode material during cycling.

[0026] Understandably, when the defined angle is too large, for example, when the crystal phase angle is defined as greater than or equal to 15° / 20° / 30° as the large-angle grain boundary that this application intends to control, some grains or crystals in the sodium ion cathode material are still subjected to significant stress from the deformation of the unit cell. At this time, controlling the proportion of the large-angle grain boundary has no significant effect on improving the structural stability of the sodium ion cathode material.

[0027] Understandably, when the proportion of large-angle grain boundaries obtained according to the definition of this application is too large, for example, greater than 50%, the extraction or insertion of sodium ions causes large deformation of the unit cell, resulting in large stress. The grains or crystals in the sodium ion cathode material are subjected to strong stress from the deformation of the unit cell, which makes the electrode material prone to cracking during cycling.

[0028] In some embodiments, the proportion of large-angle grain boundaries is 15% to 30%. For example, the proportion of large-angle grain boundaries can be 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%, or any value within the range of any two of the above values. Controlling the proportion of large-angle grain boundaries within the above range is beneficial for further reducing the stress on the grains or crystals caused by unit cell deformation, thereby improving the structural stability of the sodium-ion cathode material and further reducing the risk of cracking during cycling.

[0029] In some embodiments, M includes one or more of Zn, Ca, and Ba, and N includes one or more of Mo, W, and Cs. Controlling the selection of M and N from the above-mentioned elements is beneficial for controlling the proportion of large-angle grain boundaries within the more preferred range described above.

[0030] In some embodiments, the particle size Dv10 of the sodium-ion cathode material is from 2 μm to 15 μm. For example, the particle size Dv10 of the sodium-ion cathode material can be 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, or any value within the range of any two of the above values. It should be noted that the particle size Dv10 of the sodium-ion cathode material in this application indicates that the volume percentage of sodium-ion cathode material with a particle size smaller than Dv10 is 10%. The purpose of controlling the particle size Dv10 of the sodium-ion cathode material in this application is mainly to avoid materials with excessively small primary particles. The original grains of such materials have not yet fused and basically do not have grain boundaries.

[0031] In some embodiments, the general chemical formula of the sodium ion cathode material is Na. x M a N b Me 1-a-b O2, 0.8≤x≤1.1, 0.002≤a≤0.05, 0.002≤b≤0.05, where Me includes one or more of Ni, Fe, Mn, Cr, Cu and V.

[0032] An embodiment of this application also provides a method for preparing the above-mentioned sodium-ion cathode material, the method comprising: Step 1: Mix the sodium-ion cathode material precursor and the compound containing element M to obtain a mixture. Stir and heat the mixture to obtain the doped product. Doping with element M first is beneficial for inducing the precursor crystal to preferentially grow along one face, which helps reduce the possibility of grain boundaries forming between crystals with large phase differences.

[0033] In some embodiments, the general chemical formula of the sodium ion cathode material precursor includes one or more of MeCO3, Me(OH)2 and MeC2O4, wherein Me includes one or more of Ni, Fe, Mn, Cr, Cu and V.

[0034] In some embodiments, an alkali is further added to the mixture, and the concentration of the alkali in the mixture is from 0.1 mol / L to 1 mol / L. For example, the alkali concentration of the mixture can be 0.1 mol / L, 0.2 mol / L, 0.3 mol / L, 0.4 mol / L, 0.5 mol / L, 0.6 mol / L, 0.7 mol / L, 0.8 mol / L, 0.9 mol / L, 1 mol / L, or any value within the range of any two of the above values. Controlling the concentration of the alkali in the mixture within the above range is beneficial for controlling the crystal growth at a suitable rate.

[0035] In some embodiments, the heating temperature is 45°C to 200°C, and the time is 10 min to 200 min. For example, the heating temperature can be 45°C, 60°C, 75°C, 90°C, 105°C, 120°C, 135°C, 150°C, 165°C, 180°C, 195°C, 200°C, or any value within the range of any two of the above values. The heating time can be 10 min, 30 min, 50 min, 70 min, 90 min, 110 min, 130 min, 150 min, 170 min, 190 min, 200 min, or any value within the range of any two of the above values. Controlling the heating temperature and time within the above ranges, with stirring, is beneficial for forming more uniform M element doping.

[0036] Step 2: Mix the doped product with the sodium source to obtain a dry mixture, and sinter the dry mixture to obtain the sintered product. After doping with element M, the sodium source is initially replenished. Replenishing the sodium source in stages, compared with replenishing the sodium source all at once, can appropriately reduce the adhesion of sodium to crystal particles and is also beneficial to reducing the formation of large-angle grain boundaries.

[0037] In some embodiments, the sintering temperature of the dry mix is ​​between 650 °C and 1150 °C, and the sintering time is between 5 h and 40 h. For example, the sintering temperature of the dry mix can be 650 °C, 700 °C, 750 °C, 800 °C, 850 °C, 900 °C, 950 °C, 1000 °C, 1050 °C, or any value within the range of any two of the above values. The sintering time of the dry mix can be 5 h, 10 h, 15 h, 20 h, 25 h, 30 h, 35 h, 40 h, or any value within the range of any two of the above values. Controlling the sintering temperature and time of the dry mix within the above range helps to promote crystal growth under the influence of element M, and also helps to reduce the fusion of crystal particles and the formation of grain boundaries.

[0038] Step 3: The sintered product, the N-containing compound, and the sodium source are mixed and spray-dried to obtain a spray-dried material. This spray-dried material is then sintered to obtain the sodium-ion cathode material. Based on the effect of the nitrogen (M) element, the doped N element can accumulate on the grain surface, which helps eliminate adhesion between crystal particles and further reduces the formation of large-angle grain boundaries. Spray drying helps improve the uniformity of N doping. After N doping, the sodium source is replenished again. Stepwise replenishment of the sodium source, compared to a one-time replenishment, can appropriately reduce the adhesion effect of sodium on crystal particles and also helps reduce the formation of large-angle grain boundaries.

[0039] In some embodiments, the sintering temperature of the aerosol material is between 400 °C and 1000 °C, and the time is between 2 h and 30 h. For example, the sintering temperature of the aerosol material can be 400 °C, 450 °C, 500 °C, 550 °C, 600 °C, 650 °C, 700 °C, 750 °C, 800 °C, 850 °C, 900 °C, 950 °C, 1000 °C, or any value within the range of any two of the above values. The sintering time of the aerosol material can be 2 h, 5 h, 8 h, 11 h, 14 h, 17 h, 20 h, 23 h, 25 h, 27 h, 30 h, or any value within the range of any two of the above values. Controlling the sintering temperature and time of the aerosol material within the above ranges is also beneficial for controlling the growth of the aerosol material and forming sodium ion cathode materials with suitable particle size.

[0040] Therefore, the preparation method of this application controls the preparation process and sequentially dops specific elements and replenishes the sodium source step by step, which synergistically induces crystal growth and inhibits crystal adhesion, thus helping to control the proportion of large-angle grain boundaries.

[0041] One embodiment of this application also provides a sodium-ion secondary battery, including a positive electrode sheet comprising the aforementioned sodium-ion positive electrode material. The sodium-ion secondary battery of this application exhibits good cycle stability and safety performance.

[0042] In some embodiments, a sodium-ion secondary battery includes a casing, an electrode assembly, and an electrolyte. Both the electrode assembly and the electrolyte are located within the casing. The casing can be a packaging bag encapsulated with a film (such as an aluminum-plastic film), for example, a pouch battery. In other embodiments, it can also be a steel-cased battery, an aluminum-cased battery, etc. The electrode assembly includes electrode sheets and a separator. The electrode sheets include a positive electrode sheet and a negative electrode sheet, and the separator is used to separate the positive and negative electrode sheets and can be disposed between the positive and negative electrode sheets. In some embodiments, the electrode assembly can be a stacked structure, for example, it is formed by alternately stacking a positive electrode sheet, a separator, and a negative electrode sheet. In other embodiments, the electrode assembly can also be a wound structure, for example, it is formed by sequentially stacking a positive electrode sheet, a separator, and a negative electrode sheet and then winding them. The positive electrode sheet includes a positive current collector and a positive electrode material active layer disposed on at least one surface of the positive current collector. The positive electrode material active layer includes the aforementioned sodium-ion positive electrode material.

[0043] The present application's solution will be explained below with reference to embodiments. Those skilled in the art will understand that the following examples are for illustrative purposes only and should not be construed as limiting the present application. Unless otherwise stated, reagents, software, and instruments involved in the following embodiments that are not specifically mentioned are all conventional commercially available products or open-source materials.

[0044] Example 1: A sodium-ion cathode material, the preparation method of which includes: Step 1: Using nickel sulfate, ferrous sulfite, and manganese sulfate solutions as raw materials, sodium hydroxide is added as a precipitant and ammonia as a complexing agent. The crystallization reaction is carried out in a reactor under controlled conditions of pH 12.5, temperature 40 ℃, and stirring speed 250 rpm to obtain sodium ion cathode material precursor slurry.

[0045] Step 2: The sodium ion cathode material precursor slurry is transferred to a pressure reaction vessel, and zinc chloride solution is added. The alkalinity of the slurry is adjusted to 0.1 mol / L by NaOH solution. The reaction temperature is controlled at 95 ℃, and the mixture is stirred at 200 rpm for 60 min. Then, the material in the reaction vessel is washed, dehydrated, and dried to obtain the M-doped product.

[0046] Step 3: The M-doped product is thoroughly mixed with sodium carbonate in a ball mill or mixing tank. The mixture is then placed in a crucible and transferred to a muffle furnace for sintering at a temperature of 800 °C for 24 h to obtain the sintered product.

[0047] Step 4: Add the sintered product to a calcium nitrate solution or suspension, add sodium carbonate and stir thoroughly. Prepare a spray material using a spray drying device. Load the spray material into a crucible and transfer it to a muffle furnace for sintering at a temperature of 900 ℃ for 15 h to obtain sodium ion cathode material.

[0048] Example 2: The difference from Example 1 is that element M is Mg and element N is Nb. 5+ .

[0049] Example 3: The difference from Example 1 is that element M is Ba and element N is Mo. 6+ .

[0050] Example 4: The difference from Example 1 is that element M is Al and element N is Ta. 5+ .

[0051] Example 5: The difference from Example 1 is that element M is Sr and element N is W. 6+ .

[0052] Comparative Example 1: A sodium-ion cathode material, the preparation method of which includes: Step 1: Using a sulfate solution of nickel, iron, manganese and copper as raw material, sodium hydroxide and other precipitants are added, and ammonia water is used as a complexing agent. The crystallization reaction is carried out in a reaction vessel under controlled conditions of pH 11.5, temperature 45 ℃ and stirring speed 450 rpm to obtain a sodium ion cathode material precursor slurry. Then, the slurry is washed, dehydrated and dried to obtain the sodium ion cathode material precursor.

[0053] Step 2: The sodium ion cathode material precursor and sodium carbonate are mixed at a ratio of Na / Me=1.0 and thoroughly mixed in a ball mill jar for 3 h. The mixture is then placed in a crucible and transferred to a muffle furnace for sintering at a temperature of 850 ℃ for 10 h to obtain the sodium ion cathode material.

[0054] Comparative Example 2: The difference from Example 1 is that no N element is added in the fourth step, that is, the resulting sodium ion cathode material is not doped with N element.

[0055] Comparative Example 3: The difference from Example 1 is that the second step is not performed, that is, the resulting sodium ion cathode material is not doped with M element.

[0056] This application describes the following processing or performance testing of the sodium-ion cathode materials of Examples 1-5 and Comparative Examples 1-3: 1. This application uses backscattered electron diffraction (EBSD) technology to detect the orientation difference between adjacent grains in sodium-ion cathode materials. The specific process includes: preparing the sample using a cross-sectional argon-ion mill to obtain a flat cross-section; then inserting the sample into an electron microscope; adjusting the sample tilt angle to 70°; confirming the phase of the material to be measured; optimizing noise; using Kikuchi band information for calibration; determining the test area and step size; and starting the test. The crystallographic data obtained from the test are processed using specific software (e.g., AZtecCrystal) to obtain the grain boundary angle data. Grain boundaries with an orientation difference greater than or equal to 10° are defined as large-angle grain boundaries, thus obtaining the proportion of large-angle grain boundaries in the sodium-ion cathode material.

[0057] Please see Figure 1 Using the sodium-ion cathode material from Comparative Example 1 as the test object, a scanning electron microscope field of view was randomly selected. Through the above testing method, the percentage of grain boundaries showing large angles (>10°) in the Grain Boundaries column was found to be 87%. Please refer to... Figure 2 Using the sodium-ion cathode material of Example 1 as the test object, a scanning electron microscope (SEM) field of view was randomly selected. The Grain Boundaries column showed that the proportion of large-angle grain boundaries (>10°) was 39.8%, while the proportion of small-angle grain boundaries (2~10°) was 60.2%, indicating that the proportion of large-angle grain boundaries in the sodium-ion cathode material of this example is relatively low. This application statistically analyzed five fields of view for each test object to obtain the final proportion of large-angle grain boundaries in the sodium-ion cathode material.

[0058] 2. Particle size test of sodium ion cathode material: Weigh a small amount of sodium ion cathode material sample and add it to anhydrous ethanol and stir evenly. Then, disperse the sample in the alcohol medium for several seconds using an external ultrasonic dispersion device. Then, use a dropper to extract the dispersion slurry and add it to a laser particle size analyzer using alcohol as the test medium for testing and analysis.

[0059] 3. Structural stability test of sodium ion cathode material: Disassemble the discharged battery after 500 cycles, take the cathode sheet, clean it with DMC (dimethyl carbonate), and air dry it in a drying room; then cut the air-dried electrode sheet to an appropriate size, cure it with glue, and perform ion grinding. Finally, use an electron microscope to observe the material profile exposed on the electrode sheet after grinding and compare the crack situation of the material particle profile.

[0060] Taking Comparative Example 1 as an example, this sodium-ion cathode material has a large number of large-angle grain boundaries. Please refer to [link / reference]. Figure 3 After charge-discharge cycles, severe cracking occurs in the region at large-angle grain boundaries.

[0061] 4. Preparation of sodium-ion secondary batteries, specifically including: Preparation of the positive electrode sheet: Conductive material, acrylate rubber, and PVDF are selected according to the required proportions within the material ratio range and dispersed in NMP to prepare a composite carbon black conductive binder. In this example, the selected conductive material is conductive graphite. Then, the required weight parts of the composite carbon black conductive binder, carbon black conductive agent, and the required weight parts of the positive electrode active material are placed in a high-speed vacuum mixer and stirred at 100 rpm for 60 min. After uniform mixing, it is added to the high-speed vacuum mixer for vacuum stirring at 300 rpm for 120 min. The stirred slurry is then coated onto a 14 μm thick aluminum foil by transfer coating and dried in an oven. The coated aluminum foil is then shaped and rolled using a precision rolling mill, and after removing powder and spot welding the electrode tabs, the positive electrode sheet is formed. The coating slurry of the obtained positive electrode sheet contains 97.0 wt% positive electrode active material, 1.5 wt% conductive agent, and 1.5 wt% PVDF. Battery preparation of negative electrode sheet: The required amount of carboxymethyl cellulose (CMC) is dispersed in deionized water to prepare a slurry. Then, the required weight amounts of conductive agent and negative electrode active material are placed in a high-speed vacuum mixer and stirred at 100 rpm for 60 min. After thorough mixing, the mixture is added to the high-speed vacuum mixer and stirred under vacuum at 300 rpm for 120 min. After this, styrene-butadiene rubber (SBR) is added and stirred slowly at 20 rpm for 30 min. The resulting slurry is then transferred onto a 14 μm thick aluminum foil and dried in an oven. The coated aluminum foil is then shaped and rolled using a precision rolling mill, and after powder removal and spot welding of the tabs, a negative electrode sheet is formed. This sheet is then wound to form a battery cell, and the battery is processed using conventional methods.

[0062] 5. Cycle stability test of sodium-ion secondary batteries: Under 25 ℃ conditions, the two types of batteries were charged to 4.0 V at a current of 1.0 C, and then charged at a constant voltage to a dielectric current of 0.05 C, and then rested for 5 min; the batteries were then discharged to 2.0 V at a current of 1.0 C, and rested for 5 min. The above steps were repeated 500 times to obtain the capacity of the battery after 500 cycles when discharged to 2.0 V at a current of 1.0 C. The capacity retention rate before and after cycling was calculated using the following formula: Capacity retention rate = (Discharge capacity of the 500th cycle / Discharge capacity of the first cycle) × 100%. The test results are shown in Table 1.

[0063] Table 1. Performance test results of sodium-ion cathode materials and sodium-ion secondary batteries in Examples 1-5 and Comparative Examples 1-3 of this application. The sodium-ion cathode materials of Examples 1-5 of this application have specific doping elements. The M element induces the precursor crystal to preferentially grow along one plane, which helps reduce the possibility of grain boundaries forming between crystals with large phase differences. The N element can accumulate on the grain surface, which helps eliminate adhesion between crystal particles and reduces the formation of large-angle grain boundaries. Therefore, the aforementioned specific doping elements work synergistically to induce crystal plane growth and inhibit crystal adhesion, helping to control the proportion of large-angle grain boundaries. The large-angle grain boundaries obtained by defining angles in this application indicate that the adjacent crystals forming these large-angle grain boundaries experience significant stress from unit cell deformation. By controlling the proportion of these large-angle grain boundaries within a preset range, this application helps reduce the stress on the grains or crystals caused by unit cell deformation, thereby improving the structural stability of the sodium-ion cathode material and reducing the risk of cracking during cycling.

[0064] Compared to Examples 1-5, Comparative Example 1 did not dope with specific elements, and the resulting sodium-ion cathode material had a higher proportion of large-angle grain boundaries. Due to the large radius of sodium ions, the extraction or insertion of sodium ions during the charging and discharging process of the sodium-ion secondary battery caused large deformation of the unit cell, resulting in large stress. This made the electrode material prone to cracking or even failure during cycling, and therefore the cycle stability of the resulting secondary battery was poor.

[0065] Compared to Comparative Example 1, Comparative Example 2, doped with element M, showed a decrease in the proportion of large-angle grain boundaries in the obtained sodium-ion cathode material and an improvement in the cycle stability of the obtained secondary battery. This indicates that element M can induce the precursor crystal to preferentially grow along a specific crystal plane, which helps to reduce the possibility of grain boundaries forming between crystals with large phase differences. Furthermore, reducing the content of large-angle grain boundaries is beneficial to improving the cycle stability of the secondary battery.

[0066] Compared to Examples 1-5, Comparative Example 2 was doped with only M element and not N element, and Comparative Example 3 was doped with only N element and not M element. The effect of both on inhibiting the formation of large-angle grain boundaries was not significant, indicating that simultaneous doping with M element and N element can play a role in inducing crystal plane growth and inhibiting crystal adhesion, thereby achieving better control over the proportion of large-angle grain boundaries.

[0067] The above embodiments are only used to illustrate the technical solutions of this application and are not intended to limit it. Although this application has been described in detail with reference to the above preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions to the technical solutions of this application should not depart from the spirit and scope of the technical solutions of this application.

Claims

1. A sodium-ion cathode material, characterized in that, The sodium-ion cathode material includes doping elements, namely M and N, wherein M includes one or more of Zn, Ca, Sr, Ba, Mg, and Al, and N includes Nb, Ta, and M. O One or more of the following: W, N, P, K, Ca, Rb, Sr, Cs, and Ba; The orientation difference between adjacent grains in the sodium ion cathode material is detected by backscattering electron diffraction technology. Grain boundaries with an orientation difference greater than or equal to 10° are defined as large-angle grain boundaries. The proportion of large-angle grain boundaries in the sodium ion cathode material is less than or equal to 50%.

2. The sodium-ion cathode material as described in claim 1, characterized in that, The proportion of large-angle grain boundaries is 15% to 30%.

3. The sodium-ion cathode material as described in claim 1, characterized in that, M includes one or more of Zn, Ca, and Ba, and N includes one or more of Mo, W, and Cs.

4. The sodium-ion cathode material as described in claim 1, characterized in that, The particle size Dv10 of the sodium ion cathode material is 2 μm to 15 μm.

5. The sodium-ion cathode material as described in claim 1, characterized in that, The general chemical formula of the sodium ion cathode material is Na. x M a N b Me 1-a-b O2, 0.8≤x≤1.1, 0.002≤a≤0.05, 0.002≤b≤0.05, where Me includes one or more of Ni, Fe, Mn, Cr, Cu and V.

6. A method for preparing a sodium-ion cathode material as described in any one of claims 1 to 5, characterized in that, The preparation method includes: A sodium-ion cathode material precursor, a sodium source, and a compound containing element M are mixed to obtain a mixture. The mixture is then stirred and heated to obtain a doped product. The doped product is mixed with a sodium source to obtain a dry mixture, and the dry mixture is sintered to obtain a sintered product. The sintered product, the N-containing compound, and the sodium source are mixed and spray-dried to obtain a spray material. The spray material is then sintered to obtain the sodium ion cathode material.

7. The preparation method according to claim 6, characterized in that, The preparation method also satisfies at least one of the following conditions: (1) The mixture also contains an alkali, the concentration of which is 0.1 mol / L to 1 mol / L; (2) The heating temperature is 45 ℃ to 200 ℃ and the time is 10 min to 200 min.

8. The preparation method according to claim 6, characterized in that, The preparation method also satisfies at least one of the following conditions: (1) The sintering temperature of the dry mixture is 650 ℃ to 1150 ℃, and the time is 5 h to 40 h; (2) The temperature for sintering the spray material is 400 ℃ to 1000 ℃, and the time is 2 h to 30 h.

9. The preparation method according to claim 6, characterized in that, The general chemical formula of the sodium ion cathode material precursor includes one or more of MeCO3, Me(OH)2 and MeC2O4, wherein Me includes one or more of Ni, Fe, Mn, Cr, Cu and V.

10. A sodium-ion secondary battery, comprising a positive electrode, characterized in that, The positive electrode includes the sodium ion positive electrode material as described in any one of claims 1-5.

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