A method for reducing residual alkali on the surface of a sodium-ion battery layered oxide positive electrode material, a positive electrode sheet and applications thereof

Abstract

This invention relates to a method for reducing residual alkali on the surface of sodium-ion battery cathode materials, a cathode sheet, and its application. The method for reducing residual alkali on the surface of sodium-ion battery cathode materials includes the following steps: The layered oxide cathode material obtained from a single sintering process is placed in a corrosion-resistant heater, and HCl gas is continuously introduced at a constant temperature of 50–70°C, causing the residual alkali on the surface of the cathode material to react and generate sodium chloride; the cathode material with sodium chloride on its surface is transferred to an atmosphere sintering apparatus for a secondary sintering treatment, i.e., inert gas N2 is introduced, and after holding at a temperature for a period of time in an inert gas atmosphere, the sodium chloride is removed, and the material is cooled to obtain a layered oxide cathode material with low surface residual alkali content. By using atmosphere control and secondary sintering, the problem of excessively high residual alkali content in the first-sintering product of layered oxide cathode materials is fundamentally solved.

Description

Technical Field

[0001] This invention relates to the field of battery cathode material technology, and in particular to a method for reducing residual alkali on the surface of layered oxide cathode materials for sodium-ion batteries, cathode sheets, and their applications. Background Technology

[0002] Layered oxide cathode materials for sodium-ion batteries have become the mainstream material in current research for the industrialization of sodium-ion batteries due to their periodic structure, simple preparation method, and high specific capacity and voltage. However, the preparation of this material requires strict control of environmental conditions; otherwise, residual alkali is easily generated on the surface, affecting subsequent processing and electrochemical performance. In addition, residual alkali can react with the electrolyte, generating gas that causes battery swelling and affects safety performance.

[0003] Currently, the methods for reducing residual alkali on the surface of layered oxide cathode materials for sodium-ion batteries are similar to those for ternary lithium-ion batteries: (1) During the sintering process of layered oxide mixtures, the sodium salt ratio is reduced, and the sintering temperature and time are adjusted to reduce the sodium content on the material surface, thereby achieving the purpose of reducing the residual alkali content on the material surface. However, reducing the sodium salt ratio will lead to a decrease in material capacity. (2) The material is washed with water and then sintered a second time. However, during the water washing process, it is easy to cause Na+ in the lattice of the layered oxide material to be affected. + Leaching disrupts the surface structure of materials and also affects their electrochemical performance. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides a method for reducing the residual alkali content of layered oxide cathode materials for sodium-ion batteries, a cathode sheet, and its applications. This invention fundamentally solves the problem of excessively high residual alkali content in the first-sintering product of layered oxide cathode materials through atmosphere control and secondary sintering. In the process of reducing the residual alkali content, the surface structure of the material is not damaged, nor is its capacity affected, thus improving the material's processing performance and electrochemical performance.

[0005] The first objective of this invention is to provide a method for reducing residual alkali on the surface of layered oxide cathode materials in sodium-ion batteries, comprising the following steps:

[0006] The sodium-ion battery layered oxide cathode material obtained after a first sintering is placed in a corrosion-resistant heater and continuously purged with HCl gas at a constant temperature of 50–70°C. The residual alkali on the surface of the cathode material reacts to form sodium chloride. The cathode material with sodium chloride on its surface is then transferred to an atmosphere sintering apparatus for a second sintering treatment, i.e., inert gas N2 is introduced and the material is kept at a temperature for a period of time in an inert gas atmosphere. The sodium chloride is then removed, and the material is cooled to obtain a sodium-ion battery layered oxide cathode material with low surface residual alkali content.

[0007] In one embodiment of the present invention, in the step of obtaining the layered oxide cathode material of the sodium-ion battery by one-time sintering, the layered oxide cathode material of the sodium-ion battery is Na x Ni i Fe j Mn k M m O2, where i, j, k, and m are the molar ratios of the corresponding elements respectively, and satisfy 0 < i ≤ 0.4, 0 < j ≤ 0.5, 0 < k ≤ 0.6, 0 < m ≤ 0.2, i + j + k + m = 1, 0.6 < x ≤ 1; M is Li + 、B 3+ 、Mg 2+ 、Al 3+ 、K + 、Ca 2+ 、Co 3+ 、V 3+ 、V 4+ 、Cr 3+ 、Cu 2+ 、Zn 2+ 、Zr 4+ 、Nb 5+ 和Sn 4+ or one or more of them.

[0008] In one embodiment of the present invention, the raw materials of the layered oxide cathode material of the sodium-ion battery include Ni i Fe j Mn k M m (OH)2 precursor and a sodium source.

[0009] In one embodiment of the present invention, the sodium source is one or more of sodium carbonate, sodium hydroxide, sodium acetate, sodium oxalate, and sodium nitrate. <​​​​​​​​​​​​​​​​​In one embodiment of the present invention, the constant temperature is 50-70°C.

[0014] In one embodiment of the present invention, the flow rate and time of HCl gas introduced into the corrosion-resistant heater satisfy: Q·t=(0.3~1)·T;

[0015] Where Q is the flow rate of HCl gas, in mL / min;

[0016] t is the duration of continuous HCl gas flow, in minutes;

[0017] T represents the mass of the layered oxide cathode material for sodium-ion batteries, expressed in grams.

[0018] Furthermore, the preferred value for Q is: 10 mL / min ≤ Q ≤ 50 mL / min.

[0019] In one embodiment of the present invention, the first sintering conditions are: sintering temperature of 750-1100℃, sintering time of 4-20h, and heating rate of 1-10℃ / min; and / or, the second sintering conditions are: sintering temperature of 800-1000℃, inert gas N2 flow rate of 10-30mL / min, and holding time of 1-3h.

[0020] A second objective of this invention is to provide a positive electrode sheet comprising a sodium-ion battery layered oxide positive electrode material with low surface residual alkali content, wherein the sodium-ion battery layered oxide positive electrode material with low surface residual alkali content is prepared by the method described above.

[0021] A third objective of the present invention is to provide a sodium-ion battery comprising the aforementioned positive electrode.

[0022] The technical solution of the present invention has the following advantages compared with the prior art:

[0023] In this invention, after the first sintering is completed, HCl gas is introduced to react with the residual alkali on the surface of the layered oxide cathode material of sodium-ion battery to form NaCl. Then, the material is heated for a second sintering under N2 atmosphere, so that the NaCl reaches its melting point and is discharged in gaseous form, thereby reducing the residual alkali on the surface of the material.

[0024] The method for reducing residual alkali on the surface of layered oxide cathode materials for sodium-ion batteries provided by this invention has the following advantages: 1. This method differs from common methods such as water washing and secondary calcination to remove residual alkali, and the Na in the material... +It will not undergo a protonation reaction due to washing with water and water, resulting in the removal of sodium ions, and further damaging the surface structure of the material. Using the method for removing residual alkali from the layered oxide cathode material of a sodium-ion battery provided by the present invention, during the process of removing residual alkali, the surface structure of the material remains stable and does not affect the electrochemical performance of the material; 2. Using the method for removing residual alkali provided by the present invention, the residual alkali content and pH value of the cathode material are significantly reduced, that is, it can reduce the processing difficulty of the material during the homogenization process and prevent side reactions from occurring due to the direct contact of residual alkali with the electrolyte. 3. After removing residual alkali using the method provided by the present invention, the ionic conductivity of the contact interface between the material surface and the electrolyte is improved, and there is also an obvious improvement effect on the rate performance of the material. Brief Description of the Drawings

[0025] In order to make the content of the present invention easier to be clearly understood, the following further describes the present invention in detail according to specific embodiments of the present invention in combination with the drawings, wherein,

[0026] Figure 1 is the viscosity rebound data of the slurry processing of Example 1 and Comparative Example 1 of the present invention. Detailed Description of the Embodiments

[0027] In order to solve the technical problems pointed out in the background art, the present invention proposes a method for reducing the residual alkali on the surface of the layered oxide cathode material of a sodium-ion battery, including the following steps:

[0028] The first object of the present invention is to provide a method for reducing the residual alkali on the surface of the layered oxide cathode material of a sodium-ion battery, including the following steps:

[0029] The layered oxide cathode material of the sodium-ion battery obtained by one-time sintering is placed in a corrosion-resistant heater, and after continuously introducing HCl gas at a constant temperature of 50 - 70 °C, the residual alkali on the surface of the cathode material reacts to form sodium chloride; the cathode material with sodium chloride formed on the surface is transferred to an atmosphere sintering device for secondary sintering treatment, that is, an inert gas N₂ is introduced, and after maintaining the temperature for a period of time in an inert gas atmosphere, the sodium chloride is removed, and the layered oxide cathode material of the sodium-ion battery with a low surface residual alkali content is obtained after cooling.

[0030] In a specific embodiment, in the step of the layered oxide cathode material of the sodium-ion battery obtained by one-time sintering, the layered oxide cathode material of the sodium-ion battery is Na x Ni i Fe j Mn k M m O2, where i, j, k, m are the molar ratios of the corresponding elements respectively, and satisfy 0 < i ≤ 0.4, 0 < j ≤ 0.5, 0 < k ≤ 0.6, 0 < m ≤ 0.2, i + j + k + m = 1, 0.6 < x ≤ 1; M is Li+ , B 3+ , Mg 2+ , Al 3+ , K + , Ca 2+ , Co 3+ , V 3+ , V 4+ , Cr 3+ , Cu 2+ , Zn 2+ , Zr 4+ , Nb 5+ and Sn 4+ and one or more of the above.

[0031] In a specific embodiment, the raw materials of the layered oxide cathode material for the sodium-ion battery include Ni i Fe j Mn k M m (OH)₂ precursor and a sodium source.

[0032] Further, when 0.6 < x ≤ 0.8, the material is a layered oxide of the P2 phase; when 0.8 < x ≤ 1, the material is a layered oxide of the O3 phase.

[0033] In a specific embodiment, the sodium source is one or more of sodium carbonate, sodium hydroxide, sodium acetate, sodium oxalate, and sodium nitrate.

[0034] In a specific embodiment, the molar ratio of sodium in the sodium source to Ni i Fe j Mn k M m (OH)₂ precursor is 1 to 1.15:1.

[0035] In a specific embodiment, the temperature of the high-temperature solid-phase sintering is 750 to 1100 °C, the sintering time is 4 to 20 h, and the heating rate is 1 to 10 °C / min.

[0036] In an embodiment of the present invention, the layered oxide material for the sodium-ion battery Na x Ni i Fe j Mn k M m O₂ is prepared by the following method: Mix the Ni i Fe j Mn k M m (OH)₂ precursor and the sodium source according to the chemical formula Na x Ni i [[ID=8​​​​m After the O2 molar ratio is prepared, the mixture is placed in a ball mill jar and ball-milled at 300–800 r / min for 0.5–5 h to ensure thorough mixing. The mixed powder is then placed in a muffle furnace for high-temperature sintering for 4–20 h, followed by natural cooling and grinding to obtain the sodium-ion battery layered oxide material Na. x Ni i Fe j Mn k M m Black powder of O2.

[0037] In a specific embodiment, the constant temperature is 50–70°C. This temperature can both accelerate the reaction rate and allow the reaction byproduct H2O to volatilize; however, if the temperature is too high, the reaction rate will not be significantly improved, and it will result in excessive energy consumption.

[0038] In a specific embodiment, the flow rate and time of HCl gas introduced into the corrosion-resistant heater satisfy: Q·t=(0.3~1)·T;

[0039] Where Q is the flow rate of HCl gas, in mL / min;

[0040] t is the duration of continuous HCl gas flow, in minutes;

[0041] T represents the mass of the layered oxide cathode material for sodium-ion batteries, expressed in grams.

[0042] Furthermore, the preferred value for Q is: 10 mL / min ≤ Q ≤ 50 mL / min. If the gas flow rate is too low, the reaction time needs to be increased, increasing time costs; if the gas flow rate is too high, there is a risk of unreacted gas being discharged, resulting in wasted gas.

[0043] This invention uses HCl gas in the process of removing residual alkali because sulfuric acid and nitric acid gases are more expensive and pose greater safety risks compared to HCl gas. Furthermore, using sulfuric acid gas to remove residual alkali introduces impurities (sulfur), which are difficult to remove and thus affect material properties. A primary reason for using HCl gas in this invention is that the reaction product NaCl is easily removed in nitrogen gas, leaving no impurities.

[0044] In a specific embodiment, both the corrosion-resistant heater and the atmosphere sintering device have an air inlet and an exhaust outlet. When gas is introduced, the gas pressure inside the corrosion-resistant heater and the atmosphere sintering device must be maintained at 0 psi.

[0045] In a specific embodiment, the first sintering conditions are: a sintering temperature of 750–1100℃, a sintering time of 4–20 h, and a heating rate of 1–10℃ / min; and / or, the second sintering conditions are: a sintering temperature of 800–1000℃, an inert gas N2 flow rate of 10–30 mL / min, and a holding time of 1–3 h. Within this temperature range, reaching the melting point of NaCl is more conducive to the volatilization of NaCl; simultaneously, based on the principle of minimum Gibbs free energy, under an N2 atmosphere, NaCl will be released in the form of gaseous NaCl and gaseous Na2Cl2 after reaching its melting point, which can significantly reduce the residual alkali in the material.

[0046] A second objective of this invention is to provide a positive electrode sheet comprising a sodium-ion battery layered oxide positive electrode material with low surface residual alkali content, wherein the sodium-ion battery layered oxide positive electrode material with low surface residual alkali content is prepared by the method described above.

[0047] A third objective of the present invention is to provide a sodium-ion battery comprising the aforementioned positive electrode.

[0048] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, so that those skilled in the art can better understand and implement the present invention. However, the embodiments described are not intended to limit the present invention.

[0049] Example 1

[0050] This embodiment provides a method for reducing the residual alkali content of layered oxide cathode materials in sodium-ion batteries and its application, as detailed below:

[0051] 1.1, Ni 0.33 Fe 0.33 Mn 0.34 The (OH)₂ precursor and sodium acetate were ball-milled in a ball mill at a molar ratio of 1:1.05 for 2 hours at a speed of 300 r / min to ensure thorough mixing. The mixed powder was then placed in a muffle furnace and sintered at 800℃ for 10 hours at a heating rate of 5℃ / min. After natural cooling and grinding, the layered oxide material NaNi was obtained. 0.33 Fe 0.33 Mn 0.34 Black powder of O2.

[0052] 1.2. Take 5000g of the crushed layered oxide material NaNi 0.33 Fe 0.33 Mn 0.34O2 black powder was placed in a corrosion-resistant heater and kept at a constant temperature of 65°C. HCl gas was introduced at a flow rate of 10 mL / min for 300 min, after which the atmosphere was turned off. The material was then transferred to a sintering furnace with a heating rate of 4°C / min and an N2 atmosphere at a flow rate of 10 mL / min. Sintering was carried out at 900°C for 2.5 h. After cooling to room temperature, NaNi, a layered oxide cathode material for sodium-ion batteries with surface residual alkali removed, was obtained. 0.33 Fe 0.33 Mn 0.34 O2.

[0053] 1.3 Preparation of Sodium-ion Batteries

[0054] 1.4. The sodium-ion battery layered oxide cathode material NaNi with surface residual alkali removed. 0.33 Fe 0.33 Mn 0.34 O2, conductive agent SuperP, and binder PVDF are mixed, stirred, coated, dried, cold-pressed, and cut into sheets at a mass ratio of 97.5:1.5:1 to obtain the positive electrode sheet required for preparing the soft-pack battery. Next, the negative electrode hard carbon material, conductive agent SuperP, and CMC are weighed according to a mass ratio of 97:1.5:1.5, dissolved in a certain amount of water, stirred, coated, dried, and cut into sheets. The electrode sheets are produced using a winding process. The separator is wound 5 / 6 turns first, then the positive and negative electrodes are wound sequentially, for a total of 8 turns. Finally, the positive electrode is wound up, ensuring that the negative electrode sheet is completely encased within the positive electrode. The prepared core is welded with tabs and glued, then sealed with aluminum-plastic film. After baking in a vacuum oven for 40–120 hours, the water content is tested (H2O < 200 ppm). Then, liquid is injected according to a certain injection coefficient and ratio, sealed, aged, formed, and tested for capacity to obtain the sodium-ion battery. The electrolyte used was 1M sodium hexafluorophosphate dissolved in a solvent with a volume ratio of EC:DEC of 1:1 + 5% FEC.

[0055] Example 2

[0056] This embodiment is prepared entirely according to the scheme of Embodiment 1, except that: a method for preparing a sodium-ion battery layered oxide cathode material with surface residual alkali removed is characterized by introducing HCl gas at a flow rate of 10 mL / min for a duration of 150 min.

[0057] Example 3

[0058] This embodiment is prepared entirely according to the scheme of Embodiment 1. The difference from Embodiment 1 is that: a method for preparing a sodium-ion battery layered oxide cathode material with surface residual alkali removed is characterized by introducing HCl gas at a flow rate of 10 mL / min for a duration of 500 min.

[0059] Comparative Example 1

[0060] Ni 0.33 Fe 0.33 Mn 0.34 The (OH)₂ precursor and sodium acetate were ball-milled in a ball mill at a molar ratio of 1:1.05 for 2 hours at a speed of 300 r / min to ensure thorough mixing. The mixed powder was then placed in a muffle furnace and sintered at 800℃ for 10 hours at a heating rate of 5℃ / min. After natural cooling and grinding, the layered oxide material NaNi was obtained. 0.33 Fe 0.33 Mn 0.34 Black powder of O2.

[0061] The layered oxide material NaNi with residual alkali not removed was directly applied. 0.33 Fe 0.33 Mn 0.34 The black powder of O2 was used to make a soft-pack battery, and the battery manufacturing method is as shown in Example 1 (without removing residual alkali).

[0062] Comparative Example 2

[0063] This embodiment is prepared entirely according to the scheme of Embodiment 1, except that: a method for preparing a sodium-ion battery layered oxide cathode material with surface residual alkali removed is characterized by introducing HCl gas at a flow rate of 10 mL / min for a duration of 600 min (introducing a large amount of HCl gas).

[0064] Comparative Example 3

[0065] This embodiment is prepared entirely according to the scheme of Embodiment 1, but differs from Embodiment 1 in that: a method for preparing a sodium-ion battery layered oxide cathode material with surface residual alkali removed is characterized by not performing secondary sintering.

[0066] Comparative Example 4

[0067] This embodiment follows the same preparation method as Example 1, but differs from Example 1 in that it is a method for preparing a layered oxide cathode material for sodium-ion batteries with surface residual alkali removed. The method is characterized by sintering directly at 900°C for 2.5 hours under a N2 atmosphere without introducing HCl gas. (N2 is introduced instead of HCl)

[0068] Performance testing

[0069] 1. Viscosity testing method for the positive electrode slurry in Example 1 and Comparative Example 1: The stirred positive electrode slurry was poured into the test cup of the viscometer. According to the testing requirements, the rotation speed and measurement range were selected, and the viscometer was adjusted to appropriate parameters. The test head of the viscometer was immersed in the sample to begin the test. The testing time usually needs to be adjusted according to the characteristics of the sample and the testing requirements. After the test, the test results were recorded, and data analysis and processing were performed. The experimental results are shown in [the table below]. Figure 1 .

[0070] 2. The pH values ​​of the materials in Examples 1-3 and Comparative Examples 1-4 were tested, and the results are shown in Table 1.

[0071] 3. Capacity Testing Method: After capacity testing, the pouch battery was placed on the Xinwei Battery Testing Cabinet CTE-4064. First, it was charged at a constant current of 1C to 4V, then charged at a constant voltage of 4V until the current ≤0.05C, then discharged at 1C to 2V, and finally discharged at 0.1C to 2V. The discharge capacity of the pouch battery was collected, i.e., the 1C+0.1C discharge specific capacity, in mAh / g. Test data are shown in Table 2.

[0072] 4. Rate Performance Test: The capacity-graded pouch batteries were placed on the Xinwei Battery Testing Cabinet CTE-4064. The charge / discharge voltage range was 2.0–4.0V. For the first week, a 2C rate was used for 5 weeks of charge / discharge testing. The average value per cycle of the 2C rate charge / discharge during this period was recorded as the 2C discharge capacity. From the 6th week onwards, a 4C rate was used for 5 weeks of charge / discharge testing. The average value per cycle of the 4C rate charge / discharge during this period was recorded as the 4C discharge capacity. From the 11th week onwards, a 5C rate was used for 5 weeks of charge / discharge testing. The average value per cycle of the 5C rate charge / discharge during this period was recorded as the 5C discharge capacity. The unit is mAh / g. Test data are shown in Table 1.

[0073] Table 1. pH test data of materials from Examples 1-3 and Comparative Examples 1-4

[0074] Group pH Example 1 10.53 Example 2 10.72 Example 3 10.33 Comparative Example 1 13.27 Comparative Example 2 10.12 Comparative Example 3 8.52 Comparative Example 4 12.45

[0075] As shown in Table 1, Examples 1-3, using the method for removing residual alkali from the surface provided by the present invention, effectively reduced the residual alkali on the material surface.

[0076] Table 2 Test data for Examples 1-3 and Comparative Examples 1-4

[0077]

[0078] As shown in Table 2, in Examples 1-3 and Comparative Examples 1-4, with the same type of main material, Examples 1-3 used the method provided by this invention to remove residual alkali from the main material, resulting in materials with extremely excellent electrochemical performance. Comparative Example 1 used the same main material as Example 1, the difference being that Comparative Example 1 did not use the method of this invention to remove residual alkali; therefore, the material's electrochemical performance was limited due to the influence of surface residual alkali. Comparative Example 2 used the same main material as Example 1, the difference being that Comparative Example 2 introduced a higher content of HCl gas (i.e., not conforming to the formula Q·t=(0.3~1)·T), which consumed the excessively high sodium content in the main material, resulting in a lower discharge capacity. Comparative Example 3 and Example 1 use the same main material. The difference is that Comparative Example 3 does not undergo secondary sintering after introducing HCl gas, which increases the NaCl content on the material surface but does not improve the electrochemical performance of the material. Comparative Example 4 and Example 1 use the same main material. The difference is that Comparative Example 4 does not introduce HCl gas but directly undergoes secondary sintering under a N2 atmosphere. At high temperatures, some of the residual alkali on the material surface will volatilize as Na2O, but this is not significant and slightly improves the overall performance of the material. The test results show that the method of reducing the residual alkali on the surface of the layered oxide cathode material of sodium-ion batteries in this invention increases the specific capacity of the material and improves its rate performance and cycle stability.

[0079] Depend on Figure 1 It can be seen that the viscosity rebound of the layered cathode oxide material after removing residual alkali in Example 1 is significantly lower than that in Comparative Example 1, indicating that removing residual alkali greatly improves the processing performance of the material.

[0080] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for reducing residual alkali on the surface of layered oxide cathode material in sodium-ion batteries, characterized in that, Includes the following steps: The sodium-ion battery layered oxide cathode material obtained after a first sintering is placed in a corrosion-resistant heater and continuously purged with HCl gas at a constant temperature of 50-70°C. The residual alkali on the surface of the cathode material reacts to form sodium chloride. The cathode material with sodium chloride on the surface is then transferred to an atmosphere sintering device for a second sintering treatment, i.e., inert gas N2 is introduced and the material is kept at a temperature for a period of time in an inert gas atmosphere. The sodium chloride is then removed and the material is cooled to obtain a sodium-ion battery layered oxide cathode material with low surface residual alkali content. In the step of obtaining the layered oxide cathode material of the sodium-ion battery by one sintering, the layered oxide cathode material of the sodium-ion battery is Na x Ni i Fe j Mn k M m O2, where i, j, k, and m are the molar ratios of the corresponding elements respectively, and satisfy 0 < i ≤ 0.4, 0 < j ≤ 0.5, 0 < k ≤ 0.6, 0 < m ≤ 0.2, i + j + k + m = 1, 0.6 < x ≤ 1; M is Li + 、B 3+ 、Mg 2+ 、Al 3+ 、K + 、Ca 2+ 、Co 3+ 、V 3+ 、V 4+ 、Cr 3+ 、Cu 2+ 、Zn 2+ 、Zr 4+ 、Nb 5+ and Sn 4+ or one or more of them; the raw materials of the layered oxide cathode material of the sodium-ion battery include Ni i Fe j Mn k M m (OH)2 precursor and a sodium source; the molar ratio of sodium in the sodium source to Ni i Fe j Mn k M m (OH)2 precursor is 1 to 1.15:1; The secondary sintering conditions are as follows: sintering temperature 800~1000℃, inert gas N2 flow rate 10-30 mL / min, and holding time 1~3h.

2. The method according to claim 1, characterized in that, The flow rate and time of HCl gas introduced into the corrosion-resistant heater satisfy: Q·t = (0.3~1)·T; Where Q is the flow rate of HCl gas, in mL / min; t is the duration of continuous HCl gas flow, in minutes; T represents the mass of the layered oxide cathode material for sodium-ion batteries, expressed in grams.

3. The method according to claim 2, characterized in that, The condition Q satisfies: 10 mL / min ≤ Q ≤ 50 mL / min.

4. The method according to claim 1, characterized in that, The conditions for the first sintering are: sintering temperature of 750~1100℃, sintering time of 4~20h, and heating rate of 1~10℃ / min.

5. A positive electrode, comprising a sodium-ion battery layered oxide positive electrode material with low surface residual alkali content, characterized in that, The sodium-ion battery layered oxide cathode material with low surface residual alkali content is prepared by the method described in any one of claims 1-4.

6. A sodium-ion battery, characterized in that, Includes the positive electrode sheet as described in claim 5.

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