A sodium-based positive electrode active material and its preparation method and application

Abstract

The present invention provides a sodium-based positive electrode active material, a preparation method, and applications thereof. The sodium-based positive electrode active material comprises a core of a sodium mixed metal oxide and a coating layer covering at least a portion of the surface of the core, wherein the coating layer comprises a sodium-based borohydride. The sodium-based positive electrode active material of the present invention has a positive effect on the cycling performance of sodium-ion batteries.

Description

Technical Field

[0001] The present invention relates to a positive electrode active material, in particular to a sodium-based positive electrode active material and a preparation method and application thereof, belonging to the technical field of secondary batteries. Background Art

[0002] With the booming development of the new energy sector, the demand for energy storage devices continues to increase, and related research is also deepening and expanding. Since the 1990s, lithium batteries have dominated the secondary battery field. However, in recent years, the sharp rise in raw material prices and the increasing shortage of lithium resources have made their commercial prospects uncertain. Sodium and lithium have similar electrochemical properties, and sodium is more abundant on Earth, making sodium-ion batteries an important alternative to lithium batteries.

[0003] The positive electrode active material is a key component of the secondary battery. Its structure and characteristics will directly affect the intercalation and deintercalation of active ions, and also determine the performance of the secondary battery. The preparation process of sodium-based positive electrode active materials is basically the same as that of lithium-based active materials. Both are obtained by adding sodium compounds to the sodium metal hydroxide precursor and then calcining it. However, excessive sodium compounds often lead to a large amount of residual sodium compounds on the surface of the sodium-based positive electrode active material. Even a small amount of sodium ions that enter the sodium-based positive electrode active material during the calcination process will precipitate and come into contact with the outside air, resulting in the formation of impurities such as sodium carbonate or sodium hydroxide on the surface of the sodium-based positive electrode active material. These inactive substances accumulated on the active surface of the sodium-based positive electrode will greatly deteriorate the cycle performance of the sodium-ion battery. Summary of the Invention

[0004] The present invention provides a sodium-based positive electrode active material. By structurally modifying the sodium-based positive electrode active material and applying the sodium-based positive electrode active material to a sodium ion battery, the cycle performance of the sodium ion battery can be improved.

[0005] The present invention also provides a method for preparing a sodium-based positive electrode active material. By introducing a sodium-based borohydride raw material, a sodium-based positive electrode active material that improves the cycle performance of a sodium ion battery can be obtained.

[0006] The present invention also provides a positive electrode sheet, which includes the above-mentioned sodium-based positive electrode active material, so that the positive electrode sheet helps to improve the cycle performance of the sodium ion battery.

[0007] The present invention also provides a sodium ion battery, which includes the above-mentioned sodium-based positive electrode active material and thus has excellent cycle performance.

[0008] The present invention provides a sodium-based positive electrode active material, comprising a core of a sodium mixed metal oxide and a coating layer covering at least a portion of the surface of the core, wherein the coating layer comprises sodium-based borohydride.

[0009] The sodium-based positive electrode active material as described above, wherein the sodium-based borohydride comprises NaBH4, Na(BH4) 0.5 (NH2) 0.5 、Na2(CB9H 10 )(CB 11 H 12 ), Na2B 10 H 10 、NaCB 11 H 12 、NaCB9H 10 and at least one of Na3OBH4.

[0010] The sodium-based positive electrode active material as described above, wherein the sodium mixed metal oxide is Na 1-x M1 x M2 a M3 b O2, wherein M1 is selected from at least one of Ca and Mg, M2 is selected from at least one of Ni, Fe, and Mn, and M3 is selected from at least one of Zn, Cu, Zr, Ti, V, Cr, and Sn, and 0≤x≤0.1, 0.8≤a≤0.9, and 0.1≤b≤0.2.

[0011] The sodium-based positive electrode active material as described above, wherein the mass percentage of the sodium-based borohydride in the sodium-based positive electrode active material is 0.5-8%.

[0012] The sodium-based positive electrode active material as described above, wherein the surface free sodium ion content of the sodium-based positive electrode active material is not higher than 2000 ppm.

[0013] The sodium-based positive electrode active material as described above, wherein the thickness of the coating layer is 5-100 nm.

[0014] The sodium-based positive electrode active material as described above, wherein the particle size of the sodium-based positive electrode active material is 2-14 μm.

[0015] The present invention provides a method for preparing the sodium-based positive electrode active material as described in any one of the above, comprising the steps of sequentially drying and heat-treating a mixed solution containing the sodium-based borohydride and the sodium mixed metal oxide;

[0016] The temperature of the heat treatment is 700-1000°C.

[0017] The present invention provides a positive electrode sheet, comprising any one of the above-mentioned sodium-based positive electrode active materials.

[0018] The present invention provides a sodium ion battery, comprising any one of the sodium-based positive electrode active materials or positive electrode sheets described above.

[0019] The sodium-based positive electrode active material of the present invention comprises a core of a sodium mixed metal oxide and a coating layer comprising a sodium-based borohydride covering at least a portion of the surface of the core. The sodium-based borohydride coating layer has both high sodium ion conductivity and can reduce the alkali content of the sodium-based positive electrode active material. This improves the cycling performance of a sodium-ion battery comprising the sodium-based positive electrode active material by increasing the sodium ion intercalation and deintercalation efficiency and reducing side reactions on the surface of the sodium-based positive electrode active material. Specifically, after 200 cycles, the sodium-ion battery comprising the sodium-based positive electrode active material of the present invention exhibits a capacity retention rate of not less than 93%, and can even be as high as 96.4%. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a scanning electron microscope image of the sodium-based positive electrode active material in Example 1 of the present invention;

[0021] Figure 2 is a scanning electron microscope image of the sodium mixed metal oxide in Example 1 of the present invention;

[0022] Figure 3 : is the element distribution diagram of the B element on the surface of the sodium-based positive electrode active material in Example 1 of the present invention;

[0023] Figure 4 1 is a graph showing the cycle performance of a sodium-ion battery obtained from the sodium-based positive electrode active material in Example 1 of the present invention;

[0024] Figure 5 1 is a cycle performance diagram of a sodium ion battery obtained from the positive electrode precursor material in Example 11 of the present invention;

[0025] Figure 6 This is a cycle performance diagram of the sodium ion battery obtained from the positive electrode precursor material in Comparative Example 2 of the present invention. DETAILED DESCRIPTION

[0026] To make the objectives, technical solutions, and advantages of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts shall fall within the scope of protection of the present invention.

[0027] A first aspect of the present invention provides a sodium-based positive electrode active material, comprising a core of a sodium mixed metal oxide and a coating layer covering at least a portion of the surface of the core, wherein the coating layer comprises sodium-based borohydride.

[0028] The sodium mixed metal oxide of the present invention refers to a composite metal oxide including metallic sodium and at least one other metal, for example, it can be a common positive electrode active material used in the positive electrode sheet of a sodium ion battery in the art; the sodium-based borohydride of the present invention refers to a compound containing at least sodium, boron and hydrogen elements.

[0029] The sodium-based cathode active material of the present invention has a core-shell structure with a core and a coating layer. On the one hand, the coating layer, comprising sodium-based borohydride, acts as a physical barrier to effectively prevent side reactions between the sodium-based cathode active material and the electrolyte. This not only maintains the structural stability of the sodium-based cathode active material and ensures the normal intercalation and deintercalation of sodium ions, but also extends the service life of the electrolyte and avoids the negative impact on the cycling performance of sodium-ion batteries due to electrolyte loss or impurities generated by side reactions. On the other hand, sodium-based borohydride, as a compound with high sodium ion conductivity, can also promote the reciprocating migration of sodium ions between the positive and negative electrodes during the charge and discharge process, thus promoting the cycling performance of sodium-ion batteries to a certain extent.

[0030] In addition, the inventors also found that the surface alkali content of the sodium-based positive electrode active material of the present invention (expressed as the free sodium ion content) is significantly lower than the surface alkali content of the existing sodium-based positive electrode active material. It can be understood that the lower alkali content can prevent the sodium-based positive electrode active material from reacting with substances such as water vapor and carbon dioxide in the external environment, thereby affecting the structural stability of the sodium-based positive electrode active material, thereby further benefiting the improvement of the cycle performance of the sodium ion battery. The inventors speculate that the reason for the low surface alkali content of the sodium-based positive electrode active material of the present invention is that the internal lattice of the sodium-based borohydride has sodium vacancies, so the sodium vacancies are very easy to accept the sodium compounds that are not completely reacted during the preparation of the sodium mixed metal oxide, thereby reducing the alkali content on the surface of the sodium-based positive electrode active material by consuming the excess sodium compounds. Of course, for the sodium precipitated from the sodium mixed metal oxide, the sodium vacancies in the sodium-based borohydride will also accept it and be consumed.

[0031] The present invention does not limit the degree of coverage of the core by the coating layer. The coating layer may only cover part of the surface of the core. Of course, when the coating layer completely covers the inner surface, the sodium ion battery has more outstanding cycle performance.

[0032] In one embodiment, the sodium borohydride comprises NaBH4, Na(BH4) 0.5 (NH2) 0.5 、Na2(CB9H 10 )(CB 11 H 12 ), Na2B 10 H 10 、NaCB 11 H 12 、NaCB9H10 and Na3OBH4, in which case the sodium-based positive electrode active material has a higher sodium ion conductivity. When the sodium-based borohydride comprises at least two of the above compounds, the present invention does not limit the ratio between the individual compounds. The above sodium-based borohydrides are all commercially available.

[0033] As mentioned above, sodium mixed metal oxide refers to a common sodium-based positive electrode active material in the art. For example, the sodium mixed metal oxide is Na 1-x M1 x M2 a M3 b O2, wherein M1 is selected from at least one of Ca and Mg, M2 is selected from at least one of Ni, Fe, and Mn, and M3 is selected from at least one of Zn, Cu, Zr, Ti, V, Cr, and Sn, and 0≤x≤0.1, 0.8≤a≤0.9, and 0.1≤b≤0.2.

[0034] Reasonable control of the sodium borohydride content in the sodium-based cathode active material is beneficial for further improving the performance of sodium-ion batteries. Therefore, the mass percentage of sodium borohydride in the sodium-based cathode active material can be controlled to above 0.1%. The inventors have found that as the mass percentage of sodium borohydride in the sodium-based cathode active material increases within a certain range, the cyclability of the sodium-ion battery initially shows an increasing trend, then remains constant or even slightly decreases. Therefore, for economic and battery performance considerations, the mass percentage of sodium borohydride in the sodium-based cathode active material is generally controlled to 0.5-8%.

[0035] Of course, the use of different binders and conductive agents in the positive electrode sheet, and even different negative electrode materials and electrolytes in the battery, will all have an impact on the ultimate performance of the sodium-ion battery. Therefore, generally speaking, for different binders, conductive agents, negative electrode materials, and electrolytes, controlling the mass percentage of sodium-based borohydride in the sodium-based positive electrode active material to 1% to 5% can basically achieve optimal performance for the sodium-ion battery.

[0036] In the specific implementation process of the present invention, by matching the appropriate sodium mixed metal oxide, sodium-based borohydride and the ratio of each other, the free sodium ion content on the surface of the sodium-based positive electrode active material of the present invention can be made not higher than 2000 ppm, thereby being able to more significantly improve the cycle performance of the sodium ion battery.

[0037] In addition, the thickness of the coating layer and the particle size of the sodium-based positive electrode active material are also factors that affect the cycle performance of sodium-ion batteries.

[0038] Specifically, the coating layer acts as a physical barrier. Although it can inhibit the side reactions between the sodium-based positive electrode active material and the electrolyte, it also constitutes a physical resistance to the migration of sodium ions to a certain extent. Therefore, in order to maximize the advantages of sodium-based borohydride, the thickness of the coating layer can be controlled to 5-100nm.

[0039] When preparing a slurry containing positive electrode active materials, if the positive electrode active materials agglomerate, it is detrimental to the stable dispersion of the positive electrode active materials in the slurry, ultimately leading to polarization of the positive electrode sheet and affecting the battery's cycling performance. Therefore, the present invention controls the particle size of the sodium-based positive electrode active material to 2-14μm. At this particle size, the sodium-based positive electrode active material can be uniformly and stably dispersed in the slurry, maintaining excellent suspension properties.

[0040] A second aspect of the present invention provides a method for preparing the aforementioned sodium-based positive electrode active material, comprising the steps of sequentially drying and heat-treating a mixed solution containing the sodium-based borohydride and the sodium mixed metal oxide; the heat-treating temperature is 700-1000°C.

[0041] The invention prepares a mixed solution comprising sodium-based borohydride and sodium mixed metal oxide, and sequentially performs drying treatment and heat treatment on the mixed solution to obtain the sodium-based positive electrode active material of the invention.

[0042] When preparing the mixed solution, sodium borohydride and sodium mixed metal oxide are added to an organic solvent and stirred thoroughly to uniformly disperse the sodium borohydride and the sodium mixed metal oxide, thereby facilitating the coating of the sodium mixed metal oxide with the sodium borohydride. Ethanol can be selected as the organic solvent.

[0043] The present invention is not limited to a specific embodiment of the drying process; for example, spray drying can be used. This drying process can dry the sodium mixed metal oxide core material and the sodium-based borohydride coating material into a dispersed powder, which facilitates uniform heat transfer in subsequent heat treatment steps. Furthermore, the powdered material reduces the requirements for heat treatment equipment and facilitates operation.

[0044] After drying, the dried system is heat-treated at 700-1000°C to effectively coat the sodium mixed metal oxide with the sodium borohydride. The heat treatment time is generally controlled to be between 4 and 20 hours. The specific heat treatment time can be further determined based on the heat treatment temperature, the mass ratio of sodium borohydride to sodium mixed metal oxide, and other factors. After heat treatment, the resulting sodium-based positive electrode active material can be ground to the target particle size.

[0045] In one embodiment, the sodium borohydride is dried and ball-milled before the mixed solution is prepared. The ball-milling is used to form the dried sodium borohydride into particles with an average particle size of 1-6 μm, thereby facilitating a more uniform dispersion in the mixed solution of the sodium mixed metal oxide.

[0046] For example, the drying process is carried out at a temperature of 120 to 220° C. for about 12 hours, for example, in a vacuum drying oven, with a ball mill speed of 15 to 40 rpm for 5 to 10 hours.

[0047] The present invention is not limited to the preparation method of the sodium mixed metal oxide. For example, it can be a calcined system obtained by calcining a precursor of a sodium mixed metal hydroxide and a sodium compound. The sodium compound is, for example, one or more selected from the group consisting of sodium hydroxide, sodium sulfate, sodium nitrate, sodium chloride, sodium hypochlorite, sodium perchlorate, sodium carbonate, and sodium acetate.

[0048] The third aspect of the present invention provides a positive electrode sheet. Specifically, the positive electrode sheet of the present invention includes a positive electrode current collector and a positive electrode material attached to at least one surface of the positive electrode current collector, wherein the positive electrode material at least includes the sodium-based positive electrode active material, a conductive agent and a binder of the first aspect mentioned above.

[0049] When preparing the positive electrode sheet, the sodium-based positive electrode active material, conductive agent and binder of the first aspect mentioned above can be mixed with a solvent to obtain a positive electrode slurry, and then the positive electrode slurry is placed on at least one surface of the positive electrode current collector. After the solvent in the positive electrode slurry is volatilized, the positive electrode sheet of the present invention is obtained.

[0050] Since the positive electrode sheet of the present invention includes the sodium-based positive electrode active material described in the first aspect above, when it is used as the positive electrode sheet of a sodium ion battery, the cycle performance of the sodium ion battery can be effectively improved.

[0051] The fourth aspect of the present invention is to provide a sodium ion battery, wherein the positive electrode of the sodium ion battery adopts the positive electrode sheet of the third aspect.

[0052] The sodium ion battery of the present invention comprises at least a negative electrode sheet, an electrolyte, and the positive electrode sheet of the third aspect. Since the sodium ion battery of the present invention comprises the positive electrode sheet of the third aspect, the sodium ion battery has excellent cycle performance.

[0053] Hereinafter, the sodium-based positive electrode active material of the present invention will be described in detail through specific examples.

[0054] Example 1

[0055] The preparation method of the sodium-based positive electrode active material of this embodiment includes the following steps:

[0056] 1) NaCB 11 H 12 Place it in a vacuum drying oven at 150 ° C and dry it for 12 hours. Then use a ball mill with a ball-to-material ratio (weight ratio) of 6:1 at a speed of 30 r / min for 10 hours to obtain NaCB with a particle size of 3.37 μm. 11 H 12 particles;

[0057] 2) The ball-milled NaCB 11 H 12 With NaNi 0.8 Fe 0.05 Mn 0.05 Zn 0.1 O2 was dissolved in ethanol at a mass ratio of 3:100 and stirred to obtain a mixed solution;

[0058] 3) spray drying the mixed solution to obtain a mixed powder, and then heat treating the mixed powder in a muffle furnace at 700° C. for 10 h;

[0059] 4) Grinding the heat-treated powder to obtain a sodium-based positive electrode active material NaNi with a particle size of 11.46 μm 0.8 Fe 0.05 Mn 0.05 Zn 0.1 O2@NaCB 11 H 12 , the coating layer thickness is 16nm.

[0060] The sodium mixed metal oxide NaNi in this embodiment 0.82 Fe 0.12 Mn 0.06 The preparation method of O2 comprises the following steps:

[0061] 1) The precursor NaNi 0.8 Fe 0.05 Mn 0.05 Zn 0.1 (OH)2 and NaOH were mixed uniformly in a coulter mixer and then calcined at 950 °C for 12 h;

[0062] 2) The calcined material is crushed, sieved, and iron removed to obtain single crystal sodium-based mixed metal oxide NaNi 0.8 Fe 0.05 Mn 0.05 Zn 0.1 O2.

[0063] Figure 1 This is a scanning electron microscope image of the sodium-based positive electrode active material in Example 1 of the present invention. Figure 2 The scanning electron microscope image of the sodium mixed metal oxide in Example 1 of the present invention is shown in FIG. Figure 1 and Figure 2 It can be seen that Example 1 of the present invention achieves the coating of sodium mixed metal oxide.

[0064] Figure 3 The element distribution diagram of the B element on the surface of the sodium-based positive electrode active material in Example 1 of the present invention is shown in FIG. Figure 3 Characterization shows that the embodiment of the present invention realizes the sodium borohydride NaCB 11 H 12 Coating of sodium mixed metal oxides.

[0065] Example 2

[0066] This embodiment is basically the same as embodiment 1, except that the NaCB 11 H 12 Replaced with Na2B 10 H 10 The particle size after ball milling is 3.90 μm, and the sodium-based positive electrode active material NaNi of this embodiment is prepared. 0.8 Fe 0.05 Mn 0.05 Zn 0.1 @Na2B 10 H 10 , the coating layer thickness is 13nm.

[0067] Example 3

[0068] This embodiment is basically the same as embodiment 1, except that the NaCB 11 H 12 The sodium-based positive electrode active material NaNi was prepared by replacing NaBH4 with a particle size of 2.76 μm after ball milling. 0.8 Fe 0.05 Mn 0.05 Zn 0.1 @NaBH4, the coating thickness is 13nm.

[0069] Example 4

[0070] This embodiment is basically the same as embodiment 1, except that the NaCB 11 H 12 The sodium-based positive electrode active material NaNi was prepared by replacing it with Na3OBH4 and the particle size after ball milling was 4.08 μm. 0.8 Fe 0.05 Mn 0.05 Zn 0.1 @Na3OBH4, the coating thickness is 18nm.

[0071] Example 5

[0072] This embodiment is basically the same as embodiment 1, except that the NaNi 0.8 Fe 0.05 Mn 0.05 Zn 0.1 O2 is replaced by Na 0.95 Ca 0.05 Ni 0.8 Fe 0.05 Mn 0.05 Zn 0.1 O2, the precursor NaNi 0.8 Fe 0.05 Mn 0.05 Zn 0.1 (OH)2 is replaced by Na 0.95 Ca 0.05 Ni 0.8 Fe 0.05 Mn 0.05 Zn 0.1 (OH)2, the sodium-based positive electrode active material Na 0.95 Ca 0.05 Ni 0.8 Fe 0.05 Mn 0.05 Zn 0.1 O2@NaCB 11 H 12 , the coating layer thickness is 16nm.

[0073] Example 6

[0074] This embodiment is basically the same as embodiment 1, except that the NaNi 0.8 Fe 0.05 Mn 0.05 Zn 0.1 O2 is replaced by Na 0.92 Mg 0.08 Ni 0.85 Fe 0.02 Mn 0.02 Ti 0.11 O2, the precursor NaNi 0.8 Fe 0.05 Mn 0.05 Zn 0.1 (OH)2 is replaced by Na 0.9 2Mg 0.08 Ni 0.85 Fe 0.02 Mn 0.02 Ti 0.11 (OH)2, the sodium-based positive electrode active material Na 0.92 Mg 0.08 Ni 0.85 Fe0.02 Mn 0.02 Ti 0.11 O2@NaCB 11 H 12 , the coating layer thickness is 19nm.

[0075] Example 7

[0076] This embodiment is basically the same as embodiment 1, except that in step (2), the NaCB 11 H 12 With NaNi 0.8 Fe 0.05 Mn 0.05 Zn 0.1 The mass ratio of O2 is 0.6:100, and the sodium-based positive electrode active material NaNi of this embodiment is prepared. 0.8 Fe 0.05 Mn 0.05 Zn 0.1 O2@NaCB 11 H 12 , the coating layer thickness is 11nm.

[0077] Example 8

[0078] This embodiment is basically the same as embodiment 1, except that in step 2) the ball-milled NaCB 11 H 12 With NaNi 0.8 Fe 0.05 Mn 0.05 Zn 0.1 The mass ratio of O2 is 4:100, and the sodium-based positive electrode active material NaNi is prepared. 0.8 Fe 0.05 Mn 0.05 Zn 0.1 O2@NaCB 11 H 12 , the coating layer thickness is 17nm.

[0079] Example 9

[0080] This embodiment is basically the same as embodiment 1, except that in step 2) the ball-milled NaCB 11 H 12 With NaNi 0.8 Fe 0.05 Mn 0.05 Zn 0.1 The mass ratio of O2 is 6:100, and the sodium-based positive electrode active material NaNi is prepared. 0.8 Fe 0.05 Mn 0.05 Zn 0.1 O2@NaCB 11 H12 , the coating layer thickness is 20nm.

[0081] Example 10

[0082] This embodiment is basically the same as embodiment 1, except that in step 2) the ball-milled NaCB 11 H 12 With NaNi 0.8 Fe 0.05 Mn 0.05 Zn 0.1 The mass ratio of O2 is 8:100, and the sodium-based positive electrode active material NaNi is prepared. 0.8 Fe 0.05 Mn 0.05 Zn 0.1 O2@NaCB 11 H 12 , the coating layer thickness is 24nm.

[0083] Example 11

[0084] This embodiment is basically the same as embodiment 1, except that in step 2) the ball-milled NaCB 11 H 12 With NaNi 0.8 Fe 0.05 Mn 0.05 Zn 0.1 The mass ratio of O2 is 12:100, and the sodium-based positive electrode active material NaNi is prepared. 0.8 Fe 0.05 Mn 0.05 Zn 0.1 O2@NaCB 11 H 12 , the coating layer thickness is 29nm.

[0085] Comparative Example 1

[0086] The sodium-based positive active material of this comparative example 1 is the single crystal sodium-based mixed metal oxide NaNi with a particle size of 11.46 μm in Example 1. 0.8 Fe 0.05 Mn 0.05 Zn 0.1 O2.

[0087] Comparative Example 2

[0088] 1) Ti3(PO4)4 and NaNi 0.8 Fe 0.05 Mn 0.05 Zn 0.1 O2 was dissolved in ethanol at a mass ratio of 3:100 and stirred to obtain a mixed solution;

[0089] 2) The mixed solution was transferred to a polytetrafluoroethylene reactor and subjected to a solvothermal reaction at 180°C in a forced air drying oven for 12 h;

[0090] 3) After cooling naturally, the product was dried in a vacuum drying oven at 80°C;

[0091] 4) The obtained powder was placed in a muffle furnace and heat treated at 700°C for 8 hours to obtain a sodium fast ion conductor coated positive electrode material NaNi 0.8 Fe 0.05 Mn 0.05 Zn 0.1 O2@NaTi2(PO4)3.

[0092] Test Example 1

[0093] The sodium mixed metal oxide and the sodium-based positive electrode active material in the examples and comparative examples were dissolved in water and anhydrous ethanol, respectively. The sodium carbonate content W1 in the aqueous solution and the sodium hydroxide content W2 in the anhydrous ethanol solution were measured by potentiometric titration, and the free sodium ion content was calculated. The results are shown in Table 1. The free sodium ion content is calculated as follows:

[0094] Free sodium ion content = 22.99*2*W1 / 105.99+22.99*W2 / 39.99.

[0095] Test Example 2

[0096] The sodium-based positive electrode active materials in the above examples and comparative examples were prepared into sodium ion button batteries according to the following methods:

[0097] 1) Mix the sodium-based positive electrode active material with acetylene black and polyvinylidene fluoride (PVDF) (mass ratio of 8:2:1), then add an appropriate amount of N-methylpyrrolidone (NMP) solution and stir to form a positive electrode slurry;

[0098] 2) The positive electrode slurry was evenly coated on aluminum foil, placed in a vacuum drying oven to dry for 24 hours, and cut into positive electrode sheets with a diameter of 12 mm.

[0099] 3) After all the positive electrodes were combined one by one with the sodium metal negative electrode and the separator (the positive and negative electrodes were located on both sides of the separator, respectively), an appropriate amount of sodium ion battery electrolyte (the solute was sodium hexafluorophosphate, and the solvent was ethylene carbonate EC and diethyl carbonate DEC, and the volume ratio EC:DEC = 1:1) was added and assembled into CR2032 button batteries in an argon-filled glove box.

[0100] The cycle performance of each button cell was tested according to the following method. The results are shown in Table 1. Figure 4 1 is a graph showing the cycle performance of a sodium-ion battery obtained from the sodium-based positive electrode active material in Example 1 of the present invention;

[0101] Figure 5 1 is a cycle performance diagram of a sodium ion battery obtained from the positive electrode precursor material in Example 11 of the present invention; Figure 6 This is a cycle performance diagram of the sodium ion battery obtained from the positive electrode precursor material in Comparative Example 2 of the present invention.

[0102] The cycle performance of the button cell was tested at 25°C with 0.2C charge / 0.2C discharge at 2.0-4.0V for 200 cycles. The capacity retention after 200 cycles was calculated using the following formula.

[0103] 200-cycle capacity retention rate = 200th-cycle discharge capacity / first-cycle discharge capacity * 100%

[0104] Table 1

[0105]

[0106] According to Table 1, the sodium-based positive electrode active material of the present invention can significantly improve the cycle performance of sodium ion batteries.

[0107] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A sodium-based positive electrode active material, characterized in that The invention comprises a core of a sodium mixed metal oxide and a coating layer covering at least a portion of the surface of the core, wherein the coating layer comprises a sodium-based borohydride, and the mass percentage of the sodium-based borohydride in the sodium-based positive electrode active material is 1% to 5%; The sodium borohydride includes Na2(CB9H 10 )(CB 11 H 12 ), Na2B 10 H 10 、NaCB 11 H 12 and NaCB9H 10 At least one of; The sodium mixed metal oxide is Na 1-x M1 x M2 a M3 b O2, wherein M1 is selected from at least one of Ca and Mg, M2 is selected from at least one of Ni, Fe, and Mn, and M3 is selected from at least one of Zn, Cu, Zr, Ti, V, Cr, and Sn, and 0≤x≤0.1, 0.8≤a≤0.9, and 0.1≤b≤0.

2.

2. The sodium-based positive electrode active material according to claim 1, characterized in that The surface free sodium ion content of the sodium-based positive electrode active material is not higher than 2000 ppm.

3. The sodium-based positive electrode active material according to claim 1, characterized in that The thickness of the coating layer is 5-100 nm.

4. The sodium-based positive electrode active material according to claim 3, characterized in that The particle size of the sodium-based positive electrode active material is 2-14 μm.

5. A method for preparing the sodium-based positive electrode active material according to any one of claims 1 to 4, characterized in that: The method comprises the steps of sequentially drying and heat treating a mixed solution containing the sodium borohydride and the sodium mixed metal oxide; The temperature of the heat treatment is 700-1000°C.

6. A positive electrode sheet, characterized in that: The positive electrode sheet comprises the sodium-based positive electrode active material according to any one of claims 1 to 4.

7. A sodium ion battery, characterized in that: The sodium ion battery comprises the sodium-based positive electrode active material according to any one of claims 1 to 4 or the positive electrode sheet according to claim 6.

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