Coated modified sodium ion battery positive electrode material, positive electrode plate and preparation method of sodium ion battery

Abstract

The invention belongs to the field of sodium-ion batteries, and relates to a preparation method of a coated modified sodium-ion battery positive electrode material, a positive electrode plate and a sodium-ion battery. The positive electrode material comprises a layered transition metal oxide NaNi < 0.5 > Mn < 0.5 > O < 2 > and a multiferroic material BiFeO < 3 > coating layer coated outside the layered oxide. The BiFeO3 coating layer can be used as a protective layer to inhibit interface side reaction, so that the cycling stability of the battery is improved; meanwhile, on the basis of ferroelectricity and magnetism of BiFeO3, a physical field is introduced into a positive electrode-electrolyte interface, interface ion transport is regulated and controlled, and then the rate capability of the battery is improved.

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion batteries and relates to sodium-ion battery cathode materials. Background Technology

[0002] Sodium-ion batteries, due to the abundance and low cost of sodium resources, can serve as an effective supplement to lithium-ion batteries in large-scale energy storage. However, sodium-ion batteries suffer from low energy density and poor fast-charging performance, limiting their further application. Electrode materials are crucial to battery performance; therefore, developing electrode materials with fast-charging capabilities and high energy density is an effective way to overcome the performance bottleneck of sodium-ion batteries. Layered oxide cathodes are considered key candidates for improving the energy density of sodium-ion batteries due to their simple preparation process, high specific capacity, and low cost. However, slow sodium-ion diffusion kinetics, poor structural stability, and severe interfacial side reactions limit their cycle and rate performance. For example, comparative examples disclosed in publication CN116259743A use NaNi... 0.5 Mn 0.5 The first-week coulomb efficiency and cycle retention of O2 before coating are only about 60% of those after coating.

[0003] Surface coating is considered an effective means to improve the cycle stability of electrode materials. Currently, coating materials for sodium-ion battery cathodes generally fall into several categories: metal oxides, non-metallic element coatings, and fast ion conductors. While different types of sodium-ion battery cathode coating materials each have their advantages, they also have their own drawbacks: although oxide coatings can effectively improve the structural stability of the material, they lack electrochemical activity and cannot contribute additional capacity during charge and discharge, thus reducing the overall energy density of the electrode material; non-metallic coatings, mainly composed of carbon and nitrogen, can improve the conductivity of the electrode material, but they are difficult to effectively remove residual alkali from the surface of layered cathode materials, leading to problems such as gelation during slurry preparation and electrode processing, affecting practical applications; although fast ion conductor coatings can promote the oxidation of Na+, they also have their own drawbacks. + While fast ion conductors can transport and suppress interfacial side reactions, their synthesis process is complex and costly, making it difficult to meet the needs of large-scale production. Furthermore, fast ion conductors typically have large molecular weights, which further reduces the gravimetric energy density of the battery. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention proposes a method for preparing a coated and modified sodium-ion battery cathode material, a cathode sheet, and a sodium-ion battery.

[0005] The technical solution of this invention is implemented as follows: On the one hand, this application provides a coated modified sodium-ion battery cathode material, including layered transition metal oxide NaNi. 0.5 Mn0.5 O2 and NaNi coated with layered transition metal oxides 0.5 Mn 0.5 O2 is coated with a multiferroic material BiFeO3.

[0006] Preferably, the multiferroic material coating layer BiFeO3 and the layered transition metal oxide NaNi are mixed in the following mass ratio. 0.5 Mn 0.5 The O2 ratio is 0.7-2.8:100.

[0007] Secondly, embodiments of the present invention provide a method for preparing the above-mentioned coated and modified sodium-ion battery cathode material, wherein the preparation method is a wet chemical method, specifically including: Step 1): Dissolve the bismuth source and iron source in anhydrous ethanol and stir magnetically under water bath heating; the molar ratio of bismuth source to iron source is 1.05-1.1:0.95-1. Step 2) Add a certain amount of layered transition metal oxide, increase the heating temperature and stir to make the anhydrous ethanol evaporate due to heating. After all the anhydrous ethanol has evaporated, vacuum dry the resulting material. Step 3) The dried material powder is placed in a tube furnace for sintering to obtain the coated and modified sodium-ion battery cathode material.

[0008] The bismuth source is bismuth nitrate pentahydrate, and the iron source is ferric nitrate nonahydrate; The water bath heating temperature is 55-60 ℃, and the magnetic stirring time is 60-70 minutes; the heating stirring time is 20-30 minutes, the heating temperature is 80-85 ℃, the vacuum drying temperature is 80-85 ℃, and the drying time is 6 hours. The sintering temperature is 700-750 ℃, the sintering atmosphere is argon, and the sintering time is 2-3 hours.

[0009] The layered transition metal oxide used in this application is NaNi. 0.5 Mn 0.5 The preparation steps for O2 are as follows: First, 0.04 mol of oxalic acid is dissolved in 150 mL of deionized water. Next, 0.01 mol of anhydrous sodium carbonate, nickel hydroxide, and manganese dioxide are slowly added to the solution sequentially. The mixture is then magnetically stirred for 80 minutes in a water bath at 60°C. The temperature is then raised to 80°C and stirred for another 60 minutes. The resulting product is then transferred to a forced-air drying oven at 80°C and dried for 8 hours. Finally, the product is sintered at 900°C for 12 hours to obtain the layered transition metal oxide NaNi. 0.5 Mn 0.5 O2 powder.

[0010] Thirdly, embodiments of the present invention provide a sodium-ion battery cathode, comprising the coated and modified sodium-ion battery cathode material described in the first aspect above.

[0011] Preparation method: The modified sodium-ion battery positive electrode material, conductive agent and binder are mixed and then dissolved in a solvent to obtain a slurry. Aluminum foil is used as the current collector, and the slurry is uniformly coated on the surface of the current collector. After drying, the sodium-ion battery positive electrode sheet is obtained. The binder is polyvinylidene fluoride, the conductive agent is acetylene black, and the solvent is N-methylpyrrolidone; the mass ratio of the coated and modified sodium-ion battery cathode material to the conductive agent and binder is 75:15:10.

[0012] Fourthly, embodiments of the present invention provide a sodium-ion battery, including the sodium-ion battery positive electrode described in the third aspect above.

[0013] The sodium-ion battery is prepared according to the following steps: Negative electrode sheet: Sodium metal is compressed and cut into sheets; Electrolyte: 1M NaPF6in DME; Separator: GF-A grade Whatman glass fiber; Preparation of sodium-ion batteries: The negative electrode, separator, electrolyte and positive electrode are assembled in sequence, and the sodium-ion battery is obtained by stamping and standing.

[0014] The present invention has the following beneficial effects: The sodium-ion battery cathode material of this application has a BiFeO3 protective layer on its surface, which reduces the contact area of ​​the internal layered transition metal oxides exposed to the electrolyte, thereby reducing the occurrence of interfacial side reactions, improving the cycle stability of the material, reducing residual alkali on the material surface, and improving the air stability of the material. Simultaneously, based on the ferroelectricity and magnetism of BiFeO3, a physical field is introduced at the cathode-electrolyte interface to regulate interfacial ion transport, thereby improving the rate performance of the battery. The sodium-ion battery prepared using the cathode material of this application exhibits a performance of 113-125 mAh·g in the 2-4V voltage range. -1 The discharge specific capacity, at 0.5 C (1 C = 239 mAh g), is... -1 After cycling at a current density of 150 times, the capacity retention rate of the sodium-ion battery is greater than 85%. Attached Figure Description

[0015] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0016] Figure 1 The images show the X-ray diffraction (XRD) patterns of the sodium-ion battery cathode materials in Examples 1, 2, 3 and Comparative Example 1 of this invention.

[0017] Figure 2 This is a scanning electron microscope (SEM) image of the modified sodium-ion battery cathode material provided in Embodiment 2 of the present invention.

[0018] Figure 3 This is a transmission electron microscope (TEM) image of the modified sodium-ion battery cathode material provided in Embodiment 2 of the present invention.

[0019] Figure 4 The images show the Raman spectra of the sodium-ion battery cathode materials in Examples 1, 2, 3 and Comparative Example 1 of this invention.

[0020] Figure 5 The diagram shows the battery cycle performance of Embodiments 1, 2, 3 and Comparative Example 1 of the present invention.

[0021] Figure 6 The figures show the battery rate performance of Embodiments 1, 2, 3 and Comparative Example 1 of the present invention.

[0022] Figure 7 The interfacial ion diffusion coefficients at different peak positions were calculated based on variable scan rate CV curves in Embodiment 2 and Comparative Example 1 of the present invention.

[0023] Figure 8 The pseudocapacitive contribution rate at different scan rates is calculated based on the CV curve of variable scan rate in Embodiment 2 and Comparative Example 1 of the present invention.

[0024] Figure 9 The interfacial ion diffusion coefficient under the discharge state is calculated based on the GITT curve in Embodiment 2 and Comparative Example 1 of the present invention. Detailed Implementation

[0025] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0026] Unless otherwise specified, the experimental methods used in the following experimental examples are conventional methods; the materials and reagents used are commercially available unless otherwise specified.

[0027] The raw material of this application is the layered transition metal oxide NaNi. 0.5 Mn 0.5 The preparation steps for O2 are as follows: First, 0.04 mol of oxalic acid is dissolved in 150 mL of deionized water. Next, 0.01 mol of anhydrous sodium carbonate, nickel hydroxide, and manganese dioxide are slowly added to the solution sequentially. The mixture is then magnetically stirred for 80 minutes in a water bath at 60°C. The temperature is then raised to 80°C and stirred for another 60 minutes. The resulting product is then transferred to a forced-air drying oven at 80°C and dried for 8 hours. Finally, the product is sintered at 900°C for 12 hours to obtain the layered transition metal oxide NaNi. 0.5 Mn 0.5 O2 powder.

[0028] This invention provides a coated modified sodium-ion battery cathode material comprising: layered transition metal oxide NaNi 0.5 Mn 0.5 O2, and a multiferrous BiFeO3 coating layer covering the layered oxide. In the cathode material, the multiferrous coating layer accounts for 0.7%-2.8% of the layered transition metal oxide by mass ratio.

[0029] The cathode material of the present invention can be obtained by a wet chemical method. First, a bismuth source and an iron source are dissolved in anhydrous ethanol and magnetically stirred under water bath heating; then, a layered transition metal oxide is added, the heating temperature is increased and stirred, so that the anhydrous ethanol evaporates due to heating. After all the anhydrous ethanol has evaporated, the resulting material is vacuum dried; finally, the dried material powder is placed in a tube furnace for sintering to obtain the coated and modified sodium-ion battery cathode material.

[0030] In the above method, the bismuth source is bismuth nitrate pentahydrate, and the iron source is ferric nitrate nonahydrate; the water bath heating temperature is 55 ℃, and the magnetic stirring time is 60 minutes; the heating and stirring time is 20 minutes, the heating temperature is 80 ℃, the vacuum drying temperature is 80 ℃, and the drying time is 6 hours; the sintering temperature is 700 ℃, the sintering atmosphere is argon, and the sintering time is 2 hours.

[0031] A multiferroic BiFeO3 protective layer is formed on the material surface using a wet chemical method, reducing the internal contact area exposed to the electrolyte. This reduces interfacial side reactions, improves the material's cycle stability, and also reduces residual alkali on the material surface, enhancing its air stability. Furthermore, based on the ferroelectric and magnetic properties of BiFeO3, a physical field is introduced at the cathode-electrolyte interface to regulate interfacial ion transport, thereby improving the battery's rate performance.

[0032] The cathode material of the present invention, its preparation method, and its performance are further described in detail below through some specific examples.

[0033] Example 1 The preparation method of the coated modified sodium-ion battery cathode material in this embodiment includes the following steps: First, place 10 mL of anhydrous ethanol in a beaker. Then, add bismuth nitrate pentahydrate and ferric nitrate nonahydrate in a molar ratio of 1.05:1 to the beaker. Stir in a water bath at 55 °C for 60 minutes. Next, add NaNi. 0.5 Mn 0.5 O2 powder was added and stirred for 20 minutes. The temperature was then raised to 80 °C until the anhydrous ethanol was completely evaporated. The beaker was then dried in a vacuum drying oven at 80 °C for 6 hours. Finally, annealing was performed in an argon atmosphere in a tube furnace at 700 °C for 2 hours. The mass fraction of the BiFeO3 coating was 0.7%.

[0034] Example 2 The preparation method of the coated modified sodium-ion battery cathode material in this embodiment includes the following steps: First, place 10 mL of anhydrous ethanol in a beaker. Then, add bismuth nitrate pentahydrate and ferric nitrate nonahydrate in a molar ratio of 1.05:1 to the beaker. Stir in a water bath at 55 °C for 60 minutes. Next, add NaNi. 0.5 Mn 0.5 O2 powder was added and stirred for 20 minutes. The temperature was then raised to 80 °C until the anhydrous ethanol was completely evaporated. The beaker was then dried in a vacuum drying oven at 80 °C for 6 hours. Finally, annealing was performed in an argon atmosphere in a tube furnace at a sintering temperature of 700 °C for 2 hours. The mass fraction of the BiFeO3 coating was 1.4%.

[0035] Example 3 The preparation method of the coated modified sodium-ion battery cathode material in this embodiment includes the following steps: First, place 10 mL of anhydrous ethanol in a beaker. Then, add bismuth nitrate pentahydrate and ferric nitrate nonahydrate in a molar ratio of 1.05:1 to the beaker. Stir in a water bath at 55 °C for 60 minutes. Next, add NaNi.0.5 Mn 0.5 O2 powder was added and stirred for 20 minutes. The temperature was then raised to 80 °C until the anhydrous ethanol was completely evaporated. The beaker was then dried in a vacuum drying oven at 80 °C for 6 hours. Finally, annealing was performed in an argon atmosphere in a tube furnace at 700 °C for 2 hours. The mass fraction of the BiFeO3 coating was 2.8%.

[0036] Example 4 The preparation method of the coated modified sodium-ion battery cathode material in this embodiment includes the following steps: First, place 10 mL of anhydrous ethanol in a beaker. Then, add bismuth nitrate pentahydrate and ferric nitrate nonahydrate in a molar ratio of 1.1:1 to the beaker. Stir in a water bath at 60 °C for 65 minutes. Next, add NaNi. 0.5 Mn 0.5 O2 powder was added and stirred for 25 minutes. The temperature was then raised to 83 °C until the anhydrous ethanol was completely evaporated. The beaker was then dried in a vacuum drying oven at 80 °C for 6 hours. Finally, annealing was performed in an argon atmosphere in a tube furnace at a sintering temperature of 730 °C for 2.5 hours. The mass fraction of the BiFeO3 coating layer was 1%.

[0037] Example 5 The preparation method of the coated modified sodium-ion battery cathode material in this embodiment includes the following steps: First, place 10 mL of anhydrous ethanol in a beaker. Then, add bismuth nitrate pentahydrate and ferric nitrate nonahydrate in a molar ratio of 1.07:1 to the beaker. Stir in a water bath at 58 °C for 68 minutes. Next, add NaNi. 0.5 Mn 0.5 O2 powder was added and stirred for 28 minutes. The temperature was then raised to 85 °C until the anhydrous ethanol was completely evaporated. The beaker was then dried in a vacuum drying oven at 80 °C for 6 hours. Finally, annealing was performed in an argon atmosphere in a tube furnace at a sintering temperature of 740 °C for 2.2 hours. The mass fraction of the BiFeO3 coating layer was 2%.

[0038] Comparative Example 1 NaNi, a layered transition metal oxide cathode material, is used directly as the raw material. 0.5 Mn 0.5O2 is used as the positive electrode material for sodium-ion batteries. The specific preparation steps are as follows: First, 0.04 mol of oxalic acid is dissolved in 150 mL of deionized water. Next, 0.01 mol of anhydrous sodium carbonate, nickel hydroxide, and manganese dioxide are slowly added to the solution sequentially. The mixture is then magnetically stirred for 80 minutes in a water bath at 60°C. The temperature is then raised to 80°C and stirred for another 60 minutes. The resulting product is then transferred to a forced-air drying oven at 80°C and dried for 8 hours. Finally, it is sintered at 900°C for 12 hours to obtain the layered transition metal oxide NaNi. 0.5 Mn 0.5 O2 powder.

[0039] Implementation Results Example The X-ray diffraction test results of the sodium-ion battery cathode materials prepared in Examples 1, 2, 3 and Comparative Example 1 are as follows: Figure 1 As shown. The diffraction peaks of Comparative Example 1 and Examples 1, 2, and 3 are not only similar to those of NaNi 0.5 Mn 0.5 The O2 standard card PDF#54-0887 corresponds to this, and the peaks are strong and sharp, indicating good crystallinity. Furthermore, no impurity peaks appear after BiFeO3 coating, indicating no phase transition occurred. Simultaneously, in the magnified local image, the characteristic peaks of BiFeO3 gradually increase with increasing BiFeO3 coating amount, indicating successful coating.

[0040] Scanning electron microscopy (SEM) was performed on the cathode material of Example 2, and the results are as follows: Figure 2 As shown. Example 2 is a spherical shape assembled from irregular particles, with a size of approximately 9-11 micrometers.

[0041] Transmission electron microscopy (TEM) was performed on the cathode material of Example 3, and the results are as follows: Figure 3 As shown, a uniform and dense coating layer can be clearly seen on the surface of the material, with a thickness of approximately 2-3 nanometers.

[0042] The Raman spectroscopy results of the materials from Examples 1, 2, 3, and Comparative Example 1 are as follows: Figure 4 As shown, after BiFeO3 coating, the intensity of the Na2CO3 signal peak in the Raman spectra of Examples 1, 2, and 3 decreased, indicating that the residual alkali content on the material surface decreased and the air stability of the material improved.

[0043] The sodium-ion battery cathode materials prepared in each embodiment and comparative example were mixed with acetylene black and polyvinylidene fluoride binder at a mass ratio of 75:15:10, and N-methylpyrrolidone solution was added until a slurry was formed in a dry environment at room temperature. The prepared slurry was uniformly coated on the aluminum foil of the current collector and dried at 100 °C under vacuum for 12 hours before being cut into circular electrode sheets with a diameter of 8 mm.

[0044] The assembly of the simulated battery in this invention was carried out in a glove box under an argon atmosphere. Sodium metal was used as the counter electrode, a GF-A grade Whatman glass fiber separator was used, and a 1M NaPF6 in DME solution was used as the electrolyte to assemble a CR2032 coin cell. A constant current charge-discharge mode was used at 0.5 C (1 C = 239 mAh g⁻¹). -1 Charge-discharge tests were conducted at the specified current density. The test conditions were: discharge cut-off voltage of 2 V and charge cut-off voltage of 4 V.

[0045] The cycle performance results of the batteries assembled with the cathode materials obtained in each embodiment and comparative example are as follows: Figure 5 As shown in Table 1, the comparative results indicate that the multiferroic material BiFeO3 can serve as a protective layer, reducing the contact area of ​​the material's interior exposed to the electrolyte, reducing interfacial side reactions, and improving the material's cycle stability. The modified sodium-ion battery cathode material proposed in this invention exhibits high reversible capacity; the capacity retention rate of each embodiment after 150 cycles at a current density of 0.5 C is higher than 85%, demonstrating excellent cycle stability.

[0046] Table 1. Cyclic performance test results of comparative and example samples. The rate performance results of the batteries assembled with the cathode materials obtained in each embodiment and comparative example are as follows: Figure 6 As shown in Table 2, the comparative results demonstrate that by introducing a physical field at the cathode-electrolyte interface based on the ferroelectricity and magnetism of BiFeO3, the interfacial ion transport is regulated, thereby improving the rate performance of the material. The coated and modified sodium-ion battery cathode material proposed in this invention exhibits excellent rate performance, with each embodiment showing a capacity greater than 40% at 20 C current density / 0.1 C current density.

[0047] Table 2. Rate performance test results of comparative and example samples. Combination Figure 5 Figure 6 It can be seen that the amount of BiFeO3 coating is highly correlated with the electrochemical performance of the battery. Because BiFeO3 has poor electrochemical activity, excessive coating (Example 3) will not only reduce the electrochemical performance of NaNi...0.5 Mn 0.5 The electrochemical activity of O2 can also hinder interfacial charge transport. Furthermore, insufficient coating (Example 1) can lead to NaNi... 0.5 Mn 0.5 O2 was not fully covered, which prevented the protective effect from being fully activated, ultimately resulting in low capacity and poor rate performance.

[0048] This invention uses the CHI660E electrochemical workstation manufactured by Shanghai Chenhua Instruments Co., Ltd. to test the cyclic voltammetry (CV) curves of the battery. The test voltage range is 2-4 V, and the scan rates are 0.2, 0.4, 0.6, 0.8, and 1.0 mV s, respectively. -1 The sodium ion diffusion coefficient (D) was calculated based on cyclic voltammetry curves at different scan rates. Na+ When D Na+ Calculate using the following formula: in, I p It is the peak current (A). n This refers to the number of electrons that participate in the reaction. F It is Faraday's constant (96485 C mol) -1 ), C Expressed as volume concentration in moles. S It is the electrode area. R The gas constant is expressed as 8.314 J mol. -1 K -1 ), T It is absolute temperature. v The potential scan rate (Vs) is measured in volts per second. -1 ).

[0049] The interfacial ion diffusion coefficients of Comparative Example 1 and Example 2 were calculated using CV curves at different scan rates. The results of the interfacial ion diffusion coefficients calculated at different peak positions are shown below. Figure 7 As shown in Table 3, the comparative results indicate that the interfacial ion diffusion coefficient is improved at different peak positions based on the ferroelectricity and magnetism of BiFeO3, demonstrating that the interfacial physical field can effectively regulate interfacial ion transport.

[0050] Table 3 Comparison of interfacial ion diffusion coefficients at different peak positions (cm) 2 s -1 )×10 -11 The capacitance contribution rate of the electrode material can be calculated using the following formula: The current response can be divided into two parts: k 1 v Reflecting the contribution of capacitance, while k 2 v 1 / 2 This indicates the diffusion contribution. Pseudocapacitance is a rapid, reversible redox reaction at or near the material surface. Pseudocapacitive reactions are limited to the material surface or near the surface, and the ion diffusion and electron transfer kinetics are extremely fast.

[0051] Table 4. Pseudocapacitance contribution rate (%) at different scan rates The pseudocapacitive contribution rates of Comparative Example 1 and Example 2 at different scan rates, calculated using variable scan CV curves, are as follows: Figure 8 As shown in Table 4, the comparative results indicate that the pseudocapacitive contribution rate of the material at different scan rates is improved after BiFeO3 coating, especially at 1.0 mV / s. -1 At the specified scan rate, the pseudocapacitive contribution rate of the material increased from 85.7% to 92.9%, an increase of 7.2%. This increase in pseudocapacitive contribution rate facilitates rapid electrochemical reactions at high current densities, enabling fast charge and discharge and effectively improving the battery's rate performance.

[0052] This invention uses the Blue Lightning test system to perform GITT testing on the battery. The test voltage range is 2-4 V, and the applied repetitive current pulse is a 0.05 C current sustained for 20 minutes, with a relaxation time of 2 hours. The results are calculated based on the GITT curve. D Na+ When, calculate according to the following formula: The results of calculating the interfacial ion diffusion coefficients under the discharge state of Comparative Example 1 and Example 2 using GITT testing are as follows: Figure 9 As shown. The average discharge ion diffusion coefficient of Comparative Example 1, calculated based on the discharge state using GITT, is 3.37 × 10⁻⁶. -11 cm 2 s -1 After being coated with BiFeO3, the average discharge ion diffusion coefficient of Example 2 was 3.17 × 10⁻⁶. -10 cm 2 s -1 The average discharge ion diffusion coefficient increased by 9.4 times, indicating that the interface physical field can effectively regulate the interface ion transport.

[0053] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A coated and modified sodium-ion battery cathode material, characterized in that, Including layered transition metal oxides NaNi 0.5 Mn 0.5 O2 and NaNi coated with layered transition metal oxides 0.5 Mn 0.5 O2 is coated with a multiferroic material BiFeO3.

2. The coated and modified sodium-ion battery cathode material according to claim 1, characterized in that: The multiferroic material coating layer BiFeO3 and the layered transition metal oxide NaNi 0.5 Mn 0.5 The mass ratio of O2 is 0.7-2.8:

100.

3. The method for preparing the sodium-ion battery cathode material according to claim 1 or 2, characterized in that, The steps are as follows: (1) Dissolve bismuth nitrate pentahydrate and ferric nitrate nonahydrate in anhydrous ethanol, stir magnetically under water bath heating, then add layered transition metal oxide, heat and stir, and remove solvent to obtain solid material; (2) The solid material in step (1) is vacuum dried and sintered at high temperature to obtain a sodium-ion battery cathode material with BiFeO3 coating layer.

4. The method for preparing the sodium-ion battery cathode material according to claim 3, characterized in that: In step (1), the molar ratio of bismuth nitrate pentahydrate to ferric nitrate nonahydrate is 1.05-1.1:0.95-1; the layered transition metal oxide is NaNi. 0.5 Mn 0.5 O2.

5. The method for preparing the sodium-ion battery cathode material according to claim 4, characterized in that: The water bath heating temperature is 55-60℃, and the magnetic stirring time is 60-70 minutes; the temperature for heating and stirring is 80-85℃, and the time is 20-30 minutes.

6. The method for preparing the sodium-ion battery cathode material according to claim 3, characterized in that: In step (2), the high-temperature sintering temperature is 700-750℃, the atmosphere is argon, and the time is 2-3 hours.

7. The method for preparing the sodium-ion battery cathode material according to claim 6, characterized in that: The mass ratio of the coating layer to the layered transition metal oxide on the sodium-ion battery cathode material is 0.7-2.8:

100.

8. A sodium-ion battery positive electrode sheet prepared using the sodium-ion battery positive electrode material according to claim 1 or 2.

9. The method for preparing the positive electrode sheet of a sodium-ion battery according to claim 8, characterized in that, The steps are as follows: Sodium-ion battery positive electrode material, conductive agent and binder are mixed and then dissolved in solvent to obtain slurry. Aluminum foil is used as current collector, and the slurry is uniformly coated on the surface of the current collector. After drying, sodium-ion battery positive electrode sheet is obtained.

10. A sodium-ion battery, characterized in that: The sodium-ion battery positive electrode sheet as described in claim 8 is used as the positive electrode of the battery.

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