Preparation method of NaCrO2 single-crystal positive electrode material

Abstract

The invention discloses a preparation method of a NaCrO2 single-crystal positive electrode material, and belongs to the technical field of preparation of positive electrode materials. The invention provides a novel method for preparing a single-crystal positive electrode material for a high-performance sodium ion battery, which comprises the following steps: firstly, mixing large-size single-crystal Cr2O3 serving as a raw material (1-30 microns) with a sodium source, and carrying out one-step high-temperature roasting to obtain the NaCrO2 single-crystal positive electrode material. And the NaCrO2 single-crystal positive electrode material prepared by the method has few single-crystal structure defects and has a better long-range ordered layered structure.

Description

Technical Field

[0001] The present invention relates to the technical field of cathode material preparation, in particular to a method for preparing a NaCrO2 single crystal cathode material. Background Art

[0002] Single crystal cathode materials have many advantages, such as high mechanical strength, high compaction density, excellent electrochemical performance, thermal stability, and air stability, which can significantly improve the overall performance of batteries. At present, the synthesis methods of single crystal cathode materials for sodium ion batteries mainly include the following: (1) High temperature sintering method: By increasing the sintering temperature to promote single crystal growth, it is necessary to increase the amount of lithium salt or sodium salt to compensate for the volatilization of lithium or sodium at high temperature, and it will cause the generated single crystal particles to agglomerate; (2) Molten salt method: Adding low melting point fluxing salt to reduce the sintering temperature, accelerate ion transport and grain growth through liquid mass transfer, but there are problems with molten salt screening and cleaning of single crystal cathode materials; (3) Multi-step calcination method: It can avoid the negative effects of one-step high temperature sintering, such as grain agglomeration and impurity generation, but there are problems such as complex process and high time cost. Moreover, the above methods all convert polycrystalline sodium-ion battery positive electrode materials into single-crystalline positive electrode materials through subsequent heat treatment. However, due to the anisotropy between the particles of polycrystalline sodium-ion battery positive electrode materials, the prepared single-crystalline positive electrode materials contain more lattice defects and limit the formation of their long-range ordered structures. Summary of the Invention

[0003] The present invention aims to provide a method for preparing a single-crystal NaCrO2 cathode material to address the aforementioned problems of the prior art. The method of the present invention solves the current problems in preparing single-crystal cathode materials for sodium-ion batteries, enabling simple and efficient preparation of single-crystal cathode materials for sodium-ion batteries.

[0004] To achieve the above object, the present invention provides the following solutions:

[0005] One of the technical solutions of the present invention is a method for preparing large-sized single crystal Cr2O3, comprising the following steps:

[0006] The trivalent chromium salt and the alkaline precipitant are mixed in the form of a solution and reacted at a constant temperature to obtain a precipitation product, trivalent chromium hydroxide;

[0007] The large-sized single crystal Cr2O3 is obtained by calcining the trivalent chromium hydroxide.

[0008] Furthermore, the trivalent chromium salt is a water-soluble trivalent chromium salt; the water-soluble trivalent chromium salt includes at least one of chromium trichloride, chromium nitrate, chromium sulfate, chromium acetate, chromium citrate and potassium chromium sulfate;

[0009] The alkaline precipitant includes at least one of sodium hydroxide, potassium hydroxide, ammonia water, sodium carbonate and potassium carbonate;

[0010] The temperature of the isothermal reaction is 20 to 80° C., and the time is 2 to 12 hours;

[0011] The calcination temperature is 1200° C. to 1400° C., and the calcination time is 2 to 24 hours.

[0012] Furthermore, the diameter of the large-sized single crystal Cr2O3 is 1 to 30 μm.

[0013] The second technical solution of the present invention: a large-sized single crystal Cr2O3 prepared by the above-mentioned preparation method.

[0014] The third technical solution of the present invention: an application of the above-mentioned large-size single crystal Cr2O3 in the preparation of positive electrode materials for sodium ion batteries.

[0015] Technical solution 4 of the present invention: A method for preparing a NaCrO2 single crystal positive electrode material, comprising the following steps:

[0016] The large-sized single crystal Cr2O3 and a sodium source are mixed and then calcined to obtain the NaCrO2 single crystal positive electrode material.

[0017] Under high temperature, the sodium source will form active sodium oxide, which will gradually embed into the chromium oxide lattice under the induction of the long-range ordered single crystal structure of large-scale single crystal chromium oxide, and finally form an ordered structure to form sodium chromite single crystal.

[0018] Further, the sodium source includes at least one of NaOH, Na2CO3, NaHCO3, sodium acetate and sodium citrate;

[0019] The molar ratio of Cr in the large-sized single crystal Cr2O3 to Na in the sodium source is 1:(1-1.05);

[0020] The calcination temperature is 700° C. to 1200° C., and the calcination time is 2 to 24 hours.

[0021] The fifth technical solution of the present invention: a NaCrO2 single crystal positive electrode material prepared by the above preparation method.

[0022] Technical solution six of the present invention: an application of the above-mentioned NaCrO2 single crystal positive electrode material in the preparation of sodium ion batteries.

[0023] Technical solution seven of the present invention: A method for reducing internal defects of NaCrO2 single crystal positive electrode material, comprising the following steps:

[0024] The large-sized single crystal Cr2O3 and a sodium source are mixed and calcined to obtain a NaCrO2 single crystal positive electrode material.

[0025] Further, the sodium source includes at least one of NaOH, Na2CO3, NaHCO3, sodium acetate and sodium citrate;

[0026] The molar ratio of Cr in the large-sized single crystal Cr2O3 to Na in the sodium source is 1:(1-1.05);

[0027] The calcination temperature is 700-1200° C., and the calcination time is 2-24 hours.

[0028] The technical principles and technical problems overcome in preparing NaCrO2 single crystal cathode materials (i.e. single crystal NaCrO2 cathode materials) from large-size chromium trioxide:

[0029] The technical principle of existing single-crystal positive electrode materials is mainly to use nano-sized battery materials as precursors and achieve crystal particle growth through high-temperature calcination. The main defect is that in the process of growing single-crystalline positive electrode materials from nano-sized raw materials, due to the small particle size of the raw materials, there are a large number of grain boundaries between the particles and the anisotropy of the lattice between the particles, so that a perfect single crystal structure cannot be obtained through high-temperature treatment, and the prepared single-crystalline positive electrode materials have crystal defects.

[0030] The present invention uses complete large-sized single-crystal particle oxide (chromium oxide) as a precursor, which is mixed with a sodium source and then subjected to a high-temperature process to form active Na ions that are gradually inserted into the crystal structure of the single-crystal particle oxide, maintaining the long-range ordered structure of the single-crystal particle oxide. This overcomes the problem in the prior art that a large number of crystal defects exist inside the particles as the particles grow from small to large, causing cracking inside the particles after multiple charges and discharges.

[0031] The present invention discloses the following technical effects:

[0032] (1) The method for preparing large-sized single crystal Cr2O3 of the present invention is simple, and the size of the prepared single crystal Cr2O3 is 1 to 30 μm.

[0033] (2) The preparation method of the NaCrO2 single crystal positive electrode material of the present invention is simple and efficient, and realizes direct conversion from single crystal to single crystal, avoiding the problems existing in high-temperature sintering method, molten salt method and multi-step calcination method.

[0034] (3) The present invention proposes a new method for preparing a single-crystal cathode material for high-performance sodium-ion batteries. First, large-sized single-crystal Cr2O3 (size 1-30 μm) is used as a raw material, mixed with a sodium source, and subjected to a high-temperature calcination step to obtain a single-crystal NaCrO2 cathode material. The single-crystal NaCrO2 cathode material prepared by the method of the present invention has fewer single-crystal structural defects and a better long-range ordered layered structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0036] Figure 1 This is a SEM image of the single crystal chromium oxide prepared in Example 1 (magnified 500 times);

[0037] Figure 2 This is a SEM image of the single crystal chromium oxide prepared in Example 1 (magnified 500 times);

[0038] Figure 3 This is an SEM image of the single-crystal NaCrO2 positive electrode material prepared in Example 1 (magnified 500 times);

[0039] Figure 4 This is an SEM image of the single-crystallized NaCrO2 positive electrode material prepared in Example 1 (magnified 5000 times);

[0040] Figure 5 This is an SEM image of the single-crystal NaCrO2 positive electrode material prepared in Example 1 (magnified 130,000 times);

[0041] Figure 6 The cycling performance of a sodium ion battery assembled with the single crystal NaCrO2 cathode material prepared in Example 1 at 10C is shown;

[0042] Figure 7 The rate performance of a sodium ion battery assembled with the single crystal NaCrO2 positive electrode material prepared in Example 1;

[0043] Figure 8 This is the SEM image of the nano-chromium oxide prepared in Comparative Example 1 (magnified 50,000 times);

[0044] Figure 9 This is an SEM image of the nanoscale NaCrO2 positive electrode material prepared in Comparative Example 1 (magnified 50,000 times);

[0045] Figure 10 The cycling performance of the sodium ion battery prepared with the nano-scale NaCrO2 positive electrode material prepared in Comparative Example 1 under 2C conditions;

[0046] Figure 11 The figure shows the rate performance of the sodium ion battery prepared using the nano-scale NaCrO2 positive electrode material prepared in Comparative Example 1. DETAILED DESCRIPTION

[0047] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as limiting the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0048] It should be understood that the terms described herein are intended only to describe particular embodiments and are not intended to limit the present invention. In addition, for numerical ranges herein, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Each smaller range between any intermediate value within a stated value or stated range and any other stated value or intermediate value within the stated range is also encompassed by the present invention. The upper and lower limits of these smaller ranges may be independently included or excluded within the scope.

[0049] Unless otherwise indicated, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art. Although only preferred methods and materials are described herein, any methods and materials similar or equivalent to those described herein may also be used in the practice or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials associated with the documents. In the event of any conflict with any incorporated document, the contents of this specification shall prevail.

[0050] It will be apparent to those skilled in the art that various modifications and variations may be made to the specific embodiments described herein without departing from the scope or spirit of the invention. Other embodiments will be apparent to those skilled in the art from the description of the invention. The description and examples are intended to be exemplary only.

[0051] The words “include,” “including,” “have,” “contain,” etc. used in this document are open-ended terms, meaning including but not limited to.

[0052] It should be pointed out that the matters not described in detail in the present invention are conventional operating means in this field and are not the focus of the present invention.

[0053] Example 1

[0054] A method for preparing NaCrO2 single crystal positive electrode material:

[0055] (1) Dissolve 26.6 g of CrCl3·6H2O in 200 mL of water to prepare a trivalent chromium solution;

[0056] An alkaline solution was prepared by dissolving 14.4 g of NaOH in 50 mL of water.

[0057] (2) Add the trivalent chromium solution into a 500 mL reaction tank, and add the alkaline solution dropwise into the reaction tank under stirring to mix with the Cr in the solution.3+ A precipitation reaction was carried out, and after the alkaline solution was added dropwise, the mixed slurry was kept at a constant temperature of 50° C. for 6 hours to obtain a solid-liquid mixed slurry.

[0058] (3) The solid-liquid mixed slurry is filtered and fully washed with secondary water until the pH value of the filtrate is neutral, and then the filter cake is dried in an oven to obtain trivalent chromium hydroxide powder.

[0059] (4) The trivalent chromium hydroxide powder is calcined at 1300° C. for 6 h, cooled naturally, washed, and dried to obtain single crystal chromium oxide.

[0060] (5) Single crystal chromium oxide and Na2CO3 were mixed in a Cr:Na molar ratio of 1:1.05, and then calcined at 1100°C for 10 h in an Ar atmosphere to obtain a single crystal NaCrO2 positive electrode material.

[0061] The SEM image of the single crystal chromium oxide prepared in Example 1 (magnified 500 times) is shown in FIG. Figure 1 ; The SEM image of the single crystal chromium oxide prepared in Example 1 (magnified 500 times) is shown in Figure 2 ; The SEM image (magnified 500 times) of the single crystal NaCrO2 cathode material prepared in Example 1 is shown Figure 3 ; The SEM image (magnified 5000 times) of the single crystal NaCrO2 cathode material prepared in Example 1 is shown Figure 4 ; The SEM image of the single crystal NaCrO2 cathode material prepared in Example 1 (magnified 130,000 times) is shown Figure 5 .

[0062] from Figures 1-2 It can be seen that the particle size of the single crystal chromium oxide prepared in Example 1 is mostly above 1 micron, and the particles are large and monodisperse.

[0063] from Figures 3-5 It can be seen that the particle size of the single-crystal NaCrO2 positive electrode material prepared in Example 1 is similar to that of single-crystal chromium oxide particles, which is large and monodisperse, and a long-range ordered layered structure is differentiated inside the particles.

[0064] Example 2

[0065] A method for preparing NaCrO2 single crystal positive electrode material:

[0066] (1) Dissolve 25 g of Cr2(SO4)3·6H2O in 200 mL of water to prepare a trivalent chromium solution;

[0067] An alkaline solution was prepared by dissolving 20.2 g of KOH in 50 mL of water.

[0068] (2) Add the trivalent chromium solution into a 500 mL reaction tank, and add the alkaline solution dropwise into the reaction tank under stirring to mix with the Cr in the solution. 3+ A precipitation reaction was carried out, and after the alkaline solution was added dropwise, the mixed slurry was kept at a constant temperature of 30° C. for 12 hours to obtain a solid-liquid mixed slurry.

[0069] (3) The solid-liquid mixed slurry is filtered and fully washed with secondary water until the pH value of the filtrate is neutral, and then the filter cake is dried in an oven to obtain trivalent chromium hydroxide powder.

[0070] (4) The trivalent chromium hydroxide powder is calcined at 1350° C. for 4 hours, cooled naturally, washed, and dried to obtain single crystal chromium oxide with a size of 2 to 30 μm.

[0071] (5) Single crystal chromium oxide and NaOH were mixed in a Cr:Na molar ratio of 1:1.05, and then calcined at 1000 °C for 12 h in an Ar atmosphere to obtain a single crystal NaCrO2 positive electrode material with a size of 2 to 30 μm.

[0072] Example 3

[0073] A method for preparing NaCrO2 single crystal positive electrode material:

[0074] (1) Dissolve 40 g of Cr(NO3)3·9H2O in 200 mL of water to prepare a trivalent chromium solution;

[0075] An alkaline solution was prepared by dissolving 14.4 g of NaOH in 50 mL of water.

[0076] (2) Add the trivalent chromium solution into a 500 mL reaction tank, and add the alkaline solution dropwise into the reaction tank under stirring to mix with the Cr in the solution. 3+ A precipitation reaction was carried out, and after the alkaline solution was added dropwise, the mixed slurry was kept at a constant temperature of 30° C. for 12 hours to obtain a solid-liquid mixed slurry.

[0077] (3) The solid-liquid mixed slurry is filtered and fully washed with secondary water until the pH value of the filtrate is neutral, and then the filter cake is dried in an oven to obtain trivalent chromium hydroxide powder.

[0078] (4) The trivalent chromium hydroxide powder is calcined at 1200° C. for 24 hours, cooled naturally, washed, and dried to obtain single crystal chromium oxide with a size of 1 to 15 μm.

[0079] (5) Single crystal chromium oxide and sodium acetate were mixed in a Cr:Na molar ratio of 1:1.05, and then calcined at 900 °C for 20 h in an Ar atmosphere to obtain a single crystal NaCrO2 positive electrode material with a size of 1 to 15 μm.

[0080] Example 4

[0081] A method for preparing NaCrO2 single crystal positive electrode material:

[0082] (1) Dissolve 50 g of KCr(SO4)2·12H2O in 200 mL of water to prepare a trivalent chromium solution;

[0083] An alkaline solution was prepared by dissolving 20.2 g of KOH in 50 mL of water.

[0084] (2) Add the trivalent chromium solution into a 500 mL reaction tank, and add the alkaline solution dropwise into the reaction tank under stirring to mix with the Cr in the solution. 3+ A precipitation reaction was carried out, and after the alkaline solution was added dropwise, the mixed slurry was kept at a constant temperature of 30° C. for 12 hours to obtain a solid-liquid mixed slurry.

[0085] (3) The solid-liquid mixed slurry is filtered and fully washed with secondary water until the pH value of the filtrate is neutral, and then the filter cake is dried in an oven to obtain trivalent chromium hydroxide powder.

[0086] (4) The trivalent chromium hydroxide powder is calcined at 1250° C. for 18 h, cooled naturally, washed, and dried to obtain single crystal chromium oxide with a size of 2 to 20 μm.

[0087] (5) Single crystal chromium oxide and sodium citrate were mixed in a Cr:Na molar ratio of 1:1.05, and then calcined at 800 °C for 24 h in an Ar atmosphere to obtain a single crystal NaCrO2 positive electrode material with a size of 2 to 20 μm.

[0088] Effect Example 1

[0089] The single crystal NaCrO2 positive electrode material prepared in Example 1 was assembled into a sodium ion battery. The specific preparation steps are as follows:

[0090] Single-crystal NaCrO2 cathode material, polyvinylidene fluoride (PVDF), and acetylene black were ground and mixed in a mass ratio of 8:1:1. N-methylpyrrolidone (NMP) was added and the mixture was shaken in an oscillating ball mill at 1500 rpm for 30 minutes to form a slurry, which was then evenly coated on aluminum foil. The coated electrode was dried in a vacuum drying oven at 60°C for 2 hours, then heated to 120°C and dried for 10 hours. After drying, it was cut into 12mm diameter discs. The mass of the active material in the electrode (i.e., single-crystal NaCrO2 cathode material) was 1.5-2.5 mg cm -3A sodium metal sheet was used as the counter electrode, a glass fiber (GF / D) as the separator, and a 1.0 M sodium perchlorate (NaClO4) solution as the electrolyte. CR2025 button-type cells were assembled in an argon-filled glove box (water and oxygen contents were both less than 0.01 ppm). The assembled half-cell was left to rest for 12 hours.

[0091] Cyclic performance test, the specific steps include:

[0092] The blue battery test system was used for constant current charge and discharge, with a voltage range of 2.0-3.6V (relative to Na + / Na), the test temperature is 25 ° C, where 1C = 120mAg -1 . Carry out cycle performance test.

[0093] The cycling performance test was carried out under 10C conditions. The results are shown in Figure 6 ; and the rate performance of the sodium ion battery was measured, and the results are shown in Figure 7 .

[0094] from Figure 6 It can be seen that the single-crystal NaCrO2 positive electrode material prepared in Example 1 has a capacity retention rate of more than 80% after 1000 cycles of charge and discharge under 10C conditions, showing extremely high cycle stability.

[0095] from Figure 7 It can be seen from the figure that the single-crystal NaCrO2 positive electrode material prepared in Example 1 still has a capacity retention rate of more than 80% under the 40C rate condition, showing excellent rate performance.

[0096] Comparative Example 1

[0097] Preparation method of nano-scale NaCrO2 positive electrode material:

[0098] (1) Dissolve 26.6 g of CrCl3·6H2O in 200 mL of water to prepare a trivalent chromium solution;

[0099] An alkaline solution was prepared by dissolving 14.4 g of NaOH in 50 mL of water.

[0100] (2) Add the trivalent chromium solution into a 500 mL reaction tank, and add the alkaline solution dropwise into the reaction tank under stirring to mix with the Cr in the solution. 3+ A precipitation reaction was carried out, and after the alkaline solution was added dropwise, the mixed slurry was kept at a constant temperature of 50° C. for 6 hours to obtain a solid-liquid mixed slurry.

[0101] (3) The solid-liquid mixed slurry is filtered and fully washed with secondary water until the pH value of the filtrate is neutral, and then the filter cake is dried in an oven to obtain trivalent chromium hydroxide powder.

[0102] (4) The trivalent chromium hydroxide powder is calcined at 900° C. for 2 h, cooled naturally, washed, and dried to obtain nano-chromium trioxide.

[0103] (5) Nano-chromium oxide and Na2CO3 were mixed at a Cr:Na molar ratio of 1:1.05, and then calcined at 1100°C for 10 h in an Ar atmosphere to obtain a nano-scale NaCrO2 positive electrode material.

[0104] The SEM image of the nano chromium trioxide prepared in Comparative Example 1 (magnified 50,000 times) is shown in FIG. Figure 8 ; SEM image of nanoscale NaCrO2 cathode material prepared in Comparative Example 1 (magnified 50,000 times) Figure 9 .

[0105] Effect Example 2

[0106] The nanoscale NaCrO2 cathode material prepared in Comparative Example 1 was assembled into a sodium ion battery. The specific preparation steps are as follows:

[0107] Nanoscale NaCrO2 cathode material, polyvinylidene fluoride (PVDF), and acetylene black were ground and mixed in a mass ratio of 8:1:1. N-methylpyrrolidone (NMP) was added and the mixture was shaken in an oscillating ball mill at 1500 rpm for 30 minutes to form a slurry, which was then evenly coated on aluminum foil. The coated electrode was dried in a vacuum drying oven at 60°C for 2 hours, then heated to 120°C and dried for 10 hours. After drying, the electrode was cut into 12mm diameter discs. The mass of the active material in the electrode (i.e., the nanoscale NaCrO2 cathode material) was 1.5-2.5 mg cm -3 A sodium metal sheet was used as the counter electrode, a glass fiber (GF / D) as the separator, and a 1.0 M sodium perchlorate solution (NaClO4) as the electrolyte. CR2025 button-type cells were assembled in an argon-filled glove box (water and oxygen contents were both less than 0.01 ppm). The assembled half-cell was left to rest for 12 hours.

[0108] Cyclic performance test, the specific steps include:

[0109] The blue battery test system was used for constant current charge and discharge, with a voltage range of 2.0-3.6V (relative to Na + / Na), the test temperature is 25 ° C, where 1C = 120mAg -1 . Carry out cycle performance test.

[0110] The cycling performance test was carried out under 2C conditions. The results are shown in Figure 10 ; and the rate performance of the sodium ion battery was measured, and the results are shown in Figure 11 .

[0111] from Figures 10 and 11 It can be seen that the cycle stability and rate performance of the nano-scale NaCrO2 positive electrode material prepared in Comparative Example 1 are significantly lower than those of the single crystal NaCrO2 positive electrode material prepared in Example 1.

[0112] The embodiments described above are merely descriptions of preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Without departing from the spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by persons skilled in the art should fall within the scope of protection defined by the claims of the present invention.

Claims

1. A method for preparing large-sized single crystal Cr2O3, characterized in that: The following steps are involved: The trivalent chromium salt and the alkaline precipitant are mixed in the form of a solution and reacted at a constant temperature to obtain a precipitation product, trivalent chromium hydroxide; The large-sized single crystal Cr2O3 is obtained by calcining the trivalent chromium hydroxide.

2. The preparation method according to claim 1, characterized in that The trivalent chromium salt is a water-soluble trivalent chromium salt; and / or, the alkaline precipitant comprises at least one of sodium hydroxide, potassium hydroxide, aqueous ammonia, sodium carbonate and potassium carbonate; And / or, the temperature of the isothermal reaction is 20 to 80°C; And / or, the calcination temperature is 1200-1400° C., and the calcination time is 2-24 hours.

3. The preparation method according to claim 1, characterized in that The diameter of the large-sized single crystal Cr2O3 is 1 to 30 μm.

4. A large-sized single crystal Cr2O3 prepared by the preparation method according to any one of claims 1 to 3.

5. Use of the large-sized single crystal Cr2O3 according to claim 4 in the preparation of positive electrode materials for sodium ion batteries.

6. A method for preparing a NaCrO2 single crystal positive electrode material, characterized in that: The following steps are involved: The large-sized single crystal Cr2O3 according to claim 4 is mixed with a sodium source and then calcined to obtain the NaCrO2 single crystal positive electrode material.

7. The preparation method according to claim 6, characterized in that The sodium source includes at least one of NaOH, Na2CO3, NaHCO3, sodium acetate and sodium citrate; and / or, the molar ratio of Cr in the large-sized single crystal Cr2O3 to Na in the sodium source is 1:(1-1.05); And / or, the calcination temperature is 700-1200° C. and the calcination time is 2-24 hours.

8. A NaCrO2 single crystal positive electrode material prepared by the preparation method according to any one of claims 6 to 7.

9. Use of the NaCrO2 single crystal positive electrode material according to claim 8 in the preparation of sodium ion batteries.

10. A method for reducing internal defects of NaCrO2 single crystal cathode material, characterized in that: The following steps are involved: The large-sized single crystal Cr2O3 according to claim 4 is mixed with a sodium source and then calcined to obtain a NaCrO2 single crystal positive electrode material.

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