Carbon quantum dots coated mixed sodium-ion battery cathode material, and preparation method and application thereof

Abstract

The application discloses a carbon quantum dot coated mixed-phase sodium ion battery positive electrode material and a preparation method and application thereof, and belongs to the technical field of sodium ion batteries. The positive electrode material is a carbon quantum dot coated mixed-phase nickel-iron-manganese-based sodium ion battery positive electrode material, and the mixed-phase structure is a P2 and O3 phase mixture. The carbon quantum dot coating improves the overall conductivity of the material, inhibits the side reaction of the material with electrolyte, and enables the positive electrode material to have super-high rate performance and cycle stability, so that the positive electrode material can be used as a sodium ion battery positive electrode material. The method of the application is used for coating in a liquid phase environment, ensures uniform coating of the carbon quantum dots on the surface of the mixed-phase sodium ion battery positive electrode material, is simple, and has good coating effect. The obtained carbon quantum dot coated mixed-phase sodium ion battery positive electrode material has good crystallinity and stable structure.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of sodium ion batteries, in particular to a carbon quantum dot coated mixed-phase sodium ion battery positive electrode material and a preparation method and application thereof. BACKGROUND

[0002] Sodium ion batteries belong to secondary batteries, and secondary batteries have attracted more and more attention as one of the energy storage systems with the highest energy conversion efficiency. Among a large number of secondary battery systems, sodium ion batteries have the advantages of abundant raw material sodium source, wide distribution and low cost. Sodium does not have an alloying reaction with aluminum, and the current collector of the positive electrode and the negative electrode of the sodium ion battery can use inexpensive aluminum foil, which can further reduce the cost and has no over-discharge problem. Sodium ion batteries have excellent rate performance and high and low temperature performance; sodium ion batteries have the advantages of not catching fire, not exploding and good safety performance in safety tests. Therefore, when the energy density requirement is not high, sodium ion batteries are excellent substitutes.

[0003] One of the key components of sodium ion batteries is the positive electrode material. At present, the sodium ion battery positive electrode material has metal layered oxides, polyanions and prussian blue, among which, the layered oxide has a high theoretical capacity and is easy to synthesize, and is one of the most promising sodium ion battery positive electrode materials at present. The layered oxide sodium electrode material has a relatively stable charge and discharge platform, is low in cost and environmentally friendly, and has become a research hotspot at present. How to improve the cycle stability of the layered oxide positive electrode material and improve the energy density is a current scientific frontier problem.

[0004] Because Na + Compared with Li + Has a larger ionic radius (1.02 angstrom), its embedding and extraction in the tightly packed layered transition metal oxide material makes the structure of the positive electrode material change significantly, which leads to the gradual collapse of the material structure with the increase of the charge and discharge cycle number, which is specifically manifested as poor electrochemical stability of the active material and rapid capacity decay. Therefore, researchers are committed to developing new layered structure systems to enhance the Na + extraction and embedding reversibility. According to the Na + position of the layered oxide and the stacking order of the oxygen atom layer, the layered oxide can be divided into P2, P3, O2 and O3 phase structures. Compared with the O phase structure, the Na +The diffusion channel barrier is smaller, the structure phase transition of the material is less, and thus the rate performance and cycle stability of the material are better, but the specific capacity and average voltage of the P phase material are lower than those of the O phase layered oxide material. Therefore, most of the pure phase layered metal oxide sodium ion battery cathode materials with single crystal structure exhibit poor electrochemical performance. Recently, the method of utilizing mixed phase synergy in the layered transition metal oxide sodium ion battery cathode material and combining the electrochemical performance advantages of different structures has attracted attention. At present, the performance of the prepared material is mostly improved by taking the advantages of the two-phase layered composite material P2 and O3 mixed phase, and the performance of the prepared material cannot be comprehensively improved. How to effectively improve the ion diffusion rate and electronic conductivity of the mixed phase sodium ion battery cathode material, so as to improve the rate performance and charge-discharge cycle stability, is a technical problem to be solved in the field. SUMMARY

[0005] In view of the problems of unstable layered oxide structure of the sodium ion battery cathode material in the prior art, slow sodium ion diffusion rate, poor rate performance and poor cycle stability, the application provides a carbon quantum dot coated mixed phase sodium ion battery cathode material and a preparation method and application thereof.

[0006] The application first provides a preparation method of a carbon quantum dot coated mixed phase sodium ion battery cathode material, and has the characteristics that the method comprises the following steps:

[0007] (1) mixing a nickel-iron-manganese-based hydroxide precursor, a sodium source and a dopant, calcining, and preparing P2 phase and O3 phase sodium ion battery cathode materials by setting different proportions of the sodium source;

[0008] (2) mixing the P2 phase and O3 phase sodium ion battery cathode materials to obtain a mixed phase sodium ion battery cathode material;

[0009] (3) dissolving carbon quantum dot powder and the mixed phase sodium ion battery cathode material in a solvent, stirring, and obtaining the carbon quantum dot coated mixed phase sodium ion battery cathode material.

[0010] The cathode material is a carbon quantum dot coated mixed phase nickel-iron-manganese-based sodium ion battery cathode material, and the mixed phase structure is a P2 and O3 phase mixture. The carbon quantum dot coating improves the overall conductivity of the material, suppresses the side reaction with the electrolyte, so that the cathode material has ultra-high rate performance and cycle stability, and can be used as a sodium ion battery cathode material.

[0011] The preparation method of the carbon quantum dot coated mixed-phase sodium ion battery positive electrode material, in step (1), when the molar ratio of sodium in the nickel-iron-manganese-based hydroxide precursor and the sodium source is 1:(0.5-0.7), a P2 phase sodium ion battery positive electrode material is prepared; when the molar ratio of sodium in the nickel-iron-manganese-based hydroxide precursor and the sodium source is 1:(0.8-1), an O3 phase sodium ion battery positive electrode material is prepared;

[0012] The sodium source is sodium carbonate and / or sodium hydroxide;

[0013] The dopant is a metal oxide of at least one of Al, Ca, Mg, Ti, Cu, Zn and Zr; specifically, TiO2 and ZrO2;

[0014] The addition amount of the dopant is 500-5000 ppm of the mass of the nickel-iron-manganese-based hydroxide precursor;

[0015] Specifically, when the dopant is TiO2 and ZrO2, the Ti element is doped with 2000 ppm and the Z element is doped with 2000 ppm, with reference to the mass of the nickel-iron-manganese-based hydroxide precursor.

[0016] The preparation method of the carbon quantum dot coated mixed-phase sodium ion battery positive electrode material, in step (1), the calcination temperature is 500-1100°C; specifically, 800-1000°C; more specifically, 800°C, 850°C, 900°C, 950°C or 1000°C;

[0017] The calcination is carried out in an air atmosphere;

[0018] The calcination time is 8-15 h, specifically 15 h; the calcination heating rate is 2-10°C / min, specifically 2°C / min.

[0019] The preparation method of the carbon quantum dot coated mixed-phase sodium ion battery positive electrode material, in step (2), the mass ratio of the P2 phase and O3 phase sodium ion battery positive electrode materials is (0.2-2):1; specifically, 0.25:1;

[0020] In step (3), the carbon content contributed by the carbon quantum dots is 0.01wt%-0.3wt%, specifically 0.1%, based on the total mass of the carbon quantum dot coated mixed-phase sodium ion battery positive electrode material;

[0021] The solvent is anhydrous ethanol and / or tetrahydrofuran;

[0022] The stirring time is 0.5-2 h;

[0023] The carbon quantum dot powder and the mixed-phase sodium-ion battery positive material are dissolved in a solvent, and the solid content is 10wt%-90wt%, and specifically can be 50wt%.

[0024] The preparation method of the carbon quantum dot coated mixed-phase sodium-ion battery positive material, and the specific steps of step (3) are as follows: the carbon quantum dot powder and the mixed-phase sodium-ion battery positive material are dissolved in a solvent, nitrogen is introduced and stirred, and the obtained solid is dried to obtain the carbon quantum dot coated mixed-phase sodium-ion battery positive material.

[0025] Specifically, the introduction rate of the nitrogen is 0.5-20L / min, and specifically can be 10L / min; and the drying temperature is 100-200℃.

[0026] In the preparation method of the carbon quantum dot coated mixed-phase sodium-ion battery positive material, the molar ratio of nickel element, iron element and manganese element in the nickel-iron-manganese-based hydroxide precursor is 1:0.5-1.5:0.5-1.5; and specifically can be 1:1:1.

[0027] The nickel-iron-manganese-based hydroxide precursor is prepared by a coprecipitation method; specifically, the nickel source is one or two of nickel sulfate, nickel chloride, nickel acetate or nickel nitrate; the iron source is one or two of ferrous sulfate, ferrous chloride, ferrous acetate or ferrous nitrate; the manganese source is one or two of manganese sulfate, manganese chloride, manganese acetate or manganese nitrate; the complexing agent is ammonia water or ammonium sulfate; the precipitating agent is sodium hydroxide solution; the reaction temperature is 50-60℃, the starting liquid pH is 11.0-11.9; the concentration of the complexing agent is 2-8g / L; the concentration of the precipitating agent is 2-4mol / L; the concentration of the metal salt solution is 1-2mol / L; and the flow rate of the precipitating agent and the metal salt solution is both 2-10L / h.

[0028] In the preparation method of the carbon quantum dot coated mixed-phase sodium-ion battery positive material, the carbon quantum dots are prepared by a hydrothermal method.

[0029] Specifically, the preparation method of the carbon quantum dots comprises the following steps: at least one of citric acid, ethylenediaminetetraacetic acid and L-cysteine is prepared into an aqueous solution, and a hydrothermal reaction is performed to obtain the carbon quantum dots.

[0030] In the preparation method of the carbon quantum dot coated mixed-phase sodium-ion battery positive material, the mass percentage concentration of the aqueous solution is 20%-80%.

[0031] The temperature of the hydrothermal reaction is 150-200℃, and the time is 2-15h.

[0032] In the preparation method of the carbon quantum dots, there are steps of crystallization concentration and drying after the hydrothermal reaction.

[0033] The application also provides the carbon quantum dot coated mixed phase sodium ion battery positive electrode material prepared by the preparation method.

[0034] Finally, the application of the carbon quantum dot coated mixed phase sodium ion battery positive electrode material in the preparation of a sodium ion battery positive electrode also belongs to the protection scope of the application.

[0035] The carbon quantum dot coated mixed phase sodium ion battery positive electrode material prepared by the application is used as a positive electrode material to prepare a button cell, the electrical performance of the button cell is determined at 0.2C, 0.33C and 1C current densities respectively, and the 1C cycle stability is determined under 1C current density. The results show that the carbon quantum dot coated mixed phase sodium ion battery positive electrode material prepared by the application has excellent cycle stability.

[0036] It can be seen that the preparation method of the carbon quantum dot coated mixed phase sodium ion battery positive electrode material provided by the application can obtain carbon quantum dots by controlling the hydrothermal reaction conditions, the material has good electrical conductivity, the nickel-iron-manganese-based hydroxide precursor is prepared by the intermittent co-precipitation method, the precursor is mixed with a sodium source and calcined to obtain the mixed phase sodium ion battery positive electrode material, the carbon quantum dots and the mixed phase sodium ion battery positive electrode material are uniformly mixed in a solvent, and the composite coated material is obtained by suction filtration and drying, which improves the electrical conductivity and structural stability of the carbon quantum dot coated mixed phase sodium ion battery positive electrode material and does not affect the electrochemical performance of the carbon quantum dot coated mixed phase sodium ion battery positive electrode material itself, thereby effectively improving the rate performance and cycle stability of the carbon quantum dot coated mixed phase sodium ion battery positive electrode material.

[0037] Compared with the prior art, the application has the following advantages:

[0038] (1) The raw materials for preparing the positive electrode material are widely available, the preparation method is simple, the production cost is low, and large-scale production is easy;

[0039] (2) The mixed phase sodium ion battery positive electrode material has a P2 and O3 mixed phase structure, and the two kinds of phase structures are uniformly and alternately distributed on a micron scale. The deep mixed phase structure can inhibit the generation of irreversible phase change and improve the overall structural stability of the material. Therefore, the electrochemical performance of the material is improved in terms of cycle stability, rate performance and material capacity;

[0040] (3) The method of the application coats in a liquid phase environment, ensures uniform coating of carbon quantum dots on the surface of the mixed phase sodium ion battery positive electrode material, is simple and has good coating effect; and the carbon quantum dot coated mixed phase sodium ion battery positive electrode material obtained has good crystallinity and structural stability;

[0041] (4) The positive electrode material of the application is a carbon quantum dot coated mixed phase sodium ion battery positive electrode material. The coating material greatly inhibits the side reaction of the mixed phase sodium ion battery positive electrode material with the electrolyte, reduces the loss of reversible Na + ; and the carbon quantum dots have high carrier migration rate, good thermal stability and chemical stability, etc. advantages. The use of carbon quantum dots to coat the mixed phase sodium ion battery positive electrode material can improve the rate performance of the mixed phase sodium ion battery positive electrode material, improve the cycle performance, and also has a good effect of isolating the nickel-iron-manganese-based sodium ion battery positive electrode material and the electrolyte, which is beneficial to maintaining the structural stability of the mixed phase sodium ion battery positive electrode material, so that the material has super high rate performance and cycle stability, and can be used as a sodium ion battery positive electrode material. BRIEF DESCRIPTION OF DRAWINGS

[0042] Figure 1 SEM image of the carbon quantum dot coated mixed phase sodium ion battery positive electrode material prepared in Example 1.

[0043] Figure 2 XRD pattern of the carbon quantum dot coated mixed phase sodium ion battery positive electrode material prepared in Example 1. DETAILED DESCRIPTION

[0044] The application will be further described in detail below in conjunction with the specific embodiments. The examples given are only to illustrate the application, and are not intended to limit the scope of the application.

[0045] The experimental methods in the following examples are all conventional methods unless otherwise specified.

[0046] In the quantitative test in the following examples, three repeated experiments were set up, and the average value was taken.

[0047] The materials, reagents, etc. used in the following examples can be obtained from commercial sources unless otherwise specified.

[0048] If the specific experimental steps or conditions are not specified in the examples, the operation or conditions can be carried out according to the conventional experimental steps described in the literature in the art. If the manufacturer of the reagent or instrument is not specified, it is a conventional reagent product that can be obtained by commercial purchase.

[0049] Example 1

[0050] (1) Preparation of nickel-iron-manganese-based hydroxide precursor

[0051] The nickel-iron-manganese-based hydroxide precursor is prepared by coprecipitation, and the molar ratio of nickel element, iron element and manganese element in the nickel-iron-manganese-based hydroxide precursor is 1:1:1; wherein the raw materials of the nickel-iron-manganese-based hydroxide precursor are selected from nickel sulfate, ferrous sulfate and manganese sulfate, the complexing agent is selected from ammonia water, and the precipitant is selected from sodium hydroxide; the reaction temperature is 60°C, the starting solution is a mixed solution of ammonia water and sodium hydroxide, the pH is 11.9, and the concentration of ammonia water is 6.5g / L; the pH of the starting solution is adjusted by sodium hydroxide;

[0052] The specific preparation method is as follows: according to the molar ratio of nickel element, iron element and manganese element 1:1:1, 2mol / L sulfate solution and 4mol / L sodium hydroxide solution are prepared; the sulfate solution and the sodium hydroxide solution are both at a flow rate of 5L / h, and the hydroxide precursor is prepared by coprecipitation, and after washing and drying process, a solid black powder is obtained, which is the nickel-iron-manganese-based hydroxide precursor, and its molecular formula is Ni 0.33 Fe 0.33 Mn 0.33 (0H)2.

[0053] (2) Preparation of mixed-phase sodium-ion battery positive electrode material

[0054] The nickel-iron-manganese-based hydroxide precursor prepared above is used as raw material, the nickel-iron-manganese-based hydroxide precursor is mixed with sodium carbonate (the molar ratio of sodium element in the nickel-iron-manganese-based hydroxide precursor and sodium carbonate is 1:0.67 (P2 phase) or 1:0.95 (O3 phase)), then TiO2 and ZrO2 are added, and mixed in a high-speed mixer at a uniform speed of 3000rpm for 30min to make it uniformly mixed; wherein, taking the mass of the nickel-iron-manganese-based hydroxide precursor as reference, 2000ppm of Ti element and 2000ppm of Zr element are doped; the mixed material is transferred to a sagger and placed in a box-type atmosphere furnace for calcination at 950°C for 15h; the calcination atmosphere is air atmosphere, and the calcination heating rate is 2°C / min. After cooling, the calcined material is taken out, and is processed by crushing and sieving, and according to different proportions of sodium carbonate, P2 phase and O3 phase nickel-iron-manganese-based sodium-ion battery positive electrode materials are obtained; the P2 phase and O3 phase positive electrode materials are mixed in a mass ratio of 1:4 to obtain a mixed-phase sodium-ion battery positive electrode material;

[0055] (3) Preparation of carbon quantum dots

[0056] Take 100mL of 20% ethylenediaminetetraacetic acid aqueous solution in a polytetrafluoroethylene-lined high-pressure stainless steel reaction kettle, and prepare carbon quantum dots by hydrothermal method, with a hydrothermal temperature of 200°C and a hydrothermal time of 2h; the concentrated solution is placed in a 200°C vacuum drying oven for drying after evaporation and crystallization concentration (80°C), and carbon quantum dot solid powder is obtained.

[0057] (4) Preparation of carbon quantum dot coated mixed phase sodium ion battery cathode material

[0058] The carbon quantum dot solid powder and the mixed phase sodium ion battery cathode material (the carbon content contributed by the carbon quantum dots is 0.1wt% based on the total mass of the carbon quantum dot coated mixed phase sodium ion battery cathode material) are dissolved in anhydrous ethanol solvent (the solid content is 50wt%), 10L / min nitrogen gas is introduced, stirring for 1h, natural sedimentation, the upper liquid is extracted, the remaining thick slurry material is extracted, oven drying at 200℃, to obtain the carbon quantum dot coated mixed phase sodium ion battery cathode material.

[0059] Figure 1 SEM image of the carbon quantum dot coated mixed phase sodium ion battery cathode material prepared for this example.

[0060] Figure 2 XRD pattern of the carbon quantum dot coated mixed phase sodium ion battery cathode material prepared for this example.

[0061] Example 2

[0062] The preparation method is the same as that of Example 1, except that the calcination temperature in step (2) is 900℃.

[0063] Example 3

[0064] The preparation method is the same as that of Example 1, except that the calcination temperature in step (2) is 850℃.

[0065] Example 4

[0066] The preparation method is the same as that of Example 1, except that the calcination temperature in step (2) is 800℃.

[0067] Example 5

[0068] The preparation method is the same as that of Example 1, except that the calcination temperature in step (2) is 1000℃.

[0069] Comparative Example 1

[0070] The preparation method is the same as that of Example 1, except that in the process of coating the cathode material with carbon quantum dots, only P2 phase material is selected for coating.

[0071] Comparative Example 2

[0072] The preparation method is the same as that of Example 1, except that in the process of coating the cathode material with carbon quantum dots, only O3 phase material is selected for coating.

[0073] Comparative Example 3

[0074] The preparation method is the same as that of Example 1, except that: in this comparative example, only P2 phase material is used to prepare the sodium ion battery positive electrode material, and the P2 phase material is not coated with carbon quantum dots.

[0075] Comparative Example 4

[0076] The preparation method is the same as that of Example 1, except that: in this comparative example, only O3 phase material is used to prepare the sodium ion battery positive electrode material, and the O3 phase material is not coated with carbon quantum dots.

[0077] Comparative Example 5

[0078] The preparation method is the same as that of Example 1, except that: there is no step (3) and (4), that is, only the P2 phase and O3 phase positive electrode material is mixed in a mass ratio of 1:4 to obtain a mixed phase sodium ion battery positive electrode material, and the mixed phase sodium ion battery positive electrode material is not coated with carbon quantum dots.

[0079] Example 6, performance test

[0080] The 2016 type button cell is prepared according to the following method:

[0081] The positive electrode sheet is prepared by mixing the positive electrode material, conductive carbon black and the binder polyvinylidene fluoride (PVDF) in a mass ratio of 90:5:5, using N-methyl pyrrolidone (NMP) as the solvent to make a slurry, uniformly coating on an aluminum foil, drying at 120°C, and then rolling to a thickness of 300μm, vacuum drying at 120°C for 12h, to obtain the positive electrode sheet. The positive electrode material is prepared by Examples 1-5 and Comparative Examples 1-5, respectively.

[0082] The negative electrode is a metal sodium sheet, the separator is a polypropylene porous membrane, and the electrolyte is a 1mol / L NaPF6 solution, the solvent of which is ethylene carbonate (EC) + diethyl carbonate (DEC) + dimethyl carbonate (DMC), the volume ratio of EC:DEC:DMC is 1:1:1.

[0083] The discharge capacity of the prepared button cell is measured at 0.2C, 0.33C and 1C current density, respectively, and the cycle stability is measured at 1C current density for 50 cycles, with a voltage range of 2-4V. The test results are shown in Table 1.

[0084] Table 1 Performance test results

[0085]

[0086] As can be seen from Table 1, compared with Comparative Examples 1-5, the carbon quantum dot coated mixed phase sodium ion battery positive electrode material prepared from Example 1 exhibits excellent rate performance and cycle stability as a positive electrode material for a button cell, with a 98.70% retention rate after 50 cycles at a 1C rate; indicating that the carbon quantum dot coated mixed phase sodium ion battery positive electrode material provided by the application is particularly suitable as a cycle type sodium ion battery positive electrode material, and the electrical performance of the mixed phase is greatly improved compared with pure P2 phase or pure O3 phase positive electrode materials. Example 1 exhibits better specific capacity development and cycle stability compared with the positive electrode material of Comparative Example 5, which is not coated with carbon quantum dots on the mixed phase sodium ion positive electrode material. The application adopts a liquid phase coating method to enhance the uniformity of the coating agent on the positive electrode surface, and carbon quantum dot coating can inhibit the occurrence of side reactions between the positive electrode material and the electrolyte, thereby improving the electrical performance of the positive electrode material. Compared with Comparative Examples 1-5, the carbon quantum dot coating of Example 1 is beneficial to the material to exhibit higher specific capacity, and enhances the cycle performance of the material. As can be seen from Examples 1-5, the electrical performance is best when the calcination temperature of the positive electrode material is 950℃, which is conducive to controlling the crystal form of the sodium ion battery material, and regulating the size growth and dispersion and agglomeration of the material during the sintering process.

Claims

1. A method for preparing a carbon quantum dot-coated mixed-phase sodium-ion battery cathode material, characterized in that: The method comprises the following steps: (1) mixing a nickel-iron-manganese-based hydroxide precursor, a sodium source and a dopant, calcining, and preparing P2 phase and O3 phase sodium ion battery cathode materials by setting different proportions of the sodium source; In step (1), when the molar ratio of sodium in the nickel-iron-manganese-based hydroxide precursor and the sodium source is 1:(0.5-0.7), the P2 phase sodium ion battery cathode material is prepared; when the molar ratio of sodium in the nickel-iron-manganese-based hydroxide precursor and the sodium source is 1:(0.8-1), the O3 phase sodium ion battery cathode material is prepared; In step (1), the calcination temperature is 900-1000°C; (2) mixing the P2 phase and O3 phase sodium ion battery cathode materials to obtain a mixed-phase sodium ion battery cathode material; In step (2), the mass ratio of the P2 phase and O3 phase sodium ion battery cathode materials is (0.2-2):1; (3) dissolving carbon quantum dot powder and the mixed-phase sodium ion battery cathode material in a solvent, stirring, and obtaining the mixed-phase sodium ion battery cathode material coated with the carbon quantum dots; The preparation method of the carbon quantum dots comprises the following steps: preparing at least one of citric acid, ethylenediaminetetraacetic acid and L-cysteine into an aqueous solution, and performing hydrothermal reaction to obtain the carbon quantum dots; The hydrothermal reaction temperature is 150-200°C, and the hydrothermal reaction time is 2-15h.

2. The method for preparing the carbon quantum dot-coated mixed-phase sodium-ion battery cathode material according to claim 1, characterized in that: In step (1), the sodium source is sodium carbonate and / or sodium hydroxide; The dopant is a metal oxide of at least one of Al, Ca, Mg, Ti, Cu, Zn and Zr; The addition amount of the dopant is 500-5000ppm of the mass of the nickel-iron-manganese-based hydroxide precursor.

3. The method of claim 1, wherein the carbon quantum dots-coated intercalation sodium-ion battery cathode material is prepared by the following steps: (1) preparing a carbon quantum dots-coated sodium transition metal oxide precursor; (2) annealing the carbon quantum dots-coated sodium transition metal oxide precursor to obtain the carbon quantum dots-coated intercalation sodium-ion battery cathode material. In step (1), the calcination temperature is 950°C; The calcination is performed in an air atmosphere; The calcination time is 8-15h, and the calcination heating rate is 2-10°C / min.

4. The preparation method of the carbon quantum dot coated mixed-phase sodium ion battery cathode material according to claim 1, characterized in that: In step (3), the carbon content contributed by the carbon quantum dots is 0.01wt%-0.3wt% based on the total mass of the carbon quantum dot coated mixed-phase sodium ion battery cathode material; The solvent is anhydrous ethanol and / or tetrahydrofuran; The stirring time is 0.5-2h.

5. The method of claim 1, wherein the carbon quantum dots coated intercalation sodium-ion battery cathode material is prepared by the following steps: (1) preparing a carbon quantum dots solution; (2) mixing the carbon quantum dots solution with a sodium-ion battery cathode material to obtain a mixture; (3) drying the mixture to obtain a carbon quantum dots coated sodium-ion battery cathode material. In the nickel-iron-manganese-based hydroxide precursor, the molar ratio of nickel, iron and manganese is 1:0.5-1.5:0.5-1.5; The nickel-iron-manganese-based hydroxide precursor is prepared by a coprecipitation method.

6. The method of claim 1, wherein the carbon quantum dots coated intercalation sodium-ion battery cathode material is prepared by the following steps: (1) preparing a carbon quantum dots solution; (2) mixing the carbon quantum dots solution with a sodium-ion battery cathode material to obtain a mixture; (3) drying the mixture to obtain a carbon quantum dots coated sodium-ion battery cathode material. The mass percentage concentration of the aqueous solution is 20%-80%.

7. The carbon quantum dot coated mixed-phase sodium ion battery cathode material prepared by the preparation method in any one of claims 1-6.

8. The application of the carbon quantum dot coated mixed-phase sodium ion battery cathode material in claim 7 in the preparation of a sodium ion battery cathode.

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