Nickel-iron-manganese-based sodium battery positive electrode material coated with carbon quantum dots as well as preparation method and application of nickel-iron-manganese-based sodium battery positive electrode material

Abstract

The invention discloses a carbon quantum dot-coated nickel-iron-manganese-based sodium battery positive electrode material as well as a preparation method and application of the carbon quantum dot-coated nickel-iron-manganese-based sodium battery positive electrode material. The iron-manganese-based sodium battery positive electrode material composite material comprises an inner core and a coating layer coating the inner core, the inner core is a nickel-iron-manganese-based sodium battery positive electrode material, and the structure of the inner core is a single-crystal blocky particle structure; and the coating layer is carbon quantum dots. The nickel-iron-manganese-based sodium-ion positive electrode material is coated with the carbon quantum dots, so that the rate capability of the nickel-iron-manganese-based sodium-ion positive electrode material can be improved, the cycle performance of the nickel-iron-manganese-based sodium-ion positive electrode material can be improved, meanwhile, the side reaction of the nickel-iron-manganese-based sodium-ion positive electrode material and an electrolyte can be well isolated, the nickel-iron-manganese-based sodium-ion positive electrode material can maintain the structural stability, and the service life of the nickel-iron-manganese-based sodium-ion positive electrode material is prolonged. Therefore, the composite material has ultrahigh rate capability and cycling stability, and can be used as a sodium ion battery positive electrode material.

Description

Technical Field

[0001] The invention relates to a carbon quantum dot-coated nickel-iron-manganese-based sodium positive electrode material and a preparation method and application thereof, belonging to the technical field of sodium ion batteries. Background Art

[0002] Clean and efficient energy storage and conversion technology is the cornerstone of modern social development. As one of the energy storage systems with the highest energy conversion efficiency, secondary batteries have received increasing attention. Among the numerous secondary battery systems, lithium-ion batteries have become the best among secondary batteries due to their advantages such as high energy density, low self-discharge rate, wide operating temperature, long cycle life, environmental friendliness, and no memory effect. However, with the rise in lithium ore prices, lithium ore resources are insufficient to support long-term use. Based on this, sodium-ion batteries have attracted more and more attention from researchers. Compared with lithium-ion batteries, sodium-ion batteries have many advantages: (1) sodium resources are abundant, widely distributed, low cost, and have no development bottleneck; (2) sodium and aluminum do not undergo alloying reaction, and cheap aluminum foil can be used as the current collectors for both the positive and negative electrodes of sodium-ion batteries, which can further reduce costs and avoid over-discharge problems; (3) sodium-ion batteries have excellent rate performance and high and low temperature performance; (4) sodium-ion batteries have the advantages of not catching fire or exploding in safety tests and good safety performance.

[0003] One of the key components of sodium-ion batteries is the cathode material. Currently, the cathode materials for sodium-ion batteries include metal layered oxides, polyanions, Prussian blue, etc. Among them, layered oxides have a high theoretical capacity and are easy to synthesize, making them one of the most promising cathode materials for sodium-ion batteries. Layered oxide sodium battery cathode materials have a relatively stable charging and discharging platform, are low-cost, and are environmentally friendly, making them a hot topic in current research. How to improve the cycle stability of layered oxide cathode materials and increase their energy density is a current scientific frontier issue.

[0004] Positive electrode materials also have their own defects, such as slow ion diffusion rate and low electronic conductivity, which lead to poor rate discharge capability and low power density. In response to these two problems, the commonly used improvement method is to coat and modify the surface of the positive electrode material. The common coating material is conductive carbon material, in order to improve the conductivity of the positive electrode material of the sodium ion battery, thereby achieving a high rate of the sodium ion battery. Therefore, how to effectively improve the ion diffusion rate and electronic conductivity of the positive electrode material of the sodium ion battery, thereby improving its rate performance, is a technical problem that needs to be solved in this field. Summary of the invention

[0005] The object of the present invention is to provide a composite material for a cathode material of a nickel-iron-manganese-based sodium-ion battery. The composite material is a nickel-iron-manganese-based sodium-ion battery cathode material coated with carbon quantum dots. The layered structure reduces the diffusion resistance of sodium ions, and the coating of carbon quantum dots improves the overall conductivity and structural stability of the material, enabling the composite material to have ultra-high rate performance and cycle stability. It can be used as a cathode material for sodium-ion batteries, solving the problem of poor rate performance caused by low electronic conductivity and slow sodium-ion diffusion rate of existing layered oxide cathode materials for sodium-ion batteries.

[0006] The composite material for the nickel-iron-manganese-based sodium-ion battery cathode material provided by the present invention includes a core and a coating layer covering the core;

[0007] The core is a nickel-iron-manganese-based sodium-ion battery cathode material, and its structure is a single-crystal massive particle structure;

[0008] The coating layer is carbon quantum dots.

[0009] In the composite material of the present invention, the size of the nickel-iron-manganese-based sodium-ion battery cathode material is 4.5 ± 0.2 μm.

[0010] In the composite material of the present invention, in the nickel-iron-manganese-based sodium-ion battery cathode material, the molar ratio of nickel element, iron element, and manganese element is 0.5 - 1:0.5 - 1:0.5 - 1.

[0011] The present invention further provides a preparation method for the composite material, including the following steps:

[0012] S1. Using citric acid, ethylenediaminetetraacetic acid, and / or L-cysteine as raw materials, preparing the carbon quantum dots by a hydrothermal method;

[0013] S2. Preparing a nickel-iron-manganese ternary precursor by a coprecipitation method, and then roasting it with sodium carbonate or sodium hydroxide to obtain the nickel-iron-manganese-based sodium-ion battery cathode material;

[0014] S3. Dissolving the carbon quantum dots and the nickel-iron-manganese-based sodium-ion battery cathode material in ethanol or tetrahydrofuran, and obtaining the product after stirring and standing.

[0015] In the above preparation method, in step S1, the raw material is preferably ethylenediaminetetraacetic acid;

[0016] In step S1, the conditions of the hydrothermal method are as follows:

[0017] The temperature is 150 - 200 °C, and the time is 2 - 15 h;

[0018] Through dialysis filtration, evaporation crystallization, and freeze-drying, carbon quantum dots with good conductivity and excellent electron conduction ability are obtained.

[0019] In the above preparation method, in step S2, the raw materials used for preparing the nickel-iron-manganese ternary precursor are nickel sulfate, ferrous sulfate and manganese sulfate, the complexing agent is ammonia water or ammonium sulfate, and the precipitating agent is sodium hydroxide;

[0020] The nickel sulfate, ferrous sulfate and manganese sulfate metal salts can be respectively prepared into a 2 mol / L mixed solution, which undergoes a coprecipitation reaction with a 4 mol / L sodium hydroxide solution to obtain a particle size of 4.5 ± 0.2 μm. The slurry is pressure-filtered, washed, and dried for standby.

[0021] The conditions for the roasting are as follows:

[0022] In the above preparation method, the specific steps of step S2 are as follows:

[0023] The roasting temperature is 700 - 1200 °C, preferably 1100 - 1200 °C; the roasting time is 2 - 15 h, the roasting atmosphere is oxygen or air with carbon dioxide removed, preferably air with carbon dioxide removed, and the roasting annealing rate is 2 - 10 °C / min.

[0024] In the above preparation method, in step S2, the molar ratio of the nickel-iron-manganese ternary precursor to the sodium carbonate or the sodium hydroxide is 0.9 - 1.1:1, preferably 0.95 - 1.05:1, and more preferably 0.05:1.

[0025] In the above preparation method, the specific steps of step S3 are as follows:

[0026] After dissolution, nitrogen is introduced, stirred for 0.5 - 2 h, allowed to settle naturally, the upper liquid is drawn out, the material is suction-filtered, and dried in an oven at 100 - 200 °C.

[0027] In the above preparation method, in step S3, the molar ratio of the carbon element in the carbon quantum dots to the nickel-iron-manganese-based sodium-ion battery positive electrode material is 0.01 - 0.3:1, preferably 0.05 - 0.1:1;

[0028] The flow rate of the nitrogen is 10 - 30 mL / min;

[0029] The rotation speed of the stirring is 300 - 500 rpm.

[0030] The positive electrode sheet prepared from the composite material of the present invention and the sodium-ion battery including the positive electrode sheet also belong to the protection scope of the present invention.

[0031] Compared with the prior art, the present invention has the following beneficial technical effects:

[0032] 1. The nickel-iron-manganese-based sodium battery cathode material coated with carbon quantum dots provided by the present invention is a sodium battery layered metal oxide coated with carbon quantum dots. This coating material greatly improves the sodium ion diffusion rate, and carbon quantum dots have advantages such as high carrier migration rate, good thermal stability and chemical stability. By using carbon quantum dots to coat the nickel-iron-manganese-based sodium battery cathode material, the rate performance of the nickel-iron-manganese-based sodium battery cathode material can be improved, the cycling performance becomes better, and at the same time, it also effectively isolates the side reaction between the nickel-iron-manganese-based sodium battery cathode material and the electrolyte, which is beneficial to maintaining the structural stability of the nickel-iron-manganese-based sodium battery cathode material. Therefore, this composite material has ultra-high rate performance and cycling stability and can be used as a sodium ion battery cathode material.

[0033] 2. The preparation method of the nickel-iron-manganese-based sodium battery cathode material coated with carbon quantum dots provided by the present invention can obtain carbon quantum dots by controlling the hydrothermal reaction conditions. This material has good electrical conductivity. Then, a nickel-iron-manganese-based ternary precursor is prepared by a coprecipitation method. The ternary precursor is mixed with a sodium source and calcined to obtain the nickel-iron-manganese-based sodium battery cathode material. The carbon quantum dots and the cathode material are mixed evenly in a solvent, and after suction filtration and drying, a composite coating material is obtained, which not only improves the electrical conductivity and structural stability of the nickel-iron-manganese-based sodium battery cathode material, but also does not affect the electrochemical performance of the nickel-iron-manganese-based sodium battery cathode material itself, thus effectively improving the rate performance and cycling stability of the nickel-iron-manganese-based sodium battery cathode material. Description of the Drawings

[0034] Figure 1 SEM image of the nickel-iron-manganese-based sodium battery cathode material coated with carbon quantum dots prepared in Example 1 of the present invention. Detailed Embodiments

[0035] Unless otherwise specified, the experimental methods used in the following examples are all conventional methods.

[0036] Unless otherwise specified, the materials, reagents, etc. used in the following examples can all be obtained from commercial channels.

[0037] The preparation method of the nickel-iron-manganese-based sodium battery cathode material coated with carbon quantum dots provided by the present invention includes the following steps:

[0038] Mix the nickel-iron-manganese-based sodium-ion battery ternary precursor and the sodium source (molar ratio of 1:0.95 - 1.05) in a container, and stir evenly at a constant speed to make them well mixed. Then transfer the mixed material to a crucible and place it in a four-tray furnace. The calcination temperature is 700 - 1200 °C; the calcination time is 2 - 15 h, the calcination atmosphere is oxygen or air with carbon dioxide removed, and the calcination annealing rate is 2 - 10 °C / min. Take out the calcined material, and obtain the nickel-iron-manganese-based sodium-ion battery positive electrode material through crushing and sieving. Place the carbon quantum dots and the nickel-iron-manganese-based sodium-ion battery positive electrode material (molar ratio of 0.01 - 0.3:1) in a container, dissolve them in anhydrous ethanol or tetrahydrofuran solvent, introduce nitrogen, stir for 0.5 - 2 h, let it settle naturally, draw out the upper liquid, filter the material by suction, and dry it in an oven at 100 - 200 °C to obtain the carbon quantum dot-coated nickel-iron-manganese-based sodium-ion battery positive electrode material.

[0039] Among them, the sodium source is one or both of sodium hydroxide and sodium carbonate, and the carbon source of the carbon quantum dots is one or more of citric acid, ethylenediaminetetraacetic acid, and L-cysteine.

[0040] In the present invention, the prepared carbon quantum dot-coated nickel-iron-manganese-based sodium-ion battery positive electrode material is used as the positive electrode material to prepare a button battery. The discharge specific capacity of the button battery is measured at current densities of 0.2C, 0.33C, and 1C respectively, and it is cycled at a current density of 1C to measure its 1C cycle stability. The results show that the carbon quantum dot-coated nickel-iron-manganese-based sodium-ion battery positive electrode material prepared in the present invention exhibits excellent cycle stability.

[0041] The following combines specific examples to detail the carbon quantum dot-coated nickel-iron-manganese-based sodium-ion battery positive electrode material provided by the present invention, its preparation method and application.

[0042] Example 1

[0043] Treatment group 1

[0044] Mix the nickel-iron-manganese-based sodium-ion battery ternary precursor (molar ratio of nickel element, iron element and manganese element is 1:1:1) and the sodium source (molar ratio of 1:1.05) in a container, and stir evenly at a constant speed to make them well mixed. Then transfer the mixed material to a crucible and place it in a four-tray furnace. The calcination temperature is 1100 °C; the calcination time is 15 h, the calcination atmosphere is air with carbon dioxide removed, and the calcination annealing rate is 2 °C / min. Take out the calcined material, and obtain the nickel-iron-manganese-based sodium-ion battery positive electrode material through crushing and sieving.

[0045] Prepare carbon quantum dots. Take ethylenediaminetetraacetic acid and prepare carbon quantum dots by hydrothermal method. The hydrothermal temperature is 200 °C, the hydrothermal time is 2 h, concentrate by evaporation crystallization, and place the concentrated solution in a vacuum drying oven at 200 °C to dry to obtain solid powder carbon quantum dots.

[0046] Carbon quantum dots and a sodium - ion battery cathode material based on nickel, iron, and manganese (molar ratio 0.05:1) are placed in a container, dissolved in anhydrous ethanol solvent, nitrogen is introduced, stirred for 1 h, allowed to settle naturally, the upper liquid is drawn off, the remaining thick slurry material is subjected to suction filtration, and dried in an oven at 200 °C to obtain a carbon - quantum - dot - coated sodium - ion battery cathode material based on nickel, iron, and manganese.

[0047] Figure 1 The SEM image of the carbon - quantum - dot - coated sodium - ion battery cathode material prepared in this example is shown. From Figure 1 it can be seen that the composite material prepared in this example has a single - crystal block - like particle structure, and its planar size is about 4.50 μm.

[0048] Treatment group 2,

[0049] This treatment group refers to treatment group 1 in this example to prepare a carbon - quantum - dot - coated sodium - ion battery cathode material. The difference between this treatment group and treatment group 1 in this example is that: in the preparation process of this treatment group, citric acid is used as the raw material for preparing carbon quantum dots. Except for the above difference, the materials and process operations used in this treatment group are strictly the same as those in treatment group 1 of this example.

[0050] Treatment group 3,

[0051] This treatment group refers to treatment group 1 in this example to prepare a carbon - quantum - dot - coated sodium - ion battery cathode material. The difference between this treatment group and treatment group 1 in this example is that: in the preparation process of this treatment group, L - cysteine is used as the raw material for preparing carbon quantum dots. Except for the above difference, the materials and process operations used in this treatment group are strictly the same as those in treatment group 1 of this example.

[0052] Example 2,

[0053] Treatment group 1,

[0054] This treatment group prepares a carbon - quantum - dot - coated sodium - ion battery cathode material according to treatment group 1 in Example 1. The materials and process operations used in this treatment group are strictly the same as those in treatment group 1 of Example 1.

[0055] Treatment group 2,

[0056] This treatment group refers to treatment group 1 in this example to prepare a carbon - quantum - dot - coated sodium - ion battery cathode material. The difference between this treatment group and treatment group 1 in this example is that: in the preparation process of this treatment group, the calcination temperature is 1200 °C. Except for the above difference, the materials and process operations used in this treatment group are strictly the same as those in treatment group 1 of this example.

[0057] Treatment group 3,

[0058] This treatment group prepared the carbon quantum dot-coated sodium-ion battery cathode material based on nickel-iron-manganese by referring to Treatment Group 1 in this example. The difference between this treatment group and Treatment Group 1 in this example is that the calcination temperature in the preparation process of this treatment group is 1000 °C. Except for the above difference, the materials and process operations used in this treatment group are strictly the same as those in Treatment Group 1 in this example.

[0059] Treatment Group 4,

[0060] This treatment group prepared the carbon quantum dot-coated sodium-ion battery cathode material based on nickel-iron-manganese by referring to Treatment Group 1 in this example. The difference between this treatment group and Treatment Group 1 in this example is that the calcination temperature in the preparation process of this treatment group is 900 °C. Except for the above difference, the materials and process operations used in this treatment group are strictly the same as those in Treatment Group 1 in this example.

[0061] Treatment Group 5

[0062] This treatment group prepared the carbon quantum dot-coated sodium-ion battery cathode material based on nickel-iron-manganese by referring to Treatment Group 1 in this example. The difference between this treatment group and Treatment Group 1 in this example is that the calcination temperature in the preparation process of this treatment group is 800 °C. Except for the above difference, the materials and process operations used in this treatment group are strictly the same as those in Treatment Group 1 in this example.

[0063] Example 3

[0064] Treatment Group 1,

[0065] This treatment group prepared the carbon quantum dot-coated sodium-ion battery cathode material based on nickel-iron-manganese according to Treatment Group 1 in Example 1. The materials and process operations used in this treatment group are strictly the same as those in Treatment Group 1 in Example 1.

[0066] Treatment Group 2,

[0067] This treatment group prepared the carbon quantum dot-coated sodium-ion battery cathode material based on nickel-iron-manganese by referring to Treatment Group 1 in this example. The difference between this treatment group and Treatment Group 1 in this example is that the sodium dosage in the preparation process of this treatment group is 1.1. Except for the above difference, the materials and process operations used in this treatment group are strictly the same as those in Treatment Group 1 in this example.

[0068] Treatment Group 3,

[0069] This treatment group prepared the carbon quantum dot-coated sodium-ion battery cathode material based on nickel-iron-manganese by referring to Treatment Group 1 in this example. The difference between this treatment group and Treatment Group 1 in this example is that the sodium dosage in the preparation process of this treatment group is 1.0. Except for the above difference, the materials and process operations used in this treatment group are strictly the same as those in Treatment Group 1 in this example.

[0070] Treatment Group 4,

[0071] This treatment group prepared the sodium - ion battery cathode material based on nickel - iron - manganese coated with carbon quantum dots by referring to Treatment Group 1 in this example. The difference between this treatment group and Treatment Group 1 in this example is that: in the preparation process of this treatment group, the sodium dosage is 0.95. Except for the above difference, the materials and process operations used in this treatment group are strictly the same as those in Treatment Group 1 of this example.

[0072] Treatment Group 5

[0073] This treatment group prepared the sodium - ion battery cathode material based on nickel - iron - manganese coated with carbon quantum dots by referring to Treatment Group 1 in this example. The difference between this treatment group and Treatment Group 1 in this example is that: in the preparation process of this treatment group, the sodium dosage is 0.90. Except for the above difference, the materials and process operations used in this treatment group are strictly the same as those in Treatment Group 1 of this example.

[0074] Comparative Example 1:

[0075] Comparative Treatment Group 1

[0076] This comparative treatment group prepared the sodium - ion battery cathode material based on nickel - iron - manganese coated with carbon quantum dots by referring to Treatment Group 1 in Example 1. The difference between this comparative treatment group and Treatment Group 1 in Example 1 is that: in the preparation process of this treatment group, carbon quantum dot coating was not carried out. Except for the above difference, the materials and process operations used in this treatment group are strictly the same as those in Treatment Group 1 of Example 1.

[0077] Comparative Treatment Group 2

[0078] This comparative treatment group prepared the sodium - ion battery cathode material based on nickel - iron - manganese coated with carbon quantum dots by referring to Treatment Group 1 in Example 1 of this example. The difference between this comparative treatment group and Treatment Group 1 in Example 1 is that: in the preparation process of this treatment group, the cathode material was sintered in an air atmosphere. Except for the above difference, the materials and process operations used in this treatment group are strictly the same as those in Treatment Group 1 of Example 1.

[0079] Comparative Treatment Group 3

[0080] This comparative treatment group prepared the sodium - ion battery cathode material based on nickel - iron - manganese coated with carbon quantum dots by referring to Treatment Group 1 in Example 1 of this example. The difference between this comparative treatment group and Treatment Group 1 in Example 1 is that: in the preparation process of this treatment group, the cathode material was sintered in an oxygen atmosphere. Except for the above difference, the materials and process operations used in this treatment group are strictly the same as those in Treatment Group 1 of Example 1.

[0081] Test Example

[0082] Prepare button cells according to the following method:

[0083] The positive electrode plate is prepared by mixing a positive electrode material, conductive carbon black, and a binder polyvinylidene fluoride (PVDF) in a mass ratio of 8:1:1, using N-methylpyrrolidone (NMP) as a solvent to form a slurry, uniformly coating it on aluminum foil, drying at a suitable temperature, rolling to the required thickness, and vacuum drying at 120 °C for 12 h to obtain the positive electrode plate. Among them, the positive electrode materials are prepared from Examples 1-3 and Comparative Examples 1-2 respectively.

[0084] The negative electrode uses a sodium metal sheet, the separator is a polypropylene porous membrane, and the electrolyte is 1 mol / L NaPF6 / EC+DEC+DMC (EC:DEC:DMC = 1:1:1 by volume ratio).

[0085] The discharge specific capacity of the prepared button cells is measured at current densities of 0.2C, 0.33C, and 1C respectively, and the cycle stability is measured by cycling 50 times at a current density of 1C.

[0086] As can be seen from Table 1, compared with Comparative Example 1, the button cell using the carbon quantum dot-coated nickel-iron-manganese-based sodium battery positive electrode material prepared in Example 1 as the positive electrode material exhibits excellent rate performance and cycle stability, maintaining 99.29% after 50 cycles at a rate of 1C; indicating that the carbon quantum dot-coated nickel-iron-manganese-based sodium battery positive electrode material provided by the present invention is particularly suitable as the positive electrode material for rechargeable sodium ion batteries. And the method of coating carbon quantum dots has an obvious beneficial effect on the electrical properties of the positive electrode material, and has a better specific capacity compared with the carbon quantum dot coating prepared with citric acid as the carbon source in Comparative Example 2.

[0087] The present invention adopts a liquid-phase coating method to enhance the uniformity of the coating agent on the surface of the positive electrode, thereby improving the electrical properties of the positive electrode material. It can be seen from the comparison between Treatment Group 1 of Example 1 and Treatment Group 1 of Comparative Example 1 (without coating the positive electrode material) that the preparation method provided by the present invention can form a dense and uniform carbon coating layer on the surface of the positive electrode material. The coating of carbon quantum dots is beneficial for the material to exhibit a higher specific capacity, and enhances the cycle performance and rate performance of the material. Comparing Treatment Group 1 of Example 1 with Treatment Groups 2 and 3 of Comparative Example 1 (under different sintering atmosphere conditions), the materials in air and oxygen with carbon dioxide removed show better electrical properties. An atmosphere without carbon dioxide can reduce the generation of residual alkali during the calcination of the material, thereby improving the capacity performance and cycle stability of the material.

[0088] From the data results of Treatment Groups 1-3 of Example 1, it is shown that different carbon source selections have obvious improvement effects on the electrochemical properties of the material. Initially, ethylenediaminetetraacetic acid has a good effect as a carbon source.

[0089] The performance test results of treatment group 1 in Example 2 were compared with those of treatment groups 2-5. As can be seen from Table 1, under the condition that the preparation of coin cells, other materials and operations are the same, the difference between treatment groups 2-4 lies in setting different temperature gradients of calcination temperature during the preparation of the cathode material. The experimental data shows that the preferred calcination temperature is 1100 °C. For the prepared cathode material, the material crystallizes better during roasting, is more likely to form single crystal particles, and has more excellent electrochemical performance.

[0090] The performance test results of treatment group 1 in Example 3 were compared with those of treatment groups 2-5. As can be seen from Table 1, under the condition that the preparation of coin cells, other materials and operations are the same, the difference between treatment groups 2-4 lies in setting different sodium doping amounts during the preparation of the cathode material. When the sodium doping amount is 1.05 times the stoichiometric ratio, the prepared cathode material shows better electrical performance, indicating that when the sodium doping amount is insufficient, the content of active sodium ions in the material also decreases accordingly, and sufficient active sodium ions cannot be provided during charge and discharge. When the sodium doping amount is too high, the excess sodium cannot enter the lattice of the cathode material through calcination, resulting in too much residual alkali on the material surface, which affects the electrical performance of the material.

[0091] Therefore, the present invention reduces the nickel content in the low-nickel system to reduce production costs; for the cathode material of the sodium-ion battery with an NFM ratio of 1:1:1, it has low nickel, and at the same time, low-cost metal elements such as iron and manganese materials are used as the main elements to reduce the cost of the sodium-ion battery cathode material. The present invention uses the liquid-phase method to coat with carbon quantum dots. This material has good conductivity. Then, a nickel-iron-manganese-based ternary precursor is prepared by the coprecipitation method. The ternary precursor is mixed and roasted with a sodium source to obtain a nickel-iron-manganese-based sodium-ion battery cathode material. The carbon quantum dots and the cathode material are mixed evenly in a solvent, and after suction filtration and drying, a composite coating material is obtained, which not only improves the conductivity and structural stability of the nickel-iron-manganese-based sodium-ion battery cathode material, but also does not affect the electrochemical performance of the nickel-iron-manganese-based sodium-ion battery cathode material itself, thereby effectively improving the rate performance and cycle stability of the nickel-iron-manganese-based sodium-ion battery cathode material.

[0092] Table 1 Charge-discharge rate and cycle data of the prepared materials

[0093]

[0094]

[0095]

Claims

1. A composite material of a nickel-iron-manganese-based sodium-ion battery cathode material, comprising a core and a coating layer coating the core; The core is a nickel-iron-manganese-based sodium-ion battery cathode material, and its structure is a single-crystal massive particle structure; The coating layer is carbon quantum dots.

2. The composite material according to claim 1, wherein: The size of the nickel-iron-manganese-based sodium-ion battery cathode material is 4.5 ± 0.2 μm.

3. The composite material according to claim 1 or 2, wherein: In the nickel-iron-manganese-based sodium-ion battery cathode material, the molar ratio of nickel element, iron element and manganese element is 0.5 - 1: 0.5 - 1: 0.5 - 1.

4. A preparation method of the composite material according to any one of claims 1-3, comprising the following steps: S1. Using citric acid, ethylenediaminetetraacetic acid and / or L-cysteine as raw materials, preparing the carbon quantum dots by a hydrothermal method; S2. Preparing a nickel-iron-manganese ternary precursor by a coprecipitation method, and then roasting it with sodium carbonate or sodium hydroxide to obtain the nickel-iron-manganese-based sodium-ion battery cathode material; S3. Dissolving the carbon quantum dots and the nickel-iron-manganese-based sodium-ion battery cathode material in ethanol or tetrahydrofuran, and obtaining the product after stirring and standing.

5. The preparation method according to claim 4, wherein: In step S1, the conditions of the hydrothermal method are as follows: The temperature is 150 - 200 °C, and the time is 2 - 15 h.

6. The preparation method according to claim 4 or 5, wherein: In step S2, the raw materials used to prepare the nickel-iron-manganese ternary precursor are nickel sulfate, ferrous sulfate and manganese sulfate, the complexing agent is ammonia water or ammonium sulfate, and the precipitating agent is sodium hydroxide; The conditions of the roasting are as follows: The roasting temperature is 700 - 1200 °C; the roasting time is 2 - 15 h, the roasting atmosphere is oxygen or air with carbon dioxide removed, and the roasting annealing rate is 2 - 10 °C / min.

7. The preparation method according to claim 4 or 5, wherein: In step S2, the molar ratio of the nickel-iron-manganese ternary precursor to the sodium carbonate or the sodium hydroxide is 0.9 - 1.1:

1.

8. The preparation method according to claim 7, wherein: The specific steps of step S3 are as follows: After dissolution, nitrogen is introduced, stirred for 0.5 - 2 h, allowed to settle naturally, the upper liquid is drawn out, the material is filtered by suction, and dried in an oven at 100 - 200 °C; In step S3, the molar ratio of the carbon element in the carbon quantum dots to the nickel-iron-manganese-based sodium-ion battery cathode material is 0.01 - 0.3: 1; The flow rate of the nitrogen is 10 - 30 mL / min; The rotation speed of the stirring is 300 - 500 rpm.

9. A positive electrode sheet, prepared from the composite material according to any one of claims 1-3.

10. A sodium-ion battery, comprising the positive electrode sheet according to claim 9.

Images