Coated modified composite sodium ferric phosphate positive electrode material and preparation method thereof

Abstract

The invention relates to the technical field of sodium ion batteries, and discloses a coated modified composite sodium ferric phosphate positive electrode material and a preparation method thereof.According to the method, a nitrogen source and a titanium source are added, and the surface is coated and modified through TiN and a nitrogen-doped carbon layer; by forming a composite coating layer of titanium nitride and carbon, the conductivity of the positive electrode material is further enhanced, and meanwhile, side reaction between the material and electrolyte is inhibited; moreover, the conductivity of the positive electrode material is improved, the capacity of the battery is improved, the specific capacity of the battery is not reduced, the industrial problem is solved, a considerable improvement effect is generated, and the prepared battery is good in discharge characteristic and suitable for popularization and application.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of sodium ion batteries, in particular to a coated modified composite sodium iron phosphate positive electrode material and a preparation method thereof. BACKGROUND

[0002] The statements in this section merely provide background information related to the present disclosure and can not constitute the prior art.

[0003] The successful application of LiFePO4 in lithium ion batteries has prompted the inexpensive and environmentally friendly iron-based phosphate to become an ideal choice for sodium ion battery polyanion positive electrode materials. When NaFePO4 is synthesized by a traditional method, a non-electrochemically active triphylite phase is easily generated, but its derivative Na2FeP2O7 can be directly prepared by simple processes such as mechanical synthesis and sol-gel method, and has low cost. In addition, its crystal structure hardly distorts during the charging and discharging process, so it has excellent cycle stability, but the theoretical specific capacity of the battery prepared by taking it as a positive electrode material is only 97.1 mAh / g, and the low energy density greatly limits the potential of its large-scale commercial application. Under this background, mixed polyanion phosphate materials composed of NaFePO4 and Na2FeP2O7 in a certain molar ratio have attracted widespread attention. Such compounds combine the advantages of phosphate and pyrophosphate cathode materials and exhibit excellent electrochemical performance. Among them, Na4Fe3(PO4)2(P2O7) (NFPP, orthorhombic Pn21a) can provide a higher theoretical specific capacity (about 129 mAh / g) at an average voltage of about 3.0 V. However, polyanion materials usually have low electronic conductivity, making it difficult to achieve high-rate charging and discharging, which limits their further application. At present, ion doping and carbon coating are usually used to modify the electrochemical performance of polyanion materials. Carbon coating is an effective method to improve the electronic conductivity of materials, which not only reduces the intrinsic resistance of the material, but also reduces the occurrence of side reactions and enhances the chemical stability of the material through the introduction of a coating layer.

[0004] However, traditional carbon coating still has the problems of low electrical conductivity and the increase of carbon content will reduce the compaction density of the material, further reducing the specific capacity of the battery. Therefore, how to develop a coated composite sodium iron phosphate with high specific capacity, excellent cycle stability, good rate performance, and high electronic conductivity as a sodium ion battery positive electrode material has important research and application value. SUMMARY

[0005] The present application aims at the technical problem that the current coating method of the positive electrode material reduces the specific capacity of the battery and it is difficult to improve the specific capacity and rate performance of the battery simultaneously, and provides a coated modified composite sodium iron phosphate positive electrode material and a preparation method thereof.

[0006] The technical scheme of the present application is as follows: The present application provides a coated modified composite sodium iron phosphate positive electrode material, which has the chemical formula: Na4Fe3(PO4)2(P2O7)@M x N y , wherein M is one or more of C and Ti, N is nitrogen, X is 0.01-1, and Y is 0.01-1. , wherein M x N y is coated in an amount of 0.5-5wt%.

[0007] According to a preferred embodiment, the M x N y coating layer is a composite coating layer, and the components thereof are nitrogen-doped carbon layer, TiN (titanium nitride) from inside to outside.

[0008] Compared with the traditional carbon coating technology, the composite coating layer can further reduce the interface impedance of the material by introducing TiN with more excellent conductivity and nitrogen-doped carbon layer. Meanwhile, TiN itself has the characteristics of corrosion resistance and high hardness, which can reduce the side reaction between the material and the electrolyte to a certain extent.

[0009] Previously, TiN has not been used as a coating material for NFPP, mainly due to its limited application scenarios - it is more commonly used in surface coating, high-temperature resistant materials and electrode materials. More importantly, the traditional carbon thermal reduction nitriding method for preparing TiN needs to be carried out at a high temperature of 1000℃ or above, which is far beyond the suitable range of NFPP coating (600℃ or above will cause damage to the structure of NFPP), so it is difficult to coat TiN on the surface of NFPP by the traditional method.

[0010] The present application provides a method for coating and modifying a composite sodium iron phosphate positive electrode material, comprising the following steps: x N y Step (1): preparing a carbon-coated sodium iron pyrophosphate positive electrode material; ​Step (2): mixing the composite sodium iron phosphate material obtained in step (1) with a titanium source and a nitrogen source uniformly, so that the titanium source and the nitrogen source are coated on the outer layer of the composite sodium iron phosphate material, and then performing secondary sintering to obtain a M x N y The composite coated sodium iron pyrophosphate positive electrode material.

[0011] Preferably, step (1) specifically comprises the following sub-steps: Step (1.1): mixing a sodium source, an iron source, a phosphorus source, a carbon source and a solvent, and vacuum drying to obtain a positive electrode material precursor; Step (1.2): calcining the precursor obtained in step (1.1) under an inert gas to obtain a carbon-coated sodium iron pyrophosphate positive electrode material; Preferably, in step (2), the mass of the composite coating layer accounts for 0.5wt%-5wt% of the total raw material mass, and the total raw material is composed of a sodium source, an iron source, a phosphorus source, a titanium source, a nitrogen source and a carbon source. x N y The mass of the composite coating layer accounts for 0.5wt%-5wt% of the total raw material mass, and the total raw material is composed of a sodium source, an iron source, a phosphorus source, a titanium source, a nitrogen source and a carbon source.

[0012] Preferably, in step (2), the titanium source is at least one of titanium dioxide, tetrabutyl titanate, vinyl titanate, titanium trichloride and titanium tetrachloride.

[0013] Preferably, in step (2), the nitrogen source is at least one of urea, melamine, ethylenediamine, cyanamide and dicyandiamide.

[0014] When the mass of the coating layer is within the above range, a dense coating layer can be formed on the outer surface of the core, the surface properties of the material are optimized, direct contact between the core sodium iron pyrophosphate material and the electrolyte is inhibited, the conductivity and stability of the material can be effectively improved, and the rate and cycle stability of the material are improved. If the content of the coating layer is too low, the conductivity and surface properties of the material will be improved to a small extent, and the electronic transmission efficiency of the material cannot be well played. If the content of the coating layer is too high, the thickness of the coating layer will be too large, which will hinder the transmission speed of sodium ions, reduce the utilization rate of the sodium iron pyrophosphate bulk material, reduce the green density of the material, and greatly reduce the electrochemical performance of the material.

[0015] Preferably, in step (1), the molar ratio of each element in the added sodium source, iron source and phosphorus source corresponds to the stoichiometric ratio of each element in sodium iron pyrophosphate Na4Fe3(PO4)2(P2O7), that is, the addition amount of the sodium source, the iron source and the phosphorus source can be calculated according to the stoichiometric ratio of each element in sodium iron pyrophosphate Na4Fe3(PO4)2(P2O7).

[0016] Preferably, the sodium source in step (1) is at least one of sodium carbonate, sodium dihydrogen phosphate, sodium hydroxide, sodium nitrate, disodium hydrogen pyrophosphate, and sodium pyrophosphate.

[0017] Preferably, the iron source in step (1) is at least one of iron phosphate dihydrate, iron phosphate anhydrous, and iron oxide.

[0018] Preferably, the phosphorus source in step (1) is at least one of phosphoric acid, sodium dihydrogen phosphate, disodium hydrogen pyrophosphate, and sodium pyrophosphate.

[0019] Preferably, the carbon source in step (1) is at least one of starch, glucose, sucrose, citric acid, and ascorbic acid.

[0020] Preferably, the solid content of the solid-liquid mixed slurry in step (1) is 20-50 wt%.

[0021] Preferably, the drying method in step (1) includes air drying and spray drying, the air drying temperature is 70-100℃, and the drying time is 2-10 h.

[0022] Preferably, the spray drying inlet temperature is 150-250℃, the outlet temperature is 60-120℃, and the feeding rate is 0.5-30 mL / min.

[0023] Preferably, the inert atmosphere for sintering in step (2) includes one of nitrogen, argon, argon-hydrogen mixed gas, and nitrogen-hydrogen mixed gas.

[0024] Preferably, the sintering process in step (2) is divided into two-stage sintering or one-stage sintering, the two-stage sintering includes: the first stage pre-sintering temperature is 300-400℃, and the pre-sintering time is 2-5 h; the second stage sintering temperature is 450-600℃, and the sintering time is 2-15 h. The one-stage sintering has a sintering temperature of 450-600℃ and a sintering time of 2-15 h.

[0025] The breakthrough of the present patent is to control the preparation temperature at 580℃: since the prepared coating layer is not pure TiN, and sufficient nitrogen source is provided by dicyandiamide, the oxidation of TiCl3 can be effectively inhibited, dicyandiamide and titanium trichloride as nitrogen source and titanium source enable the reaction to be completed at a lower temperature, and the preparation method is relatively simple. Although pure TiN cannot be generated under low temperature conditions, the excess dicyandiamide as a nitrogen source can perform nitrogen doping on the formed carbon layer; at the same time, the coating temperature is lower than 600℃, which can avoid the structure of NFPP being damaged. In addition, the method of secondary coating can not only make the particle distribution of NFPP material more dense, but also improve the compaction density of the material during the coating process. It is worth noting that the TiCl3 solution needs to be prepared on site during the preparation process, and the operation must be carried out in a glove box.

[0026] Preferably, the sintering temperature rising rate is 1-10℃ / min.

[0027] Another aspect of the present application provides a sodium ion battery comprising the composite coated sodium iron phosphate pyrophosphate composite cathode material of titanium nitride and carbon as described above.

[0028] The beneficial effects of the present application compared with the prior art are: 1. A coated modified composite sodium iron phosphate cathode material, the titanium nitride coating layer is a cubic system metal nitride with high conductivity, which can improve the diffusion kinetics of Na + ; In addition, the coated modified sodium iron phosphate cathode material has good acid and alkali corrosion resistance, which can reduce the occurrence of side reactions between the material and the electrolyte and improve the cycle life of the battery; 2. A preparation method of coated modified composite sodium iron phosphate cathode material, using dicyandiamide and titanium trichloride as the nitrogen source and titanium source in the coating layer, which can further improve the compaction density on the basis of improving the conductivity of the material, and the processing technology is simple and the price is relatively low; 3. A preparation method of coated modified composite sodium iron phosphate cathode material, which is coated and modified by a simple gradient sintering process, the modification method is simple and the modification effect is excellent. BRIEF DESCRIPTION OF DRAWINGS

[0029] Figure 1 XRD curve of the sodium ion battery cathode material obtained from Example 1 and Comparative Example 1; Figure 2 The first cycle charge-discharge curve of the button cell made of the sodium ion battery cathode material obtained from Example 1 and Comparative Example 1; Figure 3 The cycle performance graph of the sodium ion battery cathode material of Example 1, 2 and 3 under different rate conditions. DETAILED DESCRIPTION

[0030] The specific examples listed in the present application are only as examples of the present application, and the present application is not limited to the specific examples described below. Any equivalent modifications and alternatives to the examples described below are also within the scope of the present application for those skilled in the art. Therefore, equivalent transformations and modifications made without departing from the spirit and scope of the present application should be encompassed within the scope of the present application. The specific conditions not mentioned in the examples are carried out according to the conventional conditions or the conditions recommended by the manufacturer. All reagents or instruments not mentioned by the manufacturer are conventional products that can be purchased on the market. In order to better illustrate the present application, numerous specific details are given in the following specific embodiments. Those skilled in the art should understand that the present application can also be implemented without certain specific details. In some embodiments, methods, means, apparatus and steps that are well known to those skilled in the art are not described in detail in order to highlight the main idea of the present application.

[0031] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. Unless otherwise indicated, all measurements used in the description herein are made in international standard units, and the numerical values and numerical ranges appearing in the present application should be understood to encompass the systematic errors that are unavoidable in industrial production.

[0032] The features and properties of the present application are further described in detail below in conjunction with the examples.

[0033] Example 1 Another aspect of the present application provides a M x N y The method for coating and modifying the composite sodium iron phosphate positive electrode material comprises the following steps: Step (1): Sodium source, iron source, and phosphorus source are weighed in a molar ratio of 4:3:4, and sodium carbonate and disodium hydrogen phosphate, iron phosphate dihydrate are weighed. 5% of the total raw material mass of anhydrous glucose is weighed as a carbon source and mixed uniformly with a solvent, and then an appropriate amount of deionized water is added, and the mixture is dispersed in a planetary ball mill at 300r for 10h; Step (2): The ball-milled precursor obtained in step (1) is dried at 80°C under vacuum conditions for 12 hours to obtain a positive electrode material precursor, and the positive electrode material precursor is crushed and sieved; Step (3): The precursor obtained in step (2) is calcined under a nitrogen sintering atmosphere, with a heating rate of 3°C / min, a sintering temperature of 350°C, and a holding time of 4h, followed by heating to 500°C at a heating rate of 3°C / min for 10h, and then naturally cooling to room temperature to obtain a carbon-coated sodium iron phosphate pyrophosphate positive electrode material; Step (4): The carbon-coated sodium iron phosphate pyrophosphate positive electrode material obtained in step (3) is crushed and sieved through a 300 mesh screen; Step (5): 0.63 g of dicyandiamide (nitrogen source) was dissolved in 50 mL of ethanol, stirred and dissolved, and then 1.2 g of TiCl3 (titanium source) was added and ultrasonically dispersed for 20 min; Step (6): 100 g of the NFPP@C powder obtained in step (4) was added to the solution obtained in step (5), stirred at 60℃ for 2 h to allow the active components to be adsorbed on the surface, and then completely dried in a vacuum environment at 80℃ and sieved; Step (7): The precursor obtained in step (6) was sintered in a nitrogen sintering atmosphere, the heating rate was 3℃ / min, the sintering temperature was 350℃, the holding time was 3h, and then the temperature was raised to 580℃ at a heating rate of 3℃ / min, and the holding time was 5h, and then naturally cooled to room temperature; Step (8): The positive electrode material obtained in step (7) was crushed and sieved through a 300 mesh sieve to obtain the final product Na4Fe3(PO4)2(P2O7)@TiN / nitrogen-doped carbon layer, and the mass of the coating layer accounted for 0.8% of the total raw material mass.

[0034] Example 2 In another aspect, the application provides a M x N y The method for coating and modifying the composite sodium iron phosphate positive electrode material comprises the following steps: Step (1): Sodium source, iron source and phosphorus source were weighed in a molar ratio of 4:3:4, i.e. sodium carbonate and disodium hydrogen phosphate, iron phosphate dihydrate; 5% of the total raw material mass of anhydrous glucose was weighed as a carbon source and mixed uniformly with a solvent, and then an appropriate amount of deionized water was added, and the mixture was dispersed in a planetary ball mill at 300r for 10h; Step (2): The ball-milled precursor obtained in step (1) was dried in a vacuum to obtain a positive electrode material precursor, and the positive electrode material precursor was crushed and sieved; Step (3): The precursor obtained in step (2) was calcined in a nitrogen sintering atmosphere, the heating rate was 3℃ / min, the sintering temperature was 350℃, the holding time was 4h, and then the temperature was raised to 500℃ at a heating rate of 3℃ / min, and the holding time was 10h, and then naturally cooled to room temperature to obtain a carbon-coated sodium iron phosphate pyrophosphate positive electrode material; Step (4): The carbon-coated sodium iron phosphate pyrophosphate positive electrode material (NFPP@C positive electrode material) obtained in step (3) was crushed and sieved through a 300 mesh sieve; Step (5): 2 g of urea (nitrogen source) was dissolved in deionized water to prepare a urea solution with a concentration of 0.5-1.0 mol / L, and stirred uniformly for use; 2.74 g of tetrabutyl titanate was slowly added to a proper amount of anhydrous ethanol, and stirred while adding to obtain a tetrabutyl titanate ethanol solution with a concentration of 0.1-0.2 mol / L; Step (6): The prepared tetrabutyl titanate ethanol solution (0.1-0.2 mol / L) was added to a 500 mL three-necked flask, which was placed in a 30°C constant-temperature water bath and mechanically stirred (300 rpm), and high-purity nitrogen was introduced (flow rate: 50 mL / min) to remove air and prevent hydrolysis of tetrabutyl titanate. The 100 g of ground NFPP@C powder was slowly added to the solution at a rate of 0.5 g / min using a peristaltic pump, and stirring was continued for 1 h, during which the particle dispersion degree was monitored in real time using a laser particle size analyzer (the particle size of agglomerates should be <20 μm). The stirring rate was kept constant, and the urea solution (0.5-1.0 mol / L) was added to the mixed system at a rate of 1 drop / s through a constant-pressure dropping funnel (the molar ratio of urea to tetrabutyl titanate was 5:1-10:1), and the pH value of the system gradually increased from weak acidity (~5.5) to neutrality (~7.0) during the dropping process, which was monitored in real time using a pH meter. After the dropping was completed, the temperature was raised to 40°C, and stirring was continued for 2.5 h to promote the preliminary condensation reaction between tetrabutyl titanate and urea (to generate a titanium-oxygen-nitrogen precursor), and the solution state was observed every 30 min during the reaction (a uniform suspension should be maintained, and no obvious precipitation should be observed). The mixed solution was subjected to ultrasonic treatment 30 min before the end of the reaction to further break the particle agglomeration and ensure that the titanium source-nitrogen source precursor was uniformly coated on the surface of the NFPP@C.

[0035] Step (7): The mixed suspension of step (6) was transferred to a rotary evaporator, and the water bath temperature was set to 60°C, the rotation speed was 80 rpm, the vacuum degree was 0.08 MPa, and the solvent (mixed vapor of ethanol and water) was evaporated under nitrogen protection. When the solution volume was reduced to 1 / 5 of the initial volume (about 2-3 h), the vacuum was closed, and the material was transferred to a glass evaporating dish and placed in a 60°C air-drying oven for further static evaporation to form a viscous sol. 2-3 drops of nitric acid (0.1 mol / L) were added to the sol to adjust the pH to 4.0-4.5 to accelerate gelation, and then the wet gel was left to stand in a fume hood for 12 h to form a uniform wet gel. The wet gel was placed in an 80°C vacuum drying oven for 12 hours to ensure complete gel solidification.

[0036] Step (8): The precursor obtained in step (7) was ground and sintered in a nitrogen sintering atmosphere, and the temperature was raised to 580°C at a rate of 3°C / min and kept for 4 h, and then naturally cooled to room temperature. Step (9): crushing and 300 mesh screen sieving the positive electrode material obtained in step (8) to obtain a titanium nitride coated Na4Fe3(PO4)2(P2O7)@TiN / nitrogen-doped carbon layer, the mass of the coating layer accounting for 1% of the total raw material mass.

[0037] Example 3 Another aspect of the present application provides a M X The method for modifying the N-coated composite sodium iron phosphate positive electrode material comprises the following steps: Step (1): sodium source, iron source, and phosphorus source are weighed in a molar ratio of 4:3:4, i.e. sodium carbonate and disodium hydrogen phosphate, iron phosphate dihydrate; 5% of the total raw material mass of anhydrous glucose is weighed as a carbon source and mixed uniformly with a solvent, and then an appropriate amount of deionized water is added, and the mixture is ball milled in a planetary ball mill at 300 r for 10 h; Step (2): the ball-milled precursor obtained in step (1) is vacuum dried to obtain a positive electrode material precursor, and the positive electrode material precursor is crushed and sieved; Step (3): the precursor obtained in step (2) is calcined under a nitrogen sintering atmosphere, the heating rate is 3 ℃ / min, the sintering temperature is 350 ℃, the holding time is 4 h, then the temperature is increased to 500 ℃ at a heating rate of 3 ℃ / min and held for 10 h, and then naturally cooled to room temperature to obtain a carbon-coated sodium iron phosphate pyrophosphate positive electrode material; Step (4): the carbon-coated sodium iron phosphate pyrophosphate positive electrode material (NFPP@C positive electrode material) obtained in step (3) is crushed and sieved through a 300 mesh screen; Step (5): 0.26 g of titanium dioxide powder is added to an appropriate amount of deionized water to form a suspension under magnetic stirring, 0.1 mol / L concentrated nitric acid is added dropwise, the pH of the suspension is adjusted to 2-3, and the suspension is placed in an ultrasonic cleaner for ultrasonic treatment for 30 min to obtain a uniform titanium dioxide sol. Then 1.6 g of melamine (to ensure sufficient nitrogen source) is weighed and added to an appropriate amount of deionized water, heated under magnetic stirring to completely dissolve to form a melamine solution with a concentration of 0.075-0.15 mol / L.

[0038] Step (6): under stirring, 10 g of ground NFPP@C powder is slowly added to the titanium dioxide solution obtained in step 5), and the mixture is mixed uniformly under magnetic stirring, and the melamine solution is added to the above solution at a rate of 0.5 mL / min through a constant pressure dropping funnel (the dropping time is greater than 30.0 min to avoid local concentration being too high to cause the precursor to precipitate), and the stirring is continued for 3 h after the dropping is completed.

[0039] Step (7): The stirring mixed solution of step (6) was transferred to an evaporation dish and heated to evaporate at 60°C, so that the solution gradually gelled. The wet gel was placed in an oven and dried at 90°C for 12h to obtain a dry gel sample.

[0040] Step (8): The precursor obtained in step (7) was ground and sintered in a nitrogen sintering atmosphere, heated to 600°C at a heating rate of 5°C / min for 4h, and naturally cooled to room temperature. Step (9): The positive electrode material obtained in step (8) was crushed and sieved through a 300 mesh screen to obtain a titanium nitride coated Na4Fe3(PO4)2(P2O7)@TiN / nitrogen-doped carbon layer, and the mass of the coating layer accounted for 2% of the total raw material mass.

[0041] Preparation method of a sodium iron phosphate pyrophosphate material Na4Fe3(PO4)2(P2O7) The difference from Example 1 is that no coating modification is performed, and the preparation method comprises the following steps: Step (1): Sodium carbonate and disodium hydrogen phosphate, iron phosphate dihydrate were weighed according to the molar ratio of Na element, Fe element and P element of 4:3:4, and an appropriate amount of deionized water was added and mixed uniformly, and then ball milled in a planetary ball mill at 300r for 10h.

[0042] Step (2): The ball milled precursor obtained in step (1) was dried and the positive electrode material precursor was crushed and sieved. Step (3): The precursor obtained in step (2) was sintered in a nitrogen sintering atmosphere, the heating rate was 3°C / min, the sintering temperature was 350°C, the holding time was 4h, then the temperature was raised to 500°C at a heating rate of 3°C / min, and the temperature was held for 10h, and then naturally cooled to room temperature. Step (4): The positive electrode material obtained in step (3) was crushed and sieved through a 300 mesh screen to obtain NFPP finished product.

[0043] Preparation method of a sodium iron phosphate pyrophosphate material Na4Fe3(PO4)2(P2O7)@C The preparation method comprises the following steps, and the difference from Example 1 is that only the surface carbon layer is coated. Step (1): Sodium carbonate and disodium hydrogen phosphate, iron phosphate dihydrate were weighed according to the molar ratio of Na element, Fe element and P element of 4:3:4, and an appropriate amount of deionized water was added and mixed uniformly, and then ball milled in a planetary ball mill at 300r for 10h.

[0044] Step (2): The ball milled precursor obtained in step (1) was dried and the positive electrode material precursor was crushed and sieved. Step (3): sintering the precursor obtained in step (2) in a nitrogen sintering atmosphere, the heating rate is 3℃ / min, the sintering temperature is 350℃, the holding time is 4h, then the temperature is increased to 500℃ at a heating rate of 3℃ / min and the holding time is 10h, and then the temperature is naturally cooled to room temperature; Step (4): crushing and sieving the positive electrode material obtained in step (3) through a 300 mesh screen to obtain the NFPP@C finished product.

[0045] The prepared positive electrode sheet and metal sodium (thickness of about 1mm) were combined to form a CR2032 button cell in an Ar-filled glove box. The separator was Whatman GF / D glass fiber, the electrolyte was a 1 mol / L NaPF6 solution, the solvent was EC (ethylene carbonate) / diethyl carbonate (DEC) (v:v = 1:1) containing 5vol% fluoroethylene carbonate (FEC) additive. The obtained CR2032 button cell was subjected to constant current charge and discharge test on a new battery test system, and the test voltage range was 2-4V. The performance data of the sodium ion battery positive electrode materials obtained in Examples 1, 2, 3, Comparative Examples 1, 2 tested under the same conditions are shown in Table 1: Table 1

[0046] As can be seen from Table 1, the electrochemical performance, electrode sheet compaction density and membrane sheet resistivity of the positive electrode materials of Examples 1-3 are significantly better than those of Comparative Examples 1 and 2.

[0047] Figure 1 The XRD image of the sodium ion battery positive electrode material obtained in Example 1 and Comparative Example 1 is shown. As can be seen from the figure, a clear peak can be observed at 36.7° and 42.6° in the sample of Example 1, which is related to the titanium nitride coating material, which confirms the presence of the titanium nitride coating layer in the sample.

[0048] Figure 2 The first cycle charge and discharge curve of the positive electrode material of Example 1 and Comparative Example 1 applied in a sodium ion battery is shown. As can be seen, under the condition of a current density of 10mA / g, the first cycle discharge specific capacity of Example 1 is 104.62mAh / g, which is significantly better than the discharge specific capacity of 84.35mAh / g of Comparative Example 1; Figure 3 The cycle performance graph of the positive electrode material of Example and Comparative Example applied in a sodium ion battery under different rate conditions is shown. As can be seen, the rate performance of the material of Example is significantly better than that of Comparative Example, which proves that the synergistic effect of titanium nitride and carbon layer can improve the electrochemical performance of the material, the titanium nitride conductive layer accelerates electron transmission, the carbon layer optimizes ion diffusion path, and effectively improves the rate performance of the material.

[0049] Comparative Example 3 A sodium iron pyrophosphate material Na4Fe3(PO4)2(P2O7)@M x N y Preparation method The difference between this comparative example and Example 1 is that the amount of the coating layer of M x N y is different, specifically: Step (1): Sodium source, iron source, and phosphorus source were weighed in a molar ratio of 4:3:4, i.e. sodium carbonate and disodium hydrogen phosphate, iron phosphate dihydrate; 5% of the total raw material mass of anhydrous glucose was weighed as a carbon source and mixed uniformly with a solvent, and then an appropriate amount of deionized water was added, and the mixture was dispersed in a planetary ball mill at 300 r for 10 h; Step (2): The ball-milled precursor obtained in step (1) was dried at 80°C under vacuum for 12 hours to obtain a positive electrode material precursor, and the positive electrode material precursor was crushed and sieved; Step (3): The precursor obtained in step (2) was calcined under a nitrogen sintering atmosphere, with a heating rate of 3°C / min, a sintering temperature of 350°C, and a holding time of 4 h, and then heated to 500°C at a heating rate of 3°C / min for 10 h, and naturally cooled to room temperature to obtain a carbon-coated sodium iron pyrophosphate positive electrode material; Step (4): The carbon-coated sodium iron pyrophosphate positive electrode material obtained in step (3) was crushed and sieved through a 300-mesh sieve; Step (5): 3.9 g of dicyandiamide (a nitrogen source) was dissolved in 50 mL of ethanol, and then 7.5 g of TiCl3 (a titanium source) was added and ultrasonically dispersed for 20 min; Step (6): 100 g of the NFPP@C powder obtained in step (4) was added to the solution obtained in step (5), stirred at 60°C for 2 h to allow the active components to be adsorbed on the surface, and then sieved after complete drying in a vacuum environment at 80°C; Step (7): The precursor obtained in step (6) was sintered in a nitrogen sintering atmosphere, with a heating rate of 3°C / min, a sintering temperature of 350°C, and a holding time of 3 h, and then heated to 580°C at a heating rate of 3°C / min for 5 h, and naturally cooled to room temperature; Step (8): The positive electrode material obtained in step (7) was crushed and sieved through a 300-mesh sieve to obtain the final product Na4Fe3(PO4)2(P2O7)@TiN / nitrogen-doped carbon layer, and the mass of the coating layer accounted for 5% of the total raw material mass.

[0050] The performance data of the obtained sodium iron pyrophosphate material Na4Fe3(PO4)2(P2O7)@TiN / nitrogen-doped carbon layer as a sodium ion battery positive electrode material tested under the same conditions are shown in Table 2: Table 2

[0051] The above embodiments only express the specific implementation of the present application, which is described in more detail and in more detail, but it cannot be understood as a limitation on the protection scope of the present application. It should be noted that for ordinary skilled in the art, without departing from the technical concept of the present application, a number of modifications and improvements can be made, which are within the scope of protection of the present application.

Claims

1. A coated modified sodium iron phosphate composite cathode material, characterized in that, Chemical formula: Na4Fe3(PO4)2(P2O7)@M x N y ; Wherein M is one or more of C, Ti, X is 0.01-1; Y is 0.01-1; wherein M x N y The coating amount of Na4Fe3(PO4)2(P2O7) is 0.5-5 wt%.

2. The coated and modified composite sodium iron phosphate cathode material according to claim 1, characterized in that, M x N y The cladding layer is a composite cladding layer, and the components thereof from inside to outside are a nitrogen-doped carbon layer and TiN.

3. The method for preparing a coated and modified composite sodium iron phosphate cathode material as described in claim 1, characterized in that, The method comprises the following steps: Step (1): a carbon-coated sodium iron phosphate pyrophosphate positive material is prepared; Step (2): the composite sodium iron phosphate material obtained in step (1) is mixed with a titanium source and a nitrogen source uniformly, and then subjected to secondary sintering to obtain a M x N y The composite coated sodium iron pyrophosphate positive electrode material.

4. The method of claim 3, wherein the coated modified sodium iron phosphate composite cathode material is prepared by the steps of: (a) mixing a sodium iron phosphate composite with a coating agent; (b) heating the mixture to a temperature of 300-600 °C; (c) cooling the mixture to room temperature; and (d) washing the mixture with water. The titanium source is at least one of titanium dioxide, tetrabutyl titanate, vinyl titanate, titanium trichloride, titanium tetrachloride.

5. The method of claim 3, wherein the coated modified sodium iron phosphate composite cathode material is prepared by the steps of: (a) mixing a sodium iron phosphate composite with a coating agent; (b) heating the mixture to a temperature of 300-600 °C; (c) cooling the mixture to room temperature; and (d) washing the mixture with water. The nitrogen source is at least one of urea, melamine, ethylenediamine, cyanamide, dicyandiamide.

6. The method of claim 3, wherein the coated modified sodium iron phosphate composite cathode material is prepared by the steps of: (a) mixing a sodium iron phosphate composite with a coating agent; (b) heating the mixture to a temperature of 600-800°C; and (c) cooling the mixture to room temperature. The iron source is at least one of iron phosphate dihydrate, anhydrous iron phosphate, iron oxide.

7. The method for preparing a coated and modified composite sodium iron phosphate cathode material according to claim 3, characterized in that, The M x N y The mass of the composite coating layer accounts for 0.5-5wt% of the mass of the sodium iron phosphate pyrophosphate, and the total raw material is composed of a sodium source, an iron source, a phosphorus source, a carbon source used for preparing the sodium iron phosphate pyrophosphate positive electrode material, and a titanium source and a nitrogen source used for preparing the composite layer.

8. The method of claim 3, wherein the coated modified sodium iron phosphate composite cathode material is prepared by the steps of: (a) mixing a sodium iron phosphate composite with a coating agent; (b) heating the mixture to a temperature of 600-800°C; (c) cooling the mixture to room temperature; and (d) washing the mixture with water. The sintering parameters of the secondary sintering are: sintering temperature is 450-700℃, calcination time is 2-15 h; the sintering heating rate is 1-10℃ / min.

9. The method of claim 3, wherein the coated modified sodium iron phosphate composite cathode material is prepared by the steps of: (a) mixing a sodium iron phosphate composite with a coating agent; (b) heating the mixture to a temperature of 300-600 °C; (c) cooling the mixture to room temperature; and (d) washing the mixture with water. The sintering inert atmosphere of the secondary sintering includes one of nitrogen, argon, argon-hydrogen mixed gas, nitrogen-hydrogen mixed gas.

10. A sodium ion battery comprising a battery positive electrode prepared by using the composite sodium iron phosphate positive material prepared according to claim 1 or 2 or the preparation method of claims 3-9.

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