Preparation method and application of ferric sodium pyrophosphate positive electrode composite material

Abstract

The invention discloses a preparation method and application of a ferric sodium pyrophosphate positive electrode composite material, and belongs to the technical field of sodium battery positive electrode materials. The method comprises the following steps: dissolving lithium fluoride and titanium aluminum carbide in concentrated hydrochloric acid, and carrying out etching reaction to obtain a micron-sized MXene material; adding the nano-scale MXene material into deionized water for hydrothermal reaction, and freeze-drying to obtain a nano-scale MXene material; the preparation method comprises the following steps: adding an inorganic salt containing sodium and phosphorus, a nanoscale MXene material, an inorganic iron salt, an organic acid carbon source and ethylene glycol into deionized water, heating and stirring, heating and evaporating to obtain a gel precursor material, and performing vacuum freeze drying and calcining to obtain an NFPP-coated MXene material; and adding the ferric sodium pyrophosphate into deionized water, adding aluminum nitrate nonahydrate, adjusting the pH value to weak acidity, adding ammonium fluoride, reacting, filtering, washing, drying and calcining to obtain the ferric sodium pyrophosphate positive electrode composite material. And the prepared composite material is used for the sodium ion battery, and has high specific capacity, good cycle performance and good rate capability.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery electrode material preparation technology, specifically to a method for preparing and applying a sodium iron pyrophosphate cathode composite material. Background Technology

[0002] The use of sustainable and clean energy sources such as wind and solar power to replace traditional fossil fuels is developing on a large scale, making the storage of highly intermittent and unschedulable energy sources a critical issue that needs to be addressed. Among the many energy storage technologies, lithium-ion batteries are widely used in portable electronic devices and electric vehicles due to their high energy density and specific capacity. However, with the depletion of lithium resources, the difficulty of mining, and the continuous price increases in recent years, their cost has become increasingly expensive. Sodium-ion batteries, due to their similar structure to lithium-ion batteries and their lower cost, are also gaining increasing attention.

[0003] Sodium-ion batteries, with their abundant resources, significant cost-effectiveness, and excellent environmental compatibility, have shown great promise for large-scale energy storage applications. As a core component of energy storage systems, their cathode materials must possess long cycle life, high safety, and excellent rate performance to meet complex operating conditions. Na4Fe3(PO4)-P2O7 (NFPP), due to its unique hybrid polyanionic framework structure, utilizes (PO4)... 3- The primitives construct a stable three-dimensional framework, while (P2O7) 4- The inductive effect of the functional group endows it with a high redox potential, combined with Fe 2+ / Fe 3+ The dual redox pair achieves a three-electron transfer reaction (theoretical specific capacity 128 mAh / g), demonstrating significant comprehensive performance advantages.

[0004] However, several key scientific problems still need to be solved in this material system: (1) The poor intrinsic conductivity results in low electron transport efficiency, and the actual reversible capacity is far less than the theoretical value; (2) The side reaction kinetics at the electrode / electrolyte interface are intensified, and the irreversible deposition of electrolyte decomposition products (such as Na2CO3 and NaF) during long-term cycling leads to poor cycling stability; (3) Volume expansion under high-rate conditions leads to lower porosity, and the increased local ion concentration polarization causes a sharp decline in rate performance (5C capacity is only 40% of 0.1C capacity). Therefore, new technologies are urgently needed to optimize the above-mentioned shortcomings so that NFPP materials can meet practical needs.

[0005] Chinese patent document CN118398810A discloses a method for preparing sodium iron pyrophosphate cathode material using sodium polyphosphate as a sodium source and its application. This method utilizes the high chelating effect of sodium polyphosphate to reduce grinding difficulty and agglomeration during the grinding process, thereby enhancing the purity of the sintered material and improving its specific capacity. After 100 cycles at 0.1C, the specific capacity is 102.5 mAh·g. -1 The retention rate was 99.2%, but it failed to change the poor conductivity of sodium iron pyrophosphate material itself and the occurrence of side reactions with the electrolyte, resulting in poor cycle stability. Summary of the Invention

[0006] The purpose of this invention is to provide a method for preparing and applying a high-capacity, high-rate, and high-cycle-stability sodium iron pyrophosphate cathode composite material.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: In a first aspect, the present invention provides a method for preparing a sodium iron pyrophosphate cathode composite material, comprising the following steps: (1) Lithium fluoride and titanium aluminum carbide were dissolved in concentrated hydrochloric acid and etched. The mixture was washed and filtered to obtain micron-sized MXene material. The micron-sized MXene material was added to deionized water for hydrothermal reaction. The mixture was centrifuged, washed, centrifuged again, and freeze-dried to obtain nano-sized MXene material. (2) Add sodium and phosphorus-containing inorganic salts, nano-sized MXene materials, inorganic iron salts, organic acid carbon sources, and ethylene glycol to deionized water, heat and stir, and evaporate at a higher temperature to obtain a gel-like precursor material. Vacuum freeze-dry and calcinate to obtain NFPP@MXene material. (3) Take NFPP@MXene material and add it to deionized water, then add aluminum nitrate nonahydrate to obtain a mixture. Adjust the pH of the mixture to weak acidity with dilute nitric acid, then add ammonium fluoride, stir the reaction, filter, wash, dry and calcine to obtain sodium iron pyrophosphate cathode composite material.

[0008] Further, in step (1), the concentration of concentrated hydrochloric acid is 5-10 mol / L; the molar ratio of lithium fluoride to titanium aluminum carbide is 1:0.668-0.0888; the etching reaction temperature is 40-60℃ and the reaction time is 24-36h; the hydrothermal reaction temperature is 100-150℃ and the reaction time is 2-6h.

[0009] Further, in step (2), the inorganic salt containing sodium and phosphorus is sodium hypophosphite or sodium dihydrogen phosphate, the inorganic iron salt is ferric nitrate or ferric chloride, and the organic acid carbon source is citric acid or glucose; the heating and stirring temperature is 50-70℃, and the time is 4-6h; the molar mass ratio of the inorganic salt containing sodium and phosphorus to the nano-sized MXene material is 1mol:2-20g, and the molar volume ratio of the inorganic salt containing sodium and phosphorus to the inorganic iron salt, organic acid carbon source, and ethylene glycol is 1mol:0.6-0.7mol:0.1-0.2mol:2-4mL; the calcination conditions are: first, pre-sintering at 250-300℃ for 2-3h under an Ar / H2 atmosphere, followed by calcination at 500-600℃ for 8-12h.

[0010] Furthermore, in step (2), freeze-drying involves cooling the gel-like precursor material to -50°C for 1 hour, followed by vacuum drying for 12-20 hours.

[0011] Further, in step (3), the pH value of the mixture after adjustment with dilute nitric acid is 4-6; the mass molar ratio of NFPP@MXene material to aluminum nitrate nonahydrate and ammonium fluoride is 1g:0.0001-0.0005mol:0.0003-0.0015mol; the drying temperature is 80-120℃, and the drying time is 2-4h; the calcination atmosphere is Ar, the calcination temperature is 400-500℃, and the calcination time is 3-5h.

[0012] The present invention also provides the application of the sodium iron pyrophosphate cathode composite material prepared by the above method in sodium-ion batteries.

[0013] Compared with the prior art, the technical solution of the present invention has the following beneficial effects: The inventors discovered that in the conventional technology, high-temperature evaporation and drying followed by calcination are used to obtain NFPP@MXene materials. This results in the collapse of pores on the material surface and severe agglomeration, leading to a decrease in specific surface area. Furthermore, it hinders the reaction between the active sites on the material surface and the electrolyte, weakens the diffusion effect of sodium ions, increases the polarization effect of the electrode material, and reduces conductivity, thus decreasing its specific capacity. In step (2) of this invention, a gel-like precursor material is first obtained through high-temperature evaporation, followed by vacuum freeze-drying and calcination to obtain NFPP@MXene material. This invention obtains a sodium iron pyrophosphate positive electrode through the synergistic effect of high-temperature evaporation and vacuum freeze-drying. Compared with the composite materials obtained by high-temperature evaporation and drying in the existing technology, the composite material can avoid the pore collapse caused by liquid phase evaporation, form a uniform wrinkled pore structure on the material surface, significantly increase the specific surface area, and suppress the agglomeration effect, making the material structure more uniform. This material structure can form a stable conductive network, reduce the bulk resistance of the material, reduce the polarization effect, and increase the exposure of active sites, promote electrolyte wetting and sodium ion diffusion, thereby improving the conductivity and specific capacity of the material. If the gel-like precursor material is not obtained by high-temperature evaporation in step (2), and the reaction liquid is directly subjected to vacuum freeze-drying, this process will result in the composite material structure being too loose.

[0014] The inventors discovered that conventional high-temperature evaporation and drying processes in existing technologies lead to the oxidation of functional groups on the surface of MXene materials, preventing MXene from forming stable chemical bonds with the electrode material NFPP and the coating layer, resulting in poor interfacial bonding and deterioration of the material's mechanical properties. This invention employs a combined process of high-temperature evaporation and freeze-drying, which can reduce the oxidation of functional groups on the surface of MXene materials caused by high temperatures and is beneficial to improving the bonding force between MXene, NFPP, and AlF3.

[0015] The inventors discovered that MXene material possesses extremely high conductivity. Existing technologies only coat the surface of electrode materials with MXene material. This coating process has two problems: first, the coating only reduces the interfacial resistance of the material, but does not reduce the overall bulk resistance of the material; second, the coating hinders the exposure of active sites on the NFPP material interface, reducing the diffusion effect of sodium ions. This invention incorporates MXene material into NFPP material. Compared with traditional coating processes, this significantly reduces the bulk resistance of NFPP material and increases its conductivity. At the same time, the layered structure of MXene enables the formation of a stable three-dimensional conductive network inside the NFPP material, allowing the electrolyte to penetrate into the electrode material and undergo redox reactions with the active substances inside the electrode material, accelerating the electron transport rate inside the electrode and thus improving the specific capacity of the electrode material.

[0016] In the prior art, the high-rate performance of NFPP materials is poor, mainly because the sodium insertion / extraction process of NFPP materials causes volume expansion under high-rate conditions, affecting the normal diffusion path of sodium ions. The inventors discovered that the MXene phase has a stable interlayer structure, which can first provide additional sodium ion transport channels, so that sodium ions also have good diffusion efficiency under high-rate conditions. In addition, the stable structure of MXene can suppress the volume expansion effect of NFPP materials, further improving the diffusion efficiency of sodium ions, thereby improving the rate performance under high-rate conditions.

[0017] The inventors discovered that AlF3, as an inert coating layer, can isolate the electrolyte from direct contact with the MXene / NFPP composite material, inhibit the corrosive effect of the electrolyte, reduce the occurrence of side reactions, and improve cycle performance. The high melting point (1040℃) and excellent thermal stability of AlF3 can enhance the structural stability of the composite material at high temperatures and improve its rate performance. However, coating only NFPP with AlF3 increases the interfacial resistance of NFPP and reduces its conductivity. Furthermore, the interfacial bonding between AlF3 and NFPP is poor, making the AlF3 layer easily detach, affecting the material's cycle performance and interfacial bonding ability. Compared to using only AlF3... Compared to AlF3 coating on the surface of NFPP, this invention first dops MXene into the NFPP material and then coats the surface of the NFPP material with an AlF3 layer. The abundant functional groups on the surface of MXene can form stable chemical bonds with the NFPP material and the AlF3 coating layer, improving the interlayer bonding force of the composite material, reducing coating layer shedding, and enhancing the cycling performance of the electrode material. At the same time, the inventors found that AlF3 can protect MXene from high-temperature oxidation at high rates, maintain the integrity of its conductive network, and avoid a decrease in conductivity during cycling. Therefore, the synergistic effect of the combination of the two can break through the performance limit of single modification and greatly enhance the electrochemical performance of the electrode composite material. Attached Figure Description

[0018] To more clearly illustrate the technical solutions of this invention, the accompanying drawings used in the embodiments will be briefly described below. Figure 1 SEM images of sodium iron pyrophosphate cathode composite materials are shown, where (a) is the sodium iron pyrophosphate cathode composite material prepared in Example 1, and (b) is the sodium iron pyrophosphate cathode composite material prepared in Comparative Example 1. Figure 2 These are the XPS curves of elements A1 (left) and F (right) for Examples 1, 5, 2, and 3. Detailed Implementation

[0019] The present invention will be further described in detail below with reference to embodiments. The accompanying drawings and embodiments are for illustrative purposes only and do not constitute an undue limitation of the present invention. Example 1

[0020] (1) Take 0.1 mol LiF and slowly pour it into 40 mL of 5 mol / L concentrated hydrochloric acid. Stir for 10 min, add 0.00888 mol Ti3AlC2, and stir at 40℃ for 36 h to complete the etching and obtain a suspension. Then pour the suspension into a centrifuge tube and wash it continuously with deionized water until the pH of the washing solution is 7. After centrifugation, obtain micron-sized MXene material. Then disperse the micron-sized MXene material in deionized water and perform hydrothermal reaction at 100℃ for 6 h. Centrifuge the reaction solution and wash it with deionized water. Repeat 3 times. After the last centrifugation, perform vacuum freeze-drying to obtain nano-sized MXene material. (2) Weigh 0.5 mol sodium dihydrogen phosphate and 1 g nano MXene into 200 mL deionized water and stir for 1 h. Then add 0.3 mol ferric nitrate nonahydrate, 0.05 mol citric acid and 1 mL ethylene glycol. Heat and stir at 50 °C for 6 h. Then heat to 100 °C to evaporate to obtain a gel material. Then place the material in a freeze dryer and pre-freeze at -50 °C for 1 h. Then vacuum to 48.8 Pa and freeze dry for 12 h. Grind the reactants evenly with a mortar and pestle and place them in a tube furnace. Pre-sinter at 250 °C for 3 h under Ar / H2 atmosphere. Then calcine at 500 °C for 12 h to obtain NFPP@MXene material. (3) Take 1g of NFPP@MXene material, disperse it in deionized water and stir. Add 0.0001mol of aluminum nitrate nonahydrate and adjust the pH to 4 with dilute nitric acid. Stir for 1h, then add 0.0003mol of ammonium fluoride and continue stirring for 3h. Then filter, wash, and dry in an oven at 80℃ for 4h to obtain the precursor. Place the precursor in a tube furnace and calcine at 400℃ for 5h under Ar atmosphere to obtain the sodium iron pyrophosphate cathode composite material, denoted as AF. 0.0001 -NFPP@MXene-2. Example 2

[0021] (1) Take 0.1 mol LiF and slowly pour it into 40 mL of 5 mol / L concentrated hydrochloric acid. Stir for 10 min, add 0.00888 mol Ti3AlC2, and stir at 40℃ for 36 h to complete the etching and obtain a suspension. Then pour the suspension into a centrifuge tube and wash it continuously with deionized water until the pH of the washing solution is 7. After centrifugation, obtain micron-sized MXene material. Then disperse the micron-sized MXene material in deionized water and perform hydrothermal reaction at 100℃ for 6 h. Centrifuge the reaction solution and wash it with deionized water. Repeat 3 times. After the last centrifugation, perform vacuum freeze-drying to obtain nano-sized MXene material. (2) Weigh 0.5 mol sodium dihydrogen phosphate and 5 g nano MXene into 200 mL deionized water and stir for 1 h. Then add 0.3 mol ferric nitrate nonahydrate, 0.05 mol citric acid and 1 mL ethylene glycol. Heat and stir at 50 °C for 6 h. Then raise the temperature to 100 °C to evaporate and obtain a gel material. Then place the material in a freeze dryer and pre-freeze at -50 °C for 1 h. Then evacuate to 48.8 Pa and freeze dry for 20 h to obtain the precursor. Grind the precursor evenly with a mortar and pestle and place it in a tube furnace. Pre-sinter at 250 °C for 3 h under Ar / H2 atmosphere. Then calcine at 500 °C for 12 h to obtain NFPP@MXene material.

[0022] (3) Take 1g of NFPP@MXene material, disperse it in deionized water and stir. Add 0.0001mol of aluminum nitrate nonahydrate and adjust the pH to 4 with dilute nitric acid. Stir for 1h, then add 0.0003mol of ammonium fluoride and continue stirring for 3h. Then filter, wash, and dry in an oven at 80℃ for 4h to obtain the precursor. Place the precursor in a tube furnace and calcine at 400℃ for 5h under Ar atmosphere to obtain the sodium iron pyrophosphate cathode composite material, denoted as AF. 0.0001 -NFPP@MXene- 10 . Example 3

[0023] (1) Take 0.1 mol LiF and slowly pour it into 40 mL of 5 mol / L concentrated hydrochloric acid. Stir for 10 min, add 0.00888 mol Ti3AlC2, and stir at 40℃ for 36 h to complete the etching and obtain a suspension. Then pour the suspension into a centrifuge tube and wash it continuously with deionized water until the pH of the washing solution is 7. After centrifugation, obtain micron-sized MXene material. Then disperse the micron-sized MXene material in deionized water and perform hydrothermal reaction at 100℃ for 6 h. Centrifuge the reaction solution and wash it with deionized water. Repeat 3 times. After the last centrifugation, perform vacuum freeze-drying to obtain nano-sized MXene material. (2) Weigh 0.5 mol sodium dihydrogen phosphate and 10 g nano MXene into 200 mL deionized water and stir for 1 h. Then add 0.3 mol ferric nitrate nonahydrate, 0.05 mol citric acid and 1 mL ethylene glycol. Heat and stir at 50 °C for 6 h. Then raise the temperature to 100 °C to evaporate and obtain a gel material. Then place the material in a freeze dryer and pre-freeze at -50 °C for 1 h. Then vacuum to 48.8 MPa and freeze dry for 15 h to obtain the precursor. Grind the precursor evenly with a mortar and place it in a tube furnace. Pre-sinter at 250 °C for 3 h under Ar / H2 atmosphere. Then calcine at 500 °C for 12 h to obtain NFPP@MXene material.

[0024] (3) Take 1g of NFPP@MXene material, disperse it in deionized water and stir. Add 0.0001mol of aluminum nitrate nonahydrate and adjust the pH to 5 with dilute nitric acid. Stir for 1h, then add 0.0003mol of ammonium fluoride and continue stirring for 3h. Then filter, wash, and dry in an oven at 80℃ for 4h to obtain the precursor. Place the precursor in a tube furnace and calcine at 400℃ for 5h under Ar atmosphere to obtain the sodium iron pyrophosphate cathode composite material, denoted as AF. 0.0001 -NFPP@MXene- 20 . Example 4

[0025] (1) Take 0.1 mol LiF and slowly pour it into 40 mL of 8 mol / L concentrated hydrochloric acid. Stir for 10 min, add 0.00768 mol Ti3AlC2, and stir at 50℃ for 30 h to complete the etching and obtain a suspension. Then pour the suspension into a centrifuge tube and wash it continuously with deionized water until the pH of the washing solution is 7. After centrifugation, obtain micron-sized MXene material. Then disperse the micron-sized MXene material in deionized water and perform hydrothermal reaction at 120℃ for 5 h. Centrifuge the reaction solution and wash it with deionized water. Repeat 3 times. After the last centrifugation, perform vacuum freeze-drying to obtain nano-sized MXene material. (2) Weigh 0.5 mol sodium hypophosphite and 1 g nano MXene into 200 mL deionized water and stir for 1 h. Then add 0.33 mol ferric chloride and 0.08 mol glucose, heat and stir at 60 °C for 5 h, and add 1.5 mL ethylene glycol. Then heat to 110 °C to evaporate the solution to obtain a gel material. Then place the material in a freeze dryer and pre-freeze at -50 °C for 1 h. Then vacuum to 48.8 Pa and freeze dry for 15 h to obtain the precursor. Grind the precursor evenly with a mortar and place it in a tube furnace. Pre-sinter at 300 °C for 2 h under Ar / H2 atmosphere, and then calcine at 550 °C for 10 h to obtain the cathode material NFPP@MXene.

[0026] (3) Take 1g of NFPP@MXene material and disperse it in deionized water and stir. Add 0.0003mol of aluminum nitrate nonahydrate and adjust the pH to 5 with dilute nitric acid. Stir for 1h, then add 0.0009mol of ammonium fluoride and continue stirring for 3h. Then filter, wash, and dry in an oven at 100℃ for 3h to obtain the precursor. Place the precursor in a tube furnace and calcine at 450℃ for 4h under Ar atmosphere to obtain the sodium iron pyrophosphate cathode composite material, denoted as AF. 0.0003 -NFPP@MXene-2. Example 5

[0027] (1) Take 0.1 mol LiF and slowly pour it into 40 mL of 8 mol / L concentrated hydrochloric acid. Stir for 10 min, add 0.00768 mol Ti3AlC2, and stir at 50℃ for 30 h to complete the etching and obtain a suspension. Then pour the suspension into a centrifuge tube and wash it continuously with deionized water until the pH of the washing solution is 7. After centrifugation, obtain micron-sized MXene material. Then disperse the micron-sized MXene material in deionized water and perform hydrothermal reaction at 120℃ for 5 h. Centrifuge the reaction solution and wash it with deionized water. Repeat 3 times. After the last centrifugation, perform vacuum freeze-drying to obtain nano-sized MXene material. (2) Weigh 0.5 mol sodium hypophosphite and 5 g nano MXene into 200 mL deionized water and stir for 1 h. Then add 0.33 mol ferric chloride and 0.08 mol glucose, heat and stir at 60 °C for 5 h while adding 1.5 mL ethylene glycol. Then raise the temperature to 110 °C to evaporate the solution to obtain a gel-like material. Then place the material in a freeze dryer and pre-freeze at -50 °C for 1 h. Then evacuate to 48.8 Pa and freeze dry for 16 h to obtain the precursor. Grind the precursor evenly with a mortar and place it in a tube furnace. Pre-sinter at 300 °C for 2 h under Ar / H2 atmosphere. Then calcine at 550 °C for 10 h to obtain the cathode material NFPP@MXene.

[0028] (3) Take 1g of NFPP@MXene material and disperse it in deionized water and stir. Add 0.0003mol of aluminum nitrate nonahydrate and adjust the pH to 5 with dilute nitric acid. Stir for 1h, then add 0.0009mol of ammonium fluoride and continue stirring for 3h. Then filter, wash, and dry in an oven at 100℃ for 3h to obtain the precursor. Place the precursor in a tube furnace and calcine at 450℃ for 4h under Ar atmosphere to obtain the sodium iron pyrophosphate cathode composite material, denoted as AF. 0.0003 -NFPP@MXene- 10 . Example 6

[0029] (1) Take 0.1 mol LiF and slowly pour it into 40 mL of 8 mol / L concentrated hydrochloric acid. Stir for 10 min, add 0.00768 mol Ti3AlC2, and stir at 50℃ for 30 h to complete the etching and obtain a suspension. Then pour the suspension into a centrifuge tube and wash it continuously with deionized water until the pH of the washing solution is 7. After centrifugation, obtain micron-sized MXene material. Then disperse the micron-sized MXene material in deionized water and perform hydrothermal reaction at 120℃ for 5 h. Centrifuge the reaction solution and wash it with deionized water. Repeat 3 times. After the last centrifugation, perform vacuum freeze-drying to obtain nano-sized MXene material. (2) Weigh 0.5 mol sodium hypophosphite and 10 g nano MXene into 200 mL deionized water and stir for 1 h. Then add 0.33 mol ferric chloride and 0.08 mol glucose, heat and stir at 60 °C for 5 h while adding 1.5 mL ethylene glycol. Then raise the temperature to 110 °C to evaporate the solution to obtain a gel-like material. Then place the material in a freeze dryer and pre-freeze at -50 °C for 1 h. Then evacuate to 48.8 Pa and freeze dry for 18 h to obtain the precursor. Grind the precursor evenly with a mortar and place it in a tube furnace. Pre-sinter at 300 °C for 2 h under Ar / H2 atmosphere. Then calcine at 550 °C for 10 h to obtain the cathode material NFPP@MXene.

[0030] (3) Take 1g of NFPP@MXene material and disperse it in deionized water and stir. Add 0.0003mol of aluminum nitrate nonahydrate and adjust the pH to 5 with dilute nitric acid. Stir for 1h, then add 0.0009mol of ammonium fluoride and continue stirring for 3h. Then filter, wash, and dry in an oven at 100℃ for 3h to obtain the precursor. Place the precursor in a tube furnace and calcine at 450℃ for 4h under Ar atmosphere to obtain the sodium iron pyrophosphate cathode composite material, denoted as AF. 0.0003 -NFPP@MXene- 20 . Example 7

[0031] (1) Take 0.1 mol LiF and slowly pour it into 40 mL of 10 mol / L concentrated hydrochloric acid. Stir for 10 min, add 0.00668 mol Ti3AlC2, and stir at 60℃ for 24 h to complete the etching and obtain a suspension. Then pour the suspension into a centrifuge tube and wash it continuously with deionized water until the pH of the washing solution is 7. After centrifugation, obtain micron-sized MXene material. Then disperse the micron-sized MXene material in deionized water and perform hydrothermal reaction at 150℃ for 2 h. Centrifuge the reaction solution and wash it with deionized water. Repeat 3 times. After the last centrifugation, perform vacuum freeze-drying to obtain nano-sized MXene material. (2) Weigh 0.5 mol sodium dihydrogen phosphate and 1 g nano MXene into 200 mL deionized water and stir for 1 h. Then add 0.35 mol ferric nitrate nonahydrate and 0.1 mol citric acid. Heat and stir at 70 °C for 4 h while adding 2 mL ethylene glycol. Then heat to 100 °C to evaporate the solution to obtain a gel-like material. Then place the material in a freeze dryer and pre-freeze at -50 °C for 1 h. Then pump to 48.8 Pa and freeze-dry under vacuum for 16 h to obtain the precursor. Grind the precursor evenly with a mortar and place it in a tube furnace. Pre-sinter at 300 °C for 2 h under Ar / H2 atmosphere. Then calcine at 600 °C for 8 h to obtain the cathode material NFPP@MXene.

[0032] (3) Take 1g of NFPP@MXene material, disperse it in deionized water and stir. Add 0.0005mol of aluminum nitrate nonahydrate and adjust the pH to 6 with dilute nitric acid. Stir for 1h, then add 0.0015mol of ammonium fluoride and continue stirring for 3h. Then filter, wash, and dry in an oven at 120℃ for 2h to obtain the precursor. Place the precursor in a tube furnace and calcine at 500℃ for 3h under Ar atmosphere to obtain the sodium iron pyrophosphate cathode composite material, denoted as AF. 0.0005 -NFPP@MXene-2. Example 8

[0033] (1) Take 0.1 mol LiF and slowly pour it into 40 mL of 10 mol / L concentrated hydrochloric acid. Stir for 10 min, add 0.00668 mol Ti3AlC2, and stir at 60℃ for 24 h to complete the etching and obtain a suspension. Then pour the suspension into a centrifuge tube and wash it continuously with deionized water until the pH of the washing solution is 7. After centrifugation, obtain micron-sized MXene material. Then disperse the micron-sized MXene material in deionized water and perform hydrothermal reaction at 150℃ for 2 h. Centrifuge the reaction solution and wash it with deionized water. Repeat 3 times. After the last centrifugation, perform vacuum freeze-drying to obtain nano-sized MXene material. (2) Weigh 0.5 mol sodium dihydrogen phosphate and 5 g nano MXene into 200 mL deionized water and stir for 1 h. Then add 0.35 mol ferric nitrate nonahydrate and 0.1 mol citric acid. Heat and stir at 70 °C for 4 h while adding 2 mL ethylene glycol. Then heat to 100 °C to evaporate the solution to obtain a gel-like material. Then place the material in a freeze dryer and pre-freeze at -50 °C for 1 h. Then vacuum to 48.8 Pa and freeze dry for 16 h to obtain the precursor. Grind the precursor evenly with a mortar and pestle and place it in a tube furnace. Pre-sinter at 300 °C for 2 h under Ar / H2 atmosphere. Then calcine at 600 °C for 8 h to obtain the cathode material NFPP@MXene.

[0034] (3) Take 1g of NFPP@MXene material, disperse it in deionized water and stir. Add 0.0005mol of aluminum nitrate nonahydrate and adjust the pH to 6 with dilute nitric acid. Stir for 1h, then add 0.0015mol of ammonium fluoride and continue stirring for 3h. Then filter, wash, and dry in an oven at 120℃ for 2h to obtain the precursor. Place the precursor in a tube furnace and calcine at 500℃ for 3h under Ar atmosphere to obtain the sodium iron pyrophosphate cathode composite material, denoted as AF. 0.0005 -NFPP@MXene- 10 . Example 9

[0035] (1) Take 0.1 mol LiF and slowly pour it into 40 mL of 10 mol / L concentrated hydrochloric acid. Stir for 10 min, add 0.00668 mol Ti3AlC2, and stir at 60℃ for 24 h to complete the etching and obtain a suspension. Then pour the suspension into a centrifuge tube and wash it continuously with deionized water until the pH of the washing solution is 7. After centrifugation, obtain micron-sized MXene material. Then disperse the micron-sized MXene material in deionized water and perform hydrothermal reaction at 150℃ for 2 h. Centrifuge the reaction solution and wash it with deionized water. Repeat 3 times. After the last centrifugation, perform vacuum freeze-drying to obtain nano-sized MXene material. (2) Weigh 0.5 mol sodium dihydrogen phosphate and 10 g nano MXene into 200 mL deionized water and stir for 1 h. Then add 0.35 mol ferric nitrate nonahydrate and 0.1 mol citric acid. Heat and stir at 70 °C for 4 h while adding 2 mL ethylene glycol. Then raise the temperature to 100 °C to evaporate the solution to obtain a gel-like material. Then place the material in a freeze dryer and pre-freeze at -50 °C for 1 h. Then evacuate to 48.8 Pa and freeze-dry for 18 h to obtain the precursor. Grind the precursor evenly with a mortar and pestle and place it in a tube furnace. Pre-sinter at 300 °C for 2 h under Ar / H2 atmosphere, and then calcine at 600 °C for 8 h to obtain the cathode material NFPP@MXene.

[0036] (3) Take 1g of NFPP@MXene material and disperse it in deionized water and stir. Add 0.0005mol of aluminum nitrate nonahydrate and adjust the pH to 6 with dilute nitric acid. Stir for 1h, then add 0.0015mol of ammonium fluoride and continue stirring for 3h. Then filter, wash, and dry in an oven at 120℃ for 2h to obtain the precursor. Place the precursor in a tube furnace and calcine at 400℃ for 5h under Ar atmosphere to obtain the sodium iron pyrophosphate cathode composite material, denoted as AF. 0.0005 -NFPP@MXene- 20 . Comparative Example 1

[0037] The difference from Example 1 is that, after evaporating the gel-like precursor material at 100°C, it was vacuum dried at 80°C, while all other conditions remained the same; the final obtained sodium iron pyrophosphate cathode composite material is denoted as AF. 0.0001 -NFPP@MXene-2-1. Comparative Example 2

[0038] (1) Take 0.1 mol LiF and slowly pour it into 40 mL of 5 mol / L concentrated hydrochloric acid. Stir for 10 min, add 0.00668 mol Ti3AlC2, and stir at 40℃ for 36 h to complete the etching and obtain a suspension. Then pour the suspension into a centrifuge tube and wash it continuously with deionized water until the pH of the washing solution is 7. After centrifugation, obtain micron-sized MXene material. Then disperse the micron-sized MXene material in deionized water and perform hydrothermal reaction at 100℃ for 6 h. Centrifuge the reaction solution and wash it with deionized water. Repeat 3 times. After the last centrifugation, perform vacuum freeze-drying to obtain nano-sized MXene material. (2) Weigh 0.5 mol sodium dihydrogen phosphate and 1 g nano MXene into 200 mL deionized water and stir for 1 h. Then add 0.3 mol ferric nitrate nonahydrate and 0.05 mol citric acid. Heat and stir at 50 °C for 6 h and add 1.5 mL ethylene glycol. Then heat to 100 °C to evaporate the solution to obtain a gel-like material. Finally, vacuum dry at 80 °C for 20 h in a vacuum drying oven to obtain the precursor. Place the precursor in a tube furnace and pre-sinter at 250 °C for 3 h in an Ar / H2 atmosphere. Then calcine at 550 °C for 10 h to obtain sodium iron pyrophosphate phosphate cathode composite material, denoted as NFPP@MXene. -2 . Comparative Example 3

[0039] (1) Weigh 0.5 mol of sodium dihydrogen phosphate into 200 mL of deionized water and stir for 1 h. Then add 0.3 mol of ferric nitrate nonahydrate and 0.05 mol of citric acid. Heat and stir at 50 °C for 6 h while adding 1.5 mL of ethylene glycol. Then raise the temperature to 100 °C to evaporate the solution to obtain a gel-like material. Then place the material in a freeze dryer and pre-freeze at -50 °C for 1 h. Then evacuate to 48.8 Pa and freeze-dry for 18 h to obtain the precursor. Place the precursor in a tube furnace and pre-sinter at 250 °C for 3 h under Ar / H2 atmosphere. Then calcine at 550 °C for 10 h to obtain the cathode material NFPP.

[0040] (2) Take 1g of NFPP@MXene material, disperse it in deionized water and stir. Add 0.0001mol of aluminum nitrate nonahydrate and adjust the pH to 5 with dilute nitric acid. Stir for 1h, then add 0.0003mol of ammonium fluoride and continue stirring for 3h. Then filter, wash and dry to obtain the precursor. Place the precursor in a tube furnace and calcine at 400℃ for 5h under Ar atmosphere to obtain sodium iron pyrophosphate cathode composite material, denoted as AF. 0.0001 -NFPP. Comparative Example 4

[0041] The amount of nano-MXene in step (2) of Example 1 was changed to 20g, while the other reaction conditions remained the same. The resulting sodium iron pyrophosphate cathode composite material was denoted as AF. 0.0001 -NFPP@MXene -40 . Comparative Example 5

[0042] In step (3) of Example 1, the amount of aluminum nitrate nonahydrate was changed to 0.001 mol, and the amount of ammonium fluoride was changed to 0.003 mol, while the other reaction conditions remained completely the same. The resulting sodium iron pyrophosphate cathode composite material was denoted as AF. 0.001 -NFPP@MXene-2. Comparative Example 6

[0043] The sodium iron pyrophosphate cathode material was prepared according to the experimental steps in patent CN118398810A. The specific reaction steps are as follows: (1) Add 0.25 mol sodium tetrapolyphosphate, 1.25 mol sodium carbonate and 3 mol ferric phosphate to a 0.3 mol / L glucose aqueous solution to obtain a mixed slurry.

[0044] (2) The mixed slurry is ground in a sand mill. The particle size of iron phosphate is used as the standard. When D50 is less than 500nm (grinding time is 0.5h), it is considered to be qualified. At this time, the precursor slurry is obtained. The precursor slurry is spray-dried to obtain the cathode material precursor.

[0045] (3) The cathode material precursor is placed in an atmosphere furnace and sintered in a nitrogen atmosphere at 550°C for 10 hours. The sintered product is then subjected to air jet pulverization to obtain sodium iron pyrophosphate cathode material, denoted as NFPP.

[0046] The composite materials prepared in the examples and comparative examples were used as positive electrodes. Sodium-ion button batteries were installed in a glove box in the order of positive electrode shell-positive electrode sheet-separator-sodium sheet-pad-spring sheet-negative electrode shell, and the electrochemical performance of each battery was tested under 0.1C conditions. The test results are shown in Table 1. At the same time, the rate performance of each battery under 20C conditions was tested. The test results are shown in Table 3.

[0047] The materials prepared in Example 1 and Comparative Example 1 were measured using a fully automated surface area and porosity analyzer (BET), and the results are shown in Table 2.

[0048] The materials prepared in the examples and comparative examples were subjected to scratch tests using a nano-scratch tester according to ASTM C1624-05 (2020) standard. A diamond indenter (tip radius 50 μm) was used, with a scratch length of 5 mm, a load range of 0-50 N, and a loading rate of 5 N / min. The critical load Lc was recorded to represent the strength of the interfacial bonding of the AlF3 coating layer. The test results are shown in Table 4. Furthermore, argon ion etching technology was used for XPS elemental analysis of the composite material to obtain and analyze the chemical bonding in the interfacial region. The results are as follows: Figure 2 As shown.

[0049] like Figure 1 As shown in Tables a and b, and in conjunction with Table 2, the specific surface area data of Example 1 and Comparative Example 1 demonstrate that changing the drying process from high-temperature drying to freeze-drying significantly increases the surface roughness of the material and reduces agglomeration, thereby increasing the surface area from 18.5 m². 2 / g increased to 30.8m 2 / g, thereby increasing the contact area between the material and the electrolyte, and enhancing the Na + The diffusion effect. As can also be seen in Table 1, the retention rate of Example 1 using freeze-drying after 2000 cycles (95.6%) is significantly better than that of Comparative Example 2 (84.2%).

[0050] from Figure 2 As can be seen from the XPS characteristic curves, compared with the precursor NFPP@MXene, the electrode material coated with AlF3 has obvious peaks for Al and F, and the peaks are more obvious as the content of AlF3 increases, which proves that AlF3 was successfully coated on the material surface. Furthermore, combined with the XPS data and scratch test data, it can be analyzed that compared with the single peak of F in Comparative Example 2, F in Examples 1 and 5 splits into two peaks. This indicates that after doping with MXene, F element forms a stable Ti-F bond with Ti element in MXene, which enhances the interfacial bonding force. In the critical load Lc data of scratch test in Table 3, it can also be seen that when MXene is doped, the Lc of the material increases significantly, indicating that MXene and AlF3 coating layer form a stable chemical structure.

[0051] As can be seen from the experimental data of the examples and comparative examples, the electrochemical performance of the composite electrode material prepared by the present invention is significantly better than that of the single NFPP material synthesized by the current existing technology. The initial specific capacity under 0.1C conditions is significantly improved, and the retention rate after 2000 cycles is also significantly improved.

[0052]

[0053] As can be seen from Table 1 and Table 2, the initial specific capacity and cycle stability of the examples are significantly improved compared with the comparative examples. This is attributed to the following factors: 1. In terms of process, high-temperature evaporation combined with vacuum freeze-drying replaces conventional heating and drying, avoiding the collapse of material pores caused by liquid phase evaporation. This can form a uniform pleated pore structure, significantly increasing the specific surface area and enhancing the wetting effect of the electrolyte. At the same time, it suppresses the agglomeration effect of the material, making the material structure more uniform. This structure can form a stable conductive network, reduce the bulk resistance of the material, reduce the polarization effect, and increase the exposure of active sites, promoting the diffusion effect of sodium ions, thereby improving the specific capacity of the material.

[0054] 2. The high conductivity and capacitance of MXene significantly improve the poor conductivity of NFPP materials, reduce the bulk resistance of NFPP materials, and improve the specific capacity of the materials.

[0055] 3. AlF3 has excellent chemical stability, which can significantly reduce the occurrence of side reactions between the electrode material surface and the electrolyte, and at the same time reduce the corrosive effect of the electrolyte on the electrode material, thus enhancing the cycle stability of the electrode material.

[0056] Combining the data in Tables 1 and 2, it can be seen that specific surface area has an impact on electrochemical performance. Comparing Example 1 and Comparative Example 1, Example 1 used a high-temperature evaporation combined with vacuum freeze-drying process, resulting in a specific surface area of ​​18.5 m². 2 / g increased to 30.8m 2 / g, correspondingly the initial specific capacity at 0.1C increased from 113.8mAh / g to 121.8mAh / g, and the retention rate after 2000 cycles increased from 84.2% to 95.6%.

[0057] Comparing the data from Example 1 and Comparative Examples 3 and 4 in Table 1, it can be seen that both excessively low and high MXene content affect the performance of the electrode material. Comparative Example 3 shows lower initial specific capacity and 2000-cycle performance at 0.1C compared to Example 1. This is because the absence of the MXene phase in the system fails to improve the inherent poor conductivity of the NFPP material, resulting in insufficient conductivity and thus lower initial specific capacity. Similarly, the data from Comparative Example 4 shows that excessively high MXene content also reduces the electrochemical performance of the material. This is because excessive MXene phase occupies too much volume, reducing the proportion of active material and thus lowering the overall energy density. Furthermore, excessive MXene leads to overly dense layered structures, hindering electrolyte penetration and ion diffusion, increasing interfacial impedance, and consequently reducing conductivity and specific capacity.

[0058] Comparing Example 1 and Comparative Examples 2 and 5 in Table 1, it can be seen that too much or too little AlF3 protective layer content can also affect the performance of composite electrode materials. The cycling performance of Comparative Example 2 at 0.1C for 2000 cycles is significantly weaker than that of Example 1. This is because when the sodium iron pyrophosphate cathode composite material system is not coated with AlF3 phase, it cannot effectively isolate the active material from the side reaction with the electrolyte, resulting in corrosion of the electrode material and a decrease in the cycling performance of the electrode material. In Comparative Example 5, the initial specific capacity and the cycling performance at 0.1C are both lower than those of Example 1. This is because an excessively high AlF3 content will block the normal diffusion of sodium ions, preventing the active material from effectively participating in the redox reaction, reducing the overall electronic conductivity of the electrode, leading to increased polarization, and also reducing the proportion of active material, thus reducing the initial specific capacity and cycling performance.

[0059] Moreover, Example 1 exhibits better electrochemical performance compared to Comparative Example 6, indicating that the electrochemical performance of the electrode material prepared by this invention is superior to that of current existing patented technologies, demonstrating its promising future.

[0060]

[0061]

[0062] Combining Tables 2 and 3, and comparing the data from Examples 1-9 in Table 3, it can be seen that the AlF3-coated and MXene-doped NFPP composites all exhibit excellent rate performance, with capacity retention rates exceeding 86% at 20°C. Compared to the NFPP material in Comparative Example 6, the rate performance is significantly improved. Comparative Examples 2 and 3, compared to Comparative Example 6, also show improved capacity retention rates at 20°C, increasing from 47.0% to 61.2% and 56.4%, respectively. This indicates that the addition of a single phase can improve the rate performance of NFPP materials, although the improvement is limited. The MXene phase, with its high conductivity and stable interlayer structure, allows the material to maintain normal Na+ performance at high rates. + It transports and suppresses volume expansion, thereby improving rate performance; while the AlF3 phase can cover the highly reactive regions on the NFPP surface, and the vacancies of fluoride ions provide Na+. + The rapid migration channel effectively prevents the self-discharge effect of the electrode material, avoids side reactions consuming active materials, and improves the effective reaction interface at high rates. It also reduces charge accumulation at the interface, suppresses concentration polarization, and inhibits the volume expansion of NFPP materials at high rates. Combined with the experimental data in Table 3, it can be seen that the co-introduction of the MXene and AlF3 phases can greatly enhance the rate performance of the material. MXene has abundant functional groups on its surface, which can form stable chemical bonds with NFPP materials and AlF3 coating layers, improving the interlayer bonding force of the composite material, reducing coating layer detachment, and further reducing side reactions of the electrolyte on the composite material. AlF3 can protect MXene from oxidation at high potentials, maintain the integrity of its conductive network, and prevent conductivity degradation during cycling. Therefore, the synergistic effect of the combination of the two can break through the performance limit of single modification. However, excessive content of either phase will weaken its rate performance. For example, in Comparative Example 4, when the MXene content is too high, it causes severe interlayer stacking, inhibiting Na... + Normal diffusion of MXene can obscure its surface reaction sites, and the functional groups on the MXene surface may react with the electrolyte, thus affecting its rate performance. In Comparative Example 5, when the AlF3 coating content is too high, it hinders the diffusion of Na+. + Normal diffusion is affected, which in turn affects the normal contact between the active material and the electrolyte. The electrochemical inertness of AlF3 will reduce the conductivity of the material, resulting in poor rate performance.

[0063] Based on the scratch test data in Table 4... Figure 2As can be seen, AlF3 can form stable Ti-F bonds with MXene, thereby enhancing the interfacial bonding strength of the composite material. The Lc values ​​of Examples 1-3 are between 5.1 and 6.3 N, indicating that there is a stable interfacial bonding strength between the layers. Furthermore, the interfacial bonding strength increases with the increase of the MXene content in the system. In contrast, the material in Comparative Example 3 does not contain MXene doping, and its Lc value is 3.3, indicating a lower bonding strength. When the MXene content is too high (Comparative Example 4), it causes the two-dimensional structure inside the material to stack, increasing brittleness, and the Lc value decreases to 4.3 N. When there is no AlF3 layer in the material (Comparative Example 2), the bonding strength is lower. The material has relatively weak mechanical properties, with an Lc of 3.6 N. When the AlF3 layer content is too high (Comparative Example 5), the coating layer becomes too thick, forming a continuous brittle phase, which reduces the toughness of the material, and the Lc decreases to 4.1. It can be seen that when the content of MXene phase and AlF3 layer in the sodium iron pyrophosphate cathode composite material system is within a certain range, MXene phase and AlF3 layer can form a stable synergistic effect. MXene has abundant functional groups on its surface, which can stably connect NFPP active material and AlF3 layer, greatly enhancing the interfacial bonding performance of sodium iron pyrophosphate cathode composite material.

[0064] .

Claims

1. A method for preparing a sodium iron pyrophosphate cathode composite material, characterized in that, Includes the following steps: (1) Lithium fluoride and titanium aluminum carbide were dissolved in concentrated hydrochloric acid and etched. The mixture was washed and filtered to obtain micron-sized MXene material. The micron-sized MXene material was added to deionized water for hydrothermal reaction. The mixture was centrifuged, washed, centrifuged again, and freeze-dried to obtain nano-sized MXene material. (2) Add sodium and phosphorus-containing inorganic salts, nano-sized MXene materials, inorganic iron salts, organic acid carbon sources, and ethylene glycol to deionized water, heat and stir, and evaporate at a higher temperature to obtain a gel-like precursor material. Vacuum freeze-dry and calcinate to obtain NFPP@MXene material. (3) Take NFPP@MXene material and add it to deionized water, then add aluminum nitrate nonahydrate to obtain a mixture. Adjust the pH of the mixture to weak acidity with dilute nitric acid, then add ammonium fluoride, stir the reaction, filter, wash, dry and calcine to obtain sodium iron pyrophosphate cathode composite material.

2. The preparation method of the sodium iron pyrophosphate cathode composite material according to claim 1, characterized in that, In step (1), the concentration of concentrated hydrochloric acid is 5-10 mol / L; the molar ratio of lithium fluoride to titanium aluminum carbide is 1:0.0668-0.0888; the etching reaction temperature is 40-60℃ and the reaction time is 24-36h; the hydrothermal reaction temperature is 100-150℃ and the reaction time is 2-6h.

3. The preparation method of the sodium iron pyrophosphate cathode composite material according to claim 1, characterized in that, In step (2), the inorganic salt containing sodium and phosphorus is sodium hypophosphite or sodium dihydrogen phosphate, the inorganic iron salt is ferric nitrate or ferric chloride, and the organic acid carbon source is citric acid or glucose; the heating and stirring temperature is 50-70℃, and the time is 4-6h; the molar mass ratio of the inorganic salt containing sodium and phosphorus to the nano-sized MXene material is 1mol:2-20g, and the molar volume ratio of the inorganic salt containing sodium and phosphorus to the inorganic iron salt, organic acid carbon source, and ethylene glycol is 1mol:0.6-0.7mol:0.1-0.2mol:2-4mL; the calcination conditions are: first, pre-sintering at 250-300℃ for 2-3h under an Ar / H2 atmosphere, followed by calcination at 500-600℃ for 8-12h.

4. The preparation method of the sodium iron pyrophosphate cathode composite material according to claim 1, characterized in that, In step (2), vacuum freeze drying involves cooling the gel-like precursor material to -50°C for 1 hour, followed by vacuum drying for 12-20 hours.

5. The method for preparing the sodium iron pyrophosphate cathode composite material according to claim 1, characterized in that, In step (3), the pH of the mixture after adjustment with dilute nitric acid is 4-6; the mass molar ratio of NFPP@MXene material to aluminum nitrate nonahydrate and ammonium fluoride is 1g:0.0001-0.0005mol:0.0003-0.0015mol; the drying temperature is 80-120℃ and the drying time is 2-4h; the calcination atmosphere is Ar, the calcination temperature is 400-500℃ and the calcination time is 3-5h.

6. An application of the sodium iron pyrophosphate cathode composite material prepared by the method according to any one of claims 1-5, characterized in that, The prepared sodium iron pyrophosphate cathode composite material was applied in sodium-ion batteries.

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