Normal-temperature sol-gel preparation method and application of novel sodium ferric sulfate pyrophosphate composite material

Abstract

The invention relates to the technical field of sodium ion battery materials, and discloses and aims to provide a normal-temperature sol-gel preparation method and application of a novel ferric sodium sulfate pyrophosphate composite material. The invention aims to prepare the sodium ferric sulfate pyrophosphate composite material through a simple and feasible normal-temperature sol-gel process so as to break through the limitation that the traditional sulfate polyanion material is poor in thermal stability and low in electronic conductivity, and the excellent electrochemical activity and stability of the sodium ferric sulfate pyrophosphate composite material are verified through experiments. Specifically, the Na2Fe2P2O7SO4 / C (15)-600 electrode material disclosed by the invention shows the first-circle discharge specific capacity of 92.87 mAh / g at the discharge rate of 0.05 C, can keep the discharge capacity of 71.83% and the coulombic efficiency of 98.91% after 900 cycles at 1C, and shows the potential value of the Na2Fe2P2O7SO4 / C (15)-600 electrode material in the application of the sodium ion battery.

Description

Technical Field

[0001] The present invention relates to a method for preparing a novel sodium iron pyrophosphate sulfate composite material by a room-temperature sol-gel method and its application, belonging to the technical field of sodium-ion batteries. Background Art

[0002] As a new energy storage technology, sodium-ion batteries have received extensive attention in recent years due to their rich resource reserves, significant cost advantages, and similar working principles to lithium-ion batteries. This technology shows great application potential in fields such as large-scale energy storage, low-speed electric vehicles, and distributed energy systems. In the research of cathode materials for sodium-ion batteries, polyanionic materials have become a key research direction due to their unique structural characteristics and excellent electrochemical performance. In particular, sulfate-based polyanionic materials, such as Alluaudite-type Na2Fe2(SO4)3 and Eldfellite-type NaFe(SO4)2, are considered to have important application values due to their high working voltage, good cycle stability, and low cost.

[0003] However, there are still many technical problems in the practical application of such materials: firstly, the FeO6 octahedrons in the material structure are connected by insulating SO4 2- groups, resulting in low electron transport efficiency and rapid capacity decay of the material under high-rate charge and discharge conditions; secondly, sulfate materials are prone to sulfate decomposition and release of SO x gas in high-temperature environments, which limits the application of high-temperature carbon coating processes; thirdly, in the traditional precursor synthesis process, common ball milling or spraying methods have problems such as high energy consumption and poor material uniformity, resulting in the easy formation of impurity phases such as Fe3O4 or Na6Fe(SO4)4 during the preparation process, seriously affecting the electrochemical performance of the material; in addition, although the conductivity can be improved by adding conductive materials to the precursor material and then sintering at low temperature, it is difficult to achieve uniform dispersion, resulting in uneven distribution of the conductive network in the composite cathode material, thereby affecting the high-rate performance and cycle stability of the battery. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for preparing a novel sodium iron pyrophosphate sulfate composite material by a room-temperature sol-gel method and its application, and innovatively propose a technical solution for preparing a high-performance sodium iron pyrophosphate sulfate composite material based on the room-temperature sol-gel method. This method significantly improves the thermal stability of sulfate by introducing pyrophosphate (P2O7 4- ), enabling in-situ coating of low-cost biochar at high temperatures, thus effectively solving the decomposition problem of traditional sulfate materials at high temperatures. The room-temperature sol-gel method has the advantages of low energy consumption, good material uniformity, and low impurity content, and can effectively overcome the deficiencies in traditional preparation processes, providing a new technical path for the development of high-performance cathode materials for sodium-ion batteries.

[0005] The object of the present invention can be achieved by the following technical solutions:

[0006] A sodium-ion battery prepared from a novel sodium iron pyrophosphate sulfate composite polyanionic cathode material includes the following steps:

[0007] Add a sodium source, a sulfate, an iron source, a phosphorus source, an antioxidant and / or a carbon source to an appropriate amount of deionized water in sequence and stir well to obtain a yellow-green gel. Vacuum-dry the gel and then grind it in an agate mortar to obtain a dark-yellow composite precursor powder. Sinter the composite precursor in a tube furnace with a protective atmosphere to obtain the sodium iron pyrophosphate sulfate composite cathode material.

[0008] Furthermore, the sodium source, the sulfate, the iron source and the phosphorus source are Na2CO3, (NH4)2SO4, Fe(NO3)3·9H2O and (NH4)2HPO4 respectively.

[0009] Furthermore, the molar ratio of Na2CO3 to (NH4)2SO4 is 1.5 - 2.5:1, the molar ratio of Fe(NO3)3·9H2O to (NH4)2SO4 is 1 - 4:1, the molar ratio of (NH4)2HPO4 to (NH4)2SO4 is 1 - 3:1, and the chemical formula of the prepared cathode material is Na a Fe b (PO4) c SO4, where 3 ≤ a ≤ 5, 1 ≤ b ≤ 4, 1 ≤ c ≤ 3.

[0010] Furthermore, the antioxidant can be citric acid or ascorbic acid, and the molar ratio of citric acid to Fe(NO3)3·9H2O is 1:10.

[0011] Furthermore, the carbon source accounts for 0 - 15% of the mass of sodium iron pyrophosphate sulfate, and the carbon source can be glucose, maltose or sucrose.

[0012] Furthermore, the mass ratio of deionized water to the mixed precursor powder is 0.2 - 1:1.

[0013] Furthermore, the vacuum drying temperature is 100°C and the drying time is 8 - 12 h.

[0014] Furthermore, the protective atmosphere is at least one of Ar, N2, and He, the sintering temperature is 500 - 700°C, the sintering time is 10 h, and the calcination heating rate is 3°C / min.

[0015] Beneficial effects

[0016] The present invention discloses a method for preparing a novel sodium iron pyrophosphate sulfate composite material by a room-temperature sol-gel method and its application. In the preparation process of this material, a technical solution for preparing a high-performance sodium iron pyrophosphate sulfate composite material based on the room-temperature sol-gel method is adopted. By introducing pyrophosphate radicals (P2O7 4- ), the thermal stability of sulfate radicals is significantly improved, enabling in-situ coating of low-cost biochar at high temperatures, thus effectively solving the decomposition problem of traditional sulfate materials at high temperatures. The room-temperature sol-gel method has advantages such as low energy consumption, good material uniformity, and low impurity content, which can effectively overcome the deficiencies in traditional preparation processes and provide a new technical route for the development of high-performance cathode materials for sodium-ion batteries. Experimental data show that the Na2Fe2P2O7SO4 / C(15)-600 material prepared by this process has an initial discharge specific capacity of 92.87 mAh / g at a discharge rate of 0.05C and can maintain 71.83% of the discharge capacity after 900 cycles at 1C. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] To more clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the drawings required for describing the embodiments.

[0018] Figure 1 is the SEM image of the Na2Fe2P2O7SO4 / C(15)-600 material in Example 1.

[0019] Figure 2 is the XRD pattern of the Na2Fe2P2O7SO4 / C(15)-600 material in Example 1.

[0020] Figure 3 is the first constant current charge-discharge performance of the Na2Fe2P2O7SO4 / C(15)-600 cathode material at 0.05C in Example 1.

[0021] Figure 4 is the rate performance test of the Na2Fe2P2O7SO4 / C(15)-600 cathode material in Example 1.

[0022] Figure 5 is the 1C cycle capacity performance of the Na2Fe2P2O7SO4 / C(15)-600 cathode material in Example 1.

[0023] Figure 6 is the 1C cycle Coulombic efficiency of the Na2Fe2P2O7SO4 / C(15)-600 cathode material in Example 1. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0024] The following further describes the present invention in detail with specific embodiments.

[0025] Example 1

[0026] Preparation of Na2Fe2P2O7SO4 / C(15)-600 material:

[0027] Weigh Na2CO3, (NH4)2SO4, Fe(NO3)3·9H2O and (NH4)2HPO4 according to the molar ratio of 1:1:2:2. Under continuous stirring, add the above substances into deionized water in turn. Add an appropriate amount of citric acid and 15wt% glucose according to the molar ratio of citric acid:Fe(NO3)3·9H2O = 10:1 to obtain a uniform yellow-green wet gel. Dry the mixed gel in a vacuum oven at 100°C for 10 h to obtain a yellow dry gel. Grind the gel in a mortar for 30 min and then place it in a tubular furnace with an Ar atmosphere and heat it to 600°C for calcination for 10 h to obtain the Na2Fe2P2O7SO4 / C(15)-600 cathode material.

[0028] Mix the Na2Fe2P2O7SO4 / C(15)-600 active material, conductive agent and binder obtained in Example 1 evenly into a black slurry in NMP solvent according to the mass ratio of 70:20:10. Coat the slurry on a 0.1 mm thick aluminum foil current collector with a scraper, dry it in a vacuum oven at 100°C for 12 h, and then use a cutting machine to cut it into a circular positive electrode plate with a diameter of 11 mm.

[0029] Half-cell assembly: Use the obtained electrode plate as the positive electrode, sodium sheet as the negative electrode, glass fiber as the separator, and 1 mol / L NaClO4 (EC:PC = 1:1 + 5% FEC) as the electrolyte. Select a battery case to assemble a button-type half-cell in a glove box with an argon atmosphere of O2 < 0.1 ppm and H2O < 0.1 ppm.

[0030] Electrochemical performance test: The constant current charge-discharge cycle test of the half-cell is carried out under the detection of the Neware system. The charging cut-off voltage is 4.5 V, the discharging cut-off voltage is 1.5 V, the test temperature is 30°C, and the current density is 0.05C - 1.0C (1C = 105 mA / g).

[0031] Figure 1 This is the SEM image of the Na2Fe2P2O7SO4 / C(15)-600 material in Example 1. It can be seen that the composite cathode material presents a spherical morphology with a uniform pore structure.

[0032] Figure 2 This is the X-ray diffraction pattern of the Na2Fe2P2O7SO4 / C(15)-600 material in Example 1. It can be seen that the composite cathode material has good crystallinity.

[0033] The electrochemical performance of the assembled battery is shown in Table 1 and Figure 3As shown, the sodium-ion battery prepared in Example 1 has an initial discharge specific capacity of 92.87 mAh / g at a current density of 0.05C, indicating that the Na2Fe2P2O7SO4 / C(15)-600 composite cathode material can achieve efficient charge and discharge of Na + .

[0034] From Figure 4 it can be seen that at each current density, the discharge specific capacity of this material performs well. This indicates that the introduction of pyrophosphate and the in-situ carbon coating process improve the inherent electronic conductivity of the polyanionic material, which is beneficial to its capacity performance under high current conditions.

[0035] From Figure 5 and Figure 6 it can be seen that the sodium-ion battery prepared in Example 1 can maintain a discharge capacity of 71.83% and a Coulombic efficiency of 98.91% after 900 cycles at 1C, which indicates that the successful preparation of the composite material has greatly improved the electrochemical stability of the polyanionic material.

[0036] Based on Figure 3 , Figure 4 , Figure 5 and Figure 6 results, it can be inferred that the Na2Fe2P2O7SO4 / C(15)-600 composite cathode material exhibits excellent crystallinity and electrochemical performance, specifically manifested as high initial discharge specific capacity and charge-discharge efficiency, as well as good cycle stability. The introduction of pyrophosphate and the in-situ carbon coating process significantly improve the electronic conductivity of the material, enabling it to maintain good capacity performance at high current densities. However, in the future, further exploration of hierarchical structure design or nanometerization strategies is still needed to further optimize the electronic transport performance and structural stability of the material, so as to comprehensively improve its overall energy storage performance.

[0037] Example 2

[0038] Prepare Na2Fe2P2O7SO4 / C(10)-600 material:

[0039] Weigh Na2CO3, (NH4)2SO4, Fe(NO3)3·9H2O and (NH4)2HPO4 according to a molar ratio of 1:1:2:2. Under continuous stirring, add the above substances into deionized water in turn. Add an appropriate amount of citric acid and 10wt% glucose according to the molar ratio of citric acid:Fe(NO3)3·9H2O = 10:1 to obtain a uniform yellow-green wet gel. Dry the mixed gel in a vacuum oven at 100°C for 10h to obtain a yellow dry gel. Grind the gel in a mortar for 30min and then place it in a tubular furnace with an Ar atmosphere and heat it to 600°C for calcination for 10h to obtain the Na2Fe2P2O7SO4 / C(10)-600 cathode material.

[0040] The active material Na2Fe2P2O7SO4 / C(10)-600 obtained in Example 2, the conductive agent, and the binder were uniformly mixed in NMP solvent according to a mass ratio of 70:20:10 to form a black slurry. The slurry was coated on a 0.1 mm thick aluminum foil current collector with a scraper and dried in a vacuum oven at 100 °C for 12 h, and then cut into circular positive electrode plates with a diameter of 11 mm using a cutting machine.

[0041] Assembly of half-cell: Using the obtained electrode plate as the positive electrode, sodium sheet as the negative electrode, glass fiber as the separator, and 1 mol / L NaClO4 (EC:PC = 1:1 + 5% FEC) as the electrolyte, a button-type half-cell was assembled using a battery case in a glove box with an argon atmosphere of O2 < 0.1 ppm and H2O < 0.1 ppm.

[0042] Electrochemical performance test: The constant current charge-discharge cycle test of the half-cell was carried out under the detection of a Neware system. The charge cut-off voltage was 4.5 V, the discharge cut-off voltage was 1.5 V, the test temperature was 30 °C, and the current density was 0.05C - 1.0C (1C = 105 mA / g).

[0043] The electrochemical performance of the assembled battery is shown in Table 1. It can be seen from the table that in Example 2, the first discharge capacity at 0.05C was 89.91 mAh / g, and the Coulomb efficiency was 77.78%.

[0044] Example 3

[0045] Preparation of Na2Fe2P2O7SO4 / C(5)-600 material:

[0046] Weigh Na2CO3, (NH4)2SO4, Fe(NO3)3·9H2O, and (NH4)2HPO4 according to a molar ratio of 1:1:2:2. Under continuous stirring, the above substances were added to deionized water in turn. Appropriate citric acid and 5 wt% glucose were added according to the molar ratio of citric acid:Fe(NO3)3·9H2O = 10:1 to obtain a uniform yellow-green wet gel. The mixed gel was dried in a vacuum oven at 100 °C for 10 h to obtain a yellow dry gel. The gel was ground in a mortar for 30 min and then placed in a tubular furnace with an Ar atmosphere and heated to 600 °C for calcination for 10 h to obtain the Na2Fe2P2O7SO4 / C(5)-600 positive electrode material.

[0047] The active material Na2Fe2P2O7SO4 / C(5)-600 obtained in Example 3, the conductive agent, and the binder were uniformly mixed into a black slurry in NMP solvent according to a mass ratio of 70:20:10. The slurry was coated on a 0.1-mm-thick aluminum foil current collector with a doctor blade, dried in a vacuum oven at 100 °C for 12 h, and then cut into circular positive electrode plates with a diameter of 11 mm using a cutting machine.

[0048] Half-cell assembly: Using the obtained electrode plate as the positive electrode, a sodium sheet as the negative electrode, a glass fiber as the separator, and 1 mol / L NaClO4 (EC:PC = 1:1 + 5% FEC) as the electrolyte, a coin-type half-cell was assembled using a battery case in a glove box with an argon atmosphere of O2 < 0.1 ppm and H2O < 0.1 ppm.

[0049] Electrochemical performance test: The constant current charge-discharge cycling test of the half-cell was carried out under the detection of a Neware system. The charge cut-off voltage was 4.5 V, the discharge cut-off voltage was 1.5 V, the test temperature was 30 °C, and the current density was 0.05 C to 1.0 C (1 C = 105 mA / g).

[0050] The electrochemical performance of the assembled battery is shown in Table 1. It can be seen from the table that in Example 3, the first discharge capacity at 0.05 C was 26.89 mAh / g, and the Coulomb efficiency was 79.13%.

[0051] Example 4

[0052] Preparation of Na2Fe2P2O7SO4 / C(5)-500 material:

[0053] Weigh Na2CO3, (NH4)2SO4, Fe(NO3)3·9H2O, and (NH4)2HPO4 according to a molar ratio of 1:1:2:2. Under continuous stirring, the above substances were successively added to deionized water. An appropriate amount of citric acid and 5 wt% glucose were added according to a molar ratio of citric acid:Fe(NO3)3·9H2O = 10:1 to obtain a uniform yellow-green wet gel. The mixed gel was dried in a vacuum oven at 100 °C for 10 h to obtain a yellow dry gel. The gel was ground in a mortar for 30 min and then placed in a tubular furnace with an Ar atmosphere and heated to 500 °C for calcination for 10 h to obtain the Na2Fe2P2O7SO4 / C(5)-500 positive electrode material.

[0054] The active material Na2Fe2P2O7SO4 / C(5)-500 obtained in Example 4, the conductive agent, and the binder were uniformly mixed into a black slurry in NMP solvent according to a mass ratio of 70:20:10. The slurry was coated on a 0.1-mm-thick aluminum foil current collector with a doctor blade, dried in a vacuum oven at 100 °C for 12 h, and then cut into circular positive electrode plates with a diameter of 11 mm using a cutting machine.

[0055] Half-cell assembly: Using the obtained electrode as the positive electrode, sodium sheet as the negative electrode, glass fiber as the separator, and 1 mol / L NaClO4 (EC:PC = 1:1 + 5% FEC) as the electrolyte, a coin-type half-cell was assembled using a battery case in a glove box with an argon atmosphere where O2 < 0.1 ppm and H2O < 0.1 ppm.

[0056] Electrochemical performance test: The constant current charge-discharge cycling test of the half-cell was carried out under the detection of a Neware system. The charge cut-off voltage was 4.5 V, the discharge cut-off voltage was 1.5 V, the test temperature was 30 °C, and the current density was 0.05 C to 1.0 C (1 C = 105 mA / g).

[0057] The electrochemical performance of the assembled battery is shown in Table 1. It can be seen from the table that in Example 4, the first discharge capacity at 0.05 C was 57.20 mAh / g, and the Coulombic efficiency was 97.49%.

[0058] Example 5

[0059] Preparation of Na2Fe2P2O7SO4 / C(5)-700 material:

[0060] Weigh Na2CO3, (NH4)2SO4, Fe(NO3)3·9H2O, and (NH4)2HPO4 according to a molar ratio of 1:1:2:2. Under continuous stirring, add the above substances to deionized water in turn. Add an appropriate amount of citric acid and 5 wt% glucose according to a molar ratio of citric acid:Fe(NO3)3·9H2O = 10:1 to obtain a uniform yellow-green wet gel. Dry the mixed gel in a vacuum oven at 100 °C for 10 h to obtain a yellow dry gel. Grind the gel in a mortar for 30 min and then place it in a tubular furnace with an Ar atmosphere and heat it to 700 °C for calcination for 10 h to obtain the Na2Fe2P2O7SO4 / C(5)-700 cathode material.

[0061] The Na2Fe2P2O7SO4 / C(5)-700 active material, conductive agent, and binder obtained in Example 5 were uniformly mixed into a black slurry in NMP solvent according to a mass ratio of 70:20:10. The slurry was coated on a 0.1 mm thick aluminum foil current collector with a scraper and dried in a vacuum oven at 100 °C for 12 h, and then cut into circular positive electrode sheets with a diameter of 11 mm using a cutting machine.

[0062] Half-cell assembly: Using the obtained electrode as the positive electrode, sodium sheet as the negative electrode, glass fiber as the separator, and 1 mol / L NaClO4 (EC:PC = 1:1 + 5% FEC) as the electrolyte, a coin-type half-cell was assembled using a battery case in a glove box with an argon atmosphere where O2 < 0.1 ppm and H2O < 0.1 ppm.

[0063] Electrochemical performance test: The constant current charge-discharge cycle test of the half-cell was carried out under the detection of the Neware system. The charge cut-off voltage was 4.5 V, the discharge cut-off voltage was 1.5 V, the test temperature was 30 °C, and the current density was 0.05 C to 1.0 C (1 C = 105 mA / g).

[0064] The electrochemical performance of the assembled battery is shown in Table 1. It can be seen from the table that in Example 5, the initial discharge capacity at 0.05 C was 45.04 mAh / g.

[0065] Example 6

[0066] Preparation of Na2Fe2P2O7SO4 / C-600 material:

[0067] Weigh Na2CO3, (NH4)2SO4, Fe(NO3)3·9H2O, and (NH4)2HPO4 according to the molar ratio of 1:1:2:2. Under continuous stirring, add the above substances to deionized water in turn. Add an appropriate amount of citric acid according to the molar ratio of citric acid:Fe(NO3)3·9H2O = 10:1 to obtain a uniform yellow-green wet gel. Dry the mixed gel in a vacuum oven at 100 °C for 10 h to obtain a yellow dry gel. Grind the gel in a mortar for 30 min and then place it in a tube furnace with an Ar atmosphere and heat it to 600 °C for calcination for 10 h to obtain the Na2Fe2P2O7SO4 / C-600 cathode material.

[0068] Mix the Na2Fe2P2O7SO4 / C-600 active material, conductive agent, and binder obtained in Example 6 in a mass ratio of 70:20:10 and uniformly mix them into a black slurry in NMP solvent. Coat the slurry on a 0.1-mm-thick aluminum foil current collector with a scraper, dry it in a vacuum oven at 100 °C for 12 h, and then use a cutting machine to cut it into a circular positive electrode plate with a diameter of 11 mm.

[0069] Half-cell assembly: Using the obtained electrode plate as the positive electrode, sodium sheet as the negative electrode, glass fiber as the separator, and 1 mol / L NaClO4 (EC:PC = 1:1 + 5% FEC) as the electrolyte, assemble a button-type half-cell in a glove box with an argon atmosphere of O2 < 0.1 ppm and H2O < 0.1 ppm using a battery case.

[0070] Electrochemical performance test: The constant current charge-discharge cycle test of the half-cell was carried out under the detection of the Neware system. The charge cut-off voltage was 4.5 V, the discharge cut-off voltage was 1.5 V, the test temperature was 30 °C, and the current density was 0.05 C to 1.0 C (1 C = 105 mA / g).

[0071] The electrochemical performance of the assembled battery is shown in Table 1. As can be seen from the table, the first discharge capacity of Example 6 at 0.05C is 55.43 mAh / g.

[0072] Comparative Example 1

[0073] Prepare Na 2+2x Fe 2-x (SO4)3-1 material:

[0074] Weigh Na2SO4 and FeSO4·7H2O according to a molar ratio of 1:1. Under continuous stirring, add the above substances to deionized water in turn. Add an appropriate amount of glucose and ascorbic acid to the mixed solution and stir to obtain a clear and homogeneous solution. Add the mixed solution dropwise to 100 ml of ethanol solution to obtain a gray-green precipitate. Wash the precipitate with ethanol and then dry it in a vacuum oven at 60°C for 12 h to obtain a precursor powder. Place the powder in a crucible and put it into a tubular furnace with an Ar atmosphere and sinter it at 350°C for 2 h to obtain Na 2+2x Fe 2-x (SO4)3-1 cathode material.

[0075] The Na 2+2x Fe 2-x (SO4)3-1 active material, conductive agent and binder obtained in Comparative Example 1 are uniformly mixed into a black slurry in NMP solvent according to a mass ratio of 70:20:10. Coat the slurry on a 0.1 mm thick aluminum foil current collector with a scraper and dry it in a vacuum oven at 100°C for 10 h, and then cut it into a circular positive electrode sheet with a diameter of 11 mm using a cutting machine.

[0076] Half-cell assembly: Use the obtained electrode sheet as the positive electrode, sodium sheet as the negative electrode, glass fiber as the separator, and 1 mol / L NaClO4 (EC:PC = 1:1 + 5% FEC) as the electrolyte. Assemble a button-type half-cell using a battery case in a glove box with an argon atmosphere where O2 < 0.1 ppm and H2O < 0.1 ppm.

[0077] Electrochemical performance test: The constant current charge-discharge cycle test of the half-cell is carried out under the detection of a Neware system. The charge cut-off voltage is 4.5 V, the discharge cut-off voltage is 2.0 V, the test temperature is 30°C, and the current density is 0.1C (1C = 100 mA / g).

[0078] The electrochemical performance of the battery assembled in Comparative Example 1 has a discharge capacity of 22.5 mAh / g at 0.1C.

[0079] Comparative Example 2

[0080] Prepare Na 2+2x Fe 2-x (SO4)3-2 material:

[0081] Weigh Na2SO4 and FeSO4·7H2O according to a molar ratio of 1:2. Add the above substances to deionized water in sequence under continuous stirring. Add appropriate amounts of glucose and ascorbic acid to the mixed solution and stir to obtain a clear and homogeneous solution. Dropwise add the mixed solution into 100 ml of ethanol solution to obtain a gray-green precipitate. Wash the precipitate with ethanol and then dry it in a vacuum oven at 60 °C for 12 h to obtain a precursor powder. Place the powder in a crucible and put it into a tube furnace with an Ar atmosphere and sinter it at 350 °C for 2 h to obtain Na 2+2x Fe 2-x (SO4)3-1 cathode material.

[0082] Mix the Na 2+2x Fe 2-x (SO4)3-1 active material, conductive agent and binder in a mass ratio of 70:20:10 uniformly in NMP solvent to form a black slurry. Coat the slurry on a 0.1 mm thick aluminum foil current collector with a scraper and dry it in a vacuum oven at 100 °C for 10 h, then cut it into a circular positive electrode plate with a diameter of 11 mm using a cutting machine.

[0083] Half-cell assembly: Use the obtained electrode plate as the positive electrode, sodium sheet as the negative electrode, glass fiber as the separator, and 1 mol / L NaClO4 (EC:PC = 1:1 + 5% FEC) as the electrolyte. Assemble a button-type half-cell using a battery case in a glove box with an argon atmosphere of O2 < 0.1 ppm and H2O < 0.1 ppm.

[0084] Electrochemical performance test: The constant current charge-discharge cycle test of the half-cell is carried out under the detection of a Neware system. The charging cut-off voltage is 4.5 V, the discharging cut-off voltage is 2.0 V, the test temperature is 30 °C, and the current density is 0.1 C (1 C = 100 mA / g).

[0085] The electrochemical performance of the battery assembled in Comparative Example 2 has a discharging capacity of 21.0 mAh / g at 0.1 C.

[0086] Table 1 Electrochemical performance test results of half-cells

[0087] Case Initial discharge capacity at 0.05C Example 1 92.87 mAh / g Example 2 89.91 mAh / g Example 3 26.89 mAh / g Example 4 57.20 mAh / g Example 5 45.04 mAh / g Example 6 55.43 mAh / g

[0088] Table 1 shows the electrochemical performance test results of the sodium-ion battery half-cells in the examples. The results show that the introduction of pyrophosphate can enhance the thermodynamic stability of sulfate, enabling in-situ coating of biochar under high-temperature conditions. The introduction of composite anions and the formation of a carbon coating layer not only improve the Na +The transport rate of ions also brings a higher discharge specific capacity. Compared with Na2Fe2(SO4)3, Na2FeP2O7SO4 and in-situ carbon-coated Na2FeP2O7SO4 / C materials exhibit more stable performance, which may be due to their superior crystal structure and Na + diffusion channels.

[0089] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and changes. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for preparing a novel sodium iron pyrophosphate sulfate composite material by a sol-gel method at room temperature and its application, characterized in that, The novel sodium iron pyrophosphate sulfate composite polyanionic cathode material for sodium-ion batteries is prepared by the following steps: Add a sodium source, a sulfate, an iron source, a phosphorus source, an antioxidant and / or a carbon source to an appropriate amount of deionized water in sequence and stir well to obtain a yellow-green gel. Vacuum-dry the gel and then grind it in an agate mortar to obtain a dark-yellow composite precursor powder. Calcinate the composite precursor in a tube furnace under the protection of an inert atmosphere to obtain the sodium iron pyrophosphate sulfate composite cathode material.

2. Preparation and application of a novel sodium-ion battery sodium iron pyrophosphate sulfate composite polyanionic cathode material according to claim 1, characterized in that, The sodium-ion battery is prepared by the following steps: Uniformly mix the cathode active material, the conductive agent and the binder in a mass ratio of 70:20:10 in an NMP solvent to form a black slurry. Coat the slurry on a 0.1-mm-thick aluminum foil current collector with a scraper, dry it in a vacuum oven at 100 °C for 12 h, and then cut it into a circular cathode electrode sheet with a diameter of 11 mm using a cutting machine. Using the obtained electrode sheet as the cathode, a sodium sheet as the anode, a glass fiber as the separator, and 1 mol / L NaClO4 (EC:PC = 1:1 + 5% FEC) as the electrolyte, assemble a coin-type half-cell in a glove box with an argon atmosphere of O2 < 0.1 ppm and H2O < 0.1 ppm. The constant-current charge-discharge cycle test of the half-cell is carried out under the detection of a Neware system. The charge cut-off voltage is 4.5 V, the discharge cut-off voltage is 1.5 V, the test temperature is 30 °C, and the current density is 0.05 C to 1.0 C (1 C = 105 mA / g).

3. A method for preparing a novel sodium ferric pyrophosphate sulfate composite material by a sol-gel method at room temperature and its application according to claim 1, characterized in that, The sodium source, the sulfate, the iron source and the phosphorus source are Na2CO3, (NH4)2SO4, Fe(NO3)3·9H2O and (NH4)2HPO4 respectively.

4. A method for preparing a novel sodium ferric pyrophosphate sulfate composite material by a sol-gel method at room temperature and its application according to claim 1, characterized in that, The molar ratio of Na2CO3 to (NH4)2SO4 is 1.5 - 2.5:1, the molar ratio of Fe(NO3)3·9H2O to (NH4)2SO4 is 1 - 4:1, the molar ratio of (NH4)2HPO4 to (NH4)2SO4 is 1 - 3:1, and the chemical formula of the prepared cathode material is Na a Fe b (PO4) c SO4, where 3 ≤ a ≤ 5, 1 ≤ b ≤ 4, and 1 ≤ c ≤ 3.

5. The preparation method and application of a novel sodium iron pyrophosphate sulfate composite material by the sol-gel method at room temperature according to claim 1, characterized in that, The antioxidant can be citric acid or ascorbic acid, and the molar ratio of citric acid to Fe(NO3)3·9H2O is 1:

10.

6. A method for preparing a novel sodium ferric pyrophosphate sulfate composite material by a sol-gel method at room temperature and its application according to claim 1, characterized in that, The carbon source accounts for 0 to 15% of the mass of sodium iron pyrophosphate sulfate. The carbon source can be glucose, maltose or sucrose, and the mass ratio of deionized water to the mixed precursor powder is 0.2 to 1:

1.

7. A method for preparing a novel sodium ferric pyrophosphate sulfate composite material by a sol-gel method at room temperature and its application according to claim 1, characterized in that, The vacuum drying temperature is 100 °C, and the drying time is 8 to 12 h.

8. A method for preparing a novel sodium ferric pyrophosphate sulfate composite material by a sol-gel method at room temperature and its application according to claim 1, characterized in that, The inert atmosphere is at least one of Ar, N2, and He.

9. A method for preparing a novel sodium ferric pyrophosphate sulfate composite material by a sol-gel method at room temperature and its application according to claim 1, characterized in that, The calcination temperature is 500 to 700 °C, the calcination time is 10 h, and the calcination heating rate is 3 °C / min.

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