A Na3.12Fe2.44(P2O7)2 / C cathode composite material, its preparation method, and its application.

Abstract

This invention discloses a Na 3.12 Fe 2.44 This invention relates to the field of sodium-ion secondary battery electrode technology, specifically the (P2O7)2 / C cathode composite material, its preparation method, and its application. The preparation method employs a sol-gel method, using sodium acetate, sodium carbonate, or sodium dihydrogen phosphate as the sodium source. A precursor solution is prepared through a reverse feeding process, followed by drying, pre-calcination, cold isostatic pressing pretreatment, and segmented calcination to obtain an ellipsoidal crystalline composite material with an aspect ratio of 1.5-2.0 and a compressive strength ≥180 MPa. Electrodes made from this composite material exhibit 97.3% structural integrity under 100T roller pressing, 91.5% capacity retention after 500 cycles at 2C rate, and a thermal runaway temperature of 182℃. This invention solves the problem of insufficient mechanical properties of high-pressure solid electrodes from the source through crystal structure control. The process is controllable and highly repeatable, and the prepared sodium-ion battery combines high energy density, long cycle life, and high safety.

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion secondary battery electrode technology, specifically a Na... 3.12 Fe 2.44 (P2O7)2 / C cathode composite material, its preparation method and application. Background Technology

[0002] With the rapid development of electric vehicles and large-scale energy storage power stations, the market has placed higher demands on the cost, energy density, cycle life, and safety of rechargeable batteries. While lithium-ion batteries are widely used, the relative scarcity and high cost of lithium resources limit their further promotion in the field of large-scale energy storage. Therefore, sodium-ion batteries, which are abundant in resources and have low cost, are considered a highly promising alternative. Among the many components of sodium-ion batteries, the cathode material is the key to determining its energy density, cycle stability, and cost. Among various cathode materials, polyanionic materials with a three-dimensional framework structure (such as phosphates and pyrophosphates) have attracted much attention due to their structural stability, long cycle life, and high operating voltage.

[0003] Among them, sodium ferric pyrophosphate (Na) with iron as the redox center x Fe y (P₂O₇)₂ materials, due to the abundance, low cost, and environmental friendliness of iron, are considered one of the ideal cathode materials for achieving low-cost, high-performance sodium-ion batteries. For example, Na 3.12 Fe 2.44 (P2O7)2 (NFP) material has an average discharge voltage of about 2.8V, showing good application prospects.

[0004] However, in the existing technology, Na 3.12 Fe 2.44 The synthesis and application of (P2O7)2 materials still face challenges: Firstly, in terms of material preparation, traditional synthesis methods such as solid-state sintering often struggle to precisely control the microstructure, crystallinity, and phase purity of the materials, easily leading to impurities or uneven particle size, thus affecting the intrinsic electrochemical performance of the materials. While the sol-gel method can improve the mixing uniformity of the precursors, specific process parameters, such as the choice of sodium source (e.g., sodium acetate, sodium carbonate, sodium dihydrogen phosphate), calcination temperature and heating rate, and atmosphere control, have a decisive impact on the phase structure, particle size distribution, and electrochemical performance of the final product. Improper process control can result in suboptimal tap density, electronic conductivity, and ion mobility, causing the actual specific capacity and rate performance to fall short of theoretical expectations. Secondly, at the electrode preparation and battery integration levels, increasing electrode compaction density in pursuit of high energy density risks degrading the mechanical properties of electrodes made from this material. On the one hand, during electrode forming processes such as roll forming, high compaction can easily lead to stress concentration and peeling between the active material layer and the current collector, or cause microcracks or even breakage in the electrode, directly increasing the risk of battery short circuits. On the other hand, during long-term cycling, the internal stress of the high-compacted electrode continuously accumulates, accelerating contact failure and structural damage between active particles, leading to rapid capacity decay. More seriously, the deterioration of the electrode's mechanical integrity severely affects battery safety: when the battery encounters external abuse such as puncture or impact, or internal local overheating, structurally unstable electrodes are more prone to triggering violent side reactions, significantly increasing the risk of thermal runaway and resulting combustion, explosion, and other safety hazards. In existing technologies, the approach to improving the performance of high-pressure compaction electrodes mainly focuses on optimizing binders or conductive agents. However, this is an aftermarket repair and does not address the mechanical integrity challenges under high-pressure compaction conditions at the source of the material's crystal structure. Therefore, there is an urgent need in this field to develop a Na process with controllable and highly repeatable characteristics. 3.12 Fe 2.44 (P2O7)2 / C cathode composite material, its preparation method and application. Summary of the Invention

[0005] The purpose of this invention is to solve the above-mentioned problems in the prior art and to provide a Na that achieves high mechanical stability through crystal structure modulation. 3.12 Fe 2.44 (P2O7)2 / C cathode composite materials, their preparation methods, and applications, with a focus on solving the problem of existing materials having a high compaction density (>2.8 g / cm³). 3 To address the issues of particle breakage, electrode fracture, and interface delamination that occur under certain conditions, a crystal-carbon composite structure with high fracture toughness is constructed. This allows the material to withstand ≥80T of rolling pressure without structural damage while maintaining high specific capacity. The specific technical solution is as follows: A type of Na 3.12 Fe 2.44 The preparation method of (P2O7)2 / C cathode composite material adopts a strategy of controlling the thermal decomposition temperature of sodium source, combined with sol-gel dual-temperature gradient sintering, and induces the formation of a multi-level porous structure through its differentiated thermal decomposition characteristics; the specific steps are as follows: Step 1: Preparation and gelation of precursor solution; using the sol-gel method, sodium, iron, phosphorus and citric acid as raw materials, a reverse feeding process is adopted, first adding iron-phosphorus source mixture and then adding sodium source, according to the molar ratio of Na:Ac:Fe:P=3.12:6:2.44:4, dissolved in deionized water, heated in a water bath at 80℃ and stirred for 3-6 hours to form a uniform sol, and then allowed to stand or evaporate to form a gel; Step 2: Drying and pre-calcination; Transfer the gel from Step 1 to a vacuum drying oven and dry it at 120℃ for 24 hours. After grinding, pre-calcinate it at 350℃ for 4 hours in an H2 / Ar mixed atmosphere at a heating rate of 2℃ / min. During the pre-calcination process, introduce a heat preservation platform at 300℃ for 2 hours. Step 3: High-temperature calcination; the pre-calcined product is ground and then compressed into tablets at 10 MPa; the tableted product is then calcined in a H2 / Ar mixed atmosphere at a programmed heating rate of 2℃ / min to 450℃-550℃, with each stage lasting 5 hours, to finally obtain Na. 3.12 Fe 2.44 (P2O7)2 / C cathode composite powder.

[0006] Preferably, in step one, the pH value of the gel is controlled within the range of 3.8-4.2, the sodium source is selected from sodium acetate, sodium carbonate or sodium dihydrogen phosphate, the iron source is ferric nitrate nonahydrate, and the phosphorus source is ammonium dihydrogen phosphate.

[0007] Preferably, the sodium source in step one is sodium acetate, and the raw material amounts are: 988.18951g of ferric nitrate nonahydrate, 460.12g of ammonium dihydrogen phosphate, 257.6056g of sodium acetate, 1152.744g of citric acid, 20kg of deionized water, and the water bath stirring time is 3 hours.

[0008] Preferably, the sodium source in step one is sodium carbonate, and the raw material amounts are: 1976.37902g of ferric nitrate nonahydrate, 920.24g of ammonium dihydrogen phosphate, 661.3776g of sodium carbonate, 2305.488g of citric acid, 40kg of deionized water, and the water bath stirring time is 6 hours.

[0009] Preferably, the sodium source in step one is sodium dihydrogen phosphate, and the raw material amounts are: 1976.37902g of ferric nitrate nonahydrate, 920.24g of ammonium dihydrogen phosphate, 748.65648g of sodium dihydrogen phosphate, 2305.488g of citric acid, 40kg of deionized water, and the water bath stirring time is 6 hours.

[0010] A type of Na 3.12 Fe 2.44 The (P2O7)2 / C cathode composite material has ellipsoidal grains with an aspect ratio of 1.5-2.0, a compressive strength ≥180MPa, and the electrode prepared from it has a structural integrity ≥97.3% under 100T roller pressure.

[0011] A type of Na 3.12 Fe 2.44 The application of (P2O7)2 / C cathode composite materials to Na 3.12 Fe 2.44The (P2O7)2 / C cathode composite material is used in the preparation of sodium-ion batteries. The specific steps are as follows: Step 1: Preparation of the positive electrode sheet; Na₂O₃ is used as the active material... 3.12 Fe 2.44 (P2O7)2 / C, binder PVDF, and conductive agent SP / CNT are mixed evenly in N-methylpyrrolidone at a ratio of 70wt%:20wt%:10wt%, and then coated onto aluminum foil. The mixture is dried in an oven at 105-145℃ for 1-2 minutes and then rolled at a pressure of 20-100T and a speed of 60-70m / min to a thickness of 190-240μm to obtain the positive electrode sheet. Step 2: Preparation of negative electrode sheet; Active material graphite, conductive agent SP, binder CMC, SBR and LA133 are mixed evenly in deionized water at a ratio of 95.8wt%:0.53wt%:0.99wt%:0.99wt%:1.69wt%, and then coated onto copper foil. The mixture is dried in an oven at 105-145℃ for 1-2 minutes, and then rolled at a pressure of 20-100T and a speed of 60-70m / min to a thickness of 190-240μm to obtain the negative electrode sheet. Step 3: Battery assembly: Stack the above positive electrode, 10+2μm wet ceramic separator and negative electrode in sequence, put them into the button cell case, inject electrolyte, and seal to obtain a button sodium-ion battery.

[0012] Preferably, the separator is a wet-process ceramic separator with a specification of 10+2μm; the electrolyte is 1M NaPF6 with a solvent ratio of EC:PC:EMC:DEC=30:3:40:27, and contains 1% FEC and 2% VC.

[0013] The sodium-ion battery has an initial discharge capacity of ≥85mAh / g at 25℃ and 10C, a capacity retention of ≥91.5% after 500 cycles at 2C, and a thermal runaway temperature of ≥182℃.

[0014] Compared with the closest existing technology, the technical solution provided by the present invention has the following beneficial effects: 1. Breakthrough in mechanical properties of this invention: By adopting a strategy of controlling the thermal decomposition temperature of sodium source, sodium acetate, sodium carbonate and sodium dihydrogen phosphate are used as sodium sources respectively. Through their differentiated thermal decomposition characteristics, a multi-level porous structure is induced. Carbon network constrains the preferential growth of crystals and pre-stress treatment, so that the compressive strength of the prepared composite material is ≥180MPa. The electrode made from it still maintains 97.3% structural integrity under 100T roller pressure, which is 35% higher than the structural integrity ratio of the prior art. It effectively solves the problems of particle breakage, electrode fracture and interface peeling under high compaction conditions.

[0015] 2. The present invention establishes a dual-temperature gradient sintering system: in the pre-sintering stage at 350℃, a three-dimensional carbon network is formed by carbonization with citric acid. In the main sintering stage at 450-550℃, the carbon network constraint effect controls the preferential growth of crystals along the (020) crystal plane to obtain ellipsoidal grains with an aspect ratio of 1.5-2.0.

[0016] 3. This invention innovatively adopts a tableting pretreatment process: after pre-calcination, the precursor is subjected to 10MPa cold isostatic pressing treatment, which generates pre-compression stress between the carbon network and the crystal precursor, thereby improving the compressive strength of the final product, which is tested to be ≥180MPa.

[0017] 4. The electrochemical stability of this invention is significantly improved: the composite material has a regular microstructure and good crystallinity, and the electronic conductivity and ion mobility are optimized. This allows the high-pressure solid electrode with a compaction density of 2.85 g / cm³ to retain 91.5% of its capacity after 500 cycles at 2C rate, and the differential capacitance curve has no phase distortion. The cycle stability is better than that of existing materials.

[0018] 5. The safety performance of this invention is greatly improved: the improved mechanical integrity of the electrode significantly enhances the safety performance of the battery. There is no delamination of the electrode in the needle penetration test, and the thermal runaway temperature is increased to 182°C, which reduces the risk of the battery burning and exploding under abuse conditions. Attached Figure Description

[0019] Figure 1 Specific embodiment 1 of the present invention: Na using sodium acetate as the sodium source at 550 °C 3.12 Fe 2.44 Powder XRD pattern of (P2O7)2 / C composite material; Figure 2 Specific embodiment 2 of the present invention: Na using sodium carbonate as the sodium source at 550 °C 3.12 Fe 2.44 Powder XRD pattern of (P2O7)2 / C composite material; Figure 3 Specific embodiment 3 of the present invention: Na using sodium dihydrogen phosphate as the sodium source at 550℃ 3.12 Fe 2.44 SEM image of (P2O7)2 / C composite material; Figure 4 This is a charge-discharge curve of a button battery assembled from a positive electrode sheet prepared with sodium carbonate as the sodium source according to the present invention, under conditions of 25 °C and 10 C. Detailed Implementation

[0020] The present invention will be described in detail below with reference to specific embodiments to enable those skilled in the art to better understand the invention; however, the invention is not limited to the following embodiments. The present invention provides a Na... 3.12 Fe2.44 (P2O7)2 / C cathode composite material, its preparation method, and its application: To make the objectives, technical solutions, and effects of this invention clearer, the invention is further described below with reference to specific embodiments and accompanying drawings. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0021] Unless otherwise specified, the experimental methods described in the implementation cases are all conventional methods; unless otherwise specified, the reagents and materials described are all commercially available.

[0022] Example 1: Preparation using sodium acetate as the sodium source Na3.12Fe2.44(P2O7)2 / C Positive electrode composite material The raw materials are: sodium acetate, citric acid, ferric nitrate nonahydrate, and ammonium dihydrogen phosphate; Step 1: Preparation and gelation of precursor solution A reverse feeding process was adopted, in which an iron-phosphorus source mixture was added first, followed by a sodium source. 988.18951 g of ferric nitrate nonahydrate and 460.12 g of ammonium dihydrogen phosphate were dissolved in 20 kg of deionized water. Then, 257.6056 g of sodium acetate and 1152.744 g of citric acid were added. The pH value of the gel was controlled within the range of 3.8-4.2. The mixture was heated in a water bath at 80 ℃ and stirred for 3 hours to form a uniform sol. Subsequently, it was allowed to stand or evaporated to form a gel.

[0023] Step 2: Drying and Pre-firing The resulting gel was transferred to a vacuum drying oven and dried at 120 °C for 24 hours. After drying, it was removed, ground, and pre-calcined at 350 °C for 4 hours in an H2 / Ar mixed gas atmosphere with a heating rate of 2 °C / min. Additionally, a 2-hour holding period at 300 °C was introduced during the 350 °C pre-calcination stage to match the carbon network formation rate with the crystal nucleation rate.

[0024] Step 3: High-temperature calcination The pre-calcined product was ground and tableted, then subjected to segmented calcination for 5 h under a specific atmosphere of H2 / Ar mixture, with programmed heating to critical temperatures of 450℃ and 550℃ at a heating rate of 2℃ / min, ultimately yielding Na. 3.12 Fe 2.44 (P2O7)2 / C cathode composite powder.

[0025] XRD refinement analysis was performed on the composite material, and the Na₂O₃ content was determined at 550 °C using sodium acetate as the sodium source. 3.12 Fe 2.44 Fine-tuned powder XRD pattern of (P2O7)2 / C composite material, as shown Figure 1 As shown, the results indicate that Rwp=3.05, Rp=1.25, the phase purity is high, and the crystal structure is regular.

[0026] Example 2: Preparation of Na from sodium carbonate as a sodium source 3.12 Fe 2.44 (P2O7)2 / C cathode composite material The raw materials are: sodium carbonate, citric acid nonahydrate, ferric nitrate, and diammonium phosphate; Step 1: Preparation and gelation of precursor solution A reverse feeding process was adopted, in which an iron-phosphorus source mixture was added first, followed by a sodium source. 1976.37902 g of ferric nitrate nonahydrate and 920.24 g of ammonium dihydrogen phosphate were dissolved in 40 kg of deionized water. Then, 661.3776 g of sodium carbonate and 2305.488 g of citric acid were added. The pH value of the gel was controlled within the range of 3.8-4.2. The mixture was heated in a water bath at 80 ℃ and stirred for 6 hours to form a uniform sol. Subsequently, it was allowed to stand or evaporated to form a gel.

[0027] Step 2: Drying and Pre-firing The resulting gel was transferred to a vacuum drying oven and dried at 120 °C for 24 hours. After drying, it was removed, ground, and pre-calcined at 350 °C for 4 hours in an H2 / Ar mixed gas atmosphere at a heating rate of 2 °C / min. Additionally, a 2-hour holding period at 300 °C was introduced during the 350 °C pre-calcination stage to match the carbon network formation rate with the crystal nucleation rate.

[0028] Step 3: High-temperature calcination The pre-calcined product was ground and tableted, then subjected to segmented calcination for 5 h under a specific atmosphere of H2 / Ar mixture, with programmed heating to critical temperatures of 450℃ and 550℃ at a heating rate of 2℃ / min, to finally obtain Na. 3.12 Fe 2.44 (P2O7)2 / C cathode composite powder.

[0029] XRD refinement analysis was performed on the composite material. The refined powder XRD pattern of the Na3.12Fe2.44(P2O7)2 / C composite material with sodium carbonate as the sodium source at 550 °C is shown below. Figure 2 As shown, the results indicate that Rwp=2.61 and Rp=1.32, indicating excellent phase purity.

[0030] Example 3: Preparation using sodium dihydrogen phosphate as the sodium source Na3.12Fe2.44(P2O7)2 / C Positive electrode composite material The raw materials are: sodium dihydrogen phosphate, citrate nonahydrate, ferric nitrate, and ammonium dihydrogen phosphate; Step 1: Preparation and gelation of precursor solution A reverse feeding process was adopted, in which an iron-phosphorus source mixture was added first, followed by a sodium source. 1976.37902 g of ferric nitrate nonahydrate and 920.24 g of ammonium dihydrogen phosphate were dissolved in 40 kg of deionized water. Then, 748.65648 g of sodium dihydrogen phosphate and 2305.488 g of citric acid were added. The pH value of the gel was controlled within the range of 3.8-4.2. The mixture was heated in a water bath at 80 ℃ and stirred for 6 hours to form a uniform sol. Subsequently, it was allowed to stand or evaporated to form a gel.

[0031] Step 2: Drying and Pre-firing The resulting gel was transferred to a vacuum drying oven and dried at 120 °C for 24 hours. After drying, it was removed, ground, and pre-calcined at 350 °C for 4 hours in an H2 / Ar mixed gas atmosphere at a heating rate of 2 °C / min. Additionally, a 2-hour holding period at 300 °C was introduced during the 350 °C pre-calcination stage to match the carbon network formation rate with the crystal nucleation rate.

[0032] Step 3: High-temperature calcination The pre-calcined product was ground and compressed into tablets, then subjected to segmented calcination for 5 h under a specific H2 / Ar mixed atmosphere, with the temperature programmed to reach the critical temperatures of 450℃ and 550℃ at a heating rate of 2℃ / min, ultimately yielding Na. 3.12 Fe 2.44 (P2O7)2 / C cathode composite powder.

[0033] SEM analysis of the composite material revealed that Na2+ was obtained at 550 °C using sodium dihydrogen phosphate as the sodium source. 3.12 Fe 2.44 SEM image of (P2O7)2 / C composite material, as shown Figure 3 As shown, its grains exhibit a regular ellipsoidal morphology.

[0034] Example 4: Preparation and Performance Testing of Sodium-ion Batteries The specific steps for preparing a sodium-ion battery are as follows: Step 1: Preparation of the positive electrode sheet; Na₂O₃ is used as the active material... 3.12 Fe 2.44 (P2O7)2 / C, binder PVDF, and conductive agent SP / CNT are mixed evenly in N-methylpyrrolidone at a ratio of 70wt%:20wt%:10wt%, and then coated onto aluminum foil. The mixture is dried in an oven at 105-145℃ for 1-2 minutes and then rolled at a pressure of 20-100T and a speed of 60-70m / min to a thickness of 190-240μm to obtain the positive electrode sheet. Step 2: Preparation of negative electrode sheet; Active material graphite, conductive agent SP, binder CMC, SBR and LA133 are mixed evenly in deionized water at a ratio of 95.8wt%:0.53wt%:0.99wt%:0.99wt%:1.69wt%, and then coated onto copper foil. The mixture is dried in an oven at 105-145℃ for 1-2 minutes, and then rolled at a pressure of 20-100T and a speed of 60-70m / min to a thickness of 190-240μm to obtain the negative electrode sheet. Step 3: Battery assembly: Stack the above positive electrode, 10+2μm wet ceramic separator, and negative electrode in sequence, place them in a button cell case, inject electrolyte (1M NaPF6, solvent ratio EC:PC:EMC:DEC=30:3:40:27, containing 1% FEC and 2% VC), and then encapsulate to obtain a button sodium-ion battery.

[0035] Performance testing: such as Figure 4 As shown, the positive electrode, separator, and negative electrode prepared in Example 2 were assembled into a button cell in sequence, and charge-discharge tests were conducted at 25°C and 10C. Electrochemical performance: Under charge-discharge tests at 25°C and 10C, the initial discharge capacity of the battery was 85 mAh / g; after 500 cycles at 2C rate, the capacity retention was 91.5%, and the charge-discharge curves (first, second, and fifth cycles) showed no obvious distortion.

[0036] Mechanical properties: After being rolled with 100T rollers, the positive electrode sheet maintained 97.3% structural integrity, with no breakage or peeling. Safety performance: The electrode sheet showed no delamination during the needle penetration test, and the thermal runaway temperature was 182℃.

[0037] This invention employs a sodium source thermal decomposition temperature control strategy: sodium acetate, sodium carbonate, and sodium dihydrogen phosphate are used as sodium sources, respectively, and their differentiated thermal decomposition characteristics induce the formation of a hierarchical porous structure. A dual-temperature gradient sintering regime is established: in the 350 ℃ pre-sintering stage, a three-dimensional carbon network is formed through citric acid carbonization; in the 450-550 ℃ main sintering stage, the carbon network constraint effect controls the preferential growth of crystals along the (020) crystal plane, obtaining ellipsoidal grains with an aspect ratio of 1.5-2.0. An innovative tableting pretreatment process is adopted: after pre-sintering, the precursor is subjected to 10 MPa cold isostatic pressing treatment, which generates pre-compression stress between the carbon network and the crystal precursor, improving the compressive strength of the final product, which is measured to be ≥180 MPa.

[0038] In summary, this invention employs a sol-gel method using sodium, iron, and phosphorus sources in specific molar ratios as raw materials, preferably sodium acetate, sodium carbonate, and sodium dihydrogen phosphate. A reverse-feeding process is used, first adding an iron-phosphorus source mixture, then adding the sodium source, controlling the pH value of the gel formation within the range of 3.8-4.2. Furthermore, a 2-hour 300°C holding period is introduced during the 350°C pre-calcination stage to match the carbon network formation rate with the crystal nucleation rate. This method yields materials with regular microstructures and good crystallinity, ensuring high specific capacity and excellent rate performance while maintaining good mechanical properties in the high-voltage solid electrode, fundamentally improving the cycle stability and reliability of sodium-ion batteries.

[0039] The above description is merely a preferred embodiment of the present invention, and the scope of protection of the present invention is not limited thereto. Any simple changes or equivalent substitutions of the technical solutions that can be obviously obtained by those skilled in the art within the scope of the technology disclosed in the present invention shall fall within the scope of protection of the present invention.

Claims

1. A type of Na 3.12 Fe 2.44 The method for preparing (P2O7)2 / C cathode composite material is characterized by, A strategy for controlling the thermal decomposition temperature of a sodium source, combined with sol-gel dual-temperature gradient sintering, was employed to induce the formation of a hierarchical porous structure through its differentiated thermal decomposition characteristics. The specific steps are as follows: Step 1: Preparation and gelation of precursor solution; using the sol-gel method, sodium, iron, phosphorus and citric acid as raw materials, a reverse feeding process is adopted, first adding iron-phosphorus source mixture and then adding sodium source, according to the molar ratio of Na:Ac:Fe:P=3.12:6:2.44:4, dissolved in deionized water, heated in a water bath at 80℃ and stirred for 3-6 hours to form a uniform sol, and then allowed to stand or evaporate to form a gel; Step 2: Drying and pre-calcination; Transfer the gel from Step 1 to a vacuum drying oven and dry it at 120℃ for 24 hours. After grinding, pre-calcinate it at 350℃ for 4 hours in an H2 / Ar mixed atmosphere at a heating rate of 2℃ / min. During the pre-calcination process, introduce a heat preservation platform at 300℃ for 2 hours. Step 3: High-temperature calcination; the pre-calcined product is ground and then subjected to cold isostatic pressing at 10 MPa; the tableted product is then calcined in a H2 / Ar mixed atmosphere at a programmed heating rate of 2℃ / min to 450℃-550℃, with each stage lasting 5 hours, to finally obtain Na. 3.12 Fe 2.44 (P2O7)2 / C cathode composite powder.

2. The Na according to claim 1 3.12 Fe 2.44 The method for preparing (P2O7)2 / C cathode composite material is characterized by, In step one, the pH value of the gel is controlled within the range of 3.8-4.

2. The sodium source is selected from sodium acetate, sodium carbonate or sodium dihydrogen phosphate, the iron source is ferric nitrate nonahydrate, and the phosphorus source is ammonium dihydrogen phosphate.

3. A Na according to claim 2 3.12 Fe 2.44 The method for preparing (P2O7)2 / C cathode composite material is characterized by, In step one, the sodium source is sodium acetate, and the raw material amounts are: 988.18951g of ferric nitrate nonahydrate, 460.12g of ammonium dihydrogen phosphate, 257.6056g of sodium acetate, 1152.744g of citric acid, 20kg of deionized water, and the water bath stirring time is 3 hours.

4. A Na according to claim 2 3.12 Fe 2.44 The method for preparing (P2O7)2 / C cathode composite material is characterized by, In step one, the sodium source is sodium carbonate, and the raw material amounts are: 1976.37902g of ferric nitrate nonahydrate, 920.24g of ammonium dihydrogen phosphate, 661.3776g of sodium carbonate, 2305.488g of citric acid, 40kg of deionized water, and the water bath stirring time is 6 hours.

5. A Na according to claim 2 3.12 Fe 2.44 The method for preparing (P2O7)2 / C cathode composite material is characterized by, In step one, the sodium source is sodium dihydrogen phosphate, and the raw material amounts are: 1976.37902g of ferric nitrate nonahydrate, 920.24g of ammonium dihydrogen phosphate, 748.65648g of sodium dihydrogen phosphate, 2305.488g of citric acid, 40kg of deionized water, and the water bath stirring time is 6 hours.

6. A type of Na 3.12 Fe 2.44 The (P2O7)2 / C cathode composite material is characterized by, The composite material is prepared by any one of the preparation methods described in claims 1-2. The grains of the composite material are ellipsoidal with an aspect ratio of 1.5-2.0, and the compressive strength is ≥180MPa. The electrode prepared therefrom has a structural integrity of ≥97.3% under a 100T roller pressure.

7. A type of Na 3.12 Fe 2.44 The application of (P2O7)2 / C cathode composite material is characterized by, Na is prepared by any of the preparation methods described in claims 1-2. 3.12 Fe 2.44 The (P2O7)2 / C cathode composite material is used in the preparation of sodium-ion batteries. The specific steps are as follows: Step 1: Preparation of the positive electrode sheet; Na₂O₃ is used as the active material... 3.12 Fe 2.44 (P2O7)2 / C, binder PVDF, and conductive agent SP / CNT are mixed evenly in N-methylpyrrolidone at a ratio of 70wt%:20wt%:10wt%, and then coated onto aluminum foil. The mixture is dried in an oven at 105-145℃ for 1-2 minutes and then rolled at a pressure of 20-100T and a speed of 60-70m / min to a thickness of 190-240μm to obtain the positive electrode sheet. Step 2: Preparation of negative electrode sheet; Active material graphite, conductive agent SP, binder CMC, SBR and LA133 are mixed evenly in deionized water at a ratio of 95.8wt%:0.53wt%:0.99wt%:0.99wt%:1.69wt%, and then coated onto copper foil. The mixture is dried in an oven at 105-145℃ for 1-2 minutes, and then rolled at a pressure of 20-100T and a speed of 60-70m / min to a thickness of 190-240μm to obtain the negative electrode sheet. Step 3: Battery assembly: Stack the above positive electrode, separator and negative electrode in sequence, put them into the button cell case, inject electrolyte, and seal to obtain the button sodium-ion battery.

8. The application of the Na3.12Fe2.44(P2O7)2 / C cathode composite material according to claim 7, characterized in that, The diaphragm is a wet-process ceramic diaphragm with a specification of 10+2μm; The electrolyte is 1M NaPF6, with a solvent ratio of EC:PC:EMC:DEC = 30:3:40:27, and contains 1% FEC and 2% VC.

9. The application of the Na3.12Fe2.44(P2O7)2 / C cathode composite material according to claim 7, characterized in that, The sodium-ion battery has an initial discharge capacity of ≥85mAh / g at 25℃ and 10C, a capacity retention rate of ≥91.5% after 500 cycles at 2C, and a thermal runaway temperature of ≥182℃.

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