A method for preparing a composite phosphate

Abstract

The application belongs to the technical field of sodium ion batteries, and discloses a composite phosphate preparation method, which comprises the following steps: S1. Preparing phosphorus source material, iron source material and sodium source material according to the nominal chemical dosage ratio of Na4Fe3(PO4)2P2O7, and simultaneously preparing carbon source material; then wet ball milling the materials to mix them uniformly, and drying to obtain a precursor; S2. Mixing the precursor with sodium vanadium phosphate powder uniformly, and then performing heat treatment at a temperature of 450 DEG C to 650 DEG C under a protective atmosphere, so that a composite material of Na4Fe3(PO4)2P2O7 and sodium vanadium phosphate is obtained, and the composite material simultaneously comprises Na4Fe3(PO4)2P2O7 phase and sodium vanadium phosphate phase. By introducing sodium vanadium phosphate NVP (not NVP precursor) into the solid phase reaction of NFPP to participate in composite formation to form a composite phosphate, the application can effectively solve the problems of difficulty in synthesizing pure phase of the existing composite sodium iron phosphate, low average voltage, high cost of sodium vanadium phosphate and low capacity.

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion battery technology, and more specifically, relates to a method for preparing composite phosphate, the obtained composite phosphate material (i.e., a composite material of Na4Fe3(PO4)2P2O7 and sodium vanadium phosphate) can be used as a positive electrode active material in sodium-ion batteries. Background Technology

[0002] In sodium-ion battery cathode materials, phosphate systems have become a research hotspot due to their excellent cycle stability, high safety, and low structural strain during charge and discharge. Among them, composite sodium iron phosphate (Na4Fe3(PO4)2P2O7, hereinafter referred to as NFPP) combines the advantages of both phosphate and pyrophosphate, possessing economic and environmental advantages. However, composite sodium iron phosphate faces challenges such as difficulty in synthesizing a pure phase, poor conductivity, and low average voltage. On the other hand, sodium vanadium phosphate (Na3V2(PO4)3, hereinafter referred to as NVP) has a high average voltage and stable structure, but its cost is high and it is toxic. To improve the average voltage and energy density of composite sodium iron phosphate, one approach is to combine composite sodium iron phosphate with sodium vanadium phosphate. However, in the solid-state synthesis process, the phase formation temperatures of sodium vanadium phosphate and composite sodium iron phosphate are approximately 750℃ and 550℃, respectively, with a temperature difference of up to 200℃, which poses a significant challenge to their combination. Summary of the Invention

[0003] In view of the above-mentioned defects or improvement needs of the existing technology, the purpose of this invention is to provide a method for preparing composite phosphate, wherein sodium vanadium phosphate NVP (instead of NVP precursor) is introduced into the solid-phase reaction of opposing NFPP to participate in the formation of composite phosphate, which can effectively solve the problems of difficulty in synthesizing pure phase of existing composite sodium iron phosphate, low average voltage, high cost and low capacity of sodium vanadium phosphate.

[0004] To achieve the above objectives, according to the present invention, a method for preparing a complex phosphate is provided, characterized by comprising the following steps:

[0005] S1. Prepare phosphorus source material, iron source material, and sodium source material according to the nominal chemical ratio of P, Fe, and Na elements in Na4Fe3(PO4)2P2O7. At the same time, prepare carbon source material according to the nominal mass ratio of Na4Fe3(PO4)2P2O7 to carbon source material of 100:(1~10). Then, wet ball mill the phosphorus source material, iron source material, sodium source material, and carbon source material to make them uniform, and dry them to obtain the precursor.

[0006] S2. The dried precursor obtained in step S1 is mixed evenly with sodium vanadium phosphate powder, and then heat-treated at a temperature of 450℃~650℃ under a protective atmosphere to obtain a composite phosphate material; the composite phosphate material is a composite material of Na4Fe3(PO4)2P2O7 and sodium vanadium phosphate, and includes both Na4Fe3(PO4)2P2O7 phase and sodium vanadium phosphate phase.

[0007] As a further preferred embodiment of the present invention, in step S2, the mass ratio of the sodium vanadium phosphate powder to the precursor is 1:(0.1-100), preferably 1:(1-10).

[0008] As a further preferred embodiment of the present invention, the iron source is one or more of ferrous acetate, ferrous oxalate, ferric pyrophosphate, ferric phosphate, and ferric oxalate.

[0009] As a further preferred embodiment of the present invention, the sodium source is one or more of sodium carbonate, sodium acetate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium oxalate, and sodium pyrophosphate.

[0010] As a further preferred embodiment of the present invention, the phosphorus source is one or more of the following: ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium pyrophosphate, ferric pyrophosphate, and ferric phosphate.

[0011] As a further preferred embodiment of the present invention, the carbon source is one or more of glucose, fructose, citric acid, ascorbic acid, polyacryl alcohol, polypropylene, and polyethylene glycol.

[0012] As a further preferred embodiment of the present invention, in step S1, the solvent for wet ball milling is one or a mixture of several of the following: water, ethylene glycol, glycerol, dimethyl sulfoxide, methanol, ethanol, and acetone.

[0013] As a further preferred embodiment of the present invention, in step S2, the protective atmosphere is one or more of nitrogen and argon;

[0014] The heat treatment time is 2 to 20 hours.

[0015] Compared with the prior art, the above-described technical solutions conceived in this invention can achieve the following beneficial effects:

[0016] 1. This invention introduces sodium vanadium phosphate (NVP) (rather than the NVP precursor) into the solid-state reaction of opposed NFPP to participate in the formation of composite phosphates. This effectively solves the problems of difficulty in synthesizing pure phases of existing composite sodium iron phosphate, low average voltage, high cost, and low capacity of sodium vanadium phosphate. Simultaneously, the sodium vanadium phosphate NVP acts as a seed crystal in the solid-state reaction, further promoting the pure-phase synthesis of induced NFPPs. The cell parameters of NVP reported in existing technologies are as follows: The unit cell parameters of NFPP are as follows: The two have significantly different unit cell parameters and are often not considered as seed crystals. However, this invention introduces sodium vanadium phosphate (NVP) into the solid-state reaction of NFPP. This forms a composite phosphate and, moreover, the NVP acts as a seed crystal in the solid-state reaction, which is beneficial for promoting the pure-phase synthesis of NFPP. (See below.) Figure 1 and Figure 3 In contrast, Example 1 yielded a composite phase of NFPP and NVP, with NFPP being the pure phase. However, Example 1 did not use any NVP particles or NVP precursors in its solid-state reaction, aiming to obtain NFPP, but the product exhibited a significant high-intensity impurity peak of NaFeP2O7. This invention uses a temperature of 450℃ to 650℃ for the NFPP solid-state reaction. The synthesis of NFPP has minimal impact on the crystal phase of sodium vanadium phosphate in the raw materials, while the presence of sodium vanadium phosphate can induce the crystallization of NFPP, thereby improving the technical difficulty of synthesizing a pure phase of NFPP.

[0017] 2. This invention combines NFPP with sodium vanadium phosphate. On one hand, sodium vanadium phosphate can increase its operating voltage, thereby improving the low average voltage of NFPP. On the other hand, NFPP has a high theoretical specific capacity, low cost, and is environmentally friendly. Combining sodium vanadium phosphate with NFPP can reduce costs and increase capacity. When the composite material prepared by this invention is used in sodium-ion batteries, the sodium-ion batteries exhibit good cycle stability, high first-cycle charge-discharge capacity, high average voltage, excellent rate performance, and high energy density, demonstrating significant practical value. Compared with NFPP, the composite material prepared by this invention shows a significant improvement in average voltage and capacity.

[0018] 3. This invention obtains a composite material by combining NFPP and sodium vanadium phosphate. Compared with single NFPP and single sodium vanadium phosphate, the composite material has a significant capacity advantage. Example 1 below will be used as an example, as will be discussed later. Figure 6 As shown, at a current density of 10C (1C = 129 mAh / g), the capacities of NFPP and NVP are both around 70 mAh / g, while the capacity of the composite phase obtained through Example 1 is close to 90 mAh / g, which is much greater than the capacities of NFPP and NVP. Furthermore, compared to NVP, the composite material exhibits higher capacity retention and superior cycling performance.

[0019] 4. Furthermore, the composite phosphate material obtained by this invention is a composite material of Na4Fe3(PO4)2P2O7 and sodium vanadium phosphate, which has a lower V content per unit mass, lower toxicity, and lower cost. Preferably, the mass ratio of sodium vanadium phosphate powder to the Na4Fe3(PO4)2P2O7 precursor is controlled at 1:(1-10), that is, the mass proportion of the Na4Fe3(PO4)2P2O7 precursor is larger, thus the proportion of sodium vanadium phosphate is smaller, resulting in a composite phosphate material with lower toxicity and lower cost (of course, other ratios can also be used, such as other values ​​within the range of 1:(0.1-100)).

[0020] Unlike the liquid-phase method (which is unsuitable for large-scale industrial production), the solid-phase method used in this invention is suitable for industrial production and has significant advantages. The solid-phase reaction temperatures of sodium vanadium phosphate (NVP) and composite sodium iron phosphate (NFPP) are approximately 750℃ and 550℃, respectively, a temperature difference of up to 200℃, making it difficult to composite them using the same solid-phase reaction (if a sintering temperature above 750℃ is used, the precursor structure of NFPP will be destroyed at that high temperature, and even with staged heat treatment, successful synthesis of NFPP is difficult). This invention, based on a solid-phase reaction, introduces finished sodium vanadium phosphate (NVP) into the solid-phase reaction of composite sodium iron phosphate (NFPP), effectively achieving the composite of NVP and NFPP to obtain a uniform composite phase. Furthermore, NVP can act as a seed crystal, further improving the crystallinity of the composite sodium iron phosphate (NFPP).

[0021] In summary, this invention proposes a method for combining sodium iron phosphate and sodium vanadium phosphate, using sodium vanadium phosphate to induce the synthesis of the pure phase of sodium iron phosphate composite, thereby obtaining a sodium-ion battery cathode material with excellent comprehensive performance. Attached Figure Description

[0022] Figure 1 The image shows the XRD pattern of the composite phase prepared in Example 1.

[0023] Figure 2 The image shows the XRD pattern of the composite phase prepared in Example 2.

[0024] Figure 3 The XRD pattern of NFPP prepared in Comparative Example 1 is shown.

[0025] Figure 4 The image shows a comparison of the first charge-discharge curves of the products obtained in Example 1 and Comparative Example 1 at 0.1C.

[0026] Figure 5 The graph shows a comparison of the median discharge voltage of the products obtained in Example 1 and Comparative Example 1 at different discharge rates.

[0027] Figure 6 The graph shows a comparison of the cycling performance of the product and NVP at 10°C between Example 1 and Comparative Example 1.

[0028] Figure 7 The XRD pattern of the composite phase prepared in Comparative Example 2 is shown.

[0029] Figure 8 The XRD pattern of the composite phase prepared in Comparative Example 3 is shown. Detailed Implementation

[0030] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0031] Example 1:

[0032] The raw materials are ferrous oxalate dihydrate FeC2O4·2H2O, sodium carbonate Na2CO3, ammonium dihydrogen phosphate NH4H2PO4 and citric acid C6H8O7, wherein ferrous oxalate dihydrate FeC2O4·2H2O is the iron source, sodium carbonate Na2CO3 is the sodium source, ammonium dihydrogen phosphate NH4H2PO4 is the phosphorus source and citric acid C6H8O7 is the carbon source;

[0033] Step 1: Add 5.40g FeC2O4·2H2O (0.03mol), 2.12g Na2CO3 (0.02mol), 4.60g NH4H2PO4 (0.04mol), and 0.062g C6H8O7 to a ball mill jar, add 150ml acetone, and ball mill at 300rpm for 5h. Place the milled material in a vacuum oven to dry at 60℃ for 14h. The molar ratio of Fe, Na, and P in the obtained precursor is 3:4:4, and the nominal mass ratio of Na4Fe3(PO4)2P2O7 to the carbon source material is 100:1.

[0034] Step 2: Add the dried precursor and sodium vanadium phosphate powder (Na3V2(PO4)3, NVP, commercially available; the same below) to a ball mill jar at a mass ratio of 1:1. The ball milling speed is 200 rpm and the ball milling time is 0.5 h (this step is dry ball milling, the same below; of course, wet ball milling can also be used, and the solvent used can be water, acetone or ethanol).

[0035] Step 3: The powder obtained in Step 2 is heat-treated in a nitrogen atmosphere and kept at 450°C for 20 hours. After cooling, the composite phosphate material (i.e., the composite phase) is obtained.

[0036] Example 2:

[0037] Using ferric phosphate (FePO4), sodium dihydrogen phosphate (NaH2PO4), sodium citrate dihydrate (C6H5Na3O7·2H2O), and glucose (C6H5PO4) 12 O6 is used as raw material, of which iron phosphate FePO4 is both an iron source and a phosphorus source, sodium dihydrogen phosphate NaH2PO4 is both a sodium source and a phosphorus source, sodium citrate dihydrate C6H5Na3O7·2H2O is a sodium source, and glucose C6H8O7 is a carbon source.

[0038] Step 1: Add 4.52g FePO4 (0.03mol), 1.20g NaH2PO4 (0.01mol), 2.94g C6H5Na3O7·2H2O (0.01mol), and 0.62g C6H5Na3O7·2H2O to the ball mill jar. 12 O6 was added to 100 ml of water and ball-milled at 400 rpm for 5 hours. The slurry after ball milling was then spray-dried at 180℃. The molar ratio of Fe, Na, and P in the resulting precursor was 3:4:4, and the nominal mass ratio of Na4Fe3(PO4)2P2O7 to the carbon source material was 10:1.

[0039] Step 2: Add the dried precursor and sodium vanadium phosphate powder to a ball mill jar at a mass ratio of 5:1. The ball milling speed is 200 rpm and the ball milling time is 0.5 h.

[0040] Step 3: The powder obtained in Step 2 is heat-treated in an argon atmosphere, held at 650℃ for 2 hours, and then cooled to obtain the composite phosphate material.

[0041] Example 3:

[0042] The raw materials are ferrous oxalate dihydrate FeC2O4·2H2O, sodium dihydrogen phosphate NaH2PO4 and ascorbic acid C6H8O6. Ferrous oxalate dihydrate FeC2O4·2H2O is the iron source, sodium dihydrogen phosphate NaH2PO4 is both the sodium and phosphorus source, and ascorbic acid C6H8O6 is the carbon source.

[0043] Step 1: Add 5.40g FeC2O4·2H2O (0.03mol), 4.80g NaH2PO4 (0.04mol), and 0.12g C6H8O6 to a ball mill jar, add 100ml of ethanol, and ball mill at 400rpm for 8h. Place the ball-milled material in a vacuum oven to dry at 80℃ for 10h. The molar ratio of Fe, Na, and P in the obtained precursor is 3:4:4, and the nominal mass ratio of Na4Fe3(PO4)2P2O7 to the carbon source material is 50:1.

[0044] Step 2: Add the dried precursor and sodium vanadium phosphate powder to a ball mill jar at a mass ratio of 10:1. The ball milling speed is 300 rpm and the ball milling time is 0.5 h.

[0045] Step 3: The powder obtained in Step 2 is heat-treated in an argon atmosphere, held at 600℃ for 8 hours, and then cooled to obtain the composite phosphate material.

[0046] Comparative Example 1:

[0047] The difference between this comparative example and Example 1 is that sodium vanadium phosphate is not used; only sodium iron phosphate is used (i.e., step 2 is omitted, and the precursor obtained in step 1 is directly heat-treated, while the atmosphere, temperature and time of the heat treatment remain unchanged).

[0048] Comparative Example 2:

[0049] The difference between this comparative example and Example 1 is that the raw material for synthesizing sodium vanadium phosphate (i.e., sodium vanadium phosphate precursor, rather than sodium vanadium phosphate itself) was directly added during the ball milling stage.

[0050] The raw materials are ferrous oxalate dihydrate FeC2O4·2H2O, sodium carbonate Na2CO3, ammonium dihydrogen phosphate NH4H2PO4, vanadium pentoxide V2O5 and citric acid C6H8O7, wherein ferrous oxalate dihydrate FeC2O4·2H2O is the iron source, sodium carbonate Na2CO3 is the sodium source, ammonium dihydrogen phosphate NH4H2PO4 is the phosphorus source, vanadium pentoxide V2O5 is the vanadium source and citric acid C6H8O7 is the carbon source;

[0051] Step 1: Add the following raw materials to the ball mill jar: 5.40g FeC2O4·2H2O (0.03mol), 2.12g Na2CO3 (0.02mol), 4.60g NH4H2PO4 (0.04mol), and 0.062g C6H8O7 (these amounts of raw materials correspond to the NFPP precursor, with a molar ratio of Fe, Na, and P of 3:4:4 and a nominal mass ratio of Na4Fe3(PO4)2P2O7 to carbon source material of 100:1), and 1.59g of the raw material required for NVP synthesis. Na₂CO₃ (0.015 mol), 1.82 g V₂O₅ (0.01 mol), and 3.45 g NH₄H₂PO₄ (0.03 mol) (these amounts of raw materials correspond to the NVP precursor, with a molar ratio of Na, V, and P of 3:2:3) were added to 200 ml of acetone and ball-milled at 300 rpm for 5 hours. The ball-milled material was then dried in a vacuum oven at 60°C for 14 hours.

[0052] Step 2: The powder obtained in Step 1 is heat-treated in a nitrogen atmosphere, kept at 450℃ for 20 hours, and then cooled to obtain the composite phosphate material.

[0053] Comparative Example 3:

[0054] The difference between this comparative example and Example 1 is that the raw materials for synthesizing sodium vanadium phosphate were directly added during the ball milling stage, and the heat treatment temperature was adjusted to the temperature required for synthesizing sodium vanadium phosphate (i.e., 750°C).

[0055] The raw materials are ferrous oxalate dihydrate FeC2O4·2H2O, sodium carbonate Na2CO3, ammonium dihydrogen phosphate NH4H2PO4, vanadium pentoxide V2O5 and citric acid C6H8O7, wherein ferrous oxalate dihydrate FeC2O4·2H2O is the iron source, sodium carbonate Na2CO3 is the sodium source, ammonium dihydrogen phosphate NH4H2PO4 is the phosphorus source, vanadium pentoxide V2O5 is the vanadium source and citric acid C6H8O7 is the carbon source;

[0056] Step 1: Add the following raw materials to the ball mill jar: 5.40g FeC2O4·2H2O (0.03mol), 2.12g Na2CO3 (0.02mol), 4.60g NH4H2PO4 (0.04mol), and 0.062g C6H8O7 (these amounts of raw materials correspond to the NFPP precursor, with a molar ratio of Fe, Na, and P of 3:4:4 and a nominal mass ratio of Na4Fe3(PO4)2P2O7 to carbon source material of 100:1), and 1.59g Na2CO3 (0.015mol) and 1.82g C6H8O7 (these amounts of raw materials correspond to the NFPP precursor, with a molar ratio of Fe, Na, and P of 3:4:4 and a nominal mass ratio of Na4Fe3(PO4)2P2O7 to carbon source material of 100:1), and the following raw materials required for NVP synthesis: V₂O₅ (0.01 mol) and 3.45 g NH₄H₂PO₄ (0.03 mol) (these amounts of raw materials correspond to the NVP precursor, with a molar ratio of Na, V, and P of 3:2:3) were added to 200 ml of acetone and ball-milled at 300 rpm for 5 hours. The ball-milled material was then dried in a vacuum oven at 60°C for 14 hours.

[0057] Step 2: The powder obtained in Step 1 is heat-treated in a nitrogen atmosphere, held at 750℃ for 8 hours, and then cooled to obtain the composite phosphate material.

[0058] Commercially available NVP powder and the composite cathode materials prepared in Example 1 and Comparative Example 1 were mixed with carbon black conductive agent and polyvinylidene fluoride binder in a mass ratio of 8:1:1, and a certain amount of N-methylpyrrolidone solvent was added to prepare a slurry. A portion of the slurry was then coated onto aluminum foil and baked in a vacuum drying oven at 120°C for 12 hours to obtain the cathode sheet. Using sodium metal as the counter electrode, glass fiber as the separator, and 1 mol / L NaClO4 EC:DMC 1:1, 5% FEC as the cathode electrolyte, coin cells were assembled in an argon glove box. Battery testing was conducted at 25°C within a voltage range of 1.8-4.2V.

[0059] In addition, the powders obtained in Examples 1-2 and Comparative Examples 1-3 were characterized by XRD, and the results are as follows:

[0060] from Figure 1 and Figure 2 It can be seen that the diffraction peaks of the composite phosphates prepared in Examples 1 and 2 match well with the standard peaks of NFPP and sodium vanadium phosphate, indicating that a pure phase NFPP and sodium vanadium phosphate composite phase was prepared.

[0061] For the product obtained in Comparative Example 1, from Figure 3 It can be seen that without combination with sodium vanadium phosphate, the synthesis of sodium iron phosphate complex is prone to impurities, making pure-phase synthesis difficult. Figure 1 , Figure 2The comparison shows that the presence of sodium vanadium phosphate has an inducing effect on the pure-phase synthesis of NFPP.

[0062] from Figure 4 It can be seen that the specific capacity of the composite phase obtained in Example 1 at 0.1C is 120 mAh / g, and the median discharge voltage is 3.01 V; while the specific capacity of NFPP obtained in Comparative Example 1 is 91 mAh / g, and the median discharge voltage is 2.85 V. Therefore, the combination of sodium vanadium phosphate and NFPP improves both its capacity and average voltage.

[0063] from Figure 5 It can be seen that the median voltage of the composite material of NFPP and sodium vanadium phosphate is significantly improved at different rates.

[0064] from Figure 6 It can be seen that the composite phase obtained in Example 1 has a specific capacity of 88.6 mAh / g at 10C, compared with the specific capacities of Example 1 and NVP at 10C, which are 67.9 mAh / g and 76.8 mAh / g, respectively. After 100 cycles at 10C, the capacity retention rate of Example 1 is 96%, which is superior to the retention rate of NVP (83%).

[0065] Regarding the product obtained in Comparative Example 2, from Figure 7 It can be seen that directly heat-treating the precursors of NFPP and NVP under the conditions for synthesizing NFPP can only synthesize NFPP, but cannot obtain NVP.

[0066] For the product obtained in Comparative Example 3, from Figure 8 It can be seen that directly heat-treating the precursors of NFPP and NVP under the conditions for synthesizing NVP can only produce NVP, not NFPP.

[0067] Depend on Figure 7 and Figure 8 The results show that a uniform composite phase of NFPP and NVP cannot be obtained directly by heat treatment of raw materials that are uniformly mixed with NFPP and NVP.

[0068] The results above show that, due to the voltage-boosting effect of sodium vanadium phosphate (NVP), the average voltage of the cathode material composed of NVP and sodium iron phosphate (i.e., the cathode material composed of NVP and NFPP) is significantly higher than that of NFPP alone (Comparative Example 1 uses only NFPP). Furthermore, after being combined with sodium vanadium phosphate, impurities are less likely to occur during NFPP synthesis, because sodium vanadium phosphate has an inductive effect on NFPP synthesis. The capacity and voltage of the material after combining NFPP with sodium vanadium phosphate are significantly improved, which is beneficial for increasing the energy density of NFPP.

[0069] The above embodiments are merely examples. For instance, the protective atmosphere used in heat treatment can be any inert atmosphere other than nitrogen or argon. Furthermore, the specific type and amount of carbon source material can be flexibly adjusted (e.g., using other carbon source materials known in the prior art), and the nominal mass ratio of Na4Fe3(PO4)2P2O7 to carbon source material can be 100:(1-10).

[0070] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a complex phosphate, characterized in that, Includes the following steps: S1. Prepare phosphorus source material, iron source material, and sodium source material according to the nominal chemical ratio of P, Fe, and Na elements in Na4Fe3(PO4)2P2O7. At the same time, prepare carbon source material according to the nominal mass ratio of Na4Fe3(PO4)2P2O7 to carbon source material of 100:(1~10). Then, wet ball mill the phosphorus source material, iron source material, sodium source material, and carbon source material to make them uniform, and dry them to obtain the precursor. S2. The dried precursor obtained in step S1 is mixed evenly with sodium vanadium phosphate powder, and then heat-treated at a temperature of 450℃~650℃ under a protective atmosphere to obtain a composite phosphate material; the composite phosphate material is a composite material of Na4Fe3(PO4)2P2O7 and sodium vanadium phosphate, and includes both Na4Fe3(PO4)2P2O7 phase and sodium vanadium phosphate phase.

2. The method for preparing the composite phosphate as described in claim 1, characterized in that, In step S2, the mass ratio of the sodium vanadium phosphate powder to the precursor is 1:(1-10).

3. The method for preparing the composite phosphate as described in claim 1, characterized in that, The iron source is one or more of ferrous acetate, ferrous oxalate, ferric pyrophosphate, ferric phosphate, and ferric oxalate.

4. The method for preparing the composite phosphate as described in claim 1, characterized in that, The sodium source is one or more of sodium carbonate, sodium acetate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium oxalate, and sodium pyrophosphate.

5. The method for preparing the composite phosphate as described in claim 1, characterized in that, The phosphorus source is one or more of the following: ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium pyrophosphate, ferric pyrophosphate, and ferric phosphate.

6. The method for preparing the composite phosphate as described in claim 1, characterized in that, The carbon source is one or more of glucose, fructose, citric acid, ascorbic acid, polyacryl alcohol, polypropylene, and polyethylene glycol.

7. The method for preparing the composite phosphate as described in claim 1, characterized in that, In step S1, the solvent for wet ball milling is one or a mixture of several of the following: water, ethylene glycol, glycerol, dimethyl sulfoxide, methanol, ethanol, and acetone.

8. The method for preparing the composite phosphate as described in claim 1, characterized in that, In step S2, the protective atmosphere is one or more of nitrogen and argon; The heat treatment time is 2 to 20 hours.

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