Composite positive electrode material of NaFePO4 modified ferric sodium phosphate pyrophosphate, preparation method of composite positive electrode material, positive plate and battery

Abstract

The invention discloses a NaFePO4 modified sodium ferric phosphate pyrophosphate composite positive electrode material, a preparation method thereof, a positive plate and a battery, the chemical formula of the composite positive electrode material is Na < 4 + x > Fe < 3 + x-y > (PO4) < 2 + x > (P2O7), 0 lt, 0 lt, 0 lt, 0 lt, 0 lt, 0 lt, 0 lt, 0 lt, 0 lt, 0 lt, 0 lt, 0 lt, 0 lt, 0 lt, 0 lt, 0 lt, 0 Xlt; 0 < = y < = 0.1. Wherein x represents the content of NaFePO4, and when 0 lt, x represents the content of NaFePO4; x is more than 0.3 and less than or equal to 0.3, the material exists in the form of a solid solution, no NaFePO4 phase appears, when x is more than 0.3 and less than 1, NaFePO4 appears in the form of a second phase, and the material is a composite phase of NaFePO4 and Na4Fe3 (PO4) 2 (P2O7). The positive electrode material provided by the invention has the characteristics of high theoretical specific capacity, long cycle life and high average working voltage in the aspect of electrochemical performance, is low in cost, environment-friendly, smooth in synthesis path and easy for mass production, and is an ideal sodium ion battery positive electrode material.

Description

Technical Field

[0001] The invention relates to the technical field of sodium ion batteries, and in particular to a composite positive electrode material of NaFePO4 modified sodium iron phosphate pyrophosphate, a preparation method thereof, a positive electrode sheet and a battery. Background Art

[0002] After the Industrial Revolution, with the continuous development of human society, the demand for energy has continued to increase, and traditional fossil energy has also entered a stage of rapid decline. Fossil energy has greatly hindered the sustainable development of the energy society due to its non-renewable properties and the continuous deterioration of the ecological environment caused by its large-scale use, which has forced people to develop new clean energy and energy storage technologies. Lithium-ion batteries are widely used in commercial applications such as energy storage devices, wearable electronic devices, and power vehicle batteries due to their advantages such as high energy density, long cycle life, and environmental friendliness. However, the scarcity of resources such as lithium and cobalt has severely restricted the further large-scale application of lithium-ion batteries. Sodium-ion batteries are considered to be candidates for the next generation of energy storage devices because of their low cost, abundant raw material resources, and similar working mechanisms to lithium-ion batteries, and they show greater competitiveness and application prospects in future large-scale energy storage systems. Despite this, the positive electrode materials of sodium-ion batteries are still in the exploration and development stage, and various performance parameters need to be improved. Among the many positive electrode materials for sodium-ion batteries,

[0003] Among polyanionic cathode materials, especially iron-based phosphate materials, sodium iron pyrophosphate Na4Fe3(PO4)2(P2O7) has always attracted extensive attention from researchers due to its advantages such as excellent cycle life, high operating voltage, and absence of high-cost precious metal elements. However, its disadvantages such as relatively low specific capacity and energy density still need to be further improved. Summary of the invention

[0004] The technical problem to be solved by the present invention is to provide a composite positive electrode material of NaFePO4 modified sodium iron phosphate pyrophosphate, a preparation method thereof, a positive electrode sheet and a battery, so that the composite positive electrode material has the characteristics of high theoretical specific capacity, long cycle life and high average working voltage.

[0005] To achieve the above object, the technical solution adopted by the present invention is: In the first aspect of the present invention, a composite positive electrode material of NaFePO4 modified sodium iron phosphate pyrophosphate is provided, and the chemical formula of the composite positive electrode material is: Na 4+x Fe 3+x-y (PO4) 2+x (P2O7), where 0 <x<1,0≤y≤0.1;

[0006] The outer layer of the composite positive electrode material is coated with a carbon layer, and the carbon content in the composite positive electrode material accounts for 1wt% to 20wt%.

[0007] The second aspect of the present invention provides a method for preparing the composite positive electrode material of the NaFePO4 modified sodium iron phosphate pyrophosphate as described above, which is a two-step sol-gel method.

[0008] Preferably, the preparation method of the composite positive electrode material of NaFePO4 modified sodium iron phosphate pyrophosphate comprises the following steps:

[0009] S1. According to the stoichiometric ratio of Na4Fe3(PO4)2(P2O7), a sodium source compound, an iron source compound and a phosphorus source compound are mixed in water, and then a chelating agent and a carbon source compound are added, and the mixture is heated to react to obtain a Na4Fe3(PO4)2(P2O7) sol;

[0010] S2, according to the stoichiometric ratio of NaFePO4, a sodium source compound, an iron source compound, and a phosphorus source compound are mixed in water, and then a chelating agent is added to obtain a raw material of NaFePO4;

[0011] S3, according to the molar ratio of Na4Fe3(PO4)2(P2O7) to NaFePO4 being 1:x, adding the raw material of NaFePO4 to the Na4Fe3(PO4)2(P2O7) sol obtained in step S1, heating and stirring until the water is evaporated to dryness, to obtain a gel precursor;

[0012] S4, drying the gel precursor and grinding it to obtain a precursor powder;

[0013] S5. The precursor powder is calcined in two steps to obtain a composite positive electrode material of NaFePO4-modified sodium iron phosphate pyrophosphate.

[0014] Preferably, the sodium source compound is selected from one or more of sodium acetate, sodium nitrate, sodium oxalate and sodium citrate; the iron source compound is selected from one or more of ferric acetate, ferric nitrate, ferric oxalate, ferric sulfate and ferrous phosphate; the phosphorus source compound is selected from one or more of phosphoric acid, ferrous phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, ammonium dihydrogen phosphate and diammonium hydrogen phosphate.

[0015] Preferably, the chelating agent is selected from one or more of citric acid, oxalic acid, tartaric acid or ethylenediaminetetraacetic acid.

[0016] The carbon source compound is one or more of glucose, sucrose, starch, citric acid and ascorbic acid.

[0017] Preferably, in step S5, both calcinations are carried out in an atmosphere of argon, nitrogen, or an argon-hydrogen mixed gas with a hydrogen concentration of 5% to 10%;

[0018] The heating rate of the first step calcination is 1-10°C / min, the calcination temperature is 300-400°C, and the holding time is 5-10h;

[0019] The heating rate of the second step calcination is 1-10°C / min, the calcination temperature is 500-600°C, and the holding time is 5-10h.

[0020] Preferably, the preparation method of the composite positive electrode material of NaFePO4 modified sodium iron phosphate pyrophosphate comprises the following steps:

[0021] S1. According to the stoichiometric ratio of Na4Fe3(PO4)2(P2O7), a sodium source compound, an iron source compound and a phosphorus source compound are mixed in water, a chelating agent is added, and then a carbon source compound is added, and the mixture is reacted at 70-90°C for 1.5-6h to obtain a Na4Fe3(PO4)2(P2O7) sol; wherein the molar ratio of the total molar amount of sodium and iron metal ions to the molar ratio of the chelating agent is 1:0.5-2;

[0022] S2. According to the stoichiometric ratio of NaFePO4, a sodium source compound, an iron source compound, and a phosphorus source compound are mixed in water, and then a chelating agent is added to obtain a raw material of NaFePO4; wherein the molar ratio of the total molar amount of sodium and iron metal ions to the chelating agent is 1:0.5-2;

[0023] S3, according to the molar ratio of Na4Fe3(PO4)2(P2O7) to NaFePO4 being 1:x, adding the raw material of NaFePO4 to the Na4Fe3(PO4)2(P2O7) sol obtained in step S1, heating and stirring at 70-90°C until the water is evaporated to dryness, to obtain a gel precursor;

[0024] S4, drying the gel precursor at 70-90° C. for 6-24 hours and then grinding to obtain a precursor powder;

[0025] S5, calcining the precursor powder in two steps in argon, nitrogen or argon-hydrogen mixed gas with a hydrogen concentration of 5% to 10% to obtain a composite positive electrode material of NaFePO4 modified sodium iron phosphate pyrophosphate;

[0026] The heating rate of the first step calcination is 1-10°C / min, the calcination temperature is 300-400°C, and the holding time is 5-10h;

[0027] The heating rate of the second step calcination is 1-10°C / min, the calcination temperature is 500-600°C, and the holding time is 5-10h.

[0028] In a third aspect of the present invention, there is provided a positive electrode sheet for a sodium-ion battery, which is prepared from a positive electrode material, a conductive additive, a binder, and a solvent. It is characterized in that the positive electrode material is selected from the composite positive electrode material of NaFePO4 modified sodium iron pyrophosphate phosphate as described above.

[0029] In a fourth aspect of the present invention, there is provided a sodium-ion battery including the positive electrode sheet for a sodium-ion battery as described above.

[0030] In a fourth aspect of the present invention, there is provided an application of the sodium-ion battery as described above in large-scale energy storage devices for electric vehicles, solar power generation, wind power generation, smart grid peak shaving, distributed power stations, or communication bases.

[0031] The beneficial effects of the present invention are as follows:

[0032] The present invention provides a preparation method of a composite positive electrode material of NaFePO4 modified sodium iron pyrophosphate phosphate, with the chemical formula of Na 4+x Fe 3+x-y (PO4) 2+x (P2O7), where 0 < x < 1 and 0 ≤ y ≤ 0.1. The positive electrode material provided by the present invention has the characteristics of high theoretical specific capacity, long cycle life, and high average working voltage in terms of electrochemical performance, and is also low in cost, environmentally friendly, has a smooth synthesis path, and is easy to mass-produce, making it an ideal positive electrode material for sodium-ion batteries. Description of the Drawings

[0033] Figure 1 It is the XRD spectrum of the target product of the positive electrode material obtained in Example 1;

[0034] Figure 2 It is the SEM image of the target product of the positive electrode material obtained in Example 1;

[0035] Figure 3 It is the charge-discharge curve of the sodium-ion battery obtained in Example 1 at a rate of 0.1C;

[0036] Figure 4 It is the cycle stability curve of the sodium-ion battery obtained in Example 1 at a rate of 1C;

[0037] Figure 5 It is the cycle stability curve of the average voltage of the sodium-ion battery obtained in Example 1 at a rate of 1C;

[0038] Figure 6 It is the cycle stability curve of the sodium-ion battery obtained in Example 1 at a rate of 10C (pre-cycled for one lap at 0.1C);

[0039] Figure 7The rate performance curve of the sodium ion battery obtained in Example 1 in the range of 0.1C-5C;

[0040] Figure 8 The XRD spectrum of the cathode material target product obtained in Example 2;

[0041] Fig. 9 The charge and discharge curve of the sodium ion battery obtained in Example 2 at a rate of 0.1C;

[0042] Fig.10 The cycle stability curve of the sodium ion battery obtained in Example 2 at a rate of 10C (one pre-cycle at 0.1C);

[0043] Fig.11 The XRD spectrum of the cathode material target product obtained in Example 3;

[0044] Fig.12 The charge and discharge curve of the sodium ion battery obtained in Example 3 at a rate of 0.1C;

[0045] Fig.13 The cycle stability curve of the sodium ion battery obtained in Example 3 at a rate of 10C (one pre-cycle at 0.1C);

[0046] Fig.14 The XRD spectrum of the cathode material target product obtained in Example 4;

[0047] Fig.15 The charge and discharge curve of the sodium ion battery obtained in Example 4 at a rate of 0.1C;

[0048] Fig.16 The cycle stability curve of the sodium ion battery obtained in Example 4 at a rate of 10C (one pre-cycle at 0.1C);

[0049] Fig.17 The XRD spectrum of the cathode material target product obtained in Example 5;

[0050] Fig.18 The charge and discharge curve of the sodium ion battery obtained in Example 5 at a rate of 0.1C;

[0051] Fig.19 This is the cycle stability curve of the sodium ion battery obtained in Example 5 at a rate of 10C (one pre-cycle at 0.1C);

[0052] Fig. 20 The XRD spectrum of the cathode material target product obtained in Comparative Example 1;

[0053] Fig.21 The results are shown in Table 1. The results are shown in Table 1. -1)Charge and discharge curves at a current density;

[0054] Fig. 22 It is the cycle stability curve of the sodium-ion battery obtained in Comparative Example 1 at a 10C rate. Specific embodiments

[0055] The following further elaborates on the present invention in conjunction with embodiments, enabling those skilled in the art to implement it with reference to the text of the specification.

[0056] It should be understood that terms such as "having", "comprising", and "including" used herein do not exclude the presence or addition of one or more other elements or their combinations.

[0057] Unless otherwise specified, the test methods used in the following examples are all conventional methods. The materials, reagents, etc. used in the following examples can all be obtained from commercial sources unless otherwise specified. For those not indicating specific conditions in the following examples, they are carried out according to conventional conditions or the conditions recommended by the manufacturer. For reagents or instruments without indicating the manufacturer, they are all conventional products that can be purchased commercially.

[0058] The present invention provides a composite cathode material of NaFePO4-modified sodium iron pyrophosphate, and the chemical formula of the composite cathode material is: Na 4+x Fe 3+x-y (PO4) 2+x (P2O7), where 0 < x < 1 and 0 ≤ y ≤ 0.1; the outer layer of the composite cathode material is coated with a carbon layer, and the addition of the carbon coating layer is used to improve the electronic conductivity of the cathode material; the carbon content in the composite cathode material accounts for 1 wt% - 20 wt%. The composite cathode material can also be expressed as Na 4+x Fe 3+x-y (PO4) 2+x (P2O7) / C, where C represents the carbon layer.

[0059] Among them, x represents the content of NaFePO4. When 0 < x ≤ 0.3, the material exists in the form of a solid solution and no NaFePO4 phase appears. When 0.3 < x < 1, NaFePO4 appears in the form of a second phase, and the material is a composite phase of NaFePO4 and Na4Fe3(PO4)2(P2O7). The introduction of the y variable is used to adjust the Na:Fe ratio in the material and effectively control the impurity content.

[0060] In a preferred embodiment, the cathode material of the sodium-ion battery provided by the present invention has a spherical morphology, with a loose and porous structure inside the sphere, and the diameter of the sphere is 10 - 50 μm.

[0061] In some specific embodiments of the present invention, the cathode material of the sodium-ion battery is Na 4.3 Fe3.3 (PO4) 2.3 (P2O7) / C has the best comprehensive performance. -1 ) and its capacity retention rate is 95.67% after 2000 cycles at a high current density. It also has high rate performance. It can maintain 76.83% of the initial capacity at a high rate of 5C and 69.00% of the original capacity at a high rate of 10C. It is an ideal positive electrode material for sodium ion battery equipment.

[0062] The present invention also provides a method for preparing the composite positive electrode material of the NaFePO4 modified sodium iron phosphate pyrophosphate as above, which is a two-step sol-gel method, specifically comprising the following steps:

[0063] S1. According to the stoichiometric ratio of Na4Fe3(PO4)2(P2O7), a sodium source compound, an iron source compound and a phosphorus source compound are mixed in water, and then a chelating agent is added, and then a carbon source compound is added, and the mixture is heated and reacted to obtain a Na4Fe3(PO4)2(P2O7) sol;

[0064] S2, according to the stoichiometric ratio of NaFePO4, a sodium source compound, an iron source compound, and a phosphorus source compound are mixed in water, and then a chelating agent is added to obtain a raw material of NaFePO4;

[0065] S3, according to the molar ratio of Na4Fe3(PO4)2(P2O7) to NaFePO4 being 1:x, the raw material of NaFePO4 is added to the Na4Fe3(PO4)2(P2O7) sol obtained in step S1, and heated and stirred until the water is evaporated to dryness to obtain a gel precursor, which is NaFePO4 containing a certain proportion of Na 4+x Fe 3+x (PO4) 2+x (P2O7) gel;

[0066] S4, drying the gel precursor and grinding it to obtain a precursor powder;

[0067] S5. The precursor powder is calcined in two steps to obtain a composite positive electrode material of NaFePO4 modified sodium iron phosphate pyrophosphate.

[0068] In a preferred embodiment, the sodium source compound is selected from one or more of sodium acetate, sodium nitrate, sodium oxalate and sodium citrate, more preferably sodium acetate.

[0069] In a preferred embodiment, the iron source compound is selected from one or more of ferric acetate, ferric nitrate, ferric oxalate, ferric sulfate and ferrous phosphate, more preferably ferric nitrate.

[0070] In a preferred embodiment, the phosphorus source compound is selected from one or more of phosphoric acid, ferrous phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate, more preferably ammonium dihydrogen phosphate.

[0071] In a preferred embodiment, the chelating agent is selected from one or more of citric acid, oxalic acid, tartaric acid or ethylenediaminetetraacetic acid, more preferably oxalic acid.

[0072] In a preferred embodiment, the carbon source compound is from one or more of glucose, sucrose, starch, citric acid, and ascorbic acid.

[0073] In a preferred embodiment, in step S5, both calcinations are performed in an atmosphere of argon, nitrogen, or an argon-hydrogen mixture with a hydrogen concentration of 5% to 10%;

[0074] The heating rate of the first step calcination is 1-10°C / min, the calcination temperature is 300-400°C, and the holding time is 5-10h;

[0075] The heating rate of the second step calcination is 1-10°C / min, the calcination temperature is 500-600°C, and the holding time is 5-10h.

[0076] In a preferred embodiment, the preparation method of the composite positive electrode material of NaFePO4 modified sodium iron pyrophosphate comprises the following steps:

[0077] S1. According to the stoichiometric ratio of Na4Fe3(PO4)2(P2O7), a sodium source compound, an iron source compound and a phosphorus source compound are mixed in water, a chelating agent is added, and then a carbon source compound is added, and the mixture is reacted at 70-90°C for 1.5-6h to obtain a Na4Fe3(PO4)2(P2O7) sol; wherein the molar ratio of the total molar amount of sodium and iron metal ions to the molar ratio of the chelating agent is 1:0.5-2;

[0078] S2. According to the stoichiometric ratio of NaFePO4, a sodium source compound, an iron source compound, and a phosphorus source compound are mixed in water, and then a chelating agent is added to obtain a raw material of NaFePO4; wherein the molar ratio of the total molar amount of sodium and iron metal ions to the chelating agent is 1:0.5-2;

[0079] S3, according to the molar ratio of Na4Fe3(PO4)2(P2O7) to NaFePO4 being 1:x, adding the raw material of NaFePO4 to the Na4Fe3(PO4)2(P2O7) sol obtained in step S1, heating and stirring at 70-90°C until the water is evaporated to dryness, to obtain a gel precursor;

[0080] S4, drying the gel precursor at 70-90° C. for 6-24 hours and then grinding to obtain a precursor powder;

[0081] S5, calcining the precursor powder in two steps in argon, nitrogen or argon-hydrogen mixed gas with a hydrogen concentration of 5% to 10% to obtain a composite positive electrode material of NaFePO4 modified sodium iron phosphate pyrophosphate;

[0082] The heating rate of the first step calcination is 1-10°C / min, the calcination temperature is 300-400°C, and the holding time is 5-10h;

[0083] The heating rate of the second step calcination is 1-10°C / min, the calcination temperature is 500-600°C, and the holding time is 5-10h.

[0084] The present invention also provides a sodium ion battery positive electrode sheet, which is prepared from a positive electrode material, a conductive additive, a binder and a solvent, wherein the positive electrode material is selected from the composite positive electrode material of the above-mentioned NaFePO4 modified sodium iron phosphate pyrophosphate.

[0085] The present invention also provides a sodium ion battery, comprising the above sodium ion battery positive electrode sheet.

[0086] The present invention also provides an application of the above-mentioned sodium ion battery in large-scale energy storage devices such as electric vehicles, solar power generation, wind power generation, smart grid peak regulation, distributed power stations or communication bases.

[0087] The present invention has the following advantages:

[0088] (1) In the composite positive electrode material of NaFePO4 modified sodium iron phosphate pyrophosphate synthesized by the present invention, the addition of NaFePO4 improves the specific capacity and cycle stability of the material, enriching the material system of sodium ion batteries.

[0089] (2) The NaFePO4-modified sodium iron phosphate pyrophosphate composite positive electrode material synthesized by the present invention has the characteristics of high theoretical specific capacity, long cycle life and high average operating voltage, and is an ideal positive electrode material for sodium ion batteries.

[0090] (3) Na synthesized in the preferred embodiment of the present invention 4.3 Fe 3.3 (PO4) 2.3 (P2O7) cathode material has the best overall performance, at 10C (1C = 120mAg -1 ) for 2000 cycles at a high current density of 95.67%, and it also has high rate performance. It can maintain 76.83% and 69.00% of the initial capacity at high rates of 5C and 10C, respectively, making it suitable for large-scale energy storage devices.

[0091] (4) The positive electrode material synthesized by the method of the present invention is low-cost, environmentally friendly, smoothly synthesized, and easy to mass-produce.

[0092] In order to further understand the present invention, the NaFePO4 modified sodium iron phosphate pyrophosphate composite positive electrode material provided by the present invention is described below in conjunction with the examples, and the protection scope of the present invention is not limited by the following examples.

[0093] Example 1

[0094] Step 1: Preparation of Na by sol-gel method 4.3 Fe 3.3 (PO4) 2.3 (P2O7) cathode material

[0095] The target product of synthesis is Na 4.3 Fe 3.3 (PO4) 2.3 (P2O7) positive electrode material, the synthetic raw materials are sodium acetate, ferric nitrate, and ammonium dihydrogen phosphate, the carbon source compound is glucose, the chelating agent is oxalic acid, and the solvent is deionized water.

[0096] S1. According to the stoichiometric ratio of Na4Fe3(PO4)2(P2O7), sodium acetate, ferric nitrate and ammonium dihydrogen phosphate were mixed in water, and then oxalic acid was added (the molar ratio of the total molar amount of sodium and iron metal ions to the chelating agent was 1:1), and glucose was added according to the mass proportion of C in the final product being 20%, and the reaction was carried out under heating in an oil bath at 80°C for 3h to obtain a Na4Fe3(PO4)2(P2O7) sol;

[0097] S2. According to the stoichiometric ratio of NaFePO4, sodium acetate, ferric nitrate and ammonium dihydrogen phosphate are mixed in water, and then oxalic acid is added (the molar ratio of the total molar amount of sodium and iron metal ions to the chelating agent is 1:1) to obtain a raw material of NaFePO4;

[0098] S3, adding the raw material of NaFePO4 to the Na4Fe3(PO4)2(P2O7) sol obtained in step S1 at a molar ratio of Na4Fe3(PO4)2(P2O7) to NaFePO4 of 1:0.3, heating and stirring in an oil bath at 80°C until the water is evaporated to dryness, to obtain a gel precursor;

[0099] S4, drying the gel precursor at 80° C. for 12 h and then grinding the precursor to obtain a precursor powder;

[0100] S5. The precursor powder is placed in a tube furnace, and the temperature is first raised to 300°C for 5h at a heating rate of 5°C / min under the conditions of an argon-hydrogen mixed gas with a hydrogen concentration of 10% and a gas flow rate of 60-80mL / min, and then calcined at 550°C for 5h to obtain the target product of the positive electrode material Na 4.3 Fe 3.3 (PO4)2.3 (P2O7).

[0101] Step 2: Preparation of Na 4.3 Fe 3.3 (PO4) 2.3 (P2O7) positive electrode

[0102] The target product Na 4.3 Fe 3.3 (PO4) 2.3 (P2O7) and Super P, binder polyvinylidene fluoride are mixed in a mass ratio of 7:2:1, and then solvent N-methylpyrrolidone is added in a ratio of the mixture to the solvent of 0.1 mg / μL. After pulping, coating, drying and other steps, a positive electrode sheet containing the target product is obtained.

[0103] Step 3: Assemble the target product Na 4.3 Fe 3.3 (PO4) 2.3 Sodium ion battery with (P2O7) as the positive electrode.

[0104] The target product cathode sheet prepared above and the metallic sodium anode were assembled into a sodium ion battery, GF / F was used as a battery separator, and the electrolyte was a carbonate electrolyte (1M NaClO4 PC solution containing 5 vol% FEC).

[0105] Performance Testing:

[0106] Figure 1 This is the XRD spectrum of the target product of the positive electrode material obtained in Example 1. It can be seen from the figure that the synthesized material has good crystallinity and a Pn21a space group structure, which is similar to the structure of Na4Fe3(PO4)2(P2O7) without the addition of NaFePO4 modification, indicating that the material forms a solid solution phase structure.

[0107] Figure 2 This is a SEM image of the target product of the positive electrode material obtained in Example 1. It can be seen from the figure that the material has a spherical morphology, a loose and porous structure inside the sphere, and the diameter of the sphere is 10-50μm.

[0108] Figure 3 The sodium ion battery obtained in Example 1 is 0.1C (1C = 120mAg -1 ) charge and discharge curves under current density. As can be seen from the figure, this material has a high specific capacity of 131.65 mAh g when applied to sodium ion batteries. -1 .

[0109] Figure 4 The cyclic stability curve of the sodium ion battery obtained in Example 1 at 1C rate is shown in the figure. As can be seen from the figure, the initial specific capacity of the target product obtained in this example is 114.52 mAh g-1 After 300 cycles, the capacity retention rate is 95.53%, which has good cycle stability.

[0110] Figure 5 The average working voltage cycle stability curve of the sodium ion battery obtained in Example 1 at 1C rate is shown in the figure. As can be seen from the figure, its initial voltage is 2.87V, and the voltage retention rate is 99.18% after 300 cycles.

[0111] Figure 6 The cyclic stability curve of the sodium ion battery obtained in Example 1 at a rate of 10C is shown in the figure. As can be seen from the figure, the initial specific capacity of the target product obtained in this example is 85.11 mAh g -1 After 2000 cycles, the capacity retention rate is 95.67%, which has good cycle stability.

[0112] Figure 7 The rate performance curve of the sodium ion battery obtained in Example 1 in the range of 0.1C-5C is shown in the figure. As can be seen from the figure, its initial capacity at a rate of 0.1C is 131.65 mAh g -1 , and can maintain 76.83% of the original capacity at a high rate of 5C, and can maintain 69.00% of the original capacity at a high rate of 10C, showing good rate performance.

[0113] Example 2

[0114] The preparation method is the same as that in Example 1, except that the raw materials are divided into the following proportions: 4.1 Fe 3.1 (PO4) 2.1 (P2O7) added in stoichiometric ratio.

[0115] Performance Testing:

[0116] Figure 8 The figure shows the XRD spectrum of the target product of the cathode material obtained in Example 2. It can be seen from the figure that the synthesized material has good crystallinity and a Pn21a space group structure, which is similar to the structure of Na4Fe3(PO4)2(P2O7) without the addition of NaFePO4 modification, indicating that the material forms a solid solution phase structure.

[0117] Fig. 9 The figure shows the sodium ion battery assembled with the positive electrode material obtained in Example 2 at 0.1C (1C = 120 mA g -1 ) charge and discharge curves under current density. It can be seen from the figure that this material has a charge and discharge capacity of 131.08 mAh g in sodium ion batteries. -1 Higher discharge specific capacity.

[0118] Fig.10The cyclic stability curve of the sodium ion battery obtained in Example 2 at a rate of 10C is shown in the figure. As can be seen from the figure, the initial specific capacity of the target product obtained in this example is 83.76 mAh g -1 After 2000 cycles, the capacity retention rate is 94.94%, which has good cycle stability.

[0119] Example 3

[0120] The preparation method is the same as that in Example 1, except that the raw materials are divided into the following proportions: 4.5 Fe 3.5 (PO4) 2.5 (P2O7) added in stoichiometric ratio.

[0121] Performance Testing:

[0122] Fig.11 The figure shows the XRD spectrum of the target product of the positive electrode material obtained in Example 3. It can be seen that the synthesized material has good crystallinity and is a Pn21a and Pnmb space group structure, corresponding to the NaFePO4 and Na4Fe3(PO4)2(P2O7) composite phase structure.

[0123] Fig.12 The sodium ion battery assembled with the positive electrode material obtained in Example 3 is shown in FIG. -1 ) charge and discharge curves at current density. As can be seen from the figure, this material has a charge and discharge capacity of 133.68 mAh g in sodium ion batteries. -1 Higher discharge specific capacity.

[0124] Fig.13 The cyclic stability curve of the sodium ion battery obtained in Example 3 at a rate of 10C is shown in the figure. As can be seen from the figure, the initial specific capacity of the target product obtained in this example is 88.78 mAh g -1 After 2000 cycles, the capacity retention rate is 85.98%, which has good cycle stability.

[0125] Example 4

[0126] The preparation method is the same as that in Example 1, except that the raw materials are divided into the following proportions: 4.7 Fe 3.7 (PO4) 2.7 (P2O7) added in stoichiometric ratio.

[0127] Performance Testing:

[0128] Fig.14Shown is the XRD spectrum of the target product of the positive electrode material obtained in Example 4. It can be seen from the figure that the synthesized material has good crystallinity and is a Pn21a and Pnmb space group structure, corresponding to the NaFePO4 and Na4Fe3(PO4)2(P2O7) composite phase structure.

[0129] Fig.15 The figure shows the sodium ion battery assembled with the positive electrode material obtained in Example 4 at 0.1C (1C = 120 mA g -1 ) charge and discharge curves at current density. As can be seen from the figure, this material has a charge and discharge capacity of 120.7 mAh g in sodium ion batteries. -1 Higher discharge specific capacity.

[0130] Fig.16 The cyclic stability curve of the sodium ion battery obtained in Example 4 at a rate of 10C is shown in the figure. As can be seen from the figure, the initial specific capacity of the target product obtained in this example is 76.35 mAh g -1 After 2000 cycles, the capacity retention rate is 83.97%, which has good cycle stability.

[0131] Example 5

[0132] The preparation method is the same as that in Example 1, except that the raw materials are divided into the following proportions: 4.9 Fe 3.9 (PO4) 2.9 (P2O7) added in stoichiometric ratio.

[0133] Performance Testing:

[0134] Fig.17 Shown is the XRD spectrum of the target product of the positive electrode material obtained in Example 5. It can be seen from the figure that the synthesized material has good crystallinity and is a Pn21a and Pnmb space group structure, corresponding to the NaFePO4 and Na4Fe3(PO4)2(P2O7) composite phase structure.

[0135] Fig.18 The figure shows the sodium ion battery assembled with the positive electrode material obtained in Example 5 at 0.1C (1C = 120 mA g -1 ) charge and discharge curves under current density. It can be seen from the figure that this material has a charge and discharge capacity of 120.91 mAh g in sodium ion batteries. -1 Higher discharge specific capacity.

[0136] Fig.19 The cyclic stability curve of the sodium ion battery obtained in Example 5 at a rate of 10C is shown in the figure. As can be seen from the figure, the initial specific capacity of the target product obtained in this example is 80.81 mAh g -1 After 2000 cycles, the capacity retention rate is 86.90%, which has good cycle stability.

[0137] Comparative Example 1

[0138] The preparation method is the same as that of Example 1, except that the raw materials are added according to the stoichiometric ratio of Na4Fe3(PO4)2(P2O7), that is, NaFePO4 is not added for modification.

[0139] Performance Testing:

[0140] Fig. 20 The figure shows the XRD spectrum of the target product of the positive electrode material obtained in Comparative Example 1. It can be seen from the figure that the synthesized material has good crystallinity and is a Pn21a space group structure, corresponding to the Na4Fe3(PO4)2(P2O7) phase structure.

[0141] Fig.21 The results are shown in Table 1. The results are shown in Table 1. -1 ) charge and discharge curves under current density. As can be seen from the figure, this material has only 124.96 mAh g in sodium ion batteries. -1 The discharge specific capacity is significantly lower than that of Example 1.

[0142] Fig. 22 The cyclic stability curve of the sodium ion battery obtained in Comparative Example 1 at a rate of 10C is shown in the figure. As can be seen from the figure, the initial specific capacity of the target product obtained in this example is 76.48 mAh g -1 After 2000 cycles, the capacity retention rate is 84.37%, which is significantly lower than that in Example 1.

[0143] Although the embodiments of the present invention have been disclosed as above, they are not limited to the applications listed in the specification and the implementation modes. They can be fully applied to various fields suitable for the present invention. For those familiar with the art, additional modifications can be easily implemented. Therefore, without departing from the general concept defined by the claims and the scope of equivalents, the present invention is not limited to specific details.

Claims

1. A composite positive electrode material of NaFePO4 modified sodium iron phosphate pyrophosphate, characterized in that: The chemical formula of the composite cathode material is: Na 4+x Fe 3+x-y (PO4) 2+x (P2O7), where 0 <x<1,0≤y≤0.1; The outer layer of the composite positive electrode material is coated with a carbon layer, and the carbon content in the composite positive electrode material accounts for 1wt% to 20wt%.

2. A method for preparing a composite positive electrode material of NaFePO4 modified sodium iron phosphate pyrophosphate according to claim 1, characterized in that: The method is a two-step sol-gel process.

3. The method for preparing the composite positive electrode material of NaFePO4 modified sodium iron phosphate pyrophosphate according to claim 2, characterized in that: The following steps are involved: S1. According to the stoichiometric ratio of Na4Fe3(PO4)2(P2O7), a sodium source compound, an iron source compound and a phosphorus source compound are mixed in water, and then a chelating agent and a carbon source compound are added, and the mixture is heated to react to obtain a Na4Fe3(PO4)2(P2O7) sol; S2, according to the stoichiometric ratio of NaFePO4, a sodium source compound, an iron source compound, and a phosphorus source compound are mixed in water, and then a chelating agent is added to obtain a raw material of NaFePO4; S3, according to the molar ratio of Na4Fe3(PO4)2(P2O7) to NaFePO4 being 1:x, adding the raw material of NaFePO4 to the Na4Fe3(PO4)2(P2O7) sol obtained in step S1, heating and stirring until the water is evaporated to dryness, to obtain a gel precursor; S4, drying the gel precursor and grinding it to obtain a precursor powder; S5. The precursor powder is calcined in two steps to obtain a composite positive electrode material of NaFePO4-modified sodium iron phosphate pyrophosphate.

4. The method for preparing the composite positive electrode material of NaFePO4 modified sodium iron pyrophosphate according to claim 3, characterized in that: The sodium source compound is selected from one or more of sodium acetate, sodium nitrate, sodium oxalate and sodium citrate; the iron source compound is selected from one or more of ferric acetate, ferric nitrate, ferric oxalate, ferric sulfate and ferrous phosphate; the phosphorus source compound is selected from one or more of phosphoric acid, ferrous phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, ammonium dihydrogen phosphate and diammonium hydrogen phosphate.

5. The method for preparing the composite positive electrode material of NaFePO4 modified sodium iron phosphate pyrophosphate according to claim 3, characterized in that: The chelating agent is selected from one or more of citric acid, oxalic acid, tartaric acid or ethylenediaminetetraacetic acid.

6. The method for preparing the composite positive electrode material of NaFePO4 modified sodium iron phosphate pyrophosphate according to claim 3, characterized in that: The carbon source compound is one or more of glucose, sucrose, starch, citric acid and ascorbic acid.

7. The method for preparing the composite positive electrode material of NaFePO4 modified sodium iron phosphate pyrophosphate according to claim 3, characterized in that: In step S5, both calcinations are carried out in an atmosphere of argon, nitrogen, or an argon-hydrogen mixed gas with a hydrogen concentration of 5% to 10%; The heating rate of the first step calcination is 1-10°C / min, the calcination temperature is 300-400°C, and the holding time is 5-10h; The heating rate of the second step calcination is 1-10°C / min, the calcination temperature is 500-600°C, and the holding time is 5-10h.

8. The method for preparing the composite positive electrode material of NaFePO4 modified sodium iron phosphate pyrophosphate according to claim 3, characterized in that: The following steps are involved: S1. According to the stoichiometric ratio of Na4Fe3(PO4)2(P2O7), a sodium source compound, an iron source compound and a phosphorus source compound are mixed in water, a chelating agent is added, and then a carbon source compound is added, and the mixture is reacted at 70-90°C for 1.5-6h to obtain a Na4Fe3(PO4)2(P2O7) sol; wherein the molar ratio of the total molar amount of sodium and iron metal ions to the molar ratio of the chelating agent is 1:0.5-2; S2. According to the stoichiometric ratio of NaFePO4, a sodium source compound, an iron source compound, and a phosphorus source compound are mixed in water, and then a chelating agent is added to obtain a raw material of NaFePO4; wherein the molar ratio of the total molar amount of sodium and iron metal ions to the chelating agent is 1:0.5-2; S3, according to the molar ratio of Na4Fe3(PO4)2(P2O7) to NaFePO4 being 1:x, adding the raw material of NaFePO4 to the Na4Fe3(PO4)2(P2O7) sol obtained in step S1, heating and stirring at 70-90°C until the water is evaporated to dryness, to obtain a gel precursor; S4, drying the gel precursor at 70-90° C. for 6-24 hours and then grinding to obtain a precursor powder; S5, calcining the precursor powder in two steps in argon, nitrogen or argon-hydrogen mixed gas with a hydrogen concentration of 5% to 10% to obtain a composite positive electrode material of NaFePO4 modified sodium iron phosphate pyrophosphate; The heating rate of the first step calcination is 1-10°C / min, the calcination temperature is 300-400°C, and the holding time is 5-10h; The heating rate of the second step calcination is 1-10°C / min, the calcination temperature is 500-600°C, and the holding time is 5-10h.

9. A sodium ion battery positive electrode sheet, prepared from a positive electrode material, a conductive additive, a binder and a solvent, characterized in that: The positive electrode material is selected from the composite positive electrode material of NaFePO4 modified sodium iron phosphate pyrophosphate described in claim 1.

10. A sodium ion battery, characterized in that: Comprising the sodium ion battery positive electrode sheet as claimed in claim 9.

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