A carbon-coated sodium iron pyrophosphate positive electrode material, a preparation method and application thereof

Abstract

The application provides a carbon-coated sodium iron pyrophosphate positive electrode material and a preparation method and application thereof. The positive electrode material has a molecular formula of Na a Fe b (PO4) c (P2O7), wherein 3

Description

Technical Field

[0001] This invention relates to the field of sodium-ion batteries, specifically to a carbon-coated sodium iron pyrophosphate cathode material, its preparation method, and its applications. Background Technology

[0002] In recent years, with the increasingly severe international challenges of energy crisis and environmental pollution, my country has proactively adapted to the global trend of green and low-carbon development, proposing the goals of "carbon peaking" and "carbon neutrality." Against this backdrop, clean energy (such as solar, wind, and tidal energy) has experienced unprecedented development. The development of these new energy forms inevitably involves the issue of energy storage. Batteries, as an energy storage medium, have received widespread attention due to their green and convenient advantages. Rechargeable lithium-ion batteries, as an energy storage device, have been extensively studied due to their high conversion efficiency and energy density. However, the scarcity and uneven distribution of lithium resources limit their future development. With the rise of large-scale energy storage, and considering the high abundance and low price of sodium resources in the Earth's crust, sodium-ion batteries have become an important supplement to lithium-ion batteries.

[0003] In sodium-ion batteries, iron-based polyanion cathode materials have attracted widespread attention due to their low cost, environmental friendliness, excellent cycle performance, and good safety. Sodium iron phosphate (Na3Fe2(PO4)3), with its monoclinic phase, cannot achieve capacity in the first charge cycle due to the trivalent oxidation state of iron, making it unsuitable as a cathode material for practical applications. While sodium ferrous phosphate (NaFePO4), with its olivine structure, exhibits good electrochemical performance, the olivine phase is thermodynamically unstable and typically requires complex synthesis processes. Thermodynamically stable sodium ferrous phosphate (NaFePO4) possesses a typical somanganese oxide structure, which is simple and readily available, but lacks effective sodium ion diffusion channels and is therefore electrochemically inactive. In recent years, sodium iron pyrophosphate has been proven to be an electrochemically active cathode material. Na4Fe3(PO4)2P2O7 and Na3Fe2PO4P2O7 cathodes with specific compositions have theoretical capacities reaching 128 mA hg, respectively. -1 and 119 mA hg -1 Sodium iron pyrophosphate, in particular, with its high sodium content, exhibits high initial capacity, low raw material cost, and good cycle performance, making it suitable for large-scale energy storage systems. However, current methods for preparing sodium-rich Na4Fe3(PO4)2P2O7 cathodes typically contain electrochemically inert NaFePO4 impurities, thus reducing their initial capacity. Controlling the sodium content to between 3 and 4% not only yields a higher theoretical capacity (120 to 128 mA hg) but also... -1 This also facilitates the production of high-purity sodium iron pyrophosphate cathode materials.

[0004] On the other hand, commercially available iron phosphate, as a precursor to lithium iron phosphate, is stable, inexpensive, and can be mass-produced. It can simultaneously provide phosphorus and iron sources, and when used in the preparation of sodium iron pyrophosphate, it exhibits high atom utilization, reducing the generation of byproducts or other gaseous or liquid phases. Furthermore, the widespread use of lithium iron phosphate batteries, due to their lifespan, inevitably leads to a large number of discarded batteries. The cathode powder from these discarded batteries contains the valuable element lithium (Li), which has significant recycling value. Currently, after selectively recovering lithium from the cathode powder, the remaining iron phosphate slag has low recycling value and is either stockpiled or requires acid dissolution, impurity removal, and further processing into iron phosphate—a complex process with high recycling costs. If the iron phosphate slag could be directly recycled into valuable materials, it would contribute to the sustainable recycling and reuse of discarded lithium iron phosphate batteries.

[0005] CN113104828A discloses a method for preparing a porous carbon-modified sodium iron pyrophosphate cathode material. The method uses ferric nitrate nonahydrate as the iron source, ammonium dihydrogen phosphate and sodium pyrophosphate as the phosphorus and sodium sources, citric acid and PVP as the carbon source, and NaCl as the template. After high-temperature sintering, a porous carbon-coated sodium iron pyrophosphate cathode material is obtained. Although introducing porous carbon coating can improve the overall conductivity of the cathode material, the method described has drawbacks: ferric nitrate nonahydrate has a low iron content, low atom utilization, and high cost; furthermore, the calcination process releases NO. x Using gases like NaCl is not environmentally friendly. Using NaCl as a template means the product needs to be rinsed with a large amount of water, making the process lengthy and unsuitable for large-scale production.

[0006] CN110061233A discloses a fluorinated carbon-coated sodium iron pyrophosphate pyrophosphate@mesoporous carbon composite material. The method uses ferric nitrate nonahydrate, ferrous acetate, ferric oxalate, or iron powder as the iron source, and adds phosphorus, sodium, fluorinated polymer, and mesoporous carbon as the carbon source. The mixture is ball-milled and then sintered at high temperature to obtain the fluorinated carbon-coated sodium iron pyrophosphate pyrophosphate@mesoporous carbon composite material. However, the iron source used in this method, such as ferric nitrate nonahydrate, ferrous acetate, or ferric oxalate, has low atom utilization and is expensive. Furthermore, the calcination process generates other gases or products, which is not environmentally friendly. Using iron powder as the iron source requires the addition of acidic reagents, which is highly corrosive and requires sophisticated equipment. In addition, the polymer and mesoporous carbon are expensive, hindering large-scale production.

[0007] CN112563484A discloses a sodium-ion battery cathode material, its preparation method, and a sodium-ion battery. The sodium-ion battery cathode material is prepared as a layered structure, and the preparation method involves controlling the reaction of a precursor mixture solution under high temperature and high pressure conditions. The preparation process is simple, but the high temperature and high pressure environment poses certain risks, and the high pressure environment places high demands on the equipment.

[0008] How to prepare a high-rate and long-cycle cathode material with low cost, simple operation and suitable for large-scale production is an important research direction in this field. Summary of the Invention

[0009] The purpose of the present invention is to provide a carbon-coated sodium iron pyrophosphate phosphate cathode material, its preparation method and application.

[0010] To achieve the purpose of this invention, the following technical solutions are adopted:

[0011] In the first aspect of the present invention, a carbon-coated sodium iron pyrophosphate phosphate cathode material is provided, and the molecular formula of the cathode material is Na a Fe b (PO4) c (P2O7), where 3 < a < 4; a = c + 2; b = c + 1; 1 < c < 2.

[0012] Among them, the value of a can be 3.1, 3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8 or 3.9, etc., and the value of c can be 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8 or 1.9, etc., but not limited to the listed values, and other unlisted values within this numerical range are equally applicable.

[0013] The present invention directly uses commercial iron phosphate (or iron phosphate slag after lithium extraction from the waste powder of the cathode of an iron phosphate lithium battery), an external phosphorus source, a sodium source and a carbon source, and prepares a series of sodium iron pyrophosphate phosphate cathode materials Na a Fe b (PO4) c (P2O7) (3 < a < 4; a = c + 2; b = c + 1; 1 < c < 2) by regulating the ratios of Na, P and Fe, which is of great significance for the future application of sodium-ion batteries in large-scale energy storage systems. Under the conditions of 3 < a < 4; a = c + 2; b = c + 1; 1 < c < 2, Na a Fe b (PO4) c(P2O7) is different from the previously reported Na4Fe3(PO4)2P2O7 and Na3Fe2PO4P2O7. When a gradually increases from 3 to 4, the larger the value of a, the higher the capacity of the material because of the high sodium ion content. However, when a exceeds 4, an inert phase of NaFePO4 is likely to form, leading to a decrease in the cycle stability of the material and limited capacity utilization. When a is smaller, the capacity of the material is lower because of the lower sodium ion content, but the cycle stability of the material is enhanced and the capacity is more easily utilized. Different from the two end compounds, the theoretical capacities of Na4Fe3(PO4)2P2O7 and Na3Fe2PO4P2O7 are 128 mA h g -1 and 119 mA h g -1 , respectively. Therefore, when 3 < a < 4, the theoretical capacity of Na a Fe b (PO4) c (P2O7) is between 128 mA h g -1 and 119 mA h g -1 . Therefore, the capacity and cycle stability of this series of compounds can be regulated by controlling the sodium content, and there must be an optimal balance point for the comprehensive performance of the material. Since the materials with different sodium contents are mutually soluble phases, the cathode material of sodium iron pyrophosphate with high cycle stability and high capacity can be synthesized without strictly controlling the conditions.

[0014] As a preferred technical solution of the present invention, the raw materials of the cathode material include iron phosphate, an additional phosphorus source, a sodium source, and an organic carbon source.

[0015] Preferably, the iron phosphate includes any one or at least two combinations of anhydrous iron phosphate, iron phosphate dihydrate, iron phosphate tetrahydrate, or iron phosphate slag after lithium extraction from the waste cathode powder of lithium iron phosphate batteries. Typical but non-limiting examples of the combinations are: the combination of anhydrous iron phosphate and iron phosphate dihydrate, the combination of iron phosphate dihydrate and iron phosphate tetrahydrate, the combination of iron phosphate dihydrate and iron phosphate slag after lithium extraction from the waste cathode powder of lithium iron phosphate batteries, or the combination of anhydrous iron phosphate, iron phosphate dihydrate, and iron phosphate slag after lithium extraction from the waste cathode powder of lithium iron phosphate batteries, etc.

[0016] Preferably, the additional phosphorus source includes any one or at least two combinations of ammonium dihydrogen phosphate, sodium ammonium dihydrogen phosphate, ammonium phosphate, sodium ammonium phosphate, diammonium hydrogen phosphate, sodium diammonium hydrogen phosphate, or phosphoric acid. Typical but non-limiting examples of the combinations are: the combination of ammonium dihydrogen phosphate and sodium ammonium dihydrogen phosphate, the combination of sodium ammonium dihydrogen phosphate and ammonium phosphate, the combination of ammonium phosphate and sodium ammonium phosphate, the combination of sodium ammonium phosphate and diammonium hydrogen phosphate, the combination of diammonium hydrogen phosphate and sodium diammonium hydrogen phosphate, or the combination of sodium diammonium hydrogen phosphate and phosphoric acid, etc.

[0017] The present invention optionally uses the leaching residue, which is mainly composed of iron phosphate, after lithium recovery from lithium iron phosphate cathode powder as raw material. This provides a valuable approach for the treatment and reuse of residue after recovering valuable lithium metal from lithium iron phosphate, and can realize the comprehensive recycling of waste lithium iron phosphate.

[0018] As a preferred technical solution of the present invention, the sodium source includes any one or a combination of at least two of sodium carbonate, sodium acetate, sodium nitrate, sodium hydroxide, or sodium oxalate. Typical but non-limiting examples of such combinations include: a combination of sodium carbonate and sodium acetate, a combination of sodium acetate and sodium nitrate, a combination of sodium nitrate and sodium hydroxide, or a combination of sodium hydroxide and sodium oxalate.

[0019] As a preferred technical solution of the present invention, the organic carbon source includes any one or a combination of at least two of citric acid, sodium citrate, oleic acid, sodium oleate, polyvinylpyrrolidone, glucose, sucrose, dopamine hydrochloride, starch, graphene, sodium bicarbonate, or Ketjen black. Typical but non-limiting examples of the combinations include: combinations of citric acid and sodium citrate, combinations of sodium citrate and oleic acid, combinations of oleic acid and sodium oleate, combinations of sodium oleate and polyvinylpyrrolidone, combinations of polyvinylpyrrolidone and glucose, combinations of glucose and sucrose, combinations of sucrose and dopamine hydrochloride, combinations of dopamine hydrochloride and starch, combinations of starch and graphene, or combinations of sodium bicarbonate and Ketjen black, etc.

[0020] As a preferred technical solution of the present invention, the molar ratio of the organic carbon source and iron phosphate is (0.5~10):1, wherein the molar ratio can be 0.5:1, 1:1, 2:1, 3:1, 4:1, 5:1, 6:1, 7:1, 8:1, 9:1 or 10:1, etc., but is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0021] A second aspect of the present invention is to provide a method for preparing a carbon-coated sodium iron pyrophosphate cathode material as described in the first aspect, the method comprising:

[0022] The raw materials and dispersing solvent of the cathode material are ground and dried to obtain a sodium iron phosphate cathode material precursor. The sodium iron phosphate cathode material precursor is then sintered in an inert atmosphere to obtain the carbon-coated sodium iron phosphate pyrophosphate cathode material.

[0023] This invention, for the first time, utilizes commercially available iron phosphate or iron phosphate slag from lithium extraction of waste lithium iron phosphate battery cathode powder as raw materials, and employs a simple mechanical ball milling method assisted by high-temperature sintering to successfully prepare a series of sodium-rich iron phosphate pyrophosphate cathode materials. Moreover, these iron phosphate pyrophosphate cathodes with different compositions all possess highly pure phases and exhibit excellent rate performance and cycle stability.

[0024] As a preferred technical solution of the present invention, the dispersing solvent includes any one or a combination of at least two of deionized water, ethanol or acetone, wherein typical but non-limiting examples of the combination include: a combination of deionized water and ethanol, a combination of ethanol and acetone, or a combination of deionized water and acetone, etc.

[0025] Preferably, the mass ratio of the dispersant in the dispersion solution to the raw material of the positive electrode material is (3~5):1, wherein the mass ratio can be 3:1, 4:1 or 5:1, etc., but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0026] As a preferred embodiment of the present invention, the grinding includes ball milling.

[0027] Preferably, the rotational speed of the ball mill is 200~1200 r / min, wherein the rotational speed can be 200 r / min, 300 r / min, 400 r / min, 500 r / min, 600 r / min, 700 r / min, 800 r / min, 900 r / min, 1000 r / min, 1100 r / min or 1200 r / min, etc., but is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0028] Preferably, the ball milling time is 0.5 to 24 hours, wherein the time can be 0.5 hours, 1 hour, 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, 10 hours, 11 hours, 12 hours, 13 hours, 14 hours, 15 hours, 16 hours, 17 hours, 18 hours, 19 hours, 20 hours, 21 hours, 22 hours, 23 hours, or 24 hours, etc., but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0029] The grinding equipment in the ball mill of the present invention includes a ball mill and / or a sand mill, wherein the mass ratio of the ball mill to the reactants is (2~3):1, wherein the mass ratio can be 2:1, 2.1:1, 2.2:1, 2.3:1, 2.4:1, 2.5:1, 2.6:1, 2.7:1, 2.8:1, 2.9:1 or 3:1, etc., but is not limited to the listed values. Other unlisted values ​​within this range are also applicable, preferably 2.5:1.

[0030] Preferably, the drying temperature is 60~130℃, wherein the temperature can be 60℃, 65℃, 70℃, 75℃, 80℃, 85℃, 90℃, 95℃, 100℃, 105℃, 110℃, 115℃, 120℃, 125℃ or 130℃, etc., but is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0031] As a preferred technical solution of the present invention, the inert atmosphere includes any one or a combination of at least two of argon, nitrogen or neon, wherein typical but non-limiting examples of the combination include: a combination of argon and nitrogen, a combination of nitrogen and neon or a combination of neon and argon, etc.

[0032] Preferably, the sintering temperature of the sintering treatment is 400~700℃, wherein the sintering temperature can be 400℃, 450℃, 500℃, 550℃, 600℃, 650℃ or 700℃, etc., but is not limited to the listed values. Other unlisted values ​​within this range are also applicable. More preferably, it is 500~600℃.

[0033] Preferably, the sintering time of the sintering treatment is 2 to 20 hours, wherein the sintering time can be 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, 10 hours, 11 hours, 12 hours, 13 hours, 14 hours, 15 hours, 16 hours, 17 hours, 18 hours, 19 hours, or 20 hours, etc., but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0034] A third aspect of the present invention is to provide an application of the carbon-coated sodium iron pyrophosphate cathode material as described in the first aspect, wherein the carbon-coated sodium iron pyrophosphate cathode material is applied in the field of sodium-ion batteries.

[0035] Compared with the prior art, the present invention has the following beneficial effects:

[0036] (1) Since iron phosphate can be used as both a phosphorus source and an iron source, it has high atom utilization, wide availability, low price, short process flow, simple operation, and is suitable for large-scale production.

[0037] (2) The present invention proposes to prepare sodium iron phosphate pyrophosphate series cathode materials using commercial iron phosphate, and uses the leaching residue with iron phosphate as the main component after lithium recovery from lithium iron phosphate cathode powder as raw material. This provides a valuable way for the treatment and reuse of the residue after recovering valuable lithium metal from lithium iron phosphate, and can realize the comprehensive recycling of waste lithium iron phosphate.

[0038] (3) The preparation method provided by this invention yields a series of sodium-rich phosphate iron pyrophosphate cathode materials with different compositions. The assembled coin cells can be charged and discharged at 0.1C, achieving an initial discharge specific capacity of over 95% of their theoretical capacity. The specific capacity retention rate at 20C is approximately 82% (compared to 0.1C), and after 200 cycles at 1C, the capacity retention rate can reach over 95%. The sodium-rich phosphate iron pyrophosphate described in this invention possesses the advantages of low cost, high rate capability, and long cycle life. Furthermore, the provided preparation method is simple to operate, has significant improvement effects, and is easy to industrialize, making it suitable for widespread use in this field. Attached Figure Description

[0039] Figure 1 This is the XRD pattern of the sodium iron pyrophosphate cathode in Example 1 of the present invention.

[0040] Figure 2 This is a charge-discharge curve of the sodium iron pyrophosphate cathode in Example 1 of the present invention at 0.1C.

[0041] Figure 3 This is a charge-discharge curve of the sodium iron pyrophosphate cathode in Example 1 of the present invention at 20C.

[0042] Figure 4 This is a cycle performance diagram of the sodium iron pyrophosphate cathode in Example 1 of the present invention at 1C. Detailed Implementation

[0043] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention.

[0044] Example 1

[0045] This embodiment provides a carbon-coated sodium iron pyrophosphate cathode material and its preparation method:

[0046] This embodiment provides Na 3.5 Fe 2.5 (PO4) 1.5 P2O7, used as a cathode material, is prepared as follows: Sodium carbonate, ferric phosphate dihydrate (CAS: 13463-10-0), ammonium dihydrogen phosphate, and glucose are placed in a ball mill jar with a molar ratio of 1.75:2.5:1, and ethanol is added as a dispersant. The molar ratio of glucose to ferric phosphate is 2:1. The ball milling speed is 1200 r / min, and the milling time is 0.5 hours. The mixture is then dried overnight in a vacuum oven at 80 degrees Celsius. The obtained precursor is ground into powder and sintered in a tube furnace under an argon atmosphere at 550 degrees Celsius for 10 hours to obtain Na. 3.5 Fe2.5 (PO4) 1.5 P2O7 cathode material.

[0047] The XRD pattern of the sodium iron pyrophosphate cathode prepared in this embodiment is as follows: Figure 1 As shown, the charge-discharge curves of the prepared sodium iron pyrophosphate cathode at 0.1C are as follows. Figure 2 As shown, the charge-discharge curves of the prepared sodium iron pyrophosphate cathode at 20 C are as follows. Figure 3 As shown. The cycling performance of the prepared sodium iron pyrophosphate cathode at 1C is as follows. Figure 4 As shown.

[0048] Example 2

[0049] This embodiment provides a carbon-coated sodium iron pyrophosphate cathode material and its preparation method:

[0050] This embodiment provides Na 3.6 Fe 2.6 (PO4) 1.6 P2O7, used as a cathode material, is prepared as follows: Sodium carbonate, ferric phosphate dihydrate (CAS: 13463-10-0), diammonium hydrogen phosphate, and glucose in a composition ratio of 1.8:2.6:1 are placed in a ball mill jar, with acetone added as a dispersant. The molar ratio of glucose to ferric phosphate is 1.5:1. The ball milling speed is 200 r / min, and the milling time is 24 hours. The mixture is then dried overnight in a vacuum oven at 70 degrees Celsius. The obtained precursor is ground into powder and sintered in a tube furnace under an argon atmosphere at 600 degrees Celsius for 2 hours to obtain Na. 3.6 Fe 2.6 (PO4) 1.6 P2O7 cathode material.

[0051] Example 3

[0052] This embodiment provides a carbon-coated sodium iron pyrophosphate cathode material and its preparation method:

[0053] This embodiment provides Na 3.7 Fe 2.7 (PO4) 1.7P2O7, used as a cathode material, is prepared as follows: Sodium hydroxide, ferric phosphate tetrahydrate (CAS: 31096-47-6), phosphoric acid, and glucose, in a composition ratio of 3.7:2.7:1, are placed in a ball mill jar, with an appropriate amount of ethanol added as a dispersant. The molar ratio of glucose to ferric phosphate is 1:1. The ball milling speed is 1000 r / min, and the milling time is 2 hours. The mixture is then dried overnight in a vacuum oven at 90 degrees Celsius. The obtained precursor is ground into powder and sintered in a tube furnace under an argon atmosphere at 620 degrees Celsius for 8 hours to obtain Na. 3.7 Fe 2.7 (PO4) 1.7 P2O7 cathode material.

[0054] Example 4

[0055] This embodiment provides a carbon-coated sodium iron pyrophosphate cathode material and its preparation method:

[0056] This embodiment provides Na 3.8 Fe 2.8 (PO4) 1.8 P2O7, used as a cathode material, is prepared as follows: Sodium acetate, anhydrous ferric phosphate (CAS: 10045-86-0), ammonium phosphate, and sucrose in a molar ratio of 3.8:2.8:1.8 are placed in a ball mill jar, with an appropriate amount of deionized water added as a dispersant. The molar ratio of sucrose to ferric phosphate is 1:1. The ball milling speed is 350 r / min, and the milling time is 20 hours. The mixture is then dried overnight in a vacuum oven at 120 degrees Celsius. The obtained precursor is ground into powder and sintered in an argon-atmospheric tube furnace at 550 degrees Celsius for 5 hours to obtain Na. 3.8 Fe 2.8 (PO4) 1.8 P2O7 cathode material.

[0057] Example 5

[0058] This embodiment provides a carbon-coated sodium iron pyrophosphate cathode material and its preparation method:

[0059] This embodiment provides Na 3.2 Fe 2.2 (PO4) 1.2P2O7, used as a cathode material, is prepared as follows: Sodium hydroxide, ferric phosphate dihydrate (CAS: 13463-10-0), phosphoric acid, and glucose in a molar ratio of 3.2:2.2:1.2 are placed in a ball mill jar, with an appropriate amount of ethanol added as a dispersant. The molar ratio of glucose to ferric phosphate is 1:1. The ball milling speed is 1000 r / min, and the milling time is 2 hours. The mixture is then dried overnight in a vacuum oven at 90 degrees Celsius. The obtained precursor is ground into powder and sintered in a tube furnace under an argon atmosphere at 600 degrees Celsius for 8 hours to obtain Na. 3.2 Fe 2.2 (PO4) 1.2 P2O7 cathode material.

[0060] Example 6

[0061] This embodiment provides a carbon-coated sodium iron pyrophosphate cathode material and its preparation method:

[0062] In this embodiment, Na is provided. 3.5 Fe 2.5 (PO4) 1.5 P2O7 is used as a cathode material. The preparation method involves using the leaching residue from the cathode powder of spent lithium iron phosphate batteries after selective lithium recovery as a raw material to prepare Na. 3.5 Fe 2.5 (PO4) 1.5 P2O7, the residue contains small amounts of residual Li, Al, carbon, conductive adhesives, and other organic matter. Based on the Fe and P content in the ferric phosphate leaching residue, and according to the molar ratio of Na:Fe:P of 3.5:2.5:3.5, a certain amount of sodium carbonate, diammonium hydrogen phosphate, and glucose are added. The mixture is placed in a ball mill jar, with ethanol added as a dispersant. The molar ratio of glucose to ferric phosphate is 2:1. The ball milling speed is 500 r / min, and the milling time is 12 hours. The mixture is then dried overnight in a vacuum oven at 90 degrees Celsius. The obtained precursor is ground into powder and sintered in an argon-atmospheric tube furnace at 55 degrees Celsius for 8 hours to obtain Na. 3.5 Fe 2.5 (PO4) 1.5 P2O7 cathode material.

[0063] Example 7

[0064] In this embodiment, the only difference is that the molar ratio of glucose to ferric phosphate is changed from 2:1 to 0.5:1. All other conditions are the same as in Example 1.

[0065] Example 8

[0066] In this embodiment, the only difference is that the molar ratio of glucose to ferric phosphate is changed from 2:1 to 10:1. All other conditions are the same as in Example 1.

[0067] Comparative Example 1

[0068] This comparative example uses Na4Fe3(PO4)2P2O7 as the cathode material, and the preparation method is as follows:

[0069] Sodium carbonate, ferric phosphate dihydrate (CAS: 13463-10-0), ammonium dihydrogen phosphate, and glucose in a molar ratio of 2:3:1 were placed in a ball mill jar, with ethanol added as a dispersant. The molar ratio of glucose to ferric phosphate was 2:1. The ball milling speed was 400 r / min, and the milling time was 15 hours. The mixture was then dried overnight in a vacuum oven at 90 degrees Celsius. The obtained precursor was ground into powder and sintered in a tube furnace under an argon atmosphere at 550 degrees Celsius for 10 hours to obtain the Na4Fe3(PO4)2P2O7 cathode material.

[0070] Comparative Example 2:

[0071] This comparative example provides Na3Fe2PO4(P2O7) as the positive electrode material, and the preparation method is as follows:

[0072] Sodium carbonate, ferric phosphate dihydrate (CAS: 13463-10-0), ammonium dihydrogen phosphate, and glucose in a molar ratio of 1.5:2:1 were placed in a ball mill jar, with ethanol added as a dispersant. The molar ratio of glucose to ferric phosphate was 1:1. The ball milling speed was 500 r / min, and the milling time was 12 hours. The mixture was then dried overnight in a vacuum oven at 80 degrees Celsius. The obtained precursor was ground into powder and sintered in a tube furnace under an argon atmosphere at 580 degrees Celsius for 8 hours to obtain the Na3Fe2PO4(P2O7) cathode material.

[0073] The carbon-coated sodium iron pyrophosphate cathode materials prepared in Examples 1-8 and Comparative Examples 1-2 were assembled into batteries. The assembly process included the following steps:

[0074] (1) Preparation of the positive electrode sheet: The prepared sodium iron pyrophosphate positive electrode material, Ketjen black, and polytetrafluoroethylene binder were ground and mixed evenly at a mass ratio of 7:2:1, and then rolled evenly with a roller mill to form a film of uniform thickness. After drying in a vacuum drying oven at 120℃ for 5 hours, the obtained positive electrode film was cut into square electrode sheets with a side length of about 6mm. After accurately weighing the material, the mass of active material in the positive electrode sheet was calculated according to the formula composition.

[0075] (2) Battery assembly:

[0076] The square positive electrode sheet, 16 mm diameter separator, 15 mm diameter sodium sheet, spring sheet and gasket obtained above are assembled into a testable 2032 button cell in a glove box (oxygen content less than 0.01 ppm, water content less than 0.01 ppm).

[0077] The batteries assembled in Examples 1-8 and Comparative Examples 1-2 were tested using the Wuhan Landian High-Performance Battery Testing System at various rates. The first-cycle discharge specific capacity at 0.1C, the first-cycle discharge specific capacity at 20C, the first-cycle discharge specific capacity at 1C, and the capacity retention rate after 200 cycles at 1C were tested. The test results are shown in Table 1.

[0078] Table 1

[0079]

[0080] Comparing Examples 1 to 6 reveals that the series of sodium iron pyrophosphates prepared using iron phosphate exhibit good electrochemical performance. Their initial discharge specific capacity reaches over 95% of their theoretical capacity, and the specific capacity retention rate at 20 C is approximately 82% (compared to 0.1 C). After 200 cycles at 1 C, the capacity retention rate reaches over 95%. Comparing Examples 1 and 7 shows that a small amount of carbon coating results in poorer electrochemical performance due to lower electronic conductivity. Comparing Examples 1 and 8 shows that excessive carbon coating increases the interfacial impedance of the cathode material and leads to more side reactions, which is also detrimental to the material's electrochemical performance. Examples 1, 1, and 2 demonstrate that higher sodium content leads to higher capacity, but also worsens capacity utilization and reduces cycle stability. Lower sodium content, while resulting in lower capacity, brings the actual capacity closer to the theoretical capacity, indicating better capacity utilization and cycle stability. Therefore, a moderate sodium content can simultaneously achieve good overall material performance.

[0081] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A carbon-coated sodium iron pyrophosphate cathode material, characterized in that, The molecular formula of the sodium iron pyrophosphate phosphate is Na a Fe b (PO4) c (P2O7), where 3 < a < 4; a = c + 2; b = c + 1; 1 < c < 2; The raw materials of the positive electrode material include iron phosphate, an additional phosphorus source, a sodium source and an organic carbon source; The iron phosphate includes any one or a combination of at least two of the following: anhydrous iron phosphate, iron phosphate dihydrate, iron phosphate tetrahydrate, or iron phosphate residue after lithium extraction from lithium iron phosphate battery cathode waste powder. The molar ratio of the organic carbon source to iron phosphate is (0.5~10):

1.

2. The cathode material according to claim 1, characterized in that, The added phosphorus source includes any one or a combination of at least two of the following: ammonium dihydrogen phosphate, sodium dihydrogen phosphate, ammonium phosphate, sodium ammonium phosphate, diammonium hydrogen phosphate, sodium diammonium hydrogen phosphate, or phosphoric acid.

3. The cathode material according to claim 1, characterized in that, The sodium source includes any one or a combination of at least two of sodium carbonate, sodium acetate, sodium nitrate, sodium hydroxide, or sodium oxalate.

4. The cathode material according to claim 2, characterized in that, The organic carbon source includes any one or a combination of at least two of the following: citric acid, sodium citrate, oleic acid, sodium oleate, polyvinylpyrrolidone, glucose, sucrose, dopamine hydrochloride, or starch.

5. A method for preparing the carbon-coated sodium iron pyrophosphate cathode material as described in claim 1, characterized in that, The preparation method includes: The raw materials and dispersing solvent of the cathode material are ground and dried to obtain a sodium iron phosphate cathode material precursor. The sodium iron phosphate cathode material precursor is then sintered in an inert atmosphere to obtain the carbon-coated sodium iron phosphate pyrophosphate cathode material.

6. The preparation method according to claim 5, characterized in that, The dispersion solvent includes any one or a combination of at least two of deionized water, ethanol, or acetone.

7. The preparation method according to claim 5, characterized in that, The grinding includes ball milling.

8. The preparation method according to claim 7, characterized in that, The ball mill rotates at a speed of 200~1200 r / min.

9. The preparation method according to claim 7, characterized in that, The ball milling time is 0.5 to 24 hours.

10. The preparation method according to claim 5, characterized in that, The drying process is carried out at a temperature of 60~130℃.

11. The preparation method according to claim 5, characterized in that, The inert atmosphere includes any one or a combination of at least two of argon, nitrogen, or neon.

12. The preparation method according to claim 5, characterized in that, The sintering temperature of the sintering process is 400~700℃.

13. The preparation method according to claim 12, characterized in that, The sintering temperature of the sintering process is 500~600℃.

14. The preparation method according to claim 5, characterized in that, The sintering time for the sintering treatment is 2 to 20 hours.

15. An application of the carbon-coated sodium iron pyrophosphate cathode material as described in any one of claims 1-4, characterized in that, The carbon-coated sodium iron pyrophosphate cathode material is used in the field of sodium-ion batteries.

Images