Polyanionic positive electrode material as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of battery positive electrode materials, and relates to a polyanionic positive electrode material as well as a preparation method and application thereof. Zirconium phosphate (B) is used as an outer coating material, the ionic conductivity is mainly improved, and the requirement for environment humidity in the manufacturing process is lowered due to the fact that the zirconium phosphate (B) is high in chemical stability and not prone to reacting with water or a solvent. More importantly, due to the existence of the B layer, the stability of the material main body in electrolyte is enhanced, the material degradation phenomenon possibly occurring in the long-term use process is reduced, and the service life of the battery is further prolonged.

Description

Technical Field

[0001] The present invention belongs to the technical field of battery positive electrode materials, and relates to a polyanion positive electrode material and a preparation method and application thereof. Background Art

[0002] Lithium-ion batteries have been widely used in portable electronic devices such as smartphones, laptops, and even electric vehicles due to their advantages such as high energy density and long cycle life. However, with the popularization of technology and the expansion of the market, people have gradually realized the importance and potential risks of cobalt, a key material in lithium-ion batteries. As an indispensable element for stabilizing battery structure and improving battery performance, cobalt has extremely limited global reserves, and most of it is concentrated in a country with a complex political and economic environment such as Congo. This uneven distribution of resources not only leads to price fluctuations, but also brings the risk of supply chain disruptions. To meet this challenge, researchers have turned their attention to sodium-ion batteries, a technical path that is similar in principle to lithium-ion batteries but uses sodium as a charge carrier.

[0003] In recent years, sodium ferric phosphate (NFPP), a polyanion cathode material in sodium-ion batteries, has attracted widespread attention due to its unique properties. First, from an environmental perspective, the preparation process of NFPP materials has little impact on the environment, which is in line with modern society's pursuit of green and sustainable development. Secondly, due to the abundance and wide distribution of sodium resources, the cost of sodium-ion-based batteries is much lower than that of cobalt-dependent lithium-ion batteries, which is crucial for promoting the large-scale application of new energy technologies. In addition, NFPP materials also exhibit excellent safety performance, which is due to the low operating voltage of the sodium-ion battery itself and the good thermal stability of the material, effectively reducing the risk of fire or explosion caused by overcharging, short circuit and other reasons.

[0004] Despite this, NFPP still faces several challenges as a cathode material for sodium-ion batteries, the most prominent of which is poor electrical conductivity. Low conductivity limits the battery's rapid charge and discharge capabilities, affecting efficiency and user experience in practical applications.

[0005] A Chinese patent document (CN117936755A) discloses a high-energy-density polyanion cathode material, its preparation method, and a sodium-ion battery cathode plate. By replacing part of the Fe element with the Mn element in the preparation of the NFPP cathode material, the battery capacity and energy density can be effectively improved. At the same time, doping a small amount of transition metal elements on this basis can improve the cycle stability performance, but there is no obvious improvement on the conductivity. Summary of the Invention

[0006] The purpose of the present invention is to address the above-mentioned problems existing in the prior art and propose a polyanion positive electrode material, which improves the electrical conductivity of the polyanion positive electrode material by doping and coating, thereby improving the electrochemical performance of the battery.

[0007] The purpose of the present invention can be achieved through the following technical solutions:

[0008] A polyanion positive electrode material, the structural formula of the positive electrode material is: Na4Fe 3-x (PO4)2P2O7A x / C y +B z , where 0.001≤x≤0.1, 0.5≤y≤5, 0.3≤z≤3;

[0009] Element A is at least one of Cu, Mg and Sn;

[0010] C is the carbon coating layer covering the outer layer of the positive electrode material;

[0011] B is a zirconium phosphate coating layer coated with an outer layer of a carbon coating layer.

[0012] The present invention improves the conductivity of the positive electrode material of a sodium ion battery by doping a small amount of element A and double coating of zirconium phosphate and carbon. This method can improve the conductivity of the material without significantly changing the basic structure of the material. Specifically, by doping element A, whose ion radius is slightly smaller than that of iron ions, lattice defects are generated in some areas of the material, providing more migration channels for sodium ions, and the carbon coating has good electronic conductivity, and the B coating has good ionic conductivity. The two work synergistically to help the embedding and de-embedding of sodium ions. In addition, the chemical properties of B are stable, insoluble in water and solvents, and resistant to acids and alkalis, making it difficult for the material to react and deteriorate with moisture in the air, and the moisture (humidity) requirements of the manufacturing environment are also lower. In addition, the B coating also makes the main body of the material more stable in the electrolyte. In this way, the conductivity and stability of the polyanion positive electrode material are improved, so that the performance of the battery is improved.

[0013] In the above-mentioned polyanionic cathode material, the thickness of the carbon coating layer is 0.6-2.6 nm.

[0014] In the above-mentioned polyanionic positive electrode material, the thickness of the zirconium phosphate coating layer is 8-15 nm.

[0015] The present invention uses a carbon layer as the inner layer and a B layer as the outer layer, with the carbon layer thickness controlled to be between 0.6-2.6nm and the B layer thickness controlled to be between 8-15nm. The carbon layer is mainly used to improve electronic conductivity; too thick a layer will prolong the sodium ion migration path. If the B layer is too thin, it will not effectively isolate the direct contact between the positive electrode material and the electrolyte, and the main material may undergo adverse side reactions with the electrolyte, thereby affecting the service life of the battery. However, if the coating layer is too thick, it will prolong the sodium ion migration path, affecting the sodium ion migration efficiency and reducing the overall energy efficiency of the battery.

[0016] The present invention also provides a method for preparing the above-mentioned polyanionic cathode material, the method comprising the following steps:

[0017] S1. Weigh a sodium source, an A source, an iron source, a phosphorus source, and a carbon source according to the molar ratio in the structural formula, mix them uniformly, sinter them in an inert gas atmosphere, and then crush them to obtain a powder;

[0018] S2. Weigh a zirconium source and a phosphorus source according to the molar ratio in the structural formula, dissolve the zirconium source in hydrofluoric acid, add the phosphorus source, and stir thoroughly to obtain a zirconium phosphate precipitate. Wash the precipitate, dry it, and crush it to obtain B;

[0019] S3. Mix the powder and B, sinter them in an inert gas atmosphere under a heat preservation condition, and obtain a polyanion positive electrode material after cooling.

[0020] In the above-mentioned method for preparing a polyanionic positive electrode material, the source A is one of copper sulfate, ferric chloride, copper nitrate, magnesium chloride, magnesium nitrate, sodium stannate, copper methanesulfonate and stannous chloride.

[0021] Preferably, the sodium source includes at least one of sodium carbonate, sodium bicarbonate, disodium hydrogen phosphate, sodium phosphate and sodium oxalate.

[0022] Preferably, the phosphorus source is at least one of diammonium phosphate, sodium tripolyphosphate and sodium pyrophosphate.

[0023] Preferably, the iron source is at least one of ferric sulfate and ferrous sulfate.

[0024] In the above-mentioned method for preparing a polyanionic cathode material, the carbon source is at least one of sucrose, citric acid, glycine, hard carbon and soft carbon.

[0025] Preferably, the zirconium source includes hydroxides and salts containing the element zirconium.

[0026] More preferably, the zirconium source is zirconium hydroxide.

[0027] In the aforementioned method for preparing a polyanionic cathode material, the sintering temperature in step S1 is 300-700°C for 16-20 hours. By controlling the sintering temperature, the present invention ensures that the material is fully sintered without generating impurities. The appropriate sintering temperature and time ensure the purity and microstructure of the cathode material, thereby ensuring the electrochemical performance of the battery.

[0028] If the sintering temperature is too low, impurities cannot be fully removed, and impurities may also be produced, affecting the purity of the material and thus affecting the battery performance. For example, impurities may reduce the conductivity of the material and affect the embedding and deintercalation of sodium ions, thereby affecting the capacity and life of the battery. If the sintering temperature is too high, such as exceeding 700°C, the microstructure of the material may be affected, such as excessive growth and agglomeration of material particles, affecting the specific surface area of the material, thereby affecting the processing performance, and may also affect the rate performance and impedance of the battery. In addition, high temperature may also change the crystal structure of the material, affecting the material performance.

[0029] Preferably, the inert gas atmosphere is a nitrogen atmosphere.

[0030] In the above-mentioned method for preparing a polyanionic positive electrode material, the sintering temperature in step S3 is 450-650° C. and the sintering time is 12-14 hours.

[0031] The present invention also provides a sodium ion battery positive electrode sheet, which includes the above-mentioned positive electrode material.

[0032] Compared with the prior art, the present invention has the following beneficial effects:

[0033] 1. The A element selected in the present invention is based on its ionic radius being slightly smaller than that of iron ions, which helps to introduce an appropriate amount of lattice defects into the material structure, increase the sodium ion migration channel, and thus improve the sodium ion transmission efficiency. In addition, the carbon coating provides a good electron conduction path and can also protect the internal material from the erosion of the electrolyte, avoiding unnecessary side reactions. Zirconium phosphate (B) as the outer coating mainly contributes to improving the ionic conductivity, and due to its high chemical stability, it is not easy to react with water or solvents, which reduces the requirements of the manufacturing process for environmental humidity. More importantly, the presence of the B layer enhances the stability of the main body of the material in the electrolyte, reduces the material degradation phenomenon that may occur during long-term use, and thus extends the service life of the battery.

[0034] 2. The present invention specifically points out that the thickness of the inner carbon coating should be controlled between 0.6-2.6nm to ensure that the sodium ion migration path is not excessively extended and the electronic conductivity is effectively improved; and the thickness of the outer layer B coating is recommended to be kept in the range of 8-15nm. This can not only effectively isolate the direct contact between the positive electrode material and the electrolyte, prevent the occurrence of adverse side reactions, but also avoid the problem of decreased sodium ion migration efficiency due to excessive thickness.

[0035] 3. The present invention describes in detail the entire process from raw material preparation to the final product, including the precise weighing of each component raw material, the initial sintering after uniform mixing to obtain a powdered precursor, the subsequent preparation of zirconium phosphate through a specific process and its mixing with the precursor powder, the subsequent sintering again under an inert gas atmosphere, and the final product after steps such as cleaning and drying. This ensures that the material is fully sintered without generating impurities or changing the material's microstructure, thereby ensuring the high quality and excellent electrochemical properties of the final product.

[0036] 4. The sodium ion battery cathode material provided by the present invention exhibits excellent electrical conductivity and cycle stability, solves the problem of poor electrical conductivity of traditional polyanion cathode materials, significantly improves the overall performance of sodium ion batteries, and has broad application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 The figures are the cycle performance of sodium ion batteries prepared with the positive electrode materials of Example 1 and Comparative Example 1, respectively, at 0.5C constant current and constant voltage charging and 1C discharge in the voltage range of 1.5-3.5V.

[0038] Figure 2 The constant current rate discharge performance of sodium ion batteries prepared with the positive electrode materials of Example 1 and Comparative Example 1 are shown. DETAILED DESCRIPTION

[0039] The following are specific embodiments of the present invention, which further describe the technical solutions of the present invention, but the present invention is not limited to these embodiments.

[0040] Example 1:

[0041] This embodiment Na4Fe 2.92 (PO4)2P2O7A 0.08 / C+B preparation method is as follows:

[0042] S1. Weigh a sodium source, an A source, an iron source, a phosphorus source, and a carbon source according to the molar ratio in the structural formula, mix them uniformly, sinter them at 500° C. in an inert gas atmosphere for 18 h, and then crush them to obtain a powder;

[0043] The sodium source is sodium carbonate;

[0044] Source A is copper sulfate;

[0045] The iron source is ferrous sulfate;

[0046] The phosphorus source is sodium pyrophosphate;

[0047] The carbon source was citric acid;

[0048] S2. Weigh a zirconium source and a phosphorus source in a molar ratio of 1:1 according to the structural formula, dissolve the zirconium source in hydrofluoric acid, add the phosphorus source, and stir thoroughly to obtain a zirconium phosphate precipitate. Wash the precipitate repeatedly with deionized water, dry it, and crush it to obtain B;

[0049] The zirconium source is zirconium hydroxide;

[0050] The phosphorus source is phosphoric acid;

[0051] S3. The powder obtained in S1 and B are sintered at 600° C. for 12 h. The sintered product after natural cooling is washed, dried and crushed to obtain a positive electrode material for a sodium ion battery.

[0052] Example 2:

[0053] This embodiment Na4Fe 2.90 (PO4)2P2O7A 0.1 / C+B preparation method is as follows:

[0054] S1. Weigh a sodium source, an A source, an iron source, a phosphorus source, and a carbon source according to the molar ratio in the structural formula, mix them uniformly, sinter them at 500° C. in an inert gas atmosphere for 18 h, and then crush them to obtain a powder;

[0055] The sodium source is sodium carbonate;

[0056] Source A is copper sulfate;

[0057] The iron source is ferrous sulfate;

[0058] The phosphorus source is sodium pyrophosphate;

[0059] The carbon source was citric acid;

[0060] S2. Weigh a zirconium source and a phosphorus source in a molar ratio of 1:1 according to the structural formula, dissolve the zirconium source in hydrofluoric acid, add the phosphorus source, and stir thoroughly to obtain a zirconium phosphate precipitate. Wash the precipitate repeatedly with deionized water, dry it, and crush it to obtain B;

[0061] The zirconium source is zirconium hydroxide;

[0062] The phosphorus source is phosphoric acid;

[0063] S3. The powder obtained in S1 and B are sintered at 600° C. for 12 h. The sintered product after natural cooling is washed, dried and crushed to obtain a positive electrode material for a sodium ion battery.

[0064] Example 3:

[0065] This embodiment Na4Fe 2.999 (PO4)2P2O7A 0.001 / C+B preparation method is as follows:

[0066] S1. Weigh a sodium source, an A source, an iron source, a phosphorus source, and a carbon source according to the molar ratio in the structural formula, mix them uniformly, sinter them at 500° C. in an inert gas atmosphere for 18 h, and then crush them to obtain a powder;

[0067] The sodium source is sodium carbonate;

[0068] Source A is copper sulfate;

[0069] The iron source is ferrous sulfate;

[0070] The phosphorus source is sodium pyrophosphate;

[0071] The carbon source was citric acid;

[0072] S2. Weigh a zirconium source and a phosphorus source in a molar ratio of 1:1 according to the structural formula, dissolve the zirconium source in hydrofluoric acid, add the phosphorus source, and stir thoroughly to obtain a zirconium phosphate precipitate. Wash the precipitate repeatedly with deionized water, dry it, and crush it to obtain B;

[0073] The zirconium source is zirconium hydroxide;

[0074] The phosphorus source is phosphoric acid;

[0075] S3. The powder obtained in S1 and B are sintered at 600° C. for 12 h. The sintered product after natural cooling is washed, dried and crushed to obtain a positive electrode material for a sodium ion battery.

[0076] Example 4:

[0077] The only difference from Example 1 is that no sintering process is performed in step S1.

[0078] Example 5:

[0079] The only difference from Example 1 is that the sintering temperature in step S1 is 200°C.

[0080] Example 6:

[0081] The only difference from Example 1 is that the sintering temperature in step S1 is 850°C.

[0082] Example 7:

[0083] The only difference from Example 1 is that the sintering temperature in step S3 is 300°C.

[0084] Example 8:

[0085] The only difference from Example 1 is that the sintering temperature in step S3 is 800°C.

[0086] Comparative Example 1:

[0087] The only difference from Example 1 is that the positive electrode material is Na4Fe3(PO4)2P2O7 / C.

[0088] Comparative Example 2:

[0089] The only difference from Example 1 is that the coating layer is only a zirconium phosphate coating layer, that is, no carbon source is added in step S1.

[0090] A sodium ion soft pack battery is prepared according to the following steps:

[0091] 1. Preparation of positive electrode sheet: Under a certain temperature and humidity environment, the positive electrode material prepared in accordance with Examples 1-8 and Comparative Examples 1-2: super-P: PVDF = 95:3:2 is homogenized, and the material is discharged when the viscosity and fineness meet the requirements. The positive electrode sheet is obtained after coating, rolling, and die-cutting.

[0092] 2. Negative electrode sheet production: Homogenize the mixture according to the ratio of hard carbon: super-P: CMC: SBR = 95:1:2:2, discharge the material when the viscosity and fineness meet the requirements, and obtain the negative electrode sheet after coating, rolling, and die-cutting. The CB value is 1.25.

[0093] 3. Diaphragm: ceramic diaphragm.

[0094] 4. Battery cell stacking: The positive electrode, diaphragm and negative electrode are stacked to form a bare battery cell, which is then sealed with aluminum-plastic film and baked in an oven at 95°C for 3 days. The moisture content of the battery cell is tested. After the moisture content passes the test, liquid injection, formation, volume separation and secondary sealing are carried out.

[0095] The assembled sodium ion battery was subjected to room temperature cycle test in the voltage range of 1.5-3.5V.

[0096] Figure 1 The 1.5-3.5V room temperature cycling curves for Example 1 and Comparative Example 1 were tested at 25±2°C under 0.5C constant current constant voltage charging, 0.05C charge cutoff current, and 1C constant current discharge. After 200 cycles, the capacity retention rate of Example 1 was 96.1%, while that of Comparative Example 1 was 95%.

[0097] Figure 2The constant current rate discharge performance of Example 1 and Comparative Example 1 is shown. The test temperature was 25±2°C, and the test conditions were 0.5C constant current constant voltage charging, with a charge cutoff current of 0.05C, and constant current discharge at the corresponding current. When calculating the discharge capacity retention rate, the 0.5C discharge capacity was used as the calculation basis. As can be seen from the figure, Example 1 can effectively improve the battery's rate discharge performance compared to Comparative Example 1.

[0098] Table 1: Performance test results of the positive electrode materials prepared in Examples 1-8 and Comparative Examples 1-2 after being used in sodium ion soft pack batteries

[0099]

[0100] In summary, the selection of element A in the present invention is based on the fact that its ionic radius is slightly smaller than that of iron ions, which helps to introduce an appropriate amount of lattice defects into the material structure, increase the sodium ion migration channel, and thus improve the sodium ion transmission efficiency. In addition, the carbon coating layer can not only provide a good electron conduction path, but also protect the internal material from the erosion of the electrolyte, avoiding unnecessary side reactions. Zirconium phosphate (B) as the outer coating mainly contributes to improving the ionic conductivity, and due to its high chemical stability, it is not easy to react with water or solvents, which reduces the requirements of the manufacturing process for environmental humidity. More importantly, the presence of the B layer enhances the stability of the material body in the electrolyte, reduces the material degradation phenomenon that may occur during long-term use, and thus extends the service life of the battery.

[0101] The parts of the embodiment herein that are not exhaustive of the midpoint values of the technical scope claimed for protection by the present invention and the new technical solutions formed by equivalent replacement of single or multiple technical features in the technical solutions of the embodiments are also within the scope claimed for protection by the present invention; at the same time, in all the embodiments listed or not listed in the solutions of the present invention, each parameter in the same embodiment merely represents an example of its technical solution (i.e., a feasible solution), and there is no strict coordination and limitation relationship between the parameters, wherein the parameters can be replaced with each other without violating the axioms and the claims of the present invention, unless otherwise stated.

[0102] The technical means disclosed in the solutions of the present invention are not limited to the technical means disclosed in the above technical means, but also include technical solutions composed of any combination of the above technical features. The above is a specific embodiment of the present invention. It should be noted that for those skilled in the art, various improvements and modifications can be made without departing from the principles of the present invention, and such improvements and modifications are also considered to be within the scope of protection of the present invention.

[0103] The specific embodiments described herein are merely illustrative of the spirit of the present invention. Persons skilled in the art may make various modifications, additions, or substitutions to the described specific embodiments without departing from the spirit of the present invention or exceeding the scope of the appended claims.

Claims

1. A polyanionic cathode material, characterized in that: The structural formula of the positive electrode material is: Na4Fe 3-x (PO4)2P2O7A x / C y +B z , where 0.001≤x≤0.1, 0.5≤y≤5, 0.3≤z≤3; Element A is at least one of Cu, Mg and Sn; C is the carbon coating layer covering the outer layer of the positive electrode material; B is a zirconium phosphate coating layer coated with an outer layer of a carbon coating layer.

2. A polyanionic cathode material according to claim 1, characterized in that: The thickness of the carbon coating layer is 0.6-2.6 nm.

3. A polyanionic cathode material according to claim 1, characterized in that: The thickness of the zirconium phosphate coating layer is 8-15 nm.

4. A method for preparing the polyanionic cathode material according to claim 1, characterized in that: The method comprises the following steps: S1. Weigh a sodium source, an A source, an iron source, a phosphorus source, and a carbon source according to the molar ratio in the structural formula, mix them uniformly, sinter them in an inert gas atmosphere, and then crush them to obtain a powder; S2. Weigh a zirconium source and a phosphorus source according to the molar ratio in the structural formula, dissolve the zirconium source in hydrofluoric acid, add the phosphorus source, and stir thoroughly to obtain a zirconium phosphate precipitate. Wash the precipitate, dry it, and crush it to obtain B; S3. Mix the powder and B, sinter them in an inert gas atmosphere under a heat preservation condition, and obtain a polyanion positive electrode material after cooling.

5. The method for preparing a polyanionic cathode material according to claim 4, characterized in that: Source A is one of copper sulfate, ferric chloride, copper nitrate, magnesium chloride, magnesium nitrate, sodium stannate, copper methanesulfonate and stannous chloride.

6. The method for preparing a polyanionic cathode material according to claim 4, characterized in that: The carbon source is at least one of sucrose, citric acid, glycine, hard carbon and soft carbon.

7. The method for preparing a polyanionic cathode material according to claim 4, characterized in that: The zirconium source is zirconium hydroxide.

8. The method for preparing a polyanionic cathode material according to claim 4, characterized in that: The sintering temperature in step S1 is 300-700° C. and the sintering time is 16-20 hours.

9. The method for preparing a polyanionic cathode material according to claim 4, characterized in that: In step S3, the sintering temperature is kept at 450-650° C. and the sintering time is 12-14 hours.

10. A sodium ion battery, characterized in that: The sodium ion battery comprises the positive electrode material according to claims 1 to 3 or the positive electrode material prepared by the preparation method according to claim 4.

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