Sodium ferric pyrophosphate phosphate positive electrode material, preparation method thereof, positive electrode and sodium ion battery

Abstract

The application relates to a sodium iron pyrophosphite phosphate positive electrode material and a preparation method, a positive electrode and a sodium ion battery thereof. The preparation method of the sodium iron pyrophosphite phosphate positive electrode material comprises the following steps: based on the stoichiometric ratio of Na4Fe3(PO4)2P2O7, dispersing an iron source, a sodium source and a phosphorus source in water, adding a carbon source to form a solid-liquid mixed slurry; performing sand milling treatment on the solid-liquid mixed slurry, and gradually adding an etching agent solution in the sand milling process to obtain a suspension slurry, the etching agent solution comprising at least one of a citric acid solution, an oxalic acid solution, an ascorbic acid solution and a malic acid solution; performing spray drying on the suspension slurry to obtain a precursor powder; and performing sintering treatment on the precursor powder in an inert atmosphere to obtain the sodium iron pyrophosphite phosphate positive electrode material. The preparation method of the sodium iron pyrophosphite phosphate positive electrode material effectively reduces the generation of impurities by dynamically and gradually adding a weak acid etching agent in the sand milling process.

Description

Technical Field

[0001] This invention relates to the field of cathode materials for ion batteries, specifically to a sodium iron pyrophosphate cathode material and its preparation method, as well as a cathode containing the material and a sodium-ion battery. Background Technology

[0002] Sodium-ion batteries are considered an ideal choice for energy storage materials due to their similar working principle to lithium batteries and the abundance and low cost of sodium resources. Existing research systems mainly include transition metal oxides, Prussian blue analogues, and polyanionic compounds. Polyanionic compounds possess ultra-long cycle durability and excellent rate performance, showing significant advantages in energy storage applications. In 2012, researchers first proposed a polyanionic compound with potential application value—the composite phosphate Na4Fe3(PO4)2P2O7 (sodium iron pyrophosphate, NFPP). This material exhibits a high theoretical specific capacity (1C = 129 mAh g⁻¹). -1 ).

[0003] From a crystallographic perspective, Na4Fe3(PO4)2P2O7 possesses a P2O7- group. 4- and PO4 3- The open polyanionic framework composed of functional groups provides it with a three-dimensional ion transport channel and lowers the activation barrier for sodium ion migration. However, the actual specific capacity of Na4Fe3(PO4)2P2O7 (approximately 100 mAh g⁻¹) is relatively low. -1 The specific capacity of Na4Fe3(PO4)2P2O7 cathodes is often lower than their theoretical specific capacity, resulting in relatively low energy density at the battery level. The capacity limitation of Na4Fe3(PO4)2P2O7 cathodes is mainly due to the formation of inactive sodium iron phosphate (NaFePO4) of the sodium phosphate ore type. Sand milling-spraying is a commonly used method for the industrial synthesis of Na4Fe3(PO4)2P2O7. However, this method suffers from the problem of easily forming sodium iron phosphate NaFePO4 impurity phases.

[0004] Therefore, developing a synthesis method that can effectively suppress the formation of NaFePO4 impurity phase and improve the electrochemical performance of NFPP materials is of great significance for promoting the development of high-performance sodium-ion batteries. Summary of the Invention

[0005] Therefore, it is necessary to address the issue of how to suppress the formation of impurity phases in sodium iron pyrophosphate and improve electrochemical performance by providing a sodium iron pyrophosphate cathode material, its preparation method, cathode, and sodium-ion battery.

[0006] In a first aspect, the present invention provides a method for preparing a sodium iron pyrophosphate cathode material, comprising the following steps:

[0007] Based on the stoichiometric ratio of Na4Fe3(PO4)2P2O7, iron, sodium, and phosphorus sources are dispersed in water, and a carbon source is added to form a solid-liquid mixture slurry.

[0008] The solid-liquid mixture is subjected to sand milling, and an etchant solution is gradually added during the sand milling process to obtain a suspension slurry. The etchant solution includes at least one of citric acid solution, oxalic acid solution, ascorbic acid solution and malic acid solution.

[0009] The suspension slurry was spray-dried to obtain precursor powder; and

[0010] The precursor powder was sintered under an inert atmosphere to obtain sodium iron pyrophosphate cathode material.

[0011] In one embodiment, the etchant solution is added gradually, either continuously or intermittently in several drops.

[0012] In one embodiment, the etchant solution is added gradually in a continuous dripping manner, and the dripping rate of the etchant solution is 0.3 g / min to 0.8 g / min.

[0013] In one embodiment, the concentration of the etchant solution is 0.05 g / mL to 0.1 g / mL.

[0014] In one embodiment, the mass ratio of the solute in the etchant solution to the iron source is (1~5):100.

[0015] In one embodiment, the sanding process takes 3 to 5 hours.

[0016] In one embodiment, the sintering temperature of the sintering process is 480°C to 550°C.

[0017] Secondly, the present invention provides a sodium iron pyrophosphate cathode material, which is prepared by any of the above-described preparation methods.

[0018] Thirdly, the present invention provides a positive electrode comprising the above-mentioned sodium iron pyrophosphate positive electrode material.

[0019] Fourthly, the present invention provides a sodium-ion battery, including the above-described positive electrode.

[0020] This invention has at least the following technical effects:

[0021] 1) The preparation method of the sodium iron pyrophosphate cathode material of the present invention involves dynamically and gradually adding an etchant solution during the sand milling process. This sand milling process achieves microscopic uniform dispersion of the etchant solution and the raw materials, ensuring sufficient contact between them. Simultaneously, the high-energy mechanical collisions during sand milling enhance reactivity, fully utilizing the etchant's effect and helping to reduce the content of impurity phases in the product. Furthermore, by adding the etchant gradually during the sand milling process, it effectively prevents excessive hydrogen ion and anion ion content in the reaction system from being added all at once, which could lead to over-etching and excessive complexation of anions, causing iron ions to accumulate or aggregate on the crystal surface, thus avoiding the formation of impurity phases.

[0022] 2) The sodium iron pyrophosphate cathode material provided by this invention effectively suppresses the formation of electrochemical inert impurity phase NaFePO4 through a specific process, resulting in high material purity. When applied to the cathode of sodium-ion batteries, it exhibits a high discharge specific capacity close to the theoretical value (e.g., ≥109mAh / g at 0.1C), excellent high-rate discharge capability (good capacity retention at 10C), and outstanding long-cycle stability (capacity retention ≥94% after 1000 cycles at 10C). Attached Figure Description

[0023] Figure 1 This is a flowchart of a method for preparing sodium iron pyrophosphate cathode material according to an embodiment of the present invention;

[0024] Figure 2 This is a scanning electron microscope (SEM) image of the sodium iron pyrophosphate cathode material sample prepared in Example 1 of the present invention;

[0025] Figure 3 The X-ray diffraction (XRD) pattern of the sodium iron pyrophosphate cathode material sample prepared in Example 1 of this invention;

[0026] Figure 4 The 0.1C rate charge-discharge curve of a sodium-ion button battery assembled from the sodium iron pyrophosphate cathode material sample prepared in Example 1 of this invention;

[0027] Figure 5 The rate performance curve of a sodium-ion button battery assembled from the sodium iron pyrophosphate cathode material sample prepared in Example 1 of this invention;

[0028] Figure 6 This is a scanning electron microscope (SEM) image of the sodium iron pyrophosphate cathode material sample prepared in Comparative Example 1 of the present invention.

[0029] Figure 7 The X-ray diffraction (XRD) pattern of the sodium iron pyrophosphate cathode material sample prepared in Comparative Example 1 of this invention;

[0030] Figure 8 The first charge-discharge curve of a sodium-ion button battery assembled from the sodium iron pyrophosphate cathode material sample prepared in Comparative Example 1 of this invention is shown at 0.1C. Detailed Implementation

[0031] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Many specific details are set forth in the following description to provide a thorough understanding of the present invention. However, the present invention can be practiced in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0032] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0033] Please see Figure 1 The preparation method of sodium iron pyrophosphate cathode material according to one embodiment of the present invention includes the following steps:

[0034] S10, based on the stoichiometric ratio of Na4Fe3(PO4)2P2O7, disperses iron, sodium and phosphorus sources in water and adds carbon source to form a solid-liquid mixture slurry.

[0035] The term "based on stoichiometry" means that the molar ratio of iron, sodium, and phosphorus in the raw materials basically conforms to the stoichiometric coefficients of each element in the Na4Fe3(PO4)2P2O7 crystal structure (i.e., Na:Fe:P = 4:3:4). It can fluctuate slightly to compensate for process losses, but the core is to use this as the target ratio for the batching.

[0036] In some embodiments, the iron source includes at least one of ferrous oxalate, ferric hydroxide, ferric hydroxyoxide, iron oxide red, and ferrous acetate.

[0037] In some embodiments, the sodium source includes at least one of sodium carbonate, sodium oxalate, sodium acetate, sodium bicarbonate, and sodium hydroxide.

[0038] In some embodiments, the phosphorus source includes at least one of sodium dihydrogen phosphate, ammonium dihydrogen phosphate, sodium acid pyrophosphate, and phosphoric acid.

[0039] In the preparation of sodium iron pyrophosphate materials, the carbon source plays a crucial multiple role: on the one hand, the carbon element formed after high-temperature decomposition or carbonization under an inert atmosphere can act as a reducing agent to remove ferric iron (Fe3+) from the raw materials. 3+ In-situ reduction to divalent iron (Fe) 2+ This ensures that the valence state of iron in the target product meets the stoichiometric requirements, thereby suppressing the formation of impurity phases. On the other hand, excess carbon will uniformly coat the surface of material particles, forming a conductive carbon layer. This can not only effectively suppress particle growth and agglomeration, but also significantly improve the electronic conductivity of the electrode material, thereby improving the rate performance and cycle stability of the material.

[0040] In some embodiments, the carbon source includes at least one of glucose, sucrose, dopamine, and polyvinylpyrrolidone.

[0041] To achieve the dual effects of reduction and coating, the amount of carbon source added needs to be controlled. In some embodiments, the carbon source accounts for 5 wt% to 10 wt% of the total mass of the iron, sodium, and phosphorus sources. This range balances the reduction requirements with the coating effect, avoiding insufficient reduction or incomplete coating due to too low a carbon content, as well as the potential decrease in tap density and obstruction of ion transport caused by too high a carbon content. Furthermore, the mass fraction of the carbon source in the total mass of the iron, sodium, and phosphorus sources can be, but is not limited to, 5 wt%, 6 wt%, 7 wt%, 8 wt%, 9 wt%, or 10 wt%.

[0042] S20. The solid-liquid mixture slurry obtained in step S10 is subjected to sand milling, and an etchant solution is gradually added during the sand milling process to obtain a suspension slurry. The etchant solution includes at least one of citric acid solution, oxalic acid solution, ascorbic acid solution and malic acid solution.

[0043] By dynamically and gradually adding the etchant solution during the sand milling process, the etchant solution and the raw material are microscopically and uniformly dispersed, ensuring sufficient contact between them. Simultaneously, the high-energy mechanical collisions during sand milling enhance reactivity, fully utilizing the etchant's effect and helping to reduce the content of impurity phases in the product. Furthermore, the gradual addition during sand milling effectively prevents excessive hydrogen and acid radical ion content in the reaction system from being caused by a single addition, thus avoiding over-etching and excessive complexation of acid radical ions, which could lead to iron ion enrichment or large-scale complexation and aggregation on the crystal surface, thereby preventing the formation of impurity phases.

[0044] The possible mechanism of this invention is as follows: Hydrogen ions in the etchant solution can gently etch the crystal structure of the iron source during the sand milling process, disrupting the original regular crystal structure and transforming the iron source from an aggregated crystal into highly active and dispersed nano / submicron-sized active sites. This results in a more uniform subsequent reaction and avoids local enrichment leading to the formation of impurity phases. Hydrogen ions in the etchant can also remove oxygen, forming oxygen vacancies. The introduction of oxygen vacancies enhances the conductivity of electrons and sodium ions. Oxygen vacancies themselves are defects that alter the local electronic structure of the material, making the valence state of Fe more flexible and contributing to the stabilization of Fe. 2+ This reduces the generation of impurities.

[0045] Anions can complex with ferrous and ferric ions, reducing the amount of free iron ions in the solvent and inhibiting their rapid reaction with phosphate ions and other ions in the system to form impurities. The above is merely a speculation on the possible mechanism of this invention and does not constitute a limitation on the scope of protection of this invention.

[0046] In one embodiment, the etchant solution is gradually added by continuous dripping or intermittent dripping. This "dynamic addition" method during the sand milling process is one of the key features of the invention. It ensures that the etchant can have sufficient and uniform contact and reaction with the raw material particles (especially the iron source) that are continuously broken and activated under the sand milling action, avoiding problems such as local over-concentration or uneven reaction.

[0047] In one embodiment, the etchant solution is gradually added dropwise, with a dropping rate of 0.3 g / min to 0.8 g / min, which may be, but is not limited to, 0.3 g / min, 0.4 g / min, 0.5 g / min, 0.6 g / min, 0.7 g / min, or 0.8 g / min. Too rapid a dropping rate will increase the concentration of hydrogen ions and anions in the system, leading to over-etching and the formation of impurity phases. Too slow a dropping rate will result in insufficient etching and complexation, ineffective removal of iron source agglomeration and crystal structure, and insufficient oxygen vacancy formation, leading to Fe... 2+ It exhibits poor stability, high impurity phase residue, and low process efficiency. Only by matching the feeding rate with the sand milling dispersion rate to achieve steady-state and controllable stepwise supply can the etching effect be maximized.

[0048] In one embodiment, the concentration of the etchant solution is 0.05 g / mL to 0.1 g / mL, and may be, but is not limited to, 0.05 g / mL, 0.06 g / mL, 0.07 g / mL, 0.08 g / mL, 0.09 g / mL or 0.1 g / mL.

[0049] In one embodiment, the mass ratio of the solute in the etchant solution to the iron source is (1~5):100, which can be, but is not limited to, 1:100, 2:100, 3:100, 4:100, or 5:100. A mass ratio that is too high may cause the effective iron source to dissolve, resulting in excessive oxygen vacancies on the surface. 2+ Oxidation leads to a decrease in product purity. A low mass ratio results in insufficient etching effect and limited product improvement.

[0050] In one embodiment, the sanding process takes 3 to 5 hours. Further, the sanding process can take, but is not limited to, 3 hours, 3.5 hours, 4 hours, 4.5 hours, or 5 hours.

[0051] S30. Spray dry the suspension slurry obtained in step S20 to obtain precursor powder.

[0052] In one embodiment, the inlet air temperature of the spray dryer is 180°C to 220°C, and the outlet air temperature is 90°C to 110°C. Further, the inlet air temperature of the spray dryer may be, but is not limited to, 180°C, 190°C, 200°C, 210°C, or 220°C, and the outlet air temperature may be, but is not limited to, 90°C, 100°C, or 110°C.

[0053] S40. The precursor powder obtained in step S30 is sintered under an inert atmosphere to obtain sodium iron pyrophosphate cathode material.

[0054] The sodium iron pyrophosphate cathode material of this invention has the chemical composition Na4Fe3(PO4)2P2O7 / C. Na4Fe3(PO4)2P2O7 is the main active material, possessing a three-dimensional open polyanionic framework crystal structure that provides stable channels for sodium ion insertion and extraction; " / C" indicates that the material contains uniformly distributed carbon components.

[0055] In one embodiment, the sintering process is carried out under a nitrogen atmosphere at a temperature of 480°C to 550°C for 6 to 12 hours. This combination of sintering temperature and time is beneficial for the full formation and crystallization of the target crystalline phase. Furthermore, the sintering temperature can be, but is not limited to, 480°C, 490°C, 500°C, 510°C, 520°C, 530°C, 540°C, or 550°C, and the sintering time can be, but is not limited to, 6 hours, 7 hours, 8 hours, 9 hours, 10 hours, 11 hours, or 12 hours. In addition, the sintering atmosphere is not limited to a nitrogen atmosphere and can be any other inert atmosphere.

[0056] The preparation method of sodium iron pyrophosphate cathode material of this invention integrates etching, dispersion and activation in one step by dynamically and gradually adding a weak acid etchant during the sand milling process, achieving in-situ and uniform modification of the iron source. The etchant primarily acts on the iron source, disrupting its crystal structure (e.g., ferrous oxalate). Hydrogen ions in the etchant carry away oxygen, forming oxygen vacancies. The introduction of oxygen vacancies enhances the conductivity of electrons and sodium ions. Oxygen vacancies themselves are defects that alter the local electronic structure of the material, making the valence state of Fe more flexible and contributing to the stability of Fe. 2+ This reduces the formation of impurity phases. Furthermore, the introduction of oxygen vacancies improves the capacity and long-cycle capability of the sodium iron pyrophosphate cathode material, making it more competitive in the energy storage field.

[0057] The sodium iron pyrophosphate cathode material of one embodiment is prepared by any of the above preparation methods.

[0058] The sodium iron pyrophosphate cathode material contains oxygen vacancies introduced by the etchant. The introduction of oxygen vacancies enhances the conductivity of electrons and sodium ions. Oxygen vacancies themselves are defects that alter the local electronic structure of the material, making the valence state of Fe more flexible and contributing to the stability of Fe. 2+ This reduces the generation of impurities.

[0059] Based on the aforementioned embodiments, the sodium iron pyrophosphate cathode material has a discharge specific capacity of not less than 109 mAh / g at a current density of 0.1C, and a capacity retention rate of not less than 94% after 1000 cycles at a current density of 10C.

[0060] The sodium iron pyrophosphate cathode material provided by this invention effectively suppresses the formation of the electrochemically inert impurity phase NaFePO4 through a specific process, resulting in high material purity and the potential introduction of beneficial oxygen vacancies. This leads to high discharge specific capacity (e.g., ≥109 mAh / g at 0.1C), excellent high-rate discharge capability (good capacity retention at 10C), and outstanding long-cycle stability (capacity retention ≥94% after 1000 cycles at 10C). Its overall performance is significantly superior to similar materials prepared by conventional methods.

[0061] One embodiment of the positive electrode includes the above-described sodium iron pyrophosphate positive electrode material.

[0062] The aforementioned sodium iron pyrophosphate cathode material serves as the positive electrode active material. Furthermore, the cathode of the present invention may also contain conductive agents and binders. The conductive agents and binders can be any substances selected in the art, and the present invention does not limit the types of the two or the proportions of the sodium iron pyrophosphate cathode material, the conductive agent, and the binder.

[0063] The cathode material described in this invention directly benefits from the high purity and excellent electrochemical performance of the active material, exhibiting high reversible capacity, excellent rate response, and extremely long cycle life. This cathode has a stable structure and a mature manufacturing process, meeting the performance requirements of high-power, long-life sodium-ion batteries for cathode plates.

[0064] One embodiment of the sodium-ion battery includes the above-described positive electrode.

[0065] The sodium-ion battery assembled using the cathode of this invention exhibits high energy density and excellent cycle life, especially maintaining stable capacity output and extremely high capacity retention even at high rates (such as 10C). This battery boasts excellent performance and controllable cost, showing broad application prospects in large-scale energy storage, electric vehicles, and other fields.

[0066] Referring to the above embodiments, in order to make the technical solution of the present invention more specific, clear and easy to understand, examples of the technical solution of the present invention are given below. However, it should be noted that the content to be protected by the present invention is not limited to the following embodiments.

[0067] Example 1

[0068] This embodiment provides a sodium iron pyrophosphate cathode material, its preparation method, cathode, and sodium-ion battery.

[0069] (1) Preparation of sodium iron pyrophosphate cathode material

[0070] Accurately weigh FePO4, Na2CO3, and Na2HPO4·2H2O according to the elemental molar ratio of Na:Fe:P of 4:3:4. Weigh 10wt% of glucose as the carbon source. Add FePO4, Na2CO3, Na2HPO4·2H2O, and glucose to deionized water, controlling the solid content to 15%, and stir to form a solid-liquid mixture.

[0071] The obtained solid-liquid mixture was subjected to sand milling for 4 hours, while citric acid solution was continuously added dropwise. The concentration of citric acid solution was 0.07 g / mL, the dropping rate of citric acid solution was 0.5 g / min, and the mass ratio of citric acid to iron source in citric acid solution was 3:100, thus obtaining a suspension slurry.

[0072] The suspension slurry obtained by sand milling was spray dried. The frequency of the spray drying fan was 40 Hz, the feed rate was 30 mL / min, the inlet air temperature was 200 ℃, and the outlet air temperature was 100 ℃ to obtain precursor powder.

[0073] The precursor powder was sintered at 500°C for 10 hours under a nitrogen atmosphere to obtain sodium iron pyrophosphate cathode material.

[0074] (2) Preparation of the positive electrode

[0075] The target product prepared above was used as the positive electrode active material, and it was weighed together with the conductive agent and binder at a mass ratio of 8:1:1. First, 240 mg of the active material and 30 mg of Super-P conductive agent were weighed and mixed in a mortar for 10 min until homogeneous. 600 mg of a 5% PVDF solution was weighed into a 5 mL beaker, and the mixed powder was added to the beaker. Three drops of NMP were added, and the beaker was sealed with plastic wrap and sealing film. The beaker was then placed on a magnetic stir bar and stirred for 4 h to ensure thorough mixing of the components. The slurry was coated onto aluminum foil and vacuum dried at 120℃ for 12 h. After drying, it was cut into circular electrode sheets to obtain the positive electrode sheet for a coin cell sodium-ion battery.

[0076] (3) Preparation of sodium-ion batteries

[0077] Using the above-mentioned electrode sheet as the positive electrode and the sodium sheet as the negative electrode, the electrolyte is 1M NaPF6EC-DEC@5% FEC (1:1, V / V). The CR2032 button cell is assembled by placing the positive electrode sheet, separator, sodium sheet, gasket, and spring sheet in that order. The packaged cell is left to stand for 24 hours to allow the electrodes to be fully wetted by the electrolyte, in preparation for subsequent electrochemical performance testing.

[0078] Figure 2 The image shows a scanning electron microscope (SEM) image of the sodium iron pyrophosphate cathode material sample prepared in Example 1. It can be seen that the primary particle size is controlled at approximately 200 nm to 400 nm, and the primary particle distribution is relatively uniform.

[0079] Figure 3 The image shows the X-ray diffraction (XRD) spectrum of the sodium iron pyrophosphate cathode material sample prepared in Example 1. It can be seen that PDF#89-0579 is the standard card for Na4Fe3(PO4)2P2O7, and its diffraction peaks correspond one-to-one with those of the sample. This means that the main phase of the material is Na4Fe3(PO4)2P2O7, and no NaFePO4 impurity phase was found. This indicates that the etching of FePO4 by citric acid effectively inhibits the formation of electrochemically inert NaFePO4.

[0080] Figure 4 The image shows the 0.1C rate charge-discharge curves of a sodium-ion button battery assembled from the sodium iron pyrophosphate cathode material sample prepared in Example 1. It can be seen that the sodium iron pyrophosphate cathode material exhibits a current density of 123.2 mAh g / g at 0.1C within a voltage range of 1.7–4.3 V. -1 It has a high discharge specific capacity and an initial coulombic efficiency of up to 95%, exhibiting excellent electrochemical performance.

[0081] Figure 5 The image shows the rate performance curve of a sodium-ion button battery assembled from the sodium iron pyrophosphate cathode material sample prepared in Example 1. It can be seen that the sodium iron pyrophosphate cathode material still maintains a rate of 95 mAh / g at a high current density of 10C. -1 It has a high discharge specific capacity and excellent rate performance.

[0082] Examples 2 to 8

[0083] The sodium iron pyrophosphate cathode material was prepared following essentially the same steps as in Example 1, except that one or more parameters were adjusted, including the type of etchant, the dropping rate of the etchant solution, the mass ratio of etchant to iron source, and the sintering temperature, as shown in Table 1 below.

[0084] The positive electrode and sodium-ion battery were prepared following essentially the same steps as in Example 1.

[0085] Comparative Example 1

[0086] This comparative example is a comparative example of Example 1, providing a sodium iron pyrophosphate cathode material and its preparation method, as well as the cathode and sodium-ion battery. The only difference from the preparation method of Example 1 is that no etching solution is added during the sand milling process, and only conventional sand milling is performed for 4 hours. Other conditions remain unchanged.

[0087] The specific preparation method of the sodium iron pyrophosphate cathode material of Comparative Example 1 is as follows:

[0088] Accurately weigh FePO4, Na2CO3, and Na2HPO4·2H2O according to a Na:Fe:P molar ratio of 4:3:4. Weigh 10wt% of glucose as the carbon source. Add the solid powder to deionized water, controlling the solid content to 15%, and stir to form a solid-liquid mixture.

[0089] The obtained solid-liquid mixture was subjected to sand milling for 4 hours to obtain a suspension slurry;

[0090] The suspension slurry obtained by sand milling was spray dried. The frequency of the spray drying fan was 40 Hz, the feed rate was 30 mL / min, the inlet air temperature was 200 ℃, and the outlet air temperature was 100 ℃ to obtain precursor powder.

[0091] The precursor powder was sintered at 500℃ for 10 hours under a nitrogen atmosphere to obtain the sodium iron pyrophosphate cathode material of Comparative Example 1.

[0092] Figure 6 The SEM image of the sodium iron pyrophosphate cathode material sample prepared in Comparative Example 1 shows that the material has a spherical morphology and the primary particle size distribution is between 300 nm and 500 nm.

[0093] Figure 7 XRD pattern of the sodium iron pyrophosphate cathode material sample prepared in Comparative Example 1. Figure 7 As can be seen, the main phase of the material is Na4Fe3(PO4)2P2O7, with a NaFePO4 impurity phase near 2θ=35°. This indicates that samples without etchant are prone to forming sodium phosphate-iron ore type NaFePO4, which adversely affects electrochemical performance.

[0094] Figure 8 The first charge-discharge curve of a sodium-ion coin cell assembled from the sodium iron pyrophosphate cathode material sample prepared in Comparative Example 1 is shown at 0.1C. Figure 8 As can be seen from this, its discharge specific capacity is 99mAh g. -1 The value is lower than that of Example 1, which incorporates an etchant.

[0095] Comparative Example 2

[0096] This comparative example is a comparative example of Example 1, providing a sodium iron pyrophosphate cathode material and its preparation method, cathode and sodium-ion battery. The only difference from Example 1 is that before the sand milling process begins, all the citric acid solution (solute to iron source mass ratio 3:100, concentration 0.07g / mL) is added to the solid-liquid mixture at once, and then the sand milling process is performed. The other conditions are the same as in Example 1.

[0097] Table 1

[0098]

[0099] Performance testing:

[0100] The performance of sodium-ion batteries assembled with sodium iron pyrophosphate cathode materials from Examples 1 to 8 and Comparative Examples 1 and 2 was tested. The test methods are as follows, and the test results are shown in Table 2.

[0101] (1) 0.1C charging specific capacity: The prepared sodium-ion battery was charged to 4.3V at 0.1C constant current at 25℃, and the charging capacity was recorded. 0.1C charging specific capacity = charging capacity / mass of positive electrode active material. The specific results are shown in Table 2 below.

[0102] (2) 0.1C discharge specific capacity: The prepared sodium-ion battery was discharged at 25°C with a constant current of 0.1C to 1.7V, and the discharge capacity was recorded. 0.1C discharge specific capacity = discharge capacity / mass of positive electrode active material. The specific results are shown in Table 2 below.

[0103] (3) Initial Coulomb efficiency: Initial Coulomb efficiency = 0.1C discharge specific capacity / 0.1C charge specific capacity * 100%, the specific results are shown in Table 2 below.

[0104] (4) Capacity retention rate after 500 cycles at 1C: At a temperature of 25±3℃, the battery is charged at a constant current of 1C to the charging cutoff voltage of 4.3V, left to stand for 30 minutes, and then discharged at a constant current of 1C to the discharge cutoff voltage of 1.7V, left to stand for 30 minutes. The above charge and discharge cycles are repeated 500 times. The ratio of the discharge capacity of the 500th cycle to the discharge capacity of the first cycle is calculated, which is the capacity retention rate after 500 cycles at 1C. The specific results are shown in Table 2 below.

[0105] (5) Capacity retention rate after 1000 cycles at 10C: At a temperature of 25±3℃, the battery is charged at a constant current of 10C to the charging cutoff voltage of 4.3V, left to stand for 30 minutes, and then discharged at a constant current of 10C to the discharge cutoff voltage of 1.7V, left to stand for 30 minutes. The above charge and discharge cycles are repeated 1000 times. The ratio of the discharge capacity of the 1000th cycle to the discharge capacity of the first cycle is calculated, which is the capacity retention rate after 1000 cycles at 10C. The specific results are shown in Table 2 below.

[0106] Table 2

[0107]

[0108] As can be seen from Table 2:

[0109] (1) Compared with Comparative Example 1, which did not use an etchant, Examples 1 to 8 showed a significant improvement in performance across the board. The 0.1C discharge specific capacity (109.1~123.3 mAh / g) and first coulombic efficiency (92.4%~95.0%) of all examples were significantly higher than those of Comparative Example 1 (98.8 mAh / g, 88.9%). This demonstrates that the introduction of the etchant solution effectively suppressed the formation of the electrochemically inert impurity phase NaFePO4, and significantly improved the utilization rate of the active material and the reversibility of the reaction.

[0110] The capacity retention rates of all embodiments after 500 cycles at 1C and 1000 cycles at 10C (≥97.2% and ≥93.5%, respectively) were significantly higher than those of Comparative Example 1 (92.6% and 88.5%). This indicates that the etchant treatment not only improved the initial capacity but, more importantly, optimized the material structure (e.g., by introducing oxygen vacancies and stabilizing Fe²⁺). + The valence state greatly enhances the structural integrity of the material under long-term, high-rate cycling.

[0111] (2) Example 1 exhibits the best overall performance, with a discharge specific capacity of 123.3 mAh g at 0.1C. -1 The theoretical specific capacity is 129 mAh g. -1 The performance of this component is 95.6%, which is significantly higher than the 98.8 mAh g of the control component (Comparative Example 1) without etchant. -1Meanwhile, the initial coulombic efficiency of Example 1 is close to and better than that of Comparative Example 1. Furthermore, the capacity retention rates of Example 1 after 500 cycles at 1C and long-term cycling at 10C are both greater than those of Comparative Example 1, indicating that the sodium iron pyrophosphate cathode material prepared by etching FePO4 with an etchant has excellent electrochemical performance.

[0112] (3) The electrochemical performance of sodium ions in Examples 1 to 8 is superior to that in Comparative Example 2, where the etchant solution was added all at once before sand milling. This proves that gradually adding the etchant solution during sand milling is beneficial for the etchant solution to gently etch the iron source, suppress the formation of impurity phases, and improve the purity of the product.

[0113] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0114] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A method for preparing a sodium iron pyrophosphate cathode material, characterized in that, Includes the following steps: Based on the stoichiometric ratio of Na4Fe3(PO4)2P2O7, iron, sodium, and phosphorus sources are dispersed in water, and a carbon source is added to form a solid-liquid mixture slurry. The solid-liquid mixture is subjected to sand milling, and an etchant solution is gradually added during the sand milling process to obtain a suspension slurry. The etchant solution includes at least one of citric acid solution, oxalic acid solution, ascorbic acid solution and malic acid solution. The suspension slurry was spray-dried to obtain precursor powder; and The precursor powder was sintered under an inert atmosphere to obtain sodium iron pyrophosphate cathode material. The etching solution is added gradually by continuous dripping, and the dripping rate of the etching solution is 0.3 g / min to 0.8 g / min; The concentration of the etching agent solution is 0.05 g / mL to 0.1 g / mL.

2. The method for preparing the sodium iron pyrophosphate cathode material according to claim 1, characterized in that, The mass ratio of the solute in the etching solution to the iron source is (1~5):

100.

3. The method for preparing the sodium iron pyrophosphate cathode material according to claim 1, characterized in that, The grinding process takes 3 to 5 hours.

4. The method for preparing the sodium iron pyrophosphate cathode material according to claim 1, characterized in that, The sintering temperature for the sintering process is 480℃~550℃.

5. A sodium iron pyrophosphate cathode material, characterized in that, It is prepared by the preparation method described in any one of claims 1 to 4.

6. A positive electrode, characterized in that, Including the sodium iron pyrophosphate cathode material as described in claim 5.

7. A sodium-ion battery, characterized in that, Includes the positive electrode as described in claim 6.

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