Sodium iron pyrophosphate precursor and its preparation method, cathode material and its preparation method, and battery

Abstract

This application relates to the field of new energy technology, and particularly to sodium iron pyrophosphate precursor and its preparation method, cathode material and its preparation method, and battery. The sodium iron pyrophosphate precursor comprises iron phosphate, iron pyrophosphate, an impurity phase, and water of crystallization. When the water of crystallization accounts for no more than 1% of the mass of the sodium iron pyrophosphate precursor, the mass percentage of pyrophosphate in the precursor is 8-30%, and the impurity phase accounts for no more than 4% of the mass of the precursor. Using a precursor with pyrophosphate content within the above range, the PO4 content in the crystal can be increased during the subsequent synthesis of sodium iron pyrophosphate. 3‑ with Fe 3+ Na + By combining the energy barrier of individual phase formation, the formation of electrochemically inactive NFP impurity phases can be suppressed, which is beneficial to improving the purity of the main phase of NFPP material and improving the specific capacity and rate performance of the battery.

Description

Technical Field

[0001] This application relates to the field of new energy technology, and in particular to sodium iron pyrophosphate precursor and its preparation method, cathode material and its preparation method, and battery. Background Technology

[0002] Sodium iron pyrophosphate (SOF) is a promising polyanion cathode material for sodium-ion batteries. It combines the structural advantages of phosphates and pyrophosphates, possessing an open three-dimensional framework that provides stable channels for sodium ion insertion and extraction. However, the synthesis of SOF readily introduces the sodium iron phosphate (NFP) phase. The NFP phase lacks continuous Na+. + The presence of NFP impurities severely restricts the improvement of phase purity in sodium iron pyrophosphate (NFPP) cathode materials, resulting in low actual specific capacity and poor rate performance in current NFPP cathode materials. Summary of the Invention

[0003] Based on this, the first aspect of this application provides a sodium iron pyrophosphate precursor, the technical solution of which is as follows: A sodium iron pyrophosphate precursor includes iron phosphate, iron pyrophosphate, an impurity phase, and water of crystallization. When the water of crystallization accounts for no more than 1% of the mass of the sodium iron pyrophosphate precursor, the mass percentage of pyrophosphate in the sodium iron pyrophosphate precursor is 8-30%, and the mass percentage of the impurity phase in the sodium iron pyrophosphate precursor is no more than 4%.

[0004] The second aspect of this application provides a method for preparing the aforementioned sodium iron pyrophosphate precursor, the technical solution of which is as follows: A method for preparing sodium iron pyrophosphate precursor includes the following steps: A mixed oxidant, iron source, phosphorus source, pH adjuster and first solvent are mixed to cause a co-precipitation reaction in the resulting mixed system to obtain a precipitate. The amount of pH adjuster is adjusted so that the pH value of the precipitate is 1.5 to 5, preferably 2 to 4. The precipitate was sintered first to obtain the sodium iron pyrophosphate precursor.

[0005] A third aspect of this application provides a sodium iron pyrophosphate cathode material, the technical solution of which is as follows: A sodium iron pyrophosphate cathode material includes a sodium iron pyrophosphate matrix and a carbon coating layer covering the sodium iron pyrophosphate matrix, wherein the chemical formula of the sodium iron pyrophosphate matrix is ​​Na. x Fe y (PO4) z (P2O7) wWherein, 3.6≤x≤4.2, 2.7≤y≤3.15, 1.8≤z≤2.1, 0.9≤w≤1.05; the mass percentage of the sodium iron phosphate phase in the sodium iron phosphate pyrophosphate cathode material is 0~1%, the mass percentage of the sodium iron pyrophosphate phase in the sodium iron phosphate pyrophosphate cathode material is 0~1%, and the mass percentage of the sodium iron phosphate pyrophosphate phase in the sodium iron phosphate pyrophosphate cathode material is 98~100%.

[0006] The fourth aspect of this application provides a method for preparing the above-mentioned sodium iron pyrophosphate cathode material, the technical solution of which is as follows: A method for preparing a sodium iron pyrophosphate cathode material includes the following steps: A mixed slurry is obtained by mixing sodium iron pyrophosphate precursor, phosphorus source, sodium source, carbon source, and a second solvent; wherein, the sodium iron pyrophosphate precursor is as described above, and the amount of sodium iron pyrophosphate precursor and the phosphorus source are adjusted to make the iron-phosphorus ratio (2.84~2.96):4, and the carbon source includes organic carboxylic acid carbon source and sugar carbon source. The mixed slurry is milled, dried, and then sintered to prepare the sodium iron pyrophosphate cathode material.

[0007] The fifth aspect of this application provides a battery comprising the sodium iron pyrophosphate cathode material as described above.

[0008] Compared with traditional solutions, this application has the following advantages: In the sodium ferric pyrophosphate precursor provided in this application, the mass percentage of pyrophosphate ions is controlled to be 8-30%. Experimental verification has shown that using a precursor with pyrophosphate ion content within the above range can increase the PO4 content in the crystals during subsequent synthesis of sodium ferric pyrophosphate. 3- with Fe 3+ Na + By combining the energy barrier of individual phase formation, the formation of electrochemically inactive NFP impurity phases can be suppressed, which is beneficial to improving the phase purity of cathode materials and improving the specific capacity and rate performance of batteries. Detailed Implementation

[0009] The present application will be further described in detail below with reference to specific embodiments. The present application can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a more thorough and complete understanding of the disclosure of this application.

[0010] 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 application belongs. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application.

[0011] Unless otherwise stated or in case of contradiction, the terms or phrases used herein shall have the following meanings: In this application, the terms "multiple", "various", "multiple times", "multi-dimensional", etc., unless otherwise specified, refer to a quantity greater than or equal to 2. For example, "one or more" means one or more or more.

[0012] In this application, "several" means at least one, such as one, two, etc., unless otherwise expressly and specifically defined.

[0013] In this application, the terms "optionally," "optionally," and "optional" refer to options that are optional, meaning they can be selected from either "with" or "without." If multiple "optional" options appear in a technical solution, unless otherwise specified and there are no contradictions or mutual constraints, each "optional" option is independent.

[0014] In this application, the terms "first aspect," "second aspect," "third aspect," and "fourth aspect," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or quantity, nor should they be construed as implicitly indicating the importance or quantity of the indicated technical features. Moreover, "first," "second," "third," and "fourth," etc., serve only a non-exhaustive enumeration purpose and should be understood not to constitute a closed limitation on quantity.

[0015] In this application, numerical intervals (i.e. numerical ranges) are involved. Unless otherwise specified, the selected numerical distributions within the above-mentioned numerical intervals are considered continuous, and include the two numerical endpoints (i.e., the minimum value and the maximum value) of the numerical range, as well as every value between the two numerical endpoints.

[0016] Unless otherwise specified, the temperature parameters in this application are permitted to be either constant-temperature treatment or variations within a certain temperature range. It should be understood that the constant-temperature treatment allows temperature fluctuations within the precision range of the instrument control, such as ±5℃, ±4℃, ±3℃, ±2℃, or ±1℃.

[0017] In this application, Dv50 refers to the particle size corresponding to a cumulative volume distribution percentage of 50%, that is, the volume content of particles smaller than or equal to this particle size accounts for 50% of the total particle volume.

[0018] In this application, NFP phase refers to sodium iron phosphate material having a Maricite phase.

[0019] In this application, the method for testing the mass ratio of water of crystallization in the sodium iron pyrophosphate precursor (hereinafter referred to as water content) is the halogen rapid moisture determination method (refer to GB / T 6284-2006); the method for testing the mass ratio of pyrophosphate and phosphate in the sodium iron pyrophosphate precursor (hereinafter referred to as pyrophosphate content and phosphate content) is ion chromatography (refer to GB 5009.256-2025); the method for testing the mass ratio of sodium ions in the impurity phase in the sodium iron pyrophosphate precursor (hereinafter referred to as sodium ion content) is ICP-OES (refer to GB / T 30902-2014); the method for testing the mass ratio of ammonium ions in the impurity phase in the sodium iron pyrophosphate precursor (hereinafter referred to as ammonium ion content) is ion chromatography (refer to GB / T 35664-2017); and the method for testing the mass ratio of hydrogen ions and hydroxide ions in the impurity phase in the sodium iron pyrophosphate precursor (hereinafter referred to as hydrogen ion content and hydroxide ion content) is thermogravimetric analysis (refer to HG / T The mass percentage of chloride, nitrate, and oxalate ions in the sodium iron pyrophosphate precursor (hereinafter referred to as chloride, nitrate, and oxalate content) is determined by ion chromatography (refer to SN / T 4310-2015); the mass percentage of iron in the sodium iron pyrophosphate precursor (hereinafter referred to as iron content) is determined by manual titration with potassium dichromate (refer to HG / T 4701-2021); the mass percentage of carbon coating and sulfate ions in the sodium iron pyrophosphate cathode material (hereinafter referred to as carbon content and sulfate content) is determined by high-frequency combustion infrared absorption method (refer to GB / T 20123-2020), wherein the sum of sodium ion content, ammonium ion content, hydrogen ion content, hydroxide ion content, chloride ion content, nitrate content, oxalate content, and sulfate content is the impurity phase content.

[0020] This application provides a sodium ferric pyrophosphate precursor, comprising ferric phosphate, ferric pyrophosphate, an impurity phase, and water of crystallization. When the water of crystallization accounts for no more than 1% of the mass of the sodium ferric pyrophosphate precursor, the mass percentage of pyrophosphate in the precursor is 8-30%, and the mass percentage of the impurity phase in the precursor is no more than 4%. For example, the mass percentage of pyrophosphate in the precursor is 8%, 10%, 15%, 20%, 25%, or 30%, and the mass percentage of the impurity phase is 4%, 3%, 2%, or 1%.

[0021] In the sodium ferric pyrophosphate precursor provided in this application, the mass percentage of pyrophosphate ions is controlled to be 8-30%. Experimental verification has shown that using a precursor with pyrophosphate ion content within the above range can increase the PO4 content in the crystals during subsequent synthesis of sodium ferric pyrophosphate.3- with Fe 3+ Na + By combining the energy barrier of individual phase formation, the formation of electrochemically inactive NFP impurity phases can be suppressed, which is beneficial to improving the phase purity of cathode materials and improving the specific capacity and rate performance of batteries.

[0022] Optionally, when the mass ratio of water of crystallization to the sodium iron pyrophosphate precursor is no more than 1%, the mass ratio of phosphate in the sodium iron pyrophosphate precursor is 30-58%, and the mass ratio of iron in the sodium iron pyrophosphate precursor is 26-35%. For example, the mass ratio of phosphate in the sodium iron pyrophosphate precursor is 30%, 35%, 40%, 45%, 50%, and 58%, and the mass ratio of iron in the sodium iron pyrophosphate precursor is 26%, 30%, and 35%.

[0023] Through sintering, the water content of the sodium iron pyrophosphate precursor is generally no more than 1%. When the water content is no more than 1%, the contents of pyrophosphate, phosphate, iron, and impurity phases are measured to be within the above-mentioned range. In some other cases, the water content of the sodium iron pyrophosphate precursor may exceed 1%. For example, if the sodium iron pyrophosphate precursor is dissolved and dried at room temperature, it may contain a large amount of water of crystallization, with a water content exceeding 1%. In this case, the water content of the sodium iron pyrophosphate precursor is controlled to no more than 1% by drying at 200°C or heat treatment, and then the contents of pyrophosphate, phosphate, iron, and impurity phases are measured to be within the above-mentioned range.

[0024] Optionally, the iron phosphate and iron pyrophosphate are randomly dispersed inside and on the surface of the sodium iron pyrophosphate precursor.

[0025] In this application, the impurity phase includes at least one of sulfate, chloride, nitrate, oxalate, sodium, ammonium, hydrogen, and hydroxide ions. It is an objective result of the liquid-phase reaction, introduced by phosphate and iron salts, and cannot be completely washed away, randomly dispersed within the sodium iron pyrophosphate pyrophosphate precursor. This impurity phase is present in small amounts and its content is stable, thus not affecting the cathode performance.

[0026] The second aspect of this application provides a method for preparing the aforementioned sodium iron pyrophosphate precursor, the technical solution of which is as follows: A method for preparing sodium iron pyrophosphate precursor includes the following steps: A mixed oxidant, iron source, phosphorus source, pH adjuster and first solvent are mixed to cause a co-precipitation reaction in the resulting mixed system to obtain a precipitate. The amount of pH adjuster is adjusted so that the pH value of the precipitate is 1.5 to 5, preferably 2 to 4. The precipitate was sintered first to obtain the sodium iron pyrophosphate precursor.

[0027] Generally, iron sources and phosphoric acid or phosphates rarely react directly to form ferric pyrophosphate. Therefore, pyrophosphate ions are usually introduced into the sodium ferric pyrophosphate precursor by a solution containing pyrophosphate ions. Unlike adding a solution containing pyrophosphate ions, this application, under conditions of oxidant and pH control, induces a co-precipitation reaction between the iron and phosphorus sources and sintersulates the precipitate, generating pyrophosphate ions in situ within the crystal structure. This directly prepares a sodium ferric pyrophosphate precursor containing pyrophosphate ions, and controls the pyrophosphate content to 8-30%, which is beneficial for increasing the PO4 content in the crystal during subsequent synthesis of sodium ferric pyrophosphate ions. 3- with Fe 3+ Na + By combining the energy barrier of individual phase formation, the formation of electrochemically inactive NFP impurity phases can be suppressed, which is beneficial to improving the phase purity of cathode materials and improving the specific capacity and rate performance of batteries.

[0028] Optionally, the oxidant is selected from at least one of hydrogen peroxide, sodium chlorate, sodium nitrate, and nitric acid. Optionally, the concentration of the oxidant in the mixed system is 0.05~1 mol / L; preferably, the concentration is 0.1~0.5 mol / L.

[0029] Optionally, the iron source includes an iron salt, which includes at least one of ferrous (Fe2+) and ferric (Fe3+) salts. The ferrous (Fe2+) salt is selected from at least one of ferrous chloride, ferrous sulfate, and ferrous oxalate; the ferric (Fe3+) salt is selected from at least one of ferric chloride, ferric nitrate, ferric sulfate, and ferric oxalate. Optionally, the concentration of iron ions in the mixed system is 0.1~1.0 mol / L; preferably, the concentration is 0.2~0.6 mol / L. When the iron salt is a ferrous (Fe2+) salt, the iron ions are ferrous (Fe2+) ions; when the iron source is a ferric (Fe3+) salt, the iron ions are ferric (Fe3+) ions; when the iron salt includes both ferrous (Fe2+) and ferric (Fe3+) salts, the iron ions include both ferrous (Fe2+) and ferric (Fe3+) ions.

[0030] Optionally, the phosphorus source includes at least one of phosphoric acid and a phosphate salt, wherein the phosphate salt includes at least one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, and trisodium phosphate; optionally, the concentration of phosphate ions in the mixed system is 0.1~1.5 mol / L. Preferably, the concentration is 0.2~0.8 mol / L.

[0031] Optionally, the pH adjuster is selected from at least one of hydrochloric acid, sulfuric acid, citric acid, oxalic acid, ammonia, and sodium hydroxide.

[0032] Optionally, the first solvent includes water.

[0033] Understandably, when mixing the oxidant, iron source, phosphorus source, pH adjuster, and first solvent, the first solvent can be mixed separately with the oxidant, iron source, and phosphorus source to prepare oxidant solution, iron source solution, and phosphorus source solution, and then the solutions are mixed. The pH adjuster can be added when mixing the solutions, or it can be added to the iron source solution and phosphorus source solution in advance.

[0034] Optionally, the temperature of the coprecipitation reaction is 20~80℃. Preferably, the temperature of the coprecipitation reaction is 35~65℃.

[0035] The first sintering can be carried out in sintering equipment such as muffle furnaces, box furnaces, kilns, and roller kilns.

[0036] Optionally, the atmosphere for the first sintering is selected from at least one of nitrogen, argon, and air.

[0037] Optionally, the temperature of the first sintering is 200~550℃, preferably 250~400℃.

[0038] Optionally, the first sintering time is 1 to 5 hours, preferably 2 to 4 hours.

[0039] A third aspect of this application provides a sodium iron pyrophosphate cathode material, the technical solution of which is as follows: A sodium iron pyrophosphate cathode material includes a sodium iron pyrophosphate matrix and a carbon coating layer covering the sodium iron pyrophosphate matrix, wherein the chemical formula of the sodium iron pyrophosphate matrix is ​​Na. x Fe y (PO4) z (P2O7) w Wherein, 3.6≤x≤4.2, 2.7≤y≤3.15, 1.8≤z≤2.1, 0.9≤w≤1.05; the mass percentage of sodium iron phosphate (NFP) phase in the sodium iron phosphate pyrophosphate cathode material is 0~1%, the mass percentage of sodium iron pyrophosphate (NFPO) phase in the sodium iron phosphate pyrophosphate cathode material is 0~1%, and the mass percentage of sodium iron phosphate pyrophosphate (NFPP) phase in the sodium iron phosphate pyrophosphate cathode material is 98~100%. The mass percentage of the NFP phase can be 0, 0.5%, or 1%. The mass percentage of the NFPO phase can be 0, 0.5%, or 1%. The mass percentage of the NFPP phase is 98%, 99%, or 100%.

[0040] Besides the NFP phase, the NFPO phase has a lower theoretical specific capacity and operates at around 2.5V (vs. Na / Na). +The discharge capacity of the cathode material cannot be fully utilized in a full-charge process. This application can simultaneously control the impurity phases of the NFP and NFPO phases to a small amount, resulting in high phase purity of the cathode material, which can even reach 100%, thus improving the specific capacity and rate performance of the battery.

[0041] The fourth aspect of this application provides a method for preparing the above-mentioned sodium iron pyrophosphate cathode material, the technical solution of which is as follows: A method for preparing a sodium iron pyrophosphate cathode material includes the following steps: A mixed slurry is obtained by mixing sodium iron pyrophosphate precursor, phosphorus source, sodium source, carbon source, and a second solvent; wherein, the sodium iron pyrophosphate precursor is as described above, and the amount of sodium iron pyrophosphate precursor and the phosphorus source are adjusted to make the iron-phosphorus ratio (2.84~2.96):4, and the carbon source includes organic carboxylic acid carbon source and sugar carbon source. The mixed slurry is milled, dried, and then sintered to prepare the sodium iron pyrophosphate cathode material.

[0042] In this application, based on the above-mentioned sodium iron pyrophosphate precursor containing 8-30% pyrophosphate, the amount of sodium iron pyrophosphate precursor and phosphorus source in the mixed slurry is adjusted to make the iron-phosphorus ratio (2.84-2.96):4 (i.e. 0.71-0.74). This can suppress the formation of the NFPO phase while suppressing the NFP phase, thereby improving the phase purity of the cathode material. At the same time, the selection of organic carboxylic acid carbon source and sugar carbon source as composite carbon source to construct uniform carbon coating is beneficial to reduce the reduction reaction between carbon and pyrophosphate, avoid local pyrophosphate consumption and phosphate enrichment, and also improve the phase purity of the cathode material, thereby improving the battery specific capacity and rate performance.

[0043] Optionally, the phosphorus source includes at least one of phosphoric acid and a phosphate salt, wherein the phosphate salt includes at least one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, and trisodium phosphate. Optionally, the sodium source is selected from at least one of sodium carbonate, sodium hydroxide, sodium citrate, sodium oxalate, and sodium nitrate; optionally, the molar ratio of sodium in the sodium source to iron in the sodium ferric pyrophosphate precursor is 4:(2.84~2.96).

[0044] Optionally, the organic carboxylic acid carbon source is selected from at least one of citric acid, sodium citrate, oxalic acid, sodium oxalate, and maleic acid; the sugar carbon source is selected from one or more combinations of glucose, sucrose, starch, and fructose. Optionally, the mass ratio of carbon in the organic carboxylic acid carbon source and the sugar carbon source is (0.1~2):1. Optionally, the carbon coating layer accounts for 1.3~2% of the mass of the sodium iron pyrophosphate cathode material.

[0045] Optionally, the second solvent includes at least one of water, methanol, ethanol, and ethylene glycol.

[0046] Optionally, the second sintering includes the following procedure: sintering at 300~350℃ for 2~5 hours, and then sintering at 500~700℃ for 8~15 hours.

[0047] The fifth aspect of this application provides a battery comprising the sodium iron pyrophosphate cathode material as described above.

[0048] The following description is further illustrated with specific embodiments and comparative examples. Unless otherwise specified, the raw materials involved in the following specific embodiments and comparative examples are all commercially available. Unless otherwise specified, the instruments used are all commercially available. Unless otherwise specified, the processes involved are conventionally selected by those skilled in the art.

[0049] Example 1 This embodiment provides a sodium iron pyrophosphate precursor and its preparation method, as well as a sodium iron pyrophosphate cathode material and its preparation method. The steps are as follows: (1) Ferrous chloride was dissolved in pure water to prepare an iron source solution, and hydrochloric acid was added dropwise to adjust the pH to 2.5. Hydrogen peroxide was dissolved in pure water to prepare an oxidant solution. Diammonium hydrogen phosphate was dissolved in pure water to prepare a phosphorus source solution, and ammonia was added dropwise to adjust the pH to 6.7. The oxidant solution and phosphorus source solution were slowly added to the iron source solution at 60℃ to obtain a mixed system, wherein the concentration of oxidant was 0.13 mol / L, the concentration of ferrous ions was 0.24 mol / L, and the concentration of phosphate ions was 0.27 mol / L. After stirring to induce a co-precipitation reaction, the mixture was centrifuged, washed, and dried to obtain a precipitate with a pH of 3.5.

[0050] (2) The precipitate was sintered in a muffle furnace at 350°C for 3 h to obtain sodium iron pyrophosphate precursor. The pyrophosphate content was measured to be 19.5%, the phosphate content was 49.1%, the iron content was 29%, the water content was 0.4%, and the impurity content was 2%.

[0051] (3) The ingredients were prepared according to the following formula: Na, Fe and P molar ratio of 4:2.9:4, carbon content of 1.7%, and C element mass ratio of sodium citrate to glucose of 2:1. Pure water, sodium iron pyrophosphate precursor, ammonium dihydrogen phosphate, sodium carbonate, sodium citrate and glucose were added to a mixing tank and mixed to obtain a mixed slurry with a solid content of 30wt%. The mixed slurry was first coarsely ground for 30min and then finely ground for 2h to pulverize to a particle size Dv50 of 0.3μm. The pulverized mixed slurry was spray-dried with an inlet air temperature of 200℃ and an outlet air temperature of 95℃ to obtain powder. The powder was sintered in stages. First, the temperature was increased to 350℃ at a heating rate of 3℃ / min and held for 2h. Then, the temperature was increased to 580℃ at a heating rate of 3℃ / min and held for 10h to obtain carbon-coated Na. 4.02 Fe 2.99 (PO4)2P2O7 polyanionic cathode material.

[0052] Example 2 This embodiment provides a sodium iron pyrophosphate precursor and its preparation method, as well as a sodium iron pyrophosphate cathode material and its preparation method, which are basically the same as those in Example 1, with the main difference being the pyrophosphate content in the sodium iron pyrophosphate precursor. The specific steps are as follows: (1) Ferrous chloride was dissolved in pure water to prepare an iron source solution, and hydrochloric acid was added dropwise to adjust the pH to 2.5. Hydrogen peroxide was dissolved in pure water to prepare an oxidant solution. Diammonium hydrogen phosphate was dissolved in pure water to prepare a phosphorus source solution, and ammonia was added dropwise to adjust the pH to 7.2. The oxidant solution and phosphorus source solution were slowly added to the iron source solution at 60℃ to obtain a mixed system, wherein the concentration of oxidant was 0.15 mol / L, the concentration of ferrous ions was 0.25 mol / L, and the concentration of phosphate ions was 0.29 mol / L. After stirring to induce a co-precipitation reaction, the mixture was centrifuged, washed, and dried to obtain a precipitate with a pH of 3.7.

[0053] (2) The precipitate was sintered in a muffle furnace at 350°C for 2 hours to obtain sodium iron pyrophosphate precursor. The pyrophosphate content was measured to be 12.4%, the phosphate content was 54.8%, the iron content was 30%, the water content was 0.8%, and the impurity content was 2%.

[0054] (3) Same as in Example 1, carbon-coated Na was obtained. 3.98 Fe 2.98 (PO4) 1.99 (P2O7) 0.99 Polyanionic cathode material.

[0055] Example 3 This embodiment provides a sodium iron pyrophosphate precursor and its preparation method, as well as a sodium iron pyrophosphate cathode material and its preparation method, which are basically the same as those in Example 1, with the main difference being the pyrophosphate content in the sodium iron pyrophosphate precursor. The specific steps are as follows: (1) Ferrous chloride was dissolved in pure water to prepare an iron source solution, and hydrochloric acid was added dropwise to adjust the pH to 2.5. Hydrogen peroxide was dissolved in pure water to prepare an oxidant solution. Diammonium hydrogen phosphate was dissolved in pure water to prepare a phosphorus source solution, and ammonia was added dropwise to adjust the pH to 6.2. The oxidant solution and phosphorus source solution were slowly added to the iron source solution at 60℃ to obtain a mixed system, wherein the concentration of oxidant was 0.12 mol / L, the concentration of ferrous ions was 0.22 mol / L, and the concentration of phosphate ions was 0.25 mol / L. After stirring to induce a co-precipitation reaction, the mixture was centrifuged, washed, and dried to obtain a precipitate with a pH of 3.2.

[0056] (2) The precipitate was sintered in a muffle furnace at 350°C for 4 hours to obtain sodium iron pyrophosphate precursor. The pyrophosphate content was measured to be 25.6%, the phosphate content was 43.9%, the iron content was 28%, the water content was 0.5%, and the impurity content was 2%.

[0057] (3) Same as in Example 1, carbon-coated Na was obtained. 3.98 Fe 2.98 (PO4) 1.98 P2O7 polyanionic cathode material.

[0058] Example 4 This embodiment provides a sodium iron pyrophosphate precursor and its preparation method, as well as a sodium iron pyrophosphate cathode material and its preparation method, which are basically the same as those in Example 1, with the main difference being the different molar ratios of Na, Fe, and P. The specific steps are as follows: (1) Same as step (1) in Example 1.

[0059] (2) Same as step (2) in Example 1.

[0060] (3) The ingredients were prepared according to the following formula: Na, Fe and P molar ratio of 4:2.82:4, carbon content of 1.7%, and C element mass ratio of sodium citrate to glucose of 2:1. Pure water, sodium iron pyrophosphate precursor, ammonium dihydrogen phosphate, sodium carbonate, sodium citrate and glucose were added to a mixing tank and mixed to obtain a mixed slurry with a solid content of 30wt%. The mixed slurry was first coarsely ground for 30min and then finely ground for 2h to pulverize to a particle size Dv50 of 0.3μm. The pulverized mixed slurry was spray-dried with an inlet air temperature of 200℃ and an outlet air temperature of 95℃ to obtain powder. The powder was sintered in stages. First, the temperature was increased to 350℃ at a heating rate of 3℃ / min and held for 2h. Then, the temperature was increased to 580℃ at a heating rate of 3℃ / min and held for 10h to obtain carbon-coated Na. 3.84 Fe 2.84 (PO4) 1.84 P2O7 polyanionic cathode material.

[0061] Example 5 This embodiment provides a sodium iron pyrophosphate precursor and its preparation method, as well as a sodium iron pyrophosphate cathode material and its preparation method, which are basically the same as those in Example 1, with the main difference being the different molar ratios of Na, Fe, and P. The specific steps are as follows: (1) Same as step (1) in Example 1.

[0062] (2) Same as step (2) in Example 1.

[0063] (3) The ingredients were prepared according to the following formula: Na, Fe and P molar ratio of 4:3:4, carbon content of 1.7%, and C element mass ratio of sodium citrate to glucose of 2:1. Pure water, sodium iron phosphate pyrophosphate precursor, ammonium dihydrogen phosphate, sodium carbonate, sodium citrate and glucose were added to a mixing tank and mixed to obtain a mixed slurry with a solid content of 30wt%. The mixed slurry was first coarsely ground for 30min and then finely ground for 2h to pulverize to a particle size Dv50 of 0.3μm. The pulverized mixed slurry was spray-dried with an inlet air temperature of 200℃ and an outlet air temperature of 95℃ to obtain powder. The powder was sintered in stages. First, the temperature was increased to 350℃ at a heating rate of 3℃ / min and held for 2h. Then, the temperature was increased to 580℃ at a heating rate of 3℃ / min and held for 10h to obtain carbon-coated Na. 3.66 Fe 2.77 (PO4) 1.89 (P2O7) 0.89 Polyanionic cathode material.

[0064] Example 6 This embodiment provides a sodium iron pyrophosphate precursor and its preparation method, as well as a sodium iron pyrophosphate cathode material and its preparation method, which are basically the same as those in Example 1, with the main difference being that a sugar is used alone as the carbon source. The specific steps are as follows: (1) Same as step (1) in Example 1.

[0065] (2) Same as step (2) in Example 1.

[0066] (3) The ingredients were prepared according to the molar ratio of Na, Fe and P of 4:2.9:4, with a carbon content of 1.7%. Pure water, sodium iron pyrophosphate precursor, ammonium dihydrogen phosphate, sodium carbonate and glucose were added to a mixing tank and mixed to obtain a mixed slurry with a solid content of 30wt%. The mixed slurry was first coarsely ground for 30min and then finely ground for 2h to pulverize to a particle size Dv50 of 0.3μm. The pulverized mixed slurry was spray-dried with an inlet air temperature of 200℃ and an outlet air temperature of 95℃ to obtain powder. The powder was sintered in stages. First, the temperature was increased to 350℃ at a heating rate of 3℃ / min and held for 2h. Then, the temperature was increased to 580℃ at a heating rate of 3℃ / min and held for 10h to obtain carbon-coated Na. 3.74 Fe 2.88 (PO4) 1.91 (P2O7) 0.91 Polyanionic cathode material.

[0067] Example 7 This embodiment provides a sodium iron pyrophosphate precursor and its preparation method, as well as a sodium iron pyrophosphate cathode material and its preparation method, which are basically the same as those in Example 1, with the main difference being that an organic carboxylic acid type carbon source is used alone. The specific steps are as follows: (1) Same as step (1) in Example 1.

[0068] (2) Same as step (2) in Example 1.

[0069] (3) The ingredients were prepared according to the molar ratio of Na, Fe and P of 4:2.9:4, with a carbon content of 1.7%. Pure water, sodium iron pyrophosphate precursor, ammonium dihydrogen phosphate, sodium carbonate and sodium citrate were added to a mixing tank and mixed to obtain a mixed slurry with a solid content of 30wt%. The mixed slurry was first coarsely ground for 30min and then finely ground for 2h to pulverize to a particle size Dv50 of 0.3μm. The pulverized mixed slurry was then spray-dried with an inlet air temperature of 200℃ and an outlet air temperature of 95℃ to obtain powder. The powder was then sintered in stages. First, the temperature was increased to 350℃ at a heating rate of 3℃ / min and held for 2h. Then, the temperature was increased to 580℃ at a heating rate of 3℃ / min and held for 10h to obtain carbon-coated Na. 3.99 Fe 3.03 (PO4) 1.98 P2O7 polyanionic cathode material.

[0070] Comparative Example 1 This comparative example provides a sodium iron pyrophosphate precursor and its preparation method, as well as a sodium iron pyrophosphate cathode material and its preparation method, which are basically the same as those in Example 1, with the main difference being the pyrophosphate content in the sodium iron pyrophosphate precursor. The specific steps are as follows: (1) Ferrous chloride was dissolved in pure water to prepare an iron source solution, and hydrochloric acid was added dropwise to adjust the pH to 2.5. Hydrogen peroxide was dissolved in pure water to prepare an oxidant solution. Diammonium hydrogen phosphate was dissolved in pure water to prepare a phosphorus source solution, and ammonia was added dropwise to adjust the pH to 8.0. The oxidant solution and phosphorus source solution were slowly added to the iron source solution at 60℃ to obtain a mixed system, wherein the concentration of oxidant was 0.13 mol / L, the concentration of ferrous ions was 0.24 mol / L, and the concentration of phosphate ions was 0.27 mol / L. After stirring to induce a co-precipitation reaction, the mixture was centrifuged, washed, and dried to obtain a precipitate with a pH of 5.5.

[0071] (2) The precipitate was sintered in a muffle furnace at 350°C for 0.5 h to obtain sodium iron pyrophosphate precursor. The pyrophosphate content was measured to be 6%, the phosphate content was 57.2%, the iron content was 34%, the water content was 0.8%, and the impurity content was 2%.

[0072] (3) Same as in Example 1, carbon-coated Na was obtained. 3.81 Fe 2.88 (PO4) 1.94 (P2O7) 0.94 Polyanionic cathode material.

[0073] Comparative Example 2 This comparative example provides a sodium iron pyrophosphate precursor and its preparation method, as well as a sodium iron pyrophosphate cathode material and its preparation method, which are basically the same as those in Example 1, except that the sodium iron pyrophosphate precursor does not contain pyrophosphate ions. The specific steps are as follows: (1) Same as step (1) in Example 1.

[0074] (2) Using the precipitate as a precursor for sodium iron pyrophosphate, carbon-coated Na was prepared according to step (3) of Example 1. 3.6 Fe 2.73 (PO4) 1.87 (P2O7) 0.87 Polyanionic cathode material.

[0075] Comparative Example 3 This comparative example provides a sodium iron pyrophosphate precursor and its preparation method, as well as a sodium iron pyrophosphate cathode material and its preparation method, which are basically the same as those in Example 1, with the main difference being the pyrophosphate content in the sodium iron pyrophosphate precursor. The specific steps are as follows: (1) Ferrous chloride was dissolved in pure water to prepare an iron source solution, and hydrochloric acid was added dropwise to adjust the pH to 2.5. Hydrogen peroxide was dissolved in pure water to prepare an oxidant solution. Diammonium hydrogen phosphate was dissolved in pure water to prepare a phosphorus source solution, and ammonia was added dropwise to adjust the pH to 5.8. The oxidant solution and phosphorus source solution were slowly added to the iron source solution at 60℃ to obtain a mixed system, wherein the concentration of oxidant was 0.13 mol / L, the concentration of ferrous ions was 0.24 mol / L, and the concentration of phosphate ions was 0.27 mol / L. After stirring to induce a co-precipitation reaction, the mixture was centrifuged, washed, and dried to obtain a precipitate with a pH of 2.9.

[0076] (2) The precipitate was sintered in a muffle furnace at 350°C for 6 hours to obtain sodium iron pyrophosphate precursor. The pyrophosphate content was measured to be 33.0%, the phosphate content was 33.7%, the iron content was 30.5%, the water content was 0.8%, and the impurity content was 2%.

[0077] (3) Same as step (3) in Example 1, to obtain carbon-coated Na 3.65 Fe 2.65 (PO4) 1.65 P2O7 polyanionic cathode material.

[0078] test: The contents of NFP, NFPO, and NFPP phases in the cathode materials of the above embodiments and comparative examples were tested, and the results are shown in Table 1.

[0079] Assembling coin cells: In a dry environment with relative humidity less than 10%, the polyanionic cathode material, conductive agent Super P, and binder PVDF of each embodiment and comparative example are dispersed in N2 at a mass ratio of 85:10:5. A positive electrode slurry was prepared by stirring in methylpyrrolidone (NMP) until homogeneous, with a solid content controlled at 50 wt%. The slurry was uniformly coated onto an aluminum foil current collector and dried in a vacuum oven at 110 °C for 4 h. Subsequently, it was rolled and stamped to obtain a polyanion positive electrode sheet. A coin cell was assembled in an argon-filled glove box, using a sodium metal sheet as the counter electrode, a PE membrane as the separator, and a mixed solution of ethylene carbonate / diethyl carbonate (EC / DEC, volume ratio 1:1) containing 1 mol / L NaClO4 as the electrolyte.

[0080] Charge to 4V at 0.2C (vs. Na / Na) +(The same applies below), discharging to 2V at 0.2C, charging to 4V at 0.5C, and discharging to 2V at 0.5C. The formula for calculating the charging specific capacity is: ,in This refers to the specific charging capacity (mAh / g). The charging current (mA) is the charging current. Charging time (h), The value represents the mass (g) of the active material. The formula for calculating the discharge specific capacity is: ,in Discharge specific capacity (mAh / g), The discharge current (mA) is the discharge current. Discharge time (h), The value represents the mass (g) of the active substance. See Table 1 for the results.

[0081] Table 1

[0082] As can be seen, in each embodiment, a sodium iron pyrophosphate precursor with a pyrophosphate content of 8-30% was used, resulting in fewer NFP and NFPO impurities in the prepared sodium iron pyrophosphate cathode material. Furthermore, controlling the iron-phosphorus ratio of the precursor to the phosphorus source while using the sodium iron pyrophosphate precursor can also reduce NFP and NFPO impurities, resulting in high phase purity of the NFPP material, even reaching 100%, which is beneficial for improving the specific capacity and rate performance of the battery.

[0083] Based on the descriptions of Examples 1 and Comparative Examples 1-3, it can be seen that pre-sintering introduces an appropriate amount of pyrophosphate ions in situ into the precursor, inhibiting the combination of iron ions, sodium ions, and phosphate ions to form the NFP impurity phase, thereby improving the specific capacity of the material; however, when the amount of pyrophosphate ions introduced is greater than 30%, some P2O7... 4- The phase does not readily depolymerize into PO4 at high temperatures. 3- This generates more NFPO phase, reducing the specific energy of the material.

[0084] As can be seen from the descriptions in Examples 1, 4, and 5, when the iron-to-phosphorus ratio is too low, NFPO impurity phase is easily generated. This is because when the iron-to-phosphorus ratio is reduced (i.e., P is excessive), the Fe and P content of the system is more matched to the 1:2 ratio of NFPO, and deviates from the 3:4 ratio of NFPP and the 1:1 ratio of NFP, thus inhibiting the nucleation driving force of the latter two phases from the source. When the iron-to-phosphorus ratio is too high, NFP impurity phase is easily generated, which reduces the specific capacity and rate of the material. This is because the system with excess Fe is more matched to the 1:1 ratio of NFP, and insufficient P content also inhibits phosphate polymerization.

[0085] As can be seen from the descriptions in Examples 1, 6, and 7, when sugars are used alone as a carbon source, carbon cannot achieve uniform coating, and an NFP phase is generated after sintering, resulting in a decrease in the specific capacity of the material. This is because the local carbon coating is relatively thick, and the excess carbon undergoes a reduction reaction with pyrophosphate, promoting the consumption of local pyrophosphate and the enrichment of phosphate. However, the added organic carboxylic acid carbon source can complex with metal ions and adsorb on the surface of primary particles to form a uniform 3D conductive network, resulting in uniform carbon coating. However, when organic carboxylic acid is used alone as a carbon source, it is not easy to form a carbon layer with a high degree of graphitization, which reduces conductivity and leads to a decrease in capacity and rate capability.

[0086] 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.

[0087] The embodiments described above are merely illustrative of several implementation methods of this application, 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 this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this patent application should be determined by the appended claims.

Claims

1. A sodium iron pyrophosphate precursor, characterized in that, It includes iron phosphate, iron pyrophosphate, impurity phase and water of crystallization. When the mass ratio of water of crystallization to the sodium iron pyrophosphate precursor is not more than 1%, the mass ratio of pyrophosphate in the sodium iron pyrophosphate precursor is 8-30%, and the mass ratio of the impurity phase to the sodium iron pyrophosphate precursor is not more than 4%.

2. The sodium iron pyrophosphate precursor according to claim 1, characterized in that, When the mass ratio of water of crystallization to the sodium iron pyrophosphate precursor is no more than 1%, the mass ratio of phosphate in the sodium iron pyrophosphate precursor is 30-58%, and the mass ratio of iron in the sodium iron pyrophosphate precursor is 26-35%.

3. The sodium iron pyrophosphate precursor according to claim 1, characterized in that, Includes at least one of the following features: (1) The iron phosphate and iron pyrophosphate are randomly dispersed inside and on the surface of the sodium iron pyrophosphate precursor; (2) The impurity phase is randomly dispersed inside the sodium iron pyrophosphate precursor; (3) The impurity phase includes at least one of sulfate, chloride, nitrate, oxalate, sodium, ammonium, hydrogen and hydroxide.

4. A method for preparing the sodium iron pyrophosphate precursor according to any one of claims 1 to 3, characterized in that, Includes the following steps: A mixed oxidant, iron source, phosphorus source, pH adjuster and first solvent are mixed to cause a co-precipitation reaction in the resulting mixed system to obtain a precipitate. The amount of pH adjuster is adjusted so that the pH value of the precipitate is 1.5 to 5, preferably 2 to 4. The precipitate was sintered first to obtain the sodium iron pyrophosphate precursor.

5. The method for preparing the sodium iron pyrophosphate precursor according to claim 4, characterized in that, Includes at least one of the following features: (1) The oxidant is selected from at least one of hydrogen peroxide, sodium chlorate, sodium nitrate and nitric acid; optionally, the concentration of the oxidant in the mixed system is 0.05~1 mol / L; preferably, the concentration is 0.1~0.5 mol / L; (2) The iron source includes iron salts, which include at least one of ferrous salts and ferric salts; the ferrous salt is selected from at least one of ferrous chloride, ferrous sulfate and ferrous oxalate; the ferric salt is selected from at least one of ferric chloride, ferric nitrate, ferric sulfate and ferric oxalate; optionally, the concentration of iron ions in the mixed system is 0.1~1.0 mol / L; preferably, the concentration is 0.2~0.6 mol / L; (3) The phosphorus source includes at least one of phosphoric acid and phosphate salt, wherein the phosphate salt includes at least one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate and trisodium phosphate; optionally, the concentration of phosphate ions in the mixed system is 0.1~1.5 mol / L; preferably, the concentration is 0.2~0.8 mol / L; (4) The pH adjuster is selected from at least one of hydrochloric acid, sulfuric acid, citric acid, oxalic acid, ammonia and sodium hydroxide; (5) The first solvent includes water; (6) The temperature of the coprecipitation reaction is 20~80℃; preferably, the temperature of the coprecipitation reaction is 35~65℃.

6. The method for preparing the sodium iron pyrophosphate precursor according to any one of claims 4 to 5, characterized in that, Includes at least one of the following features: (1) The atmosphere for the first sintering is selected from at least one of nitrogen, argon and air; (2) The temperature of the first sintering is 200~550℃, preferably 250~400℃; (3) The first sintering time is 1~5h, preferably 2~4h.

7. A sodium iron pyrophosphate cathode material, characterized in that, It includes a sodium iron pyrophosphate matrix and a carbon coating layer covering the sodium iron pyrophosphate matrix, wherein the chemical formula of the sodium iron pyrophosphate matrix is ​​Na. x Fe y (PO4) z (P2O7) w Wherein, 3.6≤x≤4.2, 2.7≤y≤3.15, 1.8≤z≤2.1, 0.9≤w≤1.05; the mass percentage of the sodium iron phosphate phase in the sodium iron pyrophosphate cathode material is 0~1%, the mass percentage of the sodium iron pyrophosphate phase in the sodium iron phosphate cathode material is 98~100%.

8. A method for preparing the sodium iron pyrophosphate cathode material according to claim 7, characterized in that, A mixed slurry is obtained by mixing sodium iron pyrophosphate precursor, phosphorus source, sodium source, carbon source, and a second solvent; wherein the sodium iron pyrophosphate precursor is as described in any one of claims 1 to 3, and the amount of the sodium iron pyrophosphate precursor and the phosphorus source is adjusted to make the iron-phosphorus ratio (2.84~2.96):4, and the carbon source includes organic carboxylic acid type carbon source and sugar carbon source; The mixed slurry is milled, dried, and then sintered to prepare the sodium iron pyrophosphate cathode material.

9. The method for preparing the sodium iron pyrophosphate cathode material according to claim 8, characterized in that, Includes at least one of the following features: (1) The phosphorus source includes at least one of phosphoric acid and phosphate salt, wherein the phosphate salt includes at least one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate and trisodium phosphate; (2) The sodium source is selected from at least one of sodium carbonate, sodium hydroxide, sodium citrate, sodium oxalate and sodium nitrate; optionally, the molar ratio of sodium in the sodium source to iron in the sodium iron pyrophosphate precursor is 4:(2.84~2.96). (3) The organic carboxylic acid carbon source is selected from at least one of citric acid, sodium citrate, oxalic acid, sodium oxalate, and maleic acid; the sugar carbon source is selected from one or more combinations of glucose, sucrose, starch, and fructose; optionally, the mass ratio of carbon elements in the organic carboxylic acid carbon source and the sugar carbon source is (0.1~2):1; optionally, the mass percentage of the carbon coating layer in the sodium iron pyrophosphate cathode material is 1.3~2%; (4) The second solvent includes at least one of water, methanol, ethanol and ethylene glycol; (5) The second sintering includes the following procedure: sintering at 300~350℃ for 2~5h, and then sintering at 500~700℃ for 8~15h.

10. A battery, characterized in that, Including the sodium iron pyrophosphate cathode material as described in claim 7.

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