A method for preparing sodium iron pyrophosphate material

Abstract

This invention relates to a method for preparing sodium iron pyrophosphate material. The method includes the following steps: extracting wet-process phosphoric acid with a choline chloride-organic acid eutectic solvent to remove impurities, separating the organic phase to obtain purified phosphoric acid; mixing an alkaline sodium source with the purified phosphoric acid to adjust the pH value, obtaining a sodium-phosphorus solution; milling and mixing an iron source, a carbon source, and the sodium-phosphorus solution to obtain a mixture; and spray-drying and sintering the mixture to obtain sodium iron pyrophosphate material. The preparation method provided by this invention achieves complete self-sufficiency in phosphorus sources, significantly reduces production costs, and significantly improves the electrochemical performance and batch consistency of the material. It effectively solves the core problems of existing technologies, such as reliance on externally purchased raw materials, poor process adaptability, and limited performance optimization, providing a complete technical path for the low-cost, large-scale, and green preparation of sodium-ion battery cathode materials.

Description

Technical Field

[0001] This invention belongs to the field of battery materials technology and relates to a method for preparing sodium iron pyrophosphate material. Background Technology

[0002] Sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7, abbreviated as NFPP) has become a research hotspot in the field of large-scale energy storage in recent years due to its comprehensive advantages, including excellent cycle performance, high thermal stability, stable operating voltage platform, and environmental friendliness. With the accelerated industrialization of sodium-ion batteries, reducing the production cost of cathode materials has become a key issue for the industry. Phosphorus source costs account for a large proportion of the raw material costs of iron phosphate-based cathode materials, and iron source costs also account for a significant share. Therefore, using inexpensive industrial waste acid to replace high-purity chemical raw materials has become an important technological breakthrough.

[0003] Currently, traditional methods for wet-process phosphoric acid purification mainly include chemical precipitation, solvent extraction, and ion exchange. Chemical precipitation separates impurity ions by adding a precipitant to form insoluble substances, but it suffers from poor selectivity and large amounts of waste. Solvent extraction separates phosphoric acid and impurities by utilizing the difference in distribution between the organic and aqueous phases, but it involves the use of organic solvents, posing risks of volatilization loss and environmental pollution. Ion exchange can achieve deep purification, but it has high processing costs and requires frequent resin regeneration. Eutectic solvents, as a new type of green solvent, exhibit unique advantages in metal extraction and separation due to their extremely low vapor pressure, designable structure, and biodegradability, but they have not yet been systematically applied to the integrated process of wet-process phosphoric acid purification and NFPP preparation.

[0004] A comprehensive analysis of existing sodium iron pyrophosphate (NFPP) preparation technologies reveals the following shortcomings and pressing issues: Current NFPP preparation technologies largely rely on purchased industrial-grade high-purity phosphorus sources, which are significantly more expensive than wet-process phosphoric acid, resulting in high overall material costs. While some attempts have attempted to utilize iron-containing waste acid as a substitute for the iron source, the phosphorus source remains entirely dependent on external purchases, failing to achieve self-sufficiency in phosphorus resources. Furthermore, the purchase of raw materials involves drying, packaging, transportation, and redissolving processes, increasing energy consumption and costs. Simultaneously, the iron source process parameters in existing technologies are relatively fixed, failing to consider the varying optimal reaction conditions resulting from differences in crystal structure, specific surface area, and surface functional groups among different iron sources. This makes it difficult to ensure that all iron sources reach their optimal reaction states, leading to low iron source conversion rates and reaction... Insufficient processing and poor product consistency negatively impact the electrochemical performance of NFPP materials. Furthermore, current technologies typically react purified phosphoric acid directly with a sodium source to generate a single sodium phosphate salt, failing to construct a multi-component compound system with flexible process parameters. When process conditions need to be adjusted or different iron sources need to be adapted, it is often necessary to completely change the raw material ratio or supplement the phosphorus and sodium sources, resulting in poor process flexibility and low raw material utilization. In terms of wet phosphoric acid purification, traditional methods suffer from poor impurity removal selectivity, large waste discharge, and organic solvent volatilization pollution. Although eutectic solvents have shown potential in the field of metal recovery, they have not yet been systematically applied to the integrated process of wet phosphoric acid purification and NFPP preparation, lacking a complete integrated design from waste acid purification to material synthesis.

[0005] To address the aforementioned issues, there is an urgent need to develop a green preparation method for NFPP that enables self-sufficiency of phosphorus source, adaptive matching of iron source activity, and flexible process control. This method would be achieved by constructing a green and efficient waste acid purification system, establishing a multi-component compounding strategy with hierarchical control, and achieving precise matching of iron source reaction conditions. This would solve the core problems of high raw material costs, poor process adaptability, and limited performance optimization in existing technologies, providing a new technical path for the low-cost, large-scale, and green preparation of sodium-ion battery cathode materials. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the present invention aims to provide a method for preparing sodium iron pyrophosphate material. This method enables complete self-sufficiency in phosphorus sources, significantly reduces production costs, and substantially improves the electrochemical performance and batch consistency of the material. It effectively solves the core problems of existing technologies, such as reliance on externally purchased raw materials, poor process adaptability, and limited performance optimization, providing a complete technical path for the low-cost, large-scale, and green preparation of sodium-ion battery cathode materials.

[0007] To achieve this objective, the present invention adopts the following technical solution:

[0008] This invention provides a method for preparing sodium iron pyrophosphate material, the method comprising the following steps:

[0009] S1. The wet-process phosphoric acid is extracted and purified by using a choline chloride-organic acid eutectic solvent to separate the organic phase and obtain purified phosphoric acid.

[0010] The choline chloride-organic acid eutectic solvent comprises choline chloride and organic acid;

[0011] S2. Mix the alkaline sodium source with the purified phosphoric acid to adjust the pH value, and obtain a sodium-phosphorus solution;

[0012] S3. The iron source, carbon source and sodium phosphorus solution are mixed by sand milling to obtain a mixture; the mixture is then spray-dried and sintered to obtain sodium iron pyrophosphate material.

[0013] The preparation method provided by this invention uses a choline chloride-organic acid eutectic solvent (DES), a mixture of choline chloride and organic acid, for efficient purification of wet-process phosphoric acid. This DES can be regenerated and recycled, significantly reducing purification costs. The purified phosphoric acid and an alkaline sodium source are neutralized to adjust the pH, achieving a compound system of phosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, and an alkaline sodium source. The molar ratio of the four substances can be flexibly adjusted by controlling the pH, enabling complete self-sufficiency of different phosphorus sources. All phosphorus sources are derived from wet-process phosphoric acid, eliminating the need to purchase any industrial raw materials, effectively reducing production costs. Therefore, the preparation method provided by this invention achieves complete self-sufficiency of phosphorus sources, significantly reduces production costs, and significantly improves the electrochemical performance and batch consistency of the materials. It effectively solves the core problems of existing technologies, such as reliance on purchased raw materials, poor process adaptability, and limited performance optimization, providing a complete technical path for the low-cost, large-scale, and green preparation of sodium-ion battery cathode materials.

[0014] In some embodiments, the molar ratio of choline chloride to organic acid in the eutectic solvent is 1:0.5 to 1:3.

[0015] In some embodiments, the organic acid includes any one or a combination of at least two of oxalic acid, citric acid, malic acid, or tartaric acid.

[0016] In some embodiments, the mass concentration of P2O5 in the wet-process phosphoric acid is 20wt% to 40wt%.

[0017] In some embodiments, the volume ratio of the choline chloride-organic acid eutectic solvent to the wet-process phosphoric acid is 1:1 to 1:5.

[0018] In some embodiments, the extraction and purification temperature is 20°C to 60°C.

[0019] In some embodiments, the extraction and impurity removal time is 10 min to 60 min.

[0020] In some embodiments, the alkaline sodium source includes any one or a combination of at least two of sodium carbonate, sodium hydroxide, sodium bicarbonate, sodium phosphate, sodium acetate, sodium oxalate, or sodium pyrophosphate.

[0021] In some embodiments, the pH value of the sodium-phosphorus solution is 0 to 12.

[0022] In some embodiments, the iron source includes any one or a combination of at least two of iron oxide, iron(II,III) oxide, iron(III) hydroxyl oxide, iron(II) oxalate, iron(II) phosphate, or iron(II) acetate.

[0023] In some embodiments, the pH value of the sand-milled mixture is 0 to 12, preferably 1 to 6.

[0024] In some embodiments, the carbon source includes inorganic carbon compounds and / or organic carbon compounds;

[0025] The inorganic carbon-containing compound includes any one or a combination of at least two of carbon black, carbon fiber, graphene, graphite or carbon nanotubes.

[0026] The organic carbon-containing compound includes any one or a combination of at least two of glucose, sucrose, isopropanol, starch, citric acid, maltose, formic acid, acetic acid, oxalic acid, or polyethylene glycol.

[0027] In some embodiments, the amount of carbon source used is 5 wt% to 15 wt% of the mixture.

[0028] In some embodiments, the spray temperature of the spray drying is 100°C to 260°C.

[0029] In some embodiments, the sintering is performed in a protective atmosphere;

[0030] The protective atmosphere uses gases including nitrogen and / or argon.

[0031] In some embodiments, the sintering temperature is 400°C to 700°C.

[0032] In some embodiments, the sintering time is 4h to 15h.

[0033] The numerical range described in this invention includes not only the point values ​​listed above, but also any point values ​​within the numerical ranges not listed above. Due to space limitations and for the sake of brevity, this invention will not exhaustively list all the specific point values ​​included in the range.

[0034] Compared with the prior art, the present invention has the following beneficial effects:

[0035] The preparation method provided by this invention uses a choline chloride-organic acid eutectic solvent (DES), a mixture of choline chloride and organic acid, for efficient purification of wet-process phosphoric acid. This DES can be regenerated and recycled, significantly reducing purification costs. The purified phosphoric acid and an alkaline sodium source are neutralized to adjust the pH, achieving a compound system of phosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, and an alkaline sodium source. The molar ratio of the four substances can be flexibly adjusted by controlling the pH, enabling complete self-sufficiency of different phosphorus sources. All phosphorus sources are derived from wet-process phosphoric acid, eliminating the need to purchase any industrial raw materials, effectively reducing production costs. Therefore, the preparation method provided by this invention achieves complete self-sufficiency of phosphorus sources, significantly reduces production costs, and significantly improves the electrochemical performance and batch consistency of the materials. It effectively solves the core problems of existing technologies, such as reliance on purchased raw materials, poor process adaptability, and limited performance optimization, providing a complete technical path for the low-cost, large-scale, and green preparation of sodium-ion battery cathode materials. Detailed Implementation

[0036] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0037] The "range" disclosed in this invention can be defined in the form of a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of the specific range. This type of range definition can include or exclude endpoints; any endpoint can be independently included or excluded, and they can be arbitrarily combined, meaning any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60~120 and 80~110 are listed for specific parameters, it is understood that ranges of 60~110 and 80~120 are also expected. Furthermore, if minimum range values ​​1 and 2 are listed, and maximum range values ​​3, 4, and 5 are also listed, then the following ranges are all expected: 1~3, 1~4, 1~5, 2~3, 2~4, and 2~5. In this invention, unless otherwise stated, the numerical range "a~b" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0~5" indicates that all real numbers between "0" and "5" have been listed in this article; "0~5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is described as an integer ≥2, it is equivalent to listing integers such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc. For instance, when a parameter is described as an integer selected from "2~10", it is equivalent to listing the integers 2, 3, 4, 5, 6, 7, 8, 9, and 10.

[0038] In this invention, "a combination of at least two" refers to a quantity greater than or equal to two, unless otherwise specified. For example, "any combination of one or at least two" means one or more or more items. It can be understood that when referring to "a combination of at least two," it refers to any suitable combination of multiple items, that is, a combination of "at least two" items carried out in a manner that does not conflict with and enables the implementation of this invention.

[0039] Unless otherwise specified, all embodiments and optional embodiments of the present invention can be combined with each other to form new technical solutions.

[0040] The term "embodiment" as used in this invention means that a specific feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment or implementation of the invention. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a mutually exclusive, independent, or alternative embodiment. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described in this invention can be combined with other embodiments.

[0041] Those skilled in the art will understand that the order in which the steps are written in the methods of the various embodiments does not imply a strict execution order. The detailed execution order of each step should be determined by its function and possible internal logic. Unless otherwise specified, all steps of the present invention may be performed sequentially or randomly, but are preferably performed sequentially. For example, if the method includes steps (a) and (b), it means that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the method may also include step (c), meaning that step (c) can be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0042] In this invention, open-ended technical features or solutions described using terms such as "comprising" do not exclude additional members beyond those listed unless otherwise specified. They can be considered as providing both closed-ended features or solutions comprised of the listed members and open-ended features or solutions that include additional members beyond the listed members. For example, A includes a1, a2, and a3. Unless otherwise specified, it may also include other members or exclude additional members. This can be considered as providing both technical features or solutions where "A is composed of a1, a2, and a3" or "A is selected from a1, a2, and a3," and technical features or solutions where "A includes not only a1, a2, and a3, but also other members."

[0043] In this invention, unless otherwise specified, the features or solutions corresponding to "and / or" include any one of two or more of the related listed items, as well as any and all combinations of the related listed items. These arbitrary and all combinations include any two related listed items, any more related listed items, or a combination of all related listed items. For example, "A and / or B" represents a group consisting of A, B, and "a combination of A and B". "Containing A and / or B" can mean "containing A, containing B, and containing A and B", or "containing A, containing B, or containing A and B", and can be appropriately understood according to the context.

[0044] Existing technologies for preparing sodium iron pyrophosphate (NFPP) from wet-process phosphoric acid suffer from several drawbacks, including reliance on externally sourced raw materials, poor process adaptability, and low resource utilization. Specifically, current technologies generally rely on externally sourced industrial-grade high-purity phosphorus sources, resulting in high costs. They fail to consider the varying optimal reaction conditions due to differences in crystal structure, specific surface area, and surface functional groups among different iron sources, leading to poor product consistency. Furthermore, purified phosphoric acid reacts directly with the sodium source to form a single sodium phosphate salt, lacking a flexible, multi-component compound system, thus limiting process adaptability. In terms of wet-process phosphoric acid purification, traditional methods suffer from poor selectivity in impurity removal, large waste emissions, or organic solvent pollution. Eutectic solvents have not yet been systematically applied in the integrated process for NFPP preparation.

[0045] Specifically, existing technologies generally use high-purity phosphorus sources such as industrial-grade sodium dihydrogen phosphate, which require external purchase and involve drying, packaging, transportation, and redissolving processes, failing to achieve self-sufficiency in phosphorus resources and resulting in high phosphorus source costs. Existing wet-process phosphoric acid purification methods generate large amounts of waste residue or cause organic solvent pollution, indicating a need to improve resource utilization. Current technologies do not consider the varying optimal reaction conditions due to differences in crystal structure, specific surface area, and surface functional groups among different iron sources, failing to establish a correlation mechanism between iron source activity and reaction conditions. This makes it impossible to control the optimal reaction pH for various iron sources, resulting in poor product consistency and consequently affecting the electrochemical performance of NFPP materials. Existing technologies directly react purified phosphoric acid with a sodium source to generate a single phosphate-sodium salt, failing to construct a multi-component complex system with flexible pH adjustment. When process conditions need to be adjusted or adapted to different iron sources, it is often necessary to completely change the raw material ratio or supplement with additional phosphorus and sodium sources, resulting in poor process flexibility.

[0046] This invention provides a method for preparing sodium iron pyrophosphate material, the method comprising the following steps:

[0047] S1. The wet-process phosphoric acid is extracted and purified by using a choline chloride-organic acid eutectic solvent to separate the organic phase and obtain purified phosphoric acid.

[0048] The choline chloride-organic acid eutectic solvent comprises choline chloride and organic acid;

[0049] S2. Mix the alkaline sodium source with the purified phosphoric acid to adjust the pH value, and obtain a sodium-phosphorus solution;

[0050] S3. The iron source, carbon source and sodium phosphorus solution are mixed by sand milling to obtain a mixture; the mixture is then spray-dried and sintered to obtain sodium iron pyrophosphate material.

[0051] The preparation method provided by this invention uses a choline chloride-organic acid eutectic solvent (DES), a mixture of choline chloride and organic acid, for efficient purification of wet-process phosphoric acid. This DES can be regenerated and recycled, significantly reducing purification costs. The purified phosphoric acid and an alkaline sodium source are neutralized to adjust the pH, achieving a compound system of phosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, and an alkaline sodium source. The molar ratio of the four substances can be flexibly adjusted by controlling the pH, enabling complete self-sufficiency of different phosphorus sources. All phosphorus sources are derived from wet-process phosphoric acid, eliminating the need to purchase any industrial raw materials, effectively reducing production costs. Therefore, the preparation method provided by this invention achieves complete self-sufficiency of phosphorus sources, significantly reduces production costs, and significantly improves the electrochemical performance and batch consistency of the materials. It effectively solves the core problems of existing technologies, such as reliance on purchased raw materials, poor process adaptability, and limited performance optimization, providing a complete technical path for the low-cost, large-scale, and green preparation of sodium-ion battery cathode materials.

[0052] A suitable molar ratio of choline chloride to organic acid enables the hydroxyl and chloride ions of choline chloride to form a stable three-dimensional network structure with the carboxyl groups of the organic acid through hydrogen bonding. The organic acid provides sufficient carboxyl coordination sites to react with Fe. 3+ Al 3+ The system forms stable complexes with metal ions, enabling efficient and selective extraction of metal impurities. Simultaneously, the system exhibits moderate viscosity, high mass transfer efficiency, short extraction equilibrium time, and high phosphoric acid recovery. Phosphoric acid molecules, due to their weak hydrogen bond affinity with DES, are primarily retained in the aqueous phase, thus achieving efficient and selective separation of metal ions from phosphoric acid. Adding excess deionized water after extraction disrupts the hydrogen bond network of DES, causing metal ions to precipitate as oxalate. The DES components can be recycled after evaporation and dehydration.

[0053] When the organic acid content is insufficient, the hydrogen bond network is incomplete and the coordination sites are few, resulting in a significant decrease in extraction efficiency and poor impurity removal effect. When the organic acid content is excessive, the system is too acidic, which may cause some phosphoric acid to form soluble complexes with metal ions and enter the DES phase, resulting in increased phosphorus loss. At the same time, the viscosity increases and the corrosiveness is enhanced, increasing the equipment requirements and operation difficulty, and the regeneration energy consumption also increases accordingly.

[0054] In some embodiments, the molar ratio of choline chloride to organic acid in the eutectic solvent is 1:0.5 to 1:3, for example, it can be 1:0.5, 1:1, 1:1.5, 1:2, 1:2.5 or 1:3, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0055] In some embodiments, the organic acid includes any one or a combination of at least two of oxalic acid, citric acid, malic acid, or tartaric acid. Typical but non-limiting combinations include combinations of oxalic acid and citric acid, combinations of citric acid and malic acid, combinations of malic acid and tartaric acid, combinations of oxalic acid, malic acid, and tartaric acid, combinations of citric acid, malic acid, and tartaric acid, or combinations of oxalic acid, citric acid, malic acid, and tartaric acid.

[0056] In some embodiments, the mass concentration of P2O5 in the wet-process phosphoric acid is 20wt% to 40wt%, for example, it can be 20wt%, 25wt%, 30wt%, 35wt% or 40wt%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0057] Those skilled in the art should know that when the mass concentration of P2O5 in the raw material used as wet-process phosphoric acid is higher than 40 wt%, it can be diluted with pure water to make the mass concentration of P2O5 20 wt% to 40 wt%.

[0058] A suitable volume ratio of choline chloride-organic acid eutectic solvent to the wet-process phosphoric acid ensures that the DES phase provides sufficient coordination sites to fully complex with metal impurity ions, while keeping phosphoric acid molecules mainly in the aqueous phase. This achieves highly efficient and selective separation of metal ions and phosphoric acid, with clear phase separation and stable interface after extraction. If the amount of DES is too small, the hydrogen bond network and coordination sites in the DES phase are insufficient, preventing sufficient contact and complexation with metal impurities in the wet-process phosphoric acid, resulting in incomplete removal of metal ion impurities and a decreased impurity removal rate. While excessive DES can ensure sufficient extraction of metal impurities, it increases process costs. Furthermore, because DES also has an affinity for phosphoric acid molecules, some phosphoric acid may be entrained into the DES phase, increasing phosphorus loss. The volume required for subsequent DES regeneration also increases, leading to higher energy consumption and operating costs.

[0059] In some embodiments, the volume ratio of the choline chloride-organic acid eutectic solvent to the wet-process phosphoric acid is 1:1 to 1:5, for example, it can be 1:1, 1:2, 1:3, 1:4 or 1:5, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0060] In some embodiments, the extraction and purification temperature is 20°C to 60°C, for example, it can be 20°C, 25°C, 30°C, 35°C, 40°C, 45°C, 50°C, 55°C or 60°C, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0061] In some embodiments, the extraction and impurity removal time is 10 min to 60 min, for example, it can be 10 min, 15 min, 20 min, 25 min, 30 min, 35 min, 40 min, 45 min, 50 min, 55 min or 60 min, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0062] In some embodiments, the alkaline sodium source includes any one or a combination of at least two of sodium carbonate, sodium hydroxide, sodium bicarbonate, sodium phosphate, sodium acetate, sodium oxalate, or sodium pyrophosphate.

[0063] In some embodiments, the pH value of the sodium-phosphorus solution is 0 to 12.

[0064] The pH value of a sodium-phosphorus solution varies, and so do the substances it contains. When the sodium-phosphorus solution is composed of pure phase substances, such as phosphoric acid, the pH value is 0-1; when it is composed of sodium dihydrogen phosphate, the pH value is 4-4.8; when it is composed of disodium hydrogen phosphate, the pH value is 8-9; and the pH value of an alkaline sodium source is 8-14. When the pH value is any other value within the range, it is an arbitrary mixture of these four substances, achieving self-supply of phosphorus and sodium sources without the need for additional supplementation of other phosphorus and sodium sources. It also corresponds to the pH range during sand milling and mixing.

[0065] In some embodiments, the iron source includes any one or a combination of at least two of iron oxide, iron(II,III) oxide, iron(III) hydroxyl oxide, iron(II) oxalate, iron(III) phosphate, or iron(II) acetate, preferably iron oxide, iron(III) hydroxyl oxide, and iron(II) phosphate.

[0066] Different iron sources have different optimal pH ranges for sand milling due to differences in crystal structure, specific surface area, and surface functional groups. In other words, a suitable iron source can be selected based on the pH value of the sodium phosphate solution. For example, when the iron source is iron oxide, the pH value for sand milling is 1-4; when the iron source is ferric hydroxide, the pH value for sand milling is 2.5-5; and when the iron source is ferric phosphate, the pH value for sand milling is 4-5.2.

[0067] In some embodiments, the pH value of the sand-milled mixture is 0 to 12, preferably 1 to 6.

[0068] In some embodiments, the carbon source includes inorganic carbon compounds and / or organic carbon compounds;

[0069] The inorganic carbon-containing compound includes any one or a combination of at least two of carbon black, carbon fiber, graphene, graphite or carbon nanotubes.

[0070] The organic carbon-containing compound includes any one or a combination of at least two of glucose, sucrose, isopropanol, starch, citric acid, maltose, formic acid, acetic acid, oxalic acid, or polyethylene glycol.

[0071] In this invention, the molar ratio of Na, Fe and P in the mixture satisfies the chemical formula of sodium iron pyrophosphate material Na4Fe3(PO4)2P2O7. When the molar amount of Na and / or P is insufficient, an alkaline sodium source and / or phosphorus source can be added 5 min to 15 min before the end of sand milling.

[0072] In some embodiments, the amount of carbon source used is 5wt% to 15wt% of the mixture, for example, it can be 5wt%, 6wt%, 7wt%, 8wt%, 9wt%, 10wt%, 11wt%, 12wt%, 13wt%, 14wt% or 15wt%, but is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0073] In some embodiments, the spray temperature of the spray drying is 100°C to 260°C, for example, it can be 100°C, 110°C, 120°C, 130°C, 140°C, 150°C, 160°C, 170°C, 180°C, 190°C, 200°C, 210°C, 220°C, 230°C, 240°C, 250°C or 260°C, but is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0074] In some embodiments, the sintering is performed in a protective atmosphere;

[0075] The protective atmosphere uses gases including nitrogen and / or argon.

[0076] If the sintering temperature is too low or the time is too short, the crystal transformation will be insufficient, the crystal phase will be impure, and the particle size distribution will be uneven. If the sintering temperature is too high or the time is too long, the crystal will agglomerate, the lattice will be distorted, and the electrochemical performance of the material will be reduced.

[0077] In some embodiments, the sintering temperature is 400°C to 700°C, for example, it can be 400°C, 450°C, 500°C, 550°C, 600°C, 650°C or 700°C, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0078] In some embodiments, the sintering time is 4h to 15h, for example, it can be 4h, 5h, 6h, 7h, 8h, 9h, 10h, 11h, 12h, 13h, 14h or 15h, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0079] As a preferred embodiment of the preparation method provided by the present invention, the preparation method includes the following steps:

[0080] S1. The wet-process phosphoric acid is extracted and purified by using a choline chloride-organic acid eutectic solvent to separate the organic phase and obtain purified phosphoric acid.

[0081] The choline chloride-organic acid eutectic solvent comprises choline chloride and organic acid in a molar ratio of 1:0.5 to 1:3;

[0082] The organic acid includes any one or a combination of at least two of oxalic acid, citric acid, malic acid or tartaric acid;

[0083] In the wet-process phosphoric acid, the mass concentration of P2O5 is 20wt%~40wt%;

[0084] The volume ratio of the choline chloride-organic acid eutectic solvent to the wet-process phosphoric acid is 1:1 to 1:5.

[0085] The extraction and impurity removal temperature is 20℃~60℃, and the time is 10min~60min;

[0086] S2. Mix the alkaline sodium source with the purified phosphoric acid to adjust the pH value, and obtain a sodium-phosphorus solution;

[0087] The alkaline sodium source includes any one or a combination of at least two of sodium carbonate, sodium hydroxide, sodium bicarbonate, sodium phosphate, sodium acetate, sodium oxalate, or sodium pyrophosphate.

[0088] The pH value of the sodium-phosphorus solution is 0~12;

[0089] S3. The iron source, carbon source and sodium phosphorus solution are mixed by sand milling to obtain a mixture; the mixture is then spray-dried and sintered to obtain sodium iron pyrophosphate material.

[0090] The pH value of the sand-milled mixture is 0~12;

[0091] The carbon source includes inorganic carbon compounds and / or organic carbon compounds; the amount of the carbon source is 5wt% to 15wt% of the mixture.

[0092] The spray temperature for the spray drying is 100℃~260℃;

[0093] The sintering is carried out in a protective atmosphere, the gas used in the protective atmosphere including nitrogen and / or argon;

[0094] The sintering temperature is 400℃~700℃, and the time is 4h~15h.

[0095] Example 1

[0096] This embodiment provides a method for preparing sodium iron pyrophosphate material, including the following steps:

[0097] S1. The wet-process phosphoric acid was extracted and impurities were removed using a choline chloride-organic acid eutectic solvent. The organic phase was separated by standing to obtain purified phosphoric acid and DES phase. Eight times the volume of deionized water was added to the DES phase, and the mixture was allowed to stand for 1 hour. The precipitate was filtered, and the aqueous phase was evaporated at 90°C to obtain regenerated DES.

[0098] The preparation of the choline chloride-organic acid eutectic solvent includes: stirring and heating choline chloride and organic acid (oxalic acid) at 80°C for 45 min in a molar ratio of 1:1.5.

[0099] In the wet-process phosphoric acid, the mass concentration of P2O5 is 30 wt%.

[0100] The volume ratio of the choline chloride-organic acid eutectic solvent to the wet-process phosphoric acid is 1:3;

[0101] The extraction and impurity removal process was carried out at a temperature of 40°C for 30 minutes.

[0102] S2. Mix the alkaline sodium source (sodium carbonate) with the purified phosphoric acid to adjust the pH value, and obtain a sodium-phosphorus solution;

[0103] The pH value of the sodium phosphorus solution is 2;

[0104] S3. The iron source (iron oxide), carbon source (glucose) and sodium phosphorus solution are mixed by sand milling. Sodium carbonate is added 10 minutes before the end of sand milling according to the sodium-phosphorus ratio of the sodium iron pyrophosphate material to obtain a mixture. The mixture is then spray-dried and sintered to obtain sodium iron pyrophosphate material Na4Fe3(PO4)2P2O7.

[0105] The amount of carbon source used is 5 wt% of the mixture;

[0106] The spray temperature for the spray drying is 180°C;

[0107] The sintering is carried out in a nitrogen atmosphere at a temperature of 530°C for 10 hours.

[0108] Example 2

[0109] This embodiment provides a method for preparing sodium iron pyrophosphate material, including the following steps:

[0110] S1. The wet-process phosphoric acid was extracted and impurities were removed using a choline chloride-organic acid eutectic solvent. The organic phase was separated by standing to obtain purified phosphoric acid and DES phase. Eight times the volume of deionized water was added to the DES phase, and the mixture was allowed to stand for 1 hour. The precipitate was filtered, and the aqueous phase was evaporated at 90°C to obtain regenerated DES.

[0111] The preparation of the choline chloride-organic acid eutectic solvent includes: stirring and heating choline chloride and organic acid (oxalic acid) at 80°C for 45 min in a molar ratio of 1:1.

[0112] In the wet-process phosphoric acid, the mass concentration of P2O5 is 30 wt%.

[0113] The volume ratio of the choline chloride-organic acid eutectic solvent to the wet-process phosphoric acid is 1:3;

[0114] The extraction and impurity removal process was carried out at a temperature of 40°C for 30 minutes.

[0115] S2. Mix the alkaline sodium source (sodium carbonate) with the purified phosphoric acid to adjust the pH value, and obtain a sodium-phosphorus solution;

[0116] The pH value of the sodium phosphorus solution is 2;

[0117] S3. The iron source (iron oxide), carbon source (glucose) and sodium phosphorus solution are mixed by sand milling. Sodium carbonate is added 10 minutes before the end of sand milling according to the sodium-phosphorus ratio of the sodium iron pyrophosphate material to obtain a mixture. The mixture is then spray-dried and sintered to obtain sodium iron pyrophosphate material Na4Fe3(PO4)2P2O7.

[0118] The amount of carbon source used is 5 wt% of the mixture;

[0119] The spray temperature for the spray drying is 180°C;

[0120] The sintering is carried out in a nitrogen atmosphere at a temperature of 530°C for 10 hours.

[0121] Example 3

[0122] This embodiment provides a method for preparing sodium iron pyrophosphate material, including the following steps:

[0123] S1. The wet-process phosphoric acid was extracted and impurities were removed using a choline chloride-organic acid eutectic solvent. The organic phase was separated by standing to obtain purified phosphoric acid and DES phase. Eight times the volume of deionized water was added to the DES phase, and the mixture was allowed to stand for 1 hour. The precipitate was filtered, and the aqueous phase was evaporated at 90°C to obtain regenerated DES.

[0124] The preparation of the choline chloride-organic acid eutectic solvent includes: stirring and heating choline chloride and organic acid (oxalic acid) at 80°C for 45 min in a molar ratio of 1:1.5.

[0125] In the wet-process phosphoric acid, the mass concentration of P2O5 is 30 wt%.

[0126] The volume ratio of the choline chloride-organic acid eutectic solvent to the wet-process phosphoric acid is 1:1;

[0127] The extraction and impurity removal process was carried out at a temperature of 40°C for 30 minutes.

[0128] S2. Mix the alkaline sodium source (sodium carbonate) with the purified phosphoric acid to adjust the pH value, and obtain a sodium-phosphorus solution;

[0129] The pH value of the sodium phosphorus solution is 2;

[0130] S3. The iron source (iron oxide), carbon source (glucose) and sodium phosphorus solution are mixed by sand milling. Sodium carbonate is added 10 minutes before the end of sand milling according to the sodium-phosphorus ratio of the sodium iron pyrophosphate material to obtain a mixture. The mixture is then spray-dried and sintered to obtain sodium iron pyrophosphate material Na4Fe3(PO4)2P2O7.

[0131] The amount of carbon source used is 5 wt% of the mixture;

[0132] The spray temperature for the spray drying is 180°C;

[0133] The sintering is carried out in a nitrogen atmosphere at a temperature of 530°C for 10 hours.

[0134] Example 4

[0135] This embodiment provides a method for preparing sodium iron pyrophosphate material, including the following steps:

[0136] S1. The wet-process phosphoric acid was extracted and impurities were removed using a choline chloride-organic acid eutectic solvent. The organic phase was separated by standing to obtain purified phosphoric acid and DES phase. Eight times the volume of deionized water was added to the DES phase, and the mixture was allowed to stand for 1 hour. The precipitate was filtered, and the aqueous phase was evaporated at 90°C to obtain regenerated DES.

[0137] The preparation of the choline chloride-organic acid eutectic solvent includes: stirring and heating choline chloride and organic acid (oxalic acid) at 80°C for 45 min in a molar ratio of 1:1.5.

[0138] In the wet-process phosphoric acid, the mass concentration of P2O5 is 30 wt%.

[0139] The volume ratio of the choline chloride-organic acid eutectic solvent to the wet-process phosphoric acid is 1:3;

[0140] The extraction and impurity removal process was carried out at a temperature of 40°C for 30 minutes.

[0141] S2. Mix the alkaline sodium source (sodium carbonate) with the purified phosphoric acid to adjust the pH value, and obtain a sodium-phosphorus solution;

[0142] The sodium phosphate solution has a pH of 4 and is composed of pure-phase sodium dihydrogen phosphate.

[0143] S3. The iron source (iron oxide), carbon source (glucose) and sodium phosphorus solution are mixed by sand milling. Sodium carbonate is added 10 minutes before the end of sand milling according to the sodium-phosphorus ratio of the sodium iron pyrophosphate material to obtain a mixture. The mixture is then spray-dried and sintered to obtain sodium iron pyrophosphate material Na4Fe3(PO4)2P2O7.

[0144] The amount of carbon source used is 5 wt% of the mixture;

[0145] The spray temperature for the spray drying is 180°C;

[0146] The sintering is carried out in a nitrogen atmosphere at a temperature of 530°C for 10 hours.

[0147] Example 5

[0148] This embodiment provides a method for preparing sodium iron pyrophosphate material, including the following steps:

[0149] S1. The wet-process phosphoric acid was extracted and impurities were removed using a choline chloride-organic acid eutectic solvent. The organic phase was separated by standing to obtain purified phosphoric acid and DES phase. Eight times the volume of deionized water was added to the DES phase, and the mixture was allowed to stand for 1 hour. The precipitate was filtered, and the aqueous phase was evaporated at 90°C to obtain regenerated DES.

[0150] The preparation of the choline chloride-organic acid eutectic solvent includes: stirring and heating choline chloride and organic acid (oxalic acid) at 80°C for 45 min in a molar ratio of 1:1.5.

[0151] In the wet-process phosphoric acid, the mass concentration of P2O5 is 30 wt%.

[0152] The volume ratio of the choline chloride-organic acid eutectic solvent to the wet-process phosphoric acid is 1:3;

[0153] The extraction and impurity removal process was carried out at a temperature of 40°C for 30 minutes.

[0154] S2. Mix the alkaline sodium source (sodium carbonate) with the purified phosphoric acid to adjust the pH value, and obtain a sodium-phosphorus solution;

[0155] The sodium phosphate solution has a pH of 4 and is composed of pure-phase sodium dihydrogen phosphate.

[0156] S3. The iron source (ferric hydroxide), carbon source (glucose) and sodium phosphorus solution are mixed by sand milling. Sodium carbonate is added 10 minutes before the end of sand milling according to the sodium-phosphorus ratio of sodium iron pyrophosphate material to obtain a mixture. The mixture is spray dried and sintered to obtain sodium iron pyrophosphate material Na4Fe3(PO4)2P2O7.

[0157] The amount of carbon source used is 5 wt% of the mixture;

[0158] The spray temperature for the spray drying is 180°C;

[0159] The sintering is carried out in a nitrogen atmosphere at a temperature of 530°C for 10 hours.

[0160] Example 6

[0161] This embodiment provides a method for preparing sodium iron pyrophosphate material, including the following steps:

[0162] S1. The wet-process phosphoric acid was extracted and impurities were removed using a choline chloride-organic acid eutectic solvent. The organic phase was separated by standing to obtain purified phosphoric acid and DES phase. Eight times the volume of deionized water was added to the DES phase, and the mixture was allowed to stand for 1 hour. The precipitate was filtered, and the aqueous phase was evaporated at 90°C to obtain regenerated DES.

[0163] The preparation of the choline chloride-organic acid eutectic solvent includes: stirring and heating choline chloride and organic acid (oxalic acid) at 80°C for 45 min in a molar ratio of 1:1.5.

[0164] In the wet-process phosphoric acid, the mass concentration of P2O5 is 30 wt%.

[0165] The volume ratio of the choline chloride-organic acid eutectic solvent to the wet-process phosphoric acid is 1:3;

[0166] The extraction and impurity removal process was carried out at a temperature of 40°C for 30 minutes.

[0167] S2. Mix the alkaline sodium source (sodium carbonate) with the purified phosphoric acid to adjust the pH value, and obtain a sodium-phosphorus solution;

[0168] The pH value of the sodium phosphorus solution is 2;

[0169] S3. The iron source (iron oxide), carbon source (glucose) and sodium phosphorus solution are mixed by sand milling. Sodium carbonate is added 10 minutes before the end of sand milling according to the sodium-phosphorus ratio of the sodium iron pyrophosphate material to obtain a mixture. The mixture is then spray-dried and sintered to obtain sodium iron pyrophosphate material Na4Fe3(PO4)2P2O7.

[0170] The amount of carbon source used is 8 wt% of the mixture;

[0171] The spray temperature for the spray drying is 180°C;

[0172] The sintering is carried out in a nitrogen atmosphere at a temperature of 530°C for 10 hours.

[0173] Example 7

[0174] This embodiment provides a method for preparing sodium iron pyrophosphate material, including the following steps:

[0175] S1. The wet-process phosphoric acid was extracted and impurities were removed using a choline chloride-organic acid eutectic solvent. The organic phase was separated by standing to obtain purified phosphoric acid and DES phase. Eight times the volume of deionized water was added to the DES phase, and the mixture was allowed to stand for 1 hour. The precipitate was filtered, and the aqueous phase was evaporated at 90°C to obtain regenerated DES.

[0176] The preparation of the choline chloride-organic acid eutectic solvent includes: stirring and heating choline chloride and organic acid (oxalic acid) at 80°C for 45 min in a molar ratio of 1:1.5.

[0177] In the wet-process phosphoric acid, the mass concentration of P2O5 is 30 wt%.

[0178] The volume ratio of the choline chloride-organic acid eutectic solvent to the wet-process phosphoric acid is 1:3;

[0179] The extraction and impurity removal process was carried out at a temperature of 40°C for 30 minutes.

[0180] S2. Mix the alkaline sodium source (sodium carbonate) with the purified phosphoric acid to adjust the pH value, and obtain a sodium-phosphorus solution;

[0181] The pH value of the sodium phosphorus solution is 2;

[0182] S3. The iron source (iron oxide), carbon source (glucose) and sodium phosphorus solution are mixed by sand milling. Sodium carbonate is added 10 minutes before the end of sand milling according to the sodium-phosphorus ratio of the sodium iron pyrophosphate material to obtain a mixture. The mixture is then spray-dried and sintered to obtain sodium iron pyrophosphate material Na4Fe3(PO4)2P2O7.

[0183] The amount of carbon source used is 5 wt% of the mixture;

[0184] The spray temperature for the spray drying is 180°C;

[0185] The sintering is carried out in a nitrogen atmosphere at a temperature of 550°C for 10 hours.

[0186] Example 8

[0187] This embodiment provides a method for preparing sodium iron pyrophosphate material, including the following steps:

[0188] S1. The wet-process phosphoric acid was extracted and impurities were removed using a choline chloride-organic acid eutectic solvent. The organic phase was separated by standing to obtain purified phosphoric acid and DES phase. Eight times the volume of deionized water was added to the DES phase, and the mixture was allowed to stand for 1 hour. The precipitate was filtered, and the aqueous phase was evaporated at 90°C to obtain regenerated DES.

[0189] The preparation of the choline chloride-organic acid eutectic solvent includes: stirring and heating choline chloride and organic acid (oxalic acid) at 80°C for 45 min in a molar ratio of 1:1.5.

[0190] In the wet-process phosphoric acid, the mass concentration of P2O5 is 30 wt%.

[0191] The volume ratio of the choline chloride-organic acid eutectic solvent to the wet-process phosphoric acid is 1:3;

[0192] The extraction and impurity removal process was carried out at a temperature of 40°C for 30 minutes.

[0193] S2. Mix the alkaline sodium source (sodium carbonate) with the purified phosphoric acid to adjust the pH value, and obtain a sodium-phosphorus solution;

[0194] The pH value of the sodium phosphorus solution is 9.5;

[0195] S3. The iron source (Fe3O4), carbon source (glucose) and sodium phosphorus solution are mixed by sand milling. Sodium carbonate is added 10 minutes before the end of sand milling according to the sodium-phosphorus ratio of the sodium iron pyrophosphate material to obtain a mixture. The mixture is then spray-dried and sintered to obtain sodium iron pyrophosphate material Na4Fe3(PO4)2P2O7.

[0196] The amount of carbon source used is 5 wt% of the mixture;

[0197] The spray temperature for the spray drying is 180°C;

[0198] The sintering is carried out in a nitrogen atmosphere at a temperature of 530°C for 10 hours.

[0199] Example 9

[0200] This embodiment provides a method for preparing sodium iron pyrophosphate material, including the following steps:

[0201] S1. The wet-process phosphoric acid was extracted and impurities were removed using a choline chloride-organic acid eutectic solvent. The organic phase was separated by standing to obtain purified phosphoric acid and DES phase. Eight times the volume of deionized water was added to the DES phase, and the mixture was allowed to stand for 1 hour. The precipitate was filtered, and the aqueous phase was evaporated at 90°C to obtain regenerated DES.

[0202] The preparation of the choline chloride-organic acid eutectic solvent includes: stirring and heating choline chloride and organic acid (oxalic acid) at 80°C for 45 min in a molar ratio of 1:0.5.

[0203] In the wet-process phosphoric acid, the mass concentration of P2O5 is 20 wt%;

[0204] The volume ratio of the choline chloride-organic acid eutectic solvent to the wet-process phosphoric acid is 1:3;

[0205] The extraction and impurity removal process was carried out at a temperature of 20°C for 60 minutes.

[0206] S2. Mix the alkaline sodium source (sodium carbonate) with the purified phosphoric acid to adjust the pH value, and obtain a sodium-phosphorus solution;

[0207] The pH value of the sodium phosphorus solution is 2;

[0208] S3. The iron source (iron oxide), carbon source (glucose) and sodium phosphorus solution are mixed by sand milling. Sodium carbonate is added 10 minutes before the end of sand milling according to the sodium-phosphorus ratio of the sodium iron pyrophosphate material to obtain a mixture. The mixture is then spray-dried and sintered to obtain sodium iron pyrophosphate material Na4Fe3(PO4)2P2O7.

[0209] The amount of carbon source used is 5 wt% of the mixture;

[0210] The spray temperature for the spray drying is 100°C;

[0211] The sintering is carried out in a nitrogen atmosphere at a temperature of 400°C for 15 hours.

[0212] Example 10

[0213] This embodiment provides a method for preparing sodium iron pyrophosphate material, including the following steps:

[0214] S1. The wet-process phosphoric acid was extracted and impurities were removed using a choline chloride-organic acid eutectic solvent. The organic phase was separated by standing to obtain purified phosphoric acid and DES phase. Eight times the volume of deionized water was added to the DES phase, and the mixture was allowed to stand for 1 hour. The precipitate was filtered, and the aqueous phase was evaporated at 90°C to obtain regenerated DES.

[0215] The preparation of the choline chloride-organic acid eutectic solvent includes: stirring and heating choline chloride and organic acid (oxalic acid) at 80°C for 45 min in a molar ratio of 1:3.

[0216] In the wet-process phosphoric acid, the mass concentration of P2O5 is 40 wt%.

[0217] The volume ratio of the choline chloride-organic acid eutectic solvent to the wet-process phosphoric acid is 1:5;

[0218] The extraction and impurity removal process is carried out at a temperature of 60°C for 10 minutes.

[0219] S2. Mix the alkaline sodium source (sodium carbonate) with the purified phosphoric acid to adjust the pH value, and obtain a sodium-phosphorus solution;

[0220] The pH value of the sodium phosphorus solution is 2;

[0221] S3. The iron source (iron oxide), carbon source (glucose) and sodium phosphorus solution are mixed by sand milling. Sodium carbonate is added 10 minutes before the end of sand milling according to the sodium-phosphorus ratio of the sodium iron pyrophosphate material to obtain a mixture. The mixture is then spray-dried and sintered to obtain sodium iron pyrophosphate material Na4Fe3(PO4)2P2O7.

[0222] The amount of carbon source used is 15 wt% of the mixture;

[0223] The spray temperature for the spray drying is 260°C;

[0224] The sintering is carried out in a nitrogen atmosphere at a temperature of 700°C for 4 hours.

[0225] Comparative Example 1

[0226] This comparative example provides a method for preparing sodium iron pyrophosphate material. Except for the absence of DES extraction for wet-process phosphoric acid purification, the method is identical to that in Example 1, including:

[0227] S1. Provide the wet-process phosphoric acid as in Example 1;

[0228] S2. Mix the alkaline sodium source (sodium carbonate) with the wet-process phosphoric acid to adjust the pH value to obtain a sodium-phosphorus solution;

[0229] The pH value of the sodium phosphorus solution is 2;

[0230] S3. The iron source (iron oxide), carbon source (glucose) and sodium phosphorus solution are mixed by sand milling. Sodium carbonate is added 10 minutes before the end of sand milling according to the sodium-phosphorus ratio of the sodium iron pyrophosphate material to obtain a mixture. The mixture is then spray-dried and sintered to obtain sodium iron pyrophosphate material Na4Fe3(PO4)2P2O7.

[0231] The amount of carbon source used is 5 wt% of the mixture;

[0232] The spray temperature for the spray drying is 180°C;

[0233] The sintering is carried out in a nitrogen atmosphere at a temperature of 530°C for 10 hours.

[0234] Performance Characterization

[0235] The sodium iron pyrophosphate material provided in the above embodiments and comparative examples was used to prepare sodium-ion batteries: sodium iron pyrophosphate material, conductive agent Super P, and binder polyvinylidene fluoride (PVDF) were accurately weighed at a mass ratio of 8:1:1, and N-methylpyrrolidone (NMP) was used as the dispersion solvent. The mixture was placed in a vacuum stirring vessel and stirred at 3000 r / min for 4 h to prepare a uniform, sedimentation-free positive electrode slurry. The positive electrode slurry was then uniformly coated onto a 16 μm thick aluminum foil current collector using a doctor blade coater, controlling the single-sided coating surface density to be 10 mg / cm². 2 First, pre-dry the electrode in a 60℃ forced-air drying oven for 2 hours, then transfer it to a 120℃ vacuum drying oven for 12 hours to completely remove the solvent and residual moisture. After drying, remove the electrode and roll it with a roller press until the compacted density is 2.0 g / cm³. 3 The electrode is punched into a circular electrode sheet with a diameter of 14mm using a punching machine, and used as the positive electrode sheet;

[0236] The electrolyte uses 1.0 mol / L sodium hexafluorophosphate as the solute, and ethylene carbonate (EC) and diethyl carbonate (DEC) mixed at a volume ratio of 1:1 as the solvent. At the same time, 5% by volume of fluoroethylene carbonate (FEC) is added as a film-forming additive.

[0237] In an argon atmosphere glove box with water and oxygen content both below 0.1 ppm, using a CR2032 button cell casing as a carrier, the positive electrode shell, the prepared positive electrode sheet, a 16 mm diameter separator, 200 μL of the electrolyte, a sodium metal sheet counter electrode (15 mm diameter, 0.5 mm thickness), a nickel foam gasket, a stainless steel gasket, and the negative electrode shell were stacked in sequence. The casing was then sealed using a battery sealing machine at a pressure of 0.8 MPa to complete the preparation of the sodium-ion half cell.

[0238] The assembled battery was placed in a constant temperature environment of 25±2℃ for 12 hours to allow the electrolyte to fully wet the electrode and separator before subsequent electrochemical performance testing.

[0239] Then, the 0.1C charging capacity, 0.1C discharging capacity, and capacity retention rate after 200 cycles at 1C were tested, and the results are shown in Table 1.

[0240] The testing of 0.1C charging capacity and 0.1C discharging capacity included constant current-constant voltage charging and constant current discharging modes, with a test voltage range of 2.0V~4.0V (vs. Na / Na). + The specific charge and discharge cycle is as follows: charge at a constant current rate of 0.1C to the cutoff voltage of 4.0V, then switch to constant voltage charging until the current drops to 0.05C, and let stand for 10 minutes; then discharge at a constant current rate of 0.1C to the cutoff voltage of 2.0V, and let stand for 10 minutes. This is one complete charge and discharge cycle; repeat the cycle 3 times, and take the constant current charging capacity of the 3rd cycle as the 0.1C charging capacity, and take the constant current discharging capacity of the 3rd cycle as the 0.1C discharging capacity.

[0241] The capacity retention test at 1C for 200 cycles includes: constant current charging at 1C to the cutoff voltage of 4.0V, switching to constant voltage charging until the current drops to 0.05C, and resting for 5 minutes; then constant current discharging at 1C to the cutoff voltage of 2.0V, and resting for 5 minutes. This constitutes one complete charge-discharge cycle. This process is repeated 200 times, and the discharge specific capacity is recorded for each cycle. The capacity retention at 1C for 200 cycles is calculated using the following formula:

[0242] Capacity retention (%) = (Specific discharge capacity at the 200th cycle / Specific discharge capacity at the first cycle) × 100%.

[0243] Table 1

[0244]

[0245] Combining the test results of Examples 1 to 3, 9 and 10 in Table 1, it can be seen that all three sets of examples exhibit high specific capacity and excellent capacity retention. The maximum discharge specific capacity is 101.8 mAh / g, indicating that under suitable low eutectic solvent feeding ratio and extraction reaction conditions, DES can selectively extract metal impurities in wet phosphoric acid solution through the synergistic effect of hydrogen bond network structure and coordination chemistry. After the purified phosphoric acid reacts with an alkaline sodium source, a suitable sodium-phosphorus compound system with trivalent iron sand milling is obtained by adjusting the pH, thereby significantly improving the electrochemical performance and batch consistency of the material.

[0246] Combining the test results of Examples 1, 4, 5, and 8 in Table 1 with those of Comparative Example 1, it can be seen that by compounding different products obtained from the neutralization reaction of purified phosphoric acid and alkaline sodium source, the pH can be flexibly controlled. Furthermore, based on the differences in crystal structure, specific surface area, and surface functional groups of different iron sources, the optimal reaction pH of the sand milling process can be precisely adapted, allowing all types of iron sources to react fully under optimal conditions, thereby significantly improving the electrochemical performance of the products.

[0247] Combining the test results of Examples 1 and 6 in Table 1, it can be seen that the more carbon source is added, the lower the proportion of active substances in the material, thus affecting the electrochemical performance.

[0248] Combining the test results of Examples 1 and 7 in Table 1, it can be seen that a suitable calcination temperature is conducive to the formation of sodium iron pyrophosphate material with good crystallinity and uniform particle size distribution, thereby improving the electrochemical performance and cycle stability of the material. If the sintering temperature is too high, the agglomeration phenomenon between material particles will become more and more intense, hindering the effective transport of ions, resulting in a decrease in cycle stability, and thus affecting the overall performance and service life of the battery.

[0249] Combining the test results of Example 1 and Comparative Example 1 in Table 1, it can be seen that compared with sodium iron pyrophosphate prepared from unremoved wet-process phosphoric acid, the sodium iron pyrophosphate prepared using the process of this application shows significant improvement in electrochemical performance, such as the first-week charge-discharge specific capacity. This indicates that the DES solvent can efficiently and cyclically remove metal ion impurities from wet-process phosphoric acid. After effective impurity removal by DES, the purity of the purified phosphoric acid is significantly improved. The NFPP material prepared in this way has a more complete crystal structure, reducing lattice distortion caused by impurity residues and providing more ordered channels for ion and electron transport. This effectively improves the ion diffusion rate and electronic conductivity of the material, ultimately resulting in a significant optimization of the electrochemical performance of the sodium iron pyrophosphate prepared by the process of this application.

[0250] In summary, the preparation method provided by this invention utilizes a choline chloride-organic acid eutectic solvent (DES), a mixture of choline chloride and organic acid, for efficient purification of wet-process phosphoric acid. This DES is regenerable and recyclable, significantly reducing purification costs. The purified phosphoric acid is neutralized with an alkaline sodium source to adjust the pH, achieving a compound system of phosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, and an alkaline sodium source. The molar ratio of these four substances can be flexibly adjusted by controlling the pH. Furthermore, the entire process achieves complete self-sufficiency in phosphorus sources, all derived from wet-process phosphoric acid, eliminating the need for purchasing any industrial raw materials. Therefore, the preparation method provided by this invention achieves complete self-sufficiency in phosphorus sources, significantly reducing production costs, and substantially improving the electrochemical performance and batch consistency of the materials. It effectively solves the core problems of existing technologies, such as reliance on purchased raw materials, poor process adaptability, and limited performance optimization, providing a complete technical path for the low-cost, large-scale, and green preparation of sodium-ion battery cathode materials.

[0251] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A method for preparing a sodium iron pyrophosphate material, characterized in that, The preparation method includes the following steps: S1. The wet-process phosphoric acid is extracted and purified by using a choline chloride-organic acid eutectic solvent to separate the organic phase and obtain purified phosphoric acid. The choline chloride-organic acid eutectic solvent comprises choline chloride and organic acid; S2. Mix the alkaline sodium source with the purified phosphoric acid to adjust the pH value, and obtain a sodium-phosphorus solution; S3. The iron source, carbon source and sodium phosphorus solution are mixed by sand milling to obtain a mixture; the mixture is then spray-dried and sintered to obtain sodium iron pyrophosphate material.

2. The preparation method according to claim 1, characterized in that, In the choline chloride-organic acid eutectic solvent, the molar ratio of choline chloride to organic acid is 1:0.5 to 1:3; And / or, the organic acid includes any one or a combination of at least two of oxalic acid, citric acid, malic acid or tartaric acid.

3. The preparation method according to claim 1, characterized in that, In the wet-process phosphoric acid, the mass concentration of P2O5 is 20wt%~40wt%; And / or, the volume ratio of the choline chloride-organic acid eutectic solvent to the wet-process phosphoric acid is 1:1 to 1:

5.

4. The preparation method according to any one of claims 1 to 3, characterized in that, The extraction and impurity removal temperature is 20℃~60℃; And / or, the extraction and impurity removal time is 10 min to 60 min.

5. The preparation method according to claim 1, characterized in that, The alkaline sodium source includes any one or a combination of at least two of sodium carbonate, sodium hydroxide, sodium bicarbonate, sodium phosphate, sodium acetate, sodium oxalate, or sodium pyrophosphate.

6. The preparation method according to claim 1 or 5, characterized in that, The pH value of the sodium phosphate solution is 0~12.

7. The preparation method according to claim 1, characterized in that, The iron source includes any one or a combination of at least two of the following: iron oxide, iron(II) oxide, iron(III) hydroxide, iron(II) oxalate, iron(II) phosphate, or iron(II) acetate. And / or, the pH value of the sand-milled mixture is 0~12.

8. The preparation method according to claim 1 or 7, characterized in that, The carbon source includes inorganic carbon compounds and / or organic carbon compounds; The inorganic carbon-containing compound includes any one or a combination of at least two of carbon black, carbon fiber, graphene, graphite or carbon nanotubes. The organic carbon-containing compound includes any one or a combination of at least two of glucose, sucrose, isopropanol, starch, citric acid, maltose, formic acid, acetic acid, oxalic acid, or polyethylene glycol; And / or, the amount of the carbon source is 5wt% to 15wt% of the mixture.

9. The preparation method according to claim 1, characterized in that, The spray temperature for the spray drying is 100℃~260℃.

10. The preparation method according to claim 1, characterized in that, The sintering is carried out in a protective atmosphere; The protective atmosphere includes nitrogen and / or argon. And / or, the sintering temperature is 400℃~700℃; And / or, the sintering time is 4h to 15h.

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