Method for directly synthesizing lithium iron phosphate by deep eutectic solvent under normal pressure

Abstract

This invention discloses a method for the direct synthesis of lithium iron phosphate (LFP) using a deep eutectic solvent under ambient pressure. The method includes the following steps: mixing iron phosphate dihydrate with the deep eutectic solvent and reacting under heating to obtain LFP; wherein the raw materials for preparing the deep eutectic solvent include a hydrogen bond donor and a hydrogen bond acceptor, the hydrogen bond acceptor being lithium chloride monohydrate, and the hydrogen bond donor being a polyol with reducing properties; the heating temperature is 60-180°C. Addressing the problems commonly found in traditional direct synthesis methods of LFP materials (such as high-temperature solid-state methods, high-pressure hydrothermal methods, or solvothermal methods), including high reaction temperatures, high energy consumption, complex processes, demanding equipment requirements, or heavy post-processing burdens, this invention proposes a method for the direct synthesis of LFP materials using a deep eutectic solvent under low-temperature, ambient-pressure conditions, thereby significantly reducing reaction condition requirements, simplifying operation, and improving production efficiency.

Description

Technical Field

[0001] This invention relates to the field of new energy technology, specifically to a method for the direct synthesis of lithium iron phosphate under normal pressure using a deep eutectic solvent. Background Technology

[0002] Lithium-ion batteries, with their advantages of high energy density, long cycle life, low self-discharge rate, and good environmental adaptability, have been widely used in new energy vehicles, energy storage systems, and consumer electronics. Lithium iron phosphate, as an important cathode material for lithium-ion batteries, possesses advantages such as high thermal stability, structural stability, low cost, good safety performance, long cycle life, and environmental friendliness, and has broad application prospects in power and energy storage batteries.

[0003] Currently, the main methods for preparing lithium iron phosphate include high-temperature solid-state methods, sol-gel methods, co-precipitation methods, hydrothermal methods, and solvothermal methods. Although these methods can all synthesize lithium iron phosphate, they generally suffer from problems such as high reaction temperatures, high energy consumption, complex processes, high equipment requirements, or heavy post-processing burdens. In particular, the hydrothermal method and its related regeneration technologies usually need to be carried out under closed high-pressure conditions, which places high demands on the pressure resistance and safety of the reaction equipment. Moreover, the reaction process is mostly intermittent, which is not conducive to process simplification and large-scale application. At the same time, the hydrothermal regeneration process often requires the addition of additional lithium sources and auxiliary reagents, and the reaction is prone to generating saline waste liquid, increasing environmental treatment costs.

[0004] Therefore, existing lithium iron phosphate synthesis and regeneration technologies still need further improvement. There is an urgent need to provide a lithium iron phosphate synthesis method that can be carried out under normal pressure and low temperature conditions, while taking into account energy saving, environmental protection and process simplification, so as to meet the actual needs of green preparation and low-cost manufacturing. Summary of the Invention

[0005] This invention aims to address at least one of the technical problems existing in the prior art. To this end, this invention proposes a method for the direct synthesis of lithium iron phosphate using a deep eutectic solvent under normal pressure. This method eliminates the need for a high-pressure reactor, significantly reducing equipment requirements and process implementation difficulty, while also simplifying the raw material system and operating procedures.

[0006] According to one aspect of the present invention, a method for directly synthesizing lithium iron phosphate under ambient pressure using a deep eutectic solvent is provided, comprising the following steps: mixing iron phosphate dihydrate with the deep eutectic solvent and reacting under heating to obtain lithium iron phosphate; wherein the raw materials for preparing the deep eutectic solvent include a hydrogen bond donor and a hydrogen bond acceptor, the hydrogen bond acceptor includes lithium chloride monohydrate, the hydrogen bond donor includes a polyol, and the polyol has reducing properties; the heating temperature is 60~180℃.

[0007] The method for direct synthesis of lithium iron phosphate using a deep eutectic solvent at low temperature and ambient pressure according to embodiments of the present invention has at least the following beneficial effects: Addressing the common problems in traditional direct synthesis methods of lithium iron phosphate materials (such as high-temperature solid-state methods, high-pressure hydrothermal methods, or solvothermal methods), including high reaction temperatures, high energy consumption, complex processes, high equipment requirements, or heavy post-processing burdens, the present invention proposes to directly synthesize lithium iron phosphate materials using a deep eutectic solvent under low-temperature (below conventional reaction temperatures) and ambient pressure conditions. This significantly reduces reaction condition requirements, simplifies operation, and improves production efficiency. Compared with the high-temperature solid-state methods, high-pressure hydrothermal methods, or solvothermal methods commonly used in the prior art, the present invention has lower requirements for the pressure resistance of reaction equipment and can complete the synthesis of the target product without completely relying on a high-pressure environment. This helps reduce equipment investment costs, simplify the operation process, and improve the convenience of process implementation. The deep eutectic solvent system used in the present invention has a stable structure during the reaction process, is not prone to irreversible decomposition or side reactions, and has low solvent component consumption. After the precursor is generated and solid-liquid separation is performed, the resulting solvent system can be replenished appropriately and reused in subsequent batches of reactions, achieving multiple recycling. This feature not only reduces solvent emissions and wastewater treatment pressure, but also further reduces process operating costs and improves overall resource utilization efficiency and environmental friendliness.

[0008] The deep eutectic solvent used in this invention serves a dual function as both a lithium source and a reaction medium: on the one hand, it provides the synthesis environment as a reaction medium, and on the other hand, it provides the necessary lithium source for the formation of the target product. Compared with traditional methods that require the separate addition of additional lithium sources such as lithium hydroxide and lithium carbonate, along with iron and phosphorus sources, this invention achieves a unified supply of lithium source and construction of the reaction environment through the deep eutectic solvent. This reduces the types of raw materials, lowers the complexity of the feeding process, and is beneficial for improving system uniformity and process integration.

[0009] This invention ingeniously utilizes lithium chloride monohydrate and a reducing polyol to construct a deep eutectic solvent system, achieving the direct low-temperature, ambient-pressure synthesis of lithium iron phosphate (LFP) material regeneration. In this system, lithium chloride monohydrate acts as a hydrogen bond acceptor, forming a stable deep eutectic structure with the reducing polyol through hydrogen bonding. This lowers the system's melting point while improving the solvent's ion solubility and mass transfer performance. Lithium chloride monohydrate acts as both a structure regulator and a lithium source. On one hand, it participates in the construction of the deep eutectic solvent's hydrogen bond network, enhancing the system's ionization environment; on the other hand, the lithium ions it provides can supplement and regulate the lithium element in the system during the reaction, thus facilitating the compositional control of the LFP precursor. The reducing polyol, acting as a hydrogen bond donor, possesses a certain mild reducing ability while forming the deep eutectic structure. Under appropriate mild temperature conditions, the reducing polyol can reduce ferric ions in the LFP system, reducing Fe... 3+ Converted to Fe 2+This facilitates the reconstruction and stabilization of the lithium iron phosphate precursor structure without requiring high-temperature phase formation. Furthermore, the polyhydroxy structure in the alcohol molecule aids in the solvation and complexation of lithium ions, enhancing their migration and mass transfer efficiency, and creating favorable conditions for the synergistic reaction of lithium, iron, and phosphorus components within the system. Through these multiple synergistic effects, the deep eutectic solvent system constructed in this invention can effectively regulate the iron, phosphorus, and lithium components under mild conditions and construct lithium iron phosphate crystals, providing a stable reaction environment for the direct regeneration of lithium iron phosphate.

[0010] Conventional techniques only use deep eutectic solvents for lithium replenishment, utilizing the existing crystal "skeleton" and lattice defects of waste materials as the driving force for lithium-ion intercalation, thus achieving lithium-ion intercalation repair. This method is highly dependent on the existing microstructure of the material. However, this invention overcomes this limitation by cleverly using lithium chloride monohydrate and polyols to formulate a deep eutectic solvent that is both rich in lithium ions and has strong reducing properties. This solvent can not only reduce iron in ferric phosphate but also actively control Fe²⁺. + The valence stability and the promotion of the formation of an ordered lattice are achieved, thus realizing the construction of a lithium iron phosphate lattice from zero to one.

[0011] According to some embodiments of the present invention, the polyol includes at least one selected from ethylene glycol, diethylene glycol, triethylene glycol, glycerol, 1,2-propanediol, and 1,3-propanediol.

[0012] According to some embodiments of the present invention, the raw materials for preparing the deep eutectic solvent are composed of lithium chloride monohydrate and polyol.

[0013] According to some embodiments of the present invention, the molar ratio of the hydrogen bond acceptor to the hydrogen bond donor is 1:2 to 6.

[0014] According to some embodiments of the present invention, the reaction is carried out under stirring at a speed of 300-600 rpm, such as 400 rpm.

[0015] According to some embodiments of the present invention, the solid-liquid ratio of the ferric phosphate dihydrate to the deep eutectic solvent is 30~120 g / L.

[0016] According to some embodiments of the present invention, the reaction time of the ferric phosphate dihydrate with the deep eutectic solvent is 3 to 48 hours.

[0017] According to some embodiments of the present invention, the method for preparing the deep eutectic solvent includes the following steps: mixing a hydrogen bond donor and a hydrogen bond acceptor at 40-100°C to form a deep eutectic solvent, with a reaction time of 0.5h-2h.

[0018] According to some embodiments of the present invention, the reaction of the deep eutectic solvent is carried out under stirring at a speed of 200-400 rpm, such as 300 rpm.

[0019] According to some embodiments of the present invention, the method further includes a step of recycling the liquid phase obtained after separating the reaction products. The liquid phase can be directly collected and recycled, making the operation simple and convenient.

[0020] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0021] Figure 1 This is the XRD pattern of ferric phosphate dihydrate, the raw material used in the embodiments of the present invention.

[0022] Figure 2 This is a schematic diagram of the process flow according to an embodiment of the present invention.

[0023] Figure 3 This is the XRD pattern of the synthesized product in Example 1 of this invention. Detailed Implementation

[0024] The following will clearly and completely describe the concept and technical effects of the present invention in conjunction with embodiments, so as to fully understand the purpose, features and effects of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, not all of them. Other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are all within the scope of protection of the present invention. Unless otherwise specified, the experimental methods used in the embodiments are conventional methods; the materials and reagents used, unless otherwise specified, are commercially available. Unless otherwise specified, the same parameter value is the same in all embodiments. The embodiments described below are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention.

[0025] In the description of this invention, references to terms such as "some embodiments" indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0026] The raw material used in the following scheme is commercially available ferric phosphate dihydrate, which was characterized by X-ray diffraction (XRD). The results are as follows: Figure 1As shown in the figure, its diffraction peaks are consistent with the standard card (JCPDS: 76-0451) of FePO4·H2O, indicating that the crystal is orthorhombic. The hydrogen bond donor in the leaching solvent is a reducing polyol (liquid phase), and the reagents for the hydrogen bond donor and acceptor are analytical grade reagents from Sinopharm Reagent Group.

[0027] Example 1 This example provides a method for the direct synthesis of iron phosphate under normal pressure using a deep eutectic solvent, such as... Figure 2 As shown. The preparation steps are as follows: Lithium chloride monohydrate was used as a hydrogen bond acceptor and mixed with ethylene glycol at a molar ratio of 1:2. The mixture was reacted in a water bath at 80°C and a stirring speed of 300 r / min for 1 h to obtain a deep eutectic solvent.

[0028] The prepared deep eutectic solvent and iron phosphate obtained from the leaching of waste lithium iron phosphate were mixed at a solid-liquid ratio of 60 g / L. The mixture was heated and stirred at 400 rpm in an oil bath at 160°C for 12 hours. After solid-liquid separation, the filtrate and the solid product of the lithium iron phosphate precursor were obtained. The separated filtrate can be recycled and reused.

[0029] Example 2 This example provides a method for the direct synthesis of iron phosphate using a deep eutectic solvent under normal pressure. The difference between this method and Example 1 is that the synthesis time is 6 hours at low temperature (160°C), while the rest of the preparation and leaching processes are the same as in Example 1.

[0030] Comparative Example 1 This example provides a method for direct high-pressure synthesis of ferric phosphate using a deep eutectic solvent. The difference between this method and Example 1 is that the deep eutectic solvent and ferric phosphate dihydrate are mixed at a solid-liquid ratio of 60 g / L and then placed in a high-pressure reactor to react at 180°C for 10 h. The solid product is then obtained by filtration and separation.

[0031] Comparative Example 2 This example provides a method for direct high-pressure synthesis of iron phosphate. Using conventional high-pressure synthesis of iron phosphate, lithium hydroxide, phosphoric acid, and ferrous sulfate heptahydrate are used as lithium, iron, and phosphorus sources, respectively, and mixed in a molar ratio of lithium:iron:phosphorus = 3:1:1. The pH is then adjusted to 8.0, and the mixture is placed in a high-pressure reactor and reacted at 180°C for 10 hours. After filtration, the solid product is obtained.

[0032] The solid products obtained in the above examples and comparative examples were characterized by XRD. The characterization results of Example 1 are as follows: Figure 3 As shown in the figure (standard card number in the figure is JCPDS: 81-1173), the product obtained is lithium iron phosphate.

[0033] Furthermore, the proportions of lithium, iron, and phosphorus in the solid products prepared in the above examples and comparative examples were measured using atomic absorption spectrometry. The results are shown in Table 1 below.

[0034] Table 1. Comparison and analysis of the proportions of lithium, iron, and phosphorus in the solid products prepared in the examples and comparative examples.

[0035] As can be seen from the table above, comparing the detection results of Example 1 and Example 2, it is evident that lithium iron phosphate can be synthesized under normal pressure using ferric phosphate dihydrate as a raw material in the ethylene glycol-lithium chloride monohydrate deep eutectic solvent system. The molar ratio of Li, Fe, and P in the obtained product is close to 1:1:1, indicating that this deep eutectic solvent can simultaneously provide an effective lithium source and construct a reaction environment conducive to precursor conversion. Furthermore, with increasing reaction temperature and reaction time, the elemental composition of the product becomes closer to the theoretical stoichiometry, indicating that increasing the reaction intensity helps to promote Li... + Mass transfer and diffusion, precursor transformation, and target crystalline phase formation are all involved. Comparative Example 1 shows that under high pressure, the deep eutectic solvent system can also achieve the preparation of lithium iron phosphate, but the product exhibits slight Fe enrichment, presumably related to insufficient local lithium sites caused by rapid crystallization under high pressure. Comparative Example 2 shows that a high-pressure synthesis system using traditional lithium, iron, and phosphorus sources can obtain products with near-theoretical stoichiometry, but this method requires more raw materials and has a longer process path. In contrast, the deep eutectic solvent method described in this invention has significant advantages in reducing process complexity, minimizing the need for external lithium sources, and achieving atmospheric pressure synthesis.

[0036] This invention utilizes a deep eutectic solvent composed of lithium chloride monohydrate and ethylene glycol, along with iron phosphate dihydrate as a raw material, to achieve a one-step synthesis of lithium iron phosphate materials. The elemental composition of the obtained product is close to the target stoichiometric ratio, indicating that the method has good synthetic feasibility. The results of the examples show that, under normal pressure conditions, the molar ratio of Li, Fe, and P in the product prepared by the method of this invention is close to the theoretical stoichiometric ratio of lithium iron phosphate (1:1:1), especially under higher temperatures and longer reaction times, the elemental ratio approaches the ideal value. This demonstrates that this invention not only possesses synthetic feasibility but also has good stoichiometric control capabilities, providing a foundation for obtaining structurally complete and stable lithium iron phosphate materials.

[0037] The embodiments of the present invention have been described in detail above, but the present invention is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present invention.

Claims

1. A method for direct synthesis of lithium iron phosphate under ambient pressure using a deep eutectic solvent, characterized in that: The process includes the following steps: mixing iron phosphate dihydrate with the deep eutectic solvent and reacting under heating to obtain lithium iron phosphate; wherein, the raw materials for preparing the deep eutectic solvent include a hydrogen bond donor and a hydrogen bond acceptor, the hydrogen bond acceptor includes lithium chloride monohydrate, the hydrogen bond donor includes a polyol, and the polyol has reducing properties; the heating temperature is 60~180℃.

2. The method for direct synthesis of lithium iron phosphate at ambient pressure using a deep eutectic solvent according to claim 1, characterized in that: The polyol includes at least one of ethylene glycol, diethylene glycol, triethylene glycol, glycerol, 1,2-propanediol, and 1,3-propanediol.

3. The method for direct synthesis of lithium iron phosphate at ambient pressure using a deep eutectic solvent according to claim 1, characterized in that: The raw materials for preparing the deep eutectic solvent consist of lithium chloride monohydrate and polyol.

4. The method for direct synthesis of lithium iron phosphate at ambient pressure using a deep eutectic solvent according to claim 1, characterized in that: The molar ratio of hydrogen bond acceptor to hydrogen bond donor is 1:2~6.

5. The method for direct synthesis of lithium iron phosphate at ambient pressure using a deep eutectic solvent according to claim 1, characterized in that: The reaction was carried out under stirring at a speed of 300-600 rpm.

6. The method for direct synthesis of lithium iron phosphate at ambient pressure using a deep eutectic solvent according to claim 1, characterized in that: The solid-liquid ratio of the iron phosphate dihydrate to the deep eutectic solvent is 30~120 g / L.

7. The method for direct synthesis of lithium iron phosphate at ambient pressure using a deep eutectic solvent according to claim 1, characterized in that: The reaction time of the ferric phosphate dihydrate with the deep eutectic solvent is 3~48h.

8. The method for direct synthesis of lithium iron phosphate at ambient pressure using a deep eutectic solvent according to claim 1, characterized in that: The method for preparing the deep eutectic solvent includes the following steps: mixing a hydrogen bond donor and a hydrogen bond acceptor at 40-100℃ to form a deep eutectic solvent, with a reaction time of 0.5h-2h.

9. The method for direct synthesis of lithium iron phosphate at ambient pressure using a deep eutectic solvent according to claim 1, characterized in that: The reaction of the deep eutectic solvent is carried out under stirring at a speed of 200-400 rpm.

10. The method for direct synthesis of lithium iron phosphate at ambient pressure using a deep eutectic solvent according to claim 1, characterized in that: The method also includes a step of recycling the liquid phase obtained after separating the reaction products.

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