One-dimensionally preferentially grown lithium iron phosphate at high salt concentration, preparation method and application thereof

Abstract

The application belongs to the technical field of lithium ion battery cathode materials, and particularly relates to one-way preferred growth of lithium iron phosphate under high salt concentration and a preparation method and application thereof. The preparation method comprises the following steps: preparing a high-concentration inorganic salt solution as a first solution; dissolving a phosphorus source and a lithium source required for a reaction in part of the first solution to form a second solution; dissolving an iron source and an additive required for the reaction in another part of the first solution to form a third solution; mixing the second solution and the third solution, and performing a hydrothermal reaction; cooling after the hydrothermal reaction is completed, and performing washing, drying and calcination treatment to obtain the lithium iron phosphate material. The method introduces high-concentration non-reaction atoms to promote one-way preferred growth of the lithium iron phosphate, widens lithium ion migration channels, improves the electrochemical performance of the material, solves the problem of low lithium ion transmission efficiency in the charging and discharging process of traditional hydrothermal lithium iron phosphate, and has important guiding significance and application value for developing high-performance phosphate cathode materials.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery cathode material technology, specifically relating to lithium iron phosphate grown unidirectionally under high salt concentration, its preparation method and application. Background Technology

[0002] In recent years, olivine lithium iron phosphate (LiFePO4) has become a preferred cathode material for lithium-ion batteries (LIBs) due to its excellent electrochemical performance, safety, environmental friendliness, and low raw material cost. LiFePO4 plays a crucial role as a cathode material in electric vehicles and energy storage. Especially with the recent rapid development of the domestic new energy vehicle market, manufacturers are not only pursuing driving performance but also addressing consumers' range anxiety. Some mid-to-high-end automakers (such as Tesla, BMW i, NIO, and XPeng) have chosen NCM ternary materials with higher energy density as cathode materials for lithium batteries. However, safety is a critical weakness of ternary materials. Therefore, achieving fast charge and discharge technology is another solution to address range anxiety, namely improving the high-rate charge and discharge performance of LiFePO4, which helps consolidate its application advantages in power batteries and energy storage.

[0003] To improve the high-rate charge-discharge performance of lithium iron phosphate, researchers generally employ a hydrothermal process to prepare nanoscale lithium iron phosphate, aiming to reduce the particle size and shorten the Li-P process time. +Ion diffusion channels are established, thereby further improving the Li-ion migration rate and two-phase transition kinetics. To date, hydrothermal strategies for preparing morphology-controllable and size-tunable C / LiFePO4 composites by adding various organic auxiliaries / surfactants to the precursor solution have achieved good results. For example, CN104201335B discloses a method of adding CTAB (hexadecyltrimethylammonium bromide) and VC (L-ascorbic acid) to a hydrothermal precursor solution to obtain morphologically regular and uniformly sized nanosheet-like lithium iron phosphate. Its unique sheet-like structure can shorten the lithium-ion transport distance during charging and discharging, thereby improving the electrochemical properties of the electrode material. However, its disadvantage is that CTAB contains halogens, posing a risk of environmental pollution from wastewater discharge. CN107628594B discloses a method of synthesizing porous 3D flower-like nanosheet lithium iron phosphate using two mixed organic solvents (oleic acid and ethylene glycol) as the active agent and solvent, respectively. Although this loose structure is beneficial for the complete wetting of the electrolyte and cathode material, shortens the lithium-ion migration and diffusion path, and improves the ion migration rate under high-rate charging and discharging of the battery... However, this significantly reduces the compaction density of lithium iron phosphate, thus lowering the volumetric energy density of lithium-ion batteries. Simultaneously, the extensive use of organic solvents increases process costs and poses certain risks to environmental pollution. CN115714199B also discloses a nano-lithium iron phosphate cathode material modified by self-polymerization initiated by acrylic acid monomers and lithium-grafted polyacrylic acid coating. Its uniform polyacrylic acid conductive coating layer improves the material's conductivity and stability, and can also avoid direct contact between the material and the electrolyte, thus preventing side reactions. This method only restricts the growth of the material's particle size and does not change the crystal structure of lithium iron phosphate; therefore, it does not fundamentally change the migration rate of lithium ions during charging and discharging.

[0004] In summary, although a series of optimizations and improvements have been made to the synthesis of lithium iron phosphate cathode materials, achieving good results, the problem of low intrinsic ion conductivity remains unresolved. In particular, the increased raw material costs and environmental pollution risks associated with hydrothermal synthesis due to the use of organic solvents are significant drawbacks. With the continuous expansion of applications in electric vehicles, lithium iron phosphate materials still require ongoing improvement, and the need to develop high-performance lithium iron phosphate materials is becoming increasingly urgent. Summary of the Invention

[0005] To address the technical problems existing in the prior art, this invention provides a method for the unidirectional preferential growth of lithium iron phosphate under high salt concentration, along with its preparation and applications. This invention controls the crystallization tendency of lithium iron phosphate by controlling the variation of salt concentration parameters during the hydrothermal process, thereby inducing it to exhibit excellent electrochemical performance.

[0006] To achieve the above objectives, the technical solution of the present invention is as follows:

[0007] The first aspect of this invention is a method for preparing lithium iron phosphate grown unidirectionally under high salt concentration, comprising the following steps:

[0008] Step 1: Prepare a high-concentration inorganic salt solution as the first solution;

[0009] Step 2: Dissolve the phosphorus and lithium sources required for the reaction in a portion of the first solution to form the second solution;

[0010] Step 3: Dissolve the iron source and additives required for the reaction in another portion of the first solution to form a third solution;

[0011] Step 4: Mix the second solution with the third solution and carry out a hydrothermal reaction;

[0012] Step 5: After the hydrothermal reaction is completed, the lithium iron phosphate material is obtained by cooling, washing, drying and calcining.

[0013] Further, in step 1, the concentration of the inorganic salt solution is 2–5 mol / L, and the inorganic salt is at least one of lithium ammonium sulfate, ammonium sulfate, ammonium chloride, and ammonium nitrate. In the above technical solution, the high-concentration inorganic salt solution can promote the reaction kinetics rate of lithium iron phosphate crystal crystallization during hydrothermal processes, catalyze the preferential growth of lithium iron phosphate along the

[200] crystal plane, broaden the lithium-ion transport channel, and reduce the internal resistance of lithium-ion migration, thereby achieving better electrochemical performance of lithium iron phosphate materials.

[0014] Furthermore, in step 2, the phosphorus source is at least one of phosphoric acid, ammonium monohydrogen phosphate, ammonium dihydrogen phosphate, ammonium phosphate, lithium phosphate, lithium monohydrogen phosphate, lithium dihydrogen phosphate, potassium phosphate, and phytic acid, and the lithium source is at least one of lithium monohydrogen phosphate, lithium dihydrogen phosphate, lithium hydroxide, lithium carbonate, and lithium acetate.

[0015] Furthermore, in step 3, the iron source is at least one of ferrous sulfate, ferrous chloride, ferrous gluconate, and ferrous citrate. The auxiliary agent mainly includes an organic iron chelating agent, and also includes at least one of a reducing agent, a nitrogen doping agent, and a surfactant. The organic iron chelating agent is at least one of glycine, DL-malic acid, and gluconic acid; the reducing agent is at least one of acetaldehyde and phenol; the nitrogen doping agent is at least one of melamine and urea; and the surfactant is at least one of SDS and ethanol. In the above technical solutions, the role of the organic iron chelating agent is to restrict iron preferentially occupying lithium sites, avoid / reduce the antisite defects of lithium iron phosphate, further reduce the internal resistance of ion migration, and improve the ion migration efficiency, which is very important for the electrochemical performance. The role of the reducing agent is, on the one hand, to prevent the divalent iron from being oxidized to trivalent iron during hydrothermal treatment, and on the other hand, the organic components are dehydrated and carbonized under the action of high temperature, high pressure, and high salt ions to form an in-situ coated conductive layer on the surface of lithium iron phosphate. The role of the nitrogen doping agent is, on the one hand, to form an in-situ nitrogen-doped carbon coating of lithium iron phosphate under hydrothermal treatment, and on the other hand, to restrict grain growth. The role of the surfactant is to disperse and prevent the agglomeration of synthesized lithium iron phosphate.

[0016] Further, the specific operation of step 2 is as follows: add the phosphorus source and lithium source to the first solution respectively, heat to 60-80℃, stir continuously until dissolved, and then mix the two to form the second solution; wherein, the molar ratio of phosphorus source and lithium source is 1:1-2, the concentration of phosphorus source in the first solution is 0.4-1 mol / L, the concentration of lithium source in the first solution is 0.4-2 mol / L, and the volume ratio of phosphorus source and lithium source in the first solution is 1:1.

[0017] Further, the specific operation of step 3 is as follows: dissolve the iron source and the additive in the remaining first solution, introduce nitrogen gas as a protective gas, heat to 60-80℃, and stir continuously until dissolved; wherein, the concentration of the iron source in the first solution is 0.4-1 mol / L, and the volume ratio of the first solution required to prepare the second and third solutions is 2:1.

[0018] Further, the specific operation of mixing the second and third solutions in step 4 is as follows: the second solution is slowly transferred to the third solution at a rate of 0.1–3 L / min and rapidly stirred at a rate of 300–1500 rap / min for 0.5–3 h; the hydrothermal reaction temperature in step 4 is 160–200 °C, and the time is 1–10 h. In the above technical solution, there are two important reaction nodes: first, the high concentration of salt promotes the unidirectional preferred orientation of lithium iron phosphate during the hydrothermal reaction. This orientation aims to broaden the lithium-ion diffusion channels of lithium iron phosphate, increase the ion conduction rate of the material, and thus improve the electrochemical performance of lithium iron phosphate; second, the iron chelating agent reduces the antisite defects of lithium iron phosphate during the hydrothermal process, while the coordinated action of the additives in the third solution can achieve a carbon-coated / nitrogen-doped carbon-coated lithium iron phosphate coating. This coating can improve the surface conductivity of the lithium iron phosphate material and limit the growth of grain size.

[0019] Furthermore, in step 5, the temperature is lowered to below 100°C, preferably 40°C or below.

[0020] Furthermore, in step 5, the drying temperature is 50°C or higher, preferably 80°C, and the drying time is 1 to 48 hours, preferably 10 hours.

[0021] Furthermore, in step 5, the calcination temperature is 650–750°C, and the time is 1–10 hours. In the above technical solution, the washing process before high-temperature calcination is to remove excess impurity ion salts, preventing the introduction of impurity ions into the lithium iron phosphate in the next high-temperature calcination stage, which would damage its structure, cause structural instability, and lead to reduced cycle performance. High-temperature calcination is to eliminate internal stresses in the material, further improve the material's crystallinity, and transform the disordered carbon coating / nitrogen-doped carbon into graphitized material, thereby improving the material's conductivity and ultimately improving the charge-discharge performance of lithium iron phosphate.

[0022] The second aspect of the present invention is lithium iron phosphate unidirectionally preferentially grown under high salt concentration obtained by the preparation method described in the first aspect, which has a flat rod-shaped structure resembling an ice cream stick, with a particle length of 0.1 to 1 μm, a width of 50 to 150 nm, and a thickness of 5 to 50 nm.

[0023] The third aspect of the present invention is the application of lithium iron phosphate grown unidirectionally with preferred concentration under high salt conditions as a cathode material for lithium-ion batteries, as described in the second aspect.

[0024] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0025] Traditional hydrothermal methods merely adjust the proportion or concentration of raw materials and solvents to alter the structure and morphology of lithium iron phosphate (LFP). Traditional nitrogen-doped carbon coating of LFP only occurs during high-temperature calcination and utilizes solvents to reduce LFP defects. This invention, however, takes a different approach, introducing a high concentration of non-reactive atoms to promote the unidirectional preferential growth of LFP, broadening lithium-ion migration channels and improving the electrochemical performance of the material—something traditional hydrothermal methods cannot achieve. The reduced LFP anti-site defects achieved in this invention are achieved by using an iron chelating agent to restrict the preferential occupation of lithium sites by iron, thus reducing defects. Simultaneously, in a high-temperature, high-pressure environment, nitrogen-containing organic matter and reducing agents, along with carbon-containing inorganic materials, achieve uniform (in-situ) nitrogen-doped carbon coating of LFP, improving the material's conductivity, limiting particle size growth, and ultimately producing superior, high-performance LFP.

[0026] The high-concentration salt used in this invention promotes the unidirectional preferential growth of lithium iron phosphate during the hydrothermal process, broadens the lithium-ion migration channel, and solves the problem of low intrinsic ion conductivity of the material. At the same time, the use of chelating agents can replace traditional organic solvents, and in-situ nitrogen doping of carbon can achieve the goals of cost reduction, efficiency improvement, and green sustainable development in the synthesis of lithium iron phosphate. Attached Figure Description

[0027] Figure 1 SEM image of lithium iron phosphate prepared in Example 1

[0028] Figure 2 XRD pattern of lithium iron phosphate prepared in Example 1 Detailed Implementation

[0029] To facilitate understanding of the present invention, a more comprehensive description will be given below. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a thorough and complete understanding of the disclosure of the present invention.

[0030] Example 1

[0031] The preparation method of lithium iron phosphate materials mainly includes the following steps:

[0032] 1) Dissolve 5 mol of ammonium sulfate in 1 L of water to obtain the first solution;

[0033] 2) Dissolve 0.3 mol ammonium dihydrogen phosphate and 0.6 mol lithium hydroxide in 1 / 3 L of the first solution, heat to 80 °C, and transfer the phosphorus solution to the lithium solution while stirring continuously to allow them to react completely, thus obtaining the second solution;

[0034] 3) Dissolve 0.3 mol of ferrous sulfate in 1 / 3 L of the first solution, then add 5 g of DL-malic acid, 8 g of phenol, 5 g of urea and 2 g of SDS, and purge with nitrogen as a protective atmosphere. Heat to 80 °C and stir continuously until dissolved to obtain the third solution.

[0035] 4) Slowly transfer the second solution obtained in step 2) to the third solution and stir rapidly. The transfer rate is 0.2 L / min, the stirring rate is 1000 rap / min, and the reaction time is 0.5 h. Then, quickly transfer the mixture to a high-pressure hydrothermal reactor with a polytetrafluoroethylene liner for high-temperature hydrothermal reaction. Set the reaction temperature to 180 °C and the reaction time to 5 h.

[0036] 5) After the hydrothermal reaction is completed, the temperature is lowered to 40℃ and centrifuged at high speed. After washing three times, it is dried in a vacuum drying oven at 80℃ for 10 hours. Then it is ground and pulverized. Under the protection of an inert atmosphere, the lithium iron phosphate precursor is subjected to high-temperature calcination treatment at 750℃ with a heating rate of 3℃ / min for 8 hours to finally obtain lithium iron phosphate material.

[0037] Figure 1 The image shows a SEM image of the lithium iron phosphate prepared in Example 1. As can be seen, the lithium iron phosphate exhibits a flat, rod-like structure resembling an ice cream stick, approximately 700–900 nm long, 150 nm wide, and 10 nm thick. The particle size distribution is uniform. Its flat structure facilitates electrolyte wetting, reduces the lithium-ion transport distance, and indirectly improves lithium-ion transport efficiency. The longer morphology also contributes to improved material conductivity (electrons tend to travel along the conductive coating on the surface of the lithium iron phosphate).

[0038] Figure 2 The XRD pattern of lithium iron phosphate prepared in Example 1 shows that the X-ray diffraction peaks of the synthesized lithium iron phosphate are basically coincident with those of the standard card (PDF#81-1173), proving that the lithium iron phosphate material prepared in this example is a typical Pnma space group and no impurity peaks are present, which means it is pure phase lithium iron phosphate. The difference is that the diffraction intensity of the synthesized lithium iron phosphate at the

[200] crystal plane is significantly enhanced compared with the standard sample (PDF#81-1173), while the diffraction intensity of other crystal planes is basically unchanged, indicating that the synthesized lithium iron phosphate has obvious unidirectional preferred orientation on the

[200] crystal plane.

[0039] Example 2

[0040] The preparation method of lithium iron phosphate materials mainly includes the following steps:

[0041] 1) Dissolve 4 mol of ammonium chloride in 0.9 L of water to obtain the first solution;

[0042] 2) Dissolve 0.4 mol of lithium dihydrogen phosphate and 0.4 mol of lithium hydroxide in 0.3 L of the first solution, heat to 80 °C, and transfer the phosphorus solution to the lithium solution while stirring continuously to allow them to react completely, thus obtaining the second solution;

[0043] 3) Dissolve 0.4 mol of ferrous sulfate in 0.3 L of the first solution, then add 5 g of gluconic acid, 15 g of melamine and 0.1 L of ethanol, and purge with nitrogen as a protective atmosphere. Heat to 80 °C and stir continuously until dissolved to obtain the third solution.

[0044] 4) The second solution obtained in step 2) is slowly transferred to the third solution and stirred rapidly. The transfer rate is 0.1 L / min, the stirring rate is 1200 rap / min, and the reaction time is 0.5 h. Then the mixture is quickly transferred to a high-pressure hydrothermal reactor with a polytetrafluoroethylene liner for high-temperature hydrothermal reaction. The reaction temperature is set at 180 °C and the reaction time is 4 h.

[0045] 5) After the hydrothermal reaction is completed, the temperature is lowered to 40℃ and centrifuged at high speed. After washing three times, it is dried in a vacuum drying oven at 80℃ for 10 hours. Then it is ground and pulverized. Under the protection of an inert atmosphere, the lithium iron phosphate precursor is subjected to high-temperature calcination treatment at 750℃ with a heating rate of 3℃ / min for 8 hours to finally obtain lithium iron phosphate material.

[0046] Example 3

[0047] The preparation method of lithium iron phosphate materials mainly includes the following steps:

[0048] 1) Dissolve 5 mol of lithium ammonium sulfate in 1 L of water to obtain the first solution;

[0049] 2) Dissolve 0.5 mol ammonium dihydrogen phosphate and 0.5 mol lithium hydroxide in 1 / 3 L of the first solution, heat to 80 °C, and transfer the phosphorus solution to the lithium solution while stirring continuously to react completely, thus obtaining the second solution;

[0050] 3) Dissolve 0.5 mol of ferrous sulfate in 1 / 3 L of the first solution, then add 5 g of glycine, 10 g of acetaldehyde and 5 g of SDS, and purge with nitrogen as a protective atmosphere. Heat to 80 °C and stir continuously until dissolved to obtain the third solution.

[0051] 4) Slowly transfer the second solution obtained in step 2) to the third solution and stir rapidly. The transfer rate is 0.1 L / min, the stirring rate is 1200 rap / min, and the reaction time is 0.5 h. Then, quickly transfer the mixture to a high-pressure hydrothermal reactor with a polytetrafluoroethylene liner for high-temperature hydrothermal reaction. Set the reaction temperature to 180 ℃ and the reaction time to 3 h.

[0052] 5) After the hydrothermal reaction is completed, the temperature is lowered to 40℃ and centrifuged at high speed. After washing three times, it is dried in a vacuum drying oven at 80℃ for 10 hours. Then it is ground and pulverized. Under the protection of an inert atmosphere, the lithium iron phosphate precursor is subjected to high-temperature calcination treatment at 750℃ with a heating rate of 3℃ / min and a holding time of 6 hours to finally obtain lithium iron phosphate material.

[0053] Comparative Example 1

[0054] The preparation method of lithium iron phosphate materials mainly includes the following steps:

[0055] 1) Dissolve 0.3 mol ammonium dihydrogen phosphate and 0.6 mol lithium hydroxide in 0.5 L of water, heat to 80 °C, and stir continuously until dissolved or evenly dispersed to obtain the first solution;

[0056] 2) Dissolve 0.3 mol of ferrous sulfate in 0.5 L of water, then add 5 g of DL-malic acid, 8 g of phenol, 5 g of urea and 2 g of SDS, and purge with nitrogen as a protective atmosphere. Heat to 80 °C and stir continuously until dissolved to obtain the second solution.

[0057] 3) Slowly transfer the first solution obtained in step 1) to the second solution and stir rapidly. The transfer rate is 0.2 L / min, the stirring rate is 1000 rap / min, and the reaction time is 0.5 h. Then, quickly transfer the mixture to a high-pressure hydrothermal reactor with a polytetrafluoroethylene liner for high-temperature hydrothermal reaction. Set the reaction temperature to 180 ℃ and the reaction time to 5 h.

[0058] 4) After the hydrothermal reaction is completed, the temperature is lowered to 40℃ and centrifuged at high speed. After washing three times, it is dried in a vacuum drying oven at 80℃ for 10 hours. Then it is ground and pulverized. Under the protection of an inert atmosphere, the lithium iron phosphate precursor is calcined at high temperature of 750℃, with a heating rate of 3℃ / min and a holding time of 8 hours to finally obtain the lithium iron phosphate material.

[0059] Comparative Example 2

[0060] The preparation method of lithium iron phosphate materials mainly includes the following steps:

[0061] 1) Dissolve 0.4 mol of lithium dihydrogen phosphate and 0.4 mol of lithium hydroxide in 0.5 L of water, heat to 80 °C, and stir continuously until dissolved or evenly dispersed to obtain the first solution;

[0062] 2) Dissolve 0.4 mol of ferrous sulfate in 0.4 L of water, then add 5 g of gluconic acid, 15 g of melamine, and 0.1 L of ethanol, and purge with nitrogen as a protective atmosphere. Heat to 80 °C and stir continuously until dissolved to obtain the second solution.

[0063] 3) Slowly transfer the first solution obtained in step 1) to the second solution and stir rapidly. The transfer rate is 0.1 L / min, the stirring rate is 1200 rap / min, and the reaction time is 0.5 h. Then, quickly transfer the mixture to a high-pressure hydrothermal reactor with a polytetrafluoroethylene liner for high-temperature hydrothermal reaction. Set the reaction temperature to 180 ℃ and the reaction time to 4 h.

[0064] 4) After the hydrothermal reaction is completed, the temperature is lowered to 40℃ and centrifuged at high speed. After washing three times, it is dried in a vacuum drying oven at 80℃ for 10 hours. Then it is ground and pulverized. Under the protection of an inert atmosphere, the lithium iron phosphate precursor is calcined at high temperature of 750℃, with a heating rate of 3℃ / min and a holding time of 8 hours to finally obtain the lithium iron phosphate material.

[0065] Comparative Example 3

[0066] The preparation method of lithium iron phosphate materials mainly includes the following steps:

[0067] 1) Dissolve 5 mol of lithium ammonium sulfate in 1 L of water to obtain the first solution;

[0068] 2) Dissolve 0.5 mol ammonium dihydrogen phosphate and 0.5 mol lithium hydroxide in 1 / 3 L of the first solution, heat to 80 °C, and transfer the phosphorus solution to the lithium solution while stirring continuously to react completely, thus obtaining the second solution;

[0069] 3) Dissolve 0.5 mol of ferrous sulfate in 1 / 3 L of the first solution, then add 10 g of acetaldehyde and 5 g of SDS, and purge with nitrogen as a protective atmosphere. Heat to 80 °C and stir continuously until dissolved to obtain the third solution.

[0070] 4) Slowly transfer the second solution obtained in step 2) to the third solution and stir rapidly. The transfer rate is 0.1 L / min, the stirring rate is 1200 rap / min, and the reaction time is 0.5 h. Then, quickly transfer the mixture to a high-pressure hydrothermal reactor with a polytetrafluoroethylene liner for high-temperature hydrothermal reaction. Set the reaction temperature to 180 ℃ and the reaction time to 3 h.

[0071] 5) After the hydrothermal reaction is completed, the temperature is lowered to 40℃ and centrifuged at high speed. After washing three times, it is dried in a vacuum drying oven at 80℃ for 10 hours. Then it is ground and pulverized. Under the protection of an inert atmosphere, the lithium iron phosphate precursor is subjected to high-temperature calcination treatment at 750℃ with a heating rate of 3℃ / min and a holding time of 6 hours to finally obtain lithium iron phosphate material.

[0072] The lithium iron phosphate materials prepared in Examples 1-3 and Comparative Examples 1-3 were used as positive electrode materials for lithium batteries and applied to CR2032 coin cells. The ratio of active material (lithium iron phosphate), binder (PVDF), and conductive agent (Ketjen Black) was 9:0.5:0.5. Charge and discharge performance tests were conducted, and the test results are shown in Table 1.

[0073] Table 1 shows the discharge capacity and cycle performance test data of the lithium iron phosphate materials prepared in Examples 1-3 and Comparative Examples 1-3.

[0074]

[0075] As shown in Table 1, the lithium iron phosphate material prepared in this embodiment exhibits excellent electrochemical performance and good rate performance when used in lithium batteries. For example, the lithium iron phosphate material prepared in Example 2 achieves a specific capacity of 161 mAh / g (theoretical capacity 170 mAh / g) under 0.2C charge / discharge conditions, and still reaches 138 mAh / g under 10C charge / discharge conditions. Here, the lithium iron phosphate materials prepared in Examples 1 and 1, as well as Examples 2 and 2, are used in lithium batteries. The electrochemical results show that, without high-concentration salt-assisted hydrothermal conditions, the rate performance of the lithium iron phosphate prepared in the comparative examples is poor. This indirectly proves that high-concentration salt-assisted hydrothermal conditions lead to a unidirectional preferred orientation of lithium iron phosphate, resulting in superior rate performance. This unidirectional preferred orientation can be achieved by… Figure 2 This has been proven. The electrochemical performance results of the lithium iron phosphate prepared in Example 3 and Comparative Example 3 show that the iron chelating agent plays an important role in optimizing the structure of lithium iron phosphate (reducing anti-site defects) and improving electrochemical performance.

[0076] The above description is only for better explaining the embodiments of the present invention and is not intended to limit them. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention shall fall within the scope of the present invention.

Claims

1. A method for preparing lithium iron phosphate grown unidirectionally under high salt concentration, characterized in that, Includes the following steps: Step 1: Prepare a high-concentration inorganic salt solution as the first solution; Step 2: Dissolve the phosphorus and lithium sources required for the reaction in a portion of the first solution to form the second solution; Step 3: Dissolve the iron source and additives required for the reaction in another portion of the first solution to form a third solution; Step 4: Mix the second solution with the third solution and carry out a hydrothermal reaction; Step 5: After the hydrothermal reaction is completed, the lithium iron phosphate material is obtained by cooling, washing, drying and calcining. The lithium iron phosphate exhibits a distinct unidirectional preferred orientation on the [200] crystal plane; In step 1, the concentration of the inorganic salt solution is 2-5 mol / L, and the inorganic salt is at least one of lithium ammonium sulfate, ammonium sulfate, ammonium chloride, and ammonium nitrate. In step 3, the iron source is at least one of ferrous sulfate, ferrous chloride, ferrous gluconate, and ferrous citrate. The auxiliary agent is an organic iron chelating agent. It also includes at least one of a reducing agent, a nitrogen doping agent, and a surfactant. The organic iron chelating agent is at least one of glycine, DL-malic acid, and gluconic acid. The reducing agent is at least one of acetaldehyde and phenol. The nitrogen doping agent is at least one of melamine and urea. The surfactant is at least one of SDS and ethanol. In step 4, the hydrothermal reaction temperature is 160~200℃ and the time is 1~10h.

2. The method for preparing lithium iron phosphate grown unidirectionally under high salt concentration according to claim 1, characterized in that, In step 2, the phosphorus source is at least one of phosphoric acid, ammonium monohydrogen phosphate, ammonium dihydrogen phosphate, ammonium phosphate, lithium phosphate, lithium monohydrogen phosphate, lithium dihydrogen phosphate, potassium phosphate, and phytic acid, and the lithium source is at least one of lithium monohydrogen phosphate, lithium dihydrogen phosphate, lithium hydroxide, lithium carbonate, and lithium acetate.

3. The method for preparing lithium iron phosphate grown unidirectionally under high salt concentration according to claim 1, characterized in that, The specific operation of mixing the second solution and the third solution in step 4 is as follows: the second solution is slowly transferred to the third solution at a rate of 0.1~3L / min and stirred rapidly at a rate of 300~1500rap / min for 0.5~3h.

4. The method for preparing lithium iron phosphate grown unidirectionally under high salt concentration according to claim 1, characterized in that, In step 5, the temperature is lowered to below 100℃; the drying temperature is above 50℃ for 1 to 48 hours; and the calcination temperature is 650 to 750℃ for 1 to 10 hours.

5. A lithium iron phosphate unidirectionally preferentially grown at high salt concentration obtained by the preparation method according to any one of claims 1-4, characterized in that, It has a flat, rod-shaped structure resembling an ice cream stick, with a particle length of 0.1~1μm, a width of 50~150nm, and a thickness of 5~50nm.

6. An application of lithium iron phosphate grown unidirectionally under high salt concentration as described in claim 5, characterized in that, As a cathode material for lithium-ion batteries.

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