A method for preparing lithium iron phosphate with ferric oxyhydroxide as an intermediate

Abstract

This invention discloses a method for preparing lithium iron phosphate using iron hydroxyl oxide as an intermediate, belonging to the field of new energy battery materials. The method includes: removing impurities from a ferrous sulfate solution, adding ammonium bicarbonate and ammonium fluoride, heating and passing an oxidizing gas to react and generate the iron hydroxyl oxide intermediate; mixing and milling this intermediate with a phosphorus source, a lithium source, and a carbon source, spray drying, and sintering under an inert atmosphere to obtain lithium iron phosphate cathode material. This invention uses iron hydroxyl oxide as the sole iron source, eliminating the need for the iron phosphate intermediate throughout the entire process. In step two, ammonium fluoride is used for in-situ doping with fluorine, and combined with the structural template effect of the nanorod-shaped iron hydroxyl oxide, uniform nanoparticles are obtained with only one milling operation, forming microporous channels. This method simplifies the production process, reduces equipment investment and energy consumption, and the resulting material exhibits high capacity, high initial efficiency, and excellent rate performance.

Description

Technical Field

[0001] This invention belongs to the field of new energy battery materials, specifically relating to a method for preparing lithium iron phosphate, a low-cost lithium-ion battery cathode material using iron hydroxyl oxide as an intermediate. Background Technology

[0002] Lithium iron phosphate (LFP) cathode material is currently the fastest-growing lithium-ion battery cathode material in China. Its raw materials are widely available and inexpensive, and it is widely used in the domestic battery industry in fields such as automobiles, power tools, energy storage equipment, emergency power supplies, and mobile power supplies. New energy electric vehicles are the primary application area, accounting for over 45% of total LFP applications. Compared with other cathode materials, LFP has advantages such as safety, environmental friendliness, low cost, long cycle life, and good high-temperature performance, making it one of the most promising lithium-ion battery cathode materials.

[0003] There are several methods for preparing lithium iron phosphate, mainly divided into solid-phase and liquid-phase methods. The solid-phase method is currently the most mature and commercially widely used method, and it can be further divided into three main categories based on the key raw materials: ferrous oxalate, iron phosphate, and iron oxide. The iron phosphate route has become the mainstream process for preparing lithium iron phosphate due to its superior cost-effectiveness, ease of processing, and high capacity. The iron phosphate route starts with ferrous sulfate and proceeds through steps such as impurity removal, crystallization, aging, drying, and crushing to obtain iron phosphate, which is then compounded with lithium and carbon sources to produce lithium iron phosphate. However, this route has a complex process flow, requires various types of equipment, and has high investment costs.

[0004] CN116354326A discloses a method for preparing lithium iron phosphate, a cathode material for lithium-ion batteries. This method uses iron hydroxyl oxide to replace a portion of the iron phosphate, and removes impurities during the preparation process. The lithium iron phosphate and lithium phosphate are combined, and the cathode material is prepared through spray drying granulation and sintering. This method saves raw material costs, is simple to operate and easy to produce, and exhibits good performance. However, this method still requires iron phosphate as a raw material, and further simplification of the production steps is needed.

[0005] CN117430104A provides a method for preparing lithium iron phosphate from low-carbon lithium iron phosphate and hydroxyl iron phosphate. The method involves purifying ferrous sulfate to form a ferrous sulfate solution, adding hydrogen peroxide, phosphoric acid, ammonium dihydrogen phosphate, and ammonia to the ferrous sulfate solution, reacting to form a mixed slurry, maintaining the slurry at room temperature for a period of time, then washing and filtering with water to form hydroxyl iron phosphate precursors with different iron-to-phosphorus ratios. These precursors are then flash-dried and sintered at high temperature to obtain hydroxyl iron phosphate precursors with different iron-to-phosphorus ratios and specific surface areas. The hydroxyl iron phosphate precursors are then pulverized and mixed to obtain the finished hydroxyl iron phosphate. High-iron-to-phosphorus ratio hydroxyl iron phosphate and low-iron-to-phosphorus ratio hydroxyl iron phosphate are mixed in a certain proportion and then proportioned with lithium phosphate and low-carbon lithium iron phosphate finished products, with the addition of carbon sources and additives to form a mixture. The mixture is then subjected to sand milling, spray drying, sintering, pulverizing, sieving, batching, and packaging to obtain the finished lithium iron phosphate. However, this method involves relatively cumbersome steps and requires further improvement in the reaction rate.

[0006] CN117819506B provides a method for one-step solid-phase synthesis of lithium iron phosphate based on iron oxide. This method requires grinding iron oxide to at least micron level, then adding lithium source, phosphorus source and carbon source to the iron oxide powder in sequence, and grinding three times to obtain a mixture. After that, the mixture is calcined, crushed and sieved to obtain lithium iron phosphate cathode material. This method uses iron oxide to synthesize lithium iron phosphate through solid-phase method, but it requires three ball milling processes, which is time-consuming and the reaction steps are more complicated than the reaction route with iron hydroxyl oxide as an intermediate.

[0007] Therefore, there is an urgent need to develop a preparation method for lithium iron phosphate cathode materials that has a shorter process flow, lower production cost, and can guarantee the comprehensive performance of the materials. Summary of the Invention

[0008] To address the shortcomings of existing technologies, the purpose of this invention is to provide a method for preparing lithium iron phosphate based on iron hydroxyl oxide as a precursor. The specific method is as follows: In a first aspect, this application provides a method for preparing lithium iron phosphate using iron hydroxyl oxide as an intermediate, comprising the following steps: Step 1: Dissolve ferrous sulfate in deionized water to obtain a ferrous sulfate solution; Step 2: Add ammonium bicarbonate and ammonium fluoride to the ferrous sulfate solution obtained in Step 1, heat and introduce oxidizing gas to react and generate ferric hydroxide intermediate. After filtration, washing and drying, ferric hydroxide powder is obtained. Step 3: Add the hydroxyl iron oxide powder obtained in Step 2 to deionized water, mix with phosphorus source, lithium source and carbon source, and mill to obtain slurry; Step 4: The slurry is dried by spray drying to obtain powder; Step 5: Sinter the powder in an inert atmosphere to obtain lithium iron phosphate cathode material.

[0009] Furthermore, in step one, after ferrous sulfate is dissolved in deionized water, Fe²⁺… + The concentration is 10–90 g / L.

[0010] Furthermore, step one also includes a purification step, which is carried out by adding dilute phosphoric acid to remove titanium, stirring and filtering, adjusting the pH to 2.5-3.5, and filtering a second time to remove aluminum and zinc.

[0011] Furthermore, in step two, the molar ratio of ammonium bicarbonate to ferrous sulfate is 1:1; the molar ratio of ammonium fluoride to ferrous sulfate is 1:120-1200.

[0012] Furthermore, in step two, the molar ratio of ammonium fluoride to ferrous sulfate is 1:(400-800).

[0013] In this step, ammonium bicarbonate acts as a precipitant, providing carbonate ions to react with Fe²⁺ ions in ferrous sulfate. + The reaction produces ferrous carbonate (FeCO3) precipitate, which is then converted to ferric hydroxide (FeOOH) by the introduction of an oxidizing gas. Theoretically, Fe²⁺... + Complete precipitation to FeCO3 requires twice the molar amount of ammonium bicarbonate. However, research has shown that using a substoichiometric ratio of 1:1 between ammonium bicarbonate and ferrous sulfate avoids excessive local alkalinity that could lead to Fe(OH)2 formation. Furthermore, with the addition of a subsequent oxidizing gas, it still efficiently converts to ferric hydroxide, resulting in a product with superior purity and crystallinity. Insufficient ammonium bicarbonate reduces the iron yield, while excess leads to side reactions.

[0014] Furthermore, the oxidizing gas in step two is one or more of compressed air, oxygen, or ozone.

[0015] Furthermore, the phosphorus source in step three is one or more of phosphoric acid, monoammonium phosphate, potassium dihydrogen phosphate, sodium dihydrogen phosphate, ammonium dihydrogen phosphate, or calcium dihydrogen phosphate; in the phosphorus source, the mass fraction of phosphoric acid is 75%~85%, and the mass fraction of monoammonium phosphate, potassium dihydrogen phosphate, sodium dihydrogen phosphate, ammonium dihydrogen phosphate, or calcium dihydrogen phosphate is greater than 98%.

[0016] Furthermore, the carbon source in step three is at least one of an organic carbon source and an inorganic carbon source. The organic carbon source is at least one of glucose, sucrose, polyethylene glycol, and starch, and the inorganic carbon source is at least one of superconducting carbon black, Ketjen black, and graphene.

[0017] Furthermore, in step three, the molar ratio of lithium source, iron hydroxyl oxide and phosphorus source, calculated by Li, Fe and P elements, is (1.02~1.08):1:(1~1.20); the amount of carbon source added is 8-30% of the total mass of phosphorus source, iron hydroxyl oxide and lithium source.

[0018] Furthermore, the drying temperature in step four is 200-300℃; the sintering temperature in step five is 600℃-800℃.

[0019] Secondly, the present invention provides a lithium-ion battery, wherein the positive electrode material of the lithium-ion battery is made of lithium iron phosphate positive electrode material prepared by any of the preparation methods described herein.

[0020] Compared with the prior art, the beneficial technical effects of this invention are reflected in: (1) such as Figure 1 As shown, this invention uses ferric hydroxide as the sole iron source intermediate to directly prepare lithium iron phosphate, without adding any iron phosphate. Compared to conventional iron phosphate... Figure 1 (Right side), the route of this invention ( Figure 1 (Left side) It saves production steps such as aging, crystallization, flash evaporation, and drying, simplifies the production process, reduces equipment investment and production energy consumption, and effectively reduces production costs; (2) The ferric hydroxide intermediate synthesized by the preparation method of the present invention exhibits a unique nanorod-like structure (see Figure 2 The surface has a large number of hydroxyl groups (-OH), which have a high specific surface area and abundant active sites. During high-temperature sintering, it can fully contact and react rapidly with phosphorus and lithium sources. After being directly mixed with phosphorus, carbon and lithium sources, it is ground, sintered and graded once to obtain the finished product. In contrast, oxidized railway lines need to be ground at least twice before entering sintering. Compared with oxidized railway lines, it reduces the number of grinding steps and has higher reactivity.

[0021] (3) The FePO4 crystals generated in situ using iron hydroxyl oxide in this invention are finer and more uniformly dispersed, and have more sufficient contact with carbon and lithium sources; and avoid the agglomeration and crystal growth that may occur in purchased FePO4 during storage; the escape of OH groups leaves microporous channels inside the particles, which is conducive to the subsequent diffusion of lithium ions, and serves as a structural template to induce the final formation of uniform nanoparticles of LiFePO4; avoids the generation of large-sized crystals, shortens the lithium ion diffusion path, and improves rate performance.

[0022] (4) The fluorine doping of the present invention is introduced in situ during the preparation stage of the iron hydroxyl oxide precursor (ammonium fluoride is added in step two), rather than surface doping or post-treatment during the sintering stage of the lithium iron phosphate finished product. This in-situ doping method allows fluorine ions to be uniformly incorporated into the iron hydroxyl oxide lattice, and in the subsequent high-temperature solid-state reaction with phosphorus and lithium sources, F- As a lattice modifier, it promotes the formation of more uniform LiFePO4 nanocrystals (such as... Figure 3 As shown, the particle size distribution is uniform, compared to Figure 4 (This avoids the agglomeration phenomenon of traditional lithium iron phosphate). At the same time, in-situ doping avoids the problem of fluorine only accumulating on the particle surface in post-doping methods, achieving uniform bulk doping and thus improving the overall electrical performance of lithium iron phosphate.

[0023] (5) The preparation method of the present invention can flexibly select the types of phosphorus source, carbon source and lithium source, has good raw material adaptability, and can flexibly adjust the ratio to optimize performance according to different application scenarios. Attached Figure Description

[0024] Figure 1 A comparison diagram of the phosphoric acid railway line and the route of this invention; Figure 2 This is a scanning electron microscope image of the iron hydroxyoxide prepared in Example 1 of the present invention; Figure 3 Here is a scanning electron microscope image of lithium iron phosphate prepared in Example 1 of this invention; Figure 4 A scanning electron microscope image of lithium iron phosphate prepared for Comparative Example 1; Figure 5 The initial charge-discharge curve of the lithium iron phosphate cathode material prepared in Example 1 after being assembled into a battery at a rate of 0.2C is shown. Figure 6 The diagram shows the state-of-charge-voltage curves of the lithium iron phosphate cathode materials prepared in Example 1 and Comparative Example 1 when assembled into batteries. Detailed Implementation

[0025] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0026] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0027] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in different places in this specification does not necessarily refer to the same embodiment, nor is it a single or selective embodiment that is mutually exclusive with other embodiments.

[0028] In addition, unless otherwise specified, the preparation processes in the following embodiments are all conventional methods in the prior art, and therefore will not be described in detail; the raw materials used in the following embodiments are all commercially available products.

[0029] The embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0030] Figure 1 The process route of this invention using ferric hydroxide as an intermediate (right side) is compared with that of the existing phosphate railway route (left side). Figure 1 As shown, this invention eliminates the cumbersome steps of crystallization, aging, drying, and crushing required for the preparation of iron phosphate. Instead, it directly mixes iron hydroxyl oxide with lithium, phosphorus, and carbon sources, mills, spray-dries, and then sinters. This significantly shortens the process flow and greatly reduces equipment investment and production energy consumption.

[0031] Example 1 A method for preparing lithium iron phosphate using iron hydroxyl oxide as an intermediate includes the following steps: Step 1: Dissolve ferrous sulfate in water to prepare Fe 2+ A 50 g / L solution was prepared and impurity removal was performed. 15 wt% dilute phosphoric acid was added to remove titanium. After stirring, the solution was filtered, and ammonia was added to adjust the pH to 2.5-3.5. Al and Zn were removed by secondary filtration to obtain a purified ferrous sulfate solution. It should be noted that the above-mentioned impurity removal steps are not absolutely necessary. When the ferrous sulfate raw material used has high purity and the content of impurities (such as titanium, aluminum, and zinc) is below the allowable range of the process, this impurity removal step can be omitted.

[0032] Step 2: Under nitrogen protection, add ammonium bicarbonate in an equal molar amount to ferrous ions to the ferrous sulfate purification solution, and then add ammonium fluoride. The amount of ammonium fluoride added is 0.25% of the mass of ferric hydroxide (calculated as Fe) (after conversion, the molar ratio of ammonium fluoride to ferrous sulfate is approximately 1:264). The temperature is raised to 60℃, and after reacting for 3 hours, oxygen is introduced and the reaction continues for 10 hours. After filtration, washing, and drying, ferric hydroxide powder is obtained. Step 3: Add iron hydroxide powder, phosphoric acid, lithium carbonate, lithium hydroxide, and glucose to deionized water, mix and mill, according to the molar ratio of n(Fe):n(P):n(Li) = 1:1.05:1.03, where the molar ratio of lithium carbonate to lithium hydroxide is 9:1; the amount of glucose added is 10% of the total mass of iron hydroxide, phosphoric acid, lithium carbonate, and lithium hydroxide. After milling, a slurry with D50 of 0.50-0.55μm is obtained. Step 4: Dry the slurry after sand milling to obtain powder by spray drying. The inlet air temperature is 260℃ and the outlet air temperature is 110℃. Step 5: Sinter the powder in an inert atmosphere at a temperature of 745℃, a heating rate of 2-3℃ / min, and a holding time of 8h. Then, grade and crush the powder to obtain lithium iron phosphate cathode material.

[0033] Example 2 A method for preparing lithium iron phosphate using iron hydroxyl oxide as an intermediate includes the following steps: Step 1: Dissolve ferrous sulfate in water to prepare Fe 2+ A 50 g / L solution was used for impurity removal. 15 wt% dilute phosphoric acid was added to remove titanium. After stirring, the solution was filtered. Ammonia was added to adjust the pH to 2.5-3.5. Al and Zn were removed by secondary filtration to obtain a purified ferrous sulfate solution. Step 2: Under nitrogen protection, add ammonium bicarbonate in an amount equal to the molar amount of ferrous ions to the ferrous sulfate purification solution, then add ammonium fluoride. The amount of ammonium fluoride added is 0.25% of the mass of ferric hydroxide (calculated as Fe). Heat to 60℃ and react for 3 hours. Then, introduce oxygen and continue to react for 10 hours. Filter, wash, and dry to obtain ferric hydroxide. Step 3: Mix ferric hydroxide, phosphoric acid, lithium carbonate, and starch in a molar ratio of n(Fe):n(P):n(Li) = 1:1.05:1.03, with the amount of starch added being 10% of the total mass of ferric hydroxide, phosphoric acid, and lithium carbonate. Add the above materials to deionized water, mix, and mill. After milling, a slurry with a D50 of 0.50-0.55 μm is obtained. Step 4: Dry the finely ground slurry by spray drying to obtain the corresponding powder. The inlet air temperature is 260℃ and the outlet air temperature is 110℃. Step 5: Sinter the powder in an inert atmosphere at a temperature of 740℃, a heating rate of 2-3℃ / min, and a holding time of 8h. Then, grade the powder to obtain lithium iron phosphate cathode material.

[0034] Example 3 A method for preparing lithium iron phosphate using iron hydroxyl oxide as an intermediate, differing from Example 1 only in the amount of ammonium fluoride added in step two. The specific steps are as follows: Step 1: Same as Step 1 in Example 1.

[0035] Step 2: Under nitrogen protection, add ammonium bicarbonate in an equal molar amount to ferrous ions to the ferrous sulfate purification solution, and then add ammonium fluoride. The amount of ammonium fluoride added is 0.055% of the mass of ferric hydroxide (calculated as Fe) (after conversion, the molar ratio of ammonium fluoride to ferrous sulfate is approximately 1:1200). The temperature is raised to 60℃, and after reacting for 3 hours, oxygen is introduced and the reaction continues for 10 hours. After filtration, washing, and drying, ferric hydroxide is obtained.

[0036] Steps three to five are the same as steps three to five in Example 1.

[0037] Example 4 A method for preparing lithium iron phosphate using iron hydroxyl oxide as an intermediate, differing from Example 1 only in the amount of ammonium fluoride added in step two. The specific steps are as follows: Step 1: Same as Step 1 in Example 1.

[0038] Step 2: Under nitrogen protection, add ammonium bicarbonate in an equal molar amount to ferrous ions to the ferrous sulfate purification solution, and then add ammonium fluoride. The amount of ammonium fluoride added is 0.55% of the mass of ferric hydroxide (calculated as Fe) (after conversion, the molar ratio of ammonium fluoride to ferrous sulfate is approximately 1:120). The temperature is raised to 60℃, and after reacting for 3 hours, oxygen is introduced and the reaction continues for 10 hours. After filtration, washing, and drying, ferric hydroxide is obtained.

[0039] Steps three to five are the same as steps three to five in Example 1.

[0040] Comparative Example 1 The difference between this comparative example and Example 1 is that lithium iron phosphate was prepared using a phosphoric acid railway wire, as follows: Step 1: Dissolve ferrous sulfate in water to remove impurities. Add 15wt% dilute phosphoric acid to remove titanium. After stirring, filter. Add ammonia to adjust the pH to 2.5-3.5. Filter a second time to remove Al and Zn to obtain purified ferrous sulfate solution. Step Two: Add the additive and oxidant sequentially to the phosphate solution and mix thoroughly. The additive is polyethylene glycol, added at 0.2-0.5% of the liquid mass, to improve the dispersibility of the precipitated particles and prevent agglomeration. The oxidant is hydrogen peroxide (H2O2), with a mass concentration of 27.5% or 50% industrial grade. The amount of hydrogen peroxide added is based on Fe²⁺. + Completely oxidized to Fe³ + The theoretical amount was calculated to be 5% excess. The mixed solution was added to the ferrous sulfate purification solution to form a phosphate mixed solution. The phosphate mixed solution was heated to 85°C and kept at 85°C for 8 hours with continuous stirring. The product after the reaction was completed was filtered and washed. Step 3: Add water and phosphoric acid to the washed filter cake and slurry it. The amount of phosphoric acid (calculated as 85% industrial phosphoric acid) added is 10% of the weight of the filter cake. After slurrying, the solid content of the slurry is controlled at 15-30%. After heating to 70℃, let it stand for aging reaction. The reaction time is about 12 hours. Step 4: Filter, wash, dry, crush and calcine the slurry obtained in Step 3. The calcination temperature is 550℃ and the holding time is 5h to obtain battery-grade iron phosphate, i.e. anhydrous iron phosphate product.

[0041] Step 5: Add anhydrous ferric phosphate, lithium carbonate, and glucose to deionized water and mix to form a slurry. The mixture is prepared according to the molar ratio of n(Fe):n(P):n(Li) = 1:1.05:1.03, with the amount of glucose added being 10% of the total mass of ferric phosphate and lithium carbonate. After sand milling, a slurry with D50 of 0.50-0.55μm is obtained. Step 6: Dry the finely ground slurry by spray drying to obtain the corresponding powder. The inlet air temperature is 260℃ and the outlet air temperature is 110℃. Step 7: Sinter the powder in an inert atmosphere at a sintering temperature of 745℃, a heating rate of 2-3℃ / min, and a holding time of 8h. Then, grade and crush the powder to obtain lithium iron phosphate cathode material.

[0042] Comparative Example 2: The difference between this comparative example and Example 1 is that lithium iron phosphate is prepared using a mixed route of iron phosphate + iron hydroxyl oxide, as follows: Step 1: Dissolve ferrous sulfate in water to remove impurities. Add 15wt% dilute phosphoric acid to remove titanium. Stir and filter. Add ammonia to adjust the pH to 2.5-3.5. Filter a second time to remove Al and Zn. Step 2: Under nitrogen protection, add ammonium bicarbonate in an amount equal to the molar amount of ferrous ions to the ferrous sulfate purification solution, then add ammonium fluoride. The amount of ammonium fluoride added is 0.02% of the mass of ferric hydroxide (calculated as Fe). Heat to 60°C and react for 3 hours. Then, introduce oxygen and continue the reaction for 10 hours. Filter, wash, and dry to obtain ferric hydroxide. Step 3: Weigh ferric hydroxide, ferric phosphate, and lithium phosphate, add them to deionized water, so that the molar ratio of total iron (from ferric hydroxide and ferric phosphate) to total phosphate (from ferric phosphate and lithium phosphate) is 0.9:1, and the molar ratio of total iron to lithium (from lithium phosphate) is 1:1. After sand milling, a slurry with a D50 of 0.50-0.55μm is obtained. Step 4: Dry the slurry by spraying to obtain the corresponding powder. The inlet air temperature is 260℃ and the outlet air temperature is 110℃. Step 5: Sinter the powder in an inert atmosphere at a temperature of 745℃, a heating rate of 2-3℃ / min, and a holding time of 8h. Then, grade the powder to obtain lithium iron phosphate cathode material.

[0043] Comparative Example 3 A method for preparing lithium iron phosphate using iron hydroxyl oxide as an intermediate, differing from Example 1 only in that ammonium fluoride is not added in step two. The specific steps are as follows: Step 1: Same as Example 1.

[0044] Step 2: Under nitrogen protection, add ammonium bicarbonate in an equal molar amount to ferrous ions to the ferrous sulfate purification solution, without adding ammonium fluoride. Heat to 60°C and react for 3 hours. Then, introduce oxygen and continue the reaction for 10 hours. Filter, wash, and dry to obtain ferric hydroxy oxide (fluorine-free).

[0045] Steps three to five: Same as in Example 1.

[0046] Comparative Example 4 A method for preparing lithium iron phosphate using iron hydroxyl oxide as an intermediate, differing from Example 1 only in the amount of ammonium fluoride added in step two. The specific steps are as follows: Step 1: Same as Example 1.

[0047] Step 2: Under nitrogen protection, add ammonium bicarbonate in an equal molar amount to ferrous ions to the ferrous sulfate purification solution, and then add ammonium fluoride. The amount of ammonium fluoride added is 1.33% of the mass of ferric hydroxide (calculated as Fe) (after conversion, the molar ratio of ammonium fluoride to ferrous sulfate is about 1:50, which is far beyond the range of 1:120-1200). The temperature is raised to 60℃, and after reacting for 3 hours, oxygen is introduced and the reaction continues for 10 hours. After filtration, washing, and drying, ferric hydroxide is obtained.

[0048] Steps three to five: Same as in Example 1.

[0049] The LiFePO4 / C cathode materials prepared in the above examples and comparative examples were assembled into CR2016 coin cells (a standard type of coin cell, specifically referring to a battery case with a diameter of 20 mm and a height of 1.6 mm) in a glove box filled with high-purity argon gas, consisting of positive and negative electrode shells, a positive electrode, an electrolyte, a separator, a negative electrode (lithium metal sheet), and a nickel mesh (or a gasket / spring). The performance indicators of the battery materials prepared in the examples and comparative examples were compared within a voltage range of 2.0–4.2 V and at charge / discharge rates of 0.2C and 1C.

[0050] The test results are shown in Table 1:

[0051] As can be seen from the data in Table 1, the lithium iron phosphate cathode material prepared in Example 1 using iron hydroxyl oxide as an intermediate has a similar compaction density and relatively better electrical performance than the lithium iron phosphate prepared in Comparative Example 1 using anhydrous iron phosphate. This proves that this method, like the commonly used iron phosphate-based lithium iron phosphate, has good physicochemical properties and better electrical performance, but the preparation method is simpler and can effectively save production costs and reaction time. The performance of lithium iron phosphate prepared by Comparative Example 2 using a mixture of the two also confirms this point. Figure 2 The image shows an electron microscope image of the iron hydroxyl oxide prepared in Example 1. As can be seen from the image, the iron hydroxyl oxide prepared in Example 1 using F...- The doped iron hydroxyl oxide has a nanorod structure, which makes the iron oxide more reactive. In the subsequent high-temperature solid-phase reaction with phosphorus and lithium sources, it can achieve more complete and uniform solid solution, thereby improving the overall electrical performance of lithium iron phosphate.

[0052] Examples 3 and 4 respectively prepared lithium iron phosphate cathode materials with upper and lower limits of doping amount. From the electrical performance data, it can be seen that compared with the undoped Comparative Example 3, the doped group can effectively improve the rate performance and energy efficiency of the battery material. However, the excessive fluorine doping in Comparative Example 2, which exceeds the solid solution limit, will cause the impurity phase formed, resulting in irregular surface of powder particles, reduced compaction, and also affected electrical performance, showing a deterioration trend.

[0053] Figure 2 The image shows an electron microscope image of the iron hydroxyl oxide prepared in this invention. As can be seen from the image, the prepared iron hydroxyl oxide has a nanorod-like structure. This structure acts as a structural template to induce the formation of uniform nanoparticles of LiFePO4 during the subsequent batching, grinding and sintering processes.

[0054] contrast Figure 3 and Figure 4 It can be seen that lithium iron phosphate prepared entirely using iron hydroxyl oxide has a more uniform particle size distribution compared to lithium iron phosphate prepared via the lithium iron phosphate route. This is because the use of F... - Doped iron hydroxyl oxide, as a structural template, can induce the formation of uniform LiFePO4 nanoparticles.

[0055] Figure 5 The figure shows the initial charge-discharge curves of the lithium iron phosphate material prepared in Example 1 after it was assembled into a battery at a rate of 0.2C. As can be seen from the figure, the lithium iron phosphate cathode material obtained in this example has a stable discharge voltage plateau and a high specific capacity of 160.39 mAh / g, with a calculated initial charge-discharge efficiency of 97.80%.

[0056] Figure 6The graph shows the state-of-charge (SOC) and voltage curves of batteries assembled from the lithium iron phosphate materials prepared in Example 1 and Comparative Example 1, reflecting the polarization phenomenon of the battery materials. The horizontal axis represents SOC% (State of Charge, often referred to as the state of charge or remaining charge) at a 0.2C rate, and the vertical axis represents voltage. The voltage difference ΔV between the charging and discharging curves can be observed. The voltage difference ΔV is a core indicator for measuring the degree of battery polarization. In the graph, the battery prepared in Example 1 (red curve) has a smaller voltage difference than that in Comparative Example 1 (black curve). A smaller voltage difference indicates better reversibility of lithium-ion insertion / extraction in the cathode material and less electrochemical polarization. Furthermore, the graph shows that the discharge curve of the lithium iron phosphate battery prepared in Example 1 maintains a stable voltage plateau within the 10%-80% range, indicating excellent voltage retention during discharge.

[0057] The above detailed description is a specific description of one of the feasible embodiments of the present invention. This embodiment is not intended to limit the patent scope of the present invention. All equivalent implementations or modifications that do not depart from the present invention should be included within the scope of the technical solution of the present invention.

[0058] It should be particularly noted that the various embodiments listed in this specification and accompanying drawings are intended to illustrate the technical solutions and advantages of the present invention, and not to limit the scope of protection of the present invention. Without departing from the core ideas and technical effects of the present invention, those skilled in the art can make any form of improvement, substitution, combination, or modification to the structural arrangement, process parameters, material selection, control logic, etc., of the described embodiments; any obvious changes based on the same concept should be considered equivalent solutions of the present invention and should be included within the scope of protection defined by the claims of the present invention. The actual scope of protection of the present invention is determined by the appended claims and should be correctly understood in conjunction with the specification and accompanying drawings.

Claims

1. A method for preparing lithium iron phosphate using iron hydroxyl oxide as an intermediate, characterized in that, Includes the following steps: Step 1: Dissolve ferrous sulfate in deionized water to obtain a ferrous sulfate solution; Step 2: Add ammonium bicarbonate and ammonium fluoride to the ferrous sulfate solution obtained in Step 1, heat and introduce oxidizing gas to react and generate ferric hydroxide intermediate. After filtration, washing and drying, ferric hydroxide powder is obtained. Step 3: Add the hydroxyl iron oxide powder obtained in Step 2 to deionized water, mix with phosphorus source, lithium source and carbon source, and mill to obtain slurry; Step 4: The slurry is dried by spray drying to obtain powder; Step 5: Sinter the powder in an inert atmosphere to obtain lithium iron phosphate cathode material.

2. The method for preparing lithium iron phosphate according to claim 1, characterized in that, In step one, after ferrous sulfate is dissolved in deionized water, Fe²⁺ + The concentration is 10–90 g / L.

3. The method for preparing lithium iron phosphate according to claim 1, characterized in that, Step one also includes a purification step, which is carried out by adding dilute phosphoric acid to remove titanium, stirring and filtering, adjusting the pH to 2.5-3.5, and filtering a second time to remove aluminum and zinc.

4. The method for preparing lithium iron phosphate according to claim 1, characterized in that, In step two, the molar ratio of ammonium bicarbonate to ferrous sulfate is 1:1; the molar ratio of ammonium fluoride to ferrous sulfate is 1:(120-1200).

5. The method for preparing lithium iron phosphate according to claim 1, characterized in that, The oxidizing gas in step two is one or more of compressed air, oxygen, or ozone.

6. The method for preparing lithium iron phosphate according to claim 1, characterized in that, The phosphorus source in step three is one or more of phosphoric acid, monoammonium phosphate, potassium dihydrogen phosphate, sodium dihydrogen phosphate, ammonium dihydrogen phosphate, or calcium dihydrogen phosphate; in the phosphorus source, the mass fraction of phosphoric acid is 75%~85%, and the mass fraction of monoammonium phosphate, potassium dihydrogen phosphate, sodium dihydrogen phosphate, ammonium dihydrogen phosphate, or calcium dihydrogen phosphate is greater than 98%.

7. The method for preparing lithium iron phosphate according to claim 1, characterized in that, The carbon source in step three is at least one of organic carbon source and inorganic carbon source. The organic carbon source is at least one of glucose, sucrose, polyethylene glycol, and starch. The inorganic carbon source is at least one of superconducting carbon black, Ketjen black, and graphene.

8. The method for preparing lithium iron phosphate according to claim 1, characterized in that, In step three, the molar ratio of lithium source, iron hydroxyl oxide and phosphorus source, calculated by Li, Fe and P elements, is (1.02~1.08):1:(1~1.20); the amount of carbon source added is 8%~30% of the total mass of phosphorus source, iron hydroxyl oxide and lithium source.

9. The method for preparing lithium iron phosphate according to claim 1, characterized in that, The drying temperature in step four is 200-300℃; the sintering temperature in step five is 600℃~800℃.

10. A lithium-ion battery, characterized in that, The positive electrode material of the lithium-ion battery is made of lithium iron phosphate positive electrode material prepared by the preparation method according to any one of claims 1 to 9.

Images