A method for preparing Cr-doped ferrophosphorus by using cast iron as iron source

Abstract

This application relates to the field of ferric phosphate preparation technology, specifically disclosing a method for preparing Cr-doped ferric phosphate using cast iron as the iron source. The method uses cast iron as raw material, which is pulverized to obtain cast iron powder, reacted with dilute phosphoric acid, and filtered to remove insoluble impurities, yielding Fe-doped ferric phosphate. 2+ Acid leaching solution; Cr-Fe doped solution is prepared by in-situ doping of Cr element naturally present in cast iron, and by controlling pH and temperature. 2+ The mixture was oxidized to Fe by an oxidizing agent. 3+ Subsequently, through crystallization, filtration, washing, drying, and pulverization, Cr-doped iron-phosphate product is obtained. This invention uses low-cost cast iron to replace the high-purity iron source, achieving in-situ doping without the need for external Cr salts, simplifying the process and reducing raw material costs by more than 30%. The prepared iron-doped iron-phosphate product is plate-like with an olivine-type crystal structure, exhibiting excellent electrochemical performance. It can be directly used in high-performance lithium iron phosphate cathode materials and is suitable for large-scale industrial production.

Description

Technical Field

[0001] This invention relates to the field of ferric phosphate preparation technology, specifically to a method for preparing Cr-doped ferric phosphate using cast iron as the iron source. Background Technology

[0002] Iron phosphate (FePO4) is an important precursor for lithium iron phosphate (LFP), the cathode material for lithium-ion batteries, and can also be used as a catalyst, ceramic pigment, and rust-preventive pigment. In recent years, with the rapid development of new energy vehicles and energy storage industries, the demand for LFP cathode materials has continued to grow, and the market demand for battery-grade LFP as its precursor has expanded accordingly. In LFP cathode materials, appropriate metal ion doping has been proven to improve electronic conductivity and lithium-ion diffusion rate, thereby enhancing electrochemical performance. Initiating doping to the preparation stage of the precursor LFP facilitates uniform mixing of dopant elements at the molecular level, better leveraging the modification effect.

[0003] Currently, the main methods for preparing iron phosphate include the ammonium method, the sodium method, and the iron method. Among them, the iron method uses zero-valent iron as the iron source. First, iron is dissolved in phosphoric acid to obtain a ferrous dihydrogen phosphate solution. Then, an oxidizing agent is used to oxidize the ferrous iron to trivalent iron, thus converting Fe²⁺ into Fe³⁺. 3+ With PO4 3- Ferric phosphate is obtained by precipitation. The iron-based process has gained increasing attention due to its short process, absence of other acidic impurities, and environmental friendliness. However, existing iron-based methods for preparing ferric phosphate still have the following shortcomings: Firstly, pure iron or high-purity iron is used as the iron source to prepare pure-phase ferric phosphate. For example, CN116462173B discloses an iron-based method for preparing ferric phosphate, in which excess pure iron with a purity greater than 99.90% is placed in dilute phosphoric acid to undergo an iron-chemical reaction, yielding a ferrous dihydrogen phosphate solution, which is then oxidized with hydrogen peroxide to obtain ferric phosphate dihydrate, without the need for pH adjustment by adding alkali throughout the process. Furthermore, existing technologies also use iron rods, iron scraps such as iron sheets, iron blocks, and iron bars, and soluble iron salts such as ferric nitrate and ferrous sulfate as iron sources. The above technical solution has the following shortcomings: First, the iron source used is pure iron or high-purity iron, resulting in high raw material costs. Furthermore, the reaction rate of pure iron blocks / rods is slow, and the iron-forming time is long, limiting production efficiency. When soluble iron salts are used as the iron source, impurity ions are easily introduced, requiring complex impurity removal processes, increasing process complexity and production costs. Second, the resulting product is pure-phase iron phosphate, free of doping elements. When used as a precursor for lithium iron phosphate, additional doping salts need to be added in subsequent processes for modification, increasing process steps and costs. Third, when preparing doped iron phosphate, soluble metal salts need to be added during the iron phosphate synthesis or conversion reaction stages. However, the pH window for iron phosphate synthesis / conversion is low, and under these conditions, certain doping metal ions, such as Cr... 3+The first method involves adding chromium salts to iron phosphate to achieve chromium doping. This is difficult because the chromium ions cannot uniformly enter the iron phosphate lattice, resulting in limited doping effects. The second method uses external chromium salts, typically soluble trivalent chromium salts such as chromium sulfate, chromium chloride, and chromium nitrate. For example, CN104024154B discloses a method for preparing metal-doped crystalline iron phosphate. First, an iron and phosphorus source are mixed and reacted to obtain amorphous iron phosphate. Then, a different metal salt, such as chromium salt CrCl3, is mixed into the amorphous iron phosphate. The amorphous iron phosphate mixed with chromium salts is then crystallized to obtain Cr-doped crystalline iron phosphate. However, the above technical solutions have the following drawbacks: First, they require additional purchase and addition of chromium salts and other dopants, increasing raw material costs and process steps. Second, the uniformity of the added chromium salt doping depends on the dispersion of the chromium salt in the reaction system. When the chromium ion concentration is locally high, it is easy to form independent chromium phases such as Cr2O3 instead of entering the iron phosphate lattice, leading to poor doping uniformity and unstable modification effects. Third, the pH window for iron phosphate synthesis and conversion is low, and under these conditions, Cr... 3+ The addition of chromium salts can easily lead to the precipitation of Cr(OH)3 or the formation of chromium phosphate precipitate, making it difficult for the chromium to enter the iron phosphate lattice effectively and uniformly. This results in the actual amount added being lower than the dosage, leading to low chromium utilization. Fourth, the addition of chromium salts introduces additional anions such as Cl-. - SO4 2- For example, a washing process is required to remove residual anions, otherwise the purity of the iron phosphate product and the subsequent electrochemical performance of lithium iron phosphate will be affected. The third type uses chromium-containing steel waste liquid as an iron source to achieve chromium doping. For example, CN112289991A discloses that steel plant pickling waste liquid is ion exchanged to obtain ferrous sulfate containing Ni and Cr, which is then oxidized and reacted with a phosphoric acid source to obtain iron phosphate dihydrate. It is then sintered with a lithium source and a carbon source to obtain Ni and Cr co-doped lithium iron phosphate. However, the composition of steel pickling waste liquid is complex and fluctuates greatly, containing a variety of heavy metal ions and residual acid. Although Cr exists in the waste liquid in ionic form, a variety of difficult-to-control impurity elements are introduced at the same time, affecting the purity of the product and batch stability. In addition, the Cr content in the pickling waste liquid cannot be controlled. The actual Cr doping amount is limited by the original composition of the waste liquid, and it is difficult to accurately control the doping amount according to product requirements.

[0004] Therefore, there is an urgent need to develop a method for preparing iron phosphate by in-situ chromium doping of cast iron with low raw material cost, simple process, and uniform and controllable chromium doping. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a method for preparing chromium-doped ferric phosphate using cast iron as the iron source. This process can solve the technical problems of high raw material costs, poor doping uniformity, and difficulty in meeting product consistency and precise doping requirements in the preparation of chromium-doped ferric phosphate in existing technologies.

[0006] In a first aspect, the present invention provides a method for preparing Cr-doped ferric phosphate using cast iron as the iron source, comprising the following steps: S1. Cast iron crushing: Cast iron is crushed and sieved to obtain cast iron powder; the cast iron is one or more of ordinary white cast iron, ductile iron, or gray cast iron; by mass percentage, the cast iron contains 90%~98% Fe, 0.5%~4.0% C, 0.1%~2.0% Si, 0.1%~1.5% Mn, and 0.01%~0.07% Cr. S2, Iron Dissolution: The cast iron powder obtained in S1 is added to dilute phosphoric acid and stirred continuously to carry out the iron dissolution reaction. Then, the insoluble impurities are removed by filtration to obtain Fe-containing... 2+ Acid leaching solution; S3, In-situ Doping: Adjust the pH and temperature of the acid leaching solution obtained in S2, and stir until homogeneous to obtain Cr-doped Fe. 2+ Mixture; S4. Oxidation: Add an oxidant to the mixture obtained in S3 and stir to carry out the oxidation reaction, obtaining Cr-doped Fe. 3+ Mixture; S5. Post-processing: The Fe obtained in S4... 3+ The mixture is crystallized, and then filtered, washed, dried and pulverized in sequence to obtain Cr-doped iron phosphate product.

[0007] Furthermore, in S1, the particle size of the cast iron powder is 100~300 mesh.

[0008] Further, in S2, the concentration of the dilute phosphoric acid is 1~4 mol / L; the solid-liquid ratio of the cast iron powder to the dilute phosphoric acid is 1 g: 2~10 mL; the temperature of the iron dissolving reaction is 55~95℃, and the reaction time is 2~10 h; the filtration is performed by suction filtration; and the continuous stirring rate is 150~500 r / min.

[0009] Furthermore, in S3, the pH value is controlled at 2.0~2.5, and pH is adjusted using hydrogen peroxide and ammonia or ammonium phosphate solution; the temperature is 40~60℃; the stirring time is 30~90min, and the stirring rate is 150~500r / min.

[0010] Further, in S4, the oxidant is one or more of hydrogen peroxide, sodium peroxide, and potassium permanganate; the amount of oxidant added is Fe. 2+ The amount required for complete oxidation is 1.0 to 1.2 times the theoretical amount; the oxidation reaction temperature is 30 to 70°C, and the oxidation reaction time is 1 to 4 hours; the stirring rate is 150 to 500 r / min.

[0011] Further, in S5, the crystallization temperature is 60~100℃, and the crystallization time is 1~8h; the washing is done with deionized water until the pH of the washing solution is 6.5~7.0; the drying temperature is 100~200℃, and the drying time is 1~10h; the pulverization is to make the particle size of the Cr-doped iron phosphate product 5-150μm.

[0012] Furthermore, in S5, the crystallization process requires stirring at a rate of 150~500 r / min.

[0013] Secondly, the present invention also provides a Cr-doped iron phosphate method, which is prepared by the method for preparing Cr-doped iron phosphate using cast iron as the iron source as described in the first aspect of this application.

[0014] Furthermore, the chemical formula of the Cr-doped iron phosphate is Fe. 1-x Cr X PO4·nH2O, where x = 0.005~0.1, n = 0.5~2.0, has an electronic conductivity of 10. -6 ~10 -4 S / cm, Li + The diffusion coefficient is 10 -11 ~10 -9 cm 2 / s; The Cr-doped iron phosphate has an olivine-type crystal structure and a primary particle size of 100-200 nm.

[0015] Thirdly, the present invention also provides the application of Cr-doped iron phosphate in lithium iron phosphate cathode materials as described in the second aspect.

[0016] Compared with existing technologies, this solution has the following advantages: 1. Replacing pure iron with cast iron significantly reduces raw material costs and improves ironmaking efficiency. This invention uses cast iron as the iron source. Cast iron contains 0.5%–4% carbon and alloying elements such as Cr, Si, and Mn. Its price is far lower than that of high-purity iron with a purity greater than 99.90%. Compared to existing technologies that use high-purity iron or iron bars / blocks with a purity greater than 99.90%, this invention pulverizes the cast iron into powder and dissolves it in dilute phosphoric acid. The specific surface area of ​​the cast iron powder is much larger than that of iron blocks or bars, which greatly increases the solid-liquid contact area in the acid leaching reaction, significantly improving the ironmaking reaction rate, effectively shortening the ironmaking time, and increasing production efficiency. Because cast iron has a high carbon content, the carbon mainly exists in the form of graphite or hard carbides, which severely damages the continuity of the matrix, making it almost ductile. During grinding, cracks propagate rapidly along graphite flakes or grain boundaries, resulting in brittle cleavage fracture and direct fragmentation into numerous irregularly shaped, angular, and fine particles. Iron blocks / bars, on the other hand, possess excellent plasticity; during grinding, the material does not easily fracture but undergoes plastic shear deformation, being cut into curled, scaly, or strip-shaped fragments. These fragments are often coarser than cast iron powder and, due to their higher toughness, are prone to "cold welding" agglomeration during grinding, further reducing the effective specific surface area. Simultaneously, cast iron, as a major industrial raw material, has a wide and stable supply, eliminating the risk of limited supply or large price fluctuations associated with pure iron. This enables the high-value utilization of low-cost industrial raw materials, significantly reducing the production cost of ferric phosphate. Compared to existing high-purity iron source preparation processes, raw material costs can be reduced by more than 30%.

[0017] 2. This invention utilizes the inherent Cr element in cast iron for in-situ doping, eliminating the need for external chromium salts, simplifying the process and avoiding the introduction of additional anions. During the dilute phosphoric acid leaching process, Cr and Fe simultaneously dissolve from the alloy phase into the solution, forming Cr-containing Fe... 2+ In the mixed solution, Cr and Fe enter the iron phosphate lattice together during the subsequent oxidation and precipitation process, achieving in-situ doping. Compared with existing technologies that use external chromium salts such as CrCl3 for doping, this invention eliminates the need for additional chromium salt procurement and addition, saving on dopant costs and avoiding the Cl-induced oxidation introduced by external chromium salts. - SO4 2- The influence of anions on the purity of ferric phosphate products is reduced, decreasing the burden of the washing process. Furthermore, the uniformity of chromium salt doping depends on the dispersion of the chromium salt in the reaction system. In this invention, Cr and Fe dissolve simultaneously from the same alloy matrix, naturally exhibiting a molecular-level uniform distribution in the solution, ensuring uniform doping from the source. Combined with the small amount of beneficial impurities such as Mn naturally present in cast iron, the electronic conductivity and lithium-ion diffusion performance of ferric phosphate are synergistically improved, increasing the electronic conductivity of the prepared Cr-doped ferric phosphate to 10. -6 ~10 -4 S / cm, Li + The diffusion coefficient was increased to 10.-11 ~10 -9 cm 2 / s, compared to pure iron phosphate, the performance is significantly improved.

[0018] 3. By adjusting the acid leaching conditions, controlled Cr dissolution and effective doping are achieved, overcoming the limitations of Cr doping under low pH conditions. 3+ The problem of Cr being difficult to penetrate the iron phosphate lattice. This invention addresses this by adjusting the pH and temperature of the acid leaching solution to control the rate and amount of Cr leaching from the alloy phase in cast iron, allowing Cr to dissolve as Fe... 2+ The Cr-ferrous oxide is introduced into the acid leaching solution via a co-dissolution process to form a homogeneous Cr-containing ferrous oxide mixture. During the subsequent oxidation and precipitation process, Cr... 3+ with Fe 3+ In a molecularly mixed state, they react with PO4 3- This allows the chromium salt to combine and effectively enter the ferric phosphate lattice. Compared to existing technologies that directly add chromium salts during the ferric phosphate synthesis / conversion stage, the lower pH window of ferric phosphate synthesis / conversion allows for more efficient addition of chromium salts under these conditions. 3+ Chromium readily undergoes hydrolysis, releasing Cr(OH)3 or forming independent chromium phosphate precipitates, making it difficult to effectively enter the iron phosphate lattice. This results in the actual doping amount being far lower than the added amount and low chromium utilization. In contrast, in this invention, Cr reacts with Fe during the acid leaching stage. 2+ Simultaneous dissolution and uniform dispersion in solution, Cr during oxidation precipitation 3+ with Fe 3+ They jointly participate in nucleation and crystal growth in a homogeneous environment, avoiding the presence of Cr. 3+ The problem of independent phase precipitation caused by excessively high local concentrations significantly improved the Cr doping rate and doping uniformity.

[0019] 4. The Cr doping amount can be precisely controlled by selecting cast iron materials and acid leaching conditions, resulting in good batch-to-batch stability. This invention, by selecting cast iron varieties with different Cr contents and adjusting the pH and temperature of the acid leaching solution, allows for flexible and precise control of Cr leaching, thus achieving precise control over the Cr doping amount in the final iron phosphate product. The one-step process of iron dissolution and doping simplifies the preparation process, reduces impurity removal steps and reagent usage, lowers process complexity and production costs, and facilitates large-scale industrial production. Compared to existing technologies utilizing Cr-containing steel pickling waste liquid, the cast iron of this invention, as a standardized industrial product, has a clearly defined Cr content and good batch-to-batch consistency. Combined with precisely controllable acid leaching process parameters, the Cr doping amount is controllable, adjustable, and repeatable, resulting in significantly better batch-to-batch stability than the waste liquid route. Furthermore, this invention eliminates the need for complex pretreatment processes such as ion exchange, concentration, and impurity removal of the waste liquid, making the process simpler and less costly.

[0020] 5. The synergistic effect of multiple alloying elements in cast iron is beneficial to improving the performance of lithium iron phosphate. Besides Cr, cast iron typically contains alloying elements such as Si and Mn. These elements partially dissolve into the solution during acid leaching and, during oxidation precipitation, enter the lithium iron phosphate lattice along with Cr, forming Cr-Si-Mn multi-element synergistic doped lithium iron phosphate. Compared to single Cr doping, multi-element synergistic doping can improve electronic conductivity while adjusting lattice parameters and lithium-ion diffusion channels, potentially producing superior electrochemical performance modification effects. Furthermore, carbon in cast iron remains in the solid phase as graphite during acid leaching, which can be removed by filtration without affecting the purity of the lithium iron phosphate product. The prepared Cr-doped lithium iron phosphate has an olivine-type crystal structure, is free of impurities, has uniform particle size, and stable performance, making it suitable for direct use in preparing high-performance lithium iron phosphate cathode materials with broad industrial application prospects. Attached Figure Description

[0021] Figure 1 This is a process flow diagram of the present invention; Figure 2 This is a SEM image of ferric phosphate from Example 1 of the present invention; Figure 3 This is a SEM image of iron phosphate in Comparative Example 1. Detailed Implementation

[0022] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0023] Example 1 This embodiment provides a method for preparing Cr-doped ferric phosphate using cast iron as the iron source, referring to... Figure 1 The specific steps are as follows: S1: Industrial gray cast iron is selected, whose main components by mass percentage are: Fe 95.2%, C 2.3%, Si 1.1%, Mn 0.8%, Cr 0.021%, and the remainder are unavoidable impurities. After being crushed by a universal pulverizer, it is sieved through a 200-mesh standard sieve to obtain cast iron powder. S2: The above cast iron powder was added to dilute phosphoric acid with a concentration of 2 mol / L, and the solid-liquid ratio of cast iron powder to dilute phosphoric acid was controlled at 1:8 (g / mL). The iron dissolution reaction was carried out for 3 hours under stirring conditions of 70℃ and 250 r / min. After the reaction, insoluble impurities, mainly unreacted C and Si elements, were removed by vacuum filtration at a rate of 2 L / min, yielding Fe-containing... 2+ Acid leaching solution; S3: Adjust the pH of the above acid leaching solution to 2.3 using hydrogen peroxide and ammonia or ammonium phosphate solution. Stir for 45 minutes at 50℃ and 200 rpm to allow in-situ doping using the Cr element naturally present in the cast iron. The Cr-doped Fe... 2+ Mixture; S4: Adding Cr-doped Fe to the above-mentioned Fe 2+ 30% hydrogen peroxide was added to the mixture as an oxidant, and the amount of oxidant added was Fe. 2 + The amount of Fe required for complete oxidation is 1.1 times the theoretical amount. Under stirring conditions of 40℃ and 200 r / min, the oxidation reaction is carried out for 1.5 h. 2+ Completely oxidized to Fe 3+ Cr-doped Fe was obtained 3+ Mixture; S5: The above-mentioned Cr-doped Fe 3+ The mixture was transferred to a reaction vessel and crystallized for 6 hours under stirring conditions of 90℃ and 180r / min. After the reaction, it was vacuum filtered and the filter cake was washed with deionized water until the pH of the washing liquid was 6.8. Then the filter cake was placed in an oven and dried at 110℃ for 8 hours. After drying, it was pulverized by a pulverizer and passed through a 300-mesh standard sieve to obtain the Cr-doped iron phosphate product.

[0024] In this embodiment, the Fe content in the molten iron is 10.57% and the Cr content is 0.0023%; 28.5g of finished iron phosphate is obtained, of which 0.0023g is Cr doped and the Cr content in the finished iron phosphate is 80ppm.

[0025] Figure 2 This is a SEM image of the ferric phosphate prepared in this embodiment. Performance tests were performed on the Cr-doped ferric phosphate product prepared in this embodiment, and the results are as follows: Chemical formula: Fe 0.98 Cr 0.02 PO4·1.2H2O has an olivine-type crystal structure, is free of impurities, and has uniformly distributed particles with a diameter of 120–180 nm; its electronic conductivity is 3.2 × 10⁻⁶. -5 S / cm, Li + The diffusion coefficient is 5.8 × 10⁻⁶. - 10 cm 2 / s; capacity is 144.5mAh / g at 0.2C rate, 115.8mAh / g at 10C rate, and capacity retention is 99.5% after 50 cycles.

[0026] Example 2 This embodiment provides a method for preparing Cr-doped ferric phosphate using cast iron as the iron source. The specific steps are as follows: S1: Select ductile iron, whose main components by mass percentage are: Fe 95.2%, C 3.0%, Si 0.3%, Mn 0.2%, Cr 0.042%, with the remainder being unavoidable impurities. After being crushed, it is passed through a 150-mesh standard sieve to obtain cast iron powder. S2: Cast iron powder was added to dilute phosphoric acid with a concentration of 1.5 mol / L, with a solid-liquid ratio of 1:8 (g / mL). The iron was dissolved for 2.5 h under stirring conditions of 65℃ and 220 r / min. Insoluble impurities were removed by vacuum filtration at a rate of 1.5 L / min, yielding a solution containing Fe. 2+ Acid leaching solution; S3: Adjust the pH of the acid leaching solution to 2.0, and stir for 30 minutes at 45℃ and 180 r / min. Utilize the Cr element naturally present in the cast iron to carry out an in-situ doping reaction to obtain Cr-doped Fe. 2+ Mixture; S4: Add sodium peroxide as an oxidant. The amount of oxidant added is Fe. 2+ The oxidation amount was 1.0 times the theoretical oxidation amount. Under stirring conditions of 35℃ and 180 r / min, the oxidation reaction was carried out for 1 h to obtain Cr-doped Fe. 3+ Mixture; S5: The mixture was crystallized at 85℃ and 160r / min for 5h, filtered and washed until pH=6.6, dried at 105℃ for 7h, pulverized and passed through a 250-mesh sieve to obtain Cr-doped iron phosphate product.

[0027] In this embodiment, the Fe content in the molten iron solution is 10.57%, and the Cr content is 0.0046%; the resulting ferric phosphate product is 28.5g, with a Cr doping amount of 0.0046g, and the Cr content in the ferric phosphate product is 161ppm.

[0028] Performance test results: Chemical formula is Fe 0.99 Cr 0.01 PO4·0.8H2O, olivine-type structure, no impurities; particle size 150~200nm; electronic conductivity 5.1×10 -5 S / cm, Li + The diffusion coefficient is 6.3 × 10⁻⁶. -10 cm 2 / s; 0.2C capacity 145.1mAh / g, 10C capacity 110.2mAh / g, 50-cycle capacity retention 99.7%.

[0029] Example 3 This embodiment provides a method for preparing Cr-doped ferric phosphate using cast iron as the iron source. The specific steps are as follows: S1: Ordinary white cast iron is selected, and its main components by mass percentage are: Fe 95.2%, C 3.5%, Si 1.8%, Mn 1.2%, Cr 0.070%, with the remainder being unavoidable impurities. After being crushed, it is passed through a 250-mesh standard sieve to obtain cast iron powder. S2: Cast iron powder was added to dilute phosphoric acid with a concentration of 2.5 mol / L, with a solid-liquid ratio of 1:8 (g / mL). The iron was dissolved for 3.5 h under stirring conditions at 75℃ and 280 r / min. Insoluble impurities were removed by vacuum filtration at a rate of 2.5 L / min, yielding a solution containing Fe. 2+ Acid leaching solution; S3: Adjust the pH of the acid leaching solution to 2.5, and stir for 50 minutes at 55℃ and 220 r / min. Utilize the Cr element naturally present in the cast iron to carry out an in-situ doping reaction to obtain Cr-doped Fe. 2+ Mixture; S4: Add potassium permanganate as an oxidant. The amount of oxidant added is Fe. 2+ The oxidation amount was 1.2 times the theoretical oxidation amount. Under stirring conditions of 45℃ and 220 r / min, the oxidation reaction was carried out for 1.8 h to obtain Cr-doped Fe. 3+ Mixture; S5: The mixture was crystallized at 95℃ and 200r / min for 7h, filtered and washed until pH=6.9, dried at 115℃ for 9h, pulverized and passed through a 350-mesh sieve to obtain Cr-doped iron phosphate product.

[0030] In this embodiment, the Fe content in the molten iron is 10.57%, and the Cr content is 0.0063%; the resulting ferric phosphate product is 28.5g, with a Cr doping amount of 0.0063g, and the Cr content in the ferric phosphate product is 221ppm.

[0031] Performance test results: Chemical formula is Fe 0.96 Cr 0.04 PO4·1.5H2O, olivine-type structure, free of impurities; particle size 100~150nm; electronic conductivity 8.7×10⁻⁶ -5 S / cm, Li + The diffusion coefficient is 9.2 × 10⁻⁶. -10 cm 2 / s; 0.2C capacity 143.8mAh / g, 10C capacity 118.5mAh / g, 50-cycle capacity retention 99.3%.

[0032] Comparative Example 1 High-purity iron powder with an Fe content ≥ 99.9% was used to replace cast iron as the iron source, and the remaining process parameters were completely consistent with those in Example 1 to prepare pure iron phosphate and Cr-doped iron phosphate products.

[0033] Performance test results: The electronic conductivity of pure iron phosphate is 8.3 × 10⁻⁶. -10 S / cm, Li + The diffusion coefficient is 4.1 × 10⁻⁶. -14 cm 2 / s; Cr-doped iron phosphate has a Cr doping amount of 2% and an electronic conductivity of 2.9 × 10⁻⁶. -5 S / cm, Li + The diffusion coefficient is 5.2 × 10⁻⁶. -10 cm 2 / s, 0.2C capacity 144.8mAh / g, 10C capacity 114.3mAh / g. For example... Figure 3 The image shown is a SEM image of the iron phosphate prepared in this comparative example.

[0034] Results analysis: 1. Increased cost: Compared with Examples 1-3, Comparative Example 1 uses high-purity iron powder, which increases the raw material cost by more than 30%.

[0035] 2. Finished Product Performance: Compared with Examples 1-3, the electronic conductivity, Li... + In terms of diffusion coefficient, 0.2C capacity, and 10C capacity, the product of Comparative Example 1 showed no significant advantage in performance, proving that the present invention, by using cast iron as the iron source, can achieve excellent product performance while reducing costs.

[0036] The above descriptions are merely embodiments of the present invention, and common knowledge regarding specific structures and characteristics is not elaborated upon here. It should be noted that those skilled in the art can make various modifications and improvements without departing from the structure of the present invention, and these should also be considered within the scope of protection of the present invention. These modifications and improvements will not affect the effectiveness of the present invention or the practicality of the patent. The scope of protection claimed in this application should be determined by the content of its claims, and the specific embodiments described in the specification can be used to interpret the content of the claims.

Claims

1. A method for preparing Cr-doped ferric phosphate using cast iron as the iron source, characterized in that: Includes the following steps: S1. Cast iron crushing: Cast iron is crushed and sieved to obtain cast iron powder; the cast iron is one or more of ordinary white cast iron, ductile iron, or gray cast iron; by mass percentage, the cast iron contains 90%~98% Fe, 0.5%~4.0% C, 0.1%~2.0% Si, 0.1%~1.5% Mn, and 0.01%~0.07% Cr. S2, Iron Dissolution: The cast iron powder obtained in S1 is added to dilute phosphoric acid and stirred continuously to carry out the iron dissolution reaction. Then, the insoluble impurities are removed by filtration to obtain Fe-containing... 2+ Acid leaching solution; S3, In-situ Doping: Adjust the pH and temperature of the acid leaching solution obtained in S2, and stir until homogeneous to obtain Cr-doped Fe. 2+ Mixture; S4. Oxidation: Add an oxidant to the mixture obtained in S3 and stir to carry out the oxidation reaction, obtaining Cr-doped Fe. 3+ Mixture; S5. Post-processing: The Fe obtained in S4... 3+ The mixture is crystallized, and then filtered, washed, dried and pulverized in sequence to obtain Cr-doped iron phosphate product.

2. The method for preparing Cr-doped ferric phosphate using cast iron as the iron source according to claim 1, characterized in that: In S1, the particle size of the cast iron powder is 100~300 mesh.

3. The method for preparing Cr-doped ferric phosphate using cast iron as the iron source according to claim 1, characterized in that: In S2, the concentration of the dilute phosphoric acid is 1~4 mol / L; the solid-liquid ratio of the cast iron powder to the dilute phosphoric acid is 1 g: 2~10 mL; the temperature of the iron dissolving reaction is 55~95℃, and the reaction time is 2~10 h; the filtration is performed by suction filtration; and the continuous stirring rate is 150~500 r / min.

4. The method for preparing Cr-doped ferric phosphate using cast iron as the iron source according to claim 1, characterized in that: In S3, the pH value is controlled at 2.0~2.5, and pH is adjusted by hydrogen peroxide and ammonia or ammonium phosphate solution; the temperature is 40~60℃; the stirring time is 30~90min, and the stirring rate is 150~500r / min.

5. The method for preparing Cr-doped ferric phosphate using cast iron as the iron source according to claim 1, characterized in that: In S4, the oxidant is one or more of hydrogen peroxide, sodium peroxide, and potassium permanganate; the amount of oxidant added is Fe 2+ The amount required for complete oxidation is 1.0 to 1.2 times the theoretical amount; the oxidation reaction temperature is 30 to 70°C, and the oxidation reaction time is 1 to 4 hours; the stirring rate is 150 to 500 r / min.

6. The method for preparing Cr-doped ferric phosphate using cast iron as the iron source according to claim 1, characterized in that: In step S5, the crystallization temperature is 60~100℃ and the crystallization time is 1~8h; the washing is done with deionized water until the pH of the washing solution is 6.5~7.0; the drying temperature is 100~200℃ and the drying time is 1~10h; the pulverization is to make the particle size of the Cr-doped iron phosphate product 5-150μm.

7. The method for preparing Cr-doped ferric phosphate using cast iron as the iron source according to claim 1, characterized in that: In step S5, the crystallization process requires stirring at a rate of 150-500 r / min.

8. A chromium-doped ferric phosphate, characterized in that: The ferric phosphate is prepared by a method for preparing Cr-doped ferric phosphate using cast iron as the iron source, according to any one of claims 1-7.

9. The Cr-doped ferric phosphate according to claim 8, characterized in that: The chemical formula of the Cr-doped iron phosphate is Fe. 1-x Cr X PO4·nH2O, where x = 0.005~0.1, n = 0.5~2.0, has an electronic conductivity of 10. -6 ~10 -4 S / cm, Li + The diffusion coefficient is 10 -11 ~10 -9 cm 2 / s; The Cr-doped iron phosphate has an olivine-type crystal structure and a primary particle size of 100-200 nm.

10. The application of Cr-doped iron phosphate as described in any one of claims 8-9 in lithium iron phosphate cathode materials.

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