A preparation method and application of lithium iron phosphate

Abstract

The present invention relates to the field of battery positive electrode materials, and in particular, to a preparation method and application of lithium iron phosphate. The preparation method of lithium iron phosphate comprises the following steps: (a) mixing a ferrous salt solution and a phosphate solution and then performing a first reaction to obtain a first mixed system; the pH of the first mixed system is 1 to 3; (b) adding the ferrous salt solution and alkaline solution to the first mixed system and then performing a second reaction to obtain a second mixed system; the second mixed system is subjected to solid-liquid separation, washing, drying and oxygen-enriched calcination to obtain a precursor; (c) mixing the precursor, a lithium source, phosphoric acid and a carbon source and then performing a third reaction to obtain a third mixed system; the third mixed system is subjected to ball milling, spray drying and roasting to obtain the lithium iron phosphate. The preparation method of lithium iron phosphate has low raw material cost, low material consumption rate, high production efficiency, simple process, high operability, and excellent performance of the prepared lithium iron phosphate.

Description

Technical Field

[0001] The present invention relates to the field of battery positive electrode materials, and in particular to a preparation method and application of lithium iron phosphate. Background Art

[0002] Since Goodenough et al. reported in 1997 that LiFePO4 with olivine structure can be used as a positive electrode material for lithium-ion batteries, it has become one of the most promising positive electrode materials due to its advantages of low price, pollution-free and good thermal stability.

[0003] Depending on the raw materials and preparation process, the industrial production of iron phosphate mainly includes the following four processes:

[0004] 1. Full wet process: lithium source, iron source, and phosphorus source are dispersed in water or other solvents, and reacted in a relatively low temperature and high pressure to synthesize lithium iron phosphate in one step through a hydrothermal or solvothermal process, and then processed. This method is simple and can produce nano lithium iron phosphate; however, the waste liquid discharge is large, high-pressure resistant equipment is required, and industrial scale-up is difficult.

[0005] 2. Ferrous oxalate process: Using ferrous oxalate (FeC2O4·2H2O) as the iron source, it is mixed with ammonium dihydrogen phosphate or diammonium hydrogen phosphate, lithium salt, and carbon source, and then pre-decomposed and calcined at high temperature under the protection of a reducing or inert atmosphere to produce lithium iron phosphate. A large amount of ammonia is generated during the preparation process, which pollutes the environment and corrodes equipment. At the same time, the raw material of ferrous oxalate is relatively expensive.

[0006] 3. Red iron oxide process: Red iron oxide (Fe2O3) is used as the iron source, mixed with lithium dihydrogen phosphate (LiH2PO4) and a carbon source by wet grinding, spray drying, and then subjected to a high-temperature carbon thermal reduction reaction under a reducing or inert atmosphere to produce lithium iron phosphate. This process is relatively environmentally friendly, but the raw material of lithium dihydrogen phosphate is expensive, and the particle size control of red iron oxide is unstable, resulting in poor batch stability of the product.

[0007] 4. Iron phosphate process: Iron phosphate (FePO4) is used as the raw material, and is evenly mixed with lithium carbonate (Li2CO3) and a carbon source, and is made into lithium iron phosphate by a high-temperature carbon thermal reduction reaction under the protection of a reducing or inert atmosphere. Since the final two-phase structure of LiFePO4 and FePO4 is during the charge and discharge process of the LiFePO4 material (the crystal structures of the two are basically the same, and the unit cell parameters do not change much), the lithium iron phosphate material made from iron phosphate usually has better performance.

[0008] When using iron phosphate as the basic raw material to prepare LiFePO4, the composition, structure and morphology of iron phosphate FePO4 will directly affect the performance of the final product. However, the current process for preparing battery-grade iron phosphate that meets the requirements of lithium iron phosphate is relatively complex, impurity control is difficult, and the process is long, which is prone to quality fluctuations, ultimately resulting in high impurities in lithium iron phosphate and unstable quality, affecting battery performance. Therefore, developing a lithium iron phosphate preparation process with a wide range of low-cost raw materials, simple process control, and excellent product performance is the direction of industry development.

[0009] In view of this, the present invention is proposed. Summary of the invention

[0010] One aspect of the present invention relates to a method for preparing lithium iron phosphate, comprising the following steps:

[0011] (a) mixing a ferrous salt solution and a phosphate solution and performing a first reaction to obtain a first mixed system;

[0012] The pH of the first mixed system is 1 to 3;

[0013] (b) adding the ferrous salt solution and alkaline solution to the first mixed system and then performing a second reaction to obtain a second mixed system; performing solid-liquid separation, washing, drying and oxygen-enriched calcination on the second mixed system to obtain a precursor;

[0014] (c) mixing the precursor, lithium source, phosphoric acid and carbon source and performing a third reaction to obtain a third mixed system; ball milling, spray drying and calcining the third mixed system to obtain the lithium iron phosphate.

[0015] The method for preparing lithium iron phosphate has low raw material cost, low material consumption rate, high production efficiency, simple process, high operability, and the prepared lithium iron phosphate has excellent performance.

[0016] Another aspect of the present invention also relates to a positive electrode material, including lithium iron phosphate prepared by the lithium iron phosphate preparation method.

[0017] Another aspect of the present invention also relates to a lithium ion battery, comprising the positive electrode material.

[0018] Compared with the prior art, the present invention has the following beneficial effects:

[0019] (1) The preparation method of lithium iron phosphate provided by the present invention uses ferrous sulfate, a by-product of the sulfuric acid method titanium dioxide production process, as raw material, thereby reducing the raw material cost; the Fe / P co-precipitation stage aims to prepare ferrous phosphate, which on the one hand reduces the consumption of phosphate and on the other hand omits the oxidation process of hydrogen peroxide or other oxidants, thereby simplifying the process, reducing material consumption and phosphorus wastewater discharge, improving production efficiency, and avoiding the environmental pressure of phosphorus wastewater discharge.

[0020] (2) The preparation method of lithium iron phosphate provided by the present invention adopts oxygen-enriched calcination for one-step high-temperature dehydration and oxidation. The process operation is simple and controllable, and is convenient for industrial implementation. The process controls the crystal form and particle morphology during the preparation of ferrous phosphate to ensure the morphology stability during the oxidation process. The process uses lower-cost lithium phosphate and phosphoric acid as part of the lithium source and phosphorus source, which reduces the amount of lithium carbonate used and can further reduce material consumption and production costs.

[0021] (3) The preparation method of lithium iron phosphate provided by the present invention solves the problem of solid waste treatment in traditional titanium dioxide production, further reduces the production consumption of lithium iron phosphate through process and raw material optimization, simplifies unit control, and improves production efficiency, which has great industrial application value. DETAILED DESCRIPTION

[0022] The technical scheme of the present invention will be clearly and completely described below in conjunction with specific embodiments, but it will be appreciated by those skilled in the art that the following described embodiments are part of embodiments of the present invention, rather than all embodiments, and are only used to illustrate the present invention, and should not be considered as limiting the scope of the present invention. Based on the embodiments in the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative work, all belong to the scope of protection of the present invention. If specific conditions are not indicated in the embodiments, they are carried out according to normal conditions or conditions recommended by the manufacturer. If the manufacturer is not indicated in the reagents or instruments used, they are all conventional products that can be purchased commercially.

[0023] One aspect of the present invention relates to a method for preparing lithium iron phosphate, comprising the following steps:

[0024] (a) mixing a ferrous salt solution and a phosphate solution and performing a first reaction to obtain a first mixed system;

[0025] The pH of the first mixed system is 1 to 3 (e.g., 1, 1.5, 2, 2.5 or 3);

[0026] (b) adding the ferrous salt solution and alkaline solution to the first mixed system and then performing a second reaction to obtain a second mixed system; performing solid-liquid separation, washing, drying and oxygen-enriched calcination on the second mixed system to obtain a precursor;

[0027] (c) mixing the precursor, lithium source, phosphoric acid and carbon source and performing a third reaction to obtain a third mixed system; ball milling, spray drying and calcining the third mixed system to obtain the lithium iron phosphate.

[0028] The method for preparing lithium iron phosphate uses ferrous sulfate, a by-product of the sulfuric acid method titanium dioxide production process, as a raw material, thereby reducing the cost of raw materials; the Fe / P co-precipitation stage aims to prepare ferrous phosphate, which reduces phosphate consumption on the one hand and omits the oxidation process using hydrogen peroxide or other oxidants on the other hand, thereby simplifying the process, reducing material consumption, and improving production efficiency; oxygen-enriched calcination is used for one-step high-temperature dehydration and oxidation, and the process operation is simple and controllable, which is convenient for industrial implementation. The process ensures morphology stability during the oxidation process by controlling the crystal form and particle morphology during the preparation of ferrous phosphate.

[0029] After the addition of phosphate solution and ferrous sulfate solution is completed, the residual Fe 2+ and very small amounts of PO4 3- , in order to increase Fe 2+ and PO4 3- To make the reaction continue, it is necessary to continue to add alkali solution to make a small amount of PO4 3- Continue with Fe 2+ Reaction to PO4 3- After the reaction is complete, there is no P residue in the system and no phosphorus waste liquid is generated; Fe 2+ It continues to react with the dilute alkali solution to form ferrous hydroxide, but the feeding speed of the dilute alkali solution needs to be controlled. Adding the alkali solution too fast will make the pH value of the reaction system too high locally, causing Fe 2+ Oxidized to Fe 3+ , forming Fe(OH)3, and causing ferrous hydroxide to precipitate too quickly, resulting in larger particles, which will affect the subsequent water washing and calcination. The alkali solution addition rate needs to be controlled to 1 / 2 to ensure that the reaction system continues to slowly generate ferrous hydroxide.

[0030] Preferably, the pH of the first mixed system is adjusted to 1-3 by adding the alkali solution, so as to ensure the stable growth of ferrous phosphate crystals and improve the particle density. The key pH should be paid attention to. When the pH is greater than 3, the addition of the alkali solution is stopped. When the pH is lower than 1, ferrous phosphate precipitation cannot be formed. When the pH is greater than 3, Fe 2+ It will oxidize quickly and form some brick-red Fe(OH)3 precipitate. At the same time, the pH is greater than 3.5, and the Ca in the ferrous solution 2+ Mg 2+ , Mn 2+Plasma will form precipitation, which will be mixed into the ferrous phosphate precipitate in large quantities, causing the precipitation product to have a high content of impurity ions. In the subsequent water washing process, the impurity ions cannot be removed by water washing, resulting in the product lithium iron phosphate impurity ion content exceeding the standard, affecting product quality.

[0031] Preferably, the temperature of the second reaction is 50-90° C., and the time of the second reaction is 0.5-7 h.

[0032] During the oxygen-enriched calcination process, ferrous phosphate and ferrous hydroxide are simultaneously dehydrated, decomposed, and oxidized to become a mixture of anhydrous ferric phosphate and ferric oxide.

[0033] Preferably, the concentration of the ferrous salt solution is 0.5-5 mol / L (eg, 0.5 mol / L, 1.5 mol / L, 2.5 mol / L, 3.5 mol / L, 4.5 mol / L or 5 mol / L).

[0034] Preferably, the preparation of ferrous salt solution is based on ferrous sulfate produced as a byproduct in the production process of titanium dioxide by sulfuric acid method, and is prepared into a 0.5-5mol / L solution after dissolving in water, and the solution temperature is controlled to be 50-80°C, and the pH of the system is adjusted to 3.0-4.5 by using a weak alkaline solution as a pH adjuster, and titanium and aluminum impurities are removed, wherein the weak alkaline solution is one of sodium carbonate, sodium bicarbonate, ammonium carbonate, and ammonium bicarbonate, and a pure ferrous sulfate solution is obtained after filtration, and an appropriate amount of dilute acid is added to ensure that the pH of the ferrous solution is 1.0-2.0, and ferrous ion oxidation is avoided, and irregular iron hydroxide crystals are formed, so that the quality of the final product is uncontrolled. Using ferrous sulfate produced as a byproduct of titanium dioxide by sulfuric acid method as a raw material reduces the cost of raw materials, and is also conducive to the efficient utilization of Fe resources in titanium dioxide production.

[0035] Preferably, the pH of the ferrous salt solution is 1.0-2.0 (eg, 1.0, 1.2, 1.4, 1.6, 1.8 or 2.0).

[0036] Preferably, the phosphate solution (in the form of PO4 3- The concentration of the hydroxyl radical (in terms of content) is 0.5 to 5 mol / L (e.g., 0.5 mol / L, 1.5 mol / L, 2.5 mol / L, 3.5 mol / L, 4.5 mol / L or 5 mol / L).

[0037] Preferably, the concentration of the ferrous salt solution is the same as the concentration of the phosphate solution.

[0038] Preferably, the alkali solution comprises a complexing agent and a base.

[0039] Preferably, the content of the complexing agent in the alkali solution is 1-2 g / L (eg, 1 g / L, 1.3 g / L, 1.7 g / L or 2 g / L).

[0040] Preferably, the complexing agent comprises at least one of ammonium citrate, sodium citrate, ammonium tartrate, sodium tartrate or ammonium maleate.

[0041] Preferably, the content of the alkali in the alkali solution is 0.25-2.5 mol / L (for example, 0.25 mol / L, 0.5 mol / L, 0.8 mol / L, 1.0 mol / L, 1.3 mol / L, 1.5 mol / L, 1.7 mol / L, 1.9 mol / L, 2.1 mol / L, 2.3 mol / L or 2.5 mol / L).

[0042] Preferably, in step (a), the ferrous salt solution and the phosphate solution are mixed according to a molar ratio of Fe to P of 1:0.5 to 1.0 (e.g., 1:0.5, 1:0.6, 1:0.7, 1:0.8, 1:0.9 or 1:1.0).

[0043] Preferably, in step (b), the volume ratio of the first mixed system to the ferrous salt solution is 1:0.5-1 (for example, 1:0.5, 1:0.6, 1:0.7, 1:0.8, 1:0.9 or 1:1).

[0044] Preferably, at the end point of the second reaction, the pH of the reaction system is 4.5-6, confirming that the iron and phosphorus in the reaction system are completely reacted to avoid the generation of phosphorus wastewater.

[0045] In step (c), the reaction equation involved in the reaction between the precursor, the lithium source, the phosphoric acid and the carbon source is as follows:

[0046] FePO4+Fe2O3+x / 2Li2CO3+(1-x / 3)Li3PO4+xH3PO4+C→3LiFePO4+nCO2

[0047] +CO+H2O, x is 0~1.

[0048] Preferably, the lithium source includes: Li2CO3 and Li3PO4.

[0049] The molar ratio of lithium carbonate and lithium phosphate is added according to the above equation. If the amount of lithium carbonate added is too much, the cost will increase; if the amount of lithium phosphate added is too much, the product will have excessive Li, resulting in excessive residual lithium in the product, making the pH value of the product too high, and the prepared lithium iron phosphate is easy to absorb water, resulting in a reduced battery cycle life.

[0050] Preferably, in step (c), the precursor, the lithium source and the phosphoric acid are mixed in a molar ratio of Li, Fe and P of 1.005 to 1.05:1:1. The materials are proportioned according to the above equation, and the excess lithium is supplemented with lithium carbonate to avoid element imbalance caused by lithium volatilization during high-temperature calcination.

[0051] Preferably, in step (c), the mass of the carbon source is 10% to 20% (eg, 10%, 12%, 14%, 16%, 18% or 20%) of the sum of the mass of the precursor, the lithium source and the phosphoric acid.

[0052] Preferably, the temperature of the oxygen-enriched calcination is 400-700°C (e.g., 400°C, 450°C, 500°C, 550°C, 600°C, 650°C or 700°C). High temperature oxidation is performed using a mixed gas of oxygen and air or oxygen-enriched air produced as a byproduct of nitrogen production by air separation, with the oxygen content of the air being 30%-35%.

[0053] The oxygen-enriched calcination involves the following reaction equation:

[0054] Fe3(PO4)2·8H2O+O2→FePO4+Fe2O3+8H2O;

[0055] 4Fe(OH)2+O2→2Fe2O3+4H2O.

[0056] Preferably, the oxygen-enriched calcination time is 2 to 6 hours (eg, 2 hours, 3 hours, 4 hours, 5 hours or 6 hours).

[0057] Preferably, the calcination temperature is 650-800°C (eg, 650°C, 680°C, 700°C, 730°C, 750°C, 780°C or 800°C).

[0058] Preferably, the calcination time is 6 to 20 h (eg, 6 h, 10 h, 14 h, 18 h or 20 h).

[0059] Preferably, the heating rate of the calcination is 1 to 10°C / min (eg, 1°C / min, 3°C / min, 5°C / min, 7°C / min, 9°C / min or 10°C / min).

[0060] Preferably, the calcination is carried out under a protective atmosphere.

[0061] The present invention uses low-cost lithium phosphate to replace part of lithium carbonate for lithium iron phosphate synthesis. Compared with the existing process, cheaper raw materials can be used, the production process is simplified, and material consumption and phosphorus wastewater discharge are reduced.

[0062] Preferably, the phosphate includes at least one of sodium hydrogen phosphate, sodium dihydrogen phosphate, ammonium phosphate or ammonium dihydrogen phosphate.

[0063] Preferably, the base comprises at least one of sodium carbonate, sodium bicarbonate, ammonium carbonate or ammonium bicarbonate.

[0064] Preferably, the carbon source comprises: one or more of glucose, sucrose, PEG2000, PEG4000 or PEG6000.

[0065] Another aspect of the present invention also relates to a positive electrode material, including lithium iron phosphate prepared by the lithium iron phosphate preparation method.

[0066] Another aspect of the present invention also relates to a lithium ion battery, comprising the positive electrode material.

[0067] The embodiments of the present invention will be described in detail below with reference to specific examples and comparative examples.

[0068] Example 1

[0069] This embodiment provides a method for preparing lithium iron phosphate, comprising the following steps:

[0070] 1. Dissolve ferrous sulfate, a by-product of titanium dioxide produced by the sulfuric acid process, in deionized water to prepare a solution with a concentration of 2 mol / L. Control the solution temperature to 60°C, add sodium bicarbonate to adjust the pH of the system to 3.5, filter and remove impurities, and add a 30% mass concentration of dilute phosphoric acid solution to the filtrate to adjust the pH of the solution to 1.5, to obtain a ferrous sulfate solution as Solution A;

[0071] 2. Dissolve sodium dihydrogen phosphate in water to prepare a 2 mol / L solution as solution B;

[0072] 3. Prepare a mixed solution containing 1.68 g / L sodium citrate and 1 mol / L sodium bicarbonate as solution C;

[0073] 4. Add solution A and solution B into the reactor at the same time for the first reaction to obtain a first mixed system. Control the solution flow rate to ensure that the molar ratio of Fe and P is 3:1.5. At the same time, use solution C to adjust the pH to ensure that the pH of the first mixed system is stable between 2 and 2.5.

[0074] 5. Add solution A and solution C to the first mixed system and then conduct a second reaction to obtain a second mixed system. The volume ratio of the first mixed system to solution A is 1:1. The pH value of the reaction system is 4.5-6 at the end of the second reaction. The feeding rate of solution C is reduced to 0.5 of the initial rate, and the feeding rate of solution A remains unchanged. The reaction temperature is controlled at 70±2°C, the feeding time is 4h, and the temperature is kept for 3h after the feeding is completed.

[0075] 6. After the insulation is completed, the second mixed system is filtered, washed, and weighed. The filter cake is dried in an oven at 150°C for 10 hours to obtain a filter cake with an average particle size of 1 to 10 μm and a tap density of ≥ 0.9 g / cm 3 , a high-density spherical mixture of ferrous phosphate and ferrous hydroxide with dispersed particles and uniform size;

[0076] 7. The reacted material is placed in an atmosphere furnace, heated to 550°C at 10°C / min for oxygen-enriched calcination for 4 hours, and then cooled to obtain a composite of iron phosphate and iron oxide, i.e., a precursor;

[0077] 8. The precursor, lithium carbonate, lithium phosphate and phosphoric acid are mixed in a molar ratio of Li, Fe and P of 1.05:1:1, glucose is added as an organic carbon source according to a solid content of 10wt% of the above-mentioned composite, and a third reaction is performed to obtain a third mixed system, alcohol is added as a ball milling medium to prepare a solution with a solid content of 35%, and high-speed ball milling is performed, and spray drying is performed to obtain a precursor powder;

[0078] 9. The precursor powder was heated to 750°C at 5°C / min in an atmosphere furnace under nitrogen protection conditions and calcined for 12 hours, then naturally cooled to room temperature, crushed, and passed through a 100-mesh sieve to obtain the final product.

[0079] Example 2

[0080] This embodiment provides a method for preparing lithium iron phosphate, comprising the following steps:

[0081] 1. Dissolve ferrous sulfate, a byproduct of titanium dioxide produced by the sulfuric acid process, in deionized water to prepare a solution with a concentration of 1.5 mol / L. Control the solution temperature to 70°C, add sodium bicarbonate to adjust the pH of the system to 3.8, filter and remove impurities, and add a 30% mass concentration of dilute phosphoric acid solution to the filtrate to adjust the pH of the solution to 1.8, to obtain a ferrous sulfate solution as Solution A;

[0082] 2. Dissolve sodium dihydrogen phosphate in water to prepare a 1.5 mol / L solution as solution B;

[0083] 3. Prepare a mixed solution containing 1.68 g / L sodium citrate and 1 mol / L sodium bicarbonate as solution C;

[0084] 4. Add solution A and solution B into the reactor at the same time to carry out the first reaction to obtain a first mixed system. Control the solution flow rate to ensure that the molar ratio of Fe and P is 3:1.5. At the same time, adjust the pH with solution C to ensure that the pH of the first mixed system is stable between 2 and 2.5;

[0085] 5. Add solution A and solution C to the first mixed system and then conduct a second reaction to obtain a second mixed system. The volume ratio of the first mixed system to solution A is 1:1. The pH value of the reaction system is 4.5-6 at the end of the second reaction. The addition rate of solution C is reduced to 0.5 of the initial rate, and the addition rate of solution A remains unchanged. The reaction temperature is controlled at 60±2°C, the feeding time is 4h, and the temperature is kept for 3h after the feeding is completed.

[0086] 6. After the insulation is completed, the second mixed system is filtered, washed, and weighed. The filter cake is dried in an oven at 150°C for 10 hours to obtain a filter cake with an average particle size of 1 to 10 μm and a tap density of ≥0.85 g / cm 3 , a high-density spherical mixture of ferrous phosphate and ferrous hydroxide with dispersed particles and uniform size;

[0087] 7. The reacted material is placed in an atmosphere furnace, heated to 550°C at 10°C / min, and calcined in oxygen for 4 hours, then cooled to obtain a composite of iron phosphate and iron oxide, i.e., a precursor;

[0088] 8. The precursor, lithium carbonate, lithium phosphate and phosphoric acid are mixed in a molar ratio of Li, Fe and P of 1.05:1:1, glucose is added as an organic carbon source according to a solid content of 10wt% of the above-mentioned composite, and a third reaction is performed to obtain a third mixed system, alcohol is added as a ball milling medium to prepare a solution with a solid content of 35%, and high-speed ball milling is performed, and spray drying is performed to obtain a precursor powder;

[0089] 9. The precursor powder was heated to 750°C at 5°C / min in an atmosphere furnace under nitrogen protection conditions and calcined for 12 hours, then naturally cooled to room temperature, crushed, and passed through a 100-mesh sieve to obtain the final product.

[0090] Example 3

[0091] This embodiment provides a method for preparing lithium iron phosphate, comprising the following steps:

[0092] 1-6 are the same as in Example 1;

[0093] 7. The reacted material is placed in an atmosphere furnace, heated to 700°C at 10°C / min, and then calcined at high temperature in oxygen-enriched conditions for 4 hours, and then cooled to obtain a composite of iron phosphate and iron oxide, i.e., a precursor;

[0094] 8. Same as Example 1;

[0095] 9. The precursor powder was heated to 650°C at 5°C / min under nitrogen protection in an atmosphere furnace and calcined for 12 hours, then naturally cooled to room temperature, crushed, and passed through a 100-mesh sieve to obtain the final product.

[0096] Example 4

[0097] This embodiment provides a method for preparing lithium iron phosphate, comprising the following steps:

[0098] 1-6 are the same as in Example 1;

[0099] 7. The reacted material is placed in an atmosphere furnace, heated to 400°C at 10°C / min, and then calcined at high temperature in oxygen-enriched conditions for 4 hours, and then cooled to obtain a composite of iron phosphate and iron oxide, i.e., a precursor;

[0100] 8. Same as Example 1;

[0101] 9. The precursor powder was heated to 800°C at 5°C / min in an atmosphere furnace under nitrogen protection conditions and calcined for 12 hours, then naturally cooled to room temperature, crushed, and passed through a 325-mesh sieve to obtain the final product.

[0102] Comparative Example 1

[0103] The only difference between this comparative example and Example 1 is that the pH of the first mixed system is stabilized at 4-5.

[0104] Comparative Example 2

[0105] The only difference between this comparative example and Example 1 is that the precursor, lithium carbonate, lithium phosphate and phosphoric acid are mixed in a molar ratio of Li, Fe and P of 1.0:1:1.

[0106] Comparative Example 3

[0107] The difference between this comparative example and Example 1 is that in step (8), the precursor, lithium carbonate and phosphoric acid are mixed in a molar ratio of Li, Fe and P of 1.05:1:1, glucose is added as an organic carbon source according to a solid content of 10wt% of the above-mentioned composite, and a third reaction is carried out to obtain a third mixed system, alcohol is added as a ball milling medium to prepare a solution with a solid content of 35%, high-speed ball milling is carried out, and spray drying is carried out to obtain a precursor powder.

[0108] Comparative Example 4

[0109] The only difference between this comparative example and Example 1 is that the temperature of oxygen-enriched calcination is 800°C.

[0110] Comparative Example 5

[0111] The only difference between this comparative example and Example 1 is that the calcination temperature is 600°C.

[0112] Experimental example

[0113] The lithium iron phosphate obtained in each embodiment and comparative example was tested, and the tap density was tested with a tap density meter; the specific capacity was tested with a button battery test data, and the button battery preparation process was as follows: PVDF, acetylene black and LiFePO4 powder were mixed in a mass ratio of 1:1:8, N-methylpyrrolidone was added to make a slurry, and it was evenly coated on aluminum foil. After vacuum drying, it was taken out and rolled and punched into a circular electrode sheet with a diameter of 12mm. The button battery was assembled in a glove box (high-purity Ar atmosphere), and the electrolyte included: 1 mol / L LiPF6, a mixed solution of DCM, EC, and EMC in a volume ratio of 1:1:1, a metal lithium sheet was used for the negative electrode, and a Celgard film was used for the diaphragm. The button battery was tested for constant current charge and discharge cycles, and the charge and discharge voltage was 2.5 to 4.2V. The results are shown in Table 1.

[0114] Table 1 Performance test of lithium iron phosphate

[0115]

[0116] It can be seen from Table 1 that the performance of Comparative Examples 1, 2, 4 and 5 are all worse than that of the embodiment, and the performance of Comparative Example 3 is equivalent to that of the embodiment, but the lithium source is all lithium carbonate, which has a high cost. This shows that the preparation method of lithium iron phosphate provided by the present invention has excellent performance and can meet the requirements of battery-grade lithium iron phosphate.

[0117] Although the present invention has been illustrated and described with specific embodiments, it should be appreciated that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Those skilled in the art should understand that the technical solutions described in the above embodiments may be modified, or some or all of the technical features thereof may be replaced by equivalents without departing from the spirit and scope of the present invention. However, these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention. Therefore, this means that all such replacements and modifications within the scope of the present invention are included in the appended claims.

Claims

1. A method for preparing lithium iron phosphate, characterized in that: The following steps are involved: (a) mixing a ferrous salt solution and a phosphate solution and performing a first reaction to obtain a first mixed system; The pH of the first mixed system is 1 to 3; (b) adding the ferrous salt solution and alkali solution to the first mixed system and then performing a second reaction to obtain a second mixed system; The second mixed system is subjected to solid-liquid separation, washing, drying and oxygen-enriched calcination to obtain a precursor; (c) mixing the precursor, lithium source, phosphoric acid and carbon source and performing a third reaction to obtain a third mixed system; ball milling, spray drying and calcining the third mixed system to obtain the lithium iron phosphate.

2. The method for preparing lithium iron phosphate according to claim 1, characterized in that: The concentration of the ferrous salt solution is 0.5-5 mol / L.

3. The method for preparing lithium iron phosphate according to claim 1, characterized in that: The pH of the ferrous salt solution is 1.0-2.

0.

4. The method for preparing lithium iron phosphate according to claim 1, characterized in that: The concentration of the phosphate solution is 0.5-5 mol / L.

5. The method for preparing lithium iron phosphate according to claim 1, characterized in that: The alkali solution comprises a complexing agent and an alkali.

6. The method for preparing lithium iron phosphate according to claim 5, characterized in that: The content of the complexing agent in the alkali solution is 1-2 g / L.

7. The method for preparing lithium iron phosphate according to claim 5, characterized in that: The complexing agent includes at least one of ammonium citrate, sodium citrate, ammonium tartrate, sodium tartrate or ammonium maleate.

8. The method for preparing lithium iron phosphate according to claim 1, characterized in that: The content of the alkali in the alkali solution is 0.25 to 2.5 mol / L.

9. The method for preparing lithium iron phosphate according to claim 1, characterized in that: In step (a), the ferrous salt solution and the phosphate solution are mixed according to a molar ratio of Fe to P of 1:0.5 to 1.

0.

10. The method for preparing lithium iron phosphate according to claim 1, characterized in that: In step (b), the volume ratio of the first mixed system to the ferrous salt solution is 1:0.5-1.

11. The method for preparing lithium iron phosphate according to claim 1, characterized in that: The endpoint pH of the second reaction is 4.5-6.

12. The method for preparing lithium iron phosphate according to claim 1, characterized in that: In step (c), the precursor, the lithium source and the phosphoric acid are mixed according to a molar ratio of Li, Fe and P of 1.005 to 1.05:1:

1.

13. The method for preparing lithium iron phosphate according to claim 1, characterized in that: In step (c), the mass of the carbon source is 10% to 20% of the sum of the masses of the precursor, the lithium source and the phosphoric acid.

14. The method for preparing lithium iron phosphate according to claim 1, characterized in that: The temperature of the oxygen-enriched calcination is 400-700°C.

15. The method for preparing lithium iron phosphate according to claim 1, characterized in that: The oxygen-enriched calcination time is 2 to 6 hours.

16. The method for preparing lithium iron phosphate according to claim 1, characterized in that: The calcination temperature is 650-800°C.

17. The method for preparing lithium iron phosphate according to claim 1, characterized in that: The calcination time is 6 to 20 hours.

18. The method for preparing lithium iron phosphate according to claim 1, characterized in that: The lithium source includes: Li2CO3 and Li3PO4.

19. The method for preparing lithium iron phosphate according to claim 1, characterized in that: The phosphate includes at least one of sodium hydrogen phosphate, sodium dihydrogen phosphate, ammonium phosphate or ammonium dihydrogen phosphate.

20. The method for preparing lithium iron phosphate according to claim 1, characterized in that: The base includes at least one of sodium carbonate, sodium bicarbonate, ammonium carbonate or ammonium bicarbonate.

Images