Method for preparing high-performance lithium iron phosphate by normal-pressure liquid-phase reflux method

Abstract

The invention discloses a method for preparing high-performance lithium iron phosphate by a normal-pressure liquid-phase reflux method, and relates to the technical field of batteries, and the method comprises the following steps: 1, taking 1 mol of iron source, adding pure water and acid liquor, stirring and dissolving to obtain an iron source solution; 2, taking 1-2.5 mol of a lithium source, adding pure water, stirring, adding 0.2-1 mol of a phosphorus source, heating, dropwise adding the iron source solution, and aging to obtain a mixed solution; 3, the mixed solution is cooled, subjected to suction filtration and dried, and a lithium iron phosphate precursor is obtained; and 4, uniformly dry-mixing the precursor, a carbon source and PEG6000, sintering, and cooling to obtain the LiFePO4-C positive electrode material. According to the present invention, the reflux method using the pure water as the medium can be achieved without high pressure and high temperature, such that the advantages of uniform raw material mixing and rapid reaction in the liquid phase method are maintained, the disadvantages of large particle size and irregular morphology in the solid phase method are avoided, the equipment cost and the operation difficulty are reduced, and the synthesis of the lithium iron phosphate under the normal pressure water phase is truly achieved.

Description

Technical Field

[0001] The invention relates to the technical field of batteries, and in particular to a method for preparing high-performance lithium iron phosphate by a normal pressure liquid phase reflux method. Background Art

[0002] In 1996, Japan's NTT first revealed the olivine structure of AyMPO4 (A is an alkali metal, M is a combination of CoFe, i.e. LiFeCoPO4) lithium battery positive electrode material. The following year, John B. Goodenough and other research groups also reported the reversible migration and extraction of lithium from LiFePO4. This discovery has attracted great attention to lithium iron phosphate materials and has led to extensive research and rapid development. The preparation methods of lithium iron phosphate mainly include high-temperature solid phase synthesis method, liquid phase method, etc.

[0003] Among them, the high-temperature solid-phase synthesis method is one of the most commonly used preparation methods at present, due to its simple operation and process route design, easy control of process parameters, stable performance of the prepared materials, and easy industrial large-scale production. However, the solid-phase method is not conducive to controlling the particle growth process and aggregation state, and the final product is usually large in particle size and irregular in morphology.

[0004] The liquid phase method is carried out in water or aqueous solution, usually including sol-gel, hydrothermal, solvothermal and other methods. The synthesis of LiFePO4@C in sol-gel, aqueous solution, organic solvent and other solutions can obtain a uniform precursor, achieve the mixing of different raw materials at the atomic or molecular scale, and generate high-purity particles. Although the liquid phase method is relatively simple, it requires specific equipment and high technology. For example, traditional liquid phase methods such as hydrothermal / solvothermal and supercritical methods have the following problems: ① Hydrothermal / solvothermal and supercritical methods require the selection of specific equipment, which is resistant to high temperature and high pressure, expensive, and has serious safety hazards; ② Solvothermal and high boiling point solvent methods use organic solvents, which have high viscosity, and it is difficult to separate the target product from the mother liquor, and it is difficult to recycle and treat, which has certain safety hazards to the environment; ③ In the organic solvent system, it is difficult to extract the valuable metal ions in the mother liquor, and the cost is high, resulting in high cost of the entire production process, which is not conducive to large-scale mass production.

[0005] In view of the above shortcomings, the traditional liquid phase synthesis process has not been widely used in the large-scale production of lithium iron phosphate positive electrode materials and is currently only suitable for laboratory research. Summary of the invention

[0006] In order to solve the above technical problems, the present invention provides a method for preparing high-performance lithium iron phosphate by a normal pressure liquid phase reflux method, firstly pouring a lithium source into a reactor, then adding appropriate phosphoric acid and phosphate, and dropping the iron liquid into the mixed solution, reacting under normal pressure reflux, synthesizing pure phase lithium iron phosphate through aging for a period of time, and then filtering and drying to obtain lithium iron phosphate material, as follows: A method for preparing high-performance lithium iron phosphate by a normal pressure liquid phase reflux method comprises the following steps: Step 1: Take 1 mol of iron source, add pure water and acid solution, stir and dissolve, and obtain an iron source solution; Step 2: Take 1-2.5 mol of lithium source and add it into a reaction kettle, add pure water and stir, add 0.2-1 mol of phosphorus source and heat to boiling, drop the iron source solution of step 1, and age for 10±2 hours to obtain a mixed solution; the dropping speed of the iron source solution is 80-120 ml / min; Step 3: naturally cool the mixed solution obtained in step 2 to below 70° C., filter to obtain a filter cake, and dry at 70° C. for 12 hours to obtain a lithium iron phosphate precursor; Step 4: Dry-mix the lithium iron phosphate precursor obtained in step 3 with the carbon source and PEG6000, sinter at 720° C. for 10 h with N2 as the protective atmosphere, and cool to finally obtain the LiFePO4-C positive electrode material.

[0007] Moreover, in step 1, the iron source is ferrous sulfate heptahydrate.

[0008] Moreover, in step 1, the acid solution is any one of hydrochloric acid, sulfuric acid, nitric acid, and phosphoric acid; the molar ratio of the iron source to pure water and the acid solution is iron source: pure water: acid solution = 1:55.5:0.024.

[0009] Moreover, in step 2, the lithium source is lithium phosphate or lithium hydroxide, and the molar ratio of the lithium source to pure water is lithium source:pure water=1:55.5.

[0010] Moreover, in step 2, the phosphorus source is any one or a combination of phosphate and concentrated phosphoric acid; the concentration of the concentrated phosphoric acid is 85%.

[0011] Moreover, in step 2, the phosphate is any one or a combination of dihydrogen phosphate and hydrogen phosphate.

[0012] Furthermore, the dihydrogen phosphate is ammonium dihydrogen phosphate.

[0013] Moreover, in the step 4, the carbon source is glucose, and the molar ratio of the lithium iron phosphate precursor, the carbon source, and PEG6000 is lithium iron phosphate precursor: carbon source: PEG6000 = 1: 0.088: 0.0288; Moreover, in the step 2, lithium iron phosphate is added as a tuberculin, specifically as follows: Step 2: Take 1-2.5 mol of lithium source and add it into the reactor, add pure water and stir, add lithium iron phosphate, add 0.2-1 mol of phosphorus source and heat to boiling, add dropwise the iron source solution of step 1, and age the reaction for 10±2 hours to obtain a mixed solution.

[0014] Moreover, the ratio of lithium iron phosphate to lithium source is 1:0.08.

[0015] Compared with the prior art, the present invention has the following beneficial effects: 1. The present invention adopts a normal pressure liquid phase reflux method to prepare LiFePO4-C positive electrode material. The reflux method using pure water as the medium can be achieved without high pressure and high temperature. It combines the advantages of the liquid phase method and the solid phase method, and not only maintains the advantages of uniform mixing of raw materials and fast reaction rate in the liquid phase method, but also avoids the disadvantages of large particle size and irregular morphology in the solid phase method, reduces equipment cost and operation difficulty, and truly realizes the synthesis of lithium iron phosphate under normal pressure water phase. The reaction process is easy to control, the steps are clear, and it is suitable for industrial large-scale production.

[0016] 2. The lithium iron phosphate positive electrode material prepared by the normal pressure liquid phase reflux method provided by the present invention has excellent electrochemical properties. This is because the lithium iron phosphate sample prepared by the liquid phase reflux method has good particle uniformity and monodispersity, and the particle size can be controlled. The particles prepared by the liquid phase method can be closely stacked and contacted, and can have a more positive effect on the electrolyte. Ions and electrons can pass more easily on the surface of the particles. The rapid deintercalation of lithium ions is mainly because the lithium iron phosphate positive electrode material synthesized by the normal pressure liquid phase aqueous phase reflux method has a better electrode interface and the impedance of charge transfer is reduced; and because the particles are relatively evenly dispersed, the migration path of lithium ions becomes shorter, the deintercalation rate of lithium ions can be accelerated, and the rate performance of the material is also improved. The normal pressure liquid phase reflux method is significantly better than the performance of the solid phase method in terms of rate performance and capacity retention rate of high rate cycles, and will be widely used, especially in the field of energy storage.

[0017] 3. In the present invention, since the Li3PO4 compound is alkaline in aqueous solution and its Ksp value is 1.0*10 -9 The pH value under boiling conditions can reach 11. In the absence of an inert atmosphere, the introduction of FeSO4 solution will inevitably produce Fe over time. 3+ Compounds other than LiFePO4 are produced. Therefore, the invention introduces an appropriate amount of H3PO4 and / or phosphate to react with Li3PO4 to generate a LiH2PO4 buffer system, which lowers the pH of the mixed system and inhibits the formation of impurity phases. The generated LiH2PO4 can react with the required Li+ and P, and react with Fe 2+The reaction generates LiFePO4, that is, under boiling conditions, a strip-shaped, amorphous intermediate product first grows in the solution and eventually transforms into a nano-rhombus-shaped crystalline LiFePO4 precursor.

[0018] 4. In the present invention, the particle size can be controlled by the droplet velocity and aging time of the iron liquid, and a lithium iron phosphate precursor of a suitable size can be customized and developed.

[0019] 5. The present invention provides two technical solutions, adding a tuberculin or not adding a tuberculin, and the prepared LiFePO4-C positive electrode material has excellent electrochemical performance and stability. After adding the tuberculin, the crystallinity of the product can be improved and the reaction time can be shortened. During this period, production capacity increased.

[0020] 6. When preparing the iron source in the present invention, acid solution is added to ensure that the ferrous ions are not oxidized, the amount of lithium source added is appropriately excessive, and the iron source is slowly added to the boiling bottom solution (80-120 ml / min) to avoid the generation of impurities in the solution. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is a structural diagram of the lithium iron phosphate precursor prepared in Example 1; Figure 2 This is a structural diagram of the LiFePO4-C positive electrode material prepared in Example 1. DETAILED DESCRIPTION

[0022] Embodiment 1 This embodiment provides a method for preparing lithium iron phosphate by a normal pressure liquid phase reflux method, comprising the following steps: Step 1: Take 1 mol of FeSO4·7H2O (as an iron source) in a beaker, add appropriate amount of pure water and acid (including one or more of phosphoric acid, sulfuric acid, hydrochloric acid, nitric acid to prevent oxidation of ferrous ions), stir thoroughly to dissolve and set aside.

[0023] Step 2: Take 1-1.4 mol of Li3PO4 (adding excess lithium phosphate will make the reaction easier and more thorough) and add it into the reactor, add appropriate amount of water and stir, and at the same time add 0.1 mol of concentrated phosphoric acid (85% concentration) and 0.1 mol of phosphate (one or more of dihydrogen phosphate MH2PO4 and hydrogen phosphate MH1PO4), then heat to boiling and start adding the iron liquid in step 1 dropwise (dropping speed is 80 ml / min), and age the reaction for 10±2 hours.

[0024] Step 3: Naturally cool the mixed solution obtained in step 2 to below 70°C (too high temperature will easily damage the filter paper, causing the filter paper to break and resulting in a decrease in yield), filter to obtain a filter cake, place it on a tray, and then put it into an oven at 70°C for 12 hours to dry (too high temperature will easily oxidize lithium iron phosphate) to obtain a filter cake, i.e., a lithium iron phosphate precursor.

[0025] Step 4: dry-mix the filter cake obtained in step 3 with glucose (7% of the weight of the filter cake, a carbon source, a substance that improves the electronic conductivity between lithium iron phosphate particles, and can also be sucrose, starch or glucose, etc.) and PEG6000 (3% of the weight of the filter cake). After fully mixing, place the mixture in a tubular furnace with N2 as a protective atmosphere, sintering at a temperature of 720°C, and keep warm for 10 hours for sintering. After sintering, cool it down to finally obtain the LiFePO4-C positive electrode material.

[0026] Embodiment 2 The main difference between this embodiment and the first embodiment is that a buffer system is formed by adding monophosphoric acid, as follows: Step 1: Same as Example 1.

[0027] Step 2: Take 1-1.4 mol of Li3PO4 (adding excess lithium phosphate will make the reaction easier and more thorough) and add it into the reactor. Add appropriate amount of water and stir. At the same time, add 0.8±0.2 mol of concentrated phosphoric acid (85% concentration) and then heat to boiling and start dripping the iron liquid in step 1 (dropping speed is 120ml / min). Age the reaction for 10±2 hours.

[0028] Steps 3 and 4: Same as in Example 1.

[0029] Embodiment 3 The main difference between this embodiment and the first embodiment is that a monophosphate is added to form a buffer system, which is as follows: Step 1: Same as Example 1.

[0030] Step 2: Take 1-1.4 mol of Li3PO4 (adding excess lithium phosphate will make the reaction easier and more thorough) and add it into the reactor, add appropriate amount of water and stir, and at the same time add 0.3±0.1 mol of phosphate (one or more of dihydrogen phosphate MH2PO4, hydrogen phosphate MH1PO4), then heat to boiling and start dripping the iron liquid in step 1 (dropping speed is 100ml / min), and age the reaction for 10±2 hours.

[0031] Steps 3 and 4: Same as in Example 1.

[0032] Embodiment 4 The main difference between this embodiment and the first embodiment is that lithium iron phosphate is added to the reaction base liquid as a tuberculin. Compared with the first embodiment, this embodiment can shorten the reaction time and improve the production capacity, as follows: Step 1: Same as Example 1.

[0033] Step 2: Take 1-1.4 mol of Li3PO4 (adding excess lithium phosphate will make the reaction easier and more thorough) and add it to the reactor, add appropriate amount of water and stir, then add 0.08 mol of lithium iron phosphate precursor prepared by liquid phase method (as a tuberculin, it can improve crystallinity and shorten the reaction time), and at the same time add 0.1 mol of concentrated phosphoric acid (85% concentration) and 0.1 mol of diammonium phosphate (single phosphoric acid and phosphate in Examples 2 and 3 can also be used), then heat to boiling and start dripping the iron liquid in step 1 (the dripping speed is 100 ml / min), and age the reaction for 5±1 hours.

[0034] Steps 3 and 4: Same as in Example 1.

[0035] Embodiment 5 The main difference between this embodiment and the fourth embodiment is that the lithium source is replaced with lithium hydroxide.

[0036] Step 1: Same as Example 1.

[0037] Step 2: Take 2±0.5 mol of LiOH as a lithium source (adding excess lithium will make the reaction easier and more thorough) and add it to the reactor, add appropriate amount of water and stir, then add 0.08 mol of lithium iron phosphate precursor prepared by liquid phase method (as a tuberculin, it can improve crystallinity and shorten the reaction time). It can also be added in the same way as in Examples 1, 2 and 3. At the same time, add 1.0±0.2 mol of phosphate (one or more of dihydrogen phosphate MH2PO4 and hydrogen phosphate MH1PO4), then heat to boiling and start dripping the iron liquid in step 1 (the dripping speed is 100 ml / min), and the rest is the same as in Example 1.

[0038] Steps 3 and 4: Same as in Example 1.

[0039] Comparative Example 1 The difference between this comparative example and the present invention is that the existing hydrothermal method is used for preparation, which is as follows: A method for preparing lithium iron phosphate by hydrothermal method comprises the following steps: Step 1: Take 0.04 mol LiOH·H2O (as lithium source) in a beaker, add 20 ml water to dissolve, add 0.015 mol phosphoric acid (85% concentration) to form a suspension, stir thoroughly, add 50 ml FeSO4·7H2O with a concentration of 0.28 mol / L, stir thoroughly, transfer to a 100 ml polytetrafluoroethylene reactor, react the reactor at 160°C for 8 hours, then cool naturally to room temperature, filter and wash with deionized water. The amount of deionized water is 5 times the mass of the precursor, wash three times, and then keep the precursor in a vacuum oven at 70°C for 12 hours to obtain a gray powder.

[0040] Step 2: Dry-mix the sample obtained in step 1 with glucose (carbon source, a substance used to improve the electronic conductivity between lithium iron phosphate particles; it can also be sucrose, starch or glucose, etc.) in an appropriate proportion (7% of the filter cake weight) and PEG6000 (3% of the filter cake weight). After thorough mixing, place the mixture in a tubular furnace with N2 as the protective atmosphere and a sintering temperature of 720°C for 10 hours for sintering. After sintering, cool the mixture to obtain the LiFePO4-C / A positive electrode material.

[0041] The main precursor included in the LiFePO4-C / A positive electrode material of this embodiment is a lithium iron phosphate precursor prepared by a hydrothermal method, and the conductive layer is an amorphous carbon layer.

[0042] This reaction is only suitable for preparation in the laboratory and needs to be carried out under high pressure and high temperature conditions. It has safety risks, high costs, and high requirements for the equipment.

[0043] Comparative Example 2 The difference between this comparative example and the present invention is that the existing high temperature solid phase synthesis method is used for preparation, which is as follows: This embodiment provides a method for preparing lithium iron phosphate by a solid phase method, comprising the following steps: Step 1: Take 1500g anhydrous FePO4, 375g Li2CO3, 105g glucose and 45g PEG6000 in a coarse grinder, add appropriate amount of pure water, sand grind until the D100 of the slurry is less than 3 microns, transfer to a sand grinder through a 200-mesh sieve until the D50 is 350±10 microns, and transfer the material to spray drying to obtain a yellow material.

[0044] Step 2: Take 200 grams of the sample obtained in step 1 and put it into a graphite crucible, place it in a tubular furnace, use N2 as the protective atmosphere, sinter at a temperature of 720°C, keep warm for 10 hours, and finally obtain the LiFePO4-C / B positive electrode material after cooling after sintering.

[0045] Experimental Section

[0046] Experiment 1 The lithium iron phosphate precursor and LiFePO4-C cathode material prepared in Example 1 were weighed, and samples were taken for electron microscopy and ICP inspection. The electron microscopy morphology of the lithium iron phosphate precursor is as follows: Figure 1 As shown, the electron microscope morphology of LiFePO4-C positive electrode material is as follows Figure 2As shown in the figure, it can be seen that the lithium iron phosphate precursor is mainly a nano-rhombus structure with a length of 100 nm-400 nm and a thickness of 50 nm-100 nm. After pyrolysis carbon coating, the particle morphology of the formed LiFePO4-C positive electrode material changes from nano-rhombus to spherical nanoparticles with a particle size of 200 nm-300 nm; the change in particle morphology is mainly due to the volume shrinkage of the nano-rhombus morphology during the heat treatment process, forming a more stable spherical morphology; the role of carbon is to enhance the electronic conductivity between particles. Through the steric hindrance effect, the carbon coating can prevent the contact between particles to a certain extent, effectively inhibit the excessive growth of grains, reduce the resistance between particles and electrode polarization, thereby improving the rate discharge performance of the material. At the same time, the conductive polymer itself has a certain lithium deintercalation function, and the conductive polymer-coated lithium iron phosphate is not easy to directly contact the electrolyte to produce side reactions, and it is not easy to form SEI film and dendrites, so that the cycle performance of the battery can be maintained and it is not easy to short-circuit due to dendrites.

[0047] The LiFePO4-C positive electrode material prepared in Example 1 includes a main precursor which is a lithium iron phosphate precursor prepared by a normal pressure liquid phase reflux method, and a conductive layer which is an amorphous carbon layer. Regarding the chemical composition of the material, the mass of the liquid phase lithium iron phosphate precursor material is taken as a benchmark (set as 100%), and the mass of the liquid phase lithium iron phosphate precursor material is taken as 100%. The ICP results show that the contents of its main components are as follows: the content of lithium (Li) is 4.05±0.02, the content of iron (Fe) is 35.15±0.1, and the content of phosphorus (P) is 18.95±0.1, which ensures that the LiFePO4-C positive electrode material has excellent electrochemical properties and stability.

[0048] Experiment 2 The positive electrode materials prepared in Examples 1 to 6 and Comparative Examples 1 and 2 were prepared into batteries, and their performances were tested. The results are shown in Table 1.

[0049] Table 1 Battery performance test results

[0050] As can be seen from Table 1, the electrical properties of the positive electrode material prepared by the present invention are comparable to those prepared by the hydrothermal method (Comparative Example 1), and are superior to those prepared by the higher temperature solid phase synthesis method (Comparative Example 2). However, the present invention is carried out under normal pressure and room temperature conditions, avoiding the disadvantages of the hydrothermal method (liquid phase method) of high pressure and high temperature, high cost, and high equipment requirements, while ensuring the same performance as the positive electrode material prepared by the hydrothermal method.

[0051] In embodiments 4 and 5, compared with embodiments 1 to 3, a tuberculin is added to improve the crystallinity, shorten the reaction time, and increase the production capacity.

Claims

1. A method for preparing high-performance lithium iron phosphate by a normal pressure liquid phase reflux method, characterized in that: The steps include: Step 1: Take 1 mol of iron source, add pure water and acid solution, stir and dissolve, and obtain an iron source solution; Step 2: Take 1-2.5 mol of lithium source and add it into a reaction kettle, add pure water and stir, add 0.2-1 mol of phosphorus source and heat to boiling, drop the iron source solution of step 1, and age for 10±2 hours to obtain a mixed solution; the dropping speed of the iron source solution is 80-120 ml / min; Step 3: naturally cool the mixed solution obtained in step 2 to below 70° C., filter to obtain a filter cake, and dry at 70° C. for 12 hours to obtain a lithium iron phosphate precursor; Step 4: After the lithium iron phosphate precursor prepared in step 3 is evenly dry-mixed with the carbon source and PEG6000, sintered at 720° C. for 10 h with N2 as the protective atmosphere, and cooled to obtain the LiFePO4-C positive electrode material.

2. The method for preparing high-performance lithium iron phosphate by a normal pressure liquid phase reflux method as claimed in claim 1, characterized in that: In the step 1, the iron source is ferrous sulfate heptahydrate.

3. The method for preparing high-performance lithium iron phosphate by a normal pressure liquid phase reflux method as claimed in claim 1, characterized in that: In step 1, the acid solution is any one of hydrochloric acid, sulfuric acid, nitric acid, and phosphoric acid; the molar ratio of the iron source to pure water and the acid solution is iron source: pure water: acid solution = 1:55.5:0.

024.

4. The method for preparing high-performance lithium iron phosphate by a normal pressure liquid phase reflux method as claimed in claim 1, characterized in that: In step 2, the lithium source is lithium phosphate or lithium hydroxide, and the molar ratio of the lithium source to pure water is lithium source:pure water=1:55.

5.

5. The method for preparing high-performance lithium iron phosphate by a normal pressure liquid phase reflux method as claimed in claim 1, characterized in that: In step 2, the phosphorus source is any one or a combination of phosphate and concentrated phosphoric acid; the concentration of the concentrated phosphoric acid is 85%.

6. The method for preparing high-performance lithium iron phosphate by a normal pressure liquid phase reflux method as claimed in claim 5, characterized in that: In the step 2, the phosphate is any one or a combination of dihydrogen phosphate and hydrogen phosphate.

7. The method for preparing high-performance lithium iron phosphate by a normal pressure liquid phase reflux method as claimed in claim 6, characterized in that: The dihydrogen phosphate is ammonium dihydrogen phosphate.

8. The method for preparing high-performance lithium iron phosphate by a normal pressure liquid phase reflux method as claimed in claim 1, characterized in that: In the step 4, the carbon source is glucose, and the molar ratio of the lithium iron phosphate precursor, the carbon source, and PEG6000 is lithium iron phosphate precursor: carbon source: PEG6000 = 1: 0.088: 0.0288.

9. A method for preparing high-performance lithium iron phosphate by a normal pressure liquid phase reflux method as claimed in any one of claims 1 to 8, characterized in that: In step 2, lithium iron phosphate is added as a tuberculin, specifically as follows: Step 2: Take 1-2.5 mol of lithium source and add it into the reactor, add pure water and stir, add lithium iron phosphate, add 0.2-1 mol of phosphorus source and heat to boiling, add dropwise the iron source solution of step 1, and age the reaction for 10±2 hours to obtain a mixed solution.

10. The method for preparing high-performance lithium iron phosphate by a normal pressure liquid phase reflux method as claimed in claim 9, characterized in that: The ratio of lithium iron phosphate to lithium source is 1:0.08.

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