A high-density lithium manganese iron phosphate cathode material and its preparation method and application

Abstract

The present invention discloses a high-density lithium iron manganese phosphate positive electrode material, and its preparation method and application. First, manganese salt, ferrous salt and phosphate are used as raw materials, and co-precipitation is carried out under different conditions to obtain ammonium manganese iron phosphate slurry. These ammonium manganese iron phosphate slurries with different properties are mixed in a certain proportion, and the solid phase product obtained by liquid-solid separation is dried, roasted and dehydrated to obtain ammonium manganese iron phosphate powder. The ammonium manganese iron phosphate powder is put into a ball mill according to a certain proportion with a lithium source, a carbon source, a ball milling aid and zirconium oxide beads. After the ball milling is completed, the zirconium beads are separated, and the obtained mixture is dried, calcined and ground to finally obtain a high-density lithium iron manganese phosphate positive electrode material. The lithium iron manganese phosphate positive electrode material has the characteristics of high compaction density and good rate performance. The method of the present invention has the advantage of being easy to industrialize.

Description

Technical Field

[0001] The present invention belongs to the field of material preparation, and in particular relates to a high-density lithium manganese iron phosphate positive electrode material and a preparation method and application thereof. Background Art

[0002] Among commercial lithium-ion batteries, the battery system based on LiFePO4 has attracted much attention in the industry due to the low cost, high safety and high rate characteristics of olivine-type cathode materials. However, LiFePO4 has a low discharge platform (3.4Vvs.Li / Li + ), low specific energy (580Wh / kg), and poor cycle performance. A low discharge platform makes it difficult to meet the requirements of new power batteries. Insufficient energy density leads to poor vehicle range and heavy vehicle loads. Poor cycle performance significantly affects the cost of electric vehicles, and the battery's capacity decreases with repeated charge and discharge, hindering its long-term use. The industry is currently in urgent need of a new material to meet the high safety and high specific energy requirements of electric power devices.

[0003] LiMnPO4 and LiFePO4 have similar structures and similar theoretical specific capacities, but the operating voltage of LiMnPO4 material is much higher than that of LiFePO4. This is because the addition of manganese atoms has the special effect of increasing the voltage platform, so its specific energy is also much higher than that of LiFePO4. However, the significant disadvantage of LiMnPO4 is that its electrical conductivity is close to that of an insulator. To overcome this problem, some scholars have proposed a co-solvent of LiFePO4 and LiMnPO4. Co-doped lithium manganese iron phosphate not only provides a higher theoretical specific capacity (170mAh / g), but also has a higher discharge platform (with 3.4V and 4.1V platforms), and a specific energy of 697Wh / kg. Therefore, as a high-voltage positive electrode material, it has received widespread attention from researchers and industries in various countries in recent years. Currently, the common lithium manganese iron phosphate structures include LiMn 0.7 Fe 0.3 PO4 / C, LiMn 0.5 Fe 0.5 PO4 / C, LiMn 0.9 Fe 0.1 Lithium manganese iron phosphate can take advantage of the synergistic effect of Mn and Fe, combining the advantages of lithium iron phosphate (stable electrochemical performance) and lithium manganese phosphate (high voltage), achieving both high energy density and high safety. At the same time, its voltage platform (4.1V) is compatible with conventional electrolytes, which provides a good opportunity to enter the market.

[0004] Although lithium manganese iron phosphate combines the advantages of high safety and high cycle stability of lithium iron phosphate with the high voltage platform of lithium manganese phosphate, it still has the disadvantages of low compaction density, low electronic conductivity and low lithium ion diffusion rate, which limit its application in high energy density and high rate performance batteries. In particular, the material's low compaction density (<2.0g / cm 3 ) results in a low load density of active materials on the positive electrode sheet of the lithium battery, making the positive electrode sheet of the battery cell thicker under the same load mass, resulting in a low volume energy density of the battery. Currently, many studies in this field on improving lithium manganese iron phosphate positive electrode materials are focused on improving the material compaction density. Current methods for improving the material compaction density are mainly concentrated on reducing the specific surface area of lithium manganese iron phosphate positive electrode powder materials, reducing carbon content, reducing interparticle gaps, and increasing the primary particle size. However, these existing methods for increasing the compaction density usually deteriorate the electrochemical properties of the material and the processes used are relatively complex, making it difficult to achieve industrial application. How to prepare lithium manganese iron phosphate positive electrode powder materials with high compaction density and excellent electrochemical properties through a simple and reliable process is a bottleneck that urgently needs to be broken through in the current field. Summary of the Invention

[0005] In view of the problems and shortcomings of the current low compaction density of lithium iron manganese phosphate positive electrode powder materials and the technology for preparing high compaction density lithium iron manganese phosphate positive electrode powder materials, the primary purpose of the present invention is to provide a method for preparing high compaction density lithium iron manganese phosphate positive electrode materials. Through this method, lithium iron manganese phosphate positive electrode powder materials with high compaction density and excellent electrochemical properties can be stably and reliably prepared, solving the bottleneck problem that urgently needs to be broken through in the current field.

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

[0007] A method for preparing a high-density lithium iron manganese phosphate cathode material comprises the following steps: first, manganese salt, ferrous salt and phosphate are used as raw materials, and co-precipitation is carried out under different conditions to obtain ammonium iron manganese phosphate slurry; these ammonium iron manganese phosphate slurries with different properties are mixed in a certain proportion; the solid phase product obtained by liquid-solid separation is dried and dehydrated to obtain ammonium iron manganese phosphate powder; the ammonium iron manganese phosphate powder is placed in a ball milling jar with a lithium source, a carbon source, a ball milling aid and grinding balls in a certain proportion and ball milled; after the ball milling is completed, the grinding balls are separated; the obtained mixture is dried, calcined and ground to finally obtain the high-density lithium iron manganese phosphate cathode material.

[0008] Furthermore, the manganese salt is at least one of manganous sulfate, manganous chloride, and manganous nitrate; the ferrous salt is at least one of ferrous sulfate, ferrous chloride, and ferrous nitrate; and the phosphate is at least one of diammonium hydrogen phosphate and ammonium dihydrogen phosphate.

[0009] Furthermore, when co-precipitation is carried out with manganese salt, ferrous salt and phosphate as raw materials, co-precipitation is carried out under the following three different conditions to obtain three co-precipitation products, ammonium manganese iron phosphate slurries with different properties; Condition ①: the molar ratio of the added manganese salt and ferrous salt is 0.25 to 4:1, and the sum of the moles of ferrous salt and manganese salt is equal to the mole of phosphate, the temperature is 20 to 50°C; the pH value is 5 to 7, and at least one of polyvinyl pyrrolidone, carboxymethyl cellulose and methyl cellulose is added with a mass concentration of 0.1 to 3 g / L; Condition ②: the added manganese salt and ferrous salt are 0.25 to 4:1, and the sum of the moles of ferrous salt and manganese salt is equal to the mole of phosphate, the temperature is 20 to 50°C; the pH value is 5 to 7, and at least one of polyvinyl pyrrolidone, carboxymethyl cellulose and methyl cellulose is added with a mass concentration of 0.1 to 3 g / L; The salt molar ratio is 0.25-4:1, and the sum of the moles of ferrous salt and manganese salt is equal to the mole of phosphate, the temperature is 50-70°C; the pH value is 7-8, and at least one of polyethylene glycol, polylactic acid, and methyl methacrylate is added with a mass concentration of 0.1-3 g / L; condition ③: the molar ratio of the added manganese salt to the ferrous salt is 0.25-4:1, and the sum of the moles of ferrous salt and manganese salt is equal to the mole of phosphate, the temperature is 70-90°C; the pH value is 8-10, and at least one of gelatin and polyvinyl alcohol is added with a mass concentration of 0.1-1 g / L.

[0010] Deoxygenated deionized water is used in the preparation of raw materials, and antioxidants such as ascorbic acid are added to the system.

[0011] Furthermore, the ammonium manganese ferrous phosphate slurries obtained under the above different conditions are mixed in a certain proportion, and the mixing conditions are: (1) if two different ammonium manganese ferrous phosphate slurries are mixed, the mixing conditions are: 20-30 wt.% of the slurry obtained under condition ① + 70-80 wt.% of the slurry obtained under condition ②, or 40-50 wt.% of the slurry obtained under condition ② + 50-60 wt.% of the slurry obtained under condition ③, or 30-40 wt.% of the slurry obtained under condition ① + 60-70 wt.% of the slurry obtained under condition ③; (2) if three different ammonium manganese ferrous phosphate slurries are mixed, the mixing conditions are: 10-20 wt.% of the slurry obtained under condition ① + 60-75 wt.% of the slurry obtained under condition ② + 15-20 wt.% of the slurry obtained under condition ③.

[0012] Furthermore, the ammonium manganese ferrous phosphate slurries obtained under the above different conditions are mixed in a certain proportion, and the solid phase product obtained after liquid-solid separation is dried and calcined for dehydration to obtain ammonium manganese ferrous phosphate powder; the drying conditions are vacuum drying, drying temperature 60-90°C, and drying time 6-12h; the calcination conditions are: inert atmosphere, temperature 300-700°C, and calcination time 30-120min.

[0013] The obtained ammonium manganese ferric phosphate powder is then ball-milled with a lithium source, a carbon source, a ball-milling aid, and zirconium oxide beads in a ball mill; the lithium source is at least one of lithium carbonate, lithium hydroxide, and lithium dihydrogen phosphate; the carbon source is at least one of glucose, sucrose, and soluble starch; and the ball-milling aid is at least one of ethanol, cyclohexane, and polyethylene glycol. During ball milling, the mass ratio of zirconium oxide balls to raw materials is 5 to 10:1, the particle size of the zirconium oxide beads is 0.6 to 0.8 mm, the solids content of the slurry is 20 to 45%, the milling time is 8 to 24 hours, the ball mill speed is 200 to 800 rpm, and the mass ratio of the added carbon source to the ammonium manganese ferric phosphate powder is 10 to 50%.

[0014] Furthermore, after the ball milling is completed, the mixed slurry is separated from the grinding balls by wet screening, and the obtained mixed slurry is then dried, calcined and ground.

[0015] Preferably, the drying method is vacuum drying, the drying temperature is 60-90° C., the drying time range is 6-12 hours, and the calcined powder is ground to a size of less than 200 mesh.

[0016] Furthermore, the atmosphere during calcination is argon or nitrogen; and two-stage calcination is adopted: first, the dried mixture is heated in the furnace to 300-400°C for pre-calcination for 2-3 hours, and then the temperature is raised to 600-800°C for secondary calcination for 5-10 hours. The lithium manganese iron phosphate powder obtained by calcination is ground after cooling in the furnace.

[0017] The second object of the present invention is to provide a high-density lithium manganese iron phosphate positive electrode material prepared by the above method.

[0018] The third object of the present invention is to provide an application of the above-mentioned high-density manganese iron phosphate lithium positive electrode material for preparing lithium-ion batteries.

[0019] The advantages of this invention over the prior art and the beneficial technical effects it brings are:

[0020] (1) The present invention provides a new technical solution that is different from the current method of improving the compaction density of lithium iron manganese phosphate positive electrode materials. Through this technical solution, lithium iron manganese phosphate positive electrode powder materials with high compaction density and high rate performance can be easily obtained from the preparation source, rather than through the method of modifying the prepared lithium iron manganese phosphate used in the current conventional method. The present invention first uses manganese salt, ferrous salt, and phosphate as raw materials, and then uses a co-precipitation method to prepare ammonium manganese iron phosphate after preparing a solution. The Mn, Fe, P and other components in the obtained ammonium manganese iron phosphate are uniformly mixed at the atomic and molecular levels. Compared with the products obtained by traditional solid-phase mixing, the electrochemical rate performance is greatly improved.

[0021] (2) Under different co-precipitation conditions, 2 to 3 co-precipitation products of ammonium manganese iron phosphate slurries with different properties are prepared. The ammonium manganese iron phosphate slurries obtained under different conditions are directly mixed in different proportions to achieve grading of ammonium manganese iron phosphate slurries with different properties such as particle size, morphology, and surface characteristics while maintaining high activity. After subsequent drying, lithium mixing, grinding, pre-burning, two-stage calcination and other steps, high-density and high-rate lithium manganese iron phosphate positive electrode powder materials can be easily obtained, which is very easy to implement industrially.

[0022] (3) After the ammonium manganese iron phosphate obtained under specific conditions is ball-milled and mixed with a lithium source, a carbon source, a ball-milling aid, and dried, a two-stage calcination method is used to prepare lithium iron manganese phosphate powder, which further achieves the uniform compounding of Li, Mn, Fe, and P components and the uniform carbon coating of lithium iron manganese phosphate particles, which is conducive to obtaining lithium iron manganese phosphate positive electrode powder materials with high rate performance.

[0023] In summary, the inventors of the present invention have proposed and constructed a new method for preparing lithium manganese iron phosphate positive electrode powder materials with high compaction density and rate performance on the basis of overcoming the practical problems encountered in the preparation of existing lithium manganese iron phosphate positive electrode powder materials, such as low compaction density, poor effect of methods for improving compaction density and difficulty in stable control. The method cleverly directly mixes the ammonium manganese iron phosphate slurry obtained by co-precipitation under different conditions in a certain proportion, and through a specific calcination mechanism, through a large number of experimental attempts and improvements, optimizes the technical solution for preparing lithium manganese iron phosphate powder, and obtains the key technical parameters for the preparation of high-density, high-rate lithium manganese iron phosphate positive electrode materials. The various steps proposed in the present invention are effectively connected, coordinated, and linked together, and the process is easy to control, obtaining technical effects that are difficult for those skilled in the art to predict, and also consuming a lot of effort from the inventors. DETAILED DESCRIPTION

[0024] The following examples are intended to further illustrate the present invention, but the present invention is not limited to the following examples.

[0025] Example 1

[0026] Take 0.1 mol of manganous sulfate and 0.1 mol of ferrous sulfate, dissolve them in 400 ml of deoxygenated deionized water, and add 1 g of ascorbic acid and 0.5 g of polyvinyl pyrrolidone to obtain a manganese-iron mixed solution. Weigh 0.2 mol of ammonium dihydrogen phosphate (NH4H2PO4) and dissolve it in 400 mL of deoxygenated deionized water to obtain an ammonium dihydrogen phosphate solution. Slowly pump the ammonium dihydrogen phosphate solution into the above-mentioned manganese-iron mixed solution, control the reaction temperature to 30°C and the pH value to 6, react for 30 minutes and age for 3 hours to obtain ammonium manganese-iron phosphate slurry ①. Take 0.2 mol of manganous sulfate and 0.1 mol of ferrous sulfate, dissolve them in 500 mL of deoxygenated deionized water, and add 2 g of ascorbic acid and 1 g of polyethylene glycol to obtain a manganese-iron mixed solution. Weigh 0.3 mol of ammonium dihydrogen phosphate (NH4H2PO4) and dissolve it in 500 ml of deoxygenated deionized water to obtain an ammonium dihydrogen phosphate solution. Slowly pump this ammonium dihydrogen phosphate solution into the above-mentioned manganese-iron mixed solution. Control the reaction temperature to 60°C and the pH to 7.5. After reacting for 40 minutes and aging for 4 hours, obtain ammonium ferromanganese phosphate slurry ②. Take 0.1 mol of manganous sulfate and 0.1 mol of ferrous sulfate and dissolve them in 400 ml of deoxygenated deionized water. Simultaneously add 1 g of ascorbic acid and 0.2 g of bone glue to obtain a manganese-iron mixed solution. Weigh 0.2 mol of ammonium dihydrogen phosphate (NH4H2PO4) and dissolve it in 400 mL of deoxygenated deionized water to obtain an ammonium dihydrogen phosphate solution. Slowly pump this ammonium dihydrogen phosphate solution into the above-mentioned manganese-iron mixed solution. Control the reaction temperature to 80°C and the pH to 8. After reacting for 30 minutes and aging for 3 hours, obtain ammonium ferromanganese phosphate slurry ③. Different amounts of the above-mentioned ammonium manganese ferrous phosphate slurry were taken and mixed in a ratio of 50 wt.% slurry ② + 50 wt.% slurry ③. After uniform mixing, liquid-solid separation was performed using a filter press. The resulting solid phase product was vacuum dried at 80°C for 12 hours. The dried ammonium manganese ferrous phosphate was then calcined at 400°C for 50 minutes under an argon atmosphere to obtain ammonium manganese ferrous phosphate powder. 4.44g of this ammonium manganese ferrous phosphate powder, 1.08g of glucose, and 0.98g of lithium carbonate were ball-milled in a jar with 15mL of cyclohexane and 20g of zirconium oxide beads for 12 hours at 400rpm. The added zirconium oxide balls had a particle size of 0.7mm and a mass of 35g. Ethanol was added as a milling aid to achieve a slurry solids content of 25%. After ball milling, the mixed slurry is separated from the zirconium beads by wet sieving, and the obtained mixed slurry is vacuum dried at 70°C for 12 hours. The obtained dry powder is calcined in two stages in a tube furnace: first, the furnace temperature is raised to 350°C for pre-calcination for 2.5 hours, and then the temperature is raised to 700°C for secondary calcination for 8 hours. The calcined lithium manganese iron phosphate powder is cooled in the furnace and then ground to less than 200 mesh to finally obtain the lithium manganese iron phosphate positive electrode powder material. The compaction density of the lithium manganese iron phosphate positive electrode powder is tested and reaches 2.9g / cm3 The electrochemical test showed that the first discharge capacity of the material was 166.41 mAh g at a current density of 0.2C. -1 At this current density, the specific capacity of the battery material after 50 cycles is 160.80 mAh g -1 , the positive electrode capacity retention rate is 96.63%.

[0027] Example 2

[0028] Take 0.3 mol of manganous sulfate and 1.0 mol of ferrous sulfate, dissolve them in 600 ml of deoxygenated deionized water, and add 5 g of ascorbic acid and 1.5 g of carboxymethyl cellulose to obtain a manganese-iron mixed solution. Weigh 1.3 mol of ammonium dihydrogen phosphate (NH4H2PO4) and dissolve it in 800 mL of deoxygenated deionized water to obtain an ammonium dihydrogen phosphate solution. Slowly pump the ammonium dihydrogen phosphate solution into the above-mentioned manganese-iron mixed solution, control the reaction temperature to 50°C and the pH value to 6, react for 30 minutes and age for 3 hours to obtain ammonium manganese-iron phosphate slurry ①. Take 0.2 mol of manganous sulfate and 0.1 mol of ferrous sulfate, dissolve them in 500 mL of deoxygenated deionized water, and add 2 g of ascorbic acid and 1.2 g of methyl methacrylate to obtain a manganese-iron mixed solution. Weigh 0.3 mol of ammonium dihydrogen phosphate (NH4H2PO4) and dissolve it in 500 ml of deoxygenated deionized water to obtain an ammonium dihydrogen phosphate solution. Slowly pump this ammonium dihydrogen phosphate solution into the above-mentioned manganese-iron mixed solution, control the reaction temperature to 70°C and the pH to 8, react for 40 minutes, and age for 4 hours to obtain ammonium ferromanganese phosphate slurry ②. Take 0.1 mol of manganous sulfate and 0.1 mol of ferrous sulfate, dissolve them in 400 ml of deoxygenated deionized water, and add 1 g of ascorbic acid and 0.3 g of polyvinyl alcohol to obtain a manganese-iron mixed solution. Weigh 0.2 mol of ammonium dihydrogen phosphate (NH4H2PO4) and dissolve it in 400 mL of deoxygenated deionized water to obtain an ammonium dihydrogen phosphate solution. Slowly pump this ammonium dihydrogen phosphate solution into the above-mentioned manganese-iron mixed solution, control the reaction temperature to 80°C and the pH to 9, react for 30 minutes, and age for 3 hours to obtain ammonium ferromanganese phosphate slurry ③. Different amounts of the above-mentioned ammonium manganese ferrous phosphate slurry were taken and mixed in the following ratio: 35wt.% slurry ① + 65wt.% slurry ③. After mixing evenly, liquid-solid separation was performed using a filter press. The resulting solid phase product was vacuum dried at 80°C for 12 hours. The dried ammonium manganese ferrous phosphate was then calcined at 500°C for 60 minutes under an argon atmosphere to obtain ammonium manganese ferrous phosphate powder. 5g of this ammonium manganese ferrous phosphate powder, 1.5g of glucose, and 1g of lithium carbonate were taken and ball-milled in a jar with 15mL of ethanol and 30g of zirconium oxide beads for 16 hours at a ball mill speed of 600rpm. The added zirconium oxide balls had a particle size of 0.6mm. After ball milling, the mixed slurry is separated from the zirconium beads by wet sieving, and the obtained mixed slurry is vacuum dried at 70°C for 8 hours. The obtained dry powder is calcined in two stages in a tube furnace: first, the furnace temperature is raised to 300°C for pre-calcination for 2.5 hours, and then the temperature is raised to 750°C for secondary calcination for 8 hours. The calcined lithium manganese iron phosphate powder is cooled in the furnace and then ground to less than 200 mesh to finally obtain the lithium manganese iron phosphate positive electrode powder material. The compaction density of the lithium manganese iron phosphate positive electrode powder is tested and reaches 2.8g / cm 3The electrochemical test showed that the first discharge capacity of the material was 167.11 mAh g at a current density of 0.2C. -1 At this current density, the specific capacity of the battery material after 100 cycles is 163.85 mAh g -1 , the positive electrode capacity retention rate is 98.05%.

[0029] Example 3

[0030] Dissolve 0.2 mol of manganous chloride and 0.1 mol of ferrous chloride in 500 mL of deoxygenated deionized water. Add 1.2 g of ascorbic acid, 0.4 g of polyvinyl pyrrolidone, and 0.4 g of methylcellulose to obtain a manganese-iron mixed solution. Weigh 0.3 mol of diammonium hydrogen phosphate and dissolve it in 400 mL of deoxygenated deionized water to obtain a diammonium hydrogen phosphate solution. Slowly pump this diammonium hydrogen phosphate solution into the above-mentioned manganese-iron mixed solution. Control the reaction temperature to 40°C and the pH to 7. React for 40 minutes and age for 4 hours to obtain ammonium manganese-iron phosphate slurry ①. Dissolve 0.1 mol of manganous nitrate and 0.1 mol of ferrous nitrate in 400 mL of deoxygenated deionized water. Add 1 g of ascorbic acid and 0.8 g of polyethylene glycol to obtain a manganese-iron mixed solution. Weigh 0.2 mol of ammonium dihydrogen phosphate (NH4H2PO4) and dissolve it in 500 mL of deoxygenated deionized water to obtain a diammonium dihydrogen phosphate solution. Slowly pump this diammonium dihydrogen phosphate solution into the above-mentioned manganese-iron mixed solution. Control the reaction temperature to 50°C and the pH to 8. After reacting for 30 minutes and aging for 3 hours, obtain ammonium ferromanganese phosphate slurry ②. Take 0.1 mol of manganous sulfate and 0.1 mol of ferrous sulfate and dissolve them in 400 mL of deoxygenated deionized water. At the same time, add 1 g of ascorbic acid and 0.3 g of bone glue to obtain a manganese-iron mixed solution. Weigh 0.2 mol of diammonium hydrogen phosphate and dissolve it in 400 mL of deoxygenated deionized water to obtain a diammonium hydrogen phosphate solution. Slowly pump this diammonium hydrogen phosphate solution into the above-mentioned manganese-iron mixed solution. Control the reaction temperature to 90°C and the pH to 9. After reacting for 30 minutes and aging for 3 hours, obtain ammonium ferromanganese phosphate slurry ③. Different amounts of the three ammonium manganese ferric phosphate slurries were taken out and mixed, and the mixing ratio was: 12wt.% slurry ① + 70wt.% slurry ② + 18wt.% slurry ③. After mixing evenly, liquid-solid separation was performed by filter press, and the obtained solid phase product was vacuum dried at 60°C for 12h. Thereafter, the dried ammonium manganese ferric phosphate was calcined at 500°C for 60min under nitrogen atmosphere to obtain ammonium manganese ferric phosphate powder. Take 6.66g of the ammonium manganese ferric phosphate powder, 1.62g of glucose, and 1.47g of lithium carbonate, and ball mill them with 20mL of cyclohexane and 25g of zirconium oxide beads in a ball mill for 20h. The ball mill speed was 300rpm, and the zirconium oxide beads had a particle size of 0.8mm. After ball milling, the mixed slurry is separated from the zirconium beads by wet sieving, and the obtained mixed slurry is vacuum dried at 80°C for 10 hours. The obtained dry powder is calcined in two stages in a tube furnace: first, the furnace temperature is raised to 400°C for pre-calcination for 2 hours, and then the temperature is raised to 800°C for secondary calcination for 6 hours. The calcined lithium manganese iron phosphate powder is cooled in the furnace and then ground to less than 200 mesh to finally obtain the lithium manganese iron phosphate positive electrode powder material. The compaction density of the lithium manganese iron phosphate positive electrode powder was tested and reached 3.1g / cm 3The electrochemical test showed that the first discharge capacity of the material was 167.52 mAh g at a current density of 0.2C. -1 At this current density, the specific capacity of the battery material after 200 cycles is 163.11 mAh g -1 , the positive electrode capacity retention rate is 97.37%.

[0031] Example 4

[0032] Take 2 mol of manganous sulfate and 2 mol of ferrous sulfate, dissolve them in 8000 mL of deoxygenated deionized water, and add 25 g of ascorbic acid and 8 g of polyvinyl pyrrolidone to obtain a manganese-iron mixed solution. Weigh 4 mol of ammonium dihydrogen phosphate (NH4H2PO4) and dissolve it in 8000 mL of deoxygenated deionized water to obtain a dihydrogen ammonium phosphate solution. Slowly pump the dihydrogen ammonium phosphate solution into the above-mentioned manganese-iron mixed solution, control the reaction temperature to 50°C and the pH value to 7, react for 90 minutes and age for 6 hours to obtain ammonium manganese-iron phosphate slurry ①. Take 2 mol of manganous sulfate and 1 mol of ferrous sulfate, dissolve them in 5000 mL of deoxygenated deionized water, and add 20 g of ascorbic acid, 8 g of polyethylene glycol and 2 g of polylactic acid to obtain a manganese-iron mixed solution. Weigh 3 mol of ammonium dihydrogen phosphate (NH4H2PO4) and dissolve it in 5000 ml of deoxygenated deionized water to obtain an ammonium dihydrogen phosphate solution. Slowly pump this ammonium dihydrogen phosphate solution into the above-mentioned manganese-iron mixed solution. Control the reaction temperature to 50°C and the pH to 7. After reacting for 60 minutes and aging for 6 hours, obtain ammonium ferromanganese phosphate slurry ②. Take 1 mol of manganous sulfate and 1 mol of ferrous sulfate and dissolve them in 4000 ml of deoxygenated deionized water. Add 10 g of ascorbic acid and 2 g of bone glue to obtain a manganese-iron mixed solution. Weigh 2 mol of ammonium dihydrogen phosphate (NH4H2PO4) and dissolve it in 4000 ml of deoxygenated deionized water to obtain an ammonium dihydrogen phosphate solution. Slowly pump this ammonium dihydrogen phosphate solution into the above-mentioned manganese-iron mixed solution. Control the reaction temperature to 70°C and the pH to 9. After reacting for 90 minutes and aging for 3 hours, obtain ammonium ferromanganese phosphate slurry ③. Different amounts of the above-mentioned ammonium manganese ferrous phosphate slurry were taken out and mixed, and the mixing ratio was: 30wt.% of the slurry obtained under condition ① + 70wt.% of the slurry obtained under condition ②. After mixing evenly, liquid-solid separation was performed by filter press, and the obtained solid phase product was vacuum dried at 80°C for 12h. Thereafter, the dried ammonium manganese ferrous phosphate was calcined at 500°C for 50min under argon atmosphere to obtain ammonium manganese ferrous phosphate powder. Take 45g of the ammonium manganese ferrous phosphate powder, 11g of glucose, and 10g of lithium carbonate, and ball mill them with 150ml of ethanol and 100g of zirconium oxide beads in a ball mill for 18h. The particle size of the zirconium oxide beads was 0.6mm, and the ball mill speed was 400rpm. After ball milling, the mixed slurry is separated from the zirconium beads by wet sieving, and the obtained mixed slurry is vacuum dried at 80°C. The obtained dry powder is calcined in two stages in a tube furnace: first, the furnace temperature is raised to 400°C for pre-calcination for 2 hours, and then the temperature is raised to 800°C for secondary calcination for 6 hours. The calcined lithium manganese iron phosphate powder is cooled in the furnace and then ground to less than 200 mesh to finally obtain the lithium manganese iron phosphate positive electrode powder material. The compaction density of the lithium manganese iron phosphate positive electrode powder was tested and reached 2.8g / cm 3The electrochemical test showed that the first discharge capacity of the material was 163.92 mAh g at a current density of 0.2C. -1 At this current density, the specific capacity of the battery material after 100 cycles is 160.15 mAh g -1 , the positive electrode capacity retention rate is 97.70%.

[0033] Comparative Example 1 (Instead of mixing several ammonium manganese ferrous phosphate slurries, as described in other existing technologies, only one ammonium manganese ferrous phosphate slurry prepared under one condition was used for subsequent treatment, resulting in a low compaction density and poor electrochemical performance of the final product)

[0034] The other conditions were the same as those in Example 1, with the only difference being that slurries ①, ②, and ③ as in Example 1 were used for subsequent treatment. The compacted densities of the resulting lithium manganese iron phosphate cathode powders were 2.01 g / cm 3 、1.92g / cm 3 , 2.13g / cm 3 (None of them have reached the current industry demand of 2.5g / cm 3 The above requirements), the first discharge capacity of the obtained lithium manganese iron phosphate positive electrode powder is less than 130mAh·g -1 , the capacity retention rate after 50 cycles is less than 70%.

[0035] Comparative Example 2 (the final product was not mixed according to the mixing ratio of the ammonium manganese iron phosphate slurry described in the present invention, and the compaction density and electrochemical performance were also poor)

[0036] The other conditions are as shown in Example 1, except that the mixing ratio of the ammonium manganese iron phosphate slurry as described in the present invention is not followed. For example, the mixing conditions are 70wt.% of the slurry obtained under condition ① + 30wt.% of the slurry obtained under condition ②, 20wt.% of the slurry obtained under condition ② + 80wt.% of the slurry obtained under condition ③, 90wt.% of the slurry obtained under condition ① + 10wt.% of the slurry obtained under condition ③, and 10wt.% of the slurry obtained under condition ① + 20wt.% of the slurry obtained under condition ② + 70wt.% of the slurry obtained under condition ③. The compacted density of the finally obtained lithium manganese iron phosphate positive electrode powder is less than 2.0g / cm 3 (It does not reach the current industry demand of 2.5g / cm 3 The above requirements), the first discharge capacity of the obtained lithium manganese iron phosphate positive electrode powder is less than 130mAh·g -1 , the capacity retention rate after 100 cycles is less than 60%.

[0037] Comparative Example 3 (After drying, the lithium manganese iron phosphate was not subjected to two-stage calcination under the conditions described in the present invention, and was not ground to less than 200 mesh. The final product had a low compaction density and poor electrochemical properties)

[0038] Other conditions were the same as those in Example 1, with the only difference being that the lithium manganese iron phosphate powder was only calcined once after drying (at 700°C for 8 h), and the resulting product was simply ball-milled and not passed through a 200-mesh sieve before being directly subjected to electrochemical testing. The final compacted density of the lithium manganese iron phosphate cathode powder was only 1.88 g / cm 3 (It does not reach the current industry demand of 2.5g / cm 3 The above requirements), the first discharge capacity of the obtained lithium manganese iron phosphate positive electrode powder is less than 130mAh·g -1 , the capacity retention rate after 100 cycles is less than 55%.

Claims

1. A method for preparing a high-density lithium manganese iron phosphate positive electrode material, characterized in that: The method comprises the following steps: first, using manganese salt, ferrous salt and phosphate as raw materials, co-precipitating under different conditions to obtain ammonium manganese ferrous phosphate slurry; mixing these ammonium manganese ferrous phosphate slurries with different properties in a certain proportion; performing a first drying and dehydration on the solid phase product obtained by liquid-solid separation to obtain ammonium manganese ferrous phosphate powder; placing the ammonium manganese ferrous phosphate powder, a lithium source, a carbon source, a ball milling aid and grinding balls in a ball mill in a certain proportion; after the ball milling is completed, separating the grinding balls; and performing a second drying, calcining and grinding on the obtained mixture to finally obtain a high-density manganese ferrous phosphate lithium cathode material; When co-precipitation is carried out with manganese salt, ferrous salt and phosphate as raw materials, co-precipitation is carried out under the following three different conditions to obtain three co-precipitation products, ammonium manganese ferrous phosphate slurries with different properties; condition ①: the molar ratio of the added manganese salt and ferrous salt is 0.25~4:1, and the sum of the moles of the ferrous salt and the manganese salt is equal to the mole of the phosphate, the temperature is 20~50°C; the pH value is 5~7, and at least one of polyvinyl pyrrolidone, carboxymethyl cellulose and methyl cellulose is added with a mass concentration of 0.1~3 g / L; condition ②: the molar ratio of the added manganese salt and ferrous salt is 0.25~4:1, and the sum of the moles of the ferrous salt and the manganese salt is equal to the mole of the phosphate, the temperature is 50~70°C; the pH value is 7~8, and the mass concentration of 0.1~3 g / L of at least one of polyethylene glycol, polylactic acid, and methyl methacrylate; Condition ③: the molar ratio of the added manganese salt to the ferrous salt is 0.25-4:1, and the sum of the moles of the ferrous salt and the manganese salt is equal to the moles of the phosphate; the temperature is 70-90°C; the pH value is 8-10, and at least one of the added gelatin and polyvinyl alcohol is added at a mass concentration of 0.1-1 g / L; The ammonium manganese ferrous phosphate slurries obtained under the above different conditions are mixed in a certain proportion, and the mixing conditions are: (1) if two different ammonium manganese ferrous phosphate slurries are mixed, the mixing conditions are: 20~30 wt.% of the slurry obtained under condition ① + 70~80 wt.% of the slurry obtained under condition ②, or 40~50 wt.% of the slurry obtained under condition ② + 50~60 wt.% of the slurry obtained under condition ③, or 30~40 wt.% of the slurry obtained under condition ① + 60~70 wt.% of the slurry obtained under condition ③; (2) if three different ammonium manganese ferrous phosphate slurries are mixed, the mixing conditions are: 10~20 wt.% of the slurry obtained under condition ① + 60~75 wt.% of the slurry obtained under condition ② + 15~20 wt.% of the slurry obtained under condition ③; The calcination is carried out in two stages: first, the dried mixture is pre-calcined by heating the furnace to 300-400°C, and then the temperature is raised to 600-800°C for secondary calcination. The lithium manganese iron phosphate powder obtained by calcination is cooled in the furnace and then ground to below 200 mesh.

2. The method according to claim 1, characterized in that The manganese salt is at least one of manganous sulfate, manganous chloride, and manganous nitrate; the ferrous salt is at least one of ferrous sulfate, ferrous chloride, and ferrous nitrate; and the phosphate is at least one of diammonium hydrogen phosphate and ammonium dihydrogen phosphate.

3. The method according to claim 1, characterized in that The ammonium manganese ferrous phosphate slurries obtained under the above different conditions are mixed in a certain proportion, and the solid phase product obtained after liquid-solid separation is subjected to a first drying and calcination dehydration to obtain ammonium manganese ferrous phosphate powder; the first drying conditions are vacuum drying, drying temperature 60~90℃, and drying time 6~24h; the calcination conditions are: inert atmosphere, temperature 300~700℃, and calcination time 30~120 min.

4. The method according to claim 1, wherein The obtained ammonium manganese ferric phosphate powder is ball-milled with a lithium source, a carbon source, a ball-milling aid, and zirconium oxide beads in a ball mill; the lithium source is at least one of lithium carbonate, lithium hydroxide, and lithium dihydrogen phosphate; the carbon source is at least one of glucose, sucrose, and soluble starch; the ball-milling aid is at least one of ethanol, cyclohexane, and polyethylene glycol; during ball milling, the mass ratio of zirconium oxide balls to raw materials is 5-10:1, the particle size of zirconium oxide beads is 0.6-0.8 mm, the solid content of the slurry is 20-45%, the ball milling time is 8-24 hours, the ball mill speed is 200-800 rpm, and the mass ratio of the added carbon source to the ammonium manganese ferric phosphate powder is 10-50%.

5. The method according to claim 1, wherein After the ball milling is completed, the mixed slurry is separated from the grinding balls by wet screening, and the obtained mixed slurry is then dried, calcined and ground for the second time.

6. The method according to claim 5, characterized in that The second drying method is vacuum drying, the drying temperature is 60-90°C, and the drying time is 6-24 hours.

7. The method according to claim 5, characterized in that The atmosphere during calcination is argon or nitrogen; the calcination is carried out in two stages: first, the dried mixture is heated with the furnace, the furnace temperature is raised to 300~400℃ for pre-calcination for 2~3 hours, and then the temperature is raised to 600~800℃ for secondary calcination for 5~10 hours. The lithium manganese iron phosphate powder obtained by calcination is ground after cooling with the furnace.

8. A high-density lithium manganese iron phosphate cathode material prepared by the method according to any one of claims 1 to 7.

9. The use of the high-density lithium manganese iron phosphate positive electrode material according to claim 8, characterized in that: Used to prepare lithium-ion batteries.