Background and overview[1-2]
The synthesis and application of transition metal phosphate crystalline functional materials with potential optical, electrical, magnetic, catalytic and other properties have been a research hotspot in the past 10 years. Among them, lithium cobalt phosphate is the cathode material used in commercial lithium-ion batteries. Its crystal has an ordered olivine structure and belongs to the orthorhombic crystal system. The lithium cobalt phosphate cathode material has a theoretical capacitance of 167mAh·g-1, good safety performance, and a potential of about 4.8V relative to the lithium electrode. It is expected to become a new generation of high-capacity and high-voltage cathode materials. Using low-heat solid-phase reaction method, zinc phosphate salts such as sodium zinc phosphate, sodium zinc hydrogen phosphate and lithium zinc phosphate have been successfully synthesized. Lithium cobalt phosphate also has some disadvantages when used as a cathode material for lithium-ion batteries. For example, poor conductivity due to a wide band gap, and at higher operating voltages (not less than 5V), the electrolyte decomposes at the electrode/electrolyte interface, resulting in unstable cycle performance. These will cause phosphoric acid to The rapid decay of the specific capacity of cobalt-lithium materials and the reduction of electrochemical activity affect the performance and service life of the cathode material. Therefore, modifying lithium cobalt phosphate materials to improve electrochemical kinetic properties and improve the stability of the electrode/electrolyte interface has become an important research direction. At present, the main methods for modifying lithium cobalt phosphate materials include surface modification and bulk doping. The main method of surface modification is coating. The purpose of coating is mainly to improve the electronic conductivity or form an isolation layer, avoid direct contact between the active material and the electrolyte, and inhibit the dissolution or oxidation of the active material. Bulk doping utilizes partial substitution of cations to increase intrinsic ion conductivity. The combination of surface coating and bulk doping is the development trend of cathode materials such as lithium cobalt phosphate.
Application
Lithium cobalt phosphate has very stable lithium ion deintercalation behavior. The theoretical discharge specific capacity of LiCoPO4 cathode material is 167mAh/g, and the electrode potential relative to lithium is 4.8V. It is expected to become a new generation of high-capacity and high-voltage cathode materials.
Preparation[1-2]
Method 1: Use CoCl2·6H2O and LiH2PO4 as raw materials, polyethylene glycol as template agent, mix in a small amount of MnSO4·H2O, neutralize with anhydrous Na2CO3, incubate at 80°C for 6 hours, and use Wash away the soluble inorganic salts with water, dry at 100°C, and burn at 600°C for 2 hours to obtain lithium cobalt phosphate, the electrode material for lithium ion batteries. The product was analyzed and characterized by XRD, IR, SEM, etc., which proved that the product is LiCoPO4 nanocrystal with an average particle size of about 36.5nm, belongs to the Orthorhombic crystal system, Pmnb (62) space group, Z=4, unit cell parameter a= 5.922゜A, b=10.206゜A, c=4.701A゜. The specific steps are as follows:
(1) Preparation of LiH2PO4·H2O: Place 6.30g of LiOH·H2O in a ceramic evaporating dish, add a small amount of distilled water and stir to make LiOH·H2O into a paste, and add it drop by drop while stirring Add an excess of 5% concentrated phosphoric acid 18.16g (9.9mL) until the pH ≈ 1 and a large number of bubbles are generated. Stir for a few minutes until bubbles no longer occur. Then place the mixture on an electric stove and slowly heat and evaporate until it is completely dry.
(2) Mix 18.0mmol (4.28g) CoCl2·6H2O and 2mmol (0.34g) MnSO4·H2O in the mortar ① and grind them into powder, then add 2 drops of PEG- and continue grinding. Then grind 20.0mol (2.44g) LiH2PO4·H2O into powder in the mortar ②, add the LiH2PO4·H2O in the mortar ② into the mortar ① containing 18.0mmol CoCl2·6H2O in three times, each time To grind evenly, grind for 30 minutes after the third addition. As grinding proceeds, the reactant first becomes hard and finally becomes waxy. Weigh 20.0mmol (2.12g) Na2CO3 and add it to the mortar ① in three times. Grind evenly each time. After the last addition, grind for 30 minutes. During the grinding process, bubbles will be released. If it becomes hard, add a few drops of distilled water and continue grinding. The final reaction product appears fluffy. Finally, transfer the reactants to the beaker, add plastic wrap and incubate it in an incubator at 80°C for 6 hours. Then wash the mixture with distilled water, and use barium chloride to check that there are no sulfate ions. Filter with suction, and rinse with absolute ethanol. Then Place the filter cake in an oven to dry at 100°C, and finally put the obtained product into a muffle furnace at 600°C and burn it for 2 hours.
Method 2: The specific steps for preparing lithium cobalt phosphate cathode material are as follows:
(1) Add polyvinylidene fluoride to N-methylpyrrolidone, stir until completely dissolved, then add modified multi-walled carbon nanotubes, ultrasonically disperse for 28 minutes, then add lithium phosphate, cobalt tetroxide, and iron oxide. Transfer to a ball mill tank for ball milling; the weight parts of each raw material are: 1 part by weight of polyvinylidene fluoride, 69 parts by weight of N-methylpyrrolidone, 5 parts by weight of modified multi-walled carbon nanotubes, 10 parts by weight of lithium phosphate, and cobalt tetroxide 12 parts by weight and 3 parts by weight of iron oxide; the modified multi-walled carbon nanotubes are hydroxylated multi-walled carbon nanotubes; the ball milling speed is 1250r/min and the time is 7h;
(2) Take out the materials in the ball mill tank, heat to remove N-methylpyrrolidone, then grind it into nanopowder, place it in a tubular Hanwha acrylic resin furnace, and perform high-temperature sintering in a nitrogen and hydrogen atmosphere to produce Composite particles of carbon nanotubes and Fe3+-doped lithium cobalt phosphate interspersed with each other were obtained; the heating temperature was 215°C; the high-temperature sintering temperature was 690°C and the time was 11 hours;
(3) Add the composite particles prepared in step (2) into ethanol, disperse with ultrasonic for 32 minutes, then add pyrrole and sodium p-toluenesulfonate, stir for 16 minutes, and then add ferric chloride hexahydrate in ethanol dropwise.The solution was stirred and reacted for 11 hours. After filtering, washing and vacuum drying, polypyrrole-coated Fe3+/LiCoPO4/MWCNT composite particles were obtained; the weight parts of each raw material were: 4 parts by weight of composite particles, 93.8 parts by weight of ethanol, and 0.7weight parts, 0.1 weight parts of sodium p-toluenesulfonate, 1.4 weight parts of ferric chloride hexahydrate; the vacuum drying temperature is 46°C and the time is 30h;
(4) Place the polypyrrole-coated Fe3+/LiCoPO4/MWCNT composite particles prepared in step (3) in a tube furnace and treat it at high temperature under a nitrogen atmosphere to obtain nitrogen-doped carbon layer-coated Fe3+ /LiCoPO4/MWCNT composite particles are lithium cobalt phosphate cathode materials used in lithium-ion batteries; the high-temperature treatment temperature is 505°C and the time is 170 minutes.
