Supercritical Method Preparation For Lithium Iron Phosphate Nanometer Material And Its Carbon-coating Research

The lithium ion battery with its high working voltage, small volume, light weight, high than energy, no memory effect, no pollution, the properities, long cycling life wait for a characteristic and attention by people and favor. At present the lithium ion battery battery anode materials as the price, safety and electrochemical properties reason, in the power of the application of the lithium ion battery is restricted. Lithium iron phosphate is cheap raw materials, non-toxic, working voltage is modest and stable structure characteristics, is considered to be the most potential lithium ion batteries battery anode materials. This paper established the supercritical water rapid continuous synthesis of lithium iron phosphate nanoparticles and the process of lithium iron phosphate coated the carbon. Study the reaction concentration, reaction temperature and the reaction pressure to factors such as the size of lithium iron phosphate, and the influence of the crystallinity of morphology and explained the relevant mechanism, and discusses the calcination temperature and carbon sources, bag of carbon way lithium iron phosphate coated carbon, the effect of using SEM, TEM and DLS and XRD analytical means such as the structure of the target product size, morphology and alloy, and at the same time with charge-discharge cycle test, the test means of materials such as current-voltage of electrochemical performance analysis.Choose two different reactor preparation LiFePO₄, through the computer simulation of two different within reactor fluid flow velocity field, the results show that the no. 2 reactors in three strands of fluid mixing of high flow velocity in can be reached 0.05m/s above, and 1 reactor mixed point in the highest velocity only 0.035 m/s, in a hybrid point 2 reactor has better mixed effect. By comparing the two kinds of reactor obtained under the same conditions the sample of the SEM figure and XRD map and the no.1 reactor income sample particles smaller, but uneven distribution, and grain abundance than 2 reactor income samples. Comprehensive comparison, choose 2 continuous hot water reactors critical method for the technology conditions of LiFePO₄ explored.Research on supercritical water hot continuous synthesis in LiFePO₄ materials of pure the best preparation technology, and the best preparation conditions of LiFePO₄ materials was successfully out the electrochemical test. Preheat temperature on the crystallinity of lithium iron phosphate have obvious influence. Low temperature, pressure to the particle effects for the performance and the influence of the crystallinity shape; Higher temperature, the lithium iron phosphate particles are good crystallinity, this time the pressure of particle influences performance for the influence of its size. The reaction temperature and reaction liquid concentration on the particle size and morphology have influence. Along with the increase of the reaction temperature, particle size change, but more uniform particle size distribution; Low reaction liquid concentration to get smaller nanoparticles. In the reaction temperature 380℃, preheat temperature of 405℃, reaction pressure of 27MP, Fe²⁺ concentration of 0.015 mol/L, the reaction time is 40s optimization condition available for the average particle size for 90 nm high crystallinity, olivine shape of lithium iron phosphate. This material for electrochemical test, in the current density of 0.1C discharge capacity can reach 103 mAh/g, through to the materials circulation current-voltage test, it can judge the materials of the electrode reversible is good.Through the direct reaction liquid in adding carbon sources, directly to the calcination lithium iron phosphate carbon-encapsulated explored, contrast calcining temperature, carbon sources, such as carbon way of LiFePO₄ bag of carbon-encapsulated optimization and the optimized samples of the electrochemical performance of the corresponding approaches. Will add to the reaction liquid carbon source in supercritical water hot direct synthesis can't get coated carbon LiFePO₄. But the reaction liquid with carbon sources, lithium iron phosphate particles will become more small and more distribution more even. Direct firing pure lithium iron phosphate samples won't change sample crystal, and samples of the average particle size particles will become smaller. Will be lithium iron phosphate glucose powder solution by ultrasound, evaporation, after drying get lithium iron phosphate and carbon source mixture, and then burning is the best bag carbon way; Samples and carbon source in under 600℃mixture calcined 2 h, carbon source can be completely carbonization and can be wrapped in lithium iron phosphate surface; Different carbon source not influence of lithium iron phosphate crystal, glucose and citric acid mixture basic all carbonation, citric acid to carbon source for the preparation of sample reunion phenomenon is relatively serious income and the particle size larger particles. Using a pure lithium iron phosphate and carbon source of successful synthesis method calcined LiFePO₄/C nanoparticles, lithium iron phosphate particles formed on the surface of about 10nm carbon layer thickness or so. This material electrochemical characterization, in 0.1C, was the first time the sample of the discharge capacity of 150.3 mAh/g, material electrode reaction reversible is very good, has the very good electrical conductivity.