Study On Hydrothermal Synthesis And Modification Of LiFePO₄

Lithium-ion batteries can be a suitable choice for new energy vehicle.Electrochemical Applications of Li-ion cells have been diversified from mobile devices tostationary energy storage. Among various cathode materials in lithium-based batteries,lithium iron phosphate is considered to be one of the most promising cathode materialsdue to its high theoretical specific capacity, cycling stability and low cost. However,LiFePO₄has low intrinsic electronic and ionic conductivity, which causes poorelectrochemical performance. To solve this problem, tremendous efforts have been madethrough reducing the particle size, coating with conductive materials and doping LiFePO₄with metal ions.LiFePO₄is prepared via hydrothermal method followed by heat treatment. ModifiedLiFePO₄were prepared via a surfactant assisted hydrothermal method and conductivepolymer coating. The samples were characterized by XRD, SEM, FITR, TGA, BET,Raman, galvanostatic charge–discharge test and electrochemical impedance spectroscopy.PEG has been applied as both carbon source and dispersant during hydrothermalprocess to control the morphology of particles. LiFePO₄/C composites are preparedfollowed by heat treatment. The effect of polyethylene glycol （PEG） molecular weight onthe crystal structure, particle size, morphology and particularly the electrochemicalperformance of LiFePO₄/C were investigated. The results show that the particle sizereduced, and the electrochemical performance of LiFePO₄/C are improved with theincreasing of the polymerization degree. The sample with PEG4000has the sphericparticles dispersed uniformly, which deliveres the best electrochemical performance. TheLiFePO₄/C composites deliver an enhanced cycling performance at0.2C,0.5Cand1Crate. However, the carbon content is5.23%.LiFePO₄/C has been synthesized via hydrothermal method whereas PPy is formed insitu by chemical oxidative polymerization of Py to achieve the PPy-LiFePO₄/Ccomposites. The results show that the olivine-type phase of LiFePO₄is not changed byPPy. It can be seen that the PPy is well coated on the surface of the LiFePO₄/C particles. The results show that the PPy-LiFePO₄/C cathode material has high graphitization degreeand a remarkable improvement in the capacity, cycling performance and rate capacitycompared with LiFePO₄/C. Electrochemical impedance measurements also show that thecoating of PPy significantly decreased the charge-transfer resistance of LiFePO₄/Celectrodes.LiFePO₄is synthesized via hydrothermal method whereas PANI is formed in situ bychemical oxidative polymerization of An to achieve the PANI-LiFePO₄composites at thedifferent PANI weight contents. The results show that PANI-LiFePO₄cathode materialmodified by7.02%polyaniline has the highest uniformity. It deliveres the capacity of154.9mAh g^-1at0.5C.