High Performance LiFePO₄Cathode Material Prepared By Modified Hydrothermal Method

Due to the various advantages of high specific capacity, excellent cycle performance,low cost, non-toxicity, environment friendly and so on, LiFePO₄is considered as one of themost potential lithium-ion battery cathode materials for commercial use, especially theapplication to electric vehicles. For the purpose of application, based on the hydrothermalmethod, we improved the low electronic conductivity by carbon coating, and obtained theLiFePO₄/graphene composite of excellent electrochemical performance successfully. Inaddition, we also tried to improve the electrochemical performance of LiFePO₄by addingdifferent organic solvents and ionic liquids during the hydrothermal process.Oliver-structured LiFePO₄cathode material was prepared by hydrothermal methodwith the starting materials LiOH, FeSO₄and H3PO₄. As is shown by XRD and SEM, themorphology showed a uniform partial size around200⁵00nm with high crystalline,resulting in excellent electrochemical performance. The optimized experimental conditionswere pH=8, T=200℃, t=600min.We studied the effect of carbon coating process using three carbon precursors ascarbon sources on the electrochemical performance LiFePO₄/C composites. The resultsrevealed that LiFePO₄/C composite showed a uniform carbon layer when sucrose was usedas carbon source with sintering temperature at750℃for5h. The LiFePO₄/C compositesshowed an excellent electrochemical performance.In-situ catalytic LiFePO₄/graphene composite was successfully synthesized by addinga few Fe²⁺into the carbon coating process. The SEM and HRTEM graphs showed that thesurface coating layer was a graphitic carbon layer around2.5nm and the graphene growingfrom the carbon layer connected the particles with each other, constructing a three- dimensional conductive network. CV, Raman, EIS and galvanostatic tests and othermeasuring methods have further proved that the LiFePO₄/graphene composites owedexcellent electronic conductivity and ionic conductivity, therefore the LiFePO₄/graphenecomposite displayed a prominent rate performance and excellent cycle life.A small addition of ionic liquid HMIMBr to the hydrothermal synthesis process ofLiFePO₄is beneficial to the stability of nano-particles in the solution. The XRD resultsillustrated that the exposed crystal face is ac plane, indicating that the thinnest part of theparticle is along the transportation path of lithium ions, stimulating the easy intercalationand deintercalation of lithium ions. LiFePO₄synthetized with the assistance of organicsolvents EC and PC showed improvement on the cycle performance to some extent whilethe rate discharge capability was slightly weakened.