Preparation And Electrochemical Properties Of Ceria Coated Li₃V₂?PO₄?₃/C Cathode Materials For Lithium-ion Batteries

Li3V2(PO4)3/C were synthesized by an improved sol-gel method of which pH value is controlled at 4. The powders were then coated with various amounts of CeO2(0, 1, 2 and 3 wt. %) by a suspension coating process assisted by polyvinyl alcohol. The composition and microstructure of these composites were evaluated by XRD, BET, SEM, HRTEM and so on. The cathode films were made by CeO2 coated Li3V2(PO4)3/C powder and then assembled into lithium batteries. The electrochemical properties of the samples at different temperature were characterized by galvanostatic charge/discharge test, cyclic voltammetry(CV), electrochemical impedance spectroscopy(EIS) and potential step chronoamperometry(PSCA).Galvanostatic electrochemical measurements indicate that surface modification with CeO2 is an effective way to improve the cycling stability of Li3V2(PO4)3 cathode materials. 2 wt. % CeO2 coated Li3V2(PO4)3/C exhibits the best electrochemical performances in the voltage range of 3.0-4.8 V, especially the cycle performance at high rates. At room temperature, the samples coated with various amounts of CeO2(0, 1, 2 and 3 wt. %) show initial discharge capacity of 75.2, 102.0, 112.7 and 99.5 mAh g-1 at 5 C, and it remains capacity retention of 60.0 %, 84.3 %, 93.5 % and 84.9 % after 100 cycles, respectively. The comparison of the cycle capability for the cathodes at-20? shows that discharge capacity improves markedly after 2 wt. % CeO2 coating. The discharge capacity at 1 C after 50 cycles of the pristine samples and the 2 wt. % CeO2 coated Li3V2(PO4)3/C is 70.0 mAh g-1 and 96.8 mAh g-1, respectively. And the capacity retention is 80.7 % and 93.8 %, respectively.EIS test shows that the charge transfer resistance(Rct) is a key factor in affecting the electrochemical properties of lithium ion battery. The 2 wt. % CeO2 coated Li3V2(PO4)3/C exhibits the smallest Rct at room temperature, 0 ? and-20 ?. The Rct of Li3V2(PO4)3/C increases from 26.65 ? to 491.80 ? as the temperature decreases from the room temperature to-20 ?. Moreover, the value of Rct reduces to 119.50 ? after 2 wt. % CeO2 coating at-20 ?, indicating that the formation of conductive network by carbon and CeO2 can increase the electrochemical performance of the cathode remarkably.