Kinetic study on Li2.8(V0.9Ge0.1)2(PO4)3 by EIS measurement

Kinetics for lithium ion transfers in the fast ionic conductor Li2.8(V0.9Ge0.1)2(PO4)3 prepared by solid-state reaction method has been studied by electrochemical impedance spectroscopy (EIS) at various temperatures and the results were correlated with observed cathodic behavior. The specific conductivities of Lix(V0.9Ge0.1)2(PO4)3 (x = 0.9–2.8) versus temperatures were analyzed from blocking-electrodes by Wagner's polarization method and the activation energy was calculated. It was observed that electronic conductivities of Lix(V0.9Ge0.1)2(PO4)3 increased with lithium contents in the materials. The compounds show a reversible capacity of 131 mAh g−1 at low current density (13 mA g−1). Modeling the EIS data with equivalent circuit approach enabled the determination of charge transfer and surface film resistances. The Li ion diffusion coefficient (DLi+DLi+) versus voltage plot shows three valleys during the first charge cycle coinciding with the irreversible plateau of the voltage versus   lithium content profiles reflecting the irreversible phase change in the compound. The obtained DLi+DLi+ from EIS varies within 10−8 to 10−7 cm2 s−1, so Li2.8(V0.9Ge0.1)2(PO4)3 shows excellent chemical diffusion performance.