Electrochemical Property for the Metal-doped Vanadium Bronze K2V8O21 as a Cathode Material of Lithium Battery

Improvement of electrochemical property of K2V8O21, a stable phase of vanadium bronzes, was attempted by redox inactive metal ion doping. From the XRD pattern and lattice parameter change, it was confirmed that Ti and Nb can be exchanged with vanadium until 2 mol%. By doping 1 mol% Nb5+ to K2V8O21, the discharged capacity was improved to 210 mAh/g from 140 mAh/g for non-dope system. In ex-situ XRD measurement, non-dope K2V8O21 became amorphous in the first discharge process. For Nb-doped system, main reflection of K2V8O21 observed after both discharge process and first discharge-charge cycle. The lattice parameter of c axis increased while that of a axis decreased after discharge. Both lattice parameters got back to the initial position after charging process. This result suggests that lithium ions are intercalating into the inter layer part along the c axis during the discharge process. Mechanical mixing active materials with acetylene black was attempted for the further improvement in the electrochemical property. The initial capacity of ball milled Nb-doped K2V8O21 was improved to 315 mAh/g. This result suggests that the capacity of K2V8O21 is determined by the numbers of lithium ion that can be intercalated into the lattice without the destruction of its crystal structure.