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

Abstract

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.