Lithium insertion chemistry of some iron vanadates

Lithium insertion into various iron vanadates has been investigated. Fe2V4O13 and Fe4(V2O7)3 · 3H2O have discharge capacities approaching 200 mAh g−1 above 2.0 V vs. Li+/Li. Although the potential profiles change significantly between the first and subsequent discharges, capacity retention is unexpectedly good. Other phases, structurally related to FeVO4, containing copper and/or sodium ions were also studied. One of these, β-Cu3Fe4(VO4)6, reversibly consumes almost 10 moles of electrons per formula unit (ca. 240 mAh g−1) between 3.6 and 2.0 V vs. Li+/Li, in a non-classical insertion process. It is proposed that both copper and vanadium are electrochemically active, whereas iron(III) reacts to form LiFeIIIO2. The capacity of the Cu3Fe4(VO4)6/Li system is nearly independent of cycling rate, stabilizing after a few cycles at 120–140 mAh g−1. Iron vanadates exhibit better capacities than their phosphate analogues, whereas the latter display more constant discharge potentials.