Li/β-VOPO4:: A new 4 V system for lithium batteries

Lithium intercalation into beta-VOPO4 has been performed, both chemically and electrochemically, with X-ray diffraction characterization of the resulting phases at various levels of intercalation-deintercalation. It is shown that intercalation occurs through a first-order transition at 3.98 V vs. Li+/L-0, which leads to a beta-LiVOPO4-like structure but with lithium ordering which doubles the a and b unit cell parameters. Starting from chemically intercalated beta-Li0.92VOPO4, the reversible process involves 0.55 electron per transition metal, i.e., 90 mAh/g, at a C/50 rate which owing to the 3.95 V potential plateau, gives an attractive specific energy of 355 Wh/kg. Fundamentally, it is shown herein that the redox energy of V5+/V4+ couple in an octahedral coordination involving (V = O)(m+) unit is included between those observed in oxides and in M-2(PO4)(3) compounds with (PO4)(3-) oxo-anions. (C) 1999 The Electrochemical Society. S0013-4651(99)04-093-8. All rights reserved.