Uncommon potential hysteresis in the Li/Li2xVO(H2−xPO4)2 (0 ≤ x ≤ 2) system

Physical and electrochemical investigations of vanadium phosphates, Li2xVO(H2−xPO4)2 (0 < x < 2), have been undertaken. H+/Li+ ionic exchange from VO(H2PO4)2 to Li2VO(HPO4)2 leads to grain decrepitation. Further ionic exchange toward formation of Li4VO(PO4)2 lowers the symmetry. As inferred from potentiodynamic cycling correlated to ex situ and in situ X-ray diffraction (XRD), the system Li/Li4VO(PO4)2 shows several phase transformations that are associated with thermodynamical potential hysteresis that span from roughly 15 mV to more than 1.8 V. Small hysteresis are associated with topotactic reactions and with VV/VIV and VIII/VII redox couples. Large potential hysteresis values (>1 V) were observed when oxidation of VIII to VIV is involved.