Hysteresis on the electrochemical lithium insertion and extraction of hexagonal tungsten trioxide - Influence of residual ammonium

Electrochemical lithium insertion property of a h-WO3, framework prepared from ammonium paratungstate decahydrate, which contained a small amount of residual ammonium (NH4+/W: 0.05), was compared with that of ammonium-free h-WO3 synthesized from WO3 1/3(H2O) according to Gerand's method. Electrochemical measurements performed in a LiClO4/propylene carbonate solution have revealed that both frameworks have almost the same level of reversible capacity (x - 1.0 in terms of LixWO3) between the cut off voltages of 4.5 and 1.8 V, and exhibit similar discharge-charge curves with very remarkable hysteresis. It was also observed for both samples that very large polarization occurred abruptly at a certain stage of the charging process near its end point, but the onset of this polarization started a little earlier for h-WO3 containing ammonium. The lattice parameters of those samples measured as a function of x (LixWO3) showed a common tendency, though the initial c-axis of ammonium-free h-WO3 was considerably longer than that of its counterpart. Therefore, the ammonium species in the h-WO3 framework does not so much affect its insertion properties, except for a slight kinetic effect. (C) 2000 Elsevier Science B.V. All rights reserved.