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.