Electrochemical and structural characteristics of tungstic acids as cathode materials for lithium batteries

Abstract

The electrochemical characteristics and structural changes associated with discharge and charge of several tungstic acids such as H2WO4 and H2WO4 · H2O have been investigated. The suitability of these substances as new cathode materials for nonaqueous lithium batteries has been assessed. H2WO4, having only coordinated water molecules, showed a discharge capacity of about 410 Ah kg−1 of acid weight and a discharge potential around 2 V vs. Li/Li+. This capacity was much higher than the 40 ∼ 180 Ah kg−1 of anhydrous WO3. H2WO4 showed a good charge-discharge cycling behavior at a capacity below 1e−/W. However, the formation of a stable phase such as Li2WO4 during the cyclings limited the cycling number. In addition, the crystal structure of H2WO4 changed from orthorhombic to tetragonal during discharge, but the original layered lattice was kept on discharge to 1.5e−/W. On the other hand, a significant decrease in the layer spacing of H2WO4 · H2O took place with discharge, due to the direct interaction between the interlayer water molecule and the lithium inserted between the layers. In this paper, in particular, the effect of the coordinated and hydrated water molecules in the acid structure on the electrochemical behavior is discussed.