Electrochemical lithium intercalation into a hexagonal WO3 framework and its structural change

Hexagonal tungsten trioxide (h-WO3) (ideally P6/mmm) has three kinds of lithium intercalation sites which are the hexagonal-window (1a), hexagonal-cavity (1b) and trigonal-cavity (2d). Calculation by the DV-Xα method using a LixW36O138(60-x)− cluster showed that inserted lithium ions were more stable at the trigonal-cavity site than at the other sites. When lithium intercalation in h-WO3 was performed electrochemically, OCV (open circuit voltage) traces exhibited large hysteresis between insertion and the extraction process. On the basis of DV-Xα calculation results and changes in lattice parameters, we concluded that lithium ions started occupying hexagonal-window and/or hexagonal-cavity sites before the trigonal-cavity site was completely filled in the insertion process, and that lithium ions at the trigonal-cavity site were extracted after the hexagonal-window and hexagonal-cavity sites were almost empty during the extraction process.