Durability evaluation of hexagonal WO3 electrode for lithium ion secondary batteries

•The degradation mechanism of h-WO3 when used as the negative electrode in LIB was clarified.•The both changes in the crystalline structure and the valence of W of h-WO3 occurred.•The deposition of a layer of reactive materials on the surface of the h-WO3 electrode was observed.•Changes in the electrolyte composition in LIB occurred.

We evaluated the durability of a hexagonal tungsten oxide (h-WO3) electrode for use as the negative electrode in lithium ion secondary batteries (LIBs). H-WO3 has attracted attention as an electrode for LIBs owing to its large hexagonal tunnels, in which Li ions can be inserted. However, when a charge-discharge cycling test was carried out in the voltage range 2.5-1.0 V (vs LiCoO2) for 500 cycles, its discharge capacity decreased by 73% compared with the initial value. To clarify the reasons for the degradation, various types of analysis were performed. The deposited layer on the electrode and changes in both the crystalline structure and the electrolyte composition are considered to be the main reasons for the degradation. The observed changes in the valence of W also affected the degradation. We also report that the voltage range plays an important role in increasing the durability.