Electron Transfer Governed Crystal Transformation of Tungsten Trioxide upon Li Ions Intercalation

Reversible insertion/extraction of foreign ions into/from a host lattice constitutes the fundamental operating principle of rechargeable battery and electrochromic materials. The insertion of foreign ions is a far more commonly observed structural evolution of the host lattice, and for the most cases such a lattice evolution is subtle. However, it has not been clear what factors control such a lattice structural evolution. Based on the tungsten trioxide (WO3) model crystal, we use in situ transmission electron microscopy (TEM) combined with density functional theory calculations to explore the nature of Li ions intercalation induced crystal symmetry evolution of WO3. We discovered that Li insertion into the octahedral cavity of the WO3 lattice will lead to a low to high symmetry transition, featuring a sequential monoclinic → tetragonal → cubic phase transition. The density functional theory results reveal that the phase transition is essentially governed by the electron transfer from Li to the WO6 octahedrons, which effectively leads to the weakening the W-O bond and modifies system band structure, resulting in an insulator-to-metal transition. The observation of the electronic effect on crystal symmetry and conductivity is significant, providing deep insights on the intercalation reactions in secondary rechargeable ion batteries and the approach for tailoring the functionalities of material based on insertion of ions in the lattice.