Yang He1 Meng Gu2 Haiyan Xiao3 Langli Luo4 Yuyan Shao1 Fei Gao5 Yingge Du1 Scott Mao6 Chongmin Wang1

1, Pacific Northwest National Laboratory, Richland, Washington, United States
2, Southern University of Science and Technology, Shenzhen, , China
3, University of Electronic Science and Technology of China, Chengdu, , China
4, Tianjin University, Tianjin, , China
5, University of Michigan, Ann Arbor, Michigan, United States
6, University of Pittsburgh, Pittsburgh, Pennsylvania, United States

Intercalation and conversion are two fundamental chemical processes for battery materials in response to ion insertion. The interplay between these two chemical processes has never been directly seen and understood at atomic scale. Here, using in situ HRTEM, we captured the atomistic conversion reaction processes during Li, Na, Ca insertion into a WO3 single crystal model electrode. An intercalation step prior to conversion is explicitly revealed at atomic scale for the first time for Li, Na, Ca. Nanoscale diffraction and ab initio molecular dynamic simulations revealed that after intercalation, the inserted ion-oxygen bond formation destabilizes the transition-metal framework which gradually shrinks, distorts, and finally collapses to an amorphous W and MxO (M=Li, Na, Ca) composite structure. This study provides a full atomistic picture of the transition from intercalation to conversion, which is of essential importance for both secondary ion batteries and electrochromic devices.