Alkali Metal Intercalation and Reduction of Layered WO2Cl2

Li, Na, K, Rb, and Cs have been intercalated into the two-dimensional (2D) van der Waals material WO2Cl2, with K, Rb, and Cs intercalation being reported for the first time. The topochemical reaction of these alkali metals causes dramatic crystallographic, optical, and electronic changes to WO2Cl2. During the reaction with each of the alkali metals, WO2Cl2 changes from a clear and transparent insulating phase to a blue partially intercalated species and finally to a lustrous bronze material. Upon intercalation, the host undergoes significant and systematic changes in interlayer spacing, as well as a 10 orders-of-magnitude change in resistivity from the base material to the intercalated bronze phase. Each of the alkali metals intercalates at a different rate and leads to specific changes in interlayer spacing and color of the bronze phase. This study provides a platform to study the electronic and optical properties of each of these new tungsten bronzes with a particular focus on lithium intercalation.