Multielectron Redox and Insulator-to-Metal Transition upon Lithium Insertion in the Fast-Charging, Wadsley-Roth Phase PNb9O25

PNb9O25, a Wadsley-Roth compound whose structure is obtained by appropriate crystallographic shear of the ReO3 structure, is a high-power electrode material that can reach 85% of the equilibrium capacity in 30 min and 67% in 6 min. Here we show that multielectron redox, as observed through X-ray absorption spectroscopy and X-ray photoelectron spectroscopy, and an insulator-to-metal transition upon lithium insertion, as suggested by a number of complementary techniques, contribute to the impressive performance. Chemically tuning the tetrahedral site between phosphorus and vanadium leads to significant changes in the electrochemistry and kinetics of lithium insertion in the structure, pointing to larger implications for the use of crystallographic shear phases as fast-charging electrode materials.