Beyond nanosizing: Crystallographic continuity promotes more homogeneous reactions in WO3 conversion anodes

Tungsten trioxide (WO 3 ) is a promising conversion anode for lithium ion batteries because of its high theoretical and volumetric capacities. In this study, WO 3 anodes with controlled particle size and crystallinity were evaluated in liquid electrolyte lithium batteries, sodium batteries, and oxide solid state lithium batteries. WO 3 underwent sequential reactions involving Li x WO 3 formation, Li−W−O intermediate phases, and subsequent W/Li 2 O nanocomposite formation. Excessively small WO 3 particles did not improve cycle performance despite the shortened diffusion distance. Instead, WO 3 particles with high crystallographic continuity showed superior electrochemical properties. Heat treatment at 700 °C improved crystallographic continuity while suppressing particle coarsening, resulting in a volumetric capacity of 2148 mA h cm −3 retained for about 270 cycles. Galvanostatic intermittent titration technique analysis, resistivity measurements, and oxygen analysis indicated that the improved cyclability could not be explained solely by Li + diffusivity, electronic conductivity, or oxygen deficiency. Rather, high crystallographic continuity likely promoted homogeneous lithiation and alleviated localized stress during conversion reactions. These findings indicate the importance of crystallographic continuity in WO 3 conversion anodes.