Strategic Off‐Stoichiometry to Accelerate Nb–W–O Lithium‐Ion Batteries

High‐power lithium‐ion batteries require durable anode materials that can store and release lithium rapidly. The achievable power densities are often limited by ionic transport, which depends on both the material diffusivity as well as its architectural length‐scale. A variety of niobium–tungsten oxides form Wadsley–Roth (WR) phases with rapid lithium‐ion diffusion. These phases are composed of ReO ₃ ‐like blocks connected at shear planes, where several stoichiometries correspond to phases with different block sizes. Here, we demonstrate a concept where off‐stoichiometric synthesis (between line compounds) enables particle size reduction since multiphase crystal growth is hindered by long‐range diffusion. Target compositions were examined between Nb ₁₂ WO ₃₃ and Nb ₁₄ W ₃ O ₄₄ (Nb _4.7 WO _14.7 ) with rapid crystallization to further limit coarsening. The use of off‐stoichiometry compositions led to smaller particle sizes measured by scanning electron microscopy (SEM) with higher surface area measured by small‐angle x‐ray scattering (SAXS). Many samples contained a variety of WR phases as indicated by whole pattern X‐ray diffraction (XRD) fitting. Batteries were prepared from these samples, where the galvanostatic capacity retention at high current densities was markedly enhanced by the off‐stoichiometry strategy, leading to a 20C capacity of 208 mAh/g where an extended 1000 cycles led to 90% capacity retention. This approach is a practical strategy for accelerating the power density of electrodes.