Understanding How the Suppression of Insertion-Induced Phase Transitions Leads to Fast Charging in Nanoscale Li_xMoO₂

Fast-charging Li-ion batteries are technologically important for the electrification of transportation and the implementation of grid-scale storage, and additional fundamental understanding of high-rate insertion reactions is necessary to overcome current rate limitations. In particular, phase transformations during ion insertion have been hypothesized to slow charging. Nanoscale materials with modified transformation behavior often show much faster kinetics, but the mechanism for these changes and their specific contribution to fast-charging remain poorly understood. In this work, we combine operando synchrotron X-ray diffraction with electrochemical kinetics analyses to illustrate how nanoscale crystal size leads to suppression of first-order insertion-induced phase transitions and their negative kinetic effects in MoO₂, a tunnel structure host material. In electrodes made with micrometer-scale particles, large first-order phase transitions during cycling lower capacity, slow charge storage, and decrease cycle life. In medium-sized nanoporous MoO₂, the phase transitions remain first-order, but show a considerably smaller miscibility gap and shorter two-phase coexistence region. Finally, in small MoO₂ nanocrystals, the structural evolution during lithiation becomes entirely single-phase/solid-solution. For all nanostructured materials, the changes to the phase transition dynamics lead to dramatic improvements in capacity, rate capability, and cycle life. This work highlights the continuous evolution from a kinetically hindered battery material in bulk form to a fast-charging, pseudocapacitive material through nanoscale size effects. As such, it provides key insight into how phase transitions can be effectively controlled using nanoscale size and emphasizes the importance of these structural dynamics to the fast rate capability observed in nanostructured electrode materials.