Asymmetric Reaction Pathways of Conversion-Type Electrodes for Lithium-Ion Batteries

Metal oxides have been actively explored as promising conversion-type electrode materials for lithium-ion batteries due to high deliverable capacity but are still notorious for poor cyclability, capacity fading, voltage hysteresis, and so forth. However, the fundamental reason for the undesirable properties of metal oxides behind the repetitive conversion process is still obscure. In this work, we take advantage of synchrotron X-ray techniques as well as transmission electron microscopy to monitor the structural changes during both conversion (lithiation) and reconversion (delithiation) reactions. The difference in diffusion rates of lithium and metal plays a decisive role in determining the reaction pathway. We find that lithium accommodation and extraction occur via different reaction routes: lithiation follows a kinetically driven way while delithiation adopts a route close to the thermodynamic ground state path. Thermodynamic structural evolution features the formation of an intermediate phase of Li-metal (M)-O, suggesting that lithium removal accompanies the Li/M ionic exchange and rearrangement of the oxygen framework. The slow diffusion of metal ions and the high kinetic and energy barrier for dissociating the intermediate phase are mainly responsible for uncompleted reconversion reaction, evidenced by the remaining Li-M-O phase at the end of charge. Imperfect reconversion reaction eventually limits the utilization of lithium ions over the repeated cycling. This work sheds light on structural changes occurring at metal oxides during both conversion and reconversion processes, which is strongly linked with the performances of conversion-type materials in applications.