Doubling Reversible Capacities in Epitaxial Li₄Ti₅O₁₂ Thin Film Anodes for Microbatteries

Despite the lower gravimetric capacity, Li₄Ti₅O₁₂ is an important alternative to graphite anodes, owing to its excellent high temperature stability, high rate capability, and negligible volume change. Although surfaces with lithium compositions exceeding Li₇Ti₅O₁₂ were observed previously during the first charge-discharge cycles, no stable reversible capacities were achieved during prolonged cycling. Here, structural engineering has been applied to enhance the electrochemical performance of epitaxial Li₄Ti₅O₁₂ thin films as compared to polycrystalline samples. Variation in the crystal orientation of the Li₄Ti₅O₁₂ thin films led to distinct differences in surface morphology with pyramidal, rooftop, or flat nanostructures for respectively (100), (110), and (111) orientations. High discharge capacities of 280-310 mAh·g^-1 were achieved due to significant surface contributions in lithium storage. The lithiation mechanism of bulk Li₄Ti₅O₁₂ thin films was analyzed by a phase-field model, which indicated the lithiation wave to be moving faster along the grain boundaries before moving inward to the bulk of the grains. The (100)-oriented Li₄Ti₅O₁₂ films exhibited the highest capacities, the best rate performance up to 30C, and good cyclability, demonstrating enhanced cycle life and doubling of reversible capacities in contrast to previous polycrystalline studies.