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.