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.