Hierarchically textured LixMn2−yO4 thin films as positive electrodes for lithium-ion batteries

Abstract

Hierarchical surface morphologies form when thin films are deposited onto preexisting templates of vertically aligned wires using a line-of-sight deposition method, providing a facile path to experimental battery electrodes with high surface-to-volume ratios. To demonstrate this, we fabricate and electrochemically cycle highly textured thin film electrodes of LixMn2−yO4 with large surface-to-volume ratios and low impedance. The active surface area of the electrodes exceeds the area of the substrate by at least a factor of five. This factor is due in part to the textured template, and in part to the effects of local shadowing during line-of-sight film deposition, resulting in a hierarchical surface morphology. The textured electrodes maintain their structural integrity for at least 30 cycles, as shown through microstructural characterization and reversible cycling against metallic lithium over the range of 2.0–4.4V. In comparison to planar thin film electrodes of equal mass, they also offer a lasting reduction in internal impedance. Overall, textured thin film electrodes of any material are readily fabricated through this templating technique and can be used to improve three-dimensional battery architectures or to simply probe electrochemical surface effects.