Enhanced Lithium Transport by Control of Crystal Orientation in Spinel LiMn₂O₄ Thin Film Cathodes

Abstract

A promising cathode material for rechargeable batteries is LiMn₂O₄, which exhibits higher operating voltage, reduced toxicity and lower costs as compared to commonly used LiCoO₂ cathodes. However, LiMn₂O₄ suffers from limited cycle life, as excessive capacity fading occurs during battery cycling due to dissolution of Mn into the acidic electrolyte. Here, we show that by structural engineering of stable, epitaxial LiMn₂O₄ thin films the electrochemical properties can be enhanced as compared to polycrystalline samples. Control of the specific crystal orientation of the LiMn₂O₄ thin films resulted in dramatic differences in surface morphology with pyramidal, rooftop or flat features for respectively (100), (110), and (111) orientations. All three types of LiMn₂O₄ films expose predominantly ⟨111⟩ crystal facets, which is the lowest energy state surface for this spinel structure. The (100)-oriented LiMn₂O₄ films exhibited the highest capacities and (dis)charging rates up to 33C, and good cyclability over a thousand cycles, demonstrating enhanced cycle life without excessive capacity fading as compared to previous polycrystalline studies.