Structural Distortion and Compositional Gradients Adjacent to Epitaxial LiMn₂O₄ Thin Film Interfaces

Thin film electrode materials are key components in the development of high rate, high capacity solid-state Li-ion batteries. Detailed knowledge of the epitaxial film/substrate(current-collector) interface structures, which provides insights into epitaxial growth mechanisms and the effects of microstructure on electrochemical properties, is essential for efficient materials and device design. Here we report the epitaxial growth mechanism of a typical cathodic LiMn₂O₄ thin film by exploring the detailed structural and compositional variations in the vicinity of a film/substrate interface using state-of-the-art scanning transmission electron microscopy. Direct observation of atom columns shows the epitaxial film forms an atomically flat and coherent heterointerface with the substrate, but that the crystal lattice is tetragonally distorted with a measurable compositional gradient from the interface to the crystal bulk. The growth mechanism is interpreted in terms of a combination of chemical and physicomechanical effects, namely a complex interplay between the internal Jahn-Teller distortions induced by oxygen non-stoichiometry and the lattice misfit strain. The direct observation of atom columns of all constituent elements at an LiMn₂O₄/Au interface using state-of-the-art ABF-STEM imaging is demonstrated. The micrograph reveals a distinctly different structure to that in the bulk film. A unit-cell thick layer of Mn- and O-deficient Mn₃O₄ is bonded to the Au substrate via a single O layer, above which tetragonally-distorted (defective) and cubic (stoichiometric) LiMn₂O₄ grows epitaxially.