Band Diagram and Rate Analysis of Thin Film Spinel LiMn₂O₄ Formed by Electrochemical Conversion of ALD‐Grown MnO

Abstract

Nanoscale spinel lithium manganese oxide is of interest as a high‐rate cathode material for advanced battery technologies among other electrochemical applications. In this work, the synthesis of ultrathin films of spinel lithium manganese oxide (LiMn 2 O 4 ) between 20 and 200 nm in thickness by room‐temperature electrochemical conversion of MnO grown by atomic layer deposition (ALD) is demonstrated. The charge storage properties of LiMn 2 O 4 thin films in electrolytes containing Li + , Na + , K + , and Mg 2+ are investigated. A unified electrochemical band‐diagram (UEB) analysis of LiMn 2 O 4 informed by screened hybrid density functional theory calculations is also employed to expand on existing understanding of the underpinnings of charge storage and stability in LiMn 2 O 4 . It is shown that the incorporation of Li + or other cations into the host manganese dioxide spinel structure (λ‐MnO 2 ) stabilizes electronic states from the conduction band which align with the known redox potentials of LiMn 2 O 4 . Furthermore, the cyclic voltammetry experiments demonstrate that up to 30% of the capacity of LiMn 2 O 4 arises from bulk electronic charge‐switching which does not require compensating cation mass transport. The hybrid ALD‐electrochemical synthesis, UEB analysis, and unique charge storage mechanism described here provide a fundamental framework to guide the development of future nanoscale electrode materials for ion‐incorporation charge storage.