Charge Storage in Thin Films of Cation-Incorporated Manganese Dioxide

Young, Matthias John

Graduate Thesis Or Dissertation

Charge Storage in Thin Films of Cation-Incorporated Manganese Dioxide Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/t435gd15m

Abstract

Of the many known stoichiometries and crystal phases of manganese oxide, a select few have been found to have remarkable properties for electrochemical charge storage. Among these are hollandite α-MnO₂ and spinel λ-MnO₂, which are studied in this work. α-Mn O₂ has been observed to have a high electrochemical charge storage capacity with incredible rate capability and cycle lifetime in aqueous alkali salt electrolytes. In these conditions, this material also exhibits the unique property of pseudocapacitance, meaning that the electrochemical behavior mimics the properties of a parallel-plate dielectric capacitor. However, the underlying mechanism for charge storage in α-MnO₂, and the origins of pseudocapacitance are generally not understood. Conversely, λ-MnO₂ is a well-studied cathode for lithium ion batteries with a high capacity and rate for charge storage, which is known to degrade quickly, especially at elevated temperatures. In this work, we study both pseudocapacitive MnO₂ and λ-MnO₂ using computational chemistry and experimental techniques. Computationally, the charge storage properties of these two materials are framed within a band-diagram and electronic structure framework to elucidate the effect of physical properties such as band gap, work function, point of zero charge, and pH on charge storage. Experimentally, thicknesses up to 200 nm of MnO grown by atomic layer deposition (ALD) are converted by electrochemical treatment into pseudocapacitive NaMn₄O₈ and spinel LiMn₂O₄. By studying the electrochemical properties of pseudocapacitive NaMn₄O₈ and spinel LiMn₂O₄ at varying thicknesses, a more detailed mechanistic understanding for charge storage in each material is established. This powerful combination of theoretical and experimental techniques provides a detailed picture of charge storage in these materials, which can be extended to other electrochemical charge storage materials.

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