Thin film manganese oxide polymorphs as anode for sodium-ion batteries: An electrochemical and DFT based study

Abstract

In the present work, we have studied phase formation behavior and electrochemical characteristics of various polymorphs of Manganese oxides in thin films forms deposited on stainless substrate by pulsed laser deposition. Investigation of these thin films using x-ray diffraction, X-ray photo electron spectroscopy and Raman spectroscopy reveals that the growth of various polymorphs of MnO2 and their mixture depend on the substrate temperatures. The films deposited at 300, 400, and 450 °C contains a mixture of γ and β, β and R, β and R MnO2 phases respectively, Whereas, film deposited at 550 and 650 °C shows a mixture of R-MnO2 and α-Mn2O3. The film containing β- and R-MnO2 phases deposited at 450 °C shows optimum electrochemical property against sodium metal electrode. The density functional theory based analysis shows that MnO2 polymorphs can accommodate sodium ions by intercalation mechanism in the ratio of 2:1 of Na and Mn, followed by a conversion mechanism into Mn metal and Na2O. Collectively, electrochemical and first principle based calculations suggests mixture of β- and R-MnO2 is suitable as anode material.