Electrochemical and chemical insertion/deinsertion of magnesium in spinel-type MgMn2O4 and lambda-MnO2 for both aqueous and non-aqueous magnesium-ion batteries

Abstract

By using both chemical and electrochemical methods, magnesium has been reversibly removed from MgMn₂O₄ (s.g. I4₁/amd) with appropiate texture to form single-phase and nanocrystalline λ-MnO₂ (s.g. Fdm). Cubic λ-MnO₂ is not stable after annealing in both air and Ar atmospheres. The oxidation of Mn(III) to Mn(IV) eliminates the tetragonal distortion of the spinel-type lattice, but λ-MnO₂ is more effectively obtained when powdered MgMn₂O₄ has a large specific surface area and a small particle size. In an aqueous solution of a magnesium salt, λ-MnO₂ is formed by galvanostatic charge of MgMn₂O₄ and continuous Ar-flowing for removing oxygen from the solution. Starting from λ-MnO₂, the tetragonal structure of MgMn₂O₄ and the cubic structure of LiMn₂O₄ are generated by electrochemical cycling in aqueous solutions containing salts of magnesium and lithium, respectively. In an aqueous solution cell, this material exhibits a reversible capacity of about 150 mA h g⁻¹ and can be used in magnesium-ion batteries demonstrating it to be competitive against their lithium counterparts. In the absence of metallic Mg, the use of carbonate-based solvents can be a good choice for veritable non-aqueous magnesium-ion batteries, for example using positive electrode materials like MgMn₂O₄ (magnesium-ion source) and negative electrode materials like V₂O₅. In non-aqueous solvents (ethylene carbonate–diethyl carbonate mixture), the cubic phase λ-MnO₂ is not formed, the tetragonal structure of Mg_xMn₂O₄ is preserved and its lattice cell is contracted.