Activation of a MnO2 cathode by water-stimulated Mg2+ insertion for a magnesium ion battery

Abstract

Magnesium batteries have been considered to be one of the promising beyond lithium ion technologies due to magnesium's abundance, safety, and high volumetric capacity. However, very few materials show reversible performance as a cathode in magnesium ion systems. We present herein the best reported cycling performances of MnO₂ as a magnesium battery cathode material. We show that the previously reported poor Mg²⁺ insertion/deinsertion capacities in MnO₂ can be greatly improved by synthesizing self-standing nanowires and introducing a small amount of water molecules into the electrolyte. Electrochemical and elemental analysis results revealed that the magnitude of Mg²⁺ insertion into MnO₂ highly depends on the ratio between water molecules and Mg²⁺ ions present in the electrolyte and the highest Mg²⁺ insertion capacity was observed at a ratio of 6H₂O/Mg²⁺ in the electrolyte. We demonstrate for the first time, that MnO₂ nanowire electrode can be “activated” for Mg²⁺ insertion/deinsertion by cycling in water containing electrolyte resulting in enhanced reversible Mg²⁺ insertion/deinsertion even with the absence of water molecules. The MnO₂ nanowire electrode cycled in dry Mg electrolyte after activation in water-containing electrolyte showed an initial capacity of 120 mA h g⁻¹ at a rate of 0.4 C and maintained 72% of its initial capacity after 100 cycles.