Activation of a MnO 2 cathode by water-stimulated Mg 2+ insertion for a magnesium ion battery

Abstract

We demonstrate that MnO 2 nanowire electrode is “activated” for efficient Mg 2+ insertion/deinsertion by introducing water molecules in to the electrolyte. 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 MnO 2 as a magnesium battery cathode material. We show that the previously reported poor Mg 2+ insertion/deinsertion capacities in MnO 2 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 Mg 2+ insertion into MnO 2 highly depends on the ratio between water molecules and Mg 2+ ions present in the electrolyte and the highest Mg 2+ insertion capacity was observed at a ratio of 6H 2 O/Mg 2+ in the electrolyte. We demonstrate for the first time, that MnO 2 nanowire electrode can be “activated” for Mg 2+ insertion/deinsertion by cycling in water containing electrolyte resulting in enhanced reversible Mg 2+ insertion/deinsertion even with the absence of water molecules. The MnO 2 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.