New electrolytes and anode materials for magnesium batteries

Abstract

Technological evolution puts up an ever-growing demand on energy storage – challenging today’s technology. Magnesium batteries are a prime candidate to those future demands – theoretically energy dense, 2205 mAh.g-1, abundant and comparatively cheap. [1.4, 1.5] Despite promising properties, magnesium batteries still face major challenges until their commercialization. The favored, high energy density, magnesium metal anode is, at the current state of research, only compatible to a handful of electrolytes, e.g. based upon either magnesium-organo-halo-aluminates or magnesium-borohydrides. [1.9, 1.10] Electrolytes based upon conventional magnesium salts e.g. such as halides, perhalates or imides seem to be incompatible to the magnesium metal surface [1.6] where the electrolyte degenerates and forms a passivating, magnesium ion impermeable, surface layer. This work presents investigations on readily mixable salt / solvent electrolytes and beyond this, the investigations on a new bis(cyclopentadienyl) magnesium electrolyte. This new electrolyte is highly suitable for a metallic magnesium anode and is comprehensively investigated. The results of electrochemical performance testing show an excellent cycling stability for over 500 cycles with a coulombic efficiency of 98 %. SEM and XRD analysis show the deposition of metallic magnesium onto a copper substrate, as well as IR and NMR studies on the fresh and cycled / aged electrolyte show no indication of electrolyte degeneration. Furthermore a comprehensive evaluation of conversion type metal electrodes and carbonaceous electrodes is showcased. The electrodes are electrochemically characterized in different electrolytes. The conversion type metal electrodes exhibit low overpotentials and high current densities in a lithium environment benchmark and no electrochemical activity in a Mg(BH4)2 / tetra-glyme electrolyte. The cyclic voltammetry and constant current testing of the carbonaceous electrodes in different magnesium electrolytes for various carbon materials presents reversible intercalation of magnesium into graphite for the first time. The obtained coulombic efficiencies are poor and the capacity is limited to 10- 20 mAh.g-1. SEM and XRD investigations, prior and subsequently to the electrochemical testing, reveal almost full degradation of the conversion electrodes and only little for the carbonaceous electrodes.