Polymer Gel Electrolytes for Magnesium Batteries: Ion Transport and Electrochemical Characterization

Abstract

The development of new rechargeable battery systems employing novel chemistries is imperative to meet the increasing energy storage demands of emerging technologies. Magnesium metal batteries are one attractive type of beyond lithium-ion system, due to the widespread abundance, gravimetric and volumetric charge capacity, and electrodeposition potential of magnesium. However, the electrolyte formulations that enable magnesium deposition and dissolution are few in comparison with lithium. Here we report on the ion transport and electrochemical properties of magnesium polymer gel electrolytes based crosslinked, ionomeric networks that are swelled. The nature of the solvent(s) and salt(s) of the liquid medium, and the charge density of the ionomer network, are all found to heavily influence the properties of the gel electrolyte. Magnesium polymer gel electrolytes are created by swelling the ionic networks in electrolytes containing AlCl3, MgCl2, and/or Mg(HMDS)2. Interestingly, higher ionic conductivities and higher current densities for magnesium deposition/dissolution are observed for some gel electrolytes compared to the properties of the pure liquid component. This is hypothesized to be due to the speciation of the free complex ions, which is altered by the presence of the ionic polymer network. These gels are further applied to the Mg-S system, and it is shown that the structure and charge density of the gels influence anionic polysulfide transport. Gel formulations are identified where polysulfides are preferentially rejected while complex magnesium cations are facilely transported across the gel. Structure-property correlations will be presented as found from the combination of SAXS/WAXS, diffusion, elemental analysis, and conductivity measurements. Single-ion conducting magnesium polymer electrolytes are also created by swelling the ionomeric networks in pure solvents. High ionic conductivities are achievable (> 10^-4 S/cm). However, even when these electrolytes have high magnesium conductivity and are electrochemical stable (non-passivating) against magnesium metal, no magnesium electrodeposition is induced at reasonable potentials. This is suspected to be due to the difficulty of the desolvation of Mg+2.