Progress in development of electrolytes for magnesium batteries

Over the last few years, there has been an increased interest in developing safe, next-generation battery systems that offer energy densities higher than those of lithium-based batteries. In this context, batteries based on multivalent-ions (Mg2+, Zn2+, Ca2+, and Al3+) have developed their own niche with their capability to achieve at least twice the energy density of monovalent-ion systems such as Li-ion and Na-ion batteries. Among the multivalent-ion battery candidates, magnesium (Mg) batteries appear to be the most viable choice to eventually replace the Li-ion technology because of the high electrode potential, superior safety, and high abundance of Mg-metal. However, the limited development in electrolytes and cathodes has prevented their commercialization to date. There is a lack of suitable electrolytes that can be used at high voltages required for Mg2+ insertion into cathode hosts. The limited compatibility of organic electrolytes with Mg-metal anode is also a challenge, which requires extensive studies of the metal/electrolyte interactions. Such studies over the last two decades were critical in developing state-of-the-art Mg electrolytes that possess voltage windows of >4.0 V and simultaneously be compatible with Mg-metal anodes. Here, we present a review on the development of Mg battery electrolytes, challenges that impede their performance, and promising strategies that have been adopted to address them. We believe that this comprehensive review covering all three categories of Mg electrolytes (liquids, polymers, and solids) would enable researchers to get a quick grasp of the prevailing challenges, and consequently motivate them to develop novel electrolyte candidates.