Advances in ionic-liquid-based eutectic electrolyte for high voltage rechargeable magnesium batteries

Rechargeable magnesium batteries (RMBs) represent a promising beyond-lithium technology for energy storage due to their high energy and power densities. However, developing suitable electrolytes compatible with both electrodes and exhibiting high thermal and electrochemical stabilities remains a significant challenge for RMBs. In this study, we present the development of a novel electrolyte for RMBs based on a eutectic mixture of 1-ethyl-3-methylimidazolium chloride and 1-ethyl-3-methylimidazolium hexafluorophosphate. This electrolyte demonstrates a high ionic conductivity of ~ 6.7 mS.cm⁻¹ at room temperature and a wide electrochemical stability window (> 4.5 V vs. Mg/Mg²⁺). We demonstrate that the present electrolyte enables the reversible operation of an Mg-graphite cell with a discharge capacity of ~ 120 mAh.g⁻¹ for over 500 cycles while maintaining a Coulombic efficiency of > 95%. Furthermore, the distinctive dual-ion transport behavior of the electrolyte is substantiated through the fabrication of a symmetric graphite cell, where both anions and cations exhibit bidirectional movement during the charge and discharge processes. This cell manifests an equivalent discharge capacity to that of Mg-graphite cells. These findings underscore the potential of further optimizing RMBs utilizing this electrolyte, offering prospects for superior energy density and enhanced performance across diverse application domains.