Electrolytes for rechargeable aluminum batteries

Rechargeable aluminum battery (RAB) is considered as one of the promising candidates for energy storage systems due to its high volumetric capacity, abundant raw materials, huge cost competitiveness and eco-friendliness. Although some improvements of RABs have been made in recent years, there are still limitations that need to be broken through before the practical applications of RABs, including the low-capacity electrode materials, fast capacity decay, instable SEI, absence of low-cost electrolytes with wide voltage window as well as stable current collectors and binders. Besides altering the intrinsic properties of the electrode materials, the modification of electrolytes is also a popular study trend in RABs, since electrolyte affects greatly on the battery cost, cell performance, the stability of cell components and the reaction mechanisms. The electrolyte plays a central role for the development of RABs. In this review, to provide a full scope of the development of the electrolytes in RABs, we summarize the properties, electrochemical performance and modified strategies of electrolytes used in RABs, aiming to help researchers to understand the limitations of the current electrolytes and guide for promoting their properties. This comprehensive review covers electrolytes have been used in RABs, including ionic liquid electrolytes, high molten temperature salt electrolytes, organic electrolytes, aqueous electrolytes and polymer electrolytes. We heavily focused on the properties of ionic liquids, which are the most frequently used electrolytes in RABs, and the diversity of compositions in ionic liquid electrolytes is also summarized. We also discussed the electrode/electrolyte interfaces in different electrolyte environments. This review aims to provide guidance for choosing appropriate electrolytes and clarify the energy storage mechanism of RABs.