Charting the course to solid‐state dual‐ion batteries

An electrolyte destined for use in a dual‐ion battery (DIB) must be stable at the inherently high potential required for anion intercalation in the graphite electrode, while also protecting the Al current collector from anodic dissolution. A higher salt concentration is needed in the electrolyte, in comparison to typical battery electrolytes, to maximize energy density, while ensuring acceptable ionic conductivity and operational safety. In recent years, studies have demonstrated that highly concentrated organic electrolytes, ionic liquids, gel polymer electrolytes (GPEs), ionogels, and water‐in‐salt electrolytes can potentially be used in DIBs. GPEs can help reduce the use of solvents and thus lead to a substantial change in the Coulombic efficiency, energy density, and long‐term cycle life of DIBs. Furthermore, GPEs are suited to manufacture compact DIB designs without separators by virtue of their mechanical strength and electrical performance. In this review, we highlight the latest advances in the application of different electrolytes in DIBs, with particular emphasis on GPEs. The dual‐ion chemistry is appealing from the perspectives of materials cost and sustainability as it allows for eliminating scarce and expensive transition metals and salts. The performance of a dual‐ion battery (DIB) is, however, still limited by the oxidative stability of electrolytes. This has inspired extensive efforts to formulate salt and solvent combinations that can afford stable cycle performance over a broad range of temperatures with low self‐discharge, and extended life. This review encapsulates recent advances in electrolytes designed for DIBs. In particular, it highlights the competitive edge that gel polymer electrolytes afford in fabricating mechanically flexible, compact DIBs with high energy and power densities.