Ever-increasing energy demands call for alternative energy storage technologies with balanced performance and cost characteristics to meet current and emerging applications. Dual-ion batteries (DIBs) are considered particularly attractive owing to the potentially high specific energy, a rich variety of charge carrier combinations, and the applicability of metal-free cathode and earth-abundant anode materials. However, their performance falls far below expectations because of a large excess of solvent needed to dissolve electroactive species that induces side reactions and contributes parasitic weight, which penalizes the reversible capacity and cell-level energy density. Herein, a solvent-free DIB utilizing a binary alkali metal molten salt based on bis(fluorosulfonyl)amide as the electrolyte to solve these issues is demonstrated. The cell (NaK-DIB) operates in a temperature range of 90-120 degrees C and exhibits high theoretical energy densities of 246 Wh kg(-1)and 533 Wh L(-1)based on active materials and capacity-matched electrolyte, far surpassing those of reported DIBs. Further improvements could realize affordable grid-scale energy storage.