We report a low-cost water-in-salt electrolyte (WiSE), of 30 m ZnCl2, which enables a dendrite-free Zn metal anode to possess a high coulombic efficiency.

The ever-increasing demand for storing renewable energy sources calls for novel battery technologies that are of sustainably low levelized energy cost. Research into battery chemistry has evolved to a stage where a plethora of choices based on earth-abundant elements can be compared during their development. One of the emerging candidates is the nonaqueous potassium-ion battery. K-ion’s unique properties as a charge carrier have aroused intense interest in exploring high-performing cathode and anode materials for this battery. Rapid progress has been made, where leading candidates of electrodes have been proposed, i.e., hard carbon as anode and Prussian white analogues as cathode. In this new battery technology’s infancy, it is our opinion that the focus should be given to potentially scalable, inexpensive electrode materials and the understanding of their cycle-life-property correlations. It may be the ultralong cycle life that differentiates potassium-ion batteries from sodium-ion batteries in the future market.

Dual-ion batteries are known for anion storage in the cathode coupled to cation incorporation in the anode. We flip the sequence of the anion/cation-storage chemistries of the anode and the cathode in dual-ion batteries (DIBs) by allowing the anode to take in anions and a cation-deficient cathode to host cations, thus operating as a reverse dual-ion battery (RDIB). The anion-insertion anode is a nanocomposite having ferrocene encapsulated inside a microporous carbon, and the cathode is a Zn-insertion Prussian blue, Zn 3 [Fe(CN) 6 ] 2 . This unique battery configuration benefits from the usage of a 30 m ZnCl 2 "water-in-salt" electrolyte. This electrolyte minimizes the dissolution of ferrocene; it raises the cation-insertion potential in the cathode, and it depresses the anion-insertion potential in the anode, thus widening the full cell's voltage by 0.35 V compared with a dilute ZnCl 2 electrolyte. RDIBs provide a configuration-based solution to exploit the practicality of cation-deficient cathode materials in aqueous electrolytes.