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.

To date, tremendous efforts of the battery community are devoted to batteries that employ Li + , Na + , and K + as charge carriers and nonaqueous electrolytes. However, aqueous batteries hold great promise for stationary energy storage due to their inherent low cost and high safety. Among metal batteries that use aqueous electrolytes, zinc metal batteries are the focus of attention. In this study, iron as an anode candidate in aqueous batteries is investigated because iron is undoubtedly the most earth-abundant and cost-effective metal anode. Reversible iron plating/stripping in a FeSO 4 electrolyte is demonstrated on the anode side and reversible topotactic (de)insertion of Fe 2+ in a Prussian blue analogue cathode is showcased. Furthermore, it is revealed that LiFePO 4 can pair up with the iron metal anode in a hybrid cell, delivering stable performance as well.

Sulfur represents one of the most promising cathode materials for next-generation batteries; however, the widely observed polysulfide dissolution/shuttling phenomenon in metal-sulfur redox chemistries has severely restricted their applications. Here it is demonstrated that when pairing the sulfur electrode with the iron metal anode, the inherent insolubility of iron sulfides renders the shuttling-free nature of the Fe-S electrochemical reactions. Consequently, the sulfur electrode exhibits promising performance for Fe 2+ storage, where a high capacity of ~1050 mAh g -1 , low polarization of ~0.16 V as well as stable cycling of 150 cycles have been realized. The Fe-S redox mechanism was further revealed as an intriguing stepwise conversion of S 8 ↔ FeS 2 ↔ Fe 3 S 4 ↔ FeS, where a low volume expansion of ~32.6% and all-solid-state phase transitions facilitate the reaction reversibility. This study suggests an alternative direction to exploit sulfur electrodes in rechargeable transition metalsulfur batteries.The pressing need for renewable energy storage entails the development of cost-effective and sustainable battery technologies. [1] Along this line, batteries that employ earth-abundant elements such Recently, our group investigated a Fe-metal battery, where a Prussian blue cathode undertakes reversible Fe 2+ (de)insertion reactions. [38] In this work, we further demonstrate Fe-S battery chemistry Received: ((will be filled in by the editorial staff))Revised: ((will be filled in by the editorial staff))

The elemental sulfur electrode with Cu2+ as the charge carrier gives a four‐electron sulfur electrode reaction through the sequential conversion of S↔CuS↔Cu2S. The Cu‐S redox‐ion electrode delivers a high specific capacity of 3044 mAh g−1 based on the sulfur mass or 609 mAh g−1 based on the mass of Cu2S, the completely discharged product, and displays an unprecedently high potential of sulfur/metal sulfide reduction at 0.5 V vs. SHE. The Cu‐S electrode also exhibits an extremely low extent of polarization of 0.05 V and an outstanding cycle number of 1200 cycles retaining 72 % of the initial capacity at 12.5 A g−1. The remarkable utility of this Cu‐S cathode is further demonstrated in a hybrid cell that employs an Zn metal anode and an anion‐exchange membrane as the separator, which yields an average cell discharge voltage of 1.15 V, the half‐cell specific energy of 547 Wh kg−1 based on the mass of the Cu2S/carbon composite cathode, and stable cycling over 110 cycles.

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