A High-Rate, Long-Life and Extremely Safe Cobalt-Ammonium Hybrid Battery

Aqueous ammonium-ion batteries (AAIBs) are attracting increasing interest because of the abundance and low cost of ammonium salts and the inherent safety of aqueous electrolytes. However, the development of AAIB anodes remains limited, and many reported anodes suffer from rapid capacity fading and unstable Coulombic efficiency, often associated with strong hydrogen-bonding effects. Here, we report the first cobalt–ammonium hybrid battery consisting of a metallic Co anode, a mixed CoSO4/(NH4)2SO4 aqueous electrolyte, and an Fe[Fe(CN)6] (FeHCF) cathode. The battery operates via coupled NH4+ insertion/extraction at the cathode and Co plating/stripping at the anode. In three-electrode tests, the optimized FeHCF/multiwalled carbon nanotube (FeHCF/MWCNT) composite cathode delivers 94 mAh g–1 at 0.2 A g–1 and retains 78.5% of its capacity with ∼100% Coulombic efficiency after 1000 cycles at 0.8 A g–1. The full cell achieves 112 mAh g–1 and 88 Wh kg–1 at 0.2 A g–1, maintaining 90% capacity with ∼100% Coulombic efficiency after 1000 cycles at 0.8 A g–1. A quasi-solid-state device further demonstrates 73 mAh g–1 at 0.2 A g–1 and stable cycling over 1000 cycles. These results demonstrate the feasibility of cobalt–ammonium hybrid chemistry for safe and durable aqueous energy storage.