Coupling dual metal active sites and low-solvation architecture toward high-performance aqueous ammonium-ion...

Aqueous rechargeable ammonium-ion batteries (AIBs) possess the characteristics of safety, low cost, environmental friendliness, and fast diffusion kinetics. However, their energy density is often limited due to the low specific capacity of cathode materials and narrow electrochemical stability windows of electrolytes. Herein, high-performance aqueous AIBs were designed by coupling Fe-substituted manganese-based Prussian blue analog (FeMnHCF) cathodes and highly concentrated NH₄CF₃SO₃ electrolytes. In FeMnHCF, Mn³⁺/Mn²⁺-N redox reaction at high potential was introduced, and two metal active redox species of Mn and Fe were achieved. To match such FeMnHCF cathodes, highly concentrated NH₄CF₃SO₃ electrolyte was further developed, where NH₄⁺ ion displays low-solvation structure because of the increased coordination number of CF₃SO₃^- anions. Furthermore, the water molecules are confined by NH₄⁺ and CF₃SO₃^- ions in their solvation sheath, leading to weak interaction between water molecules and thus effectively extending the voltage window of electrolyte. Consequently, the FeMnHCF electrodes present high reversibility during the charge/discharge process. Moreover, owing to a small amount of free water in concentrated electrolyte, the dissolution of FeMnHCF is also inhibited. As a result, the assembled aqueous AIBs exhibit enhanced energy density, excellent rate capability, and stable cycling behavior. This work provides a creative route to construct high-performance aqueous AIBs.