Boosting ammonium-ion diffusion and cycling stability in PBAs via hydrogen bonding with interstitial water

Abstract

Prussian blue analogs (PBAs) have emerged as environmentally friendly and structurally tunable cathode materials for aqueous ammonium-ion batteries (AIBs). However, the fundamental role of crystalline H₂O in regulating ammonium-ion storage and transport remains poorly understood. In this study, we present a comprehensive comparison between hydrated NH₄NiHCF-H₂O and its anhydrous counterpart NH₄NiHCF, revealing the critical contribution of interstitial water to electrochemical performance. Structural and spectroscopic analyses confirm that interstitial water forms robust hydrogen bonds with NH₄⁺ ions, stabilizing the PBA framework and mitigating structural degradation during cycling. Electrochemical measurements show that NH₄NiHCF-H₂O delivers a significantly higher specific capacity of 61 mA h g⁻¹ at 0.2 C and markedly improved rate performance compared to NH₄NiHCF (48 mA h g⁻¹ at 0.2 C). Kinetic analysis reveals that interstitial water enhances NH₄⁺ diffusion, as evidenced by higher diffusion coefficients. Furthermore, density functional theory (DFT) calculations demonstrate that crystal water acts as a hydrogen bond acceptor, preferentially interacting with NH₄⁺ and reducing the migration energy barrier, thereby facilitating fast ion transport. This work provides fundamental insights into the role of crystal water in PBAs and offers a rational design strategy for improving the kinetics, structural stability of PBAs cathodes for AIBs.