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

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.