Ammonium-Anchored Mn-Based Prussian Blue Analogues via Hydrogen Bonding for Robust Sodim-Ion Battery Cathodes

Manganese-based Prussian blue analogues (Mn-PBAs) are promising sodium-ion battery (SIB) cathodes due to their low cost and high operating voltage. However, their practical application is hindered by structural degradation, including Jahn-Teller distortion, Mn dissolution, and an irreversible cubic‑to‑tetragonal phase transition, which collectively drive rapid capacity fade. Here, we report a hydrogen-bond anchoring strategy in which tetrahedral ammonium (NH₄ ⁺) ions are incorporated into the A-site cavities of manganese hexacyanoferrate (MnHCF) to establish framework-stabilizing N─H···N hydrogen-bond interactions with cyanide ligands. This molecular-level reinforcement suppresses Jahn-Teller distortion, prevents the irreversible cubic‑to‑tetragonal transition during deep desodiation, and mitigates particle cracking, thereby enhancing structural reversibility. The NH₄ ⁺-anchored MnHCF (A-MnHCF) delivers long-term durability, retaining ∼88% of its initial capacity after 1000 cycles at 1 C. In‑situ XRD, FT-IR, and XPS demonstrate that NH₄ ⁺ remains within the framework and that N─H···N hydrogen bonding persists throughout cycling, corroborating the stabilization mechanism. This work establishes hydrogen-bond anchoring as a general paradigm for stabilizing Mn-based PBAs, paving the way for long-life, low-cost SIBs suitable for grid-scale energy storage.