Exposed crystal facet regulation of Prussian blue analogues by pre-intercalation NH₄⁺ for high-performance aqueous ammonium-ion batteries

Aqueous ammonium-ion batteries (AAIBs) represent an environmentally friendly and sustainable energy storage system. Prussian blue analogues (PBAs) have gained significant attention as electrode materials for AAIBs. In this study, NH₄⁺-rich copper‑manganese hexacyanoferrate (N-CuMnHCF) was synthesized via a co-precipitation approach and an ion-exchange method. The doped Cu enhances structural stability, while the pre-intercalation NH₄⁺ effectively regulates the exposed crystal facets and enriches (200) facets in N-CuMnHCF, facilitating the insertion/extraction of NH₄⁺ during charge and discharge processes. N-CuMnHCF exhibits a higher specific capacity of 67.23 mAh·g⁻¹, along with excellent cycling stability of 99.97 % capacity retention over 4200 cycles. The full cell comprised by N-CuMnHCF cathode and 3,4,9,10-perylenetetracarboxylic diimide (PTCDI) anode demonstrates excellent cycling stability with 96.23 % capacity retention over 10,000 cycles, a power density of 44.44 Wh·kg⁻¹, and an energy density of 197.47 W·kg⁻¹. This work proposes an efficient crystal facet regulation strategy to design promising PBAs cathode for high-performance AAIBs.