Structure engineering for organic ammonium ion batteries: A controllable distribution of Cu atoms in Mn-based Prussian blue analogues

Ammonium ion batteries (AIBs) attract extensive attention owing to the abundant resources, eco-friendly and high safety. Multivalent Prussian blue analogues (PBAs), characterized by multi-electron transfer and high operating voltage, are considered as promising cathodes for AIBs. However, their structural stability is compromised by irreversible structural evolution and metal ion dissolution. Herein, a structure engineering strategy is proposed to boost NH₄⁺ ion storage by regulating the distribution of Cu atoms in Mn-based PBAs. Cu atoms not only enrich on the surface to form a uniform layer but are doped inside the Mn-based PBAs. The resulting PBAs (noted as Cu_xMn_1-xHCF) with an optimal x value of 0.15, deliver a stable capacity of 110.1 mAh g⁻¹ at 50 mA g⁻¹ and an exceptional capacity retention of 91 % over 4000 cycles at 200 mA g⁻¹. Ex-situ characterizations and density functional theory (DFT) calculations reveal that the controllable distribution of Cu atoms significantly enhances the structural stability and suppresses the dissolution of Mn atoms. Notably, this strategy can be extended to other PBAs materials. We believe that this work advances the development of high-performance cathode materials for AIBs.