Entropy-Engineered Multi-Metallic MOFs Unlock High-Performance Ammonium-Ion Storage.

Aqueous ammonium ion batteries (AAIBs) are an emerging sustainable energy technology owing to their high ionic conductivity, safety, low cost, and non-metallic charge carrier. However, their practical applicability is restricted by low energy density, a narrower potential window, and poor cycling stability. An entropy-engineered amino group-functionalized metal-organic framework (EnMOF-NH2) is reported as a promising cathode for high-performance AAIBs. EnMOF-NH2 consists of five metal nodes (Co, Ni, Fe, Mn, Mo) and an amino-functionalized benzene dicarboxylic acid linker (BDC-NH2), which provides multiple redox centers, enhanced structural stability, and abundant hydrogen bonding sites for efficient NH4 + storage. The incorporation of Mo metal nodes into the multi-metallic MOF can strengthen the metal-ligand interaction and suppress the metal dissolution, while amino-functionalized BDC provides polarity to the framework and enables fast NH4 + storage through hydrogen bonding. EnMOF-NH2 achieves a high reversible capacity of 130.8 mAh g- 1 at 0.3 A g- 1 in 1.0 M (NH4)2SO4, outperforming the non-amino-functionalized EnMOF-BDC. It exhibits excellent rate capability, and capacity activation to 162% after 2500 cycles. A full cell coupled with an EnMOF-NH2 cathode and perylene tetracarboxylic dianhydride (PTCDA) anode retains 83% capacity over 1000 cycles. Such entropy-engineered, amino-group-functionalized MOF materials represent a promising cathode for future aqueous ammonium-ion batteries.