Ni/Co bimetallic organic frameworks nanospheres for high-performance electrochemical energy storage

In addition to their many well-known advantages (e.g., ultra-high porosity, good pore size distribution, easy functionalization, and structural tolerability), metal-organic frameworks (MOFs) are a new class of advanced functional materials. However, their backbones are highly susceptible to deformation after exposure to acidic or alkaline conditions. As a result of lithium-ion batteries embedding or detaching directly from MOFs, they irreversibly collapse. As a result, they fail to maintain their electrochemical performance. These factors have hindered the development of MOFs as direct electrode materials, making the design of MOF materials with controlled morphology and stable dimensions a new challenge. In this study, we adopted a versatile and effective method to synthesize a novel MOF material (NiCo-BP (BP = BTC/phen and BTC = 1,3,5-benzenetricarboxylic acid)) using the rigid ligands 1,10-phenanthroline and homobenzotrizoic acid, and the emergence of the Ni–O/N and Co–O/N coordination layers was observed by extended X-ray absorption fine structure (EXAFS) tests, indicating that Ni and Co were coordinated with heterocyclic N-given atoms to form a stable p–π conjugated structure. Meanwhile, the metal-ion is attached to the carboxylic acid ligand on the other side, making the metal-organic skeleton complete and robust. The nanosphere structure of NiCo-BP (~ 400 nm) allows for full exposure and utilisation of the active sites, especially the Ni, Co, and phenanthroline units, and exhibit impressively high specific capacity and cycling stability. At a high current density of 1.0 A·g⁻¹, a high discharge specific capacity of 631.6 mAh·g⁻¹ was obtained after 1000 cycles. The co-participation of two organic ligands in the coordination is in accordance with the theory of soft and hard acids and bases, which contributes to the ability of the material to maintain a high capacity in cycling as well as its cyclic stability.