Ni/Co bimetallic organic frameworks nanospheres for highperformance 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-πconjugated 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.