Remarkable improvement in the lithium storage property of Co₂(OH)₂BDC MOF by covalent stitching to graphene and the redox chemistry boosted by delocalized electron spins

Improving the insufficient rate capability of MOFs-based anodes arising from poor electrical conductivity and understanding their Li⁺ intercalation mechanism is of great significance in boosting their application in rechargeable lithium-ion batteries (LIBs). Herein, we report the fabrication of covalently reinforced Co₂(OH)₂BDC/carboxyl graphene (CGr) composites via an in-situ solvothermal process. When used as active materials in LIBs, the optimized composite (CoCGr-5) delivers a much-improved reversible capacity of 1368 mAh g⁻¹ at 100 mA g⁻¹, while also demonstrates ameliorative rate capability and long-term cyclability (818 mAh g⁻¹ at 1 A g⁻¹ after 400 repeated cycles). Ex-situ electron paramagnetic resonance (EPR) spectra demonstrate that the high-spin Co²⁺ with three localized electrons in pristine CoCGr-5 would convert to delocalized high-spin Co²⁺ possessing delocalized conducting electrons upon Li⁺ intercalation. Hence, the outstanding electrochemical performance of this CoCGr-5 hybrid electrode can be ascribed to its efficient organic-moiety-dominated Li⁺ insertion/extraction mechanism, and the bicontinuous electron/ion pathways between CGr and Co₂(OH)₂BDC.