The electrochemical Na intercalation/extraction mechanism of ultrathin cobalt(II) terephthalate-based MOF nanosheets revealed by synchrotron X-ray absorption spectroscopy

The discovery of novel metal-organic frameworks with high anodic performance and the in-depth investigation on their charge compensation mechanism is of primary significance to boost their application in sodium-ion batteries. Herein, cobalt(II) terephthalate-based MOF nanosheets (termed u-CoOHtp) with oxygen vacancies generated were fabricated via an expedient ultrasonic approach and evaluated as an active anode in Na-ion coin cells for the first time. The oxygen vacancies in u-CoOHtp could induce local built-in electric field, which is able to accelerate ion diffusion rate and thus promote reversible Na+ storage. As expected, the obtained u-CoOHtp can deliver a reversible capacity of 555 mA h g(-1) at 50 mA g(-1) and maintain remarkable cycling performance. More importantly, the valence state and local environment evolution of u-CoOHtp during Na+ intercalation/extraction were studied by a combination of hard and soft X-ray absorption spectroscopy (Co and O K-edge). The results substantiate that: (i) the pristine u-CoOHtp is converted to a mixed phase containing Co-MOF, CoOx species (0 < x < 1) and nanosized Co-0 after the first cycle; (ii) Co2+ and metallic Co are interchangeable during repeated Na+ intercalation/extraction; (iii) a certain portion of charge compensation during cycling is achieved on the carboxyl oxygen sites.