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, CoO_x species (0<x<1) and nanosized Co⁰ 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.