Lawrence Berkeley National Laboratory

Abstract 2D vanadium carbide MXene containing surface functional groups (denoted as V 2 CT x , where T x are surface functional groups) is synthesized and studied as anode material for Na‐ion batteries. V 2 CT x anode exhibits reversible charge storage with good cycling stability and high rate capability through electrochemical test. The charge storage mechanism of V 2 CT x material during Na + intercalation/deintercalation and the redox reaction of vanadium are studied using a combination of synchrotron based X‐ray diffraction, hard X‐ray absorption near edge spectroscopy (XANES), and soft X‐ray absorption spectroscopy (sXAS). Experimental evidence of a major contribution of redox reaction of vanadium to the charge storage and the reversible capacity of V 2 CT x during sodiation/desodiation process are provided through V K ‐edge XANES and V L 2,3 ‐edge sXAS results. A correlation between the CO 3 2− content and the Na + intercalation/deintercalation states in the V 2 CT x electrode observed from C and O K ‐edge in sXAS results implies that some additional charge storage reactions may take place between the Na + ‐intercalated V 2 CT x and the carbonate‐based nonaqueous electrolyte. The results of this study provide valuable information for the further studies on V 2 CT x as anode material for Na‐ion batteries and capacitors.