Na-Ion Intercalation and Charge Storage Mechanism in 2D Vanadium Carbide

© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 2D vanadium carbide MXene containing surface functional groups (denoted as V2CTx, where Txare surface functional groups) is synthesized and studied as anode material for Na-ion batteries. V2CTxanode exhibits reversible charge storage with good cycling stability and high rate capability through electrochemical test. The charge storage mechanism of V2CTxmaterial 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 V2CTxduring sodiation/desodiation process are provided through V K-edge XANES and V L2,3-edge sXAS results. A correlation between the CO32−content and the Na+intercalation/deintercalation states in the V2CTxelectrode observed from C and O K-edge in sXAS results implies that some additional charge storage reactions may take place between the Na+-intercalated V2CTxand the carbonate-based nonaqueous electrolyte. The results of this study provide valuable information for the further studies on V2CTxas anode material for Na-ion batteries and capacitors.