2D vanadium carbide MXene containing surface functional groups (denoted as V2CTx, where Tx are surface functional groups) is synthesized and studied as anode material for Na-ion batteries. V2CTx anode exhibits reversible charge storage with good cycling stability and high rate capability through electrochemical test. The charge storage mechanism of V2CTx 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 V2CTx during 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 V2CTx 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 V2CTx and the carbonate-based nonaqueous electrolyte. The results of this study provide valuable information for the further studies on V2CTx as anode material for Na-ion batteries and capacitors.

High-nickel content cathode materials offer high energy density. However, the structural and surface instability may cause poor capacity retention and thermal stability of them. To circumvent this problem, nickel concentration-gradient materials have been developed to enhance high-nickel content cathode materials' thermal and cycling stability. Even though promising, the fundamental mechanism of the nickel concentration gradient's stabilization effect remains elusive because it is inseparable from nickel's valence gradient effect. To isolate nickel's valence gradient effect and understand its fundamental stabilization mechanism, we design and synthesize a LiNi_0.8Mn_0.1Co_0.1O₂ material that is compositionally uniform and has a hierarchical valence gradient. The nickel valence gradient material shows superior cycling and thermal stability than the conventional one. The result suggests creating an oxidation state gradient that hides the more capacitive but less stable Ni³⁺ away from the secondary particle surfaces is a viable principle towards the optimization of high-nickel content cathode materials.