Understanding energy storage mechanisms in electrochemical energy storage devices lays the foundations for improving their energy and power density. Here we introduce in situ ultraviolet–visible (UV–Vis) spectroscopy method to distinguish battery-type, pseudocapacitive and electrical double-layer charge storage processes. On the basis of Ti₃C₂T_x MXene in aqueous acidic and neutral electrolytes, and lithium titanium oxide in an organic electrolyte, we found a correlation between the evolution of UV–Vis spectra and the charge storage mechanism. The electron transfer number for Ti₃C₂T_x in an acidic electrolyte was calculated using quantitative analysis, which was close to previous measurements using X-ray absorption spectroscopy. Further, we tested the methodology to distinguish the non-Faradaic process in Ti₃C₂T_x MXene in a water-in-salt electrolyte, despite well-defined peaks in cyclic voltammograms. In situ UV–Vis spectroscopy is a fast and cost-effective technique that effectively supplements electrochemical characterization to track changes in oxidation state and materials chemistry and determine the charge storage mechanism.

Achieving pseudocapacitive intercalation in MXenes with neutral aqueous electrolytes and driving reversible redox reactions is scientifically appealing and practically useful. Here, we report that the partial oxidation of MXene intensifies pseudocapacitive Li+ intercalation into Ti3C2Tx MXene from neutral water-in-salt electrolytes. An in situ X-ray absorption near-edge structure analysis shows that the Ti oxidation state changes during the Li+ intercalation, indicating the presence of a surface redox reaction. The Ti oxidation/reduction is further confirmed by an in situ extended X-ray absorption fine structure analysis, which shows a reversible contraction/expansion of the Ti-C interatomic distance. The intensified Li+ pseudocapacitive intercalation can be explained by the higher oxidation state of Ti at the open circuit potential. This work demonstrates the possibility of tuning the pseudocapacitive intercalation by adjusting the initial oxidation state of the transition metal on the MXene and offers a facile way to enhance the pseudocapacitance of various MXenes.