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Pseudocapacitance is a key charge storage mechanism to advanced electrochemical energy storage devices distinguished by the simultaneous achievement of high capacitance and a high charge/discharge rate by using surface redox chemistries. MXene, a family of layered compounds, is a pseudocapacitor‐like electrode material which exhibits charge storage through exceptionally fast ion accessibility to redox sites. Here, the authors demonstrate steric chloride termination in MXene Ti 2 C T x ( T x : surface termination groups) to open the interlayer space between the individual 2D Ti 2 C T x units. The open interlayer space significantly enhances Li‐ion accessibility, leading to high gravimetric and volumetric capacitances (300 F g −1 and 130 F cm −3 ) with less diffusion limitation. A Li‐ion hybrid capacitor consisting of the Ti 2 C T x negative electrode and the LiNi 1/3 Co 1/3 Mn 1/3 O 2 positive electrode displays an unprecedented specific energy density of 160 W h kg −1 at 220 W kg −1 based on the total weight of positive and negative active materials.
Kajiyama et al. (Fri,) studied this question.