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 Ti2CTx (Tx: surface termination groups) to open the interlayer space between the individual 2D Ti2CTx 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 Ti2CTx negative electrode and the LiNi1/3Co1/3Mn1/3O2 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.
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