Revealing High‐Rate and High Volumetric Pseudo‐Intercalation Charge Storage from Boron‐Vacancy Doped MXenes

Abstract The design of pseudocapacitive electrodes that exhibit high‐rate and high volumetric capacitances is a big challenge, since it requires subtle modulation of ion‐intercalation structures that are able to achieve high electrochemical activity, fast ion transport, and facilitated electron transfer, simultaneously. Herein, controllable and selective etching of B atoms from B‐doped Ti₃AlC₂ precursors is reported, which generates boron‐vacancy doped MXene (B‐V‐MXene) nanosheets with finely‐regulated, ion‐intercalation structures. Electrochemical studies and density‐functional‐theory calculations demonstrate that Ti around vacancies possess higher surface‐redox activity with protons than those on pristine MXenes for the improvement of capacitances. In addition, interlayer spacing can be optimized on B‐V‐MXenes in promoting proton intercalation. More importantly, the dopant B atoms can increase the electron density on Ti, facilitating the adsorption of the intercalated protons; and further, B 2p‐Ti 3d hybridized band sits closer to the Fermi energy than that of C 2p bands, which bridges the energy gap for electron transfer in the pseudo‐capacitive reaction. With synergy of all these effects, the novel B‐V‐MXene compact electrodes can deliver the previously unmatched high volumetric capacitances of 807 F cm⁻³ at 1,000 mV s⁻¹ and 1,815 F cm⁻³ at 5 mV s⁻¹, with excellent cycle stability over 10,000 cycles.