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

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.