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 Ti 3 AlC 2 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.
Layer-structured lithium cobalt oxide (LCO) is one of promising electrode materials for secondary aqueous lithium-ion batteries, yet the effect of structural proton insertion in LCO in neutral aqueous electrolytes cannot be ignored. Present study investigates the electrochemical performance of polycrystalline spherical LCO in neutral aqueous saturated Li 2 SO 4 solution. Herein, we for the first time demonstrate the dependence of LCO stability on the discharge cutoff potential. The applied LCO electrodes show good cycling stability within the potential window of 0.65–1.1 V vs. SCE, while electrochemical impedance spectrum (EIS) analysis detects no sign of proton intercalation. Moreover, the spherical LCO free from the proton intercalation exhibits a superior rate capability with 78% discharge capacity retention at 80 C. The lithium-ion chemical diffusion coefficient being seven times than that of irregular shaped LCO sample can be responsible for such significant rate capability. The cyclability testing depicts the better performance of spherical LCO in comparison with the counterpart, especially in terms of electrode activation time. Post cycling electrode characterization displays that the discharge capacity fading of LCO mainly results from the crystal grain deformation due to high potential cycling and can be alleviated by reducing the depth of charge.
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