Abstract
Improving surface morphology profiles, i.e., surface area and porosity, by nanostructure/surface engineering is effective in accommodating sodium's ionic and kinetic inadequacies. However, this strategy is limited to only activating the extrinsic pseudocapacitance in terms of improving surface-based reactions. Herein, it is aimed to improve the sodiation performance by enhancement from both intrinsic and extrinsic pseudocapacitance to maximize sodiation potential of materials. A rarely reported but highly functional spinel MnCo2S4 (MCS), is introduced and systematically analyzed using first-principles investigations, which exhibits energetically favorable charge-transfer states and strong Na-ions adsorption kinetics as well as diffusion channels (-3.65 and 0.40 eV respectively). The overall electrochemical redox profiles of the MCS nanostructure is revealed by in situ techniques, which disclose the commencing of partial and then a full conversion-type sodiation at low discharge potentials (0.52 V vs Na/Na+) with fast Na-ions diffusivity. Assisted by surface engineering technology on the intrinsically pseudocapacitive MCS, the urchin-like morphology is instrumental in boosting and realizing sodium storage performance, especially the surface capacitive behavior (from 73.4% to 94.1%), prolonged cycling stability (>800 cycles), and high-rate capability (416 mAh g(-1) at 10 A g(-1)), as well as exhibiting remarkable full cell capability (high rate at 2 A g(-1), >200 cycles at 200 mA g(-1)).