Ultraviolet Irradiation Treatment for Enhanced Sodium Storage Performance Based on Wide-Interlayer-Spacing Hollow C@MoS@CN Nanospheres.

The photochemistry and sodium storage process have been generally considered as two separated approaches without strong connection. Here, ultraviolet (UV) irradiation was applied to sodium-ion batteries to improve the electrochemical performance of MoS-based composites. C@MoS@CN nanospheres consist of double protective structures, including inner hollow carbon spheres with a thin wall (C) and outer N-doping carbon nanosheets (CNs) derived from polydopamine. The special nanostructure possesses the virtues such as wide-interlayer spacing, flexible feature with great structure integrity, and rich active sites, which endow the fast electron transfer and shorten the ion diffusion pathways. Under the excitation of UV-light, intense electrons and holes are accumulated within MoS-based composites. The excited electrons can promote the preinsertion of Na. More importantly, dense electrons promote the electrolyte to decompose and hence form a stable solid electrolyte interphase in advance. After UV-light irradiation treatment in the electrolyte, the initial Coulombic efficiency of C@MoS@CN electrodes increased from 48.2 to 79.6%, and benefiting from the fine nanostructure, the C@MoS@CN electrode with UV irradiation treatment delivered a great rate performance 116 mAh g in 20 s and  super cycling stability that 87.6% capacity was retained after 500 cycles at 500 mA g. When employed as anode for sodium-ion hybrid capacitors, it delivered a maximum power density of 6.84 kW kg (with 114.07  Wh kg energy density) and a maximum energy density of 244.15 Wh g (with 152.59 W kg power density). This work sheds new viewpoints into the applications of photochemistry in the development of energy storage devices.