Mo2C-induced solid-phase synthesis of ultrathin MoS2 nanosheet arrays on bagasse-derived porous carbon frameworks for high-energy hybrid sodium-ion capacitors

Conventional supercapacitors often suffer from low energy density. Hybrid Na-ion capacitors (NICs) are emerging as an important energy-storage device with high-energy and high-power output. They possess complementary merits of a high-capacity battery-type anode and a high-rate capacitive cathode. However, the existing anodes (e.g., carbon, TiO2, and Na2Ti2O5) are often limited by sluggish kinetics and low capacities of Na-ion storage. Here we report the fabrication of ultrathin MoS2 nanosheet arrays vertically anchored on bagasse-derived three-dimensional (3D) porous carbon frameworks (MoS2@BPC) as NIC anodes through a facile two-step solid-phase reaction strategy (BPC -> Mo2C@BPC -> MoS2@BPC). During the solid-phase synthesis process, the formation of Mo2C intermediates is the key to a successful growth of powder-type MoS2 nanosheet arrays on BPC. Meanwhile, the bagasse-derived porous cross-linked carbon structure can act as a 3D scaffold to effectively increase the conductivity, sodium-ion diffusion and structural stability of MoS2@BPC during charge-discharge processes. As a consequence, the MoS2@BPC electrode delivers a high specific capacity for Na-ion storage (490 mA h g(-1) at 0.1 A g(-1)) with superior high-rate capability and cyclability (cycled over 5000 cycles at 2 A g(-1)). Coupled with BPC as the cathode, the hybrid electrode made of MoS2@BPC enables the NIC to deliver both high energy density (112.2 W h kg(-1) at 55 W kg(-1)) and power density (8333 W kg(-1) at 53.2 W h kg(-1)) as well as long cycling stability, which may bridge the supercapacitor-battery divide.