Observation of Pseudocapacitive Effect and Fast Ion Diffusion in Bimetallic Sulfides as an Advanced Sodium-Ion Battery Anode

Sodium-ion batteries (SIBs) are promising next-generation alternatives due to the low cost and abundance of sodium sources. Yet developmental electrodes in SIBs such as transition metal sulfides have huge volume expansion, sluggish Na+ diffusion kinetics, and poor electrical conductivity. Here bimetallic sulfide (Co9S8/ZnS) nanocrystals embedded in hollow nitrogen-doped carbon nanosheets are demonstrated with a high sodium diffusion coefficient, pseudocapacitive effect, and excellent reversibility. Such a unique composite structure is designed and synthesized via a facile sulfidation of the CoZn-MOFs followed by calcination and is highly dependant on the reaction time and temperature. The optimized Co1Zn1-S(600) electrode exhibits excellent sodium storage performance, including a high capacity of 542 mA h g(-1) at 0.1 A g(-1), good rate capability at 10 A g(-1), and excellent cyclic stability up to 500 cycles for half-cell. It also shows potential in full-cell configuration. Such capabilities will accelerate the adoption of sodium-ion batteries for electrical energy applications.