Uniform Metal Sulfide@N-doped Carbon Nanospheres for Sodium Storage: Universal Synthesis Strategy and Superior Performance

Nitrogen-doped carbon-coated transition-metal sulfides (TMS@NCs) have been considered as efficient anodes for sodium-ion batteries. However, the uncontrollable morphology and weak core–shell binding forces significantly limit the sodium storage performance and life. Herein, based on the reversible ring-opening reaction of the epoxy group of the tertiary amino group-rich epoxide cationic polyacrylamide (ECP) at the beginning of hydrothermal process (acidic environment) and the irreversible ring-opening (cross-linking reactions) at the late hydrothermal period (alkaline environment), 47 nm-sized ZnS@NCs were prepared via a one-pot hydrothermal process. During this process, the covalent bonds formed between the ZnS core and elastic carbon shell significantly improved the mechanical and chemical stabilities of ZnS@NC. Benefiting from the nanosize, fast ion/electron transfer, and high stability, ZnS@NC exhibited a high reversible capacity of 421.9 mAh g⁻¹ at a current density of 0.1 A g⁻¹ after 1000 cycles and a superior rate capability of 273.8 mAh g⁻¹ at a current density of 5 A g⁻¹. Moreover, via this universal synthesis strategy, a series of TMS@NCs, such as MoS₂@NC, NiS@NC, and CuS@NC were developed with excellent capacity and cyclability.