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 MoS_(2)@NC,NiS@NC,and CuS@NC were developed with excellent capacity and cyclability.