Al₂O₃ coated metal sulfides: One-pot synthesis and enhanced lithium storage stability: Via localized in situ conversion reactions

Transition metal sulfides with high specific capacity have received increasing research interest in lithium storage. However, the low reversibility of metal sulfides usually leads to the oxidation of Li₂S into polysulfides. The dissolution of polysulfides will suppress the regeneration of sulfides due to the loss of Li₂S, which usually leads to poor cycling stability of sulfides. Herein, Al₂O₃ coated Ni₃S₄ nanoparticles (Ni₃S₄@Al₂O₃) have been rationally designed and fabricated via a one-pot hydrothermal process. The regeneration of nickel sulfides, which is the key to cycling stability of sulfides, can be promoted by the localized conversion from metal to metal sulfides because the Al₂O₃ layer can prevent the diffusion and dissolution of polysulfides. The conversion of Al₂O₃ to ion-conductive AlF₃ can enhance the quick charge transfer process of the lithium ion insertion/extraction reaction. Furthermore, the Al₂O₃/AlF₃ layer can also prevent the growth and aggregation of Ni₃S₄ nanoparticles to retain the structure of the electrodes during the cycling process. The as-prepared Ni₃S₄@Al₂O₃ exhibits a high reversible capacity of 651 mA h g^-1 at 500 mA g^-1 even after 400 cycles. This method can also be extended to other metal sulfides for improving electrochemical performances.