Template-free synthesis of Co4S3 nanotubes derived from urchin-like clusters for sustainable molten-state high-temperature lithium battery applications

Cobalt sulfides materials, such as CoS2, have been widely used in molten-state high temperature batteries (MHLBs) due to their high specific capacity. However, the disadvantages of CoS2 such as large structural variation, limited conductivity and poor cyclability at high temperatures hinder its practical applications. Herein, a novel nanotube structure composed of highly conductive Co4S3 has been designed and synthesized through a two-step hydrothermal method, which is assembled by nanospheres with the diameter of about 70 nm. The unique nanotube structure can facilitate the ion transfer while the hollow space inside can buffer the volume variation along cycling. Importantly, the as-synthesized Co4S3 nanotube shows an excellent thermal stability even at 500 °C and a higher electrical conductivity (12.08 S/m) than CoS2 (11.48 S/m), making it a kind of promising cathode materials for MHLBs. When applied in MHLBs, Co4S3 nanotubes demonstrate an excellent rate capability with a cut-off voltage of 1.5 V (240.63 mAh/g at 0.25 A/g and 119.64 mAh/g at 0.1 A/g). During the first discharging, they undergo Li ion intercalation process to form LixCoyS at the first voltage plateau of 2.13 V, and then gradually converted to the Co and S phase with continuous Li-ion uptake at the second voltage plateau of 1.82 V. Specially, the discharged material possesses a core-shell structure, with LixCoyS and Li2S in the core and Co and S in the shell, respectively. After charging, crystallized Co4S3 can be generated upon delithiation while metallic Co is still detected in the final product. Nevertheless, when the cut-off voltage is set to 1.87 V, Co4S3 nanotubes show an impressive cycling stability with an average Coulombic efficiency higher than 90 % due to the reversible Li ion intercalation/deintercalation, making MHLBs possible to be cycled.