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 Li_xCo_yS 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 Li_xCo_yS 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.