Cobalt Phosphide Nanoflake-Induced Flower-like Sulfur for High Redox Kinetics and Fast Ion Transfer in Lithium-Sulfur Batteries.

Sulfur reactivity in lithium-sulfur batteries highly depends on its distribution and morphology during cycling, which is of great significance to suppress the shuttle effect and promote conversion reaction. Herein, cobalt phosphide nanoflakes are prepared and used as a sulfur host. An improved redox kinetics from sulfur to lithium sulfide and the corresponding fast lithium-ion diffusion are observed to greatly promote the electrochemical performance of lithium-sulfur batteries. Meanwhile, for the first time, we propose "effective triple phase contact" and "insulated dead sulfur" to account for cycling performance differences of CoP@S and rGO@S batteries. The flower-like sulfur induced by CoP nanoflakes during cycling provides extra lithium-ion diffusion and electron transfer ways compared with agglomerated sulfur in the rGO@S cathode. The CoP@S battery shows good rate performance and delivers 520 mA h g after 1000 cycles with an excellent Coulombic efficiency of 99%. In contrast, no conversion reaction happens after 600 cycles in the rGO@S battery, implying no existence of reactive sulfur. This research reveals the effect of morphological evolution of sulfur on the cycling performance and affords an insight for developing high-performance lithium-sulfur batteries.