A novel insight into deterioration of heavily sulfur-loaded cathode in Li-S battery

Abstract

Active-substance (S₈ and Li₂S) deposition layer formed on polar interfaces is more reactive than that formed on apolar interfaces. However, when deposition layer reaches a certain thickness, by-path active-substance deposition takes place because of polysulfide disproportionation, which aggravates active-substance segregation to decrease the reactivity. This leads to a significant capacity decay and failure to accomplish polysulfide delithiation after certain cycles, which is understood as the results of decreased active-substance utilization and formed local micro-short-circuits, respectively. The finding of correlation between active-substance segregation and cathode deterioration inspires the design of polytetrafluoroethylene-based self-supported cathode to suppress active-substance segregation through avoiding increase of interphase contact resistance during cycling. With S-loading of 4.0 mg cm⁻², the as-prepared cathode demonstrates excellent cycleability, retaining a capacity of 300 mAh g⁻¹ at a rate of 2C after 1000 cycles. When increasing S-loading up to 36.5 mg cm⁻², the as-obtained cathode delivers an ultra-high areal capacity of 24.6 mAh cm⁻² after 100 cycles at 0.1 C. However, increasing S-loading aggravates Li₂S segregation which then narrows down Li⁺ transportation, leading to formation and growth of Li-shunts which in turn stimulates active-substance segregation. Such an inductive interaction between Li-shunt growth and active-substance segregation results in the fast formation of local micro-short-circuits, causing the constant supply of Li from Li shunts to cathode, and eventually accelerating cathode deterioration.