Synergism of Highly-Adsorptive Fe(OH) ₃ and Strongly-Catalytic FeP Coated on Separator Boosting Lithium–Sulfur Batteries

To overcome the core problem of lithium–sulfur (Li–S) batteries, the so-called shuttle effect is still a key challenge for Li–S batteries. Among many paths that have been tried, separator modification that blocks the soluble polysulfides (LiPSs) in the cathode side with a coating layer seems to be a feasible and workable strategy. Herein, Fe(OH) 3 and/or FeP are used as a coating layer to modify commercial PP separators for Li–S batteries, in which Fe(OH) 3 is highly adsorptive for LiPSs but noncatalytic for the conversion of LiPSs, while FeP is the opposite, low-adsorptive but strongly catalytic. A series of mixed materials with different component ratios of the two iron compounds are investigated. It is found that there is a clear synergism between Fe(OH) 3 and FeP. Fe(OH) 3 can effectively adsorb LiPSs in the coating layer, but the adsorbed LiPSs block the ion transport and leave from the electrochemical redox process, leading to a low rate performance and a continuous capacity decrease. FeP can effectively catalyze the conversion of LiPSs, but its low adsorption ability limits the kinetics of the electrochemical process, showing a low rate performance. Interestingly, when 80 wt % of FeP and 20 wt % of Fe(OH) 3 (80FeP/Fe(OH) 3 @PP) are used, the synergism makes the mixed coating layer have an outstanding electrochemical performance, and the corresponding Li–S battery shows the optimized electrochemical performance.