Mechanistic Understanding of Metal Phosphide Host for Sulfur Cathode in High-Energy-Density Lithium-Sulfur Batteries.

Abstract

For solving the drawbacks of low conductivity and the shuttle effect in a sulfur cathode, various nonpolar carbon and polar metal compounds with strong chemical absorption ability are applied as sulfur host materials for lithium-sulfur (Li-S) batteries. Nevertheless, previous research simply attributed the performance improvement of sulfur cathodes to the chemical adsorption ability of polar metal compounds toward lithium polysulfides (LPS), while a deep understanding of the enhanced electrochemical performance in these various sulfur hosts, especially at the molecular levels, is still unclear. Herein, for a mechanistic understanding of superior metal phosphide host in Li-S battery chemistry, an integrated phosphide-based host of CF/FeP@C (carbon cloth with grown FeP@C nanotube arrays) is chosen as the model, and this binder-free cathode can exclude interference from the binder and conductive additives. With a systematic electrochemical investigation of the loading sulfur in such oxide- and phosphide-based hosts (CF/Fe₃O₄@C and CF/FeP@C), it is found that CF/FeP@C@S shows much superior Li-S performances. The greatly enhanced performance of CF/FeP@C@S suggests that FeP can well suppress the shuttle effect of LPS and accelerate their transformation during the charge-discharge process. The first-principles calculations reveal the performance variations of Fe₃O₄ and FeP in Li-S batteries mainly because the shifts of the p band of the FeP could accelerate the interfacial electronics transfer dynamics by increasing the electronic concentration in the Fermi level of adsorbed Li₂S₄. The current work sheds light on the promising design of superior Li-S batteries from both theoretical and experimental aspects.