Mixed-Valence iron phosphate as an effective catalytic host for the High-Rate Lithium-Sulfur battery

Lithium-sulfur battery, one of the most attractive candidates for next-generation energy storage systems, commonly suffers from sluggish polysulfide conversion and detrimental shuttle mechanism. To solve these issues, finding novel catalytic hosts and strategies to facilitate polysulfide redox reaction is gaining much importance nowadays. Herein, we report oxidation-state-controlled amorphous FePO₄ as an effective catalytic host for polysulfide conversion for the first time. For this study, we fabricated the FePO₄-embedded 3D graphene composite by facile hydrothermal reaction and tailored the ratio of Fe²⁺/Fe³⁺ in the amorphous FePO₄ by a thermal treatment in the reducing H₂ atmosphere. A series of electrochemical tests showed that the mixed-valence FePO₄ demonstrates enhanced catalytic ability than Fe³⁺-dominant FePO₄. Surface and bandgap analyses indicate that such enhanced kinetics of mixed-valence FePO₄ is attributed to the combined effects of promoted electron conduction and chemical interaction. By introducing mixed-valence FePO₄ into the cathode, the battery showed robust cyclability delivering an excellent capacity of 784 mAh∙g⁻¹ at 1C after 300 cycles (retained capacity of ∼ 78%) and superior rate performance exhibiting over 872 mAh∙g⁻¹ at a high rate of 2C.