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.