In-Situ Free Radical Supplement Strategy for Improving the Redox Kinetics of Li-S Batteries

In the lithium-sulfur (Li-S) batteries with high sulfur loading and low electrolyte/sulfur (E/S) ratio, the cathodic reaction is hindered due to the insulating properties of active S and Li2S, resulting in poor reaction kinetics. Herein, the whole S8↔Li2S reaction pathway can be regulated by fluorenone (FL) molecules through an in-situ free radical supplement strategy. Firstly, FL acts as a capturer and stabilizer of trisulfur radical (S•-3) above 2.1 V. Secondly, after the reduction of S•-3 at 2.1 V, FL become an exogenous radical (FL•-) to promote the reaction kinetic through a new electron transfer pathway. The in-situ Raman spectra and theoretical calculation prove the strengthened reaction pathway. As a result, the capacities of Li-S batteries can be improved from 218.7 to 1099.7 mAh g-1 in the presence of FL at an E/S ratio of 5 µL/mgS. Moreover, the cells with FL achieve an initial capacity of 978.2 mAh g-1 with a high S loading of 5.5 mg cm-2 at 0.2 C. This work provides the fundamental understanding of S reduction reaction, and also demonstrates a new idea and method to solve the problem of poor kinetics of Li-S battery with low E/S ratio.