Optimization of the Chemical Li Predoping Process of S-Confined Microporous Activated Carbon Positive Electrodes for Li–S Batteries

Lithium–sulfur batteries are expected to be the future of high-energy-density batteries, but they require a lithium source in the cell, such as a metallic lithium negative electrode, because the sulfur positive electrode does not originally contain lithium. In this study, chemical lithium predoping was performed using lithium naphthalenide on a positive electrode with sulfur confined in microporous activated carbon. Optimization of the solvent and treatment time for the predoping was able to match the initial charge and discharge capacities, which means that the initial irreversible capacity can be eliminated. Not only the initial charge following the predoping but also the subsequent discharge and charge cycles exhibited only a single plateau in each of the voltage curves. This result indicates that soluble reactive intermediates such as lithium polysulfides were not formed during and after the chemical predoping and that the reaction proceeded as a quasi-solid-phase reaction. Excessive predoping times caused the formation of various sulfur oxides as well as undesirable extra lithium doping to the host carbon. These byproducts and phenomena were found to adversely affect the charge–discharge reaction, indicating the importance of optimizing the chemical predoping conditions.