Encapsulate lithium sulfide cathodes with carbon-doped MoS₂ for fast kinetics in lithium-sulfur batteries, a theoretical study

Li₂S-based lithium-sulfur batteries represent a novel potential energy-storage device with high energy-density and superior safety. Metal sulfide surface coating has been regarded as an effective way to improve electrical and dynamic properties of Li₂S cathode. We predict that encapsulate Li₂S with MoS₂ is a potential cathode material that outperforms Li₂S@graphene in some aspects. The poor conductivity of MoS₂ can be alleviated by doping carbon atoms, which also serve as the catalytic sites to catalyze the activation of Li₂S. The doped C atoms in MoS₂ surface greatly enhance the Li₂S binding strength via the formation of S−C bonds, a p_z-p_z interaction that is not available in pristine MoS₂. Such strong interaction facilitates the decomposition of adsorbed Li₂S, thus lowering the activation voltage in the first charge cycle. Although increase the number of C atoms catalyze the decomposition of Li₂S, the compact distribution and collective C atoms may hinder the diffusion of Li ion by forming an electrostatic potential well, split lithium polysulfides (LiPSs) by excessively strong S−C interaction, and raise the rate determining step free energy of the S₈ to Li₂S conversion during discharge process, thus a balance between them is required. Single carbon atom doped MoS₂ model balances these factors and shows an excellent electrochemical performance, including improved conductivity, enhanced LiPSs affinity, lowered Li₂S decomposition barrier, facilitated Li ion diffusion and fast kinetics of the conversion of LiPSs. This work is instructive in fabricating Li₂S cathode with metal sulfides and doping architectures for practical Li-S batteries.