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

Abstract

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 pz-pz 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.