Application of MoS2 in Lithium Sulfur Batteries

Lithium-ion batteries (LIBs) based on intercalation chemistry have been widely used in the past few decades. However, the overall energy density is approaching the ceiling due to the restriction of theoretical specific capacity of insertion-type oxide cathodes and graphite anodes. Lithium–sulfur (Li–S) batteries have great potential for applications in next-generation energy storage systems due to their higher theoretical capacity and energy density than LIBs. Apart from that, sulfur is earth-abundant and can be available at low prices. Despite these merits, the practical use of Li-S battery is facing several challenges: (1) Shuttle effect. Polysulfides are soluble in electrolyte, leading to the notorious “shuttle” effect that causes the continuous leakage of active materials, resulting in a poor cycling life and energy loss. (2) Both elemental sulfur and the final lithiation product Li₂S have very poor electrical conductivity. (3) The volume expansion about 80% while S conversion into Li₂S, resulting safety issues and affecting the cyclability. Consequently, current Li-S batteries have much lower-than-expected performance. In recent years, the concepts of catalysis have been introduced into Li-S batteries for solving the above problems. MoS₂ shows good polysulfides anchoring and catalytic effect, which can suppress shuttle effect. To further improve the anchoring and catalytic effect of MoS₂, three approaches on the modification of MoS₂ have been proposed. (1) 1T-MoS₂/carbon composite (1T-W-MoS₂/C). MoS₂ has three phases: 1T, 2H, and 3R. 1T-MoS₂ has the highest conductivity and superior catalytic ability. The intercalation of WO₄²⁻ induces the formation of 1T phase. Besides, the 3D-printed electrodes can promote ion diffusion and electrolyte penetration. (2) Strain is introduced into MoS₂ through a simple heat treatment. Strain raises parts of antibonding orbitals in Mo-S bonds above the Fermi level and weakens Li-S and S-S bonds, resulting in improved anchoring and catalytic effect which accelerates the conversion of lithium polysulfides. (3) MoS₂ arrays are vertically grown on polyaniline (PANI) in situ reduced graphene oxide (RGO). Due to the synergistic effects of the “reservoir” constructed by MoS₂ and RGO-PANI, RPM exhibits a successive “trapping–interception–conversion” behavior toward lithium polysulfides, which greatly suppresses the shuttle effect.