Incorporation of MoSx/rGO electrocatalysts in sulfur cathode to reduce polysulfide shuttling in Li-S battery

Abstract

Lithium-sulfur batteries (LSB) have emerged as one of the most promising alternatives to Li‑ion batteries due to their exceptionally high theoretical energy density, nearly an order of magnitude higher than that of Li‑ion batteries. The sulfur cathode offers a theoretical specific capacity of 1,675 mAh/g. At the same time, the Li metal anode provides a theoretical capacity of 3,860 mAh/g, making LSBs attractive for high‑energy applications such as EVs. However, the practical application of LSBs is hindered by several intrinsic challenges originating from the cathode, such as polysulfide shuttling and slow redox kinetics. This slow kinetics and polysulfide shuttling lead to sulfur loss, low coulombic efficiency, and rapid capacity fading. On the anode side, the Li metal anode undergoes dendritic growth during cycling, which causes the formation of electrically isolated dead Li, continuous consumption of electrolyte and active lithium, and ultimately short-circuits the cell. To address polysulfide shuttling and sluggish redox kinetics, crystalline, multilayered 2D MoS₂ has been widely studied as an electrocatalyst that accelerates LPS adsorption and conversion. However, due to the limited number of active sites in micron-scale MoS₂ and the multi-step nature of sulfur redox chemistry, micron-scale MoS₂ is rarely capable of optimizing all reaction pathways simultaneously. This thesis, therefore, expands the design of MoSx‑based electrocatalysts into a tunable family of nanoscale molybdenum sulfide (MoSx) materials anchored on reduced graphene oxide (rGO) sheets to enhance the active‑site density. MoSx/rGO hybrids were synthesized through a rapid microwave‑assisted reaction that initially forms 1D amorphous MoS₃. This precursor was subsequently annealed at 250, 325, and 600 °C under a 3% H₂/97% Ar atmosphere, enabling controlled structural transformation from amorphous 1D MoS₃ to few‑layer 2D crystalline MoS₂ nano‑patches approximately 5 nm in lateral size. The amorphous MoS₃ exhibits a 1D chain‑like structure with a high density of exposed Mo and S active sites, which enhances its ability to adsorb and catalytically convert LPS species. In contrast, the crystalline 2D MoS₂ shows catalytic activity concentrated at the edge sites rather than the basal plane. This work systematically investigates how structural evolution within the MoSx family governs LPS adsorption capacity, polysulfide binding strength, catalytic activity, and overall battery performance. In the first study, when MoSx/rGOs were directly incorporated into sulfur cathodes, these structural differences led to distinct electrochemical behavior. The amorphous 1D MoS₃/rGO‑250 catalyst delivers the highest specific capacity of ~1277 mAh/g at 0.1C due to its higher LPS adsorption capacity and abundant catalytic active sites. Meanwhile, the 2D MoS₂/rGO‑600 catalyst provides superior electronic conductivity, faster charge‑transfer kinetics, and excellent long‑term cycling stability, achieving a coulombic efficiency of ~97% at 1C after 300 cycles. Another independent study was conducted to develop a cathode electrolyte interphase (CEI), a protective thin layer on the cathode, using a rapid 10‑second short‑circuit method. The resulting CEI layer was selectively permeable, allowing Li⁺ ions to pass through while effectively blocking LPS diffusion. This selective transport behavior significantly improved LPS confinement within the cathode region. When combined with the MoS₃/rGO‑250‑incorporated electrode, electrochemical testing revealed 25% higher capacity retention over 300 cycles, with a coulombic efficiency increase from 71% to 92%, demonstrating that a robust physical barrier on the cathode side can substantially mitigate LPS shuttling and enhance long-cycle performance. Based on the first study’s conclusion, where MoS₃ and MoS₂ show complementary behavior in delivering both capacity and reaction kinetics, and supported by the CEI findings that clarified how optimized interfaces minimize polysulfide diffusion, a new study was introduced with the idea of using a dual‑catalyst approach that brings together the strengths of both MoS₃ and MoS₂ catalysts to achieve cooperative polysulfide catalysis. In this configuration, 2D MoS₂/rGO‑600 was integrated directly into the sulfur-carbon (S‑C) cathode to enhance initial LPS adsorption, reaction kinetics, and reduce electron transfer resistance. At the same time, 1D MoS₃/rGO‑250 was applied as a thin catalytic interlayer (denoted S‑M6\M2) to enhance remaining LPS adsorption and complete the LPS conversion near the cathode–separator interface. This dual‑catalyst architecture significantly improved LPS confinement and accelerated multi‑step redox reactions. The S‑M6/M2 system delivered an exceptional initial capacity of ~1,417 mAh/g at 0.1C and maintained ~1,000 mAh/g at 1C, with stable cycling over 500 cycles, demonstrating synergistic effects. The results of this dual-catalyst integrated cathode approach represent a substantial improvement over a mono‑catalyst design.