M0S2-S8 Composite Cathodes for Long Cycle Life High Performance Li-S Batteries Studied by FESEM and High-Resolution AEM (2017)

The development of next generation electrical energy storage driven by ever increasing demand is pushing electrochemical power sources beyond the barriers of current Li-ion batteries towards multicomponent composite electrode materials exploring new chemistries with higher energy densities, longer cycle life, and improved safety while being sustainable, "green", low cost, and reliable. With theoretical specific energy (2600 Wh kg -1 ) and specific capacity (1672 mAhg -1 ), which are the highest among all solid elemental redox couples, Li-S battery is regarded as a prospective technology for near-future commercialization in emerging applications (high-altitude pseudosatellites, long-range electric vehicles (>550 km), portable electronics, and stationary grid energy storage) Remaining obstacles for the practical realization of Li-S battery technology are, however, pronounced capacity fading, poor Coulombic efficiency, and limited cycle life, which all are to various extend related to parasitic diffusion of soluble lithium polysulfides (Li2Sx, x = 38) during cycling from a sulfur-carbon cathode to a Li metal anode (a "polysulfide shuttle" effect). In order to decrease the effective diffusion length of the polysulfides and enhance cell performance via binding Li2Sx to the cathode surface, nanostructured layered transition metal dichalcogenides MX2 (M = Mo, Ti, X = S, Se), have been employed. MoS2-elemental sulfur (-S8) composites recently introduced as a new active material for cathodes in Li-S cells exhibited extended cycle life up to 1000 cycles, high capacity retention and exceptional rate capability with the delivery of reversible capacity up to 500 mAhg -1 at 5 C rate In this work, we explore mechanistic roots for these remarkable improvements using cold-electron gun field-emission SEM (C-FESEM) and high-resolution analytical scanning/transmission electron microscopy (AS/TEM) to characterize the surface topography, morphology, interfaces, and elemental distributions in MoS2-S8 composite-based cathodes. The cathodes were fabricated by ballmilling of a mixture of MoS2-S8 composite (1:1 by mass), conductive carbon, and poly(ethylene) binder in a 75:20:5 mass ratio. Low voltage C-FESEM observations show that 10-50 m diameter and 20-500 nm-thick crystalline MoS2 platelets and sulfur microparticles are regularly distributed throughout the cathode matrix comprising carbons and PE binder practically with no cracks usually formed in S8-carbon cathodes due to mechanical stresses induced during drying of deposited cathode films (Fig. Characterization of sulfur distributions in MoS2:S8 composites by EDXS is challenging and requires appropriate standards and fitting references because of severe overlapping of MoL-series and SK-series with only a 17 eV separation between the major characteristic SK (2.307 keV) and MoL (2.293 keV) lines (Fig. Morphological, crystallographic, and compositional C-FESEM and AS/TEM analyses of the cathodes reveal 3D agglomerates formed by flakes of mechanically disrupted layered MoS2 platelets and exfoliated nanosheets with large-area interfaces primarily on basal planes surrounded by aggregated 30-60 nm carbon particles and extended micro-and mesoporosity, which serve as electron and ion transport pathways, respectively. HRTEM observations (Fig. These findings demonstrate significant implications from the incorporation of MoS2-S8 composites into the cathodes on the performance of Li-S batteries.