In this study, we explore the chemical and morphological basis for enhanced capacity retention in sulfur-carbon cathodes made from poly(sulfur-random-(1,3-diisopropenylbenzene) (poly(S-r-DIB)) copolymers for high-energy density Li-S batteries using new Li-ion focused beam (LiFIB) and high-spatial resolution analytical electron microscopy (AEM) instrumentation platforms [1, 2]. Sulfur copolymer-based composite cathodes exhibit an initial discharge capacity of 1225 mAh/g, high reversible discharge capacity, high cycle stability (1005 mAh/g at 100 cycles), and lifetimes of over 500 cycles [3, 4]. The DIB cross-linking agent not only transforms sulfur into an easy processable copolymer, but also promotes the generation of lithiated organosulfur products Li₄(S_x)4-DIB (x≈8), which effectively prevent the irreversible deposition of insoluble lower Li polysulfides and improve the cycling performance of the batteries. However, the root causes of these improvements are not yet well understood.  One of the reason is that the cathodes are randomly organized in a hierarchical 3D architecture which is composed of the poly(S-r-DIB) copolymers, aggregated conductive carbons, and a polymer binder is quite challenging to understand and characterize. Here we employ the combination of LiFIB and analytical transmission electron microscopy (TEM) and scanning TEM (STEM) techniques, coupled with multivariate statistical analysis (MSA), electron tomography, and electrical conductivity measurements to analyze the origins of this enhanced capacity retention [5]. High-resolution TEM (HRTEM) imaging and energy-dispersive X-ray (EDX) and electron energy-loss (EEL) spectroscopic analyses of the multiscale structural architectures in the cathodes up to the atomic scale reveal that the incorporation of the DIB cross-linking agent significantly improves the compatibility between the sulfur containing domains and the carbon black conductive particles in the composite cathodes, significantly decreasing the level of structural and compositional heterogeneity. Multimode STEM and MSA are used to identify statistically significant differences between spatial regions and produce an unbiased phase classification for quantifying these heterogeneities.  Valence EELS in the STEM mode has been applied to probe local phase distributions, bonding and mechanical properties (microhardness) in the poly(S-r-DIB) composite cathodes. We have found that the incorporation of the DIB into the sulfur copoloymers drastically enhances the molecular-level compatibility, which leads to a reduced propensity for cathode cracking upon cycling and provides intimate interfacial contacts between the poly(S-r-DIB) active material and onion-like carbon structure, forming random electrically conductive percolation networks within the composite cathode. As a result, a hierarchical cathode morphology is created that is electrochemically and mechanically more robust, leading to both increased specific capacity and enhanced cycle life over traditional Li-S batteries.

References

[1] K. A. Twedt et al., Ultramicroscopy 142 (2014) 24-31

[2] V.P. Oleshko et al., MRS Comm. 5 (2015) 353-364.

[3] W.J. Chung, et al., Nature Chem. 5 (2013), 518-524.

[4] A.G. Simmonds, et al., J. ACS Macro Lett. 3 (2014), 229-232.

[5] V.P. Oleshko et al., Microsc. Microanal. 21 (Suppl. 3) (2015) 143-144.