As a promising anode material due to its high theoretical capacity (1675 mAh/g) and energy density (2600 Wh/kg), lithium sulfide has been the focus of significant research.¹ In addition to being composed of non-toxic, environmentally-friendly elements, lithium-sulfur batteries offer light weight with low cost. Significant drawbacks of lithium sulfur batteries for commercial use are the short cycle life due to the polysulfide shuttle mechanism and the difficult processing required for their air-sensitivity. One approach to mitigating the shuttle mechanism is carbon encapsulation, which both physically constrains the active material and provides electrical conductivity to the insulating sulfur and lithium sulfide. Creating a fully encapsulating conformal coating of Li₂S is challenging, but can be circumvented by first infiltrating sulfur into carbon and lithiating to form Li₂S inside pre-made carbon particles. Melt-infiltration of sulfur for sulfur-based anodes has been extensively studied; however, anodes containing Li are more advantageous both from a safety perspective and because space is already made for the Li, avoiding troublesome volume expansion issues. While chemically lithiatiated sulfur and its encapsulation have been studied,² lithiation of pre-encapsulated sulfur is a unique processing method.

In this research, lithium triethylborohydride is investigated as a lithiating agent for sulfur previously melt-infiltrated into a range of nano- and meso-scale carbons. Tuning the reaction temperature, precursor ratio, and amount of solvent gives control over Li₂S:C ratio with the aim of keeping the Li₂S fully inside the carbon particles. Li₂S-C composites have been fabricated and are being evaluated for their electrochemical performance, size and location of Li₂S particles, and adequacy to protect against polysulfide dissolution. Optimization of CVD coatings as an additional protection is evaluated. In contrast to traditional methods of encapsulating lithium sulfide, this approach produces higher mass loadings with facile solution processing. In the future, additional lithiating agents, such as N-butyl lithium, and lithium aluminum hydride, will be examined for their efficacy.

(1) Wu, F.; Lee, J. T.; Fan, F.; Nitta, N.; Kim, H.; Zhu, T.; Yushin, G. Adv. Mater. 2015, 27 (37), 5579–5586.

(2) Nan, C.; Lin, Z.; Liao, H.; Song, M. K.; Li, Y.; Cairns, E. J. J. Am. Chem. Soc. 2014, 136 (12), 4659–4663.