Dynamics of lithium deposition in lithium-sulfur batteries and strategies for enhancing lithium cycling efficiencies

The lithium-sulfur (Li-S) couple is one of the most promising chemistries for realizing the next generation of high energy density rechargeable batteries. Over the last decade, there have been concerted efforts towards solving the issues with the sulfur cathode. However, maintaining reasonable cycle life in practically relevant cell designs with a limited lithium inventory and lean electrolyte supply has proven quite difficult. It can be primarily attributed to the poor reversibility of the lithium-metal anode, which undergoes high-surface area mossy growth mechanisms and severe parasitic side reactions with the electrolyte during plating and stripping. Solving this challenge is key to Li-S batteries realizing their commercial viability. Furthermore, lithium deposition in the Li-S system is uniquely convoluted by the presence of soluble polysulfide intermediates in the liquid electrolyte. In this dissertation, the dynamics of lithium deposition in Li-S batteries are systematically investigated with an anode-free full cell configuration, and the mechanisms underlying the interplay between lithium-electrolyte interfacial chemistry and degradation of lithium inventory are delineated. A generalized strategy involving molecular engineering of polysulfide species and exploiting the intrinsic shuttle effect is developed for improving efficiencies of lithium plating and stripping and cyclability under realistic limited-lithium and lean-electrolyte conditions. In Chapter 1, a general overview of the Li-S battery system is provided, including a description of the electrochemistry of sulfur cathodes, the role and dynamic behavior of lithium-metal anode, and the challenges and opportunities presented by the distinct chemistry of the polysulfide-rich electrolyte. In Chapter 2, general experimental details encompassing cathode preparation, cell assembly, electrochemical measurements, and materials characterization are outlined. In Chapter 3, the anode-free full cell configuration, which pairs a Li₂S cathode with a bare current collector and contains no excess lithium, is introduced as the ideal framework for investigating lithium deposition in Li-S batteries. In Chapter 4, the anode-free full cell configuration is used to determine a quantitative estimate of the rate of lithium inventory depletion with cycling. ToF-SIMS measurements reveal the growth of a thick electrolyte decomposition layer with cycling, which blocks ionic access to “dead” metallic lithium underneath. In Chapter 5, the chemistry of lithium-electrolyte interface and its impact on the dynamics of lithium deposition is carefully investigated. Specifically, the critical role of LiNO₃ and polysulfides in modulating lithium deposition in Li-S batteries is elucidated. In Chapter 6, the introduction of tellurium as a cathode additive is demonstrated as an effective strategy for improving the cycle life of high energy anode-free, lean-electrolyte Li-S cells. Soluble tellurium containing species are formed, which shuttle to the anode side and form a stabilizing interfacial layer in-situ on lithium surface. In Chapter 7, this molecular engineering approach is further developed by investigating the chemistry of substituting selenium and tellurium into polysulfide species. While Se substitution improves charge-transfer and redox kinetics at the sulfur cathode, Te substitution engenders a substantial improvement in the reversibility of lithium anode. Finally, Chapter 8 concludes with a summary of all the work carried out in this dissertation and a perspective on future Li-S battery research.Materials Science and Engineerin