Overcoming the kinetic limitations of lithium-sulfur batteries
Creators: name: Gupta, Abhay
name: 0000-0003-4767-4865
Publisher: name: The University of Texas at Austin
Year: 2021
Subjects: Lithium-sulfur
Electrolyte
Kinetic limitations
Polysulfide clustering

description: The adoption of lithium-ion batteries has catalyzed massive technological and societal progress over the past few decades. While lithium-ion batteries continue to show considerable promise for a large range of applications, there are several critical use-cases that require order-of-magnitude increases in the battery’s ability to store energy per unit mass. This will necessitate the development of novel battery chemistries with increased specific energy, like lithium-sulfur (Li-S) batteries. The Li-S chemistry operates in a highly distinct manner from traditional insertion electrodes; discharge of elemental sulfur produces lithium polysulfide intermediates that dissolve into the liquid electrolyte and mediate the charge-transfer process in solution. This is accompanied by unique challenges, tremendous complexity, and large opportunity in the analysis and design of Li-S batteries. In this dissertation, the unique and distinct kinetic hurdles limiting the practical implementation of Li-S batteries are mechanistically unveiled and overcome through an array of studies. First, the solution-mediated nature of Li-S batteries is modulated through the implementation of highly solvating electrolytes. These electrolytes consist of high donor number solvents that amplify the solubility, and in conjunction, the solution-mediated electrochemistry of lithium polysulfide intermediate species. This mechanistic framing is then used to diagnose and investigate the behavior of Li-S batteries at kinetically constrained low-temperature conditions. Under such conditions, despite the low freezing point and favorable ionic conductivity of the glyme-based electrolyte, Li-S batteries exhibit counterintuitively poor performance. It is shown that lithium polysulfides tend to cluster and aggregate in solution at low temperatures, which subsequently constrains the kinetics of electrochemical conversion. Methyl trifluoroacetate (CH₃TFA) is then introduced as an electrolyte additive to evaluate the joint effects of two distinct strategies: high donor number solvents/salts and organosulfur-mediated discharge. This compound is found to amplify the inherent solution-mediated kinetics of Li-S batteries to new heights, enabling an in-situ systematic molecular engineering of intermediate species for improved performance. Next, a series of investigations are performed to unify and link the kinetic constraints that arise from polysulfide clustering to the nucleation and growth behavior of Li₂S. It is explicitly shown that the drastic decrease in polysulfide diffusion coefficient arising from clustering is the dominant kinetic constraint. Finally, ammonium trifluoroacetate (NH₄TFA) is introduced as an additive to modulate and assess the coordination environment surrounding lithium polysulfides, especially at low temperatures and lean-electrolyte conditions. It is found that the simultaneous introduction of NH₄⁺ cations and TFA⁻ anions into solution amplifies electrostatic competition in solution, improving the electrochemical conversion kinetics. These mechanistic studies heighten the collective understanding of Li-S chemistry and pave a path forward towards overcoming the kinetic limits to performance