Influence of Lithium Polysulfide Clustering on the Kinetics of Electrochemical Conversion in Lithium–Sulfur Batteries

Abstract

The electrochemistry of lithium−sulfur (Li−S) batteries is heavily reliant on the structure and dynamics of lithium polysulfides, which dissolve into the liquid electrolyte and mediate the electrochemical conversion process during operation. This behavior is considerably distinct from the widely used lithium-ion batteries, necessitating new mechanistic insights to fully understand the electrochemical phenomena. Testing at low-temperature conditions presents a unique opportunity to glean new insights into the chemistry in kinetically constrained environments. Under such conditions, despite the low freezing point and favorable ionic conductivity of the glyme-based electrolyte, Li−S batteries exhibit counterintuitively poor performance. Here, we show that beyond just existing in singlemolecule conformations, lithium polysulfides tend to cluster and aggregate in solution, particularly at low-temperature conditions, which subsequently constrains the kinetics of electrochemical conversion. Energetics and coordination implications of this behavior are extended toward a new framework for understanding the solution coordination dynamics of dissolved lithium species. Based on this framework, a favorable strongly bound lithium salt is introduced in the Li−S electrolyte to disrupt polysulfide clustered networks, enabling substantially enhanced low-temperature electrochemical performance. More broadly, this mechanistic insight heightens our understanding of polysulfide chemistry irrespective of temperature, confirming the link between the solution conformation of active material and electrochemical behavior.

Designing Advanced Lithium‐Based Batteries for Low‐Temperature Conditions

“…Reproduced with permission. [64] Copyright 2020, American Chemical Society. The physical and chemical properties of the electrolyte and cathode active material are thus coupled and interdependent, and any investigation into diagnosing or optimizing the lowtemperature performance requires mechanistic insights that may be fundamentally different from those for more traditional battery chemistries.…”

“…[83,84] Individually, both polysulfide clustering and favorable growth of Li 2 S have been shown to be influenced through the use of strongly-coordinating lithium salts, and thus there is great potential in further developing the theory intrinsic to these two phenomena. [64,73,85] It is intriguing that deliberately engineered ion-coordination behavior in-solution seems to be a major driver of beneficial low-temperature behavior, not just in the Li-S system, but also in lithium-metal batteries. Thus, there is great potential in exploring future rationally designed electrolytes, including those with highly fluorinated solvents that were shown to be beneficial in reducing desolvation barriers.…”

“…[ 35 ] Indeed, such low freezing‐point solvents retain very high ionic conductivities of >4 mS cm −1 even at −40 °C, meeting the minimum requirements needed for low‐temperature optimization. [ 64 ] However, as outlined and discussed previously, the primary considerations for low‐temperature battery design can often extend far beyond just the ionic conductivity of the electrolyte at low‐temperatures, and indeed, the Li‐S battery chemistry is no exception. As shown in Figure 6 a, Li‐S batteries exhibit significant losses in discharge capacity with decreasing temperature, just as in other commonly used battery chemistries.…”

“…In our past work, we unveiled that intermediate polysulfides species like Li 2 S 4 tend to cluster and aggregate with other polysulfide units in solution at low‐temperatures, leading to an array of negative effects on attainable capacity and electrochemical kinetics. [ 64 ] By employing hybrid density functional theory calculations coupled with experimental evidence from 7 Li‐nuclear magnetic resonance (NMR) spectroscopy, we observed clear shifts to clustered aggregates at low temperatures, which from a chemical, conformational, and transport‐based perspective impede kinetics and attainable capacity. Furthermore, we see that this kinetic hurdle is particularly detrimental during discharge within the second lower voltage plateau (as seen in Figure 6), given the tendency of Li 2 S 4 specifically to aggregate (inhibiting conversion to Li 2 S).…”

The rechargeable aluminum-ion battery with different composite cathodes: A review

“…However, two negative charges are carried by polysulfide anions, and it is required to maintain electric drag between the two electrodes during the cell function [44]. It has been proven that the outward diffusion of the dissolved polysulfide from the cathode can be effectively suppressed by sulfur-carbon composites [45]. However, as a result of the incorporation of extra electrochemically inactive carbon, the improvement is accompanied by the reduction of the energy density.…”