Ultralean Electrolyte Li-S Battery by Avoiding Gelation Catastrophe

Area of Science:

• Electrochemistry
• Materials Science
• Energy Storage

Background:

• Lithium-sulfur (Li-S) batteries require dissolution and diffusion of sulfur species (S^α-) due to poor electronic conductivity of sulfur and sulfides.
• The precise requirements for dissolution and diffusion to optimize sulfur utilization and determine the minimum electrolyte/sulfur ratio (E/S)_min remain unclear.
• Electrolyte properties significantly influence polysulfide behavior and battery performance.

Purpose of the Study:

• To investigate the impact of dissolved polysulfide concentration and electrolyte donor number (DN) on Li-S battery performance.
• To identify failure mechanisms related to electrolyte gelation under lean electrolyte conditions.
• To demonstrate high energy density in Li-S batteries using optimized electrolyte formulations.

Main Methods:

• Experimental investigation of polysulfide dissolution and diffusion in various electrolytes.
• Analysis of electrolyte gelation phenomena at different polysulfide concentrations and donor numbers.
• Electrochemical testing of Li-S coin cells with high sulfur loading under lean electrolyte conditions (low E/S).

Main Results:

• Excessive dissolved polysulfide concentrations (>10-20 MS) in high-DN electrolytes lead to gelation, trapping S^α- and causing polarization.
• Low-DN electrolytes (DN = 13.9) exhibit low polysulfide solubility (0.1-0.5 MS) and prevent gelation, even at extremely low E/S ratios.
• High sulfur utilization (96% at 1600 mAh g^-1 and 78% at 1300 mAh g^-1) achieved at E/S = 2 and 1 μL mg^-1 with low-DN electrolytes and high-sulfur-loading cathodes.

Conclusions:

• Electrolyte gelation is a critical failure mode in Li-S batteries under lean conditions, exacerbated by high donor numbers.
• Low-DN electrolytes offer a promising strategy to mitigate gelation and enable high energy density in Li-S batteries.
• Optimizing electrolyte composition and managing polysulfide solubility are key to achieving high sulfur utilization and energy density in Li-S systems.