Uncontrolled growth of insulating lithium sulfide leads to passivation of sulfur cathodes, which limits high sulfur utilization in lithium-sulfur batteries. Sulfur utilization can be augmented in electrolytes based on solvents with high Gutmann Donor Number; however, violent lithium metal corrosion is a drawback. Here we report that particulate lithium sulfide growth can be achieved using a salt anion with a high donor number, such as bromide or triflate. The use of bromide leads to ~95 % sulfur utilization by suppressing electrode passivation. More importantly, the electrolytes with high-donor-number salt anions are notably compatible with lithium metal electrodes. The approach enables a high sulfur-loaded cell with areal capacity higher than 4 mA h cm−2 and high sulfur utilization ( > 90 %). This work offers a simple but practical strategy to modulate lithium sulfide growth, while conserving stability for high-performance lithium-sulfur batteries.

Despite the theoretically high energy density, the practical energy density of Li-S batteries at the moment does not meet the demand due to low sulfur (S) loading (<2 mg cm −2 ), large electrolyte amount (electrolyte/sulfur ratio >20 µL mg −1 ), and excess lithium (Li) metal use (>10 times excess). [5] In particular, large electrolyte usage (flooding) greatly diminishes the practical energy density of Li-S batteries. Due to the intrinsic solution-based redox chemistry, however, many of the challenges arise from minimizing the electrolyte/ sulfur ratio (E/S ratio). Since soluble lithium polysulfide (LiPS, Li 2 S x when 2 < x ≤ 8) intermediates are self-redox mediating, the decrease in the LiPS dissolution causes a sluggish sulfur conversion and high polarization. [6] Next, the morphology of lithium sulfide (Li 2 S) electrodeposition and the kinetics of the re-oxidation are affected by the sulfur species solubility as well. [7] Hence, uncontrolled precipitation and continual accumulation of Li 2 S limit the discharge capacity and further passivate the cathode interface throughout the cycling. [8] Reducing the electrolyte volume exacerbates not only the cathode performance but also the anode stability. A high reactivity and an infinite volume change of the Li metal anode cause the incessant decomposition of the electrolyte. Therefore, the lean electrolyte condition accelerates the increase of the cell resistance and provokes earlier performance failure compared to the flooding electrolyte system. [9] Manipulating electrolyte materials (solvents, salt anions, and additives) has a considerable impact on the electrochemical performance of Li-S batteries. There have been studies in which solvents with high Gutmann donor numbers (DNs) form strong interactions with lithium ions (Li + ) and promote the solvation of polysulfide (PS) anions. The increased LiPS solubility facilitates the solution-mediated reaction pathway, enabling fast reaction kinetics and high sulfur utilization. [10] Furthermore, the same merits can also be achieved with salt anions having high-DNs [11] or additives promoting ionic solvation. [12] Under a lean electrolyte regime, the role of highly solvating electrolytes becomes more prominent because of the limited solubility of sulfur species. For example, high-DN solvents can enhance the sulfur utilization under the reduced electrolyte amount by promoting the charge/discharge reactions. [13] Despite this fact, the Minimizing electrolyte use is essential to achieve high practical energy density of lithium-sulfur (Li-S) batteries. However, the sulfur cathode is more readily passivated under a lean electrolyte condition, resulting in low sulfur utilization. In addition, continuous electrolyte decomposition on the Li metal anode aggravates the problem, provoking rapid capacity decay. In this work, the dual functionalities of NO 3 − as a high-donor-number (DN) salt anion is presented, which improves the sulfur utilization and cycling stability of lean-electrolyte Li-S batteries. The NO 3 − anion eleva...

With the demands for better performance of polymer electrolyte membrane fuel cells, studies on controlling the distribution of ionomers have recently gained interest. Here, we present a tunable ionomer distribution in the catalyst layer (CL) with dipropylene glycol (DPG) and water mixtures as the ionomer dispersion medium. Dynamic light scattering and molecular dynamics simulation demonstrate that, by increasing the DPG content in the dispersion, the size of the ionomer aggregates in the dispersion is exponentially reduced because of the higher affinity of DPG for Nafion ionomers. The ionomer distribution of the resulting CLs dictates the dimensional feature of the ionomer dispersion. Although the ionomer distribution becomes more uniform with increasing the DPG content, an optimal power performance is obtained at a DPG content of 50 wt % regardless of feed humidity because of balanced proton and mass transports. As a guide for tuning the ionomer distribution, we suggest that the ionomer aggregates in the dispersion with a size close to that of the Pt/C aggregates form a highly connected ionomer network and maintain a porosity in the catalyst/ionomer aggregate, resulting in high power performance.

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