Unraveling the Dual Functionality of High‐Donor‐Number Anion in Lean‐Electrolyte Lithium‐Sulfur Batteries

Abstract

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...

Electrolyte solvation chemistry for lithium–sulfur batteries with electrolyte-lean conditions

“…If the LiNO 3 serves as a major lithium salt in the electrolyte, the Li + solvation shell will be dominated by NO 3 À and possibly suppress the direct contact between ether solvent and lithium metal in the SEI formation (Fig. 8b, c) [146]. The robust NO 3…”

Ultralight Electrolyte for High‐Energy Lithium–Sulfur Pouch Cells

“…[21] Raman spectroscopy was used to probe the electrolyte solvation structures.T wo peaks at % 820 and % 850 cm À1 were assigned to the uncoordinated oxide bonds of free DME. [22] As shown in Figure 3f,t he free DME peaks decreased with the introduction of the weakly coordinating MPE, indicating that Li + strongly prefers DME over MPE in the DME/MPE co-solvents.T his behavior was further confirmed by the change in the 17 ON MR spectra under different conditions. Thed isplacements of ethereal 17 On uclei in DME were far greater than those in MPE when the salt was dissolved (Figure 3h).…”

Ultralight Electrolyte for High‐Energy Lithium–Sulfur Pouch Cells

“…Meanwhile,T FSI À can be re-involved in the Li + solvation upon the consumption of NO 3 À during cycling, [25] indicating that anion-derived solvation structures are normal in our ultralight electrolyte.T his unique anion-derived solvation structure is not only conducive to sulfur utilization, but also Angewandte Chemie suppresses ether solvent decomposition on the lithium surface. [22] Generally,t he sulfur cathode has two distinct plateaus at 2.4 Vand 2.1 Vinthe solid-liquid conversion process,whose capacity ratios can reflect the conversion efficiency from polysulfides to the reduction product Li 2 S. [30] As shown in Figure 4a,t he Li-S batteries with the ultralight electrolyte displayed ah igher Q 2 /Q 1 ratio at different cycles,w hich indicated that the sulfur electrode exhibited superior reaction kinetics in the ultralight electrolyte.T his was further confirmed by its rate capability and cycling stability.L i-S batteries with an ultralight electrolyte showed excellent rate capability (Figure 4b). Thec orresponding discharge capacities were 1072, 1018, 966, 846, and 714 mAh g À1 ,atrates of 0.1, 0.2, 0.4, 0.8, and 1.0 C, respectively,a nd the capacity recovered to 1063 mAh g À1 after the rate shifted back to 0.1 C. As Figure 4c shows,L i-S batteries using the ultralight electrolyte displayed better cycling stability with ac apacity retention of 71.5 %after 200 cycles and ahigh average CE of 99.44 %, which is much higher than that of the conventional electrolytes (46.9 %a nd 98.06 %, respectively).…”