Holistic evaluation of limited voltage windows paired with artificial solid-electrolyte interphase under lean electrolyte conditions of Li[sbnd]S batteries

Abstract

Lean electrolyte conditions are detrimental to the cycling performances of LiS batteries, so electrolyte-to‑sulfur (E/S) ratios are often much higher than 15 μL mg−1. However, the energy density of such flooded cells is far inferior to those of commercial Li-ion batteries despite 8–10 times higher specific capacity of sulfur cathodes compared to the metal oxide-based cathodes. Here we propose to limit the voltage window for mitigating the detrimental effect by holistically evaluating the effects of the upper and lower cycling voltage windows to unveil the major causes of the unstable cycling behaviors. With a high total sulfur loading of 7 mg cm−2, our approach utilizing a low depth of charge/discharge (1.95–2.45 V) has enabled long and stable cycling (> 400 cycles with capacity retention of 80 %). It has mitigated irreversible polysulfide passivation on the lithium metal anode when paired with an artificial solid-electrolyte interphase (A-SEI) layer, which has been a critical problem called the shuttle effect. Electrolyte degradation such as lithium salt decomposition was suppressed by cutting off the upper voltage window. Outstanding capacity retention larger than 90 % for >180 cycles was demonstrated under a lean electrolyte condition (E/S ≈ 5 μL mg−1) with the superb actual cell-level energy density of 569 Wh L−1, which is better or at least comparable to those of commercial Li-ion batteries. More importantly, despite the reduction of the specific capacity due to the reduced voltage window, both the cumulative volumetric and gravimetric energy densities are superior to those in the literature, suggesting an effective method of extending the cycling life. Strikingly, based on the actual energy density, the material cost of our cell is considerably low (∼$29 per kWh), which is about half the cost of the popular Li-ion batteries.