Promoting the Transformation of Li 2 S 2 to Li 2 S : Significantly Increasing Utilization of Active Materials for High-Sulfur-Loading Li – S Batteries

DOI: 10.1002/adma.201901220 Lithium–sulfur batteries with a high theoretical energy density of 2600 Wh kg−1 have received great attention and have been considered as one of the most promising energy storage devices.[1–3] During the past ten years, most efforts have been focused on solving the “shuttle effect” resulting from the dissolution of polysulfides, the volumetric expansion during lithiation and low conductivity of S/Li2S. However, the sulfur utilization, another important parameter for high-energy-density was overlooked due to the well-designed materials/structures with relatively low sulfur loadings (<2 mg cm−2) and excellent capability in Li+/e− transportation.[5] In the typical Li–S batteries using prevailing ether electrolytes, the active sulfur cathode undergoes multistep electrochemical reactions through solid–liquid–solid– solid phase transformation during the discharging process.[1,6] Based on the most Lithium–sulfur (Li–S) batteries with high sulfur loading are urgently required in order to take advantage of their high theoretical energy density. Ether-based Li–S batteries involve sophisticated multistep solid–liquid–solid–solid electrochemical reaction mechanisms. Recently, studies on Li–S batteries have widely focused on the initial solid (sulfur)–liquid (soluble polysulfide)–solid (Li2S2) conversion reactions, which contribute to the first 50% of the theoretical capacity of the Li–S batteries. Nonetheless, the sluggish kinetics of the solid– solid conversion from solid-state intermediate product Li2S2 to the final discharge product Li2S (corresponding to the last 50% of the theoretical capacity) leads to the premature end of discharge, resulting in low discharge capacity output and low sulfur utilization. To tackle the aforementioned issue, a catalyst of amorphous cobalt sulfide (CoS3) is proposed to decrease the dissociation energy of Li2S2 and propel the electrochemical transformation of Li2S2 to Li2S. The CoS3 catalyst plays a critical role in improving the sulfur utilization, especially in high-loading sulfur cathodes (3–10 mg cm−2). Accordingly, the Li2S/Li2S2 ratio in the discharge products increased to 5.60/1 from 1/1.63 with CoS3 catalyst, resulting in a sulfur utilization increase of 20% (335 mAh g−1) compared to the counterpart sulfur electrode without CoS3. Lithium–Sulfur Batteries