Activated Li2s As a High-Performance Cathode for Rechargeable Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries are considered to be a promising alternative to lithium-ion (Li-ion) batteries as the next-generation rechargeable battery system. Despite significant progress in recent years, degradation of the lithium-metal anode remains a persistent challenge for Li-S batteries, resulting in safety concern for Li-S batteries and hindering their practical utility. One possible strategy to circumvent the problems associated with using metallic lithium is to use alternative, lithium-free anodes coupled with a lithium-containing sulfur-based cathode. Accordingly, Li₂S has attracted much attention as a prospective cathode material. However, Li₂S is highly insulating; either intricate synthesis to produce small Li₂S particles or a high initial charging-cut-off voltage to activate Li₂S bulk particles is required. However, these traditional methods invariably lead to increased manufacturing cost and/or instabilities in the ether-based electrolytes in Li-S batteries. We demonstrate here that the Li₂S bulk particles can be effectively activated through surface interaction with P₂S₅. Furthermore, the effective activation and the facilitated oxidation of Li₂S into polysulfides are believed to be due to the formation of shallow sulfur- and phosphorus-containing species at the surface of Li₂S particle.

In contrast to the pristine Li₂S, the P₂S₅-activated Li₂S cathodes demonstrate lower initial charging voltage plateaus. Furthermore, the lowest charging voltage plateau appears when the molar ratio between Li₂S and P₂S₅ is 7:1, resulting in an activation of ~ 80 % of the theoretical capacity of Li₂S below 3.0 V (Figure 1a). Considering that Li₂S or P₂S₅ alone lacks electrochemical activity before charging above 3.0 V, we believe the accessible charge capacity below 3.0 V is attributed to the interaction between P₂S₅ and Li₂S, leading to Li₂S activation. However, further increase in the concentration of P₂S₅ produces a gradual increase in the charging voltage plateau, likely due to the increased cell resistance. Scanning electron microscopy images reveal that the typical Li₂S particles are micron-sized spheres when the molar ratio between Li₂S and P₂S₅ is higher than 5:1. Core-shell-like morphology appears when the molar ratio between Li₂S and P₂S₅ is 5:1. In addition, X-ray photoelectron spectroscopy and Raman spectroscopy results confirm the formation of sulfur- and phosphorus-containing species at the activated Li₂S surface. Especially, when the molar ratio between Li₂S and P₂S₅ is 10:1, S 2p 3/2 peaks center at 159.8, 161.0, and 162.6 eV. The 159.8 eV peak is a characteristic peak of Li₂S. The 159.8 eV Li₂S peak is still present when the Li₂S: P₂S₅ ratio is 7:1, but disappears when the ratio reaches 5:1, suggesting the formation of a thick surface layer.

Reversible discharge capacities of ~ 800 mAh g (Li₂S)^–1 are obtained and the capacity retention is as high as 83 % after 80 cycles (Figure 1b). Coulombic efficiency remains near 100 % upon long-term cycling. This is in contrast to the pristine Li₂S that delivers limited reactivity and inferior capacity due to the incomplete oxidation reaction. The superior cycling performance of the activated Li₂S is due to the enhanced surface charge transfer reducing the electrochemically inaccessible Li₂S and ensuring cathode conductivity. This facile activation enables the direct use of Li₂S bulk particles as high-capacity cathode materials for Li-S batteries, significantly decreasing the manufacturing cost of Li-S batteries. We believe this strategy is of great significance for the safe and effective use of Li₂S as a cathode material, and is a promising step toward the low-cost fabrication of metallic lithium-free Li-S batteries.