Activated Li2s As a High-Performance Cathode for Rechargeable Lithium-Sulfur Batteries
Authors: Zu Chenxi , Klein Michael , Manthiram Arumugam 
Published: February 2020

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.

Figure 1