Delocalized Isoelectronic Heterostructured FeCoOx Sy Catalysts with Tunable Electron Density for Accelerated Sulfur Redox Kinetics in Li-S batteries

High interconversion energy barriers, depressive reaction kinetics of sulfur species, and sluggish Li+ transport inhibit the wide development of high-energy-density lithium sulfur (Li-S) batteries. Herein, differing from random mixture of selected catalysts, the composite catalyst with outer delocalized isoelectronic heterostructure (DIHC) is proposed and optimized, enhancing the catalytic efficiency for decreasing related energy barriers. As a proof-of-content, the FeCoOx Sy composites with different degrees of sulfurization are fabricated by regulating atoms ratio between O and S. The relationship of catalytic efficiency and principal mechanism in DIHCs are deeply understood from electrochemical experiments to in situ/operando spectral spectroscopies i.e., Raman, XRD and UV/Vis. Consequently, the polysulfide conversion and Li2 S precipitation/dissolution experiments strongly demonstrate the volcano-like catalytic efficiency of various DIHCs. Furthermore, the FeCoOx Sy -decorated cell delivers the high performance (1413 mAh g-1 at 0.1 A g-1 ). Under the low electrolyte/sulfur ratio, the high loading cell stabilizes the areal capacity of 6.67 mAh cm-2 at 0.2 A g-1 . Impressively, even resting for about 17 days for possible polysulfide shuttling, the high-mass-loading FeCoOx Sy -decorated cell stabilizes the same capacity, showing the practical application of the DIHCs in improving catalytic efficiency and reaching high electrochemical performance.

Direct Monitoring of Li2 S2 Evolution and Its Influence on the Reversible Capacities of Lithium-Sulfur Batteries

The polysulfide (PS) dissolution and low conductivity of lithium sulfides (Li₂ S) are generally considered the main reasons for limiting the reversible capacity of the lithium-sulfur (Li-S) system. However, as the inevitable intermediate between PSs and Li₂ S, lithium disulfide (Li₂ S₂ ) evolutions are always overlooked. Herein, Li₂ S₂ evolutions are monitored from the operando measurements on the pouch cell level. Results indicate that Li₂ S₂ undergoes slow electrochemical reduction and chemical disproportionation simultaneously during the discharging process, leading to further PS dissolution and Li₂ S generation without capacity contribution. Compared with the fully oxidized Li₂ S, Li₂ S₂ still residues at the end of the charging state. Therefore, instead of the considered Li₂ S and PSs, slow electrochemical conversions and side chemical reactions of Li₂ S₂ are the determining factors in limiting the sulfur utilization, corresponding to the poor reversible capacity of Li-S batteries.

Strong sulfur binding with conducting Magnéli-phase Ti(n)O2(n-1) nanomaterials for improving lithium-sulfur batteries

Lithium-sulfur batteries show fascinating potential for advanced energy storage systems due to their high specific capacity, low-cost, and environmental benignity. However, the shuttle effect and the uncontrollable deposition of lithium sulfide species result in poor cycling performance and low Coulombic efficiency. Despite the recent success in trapping soluble polysulfides via porous matrix and chemical binding, the important mechanism of such controllable deposition of sulfur species has not been well understood. Herein, we discovered that conductive Magnéli phase Ti4O7 is highly effective matrix to bind with sulfur species. Compared with the TiO2-S, the Ti4O7-S cathodes exhibit higher reversible capacity and improved cycling performance. It delivers high specific capacities at various C-rates (1342, 1044, and 623 mAh g(-1) at 0.02, 0.1, and 0.5 C, respectively) and remarkable capacity retention of 99% (100 cycles at 0.1 C). The superior properties of Ti4O7-S are attributed to the strong adsorption of sulfur species on the low-coordinated Ti sites of Ti4O7 as revealed by density functional theory calculations and confirmed through experimental characterizations. Our study demonstrates the importance of surface coordination environment for strongly influencing the S-species binding. These findings can be also applicable to numerous other metal oxide materials.