A Cathode-Integrated Sulfur-Deficient Co₉S₈ Catalytic Interlayer for the Reutilization of "Lost" Polysulfides in Lithium-Sulfur Batteries.

Abstract

Lithium-sulfur batteries, with their high theoretical energy density and the low material cost of sulfur, are highly promising as a post-lithium ion battery contender. Their current performance is however compromised by sulfur loss and polysulfide shuttle to result in low energy efficiency and poor cycle stability. Herein, a catalytic material (Co₉S_{8- x}/CNT, nanoparticles with a metallic Co₉S₈ core and a sulfur-deficient shell on a CNT support) was applied as an interlayer on the sulfur cathode to retain migratory polysulfides and promote their reutilization. The Co₉S_{8- x}/CNT catalyst is highly effective for the conversion of polysulfides to insoluble end products (S or Li₂S/Li₂S₂), and its deployment as a cathode-integrated interlayer was able to retain the polysulfides in the cathode for reuse. The accumulation of polysulfides in the electrolyte and the polysulfide shuttle were significantly reduced as a result. Consequently, a host-free sulfur cathode with the Co₉S_{8- x}/CNT interlayer had a low capacity fade rate of 0.049% per cycle for 1000 cycles at a 0.3C rate, a significant improvement of the capacity fade rate without it (0.28% per cycle for 200 cycles). The results here provide not only direct evidence for the contributions of sulfur deficiencies on the catalytic activity of Co₉S₈ in polysulfide conversion reactions but also the methodology on how the catalyst should be deployed in a Li-S battery for the best catalytic outcome.