Sulfur vacancies in Co9S8−x/N-doped graphene enhancing the electrochemical kinetics for high-performance lithium–sulfur batteries

Abstract

A sufficient areal capacity is necessary for achieving high-energy lithium–sulfur batteries, which require sufficiently high sulfur loading in the cathode materials. Therefore, the kinetically fast catalytic conversion of polysulfide intermediates is especially important for the full utilization of sulfur. Herein, Co₉S_8−x/N-doped graphene (Co₉S_8−x/N-G) is used as a sulfur host material and electrocatalyst in a high-sulfur-loading cathode, in which sulfur vacancies are generated via the hydrogen reduction of stoichiometric Co₉S₈. The produced sulfur vacancies in Co₉S_8−x/N-G effectively improve the adsorption ability for polysulfide anions, and theoretical calculations indicate that Co₉S_8−x has lower adsorption energy for polysulfides than Co₉S₈. Furthermore, electrochemical experiments reveal that the electrode process kinetics for the catalytic conversion of polysulfides are enhanced by sulfur vacancies. As a result, the S/Co₉S_8−x/N-G cathode with a high sulfur loading of 14.6 mg cm⁻² delivers a high areal capacity of 12.9 mA h cm⁻², and long-term cycling stability with a slow decay rate of 0.035% per cycle during 1000 cycles at 1C. Additionally, the pouch cell shows foldable flexibility and a stable areal capacity of 5.9 mA h cm⁻² at a sulfur loading of 7.1 mg cm⁻². The results show that enhancing the polysulfide conversion process kinetics using sulfur vacancies could be an effective approach for obtaining high-performance lithium–sulfur batteries.