Promoting long cycle life with honeycomb-like tri-modal porous carbon for stable lithium-sulfur polymer batteries

Abstract

Shuttling of soluble polysulfides (Li2Sn, 4 ≤ n ≤ 8) results in a low discharge capacity and unstable cycling performance of lithium–sulfur (Li–S) batteries. Furthermore, the formation of insoluble sulfides (Li2S2/Li2S) can retard the reaction kinetics, resulting in poor rate capability and short cycle life. In this study, a novel structural configuration, including a honeycomb-like porous carbon (HPC) as the sulfur host and gel polymer electrolyte (GPE), is proposed. HPC derived from waste coffee grounds possesses a tri-modal pore system. The micropore, as the main reactor, undergoes a “solid–solid” reaction mechanism in carbonate-based electrolyte, effectively preventing the generation of polysulfides. The macro- and mesopores can improve the accessibility of the electrolyte, accelerating ion transfer in the cell. Density functional theory calculations reveal that the functional groups on the HPC show strong interactions with polysulfides. These data in combination with X-ray photoelectron spectroscopy measurements indicate the presence of effective and stable mediator groups without the formation of polysulfides. The GPE provides adequate electrolyte infiltration and minimizes the leakage of flammable liquid, affording excellent cycling stability. As a result, the cell with this novel configurational shows only 0.03% capacity fading per cycle over 1500 cycles at 0.5 C-rate, providing excellent long-term cycle durability up to 10 C-rate. The excellent cycling stability and rate performance demonstrate that the novel structural configuration is effective in improving the electrochemical performance and prolonging the cycle life of Li–S batteries.