Covalent-organic frameworks rich in nitrogen and oxygen as modified separators for lithium–sulfur batteries: pore size effects

Abstract

Lithium–sulfur (Li–S) batteries are promising candidates for energy storage technology owing to their high specific capacity and relatively low raw material costs. Nevertheless, the shuttling of soluble polysulfides between electrodes remains a challenge, resulting in rapid capacity decay. Herein, we presented two porous covalent-organic framework (COF)-based battery separators (DTQ-COF and DHTA-COF) to mitigate this shuttling problem. The two COF materials exhibited same pore shapes and different pore sizes. We performed in-depth characterizations of these two COFs rich in nitrogen and oxygen using various analytical techniques and compared their pore structures and pore sizes. Notably, the DTQ-COF-modified layer with smaller pore sizes (<2 nm) efficiently inhibited the shuttling of polysulfides and simultaneously facilitated the diffusion of Li⁺ ions based on pore size effect. A cell assembled using the DTQ-COF-modified separator showed a higher ionic conductivity. Consequently, the assembled Li–S battery with the DTQ-COF-modified separator exhibited better electrochemical properties with a specific capacity of 924.9 mA h g⁻¹ at 0.1C and 476.9 mA h g⁻¹ at a higher current density of 2C. Additionally, at 1C after 500 cycles, a low fading rate of only 0.08% was observed. Thus, our work could provide guidance for suppressing the shuttling of polysulfides in Li–S batteries using the pore size sieving effect.