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

Abstract

Lithium-sulfur (Li-S) batteries are a prospective energy storage technology due to their high specific capacity and relatively low raw material costs. Nevertheless, shuttle of soluble polysulfides between electrodes remains a challenge, bringing about rapid capacity decay. Herein, we presented two porous covalent-organic framework (COF)-based battery separators (DTQ-COF and DHTA-COF) to mitigate the shuttling problem. The two COFs materials had same pore shapes and different pore sizes. We characterized these two COFs rich in nitrogen and oxygen in detail by many modern means and compared their pore structures and pore sizes. As a result, the DTQ-COF modified layer with smaller pore sizes (< 2 nm) can efficiently inhibit the shuttling of polysulfides and simultaneously allow the diffusion of Li+ under the effect of the size advantage. The cell assembled from the DTQ-COF modified separator had higher ionic conductivity. Consequently, the assembled Li-S battery with DTQ-COF modified diaphragm showed better electrochemical properties with specific capacity of 924.9 mA h g−1 at 0.1 C, and 476.9 mA h g−1 specific capacity even at 2 C high current density. Additionally, at 1 C after 500 cycles, a low fading rate of only 0.08% can be obtained. Our work will provide guide for suppressing polysulfide shuttle using pore size sieving effect for Li-S batteries.