Post-synthetic Covalent Organic Framework to Improve the Performance of Solid-State Li⁺ Electrolytes

As a new class of crystalline materials, covalent organic frameworks (COFs) have long-range ordered channels and feasibility to functionalize. The well-arranged pores make it possible to contain and transport ions. Here, we designed a novel functionalized anionic COF-SS-Li by a post-synthetic method utilizing the Povarov reaction of BDTA-COF, anchoring -SO₃^- groups to the COF backbone and converting the imine linkage to a more stable quinoline unit. The grafted -SO₃^- groups and directional channels can promote the lithium-ion transport through a hopping mechanism. As a solid-state lithium-ion electrolyte, COF-SS-Li exhibits the conductivities of 9.63 × 10^-5 S cm^-1 at 20 °C and 1.28 × 10^-4 S cm^-1 at 40 °C and a wide electrochemical window of 4.85 V. The assembled Li|COF-SS-Li|Li symmetric cell can cycle stably for 600 h at 0.1 mA cm^-2. Also, the Li|COF-SS-Li|LiFePO₄ cell delivers an initial capacity of 117 mAh g^-1 at 0.1 A g^-1 and retains a capacity rate of 56.7% after 500 cycles. The research enriches the solid-state electrolytes for lithium-ion batteries.