Intrinsic safe carboxyl-covalent organic framework gel electrolyte to anchor solvents for lithium-sulfur batteries

Compared to liquid electrolytes, gel electrolytes enhance the safety and electrochemical performance of lithium metal secondary batteries by locking the solvent within a porous polymer matrix, thereby reducing side reactions between the liquid components and the lithium metal electrode. This work designs and prepares a covalent modified organic framework gel electrolyte (Carboxyl-COF, COF-COOH), which can undergo a sol–gel transition with traditional carbonate liquid electrolytes, forming a chemically crosslinked gel via covalent bonds. The structure contains negatively charged carboxyl functional groups, which can generate strong electrostatic interactions with Li⁺, promoting the desolvation process of Li⁺ and achieving the transference number up to 0.76. Notably, by the hydrogen bond and π-π stacking interaction, carbonate solvent has been anchored in COF-COOH and, effectively suppressing side reactions between the electrolyte and the lithium anode. The Li || Li symmetric cell with the gel electrolyte shows stable cycling for over 1800 h and a low voltage hysteresis of 0.02 V and Li-SPAN batteries demonstrate stable cycling. Additionally, nonflammable gel electrolyte ensure the intrinsic safe for Li-SPAN batteries.