Squaraine-linked zwitterionic COF modified LLZTO nanoparticles for high performance polymer composite electrolytes in Li–S batteries

Abstract

Lithium–sulfur (Li–S) batteries are severely restricted for practical application due to the polysulfide shuttle effect, Li dendrites and thermal runaway. The use of PEO-based polymer composite electrolytes (PCEs) as an alternative strategy suffers from limited lithium-ion conductivity with deficient long-range transfer route. Herein, Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO) nanoparticles modified with an in situ-synthesized zwitterionic covalent organic framework layer (denoted as LLZTO@HUT4) were introduced into PEO-based PCEs. Zwitterionic HUT4 modified the lithiophobic LiOH/Li₂CO₃ layer on the surface of LLZTO nanoparticles, which could notably promote Li⁺ ion transport for superior electrochemical performance of PCEs. Additionally, the intermediate layer HUT4 located between LLZTO and PEO could further improve the mechanical properties of electrolytes due to the enhanced inorganic/organic interface compatibility and intermolecular interaction. As a result, the obtained LLZTO@HUT4-15%/PEO electrolyte exhibited a competent ionic conductivity of 0.73 mS cm⁻¹ with a Li⁺ transference number of up to 0.74 at 60 °C. The assembled S@CNT|LLZTO@HUT4-15%/PEO|Li coin cell delivered a considerable initial discharge capacity of 1018 mA h g⁻¹ at 0.2 C, with approximately 92.1% capacity retention after 100 cycles, elucidating an obviously suppressed shuttle effect.