High-Performance All-Solid-State Lithium Metal Batteries Enabled by Ionic Covalent Organic Framework Composites

Ionic covalent organic frameworks (iCOFs) are crystalline materials with stable porous structures. They hold great potential for ion transport, particularly as solid-state electrolytes (SSEs) for all-solid-state Lithium metal batteries (ASSLMBs). However, achieving an ionic conductivity of over 10⁻³ S cm⁻¹ at room temperature using pure-iCOF-based SSEs, even adding additives such as lithium salts, is challenging as the voids work as strong resistances. Thus, highly conductive iCOFs typically require quasi-solid-state configurations with organic solvents or plasticizers. In this study, we prepared composites comprising iCOFs and poly(ionic liquid) (PIL) to make all-solid-state iCOFs electrolytes with an exceptional ionic conductivity up to 1.50 × 10⁻³ S cm⁻¹ and a high Li⁺ transference number of > 0.80 at room temperature. Combined experimental and computational studies showed that the co-coordination and competitive coordination mechanism established between the PIL, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and iCOFs enabled rapid Li⁺ transport while restricting TFSI⁻ movement. ASSLMB cells, made of composite SSEs and LiFePO₄ composite cathode, demonstrated an initial discharge capacity of 141.5 mAh g⁻¹ at 1 C and r.t., with an impressive capacity retention of 87% up to 800 cycles. Overall, this work presents a breakthrough approach for developing advanced SSEs for next-generation high-energy-density ASSLMBs.