Amphoteric covalent organic framework as single Li⁺ superionic conductor in all-solid-state

As a novel class of porous crystalline solids, covalent organic frameworks (COFs) based electrolyte can combine the advantages of both inorganic and polymer electrolytes, leading to such as higher structural stability to inhibit lithium dendrites and better processing facility for improving interfacial contact. However, the ionic components of Li salt tend to be closely associated in the form of ion pairs or even ionic aggregates in the channel of COFs due to strong coulombic interactions, thus resulting in slow ionic diffusion dynamics and low ionic conductivity. Herein, we successfully designed and synthesized a novel single-ion conducting nitrogen hybrid conjugated skeleton (NCS) as all solid electrolyte, whose backbone is consisted with triazine and piperazine rings. A loose bonding between the triazine rings and cations would lower the energy barrier during ions transfer, and electrostatic forces with piperazine rings could “anchor” anions to increase the selectivity during ions transfer. Thus, the NCS-electrolyte exhibits excellent room temperature lithium-ion conductivity up to 1.49 mS·cm⁻¹ and high transference number of 0.84 without employing any solvent, which to the best of our knowledge is one of the highest COF-based electrolytes so far. Moreover, the fabricated all-solid-state lithium metal batteries demonstrate highly attractive properties with quite stable cycling performance over 100 cycles with 82% capacity reservation at 0.5 C.