Covalent organic frameworks (COFs) as fast lithium-ion transport fillers for solid polymer electrolytes

The incorporation of inorganic electrolyte fillers into solid polymer electrolytes (SPEs) provides fast Li-ion transport channels, enhancing the ionic conductivity of SPEs. However, their insufficient specific surface area and poor compatibility with the polymer matrix impede the efficient diffusion of Li-ions from polymer into fillers, thereby limiting further advancements in SPEs. To address this challenge, we propose the utilization of covalent organic frameworks (COFs), TpPa-SO3Li, as fillers due to their rapid Li+ transport properties, favorable compatibility with the polymer matrix, and ultra-high specific surface area. Validated by in-situ conductive AFM and DFT calculation, TpPa-SO3Li provides high-speed Li+ transport pathways in the polyvinylidene fluoridebased SPEs (PVDF-TpPaSO3Li). This is attributed to their higher affinity energy of -1.93 eV compared to 1.34 eV of PVDF and their low Li+ migrating barrier of 0.35 eV. Consequently, PVDF-TpPaSO3Li exhibits an outstanding ion conductivity of 1.93 mS cm-1 at 30 degrees C, an enhanced ion transfer number of 0.44, and a desirable mechanical strength of 13.5 MPa. The lithium-symmetrical batteries with PVDF-TpPaSO3Li exhibit stable Li stripping/plating cycling for 2500h at 60 degrees C and 0.1 mA cm-2. Similarly, LiFePO4/Li batteries also display stable cycling with a retention in capacity of 93.83 % after 300 cycles at 0.5C.