Confined ionic liquids in covalent organic frameworks toward the rational design of high-safety lithium metal battery

To solve the serious safety issues such as highly flammable organic electrolyte, this work developed a novel lithium metal battery containing quasi-solid-state electrolyte (QSSE) with ionic liquid (ILs)/Li salt as electrolyte and covalent organic frameworks (COFs) as hosts. The micro-scale structure and dynamics properties of several ILs confined in COFs were investigated in detail using molecular dynamics (MD) simulations. It is found that the distinctly alternate layering structure of cations and anions of ILs from the pore surface to pore center are formed inside COFs due to the strong interaction between cations/anions and pore wall of COFs. The orientational preferences of confined ILs are observed depending on the size of anions. Inside the larger pores of COFs, the bulk-like structure of confined ILs is found, which tends to enlarge the leakage risk of electrolytes, while the smaller pore size of COFs can reduce the diffusion of ILs and further limit the ionic conductivity of electrolytes. The prepared QSSE with the appropriate-size ILs confined in medium pore-size COFs shows a higher ionic conductivity (0.87 mS cm−1 at 30 °C), more efficient inhibition of lithium dendrite (above 2000 h stability) and better battery performance (150.4 mAh g−1 at 0.1C). These findings from both simulations and experiments confirm the potential of COFs for achieving nondendritic lithium growth in lithium metal batteries.