Li+ Dynamics of Liquid Electrolytes Nanoconfined in Metal-Organic Frameworks

Abstract

Metal-organic frameworks (MOFs) are excellent platforms to design hybrid electrolytes for Li batteries with liquid-like transport and stability against lithium dendrites. We report on Li+ dynamics in quasi-solid electrolytes consisting in Mg-MOF-74 soaked with LiClO4-propylene carbonate (PC) and LiClO4-ethylene carbonate (EC)/dimethyl carbonate (DMC) solutions by combining studies of ion conductivity, nuclear magnetic resonance (NMR) characterization, and spin relaxometry. We investigate nanoconfinement of liquid inside MOFs to characterize the adsorption/solvation mechanism at the basis of Li+ migration in these materials. NMR supports that the liquid is nanoconfined in framework micropores, strongly interacting with their walls and that the nature of the solvent affects Li+ migration in MOFs. Contrary to the "free'' liquid electrolytes, faster ion dynamics and higher Li+ mobility take place in LiClO4-PC electrolytes when nanoconfined in MOFs demonstrating superionic conductor behavior (conductivity σrt > 0.1 mS cm-1, transport number tLi+ > 0.7). Such properties, including a more stable Li electrodeposition, make MOF-hybrid electrolytes promising for high-power and safer lithium-ion batteries.