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 t Li+ > 0.7). Such properties, including a more stable Li electrodeposition, make MOF-hybrid electrolytes promising for high-power and safer lithium-ion batteries.

Microscopic Origin of Electrochemical Capacitance in Metal–Organic Frameworks

“…The modified interfacial interaction also greatly increases the self-diffusion coefficients of all components of the electrolyte at the PZC (Supporting Information, Figure S32), compared to those in Cu 3 (HHTP) 2 (Supporting Information, Figure S33). Cu 3 (HHTP) 2 shows an over 70% decreased self-diffusion coefficient compared to the bulk electrolyte due to the confinement effect in the pores and interactions with the electrode material. , This is consistent with previous experimental measurements on different MOF-electrolyte systems. , The self-diffusion coefficient parallel to the hexagonal pores is two times larger than that perpendicular to the pores, indicating the anisotropic nature of the self-diffusion of the in-pore electrolyte. Thus, Cu 3 (HITP) 2 also has kinetic benefits for supercapacitor systems based on the fast movement of ions inside the pores …”

“…47,48 This is consistent with previous experimental measurements on different MOF-electrolyte systems. 49,50 The self-diffusion coefficient parallel to the hexagonal pores is two times larger than that perpendicular to the pores, indicating the anisotropic nature of the selfdiffusion of the in-pore electrolyte. Thus, Cu 3 (HITP) 2 also has kinetic benefits for supercapacitor systems based on the fast movement of ions inside the pores.…”

Ionic conductivity and ion transport mechanisms of solid‐state lithium‐ion battery electrolytes: A review

“…The addition of organic solvents can help the solvation of Li + ions. 142 In addition, Li + ions can migrate in the ionic liquid phase or in the organic solvent phase. Such hybrid ion conductors may exhibit relatively high ionic conductivity, but lower Li-ion transference numbers as compared to conventional pristine MOFs and COFs.…”

“…Organic solvents are confined in nano/micropores in the framework and strongly interact with the inwalls of pores. The addition of organic solvents can help the solvation of Li + ions 142 . In addition, Li + ions can migrate in the ionic liquid phase or in the organic solvent phase.…”

On the concentration polarisation in molten Li salts and borate-based Li ionic liquids

“…1,2 Electrolytes with high t Li also suppress Li dendrite formation, thereby boosting the development of high-energy-density Li metal batteries. 3 Previous studies have aimed at improving t Li values, for instance, using metal/covalent organic frameworks (MOFs/COFs), [4][5][6] anion-receptor additives, [7][8][9][10] polyanions, [11][12][13][14][15] and highly concentrated electrolytes. [16][17][18] However, a partial Li salt concentration gradient is expected in electrochemical cells, unless single-ion conduction (t Li B 1) has been achieved.…”