Single Lithium-Ion Conducting Covalent Organic Frameworks

Lee, Sang-Young

The demands for high-energy density power sources are growing from the beginning on the advent of portable devices. Since the beginning, liquid electrolyte was adopted owing to their high ionic conductivity, excellent wetting to electrode components. However, liquid electrolyte contains risks since it exhibits highly flammable and sometimes explosive characteristics. To resolve safety issue for demanded high-energy density batteries, all-solid-state battery, which utilizes solid electrolyte instead of liquid electrolyte, hold particular promise as solid-state electrolytes feature low flammability and highly compaction nature for high-energy density battery performance.

So far, all-solid-state batteries have generally been powered using inorganic sulfides/oxidesor polymer-based electrolyte. In addition to these conventional approaches, solid-state Li-ion conductors based on porous crystalline materials (i. e. covalent organic frameworks (COFs) and metal–organic frameworks (MOFs)) have garnered attention due to their directional ion conduction pathways through the ordered pores and versatile structural design. Porous crystalline materials, however, additionally incorporate lithium salts and/or solvents inside the pores of frameworks so that these COFs or MOFs act as conducting medium, not solid-state ion conductors in specific. Herein, we demonstrate a lithium sulfonated covalent organic framework (denoted as TpPa-SO₃Li) as a solvent-free, single lithium-ion conductors. Benefiting from the well-designed directional ion channels, high number density of lithium-ions, and covalently tethered anion groups, TpPa-SO₃Li exhibits an ionic conductivity of 2.7 × 10^–5 S cm^–1 with a lithium-ion transference number of 0.9 at room temperature, and an activation energy of 0.18 eV. Moreover, unusual ion transport phenomena of TpPa-SO₃Li allow reversible and stable lithium plating/stripping on lithium metal electrodes. These features show promising results to be adopted in next-generation all-solid-state batteries and also demonstrate potential usage for lithium metal batteries.