Center for Molecular Spectroscopy and Dynamics

IBS CMSD Seminar

Prof. Hye Ryung Byon

March 25(Tue) - March 25(Tue), 2025

16:00~17:00

Seminar Room B (119)

ABSTRACT

Solid-state electrolytes have largely paid attention to eliminating fire risk and assembling compact cell configurations. Polymeric electrolytes have been developed as flexible organic solid-state electrolytes. However, their low ionic conductivity in lithium (Li)-ion batteries has limited practical use. It is attributed to the sluggish segmental motion of the polymeric backbone through Li⁺ ion transport. Presumably, a more rigid electrolyte framework can provide faster Li⁺ ion movement. Covalent organic frameworks (COFs) can be promising electrolytes with crystal structures and nanometer-scale porous channels. Immobilized anions in the channel of the COF offer the sole Li⁺ ion transport, forming a single solid-state electrolyte. It is intriguing to evaluate the Li⁺ ion conductivity in this rigid COF through anion characters.

We introduced carboxylic, sulfonate, or fluorinated sulfonimide anionic groups into COFs, providing high to low Lewis basicity. The Li⁺ ion moves one to another anionic group through the hopping process. The strength of Li⁺ and anion association/dissociation is critical to determine the ionic conductivity. We demonstrated that the weakest Lewis basicity of anion provided the superior Li⁺ ionic conductivity. The fluorinated sulfonimide-immobilized COF exhibited the highest value at 8.26 × 10^-5 S cm^-1 at room temperature, and a Li⁺ transference number was estimated to be 0.91. In comparison, the carboxylic and sulfonate anion groups in COFs provided 3.16 × 10^-6 and 2.58 × 10^-5 S cm^-1, respectively. Solid-state ⁷Li NMR spectra of all samples supported these results by a more upfield shift from softer base anion-COF. In addition, the activation energy (E_a) was decreased from 0.21 eV for the carboxylic group to 0.18 eV for the sulfonate group, and 0.14 eV for the fluorinated sulfonimide group-COF. All these results demonstrated the improved Li⁺ ion conductivity with softer anions based on the hard-soft acid-base (HSAB) theory, promoting dissociation of Li⁺/anion interaction and leading to more rapid Li⁺ ion hopping. Li|Li symmetric cells exhibited the lowest voltage hysteresis with the fluorinated sulfonimide group-COF and delivered exceeding 600 hours. I will discuss details of comparative Li⁺ ion transports among COFs and related mechanisms in this presentation.