ORS6-0227
Excellent Li+ ion conductivity of anion-immobilized single-ion solid-state electrolytes using covalent organic frameworks
When and Where
Nov 30, -0001
12:00am - 12:00am
Presenter(s)
Hye Ryung Byon (KAIST)
Co-Author(s)
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 that possess 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 characteristics.
In this study, we introduced carboxylic, sulfonate, or sulfonylimide anionic groups into COFs, providing high to low Lewis basicity. The Li+ ion moves from one anionic group to another through the hopping process. The strength of Li+ and anion association/dissociation is critical to determining the ionic conductivity. We demonstrated that the weakest Lewis basicity of the anion provided the superior Li+ ionic conductivity. The sulfonylimide-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 downfield shift from softer base anion-COF. In addition, the activation energy (Ea) was decreased from 0.21 eV for the carboxylic group to 0.18 eV for the sulfonate group, and 0.14 eV for the sulfonylimide 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 sulfonylimide group-COF and delivered for exceeding 600 hours. I will discuss details of comparative Li+ ion transport among COFs and related mechanisms.
In this study, we introduced carboxylic, sulfonate, or sulfonylimide anionic groups into COFs, providing high to low Lewis basicity. The Li+ ion moves from one anionic group to another through the hopping process. The strength of Li+ and anion association/dissociation is critical to determining the ionic conductivity. We demonstrated that the weakest Lewis basicity of the anion provided the superior Li+ ionic conductivity. The sulfonylimide-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 downfield shift from softer base anion-COF. In addition, the activation energy (Ea) was decreased from 0.21 eV for the carboxylic group to 0.18 eV for the sulfonate group, and 0.14 eV for the sulfonylimide 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 sulfonylimide group-COF and delivered for exceeding 600 hours. I will discuss details of comparative Li+ ion transport among COFs and related mechanisms.
