Methyl-group functionalization of pyrazole-based additives for advanced lithium ion battery electrolytes

Abstract

A novel methylated pyrazole derivative, namely 1-methyl-3,5-bis(trifluoromethyl)-1H-pyrazole (MBTFMP) was synthesized for the first time and comprehensively characterized for high voltage application in lithium ion batteries (LIBs). The MBTFMP reactivity and performance was compared to the known 3,5-bis(trifluoromethyl)-1H-pyrazole (BTFMP) functional additive via cyclic voltammetry (CV), constant current cycling as well as post mortem analysis techniques on the graphite and LiNi1/3Mn1/3Co1/3O2 (NMC111) electrodes, such as scanning electron microscopy (SEM) and x-ray photoelectron spectroscopy (XPS). By means of quantum chemistry (QC) calculations, reductive and oxidative stabilities of MBTFMP and BTFMP functional molecules and their reactivity with the cathode surface were determined. Both reduction and oxidation of BTFMP molecule was coupled with the intermolecular H-transfer that narrowed BTFMP containing electrolyte electrochemical stability window compared to MBTFMP functional additive. The obtained results demonstrate the benefits of hydrogen atom substitution of BTFMP by a methyl-group at the nitrogen atom that resulted in significant improvement of the NMC111||graphite cell cycling performance. This work reveals that with a smart selection of the substitution group and its position in the molecule, functional additives can be tailored in respect of vital physicochemical properties relevant for the high voltage LIB application.