Initial Stages of Oxidation Reactions of Ethylene Carbonate and Fluoroethylene Carbonate on Li <sub>x</sub> CoO <sub>2</sub> Surfaces: A DFT Study
Min Lin, Xuerui Yang, Xi Zheng, Jianming Zheng, Jun Cheng, Yong Yang
Abstract
Electrolytes based on fluorinated carbonate have been developed for improving the high−voltage performance of lithium ion batteries (LIBs). However, the functioning mechanism of fluorinated carbonate is still controversial. In this work, density functional theory (DFT) calculations are applied to investigate the initial oxidation reaction processes of ethylene carbonate (EC) and fluoroethylene carbonate (FEC) on Li x CoO 2 ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>10</mml:mn> <mml:mover accent="true"> <mml:mn>1</mml:mn> <mml:mo>¯</mml:mo> </mml:mover> <mml:mn>4</mml:mn> </mml:math> ) (x = 1, and 0.67) surfaces. The spin states of cobalt ions are examined for maintaining the comparability. It is demonstrated for the first time that FEC oxidation is more favorable than EC along with proton transfer thermodynamically and kinetically, which contrasts with the oxidation stability predicted based on isolated molecule model. These results can be attributed to stronger adsorption of fluorinated molecular fragment on the Li x CoO 2 surfaces. The fluorinated molecular fragment derived from FEC subsequently may facilitate the formation of a more stabilized/protective CEI layer, which contributes to the enhanced cathode interfacial stability and the improved high voltage cycling performance. The fundamental understanding as obtained in this work provides new insight on thinking about the effect of fluorine substitution on the oxidation stability of electrolyte solvents/additives, for developing high voltage, high energy density LIB systems.