Reductive Reactions via Li excess in Mixture Electrolytes of Li Ion Battery: Ab-initio Molecular Dynamics Study
Journal
PCCP (Physical Chemistry Chemical Physics)
Date
2019.02.12
Abstract
Electro-reduction of battery electrolytes play critical roles in the formation of solid electrolyte interphase (SEI) layer on the surface of negative electrode, which in turn heavily determines the performance of rechargeable battery cells. Using ab-initio molecular dynamics, we demonstrate computational electro-reduction of mixture electrolytes by adding certain number of Li+ excess first to form solvation structure and the same number of electrons later for reductive reactions. Our method enables direct observations of ring-opening of one cyclic carbonate followed by merging with another solvent molecule as well as gas generation. When we examined FEC- and EC-based electrolytes, we were able to see the difference in terms of reaction products. In particular, top two most generated gaseous products are in accord with recent in-situ gas measurements in the literature. Different reaction products of each electrolyte also match well with the SEI constituents that are reported experimentally. By tracing reaction pathways, we also found that Li+ ions facilitate many of electrochemical reactions presumably by lowering energy barriers otherwise not easy to occur. We also found that Li+ excess also forms cationic cluster of Li2PF6+ that enable reductive decomposition of salt anion which is not easy to occur simply by increasing electronic occupation. Based on the reaction products of FEC-based electrolytes, here we propose possible mechanism of polymerization through aldehyde intermediates that are known to form bond with surrounding radical anions.