New Insights into Structure-Property-Relationship of High-Voltage Electrolyte Components for Lithium-Ion Batteries Using a pka Value Approach
The selected electrolyte additives were calculated by Hartee-Fock based molecular orbital energy calculations in respect to relevant electronic parameters, and were characterized by linear sweep voltammetry experiments on LiMn2O4(LMO) and constant current cycling experiments in NCM as well as in graphite half-cells. Furthermore, the reaction mechanism was investigated by impedance, SEM, XPS, NMR, GC-MS and ICP-OES techniques.
The results reveal that the pre-selection of electrolyte additives by pKa value is a promising approach to identify suitable electrolyte components with high electrochemical oxidation stability. Several TMS-based electrolyte additives, which can considerably improve the capacity retention of NCM cathodes during cycling at high-voltage conditions from 53% (after 50 cycles, 1 M LiPF6 in EC/DMC electrolyte) up to 99% in the same electrolyte containing 1wt-% of TMS based additive, were identified. Due to a higher stability towards oxidation, better leaving group ability and the formation of a more effective cathode passivation layer (CEI), the capacity retention increases with lower pKa value of the leaving group of the TMS based additives. The aforementioned findings provide new insights into the structure-property relationship of certain electrolyte additives, supporting the improvement of the prospective selection procedure of new electrolyte components in lithium-ion battery research.
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