New insights into the structure-property relationship of high-voltage electrolyte components for lithium-ion batteries using the pKa value

Abstract

In pursuit of higher energy density in lithium-ion batteries (LIBs), a most promising approach focuses on cathode materials that operate at higher potentials and exhibit even higher specific charges than present LIB cathodes charged up to only 3.8 to 4.3V vs. Li/Li+. To enable a high-voltage (HV) application of the cathode, the “by-materials”, in particular the electrolyte components have to be either thermodynamically or kinetically stable. For this reason, the stability of the electrolyte components towards oxidation, in particular, depending on their HOMO energy levels, is crucial. The theoretical calculation of molecular orbital energies is a helpful and commonly used tool to predict electrochemical stability. Earlier studies demonstrated strong correlation between the HOMO energy and the pKa value, as both depend on electron affinity. Here we report on the first study referring to a pKa value based selection procedure on development of new electrolyte components for the application in lithium-ion batteries. The identified trimethylsilyl(TMS)-based additives, which are known to scavenge HF and show sufficient oxidation stability, enable the application of LiNi1/3Co1/3Mn1/3O2 (NCM) at an increased upper cut-off potential of 4.6V vs. Li/Li+ without severe degradation, leading to a 50% higher energy density. The use of pKa values is a simple, but highly effective methodology to select appropriate electrolyte components and thus helps to identify functional electrolytes beyond the typical trial and error approach or time-consuming theoretical calculations.