New Insights into Structure-Property-Relationship of High-Voltage Electrolyte Components for Lithium-Ion Batteries Using a pka Value Approach

In pursuit of higher energy density in lithium-ion batteries, the general approach is focused on cathode materials that operate at high voltages and exhibit even higher specific charges. To enable the high-voltage application of the cathode, the electrolyte interface needs 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 the molecular orbital energies is a helpful and commonly used tool to predict the electrochemical stability[1]. Earlier studies demonstrated strong correlation between the HOMO energy and the pK_a value, as both are depending on the electron affinity[2]. Having this in mind, here we report on the first study referring to a pK_a 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 oxidative stability, enable the application of LiNi_1/3Co_1/3Mn_1/3O₂ (NCM) at an increased upper cut-off potential of 4.6 V vs. Li/Li⁺ without severe degradation, leading to a 50% higher energy density[3].

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 pK_a 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 pK_a 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.

References:

[1] Y.-K. Han, K. Lee, S.C. Jung, Y.S. Huh, Computational and Theoretical Chemistry, 1031 (2014) 64-68.

[2] H.J. Soscún Machado, A. Hinchliffe, Journal of Molecular Structure: THEOCHEM, 339 (1995) 255-258.

[3] D.R. Gallus, R. Schmitz, R. Wagner, B. Hoffmann, S. Nowak, I. Cekic-Laskovic, R.W. Schmitz, M. Winter, Electrochim Acta, 134 (2014) 393-398.