(Invited) Challenges with Quantum Chemistry-Based Screening of Electrochemical Stability of Lithium Battery Electrolytes
Our results suggest that the minimal system for the examination of electrolyte oxidation stability should consist of a cluster containing two solvent molecules and an anion. While screening such clusters is computationally expensive it results in a much more realistic picture than the screening of isolated solvents and anions and leads to an improved agreement with the experimentally measured electrolyte electrochemical stabilities. Interestingly, the initial oxidation reactions of ethylene carbonate and dimethyl carbonate at the surface of the completely de-lithiated Ni0.5Mn1.5O4 high voltage cathode were found to be similar to reactions occurring in electrolyte clusters, highlighting the potential similarity between electrolyte oxidation on inert electrodes and active LiNi0.5Mn1.5O4 cathode.
Analogously, the formation of lithium fluoride (LiF) or solvent decomposition during reduction of semifluorinated solvents and anions was found to shift the electrochemical potential by as much as 1.5-2 V. Thus, inclusion of lithium in the model electrolyte cluster and consideration of the low barrier reactions within the computational methodology is important and could decrease the electrochemical stability window of electrolytes by as much as 3.5 V compared to a procedure that focuses on the isolated compounds surrounded by implicit solvent. Quantum chemistry study of the concentrated LiFSI-based electrolyte reduction presents an interesting example, when examination of the low rate reduction reaction resulted in LiF and Li-F-Li formation and predicted reduction stability of DME-LiFSI electrolyte around 1.6-2.4 V vs. Li/Li+.2 Quantum chemistry predictions were found to be in good agreement with CV measurements for this electrolyte and revealed initial reactions leading to the formation of a stable passivation layer on electrodes.2
1. O. Borodin, W. Behl and T. R. Jow, J. Phys. Chem. C, 117, 8661 (2013).
2. H. Kim, F. Wu, J. T. Lee, N. Nitta, H.-T. Lin, M. Oschatz, W. I. Cho, S. Kaskel, O. Borodin and G. Yushin, Adv. Energ. Mater., 5, 1401792 (2015).