Project
Theoretical study of the electrolyte decomposition on electrode surfaces
This research project is about the characterization of degradation mechanisms in electrolyte systems that are used in lithium-ion batteries. Due to the growing need of energy storage and the wide application in various industrial fields, such as the automotive industry, a detailed understanding of decomposition reactions is crucial for an improvement of these systems. Theoretical investigations can help to investigate molecular reactions on an atomic level, while potentially lowering the experimental workload by providing initial ideas for improvement. These types of calculations involve a large number of distinct and computationally expensive simulations, which is only feasible with a cluster of high-performance computers.
Project Details
Project term
August 4, 2022–August 25, 2023
Affiliations
RWTH Aachen University
Institute
Institute of Physical Chemistry
Principal Investigator
Methods
In this project, mainly DFT calculations in the gas phase with an implicit solvation model were used to investigate the reductive decomposition of ethylene carbonate on anode surfaces. For this purpose, the surface and the molecules were decoupled in order to treat the molecular decomposition reactions separately, which lowers the computational effort. Furthermore, initial solid state DFT calculations were performed for the ethylene carbonate decomposition on a LiCoO2 cathode.
Results
The main results of this research are the activation barriers for the decomposition of ethylene carbonate to carbonate and ethene. Furthermore, the influence of commonly used conducting salts on the activation energies was also investigated. Additionally, the decomposition pathway of the commonly used LiPF6 was characterized. Finally, test calculations of the decomposition of adsorbed ethylene carbonate on a cathode surface provided first insights into the activation barriers in the solid state.
Discussion
The ring opening reaction of the reductive decomposition of ethylene carbonate is the energetically more demanding of the two reactions steps involved. It was therefore used as the basis for further investigations. Focusing on the more crucial reaction step results in more meaningful and experimentally useful results. In the case of the addition of a conducting salt, it was found that lithium bis(oxalato)borate potentially suppresses the degradation process, which is of experimental interest and this finding could provide valuable ideas for an improved electrolyte design with regard to the commonly used LiPF6. Furthermore, in contrast to a commonly assumed reaction of LiPF6 with water, an alternative path involving a fluoride anion was found to be energetically more favorable, which could also help with a better battery design by understanding the underlying decomposition pathways in a more complete way. Additionally, the initial solid state calculations revealed that the activation barriers decrease with an increasing amount on vacancies in the material, which form naturally under the working conditions over time. An alternative approach to the composition and the associated conditions might therefore be beneficial.
Additional Project Information
DFG classification: 302-02 Physical Chemistry of Solids and Surfaces, Material Characterisation
Software: Gaussian16, VASP
Cluster: CLAIX
Publications
D. Mroz, J. Hartwig, S. Neitzel-Grieshammer, ECS Advances 2023, 2, 030506.