CSUN group shows how a-alumina ALD coating improves Li-S battery electrodes with DFT calculations
CSUN group shows how a-Al2O3 ALD coating improves Li-S battery electrodes with DFT calculations
At California State University Northridge, researchers of the TMI (Theory of Matter and Interfaces) group have been using applied computation techniques to generate atomistic simulations of novel materials, and study interface properties in energy storage and conversion devices. Led by Professor Kah Chun Lau, the group of graduates and undergraduates have recently published results that tackle challenges in battery chemistry with both molecular layer deposition and atomic layer deposition, as well as by calculating electronic and chemical properties from simulated molecular environments.
The lead author of their first paper published for the year 2022 is CSUN undergraduate physics senior and TMI group member, Jake Klorman. Published in the journal energies, Klorman works with TMI principle investigator Kah Chun Lau, and postdoc Qing Guo of Washington State university (alumni of TMI).
In this paper, the TMI group and Klorman use their talents in atomistic simulation to establish a theoretical basis for the new strategies that use ALD coatings on the electrodes to overcome critical obstacles in Li-S battery technology. Using a first-principle approach with density functional theory (DFT), the research group’s goal is to study the adsorption of common electrolyte salt and solvent molecules onto an amorphous Al2O3 ALD surface.
Understanding the function of ALD coatings at the electrode’s interfaces on the atomistic level
It’s already recognized that ALD coatings are the most practical and effective approach in tackling the biggest drawback in Li-S batteries - polysulfide shuffling. Polysulfides are formed during Li-S cycling as a result of complex redox processes at the sulfur cathode. The polysulfides diffuse from the sulfur cathode, decreasing Coulombic efficiency and capacity retention, as well as causing undesirable side reactions at the Li anode, ultimately leading to structural damage to the electrodes and cell failure over time.
Klorman et al used atomistic simulation to understand what is happening at the interface level of Al2O3 ALD films. All their calculations were done in a DFT framework using Vienna Ab Initio Simulation Package (VASP). In the software, they simulated an ALD coating layer by configuring an amorphous Al2O3 slab with a thickness of ~8.0 Å. This configuration was previously used for their earlier work studying lithium oxygen batteries and potassium metal batteries.
The following figure from the paper shows which molecules were studied for their interaction and tendency to adsorb onto a-Al2O3 ALD film:
As shown above, the computational experiment showed that all molecules of interest showed a tendency to adsorb onto the amorphous Al2O3 ALD film when in close proximity to the thin film surface. However, a trend is clearly visible in the increasing adsorption energy of lengthening polysulfide species. The culprits typically involved in polysulfide shuffling is shown from this experiment to highly favor the Al2O3 surface, as indicated by their calculated high adsorption energy values.
The researchers’ atomistic simulation shows that chemical adsorption is distinguishably selective for Li2Sx molecules at the Al2O3 thin film surface, compared to solvent and LiFSI salt molecules.
These results conceived from the DFT framework confirm the following protective effects of a-Al2O3 ALD coatings:
• Al2O3 ALD coating enhances electrode stability during cell cycling by acting as a kinetic barrier, decreasing direct contact between sulfur reactants, polysulfide species, and liquid electrolytes.
• Al2O3 ALD coating mitigates the release of polysulfides from the electrode by forming a quasi-envelope structure for the electrode
• Al2O3 ALD coating protects against shuttling effects of soluble polysulfides, in providing a more stable electrode/electrolyte interface protective layer.
When it comes to the TMI group, we can expect that theory in new applications of ALD will be put to trial by their super-computing experts.
Klorman, J.A.; Guo, Q.; Lau, K.C. First-Principles Study of Amorphous Al2O3 ALD Coating in Li-S Battery Electrode Design. Energies, (2022) DOI: 10.3390/en15010390