posted on 2024-02-21, 11:04authored byMohammed Bin Jassar, Carine Michel, Sara Abada, Theodorus De Bruin, Sylvain Tant, Carlos Nieto-Draghi, Stephan N. Steinmann
Studying
the chemical reactivity related to the solid electrolyte
interphase (SEI) in lithium-ion batteries is challenging due to system
heterogeneity (spatial and compositional). Semiempirical methods have
the potential to reduce the computational cost compared to the computationally
costly DFT computations. In this study, we have first assessed the
performance of four semiempirical methods (GFN-xtb, GFN2-xtb, PM6-D3,
and PM7-D3) to model major reactions for SEI formation and growth.
We have included the decomposition reactions of the most used solvent
(ethylene carbonate), most used salt (lithium hexafluorophosphate),
and other electrolyte species like the co-solvent 1,3-dioxolane and
the additive vinylene carbonate. We have found that PM7-D3 and GFN-xtb
are the two best performing methods for the 32 tested reactions. Finally,
we have performed PM7-D3 and GFN-xtb-based molecular dynamics for
inorganic/organic interfaces. We have found that LiF is the most rigid
salt, which barely reconstructs. In contrast, Li2O is subject
to severe reconstruction at the GFN-xtb level of theory, but significantly
less when using PM7-D3. Still, even at the PM7-D3 level of theory
Li2O readily reacts with alkyl carbonates, leading to CO2 dissociation and thus the formation of surface carbonates.
When in contact with Li2O, ethylene carbonate can undergo
partial dehydrogenation reactions and ring openings. This suggests
that Li2O is overly reactive to be in direct contact with
such organic molecules. Rather, it is surrounded by a passivating
(mono)layer of Li2CO3. Indeed, our simulations
suggest that for such a hybrid system (core of Li2O, shell
of Li2CO3, solvated with ethylene carbonate)
the organic solvent remains intact. Furthermore, for such a hybrid
system GFN-xtb produces physically meaningful results, so that this
method can be overall recommended.