A Consistent and Interactive Protocol for Generating an Atomistically Resolved Solid Electrolyte Interphase (SEI) Passivating Layer in Li-Ion Batteries

Lithium-Ion Batteries (LIBs) have received increasing attention due to their crucial role in the transition from fossil fuel to renewable energy. In supporting the energy transition process, LIBs play an essential role in a wide range of energy storage applications from the transport sector (e.g. Electric Vehicles) to the energy market, where LIBs help to improve the power grid stability [1, 2]. This high demand opens up a need to improve LIB technology, such as increasing the energy density, capacity efficiency, and safety of LIBs. One cause of the latter issue is the formation and degradation of the passivation layer called Solid Electrolyte Interphase (SEI). The SEI results from organic solvent degradation and deposition [3]. However, uncontrolled SEI growth causes an undesired and irreversible capacity fade, battery self-heating, and in the worst case to thermal runaway and explosion. Understanding SEI formation and growth is one of the main challenges for LIBs' further development [4]. The SEI's highly reactive nature and complexity of <i>in-situ</i> techniques make the experimental observation of such phenomena very challenging. A possible solution to these obstacles is using a more theoretical approach, but even this approach is not trivial because the SEI requires consideration of all the different phenomena with different time scales [5]. With this work, we propose an innovative protocol to reconstruct the morphology of the SEI polycrystalline layer with an atomic resolution. In our approach, we combine Reactive Force Field MD (ReaxFF) [6] simulations with an <i>in-house</i> routine for statistical generation and positioning in the computational box of several SEI crystalline grains. A consistent computational protocol has been developed to impose the appropriate mass density of the polycrystalline structure as well as the reactive formation of amorphous formation among the several SEI grains. Initially, the SEI grains are scattered in the simulation box, so the system undergoes a slow compression to allow the approach and orientation of the particles. Then, the formation of grain boundaries is obtained by relaxing the system with an annealing simulation. Finally, our procedure delivers consistent configurations of the SEI structure along and attached to the anode slab. Upon generation of the above SEI and SEI+Anode structures, we perform an enhanced sampling simulations campaign to extract the Free Energy Landscape (FEL) and lithium transport properties. Furthermore, our protocol is wrapped and interactively managed with the user-friendly and easy-to-use interface of Jupyter Notebooks. Thanks to python libraries and our <i>in-house </i>codes, we achieved a flexible simulation environment where the user can apply major changes to the studied system with minimal effort. In conclusion, with this protocol, we aim to pave the way for a new computational platform allowing an <i>in-silico</i> experimental campaign to shed light on the SEI formation mechanism and a more rational design of the next generation LIBs.<br/>[1] Chu, Steven, Yi Cui, and Nian Liu. "The path towards sustainable energy." Nature materials 16.1 (2017): 16-22.<br/>[2] Cano, Zachary P., et al. "Batteries and fuel cells for emerging electric vehicle markets." <i>Nature Energy</i> 3.4 (2018): 279-289.<br/>[3] Pinson, Matthew B., and Martin Z. Bazant. "Theory of SEI formation in rechargeable batteries: capacity fade, accelerated aging and lifetime prediction." <i>Journal of the Electrochemical Society</i> 160.2 (2012): A243.<br/>[4] Goodenough, John B., and Youngsik Kim. "Challenges for rechargeable batteries." <i>Journal of Power Sources</i> 196.16 (2011): 6688-6694.<br/>[5] Wang, Aiping, et al. "Review on modeling of the anode solid electrolyte interphase (SEI) for lithium-ion batteries." <i>npj Computational Materials</i> 4.1 (2018): 1-26.<br/>[6] Van Duin, Adri CT, et al. "ReaxFF: a reactive force field for hydrocarbons." <i>The Journal of Physical Chemistry A</i> 105.41 (2001): 9396-9409.