The Electrical Double Layer and Autocatalytic NaF formation in Sodium Ion Batteries

The solid electrolyte interphase (SEI) in sodium ion batteries is known to have significant concentrations of NaF, which provides excellent electrochemical stability to the cell. However, the formation mechanism of NaF is unclear. Similarly, the electrical double layer (EDL) is known to have a substantial impact on all electrochemical interfacial processes, but is challenging to model. We employ first principles calculations and kinetic Monte Carlo (kMC) modeling to determine the complex electrochemical phenomena behind the formation of NaF and elucidate the role of the EDL in SEI growth. Novel reaction mechanisms and kinetic parameters for NaPF$_6$ decomposition are determined, where hydrolysis is shown to be unfavorable and a NaF-self catalyzed mechanism is the dominant pathway. KMC simulations reveal the formation of SEIs consisting of an inner layer of Na$_2$CO$_3$ and an outer layer of NaF. Higher concentrations of the conductive salt NaPF6 lead to the formation of thinner SEIs with high fractions of NaF. Higher charging rates also generate thinner SEIs, but with higher porosity. An initial molecular-scale model of the EDL and its potential gradient is developed and integrated into the kMC framework, enabling unique insights into the role of the EDL in SEI formation and elucidating its complex relationship with salt concentration and charging rate.