Using Implicit Solvent in Ab Initio Electrochemical Modeling: Investigating Li⁺/Li Electrochemistry at a Li/Solvent Interface

This paper focuses on the use of implicit solvent in electrochemical density functional theory (DFT) calculations. We investigate both the necessity and limits of an implicit solvent polarizable continuum model (PCM). In order to recover the proper electrochemical behavior of the surface and, in particular, a proper potential scale, the solvent model is determined to be mandatory: in the limit of a high dielectric constant, the surface capacitance becomes independent of the interslab space used in the model and, therefore, the electrochemical properties become intrinsic of the interface structure. We show that the computed surface capacitance is not only dependent on the implicit solvent dielectric constant, but also on the solvent cavity parameter that should be precisely tuned. This model is then applied to the Li/electrolyte interface in order to check its ability to compute thermodynamic equilibrium properties. The use of a purely implicit solvent approach allows the recovery of a more reasonable equilibrium potential for the Li(+)/Li redox pair, compared to vacuum approaches, but a potential that it is still off by 1.5 V. Then, the inclusion of explicit solvent molecules improves the description of the solvent-Li(+) chemical bond in the first solvation shell and allows recovery of the experimental value within 100 mV. Finally, we show that the redox active center involves the first solvation shell of Li(+), suggesting a particular pathway for the observed solvent dissociation in Li-ion batteries.