Polarizable continuum model for lithium interface transitions between a liquid electrolyte and an intercalation electrode

Abstract

A model is described and used to calculate the energy of lithium when crossing the interface between a liquid electrolyte and an electrode surface. The model is based on a classical treatment of the solute–solvent interactions in terms of a polarizable continuum model (PCM), and of the lithium crystal interactions by electrostatic (Madelung) energy calculations including short-range closed shell repulsion. In addition, the coupling of Li + with the charge compensating electron on a neighbour Mn 4+ site is taken into account. A first application of the model to the Li–Mn 2 O 4 (spinel) system shows that diffusion of Li from the surface to bulk of the electrode requires an activation energy, which is higher than the one for bulk diffusion. Surface charges, as deduced from electronegativity calculations are strongly reduced compared to the bulk ones. As a result, a barrier is also formed for the solvent to the surface Li + diffusion. On the basis of the calculated energy profiles we conclude that for Li/Mn 2 O 4 bulk diffusion is considerably faster than lattice incorporation. Our results are in qualitative agreement with fits of equivalent circuits to alternating current impedance measurements for Li + intercalation in cubic and layered TiS 2 and NiO 2 and potential jump kinetic experiments on Mn 2 O 4 :Li.