Sigma-Holes in Battery Materials Using Iso-Electrostatic Potential Surfaces

The presence of highly electronegative atoms in Li-ion batteries anticipates the formation of σ-hole regions that may strongly affect the ionic conductivity. The σ-hole consists of a region of positive electrostatic potential extending in the direction of the covalent bond between atoms of groups IV–VII due to anisotropic charge distribution. Graphite electrodes in Li-ion batteries that become halogenated due to the electrolyte, as well as some solid electrolyte materials, can exhibit these σ-holes. Since Li-ions should be able to drift in any part of the battery, the fact that they can be attracted and eventually absorbed by regions of strong negative potentials produced by high-electronegativity counterions becomes detrimental to ionic conductivity. Therefore, the presence of positive well-defined regions, repulsive to the Li-ions, might act as lubricant for Li-ions drifting through electrolytes, thus improving the Li-ion conductivity. In addition, the σ-holes might also have a strong effect on the formation of the passivating layer, known as the solid electrolyte interphase (SEI) at electrode surfaces, which is of paramount importance for the performance of rechargeable batteries. Here we investigate the existence of σ-holes on surfaces of graphite anodes and of a few solid electrolytes by examining the electrostatic potentials calculated using density functional theory.