  We analyze the electronic structure of lithium ionic conductors, ${\rm
Li_3PO_4}$ and ${\rm Li_3PS_4}$, using the electronic stress tensor density and
kinetic energy density with special focus on the ionic bonds among them. We
find that, as long as we examine the pattern of the eigenvalues of the
electronic stress tensor density, we cannot distinguish between the ionic bonds
and bonds among metalloid atoms. We then show that they can be distinguished by
looking at the morphology of the electronic interface, the zero surface of the
electronic kinetic energy density.
