In a solid-state Li ion battery, the solid-state electrolyte exists principally in regions of high externally applied potentials, and this varies rapidly at the interfaces with electrodes because of the formation of electrochemical double layers. We investigate the implications of these for a model solid-state Li ion Li|Li3OCl|C battery, where C is simply a metallic intercalation cathode. We use density functional theory to calculate the potential dependence of the formation energies of the Li+ charge carriers in superionic Li3OCl. We find that Li+ vacancies are the dominant species at the cathode while Li+ interstitials dominate at the anode. With typical Mg aliovalent doping of Li3OCl, Li+ vacancies dominate the bulk of the electrolyte, as well, with freely mobile vacancies that are only ∼10–4 of the Mg doping density at room temperature. We study the repulsive interaction between Li+ vacancies and find that this is extremely short-range, typically only one lattice constant because of local structural relaxation around the vacancy, and this is significantly shorter than pure electrostatic screening. We model a Li3OCl–cathode interface by treating the cathode as a nearly ideal metal using a polarizable continuum model with an εr of 1000. There is a large interface segregation free energy of approximately −1 eV per Li+ vacancy. Combined with the short range for repulsive interactions of the vacancies, this means that very large vacancy concentrations will build up in a single layer of Li3OCl at the cathode interface to form a compact double layer. The calculated potential drop across the interface is ∼3 V for a nearly full concentration of vacancies at the surface. This suggests that nearly all the cathode potential drop in Li3OCl occurs at the Helmholtz plane rather than in a diffuse space–charge region. We suggest that our conclusions can be generalized to other superionic conductors, as well.