Lithium fluoride thin films with various thicknesses have been grown on c-plane sapphire substrates by radio-frequency sputtering. The thin films are granular with a preferential [111] orientation of the grains. Thickness-dependent measurements allow the separation of bulk and interface conductions. The normalized conductance decreases linearly with decreasing LiF layer thickness with a negative extrapolated intercept. DC polarization, AC impedance spectroscopy and electromotive force measurement indicate depletion of lithium ion vacancies as majority charge carriers and hence a negative space-charge potential. A generalized Mott-Schottky approach within the model of heterogeneous doping fully explains the entire boundary defect chemistry.