LiF has potential applications in developing intensive current negative-ion sources and neutral particle detection in space due to its wide band gap. Charge transfer processes on a LiF(100) surface have been studied for 10.5-22.5 keV negative oxygen and chlorine ions under a grazing angle of incidence. In contrast with previous studies, the electron loss of the projectile has been obviously observed. The most striking experimental result is that the positive and negative ion fractions non-monotonically vary with increasing incident angle in the scattered beam. The positive-ion production is explained by the molecular orbital promotion mechanism. The nonmonotonic positive-ion fraction is well described by the Landau-Zener formula. Besides the complete destruction from the anion sites of the insulator surface, the channel of negative-ion destruction from the cation sites has been suggested and considered in the model calculation to understand the electron loss process. It demonstrates that incident angle is a sensitive parameter for the negative ion conversion during grazing ion surface scattering.