While solution-processed halide perovskite thin films caused enormous attention when used in solar cells, thick films prepared by compressing perovskite powders are considered promising candidates for the next generation of X-ray detectors. However, X-ray detectors based on such powder-pressed perovskites typically suffer from relatively high dark currents, which were attributed to be caused by ion migration. Here we show that the dark current in 800 μm thick powder-pressed MAPbI3-pellets can be reduced by a factor of 25 when using a passivated powder. The passivation was achieved by adding 1 mol% of the ionic liquid (IL) BMIMBF4 to the precursors MAI and PbI2 during the mechanochemical synthesis of the MAPbI3 powder. NMR verified the presence of the IL, and its impact on the excited state recombination dynamics was manifested in an increase in the photoluminescence (PL) intensity and a decrease in the monomolecular (trap-assisted) recombination rate, both by about one order of magnitude. By measuring the migration of a PL quenching front upon application of an electric field in a microscope, we determine an ionic diffusivity in the typical range of iodide vacancies in the non-passivated pellet. At the same time, we observe no such PL quenching front in the passivated pellet. Concomitantly, dark I–V curves are hysteresis-free, and light-soaking effects are absent, in contrast to non-passivated pellets. Thus, our work demonstrates the effect on the optical and electrical properties when passivating mechanochemically synthesized halide perovskite powders using an IL, which will facilitate the further development of powder-based perovskite X-ray detectors.