Stable GB structures are shown in Figures and were determined using the methodology outlined in the Computational Methods section which has been shown to be predictive of experimentally observed grain boundaries. Due to the large computational cost that would be incurred by very large grain boundary models, we aim to include no more than 400 atoms in our calculations. We choose tilt angles that allow for supercell lattice vectors of at least 10 Å parallel to the GB plane whilst maintaining grains around 15 Å thick. The formation energies (Figure ) for all of the considered GBs are low (0.25-0.63 Jm -2 ). The Li 3 OCl GBs have been modelled previously (with a mirror-symmetric geometry rather than with any optimized rigid-body translation as in this study) using classical potentials, where they were also found to have low formation energies. There have been no previous atomistic studies of GBs in Li 2 OHCl, Li 3 PS 4 or Li 3 InCl 6 , but the low formation energies indicate that our models are reasonable and representative. Such low formation energies also imply that these GBs would be prevalent throughout the materials in equilibrium conditions, further highlighting the critical importance of the role they play in device performance.