When an ion can exist in multiple charge states, it is possible that polaron formation can occur. This describes a process whereby a charge carrier induces a polarization in the lattice which, in turn, causes a charge carrier to become trapped on a specific ion as a polaron; the charge carrier has effectively trapped itself. Even though the charge carriers in polarons are highly localized, it is possible for polarons to hop between sites with relatively low barriers, which may also contribute to an increase in electrical conductivity along GBs in these materials. In the case of Li 3 OCl and Li 3 PS 4 , the strong hole localization around the ions in the equilibrium geometry indicates that there may be some tendency towards forming polarons where a hole traps on either a O 2 -or S 2 -to form a O -or S -, respectively; evidence of polaron formation is reported across a wide variety of oxide and sulfide materials and previous theoretical studies have shown that polaron trapping can be favourable in the vicinity of GBs. Whilst the band edges in Li 2 OHCl are localized in a similar manner to Li 3 OCl, a polaron formed by trapping a hole on OH -to form a OH 0 is unlikely to be long-lived. Similarly, in Li 3 InCl 6 , hole polarons should not be expected to form through a hole trapping on a Cl -to form a Cl 0 , but it may be the case that electrons can trap on In 3+ to form In 2+ , which might also contribute to the formation of Li dendrites through the reaction In 2+ + Li + -→ In 3+ + Li 0 , for example. In order to determine how large of a role polaron diffusion is likely to play in leakage current at GBs of solid electrolytes, we take the case study of a hole polaron localized at the Li 3 OCl Σ3{310} GB (Figure ) and a electron polaron in Li 3 InCl 6 Σ3{112} GB (Figure ). The ionization barrier of a polaron can be estimated by calculating the potential energy surface that interpolates between the polaron geometry and the equilibrium geometry corresponding to no trap. We find that neither polaron shows any barrier to trapping, indicating that the formation of these polarons is very favourable and that band-like conduction of their respective charge carriers would be unlikely at these boundaries. Polaron hopping rates can be calculated computationally using Marcus-Emin-Holstein-Austin-Mott (MEHAM) theory. Essentially, all the information required to calculate polaron hopping rates from MEHAM theory can be obtained from the potential energy surface that interpolates between the geometry corresponding to the initial state and the geometry corresponding to the final state in which a polaron is localized on a neighbouring site; a more complete description can be found in the supplementary information. For the hole polaron in Li 3 OCl, we calculate an adiabatic activation energy of 0.30 eV and a 300 K hopping rate of 4.1 × 10 8 Hz. The electron polaron in Li 3 InCl 6 has a higher adiabatic activation energy of 0.45 eV and a correspondingly lower 300 K hopping rate of 6.4 × 10 5 Hz. Neither of these hopping rates is exceptionally high but, for the hole polaron especially, the rate is comparable that of some of the reported hopping pathways in TiO 2 which is generally considered to be a good polaronic conductor. These results highlight important considerations that need to be made when synthesizing solid electrolyte materials. Even in the absence of extrinsic dopants, a material can have free charge carriers due to 'self-doping' by intrinsic defects. For example, a recent study by Then, by analysis of defect energetics, it is determined that higher synthesis temperatures would lead to higher intrinsic conductivity in the bulk of the material due to the higher concentration of the point defects responsible for self-doping. Let us consider the example of intrinsically p-type Na 3 PS 4 . It may be tempting to reduce synthesis temperatures as far as possible to suppress the formation of acceptor defects (to reduce hole concentration and associated conductivity) but lower synthesis temperatures would also lead to smaller grain sizes and increased numbers of GBs. Given the prevalence of states above the VBM across all the materials considered in our study (especially noticeable in the closely-related sulfide Li 3 PS 4 ) we should expect that these GBs would exhibit the levels of high hole conductivity that we would seek to avoid when choosing a low synthesis temperature. It is therefore clear that there is a fine balance between the controlling the relative proportions of point defects and extended defects. In comparison, the intrinsically n-type Li 6 PS 5 Cl poses a simpler problem; GBs with states above the VBM are less deleterious in an n-type material where there are fewer holes available to fill traps, so lower synthesis temperatures, leading to a larger number of GBs, would not be as much of a concern regarding electronic conductivity. , associated with total diffusion as well as decomposed into components parallel (∥) and perpendicular (⊥) to the GB plane.