Whilst cation migration is well studied, less work has been done regarding the migration of anions in crystalline structures which may be used as solid state electrolytes (SSE) in the growing field of halide (fluoride or chloride) ion batteries. The most studied materials usable as SSEs in fluoride ion batteries (FIBs) are based on either rare-earth tysonite-type structures RF 3 (R=La, Ce, Sm) or alkaline-earth fluorite-type structures MF 2 (M=Ca, Ba, Sr). Ionic conductivity in tysonite-type fluorides happens by a vacancy mechanism via intrinsic Schottky defects, whereas it occurs in the fluorite-structures due to an anionic interstitial mechanism via intrinsic anti-Frenkel pairs. By homogeneous doping, the ionic conductivity can be significantly increased, as aliovalent cation substitution creates extrinsic defects, i.e., anionic vacancies, in case of doped tysonite-type solid solutions R 1-x M x F 3-x , or interstitials, in case of doped fluorite-type solid solutions M 1-x R x F 2+x . The first rechargeable FIB employed the nanocrystalline Ba-doped tysonite La 0.9 Ba 0.1 F 2.9 as SSE with an fluoride ion conductivity of 2.8×10 -4 S cm -1 at 160°C. By employing single crystals of La 0.9 Ba 0.1 F 2.9 , a comparable ion conductivity can already be achieved at room temperature. Yet, as single crystals are difficult to implement into batteries, attempts to improve the ionic conductivity of polycrystalline materials have been made. In detailed studies on the ion conduction mechanism in FIBs, Rongeat et al. revealed, that grain boundaries lead to noticeable blocking effects in tysonite-type fluorides and that heat treatment of the polycrystalline tysonite-type SSE can significantly enhance the ionic conductivity. In contrast, due to the different ion conduction mechanism in fluorite-type materials, the ionic conductivity of the La-doped fluorite Ba 1-x La x F 2+x can be improved by increasing the number of grain boundaries via nanostructuring, as it offers additional conduction paths for the motion of vacancies along the grain boundaries, yielding an optimum ionic conductivity for Ba 0.6 La 0.4 F 2.4 of 1.9×10 show a very high ionic conductivity (10 -3 to 10 -4 S cm -1 ) at room temperature. The superionic conductivity has been related to the disorder caused by the stereo-activity of Sn 2+ lone pairs. 222 Repulsive interactions between the electron lone pair and F ions occur, leading to a depletion of F ions in the Sn 2+ containing layers of the crystal and an increase in the fluoride ion mobility within these layers. Finally, the corner-sharing network of the octahedral building blocks in halide perovskites ABX 3 can also allow facile, vacancy-mediated anion migration. By employing a K-doped CsPbF 3 perovskite (CsPb 0.9 K 0.1 F 2.9 ) an ionic conductivity of 1.23×10 -3 S cm -1 at room temperature could be achieved by Moreover, this fluoride conducting SSE exhibits an electrochemical stability window (ESW), which is significantly wider than that of BaSnF 4 or PbSnF 4 . 225 Such a desired combination of a high ion conductivity at room tem-perature and a high electrochemical stability has also been found for the chloride perovskite CsSnCl 3 (σ=3.6×10 -4 S cm -1 , width of ESW: 6.1 V) which can be used as SSE for chloride ion batteries. In this study the creation of a high amount of nanograins, lattice strain and tin vacancies during synthesis assured the structural stability of the cubic CsSnCl