Synthesis. LNCl-I: The synthesis precursors were LiCl (Sigma-Aldrich, 99%) and Li3N (Sigma-Aldrich, >99.5%). Stoichiometric amounts of the precursors with 10% wt. excess Li3N were milled in a planetary ball mill (Jar: ZrO2 , 45 mL) with 10 mm ZrO2 balls and a ball/powder ratio of 13 at 270 rpm for 4. BM was used as the SE because it is 10 times more conductive than LNCl-I so that voltage drops in the SE becomes negligible (<0.01 V) at the current used 0.5 µA (0.64 µA cm -2 ). Conductivity measurements at different temperatures for Arrhenius fits: SS|LNCl|SS cells were kept at 30°C for 1h, then heated in 5 min to 50°C and kept at this temperature for 30 min followed by heating to 60°C in 5 min and maintaining the temperature for 30 min. This procedure was continued up to 100°C. The EIS obtained at the end of the 30 min temperature-plateaus were used for Arrhenius fits. Computational details. All DFT calculations were performed with the Vienna ab-initio simulation package VASP with computational settings consistent with those used in the Materials Project database. Calculations were done on a 2x2x2 LNCl supercell. Because of the partial occupancies in LNCl different atomic arrangements were generated. Starting point was an initial guess of a 2x2x2 supercell with all Li positions occupied (real supercell stoichiometry Li64N11Cl21) the N/Cl arrangement was optimized by minimizing the electrostatic energy using the OrderDisorderStandardTransformation tool as implemented in pymatgen. Subsequently, for the 20 arrangements with the lowest electrostatic energy, 20 further supercells were generated by removing Li, again minimizing the electrostatic energy with the PartialRemoveSpeciesTranfomation tool as implemented in pymatgen. From this pool of 400 supercells, the 40 supercells with the lowest electrostatic energy were relaxed with DFT. Additionally >10 000 supercells were generated randomly. The 40 "random" supercells with the lowest electrostatic energy were relaxed with DFT. The supercell with the lowest internal energy as calculated by DFT was taken and the N/Cl arrangement was re-optimized minimizing the electrostatic energy and the 40 supercells with the lowest electrostatic energy were relaxed with DFT. Of all supercells generated, the supercell with the lowest internal energy as calculated by DFT was adopted as the model supercell. The stoichiometry of the supercells was always Li4.82NCl1.91 (Li53N11Cl21) and approximates the Li5NCl2 (Li53.333N10.666Cl21.333) stoichiometry as good as possible given the supercell-size constraints in AIMD simulations. The electrochemical stability window and the pseudo-binaries were calculated as described in the work by Zhu and Mo. For the pseudo-binaries, the SE phase compositions with energies above the hull set to zero were added to the materials project phase space. Subsequently ED, mulutal as defined by Zhu and was calculated between LNCl and respective SEs. AIMD simulations were done in the NVT ensemble and with 2 fs time steps. The kpoint grid used was 1x1x1 and the energy cutoff was 400 eV. For the AIMD simulations the Li pseudopotential was changed from Li_sv (which was used for relaxations) to Li as this enables the of a lower energy cutoff. The simulation time was > 200 ps for every AIMD simulation and the error on the obtained diffusivities was estimated as done by He and Mo. The dissection of AIMD simulations into individual jump events and subsequent analysis of the prefactor frequency, jumpdiffusivity, jump frequencies and individual Ea, Jump values was done as first described by de Klerk and Wagemaker; 28 a comprehensive account can be found in ref 28 but crucial aspects for the understanding of the reported data is presented here. Calculation of Ea, jump values between two sites: The sites are defined around the 0 K equilibrium positions of the Li ions. At every simulation step it is recorded in which site each Li ions is located or whether it is currently between two sites. From this information the jump frequency between two site vA->B can be calculated according to equation 8: