The chemical stability of the system at hand was analysed based on the concept of the energy above hull Ehull. This entity describes the energetic difference of a certain atomic configuration (i.e. a material) to the energetically most favourable configuration with the same stoichiometry and is thus a computational measure for thermodynamic stability. Ehull = 0 eV corresponds to a thermodynamically stable compound whereas Ehull > 0 eV indicates instability. Still, due to inaccuracies of DFT and metastability, one must take these numbers with caution. Often compounds are found to be stable for values of Ehull up to about 100 meV/atom. Potential allotropes and decomposition products were taken from a database of roughly 115000 compounds, which comprises the convex hull (i.e. all compounds with Ehull = 0 eV) of the entire chemical space. More details on the calculational procedure can be found in a previous publication, the only difference being the underlying database. Note that the determination of the stability requires the choice of the appropriate reference system. Typically, the reference point for the chemical potential of chlorine 𝜇 𝐶𝑙 at standard conditions is gaseous Cl2. However, in a chloride ion battery the appropriate reference is given by the LiCl formation at the anode side which becomes further modified by the electrode potential φ of the battery. Effectively, the chemical potential of chlorine is given by 𝜇 𝐶𝑙 = 𝜇 𝐶𝑙 𝐿𝑖𝐶𝑙 + 𝑒𝜑, where 𝜇 𝐶𝑙 𝐿𝑖𝐶𝑙 is the chemical potential of chlorine in LiCl which is shifted by 4.27