Intense research efforts on transition metal chalcogenides (oxides and sulfides), pnictides (nitrides and phosphides) and fluorides have demonstrated the complex, intertwined effects of structural and chemical changes on their electrochemical response leading to intercalation, conversion or displacement reactions when reacting with lithium. Prior efforts largely left halides unexplored due to their heightened solubility in classical liquid electrolytes. In this work, we employ superconcentrated electrolytes to demonstrate the composition-and structure-dependent electrochemical reactivity of A 2 MCl 4 compounds (A = Li or Na and M = Cr, Mn, Fe and Co).

Comparing four lithiated compounds with different transition metals, we demonstrate that they all undergo conversion reaction when reacting with 2 Li + per formula unit, the reaction being associated with large polarization and limited cycling ability. Nevertheless, combining in situ XRD with post-mortem XPS and HRTEM analysis, we demonstrate that Li 2 CoCl 4 first reacts with one Li + following a displacement reaction providing a reversible capacity of 125 mAh g -1 . This reaction is enabled by the formation of a Li 6 CoCl 8 intermediate, which shares a similar anionic framework as pristine Li 2 CoCl 4 , ensuring the topotactic insertion of Li + balanced by the Co 2+ /Co 0 redox couple and the formation of metallic Co nanoparticles. Comparing these compounds, we propose that two criteria are therefore necessary to trigger the displacement reaction in A 2 MCl 4 compounds: the presence of 1D chains of edge-sharing octahedra to favor electronic delocalization and availability of a metal-deficient intermediate. Screening a multitude of A 2 MCl 4 compounds, we demonstrate the universality of these design principles which extend to Na-ion materials by demonstrating a low-polarization, reversible displacement reaction for Na 2 MnCl 4 when cycled in Na-based superconcentrated electrolyte. Overall, our work provides a broad understanding of structure-