Crystallographic Dimensionality Determines the Electrochemical Reaction Mechanism in Alkali Transition-Metal Chlorides

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 A₂MCl₄ compounds (A = Li or Na and M = Cr, Mn, Fe, and Co). Comparing four lithiated compounds, we demonstrate that they all undergo conversion reactions when reacting with 2 Li⁺ per formula unit, associated with large polarization and limited cycling ability. Nevertheless, combining in situ XRD with post-mortem XPS and STEM/EDS analysis, we demonstrate that Li₂CoCl₄ 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 Li₆CoCl₈ intermediate, which shares a similar anionic framework with pristine Li₂CoCl₄, ensuring the topotactic insertion of Li⁺ balanced by the Co²⁺/Co⁰ redox couple and the formation of metallic Co nanoparticles. Comparing these compounds, we propose that two criteria are necessary to trigger the displacement reaction in A₂MCl₄ compounds: the presence of 1D chains of edge-sharing octahedra to favor electronic delocalization and the availability of a metal-deficient intermediate. Screening numerous A₂MCl₄ compounds, we demonstrate the universality of these design principles, which extend to Na-ion materials by demonstrating a low-polarization, reversible displacement reaction for Na₂MnCl₄.