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 (e.g. chlorides, bromides, and iodides) unexplored due to their heightened solubility in classical liquid electrolytes. A significant knowledge gap for the electrochemical reactivity of transition metal halides remains, as well as a limited chemical library by which the iono-covalent bonding character in energy storage materials can be tuned. Recently, our group provided the first demonstration of electrochemical lithium ion insertion in TM halides (VX₃; X = Cl, Br, I) using superconcentrated electrolytes that suppress the solubility of halides, opening new chemistries for intercalation compounds. In this work, we employ superconcentrated electrolytes to demonstrate the composition- and structure-dependent electrochemical reactivity of a new family of halide compounds with A₂MCl₄ stoichiometry (A = Li or Na and M = Cr, Mn, Fe and Co) that were previously never studied in liquid electrolytes. Comparing four lithiated compounds with different transition metals, we demonstrate that they all undergo a conversion reaction when reacting with 2 Li⁺ per formula unit, the reaction being associated with large polarization and limited cycling ability. However, electrochemical studies reveal a unique low polarization and reversible reaction in Li₂CoCl₄. Combining in situ XRD with post-mortem XPS and TEM/EDS analysis, we demonstrate that Li₂CoCl₄ first reacts with one Li⁺ following a displacement reaction. This reaction is enabled by the formation of a Li₆CoCl₈ intermediate, which shares a similar anionic framework as 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 and availability of a metal-deficient intermediate. Screening a multitude of 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 when cycled in Na-based superconcentrated electrolyte. Overall, our work provides a broad understanding of structure-property relationships controlling the reactivity of ternary transition metal halides with alkali cations.