Cation disorder and lithium conductivity in mechanochemically synthesized chloride solid electrolytes

Abstract

Developing fast, stable, and scalable Li conductors is crucial for advancing all-solid-state batteries (ASSBs). Here, we present a rapid, one-hour mechanochemical synthesis of chloride electrolytes Li₂B_xCl₄ (B = Zn, Mg, Zr and x = 1 and 2/3) via high-energy ball milling (HEBM), achieving the targeted spinel phase without the need for any annealing steps. In Li₂ZnCl₄ electrochemical impedance spectroscopy reveals an unexpected, reversible low-temperature ionic transition at ∼75 °C, leading to a dramatic increase in total Li⁺ conductivity, from 2.95·10⁻⁹ S·cm⁻¹ at 25 °C to an extrapolated room temperature conductivity of 3.24·10⁻⁵ S·cm⁻¹ following heating to 125 °C. To elucidate the structural origins of this transition, we employ neutron powder diffraction (NPD), variable-temperature powder X-ray diffraction (PXRD), differential scanning calorimetry (DSC), and ⁶Li MAS NMR spectroscopy. We explore the stabilization of the high conducting phase via the introduction of host cation vacancies and therefore increasing the Li/B ratio based on the spinel-related compounds, Li₂Zn_1/3Zr_1/3Cl₄ and Li₂Mg_1/3Zr_1/3Cl₄, synthesized via the same one-hour mechanochemical approach. Rietveld refinement of NPD data reveals a monoclinic lattice distortion and cation disorder in both compounds, which open new Li conduction pathways. In both materials, 2–4 orders of magnitude increase of conductivity is achieved by aliovalent Zr⁴⁺-substitution compared to the undoped counterparts Li₂ZnCl₄ and Li₂MgCl₄, leading to maximum bulk conductivities up to 10⁻⁴ S·cm⁻¹ at room temperature. Notably, the investigated chloride-based solid electrolytes consist of non-critical elements and exhibit high thermal stability up to at least 190 °C which can be key for easy scalable processing. These results highlight the potential of spinel-based chloride electrolytes as candidates for next-generation solid-state battery applications, combining rapid and scalable synthesis with promising ionic transport properties.