Influence of Chloride Ion Substitution on Lithium-Ion Conductivity and Electrochemical Stability in a Dual-Halogen Solid-State Electrolyte

Li⁺ conducting halide solid-state electrolytes (SEs) are developing as an alternative to contemporary oxide and sulfide SEs for all-solid-state batteries (ASSBs) due to their high ionic conductivity, excellent chemical and electrochemical oxidation stability, and good deformability. However, the instability of halide SEs against the Li anode is still one of the key challenges that need to be addressed. Among halides, fluorides have shown a wider electrochemical stability window due to fluoride's high electronegativity and smaller ionic radius. However, the ionic conductivity of fluoride-based SEs is lower compared to other halide-based SEs. To achieve better interface stability with the Li anode, the presence of fluoride is not only advantageous for a wider potential window but also forms a stable passivation layer at the Li/SEs interface. Therefore, developing mixed halogen-based solid electrolytes, particularly fluorine and chlorine-based SEs are promising in ASSBs. Herein, we report dual halogen-based SEs, Li₂ZrF_6-xCl_x (0 ≤ x ≤ 2), synthesized via ball-milling. The X-ray diffraction results revealed that Li₂ZrF_6-xCl_x compounds crystallize in the trigonal phase (P3̅1m). Using impedance spectroscopy, an increase in Li⁺ conductivity with the increase in Cl content was observed for Li₂ZrF_6-xCl_x. Compared with x = 0, Li⁺ conductivity for the sample with x = 1 improved by ∼5 orders of magnitude. The Li⁺ conductivities for Li₂ZrF₅Cl₁ at 25 and 100 °C are 5.5 × 10^-7 and 2.1 × 10^-5 S/cm, respectively. Moreover, Li₂ZrF₅Cl₁ exhibits the widest electrochemical stability window and excellent Li interface stability. Our work indicates Li₂ZrF_6-xCl_x as an attractive material for optimization in the class of halide-based solid-state Li-ion conductors.