ZrCl ₄ co‐doping effects in Li ₆ PS ₅ Cl argyrodite solid electrolytes for enhanced ionic conductivity and stability

The argyrodite‐type Li 6 PS 5 Cl solid electrolyte was modified by ZrCl 4 co‐doping to yield the series Li ₆−₃ x P ₁− x Zr x S ₅−₄ x Cl 1+4 x (0.00 ≤ x ≤ 0.06). The dual role of Zr 4+ as a P‐site substituent and of Cl − as an additional halide source was exploited to simultaneously introduce Li‐vacancy carriers and amplify Cl − /S 2− site disorder. X‐ray diffraction analysis confirmed retention of the cubic argyrodite structure up to x = 0.04, beyond which secondary phases appear. The optimally doped composition ( x = 0.04) achieved an ionic conductivity of 7.43 mS cm −1 at room temperature with an activation energy of 0.296 eV, compared with 0.298 eV for the undoped electrolyte. Field‐emission scanning electron microscopy with energy‐dispersive X‐ray spectroscopy confirmed uniform Zr distribution within particles sized 2–5 μm, with no evidence of elemental segregation. Galvanostatic charge–discharge cycling of LiNbO 3 ‐coated NCM811|electrolyte coin cells demonstrated an initial discharge capacity of 183.8 mAh g −1 and a capacity retention of 97.3% after 50 cycles for x = 0.04, substantially exceeding the undoped counterpart (109.3 mAh g −1 after 50 cycles, 71.6% retention). Cyclic voltammetry revealed suppression of irreversible anodic peaks at high voltage upon doping. Moisture‐exposure tests at 25°C and 10% relative humidity showed a marked reduction in H₂S evolution from 0.496 to 0.177 cm 3 g −1 for x = 0.04 versus x = 0, attributed to stronger Zr–S bonding and, presumably, the formation of a Cl‐rich surface passivation layer. These results establish ZrCl 4 as a cost‐effective, earth‐abundant dopant precursor for enhancing multiple functional properties of argyrodite electrolytes concurrently.