ZrCl4 co-doping effects in Li6PS5Cl argyrodite solid electrolytes for enhanced ionic conductivity and stability

Abstract

The argyrodite-type Li₆PS₅Cl solid electrolyte was modified by ZrCl₄ co-doping to yield the series Li_₆−₃xP_₁−xZr_xS_₅−₄xCl_1+4x (0.00 ≤ x ≤ 0.06). The dual role of Zr⁴⁺ as a P-site substituent and of Cl⁻ as an additional halide source was exploited to simultaneously introduce Li-vacancy carriers and amplify Cl⁻/S²⁻ site disorder. X-ray diffraction analysis confirmed retention of the cubic argyrodite $F\overline{4}3m$ 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 LiNbO₃-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 cm³ 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 ZrCl₄ as a cost-effective, earth-abundant dopant precursor for enhancing multiple functional properties of argyrodite electrolytes concurrently.