Improved air-stability and conductivity in the 75Li₂S·25P₂S₅ solid-state electrolyte system: the role of Li₇P₃S₁₁

Doping modification is regarded as a simple and effective method for increasing the ionic conductivity and air stability of solid state electrolytes. In this work, a series of (100-x)(0.75Li₂S·0.25P₂S₅)·xP₂O₅ (mol%) (x = 0, 1, 2, 3 and 4) glass-ceramic electrolytes were synthesized by a two-step ball milling technique. Various characterization techniques (including powder X-ray diffraction, Raman and solid-state nuclear magnetic resonance) have proved that the addition of P₂O₅ can stimulate 75Li₂S·25P₂S₅ system to generate the high ionic conductivity phase Li₇P₃S₁₁. Through the doping optimization strategy, 98(0.75Li₂S·0.25P₂S₅)·2P₂O₅ glass-ceramic (2PO) not only had a 3.6 times higher ionic conductivity than the undoped sample but also had higher air stability. Its ionic conductivity remained in the same order of magnitude after 10 minutes in the air. We further investigated the reasons why 2PO has a relatively high air stability using powder X-ray diffraction and scanning electron microscopy in terms of crystal structure degradation and morphology changes. In comparison to the undoped sample, the high ionic conductivity phases (β-Li₃PS₄ and Li₇P₃S₁₁) of 2PO were better preserved, and less impurity and unknown peaks were generated over a short period of exposure time. In addition, the morphology of 2PO only changed slightly after 10 minutes of exposure. Despite the fact that the particles aggregated significantly after several days of exposure, 2PO tended to form a protective layer composed of S₈, which might allow some particles to be shielded from attack by moisture, slowing down the decay of material properties.