Enhanced Ion Conduction in Li2.5Zn0.25PS4 via Anion Doping

Abstract

Li1+2xZn1–xPS4 was computationally predicted to have superionic conductivity of over 50 mS/cm. However, experimental efforts so far have only yielded ionic conductivities on the order of 10–4 S/cm, due to difficulty in producing phase-pure crystalline products. Here, to improve phase purity and enhance ionic conductivity, Cl- doping is employed to synthesize Li1+2x–yZn1–xPS4–yCly, which is shown to stabilize Li1+2xZn1–xPS4 from decomposing to Li3PS4. Li2.5Zn0.25PS4 which shows the highest ionic conductivity among the Li1+2xZn1–xPS4 samples is chosen as an example to study. After incorporating only a small amount of Cl, the ionic conductivity increases from 0.6 mS/cm for Li2.5Zn0.25PS4 to 3.5 mS/cm for Li2.4Zn0.25PS3.9Cl0.1 at 25 °C. In addition, the activation energy is reduced from 0.33 to 0.27 eV. The phase purity and fine structure of Li1+2x–yZn1–xPS4–yCly are determined with XRD, and 7Li, 31P, 35Cl solid-state NMR characterizations. The experimental data confirm the success of Cl– doping. In addition, Li2.4Zn0.25PS3.9Cl0.1 exhibits a low electronic conductivity of 1.21 × 10–9 S/cm and a wide electrochemical stability window.