Enhanced Ion Conduction in Li_2.5Zn_0.25PS₄ via Anion Doping

Li_1+2xZn_1–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 Li_1+2x–yZn_1–xPS_4–yCl_y, which is shown to stabilize Li_1+2xZn_1–xPS₄ from decomposing to Li₃PS₄. Li_2.5Zn_0.25PS₄ which shows the highest ionic conductivity among the Li_1+2xZn_1–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 Li_2.5Zn_0.25PS₄ to 3.5 mS/cm for Li_2.4Zn_0.25PS_3.9Cl_0.1 at 25 °C. In addition, the activation energy is reduced from 0.33 to 0.27 eV. The phase purity and fine structure of Li_1+2x–yZn_1–xPS_4–yCl_y are determined with XRD, and ⁷Li, ³¹P, ³⁵Cl solid-state NMR characterizations. The experimental data confirm the success of Cl^– doping. In addition, Li_2.4Zn_0.25PS_3.9Cl_0.1 exhibits a low electronic conductivity of 1.21 × 10^–9 S/cm and a wide electrochemical stability window.