Preparation of CaI2-Doped Li7P3S11 by Liquid-Phase Synthesis and Its Application in an All-Solid-State Battery with a Graphite Anode

Here, (100 – x)Li7P3S11–xCaI2 (mol %) solid electrolytes (SEs) were prepared using the liquid-phase method with acetonitrile as the reaction medium. The prepared SEs showed an X-ray diffraction pattern assigned to the Li7P3S11 crystal phase containing Li4P2S6 and Li3PS4 as a secondary phase. The diffraction peaks originated from the Li7P3S11 crystal phase were shifted to the lower angle side by CaI2 doping, which indicated the Li7P3S11 crystal structure doped with Ca2+ and I–. Scanning electron microscopy–energy-dispersive spectroscopy (SEM–EDS) results showed that CaI2 was well-dispersed in the obtained SEs. Based on solid-state 31P nuclear magnetic resonance, the total area fraction of the peaks assigned to PS43– and P2S74– anions in the Li7P3S11 structure changed from 24.8% in x = 0 to 31.2% in x = 2. This observation indicated that the 98Li7P3S11–2CaI2 involved a higher purity of the Li7P3S11 crystal phase compared with the pristine Li7P3S11. CaI2 doping improved both ionic conductivity and activation energy, from 2.0 × 10–4 S cm–1 and 41 kJ mol–1 to 8.0 × 10–4 S cm–1 and 35 kJ mol–1, respectively, for Li7P3S11 prepared by liquid-phase synthesis (comparing samples with x = 0 and 2, respectively). Stability against lithium metal was also enhanced with CaI2 doping. Graphite–SE composites were also prepared by the liquid-phase method, in which natural graphite flakes were added to the SE precursor suspensions. SEM–EDS results indicated the formation of secondary particles composed of graphite flakes and SE nanoparticles. The stability of an all-solid-state cell employing the composite with x = 2 outperformed that with x = 0.