Introduction
   Li-ion solid electrolytes have been attracted great interest to researchers, because it could potentially replace conventional organic liquid or gel electrolytes, which are not desirable for the next generation Li-ion battery because of their flammable property. Recently, some Li-ion solid electrolytes whose conductivities exceed 10mS/cm, were reported, e.g., LGPS (Li₁₀GeP₂S₁₂) crystal with 12mS/cm [1] and Li₇P3S₁₁ glass ceramics synthesized using hot-press with 17mS/cm [2]. Despite their excellent conductivities, however, these materials are not suitable in use with Li anode because they are not stable against Li metal. Li-ion conductive glass or glass ceramics with high ionic conductivity without reaction with Li metal are eagerly desired.
   Recently, Li-argyrodite, whose structure is an analogue of chalcogenide such as the mineral Ag₈GeS₆, were reported. Rayavarapu et al. [3] revealed the high ionic conductivity of the order of 10^-4 S/cm for the crystalline Li₆PS₅X (X=Cl, Br), which is brought by the disorder of lithium and S^2-/Cl^- or S^2-/Br^-. Besides degree of disorder in crystals, the lattice volume is one of the most important parameter. In this study, we tried adding small amount of Na⁺ ions to expand the lattice volume of the Li-argyrodite and examined its ionic conductive property. 

Experiments
    We prepared the argyrodite-type glass ceramics of (Li _5.75-xNa_x)PS_4.75Cl_1.25 as a solid electrolyte using a high-energy ball-milling process. Desired amounts of Li₂S, Na₂S, LiCl and P₂S₅ were weighed and mixed in an agate mortar for about 20 min. High-energy ball milling was conducted by using a planetary ball-milling apparatus for 16.5h and the rotation speed was 380 rpm. The ball-milled powder was sealed in a quartz tube under vacuum and heated in various conditions. After heating, the quartz tube was slowly cooled down to room temperature.
   The obtained samples were characterized by means of X-ray diffraction (XRD) with Cu-Kα radiation. Simulation of the XRD data was performed using Rietveld crystal structure refinement software (Highscore plus). Electrochemical impedance (EIS) was measured using an AUTOLAB PGSTAT30 (Metrohm Autolab, Utrecht) controlled by a personal computer. Approximately 200 mg of the electrolyte powder was measured by a micro-balance, and we prepared the sample pellet by pressing at 3 tons with a die with a diameter of 13mm. Indium foils were attached to the both side of the pellets to use them as the blocking electrodes, and contained in a SUS cell. We put the cell in an incubator ESPEC TH-241 (Espec, Osaka), and measured the EIS with an amplitude of 10 mV in a frequency range of 1MHz-100mHz at 30 ºC under a normal pressure.
The cyclic voltammetry (CV) was measured with a Li/  (Li _5.75-xNa_x)PS_4.75Cl_1.25/SUS cell , where Li electrode was used either as the counter or the reference electrode and the scan range between -0.5 and 5V.

Results and Discussion
   The XRD patterns of the (Li_5.75-xNa_x)PS_4.75Cl_1.25 synthesized in the present study show that the main phases appearing in the patterns were indexed by the tetragonal crystal structure of argyrodite in space group F4-3m. Additional peaks are observed for the samples whose Na substitution levels are relatively high, i.e., x is large in (Li_5.75-xNa_x)PS_4.75Cl_1.25. These peaks were indexed by the LiCl and Li₂S phases. A continuous peak shift attributed to the argyrodite suggests the formation of the solid solutions with the composition 0<x<0.575 for (Li_5.75-xNa_x)PS_4.75Cl_1.25 . With increasing x value, the lattice volume of the argyrodite expands systematically.
   The conductivity continuously increased as the Na content increased from x = 0.0 (σ = 2.2 mS/cm) to x = 0.03 (σ = 5.6 mS/cm) in (Li_5.75-xNa_x)PS_4.75Cl_1.25. In a higher Na content above x=0.03, however, the conductivity continuously decreased. The lattice volume of the argyrodite increases systematically with increasing x value, but the amounts of LiCl and Li₂S also increases.
   In the CV, we observed solely cathodic and anodic currents corresponding to lithium deposition and dissolution. No significant currents due to the electrolyte decomposition or the redox reaction of Na⁺/Na were observed between -0.5 and 5V, indicating that this material is stable against Li metal and that the Na⁺does not contribute to the ionic current.

References
[1] Kamaya N. et al., Nature Mater. 10, 682-686 (2011)
[2] Seino Y., et al., Energy Environ. Sci. 7, 627-631 (2014)
[3] Rayavarapu P. R. et al., J. Solid State Electrochem. 16, 1807-1813 (2012)