A study on the interfacial properties between a solid glassy electrolyte, LiI‐Li3PS4 (LPSI), and graphite (MAG) composite electrodes was carried out with the aim of reducing or even eliminating the irreversible capacity during the 1st charge‐discharge cycle. The performances of all‐solid‐state MAG|LPSI|Li cells were compared with those of conventional liquid cells. To reinforce a well‐distributed conductive path in MAG as well as at the MAG/LPSI interface, the type of electron conducting additive and the pressure during cell preparation were optimized. Specific functions of the conducting additive were demonstrated, where the sub‐micrometric carbon fibers allowed better galvanostatic performance in the solid‐state configuration by virtue of their high aspect ratio. The coulombic efficiency of solid‐state cells was improved from 46 to 99 % and the reversible capacity value from 100 to 270 mAh g−1, by increasing the pressure from 2 to 4 ton. The interfacial stability of LPSI was also evaluated by impedance spectroscopy of MAG|MAG and Li|Li cells over time. Although ionic resistance of LPSI was higher than a conventional liquid electrolyte, LPSI exhibited controlled interfacial resistance. Liquid or solid? High interfacial stability of LiI‐Li3PS4 against lithium and graphite anodes was demonstrated by means of impedance spectroscopy and galvanostatic cycling. Moreover, an electron conductive additive in anodes was revealed to have a different role in all‐solid cells compared that in conventional liquid cells.