In this study, the ionic conductivity of 77.5Li 2 S•22.5P 2 S 5 and 77.5Li 2 S•(22.5-x)P 2 S 5 •xP 2 O 5 glassy solid-state electrolytes (SSEs) and the relationship of conductivity and lithium deposition behavior in a symmetric lithium metal cell in terms of P 2 O 5 concentration have been investigated. The mechanical milling method has been used to prepare the glassy electrolytes. By adding only a small amount (0.25 mol %) of P 2 O 5 , both the ionic conductivity and the longest cycling performance improve with no signs of short circuiting up to 54 cycles. All symmetric lithium metal cells using 77.5Li 2 S•(22.5-x)P 2 S 5 •xP 2 O 5 (mol %) electrolytes with P 2 O 5 addition of x < 9 display longer cycling performance compared to cells with pure Li 2 S•P 2 S 5 SSEs (x = 0), indicating the suppression of dendrite growth with the addition of P 2 O 5 . The reduced dendritic lithium growth is attributed to higher ionic conductivity and better compaction of the electrolyte pellets with P 2 O 5 addition. The better compaction is confirmed by examination of the pellet density of each SSE composition. The trends established by this study will help inform the optimal performance of 77.5Li 2 S•(22.5-x)P 2 S 5 •xP 2 O 5 glassy solid-state electrolytes (SSEs) in solid-state battery applications.