In this study, the ionic conductivity of 77.5Li(2)S center dot 22.5P(2)S(5) and 77.5Li(2)S center dot(22.5-x)P2S5 center dot 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 P2O5 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 P2O5, 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 center dot(22.5-x)P2S5 center dot xP(2)O(5) (mol %) electrolytes with P2O5 addition of x < 9 display longer cycling performance compared to cells with pure Li2S center dot P2S5 SSEs (x = 0), indicating the suppression of dendrite growth with the addition of P2O5. The reduced dendritic lithium growth is attributed to higher ionic conductivity and better compaction of the electrolyte pellets with P2O5 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 center dot(22.5-x)P2S5 center dot xP(2)O(5) glassy solid-state electrolytes (SSEs) in solid-state battery applications. (C) 2019 The Electrochemical Society.