The electrochemical reactivity of Li₁₄P₆S₂₂ (Li₇P₃S₁₁) as a sulfur‐based solid electrolyte for Li⁺ conduction was evaluated by electrochemical cell tests and ab initio calculations to determine its utility for all‐solid‐state lithium secondary batteries. Reversible removal and incorporation of lithium into Li₁₄P₆S₂₂ with a gradient of lithium concentration was confirmed as thermodynamically unfavorable. Otherwise, reductive/oxidative decomposition of Li₁₄P₆S₂₂ by the addition/removal of lithium was thermodynamically favorable. The electrochemical stability window (ESW) of Li₁₄P₆S₂₂ was 0.429 V between 1.860 and 2.289 V (Li/Li⁺). The lowest potential of Li elimination was 2.289 V and occurred as oxidative decomposition. The highest potential of lithium addition was 1.860 V as reductive decomposition. Formation of Li_14+xP₆S₂₂ and Li_14−xP₆S₂₂ could be simultaneously achieved with reductive and oxidative decomposition by applying negative and positive over‐potentials. The exposure of Li₁₄P₆S₂₂ electrodes to positive and negative electric fields generated a large amount of irreversible specific capacity, which confirmed the oxidative and reductive decomposition. Considering the results of ab initio calculations on ESW and electrochemical cell tests, Li₁₄P₆S₂₂ material should be protected from direct contact to the potential of cathode and anode so that it can appropriately serve as a solid electrolyte. The high Li⁺ conductivity of Li₁₄P₆S₂₂ might originate from temporal (kinetic) and endurable formation of Frenkel defects resulting in a Li‐deficient/excess composition of Li₁₄P₆S₂₂.