Reactivity of Li 14 P 6 S 22 as a Potential Solid Electrolyte for All‐Solid‐State Lithium‐Ion Batteries

The electrochemical reactivity of Li 14 P 6 S 22 (Li 7 P 3 S 11 ) 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 Li 14 P 6 S 22 with a gradient of lithium concentration was confirmed as thermodynamically unfavorable. Otherwise, reductive/oxidative decomposition of Li 14 P 6 S 22 by the addition/removal of lithium was thermodynamically favorable. The electrochemical stability window (ESW) of Li 14 P 6 S 22 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+ x P 6 S 22 and Li 14− x P 6 S 22 could be simultaneously achieved with reductive and oxidative decomposition by applying negative and positive over‐potentials. The exposure of Li 14 P 6 S 22 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, Li 14 P 6 S 22 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 Li 14 P 6 S 22 might originate from temporal (kinetic) and endurable formation of Frenkel defects resulting in a Li‐deficient/excess composition of Li 14 P 6 S 22 .