Reactivity of Li14P6S22 as a Potential Solid Electrolyte for All-Solid-State Lithium-Ion Batteries

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 allsolid-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 Li14+xP6S22 and Li14-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.