Reactivity of Li_(14)P6S_(22) as a Potential Solid Electrolyte for All-Solid-State Lithium-Ion Batteries

摘要

The electrochemical reactivity of Li_(14)P6S_(22) (Li7P3S_(11)) as a sulfur-based solid electrolyte for Li~+ conduction was evaluated by electrochemical cell tests and ah initio calculations to determine its utility for all-solid-state lithium secondary batteries. Reversible removal and incorporation of lithium into Li_(14)P6S_(22) with a gradient of lithium concentration was confirmed as thermodynamically unfavorable. Otherwise, reductive/oxidative decomposition of Li_(14)P6S_(22) by the addition/removal of lithium was thermodynamically favorable. The electrochemical stability window (ESW) of Li_(14)P6S_(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)P6S_(22) and Li_(14-x)P6S_(22) could be simultaneously achieved with reductive and oxidative decomposition by applying negative and positive over-potentials. The exposure of Li_(14)P6S_(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)P6S_(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)P6S_(22) might originate from temporal (kinetic) and endurable formation of Frenkel defects resulting in a Li-deficient/excess composition of Li_(14)P6S_(22).