Solid-state electrolytes are quickly rising to prominence as useful components of advanced Li battery technologies due to their excellent electrochemical stability, favorable mechanical properties, and operation over a wide temperature window. Previous investigations have resulted in multiple solid-state Li-ion conductors that exhibit favorable characteristics for application in a full electrochemical cell. A Li₁₀GeP₂S₁₂ solid-state electrolyte has been reported with conductivity rivaling that of conventional liquid electrolytes. However, the presence of Ge makes it unstable with metallic Li anodes. Despite the number of promising candidates, very few systems have been demonstrated to be successful under a full electrochemical setup as a result of interfacial kinetic limitations and electrode−electrolyte compatibility issues.

High-energy batteries use metallic Li as anode and high-voltage materials as cathode. Therefore, it is critical to develop suitable solid electrolytes with high ionic conductivity and excellent chemical stability not only against the Li anode but also at higher voltages, to facilitate high-voltage cathodes and guard against cell abuse.

—Rangasamy et al.

β-Li₃PS₄ shows the requisite characteristics, but it forms a buffer layer with the Li metal anode to produce the observed stability. Lithium halides have been shown to enhance ionic conductivity, but have been shown also to exhibit instability. The researchers sought to determine if it possible to incorporate LiI (a lithium halide) into a solid-state Li-ion conductor to enhance ionic conductivity while simultaneously eliminating the inherent oxidation of LiI (and resulting instability) and its low ionic conductivity.

They found that mixing LiI with Li₃PS₄ and applying a subsequent heat treatment, resulted in the formation of Li₇P₂S_8I—a new phase different from either parent.

In addition to the superior ionic conductivity, the new phase exhibits an electrochemical stability higher than that of state-of-the-art garnet electrolytes (9V vs Li/Li⁺ and typical sulfide electrolytes (5 V vs Li/Li⁺).

The presence of I enhances the stability of the electrolyte with metallic Li anode while demonstrating low charge-transfer resistance. These characteristics form a foundation that allows the electrolyte to exhibit excellent cycle life and stability at ambient conditions. The material property of the electrolyte allows low-temperature densification and enhanced processability, which is vital to developing industrial-scale solid electrolyte membranes. Currently investigations are underway to identify the crystal structure and mechanism of Li-ion conduction in the newly formed phase along with polymeric reinforcement for flexible solid electrolyte membranes. This opens new avenues for the development of inherently safe all-solid Li batteries.

—Rangasamy et al.

Resources

• Ezhiylmurugan Rangasamy, Zengcai Liu, Mallory Gobet, Kartik Pilar, Gayatri Sahu, Wei Zhou, Hui Wu, Steve Greenbaum, and Chengdu Liang (2015) “An Iodide-Based Li₇P₂S₈I Superionic Conductor” Journal of the American Chemical Society 137 (4), 1384-1387 doi: 10.1021/ja508723m