The Role of Metallic Protection Layers in Extending the Stability of Nasicon Electrolytes for Solid-State Batteries

Abstract

Solid-state batteries with lithium metal anodes are promising candidates for next-generation energy storage devices with improved safety and higher energy density. However, most solid electrolytes with practical ionic conductivities are thermodynamically unstable against lithium metal. Upon interfacial contact with lithium metal, unstable solid electrolytes undergo phase transformations that can be detrimental to battery performance. Lithium aluminum germanium phosphate (LAGP) is a NASICON-type solid electrolyte with relatively high ionic conductivity (10-4 - 10-3 S/cm) and chemical resistance to air and moisture. However, LAGP reacts with lithium to form an electrically-conducting interphase. This interphase continues to propagate from the interface towards the bulk of the solid electrolyte, ultimately leading to failure by causing the solid electrolyte to fracture [1]. In this work, we find that a thin layer of a metal that does not alloy with Li can extend the cycle life of LAGP in symmetric lithium cells from 30 h to more than 800 h at moderate current densities. Interestingly, this significantly improved electrochemical stability is not due to the prevention of interphase formation. Instead, the metal layer alters the morphology evolution of the interphase, creating a uniform and compact interphase. In addition, we find that the inclusion of an electrically insulating thin film, such as Al2O3, between the metal protection layer and the solid electrolyte favors lithium plating and prevents interphase formation over short periods of time. This dual layer approach also extends the electrochemical stability to more than 1000 h during cycling. However, using only the insulator without a metal is not beneficial for electrochemical stability. These results suggest that metallic protection layers may be necessary for attaining stable cycling even if suitable electrically insulating protection layers are developed to completely prevent interphase formation. Our findings are important for enabling the use of a wider range of solid electrolyte materials in solid-state batteries. [1] J.A. Lewis, et at. Interphase Morphology between a Solid-State Electrolyte and Lithium Controls Cell Failure. ACS Energy Lett. 2019, 4, 2, 591-599