Assessing Electrochemical Stability Windows of Li_1+XAl_xM_2-X(PO₄)₃ (M=Ge,Ti) Nasicon Solid Electrolytes for Their Application in All Solid-State Lithium Batteries

All-Solid-State Lithium Batteries (ASSLBs) are a new generation of lithium batteries that are developed to meet expectations in terms of safety, stability and high energy density. The liquid electrolyte of conventional Li-ion batteries is replaced in ASSLBs by a safer and more stable solid electrolyte (SE). ASSLBs are promising because they may enable the use of high potential materials as positive electrode and lithium metal as negative electrode. This is only possible through SE stated large electrochemical stability windows (ESW). Nevertheless, values for these electrochemical windows are very divergent in the published literature. Recently, several studies have come to specifically decry the frequent overestimation of SE electrochemical stability windows 1- 4 . Establishing a robust procedure to accurately determine SEs ESW has therefore become crucial. Our work is focused on using an original experimental set up to assess the ESW of two widely investigated NASICON-type SEs Li1.3Al0.3Ti1.7(PO4)3 (LATP) and Li1.5Al0.5Ge1.5(PO4)3 (LAGP). The experimental set-up provides a large contact surface between the SE and the conductive material, which maximizes the redox current signals. A combination of Potentiostatic Intermittent Titration Technique (PITT) and Electrochemical Impedance Spectroscopy (EIS) measurements allowed us to precisely determine the ESW of LATP and LAGP solid electrolytes. Using EIS and physico-chemical characterizations, we attempted to shed light on the degradation mechanisms occurring upon the solid electrolytes’ oxidation. 1 - Y. Tian et al. Energy Environ. Sci. 2017, 10, 1150. 2 - Z. Zhang et al. Energy Environ. Sci. 2018, 11, 1945-1976. 3 - F. Han et al. Adv. Energy Mater. 2016, 6(8), 1501590. 4 - T. Schwietert et al. Nature Mat. 2020, 19, 428–435.