All-solid-state lithium batteries (ASSLB) are very promising for the future development of next generation lithium battery systems due to their increased energy density and improved safety. ASSLB employing Solid Polymer Electrolytes (SPE) and Solid Composite Electrolytes (SCE) in particular have attracted significant attention. Among the several expected requirements for a battery system (high ionic conductivity, safety, mechanical stability), increasing the energy density and the cycle life relies on the electrochemical stability window of the SPE or SCE. Most published works target the importance of ionic conductivity (undoubtedly a crucial parameter) and often identify the Electrochemical Stability Window (ESW) of the electrolyte as a secondary parameter. In this review, we first present a summary of recent publications on SPE and SCE with a particular focus on the analysis of their electrochemical stability. The goal of the second part is to propose a review of optimized and improved electrochemical methods, leading to a better understanding and a better evaluation of the ESW of the SPE and the SCE which is, once again, a critical parameter for high stability and high performance ASSLB applications.

All-Solid-State Lithium Batteries (ASSLB) are promising candidates for next generation lithium battery systems due to their increased safety, stability, and energy density. Ceramic and solid composite electrolytes (SCE), which consist of dispersed ceramic particles within a polymeric host, are among the preferred technologies for use as electrolytes in ASSLB systems. Synergetic effects between ceramic and polymer electrolyte components are usually reported in SCE. Herein, we report a case study on the lithium conductivity of ceramic and SCE comprised of Li_1.4Al_0.4Ti_1.6(PO₄)₃ (LATP), a NASICON-type ceramic. An evaluation of the impact of the processing and sintering of the ceramic on the conductive properties of the electrolyte is addressed. The study is then extended to Poly(Ethylene) Oxide (PEO)-LATP SCE. The presence of the ceramic particles conferred limited benefits to the SCE. These findings somewhat contradict commonly held assumptions on the role of ceramic additives in SCE.