The guidelines for fabrication and testing of all-solid-state lithium-ion batteries as well as a correction required in the lithium transference number considering the electrical percolation behavior and variation in ion transport coefficient of various components in solid polymer electrolyte are addressed in this article.

All-solid-state batteries (ASSBs) using composite polymer electrolytes (CPEs) are a promising technology for next-generation energy storage. These electrolytes combine an inorganic solid electrolyte (filler) and a lithium source dispersed in a polymer electrolyte matrix to achieve a balance between ionic conductivity and mechanical flexibility. The progress in developing material combinations for CPEs is significant; however, the fabrication and reporting of ASSBs by various laboratories considerably vary, thereby making a direct comparison of the results tedious. Recommended standard procedures are therefore necessary to align the research on ASSBs for a common goal. By analyzing various reports, a set of guidelines for the fabrication, testing, and reporting of ASSBs is proposed. Besides, ASSBs employing CPEs often suffer from rapid capacity fade, typically failing after a few hundred charge/discharge cycles. A critical issue underlying the failure of ASSBs could be an overestimation arising from the disparity between experimentally measured average ionic conductivity and transference numbers and the local ionic transport properties within the electrolyte. This perspective also discusses the limitations arising from an unrealistic averaged ionic conductivity, the role of lithium-ion transfer cross sections, and potential remedies to improve long-term battery performance. Mathematical models and experimental evidence supporting these arguments are also presented.