In the context of global decarbonization, rechargeable alkali-ion batteries have become one of the key enabling technologies for sustainable energy storage. All-solid-state batteries (ASSBs) that incorporate inorganic solid electrolytes in place of flammable liquid electrolytes have drawn considerable attention due to their potentially improved safety, and high power density. However, there are significant challenges in developing solid electrolytes for ASSBs, especially those related to the interfacial instability with alkali-metal anodes. The interfacial stability in ASSBs largely depends on the formation and chemical and electronic properties of solid-electrolyte interphases (SEIs), which has been a central topic of battery research. Interfaces formed between solid electrolytes and electrodes can be classified into three types according to the formation and properties of the SEIs: (1) stable interfaces with no decomposition SEIs, (2) interfaces formed with SEIs that conduct both electrons and alkali ions, and (3) interfaces formed with stable SEIs that are electronically insulating. Insulating SEIs ensure that the interface is electronically passivated, i.e., it prevents progressive chemical decomposition of the electrode and electrolyte. Despite their importance, detailed characterizations of the composition, morphology, and spatial distribution of SEIs is still limited due to metrological challenges. The characterization of the electronic transport in SEIs is even more challenging, and is inferred only indirectly from devicelevel electrochemical measurements.