With the rapid popularization and development of lithium-ion batteries, associated safety issues caused by the use of flammable organic electrolytes have drawn increasing attention. To address this, solid-state electrolytes have become the focus of research for both scientific and industrial communities due to high safety and energy density. Despite these promising prospects, however, solid-state electrolytes face several formidable obstacles that hinder commercialization, including insufficient lithium-ion conduction and surge transfer impedance at the interface between solid-state electrolytes and electrodes. Based on this, this review will provide an introduction into typical lithium-ion conductors involving inorganic, organic and inorganic–organic hybrid electrolytes as well as the mechanisms of lithium-ion conduction and corresponding factors affecting performance. Furthermore, this review will comprehensively discuss emerging and advanced characterization techniques and propose underlying strategies to enhance ionic conduction along with future development trends. Graphical

Recently, the attention to sodium-ion batteries has been refocused on large-scale energy storage applications, due to sodium's low cost and infinite abundance. Sodium is one of the most abundant elements on earth and exhibits chemical properties similar to lithium. Owing to their superior sodium storage capability especially for excellent safety and stability, Ti-based compounds have been extensively investigated as both cathode and anode materials. Herein we outline the current cathodes and anodes, and emphasize the critical roles of titanium in developing advanced electrodes for sodium-ion batteries. The latest advances and progress in the exploration of Ti-based compounds with various different frameworks such as NASICON, tunnel, MXenes, spinel, and layered structures are systematically reviewed. The straightforward linking of the structure–function–property relationship for Ti-based compounds, especially for layered Na/Ti-containing oxides, is summarized and analyzed. The titanium element plays a critical role in both positive and negative electrodes, i.e., supplying the charge transfer and high safety for anodes and greatly enhancing the structural and cycling stability for cathodes. Based on the bi-functional roles of titanium, a new concept of symmetric Na-ion cells employing layered Na/Ti-containing oxides as bipolar electrodes is proposed and realized. Symmetric Na-ion cells already offered a high voltage and withstood long time charge–discharge processes, demonstrating the practicality beyond the proof of concept. The participation of titanium in sodium-based electrode materials will greatly promote the development of room-temperature sodium-ion batteries towards stationary energy storage.

A comprehensive understanding of the challenges in zinc-electrode materials and fabrication that limit the continuous development of rechargeable zinc–air batteries.