Solid-state ionics is the core discipline that deals with the transport, storage, and reactions of ions in solids. It serves as the scientific foundation for modern lithium-ion batteries, solid-state batteries, fuel cells, and related technologies, and represents a cutting-edge interdisciplinary frontier where materials, physics, and chemistry converge. Research in solid-state ionics encompasses solid electrolyte materials, mixed ionic-electronic conductors, and more. The Solid-State Ionics and Secondary Batteries Research Group at the Institute of Physics, Chinese Academy of Sciences, took the lead in China in 1978 in initiating fundamental research and technological breakthroughs on lithium batteries, thereby opening up a new field of solid-state ionics research in the country. The group organized the first national conference on fast ion conductors, promoted the establishment of the Solid-State Ionics Division of the Chinese Ceramic Society, and brought together the earliest community of solid-state ionics researchers in China. The group has achieved a series of systematic innovations in fundamental theories of solid-state ionics, key materials for lithium batteries, solid-state batteries, sodium-ion batteries, and other next-generation battery technologies. The fundamental science of solid-state ionics runs through the entire process of battery materials and technology research, laying a deep scientific foundation for China’s lithium battery industry to advance from catching up to leading, and from basic research to engineering applications.