The phenomenon of ion migration in solids forms the basis for a wide variety of electrochemical applications, ranging from power generators and chemical sensors to ionic switches. Solid-state ionics (SSI) is the field of research concerning ionic motions in solids and the materials properties associated with them. Owing to the ever-growing technological importance of electrochemical devices, together with the discoveries of various solids displaying superior ionic conductivity at relatively low temperatures, research activities in this field have grown rapidly since the 1960s, culminating in "nanoionics": the area of SSI concerned with nanometer-scale systems. This theme issue introduces key research issues that we believe are, and will remain, the main research topics in nanoionics and SSI during the 21st century. These include the application of cutting-edge experimental techniques, such as secondary ion mass spectroscopy and nuclear magnetic resonance, to investigate ionic diffusion in both bulk solids and at interfaces, as well as the use of atomic-scale modeling as a virtual probe of ionic conduction mechanisms and defect interactions. We highlight the effects of protonic conduction at the nanometer scale and how better control of interfaces can be employed to make secondary lithium batteries based on nanoionics principles. Finally, in addition to power generation and storage, the emergence of atomic switches based on cation diffusion shows great promise in developing next-generation transistors using SSI.
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