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.

Secondary ion mass spectrometry (SIMS) is a powerful analytical technique for determining elemental and isotopic distributions in solids. One of its main attractions to researchers in the field of solid-state ionics is its ability to distinguish between isotopes of the same chemical element as a function of position in a solid. With enriched stable isotopes as diffusion sources, this allows self-diffusion kinetics in solids to be studied. In this article, taking oxygen isotope diffusion in oxides as our main example, we present the standard experimental method, and, subsequently, we discuss several promising developments, in particular the opportunities offered by thin-film geometries, and the investigation of inhomogeneous systems, including possible fast diffusion along grain boundaries and making space-charge layers at interfaces “visible.” These examples demonstrate that SIMS is capable of probing mass transport processes over various length scales, ranging from some nanometers to hundreds of micrometers.

Interface engineering and the study of diffusion and transport processes through and along interfacial regions play important roles in materials science and energy research. For the latter, nanostructured materials are increasingly considered to act as powerful electrodes and solid electrolytes in sustainable energy systems, such as Li ion batteries. This is due to reduced diffusion lengths achieved when going to the nanometer scale and the fact that nanocrystalline materials with an average particle size of less than about 50 nm often show an enhanced diffusivity of their charge carriers. In this article, we show examples of how solid-state nuclear magnetic resonance (NMR) spectroscopy can be used to study the diffusion parameters of Li cations located in the interfacial regions separately from those in the interior of the grains. This article will demonstrate the future challenges and perspectives of Li NMR as a powerful tool of probing dynamic properties in functional materials.

Truly proton-conducting materials would have an immense impact on sustainable energy technologies for the 21st century, through efficient fuel cells, electrolyzers, and gas-separation membranes. However, proton conduction combined with materials stability seems difficult to achieve, and some hurdles and pathways are outlined in this article. Problems, possibilities, and artifacts of transport across and along interfaces are discussed, linked mainly to space-charge layer properties and engineering of the grain-boundary core and to water in nanovoids. The importance of protons in many semiconducting functional oxides is also explained. At lower temperatures and in humid environments, the presence of protonated cation vacancies (Ruetschi defects) is predicted and is expected to play an important role in photoelectrochemistry, catalysis, and surface transport.

Novel nanoionics devices, atomic switches, have been developed using a solid-electrochemical reaction to control the formation and annihilation of the metal filament between two electrodes. The switching operation can be achieved simply by the application of a bias voltage to precipitate metal atoms in a nanogap between the two electrodes or to dissolve them onto one of the electrodes. The small size of atomic switches enables rapid switching even though atomic motion is required. They also have several novel characteristics in that they are nonvolatile, consume less power, and have a simple structure and a low on-resistance. Logic gates and 1 kbit nonvolatile memory chips have been developed using atomic switches in order to demonstrate the possibilities for improving present-day electronic devices. Their characteristics also enable the fabrication of new types of electronic devices, such as high-performance programmable logic devices that may achieve a multitude of functions on a single chip.

Fundamental advances in solid-state ionics for energy conversion and storage are crucial in addressing the global challenge of cleaner energy sources. This review aims to demonstrate the valuable role that modern computational techniques now play in providing deeper fundamental insight into materials for solid oxide fuel cells and rechargeable lithium batteries. The scope of contemporary work is illustrated by studies on topical materials encompassing perovskite-type proton conductors, gallium oxides with tetrahedral moieties, apatite-type silicates, and lithium iron phosphates. Key fundamental properties are examined, including mechanisms of ion migration, dopant-defect association, and surface structures and crystal morphologies.

During the past decade, the electrochemical properties (energy density, power capability, and cycling stability) of practical lithium (Li) batteries have been enormously improved. Surprisingly, although the knowledge exists of how to prepare excellent batteries, a detailed understanding of how they actually work is still lacking. In particular, the impact of interfaces in electrode composites is poorly understood. Here, we collect the most advanced mechanistic studies performed in our laboratory or published in recent literature and try to embed this knowledge into the well-established concepts used in solid-state ionics for many decades. In particular, we focus on the so-called perpendicular and parallel interfacial effects. We show that much of the old wisdom can be applied to batteries, although there are several important differences. We discuss, in some detail, the effects of charge incorporation, electronic interphase contacting, electrode porosity, and heterogeneous doping in selected advanced electrode materials and emphasize the future perspectives.

The unique structural, electronic, and mechanical properties of single-walled carbon nanotubes (SWNTs) have opened the doors to developments that push the limits of science. These advancements not only further scientific discovery, but also result in the development of everyday practical applications. These applications vary from singlemolecule sensors to nano-scaled transistors to multi-modal biosensors. This article focuses on three distinct developments made as a result of recent advances in spectroscopy of SWNTs. The first system examines the use of SWNTs for molecular detection using near-infrared light to produce tunable fluorescent sensors that are highly photostable. The second system examines the use of a 4-hydroxybenzene diazonium reagent to sort SWNTs based on electronic structure to create on-chip modifications of nano-electronic devices. The third system characterizes nanotube networks for such applications as flexible electronics, exploring the irreversible binding of adsorbates onto nanotube networks using electrical transport and Raman spectroscopy.