Ionotronics is an emerging technology that exploits the coupled ionic and electronic character of materials, including the profound modifications of structure, composition, and properties achievable through external fields. While the geometry of the device and the medium of the electric field can vary, a key commonality is the importance of interfaces and the ability of ions to move along and across them. As the electronic conductivity of many metal oxides can change significantly with the oxygen vacancy concentration, understanding defect formation and migration in such materials has been the focus of many recent studies, particularly for correlated electron systems. However, the incorporation of small ions such as hydrogen or lithium can be equally or more effective depending on the material and its interface: the key is whether or not the process can be fully reversed over many cycles without interfacial degradation (much like for energy storage systems). For most device applications, the switching process should require low power and take place at high speeds, even at room temperature.

The field of ionotronics is highly interdisciplinary, involving materials science, surface science, solid state chemistry, electrochemistry, and correlated electron physics among others. In this Special Topics issue, we have asked experts in the field for their contributions, and as a result, the wide array of possible ionotronic devices are on display, ranging from electrochemical transistors and vertically aligned nanostructures to long-term data storage approaches. As may be deduced from these publications, there remains a wealth of challenges, in particular, gaining insight into the structure and behavior of electrified solid/solid, solid/liquid, and solid/gas interfaces, as well as their interactions with charged species. Understanding the fundamental science behind these field-driven diffusional and transformation processes could benefit many areas, from low-power electronics and novel sensing/energy devices to applications in neuromorphic computing. We hope that the articles here help to inspire further work in this rapidly developing field.