Preface for Special Topic: Ionotronics

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