What controls electrostatic vs electrochemical response in electrolyte-gated materials? A perspective on critical materials factors

Electrolyte-gate transistors are a powerful platform for control of material properties, spanning semiconducting behavior, insulator-metal transitions, superconductivity, magnetism, optical properties, etc. When applied to magnetic materials, for example, electrolyte-gate devices are promising for magnetoionics, wherein voltage-driven ionic motion enables low-power control of magnetic order and properties. The mechanisms of electrolyte gating with ionic liquids and gels vary from predominantly electrostatic to entirely electrochemical, however, sometimes even in single material families, for reasons that remain unclear. In this Perspective, we compare literature ionic liquid and ion gel gating data on two rather different material classes - perovskite oxides and pyrite-structure sulfides - seeking to understand which material factors dictate the electrostatic vs electrochemical gate response. From these comparisons, we argue that the ambient-temperature anion vacancy diffusion coefficient (not the vacancy formation energy) is a critical factor controlling electrostatic vs electrochemical mechanisms in electrolyte gating of these materials. We, in fact, suggest that the diffusivity of lowest-formation-energy defects may often dictate the electrostatic vs electrochemical response in electrolyte-gated inorganic materials, thereby advancing a concrete hypothesis for further exploration in a broader range of materials.