Limits on Topotactic Transformation Speed in Electrolyte-Gate La_0.5Sr_0.5CoO_3-δ Electrochemical Transistors

Voltage-driven electrochemical cycling between fully oxygenated perovskite (P) and oxygen-vacancy-ordered brownmillerite (BM) phases is now well established in electrolyte-gated complex oxides such as perovskite cobaltites (e.g., La_0.5Sr_0.5CoO_3-δ (LSCO)), enabling exceptionally wide-range reversible modulation of electronic, magnetic, thermal, and optical properties. Moving toward applications of such topotactic electrochemical transistors, progress has recently been made with cycling endurance, but the limits on operating speed remain poorly understood. We address this here in ion-gel-gate transistors based on epitaxial LSCO films, using comprehensive source-drain and gate current measurements in both frequency and time domains to assess the impact of gate voltage, side- vs top-gate geometry, and ion gel and LSCO thickness. We first establish how to rigorously define switching times in such transistors, emphasizing the inherent trade-off between ON/OFF ratio and speed. We then show unambiguously that room-temperature switching of these devices is limited by oxygen diffusion in the LSCO, not electric double layer formation in the electrolyte. Under optimized conditions with 10-unit-cell-thick P LSCO films, we thus achieve <1 s to BM formation, ∼40 s to phase-pure BM, and ∼300 s to a 5 × 10⁴ source-drain current ON/OFF ratio, orders of magnitude improved over prior work. These times scales are analyzed in terms of oxygen diffusivities in the P and BM phases of LSCO, highlighting the clear role of the phase transformation and generating critical insight into routes to improved switching speed.