Understanding the dynamics of the electrolyte-gated perovskite-brownmillerite transformation in La_0.5Sr_0.5CoO_3-δ

Voltage-driven redox cycling between fully oxygenated perovskite (P) and oxygen-vacancy-ordered brownmillerite (BM) phases is now well established in electrolyte-gated perovskite cobaltite films (e.g., La_0.5Sr_0.5CoO_3-δ, LSCO), enabling exceptional reversible modulation of electronic, magnetic, thermal, and optical properties. However, reports on the dynamics of such gated transformations are scarce, likely due to the complexity of understanding the simultaneous ion migration in both the electrolyte and oxide. Here, we systematically study the switching speed of electrolyte-gated P↔BM transformations in epitaxial LSCO films. Sweep-rate-dependent gate-voltage hysteresis loops reveal a clear LSCO-thickness-dependent trade-off between switching time and ON/OFF ratio, which is only weakly dependent on side- vs. top-gate device designs. Under optimal conditions in 10-unit-cell-thick P LSCO films, we find a time to form the first BM phase of 3 s, a time to eliminate the last P phase of 60 s, and a time to hit >10⁵ ON/OFF of ~700 s. The latter times exceed estimated times for electrolyte ion migration (1 s) and O diffusion in P LSCO (<1 s), suggesting a role for slower O diffusion in the BM and mixed P/BM phases.