High Metal–Insulator Topotactic Cycling Endurance in Electrochemically Gated La_0.5Sr_0.5CoO_3−δ Probed by Humidity-Dependent Operando Fourier Transform Infrared Spectroscopy

Electrochemical gating of perovskite La 0.5 Sr 0.5 CoO 3−δ (LSCO) films yields large room-temperature, low-power, nonvolatile, complex refractive index changes across a metal–insulator transition. However, the underlying topotactic perovskite to brownmillerite phase transformation has demonstrated limited cycling endurance, and the role of H 2 O in this electrochemical cycling is not well established. Here, we explore extended cycling endurance in ion-gel-gate LSCO electrochemical transistors through relative-humidity-dependent operando FTIR transmittance measurements, enabling correlation of the metal–insulator optical changes in LSCO to corresponding electrochemical features. We show that higher-humidity gating significantly lowers the threshold voltage for oxygen reinsertion into LSCO and thus improves optical property reversibility, but concurrently induces LSCO etching and Pt contact dewetting that limit prolonged cycling. With informed selection of a 15% relative humidity environment and Au device contacts, we then demonstrate sustained cycling of LSCO films between distinct metallic and insulating optical states for over 40 cycles, with sustained but dampened optical modulation to 100 cycles. These findings set a metal–insulator optical property endurance record for an electrochemically gated perovskite oxide undergoing a topotactic perovskite ↔ brownmillerite phase change, further motivate the use of ion-gel-gated LSCO films for tunable infrared photonic applications such as thermal camouflage and thermoregulation, and establish useful guiding principles to enhance cycling endurance in electrochemically tunable functional oxides.