The Effect of Oxygen Vacancies on Surface and Bulk Charge Storage in La_0.5Sr_0.5FeO_3−δ

X-ray photoelectron spectroscopy and electron paramagnetic resonance show that increasing δ sequentially reduces Fe⁴⁺ to Fe³⁺ and Fe²⁺, collapsing ferromagnetic double-exchange clusters and electronic percolation. Complementing these experimental findings, density functional theory (DFT) calculations on La_0.5Sr_0.5FeO_3‑δ reveal a progressive suppression of the Fe 3d and O 2p partial densities of states at the Fermi level with increasing oxygen vacancy concentration, leading to a metal-to-insulator transition at δ ≈ 0.25, consistent with the observed degradation of electronic conductivity at high defect levels.

The optimal electrochemical response thus arises from a compromise between vacancy-enhanced electrolyte ions insertion and preserved electronic conductivity. These findings provide a rational basis for defect engineering in perovskite electrodes.