The redox chemistry of La _0.5 Sr _0.5 Cr _0.2 Mn _0.8 O _{3− δ} and its application in high capacity anodes of oxygen ion batteries

Solid-state oxygen ion batteries (OIBs) are a novel technology for electrochemical energy storage, based on the exchange of oxygen between two mixed conducting oxide electrodes via an oxide ion-conducting electrolyte. Suitable electrode materials not only require good ionic and electronic conductivity, but also a highly variable oxygen non-stoichiometry δ to chemically store large amounts of charge. Another desirable characteristic for anodes is good material stability down to very reducing oxygen chemical potentials. This work focuses on the exploration of La_0.5Sr_0.5Cr_0.2Mn_0.8O_3-δ and its electrochemical and defect chemical properties, with particular focus on its applicability in anodes of oxygen ion batteries. Thin film model cells were prepared by pulsed laser deposition (PLD) of electrodes on 100-oriented Y:ZrO₂ single crystals. These planar half-cells were sealed with ZrO₂ and glass to inhibit oxygen exchange with the atmosphere. Electrode capacities of up to 930 mAh cm^-3 were achieved and confirmed to be stable over more than 70 cycles at 400 °C between -0.07 V and -2.07 V vs. 1 bar O₂. Charge/discharge curves revealed the existence of two plateaus at -0.8 V and -1.4 V. Further, electrochemical impedance measurements on samples with microelectrodes were employed to study the chemical capacitance C _chem, oxygen diffusion coefficient, and ionic resistivity of La_0.5Sr_0.5Cr_0.2Mn_0.8O_3-δ over the same range of potentials. High resolution C _chem vs. oxygen chemical potential measurements revealed two clearly separated peaks, indicating two separate redox processes, which correspond to the two distinct plateaus found in the charge/discharge curve. A defect chemical model (Brouwer diagram) was developed, based on a two stage transition: Mn⁴⁺ → Mn³⁺ → Mn²⁺. The model can quantitatively explain the location of both peaks in the chemical capacitance curve and the corresponding plateaus of the charge/discharge curve. Furthermore, X-ray photoelectron spectroscopic measurements of the Mn³⁺ → Mn²⁺ transition fully confirmed this model. Altogether, this study showed that La_0.5Sr_0.5Cr_0.2Mn_0.8O_3-δ is a highly promising anode material for oxygen ion batteries operating at high voltages.