Publication date: 10th April 2024
Mixed ionic electronic conducting (MIEC) oxides are frequently studied as electrode materials for solid oxide fuel or electrolysis cells. However, their ability to change stoichiometry depending on the oxygen chemical potential also enables charge storage typical for battery electrodes: By applying a current, formally neutral oxygen in the form of oxide ions and electron holes can be incorporated or released, thereby annihilating oxygen vacancies and creating electron holes or vice versa. This reversible electrochemical oxidation or reduction of the MIEC oxide can be exploited for energy storage, similar to lithium intercalation in lithium ion batteries.
In this contribution, such oxygen ion batteries are experimentally realized and extensively tested with different electrode materials. Thin film electrodes were grown by pulsed laser deposition on yttria stabilized zirconia single crystal electrolytes and sealed to inhibit oxygen exchange with the atmosphere. The charge–voltage characteristics of difference MIEC oxide electrodes — La0.6Sr0.4FeO3-δ (LSF), La0.6Sr0.4CoO3-δ (LSC), La0.9Sr0.1CrO3-δ (LSCr) and La0.5Sr0.5Cr0.2Mn0.8O3-δ (LSCrMn)— were studied by galvanostatic DC measurements at 350 to 500 °C, revealing specific electrode capacities up to 350 mAh/cm³ at half-cell potentials between +0.1 and -0.85 V vs. 1 bar O2 and excellent coulomb efficiency and cycle stability. LSCrMn electrodes even allow reversible charging down to voltages of ca. -2 V with capacities > 1000 mAh/cm3 Operando electrochemical impedance spectroscopy and in-situ synchrotron X-ray absorption spectroscopy were employed to investigate the defect chemical processes during charging and discharging of the electrodes, revealing the close relationship between the MIEC oxide defect chemistry (Brouwer diagram) and the charge–voltage characteristics of the corresponding electrode: Specifically, the MIEC oxide’s reducibility determines the electrode half-cell potential and its dopant concentration governs the electrode capacity. Electrodes with different reducibilities of the transition metal cation thus allow operation of oxide ion batteries. Full cell oxygen ion batteries with LSF cathodes and LSCrMn anodes were fabricated and operated at 350 to 400 °C with electrode related energy densities up to 250 J/cm³ (70 mWh/cm³), coulomb efficiencies >99 % and less than 1 % capacity loss after 100 charge cycles.
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