High-temperature electrolysis using solid oxide electrolysis cells (SOECs) is an innovative technology to temporarily store unused electrical energy from renewable energy sources. However, they show continuous performance loss during long-term operation, which is the main issue preventing their widespread use. In this work, we have performed the long-term stability tests up to 1000 h under steam and co-electrolysis conditions using commercial NiO-YSZ/YSZ/GDC/LSC single cells in order to understand the degradation process. The electrolysis tests were carried out at different temperatures and fuel gas compositions. Intermittent AC- and DC- measurements were performed to characterize the single cells and to determine the responsible electrode processes for the degradation during long-term operation. An increased degradation rate is observed at 800 °C compared to 750 °C under steam electrolysis conditions. Moreover, a lower degradation rate is noticed under co-electrolysis operation in comparison to steam electrolysis operation. Finally, the post-test analyses using SEM-EDX and XRD were carried out in order to understand the degradation mechanism. The delamination of LSC is observed under steam electrolysis conditions at 800 °C, however, such delamination is not observed during co-electrolysis operation. In addition, Ni-depletion and agglomeration are observed on the fuel electrode side for all the cells.

Achieving high‐energy‐density “anode‐free” sodium batteries remains a daunting challenge due to rapid capacity decay caused by poor reversibility and uncontrolled dendritic growth on bare current collectors. To overcome these limitations, we propose a potent interfacial engineering strategy: an aluminum current collector modified with a precisely engineered atomic‐layer‐deposited ultrathin ZnF ₂ layer. We demonstrate that the ZnF ₂ coating undergoes in situ electrochemical transformation during initial sodiation, forming a dense and conformal NaF/Na–Zn interphase. The sodiophilic Na–Zn alloy markedly lowers the nucleation overpotential, promoting highly uniform Na plating across the interface. Working synergistically, the NaF component stabilizes the solid‐electrolyte interphase by regulating Na ⁺ flux. Enabled by this dual‐functioning framework, the ZnF ₂ ‐modified aluminum current collector achieves an exceptionally high average Coulombic efficiency of ∼99.76% under aggressive plating/stripping conditions (0.5 mA cm ⁻² , 1.0 mAh cm ⁻² ). Furthermore, anode‐free full cells pairing this modified current collector with high‐mass‐loading Na ₃ V ₂ (PO ₄ ) ₃ cathodes (11.6–14.3 mg cm ⁻² ) exhibit significantly improved cycling stability. This work establishes, to the best of our knowledge, the first example of atomic‐layer‐deposited ZnF ₂ on aluminum deliberately converted into a NaF/Na–Zn interphase for anode‐free sodium batteries, providing a simple and scalable strategy for high‐performance sodium‐based energy storage systems.