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We here report that a newly discovered superior oxide-ion conductor Sr3−3xNa3xSi3O9−1.5x (x = 0.45) (SNS) demonstrates full potential to be a practical solid electrolyte for intermediate temperature-solid oxide fuel cells (IT-SOFCs). It exhibits the highest oxide-ion conductivity with the lowest activation energy among all the chemically stable solid oxide-ion conductors reported. The ionic conductivity is stable over a broad range of partial pressures of oxygen (10−30 to 1 atm) for an extended period of time. A SOFC based on a 294 μm thick SNS-electrolyte produces peak power densities of 431 and 213 mW cm−2 at 600 and 500 °C, respectively. Considering its competitive costs in materials and manufacturing and rare-earth free composition, SNS has great potential to become a new class of technologically and strategically important electrolytes for commercial IT-SOFCs.