Enhanced Sodium Dynamics in Biphasic NaSICON Solid Electrolytes

NaSICONs, or sodium superionic conductors, are promising solid electrolyte materials for Na-based all-solid-state and aqueous redox-flow battery applications. Here, the composition Na3.4Zr2Si2.4P0.6O12 has been prepared through solution-assisted microwave processing and densification through rapid induction hot pressing. The samples display remarkably high total ionic conductivities between 7 and 9 mS cm–1, competitive with liquid electrolytes. A combination of synchrotron X-ray and neutron diffraction, electrochemical impedance spectroscopy, and variable-temperature solid-state nuclear magnetic resonance studies reveals rapid Na-ion transport through the rigid skeletal framework of this solid ion conductor. Variable-temperature synchrotron X-ray diffraction reveals structural phase separation in this composition on cooling at temperatures close to 430 K with the samples at room temperature displaying a mix of monoclinic C2/c and rhombohedral R3̅c components that appear to collectively contribute to the high conductivity. The observation of very high ionic conductivity in a region of compositional space that is associated with structural instability is proposed as a design principle for superionic conduction.