Room temperature demonstration of a sodium superionic conductor with grain conductivity in excess of 0.01 S cm^-1 and its primary applications in symmetric battery cells

The lack of suitable candidate electrolyte materials for practical application limits development of all-solid-state Na-ion batteries. Na3+xZr2Si2+xP1-xO12 were the very first series of NASICONs discovered some 40 years ago; however, separation of bulk conductivity from total conductivity at room temperature is still problematic. It has been suggested that the effective Na-ion conductivity is ~10-4 S cm-1 at room temperature for Na3+xZr2Si2+xP1-xO12 ceramics; however using solution-assisted solid-state reaction for preparation of Na3+xZr2Si2+xP1-xO12, total conductivity of 5 × 10-3 S cm-1 was achieved for Na3.4Zr2Si2.4P0.6O12 at 25 °C, higher than previously reported for polycrystalline Na-ion conductors. Bulk conductivity of 1.5 × 10-2 S cm-1 was revealed by high frequency impedance spectroscopy (up to 3 GHz) and verified by low temperature impedance spectroscopy (down to -100 °C) for Na3.4Zr2Si2.4P0.6O12 at 25 °C, indicating further potential of increasing the related total conductivity. A Na/Na3.4Zr2Si2.4P0.6O12/Na symmetric cell showed low interface resistance and high cycling stability at room temperature. A full-ceramic cell was fabricated and tested at 28 °C with good cycling performance