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. Na_3+xZr₂Si_2+xP_1-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 Na_3+xZr₂Si_2+xP_1-xO₁₂ ceramics; however using solution-assisted solid-state reaction for preparation of Na_3+xZr₂Si_2+xP_1-xO₁₂, total conductivity of 5 × 10^-3 S cm^-1 was achieved for Na_3.4Zr₂Si_2.4P_0.6O₁₂ 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 Na_3.4Zr₂Si_2.4P_0.6O₁₂ at 25 °C, indicating further potential of increasing the related total conductivity. A Na/Na_3.4Zr₂Si_2.4P_0.6O₁₂/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.