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

Abstract

The lack of suitable candidate electrolyte materials for practical application limits the development of all-solid-state Na-ion batteries. Na_3+xZr₂Si_2+xP_1−xO₁₂ was 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 a solution-assisted solid-state reaction for preparation of Na_3+xZr₂Si_2+xP_1−xO₁₂, a total conductivity of 5 × 10⁻³ S cm⁻¹ was achieved for Na_3.4Zr₂Si_2.4P_0.6O₁₂ at 25 °C, higher than the values previously reported for polycrystalline Na-ion conductors. A 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 the 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.