A room-temperature full-ceramic sodium-ion battery based on Na3+xZr2Si2+xP1-xO12 as sodium superionic conductor

Despite of huge efforts, the lack of suitable candidate for electrolytes still impedes the development of all-solid-state Na batteries. Na3+xZr2Si2+xP1-xO12 (-0.2 ≤ x ≤ 0.2) are the very first composition series of NASICONs discovered 40 years ago and were reported having a total Na-ion conductivity of ~10-4 S cm-1 at room temperature. In the present study, this composition series is reconsidered and the focusing range of stoichiometry has been varied from x = 0 to x = 0.6. A solution-assisted solid-state reaction method is applied for powder preparation. Surprisingly, a total conductivity of over 5 × 10-3 S cm-1 is achieved for Na3.4Zr2Si2.4P0.6O12 at 25 °C, which is the best value of all reported polycrystalline Na-ion conductors. A bulk conductivity of about 1.5 × 10-2 S cm-1 is also revealed by high frequency impedance spectroscopy up to 3 GHz for Na3.4Zr2Si2.4P0.6O12 at 25 °C. The parameters influencing the total conductivity are discussed in detail. Benefitted from the superior total conductivity of Na3.4Zr2Si2.4P0.6O12, a full-ceramic cell has been fabricated and tested at 28 °C with good cycling performance. To our knowledge, this is the first full ceramic Na-ion battery which has been operated at room temperature