A novel NASICON-based glass-ceramic composite electrolyte with enhanced Na-ion conductivity

NASICON type material Na1+xZr2SixP3-xO12 is considered as an ideal option of solid-state electrolyte (SSE) material for solid-state Na batteries (SSNBs). However, the ionic conductivity of the material essentially needs to be further improved. Herein, a new strategy is proposed to enhance the ionic conductivity of NASICON material by adding NaF into the precursor, improving the ionic conductivity of classical Na3Zr2Si2PO12 from 4.5 × 10−4 S cm−1 to 1.7 × 10−3 S cm−1 at 25 °C with an activation energy of 0.28 eV. Furthermore, both qualitative and quantitative analysis are conducted by employing XRD, XPS, SEM, and solid-state NMR techniques to investigate the long-range and local structures as well as compositions for the prepared Na3Zr2Si2PO12-xNaF materials. It is revealed that with the addition of NaF into the precursor, the monoclinic phase of NASICON grains gradually transforms into rhombohedral phase with in-situ formed a ‘binder-like’ glassy phase around them, indicating NaF plays an important role in the modification of grain boundary and the improvement of the ionic conductivity. Moreover, the strategy is also applied to other NASICON system, achieving the optimum conductivity of 3.6 × 10−3 S cm−1 at 25 °C with an activation energy of 0.25 eV for the Na3.2Zr2Si2.2P0.8O12-0.5NaF, which is the highest ionic conductivity among previously reported NASICON-based materials. More importantly, our work provides a simple and low-cost approach to prepare high ionic conducting NASICON materials.