Publication date: 10th April 2024
Solid state sodium-ion batteries (SIBs) are seen as potentially cheaper and safer alternatives to current lithium-ion battery systems (LIBs). The replacement of the presently used liquid electrolytes by non-flammable solid electrolytes is an important avenue to create safer batteries, while the high natural abundance of sodium compared to lithium would allow for significant cost reduction. The sodium super ionic conductor, NASICON, a solid solution of general formula Na1+xZr2SixP3-xO12, is arguably the best-known sodium ion conducting solid electrolyte system. The NASICON structure, first reported by Hong [1], generally consists of a rigid three-dimensional network of ZrO6 octahedra and corner sharing PO43−/SiO44− tetrahedra. The interconnected channels in this framework provide efficient conduction pathways for Na+ ions which are distributed over two crystallographic sites. Although pure Na3Zr2Si2PO12 exhibits a high ionic conductivity of 10−4 S cm-1 at room temperature, much effort has been dedicated over the past few decades to further improve its ionic conductivity [2-7].
Using a co-doping approach, we have successfully improved the room temperature ionic conductivity of NASICON, with the highest total conductivity of 4.68 × 10−3 S cm−1 at room temperature and 3.51 × 10–2 S cm–1 at 200 °C. Rietveld analysis of neutron diffraction data reveals Na+ distributed across four crystallographic sites in the NASICON framework (Fig. 1). While conventional Rietveld analysis of this system can reveal the average Na+ ion distribution, Reverse Monte Carlo (RMC) analysis of total neutron scattering data allows for a more detailed exploration of local structure. We present details of this local structure analysis. Co-doped NASICONs demonstrate good air stability, suggesting their potential as a promising material for applications in all solid state sodium batteries.
The authors gratefully acknowledge Queen Mary University of London and the China Scholarship Councilfor a PhD Scholarship to XH. The Science and Technology Facilities Council (STFC) is thanked for a Neutron Beam Time Award at ISIS (RB2410442).