Exploring the Effect of Phosphorous Deficiency on Ionic Conductivity in NaSICON-Type Solid-Electrolytes derived from Flame-Made Nanoparticles

The battery market has recently shown that Na-ion batteries (SiB) have become a serious alternative to Li-ion batteries (LiB) [1, 2]. However, SiBs exhibit inferior energy density compared to their lithium-ion-based counterparts [3]. For further improving the energy densitv and safety of SiBs, solid-state SiBs (SSiBs) have gained a lot of interest and NaSICON-type solid-electrolytes are promising materials for such an application owing to their high ionic conductivity of 10^-4–10^-3 Scm^-1 at room temperature and chemical as well as mechanical stability [4]. Recent studies revealed that the conductivity of Na₃Zr₂Si₂PO₁₂ (NZSP) can be enhanced by increasing the silicon to phosphorous ratio [5] or doping NZSP with divalent cations [6].<br/>In this work, we present the spray-flame synthesis (SFS) of NaSICON nanoparticles as a facile method to produce powders of different compositions and stoichiometries. For morphological characterization, transmission electron microscopy is employed. Structural information is obtained by X-ray diffraction (XRD) and Raman-spectroscopy. Thermogravimetry (TG) and differential scanning calorimetry (DSC) are utilized for thermal analysis of powders. Pressed pellets are sintered at 1100°C for 3h and their ionic conductivities are measured with impedance spectroscopy.<br/>From SFS, nanoparticles with a mean diameter of 5 nm are obtained. They consist of fine ZrO₂ crystallites embedded in an amorphous phase of Na, Si and P. After a short annealing step of 1h at 1000°C, this mixture can be converted almost quantitatively into the desired rhombohedral NaSICON phase. Especially the composition Na_3+xZr₂Si_2+xP_1-xO₁₂ and Mg-doped Na_3+2yMg_yZr_2−ySi₂PO₁₂ show - with respect to the short sintering time of 3h - remarkably high ionic conductivities of 4×10^-4 Scm^-1 (for x=0.1) and 9.3×10^-4 Scm^-1 (for y=0.2), respectively. XRD reveals that the addition of Mg beyond the solubility limit in NZSP leads to the formation of Mg-phosphates, creating a P-deficiency in the lattice of NZSP and thus improving the ionic conductivity significantly.<br/>In conclusion, SFS enables the scalable production of NaSICON nanoparticles with tailored compositions allowing fast sintering. The improved ionic conductivities observed in the Mg-doped and phosphorous-deficient compositions hold promise for the development of high-performance solid-electrolytes.<br/><br/><br/>[1] Zuo, W. et al., Acc. Chem. Res., <b>56 </b>(2023), 284-296; doi.org/10.1021/acs.accounts.2c00690.<br/>[2] Tang, B. et al., Sci. Bull., <b>67</b> (2022), 2149-2153; doi.org/10.1016/j.scib.2022.10.014<br/>[3] Abraham, K. M., ACS Energy Lett., <b>5</b> (2020), 3544; doi.org/10.1021/acsenergylett.0c02181<br/>[4] Zhang, Z. et al., ACS Appl. Energy Mater., <b>3</b> (2020) 7427; doi.org/10.1021/acsaem.0c00820<br/>[5] Schuett, J. et al., Phys. Chem. Chem. Phys. <b>24</b> (2022), 22154-22167; doi.org/10.1039/D2CP03621E<br/>[6] Samiee, M. et al., J. Power Sources, <b>347</b> (2017), 229-237; doi.org/10.1016/j.jpowsour.2017.02.042