Establishing scalable, safe, and cost-efficient solutions for stationary energy storage is of key importance for leveraging contribution of renewable power supply facilities to the electrical grid. The all solid-state batteries (ASSB) are increasingly considered for this purpose with new sodium-based battery concepts emerging as a promising alternative to lithium-based ones [1]. The demonstration of ASSBs is hampered in general by the requirement to sinter the solid electrolyte and cathode materials at temperatures of 1000°C and higher. Emerging cold sintering of battery materials at temperatures below 400 °C and pressure 300-700 MPa is attracting increasing attention in recent years as a potential solution to this problem [2].
The present work is focusing on systematic investigation of the cold sintering process of the NASICON-type materials with composition close to Na₃Zr₂Si₂PO₁₂ and comparing it to conventional pressureless heat treatment of the material at different temperatures. The influence of sintering additive, additive concentration, temperature, pressure and thermal postprocessing on the microstructure and properties of the cold sintered pellets was investigated. The experiments were performed using pressure of 600 MPa and temperatures of 140 and 200 °C for 1 h using deionized water and water solutions of borax as additives. Density, microstructure, phase composition and ionic conductivity of the materials were analyzed by Archimedes method, scanning electron microscopy (SEM), x-ray diffraction (XRD) and electrochemical impedance spectroscopy (EIS), respectively. It is observed, that using water as additive results in the NASICON material with relative density not higher than 80% of theoretical value.
By optimizing cold sintering process parameters in case of borax water solution as additive, the relative densities above 90 % of theoretical value, which are comparable to those of conventionally sintered electrolytes, are achieved already at 140 °C. However, the ionic conductivities of the cold sintered materials are below 10^-5 S/cm. As a possible reason for this low ionic conductivity, SEM, XRD and EIS studies suggest formation of amorphous secondary phases in the intergranular regions, depending on the type of additive similar to the previously studied case of LATP [3]. By thermal postprocessing these secondary phases appear to be decomposing, and room temperature Na ionic conductivities of 4.1 x 10^-4 S/cm (treatment at 700 °C) and 2.4 x 10^-3 S/cm (900 °C) are achieved. This shows feasibility of realizing NASICON material with ionic conductivities suitable for battery applications by combining cold sintering and post-treatment at temperatures much lower compared to conventional sintering temperatures of 1300 °C. These promising results are being presently expanded to the NASICON tape (100 µm) using other type of additives having potential to enable material with comparable ionic conductivities directly after cold sintering bringing the demonstration of NASICON -based ASSB one step closer.
The authors are grateful to M.-T. Gerhards, E. Dashjav, and F. Tietz, FZJ, for supplying NASICON powder in frame of the BMBF project HeNa (FKZ 03XP0390A).
[1] T. Famprikis, et al, Nat. Mater. 18, 1278 (2019)
[2] J. Guo et al, Annu. Rev. Mater. Res. 49, 275 (2019)
[3] M. Vinnichenko, et al, Nanomaterials 12, 3178 (2022)