Sc, Ge co-doping NASICON boosts solid-state sodium ion batteries' performance

Solid-state sodium ion batteries (SSSIBs) have been proposed to resolve the safety hazards of traditional liquid batteries. The sodium superionic conductor (NASICON), one of the most promising solid electrolyte candidates, has received much attention. However, the practical use of NASICON has been impeded by low ion mobility at room temperature (RT) and poor interfacial connectivity. Here, the improvement of both bulk and grain boundary conductivity has been achieved simultaneously via Sc and Ge co-doping. Bulk conductivity is increased by the Sc-doping, which increases the stability of the rhombohedral phase at RT, and the Ge-doping, which reduces the monoclinic to rhombohedral phase transformation temperature. The highest total conductivity of 4.64 x 10(-3) S cm(-1) was obtained for the Na3.125Zr1.75Sc0.125Ge0.125Si2PO12 structure. The improvement in conductivity due to the stabilisation of the rhombohedral phase was verified by DFT calculations. Finally, two different solid-state batteries using Na3V2 (PO4)(3) and Sn4P3@CNT as electrodes display impressive cycling capacities of 98 and 629 mAh g(-1) after 250 cycles, respectively. This co-doping principle also provides a framework to explore multiple-doping for other materials with similar structures.