Highly Stable Quasi-Solid-State Sodium Batteries via Facile Grain Boundary Engineering

The development of ceramic solid-state electrolytes such as sodium superionic conductors (NASICON) is critical in the advancement of all-solid-state sodium batteries. However, the key fundamental issue lies in the large interfacial impedance and instability due to the mismatched mechanical properties across different battery components. Herein, we propose a novel interfacial engineering approach to tackle the challenge at the interface, where a Na⁺ conducting grain boundary complexion phase can be formed by cosintering NASICON with a thin layer of zinc oxide (ZnO) coating. The grain boundary complexion envelopes the NASICON grains and forms an ion-conducting network that enhances the ionic conductivity at the grain boundaries. Ultrastable symmetric cell cycling over 12,000 h was demonstrated, showing the efficacy in suppressing dendrite formation. Electrochemical impedance spectroscopy (EIS) measurements revealed a minimal increase in internal resistance over cycling. In addition, quasi-solid-state batteries using Na₃V₂(PO₄)₃ as the cathode, sodium metal as the anode, and cosintered NASICON as the electrolyte were assembled and tested at room temperature with no externally applied stack pressure. When cycled at 0.5 C, a high initial capacity of 116.7 mAh g^-1 was obtained, and 93.2% capacity retention was achieved at the 1300th cycle with an average Coulombic efficiency of 99.98%. Even when cycled at 2 C, the battery maintained 95.8% of the initial capacity after 1200 cycles. Overall, this work provides insights into facile and strategic approaches to interfacial modification of solid-state batteries and shows the great potential of grain boundary engineering.