Suitability of NaK alloy for the sodium or potassium metal batteries: Competition between cathode size effect and ion reaction priority effect

Sodium-potassium (NaK) alloys in a specific ratio can maintain their liquid state at room temperature, rendering them dendrite-free anodes for alkali metal batteries. Nevertheless, a controversy has arisen regarding the suitability of either sodium metal batteries (NaMBs) or potassium metal batteries (KMBs) due to the presence of dual active ions in NaK. Based on the current research, there is a contradiction regarding the feasibility of making NaK anodes suitable for NaMBs through the cathode size effect (e.g., Na3V2(PO4)3), and for KMBs due to the ion reaction priority effect of K+. In this study, Na3V2(PO4)3 (NVP) cathodes are assembled into batteries with various anodes (Na, Na-rich-NaK, NaK, K-rich-NaK, and K), to investigate the dominant role of active ions in the competing process between the cathode size effect and the ion reaction priority effect. Tests and characterization results demonstrate that the ion preference reaction spontaneously displaces Na+ with K+ in the presence of K, although the cathode size effect may restrict K+ embedding and significantly reduce battery capacity. In summary, the ion reaction priority effect dictates the reactions that render NaK anodes exclusively suitable for KMBs without altering the reaction priority of K+.