The alloy of potassium and lead is brittle, but adding sodium to the mixture softens it, and the alloy is remarkably tough. Lumps or molded pieces of the alloy may be transported and handled without any special precautions against breakage, and they suffer substantially less from impact and crushing stresses than lead alone. The addition of sodium also diminishes the reactivity of the alloy toward water and oxygen.
A liquid K/Na alloy that can remain a fluid even at -12.6 degC has demonstrated excellent electrochemical performance in reversible cycling at 4 mA cm-2 for 2000 h (K:Na = 66.3:33.7, wt%). However, the surface tension of liquid K/Na interferes with its wetting on a ceramic solid electrolyte or porous separator, and it cannot be directly shaped as a sheet electrode for practical potassium metal batteries (PMBs).
To address these problems, we prepared a “quasi-liquid” K/Na alloy by introducing only 3.5 wt% Na into liquid K. The resulting K/Na-3.5 exhibits outstanding electrochemical performance in reversible cycling, and the mechanical properties of the material are much improved relative to those of liquid K. When a 23-g weight was applied to K/Na-3.5 in order to simulate mechanical deformation, the alloy could be easily squeezed but reshaped without any loss of shape. This suggests that the quasi-liquid K/Na-3.5 can be readily fabricated as an anode in a PMB with a simple sandwich architecture. Moreover, the plating of K/Na-3.5 on copper shows no sign of dendrite growth after 20 h of charging at 500 mA cm-2, even when the plating duration is extended to 50 h.