Liquid Na-K alloy is not viable anode material for High-Performance Na-Ion batteries

Liquid sodium–potassium alloy NaxK100-x at. % (≈14 < x < 70 at room temperature) has been proposed as a dendrite-free liquid metal anode for Na-ion batteries. Nevertheless, controversy surrounds the use of NaxK100-x as a Na-ion battery anode due to the presence of the two competitive redox-active species, Na/Na+ and K/K+. To resolve this ongoing controversy, we use x-ray diffraction (XRD) and cryogenic-focused ion beam/scanning electron microscopy (cryo-FIB/SEM) to investigate liquid Na50K50 electrodes cycled in symmetric cells using Na-ion electrolytes. We find that the reaction overpotential abruptly increases after ≈ 40 cycles. Cryo-FIB/SEM reveals the precipitation of K salts, which confirms that K/K+ species are redox-active in Na-ion electrolytes. Such an effect progressively leads to irreversible leaching of K from the initial liquid Na50K50 composition, resulting in a loss of liquid metal properties. Consequently, our results suggest that NaxK100-x cannot sustainably function as a liquid metal anode in Na-ion batteries.