Electrochemical measurements and characterization of the NKO battery. (a) CV curves at 0.1 mV s−1 from 1.5 to 3.0 V. (b) Discharge/charge profiles of the initial three cycles at 250 mA g−1. (c) SEM image of the discharged SP cathode. (d) XRD patterns of the discharged and charged SP cathodes. (e, f) XPS spectra of the discharged SP cathode and the charged Na anode, respectively.