A series of acetonitrile solutions of the N(SO2CF3)2 − salts of Li, Na, K, Cs, Mg and Ca are examined in a systematic study of electrolytes based on the alkali and alkaline earth series of metal cations. Reference-electrode experiments show that the relatively large and polarisable cesium ion results in the highest capacitance at the negative electrode. We also report current collector corrosion behavior which is anomalously severe for the Mg based electrolyte. Charge-discharge kinetics are investigated using impedance spectroscopy, and it is shown that the new electrolytes have no power advantage over an organic cation based electrolyte at 0.5M concentration. Constant voltage device testing at 2.7V and 65°C reveals a very large electrochemical stability advantage for the K and Cs based devices over the hitherto more commonly studied Li systems.