A secondary power source comprising a positive electrode made mainly of activated carbon, a negative electrode made mainly of a carbon material capable of doping and undoping lithium ions and an organic electrolyte containing a solute of a lithium salt, wherein the lithium salt comprises LiN(SO.sub.2R.sup.f1) (SO.sub.2R.sup.f2) wherein each of R.sup.f1 and R.sup.f2 which are independent of each other, is a C.sub.1-6 perfluoroalkyl group except R.sup.f1.dbd.R.sup.f2.dbd.CF.sub.3.