K2FeO4 as a cathode is 30% lighter than the equivalent Fe(VI) salt BaFeO4, but had exhibited poor nonaqueous charge transfer compared to the latter. Stepwise effects improving BaFeO4 cathodic charge transfer were probed and then applied to substantially increase the observed K2FeO4 cathode capacity to beyond that of BaFeO4. In addition the potassium salt will not have environmental regulatory issues associated with the barium salt. BaFeO4 capacity increased from 28, 84, 113 to 241 mAh g(-1) when the solvent for 1 M LiClO4 electrolyte was changed from acetonitrile, ethylene carbonate-diethylene carbonate, gamma-butyrolactone to propylene carbonate-dimethoxyethane (PC-DME), respectively. LiClO4 replacement in PC-DME, yields 247 or 260 mAh g(-1) with 1 M LiBF4 or LiPF6. Polarization was 0.6, 3.0 or 5.7 mV muA(-1) cm(2) in LiI, LiBF4, or LiPF6. Carbon black enhanced Fe(VI) charge transfer compared to alternate conductors. With a threefold particle size decrease, the specific capacity increased from 241 to 290 at 25 degreesC, and to 298 mAh g(-1) at 50 degreesC. Through proper selection of electrolytes, use of carbon black as the conductor, reduction of cathode particle size, and moderate increase of temperature (from 25 to 40 degreesC), the cathodic charge transfer of Fe(VI) in nonaqueous electrolyte is enhanced, resulting in an increase of K2FeO4 specific capacity from 290 to 370 mAh g(-1). Cocathodes comprised of K2FeO4 and MnO2 exhibited high storage capacity. (C) 2003 The Electrochemical Society.