Hollandite-type alpha-MnO(2)s doped with several transition metals were synthesized by the oxidation of MnSO4 and MSO4 9M = Co or Fe) aqueous solutions with the oxidizing agent K2S2O8. The chemical analysis and X-ray diffraction measurements indicated that the metal-doped MnO(2)s are shown as alpha-KyMn(1-x)MxO(2)center dot zH(2)O (M = Co or Fe) with the hollandite-type structure and the x-value varies in the range from 0 to 0.12. Extended X-ray absorption fine structure study revealed that Co3+ ions are partly substituted for the Mn3+ ions in alpha-KyMnO2 resulting in alpha-K0.12Mn0.88Co0.12O2. Transmission electron microscopic measurement showed that the high-crystalline alpha-K-0.12 (Mn0.88Co0.12) O-2 or alpha-K (Mn7Co)O-16 particle was rod shaped with the width of 30-50 nm. The electrochemical measurements indicated that alpha-K-0.12 (Mn0.88Co0.12)O-2 electrode provides excellent performance of 296 and 193 mAh/g-active material at the first and twentieth cycles, respectively, superior to undoped alpha-MnO2 electrode (255 mAh/g at the 1st cycle and 169 mAh/g at the 20th cycle) and alpha-K-0.08 (Mn1-xFex)O-2 (x = 0.04-0.12)) electrodes (35-100 mAh/g at the 1st to 20th cycles). Moreover, the discharge performance of K-0.12(Mn1-xCox)O-2 (x = 0-0.12) electrodes was enhanced with an increase in the Co content or x-value. (c) 2005 The Electrochemical Society. All rights reserved.