Layered Li-Mn oxide-based materials (Li-2/3[NixMn1-x]O-2) With the unconventional O2 structure were prepared by ion exchanging Na for Li in the corresponding P2 structure sodium bronzes, Na-2/3(NixMn1-x)O-2. Powder X-ray diffraction (XRD) was used to confirm the purity and structure of the phase. The Bragg, peaks from La-2/3[Ni1/3Mn2/3]O-2 can be indexed based on a hexagonal unit cell with dimensions a = 2.8609 Angstrom and c = 10.061 Angstrom, and the diffraction pattern can be approximately matched assuming that the atoms are in 2a and 2b positions of space group P6(3)mc. However, the initial structure is more complex, because some observed diffraction peaks tie., Ill are forbidden in this space group. In situ XRD and electrochemical testing were used to investigate the behavior of O2-type Li-2/3[NixMn1-x]O-2 as a cathode for rechargeable lithium batteries. It exhibits a large reversible capacity of approximately 180 mAh/g divided into two plateaus centered near 2.8 and 3.9 V. In situ XRD proves that after an initial phase transformation, during which all forbidden diffraction peaks vanish, the extraction and insertion of lithium occurs without any first order phase transitions. The material remains layered and does not transform to the spinel structure during cycling. It shows good capacity retention vs. cycle number at both 30 and 55 degrees C. Differential scanning calorimetry measurements on charged electrodes show exotherms that are about one order of magnitude less intense than those from charged LixCoO2 electrodes.