O2 structure Li-2/3[Ni1/3Mn2/3]O-2: A new layered cathode material for rechargeable lithium batteries I. Electrochemical properties

Layered Li-Mn oxide-based materials (Li-2/3[NixMn1-x]O-2) With the unconven tional O2 structure were prepared by ion exchanging Na for Li in the corres ponding P2 structure sodium bronzes, Na-2/3(NixMn1-x)O-2. Powder X-ray diff raction (XRD) was used to confirm the purity and structure of the phase. Th e Bragg, peaks from La-2/3[Ni1/3Mn2/3]O-2 can be indexed based on a hexagon al unit cell with dimensions a = 2.8609 Angstrom and c = 10.061 Angstrom, a nd the diffraction pattern can be approximately matched assuming that the a toms are in 2a and 2b positions of space group P6(3)mc. However, the initia l structure is more complex, because some observed diffraction peaks tie., Ill are forbidden in this space group. In situ XRD and electrochemical test ing 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 reversib le capacity of approximately 180 mAh/g divided into two plateaus centered n ear 2.8 and 3.9 V. In situ XRD proves that after an initial phase transform ation, 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 char ged electrodes show exotherms that are about one order of magnitude less in tense than those from charged LixCoO2 electrodes. (C) 2000 The Electrochemi cal Society. S0013-4651(99)08-005-2. All rights reserved.