Portal de Periódicos da CAPES

Thermal Instability of Cycled Li x Ni 0.5 Mn 0.5 O 2 Electrodes: An in Situ Synchrotron X-ray Powder Diffraction Study

2008; American Chemical Society; Volume: 20; Issue: 15 Linguagem: Inglês

10.1021/cm800314d

1520-5002

Naoaki Yabuuchi, Yong‐Tae Kim, Hayley H. Li, Yang Shao‐Horn,

Multiferroics and related materials

Thermal instability of pristine LiNi0.5Mn0.5O2 powder and cycled LixNi0.5Mn0.5O2 electrodes in the charged (x = 0.2) and discharged (x = 0.9) states (without electrolyte) was studied by in situ synchrotron powder X-ray diffraction and thermogravimetric analysis (TGA) in the temperature range from 25 to 600 °C in inert atmosphere. Neither structural change nor oxygen loss was found in rhombohedral layered LiNi0.5Mn0.5O2 upon heating to 600 °C. LixNi0.5Mn0.5O2 electrodes were found to retain the rhombohedral, layered structure with space group R3̅m until the temperature reached ∼300 °C (∼275 °C for Li0.2Ni0.5Mn0.5O2 and ∼350 °C for Li0.9Ni0.5Mn0.5O2). Rietveld refinement revealed that a considerable amount of octahedral cations migrated from the transition metal layer to the Li layer in the rhombohedral structure of Li0.2Ni0.5Mn0.5O2 from 100 to 275 °C while no detectable change in the cation distribution was noted for Li0.9Ni0.5Mn0.5O2 up to 300 °C. TGA results showed that the weight loss from Li0.2Ni0.5Mn0.5O2 and Li0.9Ni0.5Mn0.5O2 began at ∼260 °C and ∼400 °C, respectively, and considerably higher weight loss was found for Li0.2Ni0.5Mn0.5O2 relative to Li0.9Ni0.5Mn0.5O2 upon heating. The thermal decomposition of charged Li0.2Ni0.5Mn0.5O2 was found to occur at ∼300 °C and to include the following processes: (1) the formation of a spinel-type structure (Fd3̅m) at ∼300 °C, which was shown present between 300 °C and 600 °C, and (2) the formation of a rocksalt-type phase (Fm3̅m) at ∼500 °C, which was shown to be present between 500 °C and 600 °C. In addition to these two phase transitions, recrystallization of a rhombohedral phase (having unit cell dimensions similar to those of pristine LiNi0.5Mn0.5O2 but with considerably more interlayer mixing of Li and transition metal ions) was observed upon heating Li0.9Ni0.5Mn0.5O2 to 600 °C, which was accompanied with the appearance of Ni metal. The onset temperatures of structural transitions to the spinel-type and rocksalt-type structures from Li0.2Ni0.5Mn0.5O2 were ∼100 °C and ∼230 °C higher than those previously reported for LixNiO2 (x = 0.3) measured under similar conditions in previous studies, respectively.