Thermal Instability of Cycled Li_xNi_0.5Mn_0.5O₂ Electrodes: An in Situ Synchrotron X-ray Powder Diffraction Study

Thermal instability of pristine LiNi 0.5 Mn 0.5 O 2 powder and cycled Li x Ni 0.5 Mn 0.5 O 2 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 LiNi 0.5 Mn 0.5 O 2 upon heating to 600 °C. Li x Ni 0.5 Mn 0.5 O 2 electrodes were found to retain the rhombohedral, layered structure with space group R 3̅ m until the temperature reached ∼300 °C (∼275 °C for Li 0.2 Ni 0.5 Mn 0.5 O 2 and ∼350 °C for Li 0.9 Ni 0.5 Mn 0.5 O 2 ). 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 Li 0.2 Ni 0.5 Mn 0.5 O 2 from 100 to 275 °C while no detectable change in the cation distribution was noted for Li 0.9 Ni 0.5 Mn 0.5 O 2 up to 300 °C. TGA results showed that the weight loss from Li 0.2 Ni 0.5 Mn 0.5 O 2 and Li 0.9 Ni 0.5 Mn 0.5 O 2 began at ∼260 °C and ∼400 °C, respectively, and considerably higher weight loss was found for Li 0.2 Ni 0.5 Mn 0.5 O 2 relative to Li 0.9 Ni 0.5 Mn 0.5 O 2 upon heating. The thermal decomposition of charged Li 0.2 Ni 0.5 Mn 0.5 O 2 was found to occur at ∼300 °C and to include the following processes: (1) the formation of a spinel-type structure ( Fd 3̅ m ) at ∼300 °C, which was shown present between 300 °C and 600 °C, and (2) the formation of a rocksalt-type phase ( Fm 3̅ m ) 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 LiNi 0.5 Mn 0.5 O 2 but with considerably more interlayer mixing of Li and transition metal ions) was observed upon heating Li 0.9 Ni 0.5 Mn 0.5 O 2 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 Li 0.2 Ni 0.5 Mn 0.5 O 2 were ∼100 °C and ∼230 °C higher than those previously reported for Li x NiO 2 ( x = 0.3) measured under similar conditions in previous studies, respectively.