Thermal instability of cycled LixNi0.5Mn0.5O2 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 degrees 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 degrees C. Li(x)Ni(0.5)Mn(0.5)O(2) electrodes were found to retain the rhombohedral, layered structure with space group R3m until the temperature reached similar to 300 degrees C (similar to 275 degrees C for Li(0.2)Ni(0.5)Mn(0.5)O(2) and similar to 350 degrees C for Li(0.9)N(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)W(0.5)O(2) from 100 to 275 degrees 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 degrees 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 similar to 260 degrees C and similar to 400 degrees 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 similar to 300 degrees C and to include the following processes: (1) the formation of a spinel-type structure (Fd3m) at similar to 300 degrees C, which was shown present between 300 degrees C and 600 degrees C, and (2) the formation of a rocksalt-type phase (Fm3m) at similar to 500 degrees C, which was shown to be present between 500 degrees C and 600 degrees 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 degrees 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 similar to 100 degrees C and similar to 230 degrees C higher than those previously reported for Li(x)NiO(2) (x 0.3) measured under similar conditions in previous studies, respectively.