Thermal stability in the blended lithium manganese oxide - Lithium nickel cobalt manganese oxide cathode materials: An in situ time-resolved X-Ray diffraction and mass spectroscopy study

摘要

Thermal stabilities of a series of blended LiMn_2O_4 (LMO)-LiNi_(1/3)Co_(1/3)Mn_(1/3)O_2 (NCM) cathode materials with different weight ratios were studied by in situ time-resolved X-ray diffraction (XRD) combined with mass spectroscopy in the temperature range of 25 ℃-580 ℃ under helium atmosphere. Upon heating, the electrochemically delithiated LMO changed into Mn_3O_4 phase at around 250 ℃ Formation of MnO with rock-salt structure started at 520 ℃. This observation is in contrast to the previous report for chemically delithiated LMO in air, in which a process of λ-MnO_2 transforming to β-MnO_2 was observed. Oxygen peak was not observed in all cases, presumably as a result of either consumption by the carbon or detection limit. CO_2 profile correlates well with the phase transition and indirectly suggests the oxygen release of the cathode. Introducing NCM into LMO has two effects: first, it makes the high temperature rock-salt phase formation more complicated with more peaks in CO_2 profile due to different MO (M = Ni, Mn, Co) phases; secondly, the onset temperature of CO_2 release is lowered, implying lowered oxygen release temperature. Upon heating, XRD patterns indicate the NCM part reacts first, followed by the LMO part. This confirms the better thermal stability of LMO over NCM.