Insights into the thermal decomposition of LNMO

LNMO (LiNi 0.5-x Mn 1.5+x O 4 ) is a promising cathode material for Lithium-ion batteries, which crystallizes in two structures (ordered and disordered) depending on synthesis temperature and post-synthesis annealing. In this work, the thermal stabilities of both ordered and disordered LNMO were investigated under air and argon, and the decomposition products were analyzed with XRD. The decomposition under air was also investigated by high-temperature XRD. Above approx. 700 °C, a Ni-rich rock-salt phase (Ni 1-x-y Li x Mn y )O is formed, and oxygen is released. We observed that ordered LNMO has a higher onset temperature of decomposition than disordered LNMO when heated in air. This was also reflected in the high-temperature XRD experiments, which confirmed that a higher fraction of the rock-salt phase was found in the disordered LNMO at 700 °C compared to ordered LNMO. The results indicate that disordering could facilitate the formation of (Ni 1-x-y Li x Mn y )O, driven by Ni 2+ clusters that can act as crystallization seeds. When the heat treatments are performed in air, the decomposition of both ordered and disordered LNMO is accompanied by the formation of Mn-rich LNMO, where the TM-ions can occupy the lithium position. Upon cooling, the particle morphology is maintained, but only the disordered LNMO is re-formed with a small amount of (Ni 1-x-y Li x Mn y )O. This implies that the rock-salt structure forms within domains of the crystallites and does not precipitate as a bulk phase. Under argon, both LNMO structures showed lower decomposition onset temperatures and higher total mass losses compared to when the heat treatments were performed in air. In addition to the rock-salt phase, a further irreversible decomposition occurs at higher temperatures, where the Mn-rich LNMO decomposes into Mn 3 O 4 and LiMnO 2 . In this case, however, although the secondary particle morphology is still maintained, the primary particles change significantly. They lost their regular shape and the crystallite size diminished.