Oxygen-release-related thermal stability and decomposition pathways of Li_xNi_0.5Mn_1.5O₄ cathode materials

The thermal stability of charged cathode materials is one of the critical properties affecting the safety characteristics of lithium-ion batteries. New findings on the thermal-stability and thermal-decomposition pathways related to the oxygen release are discovered for the high-voltage spinel Li _xNi_0.5Mn_1.5O₄ (LNMO) with ordered (o-) and disordered (d-) structures at the fully delithiated (charged) state using a combination of in situ time-resolved X-ray diffraction (TR-XRD) coupled with mass spectroscopy (MS) and X-ray absorption spectroscopy (XAS) during heating. Both o- and d- Li_xNi_0.5Mn_1.5O ₄, at their fully charged states, start oxygen-releasing structural changes at temperatures below 300 C, which is in sharp contrast to the good thermal stability of the 4V-spinel Li_xMn₂O₄ with no oxygen being released up to 375 C. This is mainly caused by the presence of Ni⁴⁺ in LNMO, which undergoes dramatic reduction during the thermal decomposition. In addition, charged o-LNMO shows better thermal stability than the d-LNMO counterpart, due to the Ni/Mn ordering and smaller amount of the rock-salt impurity phase in o-LNMO. Two newly identified thermal-decomposition pathways from the initial Li_xNi _0.5Mn_1.5O₄ spinel to the final NiMn ₂O₄-type spinel structure with and without the intermediate phases (NiMnO₃ and α-Mn₂O₃) are found to play key roles in thermal stability and oxygen release of LNMO during thermal decomposition.