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The thermal stability of chargedcathode materials is one of thecritical properties affecting the safety characteristics of lithium-ionbatteries. New findings on the thermal-stability and thermal-decompositionpathways related to the oxygen release are discovered for the high-voltagespinel LixNi0.5Mn1.5O4(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 absorptionspectroscopy (XAS) during heating. Both o- and d- LixNi0.5Mn1.5O4, at their fully charged states, start oxygen-releasingstructural changes at temperatures below 300 °C, which is insharp contrast to the good thermal stability of the 4V-spinel LixMn2O4with no oxygenbeing released up to 375 °C. This is mainly caused by the presenceof Ni4+in LNMO, which undergoes dramatic reduction duringthe thermal decomposition. In addition, charged o-LNMO shows better thermal stability than the d-LNMOcounterpart, due to the Ni/Mn ordering and smaller amount of the rock-saltimpurity phase in o-LNMO. Two newly identified thermal-decompositionpathways from the initial LixNi0.5Mn1.5O4spinel to the final NiMn2O4-type spinel structure with and without the intermediatephases (NiMnO3and α-Mn2O3)are found to play key roles in thermal stability and oxygen releaseof LNMO during thermal decomposition. [ABSTRACT FROM AUTHOR]