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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]

In this work, we present results from the applicationof a newin situ technique that combines time-resolved synchrotron X-ray diffractionand mass spectroscopy. We exploit this approach to provide directcorrelation between structural changes and the evolution of gas thatoccurs during the thermal decomposition of (over)charged cathode materialsused in lithium-ion batteries. Results from charged LixNi0.8Co0.15Al0.05O2cathode materials indicate that the evolution of bothO2and CO2gases are strongly related to phasetransitions that occur during thermal decomposition, specificallyfrom the layered structure (space group R3̅m) to the disordered spinel structure (Fd3̅m), and finally to the rock-salt structure(Fm3̅m). The state of chargealso significantly affects both the structural changes and the evolutionof oxygen as the temperature increases: the more extensive the charge,the lower the temperature of the phase transitions and the largerthe oxygen release. Ex situ X-ray absorption spectroscopy (XAS) andin situ transmission electron microscopy (TEM) are also utilized toinvestigate the local structural and valence state changes in Ni andCo ions, and to characterize microscopic morphology changes. The combinationof these advanced tools provides a unique approach to study fundamentalaspects of the dynamic physical and chemical changes that occur duringthermal decomposition of charged cathode materials in a systematicway. [ABSTRACT FROM AUTHOR]