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 x Ni 0.5 Mn 1.5 O 4 (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 x Ni 0.5 Mn 1.5 O 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 x Mn 2 O 4 with no oxygen being released up to 375 °C. This is mainly caused by the presence of Ni 4+ 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 x Ni 0.5 Mn 1.5 O 4 spinel to the final NiMn 2 O 4 -type spinel structure with and without the intermediate phases (NiMnO 3 and α-Mn 2 O 3 ) are found to play key roles in thermal stability and oxygen release of LNMO during thermal decomposition.