Thermal Stability of LiNixMnyCozO2 Cathode Materials

Authors

Abstract

The thermal evolution of LiNixMnyCozO2 (NMC)lithium-ion battery electrode materials is examined at various statesof charge (SOC) or lithium concentrations for a variety ofNi:Mn:Co ratios or electrode compositions. Synchrotron X-raydiffraction (XRD) combined with Rietveld analysis shows the onsetdecomposition temperatures of phases, decomposition products,lattice parameters, and phase fractions as a function of compositionand SOC. SOC impacts the lattice parameters of the NMC phase,where a collapse of the c-axis in the NMC phases is noted due tolithium extraction. Among the compositions examined, the low-NiNMC111 uncycled sample (NMC111 0% SOC) exhibited thehighest thermal stability, with a decomposition temperatureapproximately 250 C higher than that of NMC532 0% andNMC811 0%. When the SOC exceeds 50% (i.e., more than 0.4 mol of Li ions extracted), the influence of Ni content on thedecomposition temperature becomes negligible, with decomposition occurring around 250300 C for all compositions. Ni contentalso affects the decomposition pathways: NMC111 tends to first form a TM3O4-type phase, where TM represents transition metals,before transforming into a TMO-type phase, whereas most of the NMC811 samples directly decompose into the TMO phase. Thepresence of metallic phases was confirmed by both XRD and thermogravimetric-differential scanning calorimetry (TGA-DSC)analysis, as a result of heating under inert conditions. The TGA-DSC results suggest that metallic phase formation is favored at lowerSOC in samples with a higher Ni content. This work provides comprehensive insight into the thermal degradation pathways ofNMC materials as a function of composition, SOC, and temperature.