Correlating Thermal History to Structural Integrity and Cycling Stability in NMC 111 Cathodes

Abstract

Thermal treatment is critical for determining the structure and stability of layered cathode materials, which in turn affect the performance of lithium-ion batteries. This study systematically investigated the thermal evolution of NMC 111 cathode materials synthesized from prelithiated precursors. Thermogravimetric analysis and variable-temperature X-ray diffraction were employed to monitor phase transitions and mass loss over a wide temperature range. The material transformed from a calcite-type carbonate to rock salt and subsequently to the layered NMC 111 structure, with crystallization largely complete near 800 °C. Cation mixing can begin around 500 °C but remains reversible upon cooling if the sample is heated to 800 °C or lower. In contrast, prolonged thermal soaking at high temperatures (≥900 °C) causes irreversible structural degradation, including the emergence of Li-deficient spinel phases attributed to lithium volatilization. An optimal soak temperature of 800 °C was identified, balancing high crystallinity with minimal irreversible cation mixing. Material synthesized at 800 °C also exhibited greater cycling stability than materials processed at higher temperatures. A dwell time of ~6 hours is sufficient to produce high-quality NMC 111 materials. Balancing dwell time and upper-temperature thresholds is essential during synthesis. These findings offer practical guidance for optimizing thermal treatments used to manufacture layered cathodes for lithium-ion batteries.