Investigating Oxygen-Rich Alternative Calcination Approaches for NMC-811 Cathodes

The calcination of nickel-manganese-cobalt 8:1:1 hydroxide precursor to produce the LiNi_0.8Mn_0.1Co_0.1O₂ (NMC-811) is critical to maximise the practical capacity yielded from the cathode active material. This conventionally occurs in a high temperature furnace under a flow of oxygen gas for up to 15 hours at 750-850 °C, which promotes the formation of trivalent and tetravalent nickel ions which facilitate the electrochemical storage of energy in the desired α-NaFeO₂ layered LiNi_0.8Mn_0.1Co_0.1O₂ phase [1-3]. Here, we present an alternative calcination approach which utilises microwave heating to reduce the reaction time to 4 hours. Lithium peroxide (Li₂O₂) is also used as an alternative lithium precursor to act as an oxygen source during thermal decomposition. This removes the need for a separate oxygen gas source which, as well as being safer, could improve the processing conditions when moving to high-throughput syntheses.<br/><br/>This work investigates the impact of this microwave calcination procedure on the heat treatment of Ni_0.8Mn_0.1Co_0.1(OH)₂ with Li₂O₂ and a control lithium salt, lithium hydroxide (LiOH). The reaction conditions, and the material and electrochemical properties of the NMC-811 products are assessed to detail the difference between the conventional oxygen-fed calcination procedure and the alternative microwave calcination procedure. The microwave calcined approach using the Li₂O₂ salt yields initial discharge capacities of 160.5 mAh g^-¹; whereas the conventional, oxygen-fed calcination approach with LiOH produces 204.4 mAh g^-¹, when cycled between 3.0-4.3 V at C/20.<br/><br/>The cause of the disparity between the specific capacities was investigated, with high temperature in-situ X-ray diffractometry of Ni_0.8Mn_0.1Co_0.1(OH)₂ revealing the different mechanisms for the lithium integration and structural evolution during calcination with Li₂O₂ or LiOH. This, alongside morphological differences, provides insight into the challenges associated with the current microwave calcination procedures and highlights the requirements for further control over the particle morphology to minimise inter-granular resistance and capacity loss compared to conventionally heated furnaces.<br/><br/><br/><b>References: </b><br/>[1] A. Habibi, M. Jalaly, R. Rahmanifard, New J. Chem. 42 (2018) 19026-19033<br/>[2] H-J. Noh, S. Youn, C. S. Yoon, J. Power Sources. 233 (2013) 121-130<br/>[3] D.-J. Vu, J.-Y. Choi, W.-B. Kim, J. Electrochem. Soc. 164 (2017) A2670