Synthesis and characterization of core-shell NMC microparticles as cathode materials for Li-ion batteries: insights from ex situ and in situ microscopy and spectroscopy techniques

The achievement of lithium ion batteries (LiBs) with improved electrochemical performance requires advances in the synthesis of cathode materials with controlled composition and properties. In particular, NMC core-shell materials formed by a Ni-rich core and a Mn-rich shell are recently gaining interest as they allow the achievement of increased energy density and high discharge capacity values. In order to overcome some of the limitations of these NMC compounds and broaden their applicability, controlled synthesis and detailed analysis of their properties are required. In this work, NMC in the form of core and core-shell microparticles have been synthesized by an oxalate-assisted co-precipitation synthesis method which allows control of the final composition. The morphology, crystalline structure and composition of the particles have been investigated as a function of the synthesis parameters and the presence of the Mn-rich shell, by means of diverse microscopy and spectroscopy techniques. Additionally, in situ SEM and XPS measurements allow analysis of the NMC particles in variable operation environments. Aspects such as the cationic mixing in the NMC compound or the formation of a rock-salt phase as the annealing temperature increases are discussed. Finally, preliminary electrochemical tests have been performed using NMC particles as cathodes in LiBs.