Morphology of Transition Metal Carbonate Cathode Precursors

Abstract

Increasing the capacity of cathode materials used for lithium-ion batteries is desirable, as it ultimately enhances the energy density. Due to their lower cost and reversible cycling capacity of 250 – 300 mAh/g, Li- and Mn-rich LMR-NMC oxides are strong candidates as next generation cathodes used in lithium-ion batteries. Apart from the atomic structure, morphology of the cathode particles also influence their performance. LMR-NMC cathode particles are usually constructed through a two-step cathode fabrication process, which involves initial coprecipitation of the Mn-rich carbonate based cathode precursors, and later calcination of these precursors with a lithium salt at elevated temperatures. The secondary particles generally maintain their as precipitated precursor morphologies even after high temperature calcination. Even though the primary particles do change their size during calcination, the rate of oxidation and lithiation experienced by the transition metal precursors depend substantially on the primary particle morphology. Hence, it is critical to understand and control both the primary and secondary particle morphologies obtained after the coprecipitation process. In the present context, carbonate based NMC cathode precursors containing only Mn, only Ni and only Co, is precipitated, along with equal amount of the transition metals (Ni0.33Mn0.33Co0.33CO3), using conventional batch reactors. NH4HCO3 is used as the source of carbonate anions during the coprecipitation process, and the entire reaction is conducted at 50°C. The obtained particle morphologies for different transition metals are shown in Figure 1(a) as visualized using high resolution TEM techniques. Except MnCO3, all other transition metals demonstrate aggregated morphologies, which most probably form through surface growth mechanisms. Competition between growth rate and surface energies that leads to the formation of single crystalline particles for MnCO3, and particulate features for other transition metals, are demonstrated in Figure 1(b). Multiscale computational methodologies are developed to elucidate the impact of reaction kinetics and thermodynamics on determining the overall primary and secondary particle morphologies. Influence of transition metal content and ammonia concentration in determining the final particle size and size distribution will be discussed as part of this study. Figure 1