A Novel Approach for Synthesis of Nickel-Rich Layered Oxide Cathode Materials with Tailored Morphology and Their Electrochemical Performance

Abstract

The improvement in energy density of lithium ion batteries (LIBs) is primarily focused on the increase of the specific capacity of active materials and the cell voltage.[1] Transition metal (TM) oxide-based cathodes materials reach a high specific capacity through the optimization of the chemical composition with a higher Ni content and can be cycled up to 4.6 V.[2] However, the electrochemical performance of Ni-rich cathode materials depends strongly on their physical properties such as particle size, specific surface area or surface characteristics. The poor rate capability and capacity retention as well as the high first cycle irreversible capacity loss (ICL) and the low volumetric energy density of electrodes have to be improved by specific design of the particle shape. A spherical morphology with a narrow size distribution is the preferred particle shape and can only be synthesized during the co-precipitation of TM-hydroxides or -carbonates by a continuously stirred tank reactor (CSTR). Additionally, the CSTR-process allows a further improvement of the cathode materials during the synthesis of core-shell and full concentration-gradient particles.[2] In this work, the design and the operating principle of a novel Couette-Taylor-Flow-Reactor (CTFR) is evaluated regarding the synthesis of spherical shaped Ni-rich TM-precursors. The CTFR offers a continuous process and is constructed by two co‐axial arranged cylinders with a small gap and a low working volume. The rotational motion of the inner cylinder induces a turbulent Taylor‐vortex flow. Above the suitable Taylor number (Ta), a periodic and stable turbulent fluid motion is formed, which increases the fluid shear and the local concentration of educts and promotes the agglomeration of precipitates to uniform spherical particles with narrow particle size distribution.[3] Based on the co-precipitation of Ni-rich TM-precursors by a continuous CTFR, the influence of the reaction conditions on the properties of TM-precursor is investigated. The rotational speed, residence time, temperature and pH-value were varied in a broad range. The growth behavior of the particles, regarding the morphology, particle size distribution, tap density and chemical composition of TM-precursors as well as for lithiated cathode materials are investigated. The electrochemical properties of the corresponding cathode materials with their unique particle morphology in dependence of the co-precipitation conditions are comprehensively evaluated for finding the optimal synthesis conditions. [1] J. Betz, G. Bieker, P. Meister, T. Placke, M. Winter, R. Schmuch, Adv. Energy Mater. 2018, 1803170/1-18.[2] H. Li, P. Zhou, F. Liu, H. Li F. Cheng, J. Chen, Chem. Sci, DOI: 10.1039/c8sc03385d.[3] J.-M. Kim, S.-M. Chang, J.-H. Chang, W.-S- Kim, Colloids and Surfaces A: Physicochem. Eng. Aspects (2011) 384, 31-39.