Advanced Design of Ni-Rich NCM Cathode Materials synthesized by a Continuously Couette‐Taylor Flow Reactor (CTFR)

Summary:

In this work, the synthesis of Ni-rich layered cathode materials with advanced particle design (e.g., core-shell or concentration-gradient (CG) particles) by a Couette-Taylor Flow Reactor was evaluated. In order to achieve high energy densities as well as stable long-term cycling performance Ni0.9Mn0.04Co0.05Mg0.01(OH)2 and Ni0.8Mn0.1Co0.1(OH)2 were chosen as core and shell composition, respectively. By using the latest possible inlet for reagent addition (Port 4 of the reactor) to the reaction tube or rather decreasing the residence time for shell formation, the primary particles grew with thin needle-like structures instead of thicker ones. It is also expected that the pure shell precipitated at short residence times instead of encapsulating the core, leading to a synthesis of sub-micron sized particles and therefore a trimodal particle size distribution. However, the shell addition at an early state of precipitation (Port 1) led to a mixing of the core and shell and therefore to an average of both stoichiometries. The shell addition at Port 4 enabled the synthesis of particles with increasing Ni concentration from the outer to the inner particle (CG), which was tailored by changing the core:shell compositional ratio. It is worth to mention, that no clear morphological boundary between core and shell stoichiometry was visible in electron microscopy, which might relate to the intense mixing of the Couette-Taylor Flow reactor itself. An optimized lithiation temperature (735°C) was chosen with regard to morphology, surface area and Li+/Ni2+ disorder. After lithiation, the CG was still visible, even though it was slightly smoothed due to transition metal diffusion at high temperatures. The discharge capacity of cathode active materials in NMC || Li metal cells was increased with enhanced Ni content, while the stability at higher voltages decreased simultaneously. The positive impact of the CG particle design on the resulting electrochemical performance was especially visible in NMC || graphite cells. Cathodes without CG had higher initial discharge capacities (171 mAh g-1 at 1C), but also a pronounced fading. Therefore, these cathodes reached the end-of-life-criterium (80% state of health) after ~ 200 cycles, while the counterpart with CG retained 80% of their initial capacity (160 mAh g-1 at 1C) even after 300 cycles. Moreover, cathodes with a core-shell ratio of 80:20 and higher Ni content delivered higher capacities (177 mAh g-1 at 1C) and were more stable than the cathodes without CG.