Hierarchical Structuring of NMC111-Cathode Materials in Lithium-Ion Batteries: An In-Depth Study on the Influence of Primary and Secondary Particle Sizes on Electrochemical Performance

Commercially used LiNi1/3Mn1/3Co1/3O2 (NMC111) in lithium ion batteries, mainly consists of large-grained non-porous active material powder prepared by co-precipitation. However, nanomaterials are known to have extreme influence on gravimetric energy density and rate performance, but are not used on industrial scale, due to their reactivity, low tap density and diminished volumetric energy density. To overcome these problems, the build-up of hierarchically structured active materials and electrodes consisting of micro-sized secondary particles with primary particle scale in the nanometer range is preferable. In this paper the preparation and a detailed characterization of porous hierarchically structured active material with two different median secondary particle sizes, namely 9 and 37 μm, and primary particle sizes in the range between 300 and 1200 nm is presented. Electrochemical investigations by means of rate performance tests show that hierarchically structured electrodes provide higher specific capacities than conventional NMC111 and the cell performance can be tuned by adjustment of processing parameters. In particular, electrodes of coarse granules sintered at 850°C demonstrate more favorable transport parameters due to electrode build-up, i.e., the morphology of the system of active material particles in the electrode, and demonstrate superior discharge capacity. Moreover, electrodes of fine granules show an optimal electrochemical performance using NMC powders sintered at 900°C. For a better understanding of these results, i.e., of process-structure-property relationships on both, granule and electrode level, 3D imaging is performed with a subsequent statistical image analysis. Doing so, geometrical microstructure characteristics such as constrictivity quantifying the strength of bottleneck effects and descriptors for the lengths of shortest transportation paths are computed, like the mean number of particles, which have to be passed, when going from a particle through the active material to the aluminum foil. The latter one is at lowest for coarse-grained electrodes and seems to be a crucial quantity.