Commercially used LiNi1/3Mn1/3Co1/3O2 (NMC111) in lithium-ion batteries mainly consists of a large-grained nonporous active material powder prepared by coprecipitation. However, nanomaterials are known to have extreme influence on gravimetric energy density and rate performance but are not used at the industrial scale because of 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 microsized secondary particles with a primary particle scale in the nanometer range is preferable. In this paper, the preparation and detailed characterization of porous hierarchically structured active materials with two different median secondary particle sizes, namely, 9 and 37 μm, and primary particle sizes in the range 300–1200 nm are 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 because of electrode build-up, that is, 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, that is, of process-structure–property relationships at both granule and electrode levels, 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, such as 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.

We present a technique for systematically investigating electronic and ionic charge transport in single Li­(Ni1/3Co1/3Mn1/3)­O2 (NCM 111) secondary particles as a function of size. We perform electrochemical impedance spectroscopy employing ion-blocking electrodes. Micrometer-sized spherical particles are arranged in cylindrical particle traps on a patterned substrate. A specially designed electrochemical cell is used to contact and measure individual immobilized particles in a defined contact geometry. The obtained electronic and ionic resistances of the particles as a function of size are compared with model calculations based on a homogeneous sphere with finite contact areas. The modeling reveals that electronic transport mainly occurs in the bulk of the NCM 111 particles, whereas ionic transport takes place along the particle surface. The extracted material parameters are in good agreement with literature values, showing the reliability of our measurement technique and its potential for systematic studies on the single-particle level.

The quantitative influence of microstructure, porosity, surface area, and changes in the crystal lattice on the electric conduction mechanisms in cathode-active materials for lithium ion batteries and therefore on the performance of a battery cell is largely unknown. To correlate the transport properties of LiNi1/3Co1/3Mn1/3O2 (NCM-111) as model type layered cathode material with its structural properties, a systematic study of the temperature dependence of the impedance of the material was performed on a set of sintered NCM-111 pellets. By variation of the sintering temperature from 850 to 1000 °C, the porosity of the material was tuned between 2 and 45%, while the grain size of the primary particles in the pellets varied between 50 nm and 1.5 μm. A careful analysis of the impedance spectra using selectively blocking electrodes allowed for the separation of the electronic and ionic partial conductivities of NCM-111. Depending on porosity and grain size, strong variations of the electronic partial conductivity were found ranging from 1.4 × 10–6 to 6.8 × 10–9 S cm–1 accompanied by an increase in the activation energy from 0.37 to 0.61 eV. The ionic transport properties exhibit similar behavior. Rietveld refinement of the X-ray diffraction (XRD) patterns of the pellets reveals that the increase in activation energies correlates with the volume of the unit cell. A Meyer–Neldel behavior is observed for both the ionic and the electronic partial conductivities, allowing for the evaluation of the defect formation enthalpies for lithium vacancies (1.74 ± 0.56 eV) and electron holes (1.36 ± 0.59 eV). These findings illustrate the complex relationships among microstructure, morphology, and transport characteristics, highlighting the need for careful design of active materials.