Impedance Analysis of NCM Cathode Materials: Electronic and Ionic Partial Conductivities and the Influence of Microstructure

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.