With Lithium-ion batteries being the main power source in most portable electronic devices, there is always a need for further improvement of both, battery efficiency and cycle life time, to increase the overall battery performance. A big part of these improvements is to understand how the microstructure influences the electrochemical properties of the used materials, mainly the electronic and ionic conduction pathways in both electrodes. The presented work is an attempt to correlate the electrochemical properties of cathode materials on a macroscopic and a microscopic scale. Since NCM-111 is a commonly used cathode material and the specific resistance is low enough to measure the impedance of very small systems, it was chosen as a model system to compare different measurement techniques. Sintered pellets and secondary particles with a diameter between 20 and 40 µm were chosen as sample systems.
The samples were characterized via impedance spectroscopy (EIS) with temperatures varying between -60 and 100 °C. An amplitude of 50 mV was found to give optimal impedance spectra. In addition, impedance measurements were also performed using different metal electrodes to determine the contact resistance between metal electrode and active material. The spectra obtained were fitted with an equivalent circuit representing both, ionic and electronic conducting pathways (Maier et al.) including an additional RC-element representing to contact resistance of the electrode-sample interface.
The single particles were characterized using a measurement setup located inside a SEM. The particles were fixed on a sample holder with conducting silver paste as glue. Two micromanipulators were positioned on the particles and the conducting glue respectively. These micromanipulators were connected to an impedance bridge outside the SEM. The measurements were performed at room temperature and under SEM atmosphere. The conductivities estimated from these spectra are in good agreement with the values obtained from the macroscopic measurements.