Understanding the electronic and ionic transport properties inside active cathode materials and through its surroundings in lithium-ion battery cathodes is crucial for optimizing battery performance. Such studies are most often done by EIS measurements in combination with rate performance and cyclability. However, these measurements can at most link changes in performance with changes in average resistance values which can, with at least some ambiguity, be attributed to certain interfaces and interphases. For a particle composite electrode with intrinsic distribution in porosity, particle size and tortuosity, the number of interfaces and their variations are innumerable. The number and positions of the electronic contacts made to one single active material particle alone is difficult to predict. The distance between the carbon nanoparticle chains wrapped around the particle determine the resistive drop and current/potential distributions. Furthermore, the electrical contact of the (semiconductor) active material with current collector and carbon black is simply assumed to be Ohmic with inconsequential contact resistance independent of the state-of-charge. Especially at high C-rates, these electronic effects might affect battery operation. We present a methodology to build experimental models to characterize and quantify the individual interfaces using thin-film stacks and patterning of metallic contacts with controlled area and pitch. The quantitative characterization of single interfaces can be used as input parameters for mathematical modeling of simulated electrode structures.  

In this work, we will present a strategy for patterning Lithium Manganese Oxide (LMO) thin films, while preserving both the electrochemical activity and the structural morphology of the films. Having successfully established this process, we patterned metallic test structures on top of LMO films. Furthermore, employing the transmission line method [1], we calculated the conductivity of LMO and the contact resistance between LMO and the metal test structures. This approach enabled us to effectively benchmark the electron transport capabilities of different materials that serve as current collectors. Next to quantification of contact resistance, our results provided insights in the effect of grain or crystal size on conductivity of the LMO films. The grain size is controlled by the thermal budget during crystallization of the deposited films

In a further example, we address the use of titanium dioxide (TiO2) protective coating, deposited via atomic layer deposition (ALD) on our LMO films. The few nanometers thin film thickness permits lithium ions to traverse the film, while it slows down cathode degradation and suppress side reactions with electrolytes [2]. Although TiO2 is a dielectric and generally not expected to enhance electron transport directly, it can significantly affect the interface where electron transfer occurs. Our study aimed to determine the impact of TiO2 presence on contact resistance and to establish how its thickness could influence both electron transport characteristics and lithium-ion insertion/extraction dynamics. This analysis provides insights into the optimal thickness of TiO2 layers before they begin to compromise the cathode’s performance, offering a refined understanding of how such protective layers interact with both the electron and ionic transport.

The developed methodology already provides valuable insights into the effects of coatings and allows for the benchmarking and exploration of new materials. Looking ahead, this approach will enhance our understanding of the processes occurring within the composite cathode.