Elemental Mapping of Entire Thick Film Electrodes By Laser-Induced Breakdown Spectroscopy

Abstract

Next generation lithium-ion batteries require new research and development in the field of high energy / high power materials and concepts regarding cell design and electrode architectures. Particularly for automotive applications, the identification and control of degradation processes are main issues for maintaining long cell life-time and high safety standards. One promising approach for the visualization of elemental composition in lithium-based battery systems is the appliance of laser-induced breakdown spectroscopy (LIBS), which enables a rapid screening of materials, e.g., entire electrodes. So far, this diagnostic approach is being used for semi-quantitative investigation of battery components. However, at KIT, a full-quantitative analysis was developed for characterizing the lithium distribution and concentration as function of state-of-charge. From technical point of view, recording of three-dimensional (3D) elemental profiles, verification of material stoichiometry or even a quality controlled analysis during continuous roll-to-roll production can be easily performed under standard ambient conditions. In this work, we will focus on establishing LIBS for quantitative chemical mapping of entire electrodes after electrochemical cycling. Electrodes with variation in local porosity and tortuosity have been prepared by mechanical loading, while fs-laser radiation was applied for the development of 3D electrode architectures. First of all, on anode side, an increased tortuosity was realized by separator defects, consciously placed in coin cells during cell assembly. Simply saying, the separator sheet was blocked on selected regions in order to investigate post-mortem the possible emergence of local lithium plating. Secondly, compression molding was locally performed for studying lithium distribution in cathodes showing a sudden change in porosity. Two types of geometrical arrangements were applied for the formation of low and high porosity regions. Briefly, non-uniformities in porosity and tortuosity affect the electrochemical performance, especially at charging and discharging currents equal or above 1C. In an early stage of electrochemical cycling, a significant drop in specific capacity could be observed for both types of electrodes. Yielded from quantitative LIBS data, an enhanced local lithium concentration could be detected at the electrode surface, located in the neighborhood of separator defects (for anodes) and at the interface between low and high porosity regions (for cathodes). In contrast, fs-laser generated micro-pillars provide new pathways for Li+-ions, leading to a tremendous boost in diffusion kinetics. Thus, an enhanced specific capacity was observed for cells with laser-structured electrodes during high power conditions. Chemically proven by LIBS, a slight enrichment of intercalated lithium could be detected along the contour of each laser-generated micro-pillar, which indicates that laser-structuring can fulfill the requirements for next generation batteries, providing both, a contemporaneous high energy- and power density on battery-level. In summary, a full screening of entire electrodes achieved from post-mortem LIBS studies will be presented, based on influencing factors such as porosity, tortuosity, and laser-assisted surface modification.