## 1 Introduction  
During the last three decades, lithium-ion batteries (LIBs) have become the predominant energy storage for electric vehicles, portable devices, and secondary energy storage due to the high energy density. LIBs were developed in the 1980s and commercialized by Sony Corporation (Tokyo, Japan) based on a graphite anode and lithium cobalt oxide as the cathode in 1991. Since then, different cathode materials were proposed and developed in order to improve energy density and power density. However, to date, more than twenty years later, carbon-based materials are still used as the commercial anode material while the requirements for anode materials have been changed. In addition to demands of high capacities, the duration of the charging process plays an important role. Particularly in mobile applications, the ratio of battery charging time to its operation range has become a deciding factor for customers [1]. The supply of electrical charging power, as well as the technical realization of suitable charging interfaces for so-called high power charging (HPC), is essential and requires further scientific investigations, engineering, and investment costs in the near future. One promising new technical approach is based on the so-called 3D battery concept [2; 3]. The surface morphology of electrodes or active materials in a 3D battery is realized by laser printing [4; 5], chemical etching [6], chemical vapor deposition [7], physical vapor deposition [8], or micromachining [2] techniques. [PERSON] et al. [9; 10; 11] demonstrated the industrially capable ultrafast laser material processing for the generation of 3D electrode architectures. Three-dimensional architectures increase the active surface area, shorten the diffusion pathway, and the charge carriers can be inserted deeper into the electrode material within a shorter time. As a result, the high-rate capability and cell lifespan could be significantly improved. In recent research, the impact of 3D electrode architectures and cell degradation processes on the lithium distribution were studied. For this purpose, laser-induced breakdown spectroscopy (LIBS) was applied post mortem for the visualization of elemental distribution. The quantitative analysis and mapping of the lithium concentration in cathode materials were successfully realized. Local lithium plating which resulted in spontaneous cell failure could be demonstrated by LIBS [12; 13; 14].  
In this work, thick graphite electrodes with line structures were fabricated by ultrafast laser processing. The electrochemical properties of cells with unstructured and structured electrodes were analyzed by means of galvanostatic testing and cyclic voltammetry. Furthermore, cells were cycled using low and high C-rate, C/20, and 1C, respectively, at lithiated and delithiated states for LIBS measurements. Studying capacity fading as a function of the C-rate is an important measure in order to characterize LIB performances, which is a quite common procedure in battery research. Furthermore, high C-rates correspond to high-power LIB operation. LIBS is illustrated to be a suitable tool to achieve fundamental information about degradation mechanisms or diffusion kinetics as a function of the C-rate. In addition, model electrodes with micro-pillar structures were prepared for the visualization of the local lithium concentration along 3D micro-structured electrodes. The LIBS results provide a fundamental understanding of the lithium diffusion process during 3D battery operation.