## 3 Results and Discussion
### Analysis of Lithium Distribution
#### 3.3.2 Lithium Distribution within Structured and Unstructured Electrodes  
The galvanostatic measurements show that cells with the structured and unstructured electrode can deliver a similar capacity at the formation step applying a low C-rate (C/20). In this case, sufficient time is available for lithium insertion deep into the graphite layers, and the cells can reach their maximal specific capacity around 340 mAhg\({}^{-1}\) independent of lithium-ion diffusion length and electrode architectures. Figure 5 presents the 3D lithium concentration from the top of the electrode down to the current collector layer by layer. During the LIBS measurements, the laser beam was scanned over the electrode surface with a pulse shift of 100 \(\upmu\)m. The ablation rate is ~6.4 \(\upmu\)m per layer on average which depends on the local porosity and lithium amount in the electrode. The 3D mapping indicates that the first and second layer contain more lithium in comparison to the other layers. In general, a homogeneous lithium concentration can be determined within the entire unstructured electrode. The positive influence of 3D architectures on the electrochemical performance could be successfully demonstrated by rate capability tests. Model electrodes with micro-pillar structures were fabricated for the visualization of local lithium distribution at high C-rate. Regarding the result in Figure 2a, significant capacity fading is observed at 1 C. For this propose, the model cells were lithiated at 1 C applying the cycling procedure in Table 1. Three-dimensional mapping was performed on the entire electrodes. Figure 6 presents the lithium concentration on layers 2, 6, and 12, respectively. The first layer was neglected due to surface contamination. Homogenous lithium concentration could be  
Figure 4: Typical processed LIBS spectra from a lithiated graphite anode (0.01 V) used for the calibration procedure. Used elemental emission lines are labeled in the diagram. Figure 5: 3D illustration of lithium concentration within a full lithiated graphite electrode at 0.01 V.  
visualized along the first layer (layer 2) which is close to the separator. The lithium concentration in the micro-pillars is reduced from the top of the electrode down to the current collector. Long diffusion pathways for lithium-ion restrict the lithiation process. In layer 12, no lithium element could be detected in the center of some micro-pillars. Using high C- rates, lithium-ions accumulate on the interface between the electrode and electrolyte, which results in high cell polarization. A complete lithiation process takes place during the \"constant voltage\" step as a matter of fact, which can be determined by galvanostatic measurements. The visualization of the local lithium concentration in one micro-pillar is presented in Figure 7. It is obvious that the lithium concentration is increased along the contour of the structure. The ratio of nLi to nC at the fully lithiated state was 1.46 abstracted from Figure 3. The color illustration in Figure 7 indicates that the lithiated depth along the sidewall was in the range of 50-80 \(\upmu\)m. Lithium-ions prefer to diffuse directly from the electrolyte into bulk material through the laser-generated sidewall. The obtained Li-concentration depth profile is crucial information for electrode design regarding C-rate, pitch distance, and demands of the application. In addition, the Li concentration dropped dramatically in the direction to the current collector. The volume expansion of the graphite material is about 10% [20]. However, mechanical pressure on the electrode surface played a non-negligible role in the intercalation process. Additionally, the compressive load on the separator due to the coin cell design reduced its initial porosity and restricted the lithium-ion transport [14; 21]. Figure 6: Lithium concentration of a model electrode at layer 2, 6, and 12.