**4. Conclusions**

In this work, a sphere was used as a force transmitter to induce an inhomogeneous pressure distribution on a cycled LIB. The sphere shape results in a heterogenic pressure distribution on the LIB, which has its maximum in the middle point and decreases towards the edge and induce a ring of locally high electrochemical activity that leads to lithium plating. The force of 1.4 kN, which results in a maximum pressure of 20 MPa, was chosen so that only the separator closes its pores, and no influence on the positive and negative electrode should take place. The experiment in this thesis has shown that pressures in this low range are sufficient to cause defects in the active materials.

A macroscopic PMA of the cell shows that local mechanical effects change the properties of different components. Very inhomogeneous state of charge on individual layers show that these lose contact from each other in the multi-layer system. This contact loss indicates an internal bending of the layers due to different mechanical stress. Moreover, a discoloration was found on the individual two cathodes for which there is no explanation. Furthermore, the PMA showed that the load on the

separator at the pressure point was very different, which could be seen optically by different color changes or transparencies. The edge of the pressure point had the most substantial color change, which indicates a secure pore closure of the separator. The center indicated a still partially active region. A silver ring of metallic lithium around the pressure point found on all anodes during the PMA. The lithium plating ring had a width of 1.8 mm and began where the substantial color change of the separator ended. A high current density due to the pore closure of the separator generating an overpotential in this region and create conditions which favor plating created the lithium plating ring.

A microscopic examination at selected locations on the separator, anode, and cathode confirmed and showed new findings regarding the effect of the pressure point on the LIB. The examination of the separator confirmed that below the pressure area, the separator was still partially active, as partly closed and open pores found in this region. The edge area of the separator showed closed entirely pores, which led to the uneven current distribution. The findings of the cathode show that almost all particles were crushed and deformed under pressure. Although the chosen maximum pressure should theoretically be too low for this, therefore, there must have been additional pressure development. The additional pressure development during (de)lithiation of the particles can be an explanation for the extreme increase in pressure. Since the graphite expands by 5% to 11% [17,22] and the cathode expands by 1% to 2% [17,23] the increase in pressure is probably caused by the anode over the cycling.

Moreover, it assumed that the fact that the particles were soaked with electrolyte has changed their elastic properties. Therefore, the authors assume that the pressure influence of dry active materials cannot be transferred to battery cells with filled electrolytes and has a strong influence. The findings of the anode show a transition of the pressure point to the lithium plating ring. In the area of the pressure area, a new insulating layer has formed on the particles because the SEM images have become blurred due to electrostatic charging. In the area of the lithium plating ring, the morphology has changed. This microstructure could be lithium dendrites or reaction products with deposited lithium.

Overall, homogeneous pressure distribution on LIB must be considered during the battery module design, especially pressure points with small areas should be prevented that can lead to unexpected phenomena. This pressure points can result in high-pressure development within the LIB during operation, causing lithium plating and dendrite growth, which can cause short circuits by penetrating the thin separator and trigger a thermal runaway.

**Author Contributions:** Conceptualization, G.F. and D.U.S.; Formal analysis, G.F. and F.R.; Methodology, G.F.; Validation, G.F.; Investigation, G.F. and L.W.; Writing—original draft, G.F.; Writing—review & editing, G.F., L.W., F.R. and D.U.S.

**Funding:** This research received no external funding.

**Acknowledgments:** The authors would like to thank Philipp Wunderlich (Institute of Inorganic Chemistry) for the SEM images EDX analysis of the probes and Rita Graff for the ICP-OES analysis. Philipp Dechent for proofreading.

**Conflicts of Interest:** The authors declare no conflict of interest.
