Analyzing the impact of different formation strategies on large-scale format Li-ion battery cells

The formation of Li-ion batteries after production is essential for safe and reliable cell operation, however it is one of the most time-consuming and cost-intensive steps in the entire battery cell manufacturing chain. Optimized formation strategies contribute significantly to battery safety, lifetime and reliability. Forming of a high quality, homogeneous passivation layer (solid electrolyte interface, SEI) on the anode surface is especially crucial for long-term cell operation. By selecting a suitable current profile, the quality of the SEI can be considerably influenced. Formation strategies with minimized process time are preferred in order to reduce manufacturing costs while still avoiding potential degradation mechanisms or loss of quality. A significant challenge is the generation of forming gases between the electrode surfaces during the formation process, which leads to partially inhomogeneous current densities and thus can promote the undesired deposition of metallic lithium on the anode surface, the so-called Li-plating.

In this work, large-scale format battery cells are produced at the research production line (FPL) at ZSW in Ulm and different formation strategies are tested to optimize the activation process. In order to obtain a valid analysis, the electrochemical cell properties are compared during individual formation cycles as well as using a standardized end-of-line test to evaluate the cell quality. After formation, selected cells are disassembled to identify any occurring damage mechanisms.