Model-based Design and Assessment of optimized Fast Charging Strategies for the Formation of Lithium-Ion Batteries

Summary:

The formation of lithium-ion batteries is one of the most time-consuming processes during the production. Common formation methods consist of several relatively slow constant charging and discharging cycles (up to 50 hours) as it is believed to guarantee good performance of the Solid Electrolyte Interphase (SEI) on the negative electrode. The SEI has a significant impact on the performance, aging and safety of battery cells. Aiming at faster production times, cheaper production costs and better properties of lithium-ion batteries, the optimization of common formation methods is necessary. This contribution is focused on developing an optimized formation procedure by model-based design. NMC622/G cells are characterized with a three-electrode setup in order to parametrize a developed electrode equivalent circuit battery model. The output of the battery model are time and voltage optimized fast charging profiles. Theses fast charging strategies are limited by different restrictions as the maximum current, minimal electrode voltage or maximum charge amount. The optimization is designed to prevent lithium-plating during the formation which limits the maximum charging current and minimal charge time. Based on a sensitivity analysis, appropriate constraints of the optimization are selected. The optimized fast charging profile is applied to the formation procedure of new battery cells. The performance and aging of the cells with the fast charged formations are compared to cells with recent state-of-the-art formations. Furthermore, post-mortem analysis has proved the prevention of lithium-plating during the initial cycle of the fast charging formation of about 65 mins. The performance and aging of the fast charged cells show good properties while having significantly shorter formation times (>20%) compared to cells with conventional formation procedures. In addition to that, the cells with the fast formation show the best aging properties at 20°C cycling and 60°C storage. This concludes to a good SEI performance by the fast charged formation strategy.