European conglomerate thyssenkrupp, BMW and several other firms and research institutions are collaborating on a project called EffiForm, which aims to make a detailed study of the solid electrolyte interface (SEI) layer formation process in Li-ion batteries.
While the ins and outs of forming an SEI layer may seem arcane, in fact there’s a highly practical goal here. Scientists tell us that formation cycling, the final step in the battery manufacturing process, typically accounts for about a third of battery production costs.
Formation cycling refers to the electrochemical side reactions involved with creating the SEI layer, which forms during the first several charge/discharge cycles. An ideal SEI layer should be thin, minimally porous, electrochemically inert, electronically resistive, and ionically conductive.
In “Prospects for reducing the processing cost of lithium ion batteries,” published in the Journal of Power Sources, David L. Wood III and colleagues explain that, to form stable SEI layers that cover all of the electrode surface area and ensure good lithium ion conductivity and rate capability, there must be complete wetting of the electrode and separator pores.
“In practice, however, there are substantial barriers to wetting – the separator pores, the electrode binder, and the conductive carbon black additive. A period of 12-24 hours under vacuum is required to achieve adequate wetting during the cumbersome electrolyte filling process of cell assembly, and it still leaves a substantial fraction of the smallest pore volume unwetted.”
“Formation cycling is performed after a lithium-ion cell has been constructed, and it has a significant economic impact. The formation process requires that battery producers install many thousands of cycling stations to complete the process, which results in a heavy capital equipment investment, a much larger plant size, and a considerable manufacturing bottleneck.”
That all sounds very expensive, so a better understanding of formation cycling is sure to lead to opportunities for cost savings. The three-year Effiform project aims to optimize the materials and technologies used in the formation of cells for industrial applications. The next step will be translating the findings into formation systems for mass production.