Lithium-ion battery cell formation: Status and future directions towards a knowledge-based process design

The battery cell formation is one of the most critical process steps in lithium-ion battery (LIB) cell production, because it affects the key battery performance metrics, e.g. rate capability, lifetime and safety, is time-consuming and contributes significantly to energy consumption during cell production and overall cell cost. As LIBs usualy exceed the electrochemical sability window of the electrolyte, formation is required to activate and stabilise the electrochemical reactions. Enhanced battery technologies are poised to further expand voltage windows and harnessing conversion or metal electrodes to elevate energy density, thereby magnifying the significance of cell formation in the battery realm. Despite its critical importance, even the understanding of the formation process of conventional LIBs is still incomplete due to numerous influencing factors. Complex internal processes and the associated high experimental and simulation effort make it difficult to gain a thorough understanding of the process and hence to optimise it. This review paper provides a systematic overview of the formation process and its influencing factors. It is emphasized that material and cell design and the formation process are not independent, but must interlock with each other. Promising experimental and simulative methods to gain the required understanding of the interplay for a truly knowledge-based design of the formation process are highlighted. In the concluding discussion research gaps are identified and a perspective for development of tailored cell formation processes for current and future battery technologies is outlined.