Revealing the Impact of Mechanical Pressure on Lithium-Ion Pouch Cell Formation and the Evolution of Pressure During the Formation Process

The formation is a crucial step in the production process of lithium-ion batteries (LIBs), during which the solid electrolyte interphase (SEI) is formed on the surface of the anode particles to passivate the electrode. It determines the performance of the battery, including its capacity and lifetime. A meticulously designed formation protocol is essential to regulate and optimize the stability of the SEI, ultimately achieving the optimal performance of the battery. Current research on formation protocols in lithium-ion batteries primarily focuses on temperature, current, and voltage windows. However, there has been limited investigation into the influence of different initial pressures on the formation process, and the evolution of cell pressure during formation remains unclear. In this study, a pressure-assisted formation device for lithium-ion pouch cells is developed, equipped with pressure sensors. This device allows for precise adjustment of the initial pressure during cell formation and enables the recording of pressure evolution. Employing the initial pressure as a variable, five groups of pouch cells are subjected to distinct initial pressure settings for individual formation processes. Simultaneously, the pressure evolution signals during formation are meticulously recorded. The results reveal that applying an appropriate initial pressure (0.05/0.1 MPa) during the formation resulted in a capacity enhancement of over 2% for the pouch cells, accompanied by significantly improved consistency. Conversely, both inadequate and excessive pressure adversely affect the capacity of the battery. The effectiveness of the formation is analyzed using rate capability testing, electrochemical impedance spectroscopy (EIS) testing, battery disassembling, and scanning electron microscopy (SEM). Additionally, the pressure evolution during formation is scrutinized to track the growth of the SEI from a unique perspective.