Lithium-ion pouch cells offer high packaging efficiency because of their lightweight, flexible pouch foil. Due to the absence of a rigid casing, the cells are typically mechanically constrained or compressed by the battery pack housing for protection and to ensure mechanical integrity. While moderate pressure can enhance cell performance by improving electrode-particle contact, excessive or inhomogeneous pressure distribution can hinder ion transport, decrease separator porosity, and promote lithium deposition [1, 2, 3].

Conventional operando cell expansion approaches use dilatometers to measure cell thickness changes under constant force conditions, while force sensors monitor the applied force under constant dilation conditions [4]. These methods allow investigation of overall pouch cell dilation or force, but do not provide information about the local distribution. In contrast, optical, multi-directional laser scanning provides spatial dilation information [5, 6], but does not represent real operating conditions because the cell cannot be investigated under mechanically constrained conditions.

The use of thin-film measurement foils overcomes this issue and provides non-destructive, operando, localized cell pressure information. Initial measurements compare the cell pressure evolution under constant dilation conditions with the cell dilation under constant force and show comparable, characteristic signals with distinct stages. This allows us to transfer dilation signal processing methods to force and spatial pressure signals. The pressure distribution of the investigated Kokam SLPB526495 pouch cell reveals four spots of increased pressure that can be attributed to adhesive substances used during the cell assembly. With cyclic aging under high C-rate, additional high-pressure regions emerge at the cell edges that can be attributed to cell surface depositions [6]. These findings demonstrate a correlation between degradation effects and inhomogeneous pressure distribution. To further analyze inhomogeneous pressure distribution, tape strips placed on the pouch cell surface create high-pressure regions. A comparison with the surrounding low-pressure regions reveals not only the expected increased pressure but also significantly increased pressure changes of the high-pressure regions during cell cycling, which results in increased stress of the high-pressure regions. Differential pressure analysis of the high- and low-pressure regions shows characteristic peaks that align with the differential voltage analysis and can be assigned to the graphite stage transitions. However, the location and amplitude of the high- and low-pressure region signal peaks differ, which is an indicator of local lithiation differences, caused by the applied pressure inhomogeneity.

The results show that spatial pressure measurement allows detection and quantification of manufacturing-induced or evolving pressure inhomogeneities. Differential pressure analysis allows graphite stage identification and can indicate localized anode lithiation. Further pressure distribution analysis will enhance understanding of localized degradation processes and can serve as an early cell failure indicator. Improved understanding of pressure inhomogeneities and their effects on cell level will also contribute to an optimized battery pack design and can thereby enhance performance and cycle life.

References:
[1] J. Cannarella and C.B. Arnold, “Stress evolution and capacity fade in constrained lithium-ion pouch cells”, Journal of Power Sources 245 (2014) 745-751, doi: 10.1016/j.jpowsour.2013.06.165
[2] T. Deich, M. Storch, K. Steiner and A. Bund, “Effects of module stiffness and initial compression on lithium-ion cell aging”, Journal of Power Sources 506 (2021) 230163, doi: 10.1016/j.jpowsour.2021.230163
[3] H. Yu, L. Wang, Z. Zhang, Y. Li, S. Yang and X. He, “Insight Understanding of External Pressure on Lithium Plating in Commercial Lithium-Ion Batteries”, Advanced Functional Materials 34 (2024) 2406966, doi: 10.1002/adfm.202406966
[4] H. Popp, M. Koller, M. Jahn and A. Bergmann, “Mechanical methods for state determination of Lithium-Ion secondary batteries: A review”, Journal of Energy Storage 32 (2020), 101859, doi: 10.1016/j.est.2020.101859
[5] B. Rieger, S.F. Schuster, S.V. Erhard, P.J. Osswald, A. Rheinfeld, C. Willmann and A. Jossen, „Multi-directional laser scanning as innovative method to detect local cell damage during fast charging of lithium-ion cells”, Journal of Energy Storage 8 (2016) 1-5, doi: 10.1016/j.est.2016.09.002
[6] F.B. Spingler, W. Wittmann, J. Sturm, B. Rieger, A. Jossen, „Optimum fast charging of lithium-ion pouch cells based on local volume expansion criteria”, Journal of Power Sources 393 (2018) 152-160, doi: 10.1016/j.jpowsour.2018.04.095