Post-Mortem Analysis of a Commercial Li-Ion Pouch Cell and the Effects of Aging on Overall Battery State of Health (SOH)
Authors: Ostrander John , Vie Preben J. S. , Wragg David Stephen , Koposov Alexey , Lian Torleif , Wind Julia , Shearing Paul R , Wahl Markus Solberg 
Published: February 2024

In the past 20 years, the use of lithium ion batteries (LIBs) has increased exponentially. LIBs are found in electric passenger vehicles (EVs), marine craft, and are being explored for larger vehicles and even aircraft. In order to have more sustainable operation, it is necessary to better understand the aging mechanisms involved in battery degradation which affects performance and remaining capacity which are primary metrics for a battery’s state of health (SOH). When the SOH of a battery reaches a certain threshold, it may no longer be useful in its primary role, and can be unsafe to continue using. Some approaches are to utilize the cell in a second life application, but in the end there are processes that can lead to hazardous cell failure.
Here we investigate the effects of aging on large commercial Li-ion pouch cells from a post-mortem perspective. The effects of ageing are well known, but the causes are not always well understood. The deterioration of cell health can arise from a number of sources namely loss of lithium inventory (LLI)[1], physical degradation of the NMC cathodes1-2 , electrolyte degradation, graphite anodes being more resistant to lithiation3, and the electrode separator material may also become less conducive to ion transfer4 due to aging. Cumulatively, these contribute to overall deterioration of battery SOH.
This study is based on results from a battery life study performed on a large renowned Li-ion pouch cell with an initial capacity of 64 Ah. More than 100 cells were tested following a selected test matrix including different temperatures, currents, State-of-Charge and with or without mechanical constraint of the cell. All cells were tested until they reached a remaining capacity of between 60 to 80 % State-of-Health.
The investigated cells were part of a large-scale life cycle study, in which over 100 cells were tested under varying temperature, current and state-of-charge windows, both, with and without mechanical constraints of the cell. In this study we compare the post-mortem findings of three different cells: one uncycled cell, and two cells cycled at 25oC and 0-100% SoC to 80% remaining capacity (SoH). The difference between the two cycled cells was that one cell was mechanically constrained while the other cell was allowed to swell. We present electrochemical testing data for both the full cells and half cells from the recovered post-mortem electrodes as well as detailed electron microscopy, XRD and synchrotron results. Capacity losses, decreased rate capabilities, and physical degradation pheonomena are presented and discussed. Additionally, this study also looks at the calendar aging of recovered electrodes.
References
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