Lithium-ion batteries can age non-uniformly posing additional challenge in managing larger battery cells. For instance, a non-uniform distribution of solid electrolyte interphase (SEI) or plated lithium has been observed in cylindrical cells along the jelly roll length ^(1-2). The authors have suggested pressure distribution as a cause of this non-uniform ageing. This necessitates investigation of the effect of pressure on ageing. With different goals in mind, the effect of pressure on the rate of lithium-ion battery ageing has been studied previously ^(3-4).The work by Rubino et al.⁽³⁾ indicates  that the higher capacity fade in prismatic cells as compared to cylindrical cells of the same chemistry is due to the lower pressure in the former. However, the pressure was not a directly controlled parameter in this study. In contrast, the work by Arnold et al. ⁽⁴⁾ on commercial pouch cells shows that high stack pressure causes higher capacity fade, and that a small stack pressure is important to extend the life-time. However, the pouch cells were constrained along the thickness inducing non-uniform pressure on the different electrode layers and varying amplitudes of cyclic stack pressure during charge and discharge that is proportional to the external pressure. Hence, the above discrepancy of the effect of pressure calls for further investigation on ageing of model systems with single layer cells subjected to different levels of constant pressure. As an additional contribution, postmortem analysis is performed in order to understand how pressure level affects ageing mechanism.

Here we present a study on commercial NMC positive and graphite negative electrodes in small laboratory pouch cells. The pouch cells are subjected to different stack pressure levels at controlled temperature using spring loaded stainless steel plates. Additionally, in order to monitor any differences in current distribution due to pressure and ageing, groups of cells are connected in parallel as the current distribution is measured using high precision shunt resistors and a Keithly differential multimeter. Capacity and impedance are measured periodically in order to track the performance changes with aging.

A preliminary result indicates higher capacity fade in cells subjected to lower stack pressure. Results from EIS test indicate an increased ohmic resistance for high stack pressure cells, while low stack pressure cells show an increase in charge transfer resistance. Furthermore, different stack pressure on the cells of a parallel connected cell configuration results in current distribution with the cell subjected lower pressure taking slightly larger share of current. Details of the experimental techniques, and results from electrochemical and post mortem analysis will be presented and discussed during the meeting. Conclusions from this study will contribute to better understanding of the causes of non-uniform aging and suggestion for solutions to this problem through better cell design.

References

[1] M. Klett, R. Eriksson, J. Groot, P. Svens, K.S. Högstöm, R.W. Lindström, H. Berg, T. Gustafson, G. Lindbergh,
K. Edström , J.Power Sources 257 (2014) 126-137.
[2] M. Petzl, M. Kasper, M.A. Danzer, J.Power Sources 275 (2015) 799-807. 
[3] R.S. Rubino, H. Gan, E.E. Takeuchi, J. Electrochem. Soc. 148 (2001) A1029-A1033.
[4] J. Cannarella, C.B. Arnold, J.Power Sources 245 (2014) 745-751.