Post-Mortem Analysis of Calendar-Aged 16 Ah NMC/Graphite Pouch Cells for EV Application

Abstract

Application of Li-ion batteries for transportation not only requires long cycling life but also the preservation of the electrochemical performance during the resting period. For certain car usage this resting time could be predominant compared with the cycling activity and is referred to as calendar aging. To understand the aging mechanisms during calendar aging, an extensive post-mortem study was conducted on commercial 16 Ah NMC/graphite pouch cells stored at 5, 25, 45, and 60 °C. The post-mortem analyses were performed in parallel within three separate laboratories across Europe. They included visual inspection and structural and microstructural analysis along with a combination of analytical techniques to determine accurately the composition of positive (NMC) and negative (graphite) electrodes and the electrolyte. A direct correlation was established between the calendar-aging temperature and the degradation of the cells. The measurements revealed a severe deterioration phenomenon for the electrodes aged at 45 and 60 °C. These results are explained by the formation of a resistive interface on top of the negative electrodes due to a continuous and heterogeneous growth of a surface layer. Electrochemical impedance spectroscopy and electrochemical measurements confirm the resistance increase during cell degradation. At high temperatures, this occasionally leads to a Li deposition phenomenon. Nonetheless, we revealed that this degradation process does not affect the bulk structure of the materials but only the surface of the particles.

Low‐Temperature Charging and Aging Mechanisms of Si/C Composite Anodes in Li‐Ion Batteries: An Operando Neutron Scattering Study

“…This indicateda ni nhomogeneous phase distribution over the electrode depth because the ratio between LiC 6 and LiC 12 was stronger.F rom simulations, more of the LiC 6 wase xpected to be located at the anode surface( near the separator). [45] This was in accordance with measurements [20,33,42,46] and simulations [17] that have shown that Li deposition (favored by high lithiationd egrees) [18,47] is also mostly located on the anode surface.…”

“…The loss of cyclable Lih as also been observed for other cells withoutS i compound in the anode. [39][40][41][42][43][44] Therefore, ap art of the capacity loss in the cells was likely to originate from the growth of a solid electrolyte interphase (SEI) layer on the graphite particles in addition to the aging effect of the Si compound;h owever,a large part of the film growth and the low coulombic efficiency can most likely be attributed to the formation of Li silicates.…”

Microstructural Degradation of Ni‐Rich Li[NixCoyMn1−x−y]O2 Cathodes During Accelerated Calendar Aging

“…In previous calendar aging‐related literature, as‐assembled cells were stored in various conditions (e.g., SOC, temperature, charging scenarios) over periods of >1 year to obtain detectable changes in the electrode materials and cell structures . It is very likely that limited electrolyte volumes and tightly assembled cell components are subject to slow progress.…”

“…However, compared to the pristine electrode, the c ‐axis lattice parameters of both compositions are increased after aging, while the a ‐axis lattice parameters are slightly decreased. It seems that the decrease of Li content in the aged electrode materials increased the average oxidation state of the transition metal ions and the repulsive forces between slabs, yielding contracted a ‐axis and expanded c ‐axis lattice parameters . In addition, cross‐sectional SEM images of the electrodes before and after aging are observed to compare the mechanical stress incurred by the accelerated calendar aging (in Figure c–e).…”

“…Until now, most researchers have studied calendar aging effects using commercialized cells with graphite as the anodes, and low‐Ni Li[Ni x Co y Mn 1–x–y ]O 2 (NCM) materials such as NCM 333 or 523 as cathodes . Similarly to cyclic aging, the most common degradations observed from calendar aging are capacity fading and losses in power performance.…”