No abstract

The amount of free liquid electrolyte contained in commercial large format prismatic Li-ion battery cells was measured for both fresh and aged cells. Results show that cells can contain free liquid electrolyte in significant amounts -maximum average amount reaching ca. 18 g (>15 ml) in fresh cells of some well-established manufacturers and ca. 39 g (ca. 35 ml) in fresh cells of less-well established manufacturers. Cycle-aged cells were found to contain significantly lower amount of free electrolyte at the end of their life in an electric vehicle (EV) compared to the fresh cells of the same type. Preliminary results suggest that calendar ageing does not seem to noticeably reduce the amount of free liquid electrolyte in Li-ion battery cells. Comparing our findings to our previously published quantitative evaluation of the toxicity of Li-ion battery electrolytes, it can be concluded that an accidental release of the free liquid electrolyte from a single large format Li-ion battery cell can be sufficient for the formation of potentially toxic atmosphere in enclosed spaces.

This study analyzed a prototype of a pouch cell containing silicon alloy anodes with the potential to significantly increase the energy density, resulting in improved autonomy for electric vehicles. An electrical characterization campaign was performed, resulting in three main observations. Firstly, measurements showed a high energy density, although a high lower cutoff voltage (3.0 V) was used due to the prototypical nature of the cells. Further optimization would allow a decrease of the lower cutoff voltage, resulting in an even higher energy density. Secondly, a large open-circuit voltage hysteresis was observed, increasing the complexity for equivalent circuit models. Thirdly, ballooning of the pouch cell was observed, most likely caused by gas formation. This leads to a loss of active surface area, significantly reducing the cell’s capacity. This third observation was more thoroughly investigated by 3D computed tomography, which showed mechanical deformation of the layers. An extensive literature review revealed that the addition of fluoroethylene carbonate (FEC) to the electrolyte enhances the cycling stability of silicon alloy batteries but leads to the production of CO 2 as a side reaction. Furthermore, the usage of external pressure was proposed and validated as a methodology to reduce the production of CO 2 while improving the cells’ performance.

The mechanical degradation of 18650 lithium ion cells is studied by X-ray computed tomography and is correlated to the electrochemical performance. A method for the geometrical analysis of electrodes by computer tomography is developed and applied to charged and discharged cells. As shown in earlier studies, the geometry of the jelly roll is inhomogeneous leading to mechanical stress during charge/discharge cycles. This effect leads to significant deformations of the jelly roll, which can be analyzed by computed tomography. The detailed analysis reveals that expansion of the anode takes place as expected during charging, but the degree of expansion depends on the position within the battery cell: the largest expansion during charging was found within the area of strongest deformations, whereas other areas without any expansion of the jelly roll were also observed. It is reasoned that the observed inhomogeneous expansion/contraction contribute significantly to cell degradation. The strong expansion within the deformed areas leads to sharp bending of the electrodes resulting in delamination of active layers. On the other hand, the absence of anode expansion reflected by a lack of increase in thickness when charging may indicate pore clogging assuming that the additional volume of graphite with intercalated lithium has to be accommodated within the pore structure.

No abstract

The use of Si-containing negative electrodes is one of the most promising options to increase the energy density of Li-ion batteries. Nonetheless, increasing the Si content in the anode above 5-10 weight % is still a challenge because of the expansion/contraction behavior of the Si upon lithiation/de-lithiation. Due to a two- to three-fold volume increase of Si during charging, it is paramount to understand and manage structural changes from formation until the end of life. This applies not only at the electrode, but also at the cell level and specifically for cells with high electrode loadings close to mass production format. Here, we report here on the structural changes in Si-blended anode/manganese nickel cobalt oxide (NMC) 622 cathode 18650 format cells from production through formation until end of life by means of micro X-ray computed tomography. We constructed specially designed 18650 cells in which the jelly roll does not fill the full volume of the case. The volume change without external constraint led to the identification of three main deformation mechanisms at the jelly roll level and shed some light on the effect of the cell geometry on the use and performance of anodes with high Si-content.