Title: Mechanical Behaviour of Lithium Ion Pouch Cells Under the Influences of Electrochemical Aging or Mechanical Injury
Authors: Shi Lei , Kunz Ulrich 
Published: February 2020

As a major obstacle preventing the industrial application of the lithium ion batteries on a larger scale, such as in the automobile industry, the capacity retention which represents the characteristic of the E-Mobile remains still a challenge for further investigation. Among the causes leading to the capacity degradation mechanical stress is a most important inducement. Thus, it is necessary to take a look at the mechanical behaviour of a cell with serious capacity deterioration.

The mechanical stress can either be generated by the lithium insertion inside the cell during the running electrochemical process or be imposed by an external force before the cycling during the manufacture. Accordingly, the mechanical behaviour of the electrode must be observed considering electrochemical aging and mechanical injury respectively.

In our work the mechanical behaviour is investigated by monitoring the surface displacement by digital image correlation technique (DIC) parallel to the cycling according to a constant current scheme (CC) in a Voltage range between 3.0V and 4.2V. Two lithium ion pouch cells which are situated in different conditions – at the end of its cycle life and under influence of mechanical injury were selected for this research. Each samples consists of single anode (65x45mm) and cathode (70x50mm) using NMC (nickel manganese cobalt oxide) and MCMB (Meso Carbon Micro Beads) as electrode materials for cathode and anode respectively. While 12 tests according to the same procedure one cell was charged and discharged with 5 currents (6mA, 8.5mA, 11mA, 13mA, 20mA) for 3 cycles at each charging current, and 3 different charging currents (3mA, 5mA and 6mA) were applied on the other sample after it was damaged mechanically.

Normally, the lifespan of a lithium ion cell can be divided into three phases: the first two or three cycles with a considerable decrease of the capacity due to the formation of the surface film and a fierce volume expansion; the following cycling with a stable capacity retention at a steady level; the last phase exhibiting an irreversible capacity decrease to such a low level that the cell is no more useful. According to our results, however, the decrease of the capacity is not gradually, even in the last phase of the cell’s lifespan, but abruptly at a certain cycle, which is represented not only by the sudden fall of the capacity but also on the progress of the displacement. In comparison with the displacement behaviour on a cell at normal working condition, anomalous irreversible progress develops and can be recognized as increasing displacement resulting from the irreversible processes inside the cell. When the limitation of the mechanical durance of the system is exceeded, a permanent deformation is formed and then the displacement maintains a fluctuation at a low level indicating the low intensity of the remaining electrochemical reaction. Besides, the irregular behaviour is no longer located merely in the edge areas of the cell but can be observed all over the cell surface as displacement pattern with low intensity which describes the local difference of the surface displacement. By post mortem analysis of the cell it can be concluded that the weakened adhesion and the exfoliation of the electrode material on the current collector may be the major cause for this behaviour.

In comparison, the mechanical injury causes more serious damage of the electrode structure than normal cycling, resulting in capacity deterioration in only a few cycles. This is accompanied with severe permanent displacement evolution over the entire displacement pattern on the anode surface. While the displacement in the area without the imposition of the injury exhibits continuous progression on the displacement plot which illustrates the temporal evolution of the displacement, a sudden drop on the displacement plots appears in the injured area correlating to the exfoliation due to imposed mechanical damage.

To summarize our results, we can drew the conclusion that the electrochemical state of the cell can be associated with the displacement behaviour on the surface of the cell. The deterioration of material adhesion and exfoliation lead to loss of electrical connectivity which is the determined steps for severe capacity degradation. Mechanical injury brings fierce structure damage which can accelerate the degradation progress. Furthermore, we can assume that the aging of the anode caused by electrochemical cycling develops finally into structure damage, comparable as if the anode was injured by mechanical forces. The improvement of the electrode integrity is the most important measure to upgrade the capacity retention of the lithium-ion battery.

Figure 1