4.1. Cell Disassembly
In this article, cells undergo disassembly using the procedure outlined in
Section 2.3. After disassembly and the separation of the anode, cathode, and separator components, each anode and cathode sheet is divided into three segments: a front section measuring 21 cm (AF-21), a middle section measuring 30 cm (AM-30), and an end section also measuring 30 cm (AE-30), as depicted in
Figure 9. This division is necessary because capturing the entire 81 cm long electrode in a single image is challenging. Upon visual inspection, no discernible changes are noted in the cathode of the aged cells, in line with findings by Bach et al. [
27] and Xie et al. [
25] In contrast, substantial alterations are observed in the aged anode. Consequently, photographic images of the anode are provided for reference.
Previous research explored changes in the colour of aged electrodes. The authors noted that the anode’s colour transitioned to whitish, metallic grey, or metallic sheen after ageing [
25,
27,
28]. Regardless of the specific colour descriptions, the underlying cause of this change is attributed to either lithium deposition or lithium plating. However, limited literature addresses the specific shape of the regions in which colour changes occur. In most of the published articles, authors often present images of a specific small area of the electrode rather than providing a complete view of the entire electrode. When only a small portion of the electrode is examined, it can be challenging to discern the shape of the areas where the colour change has taken place. For example, when focusing on a particular region of Cell 3’s anode rather than the complete electrode, just the change in colour may be perceived, but the shape remains indistinguishable, as depicted in
Figure 9.
In Group 1, the photographic analysis (
Figure 10) reveals the distinct characteristics of the cells. Cell 1, 2, and 3 display cone-shaped black and white shaded areas, while Cell 4 exhibits a whitish oval-shaped region at the end of the jelly roll, with a few whitish dots in the middle. Notably, some copper foil is exposed at the front of all four cells, potentially occurring during the separation of the anode and separator. The photographic image analysis shows that the changes in the anodes of Cell 1, 2, and 3 are similar, while Cell 4’s anode differs from the others. This observation aligns with the findings in
Section 3.2 and
Section 3.3. It is known from the previous section that Cell 1, 2, and 3 had similar first-life ageing, with similar initial SoH and DM values. Consequently, the second-life ageing of these cells is also similar, as depicted in
Figure 5. The photographic images of these three cells confirm that their internal changes during second-life ageing align, indicating that there is a correlation, in that Cell 1, 2, and 3 indeed underwent similar ageing during their second life, while Cell 4 experienced different ageing.
Photographic images of the Group 2 cells reveal distinct characteristics. Specifically, Cell 14, 16, and 17 display oval-shaped whitish areas at the ends of their jelly rolls with irregular patterns in the middle, while the frontal part remains mostly unaffected, except for a few tiny, exposed copper foil areas. In contrast, Cell 15 and 18 have less prominent whitish cone-shaped regions at the end of their jelly rolls, with no significant colour changes in the rest. Visually, Cell 14, 16, and 17 have similar colour-change areas, suggesting extensive lithium plating [
25].
Table 1 shows that Cell 14, 16, and 17 exhibited similar ageing in their first life, sharing similar-first life SoH and DM values. Moreover, their second-life ageing patterns are also similar, as illustrated in
Figure 6. Photographic evidence of these three cells affirms that their internal changes during second-life ageing align, suggesting a correlation, and confirming that Cell 14, 16, and 17 indeed underwent similar ageing experiences during their second life. Cell 15 and 18, on the other hand, exhibit distinct ageing characteristics. An exception is Cell 5, which visually resembles Cell 14, 16, and 17, with an oval-shaped whitish area at the end of the jelly roll and irregular pattern. However, as
Table 4 notes, Cell 5’s first-life ageing differs from the other three, leading to divergent second-life ageing pattern (
Figure 6). Nevertheless, the similarity in colour change in their jelly rolls remains unexplained and requires further investigation.
Images of the Group 3 cells exhibit distinguishing characteristics. For example, Cell 7 and 8 have irregular whitish areas at the end of their jelly rolls (as depicted in
Figure 10), and vertical stripes in the middle and irregular patterns on the jelly roll’s front area. Cell 9 and 10 exhibit U-shaped whitish regions at the ends of their jelly rolls, with no visible colour change throughout the remainder of the jelly roll. Analysing the photographic images of Group 3’s cells reveals that the anode changes in Cell 7 and 8 are quite similar, while the anode changes in Cell 9 and 10 are similar but different from those in Cell 7 and 8. As described in
Table 5 and
Figure 7, Cell 7 and 8 experienced similar ageing during their first and second life. The photographic images of these cells reveal that their internal changes also correspond, indicating that Cell 7 and 8, as well as Cell 9 and 10, underwent similar ageing processes in their second life.
The photographs of Group 4’s cells suggest that at the end of the jelly roll, an irregular whitish area is observed for Cell 6, a grey and white shaded area is observed for Cell 13, a cone-shaped grey and white shaded area is observed for Cell 12, and U-shaped whitish area is observed for Cell 11. Details of the visual observation of all the cells are provided in
Figure 10. Analysing the photographic image of Group 4’s cells, it is evident that changes in the anode for Cell 6, 11, 12 and 13 are quite different. Moreover,
Table 6 and
Figure 8 suggest that the first DM and second-life ageing of Cell 6, 11, 12 and 13 are also different.
No signs of degradation were observed on the positive electrode; all the aged positive electrodes remained uniformly black. However, since the photographic images of the cathodes did not offer any valuable information about cell degradation, they were not utilized for further analysis. Summarising the above discussion, it is observed that cells had similar first-life SoH and DM and similar second-life degradation, show similar colour and shape changes at the anode. In order to gather additional evidence supporting the likeness between cells that underwent similar ageing experiences in both their first and second lives, as well as those that exhibited similar photographic images of their anodes, SEM images were acquired for a few cells, which are provided in the following section. Cell 1, 2, 3; Cell 14, 16, 17; Cell 7 and 8; and Cell 9 and 10 experienced similar first- and second-life ageing and presented with similar photographic anode images. Among them, SEM images of Cell 1, 2; Cell 14, 16, 17; and Cell 7 and 8 were collected and are described below.
4.2. Scanning Electron Microscopy (SEM)
In this article, SEM images were obtained from the end section of the jelly roll; the precise sampling location is indicated by a red circle. SEM images of Cell 1 and 2 display a spider-web-shaped area, as shown in
Figure 11a,b.
It is presumed that the lithium dendrites have become interconnected, forming a spider-web-like structure [
27]. Since Energy Dispersive X-ray (EDX) is not suitable for lithium detection, the confirmation of lithium deposition comes from the layer observed in the photographic images of the anodes in Cells 1 and 2. However, the typical appearance of lithium deposition is usually more whitish or shiny, as shown in
Figure 10 (Cell 1 and 2). The greyish deposition observed in these cells can be explained through EDX analysis. The analysis of Cell 1 and 2 (see
Appendix A,
Figure A2 and
Figure A3) indicates a nickel (Ni) transition from the cathode side, but the Ni deposition is <0.5% wt for these cells. Despite the detected nickel quantity, it is not considered a primary contributor [
26]. Bach et al. [
27] observed that the deposition appears to be less white when NMC does not play a major role. Therefore, the presence of a grey colour in Cell 1 and 2, rather than white, can be ascribed to this occurrence. Thus, a greyish deposition is noticed for Cells 1 and 2 instead of a whitish one.
SEM images of Cell 14, 16, and 17 reveal a noticeable rough surface, as illustrated in
Figure 11c–e. EDX analysis further confirms (c.f.
Figure A4,
Figure A5 and
Figure A6) the presence of nickel (Ni), manganese (Mn), and cobalt (C) with Ni concentrations exceeding 0.5% on the rough surface of Cell 14, 16, and 17. The nickel content observed in these three cells is a primary contributing factor [
26]. However, electrode images of these three cells confirm the presence of lithium deposition. Hence, it is evident that the whitish layer consists of Lithium, Ni, Mn and Co. The reason behind the white colour of the deposition on the electrode is the existence of an NMC element with lithium. As a result of this substantial electrode deposition, a rough surface layer is evident in the SEM images. Bach et al. also obtained SEM images from areas with whitish deposits, which displayed a similar rough surface texture [
27]. Small amounts of Ni, Mn and Co are recognised to dissolve from the positive electrode and subsequently accumulate on the graphite electrode [
27]. This occurs because lithiated graphite rapidly reduces many metal ions to their metallic state [
36]. It is widely believed that these deposits significantly impact the ageing of graphite and the formation of surface films.
In the case of Cell 7 and 8, some scattered mossy-like substances were observed on the surface of graphite particles (refer to
Figure 11f,g). Xie et al. [
25] also observed similar mossy-like substances. It was hypothesised that these substances predominantly consisted of lithium plating induced by the high current during charging [
37,
38]. During their first life cycling, Cell 7 and 8 were charged at a rate of 0.7 C, which corresponds to the maximum charge current specified in the data sheet for these cells. However, this high current caused the once pristine appearance of the graphite particles to become blurred, with occasional amorphous deposits forming on the graphite surfaces due to parasitic reactions between the plated lithium and the electrolyte [
25].