This poster investigates all-solid-state batteries through optical and chemical post-mortem analysis of monolayer and multilayer cells. The goal is to identify the primary aging mechanism(s) and address design challenges. Both pristine and cyclic-aged cells were opened in an argon atmosphere after obtaining ultrasound images. Structural changes within different layers and material interfaces were observed.
The investigated cells consist of several layers. The anode is a lithium metal foil, without a current collector. At the upper corner of the anode, the lithium metal (not active part) is connected to a copper foil, which is welded to the cell tab leading out of the concealed pouch bag. The cathode consists of NMC811 particles and a solid catholyte providing ionic contact between the particles, to the solid electrolyte and to the aluminum current collector. The solid electrolyte is a hybrid polymer and ceramic electrolyte. The polyethylene polymer matrix is mixed with ceramic LATP particles. To prevent Lithium dendrite growth the hybrid electrolyte is coated on a Celgard 2500 separator. The polymer matrix soaks the separator, so the polymer provides ionic contact to the lithium foil on the other side of the separator.
On CT-images, the NMC particles are the most visible due to the high contrast. On the bottom right of the poster, a cross-section CT-image of a full multilayer cell is shown. The white lines represent both cathode layers on each side of the double-coated aluminum current collector. The current collector is not visible at this resolution. The darker lines between the cathode layers are the hybrid electrolyte and the separator. Dark spots within the cathode layer are visible. These suggest air bubbles within the layer making part of the cell inactive. In the higher resolution CT-image on the top left of the poster, all layers can be distinguished. All layers show an even thickness. The hybrid electrolyte layer is much darker than the cathode layer next to it, but is still brighter than the cellgard layer due to the LATP particles showing up as bright spots.
The top right of the poster shows an SEM image (500x magnification) of an aged monolayer cell. From left to right the following different layers can be seen: current collector, NMC/catholyte, hybrid electrolyte and separator. The lithium foil was removed while preparing the sample for this measurement and the higher resolution CT-image. The current collector is a solid layer. The cathode layer shows the single NMC particles connected by the catholyte. The NMC particles are not evenly spaced and areas without any NMC particles and only catholyte are visible. The particles show no cracks and no obvious contact loss to the catholyte. The hybrid electrolyte looks similar to the cathode layer. It is distinguishable by the smaller LATP particles compared to the NMC particles. Also, the LATP particles are less bright. The polymer matrix looks the same as the catholyte and no transition is visible, so the layers look fused. The separator looks solid and no LATP particles have penetrated the separator, because they are too big.
On the left of the poster, two ultrasound images of two different multilayer cells are shown. The left image shows an uncycled cell. The main feature is a line along the outer parts of the cell. On the outside of that line, there is only a bright yellow. This suggests delamination between two different layers. While opening these cells under an argon atmosphere it was observed, that the inner parts of the lithium foil always stuck to the separator, but now always on the outer parts of the cell. Therefore, it is likely, that the delamination shown in the ultrasound image is between the lithium foil and the separator. All the layers within the line are assumed to be active. Here some dark spots are visible, which suggest air bubbles. While opening the cells some air bubbles between the separator and the cathode layer on both the outer layers of the cell could be observed. However, they didn’t quite fit the ultrasound images exactly, so there must be other contributing components. Generally, the active parts of the cell look quite homogeneous. The verticle lines are a measurement artifact. It has to be noted, that the ultrasound imaging was used as a nondestructive measurement without opening the cell. Therefore, the area from the pouch bag to the active parts of the cell is also included in this measurement and some features could be ascribed to this. The right image shows a cell cycled under pressure with low SoH. The main feature is that there are no homogeneous areas and the entire cell looks very fragmented.
On the right of the poster, two scanner images of two different multilayer cells are shown. In both pictures, the top layer is the lithium foil, which was previously in contact with the separator. Below that are the separator and the hybrid electrolyte, which are white and semi-transparent, so below the separator and the hybrid electrolyte the cathode layer is also visible. The left image shows an uncycled cell. The lithium is smooth, shiny and malleable. Because the lithium foil is larger than the cathode area the lithium is stuck to the separator at the bottom. This would normally be the anode overhang. At the top of the image, part of the lithium is ripped off, because it stuck to the other separator when the layers were separated. This is the layer where most of the lithium stuck to one of the two sides of the separator. The Adhesion between the lithium foil and the separator is the weakest of all the different layers, so it is the only point of separation while opening a cell. The Adhesion for the uncycled cell was still stronger than for the aged cell. The homogeneous lithium layer explains the homogeneous ultrasound image on the left of the poster. The right image shows a cycled cell with low SoH. While opening some of the lithium stuck to the other separator, so it is not shown in this image. The lithium looks rougher and darker than the fresh lithium. It is also much more brittle than the fresh lithium. This explains the fragmented ultrasound image on the left of the poster. At the top right of the image, some darker spots are visible. This is lithium, which was more difficult to remove than the bulk of the lithium. It seems to have grown into the polymer on top of the separator towards the lithium foil.
On the bottom left of the poster, a SEM-EDX image of the interface between the lithium foil and a polymer layer is shown. This polymer layer is unique to this cell type and was applied on top of the separator side facing the lithium foil, where previously only the soaked-through polymer was in contact with the lithium foil. The cell was not cycled, yet there is a layer forming at some parts of the interface, that is not identifiable as either Lithium or Polymer by EDX analysis. This suggests calendar aging of the lithium, if it is in contact with enough Polymer for some time.
On the bottom right of the poster above the full-cell cell CT-image three images of the same cell are shown. This cell was cycled under pressure until a very low SoH. It aged faster than two other cells of the same type under the same aging conditions. On the left is an ultrasound image that shows a few different features. On the left, it looks fragmented. A middle strip is clearly distinguishable from the left part where the cell looks very homogeneous. On the right and reaching into the middle strip is a rectangular outline. On the top right and on the right a line and an area is visible, that looks very similar to the delamination in the image on the left of the poster. The image in the middle shows the opened cell from the same angle as in the images on the right of the poster. Here also part of the lithium stuck to the other part of the separator. The same middle strip as in the ultrasound image is visible. The lithium is still stuck to the separator and it looks more homogeneous than the lithium outside of this middle strip. This phenomenon was observed for all the layers of this cell, but not for any of the other cells. This middle strip is the result of the setup, which pressurized the cells during cycling. A part of the pressure setup in the exact shape of the middle strip stuck out too far, so the middle part was under more pressure than the other parts of the cell. The rectangular outline was most likely caused by a sticker with information about the cell that was left on the pouch bag before pressurizing. The right image shows the same as the middle image except that the lithium was removed. This reveals a brown mark on top of the separator along the edge, where the middle strip was. An interpretation is that the outer parts of the cell aged the fastest and faster than cells with even pressure. This is also suggested by the more brittle and inhomogeneous lithium outside of the middle strip. After the outer parts became inactive, the current density at the edge of the still-active middle strip became very high, because a cathode overhang has been created. So parts of the cathode, which did not have lithium directly opposite, remained active by a diagonal flow of lithium ions from the outer parts of the cathode to the edge of the still active lithium because that was the shortest path.
In conclusion, the contact between the different solid layers remains challenging. Air bubbles can be trapped during mixing and assembly, which cannot be filled later due to the lack of liquid electrolyte. Dendrite growth is still a problem for polymer-based electrolytes. External and even pressure is necessary to keep electrical contact between the different layers. Post-mortem analysis is challenging since there is no natural separation between the layers.