Welcome to the honor section of this course, I'm so pleased that you decided to investigate this, as well. In this lesson, you're going to learn more about how battery cells are fabricated, especially lithium-ion battery cells. And I believe this will help you to understand how the cells actually work. Some of what you see in this lesson will be a review from what we've talked about already earlier in the course. But quite a bit will be new, and as we proceed through this week there will be a lot of new material. Remember that lithium-ion batter cells are made in a number of different form factors. Cylindrical cells look like cylinders and you can see some examples in the picture on the slide. And they're formed by wrapping an electrode structure in a spiral called a jelly roll. The second photo shows some prismatic cells which are packaged in a rectangular container, and these are formed by wrapping electrodes again in a jelly row, but in a flatter, longer jelly roll instead. And finally, pouch cells are also flat, but instead of having an internal wrapped or spiral structure, instead they comprise electrode plates stacked on top of each other. Even though cells can be packaged in different external form factors, the electrodes used inside of these cells are of a very similar form and are made by very similar processes on similar or even identical equipment. When creating an electrode structure, we start with the current collector materials which are metal foils, and we coat both sides of these metal foils with layers of the electrode materials. The top diagram shows a positive electrode current collector in gray to remind you that it's usually made from aluminum. And underneath that, you see a negative electrode current collector in orange to remind you that it's usually made from copper. Notice that both sides of the current collector are coated with active materials. And you will see that when a cell is formed either by spiral winding or by stacking plates, the electrode materials are on both sides of the current collectors and both sides are utilized. And by coding both sides instead of just one side, we get a reduction in the total volume required for the same total capacity of battery cell by not wasting one side of the current collector. The colors I used to represent the active materials are purely for illustration purposes. I consistently use the color purple when talking about positive electrodes and green when talking about negative electrodes but that's for illustrative purposes in the specialization. Whereas, in the real battery cell, these materials look pretty much like a black powder and it would be very difficult to distinguish between them with just your naked eye. As a reminder, the negative electrode active material is usually some form of graphite, and the image on the slide shows you a scanning electron microscope, or SEM, image of graphite used in a lithium-ion battery cell. As you've already learned, the electrode is not composed of a single solid piece of the active material, but instead of tiny particles because that increases the overall surface area of the electrode and it allows higher power. In general, some kind of lithium metal oxide is used as the active material in the positive electrode. And in the second photograph, I show you an SEM image of lithium manganese oxide, or LMO, material which is one of the candidate materials to be used in a positive electrode. Again, to the naked eye, both of these materials would simply look like a black powder, but under the microscope, we can see that there are distinctive shapes and patterns. And so it is actually possible often to distinguish optically what the material is at least if you've got a small list to choose from. But it's also important when we manufacture the cell not to mix these materials together or to mix them up because contamination between the materials will ruin the cell. And so for that reason, the negative and the positive electrodes themselves are usually fabricated in different rooms when the cell is being constructive, so there's no airborne contamination. You've also learned that the particle size and the shape are important when fabricating an electrode. We desire that the particles be small in order to maximize the surface areas so we can have a high current cell, a high power cell. On the other hand, we also desire a smooth spherical shapes if possible, since sharp edges are susceptible to higher electrical stresses and cause faster decomposition of this solid electrolyte interface layer, which we'll talk about later on again this week. So sharp edges and pointy edges can lead to premature aging and fail it here in the cell, and even possible thermal runaway after extended periods of degradation. When we're fabricating the cell, we begin with the electrodes, and when we fabricate the electrodes, we begin with just foils of current-collector material that are usually delivered in large spiral reels up to perhaps a half a meter in width. The negative electrode current-collector, again, is usually made from copper, and the positive current-collector is usually made from aluminum. The reels of current-collector foil are mounted directly on a machine that unwinds the foil and coats the reels of foil with the electrode active materials. You can see on this diagram to the left, how the reel is initially mounted and how the reel is unwound by the unwinding machine. And then you can see that there's a coater that coats the electrode materials onto both sides of this foil, and then the foil passes through other stages that we'll look at. Let's think about the coating process in a little more detail. When we coat the current-collector foil, we don't simply put on the active materials but we include with those some conductive agents like carbon black perhaps, and some binding agents like PVDF, for example. And all of these are mixed together, and then added to that mixture is a solvent that makes this mixture into a slurry or a paste that can be spread on top of the foils. Now this solvent that we're adding at this point is simply for the manufacturer of the electrode, and it has nothing to do with the operation of the battery cell later on in life. So once this slurry is prepared, it is spread on both sides of the foil, and there's a blade, a knife edge that's located just above the foil that scrapes off the excess materials in order to control the thickness of the electrode coating. And the thickness is designed quite carefully so that the energy storage capabilities of the negative and positive of electrodes match each other so that we would not be wasting material or volume or weight by having an excess of one material versus another. After the foil has been coated, the coated foil moves into a drying oven. And this oven bakes the electrode material onto the foil and it also evaporates the solvent that was added in the previous step to form the slurry. And at the end of the step, the electrode materials have a good mechanical connection to the foil. When the foil is completely dry, it exits the oven and it's wind on to a reel on the right hand side of the drying. And we are finished with this particular piece of equipment and we move on to the next step. The next step in processing the electrode is to perform an operation known as calendaring. This means pressing to compress something. So the reel of coated foil from the previous machine is removed from that machine and it's placed on a left-hand side of the machine that's shown in the illustration on this slide. And the electrode coated foils are once again passed through in an unwinding device, and then they're passed through large rollers that press the electrodes down to a highly calibrated thickness. And so this process is known as pressing or calendaring the electrode. The purpose of performing this calendaring is to compress the electrodes to compact out extra spaces or voids between the particles and pressing the porosity down to a calibrated level to ensure more consistent density of particles throughout the entire electrode. It also helps to ensure electronic contact between the particles that might otherwise be relatively disconnected at the end of the previous processing step. The electrode material then passes through a machine that slits the electrodes lengthwise and these narrower electrode strips are wind on to individual coils at the output of this machine having the final desired widths. All of the processes in cell manufacturing are carefully regulated and inspected at every step. In this case, we might inspect the thickness after pressing operations to make sure that the thickness is within tolerance. The slitting operation must also be done with very sharp knives and must be carefully monitored because any ragged edge, any sharp burs on the edges of the foil strips could eventually work their way through separator material and cause short circuits inside of a cell. So that's a really dangerous situation that must be avoided. And so also this calendaring and slitting machine must be very precisely manufactured and maintained. At this point in discussing how the lithium-ion cell battery is fabricated, you have learned how the electrodes themselves are fabricated. You've learned that the metal current-collector foils are first unrolled. A slurry of the electrode solid materials plus additives is deposited onto the foils. Dryers evaporate the solvents and adhere the electrode active materials onto the current-collectors. A calendaring process produces the desired porosity and ensures contact of the active materials with each other. And finally, the electrodes are split to their final widths and the narrower electrode foils are re-reeled in preparation the for next steps of cell manufacturer. So at this point you understand how the electrodes are prepared when fabricating a lithium-ion battery cell. The next step is to understand how we take those electrodes and package them in their final packaging. And that's what we look at next.
