Welcome to the second week of the course. Last week, we spent most of our effort studying background topics related to standard electrochemical cells, and how those work in general. To sum up the week, we looked at three specific examples. The first two were the copper-zinc Daniell cell, and then the lead-acid batteries cell, and finally, we looked at nickel-metal hydride cells. The Daniell cell and the lead-acid cell are standard electrochemical cells, but the nickel-metal hydride battery cell worked on a slightly different principle from a redox scheme and its negative electrode. Remember that the metal hydride and the negative electrode absorbs hydrogen a lot like a sponge absorbs water. It does this without changing its structure, or its chemical composition. And we discussed how this is a far more gentle process than a standard chemical reaction. And so, nickel-metal hydride cells tend to have much longer lifetimes than lead-acid cells, for example. This week, you're going to learn that in lithium-ion battery cells, both electrodes work on this principle which is one reason that lithium-ion cells tend to have much longer lifetimes than lead-acid cells. For example, you're fortunate if the lead-acid battery cell in your automobile lasts for more than five years. On the other hand, it's very likely that most lithium-ion battery cells that are being used for automotive applications will last the life of the vehicle, at least 10 years. Nickel-metal hydride cells work really well for hybrid electric vehicle type of applications, where there's not a huge need for a lot of energy storage, but where we do need to have high power. But if you remember our discussion of the periodic table last week, you might remember that nickel is quite a heavy element. It's in the fourth row of the table, and that means that the nickel-metal hydride type of battery pack is going to be quite a heavy battery pack. It would be nice if we could use lighter elements. In order to discuss the relative sizes and weights of otherwise similar battery technologies, we often use the terms specific energy and energy density, which measure the maximum stored energy per unit weight, or per unit volume respectively. So, for a given weight, a higher specific energy cell stores more energy. For a given storage capacity, a higher specific energy cell is lighter. For a given volume, higher energy density cells store more energy. And for a given storage capacity, higher energy density cells are smaller. In the figure on this slide, the vertical axis is showing energy density, and the horizontal axis is showing specific energy. The boxes on the plot show a rough description of the characteristics of battery cells having different chemistries. So, chemistries that you see plotted on the lower left of the plot are the largest and the heaviest for a given capacity, and as we move right on the graph, that indicates lighter cells, and moving up on the graph indicates smaller cells having the same capacity. So, we would like to move both right and up. And you'll see that the lead acid which is indicated as PbA, Pb for the element lead and A for acid. Lead acid is the largest and the heaviest of those that are shown on this plot. Nickel cadmium, NiCd, and nickel-metal hydride, NiMH are both better. But lithium-ion batteries cells are smaller and lighter than these historic chemistries. And that's the main reason that we prefer them in applications such as powering different types of electrified drivetrain vehicles, where both size and weight must be minimized. Notice that the plot also shows lithium polymer. Lithium polymer is a type of lithium ion, sometimes it's just used as a marketing term, and sometimes it's used to talk about a cell that has a solid polymer electrolyte instead of a gel electrolyte. So, just think of both of them for the time being as both being lithium-ion types of battery cells. You will notice that there are some other chemistries that are shown here that are even smaller and even lighter than lithium ion such as zinc air and lithium air, and both of those two chemistries, the negative electrode is a solid metal, either zinc or lithium, and the positive electrode is oxygen that is scavenged from air, and because air is around us everywhere, the weight of the air is not really included in the weight of the cell, and that's one reason why they can be smaller and lighter. The ultimate battery technology as far as I understand is lithium air, because you're taking lithium metal as your negative electrode, and lithium is the the third element in the periodic table, It's very small, very light. But at this present time, there are difficulties making it work robustly and repeatedly in real world applications. And there's an enormous amount of scientific research going on right now to attempt, to perfect lithium-air technologies. Similarly, zinc air has a lot of research going on right now too, to make it very repeatedly rechargeable, and so you're likely to see a lot of news releases in the future as scientists, and material scientists, and engineers, work on perfecting that technology. So, you've now learned about one possible set of advantages of lithium-ion cells versus cells having different chemistries, that is lithium-ion cells have a higher energy density and higher specific energy than most other types of secondary cells. We'll now look at a few other advantages. One is that lithium-ion cells typically operate at higher voltages per cell than other rechargeable cells. A lot of lithium-ion battery cells operate at around a nominal voltage of 3.7 volts versus about 1.2 volts for nickel-metal hydride or NiCd cells. What this means is that often in an application that requires maybe three-volt power source, I can use one lithium-ion battery cell instead of several nickel-metal hydride or NiCd cells in series. Further, lithium-ion battery cells have lower self-discharge rates than other types of rechargeable cells, which means if the cell is not connected to a load, and it's just sitting there, the level of charge in a lithium-ion cell will slowly decrease over time due to self-discharge, but it will not decrease as quickly as it does in some other technologies where the self-discharge rate is higher. And so, that is an advantage of lithium ion. Nickel-metal hydride and NiCd cells can lose anywhere from between about one percent to five percent of their charge per day, even when they're not installed in a device. And many lithium-ion cells will retain most of their charge even after months of storage. We have some in our lab that have been stored for years, and we pull them off the shelf and they are still at least 50 percent state of charge. Finally, lithium-ion cells have a long life as we've talked about because of this much gentler intercalation mechanism that they use in charging and discharging in each electrode, instead of a more standard and more harsh redox reaction. But to be fair, there are also some disadvantages to using lithium-ion cells. For example, lithium-ion batteries are presently at least more expensive than similar capacity in nickel-metal hydride and NiCd cells. Lithium-ion cells are more complicated to manufacture, and are presently manufactured in much smaller numbers than nickel-metal hydride and NiCd batteries, but that is something that is changing. For laptop and consumer electronics, they are manufactured in very high volumes. And for larger scale applications like automotive, they're beginning to be manufactured in much higher volumes too, and that is bringing the price down. Lithium-ion cells tend to be less stable electric chemically, which means that if you overcharge a lithium-ion battery cell, it's possible for that cell to catch fire. Other chemistries are usually a little bit more benign if you overcharge them. And so, for a lithium-ion battery pack, we require special safety precautions. They will need special circuitry to protect the battery from damage due to overcharge and to over discharge. Of course, that's something that you are learning about. In this specialization, you're learning about battery management, and so, this is something that you will be able to bring to any project involving designing a battery management system for lithium ion, and that will be something great for your resume. Something also which is maybe less obvious is that when you select a lithium-ion cell for a particular application, you should be aware that there can be a great diversity in the quality of cells that you find in the market, and the quality of cell materials that are used in the construction, and exactly how that cell is constructed in the factory makes an enormous difference in terms of this cell longevity, and the safety, and the performance. And so, if you are a part of a project that is looking at a large-scale application, it will be very important for you to visit the customer's site, see how the cells are prepared out of the customer site, the manufacturer's site, see how the cells are made, look at their quality controls and investigate a lot of these particular performance indicators. So, impurities in a cell can limit the performance that can be achieved, and cells from different manufacturers that on a data sheet seem to have, similar chemistries and similar construction may in fact yield quite different performance over the long term. So, to summarize this lecture topic, we've discussed in a general way the advantages and some disadvantages of lithium-ion battery cells compared to cells having different chemistries. When we talked about the benefits offered by lithium-ion cells, we introduced this idea of energy density, which is the amount of energy per unit volume, and specific energy, which is the amount of energy per unit mass. You learned that lithium-ion cells have higher energy density and higher specific energy than most other types of rechargeable cells. Also because of the gentler intercalation mechanism that's used in the electrodes of these cells, then compared with say a violent redox reaction in an electrochemical cell, that lithium-ion cells tend to have longer life than other rechargeable cells. And finally, they also can have higher per-unit voltage, higher voltage per cell, which makes the electronic designs and the battery pack designs simpler, and they also have lower self-discharge rates which means that they can be used in applications where the battery pack must be stored for a period of time, and that will work better than for other types of rechargeable cells. There are some disadvantages. The primary disadvantages are that we need to manage them very carefully to guarantee safety. But as I mentioned, the good news is that this is what you're learning about, this is a big part of what this specialization is all about. And when you've completed this specialization, you will be able to construct battery-management system algorithms that are able to keep battery cells safe, and to extend their lifetimes. A second disadvantage of lithium ion is the cost of the battery pack as compared to the cost of an equivalent capacity pack using another chemistry. This cost includes the cost of the cells plus the supporting electronics. But it seems that lithium-ion cells and electronics are becoming less expensive, and over time, we should see their price decreased significantly. And that brings us to the end of this lecture topic. And the upcoming topics this week, you're going to learn in much more detail how a lithium-ion battery cell works. Starting with a closer look at the intercalation process itself, and then looking at some of the materials, and how intercalation combines with those materials in this cell. And so that's what's coming next.
