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Hello. In this class, we will look at the Lithium-ion Batteries. You may very well be aware that lithium-ion batteries are presently extremely popular in terms of mass-market usage, and for a several, you know applications they seem to be the appropriate type of battery to use. So, for example, almost all our mobile phones are presently coming with lithium-ion batteries, and if you look at the present generation you know electric vehicles electric cars, that people are trying to you know put into the market in large scale. The battery of choice as of now seems to be the lithium-ion battery. So, there’s considerable focus on this battery chemistry and a lot of research goes on in it, a lot of R&D work goes on it and it is being used extensively by the public. So, it is in this context that we will focus on this particular battery chemistry. Although we will you know we do look at them all the chemistries in general, this is battery chemistry that is very you know current in terms of the level of interest that people have in it. In fact, the extent of interest on in this technology is so high, that specific I mean countries are countries as well as companies are actively searching for you know sources of lithium and trying to block it so, that it is now accessible only to them, to block in the sense buy exclusive rights to it. So, many companies are trying to do these even countries are trying to do this then you suggest that enough example China apparently has been spending a lot of effort trying to buy you know sources of lithium. So, to speak; so, as essentially they invest there. So, they get the exclusive right to it. So, that kind of a thing which is the typical business you know processes those kinds of things are on. So, that’s the extent to which this is the technology of interest and you know affects everybody from a common man to you know corporations that are participating in it and nations which feel that you know the future lies with the electrical energy storage associated with the electrical energy and that the lithium-ion battery is going to be a critical aspect of this energy storage process ok. So, it is in this broader context that we will look at the lithium-ion battery technology. (Refer Slide Time: 02:40) So, our learning objectives for this class are that we would like to state the advantages of the lithium battery-based chemistry, lithium-based battery chemistry. So, to get an idea what is the advantage, why are you people you know so excited about it, and you know spending so much time on it. And we would also at the same time as to indicate, what is the hazard? There is some hazard associated with lithium metal-based batteries. So, we would like to get a sense of that what is that hazard, there is one standard the issue with a lithium battery. So, that’s something that you like to look at. And I would like to say that you know even though if you are not careful they all look the same; there is actually a difference between what you would call as a lithium metal-based battery and lithium-ion battery. So, these are not the same I mean. So, that is the point that I would like to highlight here and we will see as we proceed, why this is a difference ultimately it’s the same element lithium that is being used its just the form in which it's being used that is different and therefore, that changes issues associated with it, that changes how we work with it etcetera. So, things like that. And we would there is a particular process known as intercalation. So, this is something that I will describe to you because it’s very fundamental to how these lithium-ion batteries work and may continue to play a role in you know future versions of the lithium-ion technology. Therefore, we will like to look at it. And as a general discussion, I would also like to indicate this concept of the relative position of energy levels required for the stability of the electrolyte, we will discuss it in the context of lithium-ion batteries, but it' the same for any battery system. And therefore, that’s something that we should be aware of. So, these are our learning objectives, the advantages of the lithium battery chemistry, the hazard associated with the lithium metal-based batteries how the lithium-ion batteries are overcoming that hazard that what is this process of intercalation, and the relative position of energy levels associated with these different parts of the lithium-ion battery. (Refer Slide Time: 04:52) So, those are learning objectives. So, why are people excited about lithium, why is the lithium-ion battery such an interesting technology to spend so much of time and effort and you know public interest is being built in it and it built on it. So, why is this the case; one of the most electropositive elements that you will find; so, therefore, it is very eager to hand off an electron. And therefore, if you look at the standard electrochemical series, it is you know very anodic its extremely anodic and so you, you can couple it with almost any cathode that you would like to use in this case and you will get very high voltages ok, so extremely high voltages. So, therefore, when you compare against any other battery, with lithium ion-based batteries you are looking at voltages of three volts, 4 volts etcetera something like this. So, this kind of a voltage you are looking at as the voltage you will get from the battery. So, therefore, if you draw the same amount of current right and compare it versus some other battery chemistry which is only giving you about 1.5 volts. So, for the same current, you are actually getting double the power right. So, double the power you will get. So, 3 volts if you are using a 3-volt battery versus a 1.5-volt battery. For the same amount of current that you draw out of it, you are getting double the power and therefore, power density is high. So, you know specific power is high power for per unit mole that you will get much higher with a lithium-based system than with any of the other computing systems that you might consider. The second thing is that it is very lightweight. The element is very lightweight its only 0.53 grams per centimetre cube is the density so, in fact, it is only as dense as wood some versions of wood are only this is about as dense as this. So, it is extremely light and therefore, when if you cover if you couple the high power that you will get with the low weight associated with it, you are getting high specific power ok. So, power per unit weight on a specific mass basis the power the specific power that you will get per unit mass is high ok. So, the power per unit mass which is the specific power on a mass basis is very high in this case right. So, power by weight. Therefore, that is very exciting again. So, this means with a very lightweight battery you will get a lot of power. So, first of all, for the same number of moles of the element, you get more power. So, same know same. So, when I am comparing against some other battery technology. So, you will have the same number of moles, higher power since the voltage is high okay that’s the first advantage we have. The second thing is because it is light same weight higher power. So, specific power is high ok. So, specific power is high. So, these are two very interesting concepts. So, for the same weight, you get higher power and the same numb same number of (Refer Time: 08:20) moles you get higher voltage. And for the same weight you actually have even more moles, because the element is also just you know very low in the periodic table; so hydrogen, helium, lithium. So, it is like the third element in your periodic table. So, it is going to be for the same weight the same number of grams you are going to have more moles. So, the same number of grams as some other anode material you will have more moles of lithium. The more moles of lithium will also give you power at a much higher voltage and so, you get much higher power right. So, a higher specific power in more than one way you get. So, that’s a big advantage and also it is environmentally friendly, it unlike many other chemistries that are commonly looked at, which are also chemistries that are there significantly in the mass market. So, for example, for all our car batteries most of the time we are using lead-acid batteries even as of today that is the battery that starts the car it’s not yet an electric car, but just to start the car there is always a battery even though we may have a petrol-powered vehicle in our car there is a battery. That battery is mainly went meant to start the engine when you first time start the engine as you start the engine to start I mean before you start driving that battery is typically lead-acid. The batteries which are mostly sitting in our homes as part of Uninterrupted Power Supply system’s UPS systems are also lead-acid batteries. So, there is a significant usage of lead-acid batteries around the world. So, it’s popular chemistry popular from the perspective that is very prevalent chemistry, a lot of places they use it mainly because you can draw a lot of current from it. So, that chemistry is around, but it is hazardous because you have hazardous acid and you have a hazardous lead. So, of course, they do a lot of recycling of the lead, but still, the entire industry is hazardous for that purpose for that reason. Whereas, lithium is not the case. So, it’s not at all hazardous and it is relatively safe the other popular battery system is the nickel-cadmium battery, there again you have cadmium and that is also toxic. So, in comparison to the lead-acid battery and nickel-cadmium battery, lithium-based batteries are when we very environmentally friend. So, when you look at all this the high power the that you get the high power density that you get the fact that you know its environmentally friendly all these things you take into account that is what causes this battery technology to be extremely popular right. So, that is the point about it, it's right. So, now we would also like to understand what are some issues associated with it. So, we will look at that briefly. (Refer Slide Time: 10:38) So, the point is when you have a highly reactive material like lithium or even for that matter aluminium let’s say. So, if you take that the moment they are exposed to the atmosphere they will immediately react to the atmosphere. So in fact, lithium you cannot expose to moisture and air it just rapidly reacts. So in fact, people who work with lithium-ion battery materials do not work in the open lab they will not work on a tabletop like this, you will not just have an experiment with sitting in the open-air right. So in fact, if you go to any lab that is working on lithium-ion batteries, they will have a glove box. So, glove box would be one you know it is like a big chamber which is which has transparent glass on all sides usually some you know perspex kind of glass, and it will have to be gloves through which you can we can put your hand into the glove and you can reach inside the chamber. So, both hands you can put inside the chamber, but you will be standing in front of you there is plexiglass. There is a glass or a perspex glass through which you can look inside. You can look inside and see what this inside it will be clear, but your hands will be reached and only using a glove. So, there will be a big glove with both hands which will be tied to this plexiglass. So, it is sealed the container is fully sealed and only because of this flexible gloves we can reach inside into some experiment. So, inside the container you will have an inert atmosphere you will have argon typically and it will be the dry atmosphere. So, they will actually have chambers below that you know glove box which is continuously drying that air, whatever not air the atmosphere that is inside the glove box and keeping it completely humidity free. You have to have it less than PPM of PPM level of humidity only then you can work on it, otherwise, its extremely dangerous the material react very violently with humidity. So, therefore, that is how we have to work on it. So, when people work on this lithium-ion technology and make cells which then they test we were talking about you know charge-discharge curves etcetera in our last class, but when they do all that, the all of that has to has happened only in with the cell that is sealed that that is not exposed to the atmosphere. Even the batteries that we have in our mobile phones etcetera are supposed to stay sealed they are not supposed to be opened and exposed to be the atmosphere. So, they're all sealed. So, in the same case, this is also sealed. So, normally what they would do is, at the cell would be constructed inside this glove box. So, it will be kept in a way in which it is put together in the glove box with the electrodes etcetera sticking out, but I mean or contacts to the electrodes the current collector sticking out the electrodes will be completely immersed in the electrolyte and the whole thing will be sealed. And in you know your best case scenario is such that your test equipment will be outside the glove box, just outside the glove box and wires will be connected to through the glove box into this experiment that is sitting inside the glove box. So, electronics will sit outside the cell will sit inside and through that, we will run the test or at least you should seal the cell completely and then you can take it out of the glove box. So, that it is now you know completely sealed there is no danger of atmosphere going in, and even then keep it some kind of in inert location where you have some gas flowing and then run the test. So, it is an activity that you have look carefully concerning the environment that the lithium is exposed to mainly because lithium is extremely reactive. So, what happens when we create a cell? So, you have a cell, let’s assume I am in the inert atmosphere I have made this cell in an inert atmosphere and then we have the electrolyte here. So, let’s say we just put one electrode of lithium here another electrode of something some counter electrode. So, I will just call this the lithium electrode and this is the counter electrode right. So, as soon as the lithium electrode sees the electrolyte, what happens is the surface of the electrode reacts with the electrolyte ok. So, it immediately reacts with the electrolyte and forms a passivating layer reacts meaning it gets oxidized by the electrolyte and it forms a passivating layer. This layer is called the SEI layer or Solid Electrolyte Interface layer. So, that is this Solid Electrolyte Interface layer that is this SEI. So, the SEI layer or the solid electrolyte interface layer is the layer that forms because the lithium metal reacts with the electrolyte. Now, this is both a positive and negative it is positive in the sense that what happens is this layer that forms is a continuous adherent layer and therefore, protects lithium from further reaction with the electrolyte. So, it forms on the surface of the electrode and then prevents further reaction with the rest of the electrolyte. So, for example, if this were the side view of the electrode and this is where the electrolyte is, the SEI will form at the surface. So, it will form right here. So, a structure like this will form. So, this is the SEI. SEI layer is the electrode itself ok. So, the electrode lithium electrode in contact with the electrolyte forms a solid electrolyte interface layer. So, it is good because this layer then prevents the rest of protects the rest of the metal. So, this rest of the metal is protected, only the top layer of the metal which is in contact with the electrolyte passivates like this. So, therefore, it is protected, but the bad thing is it now adds to the resistance for the path of the ion. So, the ion that is inside here. So, lithium that is sitting here which has to become lithium plus as it goes into the electrolyte, it has to pass through this layer and go through. So, it usually adds to the impedance of the cell; it adds to the impedance of the cell which is basically obstruction to the flow of current. Impedance is the most general term like you are used to resistance in an in the context of the flow of electrons, that resistance is also an obstruction to the flow of electrons. So, you can have any species that might carry a charge, in this case, an ion is carrying charge. So, Li plus is carrying the charge. So, you say more general term we say impedance which is basically an obstruction to the flow of current. So, the impedance of the cell goes up and ideally we want low impedance because otherwise, we are wasting energy going past is impedance every time there is impedance.













So, if you say there is an impedance of valuer, we are having an IR drop associated with it which is wasted electricity. That is electricity that we could that is wasted potential voltage of the cell is partly wasted in getting past this barrier instead of doing work with the whatever else that you want to do you wasted here right. So, this is the thing, but. So, this is the positive is that it protects the electrode, negative is actually two parts it is negative. One is that your adding impedance to the cell and the second thing is you are actually irreversibly consuming a little bit of the electrolyte as well as a little bit of the electrode. So, this SEI layer then is essentially a little wasted layer, its a wasted surface layer because it is used up a little bit of the electrolyte and therefore, you know irreversibly changing in the electrolyte in some way it is also used up a little bit of the metal. So, in that sense, it is also not a very positive thing, but it protects the electrodes are there that is fine. But there is another problem here so, there is the SCI. So, that is what happens when you put this lithium as soon as you put it into an electrolyte because it is so, reactivates with forms this. But let’s say you continue now having done all this you make the call. You make a cell and then you start cycling it. So, what happens is every time you send lithium-ion in and out of the cell. So, lithium-ion goes this way and then it comes back this way. So, you have some counter electrode here. So, every time the ion goes from the main from the working electrode which is lithium to the counter electrode and then comes right back the why should it come back? When you are discharging it is going on the way and your recharging it is coming back. So, this is what we are doing charging-discharging, charging-discharging that we do we keep on shuttling between the two electrodes we shuttle the ion between the two electrodes. Now every time it comes back, it is trying to come from Li plus to lithium. So, during discharge, during discharge Li goes to Li plus e minus. E minus is going with the external circuit. So, that is what a process is. During recharge you are doing the opposite reaction Li plus e minus gives you Li ok. So, this is being done by pushing electrons in from an external power source, you push electrons into the lithium electrode it will take back the lithium-ion and it will form lithium metal. So, now, for that to happen the lithium-ion has to actually go through this solid electrolyte interface and go back to that metal. So now, when this happens when you keep doing this actually sometimes it doesn’t go through properly or it goes in and forms a different kind of a structure, it doesn’t plate it in a flat manner that is the problem with the lithium metal. When it comes back and its plates on the lithium this lithium-ion comeback and plate it on the lithium metal, it would be nice if it plated back as a flat structure, unfortunately, it a thus I mean huge amount of experiment experimental evidence that shows that when lithium-ion comes back and plates back on the metal, it does not plate as a uniform flat structure instead it forms a porous structure. So, what it means is, you are actually having this lithium metal sort of form like this, some kind of a porous structure is happening. So, with the progressive cycling and on top of it again you form some solid electrolyte interface because now we have a much larger area, so this continuous. So, with every cycle, this becomes a more and more porous structure that is growing ok. So, it is continuously going with every cycle this continues to grow and there are various reasons for it. One of the reasons the reason one of the reasons that it is growing is because of the front-most point of the. So, we it is uneven surface right it's an uneven surface, the front-most point of that surface that is frontmost concerning the counter electrode. So, I have a working electrode here, I have counter electrode here. The front-most point of this electrode is the closest to the counter electrode right. So, I have a working electrode here and I have counter electrode here. So, I will say this is the counter electrode this is working electrode W E is working electrode C E is a counter electrode. Supposing this is not a uniform surface, but I have a sharp point like that. Now, this point is closest to the counter electrode. So, when the ion comes back Li plus comes back, it is easier for it to plate this point ok. So, it will continue to plate at this point. So, any rough surface it has. Multiple surfaces it will more easily plate at these points then come all the way up to here and plate. So, therefore, the moment it becomes uneven the progressive cycle makes it more and more uneven, it encourages the unevenness it builds on the unevenness. It is already uneven, it does not go back the second cycle does not make it flat the second cycle makes it even more uneven because whatever is uneven is you know is built on and that is what is progressively being built on. So, it continues to grow. So, you may have a situation where eventually it does a short circuit ok. So, that is basically what I am going to show you here, and that is what we are saying by saying that this kind of growth can result in an internal short circuit. (Refer Slide Time: 22:44) So, for example, if this is what this is just a schematic of what is happening, you have the lithium metal here you have the cathode here. So, let’s say you cycle it then what happens is, you start seeing this uneven structure here right. So, lot of. So, this is called dendritic growth, this kind of growth is called dendritic growth because it represents a structure that looks like dendrites and that is this kind of a structure this kind of you know pointy point structure which is growing in different directions; so dendritic growth. So, you can see that this is not plated back as uniformly as you see here. This is a nice uniform surface interface a flat surface which is now become you know pointed pointy surface which is highly uneven. So, if you continue this you can see how you know, for example, this is a little bit ahead this will keep growing faster with progressive cycling every cycle. So, initially, it is okay first cycle, second cycle, third cycle as you continue you will eventually reach a situation where it looks like this. You have an internal short circuit you have an internal short circuit ok. So, what this has what is happened is that with progressive cycling, the lithium metal did not plate flat flatly and then eventually found it's kept growing indefinitely and makes in internal short circuit, this is a very dangerous situation. A very dangerous situation because now there even see normally you have to connect an external circuit, normally we connect and external circuit we put a load and the electron travels like this and the ion travels like this ok. So, the moment you remove the load, the battery is an open circuit. If there is no load there that there is the battery is an open circuit the reaction stops, because the electron also has to flow, the ion also has to flow, both those steps have to happen for charge neutrality to be maintained only then this circuit is complete the reactions are complete and the reactions will take place. If you open up the circuit outside there is no path for the electron to go everything stops right. So, now, when you have an internal short circuit both of them can happen, you can have electrons just go across like that and you can have ions that go across like this. So, you no longer need an external circuit, everything is happening inside the cell. So, current will build up very fast it will flow very fast, you have a huge amount of current going like this because nothing is this is just metal, you have just metal between you know positive and negative of an electron you are just put metal. So, this is a short circuit and it is inside the cell; so its internal short circuit. So, this is actually dangerous it leads to a battery explosion because what happens if the norm in these batteries, the conductivity of the ion is usually much less than that of the electron. The ion this is, of course, the electron is in the external circuit the ion is actually in the electrolyte ok. So, the ion is flowing through the electrolyte, the electron is going through the external circuit which is what we have here. The conductivity of the ion is usually much less than the conductivity of the electron. So, that is another reason why the current is not so, huge ok. So, otherwise, the current can be huge interestingly the conductivity of electro lights our ionic conductivity of most materials increases with temperature ok; unlike electronic conductivity, electronic conductivity actually decreases with temperature ionic conductivity increases with temperature. So, now when you have an internal short circuit, you have a lot of current going through this internal short circuit will have a lot of current going through a pathway that pathway heats up. So, because of this huge current, the temperature of a cell goes up of cell increases. If the temperature of the cell increases it increases the conductivity of the ion. If we increase the conductivity of the ion the current in the cell further increases ok. So, this is called a thermal runaway this is called thermal runaway ok. So, that that is because you are having an internal short circuit, because of the internal short circuit lot of current is flowing in that path. Because a lot of currents is flowing in the path temperature of the cell is going up, temperature cell you can physically feel has started becoming warm because it has become warn the ionic conductivity has gone up which was the slowest step previously now that has gone up. If that has gone up the current in the original short circuit goes up even further ok. So, and then again it heats up even further, further ionic conductivity goes up. So, this is you know the unending cycle, it just increases rapidly and it can explode the lithium battery can explode basically it has an uncontrolled reaction it can explode. So, you have you must also understand this happens after a few cycles it is not happening at the beginning. So, the first generation lithium batteries, first-generation lithium rechargeable batteries had this problem that when we buy the battery knew it will be fine, and you keep using it for several cycles you know whatever 10 cycles or something 10, 15, 20 cycles you may not see any problem it will be working fine because at that point this is still growing this is not completed the short circuit, it will keep continuing into function suddenly one day you have the short circuit. So, suddenly one day you end up in the situation that there’s a short circuit. That time almost immediately the battery will explode. So, the first generation lithium-ion batteries with that were put in the market actually used to explode, and they had all got to be recalled they became very dangerous and you know even, in fact, some of the early factories of lithium batteries caught fire. So, that the entire factory got burnt on one of the factories got burnt on into this. So, this is a problem? So, this was a problem, but incidentally, this is not a problem with single-use batteries. So, single used lithium batteries didn’t have this problem because you never plated it back only when you plate this back you have the dendritic structure growing, which can continue to grow and make the short circuit. So, it was not an issue with lithium single used batteries or non-rechargeable batteries, but it was a problem with lithium rechargeable batteries and this was primarily because there was a plating, stripping mechanism. And this plating process was creating the situation, therefore; they decided that using lithium metal as the anode was not a safe thing to do because that required this plating and removal kind of thing. So, instead of doing putting lithium metal, they started to look for other materials which are then called as host materials, inside which lithium can sit at a potential very close to the lithium metal potential. So, now, the lithium is not sitting in the front surface, it is actually going into the structure and sitting inside that structure, and then when you discharged the cell is coming out from the structure going to complete the reaction when you recharge again it goes back into the structure ok. So, it is not on the surface it goes inside the structure. So, that kind of a host material began to be used as the anode as suppose to lithium. And in that case, you never have lithium as metal form, you only have it in ionic form and that is why the new generation batteries are called lithium-ion batteries the lithium sort of stays ionic throughout the circumstances that it is presented with.