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So, what happens for example, when you do this, you will notice this is the first this is
stage 4 this is called stage 4. So, it is happening in reverse, stage 4 simply refers to this idea that there are 4 layers of graphene between two adjacent layers of lithium ions that have intercalated into the system. So, lithium ions have intercalated into the system. So, between two adjacent layers of lithium ions, you have 4 layers of graphene and so okay, this is how. So, what I will also point out that, you see here the spacing has gone up this spacing is the little larger than this spacing that is because the ions are coming there. So, ions have come in there there is a lot of studies and I know exactly what is the status of lithium there, how much of the electron is with it, how much of the electron is with the rest of the host material etcetera. So, for our purpose, we will simply call it the lithium-ion that is there. So, lithium-ion has come in there and then it has increased the spacing between graphene layers where it is present, but these spacings are undisturbed right. So, this is the original spacing. So, if you compare again with the previous picture, this is how our picture was and you have some variation the electrolyte also because ions are moving. So, you can see the ions if you look at the electrolytes side ions are moving. If you look at the electrode side the spacing has changed, you had uniformly spaced graphene layers and you find now that some of the graphene layers have been pride a part, have been pushed apart and some of them continue to remain as per their original spacing and you have this lithium that has come inside it. So, this is stage 4; stage 3 as you can imagine concerning this description, would be a situation where we brought in a so, much lithium that on average there are three graphene layers between two lithium layers. Now, you are 4 graphene layers between two lithium layers, we introduce more lithium as they grow to go inside and inside they will readjust such that they are now you have 3 graphene layers between two lithium layer. So, this situation evolves into this situation. (Refer Slide Time: 39:36) So, suddenly you now have three layers only between every two adjacent graphene layers. Again electrolyte is continuing to change here and you continue to have a counter electrode here from where this is happening. So, this is what this is ok. So, this is 4 layered 4 layers of graphene between two lithium layers this is 3 layers of graphene between two lithium layers. So, this is stage 3. (Refer Slide Time: 40:02) We can continue this and you will have stage 2 where you have two layers of graphene between two adjacent lithium layers. (Refer Slide Time: 40:15) And then you have stage 1, where you have all the graphene layers intercalated with lithium layers lithium ions. So, these are all the stages. So, when you discharge also it will do the opposite. So, if I am if this is the charged condition and start discharging the cell it will go like this. Now all the lithium is gone to the counter electrode, you recharge it will do this ok. So, this is how the charging and discharging happens. So, this is fully charged discharging, discharging, discharging, discharging fully discharged then recharge. So, this is fully recharged. So, this is how charging-discharging happens and in this process, there is no plating of lithium here this is not happening, there is no plating here and therefore, the lithium is safe this idea of dendritic growth is not happening, it is not growing such that you will have a short circuit, so all that is not happening. So, this safety issue which was there with the lithium metal is no longer there ok. So, I will also point out that you can see here that the spacing has changed. You can see if you compare this spacing here, it is significantly higher than the original spacing between the graphene layers which is this. So, therefore, when you do x-ray diffraction of the cell, you will find that this initial spacing that you know we had this 002 spacing of 3.35 angstroms. So, when you do x-ray diffraction of this, if you do n lambda equals 2 d sin theta which is the Bragg equation right. So, this is the Bragg equation. So, this d is 3.35 angstroms. So, if you use a particular value of wave length, you will get a particular theta at which the peak will show up right. So, now, as you put lithium into it on average the spacing is increasing right. So, in other words on average the d value is going up the d value is going up. So, as the d value keeps going up, the theta at which the peak will appear keeps decreasing. Because 2 d sin theta has to be a
constant n lambda is a constant you are using the same wavelength. So, lambda is constant. So, if you keep increasing the d theta has to come down. So, the peak the position of the peak that angle at which the peak appears the XRD peak appears will keep shifting to lower values of theta. So, it will shift to a lower value of theta and you will see the peak appearing at lower and lower values and in fact, this is a clearly been observed, you can do experiments you can do you know in situ x-ray diffraction kind of experiment where you have a lithium cell inside the x-ray set up, and you can run the cell, and then you can see this you know steadily the d 002 two peaks shifting to lower and lower values of theta. So, that is what happens and this idea that it happens in stages is referred to as tagging.
This is called staging. So, intercalation happens through this process of staging in graphite and it helps safely secure the lithium and it helps create a situation where the lithium-ion battery can function safely. So, that is I think is a very key part of this whole lithium-ion technology. So, we saw what are the advantages of lithium battery, we saw what are the hazards with the lithium-ion with the lithium battery. So, advantages of the lithium you know hazards associated with the lithium battery and what is the process that they have followed to overcome that hazard, which has created the lithium-ion battery and this is the battery that is prevalent in a large scale in our current usage. This is the technology this same thing that I am showing you here is basically the idea that people have used to create the lithium-ion batteries that we are presently using. I will also point out that you know this is using graphite as the material, you can use other forms of carbon also and in fact, people actively look at other forms of carbon to do this same process and so, there is a lot of research that goes on in this area. (Refer Slide Time: 44:11) Incidentally, we should also understand what is the relationship between the electrolyte, what is being required of the electrolyte and what is being required of the electrode and how do those two relate to each other. We spoke about a specific case where you know you had the lithium we had Lithium Hexafluorophosphate right LiPF 6 and that we had with some solvent EC DEC some combination like this, which was being used as the electrolyte how did you select this how do we select this, right. So, to do that, we have to actually understand something about the energetics of that electrolyte. So, we have something called the highest occupied molecular orbital or HOMO it is called the highest occupied molecular orbital and this is the lowest unoccupied molecular orbital, this gap is very important. In an electrolyte, you will have this HOMO HOMO and LUMO these are the two terms that are used, the highest occupied molecular orbital and the lowest unoccupied molecular orbital, the Fermi energies or the chemical potentials of the anode and cathode, chemical potential of the anode and the chemical potential of the cathode should be selected should be matched with the voltage window of the permissible voltage window for the electrolyte. This gap in energy between the HOMO and the LUMO is the permissible energy window for that electrolyte. So, as you can see here, if you look at the anode and cathode this is the gap between anode and cathode, this is the open-circuit voltage. This is the open-circuit voltage between the anode and cathode and it stays within the voltage window of the electrolyte the HOMO LUMO window that you see here, this anode-cathode window should stay within that, only then this system will work stably why does why is this the case? It is because at the anode during the normal discharge process when you use the battery, the anode material is getting oxidized right. So, it is releasing electrons. Now if the LUMO of the lowest unoccupied molecular orbital of the electrolyte, if it were to be lower let’s say it was here, then this electron can directly go to the LUMO itself in which case the anode is actually reacting with the electrolyte it is reducing the electrolyte the anode is getting oxidized, but instead of releasing the electron to the external circuit it is actually releasing the electron to the electrolyte and the electrolyte itself is getting reduced. And similarly, if the cathode were lower here in the cathode as the cathode gets reduced, it will take the electrode electron from the electrolyte. So now, instead of having electrons being released to the external circuit, they will get consumed by the electrolyte itself it based on the position of the electrode right. So, you will the electrode reacting with the electrolyte and that will basically spoil the cell, you will not get energy out of the cell. And therefore, this is not something that is desired. So, we basically want a situation where this window between the anode and cathode stays within the window of the HOMO and LUMO of the electrolyte, it’s not outside of this window of the
HOMO and LUMO of the electrolyte ok. So, that is how we match the operating potentials of the anode-cathode with the operating voltage window of the electrolyte. And this is true not only for the lithium-ion system it is true for all the electrolyte electrode combinations that you may see in any other battery system. (Refer Slide Time: 47:57) So, in terms of conclusions, our main conclusions are that the lithium metal-based rechargeable batteries can develop an internal short circuit with repeated cycling. So, we saw first of all that the there are huge advantages to using the lithium metal-based battery system, and that is why there is so much interest in it, but you also so, noted that if you use lithium metal as the anode, then you can have an internal short circuit with repeated cycling and that is a not desirable situation to have. The lithium ion-based technology creates a situation, where metal is not being plated and stripped. And therefore, the dendritic structure which is the danger that happens dangerous situation that happens in the lithium metal-based system, that dendritic structure does not grow in the case of lithium-ion batteries, of course, you have to some mass balance here to ensure that you are not putting excess lithium into the electrode. If you put excess lithium then eventually you can have dendritic growth there also. So, they do some careful mass balancing to ensure that lithium is exhausted, but there is still little excess carbon available right. Because Li C 6 will form, you should not have more lithium than that if you send more lithium than that lithium will start plating on top of graphite, we don’t want that we will just keep little less lithium than possible to be held within that graphite. I mean that on the basis we overcome this plating issue. Intercalation and host compounds make lithium-ion battery safe. So, that is the idea that we explored in some detail and we also saw that it is very important to match the operating you know allowed operating window of the electrolyte with the energy values associated with the anode and cathode of the battery only when you do that the system is actually safe. And only then you will actually get current meaningfully out of the battery, otherwise, you will have the electrodes reacting with the electrolyte which is a completely understandable situation. And you are actually wasting the electrode you are also wasting the electrolyte, you won’t get any current at the external circuit. So, that is our main conclusions for this class on lithium-ion batteries. And as I said this is a very important technology in today’s world of you know portable energy, lot of research course on it. And this is what we discussed today is many of the basic concepts associated with this lithium-ion battery technology. I mean it is used as we saw in you know artificial pacemakers, all the way up to spacecraft that are out there and it includes commonplace items such as toys, household, gadgets and so on. So, many many many places we use batteries. And in fact, as I also pointed out in if you go to any in the world as it exists today if you go to any social set setting how many ever people you see their chances are that you have at least that many batteries maybe twice or three times as many batteries sitting in that same social setting because most of us are carrying two or three batteries even without us realizing it, without us consciously realizing it. Of course, we do know there is some battery there, but we don’t you know keep track of it so to speak. So, that is the state of affairs concerning energy technology and that’s the reason why we spend so much time trying to understand it, and also to the same degree when we say when we look at an issue like environmental friendliness of energy technology. This is again specific area of concern for devices such as batteries because as I said if you go to social setting you will have 2 or 3 times as many batteries as you will have people in that social setting and that could be anywhere you go to a restaurant you see people around you just count the number of people chances are there are 2 or 3 times as many batteries in that room. So, now, what happens to those batteries at the end of their lifetime? So, that those batteries are all going to last you know a month maybe, 2 months in case its mobile phone for maybe couple of years things like that there are many other devices where it is going to last loss lot less. So, there is a huge amount of you know waste that is generated which is associated with batteries. So, somehow we have not you know fully gotten into the mode where everything goes back to the manufacturer and then they can recycle it and so on. So, we are still not yet quite in that mode. Most of the time many of the things that we use once they are old we are throwing into the trash which is I mean either not safe, or it is toxic or various other issues are there and so that’s the status we are as of now we just throw things into the trash, which we should not, but unfortunately that is the reality or ground reality is many people are throwing many things into the trash. Except for specific kinds of batteries where there is a strong incentive to return the battery which we will talk about for the most part we are throwing things into the trash. Therefore, the environmental friendliness of that technology is very critical it should be such that even if you throw it into the trash you are not creating a major hazard, you are not you know distributing some poisonous material toxic material all over the landfill and all over the place. So, that is why it is very critical to understand; what is the environmental friendliness of a portable energy technology which could be very prevalent in society. So, in today’s class, we are going to look at common battery structures of different kinds because and we will see what I mean by that there are batteries come in all different shapes sizes. So, let’s at least have a brief overview of what that is, and we will also look at common battery types huge range of chemistries out there for batteries. So, we look at some of the common battery types I get to have some feel for you know what is the plus and minus of each chemistry that we are looking at. So, that’s those are our and that is the range of things that we will look at today. (Refer Slide Time: 04:04) So, learning objectives for this class are to become familiar with the different battery structures. As I said with different they come in different shapes and sizes. So, what are those shapes and sizes at least we will have a brief overview of that we would also like to become familiar with common battery types ok. So, what are common battery types we would like to become familiar with them, and of course, indicate the advantages and disadvantages of these battery types. So, now I will also point out that for both point number one and point number two which is the different battery structures and common battery types, I mean there is quite a variety that is battery structures maybe it’s a little bit limited. But you still have a range of different possible physical you know implementations of a battery and there is no end to it you can think of some new implementations also with it. So, we see what is essentially required and then on that on that on that basis we can build anything that we desire. As of now when you go to them when you go to market certain types are very commonly available and so that’s it is on that basis that we will look at some of these structures. Similarly when you talk of battery type from the perspective of chemistry, so this is the physical shape of the battery is what I am referring to as a battery structure and when you when I say battery type I am referring to battery chemistry ok. So, this is the other aspect of it. When you look at battery chemistry again it's inexhaustible, if you, if you look at even what is in the mass market there is a very large range of battery chemistries that are out there in the mass market, but if you look into research papers if you look at research papers in the area of energy technology the range of battery materials that they are looking at is huge it’s a very massive long-range of materials that they are looking at. Primarily because if you look at our electrochemical series there is a long set of materials that you could use as the anode and as a long set of materials that you could use as the cathode. So, in principle, if you can find some you know some appropriate director light that works with that anode-cathode combination and you can set up some kind of you know electrochemical cell you have a wide range of batteries that are possible and therefore, a lot of people are investigating those batteries. And in all of those batteries, there’s a lot of science involved in how the battery works and what you can do to make it work better. And for each specific battery combination that you pick one or more specific issues may be more prevalent or more you know important for that particular combination of anode and cathode, and therefore, the kind of things that you would do to improve one battery may be somewhat different from the kinds of things that you might do to improve another battery. And overall also there may be some limits within which you can you will have to work if you work with one chemistry, some other limits that you will have to work with when if you work with the other chemistry. So, given this range of you know possibilities, a lot of people are working with a very wide range of batteries. Also, there is this you know techno-economic issue associated with the fact that each manufacturer likes to patent what is their version of the battery. So, if they come up with new chemistry that is working very well, that they have figured out how to make that chemistry work well because if you look at the periodic table you can imagine all possible chemistries and if you look at the electrochemical series you can imagine all possible chemistries for the batteries. So, each manufacturer is working hard furiously to try and figure out what is that particular chemistry that works well and how you can make it work better that’s the second part, because just picking chemistry you can pick from the electrochemical series, but how to make it work well is a lot of you know fundamental research that you will have to do. So, for example, you might have to look at what should be the particle size of the material that you are using in the anode. So, you may use a paste of some form which has particles of the chemical that you want what should be the correct size of it. Is there some size that is too large? Is there some size that is too small? Things like that. What is the kind of separator you should use for the electrolyte we will take a look at that in just a moment what is the separator and then also what is the cathode material lot of other aspects associated with this? So, the battery chemistry is a fairly involved process there and so there is a wide range if you look at literature papers a very large range of combinations that people are investigating. So, what we will look at are in fact, some only some of the common battery types we are; obviously, not going to look at the whole range there’s a huge range there just to give you a flavour of what is there and what and these are types that you are you know the sort of may be aware of from common usage. And of course, in that process, we will briefly at least look at some advantages and some disadvantages of these symmetry types. So, those are our 3 or learning objectives battery structures battery types and some advantages disadvantage ok. (Refer Slide Time: 08:56) So, if you go to the store to buy a battery or you go online to buy a battery you find a wide range of shapes in which batteries are available. So, some of the shapes are listed here. We have here something called the cylindrical cell we have something else here called the button cell, prismatic cell and a pouch cell. These are four different versions that you can you know you may encounter if you just decide to explore the arena of batteries and look at what you would like to buy. Of this, the cylindrical cell is very common and is perhaps the older style version of the battery. We will see a little bit more about it, but it is perhaps the older style of the battery in all the years they let us say the 60s, 70s, 80s, 90s and all if you went to the shop and bought battery chances are you bought a cylindrical cell that’s the one that was the most commonly available. Then once they started making miniature devices and maybe the most you know the common miniature device that many of us got used to early on was the wristwatch ok. So, the electronic version of the wristwatch once they moved from the mechanical version of the wristwatch to the electronic version of the wristwatch they needed tiny batteries to sit inside that. So, those tiny batteries were typical of this kind the button cell. So, today we get used for a wide range of things the button cell gets used you know for many places where you need a tiny battery. So, for example, many times the weighing scale that you buy uses something like this you know bathroom weighing scale or whatever your household weighing scale that you buy will use some button cell many other small devices small lamps that you get which are LED lamp, LED-based lamps small lamps that you can use for decorative purposes all have this button cell. So, this miniature, miniaturization, the requirement for miniaturization helped create this button cell. So, this that is maybe relatively more recent I mean this is only in the grand scheme of things, but I am sure even from the nineties this was reasonably prevalent because we already have a fair number of devices which used such cells. Naturally, as you can imagine if you are going to make it small I told you that battery is an energy storage device ok. So, the battery is an energy storage device. So, naturally, if you go from a cylindrical cell which is I mean something that is you know significant significantly large you can hold in your hand it's one solid object that you can hold in your hand and the same chemistry same chemicals everything else you use you to create a button cell which is a tiny, thin flat disc right. So, you move from something that looks like that to something that looks like this, so disk, so cylinder becomes a disk. Naturally, everything else being the same this has more chemicals, holds more chemicals same chemicals whatever is the same chemical as you assumed it is the same chemistry relative to this there’s reactant let me say, reactants being the chemical these are the same ok. So, this holds you know cylindrical cell holds much more reactants than the button cell. So, therefore, you naturally you can expect that the button cell will not last long I mean if the cylindrical cell can last say 10 hours in some particular operating condition, the button cell might just last 15 minutes under the same operating conditions ok. So, so, but the point being, so that is why you have to keep in mind that when you miniaturized the device you should also focus on miniaturizing the or drastically reducing the power consumption. If the power consumption is high, but your device is small then you are stuck in a bad situation because you have to find a battery that is small but can still give you a considerable amount of power for a considerable amount of time ok. So, that is something that you have to be careful about. So, anyway, the button cell came about because of all this interest in miniaturization and therefore, you have a wide range of technologies which button cells, there also there are a lot of standard sizes which you can go to you know any store these days any electronics store and get yourself a button cell of some specification. Then comes the prismatic cell which is a little bit like you know flat box, flat rectangular kind of box. This was this is I would say relatively more recent, this is mainly to assist in space compaction. So, for devices such as our mobile phones, for electric vehicles etcetera, the cylindrical structure is not. So, convenient we will see that in just a moment, but the flatter structures are much more convenient too concerning space ok. I want convenience is one thing, but are much more efficient concerning space utilization and therefore, they are the prismatic cell is much more of interest for devices which are particularly trying to aim for compact spaces. So, the mobile phone is one such device, the electric vehicles are some are devices like that. So, that is the thing. Pouch cell is an again a relatively recent introduction into this process which is the very soft version of the I mean in some ways you can think of it as an evolution of from the prismatic cell it has similar kind of you know dimensions or structure physical structure, but it is made as a pouch it looks like you know some plastic bag that has been sealed and two leads are coming out of it. So, they made some interesting way in which they could connect up to those two leads and it is very lightweight and it is very flexible. So, therefore, this is a relatively recent activity. You can buy toys for example; some of the toys will have a pouch cell inside it. If you check some of these small especially if you see these small toys which are these at least today you know you have all these small helicopters that are there these toy helicopters that are there they need extremely lightweight the batteries. So, the battery structure has to be pretty lightweight and that is made using this pouch cell. So, this is the structure that we are looking at. So, these are some of the major battery structures. We will also briefly look at how they look at, look like. (Refer Slide Time: 15:22) So, for example, the button cell, the button cell has I start with this because it is very nicely laid out the three pieces that you need there you need let’s say this is the anode, this is the cathode, and this is the this is a separator which is soaked to the electrode like, separator containing the electrolyte. So, it is a separator containing the electrolyte. And with that, you can you know usually if it is a liquid electrolyte this is how you would hold it, but you can also have some kind of a polymer which is the electrolyte. So, now in a button cell what additionally you need is you have to see you have these chemicals you have to seal them in some way it has to be sealed in such a way that nothing leaks out. So, usually, you will have a cap that comes on top which and in fact, this electrolyte will also be made like this. So, that it doesn’t allow the electrodes to short circuit and then you will have some structure that looks like this. So, a structure that looks like this which is the top part of the cell and then from the bottom you will have another structure that looks like this ok. So, and here you will have a gasket. So, this is the side view, a side views so to speak of the button cell. So, you are seeing its side on. So, if you see it may be a little bit at an angle you are looking at this top structure let me just say I will put some dotted lines down here so that we can see what we are looking at. So, you are looking at a top structure that looks like that and this bottom structure is here that will give you some perspective of what we are looking at. So, we are looking at some bottom structure that looks like that and then the here is where you have the ceiling, you have that seed that goes around, which you which will not be visible you just see there will be one small ledge there and inside that ledge is where the gasket is sitting and that is how you will get your button cell. So, the button cell kind of looks like that. But basically, it has an anode and a cathode and an electrolyte.