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>> All right so I'm so thanks for the kind introduction and for the great weather today. So as you heard I am going to talk to you about those materials and their application in storage technology and here in a whisper Is that normal. Ok. So I you know I like to tell the students in the audience that this title is longer than I'm used to but there's a lot of content the intention is to address the subtext and that is that in a field that is dominated by material science and chemistry there's actually quite a lot of room for chemical engineering innovation in particular in the application of modeling tools to design materials systems so that's the sort of subtext throw throw to top so the question of why the batteries application is no it is pretty straightforward I used to have to plow through each of these contacts but not anymore right so we all kind of understand that solar and wind are not novelties anymore they're here to stay and in part they're here to stay and not because they're just here but in part because they have proven themselves as economically viable technologies both are not dispatchable which means that you can't call on the sun to give you energy when you want to so you have to have a mechanism for storing the energy then the sun is shining so that you can use it and batteries are quite important especially low cost batteries for that for that for that purpose.

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Now of course you're an audience almost filled with young people and so many of you don't know what this is but I'm pretty certain most of you know what's what the one of the end is and if you speak to people who know this field this is the microelectronics feel they will tell you that they can do much better more compact more powerful portable devices were it not for those clunky batteries that you know carry so much weight and take up so much space and then of course the very batteries that were successful in portably that Tronics are no being employed wholesale in cars in drones and increasingly in humanoid robotics and we were expecting them to perform in the same way and to have comparable impacts and the statement of this talk is that that is limited because of the choices we've made in the materials for the anode and cathode in the battery and there are opportunities with engineering innovation to potentially go beyond these choices.

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So. So a big part of my motivation comes from this light that I didn't make Actually I wish I did that's why I've added to it but this slide was made by a professor at mit. And he is the founder of the battery company a 123 and what you have to strain to understand is can batteries ever get to a level where on a material cost basis we approach 100 dollars or less per kilowatt hour you know why is that important but that's important because if you couldn't run the battery troublefree for about 3 and a half to 4 years the more ties cost for an acquisition cost that is this plus whatever systems integration costs are associated with that it's roughly at or of order $3.00 to $0.05 a kilowatt hour so that means that the storage technology will then have comparable cost benefits as the generation technology using renewables so this has been kind of like a holy grail in the batteries field how do we get the costs where we can actually you know store energy at comparable cost as we can generate from renewable sources so this chart has 2 axes so the left axis is the cost per kilowatt hour on Lower is better on the right out the bottom axis is time.

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And the time axis is not so important in this audience what is more important is the colors varied so the colors are meant to describe different battery chemistries and although you might not be able to see the names from where you're sitting the lithium ion technology sits roughly this $100.00 per kilowatt hour level.

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Which means that on the one hand it's on the more expensive side as battery technologies go but on the other hand with optimization we can get there to a few cents a kilowatt hour so this is why even the current technology has pretty substantial legs in terms of opportunities for improvement you know what caught my attention many years ago is as you come down this type of chart there are relatively inexpensive better chemistries But what is interesting is that all of them use metals as the.

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Why is that well because metals are very energy dense objects if the metal is a metal like let's say sodium or aluminum that's abundant on earth the cost is also lower and so those 2 factors combined to give you a relatively low performance metric on this this left axis.

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So if that is truly the obvious question is Why don't these batteries exist today and why they when they dominate our technology landscape. No the interesting fact is that the 1st lithium battery was in fact building this design and these batteries were developed by this man who you know the quiz question is do you know who that is.

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The answer will come in the next light and this battery was built believe it or not that Exxon Mobile is utilize a lithium metal I don't know if utilize that into collating cathode and there's this polymer member and it's called a separator whose only purpose in the world of a sickly separate the anode in the cathode from accidentally making contact with each other.

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When the battery is fully charged the lithium resides in a thin metal in the anode when you discharge the battery it gets oxidised plus onto the action of the potential gradient the lithium ion travels from the anode to the cathode through the membrane and is hosted in the cathode when you recharge the battery you want exactly the reverse to occur and what this is discovered is that this animation will show at 1st everything works pretty well charge this charge but then the anode begins to grow these structures their metallic lithium they're called androids they are very strong relative to the plastic very easily they penetrate through the plastic lair they cross over to the cathode side of the battery and a short shorts the battery internally not only keyed that is generated as a consequence of that short is often enough to ignite the volatile electrolyte that's used in the cell and this is the end of life of the cell no such cells were considered on safe and they're not in the market today because of this safety concern but what caught my attention is that every single metal does this and fails in this exact same way.

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The only exception at least until about a year ago people used to think was magnesium because during or by a very famous battery scientists that published papers saying that big museum some o. does not form dendrites but a group in Korea a year ago showed that museum fields just like everyone else you just need to drive it at a high enough current density so tells us that this question of why is it that metals do not want to form planar into faces on each other why is it they are prone to form these dendritic deposits as a fundamental interest whether you want to go for batteries that are relatively high cost to ones that are somewhat low cost in aluminum and think they will all fail in the same way.

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No of course the world didn't wait for that solution as this is the answer no to my 1st question they didn't wait for that solution in the model discovered that if you store lithium in carbon in ionic form 6 you don't have this problem of dendrite formation at least at low rates of charge and discharge and then of course good enough discovered Well if you're going to take the hit by storing lithium and carbon which basically means for every model of lithium you know carry around 6 miles of inert that doesn't contribute anything to the energy density of the battery then you need a cathode that has a high enough potential so that the energy in the battery which goes roughly is proportional to the potential and the power which goes as a potential squared could be high enough to be competitive and then of course this guy started it all and I think until the last couple of weeks they used to say you don't know who these people are but now you know you should because they're the recipients of the 21000 Nobel Prize in Chemistry for this work.

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So so why not just stop right there something wins a Nobel Prize usually means it's already pretty good and good enough you know why should smart people conceded to work on it but we continued to work on it because the questions at the core you will see in a minute are quite fundamental and there are chemical engineering questions but then we are also not satisfied that our batteries rights all of us want to be able to pull or Evie up the equivalent of the gas station and fill up on time scale equivalent of the time scale on which we can actually run a combustion engine driven car the truth is that once we get to those timescales whatever relief lithium can get by being hosted in carbon disappears because the potential difference between lithium plating the carbon and then surging in the carbon is actually just $200.00 millivolts So the lithium plates the carbon on the end consequence is that it feels just in the same as the 1st battery so it becomes a safety risk so if we're ever going to get to this fast charge technology landscape that people are talking about the need to solve this fundamental problem so my group and this is just about 4 years old we started thinking about you know why metals fail how they fail on uncovered a variety of mechanisms and I want to just spend time in this talk today highlighting you know 2 vignettes from our work that focuses on 2 mechanisms right and the 1st one occurs of low current densities and the 2nd one is high current densities and as the style of my talk into slides I'll tell you what high and low means what is the what is the speed limit that differentiates them.

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So I look around densities the the overwhelming evidence seems to be that the metals formed and writes in part because of their reactivity So the 1st step is that the metal reacts with any component in the electrolyte to put the liquid it could be salt the form this thing called the solid electrolyte interface it's a new material phase of forms on the metal know the more reactive the metal is the more heterogeneous this in a phase is the more heterogeneous the in a phase is spatially the more heterogeneous its transfer properties are with a consequence that then the metal deposits on the electrode deposits in a heterogeneous manna in what we called Euclid's now these new creates act more or less like lightning rods so you are told as children not to go into a thunderstorm at a point the pole in a similar manner these nuclear concentrate the electric field lines accelerate ions so that these in a faces then become or these nucleus then become the selective spots at which deposition a curse the end result is that you get these fractal structures that are term dendrites sort of sequence is a chemical instability leads to morphological instability the distributed bumps and that morphological instability leads to dendrites though often the fairly or isn't just the dendrites cross through the battery separator to short to sell but the failure could also be that the dendrites are fragile and they break to cause what it's called mechanical instability and this is a pretty substantial source out of fires but a premature failure of batteries and this mechanical instability is just as important as the other instabilities in this sequence no if you thought you would get away by just not you know working with reactive metals Well it turns out for all metals once you get to high enough current densities.

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Fluid mechanics takes the place of chemistry and true of mechanics creates these things called hydrodynamic instabilities or electric convection that have a very specific peculiar structure where they act to dump material at points on the surface with the end result that you create structures very much like this is nucleated structures but is caused by flow and once these structures form again in the same old as the lightning rod effect they grow in the dendrites different morphology often but same result Ok so.

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So so my talk today is actually going to cover the those 2 big topics so I'll start by talking about hydrodynamic instability talking in some detail about where it comes from and how it connects to that 1st word in the title of my talking electric kinetics to show you how one could use polymers as a very effective tool for not eliminating this instability but for managing that instability and then later on in the talk I'm going to go into the chemical in still to build the problem which is a little bit harder and show you how different types of polymers could be fact in managing in some cases even the eliminating that instability Ok so.

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So let's talk electric kinetics right so this is kind of an introductory slide so the idea is to let your kinetics basically describes how ions move in response than electric field and the system that we want to consider is a simplified system so it's a binary electrolytes that has positive ions and negative ions sitting between 2 planar electrodes held at different potentials The idea is that I polarized this system by applying a potential difference and I want to understand how the ions move.

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You know what is interesting about this particular question is that invariably the ions will do something at the end of phase so they might react at the end of phase if the interface is connected to an external circuit to become reduce the form the metal or if the end of phase is made of a membrane and iron selective membrane they might be lost by absorbing into the membrane and the end result of both processes that you end up losing Catalans the positive ions at the expense of the islands so this process generates.

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Charge gradient at the interface that can have consequences you will see in a minute including producing electric convective flow so we know how to model this process it's actually pretty straightforward we can write down the evolution equations and these are they so these are the so-called Planck equations so this is basically the conservation of mass which many of you have seen before this is the flux this flux has the usual convective term it has a diffuse If term and it has a term that's related to electro migration because this all happens in an electric field or electric potential gradient the electric field satisfies a question and you basically just solve these equations and you can actually calculate how the current in a battery or current in that or chemical cell varies with the potential So so that's good enough but what people normally say is that well the cell is closed and so I can ignore all of the terms that how of the loss of the.

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So-called stagnant electrolyte assumption. The system is at steady state so they can ignore all the terms of how time dependence Now if you do that you find that the current has a very interesting relationship with the imposed potential difference so that low potential differences the current is proportional kind like owns law which is sort of what you would expect from any linear response theory but beyond a critical dimension that's potential of order for in this analysis the current saturates In other words I am driving the system harder but the ions aren't moving and the faster you know this saturation one can show by actually solving the same set of the equations but for the concentration of violence in this plot I'm sure you get a concentration of islands what you find is that if you drive the system a drug in the low driving forces that is that the orange the on a on concentration varies a linear leave it position in the in the electrode space in other words the gradient of the concentration is a constant which means you're under what's called the fusion control the ions are moving slow enough the diffusion can always keep up with the lost of cow by hand so this is basically maintain charge neutrality at the boundary.

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No On the other hand then one gets the high driving forces the blueline you find a linear response at the low. Distances far away from the electrode surface but as you approach the electrode surface you know it is the cap and concentration goals are on this basically means that the Fusion is not fast enough to keep up with the deposition that's occurring or the last of that ions that maintain the electric neutrality so this region where you have the Senshi no Amazons because of charge you tragedy hence no cap ions It's called a space charge region and because you need ions to conduct electricity in a battery it means that the battery becomes extremely resistant.