Good morning everybody. Good morning California and good afternoon. Good evening. The rest part of the world again. This is the tree. I'm the co director of the storage X initiative right here Stanford University together with my co director William chair. Today, we will have two very outstanding speakers. As usual, to give us the storage X symposium talks. We will have professor man theorem from University of Texas of Austin. And also Professor William chair right here my co director at Stanford University to give us the two seminars. Professor man theorem is a professor and UT Austin also he's the director of Texas mysterious Institute. Very well known in over the last several decades working on exciting barely topic. His work has been widely recognized he's the fellow of electrochemical society fellow of materials research society, and a fellow of Royal Society of chemistry. The second speaker today will is absolutely our super star here at Stanford University. Since joining the faculty, he has been coming up very novel and exciting approach to characterize the batteries, particularly using for example AI approach. He's the winner of MS outstanding young investigator world. He also win the war and the electrochemicals is a society of solid state chemistry. He also now leading significant effort right here as a co director of storage X to push the new direction exciting directions of the bear is going with this. I will like to start with the presentation from Ram. Let's go ahead. Thank you for the nice introduction and for the invitation. My talk is going to be focused on richness and complexities of oxide cathodes for batteries. So it all started in 1976 when 2019 Nobel laureate professors Stanley with the ham demonstrated the first rechargeable lithium batteries that takes on cooperation in the United States. But have but by having lithium metal as the anode and titanium sulfide as the cathode. It's hard to get more than 2.5 volt because we cannot lower the cathode energy level below the top of the Salper 3P band. So the expertise of the experience and expertise on oxides, Professor John good enough to put Oxford started working on oxides because the oxide to be banned lies at a much lower energy than the Salper 3P band so you can get forward so that's how all the oxide cathodes started. So when you go from sulphide to oxide. Yes, you can increase the voltage from say approximately 2 volt to 4 volt. Now, when you go from a simple oxide like F2O3 to a poly anion oxide such as iron sulphate or phosphate or even silicates, you can increase the voltage even further in an oxide. Because in these examples for it, for example, the APO6 octahedra share corners with the XO4 tetrahedra that is sulphate or molybdenum date or phosphate. So that means the oxygen is competing with both iron on one side and the X on the other side. So if the Salper oxygen bond is very covalent, the iron oxygen bond will be less covalent or more ionic. Therefore the iron redox energy will not be raised that much compared to in a simple oxide to increase the voltage. In these examples, for example, you go from 3 volt to 3.6 volt from molybdenum to sulphate and in fact you go from less than 2.5 volt to 3.6 volt when you go from iron oxide to iron sulphate. So this is yet another way of increasing the voltage when you go from a simple oxide to a poly anion oxide. So there are now three major classes of oxide cathodes, layered, spinel and poly anion family of oxides and all the three of them were developed by Professor Gooden-Ups to started at Oxford and then also carried out at least the third part of the University of Texas at Austin. And each one of them have their advantages and disadvantages. I have listed some of them here but I will point out the major advantages. The layered oxides have high electronic and ionic conductivity. They are also highly dense so we can get much higher energy density with the layered oxides compared to the others. But there are limitations with the cobalt. You cannot charge more than 50%. With nickel there are cyclability issues with manganese stability issues. With spinels, again we get reasonably good electronic and ionic conductivity. They have good structural stability but there are limitations on the cyclability. Poly anion, they also, the main advantages, the oxygen is very strongly bound by phosphorus or sulphur or others so you get good thermal stability. There are also wide range of materials with poly anion family. You can use very low cost metal ions like iron for example. So those are all the advantages. The disadvantages are poor electronic and ionic conductivity so you have to make them nano, coat them with carbon. So at the end of the day you get very low volumetric energy density. So these were the three families or concepts developed in the 1980s involving three visiting scientists and none of them had overlap. Very interesting story. Museshima from Japan came, worked on layered oxides with young good enough, left. Then Michael Thakiri came from South Africa, worked on spinel, went back to South Africa. I came from India, started the poly anion. Then I was the one who happened to stay, come to UT Austin with good enough from Oxford. And for the past 35 years I have the privilege of seeing John almost every day. So I would say the three of us were very lucky to work with John in the 1980s to contribute to the oxide cathodes. So now today I'm going to focus on mainly the layered oxide cathodes. So the first question is why do we have three metal ions, NMC rather than one metal ion for the cathode? The answer is each one of them has its own advantages and disadvantages. There are two major aspects. The first aspect is the relative position of these redox couples with respect to the top of the oxygen 2P band. Cobalt 3 plus 4 plus overlaps with the top of the oxygen 2P band. So it has chemical instability. For example, we used to do, my group used to do a lot of chemical deletion 20 years back. So if you keep on removing lithium from the lithium cobalt oxide, the oxidation state does not increase below 0.5 lithium because you are not oxidizing cobalt anymore, you are losing oxygen. In contrast, you can go all the way to 4 plus with manganese and nickel. The other factor is the stability of these ions in the octagonal site versus the trigonal site. So if you take the difference, that will be termed as octagonal site stabilization energy. As you see here with the six electrons in the low-lying T2G band, cobalt 3 plus loves octagonal site. It hates tetragonal site. On the other hand, manganese has very low stabilization. It can move very easily from the transition metal layer through a neighboring tetragonal site to the octagonal site in the lithium layer. Transitioning from layer to spinner like faces, voltage fade, all those which we do not have with cobalt. So cobalt is a gift. There are a few other criteria. Cobalt is metallic, others are not. Manganese is abundant and environmentally benign and manganese dissolves more. Cobalt doesn't dissolve that much when you discharge. If you look at it, in all of these six criteria, nickel is in between manganese and cobalt. The important thing to note is chemical and structural stability, manganese and cobalt are exactly diametrically opposite. That's too bad, but that's what we have and we have to live with that. That's why we have a combination of NMC. Nickel can go all the way to 4 plus, so we can get much higher capacity with nickel than cobalt. Also, fortunately, when we synthesize compounds with high nickel content, we get high tap density. That's good for high volumetric energy density, even though nickel does not become metallic because we are operating with EG band rather than T2G band. EG band is still highly covalent, so you can get good rate capability. In the battery, about 50% of the material cost is from cathode. In cathode, cobalt is the most expensive and cobalt also has less supply compared to nickel. There is a lot of interest to get rid of cobalt and increase nickel, so that can also lower the cost. There are three challenges. When you increase nickel, cycle instability, thermal instability, air instability, that involves both the bulk and surface of the cathode as well as the interaction of the cathode with the electrolyte and species dissolving, interacting with the anode, so it's a complex problem and that's what my talk is going to focus. So all the samples I'm going to present data were made by co-precipitation of the hydroxides and firing with lithium hydroxide. We can actually make any composition 10 kilogram of hydroxide precursor per batch. Nickel content anywhere from 0 to 100%. We can also incorporate hard to incorporate or dope ions like manganese and aluminum during the co-precipitation. So all the data I'm going to show you, they have secondary particles of 10 to 12 micron and primary particles. One secondary particle is shown here, so you can see how the primary particles are sticking together. They also get good, nice crystalline phases. I'm also going to present all the data with pouchesel, with graphite anode, not with hapsyl lithium metal anode, so very reliable. So here I'm comparing two cathodes, NMC 819, 80% nickel and NC no manganese with 94% nickel. You can see the cyclability here, first when you go from 80% nickel to 94% you get higher capacity. That's why we are all interested to increase the nickel content, but the cyclability is getting worse when we go from 80% nickel to 94% nickel. If you look at the cycle life, if you really look at large number of cycles like 1500 cycles, then the 94, 0, 6 pays much more compared to 81, 1. So now we need to understand why. Of course, we keep the cutoff voltage 4.3 as you increase the nickel content, the voltage slightly decreases. So with the same cutoff voltage, you can get higher capacity with a higher nickel content compared to with a lower nickel content. That means when you charge it, you get lattice parameter changes in the C parameter. So that creates cracks. When you create cracks, you also create new surfaces. So interaction of the cathode surface with the electrolyte is keep on continuously increasing as we cycle. So this shows a comparison of an SEM evaluation or inspection of the two cathodes after 1500 cycles. So as you see here, initially you don't see much difference, but if you take the cathode out after 1500 cycles, there is much more cracking degradation in the 94, 0, 6 material compared to the 80% material. So more polarization, more surface reactivity. So that's just the bulk. We have also used time of flight secondary and mass spectrometry. That has the unique advantage of producing characteristic lithium varying ion fragments depending upon which compound you have. So that can help us to have a better understanding of the interfacial reaction between the electrolyte and cathode. One important aspect is supposing if you have metallic lithium on graphite, you will get a lot of Li3 clusters compared to other compounds. So that's a way we can detect whether metallic lithium is deposited on graphite or other components. In addition to that, you can spatter and we can see as you keep on spattering how the components are changing and you can also use isotope enriched cathode and you can understand the cathode-electrolyte interaction. This slide is important. So we are comparing the SEI thickness on the graphite. So the only difference is in one case, 811 cathode, another case, 94, 0, 6 cathode. Everything else is same. So we are now by spattering, see how much depth we have to see go to see the graphite as a function of cycle number. 811, after 15 minute cycle, you have to spatter this much to see the graphite. 94, 0, 6, you have to spatter that much to see the graphite. The scale bar is here, so you can kind of compare. So that means the graphite has much thicker SEI layer when it is paired with 94, 0, 6 compared to 811. Only the cathode is different. Anode is same, everything else is same. Also, the amount of nickel deposited on the graphite after tripping on the cycle is much more when the graphite is paired with 94, 0, 6 compared to when it is paired with 811. So metal dissolution increases with increasing nickel content and metal deposition on the anode because of crossover also increases with increasing nickel content. Safety or thermal. So to compare the thermal stability, we have to charge them to the same capacity, otherwise it will not be a good comparison. So this is after charging to 220 milliamp hour per gram. As you see here, when the nickel content increases, the thermal stability decreases. That is understandable. Manganese has all kinds of oxides, 2 plus, 3 plus, 4 plus oxides. Cobalt has only 2 plus and 3 plus oxides, no 4 plus. Nickel has only 2 plus oxides, not even Ni2O3. Therefore, nickel is much more unstable in contact with the electrolyte. So that's why the stability is decreased. In addition to that, the biggest headache at the industry with high nickel compounds is, lithium comes out to the surface when you keep the material at ambient condition in air to form lithium hydroxide or lithium bicarbonate or lithium carbonate. As you see here, these little green things. So that's called residual lithium. If you have too much of it, it creates problem clogging to make electrode. It also degrades the performance. So residual lithium increases exponentially with increasing nickel content. Exponential. When you go from 60 to 80, already problem, 80 to 90, much more problem, 90 to 95, even more, 95 to 100 percent, much more. Exponential as you see here in this plot. So what do we do? Are we stuck or can we do something? Yes, of course we can do something. So we are comparing here the same 9406 without any aluminum and here with 2 percent aluminum. You can see as soon as you put aluminum, you get much better cyclability. The red curves, you also get much better voltage stability. Why? If you do not have much aluminum or do not have any aluminum, you have more nickel crossover to the graphite compared to the sample, which has 2 percent aluminum. And that also lead to more trapping of active lithium on the anode compared to the material that has 2 percent aluminum. So aluminum, magnesium and even boron or other dopant do a lot of magic to have a big difference in this. I am comparing here the cyclability again, undoped versus aluminum and magnesium. Again you can see dope samples have better cyclability and dope samples also have better thermal stability compared to the undoped. I am also comparing the performance after exposing the undoped material to air. As you see here, the undoped material after exposing to air for 14 days or 30 days degrades much compared to the material with 2 percent aluminum less degradation. So what is the magic with aluminum or magnesium or other dopants? Of course there are small differences in the energy level among manganese cobalt and nickel, but they have edgyshite octahedral. So the metal ions are directly seeing each other. So there is a good flow of electron I would say highly delocalized system. Electron mobility is good. As soon as you put aluminum, you put up that electron delocalization or communication. The lattice becomes a little bit more localized, the so-called Anderson localization physics community. So because it becomes robust, the metal octogen bond becomes stronger and also mobility and electron mobility is decreased. Therefore metal dissolution is decreased. So far I talked about doping bulk stabilization. We have now surfaced stabilization by treating these oxides with octogen 18 enriched phosphoric acid. The phosphoric acid amount is less than 1 mole percent, very little. So when you do that, that react to the residual lithium and forms a nice Li3 PO4 layer on the surface. You can see from Topsim's image and the big difference you see is the cyclability there is a big difference. The sample PNC means phosphoric acid treated, NC means not treated. You can see much better cyclability when you treat the sample with phosphoric acid because the surface is covered with a nice Li, lithium ion conducting Li3 PO4. We have also examined the samples. After 1000 cycle, the rock salt formation is to a depth of 15 nanometer in the NC sample compared to 3 nanometer in the case of phosphoric acid treated sample. So degradation on the surface is decreased. This is the cathode alkylate interfacial layer analysis with Topsim's. As you see, there is much thicker CI on the sample which was not treated with phosphoric acid and much thinner 20 nanometer versus 100 nanometer on the sample treated with phosphoric acid. We have also charged the two samples to 4.4 volt and stored at 55 degrees C to see how the things change. The sample treated with phosphoric acid has much more stability on the voltage than you store at 55. And also after storing at 55, if you look at the discharge capacity, the phosphoric acid treated sample has higher discharge capacity than the other sample. So surface stabilization is important. This is the anode examination of the anode graphite anode paired with the NC and the phosphoric acid treated sample. It's very hard to go through everything, particularly I would like to pay attention to the nickel. As you see here, the sample not treated with phosphoric acid has much more quantity of nickel on the graphite anode compared to the sample which was treated with phosphoric acid. So the sample treated with phosphoric acid has a nice surface protection. So that means metal dissolution decreased. That means you see less amount of nickel on the graphite anode. And also you see much thinner SCI layer on the phosphoric acid treated sample, 10 nanometer compared to 40 nanometer thick on the untreated sample. Now the next question is yes, metal dissolution is a headache. Why do they dissolve? How do they dissolve? Would all the metal ions dissolve? The answer is no. So it is an electronically driven lattice instability that leads to the dissolution. Particularly metal ions which have one electron in the EG band, they are yantralar active. They can have long range distortion or dynamic fluctuation, yantralar distortion. If you have that, then few things can happen. The Mn3 plus ions can do charge ordering to relieve the instability. So one can become 2 plus, one can become 4 plus. Once it becomes like that, the trace amount of proton present in the material can protonate the oxide and water can be formed and that can go away. When the water goes away along with manganese, so that's how the manganese 2 plus goes into solution. And then the dissolved manganese goes to the anode and causes all these problems. So only certain ions which have electronic instability due to one electron in the EG band that causes dynamic or static lattice instability. They lead to manganese dissolution. So far I talked mainly about the cathode stabilization, both bulk and surface, but we have to also work with electrolyte. So for example I show something here, some electrolyte having EC, this does not have EC and we also change salt. So when you do not have any ethylene carbonate in the electrolyte, that gives better cyclability both at 25 degrees C as well as at 45 degrees C. It also gives better storage stability, the EC free, not only that. When you remove EC, you also get better thermal stability. So conclusion, we cannot simply work with the cathode alone. We need to work both the cathode bulk, surface as well as the electrolyte to gather good performance. So ultimate goal is to eliminate completely cobalt. So we have been successful to completely remove cobalt with a 90% nickel, 5% manganese, 5% aluminum. That just came out in advanced materials last week. So we are comparing a series of combos here. Only NMA here does not have any cobalt. Others have a little bit of 5% cobalt. You can see the NMA cycles well compared to same as 6 to 2, but 6 to 2 has lower capacity. It cycles better than some of the cobalt containing composition. Usually people will be worried about rate capability. We do not see any compromise on the rate capability when we completely eliminate the cobalt and also safety. We do not see any compromise actually impact the thermal stability is better compared to some of the cobalt containing compositions. So the answer cobalt can be completely eliminated from these layered oxides as we move forward. Now couple of slides. So far I talked about lithium layered oxides. There is a lot of work going on with sodium layered oxides. We will have a lot of similar problems as we have with lithium layered oxides on top of it. We may also have additional complication because sodium prefers both octahedral and tetrahedral trigonal prismatic coordination. Therefore they can very easily slide the layers. So you can get O3 to P means sodium is in the prismatic, O means sodium is in the octahedral. Those kind of transition can happen which is to a lesser extent in the case of lithium. Multivalent ion again oxides are useful but close packed oxides if we take multivalent ion diffusion is a big problem. Sometimes in my group we do simple chemistry experiments to learn a lot. For example, if you have LIMO2 or 3-2-NOTB or you can remove all the lithium out no problem. If you have same structure everything same you have MGM2 or 3-2-NOTB or no magnesium can be removed. You can see here you can analyze magnesium content nothing happens. Why? Magnesium 2 plus has a hard time to move from the tetrahedral side to a neighboring MTE 16C octahedral side to the next tetrahedral side so it doesn't want to move. In close packed structures diffusion will be a problem. Also in these materials if you have trace amount of water proton may be inserting in these materials rather than multivalent ion. So one needs to be careful to imply a series of different characterization techniques to have a good understanding of whether it is really proton or multivalent ion. If you go to more covalent systems like sulphide then you can have better possibility you can but then the voltage will decrease. Finally conclusion nickel is the only candidate that can eliminate cobalt without sacrificing the capacity or energy density. But they have cycle thermal and air instabilities that increase exponentially with increasing nickel content. The capacity period is due to both cracking as well as surface reaction and dissolved metal ions go to the anode and cause problem. Doping helps surface coating also helps. Again compatible helicalite will also help when you go to sodium analog similar problems can be there. Lot needs to be understood with sodium, metal dissolution, long term cyclability, multivalent ion, diffusion limitations, proton versus multivalent ion insertion we need to worry about.  I am very very grateful to all of them as well as funding agencies particularly department of energy VTO and BES. And other agencies for funding our work. Thank you very much. Well, Ram, thank you very much for the excellent talks on the cathode. The questions are coming in a lot of them. I think a lot of interest from audience. Let me start from the first one. There is a person asking how does your co-precipitation synthesis of MMC compare to industry methods? Is it a big difference or is basically the same? So industry does exactly same kind of co-precipitation of hydroxides and pyrimuclidium hydroxide. Our synthesis is exactly the same excepting. We have capability in my lab up to 10 kilogram per batch. Of course industry will do probably tons. That's the only difference. But there is no other difference. It can be translated to industry without any problem. It's the same kind of method. Okay. So the second question related to the oxidation state of nickel and lithium nickel oxide. The working couple is nickel 4 plus to 3 plus. But in MMC the nickel couple is 4 plus with 2 plus. Why doesn't the MMC involve in nickel 3 plus? So when you have MMC, when you have manganese and nickel on paper we can write manganese 3 plus and nickel 2 plus. But the nickel 3 plus will oxidize manganese 3 plus to manganese 4 plus internally and nickel 3 plus will get reduced to nickel 2 plus. This is during the synthesis. We have 10 percent nickel, 10 percent manganese, 10 percent nickel will go to 2 plus and then correspondingly manganese will go to 4 plus. So in those materials first you will when you charge oxidize the 2 plus to 3 plus then you oxidize the 3 plus to 4 plus. Okay. If you have LI Ni O 2, if there is no oxygen vacancy and if the lithium to nickel ratio is exactly one to one then everything will be 3 plus but that's very hard to get. You will always have little bit of oxygen vacancy or little bit of lithium deficiency.  Okay. The next question, Ram. So you show the data of this lithium trapping and the anode. The audience asks how does this lithium trapping link to the nickel dissolution problem from the cathode? The more nickel dissolves, the more catalytic reduction of the electrolyte. So you have more SCI pond, the more SCI pond on the anode, the more trapping of active lithium. So it all starts at the cathode. If you have more things dissolving at the cathode, they migrate crossover to the anode. When they go to the anode, there is more decomposition of the electrolyte going on because of catalysis and you have thicker SCI layer on the graphite anode. That leads to trapping of active lithium. So you keep on getting less and less cyclable lithium. That's why capacity phase. It's very clear. Yeah. Both the manganese and nickel. So Ram, just to mention one information to you, I completely agree with your observation. Recently, we use a cryoEM to look at the nickel dissolution problem and deposit on the anode. That's the case for lithium metal. And then we see a significantly thicker SCI. Once you have nickel precipitate on the anode, I presumably that would increase the impedance. Actually, we have seen that and then that can chop lithium completely agree with what you just said. And this leads to a similar question. Is nickel manganese considered worse for the anode SCI? I think the answer is yes. How does it change if you go from graphite to silicon to metallic lithium anode? How does this nickel or manganese dissolution? Okay, that has not been, we have done, the community has done a lot of work with cathode and graphite anode because we are making money. Yes, we are beginning to make some money, but I think the work has not been done. We need to do similar work that is a battery 500 job to do that with the metal and similar work should also be done with silicon. It's an interesting aspect. We need to compare graphite anode, silicon anode or silicon carbon composite anode and then lithium metal anode. How they are different? My gut feeling is lithium or silicon is not going to be any better than graphite, at least now. But I do not have the data. We are beginning to do some work pretty soon, but I don't have the data. I don't have the data, but that is something which needs to be done by the community. Yes, okay. Ram, next question. When you show this data on the air stability of MMC, when the nickel content goes higher, certainly the chemical stability will go down. This audience is asking, well, how long do you need to expose in the air? Basically, during the whole processing, for example, slowly processing and so on, how long do you need it exposed to the air? Start to see the degradation of this cathode surface. And I will add one more thing is, is it the water or is it the oxygen or maybe both that affect this chemical stability? Let me answer the second question first. You have water vapor and carbon dioxide in the atmosphere. So it is water vapor and carbon dioxide leading to the formation of lithium carbonate, bicarbonate and lithium hydroxide. The first question, how fast it happens? As the nickel content increases, it happens exponentially faster. That means if you have pure Li Ni O2, you synthesize, take it out of the furnace, keep it outside for 5 minutes, 10 minutes, you will already have residual lithium. If it is 6 to 2, much less. If it is 8 to 1, maybe it takes a little bit, not maybe, it takes a little bit longer. So the time, how long you can expose actually depends upon the nickel content and again, exponential. It is not linear. As you go to higher and higher, it goes faster and faster. So there is no simple answer. It depends upon the nickel content. Like I said, Li Ni O2 within minutes, you can see because nickel 3 plus is not stable. It wants to get reduced to nickel 2 plus by forming all these hydroxides and also lithium is mobile in the lattice structure. So it can come from the inside to the surface to do the stabilization. So let me go further on this question you mentioned. So CO2, you have water right there. So let's see if you eliminate water by going to the dry loom where reasonably dry, would you see these problems significantly suppressed? Yes, that's correct. But the question is how difficult it is to store in the dry room all tons and tons of materials, right? So that's the issue. Yeah, absolutely. Absolutely. So next question. So you talk about the dopant using magnesium and aluminum to stabilize the lattice to, you know, bounding the oxygen tighter. So what about zinc? We have done work with the zinc. Paper is being written. The zinc also has the possibility of a little bit going to the tetragonal side. Sometimes a little bit. But anyway, with the zinc, it's hard to dope too much, but the behavior is similar to aluminum and magnesium. But among all we find aluminum is the best among all the dopants. So they are all more or less similar. They all of them help, but aluminum is better than zinc. Yeah. Okay. So Ram, you mentioned particularly aluminum doping and depth right there and analyzing how aluminum affecting the electron hopping by the transport property right there. So this person asks from the audience and say aluminum doping concentration is normally very low. Why such low concentration can significantly change the electronic conductivity? It is not exactly the macroscopic conductivity. It is a matter of I'm very familiar with this because I worked on copper oxide superconductors in the 1980s with John Goodenough. If you take the equilibrium copper oxide, it will lose one oxygen when you heat from room temperature to 700 degrees C. As soon as you put a little bit of lanthanum per barium, very big decrease. That means in these materials, the oxygen band and the metal band are close. There is a lot of covalent interaction. Any time when the metal energy and the non-metal energy they are close to each other, highly covalent, it's very easy for the bonds to break. As soon as you introduce some localization, it's 1% 2% that's enough. It is introducing much more localization locally. When you do that, everything gets kind of stronger and it dissolves less. We have the data. We have not done any calculation. That explanation is my gut feeling having worked on a lot of oxides for the past 35 years. Clearly, copper oxide superconductors are highly covalent and similar to some of these. So that's my, I would say, qualitative explanation. Somebody has calculated some of the explanation. That would be nice. Yeah. So right now, there are several more questions. I think now broaden up the scope of not exactly what you are talking about, but related. So you presented, you know, just lithium-cobal oxide, just, you know, MMC, and what's your thought about new cathode without cobalt or even nickel in there, the new cathode and the, you know, prediction or insight to share? We have a lot of challenges. First of all, we have to stay with 3D transition metal series. We cannot go to 4D or 5D because weight will increase. So with the 3D, if you are staying at the left side of the periodic table, titanium and ADM, then your voltage will be lower. Their bands are up. So you have to go to the right side of the periodic table. There is no, nothing else you can do. We are lucky with the nickel. The nickel 2 plus 3 plus 3 plus 4 plus coupled overlap. So that's why that is the only metal ion I know where you can have a continuous voltage profile when you go across 2 redox couple, 2 plus 3 plus 3 plus 4 plus, you do not see a break. Of course, when ADM 3 plus 4 plus 4 plus 5 plus, you can imagine, but they will always have a stuff and that stuff can be 500 millivolt or 1 volt. So that's not good. So there is nothing else as far as I know now to beat nickel. Yes. If you want to go something else, you can go maybe poly anion or something, but you will have other challenges you have to see. So as of now, I don't know of anything which can give the same energy density as nickel is giving with lead offset. They also have high density. So you get a volumetric capacity. Yeah. Okay. So next question relates to electrolyte. This person asks, EC is the primary SCI, you know, formation chemical, right? Is there is the reason what can you remove, you know, get away from EC and having something else. So I am not an electrolyte person, but the electrolyte right now is stabilized largely based on the anode. We want to have something which works well with graphite. That's how we have been using EC. But when you put a higher nickel content, you have to always worry about compatibility of the electrolyte with both anode and cathode. EC based works well with lower nickel content and graphite. When you go to high nickel content, we actually use EMC. Okay. So we need to find out the compatibility between the anode and cathode. And of course with the silicon and lithium metal, it will be different too. Right. So that's where the challenge comes. So yes, when you remove EC, the graphite anode may be affected to some extent, but then you have to also make sure with that the anode like the high nickel also works. So the anode needs to be more done with the silicon, lithium and high nickel, the compatibility. Yeah. So, so when I'm speaking of the SEI or CEI, what's your thought like on the CEI? I mean, CEI is a highly, I think debatable. But SEI has been clearly seen. But CEI, there's people saying, we don't see it. There's people say, yeah, we see it, right? I mean, when I say CEI, I understand the definition becomes weight. If your cathode surface has chemical reaction or electrochemical reaction, the cathode material itself can degrade into something else. And then there's also electrolyte decomposition compound going on to the cathode. What I'm asking is the decomposition part of electrolyte. What's your thought about that? Did you see it? So we have done a lot of top sims analysis on the cathode with the different nickel content. When you have high nickel content, yes, you have surface buildup and they have various components. I had one of the slides earlier. You do see it is forming decomposition products formed on the cathode surface. Yeah, like you have on the anode surface. It happens on both sides. Of course, if you have low nickel and limited voltage, that will be to a lesser extent, you may not see it. So with that, I think let's move on to real chair. Thank you, Ram. And we'll bring you back at the end with our 10 minutes panel discussion. Well, now it's your turn. Thank you very much, E, and it's a great pleasure to follow Ram in this excellent and his excellent talk on cathode chemistry. So today I want to talk a little bit about the design rules for intercalation compounds for high valent and uniform redox reactions focusing on cathodes. And today I will highlight both the layer oxides and the polling and I am lifting my own phosphate as examples. So first let me acknowledge the people who actually did the work. I have the pleasure of working with very talented students and postdocs, as well as longtime collaborators I want to highlight the contribution from Martin Buzant who performed many mathematical modelings of the process I will report today. I saw the state chemistry with Linda Nazar at Waterloo density functional theory calculation with Garcedar characterization work with one of the young others at Berkeley lab, Cypher Islam and other colleagues. Ram has already given an excellent introduction to the history and some of the key problems for cathode materials for lithium ion batteries. And I want to begin my talk by highlighting some of the accepted design and engineering principles for lithium ion batteries. And throughout my talk I will come back to these design rules and hopefully offer some new insights and perhaps revisiting some of our assumptions. So the first design role I want to talk about is the atomic law, this ordering of the materials so it's usually believed that migration of atoms other than lithium is problematic for lithium ion batteries especially at the cathode. And the second design role is that intercalation is generally limited by solid state diffusion in the cathode materials and minimizing interfacial resistance, for example at the cathode electrolyte interface optimizes for power density. A separation is generally bad for the lifetime of lithium ion batteries. So I'll come to each one of these in the next half an hour. So let me begin by reminding everyone the motivation for the lithium ion battery cathodes. One of the biggest challenges that as you remove lithium, the stability the material decreases dramatically and this is why we typically can only remove about half of the lithium from cathodes like lithium cobalt oxide, the NMC materials and C materials and this has a significant limitation on the energy density. So this plot just shows you the opportunity for increasing energy density by allowing you to dilute it further. But the problem is that as you dilute it and remove the lithium the layer structure which enables the fast transport of lithium becomes quite problematic. So ideally what you want to have happen is you remove the lithium but the host structure remains stable. But in reality what happens is that as you remove the lithium, the material disorders that transition metal ions and oxygen begin to move with the material, thereby causing the material to degrade, especially in terms of lithium transport. And one of the reasons this happens is due to the electronic structure that Ram already highlighted. As you dilute the material you are oxidizing material and lowering the Fermi level. And at some point you're going to reach the non bonding oxygen orbital. And this process can lead to oxygen evolution from the material and a wide range of other degradation processes. So I want to first talk about how to stabilize the material against the oxygen from evolving and also talk about the link between oxidizing the material and the transition metals moving around. So there are three things I want to talk about. These are the three contributions to the link between the stability and what I call the high valence redox. So high valence redox means accessing very high oxidation states very deep dilatiation and very high voltage on the cathode. This is very desirable from energy density perspective. So the schematic I'm showing here, you can see the energy diagram as I remove electrons from the system, you can remove electron from the orbitals between the transition metal and oxygen. And the electrons can have various contributions from both the oxygen ligand and also the transition metal. And at first this may seem to be the full story. But it's actually quite not the case as you get to high oxidation state that atoms have a strong tendency to form strong covalent bonding to stabilize itself. And this is especially the case when you start oxidizing the oxygen ligands. And this is the driving force to form very short bonds between oxygen and between transition metal and oxygen. For example, in a typical transition metal oxide, you have about 2.6 to 2.8 instrument as the bond length between the transition metal and oxygen. But when you start oxidizing the material this bond length can shorten dramatically, you can also form very short bond between oxygen atoms, leading to the formation of what's called dimerized oxygen. When this occurs, the local distortion increases you form very large local string energy because you have large along and short bonds, and this can destabilize the material adding to the overall energy. But what really comes to save the day is that if you allow some disorder in the system, for example the transition metal migration, you can actually allow the string energy to be accommodated. And this is what I'm showing here on the right of the plot, and you can see these various modes of accommodating for the local strain you can, for example, form a higher order bond between the transition metal you can migrate between octahedral tetrahedral site, you can form a link between the oxygen atoms. In some cases you can possibly even form molecular species the material, and all of these will cause different contribution to the global energy of the system. And let me be more specific here. So let me give you the example of the lithium rich and MC material. So on the left of the screen I have the fully dilithiated material in which the atoms are in their perfect location so you can see on the top view and the site view, all the lithium have been removed. So now when we oxidize the material to a high oxidation state, the bond between nickel and oxygen or meganese and oxygen will want to shorten. But because all the atoms are in their perfect position, the energy penalties very high because there's no flexibility in the edge sharing structure in the layer oxides. So this is the destabilization cost by the distortion of the bond. But on the right, if I move one of the transition metal from the metal layer to the lithium layer, this actually opens up a vacancy in the transition metal layer. And this allows the string energy to be accommodated. So on the right can see the density functional theory calculation, the bond between the metal and the oxygen shortens dramatically. But because there's a metal vacancy created by the metal moving out of the transition metal layer, the string energy can be accommodated. So when you compare it to left to the right, although you have paid the penalty of forming a defect pair, that is the transition metal vacancy and the lithium anti site, the overall energy is more stable because you're allowing the string to be accommodated. So this is one of the key stabilization mechanisms behind why you're able to oxidize the material to a very high valence and have transition metal move around and yet have the transition metal not leave the material. And this is the ability to accommodate short oxygen bonding between metal oxygen and oxygen and oxygen. Another way to view this is to simply look at the coordination number between the metal and oxygen. So in the left structure, each of the transition metal is coordinated to two oxygen. But on the right, the transition metal is coordinated to only one oxygen. So this decreased coordination of the metal and oxygen is one of the key stabilization mechanism, allowing more flexibility in the structure. There by reducing the tendency for oxygen to depart from the material and stabilize it. So this is one of the key design rules behind how to achieve high voltage cathode. So let me come back to my slide on the design rules. So this table here shows the various possible modes of voltage degradation and hysteresis when you start increasing the voltage of the materials on the right you have very irreversible process. For example, oxygen leaving from material you have completely irreversible transformations, the battery doesn't work. Then you have partially reversible disordering so this is when your metal ions can move back and forth. You get a large hysteresis and then over time maybe some of the oxygen is lost from the material so you have voltage fading. Then moving further to the left, you could have a situation where the transition metal can move around freely going back and forth between the lithium and metal layer. So you still have the large hysteresis but there's very negligible voltage fading the material. And then finally there could be a situation where the transition metals don't move at all. So right now we are in the third column with the lithium rich material. We have some ability to move the metals around to accommodate the stabilization of the high valence state, but the hysteresis is large and you're losing material losing oxygen over time. The most optimal situation will be the first column here where you have no cation disordering. At first this seems really good, but the problem is if the transition metals are not moving around then you lose the stabilization mechanism of under coordinating to transition metal so we're actively investigating whether it's possible to stabilize oxygen in a high valence state or the transition metal in the high valence state without having the metal move around. So this is one of the curious outstanding point on high valence redox. So let me come back to my list. So migration of other atoms other than lithium is problematic. This is somewhat true, but it also can help stabilize high valence say so this is one of the design roles we are revisiting right now. For the next part of the talk, I will like to turn your attention to heterogeneity. So it's great to have high valence redox, but we also want to achieve it without heterogeneity in the material. And for this part of the talk, I'm going to focus on metal scale heterogeneity. That is focusing on heterogeneity that happens at the nanometers and the tens of nanometers and the micron length scale. So let me briefly talk about the easily explainable heterogeneity in lithium-ion batteries in the electrodes. So there are generally two types that deterministic ones arises from transport gradient so whenever you have diffusion limitation that can cause moving front within the battery whether it's within the particle within on the glomerate or within the porous electrode, you can have temperature pressure variations, you can have phase transformation imparting in compositional heterogeneity in the material. In addition to deterministic heterogeneities, you can also have stochastic heterogeneity, and Ram already alluded to this is non-uniformity in the coding the SCI. And all of these factors combined can have substantial impact on the battery performance, for example, local overcharging in the cathodes due to irreversible transformation SCI on the anode, you can have plating. As you increase the C-rate, because the cathodes expand and shrink and same for the anode, this can also lead to mechanical failure of the material. So these are what I would consider easily explainable and all of us working in the battery field are having good handle on all these problems in developing engineering solutions. But for my talk, I will focus on what I would call the non-trivial heterogeneities. And this will arise from three things that are not as commonly considered. First, I will talk about population dynamics. So lithium-ion battery electrode consists of many trillions of particle program. So we have to think about the battery, not as a single particle, but as many particle working in concert. Second, it is widely appreciated in lithium-ion battery materials, especially on the cathode, the properties are highly dependent on how much lithium you have in the system. For example, the exchange current density and the diffusion coefficients are strong functions of how much lithium remains in the material. And this has very strong connection to the heterogeneity of the system. And then finally, I would like to talk about non-equilibrium thermodynamics. So a lot of the understanding of phase transformation in battery materials, whether it's the cathode or anode, is based on negligible current. But this thermodynamics will be modified when you start flowing a current in the battery. So I won't visit that as well. So I will try to give you an overview of how mesoscale heterogeneity develops within the material due to these three factors, and then talk about some of the design rules that we can begin to apply to remove these heterogeneities. So let me begin with some unexpected observations. This is a very simple, very well-behaved system, NMC-111. And when you charge and discharge NMC-111, you would expect a solid solution behavior, which means that you can insert and remove lithium in a continuous manner without phase transitions. When you do x-ray diffraction on this material, indeed, this is what you see. This shows you the lattice constant changing with the extent of charging and discharging. The top is charging, the bottom is discharging, and you see a continuous variation in the lattice constant, and charge and discharge is fully symmetric. This is exactly what you would expect. So this particular diffraction was done in C-True at a very low rate, and we're very happy with what you see here, nothing unexpected. But when you start to dilithiate at a very high rate, you begin to see something interesting. So here we're dilithing very quickly at several C's. You begin to see what appears to be phase separation. So you see the coexistence of two lattice constant within the material, and you only see it on charging, but you don't see it on discharging. So this is a very unusual observation because, one, the material is a solid solution, and two, you are seeing this very asymmetric behavior where this phase separation or what appears to be a phase separation only occurs on dilithiation. This is not the first to see this. This has actually been reported quite a number of times in all sorts of cathodes in NMC A11, 622, NCA. The first observation were made about a decade and a half ago and was also recently reported by Karina Chapman and Claire Gray. A simple examination for this is diffusion. So here I'm showing you just a schematic. What happens when diffusion is slow within the primary or the secondary agglomerate within the cathodes, you will begin to develop a moving front in the material, and when you do extra diffraction, I'm showing the diffraction on the lower left plot here, you can see that there are emergence of two peaks as you dilithiate as you charge material. When we run a simulation on charge, we don't have this problem. You have the expected solution behavior. It's very uniformly intercalated and deintercalated. So this is the current understanding that slow diffusion within the particle can easily explain why two peaks emerge at a high rate because you're really competing with a sluggish lithium transport kinetics at the C-ray you're operating at. So this seems very plausible, but we want to dig a little bit deeper here and precisely identify the mechanism. As you can appreciate x-ray diffraction is an ensemble technique, so we measure the aggregated behavior. We actually don't know where the lithium is going into and coming from. To get better insight into that, we need to have a mesoscale level view of where the lithium goes. So to achieve that level of understanding, we've been carrying out microscopy at the many particle levels, so this is x-ray microscopy, looking at individual primary particles of NMC material. So now we can ask the question of not only what is the average behavior of where the lithium is, but also local behavior. So according to the diffusion picture, if you were to take the lithium out of the material, you have phase separation, if you let the material equilibrate, almost all the particle would have the same lithium composition from the relaxation and the equilibration of lithium within the particle. So if you don't let lithium exchange between the particle after relaxation, this is what you will see. And indeed, this is what you see when you charge slowly and discharge slowly. The histogram here basically shows you have some spread in the lithium composition represented by the colors here, and everything behaves correctly, as you would expect. But when you perform the experiment under fast charging conditions, so now we are removing lithium very quickly, you no longer see this unimodal distribution, but you see this multimodal distribution.  And this only is found when you charge very quickly, when you charge slowly, you revert back to the standard unimodal behavior. So this was very interesting because that tells us that this apparent phase separation is between particles, but not within a particle. So this directly contradicted diffusion limited picture, whereby we should find that all particle have very similar composition on average. So we were very puzzled by this behavior. So to understand what is going on here, we're also challenging one of the common design role is that transport or diffusion of lithium in the solid state is weight limiting in the battery cathodes. So when we look at this NMC material, if you plot the C rate versus the voltage, you actually see an exponential behavior which is very indicative of a charge transfer limitation. If you have a transport limitation, you will see the curve curving down rather than curving up. So this is some indication that diffusion is actually not limiting in these material. To further support that, if you perform your standard relaxation experiment, whether it's impedance or transient measurement, you can also measure the exchange current density. And there you also see the same behavior is that you have a very strongly varying exchange current density. And if you compare this to the diffusion coefficient, you find that indeed the exchange current can become limiting under extreme conditions throughout battery charging and discharging. So if the reaction is limiting rather than diffusion in NMC cathode, how can this explain the heterogeneity that increases upon the rate of dilatiation and the heterogeneity only exists on dilatiation, but not on dilatiation. And for that, I would like to introduce you to the auto catalytic behavior of the interfacial reaction. So by auto catalytic, I mean that as you remove lithium, the exchange current density is getting higher and higher. So this is very similar to diffusion coefficient, whereby as you remove lithium, the kinetics of the particles gets better. So this is what I'd like to show you here. We have a schematic of particles which have incidental inhomogeneity. This could be due to different particle size variation and coding. And what I want to see here is how does this heterogeneity amplify or decrease as you remove lithium. So as I remove lithium from the material, the kinetics is getting faster and faster and faster. This means that lithium will be preferentially extracted from particle that already has less lithium to begin with. So this causes the compositional heterogeneity between particle to grow. And this can explain the multimodal distribution of lithium as you dilate it. But as you lithiate, so as you discharge material, then the kinetics is in the opposite direction. So as you relithiate the material, then the kinetics is getting faster. It's getting slower and slower. So what this means is that the incidental hemorrhaginity actually will begin to decrease in the material. So this leads to the unimodal behavior. So in both situations, you start with the same level of inhomogeneity. But in the case of kinetics getting faster and faster on charging, then the heterogeneity is amplified. And on discharging, the same heterogeneity is diminished because the kinetics is getting slower and slower. So this is the effect of outer catalysis imparted by how the exchange current density depends on the extent of dilation. So why does this happen only on reaction limitation and not on diffusion limitation is actually very simple. When you have a reaction limited intercalation, the particles themselves have relatively homogeneous composition profile. So that means the surface composition and the ball composition are similar. And as you remove lithium, the composition on the surface changes. But when you have diffusion limitation, actually your surface of the particle remains almost in the fully dilithid state on charging due to the diffusion front. So from the perspective of the electrochemistry, although the particle lithium content is different, the surface composition is actually the same. So this is what's leading us to equalize the composition. So this is a very simple picture to understand why reaction driven process can control the growth and the retardation of heterogeneity in the layer oxide cathodes. So let me show you a video to illustrate graphically what happened. So as you remove lithium, you can see this mosaic effect where particles are preferentially dilithiated you see in the simulated diffraction pattern the bimodal distribution, you see the bimodal distribution diagram. When you run the same simulation on discharging, you can see the process proceeds very uniformly. Again, this is how the kinetics changes with the extent of the reaction. So this is done by a multi particle simulation performed by Martin Bazant and his student Humbozel. This effect actually grows. If you have transport limitation in the battery, if you have a very thick electrode, this effect is actually amplified because as you have a moving front, the local C rate actually is increased for a given C rate, and you can see this mosaic behavior occur on charging, but you do not see it on discharging. So hopefully I've given you some insights here on how heterogeneity develops within the material selectively on charging as a result of both interfacial reaction limitation and also on how the kinetics depends on the amount of lithium in the system. So coming back to some of the design rules, now we know the importance of the reaction limitation, we can make a plot of the phase diagram under kinetic condition. This is a phase diagram showing you the C rate on the y-axis and the amount of lithium extraction on the x-axis. So you can see that when you discharge, nothing happens. You're in a single phase solution uniform regime, but when you charge material, when you remove lithium, you can run into what we call this fictitious two-phase coexistence between particles under intermediate lithium regime, and this also depends very strongly on what lithium composition you access. In the layer oxide, it's very common not to fully relithiate upon discharging. So this two-phase regime will strongly depend on what lithium content you start with. So these are very interesting insight on the phase diagram for kinetic behavior of the system. So coming back to the design rule, so here I'm showing you that intercalation is not always limited by solid state diffusion. It can sometimes be limited by the interfacial reaction. And this can have strong implications on how heterogeneity developed within the material. For the last couple of minutes, I want to switch gears and talk about materials that naturally face separate. So for example, lithium iron phosphate in this material, the miscibility gap is very large. The material tend to have either full lithium occupation or no lithium. And we are asking the same questions here. How does the population of many particles behave when the material undergoes phase separation? So this is a movie that we have recorded of about 500 particles in lithium iron phosphate in the porous electrode. As we discharge, you can see that the discharging process does not occur very uniformly and occurs in hot spots. Okay, so you can see this in the dynamic video here. If I show you a static picture, you can also see that that lithium is preferentially extracted and inserted only at select particles. So the consequence is significant. This select removal and insertion of lithium in just a few particle means that very few particles a small fraction of the electrode is accommodating the current. So we find that the C rate locally and globally can differ by about 50 times. So if you are charging discharging at one C locally, you can be charging discharging at 50 C. So this is a very important consideration when you consider the stability of the material. So what's happening here? Well, let me first share you a movie. This is a simulation we have done at low rate. You have this very mosaic behavior, which means that the extraction is proceeding non uniformly. So you have what appears to be a popcorn effect. This is exactly opposite of NMC. So this happens at low rate. When you go to high rate, the battery actually becomes much more uniform. So again, the behavior is entirely opposite of NMC. So what's going on here? Well, it turns out this has everything to do with kinetics and thermodynamics. So when you have many particles in the system, you have to think about the behavior not only within a particle but between particles as well. So this is a classical nucleation and growth problem for something like lithium-araphosphate, you have a large miscibility cap of two-phase coexistence, and you have a nucleation barrier. So what happens is in order for you to maintain a constant current, for example, as you discharge the material, you have to have a certain driving force to overcome the nucleation barrier. But in order for the nucleation to happen at a finite rate, you need to keep the driving force there. But as the driving force is kept there, that means as you enter to two-phase regime, you have a fixed driving force causing the phase transformation to occur. So let me show you graphically what happens. So when you want to charge at a low rate, your potential will be slightly over the nucleation barrier. You will achieve a desired nucleation rate, but the driving force will be very large because the dashed line is considerably above the blue line. So this is the driving force for the phase separation. You get a few particle proceeding very quickly because you have a significant driving force. And as you increase the driving force, you don't increase the rate of the phase transition. Instead, you increase the rate of nucleation. So the result is that you increase the number of active particles, but you don't increase the current per-particle. So this precisely explains the population behavior we saw. We saw that the material accommodates additional current by increasing the uniformity of the reaction rather than increasing the rate of individual particle proceeding. So I'm a bit out of time here. I will just end with this one slide. The observation that phase transformation depends strongly on the rate not only occurs at the many particle level, it also occurs at the single particle level. So here I'm showing you an image of the phase separated lithium-iron phosphate at half lithium content, so lithium 0.5. And you can see the beautiful phase separation into lithium rich and lithium poor region. And this is what you expect from thermodynamics. You don't see any composition within the miscibility gap. If you do this experiment in situ, you can also see the moving front. This is what happens when you're at a C over 5 C over 6 rate, you have classical phase separation. But as you increase the rate of lithium removal and insertion, the material actually breaks thermodynamics here. So instead, you see the appearance of composition that's within the miscibility gap. So rather than forming this nucleus and growing, you actually see continuous lithium composition that goes through the miscibility gap. So this is thermodynamically forbidden. So you no longer see the phase boundary, and instead you see a continuous feeling and non-feeling. So why are we violating thermodynamics here? Well, it turns out this has everything to do with the non-equilibrium thermodynamics of the system. Briefly, what is happening here is that the reaction, the exchange current density just like in NMC, also depends very strongly on composition. The resistance is high at low and high states of lithium filling, but it is minimized at about quarter lithium filling. And if you consider this composition dependent reaction over potential, this can cause the phase behavior to change. So in lithium iron phosphate and any other phase separating system, you have this non-monotomic chemical potential or voltage that causes the lithium to want to partition into lithium rich and lithium poor distributions. But as you apply current, because the over potential it is not fixed with the composition, the over potential added to the phase diagram actually begins to skew the energy landscape. So rather than having a downhill energy diagram, as you increase current, you actually cause the energy to become uphill. And when this occurs, there's no longer a driving force to cause the lithium to separate. So what I'm highlighting in yellow here is the composition dependent over potential. So when the resistance of the battery is dependent on composition, you can actually cause a phase separating material not to phase separate in the case of NMC is exactly the opposite. So with that, let me just conclude here. And then come back to my design roles. Solid solution electrodes are more uniform than phase separating ones. This is not the case for lithium iron phosphate, especially when the current is high. Minimizing interfacial resistance doesn't always optimize for power density in the case of lithium iron phosphate. If you have a sufficient interfacial resistance, you can actually use the voltage drop to remove the heterogeneity due to phase separation. Five, as was noted in the original paper by John good enough. The separation can be a problem, but you can get rid of it by engineering the surface reaction kinetics. So with that, let me conclude and thank you very much for listening. Thank you very much real. For the really nice talk about understanding the face behavior understanding the electron induced the structure change in the castle. There are a number of questions right here. So the first one is from Chris Tracy. Well, I think Chris asked this great question. Can you comment on the role of oxygen and iron oxygen and iron repulsion. When the interlayer is depleted of lithium cath ions. To what extent does this drive the rearrangement of the transition metals. Chris thank you for the question so actually this is one of the very first hypothesis we tested so it's generally believe that removing lithium itself will drive the transition metal to migrate independent of the oxidation state of the material. What we have done here is to use simulation to guide our thinking. So we essentially remove the lithium from the interlayer, but we don't allow the charge to localize so we put the charge in the background in our density functional simulation. And actually we don't see a strong driving force. So the empty lithium layer wasn't found to be the driving force for transition metal migration. What we said we found was that the combination of the covalent re hybridization between the transition metal and the oxygen and the point defect disorder. That is what's really driving. This is not to say that the interlayer being empty has no effect. It's a contribution, but it is not the significant contribution. So you'll see that is because there's way for you to decouple the oxidation state and the amount of deletion for example you can substitute with different valence state and people do see that what is really important here is the oxidation state of the material rather than simply just the amount of lithium removed from the interlayer. So we're next question I think is for Julie I think this probably is empty Julie from MIT. What's the fiscal reason for exchange current density to increase with deletion of MMC? Is it stress? Is it CEI? And then a couple is another person's question I think it's related right this person also asks. The diffusion coefficient MMC usually increases and then decreases with the deletion process. So I think the overall these two people's questions are all about these hypothesis you have is exchange to current density depends on the degree of deletion. What's the fiscal origin of that? Well, yeah, or thanks for the great question. Let me first answer the second question. So indeed, both the diffusion coefficient and also the exchange current density or sometimes called the charge transfer resistance both depend very strongly on composition. So generally speaking, as you deletiate both diffusion coefficient and the exchange current density increases. So the question is not the dependence but the relative ratio. So it is usually assumed that diffusion always dominates in these material. But what we have seen here is that the reaction, the exchange current density actually is even lower in even more significant in contribution at extreme high levels of lithium content. In that situation, we see that the system is controlled more by reaction than diffusion though diffusion is also strongly dependent on composition as well. And I would say this is a surprising observation. These numbers are not very easy to measure. Actually, Yeming Cheng and MIT has made direct measurement and we have as well. And the electrochemical measurement as I showed is supporting this. Because if you look at the IV curve, essentially your C-ray versus your over potential, we see the resistance continues to decrease as you increase the C-ray. So this is very classical of a charge transfer limited process. If you have a mass transfer process, then you will get a current that begins to reach a diffusion limited current. So we do not see that. The question from Ju concerns, why do we have this particular functional dependence? Actually, this is just a very classical site exclusion argument. So as you start to remove lithium from the material, you essentially have a site exclusion effect that comes into the kinetic pre-factors. So this is your X over Y minus X. And this can define a peak within the material. This is a simple entropic contribution to the reaction kinetics. So this is the fundamental reason why the reaction isn't constant. You need not to invoke any other arguments. Of course, mechanical contributions will influence the reaction kinetics and its composition dependence. And the CEI will as well, but even in the absence of any of those effect just writing down a simple equation of reaction kinetics using a concentrated solution rather than a dilute solution and then counting for site exclusion on the surface. For example, lithium vacancies, then you can basically recover this very simple dependence on the lithium composition. Thank you, Will. So next question, I think blended together with my own question. It also is the audience question. So this whole analysis, what heterogeneity right there certainly can be affected by a number of external reason. For example, I think from the audience, this person is asking. So how about the heterogeneity just the lithium ion transfer through the electrolyte. I mean, I guess the thinking is, well, you know, the electrolyte ratting, right, and then as well as ion transfer during charging and discharging. I think you showed that in a thick electrode because this whole phase behavior propagate from top to bottom, just availability of lithium ion concentration and the local heterogeneity of that that's one like expect the second aspect I'm also thinking there's also electron transport heterogeneity as well, you know how good the contact between the particles. And in the whole electrodes some of them will have a little bit worse contact some of them will have better contact this also create electronic heterogeneity right this all mixed into the phase behavior. What's your thought about this. So this is thank you for the question he and this is why I showed the stimulation video. There are principally two big sources of transport limitation. Transport limitation in the electrolyte and transport limitation in the current collector so this is basically your ionic and electronic wiring. So I would say that in terms of electrolyte transport, it's very well understood all the equations governing diffusion limitation and electrolyte has been thoroughly modeled. I don't think there are many surprises there. The electronic wiring, as you pointed out is not understood at all. These materials are modestly good electronic conductors but typically carbon additives are needed to improve electronic wiring to the particle, and the distribution is not uniform. So one of the key question we're trying to think about now is, are we really always limited by ionic transport in the electrolyte, or sometimes we can be limited by electronic transport in the current collector. But to come back to my talk. What I wanted to show was, even in the absence of ionic or electronic wiring effects, even without them, heterogeneity can still occur for example in MC. So if you have no reaction, no diffusion limitation anywhere, you can still get this mosaic behavior just arising from the reaction dependence on composition in lithium ion phosphate, you can get rid of the phase separation, even without any transport limitation. So I think the main message here is transport limitation in the electrolyte and in the in the current collecting carbon network is certainly always there and that's amplified at high rates. So you have to also consider heterogeneity that can occur within the particles, even without diffusion limitation in the solid state. Yeah. Thank you will. Hi Ram, if I can bring you back to stage less. Let's have a panel discussion. So maybe we can give you let you take a one minute boy let me ask Ram a question first and in a panel discussion. Very, certainly I already know about this for a long time. You have you've been looking with John, you were John's early on postdoc right and this question is I think for the younger audience right here. And you have seen this development starting from 1970 in the middle of like 76 in the middle of that stand right of having these titanium software intercalation mechanism. You know, discover and also invented. And then later leads to oxide you mentioned that a little bit. This is a Nobel winning work, you know, on the oxide together with Stan and then our Japanese friend right there winning the Nobel. So what's your thought or what's your learning in the last several decades, you know by working with John and then later you have your own independent career and you participate in such important work. So otherwise for some of the young students or young faculties, you know what how to pick problem you know what any thought you want to share. I was fortunate to work with John. So all my degrees were in chemistry. So I am a chemist. I'm a solid state chemist. So when I came to John, I had the opportunity to work with both cathodes or electrodes for batteries as well as on superconductivity. So that opened my eyes a little bit to have the physics education that was very helpful. If you want to work on material science, you need to have both chemistry and physics. That's the first advice. You cannot just to do chemistry alone or you cannot just to do physics alone. It doesn't work very well. Actually, John's trump goddess is a physicist, but he was always fond of working with chemistry postdocs or chemistry scientists. He was always able to work at the interface between chemistry and physics. And that's how he was able to make some unique contribution compared to anybody else on earth, I would say. So that is essential. Number two, when you work, you never know which will be having an impact or which will not be having an impact. When you work, all the work we did in the 1980s, you're all interested from a point of view of publication rather than expecting that it is going to be in somebody's cell phone or car one day was not like that. That's very hard to say, but these days, everything is crowded. It's nice to students and young faculty. Maybe it's a good idea not to work on areas where there is too much crowd, then you may not be able to make an impact. You may work on some areas which may not be too hot, but you believe that it will make a big contribution as we move along. A few days later, you will be recognized much more than anybody else. So you may want to think about it. Otherwise, too many people work, too many experienced people. So it's very hard to know now who did first who did next. It's like that. So you may want to keep that in mind. So some unique areas that may be useful. Ram, this is really interesting insight. I wish I could talk to you earlier about your comment on this. Great. It doesn't apply to you. No, I'm what I'm saying is, you know, when I joined a Stanford faculty coming from the nano science community, I wanted to find an area that's not as crowded 15 years ago. It was actually the bad for you one so using my nano scale expertise. It's very interesting your comment now me let me link back to will. You're you are exactly the case. I think opposite to what ran just said. When we'll start it. I mean, this is very interesting. It's all kind of successful case when we're joining the faculty, you know when even just come to visit, I can spot right away. This is a super star. Well, you're going to area. Let's MMC. It's already many people. I think when you started many, many people. And then you go in, you have very unique insight, you know, you discover many things people didn't think about before or didn't think deeply about before and having a lot of major discovery right there. And then you're kind of like, you're kind of like with a random suggestion, you're going opposite way and jumping in and making a great impact right there. So what's your thought, you know, my my observation is you come from very different background from many folks already in the battlefield. And then you tackle the problem from different angle. So what's your thought, you know, observation, but share with everybody, particularly young students. Sure. This is a great question. I'm happy to share a little bit of my journey. So actually, Rahm and I've been working in the same field, a different parts of it. Rahm is the editor of Solacea Aionics and this is where I was trained I studied point defects and material. When I came to Stanford actually I didn't work on batteries at all was inspired by you to work on batteries. I think there are several important things to consider here. I'm always very curious about understandings of processes and materials so the first thing I did on the layer oxide is I read all the papers and see what all the understandings are and then I became curious I said well what if this is not true, or what if this is a better explanation so I think being guided by once on curiosity is very important. Then I think I also took the same approach as as Rahm and you advise which is, well we need to take a creative approach right even though you're curious, you won't get to the answer if you just take the same approach as everyone else. So there I saw an opportunity to develop characterization method and theoretical methods to try to understand these processes a little bit better. And as I showed in the talk it is possible actually to get very good and interesting understanding if you're willing to dig deeply. So I'm always a believer that as the digger you deep there's always something new there, even though, you know we take a very classical material and MC or you take a very classical material lithium iron phosphate. There's just always a huge amount of richness. That always surprises me. And I'm sure 10 years from now someone else can also dig even deeper and find something that we haven't seen or come up with a better explanation. So I think my guiding principles always never be satisfied with the explanations we have, maybe they're better ones, and try to think of new ways you can approach the problem, not just doing it the same way as before. So I think these are necessary for innovation. And then I also want to mention very briefly and I think this is also highlighted in roms talk is really important to work with others in the field both academically and industrially to understand with the real problems are so I really benefited from working with industry. They really tell me the important problems they're experiencing. And I combined that with an academic understanding, and that helps me to approach the problem in a more comprehensive way. I think it's very good. So let me summarize a little bit I think three things you, you are sharing with us this is highly valuable, while Ram, right it's interdisciplinary chemistry physics I think multi background I also wear to to certain extent, you learn from industry the problem I think this needs to be close a boundary. I think I appreciate that I'm taking your approach is going moving into an area that's not so crowded. When I move into the area not so crowded yet. So and where you are teaching us the third one is, you know, your perspective coming from different perspective different approach think deeply about it that could also can lead to success. So with that, I think I would like to conclude today's symposium. Thank you so much for two of you really amazing jobs right here will be in two weeks. So next Friday we don't have it so in two weeks August 7 the same time seven o'clock by two also outstanding speakers, Professor Vanessa Wood and Dr Robert Kostakit. I'll see everybody in two weeks. Bye now.