 Hi, everybody. My name is Zhenan Bao. I'm a KK Lee professor at the Chemical Engineering Department. Also, I have courtesy appointment with Material Science, as well as the Chemistry Department. I'm also currently the department chair for Chemical Engineering. Then in terms of research activity, I also run initiative in Stanford. It's called the Stanford Wearable Electronics Initiative, where we try to gather students and faculty on campus from different schools, ranging from School of Humanity and Science, School of Engineering, and School of Medicine to work together on building the next generation of wearable electronics. But today, I'm going to tell you about work we do in my group related to energy area. So first, since now, I can still see everybody. Just want to get a quick idea of how many of you are from chemistry background. OK. Material Science. OK. Chemical Engineering. OK. Only one. And Electrical Engineering. OK. Mechanical Engineering. All right. Civil, any civil engineering? Environmental? OK. Did I miss any? No. OK. All right. Very diverse backgrounds. I will try to make my talk accessible to everyone, but it may be a little bit more chemistry and material intensive, because that's the core of research we do in my group. So throughout this week, you will hear many faculty members talking about research related to energy. But every group has a different core fundamental expertise. In my group, the core is polymer materials. And my group is very diverse. Has one third people coming from chemistry, chemical engineering background. And one third from material science and also chemical engineering. And then another one third on electrical engineering, bioengineering, mechanical engineering. So the type of work we do ranges from synthesis design of new molecules. And usually these molecules are designed for a certain purpose. And the applications we look at ranges from sensors, electronic circuits, batteries, and either even implantable electronic devices. And then in taking the molecules to the final devices, we also need to understand how we can process the molecules and make them assemble into certain well-controlled nanostructure so that their electrical and mechanical properties can be well-controlled. So that's the part in the middle. So we have research activities in all three areas. And then the final application gives us the feedback to the materials design to allow us to better understand how we can come up with better materials. The overall research activities in my group, most of the materials and devices we're working on are in the middle part that is skin-inspired electronics, where we come up with new electronic materials that are like skin with stretchability, self-healing property, biodegradability, and tissue compatibility. And then we take these materials to explore their possible applications and they enable new applications. So those are areas like skin-inspired sensors, new forms of circuits and displays, chemical biological sensors, implantable devices, robotics. And then the materials here also allows us to think about, in addition to applications that are clearly connected to functions of human skin, what other unique applications they may enable. So that's how we started to work with battery, energy, storage materials. Basically, it was a spin-off from new type of electronic material that we developed for skin-like skins inspired electronics. But then we started to find many unique applications of these materials. And then it becomes a very important branch of research in my group. Right now, we've been working on this area for 10 years now. And we have a group typically of seven or eight students and postdocs working in this general area. And then we also have a small effort related to electrocatalysis by developing conducting porous materials, either based on these two-dimensional organic molecules or well-controlled synthesis of these carbon nanomaterials for electrocatalysis. So today, I'm going to tell you about the use of polymers specifically, the polymers that we work with and their applications for to enable high energy density batteries. And this class of polymer is called self-healing polymer. And I'm going to explain to you what they are and what's special about them and why they are particularly interesting for battery applications. So this came out of our long-term vision of making electronics beyond smartphones. This is the roadmap we give to eventually go into implantables. And I mentioned we use skin as the inspiration. And when we look at the skin as a material, the properties, flexibility, stretchability, biodegradability, and self-healing, these are the unique properties we find particularly intriguing to incorporate into future generation of electronics. So the key molecular design concept sorry, this was taped in another talk. And self-healing materials in recent years is the incorporation of dynamic crosslinks into polymer networks instead of the covalent linkages in conventional elasmers. The breakage of covalent bonds due to large strain will result in cracks and irreversible damage to the film. On the other hand, breaking dynamic bond is a mechanism for energy dissipation. In certain polymer design, dynamic bonds may form simultaneously when others are breaking to replenish them. Incorporating such an energy dissipation mechanism in addition to other ones such as chain elongations, alignment, and crystal breakage allows polymer network to gain a larger fracture energy and sustain a larger strength before damage. Dynamic polymer networks is an active area of research. There are many examples of chemistry that can allow formation of dynamic polymer networks. Here, I only list a few. We have been working with non-covalent ones due to the large range of mechanical properties we can realize. The association constant and the bonding geometry play important roles on the resulting polymer network dynamics and the mechanical properties. For example, whoopie unit pioneered by Bert Meyer has association constant of 10 to the fifths. It results in a polymer network with solid-like rheological behavior, low dynamics, and high modulus. On the other hand, the association constant of urea is only around one. The resulting polymer network may be liquid-like if highly flexible polymer backbone is chosen with high dynamics and low modulus. Okay, sorry. It was just a combining different slide deck. That happened to be two slides that I had a voice recorded. This is showing an example of the dynamic elasmer that we have made where if you cut the material and put it back together, when you cause a fracture in the material, the non-covalent dynamic bonding, I just mentioned earlier, they have weaker bonding strengths and they can break first. And then when you put the two pieces of materials together, then these chemical bonds can be formed readily at room temperature. So if you think about hydrogen bonding, that's what's holding DNA and the protein together. And these bonds can readily form at room temperature. So that's the type of bonds that we incorporate into these polymers to allow them to be able to tolerate high strain to be stretchable, at the same time be able to self heal upon applying a strain. So how does that relate to energy storage? So a little bit of motivation of why energy storage is important. Well, we're undergoing climate change and a lot of it is related to the way energy is generated because of the power plan usage, the coal burning power plan for energy generation and the cars putting out waste gas. And all of these are causing the contributing to the climate change. But if we go with the alternative energy, fortunately, their adoption is getting more and more widely spread with solar and the wind sources. The price is becoming more and more competitive. So more people are starting to use them. But solar energy is only available during the day and the wind energy is intermittent. And it's not always producing energy at a constant rate. So therefore, and also at the same time for electrical vehicles, still they're quite expensive and most of the cost comes from the battery themselves. So for all of these important alternative ways of generating energy, the energy storage is important so that whenever the energy generating source is available, we can store the energy and then use when needed. So what is lithium ion battery? This is a simple diagram showing the structure of a battery. So when here, there are two electrodes, one is the anode and here is the cathode. And then in between, there is a separator, basically currently it's a porous plastic material to prevent the cathode and the anode to be in direct contact with each other. Since they are both conductive, if they're in direct contact, then they will be shorted. So there is an insulating porous separator in between. And then there's also a liquid electrolyte that helps lithium, if it's a lithium ion battery, then lithium is the ion that's being transported through this membrane and then the anode through this membrane and in between these electrodes. So if I charge up this battery, then the lithium ion will be converted into lithium metal and deposited on the anode. And here, onto the anode and then when you discharge or use the battery, for example, lighting up this light bulb, then the lithium metal would be oxidized into the lithium ion and then transport through the liquid electrolyte and get to the cathode and then here gets reduced. So that's the very simple schematics of lithium ion battery. Here, this chart compares a different type of anode, cathode materials that, so on the bottom is the anode material and the one slash on the right side is the cathode material. So there are different combinations of choices of materials that one can use. And then depending on these materials, the amount of lithium ion that can be stored into the electrode material will vary significantly. So that can be quantified by this maximum energy density, which is a kilowatt hour per kilogram. So based on a certain weight, how much energy one would be able to store. Of course, the higher this value is, the more desirable. And the current lithium ion battery is the anode material is mostly based on the carbon or graphite. So this is the one shown on the right. And I mentioned earlier that during the charging process, lithium ion will enter the graphite and it gets reduced. In the case of graphite, it works very well. And this is used for the lithium ion battery that we use for our computers and cell phones. It works very well because the lithium is being hosted or inside the graphite. And basically each graphite, lithium intercalate because graphite has this layered structure, then lithium can intercalate into the graphite. And each carbon atom can host one lithium atom. So that determines the amount of lithium that can be stored in a certain given amount of graphite. Of course, you may say that, why don't we just put in a lot of graphite and then we can store a lot of lithium. But the energy density based on the weight is very important. As you can have the battery hold more energy, but then it will be heavier. So that's not what we want for handheld devices and also for electrical vehicle. We don't want the battery to take up most of the weight of the vehicle and then a lot of power will be consumed because of the heavy weight. So there's a limitation of how thick you can make this graphite and how much energy basically based on this amount of lithium that can go in. So with the known cathode material, these are mostly based on inorganic lithium compounds. Then the energy density is marked here and you can see that the energy density is still relatively low. Then if we look at the choices of other materials, you see mostly it's lithium, lithium, lithium. But then before the additional lithium as anode, you can see all here there's silicon. Silicon is abundant and if I remember correctly, second the most abundant element on the earth. So if this can work as cathode for lithium ion battery, then that will be really economical and also look at the energy density, it's much higher compared to that of graphite as the anode. And the reason for that is silicon can form alloy with lithium. So every single, every one silicon atom can alloy with 4.4 lithium on average. So you can see the ratio is completely reversed and a lot more lithium can be stored in silicon. So why not use silicon as the anode? Well, the problem is for silicon, so on top is the particles are representing silicon particles for anode. And then if you elicitate and there, one silicon can hold 4.4 lithium and then this lithium will take up volume. So the silicon particle will have a big volume expansion as high as four times of its original size. And when we make these electrode material, we can just put powder and the powder will just fall apart when we try to make an electrode. So we needed to have some polymer. So that's represented by the blue part, blue coating. And this blue part is the polymer that's supposed to hold the lithium particle together. It also has to have some conductivity. So we normally incorporate a little bit of carbon in it or carbon nanotube to make this blue part to be electrically conductive and ion conductive. But the problem is silicon is very rigid and brittle. So if we charge it up, then the particle will have a thermal expansion, have a expansion, volume expansion. And then when you discharge silicon, lithium will come out of the silicon. And then it will shrink. So when it expanded and shrink, just a few cycles, the particle will break and fracture into pieces. And the polymer around them, people typically have been using rigid polymer before because for carbon electrode, there's hardly any volume expansion. So they don't have to worry about the volume expansion issue. But using those polymers in the case of silicon, they will break apart as well when the silicon particles have large volume expansion. So that's when almost 10 years ago, we started working with Professor E. Trace Group. I had two postdocs at the time from my group when was developing self-healing material, one was a expert who had a battery experience. So when we thought, well, these polymers can crack, then why don't we use self-healing polymers? Self-healing polymer, even if they crack, they can still heal and recover the conduction pathway, ionic and electrical conduction pathway. So since we have been developing all these self-healing polymers in the group, why don't we apply this for the ECMI on battery for silicon particles? So we tried it and the result was quite amazing if we use the standard polymer binders that people have been using. These are, if you know some polymer names, these are some PVDF polymer. So basically like acetylene chains of polymer CH2, CH2, but some of the CH2s are replaced with CF2 and those are the typical polymer people have been using. And if we use those polymers, then the battery. So here is the cycling number. That means we charge, discharge, that's one cycle. So here we look at how many cycles we can charge and discharge the battery. And then how does the battery capacity change? We want the battery to maintain high capacity. So here close to 3,000 milliamp hour per gram, this is what silicon anode can get us. As a comparison here, the dashed gray line is what graphite can do. And you can see graphite can give much, much lower, hold much, much lower capacity compared to silicon. And then compared to these conventional polymer using self-healing polymer, we can maintain many, many cycles of charging, discharging and still be able to operate the battery. And then we subsequently studied different kinds of particle sizes because there were previously people try to solve this problem by trying to use silicon nanoparticles is when the particle is small, it will have even though it undergoes the same volume expansion. But when the size is in the nanometer size, the fracture mechanics completely changes and they become less likely to fracture. But the problem is nanoparticles are very, very expensive. They are more challenging to make. So this is the typical price, but the silicon micro particles, so these are micron sized particles or hundreds of nanometer particles, and these are basically polysilicon that's used to make solar cells and used to make silicon wafers for microelectronics industry. And these micron particles are made by just grinding the polysilicon. So they are really dirt cheap. And this is, we're not even buying in large quantity, we buy this on some website and we can get it at much lower price already, even just by a kilogram compared to the nanoparticles. And we show that for the first time at the time when we did this work, that we were able to get very good performance with the 800 nanometer particles. And this basically provides a cost effective solution and also very high performance. So that, and then we also look into the battery electrodes after charging, discharging many times. This is a scanning electron microscopy of the electrode. After charging, discharging 100 cycles, we take apart the battery. And then we take the electrode, we break it, kind of just break it, and then look at the side view, the cross section. And then what you're looking at here is the particle together with the polymer. So after charging, discharging, the shape of the particle already disappeared. It just becomes this very dense layer. But if you look at the cross section these are two other type of materials people typically use. They were useful for nanoparticles, can stabilize nanoparticles but not microparticles. So CMC is a type of cellulose material, PVDF is the other one I mentioned. And you can see here a lot of porous structure. So that's due to the fracture of the material. And also these kinds of porous structure, eventually basically the battery stopped working because the porous structure, within the porous structure, the particles lose contact with each other. And then it's no longer electrically conductive to allow lithium to be able to deposit. And the other thing is the fracture causes a lot of new exposed surfaces. So every charging, discharging, when new surface is exposed, then there's electrochemical reaction that can take place. And then slowly eat away the silicon and convert it together with the organics, convert it into some kind of insulating product that's no longer active for storing lithium. So this shows the strong contrast of using self-healing material and these other polymers. So this was the very beginning of work. My group started in exploring self-healing polymer for battery applications. And currently this concept is being explored by various other research groups around the world to look at the other type of related polymer designs. And we since then have, maybe I'll skip this, have moved to materials with even higher energy storage. So silicon is still being researched, probably next generation in five years or 10 years, silicon might be used. But then in university, we're looking at even further away kind of technology and we designed the initial concept. So we decided to see how this concept, how far this concept can go and can we really go to more advanced battery systems with even higher energy density. So currently we have a project that's actually several projects sponsored by Department of Energy, the battery material research division on the battery materials. And the goal there, the project is called Battery 500. The goal there is to double the energy density of the current battery used for Tesla electrical vehicle. And Tesla can only drive 250 miles before the next charging. And the goal is to allow it to drive 500 miles with the same size of battery. So then that means it will double the energy density. So that's the goal and probably will take 10 years or longer to realize that goal. Because there are so many challenges, but it's a great area that allows a lot of new ideas to be incorporated because this is not a new problem. It's a 20 year old problem. People have been trying to make these batteries safe and reliable for many, many years and still cannot do it. So that's why new ideas need to be incorporated. So that's why we started to shift to focus on this lithium metal based anode for batteries. Then here, if you look at how the battery operates, this is somewhat different from using graphite. In the case of graphite, you have essentially a host and the lithium is protected or is being hosted by this graphite. So they're kind of protected and they can be very stable. But the thing with lithium metal is that and also to get this high energy density, you notice the unit is per kilogram. So this per kilogram includes the weight of everything in this battery, include the weight of the electrode and this current collector, which is typically made of copper, but copper is very thick. So we're also trying to basically not use any copper and have lithium metal directly deposited to remove this weight. And then the weight of separator, weight of electrolyte, weight of the cathode, all of them are used in this equation to calculate the energy density. So in order to get this high energy density, the graphite is removed. There's only pure lithium. So you take away some weight from the anode and it's pure lithium. But the problem is there's no host to protect the lithium to make it stable. And then when lithium metal is formed, this is by electrochemical reduction of lithium ion. Then it cannot grow in a very controlled way. It forms these dendrites. There could be some non-uniformity and as soon as there's a spot that's non-uniform and have potentially a place where ion can accumulate very quickly, then the growth becomes very rapid and forms this dendrite. And then the dendrite can penetrate through the separator and if it penetrates through, lithium metal is conductive. If it touches the cathode, then it will short. If there's a shorting in the electrical circuit, what happens? It heats up. So once it heats up, then the electrolytes are flammable. Then they catch fire. And that's when you hear in a lot of the news that lithium battery catches fire. Well, that's not yet using lithium metal. That's just graphite based, but you can have defects that cause the dendrite to grow. And then there are many approaches that people have been exploring to try to eliminate the issue of forming the dendrite. People try, since the lithium is in contact with electrolyte, they try to change the chemistry of electrolyte so that the dendrite is difficult to grow. And they put a protecting layer on the surface to prevent dendrite growth. They put a host to hold the lithium so that provides some protection. And then also people have used the polymer coating on the surface. It can be called solid electrolyte interface. Can be an artificial one. And people try to put a polymer layer. That's a very, very strong so that if you form dendrite, this dendrite cannot penetrate through this polymer layer and then prevent the shorting. So when we thought that, well, these methods people have been trying them for many years and still haven't solved the problem. So we should try something different, a different idea. So then there comes the self-healing polymer again. And here our rationale is that the dendrites are formed because as lithium gets deposited, then lithium also will grow in the layer thickness. And as it grow, it will start to generate some non-uniformity at the interface with the liquid electrolyte. And this non-uniformity can cause the interface layer, I mentioned the SEI earlier, cause it to break. And once it breaks, ion can go to this opening very quickly and then lithium will grow rapidly and then immediately form the dendrite. So our thinking is our self-healing polymer, if we go to the self-healing polymer here, even though it's very soft, it's in contrary to the concept that people have been using before using tough polymer. Here we use a very soft polymer, but the thing is this polymer has the unique property. If you look at the microscope image here, if we poke a hole on the polymer, after 30 seconds, 60 seconds, this hole essentially kind of heal by itself. That's because of the polymer network that we design. It's held together by these weak hydrogen bonding. So the molecules, even though the film looks like a solid, but in the molecular scale, these molecules, so these red chains are exchanging with each other. And these clippers can break and close, in a dynamic fashion. So these red polymer chains are constantly exchanging in the molecular scale. That's why when there's a non-uniformity, so when there's a hole, the polymer can actually feel the hole on its own because it's actually mobile. And then we characterize this by using rheology to characterize. We measure the storage modulus, loss modulus, and for a liquid-like material, the loss modulus typically would be higher than the storage modulus. That's how we characterize a material that looks like a solid, but it's actually behaving like a liquid. So just to give you an example of the rheological definition of liquid, if you think about cat, there is this Nobel Prize called EGAR Nobel Prize in physics. So Harvard University awards this every year. And I think two or three years ago, they awarded to the finding that cat, do you think cat is a liquid or a solid? So it was determined to be liquid because liquid will fill in any space if you put this kind of substance in there and the cat can do that. So that's, but it looks like a solid, right? So that's the kind of material that we have. Liquid-like behavior, but looks like a solid. That's why we can use it as a coding. Okay, so that's the basic concept. I won't go into too much detail. Without this polymer coding, there are dendrites formed. You can see all these kind of chunky column-like lithium. And then with the coding, we completely change the way lithium grow. And it becomes very dense and very smooth. So this allowed us to make lithium metal. Now much more stable compared to previously possible. So our research has been since then, that was discovered in 2016. And since then, we have been trying to understand what is really going on in these systems. How does the polymer dynamics impact the growth of lithium? How does the chemistry of the polymer coding impact the growth of lithium? So now we have a group of us, including my group, Professor Yichui's group, and also Professor Jianqing from Chemical Engineering, who does molecular-level simulation. So we are now having a team working together to study this class of polymer. And we're extending it to various different kinds of structure. So for example, these are some new dynamic structures that we have designed that gives one of the best coatings that's ever reported for lithium metal protection. So now we start to understand little by little kind of the requirements for this coating design. And then this coating design, we're also expanding it to solid state electrolyte design, where the ion transport is also important. So those are, let me see, I won't go into this detail. So basically, yeah, those are the key areas we're working on in kind of design novel polymers to enable kind of first identify what are the key problems in the battery field. So I only talked about lithium metal. And then that's the anode side. There's also cathode. Cathode has basically right now for the next generation electrical vehicle. There are two choices of material, sulfur or NMC. So we're also looking at how these polymers are going to protect the cathode material and what kind of chemistry is important at the interface and also for the electrolyte. And using the unique polymer design, now we start to gain the fundamental understanding that's needed. And I think it's a quite exciting time that there are a lot of different aspects of polymer design we can study and how that impacted the energy storage and the electrochemistry. Yeah, so this is my research group. As I mentioned, we have a very diverse group having about 40 plus researchers in the group, half postdocs, half our graduate students and coming from all kinds of background and people collaborate with each other and also collaborate with other research groups. So on the battery project, Yichui and Jianqing are my main collaborators and also with Slack, formerly Dr. Mike Tony was our main collaborator, understanding the polymers and to do some institute materials study using X-ray. But currently the techniques are already set up. So we're continuing to use those advanced X-ray techniques to study our materials. And then funding is mainly funded by DOE on these projects. So I'll be very interested in talking to any student who is interested in polymer science and their application in energy storage or electronics. We have many projects that are electronics related. Okay, all right, now I'll open up to questions. Thank you, Professor Bao. We have questions for two people. So I already saw like two people raising their hand. I'll ask the first person to unmute yourself. Thank you, Professor Bao, for the wonderful talk. You alluded to some work that you're doing with Tesla and personally as someone who is really interested in electric vehicles. Could you tell me a little more about what are some of the efforts for commercialization of self-healing electrodes in batteries? Yeah, so for, well, the way I think about commercialization is either if I have a student excited about doing that, then I help the student to start a company. We have started three companies from the group and another PhD student who worked on stretchable battery and now got a Tomcat grant. And he hopes to start his company on flexible batteries. But otherwise, because I'm not going to leave Stanford, so I usually, I see my role is to help my students to become successful. Then at the same time, we work with companies, especially on technologies that requires sophisticated manufacturing. And in those cases, large companies may have advantages and there we have collaborations sponsored research with companies as well as eventually licensing the IP to companies so that company can further develop it and commercialize it. In the battery field, incorporating a new material into the commercial space takes a very long time for the battery we're using currently, they were developed more than 20 years ago and we're still using the same material. And to get any new material into the commercial space is going to take a long time. But this rate is accelerating very quickly because there's a very strong market draw that is the electrical vehicle really needs better batteries. And so I hope it will be a much shorter timeline, but DOE is looking at 10 years later for lithium metal based electrical vehicle batteries. Josh. Hi, Professor, thank you so much for the talk. I was wondering the key parameter that we've been discussing is energy density. And obviously that has massive implications for vehicles because you need to store the same amount of energy in the same amount or more energy in the same way as well as wearable technologies. Is that same parameter the key parameter for grid scale energy storage? And in general, what do you see as the future for the technology you're developing for grid scale energy storage? Yeah, for grid scale energy storage, the size and the weight are not as important consideration, but the cost is extremely important consideration because for the grid scale energy, it's competing basically it's solar energy plus the cost of energy storage, competing with what we're currently using from the power plants, co-firing power plants or natural gas power plants. So that price currently is still very low and then the grid storage price needs to be competitive combined with the wind or solar to need to be competitive compared to those other choices in order for wide adoption by consumers. So in the end, it's still the price that matters to consumer. So the using materials that are extremely low cost is very important. So in that space, my group doesn't work on it but a lot of people are working with flow batteries, for example, for grid storage. Basically you have a liquid tank and then you have redox molecules that can be oxidized or reduced and it's large in size, but the price is extremely low.