Synthesis and Performance Optimization of Sulfurized Polyacrylonitrile Materials for Practical Batteries
Cassidy Sullivan - Parallel I
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09/26/2024
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Student’s name: Cassidy Sullivan
Home Institution: University of California, Berkeley
NNCI Site: SDNI @ UC San Diego
REU Principal Investigator: Professor Ping Liu – Department of Nanoengineering, UC San Diego
REU Mentor: Dr. Shuangjie Tan - Department of Nanoengineering, UC San Diego
Abstract: The electrochemical stability of sulfurized polyacrylonitrile (SPAN) cathodes significantly influences the performance and longevity of lithium-sulfur (Li-S) batteries. This study investigates the impact of compacted density variations in SPAN cathodes on their electrochemical behavior and structural evolution during cycling. Using scanning electron microscopy (SEM) to analyze cathode morphology, we systematically vary cathode density and measure parameters including cell cycle life, capacity retention, resistance changes, and degradation indicators. Our findings aim to identify the optimal compact density that may enhance specific electrochemical properties and longevity of SPAN cathodes, thereby contributing valuable insights to the development of Li-S battery technology.
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- [00:00:00.000]Hi my name is Cassidy Sullivan and this summer I have the opportunity to work with the Professor
- [00:00:06.380]of Science and Technology at the University of Chicago working on the synthesis and performance
- [00:00:15.560]optimization of sulfurized polyacrylonitrile materials for practical batteries.
- [00:00:21.160]Now a battery I'm sure we're all very familiar with by now is the lithium ion battery.
- [00:00:28.560]The discharge mechanism for the cell occurs after the oxidation of these lithium metals.
- [00:00:35.060]After they become oxidized, the now slightly positive lithium metal ions travel through
- [00:00:40.800]the separator and are housed with these layers of graphite, while simultaneously the electrons
- [00:00:46.900]that are now completely free travel externally outside the cell and act as the power source
- [00:00:53.680]for, for example, your cell phone or your laptop.
- [00:00:56.620]Thank you very much.
- [00:00:58.460]Now the discharge mechanism of the lithium sulfur battery is slightly similar.
- [00:01:03.220]However, as you can see from the figures down below, they don't have that same interflation
- [00:01:07.840]mechanism or those layers of graphite.
- [00:01:09.780]Instead, the lithium metal travels directly across the separator and reacts with the sulfur
- [00:01:15.940]cathode to form lithium sulfides.
- [00:01:21.420]These lithium sulfides increase the energy density of lithium sulfur cells to be about
- [00:01:26.420]six times higher than the traditional lithium sulfide.
- [00:01:28.400]This is the result of the ion, which makes them the new source of investigation in terms
- [00:01:33.720]of battery storage.
- [00:01:34.720]However, of course, they are not without their challenges.
- [00:01:38.220]Unfortunately, the chemical reactions that create those lithium sulfides also have polysulfates
- [00:01:45.160]as a byproduct, which is just a long stream of sulfur atoms.
- [00:01:49.160]This can be dissolved into your electron life and not only causes a loss of active material,
- [00:01:55.620]but also creates several stability issues down the road.
- [00:01:58.340]Also, the sulfur is not very conductive, as unfortunately, sulfur batteries are not very
- [00:02:06.840]conductive as well, which I would guess is a very large problem when dealing with a device.
- [00:02:12.960]However, a really good promising solution for this is SPAN, which is an acronym for
- [00:02:18.920]sulfurized polyacrylonitrile.
- [00:02:19.920]Polyacrylonitrile is a polymer compound, and we insert the sulfur into its matrix to generate
- [00:02:28.280]sulfur.
- [00:02:29.280]This is an enhanced conductivity of our battery, as well as improving our cycling stability
- [00:02:38.240]while keeping the design with high energy density that would gain sulfur batteries eventually.
- [00:02:46.440]The investigation I completed this summer was two-fold.
- [00:02:49.920]Our first step was to synthesize a low-loading SPAN material and vertically characterize
- [00:02:55.820]their properties.
- [00:02:58.220]We focused on the impact of compact density on high-loading electric chemical behaviors
- [00:03:03.920]and we wanted to investigate the structural evolution of those cells.
- [00:03:10.180]Unfortunately, when you increase the thickness of your cathode, you require more electron
- [00:03:17.000]light, which decreases your volumetric energy density.
- [00:03:20.700]So our ultimate goal was to try and balance how different the cathode and how different
- [00:03:25.320]the energy should create with the optimal weight.
- [00:03:28.160]Our first step was to synthesize the span material that involved ball-milling a mixture
- [00:03:35.240]of our elemental sulfur and our polyacrylate nitrile.
- [00:03:38.400]We want to ball-mill it for a while to make sure that our particle size is very small
- [00:03:45.340]and that we have a homogenous mixture, and then we thermally anneal those two components
- [00:03:51.660]to create our span material, and this allows for the sulfur to become integrated into the
- [00:03:58.100]matrix.
- [00:03:59.100]Now, as I previously mentioned, our first step was to characterize the material itself.
- [00:04:06.780]We did this by taking some XEM images, and then we wanted to cycle our cell to see how
- [00:04:12.800]it would actually function.
- [00:04:15.040]This initial discharge here, we always want to make sure to discharge our batteries first
- [00:04:20.500]to get rid of some voltage that might have accumulated while we were making the cell
- [00:04:25.160]itself.
- [00:04:26.160]However, after that initial discharge, we can see that the material is very small.
- [00:04:28.040]We can see that our specific capacity is around 700, which is really great.
- [00:04:33.440]This means that we've created a great span of material and continue to see how our cell
- [00:04:41.060]stability lasts in the long term.
- [00:04:42.880]That's this graph here down below.
- [00:04:45.800]Here we have the coulombic efficiency as well as the specific capacity over about 50 cycles.
- [00:04:51.740]As you can see from that very straight line, our cycling capability and our specific capacity
- [00:04:57.980]are very constant throughout this process.
- [00:05:02.700]We also wanted to get a little bit more insight into the actual structural properties of our
- [00:05:07.320]spanning channel.
- [00:05:08.320]That's what we did here.
- [00:05:09.320]We performed Raman spectroscopy and X-ray diffraction.
- [00:05:13.420]Raman spectroscopy gave us some insight into the bonds of our sulfur with our PAN material
- [00:05:20.320]and our X-ray diffraction indicated to us the salinity of our compound.
- [00:05:27.920]We were not necessarily focused on what these graphs were and what they told us, we were
- [00:05:36.460]more focused on just comparing them to past literature so that we could ensure that what
- [00:05:40.680]we made would actually be a very good span material for our factories.
- [00:05:44.480]Some of the group's past work is and that's how we characterize those bonds.
- [00:05:54.840]Now our second part we could actually get to the process of creating the bonds.
- [00:05:57.860]So we're creating our coin cells.
- [00:06:01.320]To create our cathodes for our batteries typically you want to make a slurry first.
- [00:06:05.860]So in our case we combined carbosyl methyl cellulose as our binder as well as added carbon
- [00:06:13.300]for an extra conductive material as well as our active span material.
- [00:06:19.260]You can see here the process after that.
- [00:06:21.980]We put our slurry on a sheet of carbon cloned aluminum and we want to make a really thin
- [00:06:27.800]distributed layer of that and we let it sit and dry out on a piece of carbon.
- [00:06:35.700]Our next step is to calender and flatten out our electrodes.
- [00:06:38.880]As I said previously, this investigation focused on compact density and in order to measure
- [00:06:46.040]out our compact density, we have a simulation that equates our compact density with an estimated
- [00:06:53.200]cathode thickness based on the mass of each electrode.
- [00:06:57.740]So in order to measure the compact density of each electrode individually, we just calender
- [00:07:03.240]flatten out to a given thickness, and then we can estimate the cross of that.
- [00:07:07.630]This is a diagram of all of the electrodes we made, and these are the ones that we found that
- [00:07:16.510]were the most similar to each other and the ones that were chosen to be formed into point cells.
- [00:07:21.230]These are some SEM images of our cathodes. As you can see here, this top row is a side profile
- [00:07:30.110]of our cathode. You can see as they decrease in porosity, the thickness of our cathode also
- [00:07:36.830]decreases. We had four different porosity that we wanted to look at for this investigation. That was
- [00:07:43.150]0.7, 0.3, 0.5, and 0.1. This lower row is a top view image of them through the SEM.
- [00:07:50.750]One thing we noticed when looking at this bottom row is the more we calendar out the
- [00:07:55.870]length of jerks and the more we make them thinner, the more we get
- [00:07:59.870]rid of them. The more we calendar out the length of jerks and the more we make them thinner, the more we get rid of them.
- [00:07:59.950]The more we calendar out the length of jerks and the more we make them thinner, the more we get rid of them.
- [00:07:59.950]The more we calendar out the length of jerks and the more we make them thinner, the more we get rid of them.
- [00:08:00.030]rid of these cracks in the material, which we were not anticipating that, and that was
- [00:08:05.670]actually really great, because it means that you have less loss of active material than
- [00:08:10.990]you have with cyclical patterns. After our point cells were fabricated, they were fabricated
- [00:08:18.070]in an R.gloves block, and we were then able to run them similar to how we characterized
- [00:08:23.870]this fan material. So you can see in this graph, we did it pretty similarly. This is
- [00:08:29.890]the initial, really high specific capacity, is just the initial discharge of our battery
- [00:08:34.390]to make sure that all of the voltage accumulated while we were making those point cells is
- [00:08:39.550]not a factor in our data. And so our specific capacity of our 0.1 data was about 700, which
- [00:08:47.890]we thought was really great because that was exactly the specific capacity of our fan material
- [00:08:55.390]when we were characterizing it. And our second, our
- [00:08:59.750]second test that we wanted to run was of course the cycling stability. We can see here that
- [00:09:05.050]the coulombic efficiency and specific capacity also remained very stable throughout the cycle.
- [00:09:12.310]The coulombic efficiency is the ratio of charge to discharge. So if our coulombic efficiency
- [00:09:18.070]is relatively stable, that's a really good indicator that our battery is going to have
- [00:09:23.570]a long-term energy storage.
- [00:09:29.610]Unfortunately, we were only able to cycle our 0.1 ferocity because we ran out of time
- [00:09:34.970]before we applied this effect here. So, hopefully, when I go back, we would love to run the other
- [00:09:40.350]cells, including 0.7 and 0.5. Because we weren't able to run any of the other cells, we weren't
- [00:09:47.890]able to have a solid improvement on the impact of compact density because we don't have any
- [00:09:53.290]larger ferocities to compare it to. However, the 0.1 data did reinforce the idea that lithium
- [00:09:59.470]span batteries have an increased specific capacity, as typical lithium-ion battery cathodes
- [00:10:05.370]only have a specific capacity of around 1 in 200, and ours had about 700, which was
- [00:10:10.790]a very significant increase.
- [00:10:13.490]We also would want to run the 0.1 point cell again, not because the data was inclusive,
- [00:10:18.810]but more because usually when you're cycling your batteries, you want to cycle it for about
- [00:10:22.750]100 cycles, which means like 100 charge and discharge. Because of the time constraint,
- [00:10:29.330]we were only able to do this about 10 times. We want to make sure that our data is more
- [00:10:34.890]representative of how our cell actually functions. Those would be our next step.
- [00:10:46.090]However, the importance of lithium-span batteries is not only in its ability to be a better
- [00:10:53.050]battery technology over the lithium-ion battery cells. However, the cathodes that are used
- [00:10:59.190]in lithium-ion batteries are creating terrible ethical implications all around the world.
- [00:11:04.510]The recent mining in the Democratic Republic of Congo and the pollution in Kenya are just
- [00:11:10.930]two very recent examples that are receiving media attention. However, with the increase
- [00:11:15.450]in electrification, it's becoming ever more important to look into lithium-ion alternatives
- [00:11:22.570]for our batteries.
- [00:11:29.050]So those can be seen in this graph by the circling coin cells, oh sorry, the question
- [00:11:54.130]is which coin cells recycle or which electrons?
- [00:11:58.910]So all of these points represent electrodes that we made.
- [00:12:09.530]This was done in Python, eliminating any electrodes that we noticed had any defects from measurement
- [00:12:18.790]effectiveness, or sometimes with the higher porosity, the sand material tends to flake
- [00:12:25.770]off a little bit because it's so much larger.
- [00:12:28.770]Those data points were eliminated from the selection process, and then we wanted to make
- [00:12:33.530]sure that we had enough of each, and so we chose about three for each porosity, and we
- [00:12:40.010]tried to choose ones that were relatively similar in.
- [00:12:58.630]Yeah?
- [00:12:59.630]I think we go back to the voltage test plots.
- [00:13:02.630]For the distance or the 0.1 plots?
- [00:13:05.630]Yeah, I think the 0.1 is fine. I'm just wondering if it's a discharge outcome you have, there's
- [00:13:19.030]also a curved line going up. What does that represent?
- [00:13:24.970]For this one here?
- [00:13:25.970]The one above.
- [00:13:26.970]Oh, this one?
- [00:13:28.490]Yes.
- [00:13:29.490]Yeah, so those represent our charge and our discharge.
- [00:13:32.790]Oh, okay.
- [00:13:33.790]Yeah, so that's why the graph looks a little weird, but usually we measure the specific
- [00:13:38.370]capacity on discharge.
- [00:13:39.370]I remembered.
- [00:13:40.370]Okay.
- [00:13:41.370]I was going to ask how long a charging and discharging cycle took to run.
- [00:13:47.990]The question was how long did it typically take for a charge-discharge cycle. That typically
- [00:13:58.350]took about 10 minutes, if I'm not mistaken.
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