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Stanford team develops new approach to overcoming capacity fading in Lithium-sulfur batteries

15 February 2013

Zheng
Electrochemical performance of the modified hollow carbon nanofiber cathode. (a) Specific capacities of the PVP modified sulfur cathode at C/5, C/2 and 1C cycling rates. (b) Comparison of cycling performance at C/2 with and without the PVP modification. Credit: ACS, Zheng et al. Click to enlarge.

Lithium sulfur batteries are of great interest due to their high specific energy and relatively low cost (e.g., earlier post). However, Li-S batteries exhibit significant capacity decay over cycling. A team at Stanford University led by Profesor Yi Cui has now identified a new capacity fading mechanism of the sulfur cathodes and developed a new approach to overcoming this mechanism. A paper on their work is published in the ACS journal Nano Letters.

The new capacity fading mechanism relates to the detachment of lithium sulfide from the carbon surface during the discharge process. To overcome this mechanism, they introduced amphiphilic polymers to modify the carbon surface. The modified sulfur cathode shows excellent cycling performance with specific capacity close to 1180 mAh/g at C/5 current rate. Capacity retention of 80% is achieved over 300 cycles at C/2.

Sulfur cathode has a specific capacity of around 1673 mAh/g, which gives lithium sulfur batteries a specific energy of around 2600 Wh/kg, much higher than the conventional lithium ion batteries based on metal oxide cathodes and graphite anodes. Commercial applications of lithium sulfur batteries have not been very successful despite several decades of research. The major problems of sulfur cathode include low active material utilization, poor cycling performance and low Coulombic efficiency. Much effort has thus been put into improving the electrochemical performance of the sulfur cathode.

Recently, our group demonstrated a hollow carbon nanofiber/ sulfur composite cathode structure that exhibited a high specific capacity of around 1500 mAh/g and improved cycle life. [Earlier post.]...In this work, we investigated the structural change of the sulfur cathode using the hollow carbon nanofibers. It was observed that lithiation of sulfur resulted in the detachment of the lithium sulfide from the carbon surface, indicating the importance of interfacial effect in contributing to the sulfur cathode decay.

We performed first-principles calculations to study how lithiation changes the chemical interaction between sulfur and the carbon surface. The results showed a significant decrease in binding energy between the lithium sulfide and the carbon. In light of this new understanding, we modified the interface between the carbon and sulfur with amphiphilic polymers and showed a much-improved cycling performance of the modified electrode.

—Zheng et al.

The team fabricated a nanofiber sulfur cathode using their earlier method and assembled it into a 2032-type coin cell (MTI) with lithium metal as the counter electrode. The battery was discharged at C/5 current rate to 1.7 V and held at this voltage for another 24 h until the discharge current was smaller than 5 μA. TEM imaging showed clear shrinking of lithium sulfide away from the carbon wall along the length of the hollow nanofiber.

This observation is surprising as the density of lithium sulfide is lower than that of sulfur, which means that lithiated sulfur undergoes volumetric expansion. Separation of lithium sulfide from the carbon wall means that the intermediate polysulfides could have leaked out from the hollow carbon nanofibers through the openings. The extra Li2S could have precipitated and segregated from the carbon matrix, resulting in the loss of electrical contact and capacity decay.

—Zheng et al.

Results of DFT simulation further suggested that the interfacial effect between the lithium sulfide and the carbon can play important role in sulfur cathode degradation.

To investigate the effect of adding amphiphilic polymers—polymers composed of hydrophilic (water-loving) and hydrophobic (water-hating) parts—in modifying the interface between sulfur and the hollow carbon nanofiber, they used polyvinylpyrrolidone (PVP) due to its simple molecular structure and availability. PVP also has strong binding with a carbon surface from aqueous solution due to a strong thermodynamic driving force in eliminating the hydrophobic interface.

The electrochemical performance of the modified hollow carbon nanofiber/sulfur cathode showed marked improvement compared to the unmodified material. At C/5, a specific capacity of around 1180 mAh/g was achieved. The specific capacities were around 920 mAh/g and 820 mAh/g at C/2 and 1C, respectively.

Instead of the rapid initial decay generally observed in the unmodified electrodes, the first few cycles showed a slight increase in specific capacity from 828 to 838 mAh/g. The amphiphilic polymers provide anchoring points that allow lithium sulfides to bind strongly with the carbon surface. Subsequent cycles showed very stable performance, with less than 3% decay over the first 100 cycles. The capacity retention was over 80% for more than 300 cycles of charge/ discharge, with Coulombic efficiency at around 99%.

—Zheng et al.

Resources

  • Guangyuan Zheng, Qianfan Zhang, Judy J. Cha, Yuan Yang, Weiyang Li, Zhi Wei Seh, and Yi Cui (2013) Amphiphilic Surface Modification of Hollow Carbon Nanofibers for Improved Cycle Life of Lithium Sulfur Batteries. Nano Letters doi: 10.1021/nl304795g

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We need a battery that can take a car with a real world 200 mile range and keep it for 750 cycles to give us 150,000 miles...before you have to replace the batteries. And to get a 200 mile range, you'd need ~60kWh so we need a breakthrough that gets us down to <$100/kWh (about $6,000).
And even then, you'd have to find buyers who would look at total cost over the life of the car to pay the premium for that $6,000 worth of batteries. The average American is paying about $1,750 a year in gas right now (around $3,000 in the UK). That means a multi year payback.

As much as I like EVs, we're a long way from getting something the average person will buy. Still only for us "early adopters" for the next 5-10 years. Hopefully that will still be a few million by 2020.

The approaches for sulfur stabilization are isolation at the cathode (coated sulfur), an electrolyte that doesn't solubilize the polysulfides, and this. Not a bad result, but 300 cycles is a long way from a total success. They may need to investigate alternative coating processes for the amphiphilic polymer. Or, perhaps modification of the functional group.

Actually the 300 cycles rating is excellent and practical for high end cars (assuming cells can be manufactured in mass quantity, safe, et al). If real-world cells had a rating of around 1250Wh/kg (figure based on scaling down the cathode capacity from max theoretical 1673 mAh/g cathode and 2600Wh/kg capacity to around 825 mAh/g and 1250Wh/kg per graphs above), manufacturers would simply over-provision the EV batteries - like the Volt, your battery might have 200kWh of capacity but you only get to use 150 of it [in a Model S or X like vehicle, thats 500+ miles of range]. Extrapolating from that, a cycle life of 300 on a 200kWh pack would yield 180K miles. The bonus in this configuration is that the power draws on each cell would be lower (C/3 instead of C/2) which extends life probably to around 200K miles. Now granted, a 200kWh battery would be awfully expensive (at even $150/kWh, its $30,000 - thats Tesla/Luxury car price range), but thats honestly what I would expect the top end of the market to be at. Lower priced cars with 70kWh batteries would need batteries capable of higher cycle life to hit a 125,000 to 200,000 mile range targets, as DaveD notes above. Even if you over-provision an 85kWh battery, thats only about 75,000 miles (3 miles per kWh).

Anthony,

I agree with your logic, but I'm still looking at how to get to a good EV around $30K that will have true mass market appeal.

For me, I'd be really happy with 150 mile range as I never go over 100 miles in a day, just don't need to. I may buy a Model S or I'm thinking of building my own from a Factory Five GTM frame. Hey, I'm a geek, and it would be a fun project for me...if I can find the time. I'll probably end up just buying the Model S.

Anthony:

All long range EV components; body, ancillaries, e-motors and controllers, the 100+ kWh battery pack etc will have to be built with much lighter material to reduce total weight by 50+% and drag be reduced below 0.20. Otherwise, 200+ huge expensive battery packs may be required to match current 500 miles ICEVs range.

Alternatively, improved PHEVs will have to play that role, for the next 10+ years and/or until such time as batteries (or other type of energy storage; compressed air, flywheels etc) and other components have been improved as stated above.

It will be interesting to see the future use of re-enforced (with NCC) plastics and composites for car bodies and many components.

The 300 cycle capability is encroaching on the capability of the ubiquitous LiCoO2 cathode chemistry that is used in mobile electronics (and the Tesla). So, there should be strong commercial incentive to follow up on this research as smart phones, tablets et al with larger displays and more powerful processors come on the market.

@NorthernPiker,
You know, that's a REALLY good point. When one of these new chemistries becomes good enough to be a new standard in the laptop/phone market, then we'll get the kind of volumes that will get their cost down.

That still doesn't solve the format problem though because I just don't see thousands of little batteries as being the long term solution.

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