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New rationally designed high-performance Li-S cathode; rate performance, capacity and long life

Researchers in China report the development of a rationally designed Li−S cathode consisting of a freestanding composite thin film assembled from sulfur nanoparticles, reduced graphene oxide (rGO), and a multifunctional additive poly(anthraquinonyl sulfide) (PAQS): nano-S:rGO:PAQS.

The resulting cathode exhibits an initial specific capacity of 1255 mAh g−1 with a decay rate as low as 0.046% per cycles over 1,200 cycles. Importantly, the nano-S:rGO:PAQS batteries exhibit significant rate performances. They maintain a reversible capacity of ∼615 mAh g−1 at a rate of 13.744 A g−1 (=8 C) after more than 60 cycles at various rates and can still have a reversible capacity of ∼1000 mAh g−1 when further cycled at 0.25 C. A paper explaining their work appears in the ACS journal Nano Letters.

As one of the most intensely investigated technologies in the electrochemical energy storage field, lithium−sulfur (Li−S) batteries have observed rapid improvements in their properties in recent years. Recent work experimentally realized the theoretical specific capacity of 1672 mAh g−1 using elemental octaatomic S8 as the cathode material. Highly stable Li−S cathodes with lifetimes of more than 1000 cycles were also reported. Li−S pouch batteries have achieved high energy densities in the range of 350−450 Wh kg−1. Although the cycling stability of Li anodes is still an important issue to address, state-of-the-art performance should, in principle, enable niche applications that do not require particularly long cycling. However, such applications have not been possible with the exception of very selected cases because of the low rate performance. While 3C applications require relatively moderate discharging rates, new applications in electric vehicles, power backups, and portable power tools require much higher power densities (i.e., higher rates), which have been difficult to achieve with current Li−S technologies. Currently, the slow discharging rate inhibits the practical application of Li−S technology due to the limitations of the cathode.

In principle, the rate performance of Li−S batteries is significantly affected by polysulfide redox kinetics at the cathode as well as electron and ion transport in the electrodes and electrolyte. Previous works have shown that, due to the inherent insulating nature of elemental sulfur and the reduction product Li2S as well as poor electrode kinetics, there exists a trade-off between rate performance and energy density. To improve both properties simultaneously, sufficient redox sites inside the cathode are necessary to promote high energy density, and integrated electron/ion pathways are essential to enhance electrode kinetics. Here, we report the rational design and implementation of a Li−S cathode structure that exhibits significantly improved rate performance while maintaining a high specific capacity and long cycle lifetime.

—Chen et al.

PAQS

Freestanding cathode structures based on carbon nanomaterials that can be directly assembled into cells without using a metal current collector or carbon paste as a conducting agent have reported in a range of other work. The new cathode benefits from a holistic design approach, yielding a cathode with high energy densities, long cycle lifetimes, and excellent rate performances.

  • The nanosized sulfur particles improve the degree of sulfur utilization and the initial specific capacity. The nanosized sulfur particles are also proven to promote high rate performance.

  • The layered and porous rGO structure restricts polysulfide diffusion and also provides space to accommodate volume changes of the active material during cycling, both of which are beneficial for cycling stability.

  • The layered rGO structure ensures the electronic conductivity of the electrode, while the interconnected pores provide reservoirs for the electrolyte and constitute connected open pathways for ion transport.

  • PAQS is essential to the realization of the freestanding cathode. A PAQS polymer solution is mixed with nano-S on an rGO dispersion and vacuum filtered to assemble the freestanding film with a layered and porous structure.The PAQS additive may also restrict polysulfide diffusion and prolong the cycling lifetime. Furthermore, PAQS is also a good Li+ conductor, facilitating ion transport at high discharging rates.

  • Because no conductive carbon, no binder, nor metal current collector is needed for the free-standing thin film cathode, the composite electrode has high effective sulfur loading, 48% sulfur content in the entire cathode structure. This effective loading is far higher than that in typical C/S composite active material based cathodes.

We believe the holistic design approach targeting not only specific capacity and cycle life but also rate performance is an essential step toward the practical application of Li−S technology.

—Chen et al.

Resources

  • Hongwei Chen, Changhong Wang, Yafei Dai, Shengqiang Qiu, Jinlong Yang, Wei Lu, and Liwei Chen (2015) “Rational Design of Cathode Structure for High Rate Performance Lithium–Sulfur Batteries” Nano Letters doi: 10.1021/acs.nanolett.5b01837

Comments

kelly

If this is $/manufacturing viable, it's got the 300 cycle cell phone covered and plus 10 seconds of super-capacitor boost should run EVs as well from a ~suitcase size package.

HarveyD

Seems to be very promising.

Can it be fully developed and mass produced at an affordable price by 2020 or so?

If so, affordable extended range BEVs could become common place early in the next decade?

W. Buffet and Cos + Friends could invest another few $$B in a few mega factories to build enough of those for 1,000,000,000+ EVs?

Bob Wallace

It's only the cathode. It's not a complete solution.


"If so, affordable extended range BEVs could become common place early in the next decade?"

They can using current battery technology. Making affordable long range EVs is only a matter of ramping up production in order to bring the price of cells down further. Panasonic/Tesla are currently around $180/kWh and should be at $130/kWh when their Gigafactory is running.

$130/kWh for cells + 30% for packaging would mean $170 for battery packs. Tesla's Mod3 is expected to have a >200 mile range and use 50 kWh of storage. That puts the battery pack price at $8,500, close to the price of a new ICE and support systems. Throw in a few years of fuel savings and EVs become extremely competitive with same-model ICEs. Any battery improvements are icing on the cake.

The price for Panasonic's lithium-ion battery is expected to drop to $100/kWh over a few years. That would make price cost $130/kWh and a 50 kWh pack $6,500. Eight years of not buying $3/gallon gas for a 50 MPG hybrid (13k annual miles) would pay for the $6,500 battery pack.

Looks to me as if the ICEV is doomed. That should be very clear five years from now.

HarveyD

BW...I too hope that it will all come in the next decade or so.

Marcus

Well if it's the anode that was the problem then XG Sciences resolved that recently, see http://xgsciences.com/wp-content/uploads/2012/11/XG-Anode-Product-Sheet-Final.pdf

Li-S looks promising as the high capacity replacement for standard Li-on.

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