Las baterías de litio-azufre podrían almacenar el triple de electricidad que las de iones de litio

Y lo que es más importante, además son más ligeras y más baratas de producir.

Los diferentes estructuras atómicas de la batería y el conocimiento de la nanotecnología son las que hacen a esta batería diferente.

Utilizaron carbono mesoporoso, un material que presenta una estructura muy uniforme de los poros a nivel de nanoescala, lo que permite un diseño más eficiente.


Lithium-sulfur batteries could store triple the power of lithium-ion

A research team from the University of Waterloo has synthesized a prototype of a lithium-sulphur rechargeable battery that, thanks to its peculiar nanoscale structure, can store three times the power of a conventional lithium-ion battery in the same volume while being significantly lighter and potentially cheaper to manufacture.

When it comes to reducing our carbon footprint, a clean, long-lasting rechargeable battery could have enormous benefits in a wide range of applications, from efficient energy storage to clean transportation.

By combining lithium with the two materials, the researchers also found that batteries could be much lighter even as they store more power. A typical lithium-ion cell has 500 watt-hours per kilogram (227 per pound), but lithium-sulfur could generate 1,200 watt-hours in the same mass (545 per pound).

The challenges posed by lithium-sulfur batteries

As with lithium-ion technology, lithium-sulfur batteries store the electrical charge in one electrode during the charging phase and release it to the other during the discharge phase. However, the different atomic structure of the materials involved means the reversible chemical reactions needed are quite different and harder to obtain.

In particular, to gain high performance sulphur needs to remain in intimate contact with a conductor, such as carbon. Nazar’s team tackled the issue at the nanoscale level by employing mesoporous carbon, a material that presents a highly uniform pore structure at nanoscale level.

The team assembled a nanostructure of carbon rods separated by empty channels, sulfur was then melted to fill the tiny voids thanks to capillary forces. All the spaces were uniformly filled with sulfur, thus maximizing the surface area in direct contact with carbon and boosting battery efficiency.

Advantages and disadvantages of lithium-sulfur cells

Successfully combining lithium and sulfur delivers much higher energy densities while reducing the cost of the materials used. According to internal testing, the composite material synthesized by Nazar’s team can supply as much as 84 percent of the theoretical capacity of sulphur – three times the energy density of lithium transition cathodes. This should account for significantly more efficient batteries which will be lighter as well.

How much lighter? "We estimate the energy density of our cells to be about 1200 Wh/kg, for just the positive electrode, which would put the energy density of the cell at about 500 Wh/kg or more, but this depends on the other components of the cell," Dr. Nazar told us. "That is about a factor of 3 to 5 times more than a conventional lithium-ion battery. However, capacity fading can be more of an issue, along with lower volumetric energy and those need to be tackled more fully."

When asked whether lithium-sulfur batteries could have drawbacks at all compared to lithium-ion batteries, particularly in terms of safety, Dr. Nazar told us that because lithium-sulfur batteries typically employ a negative electrode comprised of metallic lithium, there could be safety concerns if the electrode is not adequately protected by a passivating layer. However, the research team seems optimistic that this won’t be impossible to overcome: "Others have developed such technology for negative electrode protection, and also for advanced separators which should diminish these concerns. We are not in a position to judge how effective this technology is on long-term cycling, but we are certainly hopeful."

Finally, with regard to production costs, Dr. Nazar told us that, while the material themselves are certainly cheaper than those employed in lithium-ion batteries, it would be hard to quantify how much cheaper lithium-sulfur batteries will be. "Clearly the basic raw materials for the positive electrode (sulfur and carbon) are very inexpensive, but there are costs associated with processing, electrolyte, fabrication, etc that are highly dependent on the optimization of the materials and the battery configuration."

When lithium-ion batteries were first introduced as replacements for older, heavier nickel-metal hydride (NiMH) batteries, they offered a breakthrough in greater energy density and lighter weight. This technology has made its way through the field of consumer electronics, and lithium-ion batteries are now ubiquitous. The next step in battery technology may come from the University of Waterloo, where Dr. Linda Nazar is working to develop lithium-sulfur batteries with promising characteristics including three to five times the storage of current lithium-ion batteries.

While most current electric vehicles and hybrids, including the Toyota Prius and the Honda Insight, use NiMH batteries, lithium-ion is beginning in their next generation as well. The Tesla Roadster uses lithium-ion batteries as will the Chevy Volt. A lithium-sulfur battery of comparable weight for a vehicle could significantly extend its range, allowing for more flexible use in an all-electric mode. Or, with the increased energy density available, a vehicle with a similar range could be made significantly lighter through the use of a much smaller lithium-sulfur battery.

Sulfur is currently a component in other large scale storage systems, such as sodium-sulfur batteries, but those require high temperatures and are better suited to fixed location applications, such as grid storage, rather than for portable use. Lithium-sulfur batteries may make sulfur storage energy available in a more portable form.

The lithium sulfur batteries are created by creating assemblies of carbon nanorods that are coated with molten sulfur to fill the voids. The nanoscale structure sets up conditions to keep the sulfur in contact with the carbon, allowing for the repeated charging and discharging necessary for useful rechargeability.

Lithium sulfur batteries have the potential to significantly reduce the size of batteries because they have a higher energy density than other comparable lithium-based batteries.

“This composite material can supply up to nearly 80 percent of the theoretical capacity of sulphur, which is three times the energy density of lithium transition metal oxide cathodes, at reasonable rates with good cycling stability,” said Dr. Nazar.

Sulfur’s availability and low cost may help bring this technology to market. The research team has filed for patents on their process and are working on developing it commercially. According to a press release announcing the research publication, sulfur is a less-expensive material than many others used in lithium-based batteries. "It has always showed great promise as the ideal partner for a safe, low cost, long lasting rechargeable battery, exactly the kind of battery needed for energy storage and transportation in a low carbon emission energy economy."