If any of us ever hope to complete the romantic cross-country trip across the US by electric car, the answer lies in better battery technology. Current batteries just can’t store enough energy to last trips without frequent stops to recharging stations. But there’s good news! Researchers collaborating across the Atlantic have developed new battery cathodes made of carbon nanofibers, promising to store significantly larger amounts of energy in lithium sulfur (Li-S) batteries.
The key quantities we want to look for in batteries are high specific energy, the amount of energy we can store per kilogram of material used to make the battery (y-axis above), and high energy density, energy stored per cubic meter of material (x-axis above). Li ion batteries are ubiquitous in our electronic world, used in laptops, portable electronics, even golf carts! You can see above that they have very high energy density but only relatively high specific energy compared to previous types. This means they’ll be quite heavy and require a lot of expensive material to store enough energy for a car (some data indicate that the battery is up to a third of the cost of most contemporary electric cars). Li-S batteries are fairly new on the block, first synthesized only at the beginning of the century, but hold great promise. Their theoretical specific energy and energy density, shown in the dotted area above, promise the best of both worlds. The main reason for this is that both Li and S are fairly lightweight. But we’re still working on materials that can realize this potential.
Now, Canadian and German researchers have a found a method to drastically increase specific energy of these types of batteries. The key is in developing highly porous cathodes with huge surface area per kilogram. In Li-S batteries, each sulfur at the cathode can support two Li atoms, which is the element that moves between the anode and cathode, releasing energy for use upon uncharging. In a highly porous cathode, the sulfur can fill in all the nooks and crannies and provide a very high amount of accessible bonding sites for Li, thus increasing the specific energy.
To achieve this high specific energy, the researchers developed a new processing method to create such a porous structure. First, 100 nanometer carbon nanofibers were combined with a solution of silica (SiO2) that created an interconnected template of fibers and silica. Then, the combined material was filled with surfactant and etched with hydrofluoric acid (HF) – HF happens to react very easily with silica, removing all of it and leaving pores in its wake. The template prevents the leftover nanotubes from shrinking as much as the normally would, leaving large enough pores to be useful for distributing sulfur throughout for battery applications. The result is a dense network of mesoporous carbon nanofibers as seen below:This material packs a specific surface area of 1928 square meters per gram – that’s like fitting the entire surface area of several football fields in an ice cube! The researchers tried using the material as a cathode in a Li-S battery and found a first discharge capacity of 980 mAh/g along with robust cycling capacity, the capacity asymptoting to around 600 mAh/g after 150 cycles. These are quite good numbers for such a new technology, values that are bound to improve as they optimize the pore size in the cathode. Very encouraging news for one of the most promising battery technologies!
He, G., Mandlmeier, B., Schuster, J., Nazar, L., & Bein, T. (2014). Bimodal Mesoporous Carbon Nanofibers with High Porosity: Freestanding and Embedded in Membranes for Lithium–Sulfur Batteries Chemistry of Materials DOI: 10.1021/cm403740r