Graphene Continues To Make Progress in Energy Storage Applications
Despite some lofty expectations, graphene continues to make incremental steps in energy storage applications
As we discovered in our most recent Q&A with Jari Kinaret, the director of the European Graphene Flagship, advanced batteries and supercapacitors are an early application target for graphene. In our previous newsletter, we highlighted how graphene is proving to be effective in supercapacitors as well as providing a new way to increase the capacity of the ubiquitous lithium-ion (Li-ion) battery.
In the last three months, we have continued to see developments in the application of graphene to energy storage that could pave the way for it to be one of the first fields to feel a commercial impact from graphene.
The Temptation of Supercapacitors
Unfortunately, the expectations for graphene to improve the performance properties for supercapacitors have often been beyond their capability to meet them.
To give a little background, supercapacitors inhabit the world between chemical-based batteries (like Li-ion batteries) and capacitors. Capacitors have a very high power density, meaning they can deliver a lot of power in very short bursts, but they have low energy density, which means they can’t store that much energy. While they can be charged up pretty quickly, capacitors just can store that much energy and deliver it slowly over a period of time. Chemical-based batteries are the opposite: they can store a lot of energy and release it slowly over time, but can’t deliver short bursts of power, and they take a comparatively long time to charge up.
Supercapacitors have held the promise of bridging these two by offering an energy storage medium that can be charged up relatively quickly and store almost as much energy as a chemical-based battery. But for the most part it has been just a tantalizing promise that to date has not really been delivered upon.
This is why graphene has been so hotly pursued in this area because if it could enable supercapacitors to meet the performance characteristics of both chemical-based batteries and capacitors, it could change the prospects for all electric vehicles as well as our portable electronics.
The Reality of Graphene in Supercapacitors
Unfortunately, the research to date in which graphene has been used to replace the activated carbon in the electrodes of supercapacitors has not been able to exceed the surface area of carbon. As a result, no one has yet demonstrated that graphene can actually outperform activated carbon in storing more energy.
While graphene does have superior conductivity to activated carbon, which means that it could address the market for high-frequency applications that current supercapacitors cannot, researchers around the world are still trying to see if they can get it to store more energy than activated carbon.
One of the latest attempts in this area comes out of the California NanoSystems Institute (CNSI) at UCLA where they have developed what they call a “holey graphene framework”. The CNSI team claim can significantly boost the energy density of supercapacitors.
Whether the boost is really significant enough to change the prospects for graphene in supercapacitors is unclear. Since the researchers are really trying to boost the specific energy density of supercapacitors, you need to gauge their efforts against the average energy density of lithium-ion (Li-ion) laptop battery, which is around 200 Watt-hour/kilogram (Wh/kg). Today the upper average of supercapacitors is around 28 Wh/kg. The CNSI researchers claim an energy density of a fully packaged device stack based on the holey-graphene framework is capable of 35 Wh/kg. This is not exactly the kind of boost that gets people to stand up and take notice, but it does represent a 25-percent increase on the upper average of current supercapacitors
Graphene-enabled Lithium-ion Batteries
If a 25-percent boost in performance seems significant, then recent research out of Italy should grab your attention. Researchers there were able to use graphene on the anode of a Li-ion battery that enabled it to achieve a 25-percent higher energy density than commercially available Li-ion batteries.
Professor Vittorio Pellegrini, one of the leading scientists involved in the study, which was published in the journal NanoLetters, said in the press: “These results are important for two main reasons. The first is that the anode fabrication is a relatively simple process; in fact, with the use of the spreadable graphene ink, the process can be easily scaled up at reasonable costs.”
He added: “The second is that the performance of this Li-ion battery is better than the current commercial ones. This opens to the door to many different applications. To achieve these performances, it was very important to have graphene in the form of nanoflakes; in fact this reduces remarkably the lithium ion repulsion, and allows a greater charge storage.”
Tesla CEO Looks to Graphene
No doubt encouraged by these results, and a number of other high-profile commercial endeavors to bring graphene-based Li-ion batteries to market, the CEO of electric car manufacturer, Tesla, Elon Musk, believes that graphene-based Li-ion batteries will boost an all-electric car’s range from 250 miles before it needs to be recharged to 500 miles.
While business analysts point to the high price of graphene as a hurdle in making these performance characteristics possible, this is only part of the problem and as a result only partly true.
Currently, anodes of Li-ion batteries are covered with graphite—the multilayer version of graphene. The problem with graphite is that its storage capacity is relatively low (graphite-based anodes have a capacity of around 372 milliamp-hours per gram (mAh/g). The hope has long been that silicon could be used since it has a theoretical capacity 4000 mAh/g. The problem with silicon is that as soon as you have charged and discharged the anode, the silicon would begin to expand and shrink and then crack.
For a number of years, research has focused on ways of nanostructuring silicon so it wouldn’t crack after many charge/discharge cycles. There has been some success. But of late research has turned away from nanostructured silicon to look at graphene.
So, yes, there is a problem of cost at this point, but the main obstacle is engineering the right kind of graphene-based material for the job. Once that has been found, the process of producing the graphene needed for this application is just a matter of using a bigger vat, just ramping it up. Once there, the prices for graphene will plummet quickly, getting to that point is the real obstacle.