Scientific Method / Science & Exploration

The fast and the flexible: Graphene foam batteries charge quickly

Capacity on par with existing lithium batteries, but charges in 15 minutes.

There are a variety of technologies available for storing energy, each with various tradeoffs in terms of weight, capacity, and performance. For example, capacitors are fast and lightweight but, despite their name, don't have a large capacity. Batteries can hold more charge, but are heavier and take longer to recharge. This diversity is good, in that it provides plenty of options to tailor a device to suit specific needs. But it probably leaves engineers looking longingly at the options they didn't or couldn't choose.

In yesterday's edition of PNAS, however, researchers from China describe a battery design based on graphene foam that may nicely bridge the gap. It's based on existing lithium technology, and even in experimental form, it has a similar capacity:weight ratio. But it can charge and discharge nearly as fast as a capacitor, being able to completely discharge in about 20 seconds. As an added bonus, it's flexible and works fine when you bend it.

Graphene is a single-atom thick sheet of linked carbon atoms, and its remarkable properties made it the winner of the 2010 Nobel Prize in Physics. Chief among those properties are mechanical strength and an ability to conduct electricity with very little resistance.

The focus with graphene has generally been to build larger sheets, but the process for creating graphene foam is quite distinct. To produce it, you first start with a metal foam, which is a three-dimensional mesh of metal filaments. This is used to grow graphene on the metal surface through standard techniques (vapor deposition), after which the metal is processed away. The result is a similar three-dimensional mesh, shown above, but this time constructed of graphene. The foam itself is both flexible and mechanically tough.

The resulting foam has a number of properties that make it a good battery electrode. These include a large surface area (lots of places for charge carriers to exchange electrons) and excellent conductivity (to get those electrons out of the device). It also happens to be very light-weight.

So, the authors prepared an electrode using a lithium-titanium compound (Li4Ti5O12) deposited on the surface of the graphene foam. On its own, LTO with and without graphene foam behaved identically at slow charge/discharge rates. But, as the rate of charging was increased, LTO's performance got dramatically worse, while performance for the LTO/graphene material only declined gradually. Even at a discharge rate that would completely empty the material in 18 seconds, its performance was 80 percent of what it displayed during an hour-long discharge. Plus, performance remained stable through 500 charge/discharge cycles, and the material retained the flexibility of its graphene foam skeleton.

That was good enough for the authors to decide to build a battery: with the LTO material as an anode, a cathode made of LiFePO4/graphene foam, and a standard electrolyte. The battery didn't show quite the excellent charge/discharge performance the individual electrodes had, but it still performed very well at a charge rate that could fill it in under 15 minutes. Even in this lab-based form, which undoubtedly left a number of efficiencies on the table, its energy density was about 110Whrs/kg. That's roughly in line with current lithium batteries.

And the graphene foam worked just fine while by being bent with a radius of curvature of just five millimeters. Repeated flexings only had a small impact on the performance of the battery.

The authors note they haven't optimized the battery production in any way, and expect they could get a much better energy density by implementing any of a number of things. Even in its current state however, there are plenty of potential uses for a battery that's flexible and can charge fully in 15 minutes. The real concern wouldn't seem to be the battery itself though. The production of graphene foam appears likely to be expensive and energy intensive. If the foam can't be made cheap enough, then we're not likely to see this technology commercialized, regardless of how good its performance is.

PNAS, 2012. DOI: 10.1073/pnas.1210072109  (About DOIs).

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