Graphene Poised to Transform Li-ion Batteries
Meeting a need of high capacity and ease of manufacturing, decorated graphene may hold the key to the next generation of Li-ion batteries
For generations the lithium ion (Li-ion) battery anode was pretty much ignored as far as technological development. Its material makeup stayed the same during all that time, and graphite was the electrode material that was used. But then portable devices started to proliferate and the demands put on their cells became more intense, so that soon you couldn’t get through a day using a smart phone without it needing to be recharged.
It was quickly becoming clear that the old technology was just not going to cut the mustard under the demands of a new generation of mobile electronics, and even transportation.
Is silicon the answer to high-energy Li-ion batteries?
The answer was thought to be silicon. Silicon anode material has a theoretical capacity (i.e., Li storage capability) of 4000 milliamp-hours per gram (mAh/g). This represented an enormous increase over graphite that was coming in at 372mAh/g. However, the problem with silicon was significant. It would start to break apart after a few dozen charging/discharging cycles from lithium ions expanding and contracting the Si particles, reducing conductivity and leaving the anode useless for practical applications.
But silicon was not going to lie down. Researchers discovered that if you nano-structured silicon (nanowires, nanoparticles, or nanosheets), you could get pretty close to the theoretical charge value and increase the charge/discharge cycle to as many 6000 cycles in some research.
There have even been some commercial attempts to bring nanostructured-silicon anodes in Li-ion batteries to market, such as California Lithium Battery Inc. (CalBattery). The company had entered into a Work for Others (WFO) agreement with Argonne National Laboratory (ANL) to develop just such an anode. They announced that they were able to achieve a specific anode capacity of 1,250mAh/g with their anode and have it last up to 5000 charge/discharge cycles.
Despite these efforts, not everyone is convinced that nanostructured silicon is going to be the only solution to Li-ion battery anodes. One expert, Rick Howard, Principal at Howard Battery Consulting, in Pennsylvania, believes that “decorated graphene” is the way forward for the anodes in next-generation Li-ion batteries.
Decorated graphene offers a way forward in next-generation Li-ion batteries
“’Decorated graphene’ is where you’ve got nanoparticles scattered around the surface of the graphene, a 1-atom thick sheet of carbon derived from graphite,” explains Howard. “Any element or inorganic compound that accepts Li is suitable: silicon, tin, many metal oxides, and a scattering of other ceramic-like species. There are a lot of ways to prepare decorated graphenes and many people have done it with a lot of different materials.”
Howard sees the benefits of decorated graphene as three fold. The first advantage is that by having these nanoparticles sitting on the surface of the graphene prevents the reassembly of the graphene into graphite, which if it were to occur would bring us right back to graphite-based anodes.
“Graphene by itself has a pretty decent lithium storage capacity,” says Howard. “It’s a two-dimensional sheet so when you start putting layers one on top of another you’re starting to get back towards graphite. But the nature of the anode is such that two sheets lying on top of one another is actually going to reduce capacity. The Li storage capability (i.e., energy output) of decorated graphenes is phenomenal, 2-6X better than graphite.”
The second boost that Howard sees in using graphene in the anode is that with graphene as a base for these nanoparticles you get to take advantage of the high electrical conductivity of graphene. This can translate into very high power outputs that are useful in accelerating an electrical vehicle, or running an portable tool.
The third and final advantage Howard lists for decorated graphene anodes is that because you’re working with nanoparticles you can get very close to the theoretical capacity values for each one. This translates into more Li storage per unit mass and longer run times between cell recharging.
“With graphene there’s an electron field from the carbon atom that holds these nanoparticles in place,” explains Howard. “It’s not a true chemical binding, but there’s enough of an electrical attraction so that nanoparticles don’t move around on the surface. Without that the nanoparticles will agglomerate. Then you are back to where you were with microparticles and you’ll start losing electrical contact by fragmentation.”
Manufacturability of decorated graphene anodes
As far as procedures for decorating graphene, there are several, says Howard and they’re fairly well established. None of them are terribly expensive especially compared with the graphene itself, according to Howard.
While the manufacturing processes, such as hydrothermal reactions, are fairly straight forward, at present the main obstacle for graphene in this application remains its cost.
“Most companies that are producing graphene are selling it by the gram,” says Howard “It’s going to have to come down to where it’s a commodity. You’ll have to buy it in ton quantity.
“Now having said that there are always specialty niche markets: medical and military where performance is the whole thing and it doesn’t matter what the price is, it’s got to perform. That may be the starting point for Li-ion batteries with graphene. “
Even with graphene still being fairly expensive based on low production output, it’s still more manufacturable than nanostructured silicon, according to Howard.
“There’s no way that I know to make hundreds of square meters of nanostructured silicon in a relatively acceptable amount of time,” says Howard. “Whereas with decorated graphene you can add it into polymer solution, mix it up in a bucket and coat it on a current collector, very similar to today’s processes.”
One thing that is fairly certain is that an improved anode material will need to be developed to meet the demands that are coming down the road from the high-capacity cathodes made from a combination of nickel, manganese and cobalt oxides. There are already some lower capacity versions of these Li-ion cathodes on the market; chemically-similar high capacity materials are under intense development since cathodes are the source of a battery’s energy. If you have a high capacity battery, it will translate into longer driving ranges for all-electric vehicles.
“If you’re going to have a high-capacity cathode with the next generation batteries, you’ve got to have a high-capacity anode and that’s where decorated graphene comes in,” says Howard.
It may be five years before decorated graphene anodes make their way into specialty niche markets for batteries, but that may just be the beginning of a new paradigm for the Li-ion battery.