Microscopic mountains

As anyone with a laptop or cellphone knows, battery life has made great strides over the years, but has plenty of room for improvement.

That's one of the goals behind some new research at Drexel University, where engineers have developed a new class of materials that take the form of whisper-thin "nanosheets."

The team recently reported that these layers - so thin that it would take thousands of them to match the thickness of a human hair - could be used to store a charge in a lithium-ion battery. The more sheets of material that can be packed into the battery, in theory, the more surface area there is for energy storage.

The research is a long way from store shelves, but in the meantime one of the Drexel engineers is seeing it in a different light - as art.

Using an electron microscope, Babak Anasori captures images of the nanosheets, which remind him of rocky crags, caverns, and other terrain in the full-size world. He then adds artificial color to some of the images, one of which just won the People's Choice award in an annual photo contest sponsored by the journal Science and the National Science Foundation.

"You have to sometimes put the science part aside and think about it as a piece of art," said Anasori, who is working toward a Ph.D. in materials science and engineering.

Anasori takes the pictures with an electron microscope, a dishwasher-size piece of equipment that captures images by scanning objects with a beam of electrons. Michael Naguib, a fellow Ph.D. student, fabricated the nanosheet material depicted in the prizewinning image, using a compound of titanium and carbon.

The two of them work with Drexel professors Michel Barsoum and Yury Gogotsi.

It all began 15 years ago, when Barsoum developed a new class of compounds known as MAX phase materials, each made of three elements. The M represents an "early transition metal" such as titanium. The A stands for a so-called group A element, such as aluminum or silicon. The X represents carbon or nitrogen.

These materials have a dual identity, enjoying some of the best characteristics of both metals and ceramics. In addition to being good conductors, they are machinable, lightweight and highly heat-tolerant.

Research is continuing, but Barsoum has numerous applications in mind, such as jet engines and protective industrial coatings. He recently published a paper reporting that they could be useful as components of next-generation nuclear reactors.

Then a couple of years ago, Gogotsi proposed that the materials might also be useful as electrodes for batteries, if only they could figure a way to "exfoliate" or etch away the middle A-group element, thereby creating thin, nanosheet layers.

They did so by placing a MAX-phase material in hydrofluoric acid, creating what they call MXene. The letters MX are taken from the original MAX, while the "ene" suggests that the materials are somewhat like graphene, another substance that forms ultrathin sheets.

A colleague of Barsoum's at Los Alamos National Laboratory remarked that they looked like stacks of Pringles potato chips.

This year, Gogotsi, Barsoum, Naguib, and others published evidence that one of the MXene compounds, a form of titanium carbide, could indeed be used as a battery anode - a type of electrode.

As with the current kind of rechargeable lithium-ion batteries, which use a graphite anode, the Drexel prototype stores up electrical charge by storing lithium ions, the team reported in the journal Electrochemistry Communications.

Their prototype doesn't yet have the capacity of graphite, but the engineers think it can be improved, and that their material could also allow for a longer battery life span. They also propose that it could be used in solar cells.

Bruce S. Dunn, a battery expert and engineering professor at the University of California, Los Angeles who was not involved in the Drexel research, says it looks promising.

"It's just at the research stage, but these first results are very appealing," said Dunn.

The current family of lithium-ion batteries, found in everything from cellphones to laptops, has steadily improved. The energy output per gram has increased two to three times above what it was a decade ago, said Richard Singer, a director of the International Lithium Battery Association.

Vincent Battaglia, a battery specialist and program manager at Lawrence Berkeley National Laboratory, estimated that this family of lithium-ion batteries, with graphite anodes, has room for 25 percent to 35 percent of additional improvement.

After that, new materials may be needed. MXenes, perhaps?

At the very least, the nanosheets will inspire Anasori to be creative when he's not busy with his engineering studies.

He created the award-winning image on his laptop last year while he and his wife were on an overnight camping trip in Black Moshannon State Park, in Centre County near Philipsburg.

Perhaps he was inspired by nature to tinker with the images. But he conceded that the result, with its reddish "cliffs" against a vivid blue sky, didn't look much like central Pennsylvania.

"They were like the Rocky Mountains," Anasori said.

See an image gallery

of the nanosheets magnified thousands

of times at www.philly.com/nanosheetsEndText