Lithium metal coats the hybrid graphene and carbon nanotube anode in a battery created at Rice University, thereby preventing the formation of dendrites. Image: Tour Group/Rice University.
Lithium metal coats the hybrid graphene and carbon nanotube anode in a battery created at Rice University, thereby preventing the formation of dendrites. Image: Tour Group/Rice University.

Scientists at Rice University have created a rechargeable lithium-metal battery with three times the capacity of commercial lithium-ion batteries by resolving something that has long stumped researchers: the dendrite problem.

The Rice battery stores lithium in a unique anode made from a seamless hybrid of graphene and carbon nanotubes. The material – first created at Rice in 2012 – is essentially a three-dimensional carbon surface that provides abundant area for lithium to inhabit.

The anode approaches the theoretical maximum for storage of lithium metal while resisting the formation of damaging dendrites or ‘mossy’ deposits. Dendrites are lithium deposits that can grow into the battery's electrolyte; if they bridge the anode and cathode to create a short circuit, the battery may fail, catch fire or even explode. Up to now, dendrites have bedeviled attempts to replace lithium-ion batteries with advanced lithium-metal batteries that last longer and charge faster.

Rice researchers led by chemist James Tour found that when their new batteries are charged, lithium metal evenly coats the highly-conductive carbon hybrid material, which is made up of nanotubes covalently bonded to the graphene surface. As the researchers report in ACS Nano, the hybrid is designed to replace graphite anodes in common lithium-ion batteries.

"Lithium-ion batteries have changed the world, no doubt," Tour said, "but they're about as good as they're going to get. Your cellphone's battery won't last any longer until new technology comes along."

According to Tour, the new anode's nanotube forest has a low density and high surface area, providing plenty of space for lithium particles to slip in and out as the battery charges and discharges. The lithium is evenly distributed, spreading out the current carried by ions in the electrolyte and suppressing the growth of dendrites.

Though the prototype battery's capacity is limited by the cathode, the anode material achieves a lithium storage capacity of 3351 milliamp hours per gram, close to the theoretical maximum and 10 times that of lithium-ion batteries. The low density of the nanotube carpet allows lithium to coat all the way down to the substrate, thereby ensuring maximum use of the available volume.

The researchers had their "Aha!" moment in 2014, when co-lead author Abdul-Rahman Raji, a former graduate student in Tour's lab and now a postdoctoral researcher at the University of Cambridge in the UK, began experimenting with lithium metal and the graphene-nanotube hybrid.

"I reasoned that lithium metal must have plated on the electrode while analyzing results of experiments carried out to store lithium ions in the anode material combined with a lithium cobalt oxide cathode in a full cell," Raji said. "We were excited because the voltage profile of the full cell was very flat. At that moment, we knew we had found something special."

Within a week, Raji and co-lead author Rodrigo Villegas Salvatierra, a Rice postdoctoral researcher, deposited lithium metal into a standalone hybrid anode so they could have a closer look with a microscope. "We were stunned to find no dendrites grown, and the rest is history," Raji said.

To test the anode, the Rice lab built full batteries with sulfur-based cathodes that retained 80% capacity after more than 500 charge-discharge cycles, approximately two years' worth of use for a normal cellphone user. Electron microscope images of the anodes after testing showed no sign of dendrites or the moss-like structures that have been observed on flat anodes. To the naked eye, anodes within the quarter-sized batteries were dark when empty of lithium metal and silver when full, the researchers reported.

"Many people doing battery research only make the anode, because to do the whole package is much harder," Tour said. "We had to develop a commensurate cathode technology based upon sulfur to accommodate these ultrahigh-capacity lithium anodes in first-generation systems. We're producing these full batteries, cathode plus anode, on a pilot scale, and they're being tested."

This story is adapted from material from Rice University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.