Comparison of the gravimetric capacity of GCNT-Li (with different areal capacities) with other anode materials with respect to the mass of the anode at the fully lithiated state. The areal capacities of the GCNT-Li are from 0.4 to 4 mAh cm-2, represented by GCNT-Li-0.4 to GCNT-Li-4. Credit: ACS, Raji et al.

Post-Li-ion batteries, such as Li-S and Li-air batteries, require high gravimetric capacity anodes and cathodes. Ideally, during the charging of a battery, the maximum gravimetric capacity would be achieved if Li is deposited on the anode directly as pure Li metal rather than stored in intercalation compounds such as graphite as in Li-ion batteries (LIBs). The theoretical capacity based on lithiated graphite LiC₆ is ~ 339 mAh g^-1 while pure Li metal can theoretically deliver 3860 mAh g^-1 assuming 100% of Li usage in the discharge operation.

This enormous capacity compared to commercial Li-ion anodes explain the revisiting of Li metal after more than 30 years of the first attempts to incorporate this low density metal in high energy density batteries. However, Li metal problematically forms dendrites and related unstable structures during battery operation. This results in low coulombic efficiency (CE) and cycle life and poses serious safety concerns as the dendrites can cause short circuits.

—Raji et al.

The Rice team’s GCNT, first created at Rice in 2012, is essentially a three-dimensional carbon surface that provides abundant area for lithium to inhabit. It stores large amounts of metal homogeneously distributed as a thin coating over CNT bundles—suppressing dendrite formation during reversible plating and stripping operation.

Because of the low density of the nanotube carpet, the ability of lithium to coat all the way down to the substrate ensures maximum use of the available volume, Tour said.

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, 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. We were excited because the voltage profile of the full cell was very flat. At that moment, we knew we had found something special.

—Abdul-Rahman Raji

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.

Electron microscope images of the GCNT-Li anodes from the full cell with a sulfur cathode 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. 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.

—James Tour

This holds promise for achieving superior energy density due to the near theoretical Li storage capacity and serves as the basis for the demonstrated SC||GCNT-Li full-cell in a high concentration electrolyte to produce a safe, stable, and high performance battery, thus becoming a harbinger of future systems.

—Raji et al.

Co-authors of the paper are Rice postdoctoral researcher Nam Dong Kim, visiting researchers Xiujun Fan and Junwei Sha and graduate students Yilun Li and Gladys López-Silva. Tour is the T.T. and W.F. Chao Chair in Chemistry as well as a professor of computer science and of materials science and nanoengineering at Rice.

The Air Force Office of Scientific Research Multidisciplinary University Research Initiative supported the research.

Resources

• Abdul-Rahman O. Raji, Rodrigo Villegas Salvatierra, Nam Dong Kim, Xiujun Fan, Yilun Li, Gladys A. L. Silva, Junwei Sha, and James M. Tour (2017) “Lithium Batteries with Nearly Maximum Metal Storage” ACS Nano doi: 10.1021/acsnano.7b02731