The search for high energy density electrodes is one of the central topics in lithium (Li) ion battery studies...Nanomaterials have been expected to have high storage capacities due to their high surface-to-mass ratio, as compared to three-dimensional (3D) bulk materials. For example, two-dimensional (2D) carbon−graphene, with its record surface-to-mass ratio of 2,630 m²/g, has proven to be a promising matrix for hydrogen storage. However, the experimental studies of Li storage on graphene remain controversial, and it is still not clear whether graphene could have a higher capacity than graphite, which is used commercially as an anode with a capacity of 372 mAh/g (340 mAh/g, including Li own weight).

Some experiments do show high Li capacity for graphene nanosheets, within a few charge/discharge cycles. Yet detailed examination of graphene quality attributes the Li storage to binding with defects, which are created during the fabrication of nanosheets. Furthermore, in situ Raman spectroscopy indicates that the amount of Li absorbed on monolayer graphene is greatly reduced compared to graphite, while the intercalation of Li into few-layer graphene seems to resemble that of graphite.

In order to further clarify this issue, we perform first-principles computations to assess the Li storage in the carbon (C) based nanomaterials. We start from the general description of obtaining battery characteristics from calculations, and then apply it to a Li−graphene system, which shows a distinguishing Li storage behavior compared with graphite. The feasibility of modifying graphene for the Li storage is further explored, which leads to the finding that the layered C₃B compound could be a promising storage medium.

—Liu et al.

The first-principles computations showed that the Li capacity of pristine graphene, limited by Li clustering and phase separation, is lower than that offered by Li intercalation in graphite. The Rice researchers then explored the feasibility of modifying graphene for better Li storage. They found that while certain structural defects in graphene can bind Li stably, a more efficacious approach is through substitution doping with boron (B).

A carbon/boron compound in which a quarter of the carbon atoms are replaced by boron turned out to be nearly ideal as a way to activate graphene’s ability to store lithium, Yakobson said. Boron attracts lithium ions into the matrix, but not so strongly that they can’t be pulled away from a carbon/boron anode by a more attractive cathode.

Having boron in the lattice gives very nice binding, so the capacity is good enough, two times larger than graphite. At the same time, the voltage is also right.

—Boris Yakobson

An important step will be to find a way to synthesize the carbon/boron compound in large quantities.

Co-authors of the paper are Rice research associate Vasilii Artyukhov, Rice graduate student Mingjie Liu and Avetik Harutyunyan, a chief scientist at the Honda Research Institute. Yakobson is the Karl F. Hasselmann Professor of Mechanical Engineering and Materials Science and professor of chemistry.

The Honda Research Institute and the Department of Energy (DOE) supported the research. Computations were performed on the Rice DAVinCI system and the National Institute for Computational Sciences Kraken, both funded by the National Science Foundation, and the National Energy Research Scientific Computing Center Hopper, supported by the DOE.

Resources

• Yuanyue Liu, Vasilii I. Artyukhov, Mingjie Liu, Avetik R. Harutyunyan, and Boris I. Yakobson (2013) Feasibility of Lithium Storage on Graphene and Its Derivatives. The Journal of Physical Chemistry Letters 4 (10), 1737-1742 doi: 10.1021/jz400491b