Another one from the June issue of Materials Today – can be accessed here: http://www.sciencedirect.com/science/journal/13697021/16/6
A mix of boron and graphene has been offered as a potential lithium storage solution by theorists at Rice University, leading to suggestions that high-capacity graphene batteries may not be too far off.
Graphene has been a buzz word in materials research since it was first isolated in 2004, thanks to its impressive mechanical, thermal and electrical properties. The huge surface area of these one-atom-thick sheets of carbon (C) also highlighted graphene as a potential storage material for use in lithium ion batteries. But, in practise, it was found that lithium ions did not form strong enough bonds with pristine sheets of graphene to consider as a storage medium.
However, work from a group of theoreticians at Rice University suggests that including specific boron (B) defects in the graphene lattice can hugely increase its charge capacity, making it comparable to that of graphite – the most commonly-used electrode material in today’s Li-ion batteries.
The work, published in the Journal of Physical Chemistry Letters, confirmed experimental results suggesting that the weak binding between lithium and carbon was limiting the storage capacity of graphene. The Rice researchers went on to theoretically investigate the effects of modifying graphene for better lithium storage, by mechanically stressing it and chemically doping it.
While mechanical modification did not increase graphene’s charge capacity, the addition of boron had a large effect. A stack of graphene layers in which a quarter of the carbon atoms were replaced by boron provided the optimum lithium storage capacity. The researchers found that C3B had a theoretical capacity of 857 mAh/g – over twice as large as graphite’s 372 mAh/g. The team explain that in this arrangement, the boron attracts lithium ions into the matrix, but the bond is not so strong that it inhibits lithium movement in the presence of a more attractive cathode – a key consideration for its use within batteries. These stacks of C3B were also determined to have a comparable power density to graphite and very little volume variation during discharge/charge cycles.
The Rice team say that their results help to “…clarify the fundamentals of lithium storage in low-dimensional materials”. If a method to synthesise the boron-carbon compound in large quantities can be found, it is hoped that this theoretical study can lead to a practical outcome, in the form of high-capacity lithium- C3B batteries
Journal of Physical Chemistry Letters (2013) doi:10.1021/jz400491b