Using Si2BN as an Anode in Li/Na Ion Batteries

Lithium-ion batteries are playing an ever-more prominent role in energy storage; companies like Tesla view them as the energy-storage solution for electric vehicles and solving the intermittency problem associated with renewable energy. Their large energy density and capacity for many charge cycles has rendered them almost ubiquitous in mobile devices – but they have their problems.

Sodium-ion batteries have emerged as a recent competitor; sodium is more abundantly available than lithium, and these batteries have a better safety record with fewer fires. 2D materials, with a maximal surface area to volume ratio, high electrical conductivity, and high ion diffusivities, are likely to replace bulk materials like graphite as these batteries grow more energy-dense and common.

As the field of synthesising and characterising 2D materials grows ever more advanced, these seemingly-miraculous substances are displaying fascinating and often highly useful properties. Since graphene was first synthesised through exfoliation, and new methods such as layering atoms onto a substrate with atomic layer deposition or molecular beam epitaxy have become widely used, theoreticians have been keenly exploring the potential for these materials with their calculations. Such calculations suggested in 2015 that there was the potential to synthesise a graphene-like layer of Si2BN – two atoms of silicon bound to a boron atom and a nitrogen atom.

Other graphene analogues like germanene display buckling in the crystal lattice structure, but Si2BN has a flat hexagonal structure, allowing for the creation of nanotubes. The material is predicted to be stable across a range of different physical conditions – perhaps up to temperatures of 800K or more.

Monolayer Si2BN was then subject to a number of theoretical characterisations, and has drawn a great deal of interest from the renewable energy industry. At first, it was considered that it might be useful as a means of hydrogen storage. Many renewables advocates hope to store the intermittent power from wind turbines and solar panels in the form of hydrogen, which can be produced by electrolysis during times when supply is at a peak.

One of the factors preventing the “hydrogen economy” from taking off has been a difficulty in storing hydrogen, which is not particularly energy-dense and explosive in gaseous form; consequently, efforts by research groups including the US Department of Energy seek to find materials that will chemically bond to hydrogen atoms. The presence of Silicon on the surface of the layer gave rise to hopes that it would be reactive enough to store large amounts of hydrogen – but it has also given rise to another positive effect. Si2BN is an excellent anode for lithium-ion batteries.

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A paper in Nano Energy, 2017, describes this property. The paper’s title – “The curious case of two dimensional Si2BN: A high-capacity battery anode material” – notes the unusual nature of this material. It has a theoretical capacity to adsorb and store lithium ions that may be as much as 5x greater than existing anode materials that are used in batteries today. 2D materials that have already been synthesised typically display good adsorption properties as well, but 2D Si2BN stacks up well against other such materials such as silicene, borophene, and 2D black phosphorous – as well as other materials like graphite, titanium dioxide, and molybdenum diselenide.

The paper’s calculations suggest that the key to this is the Si-Si-bond, as well as the unique response of the structure to adsorbing ions. As lithium/sodium ions are intercalcated into the material, they cause its structure to buckle; this then gives the overall structure a higher capacity than other 2D materials. This buckling, which is seen in other 2D materials like germanene, manifests itself as a phase transition when adsorption of more than one ion occurs on the surface. According to these calculations, this phase transition should be completely reversible, and aids the diffusion of ions away from the anode.

Si2BN is predicted to have the usual electronic properties that has made 2D materials a source of considerable excitement in materials science. It’s strong, flexible, has a tuneable band gap, and has high conductivity both of electrons and heat. The fact that Si2BN has a high electron mobility is crucial to its usefulness as an anode; this combines with the fact that it has a high diffusivity for ions to allow the battery to be rapidly charged and discharged. Creating a rapidly-charging battery that can reliably store a large amount of energy is important for applications in grid storage and backup, as well as in electric vehicles.

Si2BN has never been synthesised in bulk, although it’s likely that many different teams are currently working to create this 2D material. The field advances very quickly: a few years ago, borophene was considered a promising anode material. Almost as soon as it had been successfully synthesised (2015), the first predictions that Si2BN could exceed its capacity as an anode were reported. Theoretical predictions of these electronic properties allow materials scientists to explore the parameter space of what’s physically possible. The next step will be to construct battery prototypes to test the anode performance in the field, and then to refine the batteries to get ever-closer to the theoretical maximum of ion storage. As our demand for ever more capacious and flexible energy storage systems increases, such breakthroughs could be crucial to enabling continual improvement of the technology.

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