Alongside graphene, there are many other 2D materials which hold promise for various applications.
IN an article entitled ‘Enhancing cycling durability of Li-ion batteries with hierarchical structured silicon–graphene hybrid anodes’ – published in the Royal Society of Chemistry’s journal Physical Chemistry Chemical Physics (PCCP) – Dr Melanie Loveridge, from the UK’s University of Warwick, and colleagues argued that the continued global drive for a clean energy future is a compelling driving force motivating research groups across the world. This is especially true, the authors said, ‘in the electrification of transport, if this is to be a viable strategy for reducing oil dependency and the associated environmental impacts of road transportation. In parallel, to enable the increased generation of renewable electricity by way of the grid, static storage solutions will be of equal high impact. For the successful electrification of vehicles and effective, long-term grid storage opportunities batteries are required to last much longer and deliver energy at a higher rate than the current status.’
Addressing what Loveridge termed the Li-ion energy storage ‘quadrilemma’ – reasonable cost, safety (non-toxic, excellent structural/chemical stability with low heat generation), long cycle life and abundance – is crucial here when it comes to the Li-ion chemistries that need to be made from electrode materials. From an energy density perspective, the authors of the paper went on to explain that silicon (Si) has been the focus of much research due to its considerable capacity of 3,579 mA h g-1, second only to lithium metal. ‘However, its obvious drawbacks have highly limited its performance and applications, such as the significant volume expansion upon lithiation, low conductivity and consumption of Li ions by the way of continued SEI growth,’ they add.
Loveridge highlighted that attempts to improve the electrochemical performance of Si materials have been based mainly on using nanostructured Si materials, nanoporous Si frameworks, and core-shell/hollow Si structures. The paper continues: ‘Nanostructured Si materials (˂150nm) may release strain more effectively and thus experiences less structural damage and pulverisation upon lithiation as compared to bulk Si.
‘However, the drawbacks of incorporating nano-sized Si as the anode active material are obvious: (i) a higher surface area (≥100 m2 g-1) resulting in a larger initial SEI film growth (ii) electrochemical sintering processes are not avoidable, emanating from localised spikes in current/voltage which leads to performance deterioration.’
The choice of polymeric binders is considered critical to the performance of Si anodes, alongside active particles, and the PCCP paper argued that effective binders are those that can meet the following criteria: ‘(a) adhesion between the electrode and the current collector (b) interface integrity between polymer and active material surface (c) interaction of the polymer with electrolyte solvents and (d) the tensile mechanical properties. The frequently used polymers such as PAA … sodium carboxymethyl cellulose (Na-CMC) and polyimide (PI), have shown effective binding functions due to their abundant functional groups.’
Apart from polymer binders, often two or more types of conductive carbon are used to provide multi-scale conducting network, Loveridge and her colleagues explained, such as ‘small particles like acetylene black combined with larger particles with higher aspect ratio e.g. vapour-grown carbon fibre (longer range). This is carried out to highly promote the conducting network formation and enhance the rate performance of the anodes.’
The combination of graphene with Si in anodes has, of course, attracted considerable research efforts, and as Loveridge’s research paper highlighted, the fact that graphene has superior in-plane electrical conductivity, high surface area and excellent mechanical properties make it an interesting conductive additive for a more durable anode architecture. Indeed, the researchers also pointed out that graphene’s theoretical capacity ‘has been cited as 740–780 mA h g-1, which is approximately twice that of graphite, and in addition it is postulated that the diffusion pathway of the lithium ions will be less tortuous.’
In their own research, therefore, they focused on the exploitation of the conductive and mechanical properties of FLG graphene to engineer electrodes with more uniform conductive pathways and to protect against degradation.
They revealed: ‘It has been found that the electronic structure of few-layer graphene (˂10 layers) is different from that of bulk graphite, in light of this it is useful to precisely define the different types of graphene possible, i.e. single layer graphene, bilayer graphene and few-layer graphene (FLG).’
Their experiments demonstrated the enriched electrochemical performance of silicon anodes that incorporate few layer graphene in addition to conventional carbon additives. The team said: ‘Reasonable reversible lithiation behaviour is achievable at 1,800 and 2,000 mA h g-1 with the hybrid system out-performing an anode based only on a Si active mass but there is a requirement to continue to improve the coulombic efficiency of these anode systems, especially as larger format cells are developed with long-term cycling requirements.
‘Further improvements can also be made by optimising the ratio of the Si and FLG. The high capacity durability over hundreds of cycles of these anode systems derives from improving multiple cell components in unison. FLG contributes to the reversible capacity, showing reasonable electrochemical behaviour as a single active material, demonstrating reversible capacities of almost 600 mA h g-1.’
According to the scientists, the tensile strength of the electrode is potentially being enhanced by the inclusion of FLG, but further measurement and characterisation will be required to confirm this.
Extending its chain configuration with Poly(acrylic acid) incorporating an Na-counter ion is also explained to be an effective binder to generate a stable electrode architecture capable of enduring volume expansion and particle movement for ca. 200 charge-discharge cycles, Loveridge said, while there is also scope to further investigate what constitutes the optimal Si–FLG ratio for this type and particle size of Si and improve the electrode stability and performance even more.
‘Using dQ/dV analysis some interesting features, particularly at higher capacities, are seen. During lithiation, there are two main stages; break-up of the amorphous silicon structure to form large clusters, and then further break up to small clusters and isolated silicon atoms. The proportion of the total lithiation charge on the small clusters/isolation stage usually decreases during initial cycles, but can then increase again. With further optimisation of the electrode formulation and further testing in full cells there is development opportunity for an inexpensive route for making a durable anode capable of being reversibly cycled to capacities much higher than that of current graphite technologies,’ the paper concludes.
Materials other than graphene or even those which contain this material are also being investigated elsewhere. For instance, silicon nanosheets (2D layers with exceptional optoelectronic properties very similar to those of graphene) have been explored, with researchers at the Technical University of Munich (TUM), Germany, producing for the first time ever a composite material combining silicon nanosheets and a polymer that is both UV-resistant and easy to process.
Tobias Helbich from the WACKER Chair for Macromolecular Chemistry at TUM commented at the time of the announcement: “Silicon nanosheets are particularly interesting because today’s information technology builds on silicon and, unlike with graphene, the basic material does not need to be exchanged. However, the nanosheets themselves are very delicate and quickly disintegrate when exposed to UV light, which has significantly limited their application thus far.”
Helbich, in collaboration with Chair of Macromolecular Chemistry Professor Bernhard Rieger, has for the first time successfully embedded the silicon nanosheets into a polymer, protecting them from decay. At the same time, the nanosheets are protected against oxidation. This is the first nanocomposite based on silicon nanosheets.
“What makes our nanocomposite special is that it combines the positive properties of both of its components,” explained Helbich. “The polymer matrix absorbs light in the UV domain, stabilises the nanosheets and gives the material the properties of the polymer, while at the same time maintaining the remarkable optoelectronic properties of the nanosheets.”
Its flexibility and durability against external influences also makes the newly-developed material amenable to standard polymer technology for industrial processing. This puts actual applications within reach.
The composites are particularly well suited for application in the emerging field of nanoelectronics, and brings the scientists a significant step closer to industrial applications like flexible displays and photosensors.
Another 2D material with significant potential is black phosphorous – a form of phosphorus that can be separated easily into single atomic layers, known as phosphorene. Unlike graphite and graphene, however, black phosphorus is a semiconductor in both bulk and few-layer form. It also has the advantage of having a tuneable bandgap (an energy barrier, essential for controlling the flow of electrons). Black phosphorus’ bandgap varies depending on the number of black phosphorus layers: the more layers, the smaller the bandgap. This makes it interesting for the next generation of nanoelectronic and photoelectronic devices. Yet, the perceived instability of black phosphorus to oxygen and water had been something of a barrier, until a team of scientists at the Center for Multidimensional Carbon Materials (CMCM), within Korea’s Institute for Basic Science (IBS), questioned existing thinking.
The Korean scientists tested samples of black phosphorous under different conditions, and checked to see whether there is a difference between water that contains air and de-aerated water. They found that the physical and electronic properties of samples stored in de-aerated water did not degrade.
To further clarify the role of oxygen in the degradation of black phosphorus, the IBS researchers performed experiments with different oxygen isotopes (oxygen-18 and oxygen-16). They used gas with oxygen-18 and water with oxygen-16, so they could distinguish if the damage was caused by oxygen, water or both. The results confirmed that it is not water, but rather oxygen, that reacts with black phosphorous.
The Korean team also discovered that the surface of black phosphorous is hydrophobic, in contrast to previous experiments. The black phosphorous flakes absorb water as an intact molecule and, as such, the researchers were able to demonstrate that water by itself does not cause any damage. Rather, oxygen dissociates into two oxygen atoms and oxidises the flake. Once the material is oxidised by oxygen, water absorption is stronger, transforming the material from hydrophobic to hydrophilic.
These results could open new pathways for exploring applications that require contact with aqueous solutions such as solution gating, electrochemistry, and solution-phase approaches for the exfoliation, dispersion, and delivery of black phosphorus.
As both Loveridge’s paper and the other research described above, demonstrate, graphene is a material with significant potential in the field of energy storage, but there are, however, numerous alternatives – from silicon to other forms of graphene (i.e. FLG) as compared to graphene as a single layer of carbon, to black phosphorous and many others.
Indeed, there has been varied research efforts investigating materials which can be used in various applications instead of graphene (which, of course, has received significant attention and funding at both the EU and member state levels), and as this research continues around the world, the exciting field of materials – and within that, of 2D materials – and potentially disruptive applications they promise will continue to be one full of surprises.
Dr Melanie Loveridge
Senior Research Fellow
University of Warwick
This article will appear in Pan European Networks: Science & Technology issue 25, which will be published in December, 2017.