Our latest blog comes from Dr Heather Au who delivered this week’s Energy Futures Lab Lunchtime Seminar. Heather is a member of the Titirici Group based at Imperial College London. Her research focuses on understanding the fundamental mechanisms of sodium storage within hard carbons in order to develop materials with tailored hierarchical nanostructures and improved capacity for the advancement of sodium-ion technology.

The global demand for energy is growing and finding new sustainable alternatives to fossil fuels is becoming increasingly critical. Living in the UK, we are only too aware that renewable energies such as solar power are intermittent, and as non-continuous sources continue to play a larger role in our energy production, the development of effective and sustainable energy storage systems is imperative. Whilst the source of these energies may sustainable, the current technologies themselves rely heavily on critical elements. So if we talk about clean energy, it is important to think about the whole system, considering also the materials used in developing these technologies.

One way of tackling this problem is to use recycled or reclaimed material which would otherwise be lost to landfill. Every year, 88 million tonnes of food waste is generated in the EU, and in the UK alone, over 3.5 million tonnes of the total 10 million tonnes of food waste is discarded to landfill or incineration without energy recovery. This discarded biomass is mostly made up of monosaccharides or polysaccharide chains, and biopolymers (such as cellulose, chitosan and lignin). Put simply, it’s full of useful stuff, namely carbon. In landfill, uncontrolled biodegradation leads to release of methane contributing to rising carbon emissions into the atmosphere – is there a way we can reclaim that carbon?

There is. In our group, we use a method known as hydrothermal carbonisation to convert biomass precursors into useful carbon materials. At very moderate temperatures and pressures, using water as a solvent, the biomass breaks down and is transformed into a densely crosslinked carbon network. Further heat treatment can tune the structure, resulting in tailored materials which can be useful for a wide range of applications, but in particular, as electrode materials in batteries.

Rechargeable lithium-ion batteries have reformed the portable electronic industry, because of their excellent energy density (they can store 2-3 times more energy per unit weight and volume in comparison with conventional rechargeable batteries). Graphite is widely used as an anode material for lithium-ion batteries, while layered metal oxides of cobalt, manganese or nickel are typically used for the cathode. Although lithium-ion batteries are an established technology which have been extensively developed, there is concern that the lithium reserves in the Earth’s crust are insufficient to satisfy our increasing global demands. Even worse, the resources are concentrated in geographically and politically limited areas, so the search for suitable alternatives is becoming increasingly urgent.

Sodium, another s-block alkali metal, shares similar chemical properties with lithium, and sodium-ion batteries have shown potential as a cost-effective successor to lithium-ion batteries for large-scale, low cost electrical energy storage applications. There are many advantages to exploring the sodium-ion alternative: sodium is distributed evenly and abundantly around the globe; aluminium, rather than copper, may be used as the current collector significantly reducing cost; the equivalent sodium electrolytes are far cheaper; and the processing system is identical, meaning that the production of sodium-ion batteries is a ‘drop-in’ technology.

One major drawback, however, is that, unlike with Li, graphite shows very little capacity for sodium-ion storage (forming the compound NaC₆₄ vs. LiC₆). Identifying suitable alternative anode materials is, therefore, of interest to the research community. Disordered ‘hard’ carbons, such as those produced by hydrothermal conversion of biomass, are considered promising materials and provide good reversible capacity during electrochemical cycling.

Our work focuses on exploring the use of these carbons as anodes for sodium-ion batteries. Hard carbons are composed of randomly-oriented, curved and defective graphene nanosheets, turbostratically stacked with large interlayer distances, providing sodium storage capacity at defects and sheet edges, within pores, and between expanded graphene layers. In order to optimise the performance of these cells, we need to fully understand the mechanisms by which sodium is stored inside the carbon structure. Unfortunately, the insertion of sodium into hard carbons is still poorly understood, and the mechanisms are still widely debated. These processes occur over a certain voltage window, and understanding what processes occur at which voltages can facilitate improved electrode design.

We have produced and extensively characterised a range of carbons with varying porosity, graphitic structure, and defects in order to understand the sodium storage mechanisms in hard carbon. Using techniques such as in situ dilatometry and ex situ ²³Na NMR combined with DFT calculations, we have discovered that sodium first adsorbs on the carbon at defect sites and in between graphitic layers, sites with higher binding energies, before diffusing into pores and agglomerating in quasi-metallic clusters at lower voltages.

Where do we go from here? As ever, the perfect carbon anode exists in a Goldilocks zone, where the highest capacity is obtained from the optimum balance between defect concentration, expanded interlayer spacing, open diffusion pathways, and large accessible pores. These design criteria will most likely change when we consider how to reduce performance degradation after prolonged cycling and at much higher current densities, and also depending on what cathode material our carbon is paired with. Understanding how these various structural factors affect sodium storage enables us to design higher performing electrode materials for alternative battery technologies, which may one day allow us to make the full leap to 100% sustainable and clean energy.