Carbon-based materials are important for energy storage applications. Several types of carbons are used in batteries and supercapacitors. The origin and properties of these carbons vary. I’ll give in this blog text a summary of the main carbon materials that are currently used in energy storage applications, and then describe the opportunities of using biobased carbon materials in the same applications.
Graphite is a material that consists of graphene layers, which form a very organized structure. The distance between the ordered graphene layers is 3.3 Å. Graphite is currently sourced by two routes: by mining (natural graphite), or as a side stream from oil refineries (synthetic graphite). Most of the graphite that is used in Europe comes from China.
The main application area of graphite in the energy storage field is its use as the anode in lithium-ion batteries. It can be used as such - combined only with binders and conducting carbon. In addition, it is possible also to mix graphite with silicon to increase the energy density of the battery.
As the demand for graphite is constantly increasing, there are serious concerns on the supply. Battery grade natural graphite is listed as a strategic and critical raw material for Europe due to its high importance for the Green Transition.
Activated carbon is a carbon material with high porosity and surface area. Activation simply means the process to increase the surface area by creating small pores into the carbon structure. This is done either by physical or chemical activation in high temperatures.
The biggest application area of activated carbon is water and air purification. This is since the large surface area and porosity allows it to easily adsorb impurities. In the energy storage field, the main application area of activated carbon is to use it in supercapacitor electrodes. The surface area of activated supercapacitor carbons is usually around 1000 – 2000 m2/g. In comparison, graphite has much lower surface area of about 1 to a few tens of m2/g.
The raw material for the activated carbon in supercapacitors comes in large parts from coconut shells. So, these materials are already mainly biobased. We also studied how to prepare supercapacitor carbon from lignin, which would be a locally and readily available raw material here in the Nordic countries. You can find our results from this publication.
Carbon black and carbon nanotubes
Carbon black is an amorphous material that is used as the conducting additive in battery and supercapacitor electrodes. It is the pathway for electrons to travel from the active material to the current collectors. Thus, its most important property is high electronic conductivity.
This material should form a conductive network between the active material particles and the current collector. It’s amount in batteries is usually around 3 % of the weight of the electrode material. This is enough to provide conducting pathways. It is used both in the cathode and anode side as the conducting additive in Li-ion batteries.
Carbon black is mainly made from coal tar, which is a by-product of the production of coke and coal gas from coal. I have not heard many concerns on its availability so far. However, as the energy market is changing rapidly (we need to reduce usage of coal to produce electricity as it creates a lot of CO2 emissions), we should also pay attention on securing the supply chains of carbon black. But this seems to be not so urgent when compared to the other carbon-based battery materials.
Carbon nanotubes are either single-walled or multi-walled tubular graphene sheets. They can be used in combination with carbon black to improve the conductivity of the electrodes. We are using carbon nanotubes in our ongoing EU project SOLiD, where the nanotubes are planned to also improve the mechanical stability and processability of the cathode material in the extrusion process.
How to use biobased carbon in energy storage applications
Utilization of biobased carbon materials is a good way to support the supply of carbon materials for energy storage applications. The raw material can be almost any carbon-rich biomass, preferably coming from side or waste streams. At VTT, we are studying the use of lignin as the raw material. But there are also other options. For supercapacitors, the industry is already using biobased raw materials, such as coconut shells.
The main difference of biobased carbon, compared to graphite, is its structure. Graphite is a very organised material, but biobased carbon has a hard carbon type structure. This means that it contains less ordered graphitic parts, and the distance between the graphene layers is larger (around 3.7-4 Å). Biobased hard carbon is especially suitable for Na-ion batteries, as I wrote in my previous blog text. But there is much potential also to use it in Li-ion batteries. It requires still work to optimize the structure and interfaces, but I’m sure this is possible if we just put enough efforts to the research.
Biobased carbon could be used to replace any of the carbon materials in batteries. Activated carbon in supercapacitors is the easiest option, but also replacing natural or fossil-based graphite in batteries, or using biobased carbon black or even nanotubes seems to be possible.
Outlook for biocarbon
There are numerous benefits in using biobased carbon. It can be locally produced, it does not require mining, and it can help to cope with the increasing demand of graphite in a sustainable way.
However, we still need research e.g., to understand and control the properties of the material, such as porosity. We must ensure controlled quality of the biocarbon product, despite of variations in the feedstock properties. Upscaling the production in a cost-competitive way is also important. We should for example avoid extremely high temperature treatments to minimize the energy consumption during processing.
These are anyway only technical challenges, which we can overcome. I believe that biocarbon has a lot of potential to support the Green Transition.