How To Mine Graphite From the Sky.
A major graphite shortage is in store for us in the next 5 years.
Our cells should be called Nickel-Graphite, because primarily the cathode is nickel and the anode side is graphite… [there’s] a little bit of lithium in there, but it’s like the salt on the salad — Elon Musk
As we ramp up the clean-energy revolution, our capacity for energy storage has to match its growth. However, as graphite — a key component of lithium-ion batteries (LiBs) — is projected to be in shortage, it threatens to stunt the growth that we desperately need.
The need for energy storage is projected to be so high that production of key battery metals like graphite will need to ramp up to unprecedented levels. — The Assay
There is close to 15 times more graphite than lithium in lithium-ion batteries, making it the heaviest component of LiBs and one of the most important. Additionally, graphite is nearly unreplaceable due to its great chemical properties:
- The highest electrical conductivity of the non-metals,
- An incredibly high melting point of 3,927°C
- Chemically stable.
There aren’t many other materials that have similar chemical properties while being as cheap and relatively abundant as graphite. This means that we have to find a better way to produce it to fit global demands.
The status quo of graphite production is slow, costly, and dirty.
Currently, there are two methods of producing graphite: mining and synthesis. These methods produce natural and synthetic graphite respectively. Note that battery manufacturers prefer natural graphite as its structure is more crystalline making it more electrically conductive (more on this below).
Mining (Natural Graphite)
80% of battery-grade graphite comes from mining. The mining process first involves extracting various ores which contain flake graphite such as limestone. Afterwards, the ore goes through multiple iterations of milling and flotation — processes to separate the rest of the ore from the graphite — to react 90–98% purity. To get to battery grade (99.95% purity), the graphite is either crushed and mixed with acid to dissolve impurities (acid-alkali leaching) or heated to ~3000ºC (roasting). At this purity level, the graphite is further milled, shaped, and separated for the battery manufacturer.
This process is reported to take 112.5 MJ/kg of graphite and release 5.32kg CO2-eq/kg of graphite.
Synthesis (Synthetic Graphite)
The remaining 20% of battery-grade graphite is synthesized primarily from petroleum needle coke. First, green coke is collected as a byproduct of petroleum refining with several impurities. It then goes through a process called calcination where the coke is heated up to ~1300ºC with a limited supply of air to remove those impurities yielding needle coke with a purity of 97–99%. Finally, the coke is graphitized by being heated up to 2500ºC and then ground/shaped to a specific size.
This process is reported to take 89.9 MJ/kg of graphite and release 4.86kg CO2-eq/kg.
Mining the sky with carbon capture & electro-chemistry
In graphite production, quick, cheap, and clean is the name of the game. In line with my interests in carbon valorization, I wondered if there were alternatives to current graphite production that could kill two birds with one stone. In other words, can we turn ambient CO2 into battery-grade graphite?
After doing some searching, I found some interesting players such as Homeostasis, and Maple Materials who are making that possible.
Maple Materials (f.k.a Saratoga Energy) filed a patent in 2018 to create battery-grade graphite detailing their entire process and all of the electrochemistry going on behind the scenes. Their process uses an electrolyzer to generate solid carbon, specifically in the form of carbon black, graphite, or amorphous carbon, from a molten carbonate salt electrolyte (Lithium carbonate).
It’s somewhat implied that this process is used in combination with a carbon capture system. This is so you can continuously convert the ambient CO2 into carbonate and use that as a feedstock for their electrolysis process.
Before we learn about this patent, we need some background knowledge on how carbon is captured to yield carbonate.
When CO2 is in the atmosphere, it can dissolve into water (or some other alkaline solution), and start a set of chain reactions so the CO2 is eventually converted into carbonate.
First, the CO2 reacts with water to form carbonic acid (H2CO3) which later ionizes further to create carbonate ions (CO3²-) and protons.
Using this carbonate solution, an electrolyte is formed by dissolving a metal (lithium) into it at high temperatures (~600ºC). This electrolyte is more specifically known as a molten salt carbonate electrolyte and can composed of different metals such as sodium or calcium, but lithium carbonate (Li2CO3) is the main example used in the patent.
Once the electrolyte has circulated the electrolyzer, an electric field is applied, which causes an electrochemical reaction to occur at both ends of the electrolyzer.
At the cathode (305), Lithium Carbonate (Li2CO3) splits into its components where the carbonate ions are reduced to form solid carbon.
The solid carbon builds up on the cathode to be recovered later on. The leftover lithium ions and oxygen anions can react to yield lithium oxide (Li2O)
At the anode (306), generally, the oxygen anions produced at the cathode can be oxidized to yield elemental oxygen.
There can be exceptions to this if you change the input to the electrolyzer.
Once the solid carbon is formed, there’s a lengthy procedure to follow to extract the solid from the electrolyzer which essentially involves washing away or scraping off the additional products.1 Most if not all of the additional products can be recycled or sold.
Creating the perfect product.
For LiBs, the graphite should be as crystalline as possible to ensure the highest electrical conductivity. Crystallinity is measured by the number of graphene sheets stacked on top of each other to create graphite, otherwise known as crystallite height. For reference, natural and synthetic graphite have a crystallite height of 200–300nm and 10–180 nm respectively.
The patent broadly talks about the different parameters they could tweak to get their desired crystallinity level:
- The cathode’s surface could be designed as a template for the shape and size of graphite particles (and specific crystallinity by relation).
- The cathode’s surface could contain a carbon-metal compound (carbide) based on the desired properties and applications of the graphite.
- As the graphite accumulates on the cathode, its chemical properties take influence from the carbide.
- The cathode or anode could be porous which would encourage more accumulation (or mass deposition) of the graphite.
The actual settings of these parameters aren’t given to us as that’s likely an ongoing focus of their team. However, it shows how many levers you can pull and push to get your desired graphite.
The future of CO2 conversion to graphite is exciting but unknown.
Although I’m quite optimistic about this tech, I’m not sure about its economic viability. There’s nothing online about the minimum energy requirement for these reactions to occur. As I was still curious, I talked to one of my professors at Queen’s University about this patent and how to think about its energetics.
He told me that a quick way to think about this reaction is that it’s essentially the reverse reaction of combusting coal which is extremely exothermic.
Coal is so energy dense it literally powered humanity through an industrial revolution. This means that turning CO2 back into carbon would be incredibly endothermic, needing a lot of energy on the order of magnitude of coal combustion. Put simply, it’s an uphill climb against the laws of thermodynamics as many carbon capture projects are, but this one especially is a rough climb.
Personally, I’m going to still look into this space and read more about it as I think there is still a breakthrough in the making. For example, Homeostasis (mentioned above) is quite transparent about their numbers and reports that their process takes 46,800–94,500 MJ/ton of graphite. For reference, that is lower than what current graphite producers report for the energy consumption per kg of graphite.
If those numbers are true, then that is an incredible 10x breakthrough in the carbon capture space. They have the potential to completely disrupt the graphite industry and make carbon capture profitable at the same time. That’s an incredibly exciting thought that I’ll expand on in one of the next articles that I’m writing.
Thank you for reading!
— AK
