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| Fig. 1: How electrons move through graphite material. (Image Source: S. Yribarren). |
Graphite is a purely carbon-based material which possesses unique and useful chemical properties, most notably excellent thermal and electrical conductivity and high resistance to temperature without material degradation. These properties can be traced to its hexagonal, crystalline structure, ultimately facilitated by the presence of double bonds between carbon atoms in the crystal lattice. These covalent double bonds store significant energy and, due to their strength, are resistant to breakdown. [1] They are also excellent conductors due to the existence of a free valence electron on each carbon atom, as seen in Fig. 1. Graphite is the only non-metallic material that can conduct electricity. [2] As a result of its unique properties, graphite is an indispensable material for energy systems.
However, in order to form an energy-dense double bond, significant energy must be infused into the system. Ultimately, this means that graphite can be formed in two ways. All natural graphite mined today was formed from metamorphism of carbonaceous sedimentary rocks, after being exposed to high temperatures and pressures for long periods of time under the Earth's surface. Synthetic graphite is produced by humans, who can expose a variety of carbon-based sources to high temperatures and/or pressures, for a relatively short period of time, typically using fossil-fuel combustion to generate the necessary energy. [3]
Most synthetic graphite is produced through fossil-fuel-derived sources, namely petroleum coke and coal tar pitch. [4] Here we discuss the feasibility and energy input required for instead using a "green" biomass-derived source, using agricultural waste and a pyrolytic treatment process, or newer advanced methods.
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| Fig. 2: Graphite data from USGS 2024 Mineral Commodities Report [3] (Image Source: S. Yribarren). |
The main end-uses for graphite are: batteries, brake linings, lubricants, powdered metals, refractory applications, and steelmaking. [5] Demand for each of these applications is expected to steadily increase through 2028, with graphite demand for batteries almost doubling between 2024 and 2028. [6] Due to its importance in energy systems, the United States Geological Survey (USGS) has designated graphite as a Critical Mineral.
However, the US is fully dependent on global supply chains to satisfy its natural graphite demand. According to the 2024 USGS Mineral Commodity Summaries, the US did not produce any natural graphite in 2023, and maintains no government stockpile. Instead, between 2019-2022, they relied on imports from China (42%), Mexico (16%), Canada (15%), Madagascar (12%), and others (15%). [5] Fig. 2 illustrates this global dependence, showing that current natural graphite production is heavily dominated by China (around 73%), with Brazil and China containing nearly 60% of total estimated reserves. In Oct. 2023, China announced export restrictions on certain goods, which include flake graphite (natural), spherical graphite (natural and synthetic), expandable graphite, and some other synthetic graphite products, requiring licensing for export contracts. [5] Given recent geopolitical and trade tensions with China, this has raised concerns around graphite supply chain certainty in the United States and Europe. [6]
While the United States has no domestic production of natural graphite, synthetic graphite accounted for 83% of what was consumed in 2018, and 60-80% of that was domestically produced. [2] However, it notably struggles to produce battery-grade anode material, which relies on spherical graphite, leaving China with 99% of market share for synthetic electrodes. [7] Furthermore, life cycle assessments (LCAs) have demonstrated drawbacks to synthetic graphite production due to higher greenhouse gas (GHG) emissions (4.86-13.8 kg CO2-eq compared to 2.1-7.75 kg CO2-eq for natural graphite) and costs ($13/kg to $8/kg for natural graphite). [8,9] Note that synthetic graphite cost is predicted to decrease to less than $10/kg in 2025, but still contains cost drawbacks compared to natural mining. [9] In addition, synthetic graphite has different properties (such as higher porosity) which limit its suitability for application in refractories and foundries even though it has higher electrochemical performance in lithium-ion batteries. [2,10] Developing a novel method for synthetic graphite production, one that perhaps does not have material dependence on fossil-fuel industry byproducts (which will decrease in supply long-term as fossil fuels are phased out), has the potential to mitigate energy, cost, and performance concerns, though such methods are still nascent. [7]
The traditional industrial process to produce synthetic graphite requires exposure to extremely high temperatures (800-3000oC) in furnaces for weeks at a time. [10] At ambient pressure, the temperature needed to make graphite from amorphous carbon (in a reasonable timeframe for manufacture) is 2300-3000oC. High pressures can be used to bring this down, but requires excess energy in addition to safety concerns. [4] The entire process is simplified in the process flow diagram in Fig. 3. First, petroleum coke (a by-product of petroleum refining) is ground and then mixed with coal tar pitch (a byproduct of coal distillation) which is then heated to 800-1000oC in a natural gas-fired furnace. This baking or carbonization process lasts between 18-24 hours, and converts the coal tar pitch into coke. For a better final product, the mixture can be impregnated with more pitch and re-baked. Then, the coke product is sent to an electricity-powered graphitization (a.k.a. Acheson) furnace and slowly heated to 3000oC, where it remains for 3-5 days. In this furnace, graphite crystals will form, and are recovered once cooled. [10]
The two most energy-intensive processes are baking and graphitization. For baking, the natural gas intensity of the process is estimated around 6.0 MJ/kg (5.7 MMBtu/ton) of graphite produced. For the Acheson furnace, electricity intensity is estimated at roughly 12 kWh/kg (41.0 MMBtu/ton) of graphite produced (this is an underestimate since the temperature required to maintain the initial heat for days at a time is not accounted for). [10] Overall, the energy input sums up to be around 46.6 MMBtu or 55,500 MJ per ton of graphite produced.
In addition to the energy required, synthetic graphite supply depends on the availability of the raw materials, pet coke and coal tar pitch, which are byproducts of the oil and coal industries. An IEA report notes that today, there is a surplus in capacity to produce coke (mostly in China) and so in the near-term, supply of these raw materials are unlikely to be a limiting factor in graphite production. However, in "climate-driven scenarios" where oil and coal industries see long-term decline in production, these material supply considerations may change to become limiting. [7]
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| Fig. 3: Process flow diagram of synthetic graphite production from pet coke and coal tar pitch in Acheson graphitization furnace. [10] (Image Source: S. Yribarren). |
Given the high energy and material input of the traditional petrochemical-based process, a recent review points out that "biomass waste, especially that with high carbon content, has been used as a feedstock for graphite since it is ubiquitous, environmentally friendly, renewable and cheap." [11] When biomass carbon is transformed into graphite material, it is effectively sequestered there, instead of decaying and being emitted as CO2. It is also quite abundant and cheap, as discussed below. While there is primarily one widely-used method for producing graphite from pet coke/coal tar pitch as described above, there are many methodologies for converting diverse biomass sources into "green" graphite.
The two main stages of the process are (1) producing material with high carbon content from biomass through the carbonization process and (2) graphitization to restructure the amorphous carbon into crystalline graphitic carbon. [11] The most established method to achieve this is Direct Pyrolysis (DP) and Graphitization, which on a high-level mirrors the coke-based process, with a different feedstock. Of course, specifics vary, but high heat and optionally high pressure is a throughline. DP heats biomass between 400-1000oC in an inert (N2 or Ar) atmosphere, and produces biochar. (Note that biochar can be distinguished from other carbonaceous compounds, such as char and charcoal, by its unique inert pyrolysis environment that results in higher porosity/surface area and less volatility, while charcoal by contrast is often torrefied in an oxygen-rich environment that results in a less optimized properties). [12] The biochar is then heated in a graphitization process, just as with coke. This process has a very high energy input, generally producing worse lower-purity graphite powder, which is why coke-based production is the preferred synthetic method. [13] While many advanced alternative bio-graphite production methods exist that have the potential to massively reduce energy input (e.g. ultrasonic-assisted, flash Joule heating), it appears that making even simple changes in this traditional method is enough to make biomass-based graphite production at energy parity with coke-based production. [11]
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| Fig. 4: Process flow diagram of synthetic graphite production from biomass using direct pyrolysis with added catalytic graphitization step. [12] (Image Source: S. Yribarren). |
Graphitization usually requires temperatures of around 3000oC, but use of catalysts in this step can lower the temperature threshold and increase feasibility. Adding a Catalytic Graphitization step after Direct Pyrolysis (Fig. 4) is essentially the same, but with an added catalyst and recycling step that significantly lowers the graphitization temperature needed. Iron-based Fe(NO3)3 catalysts, as well as potassium carbonate K2CO3 are well-characterized choices. [14,15] One life cycle assessment found that with catalytic graphitization using (Fe(NO3)3) to produce 1 kg of battery-grade graphite powder from biomass waste, 32.71 MJ of process energy was required. [14] This equates to 32,710 MJ per ton, notably lower than traditional synthetic production methods (~55,500 MJ per ton). When accounting for the CO2 emissions saved through sequestering carbon in graphite, and using industrial ecology principles to design an efficient heat-saving process, this catalytic graphitization method is very promising for establishing a green graphite industry without drastically changing synthetic graphite production methodology. Overall, when using biomass waste and crops as raw materials, the cumulative energy demand (CED) and global warming potential (GWP) were found to be significantly lower than both natural and synthetic graphite processes. [14]
Catalytic methods often struggle as a result of cost around the acid use and purification. [13] However, catalysis is not the only, or even the most energy-efficient, way to produce synthetic graphite. Newer, less-researched methods bear some attention, to better understand both the basic science and their potential for green graphite production.
This comparison would be incomplete without a discussion of the final product quality and purity between coke-derived and biomass-derived synthetic graphite. In one catalytic graphitization process for bio-graphite (using an iron catalyst like the LCA just discussed), researchers found through numerous characterization methods that a high-quality graphitic structure had been formed, yielding comparable results to existing commercial products. [16] Thus, we might wonder why this method is not currently more prominent, given its lower energy input and similar quality product. One review points out that in the 1970s, agricultural waste was seen as one of the most important sources of fuel, but ultimately lost its commercial value after the sudden drop in oil prices in 1986. [11] Therefore, one hypothesis for why bio-graphite is not the dominant synthetic graphite method today is that low prices for pet coke, due to it being an abundant byproduct of oil refining, made it more economical, paired with logistical considerations that made coke feedstock easier to collect and process, even though energy costs were higher. However, today consumers and manufacturers are more aware of environmental concerns, and policy tools encourage internalizing the externalities of high energy consumption, so perhaps bio-graphite will see an increase in production in coming years.
Current agricultural methods generate about 998 million tonnes of waste annually around the world. [17] Thus, converting lignocellulosic biomass into energy, chemicals, or other productive end-uses is highly desirable, with graphite as one of the highest-value products and applications. [18] Agricultural biomass waste and residue are primarily plant parts like stems, stalks, leaves, roots, fruit peels, or nut shells which are regularly discarded or incinerated without valorization. [11] In the United States alone, the Office of Energy Efficiency and Renewable Energy has estimated that agricultural residues can supply approximately 130 million tonnes of biomass per year (beyond current uses) in the near-term, and up to 175 million tonnes/year in a mature-market medium scenario. [19] Utilizing this vast, mostly untapped resource to domestically produce graphite with a lower carbon footprint is a promising opportunity for establishing a stable US supply of "green" graphite.
© Sarah Yribarren. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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