Carbon materials for energy storage
Rechargable energy storage devices such as Lithium-ion batteries (LIBs) and Sodium-ion batteries (SIBs) store energy by moving Alkali-metal cations between their cathodes and their anodes. During the charging of these devices, ions are moved from cathode to anode, resulting in a current opposite to the stream of charged cations (2).
While the cathodes for these energy storage devices often comprise metal oxides or other crystal structures, their anodes are most often found to be made of carbon materials (3, 4, 5). In Lithium-ion batteries, Graphite is used (3, 4), while Sodium-ion batteries use non-graphitised carbons (Hard Carbons) (5). Other energy storage systems such as Lithium-Sulfur batteries use mixed carbon compounds (6). The reason for the use of carbon is found in the low potentials at which the Alkalimetal cations are stored in the material, allowing for overall higher cell voltages for the whole energy storage system (4). Lithiated Graphite as an example exhibits an electrochemical potential close to that of metalic Lithium, allowing for high cell voltages.
Emerging energy storage technologies such as Li-Sulfur batteries utilise amorphous carbons as well as advanced carbon materials such as carbon nanotubes (CNTs) (6).
For all of the above-mentioned energy storage technologies, carbon materials make up a significant part material composition, in the case of LIBs as high as 20 wt-% on cell level or up to 0.9 kg/kWh (7).
So far, only few cell chemistries are considering carbon free Anode systems, such as pure Silicon anodes for LIBs (3).
Nevertheless, these technologies are currently not deployed at the same scale as established LIB chemistries such as Lithium-iron phosphate (LFP) and Nickel-Manganese-Cobalt-oxide (NMC) batteries.
Climate footprint of carbon materials for energy storage
Carbon materials in energy storage
Considering that the key motivation behind electrification and the application of energy storage devices is the avoidance of CO2 emissions, it is crucial to assess the climate footprint of these devices over the course of their entire lifetime. Generally, this life-cycle assessment (LCA) considers not only feedstock materials and processes, but also any emissions related to operation and end-of-life.
Given the relative importance of carbon materials within the material mix of energy storage devices, the climate footprint of these materials will have a major impact on the battery’s overall footprint (7).
Graphite as an active material in most LIB anodes can today be sourced from two main sources. Natural Graphite is mined from geological sources while artificial Graphite is synthetized by graphitization of petroleum coke. Both types of Graphite require additional post-processes in order to achieve battery grade quality in terms of purity, surface area and particle size.
Climate footprint of state-of-the-art carbon materials
While the mining of natural Graphite emits up to 10 tons CO2-eq./ton of Graphite (8), artificial Graphite exhibits a footprint of up to 42 tons CO2-eq./ton (9). This is due to the high temperatures (up to 3000°C) required to reorder carbon atoms into the graphitic structure as well as the long timescales (several weeks) over which this process takes place.
Other highly advanced carbon materials such as carbon nanotubes (CNTs) are produced by chemical vapor deposition, essentially describing the incomplete combustion of natural gas. Given the right conditions and catalysts, CNTs are formed (10).
These processes are run at high temperatures of >1000°C and provide of rather low yield. The detailed methods of industrial CNT production are often considered trade-secrets and only few LCA studies are available to the public, nevertheless it is clear that the carbon footprint of CNTs is 20 to 480 tons CO2-eq./ton,
depending on scale and scenario.
Given that per kWh of energy stored, the average LIB requires 0.9 kg of Graphite, the impact of the climate footprint on the whole battery’s footprint can be substantial. Similar impacts are seen for CNTs, especially when these are used in higher concentrations, e.g. in Lithium-Sulfur batteries (11) or high power cathodes. For Lithium-Sulfur batteries, LCA studies have shown that CNTs can contribute to >45% of the total greenhouse gas emission footprint (11).
Climate footprint effect in applications
Considering the application of LIBs in electric vehicles, one can estimate the distance that is required to drive in order to offset the carbon footprint associated with the production of the battery. Considering the portion of the climate impact caused by carbon materials (21 wt-% Graphite, 250 Wh/kg), an average electric vehicle (EV) with a 50 kWh battery would be required to drive 21,500 km on renewable energy in order to offset the carbon emissions associated with Graphite alone, compared to an average internal combustion engine (ICE) vehicle emitting 93.6 g CO2/km (EU emission target for 2030).
Thus, the reduction of climate emissions associated with the production of carbon materials for energy storage can have a substantial impact on the overall efficacy of the overall climate impact for electrification technologies. The climate footprint for carbon materials in energy storage applications should thus be treated with similar importance as other critical metals such as Lithium or Cobalt.
Sustainable sources for carbon materials in energy storage
Given the high CO2 footprint of materials such as Graphite and CNTs, multiple approaches for more sustainable synthesis routes have been explored and partially industrialized by different market stakeholders.
For CNTs, the use of bio-methane as a feedstock can reduce carbon footprint. As biogenic Methane can be considered “bio-captured” carbon, it creates a negative carbon footprint when it is converted into solid carbon compounds, provided that these compounds are not fed into any combustion process.
Another approach is the combination of Hydrogen formation and carbon deposition via the use of catalytic conversion Methane (12, 13). Even with a fossil-sourced methane feedstock, this method does not emit any direct CO2. The resulting hydrogen can be utilized in fuel cells and other hydrogen applications such as green steel production.
For Graphite, the use of electric heating powered by renewable sources is one of the main approaches to reduce the carbon footprint of battery grade graphite (14).
Further reductions can theoretically be achieved by exchanging the fossil-fuel feedstock petroleum coke against biomass-based carbon sources. Nevertheless, most of the above-mentioned methods still exhibit a positive carbon footprint, albeit strongly reduced from 48 to 1.9 tons CO2-eq./ton.
CO2 as a direct feedstock for carbon materials
Molten carbonate CO2 electrolysis
CO2 itself contains a carbon atom, which unlike the carbon atoms in methane or elemental carbon is highly oxidized. In order to retrieve elemental carbon from CO2, the carbon atom must be reduced while the oxygen is oxidized to elemental form. Theoretically, this process can be done via electrolysis of CO2. Practically, the solubility of CO2 as well as the very limited electrochemical stability of water do not allow for CO2 electrolysis in aqueous solvents.
Nevertheless, CO2 can be dissolved in molten carbonate salts. When molten at >600°C, these salts offer high solubility of CO2 while also exhibiting high electrochemical stability.
Originally theorized and later tested by NASA (15), the so-called “molten salt carbon capture and electrochemical transformation” (MSCC-ET) process allows for direct electrolysis of CO2 into solid carbon. Unlike other electrolysis processes (16), the reduction of the carbon atom does not stop at oxidation state +II (carbon-monoxide, CO), but yields solid carbon. While factually the CO32- anion of the molten salt is electrolyzed, it is immediately replenished as long as CO2 is introduced into the molten salt. The MSCC-ET process thus is a method to yield solid carbon products directly from CO2, yielding Oxygen on the Anode of the electrolysis cell (17, 18).
As the MSCC-ET process offers high controllability of carbon deposition via control of current, voltage and temperature, it allows to direct the nature of the deposited carbon. While it is expected that Graphite can only form at commonly accepted graphitization temperatures, MSCC-ET is capable of depositing carbon as Graphite. As the process can also be used in conjunction with catalysts associated with CNT formation, it can also yield CNTs.
Carbon credits
As CO2 is a harmful greenhouse gas, there is a global price on emitting it. As of today, a system of carbon credits has been established. In a simplified explanation, carbon credits are generated by projects that reduce or remove greenhouse gases such as afforestation/reforestation or carbon capture and storage (CCS) projects. The specific projects have to be verified in terms of their long-term removal of greenhouse gases. Each carbon credit can be traded, allowing companies that remove CO2 or other greenhouse gases to earn revenue, exemplified by companies such as Tesla Inc., generating USD 2.76 Bn revenue from carbon credits in 2024 according to the company’s annual reporting.
Climate footprint of CO2 electrolysis
As every kg of solid carbon requires the electrolysis of 3.7 kg of CO2, the solid carbons formed by MSCC-ET exhibit a negative carbon footprint, provided that the utilized electricity is drawn from renewable sources. This directly impacts the life cycle assessment (LCA) of the obtained materials and allows for carbon-neutral to carbon-negative carbon materials depending on the applied scale of the technology.
A recently carried out LCA for a 10 t/a capacity pilot plant yielded a LCA value of <0.1 kg CO2 -eq./kg of deposited carbon.
In the case of an average EV, the carbon emissions associated with Graphite could be reduced from 2880 kg to just 6 kg, requiring only a distance of 30 km to be driven on renewable energy to offset. Utilizing both CNTs and Graphite made from CO2, the overall footprint of a Lithium-ion battery could be reduced by 10-15% in total.
CO2 electrolysis and carbon credits
Utilizing CO2 for the synthesis of Graphite or other carbon materials currently is not considered a permanent removal of CO2, mostly as it cannot be guaranteed that the CO2 stored in the carbon materials will not be emitted again after the battery has reached its end of life. In order to guarantee the permanent removal of CO2 from our atmosphere, it would have to be guaranteed that a battery containing CO2-derived carbons is not burned or otherwise thermally treated at its end of life. This regulatory challenge currently prohibits the possibility of carbon-credit trading for methods that turn CO2 into useful carbon materials, which otherwise would further increase the competitiveness of these materials.
One potential solution to this challenge could be a holistic recycling regulation for energy storage products, including a mandate for the non-volatile storage of carbon materials or even recycling of carbon materials. Such regulation would ensure that every CO2 molecule transformed into carbon materials is effectively removed from the planets carbon-cycle. Policies that would allow for generation of carbon credits for carbon materials made from CO2 feedstock would additionally raise the competitiveness for such solutions and accelerate their adaptation.
Industrial use case of molten salt CO2 electrolysis
MSCC-ET has not yet been scaled to large scale-production, but nevertheless has been successfully piloted at a scale of multiple tons per year by UP Catalyst, a deep-tech start-up from Estonia.
Other companies such as Bergen Carbon Solutions have set out to scale the method to multiple tons per year. As molten salt electrolysis is used at large scale for the production of Aluminium and Magnesium, challenges in scaling MSCC-ET are less related to the handling of molten salt, but rather to the separation of salt and carbon as well as the purification of the resulting carbon materials.
Conclusion
Critical raw materials such as Graphite and other carbon materials are a keystone in the localization of any battery value chain. As both the EU and the US are catching up on the creation of a localized battery value chain, three main points are to be considered: First, the cost of the localized raw materials needs to be competitive, requiring low-cost feedstocks and processes. As a consequence, the feedstock for the carbon materials needs to be local, widely available and of low cost. Finally, the raw materials’ carbon footprint should be as low as possible in order to increase the positive climate effect of each battery built.
Given that materials made directly from CO2 are of low cost, based on a widely available feedstock and essentially enable carbon-negative sourcing of Graphite and CNTs, methods such as MSCC-ET are highly promising for delivering a scaled localized value chain for energy storage applications and beyond.
This potential can be amplified by the adaptation of policies that ensure the permanent removal of carbon in the battery value chain, for example via specific recycling and end-of-life regulations for LIBs and other energy storage devices.
References and notes
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- Prajapati, Abhimanyu Kumar, and Ashish Bhatnagar. A review on anode materials for lithium/sodium-ion batteries. Journal of Energy Chemistry 83, 509-540 (2023).
- Nzereogu, P. U., et al. Anode materials for lithium-ion batteries: A review. Applied Surface Science Advances 9, 100233 (2022).
- Reddy, Mogalahalli V., et al. Brief history of early lithium-battery development. Materials 13.8, 1884 (2020).
- Lu, Wanyu, Zijie Wang, and Shuhang Zhong. “Sodium-ion battery technology: Advanced anodes, cathodes and electrolytes.” Journal of Physics: Conference Series. Vol. 2109. No. 1. IOP Publishing, (2021).
- Li, Shanshan, et al. “Review of carbon materials for lithium‐sulfur batteries.” ChemistrySelect 3.8, 2245-2260 (2018).
- Accardo, A., Dotelli, G., Musa, M. L., & Spessa, E. Life cycle assessment of an NMC battery for application to electric light-duty commercial vehicles and comparison with a sodium-nickel-chloride battery. Applied Sciences 11.3, 1160 (2021).
- Engels, Philipp, et al. “Life cycle assessment of natural graphite production for lithium-ion battery anodes based on industrial primary data.” Journal of Cleaner Production 336, 130474 (2022).
- Carrère, T., Khalid, U., Baumann, M., Bouzidi, M., & Allard, B. Carbon footprint assessment of manufacturing of synthetic graphite battery anode material for electric mobility applications. Journal of Energy Storage 94, 112356 (2024).
- Teah, H. Y., Sato, T., Namiki, K., Asaka, M., Feng, K., & Noda, S. Life cycle greenhouse gas emissions of long and pure carbon nanotubes synthesized via on-substrate and fluidized-bed chemical vapor deposition. ACS Sustainable Chemistry & Engineering, 8(4), 1730-1740 (2020).
- Teah, Heng Yi, et al. Life cycle assessment of lithium-sulfur batteries with carbon nanotube hosts: Insights from lab experiments. Sustainable Production and Consumption 48, 280-288 (2024).
- Kludpantanapan, Thunyathon, et al. Simultaneous production of hydrogen and carbon nanotubes from biogas: On the effect of Ce addition to CoMo/MgO catalyst. International Journal of Hydrogen Energy 46.77, 38175-38190 (2021).
- Parmar, Kaushal R., K. K. Pant, and Shantanu Roy. Blue hydrogen and carbon nanotube production via direct catalytic decomposition of methane in fluidized bed reactor: Capture and extraction of carbon in the form of CNTs. Energy Conversion and Management 232, 113893 (2021).
- Press announcement: https://www.vianode.com/news/vianode-sets-new-industry-standard-for-low-carbon-anode-graphite-battery-materials (Accessed May 2025)
- Hecht, Michael, et al. Mars oxygen ISRU experiment (MOXIE). Space Science Reviews 217, 1-76 (2021).
- Zhang, Gong, et al. Selective CO2 electroreduction to methanol via enhanced oxygen bonding. Nature communications 13.1, 7768 (2022).
- Remmel, Anna-Liis, et al. CO2 transformed into highly active catalysts for the oxygen reduction reaction via low-temperature molten salt electrolysis. Electrochemistry Communications 166, 107781 (2024).
- Karu, E. & Urb, G. Method for producing carbon material from raw material gas, Japan patent JP7504515B1 (2024).
Figure 1. Calculated material content for a NMC811 Lithium-ion battery cell. Relative values given in wt-%. Data taken from Ref. (7).
Figure 2. Molten Salt Carbon Capture and Electrochemical Transformation (MSCC-ET) process for the electrolysis of CO2 to O2 and carbon. Depending on synthesis conditions, Graphite or CNTs can be deposited. Scanning electron microscope (SEM) images are of MSCC-ET derived carbon products.
