Natural versus Synthetic Graphite

In battery cells we see the use of natural and synthetic graphite in the anode. What are the differences and the advantages / disadvantages.

Natural graphite anode has the advantages of lower cost, high capacity and lower energy consumption compared with the corresponding synthetic anode. But the latter performs much better in electrolyte compatibility, fast-charge turnaround and battery longevity. Fastmarkets [1]

Natural Graphite

Natural graphite is found in three forms:

1. Amorphous graphite
2. Flake graphite
3. Crystalline vein graphite

The processing of natural graphite has four fundamental stages [3]:

1. Beneficiation: Liberation of graphite flakes from the host mineral rock is achieved by crushing. Then grinding, screening and flotation processes segregate impurities and yield graphite concentrate. Flake dimensions and carbon composition significantly influence the ultimate graphite grade.
2. Spheronization: Natural graphite’s inherent anisotropy (distinct properties along different crystallographic axes) is mitigated through spheronization. This process yields a narrower particle size distribution, improved tap density and reduced specific surface area. These optimizations result in consistent electrochemical performance across diverse orientations.
3. Purification: Chemical leaching, involving acids like hydrofluoric, hydrochloric, and nitric acids, purges impurities from flake graphite. Alkaline treatment with reagents such as quicklime neutralizes residual acidic components. The outcome is a carbon content surpassing 99.95%, rendering it suitable for lithium-ion battery anode materials.
4. Coating: The purified spherical graphite particles are coated with a substance like high softening point pitch (HSP pitch). Furnace-based melting yields a uniform coating layer, which undergoes carbonization through heating in an inert atmosphere. This enhances the performance and stability of the graphite anode within lithium-ion batteries.

Synthetic Graphite

Synthetic graphite also has four fundamental steps in it’s production [3]:

1. Green Petroleum Coke Production: extracted from petroleum refining or catalytic cracking of heavy oils.
2. Calcination: The green petroleum coke undergoes calcination, a high-temperature process at around 1300-1400°C. This removes the impurities and volatile components from the coke. The output is needle coke, an enriched carbon material.
3. Graphitization: The needle coke is then placed in an electric furnace at >2500°C for graphitization. This extreme heat triggers a rearrangement of carbon atoms, transforming the coke into graphite.
4. Post-Processing: Shaping, classification, or coating can be applied to ensure optimal packaging and to improve performance within the battery anode.

This post has been built based on the support and sponsorship from: Thermo Fisher Scientific, Eatron Technologies, About:Energy and Quarto Technical Services.

For both natural and synthetic graphite the post-processing can significantly improve the performance. Hence the post-processing is often shrouded in IPR, patented and kept as trade secrets.

Material Differences

Battery Cell Comparison

The only comparison that matters is perhaps the impact on cell performance. Glazier et al [4] made NMC532/(Synthetic Graphite or Natural Graphite) pouch cells with various loadings of an electrolyte additive blend to study the effect of the graphite type. There conclusions were:

• with large electrolyte additive loadings:
  • similar coulombic efficiencies, parasitic heat flows and gas production
  • Natural Graphite cells showed worse capacity retention in long-term tests
• low additive loadings
  • Natural Graphite cells showed lower coulombic efficiency, higher capacity fade, more parasitic heat flow, and more gas production
• Stack pressure measurements showed that NMC532/NG cells irreversibly expanded during cycling while NMC532/AG cells did not.

due to the layered structure, anisotropy, and surface defects, the utilization of Natural Graphite as the anode for Lithium Ion Batteries has the following inadequacies:

In propylene carbonate (PC)-based electrolytes, large amounts of solvated lithium ions (Li⁺) co-embed in the graphite anode, resulting in carbon layer expansion and exfoliation, and the carbon layer becomes unstable. Li⁺ can only intercalate at the edge of graphite, resulting in low mass transfer efficiency, poor rate performance, and easy lithium precipitation. Carbon defects exhibit high activity and easily react with Li⁺ to produce inactive lithium, which reduces the initial coulombic efficiency (ICE). Under low-temperature and rapid-charge conditions, Li⁺ easily precipitates on the graphite surface, resulting in attenuation in battery performance. [5]

This detailed analysis backs up the high level view that synthetic graphite enables cells to have higher power charging.

References

1. Synthetic versus natural graphite debate rages on: 2023 preview, Fastmarkets
2. Natural vs Synthetic Graphite, Ceylon Graphite
3. Synthetic vs. Natural Graphite: Unveiling the Manufacturing Differences, Rijo Jacob Robin, LinkedIn
4. S. L. Glazier, Jing Li, A. J. Louli, J. P. Allen and J. R. Dahn, An Analysis of Artificial and Natural Graphite in Lithium Ion Pouch Cells Using Ultra-High Precision Coulometry, Isothermal Microcalorimetry, Gas Evolution, Long Term Cycling and Pressure Measurements, Journal of The Electrochemical Society, Volume 164, Number 14
5. Yin Zhao, Yulong Fu, Yue Meng, Zhi Wang, Junhao Liu, Xuzhong Gong, Challenges and strategies of lithium-ion mass transfer in natural graphite anode, Chemical Engineering Journal, Volume 480, 2024
6. Francis Isidore Barre, Romain Guillaume Billy, Fernando Aguilar Lopez, Daniel Beat Müller, Limits to graphite supply in a transition to a post-fossil society, Resources, Conservation and Recycling, Volume 208, 2024