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| Fig. 1: Schematic of a Lithium-Ion Battery. (Source: Wikimedia Commons) |
The lithium-ion battery has proven to be one of the most important technological advances in recent history. It is ubiquitous in our lives; nearly all the portable devices (cell phones, laptops, tablets, and consoles) we use on a daily basis are powered by the lithium-ion battery. Without the battery's phenomenal ratio of power density to volume, the pace of development of technology throughout the 21st century would likely look dramatically different. In order to better understand lithium-ion batteries and their inner workings, it is critical that we also understand the role of graphite, a carbonaceous compound that is indispensable in its superior functionality as an anode (negative battery terminal).
The basic anatomy of a lithium-ion battery is straightforward. The anode is usually made from graphite. The cathode (positive battery terminal) is often made from a metal oxide (e.g., lithium cobalt oxide, lithium iron phosphate, or lithium manganese oxide). The electrolyte is usually a lithium salt (e.g. LiPF6, LiAsF6, LiClO4, LiBF4, or LiCF3SO3) dissolved in an organic solvent (e.g. ethylene carbonate or diethyl carbonate). [1] The electrolyte is the solution through which lithium ions flow inside the cell. Fig. 1 is a schematic diagram of a simple lithium-ion battery; although the electrolyte is not shown, the general functionality of the battery is made quite clear.
In a battery charging/discharging configuration, we imagine a circuit with a device that either supplies power to the battery or takes power from the battery. The charging cycle proceeds as follows: first, electrons flow from the charging device to the anode. The ensuing surplus of negative charges at the anode causes positively charged lithium ions (Li+) to flow from the cathode through a separator (which is impermeable to electrons) in the middle of the battery, to meet and neutralize the electrons at the anode. Electrons that have been separated from the lithium at the cathode are unable to penetrate the separator. Instead, they flow in the opposite direction, through the wire, to the charging device. They join electrons added by the charging device and continue to the anode, completing the circuit. Finally, the electrons recombine with lithium ions and anode material (e.g., graphite, C6) through a chemical process called intercalation, forming LiC6 and neutralizing the positive charges of the lithium ions. When the flow of lithium cations from the cathode to the anode has stopped, the battery is fully charged. [1]
The discharging cycle is nearly identical to the charging cycle, but proceeds in reverse order. First, the device pulls electrons from the anode through a wire to itself. Some of the electrons are consumed by the device. The electrons continue to flow through the circuit, past the device, to the cathode. The net negative charge at the cathode due to the surplus of electrons there causes positively charged lithium ions to flow from the anode, through the separator, to the cathode, where their charge is neutralized by the electrons that await them there. When the flow of lithium cations from the cathode to the anode has stopped, the battery is fully depleted. [1]
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| Fig. 2: China's 650 thousand tonnes of graphite production in 2020 is over 6.5 times greater than Brazil's production of 95 thousand tonnes. [5] (Source: O. Friedman). |
Within a lithium-ion battery, graphite plays the role of host structure for the reversible intercalation of lithium cations. [2] Intercalation is the process by which a mobile ion or molecule is reversibly incorporated into vacant sites in a crystal lattice. In other words, when the lithium ions and electrons recombine with the anode material during the aforementioned charging process, the ions are being inserted into sites within the graphite lattice/frame. Intercalation is critical to the commercial success and viability of the lithium ion battery; it is the chemical process that minimizes volume change and mechanical strain on a battery during repeated insertion and extraction of ions. [3] Without graphite's remarkable efficiency during intercalation, lithium ion batteries would not function nearly as well as they do today.
According to the USGS Mineral Commodity Summaries 2021, the United States did not produce natural graphite in 2020. [4] It did, however, import approximately 41,000 tons of the material. About a third of US graphite imports came from China, with Mexico and Canada the two next leading exporters to the US at about 23% and 17%, respectively. [4] China's status as leading exporter of graphite to the US is consistent with its graphite exportation dominance worldwide. China has led the world in total graphite mine production for over 10 years, and never once produced less than 600 thousand tonnes in a single year during that time span. [5] Moreover, in 2020, it produced more than 6.5 times more graphite than the next closest country (Brazil). In 2020, China produce 650 thousand metric tons of graphite, and its growth rate per annum in production has been about 5% since 2009. [5] With no immediately available substitutes for graphite as an effective lithium-ion battery anode, China is clearly well positioned to capitalize on the continued growth of the electronic device and EV markets globally. Fig. 2 is a graph I have created in order to better visualize China's dominance in the global graphite market.
© Oliver Friedman. 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.
[1] M. Z. Jacobson, 100% Clean, Renewable Energy and Storage for Everything (Cambridge University Press, 2020).
[2] J. Asenbauer et al., "The Success Story of Graphite as a Lithium-Ion Anode Material - Fundamentals, Remaining Challenges, and Recent Developments Including Silicon (Oxide) Composites," Sustain. Energy Fuels 4, 5387 (2020).
[3] R. C. Massé et al., "Energy Storage Through Intercalation Reactions: Electrodes for Rechargeable Batteries," Natl. Sci. Rev. 4, 26 (2017).
[4] "Mineral Commodity Summaries 2021," U.S. Geological Survey, January 2021.
[5] "BP Statistical Review of World Energy 2021," British Petroleum, July 2021.
