In our graphite production business, we take little for granted. To deliver the best graphite solutions to our clients, we cannot cut corners in sourcing sound material, hiring and training knowledgeable employees, and investing in state-of-the-art machines. Doing these things right allows us to produce excellent graphite applications. If there’s one thing we do take for granted though, it’s graphite. That’s not to say we don’t study the material closely -- in fact, we’re dedicated to a deep understanding of what graphite is and how to best use it to meet our customers’ needs.
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Working with graphite applications every day, we sometimes get the feeling that this eminently versatile material has always been around. How could people live without it? But the truth is, graphite has a (partially) discoverable history and it has not always been with us. When did people start using graphite? When was it discovered in nature? When did commercial graphite production start?
We know the Aztec used it. The Celts of western Europe were using it in their pottery around the time Socrates was alive. Ancient Egyptians may have even used it thousands of years ago, though we’re not certain how or why they were using it. It was certainly mined and used in Germany in the middle ages. All of these uses predate the common myth that graphite was discovered in England, at the Borrowdale deposit in the Lake District in the 16th century, where it was used to mark sheep. The find at Borrowdale, though, is historically significant. The graphite found at Borrowdale was of remarkable quality and was discovered to hold up in very extreme conditions. It was used to cast cannonballs and eventually gave rise to one of the most well-known graphite applications--the pencil. A pencil industry was birthed around Borrowdale to make use of the brittle, dark material, which was a better marker than traditional lead (and a good bit safer too).
But what about the history of the word “graphite”? According to the New World Encyclopedia, “graphite” was coined by Abraham Gottlb Werner in the late 18th century. The choice reflected the material’s most common use at the time, which was as a writing material. The Greek base “graph” of course is found in many English words (“autograph,” “lithograph,” “stenographer” etc), all having to do with recording something down on a surface. To this day, many people associate graphite mainly with pencils.
Over time, graphite was discovered to have many other useful properties besides the ability to make markings on paper and sheep. Graphite solutions were discovered for many needs. It was found to conduct electricity, for instance, leading to its modern use in electrodes. In the 19th century, as the Industrial Revolution forged ahead in England and the U.S., graphite was found to be very slippery even when dry, and its use as an industrial lubricant soon followed. 19th century engineers and industrialists also exploited graphite’s refractory qualities -- it is not deformed or debased by high temperatures or pressures -- and put it to use in, among other things, crucibles.
Today there are many, many more graphite applications. Graphite production is an enormous worldwide industry, and the material finds itself more and more enmeshed in our daily lives, in an untold number of graphite applications.
This article is contributed by AmirReza Rouhani Esfahani
Over the past three decades, lithium-ion batteries (LIBs) have undergone a remarkable evolution, transitioning from powering small devices to fueling large-scale applications like electric vehicles (EVs) and stationary energy storage systems. An important advancement in this journey has been the adoption of graphite-based anodes, replacing soft and hard carbons. This shift has significantly improved full-cell energy densities, thanks to graphite’s low lithiation/delithiation potential and impressive (theoretical) gravimetric capacity of 372 mAh/g [1].
The improvements in graphite electrodes, shown in Fig. 1, have a long history. Since 1975, we’ve known that graphite can form a chemical compound with lithium, namely LiC6, in a reversible process. In the 1970s, however, graphite could not be used successfully for batteries due to issues with the liquid organic electrolytes, which resulted in continuous decomposition. After two decades of research and development on graphite anodes, Sony achieved a major milestone with the first lithium-ion battery in 1991, a breakthrough in battery technology [2].
Figure 1. Key achievements in the evolution of graphite negative electrodes for lithium-ion batteries [2].Since 1994, most commercial lithium-ion batteries have been manufactured with graphite as the active material for the negative electrode because of its low cost, relatively high (theoretical) gravimetric capacity of 372 mAh/g, and high coulombic efficiency. According to the 2015 anode materials market share, about 98% of anodes for LiBs are made with carbon-based materials. The remaining 2% are made with silicon (Si) and lithium titanium oxide (LTO). Large EV and cell manufacturers believe that graphite will continue to remain a vital component of LiBs either as the sole anode or in composites with other elements, such as Si [1,2].
Figure 2. Anode materials market share in 2015 [1].Carbon is the sixth element in the periodic table, with a molecular weight of 12.01 g/mol. Carbon materials in nature have two dominant structures: diamond and graphite. In graphite, sp2 hybridized graphene layers are connected via weak van der Waals forces. This bonding pattern results in a hexagonal arrangement of atoms, which are layered in sheets with an interlayer spacing of 3.354 Å, as presented in Figure 3(a). The relatively weak van der Waals forces between adjacent graphene layers facilitate intercalation — insertion into two-dimensional layered structures — of ionic and molecular species between the surfaces. The van der Waals forces allow the space between the layers to expand, eventually leading to a re-stacking of the graphene layers and enabling the reversible intercalation of lithium ions between them. The remarkable reversibility of this intercalation reaction, coupled with minimal volume changes, is responsible for graphite’s long-lasting efficiency and reliability in lithium-ion batteries and underscores graphite’s significance as an anode material [2].
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Due to its layered arrangement, graphite particles typically exhibit a flat, flake-like shape with two distinct surfaces: basal planes of dimension La and edge planes of height Lc, also known as prismatic planes. This is depicted schematically in Fig. 3 and in an SEM (scanning electron microscopy) image in Fig. 4. Accordingly, the edge planes show a higher intercalation/deintercalation reactivity than the basal planes [3].
Figure 3. Representation of graphitic crystallite structures [3].Edge planes in graphite are recognized as active sites, playing a pivotal role in various reactions. These edge sites, however, are a double-edged sword, instigating unwanted reactions that compromise the efficiency of the system. The electrochemical performance of graphite electrodes depends heavily on the ratio of basal to edge planes. While edge planes facilitate intercalation, an excess of edge planes may also lead to increased surface reactivity, including side reactions or degradation over prolonged use, which weaken the long-term stability and cycle life of the battery. Notably, different types of graphite may exhibit variations in their basal-to-edge plane ratios, influencing the overall efficiency and charge-discharge characteristics of the battery and their suitability for specific applications. This underscores the importance of tailoring graphite structures for optimized electrochemical performance in diverse battery systems [1,2].
Figure 4. SEM image of graphite flakes [2].As discussed, irreversible capacity loss stems from the breakdown of electrolytic substances, leading to unwanted reactions and the formation of a Solid Electrolyte Interface (SEI) film on the surface of the graphite. Commonly used electrolytes in lithium-ion batteries (LiBs), like propylene carbonate (PC) and ethylene carbonate (EC), react strongly with graphite, creating the SEI film. While crucial for stabilizing the electrode-electrolyte interface, the SEI film also reduces overall battery capacity due to the irreversible loss of Li ions on the graphite surface in the process of forming SEI.
Unfortunately, the SEI does not form only on pristine graphite in the first cycle; it continues to evolve during usage of the battery, spurred by ongoing interactions with the electrolyte. The cycling process induces a minor volumetric expansion in graphite through intercalation/deintercalation, resulting in microcracks in the SEI. These microcracks open the door for further SEI formation on the newly exposed surfaces, leading to an accumulation of capacity loss over time. Among various methods, coating has emerged as the most effective approach to reduce the formation of microcracks in order to address the initial capacity loss in graphitic anodes [1,2].
Figure 5. Coating of spheroidized graphite [2].The properties of the Solid Electrolyte Interface (SEI) depend largely on the electrolyte composition and graphite surface characteristics. Surface modification methods, such as coating, can be used to promote the early formation of a stable SEI. This stable interface prevents unwanted reactions and is essential for enhancing cyclability and initial coulombic efficiency (ICE). Using surface modification to improve performance was first demonstrated in 1996, when Peled et al. [4] showed a 10% improvement in initial coulombic efficiency through the mild oxidation of graphite particles at 550 °C for 1 hour. Various precursors, including metals, gases, and solid carbons, have been employed to modify the surface of LiB graphite. So far, the most investigated and most cost-effective surface coating material is carbon — and not only for graphite but for cathode as well as anode materials. In this process, the graphite is initially coated with a carbon saturated precursor such as glucose. To stabilize the coating, a heat treatment at high temperature (e.g., 900 °C) is required to carbonize (converting organic material into carbon) the precursor-coated sample. By covering the reactive sites of the graphite nanostructure, carbon mitigates negative reactions between the SEI and the electrolyte. Because these carbonaceous coatings have lower density and significantly lower energy storage capacity compared to graphite, however, they can lead to lower gravimetric and volumetric energy densities at the full-cell level.
Among carbon-based agents, coal-tar pitch (CTP) is the standard industrial coating material that provides a homogeneous amorphous carbon coating on the graphite surface while enhancing the capacity, rate capability, and initial coulombic efficiency of the graphite anode. Based on the work of Han et al. [5], CTP-coated graphite provides an initial coulombic efficiency of 90.3% as well as high-rate capability. Despite its good electrochemical features, the use of CTP has negative health and environmental implications. CTP is produced by distilling coal tar, a byproduct of carbonizing coal to make coke or gas, a process that is carbon-intensive and can release polycyclic aromatic hydrocarbons (PAHs) into the environment. In addition, the National Cancer Institute has found that exposure to CTP increases the risk of skin cancer and is linked to other types of cancer, including lung, bladder, kidney, and digestive tract cancer [6].
Researchers are currently dedicated to developing the most environmentally friendly advanced graphite anode coating material for lithium-ion batteries (LiBs). They are looking to use a biomass-derived carbon coating to create an electrochemically efficient anode that matches or surpasses the performance of CTP-coated graphite while minimizing health and environmental impacts. Despite the challenges associated with biomass-based coating agents, including higher inorganic impurity content and variable properties, the pursuit of sustainability remains a driving force in the quest for improved battery materials. While CTP offers consistency, it is not renewable, highlighting the importance of exploring alternatives with lower environmental footprints [2].
Graphite’s unique layered structure makes the material well-suited for lithium-ion intercalation. Starting from 1994, almost all commercial LIBs were (and still are) based on graphite as the active material for the negative electrode. Despite possessing excellent conductivity and stability, graphite anodes experience high first cycle capacity loss due to undesired electrolyte decomposition reactions and the uncontrolled formation of the solid electrolyte interface (SEI).
Coating of the graphite surface has been one of the most effective methods to address this issue. Balancing cost and complexity while improving the stability, efficiency, and capacity of the battery is key for advancing graphite-based anodes in batteries. Among the materials tested, disordered carbon coatings have shown the best results for preventing unwanted reactions on the graphite anode and reducing the first-cycle capacity loss.
Coal-tar pitch (CTP) is the standard industrial coating for graphitic anodes but has significant environmental and health concerns. Current research aims to develop greener alternatives, such as biomass-derived carbon coatings, to achieve an electrochemically efficient anode with reduced adverse effects.
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