Application of chlorination technology for the conversion of natural flake graphite into coated spherical purified graphite for Li-ion batteries

The global graphite market is set to expand rapidly, driven by the energy transition to electric vehicles in transportation and electric arc furnaces in steelmaking. The worldwide consumption of graphite uses both synthetic ( c. 60%) and natural ( c. 40%) graphite. Current supply risk for both natural and synthetic graphite comes from the high production concentration in China and a low end-of-life recycling input rate. Compared with natural graphite, synthetic graphite is subject to higher supply risk owing to its by-product dependency on needle coke production from petroleum. Future additional supply risk of graphite stems from uncertain energy transition demand predictions, ranging from a three- to seven-fold expansion by 2040 based on the rate of the energy transition, and a high concentration of end-use of up to c. 80% in Li-ion batteries. To reduce future supply risk of graphite in the production stage, increasing mining activities outside of China, especially in Africa, together with potential developments in recycling of spent Li-ion batteries, will be vital. In the manufacturing stage, advances in purification and modification of natural graphite to close the quality gap between the natural and synthetic graphite anode material are anticipated to decrease the share of synthetic graphite in use over time. The future resilience of the graphite supply chain will largely depend on the mining of natural graphite, particularly flake graphite for the use in Li-ion batteries. The potential of exploiting flake graphite resources is remarkable, as global flake graphite resources are an order of magnitude larger than predicted cumulative demand until 2050 and are distributed across the continents, mostly in crystalline metamorphic basement. Discovering new natural flake graphite resources to diversify and increase the supply, however, requires robust genetic models to guide mineral exploration. There is a significant knowledge gap in understanding the mineral system of flake graphite deposits. The nature and role of fluids in the transport and concentration of carbon remains poorly understood. Identifying deformational structures and alteration halos in the graphite mineral system will be essential to reconstruct the fluid pathways and constrain the structural-chemical traps. Research on world-class natural graphite deposits will be key to fill this knowledge gap and drive towards a sustainable supply of graphite.

Thermal and gas purification of natural graphite for nuclear applications

Conductive paint: U.S. Patent 5,567,357. Oct. 22, 1996 S. Wakita, assignor to Tatsuta Electric Wire & Cable Co. Ltd., Osaka, Japan

Chemical purification processes of the natural crystalline flake graphite for Li-ion Battery anodes

This work examines the processing and mechanical properties of graphite compacts electroplated by a copper layer. Natural graphite (NG) flakes have been compressed to make graphite compacts. A copper electrodeposit has been applied to the surface of the graphite compact in order to reinforce the mechanical properties and prevent the graphite particles from peeling off. The natural graphite flakes were stacked layer by layer in order within the graphite compact. The effects of different electroplating conditions on the mechanical properties of the graphite compacts were studied. The density of the compact reaches to a limit value when the pressure was above 100 MPa. The tests indicated that the compressive, flexural and shear strengths increase at least 150% with the copper layer. The damage mechanisms in the compacts were investigated by using a scanning electron microscopy.

Properties of graphite composites based on natural and synthetic graphite powders and a phenolic novolac binder

There is a current supply gap in Li-ion battery (LiB)-grade graphite, which is forecast to grow significantly over the next years. A potential source of graphite for LiBs is mined natural flake graphite (NFG). However, mined NFG must be purified to meet LiB-grade specifications. A commercially viable and environmentally acceptable (i.e. HF-free) process is needed to purify NFG produced outside China.In this context, we have recently demonstrated the technical viability of HF-free purification process (named GraphPureTM) [1]for transforming mined NFG into LiB-grade graphite product. This technology is the adaptation of a well-established process of chlorination commercially used to purify large-form graphite parts for applications in the nuclear and defense industries (e.g. rocket nose cones).In this study, it will be shown that this chlorination method can remove impurities, such as silica, iron oxide, and calcium alumina silicates, from various sources of NFG to achieve the purity specification (>99.95 wt% C) required for use in LiBs. Moreover, it will be demonstrated that this purification can be performed before or after the spheronization- stage, as in both cases the final products have comparable properties (purity, surface area, tap density, crystallinity, etc.) and electrochemical performance to existing commercial LiB-grade graphite.Reference:1. Watson K. et al., Process for purifying and producing a high purity particulate graphite material for use in lithium-ion batteries, patent WO 2023/0119361 A1

Natural graphite is currently considered as a critical raw material in EU. The demand for graphite is still increasing as it is commonly used in the anodes of the Li-ion batteries (LIBs). The total graphite content for energy storage applications such as LIBs should be more than 99.95%. Several purification processes for natural graphite exist but the requirement of high purity is challenging. Here we present the high temperature thermal treatment for natural graphite ores. Thermal treatment at 2400 °C for 15 min can produce battery-grade graphite with high purity and crystallinity needed for the optimum performance of the battery cells. In addition, the crystallinity and crystalline structure of graphite was improved during the treatment. The electrochemical studies of thermally treated graphite powders showed increased electrochemical performance compared to the untreated graphite samples. The improved performance was attributed to the increased purity and crystallinity of the thermally treated powders.Graphical Effect of thermal purification on the electrochemical performance of natural flake graphite

Graphene has been considered to be an intriguing playground for novel physics, especially since its isolation ten years ago. For the investigation of the predicted properties of graphene, however, clean samples with crystallographically defined edges are of crucial importance. Here, we explore the interaction of graphite and graphene on SiO2 and hBN substrates with hydrogen ions and radicals in two different pressure regimes, searching for a reliable fabrication method for zigzag edged graphene nanoribbons. The samples are prepared by exfoliating graphite and depositing graphitic material on the substrate of interest. Subsequently the samples are exposed to the plasma at various gas pressures and sample-plasma distances. Exposing graphite flakes to a pure hydrogen plasma at pressures around 0.03 mbar, leads to the intercalations of hydrogen atoms in between the top graphite layers where the atoms recombine to hydrogen molecules. This process is reversible as the gas molecules can be released from within the substrate when the samples are heated to elevated temperatures. In this regime a partial hydrogenation of the graphite and graphene surfaces is measured and indicates the formation of graphane. Its band structure is expected to be gapped, opening the way for atomically thin devices employable in electronic industry. Increasing the gas pressure of the plasma to 0.4-1.7 mbar, the graphitic samples are etched by hydrogen radicals at intrinsic or predefined defects, evolving into hexagonally shaped holes, indicating an anisotropic etching process. The anisotropy of the etching process is, however, strongly dependent on the substrate and the amount of graphite layers exposed. The edges of the hexagons are expected to be of zigzag type and can be employed to fabricate graphene nanoribbons with well define edges. Zigzag graphene nanoribbons were predicted to have magnetic edge states and a band gap, opening the possibility of investigating spin filter devices in graphene structures. Stimulated by the unanswered question of strongly differing c-axis resistivities (rho_c) of natural and highly oriented pyrolytic graphite (HOPG), transport experiments, following older investigation are pursued. Reducing the sample height of natural graphitic flakes to micrometer sized samples, noticeably enlarges its rho_c, approaching it to the measured HOPG values. A recent theory unveils the discrepancy in rho_c of natural graphite and HOPG, linking the density of bulk disorder with the value of rho_c. The measurement result of the micrometer sized natural graphitic samples with increased rho_c, distinctly point to a strong influence of the bulk disorder in the graphite flakes on rho_c, confirming the recent theory put forward.

In order to investigate the structural evolution of natural flake graphite during intercalation and exfoliation, natural graphite flakes were treated by intercalating, water-washing, drying and expanding. The corresponding products, graphite intercalation compound (GIC), residue GIC (expandable graphite) and expanded graphite were characterized by X-ray diffraction (XRD). The results can provide reference for the research in this field.

Electrochemical behavior of matrix graphite in nitric acid by cyclic voltammetry

The experimental temperature dependence of the electrical resistivity of a model composition based on natural flake graphite was compared with the results of a mathematical simulation performed with the use of a flake graphite polycrystal model. For increasing the section of contacts between flakes, the density of the material was increased as a result of impregnation with pitch and the subsequent calcination. The average values of the sizes of flakes in the composition and the tabular values of the conductivity tensor of a graphite quasi-single crystal served as the parameters of calculations. Based on the results of the mathematical simulation, it was found that temperature cracking due to the shrinkage of flakes on cooling should be taken into account for the correspondence of computed values to the experimental results. This cracking almost ceased at temperatures lower than 200°C regardless of the compaction of the material.

Various 2D carbons demonstrate significant effects of surface oxidation, heating, suspending–drying, cryogelation, swelling, and adsorption of polar and nonpolar compounds on the morphological, structural, and textural characteristics. Heating at 120–150 °C could result in collapse of pores not only between carbon sheets in stacks but also between neighboring stacks; therefore, the specific surface area (SSA) decreases by a factor of 30–100 for preheated graphene oxides (GO). According to the TEM and XRD data, the GO structure is rather amorphous, since only small X-ray coherent scattering regions demonstrate a certain order giving broad XRD (001) and (002) lines. In the Raman spectra, the D line (disordered defect structures with sp3 hybridized C atoms) intensity for GO is similar to that of the G line (ordered structures with sp2 hybridized C atoms). The graphite oxide (GtO) structure, which is closer to that of graphite than that of GO, is characterized by intensive G and low D lines, and the main XRD peak at 26.4° (characteristic for graphite) is broadened similar to the XRD peak of GO at 10°. Despite the GO stacks have a tendency to collapse upon heating, the collapsed stacks can be swollen not only in water (strongly) but also in liquid nitrogen (relatively weakly). Therefore, the use of GO in aqueous media can provide great SSA values in contact with the solvent and solute molecules. This could provide high efficiency of the GO use for purification of wastewater, separation of solutes, etc. MLGO produced from natural flake graphite as a precursor (flakes < 0.2 mm in size) using a modified method of ionic hydration and freeze–drying is characterized by typical light brown color, low bulk density, flexible sheet stacks easily collapsed, but its interaction with water results in strong swelling. Interaction between the carbon sheets in preheated MLGO is strong and nonpolar molecules, such as benzene, n–decane, poorly penetrate between the sheets, i.e., intercalation adsorption is small. However, water molecules can effectively penetrate (this is rather intercalation adsorption resulting in swelling) between the sheets, but the swelling effect of water adsorbed from the gas phase could be weaker than that in the aqueous suspensions. Thus, the proposed synthesis method of MLGO using natural graphite is effective and appropriate for preparation of the materials for various practical applications.

Due to their excellent thermal shock and slag resistance at high temperatures, alumina–graphite refractories are used extensively in the steel industry. The degradation of carbon based refractories through carbon depletion is an important issue and a fundamental understanding of refractory behaviour at high temperatures is crucially important. Natural flake graphite, with ash impurity levels ranging from 1 to 10%, is used extensively in the commercial preparation of alumina–carbon refractories. This study investigates the role played by ash impurities in the depletion of carbon from the refractory composite. Two natural graphites, respectively containing 2.1% and 5.26% ash, were used in this study. Substrates were prepared from mixtures of alumina and carbon over a wide concentration-range. Using a sessile drop arrangement, carbon pick-up by liquid iron from alumina–natural graphite mixtures was measured at 1 550°C and was compared with the carbon pick-up from alumina–synthetic graphite mixtures. These studies were supplemented with microscopic investigations on the interfacial region. Very high and similar levels of carbon dissolution were however observed from both alumina–natural graphite mixtures, with carbon pickup by liquid iron from mixtures with up to 30 wt% alumina reaching saturation. A sharp reduction to near zero levels was observed in the 30 to 40 wt% alumina range. Along with implications for commercial refractory applications, these results are discussed in terms of poor wettability between alumina and liquid iron, interactions between ash impurities and alumina, and formation of complexes in the interfacial region.

This study presented a new way to prepare expandable graphite (EG), which is one kind of halogen‐free flame retardant using the H2O2‐hydrothermal process. Natural graphite was immersed in H2O2 and then put in autoclave to proceed the hydrothermal process. The EG was called as H2O2‐HEG from the H2O2‐hydrothermal process. The results showed that the expanded volume of EG using the H2O2‐hydrothermal process was higher than that compared with convectional liquid phase synthesis, ultrasound irradiation, and hydrothermal method. Fourier transform infrared spectroscopy, X‐ray diffraction patterns, scanning electron microscope, and X‐ray photoelectron spectroscopy were used to analyze the structure and confirm that the EG had been prepared. Thermogravimetric analysis presents that H2O2‐HEG can improve the thermal stability of composites. The cone calorimeter show that the peak heat release rate (HRR) values of composites decrease dramatically. Limiting oxygen index (LOI) value of H2O2‐HEG composites is higher than that of high‐density polyethylene (HDPE)/natural flake graphite. HDPE composites are capable of passing the V‐0 classification and have antidripping behavior. LOI, UL‐94, and the cone calorimeter results show that the HDPE/H2O2‐HEG composite possess excellent flame retardant property. POLYM. COMPOS., 2012. © 2012 Society of Plastics Engineers