Graphite Stack Research Articles

Introducing surface adsorption lithium storage mechanism to enhance safety of graphite

The thermal stability of lithiated graphite plays an important role in the thermal safety of lithium-ion batteries (LIBs). However, a safer graphite anode usually leads to a lower energy density. This article starts from multiple hazardous factors of lithiated graphite when thermal runaway occurs, considering the impact of defects on the electrochemical performance and thermal safety of graphite electrodes. In this study, turbostratic graphite was prepared as the anode for LIBs. The stacking of graphite was altered to form narrow pores and spherical aggregates, and introducing defects. By varying the ball-milling time, the structural changes of different ball-milled graphites were analyzed. The impact mechanism of defect structure on electrode electrochemical performance and safety performance was investigated. The results show that the spherical structure and large specific surface area are beneficial to increase the active sites and delay the self-heating of the battery. The formation of defects and pores increases the adsorption sites, which makes the surface adsorption of Li+ dominant and improves the ion diffusion. In addition, the dominance of surface adsorption and desorption effectively suppresses the problem of graphite flake peeling and intensified thermal runaway caused by graphite phase transition and lithium evolution heat release at high temperature. The thermal safety test was carried out by accelerated calorimeter (ARC), and its thermodynamics and reaction kinetics were quantitatively analyzed. It was proved that the introduction of surface adsorption lithium storage method successfully delayed the thermal runaway of the battery, realizing the balance between high electrochemical performance and high safety.

Transformation of the diamond surface with a thin iron coating during annealing and transport properties of the formed conductive layer

In our study, we annealed polycrystalline diamond films with nanometer-thick iron coatings. Based on analysis of X-ray photoelectron spectroscopy data the presence of two catalytic processes was suggested. One is the continuous absorption of sp3 carbon by iron nanoparticles with subsequent extrusion of the sp2 phase from the formed iron carbide, which leads to the creation of deep tracks into the diamond bulk that are filled with graphite stacks. The other process involves a thin layer of iron carbide on the surface and results in the creation of sp2 layers, which are made up of fused graphene-like patches and are oriented mainly along the surface of the diamond film. For the sample annealed at 800 °C for 1 h, the sp2 layers formed exhibit semi-metallic conductivity with quantum corrections caused by disorders. At the same time, no evidence of iron contribution in conductivity of the annealed Fe-coated film was detected, implying that upon transforming the diamond, iron moved quite deep into the bulk. In addition, the formed conductive layer on the diamond surface shows mechanical stability, ensuring the reproducibility of conductivity measurements even after six months of storage in air. The study has revealed the potential for targeted metal-assisted formation of sp2 carbon-based conducting pathways on diamond surfaces for electronic devices.

Crystalline-topologies engineering of bio-spore-derived hard carbon for efficient low-potential potassium ion storage

Clarifying the relationship between crystalline topologies of hard carbon and electrochemical K+ storage behaviors is essential to develop low-cost anode for K+ battery. Herein, bio-mass derived hard carbons with various crystalline topologies were synthesized for the first time by one-step pyrolysis of spores of sorghum smut fungus as new precursors. The crystalline-structures of spore-derived hard carbons including graphitic-like layer number, spacing and curvature can be effectively modulated by pyrolysis temperatures, leading to significant difference between correspondingly as-synthesized hard carbons for K+ storage. The hard carbon obtained at 1400 °C presents a unique crystalline structure with fewer graphite stacking layers down to 2, achieving the excellent K+ storage performance, in which, it presents a high capacity of 495.0 mAh/g with a significant plateau capacity of 368.9 mAh/g (< 0.5 V), a good rate performance of 175.0 mAh g-1 at 500 mA g-1 and stable cycling performance (125.0 mAh g-1 at 500 mA g-1 over 600 cycles), outperforming the most of reported hard carbons derived from various precursors. Meanwhile, the DFT calculation further clarifies that the crystalline topologies of hard carbon play a critical role for efficient K+ storage. In particular, there is an interesting discovery that the number of graphite stacking layers can significantly affect the migration and diffusion of K+, in which, the fewer the number of graphite stacking layers, the lower the energy barrier of K+ diffusion between graphite layers. These results in the work provide new insights for the understanding of K+ storage behavior in hard carbon.

Beyond Conventional Carbon Activation: Creating Porosity without Etching Using Cesium Effect.

Facile synthesis of porous carbon with high yield and high specific surface area (SSA) from low-cost molecular precursors offers promising opportunities for their industrial applications. However, conventional activation methods using potassium and sodium hydroxides or carbonates suffer from low yields (<20%) and poor control over porosity and composition especially when high SSAs are targeted (>2000 m2 g-1) because nanopores are typically created by etching. Herein, a non-etching activation strategy is demonstrated using cesium salts of low-cost carboxylic acids as the sole precursor in producing porous carbons with yields of up to 25% and SSAs reaching 3008 m2 g-1. The pore size and oxygen content can be adjusted by tuning the synthesis temperature or changing the molecular precursor. Mechanistic investigation unravels the non-classical role of cesium as an activating agent. The cesium compounds that form in situ, including carbonates, oxides, and metallic cesium, have extremely low work function enabling electron injection into organic/carbonaceous framework, promoting condensation, and intercalation of cesium ions into graphitic stacks forming slit pores. The resulting porous carbons deliver a high capacity of 252 mAh g-1 (567 F g-1) and durability of 100000 cycles as cathodes of Zn-ion capacitors, showing their potential for electrochemical energy storage.

Replica exchange molecular dynamics for Li-intercalation in graphite: a new solution for an old problem.

Li intercalation and graphite stacking have been extensively studied because of the importance of graphite in commercial Li-ion batteries. Despite this attention, there are still questions about the atomistic structures of the intermediate states that exist during lithiation, especially when phase dynamics cause a disordered Li distribution. The Li migration event (diffusion coefficient of 10-5 nm2 ns-1) makes it difficult to explore the various Li-intercalation configurations in conventional molecular dynamics (MD) simulations with an affordable simulation timescale. To overcome these limitations, we conducted a comprehensive study using replica-exchange molecular dynamics (REMD) in combination with the ReaxFF force field. This approach allowed us to study the behavior of Li-intercalated graphite from any starting arrangement of Li at any value of x in LixC6. Our focus was on analyzing the energetic favorability differences between the relaxed structures. We rationalized the trends in formation energy on the basis of observed structural features, identifying three main structural features that cooperatively control Li rearrangement in graphite: Li distribution, graphite stacking mode and gallery height (graphene layer spacing). We also observed a tendency for clustering of Li, which could lead to dynamic local structures that approximate the staging models used to explain intercalation into graphite.

14C IN TREE RINGS IN THE VICINITY OF THE RBMK REACTOR NUCLEAR POWER PLANT

ABSTRACTThe paper presents the results of radiocarbon (14C) concentration measurements in tree rings in the vicinity of Kursk NPP (Russia) with four operating RBMK reactors. The sampling was carried out from the site with the highest expected accumulation of radiocarbon in vegetation. The site was determined with long-term meteorological data. The measurements of 14C concentration carried out with accelerator-mass spectrometer in Budker Institute of Nuclear Physics, Novosibirsk, Russia. The obtained results demonstrated the influence of exploitation of Kursk NPP to the concentration of 14C in tree rings. Based on the equilibrium between the 14C ratio in the tree rings and the surrounding air, retrospective estimates of the radiocarbon discharge and effective doses were made. Effective doses were calculated with two approaches: IAEA methodology and less conservative approach, considering the real food consumption in the Kursk region. The values of calculated doses by the second method (0.08–2.58 μSv) are more than 2 times less than IAEA approach (0.17–5.30 μSv). The highest difference between measured and background 14C in tree ring is 41.7 ± 5.8 pMC in 2014 during the restoration of graphite stack. The main contribution to 14С exposure in the considering period is caused by background – from 70 to 99%.

Graphenization of graphene oxide films for strongly anisotropic thermal conduction and high electromagnetic interference shielding

A macroscopic carbon film that has a “graphenic” structure (defect-free in-plane graphene lattice yet absence of Bernal type graphitic stacking order), may inherit the exotic properties of rotated/slipped few-layer graphene (e.g., high flexibility and anisotropic transport properties). However, the fabrication of such a film remains challenging and unreported. Here we show that the annealing of reduced electrochemical graphene oxide (REGO), which has fewer in-plane defects than the conventional graphene oxide from Hummers’ method, leads to a nearly intact graphene lattice (Raman ID/IG down to 0.03) without restoring the graphitic stacking order (I2D/IG > 1 and a single Lorentzian 2D peak). Atomic resolution transmission electron microscopy verifies the unique graphenic structure with random interlayer rotations. Density functional theory calculations imply the low defect density of REGO can suppress the interplanar interaction between undercoordinated carbon atoms, therefore inhibiting cross-plane migration of vacancies and the consequent establishment of stacking order. The annealed, graphenic REGO film is highly flexible and has an in-plane to cross-plane thermal conductivity ratio of 835 (among the highest reported for anisotropic thermal conductors). This high anisotropic ratio is attributed to the inhibited through-plane heat transport (0.26 W m−1 K−1) by the interlayer rotations and the high in-plane thermal conductivity (218 W m−1 K−1) enabled by intact graphene lattice. The graphenic films show high electromagnetic interference shielding effectiveness of 80.3 dB at a thickness of 53 μm and 42.4 dB at 5.7 μm, with an absolute effectiveness (>70000 dB cm2 g−1) superior to two-dimensional metal carbides (MXenes) and Al foil.

Evaluating the quality of surface carbonized woods modified with a contact charring or a gas flame charring technique

Surface carbonization, or charring, of wooden exterior cladding boards is a modification method that creates a fully organic barrier layer in resemblance to a coating. The process effectively degrades the wood and transforms it into a carbonaceous residue that protects the underlying unmodified wood from environmental stresses. The surface quality of wood modified in this manner is a combination of several factors and depends on the manufacturing method and wood species. To assess the quality of spruce and birch modified with contact and flame charring techniques, several experiments were set up from the nanoscale to macroscopic evaluation of surface resistance to different stresses. The changes in elemental composition are scaled with the modification severity with little differences between wood species. The carbon structures analyzed by high-resolution transmission electron microscopy (HR-TEM) were found to be amorphous, but the electron energy-loss spectroscopy (EELS) revealed higher ordering with what is assumed to be random graphitic stacking of carbon sheets. These carbon–carbon bonds are stable, so a higher ordering is hypothesized to induce improved resistance to exterior stresses. The scanning electron microscopy (SEM) revealed a clear difference between contact-charred and flame-charred woods. The selected contact charring temperature was not high enough to induce the transformation of cell walls from anisotropic into an isotropic material but provided other benefits such as a relatively crack-free, smooth and scratch resistant surface. Surface roughness was able to adequately predict the surface quality of the contact-charred samples, and scratch tests were found to be suitable for evaluating the mechanical stress resistance of the surface instead of abrasion. In terms of overall quality, birch instead of spruce was concluded to better respond to both charring methods, although contact charring eliminates some species-specific characteristics, resulting in more homogeneous surfaces.

Graphite rapidly forms via annihilation of screw dislocations

Graphite is the thermodynamically stable form of carbon and yet is remarkably difficult to synthesize. We show the annihilation of screw dislocations is critical for graphitization. These dislocations wind through the layers like a spiral staircase, inhibiting lateral growth of the graphenic crystallites (La) and preventing AB stacking of Bernal graphite. High-resolution transmission electron microscopy identifies screws as interdigitated fringes with narrow focal depth in graphitizing polyvinyl chloride. Molecular dynamics simulations of parallel graphenic fragments confirm that screws spontaneously form during heating, with higher annealing temperature driving screw annihilation and crystallite growth. The time evolution and kinetics of graphitization is tracked via X-ray diffraction, showing the growth of La and reduction of the interlayer spacing, consistent with screw annihilation. We find that graphite forms orders of magnitude faster than previously assumed, taking less than ten seconds at 3000 °C and just minutes at 2500 °C. This rapid transformation suggests major cost savings in synthetic graphite production, important for lithium-ion batteries and smelting electrodes. By reducing the time spent at ultra-high temperatures, energy costs and component degradation can be significantly lower.

Insertion of AlCl3 in graphite as both cation and anion insertion host for dual-ion battery

Dual-ion batteries (DIBs) are emerging energy storage devices with the unique energy storage mechanism in which both anions and cations are involved during charge-discharge processes. Graphite is usually used as both cathode and anode in DIBs, however, the limited layer spacing and the elongated diffusion channels make the ion migration resistance larger. To break the bottleneck, a strategy is proposed by incorporating AlCl3 into the layer of graphite, which increases the layer spacing while acting as a pillar to stabilize the structure. In addition, a significant fraction of defects is inducted at the edges of the folds of graphite. This engineering accelerates the electron/ion transport on the stable graphite stacking microstructure. As a result, it could be used as both cation and anion insertion host in DIBs. Electrochemical tests showed that AlCl3-GICs-5 materials could deliver a Li+ storage capacity of 169.1 mAh g−1 at 2A g−1 when cycles consistently for over 500 cycles which is much higher than graphite (60.7 mAh g−1). Furthermore, when used as a PF6− insertion cathode in a DIB, it showed a high specific capacity of 100.8 mAh g−1at 100 mA g−1. This research is anticipated to provide inspiration to design new materials for DIBs.

Calculation and Experimental Study of the Interaction Between a KZh Steel Structure and RBMK-1000 Graphite Stack

Calculation and Experimental Study of the Interaction Between a KZh Steel Structure and RBMK-1000 Graphite Stack

Обзор отечественного опыта и подходов по извлечению графита из уран-графитовых реакторов

The article overviews the existing experience in graphite removal from the stacks of Russian production, power and research uranium-graphite reactors (PUGR, RBMK, AMB, RFT, etc.). It considers the key challenges and some particular features associated with graphite stack dismantlement including the differences in reactor designs, degradation of graphite stacks due to long-term operation, etc. It presents some design solutions and approaches developed to extend the uranium-graphite reactor operation, as well as the experience in the fine-tuning of the graphite stack dismantlement methods.

Exploring interfacial interactions, dielectric, ferroelectric and piezoelectric properties of ultrahigh molecular weight polyethylene/graphene oxide nanocomposites

AbstractGraphene oxide (GO) incorporated ultra‐high molecular weight polyethylene (UHMWPE) nanocomposites were prepared by encapsulating GO by UHMWPE in an aqueous media via high‐shear mixing, which were subsequently dried and compression molded. Morphological characterizations via scanning electron microscopy revealed the intercalation of UHMWPE chains in the graphitic stacks corresponding to GO. Further, dielectric permittivity of UHMWPE/GO nanocomposite of 1 wt% GO showed a drastic increase (~61) as compared to pure UHMWPE (~2) due to an enhanced interfacial polarization. A significantly higher value of remnant polarization (~10 nC/cm2) and coercive field (~3 kV/cm) was observed in UHMWPE/GO nanocomposite of 1 wt% GO, which showed a strong hysteresis loop of polarization versus electric field plot as compared to pure UHMWPE, which displayed a very weak hysteresis loop. The piezoelectric coefficient (d33) of ~9.5 pm/V was estimated in UHMWPE/GO nanocomposite of 1 wt% GO via piezoresponse force microscopy. Nanocomposite sensor devices were also fabricated and piezoelectric output voltage of ~6 V was recorded in UHMWPE/GO nanocomposite of 1 wt% of GO. We report here for the first time the unique ferroelectric and piezoelectric properties displayed by UHMWPE/GO nanocomposites.

Possible high temperature superconducting transitions in disordered graphite obtained from room temperature deintercalated KC[formula omitted

Although progress with twisted graphene nano-devices is boosting the superconductivity that is the consequence of their Moiré flat electronic bands, the immense choice for future development is an obstacle for their optimisation. We report here that soft-chemistry deintercalation of KC8 breaks down graphite stacking generating a strong disorder that includes stacking twists and variable local doping. We obtain a bulk graphite whose individual crystallites have different stackings with arbitrary twists and doping, scanning in the same sample a huge number of stacking configurations. We perform magnetisation measurements on batches with different synthesis conditions. The disorder weakens the huge diamagnetism of graphite, revealing several phase transitions. A tiny “ferromagnetic-like” magnetisation appears with Curie temperatures T0∼ 450 K, that has to be subtracted from the measured magnetisation. Depending on sample synthesis, anomalies towards diamagnetic states appear at Tc∼ 110 K (3 samples), ∼ 240 K (4 samples), ∼ 320 K (2 samples). Electrical resistivity measurements yield anomalies for the Tc∼ 240 K transition, with one sample showing a 90% drop. We discuss the possibility that these (diamagnetic and resistive) anomalies could be due to superconductivity. As the amount of these phases is small by construction, fine-tuning of the process will be necessary to increase their fraction and select the desired phase.

Radiocarbon generation and atmospheric release assessment from nuclear power plant with RBMK type reactors

The article provides the results of the estimation of 14C nuclide generation and release from the systems and structures of RBMK-1500 reactors operated at Ignalina NPP. The main circulation circuit and gaseous circuit of this type of reactor are the primary sources of 14C generation and airborne release, evaluated to be 0.491 and 0.367 TBq per one calendar year (10.5 months of effective reactor operation at 1350 MWe power), respectively. The analysis of the operation specifics of these systems showed that they don’t provide an effective barrier to contain 14C due to the 14CO2 chemical form of radiocarbon in RBMK reactors, i.e., 100% release (0.858 TBq/year) have been evaluated available for atmospheric release. These results have been compared with the atmospheric releases evaluated from the 14C activity concentration measured in Scots pine tree (Pinus Sylvestris L.) ring samples by single stage accelerator mass spectrometer (SSAMS, NEC, USA) at Vilnius Radiocarbon facility, resulting to airborne release value of 1 TBq/year for one RBMK-1500 reactor. Slightly higher value reproduced from the tree ring samples indicates that the reactor graphite stack might be also a source of 14C releases of about 0.14–0.24 TBq/year. Based on similar design features the results of the study can be easily projected to the RBMK-1000 reactors rescaling 14C source term according to the lower reactor power.

A proposed approach for the evaluation of consequences of a large aircraft crash accident at an RBMK type reactor site during decommissioning

Nuclear power plants with RBMK reactors were designed in late sixties - early seventies and were built entirely in the European part of the Russian Federation, the Lithuanian Republic and the Ukrainian Republic. The plants in the Lithuanian and the Ukrainian Republics are in various stages of decommissioning. As currently planned, the most of RBMK reactors still operating in the Russian Federation will end their operation in this decade.The reactor building, spent nuclear fuel and radioactive waste storage facilities, built at the time of construction of RBMK plants are not specially designed to withstand the direct impact of an aircraft crash. There were no such regulatory requirements at the time the plants were designed and, no such requirements were introduced by regulatory bodies later on. But for now, in the aftermath of the September 11, 2001 terrorist attack on the United States, the event of an aircraft crash on a nuclear facility can no longer be considered as extremely unlikely.This paper considers the state of a nuclear power plant with the RBMK type reactor(s) during certain decommissioning and dismantling stages and discusses the approach for the evaluation of radiological consequences to the public in the event of a large aircraft crash on potentially hazardous objects of the plant. The damage to spent nuclear storage facilities, the reactor graphite stack and storage facilities for the combustible waste may cause a significant release of radionuclides to the environment. Key components of the approach include the evaluation of a structural damage to the facilities, the evaluation of a fire propagation and impact, the evaluation of a spent nuclear fuel cooling capability in storage pools, the evaluation of an airborne radionuclide release, the evaluation of a radionuclide dispersion in the environment and the evaluation of the public exposure. A set of widely used software and methods for the performance of the specific tasks is discussed. The paper also describes how the proposed approach was applied for the evaluation of radiological consequences from a Boeing 767-400 type aircraft crash at the Ignalina NPP site where a plant with two RBMK-1500 type reactors is decommissioned.The paper does not address modern radioactive waste and spent nuclear fuel management and storage facilities that might be built at RBMK sites as well. As a rule, assessment and mitigation of an aircraft crash accident consequences is an integral part of design of these new facilities.

Особенности пространственного распределения радионуклидов активационного происхождения в облученном графите

The paper presents the results of studies of the spatial distribution of radionuclides14C,36Cl and60Co, which are key for decommissioning uranium-graphite reactors, in graphite stacks and individual graphite blocks. The studies were carried out on a shutdown PUGR ADE-5 of «PDC UGR» JSC, which provides access to the graphite stack with the ability to extract individual graphite blocks. Based on the results of a detailed experimental study of graphite blocks retrieved in 2018, the new interpretations of some features of the spatial distribution of radionuclides in UGR graphite was proposed.

Анализ динамики выхода радионуклидов 14С и 36Сl из облученного графита РБМК-1000 в водных средах

The paper presents experimentally evaluated parameters of 14C and 36Cl leaching from irradiated RBMK-1000 graphite in aqueous media of various compositions, including distilled water and those stimulating the groundwater. To predict further radionuclide migration and to demonstrate the disposal safety, an array of initial data was formed presenting the characteristics of RBMK-1000 graphite stacks. The paper proposes an optimal approach describing numerically the function of the leaching dynamics. It also compares the calculated dynamics of 14C and 36Cl leaching from GR‑220 (PUGR) and GR-280 (RBMK-1000) graphite.

Peptide-Driven Exfoliation and Dispersion Mechanisms of Graphene in Aqueous Media.

Peptide-mediated exfoliation and suspension of graphene in aqueous media is a promising strategy for bioapplications such as drug delivery, tissue engineering, and biosensors. A few peptide sequences are known as graphene exfoliators/dispersants in water, but the mechanisms underpinning this process remain underexplored. Here, molecular simulations investigate two key steps: sheet exfoliation and subsequent sheet reunification, in aqueous media. Umbrella sampling simulations predict the energy required to separate a graphene sheet from a graphite stack in both the presence/absence of the graphene-exfoliant peptide, P1. The free-energy barrier for reunification of two P1-coated graphene sheets is similarly calculated. Under sonication, the benefit from the relatively lower free-energy barrier associated with exfoliation in the absence of the peptide is negated by its facile reunification postsonication. In contrast, although P1 slightly increases the energy barrier to exfoliation under sonication, the peptides confer high-energy barriers to sheet reunification, thus ensuring stable aqueous graphene dispersions.

First principles study on formation and migration energies of sodium and lithium in graphite

Graphite is used as an anode material in conventional lithium-ion batteries owing to its ability to form stable Li-intercalated graphite intercalation compounds (Li GICs). Its application to sodium-ion batteries has long been of great interest, but the instability of Na GICs hampers its implementation. First-principles calculations were performed to gain physical insight into the intercalation process in alkali-metal (AM) GICs, where $\mathrm{AM}=\mathrm{Li}$, Na. In this study, the structure, stability, and diffusion properties of AM GICs with various in-plane AM concentrations were systematically investigated using a van der Waals density functional simulation, and the differences between Li and Na GICs were discussed. Li GICs were found to be quite stable over a wide range of in-plane Li concentrations, with a change in the favorable stacking sequence of graphite. In terms of diffusion, the migration energy for Li in graphite increases as the graphite stacking transition occurs, suggesting that hindering the stacking transition could realize fast and uniform Li diffusion. In contrast, Na GICs are less stable than Li GICs because of following two reasons: (1) interaction between Na and carbon is less stable than that between Li and carbon, and (2) a larger amount of deformation in the interlayer distance is necessary. The Na GICs tend to be stabilized by increasing the number of Na-carbon interactions. Namely, fasted Na diffusion is expected in the Na-rich phase. Our systematic simulations of the formation energy and migration energy of Na GICs with different structures and in-plane AM concentrations suggested that the expansion of graphite layers prior to Na intercalation could achieve graphite anodes for Na-ion batteries.