Host Graphite Research Articles

Double-gradient host enabling bottom-up Li deposition towards hybrid lithium-ion/metal anode with long lifespan

The graphite-based hybrid Li-ion/metal anode holds great promise to be one of the ultimate anode choices, owing to its high specific capacity (often up to 500 mAh/g), obviously superior to 372 mAh/g of the commercial graphite anode. Unfortunately, Li deposition on the top surface of the conductive graphite host can easily drive Li dendrite growth, dead Li accumulation, and the blockage of Li+ transport pathways, leading to low host space utilization and cycling stability deterioration. Herein, a graphite host with lithiophilicity and reactive activity dual-gradient is constructed by integrating a surface insulation passivation and a bottom lithiophilicity modification to realize the “bottom-up” deposition behavior for hybrid Li-ion/metal anode. The conformal coating layer of electrical insulating and lithiophobic polymer can efficiently retard Li+ reduction and deposition on the top surface of the conductive host, while the decorated Ag nanoparticles with high lithiophilicity on the host bottom enable much lower Li nucleation barrier, thereby guiding the preferential bottom-up Li deposition. Li dendrite growth is effectively inhibited and the synergistic effects realize high space utilization of the host. Consequently, the hybrid graphite-Li anodes with 600 mAh/g of lithiation capacity (∼3.0 mAh cm−2) deliver significantly improved cycling stability over 500 cycles with a negligible capacity fading rate of 0.05 % per cycle at 1 C in LiFePO4-based full-cells (N/P ratio = 1.9).

Quantification of solvent-mediated host-ion interaction in graphite intercalation compounds for extreme-condition Li-ion batteries

Achieving simultaneous fast-charging capabilities and low-temperature adaptability in graphite-based lithium-ion batteries (LIBs) with an acceptable cycle life remains challenging. Herein, an ether-based electrolyte with temperature-adaptive Li+ solvation structure is designed for graphite, and stable Li+/solvent co-intercalation has been achieved at subzero. As revealed by in-situ variable temperature (−20 °C) X-ray diffraction (XRD), the poor compatibility of graphite in ether-based electrolyte at 25 °C is mainly due to the continuous electrolyte decomposition and the in-plane rearrangement below 0.5 V. Former results in a significant irreversible capacity, while latter maintains graphite in a prolonged state of extreme expansion, ultimately leading to its exfoliation and failure. In contrast, low temperature triggers the rearrangement of Li+ solvation structure with stronger Li+/solvent binding energy and shorter Li+–O bond length, which is conducive for reversible Li+/solvent co-intercalation and reducing the time of graphite in an extreme expansion state. In addition, the co-intercalation of solvents minimizes the interaction between Li-ions and host graphite, endowing graphite with fast diffusion kinetics. As expected, the graphite anode delivers about 84% of the capacity at room temperature at −20 °C. Moreover, within 6 min, about 83%, 73%, and 43% of the capacity could be charged at 25, −20, and −40 °C, respectively.

Highly stable aqueous Zn−I2 batteries enabled by the synergistic adsorption/conversion effect via porous graphitic iodine host

Aqueous zinc-iodide batteries (AZIBs) are considered to be the promising candidate after lithium-ion batteries due to their low cost and high safety. However, the poor electrical conductivity of iodine and the notorious shuttle effect brought about by resolvable polyiodides have seriously hindered the widespread application prospects of AZIBs. In this work, we develop a sample and effective strategy for synthesizing N-doped hierarchical porous graphitized carbon (CN-Mg-CaCO3) that shows great potential as a host material for iodine confinement. The unique porous structure with graphitic domains and synergistic effects of surface adsorption and efficient conversion contribute to the high performance of AZIBs, with notable specific capacity (185 mA h g−1 at 0.1 mA g−1), impressive rate capability (114 mA h g−1 at 10 A g−1), and excellent long-term cycling stability (85 % retention after 104 cycles at 5 A g−1). The elaborately constructed conductive carbon skeleton enhances iodine utilization, while the redox reaction of I2/I- occurs without the formation of intermediates I3-. Additionally, the presence of surface adsorbed Zn2+ leads to enhanced pseudo-capacitance. This research introduces innovative approaches and broadens new ideas for developing cathode materials for AZIBs.

Metal chloride‐graphite intercalation compounds for rechargeable metal‐ion batteries

AbstractThe typical metal chloride‐graphite intercalation compounds (MC‐GICs) inherit intercalation capacity, high charge conductivity, and high tap density from graphite, and these are considered as one of the promising alternatives of graphite anode in rechargeable metal‐ion batteries (MIBs). Notably, the special interlayer decoupling effects and the introduction of extra conversion capacity by metal chloride could greatly break the capacity limitation of graphite anodes and achieve higher energy density in MIBs. The optimization of both graphite host and metal chloride species with specific structures endows MC‐GICs with design feasibility for different application requirements of different MIBs, such as several times the actual capacity compared to graphite anodes, rapid migration of large carriers, and other properties. Herein, a brief review has been provided with the latest understanding of conductivity characteristics and energy storage mechanisms of MC‐GICs and their interesting performance features of full potential application in rechargeable MIBs. Based on the existing research of MC‐GICs, necessary improvements and prospects in the near future have been put forward.

Head-Mounted coating on graphite host enabling highly reversible Li Plating/Stripping in Lithium-ion/Lithium metal hybrid anode

Commercial graphite hosting excessive Li is a promising hybrid Li-ion/Li metal anode for high energy battery. Nevertheless, the issue of Li plating on the top surface of the graphite anode (Gra) is not well tackled, resulting in the uncontrollable growth of Li dendrites and accumulation of “dead Li”, as well as the deactivation of graphite particles. In this work, an insulating ethylene vinyl alcohol copolymer (EVOH) layer is head-mounted covered on the graphite anode surface (Gra-OH) via a simple spray coating method. Due to the electron retarding ability of EVOH film, the top surface of the graphite host is passivated and the excessive Li can be well accommodated inside the porous structure of graphite host. Meanwhile, the lithiophilic –OH group of EVOH can regulate the Li+ flux and render the uniform Li plating behavior. Featuring these, the Gra-OH anodes with a Li storage capacity of 600 mAh/g show high Coulombic efficiencies (CEs) in both coin-cell and pouch cell even under carbonate electrolyte condition. When operating in Li-ion working mode, higher capacity retentions are readily achieved in the half-cell, full-cell and 5 Ah NCM811 pouch-cell, further demonstrating the application compatibility of this surface passivated Gra-OH anode.

How Relevant Are SEI Studies from Half Cell Samples to Understand Aging Mechanisms in Potassium Batteries? An in-Depth Photoelectron Spectroscopy Study

Owing to a shared common technological basis, monovalent K-ion battery (KIB) systems follow in the footsteps of Li-ion batteries (LIBs) with the aim to develop a broader portfolio of available battery technologies and to put them on a more sustainable material basis. For example, both cations intercalate into a graphite host structure, which can be used in combination with carbonate-based electrolytes as negative electrode. Especially in half cells, the degradation of graphite electrodes in KIBs is significantly accelerated in comparison to LIBs. Previous surface studies demonstrated that the formation of a durable and protective solid electrolyte interphase (SEI), that is paramount for long cycle life in alkali-ion batteries, is a major bottleneck for KIBs because of less benign SEI properties[1][2].Recently reported crosstalk phenomena in half cell configurations due to the highly reactive potassium counter electrode make the analysis of surface layers a particular challenge [2][3]. In this presentation, a detailed characterization of the SEI formation on graphite was performed in both half and full cell configurations by in-house and synchrotron-based photoelectron spectroscopy (PES) in order to demonstrate fundamental differences in surface layer composition under the influence of a reactive counter electrode. While it is generally acknowledged that PES results show differences in SEI composition and thickness between both configurations, our results clearly demonstrate that in case of KIBs the changes become severe. This challenges interpretations of SEI layer properties made in half cells, as they are distinctly different from those in full cells. More importantly, this also affects interpretations on the major electrolyte degradation pathways in these systems. Using PES depth profiling and complementing results with gas chromatography, as well as DFT calculations, delivers new insights on the irreversible processes in KIBs. Reference s : [1] Naylor, et al., ACS Appl. Mater. Interfaces. 11 (2019) 45636–45645.[2] Allgayer et al., ACS Appl. Energy Mater. 5 (2022) 1136–1148.[3] Hosaka et al., ACS Energy Lett. 6 (2021) 3643–3649.

Acid-base encapsulation prepared N/P co-doped carbon-coated natural graphite for high-performance lithium-ion batteries

The incorporation of natural graphite as an anode material effectively enhances the lithium storage capacity of lithium-ion batteries. However, the lower specific capacity of graphite anodes and the slow dynamics limit the application of fast charging in daily life. In this work, we proposed an acid-base neutralization strategy to prepare N/P co-doped carbon coated natural graphite for high-performance lithium-ion batteries. Electrochemical testing shows that the prepared NG-PN@C electrode has a first discharge specific capacity of 429.61mAh/g and a high initial Coulombic efficiency (ICE) of 87.39 %, exhibiting a capacity three times higher than that of the commercial natural graphite at a rate of 3C. The enhanced performance can be attributed to the synergistic interplay between N/P co-doping and carbon coating. This interplay heightens the chemical reactivity of the graphite surface, an appropriate introduction of heteroatom into graphite (carbon) hosts is beneficial for Li heterogeneous nucleation and thus renders a uniform Li deposition. In addition, the stable carbon layer of about 3 nm can mitigate the volume expansion throughout cycling and improve rate capability and long-term cycling performance. This simple and low-cost acid-base neutralization strategy not only provides an eco-friendly method for the modification of graphite anodes, achieving doping of N/ P and coating a carbon layer on graphite but also offers inspiration for the advancement of alternative nanocomposites dedicated to improving their electrochemical properties.

On the Impact of Mechanics on Electrochemistry of Lithium-Ion Battery Anodes

Models exploring electrochemistry-mechanics coupling in liquid electrolyte lithium-ion battery anodes have traditionally incorporated stress impact on thermodynamics, bulk diffusive transport, and fracture, while stress-kinetics coupling is more explored in the context of all solid-state batteries. Here, we showcase the existence of strong link between active particle surface pressure and reaction kinetics affecting performance even in liquid electrolyte systems. Traction-free and immobile particle surface mechanical boundary conditions are used to delineate the varying pressure magnitudes in graphite host during cycling. Both tensile and compressive stresses are generated in traction-free case, while a fixed surface subjects the entire particle to a compression state. Pressure magnitudes are nearly two to three orders of magnitude higher for the latter resulting in significant depression of open circuit potential and improvement of exchange current densities compared to stress-free state. The results demonstrate the need for incorporating stress-kinetics linkage in models and provide a rationale for putting battery electrodes under compression to improve kinetics.

Design of High-Performance Defective Graphite-Type Anodes for Sodium-Ion Batteries

Intercalation of graphite with alkali metal ions, such as Li+, K+, Rb+, and Cs+ ions, has long been reported over past few decades. One exceptional candidate is Na+ ions in which they have very small solubility in graphite in the form of C70Nax (x ≤ 1), attributed to the positive formation energies. In this paper, we systematical design a series of high-defect graphite-type anodes through the ball-milling process. To our surprise, the electrochemically Na+ storage of the highly defective graphite was found to have a 3-fold increment, comparing to the pristine graphite. The hexagonal space group P63mmc was progressively converted into the rhombohedral space group R3̅m, attributed to the mechanical gliding during the ball-milling process. In additional large amount of atomic defects were found in two positions in the P63mmc space group. The average grain sizes of the particles would decrease with increasing the grinding time. The defective graphite samples disintegrated into the nanoscale flakes with enlarged interlayer spacings which could further promote the secondary formation of defects in the crystal structure. Some of the regions had highly distorted lattices in defective graphite. All these positive changes lead to a dramatic increase in the adsorption and intercalation of Na+ ions into the graphite host. The capacity of the high defect graphite was found to be 128.7 mAh g–1 at 0.1 A g–1, with a superior capacity retention over 8000 cycles at 5 A g–1. This work sheds some light on the design prospective of synthesizing defective graphite-type anodes for sodium-ion batteries.

Anisotropically tuning interfacial thermal conductance between graphite and poly(ethylene oxide) by lithium-ion intercalation: A molecular dynamics study

• Anisotropically enhanced thermal transport betw000een lithiated graphite and PEO. • Strong LJ interaction between lithiated graphite and PEO contributes to high ITCs. • The cross-plane ITC monotonically increases with increasing degree of lithiation. • Significantly enhanced in-plane ITC is only observed in LiC 6 /PEO system. Understanding interfacial thermal transport between lithiated two-dimensional materials and polymers can provide useful guidelines to design thermally tunable and conductive composites for efficient thermal management of energy systems. In this work, molecular dynamics simulations are conducted to investigate the effect of lithium (Li)-ion intercalation on the interfacial thermal transport between graphite and poly(ethylene oxide) (PEO). Results show that Li-ion intercalation anisotropically affects the enhancement of the interfacial thermal conductance (ITC) between graphite and PEO. The cross-plane ITC monotonically increases with increasing degree of lithiation, while a significant enhancement of the in-plane ITC is only observed when the graphite host is fully lithiated. The anisotropic effect can be attributed to the intercalation-induced strong Lennard-Jones interactions between the two contacting materials. This work may open up new opportunities to design thermally conductive polymeric composites and thermally controllable interfaces through ion intercalation of 2D materials.

(Digital Presentation) Electrochemical Measurements of Solvent Co-Intercalation Phenomenon Enabling Sodium-Ion Electrolyte Optimization for Graphite Anodes

Sodium-ion batteries (SIBs) is one of the more promising battery technologies that are starting to see commercialisation, were the low cost, abundance of necessary raw materials, higher safety and power and similar energy densities compared to lithium-ion batteries (LIBs) are often mentioned as their key advantages. SIBs, due to their great similarities with LIBs have seen a rapid development. One of the key differences, however, is the carbonaceous negative electrode, where SIBs, due to Na+ not forming stable intercalation compounds with graphite, the anode of choice for LIBs, are forces to use hard carbons. But, it was recently discovered by Jache et al. 1 that Na+ can form stable ternary graphite intercalation compounds through a solvent co-intercalation mechanism, thus enabling the use of graphite in SIBs. This sparked several follow-up studies exploring solvent co-intercalation as a way to use graphite in SIBs2, but the question of how many solvent molecules are intercalated along with the cation, and thus the stoichiometry of the redox reaction, was never settled. Here, we present a novel electrochemical method to directly measure the number of solvents that are brought along by the cation into the graphite host structure.The SIBs using the co-intercalation mechanism to enable the use of graphite has so far shown great kinetics, power densities and extreme cyclability, with great capacity retention over thousands of cycles2. They are, however, plagued by a low specific capacity, a great volume expansion, and high average voltage in half-cells vs. Na+/Na, leading to poor energy density. But, the knowledge of how many glymes entering the graphite host gained from our measurements enabled us to rationally design new electrolytes, composed of low amounts of glymes mixed with another co-solvent, that both increases the energy density of the system, by lowering the average voltage while retaining the capacity, and reduces the volume expansion, while also reducing the cost of the electrolyte (Figure 1). The impact of the new electrolytes on the structure of graphite is studies using operando dilatometry and XRD, while the local structure in the electrolyte is investigated with ab initio methods – together giving a comprehensive view of the system. Figure 1. Voltage profile of new electrolytes, showing a lowering of the average voltage a second plateau, and snap shot from ab initio molecular dynamics.

Maghemite nanoparticles decorated semiconducting graphitic carbon nitride hetero-structured nanocomposite: Facile synthesis, characterizations and its visible light active photocatalytic system for removal of hazardous organic pollutants from aqueous solutions

Hetero-structured nanocomposite consisting of graphitic carbon nitride host, decorated by γ-phase iron oxide, maghemite (γ-Fe2O3@G-CN) was synthesized in different mass contents of γ-Fe2O3 (5%, 10% and 20%) in the composite, by the method of nanosecond pulsed laser fragmentation, and defect engineering in liquid for the removal of hazardous organic pollutants from aqueous solutions. The elemental, structural, and morphological characterizations of the synthesized nanocomposites by XPS, XRD, TEM, and HR-TEM exhibited the proper anchoring of γ-Fe2O3 on the graphitic carbon nitride polymeric network. Also, the diffuse reflectance spectra, and the room temperature photoluminescence spectra respectively revealed that the presence of γ-Fe2O3 in graphitic carbon nitride brought about the enhanced and extended visible light absorption, and reduced recombination of photo induced electron hole pairs, the characteristics desired in a good photo-catalyst. Moreover, this composite material exhibited the z-scheme photo catalytic mechanism, where the more energetic electrons and holes in the composite band structure mediate the redox reaction in the photo-catalysis. The photo-catalytic efficiency of γ-Fe2O3@G-CN nanocomposite was evaluated in the photo-catalytic degradation of methyl blue (MB), and rhodamine-B (Rh-B) cationic dyes under visible light irradiations. For both the degradation processes, all the variants of γ-Fe2O3@G-CN nanocomposite consistently showed the enhanced photocatalytic efficiency with respect to pure GCN, with the maximum efficiency recorded for 10% γ-Fe2O3 in γ-Fe2O3@G-CN nanocomposite.

Comparing the Solid Electrolyte Interphases on Graphite Electrodes in K and Li Half Cells

In both Li-ion and K-ion batteries, graphite can be used as the negative electrode material. When potassium ions are stored electrochemically in the graphite host, the electrode capacities fade faster than in the lithium ion counterpart. This could be due to the high reactivity of the potassium metal counter electrode (CE) in half cells or a less stable solid electrolyte interphase (SEI) in the potassium case. Previous surface studies on graphite electrodes cycled in K half cells have focused on the SEI characteristics of different electrolyte formulations or different states of charge. In this study, we exploit the fact that graphite can store both lithium and potassium ions. Cell and component parameters have been largely maintained the same, with the only differences between Li and K half cells being the cation of the electrolyte salt and the alkali metal at the CE. The SEI layers formed under these conditions in either setup are studied using X-ray photoelectron spectroscopy with the aim to draw a direct comparison between the surface layers in both charged and discharged states. The results show a considerable crosstalk under OCV conditions between K-metal and the working electrode. Furthermore, the relative SEI layer composition after cycling varies considerably between Li and K half cells. Different dominant SEI species are present depending on the alkali metal used. The strong capacity fade observed in graphite–K half cells is likely linked to much smaller concentrations of inorganic compounds, such as KF, and increased amounts of organic compounds in the SEI.

Realizing the Multi-electron Reaction in the Na3V2(PO4)3 Cathode via Reversible Insertion of Dihydrogen Phosphate Anions.

Dual-ion battery (DIB) is an up-and-coming technology for the energy storage field. However, most of the current cathodes are still focused on the graphite hosts, which deliver a limited specific capacity. In this work, we demonstrated for the first time that H2PO4- can be used as the charge carrier for Na3V2(PO4)3 under an aqueous electrolyte, which enabled the V3+/V4+ and V4+/V5+ multielectron reactions in the Na3V2(PO4)3 electrode. The fabricated aqueous DIB delivers a high average voltage of ∼0.75 V (vs Ag/AgCl) and a high capacity of 280.7 mA h g-1. Moreover, the formed V5+-based novel cathode exhibits a capacity of 170.2 mA h g-1 in an organic sodium-ion battery. This study may open a new direction for fabricating high-voltage electrodes through the design of DIBs.

Gradient lithiation to load controllable, high utilization lithium in graphitic carbon host for high-energy batteries

The multiphase interaction between Li metal and carbon materials provides great opportunities to design advanced composite anodes for Li metal batteries. Herein, we report the conversion of single-sheet graphitic carbon papers (CPs) to function-gradient Li/C composite anodes with controllable formations of LiC6 and Li metal through controlling the thermal infiltration time. The uniform and dense lithiophilic LiC6 is formed first and gradually covers the entire CP host, which promotes the subsequent homogeneous infiltration of metallic Li into the CP. The resulting composite anode has a unique gradient structure, with one example consisting of a lithiated LiC6 coating of 5.8 mAh cm−2 and a thin Li bottom layer of 2.7 mAh cm−2. Such a gradient composite structure allows for the selective utilization of Li metal and LiC6 as the active component under different electrochemical conditions. This composite anode, with a much leaner Li loading than many reported Li/C composite anodes, has a stable porous framework and abundant lithiophilic sites, which enables uniform local electric field and stable Li metal deposition during cycling. Paired with high-capacity cathodes, the composite anode, with only a fraction of Li metal loading compared to Li foil, can provide better long-term cycling stability, and higher rate capability.

19F MAS NMR study on anion intercalation into graphite positive electrodes from binary-mixed highly concentrated electrolytes

Dual-graphite batteries (DGBs), which are based on anion intercalation into graphite positive electrodes, exhibit great potential for stationary energy storage due to use of more sustainable and low-cost electrode materials and processing routes. Binary-mixed highly concentrated electrolytes (HCEs) appeal highly suitable for the high operating voltages of DGBs, although the lack of sufficient insights into the formation of graphite intercalation compounds (GICs) limits the cell performance in terms of specific capacity and lifetime so far. Herein, anion intercalation from single-salt HCEs (LiPF 6 and LiBF 4 ) and an equimolar binary mixture of LiPF 6 /LiBF 4 are studied in graphite || Li metal cells, revealing an improved performance in terms of specific capacity and Coulombic efficiency in the order LiPF 6 > LiPF 6 /LiBF 4 > LiBF 4 . LiBF 4 -based cells exhibit an increased onset potential for anion intercalation and higher area specific impedance, suggesting an ineffective interphase formation at graphite. X-ray diffraction reveals GIC formation, while a lower stage number is achieved for the LiBF 4 -based HCE. 19 F MAS NMR spectroscopy analysis at various states-of-charge confirms no significant charge transfer between the intercalated anions and the graphite host and suggest preferred intercalation of PF 6 - compared to BF 4 - as well as a high translational and/or rotational mobility of the intercalated anions. • Anion intercalation into graphite from binary-mixed BF 4 ˉ/PF 6 - electrolytes. • Electrochemical performance: LiPF 6 /DMC > LiPF 6 /LiBF 4 /DMC > LiBF 4 /DMC. • 19 F MAS NMR spectroscopy is a powerful method to analyze BF 4 ˉ/PF 6 - intercalation. • Molar ratios of intercalated anions verify preferred PF 6 - intercalation. • High translational and/or rotational mobility of the intercalated anions.

Graphene Suspension from Modified Electrochemical Exfoliation of Graphite

Graphene has emerged as one of the most exciting materials since its discovery in 2004. Graphene is a term used for one to ten layers of sp2 hybridized carbon atoms arranged in a hexagonal pattern. Monolayer graphene is the building block of all graphitic structures and has high electron mobility, high thermal conductivity, high specific surface area, extremely high resistance to gas permeation and high optical transmittance. These exceptional properties make monolayer graphene attractive for potential applications as a catalyst or catalyst support in direct methanol fuel cells (DMFC), proton exchange membrane fuel cells (PEMFC), electrode in lithium-ion batteries, supercapacitors, as well as in thermally and electrically conductive sensors. AB-stacked bilayer and trilayer graphene have applications in the electronic industry, such as fabrication of field-effect transistors (FEts) and light detectors, due to an induced bandgap in the presence of an applied electric field. In addition, graphene finds major applications in the optoelectronic industry, such as electrode materials in solar cells, supercapacitors and transparent LCDs.Graphene is traditionally synthesized using the simple mechanical exfoliation of graphite, which is not scalable, or from chemical vapour deposition of a carbon source on a suitable substrate, which is both a complex and an expensive method. Hence, a scalable and economic graphene synthesis method is needed to harness the remarkable properties of graphene. The electrochemical exfoliation of graphite provides a promising route for the bulk production of graphene because it provides graphene flakes with the largest size and a smaller defect density than the well-known chemical synthesis methods such as Hummer or modified Hummer processes. The large size flakes resulting from electrochemical exfoliation are suitable for scalable electronic device fabrication methods. Electrochemical exfoliation proceeds via intercalation of the ions present in the electrolyte and is followed by the subsequent expansion and later exfoliation of graphite. However, this strategy produces graphene with a scatter in the layer numbers. A polydisperse suspension of graphene is undesirable because the electronic properties of graphene change with the number of layers. The intermediate graphite intercalation compounds are formed by the insertion of intercalants between the layers of the graphite host material. The intercalants can be atoms, ions or molecules. Graphite intercalation occurs in stages where the stage number indicates the number of graphene layers between the two nearest intercalant layers. In the present study, a modified electrochemical exfoliation route of graphite is explored, which proceeds through the exfoliation of a stage pure graphite intercalation compound followed by low-intensity ultrasonication. The modified electrochemical exfoliation of graphite is capable of providing a monodisperse graphene suspension

In Situ Visualization of Atmosphere-Dependent Relaxation and Failure in Energy Storage Electrodes.

Ambient atmosphere is critical for the surface/interface chemistry of electrodes that governs the operation and failure in energy storage devices (ESDs). Here, taking an Al/graphite battery as an example, both the relaxation and failure processes in the working graphite electrodes have been dynamically monitored by multiple in situ surface and interface characterization methods within various well-controlled atmospheres. Relaxation effects are manifested by recoverable stage-structure change and electronic relaxation occurring in anhydrous inert atmospheres and O2, which are induced by the anion/cation redistribution within the neighboring graphene layers and have slight influence on the long-term cycling. In contrast, rapid and unrecoverable failure behaviors happen in hydrous atmospheres as shown by the stage-structure degradation and electronic decoupling between guest ions and host graphite, which are caused by the hydrolysis between newly intercalated H2O molecules and intercalants. Consistent with the characterization results, exposure to H2O can cause nearly 100% capacity loss. The methodology and concept adopted in this work to unravel the battery mechanism under ambient conditions are universal and significant to investigate many ESDs.

Electron Microscopy Investigation of Graphite Electrodes from Fast-Charged Lithium-Ion Cells

Lithium-ion batteries (LIBs) capable of safe and reliable cycling at a fast-charge rate (i.e., >2C or <30 minutes to full charge) will have a direct impact on today’s consumer technology by improving the utility of portable electronics and reducing the barrier for widespread adoption of electric vehicles. However, at high charge rates, the long-term battery performance suffers because of heterogeneous reactivity across both electrodes (e.g., lithium metal plating on the anode surface, fracture of the cathode particles, etc.) [1]–[3]. A fundamental study of the processes responsible for battery degradation is a necessary step toward designing next-generation LIBs and is a critical research priority for the U.S. Department of Energy. Here, we demonstrate that high charging currents lead to chemical, structural, and morphological changes in the graphite anode that spans the atomic-scale to the bulk-scale. As the integrity of the graphite host is critical to lithium-ion cell performance, our findings regarding the degradation mechanism provide the essential knowledge to enable the development of viable fast-charging LIBs.Our strategies to study the graphite anode degradation are based on advanced analytical electron microscopy and diffraction. We focus on the modifications that occur in the material when subjected to charging rates up to 6C (i.e., 10 minutes to full charge). The characterization is performed post-mortem across different length-scales on as-prepared electrodes and electrodes harvested from cycled cells (represented in Figure 1). At the bulk-scale, we use scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) to probe the morphological and chemical changes in cross-sections of the graphite anode samples. At the nano- and atomic-scale, we use advanced transmission electron microscopy (TEM) and diffraction techniques to reveal changes in both the nano-scale chemistry and the graphite microstructure. Key findings from this investigation include the morphological changes to the graphite particles at the micron-scale and the partial-filling of the inter- and intra-particle gaps with solid electrolyte interphase (SEI) compounds at the meso-scale. Further, at the nano- to atomic-scale, we observe greater disorder and amorphous character in the graphite near the internal pore surfaces when compared to that of the bulk graphite regions. The nature of the lattice disorder, and the characterization methods employed to examine this disorder, will be discussed. Additional supporting evidences of graphite disorder and phase change from Raman spectroscopy and X-ray diffraction studies will also be presented. Acknowledgements: SP acknowledges support from the U.S. Department of Energy Graduate Student Research (SCGSR) program. The SCGSR program is administered by the Oak Ridge Institute for Science and Education for the U. S. Department of Energy under contract number DE‐SC0014664. This work was carried out in part in the Materials Research Laboratory Central Research Facilities, University of Illinois. DA and MTFR acknowledge support from DOE’s Vehicle Technologies Office. This document has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. References : [1] C. Heubner, K. Nikolowski, S. Reuber, M. Schneider, M. Wolter, and A. Michaelis, “Recent Insights into Rate Performance Limitations of Li-ion Batteries,” Batter. Supercaps, Nov. 2020.[2] M.-T. Fonseca Rodrigues et al., “Lithium Acetylide: A Spectroscopic Marker for Lithium Deposition During Fast Charging of Li-Ion Cells,” ACS Appl. Energy Mater., vol. 2, no. 1, pp. 873–881, Jan. 2019.[3] Y. Yang et al., “Quantification of Heterogeneous Degradation in Li-Ion Batteries,” Adv. Energy Mater., vol. 9, no. 25, p. 1900674, Jul. 2019. Figure 1

Solution Electrochemical Approach to Synthesis of Low Stage Graphite Bisulphate

Since their discovery in 1841, Graphite Intercalation Compounds (GICs) have found tremendous applications in various industrial and research fields such as supercapacitors, superconductors, heterogeneous catalysts, anode materials, hydrogen and lithium-ion storage and notably in graphene synthesis. GICs are a type of graphitic compounds formed by the insertion of a different atomic, ionic or molecular species, known as intercalants between the layers of graphite host. Intercalation occurs in stages, and the phenomenon of staging is characterised by stage index n, which is the number of carbon layers present between two successive intercalate layers. The concentration of intercalant in graphite increases as the stage index decreases.Oxyacids readily intercalate in graphite. Researchers have studied intercalation of graphite in a wide variety of such acids such as H2SO4, HNO3, H2SeO4, HClO4 and H3PO4, and concluded that sulphuric acid is the best intercalant for graphite owing to higher efficiency and lesser reaction times. Intercalation in sulphuric acid gives Graphite Bisulphate (GB). Conventional GB chemical synthesis methods, pose an environmental and safety hazard due to the usage of hazardous and explosive oxidants. Moreover, chemical methods such as Hummers’ are primarily oxidation routes for the synthesis of Graphene Oxide (GO) wherein GB is believed to be the process intermediate, which is difficult to isolate.Electrochemical synthesis of GB is inexpensive, scalable and provides an easy defect-free transfer to the application substrate due to its ability to deliver the product in a suspension. Furthermore, the electrochemical approach provides an environment- friendly solution to the synthesis of GB because it replaces harsh chemical oxidants with an external polarisation potential. The quantity of electricity passed can also be controlled by changing the applied potential to obtain desired stages of GB.However, the synthesis of low stage GB by the electrochemical process in aqueous sulphuric acid solutions is an arduous task as a result of the excess water present in the system. Water molecules react with surface C atoms to form bulky surface groups which hinder the process of intercalation. Furthermore, undissociated water molecules may also co-intercalate with the bisulphate ions from aqueous sulphuric acid. Electrochemical oxidation of this intercalated water causes reduction of bisulphate ions and ultimately results in the vigorous evolution of oxygen and sulphur dioxide, which exfoliates graphite. Thus, GB formed before exfoliation is disintegrated and lost.On the other hand, water also facilitates intercalation by opening up graphitic grain boundaries for intercalation by carrying out a nucleophilic attack on the edge sites. Thus an optimal concentration of water in aqueous sulphuric acid along with an optimal positive polarisation potential is required for the synthesis and stability of GB. The present study aims at the electrochemical synthesis and characterisation of low stage GB and determination of optimal reaction conditions for the process. A pure stage GB synthesised and isolated this way can be particularly useful for the synthesis of layer-specific graphene flakes. Intercalation reduces the force of attraction between layers of graphite and hence makes selective cleaving possible by sonication. The stage index is strongly correlated with the number of graphene layers produced upon electrochemical exfoliation and is thus a key parameter to tune the electrochemical exfoliation of graphite.