Ternary Graphite Intercalation Compounds Research Articles

Diglyme as a Promoter for the Electrochemical Formation of Quaternary Graphite Intercalation Compounds Containing two Different Types of Solvents

AbstractCo‐intercalation using ether‐based electrolytes renders graphite as a promising anode material in sodium‐ion batteries (SIBs). While most research on electrochemical solvent co‐intercalation in graphite has focused on linear ethers such as mono‐, di‐, tri‐, tetra‐, and penta‐glyme, we herein investigate the possibility of reversible electrochemical co‐intercalation with alternative solvents, especially cyclic ethers tetrahydrofuran (THF) and 1,3‐dioxolane (DOL), which show no signs of co‐intercalation on their own. We demonstrate, however, that this reaction becomes feasible when incorporating diethylene glycol dimethyl ether (2G, diglyme) as an additive. Operando X‐ray diffraction and ex‐situ ss‐NMR techniques are employed comprehensively to understand the co‐intercalation reaction of these cyclic ethers, along with Na+‐glymes, into graphite during cycling. When using these mixed electrolytes i. e., THF/2G and DOL/2G, the voltage profiles changes compared to the pure glyme‐based electrolyte, while showing comparable specific capacities and good long‐term durability. Overall, we propose that even trace amounts of diglyme prompt the co‐intercalation of THF and DOL into graphite layers. This leads to the formation of quaternary graphite intercalation compounds (q‐GICs), expanding beyond the realm of ternary graphite intercalation compounds (t‐GICs).

Co-Intercalation Batteries (CoIBs): The Use of TiS2 As Electrode for Solvent Co-Intercalation in Sodium-Ion Batteries

With the rising demand for lithium-ion batteries (LIBs), sodium-ion batteries (SIBs) have become an attractive alternative with the promise of improved safety, reduced cost, and lower environmental impact.1 However, the larger ion size of sodium cation has major consequence on the battery chemistry, which provides several challenges and opportunities for designing electrode materials and electrolyte solutions.2 Graphite is the currently most used anode material in LIBs. However, its use in SIBs, with traditional carbonate-based electrolytes, results in very poor specific capacities due to unfavorable thermodynamic processes.3 A strategy around this problem is to choose different solvents: Ethers and in particular glymes can exhibit a co-intercalation behavior into the graphite structure by forming ternary graphite intercalation compounds.4, 5 Although the co-intercalation of solvent molecules with sodium cation leads to a large increase in the graphite framework, the cycle life and rate capability of the reaction are excellent.Regardless of this, there is a lack of evidence for a comparable reaction with other electrode materials. Hence, it would be of interest to identify a cathode material that operates via this same mechanism. A battery based on solvent co-intercalation at both electrodes can have minimized charge-transfer resistances due to the absence of desolvation steps, and great cycle life. Thus, such a system can show high power density over many cycles. It has also been shown that stripping of the solvation shell can be the rate limiting step for battery operation, especially at subzero temperatures.6 Such a solvent co-intercalation battery (CoIB) could therefore be more energy efficient compared to other batteries or could enable a better low-temperature performance.Transition metal dichalcogenides (TMDs) have emerged as promising anode materials owing to their low cost, high electric conductivity, good thermal stability and environmental friendliness. Among them, titanium sulphide (TiS2), with a two-dimensional framework, exhibits several advantages such as a high conductivity (compare to other metal oxides), a larger interlayer distance (0.569 nm) compared to graphite (0.335 nm) and high stability. 7 Several papers have reported on the use of TiS2 as an anode material for SIBs, and observed that the voltage profiles and electrochemical behaviour of the system is highly dependent on the choice of electrolyte. However, this has seldomly been explicitly stated and the cause of the electrolyte dependence has never been investigated. By using operando and in-situ techniques such as Operando XRD and dilatometry, Na-diglyme complexes were showed to co-intercalate, similar as graphite, into TiS2. In contrast, no signs of solvent co-intercalation are observed in cyclic ethers and conventional carbonate-based electrolytes – where instead the redox reaction with TiS2 results in the formation of different NaxTiS2 compounds.As we identified TiS2 as “co-intercalation electrode” in glyme-based electrolytes it served to create the first ever solvent co-intercalation battery (CoIB – pronounced Co-IB) which was done by pairing TiS2 (positive electrode) with graphite (negative electrode).8 This work could enable the development of highly energy efficient batteries with minimum charge transfer resistances and References I. Hasa, S. Mariyappan, D. Saurel, P. Adelhelm, A. Y. Koposov, C. Masquelier, L. Croguennec and M. Casas-Cabanas, J. Power Sources, 2021, 482, 228872. P. Adelhelm, P. Hartmann, C. L. Bender, M. Busche, C. Eufinger and J. Janek, Beilstein Journal of Nanotechnology, 2015, 6, 1016-1055. B. Jache and P. Adelhelm, Angew. Chem., 2014, 126, 10333-10337. B. Jache, J. O. Binder, T. Abe and P. Adelhelm, Physical Chemistry Chemical Physics, 2016, 18, 14299-14316. M. Goktas, C. Bolli, J. Buchheim, E. J. Berg, P. Novák, F. Bonilla, T. f. Rojo, S. Komaba, K. Kubota and P. Adelhelm, ACS applied materials & interfaces, 2019, 11, 32844-32855. S. S. Zhang, K. Xu and T. R. Jow, Journal of Power Sources, 2003, 115, 137-140. Q. Yun, L. Li, Z. Hu, Q. Lu, B. Chen and H. Zhang, Adv. Mater., 2020, 32, 1903826. G. A. Ferrero, G. Avall, K. A. Mazzio, Y. Son, K. Janssen, S. Risse and P. Adelhelm, Adv. Energy Mater., 2022, 2202377. an improved low temperature performance, where the charge transfer can become rate limiting. Figure 1

Solvent Co-Intercalation for Electrochemical Energy Storage

One of the major differences between sodium- and lithium-ion batteries (SIBs and LIBs) is how the ions electrochemically react with graphite, where large amounts of lithium can be stored in the graphite galleries, but sodium cannot form thermodynamically stable, high capacity binary graphite intercalation compounds (b-GICs).[1] In 2014, however, it was discovered that graphite had a much greater capacity to reversibly store sodium-ions than previously thought – but only when using ether based electrolytes.[2] The increased storage capabilities in ether based electrolytes comes about by the formation of a ternary graphite intercalation compound (t-GIC), whereby the sodium-ion is intercalated along with it solvation shell. Thus, graphite can store solvated sodium-ions, but not bare ones. This unconventional mechanism is already known to occur for certain solvents, such as propylene carbonate, but was always thought to be accompanied by complete destruction of the graphite.[3]Since then, several studies have been conducted on electrochemical solvent co-intercalation for energy storage,[4] but the topic is still unexplored. Several fundamental questions have received quite conflicting answers – Especially with regards to the SEI and the reaction mechanism - the system displaying both clear staging in graphite, with at least one well defined plateau for stage I formation, but also large pseudocapacitive features. Following development on electrochemical double-layers and confined electrolytes,[5] it appears in systems where reversible electrochemical co-intercalation occurs the system will display both clear faradaic and non-faradaic properties. This is indeed what is seen, as the system offers a moderate amount of energy density, similar to batteries, while still being highly reversible and extremely fast, even at low temperatures, similar to capacitors.[6,7] Moreover, the reaction is tunable by altering the electrolyte composition - opening up for a great deal of exploration.[8] These properties, along with using abundant materials, makes the system applicable to problems where both the properties of capacitors and batteries are needed, such as in grid storage.This talk purport to summarize our research activity on electrochemical solvent co-intercalation phenomena, ranging from fundamental questions such as how to detect solvent co-intercalation, to the nature of the electrochemical reaction. We also present our work on using solvent co-intercalation in full cells, as well as a new type of battery where both the anode and cathode operate by a solvent co-intercalation mechanism – the solvent co-intercalation battery (CoIB).[9] References [1] O. Lenchuk, P. Adelhelm, D. Mollenhaur, Phys. Chem. Chem. Phys., 21 (2019) 19378-19390.[2] B. Jache, P. Adelhelm, Angewandte Chemie, 126 (2014) 10333-10337.[3] K. Xu, Chemical reviews 104 (2004), 4303-4418[4] J. Park, Z-L. Xu, K. Kang, Frontiers in Chemistry, 8 (2020), 2296-2646[5] S. Fleischmann, Y. Zhang, X. Wang, P. T. Cummings, J. Wu, P. Simon, Y. Gogotsi, V. Presser, V. Augustyn, Nature Energy, 7 (2022),222-228.[6] J. Chen, Y. Peng, Y. Yin, Z. Fang, Y. Cao, Y. Wang, X. Dong, Y. Xia, Angew. Chem. Int. Ed. 60 (2021) 23858.[7] Z-L. Xu, G. Yoon, K-Y. Park, H. Park, O. Tamwattana, S. J. Kim, W. M. Seong, K. Kang, Nat. Commun. 10 (2019) 2598.[8] B. Jache, J. O. Binder, T. Abe, P. Adelhelm, Phys. Chem. Chem. Phys., 18 (2016) 14299-14316[9] G. A. Ferrero, G. Åvall, K. A. Mazzio, Y. Son, K. Janßen, S. Risse, P. Adelhelm, Adv. Energy Mater. 12 (2022) 2202377

In Situ Pore Formation in Graphite Through Solvent Co‐Intercalation: A New Model for The Formation of Ternary Graphite Intercalation Compounds Bridging Batteries and Supercapacitors

AbstractFor Li‐ion and Na‐ion batteries, the intercalation behavior of graphite anodes is quite different. While Li‐ions intercalate, Na‐ions only co‐intercalate with solvent molecules from the electrolyte solution leading to ternary graphite intercalation compound (t‐GIC) formation along with an expansion of the graphite interlayer spacing to 1.2 nm. This large interlayer spacing represents a micropore with parallel slit geometry. Little is known about t‐GIC formation, but it is commonly believed that throughout the reaction the ion is accompanied by either a full or partial solvation shell. Here, it is elucidated for the first time, using two independent methods – mass measurements and electrochemical impedance spectroscopy – supplemented by operando microscopy, entropymetry and simulations, that the storage mechanism is far more complex. A new model for the electrochemical solvent co‐intercalation process is proposed: As soon as solvated ions enter, the graphite structure is flooded with free solvents, which are subsequently replaced by solvated ions. Close to full sodiation, few free solvents remain and structural rearrangement take place to reach the full storage capacity. Thus, t‐GICs represent a unique case of switchable microporous systems and hence appear as a bridge between ion storage in the bulk phase and in micropores, i.e., between batteries and supercapacitors.

Inner Lithium Fluoride (LiF)-Rich Solid Electrolyte Interphase Enabled by a Smaller Solvation Sheath for Fast-Charging Lithium Batteries

Fast-charging lithium-ion batteries (LIBs) have been severely hampered by the slow development of their electrolytes. Herein, we demonstrate that the size effect of solvent sheath would pose a great effect on the fast-charging performance of LIBs. Three similar ethers, including diethyl ether (DEE), dipropyl ether (DPE), and dibutyl ether (DBE), were mixed with 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) and lithium bis(fluorosulfonyl)imide (1 M) to form an electrolyte for fast-charging LIBs, respectively. The results showed that it is more difficult to form ternary graphite intercalation compounds (GICs) in the electrolyte with a larger solvation sheath. The DEE electrolyte can form stable GICs and generate an inner LiF-rich solid electrolyte interphase (SEI), lowering the diffusion barrier of Li+. Therefore, the graphite anode powered by the DEE electrolyte can maintain a capacity of 190 mAh g-1 at 4 C after 500 cycles. This kind of size effect of solvation sheath is also applicable to lithium metal batteries.

(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.

(Digital Presentation) The Role of the Electrolyte in the Charge Storage Mechanism of TiS2 in Sodium-Ion Batteries

The development of high-performance energy storage devices has recently attracted much attention for applications such as electrical vehicles, grid storage and portable devices. In this sense, sodium-ion batteries (SIBs) has appeared as an alternative to traditional lithium-ion batteries as a result of the availability, lower environmental impact and reduced cost of the battery due to the typical SIB chemistry.1 Even though graphite is a well-established and highly commercialized anode material, its application in SIBs suffers from several drawbacks, such as the practical impossibility of using traditional carbonates-based electrolytes due to unfavorable thermodynamic behavior – resulting in low specific capacities. Despite this, certain solvents (such as ethers) allow the use of graphite in SIBs through the formation of ternary graphite intercalation compounds by a co-intercalation reaction, showing excellent cyclability and kinetics. However, its capacity at 100-150 mAh g-1 is still low compare to other anode materials, in addition to showing large volume expansion. 2, 3 Transition metal dichalcogenides (TMDs) have emerged as promising anode materials owing to their low cost, high electric conductivity, good thermal stability and environmental friendliness. Among them, titanium sulphide (TiS2), with a two-dimensional framework, exhibits several advantages such as a high conductivity (compare to other metal oxides), a larger interlayer distance (0.569 nm) compared to graphite (0.335 nm) and high stability. 4 Several papers have reported on the use of TiS2 as an anode material for SIBs, and observed that the voltage profiles and electrochemical behaviour of the system is highly dependent on the choice of electrolyte, yet this has seldomly been explicitly stated, nor the cause of the electrolyte dependence investigated.Here, we demonstrate that depending on the electrolyte employed, an electrochemical co-intercalation reaction, similar to those reported with graphite as active material can also occur. Galvanostatic charge-discharge experiments were first performed to assess differences between different electrolytes (linear ethers, cyclic ethers and carbonates). Here, differences were found on the voltage profiles among the linear ethers and the rest of the solvents employed. By using in-situ and ex-situ techniques, a huge expansion on the TiS2 layers was found when diglyme was the solvent used. It was concluded here that co-intercalation of the solvent with sodium was the mechanism that occurs. On the contrary, when a cyclic ether and/or a mixture of carbonates were used, a lower expansion of the layered structure occurred and remained constant during the cycling. Dilatometry experiments were used to confirmed these mechanisms. Thus, with this technique, it is possible to monitor the variation on the thickness of the electrode during cycling, i.e. operando. In this sense, a greater expansion was observed when diglyme was used as the solvent electrolyte.Finally, DFT was used to study the solvents investigated, and their interaction with the sodium cation. It was found that among the solvents, diglyme produced by far the smallest and most stable solvation shells, making them prime candidates for co-intercalation studies. Comparing the size and stability of the solvation shell, with the interlayer binding energy of TiS2, we found it was energetically favorable to expand the lattice and allow a solvation shell into the host structure, compared to forming a bare ion. Figure 1. Illustration on the variation of the layer distance from pristine to sodiated diglyme. D. Kundu, E. Talaie, V. Duffort and L. F. Nazar, Angew. Chem. Int. Ed., 2015, 54, 3431-3448.B. Jache, J. O. Binder, T. Abe and P. Adelhelm, Phys. Chem. Chem. Phys., 2016, 18, 14299-14316.M. Goktas, C. Bolli, J. Buchheim, E. J. Berg, P. Novák, F. Bonilla, T. f. Rojo, S. Komaba, K. Kubota and P. Adelhelm, ACS applied materials & interfaces, 2019, 11, 32844-32855. Q. Yun, L. Li, Z. Hu, Q. Lu, B. Chen and H. Zhang, Adv. Mater., 2020, 32, 1903826. Figure 1

Preparation of grafted few-layer graphene from K-THF-graphite intercalation compounds by the addition of aldehyde

The preparation of graphene with large area, few layers, and without oxidation is required. Thus far, we have determined that by synthesizing graphite intercalation compounds (GICs) and adding water to them, it was possible to prepare few-layer graphene without oxidation; however, its area was small. Therefore, we investigated the addition of aldehyde (with different alkyl chain lengths containing electron-withdrawing groups, which allowed the use of an electrophilic addition reaction on carbon hexagonal layers) to GICs to prepare few-layer graphene with a large area and without oxidation. In this study, stage-1 ternary graphite intercalation compounds (K-THF-GICs) were synthesized using the solution method. Aldehyde solvents with different alkyl chain lengths were then added to K-THF-GICs, and the mixtures were stirred for 72 h. An attempt was made to prepare few-layer, small-area graphene with little oxidation. Results indicated that the addition of aldehyde containing an electron-withdrawing group to GICs promoted exfoliation with the electrophilic addition reaction on the carbon hexagonal layer, producing few-layer graphene. UV–vis (Ultraviolet･Visible Absorption Spectroscopy) results showed that solvents with long alkyl chains were more suitable for preparing few-layer graphene than solvents with short alkyl chains. According to Raman spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy, graphene exfoliated by decanal or octadecanal as three and two layers, respectively, was minimally oxidized and showed an area of several tens of μm2. Comparative analysis of the heat treatment of few-layer graphene demonstrated that it was possible to graft few-layer graphene exfoliated using decanal or octadecanal. Such grafting may be involved in exfoliation. Few-layer graphene was then prepared by adding an aldehyde solvent containing long alkyl chains to K-THF-GICs. Using octadecanal containing the longest alkyl chain among the solvents tested in this study, few-layer graphene with a thickness of 1.8 nm and an average area of 84 µm2 was obtained with little oxidation.

Graphene-Silica Hybrids Fillers for Multifunctional Solution Styrene Butadiene Rubber

A nanohybrid filler of functionalized graphene created with eco-friendly coupling agents (ECAs) and silica was fabricated by decorating silica onto graphene through a sol-gel process. To do this, relatively large high-quality graphene flakes with an extremely low oxygen content were fabricated using a ternary graphite intercalation compound method. The ECAs with hydrophobic and the hydrophilic groups mitigate graphene aggregation while also acting as an interfacial or bridging material. Furthermore, graphene-silica nanohybrids (GSNs) were incorporated by compounding to solution-styrene-butadiene rubber (SSBR) through mechanical mixing. The GSN-embedded SSBR matrix shows remarkable enhancements in the mechanical properties, thermal and electrical conductivities, and the gas barrier properties, even at a low loading rate, due to the efficient dispersion of GSNs, which enhances the interaction with the rubber matrix. The incorporation of GSNs improved the storage modulus considerably upon evolution of the filler network and with the creation of a crosslinking network of the composites.

Deciphering the paradox between the Co-intercalation of sodium-solvent into graphite and its irreversible capacity

While sodium ion (Na+) itself cannot intercalate into graphitic structure to form binary graphite-intercalation-compounds (GICs) as its alkaline brothers (lithium, potassium etc.) do, the intercalation of its solvated form [Na(solvent)x]+ can readily happen, leading to reversible formation of ternary GICs at stage 1, as long as used solvent molecules are stable against reduction, such as ethers. However, a mystery still remains: what causes the initial irreversible capacity loss given that co-intercalation requires an interphase-free surface? In this work, we answered this fundamental question by elaborately examining the solvation structures, desolvation process and the corresponding electrochemical reduction stability of ether-based solvated Na+. Based on the understanding, a simple and effective approach was proposed to significantly increase the initial coulombic efficiency of sodium-ether co-intercalation. Such precise control over solvation-sheath and interfacial structures provides molecular-level guidance to the designing of new electrolyte systems and graphite-intercalation chemistries.

Versatile and Tunable Electrical Properties of Doped Nonoxidized Graphene Using Alkali Metal Chlorides.

With the rapid development of wearable and flexible electronics, graphene has been intensively studied for the transparent, hole transport electrode layer (HTL) of field-effect transistors, light-emitting diodes, and organic photovoltaic (OPV) cells. To modulate the sheet resistance and the work function of graphene as a HTL, the surface doping is versatile while retaining high transparency. In this work, we used a chemical doping method to control the charge carrier density, band gap, and work function of graphene with minimizing the damage of the carbon network, for which metal chlorides (NaCl, KCl, and AuCl3) were used as chemical dopants. The high-quality graphene flakes were synthesized with large lateral sizes of more than 5 μm using ternary graphite intercalation compounds. Interestingly, the AuCl3-doped graphene flake film with a film thickness of about 20 nm showed the lowest reported sheet resistance of ∼249 Ω/sq with ∼75% transmittance. Furthermore, it could control the work function from 4.32 to 5.1 eV. The interfacial dipole complexes of metal cations with a low work function and the reactive radicals such as -OH were discussed to explain this result. For the practical application, an OPV device using the AuCl3-doped graphene flake film as the HTL was fabricated and it demonstrated enhanced power conversion efficiency while maintaining high optical transparency in visible light.

Tailoring sodium intercalation in graphite for high energy and power sodium ion batteries

Co-intercalation reactions make graphite as promising anodes for sodium ion batteries, however, the high redox potentials significantly lower the energy density. Herein, we investigate the factors that influence the co-intercalation potential of graphite and find that the tuning of the voltage as large as 0.38 V is achievable by adjusting the relative stability of ternary graphite intercalation compounds and the solvent activity in electrolytes. The feasibility of graphite anode in sodium ion batteries is confirmed in conjunction with Na1.5VPO4.8F0.7 cathodes by using the optimal electrolyte. The sodium ion battery delivers an improved voltage of 3.1 V, a high power density of 3863 W kg−1both electrodes, negligible temperature dependency of energy/power densities and an extremely low capacity fading rate of 0.007% per cycle over 1000 cycles, which are among the best thus far reported for sodium ion full cells, making it a competitive choice in large-scale energy storage systems.

Overview on the intercalation of gold into graphite

The intercalation of gold into graphite has been achieved thanks to a solid-liquid method using potassium metal-based molten alloys. Numerous syntheses have been performed varying the reactive alloy composition, the reaction temperature and the reaction time. The optimization of these parameters allowed to synthesize three novel ternary graphite intercalation compounds (GIC) containing poly-layered intercalated sheets, called alpha, beta and gamma. Thanks to a specific X-Ray diffraction study, the c-axis stacking sequence of the three compounds was determined. The α-GIC, which is a metastable compound, contains mixed potassium and gold two-layered intercalated sheets. The β-GIC and γ-GIC contain very thick well ordered three-layered and five-layered sheets respectively. The very high repeat distances of the latter compounds are clearly explained by the amount of intercalated metals, precisely determined using nuclear microprobe analyses.

Temperature-Induced Activation of Graphite Co-intercalation Reactions for Glymes and Crown Ethers in Sodium-Ion Batteries

The intercalation of solvated ions into graphite leads to ternary graphite intercalation compounds (t-GICs). Here, we study the impact of temperature on the electrochemical activity of graphite ele...

A New and Facile Route Using Electride Solutions To Intercalate Alkaline Earth Ions into Graphite

A series of new ternary graphite intercalation compounds (GICs) containing alkaline earth metal cations (M = Mg, Ca, Sr, Ba) and ethylenediamine (en) are reported. These GICs are deep blue to green in color and can be prepared as phase-pure compounds by the direct reaction of graphite powder with the metal in liquid en at mild temperatures (25–100 °C) under an inert atmosphere. X-ray diffraction and thermal analyses were employed to determine the structural and compositional details. [Mg2(en)2.0]C26 and [Ba2(en)2.0]C34 can be obtained as stage-1 GICs with gallery expansions of 0.55 and 0.46 nm, respectively, indicating the presence of intercalate monolayers with en cointercalates oriented perpendicular to the encasing graphene layers. Reactions with Ca and Sr metals form [Ca(en)2.0]C26 and [Sr(en)2.0]C22, which are stage-1 GICs with intercalate bilayers and gallery expansions of 0.76 nm. Titration indicates that each metal intercalate is associated with a 2-electron reduction process. Details on the effec...

Ethers Illume Sodium‐Based Battery Chemistry: Uniqueness, Surprise, and Challenges

AbstractSodium‐ion batteries (SIBs) have the potential to be practically applied in large‐scale energy storage markets. The rapid progress of SIBs research is primarily focused on electrodes, while electrolytes attract less attention. Indeed, the improvement of electrode performance is arguably correlated with the electrolyte optimization. In conventional lithium‐ion batteries (LIBs), ether‐based electrolytes are historically less practical owing to the insufficient passivation of both anodes and cathodes. As an important class of aprotic electrolytes, ethers have revived with the emerging lithium‐sulfur and lithium‐oxygen batteries in recent years, and are even booming in the wave of SIBs. Ether‐based electrolytes are unique to enabling these new battery chemistries in terms of producing stable ternary graphite intercalation compounds, modifying anode solid electrolyte interphases, reducing the solubility of intermediates, and decreasing polarization. Better still, ether‐based electrolytes are compatible with specific inorganic cathodes and could catalyze the assembly of full SIBs prototypes. This Research News article aims to summarize the recent critical reports on ether‐based electrolytes in sodium‐based batteries, to unveil the uniqueness of ether‐based electrolytes to advancing diverse electrode materials, and to shed light on the viability and challenges of ether‐based electrolytes in future sodium‐based battery chemistries.

Extremely large, non-oxidized graphene flakes based on spontaneous solvent insertion into graphite intercalation compounds

Demand for an effective strategy for exfoliating layered materials into flakes without perturbing their intrinsic structure is growing. Herein, we introduce an effective fabrication method of large-sized non-oxidized graphene flakes (NOGFs) as a representative example of a general strategy using spontaneous insertion of exfoliating medium into a layered material. We fabricated a ternary graphite intercalation compound (t-GIC) with stoichiometry of KC24(THF)2, and analyzed its morphology and electronic structure through experimental and computational approach. Interactions between the t-GIC and aprotic organic solvents with different polarities were investigated, where a unique swelling behavior was observed with dimethyl sulfoxide (DMSO). Based on the analysis of the phenomena, we demonstrate facile exfoliation of the t-GIC in polyvinyl pyrrolidone (PVP)-DMSO solution for fabrication of highly crystalline and large-sized NOGFs. The lateral size of the NOGFs ranges over 30 μm, while the 98% having thickness below 10 layers. The NOGF film exhibits supreme electrical conductivity of 3.36 × 105 S/m, which is, to our best knowledge, the highest value for a thin conductive film made of graphene flakes.

Pillared graphite anodes for reversible sodiation

There has been a major effort recently to develop new rechargeable sodium-ion electrodes. In lithium ion batteries, LiC6 forms from graphite and desolvated Li cations during the first charge. With sodium ions, graphite only shows a significant capacity when Na+ intercalates as a solvated complex, resulting in ternary graphite intercalation compounds (GICs). Although this chemistry has been shown to be highly reversible and to support high rates in small test cells, these GICs can require >250% volume expansion and contraction during cycling. Here we demonstrate the first example of GICs that reversibly sodiate/desodiate without any significant volume change. These pillared GICs are obtained by electrochemical reduction of graphite in an ether/amine co-solvent electrolyte. The initial gallery expansion, 0.36 nm, is less than half of that in diglyme-based systems, and shows a similar capacity. Thermal analyses suggest the pillaring phenomenon arises from stronger co-intercalate interactions in the GIC galleries.

Structure and Dynamic Behavior of the Na–Crown Ether Complex in the Graphite Layers Studied by DFT and 1H NMR

Diffusion of alkali metals in graphite layers is significant for the chemical and electrochemical properties of graphite intercalation compounds (GICs). Crown ethers co-intercalate into graphite with alkali metal (Na and K) cations and form ternary GICs. The structures and molecular dynamics of 15-crown-5 and 18-crown-6 ether coordinating to Na+ or K+ in GICs were investigated by DFT calculations and 1H solid state NMR analyses. DFT calculations suggest a stacked structure of crown ether–metal complex with some offset. 1H NMR shows two kinds of molecular motions at room temperature: isotropic rotation with molecular diffusion and axial rotation with fluctuation of the axis. The structure and dynamics of crown ether molecules in GIC galleries are strongly affected by the geometry of the crown ether molecules and the strength of the interaction between alkali metal and ligand molecules.

Observation of the intercalation of dimethyl sulfoxide-solvated lithium ion into graphite and decomposition of the ternary graphite intercalation compound using in situ Raman spectroscopy

Dimethyl sulfoxide (DMSO) is electrochemically co-intercalated with lithium ions into graphite to form a stable ternary graphite-intercalation-compound (GIC), Li-DMSO-GIC. To elucidate the details of the co-intercalation behavior, in situ Raman spectroscopy was performed to observe the structural variations in graphite during the potential scan. During the scan from 2.44 to 1.00 V, DMSO was co-intercalated with lithium ions into graphite, exhibiting a staged structure. The stage number decreased from 4 to 1. During the scan from 1.00 to 2.44 V, the deintercalation of DMSO-solvated lithium ions was observed. After experienced a potential cycle between 2.4 and 0 V, the graphite exhibited significant changes that are ascribed to the exfoliation of graphite, implying that the destruction of the graphite composite electrode is likely to be caused by the decomposition of DMSO within the graphite layer.