Potassium-graphite Intercalation Compounds Research Articles

Strategic approaches to observing 39K NMR signals from potassium–graphite intercalation compounds

Abstract Solid-state nuclear magnetic resonance is an invaluable tool for potassium–graphite intercalation compounds, promising anode materials of K-ion batteries, but it has not yet been applied because of several issues. We attempted 39K NMR measurements of potassium–graphite intercalation compounds sealed in glass using an 18.8 T NMR spectrometer with a nuclear magnetic resonance probe adjusted to the samples. The first observed 39K NMR signal of potassium–graphite intercalation compounds showed the possibility of evaluating intralayer density, which cannot be ascertained from Raman measurements.

Carbon coating engineering enhances the stability of graphite anode in potassium ion batteries

Graphite forms reversible potassium-graphite intercalation compounds electrochemically. Graphite anode possesses a high reversible specific capacity (279 mAh g−1) and an optimal low potential platform, rendering it an ideal anode material for potassium-ion batteries. However, the intercalation of K+ ions in graphite causes a bulk volume expansion of up to 61%, resulting in irreversible damage to the structure of graphite, which leads to inferior cycling stability. In this paper, surface-oxidized graphite (OG) with reduced particle size and increased defect densities was obtained by mechanical ball milling and chemical oxidation. Composites of soft carbon-coated OG (OG@PC) were prepared by soft carbon coating. The soft carbon coating can alleviate the bulk expansion caused by K+ intercalation/deintercalation in OG, thus slowing down the destruction of the crystalline structure of graphite after multiple charging/discharging cycles. OG@PC exhibits increased reversible specific capacity from 267 mAh g−1 to 314 mAh g−1 compared with pristine graphite at a current density of 0.05 A g−1. Moreover, at a high current density of 0.5 A g−1, a reversible specific capacity of 213 mAh g−1 was achieved after 200 cycles. This work investigates simple surface carbon coating strategies to improve the cycling stability of a graphite anode of potassium-ion batteries.

Advancements and Prospects of Graphite Anode for Potassium-Ion Batteries.

Potassium-ion batteries (KIBs) have recently attracted considerable attention owing to their resource abundance, low cost and environmental friendliness. Graphite as a mature commercial anode material for lithium-ion batteries, has been proved as a promising anode candidate for KIBs by reversible forming potassium-graphite intercalation compounds. However, large volume expansion and sluggish K+ kinetics caused by the incompatibility between large radius of K+ and the small interlayer spacing of graphite, result in the poor cycle stability and rate performances, hindering its practical application. Extensive research efforts have focused on improving the potassium storage performance of graphite anodes. This review provides an overview of recent advances in addressing these challenges and optimizing the electrochemical performance of graphite anodes for KIBs. Various strategies to improve the electrochemical performance of graphite and graphitic carbon anodes, such as microcrystalline regulation, heteroatom doping, morphological adjustment, and coating modification, are discussed, while the critical issues and challenges associated with graphite anodes and the prospects for their advancement in KIBs are highlighted. The review offers valuable guidelines for rational structural design and promotes the commercial development of high-performance graphite anode materials for KIBs.

Potassium-enriched graphite for use as stable hybrid anodes in high-efficiency potassium batteries

Potassium-ion batteries (PIBs) have attracted significant attention due to their low cost and high energy density. Despite being a promising anode candidate for PIBs, graphite is still plagued by limited capacity and fast capacity degradation with low coulombic efficiency (CE). Herein, we propose a high-capacity and highly stable potassium-enriched graphite (KRG) anode which surpasses the limitation of parent graphite. The KRG is characteristic of possessing the potassium-graphite intercalation compounds as a potassiophilic host for uniform and high-efficiency plating/stripping of potassium metal. The robust mixed conducting network possessed by the KRG is beneficial for mitigating drastic volume change of the electrode and improving ion/electron transport. A specific capacity of ∼520 mAh/g was demonstrated for KRG anode as the optimized value to maximize the capacity storage while suppressing dendrites growth. With a stable solid-electrolyte interphase buildup, the KRG/Prussian blue full cell with a controlled anode/cathode capacity ratio of 1.1:1 can deliver a high capacity of 108 mAh/g with a CE of 99% after 180 cycles at 1C. Our results provide valuable insights into the rational design of graphitic anodes for high-energy-density PIBs.

Polymerization Initiated by Graphite Intercalation Compounds Revisited: One-Pot Synthesis of Amphiphilic Pentablock Copolymers

This study reports the first attempt to employ a potassium–graphite intercalation compound (KC24) as an initiator for the one-pot synthesis of a multi-block copolymer. The results obtained show that KC24 successfully initiated the copolymerization, leading to a copolymer consisting of poly(styrene), poly(methyl methacrylate) and poly(ethylene oxide) blocks. When all three comonomers were introduced simultaneously or in a specific sequence, the resulting copolymers had molecular masses in the range between 170,000 Da and 280,000 Da. Their composition was investigated by size-exclusion chromatography with triple detection (dRI/UV/IR) and 1H-NMR. The analyses indicated that all copolymers were enriched in methyl methacrylate (50–66 mol%) despite the fact that the comonomers were added in equimolar amounts. Due to the layered structure of the initiator, the polymerization took place in the graphite interlayer spaces and lead to extensive delamination, indicating at the potential to produce in situ graphite/copolymer composite materials.

High-performance solid-solution potassium-ion intercalation mechanism of multilayered turbostratic graphene nanosheets

The solid-solution reaction between an alkali cation and an active host material is known as a single-phase redox mechanism, and it is typically accompanied by a continuous voltage change. It is distinct from the typical alkali cation intercalation reaction at an equivalent site of the active host material, which exhibits a voltage plateau. Herein, we report an unusual solid-solution potassium-ion intercalation mechanism with a low-voltage plateau capacity on multilayered turbostratic graphene nanosheets (T-GNSs). Despite the disordered graphitic structure with a broad range of d-spacings (3.65–4.18 Å), the T-GNSs showed a reversible plateau capacity of ∼ 200 mA h g−1, which is higher than that of a well-ordered graphite nanoplate (∼120 mA h g−1). In addition, a sloping capacity of ∼ 220 mA h g−1 was delivered with the plateau capacity, and higher rate capabilities, better reversibility, and a more stable cycling performance were confirmed on the turbostratic microstructure. First-principles calculations suggest that the multitudinous lattice domains of the T-GNSs contain diverse intercalation sites with strong binding energies, which could be the origin of the high-performance solid-solution potassium-ion intercalation behavior when the turbostratic graphene stacks have a d-spacing smaller than that of equilibrium potassium–graphite intercalation compounds (5.35 Å).

A Graphite Intercalation Composite as the Anode for the Potassium-Ion Oxygen Battery in a Concentrated Ether-Based Electrolyte.

Nowadays, alkali metal-oxygen batteries such as Li-, Na-, and K-O2 batteries have been investigated extensively because of their ultrahigh energy density. However, the oxygen crossover of oxygen batteries and the intrinsic drawbacks of the metal anodes (i.e., large volume changes and dendrite issues) have still been unsolved key problems. Here, we demonstrate a novel design of the K-ion oxygen battery using a graphite intercalation composite as the anode in a highly concentrated ether-based electrolyte. Instead of the metal K anode, the potassium graphite intercalation compound as the anode is depotassiated/potassiated in a binary form below 0.3 V (vs. K+/K); correspondingly, the discharged product KO2 is formed/decomposed at the carbon nanotube cathode, and an all-carbon full cell exhibits impressive cycling stability with a working voltage of 2.0 V. Furthermore, the utilization of graphite intercalation chemistry has been demonstrated to be applicable in Li-O2 batteries as well. Therefore, this study may provide a new strategy to resolve the key problems of the alkali metal-oxygen batteries.

Potassium Difluorophosphate as an Electrolyte Additive for Potassium-Ion Batteries.

The limited cyclability and inferior Coulombic efficiency of graphite negative electrodes have been major impediments to their practical utilization in potassium-ion batteries (PIBs). Herein, for the first time, potassium difluorophosphate (KDFP) electrolyte additive is demonstrated as a viable solution to these bottlenecks by facilitating the formation of a stable and K+-conducting solid-electrolyte interphase (SEI) on graphite. The addition of 0.2 wt % KDFP to the electrolyte results in significant improvements in the (de)potassiation kinetics, capacity retention (76.8% after 400 cycles with KDFP vs 27.4% after 100 cycles without KDFP), and average Coulombic efficiency (∼99.9% during 400 cycles) of the graphite electrode. Moreover, the KDFP-containing electrolyte also enables durable cycling of the K/K symmetric cell at higher efficiencies and lower interfacial resistance as opposed to the electrolyte without KDFP. X-ray diffraction and Raman spectroscopy analyses have confirmed the reversible formation of a phase-pure stage-1 potassium-graphite intercalation compound (KC8) with the aid of KDFP. The enhanced electrochemical performance by the KDFP addition is discussed based on the analysis of the SEI layer on graphite and K metal electrodes by X-ray photoelectron spectroscopy.

Synergistic Effect of a Defect-Free Graphene Nanostructure as an Anode Material for Lithium Ion Batteries.

Graphene nanosheets have been among the most promising candidates for a high-performance anode material to replace graphite in lithium ion batteries (LIBs). Studies in this area have mainly focused on nanostructured electrodes synthesized by graphene oxide (GO) or reduced graphene oxide (rGO) and surface modifications by a chemical treatment. Herein, we propose a cost-effective and reliable route for generating a defect-free, nanoporous graphene nanostructure (df-GNS) through the sequential insertion of pyridine into a potassium graphite intercalation compound (K-GIC). The as-prepared df-GNS preserves the intrinsic property of graphene without any crystal damage, leading to micro-/nano-porosity (microporosity: ~10–50 µm, nanoporosity: ~2–20 nm) with a significantly large specific surface area. The electrochemical performance of the df-GNS as an anode electrode was assessed and showed a notably enhanced capacity, rate capability, and cycle stability, without fading in capacity or decaying. This is because of the optimal porosity, with perfect preservation of the graphene crystal, allowing faster ion access and a high amount of electron pathways onto the electrode. Therefore, our work will be very helpful for the development of anode and cathode electrodes with higher energy and power performance requirements.

De-intercalation of the intercalated potassium in the preparation of activated carbons by KOH activation

In this study, the potassium formed during the activation of petroleum coke with KOH as activation agent was quantified, and three de-intercalation reagents (acetic acid, water vapor, methanol) with different flow and at different temperature were used to de-intercalate the potassium. It is suggested that the activation process yields almost the same amount potassium in the free state as that in the potassium-graphite intercalation compounds. The results also indicated that the introduction of acetic acid (250 °C, 0 .4ml/min), water vapor (200 °C, 0 .4ml/min), methanol (150 °C, 0 .4ml/min) could furthest improve the porosity and decrease the microcrystalline size as well as increase graphite layer spacing d002. Moreover, the de-intercalation reagents could accelerate the formation of oxygen-containing groups on the surface of the activated carbons. Based on these results, the de-intercalation mechanism has been proposed.

Dispersion-corrected DFT investigation on defect chemistry and potassium migration in potassium-graphite intercalation compounds for potassium ion batteries anode materials

The first-principles calculations based on the density function theory (DFT) with the dispersion-corrected DFT-D2 method were performed to investigate the structure, energetics, electronic structure, defects properties and cation migration of KC8n (n = 1, 2 and 3) and KC12n (n = 1 and 2). Compared with the standard DFT, DFT-D2 method offers good agreements with the previous experiments and calculations for c crystallographic parameters and interlayer distances. The inserted potassium atoms can significantly improve the electronic conductivity of graphite and K-GICs due to those contributions of extra electrons from the inserted potassium atoms making the Fermi levels shift to blue. Two types of potassium point defect (Schottky and Frenkel defect) were investigated and their calculated formation energies are 1.39–2.09 eV and 1.14–1.89 eV, respectively. Potassium ion migrations in the ab plane via the vacancy and Frenkel mechanism in K-GICs were researched. The calculated results of migration barrier show that potassium migration via the vacancy mechanism (0.11–1.58 eV) is absolutely prior to that via the Frenkel mechanism (2.42–7.92 eV). Besides, the occupying forms of potassium atoms have significantly influences on the defect properties and potassium migration. Compared with lithium migration in Li-GICs, potassium migration via the vacancy mechanism in K-GICs is more excellent. Thus, K-GICs are promising candidates for anode materials of potassium ion batteries.

Chemical, structural and electrical resistivity of two first stage arsenic–potassium–graphite intercalation compounds

The action on graphite of liquid arsenic–potassium alloys leads to the formation of a new series of graphite intercalation compounds. Two ternary phases of first stage have been observed and isolated. The α-type phase is obtained by action on graphite of composition alloys close to that of defined K3As compound. On the other hand, the β-phase is easily obtained by action on graphite of K–As alloys whose chemical stoichiometry is equal or near to the 37at.% As eutectic point composition.The Fourier transform analysis of the 00l reflexions family of these two phases showed that the intercalated metallic sheets between graphitic gaps in these phases are generally three-layered stacking according to K/As/K sequence along c-axis. An exam, by means classical X-ray diffraction and transmission electronic microscope (TEM) of the in-plane structure of these compounds revealed that the intercalated metallic sheets are perfectly crystallised and an equatorial stratum was defined for each of them.Electrical conductivity measurements have been carried out on these phases along both axis of graphite and showed that they are strongly anisotropic and exhibit a metallic character in parallel to graphitic plans.

Investigation of the high, stable electrical conductivity in graphite intercalation compounds prepared from flexible graphite sheets

Potassium–graphite intercalation compounds (K–GICs) prepared from flexible graphite sheets are unusually stable in air and their high electrical conductivity can be maintained for long periods. To investigate the reasons for this air-stability, we evaluated the changes in the electrical conductivity as a function of electrical carrier densities and mobilities, upon exposure of the GICs to air. These electrical carrier parameters were estimated using the measured electrical conductivity, magnetoresistance, and Hall coefficient. The decomposition processes varied depending on the characteristics of both the host graphite and the intercalated materials. In the case of K–GICs prepared from flexible graphite sheets, the electron density was decreased by the de-intercalation of K atoms. However, the electrical conductivity hardly changed, because the electron mobility was increased sufficiently to compensate for the decrease in the density. The recovery of the electron mobility was thought to be caused by the flexibility of the graphite layers. Therefore, it was found that the air-stability of GICs could be determined by considering the degree of de-intercalation as well as the flexibility of the graphite layers. Furthermore, the perfect graphite crystal is not suitable to prepare air-stable and high conductive GICs.

Preparation of few-layer graphene by the hydroxylation of a potassium–graphite intercalation compound

Few-layer graphene was prepared from the potassium–graphite intercalation compound (KC 8 ) by hydroxylation. In an oxidative atmosphere, KC 8 decomposes from its initial structural stage to a second or third stage. However, when KC 8 was hydroxylated by dropping a small amount of water onto it in a nonoxidative atmosphere, the KC 8 exfoliated with a disruption of the stage structure and agglomerated graphene was obtained. Sonication of this agglomerated graphene in water yielded few-layer graphene. It was found that dropping small quantities of water onto KC 8 is effective for achieving graphene with just a few layers. [TANSO 2014 (No. 262), 59–62.]

カリウムGICの水酸化処理によるグラフェンへの剥離

Few-layer graphene was prepared from the potassium-graphite intercalation compound (KC8) by hydroxylation. In an oxidative atmosphere, KC8 decomposes from its initial structural stage to a second or third stage. However, when KC8 was hydroxylated by dropping a small amount of water onto it in a nonoxidative atmosphere, the KC8 exfoliated with a disruption of the stage structure and agglomerated graphene was obtained. Sonication of this agglomerated graphene in water yielded few-layer graphene. It was found that dropping small quantities of water onto KC8 is effective for achieving graphene with just a few layers.

Manifestation of Charged and Strained Graphene Layers in the Raman Response of Graphite Intercalation Compounds

We present detailed multifrequency resonant Raman measurements of potassium graphite intercalation compounds (GICs). From a well-controlled and consecutive in situ intercalation and high-temperature deintercalation approach the response of each stage up to stage VI is identified. The positions of the G and 2D lines as a function of staging depend on the charge transfer from K to the graphite layers and on the lattice expansion. Ab initio calculations of the density and the electronic band structure demonstrate that most (but not all) of the transferred charge remains on the graphene sheets adjacent to the intercalant layers. This leads to an electronic decoupling of these “outer” layers from the ones sandwiched between carbon layers and consequently to a decoupling of the corresponding Raman spectra. Thus, higher stage GICs offer the possibility to measure the vibrations of single, double, and multilayer graphene under conditions of biaxial strain. This strain can additionally be correlated to the in-plane lattice constants of GICs determined by X-ray diffraction. The outcome of this study demonstrates that Raman spectroscopy is a very powerful tool to identify local internal strain in pristine and weakly charged single and few-layer graphene and their composites, yielding even absolute lattice constants.

Preparation of air-stable and highly conductive potassium-intercalated graphite sheet

Potassium–graphite intercalation compounds with chemical compositions of KC84, KC48, KC36 and KC8 were prepared from PGS graphite sheet and then treated with 6M-HCl aqueous solution. After washing with water and drying under open air, chemical composition, stage structure and electrical conductivity of the resulting air-stable compounds were determined. It was found that about 40% of potassium in the starting potassium–graphite intercalation compounds remained in the matrix after treated with 6M-HCl aqueous solution for KC84, KC48 and KC36, while 15% for KC8. The room temperature electrical conductivity of an air-stable compound, containing potassium of 0.008 in molar ratio of potassium to carbon, was determined to be 1.3×105Scm−1. This is comparable to that of iron (1.0×105Scm−1) and about 1/3 of that of aluminum (3.6×105Scm−1).

Superconductivity in Potassium-Doped Few-Layer Graphene

Here we report the successful synthesis of superconducting potassium-doped few-layer graphene (K-doped FLG) with a transition temperature of 4.5 K, which is 1 order of magnitude higher than that observed in the bulk potassium graphite intercalation compound (GIC) KC(8) (T(c) = 0.39 K). The realization of superconductivity in K-doped FLG shows the potential for the development of new superconducting electronic devices using two-dimensional (2D) graphene as a basis material.

Cathodic behavior of graphite in KF-AlF3-based melts with various cryolite ratios

The cathodic behavior of graphite in KF-AlF3-based melt with various cryolite ratios (CR; molar ratio of KF to AlF3) were investigated by means of cyclic voltammetry, sampled-current voltammetry, and chronopotentiometry. The limiting current of aluminum deposition and critical current density of potassium intercalation on graphite cathode in KF-AlF3-based melt decreased with the increase of CR of the melt. Lowering the CR of KF-AlF3-based melt suppressed the potassium intercalation but enhanced the formation of aluminum carbide. In the KF-AlF3-based melt with CR of 1.0, formation of Al4C3 seriously occurred, which resulted not only from the chemical reaction of aluminum and carbon but also from the electrochemical reduction of carbon under higher overpotential. However, both the X-ray diffraction and scanning electron microscopy failed to detect the existence of potassium–graphite intercalation compound and Al4C3 in the resultants after galvanostatic electrolysis.

An attempt to produce graphene by pulverizing graphite intercalation compounds

Stage 1 and 2 potassium-graphite intercalation compounds (GICs) suspended in N-methylpyrrolidone were ground and sonicated. After centrifuging, the supernatant solutions were analyzed photochemically. Absorption and emission spectra of the supernatant suggest that the solute is a mixture of aromatic hydrocarbons with some benzene nuclei. Graphene layers in GICs are supposed to be torn during the process including sonication. This finding is expected to provide some information on the isolation methods of graphene, which has been attracting much attention due to its fascinating physical properties.