Functionalization of 2D materials by intercalation

Since the discovery of graphene many studies focused on its functionalization by different methods. These strategies aim to find new pathways to overcome the main drawback of graphene, a missing band-gap, which strongly reduces its potential applications, particularly in the domain of nanoelectronics, despite its huge and unequaled charge carrier mobility. The necessity to contact this material with a metal has motivated a lot of studies of metal/graphene interactions and has led to the discovery of the intercalation process very early in the history of graphene. Intercalation, where the deposited atoms do not stay at the graphene surface but intercalate between the top layer and the substrate, may happen at room temperature or be induced by annealing, depending of the chemical nature of the metal. This kind of mechanism was already well-known in the earlier Graphite Intercalation Compounds (GICs), particularly famous for one current application, the Lithium-ion Battery, which is simply an application based on the intercalation of Lithium atoms between two sheets of graphene in a graphite anode. Among numerous discoveries the GICs community also found a way to obtain graphite with superconducting properties by using intercalated alkali metals. Graphene is now a playground to “revisit” and understand all these mechanisms and to discover possible new properties of graphene induced by intercalation. For example, the intercalation process may be used to decouple the graphene layer from its substrate, to change its doping level or even, in a more general way, to modify its electronic band structure and the nature of its Dirac fermions. In this paper we will focus on the functionalization of graphene by using intercalation of metal atoms but also of molecules. We will give an overview of the induced modifications of the electronic band structure possibly leading to spin-orbit coupling, superconductivity, …We will see how this concept of functionalization is also now used in the framework of other 2D materials beyond graphene and of van der Waals heterostructures based on these materials.     

锂离子电池正极材料的第一性原理研究进展

锂离子电池的发展主要依赖于电极材料的突破,解决现有电极材料存在的问题和预测新型未知材料是提高锂离子电池性能的关键,而第一性原理计算的出现能够较好的满足这一需求。本文介绍了第一性原理计算在锂离子电池正极材料研究方面的原理和应用,并对该原理在正极材料的平均嵌锂电压计算,嵌/脱锂机理、结构稳定性研究及新材料预测等方面的应用进行了详细论述,并指出了这一理论计算工具在电池材料设计过程中的重要性和局限性。     

氧缺陷TiO2-B作为可充电锂离子电池负极材料的第一性原理研究

利用对氧缺陷的TiO₂-B材料进行密度泛函理论的计算,阐述了氧空穴对于TiO₂-B材料的电化学性质的影响。计算研究主要聚焦于缺陷材料的锂离子迁移和电子导电性等基本问题。计算结果表明在低锂离子浓度下(x(Li/Ti)≤ 0.25),相比于无缺陷的TiO₂-B,氧缺陷TiO₂-B有着更高的插入电压和更低的b轴方向迁移活化能,意味着锂离子的嵌入也更容易,这对于可充电电池的充电过程是有利的。而在高浓度下(x(Li/Ti) = 1),锂饱和的氧缺陷TiO₂-B相较于无缺陷的TiO₂-B有着较低的插入电压,更有利于锂离子的脱嵌过程,这对于可充电电池的放电过程也是有利的。电子结构计算表明缺陷材料的禁带宽度在1.0-2.0 eV之间,低于无缺陷的材料的3.0 eV。主要态密度贡献者是Ti-Ov-3d,并且随着氧空穴的增加它的强度也变得更强。这就表明氧缺陷TiO₂-B有更好的电子导电性。     

The utility of photoemission spectroscopy in the study of intercalation reactions

In this brief review, the general effects of the intercalation of alkali metals (Na, Li) into V₂O₅ thin films, observed by photoemission spectroscopy (XPS and UPS), are summarized and discussed in order to better understand the involved intercalation mechanisms and surface reactions in the view points of crystal structure and electronic structure. The correlation of the change in work function and the shift in Fermi level of the host due to the intercalation of alkali metals is outlined. Finally, the contribution of different mechanisms to the batteries' voltages, as deduced from photoemission spectroscopic data, is given. Copyright © 2006 John Wiley & Sons, Ltd.     

FeOCl层状材料及其插层化合物:结构、性质与应用

氧基氯化铁(FeOCl)是一种典型的Fe基层状材料,于20世纪30年代被发现,并于20世纪70年代起作为一种优异的插层主体在超分子插层化学领域进行了大量的研究.FeOCl的层状结构赋予了其远比传统铁(氢)氧化物更加灵活的调变空间,自2013年第一次发现FeOCl具有优异的固体Fenton活性以来,围绕FeOCl及其插层...     

Nernstian Li+ intercalation into few-layer graphene and its use for the determination of K+ co-intercalation processes

Alkali ion intercalation is fundamental to battery technologies for a wide spectrum of potential applications that permeate our modern lifestyle, including portable electronics, electric vehicles, and the electric grid. In spite of its importance, the Nernstian nature of the charge transfer process describing lithiation of carbon has not been described previously. Here we use the ultrathin few-layer graphene (FLG) with micron-sized grains as a powerful platform for exploring intercalation and co-intercalation mechanisms of alkali ions with high versatility. Using voltammetric and chronoamperometric methods and bolstered by density functional theory (DFT) calculations, we show the kinetically facile co-intercalation of Li⁺ and K⁺ within an ultrathin FLG electrode. While changes in the solution concentration of Li⁺ lead to a displacement of the staging voltammetric signature with characteristic slopes ca. 54–58 mV per decade, modification of the K⁺/Li⁺ ratio in the electrolyte leads to distinct shifts in the voltammetric peaks for (de)intercalation, with a changing slope as low as ca. 30 mV per decade. Bulk ion diffusion coefficients in the carbon host, as measured using the potentiometric intermittent titration technique (PITT) were similarly sensitive to solution composition. DFT results showed that co-intercalation of Li⁺ and K⁺ within the same layer in FLG can form thermodynamically favorable systems. Calculated binding energies for co-intercalation systems increased with respect to the area of Li⁺-only domains and decreased with respect to the concentration of –K–Li– phases. While previous studies of co-intercalation on a graphitic anode typically focus on co-intercalation of solvents and one particular alkali ion, this is to the best of our knowledge the first study elucidating the intercalation behavior of two monovalent alkali ions. This study establishes ultrathin graphitic electrodes as an enabling electroanalytical platform to uncover thermodynamic and kinetic processes of ion intercalation with high versatility.

Nernstian signatures and swift voltammetry at graphene electrodes help elucidate alkali ion (co-)intercalation.     

Mechanism of lithium intercalation in titanates

First principles calculations of Li insertion in a variety of titanate structures have revealed a common mechanism underlying the intercalation behavior of these materials. The mechanism is based on the accommodation of the electron density donated upon intercalation in particular orbitals of Ti ions and is governed by a strong coupling between the structural and electronic degrees of freedom. A new predictive model is developed which relates the local structure of TiO₂ polymorphs to their phase behavior upon Li intercalation.     

Water at surfaces and interfaces: From molecules to ice and bulk liquid

The structure and growth of water films on surfaces is reviewed, starting from single molecules to two-dimensional wetting layers, and liquid interfaces. This progression follows the increase in temperature and vapor pressure from a few degrees Kelvin in ultra-high vacuum, where Scanning Tunneling and Atomic Force Microscopies (STM and AFM) provide crystallographic information at the molecular level, to ambient conditions where surface sensitive spectroscopic techniques provide electronic structure information. We show how single molecules bind to metal and non-metal surfaces, their diffusion and aggregation. We examine how water molecules can be manipulated by the STM tip via excitation of vibrational and electronic modes, which trigger molecular diffusion and dissociation. We review also the adsorption and structure of water on non-metal substrates including mica, alkali halides, and others under ambient humid conditions. We finally discuss recent progress in the exploration of the molecular level structure of solid-liquid interfaces, which impact our fundamental understanding of corrosion and electrochemical processes.     

2D Homogeneous Holey Carbon Nitride: An Efficient Anode Material for Li-ion Batteries With Ultrahigh Capacity

In present day Li-ion batteries (LIBs) is the most successful and widely used rechargeable batteries. The continuous effort is going on in finding suitable electrode material for LIBs for improved performance in terms of life-time, storage capacity etc. Computational chemistry plays an important role in identifying suitable electrode materials through electronic structure calculation. By employing state of the art density functional theory we herein explored the electronic structure of homogeneous holey carbon nitride monolayers (C_xN₃, x=10,19) to understand its suitability as electrode material for rechargeable LIB. The monolayers have shown high negative adsorption energy for Li adsorption and more interestingly the band structure of monolayers reveal Dirac semimetallic character thus would exhibit high electronic conductivity. Meanwhile, monolithiation introduces metallicity in these monolayers. The calculated average open circuit voltages of the monolayers lie in the range of 0.45 to 0.09 V, which are typically observed in high performance anode materials. Moreover, these monolayers achieve ultrahigh theoretical specific capacity upto 2092.01 mAh/g and low diffusion barrier from 0.004 to 0.44 eV. Based on our computational study we suggest that, the C_xN₃ monolayers could be a promising anode material in search of low-cost and high performance LIBs.     

Electronic Structure Manipulation of the Mott Insulator RuCl3 via Single-Crystal to Single-Crystal Topotactic Transformation

The core task for Mott insulators includes how rigid distributions of electrons evolve and how these induce exotic physical phenomena. However, it is highly challenging to chemically dope Mott insulators to tune properties. Herein, we report how to tailor electronic structures of the honeycomb Mott insulator RuCl₃ employing a facile and reversible single-crystal to single-crystal intercalation process. The resulting product (NH₄)_0.5RuCl₃⋅1.5 H₂O forms a new hybrid superlattice of alternating RuCl₃ monolayers with NH₄⁺ and H₂O molecules. Its manipulated electronic structure markedly shrinks the Mott–Hubbard gap from 1.2 to 0.7 eV. Its electrical conductivity increases by more than 10³ folds. This arises from concurrently enhanced carrier concentration and mobility in contrary to the general physics rule of their inverse proportionality. We show topotactic and topochemical intercalation chemistry to control Mott insulators, escalating the prospect of discovering exotic physical phenomena.     

锂离子电池富锂锰基正极材料的研究进展

随着新能源如电动汽车、储能电站的蓬勃发展,人们对下一代高性能锂离子电池的能量密度、功率密度和循环寿命提出了更高的要求. 而富锂锰基正极材料xLi₂MnO₃·(1-x)LiMO₂（0 < x < 1,M = Mn、Co、Ni…）具有可逆比容量高（240 ~ 280 mAh·g^-1,2.0 ~ 4.8 V）、电化学性能较佳、成本较低等优点,已吸引了研究者的关注,有望成为下一代锂离子电池用正极材料. 本实验室采用固相法和溶胶-凝胶法制备不同的富锂锰基正极材料,其中,溶胶-凝胶法制得的Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂电极首周期放电比容量277.3 mAh·g^-1,50周期循环后容量272.8 mAh·g^-1,容量保持率98.4%. 本文重点结合本实验室的研究工作,对新型富锂锰基正极材料xLi₂MnO₃·(1-x)LiMO₂的结构、合成、电化学性能改性和充放电机理等进行总结与评述.     

Layered graphitic carbon host formation during liquid-free solid state growth of metal pyrophosphates

We report a successful ligand- and liquid-free solid state route to form metal pyrophosphates within a layered graphitic carbon matrix through a single step approach involving pyrolysis of previously synthesized organometallic derivatives of a cyclotriphosphazene. In this case, we show how single crystal Mn(2)P(2)O(7) can be formed on either the micro- or the nanoscale in the complete absence of solvents or solutions by an efficient combustion process using rationally designed macromolecular trimer precursors, and present evidence and a mechanism for layered graphite host formation. Using in situ Raman spectroscopy, infrared spectroscopy, X-ray diffraction, high resolution electron microscopy, thermogravimetric and differential scanning calorimetric analysis, and near-edge X-ray absorption fine structure examination, we monitor the formation process of a layered, graphitic carbon in the matrix. The identification of thermally and electrically conductive graphitic carbon host formation is important for the further development of this general ligand-free synthetic approach for inorganic nanocrystal growth in the solid state, and can be extended to form a range of transition metals pyrophosphates. For important energy storage applications, the method gives the ability to form oxide and (pyro)phosphates within a conductive, intercalation possible, graphitic carbon as host-guest composites directly on substrates for high rate Li-ion battery and emerging alternative positive electrode materials.     

The influence of size on phase morphology and Li-ion mobility in nanosized lithiated anatase TiO2

Sustainable energy storage in the form of Li-ion batteries requires new and advanced materials in particular with a higher power density. Nanostructuring appears to be a promising strategy, in which the higher power density in nanosized materials is related to the dramatically shortened Li-ion diffusion paths. However, nanosizing materials also changes intrinsic material properties, which influence both ionic and electronic conductivity. In this work neutron diffraction is used to show that in addition to these two aspects, nanostructuring changes the phase behavior and morphology. Lithiated 40-nm TiO(2) anatase crystallites become single phase, either having the Li-poor original anatase phase, or the Li-rich Li-titanate phase, in contrast to microsized crystallites where these two phases coexist in equilibrium within one crystal particle. In addition, Li(x)TiO(2) compositions occur with stoichiometries that are not stable in micron-sized crystallites, indicating enhanced solid solution behavior. Reduced conduction electron densities at the sites of the Li ions are observed by NMR spectroscopy. This is accompanied by reduced spontaneous Li-ion mobility, suggesting a correlation between the electron density at the Li-ion site and the Li-ion mobility. The present results show that in the case of lithiated anatase TiO(2), significant effects on phase composition, morphology, and electronic configurations are induced, as well as slower intracrystallite Li diffusion.     

Phase engineering of cobalt hydroxide toward cation intercalation

Multi-cation intercalation in aqueous and neutral media is promising for the development of high-safety energy storage devices. However, developing a new host matrix for reversible cation intercalation as well as understanding the relationship between cation intercalation and the interlayer structure is still a challenge. In this work, we demonstrate layered cobalt hydroxides as a promising host for cation interaction, which exhibit high metal ion (Li⁺, Na⁺, K⁺, Mg²⁺ and Ca²⁺) storage capacities after phase transformation. Moreover, it is found that α-Co(OH)₂ with an intercalated structure is more conducive to phase transition after electrochemical activation than β-Co(OH)₂. As a result, the activated α-Co(OH)₂ delivers four times higher capacity in multi-cation storage than activated β-Co(OH)₂. Meanwhile, the α-Co(OH)₂ after activation also shows an ultralong cycle life with capacity retention of 93.9% after 5000 cycles, which is also much superior to that of β-Co(OH)₂ (∼74.8%). Thus, this work displays the relationship between cation intercalation and the interlayer structure of layered materials, which is important for designing multi-ion storage materials in aqueous media.

Phase engineering of cobalt hydroxide toward cations intercalation is explored. Among them, α-Co(OH)₂ is proven to be more conductive to phase transition than β-Co(OH)₂ during electrochemical activation, which shows superior multi-cations storage performance.     

石墨/LiCoO_2电池充放电过程电极活性材料结构演变研究

利用X射线衍射分析(XRD)详细地研究了石墨/LiCoO2体系18650型锂离子电池充放电过程中正负极活性材料的晶体结构和微结构的变化.结果发现,在电池充电过程中,锂嵌入石墨层中,优先进入碳原子六方网格面间的间隙位置,导致石墨的点阵参数a和c,以及微应变ε增加和堆垛无序度P的变化,电池充电至20%后负极中形成Li-C化合物;电池充电时,正极LiCoO2中处于(000)位的Li原子优先脱离晶体点阵,随着正极材料脱锂量的增大,其晶格参数a减小,c增大,微应变ε也随之增加.LiCoO2在整个充电和放电过程中均未发生相变.最后,讨论了锂离子电池的导电机制.发现,充电时,锂离子的迁移从负极-电解液界面开始;放电时,其迁移从正极-电解液界面开始;在充放电过程中,正负极活性材料的嵌脱锂都有一个从活性材料颗粒表面到内层的过程.电池的充放电过程不完全可逆.     

Structure,electronic, and magnetic properties of 3d transition metal intercalated borophene/BP heterostructures

The intercalation of various atoms or molecules has become one promising way to manipulate the electronic and magnetic properties of layered materials. Using density functional calculations, we explored the 3d transition metal (TM) intercalated α-borophene/black phosphorus (α-B/BP) heterostructure, TM@(α-B/BP) (TM = Sc-Ni), on their structure, electronic and magnetic properties. Our results demonstrate that TM@(α-B/BP)s can be ferromagnetic (FM), antiferromagnetic (AFM) and nonmagnetic depending on the choice of TM atoms, and most systems have large magnetic anisotropic energy. Particularly, Ti@(α-B/BP) is AFM semiconductor with Néel temperature of 470 K, which is much higher than room temperature. Moreover, the electronic and magnetic properties of TM@(α-B/BP)s can be further altered by the TM intercalation concentration. Our results provide a feasible way to design promising candidates for applications in electronic and information storage devices.     

Monolayer boron-arsenide as a perfect anode for alkali-based batteries with large storage capacities and fast mobilities

We investigate, by means of first-principles density functional theory (DFT) calculation, the possibility of using hexagonal boron-arsenide (h-BAs) as an anode material for alkali-based batteries. We show that the adsorption strength of alkali atoms (Li, Na, and K) on h-BAs in comparison with graphene and other related materials changes a little as a function of alkali atom concentration. When the separation between alkali atoms and h-BAs is less than the critical distance of ~5 Å, the adsorption energy abruptly increases showing fast adsorption without an energy barrier. Furthermore, the low energy barriers of 0.322, 0.187, and 0.0.095 eV for Li, Na, and K, respectively, ensure the fast ionic diffusivities for all the three alkali atoms. Additionally, the addition of these alkali atoms transforms the electronic properties of h-BAs from semiconducting to metallic, resulting in improved electronic conductivities. Most interestingly, the excellent storage capacities of h-BAs (~626 mAh/g) for alkali atoms make it a material of similar caliber to that of other popular anode materials. Finally, the average open circuit voltages are calculated and found to be in the desired range. In short, h-BAs possess every quality that is crucial for an anode material and thus it is interesting to see h-BAs in alkali-based battery technologies.     

Advances in two-dimensional heterostructures by mono-element intercalation underneath epitaxial graphene

Two-dimensional (2D) materials have displayed many remarkable physical properties, including 2D superconductivity, magnetism, and layer-dependent bandgaps. However, it is difficult for a single 2D material to meet complex practical requirements. Heterostructures obtained by vertically stacking different kinds of 2D materials have extensively attracted researchers’ attention because of their rich electronic features. With heterostructures, the constraints of lattice matching can be overcome. Meanwhile, high application potential has been explored for electronic and optoelectronic devices, including tunneling transistors, flexible electronics, and photodetectors. Specifically, graphene-based van der Waals heterostructures (vdWHs) by intercalation are emerging to realize various functional heterostructures-based electronic devices. Intercalating atoms under epitaxial graphene can efficiently decouple graphene from the substrate, and is expected to realize rich novel electronic properties in graphene. In this study, we systematically review the progress of the mono-element intercalation in graphene-based vdWHs, including the intercalation mechanism, intercalation-modified electronic properties, and the practical applications of 2D intercalated heterostructures. This work would inspire edge-cutting ideas in the scientific frontiers of 2D materials.     

二维材料插层改性方法研究进展

二维材料凭借其独特的电学、光学、磁学等性质引起了广泛关注,如何处理二维材料使其改性是目前的研究热点。 插层方法是目前调控二维材料性质的主要方法之一。 插层过程中,客体粒子插入主体材料的范德华层间,造成二维材料物理与化学性质的变化。 气相、液相、固相插层均可以使二维材料的性质得到提升。 本文主要介绍二维材料插层方法,分析其不同优势和限制条件,并展望如何综合应用插层方法更好地提升二维材料电学、光学等性能。     

六方氮化硼与石墨在形成层间化合物上的差异的理论研究

根据量子化学密度泛函B3LYP方法的计算结果,从六方氮化硼(h-BN）与石墨的前线轨道能级和两对离子探针（C^ 和C^-）的作用能所表现出来的在电亲和性上的差异以及金属层间化合物的电子结构,分析了h-BN不能形成金属层间化合物的原因。