Ab initio investigations of lithium diffusion in carbon nanotube systems

Phase stability of Li–Mn–O oxides as cathode materials for Li-ion batteries: insights from ab initio calculations

The Li–Mn–O phase diagram as a function of the chemical potential of Li and O and the pH.

In situ transmission electron microscopy investigation of the electrochemical lithiation-delithiation of individual Co9S8/Co-filled carbon nanotubes

Carbon nanotube (CNT)-encapsulated metal sulfides/oxides are promising candidates for application as anode materials in lithium ion battery (LIB), while their electrochemical behavior and mechanism still remain unclear. A comprehensive understanding of the lithiation mechanism at nanoscale of this type of composites will benefit the design and development of high-performance LIB materials. Here, we use Co9S8/Co nanowire-filled CNTs as a model material to investigate the lithium storage mechanism by in situ transmission electron microscopy. For a Co9S8/Co nanowire-filled closed CNT, the reaction front propagates progressively during lithiation, causing an axial elongation of 4.5% and a radial expansion of 32.4%, while the lithiated nanowire core is still confined inside the CNT. Contrastingly, for an open CNT, the lithiated Co9S8 nanowire shows an axial elongation of 94.2% and is extruded out from the open CNT. In particular, a thin graphite shell is drawn out from the CNT wall by the extruded lithiated Co9S8. The thin graphite shell confines the extruded filler and protects the filler from pulverization in the following lithiation-delithiation cycles. During multiple cycles, the Co segment remains intact while the Co9S8 exhibits a reversible transformation between Co9S8 and Co nanograins. Our observations provide direct electrochemical behavior and mechanism that govern the CNT-based anode performance in LIBs.

Feasibility of Lithium Storage on Graphene and Its Derivatives

Nanomaterials are anticipated to be promising storage media, owing to their high surface-to-mass ratio. The high hydrogen capacity achieved by using graphene has reinforced this opinion and motivated investigations of the possibility to use it to store another important energy carrier - lithium (Li). While the first-principles computations show that the Li capacity of pristine graphene, limited by Li clustering and phase separation, is lower than that offered by Li intercalation in graphite, we explore the feasibility of modifying graphene for better Li storage. It is found that certain structural defects in graphene can bind Li stably, yet a more efficacious approach is through substitution doping with boron (B). In particular, the layered C3B compound stands out as a promising Li storage medium. The monolayer C3B has a capacity of 714 mAh/g (as Li1.25C3B), and the capacity of stacked C3B is 857 mAh/g (as Li1.5C3B), which is about twice as large as graphite's 372 mAh/g (as LiC6). Our results help clarify the mechanism of Li storage in low-dimensional materials, and shed light on the rational design of nanoarchitectures for energy storage.

Catalyst-free direct growth of a single to a few layers of graphene on a germanium nanowire for the anode material of a lithium battery

Direct growth of a single to a few layers of graphene on a germanium nanowire (Gr/Ge NW; see picture) was achieved by a metal-catalyst-free chemical vapor deposition (CVD) process. The Gr/Ge NW was used as anode in a lithium ion battery. This material has a specific capacity of 1059 mA h g(-1) at 4.0 C, a long cycle life over 200 cycles, and a high capacity retention of 90%.

Applications of Carbon Nanotubes for Lithium Ion Battery Anodes

Carbon nanotubes (CNTs) have displayed great potential as anode materials for lithium ion batteries (LIBs) due to their unique structural, mechanical, and electrical properties. The measured reversible lithium ion capacities of CNT-based anodes are considerably improved compared to the conventional graphite-based anodes. Additionally, the opened structure and enriched chirality of CNTs can help to improve the capacity and electrical transport in CNT-based LIBs. Therefore, the modification of CNTs and design of CNT structure provide strategies for improving the performance of CNT-based anodes. CNTs could also be assembled into free-standing electrodes without any binder or current collector, which will lead to increased specific energy density for the overall battery design. In this review, we discuss the mechanism of lithium ion intercalation and diffusion in CNTs, and the influence of different structures and morphologies on their performance as anode materials for LIBs.

Conformal Fe3O4 sheath on aligned carbon nanotube scaffolds as high-performance anodes for lithium ion batteries

A uniform Fe(3)O(4) sheath is magnetron sputtered onto aligned carbon nanotube (CNT) scaffolds that are directly drawn from CNT arrays. The Fe(3)O(4)-CNT composite electrode, with the size of Fe(3)O(4) confined to 5-7 nm, exhibits a high reversible capacity over 800 mAh g(-1) based on the total electrode mass, remarkable capacity retention, as well as high rate capability. The excellent performance is attributable to the superior electrical conductivity of CNTs, the uniform loading of Fe(3)O(4) sheath, and the structural retention of the composite anode on cycling. As Fe(3)O(4) is inexpensive and environmentally friendly, and the synthesis of Fe(3)O(4)-CNT is free of chemical wastes, this composite anode material holds considerable promise for high-performance lithium ion batteries.

Adsorption and diffusion of Li on pristine and defective graphene

With first-principles DFT calculations, the interaction between Li and carbon in graphene-based nanostructures is investigated as Li is adsorbed on graphene. It is found that the Li/C ratio of less than 1/6 for the single-layer graphene is favorable energetically, which can explain what has been observed in Raman spectrum reported recently. In addition, it is also found that the pristine graphene cannot enhance the diffusion energetics of Li ion. However, the presence of vacancy defects can increase the ratio of Li/C largely. With double-vacancy and higher-order defects, Li ion can diffuse freely in the direction perpendicular to the graphene sheets and hence boost the diffusion energetics to some extent.

Lithiation-induced embrittlement of multiwalled carbon nanotubes

Lithiation of individual multiwalled carbon nanotubes (MWCNTs) was conducted in situ inside a transmission electron microscope. Upon lithiation, the intertube spacing increased from 3.4 to 3.6 Å, corresponding to about 5.9% radial and circumferential expansions and ∼50 GPa tensile hoop stress on the outermost tube wall. The straight tube walls became distorted after lithiation. In situ compression and tension tests show that the lithiated MWCNTs were brittle with sharp fracture edges. Such a failure mode is in stark contrast with that of the pristine MWCNTs which are extremely flexible and fail in a "sword-in-sheath" manner upon tension. The lithiation-induced embrittlement is attributed to the mechanical effect of a "point-force" action posed by the intertubular lithium that induces the stretch of carbon-carbon bonds in addition to that by applied strain, as well as the chemical effect of electron transfer from lithium to the antibonding π orbital that weakens the carbon-carbon bond. The combined mechanical and chemical weakening leads to a considerable decrease of the fracture strain in lithiated MWCNTs. Our results provide direct evidence and understanding of the degradation mechanism of carbonaceous anodes in lithium ion batteries.

Mechanism of Li adsorption on carbon nanotube-fullerene hybrid system: a first-principles study

The lithium (Li) adsorption mechanism on the metallic (5,5) single wall carbon nanotube (SWCNT)-fullerene (C(60)) hybrid material system is investigated using first-principles method. It is found that the Li adsorption energy (-2.649 eV) on the CNT-C(60) hybrid system is lower than that on the peapod system (-1.837 eV) and the bare CNT (-1.720 eV), indicating that the Li adsorption on the CNT-C(60) hybrid system is more stable than on the peapod or bare CNT system. This is due to the C(60) of high electron affinity and the charge redistribution after mixing CNT with C(60). In order to estimate how efficiently Li can utilize the vast surface area of the hybrid system for increasing energy density, the Li adsorption energy is calculated as a function of the adsorption positions around the CNT-C(60) hybrid system. It turns out that Li preferably occupies the mid-space between C(60) and CNT and then wraps up the C(60) side and subsequently the CNT side. It is also found that the electronic properties of the CNT-C(60) system, such as band structure, molecular orbital, and charge distribution, are influenced by the Li adsorption as a function of the number of Li atoms. From the results, it is expected that the CNT-C(60) hybrid system has enhanced the charge transport properties in addition to the Li adsorption, compared to both CNT and C(60).

Nanostructured hybrid silicon/carbon nanotube heterostructures: reversible high-capacity lithium-ion anodes

Lithium-ion batteries have witnessed meteoric advancement the last two decades. The anode area has seen unprecedented research activity on Si and Sn, the two anode alternatives to currently used carbon following the initial seminal work by Fuji on tin oxide nanocomposites. Recent reports on silicon nanowires, porous Si, and amorphous Si coatings on graphite nanofibers (GNF) have been very encouraging. High capacity and long cycle life anodes are still, however, elusive and much needed to meet the ever increasing energy storage demands of modern society. Herein, we report for the first time the synthesis of novel 1D heterostructures comprising vertically aligned multiwall CNTs (VACNTs) containing nanoscale amorphous/nanocrystalline Si droplets deposited directly on VACNTs with clearly defined spacing using a simple two-step liquid injection CVD process. A hallmark of these single reactor derived heterostructures is an interfacial amorphous carbon layer anchoring the nanoscale Si clusters directly to the VACNTs. The defined spacing of nanoscale Si combined with their tethered CNT architecture allow for the silicon to undergo reversible electrochemical alloying and dealloying with Li with minimal loss of contact with the underlying CNTs. The novel heterostructures thus exhibit impressive reversible stable capacities approximately 2050 mAh/g with very good rate capability and an acceptable first cycle irreversible loss approximately 20% comparable to graphitic anodes indicating their promise as high capacity Li-ion anodes. Although warranting further research, particularly with regard to long-term cycling, it can be envisaged that optimization of this simple approach could lead to reversible high capacity next generation Li-ion anodes.

New insight into carbon-nanotube electronic-structure selectivity

The fundamental role of aryl diazonium salts for post-synthesis selectivity of carbon nanotubes is investigated using extensive electronic-structure calculations. The resulting understanding for diazonium-salt-based selective separation of conducting and semiconducting carbon nanotubes shows how the primary contribution comes from the interplay between the intrinsic electronic structure of the carbon nanotubes and that of the anion of the salt. We demonstrate how the electronic-transport properties change upon the formation of charge-transfer complexes and upon their conversion into covalently attached functional groups. The results are found to correlate well with experiments and provide for the first time an atomistic description for diazonium-salt-based chemical separation of carbon nanotubes.

How does a carbon nanotube grow? An in situ investigation on the cap evolution

Catalyst-free inner growth of single-wall carbon nanotubes has been directly realized and monitored by means of in situ high-resolution transmission electron microscopy, with particular attention paid to the evolution of the cap shape. The cap of a carbon nanotube is surprisingly found to be kept closed during the growing/shrinking process, and the cap shape evolves inhomogeneously with a few particular sites growing faster during the growth, while the cap of a carbon nanotube keeps a round shape during the shrinkage process. The closed cap should be specific for noncatalytic growth of carbon nanotubes. We infer, from the results above, the possible atomistic mechanism and how the carbon network can accommodate or release the carbon atoms during the growth/shrinkage of carbon nanotubes.

Vacancy migrations in carbon nanotubes

Activities of vacancy defects in carbon nanotubes have been directly monitored by in situ high-resolution transmission electron microscopy at elevated temperatures. Adatom-vacancy pair defects are first prolific due to the knock-on damage, and then the induced vacancies indeed grow up to 1-2 nm in the size by the following Joule heating. Surprisingly, these large vacancies, or "holes", tend to migrate and coalesce with each other to form even larger ones. It suggests that the activation barrier has been substantially lowered due to the contributions of an electromigration and/or irradiation effect.

Transparent and catalytic carbon nanotube films

We report on the synthesis of thin, transparent, and highly catalytic carbon nanotube films. Nanotubes catalyze the reduction of triiodide, a reaction that is important for the dye-sensitized solar cell, with a charge-transfer resistance as measured by electrochemical impedance spectroscopy that decreases with increasing film thickness. Moreover, the catalytic activity can be significantly enhanced by exposing the nanotubes to ozone in order to introduce defects. Ozone-treated, defective nanotube films could serve as catalytic, transparent, and conducting electrodes for the dye-sensitized solar cell. Other possible applications include batteries, fuel cells, and electroanalytical devices.

Why alkali metals preferably bind on structural defects of carbon nanotubes: A theoretical study by first principles

By using ab initio calculations we investigated the interaction of alkali metal atoms and alkali metal cations with perfect and defective carbon nanotubes. Our results show that the alkali metals prefer to interact with the pentagons and heptagons that appear on the defective site of the carbon nanotube rather than with the hexagons. The alkali metals remain always positively charged not depending on their charge state (neutral, cation) or the different carbon ring that they interact with. The molecular orbital energy level splitting from a defect creation on the carbon nanotube along with the localization of charge-electron density on the defect, results in binding the alkali metals more efficient. More interestingly, metallic sodium appears to bind very weak on the nanotube compared to the rest of alkali metals. The Na anomaly is attributed to the fact that unlike the K case, sodium's inner p shell falls energetically lower than carbon nanotube's p molecular orbitals. As a result, the Na p shell is practically excluded from any binding energy contribution. In the alkali metal cation case the electronegativity trend is followed.

Lithium Adsorption on Graphite from Density Functional Theory Calculations

The structural, energetic, and electronic properties of the Li/graphite system are studied through density functional theory (DFT) calculations using both the local spin density approximation (LSDA), and the gradient-corrected Perdew-Burke-Ernzerhof (PBE) approximation to the exchange-correlation energy. The calculations were performed using plane waves basis, and the electron-core interactions are described using pseudopotentials. We consider a disperse phase of the adsorbate comprising one Li atom for each 16 graphite surface cells, in a slab geometry. The close contact between the Li nucleus and the graphene plane results in a relatively large binding energy (larger than 1.1 eV). A detailed analysis of the electronic charge distribution, density difference distribution, and band structures indicates that one valence electron is entirely transferred from the atom to the surface, which gives rise to a strong interaction between the resulting lithium ion and the cloud of pi electrons in the substrate. We show that it is possible to explain the differences in the binding of Li, Na, and K adatoms on graphite considering the properties of the corresponding cation/aromatic complexes.

Chemical Functionalization of Carbon Nanotubes by Carboxyl Groups on Stone-Wales Defects: A Density Functional Theory Study

Chemical functionalization of carbon nanotubes with Stone-Wales (SW) defects by carboxyl (COOH) groups is investigated by density functional calculations. Due to the localized donor states induced by the SW defect, the binding of the COOH group with the defective carbon nanotube is stronger than that with the perfect one. A quasi-tetrahedral bonding configuration of carbon atoms, indicating sp3 hybrid bonding, is formed in the adsorption site. The charge distribution analysis shows that, in comparison with benzoic acid, the localized or delocalized pi states on the nanotube would affect the polarities of chemical bonds of the COOH group without losing the acidity. Furthermore, it is found that the double-adsorption system (two COOH groups are respectively adsorbed on two individual carbon atoms of the SW defect) is more energetically favorable than the monoadsorption one. The adsorption of COOH groups leads to a significant change of the electronic states around the Fermi level, which is advantageous for the electrical conductivity. The functionalization by introducing functional groups on the topological defects provides a pathway for applications of carbon nanotubes in chemical sensors and nanobioelectronics.

First-Principles Calculations of Migration Energy of Lithium Ions in Halides and Chalcogenides

Migration of Li+ ions via the vacancy mechanism in LiX (X = F, Cl, Br, and I) with the rocksalt and hypothetical zinc blende structures and Li2X (X = O, S, Se, and Te) with the antifluorite structure has been investigated using first-principles projector augmented wave calculations with the generalized gradient approximation. The migration paths and energies, determined by the nudged-elastic-band method, are discussed on the basis of two idealized models: the rigid-sphere and charged-sphere models. The trajectories and energy profiles of the migration in these lithium compounds vary between these two models, depending on the anion species and crystal structure. The migration energies in LiX with both the rocksalt and hypothetical zinc blende structures show a tendency to decrease with increasing periodic number of the anion species in the periodic table. This is consistent with the widely accepted view that anion species with large ionic radii and high polarizabilities are favorable for good ionic conduction. In contrast, Li2O exhibits the lowest migration energy among Li2X compounds, although O is the smallest among the chalcogens, indicating that electrostatic attractive interactions play the dominant role in the inter-ion interactions in Li2O and, therefore, in the ion migration.

Electromigration forces on ions in carbon nanotubes

We calculate the electromigration forces on ions such as adsorbed alkali metal atoms in a carbon nanotube transistor. The forces are especially large in the turn-on regime of the transistor and much smaller in the off and on states. Electromigration in the channel is driven almost exclusively by the wind force. The sign of the "effective valence" Z* is independent of the actual charge sign but can be reversed with gate voltage, providing a dramatic illustration of the quantum character of the wind force. Our self-consistent nonequilibrium Green's function calculations treat a ballistic device within a tight-binding approximation.

Bundling up carbon nanotubes through Wigner defects

We show, using ab initio total energy density functional theory, that the so-called Wigner defects, an interstitial carbon atom right beside a vacancy, which are present in irradiated graphite, can also exist in bundles of carbon nanotubes. Due to the geometrical structure of a nanotube, however, this defect has a rather low formation energy, lower than the vacancy itself, suggesting that it may be one of the most important defects that are created after electron or ion irradiation. Moreover, they form a strong link between the nanotubes in bundles, increasing their shear modulus by a sizable amount, clearly indicating its importance for the mechanical properties of nanotube bundles.

Magnetic properties and diffusion of adatoms on a graphene sheet

We use ab initio methods to calculate the properties of adatom defects on a graphite surface. By applying a full spin-polarized description to the system we demonstrate that these defects have a magnetic moment of about 0.5micro(B) and also calculate its role in diffusion over the surface. The magnetic nature of these intrinsic carbon defects suggests that it is important to understand their role in the recently observed magnetism in pure carbon systems.

Endohedral impurities in carbon nanotubes

A generalization of the Anderson model that includes pseudo-Jahn-Teller impurity coupling is proposed to describe distortions of an endohedral impurity in a carbon nanotube. Within mean-field theory, spontaneous axial symmetry breaking is found when the vibronic coupling strength g exceeds a critical value. The effective potential is found to have O(2) symmetry, in agreement with numerical calculations. For metallic zigzag nanotubes endohedrally doped with transition metals in the dilute limit, the low-energy properties of the system may display two-channel Kondo behavior; however, strong vibronic coupling is seen to exponentially suppress the Kondo energy scale.