H2O2 adsorption on the BN and SiC nanotubes: A DFT study

We have performed a comparative density functional theory study on adsorption of hydrogen peroxide (H₂O₂) on the boron nitride and silicon carbide nanotubes (BNNT and SiCNT) in terms of energetic, geometric, and electronic properties. It has been found that the molecule is chemically adsorbed on both of the tubes so that its interaction with SiCNT (adsorption energy ∼−0.97 eV) is much stronger than that with BNNT (adsorption energy ∼−0.47 eV). The H₂O₂ adsorption on BNNT slightly decreases its work function, increasing the field electron emission from the BNNT surface while it may not affect that of the SiCNT. In addition, the adsorption process may increase the electrical conductivity of SiCNT while does not affect that of the BNNT, significantly. We believe that the SiCNT may be a potential candidate for detection of H₂O₂.     

Ab initio density functional theory investigation of Li-intercalated silicon carbide nanotube bundles

We present the results of ab initio density functional theory calculations on the energetic, and geometric and electronic structure of Li-intercalated (6,6) silicon carbide nanotube (SiCNT) bundles. Our results show that intercalation of lithium leads to the significant changes in the geometrical structure. The most prominent effect of Li intercalation on the electronic band structure is a shift of the Fermi energy which occurs as a result of charge transfer from lithium to the SiCNTs. All the Li-intercalated (6,6) SiCNT bundles are predicted to be metallic representing a substantial change in electronic properties relative to the undoped bundle, which is a wide band gap semiconductor. Both inside of the nanotube and the interstitial space are susceptible for intercalation. The present calculations suggest that the SiCNT bundle is a promising candidate for the anode material in battery applications.     

Correlation between Li+ adsorption capacity and the preparation conditions of spinel lithium manganese precursor

Spinel lithium manganese oxides can be used as Li⁺ adsorbent with topotactical extraction of lithium. In this paper, the solid state methods were introduced to prepare spinel lithium manganese precursors with Li₂CO₃ and LiOH·H₂O as different Li sources. The Li⁺ uptake was studied to clarify the correction between Li⁺ adsorption capacity and the preparation conditions of precursors, including different Li sources, Li/Mn mole ratios and heating time. The results indicated that the Li⁺-extracted materials prepared with LiOH·H₂O and MnCO₃ usually have higher Li⁺ adsorption capacity than Li₂CO₃ and MnCO₃, and an ascending trend was found in Li⁺ uptake with increasing Li/Mn mole ratio in the preparation of the precursor, but it is not proportional. The Mn₂O₃ impurities could be the primary reason for decreasing Li⁺ adsorption capacity. Furthermore, it is concluded that the Li⁺-extracted materials obtained from spinel manganese oxides synthesized with Li/Mn = 1.0 can serve as selective Li⁺ absorbents due to its high selectivity and large adsorption capacity.     

F−, Cl−, Li+ and Na+ adsorption on AlN nanotube surface: A DFT study

Adsorption of two anions (F⁻ and Cl⁻) and two cations (Li⁺ and Na⁺) on the surface of aluminum nitride nanotubes (AlNNTs) is investigated by density functional theory. The reactions are site-selective, so that the cations and anions prefer to be adsorbed atop the N and Al atoms of the tube surface, respectively. The adsorption energies of anions (−4.46 eV for F⁻ and −1.12 eV for Cl⁻) are much higher than those of cations (about −0.17 eV for Li⁺ and −0.12 eV for Na⁺) which can be explained using frontier molecular orbital theory. It was found that the adsorption of anions may facilitate the electron emission from the AlNNT surface by reducing the work function due to the charge transfer occurs from the anions to the tube. It has been predicted that in contrast to the cations the adsorption of anions also obviously increases the electrical conductivity of AlNNT.     

First principles computational investigation on the possibility of Pt-decorated SiC hexagonal sheet as a suitable material for oxygen reduction reaction

First principles calculations play a significant role in developing and optimizing new energy storage and conversion materials especially at the nanoscale. In this work, the structural, energetics and, electronic properties of adsorbed Pt atom onto two-dimensional graphene, hexagonal BN (h-BN) and SiC (h-SiC) sheets have been investigated at DFT–B3LYP level of theory using coronene molecule as a suitable model. Spin-polarization and model size effects on the Pt adsorption properties have also been evaluated. Various positions for establishing Pt atom on the selected substrates have been considered and full structural optimization was carried out for all selected systems. The adsorption energies, electronic structures and charge population analysis indicated that in all the studied structures there were strong interaction between two interacting entities. It was also found that the adsorption ability of h-SiC is much stronger than the other counterparts with adsorption energy of −3.828 eV.We have also examined the O₂ adsorption properties of Pt-decorated graphene, h-BN and h-SiC sheets for possible tunability of O₂ adsorption strength of systems under study. We found that h-SiC sheet possess a weakened O₂ adsorption energy among the selected substrates. In view of the strong stability of adsorbed Pt atom on h-SiC sheet and relatively weaker O₂ adsorption energy, one can expect that h-SiC might be a promising material for support assistant as well as increasing the catalytic activity of Pt atoms compared to graphene and h-BN substrates. This may attribute to preventing aggregating of Pt atoms due to the strong fastening nature of the h-SiC sheet and also by affording a balance in the O₂ adsorption strength that lead to enhanced catalyst turnover. Therefore, our first principles findings offer a unique opportunity for design and applications of SiC-based nanoscale supports in fuel cell technology.     

Synthesis and electrochemical characterization of M2Mn3O8 (M = Ca,Cu) compounds and derivatives

M₂Mn₃O₈ (M = Ca²⁺, Cu²⁺) compounds were synthesized and characterized in lithium cells. The M²⁺ cations, which reside in the van der Waals gaps between adjacent sheets of Mn₃O₈⁴⁻, may be replaced chemically (by ion-exchange) or electrochemically with Li. More than 7 Li⁺/Cu₂Mn₃O₈ may be inserted electrochemically, with concomitant reduction of Cu²⁺ to Cu metal, but less Li can be inserted into Ca₂Mn₃O₈. In the case of Cu²⁺, this process is partially reversible when the cell is charged above 3.5 V vs. Li, but intercalation of Cu⁺ rather than Cu²⁺ and Li⁺/Cu⁺ exchange occurs during the subsequent discharge. If the cell potential is kept below 3.4 V, the Li in excess of 4 Li⁺/Cu₂Mn₃O₈ can be cycled reversibly. The unusual mobility of + 2 cations in a layered structure has important implications both for the design of cathodes for Li batteries and for new systems that could be based on M²⁺ intercalation compounds.     

Lithium insertion into nanometer-sized rutile-type TixSn1 − xO2 solid solutions

The electrochemical properties of rutile-type Ti_xSn_1?xO₂ solid solutions (x = 0–1.0) as an anode for a lithium–ion battery were investigated using nanosized crystals prepared by an aqueous solution process. The reduction of the crystal size to nanoscale allowed a smooth lithium insertion into the rutile framework at room temperature. The lithium-insertion behavior of TiO₂, SnO₂, and the solid solutions was evaluated without any structural change of the rutile-type crystal structure in the potential range of 1.2–3.5 V (versus Li/Li⁺). The interstitial spaces for lithium ions were found to be derived from the crystal structure of the rutile framework and independent of the metal species.     

Thermodynamic analysis and experimental research on Li intercalation reactions of the intermetallic compound Al2Cu

The lithiation mechanism of the intermetallic compound Al₂Cu as anode materials for lithium-ion batteries during lithium intercalation/deintercalation was studied in this paper. The Gibbs free energy changes for five possible electrochemical reactions of Li intercalated into Al₂Cu electrode have been calculated based on the first-principles plane-wave pseudopotential method in conjunction with thermodynamic principles. The reaction Li + Al₂Cu → LiAl + AlCu that possesses the most negative value of the Gibbs free energy change per unit lithium on average among all the five reactions was claimed to be the lithiation mechanism of Al₂Cu electrode. In order to warrant the speculation, the 2032-type coin cells with Al₂Cu and lithium metal as the testing and the counter electrode, respectively, was assembled. The electrochemical performance of the cells and the phase changes in Al₂Cu electrode were examined. Finally, it is found that the experimental results were consistent with the calculated ones, indicating that the first-principles calculations can be used to investigate the lithiation mechanism of the intermetallic compounds.     

Tuning the electronic properties of single-walled SiC nanotubes by external electric field

The electronic properties of SiC nanotubes (SiCNTs) under external transverse electric field were investigated using density functional theory. The pristine SiCNTs were semiconductors with band-gaps of 2.03, 2.17 and 2.25 eV for (6,6), (8,8) and (10,10) SiCNTs, respectively. It was found the band gaps was reduced with the external transverse electric filed applied. The (8,8) and (10,10) SiCNTs changed from semiconductor to metals as the intensity of electric field reached 0.7 and 0.5 V/Å. The results indicate that the electronic properties of SiCNTs can be tuned by the transvers electric field with integrality of the nanotubes.     

Structural,electronic and magnetic properties of the 3d transition metal atoms adsorbed on boron nitride nanotubes

Adsorption configurations for a series of transition metal (TM) 3d atoms adsorbed on the zigzag (8, 0) BNNT at five different sites have been investigated using the first-principles PAW potential within DFT under GGA. The most stable adsorption sites are different for different TM atoms. Partially filled 3d metals V, Cr and Mn can bind strongly with zigzag (8, 0) BNNT, and Sc, Ti, Co and Ni can be chemically adsorbed on the (8, 0) BNNT. The binding between the Fe or Cu atom and the BNNT is only marginal. One unusual case is Zn. Its zero binding energy independent of the adsorption sites implies it can only physically adsorbed on the BNNT mainly stemmed from the van de Waals interaction. Electronic structure analyses show that: (1) for each TM atom adsorbed at five different sites, the total DOS curves of both majority and minority spins make a slightly relative shift along the energy axis, and for each site the total DOS of the minority spin shifts slightly in high energy direction with respect to that of the majority spin lead to a exchange splitting, except fully filled 3d metals Cu and Zn; (2) total DOS curves of both the majority and minority spins for the adsorbed systems shift to the lower energy region compared with that of the pristine (8, 0) BNNT. And the smaller 3d electrons number of the TM atom, the larger shift to the lower energy region of its DOS curves; (3) for V-, Mn- and Fe-adsorbed (8, 0) BNNT, only one type of electrons (either majority spin or minority spin) passes through the Fermi level implies these adsorbed systems are all half-metals.     

Hydrogen storage capacity of Li-adsorbed BC3 sheet tuned by the Li atom coverage rate

In this study, we performed first principles total energy calculations to investigate the hydrogen storage capacities of Li-adsorbed BC₃ sheets with various levels of Li atom coverage. We found that composite structures may be obtained with three levels of Li atom coverage, i.e., 8.3%, 25.0%, and 33.3%, where all the metal atoms could be adsorbed strongly on both sides of the BC₃ sheet without clustering. The results of our calculations showed that these Li-BC₃ complex structures possess hydrogen storage capacities of 5.18, 10.11, and 12.57 wt%, respectively. The latter two values satisfy the DOE requirements.     

7Li NMR study on Li+ ionic diffusion and phase transition in LixCoO2

The temperature dependence of the spin-lattice relaxation time, T₁ and the line width of the ⁷Li nucleus were measured in delithiated Li_xCoO₂ (x = 0.6, 0.8, 1.0). Two different relaxation behaviors were observed in the temperature dependence of T₁^− 1 in a x = 0.8 sample. These would have arisen from inequivalent Li sites in two coexisting phases; an original hexagonal (HEX-I) and a modified hexagonal (HEX-II) phase in the x = 0.8 sample. We analyzed using a phenomenological non Debye-type relaxation model. Motional narrowing in the line width was observed in each sample, the result revealing that Li⁺ ions begin to move at low temperature in samples with less Li content. It was found that the activation energy associating with Li⁺ ion hopping in the HEX-II phase is smaller than that in the HEX-I phase. These results show that the HEX-II phase produced in the Li deintercalation process would be suitable for Li⁺ ionic diffusion in multi-phase Li_xCoO₂, and it is expected that this would enable fast ionic diffusion. Li⁺ ionic diffusion related to phase transition is discussed from ⁷Li NMR results.     

Adsorption of oxygen molecular on pristine and defected SiC nanotubes

The structural and electronic properties of oxygen molecular adsorbed on the exterior surface of pristine and N_C or B_C defected (10,0) or (6,6) SiCNT have been investigated systematically using the first-principles projector-augmented wave potential within the density-functional theory under the generalized-gradient approximation. We find that for both pristine tubes the preferred adsorption sites of the O₂ molecule are above and nearly parallel to armchair Si-C bond whether physisorption or chemisorption. The strong chemical interaction between O₂ molecule and tube leads to not only a vanishing in magnetism of the O₂ molecule but also an outward relaxation of the underlying Si-C bond. The C atom substituted by N or B atom assists O₂ molecule adsorption above and nearly parallel to zigzag Si-N or Si-B bond as well as imparts a metallic character on the SiCNTs with higher concentration of the defects or a magnetism on the SiCNTs with lower concentration of the defects. Therefore, a combination of N or B doping followed by exposure to air may be an effective way to tune the electronic properties of the semiconducting SiCNTs. Furthermore, the lower binding energies for the pair of oxygen interstitials chemisorbed on N_C or B_C defected (10,0) or (6,6) SiCNT show that the oxygen molecule will dissociate to the pair of oxygen interstitials at the sidewall of N_C or B_C defected SiCNTs.     

Hard carbon/Li2.6Co0.4N composite anode materials for Li-ion batteries

Hard carbon/Li_2.6Co_0.4N composite anode electrode is prepared to reduce the initial high irreversible capacity of hard carbon, which hinders potential application of hard carbon in lithium ion batteries, by introducing Li_2.6Co_0.4N into hard carbon. Lithiated Li_2.6Co_0.4N provides the compensation of lithium in the first cycle, leading to a high initial coulombic efficiency of ca. 100% versus lithium. As-prepared hard carbon/Li_2.6Co_0.4N composite electrode presents initial capacity of 438 mA h g^− 1. A full cell using LiCoO₂ cathode and the composite anode shows much higher initial coulombic efficiency and capacity than those of a cell using LiCoO₂ and hard carbon anode. This paves the way to reduce the large initial irreversible capacity of hard carbon.     

Optical properties of transition metal atom adsorbed graphene: A density functional theoretical calculation

Electronic and optical properties of 3d-transition metal adsorbed graphene system, theoretically studied in the framework of density functional theory, reveals significant modification compared to the pristine system. Due to adsorption of transition metal, the emergence of closely separated electronic bands leads to substantial amount of low energy optical absorption below 2.0 eV photon energy. Very significant enhancement of static dielectric constant and large value of reflectivity in the low optical energy regime has been identified for different adsorbed systems. In the different 3d-transition metal adsorbed systems, particularly up to the half filled d-shell transition metal atom, pronounced emergence of optical absorption line in the deep ultraviolet regime beyond 30.0 eV photon energy is observed.     

Electronic and magnetic behaviors of graphene with 5d series transition metal atom substitutions: A first-principles study

The electronic structures and magnetic behaviors of graphene with 5d series transition metal atom substitutions are investigated by performing first-principles calculations. All the impurities are tightly bonded to single vacancy in a graphene sheet. The substitutions of La and Ta lead to Fermi level shifting to valence and conduction band, respectively. Both the two substitutions result in metallic properties. Moreover, the Hf, Os and Pt-substituted systems exhibit semiconductor properties, while the Re and Ir-substituted ones exhibit robust half-metallic properties. Interestingly, W-substituted system shows dilute magnetic semiconductor property. On the other hand, the substitution of Ta, W, Re and Ir induce 0.86 μ_B, 2 μ_B, 1 μ_B and 0.99 μ_B magnetic moment, respectively. Our studies demonstrate that the 5d series transition metal substituted graphene have potential applications in nanoelectronics, spintronics and magnetic storage devices.     

Synthesis and characterization of layered Li1−x(Ni0.5Co0.5)1−yFeyO2(0 ≤ (1 − x) ≤ 1 and 0 ≤ y ≤ 0.2) cathodes

Layered Li(Ni_0.5Co_0.5)_1−yFe_yO₂ cathodes with 0 ≤ y ≤ 0.2 have been synthesized by firing the coprecipitated hydroxides of the transition metals and lithium hydroxide at 700 °C and characterized as cathode materials for lithium ion batteries to various cutoff charge voltages (up to 4.5 V). While the y = 0.05 sample shows an improvement in capacity, cyclability, and rate capability, those with y = 0.1 and 0.2 exhibit a decline in electrochemical performance compared to the y = 0 sample. Structural characterization of the chemically delithiated Li_1−x(Ni_0.5Co_0.5)_1−yFe_yO₂ samples indicates that the initial O3 structure is maintained down to a lithium content (1 − x) ≈ 0.3. For (1 − x) < 0.3, while a P3 type phase is formed for the y = 0 sample, an O1 type phase is formed for the y = 0.05, 0.1 and 0.2 samples. Monitoring the average oxidation state of the transition metal ions with lithium contents (1 − x) reveals that the system is chemically more stable down to a lower lithium content (1 − x) ≈ 0.3 compared to the Li_1−xCoO₂ system. The improved structural and chemical stabilities appear to lead to better cyclability to higher cutoff charge voltages compared to that found before with the LiCoO₂ system.     

Molybdenum carbide nano-sheet as a high capacity anode material for monovalent alkali metal-ion batteries—Theoretical investigation

This contribution presents a theoretical investigation of monovalent metal-ion adsorption and diffusion on two-dimensional (2D) buckled nanostructure of molybdenum carbide (MoC) by using the first principle method. We find that buckled MoC nanostructure exhibits great stability, semiconducting electronic property, and high performance as electrode material. Interestingly, Crystal Orbital Hamilton population (COHP) method results show that buckled MoC is chemically stable in a wide range of temperatures, and various Li, Na, ions adsorbed configurations, which is beneficial for anode materials. Especially, single-layer MoC exhibits a superior theoretical capacity of 993.16 mA h g⁻¹ for Li-ions and 496.58 mA h g⁻¹ for Na/K-ions. The storage capacity of 1200 mA h g⁻¹ is found for the adsorption of ions on multilayer bulk MoC. Moreover, migration energy barriers are predicted as 0.38 eV for Li, 0.32 eV for Na, and 0.24 eV for K; these remarkable results determine the applicability of buckled MoC as ideal anode material for metal-ion battery applications.     

Origin of enhanced Ge interdiffusion at the initial stage of Ge deposition on Si(5 5 12)-2 × 1: Tensile stress induced by substrate chain structures

By combined investigation of STM and synchrotron PES on Ge/Si(5 5 12)-2 × 1 at 530 °C, it has been found that, in addition to the upward-relaxed surface Si atoms, a subsurface Si atom is also readily replaced by an arriving Ge atom at the initial adsorption stage. Such enhanced interdiffusion is due to a unique character of one-dimensional chain structures of the reconstructed substrate, such as π-bonded and honeycomb chains not existing on other low-index Si surfaces such as Si(001)-c(4 × 2) and Si(111)-7 × 7, applying a tensile surface stress to the neighbouring subsurface atoms. Interdiffusion of Ge having lower surface energy induces adsorption of the displaced Si atoms on the surface to form sawtooth-like facets composed of (113)/(335) and (113)/(112) with arriving Ge atoms until the surface is filled with those facets. Such displacive adsorption is the origin of high Si concentration of formed facets.     

Structural,XPS and impedance analysis of LixCoVO4 (x = 0.8, 1.0, 1.2)

Lithium cobalt vanadate Li_xCoVO₄ (x = 0.8; 1.0; 1.2) has been prepared by a solid state reaction method. The XRD analysis confirms the formation of the sample. A new peak has been observed for Li_1.0CoVO₄ and for Li_1.2CoVO₄ indicating the formation of a new phase. The XPS analysis indicates the reduction in the oxidation of vanadium and oxygen with the addition of Li in Li_xCoVO₄ (x = 0.8, 1.0, 1.2). The impedance analysis gives the conductivity value as 2.46 × 10^− 5, 6.16 × 10^− 5, 9 × 10^− 5 Ω^− 1 cm^− 1 for Li_xCoVO₄ (x = 0.8; 1.0; 1.2), all at 623 K. The similarity in the bulk activation energy (E_a) and the activation enthalpy for migration of ions (E_ω) indicate that the conduction in Li_1.2CoVO₄ has been due to hopping mechanism.