Search

The electronic structure of group VA impurities in PbTe is investigated by first principles calculations. Our findings show that these impurities can act as either donors or acceptors, depending on whether they occupy Pb or Te lattice sites. The impurities introduce a defect state at ∼0.3 eV above the conduction band minimum if they stay at Pb sites, whereas they introduce a state at the edge of the PbTe valence band if they stay at Te sites. The implications of our calculations on the thermoelectric efficiency of PbTe are also discussed.

Lithium-rich oxide materials are promising candidates for high-energy lithium ion batteries, but currently have critical challenges of poor cycle performance and voltage drop induced by undesirable phase transformation. To resolve these problems, it is necessary to identify the origins and mechanism of phase transformation in Li2MnO3, a key component of Li-rich oxides. In this work, the phase transformation of bulk Li2MnO3 is investigated by thermodynamic and kinetic approaches based on first-principles calculations and validated by experiments. Using the calculated thermodynamic energies, the most stable structure is determined as a function of Li extraction for Li2−xMnO3: monoclinic (x = 0.0–0.75), layered-like (x = 1.0–1.25), and spinel-like (x = 1.5–2.0) structures. The phase transformation becomes kinetically possible for Li2−xMnO3 (x > 1.0). Atomic scale origins and the mechanism of phase transformation are elucidated by the thermodynamically stable and kinetically movable tetrahedral coordination of Mn4+ in the transition state. These theoretical observations are validated by ex situ X-ray photoelectron spectroscopy combined with electrochemical experiments for Li2−xMnO3 with various Li contents upon cycling. The mechanistic understanding from theoretical calculations and experimental observations is expected to provide a fundamental solution and guidelines for improving the electrochemical performance of Li-rich oxides and, by extension, the battery performance.

As a promising hybrid energy storage system, lithium ion capacitors (LICs) have been intensively investigated regarding their practical use in various applications, ranging from portable electronics to grid support. The asymmetric LIC offers high-energy and high-power densities compared with conventional energy storage systems such as electrochemical double-layer capacitors (EDLCs) and lithium ion batteries (LIBs). To enable suitable operation of the LIC, the negative electrode should be pre-lithiated prior to cell operation, which is regarded as a key technology for developing self-sustainable LICs. In this work, we have demonstrated the potential use of Li6CoO4 as an alternative lithium source to metallic lithium. A large amount of Li+ can be electrochemically extracted from the structure incorporated into the positive electrode via a highly irreversible process. Most of the extracted Li+ is available for pre-lithiation of the negative electrode during the first charge. This intriguing electrochemical behaviour of Li6CoO4 is suitable for providing sufficient Li+ to the negative electrode. To obtain a fundamental understanding of this system, the electrochemical behaviour and structural stability of Li6CoO4 is thoroughly investigated by means of electrochemical experiments and theoretical validation based on first principles calculations.

For the over-lithiated-oxides (OLOs), a composite of layered Li2MnO3 and LiMO2 (M = Mn, Co, Ni), the Li2MnO3 part is not stable after the 1st charge–discharge cycle and partly transforms into layered LiMnO2, which in practice indicates that the phase used is actually a mixture of both Li2MnO3 and LiMnO2. In the present work, the influence of 10 cationic (Mg, Ti, V, Nb, Fe, Ru, Co, Ni, Cu, and Al) and 2 anionic (N and F) dopants on the phase stability, redox potential, ionic and electronic conductivity of both Li2MnO3 and LiMnO2 is investigated in detail using density functional theory. The calculations show that all the cationic dopants and F can be thermodynamically stable in the layered structures. The redox potential of both oxides is quite sensitive to some of the dopants, like V, Nb, and Ru, due to the appearance of gap states introduced by those dopants. The Jahn–Teller effect has a strong influence on the Li vacancy diffusion behavior in both LiMnO2 and its doped phases. Li vacancy diffusion behavior in Li2MnO3, including both interlayer and intralayer pathways, is relatively more complex and some dopants like Mg, Ti, Nb, and Ru can decrease the barriers of the diffusion paths. The calculations also show the evidence of hole polaron formation in LiMnO2 and electron polaron formation in Li2MnO3 which should be the reason why these phases have low electronic conductivities. Based on these findings, possible ways to improve the electronic conductivity through the doping process are discussed.

The experimental study of interlayer sliding at the nanoscale in layered solids has been limited thus far by the incapability of mechanical and imaging probes to simultaneously access sliding interfaces and overcome through mechanical stimulus the van der Waals and Coulombic interactions holding the layers in place. For this purpose, straightforward methods were developed to achieve interlayer sliding in molybdenum disulfide (MoS2) while under observation within a transmission electron microscope. A method to manipulate, tear, and slide free-standing atomic layers of MoS2 is demonstrated by electrostatically coupling it to an oxidized tungsten probe attached to a micromanipulator at a bias above ±7 V. A first-principles model of a MoS2 bilayer polarized by a normal electric field of 5 V/nm, emanating from the tip, demonstrates the appearance of a periodic negative sliding potential energy barrier when the layers slide into the out-of-registry stacking configuration, hinting at electrostatic gating as a means of modifying the interlayer tribology to facilitate shear exfoliation. A method to shear focused ion beam prepared MoS2 cross section samples using a nanoindenter force sensor is also demonstrated, allowing both the observation and force measurement of its interlayer dynamics during shear-induced sliding. From this experiment, the zero normal load shear strength of MoS2 can be directly obtained: 25.3 ± 0.6 MPa. These capabilities enable the site-specific mechanical testing of dry lubricant-based nanoelectromechanical devices and can lead to a better understanding of the atomic mechanisms from which the interlayer tribology of layered materials originates.

Energetics and kinetics of Li insertion into c-Si and a-Si systems are investigated using the density functional (DFT) theory calculations and kinetic Monte Carlo (KMC) simulations. DFT formation energies show the mechanism of phase separation between crystalline silicon and amorphous lithium silicide. Both crystalline and amorphous Si show similar trends in volume expansion and phase transition under lithiation, and kinetics of Li diffusion in bulk silicon (from DFT and KMC) shows a big difference between c-Si and a-Si. The Li migration barrier is 0.6 eV in c-Si, and quickly decreases to 0.4 eV under increasing Li concentration or Si volume expansion. To simulate Li diffusion in amorphous silicon using KMC, we have developed a formulation for environment dependent migration energy barriers of Li in a-Si using a volume dependent function. KMC simulations are performed for Li diffusion in both c-Si and a-Si, and the diffusion coefficient of Li in a-Si is an order of magnitude larger than in c-Si. These studies help to understand mechanisms of lithiation with atomic scale details and elucidate the phase separation between c-Si and lithium silicide.

To overcome the limitation of conventional fixed charge potential methods for the study of Li-ion battery cathode materials, a dynamic charge potential method, charge-transfer modified embedded atom method (CT-MEAM), has been developed and applied to the Li-Co-O ternary system. The accuracy of the potential has been tested and validated by reproducing a variety of structural and electrochemical properties of LiCoO2. A detailed analysis on the local charge distribution confirmed the capability of this potential for dynamic charge modeling. The transferability of the potential is also demonstrated by its reliability in describing Li-rich Li2CoO2 and Li-deficient LiCo2O4 compounds, including their phase stability, equilibrium volume, charge states and cathode voltages. These results demonstrate that the CT-MEAM dynamic charge potential could help to overcome the challenge of modeling complex ternary transition metal oxides. This work can promote molecular dynamics studies of Li ion cathode materials and other important transition metal oxides systems that involve complex electrochemical and catalytic reactions.

Transition metal (TM) modification is a common strategy for converting an earth-abundant mineral into a cost-effective catalyst for industrial applications. Among a variety of minerals, Al

Among the candidates to replace Li-ion batteries, Li-S cells are an attractive option as their energy density is about five times higher (~2,600 Wh kg

A fundamental understanding of anomalous redox mechanisms in hexacyanometallate compounds, compared with conventional NaMO2 systems (M: transition metals), is presented based on first-principles calculations and experimental validations. From theoretical calculations, we identified low-spin and high-spin states of Fe ions coordinated by the cyanide group (-CN) with the same oxidation state (Fe2+) in Na2Fe2(CN)6. Considering the site dependency of d electronic spin configurations based on the crystal field theory (CFT) of transition metals (TMs), we calculated the thermodynamic mixing energy using Na2Fe2(CN)6 and Na2Mn2(CN)6 for obtaining a thermodynamically stable phase of Na2FeMn(CN)6. The phase stabilities of Na2Fe2-xMnx(CN)6 among many atomic configurations and lattice parameters originating from octahedral structures (i.e., Fe(CN)6 and Mn(NC)6) are highly dependent on the electronic structures of TMs with spin states. From partial density of states (PDOS) and spatial electron distributions, it was observed that Fe2+ in the low-spin state (t) and Mn2+ in the high-spin states (t and e) in the stable phase lead to higher redox potentials (∼3.55 V vs. Na/Na+) with the removal of Na+ as compared to that of Na2Fe2(CN)6. In addition, lattice parameters from x = 0 to x = 1 in Na2Fe2-xMnx(CN)6 are increased due to the larger ionic radius of Mn2+ in the high-spin states. On the other hand, Fe2+ in the high-spin states (t and e) and Mn2+ in the low-spin state (t) in the most unstable phase of Na2FeMn(CN)6 would have lower redox potentials. Based on the fundamental correlation between redox potentials and CFT with spin configurations of TMs, we suggest a material design concept for intercalation compounds with higher energy densities for rechargeable battery systems.

In an effort to develop high-energy-density cathodes for sodium-ion batteries (SIBs), low-cost, high capacity Na(Li

Ni-rich LiNi0.8Co0.1Mn0.1O2 layered oxide cathodes have been highlighted for large-scale energy applications due to their high energy density. Although its specific capacity is enhanced at higher voltages as Ni ratio increases, its structural degradation due to phase transformations and lattice distortions during cycling becomes severe. For these reasons, we focused on the origins of crack generation from phase transformations and structural distortions in Ni-rich LiNi0.8Co0.1Mn0.1O2 using multiscale approaches, from first-principles to meso-scale phase-field model. Atomic-scale structure analysis demonstrated that opposite changes in the lattice parameters are observed until the inverse Li content x = 0.75; then, structure collapses due to complete extraction of Li from between transition metal layers. Combined-phase investigations represent the highest phase barrier and steepest chemical potential after x = 0.75, leading to phase transformations to highly Li-deficient phases with an inactive character. Abrupt phase transformations with heterogeneous structural collapse after x = 0.81 (~220 mAh g−1) were identified in the nanodomain. Further, meso-scale strain distributions show around 5% of anisotropic contraction with lower critical energy release rates, which cause not only micro-crack generations of secondary particles on the interfaces between the contracted primary particles, but also mechanical instability of primary particles from heterogeneous strain changes.

The effects of four typical defects on the hydroxyl groups’ migration and the conductivity of graphene have been studied using density functional theory and nonequilibrium Green’s function formalism. An obvious anisotropy of the diffusion barriers along different paths is correlated to the symmetric behavior of spin-polarized charge density around the defects. The migration energy scenario indicates that the defects effectively hinder the hydroxyl groups’ migration toward them, indicating that most hydroxyl groups could be stabilized outside the defect region in reduced graphene oxide. Through the electronic transport calculations and local density of states analysis, hydroxyl groups locating outside of the defect region will cause the transport channels near the Fermi level to disappear and reduce the conductance considerably.

Using ab-initio density-functional theory (DFT) methods, the atomic structure and electronic properties of one of the most promising family of solid electrolytes for Li-ion battery applications, lanthanum oxides with a garnet-type structure (c-Li7La3Zr2O12) are studied. The Li-ion (Li+) defects including Li/Li+ vacancies, interstitials, and vacancy–interstitial pair defect formation energy within the Li7La3Zr2O12 supercell are systematically investigated. This study is essential to understand the defect chemistry and the Li+ conductivity mechanisms. Our results indicate that the Li+ vacancy defects are thermodynamically more favorable than interstitial Li+ defects. This work will therefore be helpful to elucidate the atomic level mechanisms of Li defect formation in order to improve the ionic conductivity for future Li-ion battery applications.

We present an Al2O3 dielectric layer on molybdenum disulfide (MoS2), deposited using atomic layer deposition (ALD) with ozone/trimethylaluminum (TMA) and water/TMA as precursors. The results of atomic force microscopy and low-energy ion scattering spectroscopy show that using TMA and ozone as precursors leads to the formation of uniform Al2O3 layers, in contrast to the incomplete coverage we observe when using TMA/H2O as precursors. Our Raman and X-ray photoelectron spectroscopy measurements indicate minimal variations in the MoS2 structure after ozone treatment at 200 °C, suggesting its excellent chemical resistance to ozone.

We investigate the impact of interstitial Li and Li vacancy on the electronic structure of Li 7 P 3 S 11 and the mechanism of Li ion migration in Li 7 P 3 S 11 through first principles calculations. We find that Li 7 P 3 S 11 is a good insulator with a wide band gap of 3.5 eV. We find that both Li vacancy and interstitial Li + ion do not introduce states in the band gap hence they do not deteriorate the electronic properties of Li 7 P 3 S 11 . The calculated formation energy of Li vacancy is much larger than that of Li interstitial. The interstitial Li are found to be stable at many sites in Li 7 P 3 S 11 . More importantly, the lowest energy barrier for interstitial Li + ion diffusion in Li 7 P 3 S 11 is found to be 0.24 eV. Our calculations suggest that interstitial Li ions play a very important role on the Li 7 P 3 S 11 ionic conductivity.

Good cyclability is essential for the potential application of cathode materials. Here, we investigate the structural stability of two-dimensional (2D) Li-layered and three-dimensional (3D) structured polymorphs of Li2FeSiO4 and Li2MnSiO4 using the density functional theory calculations. We find that all 2D Li-layered polymorphs of both materials are unstable upon full delithiation owing to layer exfoliation, which can lead to an amorphous structure. However, in contrast to the fact that the amorphization of Li2FeSiO4 can be prevented by the formation of the 3D cycled structure that is energetically stable, the 3D cycled structure of Li2MnSiO4 is found to be unstable during delithiationlithiation cycling. As a result, Li2MnSiO4 easily undergoes amorphization and shows a poor cyclability.

We investigate the pathways of hydrogen migration and associative desorption of H2 on the Si(001) and Ge(001)-(2×1) reconstructed surfaces with adsorbed ad-dimers, using density functional theory m...

A first principles investigation is performed to study the structural and electrochemical properties of four polymorphs of the Li 2 FeSiO4 orthosilicate family ( Pmn2 1 , Pmnb , P2 1 /n and Pbn2 1 ), to predict the effect of the transition metal composition. Our results show that the inclusion of Mn and Ni cations helps to stabilize the layered structures, but the polymorphs with a 3D cation network arrangement ( P2 1 /n and Pbn2 1 ) still have comparable energetic stability. We also show that the decrease of the cationic repulsion with the extraction of Li (found for these polymorphs during the discharge process) reduces the voltage step between the first and second Li extraction processes. The effect of multiple interactions between Li and the transition metals is examined, and the implications of multicomponent structures to improve the design of battery cathode are also addressed.

The presence of grain boundary affects the mechanical strength, thermal dissipation, and charge transport of polycrystalline graphene flakes. There is still a debate on whether the electronic transmission is severely degraded by the grain boundary, especially between simulations and experiments. To address this issue, we performed electrical transport simulations based on π-orbital tight-binding Hamiltonian. Our results show that the intrinsic grain boundary is almost transparent for the carrier transport, but extrinsic chemical species (e.g., oxygen, hydroxyl) favor the adsorption on interdomain sites and increase the scattering substantially at the boundary region. The experiment, which shows degraded carrier transport due to grain boundary, can be plausibly explained with our theoretical results. To minimize the extrinsic effects of grain boundaries, we suggest doing electrical measurements under ultrahigh-vacuum condition after thermal annealing or applying pulsed current for desorbing the adsobates.

We compute thermal conductivity of graphene oxide at room temperature with molecular dynamics simulation. To validate our simulation model, we have investigated phonon scattering in graphene due to crystal boundary length and isotope defect, both of which are able to diagnose the behavior of long wavelength and short wavelength phonon scattering. Our simulation shows that thermal conductivity of pristine graphene has logarithmic divergence for the boundary length up to 2 μm. As compared with pristine graphene, thermal conductivity of graphene oxide can be reduced by a factor of 25 at low oxygen defect concentration. Moreover, we find that not only the concentration but also the configuration of the oxygen functional groups (e.g., hydroxyl, epoxide, and ether) has significant influence on the thermal conductivity. Through phonon mode analysis, phonon defect scattering as well as phonon localization are mainly responsible for the conspicuous reduced thermal conductivity. The simulation results have provided...

A novel phase transition, from multilayered 2H-MoTe

The rational design of semiconductor nanocrystals with well-defined surfaces is a crucial step towards the realization of next-generation photodetectors, and thermoelectric and spintronic devices. SnTe nanocrystals, as an example, are particularly attractive as a type of topological crystalline insulator, where surface facets determine their surface states. However, most of the available SnTe nanocrystals are dominated by thermodynamically stable {100} facets, and it is challenging to grow uniform nanocrystals with {111} facets. In this study, guided by surface-energy calculations, we employ a chemical vapour deposition approach to fabricate Bi doped SnTe nanostructures, in which their surface facets are tuned by Bi doping. The obtained Bi doped SnTe nanoribbons with distinct {111} surfaces show a weak antilocalization effect and linear magnetoresistance under high magnetic fields, which demonstrate their great potential for future spintronic applications.

Antimonide based III-V materials are widely used in quantum-well transistors and long wavelength optoelectronic devices benefit from their narrow bandgaps and high carrier mobilities. Interface chemistry has proven to be important in establishing reliable devices. The InSb/HfO2 stacks have been systematically studied upon atomic layer deposition (ALD) and rapid thermal annealing (RTA) at 325 °C and 400 °C, utilizing X-ray photoelectron spectroscopy (Al Kα1) and synchrotron radiation photoemission spectroscopy. No “clean up” effect was observed after the ALD process. The interface oxidization, elemental diffusion and substrate oxide desorption have been observed upon the RTA process. This work highlights the importance of substrate passivation prior to ALD process to obtain a thermally stable InSb/HfO2 interface for InSb based devices.

Currently 2D crystals are being studied intensively for use in future nanoelectronics, as conventional semiconductor devices face challenges in high power consumption and short channel effects when scaled to the quantum limit. Toward this end, achieving barrier-free contact to 2D semiconductors has emerged as a major roadblock. In conventional contacts to bulk metals, the 2D semiconductor Fermi levels become pinned inside the bandgap, deviating from the ideal Schottky-Mott rule and resulting in significant suppression of carrier transport in the device. Here, MoS2 polarity control is realized without extrinsic doping by employing a 1D elemental metal contact scheme. The use of high-work-function palladium (Pd) or gold (Au) enables a high-quality p-type dominant contact to intrinsic MoS2 , realizing Fermi level depinning. Field-effect transistors (FETs) with Pd edge contact and Au edge contact show high performance with the highest hole mobility reaching 330 and 432 cm2 V-1 s-1 at 300 K, respectively. The ideal Fermi level alignment allows creation of p- and n-type FETs on the same intrinsic MoS2 flake using Pd and low-work-function molybdenum (Mo) contacts, respectively. This device acts as an efficient inverter, a basic building block for semiconductor integrated circuits, with gain reaching 15 at VD = 5 V.

Recently, demands on energy storage systems of high capacity and efficiency are continuously increased as the growth in commercialization of new renewable energy generation, such as wind and solar power, electric vehicle and portable electronic devices. Li-ion battery(LIB) has been introduced in diverse electronic products as one of the promising energy storage devices because of good energy density and long cycle life to answer these demands. For these reasons, study on LIB actively progressed to improve the performance of LIB. However, there are critical obstacles to make advance in energy density, such as increase in voltage and capacity, of conventionally used LIB. The commercialized LIB system use liquid electrolytes as for Li-ion migration between two electrodes. This liquid electrolyte deteriorate the stability of LIB, bring about overheating and explosion as temperature changes originated mainly from increase in operating voltage. In addition, leakage of organic based liquid electrolyte from the inside of LIB could have noxious effect on health. Therefore, alternatives on liquid electrolyte have been highly required to resolve these limitations. As increase in efforts to develop new electrolyte types, Inorganic solid electrolytes have received attentions to substitute it for liquid electrolytes. Inorganic solid electrolytes is one of the promising alternatives of liquid electrolyte because of their excellent thermal stability and good Li ion conductivity. Among inorganic electrolytes, Li10GeP2S12 (LGPS) indicates the great Li ion conductivity to be comparable with Li ion conductivity of liquid conductivity. Despite of this superior Li ion conductivity, the understanding of mechanism for Li ion diffusion in LGPS has not been clearly understood. This study explains correlation between Li ion diffusion and local environments of LGPS to understand mechanism of Li ion migration by first principles calculation. LGPS has enough Li ion concentrations and two types of migration paths. Li ion could migrate in frameworks of Ge-S and P-S(fig.1a). These frameworks secure c-axis and ab-plane paths for Li ion diffusions. There are also two types of methods for first principles calculation of Nudged Elastic Band(NEB) method of Li ion diffusion. One is defect migration (single ion migration) and the other is multi ion concentration migration(fig.1). Firstly, defect migration was analyzed by NEB. We setup three local environments for understanding the correlation in c-axis direction migration. The three situations are only one Li ion without any other Li ions in LGPS(fig.1b), one Li ion in the migration path around Li ions existent in the other non-migration paths(fig.1c) and only one vacancy with existent of Li ions in the migration path(fig.1b). Although the first and second cases of Li ion migration indicates similar tendency and energy barrier for Li ion migration, the third case shows that energy barrier in same migration path is more higher than the energy barrier of the other cases. This means that the factor influencing the Li ion diffusion is not the local Li ions in non-migration path but the near Li ions in the migration path. In addition to single ion diffusion, multi ion concentration migration is examined and this type has no vacancy and assumes that the all Li ions in the path diffuse all together. The energy barrier is lower than the energy barrier in all of single ion migration cases(fig.1f). This indicates that simultaneous migration of all Li ions in migration path is energetically beneficial. The ab-plane migration has two migration paths and the energy barriers are relatively higher than the energy barrier of c-axis migration path(fig.1e). This means that Li ion migration is mainly progressed in c-axis rather than ab-plane regardless of local environments in LGPS. Therefore, this understanding of mechanism for Li ion diffusion in LGPS would be effective guide line for developing the superior inorganic solid electrolyte based on LGPS. Figure 1

With the increasing demand for electric vehicles (EVs) and portable electronic devices, interest in high energy density lithium-ion batteries (LIBs) is continuously growing. However, to enhance the overall properties of LIBs, alternatives are needed from the material aspect due to the intrinsic limitations of current commercialized LIBs such as instability of liquid electrolyte. As an alternative to mitigate thermal instability of LIBs, all-solid-state lithium batteries (ASSLBs) employing a solid-state electrolyte is receiving attention, which would mitigate flammability and internal short circuit. However, studies on the intrinsic properties of these solid-state electrolyte have been limited comparing to a growing interest on ASSLBs. Therefore, in this study, the stability of Li10GeP2S12 (LGPS), one of the sulfide solid-state electrolytes, is investigated based on first principles calculation. Designing a unit crystal structure of the LGPS to be used for first principles calculations is difficult because of the need to independently consider the Ge/P arrangements and lithium ions along c-axis arrangements in LGPS. Various researches have been suggested a complex LGPS crystal structure for first principles calculation, but all of the proposed models are slightly different. Recently, Oh et al. used the model considering Ge/P arrangements, lithium ions equi-distance arrangements, and the symmetry of the crystal system. As this model having reliability by considering all the possibilities with high throughput DFT calculations, all the calculations in this study will be conducted based on the crystal structure of LGPS in Fig 1. (a). We investigated the diffusion coefficients of lithium ions in LGPS at various temperature using ab initio molecular dynamics (AIMD) simulations. At the beginning of the simulation, the desirable temperature of the system was set by velocity scaling method. After the temperature of the system matched with assigned temperature, the AIMD simulation was executed until the diffusivity converged in the NVT ensemble with a Nosé-Hoover thermostat. With the calculated diffusivity at several high temperature organized on a table in Fig 1. (b), the diffusivity of LGPS at room temperature was extrapolated and well match with experimental results. After verifying the reliability of the unit crystal structure of LGPS, the electronic structure of LGPS was calculated to elucidate the electrochemical stability of LGPS along various temperature. As shown on a table in Fig 1. (c), the nature of lithium inside LGPS at room or high temperature is ionic state, almost +1 charged state, inside the solid-state electrolyte. Moreover, the other elements, Ge, P, and S, have the constant charge states at different temperature, showing the nature of solid-state electrolyte. Furthermore, in order to qualitatively compare the electrochemical reactivity with regard to temperature, the density of states (DOS) was plotted in Fig 1. (d). The DOS of the elements at different temperature shows the properties of insulator of LGPS, and the reduced band gap at high temperature. From the results of the electronic structure, the Bader charge and the DOS, indicate that LGPS states in constant charge, but increasing electrochemical reactivity by narrowing the band gap as temperature increase. In this study, we elucidate the stability of LGPS by temperature. The electronic structure calculation based on first principles calculation confirms that the change of electric charge of each atom is small according to temperature, but the possibility of electrochemical reaction become high. As a result, LGPS can act as a solid-state electrolyte even at high temperature, but the dangerousness of internal short circuit could be increased if operating temperature of LGPS become extremely high considering band gap variation along temperature. Figure 1

Lithium lanthanum titanate (LLTO) is considered a particularly promising candidate for use as an electrolyte in all-solid-state lithium ion batteries due its good electrochemical stability and high ionic conductivity. However, measured values for the ionic conductivity of LLTO fall far below theoretical predictions, an incongruity which has been attributed to the presence of grain boundaries. Furthermore, recent experimental work has shown that lithium dendrite growth occurs along the grain boundaries in LLTO. While experimental studies have been able to reveal the detailed microstructure of polycrystalline LLTO, but little effort has been made to identify the fundamental mechanisms at work within the grain boundaries which lead to to suppressed ionic conductivity and Li dendrite growth. Using ab-initio calculations, we examine the electronic character of LLTO grain boundaries and investigate the impact of the defect states on ion transport and dendrite formation. Based on a fundamental understanding of the atomic and electronic character of LLTO grain boundaries, material design approaches are proposed to mitigate the impact of grain boundary behavior and improve the viability of LLTO as a practical solid electrolyte. This work was supported by the International Energy Joint R&D Program (No. 20168510011350) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Ministry of Knowledge Economy, Korean government. This work is also supported by the L&F Co.’s World Class 300 Project of the Korea Institute of Advancement of Technology (KIAT) funded by the Ministry of Trade, industry and Energy (No.S2483103).

Doping is an effective strategy to modulate the electronic states of a semiconductor and improve its relevant device performance. Here, we propose a realistic monolayer two-dimensional solid electrolyte material of YBr3 to implement the carrier doping on monolayer MoS2. The stabilities, the carrier doping effect, and the electronic structures of Li-, Na-, K-, Ca-, and F-doped monolayer MoS2 through YBr3 based on the MoS2/YBr3 heterostructure have been explored by utilizing first-principles calculations. The insertion of the YBr3 layer improves the stabilities and the carrier doping effect in making monolayer MoS2 as an n-type or p-type semiconductor by looking into the binding energies and the electronic structures. More significantly, no deep impurity energy bands are introduced within the band gap of MoS2. In addition, the work function of MoS2 can be manipulated in the range from 3.59 eV to 6.58 eV due to the charge transfer and the charge redistribution caused by doping. These findings provide an effective and promising route to achieve both n- and p-type doping of monolayer MoS2.

A composite of Lanthanum manganese oxide-carbon (LaMnO3/C) has been also synthesized by a facile electrospinning approach followed by controlled heat treatment, in which the carbon form a continuous conductive network connecting the electrocatalyst LaMnO3 nanoparticles together to facilitate good electrochemical performance. Based on a correlation between theoretical (DFT) and experimental analysis on the effect of the surface atomic arrangement, the electrocatalyst (Mn-terminated LaMnO3) with uniform Mn surface termination show favourable rechargeability, and good phase and morphology stability in lithium oxygen batteries compared to La rich surface termination. Excellent cycling performance is also demonstrated, in which the terminal voltage is higher than 2.4 V after 100 cycles at limited capacity of 1000 mAh g-1 (based on composite). Figure 1

Author(s): Pan, Y; Folsch, S; Lin, YC; Jariwala, B; Robinson, JA; Nie, Y; Cho, K; Feenstra, RM | Abstract: Scanning tunneling microscopy (STM) at 5 K is used to study WSe2 layers grown on epitaxial graphene which is formed on Si-terminated SiC(0 0 0 1). Specifically, a partial hydrogenation process is applied to intercalate hydrogen at the SiC-graphene interface, yielding areas of quasi-free-standing bilayer graphene coexisting with bare monolayer graphene. We find that an abrupt and structurally perfect homojunction (band-edge offset ∼0.25 eV) is formed when WSe2 overgrows a lateral junction between adjacent monolayer and quasi-free-standing bilayer areas in the graphene. The band structure modulation in the WSe2 overlayer arises from the varying work function (electrostatic potential) of the graphene beneath. Scanning tunneling spectroscopy measurements reveal that this effect can be also utilized to create WSe2 quantum dots that confine either valence or conduction band states, in agreement with first-principles band structure calculations.

In this paper, we device stress-diffusion full coupling multiscale analysis method for battery electrode simulation. In proposed method, the diffusive and mechanical properties of electrode material depend on Li concentration are estimated using density function theory(DFT) simulation. Then, stress-diffusion full coupling continuum formulation based on finite element method(FEM) is constructed with the diffusive and mechanical properties calculated from DFT simulation. Finally, silicon nanowire anode charge and discharge simulations are performed using the proposed method. Through numerical examples, the stress-diffusion full coupling method shows more resonable results than previous one way continuum analysis.

Controlled fabrication of nanometer-scale devices such as quantum dots and nanowires requires an understanding of the initial chemisorption mechanisms involved in epitaxial growth. Vapor phase epitaxy can provide controlled deposition when using precursors that are not reactive with the H-terminated surfaces at ambient temperatures. For instance, digermane (Ge2H6) has potential as such a precursor for Ge ALE on Si(100) surfaces at moderate temperatures; yet, its adsorption configuration and subsequent decomposition pathways are not well understood. In situ Fourier transform infrared spectroscopy and first principles calculations reveal that Ge2H6 chemisorbs through a β-hydride elimination mechanism, forming Ge2H5 and H on both Si(100)-(2 × 1) and Ge(100)-(2 × 1) surfaces, instead of the previously proposed Ge–Ge bond breaking mechanism, and subsequently decomposes into an ad-dimer. The resulting coverage of Ge after a saturation exposure is estimated to be about 0.3 monolayers. Interestingly, the decompos...

First-principles calculations are performed to investigate interface properties, Li defects formation and migration mechanism across the interface between negative metal electrode and solid electrolyte (Li/γ-Li3PO4). We have analyzed the band alignment between Li and Li3PO4, interfacial charge distribution and electronic properties to elucidate the properties of the model interface. Our results show that the high Li-ion (Li+) defect formation energy is determined by the Li metal Fermi level leading to low ionic conductivity of Li metal/electrolyte interface. The electronic structure study of this Li metal/Li3PO4 interface provides information on the Li defect formation and migration, which will help us to improve the ionic conductivity for future Li-ion battery.

The contact resistance of metal-graphene junctions has been actively explored and exhibited inconsistencies in reported values. The interpretation of these electrical data has been based exclusively on a side-contact model, that is, metal slabs sitting on a pristine graphene sheet. Using in situ X-ray photoelectron spectroscopy to study the wetting of metals on as-synthesized graphene on copper foil, we show that side-contact is sometimes a misleading picture. For instance, metals like Pd and Ti readily react with graphitic carbons, resulting in Pd- and Ti-carbides. Carbide formation is associated with C-C bond breaking in graphene, leading to an end-contact geometry between the metals and the periphery of the remaining graphene patches. This work validates the spontaneous formation of the metal-graphene end-contact during the metal deposition process as a result of the metal-graphene reaction instead of a simple carbon diffusion process.

During chemical-vapor-deposited graphene transfer onto target substrates, a polymer film coating is necessary to provide a mechanical support. However, the remaining polymer residues after organic solvent rinsing cannot be effectively removed by the empirical thermal annealing in vacuum or forming gas. Little progress has been achieved in the past years, for little is known about the chemical evolution of the polymer macromolecules and their interaction with the environment. Through in situ Raman and infrared spectroscopy studies of PMMA transferred graphene annealed in nitrogen, two main processes are uncovered involving the polymer dehydrogenation below 200 °C and a subsequent depolymerization above 200 °C. Polymeric carbons over the monolayer graphitic carbon are found to constitute a fundamental bottleneck for a thorough etching of PMMA residues. The dehydrogenated polymeric chains consist of active C═C bonding sites that are readily attacked by oxidative gases. The combination of Raman spectroscopy, ...

Using density-functional theory (DFT) methods, we have investigated the structural, electronic and Li transport properties of several silicate polymorphs, some of them already synthesized and characterized in the laboratory. Our study includes the lithiated and several delithiated phases, in order to accurately describe the voltage profiles of these compounds and the effect of charge/discharge process on their structural stability. We also describe the electronic structure of these tetrahedral silicates, to examine the most favorable mechanisms for both ionic and electronic conductivities. Our kinetic studies of vacancy migration barriers show that, depending on the polymorph, some of these materials have higher diffusion barriers for Li motion than other cathode materials, but we also show a way to control the composition of the cathode to decrease such barrier and increase the ionic conductivity. Finally, we show that, with a suitable combination of transition metals (TM) in the cathode, we can tailor the position of the valence band maximum of the silicate and the d orbitals of the TM redox couple. This finding will facilitate to increase the intrinsic limit voltage of the cathode and, hence, the energy density of the Li-ion battery.

Oxygen adsorption and dissociation on a pristine graphene, nitrogen doped graphene (N-graphene), and transition metal doped graphene (M-graphene) are studied with density functional theory calculations coupled with nudged elastic band (NEB) method. Four 3 d transition metals (Fe, Co, Ni, and Cu) are selected as the doping atoms. The O binding energies on the Co-graphene and Ni-graphene have intermediate strength. The O 2 dissociation barriers for these two types of doped graphenes are also lower than that on the pristine graphene and N-graphene. The Co and Ni doped graphenes are predicted to be promising ORR catalysts.

Descriptors and quantitative structure property relationships (QSPR) were investigated for mechanical property prediction of carbon nanotubes (CNTs). 78 molecular dynamics (MD) simulations were carried out, and 20 descriptors were calculated to build quantitative structure property relationships (QSPRs) for Young's modulus and Poisson's ratio in two separate analyses: vacancy only and vacancy plus methyl functionalization. In the first analysis, C(N2)/C(T) (number of non-sp2 hybridized carbons per the total carbons) and chiral angle were identified as critical descriptors for both Young's modulus and Poisson's ratio. Further analysis and literature findings indicate the effect of chiral angle is negligible at larger CNT radii for both properties. Raman spectroscopy can be used to measure C(N2)/C(T), providing a direct link between experimental and computational results. Poisson's ratio approaches two different limiting values as CNT radii increases: 0.23-0.25 for chiral and armchair CNTs and 0.10 for zigzag CNTs (surface defects3%). In the second analysis, the critical descriptors were C(N2)/C(T), chiral angle, and M(N)/C(T) (number of methyl groups per total carbons). These results imply new types of defects can be represented as a new descriptor in QSPR models. Finally, results are qualified and quantified against experimental data.

We discover through first-principles calculations a new type of nanoporous carbon network structure formed out of small diameter nanotubes that features unique sp2-sp3 bonding hybridizations and highly ordered 1-D channels for Li-ion diffusion. Unlike other graphitic materials that are primarily bonded by intertube/interlayer van der Waals forces, the predicted sp2-sp3 hybridized carbon networks (HCNs) are held together by strong sp3 covalent bonding at the junctions, with sp2-hybridized interconnects providing conducting π-electrons near the Fermi level. With well-aligned and size-tunable 1-D nanopores, we show that besides desirable high Li capacity, stable Li-ion intercalation voltage profile, and low diffusion barriers, the volumetric change of HCN between fully lithiated/delithiated phases is