Li-intercalated Bilayer Research Articles

Computational insight into bilayer NC7 anode material for Li/Na/Mg-ion batteries

Two-dimensional (2D) materials are recognized as promising candidates for the novel materials of metal-ion batteries for their unique physical and chemical properties. Here, we explore the possibility of a 2D NC7 as an anode material for the Li/Na/Mg-ion battery using first-principles calculations. To elucidate the role of a swelling effect we considered the bilayer form of NC7 intercalated by Li, Na and Mg atoms. Our results show that pristine bilayer NC7 is dynamically and thermally stable. The intercalating atoms prefer to stay in the hollow site but the drastic volume expansion after Na-, and Mg-intercalation eliminates NC7 from the group of potential candidates for anode material for Na- and Mg-ion batteries. Against, the Li-intercalated NC7 bilayer possesses a small swelling effect, thermal stability and a high theoretical capacity of 683.14 mAh/g. In addition, the open circuit voltage and diffusion barriers of the Li ions between two NC7 layers are also quite low. These make NC7 a highly promising alternative material for Li-ion battery anode with great structural sturdiness.

Tuning electrical and interfacial thermal properties of bilayer MoS2 via electrochemical intercalation

Layered two-dimensional (2D) materials such as MoS2 have attracted much attention for nano- and opto-electronics. Recently, intercalation (e.g. of ions, atoms, or molecules) has emerged as an effective technique to modulate material properties of such layered 2D films reversibly. We probe both the electrical and thermal properties of Li-intercalated bilayer MoS2 nanosheets by combining electrical measurements and Raman spectroscopy. We demonstrate reversible modulation of carrier density over more than two orders of magnitude (from 0.8 × 1012 to 1.5 × 1014 cm−2), and we simultaneously obtain the thermal boundary conductance between the bilayer and its supporting SiO2 substrate for an intercalated system for the first time. This thermal coupling can be reversibly modulated by nearly a factor of eight, from 14 ± 4.0 MW m−2 K−1 before intercalation to 1.8 ± 0.9 MW m−2 K−1 when the MoS2 is fully lithiated. These results reveal electrochemical intercalation as a reversible tool to modulate and control both electrical and thermal properties of 2D layers.

Straintronic effect for superconductivity enhancement in Li-intercalated bilayer MoS2.

In this study, ab initio calculations were performed to show that the superconductivity in Li-intercalated bilayer MoS2 could be enhanced by applying either compressive or tensile strain. Moreover, the mechanism for superconductivity enhancement for the tensile strain case was found to be different than that of the compressive strain case. Enhanced electron phonon coupling (EPC) under tensile strain could be explained by an increase in the nesting function involved with the change in the Fermi surface topology in a wide range of Brillouin zones. The superconducting transition temperature Tc of 0.46 K at zero strain increased up to 9.12 K under a 6.0% tensile strain. Meanwhile, the enhancement in compressive strain was attributed to the increase in intrinsic electron phonon matrix elements. Furthermore, the contribution from interband scattering was large, which suggested the importance of electron pockets on the Fermi surface. Finally, 80% of the total EPC (λ = 0.98) originated from these pockets and the estimated Tc was 13.50 K.

Angle-resolved photoemission spectroscopy in low-dimensional periodic structures: a real-time first-principles simulation

We propose a new theoretical approach to simulate angle-resolved photoemission spectroscopy (ARPES) of low-dimensional periodic structures. ARPES spectra can be efficiently and accurately calculated by real-time time-dependent density functional theory. We demonstrate that the calculated ARPES spectra coincide well with the occupied band structures of the target materials graphene, silicene, and polyacetylene. The electron doping effect on ARPES is clarified by calculating the spectrum of Li-intercalated bilayer graphene.

Weak localization in bilayer graphene with Li-intercalation/desorption

We performed in-situ electrical transport measurements for bilayer graphene grown on SiC(0 0 0 1) substrate, Li-intercalated bilayer graphene, and after that desorbing Li atoms by heating. Bilayer graphene after desorbing intercalated Li atoms showed a higher resistivity and different behavior in magnetoconductance compared to pristine bilayer graphene. We observed the weak localization of carriers at low temperatures in all the three samples and analyzed the experimental results with the extended Hikami–Larkin–Nagaoka equation to investigate the transport properties. The result shows that the magnetoconductance of pristine bilayer graphene is described by the AB stacking structure model and the phase breaking scattering is dominated by the electron–electron scattering. The intra-valley scattering occurs most frequently probably due to dopants in SiC substrate. However, in Li-desorbed graphene, the magnetoconductance can be described by neither AB nor AA-stacking model, suggesting the coexistence of domains with several different stacking structures.

Superconductivity in Li-intercalated bilayer arsenene and hole-doped monolayer arsenene: a first-principles prediction

Using first-principles calculations, we find Li-intercalated bilayer arsenene with AB stacking is dynamically stable, which is different from pristine bilayer with AA stacking. Electron–phonon coupling of the stable Li-intercalated bilayer arsenene are dominated by the low frequency vibrational modes (E″(1), (1), E′(1) and acoustic modes) and lead to an superconductivity with Tc = 8.68 K with isotropical Eliashberg function. Small biaxial tensile strain (2%) can improve Tc to 11.22 K due to the increase of DOS and phonon softening. By considering the fully anisotropic Migdal–Eliashberg theory, Tc are found to be enhanced by 50% and exhibits a single anisotropic gap nature. In addition, considering its nearly flat top valence band which is favorable for high temperature superconductivity, we also explore the superconducting properties of hole-doped monolayer arsenene under different strains. the unstrained monolayer arsenene superconducts at Tc = 0.22 K with 0.1 hole/cell doping. By applying 3% biaxial strain, Tc can be lifted up strikingly to 6.69 K due to a strong Fermi nesting of the nearly flat band. Then Tc decreases slowly with strain. Our findings provide another insight to realize 2D superconductivity and suggest that the strain is crucial to further enhance the transition temperature.

Li-intercalated bilayer SnS2: A potential superconductor

Electronic structure, lattice dynamics, and electron-phonon coupling of Li-intercalated bilayer SnS2 are systematically investigated via first-principles density functional theory. The energetically stable configuration for Li-intercalated bilayer SnS2 is /AB/ stacking, which is different from /AA/ stacking for pristine bilayer. There is a charge transfer from Li to bilayer SnS2 and the change of the band structure after Li intercalation can be explained well by a rigid band model, suggesting that the intercalated Li atoms mainly play a role of charge reservoir. Our calculations show that the softening of acoustic phonon near K¯ high-symmetry point make a large contribution to electron-phonon interaction and the superconducting temperature Tc can achieve 14.0 K. Our study suggest that Li-intercalated SnS2, a potential material of lithium-ion battery, may meanwhile be a quasi two-dimensional superconductor.

Theoretical prediction of phonon-mediated superconductivity with Tc ≈ 25 K in Li-intercalated hexagonal boron nitride bilayer

A superconducting transition temperature (Tc) up to 25 K in the Li-intercalated bilayer of hexagonal boron nitride (h-BN) is predicted according to ab-initio calculations. A Tc higher than that of metal-intercalated graphene (MIG) is ascribed to the characteristic spatial distribution of electronic states near the Fermi level, which is distinctly different from that in MIG. In the Li-intercalated bilayer h-BN, the breaking of the symmetrical restriction and the increase in the overlap between the charge density and the Li in-plane motion enhance the electron–phonon coupling. Our results provide a new design guideline for two-dimensional superconductors based on intercalated layered materials.

Dynamical stability and superconductivity of Li-intercalated bilayerMoS2: A first-principles prediction

Using first-principles calculations, we find a dynamically stable structure of Li-intercalated bilayer ${\mathrm{MoS}}_{2}$, of which two monolayers are mirror images of each other. It is shown that the stable Li-intercalated bilayer ${\mathrm{MoS}}_{2}$ can become a BCS superconductor with superconducting transition temperature ${T}_{c}$ evaluated over 10 K. The partial charge transfer from Li atoms to inner S planes of two ${\mathrm{MoS}}_{2}$ monolayers leads to a semiconductor-metal transition and an ionic interlayer interaction so as to increase the screening effect within the layers, soften phonon modes, and enhance electron-phonon couplings. Our finding suggests further experimental efforts directed toward an understanding of quasi-two-dimensional superconductivity.

Superconducting Calcium-Intercalated Bilayer Graphene.

We report the direct evidence for superconductivity in Ca-intercalated bilayer graphene C6CaC6, which is regarded as the thinnest limit of Ca-intercalated graphite. We performed the electrical transport measurements with the in situ 4-point-probe method in ultrahigh vacuum under zero- or nonzero-magnetic field for pristine bilayer graphene, Li-intercalated bilayer graphene (C6LiC6) and C6CaC6 fabricated on SiC substrate. We observed that the zero-resistance state occurs in C6CaC6 with the onset temperature (T(c)(onset)) of 4 K, while the T(c)(onset) is gradually decreased upon applying the magnetic field. This directly proves the superconductivity origin of the zero resistance in C6CaC6. On the other hand, both pristine bilayer graphene and C6LiC6 exhibit nonsuperconducting behavior, suggesting the importance of intercalated atoms and its species to drive the superconductivity.

Metallization and stiffness of the Li-intercalated MoS2 bilayer

Performed density-functional theory (DFT) calculations have shown that the Li adsorption on the MoS2 (0001) surface, as well as Li intercalation into the space between MoS2 layers, transforms the semiconductor band structure of MoS2 into metallic. For the (√3×√3) – R30° Li layer, the band structures of the MoS2 bilayer with adsorbed and intercalated Li are very similar, while for higher Li concentrations, the character of metallization for the adsorbed layer substantially differs from that of the MoS2–Li–MoS2 layered system. In particular, for the adsorbed (1×1) Li monolayer, the increased density of the layer leads to the nonmetal-to-metal transition, which is evident from the appearance of the band crossing EF with an upward dispersion, pertinent to simple metals. It has been demonstrated that intercalated Li substantially increases the interlayer interaction in MoS2. Specifically, the estimated 0.12eV energy of the interlayer interaction in the MoS2 bilayer increases to 0.60eV. This result is also consistent with results of earlier DFT calculations and available experimental results for alkali-intercalated graphene layers, which have demonstrated a substantial increase in the stiffness due to intercalation of alkalis.

Prediction of superconductivity in Li-intercalated bilayer phosphorene

It is shown that bilayer phosphorene can be transformed from a direct-gap semiconductor to a BCS superconductor by intercalating Li atoms. For the Li-intercalated bilayer phosphorene, we find that the electron occupation of Li-derived band is small and superconductivity is intrinsic. With increasing the intercalation of Li atoms, both increased metallicity and strong electron-phonon coupling are favorable for the enhancement of superconductivity. The obtained electron-phonon coupling λ can be larger than 1 and the superconducting temperature Tc can be increased up to 16.5 K, suggesting that phosphorene may be a good candidate for a nanoscale superconductor.

Charge carrier density in Li-intercalated graphene

The electronic structures of bulk C6Li, Li-intercalated free-standing bilayer graphene, and Li-intercalated bilayer and trilayer graphene on SiC(0001) are studied using density functional theory. Our estimate of Young’s modulus suggests that Li-intercalation increases the intrinsic stiffness. For decreasing Li–C interaction, the Dirac point shifts to the Fermi level and the associated band splitting vanishes. For Li-intercalated bilayer graphene on SiC(0001) the splitting at the Dirac point is tiny. It is also very small at the two Dirac points of Li-intercalated trilayer graphene on SiC(0001). For all the systems under study, a large enhancement of the charge carrier density is achieved by Li intercalation.

Fabrication of Li-intercalated bilayer graphene

We have succeeded in fabricating Li-intercalated bilayer graphene on silicon carbide. The low-energy electron diffraction from Li-deposited bilayer graphene shows a sharp \documentclass[12pt]{minimal}\begin{document}$\sqrt{3}$\end{document}3×\documentclass[12pt]{minimal}\begin{document}$\sqrt{3}$\end{document}3\documentclass[12pt]{minimal}\begin{document}$\it R$\end{document}R30° pattern in contrast to Li-deposited monolayer graphene. This indicates that Li atoms are intercalated between two adjacent graphene layers and take the same well-ordered superstructure as in bulk C6Li. The angle-resolved photoemission spectroscopy has revealed that Li atoms are fully ionized and the π bands of graphene are systematically folded by the superstructure of intercalated Li atoms, producing a snowflake-like Fermi surface centered at the Γ point. The present result suggests a high potential of Li-intercalated bilayer graphene for application to a nano-scale Li-ion battery.