Thermal behaviors of lithium in and on the two layers of C150H30 graphite plane: MD simulation

doi:10.1016/S0022-3697(02)00379-7

Journal of Physics and Chemistry of Solids

Volume 64, Issue 5, May 2003, Pages 807-811

https://doi.org/10.1016/S0022-3697(02)00379-7 Get rights and content

Abstract

In order to investigate the thermal behaviors of lithium (Li) atom intercalated in and adsorbed on the graphite intercalated compound, the molecular dynamics procedure at molecular mechanics 2 level was applied to the hydrogen terminated cluster model composed of two layers of C₁₅₀H₃₀ plane. On the basis of the optimized structure, one intercalated Li atom was stabilized at the mass center and adsorbed Li was done above the center of upper plane. Both Li atoms initiate to migrate at 50 K parallel to the plane with almost the same migration rate in the temperature range from 50 to 200 K. However, their migration processes are clearly different. The former gets out of the cluster model, drawing the increasing curves accompanied by the induction period in the coordinate axes of the migration distance vs. the simulation time at every temperature. The latter, however, cannot be released from the cluster model below 150 K where it remains on the side face of the cluster model. At 50 and 100 K, it migrates from the center to the circumference periodically cannot escape from the surface of cluster model. Thus, the potential well is considered to be deeper on the surface than in the layers.

Introduction

Graphite is inherently a semiconductive material with layered structure intercalated by some atoms and molecules to form the stage structures of graphite intercalated compounds (GICs). The typical GIC is given by lithium (Li-GIC) which is widely used as the electrode of secondary battery due to the excellent storage capacity, the high electromotive force and the large current density [1], [2]. In order to develop such performance, plenty of enthusiastic works were provided. Some amorphous carbon compounds were synthesized recently [3] to store the more amount of Li than expected from the first stage of Li-GIC. Concerning the reason for excess amount of storage, lively arguments are give both experimentally [4] and theoretically [5], [6]. The charge state of Li is also calculated on the basis of ab initio molecular orbital (MO) calculation using circumcorone component [7]. According to the semiempirical MO calculation, Li atoms adsorbed on the ovalene molecule are found to form the Li cation clusters due to the interaction with the carbon lattice [8]. On the other hand, elucidation of the diffusion process and the electronic structure of Li species in graphite is necessary to get the large current density. One of the effective procedures to solve the problem is simulating the thermal behavior of Li species in terms of molecular dynamics (MD) or MO dynamics calculations, however, such works are rarely found [6] in Li-GIC.

Present authors calculated previously the trajectories of Li species in graphite cluster model in terms of MD [9] and MO dynamics [10], [11] at the temperature below 700 K. In this study, thermal behaviors are compared for two Li atoms intercalated in and adsorbed on the two layered cluster model, respectively, on the basis of the molecular mechanics 2 (MM2) method.

Section snippets

Calculation method

The cluster model, formulated as 2C₁₅₀H₃₀, is composed of two carbon staking layers one of which contains 150 carbon atoms and 30 hydrogen atoms terminating the edge sites as shown in Fig. 1. The diameter of the layer is 20.56–22.95 Å which is equivalent to the domain size of calcined poly-paraphenylene (PPP, 20–30 Å).

For the simulation of thermal behaviors of Li atom in graphite, the large sized cluster is desired, however, the application of high quality calculation is actually impossible due

Optimized structures of the cluster models

As the optimized structure of cluster model, 2C₁₅₀H₃₀, was reported previously [12], the configuration is AB stacking where two layers were 3.38 Å apart which is close to the observed value of 3.35 Å. For the cluster model intercalated by Li atom, C₁₅₀H₃₀·Li·C₁₅₀H₃₀, result of the optimization is given in Fig. 2(A) where Li atom is stabilized almost the center of mass, that is, the distance to the nearest carbon atom is 2.64 Å. The AB stacking is preserved by the intercalation. This is because

Discussion

Concerning the intercalation and the adsorption of Li atom for cluster model, 2C₁₅₀H₃₀, present results will be compared with those of H atom at the same calculation level [12]. As mentioned in Fig. 2(A), swelling was limited to the narrower central area by Li atom than by H atom in earlier work. Although no distortion is found in the host lattice in Fig. 2(B), strong covalent bond was suggested to be formed on the surface of C₁₅₀H₃₀ plane with H atom in the previous report. According to the

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Molecular dynamics simulation for sodium atom in and on the two layers of C<inf>150</inf>H <inf>30</inf> graphite plane
2003, Journal of Physics and Chemistry of Solids
Citation Excerpt :
Neither of the planes is affected by the adsorption, which is similar in the case of Li atom [16].
For inspection of thermal behaviors of sodium (Na) atom in the bulk and on the surface of two layered hydrogen terminated cluster model, 2C₁₅₀H₃₀, the molecular dynamics calculation was taken place at molecular mechanics 2 level. From the requirement of structural optimization, interlayer distance of 2C₁₅₀H₃₀ is 3.38 Å which is consistent with the observed value. In the cluster models intercalated and adsorbed by one Na atom, C₁₅₀H₃₀·Na·C₁₅₀H₃₀ and Na·2C₁₅₀H₃₀, respectively, the Na atom is stabilized beneath and above the nearest central carbon atom, C₀, in the upper layer where the distances, Na–C₀, are 2.76 and 3.16 Å, respectively. Adsorption of the Na atom to the surface has no influence on the geometrical structure of cluster model, whereas, intercalation to two layers expands the interlayer distance maximally to 5.01 Å which will be responsible for the carbon expansion of graphite electrode in cryolite melt-alumina slurries. Diffusion processes are observed above 200 K for the Na atoms stabilized in both sites. Although the Na atom migrates parallel to the layers in the range between 200 and 300 K in C₁₅₀H₃₀·Na·C₁₅₀H₃₀, it moves above the carbon layer from the center to the circumference periodically below 250 K and gets out at 300 K for Na·2C₁₅₀H₃₀. The migration rates of Na atom are almost the same irrespective of the diffusion areas.
Molecular dynamics calculation on three lithium atoms in planar cluster model C<inf>150</inf>H<inf>30</inf> for glassy carbon
2003, Journal of Physics and Chemistry of Solids
Citation Excerpt :
Tracing the positions of three Li atoms at every t, a triangle formed by three Li atoms, rotating clockwise almost parallel to the carbon plane, goes around the central area of it. This migration is far different from that of one Li atom showing the round trip on a segment between the center and the edge of the cluster model periodically below 100 K [16]. For closer inspection of the processes in Fig. 2, changes of the six parameters defined in Section 2 are presented with the lapse of t in Fig. 3.
As the preliminary step to elucidate the charge-discharge mechanism in the glassy carbon cathode of lithium (Li) secondary battery, the molecular dynamics (MD) calculation was applied at molecular mechanics 2 (MM2) level for three Li atoms stabilized in the form of regular triangle in the central area of the hydrogen terminated planar cluster, C₁₅₀H₃₀. The stabilized Li sites are given on the basis of the structural optimization. Up to 10 K, the triangular Li aggregate kept at almost the same figure as that formed by three stabilization sites, rotating parallel to the cluster plane, goes around the central area of it, whereupon interatomic vibrational stretching is observed. Below 75 K, the aggregate of three Li atoms separates to a pair of two atoms and one Li atom arbitrarily, however, reformation of the pair occurs periodically among three atoms with the lapse of simulation time. Then, three Li atoms move correlatively irrespective of the long interatomic distance of 18 Å at maximum. However, at 100 K one Li atom goes out of the cluster model directly and the rest of two atoms continues the revolutional movement with the rotation as a pair in the central area of the cluster model. Thus, three Li atoms show the appreciably stable movements in the glassy carbon by forming the aggregate or the atomic pair, which will be responsible for the hysteresis in the charge–discharge cycle of lithium secondary battery.
Molecular dynamics simulation on diffusion of lithium atom pair in C<inf>150</inf>H<inf>30</inf> cluster model for glassy carbon at very low temperatures
2003, Electrochimica Acta
Citation Excerpt :
They are given as R(L1–L2)=L1–L2, r1=L1–C0 and r2=L2–C0, respectively. In the optimized structure of planar cluster model, C150H30, the bond distances CC, 1.40 Å, CH, 1.08 Å and the bond angle of hexagonal ring, 120°, are essentially the same as those for the two layered cluster model, 2·C150H30, reported previously [16]. These parameters were influenced negligibly by the adsorption of two Li atoms in the reoptimization procedures.
As the preliminary step to elucidate the ion transfer mechanism of lithium (Li) species in the cathode electrode of glassy carbon material, molecular dynamics (MD) simulation was taken place describing the initial thermal movement of two Li atoms in the simulation time range of 10 ps maximally in the hydrogen terminated cluster model, C₁₅₀H₃₀, at molecular mechanics 2 (MM2) level. The formation of Li₂ atom pair is specified in the central area of the cluster model as the result of the structural optimization. In the low temperature range from 4 to 10 K, accompanied by the rotational and stretching vibrational motions, it goes around the central area of the cluster model. Diffusion process of the atom pair is simulated dynamically for the first time in the present study. Decomposition of the atom pair occurred at 50 K and it produces two Li atoms which go and back independently from the center to the edge of the cluster model crossing the CC bonds orthogonally. Formation and diffusion processes of Li₂ atom pair may be responsible for the charge–discharge cycles in the glassy carbon electrode in the lithium secondary battery.
Lithium adsorption on graphite from density functional theory calculations
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Thermal behaviors of lithium in and on the two layers of C150H30 graphite plane: MD simulation

Abstract

Introduction

Section snippets

Calculation method

Optimized structures of the cluster models

Discussion

J. Mol. Struct. (Theochem.)

J. Phys. Chem. Solids

J. Mol. Struct. (Theochem)

Chem. Phys. Lett.

Chem. Phys.

J. Phys. Chem. Solids

Nature

Thermal behaviors of lithium in and on the two layers of C₁₅₀H₃₀ graphite plane: MD simulation