DFT Studies of Single Lithium Adsorption on Coronene

1 Introduction

Lithium ion batteries (LIBs) have become extremely important because of the vast variety of applications in supplying power especially for high-tech electronic devices [1]. However, some limitations such as stability and capacity failures in maintaining LIBs restrict their applications for specific purposes [2]. Several attempts have been made to improve the efficiency of LIBs by constructing novel cathode and anode materials for the electrochemical batteries [3], [4], [5]. The pioneering introduction of carbon materials as appropriate anodes has led to the exploration of applications of carbon anodes to improve the efficiency of LIBs [6], [7], [8]. Graphite, which is a well-known carbon allotrope, was one of the first candidates to be employed as anodes in batteries [9]. After the development of nanotechnology, carbon nanostructures have become the target of much research for them to be characterised for possible anodic applications [10], [11], [12]. In this case, the graphene monolayer is one of the most interesting targets because of its similarities in composition with graphite and higher expectations of novel applications [13]. Investigations of atomic adsorption on pristine and oxidised forms of graphene have led to the realisation that the graphene layer is a suitable surface for adsorbing several elements of the periodic table [14], [15]. Moreover, earlier works have shown the advantage of the graphene surface for Li adsorption as an anodic material [16], [17]. Coronene, the smallest free-standing monolayer of graphene with the formula C₂₄H₁₂, could be considered another interesting target for Li adsorption for anodic application [18], [19], [20]. Coronene itself belongs to the family of polycyclic aromatic hydrocarbons (PAHs), which are very important in a variety of applications from biological to inorganic industries [21]. Atomic adsorption applications of coronene have been recognised by earlier works [22], [23], [24].

In this work, we investigate the single Li atom adsorption on a representative coronene surface by quantum chemical computations. For this purpose, we have constructed four molecular models of coronene (Fig. 1): the original (H_C), oxygen-terminated (O_C), sulfur-terminated (S_C), and oxygen/sulfur-terminated (OS_C). Next, atomic (Li) and cationic (Li⁺) adsorptions are investigated for all the constructed coronene models (Fig. 2). The evaluated electronic and structural properties at both the molecular and atomic scales are summarised in Scheme 1 and Tables 1–4. Earlier works had indicated that both oxygen and sulfur terminations could be considered for nanostructures [25], [26]; therefore, the advantage of atomic substitution has been considered for evaluating new structural properties for coronene in this work. It is worth noting that chemical and physical decorations of nanostructures are required for imparting novel properties for specific applications [26], [27], [28], [29].

Figure 1:

Singular models of coronene: (a) H-terminated (H_C), (b) O-terminated (O_C), (c) S-terminated (S_C), and (d) OS-terminated (OS_C).

Figure 2:

Central carbon zone: (a, b) Li@H_C complex model and (c, d) Li/Li⁺@Coronene complex models.

2 Computational Details

In this computational work, four molecular models of coronene (Fig. 1) were prepared: C₂₄H₁₂ H-terminated (H_C), C₁₈O₆H₆ O-terminated (O_C), C₁₈S₆H₆ S-terminated (S_C), and C₁₂O₆S₆ OS-terminated (OS_C). All models were first optimised to achieve the minimum energy structures. Afterwards, adsorptions of atomic Li and cationic Li⁺ species were individually investigated for each of the coronene models (Fig. 2). For this part, the geometries of the already optimised coronene models were kept frozen while those of Li/Li⁺ species were allowed to relax at the monolayer surface to make the Li/Li⁺@Coronene complex systems. It is important to note that the frozen geometry of coronene was considered to mimic large surfaces for the adsorption of small atoms. However, computations of all-floating structures have been included as Supplementary Information for the interested reader. Another point is that although the surface of coronene was the target of this work, computations for possible atomic adsorption at the edges of coronene have also been included as Supplementary Information. Based on the performed optimisation processes, the adsorption length (L_A), adsorption angle (A_A), adsorption energy (E_A), energy of the highest occupied molecular orbital (E_HOMO), energy of the lowest unoccupied molecular orbital (E_LOMO), energy gap (E_G), and dipole moment (D_M) were evaluated for the single and complex models (Table 1). The HOMO and LUMO distribution patterns were also evaluated for all of the investigated models based on time-dependent single-point calculations (Scheme 1). L_A, A_A, E_HOMO, E_LUMO, and D_M were directly obtained by the computed results whereas (1) and (2) were used to obtain the E_A and E_G values:

Scheme 1:

HOMO and LUMO distribution patterns.

For further investigating the models, atomic-scale nuclear quadrupole resonance (NQR) spectroscopic properties were evaluated (Tables 2–4) by calculating the electric field gradient (EFG) tensors of atoms of the optimised structures. NQR is one of the most important techniques to investigate the atomic characteristics of matter [30]. The EFG tensors are very sensitive elements to the electronic environments of atomic sites and can detect any perturbations especially in the interacting systems [31], [32]. Earlier works have indicated that the NQR parameters could very well recognise the atomic-scale properties to give insightful information on the characteristics of electronically complicated nanostructures [33], [34], [35]. To evaluate the quadrupole coupling constants (C_Q) of the NQR parameters, (3) was used, in which e, Q, q_zz, and h are the electric charge, nuclear quadrupole moment, major eigenvalue of EFG tensors, and the Planck constant [36].

All quantum chemical computations in this work were carried out based on density functional theory (DFT) employing the B3LYP exchange-correlation functional and the 6–31G* basis set as implemented in the Gaussian package [37].

3 Results and Discussion

4 Conclusion

Using DFT, we have investigated the Li/Li⁺ adsorption on representative H-, O-, S-, and OS-terminated models of coronene based on the evaluated molecular- and atomic-scale properties. Based on this work, some trends can be noted. First, in addition to H-terminated coronene (original), other O-, S- and OS-terminated models were also found to be stable and symmetric in their polarisation properties. Second, different properties were obtained for Li and Li⁺ adsorptions at the surface of coronene models, in which larger energies were obtained for Li⁺ adsorption in comparison with Li adsorption for all investigated coronene models. Third, the Li and Li⁺ species were localised at the top of central carbon zone of all monolayer coronene with the exception of Li adsorption at the surface of H-terminated coronene, in which the Li atom has moved to the edge of the monolayer. Fourth, the energy levels of HOMO and LUMO are different in the different atom-terminated models of singular coronene and also in the Li/Li⁺-adsorbed complex systems. Similar HOMO and LUMO distribution patterns have been seen for the singular and Li⁺-adsorbed complex models but different from the Li-adsorbed complex models. Fifth, the NQR parameters for Li/Li⁺ species indicated a direct relationship between the angle of adsorption and the magnitude of the C_Q values. Equivalent NQR parameters have been evaluated for the atoms of central carbon zone of the monolayer for the singular and Li⁺-adsorbed complex models of coronene, but different for the Li-adsorbed complex models. Parallel behaviours were also seen for the terminating atoms of singular and complex systems. Finally, the original and each of the O/S-terminated models of coronene can be proposed as possible surfaces for the stronger adsorption of Li⁺ than Li, in which the detailed electronic properties at the molecular and atomic scales can be tuned for applications for specific purposes.