The insertion of inorganic materials into single-walled carbon nanotubes (SWCNTs), hereinafter identified simply as CNT, enables the encapsulation of extreme nanowires (NWs) with diameters comparable to a unit cell of the parent material (Green, 1998; Sloan et al., 2002; Spencer et al., 2014). Although NWs of similar diameter can be produced using several templates, such as zeolites (Derouane, 1998), mesoporous phases (Alba-Simionesco et al., 2006; Ke et al., 2009), and metal-organic framework (MOF) (Lu et al., 2012) type materials, CNTs present many advantages as templates; they are atomically smooth, electron transparent, readily available, and can be filled by bulk infiltration to create milligram quantities of encapsulated nanowires, at least on a laboratory scale. Thus, encapsulated NW-CNT are scientifically interesting not only on their own but also as precursors to a wide range of other extreme nanowire materials.

In 2016, Ivanov et al. published a theoretical prediction of helix-shaped lithium-phosphorus nanowires encapsulated into single-walled carbon nanotubes (LiP@CNTs) (Ivanov et al., 2016). Note that helix-shaped Li_nP_n clusters (n = 5–9) had previously been reported as global minimum structures (Ivanov et al., 2012). Some solid phases consist of structural motifs like atomic clusters, i.e., in Zintl phases. This connection brings consistency to the use of models based on stable clusters to generate NWs inside nanotubes, as proposed in Ivanov’s work (Ivanov et al., 2012). The study of these clusters inside CNTs can provide relevant information about these hybrid materials, for example, about their viability (stability analysis), their structural characteristics (geometry analysis), their physical and chemical properties (analysis of their electronic structure).

Due to its excellent energy storage capacity, Si has been extensively studied experimentally as a negative electrode material for Li-ion batteries (Gao et al., 2001; Ryu et al., 2004; Li et al., 2006, Li et al., 2008, Li et al., 2009; Obrovac et al., 2007; Song et al., 2014; Shin et al., 2020). Hence, Si lithifies at high temperature (415°C) in a LiCl-KCl melt, identifying potential plateaus evidencing the crystalline phases Li₁₂Si₇, Li₇Si₃, Li₁₃Si₄, and Li₂₂Si₅ (Wen and Huggins, 1981). In particular, the binary (non-paramagnetic) Zintl-type Li₁₂Si₇ silicide contains semi-infinite sandwich-like [ Li 6 Si 5 ] ∞ 1 linear chains, consisting of Si₅ pentagons intercalated with Li atoms (see Scheme 1). Note that the unit cell of the Zintl Li₁₂Si₇ phase has been rationalized (Nesper, 1990; Chevrier et al., 2010; Köster et al., 2011; Kuhn et al., 2011a, Kuhn et al., 2011b) as (Li₆ ⁶⁺[Si₅]₆)₂ (Li₁₂ ¹⁰⁺[Si₄]₁₀)₂, with two well-defined silicon moieties: planar Si₅ rings and the Y-shaped Si₄ moiety. Such a structural pattern is justified by assigning 26 electrons (20 from 6Si + 6 from 6Li) to the Si₅ ^6- ring, favoring Hückel’s aromaticity (Hückel, 1930, Hückel, 1931a, Hückel, 1931b; Zhao et al., 2017). This aromatic character is supported by experimental evidence of an upfield shift (to -17.2 ppm) of Li (at the center of the Li₆ fragment in Scheme 1) in the corresponding magic angle NMR (MAS) spectrum (Kuhn et al., 2011b; Köster et al., 2011). It is noteworthy that the Si₅ ^6- structural motif is also present in the ternary compound Li₈MgSi₆ (Nesper et al., 1986a). On the other hand, at the cluster level, our group has identified that the global minimum (GM) of the Li₆Si₅ cluster, consists of an aromatic Si₅ ^6- pentagon surrounded by 6 Li⁺ counterions (Tiznado et al., 2009; Perez-Peralta et al., 2011; Contreras et al., 2013; Vásquez-Espinal et al., 2018). More recently, we have identified the GM structures of the oligomers (Li₆Si₅)₂ and (Li₆Si₅)₃ (Yañez et al., 2019b; Manrique-de-la-Cuba et al., 2021), which also consist of aromatic Si₅ rings surrounded by Li’s (see Scheme 1). However, the stacking of Li₆Si₅ units does not tend to form the nanowire identified in Li₁₂Si₇, suggesting that Li₁₂ ¹⁰⁺[Si₄]^10- component (with the Y-shaped Si₄ moiety) contributes decisively to the stabilization of this NW. In mentioned cluster studies, explorations of the potential energy surface have been performed by hybrid methods, including genetic algorithms (Yañez et al., 2019a, Yañez et al., 2020).

To build new class of materials with desirable properties, using atomic clusters instead of atoms as building blocks, is a remarkable possibility. However, it requires that atomic clusters must retain their identity when assembled, as Khanna and Jena first outlined when they coined the word “cluster-assembled materials” (CAMs) (Khanna and Jena, 1992). These authors argued that the clusters’ coupling would have a unique effect on both the assembled material’s electronic structure and mechanical properties, which is not possible when the assembly blocks are atoms (Khanna and Jena, 1995; Jena et al., 1996; Jena and Khanna, 1996; Claridge et al., 2009; Jena and Sun, 2018). For a more detailed and timely overview of advances in the assembly of materials from clusters, please refer to the following reviews: (Chakraborty and Pradeep, 2017; Jena and Sun, 2018; Pinkard et al., 2018; Zheng et al., 2019; Doud et al., 2020).

Given the above background, here we evaluated, in silico, the stability of the isolated [ Li 6 Si 5 ] ∞ 1 NW, as well as its electronic properties. In addition, we studied the hybrid material consisting of the NW confined in a CNT. The latter focused on identifying alternative ways to stabilize this conformation and to evaluate the effect of this association on NW electronic properties. Our density functional theory (DFT) calculations demonstrate that Li-Si@CNTs hybrid systems have excellent stability and thus have potential for experimental realization.

In the finite models (clusters), geometry optimizations and frequency calculations were performed at the PBE0 (Adamo and Barone, 1999)/Def2TZVP(Weigend and Ahlrichs, 2005) level with the Gaussian 16 program (M. J. Frisch, G. W. Trucks, H. B. Schlegel et al., 2016).

For the solid-state study, we performed first-principles calculations based on DFT (Sham and Kohn, 1966; Kohn et al., 1996) as implemented in the Vienna Ab Initio Simulation Package (VASP) (Kresse and Furthmüller, 1996). The exchange-correlation energies were calculated at PBE-D3 level (Ernzerhof and Perdew, 1998; Grimme et al., 2010). Plane-wave basis set with a kinetic energy cutoff of 400 eV, and the projector augmented-wave method for the core-valence interaction was employed (Blöchl et al., 1994). The [ L i 6 S i 5 ] ∞ 1 NWs were simulated in a large unit cell with volume (20 × 20 × z ₀) ų, considering periodic boundary conditions along the NW direction, where z ₀ is the periodicity. Within this supercell, the lateral distance between NW images is set to 15 Å. We use a (1 × 1 × 10) Monkhorst-Pack k-point mesh (Monkhorst and Pack, 1976). We also study finite (Li₆Si₅)₄ structures in the free space, also inside both an armchair and a zigzag single-walled carbon nanotube (SWCNTs). We consider armchair CNTs with chiral indexes (8,8), (9,9), and (10,10), which have diameters of 10.93, 12.27, and 13.63 Å, and zigzag CNTs with chiral indexes (14,0), (15,0), and (16,0) which have diameters of 11.04, 11.80, and 12.59 Å, respectively. For (Li₆Si₅)₄@CNT simulation, (22 × 22 × z ₀) ų volume was used, where z ₀ is the periodicity chosen for the CNTs. All studied structures were allowed to freely relax without any constraint until forces on each atom were smaller than 25 meV/Å. To gain insights on the stability of the NW models in the free space and inside the SWCNTs, we performed Born-Oppenheimer ab initio molecular dynamics (BO-AIMD) simulations within the NVT ensemble at different temperatures, over a total simulation time of 10 ps, considering a time step of 1 fs.

Using periodic DFT calculations and Born-Oppenheimer ab initio molecular dynamic simulations, we have shown that Li₆Si₅ units can be stacked one above the other, forming a one-dimensional structure linked together by Coulomb interactions. This study complements previous findings, where we demonstrated that Li₆Si₅, (Li₆Si₅)₂, and (Li₆Si₅)₃ lowest energy structures contain one, two, and three Si₅ ^6- aromatic rings stabilized by Li⁺ counterions. Additionally, the [ Li 6 Si 5 ] ∞ 1 nanowire was identified in the Zintl Li₁₂Si₇ compound but coexisting with Y-shaped Si₄ moieties. In this case, we support the stability of the isolated Si-Li-nanowire—additionally, the relaxed structure (at room temperature) exhibits metallic characteristics.

We also found that finite (Li₆Si₅)₄ systems are stable inside both armchair and zigzag carbon nanotubes of around 12 Å in diameter, preserving its stability at room temperature, supporting the viable formation of Li₆Si₅-NW inside the CNTs. Interestingly, the Li₆Si₅-NW@CNTs hybrid nanocomposite maintains the metallic character. Finally, in the Li₆Si₅-NW, the Li₆Si₅ units are connected by strong electrostatic interactions (Si₅ ^6- aromatic pentagons intercalated with the Li₆ ⁶⁺ moiety) in agreement with the Zintl ion concept. In the [Li₆Si₅-NW]@CNTs, NCI predicts that Li₆Si₅-NW interacts with the CNT walls by van der Waals interactions Frisch et al., 2016.