Elsevier

Chemical Physics Letters

Volume 629, 1 June 2015, Pages 36-39
Chemical Physics Letters

Mg intercalation into Ti2C building block

https://doi.org/10.1016/j.cplett.2015.04.015Get rights and content

Highlights

  • Li and Na are unlikely to intercalate into the Ti2C building block.

  • Mg can favorably intercalate into Ti2C building block.

  • Ti2MgC is a potential anode for Mg ion batteries.

Abstract

Generally, intercalation occurs when foreign atoms intercalate into multi-layer structures, while adsorption occurs when foreign atoms interact with monolayer structures or surfaces. We performed an investigation on the Mg intercalation into Ti2C building block (MXene) from first-principles simulation. We found that Mg can favorably intercalate into MXene, forming the stable compound Ti2MgC, which corresponds to the stage I in the Li intercalation into graphite. Based on the evaluation of the average cell potential and the energy barrier of Mg diffusion for the most energetically stable structure, our results suggest that Ti2MgC is a potential anode for Mg ion batteries.

Introduction

MAX (Mn+1AXn) phases are a large family of ternary layered cabides and nitrides, where M is an early transition metal, n = 1–3, A usually belongs to the groups IIIA and IVA, and X is carbon or nitrogen [1]. Generally, MAX phases are considered as layers of A intercalated into the Mn+1Xn building block. Recently it has been reported that element A can be selectively etched away by hydrofluoric acid, and the remaining 2 D layer structure after exfoliation is termed as MXene [2]. In 2011, Naguib et al. first realized the synthesization of MXene (Ti3C2) in experiment [3]. In viewing of the large number of already known MAX phases, considerable large family of MXene phases are expected to be synthesized in the near future.

Because of the structural similarity between MXene and graphene, MXene has a wide potential for application. Recently, Gogotsi et al., revealed the potential of MXene layers (Ti3C2) as the electrochemical energy storage, such as electrode for batteries, supercapacitors, hybrid devices [4], [5], [6]. Since then, many theoretical and experimental studies extensively reported the performance of MXene as anode material for metal ion batteries. In addition, 2D MXene phases have a variety of electronic and magnetic properties differing from their parent MAX phases, and can be used to tune the band gap of the semiconductors, e.g., MoS2 [7].

Ti2AlC is one of the most investigated MAX phases [8], from which the thinnest MXene Ti2C is expected to be realized [9]. However, in the real etching process, MXene phases have been found to be terminated with several kinds of functional group, denoted as Tin+1XnTn, where T can be F, OH and/or O [2]. It has also been reported that the electrochemical performance of MXene in metal ion batteries is strongly dependent on the functional group [10], [11], [12], [13]. In particular, recent work suggested that both Ti2C monolayer with O termination and the pure monolayer as the potential anode materials due to the large theoretical capacities and the good diffusion behavior. While Ti2C monolayer with OH termination is not designed for the anode materials from the point of large capacity demand [14]. In the mean while, several suggestions have been proposed for the treatment of termination under the guidance of the theoretical view [15]. However, it is difficult to control the termination type or even harder to get the uniquely designated termination by chemical treatment.

Graphite is a commercial anode for Li ion batteries, where Li intercalation/de-intercalation occurs during the charge/discharge process. The intercalation in batteries considered as a special case of insertion, occurs when the layered host structure taking up guest species. Li insertion into the graphite is a typical example of intercalation. Ti2AlC can be considered as layers of Al intercalated into the Ti2C building block. Due to the structure similarity of Ti2C building block with graphite, metal intercalation is also expected to occur in the Ti2C building block. Stimulated by this idea, we checked the possibilities of Li, Na, and Mg intercalations into Ti2C from first-principles simulation. We found that Mg can be intercalated into Ti2C building block, while Li and Na are unfavorable to intercalate. The performance of Ti2C as anode for Mg ion batteries such as Mg diffusion behavior, and the average cell potential is discussed in this article. It has to be mentioned that this theoretical work is the first successful try of metal intercalation into Ti2C, and it opens the door of metal intercalation in the large family of MXene. We note that ‘intercalation’ in this work is a totally different concept from ‘adsorption’ where monolayer structure was always used [10], [11], [12], [13], [14], [15]. Intercalation, defined above considers layered host structures taking up with the metals, where the bulk or at least two monolayer structures should be used. In this letter, we use the bulk model for convenience, and similar model was also adopted recently [2].

Section snippets

Computational method

The density functional theory (DFT) calculation was performed using the Vienna Ab-Initio Simulation Package [16]. The electron–ion interaction and electron–electron interaction were described by projector-augmented wave pseudopotentials and Perdew–Burke–Ernzerhof (PBE) exchange and correlation functionals, respectively [17], [18]. The valance electrons of p6d3s1, s2p2 and s2 were used for Ti atom, C atom and Mg atom, respectively. The energy cutoff of 520 eV was used for the wave functions

Results and discussion

We focused on the Mg intercalated compounds with high Mg concentration (Ti2Mg0.5C and Ti2MgC), where the stacking order of Ti2C building block from Ti2AlC was assumed unchanged. First we studied the Mg intercalated compound Ti2Mg0.5C, which corresponds to the stage II in Li intercalated graphite (Li0.5C6) [20]. In this composition, Mg is intercalated into Ti2C in every two layers. There are three types of insertion positions of Mg as shown in Figure 1. Figure 1a and c shows that Mg atoms lie on

Conclusion

In this work, we studied a new type of intercalation material Ti2C building block, which is the representative of MXene phases. By calculating the intercalation energies, we found that Li and Na are unlikely to intercalate into it, while Mg is favorable to intercalate into the Ti2C building block, forming the stable compound Ti2MgC. The layer compound Ti2MgC is similar to the Li intercalated layer compound LiC6, where Li is intercalated into graphite in each layer. The good electrochemical

Acknowledgements

This work was supported by the Natural Science Foundation of Shandong Province (no. ZR2013BM016).

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