Source: https://pubs.rsc.org/en/content/articlehtml/2018/ta/c8ta09155b?tdsourcetag=s_pcqq_aiomsg
Timestamp: 2019-04-22 04:52:32+00:00

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Two-dimensional (2D) materials as anodes exhibit unique advantages, in contrast to their bulk counterparts, such as large surface–volume ratio, broad electrochemical window, fascinating chemical activities, and excellent mechanical strength.39–42 Some 2D materials (e.g. graphene,43 transition metal oxides,44 and transition metal dichalcogenides45) have exhibited excellent performance in LIBs. Phosphorene, stripped from black P, demonstrates intriguing electron properties.46 Phosphorene as anode in SIBs shows acceptable theoretical capacity, however, its inherent non-metallic nature affects electrochemical performance.12,47 Once again, phosphorene–graphene hybrid material shows high performance, in which graphene serves as a mechanical backbone and an electrical highway.38,48 By all appearances, the combination P and C is an effective way to enhance performance of the anode. A natural and immediate thought is therefore to examine whether P and C could form stable two-dimensional compounds with high performance in SIBs.
Considering the high capacity of P as anode, we mainly focus on P-rich P–C stoichiometric monolayers. In this work, an extensive structural search was carried out with various stoichiometries (PxCy, x = 1–8 and y = 1) through first-principles swarm structural search calculations. As expected, a stable and metallic P3C monolayer with a buckled hexagonal structure was identified. More importantly, the P3C monolayer shows high adsorption capacity for sodium (i.e. P3CNa4), corresponding to a theoretical capacity of 1022 mA g h−1. The intrinsic metallicity of P3C and P3CNa4 ensures a good rate performance. The Na diffusion barrier of 0.19 eV in the P3C monolayer is comparable to that of the well-known Ti2C39 and Sb.49 Its high cohesive energy and thermodynamic stability provide good opportunities for experimental synthesis.
where EP, EC, ENa, EP3C, and EP3CNan are the total energies of a single P atom, a single C atom, a Na atom in body-centered cubic (bcc) structure, one unit cell of the P3C monolayer, and the sodiated monolayer, respectively.
A stable P3C monolayer with C2/m symmetry was identified through comprehensive structure search calculations. Its basic building blocks are P6 and P4C2 rings. The alternative arrangement of the two kinds of hexagon rings makes the P3C monolayer stabilize in a puckered single-layered structure akin to the structural configuration of layer-structured GeP3,67,68 InP3,69 and SnP3 (ref. 70) (see Fig. 1a). In more detail, each atom is three-fold coordinated with the same or other atoms, having P–P and P–C distances of 2.28 Å and 1.78 Å, which are comparable to that in black P (2.22 Å)71 and almost the same as that in β0-PC (1.77 Å).72 Their covalent bonding characters are evidenced by the presence of localized electrons between two nearest-neighbor atoms, as clearly shown in the plot of electron localization function (ELF, Fig. 1b). On the other hand, the slight torsion of two hexagon rings causes the P3C monolayer to have a thickness of 1.31 Å, which is much thinner than phosphorene (thickness of 2.10 Å)12 and GeP3 (thickness of 2.44 Å).73 Considering these characteristics, P3C monolayers have a large surface–volume ratio,74,75 facilitating intercalation of Na, as discussed later.
Fig. 1 (a) Optimized structure of P3C monolayer with C2/m symmetry. (b) Electron localization function (ELF) of P3C monolayer. (c) Decomposed PDOS of P3C monolayer, projected on four orbitals (s and px,y,z) of P and C atom.
The feasibility of experimental synthesis of the predicted two-dimensional materials correlates closely with the amount of cohesive energy.59,76 The larger the cohesive energy, the easier is experimental synthesis. The resultant cohesive energy of 4.18 eV per atom is significantly higher than that of synthesized phosphorene (3.48 eV per atom)77 or GeP3 (3.34 eV per atom),73 and is comparable to that of the 1T-NiSe2 monolayer (4.66 eV per atom)78 and the 2H–CoTe2 monolayer (4.48 eV per atom).79,80 Notably, a layer-structured GeP3/C nanocomposite has been synthesized showing excellent Li storage performance.81 Besides, the absence of the imaginary frequency mode in the whole Brillouin zone indicates the dynamical stability of the P3C monolayer (Fig. S1a and b†). As shown in projected phonon density of states (PHDOS), the appearance of extensive overlapping between P and C vibrations confirms a strong interaction in the P–C bond. To examine the temperature-dependent stability, we performed MD simulations at 1000 K. The framework of the P3C monolayer maintains its original configuration (Fig. S1c and d†) after simulation. However, the interlayer interaction caused by unique structural characters (i.e. P lone pair electrons, C dangling bond) might hinder synthesis of the P3C monolayer. This inspired us to probe the feasibility of synthesizing the P3C monolayer. According to the symmetry of P3C, five kinds of bilayer stacking patterns can be considered to investigate the exfoliation energy (Fig. S2†). After full structural relaxation, including van der Waals interaction, a stable P3C bilayer is obtained. The interlayer distance of 4.05 Å (Fig. S2d†) is much larger than for graphite (3.23 Å)82 and black phosphorus (3.10 Å).83 Moreover, the exfoliation energy of the P3C bilayer is 25.2 meV Å−2, which is smaller than hexagonal BN (28 meV Å−2) and slightly larger than graphene (21 meV Å−2) and phosphorene (22 meV Å−2).84 These results suggest that our predicted P3C has outstanding thermal and dynamical stability and favors experimental synthesis.
The P3C monolayer is expected to be a potential anode material in SIBs in view of its favorable stability, structural character, and inherent metallicity. For anode materials, spontaneously adsorbing Na atom is one of the necessary conditions. Considering the symmetry of the P3C monolayer, nine possible adsorption sites were considered (Fig. S6†). After full geometrical relaxations, there are two favorable adsorption sites (A1 and A2), localizing at the top of the C atoms (A1) and the P6 ring (A2), as shown in Fig. 2a and b. The resultant adsorption energies at A1 and A2 sites are 1.08 eV and 0.90 eV, respectively, indicating that our predicted monolayer can adsorb Na ions. The larger adsorption energy at the A1 site can be attributed to the delocalized electrons on the C surface providing a suitable habitat for Na ions. As can be seen in the ELF (Fig. S7†), the interaction type between Na and C is ionic with a Na–C distance of 2.49 Å. This is further supported by the Bader charge analysis: each Na atom transfers 0.84 electrons to the C atom.
Fig. 2 (a) Na adsorbs at top of the lower C atom, named A1. (b) Na sits at the center of a P6 ring, as A2. (c) The considered migration paths of Na diffusion on the P3C monolayer. (d) The corresponding diffusion energy barrier profiles of Path I and Path II.
The Na ion diffusion energy barrier has a great effect on the charging and circuit rate capacity of SIBs. Based on the above results, there are two different diffusion paths for Na ion (I and II, Fig. 2c) between the two nearest-neighbor low-energy adsorption sites. The calculated diffusion energy barriers are shown in Fig. 2d. Path I (i.e. A1 → A2 → A1) has the lowest diffusion energy barrier, of 0.19 eV, whereas the diffusion energy barrier of path II, which is directly from an A1 site to another nearest A1 site, is up to 0.46 eV. Based on the above analysis, the P6 ring plays a key role in accelerating Na-ion migration. Notably, the diffusion energy barrier is much lower than that of traditional MoS2 (0.28 eV)86 and comparable with that of Ti2C (0.18 eV)39 and Sb (0.21 eV).49 To clarify the migration paths I and II, the movement trails of adsorbed Na ion crossing the energy barriers are shown in Fig. S8.† Notably, the diffusion length of 7.30 Å in Path I is similar to 7.09 Å in Path II.
Fig. 3 The top and side views of optimized adsorption configuration for the four different Na concentrations (i.e. (a) P3CNa, (b) P3CNa2, (c) P3CNa3 and (d) P3CNa4).
Fig. 4 ELF map of P3C with (a) one-layer and (b) two-layer Na atoms.
Under experimental conditions, anode materials could consist of multiple monolayers. This inspired us to explore the intercalation of Na within layers and the resultant volume expansion, which closely correlates with practical applications. Here, we mainly focus on the most stable P3C bilayer, as discussed above. As shown in Fig. S11,† the P3C bilayer can adsorb two-layer Na atoms within the layer with an adsorption energy of 0.63 eV, which is larger than 0.48 eV for P3CNa3 and 0.35 eV for P3CNa4 with Na atoms on both sides of P3C monolayer. The interlayer distance of sodiation for the P3C bilayer is 9.51 Å, corresponding to a volume expansion of ∼135% with respect to the P3C bilayer.33,97 This is much smaller than the volume expansion of black phosphorus (∼400%).33 Finally, we investigated whether the sodiation P3C bilayer can be restored to its original configuration. After removing all the adsorbed Na atoms, the relaxed P3C bilayer recovered its initial structure.
In this work, a novel P3C monolayer with a puckered honeycomb structure has been found through swarm-intelligence structural search calculations, with large cohesive energy and good thermodynamic stability. The P3C monolayer can spontaneously adsorb Na ions with an unexpected stoichiometry of P3CNa4, leading to a remarkably large theoretical capacity of 1022 mA g h−1. Its Na ion diffusion barrier is as low as 0.19 eV, ensuring a rapid charge/discharge rate capacity for SIBs. The inherent metallicity of the P3C monolayer provides good electron conductivity. Its structural integrity can be well maintained in the sodiation process. By all appearances, our predicted P3C monolayer is a promising anode material for SIBs, and awaits experimental confirmation.
This research was supported by the Natural Science Foundation of China under No. 21573037, 21873017, 11704062, and 51732003, the Postdoctoral Science Foundation of China under grant 2013M541283, and the Natural Science Foundation of Jilin Province (20150101042JC), and the Fundamental Research Funds for the Central Universities (2412017QD006).
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