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TiC Monolayer with High Specific Capacity for Sodium-Ion Batteries Tong Yu, Ziyuan Zhao, Lulu Liu, Shoutao Zhang, Haiyang Xu, and Guochun Yang J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b02016 • Publication Date (Web): 25 Apr 2018 Downloaded from http://pubs.acs.org on April 25, 2018
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TiC3 Monolayer with High Specific Capacity for Sodium-Ion Batteries Tong Yu, Ziyuan Zhao, Lulu Liu, Shoutao Zhang, Haiyang Xu, and Guochun Yang* Centre for Advanced Optoelectronic Functional Materials Research and Key Laboratory for UV LightEmitting Materials and Technology of Ministry of Education, Northeast Normal University, Changchun 130024, China
ABSTRACT: Sodium-ion batteries (SIBs) have attracted considerable attention due to the intrinsic safety and high abundance of sodium. However, the lack of high-performance anode materials becomes a main obstacle for the development of SIBs. Here, we identify an ideal anode material, metallic TiC3 monolayer with not only remarkably high storage capacity of 1278 mA h g-1 but also low barrier energy and open-circuit-voltage, through first-principles swarm-intelligence structure calculations. TiC3 still keeps metallic after adsorbing two-layer Na atoms, ensuring good electrical conductivity during battery cycle. Besides, high melting point and superior dynamical stability are in favor of practical application. Its excellent performance can be mainly attributed to the presence of unusual n-biphenyl unit in TiC3 monolayer. High cohesive energy, originating from multi-bonding coexistence (e.g. covalent, ionic and metal bonds) in TiC3 monolayer, provides the strong feasibility for experimental synthesis. In comparison with TiC3, functionalized TiC3 with oxygen shows a higher storage capacity, meanwhile keeps nearly the same barrier energy. This is in sharp contrast with metal-rich MXenes. These intriguing properties make TiC3 monolayer a promising anode material for SIBs.
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1. Introduction Lithium-ion batteries (LIBs), one of the most successful clean and efficient energy storage devices, are widely used for portable electronic devices.1,2 However, the storage of lithium source on earth is rather limited. According to the current consumption rate of 21280 tons per year, the existing lithium source could be only sustained for about 65 years.3 In contrast, sodium is the fourth most abundant element in the earth’s crust, providing vast sodium source for the development of SIBs. On the other hand, sodium shares similar intercalation chemistry with lithium, leading that some of mature technologies in LIBs can be migrated to SIBs. Moreover, the safety of SIBs is much better than that of LIBs. Therefore, SIBs become one of the best candidates in energy storage devices. However, SIBs are still in initial development stage, and high performance materials and scientific challenges still need to be found and overcome.4–6 The search for high-performance anode material has become a major obstacle to the development of SIBs.7,8 One of the main reasons is that high-performance anode materials in LIBs are not applicable to SIBs. For instance, graphite, as a commercial anode material in LIBs, does not properly intercalate sodium ions due to the large atomic radius of sodium, leading to the mismatched layer spacing. Various improved C-based anode materials for SIBs have been reported (e.g. expanded graphite, hard carbon, tin nanoparticle embedded in carbon, and hierarchically porous carbon/graphene composite).9–12 However, their specific capacities (284-493 mA h g-1) are far from satisfactory. Although specific capacities of group IV and V element materials (e.g. Ge, Sn, Pb, and Sb)13–16 have been improved in some content, their poor rate capability greatly limits the performance of SIBs. Thus, for SIBs, identifying suitable anode materials with desirable properties becomes rather urgent. Two-dimensional transition-metal carbides (MXenes) are excellent anode materials in LIBs due to their high specific capacity, high electrical conductivity, high rate performance, and good structural stability.17,18 A natural thought is to apply MXenes for SIBs. However, the reported MXenes (e.g. Ti3C2, Ti2C, Mo2C) show low specific capacity19–21 because of metal-rich composition, which greatly reduces the ability of sodium absorption. For MXenes, attractive interaction between carbon and sodium is ACS Paragon Plus Environment
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mainly responsible for sodium adsorption, whereas repulsive interaction between metal and sodium ions is adverse.21 As a consequence, properly enhancing carbon chemical composition in MXenes is expected to be a promising way to elevate specific capacity, despite repulsive interaction of adjacent adsorption sodium ions allows part of carbon atoms to adsorb sodium. Two-dimensional titanium carbides have shown high stability and good performance in LIBs.22,23 Thus, we choose titanium carbides monolayer as a representative of MXenes to explore the effect of carbon-rich composition on anode material performance of SIBs. We perform our extensive structural search on sable Ti-C monolayers with four carbon-rich stoichiometries (TiCx, x = 3 - 6) through first principles swarm structural search.24–28 We identify a dynamic and thermal stable TiC3 monolayer. The calculated specific capacity reaches 1278 mA h g-1, which is the highest among the reported twodimensional materials.19–21 Its rate performance is comparable to those of the other metal-rich MXenes.21,29,30 TiC3 shows not only excellent thermal and dynamic stability but also high electron and ion conductivity. These characters clearly indicate that TiC3 monolayer is an excellent anode material for SIBs. Our work also opens up an avenue to explore other carbon-rich MXenes as high-performance anode materials for SIBs. 2. Computational Details We performed structure search by employing particle swarm optimization algorithm as implemented in the CALYPSO code,31 which can efficiently find the ground or metastable structures just depending on the given chemical compositions. Its validity has been confirmed by the application of a diverse variety of bulk24,25,32 and two-dimensional materials.33–38 A vacuum distance of ~20 Å was used to avoid interaction between adjacent layers. The structure relaxations and properties calculations were carried out by using density functional theory method,39 within the generalized gradient approximation of Perdew-Burke-Ernzerhof (GGA-PBE)40 as implemented in the Vienna ab initio simulation package (VASP).41 The ion-electron interaction was treated by the projector augmented wave (PAW) technique.42 The plane-wave cutoff energy of 800 eV was employed. The atomic positions were fully
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relaxed until the maximum force on each atomic was less than 10-3 eV/Å. Van der Waals interaction was taken into account using the semiempirical DFT-D2 approach.43 Phonon dispersion calculations were based on a supercell approach as done in the Phonopy code.44 First-principles molecular dynamics (MD) simulations in NVT ensemble using the Nosé heat bath method lasted for 10 ps with a time step of 1.0 fs.45 Detailed descriptions can be found in the supporting information. The cohesive energy of TiC3 monolayer was calculated by using the following formula: (1)
Ecoh = (ETi + 3EC − ETiC3 ) / 4
The adsorption energy of Na atom on TiC3 monolayer can be obtained by: (2)
Ead = ( ETiC3Na n − ETiC3 − nENa ) / n
The formation energies of TiC3Nan with respect to body-centered cubic (bcc) Na and TiC3 monolayer was defined as: (3)
∆E = ( ETiC3Nan − ETiC3 − nENa ) / (n + 1)
The open circuit voltage (OCV) can be evaluated from the following reaction： TiC3 + nNa + + ne− ↔ TiC3 Na n
(4) Its Gibbs free energy change originates from the contribution of internal energy (∆E), P∆V term, and entropy change (T∆S). During the sodiation process, the change of volume or entropy is rather small.46 As a consequence, we neglect the contribution of P∆V and T∆S. The average OCV between sodiation and desodiation process is expressed as in the following equation:21,47,48 Vave =
-∆E ETiC3 + nENa − ETiC3Na n = nzF nzF
(5)
where ETi , EC , ENa , ETiC , ETiC Na , F and z are the total energies of a single Ti atom, a single C atom, a 3
3
n
Na atom in body-centered cubic (bcc) structure,49 one unit cell of the TiC3 monolayer, sodiated
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monolayer, the Faraday constant and the electronic charge of Na ions in the electrolyte (z = 1), respectively. In the sodiation process, all the atoms are fully relaxed including TiC3 monolayer. 3. Results and discussion 3.1 Structure and stability After extensive structure search, we found that TiC3 monolayer with C2/m symmetry has the lowest energy, indicating a global minimum in the two-dimensional space (Figure 1a). The basic building blocks of TiC3 monolayer are n-biphenyl unit and zigzag Ti atom chain (Figure 1b). Each Ti atom connects three C6 rings on its both sides forming the four-fold coordination with Ti-C bond length of 2.10 Å. This distance is slightly shorter than 2.17 Å in face-centered cubic TiC.50 Intriguingly, Ti-Ti distance of 2.63 Å, in zigzag Ti chain, is shorter than hexagonal Ti (2.82 Å).51 The average C-C bond length of 1.45 Å is comparable to 1.42 Å in graphene.52 Here, the calculated electron localization function (ELF)53 is used to analyze the bonding character. Generally, the large ELF value (>0.5) corresponds to covalent bond or core electrons, whereas the ionic bond is represented by smaller ELF value (
