Ti2CT2 (T=F, O) Heterostructures as Promising Flexible Anodes

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MoS/TiCT (T=F, O) Heterostructures as Promising Flexible Anodes for Lithium/Sodium Ion Batteries Jie Li, Qiong Peng, Jian Zhou, and Zhimei Sun J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b01648 • Publication Date (Web): 17 Apr 2019 Downloaded from http://pubs.acs.org on April 17, 2019

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The Journal of Physical Chemistry

MoS2/Ti2CT2 (T=F, O) Heterostructures as Promising Flexible Anodes for Lithium/Sodium Ion Batteries Jie Li1,2#, Qiong Peng1,2#, Jian Zhou1,2*, and Zhimei Sun1,2* 1School

of Materials Science and Engineering, Beihang University, Beijing 100191, China

2Center

for Integrated Computational Materials Engineering, International Research Institute for

Multidisciplinary Science, Beihang University, Beijing 100191, China # These

authors contributed equally.

*Correspondence and requests for materials should be addressed to J. Zhou or Z. M. Sun: [email protected], [email protected].

Abstract Flexible batteries play a more and more important role with the increasing demands of wearable electronics and soft robots, while searching for electrode materials with high stretchability remains a great challenge. In this work, we report that the heterostructures composed of MoS2 and Ti2CT2 (T=F, O) monolayers are competitive and promising candidates as flexible anode materials for Li/Na-ion batteries through first-principles calculations. Compared with the related single-layer components, MoS2/Ti2CT2 heterostructures show more negative Li/Na adsorption energies and enhanced electrical conductivities. The theoretical capacities (over 430 mAh/g) are higher than that of the commercial anode material graphite and the diffusion barriers are as low as 0.57 eV for Li and 0.37 eV for Na. Furthermore, MoS2/Ti2CT2 heterostructures can sustain large ultimate tensile strains (>20%) and exhibit excellent mechanical flexibility.

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1. Introduction With the advantage of high energy density and wide voltage window, lithium ion batteries (LIBs) become one of the most important energy storage systems and have been widely used in portable electronic devices such as cell phones.1 However, the currently used anode material, graphite, reaches its theoretical limit. The large-scale application of LIBs is hindered by the slow speed of development of battery technologies.2,3 As an alternative to LIBs, rechargeable sodium ion batteries (NIBs), benefiting from the widespread abundance, low cost, high safety and suitable redox potential (0.3V higher than that of Li) of Na atoms, have attracted tremendous attention.4-6 But Na atom has a lager radius than Li and the currently used anode materials in LIBs may not suit for NIBs, which makes the discovering of electrode materials to host Na ion still a challenging question.7 Additionally, with the increasing demands of flexible and wearable electronics, the current electrode materials of LIBs and NIBs cannot meet the needs of future applications because of the low stretchability.8,9 Thus, there is an urgent need to search high-performance electrode materials with large reversible capacity, high cycling stability and high flexibility to accommodate both Li and Na, yet it remains a great challenge. MoS2, one of transition metal dichalcogenides (TMDs), has been confirmed to show high charge storage capacities and ion mobilities when used in metal-ion batteries.10-13 However, low electronic conductivity and large volume change during metal-ion insertion/extraction of MoS2 have affected the electrochemical properties greatly and hampered the further application in batteries.14-16 Using different two-dimensional (2D) materials to construct van der Waals (vdW) heterostructures gives us new solutions to design nanoelectronic devices which can have the advantages over both 2


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constituent materials. Many vdW heterostructures have been experimentally synthesized for battery usage and show superior performance than the individual components.17-22 Emerging transition metal carbides (MXenes) show high electrical conductivity, high capacity and good rate performance when used as anode materials in LIBs and NIBs, but the inevitable layers restacking during the preparation of MXenes severely reduces the ion mobility.23-27 Among the experimentally available MXenes, Ti2C is one of the most studied members and has a very low molar mass. During the preparation process, the surfaces of MXenes are prone to be terminated by functional groups OH, F and O. While OH group can be converted into O termination under high temperature.25 So it is a good idea to enhance the performance of MoS2 devices by forming heterostructures with functionalized Ti2C. Recently, MoS2/TiC2Tx heterostructure has already fabricated through the mechanical transfer method.28 But there is still lack of fundamental understanding of the interaction between metal-ion and the MoS2/MXene heterostructures. Meanwhile, the mechanical flexibility remains elusive. Therefore, in this work, heterostructures composed of MoS2 and Ti2CT2 (T=F, O) were systematically investigated to explore the application in LIBs and NIBs through first-principles calculations. It was found that these heterostructures bond tightly with lithium/sodium at interfaces while keep high conductivities. The theoretical capacities (over 430 mAh/g) are higher than that of graphite. Besides, the moderate Young’s modulus and large ultimate strains render the heterostructures promising for flexible batteries. 2. Computational Details All the first-principles calculations based on density functional theory are implemented in the Vienna ab initio Simulation Package(VASP).29 A plane-wave basis set with a 500 eV cut-off energy is used. For the exchange-correlation energy, we use the Perdue-Burke-Ernzerhof (PBE) of the 3



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generalized gradient approximation (GGA).30 A 5×5×1 and 11×11×1 Monkhorst−Pack (MP) k-point mesh are used for geometric optimization and the density of states (DOS) calculations, respectively. Because of the weak interaction at the interface, the van der Waals correction of Grimme (D2) is adopted.31 Over 20 Å vacuum space perpendicular to the slab is set to avoid the interaction caused by the periodic boundary conditions. The convergence criterion of 0.01 eV/Å is set to minimize the force between all the atoms. Furthermore, the climbing image nudged elastic band (CINEB) method is used to evaluate the metal ions diffusion properties.32 The interface binding energy was defined as: (1)

Eb  EMoS2 /Ti2 CT2  EMoS2  ETi2 CT2

where EMoS , ETi CT , EMoS /Ti CT are the total energies of pristine MoS2, Ti2CT2 monolayers and 2

2

2

2

2

2

MoS2/Ti2CT2 heterostructures, respectively. The adsorption energy for Li/Na intercalated systems was defined through the following equation: Ead 

1  EHetero+nM  EHetero  nEM  n

(2)

where EHetero+nM and EHetero are the total energy of the heterostructures with and without n metal atom, respectively. EM is the total energy of a Li/Na atom in its stable bcc bulk structure. According to this definition, a more negative adsorption energy means a more favorable exothermic binding of metal atom. The average adsorption energy of Li/Na in multiple-layer adsorption is defined as: Eav 

1 EHetero+(5L)M  EHetero+(3L)M  8 EM 8





(3)

Where EHetero+(5L)M and EHetero+(3L)M are the total energy of the heterostructures with five layers and three layers Li/Na atoms adsorption, respectively. The “8” in the formula means there are 8 adsorbed metal atoms in the additional two layers. 4


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The theoretical gravimetric capacity was determined from: C

nF M Hetero

(4)

where n is the number of adsorbed Li/Na adatoms, F is the Faraday constant (26801 mAh/mol), MHetero is the mole weight of MoS2/Ti2CT2 heterostructures. The open circuit voltage was obtained according to the following formula: OCV  

1  EHetero+nM  EHetero  nEM  ne

(5)

where EHetero+nM, EHetero, EM denote the total energy of MoS2/Ti2CT2 heterostructures with n metal atoms intercalation, pristine MoS2/Ti2CT2 heterostructures, and the intercalated species, respectively. e is the elementary charge. The in-plane Young’s modulus and the Poisson’s ratio can be obtained by the equation: E  C112  C12 2 / C11





(6)

  C12 / C11

(7)

3. Results and Discussion The optimized in-plane lattice constants are 3.19, 3.05, and 3.03 Å for MoS2, Ti2CF2 and Ti2CO2 monolayers, respectively, which are consistent with the previous studies.6,33 Given that the lattice mismatch between MoS2 and Ti2CT2 is rather small (20%) and Young’s modulus which is beneficial for the application of flexible batteries. Our results show that the MoS2/Ti2CT2 heterostructures are promising electrode materials for high-performance and flexible metal-ion batteries.

Supporting Information Six stacking patterns of the MoS2/Ti2CT2 and the structural parameters; adsorption sites of single Li intercalated into MoS2/Ti2CT2; binding energies of Na adsorption in MoS2/Ti2CT2; configuration of five-layer Li adsorption in MoS2/Ti2CT2.

Acknowledgments This work is financially supported by the National Key Research and Development Program of China (Materials Genome Initiative, 2017YFB0701700) and National Natural Science Foundation of China (51871009).

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Figure 1. Side views and band structures of (a, c) MoS2/Ti2CF2 and (b, d) MoS2/Ti2CO2 with the most energetically favorable configuration, respectively. The Fermi level is set to zero. The electronic orbital contribution of MoS2 is colored red. 84x84mm (300 x 300 DPI)



Figure 2. (a) Side view of adsorption active sites, taking Li adsorbed in MoS2/Ti2CO2 as an example. (b) Adsorption energies of Li on MoS2 (cyan), Ti2CT2 (yellow) and the heterostructures (brown). 177x62mm (300 x 300 DPI)


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Figure 3. Total DOS of (a, d) pristine, (b, e) a single Li-adsorbed, (c, f) a single Na-adsorbed MoS2/Ti2CF2 and MoS2/Ti2CO2, respectively. The Fermi levels are set to zero and are indicated by the dashed lines. 84x71mm (300 x 300 DPI)



Figure 4. The charge density difference of (a) one Li adsorption on the bottom surface of Ti2CO2; (b) embedding into the interlayer of MoS2/Ti2CO2; (c) adsorption on the top surface of MoS2. Yellow and blue colors indicate the electron accumulation and depletion, respectively. The isosurface value is 2×10−3 electrons/Å3. 84x54mm (300 x 300 DPI)


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Figure 5. (a) Schematic representations of the potential diffusion paths at the interlayers and (b) the diffusion barrier profiles along Path-2. 177x79mm (300 x 300 DPI)



Figure 6. Tensile stress σ as a function of uniaxial strain ε of (a, b) MoS2, (c, d) Ti2CT2, and (e, f) heterostructures along zigzag and armchair directions, respectively. The uniaxial stress-strain curve is calculated using an orthorhombic supercell, which has twice the atomic number of the original hexagonal lattice. 84x98mm (600 x 600 DPI)


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TOC graphic 85x59mm (300 x 300 DPI)


Ti2CT2 (T=F, O) Heterostructures as Promising Flexible Anodes

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