Employment of Borophene as Conductive Additive to Boost the

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C: Energy Conversion and Storage; Energy and Charge Transport

Employment of Borophene as Conductive Additive to Boost the Performance of MoS2-based Anode Materials Pan Xiang, Xianfei Chen, Jia Liu, Beibei Xiao, and Lanying Yang J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 09 Apr 2018 Downloaded from http://pubs.acs.org on April 9, 2018

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The Journal of Physical Chemistry

Employment of Borophene as Conductive Additive to Boost the Performance of MoS2-based Anode Materials Pan Xianga, Xianfei Chena,b*, Jia Liua, Beibei Xiaoc, Lanying Yanga a

College of Materials and Chemistry & Chemical Engineering, Chengdu University

of Technology, Chengdu 610059, China b

Postdoctoral Innovation Practice Base, Sichuan Konkasnow New Material Co., Ltd.,

Yaan 625400, China c

School of Energy and Power Engineering, Jiangsu University of Science and

Technology, Zhenjiang 212003, China

*

Corresponding author. Email: [email protected] 1

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Carbon-based materials including graphene, porous carbon and nanotube have been widely used as conductive additive to reduce the resistance in semiconductive anode materials of lithium-ion batteries (LIBs) toward better performance and alleviated battery overheat problem. However, these additives are usually denounced for their low lithium ion capacity. Moreover, emergence of vacant defect and heteroatom incorporation would open a sizable energy gap accompanied by reduced conductance. Here, by selecting MoS2 as a prototype system, we proclaim the utilization of emerging borophene as the conductive additive in terms of its low ion transport barrier and robust metallic conductivity against defects and external doping in addition to its high Li storage capacity. We found that substantial electrons transfer from MoS2 to borophene, producing strong electronic coupling that conduces to favorable interface bonding in combination with improved Li affinity. Incorporation of borophene also compensates the poor mechanical property of MoS2 with increased elastic modulus, ensuring the electrode integrity against pulverization. Furthermore, B/MoS2 can achieve a maximum Li storage capacity of 539 mAh/g along with low ion hopping barriers inherited from its counterparts. Our work brings new opportunities to boost the electrochemical performance of semiconductive anode materials with borophene for LIBs.

2


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1. Introduction Lithium-ion batteries (LIBs) have become the most widely used commercial electrochemical cell1 owing to higher energy density, longer cycle life and better environmental friendliness compared with other counterparts. Nevertheless, the booming development of electric vehicles and large-scale energy storage systems associated with the environmental concerns and development of new energy technologies deliver higher requirements to the performance of LIBs that highly depends on the properties of electrode materials.2 Therefore, searching for new electrode materials with satisfactory electrochemical properties including high theoretical specific capacity, lower diffusion barriers and good cycle stability is extremely urgent in the request to the surging demands for delivering better LIBs. Two-dimensional (2D) materials stand out from many potential candidates by virtue of their large specific surface area and unique geometric structure,3 enabling to afford more lithium insertion channels and helping to alleviate the volume expansion and contraction during the charging and discharging process. In the past decades, many 2D materials such as graphene & graphene-based materials,4-5 silicene,6 germanene,7 Blue-/Black-phosphorene,8-9

transition

metal

carbides

and

carbonitrides

(MXenes),10-11 transition-metal dichalcogenides (TMDCs)12-13 and h-BN14 have been widely investigated as the anode materials of LIBs both by theory and experiment, which provided us with many significant hints to further optimize the electrochemical performance of electrode materials. As a typical member of layered TMDCs, monolayer MoS2 shows great promise 3



in the application of supercapacitor electrode material,15 catalyst,16 and light emitting diode17. In the realm of electrode materials for LIBs, MoS2 has been intensively investigated as the anode, of which the particle size and morphology are closely allied to its electrochemical properties18-19. Controlling over the electrode at nanoscale indeed not only conduces to the emergence of increased active sites and interfaces in favor of complete utilization of the active materials, but also results in reduced hopping distance for Li+ intercalation/deintercalation, facilitating the achievement of outstanding rate performance. Nevertheless, the electrochemical performance of MoS2 is not as satisfied as anticipated in consideration of its poor electrical conductivity determined by a direct band gap of 1.8 eV20 and quickly fading capacity upon increased circulation. Additional additives integrated with MoS2 to form heterostructures are highly required to reduce the electron resistance induced energy loss via ohmic heats, facilitate the ion transport on electrode surface and enhance the structural stability via a “self-protected” nature because of confined reaction space21. Experimentally, carbon-based materials including graphene22, porous carbon23 and carbon nanotubes24, etc. and conductive-polymer polyaniline (PANI)25, have been commonly adopted as the components to integrate with MoS2 as the electrode materials. All these mentioned additives, however, suffer from low lithium ion capacity because of their weak affinity to Li and intrinsic storage mechanism.21 Recently, 2D monolayer borophene were successfully fabricated on Ag (111) surfaces with the aid of vapor deposition26-27 and then have been predicated, based on first principles calculations, to server as anode material of LIBs28-30 and beyond31-34. 4


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More importantly, borophene present favorable metallic conductivity combined with excellent mechanical properties, the Young’s modulus of which is comparable to or even rival than that of graphene along with obvious temperature strengthened effect which is absent in the latter35. Such temperature strengthened feather is in favor of the strain relaxation with smaller volume expansion upon lithiation, contributing to the mitigation of electrode pulverization problem. Noteworthily, the metallic conductivity of borophene is robust against lattice defect such as vacancy, H or small amount of O atoms

contamination36-38

and

external

strain39

in

light

of

unique

multi-center-two-electron bond strait40 different from graphene where emergence of these defects would open a sizable energy gap accompanied by remarkably reduced conductance. To this end, we conceive that borophene could be an alternative component to integrate with MoS2 serving as high performance anode materials for LIBs. In this work, the performances of monolayer borophene integrated 2H-MoS2 heterostructures (B/MoS2) serving as anode materials of LIBs have been considered based on Van der Waals corrected density functional theory (DFT-D). Both of borophene and MoS2 monolayer have a large surface-volume ratio and form a puckered surface, which affords more space for lithium storage. The geometrical and electronic

structure,

stress

intercalation/deintercalation

response,

properties

in

dynamical B/MoS2

stability

have

been

and

Li+

investigated

systematically. Our results demonstrate that borophene is a promise ingredient which not only boosts the electrical conductivity of MoS2 but also improves their structural 5



stability with high electrode endurance, in addition to boosting the Li+ storage in terms of synergetic effect. The results provide a steady path toward utilization of borophene as hetero-component to improve the performance of electrode materials for LIBs.

2. Computational Details Calculations were carried out in Dmol3 code41 using numerical functions on an atom-centered grid as atomic basis. Generalized gradient approximation (GGA) with Perdew-Burke-Ernzerhof (PBE) 42 were selected as the exchange correlation function with all electron including some relativistic effects as the core treatment and double numeric plus polarization (DNP) as the basis set. A vacuum space lager than 25 Å have been inserted to weaken the interactions between periodic images according to our convergence tests. The energy, force, and displacement convergence criterion of geometry optimization was set to 1.0 × 10−5 Ha, 0.002 Ha/ Å, and 0.005 Å, respectively. A smearing of 0.002 Ha was used for all the calculations. According to our test calculations, all atoms in pure monolayer borophene, MoS2 and B/MoS2 heterostructures as well as their lithiated counterparts have no retained magnetic moment. Thus, discussions in this work are based on the spin restrict results. Besides, dipole slab correction has a minor effect on the results referring to our test due to the large vacuum layer adopted and thus it is not included currently. The hopping barriers of Li+ in B/MoS2 were determined by LST/QST tools with a root-mean-square (RMS) convergence of 0.002 Ha/Å, which has been well demonstrated to identify the right transition state (TS) structure.43 Electron density difference and electron localization 6


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function (ELF) were calculated in CASTEP module. The thermal stability of primary and lithiated heterostructures were investigated by performing ab-initio molecular dynamics (AIMD) simulations in canonical ensemble (NVT) at 300 K. The adsorption energy (Ead) of Li+ on B/MoS2 is defined as: Ead = EB/MoS2Li – EB/MoS2 – µLi,

(1)

where EB/MoS2Li, EB/MoS2 indicate the total energy of lithiated and pristine B/MoS2, µLi denotes the chemical potential of lithium atoms leveled from bulk metal. The open-circuit voltage (OCV) for Li+ intercalation in B/MoS2 could be obtained by considering the common half-cell reaction: (x2-x1) Li+ + (x2-x1) e－+ Lix1B/MoS2 → Lix2B/MoS2, where x1 and x2 represents the number of Li+ inserted into the unit cell of B/MoS2. Then, the OCV could be achieved as: V=−

∆

,

(2)

where ∆G = ∆E = EB/MoS2Lix2 – EB/MoS2Lix1 – (x2 – x1) µLi. Besides, the specific energy storage capacity is given by: C =

/

,

(3)

where xmax denotes the maximum number of adsorbed lithium atoms, n = 1, F is the faraday constant, MB/MoS2 means the weight of B/MoS2 and MLi refers to the mole weight of Li. More details of the calculation methods could be found in our recent work34.

3. Results and Discussion 3.1 Atomic structure, electronic properties and stability of B/MoS2 The optimized geometrical structures of hexagonal monolayer borophene and 7



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2H-MoS2 are elucidated in Figure S1a-b, the lattice constants of which are calculated to be 3.28 and 3.20 Å, respectively, being in good accordance with previous results.26, 44

Lattice mismatche between borophene and MoS2 is only 2.37% compatible for

constructing high-quality heterostructures45. The band structure of monolayer MoS2 delivers a typical semiconducting trait with a direct band gap of 1.65 eV in accordance with the previous calculation result (1.68 eV)46, whereas that of borophene is metallic with some 2p states crossing the Fermi level as shown in Figure S1c-d, in line with previous reports.28, 47 To determine the ground atomic arrangement, eight selective sacking patterns with different displacement between the adjacent sheets for B/MoS2 have been considered in Figure S2. All configurations are subjected to structure relaxation with calculated binding energy (Ebind), charge transfer (∆q), and corresponding geometrical parameter changes tabulated in Table S1. The Ebind is evaluated by following equation: Ebind = (EB/MoS2 – EBorophenen – EMoS2)/A,

(4)

where EB/MoS2, EBorophene and EMoS2 are the total energy of combined structure, monolayer borophene and MoS2, respectively. The A denotes the interface area of the combined structure. The most energy favorable one for B/MoS2 is shown in Figure 1a-b, where the S atoms of MoS2 lie below the B-B bond of borophene spine. Besides, the energies varied with interlayer distance L of borophene and MoS2 are shown in Figure S3 with an equilibrium distance of 2.80 Å. In Table S1, the bond lengths of both B-B and Mo-S change a little (typical

Employment of Borophene as Conductive Additive to Boost the

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