Two-Dimensional Group IV Monochalcogenides: Anode Materials for

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Two Dimensional Group IV Monochalcogenides: Anode Materials for Li-Ion Battery Sharmistha Karmakar, Chandra Chowdhury, and Ayan Datta J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b04152 • Publication Date (Web): 21 Jun 2016 Downloaded from http://pubs.acs.org on June 23, 2016

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The Journal of Physical Chemistry

Two Dimensional Group IV Monochalcogenides: Anode Materials for Li-Ion Battery Sharmistha Karmakar, Chandra Chowdhury and Ayan Datta* Department of Spectroscopy, Indian Association for the Cultivation of Science, 2A and 2B Raja S. C. Mullick Road, Jadavpur – 700032, Kolkata, West Bengal, India.

ABSTRACT:

Choice of suitable electrode material is a fundamental step in Li-ion battery (LIB) to achieve enhanced performance. In the present study we have explored the feasibility of phosphorene analogs, i.e. group IV monochalcogenides (SiS, SiSe, GeS, GeSe,SnS and SnSe) monolayers to serve as anode material in LIB by density functional theory(DFT). Our exploratory study indicates lithium binds efficiently to these monolayers of which Li@SiS and Li@SiSe show appreciable stability which are comparable to phosphorene. Zero point energy corrected minimum energy pathway (MEP) for Li diffusion demonstrates high anisotropy for both SiS and SiSe with a low diffusion barrier of ~0.15eV along the zigzag direction. Inclusion of corrections due to quantum effects like the zero point energy (ZPE) and quantum mechanical tunneling (QMT) increase the diffusion rates by 6-10 % at room temperature and become increasingly significant as temperature is reduced (40-55 % increment at T=100K). The calculated theoretical capacity for SiS and SiSe are 445.7 mAhg-1 and 250.44 mAhg-1 respectively which are well above existing commercially available used anode materials. Both SiS and SiSe preserve their structural integrity upon lithiation justifying their role as host material for lithium. A semiconductor → metallic transition is observed upon full lithiation for both. All these exceptional properties including low diffusion barrier, moderate to high specific capacity, low open circuit voltage (OCV), small volume change and good electrical conductivity, suggest that monolayer SiS and SiSe could serve as a promising electrode material in LIB.

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Introduction: The ever-increasing demand of fossil fuel leads to discovery of many alternative sources of energy among which lithium ion battery (LIB) is highly promising.1-3 The high reversible capacity, high power density and long cycle life makes lithium-ion batteries (LIBs) suitable candidates for portable and green energy storage material.4-6 However the performance of LIB is greatly controlled by choice of the electrode material (cathode and anode)7,8 and electrolyte.9 Among carbon based materials, graphite has been extensively used as anode material for LIB.10,11 However poor specific capacity (372 mAhg−1) limits its application in modern electronics.3 Moving from graphite to graphene, its two dimensional analog leads to a high predicted specific capacity of ~ 955 mAhg-1. 5,12,13 Unfortunately, a high diffusion barrier and lithium nucleation over graphene renders its application in terms of durability.14 Anode materials based on silicon, the next alternative to carbon, provide substantially high theoretical capacity of 4212 mAhg−1 owing to Li alloy formation.15,16 However these materials tend to degrade as a result of large volume change.17,18 Silicene, the two dimensional Si, counterpart of graphene, forms bulked layered structure as a result of puckering in the hexagonal Si6-rings due to pseudo Jahn Teller distortion (PJT).19-21 It is has been shown theoretically that pristine silicene and silicene supported on MgX2 are promising candidates for negative electrode in Li-ion battery.22,23 Various other two-dimensional materials such as MXenes and layered transition metal dichalcogenides have also been widely studied for this purpose. 24, 25 Such new 2D materials have unusual chemical bonding and stoichiometry, which could structurally mimic graphene monolayer. 26-28 Black phosphorous, the thermodynamically most stable allotrope of phosphorous29, is a layered material like graphite and is known to possess a large theoretical capacity of 2596 mAhg−1. 30,31 However poor cycle life and irreversible structural change limits its application as negative electrode in LIB.32 Recently, phospherene, the 2D analog of black-P and phosphorene based hybrid materials have shown great potential as an anode material in Li ion battery.33,34 However high reactivity of phosphorene towards O2 and H2O still poses important technological bottlenecks.35 Recently we have shown that capping of hexagonal Boron nitride (h-BN) enhances performance of black phosphorene as an anode material.36 Therefore, the search for optimal electrode material is still at infancy and one needs to consider several aspects while design of new anode material. 2 ACS Paragon Plus Environment

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Scheme 1.Proposed Free-standing monolayer of MX; where M=Si, Ge and Sn and X=S and Se. These monolayers might be synthesized using physical techniques like sonication and mechanical exfoliation or using chemical techniques like intercalation by ionic liquids or ions.

The 2D analoges of group IV monochalcogenides; MX (M=Si, Ge, Sn and X=S or Se) display structural similarity with phosphorene and form buckled honeycomb lattice.37,38 The basic hexagonal ring structure of these systems consist of the puckered M3X3 unit similar to the P6-unit of phosphorene where each sp3 hybridized M/X atom is covalently linked with three adjacent X/M atom exhibiting

its

characteristic wave-like structure.37 The bulk form of these four mono chalcogenides (GeS, SnS, GeSe and SnSe) is known to exist in orthorhombic crystal structure similar with black-P. They belong to space group Pcmn-D2h16 whereas the space group of black P is Bmab-D2h18, as it consists of only one element.37 Preparation of their monolayers either by exfoliation from bulk material or by other chemical process (see Scheme 1) is an active area of research.39,40 Recently, silicon monosulfide and monoselenide have been predicted to form stable free standing monolayers like phosphorene and therefore, synthesis of α-SiS and α-SiSe monolayer is highly feasible through different efficient deposition technique.38,41 All of them are semiconductors with direct or indirect band gap ranging from 0.96-1.65 eV.37,38 Structural resemblance and electronic property similarity make their properties analogous. Additionally, nanostructured germanium and tin chalcogenides (GeS, GeSe, SnS and SnSe) are known to be good anode materials in rechargeable lithium ion battery.42



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Although high theoretical capacity is the main advantage for these nano-structured materials, large volume change owing to Li alloy formation (like Li4.4Ge and Li4.4Sn) during lithiation limit their performance.42,43 Lithium silicon sulfide (Li2SiS3) has also been used as negative electrode in solid state lithium batteries.44 However, the feasibility of their monolayers for Li storage materials is still unexplored. The performance of LIB also depends on the Li diffusion barrier and a fast charging/discharging rate is highly desirable to achieve fast switching result. So, a small barrier for Li-hopping between the various possible sites is highly desirable. In this work, we have investigated the suitability of 2D group-IV monochalcogenides as host material in LIB from first principal calculations.

We have identified the

favorable binding site for Li and their minimum energy pathways (MEP) for diffusion. Our results show that both SiS and SiSe monolayers have excellent potential to perform as negative electrode in LIB. Additionally, lithium being the third lightest element in the periodic table should have non-negligible contribution from quantum mechanical tunneling (QMT). In fact, Li is half the mass of carbon which has been shown to have interesting signatures of heavy atom tunneling.45-47 In this work, we consider both QMT as well as zero point energy (ZPE) effects in the context in LIB for these systems using the one-dimensional Wigner correction48,49 which takes into account both the quantum effects to provide a more accurate estimate of the activation energies.

Computational Details: All the electronic structure calculations were performed using density functional theory (DFT) as implemented in Quantum Espresso package.50 The electron-ion interaction has been described by Ultrasoft pseudo potential51 while the electron exchange-correlation interactions is treated by the generalized gradient approximation (GGA) of Perdew–Burke–Ernzerhof (PBE) approach.52 The kinetic energy cutoff of 40 Ry was used for all calculations. Structural relaxations were carried out using the Broyden–Fletcher–Goldfarb–Shanno (BFGS) quasi-Newton algorithm53 until the convergence criterion of

10−5 eV for energy and 0.01 eV/Å for force is reached. VanderWaals

interactions were taken into account by the semi-emperical correction scheme of Grimme(DFT-D2)54 which is known to give accurate description of Li binding energy and diffusion barrier. A vacuum 4 ACS Paragon Plus Environment

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space about 20 Å was employed between two adjacent layers to avoid spurious interactions induced by periodic images. The Brillouin zone was represented by a Monkhorst−Pack special k-point mesh of 5 × 5 × 1 for structural relaxation. The minimum energy path (MEP) for Li diffusion was located using climbing-image nudged elastic band (CI-NEB) method.55 Eight intermediate images were generated by linear interpolation between initial and final state along the reaction coordinate and then these images were relaxed until the largest norm of the force orthogonal to the path was smaller than 0.01eV/Å. The charge transfer between Li atom and monolayer MX has been modeled by Bader charge analysis software.56

Band structures, density of states (DOS) and phonon frequency calculations were carried out by using the projected augmented wave (PAW) potentials57 based on density functional theory (DFT-D2) as implemented in the Vienna ab initio simulation package (VASP) code 58 and the exchange correlation energy is obtained by GGA-PBE approximation. The energy convergence criterion was chosen to be 500eV and the k-point mesh as 7×7×1. Imaginary frequency of the TS along the diffusion coordinate is obtained at Ґ-point only using phonon frequency calculations using DFPT (density functional perturbation theory) resulting 3N-3 real frequencies for reactant and 3N-4 real and 1 imaginary frequency for TS, which are required for Wigner corrections into the classical barrier.

Results and Discussion: The ability of an anode material to bind Li determines its capacity in LIB. A stronger binding indicates more storage of Li atom and hence higher capacity. When the binding energy of Li to the host surface is comparable to its bulk cohesive energy (1.63 eV)59, formation of lithium cluster is thermodynamically favorable. It is well known that weak lithium binding over defect free graphene causes dendritic Li growth which eventually leads to explosion hazards3,14, limiting its application in battery technology. Hence Li binding energy (Eb) to MX monolayer is an important parameter for determining its ability to perform as an anode material.



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The adsorption energy per Li atom can be described as33-

E b  E MX  nE Li  E MX  nLi  / n Where EMX+nLi is total energy of a pristine MX layer containing nLi, EMX is the energy of MX monolayer and ELi is the energy of an isolated Li atom and n is the number of Li adsorbed. To evaluate Eb we have used 3×3 monolayer of MX containing one Li atom. A larger positive value of Eb indicates stronger binding to the surface. We have considered all possible binding sites for Li adsorption and found the hollow sites namely, H1 and H2 as the most probable one where the Li atom is coordinated with three atoms. Figure 1 shows the various binding sites of Li on the hosts. In the H1 site, Li is coordinated with two M atoms and one X atom while for the H2 site, the coordination patterns are reversed. Table 1 lists the binding energies (Eb) and amount of charge transfer (QLi) at H1 and H2 site. The data in parenthesis (see Table 1) are obtained from VASP calculations with the same geometry using PAW pseudo potential. Table 1. Binding energy (Eb) in eV and amount of charge transfer (QLi) calculated at H1 and H2 site for MX systems. MX SiS SiSe GeS GeSe SnS SnSe

Site H1 Eb in eV 1.85(2.34) 1.96(2.24) 1.04(1.86) 1.02(1.93) 0.97(1.90) 1.20(2.15)

QLi 0.99 0.99 0.99 0.99 0.99 1.00

Site H2 Eb in eV 1.71(2.3) 1.94(2.26) 1.09(2.03) 1.05(1.96) 1.07(2.05) 1.24(2.21)

QLi 0.99 0.99 0.99 0.99 1.00 1.00

From Table 1 it is found that all the monolayers have encouraging Li binding energies of which SiS and SiSe are the highest. In fact, the adsorption energy of Li to SiS/SiSe is similar to that of black phosphorene.33 Table 1 shows that the amount of charge transfer (CT) ranges from 0.99 - 1.00 e in all the cases, hence the Li atom exists in cationic state after adsorption to the MX monolayer and Coulomb forces account for stabilization of Li on the MX layer. The Coulomb interactions are directly proportional to (a) amount of charge transfer between Li to MX layer and (b) amount of fractional charge present on the M-X bond which results from their difference in electronegativity and (c) the distance between coordinated atoms (dLi-M/dLi-X). Hence, a shorter bonding distance and larger polarization in the Si-X bond (X=S and Se) facilitates stronger Li adsorption compared to the other group IV chalcogenides. However due to the heavier 6 ACS Paragon Plus Environment

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mass of the latter analogs (GeS, GeSe, SnS and SnSe), the expected specific capacity should be low irrespective of high binding towards Li. Hence, we have selected the till date, unexplored SiS and SiSe monolayers for further calculations to validate their suitability as anode material in LIB. In order to explore the binding interaction of Li with SiS and SiSe monolayer it is important to know their possible Li adsorption sites. Structural asymmetry in SiS and SiSe result in six different binding positions, which can be described as (a) above the center of SiS hexagon coordinating with 2Si and 1X(H1 site), (b) above the center of SiS hexagon coordinating with 1Si and 2X (H2 site), (c) above the mid-point of Si-X bond located near to Li (B1 site), (d) above the mid-point of Si-X bond located far from Li(B2 site), (e) on the top of Si(T1 site) and (f) on the top of X(T2 site).They are shown in Figure 1. Table 2 summarizes the calculated binding energies (Eb) and other structural parameters for different adsorption sites. As discussed above, for the hollow sites (H1 and H2) Li is coordinating with three neighboring atom. However, for sites B1/B2 and T1/T2 the coordination number is two and one respectively. For both SiS and SiSe, the hollow sites are found to be most stabilizing for Li. Although for SiS, H1 is the most stable adsorption site but for SiSe both the hollow sites (H1 and H2) share similar binding energies. Hence, we can conclude that Li atom favors binding sites which provide the maximum coordination number.

Figure 1. Top view of different stable Li adsorption sites on SiX (X=S or Se). Color code for atoms: Green=Si atom, Orange=X (S, Se) and Blue=Li.



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Table 2. Binding energy (Eb) in eV and Li-Si/Li-X bond length (dLi-Si/dLi-X) in Å at different adsorption sites for SiS and SiSe. Adsorption Site H1 H2 B1 B2 T1 T2

SiS Eb in eV 1.85 1.71 1.07 1.49 0.96 1.03

dLi-Si/dLi-S in Å 2×dLi-Si=2.64,dLi-S=2.49 dLi-Si=2.65,2×dLi-S=2.59 dLi-Si, dLi-S=2.65 dLi-Si, dLi-S=2.55 2.6 2.5

Eb in eV 1.96 1.94 1.11 1.87 1.03 0.88

SiSe dLi-Si/dLi-Se in Å 2×dLi-Si=2.65,dLi-Se=2.60 dLi-Si=2.59,2×dLi-Se=2.67 dLi-Si, dLi-Se=2.60 dLi-Si, dLi-Se=2.60 2.5 2.5

A high charging/discharging rate is highly desirable for efficient performance of Li battery, which in turn is controlled by the facile Li motion over surface. We have explored different possible diffusion pathways and their corresponding barriers for Li migration over SiS and SiSe. Structural asymmetry results in two distinct diffusion paths, one along the zigzag direction (where Li moves in the same lane) and other along armchair direction (where Li hops from one lane to the other lane). For SiS, H1 being the most stable adsorption site, Li atom hops between two neighboring H1 site. The energy profiles for Li migration over SiS have been summarized in Figure 2 (a). The diffusion barriers are estimated to be 0.17eV and 0.71eV along zigzag and armchair direction respectively. Along zigzag direction, the Li atom resides at B2 site at transition state while T2 serves as the transition state structure in armchair direction. Since, the B2 site has higher binding energy compared to T2 site, the diffusion barrier height is lower along zigzag direction. Therefore, the zigzag direction is predicted to be most suitable diffusion pathway for Li over SiS.


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’

Figure 2. Energy profiles for Li diffusion along armchair and zigzag direction for (a) SiS (H1→H 1), (b)SiSe ’

’

(H1→ H 1) and (c) SiSe (H2→ H 2).(d) Schematic representation of different Li diffusion pathways in SiX for the H1 site. For SiSe, both H1 and H2 site have similar binding energies and hence, a total of four (two for each site) diffusion directions are possible. They are shown in Figure 2(b) and Figure 2 (c). The calculated hopping barriers are 0.79 eV and 0.80 eV along armchair directions for H1 and H2 site respectively. However along the zigzag direction, the barriers are estimated to be 0.16 eV and 0.12 eV for H1 and H2 site respectively. The Boltzmann population factor for H2 site relative to H1 site is found to be 0.46. Hence, 54% of the Li atom occupy H1 site and rest of them resides at H2 site at room temperature. The Boltzmann averaged diffusion barrier turns to be 0.14eV along zigzag pathway. In both SiS and SiSe, the diffusion is quasi one-dimensional along the zigzag direction and a sufficiently low barrier ensures high rate performance vis-à-vis other commercial anodes. Next we have calculated the classical and quantum diffusion rate by incorporating the zero point energy (ZPE) and quantum mechanical tunneling (QMT) effects into the potential energy surface for Li 9 ACS Paragon Plus Environment


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hopping. Lithium being significantly less heavy than the atoms on the host surface like Si/S/Se is expected to exhibit substantial quantum mechanical effects which might consequently alter its diffusion kinetics.60-62 According to the transition state theory (TST), the classical rate constant for ion diffusion within harmonic approximation for vibrational modes can be written as63, 64

k

hTST cl

k BT Q TS  E ‡ / k BT  e IS h Q

(1)

Where, h is Plank’s constant, kB is Boltzmann constant, T symbolizes temperature and Q TS and QIS represent partition functions for transition sate and initial state, respectively. In the above equation, ∆E‡ is the classical barrier height and is obtained as the energy difference between the transition-state and the ground (initial) state as available from the typical electronic structure calculations like the NEB method without ZPE corrections. The ratio of partition functions can be described as 3 N 6

 viIS QTS h   QIS k BT

i 3 N 7

 i‡

(2)

i

Here, viIS and  i‡ represent the vibrational frequencies for the IS and TS respectively. The zero point energy correction (δEZPE) to the classical barrier height can be written as

E ZPE

h i‡ h iIS   2 2 i i

(3)

Where the first term is total ZPE of TS and second term represent the total ZPE of IS. It is known that the ZPE corrected barrier (∆E+δEZPE) is more accurate only at low temperature when all the vibrational modes are at their ground state. However at the intermediate temperature range relevant for room temperature description of materials, the Wigner correction48 describes the barrier height more accurately. In this approximation, the quantum mechanical partition functions are used for vibrational modes while their classical mechanical description is kept constant. The Wigner zero point correction to the classical barrier height can be formulated as48

EWig

 i sinh ( xiIS ) xiIS   k B T ln  ‡ ‡   i sinh ( xi ) xi 

(4)


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IS ‡ IS ‡ 2k BT , a ratio of ZPE to thermal energy at each vibrational mode. The quantum Where, xi  h i

mechanical tunneling effect on barrier height has also been employed through Wigner tunneling correction49. Figure 3(a) & 3(b) shows the variation of classical barrier (∆E‡), the ZPE corrected barrier (∆E‡+δEZPE), the Wigner ZPE corrected barrier (∆E‡+δEWig) and the Wigner tunneling and ZPE corrected barrier (∆E‡+δEWig/tunn) with temperature for SiS and SiSe in the temperature range of T=50K to 350K. It is found that the ZPE correction lowers the classical barrier height by 0.02eV for SiS and 0.014eV for SiSe in the zigzag direction. The Wigner ZPE corrected barrier (∆E‡+δEWig) lies in-between the classical barrier (∆E‡) and the ZPE corrected barrier (∆E‡+δEZPE) in the intermediate temperature range. However, it approaches the ∆E‡+δEZPE value (green line) at the low temperature limit and the ∆E‡ (black line) in the high temperature limit. The temperature dependence of the Wigner ZPE corrected barrier (∆E‡+δEWig) and the Wigner tunneling and ZPE corrected barrier (∆E‡+δEWig/tunn) are almost superimposed at high temperatures indicating that QMT plays minor role at room temperatures (∆E‡+δEWig/tunn ~ ∆E‡+δEWig). At 300K, the classical barrier height is almost similar with ZPE and tunneling corrected barrier and quantum correction lowers the barriers by only 2.7meV for SiS and 1.5meV for SiSe. But, as the temperature is reduced, ∆E‡+δEWig/tunn

Two-Dimensional Group IV Monochalcogenides: Anode Materials for

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