Energy Gap-Modulated Blue Phosphorene as Flexible Anodes for

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C: Energy Conversion and Storage; Energy and Charge Transport

Energy Gap Modulated Blue Phosphorene as Flexible Anodes for Lithium and Sodium Ion Batteries Gayatree Barik, and Sourav Pal J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11512 • Publication Date (Web): 15 Jan 2019 Downloaded from http://pubs.acs.org on January 17, 2019

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The Journal of Physical Chemistry

Energy Gap Modulated Blue Phosphorene as Flexible Anodes for Lithium and Sodium Ion Batteries Gayatree Barik1 and Sourav Pal1, 2* 1Department 2Department

of Chemistry, Indian Institute of Technology Bombay, Mumbai 400 076, India

of Chemical Sciences, Indian Institute of Science Education and Research Kolkata, Mohanpur 741 246, West Bengal, India Email: [email protected]

Abstract Effective modulation of electronic and optical properties by virtue of external control might unfold extensive variety of potential applications in nanoelectronics. In the context of this, firstprinciples DFT calculations have been evaluated, to congregate additional data about the effects of strain on electronic properties of blue phosphorene and enactment of this with regard to potential use in LIBs/SIBs. The adsorption of both Li and Na over unstrained blue phosphorene is very effective with adsorption energies of -1.77 and -1.05 eV, respectively. Incredibly the application of both compressive and tensile strain causes significant strong binding of Li/Na atoms with Li adsorption energies in the range -2.03 eV and -2.21 eV, and Na adsorption energies of -1.19 to -1.49 eV for 10% compression and 10% stretch respectively. The diffusion of these two alkali atoms over unstrained blue phosphorene is anisotropic, with an energy barrier of 0.09 eV for Li, 0.035 eV for Na in the zigzag direction and 0.350 eV for Li and 0.207 eV for Na in the armchair direction. Mere changes are observed in the diffusion of Li on application of strain, whereas the Na diffusion barrier energy is extremely sensitive to applied tensile strain both in zigzag and armchair directions. The band gap of monolayer blue phosphorene is calculated by PBE and more accurate HSE06 method. The band gap of pristine phosphorene was found to be 2.24 eV based on PBE and 2.87 eV based on HSE06 method which indicates that monolayer blue phosphorene is a semiconductor. However, the band gap of phosphorene is observed to be strain tunable and has a reciprocative effect with biaxial strain. The material exhibits indirect to direct band gap semiconductor at 10% tensile strain and semiconductorto-semi-metal transition at 10% compressive strain whereas lithiation/sodiation induces semiconductor to metal transition with unstrained phosphorene. These findings suggest that applied strain is a robust and an efficient approach to make blue phosphorene a promising material for achieving structural phase transition to meet the requirement of future electronic devices.

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1.Introduction Better endurance with sustainable performance coupled with lower manufacturing cost is the only quest for contemporary electronics entity. Thus to supplement the emerging demand, rechargeable lithium-ion batteries (LIBs)1 have become more reliable for energy storage2 systems for transistors (basic building blocks of electronics). These electrical energy storage systems,3,4 especially LIBs, have revolutionized its application in portable electronic equipment’s enabling the rise of mobile phones, laptops and more recently electric vehicles.5 In any case, there exists a dilemma that the topographically obliged lithium (20 ppm) resources in the earth outside layer and the meagre availability may not suffice to supplement the increasing demand for LIBs. Due to abundance availability of sodium (23,600 ppm)6 and its economical aspect, sodium-ion batteries (SIBs)7,8 have an equivalent prospect to act as anode in the proposed cell. Thus, it has quickened the research intrigue everywhere throughout the world and has been anticipated as a standout amongst the most appropriate alternative material to LIBs/SIBs with an equivalent level of advantage.9 The fundamental characteristics of batteries are resolved predominantly by the electrochemical performance of the anode materials and subsequently, improvement in the limit of current LIBs and SIBs innovation emerges in finding of robust electrode materials,10 particularly appropriate anode materials. However, Phosphorus,11 recently has received incredible endorsement and been anticipated as one of the most reliable candidates on account of its abundant reserve in earth crust and exceptionally high theoretical capacity for LIBs/SIBs. Phosphorus crystallizes in various allotropic forms,12–15 including white, red, violet and black, which are very familiar forms and distinguished from each other based on their energy gap with inconsistent physical and chemical properties. Among all allotropes of elemental phosphorus, black phosphorene16,17 has been intensively investigated as a versatile 2D material in the first growing research since its first manufacture in 2014.18,19 It is the most stable one which has layered structure of puckered sheets and relatively high electrical conductivity and has fascinating optical and electronic properties as well. As like graphite, it exists as layered type structure and are bonded by cohesive force i. e. inter layers are held together by weak van der Waals forces.20 It has been predicted that, the puckered structure of black phosphorus (rectangular unit cell) has the potential for alteration to another 2D allotrope with high symmetric buckled structure (hexagonal unit cell) by certain disentanglement of constituent phosphorus atoms, which is termed as the blue phosphorus.21 The recently created blue phosphorus is basically a single layer of the A7 phase of phosphorus22 and this new allotrope blue phosphorus is almost thermally stable as monolayer black phosphorene.23,24 Zhu et al.24 first anticipated that structure of blue ACS Paragon Plus Environment

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phosphorus is similar to that of graphene, yet blue phosphorus layers are non-planar and the bulk layer stacking of blue phosphorus is fundamentally the same as graphite and unlike graphene it has a wide band gap. Zhang et al.25 effectively synthesized single layer blue phosphorus on an Au(111) substrate by molecular beam epitaxy method using black phosphorus as precursor. Formation of a quasi-freestanding blue phosphorus single layer was observed by interaction of phosphorus on tellurium monolayer modified by Au(111).26 Recently surface adsorption induced indirect-to-direct band gap transition in monolayer blue phosphorus was carried out by first-principles calculations combined with GW approximation and the Bethe-Salpeter equation.27 Charge mobility and electronic structures of monolayer blue phosphorus materials (PNTs and PNRs)28 have been investigated with the usage of density functional theory combined with Boltzmann transport method. The crystal structure and electronic properties of blue phosphorene grown on an Au(111) substrate was investigated by lowtemperature STM and density functional theory (DFT) calculations.29 Blue phosphorous has attracted extensive attention as a good combinator material as it combines with other 2D materials to design new van der Waals (vdW) hetero structures which opens new opportunity to improve the performance of materials in electronics and optoelectronics. BlueP/TMDs vdW heterostructures possesses significantly improved electronic and optical properties compared to 2D materials themselves and exhibit stronger optical absorbance and energy conversion efficiency.30 Moreover, the optical absorption spectra of MXene/BlueP31 vdW heterostructures can be shifted to red (blue) region by application of compressive (tensile) strain and location of VBM and CBM can also be reversed. Black phosphorene was established to be a direct band gap semiconductor, whereas the blue phosphorene is an indirect band gap semiconductor. The phosphorous atoms have a strong adsorption capacity for alkali metals than TMDs and graphene, which make them very much useful in nanoelectronics.32 Amongst the known allotropes of phosphorous, tremendous work has been devoted towards investigating the utilization of black phosphorous as a potential anode for LIBs/SIBs both theoretically33,34 and experimentally.35 Black phosphorus involves three electron intercalation reaction and uses three Li/Na atoms to produce Li3P and Na3P,36 respectively, which corresponds to a large theoretical capacity of 2596 mAhg−1 and theoretical volume expansion ΔV/V of 216% for Li3P and 391% for Na3P.33 However, recently blue phosphorous has pulled a lot of interest and can have potential application for alkali metal ion batteries. So far, Li et al.37 recommends both monolayer blue and black phosphorene, as a promising Li ion battery anode. Additionally, monolayer blue ACS Paragon Plus Environment

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phosphorene could adsorb Na and K atoms with high binding energy resulting in high storage capacity and Na and K ions diffuses quickly over blue phosphorene with modest energy barriers.38 Blue phosphorene effectively improves the rate performance of battery when combined with other 2D materials to give stacked 2D heterostructures. For example, Peng et al.39 have proposed that blue-phosphorus /NbS2 vdW hetero structures, can provide considerably higher capacity than black phosphorene-graphene heterostructure as anode material in LIBs. Graphene/blue-phosphorus heterostructure have been investigated as an electrode material in LIBs and the rate performance of the graphene/blue-phosphorus heterostructure is better than that of monolayer graphene.40 Li et al.41 suggested that, blue-phosphorus/borophene heterostructure possesses excellent structural stability and high mechanical stiffness to be use as an potential anode material in LIBs. Recently, h-BN/blue phosphorene heterostructure has been anticipated as potential anode material for both LIBs and SIBs.41 An essential prerequisite for materials to use in nano-electronic devices is the ability to regulate the physical properties by means of external control. Applications of either an external electric field or a tensile strain are two extensively used engineering strategies to achieve tunable band gaps for 2D materials.42–44 It has been recently predicted that through compressive strain, the electronic band structure of black phosphorene can be changed to semiconductor to metal.45 Peng et al.46 has suggested that in-plane axial deformations can strongly modulate band gap and carrier effective mass of monolayer black phosphorous. Band gap tunability of semiconductors essentially influences degree of flexibility in both LIBs and SIBs. Applied strain not just regulate the electronic band structure of materials, yet additionally influences the alkali metal ion storage. For example applied tensile strain affects lithium and sodium storage behaviours of MoS2 with exceptional electrochemical performance.47 Therefore, a comprehensive methodical study is required to establish the effect of different tensile and compressive strain with varying mechanical properties, and other aspects towards production of monolayer blue phosphorene for use as an anode material in Li/Na-ion batteries. In the accompanying article, we have systematically investigated the electronic phase modulations of monolayer blue phosphorene under tensile and compressive strain by first-principles DFT calculations and applied strain yields a pronounced variation of electronic properties as well as greatly enhances its suitability to use in LIBs and SIBs. 2. Computational Details


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Plane-wave-based DFT simulations were conducted in the framework of density functional theory (DFT) as implemented in the Quantum ESPRESSO48 package for the self-consistent total energy calculations and geometry relaxation. The electron exchange-correlation was expressed using the

generalized

gradient

approximation

(GGA)49

with

Perdew-Burke-Ernzerhof

(PBE)

parametrization scheme along with the projector-augmented50 wave potentials. Besides PBE, the hybrid functional of Heyd–Scuseria–Ernzerhof 06 (HSE06) method was also adopted to correct the shortcoming of the PBE functional and to investigate the strain effect on the electronic properties of monolayer blue phosphorene.51 All atomic positions and lattice vectors were fully relaxed to obtain the optimized configuration by means of the Broyden–Fletcher–Goldfarb–Shanno (BFGS)52 method. The van der Waals interaction between the layers is considered using vdW-DF53 approach. To estimate the charge transfer between Li/Na and phosphorene, we adopted the Bader54 charge analysis method. The kinetic energy cut off for the plane wave basis set was chosen to be 435 eV. The reciprocal space was meshed at 4 × 4 × 1 using the Monkhorst-Pack55 method. A 3×3 super cell with periodic boundary condition was used to simulate a monolayer of phosphorene. A vacuum space of 16 Å was applied to minimize the interaction between layers. After structural optimization, the density of states (DOS) and electronic band structures were calculated using denser 8×8×1 k point. The diffusion barriers and MEPs are obtained by using the standard CI nudged elastic band method (NEB).56 3.1. Structural geometry of monolayer blue phosphorene. Blue phosphorene is essentially a single layer of black phosphorous by owing to its structural bonding and has a superior symmetry than black phosphorus. It possesses the D3d point group symmetry and exhibits a dual-partite honeycomb lattice structure with two different sublattices. The two phosphorus sub-lattices lie on two different atomic planes displaced along the perpendicular z-direction leads to a buckling structure. The smaller hexagonal Wigner-Seitz cell of blue phosphorus, which contains two atoms (Figure 1a), differs from the rectangular Wigner-Seitz cell of the anisotropic black phosphorus monolayer with 4 atoms per unit cell. The fully relaxed structure of a single layer blue phosphorus has an equilibrium lattice constant of 3.28 Å. The equilibrium bond length between P atoms is 2.27 Å, and the top and bottom P atoms are separated by 1.25 Å, which are consistent with previous results.38,39 The optimized geometry of monolayer blue phosphorous from top view (Figure 1b) and side view (Figure 1c) are shown in Figure 1. We apply strain by changing the in-plane lattice constant (a and b) of 2D buckled honeycomb crystal structure. In this work, two types of strains are considered: compression and stretch in the both x and y direction. The biaxial tensile strain is defined as 𝜀 = ACS Paragon Plus Environment

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𝑎′ ― 𝑎 𝑎

and biaxial


compressive strain ε is defined as 𝜀 =

𝑎 ― 𝑎′ 𝑎

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where a is the equilibrium in-plane lattice parameters and

a′ is the corresponding strained in plane lattice parameters.

Figure 1: (a) Unit cell of blue phosphorene (b) a supercell of blue phosphorene from top view (c) a supercell of blue phosphorene from side view.

It has been presumed that buckled configuration can always sustain larger tensile strain than planner one. Regardless of the certainty that a low-dimensional material predominantly sustain a higher amount of strain without fracture in comparison to bulk material, we have replicated the strain only in the range of –10% and 10%, estimating the values towards experimental realisation.57 The compressive strain is shown by a negative value while positive value of strain corresponds to the expansion. When biaxial tensile strain is applied the lattice constants are longer than that of the equilibrium state and for compressive strain the lattice constants are shorter than the equilibrium state. All atomic coordinates are fully optimized. With the increase of tensile strain, the P-P bond length (dPP) and PPP bond angle (θPPP) increases progressively, while with compressive strain, the P-P bond length and PPP bond angle decrease rapidly. The variation of P-P distance and PPP bond angle (θPPP) as a function of tensile and compressive strain (ε) are given in Table S1. Fundamental studies on the strain-induced variation of mechanical and electronic properties of monolayer blue phosphorene are pertinent to gain insight on how blue phosphorene can be productive for application as anode material in LIBs and SIBs. In this work, we explore adsorption and diffusion of Li and Na on monolayer blue phosphorene with electronic band structure of pristine and strained phosphorene by first principle DFT calculations. 3.2. Adsorption of Li and Na over blue phosphorene. With an objective to select a material to use as an electrode in alkali metal ion batteries, a prerequisite is that the material needs to adsorb alkali metal ions with relatively high adsorption energy. Consequently, to systematically study the adsorption properties of Li and Na ions on the blue phosphorene monolayer, the possible adsorption ACS Paragon Plus Environment

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sites for Li and Na ions on the monolayer blue phosphorene surface were examined. We have explored the blue phosphorene surface for Li/Na binding, by randomly placing one alkali atom at different positions above phosphorene and carried out full structural optimization by minimizing the total ground state energy. We found three most stable adsorption sites for a Li/Na atom adsorption on phosphorene which are shown in Figure 2. The three adsorption sites are namely: (a) above the centre of the phosphorene hexagon (H site) (Figure 2a), (b) above the top of the phosphorene atom (Top site), when the three nearest neighbouring “P” atoms are close to the adsorbed atom (Figure 2b), (c) below the top of the phosphorene atom (B site), where the three nearest neighbouring “p” atoms are away from the adsorbed atom(Figure 2c) .

Figure 2: Top view and side view of Li/Na atom adsorbed at different positions. (a) Hollow position, (b) Top position, (c) Below position. Blue colour represents phosphorous atoms and red colour represents Li/Na atom.

To find out Li/Na adsorption energy, a single Li/Na adatom was placed over most stable adsorption site of phosphorene and the system was relaxed. The adsorption energy ( Ea ) is calculated as

𝐸𝑎 = 𝐸Li Na + 𝑃 ― 𝐸𝑃 ― 𝐸Li Na Where 𝐸Li Na + 𝑃 and 𝐸𝑝 are total energies of Li/Na-adsorbed and pristine phosphorene, respectively, while 𝐸Li Na is the total energy of an isolated lithium or sodium atom. The adsorption energy at the three most stable adsorption site are listed in Table 1. According to this definition, negative adsorption energy corresponds to a convenient interaction and an exothermic reaction between Li/Na with pristine phosphorene. The most favourable configuration is chosen as the configuration with the largest adsorption energy i.e. minimum total energy. By comparing all


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adsorption energies at different sites, for both Li and Na adatoms, the top-site is found to be the most favoured site with a high binding energy of −1.77 eV for Li and −1.05 eV for Na with Li-P and NaP bond length was found to be 3.14 and 3.57 Å respectively. High adsorption energy and small bond length show a strong binding interaction between blue phosphorene and Li/Na and a rapid loading process. The adsorption energy at other stable adsorption sites are shown in Table 1. This binding interaction between blue phosphorene and Li/Na atom is again confirmed by charge transfer. An intercalation reaction is always driven by charge transfer from the intercalant to the host layered compound. In order to quantitatively calculate the amount of charge transfer between Li/Na and blue phosphorene, Bader charge analysis has been performed. After Li/Na adsorbed on blue phosphorene, at top position, the charges on the Li atom decrease from 3.0|e| to 2.109|e|, while the charges on the Na atoms decrease from 11.0 |e| to 10.195 |e| (Table 1). The charge transfer of about 0.891 |e| in case of Li and about 0.805 |e| in case of Na suggests a significant electron transfer from Li/Na atom to nearest phosphorene atoms, indicating an efficient binding interaction between the Li/Na atoms on blue phosphorene surface. Table 1 summarizes the calculated adsorption energies, bond lengths of all adsorption sites and quantity of charge transfer at different sites. Table 1: Single Li/Na atom insertion: Adsorption energies ( Ea ), minimum Li/Na–P distances (d), Bader charges on Li/Na atoms (Q Li/Na)

Adsorption Ea (Li) in eV Ea (Na) in eV

dLi-P (Å)

dNa-P (Å)

Q Li (|e|)

Q Na (|e|)

site H

-1.71

-1.03

2.65

2.96

0.883

0.791

T

-1.77

-1.05

3.14

3.57

0.891

0.805

B

-1.37

-0.79

2.41

2.76

0.881

0.732

The character of binding between Li/Na and blue phosphorene can be again confirmed by plotting the charge-density difference iso-surfaces. 𝛥𝜌 is defined as: 𝛥𝜌 = 𝜌𝑃 + Li Na ― 𝜌𝑃 ― 𝜌Li Na Where 𝜌𝑃 + Li Na, 𝜌𝑃 and 𝜌Li Na are total charge densities of the adatom phosphorene system, pristine phosphorene and isolated Li/Na, respectively. The large charge deficiency at Li/Na and the excess charge accumulation around nearby phosphorous, is shown by the spatial distribution of the charge ACS Paragon Plus Environment

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difference between Li/Na and phosphorene as illustrated in Figure 3. Figure 3a represents Li–P bonding and Figure 3b represents Na–P bonding. After addition of Li/Na, the charge is mainly transferred from

the alkali atom to blue phosphorene.

Figure 3: Top and side view of charge density difference plots for adsorption at the most stable sites for (a) Li–P bonding (charge transfer from Li to P) (b) Na–P bonding (charge transfer from Na to P). Green and red regions refer to electron depletion and accumulation, respectively.

In order to study the effect of strain on the adsorption energy on single-layer blue phosphorene, we have applied strain up to −10% to 10% strain along both x and y direction for both pristine and lithiated/sodiated blue phosphorene and study its effects on Li/Na adsorption energy. We explored the geometry relationship between the bond lengths and bond angles after Li/Na adsorption. We found that, application of both tensile and compressive strain affects greatly to both Li and Na adsorption energy and Li/Na-P bond length. At 10% tensile strain, the Li adsorption energy increases from -1.77 eV to -2.21 eV, and Na adsorption energy increases from -1.05 eV to -1.49 eV (Figure 4a). The increase in adsorption energy is accompanied by decrease in Li/Na-P bond length. At 10 % tensile strain Li-P bond length decreases from 3.14 Å to 2.97 Å and Na-P bond length decreases from 3.57 Å to 3.42 Å (Table S1). Similar effect has also been observed for compressive strain. Li adsorption energy increases from -1.77 eV to (without strain) to -2.03 eV (with 10% compressive strain) for Li, and from -1.05 eV (without strain) to -1.19 eV (with 10% compressive strain) (Figure 4b). At 10% compressive strain, Li-P bond length is 3.46 Å and Na-P bond length is 3.83 Å (Table S2). The adsorption energy is affected by the application of both compressive and tensile strain. We found a ACS Paragon Plus Environment

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linear increase in adsorption energy case of tensile strain whereas at compressive strain, the adsorption energy for both Li and Na first decreases and then increases and we find lowest adsorption energy at 4% compressive strain. However, the Li/Na-P bond length increases linearly. At 10% compressive strain Li-P bond length increases from 3.14 Å to 3.46 Å and Na-P bond length increases from 3.57 Å to 3.83 Å. In this case increase in bond length is not accompanied by increase of adsorption energy. However, the distance of Li/Na with the three nearest neighbouring P atoms decreases as adsorption energy increases with increasing applied compressive strain. This is because when the in-plane lattice is compressed, the ‘P’ atoms come close towards the adsorbed atom.

Figure 4: The adsorption energy of Li (blue) and Na (red) verses different magnitudes of strain (a) Tensile strain (b) Compressive strain.

Therefore, an applied strain provides an effective approach to enhance Li/Na adsorption with high tunability and is crucial for their applications in LIBs/SIBs. We find that stretch has more effect than compression strain towards adsorption of Li/Na atoms on monolayer blue phosphorene. By the application of both applied compressive and tensile strain, the distances between the atoms changes and their atomic orbitals are perturbed slightly which leads to a shift of the energy states towards fermi level. The DOS plots indicate that these states are basically originate from the ‘p’ orbitals of phosphorous atoms. An enhanced degree of interaction level is observed between the shifted ‘p’ orbital of phosphorous and the ‘s’ orbital of adsorbed Li/Na atoms which results increased adsorption. The stretch has more effect than compression strain, because when the lattice constant is stretched, the PPP bond angle (θP-P−P) increases and the surface of blue phosphorene provides larger space towards adsorbed atoms and the adsorbed atoms feel less repulsion from the three neighbouring phosphorous atoms, which gives additional stability over compression. The adsorption of alkali atoms over 2D materials is only possible because of charge transfer from adsorbed atoms to the substrates. But in the current study, strain enhances adsorption and the ACS Paragon Plus Environment

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enhanced adsorption due to strain is associated with the charge transfer from Li/Na to blue phosphorene. From Bader charge analyses results, it has been observed that for unstrained blue phosphorene 0.891 |e| charge is transferred from Li to blue phosphorene. But it increases to 0.894 |e| and 0.896 |e| for 10% tensile and 10% compressive strain respectively. Similarly, for Na adsorption, it increases from 0.805 |e| to 0.810 |e| and 0.815 |e| for 10% tensile and 10% compressive strain respectively. The enhanced charge transfer due to strain makes Li (Na) increasingly 'cationic' that ultimately increases the ionic character between Li/Na and blue phosphorene. 3.3 Li/Na diffusion and Energy profiles along the MEPs. The rate performance of batteries mostly depends on mobility of intercalated ions. To quantify this, it is essential to investigate the effect of strain on the diffusion of Li/Na on blue phosphorene surface. To have a better understanding about the minimum energy path (MEP) collectively with the diffusion barriers, we calculated Li/Na diffusion on phosphorene surface without strain, with 10% tensile strain and with 8% compressive strain. It is customary that the rate performance of the electrode material is determined by the electrical conductivity of lithium/sodium diffusion barriers. It has been found that, structural anisotropy plays a significant role in the migration of Li on monolayer black phosphorous surface.33 So it is mandatory to find out whether alkali metal ion migration on monolayer blue phosphorous is isotropic or anisotropic. While examining the symmetry of the primitive cell, there exists only two high-symmetry paths connecting two adjacent binding sites which are shown in Figure S1. Thus, for surface diffusion of one Li/Na on the monolayer blue phosphorous, we took in to account two different diffusion paths, that are considered for a single Li/Na migration. These two diffusive pathways are along the zigzag direction (Figure S1a) and the armchair direction (Figure S1b). So, the diffusion of Li/Na adatom on blue phosphorus was examined in our calculations are calculated in both zigzag (Path I) and armchair directions (Path II). In the zigzag (Path I (T-H-T)) direction, when the diffusion of Li on blue phosphorene monolayer occurs by migrating from a top site to the adjacent one, by passing through an H site (Figure 5(a)), it only necessitates to overcome a small energy barrier of 0.09 eV. Similarly, in the zigzag direction, migration of Na on monolayer phosphorene only needs to overcome a negligible energy barrier of 0.035 eV, which is significantly lower than the diffusion barrier of the Li on MoS2 or graphene. To correlate, the Li diffusion barrier on MoS2 monolayer is 0.25 eV and the energy barrier for Li diffusion in graphene surface, the most commercialised anode material is 0.22 eV,58,59 which are much larger than that along the zigzag direction on phosphorene. The low diffusion barrier signifies that blue phosphorene can be an outstanding anode material in comparison to MoS2 and graphene for


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both LIBs and SIBs. The minimum energy pathway is characterized by the two symmetrical maxima and one local minimum as shown in Figure 5a. Positions above the top sites represent the absolute energy minima, and the position at the hollow site represents the secondary adsorption minima. Therefore, an optimal path of lithium diffusion in the zigzag direction is from a top site to the adjacent top site passing through a hollow site. An additional path is the armchair (Path II (T-B-H-T)) direction in which Li/Na migrates from one top site to another through one B site and one hollow site. When Li/Na moves from one top site to the adjoining site along the armchair direction, it overcomes an energy barrier of 0.350 eV and 0.207 eV for Li and Na diffusion respectively. It is inferred that Na has a much smaller diffusion barrier than Li, suggesting much faster diffusion compared to Li on the surface of blue phosphorene and energy required for diffusion along the armchair path is relatively higher for both the metal ions. Thus Li/Na diffusion on blue phosphorene shows a strong directional anisotropy due to this buckled structure like black phosphorous.33 We have also studied the effect of both compressive and tensile strain on the energies of diffusion. When tensile strain is applied, in the zigzag direction, diffusion of Li involves an energy barrier of 0.071 eV and diffusion of Na involves an energy barrier of to 0.016 eV (Table S3) and in armchair direction, Li diffusion energy is 0.417 eV and Na diffusion energy are 0.267 eV (Figure 5b). Similarly, by compressive strain, in the zigzag direction, Li diffusion energy become 0.088 eV and Na diffusion is become 0.037 eV and in armchair direction, Li diffusion energy is 0.345 eV and Na diffusion energy is 0.145 eV (Figure 5c). Both Li and Na possesses very similar diffusion barrier energies as like pristine blue phosphorene but the migration path is affected by both compressive and tensile strain. The secondary adsorption minima at hollow position moves down when tensile strain is applied and it moves up when compressive strain is applied for both Li and Na migration on monolayer blue phosphorene. From the above discussions it has been observed that application of both compressive and tensile strain does not affect diffusion barrier energy so much (except Na diffusion at 10% tensile strain). This is because energy barrier is related to the difference between the height of maximum/minimum and the height of maximum depends on atomic arrangement of atoms. For example, in the zigzag direction, when Li/Na migrates from one top position to another through hollow position, we get maxima in between a top and hollow position. In this position, adsorption energy is less than top and hollow position because of repulson with the nearest phosphorous atom. Similarly, we found minima at top and hollow positions i.e. at the most stable adsorption sites. So,


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either it is stretched or compressed, the atomic arrangement remains unchanged. The migration path remains the same i.e. Top-Hollow-Top for zigzag direction. So, the applications of both compressive and tensile strain do not change the arrangement of atoms on the migration path in both the zigzag and armchair direction for which the barrier energy remains unaffected. However, the diffusion barrier energy of Na is very much sensitive to applied tensile strain. In case of 10% tensile strain the Na migration barrier energy was found to be negative for both in zigzag and armchair direction (Figure 5b). As discussed in section 3.1, the most stable adsorption site for Li/Na in monolayer blue phosphorene is the top site. But in case of 10% tensile strain, the adsorption energy of Na is more stable at hollow position with an adsorption energy of -1.56 eV which is at a distance of 0.5 unit in the zigzag direction (adsorption energy at top position -1.49 eV). But at this position Li adsorption energy is -2.19 eV (adsorption energy at top position -2.21 eV). For this reason, we find negative energy barrier energy for Na atom which indicates trapping of Na atoms in the hollow position which is a barrier less reactions. Similar things happened in armchair direction at a distance around 0.7 unit i.e. at hollow position. So, to confirm these observations, we calculated the barrier energy of Na migration from one hollow position to another through P-P bond and we find a positive barrier energy.


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Figure 5. Diffusion of Li (blue) and Na (red) over pristine and strained blue phosphorene along the zigzag direction(left) and armchair direction (right). Energy barrier and diffusion path of a Li/Na atom on the blue phosphorene monolayer (a) without strain, (b) with 10% tensile strain and (c) with 8% compressive strain.

3.4 Effect of strain on the band structure and band gap of blue phosphorene. In semiconductor industry, the electronic band gap is a critical quantity which determines its applications in electronic and optoelectronics and “strain engineering” is an effective approach to modulate the physical properties of electronic materials.60 Based on the first principle DFT calculations, a single layer of blue phosphorus is predicted to be as stable as monolayer black phosphorus.21 Interestingly, electronic band structure of the two allotropes differ significantly. Unlike monolayer black phosphorus, which has an intrinsic direct band gap of ∼1 eV at the Gamma point, monolayer blue phosphorus has an indirect band gap of ∼2 eV, based on GGA-PBE calculations which are conventional to undermine the band gap.21 The band gap of black phosphorus depends on the number of layers,61 strain45 and applied electric field (perpendicular to the layers). Such electronic band structure engineering in various 2D materials is indispensable.


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Figure 6: Variation of Band gap with strain (a) Tensile strain (b) Compressive strain obtained from DFT calculation within PBE and HSE06 functional.

Strain engineering is an intense and generally acknowledged tool to support the performance of materials utilized as a part of electronic, optoelectronic and spintronic devices.62–64 Hence, developing effective routes to apply tunable strain in 2D layered material such as monolayer blue phosphorene is highly essential for increasing their effectiveness for application in various nanoelectronics and nanophotonic devices. Here we investigate the electronic properties of monolayer blue phosphorene under different strain including both compressive and tensile strain. The GGA-PBE calculated band gap is 2.2 eV, reconciles with previous calculations24,25 and band gap based on HSE06 functional is 2.87 eV. Our calculation shows that band gap decreases by the application of both tensile (Figure 6a) and compressive strain (figure 6b). When tensile strain is applied, both the top of the valence band near the Gamma point and the bottom of the conduction band between the K and G points drop substantially. With 6% tensile strain the band gap reduced to 1.81 eV based on PBE (2.40 with HSE06) and the VBM lies between M and K point whereas the CBM moves towards Gamma point (Figure 7a). When tensile strain increases to 8% band gap further reduced to 1.70 eV with PBE (2.19 eV HSE 06) and the VBM comes close to M point and CBM at Gamma point but CBM comes closer towards the fermi level and it still remains an indirect band gap semiconductor. At 10% tensile strain, the CBM and VBM of phosphorene system are invariably located at the Gamma point, indicating its direct band gap character and a direct transition occurs at Gamma point having band gap of 1.46 eV based on PBE and 2.10 eV based on HSE06 functional. The calculated electronic bandgaps of the semiconducting phosphorene as function of the biaxial tensile strain are shown in Figure 7a.


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Figure 7: Energy band structures of mono-layer blue phosphorene with different amount of strain (a) applied tensile strain (b) compressive strain obtained from DFT calculation within PBE functional. The Fermi level is set at 0 eV.

When applied compressive strain increases, both CBM and VBM start moving towards the fermi Level (Figure 7b) and the band gap decreases from 2.24 eV (2.87 eV, HSE06) to 1.27 eV (1.84 eV, HSE06) at 6% compressive strain with the VBM and CBM of blue phosphorene are invariably located at the Gamma point, indicating its direct band gap character. With further increasing the applied compressive strain, the top of the valence band at the Gamma point rises again and ultimately crosses the fermi level at 8% compressive strain with a band gap of 0.78 eV (1.32 eV HSE06) and eventually the blue phosphorene monolayer turns into a semi-metal when the biaxial strain reaches 10% with VBM and CBM crosses each other at the Gamma point. The band gap drops substantially with increasing applied compressive strain in comparison to tensile strain. When the in-plane lattice is stretched, it leads to shifting of both VBM and CBM towards Gamma point and results in an indirect to direct band gap transition whereas when it is compressed, the VBM and CBM touch each other at fermi level near Gamma point and leads to semimetallic transition. But in case of TMDs, bandgap decreases as both the tensile strain and compressive strain increases, accompanying a shift of valence band maximum from K to Γ point and resulting in a direct to indirect band gap transition.42,62,65 Electronic properties of Lithiated /Sodiated Blue Phosphorene ACS Paragon Plus Environment

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To study further insight into the interactions between Li/Na and blue phosphorene, the PDOS and band structures for both pristine and lithiated/sodiated species were calculated. We found that unstrained monolayer blue phosphorene is an indirect band gap semiconductor with band gap of 2.2 eV calculated by GGA-PBE method and 2.87 eV calculated by HSE06 method (Table S1). We performed general band structure calculation for unstrained blue phosphorene. The blue phosphorene sheet was found to be an indirect band gap semiconductor with the valence band maximum (VBM) located in between Gamma and K point and the conduction band minimum (CBM) located in between the Gamma and M points. The electronic band structure and DOS plots for pristine monolayer blue phosphorene unit cell is shown in Figure 8a. Besides strain, both Li/Na adsorption, also significantly affects the electronic structures. When alkali metal ions are inserted in to the parent crystalline structure electrons are added to its band and lithium/sodium ions are fully oxidized donating electrons to the unoccupied levels of the band of the host compound. A simplified view of the intercalation phenomena that of the parent host compound with the fermi level moves up to accommodate the extra electrons.66,67 Intercalation of Li/Na to phosphorene leads up shifting of fermi level in to the bottom of conduction band region, resulting in the n-type semiconductor. The high PDOS peaks across the fermi level reveals that both lithiated (Figure 8b) and sodiated (Figure 8c) phosphorene exhibits intrinsic metallicity and high density of carriers, indicating good electronic conductivity which is required for a Li/Na-ion battery.68 Its metallic character mainly originates from contribution of phosphorous 3p orbitals. Moreover, the analysis of PDOS below the fermi level depicted that, the ‘2s’ state of Li/Na overlap with ‘3p’ state of blue phosphorene, indicating strong hybridization between blue phosphorene and Li/Na atoms. It may implicate that the 2s electron of Li/Na has been transferred to phosphorene, which was confirmed by above Bader analysis.


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Figure 8: Electronic Band structure (left) and PDOS (right) of monolayer blue phosphorene obtained from DFT calculation within PBE functional (a) Pristine blue phosphorene unit cell (b) Lithiated blue phosphorene and (c) Sodiated blue phosphorene.

3.5 Theoretical capacity and Theoretical OCV The specific capacity of anode materials depends on the quantity of adsorbed molecules and has been explored by increasing concentration of adsorbed particles. The average theoretical capacity is estimated using the given equation.69

𝐶=

nzF 𝑀

× 1000

Where z represents the valence number (z = 1 for both Li/Na), F is the Faraday constant (26.810 A h/mol), and M is the molecular mass of the blue phosphorene and n represents the maximum number of Li/Na atoms adsorbed per formula unit.


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The highest Li/Na storage capacity is achieved at maximum adsorption concentration when both sides of blue phosphorene are lithiated (Figure S2a) or sodiated (Figure S2b) with a stoichiometry of LiP and NaP, (i.e. one Li per phosphorous atom) which leads a theoretical storage capacity of 865 mAh/g well agreed with previous results38 and which is much higher than other typical anode materials.VS2 monolayer can also store Li atoms up to Li2VS2, but VS2 monolayer has a theoretical capacity of 466 mAh/g. mAh/g71 and 288 mAh/g

72

70

Correspondingly the theoretical capacity of graphene is 744

for Ti2C. Our calculated theoretical capacity for a monolayer blue

phosphorene is same as like monolayer black phosphorene, which was found to be 433 mAh/g when Li atoms are adsorbed at one side and 865 mAh/g for the both-side adsorption.73 To have a better appreciation, the electrochemical properties of blue phosphorene monolayer as an anode material, the theoretical OCV for monolayer blue phosphorene has been calculated, which could reflect the performance of the electrode material in LIBs and SIBs. The open circuit voltage can be calculated by calculating the normal voltage at different stages.74,75 The OCV is calculated by the following formulae using their formation energy.73

OCV 

 E form zF

Where F is the Faraday constant, z represents the valence number (z = 1 for both Li/Na), and E form is the formation energy, calculated as

E form 

EP Li / Na  EP  nELi / Na n

where 𝐸Li Na is the energy of a Li/Na atom in bulk body-centred cubic (bcc) structure, 𝐸𝑃 + Li Na is the energy of lithiated or sodiated blue phosphorene, 𝐸𝑃 is the energy of pristine blue phosphorene and n is the number of Li/Na atoms adsorbed. As the concentration of Li/Na atoms increased, weaker binding is observed due to the pronounced repulsive interactions between adjacent Li/Na atoms. To study the OCV, different compositions of LixP and NaxP were constructed with x taking variable values i.e. 0.08, 0.12, 0.22, 0.50 and 1. Corresponding OCV value for Li gradually decreases as demonstrated in Figure 9 and the values are observed to be 0.41, 0.32, 0.23, 0.19, and 0.16 V. Similar phenomena is also observed in the case of Na with values 0.63, 0.56, 0.53, 0.41 and 0.32. The OCV value of 0.16 eV corresponds to full lithiation and OCV value of 0.32 V corresponds to full sodiation. The above values indicate that the change of Li/Na intercalation voltage is steady and occurs in the favourable range. In addition, OCV value in the range of 0.1−1.0 V prevents cluster formation of alkali metals during the discharge/charge process.76 However, magnification of uncontrollable ACS Paragon Plus Environment

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lithium dendrite growth instigates poor cycling efficiency and has appealing safety concerns as well, dragging lithium metal batteries out of practical suitability.

Figure 9: The calculated voltage profile with respect to Li/Na. For phosphorene to be accommodated for anodes in LIBs, the formation of Li clusters should be avoided, because it will soberly reduce the charge/discharge capacity. When phosphorene is fully lithiated/sodiated, the minimal distance between Li atoms is 3.06 Å and between Na atoms 3.31 Å. These values are larger than the Li–Li bond length in Li2 dimers34 and Na-Na distance in Na2 dimer.77 Besides this, the binding energies of Li atoms on phosphorene are also larger than the formation energy of Li in Li2 dimers (-0.70 eV)34 and binding energy of Na on phosphorene is also larger than the formation energy of Na in Na2 dimers (-0.76 eV).77 Therefore, the formation of Li/Na clusters is not likely to happen on phosphorene. This has got a significant difference in comparisons with Na– graphene and Li–graphene, where in clustering of alkali atoms has been observed to pose a hinderance with respect to anode performance.78 Another critical factor to consider for anodes in LIBs/SIBs is the conceivable volume expansion caused when phosphorene is fully lithiated or sodiated. An elaborate change in volume usually tends to structural distortion of electrodes and yields perennial capacity fading. To evaluate this effect, we have optimized the lattice constant when blue phosphorene is fully lithiated/sodiated. Our results indicate that the lattice constant enlarges to 3.34 Å after lithiation and to 3.44 Å after sodiation with an increase of 1.83% and 4.98% for lithiation and sodiation respectively, which indicate that the structure of phosphorene is not affected by either lithiation or sodiation and phosphorene is robust against Li/Na insertion and does not suffer from structural changes which is a pre-requisite for practical applications.


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Finally, we compare our results with monolayer black phosphorous. Two-dimensional (2D) nanosheets have pulled in incredible consideration in LIBs and SIBs because of their high surface-volume proportion which enables them for storage of large number of foreign atoms and additionally the quick metal-ion diffusion along their surfaces. By comparing blue phosphorene with black phosphorene, the thickness of monolayer blue phosphorene is 1.25 Å (this work) whereas thickness of monolayer black phosphorene is 2.25 Å,79 which indicates that monolayer blue phosphorous can provide better surface-volume ratio in designing new types of anodes in LIBs and SIBs. Although the diffusion energy barrier energy for monolayer black and blue surface are nearly same in the zigzag direction, but in the armchair direction, the Li diffusion barrier is 0.35 eV for blue phosphorous (this work) and 0.68 eV33 for black phosphorous. Similarly, the Na diffusion barrier is 0.20 eV (this work) for blue phosphorous where as it is 0.76 eV80 for black phosphorous. The slightly flatter buckled configuration of monolayer blue phosphorene makes easier for diffusion of alkali metal ions in both the directions, in contrast to monolayer black phosphorene having puckered configuration, which restricts it to diffuse in one direction only. So, in this regard, the buckled honeycomb structured blue phosphorene is expected to be more advantageous over monolayer black phosphorene. As strain is the most immediate approach to change the atomistic and electronic structures of black phosphorene45 also, it is expected that our exhaustive understanding will give supportive guidelines for enhancing adsorption of Li/Na ions on monolayer black phosphorene in designing new types of black phosphorus anodes. Conclusion In the present study, the structural geometry, electronic property and Li/Na-ion storage and migration ability of the monolayer blue phosphorene have been examined widely based on firstprinciples DFT calculations. Monolayer blue phosphorene provides substantial adsorption sites with strong adsorption energy for both Li/Na ions and exhibits sensitive response towards applied strain, leading to both pristine and strained monolayer blue phosphorene to have high theoretical capacity. NEB simulation results show that, structural anisotropy plays a crucial role in the diffusion of both the metal ions over blue phosphorene and diffusion in the zigzag direction is faster than that of armchair direction. Overall, the energy barriers corresponding with the diffusion of Na are smaller than those for Li which implies that Na would diffuse faster than Li on monolayer blue phosphorous executing high rate capacity. The Li diffusion barrier energy is poorly affected by applied strain and the Na diffusion barrier energy is significantly affected by applied tensile strain. Electronic band structure calculations demonstrate that the bandgap decreases by applying both tensile and compressive strain, leading to a shift of valence band maximum from M to Γ point and resulting in an indirect-to-direct bandgap transition with 10% tensile strain and finally at a compressive strain of ACS Paragon Plus Environment

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10%, it becomes semi-metallic with VBM and CBM touch each other at the Gamma point. Our results indicate that both tensile and compressive strain enormously expand the adsorption energy and narrow the semiconducting gap of blue phosphorene, leading to increased stability with enhanced electrical conductivity. The PDOS plots across the fermi level indicate that both lithiated and sodiated phosphorene exhibits intrinsic metallicity and metallicity arises from phosphorous p orbitals. The high structural stability, excellent mechanical property and the strain tunable structural phase transitions in 2D monolayer blue phosphorene can give a feasible way to enhance the performance of nanodevices. Supporting Information Li/Na diffusion path on monolayer blue phosphorene from one top position to another through hollow site in both the direction. Top and side view of maximum Li/Na adsorption configuration. Calculated structural parameters by the application of tensile and compressive strain including adsorption energy, band gaps, bond lengths, bond angles and diffusion energy barriers are provided. Author Information Corresponding Author * Email: [email protected] Present address: 2Department of Chemical Sciences, Indian Institute of Science Education and Research Kolkata, Mohanpur Nadia 741 246, West Bengal, India (SP). Notes The authors declare no competing financial interest. Acknowledgements S.P. acknowledges the J.C. Bose Grant received from Dept. Of Science and Technology, Govt. Of India, New Delhi, India. G.B. acknowledges the support from Council of Scientific and Industrial Research, New Delhi, India. The authors also thankful to Institute High Performance Computing facilities of IIT Bombay, Mumbai, India. References: (1)

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