BP van der Waals Heterostructures as Promising Anode

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C: Energy Conversion and Storage; Energy and Charge Transport

GeSe/BP van der Waals Heterostructures as Promising Anode Materials for Potassium-ion Batteries Cheng He, Jiahui Zhang, Wenxue Zhang, and Tongtong Li J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b08909 • Publication Date (Web): 19 Feb 2019 Downloaded from http://pubs.acs.org on February 20, 2019

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The Journal of Physical Chemistry

GeSe/BP van der Waals Heterostructures as Promising Anode Materials for Potassium-ion Batteries C. He1, J.H. Zhang1, W.X. Zhang2, T.T. Li1

1

State Key Laboratory for Mechanical Behavior of Materials, School of Materials

Science and Engineering, Xi’an Jiaotong University, Xi’an 710049, China 2

School of Materials Science and Engineering, Chang’an University, Xi’an 710064,

China Abstract Potassium-ion batteries have attracted attention because of their abundant resources and similar electrochemistry to Li-ion batteries (LIBs). In the present work, GeSe/BP heterostructures as promising anode materials for K-ion batteries (KIBs) have been systematically investigated by first-principles calculations. The results reveal that GeSe/BP exhibits a semiconductor-to-metal transition after incorporating of K atoms, indicating the enhanced conductivity compared to monolayer GeSe. The energy barrier for K atoms diffusion on GeSe/BP surface is relatively lower than that on monolayer GeSe. In addition, GeSe/BP heterostructure can accommodate up to five layers of K atoms with negative adsorption energy, which greatly improves the storage capacity. Hence, GeSe/BP heterostructure has great potential for application in advanced electrode material in KIBs.

*Corresponding

Author: W. X. Zhang ([email protected]) 1

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Introduction Since the 21st century, Li-ion batteries (LIBs) have widely been used in the field of mobile phones, laptops, cameras and rechargeable vehicles1-3 owning to their high energy density,4-5 high reversible capacity,6-7 long cycle life,8 large electromotive force, and light weight.9-10 However, due to some intrinsic limitations such as: the high cost, scarcity of Li in the earth's crust,11 the current LIBs are less feasible for large-scale stationary energy storage systems (ESSs). In order to further facilitate the application of ESSs, K-ion batteries (KIBs) have attracted attention because of their abundant resources and similar electrochemical performance to LIBs. Moreover, K atom has a lower redox potential than Na atom (-2.92 V vs -2.71 V),12-13 which leads to higher energy density and a wide voltage window. Thus, KIBs have better prospect for application in ESSs. However, only a limited number of studies for KIBs have been reported compared to LIBs, mainly because of the limited choice of anode materials. For example, graphite can’t be used as an anode of KIBs because embedding K atoms is thermodynamically unfavorable.14-15 Hence, it is still a challenge to obtain new efficient electrode materials for KIBs system with high electronic conductivity, excellent cycling stability and rate capability. Recently, monolayer GeSe, a monolayer group-IV monochalcogenide with phosphorene-like structure has attracted wide attention because of high electronic performance and less toxicity.16 Made from earth abundant element, GeSe nanosheet with a thickness down to 1 nm has been successfully synthesized and explored for fabricating various nano-devices in the laboratory.17-20 Besides, monolayer GeSe 3


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exhibits a robust direct band gap and strong anisotropic transport property with a high hole mobility of 4.71 × 103 cm2/ V s, which makes it a promising candidate for future application in integrated circuits and optoeletronic.21-23 Moreover, due to the unique structural and excellent transport properties, monolayer GeSe is considered as an ideal anode material in rechargeable LIBs.24-25 However, monolayer GeSe has a lower storage capacity of 176.78 mA h g-1 and poorer conductivity compared to metal anode materials for KIBs. Therefore, we explored a new system using two-dimensional (2D) material as substrate and build a heterostructure, which could improve the storage capacity. Black phosphorus (BP), has entered the scope of our consideration due to its graphite-like multilayered structure and its excellent electronic properties such as high carrier mobility and high on/off ratio as field-effect transistors.26 Because of the puckered honeycomb structure of monolayer BP, a fast Li diffusion and large Li capacity can be achieved for lithium intercalation.27 The high theoretical capacity of 2596 mA h g-1 could be attained in uptake of lithium to form Li3P.28-29 In addition, monolayer BP demonstrates great mechanical and dynamical stabilities.30-32 However, BP can be easily oxidized after exposed to the air within several hours33-34 and thus cannot be used directly as an anode material. Monolayer BP could be a good substrate for GeSe as KIBs due to similar structural and electronic properties. Hence, the following study aimed at improving the performance of GeSe as an anode material. In this study, we have systematically investigated the adsorption and diffusion properties of K atoms on GeSe/BP heterostructure by using first-principles calculation. 4


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The results show that after K adsorption, the conductivity of GeSe/BP heterostructure can be greatly enhanced, the energy barrier of K diffusion on GeSe/BP surface is reduced, and the storage capacity of K is increased. This work provides key insights into GeSe/BP van der Waals (vdW) heterostructure and examines its potential applications as an anode material for KIBs with high electrical conductivity and capacity. Computational methods In the study, the first-principle calculations were performed by the Vienna Ab initio Simulation Package (VASP) with the projector augmented wave (PAW), based on the density functional theory (DFT).35-38 The Perdew-Burke-Ernzerhof scheme (PBE) within the generalized gradient approximation (GGA) was used to treat the exchangecorrelation interaction of electrons.39 The empirical correction scheme proposed by Grimme was chosen because of the nontrivial van der Waals interaction.40 The cutoff energy for the wave function was set to 500 eV and the force criterion for the structural relaxation was chosen as 0.01 eV A-1. A Monkhorst-Pack k-point mesh of 3×3×1 was respectively used to calculate properties of all studies in the 2D Brillouin zone. To avoid the interaction between neighboring layers, a vacuum spacing of 30 Å was added along the direction perpendicular to 2D nanosheet. The charge density difference was calculated by CASTEP code in this work.41 For adsorption and diffusion properties of K atoms, a 3×2×1 supercell of GeSe/BP heterostructure was chosen to guarantee enough area for K atoms to move. To find out

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the minimum energy path (MEP) and saddle points between the initial and final positions and calculate the migration energy barrier for K diffusion, the climbing image nudged elastic band (CI-NEB) method was employed.42 CI-NEB, which is a small modification of the nudged elastic band (NEB) method, is efficient in determining the transition states (TS) and MEPs for both fixed-cell and generalized solid-state transformations.43-44 Subsequently, three or four intermediate images were linearly interpolated between two stable states. Results and discussion GeSe/BP heterostructure is achieved by monolayer GeSe and BP stacked in the vertical direction with consideration of vdW interactions between layers.45-46 The calculated lattice parameters of the monolayer GeSe (a = 3.96 Å, b = 4.16 Å) and BP (a = 3.34 Å, b = 4.62 Å) are well agreement with experimental and other theoretical calculations.47 In this study, three stacking patterns are considered according to the relative position of GeSe and BP, which are shown in Figure 1. The Ge or Se atom above the center of BP hexagon (hole) is labeled as H; The monolayer GeSe directly and completely located above monolayer BP is labeled as T; The Ge or Se atom above the mid-point of P-P bond of BP is labeled as B. After structure optimization, GGAPBE calculations show that the total energy of H-style is 0.028 and 0.029 eV lower than that of T-style and B-style, and thus H-style is the most stable stacking pattern. Hence, the H-style is chosen as the stacking model in the following calculations. The mismatch in the a and b directions is a bit larger than 5%, but it won’t influence the stability of the heterostructure due to the flexible puckered structure of monolayer GeSe and BP.48 6


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To further check the stability of the heterostructure and determine the interlayer interaction intensity quantitatively, the binding energy (Eb) per unit cell is calculated by the following formula: 𝐸𝑏 = 𝐸𝐺𝑒𝑆𝑒/𝐵𝑃 ― 𝐸𝐺𝑒𝑆𝑒 ― 𝐸𝐵𝑃

(1),

where 𝐸𝐺𝑒𝑆𝑒/𝐵𝑃, 𝐸𝐺𝑒𝑆𝑒 and EBP represent the total energy of GeSe/BP heterostructure, isolated monolayer GeSe and BP, respectively. The calculated value of binding energy (Eb = -0.346 eV) is negative, which indicates the GeSe/BP heterostructure is energetically stable. Next, we have investigated the K-adsorption/diffusion behaviors on H-style GeSe/BP heterostructure to evaluate the usage as a flexible anode material for KIBs. For BP as the substrate of GeSe nanosheet, two typical adsorption sites are considered in this study: (1) K adsorbed on the surface of GeSe (labeled as K/GeSe/BP), and (2) K embedded in the interlayer of GeSe/BP (labeled as GeSe/K/BP). According to the minimum energy principle, the most stable adsorption sites of K atom on both K/GeSe/BP and GeSe/K/BP systems are very similar to pristine monolayer (K/GeSe and K/BP).27,49 For K/GeSe/BP, there is one possible adsorption site for K atom on the surface and with K adsorbed above the center of triangle Ge-Se rings (H0-site); For GeSe/BP, there are two possible adsorption sites for K atom between the layers of GeSe/K/BP: one is above the center of triangle Ge-Se rings (H1-site) and another is the adsorption above the center of the triangle P-P rings (H2-site), as shown in Figure 2. Besides, some other adsorption sites have been also considered in order to verify the

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above results, as seen in Figure S1. All adsorption sites of K atom on heterostructure are fully optimized, and no obvious structural changes are observed. In order to find out the most stable adsorption site of K adsorption, we have calculated the adsorption energy (Eads) of these three sites by the following formula: 𝐸𝑎𝑑𝑠 = (𝐸𝐺𝑒𝑆𝑒 𝐵𝑃 + 𝐾 ― 𝐸𝐺𝑒𝑆𝑒/𝐵𝑃 ― 𝑛𝐸𝐾)/𝑛

(2),

where 𝐸𝐺𝑒𝑆𝑒 𝐵𝑃 + 𝐾, 𝐸𝐺𝑒𝑆𝑒/𝐵𝑃 and 𝐸𝐾 are the total energy of K atom on GeSe/BP heterostructure, GeSe/BP heterostructure and a single K atom in the same size of supercell, respectively, and n is the number of K atoms. Based on the definition, a larger negative value of Eads suggests a stronger affinity of K atom bonded with the heterostructure and thus a more stable system, which indicate GeSe/BP heterostructure is feasible as KIBs anode. From the comparative Eads diagram of all adsorption sites in Figure S1 and Table 1, the H0-site and H1-site are the most stable adsorption sites for K/GeSe/BP and GeSe/K/BP systems, respectively. The value of Eads for H0-site is 3.501 eV, and the K adatom embedded in the interlayer of GeSe/BP prefers to locate at H1-site with -2.595 eV Eads compared with -2.319 eV of H2-site. It can be inferred that GeSe/BP heterostructure can significantly enhance the bonding strength of K atom on GeSe or BP, which can avoid the formation of K metallic clusters and thus improve the safety and reversibility of KIBs. Table 1 shows the adsorption energy (Eads), minimal distance (d), and the charge transfer (ΔQ) of a single K adsorbed on the H0, H1 and H2site of GeSe/BP heterostructure. To visualize the interaction mechanism of K atoms at the different sites, we have

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calculated the charge density difference between GeSe/BP heterostructure and K atom using the following equation:50 △ ρ(𝑟) = 𝜌(𝑟)𝐺𝑒𝑆𝑒 𝐵𝑃 + 𝐾 ― 𝜌(𝑟)𝐺𝑒𝑆𝑒 𝐵𝑃 ― 𝜌(𝑟)𝐾

(3),

where 𝜌(𝑟)𝐺𝑒𝑆𝑒 𝐵𝑃 + 𝐾 , 𝜌(𝑟)𝐺𝑒𝑆𝑒 𝐵𝑃 and 𝜌(𝑟)𝐾 are the charge densities of the K adsorbed into GeSe/BP, GeSe/BP heterostructure, and isolated K atom, respectively. Figure 3 shows the differential charge density of K atom located at the H0-site and H1site of GeSe/BP heterostructure systems (K/GeSe/BP and GeSe/K/BP). For both cases, the loss of electron is denoted by red and the gain of electron is denoted by blue. As seen from Figure 3a, the net loss of electronic charge is found directly above the K atom and the gain of electronic charge is found on their adjacent layers. Due to the electronegativity of Ge (Se) is larger than K, the electrons transfer from K atom to adjacent Ge and Se atoms forming strong ionic bonds. But, there is no electron accumulation in BP layer because of the comparatively long distance between K and BP layer. Figure 3b shows that the electrons transfer from K atom to both GeSe layer and BP layer, since the electronegativity of P is larger than K, which indicates the strong ionic bonding between the embedded K atom and the two layers. To determine the ion bond strength quantitatively, we calculate the value of ΔQ between the K atom and GeSe/BP by the Mulliken charge analysis, and the results are summarized in Table 1. As one K atom is adsorbed on outside surface of GeSe, the calculated charge on K is +0.75 |e|; while the corresponding charge of GeSe and BP is +0.23 |e| and -0.98 |e|, respectively. Hence, the charges of K atom and GeSe are

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transferred to the below BP layer. Besides, as one K atom is embedded in the interlayer of GeSe/BP, the charge transfer of K to GeSe and BP are -0.2 |e| and -1.04 |e|, respectively. BP layer gained obviously more charge compared to GeSe, which is mainly because of the stronger electronegativity of P atom. It can be seen clearly that all the configurations show the charge transfer from K to GeSe/BP heterostructure, strongly indicating the ionic interactions between K and GeSe/BP heterostructure. To investigate the conductive properties and the electronic structure of GeSe/BP upon K atom intercalation, the magnitude of quantum conductivity (G) function,51 the electron density of states (DOS) and partial density of states (PDOS) of GeSe/BP, K/GeSe/BP and GeSe/K/BP systems are calculated, as shown in Figure 4. The magnitude of G function is calculated to evaluate the conductive property after adsorption by Landauer formula:52 N

G  G0  T i

(4),

i 1

where Ti is the transmission probability of the ith channel, N is the number of propagating modes crossing Fermi energy (Ef), and G is a constant (G0=2e2/ћ, where e denotes the electronic amount and ћ is the Planck constant). In case of perfect ballistic transport, no scattering is considered, namely, Ti is always unity. According to the Landauer formula, the number of bands crossing Fermi level Ef (Figure S2) contributes to the number of conductional channels or the size of G. The calculated G values of K/GeSe/BP and GeSe/K/BP with all conductional channels are shown in Figure 4a and 4b. The G values crossing Fermi level Ef of GeSe/BP, K/GeSe/BP and GeSe/K/BP 10


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systems are 0 G0, 4 G0 and 5 G0, respectively. Therefore, K/GeSe/BP and GeSe/K/BP have higher conductivity than GeSe/BP. In Figure 4c, the largest peak of DOS below Ef is located between 0 and -1.885 eV for GeSe/BP, between -0.222 and -2.522 eV for K/GeSe/BP, and between -0.457 and 2.786 eV for GeSe/K/BP. The largest peak is slightly red-shifted and the conduction band proportions of the system increase after K adsorption, which implies that the charge transfer and the stability of the system slightly enhances. For GeSe/BP (Figure 4d), the state near the Fermi level is nearly flat and with a narrow and indirect band gap of 0.347 eV, which is a bit smaller than that of monolayer GeSe (1.100 eV) and BP (1.042 eV) (Figure S3). It is worth noting that the relative small band gap of GeSe/BP is great beneficial for the electrode conductivity. For K/GeSe/BP and GeSe/K/BP systems, both of them have no band gap and become metallic after potassiation process. Thus the problem of poor electronic conductivity can be solved by using BP nanosheets as substrate of monolayer GeSe, completing the requirement of high electronic conductivity for anode materials. This phenomenon is also found in MoS2/graphene heterostructure, which is converted from semiconducting to metallic after lithiation.53 In addition, it can be seen from Figure 4e that the orbitals of K overlap with both Ge and Se as K atoms are adsorbed on GeSe surface (K/GeSe/BP), which is the feature of covalent hybridization interactions. As K atoms are embedded into the interlayer of GeSe/BP (GeSe/K/BP), the orbitals of K overlap with both GeSe and BP, indicating the covalent hybridization of embedded K with GeSe and BP as seen in Figure 4f. Although the bond between K and GeSe/BP is dominated by ionic interactions, some covalent 11



components still cannot be ignored. Since the rapid charge and discharge capabilities of KIBs are closely related to the diffusion properties, we further analyze the diffusion energy barrier for one K atom at GeSe/BP heterostructure. The adsorption of K atom on GeSe surface and interlayer of GeSe/BP are discussed individually. The most stable K adsorption site, the possible diffusion paths and the energy barrier of K on GeSe surface are firstly studied, which is similar to that of monolayer GeSe. The path between two adjacent H1 sites along the groove of GeSe surface is chosen for K diffusion, as shown in Figure 5. As K atom diffuses on the outside surface of GeSe, it can be seen that the K atom overcomes two Ge-Se bands (band S1 and band S2), producing two similar high peaks (0.095 eV), which is slightly smaller than monolayer GeSe (0.132 eV). It can be considered that the K atom interacts only with GeSe layer because BP layer is far from the K atom. The barrier of K diffusion is relative lower on GeSe surface, mainly because the interaction of BP substrate with GeSe by van der Waals force weakens the stronger K bonds with GeSe surface. Then, we have investigated the diffusion path and the energy barrier of K atom in the interlayer of GeSe/BP. Two adjacent H1 sites are chosen as reference due to a larger negative Eads than H2-site, and the diffusion path is along the direction (H1-S1-S2-H1). There is only one peak with an energy barrier of 0.226 eV, because of the collective interaction of K with both layers, as seen in Figure 6. Consequently, the formation of the GeSe/BP heterostructure increases the energy diffusion barrier of monolayer GeSe from 0.132 to 0.226 eV. However, it is still lower than some other Li diffusion barriers 12


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in monolayer GeS (0.236 eV), graphene (0.277 eV) and GeSe (0.329 eV).49-54 The diffusion barrier suggests that the depotassiation processes on GeSe/BP heterostructure are energetically favorable, indicating that GeSe/BP is a reliable anode material. After investigating the adsorption site and diffusion path, the storage capacities of K with higher concentration are considered next. To explore the maximum storage capacity for K atoms, we have calculated the average adsorption energy of K atoms as a function of its concentration (x) on three sides of GeSe/BP heterostructure: GeSe surface, BP surface and interlayer, as seen in Figure 7. Eads of interlayer (6 K atoms), two surfaces (12 K atoms) and three sides (18 K atoms, as seen in Figure 7a) by filling all the stable adsorption sites with K atoms are -1.724, -1.449 and -1.29 eV, respectively. These negative adsorption energies can ensure good adsorption stabilities of potassiation on GeSe/BP. It can be found that Eads decreases with the increasing of K contents due to the enhancement of K-K repulsion at relatively higher K concentration. In order to further improve the level of storage capacity for K, another K adsorption layer (30 K atoms) above the remaining center of triangle Ge-Se or P-P rings have covered on outside surface of GeSe and BP separately, as seen in Figure 7b. The distances between the outer-most K layer and the second outer K layers are 3.817 Å and 3.924 Å, while the distances between the inner K layer and the host GeSe or BP are 2.625 Å and 2.508 Å, respectively. Though the outer-most two K atom layers are slightly far away from GeSe/BP, the calculated Eads is still negative with a value of 0.789 eV(Figure 7c), indicating that this system is energetically stable. The outer-most layer of K possesses a lower absorption energy (-0.346 eV per atom), indicating a 13



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weaker multilayered adsorption. Besides, G values for different composition of K atoms are calculated: G = 10, 16, 20, 28, and 20 G0 for x = 0.5, 1.0, 1.5, 2.0 and 2.5. According to our previous researches,55-56 for a system, the larger G is, the higher the structural symmetry is. Therefore, G values indicate that the structural symmetry is getting higher and the capacity tends to be saturated with the increasing of the adsorption layer. It can be inferred that GeSe/BP heterostructure can accommodate up to five K atoms layers and have lager storage capacity than monolayer GeSe. Meanwhile, the maximum 𝑥𝐹

capacity (CM) can be obtained by the following equation: 𝐶𝑀 = 𝑀𝐺𝑒𝑆𝑒/𝐵𝑃, where x is the maximum fraction of K in KxGeSe/BP, F is the Faraday constant (a value of 26.8 A h mol-1), and MGeSe/BP is the mass of GeSe/BP. The capacity of K atoms on GeSe/BP is evaluated to be 313.67 mA h g-1, which is larger than some other 2D anode materials, such as TiO2 polymorphs (Li 200 mA h g-1),57 MoS2 (Na 146 mA h g-1),58 and Ti3C2 MXenes (K 191.8 mA h g-1).12 The results show that GeSe/BP has great potential for application in high capacity KIBs electrode materials. Conclusion In this work, we have explored the potential of GeSe/BP heterostructure as an electrode material for K batteries by DFT calculations. The negative adsorption energies for K incorporation guarantee the stability of the system structure. GeSe/BP undergoes a semiconductor-to-metal transition after potassiation process, indicating its enhanced conductivity compared to monolayer GeSe, which is benefit for the free flow of electrons. Moreover, the energy barrier for depotassiation on GeSe/BP surface decreases to 0.92 eV compared to that on monolayer GeSe. In addition, K atoms on 14


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GeSe/BP heterostructure can accommodate up to five layers with negative Eads and stable structure, which greatly increases the storage capacity. These studies suggest that GeSe/BP heterostructure can be used as an excellent electrode material in potassiumion batteries.

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Supporting Information Available: Adsorption energies and band structures.

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Acknowledgments. The authors acknowledge supports by National Natural Science Foundation of China (NSFC, No. 51471124), Natural Science Foundation of Shaanxi province, China (2017JQ5045), the Fundamental Research Funds for the Central Universities (No. xjj2016018), and National Undergraduate Training Program for Innovation and Entrepreneurship (201810710128).

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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 24


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Table 1 Calculated structural and electronic properties for K atoms adsorbed on H0, H1, and H2-site of 3×2×1 supercell of GeSe monolayer and GeSe/BP heterostructure. Eads represented the adsorption energy; dK-Ge, dK-Se, and dK-P refer to minimal distances of K-Ge, K-Se and K-P. ΔQ is the charge transfer of K atoms.

System

site

Eads (eV)

dK-Ge (Å)

dK-Se (Å)

dK-P (Å)

ΔQ (e)

K/GeSe/BP

H0

-3.501

3.306

3.216

-

0.750

GeSe/K/BP

H1

-2.505

3.253

3.097

2.836

1.240

H2

-2.319

3.377

2.874

3.050

1.190

25



Figures and Captions Figure 1.

Figure 1. Top and side views of the three nonequivalent stacking patterns of GeSe/BP interface. (a) H-style (b) T-style (c) B-style

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Figure 2.

Figure 2. Top (a) and side (b) illustration views of the lattice structure and K-adsorption sites in GeSe/BP heterostructure.

27



Figure 3.

Figure 3. Differential charge densities of K adsorbed on surface (a) and in the interlayer of GeSe/BP (b). Blue and red colors indicate the electron accumulation and depletion, respectively.

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Figure 4. G of K/GeSe/BP (a) and GeSe/K/BP (b), DOS (c), PDOS of GeSe/BP heterostructure (d), K adsorbed on GeSe surface (e), K adsorbed in the interlayer of GeSe/BP (f). The Fermi level is set to zero.

29



Figure 5. Diffusion energy barrier of one K atom diffusion on monolayer GeSe and on GeSe surface of GeSe/BP.

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Figure 6.

Figure 6. Diffusion pathways (a) and energy barrier (b) of one K atom diffusion in the interlayer of GeSe/BP.

31



Figure 7.

Figure 7. Side view of one 3×2×1 GeSe/BP supercell, accommodating up to 18 K atoms (x = 1.5) (a), 30 K atoms (x = 2.5) (b). The adsorption energy of K atoms as function of x in Kx/GeSe/BP (c). The optimized stable configuration of Kx/GeSe/BP (x = 0.5, 1.0, 2.0) is shown as well.

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TOC Graphic

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BP van der Waals Heterostructures as Promising Anode

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