GeSe/BP van der Waals Heterostructures as Promising AnodeMaterials for Potassium-Ion BatteriesC. He,† J. H. Zhang,† W. X. Zhang,*,‡ and T. T. Li†

†State Key Laboratory for Mechanical Behavior of Materials, School of Materials Science and Engineering, Xi’an Jiaotong University,Xi’an 710049, China‡School of Materials Science and Engineering, Chang’an University, Xi’an 710064, China

*S Supporting Information

ABSTRACT: Potassium-ion batteries have attracted attention because of theirabundant resources and similar electrochemistry to Li-ion batteries (LIBs). In thepresent work, GeSe/black phosphorus (BP) heterostructures as promising anodematerials 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 K atoms, indicating enhanced conductivitycompared to monolayer GeSe. The energy barrier for K atom diffusion on GeSe/BP surface is relatively lower than that on monolayer GeSe. In addition, the GeSe/BP heterostructure can accommodate up to five layers of K atoms with negativeadsorption energy, which greatly improves the storage capacity. Hence, the GeSe/BP heterostructure has great potential for application in advanced electrodematerials in KIBs.

■ INTRODUCTIONSince the 21st century, Li-ion batteries (LIBs) have widelybeen used in the field of mobile phones, laptops, cameras, andrechargeable vehicles1−3 owning to their high energy density,4,5

high reversible capacity,6,7 long cycle life,8 large electromotiveforce, and light weight.9,10 However, because of some intrinsiclimitations such as high cost and scarcity of Li in the earth’scrust,11 the current LIBs are less feasible for large-scalestationary energy storage systems (ESSs). In order to furtherfacilitate the application of ESSs, K-ion batteries (KIBs) haveattracted attention because of their abundant resources andsimilar electrochemical performance to LIBs. Moreover, a Katom has a lower redox potential than a Na atom (−2.92 V vs−2.71 V),12,13 which leads to higher energy density and a widevoltage window. Thus, KIBs have a better prospect forapplication in ESSs. However, only a limited number of studiesfor KIBs have been reported compared to LIBs, mainly becauseof the limited choice of anode materials. For example, graphitecannot be used as an anode of KIBs because embedding Katoms are thermodynamically unfavorable.14,15 Hence, it is stillchallenging to obtain new efficient electrode materials for KIBsystems with high electronic conductivity and excellent cyclingstability and rate capability.Recently, monolayer GeSe, a monolayer group-IV mono-

chalcogenide with phosphorene-like structure, has attractedwide attention because of high electronic performance and lesstoxicity.16 Made from an earth-abundant element, the GeSenanosheet with a thickness less than 1 nm has beensuccessfully synthesized and explored for fabricating variousnanodevices in the laboratory.17−20 Besides, monolayer GeSeexhibits 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 futureapplication in integrated circuits and optoelectronic.21−23

Moreover, because of the unique structural and excellenttransport properties, monolayer GeSe is considered as an idealanode material in rechargeable LIBs.24,25 However, monolayerGeSe has a lower storage capacity of 176.78 mA h g−1 andpoorer conductivity compared to metal anode materials forKIBs. Therefore, we explored a new system using a two-dimensional (2D) material as the substrate and build aheterostructure, which could improve the storage capacity.Black phosphorus (BP) has entered the scope of our

consideration because of its graphite-like multilayered structureand excellent electronic properties such as high carrier mobilityand high on/off ratio as field-effect transistors.26 Because of thepuckered honeycomb structure of monolayer BP, a fast Lidiffusion and large Li capacity can be achieved for lithiumintercalation.27 The high theoretical capacity of 2596 mA h g−1

could be attained in the uptake of lithium to form Li3P.28,29 In

addition, monolayer BP demonstrates great mechanical anddynamical stabilities.30−32 However, BP can be easily oxidizedafter exposed to air within several hours33,34 and thus cannotbe used directly as an anode material. Monolayer BP could bea good substrate for GeSe as KIBs because of similar structuraland electronic properties. Hence, the following study aimed atimproving the performance of GeSe as an anode material.

Received: September 12, 2018Revised: February 13, 2019Published: February 19, 2019

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In this study, we have systematically investigated theadsorption and diffusion properties of K atoms on the GeSe/BP heterostructure by using first-principles calculation. Theresults show that after K adsorption, the conductivity of GeSe/BP heterostructure can be greatly enhanced, the energy barrierof K diffusion on GeSe/BP surface is reduced, and the storagecapacity of K is increased. This work provides key insights intoGeSe/BP van der Waals (vdW) heterostructure and examinesits potential applications as an anode material for KIBs withhigh electrical conductivity and capacity.

■ COMPUTATIONAL METHODS

In the study, the first-principle calculations were performed bythe Vienna Ab initio Simulation Package (VASP) with theprojector augmented wave based on density functional theory(DFT).35−38 The Perdew−Burke−Ernzerhof (PBE) schemewithin the generalized gradient approximation (GGA) wasused to treat the exchange−correlation interaction ofelectrons.39 The empirical correction scheme proposed byGrimme was chosen because of the nontrivial vdWinteraction.40 The cutoff energy for the wave function wasset to 500 eV and the force criterion for the structuralrelaxation was chosen as 0.01 eV A−1. A Monkhorst−Pack k-point mesh of 3 × 3 × 1 was respectively used to calculate theproperties of all studies in the 2D Brillouin zone. To avoid theinteraction between neighboring layers, a vacuum spacing of 30Å was added along the direction perpendicular to the 2Dnanosheet. The charge density difference was calculated byCASTEP code in this work.41

For adsorption and diffusion properties of K atoms, a 3 × 2× 1 supercell of GeSe/BP heterostructure was chosen toguarantee enough area for K atoms to move. To find out theminimum energy path (MEP) and saddle points between theinitial and final positions and calculate the migration energybarrier for K diffusion, the climbing image nudged elastic band(CI-NEB) method was employed.42 CI-NEB, which is a smallmodification of the NEB method, is efficient in determiningthe transition states and MEPs for both fixed cell andgeneralized solid-state transformations.43,44 Subsequently,three or four intermediate images were linearly interpolatedbetween two stable states.

■ RESULTS AND DISCUSSIONThe GeSe/BP heterostructure is achieved by monolayer GeSeand BP stacked in the vertical direction with the considerationof vdW interactions between layers.45,46 The calculated latticeparameters of monolayer GeSe (a = 3.96 Å, b = 4.16 Å) andBP (a = 3.34 Å, b = 4.62 Å) are in good agreement with theexperimental and other theoretical calculations.47 In this study,three stacking patterns are considered according to the relativeposition of GeSe and BP, which are shown in Figure 1. The Geor Se atom above the center of BP hexagon (hole) is labeledH; the monolayer GeSe directly and completely located abovethe monolayer BP is labeled T; the Ge or Se atom above themid-point of the P−P bond of BP is labeled B. After structuraloptimization, GGA-PBE calculations show that the totalenergy of H-style is 0.028 and 0.029 eV lower than that ofT-style and B-style, and thus, H-style is the most stablestacking pattern. Hence, the H-style is chosen as the stackingmodel in the following calculations. The mismatch in the a andb directions is a bit larger than 5%, but it will not influence thestability of the heterostructure because of the flexible puckeredstructure of monolayer GeSe and BP.48

To further check the stability of the heterostructure anddetermine the interlayer interaction intensity quantitatively, thebinding energy (Eb) per unit cell is calculated by the followingformula:

= − −E E E Eb GeSe/BP GeSe BP (1)

where EGeSe/BP, EGeSe, and EBP represent the total energy of theGeSe/BP heterostructure, isolated monolayer GeSe, and BP,respectively. The calculated value of binding energy (Eb =−0.346 eV) is negative, which indicates that the GeSe/BPheterostructure is energetically stable.Next, we have investigated the K adsorption/diffusion

behaviors on the H-style GeSe/BP heterostructure to evaluatethe usage as a flexible anode material for KIBs. For BP as thesubstrate of the GeSe nanosheet, two typical adsorption sitesare considered in this study: (1) K adsorbed on the surface ofGeSe (labeled K/GeSe/BP) and (2) K embedded in theinterlayer of GeSe/BP (labeled GeSe/K/BP). According to theminimum energy principle, the most stable adsorption sites ofK atoms on both K/GeSe/BP and GeSe/K/BP systems arevery similar to pristine monolayer (K/GeSe and K/BP).27,49

For K/GeSe/BP, there is one possible adsorption site for Katoms on the surface and with K adsorbed above the center of

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

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triangle Ge−Se rings (H0-site); for GeSe/BP, there are twopossible adsorption sites for K atoms between the layers ofGeSe/K/BP: one is above the center of triangle Ge−Se rings(H1-site) and the other is the adsorption above the center ofthe triangle P−P rings (H2-site), as shown in Figure 2. Besides,some other adsorption sites have also been considered in orderto verify the above results, as can be seen in Figure S1. Alladsorption sites of K atoms on the heterostructure are fullyoptimized, and no obvious structural changes are observed. Inorder to find out the most stable adsorption site of Kadsorption, we have calculated the adsorption energy (Eads) ofthese three sites by the following formula:

= − −+E E E nE n( )/ads GeSe/BP K GeSe/BP K (2)

where EGeSe/BP+K, EGeSe/BP, and EK are the total energy of Katoms on the GeSe/BP heterostructure, GeSe/BP hetero-structure, and a single K atom in the same size of the supercell,respectively, and n is the number of K atoms. On the basis ofthe definition, a larger negative value of Eads suggests a strongeraffinity of K atoms bonded with the heterostructure and thus amore stable system, which indicates that the GeSe/BPheterostructure is feasible as KIB anodes. From thecomparative Eads diagram of all adsorption sites in Figure S1and 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 theK adatom embedded in the interlayer of GeSe/BP prefers tolocate at the H1 site with −2.595 eV Eads compared with−2.319 eV of H2 site. It can be inferred that the GeSe/BPheterostructure can significantly enhance the bonding strengthof K atoms on GeSe or BP, which can avoid the formation of Kmetallic clusters and thus improve the safety and reversibilityof KIBs. Table 1 shows the adsorption energy (Eads), minimaldistance (d), and charge transfer (ΔQ) of a single K adsorbedon the H0, H1, and H2 sites of the GeSe/BP heterostructure.

To visualize the interaction mechanism of K atoms at thedifferent sites, we have calculated the charge density differencebetween the GeSe/BP heterostructure and K atom using thefollowing equation:50

ρ ρ ρ ρΔ = − −+r r r r( ) ( ) ( ) ( )GeSe/BP K GeSe/BP K (3)

where ρ(r)GeSe/BP+K, ρ(r)GeSe/BP, and ρ(r)K are the chargedensities of K adsorbed into GeSe/BP, GeSe/BP hetero-structure, and isolated K atoms, respectively. Figure 3 shows

the differential charge density of K atoms located at the H0 siteand H1 site of the GeSe/BP heterostructure systems (K/GeSe/BP and GeSe/K/BP). For both cases, the loss of electron isdenoted by red and the gain of electron is denoted by blue. Ascan be seen from Figure 3a, the net loss of electronic charge isfound directly above the K atoms and the gain of electroniccharge is found on their adjacent layers. Because theelectronegativity of Ge (Se) is larger than K, the electronstransfer from K atoms to adjacent Ge and Se atoms formingstrong ionic bonds. However, there is no electron accumu-lation in the BP layer because of the comparatively longdistance between K and BP layer. Figure 3b shows that theelectrons transfer from K atoms to both GeSe layer and BPlayer, as the electronegativity of P is larger than K, whichindicates the strong ionic bonding between the embedded Katom and the two layers.To determine the ion bond strength quantitatively, we

calculate the value of ΔQ between the K atom and GeSe/BPby the Mulliken charge analysis, and the results aresummarized in Table 1. As one K atom is adsorbed on theoutside surface of GeSe, the calculated charge on K is +0.75|e|,whereas the corresponding charge of GeSe and BP is +0.23|e|and −0.98|e|, respectively. Hence, the charges of K atoms andGeSe are transferred to the below BP layer. Besides, as one Katom is embedded in the interlayer of GeSe/BP, the chargetransfer of K to GeSe and BP is −0.2|e| and −1.04|e|,respectively. The BP layer gained obviously more charge

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

Table 1. Calculated Structural and Electronic Properties forK Atoms Adsorbed on H0, H1, and H2-Site of 3 × 2 × 1Supercell of GeSe Monolayer and GeSe/BPHeterostructurea

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 0.750GeSe/K/BP H1 −2.505 3.253 3.097 2.836 1.240

H2 −2.319 3.377 2.874 3.050 1.190aEads represented the adsorption energy; dK−Ge, dK−Se, and dK−P referto minimal distances of K−Ge, K−Se and K−P. ΔQ is the chargetransfer of K atoms.

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

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compared to GeSe, which is mainly because of the strongerelectronegativity of the P atom. It can be seen clearly that allthe configurations show the charge transfer from K to GeSe/BP heterostructure, strongly indicating the ionic interactionsbetween K and GeSe/BP heterostructure.To investigate the conductive properties and the electronic

structure of GeSe/BP upon K atom intercalation, themagnitude of quantum conductivity (G) function,51 electrondensity of states (DOS), and partial DOS (PDOS) of GeSe/BP, K/GeSe/BP, and GeSe/K/BP systems are calculated, asshown in Figure 4. The magnitude of G function is calculatedto evaluate the conductive property after adsorption by theLandauer formula52

∑==

G G Ti

N

i01 (4)

where Ti is the transmission probability of the ith channel, N isthe number of propagating modes crossing Fermi energy (Ef),and G is a constant (G0 = 2e2/ℏ, where e denotes the electronicamount and ℏ is the Planck constant). In the case of a perfectballistic transport, no scattering is considered, namely, Ti isalways unity. According to the Landauer formula, the numberof bands crossing the Fermi level Ef (Figure S2) contributes tothe number of conductional channels or the size of G. Thecalculated G values of K/GeSe/BP and GeSe/K/BP with allconductional channels are shown in Figure 4a,b. The G values

crossing the Fermi level Ef of GeSe/BP, K/GeSe/BP, andGeSe/K/BP systems are 0 G0, 4 G0, and 5 G0, respectively.Therefore, K/GeSe/BP and GeSe/K/BP have higher con-ductivity 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 −2.786eV for GeSe/K/BP. The largest peak is slightly red-shifted andthe conduction band proportions of the system increase after Kadsorption, which implies that the charge transfer and thestability of the system slightly enhances. For GeSe/BP (Figure4d), the state near the Fermi level is nearly flat and with anarrow and indirect band gap of 0.347 eV, which is slightlysmaller than that of monolayer GeSe (1.100 eV) and BP(1.042 eV) (Figure S3). It is worth noting that the relativesmall band gap of GeSe/BP is greatly beneficial for electrodeconductivity. For K/GeSe/BP and GeSe/K/BP systems, bothof them have no band gap and become metallic afterpotassiation process. Thus, the problem of poor electronicconductivity can be solved by using BP nanosheets as asubstrate of monolayer GeSe, completing the requirement ofhigh electronic conductivity for anode materials. Thisphenomenon is also found in a MoS2/graphene hetero-structure, which is converted from semiconducting to metallicafter lithiation.53 In addition, it can be seen from Figure 4e thatthe orbitals of K overlap with both Ge and Se as K atoms are

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

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

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adsorbed on the GeSe surface (K/GeSe/BP), which is thefeature of covalent hybridization interactions. As K atoms areembedded into the interlayer of GeSe/BP (GeSe/K/BP), theorbitals of K overlap with both GeSe and BP, indicating thecovalent hybridization of embedded K with GeSe and BP ascan be seen in Figure 4f. Although the bond between K andGeSe/BP is dominated by ionic interactions, some covalentcomponents still cannot be ignored.As the rapid charge and discharge capabilities of KIBs are

closely related to the diffusion properties, we further analyzethe diffusion energy barrier for one K atom at the GeSe/BPheterostructure. The adsorption of the K atom on the GeSesurface and interlayer of GeSe/BP is discussed individually.The most stable K adsorption site, the possible diffusion paths,and the energy barrier of K on the GeSe surface are firststudied, which are similar to that of monolayer GeSe. The pathbetween two adjacent H1 sites along the groove of the GeSesurface is chosen for K diffusion, as shown in Figure 5. As theK atom diffuses on the outside surface of GeSe, it can be seenthat the K atom overcomes two Ge−Se bands (band S1 andband S2), producing two similar high peaks (0.095 eV), whichis slightly smaller than monolayer GeSe (0.132 eV). It can beconsidered that the K atom interacts only with the GeSe layerbecause the BP layer is far from the K atom. The barrier of Kdiffusion is relatively lower on the GeSe surface, mainlybecause the interaction of BP substrate with GeSe by vdWforce weakens the stronger K bonds with the GeSe surface.Then, we have investigated the diffusion path and the energy

barrier of K atoms in the interlayer of GeSe/BP. Two adjacentH1 sites are chosen as reference because of a larger negativeEads than the H2 site, and the diffusion path is along thedirection (H1−S1−S2−H1). There is only one peak with anenergy barrier of 0.226 eV because of the collective interaction

of K with both layers, as can be seen in Figure 6. Consequently,the formation of the GeSe/BP heterostructure increases theenergy diffusion barrier of monolayer GeSe from 0.132 to0.226 eV. However, it is still lower than some other Li diffusionbarriers in monolayer GeS (0.236 eV), graphene (0.277 eV),and GeSe (0.329 eV).49−54 The diffusion barrier suggests thatthe depotassiation processes on the GeSe/BP heterostructureare energetically favorable, indicating that GeSe/BP is areliable anode material.After investigating the adsorption site and diffusion path, the

storage capacities of K with higher concentration areconsidered next. To explore the maximum storage capacityfor K atoms, we have calculated the average adsorption energyof K atoms as a function of its concentration (x) on three sidesof GeSe/BP heterostructure: GeSe surface, BP surface, andinterlayer, as can be seen in Figure 7. The Eads values ofinterlayer (6 K atoms), two surfaces (12 K atoms), and threesides (18 K atoms, as can be seen in Figure 7a) by filling all thestable adsorption sites with K atoms are −1.724, −1.449, and−1.29 eV, respectively. These negative adsorption energies canensure good adsorption stabilities of potassiation on GeSe/BP.It can be found that Eads decreases with the increasing of Kcontents because of the enhancement of K−K repulsion at arelatively higher K concentration. In order to further improvethe level of storage capacity for K, another K adsorption layer(30 K atoms) above the remaining center of triangle Ge−Se orP−P rings has covered on the outside surface of GeSe and BPseparately, as can be seen in Figure 7b. The distances betweenthe outermost K layer and the second outer K layers are 3.817and 3.924 Å, whereas the distances between the inner K layerand the host GeSe or BP are 2.625 and 2.508 Å, respectively.Though the outermost two K atom layers are slightly far awayfrom GeSe/BP, the calculated Eads is still negative with a value

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

Figure 7. Side view of one 3 × 2 × 1 GeSe/BP supercell, accommodating up to 18 K atoms (x = 1.5) (a) and 30 K atoms (x = 2.5) (b). Adsorptionenergy of K atoms as a 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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of −0.789 eV(Figure 7c), indicating that this system isenergetically stable. The outermost layer of K possesses a lowerabsorption energy (−0.346 eV per atom), indicating a weakermultilayered adsorption. Besides, the G values for differentcomposition of K atoms are calculated: G = 10, 16, 20, 28, and20, G0 for x = 0.5, 1.0, 1.5, 2.0, and 2.5. According to ourprevious researches,55,56 for a system, the larger the G is, thehigher the structural symmetry is. Therefore, G values indicatethat the structural symmetry is getting higher and the capacitytends to be saturated with the increasing of the adsorptionlayer. It can be inferred that the GeSe/BP heterostructure canaccommodate up to five K atom layers and have lager storagecapacity than monolayer GeSe. Meanwhile, the maximumcapacity (CM) can be obtained by the following equation:

=C xFMM

GeSe/BP, where x is the maximum fraction of K in

KxGeSe/BP, F is the Faraday constant (a value of 26.8 A hmol−1), and MGeSe/BP is the mass of GeSe/BP. The capacity ofK atoms on GeSe/BP is evaluated to be 313.67 mA h g−1,which is larger than some other 2D anode materials, such asTiO2 polymorphs (Li 200 mA h g−1),57 MoS2 (Na 146 mA hg−1),58 and Ti3C2 MXenes (K 191.8 mA h g−1).12 The resultsshow that GeSe/BP has great potential for application in high-capacity KIB electrode materials.

■ CONCLUSIONSIn this work, we have explored the potential of the GeSe/BPheterostructure as an electrode material for K batteries by DFTcalculations. The negative adsorption energies for K incorpo-ration guarantee the stability of the system structure. GeSe/BPundergoes a semiconductor-to-metal transition after potassia-tion process, indicating its enhanced conductivity compared tomonolayer GeSe, which is beneficial for the free flow ofelectrons. Moreover, the energy barrier for depotassiation onthe GeSe/BP surface decreases to 0.92 eV compared to that onmonolayer GeSe. In addition, the K atoms on the GeSe/BPheterostructure can accommodate up to five layers withnegative Eads and stable structure, which greatly increases thestorage capacity. These studies suggest that the GeSe/BPheterostructure can be used as an excellent electrode materialin potassium-ion batteries.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.jpcc.8b08909.

Adsorption energies and band structures (PDF)

■ AUTHOR INFORMATIONCorresponding Author*E-mail: wxzhang@chd.edu.cn.ORCIDC. He: 0000-0002-6612-5346W. X. Zhang: 0000-0002-9327-5761NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe authors acknowledge the support by National NaturalScience Foundation of China (NSFC, no. 51471124), NaturalScience Foundation of Shaanxi province, China

(2017JQ5045), the Fundamental Research Funds for theCentral Universities (no. xjj2016018), and the NationalUndergraduate Training Program for Innovation and En-trepreneurship (201810710128).

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DOI: 10.1021/acs.jpcc.8b08909J. Phys. Chem. C 2019, 123, 5157−5163

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