Two-Dimensional GeP3 as a High Capacity Anode Material for Non

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C: Energy Conversion and Storage; Energy and Charge Transport 3

Two-Dimensional GeP as a High Capacity Anode Material for Non-Lithium-Ion Batteries Xiaoyu Deng, Xianfei Chen, Yi Huang, Beibei Xiao, and Haiying Du J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11574 • Publication Date (Web): 04 Feb 2019 Downloaded from http://pubs.acs.org on February 4, 2019

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The Journal of Physical Chemistry

Two-dimensional GeP3 as a High Capacity Anode Material for Non-lithium-ion Batteries Xiaoyu Denga, Xianfei Chena*, Yi Huangb,c, Beibei Xiaod, Haiying Dub,c a

College of Materials and Chemistry & Chemical Engineering, Chengdu University of Technology, Chengdu 610059, China b State Environmental Protection Key Laboratory of Synergetic Control and Joint Remediation for Soil & Water Pollution, Chengdu University of Technology, Chengdu 610059, China c College of Environment and Ecology, Chengdu University of Technology, Chengdu 610059, China d School of Energy and Power Engineering, Jiangsu University of Science and Technology, Zhenjiang 212003, China

Abstract： Utilization of non-lithium-ion batteries in next-generation renewable energy storage is hindered by the lack of appropriate electrode materials with desired electrochemical performance. Motivated by low peeling-off energy (Yu Jing et. al., Nano Lett., 2017, 17 (3), 1833–1838), an experimentally available two-dimensional material, nominated as GeP3 is investigated as the anode for non-lithium-ion batteries (Na+, K+, Ca2+, Mg2+, Al3+) based on density functional theory calculations. The electrochemical properties, i.e., ion intercalation mechanism, diffusion behavior and theoretical capacities of different metal ions in GeP3, are systematically investigated. A semiconductor-to-metal transition and improved conductivity are observed due to ions intercalation in GeP3 electrode. Even though, the charge storage mechanism of Na and Ca ions is quite different, the GeP3 monolayer has exhibited a high theoretical capacity of 1295.42 mAh g-1 for both Na+ and Ca2+ ions. Furthermore, collective Na-ions transport at the phase boundary indicates that the sodiated GeP3 electrode favors well-distributed phase formation instead of separation or clustering at the nanoscale, which is beneficial in avoiding the thermal runaway issues

induced

by

dendrite

formation.

Moreover,

the

shallow

and

steady

intercalation/deintercalation resistance of Na-ion at dilute limit and phase boundary in GeP3 suggests excellent rate performance and high cyclic stability. These results provide a steady path toward further development and utilization of 2D GeP3 as an anode in non-lithium ion batteries.

*

Corresponding author. Email: [email protected]; [email protected] 1

ACS Paragon Plus Environment

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

1 Introduction Energy and environmental issues, featured as how to balance the sustainable and harmonious development of traditional fossil fuels (i.e., oil, coal, gas) and environment protection, have become one of the major concerns around the world.1-4 The current energy forms, which are turning from “burning” to conversion, also indicate the urgent requirements of public to clean energy and purging environment. Energy conversion and storage from solar energy represents the most convenient and effective way to solve the energy dilemma. However, the intermittent nature of renewable energy sources requires the development of efficient energy storage devices to store and deliver the required energy without disruptions. In the past few decades, lithium-ion batteries (LIBs) have become the most prevailing energy-storage system due to their high energy density, large working voltage and excellent cyclic performance.5-6 However, the limited Li resource and its inhomogeneous distribution impede the large-scale development and utilization of LIBs.7-8 Therefore, non-lithium ion batteries (NLIBs), i.e., Na, K, Ca, Mg and Al, are considered as cost-effective alternatives to conventional LIBs due to abundant reserves of Na, K, Ca, Mg and Al. 9-11 Similar to conventional LIBs, the successful realization of NLIBs technology also requires the development of electrode materials with a high specific capacity, excellent cyclic stability and desirable rate capability.12-13 However, the development of electrode materials, in general, and anode material, in particular, for NLIBs is hindered by the large ionic radius of corresponding metals.14 For instance, the size of Na ion is 34% larger than Li-ion. Recently, several theoretical and experimental studies have demonstrated that two-dimensional (2D) materials are promising candidates as NLIBs anodes because of their high specific surface area, excellent mechanical properties, which hinder the electrode pulverization, and rich ion insertion channels for ionic migration.15-16 Compared to the bulk electrode materials, where metal ions are stored at interstitial sites, 2D electrodes provide interlayer spacing as ions storage channels and could accommodate large volumetric changes during charge/discharge process. Consequently, several 2D materials, such as graphene17, germanene18, silicene19, black/blue phosphorus20-21, transition metal carbides and carbonitrides (MXenes)

22-24,

transition metal dichalcogenides (TMDCs)25-28, borophene and

their derivatives29-32, have been widely investigated as anode materials in Li and non-Li ion batteries. These reports offer valuable insights into the electrochemical storage of secondary ions in 2D material and provide a baseline for development of anode materials for electrochemical energy storage (EES) devices.33 Nevertheless, the proposed 2D anode materials still suffer from some intrinsic problems. For instance, even though the utilization of silicene or phosphorene as an anode material could alleviate the severe volumetric changes as compared to their bulk counterparts, the long-term stability of these 2D electrodes remains unsatisfactory34-35. On the other hand, graphene and MXenes deliver excellent cyclic stability but suffers from their low capacities.27, 36 Therefore, the pursuit of novel anode materials with high capacity and desirable structural stability is still needed for the development of NLSBs. 2


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Bulk GeP3 is superconductive, possessing puckered arsenic-type honeycomb structure, which was first reported in the 1970s.37 Recently, Jing et al. have reported that the 2D GeP3 can be prepared by peeling off layered GeP3 due to small cleavage energy .38 Moreover, extraordinarily high carrier mobility and tunable conductivity38, make 2D GeP3 a promising electrode material for EES devices. For instance, Zhang et al. have demonstrated the excellent electrochemical performance of GeP3 as an anode material in LIBs.39 Apart from GeP3, several other compositions of GePx and corresponding composites

40-42

have been investigated as anode electrodes in Li and

Na-ion batteries while the ion-storage mechanism of GePx-based electrode materials remains ambiguous. Moreover, the electrochemical performance and intercalation/deintercalation mechanism of 2D GeP3 as an anode material for NLSBs have not been investigated yet. Herein, we have carried out density functional theory (DFT) calculations to investigate the electrochemical

performance

of

2D

GeP3

as

an

anode

material

of

intercalation/deintercalation mechanism of different secondary ions, such as

Na+,

NLSBs. K+,

Ca2+,

The Mg2+

and Al3+, into GeP3 electrode, have been studied by calculating their adsorption, hopping dynamic, average open-circuit voltage and theoretical capacity. The dynamic stability of electrode has been confirmed via ab-initio molecular dynamics (AMID) simulations. In addition, ionic diffusion behavior has been studied at both dilute limit and phase boundary of sodiation state and pristine GeP3, which sheds light on collective transport behavior of Na ions during battery operation. The current study provides important guidance for future development of GeP3-based anode materials for non-Li ion battery system.

2 Computational Methods The geometry optimizations and energies are calculated in DMol3 code. The GGA with the Perdew–Burke–Ernzerh (PBE) was used to describe the electronic exchange and correlation effects.43 The DFT-D method, based on Grimme’s scheme, was selected to describe the van der Waals interactions.44 All electrons, including some relativistic effects, are explicitly considered to deal with core electrons. The double-numerical plus polarization (DNP) was employed as the basis set. The Brillouin zone was sampled using a Monkhorst−Pack method with k-point of 9 × 9 × 1 for GeP3 cell. In order to avoid the artificial interlayer interactions due to the introduction of periodic boundary conditions, the vacuum space was set to be larger than 15 Å for all calculations. The convergence tolerance of energy, force and displacement was set to be 1.0×10-5 Ha, 0.002 Ha/Å and 0.005 Å, respectively. The orbital cutoffs for Na, Mg, Al, K and Ca are selected as 5.2 Å, 4.9 Å, 4.8 Å, 5.6 Å and 5.5 Å, respectively. Linear synchronous transit (LST) and quadratic synchronous transit (QST) tool in DMol3 were used to find the transition-state structure and estimate diffusion barrier. The half-reaction for the charge/discharge process of NLSB was assumed to be: GeP3 + xMn+ + nxe- = GeP3Mx

(1)

where M refers to the secondary metal ion, x indicates the number of inserted ions and n represents the number of charges transferred during the reaction. The electronic potential during 3



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the charging/discharging process can be written in the form of Gibbs free energy 𝛥𝐺

(2)

𝑉 = ― 𝑛𝐹

where F refers to the Faraday constant of 26801 mA h mol-1 and ΔG refers to the change of Gibbs free energy in reaction (1), which can be defined as: ΔG = ΔE + PΔV– TΔS. PΔV is around 10−5 eV while TΔS is about 0.026 eV at room temperature. Hence, the contributions of PΔV and TΔS can be ignored. Then, ΔG can also be given as: ΔG = ΔE = EGeP3Mx − (xEM + EGeP3 ),45 where EGeP3Mx refers to the total energy of GeP3 supercell with x metal atoms, EM represents the chemical potential of Na, K, Ca, Mg or Al in their bulk form and E GeP3denotes the total energy of isolated GeP3. Hence, the OCV of a given NLIB can be calculated as follow: 𝛥𝐺

𝛥𝐸

(3)

𝑉 = ― 𝑛𝑒 ≈ ― 𝑛𝑒

3 Results and Discussion 3.1 Structural and Electronic Properties of Monolayer GeP3 The optimized structure of monolayer GeP3 is presented in Fig. 1a and b. 2D GeP3 is featured as a puckered hexagonal honeycomb configuration with a wrinkle height (dGeP3) of about 2.38 Å, where each Ge bonds with three neighboring P atoms, whereas each P atom forms two P−P bonds and one Ge−P, respectively. The Ge−P bond length (lGe-P) and P−P bond length (lGe-P) were calculated to be 2.52 Å and 2.19Å, respectively. The calculated lattice parameters are a = b = 6.98 Å, which are in good agreement with the previous results (a = b = 6.96 Å)

38.

Moreover, the

electronic band structure of GeP3 is shown in Figure 1c, where an indirect energy gap of 0.311eV can be readily observed, which indicates the semiconducting nature of 2D GeP3.

Fig. 1 (a) Top and (b) side view of the crystal structure of a GeP3 monolayer with possible adsorption sites for metal ions. The orchid and green balls represent the P and Ge atoms, respectively. The hexagonal unit cell is enclosed in the blue dashed line. (c) The calculated band structure of GeP3 at PBE level.

3.2 Metal-ions Adsorption on Monolayer GeP3 To determine the storage capability of GeP3 at dilute concentrations of Na, K, Ca, Mg and Al, we have considered a 2×2×1 supercell with a distance of 13.96 Å for periodic metal ions, which is 4


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proved to be large enough to weaken their interactions. Under these conditions, the chemical stoichiometry of single metal atom adsorption on GeP3 surface is GeP3M0.125. Considering the structural symmetry of GeP3 lattice, twelve feasible sites for ion adsorption are identified and shown in Fig. 1a. The T, B, H sites correspond to the adsorption of metal atom on the top of B-atom, above the bridge sites between two B-atoms and the hollow site of the puckery hexagon or the triangle, respectively. GeP3 exhibit different affinity to Na, K, Ca, Mg and Al atoms, and, most probably, the adsorption sites are related to the number of valence electrons, where Na and K preferred to occupy the T4 site, Ca and Mg inclined to adsorb on H2 site and Al adsorbed at T2 site. The calculated adsorption energy (Ead), charge transfer and adsorption height of metal atom are summarized in table 1. The Ead of metal ions are calculated by using the given relationship: 𝐸𝑎𝑑 = (𝐸𝑥𝑀 + 𝐺𝑒𝑃3 ― 𝐸𝐺𝑒𝑃3 ―𝑥𝐸𝑀)/𝑥, where ExM+GeP3 and EGeP3 refer to the total energy of GeP3 with and without absorbed metal ions, respectively, EM represents the average energy of a metal atom in bulk metal and x denotes the number of absorbed metal atoms. One should note that the negative Ead indicates the dispersive distribution of adsorbed metal atoms instead of agglomeration, which implies that the issues caused by metallic dendrite formation, such as short-circuiting, can be avoided during battery operation. Table 1 shows that the Ead of all considered metal ions is negative, which indicates that adsorption is an exothermic process. Moreover, GeP3 has shown a superior affinity towards Na, K and Ca with Ead of −1.528 eV, −2.027 eV and −2.216 eV, respectively, which are comparable or even larger than the Ead of Li adsorption on GeP3 surface (−1.67 eV).39 In general, the Ead of Na, K and Ca adsorption is also considerably higher than the adsorption of corresponding metal ions on graphene, which are positive (Na 0.464 eV, K 0.785 eV, Ca 0.607 eV)46-48. A higher Ead is desirable to achieve high theoretical capacity and enhanced stability against the formation of metallic dendrites. In contrast, moderate adsorption energy for Mg and weak bonding of Al are evidenced due to Ead of −0.927 and −0.232 eV, respectively. The adsorption height h, defined as the vertical distance of the metal atoms to the topmost surface of the GeP3, is inversely proportional to the electronegativity of metal atoms and followed the given order: K > Na > Ca > Mg > Al. On the other hand, Δq has also shown a inverse relationship with electronegativity, except for Ca, and exhibited the given order: Ca > K > Na > Mg > Al. Importantly, the potential energy surface of GeP3 is quite rough, resulting in the emergence of various metastable adsorption sites for metal-ions in Table S1 (T2, H2 and H3 for Na, H2 for K, T4 for Ca and Mg, T4, H2 and H3 for Al). However, the energy difference between all metastable and the stable sites is less than 0.24 eV except for Al (0.41 eV), which indicate that the metastable sites are possible to be occupied during ions intercalation. As discussed in the remaining part of this manuscript, these metastable adsorption sites are capable of reducing the ionic diffusion barrier and increasing the theoretical specific capacity. Table 1 Calculated adsorption energies and structural properties of single Na, K, Ca, Mg and Al ion adsorption on GeP3 electrode.The transferred charge (Δq) from metal atoms to the electrode by 5



Hirshfeld and distances between Na, K, Ca, Mg, Al atoms and GeP3 electrode are also presented. Ead(eV)

Δq(e-)

h(Å)

Na

−1.528

0.402

1.214

K

−2.027

0.501

1.783

Ca

−2.216

0.559

0.773

Mg

−0.927

0.386

0.441

Al

−0.232

0.201

0.166

To gain a deeper understanding of electronic interactions between metal ions and GeP3 electrode, we have investigated the charge density difference between metal ions and GeP3, as presented in Fig 2. The charge density difference can be expressed as: Δ𝜌 = ρGe8P24M ― ρGe8P24 ― ρM, where ρGe8P24M, ρGe8P24, and ρM refer to the charge density of GeP3 layer with adsorbed metal atoms, pristine GeP3, and the metal atoms, respectively. The yellow and blue areas represent electron depletion and accumulation, respectively. For Na, K and Ca, obvious electron localization can be observed with remarkable electron accumulation around P atoms in GeP3 and depletion around the metal atoms, which indicates the formation of the strong ionic bond (Fig. 2). However, in case of Mg and Al, more delocalized features have been observed with a large portion of electron accumulation on GeP3, which weakened the electrostatic interactions between metal atoms and GeP3 anode (Fig. 2d-e). This might be the electronic origin of the higher Ead of Na, K and Ca and lower Ead of Mg and Al, as shown in Table 1.

Fig. 2 Top-(upper) and side-(lower) views of the charge density difference for (a) Na, (b) K, (c) Ca, (d) Mg and (e) Al adsorbed system (Ge8P24 M). The yellow and blue areas represent the charge depletion and accumulation with an iso-value of 0.011 e Å-3.

3.3 Metal atoms Diffusion on Monolayer GeP3 The diffusion barrier (Ebar) is an essential parameter to estimate the performance of the desired electrode, which becomes extremely important under high current rates. Given the structural symmetry of GeP3 electrode, the elementary migration pathways are determined with metal atoms diffusing from the most stable adsorption site to the nearest equivalent one, where metastable positions are considered to reduce the barriers. The optimized migration pathways and corresponding energy profiles are presented in Fig. 3. Herein, Al has shown the largest energy barrier of 0.645 eV, whereas Mg and Ca have shared the same migration pathway with a slightly smaller Ebar of 0.527 eV and 0.585eV, respectively. In contrast, the hopping resistance of Na and K is much shallower with Ebar of 0.27 eV and 0.287eV, which are comparable to several commonly studied anode materials, such as MoS2 (0.28 eV for Na),27 MoN2 (0.56 eV for Na, 0.49 eV for K),49 silicene (0.25 eV for Na)50 and TiS2 (0.59 eV for Mg)26. Moreover, these 6


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values are superior to the Li diffusion on GeP3 surface (0.5 eV).39 The low hopping resistance could deliver high ion mobility and favorable rate performance when GeP3 is utilized as the electrode.

Fig. 3 Schematic illustrations of the migration pathway with the corresponding transition-state configurations for (a) Na+, (b) K+, (c) Ca2+, (d) Mg2+ and (e)Al3+ on GeP3 electrode and (f) the corresponding diffusion barriers. The blue, cyan, chartreuse, gold and yellow ball represent the Na+, K+, Ca2+, Mg2+ and Al3+, respectively,.

3.4 Theoretical Capacity and average OCV of Monolayer GeP3 Further, we have calculated the open circuit voltage (OCV) and the theoretical capacity of GeP3 monolayer, as an anode material in NLIBs. To reveal the intercalation mechanism, the metal ions are stepwise added on both sides of GeP3 monolayer. It is worth noting that, metal-ions occupied the metastable sites due to the complicated potential energy surface of GeP3 electrode, as mentioned in the previous section. Thus, various feasible adsorption configurations are considered to determine the theoretical capacity and OCV. Given a practical computational cost, we employ a 1×1×1 GeP3 supercell to calculate the storage capacity of Na and Ca ions, whereas a 2×2×1 GeP3 supercell was used for K, Mg and Al ions. In the case of single-layer adsorption, the calculated Ead for Na, K, Ca, Mg and Al is −0.443 eV, −0.965 eV, −0.291 eV, −0.142 eV and −0.257 eV, respectively (Table 2). Most of the metal-ions preferred to occupy the most stable adsorption sites until they are completely occupied and then, occupied the metastable sites except for the Ca2+ ions as shown in Fig. S1. The occupation of metastable sites can significantly improve the capacity of NLSBs. For instance, Na ions preferred to occupy the most stable sites (T4) and the metastable sites (T2). However, when the number of intercalated Na ions exceeded the threshold limit, which is eight ions in the first layer, the extra atom spontaneously moved to the second layer. Meanwhile, the wrinkle height of GeP3 decreased from 2.38 to 1.52 Å with a lattice expansion of 2.9%, as shown in Table 1. Nevertheless, the structure of GeP3 remained intact during Na intercalation and did not exhibit any bond breakage. Similar observations have been made for K ions, where sixteen K were 7



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stored in the first layer of 2 × 2 × 1 GeP3 supercell and when the number of added atoms exceeded the threshold limit, the extra atoms would spontaneously move to the second layer. On the other hand, Ca ions occupied newly emerged T2 sites at high concentration of Ca ions instead of H2 sites under the dilute case. After fully occupying the T2 sites, Ca ions started to reside on the metastable T4 sites. Moreover, dGeP3 further decreased to 1.496Å along with a significant lattice expansion of 5.9% (Table 1). However, it has been observed that all the metastable sites did not contribute to the intercalation of metal ions. For instance, in the case of Mg, only the most stable H2 sites are occupied by Mg ions, whereas metastable sites remained empty. Moreover, Mg ions preferred to adsorb in an asymmetrical manner, which is completely different from the symmetrical adsorption of Na, K and Ca ions on both sides of GeP3 monolayer. Due to the strong bonding of Mg to GeP3 electrode, some Ge and P atoms moved outward from the GeP3 plane, resulted in an increased dGeP3 of 3.042 Å. Similar to Ca ions, Al ions also occupied the most T2 sites. However, the GeP3 monolayer only accommodated three Al ions, which implied that Al storage capacity of GeP3 is extremely low as compared to other non-Li ions. Table 2 Calculated Ead for the saturated adsorption in the first layer, the wrinkle height (d) of GeP3 upon adsorption and the corresponding lattice expansion (ΔZ). Ead(eV)

d(Å)

ΔZ(%)

Na

-0.443

1.52

2.9

K

-0.965

2.291

1.6

Ca

-0.291

1.496

5.9

Mg

-0.142

3.042

3.0

Al

-0.257

2.599

-

Furthermore, we have estimated the maximum storage capacity of GeP3 as an anode material for NLSBs. It has been reported Ead values should be larger than −0.10 eV to avoid the formation of notorious metallic dendrites.31 Based on this criterion, Ca, Mg and Al are only capable of single-layer intercalation with a capacity of 1295.42 mAh g-1 (658.03 mAh g-1), 323.86 mAh g-1 (282.39 mAh g-1), and 182.17 mAh g-1 (171.67 mAh g-1), respectively. The values given in the parentheses are the capacity values including the weight of metal ions as calculated by previous reports13, 51. Na and K exhibited a negative Ead value of −0.103 eV and −0.116 eV for bilayer adsorption in 1×1×1 and 2×2×1 GeP3 supercell, respectively. Herein, the bilayer adsorption was considered by using the given relationship: 𝐸𝑎𝑑 = (𝐸𝑥𝑀 + 𝑦𝑀 + 𝐺𝑒𝑃3 ―𝑦𝐸𝑀 ― 𝐸𝐺𝑒𝑃3 + 𝑥𝑀)/𝑦, where y refers to the number of metal ions in the adjacent layer(s) and the energy of pristine GeP3 is substituted by metalized GeP3 as the reference, as described in previous works.13, 31 As a result, the maximum storage capacity of GeP3 as an anode material for Na and K ion batteries are determined to be 1295.42 mAhg-1 (613.59 mAhg-1)

and 485.78 mAhg-1 (284.3 mAhg-1), respectively. Fig. 4 presents the

configuration of fully intercalated GeP3 monolayers with Na, K, Ca and Mg ions. Even though the deviation exists between different calculation approaches, the storage capacities of GeP3 are superior to most of other 2D anode materials, such as Ti3C2 (Na 351.8 mAhg-1, K 191.8 mAhg-1, Ca 319.8 mAhg-1),22 Nb2C (Na 271 mAhg-1, Ca 271 mAhg-1),52 GeS (Na 512 mAhg-1, K 256 mAhg-1)25, MoS2(Na 146 mAhg-1)27, and comparable to some commonly studied electrodes, such as MoN2(Na 864 mAhg-1)49, 8


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phosphorene(Na 865 mAhg-1)53, Ca2N(Na 1138 mAhg-1)54.

Fig. 4 Side-(upper) and top-(lower) views of the fully intercalated GeP3 monolayers with a stoichiometry of (a) Ge2P6Na16, (b) Ge8P24K24, (c) Ge2P6Ca8 and (d) Ge8P24Mg8. The light blue and pink areas indicate the metal-ions adsorbed in the first and second layers. The blue, cyan, chartreuse and gold balls represents the Na+, K+, Ca2+ and Mg2+ ions,

respectively.

Fig. 5a presents the calculated OCVs of GeP3 electrode for Na, K Ca and Mg ion batteries. The average OCV for single and bilayer intercalation of Na ions is −0.44 V and −0.27 V, respectively. The average OCV values for single and bilayer intercalation of K are a little higher than Na, which are -0.97 V and -0.68 V, respectively. In the case of single-layer adsorption for Ca, Mg and Al, the average OCV values are −0.15 V, −0.07 V, −0.09 V, respectively. Moreover, the OCV of K and Mg rapidly decreased and nearly no charging or discharging platform is observed. This type of charge/discharge behavior is not suitable for steady operation of NLSBs. On the other hand, the OCV of Na and Ca decreased more smoothly and exhibited certain plateaus, which are desirable for efficient and stable battery system. The conductivity of electrode is estimated by calculating the local density of states (LDOS), as shown in Fig 5 b-f. Similar to other 2D counterparts,20, 25, 53 we have also observed a semiconductor to metal transition upon Na, K, Ca, Mg and Al intercalation with a remarkable number of electronic states pinned at Fermi level.

Figure 5 (a) The simulated OCV profiles of GeP3 as an anode material for Na, K, Ca and Mg ion batteries. (b)-(f) calculated 9



LDOS of GeP3 upon saturated ions intercalation. The Fermi level is set at zero.

3.5 Electrode Stability and Na-ion Diffusion at the Phase Boundary Owing to the high theoretical capacity and favorable OCVs of Na, Mg and Ca ion batteries, we have investigated the structural stability of fully intercalated GeP3 electrode at elevated temperature. AMID simulations are carried out in canonical ensemble (NVT) at 300K and final electrode structures, after 3000 fs, are presented in Fig. 6. One should note that no significant deformation or bond breakage have been observed in GeP3 electrode during Na and K intercalation, excepting some thermal vibrations of Ge and P atoms have been observed. The fluctuation of potential energy is smaller than 0.018 eV and 0.016eV, respectively (Fig. 6a-b). However, the obvious structural collapse happened during the intercalation of Ca ions due to the breakage of G-P bonds, as shown in Fig. 6c, which implied that Ca ions are mainly stored via conversion mechanism rather than intercalation one.

Fig. 6 Top and side views of the intercalated GeP3 electrode by (a) Na ions, (b) K ions and (c) Ca ions at the end of AMID simulations, performed at 300K. The fluctuation of potential energy as a function of simulation time is also presented.

These results indicate that GeP3 is dynamically stable with both steady OCV and shallow diffusion barrier, which make it promising anode material for Na-ion battery. It has been reported that the minimum hopping barrier is highly sensitive to the neighboring ions in bulk cathode material, which implies that collective ionic transport should be considered when ionic transport, at the phase boundary, is significantly larger than the extreme dilute limit.55 Hence, we have investigated the hopping dynamics of Na in GeP3 at the phase boundary to provide a comprehensive understanding of the charge/discharge process. Initially, the system was partially sodiated, where a portion of the periodic cell consists of NaGeP3 phase and the remaining portion is GeP3 phase (Fig. 7). To simulate the phase boundary, we have considered the intercalation of both single and bilayers Na ions on the half electrode whereas the other half of the slab remained as GeP3 electrode. The leading Na-ion hopping from NaGeP3 phase to the GeP3 phase are calculated, as illustrate in Fig. 7a. The energy barriers for Na-ion during the first hop are 0.192 eV for path-I, 0.134 eV for path-II and 0.381 eV for path-III (first-layer sodiation) and 0.214 eV for path-I and 0.221 eV for path-II (second-layer sodiation), which are a little shallower than the energy barriers in dilute concentrations. Therefore, the surrounding ions have negligible influence on the hopping of Na ions, which is in sharp contrast to the observation in bulk electrode.55 This also validates the high rate performance of GeP3, at both high and low concentrations as an anode material in Na ion battery. Moreover, the results 10


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suggest that a partially sodiated electrode prefers well-distributed phase formation instead of separation or clustering at nanoscale, which is beneficial for steady operation of a battery system.

Fig. 7 Schematic illustration of the Na-ions migration at the phase boundary of intercalated and prinstine GeP3 electrodes in the case of (a) single layer (blue area) and (b) bilayer (pink area) intercalation. (c) The as calculated hopping barriers of Na ions for path I-III (a) and path I-II (b).

4 Conclusions In summary, extensive first-principles computational investigations have been carried out to determine the electronic structure, adsorption, diffusion and storage behavior of 2D GeP3 monolayer as an anode material for secondary (Na+, Mg2+, Al3+, K+ and Ca2+) ion batteries. The result reveals that the investigated metal ions prefer to disperse on GeP3 electrode surface and avoid the metallic dendrite formation. Furthermore, the intercalation mechanism of metal-ions is determined by calculating their energetic occupation sites and hopping dynamic on electrode surface. It has been observed that metastable adsorption sites play a critical role in improving the storage capacity and reducing the hopping energy barrier. In addition, low hopping barriers of 0.27 eV, 0.287 eV, 0.585eV, 0.527 eV and 0.645eV for Na, K, Ca, Mg and Al ions indicate the moderate charge/discharge rate. Moreover, GeP3 exhibited a high theoretical Na ion capacity of 613.59 (1295.42 mAhg-1) and steady charge/discharge curves. Interestingly, Ca ions have also shown a high theoretical charge storage capacity of 658.03 mAh g-1 (1295.42 mAh g-1), however, the charge storage mechanism was found to be conversion dominated process instead of intercalation. Moreover, a transition from semiconductor-to-metal has been observed due to the intercalation of metallic-ions (Na, K, Ca, Mg and Al) in GeP3 electrode, which reduces the internal resistance-based ohmic heat and avoids thermal runaways. Owing to the high capacity, favorable ion hopping barrier and robust structural stability, we conclude that GeP3 holds great potential as a high-performance anode material for non-Li ion batteries, in general, and Na-ion batteries, in particular.

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Corresponding Author *Xianfei Chen, E-mail: [email protected]. ORCID: Xianfei Chen: 0000-0002-5078-3950 *Yi Huang, Email: [email protected]

Conflicts of interest There are no conflicts to declare.

Supporting information The calculated Ead values for metal atoms on GeP3 surface at all considered adsorption sites; top and side

view of Na, K, Ca, Mg and Al adsorption in the first-layer of GeP3.

Acknowledgments We acknowledge the supports from china postdoctoral science foundation (2017M623306XB), National Natural Science Foundation of China (41673109) and Supported from Sichuan Science and Technology Program（2018SZDZX0022 and 2017SZ0185).

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Fig. 1 (a) Top and (b) side view of the crystal structure of a GeP3 monolayer with possible adsorption sites for metal ions. The orchid and green balls represent the P and Ge atoms, respectively. The hexagonal unit cell is enclosed in the blue dashed line. (c) The calculated band structure of GeP3 at PBE level.



Fig. 2 Top-(upper) and side-(lower) views of the charge density difference for (a) Na, (b) K, (c) Ca, (d) Mg and (e) Al adsorbed system (Ge8P24 M). The yellow and blue areas represent the charge depletion and accumulation with an iso-value of 0.011 e Å-3.


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Fig. 3 Schematic illustrations of the migration pathway with the corresponding transition-state configurations for (a) Na+, (b) K+, (c) Ca2+, (d) Mg2+ and (e)Al3+ on GeP3 electrode and (f) the corresponding diffusion barriers. The blue, cyan, chartreuse, gold and yellow ball represent the Na+, K+, Ca2+, Mg2+ and Al3+, respectively.



Fig. 4 Side-(upper) and top-(lower) views of the fully intercalated GeP3 monolayers with a stoichiometry of (a) Ge2P6Na16, (b) Ge8P24K24, (c) Ge2P6Ca8 and (d) Ge8P24Mg8. The light blue and pink areas indicate the metal-ions adsorbed in the first and second layers. The blue, cyan, chartreuse and gold balls represents the Na+, K+, Ca2+ and Mg2+ ions, respectively.


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Figure 5 (a) The simulated OCV profiles of GeP3 as an anode material for Na, K, Ca and Mg ion batteries. (b)-(f) calculated LDOS of GeP3 upon saturated ions intercalation. The Fermi level is set at zero.



Fig. 6 Top and side views of the intercalated GeP3 electrode by (a) Na ions, (b) K ions and (c) Ca ions at the end of AMID simulations, performed at 300K. The fluctuation of potential energy as a function of simulation time is also presented.


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Fig. 7 Schematic illustration of the Na-ions migration at the phase boundary of intercalated and prinstine GeP3 electrodes in the case of (a) single layer (blue area) and (b) bilayer (pink area) intercalation. (c) The as calculated hopping barriers of Na ions for path I-III (a) and path I-II (b).


Two-Dimensional GeP3 as a High Capacity Anode Material for Non

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