Dual Interphase Layers In Situ Formed on a Manganese-Based Oxide Cathode Enable Stable Potassium Storage
Mn-based oxides have attracted extensive attention as electrode materials. However, the irreversible phase transition and Mn 2+ dissolution result in their structure instability and performance decay. Here, we report dual interphase layers in situ formed on P2-K 0.67 MnO 2 (P2-KMO) in 6.0 M of potassium bis(fluorosulfonyl)amide in diglyme (KFSI/G2) during charging. It is composed of a solidelectrolyte interphase (SEI) and K-poor spinel interlayer on P2-KMO, which are derived from the simultaneous decomposition of 6.0 M KFSI/G2 and disproportionation of surface Mn 3+ . They cooperatively enable the reversible phase transition of P24P 00 2 in the bulk P2-KMO and mitigate Mn loss. This leads to a high capacity retention of 90.5% and a Coulombic efficiency of 100% after 300 cycles. The investigation highlights the significance of interphase chemistry of electrode materials for potassium-ion batteries and beyond.
INTRODUCTION<!>Structure of P2-KMO<!>Reaction Mechanism<!>Electrochemical Performance<!>Interface Analysis<!>DISCUSSION<!>EXPERIMENTAL PROCEDURES<!>Electrochemical Measurements<!>Computational Details
<p>Potassium-ion batteries (KIBs) have attracted increasing interests for large-scale electric energy storage due to the abundant resources and low redox potential of K. 1,2 However, the large-sized K + (1.38 A ˚in radius) usually leads to structural damage of electrode materials during its intercalation/extraction and performance decay. 3,4 Stable electrode materials, especially cathodes that determine the energy density, are of urgency for the development of KIBs. 5 Layered transition metal oxides of K x MO 2 (0 < x % 1, M = transition metal) are promising for KIBs owing to their high volumetric energy density. [6][7][8][9] However, they often suffer from dramatic volume variation (ca. 20%) because of gliding of transition metal layers or/and loss of transition metals in cycles, which are common for its counterparts Li and Na. 10 Stabilizing the layered structure of K x MO 2 for reversible intercalation/extraction of K + is quite challenging and important for the understanding of solid-state electrochemistry.</p><p>Mn-based oxides have been widely studied as electrode materials for Li and Na storage due to their low cost and environmental friendliness. However, they are prone to the Jahn-Teller distortion in the presence of Mn 3+ and its simultaneous disproportionation (2Mn 3+ /Mn 2+ +Mn 4+ ). [11][12][13] These result in an asymmetric change in the Mn-O bond lengths of [MnO 6 ] octahedra in the [MnO 2 ] layers and dissolution of Mn 2+ into electrolytes. 12,13 To alleviate these problems, various strategies have been applied to promote the stability of layered Mn-based oxides. It is reported that the dopants of mono-or divalent cations, such as Li + , Mg 2+ , Cu 2+ , or Zn 2+ , can tune the average oxidation state of Mn and its chemical environment, and hence favor the stability of Na x MnO 2 . [14][15][16] Oxides, fluorides, or phosphates were coated onto the surface of Li(Na) x MnO 2 to mitigate the Mn loss and volume change. [17][18][19] The Bigger Picture Emerging battery technologies are indispensable to ease the growing pressure of clean and renewable energy as well as largescale energy storage driven by environment preservation and industrialization. Rechargeable potassium-ion batteries (KIBs) have attracted extensive attention owing to the abundant resources and low redox potential of K. As a key component that determines the energy density, stable cathodes are of urgency for the development of KIBs. Here, a dual interphase structure of SEI film and spinel interlayer is in situ formed and cooperatively enables stable storage of K in P2-KMO in 6.0 M KFSI/G2 for KIBs. This will provide insights into the design of high-performance electrode materials for KIBs and beyond, and promote the development of solid-state electrochemistry.</p><p>Although these are beneficial for the interfacial reactions and cycle stability, they seem to be inapplicable to the counterpart of K x MnO 2 . Therefore, new strategies for tuning the bulky and interfacial structures for the reversible redox reactions are necessitated. The interplay of electrode and electrolyte and the resulted interfacial structure play significant roles in the cycle stability of electrodes. Ether-based electrolytes are reported to favor stable anode, 20 which is rarely reported at cathodes. The elucidation of the interfacial chemistry of K x MnO 2 and ether-based electrolytes of various concentrations is crucial for understanding the stability of layered Mnbased oxides.</p><p>Herein, we investigate the interfacial property of P2-KMO in the ether-based electrolytes with different concentrations and reveal a dual interfacial structure in situ formed on the surface of P2-KMO in the electrolyte of 6.0 M of potassium bis(fluorosulfonyl)amide in diglyme (KFSI/G2). It consists of an inactive, K-poor spinel interlayer and an organic/inorganic solid-electrolyte interphase (SEI) film. They are derived from the reconstruction and disproportionation of the surface Mn 3+ , and the simultaneous decomposition of FSI À and G2 at high voltages. These enable reversible redox reactions in the bulk of P2-KMO and prevent the loss of Mn. This results in excellent electrochemical performance of P2-KMO with capacity retention and Coulombic efficiency of up to 90.5% and 100% at 50 mA g À1 , respectively, for 300 cycles.</p><!><p>The P2-KMO was synthesized via sol-gel method, as described in the Experimental Procedures. Figure 1A shows the X-ray diffraction (XRD) pattern and Rietveld-refined profile of P2-KMO. All the diffraction peaks are assigned to P2-KMO with hexagonal unit cells in a space group of P63/mmc. 21 The refinement matches well with the experimental data points, giving lattice parameters of a = b = 2.843 A ˚and c = 12.350 A ˚, as listed in Table S1. These imply a typical layered P2-type structure in P2-KMO, which is consisted of alternated [MnO 2 ] and K layers in an oxygen stacking sequence of ABBAAB, as displayed in Figure 1B. The distance between two adjacent [MnO 2 ] layers is 4.25 A ˚, which is much larger than the size of K + and favors its diffusion. As depicted in Figure 1C, K + sits in two kinds of trigonal prismatic positions: one is face-sharing (K f , 2b Wyckoff site) and the other is edge-sharing (K e , 6h Wyckoff site) with the lower and upper [MnO 6 ] octahedra, the ratio of which is 1:3. 21 The K e sites are off-center, and each one is shared by three K + . The scanning transmission electron microscopy (STEM) and scanning electron microscopy (SEM) images in Fig- ures 1D and S1 indicate that P2-KMO is of an irregular shape with micrometers in size and smooth surface. Energy dispersive spectroscopy (EDS) elemental mappings in Figures 1E-1G confirm the uniform distribution of K, Mn, and O. The well-developed lattice fringes of P2-KMO in the high-resolution transmission electron microscopy (HRTEM) image of Figure 1H suggest its high crystallinity. The d-spacing of 0.63 nm is consistent with the distance between the two adjacent Mn layers in Figure 1B and corresponds to the typical facet (002) of the P2 phase.</p><!><p>The electrochemical properties of the layered P2-KMO were investigated in CR2032 coin cells with K metal as the anode and 6.0 M KFSI/G2 as the electrolyte. The voltage window is optimized to be 1.7$4.0 V, as shown in Figure 2A. Higher voltages induce the structural instability and quickly decaying capacities of P2-KMO in Figure S2, because the electrolyte is confirmed to be stable in the applied voltage window in a half cell of K//(6.0 M KFSI/G2)//Al in Figure S3. As exhibited in Figure 2A, the charge and discharge capacities in the first cycle are 45.5 and 90.6 mAh g À1 , respectively, corresponding to the extraction of 0.19 K + and insertion of 0.36 K + . This means that 0.17 K + should be compensated via pretreated anodes or addition of reagents into electrolytes and cathodes to assemble a full K-ion battery with a graphite anode. The re-inserted excessive K + and the following reversible extraction are occupying the vacant K e sites of P2-KMO in Figure 1C. This agrees well with insertion of K + into the vacant sites of the pristine P2-KMO in Figure S4. In the following cycles in Figure 2A, the charge/discharge capacities remain 90.6 mAh g À1 with high Coulombic efficiencies of 100%. Three charge/discharge plateaus at 2.81/2.39, 3.09/2.79, and 3.81/3.52 V are attributed to the Mn 3+ /Mn 4+ redox couple. Figure 2B shows the cyclic voltammetry (CV) curves in the initial three cycles at 0.1 mV s À1 . The overlapped CV curves suggest reversible extraction and insertion of K + from and into P2-KMO. Consistent with the charge/discharge curves, the three couples of redox peaks can be observed at 2.54/2.27, 2.88/2.81, and 3.91/3.48 V, respectively. The redox peaks at 3.91/3.48 V may be assigned to phase transition, and the other two are related to K + /vacancy ordering in the K layers. 22 In situ XRD was employed to elucidate the structural evolution of P2-KMO during depotassiation/potassiation. In Figures 2C and S5, the diffraction peaks at 13.3 and 26.2 at the pristine state of P2-KMO are assigned to its characteristic facets of (002) and (004), respectively. Upon charging, the two diffraction peaks begin to shift toward low angles, suggesting an expansion of the distance between adjacent oxygen layers owing to the increased electrostatic repulsion. 23 The diffraction peak (004) splits into two asymmetrical ones in the deep charge stages, indicative of two-phase reactions. The new diffraction peak at 25.6 is assigned to the P 00 2 phase, 24,25 which gradually dominates with the decreasing P2 phase. At charge finish, the P2 phase is completely converted into P 00 2 phase. The reverse-phase evolution is monitored in the discharging process in Figure 2C. These indicate that the reversible reaction of P2-KMO involves K + intercalation/deintercalation and then phase transition of P24P 00 2, which are in agreement with the charge/discharge profiles in Figure 2A.</p><p>Rietveld refinement on the charged state of P2-KMO, namely P 00 2-KMO, is performed to reveal its crystal structure, as shown in Figure S6 and Table S2. It belongs to the same hexagonal system as the pristine P2-KMO with a space group of P63/mmc. The P 00 2 phase maintains an oxygen stacking sequence of ABBAAB, as depicted in Figure S7. The refined unit cell parameters are a = b = 2.872 A ˚and c = 12.937 A ˚. The increased lattice parameter ''c'' in comparison with that of P2 phase is corresponding to the expansion of two adjacent [MnO 2 ] layers from 4.25 to 4.45 A ˚upon extraction of K + . The Mn-O bond lengths are elongated from 1.902 and 1.903 A ˚to 1.940 A ˚in Figure 2D. Rietveld refinements on the discharged P2-KMO in Figure S8 and Table S3 indicate volume variation of 9.9%, which is beneficial to the structural stability of P2-KMO in cycles. In the ab planes of Figures 1C and  S7B, the number of K + sharing one K e site changes from 3 to 1 in the P 00 2 phase, suggesting preferential extraction of K + from the K e sites upon charging due to the large in-plane K + -K + electrostatic repulsion.</p><!><p>The cycling performance of P2-KMO in 6.0 M KFSI/G2 at a charge/discharge rate of 50 mA g À1 is displayed in Figure 3A. It delivers an initial capacity of 78.0 mAh g À1 . High capacity retention of 90.5% and Coulombic efficiency of 100% can be achieved after 300 cycles, indicating its excellent stability. The selected charge/discharge profiles over 100 cycles are illustrated in Figure 3B to present the consistent voltages. Rietveld refinements of the P2-KMO electrode after 100 cycles in Figure S9 and Table S4 indicate a volume variation of 10.1%, suggesting high cycling stability. It is found that the electrolyte composition plays a crucial role in cycling performance, as depicted in Figures 3A and S10-S14. Namely, in the carbonate-based electrolytes, P2-KMO presents quickly decaying capacities over cycles, and it shows improved cycling stability in the G2-based electrolytes. The employed 6.0 M KFSI/ G2 is not only resistant to high voltages but also beneficial for the construction of stable interphase with P2-KMO. As depicted in Figure 3C, P2-KMO in this electrolyte exhibits good rate capability. Specific capacities of 88.3, 89.0, 86.2, and 77.7 mAh g À1 are obtained from 20 mA g À1 to 50, 100, and 200 mA g À1 , respectively, of which the charge/discharge profiles are shown in Figure S15. The good rate performance is related to the high K + diffusion coefficients with orders of magnitude from 10 À11 to 10 À10 by galvanostatic intermittent titration technique (GITT), as illustrated in Figures 3D and S16, which are higher than that of commercial LiFePO 4 . 26 Such superior electrochemical performance has rarely been reported in cathode materials for KIBs, as compared in Table 8, [27][28][29][30][31][32]</p><!><p>The excellent electrochemical performance of P2-KMO is linked with the electrolyte of 6.0 M KFSI/G2. Understanding the interfacial properties will enable elucidation of the reversible behavior of P2-KMO, which empirically suffers from the Jahn-Teller distortion and loss of Mn 2+ . [11][12][13] A uniform SEI film is formed on surface of the discharged P2-KMO in 6.0 M KFSI/G2, as depicted in Figure S17. Its chemical composition is analyzed by X-ray photoelectron spectroscopy (XPS) in Figure 4. In the C1s spectra of Figure 4A, the peak at 283.2 eV stems from Super P, and the two ones at 284.9 and 289.7 eV correspond to ÀCH 2À and ÀCF 2À of the binder polyvinylidene fluoride (PVdF), respectively. After discharge, the peak of Super P is weakened with the appearance of polyether and RCH 2 OK species at 285.5 and 286.7 eV, suggesting a formation of SEI film on the surface. They are originated from decomposition of G2 molecules, as witnessed by the peaks at 531.7 and 532.6 eV in the O1s spectra of Figure 4B. The small peak at 291.9 eV is ascribed to ÀCF 3 from the decomposition of FSI À anions, which is also confirmed by the ÀSO xÀ species at 531.0 eV in the O1s spectra. 33 KF is evidenced by the peak at 682.9 eV in Figure 4C as well as the existence of N and S in Figures 4D and  4E. The N-containing species are ascribed to KFSI and KSON, and KFSI, K 2 SO 4 , K 2 SO 3 , and K 2 S 2 O 3 are responsible for the four peaks at 169.3, 168.2, 167.4, and 166.2 eV, respectively, in Figure 4E. 34 It is speculated that the SEI film on P2-KMO in 6.0 M KFSI/G2 is composed of inorganic salts (KF, KSON, KFSI, K 2 SO 4 , K 2 SO 3 , and K 2 S 2 O 3 ) and organic components (polyether and RCH 2 OK). This hybrid layer is robust and highly K + conductive to ensure high electrochemical performance. Figure 4F shows the Mn2p spectra, in which the subtle change indicates the presence of Mn 2+ after discharge.</p><p>In contrast, the SEI film in the carbonate-based electrolyte mainly consists of polyester, ROCO 2 K, and K 2 CO 3 , as revealed in Figure S18. The polyester and ROCO 2 K are unstable and decompose to result in damage of the SEI film. 20,35 The broken interphase induces more side reactions for formation of the uneven SEI film in Figure S17B and severe Mn dissolution. 36 The induced distortion and damage of crystal facets on the surface of P2-KMO after 100 cycles in the carbonate-based electrolyte are revealed by the HRTEM image in Figure The increasing characteristic absorption of Mn 2+ at 458 nm is captured in the ultraviolet (UV)-visible absorption spectra of the carbonate-based electrolyte during 100 cycles in Figure S20, while no such absorption is detected in 6.0 M KFSI/G2. This means that the uniform and robust SEI film formed in 6.0 M KFSI/G2 can suppress Mn 2+ loss.</p><p>The interphases of P2-KMO in 6.0 M KFSI/G2 at the pristine and discharged states are revealed by STEM with a high-angle annular dark-field (HAADF) detector. The Mn-atom columns are referred to the bright dots in the STEM-HAADF images in Figure 5 due to the large atomic weight. The pristine P2-KMO presents a typical atom arrangement of the P2 phase in Figures 5A and S21A. Dual interphase layers of a SEI film and an interlayer are evident in Figures 5B and S22, where a distinguishable phase boundary is visualized between the interlayer and the bulk in Figure S21B. The magnified atom arrangements in Figures 5C and 5D and the lattice fringes in Figure S22 indicate that the interlayer is reconstructed to a spinel phase from the P2 phase. The spinel interlayer begins to appear when charged to 3.7 V, as monitored in Figure S23. In comparison, no such interlayer is found in the carbonatebased electrolyte, 1.0 M KFSI/G2, or 3.0 M KFSI/G2, as revealed in Figures S19,  S24, or S25, indicating the critical role of concentration of KFSI/G2 on the formation of the dual interphase. The EDS elemental mappings of K, Mn, and O in Figures 5E-5G show that the spinel interlayer is K-poor and Mn-rich with a thickness of 10$30 nm, suggesting that it is inactive Mn 3 O 4 . The Mn 2+ identified in Figure 4F is not detectable in the Mn L-edge soft X-ray absorption spectroscopy (sXAS) in Fig- ure 5H, which may be covered by the majority of Mn 3+ and Mn 4+ due to the different detecting depths of the two techniques. It suggests the existence of Mn 2+ only in the spinel interlayer. However, Mn 2+ on P2-KMO in the carbonate-based electrolyte is detected in both the Mn2p XPS and Mn L-edge sXAS spectra in Figures S18F and  S26, which is derived from disproportionation of Mn 3+ due to the absent spinel interlayer. At the same time, a new phase of P 0 2 appears in Figure S27 with an increased Mn-O bond length of 2.019 A ˚due to the Jahn-Teller distortion, as illustrated in   37,38 The volume variation from the P2 to P 0 2 phase is up to 139%, which leads to decaying capacities of P2-KMO electrodes in carbonate-based electrolytes during cycles. These manifest that the spinel interlayer minimizes the effect of the Jahn-Teller distortion on the crystal structure of the bulk P2-KMO and guarantees its reversible (de)potassiation in cycles.</p><p>The evolution of dual interphase layers on P2-KMO in 6.0 M KFSI/G2 is schematically illustrated in Figure 6A. Upon charging, the surface K + is extracted to form vacancies, which are then occupied by partial Mn atoms due to the Jahn-Teller distortion and disproportionation of Mn 3+ . The resulted Mn 2+ moves to the 8a sites and is coordinated with four O atoms to form [MnO 4 ] tetrahedra, accompanied by gliding of the [MnO 2 ] layers and rearrangement of K + and vacancies; the Mn 3+ migrates to the 16d sites and coordinates with six O atoms to form [MnO 6 ] octahedra. 39 Simultaneously, the [MnO 2 ] layers glide to form an oxygen sequence of ABCABC with O atoms at the 32e sites, induced by a strong electronegativity of the aggregated FSI À and adsorption of G2. Density functional theory (DFT) calculations were carried out to investigate the adsorption of solvents on P2-KMO. For clarification, two simplified adsorption models, bridge and top, of solvents on the (002) facet of P2-KMO are applied in Figures 6B-6E and S29; Table S6. The adsorption energies are 0.41 and 0.40 eV in 6.0 M KFSI/G2, while they are only 0.08 and 0.10 eV in a propylene carbonate-based electrolyte. The quite strong interaction of 6.0 M KFSI/G2 with P2-KMO plays an essential role in the formation of the spinel interlayer. The [MnO 4 ] tetrahedra and [MnO 6 ] octahedra are reconstructed to a spinel interlayer via edge-and cornersharing as well as formation of a SEI film from the cooperative decomposition of FSI À and G2. A small quantity of K + at the 8a sites of spinel provides K + diffusion channels and ensures reversible redox reactions of the bulk P2-KMO. The Mn 4+ migrates to the interface between P2 and spinel. No oxygen is evolved in the charging process as monitored by gas chromatograph in Figure S30. At full charge, the thicknesses of SEI film and spinel interlayer are 10-30 nm in Figures 5 and S22. They do not increase and remain almost constant after 100 cycles in Figure S31. This suggests the essential role of the interaction between the electrolyte and P2-KMO on the formation of spinel interlayer, that is, a certain thickness of the spinel interlayer blocking the contact of the electrolyte and P2-KMO and restricting its further growth. The dual interphase layers can not only accommodate the Jahn-Teller distortion and mitigate Mn loss but also favors K + diffusion for redox reactions.</p><!><p>In summary, dual interphase layers of robust SEI film and inactive K-poor spinel interlayer on surface of P2-KMO realize the stable electrochemical performance of KIBs. The SEI film is derived from the decomposition of the aggregated FSI À anions and G2 molecules at high voltages, and the spinel interlayer is reconstructed from the Jahn-Teller distortion and disproportionation of Mn 3+ in the surface P2 phase. They cooperatively accommodate the distortion in the bulk phase and mitigate the Mn loss for the reversible phase transition of P24P 00 2 during cycles. This leads to reversible redox reactions and small volume change of 9.9% during (de) potassiation and enables its good rate capability and stable operation for 300 cycles with capacity retention and Coulombic efficiency of up to 90.5% and 100% at 50 mA g À1 , respectively. The versatile dual interphase layers on P2-KMO in 6.0 M KFSI/G2 provide insights into the design of high-performance electrode materials for KIBs and beyond and promote the development of solid-state electrochemistry.</p><!><p>Preparation of P2-KMO The P2-KMO was synthesized as follows. Firstly, 6.7 mmol of CH 3 COOK and 10.0 mmol of Mn(CH 3 COO) 2 ,4H 2 O were dissolved in 50.0 mL of deionized water. An excess of 15 wt % CH 3 COOK was added to compensate for potassium loss at high temperatures. Then, the above solution was heated at 80 C under stirring to obtain a gel and was dried overnight. The precursors were ground and pressed into a pellet under 20 MPa, which was calcined at 950 C in the air for 24 h. After cooling to 200 C, the obtained sample was transferred to an argon-filled glove box for storage.</p><p>Characterization XRD was measured on Rigaku SmartLab with Cu Ka radiation and refined with general structure analysis system (GSAS). SEM (JEOL JSM-7500F, 5kV), HRTEM (Philips Tecnai FEI, 200 kV), and STEM (FEI Titan G2 60-300, 300 kV) equipped with a probe spherical aberration corrector were used to characterize the microstructures of P2-KMO. XPS (Perkin Elmer PHI 1600 ESCA system) was employed to analyze surface properties. The synchrotron sXAS was measured at the beamline BL10B in the National Synchrotron Radiation Laboratory (NSRL) in Hefei, China. The dissolution of Mn 2+ in the electrolytes was analyzed by UV-visible absorption spectra (SPECORD 2010 plus, analytikjena). Before testing, the residual electrolyte on the electrodes was washed with dimethoxyethane in an argon-filled glove box.</p><p>Gas chromatograph (GC-2010 plus, Shimadzu) was applied to detect the loss of oxygen during cycles.</p><!><p>The electrochemical properties of P2-KMO were measured in CR2032 coin cells assembled in an argon-filled glove box (O 2 % 0.1 ppm, H 2 O % 0.1 ppm). The slurry of P2-KMO electrode was prepared by mixing 70 wt % of P2-KMO, 20 wt % of Super P, and 10 wt % of PVdF in N-methyl-2-pyrrolidone (NMP). Then, it was pasted onto an Al foil and dried at 80 C in a vacuum oven for 12 h. The electrode pellet of P2-KMO was punched to be 10 mm in diameter, on which the average mass loading of P2-KMO is 1.1 mg cm À2 . The electrolytes, 6.0 M KFSI/G2, 3.0 M KFSI/G2, 1.0 M KFSI/G2, and 6.0 M of KFSI in ethylene carbonate (EC)/diethyl carbonate (1:1, v/v) with the addition of 5 wt % of fluoroethylene carbonate (FEC) (6.0 M KFSI/EC/DEC/FEC), 1.0 M KFSI/EC/DEC/FEC, and 0.8 M of KPF 6 in EC/DEC (1:1, v/v) with 5 wt % of FEC (0.8 M KPF 6 /EC/DEC/FEC), were prepared in the glove box. 60.0 mL of the electrolyte was used in each coin cell, and K foils were applied as anodes. CV was performed on a Solartron 1470E electrochemical workstation. Galvanostatic discharge/charge tests and GITT were carried out on Land CT2001A battery instrument. In a GITT test, the charge and rest time was 20 mins and 1 h, respectively, and the applied current is 20 mA g À1 . The ionic diffusion coefficient was calculated based on the following equation:</p><p>where m B , M B , and V M are the mass, molar mass, and molar volume of P2-KMO, respectively; S is the geometric area of an electrode; DE s , DE t , and t are shown in Figure S16. All the electrochemical tests were performed at room temperature.</p><!><p>The adsorption energies of G2 and propylene carbonate solvent on P2-KMO are calculated by density functional theory (DFT) calculations, 40 as implemented in the Vienna Ab-initio Simulation Package (VASP). [41][42][43] The exchange-correlation function is described within the projector augmented wave (PAW) method 44,45 and generalized gradient approximation with the function of Perdew-Burke-Ernzerhof (GGA-PBE). 46 A P2-KMO supercell of 43431 is constructed with a 15 A ˚vacuum layers. A 23231 k-point mesh is used for all the calculations. The cutoff energy is 400 eV. The convergence criterion of the total energy and the residual Hellmann-Feynman force are 10 À5 eV/atom and 0.035 eV/A ˚, respectively.</p>