Phase Transformation and Diffusion Kinetics of V2O5 Electrode in

Article pubs.acs.org/JPCC

Cite This: J. Phys. Chem. C 2018, 122, 1513−1521

Phase Transformation and Diffusion Kinetics of V2O5 Electrode in Rechargeable Li and Mg Batteries: A First-Principle Study Renchao Xiao, Jian Xie, Ting Luo, Liting Huang, Yan Zhou, Danmei Yu, Changguo Chen, and Yuping Liu* College of Chemistry and Chemical Engineering, Chongqing University, Chongqing, 400044, China ABSTRACT: Using the density functional theory (DFT), first-principles computations were performed to investigate the intercalation site, phase transformation, electronic properties, and migration energy barrier of Li and Mg atoms in V2O5 to thoroughly illuminate the phase transformation and microscopic interaction as well as the migration mechanism of Li and Mg in V2O5. It is found that Li and Mg atoms prefer to locate at the site that is above, near the center of the quadrilateral composed of four V atoms. With the increase of the intercalation concentration (0 ≤ x ≤ 1), V2O5 undergoes a structural transformation from α-phase to ε-phase and δ-phase. Compared with ε-M0.5V2O5 (M = Li/Mg), the electronic conductivity of δ-MV2O5 (M = Li/Mg) is declined. On the basis of diffusion kinetics, Mg exhibits more difficulty in inserting and extracting in V2O5 than Li. This study can be useful for the further application of V2O5 in Mg ion batteries.

1. INTRODUCTION

improve the electrochemical performance of V2O5 in Mg ion batteries. Likewise, the intercalated-Mg mechanism of V2O5 has been recently expounded by the first-principles calculations.9−11 Zhao et al. adopted the first principles to study the properties of bulk and monolayer V2O5 in Mg ion batteries.12 They believed that the bulk and monolayer V2O5 were not very suitable as a cathode material for Mg ion batteries due to their high migration barriers. Zhou et al. found that the diffusion of Mg in α-V2O5 was a one-dimensional hopping process along the b axis.10 The migration barrier of Mg was 1.26 eV, higher than that of Li ion (0.91 eV) in α-V2O5. Even so, there is no common opinion on the intercalation site of guest ions in αV2O5.9,11,13−15 In α-V2O5, three sites are listed as follows: one site located between V and O1,15 one site located between two bridging oxygen O2,13,14 and another high-symmetry site with equal distances to four vanadyl oxygens (O1) and two bridging oxygens (O2),11 which is similar to the Wyckoff position 2b in α-type structures.10 Comparing to α-V2O5, δ-V2O5 shows better Mg mobility and higher voltage and is viewed as a promising cathode material for Mg ion batteries.9,11 Unfortunately, δV2O5 is thermodynamically stable only in the nearly fully discharged states6,12 and has difficulty in retaining its intrinsic structure at a wide intercalated-Mg concentration. Besides, Zhou et al. reported that the average voltage for Mg−V2O5 is higher than that for the Li−V2O5 system,10 which was not in

With the development of electronic devices, such as electric vehicles, a growing requirement for higher energy density devices is put forward substantially. Currently, more researchers focus on developing a new high-energy, low-cost, high-security multivalent ion battery, for example, Mg ion batteries,1 which have attracted much attention as promising candidates due to their environmentally friendly chemistry and high volumetric energy density (3833 mAh/cm3). However, the low voltage and slow kinetics of the cathode material are the main obstacles holding back development of Mg ion batteries. Among the current intercalation materials, layered V2O5, as an attractive electrode material, is characterized by high voltage and high capacity. V2O5 has three crystal structures, namely, α, ε, and δ phases. Their differences lie in the arrangements of atoms in the unit cells. Among them, α-V2O5 has been extensively used in Li ion batteries,2,3 Na ion batteries,4 and Mg ion batteries.5 In the Li ion batteries, the hollow microspheres V2O5 can deliver a specific capacity of 273 mAh·g−1 at 0.2 C.2 The phase transition may occur from α-V2O5 to the irreversible phase with the increasing of Li intercalation content.6 Surprisingly, the layered V2O5 exhibited the reversible Mg storage capacity of >150 mAh·g−1 at 2.3−2.6 V vs Mg2+/Mg.5 An unidentified new phase was examined upon magnesiation at Mg concentration of x ≤ 0.5. The relatively higher voltage plateau would render it for use as a potential cathode material in Mg ion batteries. Up to now, tremendous efforts have been dedicated to prepare various nanosized V2O5 xerogels, aerogels,7 and microcrystals,8 to © 2018 American Chemical Society

Received: November 22, 2017 Revised: December 28, 2017 Published: January 5, 2018 1513

DOI: 10.1021/acs.jpcc.7b11488 J. Phys. Chem. C 2018, 122, 1513−1521

Article

The Journal of Physical Chemistry C

3. RESULTS AND DISCUSSIONS 3.1. Geometric Structure. The graphene-like α-V2O5 is built up of layered units along the c-axis together with weak van der Waals forces, which is favorable for guest ion intercalation. In α-V2O5 (space group Pmmn),30 the layered unit consists of edge- and corner-sharing the distorted [VO5] pyramids in periodic arrangements. There are three types of inequivalent oxygen atoms: apical oxygens (O1) and bridging oxygen atoms (O2), as well as chaining oxygen atoms (O3), which act as bridges between three [VO5] square pyramids. Also, the crystal structure of ε-V2O5 is very similar to that of α-V2O5. Unlike αV2O5, the lattice parameter c of ε-V2O5 (space group Pmmn) is increased from 4.37 to 4.65 Å and a structural distortion of the V−O bonds is along the a-axis.31 As for δ-V2O5(space group Cmcm),32 the adjacent layers in the crystal are dislocated by half a unit cell parameter (b/2) along the b-axis compared to αV2O5. Three crystal structures of V2O5 are illustrated in Figure 1. The calculated lattice parameters and the experimental data are

good agreement with the previously published experimental data.2,16 Despite the above-mentioned progress, α-V2O5 still undergoes poor cathode utilization, small discharge specific capacity, and short cycle life in Mg ion batteries, which is probably related to the phase transformantion of α-V2O5 upon magnesiation. In Li ion batteries, the phase transformantion behavior greatly affects the electrochemical performance of αV2O5 with the increasing of Li insertion. The structurual evolution, electronic structure, and migration kinetics of αV2O5 during the discharge process of Mg ion batteries is not clear. Thus, a better understanding of the phase transformation and migration mechanism is indispensable for enhancing the Mg deintercalation/intercalation performance in α-V2O5. Herein, the thermodynamic and kinetic properties of V2O5 in Li and Mg ion batteries were systematically discussed by means of density functional theory (DFT). We mainly focus on intercalation site, formation energy, intercalation voltage, band structure, Mulliken charge, Mulliken population, electron density difference, and migration energy barrier to reveal the phase transformation and the microscopic interaction between the Li/Mg and O as well as the migration behavior in V2O5 polymorphs. This work achieves a fundamental understanding of the electrochemical magnesiation/lithiation mechanism, which is of significant importance to design and develop new cathode materials for Mg ion batteries with high-energy density.

2. COMPUTATIONAL METHODS In this work, all the calculations were performed using the Cambridge Serial Total Energy Package (CASTEP) code incorporated in Materials Studio 7.0.17 Ultrasoft pseudopotential was employed in our calculation. The exchange−correlation energy of electrons was described with Perdew−Burke− Ernzerhof (PBE) function in the generalized gradient approximation (GGA).18,19 A self-consistent field tolerance is set to 1.0 × 10−6 eV/atom and the cutoff energy for the planewave basis is 500 eV. The integral was applied by a 2 × 3 × 5 mesh in the reciprocal space. During the structural optimization, the convergence tolerances for the force and energy were less than 0.03 eV/Å and 10−5 eV/atom, respectively. Each single electron has a complex influence on its neighbors owing to their incompletely filled d- or f-electron shells. Then, a Hubbard U correction of 3.0 eV was added to exclude the self-interaction of d-electrons for vanadium in the GGA Hamiltonian (GGA+U).11,20,21 The calculated band gap (2.22 eV) was well-consistent with the experimental value (2.20 eV) at the Hubbard U parameter of 3.0 eV.22 To analyze the Li/Mg migration mechanism in V2O5, the linear synchronous transit and the quadratic synchronous transit (LST/QST) methods23 were used to search the transition states (TSs). We checked the minimum energy paths (MEPs) and confirmed the transition states with the nudged elastic band (NEB) method.24 It should be pointed out that a U term was not used in order to reduce the calculation time in the process of TS search and TS confirmation. It is difficult for structure optimization to be converged with GGA +U owing to its outstanding metastability of electronic states along the ion migration path. In addition, no evidence has been presented to obtain a higher accuracy of hopping energy with the GGA+U method.25−29 For the LST/QST method, the convergence criteria for the force and ionic displacement in the TS search were set as 0.05 eV/Å and 0.005 Å, respectively. For the NEB method, the convergence tolerance of the maximum energy was set to 10−5 eV/atom.

Figure 1. Crystal structure of α-V2O5 from (a) the (010) plane and (b) the (100) plane, ε-V2O5 from (c) the (010) plane and (d) the (100) plane, and δ-V2O5 from (e) the (010) plane and (f) the (100) plane. Blue balls represent the intercalated metal ions.

listed in Table 1. As can be seen in Figure 1 and Table 1, this result is consistent with the previous work.30−32 In this study, we define the longest axis of the lattice as the a-axis, the shortest axis as the b-axis, and the layer stacking direction as the c-axis. According to the literature,33 the crystal structure of αV2O5 can be stabilized only for x < 0.1 in the LixV2O5 system. From x > 0.1, the phase structure will gradually transit from αphase to ε-phase, which is stable in the range of x = 0.35−0.7.34 The pure δ-phase is produced at high Li concentration (0.88 < x < 1.0).35 Up to now, the crystallographic data of MgxV2O5 has not been reported in the previous literature as a result of the complex Mg−V−O system. By analogy with δ-LiV2O5, the δMgV2O5 structure was modeled by replacing Li atoms with Mg 1514


Article

The Journal of Physical Chemistry C Table 1. Experimental and Optimized Lattice Parameters of α-, ε-, and δ-V2O5 (Å) experimental parameters30,31 α-V2O5 ε-V2O5 δ-V2O5

optimized parameters (PBE)

a (Å)

b (Å)

c (Å)

a (Å)

b (Å)

c (Å)

space group

11.512 11.355 11.247

3.564 3.573 3.604

4.368 4.654 9.915

11.614 (0.80%) 11.463 (0.90%) 11.308 (0.50%)

3.540 (−0.60%) 3.533 (−1.10%) 3.602 (−0.05%)

4.445 (1.70%) 4.526 (−2.70%) 9.907 (−0.08%)

Pmmn Pmmn Cmcm

atoms in δ-LiV2O5. It should be noted that the intercalation voltage curves and the electronic properties were calculated with the optimized structure of Li and Mg atoms inserted into α-V2O5 lattice, which are defined as r-MxV2O5 (M = Li, Mg) and are different from the other literature.9−11 On the basis of these models, we want to intuitively simulate the structural evolution of α-V2O5 in the discharging process. The ε-Li0.5V2O5 and δ-LiV2O5 structures are constructed by 0.5 and 1.0 mol of Li insertion per unit cell of ε-V2O5 and δ-V2O5, respectively. The optimized lattice parameters of r-Li0.5V2O5 agree well with the experimental values of ε-Li0.5V2O5, while both r-LiV2O5 and δ-LiV2O5 have some differences in crystal structure and optimized lattice parameters, mainly originating from the first-principles calculation method, which does not affect our analysis. 3.2. Intercalation Sites. In order to accurately understand the intercalation sites, every possible insertion site was calculated. The most possible two sites are called P1 and P2, as seen in Figure 2, respectively. The calculated energies for Li

the resemblance between cyclic voltammogram of the Mg2+ and Li+ insertion processes.36 Additionally, small varations in lattice parameters before and after insertion of atoms into α-V2O5 suggest good structural stability of α-V 2 O 5 at dilute concentration. 3.3. Phase Transition. Although Zhao et al. has calculated the ground-state hull of metal ions in V2O5 and concluded that α-V2O5 transformed to the δ-phase upon Mg insertion,12 the formation of the ε-phase has not been confirmed during the discharge process. Therefore, in order to further clarify the phase transition of α-V2O5 upon Mg insertion, we calculated the formation energy of three V2O5 structures with the increasing of Li/Mg concentration. The formation energy was obtained from eq 1 defined as Eformation = Ex − (1 − x)E0 − xE1

(1)

The parameter x is the concentration of metal in the V2O5 cell, ranging from 0 (pure V2O5) to 1 (fully discharged state). E0, E1, and Ex are assigned to the total energy at the concentration of 0, 1, and x, respectively. Dependences of formation energy on the concentration (x = 0, 0.25, 0.5, 0.75, and 1) of Mg/Li insertion into three V2O5 phases are presented in Figure 3. From the thermodynamical viewpoint, the Mg/Li concentration greatly affects the structural stability of α-V2O5 during cycling, as shown in Figure 3. At x = 0.25, the formation energy of inserted-Mg/Li α-V2O5 is the lowest value at low concentration, indicating that the crystal structure of α-V2O5 can be maintained stably in the initial discharge state. At x = 0.5, Mg/Li intercalation into εV2O5 exhibits more stability than α-V2O5 and δ-V2O5, owing to their lower formation energy. This observation demonstrates the phase transformantion from α-phase to ε-phase as the Li/ Mg content increases. At the high concentration of x = 1, the formation energy of Mg/Li intercalation into δ-V2O5 is less than those of the other V2O5 two phases. In other words, the increase of Mg concentration results in the transformation from α-V2O5 to ε-phase at x = 0.5 and then to δ-phase in the full discharged state, similar to Li insertion. During the charging, the δ-phase may transform to the ε- and α-phases along with the subsequent removal process of Li/Mg. The strong V−O bonds have no significant changes after Li insertion and extration due to the reversible transformation of α → ε → δ phase in the range of x = 0−1,37 consistent with the experimental results.6,38 By virtue of the close ionic radius of Li and Mg atom, it can be inferred that V2O5 might transform

Figure 2. Atom configuration of Mg/Li insertion into α-V2O5: (a) along the (001) direction and (b) along the (100) direction. The P1 site is located between two bridge oxygen O2 atoms, and the P2 site is situated at the near center of the quadrilateral composed of four V atoms. Key: red, oxygen; blue, Mg or Li.

and Mg intercalation into α-V2O5 at the P1 and P2 sites are listed in Table 2. In Table 2, the total energy of insertion at the P2 site is lower than that of insertion at the P1 site (favored by −0.5 and −1.52 eV for Li and Mg, respectively). Thermodynamically, the structure of Li/Mg insertion into αV2O5 at the P2 site is more stable than that at the P1 site. This result implies that Li/Mg atoms prefer to occupy the P2 site in α-V2O5 at the dilute concentration, in agreement with a number of previous DFT studies6,18 and the experimental results.27 Imamura et al. pointed out that the Mg insertion site was the same as the Li insertion site in α-V2O5, resulting from

Table 2. Relative Energies and Optimized Lattice Parameters for Li and Mg Intercalation into α-V2O5 in the Different Sites coordinate Li Mg

optimized lattice parameters

site

a (Å)

b (Å)

c (Å)

P1 P2 P1 P2

0.25 0.25 0.25 0.25

0.625 0.375 0.625 0.375

0.35 0.35 0.35 0.35

a (Å) 11.562 11.461 11.490 11.420

(0.40%) (−0.40%) (−0.17%) (−0.70%)

1515

b (Å) 7.082 7.092 7.071 7.100

(−0.60%) (−0.50%) (−0.08%) (−0.40%)

c (Å) 4.554 4.547 5.142 4.731

(4.20%) (4.00%) (17.60%) (8.30%)

E (eV) 0.50 0 1.52 0


Article


Figure 3. Formation energy of Mg/Li insertion into α-V2O5, ε-V2O5, and δ-V2O5 at different concentrations.

from α-phase to δ-phase with the improvement of Mg intercalation up to x = 1. Significantly, it is found that the formation energies of MgxV2O5 systems are lower than those of LixV2O5 at the same concentration, implying that the structures of MgxV2O5 are more stable than those of LixV2O5. This may cause the difficult removal of Mg from MgxV2O5 upon demagnesiation. Note that only the phase transition of V2O5 in the range of 0 ≤ x ≤ 1 is considered. For α-V2O5, the irreversible transition sequence of δ → ω (x > 1) → γ (x > 2) occur at deep discharge in Li ion batteries.39 3.4. Average Voltage. Average voltage is a significant criterion to evaluate the electrochemical performance of V2O5 as cathode material for a Mg or Li battery. The average voltages of Mg/Li insertion into α-V2O5 are calculated according to eq 2 and reaction 3. V = ΔE /nz MxV2O5 + n M = M yV2O5

(2)

(M = Li, Mg)

(3) Figure 4. Calculated average voltage curves for MxV2O5 (M = Li, Mg).

Then, the following equation is deduced: ΔE = Echarge + nEM − Edischarge

by ∼0.2 V than those of other literature values,11,12 mainly attributed to different calculation methods. As for the Mg/V2O5 system, the average voltage in the range of 1.89−2.25 V vs Mg2+/Mg accords with other computational results12,40 and the experimental average voltage of ∼2.2V vs Mg2+/Mg.2,41 As can be seen, the inserted-Li voltage is higher than the intercalatedMg voltage in α-V2O5. Compared with other cathodes for Mg ion batteries, α-V2O5 is still attractive as a high-voltage cathode material because the average voltage of α-V2O5 can reach up to 2 V vs Mg2+/Mg. 3.5. Electronic Structure. On the basis of the forbidden bandwidth, the electrical conductivity of solid material can be determined from its band structure. According to the forbidden bandwidth, the solid materials are qualitatively divided into three classifications as follows: conductor, semiconductor, and insulator. Band structure can give us important information about the electronic structure of the lithiated/magnesiated V2O5. Band structure and band gap values of α-V2O5 and rMxV2O5 (M = Li or Mg) are displayed in Figure 5 and in Table

(4)

Here z is the electron transfer number (z = 1 for Li, 2 for Mg) and n is the intercalation number of the metal atom. The values of stoichiometry x and y are between 0 and 1; ΔE denotes the change in total energy; Echarge and Edischarge are the energy of the charged and discharged compounds, respectively; and EM is the calculation energy of the metal. Further, eq 2 can be turned into eq 5. V = (E MxV2O5 + nEM − E M yV2O5)/nz

(5)

The unit of ΔE is eV, so that the normalization factor (i.e., Faraday’s constant) is not introduced into eq 5. The average voltage curves of Li and Mg insertion into αV2O5 are depicted in Figure 4. Li and Mg are the reference states. In the Li/V2O5 system, the calculated average voltage ranges from 3.08 to 3.43 V vs Li+/Li. The voltage range is coincidental with the experimental values.2,3 At low Li concentration, the discharge voltage is 3.43 V vs Li+/Li, larger 1516


Article


Figure 5. Electronic band structures of α-V2O5 and r-MxV2O5 (M = Li/Mg, x = 0.5 and 1.0).

results in decreasing the band gap value and improving the conductivity for MxV2O5. Electron density difference (EDD) is often used to analyze the electron transfer and bond polarity in the process of bonding and electron coupling. EDD maps of α-V2O5 and rMxV2O5 (M = Li, Mg and x = 0.5, 1.0) have been calculated, as shown in Figure 6b−f. The positive (in blue) and negative (in red) regions corresponds to the enrichment and depletion of electron density, respectively. As seen in Figure 6b−f, the crisscross-like eletron distribution clearly shows the characteristics of d-orbitals for the donors V atoms. In contrast, the dumbbell-like eletron distribution is characterized by p-orbitals for the acceptors O atoms. It is evident that the overlap of the V3d and O2p orbitals has formed the effective orbital. In αV2O5 and r-MxV2O5, V atoms are bound with O atoms by strong covalent bonds, particularly of importance to the structure stability of electrode material.46 With the proceeding of the Li/Mg insertion, the electron transfer from Li (or Mg) to O atoms results in the decreasing of the electron density around Li (Mg) atoms, as shown in Figure 6c−f. In addition, Mg atoms lose more electrons (in deep red) than Li atoms, representing the stronger interaction of the Mg−O bonds than the Li−O bonds. This fact can explain the difficulty in the migration of Mg during cycling. To clarify the interaction between various atoms, the Mulliken populations and bond population for r-MxV2O5 (M = Li, Mg) compounds are listed for the first time in Table 4. It is know to all that the Mulliken population of an atom can display its ionization degree. To be clear, the net charge of the atoms in Table 4 denotes the average value. The interaction between three inequivalent oxygen atoms (O1, O2, and O3) and metal atoms is different in r-MxV2O5. From Table 4, the net charges of Li atoms (from +1.03 to +1.06e) in LixV2O5 are very close to their formal charge, showing the pure ionic bond between Li and O. This means that the high ionization degree is in favor of Li migration in V2O5. By comparison, the Mulliken charge of Mg ion diverges from their formal charge from +1.67e to +1.73e, which reveals that Mg is not ionized absolutely. The deviation of the V net charge (0.42e−0.45e for LixV2O5, 0.43e−0.45e for MgxV2O5) from the formal charges suggests that strong covalent bonds are formed between V and O in LixV2O5 and MgxV2O5, respectively. Also, as the inserted

3, respectively. A comparison of the band gap value (2.22 eV) for α-V2O5 in this calculation is in accordance with other Table 3. Band Gap Values of α-V2O5 and r-MxV2O5 (M = Li and Mg) method

values (eV)

compounds

values from this work (eV)

α-V2O5 (exp) VdW-DF2 LDA GGA DFT+U

2.2042 1.8710 1.7443 1.7544 2.1045

α-V2O5 r-Li0.5V2O5 r-LiV2O5 r-Mg0.5V2O5 r-MgV2O5

2.22 0.78 1.21 0.98 1.57

theoretical calculation (seen in Table 3), and experimental data (2.20 eV)42 demonstrates the rationality of the molecular model and calculation methods. From Figure 5, it is concluded that α-V2O5 is an indirect band gap semiconductor, similar to the previous reports.10,11 The band gap values of Li0.5V2O5 and LiV2O5 are 0.78 and 1.21 eV, respectively. As for Mg intercalation, the band gap values are 0.98 for x = 0.5 and 1.57 eV for x = 1, respectively. In comparison with α-V2O5, the band gap values of the lithiated/magnesiated V2O5 are decreased. This indicates that the electrons are easily excited from the valence band to the conduction band, and the increase of electronic conductivity ameliorates the electrochemical performance.5,18 As shown in Figure 5, the band structure of MxV2O5 (M = Li or Mg) shifts to the lower-energy direction, which suggests the stability of the lithiated/magnesiated V2O5. Figure 6a illustrates the corresponding density of states (DOS) of α-V2O5 and r-MxV2O5 (M = Li or Mg). As demonstrated in Figure 6a, the Fermi level of the lithiated/ magnesiated V2O5 passes through the conduction band, verifying the improvement of the electrical conductivity. The shift of the Fermi level gives rise to the decreasing of the band gap, similar to other literature.5,18 Furthermore, compared with Li1s and Mg2p bands, the O2p and V3d bands make more contribution to the DOS near the Fermi level after the lithiated/magnesiated V2O5. Finally, the strong hybridization between Li1s or Mg2p and O2p states has almost not been observed. The weak interaction between Li/Mg and its neighboring O atoms is favorable for the deintercalation and intercalation of Mg/Li. In general, the Li/Mg intercalation 1517


Article


Figure 6. Density of states (DOS) (a) and electron density difference maps along the (001) crystal plane of (b) α-V2O5, (c) r-Li0.5V2O5, (d) rLiV2O5, (e) r-Mg0.5V2O5, and (f) r-Mg0.5V2O5. The dashed lines represent the location of the Fermi level.

Table 4. Mulliken Population and Bond Population for r-MxV2O5 (M = Li, Mg) α-V2O5

M Li

Mg

Li(e) V(e) O(e) Li−O V−O Mg(e) V(e) O(e) Mg−O V−O

1.23 −0.49 0.42 1.23 −0.49 0.42

r-M0.25V2O5

r-M0.5V2O5

r-M0.75V2O5

r-MV2O5

1.03 1.18 −0.52 −0.05/0.01 0.42 1.73 1.16 −0.55 −0.36 0.43

1.05 1.12 −0.55 −0.05/0.01 0.43 1.72 1.10 −0.61 −0.36 0.45

1.06 1.07 −0.56 −0.06/0.01 0.45 1.71 1.03 −0.67 −0.37 0.45

1.06 1.02 −0.61 −0.06/0.01 0.45 1.67 0.95 −0.71 −0.39 0.45

population values of Li−O bonds (0.01, −0.05 to −0.06) are close to 0. This demonstrates that there is no obvious interaction between Li and O in LixV 2O 5 in the Li concentration range of x ≤ 1. The high ionicity of Li in LixV2O5 facilitates the reversible Li electrochemical insertion and extraction. Nevertheless, the Mg−O bond population in MgxV2O5 is more negative from −0.36 to −0.39, characterized by the strong covalent bond between Mg and O atoms. The ionicity of Mg is lower than that of Li so that Mg atoms are prone to being trapped in V2O5. This is the main reason that Mg atoms are difficult to extract from MgxV2O5 during the charge process. 3.6. Diffusivity. The migration rate of metal atoms is of significant importance for the rate performance of the electrode

lithium/magnesium content is increased, the charge variation of V (0.21e for Li, 0.28e for Mg) is more than that of O (0.12e for Li, 0.22e for Mg). During the lithiation and magnesiation processes, the redox center is V and the electrons fill in higher energy states accompanying an electron feedback from V to O. These results are in good agreement with the EDD analysis above. Bond populations have been calculated to quantitatively evaluate the bond strength, as shown in Table 4. Positive and negative bond populations represent the bonding and antibonding state, respectively.47 The greater the bond population value is, the higher the degree of covalency is, while a value close to zero denotes no significant interaction between the two atoms.47 As displayed in Table 4, the bond 1518


Article


Figure 7. Three possible pathways for a metal atom in α-V2O5: (a) pathway A, (b) pathway B, and (c) pathway C. Red balls represent oxygen atoms, gray balls represent vanadium atoms, and blue balls denote metal ions.

Figure 8. (a) The minimum-energy migration path of Li and Mg in δ-V2O5. (b) The energy barrier of lithium atoms in α-V2O5 along three possible migration pathways. (c and d) The migration energy of Li and Mg atoms in different V2O5 polymorphs, respectively.

a “V”-shaped trajectory along the b-axis direction due to its structure slipped by b/2 along the b-axis, given in Figure 8a. Therefore, in terms of the intercalation site and the migration path, we draw an analogy between Mg and Li atoms in ε-V2O5, α-V2O5, and δ-V2O5. First, the diffusion behavior of Li and Mg in α-V2O5 is investigated. Three possible diffusion paths for Li/Mg are identified to move to the neighboring vacancy P2′ in α-V2O5. The shortest path A parallels the b-axis in Figure 7a. From Figure 7b, path B refers to an approximate diagonal between the a- and b-axes. In Figure 7c, path C stands for a diffusion tunnel along the c-axis direction. As displayed in Figure 8b, the

material. In this study, the migration energies of Li/Mg atoms in α-V2O5, ε-V2O5, and δ-V2O5 were calculated to expound the diffusion mechanism. The (LST/QST) method was used to search for the transition states (TSs) between the reactants and the products, and then the NEB method was adopted to confirm the transition states and find the minimum energy pathways (MEPs). In this calculation, there is only one Li or Mg located at the P2 site to migrate to the neighboring vacancy P2 in different V2O5, as displayed in Figure 7. According to the previous literature,10,11,48 the migration behavior of Mg atom is the same as Li atom in α-V2O5, and the structures of ε-V2O5 and α-V2O5 are very similar. In δ-V2O5, the migration path is in 1519


Article

The Journal of Physical Chemistry C energy barriers of Li atoms in α-V2O5 along the three possible migration paths were computed. As for Li, the energy barriers of 0.40 eV for path A, 1.30 eV for path B, and 1.43 eV for path C are obtained. This indicates that path A in α-V2O5 is onedimensional along the b-axis, which is most favored by Li. As for Mg, path A in α-V2O5 has a higher migration barrier of 1.3 eV, close to Zhou’s value (1.26 eV).10 Second, the migration energies of Li and Mg atoms in the other two V2O5 polymorphs are illustrated in Figure 8, parts c and d, respectively. As can be seen, the migration energies of Li in α-V2O5, ε-Li0.5V2O5, and δLiV2O5 are 0.4, 0.27, and 0.17 eV, respectively. In contrast, Mg exhibits higher migration barriers of 1.30 eV for α-V2O5, 1.06 eV for ε-Mg0.5V2O5, and 0.68 eV for δ-MgV2O5. Notably, the diffusion barriers of Mg in the three V2O5 polymorphs are much larger than those of Li. This means that the magnesiation of V2O5 polymorphs should be kinetically hindered by the slow migration of Mg. It is expected that a V2O5−Mg battery has a lower power density. For Li, the lowest migration energy of δ-LiV2O5 is attributed to the high ionization degree of Li in δ-V2O5, as well as to the weak interaction between Li and the adjacent O atoms. Compared with Li atoms, the slowish diffusion of Mg atoms arises from the polarization effect of Mg2+ and the strong interaction between Mg and O atoms. At a low Mg concentration of xMg = 0.25, the energy barrier value of >1 eV in α-V2O5 reflects the slow mobility of Mg in α-V2O5 during the magnesiation and demagnesiation process. At a high Mg concentration of xMg = 1, the barrier value declines to 0.68 eV, less than 1 eV. It is significant that the energy barrier of Mg/Li atoms in V2O5 is decreased with increasing the concentration, which is closely related with the improved layer spacing of α-V2O5 for 4.36 Å, εV2O5 for 4.65 Å, and δ-V2O5 for 5.27 Å.9 For Mg, the large difference in barrier between α-V2O5 and the magnesiated δMgV2O5 has easily led to the electrochemical irreversibility of α-V2O5 upon magnesiation/demagnesiation. Although the barrier values of Mg in α-V2O5 and ε-V2O5 reach high up to 1.00 eV, the quasi-symmetric Mg intercalation/deintercalation process was realized in nanosized α-V2O5 particle at a reasonable discharge rate,49,50 which can compensate for a small diffusivity of Mg in α-V2O5 by decreasing the diffusion length.

could be compensated by the nanosized electrode material to decrease the diffusion length and improve the diffusion rate.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yuping Liu: 0000-0002-7763-7988 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This research was supported by National Natural Science Foundation of China (21406021). This work was carried out at National Supercomputing Center in Shenzhen.

■

REFERENCES

(1) Armand, M.; Tarascon, J. M. Building better batteries. Nature 2008, 451, 652−657. (2) Zhang, X.; Wang, J. G.; Liu, H.; Liu, H.; Wei, B. Facile Synthesis of V2O5 Hollow spheres as advanced cathodes for high-performance lithium-ion batteries. Materials 2017, 10, 77. (3) McNulty, D.; Buckley, D. N.; O’Dwyer, C. Synthesis and electrochemical properties of vanadium oxide materials and structures as Li-ion battery positive electrodes. J. Power Sources 2014, 267, 831− 873. (4) Su, D.; Wang, G. Single-crystalline bilayered V2O5nanobelts for high-capacity sodium-ion batteries. ACS Nano 2013, 7, 11218−11226. (5) Gershinsky, G.; Yoo, H. D.; Gofer, Y.; Aurbach, D. Electrochemical and spectroscopic analysis of Mg2+intercalation into thin film electrodes of layered oxides: V2O5 and MoO3. Langmuir 2013, 29, 10964−10972. (6) Horrocks, G. A.; Likely, M. F.; Velazquez, J. M.; Banerjee, S. Finite size effects on the structural progression induced by lithiation of V2O5: a combined diffraction and Raman spectroscopy study. J. Mater. Chem. A 2013, 1, 15265−15277. (7) Le, D. B.; Passerini, S.; Coustier, F.; Guo, J.; Soderstrom, T.; Owens, B. B.; Smyrl, W. H. Intercalation of polyvalent cations into V2O5 aerogels. Chem. Mater. 1998, 10, 682−684. (8) Shklover, V.; Haibach, T.; Ried, F.; Nesper, R.; Novak, P. Crystal Structure of the product of Mg2+ insertion into V2O5 single crystals. J. Solid State Chem. 1996, 123, 317−323. (9) Sai Gautam, G.; Canepa, P.; Abdellahi, A.; Urban, A.; Malik, R.; Ceder, G.; et al. The intercalation phase diagram of Mg in V2O5 from first-principles. Chem. Mater. 2015, 27, 3733−3742. (10) Zhou, B.; Shi, H.; Cao, R. F.; Zhang, X. D.; Jiang, Z. Y. Theoretical study on the initial stage of a magnesium battery based on a V2O5cathode. Phys. Chem. Chem. Phys. 2014, 16, 18578−18585. (11) Kulish, V. V.; Manzhos, S. Comparison of Li, Na, Mg and Al-ion insertion in vanadium pentoxides and vanadium dioxides. RSC Adv. 2017, 7, 18643−18649. (12) Zhao, X. D.; Zhang, X.; Wu, D. H.; Zhang, H. C.; Ding, F.; Zhou, Z. Ab initio investigations on bulk and monolayer V2O5 as cathode materials for Li-, Na-, K- and Mg-ion batteries. J. Mater. Chem. A 2016, 4, 16606−16611. (13) Wang, Z. G.; Su, Q. L.; Deng, H. Q. Single-layered V2O5 a promising cathode material for rechargeable Li and Mg ion batteries:an ab initio study. Phys. Chem. Chem. Phys. 2013, 15, 8705−8709. (14) Sayede, A. D.; Khelifa, S. B.; Pernisek, M.; Mathieu, C.; Aourag, H. Lithium intercalation effects on the V2O5 (001) surface. Solid State Ionics 2004, 166, 175−181. (15) Li, Z. Y.; Wu, Q. H. Electronic Structures of LixV2O5(x = 0.5 and 1): A theoretical study. ChemPhysChem 2008, 9, 300−304. (16) Delmas, C.; Cognac-Auradou, H.; Cocciantelli, J. M.; Menetrier, M.; Doumerc, J. P. The LixV2O5 system: an overview of the structure

4. CONCLUSIONS In this work, we investigated the phase transformation and diffusion kinetics of V2O5 as a cathode material for Li/Mg ion batteries by means of a first-principles calculation. Our calculational results show that Li/Mg atoms prefer to occupy in P2 sites, no matter how many inserted guest ions there are. From the calculated formtion energies, α-V2O5 will transform to ε-phase and δ-phase in turn with the increase of Li/Mg insertion content (0 → 0.5 → 1). Li/Mg insertion into α-V2O5 is favorable for increasing the electronic conductivity of electrode material, as the band gap of lithiated and magnesiated V2O5 is decreased. Bond population analysis shows that there is no obvious interaction between Li and O atoms (0.01, −0.06 to −0.05) in LixV2O5 at a Li concentration of x ≤ 1, while the Mg−O bond in MgxV2O5 (−0.36 to −0.39) is characterized by a strong covalent bond between Mg and O atoms. The high degree of covalency for the Mg−O bonds is the main reason for the trapping of Mg in V2O5. Finally, Li diffusion in V2O5 with a low energy barrier (Em ≈ 0.17−0.4 eV) is faster than for Mg (Em ≈ 0.68−1.3 eV). The energy barriers are reduced with the increasing of Li/Mg intercalation. The high migration barriers 1520


Article

The Journal of Physical Chemistry C modifications induced by the lithium intercalation. Solid State Ionics 1994, 69, 257−264. (17) Segall, M. D.; Lindan, P. J. D.; Probert, M. J.; Pickard, C. J.; Hasnip, P. J.; Clark, S. J.; Payne, M. C. First-principles simulation: ideas, illustrations and the CASTEP code. J. Phys.: Condens. Matter 2002, 14, 2717−2744. (18) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized gradient approximation made simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (19) Perdew, J. P.; Chevary, J.; Vosko, S.; Jackson, K. A.; Pederson, M. R.; Singh, D.; Fiolhais, C. Atoms, molecules, solids, and surfaces: Applications of the generalized gradient approximation for exchange and correlation. Phys. Rev. B: Condens. Matter Mater. Phys. 1992, 46, 6671−6687. (20) Anisimov, V. I.; Zaanen, J.; Andersen, O. K. Band theory and Mott insulators: Hubbard U instead of Stoner I. Phys. Rev. B: Condens. Matter Mater. Phys. 1991, 44, 943−954. (21) Zhou, F.; Cococcioni, M.; Marianetti, C.; Morgan, D.; Ceder, G. First-principles prediction of redox potentials in transition-metal compounds with LDA+U. Phys. Rev. B: Condens. Matter Mater. Phys. 2004, 70, 235121. (22) Kenny, N.; Kannewurf, C. R.; Whitmore, D. H. Optical absorption coefficients of vanadium pentoxide single crystals. J. Phys. Chem. Solids 1966, 27, 1237. (23) Govind, N.; Petersen, M.; Fitzgerald, G.; King-Smith, D.; Andzelm, J. A generalized synchronous transit method for transition state location. Comput. Mater. Sci. 2003, 28, 250−258. (24) Henkelman, G.; Jonsson, H. Improved tangent estimate in the nudged elastic band method for finding minimum energy paths and saddle points. J. Chem. Phys. 2000, 113, 9978−9985. (25) Dathar, G. K. P.; Sheppard, D.; Stevenson, K. J.; Henkelman, G. Calculations of Li-ion diffusion in olivine phosphates. Chem. Mater. 2011, 23, 4032−4037. (26) Ong, S. P.; Chevrier, V. L.; Hautier, G.; Jain, A.; Moore, C.; Kim, S.; Ma, X.; Ceder, G. Voltage, Stability and diffusion barrier differences between sodium-ion and lithium-ion intercalation materials. Energy Environ. Sci. 2011, 4, 3680−3688. (27) Lin, H.; Wen, Y.; Zhang, C.; Zhang, L.; Huang, Y.; Shan, B.; Chen, R. A GGA+U study of lithium diffusion in vanadium doped LiFePO4. Solid State Commun. 2012, 152, 999−1003. (28) Xu, B.; Meng, S. Factors affecting Li mobility in spinel LiMn2O4a first-principles study by GGA and GGA+U methods. J. Power Sources 2010, 195, 4971−4976. (29) Morgan, D.; Van der Ven, A.; Ceder, G. Li conductivity in LixMPO4 (M = Mn,Fe,Co,Ni) olivine materials. Electrochem. SolidState Lett. 2004, 7, A30. (30) Enjalbert, R.; Galy, J. A refinement of the structure of V2O5. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1986, 42, 1467−1469. (31) Satto, C.; Sciau, P.; Dooryhée, E.; Galy, J.; Millet, P. The δ-> ε-> γ-LiV2O5 ″high temperature″ phase transitions evidenced by synchrotron X-ray powder diffraction analysis. J. Solid State Chem. 1999, 146, 103−109. (32) Millet, P.; Satto, C.; Sciau, P.; Galy, J. MgV2O5 and δ-LixV2O5: a comparative structural investigation. J. Solid State Chem. 1998, 136, 56−62. (33) Horrocks, G. A.; Braham, E. J.; Liang, Y. F.; De Jesus, L. R.; Jude, J.; Velázquez, J. M.; Prendergast, D.; Banerjee, S. Vanadium KEdge X-ray absorption spectroscopy as a probe of the heterogeneous lithiation of V2O5: first-principles modeling and principal component analysis. J. Phys. Chem. C 2016, 120, 23922−23932. (34) Whittingham, M. S. Lithium Batteries and Cathode Materials. Chem. Rev. 2004, 104, 4271−4302. (35) Baddour-Hadjean, R.; Pereira-Ramos, J.-P. Raman microspectrometry applied to the study of electrode materials for lithium batteries. Chem. Rev. 2010, 110, 1278−1319. (36) Imamura, D.; Miyayama, M.; Hibino, M.; et al. Mg Intercalation Properties into V2O5 gel/carbon composites under high-rate condition. J. Electrochem. Soc. 2003, 150, A753−A758.

(37) Chernova, N. A.; Roppolo, M.; Dillon, A. C.; Whittingham, M. S. Layered vanadium and molybdenum oxides: batteries and electrochromics. J. Mater. Chem. 2009, 19, 2526−2552. (38) Delmas, C.; Cognac-Auradou, H.; Cocciantelli, J. M.; Ménétrier, M.; Doumerc. The LixV2O5 system: an overview of the structure modifications induced by the lithium intercalation. Solid State Ionics 1994, 69, 257−264. (39) Rozier, P.; Savariault, J. M.; Galy, J. A new interpretation of the LixV2O5electrochemical Behaviour for 1 < x < 3. Solid State Ionics 1997, 98, 133−144. (40) Carrasco, J. Role of van der Waals forces in thermodynamics and kinetics of layered transition metal oxide electrodes: alkali and alkaline-earth ion insertion into V2O5. J. Phys. Chem. C 2014, 118, 19599−19607. (41) Amatucci, G. G.; Badway, F.; Singhal, A.; Beaudoin, B.; Skandan, G.; Bowmer, T.; Plitz, I.; Pereira, N.; Chapman, T.; Jaworski, R. Investigation of yttrium and polyvalent ion intercalation into nanocrystalline vanadium oxide. J. Electrochem. Soc. 2001, 148, A940−A950. (42) Kenny, N.; Kannewurf, C. R.; Whitmore, D. H. Optical absorption coefficients of vanadium pentoxide single crystals. J. Phys. Chem. Solids 1966, 27, 1237−1246. (43) Eyert, V.; Hock, K. H. Electronic structure ofV2O5:role of octahedral deformations. Phys. Rev. B: Condens. Matter Mater. Phys. 1998, 57, 12727−12737. (44) Willinger, M.; Pinna, N.; Su, D. S.; Schlogl, R. Geometric and electronic structure ofγ-V2O5: Comparison between α-V2O5 and γV2O5. Phys. Rev. B: Condens. Matter Mater. Phys. 2004, 69, 155114. (45) Wang, L.; Maxisch, T.; Ceder, G. Oxidation energies of transition metal oxides within theGGA+U framework. Phys. Rev. B: Condens. Matter Mater. Phys. 2006, 73, 195107. (46) Xiao, P.; Deng, Z. Q.; Manthiram, A.; Henkelman, G. Calculations of oxygen stability in lithium-rich layered cathodes. J. Phys. Chem. C 2012, 116, 23201−23204. (47) Segall, M. D.; Shah, R.; Pickard, C. J.; Payne, M. C. Population analysis of plane-wave electronic structure calculations of bulk materials. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 16317−16320. (48) Ma, W. Y.; Zhou, B.; Wang, J. F.; Zhang, X. D.; Jiang, Z. Y. Effect of oxygen vacancy on Li-ion diffusion in a V2O5 cathode: a firstprinciples study. J. Phys. D: Appl. Phys. 2013, 46, 105306. (49) Wang, W.; Jiang, B.; Xiong, W. Y.; Sun, H.; Lin, Z.; Hu, L.; Tu, J.; Hou, J.; Zhu, H.; Jiao, S. A new cathode material for super-valent battery based on aluminium ion intercalation and deintercalation. Sci. Rep. 2013, 3, 3383. (50) Wu, N.; Yin, Y. X.; Guo, Y. G. Size-dependent electrochemical magnesium storage performance of spinel lithium titanate. Chem. Asian J. 2014, 9, 2099−2102.

1521


Phase Transformation and Diffusion Kinetics of V2O5 Electrode in

Recommend Documents