Effect of Sn-doping on Behavior of Li-intercalation in V2O5 Cathode

Subscriber access provided by Kaohsiung Medical University

Article 2

5

Effect of Sn-Doping on Behavior of Li-Intercalation in VO Cathode Materials of Li-Ion Batteries: A Computational Perspective Suwit Suthirakun, Sirichok Jungthawan, and Sukit Limpijumnong J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12321 • Publication Date (Web): 28 Feb 2018 Downloaded from http://pubs.acs.org on March 2, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 34 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

The Journal of Physical Chemistry

Effect of Sn-doping on Behavior of Li-intercalation in V2O5 Cathode Materials of Li-ion Batteries: A Computational Perspective Suwit Suthirakun*,†,‡, Sirichok Jungthawan‡,§,||, Sukit Limpijumnong§ †

School of Chemistry, Institute of Science, Suranaree University of Technology, Nakhon Ratchasima, Thailand 30000 ‡

Center of Excellence in Advanced Functional Materials, Suranaree University of Technology, Nakhon Ratchasima, Thailand 30000 §

School of Physics, Institute of Science, Suranaree University of Technology, Nakhon Ratchasima, Thailand 30000 ||Center for Theoretical Physics of Complex Systems (PCS), Institute for Basic Science (IBS), Daejeon 34126, Republic of Korea

Abstract

We utilized first-principles plane-wave calculations to obtain insight into the role of Sn-doping on the behavior of Li intercalation in V2O5 cathode materials used in Li-ion batteries. Density functional theory (DFT+U) calculations were carried out to study microscopic structures and electronic structures of a Sn-doped V2O5 system. We find that the bonding interaction between Sn and the V2O5 lattice displays mixed ionic/covalent character in which Sn donates two of its four valence electrons to the nearby V centers and shares the other two valence electrons with the surrounding lattice. The extra electrons originated from Sn insertion increase the number of charge carriers which could improve electronic conductivity of the material. In addition, Sn insertion induces structural distortion in the V2O5 lattice which in turn affects thermodynamic and kinetic properties of Li intercalation. The calculated insertion energies and diffusion barriers describe how Li intercalates into the V2O5 structure in the presence of Sn. Although the inserted Sn atom may block and trap Li-ion, most diffusion paths exhibit lower lying energy levels than those in the pure V2O5 suggesting that Sn-doping facilitates Li intercalation in V2O5-based cathode materials.

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

Page 2 of 34

1. Introduction

Rechargeable Li-ion batteries (LIBs) are considered an effective solution to the increasing need for electrochemical power storage.1-2 Thanks to their high energy-density, low self-discharge ability, long cycle lives, and environmental friendliness; LIBs provide the most efficient energystorage strategy for wide range of portable electronic devices.2-9 The development of novel and improved cathode materials can significantly enhance the overall performance of the batteries.10 Several cathode materials have been proposed and used in LIBs such as LiCoO2,11-12 LiFePO4,1216

and V2O5.17-20 Among these materials, V2O5 has been shown to be a promising candidate since

it exhibits high energy density and good safety properties.21 Its theoretical capacity of 400 mAh g– 1 22

is significantly higher than that of the commercially used cathode material, LiCoO2

(272 mAh g–1).23 Its layered structure is responsible for the high energy density. Each V2O5 layer is held together by weak van der Waals forces allowing intercalation of small elements such as Li.24 In addition, orthorhombic V2O5 has been reported to electrochemically accommodate several multivalent cations including Mg2+, Ca2+, and Y3+.25-28 It has been explored as an energy dense cathode for Mg-ion batteries.28 Although the V2O5 cathode material offers a promising Li-ion capacity, it suffers from several limitations such as poor structural stability, low electronic and ionic conductivity, and slow electrochemical kinetics.29-31 A number of synthesized approaches have been carried out to improve these drawbacks including making novel microstructures32-35 and doping foreign atoms into the lattice. Various metal elements such as Cr,36 Fe,33 Cu,37-38 Al,39 and Sn21 have been used as intercalated atoms in the V2O5 structure to boost the electrochemical performance of the cathode materials. In particular, Sn-doped V2O5 displays substantial enhancement of Li-ion storage capacity, kinetics, and cyclic stability comparing to that of the stoichiometric V2O5 film.21 In addition, experiments find that introducing small amount of Sn (5%) into the V2O5 film reduces the electrochemical reaction resistance, increases the electrochemical reaction reversibility, and improves Li-ion diffusivity.21 It was reasoned that Sn-doping facilitates the formation of reduced V ions (V4+) in the V2O5 film21 which may enhance the conductivity of the material, thereby improves the intercalation/extraction of Li-ion in V2O5 layers.40 First-principles methods, in particular density functional theory (DFT), have been used as a tool to obtain better understanding of the atomic and electronic structures and the behavior of Li intercalation in the V2O5-based cathode materials.41-45 The earlier studies based on the use of DFT with generalized gradient approximation (GGA) were successful at determining cell potentials, 2 ACS Paragon Plus Environment

Page 3 of 34 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


most stable sites for Li, and preferred Li diffusion path in the V2O5 lattice.44-45 More recent studies used a variant of the Kohn–Sham method such as DFT+U to correct, at least in part, the selfinteraction error of local or semilocal (GGA) exchange-correlation approximations.41-43,46-47 Scanlon et al. reported that the localization behavior of extra electrons in the reduced V2O5 systems can appropriately be described using the DFT+U method with the U value of 4.0 eV for V 3d electrons.41 The computed electronic structures are consistent with those obtained from ultraviolet and X-ray photoemission spectroscopy (UPS and XPS) experiments.41,48-49 Intercalation and diffusion of several alkali and alkaline-earth cations in V2O5 were modeled using the DFT+U approach.42-43,46 These theoretical works were successful at calculating ion intercalation voltages and describing atomic and electronic structures of cation-intercalated V2O5 cathode materials.4143,46

Note that, the inclusion of van der Waals interactions is crucial to quantitatively predict

intercalation energies and diffusion barriers of cation insertion in the V2O5 lattice.42,47 Beside the theoretical studies mentioned above, the roles of Sn-intercalation in the V2O5 cathode material have not been explored computationally. Thus, it is the objective of this work to use the DFT+U method to study the effects of Sn-doping on (i) the atomic and electronic structures of the V2O5 cathode and (ii) the behavior of Li intercalation in the cathode material. It is noteworthy that this study discusses the intercalation of Li in the limit of modeling the beginning of a discharge process, i.e., in the limit of dilute Li concentrations. In the following, we first describe the computational approach and the model used in this study. Next, we report the calculated results and investigate the impacts of Sn-doping on the alteration of atomic and electronic structures of the V2O5 cathode. In addition, the computed Li insertion energies and diffusion barriers of stoichiometric and Sn-doped V2O5 were analyzed to explore the effects of Sndoping on the behavior of Li intercalation. 2. Computational details

All calculations were carried out using the spin-polarized DFT+U approach with periodic supercell model implemented in the Vienna ab initio simulation package (VASP 5.3).50-52 The exchange and correlation functional was approximated using the optimized vdW-DF functional (modified versions of the vdW-DF of Dion et al.)53 as implemented in VASP, where its original GGA exchange functional was replaced by an optimized Perdew-Burke-Ernzerhof functional54 to take into account the weak van der Waals attraction between V2O5 layers. This functional has been reported to correctly predict the cell parameters and the interlayer distance of the V2O5 structure.42 3 ACS Paragon Plus Environment


Page 4 of 34

We chose the ultra soft pseudopotential with projector augmented-wave (PAW) method55-56 to describe the nuclei and core electronic states. The V 3s3p3d4s, O 2s2p, Sn 4d5s5p, and Li 2s were treated as valence electrons where their wave functions were expanded in plane-wave basis with a cutoff energy of 400 eV for electronic structure calculations and geometry optimization. We used a higher energy cutoff of 700 eV for calculating equilibrium volume of the V2O5 unit cell and the intercalated V2O5 structures. Such a high kinetic energy cutoff was found necessary to avoid an underestimation of the equilibrium volume caused by the Pulay stress. A Gaussian smearing technique ( = 0.05 eV) was used during structural relaxations in which the calculated energies were extrapolated to zero smearing width. We employed Dudarev’s approach57 for DFT+U calculations to correct for the self-interaction error inherent in current exchange-correlation functionals DFT when applied to transition metals with tightly localized d electrons, such as V in V2O5 systems. The U–J parameter of 4.0 eV was chosen for V 3d electrons which has been proposed by Scanlon et al.41 to properly describe the electronic structures of oxygen vacancies and intercalated Li in V2O5. In order to study the impact of Sn-doing toward the electrochemical performance of V2O5 cathode materials, we first optimized the lattice parameters and ion positions of the V2O5 unit cell using the Monkhorst-Pack (MP) approach58 with a dense k-point sampling of 5×11×11. The optimization was ceased when the calculated residual forces were lower than 0.02 eV/Å. The obtained unit cell was then used to construct a 1×3×3 supercell of V36O90 to further use for evaluation of Sn and Li insertions. Incorporation of metals, Li or Sn, into the V2O5 supercell were carried out where both lattice parameters and ion positions were allowed to relax to examine the effect of metal-insertion on alterations of the V2O5 lattice. These calculations were carried out using the MP k-point grid of 2×2×2 with the same convergence criterion to that of the optimization of the unit cell. To explore the electronic structures of the stoichiometric and intercalated V2O5 systems, we computed their projected density of states (PDOS) employing the tetrahedron smearing method with Bloch corrections. The k-point grid of 2×2×2 was found to be sufficient for DOS calculations, while using a denser 4×4×4 k-point results in a negligible difference of the produced DOS.


Page 5 of 34 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


3. Results and discussion 3.1. Stoichiometric V2O5

The α-V2O5 has an orthorhombic crystal structure in space group Pmmn with the lattice parameters a = 11.51 Å, b = 3.56 Å, and c = 4.37 Å.59 As depicted in Figure 1a, the unit cell consists of two formula units including four V atoms and ten O atoms. Each V atom connects to five O atoms forming distorted VO5 square-based pyramids. In the experimental structure, each VO5 unit has three inequivalent types of O atoms. One terminal oxygen (named O1) forms a V=O bond with the bond distance of 1.58 Å. One bridging oxygen (O2) connects to two adjacent vanadium centers with the V–O bond distance of 1.78 Å. Three chain-forming oxygen atoms (O3) are three-fold coordinated with V atoms, two with the bond distances of 1.88 Å, and one with the bond distance of 2.02 Å (Figure 1a and Table 1).60 These square-pyramidal VO5 units are connected via edgesharing and corner-sharing creating a layered structure oriented perpendicular to the [001] direction. Each layer contains alternating order of VO5 pairs with their terminal oxygen (O1) pointed in [001] and [001̅ ] directions. These V2O5 layers are then stacked along the [001] direction with the interlayer distance of the c lattice constant. The interaction between V2O5 layers are characterized as weak van der Waals attraction. It was found that applying the vdW-DF method to the bulk V2O5 results in accurate predictions of the lattice parameters, in particular, the interlayer distance which is characterized by the lattice parameter c.42,47 Therefore, we used the vdW-DF method in combination with the PBE+U functional to describe the interlayered weak interactions. Our calculated lattice parameters, a = 11.65 Å, b = 3.63 Å, and c = 4.44 Å, and V−O bond distances, dV−O1 = 1.61 Å, dV−O2 = 1.81 Å, dV−O3 = 1.91, 2.04 Å, are consistent with those of the experimental values and the previously calculated values as summarized in Table 1. We computed PDOS of the stoichiometric V2O5 to analyze its electronic structure. It can be seen from Figure 1b that the PBE+U calculated PDOS of the stoichiometric V2O5 exhibits semiconducting behavior with a calculated band gap of 2.26 eV which is in excellent agreement with the published PBE+U value, 2.26 eV41 (experimentally determined band gap is 2.3 eV).61 Three groups of bands were identified. The valence band starts at −4.79 eV below the Fermi level which mainly comprises O 2p states and non-negligible V 3d states. Across the band gap of 2.26 eV, a split-off conduction band with a rather narrow band width of 0.41 eV is identified. This split-off conduction band is separated from a broad higher conduction band by 0.33 eV. These conduction bands are mainly contributed by V 3d states with a substantial hybridization of O2p−V3d. The splitting of the conduction band was originated from the strong deviations of the 5 ACS Paragon Plus Environment


Page 6 of 34

VO5 square pyramids from their cubic symmetry, the VO6 octahedra, as explained in the theoretical study of the electronic structure of bulk V2O5.62 All other states that are not displayed in Figure 1b have insignificant contributions to the calculated DOS. 3.2. Li insertion

Next, we inserted a Li atom into the V2O5 supercell to explore the behavior of Li intercalation. Insertion of a Li atom into the 1×3×3 supercell of V36O90 results in a dilute concentration of Li in the V2O5 lattice corresponding to Li0.06V2O5. Computations reveal that the most stable intercalated site is at the hollow site above a ring formed by four VO5 units between V2O5 layers, as shown in Figure 2a and b. This lowest energy site is indeed the same as reported by other DFT studies.4143,63

𝑖𝑛𝑠 The energy of Li insertion (𝐸𝐿𝑖 ) in pure V2O5 was computed using the following equation 𝑖𝑛𝑠 𝐸𝐿𝑖 = 𝐸𝐿𝑖𝑉36 𝑂90 – 𝐸𝑉36 𝑂90 – 𝐸𝐿𝑖

where 𝐸𝐿𝑖𝑉36 𝑂90 and 𝐸𝑉36 𝑂90 are the DFT calculated energies of the 1×3×3 supercell of V36O90 systems in the presence and absence of the intercalated Li atom, respectively. ELi is a half of the DFT calculated energy of bulk Li (2 atoms per unit cell). Our calculated energy of Li insertion, −3.24 eV, is in good agreement with those using the optPBE+U-vdW method,42 −3.50 eV, and the PBE+U-vdW-DF2 method,46 –3.35 eV, but somewhat different from that using only the PBE+U functional, −2.78 eV41 and –3.11 eV.64 The differences of the calculated insertion energies could stem from the absence of vdW functionals. It is important to note that our calculated insertion energy is at the limit of low Li concentration. The insertion energies will become less negative as the Li concentration increases because Li–Li interaction is repulsive. At higher Li concentrations, configurations with much shorter Li–Li distances dominate which lead to less negative insertion energies. The calculated energy of Li insertion can be used to approximate the equilibrium voltage between the V 2O5 cathode and the Li metal anode.41-42,44 At dilute Li concentrations, the upper limit voltage can be 𝑖𝑛𝑠 estimated as |𝐸𝐿𝑖 | since the entropic and volume change contributions to the change in free energy

in the system are negligible comparing with the change in the internal energy.41-42,65 Our calculated upper limit voltage, 3.24 V, is in excellent agreement with that experimentally observed in LixV2O5 by Moss et al.,18 who reported a voltage of 3.25 V with a metallic Li anode, in the limit of low Li concentration. Experiments also show that the cell voltage is lower in the higher range of Li concentrations.18


Page 7 of 34 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


When Li is introduced into bulk V2O5, it ionizes to form a Li+ ion and donates an extra electron to the lattice. As depicted in Figure 2a and b, the isosurface of the calculated spin density reveals that the extra electron localizes at the V center closest to the ionized Li+ leading to a formation of a small polaron at the reduced V center. The localization behavior of the extra electron was ensured by analyzing the PDOS of the Li-intercalated V2O5 structure. As depicted in Figure 2c, the PDOS exhibits an extra state of 1 electron between the valence band and the conduction band. The gap states mainly comprise the V 3d states whose respective charge densities obtained from the band decomposition charge density calculation display localization of the electron at the nearest V center, inset Figure 2c. Combining the results from the calculated spin density and PDOS analysis, we can confirm that the extra electron originated from the ionized Li+ ion favors to localize at the nearest V center. The computed electronic structures are consistent with those reported in the previous DFT works41-42 and the observed localized state in the band gap obtained from UPS and XPS experiments.49 It is noteworthy that the Li+ ion does not position at the center of the ring but locates closer to the reduced V center due to the electrostatic attraction between the positively charged Li+ and the negatively charged polaron and the oxide anion. The deviated distance from the center was calculated to be 51 pm in which the point at the center was determined from the projected coordinates (x, y) of the four V centers (see Supporting Information S1). Insertion of the Li atom into the 1×3×3 V2O5 supercell has negligible contributions to the lattice parameters of V2O5 (Table 1); however, it creates some perturbation to the local structure of V2O5, in particular, the closest VO5 unit to the inserted Li atom. We find that the V–O bonds of the VO5 unit were lengthened with the largest elongation of 12 pm at the V–O2 bond. Slight changes of V–O bond distances were observed for V–O1 and V–O3 bonds at 5 pm and 6 pm, respectively (Table 2). Two contributions to the lattice distortion were explored which are (i) the perturbation occurred upon the interaction between the Li+ and the lattice and (ii) the distortion due to the formation of a small polaron. Insertion of a Li+ alone allows us to solely study the interaction between the Li+ ion and the V2O5 structure. It is expected that when the electron polaron is missing, the Li+ is less deviated from the center of the ring (11 pm) when compared with that when the Li+ is in the vicinity of the electron polaron (deviated distance of 51 pm, see Supporting Information S1). Furthermore, the presence of the Li+ does not contribute to the local lattice distortion as the changes in the V−O bond distances of the nearest VO5 unit are negligible with the



Page 8 of 34

maximum changes of only 2 pm for V−O1 bond (Table 2) due to the electrostatic attraction between the Li+ and the oxide anion. The lattice distortion induced by the formation of a small polaron can be examined by adding an extra electron into the V2O5 supercell. The isosurface of spin density illustrates the formation of a small polaron at a V center which causes an elongation of the V−O bonds around the reduced V center in the range between 3 and 12 pm (dV-O1 = 3 pm, dV-O2 = 12 pm, dV-O3 = 8 pm) as summarized in Table 2. The major changes of the V–O bonds are within the V2O5 layer in the x-y plane (dV-O2 = 12 pm, dV-O3 = 8 pm) whereas the elongation of V−O1 bond in the direction perpendicular to the layer ([001] direction) is not as much (dV-O1 = 3 pm). The calculated results suggest that when small amounts of Li are inserted into the V2O5 structure, the lattice must accommodate not only Li-ion but also electron polarons which play a central role in causing local lattice distortion as they impose geometric strains into the V2O5 layers. These calculated results are consistent with the earlier DFT works suggesting that formation of a small polaron induces structural reorganization in several semiconducting materials such as TiO2,66 Fe2O3,67 and LixFePO4.68 3.3. Sn-doped V2O5

Experiments show that introducing a small amount of Sn into the V2O5 cathode can significantly enhance storage capacity and cyclic stability of the electrode.21,69 It is of our central interest to better understand the role of Sn-doping on the improved structural and electrochemical properties of V2O5-based cathodes. Hence, we examined atomic and electronic structures of the Sn-doped V2O5 system and further explored the roles of Sn toward the behavior of Li intercalation. According to the preparative method of the Sn-doped V2O5 film as reported by Li et al.21, Sn incorporation in the V2O5 lattice could lead to two different scenarios – i.e., Sn could insert between the V2O5 layers or substitute at lattice V sites. The stabilities of Sn insertion and substitution were computationally examined by calculating their reaction energies. We describe the process of Sn insertion as Sn + V36O90 → SnV36O90 where the insertion energy can be calculated as follow; 𝑖𝑛𝑠 𝐸𝑆𝑛 = 𝐸𝑆𝑛𝑉36 𝑂90 – 𝐸𝑉36 𝑂90 – 𝑆𝑛

where ESnV36 O90 is the DFT calculated energy of the 1×3×3 supercell of V36O90 system in the presence of the inserted Sn atom. Sn varies from its highest value at the standard state (Sn metal at Sn-rich conditions) and limited by the stability of SnO2 in equilibrium with O2 gas as ESnO2 = 8 ACS Paragon Plus Environment

Page 9 of 34 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


Sn + 2O. The energy of Sn metal (Sn at Sn-rich conditions) is a quarter of the DFT calculated energy of bulk Sn metal in the optimized tetragonal unit cell (4 atoms per unit cell). The calculated energy of Sn insertion at Sn-rich conditions is –2.10 eV indicating that Sn can be intercalated into the V2O5 structure. Substitution at the lattice V site is another plausible way for Sn to be incorporated into V2O5. Sn substitution can be expressed as Sn + V36O90 + 5/4O2 → SnV35O90 + 1/2V2O5 with its respective reaction energy given by

where 𝐸𝑆𝑛𝑉35 𝑂90

1 5 𝑠𝑢𝑏𝑠 𝐸𝑆𝑛 = 𝐸𝑆𝑛𝑉35 𝑂90 + 𝐸𝑉2 𝑂5 – 𝐸𝑉36 𝑂90 – 𝐸𝑆𝑛 − 𝑂 2 2 is the DFT calculated energy of the 1×3×3 supercell of V36O90 system with the

substituted Sn at a lattice V site and 𝐸𝑉2 𝑂5 is a half of the DFT calculated energy of the optimized -V2O5 unit cell (2 formula units per unit cell). O reaches its highest value as the energy of O2 gas molecule (per oxygen atom at O-rich conditions) and limits by the stability of SnO2 as ESnO2 = Sn + 2O. To circumvent difficulties associated with the GGA treatment of the triplet state of gasphase O2, the O2 energy is obtained from the water splitting reaction using the experimental reaction energy and calculated DFT energies of H2 and H2O in the gas phase,70-71 𝐸𝑂𝑡𝑜𝑡 = 2[(𝐸𝐻𝐷𝐹𝑇 + 𝐸𝐻𝑍𝑃𝐸 ) − (𝐸𝐻𝐷𝐹𝑇 + 𝐸𝐻𝑍𝑃𝐸 ) − 𝐸ℎ𝑜𝑓 ] − 𝐸𝑂𝑍𝑃𝐸 , 2𝑂 2𝑂 2 2 2 2 where the experimental zero-point energies (EZPE) of H2O, H2, and O2 are 0.558, 0.273, and 0.098 eV, respectively.72 Ehof is the experimental heat of formation of a gas-phase H2O molecule (–2.505 eV),72 and EDFT is the energy calculated with optPBE+U-vdW functional. Computations reveal that Sn insertion is energetically more favorable over the considered range of chemical potentials of standards indicating that Sn prefers to be at the interstitial site between the V2O5 layers, as illustrated in Figure S2 and discussed in Supporting Information S2. Our calculated results support the experimental observations based on X-ray absorption spectroscopy (XAS) studies which suggest that doping metals, such as Cu and Zn, are found to be intercalated between the V2O5 layers in the lattice.17,20 This preferred site remains the same over the ranges of doping level for MxV2O5 from x = 0.25 to 1.0.17,20 Moreover, analysis of X-ray diffraction patterns of the 5% Sndoped V2O5 film suggests that the doped Sn metal locates between the V2O5 layers.21 Note, however, that recently Li et al.69 proposed that doped Sn ions can substitutionally replace the V ions at the lattice sites. They also proposed that Sn-doping induces the formation of oxygen vacancies which enhance the intercalation/extraction reactions of Li. While this alternative 9 ACS Paragon Plus Environment


Page 10 of 34

scenario of Sn-doped V2O5 is interesting and remained plausible, it involves more complicated defect complexes formation study and further explanation of how the model would fit with existing X-ray diffraction results. Therefore, in this work, we focus our attention only on the doped Sn atom located at the interstitial site between the V2O5 layers. 3.3.1. Behavior of Sn insertion

Insertion of a Sn atom into the 1×3×3 V2O5 supercell results in a 2.8% Sn-doped V2O5 system which corresponds to SnV36O90. We first looked for the most stable insertion site of the Sn atom within the V2O5 supercell by exploring two most plausible configurations as discussed in Supporting Information Section S2. Computations show that the lowest energy site is indeed very similar to that of Li, i.e., at the hollow site above the ring formed by four units of VO5 polyhedra. Nonetheless, the inserted Sn atom is symmetrically placed at the center of the ring while the most favorable insertion site of Li is closer to the reduced V center. Note that placing the Sn atom in the other position (Figure S2a) results in the same most stable configuration as detailed above. Our calculated results for the most favorable intercalated position of ions are consistent with the XAS study which identify the location of the inserted foreign ions to be between the V2O5 layers.17 To understand the reason behind the improved electrochemical performance of the Sn-doped V2O5 cathode, it is of great importance to study the electronic behavior of Sn in the V2O5 lattice. To do so, we evaluated in detail the electronic structures of the Sn-doped V2O5 system. We find that when Sn is inserted into bulk V2O5, it gives two electrons to the host lattice forming two small polarons at two V centers. The most stable configuration of the two reduced V centers was explored by carrying out calculations with various locations of the two polaronic sites around the inserted Sn. Since it has been computationally reported that it is more stable for the inserted cation to stay close to the polaronic site(s),42,68 we only examined arrangements of polaron pair in the vicinity of the Sn ion as schematically illustrated in Figure S3a. All symmetry inequivalent positions of the polaron pair i.e., body-diagonal, face-diagonal, and lattice-vector, were considered (see Supporting Information Section S3). Table S1 summarizes relative calculated energies of various polaron-pair configurations in the vicinity of Sn-ion. Computations suggest that it is less favorable to accommodate two electron polarons on the same V2O5 layer, i.e., the configurations V1-V2, V1-V3, and V1-V4. Formation of a small polaron creates a local lattice reorganization around the reduced V center where the major changes were found in V–O bond lengthening within the V2O5 layer. Such geometric strains make the configurations with a polaron pair on the same layer unfavorable. In contrast, forming two polarons on different V2O5 layers is more stable, i.e. 10 ACS Paragon Plus Environment

Page 11 of 34 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


V1-V8, V4-V5, V2-V5, V2-V8, and V1-V5. Interestingly, the body-diagonal arrangements of the polaron pair (V1-V8 and V4-V5), which has the largest polaron separation, are not the most stable configurations. The arrangements of the polaron pair on different V2O5 layers that allow offcentrosymmetric positions of the Sn ion are slightly more stable (up to 0.06 eV) due to the relatively short ion-polaron separation. These configurations are V2-V5 (most stable), V2-V8, and V1-V5. Note, however, that calculations carried out in this study solely based on the body-diagonal arrangement of the polaron pair (V1-V8). It has been examined that the used Sn-polaron configuration exhibits negligible differences in the computed insertion energies and diffusion barriers of Li in the Sn-doped V2O5 (up to 0.02 eV). Within the considered temperature range (300 K, kBT = 0.03 eV), thermodynamic and kinetic properties of Li intercalation are not affected by the considered Sn-polaron configurations. The used Sn-doped V2O5 model has the position of Snion symmetrically placed at the center of the ring with small deviation of only 8 pm from the center of the ring, see Supporting Information S3. Intercalation of Sn, at 2.8% Sn concentration, into bulk V2O5 hardly change the cell shape and cell volume as shown in Table 1. However, we observe some reorganization of local structures due to the formation of two small polarons and the interaction between the Sn-ion and the lattice. The inserted Sn-ion pulls the four terminal oxygen atoms in the underneath layer toward the cation resulting in the lengthening of V–O1 bonds in the range between 2 pm and 6 pm as shown in Table S2. The other important distortions are located around the two reduced VO5 units diagonally arranged nearby the inserted Sn-ion, as shown in Figure 3a (see also Supporting Information S2). The V–O bonds are stretched with the largest increase in the bond distance of 17 pm for the V–O2 bond of the reduced VO5 unit located on the upper layer (V2), see Figure 3a and Table 2. The origin of the distortion was explored by examining structural changes of one system in the presence of the Sn-ion alone and the other system in the presence of the two small polarons. As summarized in Table 2, the largest elongation involving the V–O2 bond (12 pm) and the V–O3 bond (8 pm) are due to the presence of two small polarons. The contribution of the Sn-ion to the local structure is rather small but mostly develops the lengthening of the V–O1 bond of the beneath layer (V1, 3 pm) and the V–O2 bond of the upper layer (V2, 5 pm). As previously discussed for the case of Li insertion, the formation of small polarons mostly affects the lengthening of the V–O bonds in the x-y plane within the layer (V–O2 and V–O3 bonds) while the presence of the cation induces the elongation of V–O1 bonds by pulling the terminal oxygen atoms toward it.



Page 12 of 34

The bonding character of Sn in the V2O5 lattice was further investigated by analyzing its PDOS. As depicted in Figure 3b, the PDOS of Sn-doped V2O5 exhibits localized gap states which are mainly contributed by V 3d states and non-negligible O 2p and Sn states. The overall peak area of these gap states was accounted for four electrons per supercell. As depicted in Figure 3c, the first two peaks at 0.3 eV and 0.08 eV below the Fermi energy display formation of two electron polarons at V1 and V2 sites, respectively. Immediately below the Fermi energy, there is a peak of one electron containing primarily O 2p and Sn states showing a bonding character of Sn and surrounding O atoms. Its antibonding states appear as a peak of one electron at 0.07 eV above the Fermi energy. The computed results indicate that there are in total four electrons involved in the bonding interaction between the intercalated Sn atom and the V2O5 structure. For these four electrons, two electrons are completely transferred from the Sn atom to the nearby V centers and form small polarons; suggesting the ionic bonding character between Sn-ion and the V2O5 lattice. The other two electrons, mainly contributed by O 2p and Sn states, reflect the character of covalent bonding between the inserted Sn atom and the O atoms in the host structure. The isosurface of the gap states clearly shows the formation of two small polarons at the nearby V centers (ionic character) and shared electrons between the Sn atom and the surrounding oxygen atoms (covalent character) as illustrated in the inset of Figure 3c. In addition, we carried out Bader charge analyses of Sn in V2O5, SnO2, and SnO using the program of Henkelman et al.73 to determine the amount of valence electron density around the Sn ion (see Supporting Information Section S4). The analyses reveal that the Bader charge of Sn in bulk V2O5 (+1.55) is comparable to that of SnO (+1.26) but significantly lower than that of SnO2 (+2.59) indicating that the electron density of the inserted Sn in V2O5 is similar to that of Sn(II). This agrees with the calculated electronic structures of Sn in V2O5 that the inserted Sn loses two electrons to the host lattice. These extra electrons localize at two nearby V centers which reflects through the charges on reduced V centers (~0.15 lower than that of the lattice V2O5). Our computations suggest that the interaction between Sn and bulk V2O5 exhibits mixed ionic/covalent bonding character. The experimental study based on XPS measurements reveals that the intercalated Sn in the V2O5 film displays identical behavior to that of Sn in SnO2. The XPS chemical shift of Sn in V2O5 illustrates the local bonding environment of the Sn ion with the nearest O atoms in the V2O5 lattice which is similar to that of SnO2 but is noticeably different to that of SnO. As shown in Figure S4, Sn in V2O5 is surrounded by eight O atoms with an average Sn–O bond distance of 2.54 Å. Similarly, Sn in SnO2 is surrounded by six O atoms with slightly shorter Sn–O bond distance (2.09 12 ACS Paragon Plus Environment

Page 13 of 34 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


Å). A very distinctive structure is observed in SnO which exhibits a layered structure where a Sn ion is connected with four O atoms with four Sn-O bond distance of 2.23 Å. The findings from the inspection suggest that Sn in the V2O5 sample may not be completely ionized. The bonding character of SnO2 is partly covalent with the ionicity factor, fi = 0.7474 (0  fi  1, with fi = 1 being extremely ionic), comparing to other strong ionic oxide materials, for e.g., MgO (fi = 0.84),75 CaO (fi = 0.92).75 This lower ionicity factor indicates that the inserted Sn atoms partly give some of their four valence electrons to the bulk V2O5 and share the remaining valence electrons to covalently bind to the lattice. Moreover, the calculated amounts of V4+ in the Sn-doped V2O5 system are consistent with the values obtained from the XPS study.21 Experiments reveal that introducing 5% of Sn into the V2O5 sample increases the ratio of V4+/(V4+ + V5+) by 5% which indicates that one Sn atom roughly generates one reduced V center.21 The mobility of extra electrons from Sn in V2O5 is evaluated by computing a polaron migration barrier of the Sn-doped V2O5 system employing a method based on Marcus Theory,76 see Supporting Information Section S5. We considered a polaron hopping in the [010] direction because it is the same direction as that of Li diffusion where the concomitant motion of the polaron could also lower the Li diffusion barrier and lead to improved electronic/ionic conductivity. Computations show that the polaron migration away from the Sn ion in the Sn-doped V2O5 system exhibits higher activation energy (0.32 eV) than that of the pure V2O5 system (0.25 eV), as shown in Figure S5. The calculated barriers are in reasonable agreement with other computed values at PBE+U level, Li0.083V2O5 (0.34 eV)46 and Li0.06V2O5 (0.28 eV),64 and experimentally obtained values (0.17 – 0.32 eV) at intermediate temperature range (140 – 400 K).77 Our calculated polaron migration barriers for both pure and Sn-doped systems are in the range of experimentally obtained values (0.17 – 0.32 eV) indicating that the extra electrons from Sn-insertion are not tightly bound to the Sn ion and can improve electronic conductivity of the material. From both computations and experiments, it can be concluded that Sn-doping leads to the formation of reduced V species which in turn increases the concentration of charge carrier that facilitates electronic conductivity and Li intercalation/deintercalation process in the V2O5 cathode materials. Note that increasing the Sn concentration from 2.8% to 5.6% does not alter the electronic behavior of Sn. An additional Sn atom was inserted into the 2.8% Sn-doped V2O5 system to yield the 5.6% Sn-doped V2O5 system, i.e., two Sn atoms per supercell. Three different configurations, with two Sn atoms per supercell, all display a magnetization of 4.000 B indicating that one Sn atom always donates two electrons to the host lattice. These calculated results suggest that the 13 ACS Paragon Plus Environment


Page 14 of 34

supercell size used is sufficiently large and the electronic behavior of the inserted Sn remains the same for the range of doping level from 2.8% to 5.6%. 3.3.2. Effects of Sn-doping on Li insertion

Using the 2.8% Sn-doped V2O5 model, we can explore the effects of Sn-doping on the behavior of Li intercalation in the V2O5-based cathode materials. As discussed above, when a Li atom is inserted into the V2O5 lattice, it ionizes and gives one electron to the nearby V center in the lattice. Thus, to study the effects of Sn-doping, we must examine not only the positions of the inserted Li+ ion but also the localization of the respective electron polaron at various possible V centers. Since it is known that the electron polaron favors to be near the inserted Li+ ion, we only consider the configurations consisted of the reduced V center next to the Li+ ion. In this way, we can systematically investigate the behavior of Li insertion in the Sn-doped V2O5 system. It can be seen from Figure 4a that Li-ion–polaron locations in the Sn-doped V2O5 system can be described by considering the layer or the channel that the Li atom is adsorbed. Each V2O5 layer comprises alternate pairs of VO5 pyramids with their terminal oxygen atoms pointed in the [001] and [001̅] directions as shown in Figure 4a. In this model, a Li atom can be adsorbed on three different layers, namely, Layer1 (L1), Layer2 (L2), and Layer3 (L3). Top view of each layer is schematically illustrated in Figure 4a where capital letters A–F indicate the possible positions for a Li+ ion to be adsorbed and the numbers 1–12 label the locations of the reduced V center. It is noted that for every V2O5 layer, the adsorbed positions A, B, and C correspond to the VO5 pyramids pointing downward whereas the positions D, E, and F are in accordance with the polyhedra pointing upward as illustrated in Figure 4a. When a Li+ ion is adsorbed on a V2O5 layer (position A–F), its electron may localize at one of the four nearby V centers generating a specific configuration of a Li-ion–polaron pair. For example, when a Li+ ion is adsorbed at B position, its polaronic site could be at number 3, 4, 5, or 6 which referred to as B3, B4, B5, and B6, respectively. In addition, it is important to recognize the location of the Sn-ion and its electrons in the Sn-doped V2O5 system. The inserted Sn-ion is at position E between Layer1 and Layer2 and its two electrons are at L1-10 and L2-11. The behavior of Li intercalation in the Sn-doped V2O5 system was explored by computing 𝑖𝑛𝑠 energy of Li insertion, 𝐸𝐿𝑖 , at various Li-ion–polaron locations using the equation below 𝑖𝑛𝑠 𝐸𝐿𝑖 = 𝐸𝐿𝑖/𝑆𝑛𝑉36 𝑂90 – 𝐸𝑆𝑛𝑉36 𝑂90 – 𝐸𝐿𝑖 ,


Page 15 of 34 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


where𝐸𝐿𝑖/𝑆𝑛𝑉36 𝑂90 is the DFT calculated energy of the Sn-doped V2O5 system in the presence of the intercalated Li atom. The calculated energies of Li insertion at various locations are summarized and plotted in Figure 4b. Our computations show that the Li atom favors to adsorb on the Layer1 at A, B, and C positions in the same channel as the pre-inserted Sn atom, with the lowest insertion energy of –3.50 eV for the A3 configuration. The other A, B, and C configurations for Li adsorption on Layer1 exhibit insertion energies in the range between –3.50 eV and –3.41 eV which are lower than that of the Li-insertion energy in the pure V2O5 system (–3.24 eV). The calculated results indicate that Sn-doping facilitates Li insertion by stabilizing the intercalated Li atom in the lattice. Adsorption of Li onto the other two layers, L2 and L3, are significantly less stable with the insertion energies in the range between –3.42 eV and –3.27 eV; nevertheless, they are more stable than that of the insertion in pure V2O5 (–3.24 eV). Insertion of Li into the V2O5 lattice involves accommodation of a Li+ ion and formation of a small polaron nearby the inserted ion. To explore the origin of the improved Li-insertion energies, we investigate the effects of Sn-doping on the polaron formation and the Li-ion insertion. The 𝑓,𝑟𝑒𝑙

relative polaron formation energy (𝐸𝑝𝑜𝑙 ), which indicates how much energy is needed to create a small polaron at a particular V center in the Sn-doped V2O5 system comparing with that in the pure V2O5 system, is defined as 𝑓,𝑟𝑒𝑙

𝐸𝑝𝑜𝑙 = [𝐸𝑝𝑜𝑙/𝑉2 𝑂5 – 𝐸𝑉2 𝑂5 ] − [𝐸𝑝𝑜𝑙/𝑆𝑛−𝑉2 𝑂5 – 𝐸𝑆𝑛−𝑉2 𝑂5 ], where 𝐸𝑝𝑜𝑙/𝑉2 𝑂5 and 𝐸𝑝𝑜𝑙/𝑆𝑛−𝑉2 𝑂5 are the DFT calculated energies of the optimized polaron-doped V2O5 and polaron- and Sn-doped V2O5 systems, respectively. The relative energy of Li+ insertion 𝑖𝑛𝑠,𝑟𝑒𝑙 (𝐸𝐿𝑖 ), which shows how much more stable for a Li+ ion to stay at a specific position in the Sn+

doped V2O5 system in comparison with that in the pure V2O5 system, is defined as 𝑖𝑛𝑠,𝑟𝑒𝑙 𝐸𝐿𝑖 = [𝐸𝐿𝑖 +/𝑉2 𝑂5 – 𝐸𝑉2 𝑂5 ] − [𝐸𝐿𝑖 +/𝑆𝑛−𝑉2 𝑂5 – 𝐸𝑆𝑛−𝑉2 𝑂5 ], +

where 𝐸𝐿𝑖 + /𝑉2 𝑂5 and 𝐸𝐿𝑖 + /𝑆𝑛−𝑉2 𝑂5 are the DFT calculated energies of the optimized Li+-inserted V2O5 and Li+-inserted Sn-doped V2O5 systems, respectively. These two calculated energies for selected configurations are summarized in Table 3. Since we are most interested in the effect of Sn-doping on improving stabilization of Li in the Sn-doped V2O5 lattice, we first examine the A3 configuration on Layer1 (L1-A3) which is one of the most stable configurations. It can be seen 𝑓,𝑟𝑒𝑙

from Table 3 that the 𝐸𝑝𝑜𝑙 of the L1-A3 configuration is 0.13 eV indicating that it is less stable to form a polaron at this configuration in the Sn-doped V2O5 system than that in the pure V2O5 15 ACS Paragon Plus Environment


Page 16 of 34

system. This is not surprising because the Sn-doped V2O5 system contains two polaronic sites, at L1-E10 and L2-E11, causing structural strains in the V2O5 layers. Adding an extra electron onto the Layer1 results in having the structure to accommodate two electron polarons on the same layer 𝑖𝑛𝑠,𝑟𝑒𝑙 which is energetically unfavorable. On the other hand, the 𝐸𝐿𝑖 of the L1-A3 configuration (– +

0.26 eV) indicates that Li-ion insertion is more favorable than that in the pure V2O5. Overall, Sndoping creates two electron polarons in the V2O5 lattice and imposes geometric strains into the V2O5 layers which in turn destabilizes formation of the small polaron originated from the inserted Li+ ion. However, the presence of the two electron polarons from the inserted Sn atom helps stabilize the Li+ ion in the Sn-doped V2O5 system resulting in the improved Li insertion. It is interesting that the D, E, and F configurations on Layer1 have very different energies of Li insertion. The D9 and E9 configurations are the least stable of all considered configurations with the insertion energies of –3.22 eV and –3.20 eV, respectively (Figure 4a). On the contrary, the E11 and F11 configurations exhibit the lowest insertion energies of –3.50 eV and –3.51 eV, respectively (Figure 4a). To understand the source of the differences in the Li insertion energies, we explored the contributions from Sn-doping to the stability of Li-ion and its polaron at the L1E9 and L1-E11 configurations. As listed in Table 3, it is highly unstable to form a small polaron 𝑓,𝑟𝑒𝑙

at the L1-9 configuration (𝐸𝑝𝑜𝑙 = 0.31 eV) because the additional polaronic site (L1-E9) is next to one of the existing reduced V centers (L1-10) originated from the inserted Sn atom. Creation of 𝑓,𝑟𝑒𝑙

a small polaron at the L1-11 configuration is far more stable (𝐸𝑝𝑜𝑙 = –0.06 eV) even though the polaronic site is in the same layer to that of the existed polaron at L1-10, the presence of the nearby Sn-ion dramatically stabilizes the polaron formation. While there is a large difference in the 𝑖𝑛𝑠,𝑟𝑒𝑙 polaron formation energies between the L1-E9 and L1-E11 configurations, their 𝐸𝐿𝑖 are +

equivalent (–0.04 eV) because these two configurations share the same location of the Li+ ion. The 𝑖𝑛𝑠,𝑟𝑒𝑙 𝐸𝐿𝑖 of –0.04 eV is comparable to that of the pure V2O5 system but significantly higher than +

that of the L1-A3 configuration due to the shorter Li–Sn distance for the L1-E9 and L1-E11 configurations. It is clear that the stability of both Li-ion and small polaron plays a crucial role in determining the energy of Li insertion in the Sn-doped V2O5 system. Having two polaronic sites next to one another in the same layer leads to energetically unfavorable configurations. Furthermore, the Liion does not like to be close to the pre-inserted Sn-ion due to the electrostatic repulsion between positive charges. In conclusion, at low Li concentrations Sn-doping facilitates Li insertion by 16 ACS Paragon Plus Environment

Page 17 of 34 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


stabilizing Li-ion while maintaining the stability of small polarons in the lattice. The most favorable configurations consist of the intercalated Li in the same channel as the pre-inserted Sn atom, however, the configurations with the Li-ion next to the Sn-ion are found less stable. Examples of the stable configurations are L1-A, L1-B and L1-C. The stability of Li in the Sndoped V2O5 lattice becomes lower as the Li concentration increases since Li-Li and Li-Sn interactions are repulsive. 3.3.3. Effects of Sn-doping on Li-ion migration

To gain deeper understanding on the promoting effects of Sn-doping toward Li intercalation in the V2O5 cathode, we further investigate migration processes of Li-ion in the Sn-doped V2O5 lattice. We used the CI-NEB method to locate transition states (TSs) and computed diffusion barriers of Li-ion between pairs of adjacent minima in the Sn-doped V2O5 system and compared them with that in the pure V2O5 system. Li-ion migration was considered within the interlayer spacing in the [010] direction. Other migration paths including the diffusion through a V2O5 layer along the [001] direction were determined to be at least ~0.3 eV less favorable.43,63 Note that the extra electron from the intercalated Li+ ion remains localized at one V center throughout the migration process. We first computed the diffusion barrier of Li-ion in the pure V2O5 system. The considered diffusion path includes an event of a Li+ ion travelling within close proximity to its electron polaron. Computations show that the Li+ ion has to overcome a relatively small diffusion barrier of 0.16 eV. The calculated diffusion barrier is consistent with recently published values using PBE+U42 (0.15 eV) and PBE+U-vdW-DF246 (0.16 eV) functionals, but lower than the previously calculated value using a regular DFT with PBE functional of 0.39 eV.63 The relatively high calculated barrier based on PBE functional might have its origin in the erroneously delocalized electron from the inserted Li+ ion. Interestingly, our calculated value is lower than that using the DFT+U with PBE-vdW functional (0.31 eV).42 This difference might be attributed to the use of the calculated lattice parameters of the V2O5 unit cell (Table 1) that could cause variations in the transition state (TS) structures and energies. In addition, the PBE+U barrier of 0.37 eV reported by Ma et al.43 is similar to the value calculated with the DFT-PBE method, 0.39 eV,63 but is much higher than our calculated barrier, 0.16 eV, even though the same value of U3d(V) = 4 eV was applied. The origin of such contrasting results is still unclear. We explored the behavior of Li-ion migration in the present of Sn using the 133 V2O5 supercell. Activation energies related to the Li-ion movement in every channel were computed



Page 18 of 34

except the ion movement in the Channel-Sn-R because the pre-inserted Sn atom blocks the diffusion path and prevents continuous movement of Li-ion in the lattice. The calculated energy profiles can be found in Supporting Information Section S6. Analysis of Li insertion energies and diffusion barriers at various locations helps in understanding the effect of Sn on the mobility and stability of Li. Both insertion energies and diffusion barriers vary not only with the Li-Sn distance (dLi-Sn) but also the polaron separation (polLi-polSn) and the distance between the Li ion and the nearest polaron of Sn (Li-polSn). The latter two factors tend to correlate with one another that the greater Li-polSn the larger polLi-polSn separation. Hence, we constructed contour plots that show relationships between insertion energies (diffusion barriers) and geometrical parameters (ion-ion, ion-polaron, and polaron pair separations), as shown in Figure 5. It can be seen that the improved stability and mobility of Li are obtained at intermediate separations of Li-Sn ion (550–800 pm) and Li-polSn (400–600 pm) since they exhibit low insertion energies and diffusion barriers. In contrast, the stability and mobility of Li are diminished when Li is too close or too far from the Sn ion and its polarons. When Li is too close to Sn the ion-ion repulsion dominates and there is higher chance of having an adjacent polaron pair which causes lattice strains and leads to high energy intermediates and TSs. At intermediate separations, the ion-ion repulsion is reduced, the chance of having a close polaron pair is less, then the Li-polSn attraction becomes significant leading to stabilization of intermediates and TSs. While the average insertion energy of Li in Sn-doped V2O5 (–3.36 eV, SD = 0.08 eV) is lower than that of pure V2O5 (–3.24 eV), its diffusion barrier is hardly affected (169 meV, SD = 53 meV vs 161 meV in pure V2O5). Nevertheless, the high SD value of diffusion barrier indicates inhomogeneity of Li movement affected by Sn. Overall, the calculated results suggest that, at low Li concentrations, doping V2O5 with small amounts of Sn could enhance ion intercalation by stabilizing Li-ion insertion while not diminishing its mobility in the lattice. It is interesting to also consider the mobility of Sn in the V2O5 lattice. The calculated activation energy of Sn diffusion (1.46 eV) is comparable to those of other multivalent cations calculated using PBE+U functional, Mg (1.15–1.23 eV) and Al (1.13–1.59 eV).46 Such a high barrier is expected because the multivalent Sn ion strongly interacts with the lattice oxide anions in the V2O5 framework. In addition, we examined the effect of Li intercalation on the Sn diffusion behavior where an event of Sn movement closest to Li (configuration L1-E9) is considered. Interestingly, the activation energy of Sn diffusion (1.06 eV) is greatly reduced in the vicinity of the intercalated Li. The presence of Li ion and its polaron at L1-E9 configuration induced reorganization of two VO5 units directly above Sn at TS resulting in relatively more relax TS structure and low energy, 18 ACS Paragon Plus Environment

Page 19 of 34 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


as shown in Figure S7. Nevertheless, the diffusion barriers of Sn are significantly higher than that of Li (0.17 eV on average) suggesting that Sn hardly moves during the migration process of Li. 4. Conclusions

This work employed the DFT+U method to study the effects of Sn-doping on the behavior of Li intercalation and diffusion in the V2O5 based cathode materials. Computations reveal that Sn prefers to occupy a channel between two V2O5 layers. The calculated electronic structure shows that the bonding interaction of Sn-V2O5 exhibits mixed ionic/covalent character. The inserted Sn atom donates two of its four valence electrons to the nearby V centers (ionic character) forming small polarons in the lattice whereas the other two electrons are shared with the surrounding oxygen atoms (covalent character). The mixed ionic/covalent bonding character of Sn-doped V2O5 plays an important role in enhancing Li intercalation in the material. Electron polarons generated from Sn-insertion increase the number of charge carriers in the V2O5 lattice which could improve electronic conductivity of the material. Higher electronic conductivity could in turn lead to high diffusion kinetics of Li-ion in the V2O5-based cathode. Furthermore, co-existence of Sn-ion and small polarons in the V2O5 structure imposes geometric strains into the lattice which primarily affects the behavior of Li intercalation. The calculated diffusion barriers in conjunction with the insertion energies reveal that Sn-doping promotes Li-ion insertion by stabilizing the intercalated Li-ion while maintaining ion diffusion property. Nevertheless, trapping and blocking of ion diffusion path may be expected due to the presence of Sn in the V2O5 lattice. Associated content

Supporting Information Detailed results (S1: Position of Li-ion in V2O5 lattice; S2: Formation energies of Sn insertion and Sn substitution in bulk V2O5; S3: Most stable configuration of Sn and polarons in Sn-doped V2O5; S4: Bader charges of Sn-doped V2O5 and interpretation of XPS chemical shift; S5: Polaron migration barriers; S6: Potential energy profiles of Li diffusion in the Sn-doped V2O5) Author information

Corresponding author *Email: [email protected], Tel. +66 44 224-886 Notes: The authors declare no competing financial interest.



Page 20 of 34

Acknowledgements

This work was primarily funded by the Development and Promotion of Science and Technology Talents Project (DPST Research Grant No. 016/2559). We would like to thank the Synchrotron Light Research Institute (SRLI), Thailand and National e-Science Infrastructure Consortium for computational resources. SJ was supported by Energy Conservation Promotion Fund (Energy Policy and Planning Office, Ministry of Energy), and National Science and Technology Development Agency (NSTDA), Thailand. References

(1) Goodenough, J. B.; Kim, Y. Challenges for rechargeable Li batteries. Chem. Mater. 2010, 22, 587–603. (2) Goodenough, J. B.; Park, K.-S. The Li-ion rechargeable battery: a perspective. J. Am. Chem. Soc. 2013, 135, 1167–1176. (3) Etacheri, V.; Marom, R.; Elazari, R.; Salitra, G.; Aurbach, D. Challenges in the development of advanced Li-ion batteries: a review. Energy Environ. Sci. 2011, 4, 3243–3262. (4) Marom, R.; Amalraj, S. F.; Leifer, N.; Jacob, D.; Aurbach, D. A review of advanced and practical lithium battery materials. J. Mater. Chem. 2011, 21, 9938–9954. (5) Girishkumar, G.; McCloskey, B.; Luntz, A. C.; Swanson, S.; Wilcke, W. Lithium - air battery: promise and challenges. J. Phys. Chem. Lett. 2010, 1, 2193–2203. (6) Scrosati, B.; Garche, J. Lithium batteries: status, prospects and future. J. Power Sources 2010, 195, 2419–2430. (7) Armand, M.; Tarascon, J.-M. Building better batteries. Nature 2008, 451, 652–657. (8) Kim, T.-H.; Park, J.-S.; Chang, S. K.; Choi, S.; Ryu, J. H.; Song, H.-K. The current move of lithium ion batteries towards the next phase. Adv. Energy Mater. 2012, 2, 860–872. (9) Goriparti, S.; Miele, E.; De Angelis, F.; Di Fabrizio, E.; Proietti Zaccaria, R.; Capiglia, C. Review on recent progress of nanostructured anode materials for Li-ion batteries. J. Power Sources 2014, 257, 421–443. (10) Li, H.; Wang, Z. X.; Chen, L. Q.; Huang, X. J. Research on advanced materials for Li-ion batteries. Adv. Mater. 2009, 21, 4593–4607. (11) Mizushima, K.; Jones, P. C.; Wiseman, P. J.; Goodenough, J. B. LixCoO2 (0

Effect of Sn-doping on Behavior of Li-intercalation in V2O5 Cathode

Recommend Documents