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Computational Investigation of Li Insertion in Li3VO4 M. Elena Arroyo-de Dompablo, Pedro Tartaj, J. Manuel Amarilla, and Ulises Amador Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b01519 • Publication Date (Web): 18 Jul 2016 Downloaded from http://pubs.acs.org on July 23, 2016

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Chemistry of Materials

Computational Investigation of Li Insertion in Li3VO4

M. Elena Arroyo-de Dompablo1,*, Pedro Tartaj2, J. Manuel Amarilla2 and Ulises Amador3

1

Malta Consolider Team, Departamento de Química Inorgánica, Universidad Complutense de Madrid, 28040

Madrid, (Spain) 2

Instituto de Ciencia de Materiales de Madrid, CSIC, Campus Universitario de Cantoblanco, 28049 Madrid,

(Spain) 3

Departamento de Química, Universidad San Pablo- CEU, 28668-Boadilla del Monte, (Spain).

Corresponding author: *[email protected] Abstract Parallel electrochemical reactions in low-voltage anode Li3 VO4 represent a fundamental barrier for fully reliable mechanistic studies even using the most advanced electrochemical and structural characterization techniques. Aiming to unravel the lithium insertion mechanism in Li3 VO 4 anodes, we have investigated by Density Functional Theory a total of 33 Li 3+x VO4 configurations (x = 0, 0.5, 1, 2, 2.5, 3). The key aspect of the proposed Li insertion mechanism is the structural rearrangement of the host-Li3VO4 as larger amounts of Li ions are incorporated, due to ion size and electrostatic effects. We found that for 0 < x < 2 the Li 3+x VO 4 phases are energetically stabilized by the distortion of the initial hexagonal package of the oxygen array (H1H2 transformation). Specifically, a very stable intermediate H2-Li5 VO 4 phase is formed after a biphasic region at 0.7 V. The close structural relationship between H1-Li3VO4 and H2-Li5VO5, with a moderate volume expansion of 4%, supports an insertion reaction as the main mechanism for the reversible cycling of Li3+x VO4 anodes in the range 0 < x < 2. Insertion of a third Li ion in Li3VO4 would produce a reconstructive

phase

transformation

to

an

antifluorite-type

Li 6 VO 4

structure

(H2

C

transformation). The low predicted voltage for such process (0.14 vs Li/Li+), and the major structural rearrangements (20% volume variation) make unlikely the reversible insertion of 3 Li ions in Li3VO4.

The calculated compositional-voltage profile and X-ray diffraction patterns are in

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Introduction Arguments based on specific energy require the development of battery anodes that can operate at sufficiently low voltages. However, working at low voltages comes with numerous disadvantages that affect safety, and from a more fundamental approach can obscure mechanistic studies. At low potential (below 1.0 V) electrolyte degradability is extensive, which overcomplicates mechanistic studies both in insertion and conversion electrodes. Furthermore, carbon-based conductive additives (typically carbon additives, sophisticated coatings and/or graphene-like materials) become electrochemically active, adding specific capacity that is cycle and intensity rate dependent. Thus, unequivocally determination of mechanisms is extremely difficult even using the most advanced electrochemical and structural characterization techniques. In this direction, computational methods can help to confirm/suggest plausible mechanism for energy storage in anode materials. In the present work we use Density Functional Theory (DFT) methods to elucidate the mechanism of Li insertion in the anode material Li3VO4. Li3VO4 has emerged as a potential anode material for Li ion batteries.1-3 This material can reversibly deliver a specific capacity of c.a. 400 mAh/g at a safe but still low voltage window between 0.2 and 1 V. It is generally accepted that this capacity arises from the insertion reaction of Li ions in the Li3VO4 host and the concomitant reduction of V5+ ions to lower oxidations states. The main drawback of Li3VO4 is its highly insulating character, hence a variety of electrode preparations have been utilized aiming to improve its rate capability. A good summary of the different electrode-preparations, which include synthesis approaches to tailor particle size and morphology, and carbon coating procedures, is provided in Ref. 4. For most of these electrode conformations, side reactions mask the actual specific capacity delivered by the active material. Thus, while some studies report a limit for the reversible capacity equivalent to the insertion of two Li ions 1, other studies point to the reversible insertion of a third Li ion.5 Despite the numerous efforts to improve the electrode performance,1-24 little is known about the mechanism of Li insertion in Li3VO4. Not only the maximum Li uptake is uncertain, but also the formation of intermediate Li3+xVO4 phases is a matter of debate. Recent in-situ

5,25

and ex-situ 1 XRD studies reported the occurrence of crystalline monophasic and

biphasic regions along the reaction path of Li with Li3VO4. The new peaks observed for the possible intermediate Li3+xVO4 phases differ from one work to other, and have not been interpreted on a basic of any 2 ACS Paragon Plus Environment

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crystal lattice. Other ex-situ XRD study carried out after discharging the cell at 0V and washing with ethanol and water, even found an almost amorphous state of the electrode for the first and subsequent charge/discharge processes. The authors described this result as similar to the redox reaction mechanism of NiO (a conversion electrode material).2 Regardless these discrepancies, most of these studies agree in that the peaks from the initial Li3VO4 material are recovered at the end of the first charge, supporting a reversible insertion mechanism for the Li uptake. For a reversible lithium insertion to occur, the host material (herein Li3VO4) must have, among others, an open framework structure containing empty crystallographic sites to accept lithium ions, and must contain redox centers. In this direction, XPS and XAFS experiments confirmed the reduction of V5+ to V3+ in Li//Li3VO4 cells discharged down to near 0 V.2,25 In contrast, from the existing structural data, the existence of interstitial sites for the topotactic insertion of large amounts of Li within the Li3VO4 framework structure is more questionable. The crystal structure of Li3VO4 is related to the low temperature form of Li3PO4, denoted as βLi3PO4.26 It consists of a distorted hexagonal packing of oxygen ions with half of the tetrahedral sites occupied by Li and V. The [LiO4] and [VO4] tetrahedra share corners, with all the tetrahedra pointing in the same orientation parallel to the c axis (see Figure 1a).26 In addition to the three lithium ions (Li1 in 4b and Li2 in 2a sites) already present in the Li3VO4 structure, the 2a (½, ½, 0.68) and 4b (¼ , 0.0028, ¼) sites are available for extra lithium atoms. Liang et al. performed a computational investigation of Li insertion in Li3VO4, considering the full filling of these two sites, to reach a Li6VO4 composition.5 According to these authors, the framework of Li3VO4 acts as a hollow lantern-like 3D structure capable of reversibly inserting the 3 Li ions.5 However, the authors do not report any crystallographic /energetic information of the optimized Li6VO4 structure; neither calculate the lithium insertion voltage to confront with experiments, which would assess the topotactic insertion of three Li ions. It should be noted that the 2a sites (in yellow in Figure 1a) and the 4b sites (in dark blue in Figure 1a) are located in the tunnels parallel to the z-axis of the structure in polyhedra which share faces with the LiO4 and VO4 tetrahedra of the Li3VO4 framework. Thus, a topotactic insertion of three Li ions in the Li3VO4 structure as proposed by Liang et al. is difficult to conciliate with structural arguments.5 First principles calculations can help to unravel the key aspects of the mechanism by which Li ions could be inserted in Li3VO4. In order to reach this purpose, in this first work we have investigated 3 ACS Paragon Plus Environment


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thirty Li3+x VO4 configurations with x values of 0, 0.5, 1, 2, 2.5, 3. Following the crystallographic description of the more stable Li 3+x VO 4 phases, we present the calculated voltage-composition profile. We will show that a sequence of structural phase transformations (hcp hcp ccp) constitutes the most favourable pathway for Li insertion into Li3VO4. Finally, we discuss the reversible capacity attainable assuming an insertion reaction as the only mechanism for the Li reaction with Li3VO4.

Methodology The total energy calculations and structure relaxations for pristine Li3VO4 and intercalated phases Li3+xVO4 (0 < x ≤ 3) were performed with the Vienna Ab-initio Simulation Package (VASP).27 Calculations were done within the GGA+U framework with the Projector Augmented Wave (PAW) pseudopotential28 and utilizing the PBE form of exchange-correlation functional.29 The V (3p, 3d, 4s), O (2s, 2p) and Li (1s, 2s ) were treated as valence states. DFT+U calculations were performed following the simplified rotationally invariant form proposed by Dudarev.30 Within this approach, the onsite Coulomb term U and the exchange term J, can be grouped together into a single effective parameter (U-J); the J value was fixed to 1 eV and an of U = 4 eV was used for the V(3d) states (Ueff= 3 eV). This Ueff value is appropriate for vanadium based oxides, as inferred from previous works : 3 eV for LiVOXO4,31 3.1 eV for VO/V2O5,32 and 2.72 eV for MVnO(2n+1).33 The energy cut off for the plane wave basis set was kept fix at a constant value of 600 eV throughout the calculations. The integration in the Brillouin zone is done on an appropriate set of k-points determined by the Monkhorts-Pack scheme. A convergence of the total energy close to 10 meV per formula unit is achieved with such parameters. Spin polarized calculations were performed in all cases. The atomic positions for Li3VO4 were taken from Ref. 18. Since there are two Li3VO4 formula units per unit cell, the full filling of the empty 2a and 4b sites proposed by Liang at al.5 leads to a maximum composition Li12V2O8. For compositions in between Li6V2O8 and Li12V2O8, various lithium vacancy arrangements are possible. We consider such arrangements restricted to the (1×1×1) unit-cell. Hence, there can be six different LiyV2O8 concentrations with y ranging from 6 to 12 Li ions/unit-cell. Normalized to the formula-unit of Li3+xVO4, this corresponds to x = 0, 0.5, 1, 1.5, 2, 2.5 and 3. Structural models for the Li3+xVO4 phases were constructed considering the partial filling of the 2a and/or the 4b positions suggested by Liang and co-workers.5 In a preliminary inspection, the total energy of Li3+xVO4 structural models were 4 ACS Paragon Plus Environment

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calculated allowing the relaxation of atomic positions, cell parameters and cell volume, while keeping the symmetry of the initial Li3VO4 (Pmn21). We found very unstable Li3+xVO4 structures, which result in too low or even negative Li insertion voltages. Hence, all crystal structures were fully relaxed (atomic positions, cell parameters and volume), without imposing symmetry constrains. The final energies of the optimized geometries were recalculated to correct the changes in the basis set of wave functions during relaxation. Search for pseudo symmetry in the final optimized models was done using the PLATON and the Bilbao Crystallographic Server softwares.34,35 The oxidation state around each V ion was approximated by integrating the unpaired electron density within a sphere of radius 2.0Å. Intercalation voltages were computed following the methodology described by Adynol et al.36

Results The calculated lattice parameters and volume for the optimized Li3VO4 are in good agreement with experiments (see Table S1 in S.I.). For the calculation of Li6VO4 (Li3+xVO4 with x =3) we considered two crystallographic approaches. In the first approach we started from the optimized Li3VO4 filling at once the 2a and 4b sites. In the second approach, Li ions were progressively inserted one by one in the 2b and 4b sites of the optimize Li3VO4 structure, allowing the full structure relaxation of the intermediate Li3+xVO4. Figure 1b and c show a view of the two fully relaxed Li6VO4 structures. Both structures consist of edge-sharing [LiO4] and [VO4] tetrahedra. The atomic coordinates, selected bond lengths, calculated lattice parameters and volume are given in S.I. The optimized structure for the first approach preserves the Pmn21 symmetry of Li3VO4, though the inserted Li ions displaced from their initial 2a and 4b sites to avoid face-sharing. Hereafter this structure will be denoted as H1-Li6VO4. The structural model of the second approach relaxed to a new structure (S.G. P42/nmc) , that we identified with the Li6ZnO4 structural type,37 which presents similarities to the anti-fluorite type structure (see Figure S1 in S.I.). This structure is commonly adopted by compounds where the number of cations almost doubles the number of anions, such as Li6MnO4,38 Li5FeO4, 39

or Li6CoO4.40 We found this antifluorite-Li6VO4 (or C-Li6VO4, where C stands for cubic) 0.8 eV per

formula unit more stable than H1-Li6VO4. Therefore, the calculated average voltages for the reaction Li3VO4 + 3 Li Li6VO4 are 0.2 V for H1-Li6VO4 and 0.5 V for C-Li6VO4.

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We calculated the total energy of 30 intermediate Li3+xVO4 phases with x =0.5, 1, 1.5, 2, and 2.5. Tables S1 and S2 in S.I. summarized the crystallographic information of the most stable structure found at each composition. Analysing the low energy Li3+xVO4 structures, one can extract general trends with the increasing amounts of inserted Li ions: i) volume expansion, obeying the presence of the larger reduced transition metal cations, together with the inserted lithium ions, ii) change of the vanadium coordination sphere, and iii) modifications affecting the oxygen array. For a closer view of the structural rearrangement we firstly look at the oxygen array in Li3+xVO4 phases; three types of oxygen lattices are detected, labelled as H1, H2 and C in Figure 2. At low lithium contents (x = 0.5 and 1) the structures retain the initial distorted close hexagonal package (H1, oxygen interlayer distance 2.50 Å). As the Li content increases (x = 1.5, 2 and 2.5) the hexagonal layers suffer a severe deformation. In addition, in alternate layers, oxygen ions shift out from the plane (in pink in Figure 2). The resulting oxygen array (denoted as H2) preserves the ABAB stacking sequence (interlayer distance 2.62 Å), but it clearly differs from the initial one. As discussed above, the final C-Li6VO4 has a cubic close package. The close-packed oxygen layers lying parallel to the (0-11) plane being the ABCABC stacking sequence along the [011] direction of the structure. For each of the above described oxygen arrays, the V ions form distinct sub-lattices; interestingly those in H1 and H2 are pretty similar whereas that in the cubic structure is very different (Fig.3). An analysis of all the calculated Li3+xVO4 structures reveals that in the H1-structures the V ions are exclusively in tetrahedral coordination, regardless its oxidation state. On the contrary, all the H2 structures have half of the V ions in octahedral coordination (in dark blue in Figure 3). For instance, the most stable structures at Li4.5VO3 and Li5VO3 have a H2-host. The structure of Li4.5VO3 (x = 1.5) consists of half V3+ in octahedral and half V4+, in tetrahedral coordination (S.G. P1). The ground state at x = 2 (shown in Figure 3) consists of V+3 ions, half presenting octahedral coordination, and the other in tetrahedral coordination (S.G. Pm). Figures 2 and 3 evidence that this change of coordination of vanadium ions when going from H1 structures to H2 mostly implies oxygen displacements, but some rearrangement of the cationic sub-lattice also occurs. In terms of volume variation, for the H2 structures, the predicted cell volume expansion with respect to pristine Li3VO4 is low; 3% at x = 1.5 and 4 % at x =2. On the contrary, the C structure presents a drastic increasing of the intra-layer and inter-layer oxygen distances respective to the hexagonal packages (H1 and


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H2), so as to accommodate the larger V2+ ions in tetrahedral sites and the 3 Li ions. As a consequence, CLi6VO4 expanded in 20% with respect to Li3VO4. Having the calculated energy of 33 Li3+xVO4 ( 0 ≤ x ≤ 3) configurations, the structures that are actually stable at low temperature out of this set can be found by constructing the convex hull of the formation energies. The formation energy is defined as: ∆fE = E – (x/3)E Li6VO4 - (1-x/3) ELi3VO4

(1)

where E is the total energy of the configuration per Li3+xVO4 formula unit, ELi6VO4 is the total energy of H1Li6VO4 and ELi3VO4 is the total energy of Li3VO4. Figure 4a shows the calculated formation energy, indicating the structural type present for each configuration (H1 in crosses, H2 in diamonds, and C in circles). Negative energies indicate that Li3+3xVO4 is stable with respect to a two-phase mixture of Li3VO4 and Li6VO4 at low temperatures. The convex hull, which is drawn in Figure 4a, is the line that connects the lowest energy phases along in the formation energy vs composition representation. When the energy of a particular ordered structure is above a tie line, it is unstable with respect to a mixture of the two structures that define the end points of the tie line. In the present case the vertices of the convex hull are at x= 1.5 and x= 2 structures. This results in steps in the voltage-composition profile at those compositions. Importantly, a closer look to the stability diagram shows that H2-compositions around 2.5 Li ions are very close to the convex hull line. As this initial study is constricted to the Li3VO4 unit cell (see methodology section) we can not discard that if supercells were considered a H2-Li5.5VO4 configuration might lie in the convex hull, thus, expanding the compositional region of consecutive H2 single phase regions above 2 Li ions. Figure 4b shows the calculated voltage-capacity curve of Li3VO4 at 0K. In the calculated charge profile single-phase regions appear at Li3+xVO4 with x = 1.5 and 2, as dictated by the convex hull. The insertion of 1.5 Li ions in Li3VO4 occurs at 0.73 V across a biphasic region where the initial H1 and the H2 lattices coexist. At the composition Li4.5VO4 (specific capacity 300 mAh/g), there is small voltage step of 0.05 V, which will likely vanish with temperature. Further insertion of 0.5 Li ions at 0.68 V results in the formation of the more stable phase H2-Li5VO4 (400 mAh/g). Following this single-phase region, a voltage plateau is predicted at 0.14V, to reach the final CLi6VO4 phase. Above 0 K, entropy effects would smooth the voltage-composition profile, resulting in a sloping voltage curve with the ordered phases appearing as monophasic regions of either ordered 7 ACS Paragon Plus Environment


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phases (if order persist) or becoming solid solutions domains. It should be highlighted that the stable single phases (Li3VO4, Li4.5VO4 and Li5VO4) could span in a wider compositional region as determined by phase stability (for details on phase diagrams of Li electrode materials see references 41, 42

). Construction of the phase diagram will require the calculation of around 150 Li3+xVO4

configurations, and it is out of the scope of the present work. Moreover, the actual compositional domain at which monophasic region appears also to depend on the experimental conditions.

Discussion In this initial DFT investigation of Li insertion in Li3 VO 4 anode material, we have considered a total of 33 Li3+xVO4 configurations in the unit cell of Li3 VO4. As mentioned in the Results section, three types of hostages (H1, H2 and C) can be identified in the optimized structures. Remarkably, the ground states lying on the convex hull, that actually determine the single-phase regions in the voltage-composition curve, do not retain the H1-host of the initial phase. Yet, to further discuss the topotactic insertion in Li3VO4, we have calculated the (non-equilibrium) voltage-composition profile considering only the H1-structures (see Figure S2 in S.I.). The H1-Li5VO4 will form at 0.5 V, which is too low compared with experiments. Remarkably, the formation of Li6VO4 within the H1-framework is energetically forbidden; negative Li insertion voltages are calculated for the Li insertion in H1-Li5VO4 to form H1- Li6VO4. This, undoubtedly, excludes a topotactic pathway for the insertion of 3 Li ions in Li3VO4. We found that a structural rearrangement H1 H2 C is energetically the most favourable pathway for Li insertion into Li3VO4, The driving force for the structural distortion H1 H2 is the increasing size of the reduced vanadium ions. The H2 host, with half of the V ions in square-pyramid or octahedral coordination, is energetically stable in the compositions where V ions have been reduced to the formal oxidation state of V3+. However, in the H2-host the other half V ions remain in tetrahedral coordination. Interestingly, the displacement of oxygen ions out of the plane when going from H1 to H2 (Figure 2) serves to enlarge the tetrahedral sites of the host. In conclusion, size effects drive the H1 H2 transformation. As larger amounts of Li ions are inserted in the H-host, the electrostatic repulsions increases (see Li-Li and Li-V distances in S.I.) inducing the H C transformation. This is evident comparing either the H1-Li6VO4 or H2- Li6VO4 with C-Li6VO4. In the two H-hostages, there are too short V-Li and Li-Li 8 ACS Paragon Plus Environment

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distances of about 2.5 Å and 2.1 Å, respectively. In the more stable structure of C- Li6VO3, the ccp-oxygen array (C) permits greater V-Li and Li-Li distances, of 2.72 Å and 2.32Å, respectively. Consequently, to accommodate a large amount of Li cations while minimizing electrostatic repulsions, the unit cell of CLi6VO4 has expanded in 20% with respect to the initial Li3VO4 unit cell. The fact that V2+ resides in tetrahedral coordination supposes a penalty in energy, thereby even the more stable C-Li6VO4 forms only at very low voltages (0.14 V). A legitimate question is whether the H1 H2 C transformations are reversible. With the present results we cannot give a definitive answer. The H1 H2 transformation mostly implies the distortion of the oxygen layers, associated to larger polyhedra around the V ions. The overall H1 H2 volume expansion is low, 4%, which suggest that the structural reorganization is moderate, so that this could be a reversible transformation. Computationally, when removing the Li ions from H2-Li5VO4 the relaxed Li3VO4 structure reverts to the H1 host. However, in reality phase transformations also depend on kinetics. In any case, the present results support a reversible Lithium insertion process in within the compositional range at which the hexagonal close packing (H1 or H2 structures) is stable. The more severe reconstruction H2 C (15.6% volume expansion) is very unlikely to be reversible. Computationally, when removing the Li ions from CLi6VO4 the relaxed Li3VO4 structure does not recover the H1 package. Indeed, an irreversible Li-insertion driven transformation from a hexagonal anionic packing to a close cubic packed is well documented for αFe2O3 (corundum structural type). Such irreversible shift was firstly reported by Thackeray et al.43 in Li molten-salt cells at 400ºC and later confirmed by Larcher et al.44 in Li cells discharged at room temperature. Nonetheless, even in the case of the H1 H2 C transformations being reversible, the large volume variation (20%) within H1-Li3VO4 and C-Li6VO4 hinders the continued cycling of the three lithium ions assuming a Li insertion mechanism. Yet, alternative mechanisms (say conversion) could be operating for Li uptakes beyond the compositional domain of the hexagonal Li3+xVO4 single phases. Finally, it is relevant to compare our computational findings with some experimental results. As mentioned in the Introduction, some studies combine structural and electrochemical techniques to explore the mechanism of Li insertion in Li3VO4.1,5,11,25 In principle, these studies could be ideal for comparison with DFT results. Regardless of the total capacity, or more specifically the capacity reached at any voltage, all these studies show in the first discharge curve (0.02-0.1 A/g intensity rate) a plateau region at 0.7-0.9 V 9 ACS Paragon Plus Environment


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followed by what appear to be a monophasic process or a strongly hindered electrochemical process. Interestingly, GITT (galvanostatic intermittent titration technique) studies carried out in some of these reports show the same voltage profile.5 This similarity in voltage profile despite differences in Li uptake capabilities suggests that overall Li insertion mainly occurs through a defined mechanism. Of special relevance is the fact that the more characteristic feature of the voltage profile (plateau at 0.7-0.9) is similar to our theoretical predictions corresponding to the biphasic region between H1 and H2 at c.a. 0.7 V. Furthermore, the similarity in voltage profiles suggests that the structural characterization of these samples could be used for comparison. To facilitate comparison with experiments, Figure 5 shows the calculated XRD patterns of the single-phases at H1-Li3VO4, H2-Li5VO4 and C-Li6VO4. Our predicted biphasic region at 0.7 V between Li3VO4 and H2-Li5VO4, and the predicted variations in the XRDP according with the optimized structures of H1-Li3VO4 and H2-Li5VO4, are consistent with the reported in-situ XRD experiments collected during the first discharge of Li//Li3VO4 cells.5,25 Extracting capacity data from these different samples in order to carry out reliable comparisons with the theoretical findings of this work, and so help to explain the performance of Li3VO4 anodes is a difficult task. Anodes built from these samples work at potentials in which electrolyte degradation is extensive and the different forms of carbon added to improve the electrode conductivity are electrochemically active. An adequate approach to carry out this comparison is to establish a line between those experimental works that limit the uptake to 2 Li and those that extend their uptake above 2 Li. Interestingly, the most recent studies clearly indicate that good reversibility at high rates is observed when Li uptake is limited below about 2 Li.25 In fact, the clearest experimental evidence that good reversibility takes place at around 2 Li, is given in a recent study by the groups of Mai and Zhou.10 These authors show that in properly treated mesoporous Li3VO4 anodes it is possible a reversible insertion of 2 Li, with a difference in capacity of only 5% between the first and the second cycles. In this direction, our DFT studies confirm that a reversible Li insertion reaction is a plausible mechanism within the stability range of the hexagonal close packing (H1 and H2 structures). A different scenario emerges from experiments that involve the uptake of more than 2 Li. Among these studies Ni and co-workers have recently shown that in C doped Li3VO4 nanocrystals, reversible capacities of around 550 mAh/g are attainable at slow rates after the second cycle.17 In order to explain this 10 ACS Paragon Plus Environment

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high value through our computational findings, it is important to estimate if the intrinsic response of the electrode could be considered to be around 550 mAh/g or lower as the electrodes contained around 20 wt% in carbon (doping plus carbon black) that is electrochemically active at operando conditions. Thus, a relatively modest correction of 25-50 mAh/g (5-10% of 550 mAh/g) suggests that the intrinsic response must be close to 2.5 Li (500 mAh/g). As mentioned above a closer look to the stability diagram (Figure 4) shows that compositions around 2.5 Li are very close to the convex hull line, and importantly they retain the H2packing for which a reversible lithium insertion process is plausible. An inserted phase close to 2.5 Li ions could, thus, explain experimental values around 500 mAh/g at slow rates. Alternatively, the nucleation of the Li6VO4 fluorite-type phase, with a 20% volume expansion, could certainly cause the particle cracking, a possibility already pointed out by other authors (see Ref. 17 and references therein). In that case, concomitant side reactions due to surface reactivity and SEI formation would contribute to raise the observable specific capacity above 400 mAh/g. Finally, in the line of first works by Ni et al 2, our studies cannot discard that some conversion mechanism could operate after the insertion process. This conversion mechanism could also explain the poor reversibility of anodes at high rates when more than c.a. 2.5 Li ions are involved in the reduction process.

Conclusions This first DFT study involving 33 Li3+xVO4 (0 ≤ x ≤ 3) configurations have allowed us to conclude that Li3VO4 cannot topotactically insert 3 Li ions. The DFT results, which are able to match the experimental voltage-composition curves, unravel an insertion mechanism based on the structural rearrangement of the host-Li3VO4. The energy of the Li3+xVO6 phases is minimized by a sequence of phase transitions in which the initial hexagonal package (H1) distorts to a second hexagonal package (H2) and finally transforms to a cubic-close package (C). An analysis of the calculated (H1, H2, C)-Li3+xVO4 structures indicates that the H1H2 transformation supposes a moderate structural rearrangement, while H C is a reconstructive phase transition unlikely to be reversible. These successive phase transformations (H1H2C) are driven by the combination of size and electrostatic effects. The proposed mechanism explains the experimental results that observe reversible cycling when specific capacities are close or below to the insertion of c.a. 2 Li (ca. 400 mAh/g). Specifically, DFT 11 ACS Paragon Plus Environment


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predicts that the insertion of 2 Li ions in Li3VO4 takes place at an average voltage of 0.7 V, resulting in a stable phase H2-Li5VO4 with a moderate low volume expansion (4%). We conclude that a reversible Lithium insertion mechanism is feasible in the compositional range in which the hexagonal close packing (H1 or H2 structures) is stable. Furthermore, DFT has also helped us to explain observed capacity values higher than 400 mAh/g (Li uptake > 2). Basically, DFT predicts that the insertion 3 Li ions would involve the reconstructive transformation to an antifluorite-type C-Li6VO4 structure, with a volume expansion around 20%, and at very low voltages (0.14 V). We conclude that the reversibility of such insertion reaction is highly improbable, thereby opening the possibility for alternative mechanisms operating for Li uptakes exceeding the stability range of the stable hcp single phases ( x =2, in the present study constricted to the Li3VO4 unit cell at 0 K) . These initial computational results might serve to guide experimental research.

From a

fundamental approach, the structures here determined and resolved (XRD patterns are given) can be used for further mechanistic studies involving electrochemically- or chemically-driven Li ions insertion in Li3 VO4 . As a counterpart, experimental confirmations of the proposed phasetransformations are necessary to face more detailed computational investigations, involving supercells analysis to reach a larger number of Li3+x VO4 configurations and at additional x values. Such combination of experimental and computational efforts could open new routes to improve the electrode characteristic of Li3 VO4 . A tentative target are the potential chemical substitutions to ameliorate the poor electronic conductivity of Li3VO4, tailor the output voltage, and/or improve the specific capacity of the material.

Acknowledgements We acknowledge Ministerio de Ciencia e Innovación (Spain) for grants MAT2014-53500-R, CSD2007-00045, MAT2013-46452-C4-1-R and MAT2014-54994-R. J.M. Amarilla thanks PIE201460E123 project (CSIC) for funding. M.E. Arroyo acknowledges access to computational resources from Universidad de Oviedo (MALTA-Consolider cluster) and the Spanish's national high performance computer service (I2 Basque Centre).


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Associated Content Supporting Information. Optimized structures of H1-Li6VO3 and C-Li6VO3 (CONTCAR files). Crystal structure of H2-Li5VO4 and C-Li6VO4 (cif files). Table S1. Calculated lattice parameters, cell volume and suggested space group (S.G.) for the host compound Li3VO4 and the lithium inserted phases. Table S2. Calculated bond lengths (in Å) and oxidation states (O.S.) for the most stable Li3+xVO4 phases. Figure S1 showing the comparison of the structures of (a) antifluorite-Li2O and (b,c) C- Li6VO4.

Figure S2 showing the calculated voltage-

composition profile for the insertion of Li in Li3VO4 considering only H1 structures.

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Figure Captions

Figure 1.View of the optimized crystal structures of Li3+xVO4 polymorphs. (a) Initial Li3VO4 along the [001] axis, with the vacant 2a and 4b sites indicated in dark-blue and yellow, respectively. (b) H1- Li6VO4 structure along the [001] axis and (c) C-Li6VO4 structure along the [100] axis. In the inserted phases (b-c) Li ions in yellow and dark-blue correspond to those in 2a and 4b sites, respectively, in the initial Li3VO4 host. Color code: Li light blue, dark blue and yellow, V green, O red. The crystal axes are defined in the conventional settings of the S.G. for each structure. Figure 2. Oxygen array in the H1 and H2 structural types found for Li3+xVO4 phases. The crystal axes are defined in the conventional settings of the S.G. for each structure. Figure 3. Projections of the H1, H2 and C structures along the stacking directions of close-packed oxygen layers (a, d and g). Projections along two perpendicular directions parallel to the oxygen layers showing the stacking sequences are shown (b and c; e and f, h and i). Li-O polyhedral in yellow, light and dark blue indicate polyhedra corresponding to V ions in tetrahedral and higher coordination, respectively. The vanadium array is indicated as black lines, V-V distances up to 5.5 Ǻ are shown. In every case the crystal structures are described using the conventional settings of the corresponding S.G Figure 4. (a) Formation energies of the different Li3+xVO4 configurations calculated from first principles. The black line is the constructed convex hull. (b) Calculated voltage-composition curves at 0K for Li3VO4 according to the convex-hull. Figure 5. Calculated X-Ray Diffraction Patterns (Cu Kα1) for the optimized structures of (a) H1-Li3VO4 (b) H2-Li5VO4 and (c) C-Li6VO4. For lattice parameters and atomic positions see S.I.



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Figure 3


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0.4

(a)

0.2 0.0 -0.2 -0.4 -0.6

H1-host H2-host C-Li6VO4

-0.8 -1.0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

x in Li3+xVO4 1.4

Calculated Voltage (V)

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Calculated Formation Energy (eV /f.u.)

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(b)

1 Li = 200 mAh/g

1.2 1.0

H1 + H2

0.8 0.6

Li5VO4

4 % volume expansion

0.4

H2 + C

0.2

20 % volume expansion 0.0 0

100

200

300

400

Specific Capacity (mAh/g)

Figure 4


500

600


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Figure 5


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Computational Investigation of Li Insertion in Li3VO4 TOC


Computational Investigation of Li Insertion in Li3VO4 - Chemistry of

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