Edouard Boivin

Rénald David

Jean-Noël Chotard

Tahya Bamine

Antonella Iadecola

Lydie Bourgeois

Emmanuelle Suard

François Fauth

Dany Carlier

Christian Masquelier

Laurence Croguennec
email: laurence.croguennec@icmcb.cnrs.fr

LiVPO4F1-yOy tavorite-type compositions: influence of the vanadyl-type defects concentration on the structure and electrochemical performance

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Tavorite compositions, LiMXO4Y, are positive electrode materials of interest for Lithium-ion batteries. Indeed, the working potential of these phases can be adjusted by changing the nature of the transition metal (with M = V, Fe, Mn, Ti), of the polyanionic group (XO4 with X = P or S) or of the bridging anion (with Y = O, F, OH, H2O). [START_REF] Masquelier | Phosphates, Silicates, Sulfates) Frameworks as Electrode Materials for Rechargeable Li (or Na) Batteries[END_REF] Therefore, the stabilization of new compositions is a major issue, at both the practical and fundamental level.

Vanadium based Tavorite materials LiVPO4F and LiVPO4O are the most attractive ones due to their high working voltage (i.e. 4.26 and 3.95 V vs Li + /Li respectively) and thus to their high theoretical energy density [START_REF] Ateba-Mba | Synthesis and Crystallographic Study of Homeotypic LiVPO4F and LiVPO4O[END_REF] , strongly competitive with the already commercialized LiFePO4 (655Wh/kg vs. Li of theoretical energy density for LiVPO4F against 586Wh/kg vs. Li for LiFePO4). [START_REF] Huang | Lithium Metal Phosphates, Power and Automotive Applications[END_REF] Nevertheless, among the numerous studies which report on apparently pure LiVPO4F, some discrepancies in the average structure and electrochemical properties are observed. Indeed, unit cell volumes fluctuate between V/Z = 87.00 Å 3 and V/Z = 87. 31 Å 3 whereas the difference between LiVPO4O (V/Z = 85.51 Å [START_REF] Huang | Lithium Metal Phosphates, Power and Automotive Applications[END_REF] ) and LiVPO4F is around 1.7 Å 3 . [START_REF] Ateba-Mba | Synthesis and Crystallographic Study of Homeotypic LiVPO4F and LiVPO4O[END_REF][START_REF] Zheng | Novel Synthesis of LiVPO4F Cathode Material by Chemical Lithiation and Postannealing[END_REF][START_REF] Xiao | Transport and Electrochemical Properties of High Potential Tavorite LiVPO4F[END_REF] Recently, Kang and coworkers reported the LiVPO4F~0.25O~0.75 composition whose average structure is similar to the one of LiVPO4F (despite the large O 2-/F -substitution ratio) and whose electrochemical properties are drastically different. [START_REF] Kim | High Energy Density Polyanion Electrode Material ; LiVPO4O1-xFx (X ≈ 0.25) with Tavorite Structure[END_REF] Therefore, the partial substitution of fluorine by oxygen associated with the partial oxidation of V 3+ to V 4+ =O (or {V=O} 2+ ) for charge compensation (i.e. the formation of vanadyl-type defects) could be at the origin of these discrepancies. This hypothesis has already been investigated by Nuclear Magnetic Resonance (NMR) spectroscopy and Density Functional Theory (DFT) calculations [START_REF] Messinger | Revealing Defects in Crystalline Lithium-Ion Battery Electrodes by Solid-State NMR: Applications to LiVPO4F[END_REF][START_REF] Bamine | Understanding Local Defects in Li-Ion Battery Electrodes through Combined DFT/NMR Studies: Application to LiVPO4F[END_REF] but its experimental validation was still challenging due to the low concentration of defects in LiVPO4F. Therefore, we synthetized LiVPO4F1-yOy compositions controlling the substitution ratio of oxygen for fluorine, in order to study the influence of vanadyl-type defects on the average structure as well as on the local environments around vanadium. The electrochemical study performed at high and low voltage domains reveals a strong influence of the vanadyl-type defects' concentration on the electrochemical performance in terms of energy density, cyclability and rate capability.

Experimental part

All along this study the LiV III,IV PO4F1-yOy compounds will be compared to the endmember phases (LiV III PO4F and LiV IV PO4O) whose the syntheses are different to that used to prepare the mixed valence phases (given in details in the synthesis part). The quasi stoichiometric LiVPO4F was synthesized according to the reaction (1), through a two steps carbothermal reduction, as described previously [START_REF] Ateba-Mba | Synthesis and Crystallographic Study of Homeotypic LiVPO4F and LiVPO4O[END_REF] . V2O5 (Sigma-Aldrich, > 98%) is reduced by a small excess of CSP carbon (i.e. a highly divided soot) in the presence of NH4H2PO4 (Sigma-Aldrich, > 99%). A thermal treatment at 800°C under argon flow leads to the formation of VPO4/C, in which the vanadium phosphate particles are surrounded by residual carbon nanoparticles localized in the grain boundaries. After the addition of a stoichiometric amount of LiF to VPO4/C, the mixture is annealed at 800°C under argon flow to form LiVPO4F. LiVPO4O was also synthetized by a solid state reaction from V2O3 (Cerac, ≤ 99.9%), Li3PO4 (Sigma-Aldrich, 97%) and NH4H2PO4 (Sigma-Aldrich, > 99%) as described in reference [START_REF] Ateba-Mba | Synthesis and Crystallographic Study of Homeotypic LiVPO4F and LiVPO4O[END_REF] , according to the reaction (2).

(1)

Step 1 V V 2O5 + 2 NH4H2PO4 + 2C + Cexcess 2 V III PO4/C + 2 NH3 + 2 CO + 3 H2O

Step 2 V III PO4/C + LiF  LiV III PO4F/C

(2) V III 2O3 + 4/3 NH4H2PO4 + 2/3 Li3PO4 + ½ O2  2 LiV IV PO4O + 4/3 NH3 + 2 H2O

Scanning Electron Microscopy (SEM) analysis of the samples' morphology was performed using a Hitachi S-4500 microscope. The samples were previously metallized by gold deposition.

In order to confirm the chemical compositions of the samples, the Li, V, and P contents were determined using an Inductively Coupled Plasma / Optical Emission Spectrometry (ICP-OES) spectrometer (Varian 720-ES Optical Emission Spectrometer) after complete dissolution of the powders into a mixture of hydrochloric and nitric acid solutions.

Thermogravimetric analysis coupled with mass spectrometry (TGA-MS) experiments was done on a STA449C Jupiter thermo-microbalance coupled with a QMS Aëolos 32 spectrometer commercialized by Netzsch. The thermal treatments were conducted under air and over a temperature range between 20 and 600°C, with a continuous heating rate of 5°C/min for the first experiment, measuring the concentration of HF (m/z= 20) and CO2 (m/z= 44) released during the experiments. In order to separate the different weight losses (associated to the release of carbon and fluorine) and overpass the kinetics phenomena, a second experiment was performed, with several plateaus during 1h at the characteristic temperatures identified by the first experiment (i.e. 500 and 600°C).

The average oxidation states of vanadium cations were obtained from the measurement of samples' magnetization as function of temperature in the paramagnetic domain using a Vibrating Sample Magnetometer (VSM). A 10000 Oe magnetic field was applied and the temperature was increased from 100K to 500K at a heating rate of 10K/min.

The sample holder was a sealed aluminum capsule in which the powder was immobilized with glass fibers' plug. The measured magnetic susceptibility was corrected, according to equation

(3), from the contribution of the sample holder (aluminum and glass fiber) and from the diamagnetic contribution of LiVPO4(F,O) calculated taking into account the diamagnetic susceptibility values of the constituting ions taken from reference 9 :

(3) (χpara(LiVPO4F1-yOy)= χmes -χdia (LiVPO4F1-yOy)-χblank).

High angular resolution Synchrotron X-ray powder diffraction (SXRPD) experiments were performed at the MSPD beamline of the ALBA synchrotron 10 (Barcelona, Spain). Data were collected using the MYTHEN position sensitive detector in Debye-Scherrer geometry, using a wavelength of 0.9539 Å, over a 2° -72° angular range (0.006° per step) and a total integration time of 5 minutes. The samples were sealed in a 0.5 mm diameter capillary.

Neutron diffraction (ND) was performed at the Institut Laue Langevin (ILL, Grenoble, France) on the high-resolution diffractometer D2B. The samples were contained in an 8 mm diameter vanadium tube, and the diffraction patterns were collected in transmission mode at room temperature, with a wavelength of 1.5950 Å in the 2θ angular range of [0 -150°] using a 0.05° (2θ) step with a global accumulation time of 12 h. It was necessary to correct the absorption in order to take into account a decrease of the experimental diffracted intensity compared to the expected one. The profile of the diffraction lines (ND and XRD) of the materials showing a large amount of defects had to be fitted using the phenomenological anisotropic strain broadening model provided in the Fullprof suite for triclinic P-1 symetry [START_REF] Rodriguez-Carvajal | Recent Advances in Magnetic Structure Determination by Neutron Powder Diffraction[END_REF][START_REF] Stephens | Phenomenological Model of Anisotropic Peak Broadening in Powder Diffraction[END_REF][START_REF] Rodríquez-Carvajal | Line Broadening Analysis Using FullProf: Determination of Microstructural Properties[END_REF] after deconvolution of the instrumental contribution determined from a Na2Ca3Al2F14 reference.

Vanadium K-edge X-ray absorption spectroscopy (XAS) spectra were acquired at room temperature in transmission mode at the ROCK beamline [START_REF] Briois | The New Quick-EXAFS Beamline at SOLEIL[END_REF] of the synchrotron Soleil (France), using a vanadium foil as a reference for the energy calibration. The Si (111) quick-XAS monochromator with an oscillating frequency of 2 Hz and an energy resolution of 0.2 eV was used for this experiment. The data were collected between 5330 and 6280 eV. Powder samples were mixed uniformly in a cellulose matrix and pressed into pellets of 13 mm diameter. Several X-ray absorption scans were collected to ensure the reproducibility of the spectra and to obtain high signal-to-noise ratio. The normalization, background subtraction and fit of the Extended X-Ray Absorption Fine Structure (EXAFS) oscillations were performed using the Demeter package [START_REF] Ravel | HEPHAESTUS: Data Analysis for X-Ray Absorption Spectroscopy Using IFEFFIT[END_REF] .

Infrared (IR) vibrational spectroscopy was used to prove the presence of vanadyl-type environments and to demonstrate the absence of hydroxyl groups, often formed during the synthesis of fluorinated materials in aqueous media. [START_REF] Ellis | Anion-Induced Solid Solution Electrochemical Behavior in Iron Tavorite Phosphates[END_REF] Diffuse reflectance measurements were performed in the mid-IR range (400-4000 cm -1 ) using a FTIR Nicolet 6700 spectrometer (Thermo Scientific) equipped with a DTGS detector. The samples were finely ground in a mortar with dried KBr, the mass ratio between the active material and KBr being approximately 1:50. Finally, the reflectance spectra were treated with the Kubelka-Munk law, which converts the reflectance to a signal proportional to the absorption coefficient. Raman scattering measurements were performed with a Labram HR-800 micro-spectrometer (HORIBA Jobin Yvon). Spectra were recorded with a 514.5 nm excitation wavelength of an Ar + laser, with a power adjusted to ca. 50 µW in order to avoid any degradation of the sample.

The electrochemical tests were carried out in coin cells versus Lithium. The electrodes were prepared using 80 wt.% of active material, 10 wt.% of carbon (Super P) taking into account the carbon remaining from the synthesis and 10 wt.% of polyvinylidene fluoride (PVDF). N-methyl pyrrolidone was added to this mixture and the resultant slurry was cast on an Al foil and dried at 60°C during 24h. Discs of 12 mm diameter were cut in this foil, and after a drying at 80°C under vacuum during 24h these electrodes were cycled in galvanostatic mode in coin cells vs. metallic Lithium. The electrolyte used was a 1M solution of LiPF6 in ethylene carbonate and dimethyl carbonate in a 1:1 volume ratio (LP30). Lithium cells were cycled, either in the high voltage domain between 3.0 and 4.6 V vs Li + /Li (extraction of Lithium from LiVPO4F1-yOy) or in the low voltage domain between 3.0 and 1.0 V vs Li + /Li (insertion of Lithium into LiVPO4F1-yOy).

Results

2a. Syntheses

The mixed valence (i.e. V III /V IV ) phases were obtained through one-step solid state syntheses. First, V2O5, LiF and H3PO4 were dissolved in magnetically stirred distilled water at room temperature during 1h. After water evaporation at 80°C overnight, an excess of Csp carbon (i.e. a highly divided soot) was added to the mixture. During the heating treatment (700°C, 1h under Argon) the carbon therefore acts as a reducing agent of the V 5+ to yield V n+containing phases (with an average oxidation state n such as 3 ≤ n ≤ 4). Therefore, the larger the excess of carbon introduced in the reaction medium, the smaller the oxidation state of vanadium in the final product. The samples obtained are labelled as LVPF-ε with ε referring to the molar excess of Carbon used for the syntheses (ε = 5, 25 or 50%). Let's note that the increase of carbon excess up to 50 mol.% doesn't allow to obtain a pure V 3+ -containing phase and the decrease down to 5% leads to the formation of a LiVPO4O-type phase as secondary phase. The SXRPD patterns of the three samples obtained are compared in Figure 1 to those of the end member phases (i.e. LiV IV PO4O and LiV III PO4F). All the main diffraction lines can be indexed in the Tavorite-type unit cell described in the triclinic (P-1) symmetry, with a minor impurity (< 2 wt.% of Li3V2(PO4)3 crystallizing in the anti-NASICON structure).

The corresponding SEM images (Figure S1) show that the primary particles of the mixed valence materials and of LiVPO4F are smaller compared to those of LiVPO4O (i.e. 0.5-1 m for the first against 1-3 m for LiVPO4O), most probably due to the presence of carbon which for the former ones prevents the sintering of the particles during the annealing. These primary particles are aggregated into larger agglomerates, and small grains of carbon (50-100 nm), coming from the excess of carbon used during the syntheses, surround them.

Based on ICP-OES chemical analyses, the molar ratios Li/V/P were determined for all the samples. The results, given in the S2b).

Thus, only carbon departure can be assigned to this weight loss and the carbon content determined for each sample is reported in Table 1. During the temperature plateau applied at 500°C for 1h, a third weight loss occurs accompanied by the detection of HF without any carbon release. Therefore, the weight percentage obtained allows determining the weight fraction of fluorine in the materials (also reported in Table 1).

In order to confirm the fluorine content thus obtained, which is related to the average oxidation state of vanadium, the magnetization of all materials was measured as a function of temperature in the paramagnetic domain (Figure S3). The average oxidation states of vanadium in each sample (reported in the Table 1) are found to be in good agreement with the compositions proposed based on the TGA-MS analyses and with the formation of mixed V 3+ /V 4+ valence materials: 3.75, 3.55 and 3.35 using respectively a molar excess of carbon of 5%, 25% and 50%. The compositions of LVPF-5%, LVPF-25% and LVPF-50% were thus assigned to LiVPO4F0.25O0.75, LiVPO4F0.45O0.55 and LiVPO4F0.65O0.35 respectively, while it needs to be confirmed by the diffraction study.

2b. The crystallographic structure

The cell parameters of the three samples were determined by the Le Bail refinement based on the SXRPD data and they are reported in Table 1. Considering that the cell describing LiVPO4O is twice larger than that of LiVPO4F1-yOy (y < 0.9) (Figure S4), their cell parameters cannot be directly compared. Indeed, the main crystallographic feature of the Tavorite structure is the chains of transition metal octahedra sharing a corner and forming infinite X-V n+ -X-V n+ -X sequences (X = O 2-or F -and n = 4 or 3). In LiVPO4F, the bond lengths in this sequence are constant whereas for LiVPO4O, due to the presence of V 4+ and therefore of vanadyl bonds (i.e. covalent and short V IV =O bonds), alternate short and long bond lengths are observed along the chains of octahedra. Therefore, for an easier comparison of the two different crystallographic cells, all the Tavorite-type samples are described in the LiVOPO4O's model. The evolution of the cell parameters as a function of the average oxidation state of vanadium determined by magnetic measurements is illustrated in the Figure 2. In this figure, the data from the present study (filled symbols) are listed together with data either from other authors, [START_REF] Kim | High Energy Density Polyanion Electrode Material ; LiVPO4O1-xFx (X ≈ 0.25) with Tavorite Structure[END_REF][START_REF] Onoda | Phase Transition and Spin Dynamics of the LiVFPO4 Insertion Electrode with the S = 1 Linear Chain and the Development of F -O Mixed System[END_REF] either from one of our previous study [START_REF] Boivin | Oxidation under Air of Tavorite LiVPO4F: Influence of Vanadyl-Type Defects on Its Electrochemical Properties[END_REF] (open black symbols). The cell volumes follow a continuous evolution from LiV IV PO4O (V/Z = 85.51 Å 3 ) 2 to LiV III PO4F (V/Z = 87.14 Å 3 ) 2

with the increasing oxidation state of vanadium. Considering the V/Z values obtained for the three samples LVPF-5%, LVPF-25% and LVPF-50%, their chemical compositions would be LiVPO4F0.27O0.73, LiVPO4F0.48O0.52 and LiVPO4F0.65O0.35, respectively. These compositions are in very good agreement with those previously estimated from the Curie constants. Interestingly, Figure 2 highlights also that, contrarily to the cell volume, the cell parameters (especially aLVPOtype and bLVPO-type) don't follow the Vegard's law given by the solid lines. Indeed, bLVPO-type is significantly smaller and aLVPO-type larger than predicted ones. In particular, the evolution of aLVPO-type and bLVPO-type parameters as a function of V n+ is rather linear from an average oxidation state of 3 to 3.90 for vanadium, and shows a significant deviation from linearity for higher oxidation states. Moreover, the phase transition between the cell with a multiplicity of 2 (LiVPO4F-type, 0 ≤ y < 0.9 in LiVPO4F1-yOy) and that with a multiplicity of 4 (LiVPO4O-type, y ≥ 0.9 in LiVPO4F1-yOy) that is required to describe the ordering of vanadyl-type distortion coincides with the composition for which the deviation from the Vegard's law is maximum (i.e.

an average oxidation state of vanadium ~ 3.9). Actually, the crystal field imposed by the highly ionic V-F bonds seems to prevent the ordering of the strongly covalent vanadyl bonds along the chains of octahedra (i.e. 𝑏 ⃗⃗ 𝐿𝑉𝑃𝑂-𝑡𝑦𝑝𝑒 or 𝑐 𝐿𝑉𝑃𝐹-𝑡𝑦𝑝𝑒 ). The competition between the ionicity of the V 3+ -F bond and the covalency of the V 4+ =O bond is most probably at the origin of the deviation from the Vegard's law along their propagation directions. In order to compensate the framework shrinking induced by the increasing concentration of vanadyl-type bonds along the chains, an elongation occurs along the 𝑎 ⃗⃗ ⃗ 𝐿𝑉𝑃𝑂-𝑡𝑦𝑝𝑒 (or (𝑎 + 𝑏) ⃗⃗⃗⃗ 𝐿𝑉𝑃𝐹-𝑡𝑦𝑝𝑒 ) direction whereas no significant deviation has been detected along 𝑐 ⃗⃗ 𝐿𝑉𝑃𝑂 (or ( 𝑎 -𝑏) ⃗⃗⃗⃗ 𝐿𝑉𝑃𝐹-𝑡𝑦𝑝𝑒 ).

An in-depth characterization of the average structure based on the analysis of synchrotron X-ray and neutron diffraction data was carried out in order to understand the deviation from the Vegard's law.

The profile and structure refinements have been done combining ND and SXRPD data, and considering a P-1 triclinic unit cell of LiVPO4F-type (Z = 2). Actually, both techniques are required to fully solve the structure and especially to localize the Lithium atoms and perform a careful analysis of the microstructure. Indeed, even if the angular resolution of ND is lower than that of SXRPD, the higher scattering factors at high diffraction angles allow a better differentiation of the broadening induced by size and strain effects. The absence of superstructure peaks reveals a long range disordered O 2-/F -and V 3+ /V 4+ distribution along the chains. The profiles of all these patterns are governed by a strong anisotropic strain broadening and must be refined using the corresponding model provided in the Fullprof software [START_REF] Rodríquez-Carvajal | Line Broadening Analysis Using FullProf: Determination of Microstructural Properties[END_REF] . As already observed for that kind of materials [START_REF] Boivin | Oxidation under Air of Tavorite LiVPO4F: Influence of Vanadyl-Type Defects on Its Electrochemical Properties[END_REF][START_REF] Boivin | Vanadyl-Type Defects in Tavorite-like NaVPO4F: From the Average Long Range Structure to Local Environments[END_REF] , microstrains are mainly observed along the propagation direction of the chains of octahedra (i.e.

[001] in the LiVPO4F's model), suggesting that they allow to accommodate the covalent and very short V 4+ =O bonds as well as the ionic and longer V 3+ -F ones, as disordered or even probably clustered, along this direction.

Rietveld refinements were performed considering the structural model Li2i(M(1))1a(M(2))1b{(P)2i[O2i]4}X2i already used to describe LiVPO4F and LiVPO4F1-yOy with y ~ 0.75. [START_REF] Ateba-Mba | Synthesis and Crystallographic Study of Homeotypic LiVPO4F and LiVPO4O[END_REF][START_REF] Kim | High Energy Density Polyanion Electrode Material ; LiVPO4O1-xFx (X ≈ 0.25) with Tavorite Structure[END_REF] The comparison of the experimental and calculated diffraction patterns (SXRPD and ND) is given as an example in Figure 3 The BVS value associated with the bridging anion was systemically found to be smaller A comparison between the three mixed valence samples and the end-member phases (i.e. LiV III PO4F and LiV IV PO4O) is interesting as it allows to understand the nature of the bonds observed along the chains of octahedra, depending on the bridging anion (i.e. F -or O 2-) and on the oxidation state of vanadium. In LiV III PO4F, the V-F distances along the chains built of V(1)O4F2 and V(2)O4F2 octahedra are almost constant (1.98 and 1.99 Å), with no asymmetry of the octahedra along the dz² orbitals of vanadium 2 . Conversely, due to the presence of V 4+ in LiV IV PO4O, and to the formation of the vanadyl bonds, an alternation between short bonds (vanadyl bonds) at 1.62 or 1.71 Å and longer ones at 2.17 or 2.21 Å is observed 2 . This distortion of the octahedra around vanadium is thus at the origin of the lower symmetry observed for

LiV IV PO4O, with a unit cell twice larger than that of LiVPO4F. As reported in Table 2, as for LiVPO4F, both V-X distances in a VO4X2 octahedron are symmetrically equivalent in the mixed valence materials LiVPO4F1-yOy, with as expected a slight decrease of the average distance with an increasing content in oxygen (from 1.95 Å for y = 0.35 and to 1.93 Å for y = 0.75).

The BVS values associated with vanadium atoms for the three compositions discussed here are between 3 and 4 (Table 2), in good agreement with the formation of mixed V 3+ /V 4+ phases. Nevertheless, some divergences exist between the average oxidation states of vanadium cations determined by magnetic measurements (Table 1) and their BVS values (Table 2). In fact, only tiny differences are observed between the BVS values determined for vanadium atoms in these phases, whereas their average oxidation states were shown to be significantly different. Indeed, the V 3+ -F/V 4+ =O disorder involves then the formation of inhomogeneous distances along the chains. Therefore, the BVS calculations (performed considering the average description of the structure) are invalid in that case, due to a significant discrepancy between the actual local structure and the average structure.

The high covalency within phosphate groups generally generates highly rigid P-O distances [START_REF] Huminicki | The Crystal Chemistry of the Phosphate Minerals[END_REF] , ranging here between 1.516 (7) In the absence of superstructure peaks, a disordered F-V 3+ /O=V 4+ repartition along the chains seems to be formed. Diffraction giving only access to an average long range description, no information on the V-F bonds antagonist to vanadyl-type bonds (F-(V IV =O)) and on the V-O bonds antagonist to the V-F ones (O-(V III -F)) can be obtained from this technique.

Nevertheless, it is essential to get direct information on these bonds as they may have a strong effect on the electronic configuration in vanadium's orbitals and a fortiori on the electrochemical properties. Spectroscopy experiments were thus needed to reveal the presence of the local vanadyl-type distortions predicted by DFT. 8

2c. The local structure

Vibrational spectroscopy

The analysis of the whole series of LiVPO4F1-yOy compositions was performed by mid-Infrared absorption spectroscopy and Raman scattering to evidence the absence of hydroxyl groups and probe the V-O and P-O vibrations. The corresponding IR spectra are given in Figure 5 whereas the vanadyl stretching region of the Raman scattering spectra are compared in supplementary information in Figure S5. In the 3000 -4000 cm -1 region, no contribution was detected except for LiVPO4O, for which a broad band is observed around 3400 cm -1 and assigned to the stretching vibration of adsorbed water, in agreement to the presence of the corresponding H-O-H bending mode at 1635 cm -1 . [START_REF] Nakamoto | Infrared and Raman Spectra of Inorganic and Coordination Compounds[END_REF] The absence of signal for fluorinated materials in the O-H stretching region supports the compositions proposed without any substitution of fluorine by OH group. The PO4 stretching region is located between 950 and 1200 cm -1 . [START_REF] Harrison | Microwave-Assisted Solvothermal Synthesis and Characterization of Various Polymorphs of LiVOPO4[END_REF][START_REF] Marx | Structural and Electrochemical Study of a New Crystalline Hydrated Iron(III) Phosphate FePO 4 •H 2 O Obtained from LiFePO4(OH) by Ion Exchange[END_REF] A single crystallographic site exists for phosphorus in LiVPO4F, whereas two sites are identified in LiVPO4O (due to the lower symmetry and doubling of the unit cell). This induces an increase in the number of vibration modes in the PO4 stretching region with the appearance of two bands around 1165 and 950 cm -1 , in addition to those observed for LiVPO4F

(around 1120, 1050 and 1010 cm -1 ). The PO4 stretching region signature of the mixed valence samples is similar to that of LiVPO4O, with weak additional contributions and broader signals.

Otherwise, the vanadyl stretching mode is located at 915 cm -1 for LiVPO4O. [START_REF] Harrison | Microwave-Assisted Solvothermal Synthesis and Characterization of Various Polymorphs of LiVOPO4[END_REF] This band is not expected for LiVPO4F which is theoretically a pure V 3+ -containing material, and thus a vanadyl-free composition. Nevertheless, a tiny peak is observed at 874 cm -1 , shifted versus the vanadyl stretching mode observed for LiVPO4O, but matching with the presence of local vanadyl-type defects in LiVPO4F. [START_REF] Boivin | Oxidation under Air of Tavorite LiVPO4F: Influence of Vanadyl-Type Defects on Its Electrochemical Properties[END_REF] As the average oxidation state of vanadium is increased, a slight shift towards higher wavenumbers and an increasing amplitude of the vanadyl-type signal are observed: Note that the intensity of the vanadyl vibration doesn't evolve linearly with the O 2- /F -ratio, the presence of fluorine might disrupt the formation of the vanadyl environment (O=V 4+ --O) and inhibit the corresponding vibration. Moreover, several shoulders are detected around 900, 865 and 825 cm -1 which could be induced by the broad distribution of V 4+ =O environments. Raman scattering (Figure S5) reveals contributions, assigned to the vanadyl bond [START_REF] Hardcastle | Determination of Vanadium-Oxygen Bond Distances and Bond Orders by Raman Spectroscopy[END_REF] , around 875 cm -1 for each sample. Weaker ones, at slightly higher wavenumber (~911 cm -1 ), are also observed for mixed valence materials with a broadening of the signal. That is most probably due to the wide distribution of X-V 4+ =O environments in these materials.

However, the V 4+ =O bond lengths estimated from the Raman wavenumber [START_REF] Hardcastle | Determination of Vanadium-Oxygen Bond Distances and Bond Orders by Raman Spectroscopy[END_REF] were found to be close (i.e. 1.66 Å for 875 cm -1 and 1.65 Å for 910 cm -1 vibrations) whatever the oxygen substitution ratio for fluorine. It suggests a very robust vanadyl bond, almost not really affected by its chemical environment.

IR spectroscopy and Raman scattering have confirmed the formation of the vanadyl bonds in LiVPO4F1-yOy, which is in good agreement with the predictions previously obtained from DFT calculations [START_REF] Bamine | Understanding Local Defects in Li-Ion Battery Electrodes through Combined DFT/NMR Studies: Application to LiVPO4F[END_REF] . In order to obtain further insights on the different local environments around V atoms, X-ray absorption experiments have been conducted at the V K-edge.

X-ray Absorption Spectroscopy

X-ray absorption spectra were recorded at the vanadium K-edge to probe the vanadium oxidation state and local structure around the absorber. Figure 6a displays the spectra recorded for the series of LiVPO4F1-yOy materials.

The pre-edge absorption feature P, located between 5466.0 and 5474.0 eV (inset of Figure 6a) is followed by a weak shoulder (A) on the rising absorption edge which culminates in a strong peak (B). This strong peak B is assigned as the dipole-allowed transition 1s to 4p, [START_REF] Wong | K-Edge Absorption Spectra of Selected Vanadium Compounds[END_REF] and the pre-edge feature P as the transition 1s to 3d. [START_REF] Wong | K-Edge Absorption Spectra of Selected Vanadium Compounds[END_REF] This latter is strictly forbidden in perfectly regular vanadium octahedra but becomes dipole allowed with the distortion of the local symmetry around the absorber, which leads to a 3d-4p orbital mixing and thus to an overlap of the metal 3d orbitals with the 2p orbitals of the ligands. [START_REF] Wong | K-Edge Absorption Spectra of Selected Vanadium Compounds[END_REF] Therefore, this pre-edge region is highly sensitive to the local distortions around vanadium and gives insights into its local environments. Actually, two weak contributions are distinguished in the feature P for LiVPO4F. At a first sight, these contributions can be assigned to the 1s-t2g transition at 5468.0 eV and to the 1s-eg one at 5469.7 eV resulting from the vanadium's orbitals splitting with a crystal field of 1.7 eV. Since this latter is close (i.e. shifted of only 0.3 eV) to the contribution of V 4+ involved in a vanadyl-type bond as in LiVPO4O [START_REF] Wong | K-Edge Absorption Spectra of Selected Vanadium Compounds[END_REF][START_REF] Broux | Strong Impact of the Oxygen Content in Na3V2(PO4)2F3-yOy (0 ≦ Y ≦ 0.5) on Its Structural and Electrochemical Properties[END_REF] , it could thus arise from both the contribution of vanadyl-type defects {V 4+ =O} and the 1s-eg transition of V 3+ . This contribution at 5469.7 eV grows in intensity with the rise of the vanadium oxidation state and thus the concentration of vanadyl-type environments in the materials. Figure 6b shows the evolution of the pre-edge integrated intensity and of the edge position (taken at half jump) as a function of the vanadium average oxidation state, both of them evolving not linearly between the values of the two end members, LiV III PO4F and LiV IV PO4O (dashed lines). The intensity of the pre-edge is significantly smaller than expected for a linear evolution for the intermediate compositions, whereas the shift of the edge position towards higher energy is larger. The presence of strongly ionic V-F bonds appears to inhibit the formation of "true" vanadyl-type environments (O=V 4+ --O), in good agreement with the structure of these mixed valence LiVPO4F1-yOy materials, described in a LVPF-type model and not in a LVPO-type, even for small fluorine contents. Moreover, the edge position does not evolve linearly with the vanadium average oxidation state, as it is affected by both the V 4+ /V 3+ ratio and the iono-covalent character of the V-X bonds. [START_REF] Ghatikar | Empirical Relations and the X-Ray Absorption Edge-Shifts[END_REF] It appears again that the competition between the formation of strongly ionic V 3+ -F bonds at the expense of strongly covalent {V 4+ =O} ones, already proposed to explain the offset from the Vegard's law, could be at the origin of this non-linear behavior at the electronic level.

The analysis of the EXAFS oscillations will then help us to clarify the nature of the atoms belonging to the coordination sphere of V and the distances between them. In our case, as O 2- and F -interact similarly with X-rays, only the distances between vanadium and its ligands can be probed. The complexity of the systems studied here (disordered triclinic compounds) is limiting the description of the far coordination shells. Thus, only the first coordination sphere around vanadium will be discussed here. The fits in the R space of the Fourier transforms of the k 2 -weighted V K-edge EXAFS oscillations (k-range: 3.7-11.0 Å -1 , sine window) are given in supplementary information in Figure S6. In these conditions, the number of independent parameters is limited to 6 (~ 2.Δk.ΔR/π) [START_REF] Broux | Strong Impact of the Oxygen Content in Na3V2(PO4)2F3-yOy (0 ≦ Y ≦ 0.5) on Its Structural and Electrochemical Properties[END_REF] . The local environment around vanadium in LiVPO4F can be described as 6 equivalent ligands located at 2.00(1) Å away from V atom (vs. 7 and Table S2).

We assume a constant 6-fold coordination for vanadium with the following constraints:

(i) All V 4+ forming a short vanadyl bond with an oxygen, with a probability pi = y/6, y referring to y in LiVPO4F1-yOy.

(ii) Long antagonist V 4+ --O bonds are formed along the chains with the following probability: pii = (y²/(2-y))/6.

(iii) All the other bonds (i.e. V 3+ -X (X = O or F), V 4+ -F and V 4+ -O (these latter belonging to the square plane of V 4+ O4X2)) were shown, refined independently, to be equivalent.

The probability for this distance V-X is piii = (6-y-y²/(2-y))/6. From the EXAFS point of view, the sequences of octahedra in the three mixed valence materials LiVPO4F1-yOy can be described as a sum of three distinct vanadium environments (i.e. X-V 3+ O4-X, F-V 4+ O4=O and O-V 4+ O4=O) whose proportions vary, but almost not affected in their distances whatever their surroundings. Figure 7 illustrates a schematic description considering the EXAFS results and the gap with the Vegard's law mainly observed along the propagation direction of the chains of octahedra (along 𝑏 ⃗ 𝐿𝑉𝑃𝑂 𝑜𝑟 𝑐 𝐿𝑉𝑃𝐹 ). Indeed, in a conventional solid solution, a mixture of end-member-type environments is expected whereas, in our case, a third environment is stabilized. Although X-V 3+ O4-X and O-V 4+ O4=O environments are very similar to those observed in LiVPO4F and LiVPO4O respectively, the F-V 4+ O4=O octahedron is shrunk along the dz² orbitals of vanadium. The formation of the vanadyl bond is observed, but the presence of fluorine prevents the formation of the longer antagonist bond, leading to a compaction of these V 4+ O4OF octahedra and to a shortening of the 𝑏 𝐿𝑉𝑃𝑂 (𝑜𝑟 𝑐 𝐿𝑉𝑃𝐹 ) cell parameter. Moreover, the probability for the presence of shrunk F-V 4+ O4=O octahedra (i.e. p = y-y²/(2-y)) fits very well with the amplitude of the offset from the Vegard's law observed along the 𝑏 ⃗ 𝐿𝑉𝑃𝑂 (𝑜𝑟 𝑐 𝐿𝑉𝑃𝐹 ) direction (see Figures 2 and7) as a function of the average oxidation state of vanadium.

The structural description proposed from EXAFS results describes the local environments of the disordered mixed valence materials LiVPO4F1-yOy, in good agreement with the DFT calculations. The local environments around vanadium being different from those observed in the end-member phases, the electrochemical properties should be strongly impacted by the F -/O 2-(and V 3+ /V 4+ ) ratio.

2d. The electrochemical properties

We suspect that all LiVPO4F reported in literature present various amount of vanadyltype defects influencing the electrochemical performance. [START_REF] Messinger | Revealing Defects in Crystalline Lithium-Ion Battery Electrodes by Solid-State NMR: Applications to LiVPO4F[END_REF] In fact, the electrochemical performance might closely depend on their concentrations/distributions in a similar way than the carbon coating or the particles' sizes. In this part we will discuss about the influence of the vanadyl-type environments on the electrochemical properties. The ability of vanadium to be stabilized in various oxidation states allows to play with the V 2+ /V 3+ , V 3+ /V 4+ and V 4+ /V 5+ redox couples, by either inserting or extracting Lithium into/from LiVPO4F1-yOy. Recently, this system has been investigated as multi-electron materials 6 (exchanging ~1.6 electron per transition metal), but due to the complex processes involved in the two reactions (i.e. insertion and extraction) we decided to investigate them separately in the low and high voltage domains as shown in Figure 8.

Low voltage signature as a local probe

In the low voltage region (i.e. 3.0 -1.5 V vs. Li + /Li), the Lithium insertion into LiVPO4F occurs at 1.8 V according to a biphasic process between LiVPO4F and Li2VPO4F involving the V III /V II redox couple [START_REF] Ateba Mba | Lithium Insertion or Extraction From/into Tavorite-Type LiVPO4F: An In Situ X-Ray Diffraction Study[END_REF] . The Lithium insertion into LiVPO4O leads to the reduction of V IV to V III according to a complex series of phase transitions occurring at 2.48 V, 2.31 V and 2.03 V vs.

Li + /Li, to reach the composition Li2V III PO4O. [START_REF] Bianchini | Multiple Phases in the ε-VPO4O-LiVPO4O-Li2VPO4O System: A Combined Solid State Electrochemistry and Diffraction Structural Study[END_REF][START_REF] Lin | Thermodynamics, Kinetics and Structural Evolution of ε -LiVOPO4 over Multiple Lithium Intercalation[END_REF] Figure 8b shows the galvanostatic curves of the mixed valence materials cycled in the 3.0 -1.0 V vs. Li + /Li voltage range (involving the V II /V III and V III /V IV =O redox couples) at C/10. The voltage profile of the mixed valence samples LiVPO4F1-yOy can be described as a combination of a LiVPO4O-type signature above 1.9 V, a LiVPO4F-type signature between 1.9 and 1.6 V and a third unknown contribution at lower potential (i.e. 1.4 V vs. Li + /Li, see inset of Figure 8b). Moreover, the relative proportions of each contribution evolve as a function of the O 2-/F -ratio. Indeed, by increasing the fluorine content, the length of the LiVPO4F-type pseudo-plateau increases. Regarding the third unknown contribution, it could correspond either to the further reduction of V 3+ in an oxygenrich environment (i.e. O-V 3+ -O to O-V 2+ -O) or to the reduction of V 3+ in a mixed fluorine-oxygen

environment (i.e. O-V 3+ -F to O-V 2+ -F).
To the best of our knowledge, the further insertion of Lithium into Li2V III PO4O has never been reported and the corresponding voltage is thus unknown. However, the pseudo-plateau observed for LiVPO4F1-yOy samples appears at a voltage close to the one measured for the V 2+ /V 3+ redox couple in Li1+xVPO4OH (i.e. 1.40 V against 1.35 V vs. Li + /Li in LiVPO4OH 32 ). This contribution at 1.40 V vs. Li + /Li cannot be assigned to the presence of OH groups whose absence has already been demonstrated by IR spectroscopy. However, the voltage of an eventual reduction of V 3+ in oxygen-rich environment should be located close to 1.35 V vs. Li + /Li. Another hypothesis explaining this contribution is to consider the reduction of V 3+ in a mixed fluorine-oxygen environment, whose redox potential is expected to be located at slightly lower voltage than V 2+ /V 3+ redox couple in Li1+xVPO4F (i.e. 1.8 V vs. Li + /Li) due to the lower electronegativity of O 2-compared to F -.

Although the Lithium insertion reaction isn't interesting from an application's point of view because of its low voltage, the corresponding electrochemical signature appears as a local probe, which supports the local structural description of the materials that we previously proposed and it is thus useful to estimate easily the amount of vanadyl-type defects in the materials.

High voltage

Figure 8a gathers the galvanostatic data and the corresponding derivative curves obtained for the mixed valence materials cycled in the 3.0 -4.6 V vs. Li + /Li voltage range (involving the V 3+ /V 4+ and V 4+ /V 5+ redox couples) at C/10. In this voltage region, the Lithium extraction from LiVPO4F corresponds to the oxidation of V III to V IV whereas the V IV /V V redox couple is involved for LiVPO4O. The Lithium deintercalation from LiVPO4F occurs at an average voltage of 4.25 V vs. Li + /Li while it occurs at a lower potential (3.95 V vs. Li + /Li for V IV /V V ) for LiVPO4O [START_REF] Ateba-Mba | Synthesis and Crystallographic Study of Homeotypic LiVPO4F and LiVPO4O[END_REF] . Actually, for that latter, due to the presence of vanadyl bonds, the redox couple involved is the {V=O} 2+ /{V=O} 3+ one and the presence of this strongly covalent bond acts as a "reverse inductive effect" which tends to decrease significantly the voltage of the V 4+ /V 5+ redox couple in LiVPO4O. From LiVPO4O to VPO4O, the extraction mechanism involved is a single biphasic reaction [START_REF] Bianchini | Multiple Phases in the ε-VPO4O-LiVPO4O-Li2VPO4O System: A Combined Solid State Electrochemistry and Diffraction Structural Study[END_REF] . For LiVPO4F, a more complex mechanism occurs with the formation of an intermediate Li0.67VPO4F composition during charge which isn't observed during discharge [START_REF] Ateba Mba | Lithium Insertion or Extraction From/into Tavorite-Type LiVPO4F: An In Situ X-Ray Diffraction Study[END_REF] . Similarly to the electrochemical process involved for LiVPO4F, those associated with LiVPO4F1-yOy samples seem to be asymmetric. For instance, three peaks are observed at Li + /Li respectively for y = 0.75, 0.55 and 0.35 in LiVPO4F1-yOy). From these observations, it appears that the fluorine rich mixed valence materials deliver the better performance, to be compared with that of LiVPO4F. However, the performance delivered by our LiVPO4F sample are rather poor compared to the better ones reported in literature [START_REF] Huang | Lithium Metal Phosphates, Power and Automotive Applications[END_REF][START_REF] Kim | Fast-Rate Capable Electrode Material with Higher Energy Density than LiFePO4 : 4.2V LiVPO4F Synthesized by Scalable Single-Step Solid-State Reaction[END_REF] . The too low amount of defects in our LiVPO4F samples could be the reason. But, in such kind of materials with low electronic conductivity, the optimization of the carbon coating and of the particles' size is essential and there is no doubt that they aren't optimal in our samples. In any case, the materials compared in this work present similar coating and particle's sizes and the performance delivered by the mixed valence samples is better than those of the stoichiometric material and even better when their fluorine content increases. Therefore, it would seem that an optimal vanadyl-type defects' concentration (located between the LiVPO4F0.65O0.35 and the LiVPO4F compositions) exists which would improve the transport properties. Actually, for high voltage materials such as LiVPO4F, the charge capacity is often limited by the polarization which makes that the cut-off voltage (i.e. 4.6 V vs. Li + /Li, stability limit of the classical electrolytes) is reached before the reaction is completed. Indeed, many works reported high discharge capacity at high rate after low charge rate [START_REF] Xiao | Transport and Electrochemical Properties of High Potential Tavorite LiVPO4F[END_REF][START_REF] Wang | Long-Life and High-Rate LiVPO4F/C Nanocrystals Modified with Graphene as Cathode Material for Lithium-Ion Batteries[END_REF] . Therefore, the decrease of the polarization and voltage by introducing vanadyl-type defects in the LiVPO4F structure benefits to the rate capability.

The decrease of the polarization can originate from the enhancement of either electronic or cationic mobility. Typically, the mixed valence states of the transition metals can increase the electronic conductivity by enhancing the hopping process between cations with different oxidation states. This was recently observed in the mixed valence phase Na3V2(PO4)2F2.5O0.5 for which a significant improvement of the electronic conductivity has been evidenced compared to that of the pure V 3+ phase (i.e. Na3V2(PO4)2F3) [START_REF] Broux | Temperature Dependence of Structural and Transport Properties for Na3V2(PO4)2F3 and Na3V2(PO4)2F2.5O0.5[END_REF] . Furthermore, the distortion of the Tavorite network due to the competition between V 3+ -F and V 4+ =O (at the origin of the non-Vegard's law behavior) could open new diffusion paths for Lithium and thus improve the ionic transport properties. Finally, the complex series of phase transitions and the corresponding redox mechanisms involved in LixVPO4F1-yOy upon Lithium deintercalation and re-intercalation could also influence greatly their electrochemical properties. These mechanisms are under study, in order to determine the origin of the performance improved with the introduction of vanadyl-type defects in LiVPO4F: it will be the subject of an up-coming article.

Conclusion

This work is part of a series of studies conducted in our groups and dealing with vanadyltype defects in vanadium fluoride phosphate materials (LiVPO4F 7,8,17 and Na3V2(PO4)2F3 [START_REF] Broux | Strong Impact of the Oxygen Content in Na3V2(PO4)2F3-yOy (0 ≦ Y ≦ 0.5) on Its Structural and Electrochemical Properties[END_REF][START_REF] Broux | Temperature Dependence of Structural and Transport Properties for Na3V2(PO4)2F3 and Na3V2(PO4)2F2.5O0.5[END_REF] ). The combination of advanced characterizations by diffraction and spectroscopy allowed an in-depth understanding of the LiVPO4F1-yOy phases, from their long range average structure to their local one. Beyond the fundamental interest offered by the rich crystal chemistry induced by the competition between the ionicity of the V 3+ -F bond and the covalency of the V 4+ =O bond, these phases attract a real interest as positive electrode materials for Li-ion batteries. We found that the electrochemical properties are correlated to the vanadyl type defects' concentration, and the careful control of the composition thus allows a significant improvement of the performance in batteries. Many studies dealing with LiVPO4F mention coating or particles' size as the only parameters impacting its electrochemical behavior, and defects were rarely reported as such.

This study also demonstrates that the amount of vanadyl-type defects in LiVPO4F-type phases can be easily evaluated by the low voltage signature (i.e. activation of the V 3+ /V 2+ redox couple of LiVPO4F). In the future, in order to be more critical on the contribution of the vanadyl-type defects into the performance reported for LiVPO4F and to compare more easily the LiVPO4F-type materials between them, the authors should thus provide the low voltage signature.

Table captions

Table 1: cell parameters determined from Le Bail refinement on Synchrotron XRPD data with the corresponding chemical analyses performed by ICP-OES (Li, V and P) or TGA (C and F content), and magnetic parameters determined from the 1/m = f(T) curves (m = magnetic susceptibility, T = temperature) at a magnetic field H of 10 kOe. with N the number of bond distances considered, di the distance between the atom i and the central atom of the polyhedron and ‹d› the average of the distances considered. Those appearing as filled and colored symbols are at the center of this paper, whereas those appearing as black and empty symbols are already reported in literature, either by other authors [START_REF] Kim | High Energy Density Polyanion Electrode Material ; LiVPO4O1-xFx (X ≈ 0.25) with Tavorite Structure[END_REF][START_REF] Onoda | Phase Transition and Spin Dynamics of the LiVFPO4 Insertion Electrode with the S = 1 Linear Chain and the Development of F -O Mixed System[END_REF] or by us [START_REF] Boivin | Oxidation under Air of Tavorite LiVPO4F: Influence of Vanadyl-Type Defects on Its Electrochemical Properties[END_REF] . The red-filled area corresponds to the range of compositions for which the unit cells are of LiVPO4F-type (Z = 2, with centrosymmetric vanadium octahedra) and the blue-filled area corresponds to the range of compositions for which the unit cells are of LiVPO4O-type (Z = 4, with distorted vanadium octahedra). The Vegard's law is given for each parameter as a line.

  for the LiVPO4F0.45O0.55 (LVPF-25%) composition. The corresponding cell parameters and atomic positions are gathered in Table S1. The crystal structure, illustrated in Figure 4, can be described as VO4X2 (with X = O or F) octahedra sharing common corners and forming [𝑋 -𝑉𝑂 4 -𝑋] ∞ chains along the [001] direction. These chains of octahedra are connected to each other via PO4 tetrahedra, in such a way that each oxygen atom belonging to the VO4 square plane of the VO4X2 octahedra is involved in a different phosphate group. The three-dimensional network thus generated accommodates Lithium atoms within the tunnels along the [100] direction. Since oxygen and fluorine are not differentiated either by X-ray or neutron diffraction, the Bond Valence Sums 20 (BVS) were calculated to determine the nature of the atoms which links the octahedra between them.

( 1 .

 1 37(2)/1.60(2), 1.23(1)/1.42(1) and 1.22(1)/1.40(1) for F(1)/O(5) for y = 0.75, 0.55 and 0.35 in LiVPO4F1-yOy respectively) than those of the four other oxygen atoms (in average 1.94(2), 1.95(1), 1.91(1)). This result is in agreement with the structure already reported for other existing Tavorite-like fluoride phosphate phases, 2 with a fluorine atom bridging adjacent MO4F2 octahedra along the chains.

  and 1.560(8) Å, and small tetrahedral distortion (Δ = 8.51 x10 -5 ). The Li site observed in the structure is surrounded by four oxygen atoms belonging to the square plane of four different VO4X2 octahedra and to one bridging anion, X (X = O or F), in a distorted LiO4X polyhedron. The BVS values associated with Li in this site don't correspond to the theoretical value (i.e. 1, vs. 0.86(3), 0.89(3) and 0.88(3) respectively for LiVPO4F0.25O0.75, LiVPO4F0.45O0.55 and LiVPO4F0.65O0.35) and the Debye-Waller factors (Biso) associated are rather high (6.6(3), 3.7(3) and 3.0(3) respectively). Actually, the Li position might be impacted by the V 3+ /V 4+ and F -/O 2-local disorder along the chains. The diffraction techniques, giving only an average position, balance the effect of a broad distribution for Li positions by artificially high atomic displacement parameters, which are probably not correlated to the thermal motion of Lithium atoms around their equilibrium positions.

4. 21 , 4 .

 214 Figure 8areveals, additionally, a clear decrease of the irreversible capacity as the fluorine content in the materials is increased conferring to the fluorine rich materials a higher reversible capacity. Moreover, the polarization tends to decrease (i.e. 130 mV, 100 mV and 70

Figure 9

 9 shows the evolution of the discharge capacity as a function of the cycling number at various C-rates (i.e. 5 cycles at C/10, C/5, C/2, C, 2C, C, C/2, C/5, C/10 and then 50 cycles at C) for LiVPO4F1-yOy materials (with y between 0 and 0.75). The LiVPO4F given for comparison in this figure is a quasi-stoichiometric one obtained by a different synthesis (described in the experimental part) whose morphology and "Carbon coating" are very similar to those of mixed valence phases (see FigureS1). Even if the initial reversible capacity at low rate of LiVPO4F is the highest of the studied materials (122, 109, 102, 89 mAh/g respectively from y = 0 to 0.75), its capacity retention and rate capability decrease faster. Indeed after 5 cycles at C/10 a loss of 20% of the reversible capacity is observed for LiVPO4F whereas no significant decrease is observed for the mixed valence materials. Moreover, as the rate is increased, the reversible capacity delivered by LiVPO4F decreases strongly until reaching 15mAh/g at 2C whereas at this rate the capacity provided by the LiVPO4F1-yOy samples is several times larger (62, 55 and 40 mAh/g respectively for y = 0.35 to 0.75).

Figure 1 :

 1 Figure 1: Synchrotron XRD patterns of the LiVPO4F1-yOy series of materials.

Figure 2 :

 2 Figure2: Evolution of the cell parameters (a, b and c as triangles, squares and circles respectively) and of the cell volumes (V/Z as stars) in a LiVPO4O-type model (Z = 4) as a function of the average oxidation state of vanadium for LiVPO4F1-yOy compositions. Those appearing as filled and colored symbols are at the center of this paper, whereas those appearing as black and empty symbols are already reported in literature, either by other authors[START_REF] Kim | High Energy Density Polyanion Electrode Material ; LiVPO4O1-xFx (X ≈ 0.25) with Tavorite Structure[END_REF][START_REF] Onoda | Phase Transition and Spin Dynamics of the LiVFPO4 Insertion Electrode with the S = 1 Linear Chain and the Development of F -O Mixed System[END_REF] or by us[START_REF] Boivin | Oxidation under Air of Tavorite LiVPO4F: Influence of Vanadyl-Type Defects on Its Electrochemical Properties[END_REF] . The red-filled area corresponds to the range of compositions for which the unit cells are of LiVPO4F-type (Z = 2, with centrosymmetric vanadium octahedra) and the blue-filled area corresponds to the range of compositions for which the unit cells are of LiVPO4O-type (Z = 4, with distorted vanadium octahedra). The Vegard's law is given for each parameter as a line.

Figure 3 :

 3 Figure 3: Rietveld refinement of the structure of LVPF-25% (i.e. LiVPO4F0.45O0.55) based on the combined analysis of Synchrotron X-ray (a) and Neutron (b) diffraction patterns. The observed intensities are plotted as red points, the theoretical Bragg positions are plotted as purple marks for the main phase and as grey ones for the secondary phase (i.e. Li3V2(PO4)3), the calculated intensities and the difference between observed and calculated intensities are plotted as black lines.

Figure 4 :

 4 Figure 4: Structure of LiVPO4F1-yOy described in the LVPF-type unit cell (in the P-1 space group): VO4X2 (X = O, F) octahedra in blue, PO4 tetrahedra in pink and Lithium in purple.

Figure 5 :

 5 Figure 5: Diffuse reflectance infrared spectra of LiVPO4O (grey line), LiVPO4F0.25O0.75 (LVPF-5%, blue line), LiVPO4F0.45O0.55 (LVPF-25%, purple line), LiVPO4F0.65O0.35 (LVPF-50%, red line) and LiVPO4F (black line).

Figure 6 :Figure 7 :

 67 Figure 6: (a) V K-edge XANES spectra of the whole series of LiVPO4F1-yOy compounds with the enlargement of the pre-edge region in inset, showing the gradual growth of the vanadyl-type environment. (b) Evolution of the edge position and pre-edge integrated intensity as a function of the average oxidation of vanadium

Figure 8 :

 8 Figure 8: Galvanostatic profiles obtained versus Lithium at a C/10 rate for LiVPO4F0.25O0.75 (LVPF-5%, blue line), LiVPO4F0.45O0.55 (LVPF-25%, purple line) and LiVPO4F0.65O0.35 (LVPF-50%, red line): (a) in the high voltage domain (i.e. between 3.0 and 4.6 V vs Li + /Li) and (b) in the low voltage domain (i.e. between 3.0 and 1.0 V vs Li + /Li). The derivative curves of their first cycle are given in inset.

Figure 9 :

 9 Figure 9: Evolution of the discharge capacity of the Li//LiVPO4F1-yOy cells at different C-rates (from C/10 to 2C) as a function of the cycle number.

Figure 1 Figure 2 Figure 3 Figure 6

 1236 Figure 1

Figure 7

 7 Figure 7

Table 1

 1

	, are in good agreement with the expected

Table 2 :

 2 Significant bond lengths (Å) obtained for the LiVPO4F1-yOy materials from the combined Rietveld refinement based on the SXRPD and ND data. Polyhedral distortion is

	calculated as Δ= N 1	∑	N i=0	(di-‹d›)² ‹d›²

Table 2

 2 LiVPO 4 F 0.65 O 0.35

					LiVPO 4 F 0.45 O 0.55	LiVPO 4 F 0.25 O 0.75
		LVPF-50%	LVPF-25%	LVPF-5%
	Coordination	V(1)		V(2)	V(1)	V(2)	V(1)	V(2)
	O(1) (x2)	/		1.936(8)	/	1.945(9)	/	1.920(9)
	O(2) (x2)	/		1.997(6)	/	1.988(6)	/	2.012(9)
	O(3) (x2)	1.991(6)	/	2.004(6)	/	1.985(9)	/
	O(4) (x2)	1.964(6)	/	1.975(6)	/	1.988(9)	/
	O(5)/F(1) (x2) 1.972(9) 1.929(7) 1.934(9)	1.945(9)	1.937(9)	1.922(9)
	Average	1.976(7) 1.954(7) 1.971(7)	1.959(8)	1.972(9)	1.952(9)
	Δ	6.5x10	-5	4.8x10 -4	5.3x10 -5	5.2x10 -4	2.8x10 -4	9.4x10	-4
	BVS	3.13(1)	3.33(1)	3.15(1)	3.35(1)	3.22(1)	3.39(1)

Acknowledgment

The authors thank Michel Ménétrier, Marie Guignard, Patrick Rosa, Cathy Denage, Laetitia Etienne and Eric Lebraud at ICMCB and Matthieu Courty at LRCS for discussions and support in running some technical experiments, as well as ALBA (Barcelona, Spain) for Synchrotron Xray diffraction experiments on MSPD beamline, ILL (Grenoble, France) for neutron diffraction experiments on the D2B diffractometer, and SOLEIL (Gif-sur-Yvette, France) for X-ray spectroscopy experiments on ROCK beamline (financed by the French National Research Agency (ANR) as a part of the "Investissements d'Avenir" program, reference: ANR-10-EQPX-45; proposal number 20160282). The authors also acknowledge FEDER, the Région Hauts-de-France and the RS2E Network for the funding of EB's PhD thesis, as well as the financial support of Région Nouvelle Aquitaine and of the French National Research Agency (STORE-EX Labex Project ANR-10-LABX-76-01 and HIPOLITE Progelec project ANR-12-PRGE-0005-02).

Figure 8