Article Cite This: Chem. Mater. 2018, 30, 5682−5693
pubs.acs.org/cm
LiVPO4F1−yOy Tavorite-Type Compositions: Influence of the Concentration of Vanadyl-Type Defects on the Structure and Electrochemical Performance Edouard Boivin,†,‡,∇ Rénald David,‡,∇ Jean-Noël Chotard,‡,∇,○ Tahya Bamine,†,∇ Antonella Iadecola,∇ Lydie Bourgeois,§,∥ Emmanuelle Suard,⊥ François Fauth,# Dany Carlier,†,∇ Christian Masquelier,‡,∇,○ and Laurence Croguennec*,†,∇,○ †
CNRS, Université Bordeaux, Bordeaux INP, ICMCB UMR 5026, Pessac F-33600, France Laboratoire de Réactivité et de Chimie des Solides, CNRS UMR 7314, Université de Picardie Jules Verne, Amiens F-80039 Cedex 1, France § Université de Bordeaux, ISM, Groupe de Spectroscopie Moléculaire, Talence F-33405, France ∥ Bordeaux INP, ISM, CNRS UMR 5255, Talence F-33405, France ⊥ Institut Laue-Langevin, 71 Avenue des Martyrs, Grenoble F-38000, France # CELLS - ALBA Synchrotron, E-08290 Cerdanyola del Vallès, Barcelona, Spain ∇ RS2E, Réseau Français sur le Stockage Electrochimique de l’Energie, FR CNRS 3459, Amiens F-80039 Cedex 1, France ○ ALISTORE-ERI European Research Institute, FR CNRS 3104, Amiens F-80039 Cedex 1, France
Chem. Mater. 2018.30:5682-5693. Downloaded from pubs.acs.org by UNIV OF ROCHESTER on 10/21/18. For personal use only.
‡
S Supporting Information *
ABSTRACT: Mixed-valence LiVPO4F1−yOy materials were obtained for the first time over a large composition range (here 0.35 ≤ y ≤ 0.75) through a single-step solid-state synthesis. Interestingly, the competition between the ionic character of the V3+−F bond and the strong covalency of the V4+O vanadyl bond originates complex crystal chemistry at the local scale, which allows stabilization of a solid solution between LiVPO4F and LiVPO4O despite a significant deviation from Vegard’s law for the cell parameters. A combined study using IR, Raman, and X-ray absorption spectroscopies highlights the effect of the vanadyl environment on the electronic structure of the vanadium orbitals and a fortiori the electrochemical behavior. Our results underline that the electrochemical performance of LiVPO4F1−yOy-type materials can be controlled by tuning the concentration of vanadyl-type defects, i.e., by playing on the competition between the ionic V3+−F bond and the covalent V4+O bond.
1. 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), the polyanionic group (XO4 with X = P, S), or the bridging anion (with Y = O, F, OH, H2O).1 Therefore, the stabilization of new compositions is a major issue at both the practical and fundamental level. The vanadium-based tavorite materials LiVPO4F and LiVPO4O are the most attractive ones because of their high working voltages (i.e., 4.26 and 3.95 V vs Li+/Li, respectively) and thus their high theoretical energy densities,2 which are strongly competitive with the already commercialized LiFePO4 (theoretical energy density of 655 W h/kg vs Li for LiVPO4F compared with 586 W h/kg vs Li for LiFePO4).3 Nevertheless, among the numerous studies reporting on © 2018 American Chemical Society
apparently pure LiVPO4F, some discrepancies in the average structure and electrochemical properties are observed. Indeed, the unit cell volumes fluctuate between V/Z = 87.00 Å3 and V/Z = 87.31 Å3, whereas the difference between LiVPO4O (V/Z = 85.51 Å3) and LiVPO4F is around 1.7 Å3.2,4,5 Recently, Kang and co-workers reported the LiVPO4F∼0.25O∼0.75 composition, whose average structure is similar to that of LiVPO4F (despite the large O2−/F− substitution ratio) and whose electrochemical properties are drastically different.6 Therefore, the partial substitution of fluorine by oxygen associated with the partial oxidation of V3+ to V4+O (or {VO}2+) for charge compensation (i.e., the formation of vanadyl-type defects) Received: May 21, 2018 Revised: July 7, 2018 Published: July 11, 2018 5682
DOI: 10.1021/acs.chemmater.8b02138 Chem. Mater. 2018, 30, 5682−5693
Article
Chemistry of Materials
The average oxidation states of vanadium cations were obtained from measurements of the samples’ magnetizations as functions of temperature in the paramagnetic domain using a vibrating sample magnetometer (VSM). A 10 000 Oe magnetic field was applied, and the temperature was increased from 100 to 500 K at a heating rate of 10 K/min. The sample holder was a sealed aluminum capsule in which the powder was immobilized with a glass fiber plug. As shown in eq 3, the measured magnetic susceptibility was corrected for the contribution of the sample holder (aluminum and glass fiber) and the diamagnetic contribution of LiVPO4(F,O), which was calculated by taking into account the diamagnetic susceptibilities of the constituting ions taken from ref 9:
could be the origin of these discrepancies. This hypothesis has already been investigated by NMR spectroscopy and density functional theory (DFT) calculations,7,8 but its experimental validation was still challenging because of the low concentration of defects in LiVPO4F. Therefore, we synthesized LiVPO4F1−yOy compositions with control of 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 environment around vanadium. Electrochemical studies performed in the high- and low-voltage domains revealed a strong influence of the concentration of vanadyl-type defects on the electrochemical performance in terms of energy density, cyclability, and rate capability.
χpara (LiVPO4 F1 − yOy ) = χmeas − χdia (LiVPO4 F1 − yOy ) − χblank (3)
2. EXPERIMENTAL SECTION
High-angular-resolution synchrotron X-ray powder diffraction (SXRPD) experiments were performed on the MSPD beamline of the ALBA synchrotron10 (Barcelona, Spain). Data were collected using the MYTHEN position-sensitive detector in the Debye− Scherrer geometry at a wavelength of 0.9539 Å over an angular range of 2−72° (0.006° per step) and a total integration time of 5 min. The samples were sealed in a 0.5 mm diameter capillary. Neutron diffraction (ND) was performed at Institut Laue Langevin (ILL) (Grenoble, France) on the high-resolution D2B diffractometer. The samples were contained in an 8 mm diameter vanadium tube, and the diffraction patterns were collected in transmission mode at room temperature at a wavelength of 1.5950 Å over the 2θ angular range of 0−150° using a 2θ step of 0.05° with a global accumulation time of 12 h. It was necessary to correct the absorption in order to take into account a decrease in the experimental diffracted intensity compared with the expected one. The profile of the diffraction lines (ND and XRD) of the materials showing a large number of defects had to be fitted using the phenomenological anisotropic strain broadening model provided in the FullProf suite for triclinic P1̅ symmetry11−13 after deconvolution of the instrumental contribution determined from a Na2Ca3Al2F14 reference. Vanadium K-edge X-ray absorption spectroscopy (XAS) was performed at room temperature in transmission mode on the ROCK beamline14 of Synchrotron Soleil (Saint-Aubin, 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 with a diameter of 13 mm. Several X-ray absorption scans were collected to ensure the reproducibility of the spectra and to obtain high signal-tonoise ratio. The normalization, background subtraction, and fitting of the extended X-ray absorption fine structure (EXAFS) oscillations were performed using the Demeter package.15 Infrared (IR) vibrational spectroscopy was used to prove the presence of vanadyl-type environments and to demonstrate the absence of hydroxyl groups, which are often formed during the synthesis of fluorinated materials in aqueous media.16 Diffuse reflectance measurements were performed in the mid-IR range (400−4000 cm−1) using a Nicolet 6700 FT-IR 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 was 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 microspectrometer (HORIBA Jobin Yvon). Spectra were recorded with an excitation wavelength of 514.5 nm from an Ar+ laser with the 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 % active material, 10 wt % Super P carbon (taking into account the carbon remaining from the synthesis), and 10 wt % poly(vinylidene fluoride) (PVDF). N-Methylpyrrolidone was added to this mixture, and the resultant
All along this study the LiVIII,IVPO4F1−yOy compounds will be compared to the end-member phases (LiVIIIPO4F and LiVIVPO4O), whose syntheses are different from that used to prepare the mixedvalence phases (described in detail in section 3a). Quasi-stoichiometric LiVPO4F was synthesized through a two-step carbothermal reduction as described previously2 (eq 1): step 1:
VV 2O5 + 2NH4H 2PO4 + 2C + Cexcess → 2VIIIPO4 /C + 2NH3 + 2CO + 3H 2O step 2:
VIIIPO4 /C + LiF → LiVIIIPO4 F/C
(1)
V2O5 (Sigma-Aldrich, >98%) was reduced by a small excess of CSP carbon (i.e., a highly divided soot) in the presence of NH4H2PO4 (Sigma-Aldrich, >99%). Thermal treatment at 800 °C under argon flow led to the formation of VPO4/C, in which the vanadium phosphate particles are surrounded by residual carbon nanoparticles localized at the grain boundaries. After the addition of a stoichiometric amount of LiF to VPO4/C, the mixture was annealed at 800 °C under an argon flow to form LiVPO4F. LiVPO4O was synthesized from V2O3 (Cerac, ≥99.9%), Li3PO4 (Sigma-Aldrich, 97%), and NH4H2PO4 (Sigma-Aldrich, >99%) by a solid-state reaction as described in ref 2 (eq 2): 4 2 1 NH4H 2PO4 + Li3PO4 + O2 3 3 2 4 IV → 2LiV PO4 O + NH3 + 2H 2O 3
VIII 2O3 +
(2)
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 by inductively coupled plasma/optical emission spectrometry (ICP-OES) on a Varian 720-ES optical emission spectrometer after complete dissolution of the powders in a mixture of hydrochloric acid and nitric acid. Thermogravimetric analysis coupled with mass spectrometry (TGA−MS) experiments were done on a STA449C Jupiter thermomicrobalance coupled with a QMS Aëolos 32 spectrometer commercialized by Netzsch. The thermal treatments were conducted under air over the temperature range between 20 and 600 °C with a continuous heating rate of 5 °C/min for the first experiment, measuring the concentrations of HF (m/z 20) and CO2 (m/z 44) released during the experiments. In order to separate the different weight losses (associated with the release of carbon and fluorine) and overpass the kinetic phenomena, a second experiment was performed, with several plateaus for 1 h at the characteristic temperatures identified by the first experiment (i.e., 500 and 600 °C). 5683
DOI: 10.1021/acs.chemmater.8b02138 Chem. Mater. 2018, 30, 5682−5693
Article
Chemistry of Materials slurry was cast on an Al foil and dried at 60 °C for 24 h. Discs with a diameter of 12 mm were cut from this foil and dried at 80 °C under vacuum for 24 h, after which these electrodes were cycled in galvanostatic mode in coin cells versus metallic lithium. The electrolyte used was a 1 M solution of LiPF6 in 1:1 v/v ethylene carbonate/dimethyl carbonate (LP30). Lithium cells were cycled either in the high-voltage domain between 3.0 and 4.6 V vs Li+/Li (extraction of Li from LiVPO4F1−yOy) or in the low-voltage domain between 3.0 and 1.0 V vs Li+/Li (insertion of Li into LiVPO4F1−yOy).
3. RESULTS 3a. Syntheses. The mixed-valence (i.e., VIII/VIV) 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 1 h. 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, 1 h under argon), the carbon acts as a reducing agent of V5+ to yield Vn+-containing phases (with an average oxidation state n such as 3 ≤ n ≤ 4). Therefore, the larger the excess of carbon introduced into the reaction medium, the lower the oxidation state of vanadium in the final product. The samples obtained are labeled as LVPF-ε, where ε refers to the molar excess of carbon used for the synthesis (ε = 5, 25, or 50%). Let us note that increasing the carbon excess to 50 mol % did not allow us to obtain a pure V3+-containing phase and that decreasing the excess to 5% led to the formation of a LiVPO4O-type phase as a secondary phase. The SXRPD patterns of the three samples obtained are compared in Figure 1 with those of the end-member phases (i.e., LiVIVPO4O and LiVIIIPO4F). All of the main diffraction lines can be indexed in the tavorite-type unit cell described in the triclinic (P1̅) symmetry, with a minor impurity (
