Karim Karoui*a,
Abdelfattah Mahmoudb,
Abdallah Ben Rhaiema and
Frèdéric Boschinib
aLaboratory of Condensed Matter, Faculty of Science of Sfax, University of Sfax, BP1171 – 3000 Sfax, Tunisia. E-mail: karouikarim36@yahoo.com
bGREENMAT, CESAM Research Unit, University of Liege, Chemistry Institute B6, Quartier Agora, Allée du 6 août, 13, B-4000 Liege, Belgium
First published on 27th February 2019
Li2M(WO4)2 (M = Co, Cu or Ni) materials have been synthesized using the solid-state reaction method. X-ray diffraction measurements confirmed the single phase of the synthesized compounds in the triclinic crystal system (space group P). The SEM analyses revealed nearly spherical morphology with the particle size in the range of 1–10 μm. The IR spectra confirm the presence of all modes of WO42−. The impedance spectroscopy measurements showed the presence of grain boundaries and allow determination of the conductivity of the synthesized materials at room temperature. As positive electrode materials for lithium ion batteries, Li2M(WO4)2 (M = Co, Cu or Ni) cathode materials deliver initial discharge capacities of 31, 33 and 30 mA h g−1 for cobalt, nickel, and copper, respectively.
Double tungstates materials have been considered to be attractive materials basically thanks to their interesting luminescence properties and possible application in the field of solid-state lasers.7–9 These materials have a general formula A2M(WO4)2 where A is a monovalent cation and M (Ni, Co and Cu…) is a divalent metal. Two different coordination have been found for tungsten: (i) tetrahedral for the scheelite-like double tungstates as in NaLa(WO4)2 (ref. 10) or (ii) octahedral for those adopting the wolframite type of structure as in LiFe(WO4)2.11 Nevertheless, a few double tungstates have been structurally investigated and not much has been study about their chemical and physical properties. The wolframite type structure is very common among MWO4 compounds (where M is a transition metal), and it can be described as made up of hexagonal close-packed oxygens with certain octahedral sites filled by M2+ and W6+ cations in an ordered way.12 As though the available information about this type of tungstates remains scarce, we explored the existence of new lithium metal(II) double tungstates and to investigate the electrical and electrochemical properties for which the previously published information was incomplete. The aim of this work is, to investigate and study the electrical and electrochemical properties of Li2M(WO4)2 (M = Ni, Co and Cu) compounds at room temperature. We report here for the first time the electrochemical tests of Li2M(WO4)2 as positive electrode material for Li-ion batteries and we investigate the electrical and SEM properties at room temperature to combine the electrochemical properties and conductivity measurements with morphological properties.
Stoichiometric amounts of the starting reagents were mixed and intimately ground in an agate mortar. An excess of lithium was used during these preparations to compensate for the evaporation of the latter at high temperature. Initially the samples were fired at 550 °C under air for 60 hours to allow decarbonation and then between 650 and 700 °C for 160 hours as summarized in Table 1. After heat treatment, samples have been grounded and then reheated to ensure homogeneity. The colors of these synthesized samples, which depend on the used metal, are in good agreement with the reported colors in the literatures as indicated in Table 1.
Sample | Synthesis | Color |
---|---|---|
Li2Ni(WO4)2 | 550 °C/60 h, 700 °C/160 h | Yellow |
Li2Cu(WO4)2 | 550 °C/60 h, 700 °C/160 h | Green-yellow |
Li2Co(WO4)2 | 550 °C/60 h, 650 °C/146 h | Purple |
The phase purity has been examined and confirmed using powder diffraction. Powder X-ray diffraction (XRD) data were collected with a Bruker D8 ECO powder diffractometer using Cu Kα radiation (λ = 1.5418 Å), operating from 2θ = 15 to 80°. The crystal structure was refined by the Rietveld method, starting from the observed powder diffraction pattern and using Fullprof software. The morphology and particle size were studied with a scanning electron microscope (XL 30 FEGESEM, FEI) with an accelerating voltage of 15 kV under high vacuum. Samples were deposited on carbon tapes. Sputtering deposition was done with gold target under argon atmosphere (Balzers, SCD004, Sputter coater).
To determine the particle size distribution, at least 10 micrographs were taken for each LTO-sample in different regions of the holder, and about 100 particles were analyzed using the commercial software IMAGEJ.13
The IR measurements were performed using a Perkin Elmer 1600FT spectrometer. Samples were dispersed with spectroscopic KBr and pressed into a pellet. Scans were run over the range 400–4000 cm−1 at room temperature.
The ATG analyze was carried out using a thermogravimetric balance TGA-50 Shimadzu in the temperature range from 30 to 600 °C under O2 atmosphere with a heating rate of 10 °C min−1.
The real (Z′) and imaginary (Z′′) parts of electrical impedance data were measured by means of Solartron1260A Impedance Analyzer coupled with a 1296A Dielectric Interface in the 1–106 Hz frequency range at room temperature. This measurement is realised on pellet disks of about 8 mm in diameter and 1.2 mm in thickness.
The working electrodes were prepared by dispersing 60 wt% active material (complex), 20 wt% conductive carbon, and 20 wt% binders (polyvinylidene fluoride) in 1-methyl-2-pyrrolidinone (NMP) to stir into homogeneous slurry during 2 hours. The slurry was coated on aluminum foil, dried at 110 °C in a vacuum oven for 8 h. The active material loading of electrodes was in the range of 1–2 mg cm−2. The electrochemical properties were studied using coin cells (2032, R-type), which were assembled in the Ar-filled glove box. Lithium metal (Aldrich) was used as counter and reference electrode, 1 M LiPF6 in ethylene carbonate and dimethylcarbonate (1/1, v/v) as electrolyte solution and celgard as separator.
The cyclic voltammetry (CV) analysis was performed in the range of 1.5–4.5 V at 0.1 mV s−1. The galvanostatic charge/discharge curves were measured using a multichannel Biologic potentiostat (VMP3) in the voltage range 1.5–4.5 V vs. Li+/Li0 at C/20 rate.
Sample | System/space group | Parameter |
---|---|---|
Li2Ni(WO4)2 | Triclinic/P | a = 4.903/b = 5.597/c = 5.836 |
α = 70.881/β = 88.548/γ = 115.436 | ||
Li2Cu(WO4)2 | Triclinic/P | a = 4.962/b = 5.492/c = 5.884 |
α = 70.741/β = 85.995/γ = 66.041 | ||
Li2Co(WO4)2 | Triclinic/P | a = 4.916/b = 5.660/c = 5.875 |
α = 69.491/β = 91.450/γ = 116.153 |
Fig. 2 (a) SEM micro-graphs and (b) the corresponding histograms for statistical calculation of the particle size of Li2Co(WO4)2. |
Fig. 3 (a) SEM micro-graphs and (b) the corresponding histograms for statistical calculation of the particle size of Li2Ni(WO4)2. |
Fig. 4 (a) SEM micro-graphs and (b) the corresponding histograms for statistical calculation of the particle size of Li2Cu(WO4)2. |
The analysis of SEM images using the software (image J) leads to the determination of the particle size distribution of the elementary particles. The statistical determination of the sizes was carried out on a hundred of the particles. Fig. 2b, 3b and 4b shows the histogram of the statistical calculation of the particle size. The size distribution of the particles is of Gaussian type and is relatively homogeneous with an average particle size of about 2.1 μm, 3.46 μm, and 9.81 μm for the Li2Co(WO4)2, Li2Ni(WO4)2, and Li2Cu(WO4)2 respectively. This result confirm the SEM micro-graphs (Fig. 3) and indicates that the nature of the metal has a strong effect on the average particle size and the particle size distribution of the primary particles of Li2M(WO4)2 materials.
IR frequency (cm−1) | Symmetry and assignments | |||
---|---|---|---|---|
Li2Ni(WO4)2 | Li2Cu(WO4)2 | Li2Co(WO4)2 | ||
915 | 908 | 910 | Au | νs(W–O) |
826 | 816 | 822 | Bu | νas(W–O) |
751 | 748 | 750 | Au | νas(WOOW) |
594 | 581 | 584 | Bu | νas(WOOW) |
502 | 486 | 493 | Au | νas(WOOW) |
454 | 450 | 450 | Bu | νas(WOOW) |
Fig. 8 Complex impedance spectra as a function of temperature with electrical equivalent circuit of Li2M(WO4)2 (M = Co, Ni and Cu). |
The calculate value of conductivity are listed in Table 4 which indicate the weak conductivity of these materials at room temperature. We can also notice that the highest conductivity among the three synthesized compounds is attributed to the Ni-based compound and the lowest value attributed to the Li2Co(WO4)2. This result can be explain by the agglomeration observed in the SEM micro-graphs of the Li2Co(WO4)2 compound (Fig. 3) and not observed in the SEM micro-graphs of the of Ni and Cu based compounds.
Sample | Conductivity at 300 K |
---|---|
Li2Ni(WO4)2 | 1.46 × 10−8 Ω−1 cm−1 |
Li2Cu(WO4)2 | 1.17 × 10−8 Ω−1 cm−1 |
Li2Co(WO4)2 | 0.29 × 10−8 Ω−1 cm−1 |
Cyclic voltammetry (CV) is a convenient technique for evaluation of the electrochemical performance and electrode kinetics of oxide material. Fig. 9 displays the CV curves of Li2M(WO4)2 for cobalt, nickel, and copper at room temperature in the voltage range of 1.5–4.5 V at scan rate of 0.1 mV s−1. The CV curves show only one pair symmetrical and large redox peaks corresponding to the intercalation/deintercalation of Li+ ions into/from the structure of Li2M(WO4)2 materials. The anodic and cathodic peaks occur at about 4.35 and 1.8 V, respectively. To determine the electrochemical reaction reversibility, an important parameter of the separation potential (ΔEp) between anodic and cathodic peaks should be taking into account. The ΔEp of the Li2Co(WO4)2 is 2.15 V, for the Li2Cu(WO4)2 is 1.98 V and for the Li2Ni(WO4)2 is 1.93 V. This result confirm that the material Li2Ni(WO4)2 shows the best reversibility of lithium extraction/insertion during charge/discharge process.
Fig. 9 The cyclic voltammetry curves of Li2M(WO4)2 (M = Co, Ni and Cu) cathode materials in the voltage range 1.5–4.5 V. |
To study the influence of the nature of metal of Li2M(WO4)2 materials on their cycling performances, The electrode materials were galvanostatically evaluated in the voltage range of 1.5–4.5 V at room temperature.
The electrochemical charge/discharge measurements were carried at the charge/discharge rate C/20 for the Li2Ni(WO4)2, Li2Co(WO4)2 and Li2Cu(WO4)2 compounds. Initial charge/discharge curves recorded at room temperature are presented in Fig. 10 for the studied cathode materials. The three materials show a continuous increase of the voltage with Li-extraction (charge), than a pseudo plateau starting from 4.35 V with low polarization is observed for all samples, which corresponds to approximately 0.1–0.2 Li+-ion de-insertion during the charge processes. During the discharge, the reversible process is observed. The voltage decreases continuously until the appearance of small plateau at around 1.8 V in good agreement with the CV measurement. The analysis of the charge/discharge curves suggests the existence of the of a solid solution domain during the first charge and discharge process of Li2Ni(WO4)2, Li2Co(WO4)2 and Li2Cu(WO4)2 materials.22–24 Fig. 11 shows the evolution of the discharge capacity as function of the number of cycles during 50 cycles at C/20. Li2Ni(WO4)2, Li2Co(WO4)2 and Li2Cu(WO4)2 materials show initial discharge capacities of 35, 32 and 30 mA h g−1 respectively. However the three materials show different cycling behaviors. Noticeable capacity drop until the 15th cycle is observed Li2Co(WO4)2 and Li2Cu(WO4)2 electrode materials. However, Li2Ni(WO4)2 electrode material shows stable cycling during 15 cycles with a reversible discharge capacity of about 30 mA h g−1. This rapid decrease in capacity for Li2Co(WO4)2 and Li2Cu(WO4)2 materials is mainly due to their high particle size and to the strong agglomeration of the particles of these materials and was confirmed by the results obtained by SEM analyses. Li2Ni(WO4)2, Li2Co(WO4)2 and Li2Cu(WO4)2 materials deliver discharge capacity of about 23, 13 and 11 mA h g−1 respectively after 50 cycles. As shown, After 50 cycles, the sample Li2Ni(WO4)2 shows the best cycling behavior. This electrode exhibits the highest rechargeable capacity (30 mA h g−1) during 15 cycles, with highest capacity retention after 50 cycles (approximately 50% of capacity retention).
Fig. 10 Selected voltage profiles vs. capacity of Li2Ni(WO4)2, Li2Co(WO4)2 and Li2Cu(WO4)2 cathode material for Li-ion batteries in the voltage range 1.5–4.5 V at C/20. |
Fig. 11 Evolution of the discharge capacity vs. cycle number the of Li2M(WO4)2 (M = Co, Ni and Cu) cathode materials for Li-ion batteries in the voltage range 1.5–4.5 V at C/20. |
It is easily understood that the capacity drop after first discharge for the samples based on Co and Cu should be related to their high particle size which leads to the unfavorable interaction with liquid electrolyte.25–27 Interestingly, Li2Ni(WO4)2 illustrates the best capacity retention compared to Li2Co(WO4)2 Li2Cu(WO4)2 during cycling. Indeed, among the studied samples, the Li2Ni(WO4)2 shows the best electrochemical performances due to its good crystallinity, the smallest particle size which benefits Li+-diffusion in Li2Ni(WO4)2 structure.
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