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Effects of fluorine doping on the electrochemical properties of LiV3O8 cathode material
Electrochimica Acta 54 (2009) 3184–3190
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Effects of fluorine doping on the electrochemical properties of LiV3 O8 cathode material Yongmei Liu, Xuechou Zhou, Yonglang Guo ∗ College of Chemistry and Chemical Engineering, Fuzhou University, Fuzhou 350108, PR China
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Article history: Received 31 October 2008 Received in revised form 21 November 2008 Accepted 22 November 2008 Available online 30 November 2008 Keywords: Cathode material Fluorine doping Lithium batteries Lithium trivanadate Solid-state reaction
a b s t r a c t A series of fluorine-doped lithium trivanadates LiV3 O8−y Fz (z = 0, 0.03, 0.05, 0.1, 0.15, 0.2 and 0.5) were synthesized by the solid-state reaction. X-ray diffraction (XRD), Fourier transform infrared (FTIR) and scanning electron microscope (SEM) tests show that a proper amount of fluorine substituting for oxygen in LiV3 O8 can modify its structure and surface morphology. Charge–discharge tests show that the doped samples with a proper amount of fluorine display good cycling stability, high coulombic efficiency and good rate capability, compared with undoped sample. The cyclic voltammetry (CV), area-specific impedance (ASI) and electrochemical impedance spectroscopy (EIS) tests indicate that the doped samples with a low fluorine content can stabilize the interface between the surface layer of the active particles and the electrolyte after cycling, while a high fluorine content form an unstable interface. The fluorine substitution is a convenient and effective method for improving the electrochemical performances of LiV3 O8 . © 2008 Elsevier Ltd. All rights reserved.
1. Introduction Lithium trivanadate is an attractive candidate cathode material for rechargeable lithium batteries due to its high capacity, low cost, long cycle life and facile preparation [1–4]. More than three Li+ ions can be accommodated in each Li1+x V3 O8 unit, and even it reaches 4.5 Li+ ions in its amorphous form [5]. It is well known that the preparation method for Li1+x V3 O8 strongly influences its electrochemical properties. Traditional synthesis method of Li1+x V3 O8 was that Li2 CO3 reacted with V2 O5 at 680 ◦ C [6]. At such high reaction temperature, it is difficult to control the accurate composition of the final products. Meanwhile, the product has a low capacity of 180 mAh g−1 in the range of 1.8–4.0 V, and the molten V2 O5 will cause corrosion to the crucible. To improve the electrochemical properties of this material, much attention has been paid to the new synthetic methods and post treatments, mainly the sol–gel method [7–11], hydrothermal reaction [4], microwave-assisted synthesis [12], rheological phase reaction method [2], ultrasonic treatment [13,14], spray-drying synthesis [15] and intercalation of inorganic molecules between the interlayers [1]. These strategies can improve the electrochemical performance of the material. However, there are some disadvantages such as long reaction time or complex post treatment steps. So they are not suitable for large-scale production. Substitution of
∗ Corresponding author. Fax: +86 591 8807 3608. E-mail address: [email protected] (Y. Guo). 0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2008.11.045
lithium with other cations such as Na+ , K+ , Mg2+ , Ca2+ , Cu+ , Ag+ [16–20] and that of vanadium with Mn, Mo, W [21,22] have been studied, but they do not significantly improve the electrochemical performances of lithium trivanadates. According to Jouanneau et al. [23], the capacity fading of lithium trivanadate was ascribed to the local damage of the crystal structure, which was caused by the great change in cell lattice and the partial dissolution of the active material in the liquid electrolyte during the cycling. Oxygen substitution with fluorine was reported to have improved the electrochemical performances of layered Li(Ni,Mn)O2 and spinel LiMn2 O4 [24–27]. For example, Oh et al. [26] believed that F-substituted LiNi0.5 Mn1.5 O4−x Fx has a smaller lattice variation during the Li+ ion insertion/extraction and exhibits good resistance to the attack of HF existing in the electrolyte. Choi and Manthiram [27] had also a similar report. However, its effect on the properties of lithium trivanadate has not been reported. In this work, the effect of oxygen substitution with fluorine on the electrochemical performance of LiV3 O8 has been investigated. 2. Experimental LiV3 O8−y Fz (z = 0, 0.03, 0.05, 0.1, 0.15, 0.2 and 0.5) samples were prepared as described in Ref. [28] by the solid-state reaction containing Li2 CO3 , NH4 VO3 and LiF. The stoichiometrically weighted starting materials were well mixed in an agate mortar. The mixture was pressed into pieces, transferred to a porcelain crucible, and then heated to 600 ◦ C at the rate of 1 ◦ C min−1 in dry air using a SCQ-8-12B tube furnace. The quasi melted compounds were quickly
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Fig. 1. XRD patterns of LiV3 O8−y Fz (z = 0, 0.03, 0.05, 0.1 and 0.5).
cooled by putting the crucible rapidly in an airproof stainless steel container in ice bath, ground in an agate mortar and then dried in vacuum environment at 120 ◦ C for 4 h to obtain the products used to prepare the positive electrode. The X-ray diffraction (XRD) experiments were carried out using a X’pert-MPD X-ray Diffractometer with Cu K␣ radiation ( = 1.54056 Å) in the range of 10–70◦ to identify the phases and the structure of the synthesized materials. Fourier transform infrared (FTIR) absorption spectra used to determine the bonding nature were obtained by the KBr disk method using a SPECTRUM 2000 Fourier transform infrared spectrometer. Their morphologies were observed by a scanning electron microscope (SEM, Philips, XL30 ESEM-TEP). The electrochemical performance of the synthesized materials was evaluated in model CR2025 coin cells. The paste consisted of the positive active material prepared above, super P carbon black and polyvinylidene fluoride (PVDF) binder in a weight ratio of 85:10:5. The positive electrode was prepared by coating the paste on Al collector. Lithium foil was served as the negative and reference electrodes. The separator was Celgard 2320 membrane and the electrolyte was 1 M LiPF6 in ethylene carbonate (EC) and dimethyl carbonate (DMC) in volume ratio of 1:1. The cyclic tests, the open circuit voltage (OCV) data measurements and the area-specific impedance (ASI) were performed by Land automatic battery tester (Wuhan, China). Cyclic tests were conducted between 2.0 and 4.0 V at different charge–discharge rates. Cyclic voltammograms (CV) and the electrochemical impedance spectroscopy (EIS) on the positive electrode in the cells described above were performed by a CHI 660C electrochemical workstation. The CV scan rate was 0.1 mV s−1 . The EIS measurements were carried out by applying an ac voltage of 2 mV in the frequency range of 10 mHz–100 kHz before and after 3 and 20 cycles. All cells were assembled in a glove box filled with purified argon gas.
Fig. 2. The lattice parameters and d100 value of LiV3 O8−y Fz (z = 0, 0.03, 0.05, 0.1 and 0.5) calculated from Fig. 1.
V5+ in LiV3 O8 is reduced to V4+ and even V3+ [29]. With the increase of fluorine content, it is difficult to further reduce the oxidation state of vanadium. Consequently, the lithium content in LiV3 O8 decreases to balance the anion valence. This results in the structure collapse and the formation of Li0.3 V2 O5 and even Li0.04 V2 O5 phases. Based on this, it can be concluded that only when the doping amount is low, can F− substitute O2− and locate in the crystal lattice sites of O2− , and form the LiV3 O8−y Fz (z = y) isomorphous solid solution. Fig. 2 shows the changes in the lattice parameters and the interlayer distance of (1 0 0) crystal plane calculated from the XRD patterns in Fig. 1. The lattice parameters a, b, c,ˇ and d100 values decrease with small amount of fluorine substitution (z < 0.05). When the fluorine doping increases (z ≥ 0.05), however, the lattice parameters and d100 values become large and then remain almost constant. These results indicate that only primary single-phase exists when z < 0.05. Similar phenomenon was reported by Oh et al. [26] on the investigation of LiNi0.5 Mn1.5 O4−x Fx . The shrink of the lattice parameters in LiV3 O8−y Fz (z < 0.05) samples may be due to the small ionic radius of F− (r = 1.33 Å), compared with that of O2− (r = 1.40 Å) [30]. FTIR spectra of LiV3 O8−y Fz (z = 0, 0.03, 0.05, 0.1 and 0.5) are shown in Fig. 3. The FTIR spectra of the oxyfluoride compounds are very similar to that of the fluorine-free oxide. The two peaks at 956 and 995 cm−1 and the peak near 740 cm−1 are attributed to the stretching vibration of the short range (V O) and long range
3. Results and discussion The XRD patterns of LiV3 O8−y Fz (z = 0, 0.03, 0.05, 0.1 and 0.5) are shown in Fig. 1. All samples with different amount of F− doping have very strong diffraction peaks and well-developed crystal structure of layered LiV3 O8 (JCPDS card: No. 18-754). But some impurity phases are found when z ≥ 0.05, that is a weak peak (2 = 12.3◦ ) ascribed to Li0.3 V2 O5 (marked by ) for samples z = 0.05 and 0.1, and peaks ascribed to Li0.3 V2 O5 and Li0.04 V2 O5 (marked by and ♦, respectively) for sample z = 0.5. This phenomenon is related to the charge compensation [21]. Since no peaks of LiF compound appear (its strongest peak is located at 45◦ ), it is deduced that the partial O2− in LiV3 O8 is replaced by F− . To balance the anion valance, partial
Fig. 3. FTIR spectra of LiV3 O8−y Fz (z = 0, 0.03, 0.05, 0.1 and 0.5).
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Fig. 5. The dependence of specific capacity on the cycle number at the charge and discharge rates of 0.25 C on LiV3 O8−y Fz (z = 0, 0.05, 0.1, 0.15, 0.2 and 0.5) positive materials.
Fig. 4. SEM photographs of LiV3 O8−y Fz . (a) z = 0; (b) z = 0.1; (c) z = 0.2.
(V–O–V) order between vanadium and oxygen atoms, respectively [31]. No Li–F absorption band is observed in FTIR spectra. These phenomena and XRD data indicate the existence of the order configuration of VO6 unit but no LiF phase in LiV3 O8−y Fz compounds. With F− doping, both peaks located at about 1000 and 740 cm−1 become narrow, indicating that F− substitutes O2− both in VO6 octahedrons and VO5 trigonal bipyramids. Fig. 4 shows the SEM of LiV3 O8−y Fz (z = 0, 0.1 and 0.2). All samples present ordered crystals. The particle size of LiV3 O8−y Fz increases with the increase of the F− content and the surface of the grains becomes smooth. These morphological differences can affect the contact area with the electrolyte and the vanadium dissolution reaction, which are very important to both the specific capacity and cycle ability of the materials. Fig. 5 shows the discharge capacity of LiV3 O8−y Fz materials with different contents of fluorine doping. All cycle ability is improved by the fluorine substitution, of which the performances for sample z = 0.1 is best. The capacity of undoped material declines continually during cycling and decreases by 56% after 50 cycles. For sample z = 0.1, although the discharge capacity reduces about 23 mAh g−1 in the initial 10 cycles, only a little capacity falling is observed in
the subsequent 40 cycles. This may be related to the difference of the morphology. The smooth surface can restrain the vanadium dissolution in electrolyte. For samples z = 0.15 and 0.2, the maximum capacities appear after several cycles. This arises from a slight fragmentation of the grains (induced by electrochemical grinding), leading to smaller grain size and thus to shorter diffusion length for Li+ ions [32]. For sample z = 0.5, its discharge capacity loss mainly occurs during the first 10 cycles and reaches about 60 mAh g−1 . This is related to the formation of the unstable interface on cathode surface, which is caused by the generated Lix V2 O5 as shown in XRD. Fig. 6 shows the coulombic efficiency of LiV3 O8−y Fz materials under the charge and discharge conditions of Fig. 5. The coulombic efficiency of undoped sample decreases gradually in the initial 13 cycles and then increases slowly. And it is much lower than that of all other samples except for sample z = 0.5 during cycling. This phenomenon with low coulombic efficiency and continual capacity falling for undoped sample is caused by the destruction of the crystal structure and vanadium dissolution in electrolyte induced by the variation of the cell lattice in the processes of lithium ion insertion/extraction [16]. Fig. 7 shows the cycle performance of samples z = 0 and 0.1 at different temperatures at the charge and discharge rates of 1.0 C. It can be seen from Fig. 7 that the positive effect of the fluorine dopant on the cycle performance is more noticeable at elevated
Fig. 6. The dependence of coulombic efficiency of LiV3 O8−y Fz on the cycle number at the charge and discharge rates of 0.25 C.
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Fig. 7. The dependence of specific capacity of LiV3 O8−y Fz on the cycle number at the charge and discharge rate of 1.0 C and at different temperatures.
temperature. The capacity of undoped material shows significant decline with cycling while the fluorine-doped material has good cycling performance at 55 ◦ C. Fig. 8 shows the rate capability of samples z = 0 and 0.1. The cells were cycled in the range of 2.0–4.0 V at room temperature and their charge and discharge rates were changed successively from 5 to 0.05 C after the 12th cycle. The capacity fading is greater at low charge and discharge rates than that at high rates. This can be attributed to the exposure-time effects of vanadate dissolution at high voltage [25]. Furthermore, the effect of different charge and discharge rates on the capacity of the doped sample is less obvious than that of the undoped sample. And the average capacity fading for the undoped sample is approximately 1.8 mAh g−1 per cycle while it is 1.2 mAh g−1 for sample z = 0.1 (after 62 cycles). The results above indicate that the appropriate amount of fluorine substitution can restrain the occurrence of side reaction and improve the cycle properties of the materials. Fig. 9 shows the 1st discharge curves of Li/LiV3 O8−y Fz (z = 0, 0.05, 0.1 and 0.2) cells at the discharge rate of 0.1 C. The initial open circuit voltage (OCV) and the 1st discharge capacity are given in Table 1. The shape of the discharge curves of the doped samples is similar to that of the undoped sample. Their differences occur mainly at the
Fig. 8. Plots of rate-capability tests on LiV3 O8−y Fz positive materials when the cells were cycled at different charge and discharge rates in the range of 2.0–4.0 V at room temperature.
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Fig. 9. The 1st discharge curves of LiV3 O8−y Fz with different fluorine contents at 0.1 C discharge rate. Table 1 The initial OCV and the 1st discharge capacity of LiV3 O8−y Fz (z = 0, 0.05, 0.1 and 0.2) positive materials at 0.1 C discharge rate.
Initial OCV (V) The 1st discharge capacity (mAh g−1 )
z=0
z = 0.05
z = 0.1
z = 0.15
z = 0.2
3.532 254
3.602 252.6
3.759 250.4
3.799 244.8
3.810 223.5
initial stage of the discharge curves. With the increase of fluorine content, the initial OCV increases, but the discharge voltage drops sharply and the 1st discharge capacity decreases. These phenomena might be caused by the effect of the impurities (Li0.3 V2 O5 and Li0.04 V2 O5 phases caused by the excessive fluorine substitution) on the electrode or the difference of the morphology such as particle size. Fig. 10a shows the relationship between the discharge voltage (DV) and open-circuit voltage (OCV) in the 2nd cycle at 0.1 C discharge rate for undoped sample. Fig. 10b presents the overvoltage of the samples versus composition x in Li1+x V3 O8−y Fz (z = 0, 0.05, 0.1 and 0.2) at 0.1 C discharge rate. The tests were carried out after the pristine cells were discharged to 2.0 V and then charged to 4.0 V at
Fig. 10. (a) The change of discharge voltage (DV) and open-circuit voltage (OCV) in the 2nd cycle at 0.1 C for undoped sample. (b) Overvoltage evolution of LiV3 O8−y Fz (z = 0, 0.05, 0.1 and 0.2) obtained by the difference between DV and OCV according to figure (a).
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Fig. 11. Variation of ASI at 50% SOD with cycling, measured from Li/LiV3 O8−y Fz (z = 0, 0.05, 0.1 and 0.2) cells at 0.25 C charge and discharge rates.
0.1 C rate. The overvoltages in Fig. 10b were obtained from the difference between DV and OCV according to the experiments in Fig. 10a. The overvoltage, , can be expressed by = EOCV − EDV . is related to internal resistance of the cell and higher internal resistance will result in higher , which is disadvantageous to the charge and discharge capacity. Fig. 10b shows that the difference of for different samples becomes obvious after x > 1.5. of samples z = 0.05 and 0.1 is smaller than that of the undoped sample, while for sample z = 0.2 is higher. The change in the overvoltage for different samples may be caused by the presence of F− ions in the lithium trivanadate structure. Fig. 11 shows the variation of area-specific impedance (ASI) at 50% state-of-discharge (SOD) with cycle number of Li/LiV3 O8−y Fz (z = 0, 0.05, 0.1 and 0.2) cells at 0.25 C charge and discharge rate. The ASI includes the ohmic resistance, electrode kinetic processes and Li+ ions diffusion through the electrolyte and within electrode. It is a very important property of the electrode because it provides information on the nature and magnitude of the electrochemical and mass-transport limitation [33]. The ASI is determined by A × V/I, where A is the cross-sectional area of the electrodes, V is the voltage change after current interruption for 60 s at each SOD and I is the applied current during cycling. As shown in Fig. 11, the fluorine addition and its content affect not only the initial ASI, but also its cycling stability. The initial ASI increases with F− dopant. The ASI of the 3rd cycle is 85 cm2 for sample z = 0 while it is 144, 133 and 194 cm2 for sample z = 0.05, 0.1 and 0.2, respectively. The ASI for samples z = 0.05 and 0.1 changes a little with cycling, while the ASI for samples z = 0 and 0.2 increase. After about 15 cycles, the ASI for undoped sample is higher than that for sample z = 0.1. So the proper amount of fluorine substitution can retain their structural stability, although it increases their initial ASI. To further understand the effect of fluorine dopant on the electrochemical behavior of synthesized samples, Fig. 12 shows the CV curves of Li/LiV3 O8−y Fz (z = 0, 0.1 and 0.2) cells at a scan rate of 0.1 mV s−1 in the voltage range of 2.0–4.0 V at room temperature. The CV behavior of fluorine-doped samples is similar to that of fluorine-free sample, and they all have main cathodic peaks at potentials of about 3.6, 2.8 and 2.5 V and main anodic peaks at 2.85 and 3.7 V, indicating that fluorine-doped samples have similar electrochemical behavior as a positive electrode of secondary lithium batteries. From Fig. 12a and b, only slight differences are observed in two cycles for samples z = 0 and 0.1, suggesting that the incorporation of lithium ions into the host lattice almost does not change their original structure. For sample z = 0.2, however, the voltammograms change obviously in the two cycles in Fig. 12c,
Fig. 12. Cyclic voltammograms of Li/LiV3 O8−y Fz cells (a) z = 0; (b) z = 0.1; (c) z = 0.2. Scan rate: 0.1 mV s−1 .
thus structural modifications may take place during the first charge and discharge operations. Fig. 13 presents the CV of Li/LiV3 O8−y Fz (z = 0, 0.1 and 0.2) cells after 20 cycles between 2.0 and 3.5 V at the scan rate of 0.1 mV s−1 . Three samples have the similar general voltammetric features, but the peak position and its intensity are different. The anodic peaks of sample z = 0.1 are larger, higher and shifted toward lower potential than those of undoped sample, indicating that sample z = 0.1 has more sites and faster kinetics for Li+ insertion/extraction. The anodic peaks of sample z = 0.2 are broader
Fig. 13. Cyclic voltammograms of Li/LiV3 O8−y Fz (z = 0, 0.1 and 0.2) cells after 20 cycles. Scan rate: 0.1 mV s−1 .
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diameters of the semicircles decrease significantly for all samples after 3 cycles. After 20 cycles, however, the diameters of the semicircles almost unchange for samples z = 0.05 and 0.1, while the diameters of the semicircles increase for samples z = 0 and 0.2 (Fig. 14c). Since the diameters of the semicircles are related to the conductivity of the solid electrolyte interface (SEI) film, the thicker the SEI film, the larger the resistance and the larger the diameters of the semicircles. According to Ref. [34], the pristine composite electrode is covered with a surface film, possibly Li2 CO3 or lithium alkaline oxides, and its presence causes a noticeable decline of the surface conductivity. The high fluorine content in LiV3 O8−y Fz significantly increases the film resistance (see Fig. 14a), which is probably related to the formation of a new interface mass resulting from the non-equivalent substitution. When the charging/discharging starts, the current flow can destroy or replace the passive surface film covering the electrode by active mass, reducing the surface film resistance (see Fig. 14b) and making the charge transfer more easily at the electrode/electrolyte interface [35]. With cycling, the electrolyte decomposition and vanadium dissolution accompanied by the capacity fading cause the surface film to be thicker, which results in the increase of the resistance. So the substitution of fluorine for oxygen can stabilize the interface between the electrolyte and the surface layer of the particles during cycling when the fluorine content is low. However, this interface becomes unstable when the fluorine content is high. 4. Conclusion LiV3 O8−y Fz (0 ≤ z ≤ 0.5) was prepared by a solid-state reaction and the impacts of the fluorine substitution for oxygen in LiV3 O8 on its crystal structures, surface properties and electrochemical characteristics have been systematically investigated. XRD, FTIR and SEM tests reveal that the proper amount of fluorine-doped for oxygen in LiV3 O8 can modify its structure and surface morphology. Charge–discharge tests indicate that sample z = 0.1 displays good cycling stability, high coulombic efficiency and good rate capability compared with undoped sample, even at a high temperature (55 ◦ C). Moreover, overvoltage, ASI, CV and EIS studies confirm that sample z = 0.1 has the lower impedance and better stability during cycling, compared with undoped sample. Thus, fluorine substitution is an effective way to improve the electrochemical performance of LiV3 O8 material. References
Fig. 14. EIS plots of LiV3 O8−y Fz electrodes before charge and after different cycles.
and shifted toward higher potential than those of undoped sample, suggesting that a portion of the Li+ ions need more energy to be removed from sample z = 0.2. Fig. 14 shows the impedance spectra of the Li/LiV3 O8−y Fz (z = 0, 0.05, 0.1 and 0.2) cells before and after 3 and 20 cycles. The cycled cells were measured on the open circuit voltage after charging to 4.0 V. A semicircle in the high frequencies and a straight slopping line in low frequencies can be observed for all samples. The semicircle in the high frequencies reflects the resistance for Li+ ions migration through the interface between the surface layer of the particles and the electrolyte. The slopping line in the low frequencies corresponds to Li+ ions solid-state diffusion within the electrode. For fresh cells, the diameters of the semicircles increase with the amount of doped fluorine (Fig. 14a). Fig. 14b shows the
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Effects of fluorine doping on the electrochemical properties of LiV3 O8 cathode material Yongmei Liu, Xuechou Zhou, Yonglang Guo ∗ College of Chemistry and Chemical Engineering, Fuzhou University, Fuzhou 350108, PR China
a r t i c l e
i n f o
Article history: Received 31 October 2008 Received in revised form 21 November 2008 Accepted 22 November 2008 Available online 30 November 2008 Keywords: Cathode material Fluorine doping Lithium batteries Lithium trivanadate Solid-state reaction
a b s t r a c t A series of fluorine-doped lithium trivanadates LiV3 O8−y Fz (z = 0, 0.03, 0.05, 0.1, 0.15, 0.2 and 0.5) were synthesized by the solid-state reaction. X-ray diffraction (XRD), Fourier transform infrared (FTIR) and scanning electron microscope (SEM) tests show that a proper amount of fluorine substituting for oxygen in LiV3 O8 can modify its structure and surface morphology. Charge–discharge tests show that the doped samples with a proper amount of fluorine display good cycling stability, high coulombic efficiency and good rate capability, compared with undoped sample. The cyclic voltammetry (CV), area-specific impedance (ASI) and electrochemical impedance spectroscopy (EIS) tests indicate that the doped samples with a low fluorine content can stabilize the interface between the surface layer of the active particles and the electrolyte after cycling, while a high fluorine content form an unstable interface. The fluorine substitution is a convenient and effective method for improving the electrochemical performances of LiV3 O8 . © 2008 Elsevier Ltd. All rights reserved.
1. Introduction Lithium trivanadate is an attractive candidate cathode material for rechargeable lithium batteries due to its high capacity, low cost, long cycle life and facile preparation [1–4]. More than three Li+ ions can be accommodated in each Li1+x V3 O8 unit, and even it reaches 4.5 Li+ ions in its amorphous form [5]. It is well known that the preparation method for Li1+x V3 O8 strongly influences its electrochemical properties. Traditional synthesis method of Li1+x V3 O8 was that Li2 CO3 reacted with V2 O5 at 680 ◦ C [6]. At such high reaction temperature, it is difficult to control the accurate composition of the final products. Meanwhile, the product has a low capacity of 180 mAh g−1 in the range of 1.8–4.0 V, and the molten V2 O5 will cause corrosion to the crucible. To improve the electrochemical properties of this material, much attention has been paid to the new synthetic methods and post treatments, mainly the sol–gel method [7–11], hydrothermal reaction [4], microwave-assisted synthesis [12], rheological phase reaction method [2], ultrasonic treatment [13,14], spray-drying synthesis [15] and intercalation of inorganic molecules between the interlayers [1]. These strategies can improve the electrochemical performance of the material. However, there are some disadvantages such as long reaction time or complex post treatment steps. So they are not suitable for large-scale production. Substitution of
∗ Corresponding author. Fax: +86 591 8807 3608. E-mail address: [email protected] (Y. Guo). 0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2008.11.045
lithium with other cations such as Na+ , K+ , Mg2+ , Ca2+ , Cu+ , Ag+ [16–20] and that of vanadium with Mn, Mo, W [21,22] have been studied, but they do not significantly improve the electrochemical performances of lithium trivanadates. According to Jouanneau et al. [23], the capacity fading of lithium trivanadate was ascribed to the local damage of the crystal structure, which was caused by the great change in cell lattice and the partial dissolution of the active material in the liquid electrolyte during the cycling. Oxygen substitution with fluorine was reported to have improved the electrochemical performances of layered Li(Ni,Mn)O2 and spinel LiMn2 O4 [24–27]. For example, Oh et al. [26] believed that F-substituted LiNi0.5 Mn1.5 O4−x Fx has a smaller lattice variation during the Li+ ion insertion/extraction and exhibits good resistance to the attack of HF existing in the electrolyte. Choi and Manthiram [27] had also a similar report. However, its effect on the properties of lithium trivanadate has not been reported. In this work, the effect of oxygen substitution with fluorine on the electrochemical performance of LiV3 O8 has been investigated. 2. Experimental LiV3 O8−y Fz (z = 0, 0.03, 0.05, 0.1, 0.15, 0.2 and 0.5) samples were prepared as described in Ref. [28] by the solid-state reaction containing Li2 CO3 , NH4 VO3 and LiF. The stoichiometrically weighted starting materials were well mixed in an agate mortar. The mixture was pressed into pieces, transferred to a porcelain crucible, and then heated to 600 ◦ C at the rate of 1 ◦ C min−1 in dry air using a SCQ-8-12B tube furnace. The quasi melted compounds were quickly
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Fig. 1. XRD patterns of LiV3 O8−y Fz (z = 0, 0.03, 0.05, 0.1 and 0.5).
cooled by putting the crucible rapidly in an airproof stainless steel container in ice bath, ground in an agate mortar and then dried in vacuum environment at 120 ◦ C for 4 h to obtain the products used to prepare the positive electrode. The X-ray diffraction (XRD) experiments were carried out using a X’pert-MPD X-ray Diffractometer with Cu K␣ radiation ( = 1.54056 Å) in the range of 10–70◦ to identify the phases and the structure of the synthesized materials. Fourier transform infrared (FTIR) absorption spectra used to determine the bonding nature were obtained by the KBr disk method using a SPECTRUM 2000 Fourier transform infrared spectrometer. Their morphologies were observed by a scanning electron microscope (SEM, Philips, XL30 ESEM-TEP). The electrochemical performance of the synthesized materials was evaluated in model CR2025 coin cells. The paste consisted of the positive active material prepared above, super P carbon black and polyvinylidene fluoride (PVDF) binder in a weight ratio of 85:10:5. The positive electrode was prepared by coating the paste on Al collector. Lithium foil was served as the negative and reference electrodes. The separator was Celgard 2320 membrane and the electrolyte was 1 M LiPF6 in ethylene carbonate (EC) and dimethyl carbonate (DMC) in volume ratio of 1:1. The cyclic tests, the open circuit voltage (OCV) data measurements and the area-specific impedance (ASI) were performed by Land automatic battery tester (Wuhan, China). Cyclic tests were conducted between 2.0 and 4.0 V at different charge–discharge rates. Cyclic voltammograms (CV) and the electrochemical impedance spectroscopy (EIS) on the positive electrode in the cells described above were performed by a CHI 660C electrochemical workstation. The CV scan rate was 0.1 mV s−1 . The EIS measurements were carried out by applying an ac voltage of 2 mV in the frequency range of 10 mHz–100 kHz before and after 3 and 20 cycles. All cells were assembled in a glove box filled with purified argon gas.
Fig. 2. The lattice parameters and d100 value of LiV3 O8−y Fz (z = 0, 0.03, 0.05, 0.1 and 0.5) calculated from Fig. 1.
V5+ in LiV3 O8 is reduced to V4+ and even V3+ [29]. With the increase of fluorine content, it is difficult to further reduce the oxidation state of vanadium. Consequently, the lithium content in LiV3 O8 decreases to balance the anion valence. This results in the structure collapse and the formation of Li0.3 V2 O5 and even Li0.04 V2 O5 phases. Based on this, it can be concluded that only when the doping amount is low, can F− substitute O2− and locate in the crystal lattice sites of O2− , and form the LiV3 O8−y Fz (z = y) isomorphous solid solution. Fig. 2 shows the changes in the lattice parameters and the interlayer distance of (1 0 0) crystal plane calculated from the XRD patterns in Fig. 1. The lattice parameters a, b, c,ˇ and d100 values decrease with small amount of fluorine substitution (z < 0.05). When the fluorine doping increases (z ≥ 0.05), however, the lattice parameters and d100 values become large and then remain almost constant. These results indicate that only primary single-phase exists when z < 0.05. Similar phenomenon was reported by Oh et al. [26] on the investigation of LiNi0.5 Mn1.5 O4−x Fx . The shrink of the lattice parameters in LiV3 O8−y Fz (z < 0.05) samples may be due to the small ionic radius of F− (r = 1.33 Å), compared with that of O2− (r = 1.40 Å) [30]. FTIR spectra of LiV3 O8−y Fz (z = 0, 0.03, 0.05, 0.1 and 0.5) are shown in Fig. 3. The FTIR spectra of the oxyfluoride compounds are very similar to that of the fluorine-free oxide. The two peaks at 956 and 995 cm−1 and the peak near 740 cm−1 are attributed to the stretching vibration of the short range (V O) and long range
3. Results and discussion The XRD patterns of LiV3 O8−y Fz (z = 0, 0.03, 0.05, 0.1 and 0.5) are shown in Fig. 1. All samples with different amount of F− doping have very strong diffraction peaks and well-developed crystal structure of layered LiV3 O8 (JCPDS card: No. 18-754). But some impurity phases are found when z ≥ 0.05, that is a weak peak (2 = 12.3◦ ) ascribed to Li0.3 V2 O5 (marked by ) for samples z = 0.05 and 0.1, and peaks ascribed to Li0.3 V2 O5 and Li0.04 V2 O5 (marked by and ♦, respectively) for sample z = 0.5. This phenomenon is related to the charge compensation [21]. Since no peaks of LiF compound appear (its strongest peak is located at 45◦ ), it is deduced that the partial O2− in LiV3 O8 is replaced by F− . To balance the anion valance, partial
Fig. 3. FTIR spectra of LiV3 O8−y Fz (z = 0, 0.03, 0.05, 0.1 and 0.5).
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Fig. 5. The dependence of specific capacity on the cycle number at the charge and discharge rates of 0.25 C on LiV3 O8−y Fz (z = 0, 0.05, 0.1, 0.15, 0.2 and 0.5) positive materials.
Fig. 4. SEM photographs of LiV3 O8−y Fz . (a) z = 0; (b) z = 0.1; (c) z = 0.2.
(V–O–V) order between vanadium and oxygen atoms, respectively [31]. No Li–F absorption band is observed in FTIR spectra. These phenomena and XRD data indicate the existence of the order configuration of VO6 unit but no LiF phase in LiV3 O8−y Fz compounds. With F− doping, both peaks located at about 1000 and 740 cm−1 become narrow, indicating that F− substitutes O2− both in VO6 octahedrons and VO5 trigonal bipyramids. Fig. 4 shows the SEM of LiV3 O8−y Fz (z = 0, 0.1 and 0.2). All samples present ordered crystals. The particle size of LiV3 O8−y Fz increases with the increase of the F− content and the surface of the grains becomes smooth. These morphological differences can affect the contact area with the electrolyte and the vanadium dissolution reaction, which are very important to both the specific capacity and cycle ability of the materials. Fig. 5 shows the discharge capacity of LiV3 O8−y Fz materials with different contents of fluorine doping. All cycle ability is improved by the fluorine substitution, of which the performances for sample z = 0.1 is best. The capacity of undoped material declines continually during cycling and decreases by 56% after 50 cycles. For sample z = 0.1, although the discharge capacity reduces about 23 mAh g−1 in the initial 10 cycles, only a little capacity falling is observed in
the subsequent 40 cycles. This may be related to the difference of the morphology. The smooth surface can restrain the vanadium dissolution in electrolyte. For samples z = 0.15 and 0.2, the maximum capacities appear after several cycles. This arises from a slight fragmentation of the grains (induced by electrochemical grinding), leading to smaller grain size and thus to shorter diffusion length for Li+ ions [32]. For sample z = 0.5, its discharge capacity loss mainly occurs during the first 10 cycles and reaches about 60 mAh g−1 . This is related to the formation of the unstable interface on cathode surface, which is caused by the generated Lix V2 O5 as shown in XRD. Fig. 6 shows the coulombic efficiency of LiV3 O8−y Fz materials under the charge and discharge conditions of Fig. 5. The coulombic efficiency of undoped sample decreases gradually in the initial 13 cycles and then increases slowly. And it is much lower than that of all other samples except for sample z = 0.5 during cycling. This phenomenon with low coulombic efficiency and continual capacity falling for undoped sample is caused by the destruction of the crystal structure and vanadium dissolution in electrolyte induced by the variation of the cell lattice in the processes of lithium ion insertion/extraction [16]. Fig. 7 shows the cycle performance of samples z = 0 and 0.1 at different temperatures at the charge and discharge rates of 1.0 C. It can be seen from Fig. 7 that the positive effect of the fluorine dopant on the cycle performance is more noticeable at elevated
Fig. 6. The dependence of coulombic efficiency of LiV3 O8−y Fz on the cycle number at the charge and discharge rates of 0.25 C.
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Fig. 7. The dependence of specific capacity of LiV3 O8−y Fz on the cycle number at the charge and discharge rate of 1.0 C and at different temperatures.
temperature. The capacity of undoped material shows significant decline with cycling while the fluorine-doped material has good cycling performance at 55 ◦ C. Fig. 8 shows the rate capability of samples z = 0 and 0.1. The cells were cycled in the range of 2.0–4.0 V at room temperature and their charge and discharge rates were changed successively from 5 to 0.05 C after the 12th cycle. The capacity fading is greater at low charge and discharge rates than that at high rates. This can be attributed to the exposure-time effects of vanadate dissolution at high voltage [25]. Furthermore, the effect of different charge and discharge rates on the capacity of the doped sample is less obvious than that of the undoped sample. And the average capacity fading for the undoped sample is approximately 1.8 mAh g−1 per cycle while it is 1.2 mAh g−1 for sample z = 0.1 (after 62 cycles). The results above indicate that the appropriate amount of fluorine substitution can restrain the occurrence of side reaction and improve the cycle properties of the materials. Fig. 9 shows the 1st discharge curves of Li/LiV3 O8−y Fz (z = 0, 0.05, 0.1 and 0.2) cells at the discharge rate of 0.1 C. The initial open circuit voltage (OCV) and the 1st discharge capacity are given in Table 1. The shape of the discharge curves of the doped samples is similar to that of the undoped sample. Their differences occur mainly at the
Fig. 8. Plots of rate-capability tests on LiV3 O8−y Fz positive materials when the cells were cycled at different charge and discharge rates in the range of 2.0–4.0 V at room temperature.
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Fig. 9. The 1st discharge curves of LiV3 O8−y Fz with different fluorine contents at 0.1 C discharge rate. Table 1 The initial OCV and the 1st discharge capacity of LiV3 O8−y Fz (z = 0, 0.05, 0.1 and 0.2) positive materials at 0.1 C discharge rate.
Initial OCV (V) The 1st discharge capacity (mAh g−1 )
z=0
z = 0.05
z = 0.1
z = 0.15
z = 0.2
3.532 254
3.602 252.6
3.759 250.4
3.799 244.8
3.810 223.5
initial stage of the discharge curves. With the increase of fluorine content, the initial OCV increases, but the discharge voltage drops sharply and the 1st discharge capacity decreases. These phenomena might be caused by the effect of the impurities (Li0.3 V2 O5 and Li0.04 V2 O5 phases caused by the excessive fluorine substitution) on the electrode or the difference of the morphology such as particle size. Fig. 10a shows the relationship between the discharge voltage (DV) and open-circuit voltage (OCV) in the 2nd cycle at 0.1 C discharge rate for undoped sample. Fig. 10b presents the overvoltage of the samples versus composition x in Li1+x V3 O8−y Fz (z = 0, 0.05, 0.1 and 0.2) at 0.1 C discharge rate. The tests were carried out after the pristine cells were discharged to 2.0 V and then charged to 4.0 V at
Fig. 10. (a) The change of discharge voltage (DV) and open-circuit voltage (OCV) in the 2nd cycle at 0.1 C for undoped sample. (b) Overvoltage evolution of LiV3 O8−y Fz (z = 0, 0.05, 0.1 and 0.2) obtained by the difference between DV and OCV according to figure (a).
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Fig. 11. Variation of ASI at 50% SOD with cycling, measured from Li/LiV3 O8−y Fz (z = 0, 0.05, 0.1 and 0.2) cells at 0.25 C charge and discharge rates.
0.1 C rate. The overvoltages in Fig. 10b were obtained from the difference between DV and OCV according to the experiments in Fig. 10a. The overvoltage, , can be expressed by = EOCV − EDV . is related to internal resistance of the cell and higher internal resistance will result in higher , which is disadvantageous to the charge and discharge capacity. Fig. 10b shows that the difference of for different samples becomes obvious after x > 1.5. of samples z = 0.05 and 0.1 is smaller than that of the undoped sample, while for sample z = 0.2 is higher. The change in the overvoltage for different samples may be caused by the presence of F− ions in the lithium trivanadate structure. Fig. 11 shows the variation of area-specific impedance (ASI) at 50% state-of-discharge (SOD) with cycle number of Li/LiV3 O8−y Fz (z = 0, 0.05, 0.1 and 0.2) cells at 0.25 C charge and discharge rate. The ASI includes the ohmic resistance, electrode kinetic processes and Li+ ions diffusion through the electrolyte and within electrode. It is a very important property of the electrode because it provides information on the nature and magnitude of the electrochemical and mass-transport limitation [33]. The ASI is determined by A × V/I, where A is the cross-sectional area of the electrodes, V is the voltage change after current interruption for 60 s at each SOD and I is the applied current during cycling. As shown in Fig. 11, the fluorine addition and its content affect not only the initial ASI, but also its cycling stability. The initial ASI increases with F− dopant. The ASI of the 3rd cycle is 85 cm2 for sample z = 0 while it is 144, 133 and 194 cm2 for sample z = 0.05, 0.1 and 0.2, respectively. The ASI for samples z = 0.05 and 0.1 changes a little with cycling, while the ASI for samples z = 0 and 0.2 increase. After about 15 cycles, the ASI for undoped sample is higher than that for sample z = 0.1. So the proper amount of fluorine substitution can retain their structural stability, although it increases their initial ASI. To further understand the effect of fluorine dopant on the electrochemical behavior of synthesized samples, Fig. 12 shows the CV curves of Li/LiV3 O8−y Fz (z = 0, 0.1 and 0.2) cells at a scan rate of 0.1 mV s−1 in the voltage range of 2.0–4.0 V at room temperature. The CV behavior of fluorine-doped samples is similar to that of fluorine-free sample, and they all have main cathodic peaks at potentials of about 3.6, 2.8 and 2.5 V and main anodic peaks at 2.85 and 3.7 V, indicating that fluorine-doped samples have similar electrochemical behavior as a positive electrode of secondary lithium batteries. From Fig. 12a and b, only slight differences are observed in two cycles for samples z = 0 and 0.1, suggesting that the incorporation of lithium ions into the host lattice almost does not change their original structure. For sample z = 0.2, however, the voltammograms change obviously in the two cycles in Fig. 12c,
Fig. 12. Cyclic voltammograms of Li/LiV3 O8−y Fz cells (a) z = 0; (b) z = 0.1; (c) z = 0.2. Scan rate: 0.1 mV s−1 .
thus structural modifications may take place during the first charge and discharge operations. Fig. 13 presents the CV of Li/LiV3 O8−y Fz (z = 0, 0.1 and 0.2) cells after 20 cycles between 2.0 and 3.5 V at the scan rate of 0.1 mV s−1 . Three samples have the similar general voltammetric features, but the peak position and its intensity are different. The anodic peaks of sample z = 0.1 are larger, higher and shifted toward lower potential than those of undoped sample, indicating that sample z = 0.1 has more sites and faster kinetics for Li+ insertion/extraction. The anodic peaks of sample z = 0.2 are broader
Fig. 13. Cyclic voltammograms of Li/LiV3 O8−y Fz (z = 0, 0.1 and 0.2) cells after 20 cycles. Scan rate: 0.1 mV s−1 .
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diameters of the semicircles decrease significantly for all samples after 3 cycles. After 20 cycles, however, the diameters of the semicircles almost unchange for samples z = 0.05 and 0.1, while the diameters of the semicircles increase for samples z = 0 and 0.2 (Fig. 14c). Since the diameters of the semicircles are related to the conductivity of the solid electrolyte interface (SEI) film, the thicker the SEI film, the larger the resistance and the larger the diameters of the semicircles. According to Ref. [34], the pristine composite electrode is covered with a surface film, possibly Li2 CO3 or lithium alkaline oxides, and its presence causes a noticeable decline of the surface conductivity. The high fluorine content in LiV3 O8−y Fz significantly increases the film resistance (see Fig. 14a), which is probably related to the formation of a new interface mass resulting from the non-equivalent substitution. When the charging/discharging starts, the current flow can destroy or replace the passive surface film covering the electrode by active mass, reducing the surface film resistance (see Fig. 14b) and making the charge transfer more easily at the electrode/electrolyte interface [35]. With cycling, the electrolyte decomposition and vanadium dissolution accompanied by the capacity fading cause the surface film to be thicker, which results in the increase of the resistance. So the substitution of fluorine for oxygen can stabilize the interface between the electrolyte and the surface layer of the particles during cycling when the fluorine content is low. However, this interface becomes unstable when the fluorine content is high. 4. Conclusion LiV3 O8−y Fz (0 ≤ z ≤ 0.5) was prepared by a solid-state reaction and the impacts of the fluorine substitution for oxygen in LiV3 O8 on its crystal structures, surface properties and electrochemical characteristics have been systematically investigated. XRD, FTIR and SEM tests reveal that the proper amount of fluorine-doped for oxygen in LiV3 O8 can modify its structure and surface morphology. Charge–discharge tests indicate that sample z = 0.1 displays good cycling stability, high coulombic efficiency and good rate capability compared with undoped sample, even at a high temperature (55 ◦ C). Moreover, overvoltage, ASI, CV and EIS studies confirm that sample z = 0.1 has the lower impedance and better stability during cycling, compared with undoped sample. Thus, fluorine substitution is an effective way to improve the electrochemical performance of LiV3 O8 material. References
Fig. 14. EIS plots of LiV3 O8−y Fz electrodes before charge and after different cycles.
and shifted toward higher potential than those of undoped sample, suggesting that a portion of the Li+ ions need more energy to be removed from sample z = 0.2. Fig. 14 shows the impedance spectra of the Li/LiV3 O8−y Fz (z = 0, 0.05, 0.1 and 0.2) cells before and after 3 and 20 cycles. The cycled cells were measured on the open circuit voltage after charging to 4.0 V. A semicircle in the high frequencies and a straight slopping line in low frequencies can be observed for all samples. The semicircle in the high frequencies reflects the resistance for Li+ ions migration through the interface between the surface layer of the particles and the electrolyte. The slopping line in the low frequencies corresponds to Li+ ions solid-state diffusion within the electrode. For fresh cells, the diameters of the semicircles increase with the amount of doped fluorine (Fig. 14a). Fig. 14b shows the
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