J. Phys. Chem. C 2007, 111, 10707-10711
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Structural and Electrochemical Studies on β-LixV2O5 as Cathode Material for Rechargeable Lithium Batteries Jun Jiang, Zhaoxiang Wang,* and Liquan Chen Laboratory for Solid State Ionics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100080, China ReceiVed: February 28, 2007; In Final Form: May 7, 2007
Lithium vanadium pentoxide β-LixV2O5 (x ≈ 0.32) was synthesized by solid-state reaction and evaluated as a cathode material for rechargeable lithium battery. Its electrochemical performances were characterized by galvanostatic cycling between 4.4 V versus Li/Li+. The lithium diffusion in this material is studied by first principles calculations.
Introduction Layer-structured LiCoO2, spinel LiMn2O4, and polyanions such as LiFePO4 have been well studied as cathode materials for lithium ion batteries. However, the available capacities of these materials are only 150-180 mA h/g. Energy-storage and power-type rechargeable lithium (ion) batteries require cathode materials with higher energy density and/or power density as well as longer cycle life. Vanadium oxides are attractive alternatives as vanadium is known to exist in a wide range of oxidation states from +2 as in VO to +5 as in V2O5. In addition, the vanadium oxides have the potential to offer much higher capacities. Depending on its oxygen contents, lithium vanadium pentoxides have various phases such as R-, β-, -, δ-, ν-, γ-, and F- phases, etc., some of which have been studied in recent years.1-4 The γ-LiV2O5 prepared by carbothermal reduction showed a capacity of 100 mA h/g.5 Wadsley proposed Li1+xV3O8 as a cathode material of lithium ion battery.6 Leger et al. used ω-LixV2O5 (0.4 E x E 3) as cathode material for rechargeable lithium batteries. Its initial discharge capacity reaches 335 mA h/g, but it drops to 310 mA h/g after 30 cycles at C/20 rate.7 All these pentoxides are potential candidate cathode materials for lithium batteries. Galy et al.8 investigated the crystalline structure of β-LixV2O5 (0.3 e x e 0.4; Figure 1). β-LixV2O5 belongs to the monoclinic system of C2/m space group constructed with the network of [VO6] octahedrons. The [VO6] octahedrons in one layer form a group and the groups build the network by sharing edges and acmes with each other. The lithium is inserted along the b-axis and stays in the [VO6] octahedrons. In this article, we show that β-LixV2O5 (x ≈ 0.32 here) is a promising cathode material for rechargeable lithium batteries based on our experimental and theoretical investigations. Experimental Section β-LixV2O5 was synthesized with δ-LiV2O5 and commercial V2O5 as the precursors. The precursors were milled in a mortar and then pressed into pellets. The pellets were sealed in a vacuum quarts tube and heated at 500 °C for 7 days. The pellets changed from yellow to black during this process. δ-LixV2O5 was prepared by refluxing commercial V2O5 in an 8 M LiI/ acetonitrile solution at ca. 80 °C. δ-LixV2O5 was obtained by washing and filtering the mixture with acetonitrile. * Corresponding author.
Figure 1. The cell structure of β-LixV2O5.
Figure 2. The XRD pattern of β-LixV2O5 (upper for experimental and lower for the standard PDF).
Test cells were assembled using β-LixV2O5 (cast on Al foil with 5 w/w % PVdF as the binder and 10 w/w % carbon black as the conduction reagent), fresh lithium as the counter electrode and Celgard 2300 as the separator. A 1 mol/L sample of LiPF6 dissolved in EC/DMC was used as the electrolyte. Results and Discussion X-ray diffraction pattern of the material agrees rather well with the standard XRD pattern of β-LixV2O5 (Figure 2). The peak at 14.05° (2θ) belongs to Li1.11V3O7.89 (PDF card number: 44-0400). Scanning electron microscopy (SEM, Figure 3) imaging shows that most particles are bar-like and their aspect ratios are around 5. Some smaller fractures are also observed because the sample was mechanically milled after synthesis.
10.1021/jp071635o CCC: $37.00 © 2007 American Chemical Society Published on Web 06/23/2007
10708 J. Phys. Chem. C, Vol. 111, No. 28, 2007
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Figure 6. The XRD patterns while discharging to different voltages.
Figure 7. The calculated cell structure of β-LixV2O5.
Figure 3. The SEM images of β-LixV2O5.
Figure 8. The calculated density of states (DOS) of β-LixV2O5.
TABLE 1: The Calculated Cell Parameters of β-LixV2O5 and Data in PDF Database Figure 4. The galvanostatic voltage profiles of β-LixV2O5 between 4.5 and 1.5 V vs Li+/Li.
Figure 5. The cycling performance of β-LixV2O5 between 4.5 and 1.5 V vs Li+/Li.
The electrochemical performances of the material are evaluated by galvanostatic cycling. Figure 4 shows the chargedischarge curves in the first few cycles at 20 mA/g. As many as 5 charge plateaus are recognized, corresponding to the
calcd PDF data (19-0732)
a (Å)
b (Å)
c (Å)
V
b
10.13 10.03
3.61 3.60
15.51 15.38
533.50 545.86
110.3° 110.6°
complicated phase transition of the material between 4.5 and 1.5 V. The capacity of this material in this voltage range is as high as 320 mA h/g or an energy density of 864 W h/kg. The main plateau at around 2.80 V contains ca. 130 mA h/g (40% of the total capacity) in the first cycle. The capacity between 2.7-4.5 V is about 80% (250 mA h/g) of the total capacity in the first cycle. So the capacity of β-LixV2O5 is much higher than that of LiCoO2. The capacity at around the 2.20 V plateau may find its application in the future. Actually 1.5 V Li-ion batteries with high safety, high rate performance, and long cycle life have been developed with LiFePO4 and Li4Ti5O12 as the active electrode materials (ATL, Dongguan, China). Therefore, a lithium battery with wide operation voltage but high-energy density may still has its practical application in the future. All the discharge plateaus are very similar to their charge counterparts in length and in potential, indicating the low polarization and the good structural reversibility of the material within this potential range.
β-LixV2O5 as Cathode Material
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Figure 9. The simulated lithium diffusion pathway when (a) zero position, (b) 1 position, (c) 3 positions, and (d) 7 positions are inserted between A and B positions.
Figure 5 shows the cycling performance of the material. Its reversible capacity in the first cycle is 320 mAh/g or an energy density of 864 W h/kg, much higher than that of LiCoO2 (ca. 600 Wh/kg). The reversible capacity fades to 260 mA h/g after 50 cycles, demonstrating the rather good structural stability of
the material between 4.5 and 1.5 V (note: neither LiCoO2 nor LiMn2O4 can be discharged to 1.5V because of irreversible reduction for LiCoO2 and the Jahn-Teller effect for LiMn2O4). The reason for the capacity fading is complicated, probably including (1) unrecognized structural degradation of the material,
10710 J. Phys. Chem. C, Vol. 111, No. 28, 2007 (2) electrolyte decomposition at high charge voltages, (3) loss of some volatile component in the electrolyte (DMC, for example), and so on. As β-LixV2O5 is lithium-deficient in its pristine state, further experiment was carried out to find out if the initial charging or discharging process has any impacts on its capacity and cycling performances. It is found that the reversible capacity of the material does not depend on the initial charging or discharging process. This demonstrates the good reversibility of the structure of the material from another side. We tried to investigate the phase transition of the material during lithium insertion and extraction. The Li/β-LixV2O5 cells were charged to various voltages after initially charged to 4.5 V. As shown in Figure 6, the wide peak at around 25.9° belongs to the Pylar film masking the sample from contacting with air while the sharp peaks at ca. 65.0° comes from the Al foil.9 Besides those peaks, many new peaks appear in the XRD pattern of the discharged samples, indicating the complicated phase transition during discharge. We can only recognize phases like LiV3O8 and LiV2O4 except for the diffraction peaks of pristine β-LixV2O5. It is interesting that the main diffraction peaks of β-LixV2O5 are kept until the end of discharge, but their positions shift toward the left with decreasing potential, meaning that the lattice parameters of the material increases with lithium intercalation. Calculations In order to understand the electronic property of β-LixV2O5 and diffusion of lithium in it, first principles calculations are carried out based on density function theory (DFT) within pseudopotential approximation and generalized gradient approximation (GGA), as implemented in the Vienna Ab initio Simulation Package (VASP).10,11 The plane wave basis set is adopted and the correlation between the nucleus and the electrons is treated with ultrasoft pesudopotentials. The energy cutoff is set at 450 eV and the total energy converged less than 5 meV. The calculated supercell contains 6 units of β-LixV2O5 (Li2V12O30). The initial lattice parameters and positions of the atoms are supposed the same as in Figure 1. For the relaxation calculations, 32 irreducible k-points are automatically generated by a Monkhorst-Pack scheme. Figure 7 shows the lattice structure of β-LixV2O5 after full relaxation. The cell parameters are shown in Table 1. Comparing the relaxed structure with the initial one, it is found that the [VO6] octahedrons twist a little around the b-axis in the a-c plane and change to [VO5], indicating that the lithium ion impacts the positions of the neighboring atoms due to the Coulombic attraction The density of states (DOS) is shown in Figure 8, and the Fermi level is set to 0 eV. Figure 8 shows that the band gap of β-LixV2O5 is 1.20 eV and the Fermi level is slightly above the bottom of the conduction band. The inset in Figure 10 shows the DOS of V2O5, which is quite similar to the DOS pattern of β-LixV2O5. The Fermi level of V2O5 is slightly above the top of the valence band. The main difference between the DOS of β-LixV2O5 and V2O5 is that the Fermi level of β-LixV2O5 is higher than that of V2O5. The integral of the DOS of β-LixV2O5 indicates that there are two electrons between the bottom of the conduction band and the Fermi level. This implies that these parts of DOS are filled up by electrons of the lithium atom. The band structure indicates that β-LixV2O5 has good electronic conductivity. The lithium diffusion in β-LixV2O5 is calculated by the molecular dynamics method (MD). The lithium atoms stay at
Jiang et al.
Figure 10. Energy barriers corresponding to the above pathways.
the positions shown in Figure 7. Considering the crystalline structure of the material (Figure 1 and Figure 7), the diffusion pathway is supposed to be one dimension and along the b-axis only. Calculation shows that it is very difficult for the lithium ions to transport from site A to site B in a linear pathway along the b-axis (Figure 9a) because it has to get over an energy barrier over 100 eV. When one lithium site is inserted between A and B (site C′, the center of the A-B line, Figure 9b), the lithium atom is relaxed in the plane vertical to the A-B line and stays at site C shown in Figure 9b (CC′ ≈ 0.219 Å). The new pathway of lithium is A f C f B. The diffusion barrier along the A f C f B pathway is calculated. Clearly the diffusion barrier is drastically decreased to 2.0 eV upon the insertion of site C. The same method is applied to insert more lithium sites between A and B. Panels c and d of Figure 9 show the pathways with five sites and nine sites, respectively. The pathway in Figure 9d is mostly similar to the actual pathway of lithium diffusion in β-LixV2O5. Figure 10 summarizes the calculated energy barriers when the lithium diffuses along these above three pathways (b, c, d). The energy barrier decreases to 0.45 eV for lithium to diffuse along the b-axis when seven sites are inserted between A and B. These nine points collect to become a crooked pathway. Clearly the strong Coulombic repulsion of V5+ ions at the top of the [VO6] octahedrons to the lithium ions is responsible for the crook of the pathway. The lithium ions have to circumambulate the V atom and follow an S-like route in the b-axis direction in β-LixV2O5. The lithium diffusion coefficient for 1D chain can be simply described as D ) Γa2,12 where Γ is the hopping frequency from a occupied site to a vacant one and a is the hopping length. Γ is denoted as Γ ≈ ν* exp(-E/kBT) through the transition state theory,13 where ν* is the attempt frequency and E is the energy barrier that the lithium atom has to hop over. Therefore, the diffusion coefficient can be described as
D ) a2ν* exp(-E/kBT)
(1)
where a ) 3 Å and ν* ) 1012 Hz.14 The calculated diffusion coefficient is 10-11 cm2/s. Comparing with the lithium cathode materials such as LiCoO2, LiFePO4 and LiMn2O4, which have the diffusion coefficients of 10-7 cm2/s,15 10-13-10-14 cm2/s,16 and 10-9-10-11 cm2/s,17 respectively, β-LixV2O5 is a promising cathode material for power-type rechargeable lithium batteries. Conclusion β-LixV2O5 (x ≈ 0.32) was synthesized by solid-state reaction. Its specific capacity is as high as 320 mA h/g when cycled between 1.5V and 4.5V. Its structure is stable within this potential range though it experiences a series of phase transition during cycling. First-principles calculations indicate that its diffusion barrier is only 0.45 eV, predicting that this material
β-LixV2O5 as Cathode Material has a very good rate performance and can be a promising candidate of cathode material for power-type as well as energystorage type rechargeable lithium batteries. Acknowledgment. This work was financially supported by the National 973 Program of China (No. 2002CB211800) and the Beijing Key Laboratory for Nano-Photonics and NanoStructures. References and Notes (1) Murphy, D. W.; Christian, P. A.; Disalvo, F. J.; Waszczak, J. V. Inorg. Chem. 1979, 18, 2800. (2) Dickens, P. G.; French, S. J.; Hight, A. T.; Pye, M. F. Mater. Res. Bull. 1979, 14, 1295. (3) Enjalbert, R.; Galy, J. Acta Crystallogr., Sect. C 1986, 42, 1467. (4) Galy, J. J. Solid State Chem. 1992, 100, 229. (5) Barker, J.; Saidi, M. Y.; Swoyer, J. L. J. Elec. Soc. 2003, 150, A1267.
J. Phys. Chem. C, Vol. 111, No. 28, 2007 10711 (6) Wadsley, A. D. Acta Crystallogr. 1957, 10, 261. (7) Leger, C.; Bach, S.; Soudan, P.; Pereira-Romas, J. P. J. Electrochem. Soc. 2005, 152, A236. (8) Galy, J.; Savariault, J.; Roucau, C. Aust. J. Chem. 1996, 49, 1009. (9) Wang, Z. X.; Sun, Y. C.; Chen, L. Q.; Huang, X. J. J. Electrochem. Soc. 2004, 151, A914. (10) Kresse, G.; Hafner, J. Phys. ReV. B 1993, 47, 558 and 1994, 49, 14251. (11) Kresse, G.; Furthmo¨ller, J. Comput. Mater. Sci. 1996, 6, 15; Phys. ReV. B 1996, 54, 11169. (12) Kutner, R. Phys. Lett. A 1981, 81, 239. (13) Vineyard, G. H. J. Phys. Chem. Solids. 1957, 3, 121. (14) Morgan, D.; Van der Ven, A.; Ceder, G. Electrochem. Solid-State Lett. 2004, 7 (2), A30. (15) Van der Ven, A.; Ceder, G. J. Power Sources. 2001, 97-98, 529. (16) Prosini, P. P.; Lisi, M.; Zane, D.; Pasquali, M. Solid State Ionics. 2002, 148, 45. (17) Cao, F.; Prakash, J. Electrochim. Act. 2002, 47, 1607.
