Effect of AlPO4 Nanowire Coating on the Electrochemical Properties of

The AlPO4 compound is a good coating material for many cathode materials, such as ... The discharge−charge cycle tests were run at a current density...
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J. Phys. Chem. C 2008, 112, 18249–18254

18249

Effect of AlPO4 Nanowire Coating on the Electrochemical Properties of LiV3O8 Cathode Material Lifang Jiao,* Li Liu, Junli Sun, Lin Yang, Yanhui Zhang, Huatang Yuan,* Yongmei Wang, and Xingdi Zhou Institute of New Energy Material Chemistry, Engineering Research Center of Energy Storage & ConVersion (Ministry of Education) and Key Laboratory of Energy-Material Chemistry (Tianjin), Nankai UniVersity, Tianjin 300071, P.R. China ReceiVed: June 13, 2008; ReVised Manuscript ReceiVed: September 11, 2008

In this study, AlPO4 nanowires were coated on the surfaces of LiV3O8 powders successfully. The influence of the coating on the electrochemical behavior of LiV3O8 was discussed. The surface morphology was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). AlPO4coated LiV3O8 cathode materials exhibited distinct surface morphology. AlPO4 nanowires were clearly observed on the surfaces of LiV3O8. The structural changes of the cathode materials before and after coating were revealed by X-ray diffraction spectroscopy (XRD). The 1 wt % AlPO4 coating has been found to reduce the capacity fade of LiV3O8 significantly. Cyclic voltammetry (CV) shows that the characteristic phase transitions during cycling exhibited by the uncoated material are suppressed by the 1 wt % AlPO4 coating. This behavior implies that AlPO4 inhibits structural change of LiV3O8 during cycling. In addition, the AlPO4 coating on LiV3O8 significantly suppresses the increase of charge-transfer impedance (Rct) in cycling. Comparative data for the coated and uncoated materials are presented and discussed. 1. Introduction The lithium vanadate oxide Li1+xV3O8 is well-known as a promising cathode material for rechargeable lithium batteries because of its high capacity, acceptable cyclability, and high rate capability.1,2 Li1+xV3O8 materials have a layered structure where the adjacent layers are held together by Li+ ions in octahedral coordination. In spite of their good lithium insertion behavior, the two-phase transition occurring upon lithiation in the range Li3-Li4V3O8 corresponds to a slight rearrangement of the oxygen packing and induces some capacity loss upon cycling due to a sudden change in structural parameters. Doping and coating are two important methods to improve the electrochemical performances of cathode materials. Modification of the crystal structure by doping is an effective method to improve the electrochemical performances of Li1+ xV3O8. Substitution of various metal ions (Mn, Mo, W, etc.) for V has been tried and proved effectively.3-5 However, the studies about surface modification on the surface of Li1+xV3O8 are few. The AlPO4 compound is a good coating material for many cathode materials, such as LiCoO2,6-8 LiNi0.8Co0.1Mn0.1O2,9 and LiMn2O4.10 But as far the authors know, the AlPO4 nanowire coating has not been reported before. AlPO4 nanowire coating has been utilized on the surfaces of LiV3O8 in this work. The effect of the AlPO4 nanowire coating on the electrochemical properties of LiV3O8 was investigated in detail. 2. Experimental 2.1. Preparation of Cathode Materials. Pristine LiV3O8 powders were synthesized by a peroxide sol-gel method. Ten percent (v/v) peroxide solution was slowly added to a reaction * Corresponding author. Tel.: +86-22-23498089. Fax: +86-22-23502604. E-mail addresses: [email protected] (L.F. Jiao); [email protected] (H.T. Yuan).

Figure 1. XRD patterns of LiV3O8 and AlPO4-coated LiV3O8.

vessel with a certain amount of V2O5, and the obtained solution was vigorously stirred for 2 h at room temperature to get a clear orange solution. Stoichiometric amounts of LiOH · H2O were added to the above orange solution with vigorous stirring. The resulting solution was mixed with a magnetic stirrer at 80-90 °C and then kept stirring to evaporate water for about 12 h. Red brown viscous gels or colloidal solutions were formed. The gel was dried in a vacuum oven at 120 °C for 24 h. Then the dry gel was put into a Mulff oven for calcination at 300 °C for 16 h in air. Lastly, the products were cooled and ground to a fine polycrystalline powder. To prepare AlPO4-coated LiV3O8 powders, Al (NO3)3 · 9H2O and (NH4)2HPO4 were slowly dissolved in distilled water until a white AlPO4 nanoparticle suspension was observed. The active material LiV3O8 was slowly poured into the suspension. The

10.1021/jp805200d CCC: $40.75  2008 American Chemical Society Published on Web 10/25/2008

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Jiao et al.

Figure 2. SEM images of LiV3O8 and AlPO4-coated LiV3O8.

concentration of the AlPO4-nanoparticle solution to LiV3O8 was varied to control the amount of AlPO4 nanoparticle coating. The suspension containing the active material was constantly stirred for 5 h, accompanied by a slow evaporation of solvent, and dried at 120 °C. Then powders were calcined for 3 h at 300 °C in air. The coated samples with ratio AlPO4/LiV3O8 ) 0.01, 0.02, and 0.03 in weight were synthesized. 2.2. Structure Characterization and Electrochemical Measurements. The synthesized samples were characterized by X-ray diffraction. X-ray powder diffraction data were obtained using a Rigaku D/MAX-2500 powder diffractometer with a graphite monochromatic and Cu KR radiation (λ ) 0.15418 nm) in the 2θ range of 3-80°. The morphology was investigated with a scanning election microscope (SEM, FEG SEM Sirion scanning election micro-

scope) and a transmission election microscope (TEM, FEI TECNAI-20). The cathodes for lithium cells were fabricated by mixing the active material, carbon black, and polytetrafluoroethylene (PTFE) binder in a weight ratio of 85:10:5. The testing cells were assembled with the cathodes thus fabricated, a metallic lithium anode, a Celgard 2300 film separator, and 1 M LiPF6 in 1:1 ethylene carbonate (EC)/dimethyl carbonate (DMC) electrolyte. The assembly of the testing cells was carried out in an argon-filled glovebox (homemade), where water and oxygen concentration was kept at less than 5 ppm. The discharge-charge cycle tests were run at a current density of 0.5 C or 1 C (300 mA/g was assumed to be 1 C rate) between 4.0 and 1.8 V. All the tests were performed at room temperature.

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Figure 3. TEM/EDX images of 1 wt % AlPO4-coated LiV3O8.

Figure 4. Initial discharge curves of LiV3O8 and AlPO4-coated LiV3O8 at a current density of 0.5 C.

Cyclic voltammetry (CV) tests were performed on a CHI660B electrochemical workstation at a scan rate of 0.1 mV/s on the potential interval 1.8-4.0 V (vs Li/Li+). EIS experiments were carried out using a frequency response analyzer (Solartron 1250) and an electrochemical interface (Solartron 1280) controlled by a computer. The ac perturbation signal was ( 5 mV, and the frequency range was from 10 mHz to 105 Hz. EIS experiments were carried out at the same discharge terminated voltage of 1.8 V for various cycles. The charge-discharge rate is 0.5 C. The impedance spectra were analyzed by using Z-View software from Scribner Associates. 3. Results and Discussion 3.1. Phase Analysis by XRD. XRD patterns of synthesized compounds are show in Figure 1. The bare LiV3O8 shows good

Figure 5. Cycling stability curves of LiV3O8 and AlPO4-coated LiV3O8 with a 0.5 C current in the range of 1.8-4.0 V.

crystal structure. Most of the diffractive peaks are well attributed (JCPDS: 72-1193). This compound has monoclinic structures with the space group P21/m. AlPO4-coated materials also can be attributed to LiV3O8 structure, and the AlPO4 phase was not observed in these patterns. This is due to a small amount of AlPO4. It is shown that weak X-ray diffraction peaks are observed for AlPO4-coated materials. The AlPO4-coated materials exhibit microcrystal structures. It is known that LiV3O8 as a cathode material will benefit from lower crystallization.11 Better performances of the coated samples can be expected. 3.2. Surface Morphology Studies. Figure 2 shows the morphology of LiV3O8 and AlPO4-coated LiV3O8. The range of bare LiV3O8 particles size is 1-4 µm. For the AlPO4-coated LiV3O8 powders, AlPO4 nanowires are clearly observed on the surfaces of LiV3O8 particles. The morphologies of 1.0 wt % AlPO4-coated LiV3O8 and 2.0 wt % AlPO4-coated LiV3O8 are

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Figure 6. Cycling stability curves of AlPO4-coated LiV3O8 with a 1 C current in the range of 1.8-4.0 V.

much different to bare LiV3O8. LiV3O8 particles are coated by AlPO4 nanowires uniformly at 1.0 wt % AlPO4 coating. When the coating amount increases to 2.0 wt %, many AlPO4 nanowires can be observed on the surface of LiV3O8 particles, but it also can be found that some AlPO4 compound is formed as little particles. In the case of the 3.0 wt % AlPO4 coating, LiV3O8 particles are coated mainly by AlPO4 particles, and only a few AlPO4 nanowires can be observed. Its morphology is similar to bare LiV3O8. The reason is that a high concentration of the AlPO4-nanoparticle solution will not be propitious to formation of AlPO4 nanowires. It has been reported by many researchers that surface morphology is an important factor for electrochemical performances of LiV3O8. The TEM micrographs of 1.0 wt % AlPO4-coated LiV3O8 are shown in Figure 3. AlPO4 coated on its surface presents a

Figure 7. Cyclic voltammograms of LiV3O8 and AlPO4-coated LiV3O8.

Jiao et al. good nanowire (about Φ 40 nm and 0.5∼2 µm long) morphology. One end of AlPO4 nanowires is outward and visible, while another end of it is invisible and integrates into the LiV3O8 matrix. It can be concluded that AlPO4 can partially react with LiV3O8 to form a solid solution on the surface of the LiV3O8 matrix. The composition in Figure 3c is confirmed by the EDX spectrum in Figure 3d. The Cu and C elements are attributed to Cu foil. The atomic ratio of Al/P is 1.1:1, approximate to the theoretical value. 3.3. Electrochemical Characterization. 3.3.1. Charge/ Discharge Studies. The initial discharge curves are illustrated in Figure 4. The shapes of these curves are similar. The initial discharge specific capacity for LiV3O8 is 329 mAh/g. At 1.0 wt % AlPO4 coating, the initial discharge capacity is sharply decreased to 240 mAh/g. Then an increase of discharge capacity can be observed with the increase of coating amount. At 2.0 wt % and 3.0 wt % coating, the discharge capacities are 271 and 283 mAh/g, respectively. This can be related to the surface morphology after coating. LiV3O8 was coated by AlPO4 nanowires most sufficiently with 1.0 wt % AlPO4 coating. So AlPO4 coating affects the initial discharge capacity significantly, but with 3.0 wt % AlPO4 coating, the morphology is similar to uncoated LiV3O8. The cycle performances of the LiV3O8 and AlPO4-coated LiV3O8 materials cycled between 1.8 and 4.0 V at 0.5 C are shown in Figure 5. The discharge capacities at the first and 30th cycles of bare LiV3O8 are 329 and 160 mAh/g, respectively, and the discharge capacity decreased acutely. The cycling performance of AlPO4-coated materials improved remarkably. At 2.0 and 3.0 wt % coating, the discharge capacities remain 207 and 218 mAh/g after 30 cycles, respectively. At 1.0 wt % coating, the sample shows the excellent cycling performance, and the first discharge capacity was 240 mAh/g and increased to 269 mAh/g at the ninth cycle. It remained 256 mAh/g after

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Figure 8. Nyquist plots of LiV3O8 and AlPO4-coated LiV3O8 at different cycles and the equivalent circuit model.

TABLE 1: Rct Value for LiV3O8 and AlPO4-Coated LiV3O8 after Different Cycles sample

Rct (Ω,first)

Rct (Ω, fifth)

Rct (Ω, tenth)

LiV3O8 1 wt % coated 2 wt % coated 3 wt % coated

160.9 113.3 153.3 159.5

210.2 138.8 186.5 192.1

337.1 160.3 214.9 300.3

30 cycles. Though AlPO4 coating affects the initial discharge capacity, the cycle stability distinctly improved which is the most important property for secondary batteries. The cycle performances of AlPO4-coated LiV3O8 materials cycled between 1.8 and 4.0 V at larger current density (1 C) are shown in Figure 6. The discharge capacities of AlPO4-coated LiV3O8 decrease with increasing current density. The initial discharge capacity of uncoated LiV3O8 is only 245 mAh/g and much lower than that at 0.5 C rates. The coated cathodes show better rate capability. 3.0 wt % AlPO4-coated LiV3O8 has the highest initial discharge capacity, but the capacity retention for this material is very poor. The initial discharge capacity is 269 mAh/g and sharply decreases to 116 mAh/g at the 60th cycle, and it is noticeable that its cycle performance is similar to bare LiV3O8 after 50 cycles. The 2.0 wt % AlPO4-coated LiV3O8 has a slightly lower initial discharge capacity (about 264 mAh/ g), and slower capacity fade is observed up to 60 cycles. It is also clear that 1.0 wt % AlPO4 coating reduces the initial discharge capacity of the materials greatly. However, the 1.0 wt % AlPO4-coated LiV3O8 shows excellent cycling performance, and the initial discharge capacity is 202.9 mAh/g. After 14 cycles, the discharge capacity increases to 232 mAh/g, and

it still remains 224 mAh/g after 60 cycles. This also should be attributed to 1.0 wt % AlPO4 coating, which is the most effective coating amount. It is found from Figure 5 and Figure 6 that 1.0 wt % AlPO4-coated LiV3O8 shows an obvious activation process during the first several cycles. 3.3.2. Cyclic Voltammetry. Cyclic voltammograms of the LiV3O8 and AlPO4-coated LiV3O8, starting with an oxidation, followed by a subsequent cycle between 4.0 and 1.8 V, are presented in Figure 7. The open circuited potential (OCP) of the starting material is approximately 3.65 V. The first voltammogram curves are rather different from the rest; some structural modifications have probably taken place during the first charge and discharge operations. The features of voltammogram curves of 1.0 wt % AlPO4- and 2.0 wt % AlPO4-coated LiV3O8 are much different from bare LiV3O8. There are three anodic peaks at 2.49, 2.81, and 2.90 V for LiV3O8; however, when 1.0 wt % AlPO4 coated it, the peak at 2.49 V disappeared, and there are two anodic peaks of 2.76 and 2.95 V at the second cycle. Then, these two peaks merge into one peak at 2.90 V after two cycles. For 2.0 wt % AlPO4-coated LiV3O8, it also has two anodic peaks at 2.88 and 3.0 V at the second cycle. However, with the increase of the cycle, the anodic peaks convert to three peaks of 2.46, 2.79, and 2.95 V at the sixth cycle. The voltammograms curves of 3.0 wt % AlPO4-coated LiV3O8 are similar to bare LiV3O8, and three anodic peaks at 2.42, 2.76, and 2.91 V are presented. Three main cathodic peaks at 2.76, 2.47, and 2.15 V appear in the voltammogram curves of LiV3O8, which indicate that multiple discharge plateaus exhibit the typical steps of the crystal LiV3O8 phase. Less

18254 J. Phys. Chem. C, Vol. 112, No. 46, 2008 cathodic peaks are presented with 1.0 wt % and 2.0 wt % AlPO4coated LiV3O8. However, for 3.0 wt % AlPO4-coated LiV3O8, three cathodic peaks are presented too. It is found that the 1.0 wt % AlPO4-coated LiV3O8 shows the best structure stability and reversibility during cycling, which is due to the fact that AlPO4 nanowire coating can suppress the phase transition of LiV3O8 during lithium deintercalation and intercalation processes. It is due to the uniform AlPO4 coating on LiV3O8 when the coating amount is 1.0 wt %. But with 3.0 wt % AlPO4 coating, LiV3O8 is only coated by a few AlPO4 nanowires, and its morphology is similar to bare LiV3O8. It is obvious that the current densities of bare LiV3O8 decrease in cycling while the current densities of 1.0 wt % AlPO4-coated LiV3O8 increase. The larger current densities in the cyclic voltammograms agree with the larger capacities for the respective materials. This phenomenon corresponds to Figures 5 and 6. 3.3.3. Electrochemical Impedance Spectroscopic Studies. EIS experiments were carried out at the same terminated voltage of 1.8 V at the first, fifth, and tenth cycles. The ac impedance of coin cells with LiV3O8 and AlPO4-coated LiV3O8 cathodes are illustrated in Figure 8. The impedance spectra of LiV3O8 and AlPO4-coated LiV3O8 show a high frequency semicircle and a low frequency tail, which indicates the double-layer response at the electrode/sample interface and the diffusion of lithium ions in the solid matrix. The impedance plots are fitted using the equivalent circuit model (Figure 8e), and the fitted Rct values are listed in Table 1. The equivalent circuit model includes electrolyte resistance Rct, a constant phase element (CPE) associated with the interfacial resistance, charge transfer resistance Rct, and the Warburg impedance (Zw) that is related to the diffusion of lithium ions in the solid oxide matrix. According to Chen et al.’s12 studies on EIS of lithium-ion cells, the cell impedance is mainly attributed to cathode impedance, especially charge-transfer resistance. It is clear from the Table 1 that the Rct value of the bare cathode is 160.9 Ω at the first cycle and then enlarges drastically from 210.2 Ω at the fifth cycle to 337.1 Ω at the tenth cycle, and it is found that the Rct value for 3.0 wt % AlPO4-coated LiV3O8 increased quickly too. By contrast, Rct values of 1.0 wt % AlPO4-coated and 2.0 wt % AlPO4-coated cathodes increased slowly. Especially for 1.0 wt % AlPO4-coated LiV3O8, the Rct value only increased from

Jiao et al. 113.3 Ω at the first cycle to 160.3 Ω at the tenth cycle. It shows that a suitable amount of AlPO4 coating has an efficacious effect on restraining the increase of charge transfer impedance of cathode during cycling. 4. Conclusions With 1.0 wt % AlPO4 coating, LiV3O8 was coated by AlPO4 nanowires the most uniformly. Though the high initial discharge capacity for uncoated LiV3O8 could not be reproduced, the improvement in the cycle performance due to effective coating of 1.0 wt % AlPO4 especially at relatively high current densities (1 C) has been demonstrated. The slow degradation of the electrochemical property of AlPO4-coated LiV3O8 during cycling can be explained by the greater reversibility and smaller chargetransfer impedance than the uncoated sample. These improved properties are attributed to the change of structure and morphology after coating. Acknowledgment. This work was supported by the National Science Fund of China (Project 20673062, 20801059), the Natural Science Fund of Tianjin (06YFJMJC04900), and 973 Program (2002CB 211800). References and Notes (1) Pistoia, G.; Di Vona, M. L.; Tagliatesta, P. Solid State Ionics 1987, 24, 103. (2) Pistoia, G.; Pasquali, M.; Tocci, M.; Moshtev, R. V.; Manev, V. J. Electrochem. Soc. 1985, 132, 281. (3) Kawakita, J.; Katagiri, H.; Takashi, M.; Kishi, T. J. Power Sources 1997, 68, 680. (4) Pouchko, S. V.; Ivanov-Schitz, A. K.; Ooms, F. G. B.; Schoonman, J. Solid State Ionics 2001, 144, 151. (5) Pouchko, S. V.; Ivanov-Schitz, A. K.; Kulova, T. L.; Skundin, A. M.; Turevskaya, E. P. Solid State Ionics 2002, 151, 129. (6) Cho, J.; Kim, H.; Park, B. J. Electrochem. Soc. 2004, 151, A1707. (7) Cho, J.; Kim, B.; Lee, J. G.; Kim, Y. W.; Park, B. J. Electrochem. Soc. 2005, 152, A32. (8) Kim, B.; Lee, J. G.; Choi, M.; Cho, J.; Park, B. J. Power Sources 2004, 126, 190. (9) Cho, J.; Kim, T. J.; Kim, J.; Noh, M.; Park, B. J. Electrochem. Soc. 2004, 151, A1899. (10) Liu, D.; He, Z.; Liu, X. Mater. Lett. 2007, 61, 4703. (11) Jouanneau, S,; Salle, A. L. G. L.; Verbaere, A.; Deschamps, M.; Lascaud, S,; Guyomard, D. J. Mater. Chem. 2003, 13, 921. (12) Chen, C. H.; Liu, J.; Amine, K. J. Power Sources 2001, 96, 321.

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