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Manganese vanadium oxides as cathodes for lithium batteries
Solid State Ionics 176 (2005) 307 – 312 www.elsevier.com/locate/ssi
Manganese vanadium oxides as cathodes for lithium batteries Heai-Ku Park* Department of Chemical System Engineering, Keimyung University, 1000 Shindang-Dong Dalseo-Gu, Daegu 704-701, South Korea Received 6 February 2004; received in revised form 25 June 2004; accepted 26 July 2004
Abstract Mnx V2O5 (x=0.02, 0.04, 0.09, and 0.19) was prepared by controlling the manganese content in V2O5 using an ion exchange resin via the sol-gel process and examined with an emphasis on characterizing its electrochemical properties, such as the current–voltage correlation, cycling performance, rate capability, and impedance characteristics. The electrochemical performance of the Mn-doped-V2O5 was better than the undoped vanadium pentoxides. The specific capacity and average voltage of the V2O5 cathodes increased gradually as the manganese doping concentration increases. D 2004 Elsevier B.V. All rights reserved. Keywords: Vanadium oxides; Manganese vanadium oxides; Lithium secondary batteries; Lithium battery cathodes
1. Introduction Since Murphy and Christian [1] first introduced vanadium oxides as a promising cathode material in lithium secondary batteries, considerable efforts have been made to understand the underlying mechanism and discover new intercalation materials. Currently, LiCoO2, LiNiMnO2, and LiMn2O4 are the most widely used cathodes in lithium ion secondary batteries due to their intrinsic high open circuit voltage and reversible cell performance. Although amorphous V2O5 has a wide reversible lithium insertion range and a high specific capacity (1600 C/g) [2], it suffers from a relatively low average cell voltage compared with other inorganic intercalation materials. Several studies on vanadium oxides [3,4] have reported that the electrochemical properties of a V2O5 xerogel can be improved progressively by doping transition metal ions (e.g. Cu, Ag) into its matrix. In order to obtain a better electrochemical performance of V2O5 as a cathode, the coordination around vanadium is important for the insertion and release of a cation. A vanadium oxygen octahedral structure with a +5 oxidation state is preferred for secondary
batteries, and the coordination number is largely dependent on the pH during synthesis. If a metal, such as Zn, Fe, Ag, or Mn, are doped between the double sheets of V2O5 using hydrothermal synthesis, the vanadium adopts a distorted octahedral geometry, and the metal-doped vanadium oxides become more stable and reversible for redox reactions [5]. It has also been reported that the 0.33 H3O+ ions per mole of vanadium pentoxide xerogel prepared using an ion exchange resin will be exchanged with other cations, which improves the performance of heat-treated vanadium pentoxide bronze [6,7]. Therefore, it is believed that doping inside the vanadium oxide lattice by redox active ions at the appropriate pH would be an attractive method for improving its electrochemical properties as a secondary battery cathode. The aim of this study was to synthesize metal-doped vanadium oxide, in particular, Mnx V2O5, via the bsol-gel processQ using an ion exchange method and to characterize the effects of doping on the electrochemical properties of this material as a cathode for secondary lithium batteries.
2. Experimental * Tel.: +82 53 5805456; fax: +82 53 5805165. E-mail address: [email protected]. 0167-2738/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2004.07.014
The Mnx V2O5 sols were prepared by the protonation of sodium metavanadate (Aldrich) using an ion exchange resin
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(Dowex 50WX2, 50–100 mesh, Fluka) and then by consecutive manganese ion exchange with peroxovanadates species using the Dowex 50WX2 resin, which had been regenerated with Mn2+ ions using 3 M MnCl2 (Aldrich). The gels were aged for at least a month at 30 8C in order to insure homogenization before being dip-coated onto a SUS expanded metal for the electrochemical tests. The Mn/V ratios were controlled deliberately by varying the volume of the ion exchange resin. The chemicals used in this study were all reagent grade and used without further purification. The electrochemical intercalation and deintercalation of lithium was achieved at room temperature using a conventional 3-electrode configuration in 1 M LiClO4-propylene carbonate (Merck) electrolyte. Li metal (Foote) was used as both the reference and auxiliary electrode, and dip-coated Mnx V2O5 was used as the working electrode. Gas tight electrochemical cells were assembled and tested in an Arfilled glove box (Vac). A Solartron 1287 electrochemical interface was used for the electrochemical control along with a Solartron 1260 FRA. The cycling test was carried out using a Wonatech cycle tester (XENO3000). The chemical formula and water content of the vacuumdried Mnx V2O5 were determined using ICP (Jovin ybon, JY385) and thermogravimetric analysis (Shimadzu TGA50). The surface morphology of the dip-coated samples was observed using scanning electron microscopy (SEM, JEOL JSM5410). The X-ray diffraction patterns (XRD) of the dip-coated xerogels were obtained using a Philips PW3710 X-ray diffractometer with Cu Ka radiation. Fig. 2. XRD patterns of the Mnx V2O5 xerogels; (a) at 25 8C, (b) heattreated at 400 8C. * sign represents the diffraction peaks related to MnV2O6.
3. Results and discussion The manganese-doped vanadium oxides were successfully synthesized via a sol-gel process using an ion-exchange resin. The Mn/V ratios were varied deliberately in order to obtain the desired compositions of the compounds at pH 2.0. At a low doping level, the manganese-doped vanadium oxides became a gel through the sol. However, one of the
Fig. 1. TGA curves of the V2O5 and Mnx V2O5 xerogels.
samples precipitated as the Mn concentration (Mn0.19V2O5) in the sols was increased. Chemical analysis of the Mn-doped vanadium oxides showed that the vanadium oxides were doped with approximately 0.02, 0.04, 0.09, and 0.19 mol of the Mn per mole of V2O5 resulting in the formulae Mn0.02V2O5, Mn0.04V2O5, Mn0.09V2O5, and Mn0.19V2O5, respectively. The water content of the vacuum-dried Mnx V2O5 at room temperature were determined using thermogravimetric analysis. The TGA curves are shown in Fig. 1. Thermal analysis revealed that the interlamellar water (loosely and tightly bound water) could be removed in a two-step process upon heating without any significant structural changes. The amount of interlayer water was measured to be 1.4–1.6 mol regardless of the Mn doping composition in the Mnx V2O5, which is mainly dependent on the drying and atmospheric conditions. The onset temperature of water removal in Mn0.19V2O5 was different from that of the others, suggesting that the binding energy of the interlayer water was slightly different. Fig. 2 shows the XRD patterns of Mnx V2O5, indicating that the manganese-doped vanadium oxides are amorphous, and a set of 00L (L=1, 3, 4, 5) diffraction peaks can be seen at low Mn doping concentrations (see Fig. 2a). This confirms
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Fig. 3. SEM images of (a) Mn0.02V2O5 and (b) Mn0.19V2O5 (20 kV, 2000).
that it retains an ordered layered structure at room temperature. The interlayer distance of the Mnx V2O5 (x=0.02, 0.04, 0.09) xerogels is approximately 11.4 2, which is similar to that of the V2O5 xerogel [8,13]. However, the interlamellar spacing of the Mn0.19V2O5 is 13.2 2, and the sol was precipitated resulting in a powdered product instead of xerogel when dried. The XRD pattern of the Mn0.19V2O5 revealed that ordered stacking of the ribbons is slightly disturbed and is quite similar to that of the hydrothermally prepared potassium manganese-vanadium oxide (K0.02Mn0.18V2O5.19d 0.48H2O, Mn/V ratio: 0.2) [4]. As the manganese doping content is increased, manganese ions bind the V2O5 layers together strongly in the sol and probably squeeze out the interstitial water. This is then followed by precipitation, which was also observed on the hydrothermal preparation of vanadium oxide bronzes as well [9]. Giorgetti et al. [10] reported that the doped Zn has an octahedral coordination in Znx V2O5. The ZnO6 octahedron consists of two apical oxygen atoms from two V2O5 layers and by four coplanar oxygen atoms that belong to four interstitial water molecules. It is believed that manganesedoped vanadium oxides have a similar coordination to oxygen to Znx V2O5. It is likely that the water bound by Mn would be removed at higher temperatures than the ordinary interstitial water. This was evidenced by the TGA curves.
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Heat-treated Mnx V2O5 (x=0.02, 0.04, 0.09) xerogels transform into an orthorhombic crystal that is typical in c-V2O5, as is shown in Fig. 2b. In the case of Mn0.19V2O5, the structure of MnV2O6 appears in addition to the orthorhombic V2O5 when heat-treated N400 8C. Zhang and Whittingham [11] reported the presence of both V2O5 and Mn(VO3)2 in y-Mn0.15V2O5, which was prepared from [N(CH3)4]zMnyV2O5d nH2O (monoclinic unit cell) and heat-treated at 600 8C in oxygen by the hydrothermal synthesis method. This indicates that above a certain doping level, two phases coexist, orthorhombic V2O5 and MnV2O6. Baffier et al. [6,7] reported that the replacement of the H3O+ in vanadium pentoxide xerogels by more than 0.16 mol per V2O5 also causes in a change in the structure. Fig. 3 shows a SEM image of the dip-coated Mnx V2O5. The figure shows features that could be associated with the corrugated sheet-like features of the V2O5 xerogel on the surface. No abrupt change in the morphology as a function of the dopant concentration was observed when the doping level was low. However, if the amount of dopant exceeds a certain value (x=0.19 in Mnx V2O5), powdered vanadium oxides instead of gels are obtained, as shown in Fig. 3b. The linear sweep voltammograms of Mnx V2O5 measured at a sweep rate of 0.1 mV/s are shown in Fig. 4. Lithium insertion occurs with peaks at slightly different potentials depending upon the Mn content. As the Mn concentration is increased, the first cathodic peak gradually shifts to a more positive voltage range, which leads to an increase of the overall average cell potential and an increase in the energy density compared with V2O5. In the case of Mn0.09V2O5, several cathodic peaks appear between the 2.9 and 3.6 V range, which were not observed in the dip-coated V2O5 xerogel (see Fig. 4). It appears that the additional broad peak located approximately at the 3.2 V range is due to Mn doping. The redox peaks intensity of the xerogels around 2.5 V decreased, as the Mn doping in the V2O5 decreases. This suggests that there may not be sites for Li+ ions insertion, and/or there is some variation in the conduction
Fig. 4. Linear sweep voltammograms of the Mnx V2O5 xerogels at a 0.1 mV/s sweep rate.
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path of the dip-coated electrode as more Li+ ions are inserted. However, the redox peaks below 2.5 V begins to appear in the Mn0.09V2O5 and Mn0.19V2O5. In general, the V2O5 xerogel layers are stacked parallel to the substrate when it is dip-coated onto it. In the case of the spin-coated V2O5 xerogel, it is believed that those layers are randomly distributed during the spin-coating process and its stacking is perturbed. The doped Mn in V2O5 leads to an enhancement in the conducting path and/or provides sites for insertion. Therefore, there appears an additional peak below 2.5 V resulting in the capacity enhancement of Mnx V2O5. If it is considered that no additives such as binders and conducting compounds are added to all the working electrode, the development of a cathodic peak around 2.5 V of the Mnx V2O5, which was also developed in the spincoated V2O5 xerogel [2], can be a distinguished result. As Li+ ions are inserted into the layers, electrons are accommodated in the xerogel lattice reducing V5+ to V4+ and finally to V3+. The peak around 3.2 V appears as more Mn is doped. It is likely that there is another site for Li intercalation because of the Mn doping. However, it is not yet clear that the doped Mn is involved in the redox reactions in the course of Li+ intercalation and release. Therefore, further investigation using XPS would be necessary to clarify this. Fig. 5 shows the discharge profile of Mn x V 2 O 5 performed at a 9–15 mA/g discharge current. Plateau-like curves, which are typical in the amorphous materials, appear as a result of the distinguishable quasi-ordered sites where the Li+ ions reside [2]. A sharp decrease in the cell potential below 2.5 V was observed, which was already predicted in the potential–current relationship of linear sweep voltammetry. The cell capacity and average potential were increased as the manganese doping concentration increased. The average cell potential of the Mn0.09V2O5 is approximately 3.12 V, making it attractive as a cathode in 3-V lithium batteries. A plot of the discharge profiles at different current densities of the Mnx V2O5 (x=0.09 and 0.19) is shown in Figs. 6 and 7, respectively. The Mn0.09V2O5 xerogel
Fig. 5. Discharge profiles of the Mnx V2O5 electrode.
Fig. 6. Discharge profiles of the Mn0.09V2O5 xerogel electrode at different discharge rates.
electrode tolerates discharge at these rates reasonably and shows a good rate capability. The capacity (specific charge) of the cell decreased with increasing current rates generally, and the capacity fading of the Mn0.19V2O5 at higher rates was more pronounced. The rate capability of the Mn0.19V2O5, which has a higher specific charge, appears to be poorer than that of Mn0.09V2O5 considering the similar mass loading (about 1.0 mg). In general, a gradual increase in the Mn doping concentration in vanadium oxides enhances the specific capacity approximately 20%. The extent of these changes in the vanadium oxides also affects the utilization of these materials. The utilization of the cathode materials depends not only on the discharge/charge rate, but also on the mass load if the conductivity and the diffusion coefficient of the electrode is not high. Some variation in the dopant concentration will substantially improve the overall electrochemical performance of the electrode material. Further optimization of the doping contents in the material may enable the use of thicker electrodes for practical applications in batteries with no additives.
Fig. 7. Discharge profiles of the Mn0.19V2O5 xerogel electrode at different discharge rates.
H.-K. Park / Solid State Ionics 176 (2005) 307–312
Fig. 8. Cycling performance of Mnx V2O5.
The Mn-doped vanadium oxides were subjected to repeated insertion and release cycles (50 mA/g current was applied) using a cycling tester, and the results are shown in Fig. 8. There was some capacity loss of all the cells during the consecutive cycles. Ninety-one percent of the original electrode capacity was still available after 30 consecutive cycles in the case of the Mn0.09V2O5 xerogel electrode and at least 120 mA/g was still available. In order to monitor the evolution of the kinetic parameters, impedance analysis was performed to characterize the insertion processes at different Li compositions (seven different potentials). The Cole–Cole plot is shown in Fig. 9. The inset shows an expanded Cole–Cole plot for the two semicircles. Each equilibrium potential of the electrode was achieved potentiostatically, and a small ac perturbation (10 mV) was coupled to each equilibrium potential for this purpose. The equilibrium potential applied corresponds to the voltage, which is located after the peak potential in Fig. 4. Fig. 9 shows at least three regions due to the different
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relaxation times in the process of Li+ ion insertion: (i) interfacial resistance at a very high frequency (first semicircle), (ii) charge transfer at the interface at a high to medium frequency (second semicircle), (iii) diffusion of Li+ ions within the electrode lattice at a low frequency (458 slope). In all cases, a 908 slope caused by charge saturation was not observed at low frequencies, which was attributed to diffusion. The first semicircle does not change regardless of the applied potential. This indicates the formation of interphases between the electrolyte and the electrode, which was also presented by Shembel et al. [12]. The radii of the second semicircles varied dramatically according to the equilibrium potentials. This suggests that the charge transfer resistance changes as a function of the applied potential, which is related to the quantized site energy in the vanadium lattices. The locus of the second semicircle is reduced, beginning from 3.8 to 3.0 V, and increased at 2.5 V again as Li+ is inserted. At the reverse potentials, 3.0, 3.5, and 3.8 V, the locus of the second semicircle was recovered as the cell was charged. This illustrates the high reversibility of the charge/discharge process, which is an indispensable property for cathode materials to be used in a secondary battery. Therefore, the second semicircle results from the charge transfer of Li at the electrode, and the change of the second arcs is likely to be due to the charge transfer resistance of the Mnx V2O5 electrode. Its variation arises from attractive and/ or repulsive interactions between the intercalated lithium species and/or the quantized site energy of the vanadium oxide lattices. The remarkable change in the complex impedance plot appears at high Li concentrations, i.e. at low potential (2.5 V), which may be the cause for the precipitous reduction of the cell voltage in Fig. 5. The complex impedance plots at the potentials, OCV (3.8 V) and charge (3.8 V), show only two semicircles. This suggests that there are other processes that have a relaxation
Fig. 9. Cole–Cole plot for Mn0.09V2O5 at different potentials (frequency ranges from 75 MHz to 0.1 Hz).
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time very close to that of charge transfer at the high frequencies. This shows that Li insertion at these potentials is constrained, as is the case at 2.5 V. A redox reaction of the electrolyte instead of Li+ ion insertion occurs on the surface of the Mn0.09V2O5 xerogel at a high potential.
4. Conclusions Mnx V2O5 (x=0.02, 0.04, 0.09, and 0.19) were prepared successfully via the sol-gel method, and the manganese content in vanadium oxides could be controlled deliberately using an ion exchange resin. The Mn-doped vanadium oxides appear to be amorphous and retain an ordered layered structure at room temperature. However, heattreated Mnx V2O5 shows a different crystalline structure and surface features according to the Mn doping content. The electrochemical performance of the Mn-doped vanadium oxides was enhanced compared with that of the V2O5 xerogel. In general, the cell capacity and average voltage increases as the amount of Mn doping in V2O5 increases. However, in order to achieve a higher cycling performance, specific capacity, and average cell voltage, the manganese content in vanadium oxides needs to be optimized. Impedance analysis showed that two different relaxation processes occur, an interfacial reaction due to the electrolyte and charge transfer of Li+ ions. The charge transfer resistance, which is related to the Li+ insertion, varies as more Li+ ions are inserted into the xerogel
structure, and this corresponds to the quantized site energy in the lattices.
Acknowledgement This research was supported by the KEMCO, Contract No. 2001-E-EL03-P-01.
References [1] D.W. Murphy, P.A. Christian, Science 205 (1979) 651. [2] H.-K. Park, W.H. Smyrl, M.D. Ward, J. Electrochem. Soc. 142 (1995) 1068. [3] F. Coustier, J. Hill, B.B. Owens, S. Passerini, W.H. Smyrl, J. Electrochem. Soc. 146 (1999) 1355. [4] P. Liu, J.-G. Zhang, J.A. Turner, J. Power Sources 92 (2001) 204. [5] M.S. Whittingham, P.Y. Zavalij, Int. J. Inorg. Mater. 3 (2001) 1231. [6] P. Soudan, J.P. Pereira-Ramos, J. Farcy, G. Gregoire, N. Baffier, Solid State Ionics 135 (2000) 291. [7] J. Farcy, S. Maingot, P. Soudan, J.P. Pereira-Ramos, N. Baffier, Solid State Ionics 99 (1997) 61. [8] J. Livage, Mater. Res. Bull. 26 (1991) 1173. [9] T. Chirayil, P.Y. Zavalij, M.S. Whittingham, Chem. Mater. 10 (1998) 2629. [10] M. Giorgetti, S. Passerini, W.H. Smyrl, Chem. Mater. 11 (1999) 2257. [11] F. Zhang, M.S. Whittingham, Electrochem. Commun. 2 (2000) 69. [12] E. Shembel, R. Apostolova, V. Nagirny, D. Aurbach, B. Markovsky, J. Power Sources 80 (1999) 90. [13] T. Yao, Y. Oka, N. Yamamoto, Mater. Res. Bull. 27 (1992) 669.
Manganese vanadium oxides as cathodes for lithium batteries Heai-Ku Park* Department of Chemical System Engineering, Keimyung University, 1000 Shindang-Dong Dalseo-Gu, Daegu 704-701, South Korea Received 6 February 2004; received in revised form 25 June 2004; accepted 26 July 2004
Abstract Mnx V2O5 (x=0.02, 0.04, 0.09, and 0.19) was prepared by controlling the manganese content in V2O5 using an ion exchange resin via the sol-gel process and examined with an emphasis on characterizing its electrochemical properties, such as the current–voltage correlation, cycling performance, rate capability, and impedance characteristics. The electrochemical performance of the Mn-doped-V2O5 was better than the undoped vanadium pentoxides. The specific capacity and average voltage of the V2O5 cathodes increased gradually as the manganese doping concentration increases. D 2004 Elsevier B.V. All rights reserved. Keywords: Vanadium oxides; Manganese vanadium oxides; Lithium secondary batteries; Lithium battery cathodes
1. Introduction Since Murphy and Christian [1] first introduced vanadium oxides as a promising cathode material in lithium secondary batteries, considerable efforts have been made to understand the underlying mechanism and discover new intercalation materials. Currently, LiCoO2, LiNiMnO2, and LiMn2O4 are the most widely used cathodes in lithium ion secondary batteries due to their intrinsic high open circuit voltage and reversible cell performance. Although amorphous V2O5 has a wide reversible lithium insertion range and a high specific capacity (1600 C/g) [2], it suffers from a relatively low average cell voltage compared with other inorganic intercalation materials. Several studies on vanadium oxides [3,4] have reported that the electrochemical properties of a V2O5 xerogel can be improved progressively by doping transition metal ions (e.g. Cu, Ag) into its matrix. In order to obtain a better electrochemical performance of V2O5 as a cathode, the coordination around vanadium is important for the insertion and release of a cation. A vanadium oxygen octahedral structure with a +5 oxidation state is preferred for secondary
batteries, and the coordination number is largely dependent on the pH during synthesis. If a metal, such as Zn, Fe, Ag, or Mn, are doped between the double sheets of V2O5 using hydrothermal synthesis, the vanadium adopts a distorted octahedral geometry, and the metal-doped vanadium oxides become more stable and reversible for redox reactions [5]. It has also been reported that the 0.33 H3O+ ions per mole of vanadium pentoxide xerogel prepared using an ion exchange resin will be exchanged with other cations, which improves the performance of heat-treated vanadium pentoxide bronze [6,7]. Therefore, it is believed that doping inside the vanadium oxide lattice by redox active ions at the appropriate pH would be an attractive method for improving its electrochemical properties as a secondary battery cathode. The aim of this study was to synthesize metal-doped vanadium oxide, in particular, Mnx V2O5, via the bsol-gel processQ using an ion exchange method and to characterize the effects of doping on the electrochemical properties of this material as a cathode for secondary lithium batteries.
2. Experimental * Tel.: +82 53 5805456; fax: +82 53 5805165. E-mail address: [email protected]. 0167-2738/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2004.07.014
The Mnx V2O5 sols were prepared by the protonation of sodium metavanadate (Aldrich) using an ion exchange resin
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(Dowex 50WX2, 50–100 mesh, Fluka) and then by consecutive manganese ion exchange with peroxovanadates species using the Dowex 50WX2 resin, which had been regenerated with Mn2+ ions using 3 M MnCl2 (Aldrich). The gels were aged for at least a month at 30 8C in order to insure homogenization before being dip-coated onto a SUS expanded metal for the electrochemical tests. The Mn/V ratios were controlled deliberately by varying the volume of the ion exchange resin. The chemicals used in this study were all reagent grade and used without further purification. The electrochemical intercalation and deintercalation of lithium was achieved at room temperature using a conventional 3-electrode configuration in 1 M LiClO4-propylene carbonate (Merck) electrolyte. Li metal (Foote) was used as both the reference and auxiliary electrode, and dip-coated Mnx V2O5 was used as the working electrode. Gas tight electrochemical cells were assembled and tested in an Arfilled glove box (Vac). A Solartron 1287 electrochemical interface was used for the electrochemical control along with a Solartron 1260 FRA. The cycling test was carried out using a Wonatech cycle tester (XENO3000). The chemical formula and water content of the vacuumdried Mnx V2O5 were determined using ICP (Jovin ybon, JY385) and thermogravimetric analysis (Shimadzu TGA50). The surface morphology of the dip-coated samples was observed using scanning electron microscopy (SEM, JEOL JSM5410). The X-ray diffraction patterns (XRD) of the dip-coated xerogels were obtained using a Philips PW3710 X-ray diffractometer with Cu Ka radiation. Fig. 2. XRD patterns of the Mnx V2O5 xerogels; (a) at 25 8C, (b) heattreated at 400 8C. * sign represents the diffraction peaks related to MnV2O6.
3. Results and discussion The manganese-doped vanadium oxides were successfully synthesized via a sol-gel process using an ion-exchange resin. The Mn/V ratios were varied deliberately in order to obtain the desired compositions of the compounds at pH 2.0. At a low doping level, the manganese-doped vanadium oxides became a gel through the sol. However, one of the
Fig. 1. TGA curves of the V2O5 and Mnx V2O5 xerogels.
samples precipitated as the Mn concentration (Mn0.19V2O5) in the sols was increased. Chemical analysis of the Mn-doped vanadium oxides showed that the vanadium oxides were doped with approximately 0.02, 0.04, 0.09, and 0.19 mol of the Mn per mole of V2O5 resulting in the formulae Mn0.02V2O5, Mn0.04V2O5, Mn0.09V2O5, and Mn0.19V2O5, respectively. The water content of the vacuum-dried Mnx V2O5 at room temperature were determined using thermogravimetric analysis. The TGA curves are shown in Fig. 1. Thermal analysis revealed that the interlamellar water (loosely and tightly bound water) could be removed in a two-step process upon heating without any significant structural changes. The amount of interlayer water was measured to be 1.4–1.6 mol regardless of the Mn doping composition in the Mnx V2O5, which is mainly dependent on the drying and atmospheric conditions. The onset temperature of water removal in Mn0.19V2O5 was different from that of the others, suggesting that the binding energy of the interlayer water was slightly different. Fig. 2 shows the XRD patterns of Mnx V2O5, indicating that the manganese-doped vanadium oxides are amorphous, and a set of 00L (L=1, 3, 4, 5) diffraction peaks can be seen at low Mn doping concentrations (see Fig. 2a). This confirms
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Fig. 3. SEM images of (a) Mn0.02V2O5 and (b) Mn0.19V2O5 (20 kV, 2000).
that it retains an ordered layered structure at room temperature. The interlayer distance of the Mnx V2O5 (x=0.02, 0.04, 0.09) xerogels is approximately 11.4 2, which is similar to that of the V2O5 xerogel [8,13]. However, the interlamellar spacing of the Mn0.19V2O5 is 13.2 2, and the sol was precipitated resulting in a powdered product instead of xerogel when dried. The XRD pattern of the Mn0.19V2O5 revealed that ordered stacking of the ribbons is slightly disturbed and is quite similar to that of the hydrothermally prepared potassium manganese-vanadium oxide (K0.02Mn0.18V2O5.19d 0.48H2O, Mn/V ratio: 0.2) [4]. As the manganese doping content is increased, manganese ions bind the V2O5 layers together strongly in the sol and probably squeeze out the interstitial water. This is then followed by precipitation, which was also observed on the hydrothermal preparation of vanadium oxide bronzes as well [9]. Giorgetti et al. [10] reported that the doped Zn has an octahedral coordination in Znx V2O5. The ZnO6 octahedron consists of two apical oxygen atoms from two V2O5 layers and by four coplanar oxygen atoms that belong to four interstitial water molecules. It is believed that manganesedoped vanadium oxides have a similar coordination to oxygen to Znx V2O5. It is likely that the water bound by Mn would be removed at higher temperatures than the ordinary interstitial water. This was evidenced by the TGA curves.
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Heat-treated Mnx V2O5 (x=0.02, 0.04, 0.09) xerogels transform into an orthorhombic crystal that is typical in c-V2O5, as is shown in Fig. 2b. In the case of Mn0.19V2O5, the structure of MnV2O6 appears in addition to the orthorhombic V2O5 when heat-treated N400 8C. Zhang and Whittingham [11] reported the presence of both V2O5 and Mn(VO3)2 in y-Mn0.15V2O5, which was prepared from [N(CH3)4]zMnyV2O5d nH2O (monoclinic unit cell) and heat-treated at 600 8C in oxygen by the hydrothermal synthesis method. This indicates that above a certain doping level, two phases coexist, orthorhombic V2O5 and MnV2O6. Baffier et al. [6,7] reported that the replacement of the H3O+ in vanadium pentoxide xerogels by more than 0.16 mol per V2O5 also causes in a change in the structure. Fig. 3 shows a SEM image of the dip-coated Mnx V2O5. The figure shows features that could be associated with the corrugated sheet-like features of the V2O5 xerogel on the surface. No abrupt change in the morphology as a function of the dopant concentration was observed when the doping level was low. However, if the amount of dopant exceeds a certain value (x=0.19 in Mnx V2O5), powdered vanadium oxides instead of gels are obtained, as shown in Fig. 3b. The linear sweep voltammograms of Mnx V2O5 measured at a sweep rate of 0.1 mV/s are shown in Fig. 4. Lithium insertion occurs with peaks at slightly different potentials depending upon the Mn content. As the Mn concentration is increased, the first cathodic peak gradually shifts to a more positive voltage range, which leads to an increase of the overall average cell potential and an increase in the energy density compared with V2O5. In the case of Mn0.09V2O5, several cathodic peaks appear between the 2.9 and 3.6 V range, which were not observed in the dip-coated V2O5 xerogel (see Fig. 4). It appears that the additional broad peak located approximately at the 3.2 V range is due to Mn doping. The redox peaks intensity of the xerogels around 2.5 V decreased, as the Mn doping in the V2O5 decreases. This suggests that there may not be sites for Li+ ions insertion, and/or there is some variation in the conduction
Fig. 4. Linear sweep voltammograms of the Mnx V2O5 xerogels at a 0.1 mV/s sweep rate.
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path of the dip-coated electrode as more Li+ ions are inserted. However, the redox peaks below 2.5 V begins to appear in the Mn0.09V2O5 and Mn0.19V2O5. In general, the V2O5 xerogel layers are stacked parallel to the substrate when it is dip-coated onto it. In the case of the spin-coated V2O5 xerogel, it is believed that those layers are randomly distributed during the spin-coating process and its stacking is perturbed. The doped Mn in V2O5 leads to an enhancement in the conducting path and/or provides sites for insertion. Therefore, there appears an additional peak below 2.5 V resulting in the capacity enhancement of Mnx V2O5. If it is considered that no additives such as binders and conducting compounds are added to all the working electrode, the development of a cathodic peak around 2.5 V of the Mnx V2O5, which was also developed in the spincoated V2O5 xerogel [2], can be a distinguished result. As Li+ ions are inserted into the layers, electrons are accommodated in the xerogel lattice reducing V5+ to V4+ and finally to V3+. The peak around 3.2 V appears as more Mn is doped. It is likely that there is another site for Li intercalation because of the Mn doping. However, it is not yet clear that the doped Mn is involved in the redox reactions in the course of Li+ intercalation and release. Therefore, further investigation using XPS would be necessary to clarify this. Fig. 5 shows the discharge profile of Mn x V 2 O 5 performed at a 9–15 mA/g discharge current. Plateau-like curves, which are typical in the amorphous materials, appear as a result of the distinguishable quasi-ordered sites where the Li+ ions reside [2]. A sharp decrease in the cell potential below 2.5 V was observed, which was already predicted in the potential–current relationship of linear sweep voltammetry. The cell capacity and average potential were increased as the manganese doping concentration increased. The average cell potential of the Mn0.09V2O5 is approximately 3.12 V, making it attractive as a cathode in 3-V lithium batteries. A plot of the discharge profiles at different current densities of the Mnx V2O5 (x=0.09 and 0.19) is shown in Figs. 6 and 7, respectively. The Mn0.09V2O5 xerogel
Fig. 5. Discharge profiles of the Mnx V2O5 electrode.
Fig. 6. Discharge profiles of the Mn0.09V2O5 xerogel electrode at different discharge rates.
electrode tolerates discharge at these rates reasonably and shows a good rate capability. The capacity (specific charge) of the cell decreased with increasing current rates generally, and the capacity fading of the Mn0.19V2O5 at higher rates was more pronounced. The rate capability of the Mn0.19V2O5, which has a higher specific charge, appears to be poorer than that of Mn0.09V2O5 considering the similar mass loading (about 1.0 mg). In general, a gradual increase in the Mn doping concentration in vanadium oxides enhances the specific capacity approximately 20%. The extent of these changes in the vanadium oxides also affects the utilization of these materials. The utilization of the cathode materials depends not only on the discharge/charge rate, but also on the mass load if the conductivity and the diffusion coefficient of the electrode is not high. Some variation in the dopant concentration will substantially improve the overall electrochemical performance of the electrode material. Further optimization of the doping contents in the material may enable the use of thicker electrodes for practical applications in batteries with no additives.
Fig. 7. Discharge profiles of the Mn0.19V2O5 xerogel electrode at different discharge rates.
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Fig. 8. Cycling performance of Mnx V2O5.
The Mn-doped vanadium oxides were subjected to repeated insertion and release cycles (50 mA/g current was applied) using a cycling tester, and the results are shown in Fig. 8. There was some capacity loss of all the cells during the consecutive cycles. Ninety-one percent of the original electrode capacity was still available after 30 consecutive cycles in the case of the Mn0.09V2O5 xerogel electrode and at least 120 mA/g was still available. In order to monitor the evolution of the kinetic parameters, impedance analysis was performed to characterize the insertion processes at different Li compositions (seven different potentials). The Cole–Cole plot is shown in Fig. 9. The inset shows an expanded Cole–Cole plot for the two semicircles. Each equilibrium potential of the electrode was achieved potentiostatically, and a small ac perturbation (10 mV) was coupled to each equilibrium potential for this purpose. The equilibrium potential applied corresponds to the voltage, which is located after the peak potential in Fig. 4. Fig. 9 shows at least three regions due to the different
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relaxation times in the process of Li+ ion insertion: (i) interfacial resistance at a very high frequency (first semicircle), (ii) charge transfer at the interface at a high to medium frequency (second semicircle), (iii) diffusion of Li+ ions within the electrode lattice at a low frequency (458 slope). In all cases, a 908 slope caused by charge saturation was not observed at low frequencies, which was attributed to diffusion. The first semicircle does not change regardless of the applied potential. This indicates the formation of interphases between the electrolyte and the electrode, which was also presented by Shembel et al. [12]. The radii of the second semicircles varied dramatically according to the equilibrium potentials. This suggests that the charge transfer resistance changes as a function of the applied potential, which is related to the quantized site energy in the vanadium lattices. The locus of the second semicircle is reduced, beginning from 3.8 to 3.0 V, and increased at 2.5 V again as Li+ is inserted. At the reverse potentials, 3.0, 3.5, and 3.8 V, the locus of the second semicircle was recovered as the cell was charged. This illustrates the high reversibility of the charge/discharge process, which is an indispensable property for cathode materials to be used in a secondary battery. Therefore, the second semicircle results from the charge transfer of Li at the electrode, and the change of the second arcs is likely to be due to the charge transfer resistance of the Mnx V2O5 electrode. Its variation arises from attractive and/ or repulsive interactions between the intercalated lithium species and/or the quantized site energy of the vanadium oxide lattices. The remarkable change in the complex impedance plot appears at high Li concentrations, i.e. at low potential (2.5 V), which may be the cause for the precipitous reduction of the cell voltage in Fig. 5. The complex impedance plots at the potentials, OCV (3.8 V) and charge (3.8 V), show only two semicircles. This suggests that there are other processes that have a relaxation
Fig. 9. Cole–Cole plot for Mn0.09V2O5 at different potentials (frequency ranges from 75 MHz to 0.1 Hz).
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time very close to that of charge transfer at the high frequencies. This shows that Li insertion at these potentials is constrained, as is the case at 2.5 V. A redox reaction of the electrolyte instead of Li+ ion insertion occurs on the surface of the Mn0.09V2O5 xerogel at a high potential.
4. Conclusions Mnx V2O5 (x=0.02, 0.04, 0.09, and 0.19) were prepared successfully via the sol-gel method, and the manganese content in vanadium oxides could be controlled deliberately using an ion exchange resin. The Mn-doped vanadium oxides appear to be amorphous and retain an ordered layered structure at room temperature. However, heattreated Mnx V2O5 shows a different crystalline structure and surface features according to the Mn doping content. The electrochemical performance of the Mn-doped vanadium oxides was enhanced compared with that of the V2O5 xerogel. In general, the cell capacity and average voltage increases as the amount of Mn doping in V2O5 increases. However, in order to achieve a higher cycling performance, specific capacity, and average cell voltage, the manganese content in vanadium oxides needs to be optimized. Impedance analysis showed that two different relaxation processes occur, an interfacial reaction due to the electrolyte and charge transfer of Li+ ions. The charge transfer resistance, which is related to the Li+ insertion, varies as more Li+ ions are inserted into the xerogel
structure, and this corresponds to the quantized site energy in the lattices.
Acknowledgement This research was supported by the KEMCO, Contract No. 2001-E-EL03-P-01.
References [1] D.W. Murphy, P.A. Christian, Science 205 (1979) 651. [2] H.-K. Park, W.H. Smyrl, M.D. Ward, J. Electrochem. Soc. 142 (1995) 1068. [3] F. Coustier, J. Hill, B.B. Owens, S. Passerini, W.H. Smyrl, J. Electrochem. Soc. 146 (1999) 1355. [4] P. Liu, J.-G. Zhang, J.A. Turner, J. Power Sources 92 (2001) 204. [5] M.S. Whittingham, P.Y. Zavalij, Int. J. Inorg. Mater. 3 (2001) 1231. [6] P. Soudan, J.P. Pereira-Ramos, J. Farcy, G. Gregoire, N. Baffier, Solid State Ionics 135 (2000) 291. [7] J. Farcy, S. Maingot, P. Soudan, J.P. Pereira-Ramos, N. Baffier, Solid State Ionics 99 (1997) 61. [8] J. Livage, Mater. Res. Bull. 26 (1991) 1173. [9] T. Chirayil, P.Y. Zavalij, M.S. Whittingham, Chem. Mater. 10 (1998) 2629. [10] M. Giorgetti, S. Passerini, W.H. Smyrl, Chem. Mater. 11 (1999) 2257. [11] F. Zhang, M.S. Whittingham, Electrochem. Commun. 2 (2000) 69. [12] E. Shembel, R. Apostolova, V. Nagirny, D. Aurbach, B. Markovsky, J. Power Sources 80 (1999) 90. [13] T. Yao, Y. Oka, N. Yamamoto, Mater. Res. Bull. 27 (1992) 669.