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Synthesis of nitrided MoO2 and its application as anode materials for lithium-ion batteries
Journal of Alloys and Compounds 536 (2012) 179–183
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Synthesis of nitrided MoO2 and its application as anode materials for lithium-ion batteries Sukeun Yoon ⇑, Kyu-Nam Jung, Chang Soo Jin, Kyung-Hee Shin New and Renewable Energy Research Division, Korea Institute of Energy Research, Daejeon 305-343, Republic of Korea
a r t i c l e
i n f o
Article history: Received 2 April 2012 Received in revised form 24 April 2012 Accepted 27 April 2012 Available online 8 May 2012 Keywords: Lithium-ion batteries Anode Molybdenum nitride Molybdenum oxynitride Nitridation
a b s t r a c t Nitrided MoO2 has been synthesized by hydrothermal processing followed by post-nitridation with NH3 and investigated as alternative anode materials for rechargeable lithium batteries. Characterization data reveal the presence of molybdenum nitride (c-Mo2N and d-MoN) and molybdenum oxynitride (MoOxNy). The nitrided MoO2 exhibits a capacity of >420 mAh/g after 100 cycles and good rate capability. The improved electrochemical performance of the nitrided MoO2 compared to that of molybdenum oxide (MoO2) is attributed to high electrical conductivity provided by nitrogen doping/or substitution in the oxygen octahedral site of MoO2 structure. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Molybdenum dioxide (MoO2) is an attractive material among the transition metal oxides due to its unusual chemistry, such as metallic electrical resistivity (8.8 105 X cm at 300 K in bulk samples), high melting point, and high chemical stability, produced by the multiple valence states [1]. It has been suggested as a promising candidate material in a wide range of field like solar energy conversion, photocatalysis, photochromic devices, gas sensing, and energy storage. Particularly, polycrystalline MoO2 with disordered rutile structure is an intensely appealing as an anode material in lithium ion batteries because it shows theoretical capacity (838 mAh/g) that is higher than that of the currently used graphite (372 mAh/g) anode [2–4]. Unfortunately, however, the reaction of MoO2 with lithium is accompanied by a large volume change, which results in cracking and crumbling of the particles, disconnection of the electrical contact between the particles and current collectors, and consequent capacity fade during cycling. In addition, the electrochemical performance of MoO2 strongly depends on the size and shape of the particles [5]. To alleviate this problem, a wide variety of approaches have been pursued over the years for the synthesis of nanostructured MoO2, including hydro/solvothermal reaction, H2 reduction process, solution-phase reaction, templating methods, and chemical vapor deposition (CVD) methods [3,4,6–10]. Doping in metal oxides can provide an impact on electronic, optical, photoelectrical, and electrochemical performance. It has ⇑ Corresponding author. Tel./fax: +82 42 860 3526. E-mail address: [email protected] (S. Yoon). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.04.116
been generally assumed that the effect of aliovalent ions on the electrical conductivity corresponds to the amount of electronic charge carriers [11]. Accordingly, among the doping materials, nitrogen doping in metal oxides (i.e., nitridation) in the field of photocatalyst has been an intensively research because N 2p states created by substitutional doping of the O-sites are located above the valence band edge, so that these states cause a decrease in the band gap, which shift the photocatalytic activity from UV to visible light [12]. Recently, Cabana et al. reported lithium transition metal nitrides and oxynitride materials, which show interesting performance as attractive candidates to replace currently used anode materials such as graphite because of physicochemical properties [13]. Antifluorite-type Li7MnN4 and Li7.9MnN3.2O1.6 exhibit specific gravimetric capacities in excess of 300 mAh/g with excellent cyclability corresponding to the small volume changes during the cycling and the significantly enhanced lithium mobility through their framework. We present here a hydrothermal reaction and post-annealed nitridation process to obtain a nitrided MoO2. The as-synthesized sample is investigated as negative electrode materials for lithium-ion batteries that exhibits superior electrochemical properties compared to pristine MoO2. 2. Experimental The MoO2 particles were prepared by a hydrothermal reaction without any surfactants according to a method previously reported by Lou et al. [14]. In a typical experiment, 5 mL saturated ammonium heptamolybdate tetrahydrate ((NH4)6Mo7 O244H2O, Alfa Aesar) solution was stored in 10 mL nitric acid (2.2 M HNO3, Aldrich) for 1 month. The resultant solution, together with some white precipitate, was
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S. Yoon et al. / Journal of Alloys and Compounds 536 (2012) 179–183 bonate (EC)/diethyl carbonate (DEC) (1:1 v/v) as the electrolyte. The charge–discharge experiments were performed galvanostatically at a constant current density of 120 mA/g of active material within the voltage range of 0.01–3 V vs. Li+/Li. With an aim to understand the evolution of lithium diffusivity as a function of cell potential, the galvanostatic intermittent titration technique (GITT) was employed. A current pulse of 50 mA/g was applied for 5 min to take the closed-circuit voltage (CCV) and turned off for 10 min to obtain the quasi-open-circuit voltage (QOCV). The sequential current pulse was applied for both discharge and charge period in the range of 0.01–3 V vs. Li+/Li. The electrochemical impedance spectroscopic analysis (EIS) was carried out with a Zahner zennium instrument by applying a 10 mV amplitude signal in the frequency range of 10–0.01 Hz. In the EIS measurements, the nitrided MoO2 with an active material content of 3 mg served as the working electrode and lithium foil served as the counter and reference electrodes. The impedance response was measured after 20 cycles.
3. Results and discussion
Fig. 1. XRD patterns of MoO2 at various nitridation temperatures.
transferred into a Teflon-lined autoclave and heated at 200 °C for 35 h with a heating/cooling rate of 2 °C/min. The product precipitate was filtered and washed with deionized water before drying in a vacuum oven. In order to obtain the nitrided molybdenum oxide particles, the as-synthesized MoO2 powders were carried out the nitridation by NH3 gas in a vertical quartz-tube furnace when the temperature reached at 550, 600, and 650 °C with NH3 flowing rate of 200 mL/min for 12 h. The phase analysis of the synthesized samples was performed with a Bruker Xray diffractometer with Cu Ka radiation and a Thermo MultiLab 2000 X-ray photoelectron spectrometer (XPS). The morphology, microstructure, and composition of the synthesized powders were examined with a Hitachi S4000 scanning electron microscope (SEM) in conjunction with energy dispersive X-ray spectroscopy (EDS). The electrodes for the electrochemical evaluation were prepared by mixing 70 wt.% active material (nitrided MoO2) powders, 15 wt.% carbon black (Super C65) as a conducting agent, and 15 wt.% polyvinylidene fluoride (PVDF) dissolved in N-methylpyrrolidinone (NMP) as a binder to form a slurry, followed by coating on copper foil, pressing, and drying at 120 °C for 2 h under vacuum. The CR2032 coin cells were assembled in an Ar-filled glove box using Celgard polypropylene as a separator, lithium foil as the counter electrode, and 1 M LiPF6 in ethylene car-
The XRD patterns of the as-synthesized samples obtained via nitridation of MoO2 at various temperatures are shown in Fig. 1. In case of nitridation at 550 and 600 °C, all the diffraction peaks could be indexed based on the monoclinic MoO2 (JCPDS No. 781072) phase. In contrast, the XRD pattern shows reflections that correspond to the formation of molybdenum nitrides and intermediate phase (MoOxNy) in case of the nitridation at 650 °C [15]. All the peaks are shifted to lower diffraction angles when they are compared to those of the c-Mo2N (JCPDS No. 25-1366) phase. The d-MoN phase is a metastable state and has a higher lattice constant as a result of the nitrogen incorporation [16]. To evaluate the surface composition of the nitrided MoO2 sample (nitridation at 650 °C), we carried out XPS studies. The XPS survey spectra of nitrided MoO2 are shown in Fig. 2(a). The peaks of Mo 3d, N 1s, and O 1s can be clearly seen [17,18]. The peak given in Fig 2(b) corresponding with Mo 3d exhibits a complex signal that consists of the superimposed doublets of several oxidation states. It is possible to fit the broadened spectra with sets of doublets related to spin orbit split to 3d5/2 and 3d3/2, representing individual Mo2+, Mo3+, Mo4+, Mo5+, and Mo6+, respectively [17]. The
Fig. 2. (a) XPS survey and (b–d) high-resolution XPS spectra of the nitrided MoO2 sample at Mo 3d, N 1s, and O 1s electron.
S. Yoon et al. / Journal of Alloys and Compounds 536 (2012) 179–183
Fig. 3. (a) SEM image and (b) elemental distribution of the nitrided MoO2 sample.
Fig. 4. The first discharge–charge profile of the nitrided MoO2 sample. The inset indicates DCPs at various cycles.
binding energy of Mo 3d5/2, Mo2+, Mo3+, Mo4+, Mo5+, and Mo6+ are 228.1, 229.1, 231.1, 232.3, and 233.6 eV, respectively. The peak at 234.8 eV is assigned to Mo 3d3/2 component of Mo6+, while that at 228.7 eV to the Mo 3d5/2 component of Mo2N. The binding energy of Mo6+ indicates that it maybe formed due to the sample was oxidized at the surface by the exposure to air, leading to the
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Fig. 5. (a) GITT voltage profile and (b) lithium diffusivities of the nitrided MoO2 sample as a function of the cell potential during 2nd and 10th cycles, which were determined by GITT during Li de-intercalation (charging).
presence of some MoO3 [19]. The value of Mo 3d5/2 is higher than that of Mo2+ (228.1 eV) but slightly lower than that of Mo4+ (231.1 eV). Accordingly, we denote the Mo species of Mo2N as Mod+, where 2 < d < 4 [20,21]. Fig. 2(c) shows that the N 1s peaks overlap severely with the Mo 3p3/2 peaks. The N 1s peak is at 397.5 and 401 eV, which is attributed to the formation of molybdenum nitride phases and the O–Mo–N linkages in MoO2 lattice related with molybdenum oxynitride, respectively [20,22]. Additionally, curve fitting at 394.6 eV shows a shoulder corresponding wtih Mo 3p and its oxidation state is between 0 and 4 associated with molybdenum nitride phases [19,21]. While this assignment is consistent with the spectra of O 1s electrons shown in Fig. 2(d), composed of two components centered at 530.2 and 531.8 eV, characteristic of molybdenum oxide and molybdenum oxynitride, respectively. Therefore, these are deduced that nitrogen was doped into MoO2 lattices by substituting the oxygen atoms. Furthermore, in order to know the composition of nitrided MoO2 sample, it was determined by induced-couple plasma (ICP) and elemental analysis; the data indicated the atomic ratio of Mo, O, and N is 1, 1.8 and 0.2, respectively, although it is not containing an optimum level of N doping. The SEM image of the nitrided MoO2 sample (nitridation at 650 °C) provided in Fig. 3(a) exhibits hexagonal prism morphologies with an average particle size of 15 lm (i.e., mean size of the counted 35 particles). A selected individual single particle (Fig. 3(b)) clearly reveals a homogeneous distribution of Mo, O, and N elements in the hexagonal nitrided MoO2 particle.
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Fig. 6. (a) Cycling performances the nitrided MoO2 and (b) comparison of capacity retention at various C rates, illustrating the rate capability of the nitrided MoO2 with those of pristine MoO2.
Fig. 4 gives the first discharge–charge profile of the nitrided MoO2 sample (nitridation at 650 °C). The first discharge and charge capacities are, respectively, 714 and 447 mAh/g, implying an initial Coulombic efficiency of 63%. The initial capacity loss may result from the incomplete conversion reaction and the irreversible lithium loss due to the formation of a solid electrolyte interphase (SEI) layer. The differential capacity plots (DCPs) shown in the inset of Fig. 4 display features characteristic of the voltage plateau during discharge at 1.28 V and then decrease slowly to 0.01 V, associated with the complete reduction reaction of Mo with lithium. While main anodic peak 1.4 V during the charge process is ascribed to the oxidation reaction. In this operating voltage window, 3.4 lithium reacts with the nitride MoO2 sample. Additionally, the voltage plateau during the discharge process, with the increase of cycling, slightly changes from 1.28 to 1.6 V, indicating a different electrochemical reaction. This change of voltage plateau maybe has an effect on the capacity retention after the 25th cycles. The GITT as shown in Fig. 5(a) was employed to investigate the evolution of lithium diffusivity as a function of cell potential. Weppner and Huggins derived a simple expression for lithium diffusivity in the electrode as below [23]:
DLi ¼
(nitridation at 650 °C) was used in implementing this equation. Fig. 5(b) shows the lithium diffusivities as a function of cell potential of the nitrided MoO2 sample during the charge process. The lithium diffusivity of the 2nd cycles shows between 1 109 and 4 1010 cm2/sec. The lithium diffusivity still presents high values of 1 1010 after the 10th cycles due to less the degradation of the nitrided MoO2 sample. This result suggests that the nitrided MoO2 sample can tolerate electrochemical cycling with volume expansion–contraction. Fig. 6(a) presents the cyclability of the nitrided MoO2 sample (nitridation at 650 °C) between 0.01 and 3 V at a constant current of 120 mA/g (C/5 rate). In general, Li storage behavior for MoO2 strongly depends on the size and shape of the particles because morphology is related with electrochemical activity. Interestingly, the bulk nitrided MoO2 sample shows good cyclability, retaining 93% of the capacity after 100 cycles. The Fig. 6(b) compares the rate capabilities at various C rates from 0.2C to 5C rates. The nitrided MoO2 sample exhibits improved rate capability due to the molybdenum nitride phases and molybdenum oxynitride, which has electrical conductivity better than that of oxides. Particularly, it retains a high capacity of 200 and 80 mAh/g, respectively, at 3C and 5C rates with stable cycling, which is approximately two times higher than that value for the pristine MoO2 electrode. In order to gain further insight into electrochemical performances, EIS measurements were carried out at 3 V vs. Li+/Li with the nitrided MoO2 sample (nitridation at 650 °C) after 20th cycles and they were fitted with use of the Zview software. In general, the EIS spectrum can be divided into three frequency regions: low, medium to low, and high frequency regions, which correspond to cell geometric capacitance, charge transfer reaction, and lithium-ion diffusion through the surface layer, respectively. The EIS data were analyzed based on an equivalent circuit given in the Fig. 7 [24]. The EIS plots of pristine of MoO2 and nitrided MoO2 are composed by two semicircles and a line. The small diameter of the first semicircle (at high frequency region) is a measure of the surface layer resistance Rs, which is ascribed to lithium-ion diffusion through the surface layer, and the diameter of the second semicircle (at medium–low frequency region) is a measure of the charge transfer resistance Rct, which is related to the contact between the particles or between the electrode and the electrolyte. The sloping line is related to lithium-ion diffusion in the bulk of the active material. The pristine MoO2 show surface resistances of 387 X and a charge transfer resistance of 640 X. The EIS spectra of the nitrided MoO2, on the other hand, reveals surface resistances and a charge transfer resistances of 288 and 495 X, respectively, due to the molybdenum nitride phases and molybdenum oxynitride materials which have high electrical conductivity.
2 4L2 DEs ps DEs
where L refers to the electrode thickness, s refers to interval time of the current pulse (300 s), and DEs and DEs refer to voltage changes during, respectively, the applied current pulse and the turned off current pulse. The ideal density for the nitrided MoO2 sample
Fig. 7. Electrochemical impedance spectra (EIS) of pristine MoO2 and the nitrided MoO2 after 20th cycles. Equivalent circuit shown in the inset.
S. Yoon et al. / Journal of Alloys and Compounds 536 (2012) 179–183
4. Conclusions The nitrided MoO2 has been synthesized by a hydrothermal processing followed by post-nitridation and characterized by XRD, XPS, and electrochemical analysis. It is composed by molybdenum nitrides and molybdenum oxynitride. The nitrided MoO2 sample exhibits a high capacity with good cyclability and high rate capability. These characteristics are attributed to the good electrical contact and smooth accommodation of the strain, resulting in facile lithium-ion diffusion. The nitrided MoO2 shows considerable promise as a candidate for high performance lithium-ion batteries. Acknowledgements This work was supported by the Energy Efficiency & Resources of the Korea Institute of Energy Technology Evaluation and Planning (No. 20112010100110) Grant funded by the Korea government Ministry of Knowledge Economy. References [1] D.B. Rogers, R.D. Shannon, A.W. Sleight, J.L. Gillson, Inorg. Chem. 8 (1969) 841. [2] J.J. Auborn, Y.I. Barberio, J. Electrochem. Soc. 134 (1987) 638. [3] R.L. Gitzendanner, F.J. DiSalvo, J. Alloy. Compd. 218 (1995) 9.
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[4] L.C. Yang, Q.S. Gao, Y.H. Zhang, Y. Tang, Y.P. Wu, Electrochem. Commun. 10 (2008) 118. [5] J.Y. Luo, J.J. Zhang, Y.Y. Xia, Chem. Mater. 18 (2006) 5618. [6] P.A. Spevack, N.S. McIntyre, J. Phys. Chem. 97 (1993) 11020. [7] B.C. Satishkumar, A. Govindaraj, M. Nath, C.N.R. Rao, J. Mater. Chem. 10 (2000) 2115. [8] J. Zhou, N.S. Xu, S.Z. Deng, J. Chen, J.C. She, Z.L. Wang, Adv. Mater. 15 (2003) 1835. [9] S.V. Pol, V.G. Pol, V.G. Kessler, G.A. Seisenbaeva, M. Sung, S. Asai, A. Gedanken, J. Phys. Chem. B 108 (2004) 6322. [10] Y.G. Liang, Z.H. Yi, S.J. Yang, L.Q. Zhou, J.T. Sun, Y.H. Zhou, Solid State Ionics 177 (2006) 501. [11] Y. Wang, T. Chen, Q. Mu, J. Mater. Chem. 21 (2011) 6006. [12] J. Wang, Z. Wang, H. Li, Y. Cui, Y. Du, J. Alloy. Compd. 494 (2010) 372. [13] J. Cabana, C.M. Ionica-Bousquet, C.P. Grey, M.R. Palcín, Electrochem. Commun. 12 (2010) 315. [14] X.W. Lou, H.C. Zeng, Chem. Mater. 14 (2002) 4781. [15] S.L. Robertson, D. Finello, R.F. Davis, Mater. Sci. Eng., A 248 (1998) 198. [16] J. Barbosa, L. Cunha, L. Rebouta, C. Moura, F. Vaz, S. Carvalho, E. Alves, E.L. Bourhis, P. Goudeau, J.P. Rivière, Thin Solid Films 494 (2006) 201. [17] W. Grünert, A.Y. Stakheev, R. Feldhaus, K. Anders, E.S. Shpiro, K.M. Minachev, J. Phys. Chem. 95 (1991) 1323. [18] M. Zukalova, J. Prochazka, Z. Bastl, J. Duchoslav, L. Rubacek, D. Havlicek, L. Kavan, Chem. Mater. 22 (2010) 4045. [19] J.-G. Choi, L.T. Thompson, Appl. Surf. Sci. 93 (1996) 143. [20] Y. Li, Y. Fan, J. He, B. Xu, H. Yang, J. Miao, Y. Chen, Chem. Eng. J. 99 (2004) 213. [21] B.V. Crist (Ed.), Handbook of Monochromatic XPS Spectra, Wiley, 2000. [22] D. Mckay, J.S.J. Hargreaves, R.F. Howe, Catal. Lett. 112 (2006) 109. [23] W. Weppner, R.A. Huggins, J. Electrochem. Soc. 124 (1977) 1569. [24] J. Liu, A. Manthiram, Chem. Mater. 21 (2009) 1695.
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Synthesis of nitrided MoO2 and its application as anode materials for lithium-ion batteries Sukeun Yoon ⇑, Kyu-Nam Jung, Chang Soo Jin, Kyung-Hee Shin New and Renewable Energy Research Division, Korea Institute of Energy Research, Daejeon 305-343, Republic of Korea
a r t i c l e
i n f o
Article history: Received 2 April 2012 Received in revised form 24 April 2012 Accepted 27 April 2012 Available online 8 May 2012 Keywords: Lithium-ion batteries Anode Molybdenum nitride Molybdenum oxynitride Nitridation
a b s t r a c t Nitrided MoO2 has been synthesized by hydrothermal processing followed by post-nitridation with NH3 and investigated as alternative anode materials for rechargeable lithium batteries. Characterization data reveal the presence of molybdenum nitride (c-Mo2N and d-MoN) and molybdenum oxynitride (MoOxNy). The nitrided MoO2 exhibits a capacity of >420 mAh/g after 100 cycles and good rate capability. The improved electrochemical performance of the nitrided MoO2 compared to that of molybdenum oxide (MoO2) is attributed to high electrical conductivity provided by nitrogen doping/or substitution in the oxygen octahedral site of MoO2 structure. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Molybdenum dioxide (MoO2) is an attractive material among the transition metal oxides due to its unusual chemistry, such as metallic electrical resistivity (8.8 105 X cm at 300 K in bulk samples), high melting point, and high chemical stability, produced by the multiple valence states [1]. It has been suggested as a promising candidate material in a wide range of field like solar energy conversion, photocatalysis, photochromic devices, gas sensing, and energy storage. Particularly, polycrystalline MoO2 with disordered rutile structure is an intensely appealing as an anode material in lithium ion batteries because it shows theoretical capacity (838 mAh/g) that is higher than that of the currently used graphite (372 mAh/g) anode [2–4]. Unfortunately, however, the reaction of MoO2 with lithium is accompanied by a large volume change, which results in cracking and crumbling of the particles, disconnection of the electrical contact between the particles and current collectors, and consequent capacity fade during cycling. In addition, the electrochemical performance of MoO2 strongly depends on the size and shape of the particles [5]. To alleviate this problem, a wide variety of approaches have been pursued over the years for the synthesis of nanostructured MoO2, including hydro/solvothermal reaction, H2 reduction process, solution-phase reaction, templating methods, and chemical vapor deposition (CVD) methods [3,4,6–10]. Doping in metal oxides can provide an impact on electronic, optical, photoelectrical, and electrochemical performance. It has ⇑ Corresponding author. Tel./fax: +82 42 860 3526. E-mail address: [email protected] (S. Yoon). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.04.116
been generally assumed that the effect of aliovalent ions on the electrical conductivity corresponds to the amount of electronic charge carriers [11]. Accordingly, among the doping materials, nitrogen doping in metal oxides (i.e., nitridation) in the field of photocatalyst has been an intensively research because N 2p states created by substitutional doping of the O-sites are located above the valence band edge, so that these states cause a decrease in the band gap, which shift the photocatalytic activity from UV to visible light [12]. Recently, Cabana et al. reported lithium transition metal nitrides and oxynitride materials, which show interesting performance as attractive candidates to replace currently used anode materials such as graphite because of physicochemical properties [13]. Antifluorite-type Li7MnN4 and Li7.9MnN3.2O1.6 exhibit specific gravimetric capacities in excess of 300 mAh/g with excellent cyclability corresponding to the small volume changes during the cycling and the significantly enhanced lithium mobility through their framework. We present here a hydrothermal reaction and post-annealed nitridation process to obtain a nitrided MoO2. The as-synthesized sample is investigated as negative electrode materials for lithium-ion batteries that exhibits superior electrochemical properties compared to pristine MoO2. 2. Experimental The MoO2 particles were prepared by a hydrothermal reaction without any surfactants according to a method previously reported by Lou et al. [14]. In a typical experiment, 5 mL saturated ammonium heptamolybdate tetrahydrate ((NH4)6Mo7 O244H2O, Alfa Aesar) solution was stored in 10 mL nitric acid (2.2 M HNO3, Aldrich) for 1 month. The resultant solution, together with some white precipitate, was
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S. Yoon et al. / Journal of Alloys and Compounds 536 (2012) 179–183 bonate (EC)/diethyl carbonate (DEC) (1:1 v/v) as the electrolyte. The charge–discharge experiments were performed galvanostatically at a constant current density of 120 mA/g of active material within the voltage range of 0.01–3 V vs. Li+/Li. With an aim to understand the evolution of lithium diffusivity as a function of cell potential, the galvanostatic intermittent titration technique (GITT) was employed. A current pulse of 50 mA/g was applied for 5 min to take the closed-circuit voltage (CCV) and turned off for 10 min to obtain the quasi-open-circuit voltage (QOCV). The sequential current pulse was applied for both discharge and charge period in the range of 0.01–3 V vs. Li+/Li. The electrochemical impedance spectroscopic analysis (EIS) was carried out with a Zahner zennium instrument by applying a 10 mV amplitude signal in the frequency range of 10–0.01 Hz. In the EIS measurements, the nitrided MoO2 with an active material content of 3 mg served as the working electrode and lithium foil served as the counter and reference electrodes. The impedance response was measured after 20 cycles.
3. Results and discussion
Fig. 1. XRD patterns of MoO2 at various nitridation temperatures.
transferred into a Teflon-lined autoclave and heated at 200 °C for 35 h with a heating/cooling rate of 2 °C/min. The product precipitate was filtered and washed with deionized water before drying in a vacuum oven. In order to obtain the nitrided molybdenum oxide particles, the as-synthesized MoO2 powders were carried out the nitridation by NH3 gas in a vertical quartz-tube furnace when the temperature reached at 550, 600, and 650 °C with NH3 flowing rate of 200 mL/min for 12 h. The phase analysis of the synthesized samples was performed with a Bruker Xray diffractometer with Cu Ka radiation and a Thermo MultiLab 2000 X-ray photoelectron spectrometer (XPS). The morphology, microstructure, and composition of the synthesized powders were examined with a Hitachi S4000 scanning electron microscope (SEM) in conjunction with energy dispersive X-ray spectroscopy (EDS). The electrodes for the electrochemical evaluation were prepared by mixing 70 wt.% active material (nitrided MoO2) powders, 15 wt.% carbon black (Super C65) as a conducting agent, and 15 wt.% polyvinylidene fluoride (PVDF) dissolved in N-methylpyrrolidinone (NMP) as a binder to form a slurry, followed by coating on copper foil, pressing, and drying at 120 °C for 2 h under vacuum. The CR2032 coin cells were assembled in an Ar-filled glove box using Celgard polypropylene as a separator, lithium foil as the counter electrode, and 1 M LiPF6 in ethylene car-
The XRD patterns of the as-synthesized samples obtained via nitridation of MoO2 at various temperatures are shown in Fig. 1. In case of nitridation at 550 and 600 °C, all the diffraction peaks could be indexed based on the monoclinic MoO2 (JCPDS No. 781072) phase. In contrast, the XRD pattern shows reflections that correspond to the formation of molybdenum nitrides and intermediate phase (MoOxNy) in case of the nitridation at 650 °C [15]. All the peaks are shifted to lower diffraction angles when they are compared to those of the c-Mo2N (JCPDS No. 25-1366) phase. The d-MoN phase is a metastable state and has a higher lattice constant as a result of the nitrogen incorporation [16]. To evaluate the surface composition of the nitrided MoO2 sample (nitridation at 650 °C), we carried out XPS studies. The XPS survey spectra of nitrided MoO2 are shown in Fig. 2(a). The peaks of Mo 3d, N 1s, and O 1s can be clearly seen [17,18]. The peak given in Fig 2(b) corresponding with Mo 3d exhibits a complex signal that consists of the superimposed doublets of several oxidation states. It is possible to fit the broadened spectra with sets of doublets related to spin orbit split to 3d5/2 and 3d3/2, representing individual Mo2+, Mo3+, Mo4+, Mo5+, and Mo6+, respectively [17]. The
Fig. 2. (a) XPS survey and (b–d) high-resolution XPS spectra of the nitrided MoO2 sample at Mo 3d, N 1s, and O 1s electron.
S. Yoon et al. / Journal of Alloys and Compounds 536 (2012) 179–183
Fig. 3. (a) SEM image and (b) elemental distribution of the nitrided MoO2 sample.
Fig. 4. The first discharge–charge profile of the nitrided MoO2 sample. The inset indicates DCPs at various cycles.
binding energy of Mo 3d5/2, Mo2+, Mo3+, Mo4+, Mo5+, and Mo6+ are 228.1, 229.1, 231.1, 232.3, and 233.6 eV, respectively. The peak at 234.8 eV is assigned to Mo 3d3/2 component of Mo6+, while that at 228.7 eV to the Mo 3d5/2 component of Mo2N. The binding energy of Mo6+ indicates that it maybe formed due to the sample was oxidized at the surface by the exposure to air, leading to the
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Fig. 5. (a) GITT voltage profile and (b) lithium diffusivities of the nitrided MoO2 sample as a function of the cell potential during 2nd and 10th cycles, which were determined by GITT during Li de-intercalation (charging).
presence of some MoO3 [19]. The value of Mo 3d5/2 is higher than that of Mo2+ (228.1 eV) but slightly lower than that of Mo4+ (231.1 eV). Accordingly, we denote the Mo species of Mo2N as Mod+, where 2 < d < 4 [20,21]. Fig. 2(c) shows that the N 1s peaks overlap severely with the Mo 3p3/2 peaks. The N 1s peak is at 397.5 and 401 eV, which is attributed to the formation of molybdenum nitride phases and the O–Mo–N linkages in MoO2 lattice related with molybdenum oxynitride, respectively [20,22]. Additionally, curve fitting at 394.6 eV shows a shoulder corresponding wtih Mo 3p and its oxidation state is between 0 and 4 associated with molybdenum nitride phases [19,21]. While this assignment is consistent with the spectra of O 1s electrons shown in Fig. 2(d), composed of two components centered at 530.2 and 531.8 eV, characteristic of molybdenum oxide and molybdenum oxynitride, respectively. Therefore, these are deduced that nitrogen was doped into MoO2 lattices by substituting the oxygen atoms. Furthermore, in order to know the composition of nitrided MoO2 sample, it was determined by induced-couple plasma (ICP) and elemental analysis; the data indicated the atomic ratio of Mo, O, and N is 1, 1.8 and 0.2, respectively, although it is not containing an optimum level of N doping. The SEM image of the nitrided MoO2 sample (nitridation at 650 °C) provided in Fig. 3(a) exhibits hexagonal prism morphologies with an average particle size of 15 lm (i.e., mean size of the counted 35 particles). A selected individual single particle (Fig. 3(b)) clearly reveals a homogeneous distribution of Mo, O, and N elements in the hexagonal nitrided MoO2 particle.
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Fig. 6. (a) Cycling performances the nitrided MoO2 and (b) comparison of capacity retention at various C rates, illustrating the rate capability of the nitrided MoO2 with those of pristine MoO2.
Fig. 4 gives the first discharge–charge profile of the nitrided MoO2 sample (nitridation at 650 °C). The first discharge and charge capacities are, respectively, 714 and 447 mAh/g, implying an initial Coulombic efficiency of 63%. The initial capacity loss may result from the incomplete conversion reaction and the irreversible lithium loss due to the formation of a solid electrolyte interphase (SEI) layer. The differential capacity plots (DCPs) shown in the inset of Fig. 4 display features characteristic of the voltage plateau during discharge at 1.28 V and then decrease slowly to 0.01 V, associated with the complete reduction reaction of Mo with lithium. While main anodic peak 1.4 V during the charge process is ascribed to the oxidation reaction. In this operating voltage window, 3.4 lithium reacts with the nitride MoO2 sample. Additionally, the voltage plateau during the discharge process, with the increase of cycling, slightly changes from 1.28 to 1.6 V, indicating a different electrochemical reaction. This change of voltage plateau maybe has an effect on the capacity retention after the 25th cycles. The GITT as shown in Fig. 5(a) was employed to investigate the evolution of lithium diffusivity as a function of cell potential. Weppner and Huggins derived a simple expression for lithium diffusivity in the electrode as below [23]:
DLi ¼
(nitridation at 650 °C) was used in implementing this equation. Fig. 5(b) shows the lithium diffusivities as a function of cell potential of the nitrided MoO2 sample during the charge process. The lithium diffusivity of the 2nd cycles shows between 1 109 and 4 1010 cm2/sec. The lithium diffusivity still presents high values of 1 1010 after the 10th cycles due to less the degradation of the nitrided MoO2 sample. This result suggests that the nitrided MoO2 sample can tolerate electrochemical cycling with volume expansion–contraction. Fig. 6(a) presents the cyclability of the nitrided MoO2 sample (nitridation at 650 °C) between 0.01 and 3 V at a constant current of 120 mA/g (C/5 rate). In general, Li storage behavior for MoO2 strongly depends on the size and shape of the particles because morphology is related with electrochemical activity. Interestingly, the bulk nitrided MoO2 sample shows good cyclability, retaining 93% of the capacity after 100 cycles. The Fig. 6(b) compares the rate capabilities at various C rates from 0.2C to 5C rates. The nitrided MoO2 sample exhibits improved rate capability due to the molybdenum nitride phases and molybdenum oxynitride, which has electrical conductivity better than that of oxides. Particularly, it retains a high capacity of 200 and 80 mAh/g, respectively, at 3C and 5C rates with stable cycling, which is approximately two times higher than that value for the pristine MoO2 electrode. In order to gain further insight into electrochemical performances, EIS measurements were carried out at 3 V vs. Li+/Li with the nitrided MoO2 sample (nitridation at 650 °C) after 20th cycles and they were fitted with use of the Zview software. In general, the EIS spectrum can be divided into three frequency regions: low, medium to low, and high frequency regions, which correspond to cell geometric capacitance, charge transfer reaction, and lithium-ion diffusion through the surface layer, respectively. The EIS data were analyzed based on an equivalent circuit given in the Fig. 7 [24]. The EIS plots of pristine of MoO2 and nitrided MoO2 are composed by two semicircles and a line. The small diameter of the first semicircle (at high frequency region) is a measure of the surface layer resistance Rs, which is ascribed to lithium-ion diffusion through the surface layer, and the diameter of the second semicircle (at medium–low frequency region) is a measure of the charge transfer resistance Rct, which is related to the contact between the particles or between the electrode and the electrolyte. The sloping line is related to lithium-ion diffusion in the bulk of the active material. The pristine MoO2 show surface resistances of 387 X and a charge transfer resistance of 640 X. The EIS spectra of the nitrided MoO2, on the other hand, reveals surface resistances and a charge transfer resistances of 288 and 495 X, respectively, due to the molybdenum nitride phases and molybdenum oxynitride materials which have high electrical conductivity.
2 4L2 DEs ps DEs
where L refers to the electrode thickness, s refers to interval time of the current pulse (300 s), and DEs and DEs refer to voltage changes during, respectively, the applied current pulse and the turned off current pulse. The ideal density for the nitrided MoO2 sample
Fig. 7. Electrochemical impedance spectra (EIS) of pristine MoO2 and the nitrided MoO2 after 20th cycles. Equivalent circuit shown in the inset.
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4. Conclusions The nitrided MoO2 has been synthesized by a hydrothermal processing followed by post-nitridation and characterized by XRD, XPS, and electrochemical analysis. It is composed by molybdenum nitrides and molybdenum oxynitride. The nitrided MoO2 sample exhibits a high capacity with good cyclability and high rate capability. These characteristics are attributed to the good electrical contact and smooth accommodation of the strain, resulting in facile lithium-ion diffusion. The nitrided MoO2 shows considerable promise as a candidate for high performance lithium-ion batteries. Acknowledgements This work was supported by the Energy Efficiency & Resources of the Korea Institute of Energy Technology Evaluation and Planning (No. 20112010100110) Grant funded by the Korea government Ministry of Knowledge Economy. References [1] D.B. Rogers, R.D. Shannon, A.W. Sleight, J.L. Gillson, Inorg. Chem. 8 (1969) 841. [2] J.J. Auborn, Y.I. Barberio, J. Electrochem. Soc. 134 (1987) 638. [3] R.L. Gitzendanner, F.J. DiSalvo, J. Alloy. Compd. 218 (1995) 9.
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