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NdFeB alloy as a magnetic electrode material for lithium-ion batteries
Journal of Alloys and Compounds 391 (2005) 212–216
NdFeB alloy as a magnetic electrode material for lithium-ion batteries J. Zhang, J.L. Shui, S.L. Zhang, X. Wei, Y.J. Xiang, S. Xie, C.F. Zhu, C.H. Chen∗ Department of Materials Science and Engineering, University of Science and Technology of China, Anhui Hefei 230026, PR China Received 23 July 2004; accepted 6 August 2004
Abstract The search for a reliable indicator of state of charge and even the remaining energy of a lithium-ion cell is of great importance for various applications. This study was an exploratory effort to use magnetic susceptibility as the indicator. In this work, for the first time the change of ac susceptibility of cells was in situ monitored during charge–discharge process. A strong permanent magnetic material, NdFeB alloy, was investigated as an anode material for rechargeable lithium batteries. Both original and partially oxidized NdFeB powders were made into electrodes. Structural characterization was performed on the NdFeB electrodes by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis. An alloy (core)–oxide (shell) structure was found for those partially oxidized samples. The electrochemical cycling of cells made of the NdFeB electrodes against lithium was measured. The first lithium intercalation capacity of a treated NdFeB can be up to about 831 mAh/g, while a rather reversible capacity of up to 352 mAh/g can be obtained. With a specially designed cell, we were able to monitor in situ the change of relative ac susceptibility during charge and/or discharge steps. A clearly monotonous relationship is found between the ac susceptibility of a cell and its depth-of-discharge (DOD). A mechanism based on skin effect and eddy current change is proposed to explain this susceptibility versus DOD relationship. © 2004 Elsevier B.V. All rights reserved. Keywords: Magnetically ordered materials; X-ray diffraction; Scanning electron microscopy; Electrochemical reaction; Magnetic measurements
1. Introduction In a short period of less than a quarter of century, the industry of lithium ion batteries has brought great convenience to human being. We are now looking forward to further exploiting this technology to have rechargeable batteries with longer cycle life, higher energy density and power capability, and wider applications. As the electrode materials (including cathode and anode) determine largely the capacity and cycle life of a cell, the search for new materials with excellent electrode performance has always been one of the most important motivations in this field. In current lithium-ion technologies, the state-of-the-art electrodes are a lithium metal oxide (e.g. LiCoO2 and LiMn2 O4 ) as the cathode and a carbonaceous material (graphite or hard carbon) as the anode. At the same time, owing to the usually high capacity yet too large volume change during charge–discharge process of some metal anodes such as Sn [1,2] and Al [3,4], intensive efforts have ∗
Corresponding author. Tel.: +86 551 3602938; fax: +86 551 3602940. E-mail address: [email protected] (C.H. Chen).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.08.083
also been made on exploring alloys as the alternative anodes. Examples of the investigated alloys are SnSb [5], Mg2 Si [6], Mg2 Sn [7], Snx Agw [8], Cu6 Sn5 [9], CoSb3 [10] and Sn2 Fe [11]. In addition to the capacity and electrode integrity, some magnetic alloys have also attracted our attention because they may eventually provide a state-of-charge indicator for the practical lithium-ion batteries. In our previous study on magnetic electrode material, the permanent magnetic material Alnico was explored [12]. Both pristine and surface-oxidized Alnico powders were investigated as potential electrode materials. A relationship between the depth-of-discharge (DOD) of cells and the ac magnetic susceptibility of the Alnico electrode has been revealed. However, the measurement of ac magnetic susceptibility was conducted ex situ by disassembling cells at different DOD. For practical applications, it would be much more convenient to monitor the change of magnetic susceptibility of a cell in situ. NdFeB, a stronger permanent magnetic alloy than Alnico, contains metal elements Nd and Fe, and non-metal element B. With the strategy developed in our Alnico study [12], we
J. Zhang et al. / Journal of Alloys and Compounds 391 (2005) 212–216
213
Table 1 The electrodes of original and treated NdFeB powders Electrodes
Treatment temperature (◦ C)
NdFeB (wt.%)
Acetylene black (wt.%)
PVDF (wt.%)
N0 N1 N2 N3
– 400 550 650
96 84 84 84
0 8 8 4
4 8 8 8
could study both the pristine form and its surface-treated form. Naturally, it is of great curiosity to explore the possibility of using them as magnetic anode materials. In this paper, we will show that the original NdFeB itself is nearly inert to the reaction with lithium. Nevertheless, an oxidation treatment has resulted in an anode material with rather high specific capacity. By measuring the in situ ac susceptibility change of cells using the oxidized NdFeB as anode material in a specially designed cell fixture, we propose a possible way to monitor the remaining energy of a cell through its magnetic signature.
2. Experimental aspects We selected one kind of commercial magnetic NdFeB powder (Nd: 26.6 wt.%; Fe: 72.4 wt.%; B: 1.0 wt.%) as an electrode material. In addition, different levels of oxidation treatment were applied to the powder by heating it at 400 ◦ C for 1 h, 550 ◦ C for 1 h, and 650 ◦ C for 3 h in air, respectively. The original and treated NdFeB powders were coded in this study as N0–N3, respectively. The electrode laminates on copper foil were fabricated from these NdFeB powders with tape-casting technique. The powder treatment temperature and composition of these electrodes are shown in Table 1. Coin cells (CR2032) were assembled in an argon-filled glove box (MBraun Lab Master 130) with these electrodes against metallic lithium. The electrolyte was 1 M LiPF6 in a mixture of 1:1 (w/w) ethylene carbonate (EC) and diethyl carbonate (DEC) while Celgard 2400 microporous polypropylene membrane was used as separator. These cells were cycled galvanostatically between 0.01 and 3 V at room temperature on a multi-channel battery test system (Shenzhen Neware BTS-610) to analyze the electrochemical response. The crystal structure of the NdFeB powders was analyzed using X-ray diffraction (XRD) (Philips X’Pert Pro Super, Cu K␣ radiation) with a 2θ scan from 20◦ to 70◦ , while their particle morphology was studied using a scanning electron microscope (Hitachi X-650). For N3 electrode, XRD was also performed on the samples at different DOD. Before measuring X-ray diffraction of the electrodes, we disassembled the cells in the glove box, washed the electrodes in ethylene carbonate and diethyl carbonate, and sealed them in thin, transparent polyethylene bags, respectively, after drying. The in situ susceptibility measurements were carried out with a specially designed cell fixture that comprised of wires
Fig. 1. Schematics of the homemade cell, which is used to measure in situ relative ac susceptibility.
of primary and secondary loops (Fig. 1). The scheme of measurement instrument was reported previously [12]. The frequency of the ac signal was set at 1000 Hz.
3. Results and discussion Fig. 2 shows the X-ray diffraction patterns of NdFeB powders before and after oxidation treatment at different temperatures. It is clear that the component of the original powder is Nd2 Fe14 B. After oxidation, the peaks of Nd2 Fe14 B phase become weak, while the peaks of oxides such as Fe3 O4 , Fe2 O3 , B2 O3 and Nd2 O3 appear. This result is consistent to [13]. With the increase in the treatment temperature, the intensity
Fig. 2. XRD patterns of NdFeB powders before and after heat-treated at various temperatures in air.
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Fig. 3. The typical SEM micrographs of the original powder N0 and treated powder N1, left micrographs with the magnification of 2500×, right micrographs with the magnification of 10 000×.
of the diffraction peaks from the oxides becomes stronger. Nevertheless, the primary peaks of Nd2 Fe14 B phase between 41◦ and 45◦ are always there [14]; we conclude that there are still remains of the magnetic phase in these oxidized powders. The SEM analysis of these powders (Fig. 3) reveals that they consist of particles of irregular shapes with particle size ranging from 2 to 12 m. Comparing the morphology of the powders before and after oxidation (Fig. 3b and d) we can see that the surface of treated particles became fluffy. Probably, a thin oxides layer surrounded original NdFeB particles, which is detected by the X-ray diffraction (Fig. 2) and also evidenced by TEM observation in [13]. Therefore, the oxidation treatment leads to the formation of powders with a core–shell structure, where the shell is composed of Fe3 O4 , Fe2 O3 , Nd2 O3 and B2 O3 while the core is magnetic NdFeB. The voltage profile of NdFeB/Li cell (Fig. 4) shows a rather reversible charge–discharge behavior between 0.01 and 3.0 V. Nevertheless, its specific capacity is very low, only about 1.7 mAh/g. Because the charge–discharge curves are similar to those of transition-metal oxides (MO, where M is Fe, Co, Ni, or Cu) [14–16], it is believed that lithium is not intercalated into the lattice of NdFeB but rather into an oxide layer on the surface. This oxide layer is likely formed by a
mild oxidation when the NdFeB powder is exposed to ambient atmosphere during the preparation of electrode laminate. The composition of this thin surface layer should be the oxidation products, such as Fe3 O4 , Fe2 O3 , Nd2 O3 , and B2 O3 . Note that, depending on the cells, the specific capacity can range from 1 to 6 mAh/g due probably to the different extent of surface oxidation. To improve the capacity of cells, partial oxidation treatments can be applied to original NdFeB powder (Table 1).
Fig. 4. The charge–discharge curves of the electrode N0/Li cell. The charge–discharge was operated in the voltage range between 3.0 and 0.01 V at the current density of 0.02 mA/cm2 , the cycle numbers are indicated.
J. Zhang et al. / Journal of Alloys and Compounds 391 (2005) 212–216
215
Fig. 5. The charge–discharge curves of the electrode N1/Li cells. The charge–discharge was operated in the voltage range between 3.0 and 0.01 V at the current density of 0.20 mA/cm2 , the cycle numbers are indicated.
After the treatments, the amount of surface oxides layer is increased. In addition, acetylene black (AB) as a conductive additive can be added into the electrode to lower the cell impedance. A reversible capacity of about 150 mAh/g can be contributed from AB in the potential range between 0.01 and 3.0 V versus Li [12]. This contribution should be taken into account when calculating the capacity of treated NdFeB electrodes in Table 1. Fig. 5 shows voltage profile and specific capacity of the cells using treated NdFeB electrodes against lithium. It is found that the voltage profile (Fig. 5) is similar to the profiles of transition-metal oxides (MO, where M is Fe, Co, Ni, or Cu) electrode [15–17]. The first intercalation capacity reaches about 352 mAh/g. After suffering irreversible capacity losses in the subsequent 30 cycles, a rather reversible capacity of about 180 mAh/g can be achieved. Furthermore, the specific capacity of treated NdFeB powder increases with increasing the extent of oxidation (Fig. 6, curves for the electrodes N1–N3). These results also support the core–shell model discussed above. Besides, Shui et al. [12] in our group investigated the effect of AB, it was observed that the capacity increases from 40 to 140 mAh/g after the addition of AB in electrodes. The same effect is expected to exist in the NdFeB system. Hence, we only studied here the electrodes with AB as the conducting additive. Fig. 7 shows the discharge curve and in situ relative ac susceptibility data of the electrodes N0 and N3/Li homemade cells at different DOD on the first discharge. It can be seen that the in situ ac susceptibility of electrode decreases steadily
Fig. 6. Specific capacity as a function of cycle number for treated powders/Li cells. The testing conditions are given in the caption of Fig. 5 (note: the capacity contribution of conductive additive-acetylene black was already removed).
Fig. 7. The discharge curve and in situ relative ac susceptibility data of the electrodes N0/Li and N3/Li cells at different DOD on the first discharge. The discharge was operated in the voltage range from OCV to 0.01 V at the current density of 0.10 mA/cm2 for N0/Li cell, and 0.5 mA/cm2 for N3/Li cell.
with the increase of DOD and amount of intercalated lithium. This relationship indicates the possibility of using magnetic susceptibility as the indicator of DOD and remaining energy of a cell. This result can be explained by the formation of a metallic shell on the magnetic NdFeB core. When lithium is “pumped” into the metal oxides layer on the surface of NdFeB grains during the discharge step, the metal oxides are reduced to form metal nanoparticles [15]. These nanoparticles actually constitute a metallic shielding layer on individual NdFeB grains. Due to the skin effect, there must be an eddy current running in this shell layer. This eddy current increases with increasing the concentration of metal component in the shell. As a result, the ac susceptibility of the electrode mea-
Fig. 8. The discharge curve and X-ray diffraction patterns of treated NdFeB powders: (a) the voltage vs. DOD curve of N3/Li cells for the first discharge; (b) X-ray diffraction patterns of the electrode N3 collected at different DOD (A: 5%, B: 50%, C: 70%, D: 100%). The DOD and voltage of the cell corresponding to these patterns are marked in (a).
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sured is decreased with the lithium intercalation. More deeply the NdFeB oxidized, more significantly the ac susceptibility varies with the increase of DOD. Nevertheless, owing to the small capacity of original NdFeB powder, it cannot be directly used as an electrode material in practical applications, but considering the relatively high capacity of these treated NdFeB powders, they are possibly used as magnetic anode materials in lithium-ion batteries. In addition to that, during the discharge, the iron nanoparticles reduced from Fe2 O3 and Fe3 O4 remain magnetic, in that way, the ac susceptibility cannot vary greatly for oxidized NdFeB electrodes. So NdFeB is not an excellent system as an indicator. In future work, our group will focus on exploring an appropriate magnetic material. Fig. 8 shows the evolution of the phase structure of the electrode N3 during cycling. As lithium reacts continuously with heat-treated NdFeB during the first discharge (Fig. 8a), we observed a steady decrease in the intensity of diffraction peaks for oxides until these peaks completely disappear at the voltage of 0.01 V, the peak at 36.6◦ is from polyethylene bags. On the other hand, the peaks around 41–45◦ belonging to NdFeB remain unchanged in these diffraction patterns. Combining with the SEM analysis (Fig. 3), we can conclude that the core–shell structure model is validated.
4. Conclusions The feasibility of commercial NdFeB powder as a possible magnetic electrode in lithium battery is investigated. It is found that the original NdFeB is nearly inert to lithium intercalation. After partial oxidation treatments, however, the treated NdFeB may be possibly used as magnetic anode materials with the first intercalation capacity of about 352 mAh/g and rather reversible capacity up to 180 mAh/g. The treated NdFeB powders have a core–shell structure, where NdFeB particles are located in the core of grains while a layer of oxides covers the core. We designed a homemade cell to measure the change of in situ ac susceptibility for the first time; the remaining energy of a cell can be monitored. The mecha-
nism of lithium intercalation in the treated NdFeB electrode is due to the reaction of lithium with the surface oxide layer.
Acknowledgement This study was supported by 100 Talents Program of Academia Sinica and National Science Foundation of China (Grant No. 50372064). We are also grateful to the China Education Council PhD Foundation (Grant No. 20030358057).
References [1] C.J. Wen, R.A. Huggins, J. Solid State Chem. 35 (1980) 376. [2] C.J. Wen, R.A. Huggins, J. Electrochem. Soc. 128 (1981) 1181. [3] N.P. Yao, L.A. Heredy, R.C. Saunders, J. Electrochem. Soc. 118 (1971) 1039. [4] R.A. Huggins, J. Power Sources 81 (1999) 13. [5] J. Yang, M. Wachtler, M. Winter, J.O. Besenhard, Electrochem. Solid State Lett. 2 (1999) 161. [6] H. Kim, J. Choi, H.J. Sohn, T. Kang, J. Electrochem. Soc. 146 (1999) 4401. [7] H. Kim, Y.J. Kim, D.G. Kim, H.J. Sohn, T. Kang, Solid State Ionics 144 (2001) 41. [8] M.M. Thackeray, J.T. Vaughey, A.J. Kahaian, K.D. Kepler, R. Benedek, Electrochem. Commun. 1 (1999) 111. [9] K.D. Kepler, J.T. Vaughey, M.M. Thackeray, Electrochem. Solid State Lett. 2 (1999) 307. [10] R. Alcantara, F.J. Fernandez-Madrigal, P. Lavela, J.L. Tirado, J.C. Jumas, J. Olivier-Fourcade, J. Mater. Chem. 9 (1999) 2517. [11] O. Mao, R.A. Dunlap, I.A. Courtney, J.R. Dahn, J. Electrochem. Soc. 145 (1998) 4195. [12] J.L. Shui, S.L. Zhang, W.L. Liu, Y. Yu, G.S. Jiang, S. Xie, C.F. Zhu, C.H. Chen, Electrochem. Commun. 6 (2004) 33–38. [13] Y. Li, H.E. Evans, I.R. Harris, I.P. Jones, Oxid. Met. 59 (2003) 167–181. [14] D. Givord, S.H. Li, J.M. Moreau, Solid State Commun. 50 (1984) 497. [15] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.-M. Tarascon, Nature 407 (2000) 496–499. [16] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.-M. Tarascon, J. Power Sources 97–98 (2001) 235–239. [17] G.X. Wang, Y. Chen, K. Konstantinow, M. Lindsay, H.K. Liu, S.X. Dou, J. Power Sources 109 (2002) 142–147.
NdFeB alloy as a magnetic electrode material for lithium-ion batteries J. Zhang, J.L. Shui, S.L. Zhang, X. Wei, Y.J. Xiang, S. Xie, C.F. Zhu, C.H. Chen∗ Department of Materials Science and Engineering, University of Science and Technology of China, Anhui Hefei 230026, PR China Received 23 July 2004; accepted 6 August 2004
Abstract The search for a reliable indicator of state of charge and even the remaining energy of a lithium-ion cell is of great importance for various applications. This study was an exploratory effort to use magnetic susceptibility as the indicator. In this work, for the first time the change of ac susceptibility of cells was in situ monitored during charge–discharge process. A strong permanent magnetic material, NdFeB alloy, was investigated as an anode material for rechargeable lithium batteries. Both original and partially oxidized NdFeB powders were made into electrodes. Structural characterization was performed on the NdFeB electrodes by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis. An alloy (core)–oxide (shell) structure was found for those partially oxidized samples. The electrochemical cycling of cells made of the NdFeB electrodes against lithium was measured. The first lithium intercalation capacity of a treated NdFeB can be up to about 831 mAh/g, while a rather reversible capacity of up to 352 mAh/g can be obtained. With a specially designed cell, we were able to monitor in situ the change of relative ac susceptibility during charge and/or discharge steps. A clearly monotonous relationship is found between the ac susceptibility of a cell and its depth-of-discharge (DOD). A mechanism based on skin effect and eddy current change is proposed to explain this susceptibility versus DOD relationship. © 2004 Elsevier B.V. All rights reserved. Keywords: Magnetically ordered materials; X-ray diffraction; Scanning electron microscopy; Electrochemical reaction; Magnetic measurements
1. Introduction In a short period of less than a quarter of century, the industry of lithium ion batteries has brought great convenience to human being. We are now looking forward to further exploiting this technology to have rechargeable batteries with longer cycle life, higher energy density and power capability, and wider applications. As the electrode materials (including cathode and anode) determine largely the capacity and cycle life of a cell, the search for new materials with excellent electrode performance has always been one of the most important motivations in this field. In current lithium-ion technologies, the state-of-the-art electrodes are a lithium metal oxide (e.g. LiCoO2 and LiMn2 O4 ) as the cathode and a carbonaceous material (graphite or hard carbon) as the anode. At the same time, owing to the usually high capacity yet too large volume change during charge–discharge process of some metal anodes such as Sn [1,2] and Al [3,4], intensive efforts have ∗
Corresponding author. Tel.: +86 551 3602938; fax: +86 551 3602940. E-mail address: [email protected] (C.H. Chen).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.08.083
also been made on exploring alloys as the alternative anodes. Examples of the investigated alloys are SnSb [5], Mg2 Si [6], Mg2 Sn [7], Snx Agw [8], Cu6 Sn5 [9], CoSb3 [10] and Sn2 Fe [11]. In addition to the capacity and electrode integrity, some magnetic alloys have also attracted our attention because they may eventually provide a state-of-charge indicator for the practical lithium-ion batteries. In our previous study on magnetic electrode material, the permanent magnetic material Alnico was explored [12]. Both pristine and surface-oxidized Alnico powders were investigated as potential electrode materials. A relationship between the depth-of-discharge (DOD) of cells and the ac magnetic susceptibility of the Alnico electrode has been revealed. However, the measurement of ac magnetic susceptibility was conducted ex situ by disassembling cells at different DOD. For practical applications, it would be much more convenient to monitor the change of magnetic susceptibility of a cell in situ. NdFeB, a stronger permanent magnetic alloy than Alnico, contains metal elements Nd and Fe, and non-metal element B. With the strategy developed in our Alnico study [12], we
J. Zhang et al. / Journal of Alloys and Compounds 391 (2005) 212–216
213
Table 1 The electrodes of original and treated NdFeB powders Electrodes
Treatment temperature (◦ C)
NdFeB (wt.%)
Acetylene black (wt.%)
PVDF (wt.%)
N0 N1 N2 N3
– 400 550 650
96 84 84 84
0 8 8 4
4 8 8 8
could study both the pristine form and its surface-treated form. Naturally, it is of great curiosity to explore the possibility of using them as magnetic anode materials. In this paper, we will show that the original NdFeB itself is nearly inert to the reaction with lithium. Nevertheless, an oxidation treatment has resulted in an anode material with rather high specific capacity. By measuring the in situ ac susceptibility change of cells using the oxidized NdFeB as anode material in a specially designed cell fixture, we propose a possible way to monitor the remaining energy of a cell through its magnetic signature.
2. Experimental aspects We selected one kind of commercial magnetic NdFeB powder (Nd: 26.6 wt.%; Fe: 72.4 wt.%; B: 1.0 wt.%) as an electrode material. In addition, different levels of oxidation treatment were applied to the powder by heating it at 400 ◦ C for 1 h, 550 ◦ C for 1 h, and 650 ◦ C for 3 h in air, respectively. The original and treated NdFeB powders were coded in this study as N0–N3, respectively. The electrode laminates on copper foil were fabricated from these NdFeB powders with tape-casting technique. The powder treatment temperature and composition of these electrodes are shown in Table 1. Coin cells (CR2032) were assembled in an argon-filled glove box (MBraun Lab Master 130) with these electrodes against metallic lithium. The electrolyte was 1 M LiPF6 in a mixture of 1:1 (w/w) ethylene carbonate (EC) and diethyl carbonate (DEC) while Celgard 2400 microporous polypropylene membrane was used as separator. These cells were cycled galvanostatically between 0.01 and 3 V at room temperature on a multi-channel battery test system (Shenzhen Neware BTS-610) to analyze the electrochemical response. The crystal structure of the NdFeB powders was analyzed using X-ray diffraction (XRD) (Philips X’Pert Pro Super, Cu K␣ radiation) with a 2θ scan from 20◦ to 70◦ , while their particle morphology was studied using a scanning electron microscope (Hitachi X-650). For N3 electrode, XRD was also performed on the samples at different DOD. Before measuring X-ray diffraction of the electrodes, we disassembled the cells in the glove box, washed the electrodes in ethylene carbonate and diethyl carbonate, and sealed them in thin, transparent polyethylene bags, respectively, after drying. The in situ susceptibility measurements were carried out with a specially designed cell fixture that comprised of wires
Fig. 1. Schematics of the homemade cell, which is used to measure in situ relative ac susceptibility.
of primary and secondary loops (Fig. 1). The scheme of measurement instrument was reported previously [12]. The frequency of the ac signal was set at 1000 Hz.
3. Results and discussion Fig. 2 shows the X-ray diffraction patterns of NdFeB powders before and after oxidation treatment at different temperatures. It is clear that the component of the original powder is Nd2 Fe14 B. After oxidation, the peaks of Nd2 Fe14 B phase become weak, while the peaks of oxides such as Fe3 O4 , Fe2 O3 , B2 O3 and Nd2 O3 appear. This result is consistent to [13]. With the increase in the treatment temperature, the intensity
Fig. 2. XRD patterns of NdFeB powders before and after heat-treated at various temperatures in air.
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Fig. 3. The typical SEM micrographs of the original powder N0 and treated powder N1, left micrographs with the magnification of 2500×, right micrographs with the magnification of 10 000×.
of the diffraction peaks from the oxides becomes stronger. Nevertheless, the primary peaks of Nd2 Fe14 B phase between 41◦ and 45◦ are always there [14]; we conclude that there are still remains of the magnetic phase in these oxidized powders. The SEM analysis of these powders (Fig. 3) reveals that they consist of particles of irregular shapes with particle size ranging from 2 to 12 m. Comparing the morphology of the powders before and after oxidation (Fig. 3b and d) we can see that the surface of treated particles became fluffy. Probably, a thin oxides layer surrounded original NdFeB particles, which is detected by the X-ray diffraction (Fig. 2) and also evidenced by TEM observation in [13]. Therefore, the oxidation treatment leads to the formation of powders with a core–shell structure, where the shell is composed of Fe3 O4 , Fe2 O3 , Nd2 O3 and B2 O3 while the core is magnetic NdFeB. The voltage profile of NdFeB/Li cell (Fig. 4) shows a rather reversible charge–discharge behavior between 0.01 and 3.0 V. Nevertheless, its specific capacity is very low, only about 1.7 mAh/g. Because the charge–discharge curves are similar to those of transition-metal oxides (MO, where M is Fe, Co, Ni, or Cu) [14–16], it is believed that lithium is not intercalated into the lattice of NdFeB but rather into an oxide layer on the surface. This oxide layer is likely formed by a
mild oxidation when the NdFeB powder is exposed to ambient atmosphere during the preparation of electrode laminate. The composition of this thin surface layer should be the oxidation products, such as Fe3 O4 , Fe2 O3 , Nd2 O3 , and B2 O3 . Note that, depending on the cells, the specific capacity can range from 1 to 6 mAh/g due probably to the different extent of surface oxidation. To improve the capacity of cells, partial oxidation treatments can be applied to original NdFeB powder (Table 1).
Fig. 4. The charge–discharge curves of the electrode N0/Li cell. The charge–discharge was operated in the voltage range between 3.0 and 0.01 V at the current density of 0.02 mA/cm2 , the cycle numbers are indicated.
J. Zhang et al. / Journal of Alloys and Compounds 391 (2005) 212–216
215
Fig. 5. The charge–discharge curves of the electrode N1/Li cells. The charge–discharge was operated in the voltage range between 3.0 and 0.01 V at the current density of 0.20 mA/cm2 , the cycle numbers are indicated.
After the treatments, the amount of surface oxides layer is increased. In addition, acetylene black (AB) as a conductive additive can be added into the electrode to lower the cell impedance. A reversible capacity of about 150 mAh/g can be contributed from AB in the potential range between 0.01 and 3.0 V versus Li [12]. This contribution should be taken into account when calculating the capacity of treated NdFeB electrodes in Table 1. Fig. 5 shows voltage profile and specific capacity of the cells using treated NdFeB electrodes against lithium. It is found that the voltage profile (Fig. 5) is similar to the profiles of transition-metal oxides (MO, where M is Fe, Co, Ni, or Cu) electrode [15–17]. The first intercalation capacity reaches about 352 mAh/g. After suffering irreversible capacity losses in the subsequent 30 cycles, a rather reversible capacity of about 180 mAh/g can be achieved. Furthermore, the specific capacity of treated NdFeB powder increases with increasing the extent of oxidation (Fig. 6, curves for the electrodes N1–N3). These results also support the core–shell model discussed above. Besides, Shui et al. [12] in our group investigated the effect of AB, it was observed that the capacity increases from 40 to 140 mAh/g after the addition of AB in electrodes. The same effect is expected to exist in the NdFeB system. Hence, we only studied here the electrodes with AB as the conducting additive. Fig. 7 shows the discharge curve and in situ relative ac susceptibility data of the electrodes N0 and N3/Li homemade cells at different DOD on the first discharge. It can be seen that the in situ ac susceptibility of electrode decreases steadily
Fig. 6. Specific capacity as a function of cycle number for treated powders/Li cells. The testing conditions are given in the caption of Fig. 5 (note: the capacity contribution of conductive additive-acetylene black was already removed).
Fig. 7. The discharge curve and in situ relative ac susceptibility data of the electrodes N0/Li and N3/Li cells at different DOD on the first discharge. The discharge was operated in the voltage range from OCV to 0.01 V at the current density of 0.10 mA/cm2 for N0/Li cell, and 0.5 mA/cm2 for N3/Li cell.
with the increase of DOD and amount of intercalated lithium. This relationship indicates the possibility of using magnetic susceptibility as the indicator of DOD and remaining energy of a cell. This result can be explained by the formation of a metallic shell on the magnetic NdFeB core. When lithium is “pumped” into the metal oxides layer on the surface of NdFeB grains during the discharge step, the metal oxides are reduced to form metal nanoparticles [15]. These nanoparticles actually constitute a metallic shielding layer on individual NdFeB grains. Due to the skin effect, there must be an eddy current running in this shell layer. This eddy current increases with increasing the concentration of metal component in the shell. As a result, the ac susceptibility of the electrode mea-
Fig. 8. The discharge curve and X-ray diffraction patterns of treated NdFeB powders: (a) the voltage vs. DOD curve of N3/Li cells for the first discharge; (b) X-ray diffraction patterns of the electrode N3 collected at different DOD (A: 5%, B: 50%, C: 70%, D: 100%). The DOD and voltage of the cell corresponding to these patterns are marked in (a).
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sured is decreased with the lithium intercalation. More deeply the NdFeB oxidized, more significantly the ac susceptibility varies with the increase of DOD. Nevertheless, owing to the small capacity of original NdFeB powder, it cannot be directly used as an electrode material in practical applications, but considering the relatively high capacity of these treated NdFeB powders, they are possibly used as magnetic anode materials in lithium-ion batteries. In addition to that, during the discharge, the iron nanoparticles reduced from Fe2 O3 and Fe3 O4 remain magnetic, in that way, the ac susceptibility cannot vary greatly for oxidized NdFeB electrodes. So NdFeB is not an excellent system as an indicator. In future work, our group will focus on exploring an appropriate magnetic material. Fig. 8 shows the evolution of the phase structure of the electrode N3 during cycling. As lithium reacts continuously with heat-treated NdFeB during the first discharge (Fig. 8a), we observed a steady decrease in the intensity of diffraction peaks for oxides until these peaks completely disappear at the voltage of 0.01 V, the peak at 36.6◦ is from polyethylene bags. On the other hand, the peaks around 41–45◦ belonging to NdFeB remain unchanged in these diffraction patterns. Combining with the SEM analysis (Fig. 3), we can conclude that the core–shell structure model is validated.
4. Conclusions The feasibility of commercial NdFeB powder as a possible magnetic electrode in lithium battery is investigated. It is found that the original NdFeB is nearly inert to lithium intercalation. After partial oxidation treatments, however, the treated NdFeB may be possibly used as magnetic anode materials with the first intercalation capacity of about 352 mAh/g and rather reversible capacity up to 180 mAh/g. The treated NdFeB powders have a core–shell structure, where NdFeB particles are located in the core of grains while a layer of oxides covers the core. We designed a homemade cell to measure the change of in situ ac susceptibility for the first time; the remaining energy of a cell can be monitored. The mecha-
nism of lithium intercalation in the treated NdFeB electrode is due to the reaction of lithium with the surface oxide layer.
Acknowledgement This study was supported by 100 Talents Program of Academia Sinica and National Science Foundation of China (Grant No. 50372064). We are also grateful to the China Education Council PhD Foundation (Grant No. 20030358057).
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