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Exploration of Alnico alloy as a magnetic electrode material for lithium-ion batteries
Electrochemistry Communications 6 (2004) 33–38 www.elsevier.com/locate/elecom
Exploration of Alnico alloy as a magnetic electrode material for lithium-ion batteries J.L. Shui, S.L. Zhang, W.L. Liu, Y. Yu, G.S. Jiang, 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 17 September 2003; received in revised form 6 October 2003; accepted 7 October 2003 Published online: 4 November 2003
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. Magnetic Alnico alloy was investigated as an anode material for rechargeable lithium batteries. Both pristine and partially oxidized Alnico powders were made into electrodes. Structural characterization was performed on the Alnico electrodes by means of X-ray diffraction, scanning electron microscopy, and inductively coupled plasma analysis. The electrochemical cycling of cells made of the Alnico electrodes against lithium was measured. The first lithium intercalation capacity of a treated Alnico can be up to about 600 mAh/g, while a rather reversible capacity of up to 180 mAh/g can be obtained. The capacity increases with the extent of oxidation of Alnico. It was observed that the ac susceptibility of an electrode changes with depth-of-discharge (DOD). We have proposed an electrode model with a core-shell structure, which can explain this susceptibility vs. DOD relationship. Ó 2003 Elsevier B.V. All rights reserved. Keywords: State-of-charge; Magnetic susceptibility; Lithium batteries; Electrode; Oxidation
1. Introduction The last decade of the 20th century has witnessed the rapid development and commercial success of rechargeable lithium-ion batteries as the excellent energy conversion and storage system. These batteries of various forms and sizes are now widely used to power portable electronic devices such as cellular phones and notebook computers. Battery blocks with larger size and greater integration are also very promising to be used in automobiles in the near future. However, a practical issue has been puzzling the battery manufacturers these years; namely, we are lack of an accurate indicator of remaining energy of cells. At this moment, the opencircuit-voltage (OCV) of a cell is usually taken for this purpose because it can indicate to the users approximate state-of-charge (SOC) of a cell. Nevertheless, the correctness of this alarming approach is based upon two *
Corresponding author. Fax: +86-551-3602940. E-mail address: [email protected] (C.H. Chen).
1388-2481/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2003.10.006
assumptions, i.e. the relationship between OCV and SOC keeps unchanged, and meanwhile, there is no capacity fading. Obviously, these two assumptions are not valid for cells that have been used for some time already. On the other hand, erroneous indication of remaining energy may lead to mis-alarming, overcharge, over-discharge, or even safety problems. Therefore, it is necessary to search for alternative indicators. Basically, a correct indication of the remaining energy in a cell is equivalent to probing effectively the amount of active delithiated cathode material or lithiated anode material. Since it is very unlikely to really monitor the mass variation of either electrode material, we need to correlate the change of remaining energy with a certain physical property that is easily measurable. As a cathode material always contains at least one transition-metal element, one might consider the correlation of the change of oxidation-state of the transition metal with its magnetic property during the charge– discharge cycling. For example, lithium manganese oxide spinel (LiMn2 O4 ) is antiferromagnetic while
34
J.L. Shui et al. / Electrochemistry Communications 6 (2004) 33–38
Li4 Mn5 O12 is ferromagnetic [1]. However, the total magnetic susceptibility of the cathode of a cell is too small to be detected conveniently by a simple measurement device, especially at ambient temperature [2]. Consequently, it is very difficult to use the very minor change of the susceptibility as the signature of remaining energy. On the other hand, if we can find a suitable anode material that has strong magnetic property, we may be able to detect the remaining energy of a cell by monitoring the change of magnetic susceptibility of a cell. This study is an exploratory effort toward this direction. Since lithium can be intercalated into some metals such as Sn, Al, Mg, Zn, Ni, Mo, Fe, Cu or alloys containing these metals [3–5] at potentials under 1 V vs. Li, they are potential anode materials in place of carbonaceous materials (graphite, hard carbon, etc.). In particular, aluminum–lithium intermetallic compound LiAl is found to be a good anode material with large specific capacity and relatively small volume change (97%), while lithiation of many other metals could result in very substantial volume change for instance 676% for Li22 Sn5 [6]. Because Alnico, a permanent magnetic alloy, also contains aluminum, it is natural to explore the possibility of using it as a magnetic anode material. In this paper, we will show that the pristine Alnico 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 susceptibility change of cells using the oxidized Alnico as anode material, we propose a possible way to monitor the remaining energy of a cell through its magnetic signature.
2. Experimental We selected one kind of commercial magnetic Alnico powder (Fe: 58%, Ni: 26%, Al: 13%, Cu: 3%) 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 crystalline structure of these powders was analyzed with X-ray diffraction (XRD) (Philips XÕPert PRO SUPER, Cu Ka radiation), while the particle morphology of the powders was studied using a scanning electron microscope (HITACHI X-650). Several electrode laminates were fabricated from the pristine and treated Alnico powders with tape-casting technique. Copper foil was used as the current collector. 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 Matster 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
Table 1 Five Alnico electrodes with different compositions and temperatures for Alnico powder treatment Electrode
I II III IV V
Treatment temperature (°C) 400 550 650 550
AlNiCo wt.(%)
CB wt.(%)
PVDF wt.(%)
97 84 84 84 97
0 8 8 8 0
3 8 8 8 3
Fig. 1. Sketch map of susceptibility measurements instrument.
polypropylene membrane was used as separator. These cells were cycled galvanostatically between 0 and 3 V at room temperature on a multi-channel battery test system (NEWARE BTS-610) to analyze the electrochemical response. The susceptibility measurements were carried out at 25 °C with a home-built instrument consisted of primary and secondary copper-wire loops, a 3325A function generator, a SR530 lock-in amplifier and a control computer (Fig. 1). The real part (v0 ) and imaginary part (v00 ) of the ac susceptibility of a sample at a certain frequency can be measured. The frequency of the ac signal in this study was set at 1000 Hz. Before measuring ac susceptibility and XRD 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 amount of lithium in electrodes was analyzed by inductively coupled plasma atomic emission spectrometer (ICP-AES) (Atomscan Advantage Inc.).
3. Results and discussion Fig. 2 shows the XRD patterns of AlNiCo powders before and after oxidation treatment at various temperatures. It is clear that the Alnico powder shows a
J.L. Shui et al. / Electrochemistry Communications 6 (2004) 33–38
Fig. 2. XRD patterns of AlNiCo powder before and after heat-treated at various temperatures in air: (a) pristine powder, (b) 400 °C for 1 h, (c) 550 °C for 1 h and (d) 650 °C for 3 h.
diffraction pattern with the strongest peak at 44.7°. After oxidation, the powders contain corresponding phases of metal oxides including Fe3 O4 , Fe2 O3 and NiO. With the increase in the treatment temperature, the intensity of the diffraction peaks from the oxides becomes stronger. Nevertheless, the peak at 44.7° that is from the diffraction of Alnico is always there, meaning that there
35
exists a 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 5 to 30 lm. Comparing the morphology of the powders before and after oxidation (Fig. 3(b) and (d)) we can see that a coarse film is formed on the particle surface after oxidation treatment. 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 and NiO while the core is magnetic Alnico. The voltage profile of Alnico/Li cell (Fig. 4) shows a rather reversible charge–discharge behavior between 0 and 3.0 V. Nevertheless, its specific capacity is very low, only about 1 mAh/g. Because the charge–discharge curves are similar to those of transition-metal oxides (MO, where M is Fe, Co, Ni, or Cu) [6], It is believed that lithium is not intercalated into the lattice of Alnico but rather into an oxide layer on the surface. This oxide layer is likely formed by a mild oxidation when the Alnico powder is exposed to ambient atmosphere during the preparation of electrode laminate. The composition of this thin surface layer should be the oxidation products of alloy components, such as Fe3 O4 , Fe2 O3 and
Fig. 3. SEM pictures of AlNiCo pristine powder (a), (b) before and (c), (d) after heat-treated at 400 °C in air for 1 h.
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J.L. Shui et al. / Electrochemistry Communications 6 (2004) 33–38
15th
10th
3.0
2nd
2.5
2.5
1st
Voltage (V)
Voltage (V)
3.0
10th 5th 2nd
2.0 1.5 1.0
1st
2.0 1.5 1.0
0.5
0.5
0.0
0.0
1st
15th 10th 5th 2nd
0.0
0.5
1.0 1.5 Capacity (mAh/g)
1st
10th
0
2.0
Fig. 4. Voltage profile of a cell: the electrode (I)/Li, current density was 0.02 mA/cm2 , the cycle numbers are indicated.
NiO. 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. The ac susceptibility of Alnico electrode at 25 °C and the amount of lithium consumed by per unit weight of Alnico alloy at different depth-of-discharge (DOD) are shown in Table 2. Because the trend of change is the primary concern, relative susceptibility against that of pristine Alnico is used here. It can be seen that the ac susceptibility of electrode decreases steadily 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 Alnico core. When lithium is ‘‘pumped’’ into the metal oxides layer on the surface of Alnico grains during the discharge step, the metal oxides are reduced to form metal nanoparticles [7]. These nanoparticles actually constitute a metallic shielding layer on individual Alnico 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 measured is decreased with the lithium intercalation (Table 2). Nevertheless, owing to the small capacity of pristine Alnico powder, it cannot be directly used as an electrode material in practical applications. To improve the capacity of cells, partial oxidation treatments can be applied to pristine Alnico powder (Table 1). After the treatments, the amount of surface
50
2nd
100 150 200 Capacity (mAh/g)
250
Fig. 5. Voltage profile of a cell: CB/Li, current density was 0.2 mA/ cm2 , the cycle numbers are indicated.
oxides layer is increased. In addition, carbon black (CB) as a conductive additive can be added into the electrode to lower the cell impedance. Fig. 5 shows the voltage profile of a CB/Li cell. Hence, a reversible capacity of about 150 mAh/g can be contributed from CB in the potential range between 0 and 3.0 V vs. Li. This contribution should be taken into account when calculating the capacity of treated Alnico electrodes in Table 1. Fig. 6 shows voltage profile and specific capacity of the cells using treated Alnico electrodes against lithium. It is found that the voltage profile (Fig. 6(a)) is similar to the profiles of transition-metal oxides (MO, where M is Fe, Co, Ni, or Cu) electrode [7–9]. The first intercalation capacity reaches about 600 mAh/g. After suffering irreversible capacity losses in the subsequent 20 or so cycles, a rather reversible capacity of about 180 mAh/g can be achieved. Furthermore, the specific capacity of treated Alnico powder increases with increasing the extent of oxidation (Fig. 6(b), curves for the electrodes II–IV). These results also support the core-shell model discussed above. Besides, the effect of CB is also confirmed by comparing the capacity of (electrode III)/Li cell with that of (electrode V)/Li cell. The capacity increases from 40 to 140 mAh/g after the addition of CB in electrodes for these two cells. Nevertheless, a detailed study on the effect of CB addition is beyond the scope of this paper. Fig. 7 shows the evolution of the phase structure of the electrode IV during cycling. As lithium reacts continuously with heat-treated Alnico during the discharge step, we observed a steady decrease in the intensity of diffraction peaks for metal oxides until these peaks completely disappear at the voltage of 0 V. On the other
Table 2 The relative ac susceptibility of pristine Alnico electrode (Type I in Table 1) at 25 °C and ICP analysis results at three different depth-of-discharge Voltage (V) Relative ac susceptibilitya v=v0 ICP analysis of electrode (I) mgLi/g
2.19 100% 0
0.65 92.8% 0.47
0.2 88.0% 0.65
a All ac susceptibility data listed are relative value of the real part (v) of susceptibility measured at 1000 Hz against that of a pristine Alnico electrode (v0 ) at potential 2.19 V vs. Li. At this potential, no lithium is in the electrode.
J.L. Shui et al. / Electrochemistry Communications 6 (2004) 33–38 20th 30th
3.0
10th
5nd
Table 3 The relative ac susceptibility of electrode IV at different depth-ofdischarge
1st
2.5
Voltage (V) Relative ac susceptibilitya v=v0
Voltage (V)
2.0
2.50 90.4%
0.73 70.7%
0.08 52.7%
a All ac susceptibility data listed are relative value of the real part (v) of susceptibility measured at 1000 Hz against that of a pristine Alnico electrode (v0 ) at potential 2.19 V vs. Li. At this potential, no lithium is in the electrode.
1.5 1.0 0.5 1st
0.0
30th 20th 10th 5nd
0 (a)
change as in the Alnico/Li cell (Table 2). Because of the thicker surface oxide layer and thus stronger shielding effect, the ac susceptibility measured here is less than that for untreated Alnico electrode. Considering the relatively high capacity of these treated Alnico powders, they are possibly used as magnetic anode materials in lithium-ion batteries.
100 200 300 400 500 600 Capacity (mAh/g)
400˚C/1h 550˚C/1h 650˚C/3h 550˚C/1h
600
capacity (mAh/g)
37
500 400 300
4. Conclusions
200 100 0
0
5
(b)
10 15 20 cycle number
25
30
Fig. 6. (a) Voltage profile of a cell: electrode (IV)/Li, (b) the comparison of the specific capacity fading for the cell: electrode (II: j, III: d, IV: N, V: s)/Li. Note: the capacity contribution of conductive additive-CB was already removed.
d
The feasibility of commercial Alnico alloy powder as a possible magnetic electrode in lithium battery is investigated. It is found that the pristine Alnico is nearly inert to lithium intercalation. After partial oxidation treatments, however, the treated Alnico may be possibly used as magnetic anode materials with the first intercalation capacity of about 600 mAh/g and rather reversible capacity up to 180 mAh/g. The treated Alnico powders have a core-shell structure, where Alnico alloy is located in the core of grains while a layer of metal oxides covers the core. The remaining energy of a cell can be monitored by measuring the change of susceptibility. The mechanism of lithium intercalation in the treated Alnico electrode is due to the reaction of lithium with the surface oxide layer.
c
Acknowledgements
b
a 30
40
50
60
70
80
Fig. 7. XRD patterns of electrodes (IV) collected at various states of discharge on the first cycle: (a) pristine powder treated at 650 °C in air for 3 h, (b) discharged to 1.0 V, (c) discharged to 0.6 V and (d) discharged to 0 V.
hand, the peak at 44.7° belonging to Alnico remains unchanged in these diffraction patterns. Combining with the SEM analysis (Fig. 3), we can conclude that the coreshell structure model is validated. The ac susceptibility of electrode IV at different cut-off voltage is shown in Table 3. This result shows the same trend of susceptibility
This study was supported by 100 Talents Program of Academia Sinica. The authors are also grateful to Hangzhou Permanent Magnet Group for providing commercial Alnico powder.
References [1] C. Masquelier, M. Tabuchi, K. Ado, R. Kanno, Y. Kobayashi, Y. Maki, O. Nakamura, J.B. Goodenough, J. Solid State Chem. 123 (1996) 255. [2] A.S. Wills, N.P. Raju, J.E. Greedan, Chem. Mater. 11 (1999) 1510. [3] J. Chouvin, C. Branci, J. Sarradin, J. Olivier-Fourcade, J.C. Jumas, B. Simon, Ph. Biensan, J. Power Sources 81–82 (1999) 277–281. [4] K.D. Kepler, J.T. Vaughey, M.M. Thackeray, J. Power Sources 81–82 (1999) 383. [5] G-J. Jeong, Y.U. Kim, H-J. Sohn, T. Kang, J. Power Sources 101 (2001) 201.
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[6] J.O. Besenhard, M. Hess, P. Komenda, Solid State Ionics 40–41 (1990) 525. [7] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J-M. Tarascon, Nature 407 (2000) 496.
[8] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J-M. Tarascon, J. Power Sources 97–98 (2001) 235. [9] G.X. Wang, Y. Chen, K. Konstantinow, M. Lindsay, H.K. Liu, S.X. Dou, J. Power Sources 109 (2002) 142.
Exploration of Alnico alloy as a magnetic electrode material for lithium-ion batteries J.L. Shui, S.L. Zhang, W.L. Liu, Y. Yu, G.S. Jiang, 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 17 September 2003; received in revised form 6 October 2003; accepted 7 October 2003 Published online: 4 November 2003
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. Magnetic Alnico alloy was investigated as an anode material for rechargeable lithium batteries. Both pristine and partially oxidized Alnico powders were made into electrodes. Structural characterization was performed on the Alnico electrodes by means of X-ray diffraction, scanning electron microscopy, and inductively coupled plasma analysis. The electrochemical cycling of cells made of the Alnico electrodes against lithium was measured. The first lithium intercalation capacity of a treated Alnico can be up to about 600 mAh/g, while a rather reversible capacity of up to 180 mAh/g can be obtained. The capacity increases with the extent of oxidation of Alnico. It was observed that the ac susceptibility of an electrode changes with depth-of-discharge (DOD). We have proposed an electrode model with a core-shell structure, which can explain this susceptibility vs. DOD relationship. Ó 2003 Elsevier B.V. All rights reserved. Keywords: State-of-charge; Magnetic susceptibility; Lithium batteries; Electrode; Oxidation
1. Introduction The last decade of the 20th century has witnessed the rapid development and commercial success of rechargeable lithium-ion batteries as the excellent energy conversion and storage system. These batteries of various forms and sizes are now widely used to power portable electronic devices such as cellular phones and notebook computers. Battery blocks with larger size and greater integration are also very promising to be used in automobiles in the near future. However, a practical issue has been puzzling the battery manufacturers these years; namely, we are lack of an accurate indicator of remaining energy of cells. At this moment, the opencircuit-voltage (OCV) of a cell is usually taken for this purpose because it can indicate to the users approximate state-of-charge (SOC) of a cell. Nevertheless, the correctness of this alarming approach is based upon two *
Corresponding author. Fax: +86-551-3602940. E-mail address: [email protected] (C.H. Chen).
1388-2481/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2003.10.006
assumptions, i.e. the relationship between OCV and SOC keeps unchanged, and meanwhile, there is no capacity fading. Obviously, these two assumptions are not valid for cells that have been used for some time already. On the other hand, erroneous indication of remaining energy may lead to mis-alarming, overcharge, over-discharge, or even safety problems. Therefore, it is necessary to search for alternative indicators. Basically, a correct indication of the remaining energy in a cell is equivalent to probing effectively the amount of active delithiated cathode material or lithiated anode material. Since it is very unlikely to really monitor the mass variation of either electrode material, we need to correlate the change of remaining energy with a certain physical property that is easily measurable. As a cathode material always contains at least one transition-metal element, one might consider the correlation of the change of oxidation-state of the transition metal with its magnetic property during the charge– discharge cycling. For example, lithium manganese oxide spinel (LiMn2 O4 ) is antiferromagnetic while
34
J.L. Shui et al. / Electrochemistry Communications 6 (2004) 33–38
Li4 Mn5 O12 is ferromagnetic [1]. However, the total magnetic susceptibility of the cathode of a cell is too small to be detected conveniently by a simple measurement device, especially at ambient temperature [2]. Consequently, it is very difficult to use the very minor change of the susceptibility as the signature of remaining energy. On the other hand, if we can find a suitable anode material that has strong magnetic property, we may be able to detect the remaining energy of a cell by monitoring the change of magnetic susceptibility of a cell. This study is an exploratory effort toward this direction. Since lithium can be intercalated into some metals such as Sn, Al, Mg, Zn, Ni, Mo, Fe, Cu or alloys containing these metals [3–5] at potentials under 1 V vs. Li, they are potential anode materials in place of carbonaceous materials (graphite, hard carbon, etc.). In particular, aluminum–lithium intermetallic compound LiAl is found to be a good anode material with large specific capacity and relatively small volume change (97%), while lithiation of many other metals could result in very substantial volume change for instance 676% for Li22 Sn5 [6]. Because Alnico, a permanent magnetic alloy, also contains aluminum, it is natural to explore the possibility of using it as a magnetic anode material. In this paper, we will show that the pristine Alnico 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 susceptibility change of cells using the oxidized Alnico as anode material, we propose a possible way to monitor the remaining energy of a cell through its magnetic signature.
2. Experimental We selected one kind of commercial magnetic Alnico powder (Fe: 58%, Ni: 26%, Al: 13%, Cu: 3%) 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 crystalline structure of these powders was analyzed with X-ray diffraction (XRD) (Philips XÕPert PRO SUPER, Cu Ka radiation), while the particle morphology of the powders was studied using a scanning electron microscope (HITACHI X-650). Several electrode laminates were fabricated from the pristine and treated Alnico powders with tape-casting technique. Copper foil was used as the current collector. 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 Matster 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
Table 1 Five Alnico electrodes with different compositions and temperatures for Alnico powder treatment Electrode
I II III IV V
Treatment temperature (°C) 400 550 650 550
AlNiCo wt.(%)
CB wt.(%)
PVDF wt.(%)
97 84 84 84 97
0 8 8 8 0
3 8 8 8 3
Fig. 1. Sketch map of susceptibility measurements instrument.
polypropylene membrane was used as separator. These cells were cycled galvanostatically between 0 and 3 V at room temperature on a multi-channel battery test system (NEWARE BTS-610) to analyze the electrochemical response. The susceptibility measurements were carried out at 25 °C with a home-built instrument consisted of primary and secondary copper-wire loops, a 3325A function generator, a SR530 lock-in amplifier and a control computer (Fig. 1). The real part (v0 ) and imaginary part (v00 ) of the ac susceptibility of a sample at a certain frequency can be measured. The frequency of the ac signal in this study was set at 1000 Hz. Before measuring ac susceptibility and XRD 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 amount of lithium in electrodes was analyzed by inductively coupled plasma atomic emission spectrometer (ICP-AES) (Atomscan Advantage Inc.).
3. Results and discussion Fig. 2 shows the XRD patterns of AlNiCo powders before and after oxidation treatment at various temperatures. It is clear that the Alnico powder shows a
J.L. Shui et al. / Electrochemistry Communications 6 (2004) 33–38
Fig. 2. XRD patterns of AlNiCo powder before and after heat-treated at various temperatures in air: (a) pristine powder, (b) 400 °C for 1 h, (c) 550 °C for 1 h and (d) 650 °C for 3 h.
diffraction pattern with the strongest peak at 44.7°. After oxidation, the powders contain corresponding phases of metal oxides including Fe3 O4 , Fe2 O3 and NiO. With the increase in the treatment temperature, the intensity of the diffraction peaks from the oxides becomes stronger. Nevertheless, the peak at 44.7° that is from the diffraction of Alnico is always there, meaning that there
35
exists a 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 5 to 30 lm. Comparing the morphology of the powders before and after oxidation (Fig. 3(b) and (d)) we can see that a coarse film is formed on the particle surface after oxidation treatment. 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 and NiO while the core is magnetic Alnico. The voltage profile of Alnico/Li cell (Fig. 4) shows a rather reversible charge–discharge behavior between 0 and 3.0 V. Nevertheless, its specific capacity is very low, only about 1 mAh/g. Because the charge–discharge curves are similar to those of transition-metal oxides (MO, where M is Fe, Co, Ni, or Cu) [6], It is believed that lithium is not intercalated into the lattice of Alnico but rather into an oxide layer on the surface. This oxide layer is likely formed by a mild oxidation when the Alnico powder is exposed to ambient atmosphere during the preparation of electrode laminate. The composition of this thin surface layer should be the oxidation products of alloy components, such as Fe3 O4 , Fe2 O3 and
Fig. 3. SEM pictures of AlNiCo pristine powder (a), (b) before and (c), (d) after heat-treated at 400 °C in air for 1 h.
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J.L. Shui et al. / Electrochemistry Communications 6 (2004) 33–38
15th
10th
3.0
2nd
2.5
2.5
1st
Voltage (V)
Voltage (V)
3.0
10th 5th 2nd
2.0 1.5 1.0
1st
2.0 1.5 1.0
0.5
0.5
0.0
0.0
1st
15th 10th 5th 2nd
0.0
0.5
1.0 1.5 Capacity (mAh/g)
1st
10th
0
2.0
Fig. 4. Voltage profile of a cell: the electrode (I)/Li, current density was 0.02 mA/cm2 , the cycle numbers are indicated.
NiO. 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. The ac susceptibility of Alnico electrode at 25 °C and the amount of lithium consumed by per unit weight of Alnico alloy at different depth-of-discharge (DOD) are shown in Table 2. Because the trend of change is the primary concern, relative susceptibility against that of pristine Alnico is used here. It can be seen that the ac susceptibility of electrode decreases steadily 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 Alnico core. When lithium is ‘‘pumped’’ into the metal oxides layer on the surface of Alnico grains during the discharge step, the metal oxides are reduced to form metal nanoparticles [7]. These nanoparticles actually constitute a metallic shielding layer on individual Alnico 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 measured is decreased with the lithium intercalation (Table 2). Nevertheless, owing to the small capacity of pristine Alnico powder, it cannot be directly used as an electrode material in practical applications. To improve the capacity of cells, partial oxidation treatments can be applied to pristine Alnico powder (Table 1). After the treatments, the amount of surface
50
2nd
100 150 200 Capacity (mAh/g)
250
Fig. 5. Voltage profile of a cell: CB/Li, current density was 0.2 mA/ cm2 , the cycle numbers are indicated.
oxides layer is increased. In addition, carbon black (CB) as a conductive additive can be added into the electrode to lower the cell impedance. Fig. 5 shows the voltage profile of a CB/Li cell. Hence, a reversible capacity of about 150 mAh/g can be contributed from CB in the potential range between 0 and 3.0 V vs. Li. This contribution should be taken into account when calculating the capacity of treated Alnico electrodes in Table 1. Fig. 6 shows voltage profile and specific capacity of the cells using treated Alnico electrodes against lithium. It is found that the voltage profile (Fig. 6(a)) is similar to the profiles of transition-metal oxides (MO, where M is Fe, Co, Ni, or Cu) electrode [7–9]. The first intercalation capacity reaches about 600 mAh/g. After suffering irreversible capacity losses in the subsequent 20 or so cycles, a rather reversible capacity of about 180 mAh/g can be achieved. Furthermore, the specific capacity of treated Alnico powder increases with increasing the extent of oxidation (Fig. 6(b), curves for the electrodes II–IV). These results also support the core-shell model discussed above. Besides, the effect of CB is also confirmed by comparing the capacity of (electrode III)/Li cell with that of (electrode V)/Li cell. The capacity increases from 40 to 140 mAh/g after the addition of CB in electrodes for these two cells. Nevertheless, a detailed study on the effect of CB addition is beyond the scope of this paper. Fig. 7 shows the evolution of the phase structure of the electrode IV during cycling. As lithium reacts continuously with heat-treated Alnico during the discharge step, we observed a steady decrease in the intensity of diffraction peaks for metal oxides until these peaks completely disappear at the voltage of 0 V. On the other
Table 2 The relative ac susceptibility of pristine Alnico electrode (Type I in Table 1) at 25 °C and ICP analysis results at three different depth-of-discharge Voltage (V) Relative ac susceptibilitya v=v0 ICP analysis of electrode (I) mgLi/g
2.19 100% 0
0.65 92.8% 0.47
0.2 88.0% 0.65
a All ac susceptibility data listed are relative value of the real part (v) of susceptibility measured at 1000 Hz against that of a pristine Alnico electrode (v0 ) at potential 2.19 V vs. Li. At this potential, no lithium is in the electrode.
J.L. Shui et al. / Electrochemistry Communications 6 (2004) 33–38 20th 30th
3.0
10th
5nd
Table 3 The relative ac susceptibility of electrode IV at different depth-ofdischarge
1st
2.5
Voltage (V) Relative ac susceptibilitya v=v0
Voltage (V)
2.0
2.50 90.4%
0.73 70.7%
0.08 52.7%
a All ac susceptibility data listed are relative value of the real part (v) of susceptibility measured at 1000 Hz against that of a pristine Alnico electrode (v0 ) at potential 2.19 V vs. Li. At this potential, no lithium is in the electrode.
1.5 1.0 0.5 1st
0.0
30th 20th 10th 5nd
0 (a)
change as in the Alnico/Li cell (Table 2). Because of the thicker surface oxide layer and thus stronger shielding effect, the ac susceptibility measured here is less than that for untreated Alnico electrode. Considering the relatively high capacity of these treated Alnico powders, they are possibly used as magnetic anode materials in lithium-ion batteries.
100 200 300 400 500 600 Capacity (mAh/g)
400˚C/1h 550˚C/1h 650˚C/3h 550˚C/1h
600
capacity (mAh/g)
37
500 400 300
4. Conclusions
200 100 0
0
5
(b)
10 15 20 cycle number
25
30
Fig. 6. (a) Voltage profile of a cell: electrode (IV)/Li, (b) the comparison of the specific capacity fading for the cell: electrode (II: j, III: d, IV: N, V: s)/Li. Note: the capacity contribution of conductive additive-CB was already removed.
d
The feasibility of commercial Alnico alloy powder as a possible magnetic electrode in lithium battery is investigated. It is found that the pristine Alnico is nearly inert to lithium intercalation. After partial oxidation treatments, however, the treated Alnico may be possibly used as magnetic anode materials with the first intercalation capacity of about 600 mAh/g and rather reversible capacity up to 180 mAh/g. The treated Alnico powders have a core-shell structure, where Alnico alloy is located in the core of grains while a layer of metal oxides covers the core. The remaining energy of a cell can be monitored by measuring the change of susceptibility. The mechanism of lithium intercalation in the treated Alnico electrode is due to the reaction of lithium with the surface oxide layer.
c
Acknowledgements
b
a 30
40
50
60
70
80
Fig. 7. XRD patterns of electrodes (IV) collected at various states of discharge on the first cycle: (a) pristine powder treated at 650 °C in air for 3 h, (b) discharged to 1.0 V, (c) discharged to 0.6 V and (d) discharged to 0 V.
hand, the peak at 44.7° belonging to Alnico remains unchanged in these diffraction patterns. Combining with the SEM analysis (Fig. 3), we can conclude that the coreshell structure model is validated. The ac susceptibility of electrode IV at different cut-off voltage is shown in Table 3. This result shows the same trend of susceptibility
This study was supported by 100 Talents Program of Academia Sinica. The authors are also grateful to Hangzhou Permanent Magnet Group for providing commercial Alnico powder.
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