- Email: [email protected]
New mixed transition metal oxysalts as negative electrode materials for lithium-ion batteries
Solid State Ionics 225 (2012) 518–521
Contents lists available at SciVerse ScienceDirect
Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
New mixed transition metal oxysalts as negative electrode materials for lithium-ion batteries B. León, C. Pérez Vicente, J.L. Tirado ⁎ Laboratorio de Química Inorgánica, Universidad de Córdoba, Edificio C3, Campus de Rabanales, Córdoba, Spain
a r t i c l e
i n f o
Article history: Received 8 September 2011 Received in revised form 15 December 2011 Accepted 16 December 2011 Available online 5 January 2012 Keywords: Lithium batteries Conversion electrode materials Transition metal oxalates
a b s t r a c t Different solid solutions in the MxM´1-xC2O4 ·2H2O (MM′: FeMn, CoMn; 0 ≤ x ≤ 1) system have been prepared in the form of nanoribbons by a reverse micelles method. The resulting di-hydrated oxalates form solid solutions that crystallize in two forms: the α monoclinic form with space group C2/c and the β orthorhombic form with space group Cccm. Anhydrous mixed oxalates are used as high-capacity (ca. 600 mA h g− 1) lithium storage materials, with good capacity retention after 75 cycles. The lithium storage mechanism is complex and involves conversion reactions together with double layer charge storage and reversible reactions with the electrolyte. The low temperature synthesis of these materials makes them attractive materials for low-cost Li-ion electrodes. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Ten years ago, it was difficult to predict the huge number of compounds studied as new active electrode materials in lithium batteries. Among them, fluorides [1], oxides [2,3], nitrides [4] phosphides [5] and oxysalts [6–8] have shown promising responses. Also, the development of suitable synthesis procedures to produce particle size and morphologies in the nano scale, thus improving the electrode performance, has taken place. Conversion electrode materials show a reduction of the metal ions to the metallic state together with the formation of lithium oxide, fluoride or oxysalts. Recently, the possibility of extending the range of new materials to submicrometric particles of transition metal oxysalt was scrutinized. The promising results on cobalt oxalate [7] are tarnished by the cost and environmental problems associated to cobalt, while the poorer response of the iron analogue [6] prompted us to evaluate solid solutions [8]. In this work, two mixed transition metal oxalate series with MxM´1-xC2O4 (MM`: CoMn and FeMn stoichiometries system have been compared. The structural and electrochemical characteristics of these solids as active electrode material in lithium test cells are discussed with a view to their possible application in the anode of future Li-ion batteries.
2. Experimental The synthesis of the mixed transition metal oxalates was carried out by reverse micelles method. First, two microemulsions (I and II) ⁎ Corresponding author. E-mail address: [email protected] (J.L. Tirado). 0167-2738/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2011.12.012
were obtained under an argon atmosphere. Microemulsion I contained cetyl-trimethylammonium bromide (CTAB) as the surfactant (16,76%), hexanol as the cosurfactant (13,9%), isooctane as the hydrocarbon phase (59,29%), and 0.3 M solution of the mixture of divalent metal ions in the desired proportion as the aqueous phase (10,05%). Microemulsion II has the same constituents as above except for having 0.3 M ammonium oxalate as the aqueous phase. In a second step, both microemulsions were slowly mixed and stirred. The resulting precipitate was separated by centrifugation followed by rinsing with a 1:1 mixture of methanol and chloroform. The different hydrated oxalates particles were subjected to a careful thermal decomposition at 200 °C under vacuum to yield anhydrous compound. X-ray diffraction (XRD) patterns were recorded on a Siemens D5000, using Cu Kα radiation and a graphite monochromator. Transmission electron microscopy (TEM) images were obtained with a JEOL 200CX microscope. Thermogravimetric (TG) and differential thermal analysis (DTA) curves were obtained in a SHIMADZU DTG-60AH instrument in both air and argon atmosphere. 57Fe Mössbauer spectra were recorded under the classical constant acceleration working mode using a conventional spectrometer. The source was 57Co(Rh). The velocity scale was calibrated using the sextet of a high-purity iron foil. All spectra were recorded at room temperature. Spectra were fitted to Lorentzian profiles by the least-squares method. For electrochemical tests, two-electrode Swagelok-type lithium test cells were used. The electrodes were prepared by blending the powdered active material (60%) with carbon black (30%) and polyvinylidene fluoride (10%) dissolved in N-methyl-pyrrolidone. The slurry was cast onto a Cu foil and vacuum-dried at 120 °C. As the negative electrode, a lithium foil was used. The electrolyte was a solution of ethylene
B. León et al. / Solid State Ionics 225 (2012) 518–521
carbonate-diethyl carbonate in 1:1 weight proportion, including 1 M LiPF6, supported by a porous glass-paper disk. The cells were assembled/disassembled in an Ar-filled glove box (H2O, O2 b 1 ppm). Galvanostatic charge/discharge cycles were carried out at different C rates (C = 1 Li· h − 1 · mol − 1, i.e.,≈ 0.22 mA·cm − 2 current density). Step potential electrochemical spectra were obtained at different sweep rates.
Dihydrated transition metal oxalates crystallize in the α monoclinic form with space group C2/c and the β orthorhombic form with space group Cccm [9,10]. The MxM´1-xC2O4 ·2H2O systems obtained in this work showed an orthorhombic lattice. Fig. 1, shows the XRD pattern of a typical example of CoC2O4·2H2O and mixed metal oxalates. The patterns of all compounds presented similar reflections and they are in agreement with the reflections of MnC2O4 ·2H2O, FeC2O4·2H2O and CoC2O4 ·2H2O from previous works [6,7,11]. The orthorrhombic unit cell parameters are included in Table 1. While a and b increase with manganese content, the changes in c are not significant. The overall increase in unit cell dimension is in agreement with the higher ionic radius of Mn 2 + vs. Co 2 +. In order to estimate the content of crystallization water in the mixed transition metals oxalates, thermogravimetric (TG) curves were recorded. These results confirmed that the new mixed oxalates also crystallize with two water molecules. In addition, the TG results were helpful to locate the best temperature and experimental conditions at which the two water molecules are released from MxM´1xC2O4·2H2O without breakage of the oxalate anion. After the initial weight loss of adsorbed water below 200 °C, the calculated weight loss agrees to the theoretical value for each compound for the release
Co0.5Mn0.5C2O4
CoC2O4
Co0.5Mn0.5C2O4·2H2O
40
Scattering angle / °2θ Fig. 1. XRD pattern for CoxMn1-xC2O4 · 2H2O.
602 026
315, 223
224
022
30
206
CoC2O4·2H2O
400, 312
112
004
111, 202
Co0.7Mn0.3C2O4·2H2O
20
Table 1 Unit cell parameters (Å) for CoxMn1-xC2O4 · 2H2O. x
a
b
c
1.0 0.7 0.5
11.931 (7) 12.03 (1) 12.067 (7)
5.427 (3) 5.471 (5) 5.542 (2)
15.61 (1) 15.59 (2) 15.62 (1)
of two water molecules, as described in Eq. (1). The weight lost is accompanied by an endothermic DTA peak at ca. 160 °C
3. Results and discussion
10
519
50
Mx M′1x C2 O4 2H2 O→Mx M′1x C2 O4 þ 2H2 O
ð1Þ
Following dehydration, a second weight loss effect in the 180–270 °C temperature range was visible in the TG curves. This effect is ascribable to oxalate decomposition, according to either Eq. (2) in an inert atmosphere or Eq. (3) in the presence of oxygen. Globally, these processes account for the main exotherm in the DTA curve and the weight loss at ca. 260 °C [12]. MC2 O4 →M þ 2CO2
ð2Þ
MC2 O4 →MO þ CO þ CO2
ð3Þ
Previously to the electrochemical experiments, MxM1-xC2O4·2H2O were dehydrated at 200 °C under vacuum. In order to confirm that the dehydration of iron oxalate took place without breakage of the oxyanion, the XRD patterns of the dehydrated mixed metal oxalates were recorded (Fig. 1). The results were congruent with a poorly crystalline solid with divalent metals oxalate composition [11]. Moreover, the TG–DTA experiments of the dehydrated materials exactly reproduced the second part of the curves corresponding to the dihydrates. TEM micrographs of the dehydrated micellar oxalates showed ultrafine particles (Fig. 2) with elongated shapes. Nanoribbons around 30 to 80 nm wide are observed for all samples. Every product dehydrated at 200 °C showed a complex porous texture with pores (ca. 5 nm) probably created during water release. The mesoporous material is then obtained by a top-down approach, which has been recently found in conversion electrodes [13]. Previous studies on the 57Fe Mössbauer spectra of iron-containing oxalates [7,14,15] showed a single quadrupole split signal with an isomer shift of ca. 1.20 mm s − 1 and quadrupole splitting of ca. 1.62 mm/s for hydrated oxalates, which is characteristic of Fe2 + ions in an environment with electric field gradient. For anhydrous oxalates, the Mössbauer spectra [14,15] showed a main signal with values of isomer shift consistent with Fe2 + in high-spin configuration, which are also indicative of little changes in the oxidation state of iron during the dehydration reaction. For anhydrous cobalt oxalate, XANES measurements showed that the Co2 + ions were preserved after dehydration [6]. From these previous results, it is expected that the anhydrous metal oxalates studied here contain the transition metal elements in a divalent oxidations state. The galvanostatic discharge/charge cycling experiments of these materials in lithium test cells showed similar charge–discharge profiles (see Fig. 3 as a representative example) and high capacities for the first cycle [6,7,14]. The electrochemical reaction of oxalates with lithium took place with an initial abrupt decrease of the voltage to 1.2 V followed by a smooth decrease in the voltage until the end of the discharge (0 V), as shown in Fig. 3. XRD patterns of the fully reduced electrode showed an X-ray amorphous product. As commonly found in conversion electrodes [16], the initial irreversible capacity is large. Further cycles displayed reversible capacity values above 600 mA h g− 1 in several samples, much higher than theoretical capacity of graphite (372 mA h g− 1).
520
B. León et al. / Solid State Ionics 225 (2012) 518–521
1 µm
Fig. 2. TEM images of Fe0.5Mn0.5C2O4 at different magnifications.
Voltage vs. Li+/Li / V
3 20 5 2 1
2 2 1
1 20 5
0 0
200
400
600
Capacity /
800
1000
1200
mAhg-1
Fig. 3. Voltage profiles for lithium test cells using Fe0.5Mn0.5C2O4 electrode.
a
b 1200
Fe0.8Mn0.2C2O4
Co0.7Mn0.3C2O4
1000
CoC2O4 MnC2O4
800 600 400 200 0
1200
Co0.5Mn0.5C2O4
Capacity / mAhg-1
Capacity / mAhg-1
Fig. 4 shows the cycling experiments at 2C rate for the manganese–cobalt and manganese–iron series of dehydrated oxysalts. As compared with iron or cobalt oxalates, the reversible capacity of the manganese compound is significantly lower, although capacity retention is better. In mixed oxalates, the initial reversible capacities increase on decreasing manganese content. The incorporation of manganese neither increases the reversible capacity of pure iron and cobalt oxalates, nor improves the capacity retention as of pure manganese oxalate. However, intermediate compositions, such as Co0.7Mn0.3C2O4 show a good compromise between both parameters, retaining capacities above 600 mAhg − 1 for more than 50 cycles. The values are still lower than those of the pure cobalt oxalate compound but the reduction in cobalt content could be useful to reduce production costs and/or toxicity keeping simultaneously the good capacity retention upon cycling shown by the pure iron oxalate (Fig. 5).
0
10
20
30
40
Cycles / n
50
60
70
1000
Fe0.5Mn0.5C2O4 FeC2O4
800
MnC2O4
600 400 200 0
0
10
20
30
40
50
60
Cycles / n
Fig. 4. Cycling performance for a) manganese–cobalt oxalates and b) manganese–iron oxalates at 2 °C rate.
70
B. León et al. / Solid State Ionics 225 (2012) 518–521
Current contribution / %
100
4. Conclusions The main conclusion of this study is that first-row transition metal mixed oxalates are potential candidates for the active material of the negative electrode of lithium-ion batteries. The low temperature dehydration synthesis of these solids makes them an inexpensive option to obtain high-capacity active materials. The best electrochemical performance was achieved for manganese–cobalt oxalates where high capacity values (ca. 600 mA h g − 1) are reached, with a very good capacity retention after 75 cycles.
75
50
25
0
521
Acknowledgments The authors are indebted to MICINN (MAT2008-05880) and Junta de Andalucía (FQM-288).
0.5
2
5
Sweep rate / mVs-1
10
20 Faradaic Capacitive
References
Fig. 5. Relative intensity of capacitive and faradic current contribution for Fe0.8Mn0.2C2O4.
For a theoretical full conversion reaction with 0 ≤ x ≤ 1, M or M′: Mn, Fe, Co þ
Mx M′1x C2 O4 þ 2Li þ 2e →xM þ ð1 xÞM′ þ Li2 C2 O4
ð4Þ −1
range. The calculated capacity would be in the 364–374 mAhg To understand the origin of the extra capacity found in Fig. 4, we have carried out a study of the possible contribution of Faradic and non-Faradic (capacitance) to the global observed values. This study follows the model recently described by Brezesinski et al. for TiO2 electrodes [17]. An example is shown in Fig. 5. The faradic current presents the larger values during all the experiments, indicating that the reaction with lithium is favored especially for those experiments at low rate of discharge. The relative capacitive contribution presents the lower contribution at low sweep rate, although the values are increasing when the sweep rate is increasing. Still the Faradic contribution is higher than expected from Eq. (4). This result implies that part of the Faradic contribution comes from a different reaction such as side reactions with the electrolyte and/or the reversible formation of a polymer-like layer, as described by Tarascon group [2].
[1] F. Badway, A.N. Mansour, N. Pereira, J.F. Al-Sharab, F. Cosandey, I. Plitz, G.G. Amatucci, Chem. Mater. 19 (2007) 4129. [2] P. Poizot, S. Laruelle, S. Grugeon, J.M. Tarascon, J. Electrochem. Soc. 149 (2002) A1212. [3] R. Alcántara, M. Jaraba, P. Lavela, J.L. Tirado, Chem. Mater. 14 (2002) 2847. [4] J. Cabana, Z. Stoeva, J.J. Titman, D.H. Gregory, M.R. Palacin, Chem. Mater. 20 (2008) 1676. [5] R. Alcántara, J.L. Tirado, J.C. Jumas, L. Monconduit, J. Olivier-Fourcade, J. Power Sources 109 (2002) 308. [6] M.J. Aragón, B. León, C. Pérez Vicente, J.L. Tirado, A.V. Chadwick, A. Berko, S.Y. Beh, Chem. Mater. 21 (2009) 1834. [7] M.J. Aragón, B. León, C. Pérez-Vicente, J.L. Tirado, Inorg. Chem. 47 (2008) 10366. [8] M.J. Aragón, B. León, T. Serrano, C. Pérez Vicente, J.L. Tirado, J. Mater. Chem. 21 (2011) 10102. [9] R. Deyrieux, A. Peneloux, Bull. Soc. Chim. Fr. 8 (1969) 2675. [10] I. Sledzinska, A. Murasik, M. Piotrowski, Physica B (Amsterdam) 138 (1986) 315. [11] R.A. Brown, S.C. Bevan, J. Inorg. Nucl. Chem. 28 (1966) 387. [12] R.C. Mackenzie, Differential Thermal Analysis, vol. 1, Academic Press, 1970, chap. 13. [13] Y.S. Hu, Y.G. Guo, W. Sigle, S. Hore, P. Balaya, J. Maier, Nat. Mater. 5 (2006) 713. [14] M.J. Aragón, B. León, T. Serrano, C. Pérez Vicente, J.L. Tirado, J. Mater. Chem. 21 (2011) 10102. [15] M. Hermanek, R. Zboril, M. Mashlan, L. Machala, O. Schneeweiss, J. Mater. Chem. 16 (2006) 1273. [16] A.R. Armstrong, G. Armstrong, J. Canales, R. Garcia, P.G. Bruce, Adv. Mater. 17 (2005) 862. [17] T. Brezesinski, J. Wang, J. Polleux, B. Dunn, S.H. Tolbert, J. Am. Chem. Soc. 131 (2009) 1802.
Contents lists available at SciVerse ScienceDirect
Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
New mixed transition metal oxysalts as negative electrode materials for lithium-ion batteries B. León, C. Pérez Vicente, J.L. Tirado ⁎ Laboratorio de Química Inorgánica, Universidad de Córdoba, Edificio C3, Campus de Rabanales, Córdoba, Spain
a r t i c l e
i n f o
Article history: Received 8 September 2011 Received in revised form 15 December 2011 Accepted 16 December 2011 Available online 5 January 2012 Keywords: Lithium batteries Conversion electrode materials Transition metal oxalates
a b s t r a c t Different solid solutions in the MxM´1-xC2O4 ·2H2O (MM′: FeMn, CoMn; 0 ≤ x ≤ 1) system have been prepared in the form of nanoribbons by a reverse micelles method. The resulting di-hydrated oxalates form solid solutions that crystallize in two forms: the α monoclinic form with space group C2/c and the β orthorhombic form with space group Cccm. Anhydrous mixed oxalates are used as high-capacity (ca. 600 mA h g− 1) lithium storage materials, with good capacity retention after 75 cycles. The lithium storage mechanism is complex and involves conversion reactions together with double layer charge storage and reversible reactions with the electrolyte. The low temperature synthesis of these materials makes them attractive materials for low-cost Li-ion electrodes. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Ten years ago, it was difficult to predict the huge number of compounds studied as new active electrode materials in lithium batteries. Among them, fluorides [1], oxides [2,3], nitrides [4] phosphides [5] and oxysalts [6–8] have shown promising responses. Also, the development of suitable synthesis procedures to produce particle size and morphologies in the nano scale, thus improving the electrode performance, has taken place. Conversion electrode materials show a reduction of the metal ions to the metallic state together with the formation of lithium oxide, fluoride or oxysalts. Recently, the possibility of extending the range of new materials to submicrometric particles of transition metal oxysalt was scrutinized. The promising results on cobalt oxalate [7] are tarnished by the cost and environmental problems associated to cobalt, while the poorer response of the iron analogue [6] prompted us to evaluate solid solutions [8]. In this work, two mixed transition metal oxalate series with MxM´1-xC2O4 (MM`: CoMn and FeMn stoichiometries system have been compared. The structural and electrochemical characteristics of these solids as active electrode material in lithium test cells are discussed with a view to their possible application in the anode of future Li-ion batteries.
2. Experimental The synthesis of the mixed transition metal oxalates was carried out by reverse micelles method. First, two microemulsions (I and II) ⁎ Corresponding author. E-mail address: [email protected] (J.L. Tirado). 0167-2738/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2011.12.012
were obtained under an argon atmosphere. Microemulsion I contained cetyl-trimethylammonium bromide (CTAB) as the surfactant (16,76%), hexanol as the cosurfactant (13,9%), isooctane as the hydrocarbon phase (59,29%), and 0.3 M solution of the mixture of divalent metal ions in the desired proportion as the aqueous phase (10,05%). Microemulsion II has the same constituents as above except for having 0.3 M ammonium oxalate as the aqueous phase. In a second step, both microemulsions were slowly mixed and stirred. The resulting precipitate was separated by centrifugation followed by rinsing with a 1:1 mixture of methanol and chloroform. The different hydrated oxalates particles were subjected to a careful thermal decomposition at 200 °C under vacuum to yield anhydrous compound. X-ray diffraction (XRD) patterns were recorded on a Siemens D5000, using Cu Kα radiation and a graphite monochromator. Transmission electron microscopy (TEM) images were obtained with a JEOL 200CX microscope. Thermogravimetric (TG) and differential thermal analysis (DTA) curves were obtained in a SHIMADZU DTG-60AH instrument in both air and argon atmosphere. 57Fe Mössbauer spectra were recorded under the classical constant acceleration working mode using a conventional spectrometer. The source was 57Co(Rh). The velocity scale was calibrated using the sextet of a high-purity iron foil. All spectra were recorded at room temperature. Spectra were fitted to Lorentzian profiles by the least-squares method. For electrochemical tests, two-electrode Swagelok-type lithium test cells were used. The electrodes were prepared by blending the powdered active material (60%) with carbon black (30%) and polyvinylidene fluoride (10%) dissolved in N-methyl-pyrrolidone. The slurry was cast onto a Cu foil and vacuum-dried at 120 °C. As the negative electrode, a lithium foil was used. The electrolyte was a solution of ethylene
B. León et al. / Solid State Ionics 225 (2012) 518–521
carbonate-diethyl carbonate in 1:1 weight proportion, including 1 M LiPF6, supported by a porous glass-paper disk. The cells were assembled/disassembled in an Ar-filled glove box (H2O, O2 b 1 ppm). Galvanostatic charge/discharge cycles were carried out at different C rates (C = 1 Li· h − 1 · mol − 1, i.e.,≈ 0.22 mA·cm − 2 current density). Step potential electrochemical spectra were obtained at different sweep rates.
Dihydrated transition metal oxalates crystallize in the α monoclinic form with space group C2/c and the β orthorhombic form with space group Cccm [9,10]. The MxM´1-xC2O4 ·2H2O systems obtained in this work showed an orthorhombic lattice. Fig. 1, shows the XRD pattern of a typical example of CoC2O4·2H2O and mixed metal oxalates. The patterns of all compounds presented similar reflections and they are in agreement with the reflections of MnC2O4 ·2H2O, FeC2O4·2H2O and CoC2O4 ·2H2O from previous works [6,7,11]. The orthorrhombic unit cell parameters are included in Table 1. While a and b increase with manganese content, the changes in c are not significant. The overall increase in unit cell dimension is in agreement with the higher ionic radius of Mn 2 + vs. Co 2 +. In order to estimate the content of crystallization water in the mixed transition metals oxalates, thermogravimetric (TG) curves were recorded. These results confirmed that the new mixed oxalates also crystallize with two water molecules. In addition, the TG results were helpful to locate the best temperature and experimental conditions at which the two water molecules are released from MxM´1xC2O4·2H2O without breakage of the oxalate anion. After the initial weight loss of adsorbed water below 200 °C, the calculated weight loss agrees to the theoretical value for each compound for the release
Co0.5Mn0.5C2O4
CoC2O4
Co0.5Mn0.5C2O4·2H2O
40
Scattering angle / °2θ Fig. 1. XRD pattern for CoxMn1-xC2O4 · 2H2O.
602 026
315, 223
224
022
30
206
CoC2O4·2H2O
400, 312
112
004
111, 202
Co0.7Mn0.3C2O4·2H2O
20
Table 1 Unit cell parameters (Å) for CoxMn1-xC2O4 · 2H2O. x
a
b
c
1.0 0.7 0.5
11.931 (7) 12.03 (1) 12.067 (7)
5.427 (3) 5.471 (5) 5.542 (2)
15.61 (1) 15.59 (2) 15.62 (1)
of two water molecules, as described in Eq. (1). The weight lost is accompanied by an endothermic DTA peak at ca. 160 °C
3. Results and discussion
10
519
50
Mx M′1x C2 O4 2H2 O→Mx M′1x C2 O4 þ 2H2 O
ð1Þ
Following dehydration, a second weight loss effect in the 180–270 °C temperature range was visible in the TG curves. This effect is ascribable to oxalate decomposition, according to either Eq. (2) in an inert atmosphere or Eq. (3) in the presence of oxygen. Globally, these processes account for the main exotherm in the DTA curve and the weight loss at ca. 260 °C [12]. MC2 O4 →M þ 2CO2
ð2Þ
MC2 O4 →MO þ CO þ CO2
ð3Þ
Previously to the electrochemical experiments, MxM1-xC2O4·2H2O were dehydrated at 200 °C under vacuum. In order to confirm that the dehydration of iron oxalate took place without breakage of the oxyanion, the XRD patterns of the dehydrated mixed metal oxalates were recorded (Fig. 1). The results were congruent with a poorly crystalline solid with divalent metals oxalate composition [11]. Moreover, the TG–DTA experiments of the dehydrated materials exactly reproduced the second part of the curves corresponding to the dihydrates. TEM micrographs of the dehydrated micellar oxalates showed ultrafine particles (Fig. 2) with elongated shapes. Nanoribbons around 30 to 80 nm wide are observed for all samples. Every product dehydrated at 200 °C showed a complex porous texture with pores (ca. 5 nm) probably created during water release. The mesoporous material is then obtained by a top-down approach, which has been recently found in conversion electrodes [13]. Previous studies on the 57Fe Mössbauer spectra of iron-containing oxalates [7,14,15] showed a single quadrupole split signal with an isomer shift of ca. 1.20 mm s − 1 and quadrupole splitting of ca. 1.62 mm/s for hydrated oxalates, which is characteristic of Fe2 + ions in an environment with electric field gradient. For anhydrous oxalates, the Mössbauer spectra [14,15] showed a main signal with values of isomer shift consistent with Fe2 + in high-spin configuration, which are also indicative of little changes in the oxidation state of iron during the dehydration reaction. For anhydrous cobalt oxalate, XANES measurements showed that the Co2 + ions were preserved after dehydration [6]. From these previous results, it is expected that the anhydrous metal oxalates studied here contain the transition metal elements in a divalent oxidations state. The galvanostatic discharge/charge cycling experiments of these materials in lithium test cells showed similar charge–discharge profiles (see Fig. 3 as a representative example) and high capacities for the first cycle [6,7,14]. The electrochemical reaction of oxalates with lithium took place with an initial abrupt decrease of the voltage to 1.2 V followed by a smooth decrease in the voltage until the end of the discharge (0 V), as shown in Fig. 3. XRD patterns of the fully reduced electrode showed an X-ray amorphous product. As commonly found in conversion electrodes [16], the initial irreversible capacity is large. Further cycles displayed reversible capacity values above 600 mA h g− 1 in several samples, much higher than theoretical capacity of graphite (372 mA h g− 1).
520
B. León et al. / Solid State Ionics 225 (2012) 518–521
1 µm
Fig. 2. TEM images of Fe0.5Mn0.5C2O4 at different magnifications.
Voltage vs. Li+/Li / V
3 20 5 2 1
2 2 1
1 20 5
0 0
200
400
600
Capacity /
800
1000
1200
mAhg-1
Fig. 3. Voltage profiles for lithium test cells using Fe0.5Mn0.5C2O4 electrode.
a
b 1200
Fe0.8Mn0.2C2O4
Co0.7Mn0.3C2O4
1000
CoC2O4 MnC2O4
800 600 400 200 0
1200
Co0.5Mn0.5C2O4
Capacity / mAhg-1
Capacity / mAhg-1
Fig. 4 shows the cycling experiments at 2C rate for the manganese–cobalt and manganese–iron series of dehydrated oxysalts. As compared with iron or cobalt oxalates, the reversible capacity of the manganese compound is significantly lower, although capacity retention is better. In mixed oxalates, the initial reversible capacities increase on decreasing manganese content. The incorporation of manganese neither increases the reversible capacity of pure iron and cobalt oxalates, nor improves the capacity retention as of pure manganese oxalate. However, intermediate compositions, such as Co0.7Mn0.3C2O4 show a good compromise between both parameters, retaining capacities above 600 mAhg − 1 for more than 50 cycles. The values are still lower than those of the pure cobalt oxalate compound but the reduction in cobalt content could be useful to reduce production costs and/or toxicity keeping simultaneously the good capacity retention upon cycling shown by the pure iron oxalate (Fig. 5).
0
10
20
30
40
Cycles / n
50
60
70
1000
Fe0.5Mn0.5C2O4 FeC2O4
800
MnC2O4
600 400 200 0
0
10
20
30
40
50
60
Cycles / n
Fig. 4. Cycling performance for a) manganese–cobalt oxalates and b) manganese–iron oxalates at 2 °C rate.
70
B. León et al. / Solid State Ionics 225 (2012) 518–521
Current contribution / %
100
4. Conclusions The main conclusion of this study is that first-row transition metal mixed oxalates are potential candidates for the active material of the negative electrode of lithium-ion batteries. The low temperature dehydration synthesis of these solids makes them an inexpensive option to obtain high-capacity active materials. The best electrochemical performance was achieved for manganese–cobalt oxalates where high capacity values (ca. 600 mA h g − 1) are reached, with a very good capacity retention after 75 cycles.
75
50
25
0
521
Acknowledgments The authors are indebted to MICINN (MAT2008-05880) and Junta de Andalucía (FQM-288).
0.5
2
5
Sweep rate / mVs-1
10
20 Faradaic Capacitive
References
Fig. 5. Relative intensity of capacitive and faradic current contribution for Fe0.8Mn0.2C2O4.
For a theoretical full conversion reaction with 0 ≤ x ≤ 1, M or M′: Mn, Fe, Co þ
Mx M′1x C2 O4 þ 2Li þ 2e →xM þ ð1 xÞM′ þ Li2 C2 O4
ð4Þ −1
range. The calculated capacity would be in the 364–374 mAhg To understand the origin of the extra capacity found in Fig. 4, we have carried out a study of the possible contribution of Faradic and non-Faradic (capacitance) to the global observed values. This study follows the model recently described by Brezesinski et al. for TiO2 electrodes [17]. An example is shown in Fig. 5. The faradic current presents the larger values during all the experiments, indicating that the reaction with lithium is favored especially for those experiments at low rate of discharge. The relative capacitive contribution presents the lower contribution at low sweep rate, although the values are increasing when the sweep rate is increasing. Still the Faradic contribution is higher than expected from Eq. (4). This result implies that part of the Faradic contribution comes from a different reaction such as side reactions with the electrolyte and/or the reversible formation of a polymer-like layer, as described by Tarascon group [2].
[1] F. Badway, A.N. Mansour, N. Pereira, J.F. Al-Sharab, F. Cosandey, I. Plitz, G.G. Amatucci, Chem. Mater. 19 (2007) 4129. [2] P. Poizot, S. Laruelle, S. Grugeon, J.M. Tarascon, J. Electrochem. Soc. 149 (2002) A1212. [3] R. Alcántara, M. Jaraba, P. Lavela, J.L. Tirado, Chem. Mater. 14 (2002) 2847. [4] J. Cabana, Z. Stoeva, J.J. Titman, D.H. Gregory, M.R. Palacin, Chem. Mater. 20 (2008) 1676. [5] R. Alcántara, J.L. Tirado, J.C. Jumas, L. Monconduit, J. Olivier-Fourcade, J. Power Sources 109 (2002) 308. [6] M.J. Aragón, B. León, C. Pérez Vicente, J.L. Tirado, A.V. Chadwick, A. Berko, S.Y. Beh, Chem. Mater. 21 (2009) 1834. [7] M.J. Aragón, B. León, C. Pérez-Vicente, J.L. Tirado, Inorg. Chem. 47 (2008) 10366. [8] M.J. Aragón, B. León, T. Serrano, C. Pérez Vicente, J.L. Tirado, J. Mater. Chem. 21 (2011) 10102. [9] R. Deyrieux, A. Peneloux, Bull. Soc. Chim. Fr. 8 (1969) 2675. [10] I. Sledzinska, A. Murasik, M. Piotrowski, Physica B (Amsterdam) 138 (1986) 315. [11] R.A. Brown, S.C. Bevan, J. Inorg. Nucl. Chem. 28 (1966) 387. [12] R.C. Mackenzie, Differential Thermal Analysis, vol. 1, Academic Press, 1970, chap. 13. [13] Y.S. Hu, Y.G. Guo, W. Sigle, S. Hore, P. Balaya, J. Maier, Nat. Mater. 5 (2006) 713. [14] M.J. Aragón, B. León, T. Serrano, C. Pérez Vicente, J.L. Tirado, J. Mater. Chem. 21 (2011) 10102. [15] M. Hermanek, R. Zboril, M. Mashlan, L. Machala, O. Schneeweiss, J. Mater. Chem. 16 (2006) 1273. [16] A.R. Armstrong, G. Armstrong, J. Canales, R. Garcia, P.G. Bruce, Adv. Mater. 17 (2005) 862. [17] T. Brezesinski, J. Wang, J. Polleux, B. Dunn, S.H. Tolbert, J. Am. Chem. Soc. 131 (2009) 1802.