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Electrochemical deintercalation of lithium ions from lithium iron chloride spinel
Solid State Ionics 152–153 (2002) 295 – 302 www.elsevier.com/locate/ssi
Electrochemical deintercalation of lithium ions from lithium iron chloride spinel Akihisa Kajiyama a,b,*, Kazunori Takada a, Taro Inada a,c, Masaru Kouguchi a,d, Shigeo Kondo a, Mamoru Watanabe a, Mitsuharu Tabuchi e a
Advanced Materials Laboratory, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan b Toda Kogyo Corp., Hiroshima, Japan c Denki Kagaku Kogyo K. K., Tokyo, Japan d Japan Storage Battery Co., Ltd., Kyoto, Japan e Green Life Technology, National Institute of Advanced Industrial Science and Technology, 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan Accepted 15 March 2002
Abstract A lithium iron chloride inverse spinel was prepared to examine the electrochemical Li+ deintercalation from the chloride spinel in an all-solid state cell. While the chloride spinel has a high ionicity, it was confirmed that Li ions could be extracted from the structure without dissolving with a potential of 3.6 V vs. Li/Li+. Powder X-ray diffraction of the material showed coexistence of two kinds of spinels with different lattice constants. Mo¨ssbauer spectra revealed that the deintercalation of Li+ was accompanied with the oxidation of iron from Fe2+ to Fe3+. The amount of deintercalated lithium estimated from the electrical capacity showed large value of 1.2 per formula unit, which indicates both Li+ in 8a and 16d sites were deintercalated. D 2002 Elsevier Science B.V. All rights reserved. PACS: 71.20.Tx; 82.75.Fq; 84.60.Dn Keywords: Chloride spinel; Lithium battery; Spinel; Deintercalation; Mo¨ssbauer spectroscopy
1. Introduction Double chlorides, Li2 2xM1+xCl4 (M=Mg, V, Co, Mn, Fe, Cd), with spinel structure have been studied as lithium ion conductors [3 – 7]. When M is a transition element, it is considered available for an electrode material at first glance. For instance, Li2 2x* Corresponding author. Advanced Materials Laboratory, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan. Tel.: +81-298-51-3354x673; fax: +81-298-549061. E-mail address: [email protected] (A. Kajiyama).
Fe1+xCl4 shows fast lithium ionic conduction [3]. In addition, it contains Fe2+, which will play a role in redox couple. Therefore, it is expected to be available as an active electrode material. Moreover, the large amount of Li ions in the material implies large electrical capacity when it is used as an active electrode material of lithium ion batteries. Actually, the deintercalation properties of Li ion have not been investigated because the chloride is easily dissolved into the liquid electrolyte due to its high ionicity. In this study, we used a solid state electrolyte glass as an Li+ conducting electrolyte instead of a liquid electrolyte to prevent the dissolution of it, and investigated the Li+ deintercalation from the Li – Fe chloride spinel.
0167-2738/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 2 7 3 8 ( 0 2 ) 0 0 3 1 4 - 4
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2. Experimental 2.1. Sample preparation Lithium iron chlorides were prepared by solid state reaction as reported previously [3 – 5]. Anhydrous lithium chloride and iron dichloride (LiCl: Kanto Chemical >99% purity; FeCl2: High Purity Chemicals >99.9% purity) were used as the starting materials. Predetermined amount (x in Li(2 2x)Fe(1+x)Cl4=0, 0.05, 0.10, 0.15, 0.20) of starting materials were thoroughly mixed in an agate mortar, pressed into a pellet in an argon-filled glove box. The pellet was crushed in appropriate size to seal with an evacuated Pyrex tube to avoid the contact with moisture. The tube was then heated at 450 jC for a week, followed by furnace cooling down to room temperature. 2.2. Characterization The X-ray powder diffraction measurement (RINT2100S CuKa, Rigaku) was used to identify the product phases. Diffraction data were collected by a step scan over a range of 10j<2h<120j with an increment angle of 0.02j. The sample was kept under dry argon atmosphere at room temperature during the measurement. The structural refinement from the diffraction data was performed with the Rietveld analysis computer program RIETAN-h [1]. 2.3. Electrochemical performance Li ions were electrochemically deintercalated from and intercalated to the chlorides in all-solid state Li battery cell, which has a diameter of 10 mm. Electrolyte used was an oxysulfide with a composition of 0.01Li3PO4 – 0.63Li2S – 0.36SiS2 [2]. The working electrode was a mixture of Li – Fe chloride, the solid electrolyte and acetylene black at a weigh ratio of 49:49:1. The counter electrode was In – Li alloy, which quantity gives excess capacity to that of working electrode. A solid state electrolyte layer, which thickness was approximately 1.1 mm, was fabricated between the electrodes, which also play a role in a separator. The cell was pressed at once before electrochemical measurement approximately at 0.02 N/m2 to have good electrical contact within the cell. The quasiopen circuit voltage profiles were measured in an
argon atmosphere using intermittent charging/discharging as follows. A charging current of 15 AA was supplied for 2 h, followed by a rest time of 2 h. The quasi-open circuit voltage was recorded after the relaxation time. The charging process was stopped when the closed circuit voltage reached 4.0 V vs. Li/ Li+, and then the cell was discharged at intermittent current in the same way down to 1.0 V. After the electrochemical test, the apparent lithium content in the chloride was calculated from the quantity of electrical charge. The working electrodes after the electrochemical test were taken out from the cell and investigated by X-ray diffraction. 2.4. Mo¨ssbauer spectroscopy Mo¨ssbauer spectra of 57Fe in the samples were taken using a constant acceleration spectrometer and a 57 Co/Th source at room temperature in zero magnetic fields. Pure Fe was used as standard and the velocity correction was made. The data collected were fitted assuming the existence of hyperfine components made of absorption peaks with Lorentzian line shape.
3. Results and discussion Fig. 1 shows the powder X-ray diffraction patterns of Li2 2xFe1+xCl4 (x=0, 0.05, 0.10, 0.15, 0.20). All the patterns could be indexed with cubic spinel structure except for the sample with x=0, which was assignable to an orthorhombic unit cell. This is consistent with the report by Kanno et al. [3]. Furthermore, sight shift of each peak to the higher diffraction angle, which indicates the lattice contraction, was also observed with increase in x value. In Fig. 2, the result of structural refinement for Li1.9Fe1.05Cl4 is shown on the basis of the cubic unit ˚ and Fd3¯m. The refined lattice cell with a=10.37 A parameter, atomic coordinates, and thermal parameters are listed in Table 1. Fig. 3 shows a 57Fe Mo¨ssbauer spectrum for Li1.9Fe1.05Cl4. Lutz et al. [10] reported that Mo¨ssbauer spectra were different among structural modifications. The high temperature cubic phase of Li2FeCl4 showed five doublets, while the room temperature orthorhombic phase gave two doublets. The difference was attributed to cation
A. Kajiyama et al. / Solid State Ionics 152–153 (2002) 295–302
Fig. 1. X-ray diffraction patterns for Li2
2xFe1+xCl4
distribution in 16d sites. Li1.9Fe1.05Cl4 in the present study showed a quite similar spectrum to the high temperature phase. These results suggested that Li1.9Fe1.05Cl4 had a cubic spinel structure with a random cation distribution in 16d sites. An increasing x resulted in the contraction of unit cell. Soubeyroux et al. had confirmed [Li1 x+5x]8a [Fe1+x2+Li1 x+]16dCl4 for the chemical formula, where 5: vacancy [9]. Since the cation vacancies at the 8a tetrahedral site increase with an increase in Fe2+ to compensate the charge valance of the formula, the increase in vacancy seemed to cause the above structural contraction. In the composition range where the spinel phase was observed alone, Li1.9Fe1.05Cl4 is of great interest as an electrode active material because of its largest Li content, but this chloride spinel is easily dissolved into liquid electrolyte owing to their high ionicity. Solid state electrolyte has made it possible to study their electrode behavior. A potential profile of lithium iron chloride spinel during the electrochemical dein-
297
prepared at ranging of x from 0 to 0.20.
tercalation and following intercalation of Li ions is shown in Fig. 4. The long plateau was observed around 3.6 V vs. Li/Li+ during the deintercalation, resulting in about 1.2 for the maximum amount of extracted Li ions per formula unit. There have been many studies on electrode materials with spinel structure. In normal spinels such as LiMn2O4, Li ions occupy 8a tetrahedral sites, and Mn ions occupy 16d octahedral sites. Those Li ions were deintercalated from the 8a sites via 16c octahedral sites, that is, Li ions at 8a sites and 16c sites undergo easy deintercalation. The present spinel has [Li0.95]8a [Fe1.05Li0.95]16dCl4 as the apparent cation distribution before the deintercalation. By analogy with the normal oxide spinels, the maximum amounts of deintercalated Li ions are expected to be 0.95, which is smaller than the observed value. Probably, it could be thought that Li ions at 16d sites besides those at 8a sites were also deintercalated, as observed in LiNiVO4. LiNiVO4 has an inverse spinel structure, where Li ions occupy half of the 16d octahedral sites, and it could be extracted
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Fig. 2. Rietveld refinement profiles for the Li1.9Fe1.05Cl4 sample. Dots and solid line denote the observed and calculated intensities, respectively. Difference of the observed and calculated intensities was plotted at the bottom. The bar code indicates the positions of all possible Bragg reflections.
[11]. To our knowledge, this is the first result to show the extraction of both lithium ions at 8a and 16d sites, which gives a large capacity more than 150 mA h/g. Although 1.2 Li ions were deintercalated at ca. 3.6 V vs. Li+/Li, the amount of re-intercalated Li ions at the same potential was 0.5. Some structural modification caused by the first deintercalation was considered to result in the change in the intercalation potential. The flatness of the potential plateau suggests the two-phase reaction of the lithium extraction.
Fig. 5 shows the X-ray diffraction patterns of the positive electrode after the electrochemical Li extraction. The extraction decreased peak intensities as compared to the original phase, and additional peaks to that of original phase were appeared at 2h=26.3j, 32.9j, 36.3j, and 39.1j instead, which were regarded to correspond to a new phase produced by the lithium extraction. Therefore, it would be considered that the diffraction patterns for the 0.5 Li+ extracted material implied the coexistence of original and new phases.
Table 1 Refined lattice parameter, atomic coordinates, and thermal parameters for the Li1.9Fe1.05Cl4 Atom
Site
g b
x
y
z
Beqa
Fe Cl
16d 32e
0.525 1.000b
1/2 0.2558(2)
1/2 0.2558(2)
1/2 0.2558(2)
0.580 1.021
Atom
˚ 2] U11 [A
˚ 2] U22 [A
˚ 2] U33 [A
˚ 2] U12 [A
˚ 2] U13 [A
˚ 2] U23 [A
Fe Cl
1347 2370
=U11 =U11
=U11 =U11
=U11 =U11
=U11 =U11
997 1625
˚ , Rwp=14.78, S=1.666. Space group Fd3¯m, a=10.3769(2) A a Equivalent isotropic thermal parameter. b Constrains on the site occupancy, 0.525 and 1.00 for Fe and Cl, respectively, were applied.
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Fig. 3. Mo¨ssbauer spectra of Li1.9 xFe1.05Cl4.
Fig. 4. Potential profile of the Li1.9 xFe1.05Cl4 during the electrochemical Li+ deintercalation test.
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Fig. 5. Powder X-ray diffraction pattern for the composite positive electrodes during the Li+ deintercalation.
Fig. 6. Mo¨ssbauer spectra of the samples with Li1.9 xFe1.05Cl4 during the Li+ extraction.
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An accurate structure of the Li+ extracted phase could not be undertaken because of the high background from the acetylene black and the solid electrolyte in the electrode. However, they gave important deductions on structural features of the Li+ extracted phase. Spinel structures consist of three-dimensional network, and in general, Li ions are extracted electrochemically without a large change in the structures. Therefore, the generated phase seems to have a deformed structure of the original spinel phase. Actually, the additional peaks could be indexed with cubic spinel structure on the assumption that the new phase maintained its spinel structure. The cell parameter of that was smaller than that of the original phase, suggesting that Li+ extraction from the Li –Fe chloride spinel resulted in a volume contraction of the material. In addition, the intensity ratio I220/I400 for the Li+ extracted one became larger than that of the original spinel phase. The I220/I400 ratio is known to be influenced by the scattering strength of cations at 8a tetrahedral sites in the spinel structure, i.e., it becomes larger when strong-scattering cations like transition metal ions occupy the 8a sites and smaller in case of weak-scattering cations such as Li ions [8]. Hence, the larger ratio inferred that some parts of iron atoms were displaced to 8a sites. This displacement is considered to present result in the short 3.6 V plateau during the reduction. The valence of the iron ions was investigated by 57 Fe Mo¨ssbauer spectroscopy. The Mo¨ssbauer spectrum of the sample, in which 1.0 Li ion was deintercalated, is shown in Fig. 6. It showed an additional doublet to those observed in the sample before the deintercalation. The isomer shift of the doublet has a smaller value (0.4 mm/s) than those of the original doublets. The lithium deintercalation should accompany the oxidation of Fe2+ to a higher valence state, i.e., the removal of 3d electrons. It would decrease the screening effect of the inner electrons, contract the wave function of 3s electrons, and thus increased the charge density at the nuclei. It was considered to result in the appearance of the additional doublet with the smaller isomer shift. The decrease in the screening effect would also cause the decrease in the ionic radius. In spinel structures, such small cations preferably occupy the 8a tetrahedral sites to Li ion. For instance, a lithium ferrite, LiFe5O8, has a cubic inverse spinel structure ([Fe3+]8a[Li+0.5Fe3+1.5]16dO4), in which 8a
301
tetrahedral sites are not occupied with lithium ions but ferric ions [12]. Therefore, the resultant Fe ions with higher valence would be displaced to the tetrahedral sites as discussed above.
4. Conclusions Electrochemical deintercalation of Li+ from Li – Fe chloride inverse spinel was performed in an allsolid lithium battery cell. According to charge capacity measurement, approximately 1.2 Li+ per formula unit could be extracted. The large amount indicated that both 8a and 16d lithium ions were extracted. Mo¨ssbauer spectra showed the oxidation of iron ions was accompanied with lithium deintercalation. It is expected that the redox couple between Fe2+ and Fe3+ generated the potential of 3.6 V vs. Li/Li+. The large electric capacity seems to promise for high capacity electrode material of secondary batteries.
Acknowledgements Authors are grateful to professor R. Kanno of Tokyo Institute of Technology for his fruitful suggestion and discussion. This work has been supported by the ‘Combinatorial Materials Exploration and Technology Joint Project’ from Ministry of Education, Culture, Sports, Science and Technology, Japan.
References [1] F. Izumi, in: R.A. Young (Ed.), The Rietveld Method, Oxford Univ. Press, Oxford, 1993, Chap. 13. [2] K. Takada, K. Iwamoto, S. Kondo, Solid State Ionics 117 (1999) 273. [3] R. Kanno, Y. Takeda, K. Takada, O. Yamamoto, Solid State Ionics 9 – 10 (1983) 153. [4] R. Kanno, Y. Takeda, A. Takahashi, O. Yamamoto, J. Solid State Chem. 72 (1988) 363. [5] R. Kanno, Y. Takeda, M. Mori, O. Yamamoto, Chem. Lett. (1987) 1465. [6] H.D. Lutz, A. Pfitzner, J.K. Cockcroft, J. Solid State Chem. 107 (1993) 245. [7] M. Schieber, J. Inorg. Nucl. Chem. 26 (1964) 1363.
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[8] T. Sano, Y. Tamaura, Mater. Res. Bull. 34 (1999) 389. [9] J.L. Soubeyroux, C. Cros, W. Gang, R. Kanno, M. Pouchard, Solid State Ionics 15 (1985) 293. [10] H.D. Lutz, A. Pfitzner, W. Schmidt, Z. Naturforsch. 44a (1989) 756.
[11] G.T. Fey, K.S. Wang, S.M. Yang, J. Power Sources 68 (1997) 159. [12] L.A. de Picciotto, M.M. Thackeray, Mater. Res. Bull. 21 (1986) 583.
Electrochemical deintercalation of lithium ions from lithium iron chloride spinel Akihisa Kajiyama a,b,*, Kazunori Takada a, Taro Inada a,c, Masaru Kouguchi a,d, Shigeo Kondo a, Mamoru Watanabe a, Mitsuharu Tabuchi e a
Advanced Materials Laboratory, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan b Toda Kogyo Corp., Hiroshima, Japan c Denki Kagaku Kogyo K. K., Tokyo, Japan d Japan Storage Battery Co., Ltd., Kyoto, Japan e Green Life Technology, National Institute of Advanced Industrial Science and Technology, 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan Accepted 15 March 2002
Abstract A lithium iron chloride inverse spinel was prepared to examine the electrochemical Li+ deintercalation from the chloride spinel in an all-solid state cell. While the chloride spinel has a high ionicity, it was confirmed that Li ions could be extracted from the structure without dissolving with a potential of 3.6 V vs. Li/Li+. Powder X-ray diffraction of the material showed coexistence of two kinds of spinels with different lattice constants. Mo¨ssbauer spectra revealed that the deintercalation of Li+ was accompanied with the oxidation of iron from Fe2+ to Fe3+. The amount of deintercalated lithium estimated from the electrical capacity showed large value of 1.2 per formula unit, which indicates both Li+ in 8a and 16d sites were deintercalated. D 2002 Elsevier Science B.V. All rights reserved. PACS: 71.20.Tx; 82.75.Fq; 84.60.Dn Keywords: Chloride spinel; Lithium battery; Spinel; Deintercalation; Mo¨ssbauer spectroscopy
1. Introduction Double chlorides, Li2 2xM1+xCl4 (M=Mg, V, Co, Mn, Fe, Cd), with spinel structure have been studied as lithium ion conductors [3 – 7]. When M is a transition element, it is considered available for an electrode material at first glance. For instance, Li2 2x* Corresponding author. Advanced Materials Laboratory, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan. Tel.: +81-298-51-3354x673; fax: +81-298-549061. E-mail address: [email protected] (A. Kajiyama).
Fe1+xCl4 shows fast lithium ionic conduction [3]. In addition, it contains Fe2+, which will play a role in redox couple. Therefore, it is expected to be available as an active electrode material. Moreover, the large amount of Li ions in the material implies large electrical capacity when it is used as an active electrode material of lithium ion batteries. Actually, the deintercalation properties of Li ion have not been investigated because the chloride is easily dissolved into the liquid electrolyte due to its high ionicity. In this study, we used a solid state electrolyte glass as an Li+ conducting electrolyte instead of a liquid electrolyte to prevent the dissolution of it, and investigated the Li+ deintercalation from the Li – Fe chloride spinel.
0167-2738/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 2 7 3 8 ( 0 2 ) 0 0 3 1 4 - 4
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2. Experimental 2.1. Sample preparation Lithium iron chlorides were prepared by solid state reaction as reported previously [3 – 5]. Anhydrous lithium chloride and iron dichloride (LiCl: Kanto Chemical >99% purity; FeCl2: High Purity Chemicals >99.9% purity) were used as the starting materials. Predetermined amount (x in Li(2 2x)Fe(1+x)Cl4=0, 0.05, 0.10, 0.15, 0.20) of starting materials were thoroughly mixed in an agate mortar, pressed into a pellet in an argon-filled glove box. The pellet was crushed in appropriate size to seal with an evacuated Pyrex tube to avoid the contact with moisture. The tube was then heated at 450 jC for a week, followed by furnace cooling down to room temperature. 2.2. Characterization The X-ray powder diffraction measurement (RINT2100S CuKa, Rigaku) was used to identify the product phases. Diffraction data were collected by a step scan over a range of 10j<2h<120j with an increment angle of 0.02j. The sample was kept under dry argon atmosphere at room temperature during the measurement. The structural refinement from the diffraction data was performed with the Rietveld analysis computer program RIETAN-h [1]. 2.3. Electrochemical performance Li ions were electrochemically deintercalated from and intercalated to the chlorides in all-solid state Li battery cell, which has a diameter of 10 mm. Electrolyte used was an oxysulfide with a composition of 0.01Li3PO4 – 0.63Li2S – 0.36SiS2 [2]. The working electrode was a mixture of Li – Fe chloride, the solid electrolyte and acetylene black at a weigh ratio of 49:49:1. The counter electrode was In – Li alloy, which quantity gives excess capacity to that of working electrode. A solid state electrolyte layer, which thickness was approximately 1.1 mm, was fabricated between the electrodes, which also play a role in a separator. The cell was pressed at once before electrochemical measurement approximately at 0.02 N/m2 to have good electrical contact within the cell. The quasiopen circuit voltage profiles were measured in an
argon atmosphere using intermittent charging/discharging as follows. A charging current of 15 AA was supplied for 2 h, followed by a rest time of 2 h. The quasi-open circuit voltage was recorded after the relaxation time. The charging process was stopped when the closed circuit voltage reached 4.0 V vs. Li/ Li+, and then the cell was discharged at intermittent current in the same way down to 1.0 V. After the electrochemical test, the apparent lithium content in the chloride was calculated from the quantity of electrical charge. The working electrodes after the electrochemical test were taken out from the cell and investigated by X-ray diffraction. 2.4. Mo¨ssbauer spectroscopy Mo¨ssbauer spectra of 57Fe in the samples were taken using a constant acceleration spectrometer and a 57 Co/Th source at room temperature in zero magnetic fields. Pure Fe was used as standard and the velocity correction was made. The data collected were fitted assuming the existence of hyperfine components made of absorption peaks with Lorentzian line shape.
3. Results and discussion Fig. 1 shows the powder X-ray diffraction patterns of Li2 2xFe1+xCl4 (x=0, 0.05, 0.10, 0.15, 0.20). All the patterns could be indexed with cubic spinel structure except for the sample with x=0, which was assignable to an orthorhombic unit cell. This is consistent with the report by Kanno et al. [3]. Furthermore, sight shift of each peak to the higher diffraction angle, which indicates the lattice contraction, was also observed with increase in x value. In Fig. 2, the result of structural refinement for Li1.9Fe1.05Cl4 is shown on the basis of the cubic unit ˚ and Fd3¯m. The refined lattice cell with a=10.37 A parameter, atomic coordinates, and thermal parameters are listed in Table 1. Fig. 3 shows a 57Fe Mo¨ssbauer spectrum for Li1.9Fe1.05Cl4. Lutz et al. [10] reported that Mo¨ssbauer spectra were different among structural modifications. The high temperature cubic phase of Li2FeCl4 showed five doublets, while the room temperature orthorhombic phase gave two doublets. The difference was attributed to cation
A. Kajiyama et al. / Solid State Ionics 152–153 (2002) 295–302
Fig. 1. X-ray diffraction patterns for Li2
2xFe1+xCl4
distribution in 16d sites. Li1.9Fe1.05Cl4 in the present study showed a quite similar spectrum to the high temperature phase. These results suggested that Li1.9Fe1.05Cl4 had a cubic spinel structure with a random cation distribution in 16d sites. An increasing x resulted in the contraction of unit cell. Soubeyroux et al. had confirmed [Li1 x+5x]8a [Fe1+x2+Li1 x+]16dCl4 for the chemical formula, where 5: vacancy [9]. Since the cation vacancies at the 8a tetrahedral site increase with an increase in Fe2+ to compensate the charge valance of the formula, the increase in vacancy seemed to cause the above structural contraction. In the composition range where the spinel phase was observed alone, Li1.9Fe1.05Cl4 is of great interest as an electrode active material because of its largest Li content, but this chloride spinel is easily dissolved into liquid electrolyte owing to their high ionicity. Solid state electrolyte has made it possible to study their electrode behavior. A potential profile of lithium iron chloride spinel during the electrochemical dein-
297
prepared at ranging of x from 0 to 0.20.
tercalation and following intercalation of Li ions is shown in Fig. 4. The long plateau was observed around 3.6 V vs. Li/Li+ during the deintercalation, resulting in about 1.2 for the maximum amount of extracted Li ions per formula unit. There have been many studies on electrode materials with spinel structure. In normal spinels such as LiMn2O4, Li ions occupy 8a tetrahedral sites, and Mn ions occupy 16d octahedral sites. Those Li ions were deintercalated from the 8a sites via 16c octahedral sites, that is, Li ions at 8a sites and 16c sites undergo easy deintercalation. The present spinel has [Li0.95]8a [Fe1.05Li0.95]16dCl4 as the apparent cation distribution before the deintercalation. By analogy with the normal oxide spinels, the maximum amounts of deintercalated Li ions are expected to be 0.95, which is smaller than the observed value. Probably, it could be thought that Li ions at 16d sites besides those at 8a sites were also deintercalated, as observed in LiNiVO4. LiNiVO4 has an inverse spinel structure, where Li ions occupy half of the 16d octahedral sites, and it could be extracted
298
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Fig. 2. Rietveld refinement profiles for the Li1.9Fe1.05Cl4 sample. Dots and solid line denote the observed and calculated intensities, respectively. Difference of the observed and calculated intensities was plotted at the bottom. The bar code indicates the positions of all possible Bragg reflections.
[11]. To our knowledge, this is the first result to show the extraction of both lithium ions at 8a and 16d sites, which gives a large capacity more than 150 mA h/g. Although 1.2 Li ions were deintercalated at ca. 3.6 V vs. Li+/Li, the amount of re-intercalated Li ions at the same potential was 0.5. Some structural modification caused by the first deintercalation was considered to result in the change in the intercalation potential. The flatness of the potential plateau suggests the two-phase reaction of the lithium extraction.
Fig. 5 shows the X-ray diffraction patterns of the positive electrode after the electrochemical Li extraction. The extraction decreased peak intensities as compared to the original phase, and additional peaks to that of original phase were appeared at 2h=26.3j, 32.9j, 36.3j, and 39.1j instead, which were regarded to correspond to a new phase produced by the lithium extraction. Therefore, it would be considered that the diffraction patterns for the 0.5 Li+ extracted material implied the coexistence of original and new phases.
Table 1 Refined lattice parameter, atomic coordinates, and thermal parameters for the Li1.9Fe1.05Cl4 Atom
Site
g b
x
y
z
Beqa
Fe Cl
16d 32e
0.525 1.000b
1/2 0.2558(2)
1/2 0.2558(2)
1/2 0.2558(2)
0.580 1.021
Atom
˚ 2] U11 [A
˚ 2] U22 [A
˚ 2] U33 [A
˚ 2] U12 [A
˚ 2] U13 [A
˚ 2] U23 [A
Fe Cl
1347 2370
=U11 =U11
=U11 =U11
=U11 =U11
=U11 =U11
997 1625
˚ , Rwp=14.78, S=1.666. Space group Fd3¯m, a=10.3769(2) A a Equivalent isotropic thermal parameter. b Constrains on the site occupancy, 0.525 and 1.00 for Fe and Cl, respectively, were applied.
A. Kajiyama et al. / Solid State Ionics 152–153 (2002) 295–302
Fig. 3. Mo¨ssbauer spectra of Li1.9 xFe1.05Cl4.
Fig. 4. Potential profile of the Li1.9 xFe1.05Cl4 during the electrochemical Li+ deintercalation test.
299
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Fig. 5. Powder X-ray diffraction pattern for the composite positive electrodes during the Li+ deintercalation.
Fig. 6. Mo¨ssbauer spectra of the samples with Li1.9 xFe1.05Cl4 during the Li+ extraction.
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An accurate structure of the Li+ extracted phase could not be undertaken because of the high background from the acetylene black and the solid electrolyte in the electrode. However, they gave important deductions on structural features of the Li+ extracted phase. Spinel structures consist of three-dimensional network, and in general, Li ions are extracted electrochemically without a large change in the structures. Therefore, the generated phase seems to have a deformed structure of the original spinel phase. Actually, the additional peaks could be indexed with cubic spinel structure on the assumption that the new phase maintained its spinel structure. The cell parameter of that was smaller than that of the original phase, suggesting that Li+ extraction from the Li –Fe chloride spinel resulted in a volume contraction of the material. In addition, the intensity ratio I220/I400 for the Li+ extracted one became larger than that of the original spinel phase. The I220/I400 ratio is known to be influenced by the scattering strength of cations at 8a tetrahedral sites in the spinel structure, i.e., it becomes larger when strong-scattering cations like transition metal ions occupy the 8a sites and smaller in case of weak-scattering cations such as Li ions [8]. Hence, the larger ratio inferred that some parts of iron atoms were displaced to 8a sites. This displacement is considered to present result in the short 3.6 V plateau during the reduction. The valence of the iron ions was investigated by 57 Fe Mo¨ssbauer spectroscopy. The Mo¨ssbauer spectrum of the sample, in which 1.0 Li ion was deintercalated, is shown in Fig. 6. It showed an additional doublet to those observed in the sample before the deintercalation. The isomer shift of the doublet has a smaller value (0.4 mm/s) than those of the original doublets. The lithium deintercalation should accompany the oxidation of Fe2+ to a higher valence state, i.e., the removal of 3d electrons. It would decrease the screening effect of the inner electrons, contract the wave function of 3s electrons, and thus increased the charge density at the nuclei. It was considered to result in the appearance of the additional doublet with the smaller isomer shift. The decrease in the screening effect would also cause the decrease in the ionic radius. In spinel structures, such small cations preferably occupy the 8a tetrahedral sites to Li ion. For instance, a lithium ferrite, LiFe5O8, has a cubic inverse spinel structure ([Fe3+]8a[Li+0.5Fe3+1.5]16dO4), in which 8a
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tetrahedral sites are not occupied with lithium ions but ferric ions [12]. Therefore, the resultant Fe ions with higher valence would be displaced to the tetrahedral sites as discussed above.
4. Conclusions Electrochemical deintercalation of Li+ from Li – Fe chloride inverse spinel was performed in an allsolid lithium battery cell. According to charge capacity measurement, approximately 1.2 Li+ per formula unit could be extracted. The large amount indicated that both 8a and 16d lithium ions were extracted. Mo¨ssbauer spectra showed the oxidation of iron ions was accompanied with lithium deintercalation. It is expected that the redox couple between Fe2+ and Fe3+ generated the potential of 3.6 V vs. Li/Li+. The large electric capacity seems to promise for high capacity electrode material of secondary batteries.
Acknowledgements Authors are grateful to professor R. Kanno of Tokyo Institute of Technology for his fruitful suggestion and discussion. This work has been supported by the ‘Combinatorial Materials Exploration and Technology Joint Project’ from Ministry of Education, Culture, Sports, Science and Technology, Japan.
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