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Lithium iron sulfide as an electrode material in a solid state lithium battery
Solid State Ionics 117 (1999) 273–276
Lithium iron sulfide as an electrode material in a solid state lithium battery Kazunori Takada*, Kazuya Iwamoto, Shigeo Kondo Technology Laboratory, Matsushita Battery Industrial Co., Ltd., 1 -1, Matsushita-cho, Moriguchi, Osaka 570 -8511, Japan Received 10 April 1998; accepted 30 August 1998
Abstract The electrochemical properties of Lix FeS 2 in a Li 1 ion conductive glass were investigated. The electrode reaction shows two potential plateaus within the range 0 # x # 4, and is reversible. The reduction of FeS 2 by 4e 2 or Li 2 FeS 2 by 2e 2 did not proceed to form Fe metal as reported in earlier studies. Li x FeS 2 is promising as an electrode active material in a solid state lithium battery. 1999 Elsevier Science B.V. All rights reserved. Keywords: Anode material; Lithium battery; Lithium iron sulfide; Solid electrolyte
1. Introduction Many transition metal sulfides show more than one potential plateau during reduction in Li 1 conductive electrolytes [1]. The equivalent potential of one of the plateaus of each sulfide is lower than 2 V vs. Li 1 / Li. With the decreasing operation voltage of semiconductor devices and the development of high voltage cathodes such as Li 12x CoO 2 and Li 12x Mn 2 O 4 , they have become available as negative electrode materials, even though the potential is more noble than C 6 Li x or Li metal. However, the reversibility of the electrode reactions are not adequate for practical use. Iron sulfide has been investigated as a positive electrode material in thermal batteries [2]. The studies revealed that FeS 2 has a theoretical capacity *Corresponding author. Tel.: 181-6-994-4552; fax: 181-6998-3179; e-mail: [email protected]
as high as 894 mAh / g (4e 2 / FeS 2 ). It shows potential plateaus at 2.3 and 1.6 V [3]. Previous studies of the charge–discharge behavior of FeS 2 in a liquid electrolyte or a polymer electrolyte [3,4] indicated a reversible reaction through the 2.3 V plateau. However, the electrode reaction of the 1.6 V plateau, the potential of which fits a negative electrode in a lithium battery, is reported to be irreversible. We investigated the electrochemical properties of iron sulfides using a Li 1 conductive sulfide glass [5].
2. Experimental
2.1. Synthesis of materials Li 2 FeS 2 was synthesized as follows. Stoichiometric amounts of Li 2 S and FeS were mixed. The mixture was heated in a vitreous carbon crucible under Ar gas flow at 9508C, and then the molten
0167-2738 / 99 / $ – see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S0167-2738( 98 )00413-5
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K. Takada et al. / Solid State Ionics 117 (1999) 273 – 276
mixture was cooled to room temperature in a furnace. LiCoO 2 was used as a positive electrode material in electrochemical cells to investigate the electrochemical properties of FeS 2 and Li 2 FeS 2 . The starting materials, Li 2 CO 3 and Co 3 O 4 , were mixed and heated at 9508C in air. Lithium ion conductive glass, 0.01Li 3 PO 4 –0.63Li 2 S–SiS 2 , was synthesized as described in a previous paper [5].
2.2. Electrochemical measurements FeS 2 or Li 2 FeS 2 was mixed with the ground glass in a weight ratio of 1:1. The mixture was used as a negative electrode in electrochemical cells. Positive electrodes consisted of a mixture of LiCoO 2 and the solid electrolyte powder in the weight ratio 3:2 mixed in a ball mill. Three layered pellets were constructed with appropriate amounts of the mixtures and the solid electrolyte powder. The mixture containing iron sulfide and that containing LiCoO 2 were pressed together with the solid electrolyte between them in the hole of an insulated tube. The positive electrode consisted of 63 mg LiCoO 2 and 42 mg glass powder. The negative electrode consisted of 10 mg of the mixture containing FeS 2 or 20 mg in the case of Li 2 FeS 2 . The diameter of the hole was 10 mm. Current collectors made of stainless steel were attached to both faces of the pellet. The electrochemical cells were charged and discharged at a constant current of 150 mA at 208C. The crystal structures of the products of the negative electrode were investigated by X-ray diffraction. The electrochemical cell was disassembled after the charge or discharge. The negative electrode was ground and filled into an air-tight sample holder with an Al window. X-ray diffraction was carried out using a Cu Ka source.
3. Results and discussion In the following sections, the reduction reaction of FeS 2 is represented as the formation of Li x FeS 2 as shown in Eq. (1) in order to simplify the discussion, where x corresponds to the depth of the reduction FeS 2 1 xLi 1 1 xe 2 → Li x FeS 2 .
(1)
Fig. 1. Charge-–discharge curves of the cell FeS 2 / LiCoO 2 . Positive electrode: 63 mg LiCoO 2 142 mg solid electrolyte. Negative electrode: 5 mg FeS 2 15 mg solid electrolyte. Charge–discharge current 150 mA.
3.1. Electrochemical behavior Charge–discharge curves of the cell, FeS 2 / LiCoO 2 , are shown in Fig. 1. The first charge curve is quite different from the consecutive curves. It consists of one plateau, but the others consist of two. This result is consistent with that reported by Fong et al. [4]. They also reported that the first reduction of FeS 2 occurs at 1.6 V in the range 0#x#4, and the following charge–discharge curves show plateaus at 2.3 and 1.6 V in the range 0#x#2 and 2#x#4, respectively. In this study, plateaus A and B in the charging curves also correspond to the reduction from x50 to 2 and from x52 to 4, respectively. Fig. 2 shows that the cell can be cycled reversibly. The electrochemical behaviors are quite similar when starting from Li 2 FeS 2 synthesized from Li 2 S and FeS. Fig. 3 shows the charge–discharge curves of the cell consisting of Li 2 FeS 2 . The reaction range of Li x FeS 2 in this cell is 2#x#4. Therefore, the charge–discharge curves consist of only one plateau, which corresponds to B or B9 in Fig. 1. The electrochemical reaction is also completely reversible, as indicated in Fig. 4. These results indicate that Li 2 FeS 2 is available as a negative electrode material in a solid state lithium battery.
K. Takada et al. / Solid State Ionics 117 (1999) 273 – 276
Fig. 2. Cycling property of the FeS 2 / LiCoO 2 cell.
275
Fig. 4. Cycling property of the Li 2 FeS 2 / LiCoO 2 cell.
studies, Fe metal is the final product of the reduction of FeS 2 FeS 2 1 2Li 1 1 2e 2 → Li 2 FeS 2 ,
(2)
Li 2 FeS 2 1 2Li 1 1 2e 2 → 2Li 2 S 1 Fe.
(3)
X-ray diffraction patterns of the electrodes are shown in Fig. 5. After reduction to x54, i.e. reduction of Li 2 FeS 2 with 401 mAh / g, some diffraction peaks became broad, and their intensities decreased sig-
Fig. 3. Charge-–discharge curves of the cell Li 2 FeS 2 / LiCoO 2 . Positive electrode: 63 mg LiCoO 2 142 mg solid electrolyte. Negative electrode: 10 mg FeS 2 110 mg solid electrolyte. Charge–discharge current 150 mA.
3.2. Identification of the reduction product The equilibrium phase diagram of Li–Fe–S at 4508C was investigated by Tomczuk et al. [6] in a LiCl–KCl molten electrolyte. The reported phases were also observed in the reduction of FeS 2 when using a liquid electrolyte [4]. It was concluded that the reduction of FeS 2 follows the two-step reaction shown in Eqs. (2) and (3). According to the previous
Fig. 5. X-ray diffraction patterns of the negative electrodes. Li 2 FeS 2 and the solid electrolyte was mixed in the weight ratio 1:1. Li 4 FeS 2 was obtained by the reduction of Li 2 FeS 2 with 401 mAh / g. Li 2.27 FeS 2 was obtained by the oxidation of Li 4 FeS 2 with 347 mAh / g.
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K. Takada et al. / Solid State Ionics 117 (1999) 273 – 276
nificantly. This indicates that the reduction product consists of small particles or amorphous phase. The formation of very small particles of Fe metal was also reported by Jones et al. [7]. Assuming that the final product is Fe metal in this case also, the Fe metal formed by the reduction cannot be oxidized again, because Fe metal is not active in the solid electrolyte (Fig. 1 of Ref. [8]). Actually, the reactions are reversible, as indicated in Figs. 2 and 4. It is concluded that the reduction of FeS 2 leads to the formation of very small particles or amorphous phase, but they do not consist of Fe metal. Only Li 1 is mobile in the glass. We assume that this ion selectivity plays a very important role in suppressing the formation of Fe metal during reduction. The diffraction pattern of Li 2.27 FeS 2 , which was obtained by oxidation with 347 mAh / g following the reduction of Li 2 FeS 2 with 401 mAh / g, is shown in Fig. 5. Broad peaks at around 2u 5308, 408, and 478 were observed in both diffraction patterns for x54 and 2.27. The electrode reactions within the range 2#x#4 may be similar to the Li 1 intercalation reaction observed in Li x TiS 2 [9], because of the structural similarity.
4. Conclusion We investigated the electrode reactions of FeS 2 or Li 2 FeS 2 in Li 1 conductive glass. When they are
reduced to the composition of Li / Fe54, the final product(s) is not Fe metal. The potential within 2#Li / Fe#4 was ca. 1.6 V, and the electrochemical reactions were completely reversible. These results suggest that they are available as an electrode material in solid state lithium batteries. The reduction mechanism, however, is not yet clear. Further study on the reduction product is necessary.
References [1] D.W. Murphy, J.N. Carides, J. Electrochem. Soc. 126 (1979) 349. [2] R.A. Guidotti, F.W. Reinhardt, in: Proceedings of the 9th International Symposium on Molten Salts, 1994 (1994) 820. ´ Mater. Res. Bull. 15 [3] R. Brec, A. Dugast, A.L. Mehaute, (1980) 619. [4] R. Fong, J.R. Dahn, C.H.W. Jones, J. Electrochem. Soc. 136 (1989) 3206. [5] N. Aotani, K. Iwamoto, K. Takada, S. Kondo, Solid State Ionics 68 (1994) 35. [6] Z. Tomczuk, B. Tani, N.C. Otta, M.F. Roche, D.R. Vissers, J. Electrochem. Soc. 129 (1982) 925. [7] C.H.W. Jones, P.E. Kovacs, R.D. Sharma, R.S. McMillan, J. Phys. Chem. 94 (1990) 832. [8] K. Takada, N. Aotani, K. Iwamoto, S. Kondo, Solid State Ionics 86–88 (1996) 877. [9] R.J. Batchelor, F.W.B. Einstein, C.H.W. Jones, R. Fong, J.R. Dahn, Phys. Rev. B 37 (1988) 3699.
Lithium iron sulfide as an electrode material in a solid state lithium battery Kazunori Takada*, Kazuya Iwamoto, Shigeo Kondo Technology Laboratory, Matsushita Battery Industrial Co., Ltd., 1 -1, Matsushita-cho, Moriguchi, Osaka 570 -8511, Japan Received 10 April 1998; accepted 30 August 1998
Abstract The electrochemical properties of Lix FeS 2 in a Li 1 ion conductive glass were investigated. The electrode reaction shows two potential plateaus within the range 0 # x # 4, and is reversible. The reduction of FeS 2 by 4e 2 or Li 2 FeS 2 by 2e 2 did not proceed to form Fe metal as reported in earlier studies. Li x FeS 2 is promising as an electrode active material in a solid state lithium battery. 1999 Elsevier Science B.V. All rights reserved. Keywords: Anode material; Lithium battery; Lithium iron sulfide; Solid electrolyte
1. Introduction Many transition metal sulfides show more than one potential plateau during reduction in Li 1 conductive electrolytes [1]. The equivalent potential of one of the plateaus of each sulfide is lower than 2 V vs. Li 1 / Li. With the decreasing operation voltage of semiconductor devices and the development of high voltage cathodes such as Li 12x CoO 2 and Li 12x Mn 2 O 4 , they have become available as negative electrode materials, even though the potential is more noble than C 6 Li x or Li metal. However, the reversibility of the electrode reactions are not adequate for practical use. Iron sulfide has been investigated as a positive electrode material in thermal batteries [2]. The studies revealed that FeS 2 has a theoretical capacity *Corresponding author. Tel.: 181-6-994-4552; fax: 181-6998-3179; e-mail: [email protected]
as high as 894 mAh / g (4e 2 / FeS 2 ). It shows potential plateaus at 2.3 and 1.6 V [3]. Previous studies of the charge–discharge behavior of FeS 2 in a liquid electrolyte or a polymer electrolyte [3,4] indicated a reversible reaction through the 2.3 V plateau. However, the electrode reaction of the 1.6 V plateau, the potential of which fits a negative electrode in a lithium battery, is reported to be irreversible. We investigated the electrochemical properties of iron sulfides using a Li 1 conductive sulfide glass [5].
2. Experimental
2.1. Synthesis of materials Li 2 FeS 2 was synthesized as follows. Stoichiometric amounts of Li 2 S and FeS were mixed. The mixture was heated in a vitreous carbon crucible under Ar gas flow at 9508C, and then the molten
0167-2738 / 99 / $ – see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S0167-2738( 98 )00413-5
274
K. Takada et al. / Solid State Ionics 117 (1999) 273 – 276
mixture was cooled to room temperature in a furnace. LiCoO 2 was used as a positive electrode material in electrochemical cells to investigate the electrochemical properties of FeS 2 and Li 2 FeS 2 . The starting materials, Li 2 CO 3 and Co 3 O 4 , were mixed and heated at 9508C in air. Lithium ion conductive glass, 0.01Li 3 PO 4 –0.63Li 2 S–SiS 2 , was synthesized as described in a previous paper [5].
2.2. Electrochemical measurements FeS 2 or Li 2 FeS 2 was mixed with the ground glass in a weight ratio of 1:1. The mixture was used as a negative electrode in electrochemical cells. Positive electrodes consisted of a mixture of LiCoO 2 and the solid electrolyte powder in the weight ratio 3:2 mixed in a ball mill. Three layered pellets were constructed with appropriate amounts of the mixtures and the solid electrolyte powder. The mixture containing iron sulfide and that containing LiCoO 2 were pressed together with the solid electrolyte between them in the hole of an insulated tube. The positive electrode consisted of 63 mg LiCoO 2 and 42 mg glass powder. The negative electrode consisted of 10 mg of the mixture containing FeS 2 or 20 mg in the case of Li 2 FeS 2 . The diameter of the hole was 10 mm. Current collectors made of stainless steel were attached to both faces of the pellet. The electrochemical cells were charged and discharged at a constant current of 150 mA at 208C. The crystal structures of the products of the negative electrode were investigated by X-ray diffraction. The electrochemical cell was disassembled after the charge or discharge. The negative electrode was ground and filled into an air-tight sample holder with an Al window. X-ray diffraction was carried out using a Cu Ka source.
3. Results and discussion In the following sections, the reduction reaction of FeS 2 is represented as the formation of Li x FeS 2 as shown in Eq. (1) in order to simplify the discussion, where x corresponds to the depth of the reduction FeS 2 1 xLi 1 1 xe 2 → Li x FeS 2 .
(1)
Fig. 1. Charge-–discharge curves of the cell FeS 2 / LiCoO 2 . Positive electrode: 63 mg LiCoO 2 142 mg solid electrolyte. Negative electrode: 5 mg FeS 2 15 mg solid electrolyte. Charge–discharge current 150 mA.
3.1. Electrochemical behavior Charge–discharge curves of the cell, FeS 2 / LiCoO 2 , are shown in Fig. 1. The first charge curve is quite different from the consecutive curves. It consists of one plateau, but the others consist of two. This result is consistent with that reported by Fong et al. [4]. They also reported that the first reduction of FeS 2 occurs at 1.6 V in the range 0#x#4, and the following charge–discharge curves show plateaus at 2.3 and 1.6 V in the range 0#x#2 and 2#x#4, respectively. In this study, plateaus A and B in the charging curves also correspond to the reduction from x50 to 2 and from x52 to 4, respectively. Fig. 2 shows that the cell can be cycled reversibly. The electrochemical behaviors are quite similar when starting from Li 2 FeS 2 synthesized from Li 2 S and FeS. Fig. 3 shows the charge–discharge curves of the cell consisting of Li 2 FeS 2 . The reaction range of Li x FeS 2 in this cell is 2#x#4. Therefore, the charge–discharge curves consist of only one plateau, which corresponds to B or B9 in Fig. 1. The electrochemical reaction is also completely reversible, as indicated in Fig. 4. These results indicate that Li 2 FeS 2 is available as a negative electrode material in a solid state lithium battery.
K. Takada et al. / Solid State Ionics 117 (1999) 273 – 276
Fig. 2. Cycling property of the FeS 2 / LiCoO 2 cell.
275
Fig. 4. Cycling property of the Li 2 FeS 2 / LiCoO 2 cell.
studies, Fe metal is the final product of the reduction of FeS 2 FeS 2 1 2Li 1 1 2e 2 → Li 2 FeS 2 ,
(2)
Li 2 FeS 2 1 2Li 1 1 2e 2 → 2Li 2 S 1 Fe.
(3)
X-ray diffraction patterns of the electrodes are shown in Fig. 5. After reduction to x54, i.e. reduction of Li 2 FeS 2 with 401 mAh / g, some diffraction peaks became broad, and their intensities decreased sig-
Fig. 3. Charge-–discharge curves of the cell Li 2 FeS 2 / LiCoO 2 . Positive electrode: 63 mg LiCoO 2 142 mg solid electrolyte. Negative electrode: 10 mg FeS 2 110 mg solid electrolyte. Charge–discharge current 150 mA.
3.2. Identification of the reduction product The equilibrium phase diagram of Li–Fe–S at 4508C was investigated by Tomczuk et al. [6] in a LiCl–KCl molten electrolyte. The reported phases were also observed in the reduction of FeS 2 when using a liquid electrolyte [4]. It was concluded that the reduction of FeS 2 follows the two-step reaction shown in Eqs. (2) and (3). According to the previous
Fig. 5. X-ray diffraction patterns of the negative electrodes. Li 2 FeS 2 and the solid electrolyte was mixed in the weight ratio 1:1. Li 4 FeS 2 was obtained by the reduction of Li 2 FeS 2 with 401 mAh / g. Li 2.27 FeS 2 was obtained by the oxidation of Li 4 FeS 2 with 347 mAh / g.
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K. Takada et al. / Solid State Ionics 117 (1999) 273 – 276
nificantly. This indicates that the reduction product consists of small particles or amorphous phase. The formation of very small particles of Fe metal was also reported by Jones et al. [7]. Assuming that the final product is Fe metal in this case also, the Fe metal formed by the reduction cannot be oxidized again, because Fe metal is not active in the solid electrolyte (Fig. 1 of Ref. [8]). Actually, the reactions are reversible, as indicated in Figs. 2 and 4. It is concluded that the reduction of FeS 2 leads to the formation of very small particles or amorphous phase, but they do not consist of Fe metal. Only Li 1 is mobile in the glass. We assume that this ion selectivity plays a very important role in suppressing the formation of Fe metal during reduction. The diffraction pattern of Li 2.27 FeS 2 , which was obtained by oxidation with 347 mAh / g following the reduction of Li 2 FeS 2 with 401 mAh / g, is shown in Fig. 5. Broad peaks at around 2u 5308, 408, and 478 were observed in both diffraction patterns for x54 and 2.27. The electrode reactions within the range 2#x#4 may be similar to the Li 1 intercalation reaction observed in Li x TiS 2 [9], because of the structural similarity.
4. Conclusion We investigated the electrode reactions of FeS 2 or Li 2 FeS 2 in Li 1 conductive glass. When they are
reduced to the composition of Li / Fe54, the final product(s) is not Fe metal. The potential within 2#Li / Fe#4 was ca. 1.6 V, and the electrochemical reactions were completely reversible. These results suggest that they are available as an electrode material in solid state lithium batteries. The reduction mechanism, however, is not yet clear. Further study on the reduction product is necessary.
References [1] D.W. Murphy, J.N. Carides, J. Electrochem. Soc. 126 (1979) 349. [2] R.A. Guidotti, F.W. Reinhardt, in: Proceedings of the 9th International Symposium on Molten Salts, 1994 (1994) 820. ´ Mater. Res. Bull. 15 [3] R. Brec, A. Dugast, A.L. Mehaute, (1980) 619. [4] R. Fong, J.R. Dahn, C.H.W. Jones, J. Electrochem. Soc. 136 (1989) 3206. [5] N. Aotani, K. Iwamoto, K. Takada, S. Kondo, Solid State Ionics 68 (1994) 35. [6] Z. Tomczuk, B. Tani, N.C. Otta, M.F. Roche, D.R. Vissers, J. Electrochem. Soc. 129 (1982) 925. [7] C.H.W. Jones, P.E. Kovacs, R.D. Sharma, R.S. McMillan, J. Phys. Chem. 94 (1990) 832. [8] K. Takada, N. Aotani, K. Iwamoto, S. Kondo, Solid State Ionics 86–88 (1996) 877. [9] R.J. Batchelor, F.W.B. Einstein, C.H.W. Jones, R. Fong, J.R. Dahn, Phys. Rev. B 37 (1988) 3699.