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Electrochemical behavior of LixMO2 (M = Co, Ni) in all solid state cells using a glass electrolyte
SOLID
ELSEWIER
STATE IONICS
Solid State tonics 79 (1995) 284-287
Electrochemical behavior of Li,MO, ( M = Co, Ni) in all solid state cells using a glass electrolyte Kazunori Takada *, Noboru Aotani, Kazuya Iwamoto, Shigeo Kondo Technology Laboratory, Matsushita Battery Industrial, Co. Ltd., 1, Matsushita-cho, Moriguchi, Osaka 570, Japan
Abstract Electrochemical behavior of transition metal oxides, Li,MO, (M = Co, Ni) was examined in all solid state cells using a lithium ion conductive glass, Li,PO,-Li,S-SiS,, as the electrolyte and Li metal as the negative electrode. Complex impedance analysis showed that the electrochemical reaction rate at Li,CoO, electrode was mainly controlled by diffusion for x > 0.95 and by charge transfer for x < 0.95. Coulometric titration showed that these electrode materials had specific capacities of 80-90 mAh/g, coulombic efficiencies of lOO%, and equivalent voltages of around 4 V versus Li, so they are promising candidates for positive electrode materials for all solid state lithium batteries. Keywords: Solid electrolyte;
Lithium cobalt oxide; Sulfide glass; Complex impedance;
1. Introduction Solid state lithium batteries are of great interest because of their high performance as well as their high reliabilities. One of the reasons that have prevented them from practical use is their small operating current densities, which are caused by the poor ionic conductivities of lithium ion conductive solid electrolytes. It is necessary to employ a solid electrolyte with a high ionic conductivity to develop practical batteries. Lithium iodide doped sulfide glasses showed conductivities as high as 1O-3 S/cm [l--3]. The iodide ions in the electrolytes are, however, considered to be oxidized at a potential greater than 3 V; transition metal sulfides, for example, were studied for positive
* Corresponding 998-3179.
author:
Tel
+81-6-994-45.52;
Fax
+81-6-
0167-2738/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0167-2738(95)00075-5
Electrode reaction
electrodes in solid state batteries whose voltages seldom exceeded 3 V [1,4-61. On the other hand, lithium transition metal oxides including lithium cobalt oxide are of interest to study as positive electrode materials in rechargeable lithium batteries with organic electrolytes, because of their high equivalent voltages, about 4 V [7,8]. The voltages are so high that they will not be employed as the positive electrode materials in combination with the solid electrolytes. The authors reported on Li/TiS, batteries with a lithium ion conductive solid electrolyte, Li,PO,Li,S-SiS,, which were operated at 2 V [9]. High ionic conductivity of the electrolyte enabled the battery to be operated at a high current density up to 600 kA/cm*, and this solid electrolyte had a wide electrochemical window [lo]. The high stability of the electrolyte makes possible the use of transition metal oxides as positive electrodes.
K. Takada et al. /Solid
285
State Ionics 79 (1995) 284-287
In this study, we investigated the electrochemical behaviors of Li,CoO, and Li,NiO, in solid state batteries using the lithium ion conductive sulfide glass.
2. Experimental The lithium ion conductive glass, O.OlLi,PO,0.63 Li,S-0.36 SiS,, was synthesized by twin-roller quenching as reported previously [ll]. The obtained glass was ground with a planetary ball mill to construct electrochemical cells. LiCoO, was obtained by heating a mixture of stoichiometric amounts of Li,CO, and Co,O, at 900°C for 10 h. LiNiO, was synthesized from LiOH and Ni(OH),. The mixture of the starting materials was heated at 550°C for 15 h and then heated at 750°C for 2 h. An electrochemical cell was constructed as follows. The working electrode consisted of a mixture of 20 mg LiCoO, and 30 mg solid electrolyte. For the LiNiO, electrode, the working electrode consisted of a mixture of 37 mg LiNiO, and 24 mg solid electrolyte. The electrode material and solid electrolyte were stacked and pressed together inside a hole of an insulator tube. A lithium foil was attached to the face of the solid electrolyte layer as a counter electrode. The diameter of the cell was 10 mm. Coulometric tritration curves were obtained at a constant current of 50 p,A for LiCoO, and 100 PA for LiNiO,. The electrochemical cell with LiCoO, was also charged by intermittent current pulses at 50 p.A for 30 min. Open circuit voltage was measured as the cell voltage at one hour after the current pulse. Overvoltage (AV) was measured as a voltage difference between the end of the rest period and the beginning of the applied current (see Fig. 1). The three-electrode cell for impedance measurements was almost the same as the two-electrode cell, except for embedding a silver wire in the solid electrolyte layer as a reference electrode. The working electrode consisted of a mixture of 36 mg LiCoO, and 55 mg solid electrolyte. The cell was polarized at a constant current to deintercalate appropriate amounts of lithium from LiCoO,. The ac impedance of the Li,CoO, electrode was measured in the frequency range of 106-10-3 Hz using a Solartron
4.11w /h
129
Time
Fig. 1. Cell voltage of the Li/Li,CoO, tent pulse charging.
cell during the intermit-
1286 electrochemical interface connected to a Solartron 1260 frequency response analyzer. The signal across the cell was 10 mV.
3. Results and discussion Fig. 2 shows coulometric titration curves of the of the 1st and 4th charge-discell, Li/LiXCo02, charge cycles. The horizontal axis of the figure indicates lithium composition x in Li,CoO, calculated from the electrical charge that passed through the cell. The titration curve at each cycle had a plateau at 4 V with a specific capacity of 80 mAh/g. Coulombic efficiencies were almost 100% except for the 1st cycle. Fig. 3 shows the composition dependence of the open circuit voltage and AV measured by the inter-
Electrical charge / mAb g.1
5w 4.5 1st cycle
> L S -g 3.5 i u
4th cycle
3 2.5
.
42 u 0.5
0.6
0.7
0.8
0.9
1
x in LixCoO,
Fig. 2. Coulometric
titration curves of Li,CoO,.
K. Takada et al. /Solid State Ionics 79 (1995) 284-287
286
0.3
0.4
0.5
0.6 x
0.7
0.8
0.9
I
in LixcOO,
Fig. 3. Quasi open circuit voltage and overvoltage Li/Li,CoO,, charged by intermittent current pulses.
of the cell,
mittent charge. The overvoltage decreased from 2.0 V to 0.1 V with deintercalation from x = 1.0 to 0.97, and remained constant for 0.97 > x > 0.4. For x < 0.4, the overvoltage increased from 0.1 V to 1.4 V, and the cell voltage decreased from 4.6 V to 4.4 V. Fig. 4 shows the relationship between log w and log Z for Li.CoO, with x 2 0.975. In the low frequency range of 10-l to 10e3 Hz, the impedance Z was proportional to w-l/2 for Li,CoO, with 1.0 2 x 2 0.985, which indicates that the rate of electrode reaction was mainly controlled by diffusion at the low frequency. The diffusion-controlled impedance decreased drastically from x = 1.0 to x
Fig. 4. Impedances of the Li,CoO, electrodes with various lithium compositions as a function of applied frequency.
Fig. 5. Complex impedance plots of the Li,CoO, various lithium compositions.
electrodes with
= 0.975, which resulted in the rapid change in overvoltage observed in the range of x 2 0.95. Fig. 5 shows the complex impedance plots of the Li,CoO, with 0.4 I x < 0.95. Each impedance plot consisted of two semicircles. One corresponds to the impedance of solid electrolyte (Z,) and the other to that of the electrode controlled by charge transfer (Z,). Diffusion controlled impedances were small in this composition range. The charge transfer resistance of Z, increased rapidly as lithium ions were deintercalated to x I 0.5, which resulted in the large overvoltage observed in this composition range. Electrode properties of Li,CoO, with liquid organic electrolytes have been reported by many researchers. The change in crystal structure of Li.CoO, from hexagonal to monoclinic symmetry was reported at x = 0.5 [12]. The composition ranges of reversible electrode reaction were reported to be x 2 0.5 [7] and x > 0.4 [12] in Li.CoO,. Lithium ions were completely removed electrochemically [7]. In the present study using solid electrolyte cells, a reversible electrode reaction was observed for the composition range of 1.0 2 x 2 0.5 in Li,CoO,, and the increase in the charge transfer resistances prevented the deintercalation at x > 0.5 due to the large overvoltage. Electrode behaviors of lithium nickel oxide were also investigated by coulometric titration. Coulometric titration curves of the cell, Li/Li,NiO,, during
K. Takmia et al. /Solid
State Ionics 79 (199.5) 284-287
Electrical charge / mAh g.’ 0
20 40 60 80 100 120 140 160
‘r-----l
287
(3) lithium cobalt oxide and lithium nickel oxide are promising candidates for positive electrode materials in solid state lithium batteries, due to their characteristics of high voltage, large capacity, and high reversibility.
References ill
21 0.4
0.5
0.6
0.7
0.8
0.9
1
x in LixNiO,
Fig. 6. Coulometric
titration curves of Li .NiO,
the 1st and 4th cycles are shown in Fig. 6. The curve also had a plateau at 4 V with a specific capacity of 90 mAh g-’ . The overvoltage was slightly larger than that of lithium cobalt oxide.
4. Conclusions (1) The electrode reaction rate at Li,CoO, electrode is mainly controlled by diffusion of lithium ion for x > 0.95 and by charge transfer for x < 0.95; (2) the increase in charge transfer impedance observed in x I 0.5 limits the specific capacity of Li,CoO,, and
R. Mercier, J.P. Malugani, B. Fahys and G. Robert, Solid State Ionics 5 (1981) 663. Dl H. Wada, M. Menetrier, A. Levasseur and P. Hagenmuller, Mater. Res. Bull. 18 (1983) 189. [31 S. Sahami, S.W. Shea and J.H. Kennedy, J. Electrochem. Sot. 132 (1985) 985. [41 J.P. Malugani, B. Fahys, R. Mercier, G. Robert, J.P. Duchange, S. Baudry, M. Broussely and J.P. Gabano, Solid State Ionics 9/10 (1983) 659. 151J.H. Kennedy, Z. Zhang and H. Eckert, J. Non-Cryst. Solids 123 (1990) 328. [61SD. Jones and J.R. Akridge, J. Power Sources 43-44 (1993) 505. t71 K. Mizushima, P.C. Jones, P.J. Wiseman and J.B. Goodenough, Mater. Res. Bull. 17 (1980) 785. [Sl M.G.S.R. Thomas, W.I.F. David, J.B. Goodenough and P. Graves, Mater. Res. Bull. 20 (1985) 1137. 191 K. Iwamoto, N. Aotani, K. Takada and S. Kondo, Denki Kagaku 63 (1995) 25. ml S. Kondo, K. Takada and Y. Yamamura, Solid State Ionics 53-56 (1992) 1183. Ill1 N. Aotani, K. Iwamoto, K. Takada and S. Kondo, Solid State Ionics 68 (1994) 35. 1121J.N. Reimers and J.R. Dahn, J. Electrochem. Sot. 139 (1992) 2091.
ELSEWIER
STATE IONICS
Solid State tonics 79 (1995) 284-287
Electrochemical behavior of Li,MO, ( M = Co, Ni) in all solid state cells using a glass electrolyte Kazunori Takada *, Noboru Aotani, Kazuya Iwamoto, Shigeo Kondo Technology Laboratory, Matsushita Battery Industrial, Co. Ltd., 1, Matsushita-cho, Moriguchi, Osaka 570, Japan
Abstract Electrochemical behavior of transition metal oxides, Li,MO, (M = Co, Ni) was examined in all solid state cells using a lithium ion conductive glass, Li,PO,-Li,S-SiS,, as the electrolyte and Li metal as the negative electrode. Complex impedance analysis showed that the electrochemical reaction rate at Li,CoO, electrode was mainly controlled by diffusion for x > 0.95 and by charge transfer for x < 0.95. Coulometric titration showed that these electrode materials had specific capacities of 80-90 mAh/g, coulombic efficiencies of lOO%, and equivalent voltages of around 4 V versus Li, so they are promising candidates for positive electrode materials for all solid state lithium batteries. Keywords: Solid electrolyte;
Lithium cobalt oxide; Sulfide glass; Complex impedance;
1. Introduction Solid state lithium batteries are of great interest because of their high performance as well as their high reliabilities. One of the reasons that have prevented them from practical use is their small operating current densities, which are caused by the poor ionic conductivities of lithium ion conductive solid electrolytes. It is necessary to employ a solid electrolyte with a high ionic conductivity to develop practical batteries. Lithium iodide doped sulfide glasses showed conductivities as high as 1O-3 S/cm [l--3]. The iodide ions in the electrolytes are, however, considered to be oxidized at a potential greater than 3 V; transition metal sulfides, for example, were studied for positive
* Corresponding 998-3179.
author:
Tel
+81-6-994-45.52;
Fax
+81-6-
0167-2738/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0167-2738(95)00075-5
Electrode reaction
electrodes in solid state batteries whose voltages seldom exceeded 3 V [1,4-61. On the other hand, lithium transition metal oxides including lithium cobalt oxide are of interest to study as positive electrode materials in rechargeable lithium batteries with organic electrolytes, because of their high equivalent voltages, about 4 V [7,8]. The voltages are so high that they will not be employed as the positive electrode materials in combination with the solid electrolytes. The authors reported on Li/TiS, batteries with a lithium ion conductive solid electrolyte, Li,PO,Li,S-SiS,, which were operated at 2 V [9]. High ionic conductivity of the electrolyte enabled the battery to be operated at a high current density up to 600 kA/cm*, and this solid electrolyte had a wide electrochemical window [lo]. The high stability of the electrolyte makes possible the use of transition metal oxides as positive electrodes.
K. Takada et al. /Solid
285
State Ionics 79 (1995) 284-287
In this study, we investigated the electrochemical behaviors of Li,CoO, and Li,NiO, in solid state batteries using the lithium ion conductive sulfide glass.
2. Experimental The lithium ion conductive glass, O.OlLi,PO,0.63 Li,S-0.36 SiS,, was synthesized by twin-roller quenching as reported previously [ll]. The obtained glass was ground with a planetary ball mill to construct electrochemical cells. LiCoO, was obtained by heating a mixture of stoichiometric amounts of Li,CO, and Co,O, at 900°C for 10 h. LiNiO, was synthesized from LiOH and Ni(OH),. The mixture of the starting materials was heated at 550°C for 15 h and then heated at 750°C for 2 h. An electrochemical cell was constructed as follows. The working electrode consisted of a mixture of 20 mg LiCoO, and 30 mg solid electrolyte. For the LiNiO, electrode, the working electrode consisted of a mixture of 37 mg LiNiO, and 24 mg solid electrolyte. The electrode material and solid electrolyte were stacked and pressed together inside a hole of an insulator tube. A lithium foil was attached to the face of the solid electrolyte layer as a counter electrode. The diameter of the cell was 10 mm. Coulometric tritration curves were obtained at a constant current of 50 p,A for LiCoO, and 100 PA for LiNiO,. The electrochemical cell with LiCoO, was also charged by intermittent current pulses at 50 p.A for 30 min. Open circuit voltage was measured as the cell voltage at one hour after the current pulse. Overvoltage (AV) was measured as a voltage difference between the end of the rest period and the beginning of the applied current (see Fig. 1). The three-electrode cell for impedance measurements was almost the same as the two-electrode cell, except for embedding a silver wire in the solid electrolyte layer as a reference electrode. The working electrode consisted of a mixture of 36 mg LiCoO, and 55 mg solid electrolyte. The cell was polarized at a constant current to deintercalate appropriate amounts of lithium from LiCoO,. The ac impedance of the Li,CoO, electrode was measured in the frequency range of 106-10-3 Hz using a Solartron
4.11w /h
129
Time
Fig. 1. Cell voltage of the Li/Li,CoO, tent pulse charging.
cell during the intermit-
1286 electrochemical interface connected to a Solartron 1260 frequency response analyzer. The signal across the cell was 10 mV.
3. Results and discussion Fig. 2 shows coulometric titration curves of the of the 1st and 4th charge-discell, Li/LiXCo02, charge cycles. The horizontal axis of the figure indicates lithium composition x in Li,CoO, calculated from the electrical charge that passed through the cell. The titration curve at each cycle had a plateau at 4 V with a specific capacity of 80 mAh/g. Coulombic efficiencies were almost 100% except for the 1st cycle. Fig. 3 shows the composition dependence of the open circuit voltage and AV measured by the inter-
Electrical charge / mAb g.1
5w 4.5 1st cycle
> L S -g 3.5 i u
4th cycle
3 2.5
.
42 u 0.5
0.6
0.7
0.8
0.9
1
x in LixCoO,
Fig. 2. Coulometric
titration curves of Li,CoO,.
K. Takada et al. /Solid State Ionics 79 (1995) 284-287
286
0.3
0.4
0.5
0.6 x
0.7
0.8
0.9
I
in LixcOO,
Fig. 3. Quasi open circuit voltage and overvoltage Li/Li,CoO,, charged by intermittent current pulses.
of the cell,
mittent charge. The overvoltage decreased from 2.0 V to 0.1 V with deintercalation from x = 1.0 to 0.97, and remained constant for 0.97 > x > 0.4. For x < 0.4, the overvoltage increased from 0.1 V to 1.4 V, and the cell voltage decreased from 4.6 V to 4.4 V. Fig. 4 shows the relationship between log w and log Z for Li.CoO, with x 2 0.975. In the low frequency range of 10-l to 10e3 Hz, the impedance Z was proportional to w-l/2 for Li,CoO, with 1.0 2 x 2 0.985, which indicates that the rate of electrode reaction was mainly controlled by diffusion at the low frequency. The diffusion-controlled impedance decreased drastically from x = 1.0 to x
Fig. 4. Impedances of the Li,CoO, electrodes with various lithium compositions as a function of applied frequency.
Fig. 5. Complex impedance plots of the Li,CoO, various lithium compositions.
electrodes with
= 0.975, which resulted in the rapid change in overvoltage observed in the range of x 2 0.95. Fig. 5 shows the complex impedance plots of the Li,CoO, with 0.4 I x < 0.95. Each impedance plot consisted of two semicircles. One corresponds to the impedance of solid electrolyte (Z,) and the other to that of the electrode controlled by charge transfer (Z,). Diffusion controlled impedances were small in this composition range. The charge transfer resistance of Z, increased rapidly as lithium ions were deintercalated to x I 0.5, which resulted in the large overvoltage observed in this composition range. Electrode properties of Li,CoO, with liquid organic electrolytes have been reported by many researchers. The change in crystal structure of Li.CoO, from hexagonal to monoclinic symmetry was reported at x = 0.5 [12]. The composition ranges of reversible electrode reaction were reported to be x 2 0.5 [7] and x > 0.4 [12] in Li.CoO,. Lithium ions were completely removed electrochemically [7]. In the present study using solid electrolyte cells, a reversible electrode reaction was observed for the composition range of 1.0 2 x 2 0.5 in Li,CoO,, and the increase in the charge transfer resistances prevented the deintercalation at x > 0.5 due to the large overvoltage. Electrode behaviors of lithium nickel oxide were also investigated by coulometric titration. Coulometric titration curves of the cell, Li/Li,NiO,, during
K. Takmia et al. /Solid
State Ionics 79 (199.5) 284-287
Electrical charge / mAh g.’ 0
20 40 60 80 100 120 140 160
‘r-----l
287
(3) lithium cobalt oxide and lithium nickel oxide are promising candidates for positive electrode materials in solid state lithium batteries, due to their characteristics of high voltage, large capacity, and high reversibility.
References ill
21 0.4
0.5
0.6
0.7
0.8
0.9
1
x in LixNiO,
Fig. 6. Coulometric
titration curves of Li .NiO,
the 1st and 4th cycles are shown in Fig. 6. The curve also had a plateau at 4 V with a specific capacity of 90 mAh g-’ . The overvoltage was slightly larger than that of lithium cobalt oxide.
4. Conclusions (1) The electrode reaction rate at Li,CoO, electrode is mainly controlled by diffusion of lithium ion for x > 0.95 and by charge transfer for x < 0.95; (2) the increase in charge transfer impedance observed in x I 0.5 limits the specific capacity of Li,CoO,, and
R. Mercier, J.P. Malugani, B. Fahys and G. Robert, Solid State Ionics 5 (1981) 663. Dl H. Wada, M. Menetrier, A. Levasseur and P. Hagenmuller, Mater. Res. Bull. 18 (1983) 189. [31 S. Sahami, S.W. Shea and J.H. Kennedy, J. Electrochem. Sot. 132 (1985) 985. [41 J.P. Malugani, B. Fahys, R. Mercier, G. Robert, J.P. Duchange, S. Baudry, M. Broussely and J.P. Gabano, Solid State Ionics 9/10 (1983) 659. 151J.H. Kennedy, Z. Zhang and H. Eckert, J. Non-Cryst. Solids 123 (1990) 328. [61SD. Jones and J.R. Akridge, J. Power Sources 43-44 (1993) 505. t71 K. Mizushima, P.C. Jones, P.J. Wiseman and J.B. Goodenough, Mater. Res. Bull. 17 (1980) 785. [Sl M.G.S.R. Thomas, W.I.F. David, J.B. Goodenough and P. Graves, Mater. Res. Bull. 20 (1985) 1137. 191 K. Iwamoto, N. Aotani, K. Takada and S. Kondo, Denki Kagaku 63 (1995) 25. ml S. Kondo, K. Takada and Y. Yamamura, Solid State Ionics 53-56 (1992) 1183. Ill1 N. Aotani, K. Iwamoto, K. Takada and S. Kondo, Solid State Ionics 68 (1994) 35. 1121J.N. Reimers and J.R. Dahn, J. Electrochem. Sot. 139 (1992) 2091.