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Salting-out in intercalation compounds: Removing copper from Cu3Mo6S8 by intercalating lithium
0038-1098/84 $3.00 + .OO Pergamon Press Ltd.
Solid State Communications,Vo1.52,No.3, pp.245-248, 1984. Printed in Great Britain.
SALTING-GUI IN INTERCALATION COMPOUNDS: REMOVING COPPER FROM Cu3M06S, BY INTERCALATING LITHIUM W.R. McKinnon and J.R. Dahn CT Solid State Chemistry, Chemistry Division, National Research Council of Canada, Ottawa KlA OR9 CANADA
(Received 15 June 1984 by R. Barrie)
We study the removal of the copper in Cu3M06SB by the intercalation of lithium. For ol, the copper atoms are forced out of the Mo6SS host. We present a lattice-gas model which shows how the intercalation of Li can force the Cu out of the host.
Most of the ternary Chevrel compounds, A Mo6XB (A = metal, X = S, Se or Te) can be pzepared by direct reaction of the elements at high temperatures (1,Z). Of the binary Chevrel compounds, only Mo6SB cannot be prepared in this way. To prepare Mo6SB the A atoms in MO6SB (eg. A = Cu) are removed by chemical re $ ctions, like acid leaching, at room temperature (3,4,5). Recently (5,6) it has been found that the Cu in Cu 06SS can be displaced from the host by inrercalating lithium to produce Li4MogSB and elemental copper. Subsequent de-intercalation of the lithium produces Mo6SS. However, these studies did not report the details of the copper displacement. Here we report the intercalation of Li in LixCu3M06SB. Using Li/LixCu3M06Ss electrochemical cells and in-situ x-ray diffraction (7), we found that copper stays in the host for xc1 but comes out for x>l. We discuss how the added Li can change the chemical potential of Cu so as to force it out of the host. Highly crystalline Cu3M06SS powder was prepared by direct reaction and Cu S, MO and MoS2 in sealed quarts ampoules at f15O'C. The rhombohedral lattice parameters of our CugMogSB, a = 6.571 A and a - 95.56' are in good agreement with those reported by Flukiger et al (8). We measured the voltage V as a function of x in Li/LixCu3M06SB cells described previously (9). The cells were charged or discharged at a constant'current while V was measured (10). The structure of LixCu3M06SB was studied by in-situ x-ray diffraction of Li/LixCu3M06Se cells with beryllium x-ray windows (7). Homogeneously intercalated samples were prepared by fixing the cell's voltage and waiting for the intercalation to reach equilibrium. For these samples, x was calculated either from the charge transferred during equilibration and the mass of Cu3M06SB in the cell or from V(x). In this in-situ technique, changes in the intensity of Bragg peaks reflect directly changes in the
intercalation electrode, not simply changes in the amount of material in the x-ray beam. Figure 1 shows V(x) for two Li/LixCu3M06SS cells at 28OC for O
2.2 E &J 2 9
2.0
. 1.8 0.0
I
0.2
I
I
0.4 x in
Figure 1
I
I
0.6
I
I
0.0
I
J 1.0
Lti,Cu,Mo,S,
V(x) for Li/LixCu3M06SS cells. Solid curve - discharge, dashed curve - charge and l - equilibrium
successively lower voltages as described above and x was calculated from the charge in the cell. The agreement between the data shows the reversibility of intercalation in this material and that these cells are near even at 25 hour rates. At each of the data points in figure 1, x-ray profiles were taken and the lattice parameters determined by least squares to the positions of at least 15 Bragg peaks. Table 1 shows typical results. Figure 2 shows a, a and the rhombohedral LfxCU3MOg&3. The
246
SALTING-OUT IN INTERCALATIONCOMPOUNDS
Table 1 Least squares refinement of the Bragg peak positions to obtain a and a. For peaks with scattering angles greater than 55', the Cu Ra doublet is resolved and we give the position of the Kal peak. The al wavelength has been used to compute the plane spacings for these peaks. The refined lattice parameters were a=6.6036 A and a=95.348O for Li_50Cu3M06S8. hkl
28 observed
28 calculated
100 ll-0 110 2TO 2T1 211 270 271 3T0 370 371 4T0 41T 4fT 437 430 530 520 521 43-o
13.564 18.177 20.190 29.265 32.963 36.400 36.793 39.395 42.313 47.670 50.221 56.548 59.595 61.689 74.019 76.246 81.818 82.053 85.927 91.337
13.539 18.170 20.164 29.251 32.967 36.417 36.818 39.398 42.332 47.659 50.201 56.553 59.581 61.679 74.022 76.233 81.822 82.045 85.944 91.343
d (A) calculated 6.540 4.882 4.404 3.053 2.717 2.467 2.441 2.2870 2.1350 1.9066 1.8158 1.6260 1.5504 1.5026 1.2796 1.2479 1.1762 1.1736 1.1300 1.0768
1
a 3 8
98.5
. .
98.4
. .
i
"
l.
.
-I
0.0
0.2
0.4
0.6
0.8
x in Li&u,Mo6S8 Figure 2
Rhombohedral lattice parameters and unit cell volume versus x in LixCu3MosSs.
1.0
Vol. 52, No. 3
lattice parameters change reversibly with x for x
peak in curve
5 is
just
as large
as
SALTING-OUT IN INTERCALATION COMPOUNDS
Vol. 52, No. 3 I
I
247
2.0
,
a-
2.5 t I l ‘\
‘.
------
1.5
b-
1.0
0.5
2.0
41.0
43.0
42.0
44.0
Scattering Angle (deg) l.5 2.5
/ \
Figure 4
d-
Portions of in-situ x-ray diffraction profiles (see text). i) Li_9CuSMo6Ss 2) Li gCugMo6Sg, Li3 7M06SB and Cu 3) Li,)io6Ssand Cu 4) Mo6SB, Cu4MogSS and Cu 5) Mo,S, and Cu The $l"ler indices of the Bragg peaks are indicated. Peaks A and B are from Li4M06Ss (12). Note that curve 3 is plotted twice. The beryllium oxide peak comes from the x-ray window. l
2.0
1.5 0
2
1
x Figure 3
in
3
4
Li&up0(&
V(x) for an Li/LixCuSMo6SS cell which was charged (dashed curves) and discharged (solid curves) with currents which took 3 hours to change x by 1. a) first cycle, b) second cycle, c) third cycle, d) eleventh cycle.
in curve 3, showing that eventually all the Cu stays out of the host even when all the Li is de-intercalated. Bragg peaks from Mo6Ss are observed in curve 5. In the displacement of (XIby Id, Li pushes Cu out of the host onto the surface where the Cu forms copper metal. This displacement is not an ion-exchange process of the type Li+(solution) + Cucintercalated) z Cu+(solution) + Licintercalated). An ion-exchange process would not give metallic copper; if Cu+ was not soluble, a copper salt would precipitate. In addition, in-situ x-ray experiments show that Cu displacement ceases when the discharge current is stopped while the cell is on the plateau at 1.852 V. By contrast, ion-exchange would proceed with no discharge current.
The displacement of Cu by Li can only occur if the Li changes the chemical potential of Cu in the host. When the Cu is inside the host, its chemical potential (relative to Cu metal) is negative. In order for Li to force the Cu to leave the host, the Li must increase the chemical potential of the Cu to zero. Such an increase can occur even in the simplest lattice-gas models. Consider a lattice of 4N sites which are filled by xN Li atoms and yN Cu atoms, where (kx<4 and O(yt4. If we assume the intercalated atoms do not interact with each other, the Helmholtz free energy, F, is
F =ELx + Ecy + kT [y log(y/4) N + x log(x/4) + (4-x-y) log((4-x-y)/4)]
[l]
EL and E are the differences in energy between a Li or 6 atom in a lattice site and in Li or Cu metal respectively. The third term in equation (1) is the configurationalentropy times the temperature T; k is Boltzman's constant. The chemical potentials of Li and Cu, nL and u, are (13) pL = +g, 0 and
= EL + kT log[x/4-xy)]
248
PC
SALTING-OUT IN INTERCALATIONCOMPOUNDS
0=
= L dF
Ndyx
E, + kT
log[ y/(4-x-y)]
131
Note that the chemical potential of Cu depends on x as well as on y. The values of EL and E, must be about -2eV and -0.2 eV respectively, from measurements made on Li/LlxMo6SB and Cu/Cu Mo6Ss cells (14). With such values for EL an3 g,, uLcwill reach zero before uL does as x increases, so the Cu wfll come out of the host. Several processes in chemistry arise because adding one component to a solution changes the chemical potential of the other components (15). For example, the chemical potential of water is lower in a salt solution than in pure water. If a salt solution is separated from pure water by a semi-permeable membrane that passes only water, water will flow to the salt solution until the pressure difference (the osmotic pressure) between the two liquids makes the chemical potentials of water on each side of the membrane equal. An analogy closer to our problem is salting-out (15): salt is more soluble In water than in alcohol, 60 alcohol added to salt water can precipatate the salt. The salt precipitates when the chemical potential of salt in solution equals the chemical potential of solid salt, in the same way that Cu leaves the Mo6SB host when its chemical potential In the host is equal to the chemical potential of Cu in Cu metal.
Vol. 52, No. 3
In the model of equations (l-3), Cu comes out of the host continuouslywhile the host remains a single phase. Experimentally, however, two phases of the host coexist with metallic Cu during the plateau at 1.852V. (If we consider the host, Mo6SR, as a single component, then we can regard the system Li-Cu-Mo6SB as a ternary system. When three phases coexist in a ternary system the chemical potential of each of the components is constant (151, so we expect a plateau in the voltage as observed.) We are still studying more complicated models which consider interactions between the guest atoms, as well as models which consider the real lattice of available sites In Mo6SB; these models may explain this experimental observation. These complications, however, do not alter the basic notion that Li changes the chemical potential of Cu and so causes the Cu to leave the host. The above arguments imply that the copper should go back into the host when the Li is de-intercalated. Experimentally,we see that some Cu goes back into the host the first few times Li/LixCuBMogSg cells are recharged but eventually all the Cu stays out of the host. This implies that the Cu metal becomes detached from the particles of the Chevrel compounds. Acknowledgement: We thank P.J. Mulhern for providing us with the CuBMo6SB sample.
References
1. K. Yvon, in Current Topics In Materials Science, edited by E. Kaldls (North-Holland,Amsterdam 1979) Vol 3. p.53. 2. 0. Fischer, Appl. Phys. '6, 1 (1978). 3. R. Schollhorn, M. Kumpers and J.O. Besenhard, Mat. Res. Bull. 12, 781 (1977) 4. R. Chevrel, M. Sergent and J. Prigent, Mat. Res. Bull. 2, 1487 (1974). 5. J.M. Tarascon, F.J. DiSalvo, D.W. Murphy, G.W. Hull and J.V. Waszczak, in press. 6. P.J. Mulhem and R.R. Haering, Can. J. Phys. in press. 7. J.R. Dahn, M.A. Py and R.R. Baering, Can. J. Phys. E, 307 (1982).
a.
9. 10. 11. 12. 13.
14.
R. Flukiger, R. Balllif, J. Muller and K. Yvon, J. Less Common Metals c, 193 (1980). D.C. Dahn and R.R. Raering, Solid State comm. E, 29 (1982). J.R. Dahn and W.R. McKinnon, J. Electrochem. Sot. 131, 1823 (1984). R.D. Shannon, Acta Cryst. &, 751 (1976). W.R. McKinnon and J.R. Dahn to be published. W.R. McKinnon and R.R. Raering in Modern Aspects of ElectrochemistryVol. 15, edited by R.E. White, J.O'M. Bockris and B.E. Conway, Plenum Press (1983). M. Tovar, L.E. Delong, D.C. Johnston and M.B. Maple, Solid State Comm. 30, 551 (1979).
Solid State Communications,Vo1.52,No.3, pp.245-248, 1984. Printed in Great Britain.
SALTING-GUI IN INTERCALATION COMPOUNDS: REMOVING COPPER FROM Cu3M06S, BY INTERCALATING LITHIUM W.R. McKinnon and J.R. Dahn CT Solid State Chemistry, Chemistry Division, National Research Council of Canada, Ottawa KlA OR9 CANADA
(Received 15 June 1984 by R. Barrie)
We study the removal of the copper in Cu3M06SB by the intercalation of lithium. For o
Most of the ternary Chevrel compounds, A Mo6XB (A = metal, X = S, Se or Te) can be pzepared by direct reaction of the elements at high temperatures (1,Z). Of the binary Chevrel compounds, only Mo6SB cannot be prepared in this way. To prepare Mo6SB the A atoms in MO6SB (eg. A = Cu) are removed by chemical re $ ctions, like acid leaching, at room temperature (3,4,5). Recently (5,6) it has been found that the Cu in Cu 06SS can be displaced from the host by inrercalating lithium to produce Li4MogSB and elemental copper. Subsequent de-intercalation of the lithium produces Mo6SS. However, these studies did not report the details of the copper displacement. Here we report the intercalation of Li in LixCu3M06SB. Using Li/LixCu3M06Ss electrochemical cells and in-situ x-ray diffraction (7), we found that copper stays in the host for xc1 but comes out for x>l. We discuss how the added Li can change the chemical potential of Cu so as to force it out of the host. Highly crystalline Cu3M06SS powder was prepared by direct reaction and Cu S, MO and MoS2 in sealed quarts ampoules at f15O'C. The rhombohedral lattice parameters of our CugMogSB, a = 6.571 A and a - 95.56' are in good agreement with those reported by Flukiger et al (8). We measured the voltage V as a function of x in Li/LixCu3M06SB cells described previously (9). The cells were charged or discharged at a constant'current while V was measured (10). The structure of LixCu3M06SB was studied by in-situ x-ray diffraction of Li/LixCu3M06Se cells with beryllium x-ray windows (7). Homogeneously intercalated samples were prepared by fixing the cell's voltage and waiting for the intercalation to reach equilibrium. For these samples, x was calculated either from the charge transferred during equilibration and the mass of Cu3M06SB in the cell or from V(x). In this in-situ technique, changes in the intensity of Bragg peaks reflect directly changes in the
intercalation electrode, not simply changes in the amount of material in the x-ray beam. Figure 1 shows V(x) for two Li/LixCu3M06SS cells at 28OC for O
2.2 E &J 2 9
2.0
. 1.8 0.0
I
0.2
I
I
0.4 x in
Figure 1
I
I
0.6
I
I
0.0
I
J 1.0
Lti,Cu,Mo,S,
V(x) for Li/LixCu3M06SS cells. Solid curve - discharge, dashed curve - charge and l - equilibrium
successively lower voltages as described above and x was calculated from the charge in the cell. The agreement between the data shows the reversibility of intercalation in this material and that these cells are near even at 25 hour rates. At each of the data points in figure 1, x-ray profiles were taken and the lattice parameters determined by least squares to the positions of at least 15 Bragg peaks. Table 1 shows typical results. Figure 2 shows a, a and the rhombohedral LfxCU3MOg&3. The
246
SALTING-OUT IN INTERCALATIONCOMPOUNDS
Table 1 Least squares refinement of the Bragg peak positions to obtain a and a. For peaks with scattering angles greater than 55', the Cu Ra doublet is resolved and we give the position of the Kal peak. The al wavelength has been used to compute the plane spacings for these peaks. The refined lattice parameters were a=6.6036 A and a=95.348O for Li_50Cu3M06S8. hkl
28 observed
28 calculated
100 ll-0 110 2TO 2T1 211 270 271 3T0 370 371 4T0 41T 4fT 437 430 530 520 521 43-o
13.564 18.177 20.190 29.265 32.963 36.400 36.793 39.395 42.313 47.670 50.221 56.548 59.595 61.689 74.019 76.246 81.818 82.053 85.927 91.337
13.539 18.170 20.164 29.251 32.967 36.417 36.818 39.398 42.332 47.659 50.201 56.553 59.581 61.679 74.022 76.233 81.822 82.045 85.944 91.343
d (A) calculated 6.540 4.882 4.404 3.053 2.717 2.467 2.441 2.2870 2.1350 1.9066 1.8158 1.6260 1.5504 1.5026 1.2796 1.2479 1.1762 1.1736 1.1300 1.0768
1
a 3 8
98.5
. .
98.4
. .
i
"
l.
.
-I
0.0
0.2
0.4
0.6
0.8
x in Li&u,Mo6S8 Figure 2
Rhombohedral lattice parameters and unit cell volume versus x in LixCu3MosSs.
1.0
Vol. 52, No. 3
lattice parameters change reversibly with x for x
peak in curve
5 is
just
as large
as
SALTING-OUT IN INTERCALATION COMPOUNDS
Vol. 52, No. 3 I
I
247
2.0
,
a-
2.5 t I l ‘\
‘.
------
1.5
b-
1.0
0.5
2.0
41.0
43.0
42.0
44.0
Scattering Angle (deg) l.5 2.5
/ \
Figure 4
d-
Portions of in-situ x-ray diffraction profiles (see text). i) Li_9CuSMo6Ss 2) Li gCugMo6Sg, Li3 7M06SB and Cu 3) Li,)io6Ssand Cu 4) Mo6SB, Cu4MogSS and Cu 5) Mo,S, and Cu The $l"ler indices of the Bragg peaks are indicated. Peaks A and B are from Li4M06Ss (12). Note that curve 3 is plotted twice. The beryllium oxide peak comes from the x-ray window. l
2.0
1.5 0
2
1
x Figure 3
in
3
4
Li&up0(&
V(x) for an Li/LixCuSMo6SS cell which was charged (dashed curves) and discharged (solid curves) with currents which took 3 hours to change x by 1. a) first cycle, b) second cycle, c) third cycle, d) eleventh cycle.
in curve 3, showing that eventually all the Cu stays out of the host even when all the Li is de-intercalated. Bragg peaks from Mo6Ss are observed in curve 5. In the displacement of (XIby Id, Li pushes Cu out of the host onto the surface where the Cu forms copper metal. This displacement is not an ion-exchange process of the type Li+(solution) + Cucintercalated) z Cu+(solution) + Licintercalated). An ion-exchange process would not give metallic copper; if Cu+ was not soluble, a copper salt would precipitate. In addition, in-situ x-ray experiments show that Cu displacement ceases when the discharge current is stopped while the cell is on the plateau at 1.852 V. By contrast, ion-exchange would proceed with no discharge current.
The displacement of Cu by Li can only occur if the Li changes the chemical potential of Cu in the host. When the Cu is inside the host, its chemical potential (relative to Cu metal) is negative. In order for Li to force the Cu to leave the host, the Li must increase the chemical potential of the Cu to zero. Such an increase can occur even in the simplest lattice-gas models. Consider a lattice of 4N sites which are filled by xN Li atoms and yN Cu atoms, where (kx<4 and O(yt4. If we assume the intercalated atoms do not interact with each other, the Helmholtz free energy, F, is
F =ELx + Ecy + kT [y log(y/4) N + x log(x/4) + (4-x-y) log((4-x-y)/4)]
[l]
EL and E are the differences in energy between a Li or 6 atom in a lattice site and in Li or Cu metal respectively. The third term in equation (1) is the configurationalentropy times the temperature T; k is Boltzman's constant. The chemical potentials of Li and Cu, nL and u, are (13) pL = +g, 0 and
= EL + kT log[x/4-xy)]
248
PC
SALTING-OUT IN INTERCALATIONCOMPOUNDS
0=
= L dF
Ndyx
E, + kT
log[ y/(4-x-y)]
131
Note that the chemical potential of Cu depends on x as well as on y. The values of EL and E, must be about -2eV and -0.2 eV respectively, from measurements made on Li/LlxMo6SB and Cu/Cu Mo6Ss cells (14). With such values for EL an3 g,, uLcwill reach zero before uL does as x increases, so the Cu wfll come out of the host. Several processes in chemistry arise because adding one component to a solution changes the chemical potential of the other components (15). For example, the chemical potential of water is lower in a salt solution than in pure water. If a salt solution is separated from pure water by a semi-permeable membrane that passes only water, water will flow to the salt solution until the pressure difference (the osmotic pressure) between the two liquids makes the chemical potentials of water on each side of the membrane equal. An analogy closer to our problem is salting-out (15): salt is more soluble In water than in alcohol, 60 alcohol added to salt water can precipatate the salt. The salt precipitates when the chemical potential of salt in solution equals the chemical potential of solid salt, in the same way that Cu leaves the Mo6SB host when its chemical potential In the host is equal to the chemical potential of Cu in Cu metal.
Vol. 52, No. 3
In the model of equations (l-3), Cu comes out of the host continuouslywhile the host remains a single phase. Experimentally, however, two phases of the host coexist with metallic Cu during the plateau at 1.852V. (If we consider the host, Mo6SR, as a single component, then we can regard the system Li-Cu-Mo6SB as a ternary system. When three phases coexist in a ternary system the chemical potential of each of the components is constant (151, so we expect a plateau in the voltage as observed.) We are still studying more complicated models which consider interactions between the guest atoms, as well as models which consider the real lattice of available sites In Mo6SB; these models may explain this experimental observation. These complications, however, do not alter the basic notion that Li changes the chemical potential of Cu and so causes the Cu to leave the host. The above arguments imply that the copper should go back into the host when the Li is de-intercalated. Experimentally,we see that some Cu goes back into the host the first few times Li/LixCuBMogSg cells are recharged but eventually all the Cu stays out of the host. This implies that the Cu metal becomes detached from the particles of the Chevrel compounds. Acknowledgement: We thank P.J. Mulhern for providing us with the CuBMo6SB sample.
References
1. K. Yvon, in Current Topics In Materials Science, edited by E. Kaldls (North-Holland,Amsterdam 1979) Vol 3. p.53. 2. 0. Fischer, Appl. Phys. '6, 1 (1978). 3. R. Schollhorn, M. Kumpers and J.O. Besenhard, Mat. Res. Bull. 12, 781 (1977) 4. R. Chevrel, M. Sergent and J. Prigent, Mat. Res. Bull. 2, 1487 (1974). 5. J.M. Tarascon, F.J. DiSalvo, D.W. Murphy, G.W. Hull and J.V. Waszczak, in press. 6. P.J. Mulhem and R.R. Haering, Can. J. Phys. in press. 7. J.R. Dahn, M.A. Py and R.R. Baering, Can. J. Phys. E, 307 (1982).
a.
9. 10. 11. 12. 13.
14.
R. Flukiger, R. Balllif, J. Muller and K. Yvon, J. Less Common Metals c, 193 (1980). D.C. Dahn and R.R. Raering, Solid State comm. E, 29 (1982). J.R. Dahn and W.R. McKinnon, J. Electrochem. Sot. 131, 1823 (1984). R.D. Shannon, Acta Cryst. &, 751 (1976). W.R. McKinnon and J.R. Dahn to be published. W.R. McKinnon and R.R. Raering in Modern Aspects of ElectrochemistryVol. 15, edited by R.E. White, J.O'M. Bockris and B.E. Conway, Plenum Press (1983). M. Tovar, L.E. Delong, D.C. Johnston and M.B. Maple, Solid State Comm. 30, 551 (1979).