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Lithiation characteristics of Cu5V2O10
Hecwochimico Acta. Vol. 41, No. 6. pp. 857-861. 1996 Copyright C 1996 Elsevier Science Ltd. Printed in Great Britain. All rights rcscrvcd 0013~4686/96 $15.00 + 0.00
Pergamon
LITHIATION
CHARACTERISTICS
OF Cu5V2010
MIKA EGUCHI,* AYAKO KOMAMURA,TAKASHI MIURA and TOMIYA KIWI Department of Applied Chemistry, Faculty of Science and Technology, Keio University, Hiyoshi 3-14-l. Kohoku-ku, Yokohama 223, Japan (Received 22 May 1995; in revised form 5 September
1995)
Abstract-Lithiation characteristics of a complex oxide containing V(V) and Cu(II), Cu,V,O,, , were investigated. Lithiation was carried out chemically using an n-BuLi/hexane solution. The valence states of copper and vanadium and the structural changes upon lithiation were analyzed using esr, XRD and ir measurements. It was revealed that reduction processes occurred consecutively with an increasing extent of lithiation, x = Li/(S Cu + 2V). Up to x = 3, preferential reduction of Cu(I1) to Cu(I) occurred in the single phase. At 3 < x, Cu(I1) was reduced to Cu(0) and Cu(I1) (LiCuVO,) in the multiphase system. Above about x = 6, LiCuVO, was further reduced to metallic copper. The open-circuit potentials of lithiated Cu,V,O,, decreased with x gradually at 0 < x < 3, and has a plateau at x > 3. Key words: Cu,V,O,,
, lithium insertion, esr spectra, ir spectra, open-circuit potential, discharge curve
INTRODUCTION having the formula of oxides nCu0. V,O, ,which contain more than one redox component upon lithiation, are of interest as an insertion electrode for rechargeable lithium batteries. The authors have investigated the lithiation characof a-@ = 2)[1], /!?-Cu,V,O,(n = 2)[2], teristics Cu,,V,O,,(n = 11/3)[3] and also of a-Zn,V,0,[4] as a reference material. For these oxides, it was revealed that reduction of Cu(II) comes before that of V(V) component upon lithiation and the sequence and relative extent of reduction of each component depend on the crystal structures of the oxides. In a series of the oxides nCu0. V,O,, the oxide with is known to exist as the copper n = 5, cu,v,o,,, richest compound in the phase diagram of the CuO *V,O, system[5], and the crystal structure of Cu,V,O,, was determined by Shannon et aI.[6]. In the structure of Cu,V,O,, , octahedrally coordinated (Jahn-Teller distorted) CuO, and CuO, trigonal bipyramids form a sheet and the sheets are cross-linked by VO, tetrahedra. Each VO, tetrahedron does not share common oxygen atoms with each other. The electrochemical charge and discharge behaviours of CusV,O10 were reported by Yamaki et aI.[7]. In this investigation, CusV,O,, was prepared by a solid state reaction and changes in the valence states and in the crystal structure of CusV,O,,, upon lithiation were examined. Complex
EXPERIMENTAL Powder samples of Cu,V,O,, were prepared by a solid state reaction of reagent grade CuO and V,Os powders. CuO and V,O,, preheated at 750 and 650°C respectively for 24 h, were mixed at a molar * Author to whom correspondence should be addressed.
ratio oi 5 : 1 in an agate mortar and the mixture was pressed (200 MPa) into pellets. Pellets were heated at 740°C on a platinum plate for 67 h under air atmosphere[6] and then cooled down to room temperature at a rate of 2”Cmin- ‘. The samples were identified by powder X-ray diffraction (XRD) measurement to be a single phase Cu,V,O,,, referring to JCPDS files (No. 33-504). For chemical lithiation of Cu,V,O,,, n-BuLi/nhexane was added to Cu,V,O,,, powder dispersed in n-hexane at 25°C under a nitrogen atmosphere. The concentration of n-BuLi was varied from 0.151.25 moldmW3 to control the content of lithiation, which was determined by analyzing lithium in the samples by the flame photometric method using an aqueous LiVO, solution as a standard. The extent of lithiation was designated by x = Li/(5 Cu + 2 V). To prepare the electrode for the open circuit potential measurement in an inert atmosphere at 25”C, lithiated Cu,V,O,, was powdered and pressed (200MPa) into a pellet, the back face of which was connected to a lead wire with silver paste, and was covered with silicone rubber. For the electrochemical lithiation, a mixture of Cu,V,O,,, acetylene black and poly (tetrafluoroethylene) powders in a weight ratio of 70 : 25 : 5 was pressed into a pellet on a porous nickel sheet, and offered to the electrode fabrication as described above. A 1 mol dme3 LiClO, solution of propylene carbonate (PC) was used as the electrolyte. The reference and counter electrodes were metallic lithium wires. Electron spin resonance (esr) and infra-red absorption (ir) (using KBr disc) spectral, together with XRD (with CuKa, radiation) measurements, were carried out at 25°C. RESULTS AND DISCUSSION Esr spectra of chemically lithiated Cu,V,O,, The extent of chemical lithiation of CU,V,O,~ increased almost linearly with the concentration of
M. EGUCHIet al.
858
BuLi in the solution up to about x = 5, and then reached asymptotically a maximum value of x = 7.15. This maximum value of x seems to be in a good accordance with the inflection point of ocv found by Yamaki et aI.[7]. For lithiation of Cu,V*O,fJ f the valence states of both V(V) and Cu(II) become lower, and hence esr analysis offers a tool to detect these changes, as both V(N) and Cu(I1) are esr-active. Some examples of esr spectra of the lithiated oxides with that of the starting material are shown in Fig 1. The starting material Cu,V,O,, was esr-silent owing to its antiferromagnetic nature, like as in the case of antiferromagnetic CuO containing only Cu(II)[8]. The same situation was observed for Cu,,V,O,,[3]. For lithiated oxides a broad singlet signal ascribed to Cu(II)[9] became observable, and the signal intensities increased gradually with increasing x. The phenomenon is a result of a chemical “doping” by introducing Cu(1) in the lattices through the reduction of Cu(I1) to Cu(1). Introduction of Cu(1) in the lattice caused a breakoff of the antiferromagnetic nature, and Cu(I1) became esr active. Therefore, increase in the intensity of the signal was regarded as increase in Cu(1) in the lattice. The shapes of broad singlet signal, typical of those under a spin exchange interaction between adjacent ions in the lattice, remained almost unchanged up to about x = 3, and the g-values of these signals were about 2.15 almost independent of x at x < 3, as seen in Fig. 2. Above x = 3 hyperfine signals which could be ascribed to Cu(I1) appeared clearly overlapping with the broad singlet signal of Cu(II), as shown in Fig. 1. The g-value of the broad singlet signal
6 5
0 it E -
I”“““1 0
2
4
6
8
x (= Li / (5Cu + 2V)) Fig. 2. Change in the g-values of Cu(II) with x in lithiated Cu,V&. (0) &Cu(II)),(A) g&u(II)) and (0) g,(Cu(lIN.
changed discontinuously at about x = 3 and became about 2.09. These facts indicate that the coordination of Cu(I1) in the oxide changes at x = 3. The hyper!ine structure of Cu(I1) had the average g,,- and g,-values of 2.262 and 2.038 (A,, = 17.37 and A, = 3.096) respectively (for the assignment, see Fig. 3), and were almost independent of x, as shown in Fig. 2. While the signal of V(N) was not observed, this indicated that the reduction of V(V) to V(IV) did not occur in this region of x. This is in contrast to the case, where formation of V(IV) was observed by esr signals, from x = 1, 1, 2 and 3 for a-CuV,O,, aand Cu,,V,O,,[l-33, 8-C~,V*O, respectively. For electrochemical lithiation using a and acetylene black and mixture of Cu,V,O,, poly(tetrafluoroethylene), esr signal of V(IV) was not observed even at about x = 10. To estimate the concentration of Cu(I1) in esr active state, the integrated intensities of these signals were shown in Fig. 4, where the intensity increased slightly with x up to x = 3. This means that the reduction of Cu(I1) to Cu(1) occurs, resulting in an increase in the concentrations of Cu(I1) in the esractive state and also Cu(I). Above x = 3, Cu(I1) with the different coordination in the oxides was formed remarkably with lithiation, and at 6 < x its integrated intensity decreased again.
cu,v,o,,
4
2 1 1
186
I
336 Magnetic field I mT
1
ULI’
486
Fig. 1. Esr spectra of lithiated Cu,V,O,,. 1: x = 0 (unlithiated), 2: x = 0.85, 3: x = 1.66, 4: x = 2.79, 5: x = 3.48, 6: x = 4.65, 7: x = 5.99, 8: x = 6.72, with x = Li/(5 Cu + 2 V).
g1 I
I
1
186
336 Magneticfield I mT
486
Fig. 3. Assignment of hyperfine signals in the spectrum of lithiated Cu,V,O,, (x = 4.36).
Lithiation characteristics of Cu5V,0,,
i
859
The lattice parameters determined from some clear diffraction lines were plotted against x, as shown in Fig. 6. The lattice parameters of a, b, c and /? were unchanged and almost independent of x at x < 3, but no diffraction lines other than original ones were observed. The latter fact indicates that lithium insertion reaction in the single phase occurs at x < 3, as given in equation (1).
6
d .
Cu,V,O,,
+ xLi+ + xe- + Li,Cu,V,O,,(x
< 3). (1)
2
0
4
6
I 8
x (= Li / (5Cu + 2V))
Fig. 4. Dependence of integrated intensity of esr signal of Cu(I1) on x.
XRD patterns of chemically lithiated Cu,V,O,,
The structural changes of Cu,V,O,, upon lithiation were investigated by XRD measurements, and typical XRD patterns were given in Fig. 5. With increasing x, XRD lines, though remaining at the original positions, became broader gradually and the crystal lattice tended to be disordered up to x = 3.
-7
Li,Cu5V,0,,
l
.
q6
.
The causes of constancy of the lattice parameters upon lithiation were not clear, as sites for inserted Li+ ions were not identified. In the structure of Cu,V*O,, 9 double chains of CuO, octahedra running along the y axis are connected by edgesharing with CuO, and CuO, polyhedra running along the z axis as zigzag chains. These two chains cross link also by VO1 tetrahedra, and this structure forms a void space between two adjacent V04 tetrahedra[7]. These voids will offer insertion sites for Li+ ions, and this specific structure can be a cause of no expansion or contraction of the lattice upon lithiation. But further discussion will be necessary. At 3
.
+ x’Li+
+ x’e- + (3 - x’)/3Li,Cu~‘rV,,O,, + (2/3)x’LiCu”VO, + X’CU+ (2/3)x’Li,O.
(2)
Metallic copper and L&O appearing in equation (2) was not observed in the XRD patterns presumably because of the finely dispersed form of copper and of insertion of L&O into LiCuVO, or the amorphous nature of Li,O, respectively. Steep increase in the intensity of the esr signal of Cu(I1) at 3 < x will be due to the formation of LiCuVO,. Decrease in the intensity of Cu(II) signal at 6 < x, is a result of consumption of Cu(II) in LiCuVO,, as the lithiated
2
5oocps I
I 8
I 20
I 40
I 60
I 8;
20 I deg 1: x = 0 Fig. 5. XRD patterns of lithiated Cu,V,O,,. (unlithiated), 2: x = 0.85, 3: x = 1.66, 4: x = 3.48, 5: x = 5.99, 6: x = 6.85, 7: x = 7.15, with x = Li/(5 Cu + 2V).
-
2’
I
0
1
I
1
I
i
2
1
1
Igo
3
x (= Li / (5Cu + 2V)) Fig. 6. Change in the lattice parameters a,b,c and p with x. (0) a, (A) b, (0) c and (0) P.
860
M.
EGUCHI et al.
mother phase disappears at x = 6(x’ = 3). Above about x > 7, the diffraction lines of LiCuVO, tended to disappear in XRD patterns suggesting that further reduction of LiCuVO, occurred. At the same time, broad diffraction lines observed at 28 = 43”, 51” and 74” corresponded to those for metallic copper (JCPDS No. 4-836), indicating that LiCuVO, was reduced to metallic copper. This suggests that Cu(I) is unstable in the newly formed phases. The open-circuit potential of chemically lithiated C~,V,Ol0
The open-circuit potentials of lithiated oxides in a 1 moldm-‘/PC solution were plotted against x in Fig. 7. In the composition region x < 3, the electrode potential decreased with increasing x, as typical of a solid solution electrode, and at 3 < x the electrode potential vs x curve became more or less flat at about 2.8V, as typical of a multiphase electrode. These dependencies are consistent with the results of XRD measurements. At 0 < x < 3 the open circuit potential was plotted after the equation proposed by Armand (equation (3))[ lo], E = R” - F
ln &
+ G, c = xix,.
(3)
ie, E + RT/F In t/(1 - {) was plotted against {, as shown in Fig. 8. In equation (3), E, E”, xL and K were reversible electrode potential, standard electrode potential, limiting x value of the solid solution region and a constant originated from interactions between inserted lithium ions, respectively and R, T and F had their usual meanings. Constancy of E + RT/F In t/(1 - <), ie, E” (K = 0) indicated that the insertion compound behaved as an ideal solid solution, and E” was about 3.1 V vs. Li+/Li. Open circuit potentials of lithiated Cu,V,O,, reported previously by Yamaki et al.[7] were about 0.4 to OSV negative to the value obtained in this work. The former values were obtained as quasi-oco by the electrochemical method with 1 h of recuperation time. This recuperation time would not be enough to obtain the reversible potentials. Quasi-ecu-x curves had a inflection point at about x = 2.5 and at 6 > x > 2.5 quasi-ecu decreased almost linearly. These may correspond to the solid solution and two
3.5
I
I
I
I
I
I
I
3.4
0
I
I
2
I
I
I
I
4 6 x (= Li / (5Cu + 2V))
1
I
I
I
I
0
”
+; 9 > . z Y 5 g = k &
3.2 0
tr
0
3.0
z
2.8
1
I
I
I
I
0.0 0.2 0.4 0.6 0.8
I
1.0 1.2
5 (=x/Ad Fig. 8. Relation between E + (RT/F)ln(S/(l - 0) and {( = x/xL)with xL = 3.0 for Cu,V,O,, phase regions respectively obtained in this work, though the quasi-ecu would contain some polarization terms. Ir spectra of chemically lithiated Cu,V,O,, Some typical ir spectra of lithiated oxides were shown in Fig. 9. Absorption bands at about lOOf6OOcm-’ were ascribed to the vibrational modes of V04 unit in oxides[ll]. At x = 1, the ir spectra became broader rather abruptly and absorption
I
;; I
I
I
? :.d . 3.0 : > 2.5
~ .-
I
6
Fig. 7. Dependence of the open circuit potential on x.
I
1000
1
I
800
I
I
600
I
I
400
Wave numbers I cm” Fig. 9. Ir spectra of lithiated Cu,V,O,,. 1: x = 0 (unlithiated), 2: x = 0.85, 3: x = 1.66, 4: x = 3.48, 5: x = 4.63, 6: x = 5.99, 7: x = 7.15, with x = Li/(SCu + 2V).
861
Lithiation characteristics of Cu,VIO,,
caused by a low electronic conductivity of the electrode. The limiting value of x = 1 found in the discharge curves has a corresponding change in ir spectra, where an abrupt change was observed at about x = 1, though no evident phenomena were found in XRD, esr and ocv data. This phenomenon could, therefore, presumably attribute a possible change in diffusion paths of lithium ions as a result of occupation of lithium ions at most stable sites at x = 1, but further examination must be necessary.
3.0 3 :
il
uj 2.5 >
>
2 2.0
1.5~
CONCLUSION
’
0
’
’
’
2
’
4
’
’
6
’
’
8
’
1
IO
x=Lil(EiCu+2V) Fig. 10. Discharge curves for Cu,V,O,,. 1: 30pAcme2, 2: SO~tAcrn-~, 3: l@OpAcm-*. Inset: log j-log t relation for the first plateau.
intensities of the bands at about 950, 900 and 72Ocm-’ decreased relative to that of the band at SOOcm-‘, though the positions of the absorption bands of CusVtO,, remained unchanged. As a corresponding change in XRD patterns was not observed at x = 1, the change in ir spectra can be attributed to a local disorder of VO, units. This fact is due presumably to a change in the sites for inserted lithium ions at x = 1, though it was not detected also in the open-circuit potential measurements. The absorption band at about 86Ocm-’ appearing at 3 < x will be due to the formation of LiCuVO, detected by XRD analysis. Discharge characteristics
Some examples of galvanostatic discharge curves, as shown in Fig. 10, had two plateaus, and the first one ended at about x = 1, depending on the discharge current density (cd) (a relation i” *t = constant with m = 1.16 was found as shown in the inset of Fig. 10) and the second one continued with a smaller slope up to about x = 10. In the previous report by Yamaki et al.[7], only one plateau could be found at 0 < x < -0.85, corresponding to the second one in this work. Relatively large potential drop at the start of polarization was presumably
For Cu,V,O,, preferential reduction of Cu(II) over V(V) components occurs up to about x = 7. The single phase limits exist at x = 1 and 3, depending presumably on the different insertion sites of lithium ions, and the former limit may be controlled by the insertion paths provided by adjacent VO, units. At x > 3 further reduction of Cu(II) occurs in the multiphase and Cu(0) and Cu(T1) (LiCuVO,) coexist, because of instability of Cu(1). At about x > 7 LiCuVO, is further reduced. It is revealed that the preferential reduction of copper component is most remarkable in other nCu0. V205’s examined.
REFERENCES 1. M. Eguchi, M. Yamaguchi, T. Miura and T. Kishi, Denki Kogaku 61, 1347 (1993). 2. M. Eguchi, I. Furusawa, T. Miura and T. Kishi, Solid State Ionics 6% 159 ( 1994). 3. M. Eguchi, T. Yokoyama, T. Miura and T. Kishi, Solid State lonics, 74,269 (1994). 4 M. Eguchi, T. Miura and T. Kishi, Denki Kagaku 62, 504 (1994). 5 P. Fleury, C. R. Acad. Sci., Ser. C, 263, 1375 (1966). 6 R. D. Shannon and C. Calvo, Acta Cryst. B29, 1338 (1973). 7 Y. Sakurai, H. Ohtsuka and J. Yamaki, J. Efectrochem. Sot.
135,32 (1988).
a. P. A. Cox, Transition Metal Oxides, p. 250. Clarendon Press, Oxford ( 1992). 9. C. S. Sunandana, Proc. Nucl. Phys. Solid State Phys. 25, Cl97 (1969). 10. M. B. Armand, Materials fir Advanced Batteries, (Edited by D. W. Murphy, J. Broadhead and B. C. H. Steele), p. 145. Plenum Press, New York (1980). 11. V. D. Zhuravlev, Yu. A. Velikodnyi and L. V. Kristallov, Zh. Neorg. Khim. 32, 3060 (1987).
Pergamon
LITHIATION
CHARACTERISTICS
OF Cu5V2010
MIKA EGUCHI,* AYAKO KOMAMURA,TAKASHI MIURA and TOMIYA KIWI Department of Applied Chemistry, Faculty of Science and Technology, Keio University, Hiyoshi 3-14-l. Kohoku-ku, Yokohama 223, Japan (Received 22 May 1995; in revised form 5 September
1995)
Abstract-Lithiation characteristics of a complex oxide containing V(V) and Cu(II), Cu,V,O,, , were investigated. Lithiation was carried out chemically using an n-BuLi/hexane solution. The valence states of copper and vanadium and the structural changes upon lithiation were analyzed using esr, XRD and ir measurements. It was revealed that reduction processes occurred consecutively with an increasing extent of lithiation, x = Li/(S Cu + 2V). Up to x = 3, preferential reduction of Cu(I1) to Cu(I) occurred in the single phase. At 3 < x, Cu(I1) was reduced to Cu(0) and Cu(I1) (LiCuVO,) in the multiphase system. Above about x = 6, LiCuVO, was further reduced to metallic copper. The open-circuit potentials of lithiated Cu,V,O,, decreased with x gradually at 0 < x < 3, and has a plateau at x > 3. Key words: Cu,V,O,,
, lithium insertion, esr spectra, ir spectra, open-circuit potential, discharge curve
INTRODUCTION having the formula of oxides nCu0. V,O, ,which contain more than one redox component upon lithiation, are of interest as an insertion electrode for rechargeable lithium batteries. The authors have investigated the lithiation characof a-@ = 2)[1], /!?-Cu,V,O,(n = 2)[2], teristics Cu,,V,O,,(n = 11/3)[3] and also of a-Zn,V,0,[4] as a reference material. For these oxides, it was revealed that reduction of Cu(II) comes before that of V(V) component upon lithiation and the sequence and relative extent of reduction of each component depend on the crystal structures of the oxides. In a series of the oxides nCu0. V,O,, the oxide with is known to exist as the copper n = 5, cu,v,o,,, richest compound in the phase diagram of the CuO *V,O, system[5], and the crystal structure of Cu,V,O,, was determined by Shannon et aI.[6]. In the structure of Cu,V,O,, , octahedrally coordinated (Jahn-Teller distorted) CuO, and CuO, trigonal bipyramids form a sheet and the sheets are cross-linked by VO, tetrahedra. Each VO, tetrahedron does not share common oxygen atoms with each other. The electrochemical charge and discharge behaviours of CusV,O10 were reported by Yamaki et aI.[7]. In this investigation, CusV,O,, was prepared by a solid state reaction and changes in the valence states and in the crystal structure of CusV,O,,, upon lithiation were examined. Complex
EXPERIMENTAL Powder samples of Cu,V,O,, were prepared by a solid state reaction of reagent grade CuO and V,Os powders. CuO and V,O,, preheated at 750 and 650°C respectively for 24 h, were mixed at a molar * Author to whom correspondence should be addressed.
ratio oi 5 : 1 in an agate mortar and the mixture was pressed (200 MPa) into pellets. Pellets were heated at 740°C on a platinum plate for 67 h under air atmosphere[6] and then cooled down to room temperature at a rate of 2”Cmin- ‘. The samples were identified by powder X-ray diffraction (XRD) measurement to be a single phase Cu,V,O,,, referring to JCPDS files (No. 33-504). For chemical lithiation of Cu,V,O,,, n-BuLi/nhexane was added to Cu,V,O,,, powder dispersed in n-hexane at 25°C under a nitrogen atmosphere. The concentration of n-BuLi was varied from 0.151.25 moldmW3 to control the content of lithiation, which was determined by analyzing lithium in the samples by the flame photometric method using an aqueous LiVO, solution as a standard. The extent of lithiation was designated by x = Li/(5 Cu + 2 V). To prepare the electrode for the open circuit potential measurement in an inert atmosphere at 25”C, lithiated Cu,V,O,, was powdered and pressed (200MPa) into a pellet, the back face of which was connected to a lead wire with silver paste, and was covered with silicone rubber. For the electrochemical lithiation, a mixture of Cu,V,O,,, acetylene black and poly (tetrafluoroethylene) powders in a weight ratio of 70 : 25 : 5 was pressed into a pellet on a porous nickel sheet, and offered to the electrode fabrication as described above. A 1 mol dme3 LiClO, solution of propylene carbonate (PC) was used as the electrolyte. The reference and counter electrodes were metallic lithium wires. Electron spin resonance (esr) and infra-red absorption (ir) (using KBr disc) spectral, together with XRD (with CuKa, radiation) measurements, were carried out at 25°C. RESULTS AND DISCUSSION Esr spectra of chemically lithiated Cu,V,O,, The extent of chemical lithiation of CU,V,O,~ increased almost linearly with the concentration of
M. EGUCHIet al.
858
BuLi in the solution up to about x = 5, and then reached asymptotically a maximum value of x = 7.15. This maximum value of x seems to be in a good accordance with the inflection point of ocv found by Yamaki et aI.[7]. For lithiation of Cu,V*O,fJ f the valence states of both V(V) and Cu(II) become lower, and hence esr analysis offers a tool to detect these changes, as both V(N) and Cu(I1) are esr-active. Some examples of esr spectra of the lithiated oxides with that of the starting material are shown in Fig 1. The starting material Cu,V,O,, was esr-silent owing to its antiferromagnetic nature, like as in the case of antiferromagnetic CuO containing only Cu(II)[8]. The same situation was observed for Cu,,V,O,,[3]. For lithiated oxides a broad singlet signal ascribed to Cu(II)[9] became observable, and the signal intensities increased gradually with increasing x. The phenomenon is a result of a chemical “doping” by introducing Cu(1) in the lattices through the reduction of Cu(I1) to Cu(1). Introduction of Cu(1) in the lattice caused a breakoff of the antiferromagnetic nature, and Cu(I1) became esr active. Therefore, increase in the intensity of the signal was regarded as increase in Cu(1) in the lattice. The shapes of broad singlet signal, typical of those under a spin exchange interaction between adjacent ions in the lattice, remained almost unchanged up to about x = 3, and the g-values of these signals were about 2.15 almost independent of x at x < 3, as seen in Fig. 2. Above x = 3 hyperfine signals which could be ascribed to Cu(I1) appeared clearly overlapping with the broad singlet signal of Cu(II), as shown in Fig. 1. The g-value of the broad singlet signal
6 5
0 it E -
I”“““1 0
2
4
6
8
x (= Li / (5Cu + 2V)) Fig. 2. Change in the g-values of Cu(II) with x in lithiated Cu,V&. (0) &Cu(II)),(A) g&u(II)) and (0) g,(Cu(lIN.
changed discontinuously at about x = 3 and became about 2.09. These facts indicate that the coordination of Cu(I1) in the oxide changes at x = 3. The hyper!ine structure of Cu(I1) had the average g,,- and g,-values of 2.262 and 2.038 (A,, = 17.37 and A, = 3.096) respectively (for the assignment, see Fig. 3), and were almost independent of x, as shown in Fig. 2. While the signal of V(N) was not observed, this indicated that the reduction of V(V) to V(IV) did not occur in this region of x. This is in contrast to the case, where formation of V(IV) was observed by esr signals, from x = 1, 1, 2 and 3 for a-CuV,O,, aand Cu,,V,O,,[l-33, 8-C~,V*O, respectively. For electrochemical lithiation using a and acetylene black and mixture of Cu,V,O,, poly(tetrafluoroethylene), esr signal of V(IV) was not observed even at about x = 10. To estimate the concentration of Cu(I1) in esr active state, the integrated intensities of these signals were shown in Fig. 4, where the intensity increased slightly with x up to x = 3. This means that the reduction of Cu(I1) to Cu(1) occurs, resulting in an increase in the concentrations of Cu(I1) in the esractive state and also Cu(I). Above x = 3, Cu(I1) with the different coordination in the oxides was formed remarkably with lithiation, and at 6 < x its integrated intensity decreased again.
cu,v,o,,
4
2 1 1
186
I
336 Magnetic field I mT
1
ULI’
486
Fig. 1. Esr spectra of lithiated Cu,V,O,,. 1: x = 0 (unlithiated), 2: x = 0.85, 3: x = 1.66, 4: x = 2.79, 5: x = 3.48, 6: x = 4.65, 7: x = 5.99, 8: x = 6.72, with x = Li/(5 Cu + 2 V).
g1 I
I
1
186
336 Magneticfield I mT
486
Fig. 3. Assignment of hyperfine signals in the spectrum of lithiated Cu,V,O,, (x = 4.36).
Lithiation characteristics of Cu5V,0,,
i
859
The lattice parameters determined from some clear diffraction lines were plotted against x, as shown in Fig. 6. The lattice parameters of a, b, c and /? were unchanged and almost independent of x at x < 3, but no diffraction lines other than original ones were observed. The latter fact indicates that lithium insertion reaction in the single phase occurs at x < 3, as given in equation (1).
6
d .
Cu,V,O,,
+ xLi+ + xe- + Li,Cu,V,O,,(x
< 3). (1)
2
0
4
6
I 8
x (= Li / (5Cu + 2V))
Fig. 4. Dependence of integrated intensity of esr signal of Cu(I1) on x.
XRD patterns of chemically lithiated Cu,V,O,,
The structural changes of Cu,V,O,, upon lithiation were investigated by XRD measurements, and typical XRD patterns were given in Fig. 5. With increasing x, XRD lines, though remaining at the original positions, became broader gradually and the crystal lattice tended to be disordered up to x = 3.
-7
Li,Cu5V,0,,
l
.
q6
.
The causes of constancy of the lattice parameters upon lithiation were not clear, as sites for inserted Li+ ions were not identified. In the structure of Cu,V*O,, 9 double chains of CuO, octahedra running along the y axis are connected by edgesharing with CuO, and CuO, polyhedra running along the z axis as zigzag chains. These two chains cross link also by VO1 tetrahedra, and this structure forms a void space between two adjacent V04 tetrahedra[7]. These voids will offer insertion sites for Li+ ions, and this specific structure can be a cause of no expansion or contraction of the lattice upon lithiation. But further discussion will be necessary. At 3
.
+ x’Li+
+ x’e- + (3 - x’)/3Li,Cu~‘rV,,O,, + (2/3)x’LiCu”VO, + X’CU+ (2/3)x’Li,O.
(2)
Metallic copper and L&O appearing in equation (2) was not observed in the XRD patterns presumably because of the finely dispersed form of copper and of insertion of L&O into LiCuVO, or the amorphous nature of Li,O, respectively. Steep increase in the intensity of the esr signal of Cu(I1) at 3 < x will be due to the formation of LiCuVO,. Decrease in the intensity of Cu(II) signal at 6 < x, is a result of consumption of Cu(II) in LiCuVO,, as the lithiated
2
5oocps I
I 8
I 20
I 40
I 60
I 8;
20 I deg 1: x = 0 Fig. 5. XRD patterns of lithiated Cu,V,O,,. (unlithiated), 2: x = 0.85, 3: x = 1.66, 4: x = 3.48, 5: x = 5.99, 6: x = 6.85, 7: x = 7.15, with x = Li/(5 Cu + 2V).
-
2’
I
0
1
I
1
I
i
2
1
1
Igo
3
x (= Li / (5Cu + 2V)) Fig. 6. Change in the lattice parameters a,b,c and p with x. (0) a, (A) b, (0) c and (0) P.
860
M.
EGUCHI et al.
mother phase disappears at x = 6(x’ = 3). Above about x > 7, the diffraction lines of LiCuVO, tended to disappear in XRD patterns suggesting that further reduction of LiCuVO, occurred. At the same time, broad diffraction lines observed at 28 = 43”, 51” and 74” corresponded to those for metallic copper (JCPDS No. 4-836), indicating that LiCuVO, was reduced to metallic copper. This suggests that Cu(I) is unstable in the newly formed phases. The open-circuit potential of chemically lithiated C~,V,Ol0
The open-circuit potentials of lithiated oxides in a 1 moldm-‘/PC solution were plotted against x in Fig. 7. In the composition region x < 3, the electrode potential decreased with increasing x, as typical of a solid solution electrode, and at 3 < x the electrode potential vs x curve became more or less flat at about 2.8V, as typical of a multiphase electrode. These dependencies are consistent with the results of XRD measurements. At 0 < x < 3 the open circuit potential was plotted after the equation proposed by Armand (equation (3))[ lo], E = R” - F
ln &
+ G, c = xix,.
(3)
ie, E + RT/F In t/(1 - {) was plotted against {, as shown in Fig. 8. In equation (3), E, E”, xL and K were reversible electrode potential, standard electrode potential, limiting x value of the solid solution region and a constant originated from interactions between inserted lithium ions, respectively and R, T and F had their usual meanings. Constancy of E + RT/F In t/(1 - <), ie, E” (K = 0) indicated that the insertion compound behaved as an ideal solid solution, and E” was about 3.1 V vs. Li+/Li. Open circuit potentials of lithiated Cu,V,O,, reported previously by Yamaki et al.[7] were about 0.4 to OSV negative to the value obtained in this work. The former values were obtained as quasi-oco by the electrochemical method with 1 h of recuperation time. This recuperation time would not be enough to obtain the reversible potentials. Quasi-ecu-x curves had a inflection point at about x = 2.5 and at 6 > x > 2.5 quasi-ecu decreased almost linearly. These may correspond to the solid solution and two
3.5
I
I
I
I
I
I
I
3.4
0
I
I
2
I
I
I
I
4 6 x (= Li / (5Cu + 2V))
1
I
I
I
I
0
”
+; 9 > . z Y 5 g = k &
3.2 0
tr
0
3.0
z
2.8
1
I
I
I
I
0.0 0.2 0.4 0.6 0.8
I
1.0 1.2
5 (=x/Ad Fig. 8. Relation between E + (RT/F)ln(S/(l - 0) and {( = x/xL)with xL = 3.0 for Cu,V,O,, phase regions respectively obtained in this work, though the quasi-ecu would contain some polarization terms. Ir spectra of chemically lithiated Cu,V,O,, Some typical ir spectra of lithiated oxides were shown in Fig. 9. Absorption bands at about lOOf6OOcm-’ were ascribed to the vibrational modes of V04 unit in oxides[ll]. At x = 1, the ir spectra became broader rather abruptly and absorption
I
;; I
I
I
? :.d . 3.0 : > 2.5
~ .-
I
6
Fig. 7. Dependence of the open circuit potential on x.
I
1000
1
I
800
I
I
600
I
I
400
Wave numbers I cm” Fig. 9. Ir spectra of lithiated Cu,V,O,,. 1: x = 0 (unlithiated), 2: x = 0.85, 3: x = 1.66, 4: x = 3.48, 5: x = 4.63, 6: x = 5.99, 7: x = 7.15, with x = Li/(SCu + 2V).
861
Lithiation characteristics of Cu,VIO,,
caused by a low electronic conductivity of the electrode. The limiting value of x = 1 found in the discharge curves has a corresponding change in ir spectra, where an abrupt change was observed at about x = 1, though no evident phenomena were found in XRD, esr and ocv data. This phenomenon could, therefore, presumably attribute a possible change in diffusion paths of lithium ions as a result of occupation of lithium ions at most stable sites at x = 1, but further examination must be necessary.
3.0 3 :
il
uj 2.5 >
>
2 2.0
1.5~
CONCLUSION
’
0
’
’
’
2
’
4
’
’
6
’
’
8
’
1
IO
x=Lil(EiCu+2V) Fig. 10. Discharge curves for Cu,V,O,,. 1: 30pAcme2, 2: SO~tAcrn-~, 3: l@OpAcm-*. Inset: log j-log t relation for the first plateau.
intensities of the bands at about 950, 900 and 72Ocm-’ decreased relative to that of the band at SOOcm-‘, though the positions of the absorption bands of CusVtO,, remained unchanged. As a corresponding change in XRD patterns was not observed at x = 1, the change in ir spectra can be attributed to a local disorder of VO, units. This fact is due presumably to a change in the sites for inserted lithium ions at x = 1, though it was not detected also in the open-circuit potential measurements. The absorption band at about 86Ocm-’ appearing at 3 < x will be due to the formation of LiCuVO, detected by XRD analysis. Discharge characteristics
Some examples of galvanostatic discharge curves, as shown in Fig. 10, had two plateaus, and the first one ended at about x = 1, depending on the discharge current density (cd) (a relation i” *t = constant with m = 1.16 was found as shown in the inset of Fig. 10) and the second one continued with a smaller slope up to about x = 10. In the previous report by Yamaki et al.[7], only one plateau could be found at 0 < x < -0.85, corresponding to the second one in this work. Relatively large potential drop at the start of polarization was presumably
For Cu,V,O,, preferential reduction of Cu(II) over V(V) components occurs up to about x = 7. The single phase limits exist at x = 1 and 3, depending presumably on the different insertion sites of lithium ions, and the former limit may be controlled by the insertion paths provided by adjacent VO, units. At x > 3 further reduction of Cu(II) occurs in the multiphase and Cu(0) and Cu(T1) (LiCuVO,) coexist, because of instability of Cu(1). At about x > 7 LiCuVO, is further reduced. It is revealed that the preferential reduction of copper component is most remarkable in other nCu0. V205’s examined.
REFERENCES 1. M. Eguchi, M. Yamaguchi, T. Miura and T. Kishi, Denki Kogaku 61, 1347 (1993). 2. M. Eguchi, I. Furusawa, T. Miura and T. Kishi, Solid State Ionics 6% 159 ( 1994). 3. M. Eguchi, T. Yokoyama, T. Miura and T. Kishi, Solid State lonics, 74,269 (1994). 4 M. Eguchi, T. Miura and T. Kishi, Denki Kagaku 62, 504 (1994). 5 P. Fleury, C. R. Acad. Sci., Ser. C, 263, 1375 (1966). 6 R. D. Shannon and C. Calvo, Acta Cryst. B29, 1338 (1973). 7 Y. Sakurai, H. Ohtsuka and J. Yamaki, J. Efectrochem. Sot.
135,32 (1988).
a. P. A. Cox, Transition Metal Oxides, p. 250. Clarendon Press, Oxford ( 1992). 9. C. S. Sunandana, Proc. Nucl. Phys. Solid State Phys. 25, Cl97 (1969). 10. M. B. Armand, Materials fir Advanced Batteries, (Edited by D. W. Murphy, J. Broadhead and B. C. H. Steele), p. 145. Plenum Press, New York (1980). 11. V. D. Zhuravlev, Yu. A. Velikodnyi and L. V. Kristallov, Zh. Neorg. Khim. 32, 3060 (1987).