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Li+ ionic diffusion in Li–Cu–O compounds
Solid State Ionics 177 (2006) 2775 – 2778 www.elsevier.com/locate/ssi
Li + ionic diffusion in Li–Cu–O compounds Koichi Nakamura a,⁎, Kenta Kawai b , Koji Yamada c , Yoshitaka Michihiro a , Toshihiro Moriga b , Ichiro Nakabayashi b , Tatsuo Kanashiro a b
a Department of Physics, Faculty of Engineering, The University of Tokushima, 2-1 Minami-Josanjima-Cho, Tokushima 770-8506, Japan Department of Chemical Science and Technology, Faculty of Engineering, The University of Tokushima, 2-1 Minami-Josanjima-Cho, Tokushima 770-8506, Japan c Department of Chemistry, Faculty of Science, Hiroshima University, Higashi-Hiroshima 739-8511, Japan
Received 1 July 2005; received in revised form 10 March 2006; accepted 29 March 2006
Abstract Ionic diffusion and conductivity in Li2CuO2 and LiCuO2 have been studied by complex impedance measurements. The conductivity of both samples is largely enhanced with increasing temperature and reaches 10− 2–10− 1 S/m at around 450 K. X-ray diffraction measurements show that a small amount of CuO phase appears with increasing temperature in both samples. High electric conduction would be ascribed to a complicated mechanism containing the ionic and hole conduction. © 2006 Elsevier B.V. All rights reserved. Keywords: Li+ ionic diffusion; Ionic conductivity; Complex impedance
1. Introduction LiMO2 (M is a transition metal) compounds with the layered structure, which is considered to be advantageous for Li+ ionic diffusion, have attracted a great interest concerning an application as the cathode materials in lithium secondary batteries with high energy density, capacity and power rate [1]. Especially Li–Cu–O compounds are candidates for a new cathode material because copper is much more abundant, and is less expensive and less toxic in comparison with cobalt. LiCuO2 and Li3Cu2O4 have been successively synthesized since Li2CuO2 was synthesized in studies on high-Tc superconductors [2–5]. These compounds commonly have the layered structure consisting of two-dimensional [CuO4] sheets and Li layers. Cu ions are trivalent in non-magnetic LiCuO2 and divalent in magnetic Li2CuO2 with Néel temperature, TN = 8.3 K [2,6]. LiCuO2 has the monoclinic NaCuO2 structure of the space group, C2/m with a = 5.733 Å, b = 2.7176 Å, c = 5.522 Å, and β = 120.68° and has Li layers separated by edge-sharing [CuO4] sheets. Li ions form a slightly distorted triangular lattice. In
⁎ Corresponding author. E-mail address: [email protected] (K. Nakamura). 0167-2738/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2006.03.046
LiCuO2, the Li layers and [CuO4] sheets are alternately stacked in the direction of the c-axis as shown in Fig. 1(a) [5,7]. Li2CuO2 has the orthorhombic structure of the space group, Immm with a = 3.659 Å, b = 2.861 Å, c = 9.387 Å. This structure is consisting of edge-sharing [CuO4] sheets lying on the bc plane, which are linked along the b-axis, and Li layers being situated between the [CuO4] sheets and making up zigzag layers themselves, as shown in Fig. 1(b). For designing and developing Li–M–O compounds as a new cathode material, it is important to obtain more physical and chemical insight into the Li+ ionic diffusion in Li–Cu–O compounds. We report the temperature dependence of electrical conductivity of LiCuO2 and Li2CuO2, and give a discussion about the Li+ ionic diffusion in these compounds. 2. Experimental Li2CuO2 was prepared from stoichiometric powders of LiOH–H2O and CuO. The mixture was ground, pelletized and sintered at 973 K in air. Polycrystalline LiCuO2 was prepared by extracting Li from Li2CuO2 with I2 in acetonitrile at 333 K by dissolving tetra n-butyl ammonium iodide as a catalyst. Complex impedance measurements were performed using a compressed powder pellet, which was mounted in a sample cell
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Li 2CuO2 CuO
Intensity ( arb. units )
513K
473K
433K
393K
RT
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2θ ( deg. ) Fig. 3. XRD patterns of Li2CuO2 at RT, 393, 433, 473, and 513 K.
Fig. 1. Crystal structures of (a) LiCuO2 and (b) Li2CuO2. Small and large spheres represent lithium and oxygen atoms, respectively. Each copper atom locates in a centre of a gray square sheet in both structures.
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3. Results and discussion Fig. 2 shows the temperature dependence of electrical conductivity of Li2CuO2 and LiCuO2. In Li2CuO2, conductivity increases up to about 3 × 10− 1 S/m with increasing temperature as shown by closed (the 1st cycle) and open (the 2nd cycle)
LiCuO2
CuO 513K
Intensity ( arb. units )
σ0 ( S / m )
and held between two spring-loaded stainless steel electrodes covered with carbon. The impedance ∣Z⁎∣ and phase θ were measured in the frequency range from 42 Hz to 5 MHz and the temperature range from 300 to 500 K.
The obtained polycrystalline powder samples were confirmed to be a single-phase by X-ray diffraction at room temperature (RT). The X-ray diffraction patterns of LiCuO2 and Li2CuO2 have been recorded using a Rigaku RINT 2000 X-ray diffractometer with monochromatized CuKα radiation at 40 kV and 150 mA at temperatures of RT, 393, 433, 473, and 513 K.
Li2CuO2-1st cycle Li2CuO2-2nd cycle LiCuO2 -1st cycle LiCuO2 -2nd cycle
473K
433K
393K
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1000 / T ( K-1 )
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2θ ( deg. ) Fig. 2. Temperature dependence of electrical conductivity of Li2CuO2 and LiCuO2.
Fig. 4. XRD patterns of LiCuO2 at RT, 393, 433, 473, and 513 K.
K. Nakamura et al. / Solid State Ionics 177 (2006) 2775–2778 0
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circles. A dip observed between 343 and 373 K for the 1st cycle might arise from the desorption of H2O contained in the sample. It is noted that the conductivity observed in a subsequent cooling process keeps high and is 10− 2 S/m at RT. The behavior in the temperature dependence of conductivity of LiCuO2 is similar to that of Li2CuO2 except the very steep change in the 1st heating process. The conductivity of LiCuO2 is about ten times higher than that of Li2CuO2 over the measured temperature range. The conductivity after the heating process in each sample is comparable to the electrical conductivity ∼ 0.1 S/m of LiCoO2 at RT [8]. The temperature hysteresis in conductivity is found in both samples. The conductivity in the 2nd heating process increases gradually from the value at RT in both samples. In a re-measurement after several days, however, the conductivity at RT decreases down to about 10− 4 S/m in Li2CuO2, which indicates that the same temperature dependence as the observed result in Fig. 2 will be repeated again in the 1st and 2nd cycles. The hygroscopic effect in the conductivity has yet to be known in these samples. The activation energies in the 1st heating process have been evaluated in Li2CuO2 and LiCuO2. The activation energy in Li2CuO2 is evaluated to be about 0.98 eV from a steep slope in the temperature range from 390 to 440 K in Fig. 2. That in LiCuO2 is evaluated to be 0.23 eV from the slope in the range from RT to 400 K. This value is consistent with the activation energy of 0.23 eV reported in a previous NMR study on LiCuO2 [9]. These activation energies evaluated from the 1st heating process reflect a mechanism of conduction in these samples, because every initial heating measurement in the virgin sample shows almost the same temperature dependence of conductivity. We discuss the mechanism of the high conductivity in Li2CuO2 and LiCuO2. The conductivity except the 1st heating process exhibits weak temperature dependence in both samples. The temperature dependence of X-ray diffraction pattern has been measured to elucidate the relationship between the crystal structure and the high conductivity after heating process, and is shown in Figs. 3 and 4. There is no special change originating from a phase transition in the measured temperature range in both samples. Small additional peaks arising from CuO appear around 35° and 38° with increasing temperature in the patterns of Li2CuO2. Similar peaks from CuO appear also in LiCuO2 with increasing temperature. This increasing CuO phase is ascribed to the loss of oxygen. Diffraction peaks of LiCuO2 are broader than those of Li2CuO2 and the small CuO peaks are observed even at RT. These results indicate that polycrystalline LiCuO2, which is obtained in acetonitrile, is not stoichiometrically crystallized well as compared with Li2CuO2, which has been sintered at high temperature. Fig. 5 shows the frequency dependence of conductivity at 330 K. The conductivity at low temperature in the 1st heating process depends on frequency, while the conductivity depends slightly on frequency in the 2nd heating process. It is noted that the frequency independent conductivity is usually observed in semiconductors. Suda et al. revealed that in CuO a hole is generated to compensate the negative charge introduced by substituting Li+ for Cu2+ and yields p-type semiconductivity
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ω ( rad / s ) Fig. 5. Frequency dependence of electrical conductivity of Li2CuO2 and LiCuO2.
[10,11]. Therefore, it is likely that the conductivity of both samples after the 1st heating process contains the semiconducting contribution as well as the ionic conductivity. This suggests that the large activation energy in Li2CuO2 is not ascribed only to the ionic hopping. However, the change over four orders of magnitude in the conductivity of Li2CuO2 in the 1st process is much larger than the increase of one order of magnitude measured from RT to 500 K in pure CuO. Additionally, the fact that 7Li NMR measurements exhibit motional narrowed spectra in both samples at high temperatures shows that the conductivity of Li2CuO2 and LiCuO2 are not only attributed to semiconductivity but also to a contribution of conduction mechanism due to Li+ ionic diffusion [9]. Further NMR and conductivity measurements are now going to clarify the conduction mechanism in LiCuO2 and Li2CuO2. 4. Summary The high electrical conductivity has been observed in Li2CuO2 and LiCuO2. The large activation energy of Li2CuO2 observed in the 1st heating cycle is not considered to directly reflect the contribution from ionic hopping. The activation energy of LiCuO2 is estimated to be 0.23 eV from the fitting in the middle temperature range of the 1st heating process. This activation energy evaluated from the impedance in LiCuO2 is comparable with that from NMR in LiCoO2. In LiCuO2 and Li2CuO2, the high electrical conductivity after the 1st heating process would be ascribed to the complicated conduction mechanism, which arises from the Li+ ionic hopping and semiconductivity originating from CuO phase. References [1] M. Winter, J.O. Besenhard, M.E. Spahr, P. Novák, Adv. Mater. 10 (1998) 725. [2] K. Imai, M. Koike, H. Takei, H. Sawa, D. Shiomi, K. Nozawa, M. Kinoshita, J. Phys. Soc. Jpn. 61 (1992) 1819. [3] T.A. Hewston, B.L. Chamberland, J. Phys. Chem. Solids 48 (1987) 97.
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[4] R. Berger, J. Less-Common Met. 169 (1991) 33. [5] H. Arai, S. Okada, Y. Sakurai, J. Yamaki, Solid State Ionics 106 (1998) 45. [6] F. Sapiña, J. Rodríguez-Carvajal, M.J. Sanchis, R. Ibáñez, A. Beltrán, D. Beltrán, Solid State Commun. 74 (1990) 779. [7] R. Berger, L. Tergenius, J. Alloys Compd. 203 (1994) 203. [8] J. Molenda, A. Stokłosa, T. Bąk, Solid State Ionics 36 (1989) 53.
[9] K. Nakamura, T. Moriga, A. Sumi, Y. Kashu, Y. Michihiro, I. Nakabayashi, T. Kanashiro, Solid State Ionics 176 (2005) 837. [10] S. Suda, S. Fujitsu, K. Koumoto, H. Yanagida, Jpn. J. Appl. Phys. 31 (1992) 2488. [11] S. Suda, T. Aoyama, K. Kanamura, T. Umegaki, Jpn. J. Appl. Phys. 39 (2000) 3566.
Li + ionic diffusion in Li–Cu–O compounds Koichi Nakamura a,⁎, Kenta Kawai b , Koji Yamada c , Yoshitaka Michihiro a , Toshihiro Moriga b , Ichiro Nakabayashi b , Tatsuo Kanashiro a b
a Department of Physics, Faculty of Engineering, The University of Tokushima, 2-1 Minami-Josanjima-Cho, Tokushima 770-8506, Japan Department of Chemical Science and Technology, Faculty of Engineering, The University of Tokushima, 2-1 Minami-Josanjima-Cho, Tokushima 770-8506, Japan c Department of Chemistry, Faculty of Science, Hiroshima University, Higashi-Hiroshima 739-8511, Japan
Received 1 July 2005; received in revised form 10 March 2006; accepted 29 March 2006
Abstract Ionic diffusion and conductivity in Li2CuO2 and LiCuO2 have been studied by complex impedance measurements. The conductivity of both samples is largely enhanced with increasing temperature and reaches 10− 2–10− 1 S/m at around 450 K. X-ray diffraction measurements show that a small amount of CuO phase appears with increasing temperature in both samples. High electric conduction would be ascribed to a complicated mechanism containing the ionic and hole conduction. © 2006 Elsevier B.V. All rights reserved. Keywords: Li+ ionic diffusion; Ionic conductivity; Complex impedance
1. Introduction LiMO2 (M is a transition metal) compounds with the layered structure, which is considered to be advantageous for Li+ ionic diffusion, have attracted a great interest concerning an application as the cathode materials in lithium secondary batteries with high energy density, capacity and power rate [1]. Especially Li–Cu–O compounds are candidates for a new cathode material because copper is much more abundant, and is less expensive and less toxic in comparison with cobalt. LiCuO2 and Li3Cu2O4 have been successively synthesized since Li2CuO2 was synthesized in studies on high-Tc superconductors [2–5]. These compounds commonly have the layered structure consisting of two-dimensional [CuO4] sheets and Li layers. Cu ions are trivalent in non-magnetic LiCuO2 and divalent in magnetic Li2CuO2 with Néel temperature, TN = 8.3 K [2,6]. LiCuO2 has the monoclinic NaCuO2 structure of the space group, C2/m with a = 5.733 Å, b = 2.7176 Å, c = 5.522 Å, and β = 120.68° and has Li layers separated by edge-sharing [CuO4] sheets. Li ions form a slightly distorted triangular lattice. In
⁎ Corresponding author. E-mail address: [email protected] (K. Nakamura). 0167-2738/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2006.03.046
LiCuO2, the Li layers and [CuO4] sheets are alternately stacked in the direction of the c-axis as shown in Fig. 1(a) [5,7]. Li2CuO2 has the orthorhombic structure of the space group, Immm with a = 3.659 Å, b = 2.861 Å, c = 9.387 Å. This structure is consisting of edge-sharing [CuO4] sheets lying on the bc plane, which are linked along the b-axis, and Li layers being situated between the [CuO4] sheets and making up zigzag layers themselves, as shown in Fig. 1(b). For designing and developing Li–M–O compounds as a new cathode material, it is important to obtain more physical and chemical insight into the Li+ ionic diffusion in Li–Cu–O compounds. We report the temperature dependence of electrical conductivity of LiCuO2 and Li2CuO2, and give a discussion about the Li+ ionic diffusion in these compounds. 2. Experimental Li2CuO2 was prepared from stoichiometric powders of LiOH–H2O and CuO. The mixture was ground, pelletized and sintered at 973 K in air. Polycrystalline LiCuO2 was prepared by extracting Li from Li2CuO2 with I2 in acetonitrile at 333 K by dissolving tetra n-butyl ammonium iodide as a catalyst. Complex impedance measurements were performed using a compressed powder pellet, which was mounted in a sample cell
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Li 2CuO2 CuO
Intensity ( arb. units )
513K
473K
433K
393K
RT
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2θ ( deg. ) Fig. 3. XRD patterns of Li2CuO2 at RT, 393, 433, 473, and 513 K.
Fig. 1. Crystal structures of (a) LiCuO2 and (b) Li2CuO2. Small and large spheres represent lithium and oxygen atoms, respectively. Each copper atom locates in a centre of a gray square sheet in both structures.
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3. Results and discussion Fig. 2 shows the temperature dependence of electrical conductivity of Li2CuO2 and LiCuO2. In Li2CuO2, conductivity increases up to about 3 × 10− 1 S/m with increasing temperature as shown by closed (the 1st cycle) and open (the 2nd cycle)
LiCuO2
CuO 513K
Intensity ( arb. units )
σ0 ( S / m )
and held between two spring-loaded stainless steel electrodes covered with carbon. The impedance ∣Z⁎∣ and phase θ were measured in the frequency range from 42 Hz to 5 MHz and the temperature range from 300 to 500 K.
The obtained polycrystalline powder samples were confirmed to be a single-phase by X-ray diffraction at room temperature (RT). The X-ray diffraction patterns of LiCuO2 and Li2CuO2 have been recorded using a Rigaku RINT 2000 X-ray diffractometer with monochromatized CuKα radiation at 40 kV and 150 mA at temperatures of RT, 393, 433, 473, and 513 K.
Li2CuO2-1st cycle Li2CuO2-2nd cycle LiCuO2 -1st cycle LiCuO2 -2nd cycle
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2θ ( deg. ) Fig. 2. Temperature dependence of electrical conductivity of Li2CuO2 and LiCuO2.
Fig. 4. XRD patterns of LiCuO2 at RT, 393, 433, 473, and 513 K.
K. Nakamura et al. / Solid State Ionics 177 (2006) 2775–2778 0
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circles. A dip observed between 343 and 373 K for the 1st cycle might arise from the desorption of H2O contained in the sample. It is noted that the conductivity observed in a subsequent cooling process keeps high and is 10− 2 S/m at RT. The behavior in the temperature dependence of conductivity of LiCuO2 is similar to that of Li2CuO2 except the very steep change in the 1st heating process. The conductivity of LiCuO2 is about ten times higher than that of Li2CuO2 over the measured temperature range. The conductivity after the heating process in each sample is comparable to the electrical conductivity ∼ 0.1 S/m of LiCoO2 at RT [8]. The temperature hysteresis in conductivity is found in both samples. The conductivity in the 2nd heating process increases gradually from the value at RT in both samples. In a re-measurement after several days, however, the conductivity at RT decreases down to about 10− 4 S/m in Li2CuO2, which indicates that the same temperature dependence as the observed result in Fig. 2 will be repeated again in the 1st and 2nd cycles. The hygroscopic effect in the conductivity has yet to be known in these samples. The activation energies in the 1st heating process have been evaluated in Li2CuO2 and LiCuO2. The activation energy in Li2CuO2 is evaluated to be about 0.98 eV from a steep slope in the temperature range from 390 to 440 K in Fig. 2. That in LiCuO2 is evaluated to be 0.23 eV from the slope in the range from RT to 400 K. This value is consistent with the activation energy of 0.23 eV reported in a previous NMR study on LiCuO2 [9]. These activation energies evaluated from the 1st heating process reflect a mechanism of conduction in these samples, because every initial heating measurement in the virgin sample shows almost the same temperature dependence of conductivity. We discuss the mechanism of the high conductivity in Li2CuO2 and LiCuO2. The conductivity except the 1st heating process exhibits weak temperature dependence in both samples. The temperature dependence of X-ray diffraction pattern has been measured to elucidate the relationship between the crystal structure and the high conductivity after heating process, and is shown in Figs. 3 and 4. There is no special change originating from a phase transition in the measured temperature range in both samples. Small additional peaks arising from CuO appear around 35° and 38° with increasing temperature in the patterns of Li2CuO2. Similar peaks from CuO appear also in LiCuO2 with increasing temperature. This increasing CuO phase is ascribed to the loss of oxygen. Diffraction peaks of LiCuO2 are broader than those of Li2CuO2 and the small CuO peaks are observed even at RT. These results indicate that polycrystalline LiCuO2, which is obtained in acetonitrile, is not stoichiometrically crystallized well as compared with Li2CuO2, which has been sintered at high temperature. Fig. 5 shows the frequency dependence of conductivity at 330 K. The conductivity at low temperature in the 1st heating process depends on frequency, while the conductivity depends slightly on frequency in the 2nd heating process. It is noted that the frequency independent conductivity is usually observed in semiconductors. Suda et al. revealed that in CuO a hole is generated to compensate the negative charge introduced by substituting Li+ for Cu2+ and yields p-type semiconductivity
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ω ( rad / s ) Fig. 5. Frequency dependence of electrical conductivity of Li2CuO2 and LiCuO2.
[10,11]. Therefore, it is likely that the conductivity of both samples after the 1st heating process contains the semiconducting contribution as well as the ionic conductivity. This suggests that the large activation energy in Li2CuO2 is not ascribed only to the ionic hopping. However, the change over four orders of magnitude in the conductivity of Li2CuO2 in the 1st process is much larger than the increase of one order of magnitude measured from RT to 500 K in pure CuO. Additionally, the fact that 7Li NMR measurements exhibit motional narrowed spectra in both samples at high temperatures shows that the conductivity of Li2CuO2 and LiCuO2 are not only attributed to semiconductivity but also to a contribution of conduction mechanism due to Li+ ionic diffusion [9]. Further NMR and conductivity measurements are now going to clarify the conduction mechanism in LiCuO2 and Li2CuO2. 4. Summary The high electrical conductivity has been observed in Li2CuO2 and LiCuO2. The large activation energy of Li2CuO2 observed in the 1st heating cycle is not considered to directly reflect the contribution from ionic hopping. The activation energy of LiCuO2 is estimated to be 0.23 eV from the fitting in the middle temperature range of the 1st heating process. This activation energy evaluated from the impedance in LiCuO2 is comparable with that from NMR in LiCoO2. In LiCuO2 and Li2CuO2, the high electrical conductivity after the 1st heating process would be ascribed to the complicated conduction mechanism, which arises from the Li+ ionic hopping and semiconductivity originating from CuO phase. References [1] M. Winter, J.O. Besenhard, M.E. Spahr, P. Novák, Adv. Mater. 10 (1998) 725. [2] K. Imai, M. Koike, H. Takei, H. Sawa, D. Shiomi, K. Nozawa, M. Kinoshita, J. Phys. Soc. Jpn. 61 (1992) 1819. [3] T.A. Hewston, B.L. Chamberland, J. Phys. Chem. Solids 48 (1987) 97.
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[4] R. Berger, J. Less-Common Met. 169 (1991) 33. [5] H. Arai, S. Okada, Y. Sakurai, J. Yamaki, Solid State Ionics 106 (1998) 45. [6] F. Sapiña, J. Rodríguez-Carvajal, M.J. Sanchis, R. Ibáñez, A. Beltrán, D. Beltrán, Solid State Commun. 74 (1990) 779. [7] R. Berger, L. Tergenius, J. Alloys Compd. 203 (1994) 203. [8] J. Molenda, A. Stokłosa, T. Bąk, Solid State Ionics 36 (1989) 53.
[9] K. Nakamura, T. Moriga, A. Sumi, Y. Kashu, Y. Michihiro, I. Nakabayashi, T. Kanashiro, Solid State Ionics 176 (2005) 837. [10] S. Suda, S. Fujitsu, K. Koumoto, H. Yanagida, Jpn. J. Appl. Phys. 31 (1992) 2488. [11] S. Suda, T. Aoyama, K. Kanamura, T. Umegaki, Jpn. J. Appl. Phys. 39 (2000) 3566.