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Low temperature synthesis of highly ion conductive Li7La3Zr2O12–Li3BO3 composites
Electrochemistry Communications 33 (2013) 51–54
Contents lists available at SciVerse ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Short communication
Low temperature synthesis of highly ion conductive Li7La3Zr2O12–Li3BO3 composites Kiyoharu Tadanaga ⁎, Ryohei Takano, Takahiro Ichinose, Shigeo Mori, Akitoshi Hayashi, Masahiro Tatsumisago Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, Naka-ku, Sakai, Osaka 599-8531, Japan
a r t i c l e
i n f o
Article history: Received 21 February 2013 Received in revised form 2 April 2013 Accepted 3 April 2013 Available online 17 April 2013 Keywords: Li7La3Zr2O12 Sol–gel method Garnet oxide Liquid phase sintering Solid electrolyte
a b s t r a c t Highly lithium ion conductive composites with Al-doped Li7La3Zr2O12 (LLZ) and amorphous Li3BO3 were prepared from sol–gel derived precursor powders of LLZ and Li3BO3. Precursor LLZ powders with cubic phase were obtained by a heat treatment of the precursor dried gel at 600 °C. Pellets of the mixture of the obtained LLZ and Li3BO3 were first held at 700 °C, and then successively sintered at 900 °C. Density of the sintered pellet with Li3BO3 was larger than that of the pellet without Li3BO3. From the TEM observation, the pellets were found to consist of cubic LLZ and amorphous Li3BO3. Total electrical conductivity of the obtained LLZ–Li3BO3 composite was 1 × 10 −4 Scm −1 at 30 °C. © 2013 Elsevier B.V. All rights reserved.
1. Introduction All-solid-state lithium ion secondary batteries with inorganic solid electrolytes attract attention because of their high safety, reliability and energy density [1]. We have been developing sulfide-based electrolytes such as Li2S–P2S5 glasses and glass ceramics with high lithium ion conductivities of over 10 −3 Scm −1 at room temperature. Compared with sulfide-based materials, lithium ion conductive oxide glasses have rather low conductivity [2]. However, oxide materials have advantages such as their chemical stability and handling. Recently, garnet-type Li7La3Zr2O12 (LLZ) has been studied extensively because LLZ has high lithium ion conductivity (σtotal > 10−4 Scm−1 at room temperature) in cubic phase and chemical stability against lithium metal [3–6]. Although the bulk conductivity is close to 10 −3 Scm−1, very high temperature is needed for the sintering to reduce grain boundary resistance. To obtain a dense pellet with cubic LLZ, a heat-treatment at around 1200 °C is required in the conventional solid state reaction method. Such a heat-treatment at high temperatures causes a lithium loss, and to suppress the lithium loss, samples must be covered with mother powders. Thus, low temperature synthesis and sintering of cubic LLZ are desired. Addition of γ-Al2O3 to LLZ [7–9] or synthesis by sol–gel process [10–12] has been reported to lower the sintering temperature. Nevertheless, the densification of cubic LLZ at rather low temperature has not been achieved. To lower the sintering temperature, the addition of sintering additives that have low melting points and make liquid phases below ⁎ Corresponding author. Tel./fax: +81 72 254 9333. E-mail address: [email protected] (K. Tadanaga). 1388-2481/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.elecom.2013.04.004
sintering temperature is effective. The liquid phase promotes the densification and coarsening at low temperature [13,14]. Here, we propose a composite electrolyte of LLZ and lithium borate glass as a novel solid electrolyte with high lithium ion conductivity, and with the maximum process temperature of 900 °C. In the heat-treatment at 900 °C, the lithium loss during heat-treatment is assumed to be suppressed. During sintering of precursor powders of LLZ and lithium borate, the liquid phase of lithium borate acts as an accelerator for inter-reaction of grains at grain boundaries and the grain growth. In the obtained composite, amorphous lithium borate would be formed as a thin layer at the grain boundaries. These glassy phases will reduce the grain boundary resistance, and thus the composite can achieve high total lithium ion conductivity. When this composite is used for the lithium ion conductive path in composite electrodes with active materials, the lithium borate glass can act as a binder for electrolyte and active materials, and thus a small interfacial resistance will be expected. In the present study, we have prepared a composite of Al-doped LLZ and Li2O–B2O3 glass. The heat treatment of the LLZ precursor particles and Li3BO3 at 700 °C and successive sintering at 900 °C led to the formation of a dense LLZ and Li3BO3 glass composite, and the total ion conductivity of this composite is about 1 × 10 −4 Scm −1 at 30 °C.
2. Experimental Al-doped LLZ were prepared by a sol–gel process. Firstly, LiNO3 and La(NO3)3·6H2O were dissolved in ethanol. Separately, Zr(O-n-C3H7)4 and Al(O-sec-C4H9)3 were reacted with Ethylacetoacetete (EAcAc).
52
K. Tadanaga et al. / Electrochemistry Communications 33 (2013) 51–54
Cubic LLZ La2Zr2O7
a) 600oC
b) +Li3BO3 +900oC
15
20
25
30
35
40
45
50
55
2θ / o (CuKα) Fig. 1. XRD patterns of (a) Al-doped LLZ powder calcined at 600 °C and (b) sintered pellet with Li3BO3 at 700 and then 900 °C.
is 7.7:3:2:0.3:1.6:50. The gel was dried at 80 °C for 12 h, and then successively at 150 °C for 5 h. The dry gel was ground well and calcinated at 600 °C for 5 h. Al-doped LLZ powders obtained at 600 °C were ball-milled with zirconia balls for 12 h. Li3BO3 and the milled powders were mixed in an agate mortar, where the mole ratio of Li3BO3/LLZ was 0.68. Effects of Li3BO3 contents will be reported elsewhere [15]. Mixed powders were pressed into pellets at a pressure of 200 MPa, and the pellets were first held at 700 °C for 5 h, and then successively sintered at 900 °C for 36 h in an alumina crucible. X-ray diffraction (XRD) patterns were obtained by X-ray diffractometer (XRD-6000, Shimadzu). A scanning electron microscope (SEM) (JSM-6610A, JEOL) was used for the observation of the particles or polished fracture surface of sintered pellets. The sintered pellets were polished with sandpapers (#1000) and Au (0.5 cm 2) was sputter-coated onto the both surfaces of the pellets. AC impedance was measured by an impedance analyzer (SI 1260; Solartron) in a frequency range of 0.1Hz to 1 MHz. Porosity was calculated by the density of the pellets determined from the weight and physical dimensions. A symmetrical cell with Li/LLZ–Li3BO3 composite/Li configuration was fabricated through the deposition of lithium by vacuum-evaporation on both sides of the composite, and the reversibility of its dissolution and deposition was evaluated by galvanostatic cycling of 9 × 10 3 C. This corresponds to the dissolution and deposition of 9.3 × 10−8 mol of Li (about 3.4% of the deposited Li). 3. Results and discussion
These two solutions were mixed, and the obtained solution was stirred at room temperature for 1 h to obtain gel. The mole ratio of LiNO3:La(NO3)3·6H2O:Zr(O-n-C3H7)4:Al(O-sec-C4H9)3:EAcAc:ethanol
Fig. 1 shows XRD patterns of (a) Al-doped LLZ powder calcined at 600 °C and (b) sintered pellet with Li3BO3 at 700 and then 900 °C. In
a 600 oC
1 μm
b
magnified
10 µm
5 µm
Fig. 2. SEM image of Al-doped LLZ powder calcined at 600 °C (a), and the cross-sectional SEM image of the LLZ pellet sintered at 900 °C with Li3BO3 (b).
K. Tadanaga et al. / Electrochemistry Communications 33 (2013) 51–54
Fig. 1(a), diffraction peaks attributed to the cubic phase of LLZ are mainly observed, and a very small peak assigned to La2Zr2O7 is also observed. In the pellet sintered with Li3BO3, Fig. 1(b), peaks assigned to the cubic phase of LLZ are only observed. The peaks of Li3BO3 crystals are not observed, indicating that Li3BO3 exists as amorphous phase. Fig. 2 shows (a) SEM image of Al-doped LLZ powder calcined at 600 °C and (b) the cross-sectional SEM image of the LLZ pellet sintered at 900 °C with Li3BO3. The particle size of the precursor powders calcined at 600 °C is about 0.5 to 1.0 μm. Fig. 2(b) reveals that the pellet sintered with Li3BO3 has larger grain size. The dark areas are lithium borate region. The density of the sintered pellet relative to the theoretical density, determined from the weight and physical dimensions, was about 92%. During the sintering process, Li3BO3 should form a liquid phase, and rearrangement of LLZ particles and grain growth must be promoted by the liquid phase. Thus, dense structure was obtained after the sintering. Fig. 3 shows TEM and electron diffraction images of the sintered pellet with Li3BO3. The grains are confirmed to be the cubic phase of LLZ, and amorphous phase is observed in the triple point grain boundary. Li3BO3 amorphous phase can also exist at the grain boundary between LLZ particles as very thin layer. From the d-value of the halo in
53
the electron diffraction image, the amorphous phase is assumed to be lithium borate glass. These results are well consistent with the XRD result shown in Fig. 1(b), where no diffraction peaks attributed to Li3BO3 crystal were observed. Fig. 4 shows (a) the Nyquist plots for the composite electrolyte with Li3BO3 at 30 °C, (b) the total ionic conductivity determined by the AC impedance measurements, as a function of temperature, and (c) lithium dissolution and deposition curves in the Li/LLZ–Li3BO3 composite/Li cell at 0.02 mA cm −2. The high frequency arc and a low frequency tail are observed in the Nyquist plots. In Fig. 4(a), the frequency at 5000 Ω on the Z-real axis is 100 Hz, and no apparent resistance was observed up to 0.1 Hz. The impedance spectra similar to that shown in Fig. 4(a) were observed at the temperatures studied here. The total resistance R was determined from the real part of impedance Z value at the minimum between the high frequency arc and the low frequency tail. From the Arrhenius plots of the ionic conductivity, the total electrical conductivity at 30 °C and activation energy for the conduction of the LLZ–Li3BO3 composite were calculated to be 1 × 10 −4 Scm −1 and 35 kJ mol −1 (0.36 eV), respectively. The conductivity and activation energy are comparable to that of Al-doped LLZ pellet sintered at 1230 °C with usual solid state reaction process (3–4 × 10 −4 Scm −1 and 0.31 eV) [9]. The relative density of
LLZ
Li3BO3
LLZ
Fig. 3. TEM and electron diffraction images of the sintered pellet with Li3BO3.
54
K. Tadanaga et al. / Electrochemistry Communications 33 (2013) 51–54
a
b
observation, LLZ particles are basically the continuous phase in the composite, and Li3BO3 exists at the triple grain boundary as amorphous phase or at the grain boundaries as a thin layer between LLZ particles. Li3BO3 should form a liquid phase during heat treatment at 700 °C, and promoted the rearrangement of LLZ particles. In the sintering at 900 °C, the grain growth of LLZ particles must be promoted with the liquid phase, and the lithium loss must be also suppressed by the liquid phase. After sintering and cooling, Li3BO3 exists at grain boundaries as amorphous phase to form the composite. The volume of amorphous phase is estimated to be about 15% in the composite from the density of LLZ and Li3BO3. As well as densification of the pellet, the amorphous Li3BO3 is assumed to reduce the grain boundary resistance, and thus the composite can achieve high total lithium ion conductivity. At the same time, the conductivity of amorphous Li3BO3 is much lower than LLZ. This causes the rather low ionic conductivity of the composite compared with LLZ bulk conductivity. In the repeated lithium dissolution–deposition cycle test with the Li/LLZ–Li3BO3 composite/Li cell, the cell exhibits stable voltage of ± 60 mV during lithium dissolution or deposition for 5 cycles as shown in Fig. 4(c). The cycle test proved that this composite has very high chemical stability against lithium metal. The total conductivity estimated from the cycle test agrees with the result of the AC impedance measurement. Li/LLZ interfacial impedance was about 1000 Ω cm 2, which agrees with the previous reported value [9]. Thus, the LLZ–Li3BO3 composite is a promising oxide solid electrolyte for the all solid-state lithium batteries.
4. Conclusions High lithium ion conductive composite electrolyte with Li7La3Zr2O12 (LLZ) and amorphous Li3BO3 was prepared from sol–gel derived precursor powder of LLZ and Li3BO3. SEM and TEM observations prove that LLZ particles are continuous phase in the dense composite, and Li3BO3 exists at the grain boundary as amorphous phase. The total electrical conductivity of the composite at 30 °C was 1 × 10−4 Scm−1, which is comparable to that of Al-doped LLZ pellet sintered at 1230 °C without Li3BO3.
c
Fig. 4. The Nyquist plots for the composite electrolyte with Li3BO3 (a), the total ionic conductivity determined by the AC impedance measurements, as a function of temperature (b), and lithium dissolution and deposition curves in the Li/composite electrolyte/ Li cell at 0.02 mA cm−2(c).
the Al-doped LLZ without lithium borate was about 58%, and the conductivity was 1.6 × 10 −6 Scm −1. As indicated from SEM and TEM
References [1] M. Tatsumisago, A. Hayashi, Journal of Non-Crystalline Solids 354 (2008) 1411. [2] M. Tatsumisago, K. Yoneda, N. Machida, T. Minami, Journal of Non-Crystalline Solids 95 (96) (1987) 857. [3] R. Murugan, V. Thangadurai, W. Weppner, Angewandte Chemie International Edition 46 (2007) 7778. [4] J. Awaka, N. Kijima, H. Hayakawa, J. Akimoto, Journal of Solid State Chemistry 182 (2009) 2046. [5] M. Huang, T. Liu, Y. Deng, H. Geng, Y. Shen, Y. Lin, W. Nan, Solid State Ionics 204 (2011) 41. [6] C. Geiger, E. Alekseev, B. Lazic, M. Fisch, T. Armbruster, R. Langner, M. Fechtelkord, N. Kim, T. Pettke, W. Weppner, Inorganic Chemistry 50 (2011) 1089. [7] E. Rangasamy, J. Wolfenstine, J. Sakamoto, Solid State Ionics 206 (2012) 28. [8] M. Kotobuki, K. Kanamura, Y. Sato, T. Yoshida, Journal of Power Sources 196 (2011) 7750. [9] H. Buschmann, J. Dölle, S. Berendts, A. Kuhn, P. Bottke, M. Wilkening, P. Heitjan, A. Senyshyn, H. Ehrenberg, A. Lotnyk, V. Duppel, L. Kienle, J. Janek, Physical Chemistry Chemical Physics 13 (2011) 19378. [10] I. Kokal, M. Somer, L. Notten, T. Hintzen, Solid State Ionics 185 (2011) 42. [11] N. Janani, S. Ramakumar, L. Dhivya, C. Deviannapoorani, K. Saranya, R. Murugan, Ionics 17 (2011) 575. [12] H. Xie, Y. Li, B. Goodenough, Materials Research Bulletin 47 (2012) 1229. [13] S.M. Rhim, S. Hong, H. Bak, O.K. Kim, Journal of the American Ceramic Society 83 (2000) 1145. [14] H. Aono, E. Sugimoto, Y. Sadaoka, N. Imanaka, G.-Y. Adachi, Solid State Ionics 47 (1991) 257. [15] R. Takano, K. Tadanaga, A. Hayashi, M. Tatsumisago, submitted.
Contents lists available at SciVerse ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Short communication
Low temperature synthesis of highly ion conductive Li7La3Zr2O12–Li3BO3 composites Kiyoharu Tadanaga ⁎, Ryohei Takano, Takahiro Ichinose, Shigeo Mori, Akitoshi Hayashi, Masahiro Tatsumisago Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, Naka-ku, Sakai, Osaka 599-8531, Japan
a r t i c l e
i n f o
Article history: Received 21 February 2013 Received in revised form 2 April 2013 Accepted 3 April 2013 Available online 17 April 2013 Keywords: Li7La3Zr2O12 Sol–gel method Garnet oxide Liquid phase sintering Solid electrolyte
a b s t r a c t Highly lithium ion conductive composites with Al-doped Li7La3Zr2O12 (LLZ) and amorphous Li3BO3 were prepared from sol–gel derived precursor powders of LLZ and Li3BO3. Precursor LLZ powders with cubic phase were obtained by a heat treatment of the precursor dried gel at 600 °C. Pellets of the mixture of the obtained LLZ and Li3BO3 were first held at 700 °C, and then successively sintered at 900 °C. Density of the sintered pellet with Li3BO3 was larger than that of the pellet without Li3BO3. From the TEM observation, the pellets were found to consist of cubic LLZ and amorphous Li3BO3. Total electrical conductivity of the obtained LLZ–Li3BO3 composite was 1 × 10 −4 Scm −1 at 30 °C. © 2013 Elsevier B.V. All rights reserved.
1. Introduction All-solid-state lithium ion secondary batteries with inorganic solid electrolytes attract attention because of their high safety, reliability and energy density [1]. We have been developing sulfide-based electrolytes such as Li2S–P2S5 glasses and glass ceramics with high lithium ion conductivities of over 10 −3 Scm −1 at room temperature. Compared with sulfide-based materials, lithium ion conductive oxide glasses have rather low conductivity [2]. However, oxide materials have advantages such as their chemical stability and handling. Recently, garnet-type Li7La3Zr2O12 (LLZ) has been studied extensively because LLZ has high lithium ion conductivity (σtotal > 10−4 Scm−1 at room temperature) in cubic phase and chemical stability against lithium metal [3–6]. Although the bulk conductivity is close to 10 −3 Scm−1, very high temperature is needed for the sintering to reduce grain boundary resistance. To obtain a dense pellet with cubic LLZ, a heat-treatment at around 1200 °C is required in the conventional solid state reaction method. Such a heat-treatment at high temperatures causes a lithium loss, and to suppress the lithium loss, samples must be covered with mother powders. Thus, low temperature synthesis and sintering of cubic LLZ are desired. Addition of γ-Al2O3 to LLZ [7–9] or synthesis by sol–gel process [10–12] has been reported to lower the sintering temperature. Nevertheless, the densification of cubic LLZ at rather low temperature has not been achieved. To lower the sintering temperature, the addition of sintering additives that have low melting points and make liquid phases below ⁎ Corresponding author. Tel./fax: +81 72 254 9333. E-mail address: [email protected] (K. Tadanaga). 1388-2481/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.elecom.2013.04.004
sintering temperature is effective. The liquid phase promotes the densification and coarsening at low temperature [13,14]. Here, we propose a composite electrolyte of LLZ and lithium borate glass as a novel solid electrolyte with high lithium ion conductivity, and with the maximum process temperature of 900 °C. In the heat-treatment at 900 °C, the lithium loss during heat-treatment is assumed to be suppressed. During sintering of precursor powders of LLZ and lithium borate, the liquid phase of lithium borate acts as an accelerator for inter-reaction of grains at grain boundaries and the grain growth. In the obtained composite, amorphous lithium borate would be formed as a thin layer at the grain boundaries. These glassy phases will reduce the grain boundary resistance, and thus the composite can achieve high total lithium ion conductivity. When this composite is used for the lithium ion conductive path in composite electrodes with active materials, the lithium borate glass can act as a binder for electrolyte and active materials, and thus a small interfacial resistance will be expected. In the present study, we have prepared a composite of Al-doped LLZ and Li2O–B2O3 glass. The heat treatment of the LLZ precursor particles and Li3BO3 at 700 °C and successive sintering at 900 °C led to the formation of a dense LLZ and Li3BO3 glass composite, and the total ion conductivity of this composite is about 1 × 10 −4 Scm −1 at 30 °C.
2. Experimental Al-doped LLZ were prepared by a sol–gel process. Firstly, LiNO3 and La(NO3)3·6H2O were dissolved in ethanol. Separately, Zr(O-n-C3H7)4 and Al(O-sec-C4H9)3 were reacted with Ethylacetoacetete (EAcAc).
52
K. Tadanaga et al. / Electrochemistry Communications 33 (2013) 51–54
Cubic LLZ La2Zr2O7
a) 600oC
b) +Li3BO3 +900oC
15
20
25
30
35
40
45
50
55
2θ / o (CuKα) Fig. 1. XRD patterns of (a) Al-doped LLZ powder calcined at 600 °C and (b) sintered pellet with Li3BO3 at 700 and then 900 °C.
is 7.7:3:2:0.3:1.6:50. The gel was dried at 80 °C for 12 h, and then successively at 150 °C for 5 h. The dry gel was ground well and calcinated at 600 °C for 5 h. Al-doped LLZ powders obtained at 600 °C were ball-milled with zirconia balls for 12 h. Li3BO3 and the milled powders were mixed in an agate mortar, where the mole ratio of Li3BO3/LLZ was 0.68. Effects of Li3BO3 contents will be reported elsewhere [15]. Mixed powders were pressed into pellets at a pressure of 200 MPa, and the pellets were first held at 700 °C for 5 h, and then successively sintered at 900 °C for 36 h in an alumina crucible. X-ray diffraction (XRD) patterns were obtained by X-ray diffractometer (XRD-6000, Shimadzu). A scanning electron microscope (SEM) (JSM-6610A, JEOL) was used for the observation of the particles or polished fracture surface of sintered pellets. The sintered pellets were polished with sandpapers (#1000) and Au (0.5 cm 2) was sputter-coated onto the both surfaces of the pellets. AC impedance was measured by an impedance analyzer (SI 1260; Solartron) in a frequency range of 0.1Hz to 1 MHz. Porosity was calculated by the density of the pellets determined from the weight and physical dimensions. A symmetrical cell with Li/LLZ–Li3BO3 composite/Li configuration was fabricated through the deposition of lithium by vacuum-evaporation on both sides of the composite, and the reversibility of its dissolution and deposition was evaluated by galvanostatic cycling of 9 × 10 3 C. This corresponds to the dissolution and deposition of 9.3 × 10−8 mol of Li (about 3.4% of the deposited Li). 3. Results and discussion
These two solutions were mixed, and the obtained solution was stirred at room temperature for 1 h to obtain gel. The mole ratio of LiNO3:La(NO3)3·6H2O:Zr(O-n-C3H7)4:Al(O-sec-C4H9)3:EAcAc:ethanol
Fig. 1 shows XRD patterns of (a) Al-doped LLZ powder calcined at 600 °C and (b) sintered pellet with Li3BO3 at 700 and then 900 °C. In
a 600 oC
1 μm
b
magnified
10 µm
5 µm
Fig. 2. SEM image of Al-doped LLZ powder calcined at 600 °C (a), and the cross-sectional SEM image of the LLZ pellet sintered at 900 °C with Li3BO3 (b).
K. Tadanaga et al. / Electrochemistry Communications 33 (2013) 51–54
Fig. 1(a), diffraction peaks attributed to the cubic phase of LLZ are mainly observed, and a very small peak assigned to La2Zr2O7 is also observed. In the pellet sintered with Li3BO3, Fig. 1(b), peaks assigned to the cubic phase of LLZ are only observed. The peaks of Li3BO3 crystals are not observed, indicating that Li3BO3 exists as amorphous phase. Fig. 2 shows (a) SEM image of Al-doped LLZ powder calcined at 600 °C and (b) the cross-sectional SEM image of the LLZ pellet sintered at 900 °C with Li3BO3. The particle size of the precursor powders calcined at 600 °C is about 0.5 to 1.0 μm. Fig. 2(b) reveals that the pellet sintered with Li3BO3 has larger grain size. The dark areas are lithium borate region. The density of the sintered pellet relative to the theoretical density, determined from the weight and physical dimensions, was about 92%. During the sintering process, Li3BO3 should form a liquid phase, and rearrangement of LLZ particles and grain growth must be promoted by the liquid phase. Thus, dense structure was obtained after the sintering. Fig. 3 shows TEM and electron diffraction images of the sintered pellet with Li3BO3. The grains are confirmed to be the cubic phase of LLZ, and amorphous phase is observed in the triple point grain boundary. Li3BO3 amorphous phase can also exist at the grain boundary between LLZ particles as very thin layer. From the d-value of the halo in
53
the electron diffraction image, the amorphous phase is assumed to be lithium borate glass. These results are well consistent with the XRD result shown in Fig. 1(b), where no diffraction peaks attributed to Li3BO3 crystal were observed. Fig. 4 shows (a) the Nyquist plots for the composite electrolyte with Li3BO3 at 30 °C, (b) the total ionic conductivity determined by the AC impedance measurements, as a function of temperature, and (c) lithium dissolution and deposition curves in the Li/LLZ–Li3BO3 composite/Li cell at 0.02 mA cm −2. The high frequency arc and a low frequency tail are observed in the Nyquist plots. In Fig. 4(a), the frequency at 5000 Ω on the Z-real axis is 100 Hz, and no apparent resistance was observed up to 0.1 Hz. The impedance spectra similar to that shown in Fig. 4(a) were observed at the temperatures studied here. The total resistance R was determined from the real part of impedance Z value at the minimum between the high frequency arc and the low frequency tail. From the Arrhenius plots of the ionic conductivity, the total electrical conductivity at 30 °C and activation energy for the conduction of the LLZ–Li3BO3 composite were calculated to be 1 × 10 −4 Scm −1 and 35 kJ mol −1 (0.36 eV), respectively. The conductivity and activation energy are comparable to that of Al-doped LLZ pellet sintered at 1230 °C with usual solid state reaction process (3–4 × 10 −4 Scm −1 and 0.31 eV) [9]. The relative density of
LLZ
Li3BO3
LLZ
Fig. 3. TEM and electron diffraction images of the sintered pellet with Li3BO3.
54
K. Tadanaga et al. / Electrochemistry Communications 33 (2013) 51–54
a
b
observation, LLZ particles are basically the continuous phase in the composite, and Li3BO3 exists at the triple grain boundary as amorphous phase or at the grain boundaries as a thin layer between LLZ particles. Li3BO3 should form a liquid phase during heat treatment at 700 °C, and promoted the rearrangement of LLZ particles. In the sintering at 900 °C, the grain growth of LLZ particles must be promoted with the liquid phase, and the lithium loss must be also suppressed by the liquid phase. After sintering and cooling, Li3BO3 exists at grain boundaries as amorphous phase to form the composite. The volume of amorphous phase is estimated to be about 15% in the composite from the density of LLZ and Li3BO3. As well as densification of the pellet, the amorphous Li3BO3 is assumed to reduce the grain boundary resistance, and thus the composite can achieve high total lithium ion conductivity. At the same time, the conductivity of amorphous Li3BO3 is much lower than LLZ. This causes the rather low ionic conductivity of the composite compared with LLZ bulk conductivity. In the repeated lithium dissolution–deposition cycle test with the Li/LLZ–Li3BO3 composite/Li cell, the cell exhibits stable voltage of ± 60 mV during lithium dissolution or deposition for 5 cycles as shown in Fig. 4(c). The cycle test proved that this composite has very high chemical stability against lithium metal. The total conductivity estimated from the cycle test agrees with the result of the AC impedance measurement. Li/LLZ interfacial impedance was about 1000 Ω cm 2, which agrees with the previous reported value [9]. Thus, the LLZ–Li3BO3 composite is a promising oxide solid electrolyte for the all solid-state lithium batteries.
4. Conclusions High lithium ion conductive composite electrolyte with Li7La3Zr2O12 (LLZ) and amorphous Li3BO3 was prepared from sol–gel derived precursor powder of LLZ and Li3BO3. SEM and TEM observations prove that LLZ particles are continuous phase in the dense composite, and Li3BO3 exists at the grain boundary as amorphous phase. The total electrical conductivity of the composite at 30 °C was 1 × 10−4 Scm−1, which is comparable to that of Al-doped LLZ pellet sintered at 1230 °C without Li3BO3.
c
Fig. 4. The Nyquist plots for the composite electrolyte with Li3BO3 (a), the total ionic conductivity determined by the AC impedance measurements, as a function of temperature (b), and lithium dissolution and deposition curves in the Li/composite electrolyte/ Li cell at 0.02 mA cm−2(c).
the Al-doped LLZ without lithium borate was about 58%, and the conductivity was 1.6 × 10 −6 Scm −1. As indicated from SEM and TEM
References [1] M. Tatsumisago, A. Hayashi, Journal of Non-Crystalline Solids 354 (2008) 1411. [2] M. Tatsumisago, K. Yoneda, N. Machida, T. Minami, Journal of Non-Crystalline Solids 95 (96) (1987) 857. [3] R. Murugan, V. Thangadurai, W. Weppner, Angewandte Chemie International Edition 46 (2007) 7778. [4] J. Awaka, N. Kijima, H. Hayakawa, J. Akimoto, Journal of Solid State Chemistry 182 (2009) 2046. [5] M. Huang, T. Liu, Y. Deng, H. Geng, Y. Shen, Y. Lin, W. Nan, Solid State Ionics 204 (2011) 41. [6] C. Geiger, E. Alekseev, B. Lazic, M. Fisch, T. Armbruster, R. Langner, M. Fechtelkord, N. Kim, T. Pettke, W. Weppner, Inorganic Chemistry 50 (2011) 1089. [7] E. Rangasamy, J. Wolfenstine, J. Sakamoto, Solid State Ionics 206 (2012) 28. [8] M. Kotobuki, K. Kanamura, Y. Sato, T. Yoshida, Journal of Power Sources 196 (2011) 7750. [9] H. Buschmann, J. Dölle, S. Berendts, A. Kuhn, P. Bottke, M. Wilkening, P. Heitjan, A. Senyshyn, H. Ehrenberg, A. Lotnyk, V. Duppel, L. Kienle, J. Janek, Physical Chemistry Chemical Physics 13 (2011) 19378. [10] I. Kokal, M. Somer, L. Notten, T. Hintzen, Solid State Ionics 185 (2011) 42. [11] N. Janani, S. Ramakumar, L. Dhivya, C. Deviannapoorani, K. Saranya, R. Murugan, Ionics 17 (2011) 575. [12] H. Xie, Y. Li, B. Goodenough, Materials Research Bulletin 47 (2012) 1229. [13] S.M. Rhim, S. Hong, H. Bak, O.K. Kim, Journal of the American Ceramic Society 83 (2000) 1145. [14] H. Aono, E. Sugimoto, Y. Sadaoka, N. Imanaka, G.-Y. Adachi, Solid State Ionics 47 (1991) 257. [15] R. Takano, K. Tadanaga, A. Hayashi, M. Tatsumisago, submitted.