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Garnet-type Li6.75La3Zr1.75Nb0.25O12 synthesized by coprecipitation method and its lithium ion conductivity
SOSI-13148; No of Pages 4 Solid State Ionics xxx (2013) xxx–xxx
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Garnet-type Li6.75La3Zr1.75Nb0.25O12 synthesized by coprecipitation method and its lithium ion conductivity Haruo Imagawa ⁎, Shingo Ohta, Yuki Kihira, Takahiko Asaoka TOYOTA Central Research and Development Labs., Inc., Nagakute, Aichi 480-1192, Japan
a r t i c l e
i n f o
Article history: Received 17 May 2013 Received in revised form 23 October 2013 Accepted 28 October 2013 Available online xxxx Keywords: Garnet-type oxides Coprecipitation Solid state electrolytes Lithium ion conductivity
a b s t r a c t Li6.75La3Zr1.75Nb0.25O12 (LLZNb) was synthesized using the coprecipitation method. X-ray powder diffraction patterns of LLZNb after calcination at 600 °C or higher indicate a cubic phase with a garnet-type structure. Transmission electron microscopy revealed primary LLZNb particles formed by coprecipitation with sizes in the range of 50–100 nm. Sintering of LLZNb was possible at relatively low temperatures of 1000 and 1100 °C under an air atmosphere due to the small-sized primary particles, although LLZNb powder synthesized by solid-state reaction did not sinter under the same conditions due to the large size of the primary particles. The total lithium ion conductivity of an LLZNb pellet sintered at 1000 °C was measured using AC impedance to be 1.0 × 10− 4 S cm−1. © 2013 Elsevier B.V. All rights reserved.
1. Introduction All-solid-state lithium ion batteries are attracting much attention, especially with respect to safety, due to their non-flammability, unlike the liquid organic electrolytes used in conventional lithium ion batteries. Garnet-type Li7La3Zr2O12 (LLZ) is one metal oxide with excellent lithium ion conductivity (N 10− 4 S cm− 1) as a solid electrolyte, which was first reported by Weppner et al. [1]. LLZ has been intensively investigated [2–7] due to its high lithium ion conductivity, wide electrochemical window, and excellent stability against lithium metal, which makes metallic lithium available as an anode material. Doping of other elements into the LLZ structure has included the partial substitution of zirconium with niobium, which was reported to be effective to increase the lithium ion conductivity [2]. Some fabrication trials of all-solid-batteries using LLZ have already been conducted, with particular focus on formation at the interface between LLZ and various cathode materials [8,9]. LLZ forms cubic [1–5] or tetragonal [4–7] phases, depending on the preparation conditions such as composition, synthesis method and calcination temperature. Cubic phase LLZ tends to exhibit excellent lithium
⁎ Corresponding author at: 41-1 Yokomichi, Nagakute, Aichi 480-1192, Japan. Tel.: +81 561 71 7226; fax: +81 561 63 5328. E-mail address: [email protected] (H. Imagawa).
ion conductivity compared with that for tetragonal LLZ, mainly due to the suitable occupation of lithium sites of 24d and 96 h in the cubicphase framework [10]. Therefore, the selective formation of cubic phase LLZ is favorable for an increase of the lithium ion conductivity. One drawback in the formation of bulk LLZ for use as a solid electrolyte is the high sintering temperature required, which is typically around 1200 °C, to obtain sufficient density for high lithium ion conductivity. Thus, there is a difficulty in achieving a one-step fabrication of allsolid-state batteries, because the calcination temperature of the various active electrode materials is far lower than the temperature required to sinter LLZ. Recently, the synthesis of LLZ based on wet chemical methods was reported [4,5]. The formation of garnet-type LLZ as a powder is possible between 700 and 800 °C using wet chemical methods. These reports are promising with respect to decreasing the sintering temperature of LLZ, although the lithium ion conductivity of the assintered LLZ is still low due to the formation of tetragonal phase during low temperature sintering. Here we report the synthesis of Li6.75La3Zr1.75Nb0.25O12 (LLZNb) using the coprecipitation method for the selective formation of the cubic phase with a garnet-type structure, and low temperature sintering while still obtaining high lithium ion conductivity. The concept for this synthesis is the formation of small-sized LLZNb particles by a wet chemical method and enhanced sintering of the LLZNb at lower temperatures (1000–1100 °C) than those used to sinter LLZNb synthesized by solid-state reaction. Cubic phase garnet-type LLZNb is expected to form at low temperature using this synthesis method with Nb doping. Furthermore, the small-
0167-2738/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ssi.2013.10.059
Please cite this article as: H. Imagawa, et al., Solid State Ionics (2013), http://dx.doi.org/10.1016/j.ssi.2013.10.059
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H. Imagawa et al. / Solid State Ionics xxx (2013) xxx–xxx
2. Experimental
:LLZNb :La2Zr2O7
2.1. Synthesis LLZNb was prepared by the coprecipitation method. All chemicals used in the synthesis were of regular grade purity and purchased from Wako Pure Chemical Industries. LiNO3, La(NO3)3, ZrO(NO3)2 and NbCl5 in HCl were dissolved in ion-exchanged water, and then ammonia solution was added for coprecipitation. The obtained precursor was dried at 400 °C for 5 h in air and calcined at 600–950 °C for 24 h. The amount of Li in the calcined powder was almost 15 wt.% excess relative to the original target LLZNb composition. The powder calcined at 950 °C was pressed and sintered within a bed of the LLZNb mother powder at 1000, 1100, and 1180 °C for 36 h. Li6.75La3Zr1.75Nb0.25O12 was also synthesized by solid-state reaction according to a literature method [4] (denoted Ref) and used as a reference.
Intensity (a.u.)
(C)
(B)
(A) 10
2.2. Characterization of LLZNb 20
30
40
50
60
70
80
2θ(deg.) Fig. 1. XRD patterns of LLZNb calcined at (A) 550, (B) 600, and (C) 950 °C.
100
Exotherm
2.3. Lithium ion conductivity
60
40
Endotherm
Weight loss (%)
80
20
0 400
The crystalline structure was characterized using powder X-ray diffraction (XRD; Rigaku RINT-2200) with Cu Kα radiation (λ = 1.5418 Å) and operating conditions of 40 kV and 30 mA. Transmission electron microscopy (TEM; JEOL JEM-2100F) was employed to analyze the structure of LLZNb. Thermogravimetry–differential thermal analysis (TG–DTA; Mac Science TG–DTA2000) was used to determine the transition state of LLZNb with increasing temperature. The samples used for TG–DTA analysis were dried at 400 °C for 5 h after coprecipitation. Scanning electron microscopy (SEM; Hitachi S-3600N) was used to examine cross-section specimens of sintered pellets.
600
800
1000
Temperature ( )
The lithium ion conductivity of sintered pellets with lithium ionblocking gold electrodes was measured using an AC impedance analyzer (Agilent 4294A) in the frequency range of 40–110 MHz at 25 °C. The obtained Nyquist plots are composed of two half circular components at the higher and lower frequencies, which represent bulk resistance Rbulk and grain boundary resistance Rgb respectively [1,4]. Bulk and total lithium ion conductivities were calculated based on resistivity in Nyquist plots. Effective conductivity (σe), which reflects the effect of porosity of pellets, was calculated from pellet density and bulk conductivity according to Bruggeman equation [11].
Fig. 2. TG–DTA curves for LLZNb dried at 400 °C.
3. Results and discussion 3.1. Synthesis and characterization of LLZNb powders sized LLZNb particles are expected to accelerate the sintering, even at low temperatures, due to the lowering of the surface free energy by increasing the contact interfaces between the particles.
(A)
(B)
2µm
100nm
All precursor salts were dissolved in water and a precipitate was formed after the addition of the ammonia solution. Most of the precipitate
(C)
2µm
Fig. 3. TEM images of the (A) LLZNb, (B) primary particles of LLZNb, and (C) Ref powders calcined at 950 °C.
Please cite this article as: H. Imagawa, et al., Solid State Ionics (2013), http://dx.doi.org/10.1016/j.ssi.2013.10.059
H. Imagawa et al. / Solid State Ionics xxx (2013) xxx–xxx
consists of metal hydroxides immediately after synthesis, except for the Li compound. Fig. 1(A) shows an XRD pattern of the powder after calcination at 550 °C. Peaks derived from La2Zr2O7 were observed and these peaks were already present after calcination at 500 °C (data not shown). However, the garnet-type LLZNb also began to form at around 550 °C, as evident in Fig. 1(A). Cubic phase garnet-type LLZNb [12] was selectively obtained at 600 °C, as shown in Fig. 1(B). The formation temperature of cubic phase LLZNb was more than 300 °C lower than that synthesized by solid-state reaction [1,2]. Furthermore, selective formation of cubic LLZNb was possible at temperatures from 600 to 950 °C (Fig. 1(C)), although some LLZ synthesized by wet chemical methods formed the tetragonal phase after calcination at temperatures less than 1000 °C [4,5]. One of the reasons for maintaining cubic phase may be the lithium content in LLZNb, which has maintained lithium content as less than 7 for the LLZ composition [10] in Li6.75La3Zr1.75Nb0.25O12. Fig. 2 shows the TG–DTA curves measured for LLZNb dried at 400 °C. A large weight loss and endothermic reaction started from approximately 550 °C, which corresponds well with the XRD results for the formation of garnet-type LLZNb. Therefore, the estimated reaction mechanism for the formation of LLZNb by the coprecipitation method, according to the XRD and TG–DTA results is as follows. Firstly, metal hydroxides are coprecipitated with a Li complex. La2Zr2O7 and Li compounds such as Li2CO3 or Li2O form during heating, and then finally react together to form cubic phase LLZNb with the garnet-type structure. Fig. 3(A) and (B) shows a TEM image of LLZNb after calcination at 950 °C. The LLZNb sample is composed of fine 50–100 nm primary particles formed during coprecipitation, and these agglomerate to form secondary particles. The crystallite diameter estimated from the XRD pattern using Scherrer's equation was ca. 40 nm. In contrast, the Ref powder has large primary particles, as shown in Fig. 3(C), which resulted from the solid-state reaction of precursor metal salts and oxides. The formation of small-sized primary particles is effective for the low-temperature sintering of LLZNb due to the stabilization of the surface free energy of the particles by an increase of the contacting interfaces between particles.
3
Fig. 4. Cross-sectional SEM images of LLZNb pellets sintered at (A) 1100 and (B) 1000 °C.
The density of the LLZNb pellet sintered at 1180 °C was slightly less than that sintered at 1100 °C. Moreover, the peak intensities of XRD pattern for LLZNb sintered at 1180 °C are slightly different from the other samples, which may be due to a loss of lithium content [13]. Therefore, the best sintering condition for LLZNb seems to shift by almost 80 °C relative to that prepared by solid-state reaction. This tendency is due to the difference in the size of the primary and secondary particles, where smaller LLZNb particles enhance sintering due to the lowering of the surface free energy by increased contacting interfaces.
3.2. Sintering behavior of LLZNb The sintering behavior of LLZNb at 1000, 1100, and 1180 °C was investigated and the resulting densities are shown in Table 1. Sintering of LLZNb at 1000 °C resulted in 75% theoretical density, although the Ref sample did not sinter well at 1100 °C or less. Cross-sectional SEM images of LLZNb pellets sintered at 1000 and 1100 °C are shown in Fig. 4. LLZNb is almost fully-sintered at 1100 °C, and many grain boundaries are observed in LLZNb sintered at 1000 °C. Fig. 5 shows XRD patterns of the sintered LLZNb pellets. Garnet-type LLZNb with selective formation of the cubic phase was observed throughout the sintering temperatures and no transitional change to other phases was observed. The lattice parameter of LLZNb sintered at 1100 °C was 12.96 Å, which is almost the same as that for the Ref sample. Therefore, Nb5+ was successfully substituted at Zr4+ sites in LLZNb [2].
3.3. Lithium ion conductivity The lithium ion conductivities of the sintered LLZNb pellets are shown in Table 1. The total lithium ion conductivity of LLZNb sintered at 1100 °C was 5.2 × 10−4 S cm−1, which is almost identical to Ref
Table 1 Densitiesa and lithium ion conductivities of LLZNb and Ref sintered at various temperatures. LLZNb Sintering temperature (°C)
Density (g cm−3)
Ref Lithium ion conductivity (S cm−1)
a b
4.3 4.5 3.9
σeb
σtotal
σbulk 1180 1100 1000
Density (g cm−3)
−4
7.9 × 10 7.8 × 10−4 2.3 × 10−4
−4
5.2 × 10 5.2 × 10−4 1.0 × 10−4
Lithium ion conductivity (S cm−1) σbulk
−4
5.9 × 10 6.3 × 10−4 1.5 × 10−4
4.6 – –
σeb
σtotal −4
7.9 × 10 – –
Estimated theoretical density is 5.2 g cm−3. σe: Effective conductivity calculated from Bruggeman equation.
Please cite this article as: H. Imagawa, et al., Solid State Ionics (2013), http://dx.doi.org/10.1016/j.ssi.2013.10.059
−4
5.3 × 10 – –
6.6 × 10−4 – –
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H. Imagawa et al. / Solid State Ionics xxx (2013) xxx–xxx
-3000
:LLZNb
(C)
,, Z / cm2
Intensity (a.u.)
-2000
(B)
Rbulk -1000
0
(A) 10
20
30
40
50
60
70
Rgb
Rbulk
0
500
1000
1500
, Z / cm 2
2000
2500
80
2θ(deg.)
Fig. 6. Nyquist plots of LLZNb sintered at (○) 1180, (◊) 1100, and (×) 1000 °C.
Fig. 5. XRD patterns of LLZNb pellets sintered at (A) 1000, (B) 1100, and (C) 1180 °C.
4. Conclusion sintered at 1180 °C. Furthermore, LLZNb sintered at 1000 °C still shows the total lithium ion conductivity of 1.0 × 10−4 S cm−1, despite the relatively low density. The conductivity is almost 5 times lower than that of Ref sintered at 1180 °C, but the Ref sample, which was synthesized by solid-state reaction, did not sinter at 1000 °C at all. Thus, LLZNb with small-sized particles is effective to decrease the sintering temperature with maintaining lithium ion conduction. The contribution of resistance from the bulk and grain-boundary was determined from Nyquist plots, as shown in Fig. 6. The grainboundary contribution to the total resistance of LLZNb sintered at 1000 °C is clearly larger than that of LLZNb sintered at 1100 °C or more (Rgb/Rbulk + Rgb, 1000 °C: 30%, 1100 °C: 10%). As shown in Fig. 4, many grain boundaries are observed in LLZNb sintered at 1000 °C, which causes the large grain-boundary resistance with the influence on the total resistance. Sintered LLZNb pellets show the difference of pellets density derived from porosity as a geometrical effect depending on sintering temperature. In order to clarify the effect of grain-boundary resistance and porosity on total conductivity, effective conductivity of LLZNb based on Bruggeman equation was shown in Table 1. LLZNb pellets sintered at 1100 and 1180 °C show that the difference between the effective conductivity and the total conductivity is within 20%, whereas the difference of LLZNb pellet sintered at 1000 °C is almost 50%. Therefore, the large effect of grain-boundary resistance on the decrease of total conductivity occurs in the sample sintered at 1000 °C as well as the simple geometrical effect of porosity. While small-sized particles enable LLZNb sintering to obtain the cubic phase at relatively low temperatures, a further increase of the grain-boundary conductivity in addition to the bulk conductivity is necessary, in addition to the use of other sintering methods such as hot-pressing [7] or high-pressure sintering to realize decreased sintering temperatures.
LLZNb was synthesized by the coprecipitation method. Cubic phase LLZNb with the garnet-type structure was obtained after calcination at 600 °C or higher. The LLZNb precipitate powder has fine 50–100 nm primary particles as a result of the coprecipitation method. Sintering of LLZNb was possible at 1000 and 1100 °C under air due to the small-sized primary particles, although powder synthesized by solid-state reaction did not sinter under the same conditions. The total lithium ion conductivity of the LLZNb pellet sintered at 1000 °C was 1 × 10− 4 S cm− 1. The realization of cubic phase LLZNb sintered at low temperatures is promising for the fabrication of all-solid-state batteries by one-step heating without reaction of the electrode materials, although further decrease in sintering temperature is still required. References [1] R. Murugan, V. Thangadurai, W. Weppner, Angew. Chem. Int. Ed. 46 (2007) 7778. [2] S. Ohta, T. Kobayashi, T. Asaoka, J. Power Sources 196 (2011) 3342. [3] C.A. Geiger, E. Alekseev, B. Lazic, M. Fisch, T. Armbruster, R. Langner, M. Fechtelkord, N. Kim, T. Pettke, W. Weppner, Inorg. Chem. 50 (2011) 1089. [4] Y. Shimonishi, A. Toda, T. Zhang, A. Hirano, N. Imanishi, O. Yamamoto, Y. Takeda, Solid State Ionics 183 (2011) 48. [5] I. Kokal, M. Somer, P.H.L. Notten, H.T. Hintzen, Solid State Ionics 185 (2011) 42. [6] J. Awaka, N. Kijima, K. Kataoka, H. Hayakawa, J. Akimoto, J. Solid State Chem. 183 (2010) 180. [7] J. Wolfenstine, E. Rangasamy, J.L. Allen, J. Sakamoto, J. Power Sources 208 (2012) 193. [8] K.-H. Kim, Y. Iriyama, K. Yamamato, S. Kumazaki, T. Asaka, K. Tanabe, C.A.J. Fisher, T. Hirayama, R. Murugan, Z. Ogumi, J. Power Sources 196 (2011) 764. [9] S. Ohta, T. Kobayashi, J. Seki, T. Asaoka, J. Power Sources 202 (2012) 332. [10] A. Logéat, T. Köhler, U. Eisele, B. Stiaszny, A. Harzer, M. Tovar, A. Senyshyny, H. Ehrenberg, B. Kozinsky, Solid State Ionics 206 (2012) 33. [11] X. Weng, D. Brett, V. Yufit, P. Shearing, N. Brandon, M. Reece, H. Yan, C. Tighe, J.A. Darr, Solid State Ionics 181 (2010) 827. [12] H. Hyooma, K. Hayashi, Mater. Res. Bull. 23 (1988) 1399. [13] E. Rangasamy, J. Wolfenstine, J. Sakamoto, Solid State Ionics 206 (2012) 28.
Please cite this article as: H. Imagawa, et al., Solid State Ionics (2013), http://dx.doi.org/10.1016/j.ssi.2013.10.059
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Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
Garnet-type Li6.75La3Zr1.75Nb0.25O12 synthesized by coprecipitation method and its lithium ion conductivity Haruo Imagawa ⁎, Shingo Ohta, Yuki Kihira, Takahiko Asaoka TOYOTA Central Research and Development Labs., Inc., Nagakute, Aichi 480-1192, Japan
a r t i c l e
i n f o
Article history: Received 17 May 2013 Received in revised form 23 October 2013 Accepted 28 October 2013 Available online xxxx Keywords: Garnet-type oxides Coprecipitation Solid state electrolytes Lithium ion conductivity
a b s t r a c t Li6.75La3Zr1.75Nb0.25O12 (LLZNb) was synthesized using the coprecipitation method. X-ray powder diffraction patterns of LLZNb after calcination at 600 °C or higher indicate a cubic phase with a garnet-type structure. Transmission electron microscopy revealed primary LLZNb particles formed by coprecipitation with sizes in the range of 50–100 nm. Sintering of LLZNb was possible at relatively low temperatures of 1000 and 1100 °C under an air atmosphere due to the small-sized primary particles, although LLZNb powder synthesized by solid-state reaction did not sinter under the same conditions due to the large size of the primary particles. The total lithium ion conductivity of an LLZNb pellet sintered at 1000 °C was measured using AC impedance to be 1.0 × 10− 4 S cm−1. © 2013 Elsevier B.V. All rights reserved.
1. Introduction All-solid-state lithium ion batteries are attracting much attention, especially with respect to safety, due to their non-flammability, unlike the liquid organic electrolytes used in conventional lithium ion batteries. Garnet-type Li7La3Zr2O12 (LLZ) is one metal oxide with excellent lithium ion conductivity (N 10− 4 S cm− 1) as a solid electrolyte, which was first reported by Weppner et al. [1]. LLZ has been intensively investigated [2–7] due to its high lithium ion conductivity, wide electrochemical window, and excellent stability against lithium metal, which makes metallic lithium available as an anode material. Doping of other elements into the LLZ structure has included the partial substitution of zirconium with niobium, which was reported to be effective to increase the lithium ion conductivity [2]. Some fabrication trials of all-solid-batteries using LLZ have already been conducted, with particular focus on formation at the interface between LLZ and various cathode materials [8,9]. LLZ forms cubic [1–5] or tetragonal [4–7] phases, depending on the preparation conditions such as composition, synthesis method and calcination temperature. Cubic phase LLZ tends to exhibit excellent lithium
⁎ Corresponding author at: 41-1 Yokomichi, Nagakute, Aichi 480-1192, Japan. Tel.: +81 561 71 7226; fax: +81 561 63 5328. E-mail address: [email protected] (H. Imagawa).
ion conductivity compared with that for tetragonal LLZ, mainly due to the suitable occupation of lithium sites of 24d and 96 h in the cubicphase framework [10]. Therefore, the selective formation of cubic phase LLZ is favorable for an increase of the lithium ion conductivity. One drawback in the formation of bulk LLZ for use as a solid electrolyte is the high sintering temperature required, which is typically around 1200 °C, to obtain sufficient density for high lithium ion conductivity. Thus, there is a difficulty in achieving a one-step fabrication of allsolid-state batteries, because the calcination temperature of the various active electrode materials is far lower than the temperature required to sinter LLZ. Recently, the synthesis of LLZ based on wet chemical methods was reported [4,5]. The formation of garnet-type LLZ as a powder is possible between 700 and 800 °C using wet chemical methods. These reports are promising with respect to decreasing the sintering temperature of LLZ, although the lithium ion conductivity of the assintered LLZ is still low due to the formation of tetragonal phase during low temperature sintering. Here we report the synthesis of Li6.75La3Zr1.75Nb0.25O12 (LLZNb) using the coprecipitation method for the selective formation of the cubic phase with a garnet-type structure, and low temperature sintering while still obtaining high lithium ion conductivity. The concept for this synthesis is the formation of small-sized LLZNb particles by a wet chemical method and enhanced sintering of the LLZNb at lower temperatures (1000–1100 °C) than those used to sinter LLZNb synthesized by solid-state reaction. Cubic phase garnet-type LLZNb is expected to form at low temperature using this synthesis method with Nb doping. Furthermore, the small-
0167-2738/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ssi.2013.10.059
Please cite this article as: H. Imagawa, et al., Solid State Ionics (2013), http://dx.doi.org/10.1016/j.ssi.2013.10.059
2
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2. Experimental
:LLZNb :La2Zr2O7
2.1. Synthesis LLZNb was prepared by the coprecipitation method. All chemicals used in the synthesis were of regular grade purity and purchased from Wako Pure Chemical Industries. LiNO3, La(NO3)3, ZrO(NO3)2 and NbCl5 in HCl were dissolved in ion-exchanged water, and then ammonia solution was added for coprecipitation. The obtained precursor was dried at 400 °C for 5 h in air and calcined at 600–950 °C for 24 h. The amount of Li in the calcined powder was almost 15 wt.% excess relative to the original target LLZNb composition. The powder calcined at 950 °C was pressed and sintered within a bed of the LLZNb mother powder at 1000, 1100, and 1180 °C for 36 h. Li6.75La3Zr1.75Nb0.25O12 was also synthesized by solid-state reaction according to a literature method [4] (denoted Ref) and used as a reference.
Intensity (a.u.)
(C)
(B)
(A) 10
2.2. Characterization of LLZNb 20
30
40
50
60
70
80
2θ(deg.) Fig. 1. XRD patterns of LLZNb calcined at (A) 550, (B) 600, and (C) 950 °C.
100
Exotherm
2.3. Lithium ion conductivity
60
40
Endotherm
Weight loss (%)
80
20
0 400
The crystalline structure was characterized using powder X-ray diffraction (XRD; Rigaku RINT-2200) with Cu Kα radiation (λ = 1.5418 Å) and operating conditions of 40 kV and 30 mA. Transmission electron microscopy (TEM; JEOL JEM-2100F) was employed to analyze the structure of LLZNb. Thermogravimetry–differential thermal analysis (TG–DTA; Mac Science TG–DTA2000) was used to determine the transition state of LLZNb with increasing temperature. The samples used for TG–DTA analysis were dried at 400 °C for 5 h after coprecipitation. Scanning electron microscopy (SEM; Hitachi S-3600N) was used to examine cross-section specimens of sintered pellets.
600
800
1000
Temperature ( )
The lithium ion conductivity of sintered pellets with lithium ionblocking gold electrodes was measured using an AC impedance analyzer (Agilent 4294A) in the frequency range of 40–110 MHz at 25 °C. The obtained Nyquist plots are composed of two half circular components at the higher and lower frequencies, which represent bulk resistance Rbulk and grain boundary resistance Rgb respectively [1,4]. Bulk and total lithium ion conductivities were calculated based on resistivity in Nyquist plots. Effective conductivity (σe), which reflects the effect of porosity of pellets, was calculated from pellet density and bulk conductivity according to Bruggeman equation [11].
Fig. 2. TG–DTA curves for LLZNb dried at 400 °C.
3. Results and discussion 3.1. Synthesis and characterization of LLZNb powders sized LLZNb particles are expected to accelerate the sintering, even at low temperatures, due to the lowering of the surface free energy by increasing the contact interfaces between the particles.
(A)
(B)
2µm
100nm
All precursor salts were dissolved in water and a precipitate was formed after the addition of the ammonia solution. Most of the precipitate
(C)
2µm
Fig. 3. TEM images of the (A) LLZNb, (B) primary particles of LLZNb, and (C) Ref powders calcined at 950 °C.
Please cite this article as: H. Imagawa, et al., Solid State Ionics (2013), http://dx.doi.org/10.1016/j.ssi.2013.10.059
H. Imagawa et al. / Solid State Ionics xxx (2013) xxx–xxx
consists of metal hydroxides immediately after synthesis, except for the Li compound. Fig. 1(A) shows an XRD pattern of the powder after calcination at 550 °C. Peaks derived from La2Zr2O7 were observed and these peaks were already present after calcination at 500 °C (data not shown). However, the garnet-type LLZNb also began to form at around 550 °C, as evident in Fig. 1(A). Cubic phase garnet-type LLZNb [12] was selectively obtained at 600 °C, as shown in Fig. 1(B). The formation temperature of cubic phase LLZNb was more than 300 °C lower than that synthesized by solid-state reaction [1,2]. Furthermore, selective formation of cubic LLZNb was possible at temperatures from 600 to 950 °C (Fig. 1(C)), although some LLZ synthesized by wet chemical methods formed the tetragonal phase after calcination at temperatures less than 1000 °C [4,5]. One of the reasons for maintaining cubic phase may be the lithium content in LLZNb, which has maintained lithium content as less than 7 for the LLZ composition [10] in Li6.75La3Zr1.75Nb0.25O12. Fig. 2 shows the TG–DTA curves measured for LLZNb dried at 400 °C. A large weight loss and endothermic reaction started from approximately 550 °C, which corresponds well with the XRD results for the formation of garnet-type LLZNb. Therefore, the estimated reaction mechanism for the formation of LLZNb by the coprecipitation method, according to the XRD and TG–DTA results is as follows. Firstly, metal hydroxides are coprecipitated with a Li complex. La2Zr2O7 and Li compounds such as Li2CO3 or Li2O form during heating, and then finally react together to form cubic phase LLZNb with the garnet-type structure. Fig. 3(A) and (B) shows a TEM image of LLZNb after calcination at 950 °C. The LLZNb sample is composed of fine 50–100 nm primary particles formed during coprecipitation, and these agglomerate to form secondary particles. The crystallite diameter estimated from the XRD pattern using Scherrer's equation was ca. 40 nm. In contrast, the Ref powder has large primary particles, as shown in Fig. 3(C), which resulted from the solid-state reaction of precursor metal salts and oxides. The formation of small-sized primary particles is effective for the low-temperature sintering of LLZNb due to the stabilization of the surface free energy of the particles by an increase of the contacting interfaces between particles.
3
Fig. 4. Cross-sectional SEM images of LLZNb pellets sintered at (A) 1100 and (B) 1000 °C.
The density of the LLZNb pellet sintered at 1180 °C was slightly less than that sintered at 1100 °C. Moreover, the peak intensities of XRD pattern for LLZNb sintered at 1180 °C are slightly different from the other samples, which may be due to a loss of lithium content [13]. Therefore, the best sintering condition for LLZNb seems to shift by almost 80 °C relative to that prepared by solid-state reaction. This tendency is due to the difference in the size of the primary and secondary particles, where smaller LLZNb particles enhance sintering due to the lowering of the surface free energy by increased contacting interfaces.
3.2. Sintering behavior of LLZNb The sintering behavior of LLZNb at 1000, 1100, and 1180 °C was investigated and the resulting densities are shown in Table 1. Sintering of LLZNb at 1000 °C resulted in 75% theoretical density, although the Ref sample did not sinter well at 1100 °C or less. Cross-sectional SEM images of LLZNb pellets sintered at 1000 and 1100 °C are shown in Fig. 4. LLZNb is almost fully-sintered at 1100 °C, and many grain boundaries are observed in LLZNb sintered at 1000 °C. Fig. 5 shows XRD patterns of the sintered LLZNb pellets. Garnet-type LLZNb with selective formation of the cubic phase was observed throughout the sintering temperatures and no transitional change to other phases was observed. The lattice parameter of LLZNb sintered at 1100 °C was 12.96 Å, which is almost the same as that for the Ref sample. Therefore, Nb5+ was successfully substituted at Zr4+ sites in LLZNb [2].
3.3. Lithium ion conductivity The lithium ion conductivities of the sintered LLZNb pellets are shown in Table 1. The total lithium ion conductivity of LLZNb sintered at 1100 °C was 5.2 × 10−4 S cm−1, which is almost identical to Ref
Table 1 Densitiesa and lithium ion conductivities of LLZNb and Ref sintered at various temperatures. LLZNb Sintering temperature (°C)
Density (g cm−3)
Ref Lithium ion conductivity (S cm−1)
a b
4.3 4.5 3.9
σeb
σtotal
σbulk 1180 1100 1000
Density (g cm−3)
−4
7.9 × 10 7.8 × 10−4 2.3 × 10−4
−4
5.2 × 10 5.2 × 10−4 1.0 × 10−4
Lithium ion conductivity (S cm−1) σbulk
−4
5.9 × 10 6.3 × 10−4 1.5 × 10−4
4.6 – –
σeb
σtotal −4
7.9 × 10 – –
Estimated theoretical density is 5.2 g cm−3. σe: Effective conductivity calculated from Bruggeman equation.
Please cite this article as: H. Imagawa, et al., Solid State Ionics (2013), http://dx.doi.org/10.1016/j.ssi.2013.10.059
−4
5.3 × 10 – –
6.6 × 10−4 – –
4
H. Imagawa et al. / Solid State Ionics xxx (2013) xxx–xxx
-3000
:LLZNb
(C)
,, Z / cm2
Intensity (a.u.)
-2000
(B)
Rbulk -1000
0
(A) 10
20
30
40
50
60
70
Rgb
Rbulk
0
500
1000
1500
, Z / cm 2
2000
2500
80
2θ(deg.)
Fig. 6. Nyquist plots of LLZNb sintered at (○) 1180, (◊) 1100, and (×) 1000 °C.
Fig. 5. XRD patterns of LLZNb pellets sintered at (A) 1000, (B) 1100, and (C) 1180 °C.
4. Conclusion sintered at 1180 °C. Furthermore, LLZNb sintered at 1000 °C still shows the total lithium ion conductivity of 1.0 × 10−4 S cm−1, despite the relatively low density. The conductivity is almost 5 times lower than that of Ref sintered at 1180 °C, but the Ref sample, which was synthesized by solid-state reaction, did not sinter at 1000 °C at all. Thus, LLZNb with small-sized particles is effective to decrease the sintering temperature with maintaining lithium ion conduction. The contribution of resistance from the bulk and grain-boundary was determined from Nyquist plots, as shown in Fig. 6. The grainboundary contribution to the total resistance of LLZNb sintered at 1000 °C is clearly larger than that of LLZNb sintered at 1100 °C or more (Rgb/Rbulk + Rgb, 1000 °C: 30%, 1100 °C: 10%). As shown in Fig. 4, many grain boundaries are observed in LLZNb sintered at 1000 °C, which causes the large grain-boundary resistance with the influence on the total resistance. Sintered LLZNb pellets show the difference of pellets density derived from porosity as a geometrical effect depending on sintering temperature. In order to clarify the effect of grain-boundary resistance and porosity on total conductivity, effective conductivity of LLZNb based on Bruggeman equation was shown in Table 1. LLZNb pellets sintered at 1100 and 1180 °C show that the difference between the effective conductivity and the total conductivity is within 20%, whereas the difference of LLZNb pellet sintered at 1000 °C is almost 50%. Therefore, the large effect of grain-boundary resistance on the decrease of total conductivity occurs in the sample sintered at 1000 °C as well as the simple geometrical effect of porosity. While small-sized particles enable LLZNb sintering to obtain the cubic phase at relatively low temperatures, a further increase of the grain-boundary conductivity in addition to the bulk conductivity is necessary, in addition to the use of other sintering methods such as hot-pressing [7] or high-pressure sintering to realize decreased sintering temperatures.
LLZNb was synthesized by the coprecipitation method. Cubic phase LLZNb with the garnet-type structure was obtained after calcination at 600 °C or higher. The LLZNb precipitate powder has fine 50–100 nm primary particles as a result of the coprecipitation method. Sintering of LLZNb was possible at 1000 and 1100 °C under air due to the small-sized primary particles, although powder synthesized by solid-state reaction did not sinter under the same conditions. The total lithium ion conductivity of the LLZNb pellet sintered at 1000 °C was 1 × 10− 4 S cm− 1. The realization of cubic phase LLZNb sintered at low temperatures is promising for the fabrication of all-solid-state batteries by one-step heating without reaction of the electrode materials, although further decrease in sintering temperature is still required. References [1] R. Murugan, V. Thangadurai, W. Weppner, Angew. Chem. Int. Ed. 46 (2007) 7778. [2] S. Ohta, T. Kobayashi, T. Asaoka, J. Power Sources 196 (2011) 3342. [3] C.A. Geiger, E. Alekseev, B. Lazic, M. Fisch, T. Armbruster, R. Langner, M. Fechtelkord, N. Kim, T. Pettke, W. Weppner, Inorg. Chem. 50 (2011) 1089. [4] Y. Shimonishi, A. Toda, T. Zhang, A. Hirano, N. Imanishi, O. Yamamoto, Y. Takeda, Solid State Ionics 183 (2011) 48. [5] I. Kokal, M. Somer, P.H.L. Notten, H.T. Hintzen, Solid State Ionics 185 (2011) 42. [6] J. Awaka, N. Kijima, K. Kataoka, H. Hayakawa, J. Akimoto, J. Solid State Chem. 183 (2010) 180. [7] J. Wolfenstine, E. Rangasamy, J.L. Allen, J. Sakamoto, J. Power Sources 208 (2012) 193. [8] K.-H. Kim, Y. Iriyama, K. Yamamato, S. Kumazaki, T. Asaka, K. Tanabe, C.A.J. Fisher, T. Hirayama, R. Murugan, Z. Ogumi, J. Power Sources 196 (2011) 764. [9] S. Ohta, T. Kobayashi, J. Seki, T. Asaoka, J. Power Sources 202 (2012) 332. [10] A. Logéat, T. Köhler, U. Eisele, B. Stiaszny, A. Harzer, M. Tovar, A. Senyshyny, H. Ehrenberg, B. Kozinsky, Solid State Ionics 206 (2012) 33. [11] X. Weng, D. Brett, V. Yufit, P. Shearing, N. Brandon, M. Reece, H. Yan, C. Tighe, J.A. Darr, Solid State Ionics 181 (2010) 827. [12] H. Hyooma, K. Hayashi, Mater. Res. Bull. 23 (1988) 1399. [13] E. Rangasamy, J. Wolfenstine, J. Sakamoto, Solid State Ionics 206 (2012) 28.
Please cite this article as: H. Imagawa, et al., Solid State Ionics (2013), http://dx.doi.org/10.1016/j.ssi.2013.10.059