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Microstructure and ionic conductivity of Li1.5Al0.5Ge1.5(PO4)3 solid electrolyte prepared by spark plasma sintering
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Microstructure and ionic conductivity of Li1.5Al0.5Ge1.5(PO4)3 solid electrolyte prepared by spark plasma sintering Huan Tonga, Jingru Liua, Jian Liub, Yulong Liuc, Dawei Wangc, Xueliang Sunc, Xiping Songa,∗ a
State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing, 100083, PR China School of Engineering, Faculty of Applied Science, University of British Columbia, Kelowna, BC, V1V 1V7, Canada c Department of Mechanical and Materials Engineering, Western University, London, Ontario, N6A 5B9, Canada b
A R T I C LE I N FO
A B S T R A C T
Keywords: Li1.5Al0.5Ge1.5(PO4)3 Ionic conductivity Spark plasma sintering Carbon contamination
In this paper, the microstructure and ionic conductivity of Li1.5Al0.5Ge1.5(PO4)3 (LAGP) solid electrolytes prepared by spark plasma sintering (SPS) were investigated by XRD, SEM, TEM and EIS, respectively. The results showed that as the sintering temperature was increased, both the relative density and the ionic conductivity of the sintered LAGP samples first increased and then decreased, achieving a maximum value of 97% and 2.12 × 10−4 S cm−1 simultaneously at 700 °C. At the same time, the crystallinity of the sintered samples was improved, while a few impurity phases, such as AlPO4 and GeO2, appeared in the samples. It was also found that carbon contamination and oxycarbide gas was be brought in during SPS. Carbon contamination could produce an extra grain boundary impedance to the samples and could be removed by annealing at 500 °C in an air atmosphere. Oxycarbide gas could affect the relative density of the sintered LAGP samples and could be mitigated by choosing a suitable SPS process. Moreover, the shear modulus of the sintered LAGP was measured to be 49.6 GPa, which exceeded the minimum value of 8.5 GPa that was necessary to suppress Li dendrite growth.
1. Introduction Lithium-ion batteries (LIBs) are important energy storage devices for electric vehicles and smart grids due to their high energy density, high powder density and long cycle life. However, at present LIBs have serious safety issues due to the use of flammable organic liquid electrolytes [1]. Therefore, all solid-state lithium batteries (ASSLBs) using solid electrolytes to replace organic liquid electrolytes have been proposed and received increasing attention in recent years [2]. As a key part of ASSLBs, solid electrolytes have several advantages, such as nonflammability, nonvolatility, and high mechanical strength [3]. Nonetheless, compared with organic electrolytes, solid electrolytes still have some problems, such as low conductivity and large interface impedance which need to be addressed [4,5]. Among all the solid electrolytes, the Li1.5Al0.5Ge1.5 (PO4)3 (LAGP) is regarded as one of the most promising candidates due to its high ionic conductivity [6] and good chemical stability against metallic lithium [7]. In the conventional sintering of LAGP solid electrolytes, it has been found that the relative density of sintered samples is low (70–80%) [8], due to the pressureless sintering environment. SPS is considered to be an efficient way to deal with this problem. This method can densify samples to near theoretical density and suppress grain coarsening, both
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of which are beneficial to the ionic conductivity of samples [9,10]. In previous studies, Li1.3Al0.3Ti1.7(PO4)3 [11] and Li7La3Zr2O12 [12] solid electrolytes have been prepared by SPS, and achieved relative high density and ion conductivity. As for LAGP, Bouchet's group obtained LAGP solid electrolytes with a relative density of 93% and ionic conductivity of over 10−4 S cm−1 by SPS [13,14]. However, their studies did not consider the existence of carbon contamination which is inevitable during SPS. In this paper, the effect of carbon contamination on the microstructure and ionic conductivity of the samples is investigated. Furthermore, the shear modulus of LAGP prepared by SPS is also determined in order to evaluate its ability to inhibit Li dendrite. 2. Experiments 2.1. Sample preparation The LAGP powder used in this study was purchased from MTI CO., LTD (Hefei, China). The particle size of the LAGP was about 300–500 nm. Pellet samples with a diameter of 12.5 mm and a thickness of 2 mm were sintered by the spark plasma sintering system (Dr. sinter SPS-1050) using the LAGP powder. Two different sintering processes were adopted to prepare these samples. For sintering process Ⅰ
Corresponding author. E-mail address: [email protected] (X. Song).
https://doi.org/10.1016/j.ceramint.2019.11.264 Received 17 September 2019; Received in revised form 18 November 2019; Accepted 28 November 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Huan Tong, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.11.264
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(SP Ⅰ), the samples were heated to the target sintering temperature with a rate of pressure increase. The pressure and temperature simultaneously achieved their target values. For sintering process Ⅱ (SP Ⅱ), the samples were first heated to the target sintering temperature, and then the pressure was loaded to the target value within 3 min. In both sintering processes I and II, the target sintering temperatures were 600 °C, 650 °C, 700 °C, 750 °C, 800 °C and 850 °C, respectively, the target pressure was 60 MPa, the heating rate was 100 °C min−1 and the sintering time was 2 min. In order to remove carbon contamination, the sintered samples were annealed at 500 °C for 8 h in air. Their relative density was calculated based on the ratio of measure density to theoretical density (3.41 g cm−3). The measure density was determined based on the weight and volume of the LAGP pellets, while the theoretical density was calculated by the LAGP cell parameters and molecular weight [15]. 2.2. Phase and microstructure characterizations
Fig. 1. XRD patterns of the LAGP powders and the sintered LAGP pellets.
The phases of both the LAGP powder and the sintered LAGP samples were examined by X-ray diffraction (XRD, TTR Ⅲ), using Cu Kα1 radiation in the 2θ range from 10° to 80° with a step of 0.02°. The morphology and microstructure of the LAGP samples were examined by field emission scanning electron microscopy (SEM, Zeiss Auriga) and transmission electron microscopy (TEM, FEI F30). The elemental distribution of the samples was investigated by energy-dispersive X-ray spectroscopy (EDS). To prepare the TEM samples, the sintering samples were first ground into powders and then the powders were dispersed into acetone. The TEM samples were prepared by dropping the acetone onto a carbon films holder.
indicates that the orientation of the LAGP grains has been rearranged during the SPS. The diffraction peaks of the LAGP sintered samples between sintering processes Ⅰ and Ⅱ are almost the same, suggesting that the sintering processes have no influence on the phase composition. Fig. 2(a) shows the XRD patterns of the SP Ⅰ-650 sample before and after annealing. The diffraction peaks remain unchanged after annealing, implying that the annealing process has no influence on the phase composition of the samples. But the colors are different for the sintered sample SP Ⅰ-650 before and after annealing, as shown in Fig. 2(b). The sintered sample prepared by SPS at 650 °C is black, which is mainly caused by carbon contamination during the SPS. After annealing in air, the carbon is oxidized and the sample becomes white, similar to the initial color of the LAGP powders. By checking the color of samples, it is determined that the carbon contamination is removed after annealing process. Fig. s 2 (b) and (c) show the microstructure of the SP Ⅰ-650 sample before and after annealing. The average grain size of the SP Ⅰ-650 sample is about 500 nm and remains unchanged after annealing. Hence, the annealing process can remove carbon contamination, without affecting the phase composition and microstructure of the LAGP pellets. Fig. s 3(a), (c) and (e) show SEM images of the LAGP samples prepared by sintering process Ⅰ at 600 °C, 700 °C and 800 °C, respectively. It can be seen that the pores of the samples significantly increase as the temperature increases. In the higher magnification images in Fig. s 3(b), (d) and (f), the LAGP grains appear as rectangles and gradually get larger as the temperature increases, indicating increased crystallinity. In addition to the rectangular grains, spherical particles can also be observed in the sintered samples. As EDS mapping results show in Fig. 3(g), O and P elements are homogeneously distributed in the spherical particle and rectangular grains, while Al element is heavily segregated in the spherical particle region, and Ge disappears in this region. Based on the results of XRD, it can be inferred that the spherical particle is attributable to the impurity of AlPO4. It is concluded that LAGP solid electrolyte can be sintered into pellets by SPS, but the samples sintered by sintering process Ⅰ are porous. Fig. 4 shows the XRD patterns of the LAGP samples sintered by SPS from 600 °C to 850 °C by sintering process Ⅱ. The peak intensity of impurity phases AlPO4 and GeO2 increases with the temperature, as shown in the inset, which suggests the amount of impurities is increasing. However, this impurity peak suddenly decreases and the peak of Al2O3 appears at 850 °C, which may be related to the decomposition of AlPO4 at 850 °C during the SPS. Fig. 5 displays SEM images of LAGP samples sintered at different temperatures by sintering process Ⅱ. As can be seen in Fig. s 5(a) and (b), the LAGP grains are irregular after sintering at 600 °C and 650 °C, demonstrating a low crystallinity in the samples. As the sintering
2.3. Electrochemical characterizations The AC impedance spectroscopy was measured by electrochemical workstation (CHI660E) in a frequency range of 10−2-106 Hz with an amplitude of 5 mV AC at ambient temperature. Before the measurement, both sides of the LAGP pellets were painted with silver paste as blocking electrodes and were dried at 200 °C for 2h. The AC impedance spectroscopy was fitted with Z-view software, and the ionic conductivity (σ) was calculated according to Eq. (1)
σ=
d (Rb + Rgb)⋅S
(1)
where Rb was the bulk impedance of the samples, Rgb was the grain boundary impedance of the samples, d was the thickness of the samples and S was the cross-section area of the samples. 2.4. Elastic modulus measurement The elastic modulus was measured using nanoindentation (MTS, Nanolndenter XP) based on the continuous stiffness method (CSM). Test points were randomly selected with an indentation depth of 500 nm. The elastic modulus E and shear modulus G were calculated based on load−displacement stiffness data. 3. Results and analysis Fig. 1 shows the XRD patterns of the LAGP powders and sintered LAGP pellets prepared by sintering processes Ⅰ and Ⅱ. The diffraction peaks of the LAGP powders and sintered LAGP pellets are indexed to the LiGe2(PO4)3 phase with NASICON structure (PDF# 80–1924). Some diffraction peaks of impurity phases, AlPO4 and GeO2, are also observed in the sintered samples, and this may be the result of the volatilization of lithium. In addition, the diffraction peak located at about 15° in the LAGP powders disappears in the sintered samples but can be observed again when the sintered samples are ground into powders. This 2
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Fig. 2. (a) XRD patterns of the sintered sample SP Ⅰ-650 before and after annealing; (b) optical images of the sintered sample SP Ⅰ-650 before and after annealing; SEM images of the sintered sample SP Ⅰ-650 before(c) and after (d) annealing.
tail related to blocking electrodes in the low frequency region. (For the ceramic solid electrolytes, the impedance semicircle consists of grain impedance semicircle and grain boundary impedance semicircle. As reported, the grain impedance (bulk impedance) semicircle can be observed at an ultra-high frequency of 10 MHz, while the grain boundary impedance semicircle can be observed at the high frequency of 1 MHz [16]. In this study, the measuring frequency is below 1 MHz, thus the measured impedance semicircle is related to the grain boundary impedance, not the grain impedance. Comparing the impedance plots of sample SP Ⅰ-650 before and after annealing, it is found that the annealing process significantly reduces the impedance of the samples. Comparing the impedance plots of sample SP Ⅱ-700 before and after annealing, it is also found that the annealing process significantly reduces the impedance of the samples. Comparing the impedance plots of SP Ⅰ-650 and SP Ⅱ-650 after annealing, the impedance of SP Ⅱ-650 is lower than that of SP Ⅰ-650, which may be a result of their different relative densities. Fig. 7(c) shows the complex impedance plots of the LAGP samples prepared at different temperatures by sintering process Ⅱ and subsequently annealed. In the complex impedance plots, the intercept of the impedance semicircle in the high frequency region represents the bulk impedance of the samples, and the diameter of the impedance semicircle represents the grain boundary impedance [17]. As can be seen, each sample has a similar intercept value and very different impedance semicircles. This means that the bulk impedance of LAGP is relatively insensitive to the sintering temperature, but the grain boundary impedance is significantly affected by the sintering temperature. Fig. 8 shows the elastic modulus of the SP Ⅱ-700 LAGP sample as a function of nanoindentation displacement measured by nanoindentation. The most accurate estimate of E was obtained from an indentation depth range of approximately 140−300 nm with a value of 124 GPa. Data from depths shallower than about 140 nm is more susceptible to errors from thermal drift, and contaminants on the surface of the specimen. Data from depths deeper than ~300 nm is influenced by the averaging grain volume. The Poisson's ratio, υ, refers to 0.25, which is Poisson's ratio of Li1.3Al0.3Ti1.7(PO4)3 [18]. The shear modulus, G, is 49.6 GPa calculated by Eq. (2)
temperature increases to 700 °C, the LAGP grains become more regular and turn into rectangles. Fig. 5(c) shows the densest microstructure in SP Ⅱ-700, and it is difficult to distinguish the grain boundaries from each other. With the further increase of temperature, rectangular grains grow up gradually and grain boundaries become apparent. The insets in Fig. 5 are optical images of the corresponding samples. As can be seen, the color of the sintered samples is gray at 600 °C and 650 °C, and becomes black at 700 °C and 750 °C, then turns to white at 800 °C and 850 °C. It can be speculated that the carbon contamination is sensitive to sintering temperatures. Fig. s 6(a)–(c) are TEM images of the LAGP powders, sintered sample SP Ⅱ-700 and SP Ⅱ-850, respectively. Before sintering, the LAGP are irregular particles. After sintering, LAGP grains become regular and partially turn to rectangles. Irregular particles and rectangular particles can both be observed in the sintered samples. From SAED and EDS analysis of these particles, it can be seen that both the irregular particles and the rectangular particles are LAGP particles. Fig. 6(e) is the result of elements mapping, each element distributes homogeneously in both particles, while a black spot in the HADDF image can also be observed, which may be an adhered AlPO4 impurity particle. It can conclude that the increase of temperature promotes the crystallinity of the LAGP sintered samples while each element is uniformly distributed in the LAGP grains. Fig. 7(a) shows the relative density of LAGP samples sintered at different temperatures with two sintering processes and the ionic conductivity of the LAGP samples sintered with sintering process Ⅱ. The relative density of the samples prepared by both sintering processes Ⅰ and Ⅱ first increases and then decreases. The maximum relative density reaches 80% at 650 °C in sintering process Ⅰ and 97% at 700 °C in sintering process Ⅱ. Meanwhile, the conductivity of the samples prepared by sintering process Ⅱ shows a similar tendency in respect of relative density, and the maximum ionic conductivity is 2.12 × 10−4 S cm−1 at the sintering temperature of 700 °C. Fig. 7(b) shows effects of the annealing process and different sintering processes on the impedance of the samples. As can be seen, each complex impedance plot shows a single depressed semicircle related to the impedance of grain boundaries in the high frequency region and a 3
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Fig. 3. SEM images of the LAGP samples: (a, b) SP Ⅰ-600; (c, d) SP Ⅰ-700; (e, f) SP Ⅰ-800, (g) EDS element mapping of different grains for the SP Ⅰ-800.
G=
E 2(1 + υ )
prepared by spark plasma sintering technology, the maximum of ion conductivity is 2.12 × 10−4 S cm−1 which is significantly higher than that of the general LAGP in the literature [6]. In addition, the effects of different sintering processes and carbon contamination on the microstructure and ionic conductivity of sintered samples have been analyzed. The results show that the relative density of samples prepared by both sintering processes Ⅰ and Ⅱ first increases and then decreases as the temperature increases. It is contended that the increase of relative density is attributable to high temperature sintering, and that the decrease of relative density is related to the gas generated during sintering. Due to the use of graphite dies, carbon atoms react with oxygen atoms which exist inside samples and form oxycarbide gas during
(2)
Linear elasticity analyses performed by Monroe and Newman suggest that the shear modulus of solid electrolytes needs to exceed a value that is at least twice the Li metal shear modulus (~8.5 GPa) to prevent dendrite nucleation [19]. The G value of LAGP in this work is 49.6 GPa, which far exceeds the minimum value calculated by Monroe and Newman. Hence, the LAGP solid electrolyte is capable of inhibiting Li dendrite nucleation and penetration. 4. Discussions In this study, the LAGP solid electrolyte has been successfully 4
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significant difference between sintering processes Ⅰ and Ⅱ. The relative density is only 80% in sintering process Ⅰ, while it is up to 97% in sintering process Ⅱ. This difference may be related to the different pressurization processes. In sintering process Ⅰ, samples are heated up and pressurized simultaneously. Due to the effect of pressure, the generated gas finds it difficult to escape from the samples and finally remains in them as pores, which hinders their densification. In sintering process Ⅱ, the samples are first heated up and then pressurized. The generated gas can easily escape from the samples, which can be sufficiently densified during the pressurization process. It has been reported that carbon contamination can darken samples [23,24]. In this study, it is found that samples present different colors at different sintering temperatures, which may be a result of the degree of carbon contamination. As shown in Fig. 5, at 600 °C and 650 °C, the degree of carbon contamination is low and the samples are gray. When the temperature rises to 700 °C and 750 °C, the degree of carbon contamination increases and the samples become black. As the temperature increases to 800 °C and 850 °C, the carbon contamination can be oxidized and eliminated during the sintering process. As a result, the samples turn white again. In addition to affecting the color, the carbon can also affect the grain boundary impedance. As shown in Fig. 7(b), the impedance semicircles of SP Ⅰ-650 and SP Ⅱ-700 can be significantly reduced after annealing. It has also been found that the annealing process can remove carbon contamination significantly without any effects on the composition of
Fig. 4. XRD patterns of LAGP pellets sintered by SPS at different temperatures.
sintering [20,21]. The lithium element of samples can also volatilize as gas at high temperatures [22]. These gases produce a number of pores in the samples and result in the decrease of their relative density. As can be seen in Fig. 7(a), the maximum relative density shows a
Fig. 5. SEM images of the LAGP samples sintered at different temperatures by SPS: (a) SP Ⅱ-600; (b) SP Ⅱ-650; (c) SP Ⅱ-700; (d) SP Ⅱ-750; (e) SP Ⅱ-800; (f) SP Ⅱ-850, the insets are optical images of the corresponding samples. 5
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Fig. 6. TEM images of the LAGP samples: (a) LAGP initial powder; (b) SP Ⅱ-700; (c) SP Ⅱ-850. (d) a higher magnification TEM image, SAED and EDS of SP Ⅱ-700; (e) HADDF image and elements mapping of (d).
has been reported that carbon contamination can affect the electrochemical properties of garnet-type solid electrolyte by decomposing the electrolytes [25]. However, in this study, it is found that the carbon contamination has a direct impact on the grain boundary impedance of sintered LAGP samples as a result of the presence of local electronic conduction. As shown in Fig. 7(a), the conductivity of the samples is highly dependent on their relative density. However, after 700 °C, the relative density decreases slowly while the ionic conductivity decreases
phases and microstructures. Therefore, these results indicate that the annealing process can remove the carbon contamination in both sintering process Ⅰ and Ⅱ, and it can be deduced that the decrease of impedance is due to the elimination of carbon contamination. In other words, the carbon existing in the samples creates an extra grain boundary impedance. The reason for this is that the carbon can produce local electronic conduction at the grain boundary. With the existence of local electronic conduction, the migration of ions can be blocked, causing an increase of grain boundary impedance. In previous studies, it 6
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Fig. 8. Elastic modulus of sample SP Ⅱ-700 as a function of nanoindentation displacement.
In summary, LAGP solid electrolytes can be quickly densified by SPS to a high relative density, but carbon contamination is inevitable during sintering. Carbon plays an important role because it generates oxycarbide gas which affects the relative density and microstructure of the sintered samples. Furthermore, it also creates an extra grain boundary impedance in the LAGP samples. Therefore, it is also important to consider the effect of carbon contamination. 5. Conclusions By means of SPS, LAGP solid electrolytes have been synthesized. The effects of carbon contamination on their microstructure and ionic conductivity have been explored. In addition, the elastic properties of LAGP have also been characterized. The conclusions are as follows: (1) Both the relative density and ionic conductivity of LAGP samples prepared by SPS first increase and then decrease as the temperature increases, achieving a maximum value of 97% and 2.12 × 10−4 S cm−1 simultaneously at 700 °C; (2) During sintering, the carbon from graphite dies reacts with oxygen in the samples to generate oxycarbide gas which results in the formation of pores and in the decrease of the relative density of the samples. This unfavorable effect can be mitigated by optimizing the pressurization method of the sintering process. (3) Carbon contamination is sensitive to the sintering temperature, which causes the samples to change color. Carbon contamination can also create an extra grain boundary impedance. This unfavorable effect can be removed by means of annealing at 500 °C in air atmosphere. (4) The shear modulus of the LAGP solid electrolyte is measured to be 49.6 GPa, which means the LAGP prepared by SPS has the ability to inhibit Li dendrite nucleation and penetration. Declaration of competing interestCOI The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Fig. 7. (a) Relative density and ionic conductivity of LAGP samples sintered at different temperatures; (b) complex impedance plots of LAGP samples treated with different processes; (c) complex impedance plots of LAGP samples sintered at different temperatures.
Acknowledgements
significantly. This indicates that the conductivity not only depends on the relative density of the samples, but also relates to other factors, such as impurities. As shown in Fig. 4, the impurities of AlPO4 and GeO2 increase with the temperature, which may damage the ionic conductivity of samples.
This work was supported by State Key Lab of Advanced Metals and Materials [No.2019-ZD06], National Natural Science Foundation of China [grant No.51271021 and 21171018], and the Beijing Natural Science Foundation [grant No. 2162025]. 7
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Microstructure and ionic conductivity of Li1.5Al0.5Ge1.5(PO4)3 solid electrolyte prepared by spark plasma sintering Huan Tonga, Jingru Liua, Jian Liub, Yulong Liuc, Dawei Wangc, Xueliang Sunc, Xiping Songa,∗ a
State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing, 100083, PR China School of Engineering, Faculty of Applied Science, University of British Columbia, Kelowna, BC, V1V 1V7, Canada c Department of Mechanical and Materials Engineering, Western University, London, Ontario, N6A 5B9, Canada b
A R T I C LE I N FO
A B S T R A C T
Keywords: Li1.5Al0.5Ge1.5(PO4)3 Ionic conductivity Spark plasma sintering Carbon contamination
In this paper, the microstructure and ionic conductivity of Li1.5Al0.5Ge1.5(PO4)3 (LAGP) solid electrolytes prepared by spark plasma sintering (SPS) were investigated by XRD, SEM, TEM and EIS, respectively. The results showed that as the sintering temperature was increased, both the relative density and the ionic conductivity of the sintered LAGP samples first increased and then decreased, achieving a maximum value of 97% and 2.12 × 10−4 S cm−1 simultaneously at 700 °C. At the same time, the crystallinity of the sintered samples was improved, while a few impurity phases, such as AlPO4 and GeO2, appeared in the samples. It was also found that carbon contamination and oxycarbide gas was be brought in during SPS. Carbon contamination could produce an extra grain boundary impedance to the samples and could be removed by annealing at 500 °C in an air atmosphere. Oxycarbide gas could affect the relative density of the sintered LAGP samples and could be mitigated by choosing a suitable SPS process. Moreover, the shear modulus of the sintered LAGP was measured to be 49.6 GPa, which exceeded the minimum value of 8.5 GPa that was necessary to suppress Li dendrite growth.
1. Introduction Lithium-ion batteries (LIBs) are important energy storage devices for electric vehicles and smart grids due to their high energy density, high powder density and long cycle life. However, at present LIBs have serious safety issues due to the use of flammable organic liquid electrolytes [1]. Therefore, all solid-state lithium batteries (ASSLBs) using solid electrolytes to replace organic liquid electrolytes have been proposed and received increasing attention in recent years [2]. As a key part of ASSLBs, solid electrolytes have several advantages, such as nonflammability, nonvolatility, and high mechanical strength [3]. Nonetheless, compared with organic electrolytes, solid electrolytes still have some problems, such as low conductivity and large interface impedance which need to be addressed [4,5]. Among all the solid electrolytes, the Li1.5Al0.5Ge1.5 (PO4)3 (LAGP) is regarded as one of the most promising candidates due to its high ionic conductivity [6] and good chemical stability against metallic lithium [7]. In the conventional sintering of LAGP solid electrolytes, it has been found that the relative density of sintered samples is low (70–80%) [8], due to the pressureless sintering environment. SPS is considered to be an efficient way to deal with this problem. This method can densify samples to near theoretical density and suppress grain coarsening, both
∗
of which are beneficial to the ionic conductivity of samples [9,10]. In previous studies, Li1.3Al0.3Ti1.7(PO4)3 [11] and Li7La3Zr2O12 [12] solid electrolytes have been prepared by SPS, and achieved relative high density and ion conductivity. As for LAGP, Bouchet's group obtained LAGP solid electrolytes with a relative density of 93% and ionic conductivity of over 10−4 S cm−1 by SPS [13,14]. However, their studies did not consider the existence of carbon contamination which is inevitable during SPS. In this paper, the effect of carbon contamination on the microstructure and ionic conductivity of the samples is investigated. Furthermore, the shear modulus of LAGP prepared by SPS is also determined in order to evaluate its ability to inhibit Li dendrite. 2. Experiments 2.1. Sample preparation The LAGP powder used in this study was purchased from MTI CO., LTD (Hefei, China). The particle size of the LAGP was about 300–500 nm. Pellet samples with a diameter of 12.5 mm and a thickness of 2 mm were sintered by the spark plasma sintering system (Dr. sinter SPS-1050) using the LAGP powder. Two different sintering processes were adopted to prepare these samples. For sintering process Ⅰ
Corresponding author. E-mail address: [email protected] (X. Song).
https://doi.org/10.1016/j.ceramint.2019.11.264 Received 17 September 2019; Received in revised form 18 November 2019; Accepted 28 November 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Huan Tong, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.11.264
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(SP Ⅰ), the samples were heated to the target sintering temperature with a rate of pressure increase. The pressure and temperature simultaneously achieved their target values. For sintering process Ⅱ (SP Ⅱ), the samples were first heated to the target sintering temperature, and then the pressure was loaded to the target value within 3 min. In both sintering processes I and II, the target sintering temperatures were 600 °C, 650 °C, 700 °C, 750 °C, 800 °C and 850 °C, respectively, the target pressure was 60 MPa, the heating rate was 100 °C min−1 and the sintering time was 2 min. In order to remove carbon contamination, the sintered samples were annealed at 500 °C for 8 h in air. Their relative density was calculated based on the ratio of measure density to theoretical density (3.41 g cm−3). The measure density was determined based on the weight and volume of the LAGP pellets, while the theoretical density was calculated by the LAGP cell parameters and molecular weight [15]. 2.2. Phase and microstructure characterizations
Fig. 1. XRD patterns of the LAGP powders and the sintered LAGP pellets.
The phases of both the LAGP powder and the sintered LAGP samples were examined by X-ray diffraction (XRD, TTR Ⅲ), using Cu Kα1 radiation in the 2θ range from 10° to 80° with a step of 0.02°. The morphology and microstructure of the LAGP samples were examined by field emission scanning electron microscopy (SEM, Zeiss Auriga) and transmission electron microscopy (TEM, FEI F30). The elemental distribution of the samples was investigated by energy-dispersive X-ray spectroscopy (EDS). To prepare the TEM samples, the sintering samples were first ground into powders and then the powders were dispersed into acetone. The TEM samples were prepared by dropping the acetone onto a carbon films holder.
indicates that the orientation of the LAGP grains has been rearranged during the SPS. The diffraction peaks of the LAGP sintered samples between sintering processes Ⅰ and Ⅱ are almost the same, suggesting that the sintering processes have no influence on the phase composition. Fig. 2(a) shows the XRD patterns of the SP Ⅰ-650 sample before and after annealing. The diffraction peaks remain unchanged after annealing, implying that the annealing process has no influence on the phase composition of the samples. But the colors are different for the sintered sample SP Ⅰ-650 before and after annealing, as shown in Fig. 2(b). The sintered sample prepared by SPS at 650 °C is black, which is mainly caused by carbon contamination during the SPS. After annealing in air, the carbon is oxidized and the sample becomes white, similar to the initial color of the LAGP powders. By checking the color of samples, it is determined that the carbon contamination is removed after annealing process. Fig. s 2 (b) and (c) show the microstructure of the SP Ⅰ-650 sample before and after annealing. The average grain size of the SP Ⅰ-650 sample is about 500 nm and remains unchanged after annealing. Hence, the annealing process can remove carbon contamination, without affecting the phase composition and microstructure of the LAGP pellets. Fig. s 3(a), (c) and (e) show SEM images of the LAGP samples prepared by sintering process Ⅰ at 600 °C, 700 °C and 800 °C, respectively. It can be seen that the pores of the samples significantly increase as the temperature increases. In the higher magnification images in Fig. s 3(b), (d) and (f), the LAGP grains appear as rectangles and gradually get larger as the temperature increases, indicating increased crystallinity. In addition to the rectangular grains, spherical particles can also be observed in the sintered samples. As EDS mapping results show in Fig. 3(g), O and P elements are homogeneously distributed in the spherical particle and rectangular grains, while Al element is heavily segregated in the spherical particle region, and Ge disappears in this region. Based on the results of XRD, it can be inferred that the spherical particle is attributable to the impurity of AlPO4. It is concluded that LAGP solid electrolyte can be sintered into pellets by SPS, but the samples sintered by sintering process Ⅰ are porous. Fig. 4 shows the XRD patterns of the LAGP samples sintered by SPS from 600 °C to 850 °C by sintering process Ⅱ. The peak intensity of impurity phases AlPO4 and GeO2 increases with the temperature, as shown in the inset, which suggests the amount of impurities is increasing. However, this impurity peak suddenly decreases and the peak of Al2O3 appears at 850 °C, which may be related to the decomposition of AlPO4 at 850 °C during the SPS. Fig. 5 displays SEM images of LAGP samples sintered at different temperatures by sintering process Ⅱ. As can be seen in Fig. s 5(a) and (b), the LAGP grains are irregular after sintering at 600 °C and 650 °C, demonstrating a low crystallinity in the samples. As the sintering
2.3. Electrochemical characterizations The AC impedance spectroscopy was measured by electrochemical workstation (CHI660E) in a frequency range of 10−2-106 Hz with an amplitude of 5 mV AC at ambient temperature. Before the measurement, both sides of the LAGP pellets were painted with silver paste as blocking electrodes and were dried at 200 °C for 2h. The AC impedance spectroscopy was fitted with Z-view software, and the ionic conductivity (σ) was calculated according to Eq. (1)
σ=
d (Rb + Rgb)⋅S
(1)
where Rb was the bulk impedance of the samples, Rgb was the grain boundary impedance of the samples, d was the thickness of the samples and S was the cross-section area of the samples. 2.4. Elastic modulus measurement The elastic modulus was measured using nanoindentation (MTS, Nanolndenter XP) based on the continuous stiffness method (CSM). Test points were randomly selected with an indentation depth of 500 nm. The elastic modulus E and shear modulus G were calculated based on load−displacement stiffness data. 3. Results and analysis Fig. 1 shows the XRD patterns of the LAGP powders and sintered LAGP pellets prepared by sintering processes Ⅰ and Ⅱ. The diffraction peaks of the LAGP powders and sintered LAGP pellets are indexed to the LiGe2(PO4)3 phase with NASICON structure (PDF# 80–1924). Some diffraction peaks of impurity phases, AlPO4 and GeO2, are also observed in the sintered samples, and this may be the result of the volatilization of lithium. In addition, the diffraction peak located at about 15° in the LAGP powders disappears in the sintered samples but can be observed again when the sintered samples are ground into powders. This 2
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Fig. 2. (a) XRD patterns of the sintered sample SP Ⅰ-650 before and after annealing; (b) optical images of the sintered sample SP Ⅰ-650 before and after annealing; SEM images of the sintered sample SP Ⅰ-650 before(c) and after (d) annealing.
tail related to blocking electrodes in the low frequency region. (For the ceramic solid electrolytes, the impedance semicircle consists of grain impedance semicircle and grain boundary impedance semicircle. As reported, the grain impedance (bulk impedance) semicircle can be observed at an ultra-high frequency of 10 MHz, while the grain boundary impedance semicircle can be observed at the high frequency of 1 MHz [16]. In this study, the measuring frequency is below 1 MHz, thus the measured impedance semicircle is related to the grain boundary impedance, not the grain impedance. Comparing the impedance plots of sample SP Ⅰ-650 before and after annealing, it is found that the annealing process significantly reduces the impedance of the samples. Comparing the impedance plots of sample SP Ⅱ-700 before and after annealing, it is also found that the annealing process significantly reduces the impedance of the samples. Comparing the impedance plots of SP Ⅰ-650 and SP Ⅱ-650 after annealing, the impedance of SP Ⅱ-650 is lower than that of SP Ⅰ-650, which may be a result of their different relative densities. Fig. 7(c) shows the complex impedance plots of the LAGP samples prepared at different temperatures by sintering process Ⅱ and subsequently annealed. In the complex impedance plots, the intercept of the impedance semicircle in the high frequency region represents the bulk impedance of the samples, and the diameter of the impedance semicircle represents the grain boundary impedance [17]. As can be seen, each sample has a similar intercept value and very different impedance semicircles. This means that the bulk impedance of LAGP is relatively insensitive to the sintering temperature, but the grain boundary impedance is significantly affected by the sintering temperature. Fig. 8 shows the elastic modulus of the SP Ⅱ-700 LAGP sample as a function of nanoindentation displacement measured by nanoindentation. The most accurate estimate of E was obtained from an indentation depth range of approximately 140−300 nm with a value of 124 GPa. Data from depths shallower than about 140 nm is more susceptible to errors from thermal drift, and contaminants on the surface of the specimen. Data from depths deeper than ~300 nm is influenced by the averaging grain volume. The Poisson's ratio, υ, refers to 0.25, which is Poisson's ratio of Li1.3Al0.3Ti1.7(PO4)3 [18]. The shear modulus, G, is 49.6 GPa calculated by Eq. (2)
temperature increases to 700 °C, the LAGP grains become more regular and turn into rectangles. Fig. 5(c) shows the densest microstructure in SP Ⅱ-700, and it is difficult to distinguish the grain boundaries from each other. With the further increase of temperature, rectangular grains grow up gradually and grain boundaries become apparent. The insets in Fig. 5 are optical images of the corresponding samples. As can be seen, the color of the sintered samples is gray at 600 °C and 650 °C, and becomes black at 700 °C and 750 °C, then turns to white at 800 °C and 850 °C. It can be speculated that the carbon contamination is sensitive to sintering temperatures. Fig. s 6(a)–(c) are TEM images of the LAGP powders, sintered sample SP Ⅱ-700 and SP Ⅱ-850, respectively. Before sintering, the LAGP are irregular particles. After sintering, LAGP grains become regular and partially turn to rectangles. Irregular particles and rectangular particles can both be observed in the sintered samples. From SAED and EDS analysis of these particles, it can be seen that both the irregular particles and the rectangular particles are LAGP particles. Fig. 6(e) is the result of elements mapping, each element distributes homogeneously in both particles, while a black spot in the HADDF image can also be observed, which may be an adhered AlPO4 impurity particle. It can conclude that the increase of temperature promotes the crystallinity of the LAGP sintered samples while each element is uniformly distributed in the LAGP grains. Fig. 7(a) shows the relative density of LAGP samples sintered at different temperatures with two sintering processes and the ionic conductivity of the LAGP samples sintered with sintering process Ⅱ. The relative density of the samples prepared by both sintering processes Ⅰ and Ⅱ first increases and then decreases. The maximum relative density reaches 80% at 650 °C in sintering process Ⅰ and 97% at 700 °C in sintering process Ⅱ. Meanwhile, the conductivity of the samples prepared by sintering process Ⅱ shows a similar tendency in respect of relative density, and the maximum ionic conductivity is 2.12 × 10−4 S cm−1 at the sintering temperature of 700 °C. Fig. 7(b) shows effects of the annealing process and different sintering processes on the impedance of the samples. As can be seen, each complex impedance plot shows a single depressed semicircle related to the impedance of grain boundaries in the high frequency region and a 3
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Fig. 3. SEM images of the LAGP samples: (a, b) SP Ⅰ-600; (c, d) SP Ⅰ-700; (e, f) SP Ⅰ-800, (g) EDS element mapping of different grains for the SP Ⅰ-800.
G=
E 2(1 + υ )
prepared by spark plasma sintering technology, the maximum of ion conductivity is 2.12 × 10−4 S cm−1 which is significantly higher than that of the general LAGP in the literature [6]. In addition, the effects of different sintering processes and carbon contamination on the microstructure and ionic conductivity of sintered samples have been analyzed. The results show that the relative density of samples prepared by both sintering processes Ⅰ and Ⅱ first increases and then decreases as the temperature increases. It is contended that the increase of relative density is attributable to high temperature sintering, and that the decrease of relative density is related to the gas generated during sintering. Due to the use of graphite dies, carbon atoms react with oxygen atoms which exist inside samples and form oxycarbide gas during
(2)
Linear elasticity analyses performed by Monroe and Newman suggest that the shear modulus of solid electrolytes needs to exceed a value that is at least twice the Li metal shear modulus (~8.5 GPa) to prevent dendrite nucleation [19]. The G value of LAGP in this work is 49.6 GPa, which far exceeds the minimum value calculated by Monroe and Newman. Hence, the LAGP solid electrolyte is capable of inhibiting Li dendrite nucleation and penetration. 4. Discussions In this study, the LAGP solid electrolyte has been successfully 4
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significant difference between sintering processes Ⅰ and Ⅱ. The relative density is only 80% in sintering process Ⅰ, while it is up to 97% in sintering process Ⅱ. This difference may be related to the different pressurization processes. In sintering process Ⅰ, samples are heated up and pressurized simultaneously. Due to the effect of pressure, the generated gas finds it difficult to escape from the samples and finally remains in them as pores, which hinders their densification. In sintering process Ⅱ, the samples are first heated up and then pressurized. The generated gas can easily escape from the samples, which can be sufficiently densified during the pressurization process. It has been reported that carbon contamination can darken samples [23,24]. In this study, it is found that samples present different colors at different sintering temperatures, which may be a result of the degree of carbon contamination. As shown in Fig. 5, at 600 °C and 650 °C, the degree of carbon contamination is low and the samples are gray. When the temperature rises to 700 °C and 750 °C, the degree of carbon contamination increases and the samples become black. As the temperature increases to 800 °C and 850 °C, the carbon contamination can be oxidized and eliminated during the sintering process. As a result, the samples turn white again. In addition to affecting the color, the carbon can also affect the grain boundary impedance. As shown in Fig. 7(b), the impedance semicircles of SP Ⅰ-650 and SP Ⅱ-700 can be significantly reduced after annealing. It has also been found that the annealing process can remove carbon contamination significantly without any effects on the composition of
Fig. 4. XRD patterns of LAGP pellets sintered by SPS at different temperatures.
sintering [20,21]. The lithium element of samples can also volatilize as gas at high temperatures [22]. These gases produce a number of pores in the samples and result in the decrease of their relative density. As can be seen in Fig. 7(a), the maximum relative density shows a
Fig. 5. SEM images of the LAGP samples sintered at different temperatures by SPS: (a) SP Ⅱ-600; (b) SP Ⅱ-650; (c) SP Ⅱ-700; (d) SP Ⅱ-750; (e) SP Ⅱ-800; (f) SP Ⅱ-850, the insets are optical images of the corresponding samples. 5
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Fig. 6. TEM images of the LAGP samples: (a) LAGP initial powder; (b) SP Ⅱ-700; (c) SP Ⅱ-850. (d) a higher magnification TEM image, SAED and EDS of SP Ⅱ-700; (e) HADDF image and elements mapping of (d).
has been reported that carbon contamination can affect the electrochemical properties of garnet-type solid electrolyte by decomposing the electrolytes [25]. However, in this study, it is found that the carbon contamination has a direct impact on the grain boundary impedance of sintered LAGP samples as a result of the presence of local electronic conduction. As shown in Fig. 7(a), the conductivity of the samples is highly dependent on their relative density. However, after 700 °C, the relative density decreases slowly while the ionic conductivity decreases
phases and microstructures. Therefore, these results indicate that the annealing process can remove the carbon contamination in both sintering process Ⅰ and Ⅱ, and it can be deduced that the decrease of impedance is due to the elimination of carbon contamination. In other words, the carbon existing in the samples creates an extra grain boundary impedance. The reason for this is that the carbon can produce local electronic conduction at the grain boundary. With the existence of local electronic conduction, the migration of ions can be blocked, causing an increase of grain boundary impedance. In previous studies, it 6
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Fig. 8. Elastic modulus of sample SP Ⅱ-700 as a function of nanoindentation displacement.
In summary, LAGP solid electrolytes can be quickly densified by SPS to a high relative density, but carbon contamination is inevitable during sintering. Carbon plays an important role because it generates oxycarbide gas which affects the relative density and microstructure of the sintered samples. Furthermore, it also creates an extra grain boundary impedance in the LAGP samples. Therefore, it is also important to consider the effect of carbon contamination. 5. Conclusions By means of SPS, LAGP solid electrolytes have been synthesized. The effects of carbon contamination on their microstructure and ionic conductivity have been explored. In addition, the elastic properties of LAGP have also been characterized. The conclusions are as follows: (1) Both the relative density and ionic conductivity of LAGP samples prepared by SPS first increase and then decrease as the temperature increases, achieving a maximum value of 97% and 2.12 × 10−4 S cm−1 simultaneously at 700 °C; (2) During sintering, the carbon from graphite dies reacts with oxygen in the samples to generate oxycarbide gas which results in the formation of pores and in the decrease of the relative density of the samples. This unfavorable effect can be mitigated by optimizing the pressurization method of the sintering process. (3) Carbon contamination is sensitive to the sintering temperature, which causes the samples to change color. Carbon contamination can also create an extra grain boundary impedance. This unfavorable effect can be removed by means of annealing at 500 °C in air atmosphere. (4) The shear modulus of the LAGP solid electrolyte is measured to be 49.6 GPa, which means the LAGP prepared by SPS has the ability to inhibit Li dendrite nucleation and penetration. Declaration of competing interestCOI The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Fig. 7. (a) Relative density and ionic conductivity of LAGP samples sintered at different temperatures; (b) complex impedance plots of LAGP samples treated with different processes; (c) complex impedance plots of LAGP samples sintered at different temperatures.
Acknowledgements
significantly. This indicates that the conductivity not only depends on the relative density of the samples, but also relates to other factors, such as impurities. As shown in Fig. 4, the impurities of AlPO4 and GeO2 increase with the temperature, which may damage the ionic conductivity of samples.
This work was supported by State Key Lab of Advanced Metals and Materials [No.2019-ZD06], National Natural Science Foundation of China [grant No.51271021 and 21171018], and the Beijing Natural Science Foundation [grant No. 2162025]. 7
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