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Effect of different treatment methods on the electrochemical properties of LiV3O8 at elevated temperatures
Ceramics International xx (xxxx) xxxx–xxxx
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Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Effect of different treatment methods on the electrochemical properties of LiV3O8 at elevated temperatures ⁎
Changrong Zhonga,b, , Xunjia Sua, Genliang Houa, Fushan Yuc, Song Bia, Zhaohui Liua, Hao Lia a b c
Xi'an Research Institute of High-tech, Xi'an, Shaanxi 710025, China Baoji Research Institute of High-tech, Baoji, Shaanxi 721013, China Xi'an Qinghua Cooperation, North Special Energy Group Limited Company, Xi'an, Shaanxi 710025, China
A R T I C L E I N F O
A BS T RAC T
Keywords: Sol-gel method LiV3O8 Thermal battery Treatment methods Electrochemical properties
A lithium vanadium oxide cathode material was synthesized via sol-gel processing using citric acid as the chelating agent. The cathode powder was treated with two different processes. The composition, morphology and electrochemical performance of the samples were characterized by X-ray diffraction, scanning electron microscopy, open circuit potential monitoring and galvanostatic discharge testing. Results show that the sample sintered at 450 °C has even micron-sized crystallites and its corresponding mixed electrode exhibits the best electrochemical performance. Although the impurity of the active material was changed by the adsorption, leading to sharp initial voltage spikes, the adsorbed electrodes show more potential as cathodes due to their stable discharge curve and comparable special capacity.
1. Introduction Thermally activated (“thermal”) batteries are mainly used for military purposes that require a high level of reliability [1]. They are able to operate in harsh conditions and can survive high shock environments. In addition, they are hermetically sealed and can remain in weapons systems for 25 years or more, over a wide range of storage temperatures (typically, −55 to +75 °C) without degradation [2]. The size of a thermal battery is an important factor in small systems engineering, and thus, decreasing the battery size enables design flexibility. Researchers have adopted two methods to achieve this goal. One method is using thin films instead of thick disc electrodes. In traditional pellet technology, the eutectic salt powder and active material are evenly mixed as the raw material of the positive, and then the cathode pellet is press-formed by a fixed mold [3]. Some literature studies have reported improved methods on positive preparing that have not yet implemented for scale production, such as electrodepositing, screen printing, plasma spraying and coating methods [4–7]. The other method is increasing the overall working cell potential. The development of cathode materials that can deliver a higher working potential will significantly increase the cell energy density, leading to a substantial decrease in the battery weight and dimensions. Disulfide cathodes are currently regarded as the main materials for thermal batteries, but their discharge potential plateaus are relatively low (below 2 V). Thus, a number of transition metal oxides and halides
⁎
have been researched as thermal battery cathodes [8]. Among these compounds, LiV3O8 has been extensively investigated as a potential and promising substituted cathode material due to its attractive electrochemical properties that include high voltage, high discharge capacity, high temperature stability, low cost and facile preparation [9,10]. In a previous study [11], a sol-gel method was introduced to prepare the positive material, which had adsorbed molten eutectic salt before it was pressed into a pellet. However, the study did not mention any information about the composition and morphology of the adsorbed material. In the present work, LiV3O8 crystallites have been synthesized via sol-gel processing using citric acid as the chelating agent. Taking into account the three-dimensional tunnel structure existing in the gel, the positive pellets were prepared by the active material adsorbed molten eutectic salt. By comparing with the traditional mixed method, the differences between the two positive pellets were analyzed in the context of their microstructure and electrochemical performance. 2. Experiment 2.1. Material preparation LiV3O8 was synthesized by a normal sol-gel method. However, the reagents used in the experiments were vanadium pentoxide (V2O5),
Corresponding author at: Xi'an Research Institute of High-tech, Xi'an, Shaanxi 710025, China. E-mail address: [email protected] (C. Zhong).
http://dx.doi.org/10.1016/j.ceramint.2016.09.174 Received 20 July 2016; Received in revised form 21 September 2016; Accepted 24 September 2016 Available online xxxx 0272-8842/ © 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: ZHONG, C., Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.09.174
Ceramics International xx (xxxx) xxxx–xxxx
C. Zhong et al.
could be easily comminuted. LVO400A was brown block and very hard to comminute. LVO400A, LVO500A and LVO550A were reground and sifted through a 100 mesh sieve, and then all the adsorbed powder was used to prepare the cathode pellet.
lithium hydroxide (LiOH·H2O) and citric acid (C6H8O7·H2O). All the chemical reagents used in the experiments were of analytical grade and could be used without any purification. The desired amount of V2O5 was dumped into deionized water kept in a water bath at 80 °C. Then, a precise amount of LiOH·H2O with the designed Li/V molar ratio of 1.05:3 was added to form a yellow suspension by continuous stirring. The ratio of the total amount of metal ions to citric acid was 1:2. Next, the citric acid was dissolved in deionized water. This solution of citric acid, with an appropriate concentration, was slowly dripped into the reaction system under stirring. Accompanied by the release of pungent gases, the color of the reaction system changed according to the following order: yellow→ light green→dark green→blue. After stirring for 1 h and drying the sol at 80 °C in air, a green gel was obtained. The obtained gel was dried in a vacuum oven at 120 °C for 6 h to form the bulky honeycombed precursor. The precursor was grinded through a 100 mesh sieve and divided into four portions. The four sieved powders were heated at 400, 450, 500 and 550 °C for 12 h in a muffle furnace, respectively (sequentially labeled as LVO400, LVO450, LVO500 and LVO550). The eutectic salt used in the thermal battery was made up of LiCl and KCl (45:55, wt%). The cathode powder was fabricated by a mixture of 80 wt% gel powder and 20 wt% eutectic salt. Half of the cathode powder was directly used to prepare the cathode pellet, labeled as the mixed electrode (LVO400M, LVO450M, LVO500M and LVO550M), and the other half of the powder was sintered at 400 °C for 1 h in a muffle furnace, labeled as the adsorbed electrode (LVO400A, LVO450A, LVO500A and LVO550A), so that the eutectic salt would be evenly adsorbed by the gel powder. After the sintering process, LVO450A did not need to be reground and could be completely sifted through a 100 mesh sieve. LVO500A was slightly yellow block and
2.2. Material characterization and battery assembly The crystal structures of the samples were determined using an Oxford Instruments type 7718 X-ray spectrometer in the range of 10– 80°. The morphology of the samples was observed with a Tescan VEGA 2 XMU SEM. The cell was assembled from a pelletized cathode material, with a 1 mm thickness and a diameter of 20 mm, a Li-Si anode, eutectic salt/ MgO (weight ratio of 65:35), a separator and a nickel collector. All components comprising the anode, separator and cathode were captured by a ceramic ring to prevent spillage and were mechanically isolated (top and bottom) with mica. The assemblage and measurements were conducted under a dry air atmosphere (1% humidity). The cells were introduced into a home-built instrument set to a test temperature of 500 °C. During heating, the electrolyte melts and the voltage begins to rise. For stabilization, the cell was placed for a short duration of 60 s and then a current load was applied. The open circuit potential (OCP) and elevated temperature discharge measurements were recorded on a CorrTest CS 350 electrochemical workstation. 3. Results and discussion 3.1. X-ray diffraction (XRD) analysis Fig. 1 shows the XRD patterns of the sintered sample and the two
Fig. 1. XRD patterns of the (a) sintered, (b) mixed and (c) adsorbed samples.
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3.2. Morphological characterization
cathode powders. The composition and crystallinity of the materials synthesized using a sol-gel method followed by different heat treatments (400, 450, 500 or 550 °C) are shown in Fig. 1(a). We find that the sintering temperature has an important effect on the component and crystallinity. The diffractions peaks at 2θ of 13.9° and 28.5° can be identified as the typical layered structure corresponding to LiV3O8, which means that the as-synthesized LiV3O8 (JCPDS No. 72-1193) crystallites have a monoclinic crystalline structure and belong to the P21/m space group. However, the pattern of the prepared sample also shows a small impurity peak at 2θ of 12.3°, which can be attributed to Li0.3V2O5 (JCPDS No. 18-0755). This Li0.3V2O5 impurity phase has also been observed in the synthesis of LiV3O8 by other methods [12– 14]. There is a possible reason for the presence of the Li0.3V2O5 impurity. In the precursor, the ratio of Li/V should be under 1:3 at some local areas, because the citric acid is not completely complexed with the metal ions, which leads to insufficient Li+ ion diffusion into the V3O8 layers. Fig. 1(a) shows that the four patterns are mostly similar to each other. The main difference is the relative intensity of some peaks of LiV3O8 and Li0.3V2O5. When the sintered temperature was increased to 500 °C, the diffraction peaks of the samples became sharper and higher than for other samples, which indicates an increase in the crystallinity and grain size of the powders. The diffraction peaks of the sample sintered at 550 °C are relatively mild. The presence of the KCl (JCPDS No. 41-1476) phase was detected in the samples of the mixed electrode, as shown in the inset of Fig. 1(b). We previously noted that there was 20 wt% eutectic salt in the cathode powder of the mixed electrode, so it is natural for the presence of KCl and LiCl phases. The disappearance of the LiCl phase may be because the percentage of LiCl is low in the cathode powder. Fig. 1(c) shows the XRD patterns of the adsorbed electrode. In contrast with Fig. 1(b), the diffractions peaks of Li0.3V2O5 are not present and the diffractions peaks of LiVO3 (JCPDS No. 70-1545) emerge. This phenomenon indicates that LiVO3 may be produced by Li0.3V2O5 with LiCl in the eutectic salt, so the molar ratio of LiVO3 should coincide with that of Li0.3V2O5. The diffraction peaks of LiVO3 also confirm this assumption.
Figs. 2–4 show the SEM images of the sintered samples and adsorbed electrode. The sintered samples shown in Fig. 2 indicate that the morphology and size of the as-synthesized LiV3O8 are significantly influenced by the heat-treatment temperature. Pristine LVO400 appears as large numbers of smaller particles with an undeveloped crystalline shape, exhibiting a wide size distribution. In contrast, LVO450 exhibits a relatively regular geometric shape and even size distribution. Moreover, the rod-like grains show a gradual increase in dimension with increasing temperature and achieve a maximum at 500 °C. LVO550 shows small rod-like crystals which may be produced by the broken grains, consistent with the changes in diffraction peak intensity observed in the XRD patterns.. The SEM images of the mixed electrode are shown in Fig. 3, which is composited of 80 wt% active material and 20 wt% eutectic salt. The cathode and eutectic salt can be roughly distinguished by the surface morphology of the particle. There are some large particles of eutectic salt that may be caused by the agglomerated eutectic salt in a wet air atmosphere. Fig. 4 shows the morphology of the adsorbed electrode specimen. The morphology of LVO400A is different from the other adsorbed samples, may be because this sample was produced by ball milling for a long time. The definition of adsorbed images is lower than the sintered samples since the sintered sample is higher conductible than the eutectic salt coated on the surface of the particle. This also indicates that the sintered samples were completely coated with eutectic salt. Moreover, the crystal grains of the adsorbed electrode are similar to the sintered samples, except for LVO400A. 3.3. Open circuit potential (OCP) The single cells were introduced into a home-built instrument set to 500 °C. During heating, the OCP value starts to increase and maintains a constant value for a short duration, then, the value decreases in the continuing process. The peak value of OCP was recorded and is shown in Table 1.
Fig. 2. SEM images of the sintered samples: LVO400, LVO450, LVO500 and LVO550.
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Fig. 3. SEM images of the mixed electrode: LVO400M, LVO450M, LVO500M and LVO550M.
Fig. 4. SEM images of the adsorbed electrode: LVO400A, LVO450A, LVO500A and LVO550A.
By combining with XRD analysis, we find that the peak value is seriously affected by the composition and its percentage. Among mixed oxide materials with multi redox systems, LiV3O8 has an appreciable advantage due to the preferential reduction of V5+ over V4+ during the oxidation of the lithium anode in the first stage of discharge, followed by a dual redox of V5+→V4+ and V4+→V3+ reactions in Li0.3V2O5. Thus, the content of Li0.3V2O5 may affect the OCP of the cathode materials, and lead to the difference on the peak value of the OCP. It can be expected that the higher voltage may bring about a longer working time
Table 1 Peak value of OCP. Sample
Voltage (V)
Sample
Voltage (V)
LVO400M LVO450M LVO500M LVO550M
3.25 3.51 3.53 3.29
LVO400A LVO450A LVO500A LVO550A
3.38 3.39 3.38 3.37
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Fig. 5. Discharge curves of (a) LVO400M, (b) LVO400A, (c) LVO450M, (d) LVO450A, (e) LVO500M, (f) LVO500A, (g) LVO550M and (h) LVO550A cathode materials at 500 °C.
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of the increased contact area of the reaction. The network structure is excellent for Li+ fast diffusion, thus the adsorbed electrode exhibits more stable discharge performance. Overall, the adsorbed electrode can be regarded as a promising cathode material if its initial voltage spikes can be eliminated. According to the previous analysis, LVO450M, LVO500M, LVO450A and LVO500A can be used as promising cathodes for small current densities (100 mA/cm2) and medium durations (4–12 min) in thermal battery applications. LVO450M, LVO450A and LVO500A can be used as potential cathodes for medium current densities (200 mA/cm2) and short durations (1–4 min) in thermal battery applications. LiV3O8 treated by the traditional method is not fit to be used for large current density thermal batteries. Whereas, the adsorbed electrode can be used for large current densities (300 mA/cm2), high potentials (2.5–2.0 V) and short duration (30–70 s) thermal batteries.
Table 2 Specific capacity of the single cells. Current density
100 mA/cm2
200 mA/cm2
300 mA/cm2
LVO400M LVO450M LVO500M LVO550M LVO400A LVO450A LVO500A LVO550A
205.4 301.1 260.9 201.9 255.3 248.8 267.6 280.2
156.3 243.9 166.7 205.0 191.9 207.5 205.4 191.4
188.6 205.0 182.5 128.4 199.9 157.7 190.5 183.5
mA h/g mA h/g mA h/g mA h/g mA h/g mA h/g mA h/g mA h/g
mA h/g mA h/g mA h/g mA h/g mA h/g mA h/g mA h/g mA h/g
mA h/g mA h/g mA h/g mA h/g mA h/g mA h/g mA h/g mA h/g
and higher specific capacity. The adsorbed electrode is fabricated by a mixture of LiV3O8 and LiVO3, which exhibit a stable peak value. It was mentioned previously that the LiVO3 may be produced by the reacted Li0.3V2O5 and LiCl, so the content of LiVO3 should be different in the electrode. This phenomenon indicates that the existence of LiVO3 can evidently reduce the peak value of the electrode disregard the exact percentage.
4. Conclusions We have synthesized LiV3O8 powder via a sol-gel process with citric acid as the chelating agent. The performance of the cathode materials treated by two methods was tested under the same conditions. Results show that the traditional mixed electrode LVO450M, made by the active material sintered at 450 °C, exhibits the best electrochemical performance. Compared with the traditional mixed electrode, the adsorbed electrodes exhibit comparable special capacity and more stable discharge curves, which indicate that the effective working time of the thermal battery would be prolonged. In addition, the adsorbed electrode can be used in wider situations than the mixed ones. However, the impurity of the active material changed in the adsorption process, leading to sharp initial voltage spikes. In future experiments, we should adjust the technical parameters in order to lessen the impurity content so that the true performance of LiV3O8 treated by two different methods can be evaluated.
3.4. Galvanostatic discharge properties The voltage-time curves of the samples at current densities of 100, 200 and 300 mA/cm2 are illustrated in Fig. 5 (the terminated voltage is 0.2 V). In the contrast test, the cathode powders were test at the same conditions, such as anode, electrolyte, isolation layer, collector and experimental environment. The difference in electrochemical performance should be caused by the different composition the of cathode powder. The effective area of the cell is 3.14 cm2. In addition, the mass of the active material is about 0.3 g. Then, the specific capacity of the electrode material can be calculated according to the current density and discharge time. Table 2 lists the calculated capacities of each thermal battery. The performance of the mixed electrodes was quite similar to that of the adsorbed counterpart under these conditions. Firstly, the discharge voltage and working time both decrease with increasing current density. The larger the current density is, the less the working time is. For example, LVO450 can discharge for more than 1000 s at a current density of 100 mA/cm2, and reaches a specific capacity of 301.1 mA h/g, while 429 s and 243.9 mA h/g at 200 mA/cm2. The discharge property of the adsorbed electrodes is comparable to the mixed ones. Secondly, the two cathode materials can be defined as thermal battery cathode promising candidates for both medium and small current densities in thermal battery applications, which exhibit a higher operating voltage range than commonly used disulfide cathodes. There is some difference that can be easily detected from the discharge curves. The initial voltage spikes were more obvious at the start of discharge curves of adsorbed electrodes, which means that the presence of LiVO3 is harmful to the galvanostatic discharge. In addition, the discharging time of the adsorbed electrodes is basically equal to that of the mixed electrodes, but the discharge curves of the adsorbed electrodes are more stable than those of the mixed electrodes, which indicates that the effective working time of the thermal battery is prolonged. The stable discharge curves of the adsorbed electrodes should be attributed to its structure. It is well known that there is a number of three-dimensional network structures in the gel. However, the network structure of adsorbed electrodes was filled with eutectic salt in the adsorption process. In contrast, the mixed electrode needs to experience a molten diffusion process for the eutectic salt. Combined with the previous analysis, the eutectic salt adsorbed into the gel can shorten the average trip of Li+ diffusion in discharge. Simultaneously, the current density is reduced on the surface because
References [1] P. Masset, R.A. Guidotti, Thermal activated (thermal) battery technology Part II. Molten salt electrolytes, J. Power Sources 164 (2007) 397–414. [2] R.A. Guidotti, P. Masset, Thermally activated (“thermal”) battery technology Part I: an overview, J. Power Sources 161 (2006) 1443–1449. [3] Ruisheng Lu, Xiaojiang Liu, Thermal Battery, 1rd, National Defense Industry Press, Beijing, 2005. [4] Xueqin Wang, Gang Wang, Jinwei Chen, et al., Pyrite thin films prepared for thermal batteries via sulfuring electrodeposited iron sulfide films: structure and physical properties, Mater. Lett. 110 (2013) 144–147. [5] Kun Lv, Shaohua Yang, Ping Zhao, et al., Research of performance influence factors of FeS2 thin-film cathodes for LiSi/FeS2 thermal batteries, CN J. Power Sources 38 (2014) 1519–1522. [6] R.A. Guidotti, F.W. Reinhardt, J. Dai, Performance of thermal cells and batteries made with plasma-sprayed cathode and anode, J. Power Sources 160 (2006) 1456–1464. [7] Yongqiang Yuan, Rui Gao, Wei Lan, et al., Fabrication and electrochemical properties of thin film electrode materials for thermal batteries, J. Terahertz Sci. Electron. Inf. Technol. 13 (2015) 833–836. [8] R.A. Guidotti, P. Masset, Thermal activated (“thermal”) battery technology Part IIIb. Sulfur and oxide-based cathode materials, J. Power Sources 178 (2008) 456–466. [9] Zhao-jin Wu, Yuan Zhou, Effect of Ce-doping on the structure and electrochemical performance of lithium trivanadate prepared by a citrate sol–gel method, J. Power Sources 199 (2012) 300–307. [10] Miao Shui, Weidong Zheng, Jie Shu, et al., Synthesis and electrochemical performance of Li1+xV3O8 as cathode material prepared by citric acid and tartaric acid assisted sol-gel processes, Curr. Appl. Phys. 13 (2013) 517–521. [11] Li Zhi-you, Huang Bai-yun, Tang Chun-feng, et al., Synthesis of LiV3O8 by sol-gel method and its cathodical discharge behavior at 500 °C, Funct. Mater. 32 (2001) 181–183. [12] S. Sarkar, H. Banda, S. Mitra, High capacity lithium-ion battery cathode using LiV3O8 nanorods, Electrochim. Acta 99 (2013) 242–252. [13] Kyungho Kim, Su. Han Park, Tae Hyung Kwon, et al., Characterization of Li–V–O nanorod phases and their effect on electrochemical properties of Li1+xV3O8 cathode materials synthesized by hydrothermal reaction and subsequent heat treatment, Electrochim. Acta 89 (2013) 708–716. [14] Dunqiang Wang, Liyun Caon, Jianfeng Huang, et al., Effects of different chelating agents on the composition, morphology and electrochemical properties of LiV3O8 crystallites synthesized via sol–gel method, Ceram. Int. 39 (2013) 3759–3764.
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Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Effect of different treatment methods on the electrochemical properties of LiV3O8 at elevated temperatures ⁎
Changrong Zhonga,b, , Xunjia Sua, Genliang Houa, Fushan Yuc, Song Bia, Zhaohui Liua, Hao Lia a b c
Xi'an Research Institute of High-tech, Xi'an, Shaanxi 710025, China Baoji Research Institute of High-tech, Baoji, Shaanxi 721013, China Xi'an Qinghua Cooperation, North Special Energy Group Limited Company, Xi'an, Shaanxi 710025, China
A R T I C L E I N F O
A BS T RAC T
Keywords: Sol-gel method LiV3O8 Thermal battery Treatment methods Electrochemical properties
A lithium vanadium oxide cathode material was synthesized via sol-gel processing using citric acid as the chelating agent. The cathode powder was treated with two different processes. The composition, morphology and electrochemical performance of the samples were characterized by X-ray diffraction, scanning electron microscopy, open circuit potential monitoring and galvanostatic discharge testing. Results show that the sample sintered at 450 °C has even micron-sized crystallites and its corresponding mixed electrode exhibits the best electrochemical performance. Although the impurity of the active material was changed by the adsorption, leading to sharp initial voltage spikes, the adsorbed electrodes show more potential as cathodes due to their stable discharge curve and comparable special capacity.
1. Introduction Thermally activated (“thermal”) batteries are mainly used for military purposes that require a high level of reliability [1]. They are able to operate in harsh conditions and can survive high shock environments. In addition, they are hermetically sealed and can remain in weapons systems for 25 years or more, over a wide range of storage temperatures (typically, −55 to +75 °C) without degradation [2]. The size of a thermal battery is an important factor in small systems engineering, and thus, decreasing the battery size enables design flexibility. Researchers have adopted two methods to achieve this goal. One method is using thin films instead of thick disc electrodes. In traditional pellet technology, the eutectic salt powder and active material are evenly mixed as the raw material of the positive, and then the cathode pellet is press-formed by a fixed mold [3]. Some literature studies have reported improved methods on positive preparing that have not yet implemented for scale production, such as electrodepositing, screen printing, plasma spraying and coating methods [4–7]. The other method is increasing the overall working cell potential. The development of cathode materials that can deliver a higher working potential will significantly increase the cell energy density, leading to a substantial decrease in the battery weight and dimensions. Disulfide cathodes are currently regarded as the main materials for thermal batteries, but their discharge potential plateaus are relatively low (below 2 V). Thus, a number of transition metal oxides and halides
⁎
have been researched as thermal battery cathodes [8]. Among these compounds, LiV3O8 has been extensively investigated as a potential and promising substituted cathode material due to its attractive electrochemical properties that include high voltage, high discharge capacity, high temperature stability, low cost and facile preparation [9,10]. In a previous study [11], a sol-gel method was introduced to prepare the positive material, which had adsorbed molten eutectic salt before it was pressed into a pellet. However, the study did not mention any information about the composition and morphology of the adsorbed material. In the present work, LiV3O8 crystallites have been synthesized via sol-gel processing using citric acid as the chelating agent. Taking into account the three-dimensional tunnel structure existing in the gel, the positive pellets were prepared by the active material adsorbed molten eutectic salt. By comparing with the traditional mixed method, the differences between the two positive pellets were analyzed in the context of their microstructure and electrochemical performance. 2. Experiment 2.1. Material preparation LiV3O8 was synthesized by a normal sol-gel method. However, the reagents used in the experiments were vanadium pentoxide (V2O5),
Corresponding author at: Xi'an Research Institute of High-tech, Xi'an, Shaanxi 710025, China. E-mail address: [email protected] (C. Zhong).
http://dx.doi.org/10.1016/j.ceramint.2016.09.174 Received 20 July 2016; Received in revised form 21 September 2016; Accepted 24 September 2016 Available online xxxx 0272-8842/ © 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: ZHONG, C., Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.09.174
Ceramics International xx (xxxx) xxxx–xxxx
C. Zhong et al.
could be easily comminuted. LVO400A was brown block and very hard to comminute. LVO400A, LVO500A and LVO550A were reground and sifted through a 100 mesh sieve, and then all the adsorbed powder was used to prepare the cathode pellet.
lithium hydroxide (LiOH·H2O) and citric acid (C6H8O7·H2O). All the chemical reagents used in the experiments were of analytical grade and could be used without any purification. The desired amount of V2O5 was dumped into deionized water kept in a water bath at 80 °C. Then, a precise amount of LiOH·H2O with the designed Li/V molar ratio of 1.05:3 was added to form a yellow suspension by continuous stirring. The ratio of the total amount of metal ions to citric acid was 1:2. Next, the citric acid was dissolved in deionized water. This solution of citric acid, with an appropriate concentration, was slowly dripped into the reaction system under stirring. Accompanied by the release of pungent gases, the color of the reaction system changed according to the following order: yellow→ light green→dark green→blue. After stirring for 1 h and drying the sol at 80 °C in air, a green gel was obtained. The obtained gel was dried in a vacuum oven at 120 °C for 6 h to form the bulky honeycombed precursor. The precursor was grinded through a 100 mesh sieve and divided into four portions. The four sieved powders were heated at 400, 450, 500 and 550 °C for 12 h in a muffle furnace, respectively (sequentially labeled as LVO400, LVO450, LVO500 and LVO550). The eutectic salt used in the thermal battery was made up of LiCl and KCl (45:55, wt%). The cathode powder was fabricated by a mixture of 80 wt% gel powder and 20 wt% eutectic salt. Half of the cathode powder was directly used to prepare the cathode pellet, labeled as the mixed electrode (LVO400M, LVO450M, LVO500M and LVO550M), and the other half of the powder was sintered at 400 °C for 1 h in a muffle furnace, labeled as the adsorbed electrode (LVO400A, LVO450A, LVO500A and LVO550A), so that the eutectic salt would be evenly adsorbed by the gel powder. After the sintering process, LVO450A did not need to be reground and could be completely sifted through a 100 mesh sieve. LVO500A was slightly yellow block and
2.2. Material characterization and battery assembly The crystal structures of the samples were determined using an Oxford Instruments type 7718 X-ray spectrometer in the range of 10– 80°. The morphology of the samples was observed with a Tescan VEGA 2 XMU SEM. The cell was assembled from a pelletized cathode material, with a 1 mm thickness and a diameter of 20 mm, a Li-Si anode, eutectic salt/ MgO (weight ratio of 65:35), a separator and a nickel collector. All components comprising the anode, separator and cathode were captured by a ceramic ring to prevent spillage and were mechanically isolated (top and bottom) with mica. The assemblage and measurements were conducted under a dry air atmosphere (1% humidity). The cells were introduced into a home-built instrument set to a test temperature of 500 °C. During heating, the electrolyte melts and the voltage begins to rise. For stabilization, the cell was placed for a short duration of 60 s and then a current load was applied. The open circuit potential (OCP) and elevated temperature discharge measurements were recorded on a CorrTest CS 350 electrochemical workstation. 3. Results and discussion 3.1. X-ray diffraction (XRD) analysis Fig. 1 shows the XRD patterns of the sintered sample and the two
Fig. 1. XRD patterns of the (a) sintered, (b) mixed and (c) adsorbed samples.
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3.2. Morphological characterization
cathode powders. The composition and crystallinity of the materials synthesized using a sol-gel method followed by different heat treatments (400, 450, 500 or 550 °C) are shown in Fig. 1(a). We find that the sintering temperature has an important effect on the component and crystallinity. The diffractions peaks at 2θ of 13.9° and 28.5° can be identified as the typical layered structure corresponding to LiV3O8, which means that the as-synthesized LiV3O8 (JCPDS No. 72-1193) crystallites have a monoclinic crystalline structure and belong to the P21/m space group. However, the pattern of the prepared sample also shows a small impurity peak at 2θ of 12.3°, which can be attributed to Li0.3V2O5 (JCPDS No. 18-0755). This Li0.3V2O5 impurity phase has also been observed in the synthesis of LiV3O8 by other methods [12– 14]. There is a possible reason for the presence of the Li0.3V2O5 impurity. In the precursor, the ratio of Li/V should be under 1:3 at some local areas, because the citric acid is not completely complexed with the metal ions, which leads to insufficient Li+ ion diffusion into the V3O8 layers. Fig. 1(a) shows that the four patterns are mostly similar to each other. The main difference is the relative intensity of some peaks of LiV3O8 and Li0.3V2O5. When the sintered temperature was increased to 500 °C, the diffraction peaks of the samples became sharper and higher than for other samples, which indicates an increase in the crystallinity and grain size of the powders. The diffraction peaks of the sample sintered at 550 °C are relatively mild. The presence of the KCl (JCPDS No. 41-1476) phase was detected in the samples of the mixed electrode, as shown in the inset of Fig. 1(b). We previously noted that there was 20 wt% eutectic salt in the cathode powder of the mixed electrode, so it is natural for the presence of KCl and LiCl phases. The disappearance of the LiCl phase may be because the percentage of LiCl is low in the cathode powder. Fig. 1(c) shows the XRD patterns of the adsorbed electrode. In contrast with Fig. 1(b), the diffractions peaks of Li0.3V2O5 are not present and the diffractions peaks of LiVO3 (JCPDS No. 70-1545) emerge. This phenomenon indicates that LiVO3 may be produced by Li0.3V2O5 with LiCl in the eutectic salt, so the molar ratio of LiVO3 should coincide with that of Li0.3V2O5. The diffraction peaks of LiVO3 also confirm this assumption.
Figs. 2–4 show the SEM images of the sintered samples and adsorbed electrode. The sintered samples shown in Fig. 2 indicate that the morphology and size of the as-synthesized LiV3O8 are significantly influenced by the heat-treatment temperature. Pristine LVO400 appears as large numbers of smaller particles with an undeveloped crystalline shape, exhibiting a wide size distribution. In contrast, LVO450 exhibits a relatively regular geometric shape and even size distribution. Moreover, the rod-like grains show a gradual increase in dimension with increasing temperature and achieve a maximum at 500 °C. LVO550 shows small rod-like crystals which may be produced by the broken grains, consistent with the changes in diffraction peak intensity observed in the XRD patterns.. The SEM images of the mixed electrode are shown in Fig. 3, which is composited of 80 wt% active material and 20 wt% eutectic salt. The cathode and eutectic salt can be roughly distinguished by the surface morphology of the particle. There are some large particles of eutectic salt that may be caused by the agglomerated eutectic salt in a wet air atmosphere. Fig. 4 shows the morphology of the adsorbed electrode specimen. The morphology of LVO400A is different from the other adsorbed samples, may be because this sample was produced by ball milling for a long time. The definition of adsorbed images is lower than the sintered samples since the sintered sample is higher conductible than the eutectic salt coated on the surface of the particle. This also indicates that the sintered samples were completely coated with eutectic salt. Moreover, the crystal grains of the adsorbed electrode are similar to the sintered samples, except for LVO400A. 3.3. Open circuit potential (OCP) The single cells were introduced into a home-built instrument set to 500 °C. During heating, the OCP value starts to increase and maintains a constant value for a short duration, then, the value decreases in the continuing process. The peak value of OCP was recorded and is shown in Table 1.
Fig. 2. SEM images of the sintered samples: LVO400, LVO450, LVO500 and LVO550.
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Fig. 3. SEM images of the mixed electrode: LVO400M, LVO450M, LVO500M and LVO550M.
Fig. 4. SEM images of the adsorbed electrode: LVO400A, LVO450A, LVO500A and LVO550A.
By combining with XRD analysis, we find that the peak value is seriously affected by the composition and its percentage. Among mixed oxide materials with multi redox systems, LiV3O8 has an appreciable advantage due to the preferential reduction of V5+ over V4+ during the oxidation of the lithium anode in the first stage of discharge, followed by a dual redox of V5+→V4+ and V4+→V3+ reactions in Li0.3V2O5. Thus, the content of Li0.3V2O5 may affect the OCP of the cathode materials, and lead to the difference on the peak value of the OCP. It can be expected that the higher voltage may bring about a longer working time
Table 1 Peak value of OCP. Sample
Voltage (V)
Sample
Voltage (V)
LVO400M LVO450M LVO500M LVO550M
3.25 3.51 3.53 3.29
LVO400A LVO450A LVO500A LVO550A
3.38 3.39 3.38 3.37
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Fig. 5. Discharge curves of (a) LVO400M, (b) LVO400A, (c) LVO450M, (d) LVO450A, (e) LVO500M, (f) LVO500A, (g) LVO550M and (h) LVO550A cathode materials at 500 °C.
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of the increased contact area of the reaction. The network structure is excellent for Li+ fast diffusion, thus the adsorbed electrode exhibits more stable discharge performance. Overall, the adsorbed electrode can be regarded as a promising cathode material if its initial voltage spikes can be eliminated. According to the previous analysis, LVO450M, LVO500M, LVO450A and LVO500A can be used as promising cathodes for small current densities (100 mA/cm2) and medium durations (4–12 min) in thermal battery applications. LVO450M, LVO450A and LVO500A can be used as potential cathodes for medium current densities (200 mA/cm2) and short durations (1–4 min) in thermal battery applications. LiV3O8 treated by the traditional method is not fit to be used for large current density thermal batteries. Whereas, the adsorbed electrode can be used for large current densities (300 mA/cm2), high potentials (2.5–2.0 V) and short duration (30–70 s) thermal batteries.
Table 2 Specific capacity of the single cells. Current density
100 mA/cm2
200 mA/cm2
300 mA/cm2
LVO400M LVO450M LVO500M LVO550M LVO400A LVO450A LVO500A LVO550A
205.4 301.1 260.9 201.9 255.3 248.8 267.6 280.2
156.3 243.9 166.7 205.0 191.9 207.5 205.4 191.4
188.6 205.0 182.5 128.4 199.9 157.7 190.5 183.5
mA h/g mA h/g mA h/g mA h/g mA h/g mA h/g mA h/g mA h/g
mA h/g mA h/g mA h/g mA h/g mA h/g mA h/g mA h/g mA h/g
mA h/g mA h/g mA h/g mA h/g mA h/g mA h/g mA h/g mA h/g
and higher specific capacity. The adsorbed electrode is fabricated by a mixture of LiV3O8 and LiVO3, which exhibit a stable peak value. It was mentioned previously that the LiVO3 may be produced by the reacted Li0.3V2O5 and LiCl, so the content of LiVO3 should be different in the electrode. This phenomenon indicates that the existence of LiVO3 can evidently reduce the peak value of the electrode disregard the exact percentage.
4. Conclusions We have synthesized LiV3O8 powder via a sol-gel process with citric acid as the chelating agent. The performance of the cathode materials treated by two methods was tested under the same conditions. Results show that the traditional mixed electrode LVO450M, made by the active material sintered at 450 °C, exhibits the best electrochemical performance. Compared with the traditional mixed electrode, the adsorbed electrodes exhibit comparable special capacity and more stable discharge curves, which indicate that the effective working time of the thermal battery would be prolonged. In addition, the adsorbed electrode can be used in wider situations than the mixed ones. However, the impurity of the active material changed in the adsorption process, leading to sharp initial voltage spikes. In future experiments, we should adjust the technical parameters in order to lessen the impurity content so that the true performance of LiV3O8 treated by two different methods can be evaluated.
3.4. Galvanostatic discharge properties The voltage-time curves of the samples at current densities of 100, 200 and 300 mA/cm2 are illustrated in Fig. 5 (the terminated voltage is 0.2 V). In the contrast test, the cathode powders were test at the same conditions, such as anode, electrolyte, isolation layer, collector and experimental environment. The difference in electrochemical performance should be caused by the different composition the of cathode powder. The effective area of the cell is 3.14 cm2. In addition, the mass of the active material is about 0.3 g. Then, the specific capacity of the electrode material can be calculated according to the current density and discharge time. Table 2 lists the calculated capacities of each thermal battery. The performance of the mixed electrodes was quite similar to that of the adsorbed counterpart under these conditions. Firstly, the discharge voltage and working time both decrease with increasing current density. The larger the current density is, the less the working time is. For example, LVO450 can discharge for more than 1000 s at a current density of 100 mA/cm2, and reaches a specific capacity of 301.1 mA h/g, while 429 s and 243.9 mA h/g at 200 mA/cm2. The discharge property of the adsorbed electrodes is comparable to the mixed ones. Secondly, the two cathode materials can be defined as thermal battery cathode promising candidates for both medium and small current densities in thermal battery applications, which exhibit a higher operating voltage range than commonly used disulfide cathodes. There is some difference that can be easily detected from the discharge curves. The initial voltage spikes were more obvious at the start of discharge curves of adsorbed electrodes, which means that the presence of LiVO3 is harmful to the galvanostatic discharge. In addition, the discharging time of the adsorbed electrodes is basically equal to that of the mixed electrodes, but the discharge curves of the adsorbed electrodes are more stable than those of the mixed electrodes, which indicates that the effective working time of the thermal battery is prolonged. The stable discharge curves of the adsorbed electrodes should be attributed to its structure. It is well known that there is a number of three-dimensional network structures in the gel. However, the network structure of adsorbed electrodes was filled with eutectic salt in the adsorption process. In contrast, the mixed electrode needs to experience a molten diffusion process for the eutectic salt. Combined with the previous analysis, the eutectic salt adsorbed into the gel can shorten the average trip of Li+ diffusion in discharge. Simultaneously, the current density is reduced on the surface because
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