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Synthesis of tetragonal solid-state electrolyte Li7La3Zr2O12
Materials Today: Proceedings xxx (xxxx) xxx
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Synthesis of tetragonal solid-state electrolyte Li7La3Zr2O12 D.S. Aleksandrov a,b,⇑, A.A. Popovich b, W. Qingsheng a, P.A. Novikov a,b a b
CHN/RUS New Energy and Material Technology Research Institute, Changxing 313100, China Peter the Great St. Petersburg Polytechnic University, Saint Petersburg 195251, Russia
a r t i c l e
i n f o
Article history: Received 12 December 2019 Accepted 6 January 2020 Available online xxxx Keywords: Solid-state electrolyte Lithium ion battery Tetragonal structure X-ray powder diffraction Wet grinding
a b s t r a c t Solid-state electrolyte (SSE) with tetragonal structure Li7La3Zr2O12 (LLZO) is prepared by solid state reaction. Investigation of heat treatment temperature and time are observed. Increasing temperature from 700 °C to 950 °C improves structure purity and stability. Synthesis at 700 °C show appearance of La2Zr2O7 phase, however 750 °C synthesis sample material is free of that impurity. Heat treatment time exhibits the biggest sintering time provide small amount of Li2CO3 impurity. Crucible material influences by formation of Li2ZrO3 and Li2CO3 impurities at 900 °C and 6 h synthesis. Ó 2020 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of the scientific committee of the Materials Science: Composites, Alloys and Materials Chemistry.
1. Introduction Lithium-ion batteries (LIB) use in lot of the most popular portable devices since their commercialization almost 30 years ago by Sony in 1990 as they started a mass-production manufacturing [1]. LIB can be used in all spheres of our life: cellphones, MP3player, electric vehicles, laptops etc. Novel technologies of LIB manufacturing dictates high quality requirements, such as energy density [2–5], long cycle life [6], structure stability of electrodes materials, especially for cathode materials [6–9], big potential window studies [10,11], thin films materials synthesis technologies [5,12–16], lithium enriched materials [17], application of new materials [18,19], nanosized materials technologies [20–27], low price ($250 for 1 kWh till 2020 [28]) etc. There are several safety problems of LIB production nowadays. First of all, lithium-ion batteries use of highly flammable organic liquid electrolyte or polymer electrolyte, which has low flame point and low thermal stability. These issues can lead to fire and explosion during car driving or using your cellphone [29]. Therefore, it is recommended use all-solid-state batteries (ASSBs), because ASSB use inorganic solid-state electrolyte (SSE) instead of liquid highly flammable organic electrolyte. Besides, all-solidstate batteries have another advantages: 1) decreasing of dead weight of lithium-ion battery and increasing of energy density as a result; 2) compare to liquid organic electrolyte, solid inorganic ⇑ Corresponding author at: Peter the Great St. Petersburg Polytechnic University, Saint Petersburg 195251, Russia. E-mail address: [email protected] (D.S. Aleksandrov).
electrolyte has higher electrochemical stability, wide electrochemical stability window (higher than liquid electrolyte has) and can be used with higher potential cathode materials to increase energy density; 3) all-solid-state batteries have the best mechanical properties between all of lithium-ion batteries. Working principle of all-solid-state battery is same as basic lithium-ion battery has. Discharging is occurred, when Li-ions are deintercalated from anode and diffuses through anodeelectrolyte interface, electrolyte and electrolyte-cathode interface into cathode material, and electrons flows through external circuit, e.g. electric engine of EV, lamp in a flashlight, for the screen lightning of phone etc. During charging, these processes are reversed. Main functions of solid-state electrolyte are the same as liquid electrolyte and separator have: SSE allows diffusion of Li-ions between cathode and anode, prevent electron diffusion, and, as a result, prevent short circuit. Therefore, there are some requirements for solid-state electrolyte in Li-ion batteries [30]: High ionic conductivity (nearly 10 4 10 3 Sm cm 1) at room temperature; Prevent short circuit, i.e. SSE can’t have electron conductivity; Have wide potential window.
1.1. General characteristics of solid-state electrolyte There are several types of solid-state electrolyte with different structure for all-solid-state battery, including garnet, perovskite, LISICON, LiPON, Li3N, sulfide etc.
https://doi.org/10.1016/j.matpr.2020.01.142 2214-7853/Ó 2020 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of the scientific committee of the Materials Science: Composites, Alloys and Materials Chemistry.
Please cite this article as: D. S. Aleksandrov, A. A. Popovich, W. Qingsheng et al., Synthesis of tetragonal solid-state electrolyte Li7La3Zr2O12, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.142
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This paper is about solid-state electrolyte with garnet-type structure Li7La3Zr2O12, because it is one of the most promising solid-state electrolyte for ASSBs and mass production. Common formula of garnets is A3B2(C2+ 3 ) O12, where A = Ca, Fe etc., B = Al, Cr etc., C = Si, As, V etc., it’s space group is m3m (4/m 3 2/m): Ia3d [31]. Garnet-type composition of Li3Ln3M2O12 (M = Te, W; Ln = Y, Pr, Nd, Sm, Eu, Gd etc.) with Li content were firstly discovered at 1968 [32]. Next development step of solid-state electrolytes with garnettype structure have been study compositions of Li7La3Zr2O12. This composition has cubic (space group is Ia-3d) [30,33] and tetragonal (space group is I41/acd) structures. Cubic structure of LLZO is shown at Fig. 1(a). Cubic structure of LLZO has two types of Li atoms: Li1 and Li2 (Fig. 1b). It consists of octahedral ZrO6 and dodecahedral LaO8. Li1 and Li2 atoms occupy tetrahedral and distorted octahedral sites, and their total occupation is 0.35 and 0.94, respectively. Hence, local framework structure of two Li2 atoms may be shaped as LiO4 [35]. Thus, disordering and partial occupation of Li2 atoms was reported as a key role of Li-ion conduction in this type of Li-ion conductor and provide high ion conductivity almost 1 mSm cm 1 [36]. Li2 site occupancy of Li7La3Zr2O12 and cubic garnet-type structures, such as Li6SrLa2Nb2O12 and Li6CaLa2Nb2O12 are in a rough agreement [34,37]. Despite this, Li1 atoms site occupation (0.94) in composition of Li7La3Zr2O12 is the biggest between all known solid-state electrolytes with garnet-related structure. It means, that the biggest content of Li atoms with amount of 7 in structure of Li7La3Zr2O12 just increase occupation of Li1 sites. Li1 and Li2 types of atoms in cubic Li7La3Zr2O12 connected with each other and construct a loop. This loop links to another one, where only the Li1 site was shared by two loops as a junction, which forms a three-dimensional network (Fig. 1c, e), that allows Li-ion diffusion in three directions. Thus, this framework of Li ions provides high ion conductivity of cubic Li7La3Zr2O12. Tetragonal Li7La3Zr2O12 (Fig. 2) has three types of Li atoms: Li1, Li2 and Li3. Tetrahedral Li1O4 and distorted octahedral Li2O6 shared with the face, which result in the very short Li-Li distance, because of full occupation of tetrahedral sites (occupied by Li1 atom and vacancy) and distorted octahedral sites (occupied by Li2 and Li3 atoms). Therefore, ion conductivity of tetragonal Li7La3Zr2O12 is much lower, then cubic structure has. However, ion conductivity of cubic Li7La3Zr2O12 is lower compared to liquid electrolytes because of crystal structure, grain
Fig. 2. Crystal structure of tetragonal Li7La3Zr2O12. Square is one of the unit cell planes [35].
boundaries and high synthesis temperature of solid-state electrolyte. Hence, some suggestions for doping elements are occurred to improve total Li-ion conductivity of these types of electrolytes. Thus, a lot of works are related for doping with next elements: Ta5+ [37,39,46,49], Te6+ [39], Nb5+ [44], Al3+ [37–42], Ti4+ [43], Ga3+[37,40], Ca2+ [45,46], Mg2+ [47,50], Sc3+ [47], Zn2+ [47], Mo6+ [47,48], Ba2+ [49,50] etc. 2. Materials and methods 2.1. Analysis methods X-ray phase analysis have been made by BRUKER D8 ADVANCE with vertical goniometer, Cu-Ka radiation (wave length 1.5406 Å), diffraction angles 2h = 10–60° with angle approximately 0.02°. Atoms coordinates data for quantitative phase analysis have been used from [35]. DIFFRAC.EVA and TOPAS5 software have been used for pattern determination. Images of tetragonal LLZO powder have been made with scanning electron microscope model TESCAN MAIA 3 with ultrahigh
Fig. 1. (a) Crystal structure of cubic Li7La3Zr2O12; (b) coordination polyhedra around the Li1 and Li2 [34]; loop structure consisted of Li atoms in cubic (c) and tetragonal (d) structures in Li7La3Zr2O12; (e) diffusion scheme of Li atoms in cubic Li7La3Zr2O12. ‘‘g” is site occupation of each type of Li atoms [34].
Please cite this article as: D. S. Aleksandrov, A. A. Popovich, W. Qingsheng et al., Synthesis of tetragonal solid-state electrolyte Li7La3Zr2O12, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.142
D.S. Aleksandrov et al. / Materials Today: Proceedings xxx (xxxx) xxx
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2.2. Synthesis description
Fig. 3. XRD pattern of solid-state electrolyte Li7La3Zr2O12 with synthesis temperature 900 °C for 6 h. Arrows shows peaks of lanthanum oxide (III).
Solid-state synthesis has been chosen as formation method of tetragonal phase of LLZO, because it is one of the cheapest methods for mass production. Initial materials are lithium carbonate Li2CO3 (Xilong Sci., 98%), lanthanum oxide La2O3 (ReLAB, 99.99%) and zirconium acetate Zr(CH3COO)4 (Sinopharm). These materials dissolved in acetic acid and magnetic stirred during 12 h with 60– 80 °C to get homogeneous solution. Then after evaporation of acid, dried precursor milled at agate mortar, then in milling machine and dried again in vacuum furnace. Heat treatment had next parameters: heated till 400 °C (heat rate is 1 °C/min) for 2 h in a muffle furnace. Afterward the temperature was slowly increased to 900 °C and maintained for 6 h to produce calcined powders. Electrolyte of this precursor can be produced from this chemical formula:
4ZrO2 + 3La2 O3 + 7Li2 CO3 = 2Li7 La3 Zr2 O12 + 7CO2 Synthesis methods described in this work allow to produce garnet-type tetragonal Li7La3Zr2O12 type of solid-state electrolyte. Generally, solid-state synthesis is used for this type of electrolyte and for compounds of this electrolyte, but sol-gel method also can be used. Solid-state synthesis requires high temperature and long time, also synthesized powder may have a lot of undesirable pores. Thus, it is possible to use another method, such as spark plasma synthesis (SPS), hot pressing etc. By comparing synthesis methods and results of this work, it can be resulted as: Lithium loss is high during synthesis, that lead oxides formation of remaining system elements Different phases – cubic and tetragonal – can be formed depending on temperature and synthesis time. 3. Results and discussion
Fig. 4. Influence of synthesis temperature on electrolyte structure. All samples heated for 6 h.
resolution mode, landing energy 10 keV with detector of secondary electrons. Synthesized powder milled with wet grinding by NETZSCH MiniCer.
Synthesis parameters for high purity electrolyte (98.5% mass) are the next: heat till 400 °C for 2 h (heat rate is 1 °C/min), heat till 900 °C for 6 h (heat rate 5 °C/min). XRD pattern of electrolyte powder after synthesis with these parameters is shown on Fig. 3. Solid-state synthesis of electrolyte carried out in air atmosphere with different synthesis parameters (time, temperature, crucible material). Influence of different synthesis temperatures on electrolyte structure is shown on Fig. 4. Electrolyte phase can’t be formed at 700 °C, but it is started forming at 750 °C (with impurity of lanthanum oxide) till 900 °C incl., and besides quantity of electrolyte phase increasing linearly for 1–1.5%
Fig. 5. Influence of crucible material (a) and calcined time (b) on structure of Li7La3Zr2O12 heated at 900 °C.
Please cite this article as: D. S. Aleksandrov, A. A. Popovich, W. Qingsheng et al., Synthesis of tetragonal solid-state electrolyte Li7La3Zr2O12, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.142
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Fig. 6. SEM images of polyhedral electrolyte particles (a) and electrolyte particles with impurity particles of La2O3.
Fig. 7. XRD patterns (a) and SEM images after 1 h (b), 2 h (c) and 3 h (d) of wet grinding of solid-state electrolyte Li7La3Zr2O12.
(mass) with every 50 °C and achieve maximum at 900 °C with 98% of tetragonal Li7La3Zr2O12 and impurity of lanthanum oxide (III) La2O3. Also, crucible material influences on electrolyte structure. Crucible made of Ni, Al2O3 and SiO2 used in this work. XRD patterns of synthesis with different crucible materials are shown at Fig. 5a. Samples from Al2O3 and SiO2 have more impurities in comparison with Ni crucible, such as Li2CO3 and Li2ZrO3. Thus, synthesis at Ni crucible allow to get 98% pure tetragonal Li7La3Zr2O12 with La2O3 impurity. Synthesis at Al2O3 and SiO2 crucibles electrolyte phase quantity is less than 80%. Structure of
electrolyte with different calcined time (Fig. 5b) changed slightly: from 96% at 3 h to 98% at 6 h synthesis time. Morphology of electrolyte particles is shown on Fig. 6. Polyhedral particles are electrolyte particles, laminated are particles of La2O3. Amount of 80% particles has sizes 1–3 lm. Synthesized solid-state electrolyte wet grinded for 1, 2 and 3 h. XRD patterns and SEM images of milled electrolyte are shown on Fig. 7. Structure of electrolyte became partially amorphous after wet grinding. Amorphicity grade increases with milling time (peak intensity decreasing with milling time). SEM images shows the difference between 1, 2 and 3 h of milling. Actually, there is no
Please cite this article as: D. S. Aleksandrov, A. A. Popovich, W. Qingsheng et al., Synthesis of tetragonal solid-state electrolyte Li7La3Zr2O12, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.142
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big difference between 2 and 3 h (Fig. 7c and d) of milling in comparison with 1 and 2 h of milling (Fig. 7b and c). 4. Conclusions This article is about synthesis investigation of tetragonal Li7La3Zr2O12 solid-state electrolyte for lithium ion batteries. Crucible material, calcined temperature and time were studied to develop their influence on material structure and quantity of impurities. Scanning electron microscopy technique showed particles morphology, its’ sizes and impurities morphology. Also, wet grinding influence on material structure at different grinding time was studied. Declaration of Competing Interest 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. Acknowledgements We thank ENV Zhejiang Changxing CHN/RUS New Energy and Material Technology Research Institute for their laboratories, materials, funding and help through the all steps of research. References [1] Z. Li, J. Huang, B.Y. Liaw, V. Metzler, J. Zhang, J. Power Sources 254 (2014) 168– 182. [2] A.A. Popovich, M.Y. Maximov, A.M. Rumyantsev, P.A. Novikov, Russ. J. Appl. Chem. 88 (5) (2015) 898–899. [3] Y. Zhao, L. Wang, L. Huang, M. Maximov, M. Jin, Y. Zhang, G. Zhou, Nanomaterials 7 (11) (2017) 402. [4] Y. Zhao, Z. Liu, Y. Zhang, A. Mentbayeva, X. Wang, M.Y. Maximov, F. Yin, Nanoscale Res. Lett. 12 (1) (2017) 459. [5] D.V. Nazarov, M.Y. Maximov, P.A. Novikov, A.A. Popovich, A.O. Silin, V.M. Smirnov, A.M. Rumyantsev, J. Vacuum Sci. Technol. A: Vacuum Surf Films 35 (1) (2017) 01B137. [6] A.A. Popovich, M.Y. Maximov, P.A. Novikov, A.O. Silin, D.V. Nazarov, A.M. Rumyantsev, Russ. J. Appl. Chem. 89 (4) (2016) 679–681. [7] Q.S. Wang, Y.Y. Bao, L.Y. Zheng, Z.L. Yang, A.A. Popovich, P.A. Novikov, A.O. Silin, Gongneng Cailiao 45 (13) (2014), 13098-1310113107. [8] A.A. Popovich, P.A. Novikov, A.O. Silin, N.G. Razumov, Q.S. Wang, Russ. J. Appl. Chem. 87 (9) (2014) 1268–1273. [9] P. Novikov, A. Silin, Q.S. Wang, A. Popovich, in: Advanced Materials Research, Trans Tech Publications, 2015, pp. 132–136. [10] K.A. Pushnitsa, A.E. Kim, A.A. Popovich, Q. Wang, P.A. Novikov, J. Electron. Mater. 48 (10) (2019) 6694–6699. [11] P.A. Novikov, A.E. Kim, K.A. Pushnitsa, W. Quingsheng, M.Y. Maksimov, A.A. Popovich, Russ. J. Appl. Chem. 92 (7) (2019) 1013–1019. [12] Q. Wang, A.A. Popovich, V.V. Zhdanov, P.A. Novikov, M.Y. Maximov, Y.M. Koshtyal, A.O. Silin, Russ. J. Appl. Chem. 91 (1) (2018) 53–57. [13] M.Y. Maximov, P.A. Novikov, D.V. Nazarov, A.M. Rymyantsev, A.O. Silin, Y. Zhang, A.A. Popovich, J. Electron. Mater. 46 (11) (2017) 6571–6577.
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Please cite this article as: D. S. Aleksandrov, A. A. Popovich, W. Qingsheng et al., Synthesis of tetragonal solid-state electrolyte Li7La3Zr2O12, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.142
Contents lists available at ScienceDirect
Materials Today: Proceedings journal homepage: www.elsevier.com/locate/matpr
Synthesis of tetragonal solid-state electrolyte Li7La3Zr2O12 D.S. Aleksandrov a,b,⇑, A.A. Popovich b, W. Qingsheng a, P.A. Novikov a,b a b
CHN/RUS New Energy and Material Technology Research Institute, Changxing 313100, China Peter the Great St. Petersburg Polytechnic University, Saint Petersburg 195251, Russia
a r t i c l e
i n f o
Article history: Received 12 December 2019 Accepted 6 January 2020 Available online xxxx Keywords: Solid-state electrolyte Lithium ion battery Tetragonal structure X-ray powder diffraction Wet grinding
a b s t r a c t Solid-state electrolyte (SSE) with tetragonal structure Li7La3Zr2O12 (LLZO) is prepared by solid state reaction. Investigation of heat treatment temperature and time are observed. Increasing temperature from 700 °C to 950 °C improves structure purity and stability. Synthesis at 700 °C show appearance of La2Zr2O7 phase, however 750 °C synthesis sample material is free of that impurity. Heat treatment time exhibits the biggest sintering time provide small amount of Li2CO3 impurity. Crucible material influences by formation of Li2ZrO3 and Li2CO3 impurities at 900 °C and 6 h synthesis. Ó 2020 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of the scientific committee of the Materials Science: Composites, Alloys and Materials Chemistry.
1. Introduction Lithium-ion batteries (LIB) use in lot of the most popular portable devices since their commercialization almost 30 years ago by Sony in 1990 as they started a mass-production manufacturing [1]. LIB can be used in all spheres of our life: cellphones, MP3player, electric vehicles, laptops etc. Novel technologies of LIB manufacturing dictates high quality requirements, such as energy density [2–5], long cycle life [6], structure stability of electrodes materials, especially for cathode materials [6–9], big potential window studies [10,11], thin films materials synthesis technologies [5,12–16], lithium enriched materials [17], application of new materials [18,19], nanosized materials technologies [20–27], low price ($250 for 1 kWh till 2020 [28]) etc. There are several safety problems of LIB production nowadays. First of all, lithium-ion batteries use of highly flammable organic liquid electrolyte or polymer electrolyte, which has low flame point and low thermal stability. These issues can lead to fire and explosion during car driving or using your cellphone [29]. Therefore, it is recommended use all-solid-state batteries (ASSBs), because ASSB use inorganic solid-state electrolyte (SSE) instead of liquid highly flammable organic electrolyte. Besides, all-solidstate batteries have another advantages: 1) decreasing of dead weight of lithium-ion battery and increasing of energy density as a result; 2) compare to liquid organic electrolyte, solid inorganic ⇑ Corresponding author at: Peter the Great St. Petersburg Polytechnic University, Saint Petersburg 195251, Russia. E-mail address: [email protected] (D.S. Aleksandrov).
electrolyte has higher electrochemical stability, wide electrochemical stability window (higher than liquid electrolyte has) and can be used with higher potential cathode materials to increase energy density; 3) all-solid-state batteries have the best mechanical properties between all of lithium-ion batteries. Working principle of all-solid-state battery is same as basic lithium-ion battery has. Discharging is occurred, when Li-ions are deintercalated from anode and diffuses through anodeelectrolyte interface, electrolyte and electrolyte-cathode interface into cathode material, and electrons flows through external circuit, e.g. electric engine of EV, lamp in a flashlight, for the screen lightning of phone etc. During charging, these processes are reversed. Main functions of solid-state electrolyte are the same as liquid electrolyte and separator have: SSE allows diffusion of Li-ions between cathode and anode, prevent electron diffusion, and, as a result, prevent short circuit. Therefore, there are some requirements for solid-state electrolyte in Li-ion batteries [30]: High ionic conductivity (nearly 10 4 10 3 Sm cm 1) at room temperature; Prevent short circuit, i.e. SSE can’t have electron conductivity; Have wide potential window.
1.1. General characteristics of solid-state electrolyte There are several types of solid-state electrolyte with different structure for all-solid-state battery, including garnet, perovskite, LISICON, LiPON, Li3N, sulfide etc.
https://doi.org/10.1016/j.matpr.2020.01.142 2214-7853/Ó 2020 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of the scientific committee of the Materials Science: Composites, Alloys and Materials Chemistry.
Please cite this article as: D. S. Aleksandrov, A. A. Popovich, W. Qingsheng et al., Synthesis of tetragonal solid-state electrolyte Li7La3Zr2O12, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.142
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This paper is about solid-state electrolyte with garnet-type structure Li7La3Zr2O12, because it is one of the most promising solid-state electrolyte for ASSBs and mass production. Common formula of garnets is A3B2(C2+ 3 ) O12, where A = Ca, Fe etc., B = Al, Cr etc., C = Si, As, V etc., it’s space group is m3m (4/m 3 2/m): Ia3d [31]. Garnet-type composition of Li3Ln3M2O12 (M = Te, W; Ln = Y, Pr, Nd, Sm, Eu, Gd etc.) with Li content were firstly discovered at 1968 [32]. Next development step of solid-state electrolytes with garnettype structure have been study compositions of Li7La3Zr2O12. This composition has cubic (space group is Ia-3d) [30,33] and tetragonal (space group is I41/acd) structures. Cubic structure of LLZO is shown at Fig. 1(a). Cubic structure of LLZO has two types of Li atoms: Li1 and Li2 (Fig. 1b). It consists of octahedral ZrO6 and dodecahedral LaO8. Li1 and Li2 atoms occupy tetrahedral and distorted octahedral sites, and their total occupation is 0.35 and 0.94, respectively. Hence, local framework structure of two Li2 atoms may be shaped as LiO4 [35]. Thus, disordering and partial occupation of Li2 atoms was reported as a key role of Li-ion conduction in this type of Li-ion conductor and provide high ion conductivity almost 1 mSm cm 1 [36]. Li2 site occupancy of Li7La3Zr2O12 and cubic garnet-type structures, such as Li6SrLa2Nb2O12 and Li6CaLa2Nb2O12 are in a rough agreement [34,37]. Despite this, Li1 atoms site occupation (0.94) in composition of Li7La3Zr2O12 is the biggest between all known solid-state electrolytes with garnet-related structure. It means, that the biggest content of Li atoms with amount of 7 in structure of Li7La3Zr2O12 just increase occupation of Li1 sites. Li1 and Li2 types of atoms in cubic Li7La3Zr2O12 connected with each other and construct a loop. This loop links to another one, where only the Li1 site was shared by two loops as a junction, which forms a three-dimensional network (Fig. 1c, e), that allows Li-ion diffusion in three directions. Thus, this framework of Li ions provides high ion conductivity of cubic Li7La3Zr2O12. Tetragonal Li7La3Zr2O12 (Fig. 2) has three types of Li atoms: Li1, Li2 and Li3. Tetrahedral Li1O4 and distorted octahedral Li2O6 shared with the face, which result in the very short Li-Li distance, because of full occupation of tetrahedral sites (occupied by Li1 atom and vacancy) and distorted octahedral sites (occupied by Li2 and Li3 atoms). Therefore, ion conductivity of tetragonal Li7La3Zr2O12 is much lower, then cubic structure has. However, ion conductivity of cubic Li7La3Zr2O12 is lower compared to liquid electrolytes because of crystal structure, grain
Fig. 2. Crystal structure of tetragonal Li7La3Zr2O12. Square is one of the unit cell planes [35].
boundaries and high synthesis temperature of solid-state electrolyte. Hence, some suggestions for doping elements are occurred to improve total Li-ion conductivity of these types of electrolytes. Thus, a lot of works are related for doping with next elements: Ta5+ [37,39,46,49], Te6+ [39], Nb5+ [44], Al3+ [37–42], Ti4+ [43], Ga3+[37,40], Ca2+ [45,46], Mg2+ [47,50], Sc3+ [47], Zn2+ [47], Mo6+ [47,48], Ba2+ [49,50] etc. 2. Materials and methods 2.1. Analysis methods X-ray phase analysis have been made by BRUKER D8 ADVANCE with vertical goniometer, Cu-Ka radiation (wave length 1.5406 Å), diffraction angles 2h = 10–60° with angle approximately 0.02°. Atoms coordinates data for quantitative phase analysis have been used from [35]. DIFFRAC.EVA and TOPAS5 software have been used for pattern determination. Images of tetragonal LLZO powder have been made with scanning electron microscope model TESCAN MAIA 3 with ultrahigh
Fig. 1. (a) Crystal structure of cubic Li7La3Zr2O12; (b) coordination polyhedra around the Li1 and Li2 [34]; loop structure consisted of Li atoms in cubic (c) and tetragonal (d) structures in Li7La3Zr2O12; (e) diffusion scheme of Li atoms in cubic Li7La3Zr2O12. ‘‘g” is site occupation of each type of Li atoms [34].
Please cite this article as: D. S. Aleksandrov, A. A. Popovich, W. Qingsheng et al., Synthesis of tetragonal solid-state electrolyte Li7La3Zr2O12, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.142
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2.2. Synthesis description
Fig. 3. XRD pattern of solid-state electrolyte Li7La3Zr2O12 with synthesis temperature 900 °C for 6 h. Arrows shows peaks of lanthanum oxide (III).
Solid-state synthesis has been chosen as formation method of tetragonal phase of LLZO, because it is one of the cheapest methods for mass production. Initial materials are lithium carbonate Li2CO3 (Xilong Sci., 98%), lanthanum oxide La2O3 (ReLAB, 99.99%) and zirconium acetate Zr(CH3COO)4 (Sinopharm). These materials dissolved in acetic acid and magnetic stirred during 12 h with 60– 80 °C to get homogeneous solution. Then after evaporation of acid, dried precursor milled at agate mortar, then in milling machine and dried again in vacuum furnace. Heat treatment had next parameters: heated till 400 °C (heat rate is 1 °C/min) for 2 h in a muffle furnace. Afterward the temperature was slowly increased to 900 °C and maintained for 6 h to produce calcined powders. Electrolyte of this precursor can be produced from this chemical formula:
4ZrO2 + 3La2 O3 + 7Li2 CO3 = 2Li7 La3 Zr2 O12 + 7CO2 Synthesis methods described in this work allow to produce garnet-type tetragonal Li7La3Zr2O12 type of solid-state electrolyte. Generally, solid-state synthesis is used for this type of electrolyte and for compounds of this electrolyte, but sol-gel method also can be used. Solid-state synthesis requires high temperature and long time, also synthesized powder may have a lot of undesirable pores. Thus, it is possible to use another method, such as spark plasma synthesis (SPS), hot pressing etc. By comparing synthesis methods and results of this work, it can be resulted as: Lithium loss is high during synthesis, that lead oxides formation of remaining system elements Different phases – cubic and tetragonal – can be formed depending on temperature and synthesis time. 3. Results and discussion
Fig. 4. Influence of synthesis temperature on electrolyte structure. All samples heated for 6 h.
resolution mode, landing energy 10 keV with detector of secondary electrons. Synthesized powder milled with wet grinding by NETZSCH MiniCer.
Synthesis parameters for high purity electrolyte (98.5% mass) are the next: heat till 400 °C for 2 h (heat rate is 1 °C/min), heat till 900 °C for 6 h (heat rate 5 °C/min). XRD pattern of electrolyte powder after synthesis with these parameters is shown on Fig. 3. Solid-state synthesis of electrolyte carried out in air atmosphere with different synthesis parameters (time, temperature, crucible material). Influence of different synthesis temperatures on electrolyte structure is shown on Fig. 4. Electrolyte phase can’t be formed at 700 °C, but it is started forming at 750 °C (with impurity of lanthanum oxide) till 900 °C incl., and besides quantity of electrolyte phase increasing linearly for 1–1.5%
Fig. 5. Influence of crucible material (a) and calcined time (b) on structure of Li7La3Zr2O12 heated at 900 °C.
Please cite this article as: D. S. Aleksandrov, A. A. Popovich, W. Qingsheng et al., Synthesis of tetragonal solid-state electrolyte Li7La3Zr2O12, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.142
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Fig. 6. SEM images of polyhedral electrolyte particles (a) and electrolyte particles with impurity particles of La2O3.
Fig. 7. XRD patterns (a) and SEM images after 1 h (b), 2 h (c) and 3 h (d) of wet grinding of solid-state electrolyte Li7La3Zr2O12.
(mass) with every 50 °C and achieve maximum at 900 °C with 98% of tetragonal Li7La3Zr2O12 and impurity of lanthanum oxide (III) La2O3. Also, crucible material influences on electrolyte structure. Crucible made of Ni, Al2O3 and SiO2 used in this work. XRD patterns of synthesis with different crucible materials are shown at Fig. 5a. Samples from Al2O3 and SiO2 have more impurities in comparison with Ni crucible, such as Li2CO3 and Li2ZrO3. Thus, synthesis at Ni crucible allow to get 98% pure tetragonal Li7La3Zr2O12 with La2O3 impurity. Synthesis at Al2O3 and SiO2 crucibles electrolyte phase quantity is less than 80%. Structure of
electrolyte with different calcined time (Fig. 5b) changed slightly: from 96% at 3 h to 98% at 6 h synthesis time. Morphology of electrolyte particles is shown on Fig. 6. Polyhedral particles are electrolyte particles, laminated are particles of La2O3. Amount of 80% particles has sizes 1–3 lm. Synthesized solid-state electrolyte wet grinded for 1, 2 and 3 h. XRD patterns and SEM images of milled electrolyte are shown on Fig. 7. Structure of electrolyte became partially amorphous after wet grinding. Amorphicity grade increases with milling time (peak intensity decreasing with milling time). SEM images shows the difference between 1, 2 and 3 h of milling. Actually, there is no
Please cite this article as: D. S. Aleksandrov, A. A. Popovich, W. Qingsheng et al., Synthesis of tetragonal solid-state electrolyte Li7La3Zr2O12, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.142
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big difference between 2 and 3 h (Fig. 7c and d) of milling in comparison with 1 and 2 h of milling (Fig. 7b and c). 4. Conclusions This article is about synthesis investigation of tetragonal Li7La3Zr2O12 solid-state electrolyte for lithium ion batteries. Crucible material, calcined temperature and time were studied to develop their influence on material structure and quantity of impurities. Scanning electron microscopy technique showed particles morphology, its’ sizes and impurities morphology. Also, wet grinding influence on material structure at different grinding time was studied. Declaration of Competing Interest 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. Acknowledgements We thank ENV Zhejiang Changxing CHN/RUS New Energy and Material Technology Research Institute for their laboratories, materials, funding and help through the all steps of research. References [1] Z. Li, J. Huang, B.Y. Liaw, V. Metzler, J. Zhang, J. Power Sources 254 (2014) 168– 182. [2] A.A. Popovich, M.Y. Maximov, A.M. Rumyantsev, P.A. Novikov, Russ. J. Appl. Chem. 88 (5) (2015) 898–899. [3] Y. Zhao, L. Wang, L. Huang, M. Maximov, M. Jin, Y. Zhang, G. Zhou, Nanomaterials 7 (11) (2017) 402. [4] Y. Zhao, Z. Liu, Y. Zhang, A. Mentbayeva, X. Wang, M.Y. Maximov, F. Yin, Nanoscale Res. Lett. 12 (1) (2017) 459. [5] D.V. Nazarov, M.Y. Maximov, P.A. Novikov, A.A. Popovich, A.O. Silin, V.M. Smirnov, A.M. Rumyantsev, J. Vacuum Sci. Technol. A: Vacuum Surf Films 35 (1) (2017) 01B137. [6] A.A. Popovich, M.Y. Maximov, P.A. Novikov, A.O. Silin, D.V. Nazarov, A.M. Rumyantsev, Russ. J. Appl. Chem. 89 (4) (2016) 679–681. [7] Q.S. Wang, Y.Y. Bao, L.Y. Zheng, Z.L. Yang, A.A. Popovich, P.A. Novikov, A.O. Silin, Gongneng Cailiao 45 (13) (2014), 13098-1310113107. [8] A.A. Popovich, P.A. Novikov, A.O. Silin, N.G. Razumov, Q.S. Wang, Russ. J. Appl. Chem. 87 (9) (2014) 1268–1273. [9] P. Novikov, A. Silin, Q.S. Wang, A. Popovich, in: Advanced Materials Research, Trans Tech Publications, 2015, pp. 132–136. [10] K.A. Pushnitsa, A.E. Kim, A.A. Popovich, Q. Wang, P.A. Novikov, J. Electron. Mater. 48 (10) (2019) 6694–6699. [11] P.A. Novikov, A.E. Kim, K.A. Pushnitsa, W. Quingsheng, M.Y. Maksimov, A.A. Popovich, Russ. J. Appl. Chem. 92 (7) (2019) 1013–1019. [12] Q. Wang, A.A. Popovich, V.V. Zhdanov, P.A. Novikov, M.Y. Maximov, Y.M. Koshtyal, A.O. Silin, Russ. J. Appl. Chem. 91 (1) (2018) 53–57. [13] M.Y. Maximov, P.A. Novikov, D.V. Nazarov, A.M. Rymyantsev, A.O. Silin, Y. Zhang, A.A. Popovich, J. Electron. Mater. 46 (11) (2017) 6571–6577.
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Please cite this article as: D. S. Aleksandrov, A. A. Popovich, W. Qingsheng et al., Synthesis of tetragonal solid-state electrolyte Li7La3Zr2O12, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.142