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Structural and Electrochemical Consequences of Al and Ga Cosubstitution in Li7La3Zr2O12 Solid Electrolytes.

Rettenwander D, Redhammer G, Preishuber-Pflügl F, Cheng L, Miara L, Wagner R, Welzl A, Suard E, Doeff MM, Wilkening M, Fleig J, Amthauer G - Chem Mater (2016)

Bottom Line: The phase transition was located at an Al:Ga substitution ratio of 0.05:0.15 and is accompanied by a significant lowering of the activation energy for Li-ion transport to 0.26 eV.The increase in Ga content is also associated with better densification and smaller grains and is accompanied by a change in the area specific resistance (ASR) from 78 to 24 Ω cm(2), the lowest reported value for LLZO so far.These results illustrate that understanding the structure-properties relationships in this class of materials allows practical obstacles to its utilization to be readily overcome.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemistry and Physics of Materials, University of Salzburg , 5020, Salzburg, Austria.

ABSTRACT

Several "Beyond Li-Ion Battery" concepts such as all solid-state batteries and hybrid liquid/solid systems envision the use of a solid electrolyte to protect Li-metal anodes. These configurations are very attractive due to the possibility of exceptionally high energy densities and high (dis)charge rates, but they are far from being realized practically due to a number of issues including high interfacial resistance and difficulties associated with fabrication. One of the most promising solid electrolyte systems for these applications is Al or Ga stabilized Li7La3Zr2O12 (LLZO) based on high ionic conductivities and apparent stability against reduction by Li metal. Nevertheless, the fabrication of dense LLZO membranes with high ionic conductivity and low interfacial resistances remains challenging; it definitely requires a better understanding of the structural and electrochemical properties. In this study, the phase transition from garnet (Ia3̅d, No. 230) to "non-garnet" (I4̅3d, No. 220) space group as a function of composition and the different sintering behavior of Ga and Al stabilized LLZO are identified as important factors in determining the electrochemical properties. The phase transition was located at an Al:Ga substitution ratio of 0.05:0.15 and is accompanied by a significant lowering of the activation energy for Li-ion transport to 0.26 eV. The phase transition combined with microstructural changes concomitant with an increase of the Ga/Al ratio continuously improves the Li-ion conductivity from 2.6 × 10(-4) S cm(-1) to 1.2 × 10(-3) S cm(-1), which is close to the calculated maximum for garnet-type materials. The increase in Ga content is also associated with better densification and smaller grains and is accompanied by a change in the area specific resistance (ASR) from 78 to 24 Ω cm(2), the lowest reported value for LLZO so far. These results illustrate that understanding the structure-properties relationships in this class of materials allows practical obstacles to its utilization to be readily overcome.

No MeSH data available.


Related in: MedlinePlus

(a) Crystalstructure of cubic LLZO with space group Ia3̅d (No. 230). Blue dodecahedra (24c) areoccupied by La3+, green octahedra (16a) by Zr4+. Li+ are distributed overthree sites, viz., tetrahedrally coordinated (24d) sites represented by red spheres, octahedrally coordinated (48g) sites represented by yellow spheres, and distorted 4-foldcoordinated (96h) sites represented by orange spheres.The corresponding Li-ion diffusion pathway is shown in (b). (c) Crystalstructure of cubic LLZO with space group I4̅3d (No. 220). Blue dodecahedra (24d) areoccupied by La3+, green octahedra (16c) by Zr4+. Li+ are distributed over three sites,two tetrahedrally coordinated sites 12a and 12b (equivalent to 24d in Ia3̅d)) represented by red and orange spheres,respectively, and octahedrally coordinated (48e) sites representedby yellow spheres. The corresponding Li-ion diffusion pathway is shownin (d).
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fig1: (a) Crystalstructure of cubic LLZO with space group Ia3̅d (No. 230). Blue dodecahedra (24c) areoccupied by La3+, green octahedra (16a) by Zr4+. Li+ are distributed overthree sites, viz., tetrahedrally coordinated (24d) sites represented by red spheres, octahedrally coordinated (48g) sites represented by yellow spheres, and distorted 4-foldcoordinated (96h) sites represented by orange spheres.The corresponding Li-ion diffusion pathway is shown in (b). (c) Crystalstructure of cubic LLZO with space group I4̅3d (No. 220). Blue dodecahedra (24d) areoccupied by La3+, green octahedra (16c) by Zr4+. Li+ are distributed over three sites,two tetrahedrally coordinated sites 12a and 12b (equivalent to 24d in Ia3̅d)) represented by red and orange spheres,respectively, and octahedrally coordinated (48e) sites representedby yellow spheres. The corresponding Li-ion diffusion pathway is shownin (d).

Mentions: LLZO garnetscrystallize in a highly conductive cubic modification(SG: Ia3̅d, No. 230)5 and a less conductive tetragonal polymorph (spacegroup (SG): I41/acd,No. 142).6 The former is stabilized atroom temperature (RT) by supervalent substitution at the Li, La, orZr position in LLZO.7,8 The most promising, and extensivelystudied, supervalent cations are Al and Ga, generally substitutedon the Li sites.7−9 Much experimental and theoretical effort has beenexpended to elucidate the site preferences of Al and Ga and theirinfluence on Li-ion dynamics/conduction in LLZO garnets.10 Additionally, it has been shown that the Li-ionconductivity of LLZO stabilized with Ga is twice that compared toLLZO stabilized with Al.3,11−17 In order to understand this behavior better, cubic LLZO was synthesizedby simultaneous substitution of Al and Ga in different ratios.10 In the corresponding 7Li NMR lineshape measurements an increase in Li-ion dynamics with increasingGa is observed, as yet the origin of this phenomenon remains, however,unexplained.10 A possible explanation wasfound by some recent investigations on single crystals of Li7–3xAlxLa3Zr2O12, with x = 0.1–0.4 andLi7–3yGayLa3Zr2O12, with y = 0.1–0.6 by means of single crystal X-ray diffraction (SC-XRD).18 It was demonstrated that Ga-stabilized LLZOcrystallizes in the acentric “non-garnet” cubic spacegroup I4̅3d, No. 220, in contrastto LLZO (see Figure 1 for structural details).18


Structural and Electrochemical Consequences of Al and Ga Cosubstitution in Li7La3Zr2O12 Solid Electrolytes.

Rettenwander D, Redhammer G, Preishuber-Pflügl F, Cheng L, Miara L, Wagner R, Welzl A, Suard E, Doeff MM, Wilkening M, Fleig J, Amthauer G - Chem Mater (2016)

(a) Crystalstructure of cubic LLZO with space group Ia3̅d (No. 230). Blue dodecahedra (24c) areoccupied by La3+, green octahedra (16a) by Zr4+. Li+ are distributed overthree sites, viz., tetrahedrally coordinated (24d) sites represented by red spheres, octahedrally coordinated (48g) sites represented by yellow spheres, and distorted 4-foldcoordinated (96h) sites represented by orange spheres.The corresponding Li-ion diffusion pathway is shown in (b). (c) Crystalstructure of cubic LLZO with space group I4̅3d (No. 220). Blue dodecahedra (24d) areoccupied by La3+, green octahedra (16c) by Zr4+. Li+ are distributed over three sites,two tetrahedrally coordinated sites 12a and 12b (equivalent to 24d in Ia3̅d)) represented by red and orange spheres,respectively, and octahedrally coordinated (48e) sites representedby yellow spheres. The corresponding Li-ion diffusion pathway is shownin (d).
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Related In: Results  -  Collection

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fig1: (a) Crystalstructure of cubic LLZO with space group Ia3̅d (No. 230). Blue dodecahedra (24c) areoccupied by La3+, green octahedra (16a) by Zr4+. Li+ are distributed overthree sites, viz., tetrahedrally coordinated (24d) sites represented by red spheres, octahedrally coordinated (48g) sites represented by yellow spheres, and distorted 4-foldcoordinated (96h) sites represented by orange spheres.The corresponding Li-ion diffusion pathway is shown in (b). (c) Crystalstructure of cubic LLZO with space group I4̅3d (No. 220). Blue dodecahedra (24d) areoccupied by La3+, green octahedra (16c) by Zr4+. Li+ are distributed over three sites,two tetrahedrally coordinated sites 12a and 12b (equivalent to 24d in Ia3̅d)) represented by red and orange spheres,respectively, and octahedrally coordinated (48e) sites representedby yellow spheres. The corresponding Li-ion diffusion pathway is shownin (d).
Mentions: LLZO garnetscrystallize in a highly conductive cubic modification(SG: Ia3̅d, No. 230)5 and a less conductive tetragonal polymorph (spacegroup (SG): I41/acd,No. 142).6 The former is stabilized atroom temperature (RT) by supervalent substitution at the Li, La, orZr position in LLZO.7,8 The most promising, and extensivelystudied, supervalent cations are Al and Ga, generally substitutedon the Li sites.7−9 Much experimental and theoretical effort has beenexpended to elucidate the site preferences of Al and Ga and theirinfluence on Li-ion dynamics/conduction in LLZO garnets.10 Additionally, it has been shown that the Li-ionconductivity of LLZO stabilized with Ga is twice that compared toLLZO stabilized with Al.3,11−17 In order to understand this behavior better, cubic LLZO was synthesizedby simultaneous substitution of Al and Ga in different ratios.10 In the corresponding 7Li NMR lineshape measurements an increase in Li-ion dynamics with increasingGa is observed, as yet the origin of this phenomenon remains, however,unexplained.10 A possible explanation wasfound by some recent investigations on single crystals of Li7–3xAlxLa3Zr2O12, with x = 0.1–0.4 andLi7–3yGayLa3Zr2O12, with y = 0.1–0.6 by means of single crystal X-ray diffraction (SC-XRD).18 It was demonstrated that Ga-stabilized LLZOcrystallizes in the acentric “non-garnet” cubic spacegroup I4̅3d, No. 220, in contrastto LLZO (see Figure 1 for structural details).18

Bottom Line: The phase transition was located at an Al:Ga substitution ratio of 0.05:0.15 and is accompanied by a significant lowering of the activation energy for Li-ion transport to 0.26 eV.The increase in Ga content is also associated with better densification and smaller grains and is accompanied by a change in the area specific resistance (ASR) from 78 to 24 Ω cm(2), the lowest reported value for LLZO so far.These results illustrate that understanding the structure-properties relationships in this class of materials allows practical obstacles to its utilization to be readily overcome.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemistry and Physics of Materials, University of Salzburg , 5020, Salzburg, Austria.

ABSTRACT

Several "Beyond Li-Ion Battery" concepts such as all solid-state batteries and hybrid liquid/solid systems envision the use of a solid electrolyte to protect Li-metal anodes. These configurations are very attractive due to the possibility of exceptionally high energy densities and high (dis)charge rates, but they are far from being realized practically due to a number of issues including high interfacial resistance and difficulties associated with fabrication. One of the most promising solid electrolyte systems for these applications is Al or Ga stabilized Li7La3Zr2O12 (LLZO) based on high ionic conductivities and apparent stability against reduction by Li metal. Nevertheless, the fabrication of dense LLZO membranes with high ionic conductivity and low interfacial resistances remains challenging; it definitely requires a better understanding of the structural and electrochemical properties. In this study, the phase transition from garnet (Ia3̅d, No. 230) to "non-garnet" (I4̅3d, No. 220) space group as a function of composition and the different sintering behavior of Ga and Al stabilized LLZO are identified as important factors in determining the electrochemical properties. The phase transition was located at an Al:Ga substitution ratio of 0.05:0.15 and is accompanied by a significant lowering of the activation energy for Li-ion transport to 0.26 eV. The phase transition combined with microstructural changes concomitant with an increase of the Ga/Al ratio continuously improves the Li-ion conductivity from 2.6 × 10(-4) S cm(-1) to 1.2 × 10(-3) S cm(-1), which is close to the calculated maximum for garnet-type materials. The increase in Ga content is also associated with better densification and smaller grains and is accompanied by a change in the area specific resistance (ASR) from 78 to 24 Ω cm(2), the lowest reported value for LLZO so far. These results illustrate that understanding the structure-properties relationships in this class of materials allows practical obstacles to its utilization to be readily overcome.

No MeSH data available.


Related in: MedlinePlus