First-Principles Study of Chemical Stability of the Lithium Oxide

Li-ion conductivity and crystal structure of garnet-type Li6.5La3M1.5Ta0.5O12 (M ...
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First-Principles Study of Chemical Stability of the Lithium Oxide Garnets Li7La3M2O12 (M = Zr, Sn, or Hf) Sung Gu Kang and David S. Sholl* School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive, Atlanta, Georgia 30332-0100, United States ABSTRACT: Li-oxide garnet-related structures are promising solid-state Li-ion electrolytes in Li-ion batteries. However, garnet-type structures are susceptible to carbonate and hydroxide formation in environments containing gaseous CO2 and H2O. Therefore, in considering garnets for Li-ion conducting applications, chemical stability is an important issue. We examine the chemical stability of Li7La3Zr2O12, Li7La3Sn2O12, and Li7La3Hf2O12 with respect to carbonate and hydroxide formation reactions using density functional theory (DFT) calculations. From these studies, we rank the chemical stability of Li-oxide garnet-related structures against CO2 and H2O. The ranking of these materials by their chemical stability with respect to carbonate and hydroxide formation changes at higher partial pressures of CO2 and H2O.

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related fast Li-ion conductor.16 In this study, we examine the chemical stability of three garnet-related structures, Li7La3Zr2O12, Li7La3Sn2O12, and Li7La3Hf2O12, with respect to carbonate and hydroxide formation using DFT calculations. They are the only available crystalline compounds with the stoichiometry of Li7La3M2O12 (where M = tetravalent metal) in the Inorganic Crystal Structure Database (ICSD).17 Our results give information about these specific electrolyte materials and also illustrate an approach that could be used to compliment experimental studies of related materials in the future.

INTRODUCTION Solid-state Li-ion electrolytes have been considered as potential replacements for liquid organic electrolytes because of their safety and low cost.1 However, the lithium ionic conductivity of solid-state ion conductors is lower than that of liquid organic electrolytes. Therefore, it is important to improve ionic conductivity of solid-state electrolytes.2 A longstanding aim in the development of electrolytes has been to find conductors that give high Li-ion conductivity coupled with low electronic conductivity.1,2 Aside from conductivity, chemical stability is also important for electrolytes in solid-state Li-ion batteries.3 Lithium ion conduction has been studied for a wide range of crystalline metal oxides. Lithium phosphorus oxynitride (LiPON) has been used as a thin-film solid-state electrolyte, but it has low lithium ion conductivity.4−7 Li14ZnGe4O16 (LISICON) shows high lithium ion conductivity, but the conductivity decreases with time; the material is highly reactive with Li-metal, and its CO 2 stability is limited. 8 Li1.3Ti1.7Al0.3(PO4)3 (NASICON) is not stable with Li-metal.8 Perovskite-type (Li, La)TiO3 has very high lithium ion conductivity compared to other oxides, but this material is not stable against Li.9−11 Li-ion conductors with garnet-like structures have been considered as potential electrolyte materials because of their high conductivity and electrochemical stability.1,12 For example, Li5La3Nb2O12 and Li5La3Ta2O12 have been examined as Li-ion conductors.13 Li5La3Nb2O12 and Li5La3Ta2O12 have higher ionic conductivities than LiPON, Li9AlSiO8, and Li-β-alumina.13 Li7La3Zr2O12, with a cubic garnet-related-type structure, has been considered a promising solid electrolyte, because it has high Li-ion conductivity as well as high chemical stability.12 The structure of Li7La3Sn2O12 has been reported as tetragonal with a low Li-ion conductivity.14,15 Tetragonal Li7La3Hf2O12 has also been studied as a new garnet© 2014 American Chemical Society

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CALCULATION METHODS Plane wave DFT calculations were performed with the Vienna ab initio Simulation Package (VASP) using the PW91 generalized gradient approximation (GGA) functional. All calculations are done using projector augmented wave (PAW) pseudopotentials to describe core electrons.18 Plane wave basis sets are used with a cutoff of 500 eV. k-points are obtained using the Monkhorst−Pack method,19 with the number of k-points chosen to give a spacing of ∼0.028 Å−1 along the axes of the reciprocal unit cells in calculations for bulk materials. We use a single crystallographic unit cell to optimize the bulk crystal structure of each solid oxide, hydroxide, and carbonate compound we study. The initial structures for geometry relaxations are obtained from the experimental data available from ICSD.17 Geometries were relaxed until the forces on all atoms are Li7La3Zr2O12 > Li7La3Hf2O12. In the pressure range of H2O from 0.01 to 10.0 bar, the chemical stability ranking with respect to H2O is Li7La3Sn2O12 > Li7La3Hf2O12 > Li7La3Zr2O12. The differences in stability between the three materials, however, are small. Because the stabilities of the Zr-, Sn-, and Hf-containing materials are similar, the ranking of their predicted stability depends on the partial pressure of CO2 or H2O. At low CO2 pressures, Li7La3Sn2O12 has a lower critical temperature than Li7La3Zr2O12. Li7La3Sn2O12 also has a lower critical temperature than Li7La3Zr2O12 at all H2O pressures we examined. Experimentally, the tetragonal Li7La3Sn2O12 has the phase transition to the cubic phase during the Li+/H+ ion exchange.15 Another experiment with Li5La3Ta2O12 demonstrated its instability in air.34 Our results indicated that all three garnetrelated materials we investigated are unstable in ambient atmosphere. Under atmospheric dry air conditions (PCO2 = 400 ppm), the rankings of the chemical stability of Li7La3M2O12 with respect to CO2 are Li7La3Sn2O12 (T* = 817 K) > Li7La3Zr2O12 (T* = 821 K) > Li7La3Hf2O12 (T* = 835 K). The vapor pressure of water (PH2O) increases as temperature increases based on the Antoine equation,35 which gives vapor pressure of water as a function of temperature. For example, saturated water vapor has PH2O = 2.44 bar at T = 400 K. Under these conditions, the rankings of chemical stability of Li7La3M2O12 with respect to H2O are Li7La3Sn2O12 (T* = 444 K) > Li7La3Hf2O12 (T* = 452 K) > Li7La3Zr2O12 (T* = 472 K). Because the T* of these three materials are higher than the temperature we used, all three are unstable against H2O at this vapor pressure of water.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (404) 894-2822. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by funding from the Department of Energy NEUP Program.

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REFERENCES

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CONCLUSIONS In this paper, we used DFT calculations to examine the chemical stability of Li 7 La 3 Zr 2 O 12 , Li 7 La 3 Sn 2 O 12 , and Li7La3Hf2O12 with respect to carbonate and hydroxide formation reactions in environments with CO2 and H2O. The chemical stability of Li7La3Sn2O12 is higher than those of Li7La3Zr2O12 and Li7La3Hf2O12 with respect to both carbonate and hydroxide formation reactions at 1 bar of CO2 partial pressure. The chemical stabilities of Li7La3Zr2O12 and Li7La3Sn2O12, however, are very similar to each other in terms of their carbonate formation reactions at 1 bar of CO2 partial pressure. Because the chemical stabilities of the Li7La3Zr2O12, Li7La3Sn2O12, and Li7La3Hf2O12 are relatively similar, the ranking of their predicted chemical stabilities depends on the partial pressure of CO2 or H2O. Therefore, 17405

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