Elsevier

Journal of Power Sources

Volume 391, 1 July 2018, Pages 26-33
Journal of Power Sources

Stabilization of cubic Li7La3Hf2O12 by Al-doping

https://doi.org/10.1016/j.jpowsour.2018.04.066Get rights and content

Highlights

  • Cubic Li7La3Hf2O12 can be stabilized by Al3+ doping.

  • Polycrystalline Al-doped Li7La3Hf2O12 was prepared by a modified solid state method.

  • Powder consists of μ-size grains encapsulated by a glassy phase.

  • Al-doped Li7La3Hf2O12 crystallizes in garnet-related structure with cubic unit cell.

  • Total conductivity is close to values obtained for cubic Li7La3Zr2O12 garnets.

Abstract

In this paper we report on the stabilization of cubic Li7La3Hf2O12 by Al3+ doping and present a detailed crystal structure study and lithium ion conductivity measurements of the obtained compound. Polycrystalline Al-doped Li7La3Hf2O12 was prepared by a modified solid state method. The compound consists of micrometer size grains encapsulated by a glassy phase, which helps preventing the volatilization of lithium during annealing. Al-doped Li7La3Hf2O12 crystallizes in the garnet-related structure with a cubic unit cell (sp. gr. Ia3¯d (230)). A structural refinement using X-ray and neutron powder diffraction data showed that the Al3+ ions occupy only tetrahedral Li+ sites in the structure. The presence of overextended leading edges of the peaks on the XRD and NPD data is described by the introduction of an additional phase with rhombohedral distortion that occurs through a stretching of the cubic phase along the body diagonal. The activation energy as well as the total conductivity at room temperature are close to values obtained for un-doped cubic Li7La3Zr2O12 and Li7La3Hf2O12 garnets, which make Al-doped Li7La3Hf2O12 a potential candidate for the application as solid electrolyte in solid-state rechargeable lithium-ion batteries.

Introduction

In recent years, Li-containing garnets LixA3M2O12 (A = La, Y; M = Zr, Nb, Sn, Sb, Te, Hf, Ta; x = 3–7) have attracted a lot of interest mainly as potential electrolytes in solid-state lithium ion batteries [1,2]. The ionic conductivity in these compounds depends on several factors: overall Li content, polymorph structure, Li occupancy on the different sites (octahedral and tetrahedral coordination) within these structures and the Li/vacancy ratio. For example: the ionic conductivity of the LixA3M2O12 (x = 3–7) cubic garnets generally increases with increasing Li ion content [1,3,4]; Li3A3Te2O12 garnets exhibit a low conductivity of 10−5 S cm−1 at 600 °C, which is explained by the presence of only tetrahedral coordinated Li ions in the structure [1,2]; LixLa3M2O12 (M = Zr, Nb, Sn, Sb, Hf, Ta; x = 5–7) [1,2,4] garnets have Li ions in the octahedral site displaying high Li ion conduction.

One of the most extensively studied compounds of this family is lithium zirconate Li7La3Zr2O12. Two stable phases of Li7La3Zr2O12 with cubic (high temperature polymorph) and tetragonal (low temperature polymorph) unit cells have been reported [[5], [6], [7]]. Long-term high-temperature heat treatment at above 1200 °C is required for the synthesis of the cubic Li7La3Zr2O12 polymorph whereas the tetragonal phase forms at 980 °C. Cubic Li7La3Zr2O12 exhibits high bulk conductivity around 10−4 S cm−1 at room temperature, and has good thermal and chemical stability against molten lithium [5]. The tetragonal phase is thermodynamically stable relative to lithium metal, but exhibits an ionic conductivity two–three orders of magnitude lower than the cubic phase [7,8]. Both polymorphs show a similar structural motif, but in the tetragonal structure Li atoms and vacancies are ordered in the tetrahedral and octahedral sites, whereas these sites in the cubic structure have mixed occupancies [6,7].

Since cubic garnets show a higher conductivity compared to tetragonal ones a large number of studies has been devoted to the stabilization of high temperature cubic phases Li7La3M2O12 (M = Zr, Sn) [[9], [10], [11], [12], [13], [14]]. Li7La3Zr2O12 was synthesized in the absence of any dopant by H. Xie et al. [15], N. Janani et al. [16] and Kokal et al. [17] by modified sol-gel methods at low temperatures as well as by J. Awaka et al. who prepared a Li7La3Zr2O12 single crystal by a self-flux method using LiNO3 at 1150 °C [18]. The conductivity of these samples was significantly lower than that of compounds obtained by the solid-state method at high temperatures. The subsequent increase in annealing temperature to 800–900°С leads to the formation of a tetragonal modification [17]. Recently, it was found that in the case of highly conducting cubic Li7La3Zr2O12, Al contamination from the alumina crucibles at high temperatures leads to the incorporation of small amounts of aluminium (∼0.58–0.61 wt%) in the garnet structure and the stabilization of the cubic phase [10,11]. Rangasamy et al. studied the effect of cubic phase Li7La3Zr2O12 stabilization by Al3+ and concluded that at least 0.204 mol of Al is required to stabilize the cubic Li7La3Zr2O12 phase [19]. As the Al concentration is increased above to 0.389 mol, the impurity phase LaAlO3 forms. It has been shown in many publications using different experimental techniques, in particular neutron powder diffraction and nuclear magnetic resonance that Al3+ substitutes for Li+ in the garnet related structures [[10], [11], [12], [13], [14],19]. This substitution creates charge compensating Li vacancies and it is these Li vacancies which determine phase stability of cubic phase [19]. A significant improvement of conductivity has been recently achieved through substitution of Li+ and Zr4+ in Li7La3Zr2O12 for Al3+, Ga3+ [[19], [20], [21]], and for Nb5+ and Ta5+ [[21], [22], [23], [24], [25]], respectively. It is worth to mention that substitution on the M4+ sites does not block the lithium ion pathways and thus leads to higher conductivity than that observed for the Al3+ and Ga3+ substitutions [21].

Recently, Li7La3Hf2O12 with tetragonal and cubic structure has been synthesized using the same temperature treatment as for Li7La3Zr2O12 [26,27]. The crystal structure of the tetragonal polymorph was refined using neutron powder diffraction data. Its Li-ion conductivity was found to be slightly lower than that of tetragonal Li7La3Zr2O12. The highest Li+-ion conductivity, 3.45 × 10−4 S cm−1 at room temperature with an activation energy of 0.43 eV, has been reported for a tantalum doped Li7La3Hf2O12 [28]. However, to the best of our knowledge, neither a detailed crystal structure study of cubic Li7La3Hf2O12 nor an investigation of the Li+ substitution for other cations has been reported. In this paper we report a detailed structural study and lithium ion conductivity measurements of cubic Li7La3Hf2O12 stabilized by Al3+ doping.

Section snippets

Experimental

A polycrystalline sample of Al-doped Li7La3Hf2O12 (Al amount is ∼0.61 wt%, which corresponds to the formula Li6.04Al0.32La3Hf2O12) was prepared by a modified solid-state method. La2O3 (99.99%), which was calcined beforehand at 900°С for 5 h, HfO2 (99.99%), Li2CO3 (99.99%) and α-Al2O3 (99.9%) were used as initial compounds. A 20% molar excess of Li2CO3 was added to a stoichiometric mixture in order to compensate lithium loss during subsequent high-temperature heat treatments. The main part of

Results and discussion

The results of the thermal analysis (Fig. 1) show the stepped nature of the weight change of tetragonal Al-doped Li7La3Hf2O12 prepared at 900 °C. The DTA curve recorded during heating from 30 °C to 1000 °C shows four endothermic effects at temperatures of about 80, 310, 500 and 710 °C. The first effect in the temperature range 50–120 °C corresponds to the weight loss (0.38 wt%) associated with the evaporation of adsorbed surface water [39]. Further heating leads to weight gain (0.45 wt%) at

Conclusions

We showed that cubic Li7La3Hf2O12 can be stabilized by Al3+ doping. Al-doped Li7La3Hf2O12 powder was found to consist of micrometer size grains encapsulated by a glassy phase, which helps preventing the volatilization of lithium during annealing. A crystal structure refinement based on XRPD and NPD data as well as an analysis of electron diffraction patterns proved the formation of garnet-related structure with cubic unit cell (sp. gr. Ia3¯d (230)). Al3+ ions occupy tetrahedral Li+ sites in the

Acknowledgements

This work was partially supported by the FASO Programs (No. A16-116122810214-9, A16-116122810218-7) and RFBR Grant (No. 15-03-03951a). The crystallographic study was carried out at the multiple–access center for X–ray structure analysis at the Institute of Solid State Chemistry, UB RAS (Ekaterinburg, Russia). The authors are grateful to Dr. Irina Е. Animitsa (Ural Federal University, Ekaterinburg, Russia) for fruitful discussions.

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