Effect of High-Pressure Treatment on the Crystal Structure and Li+ Conduction Properties of Li3AlF6
Authors: Kohama Yamato , Miyazaki Reona , Hihara Takehiko 
Published: December 2024

All-solid-state batteries which are attracting attention as next-generation energy storage devices overcome the flammability of conventional lithium-ion batteries. These batteries achieve high safety and energy density through the use of non-flammable solid electrolytes, and could be used as the power sources for electric vehicles and electronic devices in the future. However, several challenges are remained for the practical application of all-solid-state batteries, which require the further development of solid electrolytes.
Sulfide-based solid electrolytes, which are currently extensively studied, exhibit high Li+ ionic conductivity[1]. However, the sulfides have problems with atmospheric and electrochemical stability[1]. In 2018, Asano et al. have reported that lithium metal chlorides and bromides (Li3YCl6, Li3YBr6) exhibit high ionic conductivity, a wide electrochemical stability window, and excellent electrode interface stability[2]. However, these lithium metal chlorides and bromides have problems with atmospheric stability[3].
My research focused on the lithium metal fluoride materials, which have excellent electrochemical stability and high air stability[4]. On the other hand, their low ionic conductivity due to the strong electronegativity of F- is challenging for the application. Li3AlF6, which has a monoclinic structure at room temperature, has been reported as the lithium ionic conductor[5]. The solid solution of Li3AlF6-Li2SiF6 in which the host Al3+ is substituted by Si4+ exhibited moderately high Li+ ionic conductivity of 3×10-5 S/cm at room temperature[6]. Li3AlF6-Li2SiF6 solid solution was mixture of monoclinic and orthorhombic Li3AlF6 and the positive relationship between the Li+ conductivity and the orthorhombic fraction was suggested[6].
Therefore, we consider that Li+ conductivity can be further improved for other crystalline phases of Li3AlF6. In this study, Li3AlF6 is heated under 2 ~ 8 GPa (high-pressure treatment) to investigate the variation of the polymorphs under high-pressure. The Li+ conductivities of Li3AlF6 after high-pressure treatment are evaluated.
Experimental method.
LiF and AlF3 were mixed in a molar ratio of 3:1 and fused at 900℃ for 15min in the electric furnace set in a glovebox. Li3AlF6 was ball-milled using a ZrO2 vessel (45 mL) for 50 h. The high-pressure treatment was conducted using a cubic anvil-type apparatus. Li3AlF6 pellet with 5.6 mm was placed in the BN capsule (4.4 mm in diameter). Subsequently, this BN capsule was further inserted in the carbon tube heater(5.4 mm in diameter). This all-in-one capsule was embedded in the pyrophyllite cube (pressure-transmitting media), which was subjected to the high-pressure treatment. Li3AlF6 was heated to 500 ~ 700°C under 2 ~ 8 GPa. After high-pressure treatment, the crystalline phase of the sample was identified by XRD. For the conductivity measurement, the sample was grounded and pelletized at 450 MPa in the PEEK cylinder. The impedance was measured in a frequency range between 1 MHz to 1 Hz.
Result.
Fig. 1 shows the XRD patterns of the Li3AlF6 after high-pressure treatment at room temperature (bottom) and 500℃ (top). As show in Fig. 1(top) the diffraction pattern of Li3AlF6 (8 GPa, 500℃) was fitted by orthorhombic (Pna21), indicating the stabilization of the orthorhombic Li3AlF6 at ambient condition. The densities of monoclinic (C2/c) and orthorhombic (Pna21) Li3AlF6 are 2.84 g/cm3 and 2.90 g/cm3, respectively. Therefore, orthorhombic Li3AlF6 is considered to be more stable under high pressure condition. The crystal structure of Li3AlF6 is known as monoclinic structure at room temperature[6]. The lattice volume of Li3AlF6 doped with Si was decreased and the crystal structure was the mixed phase of orthorhombic and monoclinic[6]. Therefore, decreasing the lattice volume can be effective for the stabilization of orthorhombic Li3AlF6. On the other hand, the crystal structure of Li3AlF6(8 GPa, RT) was monoclinic(C2/c). It is considered that by heating at high pressure, Li3AlF6 maintained its high temperature phase[7]. The Li+ conductivity of high-pressure treated samples will be presented in the poster.

Acknowledgement

This work was supported by the NGK Environment Innovation Laboratory and JSPS KAKENHI Grant JP23H23447

Reference

[1] J. Wu et al., Advanced Materials, vol. 33, no. 6. 2021.
[2] T. Asano et al., Advanced Materials, vol. 30, no. 44, 2018
[3] X. Li et al., Nano Lett, vol. 20, no. 6, pp. 4384–4392, 2020
[4] M. Jin et al., Advanced Engineering Materials, vol. 25, no. 8. John Wiley and Sons Inc, 2023.
[5] T et al., Solid State Ion, vol. 34, no. 3, pp. 201–205, 1989.
[6] R. Miyazaki et al., ACS Appl Energy Mater, 2024,.
[7] Jan L. Holm et al., Acta Chem. Scand. 23, 1969

Figure 1