Li+ Conduction Properties and Local Structures of Sodium Iodide Doped with Li+ and BH4 -

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Background The most of the solid electrolytes for all-solid-state Li and Na batteries have been developed based on the single alkali compounds in which other alkali ions do not coexist in the same lattice. The mixing of two alkali cations in the same phase results in the deterioration of the ion conductivity (mixed cation effect) [1]. Hence, the mixing of two alkali cations has been avoided for the preparation of solid electrolytes. On the other hand, our group has focused on the conduction of the “doped” Li+ in Na compounds, particularly for NaI. NaI-NaBH4-LiI is the NaI-based solid solution where the host Na+ and I- are replaced by Li+ and BH4 -, respectively [2, 3]. Although the concentration of Li+ is less than 10 mol%, the dominant conduction ions in NaI-NaBH4-LiI has been proven to be the doped Li+ [4]. The Li+ conduction mechanism was also discussed from the atomic point of view; “off-centered” Li+ was considered as one of the origins for the dominant Li+ conduction in NaI-LiI [5]. In this study, the local structure of Li+ in NaI-NaBH4-LiI is investigated from the perspective of Li+ conduction. Reverse Monte Carlo (RMC) simulation was performed based on the neutron total scattering data. During the simulation, swapping among I- and BH4 - was considered to represent the random distribution of BH4 -. The local environment of Li+ in NaI-NaBH4-LiI will be extracted by partial pair distribution function of the modeling structure. Experimental The sample fabrication and storage were conducted in Ar-filled glove box. NaI, NaBH4 and LiI were mixed in a given molar ratio and mixed by hand. The mixed powder was sealed in the chrome steel vessel with 10 pieces of balls (10 mm in diameter) and ball-milled for 5 hours. In this study, the concentration of Li+ and BH4 - were 10 and 5 mol %, respectively. The sample was post-annealed at 250°C in the glove box. For the conductivity measurement, the ball-milled NaI-NaBH4-LiI was pelletized by uniaxial pressing at approximately 270MPa. The pellet (10 mm and 1mm in diameter and thickness, respectively) was set in the PEEK cylinder with the inner diameter of 10 mm. Stainless steel (S.S.) disks were used for the blocking electrodes. The PEEK cylinder with the solid electrolyte pellet sandwiched by two S.S. disks was set in the hermetic cell. The conductivity was measured by AC impedance method (1 MHz ~ 1 Hz) with the temperature range of 250°C and 30°C. Neutron total scattering measurements of NaI-NaBH4-LiI were performed at NOVA, beamline BL21 with a 90 deg. (Q (= 2π/d = 4πsinθ/λ) = 1 ~ 82 Å-1) detector bank at Japan Proton Accelerator Research Complex (J-PARC). To avoid the absorption of neutrons, 7LiI and Na11BD4 were used for the raw materials. The atomic configuration of NaI-Na11BD4-7LiI was modeled by an RMC simulation using RMCProfile [6]. During the simulation, the swapping among I- and 11BD4 - tetrahedron was considered to represent the random distribution of 11BD4 -. Results and Discussions The results of RMC simulation are summarized in the Figure. The Bragg profile, the structure factor, (S(Q)) and the reduced pair distribution function (G(r)) are well fitted. Hence, not only the long-range periodic structure but the local structures which are often hidden in the background of the diffraction data were reproduced. The partial pair distribution functions (gij (r)) of Na-11B and 7Li-11B are calculated based on the modeling structure (Figure (d) and (e)). The Na-11B correlation was observed over a long-range, indicating that the position of Na and BD4 - is randomly deviated from the ideal crystallographic sites. On the other hand, the nearest neighbor distance between 7Li and 11B was not uniform and the long-range correlation was not confirmed. These results indicate that the doped Li+ was trapped by BH4 -. The conduction properties of NaI-NaBH4-LiI will be discussed based on its local structure.