Synthesis, Conductive Properties, and Crystal and Electronic Structures of Nasicon-Type Li1.5Al0.5Ge1.5-XMx(PO4)3[M=Mn, V, Mo]

Introduction All-solid-state lithium-ion batteries are expected to offer excellent stability and high energy density over a wide temperature range with a low risk of fire. Therefore, vigorous development efforts have been made in recent years, particularly with the increasing demand for electric vehicles. However, to improve the performance, novel solid electrolytes need to be discovered. Consequently, NASICON-type Li 5 Al 0.5 Ge 1.5 (PO 4 ) 3 materials have garnered attention because these materials exhibit higher chemical stability in air and water compared to other solid electrolytes and possess relatively high ion conductivity at room temperature. Nonetheless, further enhancement of conductivity is necessary for practical applications. In this study, Li 1.5 Al 0.5 Ge 1.5-x M x (PO 4 ) 3 (M=Mn, V, Mo ; x=0.05, 0.1, 0.2, 0.3) was synthesized, and the composition-dependency of conductivity characteristics were investigated. Additionally, through average and electronic structure analyses using synchrotron X-ray and neutron diffraction, the correlation between composition, average/electronic structure, and conductivity was examined. Experiment Li 5 Al 0.5 Ge 1.5-x M x (PO 4 ) 3 was synthesized by a solid-state reaction. 1) Substitution at the Ge site was achieved using MnO 2 , V 2 O 5 , and MoO 2 . Phase identification was conducted through powder X-ray diffraction measurements, while the metallic composition was determined using an inductively coupled plasma optical emission spectrometer (ICP-OES). The sintering properties were evaluated using scanning electron microscopy (SEM, HITACHI, S-2600N) and true density measurements (Microtrac, BELPycno). The electrical conductivity characteristics were evaluated using AC impedance spectroscopy (SP-300, Bio-Logic). Additionally, the average structure was examined through Rietveld refinement (RIETAN-FP, Z-Code) using data obtained from synchrotron X-ray diffraction (BL19B2, SPring-8) and neutron diffraction (iMATERIA, J-PARC). The electronic structure was analyzed using the Maximum Entropy Method (MEM: Dysnomia). Results and Discussion Powder X-ray diffraction measurements confirmed that most peaks of the synthesized Li 5 Al 0.5 Ge 1.5-x M x (PO 4 ) 3 (x=0, 0.05, 0.1, 0.2, 0.3) were attributed to the R-3c space group, indicating a NASICON-type structure (Fig.1). In the case of Mn, V, and Mo substitutions (x=0.05, 0.1), no diffraction peaks from starting materials were observed, suggesting a successful substitution of Mn, V, and Mo for the Ge site. Furthermore, it was evident that the lattice constant of the a-axis and c-axis increased and decreased, respectively, due to the substitution. To assess the influence of the sintering process, a spark plasma sintering (SPS) was conducted in addition to a conventional atmospheric sintering, using finely ground powders obtained after the initial sintering. The results revealed that the density of the sintered bodies improved with the application of SPS. The temperature dependence of conductivity was measured for the obtained samples. It was found that as the substitution level of Mn, V, and Mo increased, the conductivity decreased, and the activation energy increased. Additionally, it was observed that some of the samples exhibited higher conductivity by SPS, likely due to the higher density of the sintered bodies. To investigate crystal and electronic structures, synchrotron X-ray diffraction measurements were conducted, followed by Rietveld refinement. The crystal structure was refined assuming the R-3c space group, with the substitution species M (Mn, V, Mo) occupying the 12c sites (Ge site). The results suggested that Li occupied two sites, 6b and 36f. Moreover, it was observed that substitution of metal (M) at the 12c sites led to reduced distortion of the polyhedra. This implies that the polyhedral distortion affects the conductivity. Regarding electronic structure, the bond strength between each metal and oxygen was evaluated based on electron density distributions obtained from MEM, and the correlation with lithium ion conductivity was examined. Reference 1) J. Ock et al., ACS Omega , 6 , 16187-16193 (2021). Figure 1