A NASICON-type Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP) electrolyte sheet for all-solid-state batteries was fabricated by a cold sintering process (CSP). The microstructure of the LATP sheet was controlled to improve the wettability which is an essential factor in CSP. The porous sheets of LATP were prepared by calcination the green sheets to remove the organic components and form the porous structure. By the CSP using the porous sheets, the densification of grain boundary was observed and further densified with increasing reaction time. The total conductivity of the prepared LATP sheet was improved from 3.0 × 10⁻⁶ S/cm to 3.0 × 10⁻⁴ S/cm due to the formation of necks between the particles at the grain boundary.

Thin and dense solid electrolytes are required to improve the performance of oxide-based all-solid-state batteries. We investigated the effect of the uniformity of dopant diffusion on the sinterability of the electrolyte, fabricated a garnet-type electrolyte sheet composed with a single grain along with thickness direction using the tape casting method. The electrolyte sheet using raw powder prepared by the precursor method exhibits a relatively high Li-ion conductivity of 6.7 × 10⁻⁴ S·cm⁻¹ at 25°C and low activation energy of 0.33 eV. We then investigated the factors responsible for the densification of the obtained sheets by cross-sectional morphological observation and compositional analysis.

A garnet-type electrolyte sheet for all-solid-state batteries was fabricated by modified cold sintering method. The pores in the electrolyte sheet were closed during the cold sintering process, and the grain-boundary resistance was reduced. The impurity phase formed after cold sintering was removed by annealing treatment at higher temperatures.

We successfully synthesized a garnet-type Li6.5La3Zr1.5Ta0.5O12 polycrystalline sample at a relatively low temperature of 700°C using pyrochlore-type La2Zr2O7 and weberite-type La3TaO7 as precursor oxides. The cubic garnet-type structure with the lattice parameter of a = 1.29577(1) nm was confirmed using powder X-ray diffraction data. The primary particle size of the present Li6.5La3Zr1.5Ta0.5O12 sample was about 2 µm, which is considerably smaller than that prepared using conventional solid-state reaction at higher temperatures. Thermogravimetry differential thermal analysis data confirmed that the reaction between LiOH·H2O and precursor oxides occurred above 500°C after the dehydration reaction of LiOH·H2O.

Lithium cobalt oxide (LCO) is a highly significant material for the positive electrodes of lithium-ion batteries. Due to the correlation between crystal morphology and electrochemical performance in the layered rock-salt structure, LCO with crystal morphology exhibiting anisotropy demonstrates superior charge-discharge characteristics. In this study, various morphologies of LCO were synthesized via hydrothermal synthesis using a plate-like precursor. Under conditions without agitation, a hexagonal plate-like LCO was synthesized, while a spherical LCO was obtained with agitation during synthesis. The particle morphology was investigated by using X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Furthermore, in the performance evaluation of positive electrode materials for lithium-ion batteries, the hexagonal plate-like LCO exhibited a larger charge-discharge capacity compared to the spherical LCO.

Oxide type all-solid-state battery has attracted a great deal of attention its high energy density and safety. Perovskite-type (Li_3x,La_2/3-x)TiO₃ (LLTO) has high ionic conductivity and it is one of the candidate electrolyte for an all-solid-state battery. Various efforts are underway to improve the properties. Facet-controlled nanocrystals can assemble into highly ordered nanostructures, and their orientation control and epitaxial grain growth during sintering are expected to improve their electrical properties. In this study, we focused on the orientation of LLTO ceramics. To control the orientation of electrolyte, we tried to fabricate perovskite LLTO platelet nanocrystals (nanoplates). LLTO nanoplates were synthesized by hydrothermal reaction using aqueous Ti complex as a starting material and NaOH as a mineralizer [1]. The pellet made from LLTO nanoplates is sintered at 1000 ºC which is lower sintering temperature than LLTO microparticles. However, the ionic conductivity is as low as 10^-6 S/cm. STEM-EDX analysis revealed that this is due to the Na ions remaining at the Li site of the perovskite structure. We found that Ruddlesden-Popper (R-P) type layered perovskite Li₂La₂Ti₃O₁₀ nanoplates can be synthesized by selecting LiOH instead of NaOH as the mineralizer to prevent substitution of Li to Na ion. The R-P nanoplate is a single crystal having c-plane of R-P phase. The pellet made from R-P nanoplate has highly orientation of c-axis along to the press direction. This pellet is also able to sinter relatively low temperature at 1200 ºC with keeping orientation, but R-P phase shows extremely low ionic conductivity of 3.2x10^-8 S/cm. As Li site of R-P phase is fully occupied by Li ion, Li ions are difficult to move. However, ionic conductivity of the direction of c-plane is higher than that of the c-axis direction. This result suggests that Li ion is located interlayer and Li ion is easy to move in c-plane of R-P structure. When this pellet is sintered at 1400 ºC, R-P phase has transformed to perovskite LLTO with keeping orientation. Transformation to perovskite phase is caused by vaporization of Li ion from R-P structure and it shows good conductivity of 1.1x10^-4 S/cm. Reference [1] K. Mimura, N. Hamao, H. Itasaka, Z. Liu, K. Hamamoto, J. Sol-Gel Sci. Tech. 104, 599 (2022). Acknowledgement: This study is based on results obtained from a project, JPNP20005, subsidized by the New Energy and Industrial Technology Development Organization (NEDO).