Co-Sintering Anode Materials with Garnet Electrolytes for Solid-State Batteries
Authors: He Zongqi , Watanabe Ken , Suematsu Koichi , Shimanoe Kengo 
Published: December 2024

Li7La3Zr2O12 (LLZ) has garnered considerable attention as the solid electrolyte among oxide-based materials due to its notable Li-ion conductivity and stability against Li metal. Nevertheless, LLZ necessitates high-temperature sintering to achieve enhanced ionic conductivity, which inadvertently fosters the formation of undesired interphases between the LLZ electrolyte and electrode materials. Addressing this challenge necessitates a reduction in sintering temperature. While there are numerous studies on co-sintering LLZ with cathode materials, the analogous investigations regarding anode materials remain scarce. Consequently, the selection of anode materials suitable for LLZ-based all-solid-state batteries has yet to progress significantly. Thus, this study endeavors to identify anode materials compatible with LLZ for co-sintering and enhancing electrode performance. In the realm of anode materials, achieving both reaction suppression during sintering and high energy density is paramount. Predicting reactions between different materials typically relies on thermodynamic calculations. This study focuses on the Li-concentration between diverse materials, serving as the driving force behind the reaction rate. Hence, we investigate the relationship between the lithium concentration in the electrode material and LLZ reactivity.

Bi5Nb3O15, TiNb2O7, CeO2-SrLi2Ti6O14, SrLi2Ti6O14, Li2Ni(WO4)2, Li3Ni2SbO6, Li2SnO3, Li2TiO3 and Li3SbO4 were selected as the anode candidate. All anode materials and Bi-substituted LLZ (Bi-LLZ) were synthesized via solid-state reaction. The resulting anode material and LLZ were mixed at a volume ratio of 1:0.85 and press-formed into the disk, which was subsequently sintered at temperatures ranging from 650°C–850°C. The crystal structure of the sintered disk was characterized via X-ray diffraction, while microstructure observations were conducted using SEM-EDS and TEM-EDS. The electrical properties of the composite anode disks were evaluated using AC impedance spectroscopy and DC polarization methods. To assess the electrode properties of the composite anode disks, the charge-discharge tests were performed using a liquid electrolyte.

XRD measurement post co-sintering was conducted to evaluate the reactivity of anode candidates with Bi-LLZ. The relationship between the Li-concentration of anode materials (C
Li: mol/cm3) and the impurity phase ratio post co-sintering was investigated. The reaction between anode materials and LLZ can be categorized into two groups based on lithium concentration. Highly reactive materials exhibit lower C
Li than Bi-LLZ, while low-reactive materials possess higher C
Li than LLZTB. When the Li-concentration in the anode material was low, both LLZ and the anode material vanished entirely, and new reaction products emerged. Conversely, high Li-concentration materials maintained both peaks, indicating minimal reaction progress. Furthermore, we demonstrated that the composite anode based on higher CLi materials performs effectively. Thus, increasing the Li-concentration in the anode material to the same level or higher than that in LLZO can suppress the reaction during sintering.