Characterization of Garnet-Type Li₆SrLa₂Bi₂O₁₂ Solid Electrolyte for LiCoO₂ Cathode Composite in All-Solid-State Battery

Together with the development of inorganic solid lithium (Li) ion conducting materials for the use as solid electrolytes, the formation of intimate solid-solid interface between electrode active material and solid electrolyte is pivotal for realizing high performance all-solid-state Li-ion batteries (LiBs) with high safety and reliability. Oxide-based solid electrolytes have the advantages of good chemical stability and ease of handling, but for many of them, the high temperature processing is required for the densification, which may cause undesired side reactions with electrode active materials during the processing. Although garnet-type oxide solid electrolyte Li6SrLa2Bi2O12 (LSLBO) has lower ionic conductivity than widely studied garnet-type Li7La3Zr2O12 (LLZO), LSLBO can be sintered at much lower temperatures than LLZO [1,2]. Therefore, there is a possibility to suppress the side reaction with electrode active materials during high-temperature sintering for fabricating oxide-based all-solid-state LiBs. In this study, we prepared dense LSLBO or composites of LiCoO2 (LCO) cathode and LSLBO by hot pressing (HP). LSLBO densified at 750 °C for 1 hour showed high relative density of 95%. The densification temperature of LSLBO by HP confirmed in this work is 100–150 °C lower than Bi-doped LLZO with the formula of Li6La3ZrBiO12 (LLZBO) [3, 4]. Bulk and total (bulk + grain conductivity) ionic conductivity of LSLBO fabricated by HP attained to 2.1 × 10-4 and 1.1 × 10-4 S/cm at room temperature, which is much higher than LSLBO prepared by a conventional furnace sintering (FS) [1] and spark plasma sintering (SPS) [2]. The electrochemical stability of LSLBO at an electrical potential from 3 to 6 V vs. Li/Li+ was also confirmed by measuring the cyclic voltammogram. From XRD analysis of mixed LSLBO and LCO powders after annealing, LSLBO and LCO were found to be thermally compatible at temperature below 800 °C and LCO-LSLBO composite densified at 700 °C for 1 hour by HP showed mixed conducting properties with both electronic and ionic conductivity of 10-5-10-4 S/cm at room temperature. From galvanostatic charge and discharge testing of sintered LCO-LSLBO composite sheet, we found that electrochemical performance of LCO is not degraded by co-sintering with LSLBO. These results suggest that LSLBO solid electrolyte can be applied for LCO cathode composite in oxide-based all-solid-state LiBs. References 1. R. Murugan, W. Weppner, P. Schmid-Beurmann and V. Thangadurai, Mater. Sci. Eng. B 143, 14–20 (2007). 2. Y. Gao, X. Wang, Q. Sun, Z. Zhuang and Q. Fang, Front. Mater. Sci. 6(3), 216–224 (2012). 3. M. Wang and J. Sakamoto, Ionics 24, 1861–1868 (2018). 4. D. K. Schwanz, A. Villa, M. Balasubramanian, B Helfrecht and E. E. Marinero, AIP Adv. 10, 035204 (2020).