Electrochemical Property of Li3x
La2/3-X
TiO3 Sintered Anode for Solid-State Batteries
Authors: Takeno Shinichi , Watanabe Ken , Suematsu Koichi , Shimanoe Kengo 
Published: December 2024

Oxide-typed solid-state batteries have garnered significant attention owing to their high energy density, extended lifespan, and enhanced safety features. Achieving a densely packed electrolyte based on Li-ion conducting oxides typically necessitates sintering at temperatures exceeding 1000°C. However, such high-temperature sintering often results in undesirable interphases between electrolytes and electrode materials. Nevertheless, employing high-temperature sintering facilitates the densification of active materials through co-sintering, rendering them suitable for electrodes and contributing to increased energy density. In our prior research, we demonstrated the tunability of electrode properties in LiCoO2 sintered electrodes by adjusting the excess Li content. Additionally, we identified the presence of excess Li in the form of anti-site Li defects, thereby enhancing Li-ion conductivity. These findings underscore the importance of enhancing the mixed conductivity of active materials to achieve sintered electrodes without the need for additives. For LiCoO2, Li-ion conductivity plays a crucial role in electrode performance. Hence, this study shifts focus to Li3x
La2/3-x
TiO3 (LLTO) as the active material for sintered anodes. LLTO exhibits a high Li-ion conductivity exceeding 10-4 S/cm, while its electronic conductivity remains below 10-8 S/cm. Previous reports indicate that LLTO particles can serve as the anode active material, operating at a low potential under 1V with a high capacity of 225 mAh/g. Here, our objective is to elucidate the electrochemical properties of LLTO sintered anodes.

Li0.33La0.56TiO3 was synthesized via a solid-state reaction using high-quality Li2CO3, TiO2, and La(OH)3 as starting materials. These materials were mixed with isopropanol using a planetary ball mill at 450 rpm for 15 hours. The resulting powder underwent grinding and calcination in air at 800°C for 4 hours. The calcined powder was then ground and press-formed into a disk, which was subsequently sintered at 1100°C for 15 hours in air. Characterization involved X-ray diffraction (XRD) for crystal structure and scanning electron microscopy (SEM) for microstructure observation. Electric and Li-ionic conductivities were measured using DC polarization measurement and AC impedance spectroscopy, respectively. Electrode performance as the anode for a Li-ion battery was evaluated through a constant-current charge-discharge test using LiPF6 (EC: DMC=1:2 v/v%) as the electrolyte and Li metal as the counter electrode.

Following sintering, a dense LLTO disk was obtained, with electronic and Li-ion conductivities measured at 1.1×10-9 S/cm and 1.9×10-5 S/cm, respectively. Initial lithiation during charge-discharge testing exhibited lower operating voltage and significantly lower capacity of 37 mAh/g compared to previous reports. However, charge/discharge capacity gradually increased with cycle number until the 11th cycle. Examination of the LLTO sintered electrode after lithiation at 0.03 mAh/g and 5 mAh/g revealed a color change to black and an increase in electronic conductivity to 1.5×10-4 S/cm due to the reduction of Ti4+ to Ti3+. Moreover, at 0.03 mAh/g lithiation, a color change was observed at the triple phase boundary at the current collector/LLTO/electrolyte interface. These results suggest that electrode reactions primarily occur at the triple phase boundary, indicating that electronic conduction limits the reaction in LLTO-sintered anodes. Thus, enhancing electronic conductivity could improve the initial electrode performance of LLTO-sintered anodes.