High-Temperature Chemical Stability of Li1.4Al0.4Ti1.6(PO₄)₃ Solid Electrolyte with Various Cathode Materials for Solid-State Batteries

In a solid-state battery (SSB) system, undesirable electrode–electrolyte interfacial reactions lead to a significant performance degradation. Herein, we performed a systematic study on the chemical stabilities between Li 1.4 Al 0.4 Ti 1.6 (PO 4 ) 3 (LATP) solid electrolyte and various cathode materials at their adhesion temperatures of 500–900 °C. Quantitative analysis of X-ray diffraction (XRD) data using Rietveld refinement revealed that Li-concentration disparity between LATP and oxide cathode materials (e.g., layered and spinel phases) is the root cause of phase degradation at high temperatures. For example, Li migration from oxide cathodes to LATP produces multiple secondary phases including LiMPO 4 olivine. In contrast, the LiFePO 4 cathode severely reacted with LATP at low temperature ( T < 500 °C) and produced an Fe-rich NASICON phase (e.g., Li 3 M 2 (PO 4 ) 3 ). The onset temperature of the phase decomposition varies with chemical compositions and crystal phases of cathodes. Increasing the cathode/electrolyte adhesion temperature offers a trade-off between the specific capacity and cycle life, as exemplified by the LiCoO 2 (LCO) + LATP composite cathodes. The results in this study offer a fundamental understanding of the LATP/cathode reaction mechanism, which will serve as guidance for designing interfaces and controlling the fabrication processes of SSB cells.