Probing the Interface Evolution in Co-sintered All-Phosphate Cathode-Solid Electrolyte Composites

To achieve cohesive interfaces of solid components in next-generation Li-ion batteries (LIBs) composed of oxide cathodes and solid electrolytes (SEs) high-temperature sintering is necessary which leads to material degradation and phase decompositions. Thermochemically compatible components can address these issues. In this work, advanced electron microscopy techniques are employed to investigate the structural and chemical evolution taking place at the interfaces between a LiFePO4 (LFP) cathode and an in-house optimized Li1.3Al0.3Ti1.7(PO4)3 (LATP) SE co-sintered between 650 °C- 850 °C. While the LFP particles exhibit excellent structural stability, the LATP particles phase transforms at elevated temperatures. At 650 °C, FeLi anti-site defect is observed at the LFP grain boundaries. Between 750 and 850 °C, the LATP (R-3c) transforms into two orthorhombic phases (Pbca and Pbcn) of the type Li1.3+xAl0.3FexTi1.7-x(PO4)3 depending on Fe substitution while forming the cohesive interfaces. The discharge capacity of the composite decreases from 148 mAh g-1 at 650 °C to 130 mAh g-1 at 850 °C confirming that the new phases are still electrochemically active. This study provides a spatially resolved insight into the formation of cohesive interfaces highlighting the advantage of employing components with similar anionic structures for further improvements in LIBs.