Li-ion all-solid-state batteries (ASSBs) employing solid electrolytes (SEs) can address the energy density and safety issues that plague the current state-of-the-art Li-ion battery (LIB) architecture. To that end, intimate physical and chemical bonding has to be established between high-performance cathodes and high-voltage stable SEs to facilitate high Li+ transfer. The production of intimate interfaces in oxide cathode–solid electrolyte composites requires high-temperature (>1000 °C) processing, which results in a range of degradation products. Here, we report the morphological, structural, and chemical changes that occur in commercial Ni-rich layered LiNi0.6Co0.2Mn0.2O2 (NCM622) cathode in contact with oxide SE Li1.3Al0.3Ti1.7(PO4)3 (LATP) when cosintered between 550 °C and 650 °C. The structural evolution of pristine NCM622 heat-treated at a temperature of 650 °C is contrasted with the NCM622 from the composites using aberration-corrected scanning transmission electron microscopy (AC-STEM). At high spatial resolutions, the degradation of NCM particles in the composites proceeds via phase transitions from R3̅m (layered) to Fd3̅m (spinel) to Fm3̅m (rocksalt) to amorphous at the grain boundaries and via pit formations and intragranular crack nucleation and propagation in the bulk. Automated crystal orientation mapping (ACOM) in combination with low-dose TEM was used to investigate the beam-sensitive cathode–solid electrolyte interfaces. To provide statistical relevance to the investigations undertaken, ACOM-TEM was used in combination with time-of-flight secondary ion mass spectroscopy (ToF-SIMS). By combining these techniques, we show that the phase transitions of the NCM particles are correlated with simultaneous lithium transfer from NCM regions to LATP regions with evolving temperature.
