Advanced Analytical Characterization of Interface Degradation in Ni-Rich NCM Cathode Co-Sintered with LATP Solid Electrolyte

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 LiNi 0.6 Co 0.2 Mn 0.2 O 2 (NCM622) cathode in contact with oxide SE Li 1.3 Al 0.3 Ti 1.7 (PO 4 ) 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 R 3̅ m (layered) to Fd 3̅ m (spinel) to Fm 3̅ m (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.