Pressure‐Dependent Electronic and Ionic Tortuosities During Fabrication of Composite Cathodes for All‐Solid‐State Batteries: Influence of Active Material Particle Morphology, Volume Fraction, and Coating

The cathode of bulk‐type all‐solid‐state batteries (ASSBs) is a composite consisting of cathode active material (CAM) particles and solid electrolyte (SE) particles. Since ASSBs are usually fabricated and cycled under pressure, it is important to achieve a better understanding of the influence of pressure on the electronic and ionic transport in CAM/SE composites. However, experimental values for pressure‐dependent electronic and ionic conductivities of CAM/SE composites are scarce. Therefore, we have carried out pressure‐dependent impedance spectroscopic measurements during compaction of composite powders under an increasing fabrication pressure up to 663 MPa. The composites contained four different types of LiNi 0.6 Mn 0.2 Co 0.2 O 2 (NMC622) cathode active material particles: (i) single‐crystalline and uncoated, (ii) single‐crystalline and coated with LiNbO 3 , (iii) polycrystalline and uncoated, and (iv) polycrystalline and coated with LiNbO 3 . In addition, the volume fractions of CAM particles and SE particles were varied. The physical meaning of the resistances extracted from the impedance spectra were analyzed, and electronic and ionic tortuosities were calculated. A strong influence of the CAM particle morphology and coating on the microstructure of the composites was observed. Based on the results, the interrelation between composite microstructure and tortuosities is discussed.