Origin of High Interfacial Resistances in Solid‐State Batteries: Interdiffusion and Amorphous Film Formation in Li 0.33 La 0.57 TiO 3 /LiMn 2 O 4 Half Cells

The large interfacial resistance between electrolyte and electrodes poses a significant roadblock for the application of all‐solid‐state batteries. The formation of interfacial phases (interphases) has been identified as one of the most significant sources for such high resistance. Therefore, studying the mechanism of interphase formation, along with investigating its effect on ionic conductivity, could lead to the discovery of avenues towards designing high‐performance all‐solid‐state batteries. In this work, we studied the interphase formation in the perovskite electrolyte Li0.33La0.57TiO3 (LLTO) and spinel cathode LiMn2O4 (LMO) pair by co‐sintering experiments via spark plasma sintering (SPS), as well as conventional sintering. Although the processing method has an influence on the electrode/electrolyte contact, the formation of an interphase could not be avoided. At the LLTO/ LMO interface, we observed both an interphase formed by interdiffusion, as well as a complexion‐like amorphous layer. We directly characterized the complexion layer morphology by using HRTEM. Analytical TEM and SEM were used to reveal the elemental composition of the interphase and the interdiffusion layer. Furthermore, we used impedance spectroscopy to measure the electrical properties of the LLTO/LMO interphase and identified the interfacial resistance from the interdiffusion induced interphase to be larger than the individual phases by a factor of 40, whereas the amorphous layer was not visible in the impedance