How interdiffusion affects the electrochemical performance of LiMn2O4 thin films on stainless steel

The sequential production process of thin-film solid-state batteries (TF-SSB) requires high temperatures up to 700 °C in order to achieve crystallized cathode films with high capacities and ceramic electrolytes with high ionic conductivities. This presents challenges for thermal interfacial stability between the electrode and electrolyte as well as electrode and substrate. Interdiffusion processes between the substrate and electrode at elevated temperatures are critical and may explain the low utilization of cathode active materials in TF-SSBs. This study examines the interdiffusion that occur between LiMn2O4 (LMO) thin films and the flexible stainless steel (StSt) substrate. Grazing incidence X-ray diffraction spectroscopy, time-of-flight secondary ion mass spectrometry and X-ray photoelectron spectroscopy reveal that during the crystallization at 600 °C and 700 °C, LMO undergoes lithium and manganese depletion whereas elements from the substrate (Fe, Cr, Al, Pt) diffuse into the LMO spinel structure. The combination of these effects, which are exacerbated by the higher crystallization temperature, results in a significant capacity loss during galvanostatic cycling. Various interlayers, including gold, platinum and indium tin oxide (ITO) were tested as conductive interdiffusion barriers. The use of ITO, unlike gold and platinum, as a barrier layer essentially prevents the interdiffusion, leading to an improved discharge capacity of 4.2 mA h cm-2 (600 °C) and 5.1 mA h cm-2 (700 °C). The results indicate that ITO is an effective interdiffusion barrier on flexible StSt current collector foils, which can enable higher performance TF-SSBs as well as cost effective roll-to-roll manufacturing.