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How interdiﬀusion aﬀects the electrochemical performance of LiMn 2 O 4 thin films on stainless steel †

The sequential production process of thin-film solid-state batteries (TF-SSB) requires high temperatures up to 700 1 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 LiMn 2 O 4 (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 1 C and 700 1 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 m A h cm (cid:2) 2 (600 1 C) and 5.1 m A h cm (cid:2) 2 (700 1 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.