Anodic Dissolution in Dual-Ion Batteries: Development of Protection Layers for Current Collectors

One aspect of the current research on lithium ion batteries is the increase of the cell voltage to improve the energy density. In innovative dual-ion cells, graphite intercalation compounds are used for both electrodes. Consequently, a simultaneous intercalation of lithium ions into the anode and the corresponding salt anions into the cathode is possible and enables voltage values above 5 V vs. Li/Li⁺. Established liquid electrolytes, consisting of carbonates and lithium hexafluorophosphate, do not resist these conditions. Therefore, we replace this mixture by ethyl methanesulfonate and organic lithium salts, having a good electrochemical performance also at higher temperatures. Now, it is the challenge to overcome the anodic dissolution of the aluminum current collector triggered by fluorinated anions like bis(trifluoromethylsulfonyl)imide (TFSI^-). Our approach to protect the metal against anodic dissolution is the deposition of an only electronically conductive, defect-free and mechanically as well as electrochemically stable layer. We test different material compositions in the range of oxidic ceramics. First of all semiconductors like alumina doped zinc oxide show promising results to meet the requirements.  The preparation is based on a sol-gel route combined with several wet-chemical coating methods, because these are easily adjustable to different substrate dimensions. Ceramic thin layers of around 100 nm thickness are deposited on aluminum foil and mainly investigated by scanning electron microscopy, X-ray diffraction and secondary ion mass spectrometry. Additionally, we present conductivity measurements and the electrochemical performance tested with cyclic voltammetry.