Corrosion investigation in polymer lithium sulphur batteries

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Current battery technology is searching different alternatives for Li-Ion batteries. One of the promising type are lithium sulfur batteries where cathode is reaching extremely high theoretical capacity by replacement of lithium transition metal oxides by elemental sulphur. Considering challenges related with polysulfides diffusion polymer electrolytes are attractive candidates in the final cells preventing unwanted diffusion of polysulfides. The research topic proposed covers the comparative study of the corrosion on the aluminum current collector in polymer electrolytes tested with two different salts. The imide (lithium bis(trifluoromethanesulfonyl)imide LiTFSI) is well known and standardized in polymer electrolytes. Unfortunately LiTFSI propagate corrosion of Al current collector. On the other side the imidazole (lithium 2-trifluoromethyl-4,5-dicyanoimidazole LiTDI) might be a candidate preventing corrosion acting as active organic corrosion inhibitor by interaction of imidazole ring structure with metal surface. Both salts solutions are tested in PEO liquid analogue (PEG DME 500) at two different temperatures (RT and 70oC). Additionally in situ synthesis in the electrolyte of polysulfides allows to follow the influence of electrode reaction products on corrosion. All considered parameters are crucial for operation of solid state lithium sulphur batteries. Different electrochemical tests including two and three electrode configurations were supported by real Li-S batteries tests tor two different salts. Additionally the in situ Raman spectroscopy allows to follow the changes on Al electrode under potential sweep. The experimental data were supported by computational modelling.
The applied electrochemical characterizations do not directly confirm corrosion of Al. The corrosion current is vanishing in the total anodic current giving almost the same voltage/current profile for all samples. However polysulfides present in the electrolyte may prevent corrosion by re-passivation of Al electrode especially at elevated temperature. Additionally LiTDI salt rise the corrosion potential much above Li-S operational potential, however the corrosion current is rising for LiTDI salt containing electrolytes. This is with agreement to computational modelling. Two electrode configuration do not allow to drawn clear conclusion about corrosion potential however all Al electrodes have much lower evidences of pitting corrosion for LiTDI containing electrolytes. Unfortunately real cell tests indicates rather poor electrochemical performance for electrolyte with LiTDI despite corrosion elimination. In Situ Raman allow to follow changes on the electrode by visual inspection. All spectrums are quite similar. To support the observation the immersion static tests were processed. Again LiTDI most probably prevents corrosion while at elevated temperature pitting corrosion is practically stopped.

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