loss of passivation under rest and cycling conditions. Thus, additional functionalities are required, with one such example being the addition of urea, which acts as chaotropic agent, disrupting the hydrogen bonding network (HBN) of H2O, thus reducing water reactivity while also decomposing to add a polymeric organic component to the SEI. Interestingly, the Hofmeister series, which characterizes ion interactions with the HBN of water, suggests that Li + cations are kosmotropic, hence strengthening HBN. Thus, an interesting prospect would be to combine TFSI with a chaotropic cation to form a salt that could benefit from the aforementioned functionality from TFSI, while also potentially displaying similar effects to urea, to be used as a co-salt with a conventional alkali-metal based salt. Recently, co-salts have been employed in the development of 'high-entropy' aqueous electrolytes, resulting in intriguing desolvation properties at the electrode-electrolyte interface. However, we anticipate distinct roles for entropic and chaotropic effects in solution, with the primary focus of this communication being on the latter. While the true effects of chaotropic ions on HBN and the consequences of such interactions are still unclear within the field of macro-molecular science, it has been demonstrated that chaotropicity plays a role in the reactivity of aqueous electrolytes. A chaotropic cation of particular interest is guanidinium (Gdm) (Fig. ). Gdmderived salts have been studied with a wide range of different anions, including TFSI, for the design of ionic liquids. However, the non-functionalised GdmTFSI (Fig. ) has, to the best of our knowledge, only been reported twice, while partially characterised; its structure was not resolved and properties as a salt in aqueous electrolytes were not studied. Herein, we report a synthesis procedure for GdmTFSI, solve its crystal structure, and provide chemical and electrochemical characterisation of the salt for potential use as co-salt for aqueous batteries.