Kosmotropic anion for improving cycling stability of aqueous lithium-ion batteries in salt-in-water electrolytes

Abstract The incompatibility between Li+-intercalated electrodes and water limits the practical feasibility of aqueous lithium-ion batteries (LIBs), which are economical and environmentally benign energy storage systems. Tremendous amounts of salts dissolved in water (water-in-salt) have been utilized to mitigate the access of water to the electrode/electrolyte interface and to extend the electrochemical potential window of aqueous LIBs. However, this approach has low viability owing to the expense of the salts. Here, we show that kosmotropic anions with moderate concentrations (0.5 ~ 3 mol kg− 1) protect the LiCoO2 electrode by harnessing water molecules. The sulfates of kosmotropic anions develop rigid water-solvation shells and also form ion pairs with Li+. All-atomic-level multiscale simulation revealed that sulfates tied with Li+ and the water shell are highly concentrated at the interface, thus decreasing the density of free water. The suppressed water activity explains the superior cell performance achieved with 0.5 mol kg− 1 sulfate relative to that in cells with 1 mol kg− 1 of chaotropic anions such as nitrate, perchlorate, and bis(trifluoromethylsulfonyl)imide. The formation of a liquid-phase protective layer is a new concept for developing stable aqueous batteries without the requirement for a solid-state electrolyte or an artificial protective layer on the electrode.