Nowadays, lithium ion batteries (LIB) with safer and sustainable properties are getting particular attention, especially in the field of stationary energy storage. Aqueous electrolytes offer a high safety aspect due to the intrinsic non-flammability of the solvent water, accompanied by its environmental friendliness and affordability. However, the application of aqueous electrolytes is limited by the narrow electrochemical stability window (ESW) of water. Therefore, concepts as “water-in-salt” electrolytes (WiSE) with high amounts of conducting salts are common in order to broaden the ESW and allow the use of aqueous electrolytes in LIB. [1,2] In WiSE the water molecules are coordinated by the Li+ cations of the conducting salt and the high amounts lead to a disruption of the continuous water network. [3] Nevertheless, large amounts of cost intensive conducting salts are required for that.
In our study, we investigated the addition of non-toxic and cost-effective sugar alcohols to aqueous electrolytes, here erythritol, xylitol and sorbitol in 1 m LiTFSI-H2O, to disrupt water network by interactions and thereby successfully reduce the amount of lithium conducting salt needed. Cyclic voltammetry shows that the concentration of sugar alcohol added has no impact on the ESW, in contrast to the concentration of LiTFSI. This is confirmed by vibrational spectroscopy techniques, where no impacts on the water coordination are observed. The electrolyte behaves like a low-concentrated one with a network of free water molecules. This is caused by the reductive instability of OH-groups of sugar alcohol and the formation of bicontinuous phases. By adding modifications of such sugar alcohols to the electrolyte, a shift of water coordination environment in infrared spectroscopy towards higher coordination degree and a drastically increased ESW are observed.

[1] L. Suo et al., Science 350, 2015, 938.
[2] H. Zhang et al., Angew. Chem. 133, 2021, 608.
[3] Y. Zhang et al., J. Phys. Chem. B 2021, 125, 4501.