One challenge in developing the next generation of lithium ion batteries is the replacement of organic electrolytes, which are flammable and most often contain toxic and thermally unstable lithium salts, with safer, more environmentally friendly options. Water-in-Salt Electrolytes (WiSE) are a promising class of electrolytes that have been recently developed to overcome this challenge. Many of the desirable properties of WiSE, such as their expanded electrochemical stability window, arise due to physics occurring at the electrochemical interface. One interesting observation seen in molecular simulations of WiSE, is that water is accumulated next to negative electrodes, but depleted next to positive electrodes. In this work, we develop a simple modified Poisson-Boltzmann theory, which is able to explain this asymmetric electrosorption of water in WiSE in terms of the competing solvation and polarization forces acting on water molecules. Additionally, the theory is parametrized entirely from molecular simulations and is able reproduce the electrical double layer (EDL) structure of WiSE electrolytes observed in those molecular simulations with surprising accuracy. Furthermore, we construct a Marcus-like theory of electron transfer, which accounts for the EDL-induced heterogeneity of the solution. By including the effects of the EDL as well as the reduced chemical potential of water, our theory is able to predict a suppression of oxygen evolution in WiSE that is consistent with experimental measurements.

The electrical double layer and stability window of water-in-salt electrolytes

256th National Meeting (2018)

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