Li-ion batteries (LiBs) are widely used in very diverse set of applications from portable electronics, to electric vehicle transportation. Hence, improving LiBs’ power and energy while not compromising safety, remains a hot research topic. Organic solvents permit higher operating voltages; however, they have the disadvantage of being volatile and flammable. Aqueous solvents, on the other hand, are inexpensive and non-flammable but they are typically stable only at low voltages (<1V) resulting in relatively low energy density (<100Wh/kg). Recently, an intriguing paradigm shift regarding the extending electrochemical stability window of aqueous electrolytes was reported for the aqueous electrolytes.(1-4) Specifically, highly concentrated aqueous solutions of electrolytes (i.e. “water in salt”) were shown to have electrochemical stability window above 3V, despite the presence of water in the system.

In this presentation we explore an intriguing supposition that the electrochemical stability of these systems can be correlated with the electrolyte structure near surface. Using classical molecular dynamics simulations we investigate the electrolyte partition near the electrode, as a function of the applied potential. The studied system consisted of a highly concentrated solution of two salts LiTFSI and LiCF₃SO₃in water that was recently shown to have one wide electrochemical stability.(2) The electrolyte structuring near the electrode was found to be strongly dependent on the applied potential. Specifically, at the positively charged electrode water is displaced from the surface by the voluminous anions, while at the negatively charged electrode a large accumulation of water is observed in the interfacial layer. Interestingly, the water is displaced rapidly from the negative surface as the potential increases from -2V to 0V, and surprisingly elevated densities of F groups were found next to surface at -1V.

In such concentrated electrolytes, the Li ions are only partly coordinated with water (2-2.6 water molecules per Li). The partly desolvated Li⁺ can drag the TFSI anion near the negative electrode. In agreement with this observation, our simulations confirmed the presence of F groups in the proximity of the interfacial Li⁺ at the negative electrode. The presence of Li⁺and F next to the negative charged surface can trigger electrochemical reactions that kinetically passivate the negative electrode surface.

References

1. L. Suo, O. Borodin, T. Gao, M. Olguin, J. Ho, X. Fan, C. Luo, C. Wang and K. Xu, Science, 350, 938 (2015).

2. L. Suo, O. Borodin, W. Sun, X. Fan, C. Yang, F. Wang, T. Gao, Z. Ma, M. Schroeder, A. von Cresce, S. M. Russell, M. Armand, A. Angell, K. Xu and C. Wang, Angew. Chem. Int. Ed, 55, 7136 (2016).

3. F. Wang, Y. Lin, L. Suo, X. Fan, T. Gao, C. Yang, F. Han, Y. Qi, K. Xu and C. Wang, Energy Environ. Sci.,, 9, 3666 (2016).

4. Y. Yamada, K. Usui, K. Sodeyama, S. Ko, Y. Tateyama and A. Yamada, Nature Energy, 1, 16129 (2016).