Computational Screening and Design of Fluorinated Solvents for High-Voltage Li-Ion Batteries

As societal needs move towards more sustainable and efficient energy landscapes, electrical energy storage is taking a center stage. The need for devices with greater safety, higher energy and power densities, and longer lifetimes is ever increasing, with applications ranging from stationary storage for intermittent sources (e.g. wind and solar) and grid load-leveling, to electrification of transportation and consumer electronics.

The development of high voltage Li-ion batteries is an important goal for the large-scale deployment of electrified transportation, and consumer electronics with improved battery life.  A key requirement for these devices is an improved solvent-electrolyte system, capable of withstanding the oxidizing environment present at state of the art high-voltage cathode materials. Fluorinated solvents have been studied for these applications due to the electron withdrawing character of fluorine substituents, resulting in higher oxidative stability and, in some case, flame retardancy.^1,2 However, fluorination can also lead to problems for reductive stability and conductivity.

We have undertaken a project to explore the feasibility of fluorinated solvents for high-voltage Li-ion batteries. This project capitalizes on DuPont core competencies in fluorine chemistry, electrochemistry and computational chemistry.

Computational methods, in particular, are well suited for an initial pass at molecules of potential interest. Several methods have been proposed for using computational methods to predict the oxidative properties of electrolytes solvents from single molecules, with varying degrees of success.^3,4,5 However, the presence of fluorine requires special considerations from the perspective of computational methods, as well as screening considerations around compatibility with standard carbonates and lithium salts.

We will present an overview of our solvent discovery method with computational data from an initial library of ~2000 fluorinated and reference molecules. Anion and cation effects, observations of in-silico LiF formation, and comparisons to experimental electrochemical data will specifically be addressed. Using our computational approach, we have reduced the search field by over 90%, as well as identified several candidates for synthesis and testing.

Finally, cycle life data for some DuPont fluorinated electrolyte blends will also be presented, which demonstrate high conductivity and significantly improved cycle life when compared to standard carbonate electrolyte blends cycled at 55 oC in high-voltage Li-ion full cells.

References:

(1)      Achiha, T.; Nakajima, T.; Ohzawa, Y.; Koh, M.; Yamauchi, A.; Kagawa, M.; Aoyama, H. J. Electrochem. Soc. 2010, 157, A707.

(2)      Zhang, Z.; Hu, L.; Wu, H.; Weng, W.; Koh, M.; Redfern, P. C.; Curtiss, L. a.; Amine, K. Energy Environ. Sci. 2013, 6, 1806.

(3)      Borodin, O.; Jow, T. R. ECS Trans. 2011, 33, 77–84.

(4)      Zhang, X.; Pugh, J. K.; Ross, P. N. J. Electrochem. Soc. 2001, 148, E183.

(5)      Halls, M. D.; Tasaki, K. J. Power Sources 2010, 195, 1472–1478.