(352c) Designing Electrolytes for Beyond Li-Ion Batteries Using Coupled High Throughput Ab Intio Calculations and MD Simulations

Transformative outcomes for transportation and electrical grid require high performance, low cost energy storage systems beyond Li-ion technology and innovations in electrodes and electrolytes, alike. Multivalent batteries, chemical transformation and redox flow batteries are promising candidates for next generation electricity storage technologies. An automatic High-throughput computational infrastructure has been constructed for the electrolyte genome project supported by the US Joint Center for Energy Storage Research (JCESR) coupled with experimental analysis with the aim to design electrolytes for beyond Li-ion battery technology.¹ A non-aqueous multivalent (e.g., Mg²⁺, Ca²⁺ and Zn²⁺) metal cell is one of the potential candidates for a post-lithium-ion battery. The theoretical volumetric capacity of a metal anode coupled with the lack of dendrite formation at a multivalent metal anode provide an attractive opportunity in energy storage.² However, development and commercialization of multivalent batteries are fraught with complications. Thus a fundamental understanding of molecular level properties of these electrolytes is required to improve the electrochemical stability and the charge transfer properties. In this work, we present a multi-scale modelling approach for multivalent salts in various solvents. We uncover a novel effect between concentration dependent ion pair formation and anion stability at reducing potential, e.g., at the metal anode.³ We find that both Mg and Zn electrolytes are highly prone to ion pair formation, even at modest concentrations, for a wide range of solvents with different dielectric constants, which have implications for dynamics as well as charge transfer.^3,4 Specifically, we observe that, at Mg metal potentials, the ion pair undergoes partial reduction at the Mg cation center (Mg²⁺→Mg⁺), which competes with the charge transfer mechanism and can activate the anion to render it susceptible to decomposition.³ It was observed that Zn electrolytes are also prone to similar TFSI^- anion decomposition but with much higher transition state barrier, which results in better reversible Zn deposition on Zn metal anode in non-aqueous Zn electrolytes as compared to Mg electrolytes. Another category of battery that has gained much attention recently includes chemical transformation systems such as Li-S batteries, which stores energy through chemical bonds instead of intercalation. Here, we study the effect of salt anion and the solvent on the solvation structure and dynamics of Li-polysulfide in the solution. It is observed that the polysulfide chain length has a significant effect on the ion-ion and ion-solvent interaction as well as on the diffusion coefficient of the ionic species in solution. In particular, optimal solvent/salt ratio is essential to control the solubility and conductivity as the addition of Li salt increases the solubility but decreases the mobility of the ionic species.⁵ The third energy storage concept is of redox flow batteries, which have shown outstanding promise for grid-scale energy storage with improved grid stability. Here we study ionic liquid tethered ferrocene catholyte used as redox center utilizing the Fc/Fc⁺ reaction for improving the performance of non-aqueous redox flow battery materials. We observed that solubility of Fc/Fc⁺ redox center can be increased up to eight-fold in a mixture of carbonate electrolytes through the modification Fc with a polar tetra alkylammonium functional group containing TFSI^- anion. It was also observed that at solubility limit, the precipitation of solute is initiated through agglomeration of contact-ion pairs due to overlapping solvation shells.^6-7 This works shows that the combination of multi-scale modeling with experimental techniques provides unprecedented insight into the origin of the electrochemical, structural, and transport properties of electrolytes, which is crucial in designing electrolytes for beyond Li-ion batteries.

References

1. Qu, X.; Jain, A.; Rajput, N. N.; Cheng, L.; Zhang, Y.; Ong, S. P.; Brafman, M.; Maginn, E.; Curtiss, L. A.; Persson, K. A., The Electrolyte Genome project: A big data approach in battery materials discovery. Computational Materials Science 2015, 103, 56-67.

2. Muldoon, J.; Bucur, C. B.; Gregory, T., Quest for nonaqueous multivalent secondary batteries: magnesium and beyond. Chem. Rev. 2014, 114 (23), 11683-11720.

3. Rajput, N. N.; Qu, X.; Sa, N.; Burrell, A. K.; Persson, K. A., The Coupling between Stability and Ion Pair Formation in Magnesium Electrolytes from First-Principles Quantum Mechanics and Classical Molecular Dynamics. J. Am. Chem. Soc. 2015, 137 (9), 3411-3420.

4. Han, S.-D.; Rajput, N. N.; Qu, X.; Pan, B.; He, M.; Ferrandon, M. S.; Liao, C.; Persson, K. A.; Burrell, A. K., Origin of Electrochemical, Structural, and Transport Properties in Nonaqueous Zinc Electrolytes. ACS Applied Materials & Interfaces 2016.

5. Rajput, N. N.; Murugesan, V.; Shin, Y.; Han, K. S.; Lau, K. C.; Chen, J.; Liu, J.; Curtiss, L. A.; Mueller, K. T.; Persson, K. A., Elucidating the Solvation Structure and Dynamics of Lithium Polysulfides Resulting from Competitive Salt and Solvent Interactions. Chem. Mater. 2017.

6. Han, K. S.; Rajput, N. N.; Wei, X.; Wang, W.; Hu, J. Z.; Persson, K. A.; Mueller, K. T., Diffusional motion of redox centers in carbonate electrolytes. J. Chem. Phys. 2014, 141 (10), 104509.

7. Han, K. S.; Rajput, N. N.; Vijayakumar, M.; Wei, X.; Wang, W.; Hu, J.; Persson, K. A.; Mueller, K. T., Preferential Solvation of an Asymmetric Redox Molecule. J. Phys. Chem. C 2016, 120 (49), 27834-27839.