(625d) Designing Optimal Electrolytes for Next Generation Energy Storage

Authors: 
Rajput, N. N., Tufts University
Persson, K., Lawrence Berkeley Lab
Today, the pursuit of transformative gains in the performance of electrical energy storage (EES) systems and industrial technologies are intrinsically a material’s problem, which requires development of novel electrode materials, electrolytes, and architecture. Such innovations of novel electrode as well as electrolytes for future EES systems lie inherently in the computationally driven design of materials by obtaining a fundamental understanding of the interplay between events scaling over wide spatial and temporal ranges. My work comprises of developing infrastructure for high throughput computations for the Joint Center for Energy Storage (JCESR) and Electrolyte Genome (Fig.1), which is a first-principle code-base for rapid understanding and design of optimal electrolytes. The composition of electrolytes has critical implications for the performance of current and future energy storage systems; from the formation and stability of the electrode-electrolyte interface to the transport properties, speciation, and viscosity of the bulk electrolyte. In this work, I present a multi-scale modeling approach for salts and solvents important to multivalent (e.g., Mg2+, Ca2+ and Zn2+), chemical transformation (e.g., Li-S), and redox flow batteries. We uncover a novel effect between concentration dependent ion pair formation and anion stability at reducing potential, e.g., at the metal anode. 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. 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.1-2 We also study the effect of salt anion and the solvent on the solvation structure and dynamics of Li-polysulfide in the solution for application in Li-S battery.3 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. We considered ionic liquid tethered ferrocene catholyte as redox center utilizing the Fc/Fc+ reaction for improving the performance of non-aqueous redox flow battery materials.4 It was observed that at solubility limit, the precipitation of solute is initiated through agglomeration of contact-ion pairs due to overlapping solvation shells. This works shows that the combination of modeling with experimental techniques provides unprecedented insight into the origin of the electrochemical, structural, and transport properties of electrolytes, which is crucial in designing optimal electrolytes for beyond Li-ion batteries.

References

  1. 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. Journal of the American Chemical Society 2015.
  2. Hu, J. Z.; Rajput, N. N.; Wan, C.; Shao, Y.; Deng, X.; Jaegers, N. R.; Hu, M.; Chen, Y.; Shin, Y.; Monk, J.; Chen, Z.; Qin, Z.; Mueller, K. T.; Liu, J.; Persson, K. A., 25Mg NMR and computational modeling studies of the solvation structures and molecular dynamics in magnesium based liquid electrolytes. Nano Energy 2018.
  3. 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.
  4. 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.