Localized High Concentration Electrolytes for High Voltage Lithium-Metal Batteries: Correlation between Salt, Solvent, and Diluent Contents, and Reductive Stability of the Electrolytes
Authors: Perez-Beltran Saul , Balbuena Perla B. , Cao Xia , Zhang Jason 
Published: May 2020

The quest for energy storage devices with high energy densities leads researches to drift away from conventional electrodes for novel architectures able to withstand high lithium loadings and deliver high discharge voltages. Two of the most promising electrode architectures to achieve these goals are the lithium metal anodes and the so-called high-Ni LiNixCoyMn1-x-yO2 (x ≥ 0.6) (NMC) cathodes based on mixed transition metal oxides. These electrodes, however, produce strong reducing/oxidizing environments at the interface with the electrolyte. Dilute electrolyte formulations (dominant in conventional lithium-ion batteries) are inadequate to resist uncontrolled electrolyte decomposition, triggering Li dendritic growth, and prompting low Coulombic efficiency (CE).

There is an urgent need to work on new electrolyte formulations. In this sense, high concentrated electrolytes (HCE) gained attention due to their enhanced reductive and oxidative stability and high carrier density but still suffer from low ionic conductivity, high viscosity, and poor wettability. A promising strategy to overcome these drawbacks is diluting the electrolyte without affecting the original lithium salt – solvent coordination from the HCE, which has proven effective in increasing the CE and producing thinner lithium deposition patterns on the anode compared to dilute electrolytes.

In this work, we examine the liquid structure and electronic properties in dilute, HCE, and localized high concentration electrolytes (LHCE), in an attempt to uncover changes in the solvated complexes based on salt, solvent, and diluent contents, focusing on an electrolyte formulation proven effective in experiments with lithium bis(fluorosulfonyl)imide (LiFSI), dimethyl carbonate (DMC), and bis(2,2,2-triflouroethyl) ether (BTFE) as the diluent. In our calculations, we modeled the liquid electrolytes within a unit cell with periodic boundaries along the x-, y-, and z-coordinates, and followed the formation of solvated structures performing ab-initio molecular dynamics (AIMD).

Examining the dilute and the HCE formulations, besides the depletion of free solvent DMC molecules, we found a change in the coordination behavior of the FSI- anions that trigger the formation of a three-dimensional solution structure. Each FSI- anion coordinates with multiple lithium-ions, shifting the anion LUMO levels lower than the solvent molecules, which explains the improved reductive stability of the HCE formulation and the salt-dominant composition of the SEI films in experiments. Different from previous explanations, however, we do not attribute this improvement to the depletion of free solvent molecules but to lithium-salt pairing mechanisms modified in HCE electrolytes.

For the LHCE formulations, we found that dilution does not influence the three-dimensional solution structure from the equivalent HCE. However, we found there should be a limit beyond which the solvation complexes that exist in the equivalent HCE formulation disappear. In summary, our approach allows for the development of a screening methodology that allows finding a correlation between the salt, solvent, and diluent contents, and the reductive properties of the electrolyte formulation. We believe this methodology constitutes an effective tool to explore further LHCE electrolyte formulations for a new generation of high energy density rechargeable batteries.