Revealing the Nanostructures of Liquid Electrolytes By X-Ray Scattering

Abstract

The solution structure of the liquid electrolyte directly affects its ion migration ability and interfacial insertion chemistry, which ultimately determines the performance of batteries. Herein, small-angle X-ray scattering-wide angle X-ray scattering (SAXS-WAXS) is employed to investigate the diffuse structure of carbonate electrolytes containing lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and lithium hexafluorophosphate (LiPF6), respectively. As the salt concentration increases, a upheave behavior is observed in both electrolyte systems at the high q part, which is caused by the cation-solvent interaction and charge ordering formation. Notably, at the low q part, we observe another gel polymer-like structure in the LiPF6 electrolyte system while the LiTFSI electrolytes do not have. It is believed that the anions cause this polymer-like structure, that is PF6- have strong hydrogen bonding ability than TFSI anions and, to some extent, cross-links solvent molecules. Raman, nuclear magnetic resonance (NMR), and molecular dynamics (MD) simulation further support the proposed structures. The structure-electrochemical performance relationship study suggests that LiPF6 electrolytes is more conducive to form stable SEI and reduce solvent-electrode side reactions. This work highlights the fundamental structure analysis in nanoscale, which will prompt better electrolytes in the future.