The Anion Effect on Li+ Ion Coordination Structure in Ethylene Carbonate Solutions

TL;DR

This study reveals that in lithium carbonate electrolytes, the first solvation shell of Li+ ions contains only two ethylene carbonate molecules, influenced by anions, which impacts ionic conductivity at high concentrations.

Contribution

It provides the first experimental evidence of anion effects on Li+ solvation structure in carbonate electrolytes, challenging previous assumptions of tetrahedral coordination.

Findings

Li+ solvation shell contains only two EC molecules at concentrations above 0.5 M

Anions in the first shell modify the typical tetrahedral structure

The structure explains ionic conductivity decline at high electrolyte concentrations

Abstract

Rechargeable lithium ion batteries are an attractive alternative power source for a wide variety of applications. To optimize their performances, a complete description of the solvation properties of the ion in the electrolyte is crucial. A comprehensive understanding at the nanoscale of the solvation structure of lithium ions in nonaqueous carbonate electrolytes is, however, still unclear. We have measured by femtosecond vibrational spectroscopy the orientational correlation time of the CO stretching mode of Li+-bound and Li+-unbound ethylene carbonate molecules, in LiBF4, LiPF6, and LiClO4 ethylene carbonate solutions with different concentrations. Surprisingly, we have found that the coordination number of ethylene carbonate in the first solvation shell of Li+ is only two, in all solutions with concentrations higher than 0.5 M. Density functional theory calculations indicate that the presence of anions in the first coordination shell modifies the generally accepted tetrahedral structure of the complex, allowing only two EC molecules to coordinate to Li+ directly. Our results demonstrate for the first time, to the best of our knowledge, the anion influence on the overall structure of the first solvation shell of the Li+ ion. The formation of such a cation/solvent/anion complex provides a rational explanation for the ionic conductivity drop of lithium/carbonate electrolyte solutions at high concentrations.