The electrolyte plays an essential role in lithium-ion batteries. It is composed of a solvent such as ethylene carbonate, in which a salt is dissolved (LiPF6 for instance). Its function is to promote the transportation of matter (Li+ ions) from one electrode to the other, a process which drives, in principle, the amount of energy released by the accumulator. Last on the list of travel preparations is a first solvation layer that forms around lithium ions, comprising solvent molecules and anions (negatively charged ions associated to the salt). Yet the evolution of the structure of this first envelop under varying salt concentrations still remains poorly known.
This topic is what researchers from INAC and LITEN have decided to study. They performed computer simulations, which they compared to experimental results from a Chinese-American team. They, then, were able to explain why the electrolyte ionic conductivity collapses when anion concentration increases: the structure of the complex that traps lithium ions has half as much ethylene carbonate! The available space, therefore, is occupied by the anion, which neutralizes the charge of solvated lithium ions, thus significantly reducing its mobility under applied potential.
The vibrational spectra of complexes were carried out using an ultrashort pulse laser (femtosecond, 10-15 s) at Rice University in Houston, USA.