A brief historical review introduced Michaël Faraday's first observations of ionic conduction in solids in 1834, and mentioned the theory of electrolytic dissociation proposed in 1884. Liquid electrolytes, which are substances in which ions appear surrounded by a sphere of solvation, were then introduced, while defining some of the key physico-chemical parameters that characterize them.
Thus, the mobility (μi) of an ion and its transport number (tli+) were defined, as were a solvent's dielectric constant (ε), which controls its dissociative power, and its viscosity (η). The ambivalent effect of these last two parameters on the ionic conductivity of an electrolyte (solute + solvent) has been illustrated, and the harmful phenomenon of ion aggregation on conductivity described.
On the basis of these fundamental notions for establishing the specifications of an ideal solvent and solute, the intellectual path leading to the production of current electrolytes used in Li ion batteries was detailed. The magic electrolyte to date, based mainly on the use of a Li salt (LiPF6), whose anion has a delocalized charge to be less reactive and is dissolved in a mixture of polar aprotic solvents such as cyclic and acyclic carbonates, was presented.
Current research into better salts and solvents was also reported. This research obviously integrates the electrochemical compatibility of these electrolytes with electrode materials, introducing notions of electrolyte thermodynamic stability via the determination of their HUMO and LOMO orbital boundaries.
Based on these considerations, the notion of SEI(solid electrolyte interface ), whose formation results from parasitic reactions, has been introduced.
