Investigating the abnormal conductivity behaviour of divalent cations in low dielectric constant tetraglyme-based electrolytes

International audienceSolutions made of tetraglyme (G4) containing Ca(TFSI)2 have been studied as models to understand thesolvation structure and the conductivity properties of multivalent ions in low dielectric constant etherealelectrolytes. These solutions have been characterised using electrochemical impedance spectroscopy,rheological measurement, and Raman spectroscopy. The ionic conductivity of these electrolytes showsan intriguing non-monotonic behaviour with temperature which deviates from the semi-empiricalVogel–Tammann–Fulcher equation at a critical temperature. This behaviour is observed for bothMg(TFSI) 2 and Ca(TFSI)2, but not LiTFSI, indicating a difference in the solvation structure and thethermodynamic properties of divalent ions compared to Li +. The origin of this peculiar behaviour isdemystified using temperature-controlled Raman spectroscopy and first-principles calculationscombined with a thermodynamic analysis of the chemical equilibrium of Ca2+ ion-pairing versussolvation. As long-range electrostatic interactions are critical in solutions based on low dielectricethereal solvents, a periodic approach is here proposed to capture their impact on the solvationstructure of the electrolyte at different salt concentrations. The obtained results reveal that thethermodynamic and transport properties of Ca(TFSI)2/G4 solutions stem from a competition betweenenthalpic (ionic strength) and entropic factors that are directly controlled by the solution concentrationand temperature, respectively. At high salt concentrations, the ionic strength of the solution favours theexistence of free ions thanks to the strong solvation energy of the polydentate G4 solvent conjugatedwith the weak complexation ability of TFSI. At elevated temperatures, the configurational entropyassociated with the release of a coordinated G4 favours the formation of contact ion-pairs due to its flatpotential energy surface (weak strain energy), offering a large configuration space. Such a balancebetween ion-pair association and dissociation not only rationalises the ionic conductivity behaviourobserved for Ca(TFSI)2/G4 solutions, but also provides valuable information to extrapolate the ionictransport properties of other electrolytes with different M(TFSI)n salts dissolved in longer-chain glymesor even poly(ethylene oxide). These findings are essential for the understanding of solvation structuresand ionic transport in low-dielectric media, which can further be used to design new electrolytes forLi-ion and post Li-ion batteries as well as electrocatalysts