posted on 2017-12-04, 00:00authored byPieter Geysens, Vijay Shankar Rangasamy, Savitha Thayumanasundaram, Koen Robeyns, Luc Van Meervelt, Jean-Pierre Locquet, Jan Fransaer, Koen Binnemans
Electrolytes
consisting of sodium bis(fluorosulfonyl)imide (NaFSI)
dissolved in glymes (monoglyme, diglyme, and triglyme) were characterized
by FT-Raman spectroscopy and 13C, 17O, and 23Na NMR spectroscopy. The glyme:NaFSI molar ratio was varied
from 50:1 to 1:1, and it was observed that, in the dilute electrolytes,
the sodium salt is completely dissociated into solvent separated ion
pairs (SSIPs). However, contact ion pairs (CIPs) and aggregates (AGGs)
become the predominant species in more concentrated solutions. Some
of the electrolytes with the highest concentrations can be classified
as solvate ionic liquids (SILs), where all of the solvent molecules
are coordinated to sodium cations. Therefore, these electrolytes are
fundamentally different from more dilute electrolytes which are typically
used in commercially available secondary batteries. The melting point
or glass transition temperature, dynamic viscosity, density, sodium
concentration, and ionic conductivity of these solvate ionic liquids
are reported as well as the crystal structures of [Na(G3)][FSI] and
[Na(G3)2][FSI]. Galvanostatic cycling experiments were
performed in coin-type cells with a Na2/3[Mn0.55Ni0.30Co0.15]O2 cathode to study
the influence of these electrolytes on the electrochemical stability
and charge/discharge behavior.