Effect of Anion Size on Conductivity and Transference Number of Perfluoroether Electrolytes with Lithium Salts

Abstract

Mixtures of perfluoropolyethers (PFPE) and lithium salts with fluorinated anions are a new class of electrolytes for lithium batteries. Unlike conventional electrolytes wherein electron-donating oxygen groups interact primarily with the lithium cations, the properties of PFPE-based electrolytes appear to be dependent on interactions between the fluorinated anions and the fluorinated backbones. We study these interactions by examining a family of lithium salts wherein the size of the fluorinated anion is systematically increased: lithium bis(fluorosulfonyl)imide (LiFSI), bis(trifluoromethanesulfonyl)imide (LiTFSI) salts and lithium bis(pentafluoroethanesulfonyl)imide (LiBETI). Two short chain perfluoroethers (PFE), one with three repeat units, C6-DMC, and another with four repeat units, C8-DMC were studied; both systems have dimethyl carbonate end groups. We find that LiFSI provides the highest conductivity in both C6-DMC and C8-DMC. These systems also present the lowest interfacial resistance against lithium metal electrodes. The steady-state transference number (t + ss ) was above 0.6 for all of the electrolytes and was an increasing function of anion size. The product of conductivity and the steady-state transference number, a convenient measure of the efficacy of the electrolytes for lithium battery applications, exhibited a maximum at about 20 wt% salt in all electrolytes. Amongst the systems studied, LiFSI/PFE mixtures were the most efficacious electrolytes.

Fluorine and Lithium: Ideal Partners for High‐Performance Rechargeable Battery Electrolytes

“…This effect was systematically considered for systems based on LiFSI, LiTFSI, and lithium bis(pentafluoroethanesulfonyl)imide (LiBETI). [26,34] Angewandte Chemie Reviews 2.2. Thermal Stability One of the crucial parameters for battery performance is the thermal stability of the electrolyte salt, especially at elevated/high temperatures,a st hermally induced decomposition products may alter the physicochemical properties of the electrolyte and interfere with reactions taking place at electrodes.…”

Multivalent ion conduction in solid polymer systems

“…It is also possible to design polymers in which the solvating groups primarily interact with the anion rather than cation species through the addition of either Lewis-acidic moieties into the polymer [88][89] or other groups that preferentially solvate specific anions, such as fluorination of the polymer backbone to sequester fluorinated anions. 90 A PEO-based network with boroxine ring crosslinkers acts as an anion trap and increases Li + transference numbers to 0.75 for bulky salts such as LiCF3SO3 and LiBF4, with additional improvement to 0.88 for LiCl. 91 Similar studies were performed for borate ester networks mixed with Mg 2+ salts with perchlorate, triflate and TFSIanions.…”

Lithium and magnesium polymeric electrolytes prepared using poly(glycidyl ether)-based polymers with short grafted chains

“…The differences in the trends between the conductivities of the LiTFSI and LiCl complexes may also reflect the differences in the interactions of the two anions with the host polymers. 30,56,58 The strong dipole interaction of the sulfone group plays a major role in the conductivity of PEsulfoPGE, outperforming even PAGE which has a lower T g of about 40°C.…”