Journal
COLLECTION OF CZECHOSLOVAK CHEMICAL COMMUNICATIONS
Volume 73, Issue 12, Pages 1777-1798Publisher
INST ORGANIC CHEM AND BIOCHEM
DOI: 10.1135/cccc20081777
Keywords
LiTFSI; Dilithium N,N '-bis(trifluoromethanesulfonyl)perfluoroalkane-1,omega-disulfonamide salts; Cross-linked poly(ethylene glycol); Solid polymer electrolyte; Ionic conductivity; PEG
Categories
Funding
- DoD [DAAG 55-98-1-Q004]
- DOE [DE-AC03-76SF00098]
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The present work consists of a series of studies with regard to the structure and charge transport in solid polymer electrolytes (SPE) prepared using various new bis(trifluoromethanesulfonyl)imide (TFSI)-based dianionic dilithium salts in crosslinked low-molecular-weight poly(ethylene glycol). Some of the thermal properties (glass transition temperature, differential molar heat capacity) and ionic conductivities were determined for both diluted (EO/Li = 30:1) and concentrated (EO/Li = 10:1) SPEs. Trends in ionic conductivity of the new SPEs with respect to anion structure revealed that while for the dilute electrolytes ionic conductivity is generally rising with increased length of the perfluoroalkylene linking group in the dianions, for the concentrated electrolytes the trend is reversed with respect to dianion length. This behavior could be the result of a combination of two factors: on one hand a decrease in dianion basicity that results in diminished ion pairing and an enhancement in the number of charge carriers with increasing fluorine anion content, thereby increasing ionic conductivity while on the other hand the increasing anion size and concentration produce an increase in the friction/entanglements of the polymeric segments which lowers even more the reduced segmental motion of the crosslinked polymer and decrease the dianion contribution to the overall ionic conductivity. DFT modeling of the same TFSI-based dianionic dilithium salts reveals that the reason for the trend observed is due to the variation in ion dissociation enthalpy, derived from minimum-energy structures, with respect to perfluoroalkylene chain length.
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