Effect of fluorinated polymer matrix type in the performance of solid polymer electrolytes based on ionic liquids for solid-state lithium-ion batteries

Solid polymer electrolytes (SPEs) have been produced using an ionic liquid (IL) (1-Methyl-1-propylpyrrolidinium bis(trifluoromethylsulfonyl)imide - [PMPyrr][TFSI]) embedded in different fluorinated polymer matrices, including poly(vinylidene-fluoride) - PVDF, poly(vinylidene fluoride-co-hexafluoropropylene) - P(VDF-HFP), poly(vinylidene fluoride-co-trifluoroethylene) - P(VDF-TrFE) and poly(vinylidene fluoride-co-trifluoroethylene-chlorofluoroethylene) - P(VDF-TrFE-CFE), in order to evaluate the effect of the polymer chain polarity, degree of crystallinity and dielectric constant on the electrochemical properties and battery performance. It is shown that the use of the different polymers for SPE development does not have significant influence on the morphology, and thermal properties of the samples. The degree of crystallinity is significantly reduced for the P(VDF-TrFE-CFE) sample. The mechanical characteristics (Young modulus and yield strength and strain) are also associated with the crystallinity degree, being reduced for lower crystallinities. Regarding the electrochemical parameters, the samples present considered high ionic conductivity, with a maximum room temperature value of 6.2 x 10-5 S cm-1 for the P(VDF-HFP) sample. The lithium transference number for all samples show their suitability for application in lithium-ion batteries with an outstanding value of 0.71 for the P (VDF-TrFE-CFE) sample associated with the higher dielectric constant and lower degree of crystallinity of this polymer. Battery cycling tests show a maximum initial discharge capacity of 146 mAh/g for the P(VDF-TrFE) sample at room temperature and C/10 rate. Furthermore, the P(VDF-TrFE-CFE) sample presents an excellent ability to cycle at high discharge rates, achieving 91 mAh/g at 1C rate, an maintaining a high stability after 60 cycles at room temperature, showing excellent potential for application in lithium-ion batteries due to its low crystallinity and high dielectric constant. This work proves the suitability of fluorinated polymer based SPE with embedded ionic liquids for the next generation of solid-state batteries and provides a valuable insight on the role of the polymer dielectric constant and degree of crystallinity on battery performance.