High Performance Ternary Solid Polymer Electrolytes Based on High Dielectric Poly(vinylidene fluoride) Copolymers for Solid State Lithium-Ion Batteries

Renewable energy sources require efficient energy storagesystems.Lithium-ion batteries stand out among those systems, but safety andcycling stability problems still need to be improved. This can beachieved by the implementation of solid polymer electrolytes (SPE)instead of the typically used separator/electrolyte system. Thus,ternary SPEs have been developed based on poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) and poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene),P(VDF-TrFE-CFE) as host polymers, clinoptilolite (CPT) zeolite addedto stabilize the battery cycling performance, and ionic liquids (ILs)(1-butyl-3-methylimidazolium thiocyanate ([BMIM][SCN])), 1-methyl-1-propylpyrrolidiniumbis(trifluoro-methylsulfonyl)-imide ([PMPyr][TFSI]) or lithiumbis(trifluoromethane-sulfonyl)-imide (LiTFSI), incorporatedto increase the ionic conductivity. The samples were processed bydoctor blade with solvent evaporation at 160 & DEG;C. The nature ofthe polymer matrix and fillers affect the morphology and mechanicalproperties of the samples and play an important role in electrochemicalparameters such as ionic conductivity value, electrochemical windowstability, and lithium-transference number. The best ionic conductivity(4.2 x 10(-5) S cm(-1)) and lithiumtransference number (0.59) were obtained for the PVDF-HFP-CPT-[PMPyr][TFSI]sample. Charge-discharge battery tests at C/10 showed excellentbattery performance with values of 150 mAh g(-1) after50 cycles, regardless of the polymer matrix and IL used. In the rateperformance tests, the best SPE was the one based on the P(VDF-TrFE-CFE)host polymer, with a discharge value at C-rate of 98.7 mAh g(-1), as it promoted ionic dissociation. This study proves for the firsttime the suitability of P(VDF-TrFE-CFE) as SPE in lithium-ion batteries,showing the relevance of the proper selection of the polymer matrix,IL type, and lithium salt in the formulation of the ternary SPE, inorder to optimize solid-state battery performance. In particular,the enhancement of the ionic conductivity provided by the IL and theeffect of the high dielectric constant polymer P(VDF-TrFE-CFE) inimproving battery cyclability in a wide range of discharge rates mustbe highlighted.

Automated summary

Renewable energy sources require efficient energy storage systems, and lithium-ion batteries are a standout option. However, there is still room for improvement in terms of safety and cycling stability. By implementing solid polymer electrolytes (SPEs) instead of the commonly used separator/electrolyte system, these issues can be addressed. This study focuses on the development of ternary SPEs based on specific host polymers, fillers, and ionic liquids to enhance the performance of lithium-ion batteries in terms of conductivity, stability, and cyclability.