Quasi-Solid State Polymer Electrolytes Based on PVdF-HFP Host Polymer for Sodium-Ion Secondary Batteries

Prices of lithium raw materials keep on increasing exponentially due to their heavy consumption for lithium batteries used in portable electronic devices as well as automobiles. Also, the global lithium deposits are very limited. Hence, sodium-ion batteries (SIBs) have been heavily investigated as cheaper alternatives to expensive lithium-ion batteries, mainly due to the abundance of sodium raw materials. However, one of the major bottlenecks faced by the material research community to commercialize SIBs is the poor ionic conductivity of sodium-ion conducting electrolytes at ambient temperature, especially in the solid-state. Very recently, quasi-solid state polymer electrolytes (QSSPEs) have been proposed to overcome this challenge. In this work, a set of QSSPEs have been synthesized by using poly (vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP) host polymer and NaBF4 ionic salt dissolved in EC/PC plasticizer/solvent mixture. The highest conducting composition; 6 PVdF-HFP: 14 NaBF4: 40 EC: 40 PC (wt.%); showed an ambient temperature ionic conductivity of 4.1x10-3 S cm-1. The activation energy is almost same for all the sample compositions studied in this work suggesting that the activation process is mainly controlled by EC/PC. DC polarization test on highest conducting electrolyte composition with a configuration of SS/QSSPE/SS revealed that the electrolyte is predominantly ionic conductor with negligible electronic conductivity; a much desired property for a good electrolyte. Linear sweep voltammetric studies confirmed that the electrochemical stability window of the highest conducting electrolyte is about 3.6 V. This highest conducting electrolyte composition is found to be highly suitable for practical applications in sodium batteries.