Study on the electrochemical characteristics of the advanced electrolyte system based on the functional polymer membrane

Abstract

Polyethylene separator commercialized as the separator for lithium rechargeable battery has advantages of excellent porous structure and high mechanical strength. However, it has low affinity with liquid electrolytes and high cost for materials and process. Therefore, Many studies have been progressed for substitution with other materials or complement its property. In this study, novel polymer matrix membrane was fabricated by phase inversion method, and functional coating system containing of anion receptor and linear polymer was developed. In addition, their electrochemical property was investigated. As the component of matrix, PVC was introduced for mechanical strength of polymer membrane and P(VdF-co-HFP) and PMMA is for enhancement of affinity with liquid electrolyte and of elongation property of membrane. These polymers was dissolved in solvent (DMF/THF). Prepared polymer solution was casted on the glass plate and then immersed into bath of nonsolvent (Water+THF) for phase inversion method. In the case of membrane prepared in the composition of Water/THF 40/60 v/v, ionic conductivity was $1.9 x 10^{-4}S/cm$ and tensile strength was 8.6MPa. To enhance mechanical strength and ionic conductivity, high temperature drawing was introduced to prepared porous membrane. It was performed at $90^\circ C$ (PVC : Tg~80^\circ C)$ and 5mm/min rate. Through high temperature drawing, the crystallinity and porosity was increased, and symmetric membrane structure was formed. After 500% drawing, its tensile strength was 53MPa and ionic conductivity was $1.9 x 10^{-4}S/cm$. The prepared membrane immersed in liquid electrolyte was stabilized up to 4.7V. and for the cycle properties, the discharge capacity of 0.5C rate was 85% of 0.1C rate at initial cycle. In addition, It was maintained to discharge capacity of 95% at $20^{th}$ cycles. To develop novel coating system for PE separator, PVAc and PEG-Borate was selected. PVAc has a good affinity with liquid electrolytes, and PEG...