Supercapacitors Using Separators With Piezoelectric Effect For Reduced Self-discharge

Chemical double electric layer capacitor(EDLC)is an important electrochemical energy storage device with high energy density,fast charging and discharging speed,long cycle life,excellent safety performance.However,self-discharge is a problem that cannot be ignored for the practical application of supercapacitors.The self-discharge pathway of supercapacitor mainly includes charge redistribution,Ohmic leakage and Faraday reaction.Usually,supercapacitors exhibit a greater self-discharge rate than most batteries,which not only results in the loss of energy storage efficiency,but also affects the reliability of the whole system containing the supercapacitor,severely limiting the practical application and further development of supercapacitors.In this work,we investigate the effect of diaphragm with piezoelectric effect on the self-discharge of supercapacitor,and PVDF porous film with piezoelectric properties is selected as the object of study.Combining PVDF porous films with piezoelectric properties and supercapacitor self-discharge provides a new idea for the study of supercapacitor self-discharge.This paper focuses on the preparation of PVDF porous films with piezoelectric properties and its study of the effect of its use as a diaphragm on supercapacitor self-discharge,as follows.(1)Different methods for the preparation of PVDF porous films were investigated,and the optimal parameters for the preparation of PVDF films by phase transition method were investigated,and uniform PVDF porous films were produced by this method.Then PVDF porous films with piezoelectric properties were obtained by polarization treatment.Finally,the morphology and crystallization were characterized by SEM and XRD,and the piezoelectric properties were also tested,proving that the films have excellent piezoelectric properties.(2)The piezoelectric PVDF diaphragm is used in supercapacitors and its electrochemical performance is tested.The experimental results show that the use of P-PVDF diaphragm as a supercapacitor diaphragm does not cause any loss of electrochemical performance,but still has good capacitive performance,excellent charging and discharging efficiency and long cycle stability.In addition,the piezoelectric PVDF diaphragm exhibits an ultra-low leakage current(3.8 ?A)and low self-discharge behavior,with an OCV retention rate of 77% after the capacitor has been charged and left in open circuit for 24 h.Based on the analysis of the self-discharge mechanism of the supercapacitor,it is speculated that the greatly reduced self-discharge rate may be due to the PVDF diaphragm with piezoelectric properties that can bi-directionally inhibit the ion shuttle inside the capacitor,resulting in a slower ion diffusion rate.Therefore,the self-discharge of the dual-layer supercapacitor,which is mainly caused by ion diffusion,is effectively reduced.