Giant energy storage density in PVDF with internal stress engineered polar nanostructures

Xintong Ren,Nan Meng,Haixue Yan,Han Zhang,Jiyue Wu,Michael John Reece,Emiliano Bilotti,Isaac Abrahams

doi:10.1016/j.nanoen.2020.104662

Abstract

Abstract High power dielectric capacitors with high energy density are needed in order to develop modern electronic and electrical systems, including hybrid vehicles, telecommunication infrastructures and portable electronic devices. Relaxor ferroelectric polymers (RFP) are considered to be the most promising candidates for the next generation of capacitors owing to their relatively high energy storage density. However, the commercialization of RFP capacitors in power systems is hindered by their high cost and low dielectric breakdown strength. In this study, inexpensive, free-standing nano-crystalline (~3.3 nm) poly (vinylidene fluoride) (PVDF) films with high β phase content (~98%), “relaxor-like” ferroelectric behaviour and high breakdown strength (880 kV/mm) were fabricated using the facile Press & Folding (P&F) technique. An internal stress dominated polarization switching model is proposed to explain the origin of the relaxor-like ferroelectric behaviour. The internal stress generated during pressing alters the intermolecular chain distance of the (200) plane of β-PVDF from 4.24 A in internal stress free films to 4.54 A in P&F films, corresponding to a tensile strain and residual stress of 7.11% and 142 MPa, respectively. The internal stress acts to partially reverse the polarization on reversal of the applied electric field. This, combined with preferred in-plane orientation of the crystallites, results in a polar nanostructure with high polarization reversibility at high electric fields. A giant discharged energy storage density of 39.8 J/cm3 at 880 kV/mm was achieved for P&F films, which surpasses all previously reported polymer-based materials.

Highlights

Among all of the energy storage methods, dielectric capacitors excel in terms of stability, ultrahigh power density and fast charging-discharging speed (~10 ns – 1 ms)
We report the mechanism by which Press & Folding (P&F) produces relaxor-like ferroelectric behaviour in Polyvinylidene fluoride (PVDF), and use this knowledge to optimise its energy storage properties
The pristine hot-pressed PVDF films behaved like linear dielectrics, with a maximum field-induced polari zation Pin-max of only 0.032 C/m2, which is explained by their predom inantly non-polar α phase content that requires a much higher applied electric field (500 kV/mm) to achieve the ferroelectric β phase trans formation [48]

Summary

Introduction

Among all of the energy storage methods, dielectric capacitors excel in terms of stability, ultrahigh power density (up to 108 W/kg) and fast charging-discharging speed (~10 ns – 1 ms). They have great potential in advanced electronic and electrical systems, wearable devices, medical apparatus and electric vehicles [1]. PVDF has a high remnant polarization Pr produced by ferroelectric switching, which severely decreases its charge-discharge efficiency and restricts it from achieving high Ue [12] To overcome these barriers, methods for producing relaxor ferroelectric behaviour with more reversible polar structures under high fields have been explored, including chemical modification

Methods

Results

Conclusion