Roushanak Nejat

The influence of high-intensity electric fields on the stability of polymeric materials is a problem of interest in the design of next-generation energy storage and electronic devices, and for understanding the limits of stability of polymer films exposed to large electric fields generally. Here, we show that the dielectric strength of entangled glassy polymer films increases as an inverse power-law of the film thickness h for "ultrathin" films below a micron in thickness. The dielectric strength enhancement in these polymer films becomes as large as ≈2 GV/m in films thinner than 100 nm, but in this thickness regime, the increase of the dielectric strength depends strongly on the polymer mass, sample aging time, and the method of film preparation. The enhancement of the dielectric breakdown strength is attributed to the mechanical instability of the elastic film subjected to sufficiently large electric fields and a large, but not generally well-understood, enhancement of effective stiffness of entangled glassy polymer films subject to large deformations, an effect that has previously been observed to become greatly enhanced when such films are made thinner. As a proof of principle regarding applications, we utilize ultrathin glassy polymer films of the type studied in our paper to fabricate polymeric nanocapacitors having ultrahigh discharge energy densities (U _d ^max) as large as 27 J/cm³ and having efficiencies greater than 80%. These efficiency values at comparable charge densities are significantly higher than those of competing ferroelectric polymer materials, and we anticipate that our observations will inspire the creation of practical high-energy density nanocapacitor devices for advanced energy storage applications.

Dielectric capacitors are critical components in electronics and energy storage devices. The polymer-based dielectric capacitors have the advantages of device flexibility, fast charge-discharge rates, low loss, and graceful failure. Elevating the use of polymeric dielectric capacitors for advanced energy applications such as electric vehicles (EVs), however, requires significant enhancement of their energy densities. Here, we report a polymer thin film heterostructure-based capacitor of poly(vinylidene fluoride)/poly(methyl methacrylate) with stratified 2D nanofillers (Mica or h-BN nanosheets) (PVDF/PMMA-2D fillers/PVDF), that shows enhanced permittivity, high dielectric strength, and an ultrahigh energy density of ≈75 J/cm³ with efficiency over 79%. Density functional theory calculations verify the observed permittivity enhancement. This approach of using oriented 2D nanofillers-based polymer heterostructure composites is expected to be versatile for designing high energy density thin film polymeric dielectric capacitors for myriads of applications.

The maximum capacitive energy stored in polymeric dielectric capacitors, which are ubiquitous in high-power-density devices, is dictated by the dielectric breakdown strength of the dielectric polymer. The fundamental mechanisms of the dielectric breakdown, however, remain unclear. Based on a simple free-volume model of the polymer fluid state, we hypothesized that the free ends of linear polymer chains might act as "defect" sites, at which the dielectric breakdown can initiate. Thus, the dielectric breakdown strength of cyclic polymers should exhibit enhanced stability in comparison to that of their linear counterparts having the same composition and similar molar mass. This hypothesis is supported by the ∼50% enhancement in the dielectric breakdown strength and ∼80% enhancement in capacitive energy density of cyclic polystyrene melt films in comparison to corresponding linear polystyrene control films. Furthermore, we observed that cyclic polymers exhibit a denser packing density than the linear chain melts, an effect that is consistent with and could account for the observed property changes. Our work demonstrates that polymer topology can significantly influence the capacitive properties of polymer films, and correspondingly, we can expect polymer topology to influence the gas permeability, shear modulus, and other properties of thin films dependent on film density.