Materials science

Composite for energy storage takes the heat

A polymer-based material has been discovered that breaks the rules — it has the right combination of properties for use in energy-storage devices called dielectric capacitors, and can function at high temperatures. See Letter p.576

Devices known as dielectric capacitors have a crucial role in applications that require short, intense power pulses or the conversion of direct current to alternating current. These applications include electronic systems for the integration of energy from renewable sources into power grids1, transport2 and military weapon systems3. They depend on electrically insulating materials known as dielectrics, which come in several types. Polymeric dielectrics offer advantages for large capacitors, but suffer from low operating temperatures (usually well below 150 °C) and low energy density (which means that devices that use polymeric dielectrics occupy large volumes). On page 576 of this issue, Li et al.4 report that a composite of a polymer and nanometre-scale sheets of boron nitride provides more than a 40% improvement in energy density compared with the best-available polymer dielectric, as well as remarkable stability at temperatures up to 300 °C across a wide range of electric-field frequencies.

Dielectric capacitors achieve the highest rate of energy transfer (termed the power or rate capability) of all capacitor types. They store energy through a variety of molecular and nanoscale electron-polarization mechanisms5,6 that create oriented dipoles and associated dipolar electric fields. For high energy density, dielectric materials must have a high density of dipoles that have large induced dipole moments (which provide a measure of a charged system's polarity). A dielectric's rate capability depends on how fast charges polarize and depolarize — how fast the dipoles reorient — as an applied electric field varies. Invariably, not all of the energy stored in dipolar electric fields is recovered on depolarization; some is transferred into molecular translation and vibration (thermal energy) and is lost as heat, a process called dielectric loss.

When and how polarized electrons begin to 'leak' (conduct) through a dielectric depends on a property called the dielectric breakdown field strength (Eb). Relatively small leakage currents may occur at field strengths below Eb. Once the field reaches Eb, it promotes a cascade of electrons into the material's conduction band, resulting in catastrophic breakdown as the dielectric is transformed from an insulator into a conductor — which is bad news for dielectric capacitors. Leakage current also converts electrical energy into thermal energy. If this heat is not efficiently removed, the dielectric's internal temperature rises, amplifying molecular motions and degrading the material's mechanical properties to the point that electromechanical stresses create another mechanism for dielectric breakdown, shifting Eb to lower values.

The search for dielectrics that have high energy density, high rate capability and low conversion of electrical energy into heat has followed two distinct, yet intersecting paths. The first has led towards pure, homogeneous materials that can be synthesized and fabricated into large capacitors at minimal cost. The second has led to heterogeneous, multiphase composites, which sacrifice some of the ease of manufacturing simplicity for an optimal compromise between properties and performance. Homogeneous materials include inorganics such as barium titanate (BTO) and organic polymers such as biaxially oriented polypropylene (BOPP, currently the best polymer dielectric) and polyvinylidiene fluoride (PVDF).

Polymers are the preferred choice for large capacitors because of the ease with which they can be processed and their defects controlled. However, the Moss rule, which originated in the semiconductor field7,8,9, constrains the dielectric properties of homogeneous materials: an increased polarizability is invariably accompanied by a decreased Eb, which in turn lowers the maximum-attainable volumetric energy density, Ǔ.

Heterogeneous materials might be able to get around the Moss rule. Most studies of such materials are variations on a theme: dispersing a suitable filler material (such as BTO) in a polymer may yield a composite with both high polarizability and high energy density, as well as adequate processability for fabrication into large, reliable capacitors. But often, the addition of the filler dramatically reduces Eb, eliminating any advantage. Dielectric losses and thermal management are also unresolved issues — even though these are key issues in the engineering of large capacitors.

Li and colleagues' work addresses a different set of issues that may enable polymers to bend the Moss rule, if not to break it. Scientists from the same research group previously reported10 that blends of boron nitride nanosheets (BNNS) with a PVDF-based polymer resulted in remarkable increases in Eb, Ǔ, charge–discharge efficiency (the fraction of stored energy released on discharge), stiffness and thermal conductivity compared with the pure polymer. These improvements were attributed to suppression of leakage currents by the BNNS. Those nanocomposites achieved Ǔ values up to ten times that of BOPP, but still suffered from high dielectric losses associated with the host polymer.

In the current work, Li et al. blended BNNS with a compound called divinyltetramethyldisiloxane-bis(benzocyclobutene) (BCB), and then reacted the BCB molecules to produce nanocomposites of BNNS in polymeric cross-linked BCB (c-BCB; Fig. 1). The dielectric properties of these composites are remarkably stable over a wide range of temperatures (from room temperature to 300 °C) and field frequencies (100 hertz to 1 MHz). The volumetric energy density and charge–discharge efficiency of c-BCB/BNNS composites at 150 °C greatly exceed those of other polymers designed to work at high temperatures, and maintain meaningful values even at 300 °C — more than 200 °C higher than BOPP's thermal limit for practical use. None of the other candidate high-temperature polymers studied by Li et al. approaches this level of dielectric performance at such high temperatures.

Figure 1: A high-temperature dielectric material.
figure1

Li et al.4 have made a composite material in which boron nitride nanosheets (BNNS) are suspended in a polymeric material, cross-linked divinyltetramethyldisiloxane-bis(benzocyclobutene). In this micrograph, the BNNS are visible as lighter flecks against the darker background of the polymer. The composite has better properties as a dielectric material for energy-storage applications than the best-available polymer dielectrics, and operates at higher temperatures. Scale bar, 5 micrometres. (Image from ref. 4.)

Li and co-workers observed that BNNS suppress leakage currents in the nanocomposite by about a factor of ten compared with pristine c-BCB, even at high temperatures. Remarkably, BNNS also increase the polymer's Eb value by 30–50%. More work should be carried out to better understand the underlying mechanisms for suppression of current leakage and dielectric breakdown by BNNS, which form the basis by which c-BCB/BNNS apparently circumvents the Moss rule. Finally, the researchers report that BNNS increase the polymer's thermal conductivity sixfold, and double its stiffness at temperatures above 150 °C. These attributes will help c-BCB/BNNS composites to stay cool and retain electromechanical stability under continuous charge–discharge cycling, reducing or eliminating the need for auxiliary thermal-management systems.

One drawback of the new composites is that BCB will be more expensive to produce than BOPP's monomeric precursor, and blending BNNS into c-BCB represents an additional processing step compared with the manufacture of a pure polymer. It also remains to be seen what effects the BNNS will have on the number of defects and long-term reliability of the composites. Nonetheless, the relatively low filler loading of the nanocomposites, and the fact that irreversible cross-linking can be induced on heating or irradiation with ultraviolet light, will facilitate the development of 'roll-to-roll' processes for producing dielectric polymer films, which may help to control manufacturing costs for capacitors. If the advantages of the BNNS filler can be translated to other dielectric polymers that have higher polarizabilities than c-BCB, then even more remarkable advances in discharged energy density can be anticipated — as well as a reduction in the size of dielectric capacitors for electronics in power systems.Footnote 1

Notes

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Correspondence to Harry J. Ploehn.

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Ploehn, H. Composite for energy storage takes the heat. Nature 523, 536–537 (2015). https://doi.org/10.1038/523536a

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