Sandwiching energy storage success

Materials World magazine
,
3 Oct 2016

The transition from 2D nanocomposite films to 3D sandwich structures has enabled for more efficient energy storage materials. Khai Trung Le reports.

A 3D sandwich-like structure is reportedly the key to creating a polymer dielectric material with high energy and power density for electric vehicles, according to recent findings from a Pennsylvania State University, USA, research team.

Qing Wang, Professor of Materials Science at Penn State, said, ‘Polymers are ideal for energy storage for transportation due to their light weight, scalability and high dielectric strength.’ However, biaxially oriented polypropylene (BOPP), a 2D polymer film and the most widely commercially used polymer in hybrid and electric vehicles, is unable to withstand the high operating temperatures without additional cooling equipment that adds weight and cost.

Noting that increasing the dielectric constant and strength of the electric field is at odds with stability and charge-discharge efficiency, increasing the likelihood of leaking energy as heat, the Penn State research team turned to the 3D structure. ‘That’s why we developed this sandwich structure. We have the top and bottom layers that block charge injection from the electrodes. Then in the central layer we can put all of the high dielectric constant ceramic/polymer filler material that improves the energy and power density,’ Wang continued.

The outer layers comprise boron nitride nanosheets in a polymer matrix, surrounding the barium titanate central layer. Wang noted, ‘We can operate this material at high temperature for 24 hours straight over more than 30,000 cycles and it shows no degradation.’

3>2

The research team originally sought to mix different materials to resolve the divide between charge-discharge efficiency and energy leak in the 2D form. However, while they were able to increase the energy capacity, the 2D film broke down at high temperatures when electrons moved away from the electrodes and into the polymer, limiting its application.

With the sandwich structure, named SSN-x, the team was able to integrate the polymer nanocomposites to enable both the ‘complementary properties of spatially organised multicomponents in a synergistic fashion to raise dielectric constant, and subsequently greatly improve discharged energy densities, while retaining low loss and high charge-discharge efficiency at elevated temperatures’. This is detailed in the paper, Sandwich-structured polymer nanocomposites with high energy density and great charge-discharge efficiency at elevated temperature, published in PNAS.

SSN-x has a similar charge-discharge energy at 150˚C and 200 MV/m-1, as BOPP reaches the same degree of charge-discharge energy at 70˚C, outperforming current top-level 2D polymer-based dielectrics at operating conditions suitable for electric vehicles. The higher energy density also enables weight and size reduction suitable for aerospace application.

Explicitly, the Penn State team claims that its findings may pave the way for power modules in harsh operating temperatures, with Wang commenting, ‘Our next step is to work with a company or with more resources to do processability studies to see if the material can be produced at a larger scale at a reasonable cost. We have demonstrated the materials performance in the lab. We are developing a number of state-of-the-art materials working with our theory colleague Long-Quen Chen in our department. Because we are dealing with a 3D space, it is not just selecting the materials, but how we organise the multiple nanosized materials in specific locations. Theory helps us design materials in a rational fashion.’