Qi Li Research Group
Lab of Advanced Nanodielectrics for Energy Storage, Control and Conversion

High-Temperature Dielectrics

Polymer dielectrics are materials of choice for many power electronics, power conditioning, and pulsed power applications. For example, biaxially oriented polypropylene (BOPP) film capacitors are currently used in hybrid and electric vehicles to control and convert DC current from vehicle batteries into AC current required to power the motor. However, dielectric polymers are limited to relatively low working temperatures. To accommodate BOPPs in electric vehicles where temperature can go up to 140 °C, an additional cooling system besides the cooling for electric motor has to be employed to decrease the temperature to ~70 °C; this unfortunately brings extraweight, volume, cost and energy consumption to the integrated power system. The upsurge in lightweight and flexible electronic devices has also created atremendous demand for high temperature dielectric polymers, as heat generated by electronics increase exponentially with miniaturization and functionality of the circuits. We aim to develop novel polymer-based dielectrics with extraordinary high-temperature and high-electric-field dielectric and energy storage properties.

Smart Dielectrics

Flexible electronics that can be bent, stretched and conformed to intricate surfaces is emerging as disruptive technology in numerous applications such as wearable electronics, implantable sensors and alternative energy devices. The functional materials used inflexible devices, however, are susceptible to mechanical deformation induced damage, resulting in loss of functionality that seriously limits device reliability and lifetime. While currents efforts are mainly focused on self healable electrical conductors, it is noted that, besides electrical conductors, there are a variety of requisite functional components in electronic devices, which also need to be developed for future integrated function devices with self healing features. One of such indispensable elements in microelectronics is dielectric materials utilized in electronic insulation and packaging. Besides, the reliability and service lifetime under high voltage conditions is another critical issue for the next generation dielectric materials. For example, it would be necessary to develop smart insulation materials that not only possess good insulating properties, but also have the ability to repair the electrical damage and eventually restore the insulating properties. Another attractive aspect of smart dielectrics would be their capability for energy interconversion. Dielectrics of this class would find applications in energy harvesting, soft robotics, flexible electronics, among others.

High-Energy-Density Dielectrics

Electrical energy storage plays an essential role in advanced electronics and electrical power systems. Among currently available electrical energy storage devices, dielectric capacitors possess the highest power density because of their fast discharge capability, yet they are limited by energy densities that are at least an order of magnitude lower than those of the electrochemical devices such as batteries and electrochemical capacitors. For instance, the energy density of most commercially available electrochemical capacitors to date is in the range of 5 to 8 Wh/L, while that of the best commercial capacitor film represented by biaxially oriented polypropylenes (BOPP) is only about 0.3 Wh/L. Since capacitors can contribute more than 25% of the volume and weight to power electronics and pulsed power systems, dramatic improvement of the energy density of capacitors would be essential to realize their full potential as an enabling technology. It is envisaged that the integration of complementary elements such as large dielectric constant (K) from inorganic materials and high breakdown strength (Eb) from polymers in the polymer nanocomposites could lead to an enhanced energy storage capacity. However, it remains elusive how the contradictory criteria of enhancing K while maintaining high Eb could be balanced in the nanocomposites. We have been dedicated to developing novel material approaches to simultaneous optimization of K and Eb, which can eventually lead to large energy densities at high energy extraction efficiency.

In-Situ Study of Interface

Dielectric polymer nanocomposites are widely explored for electrical insulation, capacitive energy storage, piezoelectric and pyroelectric sensing, and mechanical energy harvesting due to their exceptional electric property and ease of fabrication. However,while it is generally considered that the abnormal performance of dielectric polymer nanocomposites stems from the interfacial region between the polymer matrix and embedded nanoparticles, the structure property-correlation of the interfacial region in these materials remains mostly unknown. We take the advantage of scanning probe microscopy-based techniques for in-situ study of the interfacial property in dielectric polymer nanocomposites, which significantly deepens the understanding of this class of materials.

Recyclable Polymer Insulation for Power Cables

Power cable is an important partof power transmission system. With the acceleration of urbanization and thedevelopment of renewable resources, the demand for power cables is continuously increasing. Particularly the development of flexible DC transmission drives the rapid development of high voltage direct current (HVDC) cable. Nowadays cross-linked polyethylene (XLPE) has been extensively used as the main insulating material in HVDC cables. However, the operating temperature of XLPE-insulated HVDC cable is limited within 70 ºC, which cannot meet the demand of further rising capacity of transmission system. In addition, XLPE is a thermosetting polymer, so that it cannot be recycled after the cable decommissioning. We aim to develop recyclable polymer insulations for HVDC power transmission cables, with tailored electrical, mechanical and thermal properties.