On October 03, 2018, both Associate Prof. Li Qi and Prof. He Jinliang in Department of Electrical Engineering, Tsinghua University, together with their collaborators, published a research paper in Advanced Materials magazine entitled A Scalable, High-Throughput and Environmentally Benign Approach to Polymer Dielectrics Exhibiting Significantly Improved Capacitive Performance at High Temperatures. In this paper, a large-scale preparation method of high-temperature polymer capacitor film is proposed, which can greatly improve the dielectric energy storage characteristics of polymer capacitor film at high temperature. It is expected that the approach can be combined with the existing production line of polymer capacitor film to realize industrializing production and solve the problem of overheating damage to the capacitors in electronic control systems of power electronics, aerospace and electric vehicles.

Dielectric capacitors (DCs) have very fast charging and discharging efficiency and high power density. As a kind of extremely important power-type energy storage devices, they play a key role in power network frequency modulation, electromagnetic weapons, power electronic converters, new energy vehicles and pulsed power systems. However, the thin film capacitors based on polymer dielectric materials present poor thermal stability, and they can not work stably at high temperature. Especially under the action of high electric field, the leakage current in polymer dielectrics will increase exponentially with the increase of temperature, resulting in a sharp decline in charge-discharge efficiency and energy storage density, which can not meet the application requirements. What is more serious is that the leakage current transforms into Joule heat, which makes the capacitor temperature rise continuously and eventually gets the capacitor damaged. For a long time, scholars at home and abroad mainly use nano-doping to improve the high temperature dielectric energy storage performance of capacitive thin films, but at present it is impossible to achieve its large-scale preparation and application. The industrial solution is to introduce a cooling system to reduce the working environment temperature to below the maximum operating temperature of dielectric materials. For example, the electric control system of Toyota Prius Hybrid Electric Vehicle uses cooling system to reduce ambient temperature from 120-140 ℃ to 70-80℃. However, the existence of cooling system will undoubtedly increase the mass and volume of power system and reduce fuel efficiency.

In order to solve the above problems, the research team proposed to use the plasma enhanced chemical vapor deposition (PECVD) technology to rapidly deposit nano-insulating layers with wide bandgap on the surface of polymer films. This approach can improve the charge injection barrier at the interface between electrodes and dielectrics, to inhibit the leakage current of polymer dielectrics at high temperatures, and greatly improve the energy storage characteristics of polymer dielectric films at high temperatures and high electric fields. Moreover, this approach can realize rapid deposition under atmospheric pressure and has the ability of continuous treatment. Its room temperature deposition characteristics make this approach directly applicable to any polymer dielectric film. By introducing coil-to-coil film processing technology and dynamic deposition, large-scale and continuous production can be realized. The approach is pollution-free, simple, efficient and low-cost, and compatible with the existing polymer capacitor film production line. At present, the research team has applied for many domestic patents and PCT patents in this field, and is jointly conducting industrializing development with related enterprises.

In recent years, Prof. Li Qi has focused on the basic research and industrial development of advanced dielectric materials, and has made many important achievements in the field of material structure design and processing methods. His relevant works were published in the journals Nature, PNAS, Advanced Materials and the Annual Review of Materials Research.

The first author of this paper is Zhou Yao, a Ph.D. student of 2014 in the Department of Electrical Engineering Tsinghua University. The correspondence authors are Associate Prof. Li Qi, Prof. He Jinliang of Tsinghua University, and Prof. Wang Qing of Pennsylvania State University. The co-authors include Prof. Zeng Rong, Associate Prof. Hu Jun of Department of Electrical Engineering, and Prof. Shao Tao of Electrical Engineering Institute of Tsinghua University. The research results are funded by the National Natural Fund and Beijing Natural Fund.