Design, Preparation And Performance Of High-Temperature Polyimide Films For Energy Storage Applications

Polymer film capacitors have a wide range of applications in electronic power systems with advantages such as high power density,great self-healing and ease of mass production.With the booming development of electric vehicles,aerospace and other fields,the demand for polymer film capacitors that operate under extreme conditions has increased.Under the coupling of thermal and electrical fields,the conduction loss of traditional polymer films increases rapidly,leading to a serious deterioration of their electrical insulation and energy storage performance.The main objective of this thesis is to investigate polymer capacitor films at high temperatures and high fields.The design and preparation of polymers in terms of functional groups,chain structures and multilayer structures is based on the multi-level structure of polymers to achieve synergistic optimization of multiple parameters such as dielectric permittivity,breakdown strength and dielectric loss.The focus is on the high temperature conduction losses of polymers,which are caused by charge injection at the electrode/dielectric interface and electrically and thermally excited charge carrier transport within the dielectrics.This study explores the mechanism of the influence of multi-level structural design on the internal charge transfer characteristics of polymers,solves the issues of low energy density(Ue)and high energy loss in polymer films at high temperatures and high fields,and realizes comprehensive regulation of the high temperature energy storage performance of polymer films.(1)The alteration of the molecular structure of polymers caused by changes in the type and content of functional groups,which is one of the key factors affecting the high temperature energy storage performance of polymers.Optimisation of the molecular structure of polyimides(PI)is achieved by taking advantage of the reaction kinetics of polyamic acid to modulate the polar COOH/CONH-groups in the PI molecular chains.The appropriate amount of-COOH/CONH-groups in the "partially imidized" polyimide films exhibite a suppression of space charge accumulation and improved electric field distortion.An Ue of 3.9 J/cm3 is obtained at 150℃.Compared to the "completely imidized" polyimide film,the "partially imidized" polyimide film exhibits an Ue increase of 130%and a processing temperature reduction of 100℃.This work demonstrates the versatility of the reaction kinetics regulation strategy by synthesising a variety of polyimides with different structures.The synthesis process and structural composition of polyimide for obtaining synergistic enhancement of dielectric permittivity and breakdown strength are clarified.The mechanism of the influence of charge transfer characteristics on the high temperature conductivity and energy storage performance of polyimide films is elucidated from the perspective of trap theory.(2)The amorphous rigid polyimide has a wide chain spacing,resulting in a large free volume that reduces insulation and energy storage performances.By optimizing the distribution of functional groups in the molecular chains and introducing flexible thermoplastic polyurethane(TPU)fillers,the inter-chain interactions and spatial structure of the polyimide can be optimized.The obtained all-organic composite film reduces the chain spacing of the polymer,effectively hindering charge carrier transport and mitigating electric field distortion,resulting in excellent high-temperature energy storage performance.At 150℃,the composite film achieves a high Ue of 4.1 J/cm3 and a charge-discharge efficiency(η)of 70%.Compared to traditional polyimide,the Ue and η are increased by 583%and 400%respectively.This work illustrates the inhibition mechanism of charge transport within all-organic composite films by functional group distribution and molecular chain morphology,and reveals the influence of polymer chain spacing on the high temperature energy storage performance of all-organic composite films.(3)The charge injection barrier at the dielectric/electrode interface decreases significantly with increasing temperature,leading to rapidly increasing conduction losses,which is the cause of the dramatic degradation of the energy storage performance of polymer films in high temperature environments.A bilayer structured polyimide composite film with a wide band gap inorganic layer as a charge blocking layer is designed to reduce high temperature conduction losses by regulating the electrode charge injection barrier.The inorganic layer improves the charge trapping ability and adjusts the charge mobility at the electrode/dielectric interface.The obtained composite films exhibit a significant suppression of conduction losses and excellent capacitive performance at high temperatures.A excellent Ue of 4.37 J/cm3 and η of 92%are obtained for the composite films at 200℃ and 500 MV/m,which are 127%and 207%higher than those of pure PI film,respectively.This work illustrates the major role of inorganic layer thickness and inorganic/polymer interface structure on the suppression of charge injection,reveals the suppression mechanism of current density and electric field distribution on conduction loss at high temperature,and optimizes the method of regulating the high temperature energy storage characteristics of bilayer structured composite films.