Key Takeaways & Executive Findings
- •• • The optimized FPE/HAT-CN composite achieves discharged energy densities of 7.31 J cm−3 at 150 °C and 6.14 J cm−3 at 200 °C with energy efficiency ≥90%, surpassing conventional high-temperature polymers and enabling reliable operation in electrified transportation and pulsed power systems. • • HAT-CN is identified as a rare organic filler that simultaneously exhibits high electron affinity (deep-level trapping), large bandgap (strong insulation), and high thermal stability, addressing the fundamental trade-off that has limited previous filler designs. • • The all-organic composite preserves mechanical robustness while suppressing high-temperature conduction loss, offering a scalable and low-cost alternative to inorganic-filled composites that often suffer from processing and reliability issues. • • The synergistic filler design strategy provides a potent pathway to break the longstanding high-temperature performance bottleneck in polymer dielectrics, with potential for industrial adoption in film capacitors operating at ≥150 °C.
Abstract
Polymer composite dielectrics are key materials for high-temperature film capacitors, yet their energy storage capability is severely constrained at elevated temperatures. Molecular fillers that simultaneously integrate deep-level trapping (high electron affinity, Ea), strong insulation (large bandgap, Eg), and high thermal stability are rarely available, posing a major challenge for improving high-temperature energy storage performance. To address this challenge, we screen and identify hexaazatriphenylene hexacarbonitrile (HAT-CN) as a promising candidate that fulfills the above critical requirements from numerous commercial organic molecules. When incorporated into a high glass transition temperature (Tg) polymer fluorene polyester (FPE), the resulting all-organic composite exhibits simultaneously suppressed high-temperature conduction loss and preserved mechanical robustness. Consequently, the optimized composite achieves record-high discharged energy densities of 7.31 J cm−3 at 150 °C and 6.14 J cm−3 at 200 °C (η≥90%) with a low cost and scalable process. This work demonstrates that the filler design based on synergistic key properties provides a potent pathway to break the longstanding high-temperature performance bottleneck in polymer dielectrics.
1. Introduction
Polymer-based film capacitors are cornerstone technologies for advanced energy storage, prized for their high energy density, rapid charge-discharge dynamics, and intrinsic safety. However, the demand for reliable operation at temperatures ≥150 °C in electrified transportation and pulsed power systems has exposed the limitations of commercial materials. Biaxially oriented polypropylene (BOPP), the industry standard, is restricted to operating temperatures below 85 °C; beyond this, it suffers from significant dielectric loss and a sharp drop in discharge energy density and efficiency. Even high-temperature polymers like polyetherimide (PEI) exhibit poor capacitive performance at elevated temperatures, and reducing dielectric loss in high-Tg polymers such as polyimide (PI) or ferroelectric polymers like PVDF remains challenging.
Incorporating fillers with high electron affinity (Ea) has been a promising strategy to engineer charge traps and reduce leakage currents, but recent work by Shen et al. established that effective charge-trapping fillers must also possess a large bandgap (Eg) to robustly block charge injection and stabilize trapped charges. Yet, organic molecules that simultaneously satisfy high Ea, wide Eg, and high thermal stability are exceptionally rare due to fundamental design conflicts: strong electron-withdrawing groups that raise Ea often extend conjugation and narrow Eg. This study identifies hexaazatriphenylene hexacarbonitrile (HAT-CN) as a filler that meets all three criteria, and when blended with fluorene polyester (FPE), it achieves record-high energy densities at 150 °C and 200 °C, offering a scalable, low-cost path to high-temperature capacitive energy storage.
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WANG Yuanqi, WU Hangyao, CHEN Lan, LIN Sinan, LI Chenyi, ZHANG Yun, LI Yang, ZHOU Huamin, LIU Yang (2026). Integrated Molecular Engineering Strategy in All-Organic Dielectrics for Ultrahigh-Temperature Capacitive Energy Storage. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-026-4118-0
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Frequently Asked Questions
What are the specific failure mechanisms that limit conventional polymer dielectrics at temperatures above 150 °C, and how does the FPE/HAT-CN composite address them?
At elevated temperatures, conventional polymers like BOPP suffer from exponential increases in conduction loss due to enhanced charge injection and transport, leading to reduced discharge energy density and efficiency. The FPE/HAT-CN composite mitigates this by incorporating HAT-CN, which has a high electron affinity to create deep traps that immobilize injected charges, and a wide bandgap to suppress charge injection. This dual mechanism effectively reduces leakage current, as evidenced by the composite maintaining η≥90% at 200 °C, a condition where BOPP would be inoperable.
How does the energy density of the FPE/HAT-CN composite compare to existing high-temperature capacitor films, and what is the cost implication?
The optimized composite achieves discharged energy densities of 7.31 J cm−3 at 150 °C and 6.14 J cm−3 at 200 °C, which are record-high values for all-organic dielectrics at these temperatures. For context, commercial high-temperature polymers like PEI typically deliver less than 2 J cm−3 at 150 °C with lower efficiency. The all-organic nature and solution-processable fabrication suggest a low-cost, scalable route compared to inorganic-filled composites that often require complex processing and may suffer from filler agglomeration.
What is the thermal stability of HAT-CN, and how does it affect the long-term reliability of the composite at high operating temperatures?
HAT-CN was specifically selected for its high thermal stability, which is critical for maintaining filler integrity and trap characteristics during prolonged exposure to high temperatures. The composite's ability to sustain high energy densities and efficiency at 200 °C indicates that HAT-CN does not degrade or migrate, preserving the dielectric performance. This is essential for industrial applications where capacitors must operate reliably for thousands of hours.
Can the FPE/HAT-CN composite be processed using existing manufacturing techniques for film capacitors?
Yes, the composite is all-organic and can be processed via conventional solution casting or melt extrusion methods used for polymer films. The low cost and scalability of the process are highlighted in the paper, suggesting that it can be readily integrated into roll-to-roll manufacturing lines, unlike some nanocomposite systems that require specialized dispersion techniques.
What are the key material parameters that determine the high-temperature performance, and how were they optimized in this study?
The key parameters are electron affinity (Ea) for deep-level trapping, bandgap (Eg) for insulation strength, and thermal stability. HAT-CN was identified through screening of commercial molecules to simultaneously possess high Ea, wide Eg, and high thermal stability. When blended with FPE, the composite's performance was optimized by adjusting the filler loading to balance charge trapping and mechanical integrity, achieving the record energy densities.
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