Overcoming polarization-breakdown trade-off via hydrogen bonding-modulated molecular stacking in high-temperature high-energy-density flexible dielectric

Abstract

Flexible dielectrics suffer from an inherent polarization-breakdown trade-off. Herein, we propose a hydrogen bonding-modulated molecular stacking strategy to improve high-temperature capacitive energy storage performance. A series of polyetherimide copolymers with tunable hydrogen bond donors and acceptors was synthesized. Highly polarizable hydroxyl groups boost dielectric constant and increase hydrogen bond donor density. The constructed hydrogen bonding network strengthens intermolecular interactions, thermal stability, and breakdown strength. Bulky trifluoromethyl substituents enlarge interchain spacing and suppress interchain charge transport. Ultimately, the copolymer with optimal molecular stacking delivers a maximum discharged energy density of 7.34 J/cm³ with efficiency exceeding 90% at 680 MV/m and 200 °C, representing a 395.95% improvement over the pristine film and outperforming most reported flexible dielectrics. This intrinsic molecular modulation route offers a facile design strategy for designing high-temperature high-energy-density flexible dielectrics.

The authors propose a molecular stacking strategy regulated by hydrogen bonding to enhance the high-temperature capacitive energy storage performance of polymer dielectrics. This special structural design achieves an energy density of 7.34 J/cm³ and an efficiency of 90% at 200 °C.

Introduction

Electrostatic capacitors are an indispensable component in modern power electronics and advanced electrical systems owing to their ultrahigh power density, rapid charge-discharge capability and exceptional cycle stability^1–5. Polymer dielectrics exhibit high breakdown strength, stable dielectric frequency response and favorable mechanical flexibility, thereby emerging as the most competitive dielectrics for high-performance capacitors^6–10. Unfortunately, polymer dielectrics suffer from an inherently low dielectric constant, which limits their energy storage density and hinders their application in miniaturized compact energy storage systems^11–15. Additionally, the inferior heat resistance and sharply degraded insulation of polymer dielectrics at elevated temperatures hinder their utilization in emergent extreme environments above 200 °C, including electric vehicles, oil and gas exploitation, and aerospace industries^16–19. For instance, as a typical industrial benchmark for dielectric capacitor, commercially available biaxially oriented polypropylene (BOPP) can merely deliver a discharged energy density (U_d) below 5.0 J/cm³ at room temperature, owing to its low dielectric constant (2.2 at 10³ Hz), despite its exceptional breakdown strength (E_b) of up to 700 MV/m^20,21. Moreover, BOPP film undergoes severe capacitive degradation once the temperature rises above 105 °C due to an abrupt increase in conduction loss. At 120 °C, E_b and U_d drop to below 500 MV/m and 0.1 J/cm³, respectively^22–24. On this basis, exploring engineering polymers with high glass transition temperatures, including fluorene polyester (FPE)^25–27, polyimide (PI)^28–30 and polyetherimide (PEI)^31–33, has been extensively pursued to address the degradation of insulation and capacitive performances at elevated temperatures. Nevertheless, the conjugated aromatic moieties in these engineering polymers may facilitate electron delocalization, thereby resulting in reduced breakdown strength and increased conduction loss. Therefore, overcoming the polarization-breakdown trade-off of high-T_g polymer dielectrics is of great importance for high-temperature capacitive performance.

Two main approaches, namely extrinsic modification by adding fillers and intrinsic optimization via molecular stacking modulation, have been extensively explored to break the trade-off between polarization and breakdown^6,34–36. For filler incorporation, inorganic ceramic oxides with a large bandgap (E_g) and organic molecular semiconductors with high electron affinity (E_a) have been widely adopted to construct charge carrier traps^12,34. These traps can immobilize charge carriers and restrain their participation in electrical conduction, thus contributing to the inhibition of conduction loss. However, a high loading fraction of inorganic fillers is usually required to achieve a considerable dielectric constant. Combined with their poor compatibility with polymers, such a high loading easily results in severe aggregation, which may act as structural defects and give rise to premature breakdown failure. More importantly, the incorporation of ceramic fillers inevitably impairs the flexibility of polymers, which is a great concern for industrial roll-to-roll film manufacturing. Although the incorporation of molecular semiconductors can avoid the aforementioned issues owing to their excellent compatibility with polymer matrix, their low thermal stability and narrow bandgap severely restrict the high-temperature operational capability of polymer composite dielectrics. On the other hand, modulating molecular stacking is an intrinsic optimization method to improve the high-temperature capacitive performance of polymer dielectrics^37–41. The modulation of molecular stacking primarily relies on two approaches, including covalent crosslinking and intermolecular non-covalent interactions. The formation of a crosslinking network can constrain segmental motion and diminish free volume, thus simultaneously enhancing the thermal stability and breakdown strength⁸. Unfortunately, crosslinking inevitably constrains dipole polarization, ultimately lowering the dielectric constant of polymer dielectrics. Furthermore, the permanent covalent network can not be dissociated easily under external stimuli, which severely restricts the recyclability of crosslinked polymer dielectrics and poses a heavy environmental burden. Different from permanently crosslinked covalent networks, intermolecular non-covalent interactions including π-π stacking⁴², coordination¹⁰ and hydrogen bonding³⁷ only occur between polymer chains without damaging the intrinsic covalent backbone and possess favorable reprocessability and recyclability. Nevertheless, the π-π stacking highly relies on aromatic moieties, which may aggravate intermolecular charge transfer and thus result in unsatisfactory conduction loss. The coordination interaction depends strongly on metal ions and organic ligands. The introduced metal species in polymer dielectrics may also intensify intermolecular charge transfer and thereby raise conduction loss. Moreover, a specific metal-ligand pair is indispensable for coordination interaction, which restricts the flexibility in molecular design. Accordingly, it is of great significance to explore a feasible strategy for the effective modulation of molecular stacking. Rational design of chain packing structures is particularly critical to address the polarization-breakdown trade-off. Although compact chain packing enhances thermal stability, it simultaneously accelerates interchain charge transport and deteriorates the polarization capability of polymer segments.

Hence, we report a hydrogen bonding-modulated molecular stacking approach to tackle the trade-off between dielectric constant and breakdown strength of high-temperature flexible dielectrics. Compared to its counterparts of π-π stacking and coordination interactions, the intermolecular hydrogen bonding can not only enhance the thermal stability and breakdown strength, but also elevate the dielectric constant due to abundant hydrogen donor and acceptor groups with high polarizability, thereby improving high-temperature capacitive performance. In this contribution, hydrogen bonding with adjustable strength and density is rationally constructed in a PEI copolymer dielectric by employing 4,4’-oxydianiline (ODA) and 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane (6FAP) as diamines, along with 4,4’-(4,4’-isopropylidenediphenoxy)bis-(phthalic anhydride) (BPADA) as dianhydride. The presence of highly polarizable hydroxyl (-OH) and bulky trifluoromethyl (-CF₃) groups in 6FAP can effectively modulate the intermolecular stacking of PEI copolymer through hydrogen bonding, where -OH and -CF₃ act as hydrogen donors and acceptors, respectively. In addition, -OH groups can also enhance the dielectric constant owing to their high polarizability. Meanwhile, -CF₃ groups can expand interchain spacing via steric hindrance, which effectively restrains interchain charge transport. The synergistic effect between hydrogen bonding-induced chain assembly and steric hindrance of -CF₃ moieties endows the copolymer with optimal intermolecular stacking, which effectively addresses the polarization-breakdown trade-off. Remarkably, the PEI copolymer dielectric achieves a high discharged energy density (U_e) of 7.34 J/cm³ with a charge-discharge efficiency (η) above 90% at 680 MV/m and 200 °C. The proposed methodology opens up a new avenue for designing intrinsic polymer dielectrics toward high-temperature capacitive energy storage.

Results

The incorporation of comonomers represents a promising route to design intrinsic high-temperature polymer dielectrics, which enables precise regulation of molecular stacking and further improves high-temperature capacitive energy storage performance. To tackle the aforementioned polarization-breakdown trade-off, three additional comonomers, namely 2,2-bis (4-aminophenyl) hexafluoropropane (6FDAM), 2,2-bis (3-amino-4-hydroxyphenyl) propane (BAP), and 2,2-bis (3-amino-4-hydroxyphenyl) hexafluoropropane (6FAP), were incorporated into the synthesis of polyetherimide (PEI) dielectrics. The pristine PEI was synthesized from commercially available dianhydride 4,4’-(4,4’-isopropylidenediphenoxy) diphthalic anhydride (BPADA) and diamine 4,4’-oxydianiline (ODA). The molecular structures and corresponding synthetic routes of various PEI-based dielectrics are shown in Fig. 1a and Supplementary Fig. 1. The introduction of highly polarizable hydroxyl groups (-OH) into the polymer backbone can not only boost the dielectric constant via strong polarizability, but also increase the density of hydrogen bond donors, facilitating the construction of a robust hydrogen bonding network. Hydrogen bonding strengthens intermolecular interactions and confines polymer segmental motion, which favors the reduction of free volume and the improvement of thermal stability as well as breakdown strength. Nevertheless, overly high hydrogen bonding density accelerates interchain charge transport and thus increases the conduction loss at elevated temperatures. From this, bulky -CF₃ substituents expand interchain distance through steric hindrance and weaken hydrogen bonding interaction, accordingly contributing to the inhibition of interchain charge migration. Additionally, -CF₃ moieties constrain the mobility of polymer segments and thereby improve thermal stability. Nevertheless, the fluorine (F) atoms within -CF₃ can act as hydrogen bond acceptors, which inevitably increases the overall hydrogen bonding density. Accordingly, constructing an optimized interchain stacking structure is critical for developing intrinsic high-temperature polymer dielectrics, which can be achieved by tuning the molar fraction of comonomers. On this basis, 6FAP diamine containing both -OH and -CF₃ groups was chosen as the optimal copolymerization monomer, while 6FDAM and BAP diamines, which only contain -CF₃ or -OH separately, were adopted as reference comonomers. Hence, we systematically investigate the structural discrepancies between pristine PEI and the corresponding PEI-based dielectrics with 100 mol% comonomer substitution. As presented in Fig. 1b, density functional theory (DFT) calculations reveal that the energy bandgap rises from 3.79 eV of pristine PEI to 3.94 eV of P6FDAM, which is attributed to the reduced intermolecular electron coupling and suppressed π-electron delocalization induced by bulky -CF₃ groups. In contrast, the incorporation of -OH groups narrows the bandgap, which decreases from 3.79 eV for pristine PEI to 3.18 eV for PBAP. Notably, the simultaneous introduction of -CF₃ and -OH groups leads to a marginal reduction in the bandgap of P6FAP compared to pristine PEI, and its bandgap falls between those of PBAP and P6FDAM dielectrics. To further investigate the effects of -CF₃ and -OH groups on intermolecular interactions, electrostatic potential (ESP) simulations were conducted on various PEI-based dielectrics (Fig. 1c,d and Supplementary Fig. 2). The regions with an absolute ESP value exceeding 10 kcal/mol are defined as strong electron-interaction regions in the ESP mapping, in which red and blue regions represent positive ESP and negative ESP, respectively²⁹. A larger area of strong electron-interaction region generally corresponds to intensified electrostatic intermolecular interactions. The -CF₃ groups display strong electron-withdrawing capability owing to their higher electronegativity, while -OH moieties deliver a strong electron-donating effect via conjugation. As summarized in Supplementary Fig. 3, P6FAP possesses the largest area value among all PEI-based dielectrics, demonstrating the construction of compact molecular stacking. In addition, molecular dynamics (MD) simulations were also implemented to analyze hydrogen bonding behaviors and molecular stacking architectures (Fig. 1e). As summarized in Fig. 1f, pristine PEI and P6FDAM dielectrics barely form hydrogen bonding due to the absence of hydrogen bond donors. Notably, P6FAP exhibits a higher hydrogen bonding density compared to PBAP because it contains both hydrogen donors and acceptors, which agrees well with the strong intermolecular interactions observed in ESP calculations. Furthermore, the fractional free volume (FFV) derived from MD simulations demonstrates that bulky -CF₃ groups greatly disrupt compact chain packing and generate extra free volume inside the polymer matrix, which is verified by the higher FFV of P6FDAM relative to pristine PEI (Fig. 1g). In contrast, PBAP exhibits a remarkable increase in density and reduction in FFV, revealing that hydrogen bonding effectively induces the formation of compact molecular packing. As for P6FAP, although the incorporation of -CF₃ groups increases the concentration of hydrogen acceptors and promotes the formation of hydrogen bonding, steric hindrance enlarges interchain spacing, consequently leading to a slight increase in FFV.

Fig. 1. Structural design of PEI-based dielectrics.

a Molecular structures of pristine PEI, P6FDAM, PBAP and P6FAP. b Corresponding Energy band structures of PEI-based dielectrics. ESP distributions of c pristine PEI and d P6FAP. e MD simulations of PEI-based dielectrics. Comparison of the f hydrogen bonding number, g FFV and density of PEI-based dielectrics, which are deduced from MD simulations.

To validate the above theoretical analysis, structural and electrical characterizations were performed on the as-prepared PEI-based dielectrics. As displayed in Fig. 2a, a peak at 1715 cm⁻¹ attributed to the C = O symmetric stretching of imide rings can be observed in the FT-IR spectra of all PEI-based dielectrics³¹. The characteristic peak at 1150 cm⁻¹, corresponding to the stretching vibration of -CF₃ groups, only emerges for P6FDAM and P6FAP, confirming that the -CF₃-containing comonomers have been successfully embedded into the polymer backbone⁴³. Additionally, PBAP and P6FAP exhibit a broad band ranging from 3100 to 3600 cm⁻¹, which corresponds to the stretching vibrations of -OH groups⁴⁴. More importantly, the broad -OH band of P6FAP shifts to lower wavenumbers relative to PBAP, revealing enhanced hydrogen bonding intensity, which is consistent with MD simulation results. XRD results show that the interchain spacing (d) increases from 4.77 Å for pristine PEI to 5.68 Å for P6FDAM due to the bulky steric effect of -CF₃ groups (Fig. 2b). In contrast, PBAP displays a slightly reduced d-spacing relative to pristine PEI, which confirms compact intermolecular stacking driven by hydrogen bonding after -OH incorporation. Although the coexistence of -OH and -CF₃ groups effectively increases the concentration of hydrogen donors and acceptors, the large steric hindrance of -CF₃ moieties disrupts the ordered packing of polymer chains, thereby yielding a larger d-spacing of 5.11 Å for P6FAP. Supplementary Fig. 4 presents the optical bandgap values of PEI, P6FDAM, PBAP, and P6FAP, as determined from UV-vis spectra. The measured bandgaps are 3.37 eV for PEI, 3.41 eV for P6FDAM, 3.30 eV for PBAP, and 3.31 eV for P6FAP, respectively. Notably, these values exhibit the same trend as those predicted by DFT simulations, thereby confirming the reliability of both the experimental approach and the theoretical model. Furthermore, UPS measurements demonstrate that P6FAP possesses the highest electron affinity among the PEI-based dielectrics, promoting the formation of deep traps for charge carrier capture (Supplementary Fig. 5 and S6). Due to the inherent hydrophobicity of the -CF₃ groups and the hydrophilicity associated with -OH moieties, P6FDAM exhibits the largest contact angle, while PBAP displays the smallest contact angle values (Supplementary Fig. 7). In addition, DSC was utilized to investigate variations in thermal resistance. As shown in Fig. 2c, P6FAP possesses the highest T_g among all PEI-based dielectrics. This behavior is attributed to the synergistic effect of enhanced hydrogen bonding and increased steric hindrance. Polymer dielectrics with higher T_g values display suppressed segmental relaxation at elevated temperatures, resulting in superior thermal stability and reduced charge transport²⁰. As shown in Fig. 2d, all PEI-based dielectrics exhibit stable dielectric spectra across the frequency range from 10² to 10⁶Hz. PBAP demonstrates the highest dielectric constant of 3.66 at 10³Hz and 200 °C, which is ascribed to the strong polarizability of -OH moieties. Upon incorporation of -CF₃ groups, P6FAP displays a slightly reduced dielectric constant owing to the restricted polymer segment mobility and increased free volume. Furthermore, the dielectric breakdown strength (E_b) of PEI-based dielectrics was evaluated using a two-parameter Weibull statistical analysis⁶. As shown in Fig. 2e, E_b significantly increases from 414 MV/m for pristine PEI to 573 MV/m for PBAP at 200 °C. Notably, P6FAP, which incorporates both -CF₃ and -OH moieties, exhibits a further enhanced breakdown strength, reaching 604 MV/m. The enhanced high-temperature E_b is attributed to improved thermal stability and optimized intermolecular stacking, which originates from the synergistic effect of hydrogen-bonding networks and steric hindrance. Combined with the enhanced dielectric constant, P6FAP thereby delivers superior high-temperature capacitive performance. The enhanced high-temperature E_b can be attributed to improved thermal stability and optimized intermolecular stacking, stemming from the synergistic effect of hydrogen bonding networks and increased steric hindrance. Combined with the elevated dielectric constant, P6FAP consequently exhibits superior capacitive performance at high temperatures. Supplementary Fig. 8 presents the unipolar D-E loops for all PEI-based dielectrics as a function of electric field at 200 °C. As summarized in Supplementary Fig. 9 and Supplementary Fig. 10, P6FAP exhibits the slimmest D-E loop and the lowest energy loss at 400 MV/m and 200 °C. Consequently, P6FAP achieves a maximum discharged energy density (U_e) of 6.22 J/cm³ with a charge-discharge efficiency (η) exceeding 90% at 200 °C, outperforming all other PEI-based dielectrics. This outstanding performance confirms the structural advantages of 6FAP monomer, which enables the formation of stable hydrogen bonding networks and well-regulated intermolecular stacking (Fig. 2f).

Fig. 2. Structural characteristics and electrical properties of designed PEI-based dielectrics.

a FT-IR spectra, b XRD patterns and c DSC curves of pristine PEI, P6FDAM, PBAP and P6FAP. d Dielectric spectra, e Weibull distribution of breakdown strength and f discharged energy density and charge-discharge efficiency versus electric field at 200 °C for the designed PEI-based dielectrics.

Based on the above analysis, 6FAP was selected as the copolymerization monomer to synthesize P6FAP dielectrics containing varying mole fractions of 6FAP within the total mixed diamine of ODA and 6FAP. The effect of 6FAP concentration on the structural characteristics and electrical performance of the resulting polymer dielectrics was systematically investigated, leading to the identification of an optimal molar ratio for 6FAP. We have systematically monitored the imidization process by analyzing the evolution of amide and imide characteristic peaks in the FT-IR curves. As shown in Supplementary Fig. 11, the amide peak gradually diminishes, while the imide peak emerges as the temperature rises from 80 °C to 250 °C. More importantly, there is no obvious weight loss below 250 °C, demonstrating that residual solvent has been thoroughly removed. Figure 3a and Supplementary Fig. 12 show that the characteristic peak intensities corresponding to -CF₃ and -OH groups increase monotonically with the molar fraction of 6FAP monomer. Furthermore, XRD analysis indicates that the d-spacing gradually decreases as the loading of 6FAP units increases, which is attributed to the reinforced hydrogen bonding network (Fig. 3b). Compared with the pristine PEI, a significant increase in d-spacing is observed in the P6FAP copolymer with 25 mol% 6FAP monomer content owing to the large steric hindrance of bulky -CF₃ groups. Additionally, the copolymerized P6FAP demonstrates a gradual increase in T_g with increasing 6FAP molar concentration (Fig. 3c), indicating significantly enhanced thermal stability. Interestingly, the contact angle decreases from 76.5° for pristine PEI to 60.7° for P6FAP as the 6FAP content rises (Fig. 3d). This effect primarily stems from the predominant hydrophilic interactions of -OH groups, which overwhelm the hydrophobic influence of -CF₃ substituents and thus governs the surface wettability. Variable temperature infrared spectra (VT-IR) was conducted to verify the formation of hydrogen bonding in P6FAP dielectrics with varying mole fractions of the copolymerized monomer (Supplementary Fig. 13). As illustrated in Fig. 3e, the characteristic absorption band of -OH groups exhibits a blue shift accompanied by a narrowing of the peak profile. Meanwhile, the intensity of the free -OH peaks increases as the temperature rises from 40 °C to 400 °C. High temperature induces intensified thermal motion of macromolecular chains, which gradually weakens and ruptures hydrogen bonding interactions³⁷. Upon dissociation, hydrogen bonding liberates free -OH groups, resulting in an intensified stretching signal for free -OH as observed in the VT-IR curve. Furthermore, the dielectric spectra and electrical breakdown strength at elevated temperatures were systematically analyzed. As shown in Fig. 3f, the dielectric constant progressively increases with the concentration of 6FAP monomer, which is attributed to the high polarizability of -OH moieties. Meanwhile, the breakdown strength increases from 414 MV/m for pristine PEI to 698 MV/m for P6FAP containing 75 mol% 6FAP monomer (75-P6FAP), owing to the formation of a stable hydrogen bonding network (Fig. 3g). However, a further increase in the 6FAP molar fraction leads to a decrease in breakdown strength to 604 MV/m, which is ascribed to the excessively high density of hydrogen bonding. A dense hydrogen bonding network can aggravate intermolecular charge transfer, thereby lowering the breakdown strength. At a low 6FAP content, the bulky -CF₃ groups dominate molecular stacking and charge transport behavior owing to the limited hydrogen bonding density. As the 6FAP content gradually increases, the -OH moieties become predominant in regulating microstructural evolution and charge transport by forming a robust intermolecular hydrogen bonding network. Therefore, optimizing the molar ratio of 6FAP monomer is crucial for achieving the simultaneous enhancement of polarization and breakdown strength. As a result, the 75-P6FAP dielectric exhibits a discharged energy density of 7.34 J/cm³ with a charge-discharge efficiency exceeding 90% at 680 MV/m and 200 °C due to suppressed energy loss, which significantly surpasses the performance of other P6FAP-based dielectrics (Fig. 3h and Supplementary Fig. 14-Supplementary Fig. 16).

Fig. 3. Structural characteristics and electrical performance of P6FAP dielectrics with varying the mole fraction of 6FAP diamine monomer.

a FT-IR spectra, b XRD patterns, c DSC curves, d contact angles and e maximized VT-IR of P6FAP dielectrics with different mole fractions of 6FAP monomer with temperature ranging from 40 °C to 400 °C. f Dielectric spectra, g Weibull distribution of breakdown strength and h discharged energy density and charge-discharge efficiency of P6FAP dielectrics with different mole fractions of 6FAP monomer as a function of electric field at 200 °C.

Furthermore, we systematically analyzed the influence of the molar ratio of copolymerization monomers on the insulation and capacitive performance of P6FDAM and PBAP-based dielectrics. As shown in Supplementary Fig. 17, the breakdown strength increases monotonically with the molar ratio of 6FDAM and BAP monomers, reaching a maximum value at 100 mol% 6FDAM and BAP monomer content. As a strong electron-withdrawing group, the -CF₃ groups in P6FDAM reduce the π-electron density of aromatic chains and suppress the formation of intermolecular charge-transfer complexes (CTCs). This effect disrupts continuous π-π stacking, blocks charge tunneling, and enlarges the band gap of polymers. Additionally, the incorporated -CF₃ groups introduce abundant deep traps, which effectively capture charge carriers and lower their mobility. Furthermore, the bulky -CF₃ moieties create significant steric hindrance, thereby disturbing ordered molecular chain packing and restricting segmental motion. These synergistic effects greatly inhibit the formation of intermolecular CTCs and improve thermal stability. Consequently, P6FDAM with 100 mol% 6FDAM monomer content exhibits optimal insulation performance among all P6FDAM-based copolymers. As for the BAP monomer, the -OH groups induce the formation of abundant intra- and intermolecular hydrogen bonds between -OH and imide carbonyl groups. The constructed hydrogen bonding contributes to tight intermolecular stacking, thereby reducing structural free volume and improving thermal stability. Coupled with the high dielectric constant ascribed to the high polarizability of -OH moieties, PBAP with 100 mol% BAP monomer content exhibits optimal dielectric and insulation performance among all PBAP-based copolymers. Therefore, the optimal molar ratio for P6FDAM and PBAP dielectrics is 100%. Accordingly, the electrical performance of pristine PEI, P6FDAM, PBAP, and 75-P6FAP was comprehensively evaluated. As illustrated in Fig. 4a, b, 75-P6FAP exhibits a dielectric constant comparable to PBAP, while achieving the highest breakdown strength among all PEI-based dielectrics. The simultaneous enhancement of ε_r and E_b can be ascribed to the synergistic effect between the high polarizability of -OH groups and the optimal hydrogen bonding density. Excessive hydrogen bonding density exacerbates intermolecular charge transfer, thereby diminishing the insulation properties. Conversely, insufficient hydrogen bonding density results in the formation of cavities or defects, which can cause premature failure in polymer dielectrics. Thus, achieving an optimal hydrogen bonding density enables the formation of well-packed molecular chains, which is essential for attaining superior high-temperature capacitive energy storage performance. A significant increase in Young’s modulus is observed for 75-P6FAP, which can be correlated with the enhanced breakdown strength (Fig. 4c and Supplementary Fig. 18a)¹². This feature is attributed to the dense hydrogen bonding network and large steric hindrance of -CF₃ groups. Nevertheless, the 75-P6FAP copolymer film still maintains outstanding flexibility even after repeated bending cycles (Supplementary Fig. 18b). The leakage current densities of PEI-based dielectrics were measured as a function of electric field. As shown in Fig. 4d, 75-P6FAP exhibits the lowest leakage current density among all samples, and the hopping distance, calculated from the fitting of the hopping conduction model, decreases from 0.994 nm for pristine PEI to 0.625 nm for 75-P6FAP. This result further confirms the presence of deep traps capable of immobilizing charge carriers. To further verify this charge-trapping effect, thermally stimulated depolarization current (TSDC) measurements were conducted (Fig. 4e). As indicated by the peak temperatures, an identical trend is observed with the DSC results, wherein 75-P6FAP exhibits the highest T_g. Moreover, compared with other PEI-based dielectrics, 75-P6FAP demonstrates a pronounced increase in the intensity of TSDC peak, indicating the formation of deep carrier traps capable of capturing charge carriers and effectively suppressing conduction loss at elevated temperatures. Furthermore, the unipolar D-E loops of P6FDAM and PBAP dielectrics with varying molar fractions of copolymerization monomers were systematically analyzed (Supplementary Figs. 19, 20). As depicted in Fig. 4f, 75-P6FAP demonstrates the slimmest D-E loop among all PEI-based samples, indicating a superior charge-discharge efficiency. Consequently, 75-P6FAP achieves a maximum discharged energy density of 7.34 J/cm³ with a charge-discharge efficiency exceeding 90% at 680 MV/m and 200 °C (Fig. 4g). Notably, 75-P6FAP maintains exceptional cyclic capacitive stability after 5 × 10⁴ charge-discharge cycles at 200 °C and 200 MV/m (Fig. 4h). Ultimately, the discharged energy density of 75-P6FAP with efficiency above 90% at 200 °C surpasses that of previously reported polymer dielectrics (Fig. 4i)^{2–5,7,10,12,28,30–32,34,38,45–50}. Additionally, gel permeation chromatography (GPC) measurements were carried out to analyze the molecular weight characteristics of various polymers. As shown in Table. S1, pristine PEI possesses the highest molecular weight, while P6FAP shows the lowest value among all samples. Owing to the synergistic effects of steric hindrance and electronic induction from -CF₃ and -OH groups, the 6FAP monomer exhibits the weakest electron-donating ability and chemical reactivity. This further reduces its degree of polymerization and molecular weight, thereby endowing the P6FAP polymer with the lowest molecular weight among all samples. Although P6FAP-based polymers possess a relatively low molecular weight, their mechanical strength, thermal stability, and dielectric performance are significantly improved. This further demonstrates that the constructed hydrogen bonding network effectively facilitates molecular stacking, compensating for the deficiencies caused by low molecular weight and promoting dielectric performance enhancement. Furthermore, we have prepared additional samples with different thicknesses and systematically compared their insulation performance and structural evolution as a function of film thickness. As illustrated in Supplementary Fig. 21a, the thinner film indeed exhibits higher breakdown strength relative to the thicker counterpart. Uniform thermal imidization is achieved for both thin and thick samples, as confirmed by the disappearance of the amide characteristic peak at 1680 cm⁻¹ (Supplementary Fig. 21b). We further performed variable-temperature infrared spectroscopy (VT-IR) to investigate the hydrogen bonding network in thin and thick films. As displayed in Supplementary Fig. 21c-d, the characteristic absorption band assigned to -OH groups of both samples undergoes a blue shift when the temperature increases from 40 °C to 400 °C. Meanwhile, the peak intensity corresponding to free -OH species rises with temperature. The above results verify that the thermal treatment procedure adopted in this work enables uniform imidization, and the construction of hydrogen bonding network can be achieved even for thicker films. Since there is no permanent covalent crosslinking, we have recycled the 75-P6FAP copolymer in NMP solvent and compared its dielectric and energy storage performance before and after reprocessing. As displayed in Supplementary Fig. 22, compared with the original sample (75-P6FAP), the recycled film (R-75-P6FAP) exhibits a slight increase in dielectric constant, which is attributed to the partial destruction of hydrogen bonding network and the generation of free -OH groups. In contrast, the breakdown strength slightly decreases owing to the disruption of hydrogen bonding network. Furthermore, the recycled film achieves a maximum discharged energy density of 6.78 J/cm³ with an efficiency above 90% at 650 MV/m and 200 °C, retaining 92.37% of the energy density of the original film (Supplementary Fig. 23). The above analysis confirms that the as-prepared copolymer remains soluble and the recycled sample exhibits excellent capacitive energy storage performance. These findings confirm that the polarization-breakdown trade-off can be effectively resolved via hydrogen bonding-modulated molecular stacking, offering a viable strategy for the design of high-performance polymer dielectrics.

Fig. 4. Comparison of the electrical performance of PEI-based dielectrics.

a Dielectric spectra, b Weibull distribution of breakdown strength, c Young’s modulus and d leakage current density as a function of electric field for pristine PEI, P6FDAM, PBAP and 75-P6FAP at 200 °C. e TSDC curves of pristine PEI, P6FDAM, PBAP and 75-P6FAP. f Unipolar D-E hysteresis loops of pristine PEI, P6FDAM, PBAP and 75-P6FAP at 200 °C and 400 MV/m. g Discharged energy density and charge-discharge efficiency of pristine PEI, P6FDAM, PBAP and 75-P6FAP as a function of electric field at 200 °C. h Cycling energy storage performance of 75-P6FAP at 200 °C and 200 MV/m. i Comparison of the discharged energy density above 90% efficiency in this work with other representative polymer dielectrics at 200 °C^{2–5,7,10,12,28,30–32,34,38,45–50}.

Discussion

This work develops an intrinsic molecular engineering strategy to break the polarization-breakdown trade-off of flexible dielectrics for high-temperature energy storage via hydrogen bonding-modulated intermolecular stacking. The dual-functional 6FAP comonomer containing -OH as the hydrogen donors and -CF₃ as the acceptors is copolymerized into macromolecular chains, which balances dipole polarization and interchain charge migration unavailable for single-functional 6FDAM or BAP monomer. The results verify that hydrogen bonding improves dipole density, thermal stability and mechanical strength, while steric hindrance of -CF₃ widens interchain spacing to restrain charge transport and conduction loss. The optimized 75-P6FAP with 75 mol% 6FAP achieves a high discharged energy density of 7.34 J/cm³ above 90% efficiency at 200 °C, associated with a breakdown strength of 698 MV/m and stable cycling over 5 × 10⁴ times. This hydrogen bonding-regulated chain stacking strategy offers a universal and scalable route to design high-performance flexible dielectrics for extreme-temperature capacitors.

Methods

Materials

Bisphenol A diether dianhydride (BPADA), 4,4’-oxydianiline (ODA), 2,2-bis(4-aminophenyl)hexafluoropropane (6FDAM), 2,2-bis(3-amino-4-hydroxylphenyl) propane (BAP) and 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane (6FAP), were purchased from Macklin. N-Methyl-2-pyrrolidone (NMP) and other solvents were purchased from Beijing Chemical Reagents Company and utilized directly without extra purification.

Preparation of pristine PEI, P6FDAM, PBAP and P6FAP dielectric films

The fabrication of pristine PEI and other PEI-based dielectric films primarily involves pre-polymerization and subsequent thermal imidization. Taking the P6FAP as an example, the predetermined amounts of 6FAP and ODA diamines were first dissolved in NMP to form a homogeneous solution. Subsequently, an equimolar amount of BPADA was added to the solution in five portions to maintain an amine-to-anhydride molar ratio of 1:1. The resulting mixture was magnetically stirred at room temperature for 9 hours, yielding a homogeneous, transparent, and viscous poly(ether amide acid) (PEAA) precursor solution. The PEAA solution was uniformly cast onto a clean glass substrate, followed by sequential thermal treatment at 80 °C for 2 h, 120 °C for 1 h, 150 °C for 1 h, 200 °C for 1 h, and 250 °C for 1 h under an air atmosphere to complete the thermal imidization process, thereby yielding the final P6FAP film. Following the same synthetic protocol, a series of P6FAP copolymer films with gradient compositions was obtained by adjusting the feed molar ratio of 6FAP to ODA to 25:75, 50:50, 65:35, 75:25, 85:15, and 100:0. The resulting copolymer films were correspondingly named 25-P6FAP, 50-P6FAP, 65-P6FAP, 75-P6FAP, 85-P6FAP, and P6FAP. Similarly, pristine PEI, gradient P6FDAM and PBAP films were fabricated via identical procedures. Pristine PEI film were synthesized using only BPADA and ODA as monomers and subjected to the identical thermal imidization. The thicknesses of all as-prepared PEI-based dielectric films were maintained at 10–15 μm.

Recycling of 75-P6FAP dielectric film

The 75-P6FAP copolymer film was cut into regular rectangular specimens. A portion of these specimens was dissolved in NMP at 50 °C under continuous stirring for 24 h to yield a homogeneous solution. The as-prepared homogeneous solution was spin-coated onto glass substrates with a rotation speed of 300 rpm and subjected to a stepwise thermal curing at 80 °C for 2 h, 120 °C for 1 h, 150 °C for 1 h, 200 °C for 1 h, and 250 °C for 1 h under an air atmosphere. The thickness of the recycled film was approximately 10–15 μm.

Characterization

Ultraviolet visible (UV-vis) spectra were recorded on a PerkinElmer LAMBDA 750 spectrophotometer. Ultraviolet photoelectron spectroscopy (UPS) measurements were carried out using a Thermo Fisher Scientific Nexsa G2 system with He-I (21.2 eV) as the excitation, and the Fermi level was calibrated using clean Au. Fourier transform infrared (FT-IR) spectra were acquired with a PerkinElmer Spectrum 3TM spectrometer. X-ray diffraction (XRD) patterns were collected on a Rigaku SmartLab diffractometer. Gel permeation chromatography (GPC) measurement was performed on an Agilent PL-GPC 50 system. Differential scanning calorimetry (DSC) was performed on a Netzsch DSC 214 Polyma thermal analyzer under a nitrogen atmosphere at a heating rate of 5 °C/min from 50 °C to 300 °C. Glass transition temperature (T_g) was determined from the inflection point of the second heating scan following one preliminary heating-cooling cycle to eliminate thermal history. Thermogravimetric analysis (TGA) was carried out on a TA TGA 550 instrument at a heating rate of 10 °C/min from 30 °C to 800 °C under N₂ flow. Contact angle measurements were performed using a Krüss DSA25S Drop Shape Analyzer. For each sample, the average contact angle was calculated from five droplets at different locations. Variable temperature infrared (VT-IR) spectra were acquired in attenuated total reflectance (ATR) mode using a Spectrum Two spectrometer (PerkinElmer). The tensile Young’s modulus was determined with an INSTRON 3343 tester at a rate of 200 mm/min, utilizing strip-shaped specimens having a width of 8 mm and an effective gauge length of 10 mm. Bending cycles were conducted on an INSTRON 3343 tester.

Gold electrodes with a diameter of 20 mm were sputtered onto both sides of the films for dielectric spectra and thermally stimulated depolarization current (TSDC) measurements. Dielectric properties were measured using a Novocontrol Concept 80 broadband dielectric spectrometer at 200 °C over a frequency range of 10²−10⁶Hz. TSDC tests were performed on the Novocontrol platform integrated with a Keysight 2985B electrostatic/high-resistance analyzer. Samples were polarized at 200 °C under a DC field of 30 MV/m for 30 min and subsequently quenched to −10 °C while maintaining the applied electric field. Subsequently, the samples were maintained at −10 °C for 3 min before the applied electric field was removed. The samples were then short-circuited and heated to 250 °C at a rate of 3 °C/min, with the transient depolarization current continuously recorded throughout the heating process. For the evaluation of breakdown strength, D-E hysteresis loops, leakage current and cyclic charge-discharge performance, gold electrodes with a diameter of 3 mm were sputtered onto both sides of each film. Breakdown strength was determined using a BOHER high-voltage supply with a voltage ramp rate of 500 V/s, applying voltage until electrical breakdown occurred. The breakdown data were subsequently statistically analyzed using a two-parameter Weibull distribution. Leakage current measurements were carried out on a test platform comprising a Keithley 2290-10 high-voltage power supply and a Keithley 2635B meter. D-E hysteresis loops and cyclic charge-discharge performance were recorded using a Premier II ferroelectric test system with a unipolar waveform at 10²Hz (Radiant Technologies Inc, USA).

Simulation

Density functional theory (DFT) calculations were carried out with the ORCA 6.0.0 software package. The BLYP functional was employed in conjunction with the D3 version of Grimme’s dispersion, using Becke-Johnson damping (DFT-D3BJ). Geometry optimization was carried out using the def2-SVP basis set together with the auxiliary def2/J basis set. Single-point energy calculations were subsequently performed employing the def2-TZVP basis set and the auxiliary def2/J basis set. The Solvation Model Based on Density (SMD) implicit solvation approach was employed to account for the solvation effect during geometry optimization and frequency calculations, with N-methylpyrrolidone specified as the solvent. Frontier molecular orbital analyses were conducted using the Multiwfn and VMD software packages. Molecular dynamics (MD) simulations were performed to theoretically analyze the density and fractional free volume (FFV) of samples. The unit cells for pristine PEI, P6FDAM, PBAP and P6FAP dielectrics, each containing 15 chains with 8 repeat units per chain, were constructed using Materials Studio (MS) 8.0 software developed by BIOVIA Systems. Geometry and dynamics optimizations of the above models were performed using the Forcite module with the universal force field. All systems underwent a dynamic optimization at 300 K for 500 ps with a timestep of 1 fs. Electrostatic interactions were treated using the Ewald summation method, and the van der Waals interactions were calculated using an atom-based approach. Temperature was controlled using a Nose-Hoover thermostat, while pressure regulation was achieved with a Berendsen barostat. The total system volume comprises two components: the volume occupied by the molecule (V_o) and the free volume (V_f). The FFV was calculated according to the following equation:

$$\mathrm{FFV}=\frac{V_f}{V_f+V_o}\times 100\%$$ 1

The values of V_o and V_f were determined using the Atom Volumes & Surfaces tool within the MS software.

Author contributions

M.Y., J.L., B.T., and Z.M.D. conceived the idea. M.Y. and H.S. designed the experiments. M.Y., H.S., J.W., S.Y., Z.Z., S.Z., and H.Y. carried out the experiments. H.S. and J.Q. performed the simulations. M.Y., H.S., J.W., S.Y., Z.Z., S.Z., H.Y., J.Q., J.L., B.T., and Z.M.D. analyzed the data. M.Y., H.S., J.L., B.T., and Z.M.D. wrote the manuscript. All authors discussed the results and revised the manuscript.

Peer review

Peer review information

Nature Communications thanks Linping Wang and the other anonymous reviewer(s) for their contribution to the peer review of this work. A peer review file is available.

Funding

This work was supported by the Smart Grid-National Science and Technology Major Project (Grant no. 2025ZD0808403 to M.Y.) and National Natural Science Foundation of China (Grant no. 52577025 to M.Y., Grant no. 52527803 to Z.M.D. and Grant no. T2222025 to B.T.).

Data availability

All data supporting the findings of this study are available from the corresponding author upon reasonable request (minhao.yang@ncepu.edu.cn and dangzm@tsinghua.edu.cn).

Competing interests

The authors declare no competing interests.

Supplementary information

The online version contains supplementary material available at https://doi.org/10.1038/s41467-026-77564-3.