Polymer Capacitor Films with Nanoscale Coatings for Dielectric Energy Storage: A Review

Abstract

Enhancing the energy storage properties of dielectric polymer capacitor films through composite materials has gained widespread recognition. Among the various strategies for improving dielectric materials, nanoscale coatings that create structurally controlled multiphase polymeric films have shown great promise. This approach has garnered considerable attention in recent years due to its effectiveness. This review examines surface-coated polymer composites used for dielectric energy storage, discussing their dielectric properties, behaviors, and the underlying physical mechanisms involved in energy storage. The review thoroughly examines the fabrication methods for nanoscale coatings and the selection of coating materials. It also explores the latest advancements in the rational design and control of interfaces in organic–inorganic, organic–organic, and heterogeneous multiphase structures. Additionally, the review delves into the structure–property relationships between different interfacial phases and various interface structures, analyzing how nanoscale coatings the impact dielectric constant, breakdown strength, conduction and charge transport mechanisms, energy density and efficiency, thermal stability, and electrothermal durability of polymeric capacitor films. Moreover, the review summarizes relevant simulation methods and offers computational insights. The potential practical applications and characteristics of such nanoscale coating techniques are discussed, along with the existing challenges and practical limitations. Finally, the review concludes with a summary and outlook, highlighting potential research directions in this rapidly evolving field.

1. Introduction

Dielectric capacitors are broadly used in areas including new energy power systems, modern electronics, electric transportation, etc. (see Figure 1a) [1,2,3,4,5,6,7,8,9,10,11], owing to their ultra-high power density compared to other energy storage devices, such as batteries, electrochemical capacitors, fuel cells, etc. (see Figure 1b). Compared to ceramic capacitors [12,13,14,15,16], polymer-based dielectrics are particularly promising for flexible dielectric capacitors, due to their excellent mechanical flexibility, lightweight nature, low cost, and ease of processing [17,18,19,20,21,22,23,24,25]. The industry-used film capacitors are typically based on commercially-available polymers like biaxially oriented polypropylene (BOPP), polyvinylidene fluoride (PVDF) [26], polyimide (PI), and polyetherimide (PEI), which, while offering good operational stability, still have room to further improve performance in terms of dielectric breakdown strength (E_b), discharged energy density (U_d), and charge–discharge efficiency (η), especially under harsh electric fields and temperature conditions (see Table 1). Especially for applications in high-temperature environments, such as electrified aircraft, new energy power systems, hybrid vehicles, and integrated microelectronics, the thermally-assisted charge injection and conduction can lead to increased electrical conductivity and reduced E_b, ultimately decreasing energy storage performance (see Table 1). The fundamental challenge lies in the inability of current polymer materials to meet the growing demands for electrostatic energy storage in such conditions, making the development of robust polymer dielectrics with excellent insulation performance at high temperatures critically important [27,28,29,30,31,32].

The stored energy density U_s is defined as the integration of the applied electric field (E_a) with electric displacement (D), as follows:

$$U_{\mathrm{s}}={\displaystyle \int E_{\mathrm{a}}\mathrm{d}D}$$

(1)

Considering η, with D determined by dielectric constant (K), U_d can be expressed as (scheme as shown in Figure 1c):

$$U_{\mathrm{d}}=\eta {\displaystyle \int K{E}_{\mathrm{a}}\mathrm{d}E}$$

(2)

From Equation (2), the key to improving energy storage performance (increasing U_d) relies on the three parameters η, K, and E_a (limited by E_b), which will be introduced in detail later.

Recent research has focused on improving the U_d of polymer-based dielectric capacitors through designing new polymers [33,34,35,36,37,38,39,40,41,42], polymer nanocomposites [43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61], constructing multilayered structures [62], blending polymers [63,64,65], etc. Among these, the nanoscale coating methods can effectively achieve well-controlled hierarchical nanostructures in polymeric films, showing significant potential to simultaneously enhance E_b, U_d, and η. Motivated by the ease of fabrication provided by nanotechnology and the potential for targeted performance in practical applications, this article summarizes recent advances in using nanocoating to improve the dielectric energy storage performance of polymer film capacitors.

The review begins by introducing the fundamental principles of energy storage in polymer dielectrics. It then systematically covers various coating methods and materials, including (i) chemical fabrication techniques like chemical vapor deposition (CVD), atomic layer deposition (ALD), and in situ growth of surface coating; (ii) physical preparation methods like physical vapor deposition (PVD), spraying, and hot pressing; (iii) molecular-level surface modifications such as irradiation and surface grafting; and (iv) the coating of polymers and/or the fabrication of multilayered polymer composites, including physical force-driven self-assembly of layered structured films. In particular, the article elucidates the relationship between coating structures and dielectric properties under high temperatures and high electric fields. The concluding section offers insights into the challenges and opportunities for future developments in nanocoating-reinforced polymer dielectric films, emphasizing enhanced E_b, U_d, η, and operational stability.

Table 1. Overview of key dielectric and energy storage properties of commercial polymer dielectrics, in which RT represents room temperature.

Polymer	Condition	K	Eb (MV m−1)	Ud (J cm−3)	η (%)	Ref.
BOPP	RT	2.3	694	5.9	91	[66]
	120 °C	2.3	428	2.0	85
Kapton® PI	150 °C	3.1	/	0.5	15	[67]
	200 °C	3	/	0.2	35
Ultem® PEI	RT	3.2	501	5.5	85	[43]
	150 °C	3.2	470	3.1	75	[56]
	200 °C	3.2	440	2.5	65

Table 1. Overview of key dielectric and energy storage properties of commercial polymer dielectrics, in which RT represents room temperature.

Polymer	Condition	K	Eb (MV m−1)	Ud (J cm−3)	η (%)	Ref.
BOPP	RT	2.3	694	5.9	91	[66]
	120 °C	2.3	428	2.0	85
Kapton® PI	150 °C	3.1	/	0.5	15	[67]
	200 °C	3	/	0.2	35
Ultem® PEI	RT	3.2	501	5.5	85	[43]
	150 °C	3.2	470	3.1	75	[56]
	200 °C	3.2	440	2.5	65

2. Fundamentals of Energy Storage in Polymer Dielectrics

2.1. Dielectric Constant and Dielectric Loss

The polarization processes in these materials are correlated with the displacement of molecular chains, whether through movement or rotation, and are influenced by both frequency and temperature. The combined effects of various polarization processes determine the K value of polymer dielectrics used in energy storage capacitors. The overall dielectric loss (P_l) is defined as the sum of the conduction loss (P_c) and the relaxation polarization loss (tan δ) as follows:

$$P_{\mathrm{l}}=P_{\mathrm{c}}+\tan \delta ={\displaystyle \frac{\gamma }{\omega {\epsilon }_{0}K}}+{\displaystyle \frac{({\epsilon }_{\mathrm{s}}-{\epsilon }_{\infty })\omega \tau }{K(1+{\omega }^2{\tau }^2)}}$$

(3)

where γ is the electrical conductivity in S m⁻¹, ω is the angular frequency in s⁻¹, ε_s is the static dielectric constant under a constant electric field, and ε_∞ is the dielectric constant at optical frequency.

Polarization processes play a crucial role in determining K, which in turn influences the U_d of polymer dielectrics. Except for that, relaxation processes affect U_l. Therefore, achieving a high K and minimizing P_l are key to attaining both high U_d and high η in polymer dielectrics.

2.2. Dielectric Breakdown

As previously mentioned, in addition to K, E_b is another crucial factor. For polymers, a higher E_b allows for a greater voltage rise margin, which is essential for the lightweight design and miniaturization of dielectric capacitors. The dielectric breakdown process is complex, and the primary dielectric breakdown mechanisms in polymer dielectrics at high temperatures consist of electronic breakdown, thermal breakdown, and electromechanical breakdown (see Figure 2a–c). The dominant mechanisms of dielectric breakdown, however, are still not fully understood, as they are influenced by complex factors such as film thickness, voltage type, etc. To evaluate dispersed breakdown data, the two-parameter Weibull distribution is commonly used, which is expressed as:

$$P(E_{\mathrm{a}})=1-e^{-\beta ({\displaystyle \frac{E_{\mathrm{a}}}{E_{\mathrm{b}}}})}$$

(4)

where E_b is the Weibull breakdown strength corresponding to a 63.2% probability of breakdown and β is the shape parameter that evaluates data dispersity.

2.3. Charge Injection and Migration

Both E_b and P_l are affected by charge injection and migration processes in polymer dielectrics [68,69,70,71]. Under high electric fields and elevated temperatures, charges can surmount barriers and gain sufficient energy to be injected from electrodes. This phenomenon is known as Schottky injection, which can be described as follows:

$$J_{\mathrm{in}}=A{T}^2\exp ({\displaystyle \frac{-{\omega }_{\mathrm{in}}}{K_{\mathrm{B}}T}})\exp ({\displaystyle \frac{e}{K_{\mathrm{B}}T}}\sqrt{{\displaystyle \frac{e\left|E_{\mathrm{a}}\right|}{4\pi {\epsilon }_{0}K}}})$$

(5)

where J_in is the injection current density, A is the Richardson constant, e is the elementary charge, and ω_in is the Schottky injection barrier.

Electrical conduction current is generated due to charge migration under E_a, following Ohm’s law. If the injection current surpasses the conduction current at higher electric fields, space charge limited current (SCLC) occurs, comprising drift and diffusion currents. In many semi-crystalline polymer dielectrics, charges tend to “hop” between conduction bands under harsh electric field and temperature conditions, and the hopping conduction current (J_con) is given by:

$$J_{\mathrm{con}}=2Ne\lambda \nu \exp (-{\displaystyle \frac{{\omega }_{\mathrm{con}}}{K_{\mathrm{B}}T}})\sinh ({\displaystyle \frac{\lambda e{E}_{\mathrm{a}}}{2K_{\mathrm{B}}T}})$$

(6)

where λ is the hopping distance, ω_con is the trap depth, and ν is the attempt-to-escape frequency.

The schematics of the aforementioned charge injection and migration processes are illustrated in Figure 2d,e. These conduction processes cause some of the stored charges to be dissipated as electrical conduction current across the polymer dielectrics. This results in an increase in U_l and a decrease in η. Additionally, the significant rise in electrical conduction current could lead to a reduced E_b.

3. Effect of Coating Methods and Materials on Energy Storage Performance

To improve energy storage capabilities, researchers have investigated various methods to increase K, improve E_b, and reduce the electrical conductivity γ. Restricting charge migration and inhibiting conduction current are widely recognized as key strategies for enhancing E_b and η. One effective approach involves introducing functional layers onto dielectric polymers to prevent charge injection from the electrode–polymer interface or to impede charge transport. This section will review different strategies for constructing surface-coated or functionalized polymer composites and their impact on improving the energy storage performance of polymer dielectric films.

3.1. Chemical Methods for Coating

The first category is chemical fabrication methods, including chemical vapor deposition (CVD) and atomic layer deposition (ALD). In situ growth/self-assembly is crucial for surface deposition on polymer dielectrics, with wide applications in electronics and materials science [72]. CVD involves the reaction of volatile precursor gases on a substrate’s surface to form a solid film, allowing for precise control over film thickness and composition, making it widely used in semiconductor manufacturing. ALD, actually a special branch of CVD, employs sequential, self-limiting chemical reactions, introducing precursors one at a time to achieve atomic-scale control of film thickness. This technique is particularly valuable for depositing ultra-thin, conformal coatings, essential for advanced semiconductor devices and nanotechnology applications. Both methods play a critical role in producing high-quality films for energy storage capacitors. Researchers have also developed ways to carry out the in situ growth of functional components onto polymers. A scheme of CVD, ALD, and in situ growing methods is presented in Figure 3.

Most researchers put their focus on conducting surface modifications on the most used commercially available polymers, for example, PEI and BOPP. Zhou et al. successfully combined plasma-enhanced CVD (PECVD) and roll-to-roll processing to provide a general and scalable method of producing surface-coated BOPP polymer composites (scheme as shown in Figure 4a) [73]. The introduced nanoscale wide-bandgap SiO₂ layer (shown in Figure 4a) increases the potential barrier at the electrode–dielectric interface, and consequently, impedes the charge injection from electrodes and yields much reduced space charge densities (shown in Figure 4b). The SiO₂-coated films exhibit substantially improved high-temperature capacitive performance in comparison to the pristine polymer BOPP (shown in Figure 4c). The same methods were also performed on other commercial polymers, such as PEI, polyethylene naphthalate (PEN), PI, polycarbonate (PC), and fluorene polyester (FPE), all showcasing obvious enhancement in η and U_d with SiO₂ coatings (shown in Figure 4d), proving the generality of this approach. In another work, a parylene layer was deposited (using CVD) on PP, showcasing a great enhancement in both E_b and U_d. Similar results were found in a later work, and a maximum U_d of 5.52 J·cm⁻³ with an η of over 90% at 120 °C was achieved in modified BOPP films.

To investigate the effect of surface coating materials on the enhancement effect in energy storage performance, the Li group selected a series of deposition materials (SiO₂, Al₂O₃, HfO₂, TiO₂) with distinct band structures and dielectric properties [76], as shown in Figure 4e. The authors considered the surface-coated metal oxide layer as a normal RC circuit, and played the role of the interfacial potential that elevated the Schottky injection barrier, as illustrated in Figure 4f–h. Unlike what would be considered common sense, that the larger the bandgap the better, the authors pointed out that a well-balanced bandgap (E_g), K, and γ of the nanoscale deposition layer is desirable for suppressing charge injection and therefore improving energy storage performance. The resultant Al₂O₃ surface-coated PEI composite film gives rise to a concurrent high U_d (2.8 J·cm⁻³) and η (90%) up to 200 °C, with an optimized coating thickness of 150 nm. The high-insulating (bandgap ~5.97 eV [77]) and thermal conductive BN also showed great potential in enhancing the energy storage performance of PEI. Based on this, researchers are further designing functional layered structures; for example, the Chi group designed BN/SiO₂ heterojunctions on the surface of PC films [78], detecting a synergistic suppression of the carrier injection and transport, achieving an excellent U_d of 5.22 J·cm⁻³ at 150 °C.

Recently, the construction of multilayered polymer thin films with nanoscale coatings showed great potential to further improve energy storage properties in both U_d and η. The Tan group took the strategy of combining the solution casting and ALD technology to make three-layered PEI/Al₂O₃ composites, achieving improved E_b from a weakened electrical field and faster heat dissipation [79]. A very recently published study using a similar method provided an exciting new point that ultra-thin film devices will benefit from the nanoconfinement effect and showed extraordinary energy storage performance, achieving an ultrahigh U_d of 18.9 J·cm⁻³ with a high η of ~91% at 200 °C [74]. The positive effects of nanoconfinement and interfacial trapping within the nanolaminates in the fabricated seven-layered PEI/Al₂O₃ composites (as shown in Figure 4i–k) synergistically improved E_b and enhanced energy storage properties at temperatures up to 250 °C. Note that even though this is not a case of a self-standing film, it still showed great potential in the nanoscale fabrication of high-performance energy storage capacitors for potential usage in modern electronics.

3.2. Physical Methods for Coating

Physical preparation methods, including physical vapor deposition (PVD) [80,81], spraying [82], and hot pressing [83], are essential for creating thin films on substrates, widely used in electronics and coatings. In PVD, material from a solid source is vaporized and deposited onto a substrate, with magnetic sputtering being a common variant where ions bombard a target material, ejecting atoms that form the thin film. Spraying involves dissolving or suspending a material in a solvent and then spraying it onto the substrate, where the solvent evaporates, generating a thin coating and making it a simple and cost-effective method for coating large areas. Hot pressing combines heat and pressure to densify powder material on a substrate, forming a uniform thin film, commonly used in the fabrication of ceramics and composites. These methods are integral in producing thin films with specific mechanical, electrical, and optical properties, applied in microelectronics, protective coatings, and advanced materials. A scheme of PVD, spraying, and hot pressing methods is presented in Figure 5.

Several works have been carried out utilizing physical deposition methods to enhance the energy storage performance of BOPP. Magnetron sputtered Al₂O₃ (~300 nm thick) showed a higher K, a lower electrical conduction loss, and stronger mechanical properties [83], which helped reduce the Joule heating accumulation and was beneficial for the reduction in the conduction loss and increase in U_d (0.45 J·cm⁻³ under 200 MV/m at 125 °C). The magnetron sputtered wide-bandgap SiO₂ layer [84] showed a similar effect with the CVD-coated one, in enhancement in E_b and energy storage performance in BOPP. Different from magnetron sputtering of metal oxides, Nan and coworkers used low-cost spray coating to decompose 2-D calcium niobate (CaNb₂O₆) nanosheets onto BOPP [85], achieving an ultra-high U_d of 11.6 J·cm⁻³ with a high η of 90%. A very recently published study conducted a broad exploration of the effect of different coating materials on the high-temperature energy storage performance of surface-coated BOPP. Through comparing AlN, SiO₂, BN, and PbZrTiO₃, the author concluded that adding two outer inorganic nanoscale coating layers with a wide bandgap, moderate K, and optimal thickness to the surface of BOPP films significantly raises the barrier height at the electrode–dielectric interface. Similar positive effects were found in other high-temperature polymers, such as PEI, PI, PC, and polyamide-imides (PAI), with surface coating materials of SiO₂ [86], BN [87,88], montmorillonite (MMT) [89,90], etc. The Chi group succeeded in combining electrospinning, spraying, and hot pressing to make surface-coated BN/PC/BN composites [82], as shown in Figure 6a. The BN layer with a large bandgap has a high surface barrier to impede charge migration and therefore lower the leakage current under high temperatures. Moreover, the high thermal conductivity of BN allows heating to be conducted faster in the in-plane direction, effectively lowering the temperature of the film and inhibiting the thermal runaway of the film (as shown in Figure 6b).

Compared with linear polymers such as BOPP and PEI, the ferroelectric polymers, represented by PVDF, possess higher K and attracted lots of research interest. However, they suffer from high U_l due to the inherent ferroelectric loss under applied electric fields. To enhance the energy storage properties of PVDF-based polymers, physical-based coating methods were explored in fabricating surface-coated PVDF-based polymer composites. Both BN [91] and SiO₂ [92] showed great results in lowering U_l and improving the energy storage performance of PVDF-based polymer composites. Liu et al. used Langmuir–Blodgett (LB) and spray coating technologies (Figure 6c), with an emphasis on how the thickness of a Ca₂Nb₃O₁₀ (CNO) perovskite nanosheets nanocoating influences the dielectric and energy storage properties of PVDF-based polymers [93]. The study revealed that CNO nanosheet-based high-insulation layers effectively block charge injection at the electrode–dielectric interface, which greatly decreases the leakage current density within the polymer matrix. As a result, U_d in the CNO-coated PVDF films was substantially enhanced, reaching 25.1 J·cm⁻³ (Figure 6d).

Figure 6. (a) Flow chart of preparation of sandwich structure film; working principal diagram of surface h-BN functional layer. (b) A schematic representation of a real capacitor with a height of H = 40 mm and a diameter of D = 40 mm made by winding BN/PC/BN nanocomposite films. Internal temperature distribution when the ambient temperature is 80 °C, operating at an applied electric field of 100 MV/m in different capacitors made by composite films of PC/BN composites (reused with permission, [82] © 2020, Elsevier). (c) Schematic illustration of the fabrication process of sandwich-structured nanocomposite films. (d) Comparison of U_d and η (reused with permission, [93] © 2023, Elsevier).

Figure 6. (a) Flow chart of preparation of sandwich structure film; working principal diagram of surface h-BN functional layer. (b) A schematic representation of a real capacitor with a height of H = 40 mm and a diameter of D = 40 mm made by winding BN/PC/BN nanocomposite films. Internal temperature distribution when the ambient temperature is 80 °C, operating at an applied electric field of 100 MV/m in different capacitors made by composite films of PC/BN composites (reused with permission, [82] © 2020, Elsevier). (c) Schematic illustration of the fabrication process of sandwich-structured nanocomposite films. (d) Comparison of U_d and η (reused with permission, [93] © 2023, Elsevier).

3.3. Molecular-Level Surface Modification

Based on the fact that the difference in multi-physical and -chemical properties between the inorganic layers and organic polymers may cause an increase in structural defects on the surface of the film, researchers explored molecular-level surface modifications, such as irradiation treatments and surface grafting, that can inherently modify the metal–polymer interface (scheme as shown in Figure 7). The Du group first carried out surface grafting onto BOPP using a photocatalytic oxidation reaction [94]. It was observed that grafting C–OH groups onto the film’s surface creates deeper traps that capture charges injected from the electrode. This process reduces the electric field at the interface and provides a protective barrier against charge injection processes. In another work from the same group [95], the surface-grafted BOPP showed great improvement in its high-temperature energy storage performance, with U_d going up from 1.45 to 2.77 J·cm⁻³ at 85 ℃.

Some recent studies exhibited exciting results for the irradiation treatments of polymers. Wang et al. found that high-energy and strong penetrating γ-irradiation significantly enhances the energy storage performance of polymer dielectrics [96]. At 125 °C, the γ-irradiated BOPP film maintains a significant U_d of 5.88 J·cm⁻³ with a 90% of η at 770 MV m⁻¹. Research and theoretical evaluations suggest that this remarkable performance is due to the γ-irradiation-induced polar functional groups with high electron affinity (E_A) in the polymer matrix. These groups generate deep energy traps that effectively obstruct charge transport, thereby improving the film’s energy storage performance. (Figure 8a–c). Huang et al. took a different path and utilized very low-dose ultra-violet (UV) irradiation from KrCl (222 nm) and Xe₂ (172 nm) excimers to conduct molecular-level surface modifications on BOPP [66]. The enhanced interface between the electrode and polymer dielectric effectively reduces space charge injection and mitigates electric field distortion. High-energy UV photons, which can cleave BOPP chains and break O₂ molecules, promote the development of a more thermally stable oxygen-containing structure (Figure 7), which could further form deep traps and suppress the migration of charge carriers. UV irradiation has also proven effective in enhancing the high-temperature energy storage performance of PEI films by some researchers [97,98]. A very recently published work had some new findings that showed that the injected charge is demonstrated to be the main culprit for U_l during the charge–discharge process at elevated temperatures and high electric fields [99] (Figure 8d). Free radicals introduced by UV irradiation can act as deep traps to suppress the charge injection from the electrode (Figure 8e). The resultant dielectric films exhibit a high capacitive performance of U_d of 3.2 J·cm⁻³ and over 90% η at 480 MV·m⁻¹, 200 °C (Figure 8f).

3.4. Polymeric Coatings and Multilayered Polymeric Films

Besides applying coatings and intercalating inorganic layers in dielectric polymer films, multilayered polymer composites incorporating nanofillers within their layers have also demonstrated enhanced performance over unmodified polymers. As illustrated in Figure 9a, the left side depicts a cross-sectional schematic of a polymer-based multilayer film, while the right side shows a three-dimensional structure of a dielectric capacitor connected to an external circuit with power and load. The gold-colored outer layers represent electrodes, the dark gray layers embedded with green spheres are nanocomposite material layers, and the light gray layers are pure polymer layers. This work employed a convenient entropy-driven self-assembly method to successfully fabricate block copolymer-based supramolecular composite films with a layered structure [62]. Notably, the ZrO₂ nanoparticles, with a wide bandgap, are precisely controlled by self-assembly to be distributed near the outer layers close to the metal electrodes. These nanoparticles within the nanocomposite layers effectively hinder dielectric breakdown pathways (represented by pink branches).

Besides employing block copolymer-directed layer-by-layer self-assembly techniques to achieve multilayered film structures, researchers have reported numerous macro- and micro-interface control methods to prepare dielectric nanocomposite films with organic coatings and/or multilayered structures [100,101,102]. One representative approach involves the layer-by-layer process using the solution casting or blade casting methods.

Boron nitride nanosheets (BNNSs), a wide-bandgap two-dimensional material with high electrical insulation, are widely used as fillers in polymer matrices to enhance the insulating properties of polymers [67,103,104,105,106]. Wang et al. designed a sandwich-structured nanocomposite using solution casting, employing crosslinked divinyltetramethyldisiloxane-bis (benzocyclobutene) (BCB) or PVDF as the polymer matrix [107,108]. BNNS was used as the filler for the outer layers, and BaTiO₃ as the filler for the middle layer. Adding BNNS to the outer layers effectively prevents charge injection from the electrodes, blocking current paths and reducing conductivity, while the introduction of BaTiO₃ nanoparticles in the middle layer significantly enhances the K value. The coupling effect between different layers in the sandwich-structured composite is responsible for the synergistic enhancement of K, E_b, and capacitive energy storage performance. A similar strategy was adopted to prepare (PEI/BNNS)/(PEI/SrTiO₃)/(PEI/BNNS) composites via solution casting [109]. This sandwich-structured polymer can balance the two opposing parameters, K and E_b, even at high temperatures. BNNS in the outer composite layers suppresses charge injection, while SrTiO₃ nanoparticles in the inner layers increase K. Zhang and coworkers designed (DPAES/BN)/(DPAES/BT)/(DPAES/BN) composites based on poly(arylene ether sulfone) (DPASE) using BCB groups as organic shells grafted onto BN and BT fillers [110]. It is speculated that the introduced dielectric transition layer acts as a “dielectric gradient”, mitigating local electric field distribution and preventing electrical tree growth. Additionally, the overall mechanical strength of the composite is improved due to the presence of interlayer crosslinked networks, enhancing resistance to electromechanical failure.

In addition to BN, wide-bandgap metal oxides like Al₂O₃, MgO, and HfO₂ also possess high insulation properties and have been proven to be effective fillers for dielectric film capacitors [48,111]. As shown in Figure 9b, Li et al. used polystyrene/Al₂O₃ nanoplate composites (orange) as a charge-blocking layer and polystyrene/TiO₂ nanoparticle composites (green) as a K enhancement layer [112]. The bi-layered nanocomposite films, designed using a simple solution casting method, exhibit synergistic improvements in K and E_b, achieving an U_e that is double that of pure polystyrene. Cheng et al. developed multilayered poly(methyl methacrylate) (PMMA)-based nanocomposites with extremely low nanofiller concentrations to overcome the previously mentioned trade-off [113]. Specifically, they incorporated 0.2 vol% Al₂O₃ nanoparticles in the middle layer to achieve high polarization and 2 vol% Al₂O₃ nanoparticles in the outer layers to enhance E_b and prevent charge injection. This innovative three-layer nanocomposite design demonstrated a significantly improved U_d of 25.1 J·cm⁻³ and an outstanding η of 93.8% at room temperature.

Moreover, many studies have used pure polymers as coating layers [114,115]. Feng et al. used a low-loss linear polyfluorene ester as the two outer layers and a high-K poly(vinylidene fluoride-co-hexafluoropropylene) [P(VDF-HFP)] as the middle layer, fabricating sandwich-structured films using a simple blade casting followed by the hot pressing method (see Figure 9c) [116]. Compared to single-layer P(VDF-HFP), the FPE on both sides of the composite significantly enhanced the E_b and inhibited dielectric loss.

Figure 9d presents a more refined structure of multilayer all-organic polymer films [117]. Zhang et al. developed ferroelectric PVDF-based all-organic dielectric polymer films featuring a continuous compositional gradient structure. This gradient was created by adjusting the spatial distribution of PMMA components using a straightforward and scalable additive manufacturing technique. The continuous out-of-plane composition gradient enables precise control over the films’ electrical and mechanical properties, significantly improving E_b by optimizing the interaction between the local electric field and stress. The gradient polymer films achieve an impressive U_d of 38.8 J·cm⁻³ and a high η of over 80% at an electric field of 800 kV·mm⁻¹.

Figure 9. Schematic of (a) polymeric coating in polymer composites, in which coated layer embedded with nanoparticles (green colored circles) could effectively impede dielectric breakdown pathway. [62] © 2024 John Wiley & Sons. Schematic of polymeric coating methods including (b) successive drop-casting (reused with permission, [112] © 2020, American Chemical Society), (c) hot pressing (reused with permission, [116] © 2022, Elsevier), (d) electrospinning (reused with permission, [117] © 2024, John Wiley & Sons), and (e) dip coating (reused with permission, [118] © 2023, Elsevier) methods for deposition.

Figure 9. Schematic of (a) polymeric coating in polymer composites, in which coated layer embedded with nanoparticles (green colored circles) could effectively impede dielectric breakdown pathway. [62] © 2024 John Wiley & Sons. Schematic of polymeric coating methods including (b) successive drop-casting (reused with permission, [112] © 2020, American Chemical Society), (c) hot pressing (reused with permission, [116] © 2022, Elsevier), (d) electrospinning (reused with permission, [117] © 2024, John Wiley & Sons), and (e) dip coating (reused with permission, [118] © 2023, Elsevier) methods for deposition.

Using dip coating to prepare a layer of polymer composite coating on the surface of polymer films is also an effective method to enhance the energy storage properties of the films [119]. As shown in Figure 9e, commercial BOPP is used as the base film [118], and high-K particles are loaded on the outer surface of the pure BOPP film through plasma-enhanced coating to address these issues. BaTiO₃ particles are mixed with polyvinyl alcohol (PVA) to prepare the high-K coating. Simulations reveal that the surface coating can eliminate the high electric fields in traditional composites and result in a higher E_b.

Niu et al. reported the designed all-polymer sandwiched films consisting of PI and PEIs, with a gradient change molecular stacking structure between different layers [120] (Figure 10a,b). It was found that high-field electrical conduction was effectively suppressed while high-temperature energy storage performance was greatly improved. Li et al. took a different strategy of self-assembly, and as seen in Figure 10c, a mixture of polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) polymer, 3-n-pentadecylphenoland (PDP) molecules, and ZrO₂ nanofillers in chloroform solvent was used; by controlling the evaporation rate of the chloroform, a highly ordered layered structure was obtained [62]. PDP molecules serve as an entropy source within the system; when entering the PS phase, they reduce the enthalpic barrier for diffusion at the microdomain boundaries. When microdomains solidify into a layered structure, the ZrO₂ nanoparticles are squeezed out of the PS phase due to tighter polymer conformation. The P4VP phase is compatible with both PDP molecules and ZrO₂ nanoparticles, allowing the nanoparticles to remain separated within alternating layered structures. The film features highly ordered interlayer alignment and precise nanoparticle distribution, with an extremely low defect density. The simulated leakage current distributions, shown in Figure 10d, demonstrate that in the controlled-drying model, the P4VP(PDP)/ZrO₂ lamellae are continuous and aligned perpendicular to the applied electric field. This alignment significantly reduces leakage current by obstructing electrical conduction paths in the vertical direction. In contrast, randomly dispersed ZrO₂ nanoparticles in the uncontrolled drying model lead to higher leakage currents due to the formation of percolation pathways along the direction of the electric field. The entropy-driven self-assembled multilayer nanocomposite film doped with ZrO₂ achieves a synergistic enhancement in K and E_b, resulting in a high U_d of 6.2 J·cm⁻³ at an electric field of 650 MV·m⁻¹, while maintaining a high η of >90%. This work reveals the close relationship between the microstructure of polymer nanocomposites and their macroscopic properties, providing a novel approach for the controlled design and fabrication of organic–inorganic composite materials for energy storage film capacitors. In addition, the controlled dispersion of inorganic nanoparticles in hierarchical-structured polymer composite films was also effective in hindering electric breakdown paths, as illustrated by some earlier simulation works [100,121] (Figure 10e).

4. Summary and Perspectives

High-energy-density dielectric materials play a crucial role in advanced energy storage devices for emerging electronic and power applications. However, most existing polymer dielectrics for film capacitors still struggle to meet the trade-off between high U_d and high η. For instance, as mentioned earlier, low-K BOPP excels in η at room temperature but offers limited U_d. Conversely, high-K dielectric polymers like PVDF provide greater U_d, but the inherent ferroelectric loss compromises η. Although efforts to design and develop new polymer structures for enhanced dielectric energy storage have been ongoing, progress has been slow, with innovations largely restricted to modifications of existing chemical structures. Consequently, researchers have continuously explored composite materials, aiming to achieve the goal of ‘1+1 greater than 2’ in terms of material performance.

Traditional polymer-based composites typically incorporate high-K ceramic fillers into the polymer matrix, but this often results in uneven electric field distribution at the organic–inorganic interfaces, significantly reducing E_b. The conflict between K and E_b is a key limitation in improving U_d. In response, research has increasingly focused on integrating wide-bandgap ceramic fillers, such as Al₂O₃, SiO₂, MgO, ZrO₂, and BN, into polymer matrices. This approach helps improve E_b while reducing dielectric loss and enhancing η. However, achieving a good interfacial match between inorganic nanofillers and organic matrices remains a significant challenge. Despite advancements in surface treatment methods for nanofillers, such as organic grafting or core-shell structure construction, uniform and controlled dispersion of nanoparticles within the organic matrix is still difficult to achieve. Even the minor agglomeration of fillers can lead to local electric field distortions under high electric field conditions, initiating breakdown channels that can cause material degradation or even device failure. Thus, achieving controllable nanostructures is a critical focus in composite material design.

Encouraging progress has been made in the field of polymer nanocomposites. Surface-coated or functionalized polymer composite films and multilayer polymeric films with various coatings are widely regarded as promising materials. Whether through physical or chemical methods, using organic, inorganic, or hybrid coatings, most reported hierarchical-structured dielectric films demonstrate highly controllable nanostructures. These nanocoating designs are superior to traditional blended organic–inorganic nanocomposites in studying structure–property relationships, enabling controlled production and offering potential practical applications. While significant increases in K are uncommon in most reported films (an exception being the recent work by Zhang et al. [74], where nano-confinement using oxide films increased the K by over 100%), thin nanocoatings can effectively limit charge transport, particularly by preventing charge injection from electrodes into polymer films. Numerous studies have shown that these nanocoatings can reduce leakage current, improve E_b, and enhance both U_d and η, achieving synergistic improvements in dielectric properties and energy storage performance, as summarized in

Table 2. Summary of key dielectric and energy storage parameters of surface-coated polymer composites. RT refers to room temperature, U_d-90% refers to U_d at η over 90%, U_d-max refers to maximum U_d.

Polymer	T (°C)	Ud-90% (J cm−3)	Ud-max (J cm−3)	Ref.
BOPP-SiO2	120	1.3	1.3	[73]
PEI-150 nm Al2O3	150	3.6	4.0	[76]
	200	2.8	3.4
PEI/4-layer Al2O3	RT	8.0	8.0	[79]
PC-BN-SiO2	RT	9.8	12.2	[78]
	120	3.0	6.4
	150	1.0	5.0
BOPP-F	120	6.2	8.0	[115]
PI/BNNS-2	150	5.2	5.2	[122]
	200	4.4	4.4
Al2O3-PI-Al2O3-PI-Al2O3	150	2.3	3.0	[123]
	200	1.5	2.2
t-BPB-8	RT	7.5	7.5	[124]
	200	3.7	4.0
Al2O3-BT@SiO2/PI-Al2O3	150	2.3	3.6	[125]
	200	1.6	2.6
UV-Al2O3/PEI	150	5.5	5.8	[97]
	200	3.5	4.2
BOPP/γ-150 kGy	RT	10.2	10.2	[96]
P-PEI	150	4.1	4.7	[99]
	200	3.1	3.2
P5B4 (0.75 mg)	RT	4.9	4.9	[88]
	150	2.5	4.0
bothM-LN	150	2.7	4.3	[89]
	200	2.0	4.1
BOPP-UV-CN	RT	11.2	11.2	[85]
P-0.5%E	150	1.8	1.8	[126]
PS/TiO2/Al2O3	RT	3.7	4.4	[112]
PEIs/PIs/PEIs	RT	4.5	4.5	[120]
	150	2.9	3.2
	200	2.0	2.5
P-1%E	RT	3.6	3.6	[127]
	150	2.2	2.2
PS-b-P4VP(PDP) /9 vol% ZrO2	RT	6.2	6.2	[62]

.

Despite these advancements, several key challenges remain. These include gaining a deeper understanding of electrical breakdown mechanisms, optimizing nanocoating structures, and developing scalable manufacturing methods for hierarchical polymer composite films. Generally, layered polymer composites and all-polymer films demonstrate higher E_b than their single-layer counterparts. Various dielectric breakdown mechanisms, including electronic, thermal, and mechanical theories, can be theoretically modeled using finite element and phase-field simulations. Additionally, temperature distribution and mechanical strain in hierarchical multilayer composite films under applied electric fields have been explored. However, comprehensive simulation methods that account for all breakdown theories are still in their early stages. Moreover, most existing simulations only qualitatively describe the breakdown process, and quantitatively predicting E_b remains highly challenging. The thin nature of polymer films used in dielectric applications also makes it difficult to observe internal breakdown processes experimentally, complicating the validation of simulation results.

In addition, while coated polymer films generally show good structural integrity and reproducibility, scaling up the production of capacitor films remains challenging. Techniques like CVD and PVD often require expensive equipment and harsh processing environments, such as vacuum conditions, plasma assistance, or electromagnetic fields. Moreover, purely organic coatings and some solution-processed organic–inorganic composite nanocoatings involve the use of organic solvents, raising concerns about solvent recycling and environmental impact. While some methods have successfully developed centimeter-scale or A4-sized nano-coated polymer films, producing defect-free films tens or even hundreds of meters long and rolling them into film capacitors is still difficult.

Joule heating, resulting from energy loss in polymers, is considered one of the primary causes of dielectric failure. Although wide-bandgap nanocoatings have been shown to reduce leakage current under high temperatures and electric fields, most dielectric polymer-based films, except those with high crystallinity, are difficult to use above the polymer’s glass transition temperature (T_g). The dielectric and energy storage properties of most polymer composite films still decline significantly as the test temperature increases. Moreover, under high-temperature and high-field conditions, the physical properties in such finely structured films, such as electric treeing and charge transport mechanisms, become increasingly complex and remain poorly understood.

Polymer-based materials also face limitations when used as dielectric energy storage capacitors, primarily due to their relatively low K, which remains around ~3—significantly lower than that of ceramic materials, which can exceed 10 or even 100. Additionally, an intrinsic trade-off exists between insulating and thermal properties. Heat-resistant polymers, typically composed of aromatic structures, tend to have a lower bandgap, which consequently reduces their voltage-endurance capability. Addressing these challenges requires innovative approaches, for example, designing polymer–ceramic blending [128], to overcome these limitations and further improve polymer performance. Recent advances in inorganic polymer-like substances, such as poly(SiO₂) and poly(Al₂O₃), have shown promising high K while exhibiting mechanical flexibility properties similar to organic polymers, making them potential candidates for energy storage applications.

In conclusion, this review article highlights the significant achievements in the development of nanoscale-coated dielectric films for high-energy-density capacitors. Further progress in this highly interdisciplinary field will depend on successful collaboration among synthetic chemists, physicists, materials scientists, and electrical engineers. It is anticipated that, in the near future, well-designed layered capacitor films will lead to greatly improved fundamental understanding, new simulation methods, and breakthrough energy storage performance, especially under demanding operational conditions such as high temperatures and high electric fields.