2D Nanosheet Spray Coating for Scalable Processing of High‐Energy‐Density Dielectric Polymer Films

Dielectric film capacitors with high power density and rapid charge–discharge capability are widely used as key components in modern electronic and electrical systems, and polymers are primary dielectric for film capacitors due to their low cost, flexibility, and ease of processing. Here, a surface engineering approach is reported to improve the energy storage properties of polymer films by directly spray coating 2D nanosheets on the polymer film surface. The spraying of 2D calcium niobate nanosheets on the surface of biaxially oriented polypropylene (BOPP) films leads to remarkably increased breakdown strength and dielectric constant, resulting in a maximum 64% enhancement of energy density compared to the pristine BOPP films. Ultraviolet irradiation is further employed to improve the adhesion of nanosheets to the BOPP film surface, leading to an ultrahigh energy density of 11.6 J cm−3 with a high energy efficiency of 90%, which is the highest energy density ever achieved in polypropylene‐based films. This work provides a universal, cost‐effective, and scalable approach to improve the energy density of dielectric polymer films, which is of great significance for the application of high‐energy‐density polymer films in compact and efficient power systems.


Introduction
With the development of modern electronic and electrical systems, electrostatic dielectric capacitors have attracted extensive attention for their superior power density, high energy efficiency ( ), and operational reliability. [1][2][3][4][5] Dielectric capacitors are widely used as energy storage devices in pulse power generation, hybrid vehicles, smart grids, etc. The dielectric layer, serving as the DOI: 10.1002/aelm.202300187 key component of electrostatic capacitors, is usually an oxide ceramic or polymer dielectric. [6] Compared with oxide ceramics, polymer film dielectrics have superior physicochemical properties of polymers and unique advantages in breakdown strength (E b ), low cost, and high reliability. [7][8][9] In addition, polymerbased film capacitors can be easily processed into large-size capacitors and customized into various forms, due to the polymers having good mechanical flexibility and machinability. However, the low energy density of current dielectric polymers makes capacitors bulky and heavy, which cannot meet the requirements of compact and efficient power systems. [10][11][12][13][14] For example, the most advanced commercially available dielectric polymer, biaxially oriented polypropylene (BOPP), has an energy density of only 2 J cm −3 . [15,16] The discharged energy density (U e ) of the dielectric films can be calculated using U e = ∫ E dD, where E and D represent the applied electric field and electric displacement, respectively. For linear polymer dielectrics, the U e can be calculated by a simplified calculation U e = 1 2 0 r E 2 , where ɛ 0 is the vacuum dielectric permittivity. Therefore, the maximum U e of a film capacitor is determined by the dielectric constant (ɛ r ) and E b of the dielectric polymer. [17] However, the ɛ r and E b are always mutually constrained in single-phase polymer or ceramic dielectric materials, which can only be increased at the expense of each other, which severely limits the further enhancement of their energy density. [18,19] In the past decade, polymer-ceramic dielectric nanocomposites combining their respective advantages, i.e., the high E b of the polymer matrix and the high ɛ r of the ceramic filler, have been actively studied and believed to be an effective approach to improve the energy storage performance of dielectric polymer films. [20,21] In addition to the bulk phase of the dielectric polymer film, the surface physicochemical properties, including surface morphology, surface electrical properties, and charge transport behavior, also have a great influence on the dielectric and energy storage properties. [22,23] It has been demonstrated that ceramic nanocoatings onto polymer film surfaces, such as ceramic nanofilms and nanosheets, can effectively impede charge injection from the electrode to the films, exhibiting great promise in the development of high-performance polymeric dielectrics. [24,25] At present, the surface nanocoating technologies of polymer films are mainly enabled by atomic layer deposition (ALD), chemical vapor deposition (CVD), physical vapor deposition (PVD), etc. [26][27][28] which involve complicated and expensive processes. Therefore, the engineering of surface nanocoating is constrained by the conflict between the improvement of energy storage performance and the facile scale-up process. [29][30][31] Here, we describe a low-cost and facile scalable approach to modify the polymer surface with 2D nanosheets by a waterbased spray coating process. Nanocoatings of calcium niobate (CN) have been successfully attached to the surface of BOPP and polyimide (PI) films, demonstrating the universality of the proposed surface nanocoating strategy. Based on this, the effect of 2D nanosheets on the dielectric and energy storage properties of polymer films has been systematically investigated. It is found that CN coatings can simultaneously improve the E b and ɛ r of polymer films. Of particular importance is that ultraviolet (UV) irradiation further improves the surficial hydrophilicity of BOPP film, which facilitates the spread of nanosheets on the films, being conducive to enhanced energy storage properties. The UVirradiated BOPP film sprayed with CN output an ultrahigh U e of 11.6 J cm −3 with a high of 90%.

Results and Discussion
The process of spray coating nanosheets onto polymer films is shown in Figure 1a. The nanosheets are dispersed in an aqueous solution and then sprayed onto the surface of polymer films by a spray gun. After the water dries, the nanosheets are coated on the polymer surface and stacked layer by layer as the number of nanosheets increases, forming the nanocoatings on the surface of polymer films. The concentration of nanosheets in the aqueous solution is changed to control the resultant number of nanosheets on the film surface. Chalcogenide type 2D nanosheets CN with intrinsically high ɛ r (Figure 1b,c) were selected as the spray coating materials for polymer films, and commercially available BOPP and PI films were chosen to verify the generalizability and scalability of this method. BOPP film sprayed with CN is denoted as BOPP-CN for example. The surface contact angles of the polymer films were tested to verify the change of surface states before and after the spraying, where a tight and uniform distribution of nanosheets with smooth surface favors a smaller contact angle. [32][33][34] The proposed spray coating process was found to be suitable for polymer film with both hydrophobic and hydrophilic surfaces. As shown in Figure 1d and Figure S1 (Supporting Information), the commercial BOPP film has a hydrophobic surface with a contact angle of 101°, decreasing to 79°for BOPP-CN after nanosheets coating, indicating the surface hydrophilicity of the polymer film is improved by the coated nanosheets. On the other hand, for PI film with a hydrophilic surface (with a contact angle of 65°), the contact angle is also significantly reduced after the coating of nanosheets, where the PI-CN has a much smaller contact angle of ≈11°, revealing that the hydrophilic polymer surface is more conducive to the spreading and alignment of nanosheets on the surface. This is due to the fact the existence of polar groups on the hydrophilic surface may strengthen their adhesion with nanosheets. [35][36][37] Improving the surface hydrophilicity of BOPP polymer films may be more conducive to the spreading and alignment of nanosheets on the polymer surface. UV light irradiation has been proposed as a simple and effective approach to modify the surface state of polymer films. [38,39] Under UV irradiation, ozone can react with free radicals generated on the surface of polymer films to form oxygen-containing functional groups, thus improving the hydrophilicity of the films. Therefore, the pristine commercial hydrophobic BOPP was exposed to UV light irradiation (denoted as BOPP-UV). After UV light irradiation, the BOPP-UV exhibits a smaller contact angle of 87°than the pristine BOPP film, indicating the surface state has been changed from hydrophobicity of commercial BOPP to hydrophilicity of BOPP-UV. The contact angle of BOPP-UV with CN nanocoating (denoted as BOPP-UV-CN) is notably reduced to 48°, much smaller than the BOPP-CN without UV irradiation, indicating the BOPP-UV-CN has a more uniform and smooth distribution of CN on the surface. According to the cross-sectional SEM image (Figure 1e), the spraying of CN with 0.4 mg mL −1 on the BOPP-UV film forms a dense and uniform nanocoating layer with a thickness of about 12 nm. It is noted that the resultant BOPP-UV-CN film still exhibits excellent flexibility and transparency even after the formation of CN nanocoatings ( Figure 1f). The distribution of CN on the polymer film surfaces is also evidenced by the surface SEM image and corresponding element mapping images. As shown in Figure 1g-j, the CN nanosheets are successfully coated and uniformly distributed on the surface of both BOPP and BOPP-UV. It is noted that some wrinkles still exist on the BOPP-CN, in contrast, BOPP-UV-CN has a smooth surface indicating better adhesion and arrangement of nanosheets on the BOPP-UV surface.
To investigate the effect of spray-coated nanosheets on the properties of polymer films, we first studied the dielectric response of these films. The concentration of nanosheets in the aqueous solution was changed from 0 to 0.8 mg mL −1 to control the number of nanosheets on the surface. We first studied the electrical and breakdown properties of these BOPP-based films. Figure S2 (Supporting Information) gives the E b of polymer films analyzed by the two-parameter Weibull distribution function: is the cumulative probability of electrical failure, E is the experimental breakdown strength, E b is a scaling parameter referring to the breakdown strength at a cumulative failure probability of 63.2%, also known as the characteristic breakdown strength, and is the Weibull modulus associated with a linear regression fit of the distribution. [22,40,41] The obtained E b for different BOPP polymer films with CN nanocoating is shown in Figure 2a. As seen, the CN nanocoating induces enhanced E b compared to the pristine BOPP films with increasing the concentration of nanosheets, which reaches the highest value at the concentration of 0.4 mg mL −1 , being on the order of 820 kV mm −1 , above which, the E b decreases with further increasing the concentration of nanosheet, being ascribed to the possible agglomeration of the nanosheets at higher concentrations. Of particular significance is that the E b is further improved in BOPP-UV-CN film, being dramatically increased to 970 kV mm −1 , with 40% enhancement over the BOPP-CN without UV irradiation. The improved E b in the polymer films coated with nanosheets is first ascribed to the suppressed leakage current density. Leakage currents play an important role in determining the breakdown behavior of dielectrics, since the Joule heating induced by the current may lead to thermal breakdown of the polymer. [42,43] As shown in Figure 2b and Figure S3 (Supporting Information), BOPP-CN film exhibits a much lower leak-age current compared to pristine BOPP film, while the leakage current is further suppressed in BOPP-UV-CN film, being one order lower than that of the pristine counterpart. It should be noted that the dielectric insulating properties of polymers fundamentally depend on the injection and transport of charge carriers, the coated nanosheets are highly insulating in the thickness direction of the polymer, which blocks the charge injection from the electrodes and regulates charge transport, thereby inhibiting the conduction across the thickness of the film. [44,45] The modulated charge injection and transfer behaviors by the CN nanolayers were further evidenced by the surface potential changes. The noncontact Kelvin probe force microscope (KPFM) was employed to test the potential changes of the films after applying a 10 V voltage. As shown in Figure 2c, the normalized extremal contact potential difference (CPD) of the BOPP films shows only a slight decay within 10 min and the process continued for 1 h. In contrast, the normalized extremal CPD of the BOPP-CN film decreases significantly within 10 min and achieves complete charge dissipation within 30 min. The higher rate of charge dissipation by the presence of CN nanosheet layer is mainly ascribed to the accumulation of the injected charges at the nanosheet-polymer interface, where a built-in electric field in the opposite direction of the applied field can be induced to provide the driving force for rapid charge dissipation in the nanosheet layer, which further repels the net inflow of electrons from the electrode. [46] In addition to the leakage current, the breakdown behavior of polymer films also depends on the mechanical strength, where a higher modulus of elasticity generally leads to a stronger electrostatic tolerance. [25,42,47] The mechanical properties of these BOPP films were tested and shown in Figure 2d and Figure S4 (Supporting Information), as expected, the Young's modulus of polymer films coated with nanosheets is increased by about 30% compared to the pristine BOPP film, which is conducive to the higher E b .
In addition to the increased E b , the ɛ r of the polymer films after nanosheet coating is also improved. The ɛ r of different BOPP films spray-coated with CN nanosheets as a function of nanosheet concentrations is shown in Figure 2e. The frequency dependence of the ɛ r and dielectric loss is shown in Figure S5 (Supporting Information). The ɛ r of the BOPP films increases after spray-coating the nanosheets. The increased ɛ r is ascribed to the intrinsic large ɛ r of CN nanosheet layer (ɛ r > 400) [48] and the emergence of interfacial polarization at CN/BOPP interfaces. It can be seen that the BOPP-CN reaches a maximum ɛ r of 2.7 at the concentration of 0.4 mg mL −1 , being more than 20% higher than that of the pristine BOPP film (ɛ r = 2.2). Notably, BOPP-UV-CN exhibits further increased ɛ r compared to BOPP-CN, due to the more uniform and tightly distributed nanosheets reduces possible interfacial voids between nanosheets layers. The dielectric loss of the BOPP-CN films with nanocoatings slightly increases as the concentration of the sprayed nanosheets increases. In contrast, the dielectric loss of BOPP-UV-CN is remarkably suppressed below 0.1, due to the tightly and uniformly distributed nanosheets on the BOPP-UV surface reducing structural defects.
To investigate the energy storage performance of the studied polymer films, D-E loops of different BOPP-based films were measured and shown in Figure 3a and Figure S6 (Supporting Information). The U e and of polymer films were calculated based on the D-E loops and given in Figure 3b and Figure S7 (Supporting Information), and the highest energy densities of the studied films were compared in Figure 3c. Compared with pristine BOPP, the coating of CN nanosheet leads to improved energy density. The BOPP-CN with enhanced ɛ r and E b achieves a higher U e of 7.97 J cm −3 at an electric field of 820 kV mm −1 , which is about 64% higher than pristine BOPP film while maintaining a high more than 87%. Of particular significance is that the U e of the BOPP-UV-CN was further increased to a record-high value of 11.6 J cm −3 (at an electric field of 960 kV mm −1 ), with a 138% enhancement over that of pristine BOPP film, meanwhile possessing an ultrahigh of 90%, being superior to all the actively studied BOPP-based polymers and nanocomposites, as compared in Figure 3d. [49][50][51][52][53][54][55][56][57] In addition to high U e and , the stability and reliability of the dielectric film are also important in the practical capacitor applications. We further characterized the charge-discharge performance of BOPP-based polymer films. As shown in Figure 4a, the U e and of polymer films spray-coated with nanosheets maintain stable even after 10 6 successive cycles without any degradation. In addition, the BOPP-CN film exhibits no change in energy storage performances after 15 days of resting treatment ( Figure S8, Supporting Information), indicating excellent stability of the CN on the BOPP film surface. It is worth noting that the proposed High power density is one of the main advantages of dielectric capacitors in energy storage devices. To study the effect of spraying nanosheets on the power performance, the polymer films were charged at 300 kV mm −1 and then discharged to a load resistance of 100 kΩ, and the discharge time is determined to be at 90% of the U e , as shown in Figure 4c,d. The energy density released by the polymer films after spraying the nanosheets is increased compared to pristine polymer films. The pristine BOPP film releases an energy density of 0.8 J cm −3 in 9.8 μs with a maximum power density of 0.22 MW cm −3 , while the BOPP-UV-CN releases an energy density of 1.1 J cm −3 in 13 μs with a maximum power density of 0.24 MW cm −3 , being comparable to its pristine counterpart.
To verify the generalizability of the proposed nanosheet spraying method, the dielectric and energy storage properties of PI films sprayed with CN were investigated. As shown in Figure 5a, the ɛ r of PI films also increases after spraying CN nanosheets. Figure 5b shows the Weibull distribution of E b for pristine PI and PI-CN films, the E b of PI is increased after the spraying of CN nanosheets. For instance, the E b is increased from 420 kV mm −1 for pristine PI to 550 kV mm −1 for PI-CN with 0.4 mg mL −1 . Sim-ilar to the BOPP-based films, the improved E b in PI-CN films is mainly caused by the suppressed leakage current and increased Young's modulus ( Figures S9 and S10, Supporting Information). Contributed by the improved E b and ɛ r , as shown in Figure 5c,d and Figure S11 (Supporting Information), the U e of PI-CN film reaches 5.17 J cm −3 ( = 85%), which is 118% enhancement over the pristine PI film (U e = 2.37 J cm −3 , = 87%). In addition to the improved energy density, it is noted that the PI-CN film also exhibits excellent power performances ( Figure S12, Supporting Information), which releases an energy density of 1.44 J cm −3 in 15 μs with a maximum power density of 0.25 MW cm −3 .

Conclusions
In summary, a general and scalable method is proposed to improve the energy storage properties of polymer films by direct spray coating of nanosheets, which is broadly applicable to polymer films with both hydrophobic and hydrophilic surfaces. Spraying of 2D nanosheets onto the surface of polymer film results in enhanced ɛ r and E b , leading to a remarkably improved U e of BOPP and PI films. The adhesion of the nanosheets to the BOPP film surface can be further improved with UV irradiation treatment, and the BOPP-UV-CN film outputs a recordhigh U e of 11.6 J cm −3 along with a high of 90% at E b of 960 kV mm −1 . This work provides an effective path to explore the commercial-scale fabrication of low-loss, high-efficiency, and high-energy-density dielectric polymer films.

Experimental Section
Preparation of Polymer Films: The commercial BOPP film was purchased from Anhui Tongfeng Electronics and its thickness is 8 μm. The commercial PI film was purchased from Kaneka Corporation and its thickness is 8 μm.
Aqueous solutions of calcium niobate nanosheets with concentrations of 0.2, 0.4, 0.6, and 0.8 mg mL −1 were prepared. The spraying process was carried out at a fixed spraying distance of 30 cm, 5 mL of aqueous nanosheet solution with different nanosheet concentrations was sprayed on the surface of a 10 × 5 cm 2 film within 10 min, and then dried at a drying temperature of 75°C.
UV Irradiation Treatment: The BOPP film was exposed to UV light from the source at ambient temperature. The UV irradiation was applied by a UV lamp system (BOT-UV200WT) whose power is 200 W and wavelength is 185 and 254 nm. The water contact angles of BOPP films were characterized by contact angle meter (Dataphysics OCA 35).
Characterization: The morphologies of the nanosheets were analyzed by AFM (Cypher ES, Asylum Research, USA) and TEM (Talos F200S). KPFM images were acquired with an Asylum Research Cypher ES atomic force microscope. Free-standing polymeric film samples were attached onto the sample stage and silver paint was used to ensure electrical contact. For KPFM images, a low-frequency electrostatic excitation (≈2 kHz) is applied on the metallized AFM tip (Asylum Research ASYELEC-01-R2 with typical stiffness ≈2 N m −1 and resonance frequency ≈75 kHz). A feedback loop controls an additional DC voltage applied to the tip in order to keep the electrostatic tip-sample interaction as small as possible. This applied DC voltage provides a direct measure of the contact potential difference (CPD) between the tip and the sample. For charge injection, a typical conductive-AFM setup was used with a high-voltage source being connected to the tip and the back of the sample. The morphologies of the polymer films were characterized by SEM (JSM-7610FPlus, JEOL, Japan) and EDS (Oxford X-Max50). For the electrical measurements, copper electrodes (4 mm in diameter and 50 nm in thickness) were deposited on both sides of the films. ɛ r and loss were analyzed with a Precision Impedance Analyzer (4294A, Agilent, USA) at room temperature in the frequency range from 10 2 to 10 6 Hz at 1Vrms. D-E loops were measured at 10 Hz with a multiferroic ferroelectric test system (Premier II, Radiant Technologies, Inc.) at room temperature, the DC leakage current densities (in A cm −2 ) were also analyzed by ∇D•Area/∇t with this ferroelectric test system. E b tests were performed with a Withstand Voltage Test System (Shenzhen Rek Electronic Technology Co., Ltd, China) at a ramping rate of 200 V s −1 with a limiting current of 5 mA. Young's modulus was measured at a speed of 1 mm min −1 with Electromechanical Universal Testing Machine (MTS Systems Co., Ltd, China). The charge and discharge behaviors were measured by RC circuit performed by a capacitor chargedischarge test system (PKCPR1701, PolyK Technologies, USA) with a highvoltage MOSFET switch, where the sample was charged by an electric field of 300 kV mm −1 through the MOSFET switch and the stored energy was discharged to a load resistor of 100 kΩ.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.