Significantly Improved High‐Temperature Energy Storage Performance of BOPP Films by Coating Nanoscale Inorganic Layer

Biaxially oriented polypropylene (BOPP) is one of the most commonly used commercial capacitor films, but its upper operating temperature is below 105 °C due to the sharply increased electrical conduction loss at high temperature. In this study, growing an inorganic nanoscale coating layer onto the BOPP film's surface is proposed to suppress electrical conduction loss at high temperature, as well as increase its upper operating temperature. Four kinds of inorganic coating layers that have different energy band structure and dielectric property are grown onto the both surface of BOPP films, respectively. The effect of inorganic coating layer on the high‐temperature energy storage performance has been systematically investigated. The favorable coating layer materials and appropriate thickness enable the BOPP films to have a significant improvement in high‐temperature energy storage performance. Specifically, when the aluminum nitride (AlN) acts as a coating layer, the AlN‐BOPP‐AlN sandwich‐structured films possess a discharged energy density of 1.5 J cm−3 with an efficiency of 90% at 125 °C, accompanying an outstandingly cyclic property. Both the discharged energy density and operation temperature are significantly enhanced, indicating that this efficient and facile method provides an important reference to improve the high‐temperature energy storage performance of polymer‐based dielectric films.


Introduction
[3][4][5][6] The mainstream dielectric materials used for electrostatic capacitors are inorganic ceramic or organic polymer films.[12] However, the sharp increase in electrical conduction as temperature increasing dramatically degrades the high-temperature capacitive performance, [13,14] which is difficult to meet the requirement in the case of high temperature.For example, the temperature of inverters near the combustion engines in hybrid electrical vehicles (HEVs) exceeds 140 °C, [15,16] but biaxially oriented polypropylene (BOPP) films as the most popular commercial dielectric films can only suffer from an upper limit temperature of 105 °C, [17][18][19] so the cooling systems have to incorporate to maintain the capacitor working in the allowable temperature range, which brings the increase of manufacturing cost and device volume.Moreover, the cooling system is not practical in some particular situation such as in an underground oil and gas exploration, which requires the capacitors to withstand the temperature above 200 °C. [20,21]n order to meet the development of the miniaturization and lightweight of power electronic systems, the high-temperature resistant polymer films, such as polyimide (PI), polyetherimide (PEI), polyetheretherketone (PEEK), polycarbonate (PC), and benzocyclobutene (BCB), have received great attention due to their high glass transition temperature (T g ). [5]However, the high-temperature energy storage density of these dielectric films is still unsatisfied due to the serious conduction loss.In recent years, many effective ways have been proposed to reduce electrical conduction loss at high temperature, for example, by filling the inorganic fillers with wide bandgaps [22][23][24][25][26] or semiconducting organic fillers with high electron affinity [27] are beneficial to incorporating trap centers and limiting charge transport in the films.Besides the introduction of charge traps, by growing BNNSs and MgO that have wide bandgap structure as intermediate barrier layers in the polymer films can also build topological potential barrier and electric charge scattering centers, which also can hinder carrier transport, reduce leakage current, and improve high-temperature energy storage performance. [28,29]However, under the high temperature and high electric field, the barrier height at the electrode/polymer interface decreases and Schottky-emitting carriers increase, this is the main obstacle to achieve excellent energy storage performance at elevated temperature.The Schottky emission can be expressed by Li et al., [5] Zhou et al., [16] and Lengyel: [30] where J s is the leakage current density, E is the electric field, A is the Richardson constant, T is the temperature, ϕ is the barrier height at the electrode/dielectric interface, ε r is the dielectric constant, and k is the Boltzmann constant.The leakage current density mainly depends on the height of interfacial barrier, temperature, and applied electric field.][33] It was reported that the introduction of Al 2 O 3 coating layer in PI films decreases the leakage current from 1.1 × 10 −7 A cm −2 to 5.53 × 10 −9 A cm −2 at 200 MV m −1 and 150 °C. [34]Similarly, the Al 2 O 3 -PEI-Al 2 O 3 films achieve a maximum U e of 2.8 J cm −3 with η > 90% at 200 °C, which is much superior to that of the films without the coating layer. [32]In addition, both phase-field simulations and experimental results have shown that the coating layer can effectively inhibit charge injection and hinder the growth of electric trees. [16,34]Although the inorganic coating layer has shown advantages in improving the high temperature of polymer dielectric films, the effect of inorganic coating layer's bandgap structures on the suppression of conduction loss is less studied; furthermore, a facile, large-scale, lowcost technique for growing inorganic coating layer is also in great need.More importantly, despite many kinds of high-temperature resistant polymers having great potential to be used as high-temperature dielectric films due to their high T g , their self-healing capability as well as a high dielectric loss should be resolved for application in the future.Therefore, how to improve the hightemperature performance of commercial BOPP films has become the concerned issue.In this study, the magnetron sputtering technique is performed to grow highquality coating layer, as shown in Figure S1, Supporting Information, [35,36] four kinds of inorganic coating layers with different bandgap structures, such as AlN (6.2 eV), BN (5.8 eV), SiO 2 (9 eV), and PZT (3.5 eV), [15,16,[37][38][39] are grown onto the surface of commercial BOPP films to construct the sandwich-structured films, named as A-B-A, B-B-B, S-B-S, and P-B-P, respectively.By utilizing the wide bandgap of the inorganic coating layer and the increased interfacial barrier in the sandwich-structured films, both the electron injection and charge transport can be significantly suppressed; concurrently, the effect of inorganic coating layer's thickness on the high-temperature energy storage performance is also investigated.The favorable coating layer materials and appropriate thickness enable the BOPP films to have a significant improvement in high-temperature energy storage performance.Specifically, when the aluminum nitride (AlN) acts as a coating layer, the AlN-BOPP-AlN sandwich-structured films possess a discharged energy density of 1.5 J cm −3 with an efficiency of above 90% at 125 °C, accompanying an outstandingly cyclic property.

Structure Characterization
The successful deposition of inorganic coating layer is demonstrated by SEM and XPS.As shown in Figure 1a and Figure S2, Supporting Information, it is clearly seen that the inorganic coating layer with a thickness of ~260 nm AE 10 nm is well bonded onto the BOPP films without any delamination or other cracks.It should be mentioned that the fibrous stripes morphology in the commercial BOPP films may be induced by the stretching process.To further demonstrate the successful preparation of inorganic coating layer, the surface element was tested by XPS, as given in Figure 1b and Figure S3, Supporting Information.For the A-B-A sandwich-structured films, the binding energy of 74.3 eV and 399.7 eV for the Al 2p and N 1s spectra reflects the presence of AlN, while some oxide-N pollution around 402.8 eV is observed in AlN coating layer, which may be caused by the residual air at a nonperfect vacuum. [40,41]The detailed XPS results for B-B-B, S-B-S, and P-B-P films are provided in Figure S3, Supporting Information, meaning that SiO 2 , BN, and PZT inorganic coating layers are also successfully coated onto the BOPP film's surface.It can be seen from Figure S4, Supporting Information of the XRD patterns that BOPP films exhibit the typical α-crystalline structure, and the growth of coating layer does not have an obvious influence on the crystal structure of BOPP films.Besides, there are no diffraction peaks corresponding to the coating layers can be found, demonstrating their amorphous characteristics.The barrier height between the electrode/dielectric is the most important factor to access the effectiveness of the coating layer on the suppression of charge injection.The energy band structures of AlN, SiO 2 , BN, and  S5 and S6, Supporting Information.The binding energies obtained from the UPS spectra for AlN, SiO 2 , BN, and PZT coating layers are 8.83, 11.99, 9.65, and 7.73 eV, respectively. [16]The barrier height of hole and electron injection are 3.63, 5.79, 4.45, 2.53 eV and 2.57, 3.21, 1.45, 0.87 eV, respectively, for A-B-A, S-B-S, B-B-B, P-B-P sandwich-structured films.From the above-mentioned results, the largest barrier height is obtained in the S-B-S sandwichstructured films due to the wider bandgap of SiO 2 coating layer, benefiting to suppress the conduction loss and optimize the energy storage performance.However, it should be also noted that in addition to the coating layer's bandgap, the dielectric constant of inorganic coating layer also plays an important role in the leakage current at an elevated temperature according to Equation (1). [32]

Dielectric Performance
The dielectric properties of the BOPP films with different coating layers were investigated, and the dielectric constant (ε r ) and loss factor (tanδ) versus frequencies and temperatures are given in Figure 2a,b and Figure S7, Supporting Information.The dielectric constant and loss factor show a weak dependence on the measured frequency and temperature.But the dielectric constant of sandwichstructured films is slightly higher than pristine BOPP films (ε r ~2.2 at 1 kHz), which gradually increases with the increase of the coating layer's dielectric constant, herein ε r of PZT > AlN > SiO 2 > BN. [32,[42][43][44] Nevertheless, the dielectric loss of sandwich-structured films is slightly higher compared to pristine BOPP films, which may be caused by the ceramic target deposition-induced broken bonds of molecular chains or generation of reactive radicals on the BOPP film's surface.As shown in Figure 2b, the dielectric parameters of the sandwichstructured films exhibit excellent thermal stability, the dielectric constant is almost unchanged and exhibits excellent thermal stability from 25 to 125 °C, where the dielectric loss slightly increases above 100 °C, mainly contributing to the thermal-induced conduction loss.
The electric breakdown strength is an important parameter to assess the insulation of dielectric films.The two-parameter Weibull distribution is commonly used to analyze the breakdown strength of polymer dielectric films according to Equation (2).
where P(E i ) is the probability of electric breakdown, E i is the measured electric breakdown strength, β is the shape parameter that evaluates the scatter of data, and E 0 is the Weibull characteristic breakdown strength, representing the breakdown strength at failure probability of 63.2%. [45]It can be seen that by growing inorganic coating layer enhances the breakdown strength of the sandwichstructured films, the maximum E 0 of 589.8 MV m −1 at 125 °C is obtained in the A-B-A sandwich-structured films, which is 1.18 times that of pristine BOPP films.Meanwhile, for S-B-S, B-B-B, and P-B-P sandwich-structured films, E 0 value gradually decreases but is still superior to pristine BOPP films, as given in Figure 2c and Figure S8, Supporting Information.As we know, the wide bandgap would induce a high potential barrier for electron injection, as well as the high dielectric constant also help to inhibit the reduction of the barrier height with the applied electric field increase. [5]The nanoscale coating layer has a good balance of bandgap and dielectric constant, which is beneficial for suppressing charge injection.The AlN coating layer possesses both wide bandgap and high dielectric constant of (ε r ~9), resulting in the significant suppression of conduction loss and improvement of breakdown strength in the A-B-A sandwichstructured films.Energy Environ.Mater.2024, 7, e12549 The leakage current of sandwich-structured films is characterized by different electric field and temperature, as shown in Figure 2e and Figure S9, Supporting Information, which of pristine BOPP, P-B-P, B-B-B, S-B-S, and A-B-A films are 2.49 × 10 −7 A cm −2 , 1.21 × 10 −7 A cm −2 , 7.02 × 10 −8 A cm −2 , 3.50 × 10 −8 A cm −2 , and 2.54 × 10 −8 A cm −2 at 125 °C and 100 MV m −1 , respectively.As expected, the inorganic coating layer effectively reduces the leakage current density and conduction loss of BOPP films, which is beneficial to decrease the energy loss at high temperature.The suppressed electric charge injection can be further verified according to the Schottky conduction mechanism of Equation ( 3), the fitting results in Figure 2f show a linear relationship between Ln of current density/sq (temperature) and sqrt (electric field). [46]The intercept of the fitting curve corresponds to the barrier height, where A-B-A sandwich-structured films have a much higher barrier height, probably because the difference in dielectric constant between AlN and BOPP is larger than that of SiO 2 and BOPP, the enhanced interfacial potential barrier is more helpful in suppressing electric charge injection. [47,48]Conversely, although the dielectric constant of PZT coating layer is the highest one in this work, the much lower bandgap leads to the inferior barrier height.

Energy Storage Performance
The energy storage performance of the films was analyzed by measuring electric displacement-electric field (D-E) hysteresis loops.The discharge energy density U e and efficiency η can be obtained by calculating the integral area in the hysteresis loop, as shown in Figure S10, Supporting Information, where the purple area of D-E hysteresis loops describes the energy loss in the charge-discharge process. [49]The polarization behaviors of the sandwich-structured films at different electric field and high temperature were measured by D-E hysteresis loops, as shown in Figure S11, Supporting Information.Obviously, the D-E hysteresis loops of the pristine BOPP films become much fatter, and remnant polarization (P r ) also exhibits a slight increasing trend with the increase of electric field, indicating that the energy loss increases during the charge and discharge process.On the contrary, the D-E hysteresis loops of the sandwichstructured films are much slimmer, meaning that the introduction of inorganic coating layer reduces the energy loss.
In Figure 3a-c, an obvious characterization can be found that η of the sandwichstructured films is much higher than that of BOPP films at high temperature and high electric field.The maximum discharge energy density (U emax ) above η > 90% is the key parameter to access the film's hightemperature energy storage performance. [50]he U emax of A-B-A, S-B-S, B-B-B, and P-B-P films are 3.7, 3.1, 2.42, and 1.95 J cm −3 , respectively, which are much higher than 0.85 J cm −3 at 100 °C of pristine BOPP films.It has also been demonstrated that the improved barrier height not only requires the coating layer to have a wide bandgap but also possesses an appropriate dielectric constant, thus resulting in optimal capacitive performance.At 125 °C, the U emax of the A-B-A films decreases significantly but still reaches 1.5 J cm −3 , which is 16 times higher than BOPP films.More encouragingly, as shown in Figure 3d, the U e and η of the sandwich-structured films are almost unchanged after 50 000 charge/discharge cycles at 125 °C and 200 MV m −1 (i.e., the operating electric field of an electric vehicle power inverter).It means that the energy storage performance of sandwich-structured films has excellent cycling stability and structural integrity at high temperature.
The above experimental results demonstrate that by growing inorganic coating layer can effectively reduce the conduction loss and enhance the high-temperature energy storage performance, the possible mechanism is given below.Under the externally applied electric field, the metal electrodes will emit electrons or holes into the films due to the Schottky effect, respectively, [5,51] and the injected charge will transfer and accumulate at the interface between the metal electrode and the polymer films, as shown in Figure 4a.Under high temperature and high electric field, the potential barrier height decreases, and the Schottky injection charges increase, as shown in Figure 4b and Figure S12, Supporting Information. [5]Interestingly, the inorganic coating layer with a wide bandgap and medium dielectric constant is grown onto the polymer/electrode interface to build a higher barrier, leading to the suppression of charge injection.According to the equivalent circuit model, the sandwich-structured films are equal to the non-ideal series capacitors, as shown in Figure 4d.Taking AlN as an example, AlN coating layer could share much lower electric field due to its higher dielectric constant compared to BOPP, the lower local electric field and the increase in the potential height may induce less electric charge injection.In addition, the binding charges induced by the BOPP polarization will accumulate at the interfaces of the coating layer and BOPP films, which also traps and neutralizes the injection charges and limits their transport into the BOPP films, thus resulting in the reduction of leakage current density and conduction loss, as displayed in Figure 4c. [47]

Optimal Thickness of Inorganic Coating Layer
Besides, the electrical parameters, such as bandgap and dielectric constant, have a deep influence on the suppression of conduction loss and the improvement of high-temperature energy storage performance of BOPP films, the thickness of the inorganic coating layer should be also attracted much attention. [52,53]Herein, the sandwich-structured films with different thicknesses of AlN coating layer have been prepared by changing the deposition time, abbreviated as A-B-A-x.From the crosssectional SEM images, as displayed in Figure 5a-e, it can be seen that there are no obvious structural defects in AlN layer, and the thickness of AlN coating layer conforms to a linear relationship with the deposition time, as shown in Figure 5f, which increases from 163 to 475 nm for A-B-A-0.5 films and A-B-A-2 films, respectively.To further explore the relationship between the surface morphology and sputtering time, the surface roughness (RMS) was characterized by AFM, as shown in Figure 5h-k, the RMS of the AlN layer is only 6.68 nm at a short sputtering time of 0.5 h, which is close to that of the pristine BOPP films (5.17 nm); however, with the further increase in the sputtering time, the RMS of AlN coating layer markedly increases, as shown in Figure 5l.When the sputtering time reaches 2 h, the RMS increases to 13.6 nm.The main reason may be that the BOPP films are used as a growth template during the initially sputtering AlN, resulting in a relatively low roughness of the AlN layer due to the smooth surface of BOPP films.As the sputtering time increases, the AlN itself acts as a growth template and the "island" growth and leads to increased surface roughness.It can be seen that the RMS rises slightly when the sputtering time is higher than 1.5 h.To be highlighted, the higher surface roughness may lead to "concave-convex" morphology, which may induce an inhomogeneous distribution of the electric field and degradation of electric breakdown strength. [54]s far as we know, the AlN coating layer which exhibits a large bandgap and medium dielectric constant is the first time to be used to suppress the charge injection of polymer capacitor films.As shown in Figure 6a and Figure S13, Supporting Information, the dielectric constant of the A-B-A sandwichstructured films is slightly higher than pristine BOPP (ε r ~2.22) at 1 kHz, due to the much higher dielectric constant of AlN layer as well as the interfacial polarization effect.In addition, both the dielectric constant and dielectric loss factor increase slightly as the increase in the thickness of AlN coating layer.It can be seen that the dielectric constant of all the samples exhibits excellent temperature stability from 25 °C to 125 °C.As shown in Figure 6b, the breakdown field strength of the sandwich-structured films with different AlN thicknesses gradually decreases as the temperature increases, mainly due to the reduction of the interfacial barrier height, leading to the increase in the injection charge  density and decrease in the electric breakdown strength.The largest breakdown field strength of 589 MV m −1 is obtained in the A-B-A-1 sandwich-structured films at 125 °C, and the β value increases from 16.14 to 17.61, indicating its higher dielectric reliability, as given in Figure 6c and Figure S14, Supporting Information.However, with the further increase of AlN coating layer's thickness, both the breakdown field strength and β values start to decrease, according to SEM and AFM image analyses, this phenomenon may be related to the AlN coating layer's quality due to the much longer sputtering time. [52]t was reported that the introduction of an inorganic coating layer on the surface of the polymer films could increase Young's modulus, improve the films resistance to Coulomb forces, reduce the possibility of electromechanical breakdown, and enhance the breakdown strength, [33] this may be also one of the reasons for the excellent insulation properties achieved in the A-B-A sandwichstructured films.The leakage current density variation is similar to that of breakdown field strength, as displayed in Figure 6d and Figure S15, Supporting Information.When the sputtering time is <1 h, the leakage current density decreases first and then increases with the further increase of sputtering time.At 125 °C and 100 MV m −1 , the leakage current density of A-B-A-1 sandwichstructured films is only 1.10 × 10 −8 A cm −2 , which is 88% lower than that of pristine BOPP films, demonstrating the suppression of high-temperature conduction loss.It should be noted that the tunneling current has an effect on the overall leakage current in the films at high electric fields, while the tunneling current decreases exponentially with the increase of the barrier height and thickness; therefore, an effective energy barrier can be established in response to wide bandgap, medium dielectric constant as well as appropriate thickness. [55,56]e unipolar hysteresis loops, discharge energy density, and efficiency of the A-B-Ax sandwich-structured films at different temperatures are given in Figure 7a,b and Figure S16, Supporting Information.With the sputtering time increasing, the remnant polarization of the coated films increases and then decreases, the lowest remnant polarization is obtained when the sputtering time is 1 h, representing that the A-B-A-1 sandwich-structured films have a much lower conduction loss and higher η.It can be clearly seen that η value of the pristine BOPP films drops below 90 and 76.2% at an electric field of 100 and 400 MV m −1 , respectively.In contrast, η value of the A-B-A-1 sandwich-structured films remains above 90% even at an electric field of 400 MV m −1 , mainly contributing to the introduction of the AlN coating layer that increases the electron injection barrier height and suppresses the conduction loss at high temperature.For film capacitors, low η means a large amount of energy loss that will be converted into Joule heat, the accumulation of heat will cause thermal runaway of the capacitor and result in device failure. [57]The AlN coating layer can inhibit the charge injection and decrease the generation of Joule heat, besides, the much higher thermal conductivity of AlN coating layer may be also beneficial to promoting Joule heat dissipation and reducing the internal temperature rising of the capacitor.As shown in Figure 7b and Figure S17, Supporting Information, both U e and η of the pristine BOPP films drop sharply with the increasing temperature from 25 °C to 125 °C, while the A-B-A-x sandwich-structured films show both high U e and η at high temperature and high electric field.For example, at 125 °C with an efficiency above 90%, the U emax of BOPP, A-B-A-0.5, A-B-A-1, A-B-A-1.5, and A-B-A-2 films are 0.08, 0.8, 1.5, 1.1, and 1.1 J cm −3 , respectively.More interestingly, the U emax (1.5 J cm −3 ) of A-B-A-1 sandwich-structured films at η > 90% and 125 °C is close to U emax of BOPP films at 75 °C (1.56 J cm −3 ), demonstrating that the maximum operating temperature of BOPP films can increase to 125 °C.Notably, over a continuous 50 000 cycles of charge/discharge at 125 °C and 200 MV m −1 , as shown in Figure 7c, the excellent stability of U e and η for the A-B-A-1 sandwich-structured films indicates the remarkable long-term reliability of the capacitive films.Moreover, a fast discharge test has been performed to study the charge/discharge power density.The discharge time is defined as the time when the discharge energy in the load resistor reaches 95% of the total discharge energy density, and the power density is the ratio of 95% of the total discharge energy density to the discharge time. [58]As shown in Figure 7d, the released energy for A-B-A-1 sandwich-structured films at 125 °C is 0.31 J cm −3 in 2.476 μs, while the energy released from BOPP films is only 0.29 J cm −3 in 2.48 μs.It shows that the introduction of an inorganic coating layer would increase the U e rather than delay the released energy time.Most encouragingly, the A-B-A-1 sandwich-structured films show an excellent release power density of 0.12 MW cm −3 at 125 °C, which is about a 10% increase compared to the pristine BOPP films Energy Environ.Mater.2024, 7, e12549 (0.11 MW cm −3 ).Although heat-resistant polymers and their nanocomposites have been reported to show excellent hightemperature capacitive performances, [22,24,26,59] these raw materials are expensive relative to polypropylene as well as the complex preparation or modification methods that may be much more difficult to accord with the industrial large-scale production, whereas the magnetron sputtering treatment used in this study meets well with the mature production technology of polymer capacitor films, exhibiting good potential to be used in the industrial production in the future.

Conclusion
In summary, an efficient and facile method has been proposed to improve the high-temperature energy storage performance for commercial BOPP films.This work demonstrates that with the introduction of inorganic nanoscale coating layer with wide bandgap, medium dielectric constant, and appropriate thickness on the surface of BOPP films, the barrier height can be significantly increased at the electrode/dielectric interface, which effectively suppresses the conduction loss and improves the energy storage performance at the high electric field and high-temperature conditions.The obtained A-B-A-1 sandwich-structured films exhibit excellent capacitive performances, U e at η > 90% and 125 °C is 1.5 J cm −3 which is almost 16 times that of pristine BOPP films.Moreover, the coated BOPP films have ultra-high breakdown strength and excellent charge/discharge cycle stability at high temperature.The nanoscale coating layer can be fabricated efficiently in industrial production using the facile magnetron sputtering technique, which exhibits great potential to be used in the large-scale manufacturing of polymer-based capacitor films.

Experimental Section
Materials: BOPP films were purchased from PolyK Technologies.The ceramic targets of AlN, SiO 2 , BN, and PZT (3 mm in thickness, 50 mm in diameter) were purchased from Hefei Crystal Material Technology Co., Ltd.Preparation of sandwich-structured films: The schematic diagram of the preparation process is shown in Figure S1a, Supporting Information.The deposition process was carried out by a high vacuum magnetron sputtering coating machine (JCP350; Beijing Taico Technology Co., Ltd.) and the inorganic coating layer was deposited on both sides of BOPP films.First, the pristine BOPP films were washed with anhydrous ethanol and distilled water to remove surface impurities and oil contamination.Then, BOPP films were fixed on the stainless steel substrate in the vacuum chamber, the ceramic target was also put into the target carrier.The vacuum level was 2 × 10 −4 Pa, and DC bias power supply was turned on and modulated to 500 V.The flow rate ratio of argon and oxygen was set as 14:3, the sputtering gas pressure was 1.3 Pa.The substrate rotation speed and sputtering power were 6 r min −1 and 80 W, respectively.The thickness of AlN coating layer was controlled by adjusting the sputtering time to 0.5, 1, 1.5, and 2 h, respectively.The AlN-BOPP-AlN films with the different sputtering time corresponded to different thicknesses in AlN coating layer, labeled as "A-B-A-x", where "x" represents the sputtering time.BN-BOPP-BN (labeled as B-B-B) and SiO 2 -BOPP-SiO 2 (labeled as S-B-S) and PZT-BOPP-PZT (labeled as P-B-P) sandwich-structured films were prepared by the same process.The large-scale sandwich-structured films are successfully prepared by a "roll-toroll" industrial production process, as shown in Figure S1b, Supporting Information.Structure characterization: The morphology of the inorganic coating layer was determined by field emission SEM (SU 8020).The size of the surface roughness film samples was characterized by AFM (CSPM, China).The crystal structure of the sandwich-structured films was analyzed by XRD (Panalytical Empyrean), which adopted copper target as the radiation source, and the operating voltage and current are 40 kV and 40 mA.The elemental characterization of the inorganic coating layer was observed by XPS (PHI 5700 ESCA elemental characteristics).X-ray photoelectron spectrometer (Ultra DLD ; KRATOS) was used for XPS measurement.The film powder is pasted onto the conductive adhesive.The vacuum of the analysis chamber was 9.8 × 10 −10 Torr, and the excitation source was Al ka ray (hν = 1486.6eV).The full-spectrum acquisition voltage was 15 kV, the filament current was 5 mA, and the test energy pass energy was 160 eV; the fine spectrum voltage was 15 kV, the filament current was 10 mA, and the test energy pass energy was 40 eV.Ultraviolet photoelectron spectroscopy (UPS) was tested by an X-ray photoelectron spectrometer (ESCALAB 250Xi; ThermoFisher).The film was directly adhered to the conductive adhesive and then adhered to the sample table.The vacuum of the analysis chamber was about 2 × 10 −8 mbar, the excitation source was He I ultraviolet light, the excitation source energy was 21.22 eV, the working voltage was 12.5 kV, the filament current was 16 mA, and the signal accumulation was carried out for 5-10 cycles.Electrical performance measurements: It is necessary to evaporate metal electrodes on both sides of sandwich-structured films to facilitate electrical performance measurements.The dielectric properties with wide frequencies from 10 2 to 10 6 Hz were studied using a broadband dielectric spectrometer (Novocontrol GmbH).The electric displacement-electric field (D-E) loops were measured using a modified Sawyer Tower circuit (Poly K) with a triangular monopole wave at a frequency of 10 Hz.The leakage current-electric field (I-V) relationship curve of composites was tested by the current test module of ferroelectric comprehensive measurement system (Radiant Technologies).Starting from 10 MV m −1 , each step increment is 10 MV m −1 , adding up to a maximum field strength of 200 MV m −1 ; and sampling multiple times at each field strength until the current reaches a steady state.Furthermore, the breakdown strength of thin films was tested by DC breakdown test system (Poly K) with a ramping rate of 300 V s −1 , and the cumulative breakdown probability was calculated using a Weibull distribution.The cyclic fast charge-discharge tests were performed using a modified Sawyer-Tower circuit.The fast discharge tests were performed through a capacitor charge-discharge test system with a load resistor of 10 kΩ.

Figure 1 .
Figure 1.Microstructures of the BOPP films coated with different inorganic layers.a) Cross-sectional SEM images.b) XPS spectra.c) Energy band diagrams at the interfaces.

Figure 2 .
Figure 2. Dielectric properties of the BOPP films coated with different inorganic layers.a) Frequencydependent and b) temperature-dependent dielectric property.c) Two-parameter Weibull distribution.d) Comparison of electric breakdown strength.e) Leakage current density.f) Schottky fitting plot corresponding to (e).

Figure 3 .
Figure 3. Energy storage performance of the BOPP films with different coating layers.Charge-discharge efficiency and discharged energy density of sandwich-structured films at a) 100 °C and b) 125 °C.c) Comparison of the maximum discharged energy density with an efficiency above 90% for the sandwichstructured films.d) Cyclic energy storage at 125 °C.

Figure 4 .
Figure 4. Schematic diagram of the suppression mechanism for leakage current and conduction loss.a) Difference in the charge injection of the sandwich-structured films and pristine BOPP films.b) Energy-level diagram shows the lowered potential barrier owing to the image force and the applied electric field.c) Charge accumulation process in the sandwich-structured films.d) Illustration of the sandwich structure and the equivalent circuit.

Figure 5 .
Figure 5. Cross-sectional and superficial structures of the A-B-A sandwich-structured films with different thicknesses of AlN coating layer.a-e) Cross-sectional SEM images.f) Thickness of the AlN coating layer versus sputtering time.g-k) AFM images.l) RMS variation at a different sputtering time.

Figure 6 .
Figure 6.Dielectric properties of the A-B-A sandwich-structured films with different thicknesses of AlN coating layer.a) Temperature-dependent dielectric properties of A-B-A-x sandwich-structured films measured at 1 kHz.b) Characteristic electric breakdown strength at different temperatures and c) Weibull breakdown strength distribution.d) leakage current density measured at 125 °C.

Figure 7 .
Figure 7. Energy storage performance of the A-B-A-x sandwich-structured films with different thicknesses of AlN coating layer.a) Charge-discharge efficiency and discharged energy density at 125 °C.b) Comparison of the U emax when η > 90% of A-B-A-1 sandwich-structured films at 125 °C.c) Cyclic charge/ discharge performance at 200 MV m −1 and 125 °C.d) Discharged energy density as a function of time measured from the direct discharge to a 10 kΩ resistor load.