3.1 Characterization
Hydrothermal reaction is a common method to prepare BTO NWs [28–31]. The morphology of BTO NWs, e.g., coral-like nanostructures of assembled nanorods, starfish-like nanostructures, and sword-like nanostructures, is highly dependent on the reaction conditions of the second step such as the concentration of Ba(OH)2, the temperature, and the nature of the precursors [31]. As indicated by the SEM images (Fig. 2a), by employing good precursors and shortened reaction time, the high-aspect ratio BTO NWs with length of several micrometers and diameter of several hundreds of nanometers were successfully prepared. The peaks in the XRD pattern of BTO NWs are consistent well with the standard pattern of BTO perovskite crystal structure (Fig. 2b), indicating that few byproducts were obtained. The HRTEM image shows that the single-crystalline BTO NWs was successfully coated with 5 nm thick amorphous SiO2 layers (Fig. 2c). The successful coating was further verified by the FTIR spectra of BTO NWs and BTO@SiO2 NWs. The absorption peak at around 550 cm-1 is attributed to the vibration of Ti-O and the absorption peaks at 1077 and 1229 cm-1 are corresponding to Si-O-Si and Ti-O-Si, respectively (Fig. 2d). It is obvious that the coating layer mainly consists of SiO2.
Figure 3a, 3b show the cross-sectional SEM images of z-aligned and random-aligned nanocomposites. Owing to the gravity, NWs tended to be parallel to the surface of the film in random-aligned nanocomposites. However, NWs moved parallel to rather than perpendicular to the axis in the inner surface of glass tube due to the lower resistance force, and thus they tended to align perpendicular to the surface in the resultant nanocomposite [27]. As for coating layers, Al2O3, which is supposed to be the best material to suppress the charge injection according to our previous works [32], was chosen as the material. The optimal thickness of 150 nm was achieved by controlling the deposition time. The SEM image (Fig. 3c) reveals that Al2O3 layers are dense and no visible defects are observed either within the film or at the interface between Al2O3 deposition layer and nanocomposite. Peaks corresponding to Al2p, Al2s, O1s, O2s orbits are all observed in XPS spectra (Fig. 3d), and the atomic ratio of Al:O derived from XPS results is 1:1.63, approximately consistent with the stoichiometric ratio of Al2O3, showing the Al2O3 deposition layer is in good quality.
3.2 Dielectric Performance
Here four different types of samples, i.e., polymer nanocomposites containing random-aligned BTO NWs, random-aligned BTO@SiO2 NWs, z-aligned BTO@SiO2 NWs, and surface-coated polymer nanocomposites with z-aligned BTO@SiO2 NWs were prepared for comparison, labelled as PEI-BTO NWs, PEI-BTO@SiO2 NWs, PEI-z-aligned BTO@SiO2 NWs, PEI-Al2O3-z-aligned BTO@SiO2 NWs, respectively. Dielectric performance of dielectric materials is characterized by dielectric permittivity and dissipation factor. For linear dielectrics, the delivered energy density is expressed as
where ε0 is the vacuum dielectric permittivity, εr is the relative dielectric permittivity and E is the applied electric field. Obviously, high dielectric permittivity and high breakdown strength are prerequisite for dielectric materials gaining a high energy density. Dissipation factor reflects the energy loss in dielectrics during charge-discharge cycles, which leads to the inner temperature rising and compromised dischargeable energy density. Dielectric performance of neat PEI and polymer nanocomposites was measured at 150°C and a wide range of frequency (Fig. S1).
Figure 4a shows the dielectric constant and dissipation factor of polymer nanocomposites with different volume fraction of NWs at 150°C and 1 kHz. It is seen that nanofillers at low doping concentration effectively promotes the dielectric constant and the dielectric constant of all samples increases with the increasing volume fraction. For example, only 1 vol% random-aligned BTO NWs promotes the dielectric constant of PEI from 3.2 to 4.63, around a 44.7% enhancement. Due to the compromised interfacial polarization induced by the improved interfacial compatibility between polymer matrix and NWs as well as the low dielectric constant of SiO2, PEI-1 vol% BTO@SiO2 NWs exhibits a dielectric constant of 4.43, slightly lower than that of PEI-1 vol% BTO NWs. Remarkably, the z-alignment of NWs further substantially elevates the dielectric constant of nanocomposites, e.g., the dielectric constant of PEI-1 vol% z-aligned BTO@SiO2 NWs reaches as high as 5.51, 18.9% and 72.2% higher than that of PEI-1 vol% BTO@SiO2 NWs and neat PEI, respectively. Since the Al2O3 coating layers are too thin to influence the dielectric response, PEI-Al2O3-z-aligned BTO@SiO2 NWs shows a dielectric constant approximately equal to that of PEI-z-aligned BTO@SiO2 NWs. Frequency-dependent dielectric spectra show the dielectric constant retains approximately unchanged over a wide range of frequency (Fig. 4b).
As for as dissipation factor, all the samples exhibit slightly increased dissipation factor as the volume fraction of BTO NWs increases (Fig. 4a). Moreover, z-alignment of NWs leads to slightly increased dissipation factor, whereas the SiO2 coating can reduce it. For example, the dissipation factor of neat PEI, PEI-1 vol.% BTO NWs, PEI-1 vol.% BTO@SiO2 NWs and PEI-1 vol.% z-aligned BTO@SiO2 NWs at 1kHz is 0.00313, 0.00502, 0.00436, and 0.00539, respectively. Al2O3 deposition layers cannot also reduce the dissipation factor since energy loss induced by polarization-depolarization rather than charge conduction is the main source of dielectric loss at relative high frequency. Note that either the increased dissipation factor induced by the introduction of NWs or the reduced one resulting from the SiO2 coating and Al2O3 deposition is more significant at relatively low frequency, revealing the design of nanocomposite in this work mainly influences the conduction loss (Fig. 4b).
Overall, the nanocomposite with optimal structure exhibits a significantly promoted dielectric constant at the cost of slightly increased dielectric loss.
3.3 Insulating Performance
Insulating performance of the dielectrics includes leakage current (or electrical resistance) and breakdown strength. Breakdown strength determines the maximum energy density dielectric materials can deliver, while leakage current is the dominant part of dielectric loss under high temperatures.
Figure 5a shows the leakage current of neat PEI, PEI-1 vol% BTO NWs, PEI-1 vol% BTO@SiO2 NWs and PEI-1 vol% z-aligned BTO@SiO2 NWs. It is seen that PEI-1 vol% BTO NWs exhibits sharply increased leakage current compared to neat PEI, which may result from 3 reasons. First, the high surface energy leads to the agglomeration of NWs, providing long transmission path for charge carriers. Second, the large contrast of dielectric constant between NWs and polymer matrix induces electric field distortion, generating high field regions in the composite and enhancing the movement of charge carriers. Third, unavoidable defects are introduced at the interfaces between NWs and PEI matrix due to the poor compatibility, deteriorating the insulating properties. The leakage current can be effectively reduced by coating NWs with SiO2 since it can reduce the surface energy of NWs, and thus relieve the agglomeration. Moreover, the moderate dielectric constant of SiO2 eases the distortion of electric fields. The leakage current can be further reduced by the Al2O3 deposition layers, which enhances the energy barrier height at the electrode/dielectric interfaces, preventing the charge carriers from injecting into the dielectric films. Note that PEI-1 vol% z-aligned BTO@SiO2 NWs shows higher leakage current in relative to PEI-1 vol% BTO@SiO2 NWs because NWs arranged parallel to the surface serve as topologic barrier, prolonging the propagation path and restricting the movement of charge carriers, whereas there is no barrier effect for NWs arranged perpendicular to the surface. The leakage current increases with elevated volume fraction of NWs because either the agglomeration of NWs or the field disorder is more significant at higher content of nanofillers (Fig. 5b). Remarkably, PEI-1 vol% z-aligned BTO@SiO2 NWs shows an electrical resistance around equal to that of neat PEI, and PEI-0.5 vol% z-aligned BTO@SiO2 NWs even shows a higher one (Fig. S2).
The measured breakdown strength is analyzed using 2-parameter Weibull static. For a given electric field E, the probability dielectric failure occurs (P(E)) is expressed by
where Eb is the Weibull breakdown strength, at which there is a 63.2% probability for the dielectric material to breakdown. β is the shape parameter, reflecting the scatter of measured data. As thermal runaway caused by temperature rising in dielectric materials is believed the dominant mechanism accounting for the dielectric failure at high temperatures and conduction loss is the major source of heat generation, the change trend in Weibull breakdown strength of the samples investigated in this work is consistent well with that in leakage current (Fig. 5c). Namely, the introduction of BTO NWs and higher doping amount lead to decrease breakdown strength, while SiO2 coating and Al2O3 deposition result in improved dielectric strength. z-alignment of NWs also slightly degrades the breakdown strength of nanocomposites. Notably, despite PEI-Al2O3-0.5 vol% z-aligned BTO@SiO2 NWs exhibits lower leakage current compared to neat PEI, its breakdown strength is lower too, indicating that electrical conduction is not the only factor influencing the breakdown strength of polymer nanocomposites and the local high electric field stemming from the field disorder may contribute to early breakdown (Fig. 5d). The breakdown strength of PEI-1 vol% z-aligned BTO@SiO2 NWs maintains as high as 502 MV/m at 150°C, slightly (5.6%) lower than that of neat PEI.
3.4 Energy Storage Performance
Energy storage performance of dielectric materials is evaluated with charge-discharge efficiency and discharged energy density. High dischargeable energy density means a certain energy density is achievable for a capacitor with relatively small size and weight, which is of great significance to the miniaturization and compactness of advanced electronic devices [12]. Charge-discharge efficiency reflects the energy loss in charge-discharge cycles, directly determining the discharged energy density and being closely related to thermal runaway of capacitors during continuous operation. At high temperatures, discharged energy density with a charge-discharge efficiency above 90% is believed worthy of attention.
Figure 6a,6b and Fig. S3 show the energy storage performance of PEI-BTO NWs, PEI-BTO@SiO2 NWs, PEI-z-aligned BTO@SiO2 NWs and PEI-Al2O3-z-aligned BTO@SiO2 NWs containing various volume fraction of NWs. Considering that conduction loss is dominant in energy loss at elevated temperatures, nanocomposites possessing lower leakage current exhibit higher charge-discharge efficiency. At 300 MV m-1, for example, the charge-discharge efficiency of neat PEI, PEI-1vol% BTO NWs, PEI-1 vol% BTO@SiO2 NWs, PEI-1 vol% z-aligned BTO@SiO2 NWs and PEI-Al2O3-1 vol% z-aligned BTO@SiO2 NWs is 92.3%, 73.6%, 86.2%, 82.4%, and 90.2% respectively (Fig. 6c). Moreover, the sample with higher dielectric constant tends to deliver larger discharged energy density unless the charge-discharge efficiency is too low. For optimal-structured nanocomposites with low volume fraction of NWs, high discharged energy density is achieved without a decline in charge-discharge efficiency (Fig. 6b). Notably, PEI-Al2O3-1 vol% z-aligned BTO@SiO2 NWs has similar resistance, breakdown strength and charge-discharge efficiency to neat PEI, while it exhibits more than twice discharged energy density at different electric fields attributed to higher dielectric constant.
It is seen from Fig. 6d that PEI-BTO NWs, PEI-BTO@SiO2 NWs and PEI-z-aligned BTO@SiO2 NWs show lower maximum discharged energy density above 90% than PEI due to the high energy loss, despite their high dielectric constant. In corporation of NWs would concurrently promote the dielectric constant and reduce the charge-discharge efficiency, and thus an optimal doping content is required to obtain high discharged energy density above 90%. PEI-Al2O3-1 vol% z-aligned BTO@SiO2 NWs exhibits the highest high discharged energy density above 90% of 2.73 J cm-3 at relatively low electric field of 325 MV m-1, 42% higher than neat PEI.
At 200 MV m-1, the operating electric field of BOPP film capacitors in electric vehicles, although the nanocomposites show charge-discharge efficiency approximately equal to neat PEI, they deliver much higher energy density due to the high dielectric constant, e.g., the discharged energy density of PEI-Al2O3-1 vol% z-aligned BTO@SiO2 NWs and PEI-Al2O3-2 vol% z-aligned BTO@SiO2 NWs is as high as 1.13 J cm-3 and 1.29 J cm-3, 113% and 143% higher than that of neat PEI, respectively (Fig. 7a). The energy storage performance of optimal-structured nanocomposites at 150°C retains stable over 50000 charge-discharge cycles (Fig. 7b).