Enhanced dielectric performance and energy storage of PVDF‐HFP‐based composites induced by surface charged Al 2 O 3

Atomic-Level Matching Metal-Ion Organic Hybrid Interface to Enhance Energy Storage of Polymer-Based Composite Dielectrics

In this work, a distinctive "metal-ion organic hybrid interface" (MOHI) between polyimide (PI) and calcium niobate (CNO) nanosheets is designed. The metal ions in the MOHI can achieve atomic-level matching not only with the inorganic CNO, but also with the PI chains, forming uniform and strong chemical bonds. These results are demonstrated by experiment and theory calculations. Significantly, the MOHI reduces the free volume and introduces deep traps across the filler-matrix interfacial area, thus suppressing the electric field distortion in PI-based composite dielectrics. Consequently, PI-based dielectric containing the MOHI exhibits excellent energy storage performance. The energy storage densities (Ue) of the composite dielectric reach 9.42 J cm-3 and 4.75 J cm-3 with energy storage efficiency (η) of 90% at 25 °C and 150 °C respectively, which are 2.6 and 11.6 times higher than those of pure PI. This study provides new ideas for polymer-based composite dielectrics in high energy storage.

Thermally activated dynamic bonding network for enhancing high-temperature energy storage performance of PEI-based dielectrics

To address the paradox of mutually exclusive confusions between the breakdown strength and polarization of the polymer-based composites at high-temperature, a dynamic multisite bonding network is constructed by connecting the -NH2 groups of polyetherimide (PEI) and Zn2+ in metal-organic frameworks (MOFs). Owing to the multisite bonding network being dynamically stable at high-temperature, the composites possess a high breakdown strength of 588.1 MV m-1 at 150 °C, which is 85.2% higher than that of PEI. Importantly, the multisite bonding network could be thermally activated at high-temperature to generate extra polarization, which is because the Zn-N coordination bonds are evenly stretched. At similar electric fields, the composites show higher energy storage density at high-temperature compared with that at room temperature, and present excellent cycling stability even with increased electrode size. Finally, the reversible stretching of the multisite bonding network against temperature variation is confirmed by the in situ X-ray absorption fine structure (XAFS) and theoretical calculations. This work presents a pioneering example of the construction of self-adaptive polymer dielectrics in extreme environments, which might be a potential method for designing recyclable polymer-based capacitive dielectrics.

Expedient secondary functions of flexible piezoelectrics for biomedical energy harvesting

Flexible piezoelectrics realise the conversion between mechanical movements and electrical power by conformally attaching onto curvilinear surfaces, which are promising for energy harvesting of biomedical devices due to their sustainable body movements and/or deformations. Developing secondary functions of flexible piezoelectric energy harvesters is becoming increasingly significant in recent years via aiming at issues that cannot be addressed or mitigated by merely increasing piezoelectric efficiencies. These issues include loose interfacial contact and pucker generation by stretching, power shortage or instability induced by inadequate mechanical energy, and premature function degeneration or failure caused by fatigue fracture after cyclic deformations. Herein, the expedient secondary functions of flexible piezoelectrics to mitigate above issues are reviewed, including stretchability, hybrid energy harvesting, and self-healing. Efforts have been devoted to understanding the state-of-the-art strategies and their mechanisms of achieving secondary functions based on piezoelectric fundamentals. The link between structural characteristic and function performance is unravelled by providing insights into carefully selected progresses. The remaining challenges of developing secondary functions are proposed in the end with corresponding outlooks. The current work hopes to help and inspire future research in this promising field focusing on developing the secondary functions of flexible piezoelectric energy harvesters.

From Triboelectric Nanogenerator to Multifunctional Triboelectric Sensors: A Chemical Perspective toward the Interface Optimization and Device Integration

Triboelectric nanogenerators (TENGs) have intrigued scientists for their potential to alleviate the energy shortage crisis and facilitate self-powered sensors. Triboelectric interfaces containing triboelectric functionalized molecular groups and tunable surface charge densities are important for improving the electrical output capability of TENGs and the versatility of future electronics. In this review, following an introduction to the fundamental progress of TENG systems for mechanic energy harvesting, surface modifications that aim to increase the surface charge density and functionality are highlighted, with an emphasis on interfacial chemical modification and triboelectric energetics/dynamics optimization for efficient electrostatic induction and charge transfer. Recent advances in assemblies of multifunctional triboelectric sensing are briefly introduced, and future challenges and chemical perspectives in the field of TENG-based electronics are concisely reviewed. This review presents and advances the understanding of the state-of-the-art chemical strategies toward rational triboelectric interface engineering and system assembly and is expected to guide the rational design of highly efficient and versatile triboelectric sensing.

Free volume dependence of the dielectric constant of poly(vinylidene fluoride) nanocomposite films

The free volume effects on the dielectric properties of the polymer are ambiguous, and the quantitative effect of free volume on the dielectric properties has rarely been systematically studied, especially in the high-elastic state dipolar (HESD) polymer. In this work, the free volume of dipolar poly(vinylidene fluoride) (PVDF) is regulated by the addition of Al2O3, which greatly increase the size of free volume holes. Then the effect of free volume on the dielectric properties of PVDF/Al2O3 composites is discussed. The greatly enlarged size of free volume holes is believed to potentially generate disparate effects on dielectric constant under different frequencies in such kinds of HESD polymer-based composites, bringing about more remarkable frequency dependence of the dielectric constant. The influence of atomic-scale microstructure based on free volume further clarifies the free volume effects on the dielectric properties and provides valuable insights for the research of dielectric behaviour of polymer composites, which is constructive to design novel dielectric materials and further optimize the dielectric properties of dipolar dielectric polymer composites.

Obtaining Greatly Improved Dielectric Constant in BaTiO3-Epoxy Composites with Low Ceramic Volume Fraction by Enhancing the Connectivity of Ceramic Phase

Ceramic-polymer dielectric composites show promising potential as embedded capacitors, whereas it is a great challenge to obtain a high dielectric constant (ε_r) at a low ceramic volume fraction (V_c). This work demonstrates a strategy for overcoming this challange. By employing a high sintering temperature (T_s) and introducing porogen, BaTiO₃ ceramics with both great connectivity and high porosity are obtained, and the composites with improved ε_r at a low V_c are prepared after curing the epoxy monomer, which is infiltrated into the porous ceramic bodies. For the composite with a T_s of 1300 °C and a V_c of 38.1%, the ε_r is as high as 466.8 at 1 kHz, which is improved by about nine times compared to the 0-3 counterpart with a higher V_c of 60.8%. Furthermore, the composite exhibits low dielectric loss and good frequency and temperature stability of ε_r, indicating the great potential for practical applications. Finite element simulation shows that the enhanced connectivity of BaTiO₃ increases the electric field intensity in high-ε_r BaTiO₃ dramatically and therefore plays a key role in the dielectric response of the composite. This work not only sheds light on the high-ε_r ceramic-polymer composites but also deepens the understanding on the relationship between their properties and microstructures.