Improving Dielectric and Piezoelectric Property of Flexible Composite for Energy Applications Using CNC and Related Fundamentals

Abstract

With the rapid advancement of modern electronic devices, there is a growing demand for miniaturized, high-energy-output, and flexible energy storage and generation/conversion systems. Among energy storage technologies, dielectric capacitors are widely adopted due to their ultrafast charge–discharge capability compared to conventional electrochemical batteries. However, their application is often limited by its low energy density, restricting their use to "temporary" energy storage functions. The key to achieving higher energy storage performance lies in simultaneously enhancing the dielectric breakdown strength and the dielectric permittivity.

In the first part of this study, a new methodology is proposed to eliminate the intrinsic thickness dependence of dielectric breakdown strength (DBS) and recoverable energy storage density in ceramic dielectric materials. This method references all DBS and recoverable energy storage density measurements to a standardized value, thereby providing a unified benchmark for comparing the DBS and energy storage performance of ceramic films across different materials and fabrication methods. It can also be extended to polymer dielectrics, enabling a more accurate evaluation of the true energy storage potential of thicker polymer films.

For improving the DBS of polymer-based material, fluorinated trichlorosilane was identified as a highly effective organic additive for polymer matrices, including P(VDF-HFP), P(VDF-CTFE), and PMMA. Incorporating this fluorinated silane compound resulted in up to a 40% increase in dielectric permittivity and an 80% enhancement in DBS compared to unmodified polymers. This demonstrates its dual functionality in improving both polarization response and breakdown strength, independently of inorganic filler surface interactions.

Furthermore, due to environmental concerns related to the potential chemical pollution caused by small molecule fluorinated silane compounds, biofriendly and renewable cellulose nanocrystals (CNC) were explored as fillers for PVDF copolymers. During the development of CNC–P(VDF-HFP) composites, a novel low-water-content fabrication process was established. This process enabled the incorporation of CNC into P(VDF-HFP) while maintaining low moisture levels, resulting in approximately 40% improvements in both DBS and maximum chargeable energy density.

As for the energy conversion, piezoelectric nanogenerators (PENG) have garnered considerable attention for energy‐harvesting applications because their compact size and power output are sufficient to operate modern electronic sensors. Cellulose nanocrystals (CNC) are promising candidates for PENG fabrication due to their high intrinsic piezoelectric coefficient (e.g., d_25~210 pC/N) and inherent flexibility. To fully exploit this large coefficient, CNC–polymer composites with well-aligned CNC were produced by shear casting using the low-water-content methods described earlier. Computational analyses confirm that aligned CNC make a substantial contribution to the composite’s overall piezoelectric response. Experimental measurements revealed that incorporating CNC into P(VDF-TrFE) results in an approximately eightfold increase in piezoelectric output.

Taken together, these four complementary strategies—thickness normalization, fluorinated silane additive incorporation, CNC-based bio-fillers, and alignment-enhanced CNC composites—offer a robust and integrative framework for both the accurate evaluation and targeted enhancement of dielectric and piezoelectric materials. These advancements pave the way for the development of next-generation high-energy-density capacitors and advanced piezoelectric nanogenerators.