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Electrostatic capacitors store energy via dielectric polarization. This energy storage mechanism endows them inherently high power density. They have attracted a lot of interest as the demand for miniaturized and integrated power electronics increases. Metal oxides are traditional dielectrics that have high polarizations but suffer from low breakdown strengths and limited flexibility. Additionally, most of the dielectric polymers today are made from non-biodegradable, non-renewable synthetic compounds or petroleum-derived components. As a result, their manufacturing process is unsustainable and the final disposal of discarded electronics is not environmentally friendly, raising environmental pollution issues. The development of innovative high energy density, compostable or biodegradable dielectrics has long been a challenge.

The researchers from CRPP (CNRS/University of Bordeaux), with their American partner (University of Washington) have developed a flexible and transparent dielectric film composed of TEMPO oxidized cellulose nanofibrils with tailored charge densities and colloidal nanoparticles of poly(fluoride of vinylidene). Compared to other biopolymers and ceramic dielectrics, cellulose-based nanocomposites sandwiched between two thin layers of polyvinyl alcohol achieve a high energy density of 7.22 J∙cm^-3 at an operating electric field of 388 MV∙m^-1. Additionally, the energy stored in the laminated composite is released at a rate of 1.60 microseconds, resulting in a stable power density of approximately 3 MW∙cm^-3 under an applied field of 300 MV∙m^-1 over 1000 charge/discharge cycles, more than ten times that of benchmark bi-oriented polypropylene dielectric films.