Water-Processable Cellulosic Nanocomposites as Green Dielectric Films for High-Energy Storage

With the depletion of fossil resources and the ever-increasing energy demand, it becomes crucial to address the global challenge of sustainable routes to renewable dielectric materials, which can store energy electrostatically for flexible electronics and pulsed power applications. Here, TEMPO-oxidized cellulose nanofibrils with tailored charge density are synthesized and mixed with colloidal poly(vinylidene fluoride) nanoparticles using nontoxic water as solvent to produce flexible and transparent dielectric films. The as-prepared nanomaterials and resulting composite films were extensively characterized. Compared to other biopolymer and ceramic dielectrics, the cellulose-based nanocomposites sandwiched between two thin polyvinyl alcohol layers achieve a high energy density of 7.22 J·cm−3 at breakdown strength of 388 MV·m−1. Furthermore, the stored energy in the laminated composite is released at a rate of 1.60 microseconds, yielding a stable power density of ∼3 MW·cm−3 under an applied field of 300 MW·m−1 over 1000 charge/discharge cycles, which is more than ten times greater than that of biaxially-oriented polypropylene. Significantly, these findings pave the way toward environmentally-benign processing of naturally-derived materials for applications in flexible and transparent energy storage devices.