Fig. 1

Pressed and folded poly(vinylidene fluoride) compared with stretched samples. a Schematic demonstration of P&F technique. b Cross-sectional SEM images of P&F samples folded at 165 °C and 300 kN after different numbers of folding cycles. A fine and discrete layered structure is generated during P&F. c The evolution of crystalline phases revealed by the FTIR absorbance spectrum. The initial hot-pressed (HP) films mainly crystallized into α-phase with characteristic peaks at 764, 975 and 1212 cm⁻¹ highlighted by asterisks and transformed to about 98% β-phase (strong peak at 840 cm⁻¹) after seven folding cycles. The horizontal green shaded area indicates the reported values of fraction of β-phase in commonly stretched PVDF films, the blue dashed line represents the fraction of β-phase for the stretched film in this work and the continuous line is the best fit of the experimental P&F data with Eq. 2 using c as free parameter (f_β0 and h were set at 8% and 0.6, respectively). d A comparison of electric energy storage properties of P&F and stretched films, which includes unipolar ferroelectric hysteresis loops, schematic calculations of stored energy of ferroelectric materials and the recoverable energy density U_rec and energy efficiency η of P&F and stretched films