Preparation And Energy Storage Properties Of Poly(Vinylidene Fluoride)-Based Nanocomposite Dielectrics

Dielectric capacitor is one of the mainstream energy storage devices,and its energy storage properties depend on the dielectric.Compared with ceramic dielectrics,polymer dielectrics have advantages of easy processing,low density and high breakdown strength（E_b）.Therefore,they are widely used as the core materials of electrical equipment components such as inverter,pulse power supply and high-power energy storage system,but their low dielectric constant（ε_r）results in low energy density（U_d）.With the development of electronic devices and electrical equipment towards miniaturization,lightweight and integration,higher requirements are put forward for the U_d of polymer dielectrics.Introducing high-ε_r nanofiller to polymer matrix to prepare polymer-based nanocomposite dielectric is an effective way to enhance itsε_r and hence U_d.However,the effective improvement of U_d usually achieves at a high filler ratio,which affects not only the mechanical and optical properties of nanocomposite dielectrics,but also their processability.Among various polymer dielectrics,poly（vinylidene fluoride）（PVDF）has the advantages of relatively highε_r and U_d and easy regulation of energy storage properties,thus attracting much attention in the field of nanocomposite dielectrics.In this thesis,PVDF was chosen as matrix.By combining the regulation ofε_r difference between nanofillers and matrix and the utilization of structural advantages of anisotropic fillers to enhance the E_b of PVDF-based nanocomposite dielectrics,so as to enhance their U_d at relatively low filler ratios.High-ε_r 0.85BaTiO₃-0.15Bi(Mg_0.5Zr_0.5)O₃ nanofibers（BT-BMZ＿nfs）were prepared via electrospinning.The doping of Bi(Mg_0.5Zr_0.5)O₃ can make Ba TiO₃ ferroelectric transform into relaxor ferroelectric,reducing itsε_r and dielectric loss and enhancing its E_b.As fillers,BT-BMZ＿nfs enhance the E_b of PVDF-based nanocomposite dielectrics,and E_b of the 6 wt%BT-BMZ＿nf/PVDF nanocomposite film reaches 394 MV/m.To improve the insulating property,BT-BMZ＿nfs were coated with Si O₂ via St（?）ber method to obtain BT-BMZ@Si O₂＿nfs.As fillers,they further enhance the E_bof PVDF-based nanocomposite dielectrics.When the filler ratio is 6 wt%,E_b of the BT-BMZ@Si O₂＿nf/PVDF nanocomposite film reaches the maximum value of 405 MV/m,which is 1.23 times that of pristine PVDF dielectric film;its maximum energy density(U_max)reaches 8.4 J/cm³,which is1.61 times that of pristine PVDF dielectric film.High-ε_r BaZr_0.15Ti_0.85O₃@Ba Zr_0.35Ti_0.65O₃ core-shell nanofibers（BZT15@BZT35＿nfs）were prepared via coaxial electrospinning,and then coated with polydopamine（PDA）via dopamine oxidative self-polymerization.The as-obtained BZT15@BZT35@PDA＿nfs were utilized as fillers in PVDF.BZT15 and BZT35 have similar components,and they show higher and lowerε_r than that of Ba TiO₃,respectively.These characteristics can not only inhibit the formation of additional defects in the process of calcination and crystallization,but also make BZT15@BZT35＿nfs maintain a highε_r.Meanwhile,the BZT35 shell alleviates the difference ofε_r between the filler and PVDF,and PDA coating improves the compatibility between the filler and PVDF.The above design realizes the synchronous enhancement of E_bandε_r of BZT15@BZT35@PDA＿nf/PVDF nanocomposite dielectric films,leading to increased U_d.When the filler ratio is 2 wt%,E_b and U_max of the BZT15@BZT35@PDA＿nf/PVDF nanocomposite dielectric film reach 407 MV/m and 8.2J/cm³,respectively.Furthermore,sandwich-structural nanocomposite dielectric films with pristine PVDF as the top and bottom layers and BZT15@BZT35@PDA＿nf/PVDF composite as the middle layer were prepared via a layer-by-layer solution casting method.The field strength redistributes in each layer according to theε_r,further enhancing E_b of the nanocomposite dielectrics because of the barrier effect of interface layers.When the filler ratio of middle layer is 2 wt%,E_b and U_max of the sandwich-structural nanocomposite film reach 450 MV/m and 9.9 J/cm³,respectively.High-ε_rtitanoniobate was introduced to PVDF to prepare nanocomposite dielectrics because of its lamellar structure and negatively charged characteristic.Ti₂Nb O₇ nanosheets（Ti₂Nb O₇＿nss）with a thickness of less than 5 nm were prepared via a wet chemical exfoliation method,and then modified by PDA to obtain Ti₂Nb O₇@PDA＿ns fillers.They show relatively high insulation,negatively charged characteristic,large aspect ratio,in-plane orientation in the nanocomposite film and highε_r,which make them can effectively enhance the E_b andε_r of polymer-based nanocomposite dielectrics.Therefore,their U_d can be effectively improved at ultralow filler ratios.When the filler ratio is 0.2 wt%,E_b and U_max of the Ti₂Nb O₇@PDA＿ns/PVDF nanocomposite film reach 416 MV/m and 8.6 J/cm³,respectively.Based on the above two-phase nanocomposite dielectric,10 vol%linear dielectric polymethylmethacrylate（PMMA）with high E_b was introduced to further enhance the E_b and dishcarge efficiency,so as to enhance U_d.When the filler ratio is 0.2 wt%,E_b and U_max of the Ti₂Nb O₇@PDA＿ns/PMMA/PVDF three-phase nanocomposite dielectric film reach 481 MV/m and 11.5 J/cm³,respectively.