Preparation And Performance Research Of High Energy Storage Polypropylene Capacitor Films

With the increasing energy demand and fossil fuel depletion worldwide,renewable resources,such as wind,tidal,and solar energy,have received increasing attention.Converting these resources to electricity is a common strategy to maximize their use,and efficient and reliable electrical energy storage devices are essential in this process.Compared with batteries and electrochemical capacitors,dielectric capacitors exhibit higher working voltage and power density,longer life-time,and greater cycling stability.The polymer-based film capacitor is a typical electrostatic capacitor that boasts distinct preponderance in the operation of energy storage devices owing to the superior electrical breakdown strength(Eb)provided by the polymer film.The polypropylene(PP)film is the most commonly used polymer-based film,accounting for 70%of the total product market share.It has a relatively high charge-discharge efficiency(?)owing to its intrinsically low dielectric loss.However,its inherently low dielectric permittivity(?)(2.2 at 1k Hz)severely hinders the increase in energy storage density in use(1–2 J/cm~3).Therefore,it needs to study preparation and performance of high energy storage dielectric capacitor films.In this paper,organic conducting polymers such as polyaniline(PANI)and inorganic ceramic particles such as spherical boron nitride(s-BN)and boron nitride nanosheets(BNNSs)were used as fillers,and polypropylene(PP)was used as the matrix with maleic anhydride grafted polypropylene(PP-g-MAH)as the intermediate phase.The composite films PANI/PP-g-MAH/PP,s-BN/PP,BNNSs/PP and BNNSs/PP-g-MAH/PP with different filler contents were prepared respectively by melt blending and hot pressing cross-linking,and then their dielectric and storage properties were measured and analysed.The main contents are as follows:Two different morphologies of polyaniline particles were prepared by the solution polymerization and freezing method,one is a rectangular hollow tubular structure of polyaniline particles and the other is a laminated irregular cluster of polyaniline.The conductivity of the polyaniline particles was tested by the four-probe method,and the conductivity reached 1.18 S/cm and 1.52 S/cm,respectively.the conductivity mechanism is mainly due to the protonation of the internal groups of polyaniline in HCl solution,followed by the formation of cavities in the molecular chains to form a stable emerald green imine atomic group.The positive charge in this atomic group is dispersed to the neighboring atoms by conjugation,and then the resonance of conjugated?-electrons under the action of an applied electric field allows the holes to move throughout the chain segment and finally exhibit electrical conductivity.PANI/PP two-phase composite films with a thickness of 100?m and PANI/PP-g-MAH/PP three-phase composite films were prepared with conductive polyaniline as filler,respectively.The distribution of conductive PANI in PANI/PP two-phase composite films was poor and the agglomeration was serious,and when the content of conductive PANI was large,the composite films were filled with internal pores and gaps,while in PANI/PP-g-MAH/PP three-phase composite films,the distribution of conductive PANI was improved considerably.Under the effect of percolation theory,the dielectric permittivity of the composite films all increase with the increase of the content of conducting PANI.The dielectric loss of the PANI/PP two-phase composite films also increase with the increase of the content of conducting PANI.While under the effect of interfacial polarization and conductivity loss,the dielectric loss of the PANI/PP-g-MAH/PP three-phase composite films increases with the increase of the content of conducting PANI content first increases and then decreases.The breakdown strength of the conductive filler composites was reduced substantially due to the electrical imbalance between the conductive filler and the matrix.With 0D s-BN and 2D BNNSs as fillers,s-BN/PP,BNNSs/PP and BNNSs/PP-g-MAH/PP composite films were prepared.The breakdown field strength of the s-BN/PP composite film decreases sharply due to the destruction and serious agglomeration of s-BN in PP matrix.In contrast,BNNSs were uniformly distributed in the PP matrix,and the breakdown field strength of BNNSs/PP composite films increased by 15%compared to that of PP.In BNNSs/PP-g-MAH/PP three-phase composite films,the flexible polymer chains of PP-g-MAH themselves affect the polarization of the interface,resulting in a more uniform distribution of BNNSs in the PP matrix and a lower density of defects inside the film.Therefore,the PP/PP-g-MAH/BNNSs three-phase system significantly improves the breakdown and energy storage performance of polymer nanocomposites.