Translated Abstract
Limited fossil fuel supplies and negative environmental of non-renewable energy resources have spurred the development of renewable energies as well as advanced energy storage technologies. Among the currently available energy storage technologies, dielectric energy storage has advantages of high power density and fast charge-discharge capability, which has a promising application prospect in the area of intermittent energy storage, manufacturing of power devices, etc. Polymer dielectric materials are flexible and can sustain high electric fields, making them one of the preferred candidates for dielectric energy storage devices. However, limited by their low dielectric constants, the energy density of polymer dielectrics is at a relatively low level, which is at least an order of magnitude lower than that of electrochemical counterparts such as batteries and supercapacitors. Ceramic/polymer nanocomposites recently have become a strenuous topic of research for improving the dielectric constant of polymer by introducing high-k ceramic nanofillers. Nevertheless, high volume fraction of ceramic increases the electrical conductivity of nanocomposites, thus leading to an improvement in dielectric constant along with the cost of the sharp reduction in breakdown strength.
To address this issue, the macro-structural design is introduced in this dissertation to design and manufacture a 2-2 type layered nanocomposite. By tailoring the filler contents in each layer, the electric field will be redistributed and weak electric field regions will be formed in the layered nanocomposite. As a result, the growth of the electrical trees could be dramatically hindered, resulting in an improved breakdown strength of the nanocomposite. Besides, the dielectric energy storage properties of the nanocomposites, including energy density, charge-discharge efficiency, etc., can be further improved by optimizing the filler content and the structural design. This method can promote the development and industrial application of high-energy-density dielectric materials and devices.
Firstly, based on the layered structure design, an A-B-A type sandwich-structured BaTiO3/PVDF nanocomposite is manufactured by solution casting method. In this structure, B layer with high breakdown strength is in the middle of two A layers with high dielectric constant. By adjusting the ceramic filler contents in A layer, the breakdown strength of the nanocomposite is well improved by 1.5 times higher than that of pristine polymer matrix, and its energy density is enhanced to 18.8 J·cm-3. Due to the difference of dielectric constants between A layer and B layer, large amount of applied electric voltage concentrates on B layer, and local regions of weak electric field are formed in A layer, which is believed to be the main reason for achieving much higher breakdown strength in sandwich-structured nanocomposite.
The charge-discharge efficiency of sandwich-structured nanocomposites can be improved by surface modification of ceramic fillers and adjustment of filler content. Dopamine is adopted as the surface modifier to alleviate the fillers aggregation and impede the formation of conductive paths. The redistribution of electric field in layered structure has a significant influence on the leakage current distribution. The low conductivity in the weak electric field regions can decrease the conductivity and the energy loss of nanocomposites, and a high energy density over 15 J·cm-3 accompanied by a high charge-discharge efficiency of 70% are achieved. The electrical insulation properties of nanocomposites are related to the electric field distribution, which has a strong relationship with the filler content.
Considering the negative effect of electrode charge injection on breakdown strength and charge-discharge efficiency, a reverse-sandwich-structured B-A-B type nanocomposite is designed and manufactured. In this structure, the insulation layers with high breakdown strength are placed nearby the electrodes to effectively impede the charge injection from A layer with high dielectric constant. Consequently, the breakdown strength is effectively improved and the energy density can be enhanced to 26.4 J·cm-3. Moreover, the charge-discharge efficiency also increases to 72% due to the enhancement of electrical insulation. This is by far the best comprehensive performance among tri-layered polymer nanocomposites.
To decrease the high driving voltage of dielectric nanocomposites, an A-B-C type gradient-layered nanocomposite is designed, where the ceramic fillers content gradually increases from top layer to bottom layer. On the one hand, the electric field is also redistributed in the gradient-layered nanocomposite just like the case in the sandwich-structured materials. On the other hand, this structural design guarantees that a higher filler content can be introduced into the polymer matrix without impairing its breakdown strength. High filler content leads to a high dielectric constant of nanocomposite and increases a high energy density over 15 J·cm-3 under a low driving voltage of 390 MV·cm-1. Due to the structural characteristic, the electric field strength increases from top layer to bottom layer. The gradient of electric field strength between adjacent layers shows an important influence on the growth of electrical trees.
Ceramic nanowires show higher dielectric constants than nanoparticles, and the breakdown strength of composite can be enhanced when the nanowires are introduced as fillers and arrays perpendicularly to the direction of applied electric field. Thus, 1D ceramic nanowires are adopted in the gradient-layered nanocomposite to further enhance its energy storage capability. Particularly, a thin buffer layer of pure polymer is placed near the layer containing high filler content to impede the electrode charge injection. This method has apparently improved the breakdown strength to 510 MV·cm-1, thus leading to high energy density of 17.5 and charge-discharge efficiency over 70%. The gradients of electric field strength between adjacent layers can be well adjusted by tailoring the filler content in the middle layer. It should be noticed that only if the electric field gradients of each interface are at a high value, can the insulating capability of nanocomposites be the best.
Translated Keyword
[Dielectric properties, Energy density, Nanocomposites, Structural design]
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