Understanding the breakdown mechanisms of polymer-based dielectrics is critical to achieving high-density energy storage. Here a comprehensive phase-field model is developed to investigate the electric, thermal, and mechanical effects in the breakdown process of polymer-based dielectrics. High-throughput simulations are performed for the P(VDF-HFP)-based nanocomposites filled with nanoparticles of different properties. Machine learning is conducted on the database from the high-throughput simulations to produce an analytical expression for the breakdown strength, which is verified by targeted experimental measurements and can be used to semiquantitatively predict the breakdown strength of the P(VDF-HFP)-based nanocomposites. The present work provides fundamental insights to the breakdown mechanisms of polymer nanocomposite dielectrics and establishes a powerful theoretical framework of materials design for optimizing their breakdown strength and thus maximizing their energy storage by screening suitable nanofillers. It can potentially be extended to optimize the performances of other types of materials such as thermoelectrics and solid electrolytes.

are subject to thermal degradation due to the heat generation from electrical power dissipation, namely, Joule heating. If this does not cause an insulation failure, the temperature in the polymer-based dielectrics will continue to rise until the cooling of the material is equal to the electrical power dissipation and a steady-state heat flow is established. In many circumstances, this causes an insulation failure, either because the intrinsic breakdown strength is lowered due to temperature rise, or because the conductivity and hence the electrical power dissipation in the dielectrics increases causing a further rise in temperature and thermal degradation.The insulation failure due to the temperature rise is called the thermal breakdown, which is accompanied by either a glass transition (T > T g ) or melting of the polymer (T > T m ), or the avalanche multiplication of electronic charge carriers. [28][29][30] Compared with the electric breakdown and the electromechanical breakdown, the thermal breakdown is perhaps the most obvious form of breakdown mechanism for polymer-based dielectrics. Unfortunately, there is a lack of a quantitative understanding of the thermal effects on breakdown. For example, there can be two approaches to preventing the thermal breakdown. First, the electrical conductivity can be decreased to reduce the Joule heating. Second, the thermal conductivity can be increased to improve the efficiency of the heat dissipation from the materials to the surrounding, so as to cool the materials. However, in many cases, materials with high thermal conductivity also tend to have high electrical conductivity, partly because free carriers carry charges as well as heat, and this is also the fundamental paradox needed to be decoupled in thermoelectrics. [31,32] Therefore, if only one of these two properties is to be optimized, which one is more effective for preventing the thermal breakdown?In a previous work, a phase-field model was developed to simulate the dielectric breakdown process in polymer nanocomposites. [33] It incorporated the phase separation energy, gradient energy, and electric energy. However, thermal energy is not included in the existing model. Therefore, the predicted breakdown strengths and energy densities of various polymer nanocomposites correspond to room temperature. In this work, we extend the phase-field model to include the thermal energy contribution from Joule heating (see Experimental Section and the Supporting Information). We then investigated the thermal Polymer-based dielectrics are attracting increasing attention due to their high-density energy storage. However, mitigating the heat generation in real capacitors has been a challenge. Here an electrothermal breakdown phasefield model is developed to fundamentally understand the thermal effects on the dielectric breakdown of polymer-based dielectrics in real capacitor configurations including the increase in the dielectric loss and the decrease in the breakdown strength. While both enhancing the thermal conductivity and re...

Nanoporous holey-graphene (HG) shows potential versatility in several technological fields, especially in biomedical, water filtration, and energy storage applications. Particularly, for ultrahigh electrochemical energy storage applications, HG has shown promise in addressing the issue of low gravimetric and volumetric energy densities by boosting of the ion-transport efficiency in a high-mass-loaded graphene electrode. However, there are no studies showing complete control over the entire pore architecture and density of HG and their effect on high-rate energy storage. Here, we report a unique and cost-effective method for obtaining well-controlled HG, where a copper nanocatalyst assists the predefined porosity tailoring of the HG and leads to an extraordinary high pore density that exceeds 1 × 10 3 μm −2 . The pore architectures of the hierarchical and homogenous pores of HG were realized through a rationally designed nanocatalyst and the annealing procedure in this method. The HG electrode with a high mass loading results in improved supercapacitor performance that is at least 1 order of magnitude higher than conventional graphene flakes (reduced electrochemically exfoliated graphene (rECG)) in areal capacitance (∼100% retention of capacitance until 15 000 cycles), energy density, and power density. The diffusion coefficient of the HG electrode is 1.5-fold higher than that of rECG at a mass loading of 15 mg cm −2 , indicating excellent iontransport efficiency. The excellent ion-transport efficiency of HG is further proved by nearly 4-fold magnitude lowering of its R ion (the ionic resistance in the electrolyte-filled pores) value as compared with rECG when estimated for equivalent high-massloaded electrodes. Furthermore, the HG exhibits a packing density that is 2 orders of magnitude higher than rECG, revealing the utility of the maximum electrode mass and possessing higher volumetric capacitance. The perfect tailoring of HG with optimized porosity allows the achievement of high areal capacitance and excellent cycling stability due to the facile ion-and charge-transport at high-mass-loaded electrodes, which could open a new avenue for addressing the long-existing issue of practical application of graphene-based energy storage devices.