Elsevier

Nano Energy

Volume 44, February 2018, Pages 364-370
Nano Energy

Full paper
Ultrahigh energy density and greatly enhanced discharged efficiency of sandwich-structured polymer nanocomposites with optimized spatial organization

https://doi.org/10.1016/j.nanoen.2017.12.018Get rights and content

Highlights

  • A newly designed sandwich-structuredBaTiO3/P(VDF-HFP) nanocomposite is proposed.

  • An ultrahigh discharged energy density of 26.4 J cm−3 and a superior discharged efficiency of 72% have been obtained.

  • Finite element simulations reveal that the designed structure can enhance the insulating and electrical properties by preventing the charge injection from electrodes.

Abstract

Sandwich-structured polymer nanocomposites that provide a pathway to overcome the paradox between permittivity and breakdown strength ever existing in dielectric materials are receiving increasing attentions for their superior energy storage performance. Despite certain advances obtained in previous effort, further enhancement of the energy density by structure optimizing is still a challenge. Herein, we present a newly designed sandwich-structured barium titanate/poly(vinylidene fluoride-co-hexafluoropropylene) (BaTiO3/P(VDF-HFP)) nanocomposite via layer-by-layer tape casting process, where high contents of BaTiO3 nanoparticles are dispersed in the middle layer to offer high permittivity, while two outer layers containing small amounts of BaTiO3 provide favorable breakdown strength. The solution-processed nanocomposites with an optimal composition exhibits an ultrahigh discharged energy density of 26.4 J cm−3 and a superior discharged efficiency of 72%, which are by far the highest values ever achieved in sandwich-structured dielectric polymer composites. It is revealed that the designed structure can enhance the breakdown strength and discharged efficiency by preventing the charge injection from electrodes and impeding the development of electrical tress during breakdown process, as confirmed by the leakage current and thermally stimulated depolarization current measurements, as well as the finite element simulations. This work represents a new design paradigm to exploit advanced dielectric materials for electrical energy storage applications.

Introduction

The ever-increasing global appetite for energy supply accompanying with rapid fossil fuels consumption presents a grand challenge that stimulates the development of renewable energy as well as advanced energy storage technologies [1], [2]. Among the currently available energy storage devices, dielectric capacitors possess the highest power densities because of their ultrafast charging-discharging capability, which makes them one of the primary enablers for intermittent energy (solar, wind, etc.) collection and conversion [3], [4]. Yet they are much limited by their energy densities that are at least an order of magnitude lower than their electrochemical counterparts such as batteries and electrochemical capacitors, etc [5], [6]. For instance, the energy density of the best commercially available film capacitors represented by biaxially oriented polypropylenes (BOPPs) is only ~ 2 J cm−3, as compared with ~ 20 J cm−3 of electrochemical capacitors [7]. Therefore, it is crucial to explore novel approaches that can significantly increase the energy density of dielectric capacitors to meet the urgent needs of miniaturization and cost-reduction of electrical power systems [8], [9].

Electrical energy can be stored in and released from dielectric capacitors when the dielectric materials are polarized and depolarized upon application and removal of electrical field. Their energy densities are governed by the opposite static charges on two electrodes separated by the dielectric materials, which can be expressed as:U=EdDwhere E is the applied electric field, and D is the produced electric displacement. For linear dielectrics,U=12ε0εrE2where ε0 is the vacuum permittivity and εr is the relative permittivity [10]. As U is quadratic dependent on E, the maximum U value is largely determined by the breakdown strength (Eb) that signifies the highest electric field applicable to dielectric materials. Therefore, polymers are the preferred candidates for high-energy density capacitors due to their high Eb and graceful failure with an open circuit [3], [4], [11]. However, the state-of-the-art polymer dielectrics deliver relatively low energy densities, which are limited by their low εr. To raise εr and subsequent U, inorganic ceramic nanofillers with high εr ranging from hundreds to thousands, such as titanium oxide (TO) [12], [13], barium titanate (BT) [14], [15], [16], [17], [18], barium strontium titanate (BST) [19], [20], are introduced into the polymer matrix to form dielectric polymer composites. Yet, a general drawback of the nanocomposite approach is that an increased εr is usually achieved at the cost of substantially decreased Eb owing to inhomogeneous field distribution caused by the large contrast of εr between the ceramic nanofiller and polymer matrix, thus leading to a moderate improvement of U [14], [21], [22], [23], [24], [25]. Therefore, it remains a great challenge to concomitantly enhance Eb and εr in order to fully explore the potential of polymer composites for high energy density capacitors [26].

The emerging of sandwich structures provides a new route to solve the paradox between high Eb and high εr in dielectric polymer composites [27], [28], [29], [30], [31], [32], [33], [34], [35]. In the so-called 2-2 type topologically structured composites, the polarization layer (PL) with high εr and the insulation layer (IL) with high Eb are stacked layer-by-layer to provide synergistically combined advantages for great enhancement in U. The electric field is redistributed as a result of the difference of εr between PL and IL, which alleviates the electric field strength in PL to prevent the sandwich composite from complete breakdown under high applied voltages, thus leading to a much improved Eb which is even higher than the intrinsic Eb of each component layer. Although good properties have been obtained in sandwich-structured composites, the enhancement of their energy densities are still moderated due to the less study of structural optimization. One of the challenges is that the charge carrier injection and associated electrical conduction cannot be remarkably suppressed, which substantially limit their energy storage capability, especially under high electric field. For example, to the best of our knowledge, the Eb values of the most reported sandwich-structured polymer composites are lower than 450 MV m−1 [29], [31], [32], [33], [34], [35]. Moreover, this unsatisfied electrical characteristic gives rise to a low electrical resistivity and a high energy loss during charge-discharge process. As reflected in the ferroelectric hysteresis loops, a high remanent polarization over 2 μC cm−2 has always been observed in the relevant literature when the maximum polarizations are enhanced to 10 μC cm−2. Consequently, the reported U and discharged efficiency (η) of sandwich-structured polymer composites are rarely higher than 20 J cm−3 and 70%, respectively [27], [29], [30], [31], [32], [33], [34], [35].

In this contribution, we present newly designed sandwich-structured nanocomposites with optimized spatial organizations of barium titanate/poly(vinylidene fluoride-co-hexafluoropropylene) (BT/P(VDF-HFP)) to break the above mentioned upper limits of U and η. The crystallite size and the dielectric loss of the polymer matrix can be reduced as the bulky comonomer of HFP is incorporated in the PVDF main chain [36], [37]. Different from most of the previous work where PLs are always adopted as the outer layers [27], [29], [31], [32], [35], a PL with high filler contents is herein clamped by two outer ILs that are functionalized with small amounts of BT nanoparticles (NPs). The designed structure in which ILs are placed near the electrodes can realize the full potential of ILs to prevent the charges injection, and thus lead to a significantly improved Eb of 526 MV m−1. Additionally, the preferred insulation properties of the designed sandwich films are also reflected by the much suppressed energy loss that the remanent polarization is successfully limited within 2 μC cm−2 even when the maximum polarization is as high as 13 μC cm−2. As a result, an ultrahigh U of 26.4 J cm−3 along with a much enhanced η of 72% has been achieved near the breakdown strength, which represents an enhancement of ~ 1300% over that of the benchmark BOPP (~ 2 J cm−3). The values of U and η are by far the highest ever achieved in the sandwich-structured polymer nanocomposites, dramatically beyond the current upper limits (i.e. ~ 20 J cm−1 of U and ~ 70% of η). Furthermore, the sandwich-structured BT/P(VDF-HFP) nanocomposites, composed of easily obtained raw materials (BT NPs, P(VDF-HFP)), can be manufactured through a versatile and low-cost solution casting process, which provides the possibility for industrial mass production.

Section snippets

Fabrication of the nanocomposites

For the fabrication of single layer BT/P(VDF-HFP) nanocomposites, BT nanoparticles with an average diameter of 100 nm (HBT-010, Sinocera, Ltd.) were first dispersed into N,N-dimethylformamide (DMF) by ultrasonication for 1 h. Then P(VDF-HFP) powders (Arkema, with 10 wt% HFP, molecular weight = 470,000, glass transition temperature = −35 °C) were dissolved in the former solution and stirred for 12 h at 30 °C. The mixture was cast into films on glass plate. The sandwich-structured BT/P(VDF-HFP)

Results and discussion

In order to determine the optimized contents of BT NPs contents in the ILs and PL respectively, single layer BT/P(VDF-HFP) nanocomposites were prepared and their dielectric and electrical properties were investigated as the first step. The data of εr and Eb shown in Table S1 are suggestive of the significant drawback of single-layer polymer composites in which that the enhancement of εr attributed by increasing the filler content is obtained at the cost of a sharp decrease of Eb. Compared with

Conclusions

In summary, the sandwich structured BT/(P(VDF-HFP)) nanocomposites composed of a middle polarization layer and two outer insulation layers have been developed toward high-energy-density high-discharged-efficiency dielectric energy storage. Different from most of the current designed sandwich-structured composites, the insulation layers with high Eb are placed nearby the electrodes to effectively impede the charge injection and introduce deep traps for charge carriers. Simulated results indicate

Acknowledgements

The work was supported by National 973 Projects of China (No. 2015CB654603) and the National Natural Science Foundation of China (Nos. 61471290, 61631166004).

Yifei Wang is a Ph.D. candidate of electronic science and technology at Xi’an Jiaotong University. He received his B.S. degree in electronic science and technology from Xi’an Jiaotong University, Xi’an, China, in 2013. His research focuses on polymer nanocomposites for dielectric energy storage applications.

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    Yifei Wang is a Ph.D. candidate of electronic science and technology at Xi’an Jiaotong University. He received his B.S. degree in electronic science and technology from Xi’an Jiaotong University, Xi’an, China, in 2013. His research focuses on polymer nanocomposites for dielectric energy storage applications.

    Linxi Wang is currently a Master candidate in the School of electronic science and technology in Xi’an Jiaotong University. She received the B.E. degree in electronic science and technology from the Xi’an Jiaotong University in 2015. Her research interest is focused on the modeling of dielectric polymer nanocomposites for high electric field applications.

    Qibin Yuan received the B.S. degree in materials physics and the M. E. degree in materials engineering from the shaanxi university of science and technology, China in 2009 and 2013, respectively. He is currently a doctoral candidate in electronic science and technology in Xi’an Jiaotong university Xi’an, China. His research interest is focused on high energy storage density ceramics and devices.

    Jie Chen is currently a Ph.D. candidate in electronic science and technology in Xi’an Jiaotong University. He received the B.S. and M.S. in materials science and Engineering from the Xi’an University of Architecture and Technology and Guilin University of Technology in 2012 and 2015, respectively. His research interest is on the polymer dielectrics with high energy storage density.

    Yujuan Niu received the B.S. degree in Chemistry and the M.S. degrees in Physical Chemistry from Northwest University, Xi’an, China, in 2007 and 2010, respectively, and received the Ph.D. degree in Electronic Science and Technology from Xi’an Jiaotong University. She is currently working Center for Advanced Functional Materials as a postdoctoral at Xi’an Jiaotong University, Xi’an, China. Her research interests are focused on polymer matrix composites materials for energy storage and surface modification for composites.

    Xinwei Xu is a Ph.D. candidate of electronic science and technology at Xi’an Jiaotong University. He received his B.S. degree in electronic science and technology from Xi’an Jiaotong University, Xi’an, China, in 2015. His research focuses on high-temperature dielectric energy storage applications.

    Yatong Cheng is a Ph.D. candidate of electronic science and technology at Xi’an Jiaotong University. He received his B.S. degree in electronic science and technology from Xi’an Jiaotong University, Xi’an, China, in 2014. His research focuses on polymer-based composites for magneto-dielectric applications.

    Bin Yao is a Ph.D. student of materials science and engineering at Xi’an Jiaotong University. He received his M.S degree in materials science and engineering from Northwestern Polytechnical University, and obtained his B.S. degree from Central South University. His research focuses on stretchable composites in the field of flexible electronics.

    Qing Wang is Professor of Materials Science and Engineering at The Pennsylvania State University, University Park, PA, USA. He received his Ph.D. in 2000 at University of Chicago. Prior to joining the faculty at Penn State in 2002, he was a postdoctoral fellow at Cornell University. His research programs are centered on using chemical and material engineering approaches towards the development of novel functional polymers and polymer nanocomposites with unique dielectric, electronic and transport properties for applications in energy harvesting and storage.

    Hong Wang is a chair professor of Materials Science and Engineering at Southern University of Science and Technology, Shenzhen, China. Her main research interests include dielectric materials, ceramic-polymer composites, and dielectric measurements for applications in passive integration and electronic devices. She has authored and co-authored more than 220 peer-reviewed papers and 2 book chapters. She holds 27 Chinese patents and 1 U.S. patent and has presented over 40 invited talks in international academic conferences. She is a senior member of IEEE, the chair of the Executive committee of the Asian Electroceramic Association (AECA), and a member of IEEE UFFC society’s Ferroelectric committee.

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