此式表明，电介质材料的击穿场强(Eb)、剩余电位移强度(Dr)和最大电位移(Dmax)是影响电介质薄膜储能密度的关键因素。因此，提高电介质材料储能密度的关键，是降低Dr和提高其击穿场强和Dmax。PP、PC等线性介电聚合物虽然具有较大的击穿场强和较大的充放电速率，但是其非极性本质使其极化值较低、Dmax小和可释放储能密度较低。以PVDF为代表的铁电聚合物极化值较高，能提供较高的可释放能量密度[12,13]。但是，PVDF固有的高介电损耗使其充放电效率较低。这意味着，在能量转换过程中很大一部分转换为热能，使电容器升温和失效，不利于电容器的安全运行[14]。减少PVDF能量损失的方法，包括纳米复合、化学改性和聚合物共混等[15~17]。其中聚合物共混策略是一种既简单又经济有效的方法，能在不牺牲PVDF基聚合物可释放储能密度的情况下降低其能量损失[18]。线性介电聚合物/PVDF二元共混物受到了极大的关注。这种二元共混物，在理论上是一种低损耗线性聚合物。此外，线性介电聚合物能减弱相邻PVDF铁电体之间的耦合域，最大限度地减少铁电损失和能量损失。Yang等[18]将ABS与PVDF共混制备出均匀的复合薄膜，实现了性能的优化。本文选用具有优异的机械性、耐化学性、热稳定性的聚酰亚胺(PI)，将共沉淀法和热压法相结合制备PI/PVDF全有机复合薄膜，研究其储能性能。
1 实验方法
1.1 薄膜的制备
图1给出了全有机复合薄膜的制备流程。制备步骤：(1)将一定量的聚偏氟乙烯(PVDF)粉末和热塑型聚酰亚胺(PI)加入容积为4 mL的N,N-二甲基甲酰胺(DMF，分析纯)中，将其置于65℃的加热台上使其完全溶解；(2) 在500 mL烧杯中倒入200 mL纯水及200 mL无水乙醇，用磁力搅拌器搅拌，转速为550 r/min；(3) 将步骤(1)中的混合溶液缓慢滴加入步骤(2)的烧杯中，收集析出的絮状物；(4) 将絮状物抽滤(SHZ-D(III)循环水式多用真空泵)、烘干(烘箱，DZF-6020)后热压(热压机，YLJ-HP300)，烘干温度为60℃，时间为12 h，热压温度为155℃，热压时间为2 h。热压后得到全有机复合薄膜。
图1
改变PI的加入量，可制备出不同配比的全有机复合薄膜。PI的加入量(质量分数)分别为PVDF的0%，5%，10%，15%，20%，100%，将制备出的样品分别标记为0/100，5/95，10/90，15/85，20/80，100/0。
1.2 性能表征
用场发射扫描电子显微镜(SEM，Hitachi SU8010)分析复合薄膜的截面；用X-射线衍射(XRD)仪表征不同薄膜的晶体结构，测试条件为：Cu-Kα靶，波长0.154 nm，扫描角2θ的变化范围为5°~60°，扫描速率为0.1 (°)·s-1。用差示扫描量热法(DSC7020)记录复合材料的熔融与结晶行为，温度测试范围为90℃~190℃，加热速率为10℃·min-1。用阻抗分析仪(HP4294，Agilent)测试复合材料的室温介电性能。用介电耐压测试仪测试复合材料的击穿场强。用铁电测试系统(TF2000，Trek 10/10B-HS)测试位移-电场(D-E)回线。
2 结果和讨论
2.1 全有机复合薄膜的微观结构
图2给出了PI/PVDF全有机复合薄膜的截面SEM照片。从图2a~e可见，用该方法制备的全有机薄膜的厚度约为18 μm。与纯PVDF薄膜的截面(图2a)相比，PI的加入没有产生明显的空隙和孔洞(图2b~e)，复合薄膜的结构依旧比较致密，验证了共沉淀法与热压法相结合的优越性。增大PI的添加量则PI线性介电材料的特征更加明显，可释放储能密度急剧降低，因此只讨论PI在低添加量时的情况。图2f~i给出了20/80组分的SEM元素映射图，C元素和F元素属于PVDF，O元素和N元素属于PI。与预期的一样，O元素和N元素在20/80复合材料的断口处的分散相当均匀。综上所述，SEM测试结果表明，共沉淀法与热压法相结合制备的全有机复合薄膜结构均匀、致密。
图2
图2
PI/PVDF复合薄膜截面的SEM形貌和(f-i)20/80组分的SEM元素映射图
Fig.2
Cross-sectional SEM of PI/PVDF composite film (a~e) and SEM element mapping of the 20/80 component (f~i)
PVDF薄膜的性能与其晶相结构紧密相关，PVDF 主要有α、β与γ相，其中α和γ相极性较小，铁电损耗较小，适用于储能领域[19~21]。全有机复合薄膜的晶相结构，如图3所示。可以看出，在纯PVDF衍射谱的18.4°和19.8°处出现了两个衍射强峰，分别对应(020)晶面和(021)晶面的α相，说明纯PVDF具有以α相为主的相结构。由PI/PVDF共混膜的XRD谱可见，PI的加入使18.4°处的衍射峰分裂成17.7°和18.5°这两个小衍射峰，分别归属于(100)晶面的α相衍射和(020)晶面的γ相衍射。PI的加入对PVDF薄膜的相结构没有较大影响，复合薄膜依旧是α相为主导，意味着复合薄膜应该较好的储能性能。
图3
图4
图4
PI/PVDF复合薄膜的DSC曲线和结晶度
Fig.4
DSC curve and crystallinity of PI/PVDF composite film (a) The melting DSC traces of samples, (b) Crystallinity of samples
根据DSC测试结果，可计算材料的结晶度[24]
其中
2.2 全有机复合薄膜的电学性能
图5
图5
PI/PVDF复合薄膜的介电和铁电性能
Fig.5
Dielectric and ferroelectric performance of PI/PVDF composite film (a) room temperature dielectric constant εr and dielectric loss tanδ versus frequency, (b) weibull distribution, (c) D-E loops, (d) discharged energy density and charge-discharge efficiencies
其中E为测试时薄膜的击穿强度，P为在E下发生击穿的概率，Eb为击穿概率为63.2%时电场强度的大小，β为拟合直线斜率。由图5b可见，纯PVDF的击穿场强Eb为354 MV·m-1，PI的加入略微降低了薄膜的击穿场强，但是影响不大，因为低添加量时PI与PVDF良好的结合性，材料的致密度较高。
图5c给出了PI/PVDF全有机复合薄膜在300 MV·m-1电场下的D-E曲线。可以看出，PI的加入使剩余电位移降低，最大电位移增大，且在PI/PVDF为5/95时达到饱和最大电位移。在300 MV·m-1电场下5/95全有机复合薄膜的Dr为1.3 μC·cm-2，Dmax为7.2 μC·cm-2，而在相同情况下纯PVDF薄膜的Dr为2.4 μC·cm-2，Dmax为6.5 μC·cm-2。Dr的减小反映了全有机复合薄膜内部较低的铁电损耗和电导损耗，因为PI和PVDF之间强的相互作用和PI较低的铁电损耗。XRD测试结果表明，复合薄膜中还有少量的γ相结构，有利于抑制薄膜的铁电损耗。因此，添加PI使Dr明显减小[22]。同时，PI的加入提高了Dmax。根据单极D-E曲线计算出PI/PVDF全有机薄膜的储能密度、可释放储能密度及充放电效率，结果在图5d中给出。纯PVDF在300 MV·m-1时可释放储能密度约为4.67 J·cm-3，5/95复合薄膜在300 MV·m-1时可释放储能密度可达6.52 J·cm-3，是纯PVDF的1.4倍。同时，PI/PVDF全有机复合薄膜的放电效率优于纯PVDF，在300 MV·m-1内PI/PVDF全有机复合薄膜的充放电效率可保持在50%以上，而纯PVDF的充放电效率在200 MV·m-1就急剧下降到50%。例如，在300 MV·m-1时5/95复合薄膜的充放电效率为50.4%，而纯PVDF的充放电效率仅为38.21%。PI/PVDF全有机复合薄膜的高充放电效率，伴随着较高的放电能量密度。
3 结论
(1) 将共沉淀法与热压法相结合制备的PI/PVDF薄膜，具有致密的结构。
(2) 添加量较低的PI分散性良好且具有界面极化效应，加入PI使薄膜的εr略微降低、tanδ的变化较小。
(3) PI的加入提高了PVDF薄膜的可释放储能密度，PI添加量为5%的复合薄膜在300 MV·m-1电场下可释放储能密度达到6.52 J·cm-3。在300 MV·m-1条件下5/95复合薄膜的充放电效率为50.4%。
参考文献
Study on energy storage properties of PMMA/PVDF based composites filled with core-shell nanofibers
[J].
核壳结构纳米纤维掺杂PMMA/PVDF基复合介质的储能特性研究
[J].
Recent progress in dielectric energy storage materials with high energy density
[J].
高能量密度介电储能材料研究进展
[J].
Study on energy storage performance of ternary TiO2/MXene/PVDF nanocomposite films enhanced by adding low concentration of MXene
[J].
低浓度MXene填充增强三元TiO2/MXene/PVDF纳米复合膜的储能性能研究
[J].
High-energy-density ceramic/PVDF-based nanocomposites dielectrics
[J].
高储能陶瓷/PVDF基复合电介质材料的研究进展
[J].
Largely enhanced energy storage density of poly(vinylidene fluoride) nanocomposites based on surface hydroxylation of boron nitride nanosheets
[J].
Ultrahigh Energy Density of Polymer Nanocomposites Containing BaTiO3@TiO2 Nanofibers by Atomic-Scale Interface Engineering
[J].
Montmorillonite/PVDF-HFP-based energy conversion and storage films with enhanced piezoelectric and dielectric properties
[J].
Energy Storage Properties of Poly(vinylidene fluoride)-based Copolymers
[J].
聚偏氟乙烯基含氟聚合物介电和储能研究进展
[J].
A Novel Flexible Supercapacitor Based on Cross-Linked PVDF-HFP Porous Organogel Electrolyte and Carbon Nanotube Paper@π-Conjugated Polymer Film Electrodes
[J].
Synchronously promoted discharged energy density and discharged efficiency in PVDF films through a linear dielectric polymer PP blending approach
[J].
Research progress of polymer based dielectrics for high-temperature capacitor energy storage
[J].
面向高温介电储能应用的聚合物基电介质材料研究进展
[J].
Manipulating microstructures and electric properties in PVDF-based multifunctional ferroelectric polymers
[D].
多功能铁电共聚物微结构的调控及其电性能的调制
[D].
Polymer/molecular semiconductor all-organic composites for high-temperature dielectric energy storage
[J].
Dielectric polymers for electrostatic energy storage suffer from low energy density and poor efficiency at elevated temperatures, which constrains their use in the harsh-environment electronic devices, circuits, and systems. Although incorporating insulating, inorganic nanostructures into dielectric polymers promotes the temperature capability, scalable fabrication of high-quality nanocomposite films remains a formidable challenge. Here, we report an all-organic composite comprising dielectric polymers blended with high-electron-affinity molecular semiconductors that exhibits concurrent high energy density (3.0 J cm) and high discharge efficiency (90%) up to 200 °C, far outperforming the existing dielectric polymers and polymer nanocomposites. We demonstrate that molecular semiconductors immobilize free electrons via strong electrostatic attraction and impede electric charge injection and transport in dielectric polymers, which leads to the substantial performance improvements. The all-organic composites can be fabricated into large-area and high-quality films with uniform dielectric and capacitive performance, which is crucially important for their successful commercialization and practical application in high-temperature electronics and energy storage devices.
Simultaneously realizing ultra-high energy density and discharge efficiency in PVDF composites loaded with highly aligned hollow MnO2 microspheres
[J].
Interface modification and energy storage properties of barium titanate-based/polyvinylidene fluoride composite
[J].
钛酸钡基/聚偏氟乙烯复合介质材料的界面改性与储能性能
[J].
Preparation and properties of polyphenylthiourea-poly(vinylidenefluride) composites modified by do-pamine
[J].
多巴胺改性聚苯硫脲与聚偏氟乙烯共混复合材料的制备与性能
[J].
Study on preparation and dielectric properties of polyimide-based all-organic composite materials
[D].
聚酰亚胺基全有机复合材料的制备及其介电性能研究
[D].
Poly(acrylonitrile butadiene styrene)/poly(vinylidene fluoride) binary blends films with superior breakdown strength and discharge efficiency
[J].
Interface design for high energy density polymer nanocomposites
[J].This review provides a detailed overview on the latest developments in the design and control of the interface in polymer based composite dielectrics for energy storage applications. The methods employed for interface design in composite systems are described for a variety of filler types and morphologies, along with novel approaches employed to build hierarchical interfaces for multi-scale control of properties. Efforts to achieve a close control of interfacial properties and geometry are then described, which includes the creation of either flexible or rigid polymer interfaces, the use of liquid crystals and developing ceramic and carbon-based interfaces with tailored electrical properties. The impact of the variety of interface structures on composite polarization and energy storage capability are described, along with an overview of existing models to understand the polarization mechanisms and quantitatively assess the potential benefits of different structures for energy storage. The applications and properties of such interface-controlled materials are then explored, along with an overview of existing challenges and practical limitations. Finally, a summary and future perspectives are provided to highlight future directions of research in this growing and important area.
Electroactive phases of poly(vinylidene fluoride): Determination, processing and applications
[J].
Exploring strategies for high dielectric constant and low loss polymer dielectrics
[J].Polymer dielectrics having high dielectric constant, high temperature capability, and low loss are attractive for a broad range of applications such as film capacitors, gate dielectrics, artificial muscles, and electrocaloric cooling. Unfortunately, it is generally observed that higher polarization or dielectric constant tends to cause significantly enhanced dielectric loss. It is therefore highly desired that the fundamental physics of all types of polarization and loss mechanisms be thoroughly understood for dielectric polymers. In this Perspective, we intend to explore advantages and disadvantages for different types of polarization. Among a number of approaches, dipolar polarization is promising for high dielectric constant and low loss polymer dielectrics, if the dipolar relaxation peak can be pushed to above the gigahertz range. In particular, dipolar glass, paraelectric, and relaxor ferroelectric polymers are discussed for the dipolar polarization approach.
FTIR studies of β-phase crystal formation in stretched PVDF films
[J].
Determination of the α, β, and γ crystalline phases of poly(vinylidene fluoride) films prepared at different conditions
[J].
The fabrication and electric properties of poly(vinylidene fluoride)-based ferroelectric composites
[D].
聚偏氟乙烯基铁电复合材料的制备及其电性能研究
[D].
