Dialkyl carbonates enforce energy storage as new dielectric liquids

Highlights

• Dialkyl carbonates exhibit a decent affinity to dielectric polymers.

• Thermodynamics and structural properties favor the usage of dialkyl carbonates in electrical capacitors.

• Dialkyl carbonates improve capacitors by fostering uniform densities of the dielectric polymers.

• Dielectric liquids possessing both polar and non-polar moieties wet cellulose and PET better than mineral oils.

Abstract

The performance of energy storage devices depends essentially on the quality and construction of their components. Capacitors store electrical charges being the most basic components of electronics. The application of suitable materials in capacitors may boost performance and suppress energy losses. In the present work, we employ atomistically precise simulations to investigate the improvement of physicochemical properties of an electrical capacitor by using novel dielectric liquids, such as dialkyl carbonates (DACs), to reinforce the conventional polymer dielectrics. We unveil and characterize molecular interactions between three DACs (diethyl carbonate, dioctyl carbonate, and didodecyl carbonate) and dielectric polymers – cellulose (CEL), polyethylene terephthalate (PET), and polypropylene (PP) – applied widely in modern capacitors. The obtained vast variety of structure and thermodynamic properties suggest that DACs are strongly enough coupled to the dielectric polymers. Furthermore, the length of the side alkyl chain can be successfully used to modulate the binding of DACs to CEL, PET, and PP separately. Therefore, the novel task-specific DACs for combined dielectric usage with the insulating polymers are hereby reported. The newly acquired physicochemical insights foster the development of more productive and sustainable energy storage devices.

Introduction

The world of humanity is permanently hungry for energy [1], [2], [3], [4], [5], [6], [7], [8]. Technologies behind the production, storage, and exchange of energy are at the cutting edge nowadays [9], [10], [11], [12], [13], [14], [15], [16]. A capacitor is a device used to preserve the generated electrostatic charge and electric field whose performance varies depending on engineering solutions and materials employed [17], [18], [19], [20]. Capacitors are omnipresent in electronics as energy storage components and have a host of various applications. For instance, the capacitors are necessary for signal coupling or decoupling, electronic noise filtering, power conditioning, and remote sensing. Modern consumer electronic devices are unimaginable without different implementations of electrical capacitors. Significant technological progress is achieved by picking up robust combinations of the energy capacitor’s constituents and identifying suitable exploitation conditions (Table S1) [21], [22], [23], [24], [25], [26], [27], [28], [29], [30].

The dielectric material of a capacitor is a cornerstone of its technological advantages. The capacity, reliability, energy storage efficacy, and specific electrical properties depend on the properties of the dielectric material [31], [32], [33]. The capacitors with a solid organic dielectric are used in power factor correction and voltage support, in high harmonic filters and to launch capacitor motors [34], [35], [36], [37]. Cellulose (CEL)-based materials (papers) and synthetic polymer films are used in such capacitors as a dielectric [38].

Polymer papers are traditionally employed in high-voltage capacitors. They can be a single dielectric in a device but they can also be combined with polymer films. Papers per se exhibit satisfactory electrical insulating and technological properties. Furthermore, papers are cheaper than other types of solid organic dielectrics. The thickness of the dielectric is in direct proportion to the operating field strength. Therefore, the large values of the specific capacitance and power density of capacitors are attainable with small dielectric constants ε and modest dimensions of the fabricated device.

The core of the capacitor’s paper is α-cellulose whose content ranges between 85 and 95 %. Its macromolecule consists of several thousand glucose rings connected by oxygen bridges (Fig. 1). CEL represents a flexible thread that folds and self-assembles into bundles. The bundles form fibrils [39], [40], [41], [42]. Out of fibrils, a cellulose tube-like fiber is formed. CEL is significantly porous due to its molecular structure and abundant competing electrostatic interactions among adjacent bundles and fibrils [43], [44], [45], [46]. As a result, the untreated cellulose paper routinely adsorbs 20-–25 % of air by volume. This phenomenon results in the heterogeneity of the material and is harmful to its insulating performance. Polymeric synthetic films are superior to cellulose in terms of electrical strength. By introducing films, it becomes, therefore, possible to make the distance between the charged plates smaller and reduce the size of the capacitor.

The difference between polymer films and cellulose is that polymeric film is a relatively monolithic system. The state-of-the-art technology of capacitor production involves winding polymer insulation and metal electrodes in layers. Air inclusions are unavoidable and occur between the layers. The polymeric films differ by polarity. They are formally divided into non-polar ones (2 < ε < 2.7) and polar ones (ε > 3). The polarity of the dielectric material determines the field of industrial application of the resulting capacitor.

Isotactic polypropylene (PP) exhibits good mechanical strength, chemical resistance to most impregnating compounds, necessary electrical properties, and thermal stability [47], [48], [49], [50]. The structure of isotactic PP is shown in Fig. 1(b). Polyethylene terephthalate (PET), Fig. 1(c), has a higher dielectric constant than that of PP [51], [52], [53]. The latter increases the capacitance of the capacitor. However, as the dielectric constant increases, the energy dissipation factor increases proportionally. This is particularly sensitive at higher frequencies of the current and working temperatures. Therefore, PET is poorly suitable for high pulse power applications. The technical parameters of CEL, PET, and PP used in electrical capacitors are summarized in Table S1.

The porosity is a highly undesirable feature of the dielectric with a prospective usage in capacitors. The discussed pores are normally of macroscopic sizes originating from the peculiarities of the polymer structure properties. Such pores are spontaneously filled with air. The breakdown of a dielectric with high porosity begins with the breakdown of air gaps therein. Therefore, the dielectric strength of a non-treated porous dielectric is low unless catastrophically small by modern standards. There is always a certain quantity of free electrons in the air. These supply the initial charges leading to the breakdown of the gas in a sufficiently strong field. Under the action of an electric field, electrons acquire energy sufficient to ionize the gas molecules via thermally induced collisions. When the free electrons collide with atoms and molecules, they generate new electrons. The newly liberated electrons, in turn, cause the ionization of gas molecules. Ultimately, the local breakdown of the dielectric takes place.

As a matter of fact, the papers contain conductive inclusions. They get activated in humid environments and conduct electricity. The ionic conductivity of the material increases. For the above reason, the polymers are routinely dried before impregnation. Qu and coworkers [54] reported that the dielectric strength of the impregnated cellulose is five times larger than that of pure cellulose. The nanoscale adsorption of a liquid by the polymer affects the charge separation and produces the traps to profoundly hinder electromigration under an electric field. Such a robust structure enhancement is responsible for the observed difference in dielectric strength.

The voids are filled with a liquid to extrude air from the insulating papers and the gaps between the electrodes and the polymeric films [55], [56]. Currently, mineral oils are used as such a liquid [57], [58], [59]. The mineral oils represent the mixtures of higher alkanes, largely cycloalkanes, obtainable from a distillate of petroleum. The chemical composition of the mineral oils differs depending on their sources. The specific density of mineral oils ranges between 800–900 kg/m³. The mineral oils, such as natural derivatives of naphthenes, are cheap, thermally, electrochemically, and chemically stable. However, their structures are highly lyophilic. As a result, mineral oils form an ideal additive to most non-polar insulators, such as PP. However, they do not account for the existence of polar moieties in other dielectric materials, such as PET and CEL. Furthermore, CEL gave rise to numerous natural and artificial modifications which retained its inherent polarity. The technological flaw resulting from the large-scale application of the mineral oils is their mediocre efficiency in eliminating voids. Due to non-specific physical interactions between an oil and a polymer, the corresponding wetting is not perfect. The weak van der Waals coupling of the mineral oil and the polymer is largely the same as that of the gas molecules and the polymer.

An ideal liquid dielectric should exhibit high dielectric strength, strong thermal conductivity, low shear viscosity in the operating temperature range, low toxicity, electrochemical stability, high fluidity, and chemical compatibility with the remainder of the system. Naturally, the main property of a suitable liquid dielectric is its physical affinity to a solid dielectric to maintain a pore-free insulating structure. It is, therefore, desirable to enhance the wetting of the inherent polymer pores and the regions of contact with an electrode [60]. Consequently, the access of the gas molecules must be blocked and the electric breakdown at lower voltages must be excluded.

An affinity enhancement of the insulating liquid to a polymer can be achieved by using an insulating liquid with certain local polarities in its structure. Hereby, we introduce organic acyclic dialkyl carbonates (DACs). DACs possess peculiar structures combining somewhat polar carbonate heads and non-polar alkyl chains (Fig. 2). Due to the unusual geometry dictated by the C(+4) electronic hybridization, DACs self-associate weakly [61], [62], [63], [64], [65]. Their shear viscosities are surprisingly low equaling several centipoises even for long side chains (vide infra). The physicochemical properties of DACs reveal their potential as superior low-polar solvents and electrochemically stable substances.

In the present work, we have probed diethyl carbonate (D2C), dioctyl carbonate (D8C), and di-dodecyl carbonate (D12C) as the three dielectric liquids of somewhat different polarity but the same-type structures (Fig. 2). The specific densities of DACs are around 1000 kg/m³, whereas the boiling points start from 126 °C in D2C and get gradually higher in the longer-chained DACs. The compatibilities of D2C, D8C, and D12C with CEL, PET, and PP were comprehensively assessed in terms of a vast set of thermodynamic and structural properties.