Electrical and Electronic Devices, Circuits, and Materials. Группа авторов
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3 (3) Hybrid ESs, is a combination of the two: electrical double-layer (EDL) and faradaic mechanisms. It is also termed as an asymmetric supercapacitor. While, if one electrode material is a battery type such as PbO2, then the device is a hybrid SC.
SC has a high power density and low energy density. So, various strategies have been adopted by researchers to improve the energy density of the SC cell. Novel cell design (symmetric, asymmetric, hybrid), cell voltage (E ∝ V2) and synthesizing new electrode nanostructures and electrolyte material opens new doors of opportunity to researchers. Figure 3.3 depicts the overview of the different strategies used to improve the energy density of the SC cell [13].
Figure 3.1 (a) The development of supercapacitors in different countries [Reproduced with permission from Ref. [6], © AIP Publishing 2019]. (b) An illustration of key performance metrics, test methods, major affecting factors for the evaluation of SCs [Reproduced with permission from Ref. [7], © Wiley 2014].
Figure 3.2 Schematic diagram of (a) an electrostatic capacitor, (b) an electric double-layer capacitor, (c) a pseudocapacitor, and (d) a hybrid-capacitor [Reproduced with permission from Ref. [12], © Royal Society of Chemistry 2015].
3.1.2 Key Characteristics of the Electrolyte
In general, the electrolytes for the SC application need to follow some requirements: (1) broad potential window; (2) high ionic conductivity; (3) broad operating temperature range; (4) non-volatile and non-flammable nature; (5) better chemical and electrochemical stability; (6) chemically inert toward SC cell components such as electrodes, current collectors; and (7) cost-effective and environmentally friendly. The prepared polymer electrolyte needs to be examined on the basis of the characteristic parameter that influences the morphological, structural and electrical properties [14–16]. These important parameters are influenced by the host polymer, salt and nanofiller addition. This section discusses the important parameters.
Morphology and Crystallinity
Fast ion dynamics in polymer electrolytes is facilitated by high amorphous content and is examined before going ahead for electrical properties. The X-ray diffraction (XRD) and differential scanning calorimetry (DSC) are techniques to estimate the degree of crystallinity (ΧC).
Figure 3.3 Strategies for improving the energy density of supercapacitors [Reproduced with permission from Ref. [13], © Springer Nature 2016].
It provides information about the crystalline and amorphous content in the polymer matrix. From XRD degree of crystallinity is evaluated from the area of crystalline (AC) and amorphous peaks (AA) using expression;
Ionic Conductivity
The ion dynamics in the polymer matrix is examined by evaluating the ionic conductivity and is expressed by relation;
The ionic conductivity also varies with temperature. The increase of temperature thermally activates the charge carriers and lowers the activation energy or potential barrier required for ion migration. The variation of ionic conductivity with temperature follows three behaviors depending upon temperature range: (i) Arrhenius behavior, (ii) Vogel-Tamman-Fulcher (VTF) behavior, and (iii) Williams-Landel-Ferry (WLF) behavior [17–22].
Arrhenius behavior
The increase of temperature in the polymer matrix thermally activates the charge carriers and increase in flexibility leads to fast ion migration via coordinating sites. This collectively favors the ion dynamics and Arrhenius’s behavior suggests the ion transport occurs via hopping mechanism. This behavior dominates when the temperature is lower than the glass transition temperature (Tg) [18]. To explore it further, activation energy is evaluated and the lower value of the activation energy is favorable for fast ion dynamics and hence promotes higher ionic conductivity. The activation energy (Ea) is slope of linear-least square fitting of the log σvs. 1/T plot by Arrhenius equation and is expressed as;
Vogel-Tamman-Fulcher (VTF) behavior
The VTF σ vs. 1/T plot is the non-linear plot and ion transport occurs via the segmental motion of the polymer chain coupled with hopping. The ion diffusion within the polymer matrix occurs via the availability of free volume that is delivered by the polymer chains. The thermally activated charge carriers cross the potential barrier and contribute to conduction [19, 20]. The VTF equation is;
Cation/Ion Transference Number
As in polymer electrolytes, the main contribution is from the ion migration. So, the cation (t+) and ion (tion) transference