Polymer gel electrolytes for flexible supercapacitors: Recent progress, challenges, and perspectives

Abstract

With the rapid development of portable electronic products, wearable flexible energy storage devices such as flexible supercapacitors (FSCs) have attracted much attention. FSC devices possess various advantages, such as small size, flexibility and lightness, smooth operation, wide operating temperature range, and high energy density and power density. However, there are still significant challenges to make FSC devices in large-scale applications, such as poor gel electrolyte/electrode interface compatibility, poor cyclic stability under bending test conditions and low ionic conductivity of the gel electrolyte. Therefore, to develop FSC devices with outstanding characteristics, many efforts have been devoted to the development of polymer gel electrolytes which have excellent mechanical properties and electrochemical performance. This review aims to provide an overview of recent progress towards the development of advanced FSCs based on aqueous, non-aqueous, ionic liquid-based and redox gel electrolytes. We systematically discussed the challenges and future development of various gel electrolytes for FSC devices. Moreover, the development and performance evaluation of FSCs are also discussed.

Introduction

In recent years, the increasing demand for electronic devices such as Google glasses, smartwatches and bracelets has promoted the rapid development of flexible wearable electronic devices [1], [2], [3]. Compared with traditional electronic devices, the flexible wearable electronic devices have the unique advantages of flexibility, lightweight, and comfortability, which has triggered their applications in flexible sensors, artificial electronic skin and other wearable microelectronic devices [4], [5], [6], [7], [8]. With this perspective, flexible wearable power sources are critical and in urgent need for these devices.

The flexible wearable powers can be classified into two categories: flexible electrochemical energy storage devices (FEESDs) including flexible batteries [9] and FSCs [10], and the non-electrochemical energy storage devices such as flexible photovoltaic cells [11]. Currently, the FEESDs are the mainstream of flexible energy storage devices because of their mature preparation process, no time limit, and easy commercialization. However, due to the differences in materials utilization and assembly methods, FEESDs also have their disadvantages, such as thermal runaway and low energy density. Table 1 lists the performance and basic parameters of flexible batteries and FSCs [12]. As can be seen, FSCs have several advantages, such as high power density, fast charge and discharge speed, wide operating temperature range, high efficiency, and long cycle life, compared with flexible batteries [13]. FSCs also provide higher security than flexible batteries, making them substantial competitors in the field of flexible energy storage. FSCs are mainly composed of flexible electrode materials, gel electrolyte, and packaging materials. The difference between traditional supercapacitors and FSCs lies in the electrode materials and electrolytes used which exhibit excellent flexibility. The flexibility property of the electrolytes allows the device having a variety of shapes, lightweight, strong deformation ability and small size advantages [14,15], to be widely used in flexible and wearable electronic devices.

However, to achieve FSC devices that can continuously provide stable electrochemical performance under repeated bending and folding conditions, the development of flexible components such as electrodes, separators, electrolytes, and housings is crucial. The key factors of the FSCs are: (i) the necessity of the flexible electrodes to have multiple functions (such as a conductive electronic skeleton, an ion-conducting skeleton, and an active material carrier), and (ii) the electrolyte requirements include outstanding ionic conductivity, excellent contact between electrode and electrolyte, low electrode/electrolyte interfacial resistance, flexibility and high stability under elevated temperatures [16], [17], [18], [19], [20]. Some researchers have summarized the progress of the electrode materials for SCs. Carbon-based electrode materials have been widely concerned by researchers in the field of energy storage applications due to their advantages including high specific surface area, good electrical conductivity, large power density, and excellent chemical stability. For instance, Xia et al. [21] systematically summarized the latest research progress and electrochemical performance of graphene-based materials in terms of graphene synthesis methods, corresponding composite materials, structural characterization, and design concepts. Chen et al. [22] resumed advanced flexible solid-state supercapacitors depending on carbon nanomaterials and introduced the mechanism, structure and electrochemical performances for FSCs. Huang et al. [23] summarized the fabrication, electrochemical properties and novel devices for the flexible polypyrrole-based electrode. Apart from carbon materials, metal-based electrode materials also have drawn great attention from scientific researchers due to their excellent electrode/electrolyte contact area, fast ion transmission channel and good electrical conductivity. To better understand the development of metal oxide electrodes, Fan et al. [24] give an outline of solution-based methods for the controllable synthesis of metal oxides and applications of corresponding materials in electrochemical energy storage. Moreover, Zhong et al. [25] resumed recent progress in the synthesis and electrochemical application of transition metal carbides (TMCs) and nitrides (TMNs) electrode for SCs, which opened the door for new high-performance electrode materials in energy storage and conversion.

In this review, we mainly focus on the review of the most recent processes of the utilization of various electrolytes, which are an essential component of FSCs due to numerous reasons [26]. First of all, the electrolyte plays a crucial role in an electrochemical reaction. During the FSC discharge progress, the electrode releases electrical energy by absorbing the solute in the electrolyte. However, this step is not possible in the absence of the electrolyte, and therefore the FSC cannot release electrical energy. Secondly, since the FSC electrodes are not in contact, the current loop cannot be formed without the presence of an electrolyte. Therefore, the electrolyte plays the essential role of the conductive material in FSC devices. Thirdly, the presence of the electrolyte determines the operating voltage of the FSCs, which affects both the energy and power density of the flexible supercapacitor. However, the use of gel electrolytes in FSC devices has several challenges, such as poor gel electrolyte/electrode interface compatibility, high volatility at high temperature and low ionic conductivity, which need to be addressed to obtain effective FSC devices. To solve these problems, researchers have taken much effort on the further development of high-performance electrolytes [27], [28], [29], [30], [31].

Herein, we will first demonstrate the development of electrolytes, the reaction mechanism, the mechanical properties (e.g. length (L), bending angle (θ) and radius (R)) and the electrochemical performance of FSCs in different electrolyte systems, which will help to further understand the design principles of gel electrolytes and promote the commercial application of FSC devices. The recent scientific development of gel electrolytes for FSC devices is separated, into four categories as observed in Fig. 1, which will be discussed throughout this review. Firstly, the development of aqueous gel electrolytes using a conductive polymer as the framework and acid/alkaline/salt as the electrolyte solute is discussed. Secondly, the research progress in FSC devices based on non-aqueous gel electrolytes is presented. Thirdly, a summary of the development of ionic liquid gel electrolytes with a series of categories is presented. Further, the scientific development of redox gel electrolyte is discussed. Besides, we discuss the standard performance evaluation for FSCs from the perspective of energy density, power density, and mechanical properties.