A Review of Electrolyte Materials and Compositions for Electrochemical Supercapacitors

Chemical Society Reviews A Review of Electrolyte Materials and Compositions for electrochemical supercapacitors Journal: Chemical Society Reviews Manuscript ID: CS-REV-04-2015-000303 Article Type: Review Article Date Submitted by the Author: 13-Apr-2015 Complete List of Authors: Zhong, Cheng; Tianjin University, School of Materials Science and Engineering Yida, Deng; Tianjin University, School of Materials Science and Engineering Hu, Wenbin; Tianjin University, School of Materials Science and Engineering Qiao, Jinli; Donghua University, School of Environmental Engineering Zhang, Lei; National Research Council of Canada, Zhang, Jiujun; Inst Fuel Cell Innovat, National Research Council Canada Page 1 of 114 Chemical Society Reviews A review of electrolyte materials and compositions for electrochemical supercapacitors Cheng Zhong, a Yida Deng, b Wenbin Hu, a,b, ∗ Jinli Qiao, c Lei Zhang, d Jiujun Zhang d,* a Key Laboratory of Advanced Ceramics and Machining Technology (Ministry of Education), School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China b Tianjin Key Laboratory of Composite and Functional Materials, Tianjin University, Tianjin 300072, China c School of Environmental Engineering, Donghua University, Shanghai, China d Department of Chemical & Biochemical Engineering, University of British Columbia, Vancouver, BC, Canada Abstract: Electrolytes have been identified as one of the most influential components in the performance of electrochemical supercapacitors (ESs), which include: electrical double-layer capacitors, pseudocapacitors and hybrid supercapacitors. This paper reviews recent progress in the research and development of ES electrolytes. The electrolytes are classified into several categories, including: aqueous, organic, ionic liquids, solid-state or quasi-solid-state, as well as redox-active electrolytes. Effects of electrolyte properties on ES performance are discussed in detail. The principles and methods of designing and optimizing electrolytes for ES performance and application are highlighted through a comprehensive analysis of the literature. Interaction among the electrolytes, electro-active materials and inactive components (current collectors, binders, and separators) are discussed. The challenges of producing high-performing electrolytes are analyzed. Several possible research directions to overcome these challenges are proposed for future efforts, with the main aim to improve ESs’ energy density without sacrificing existing advantages (e.g., high power density and long cycle-life). (507 references) 1. Introduction Significant worldwide increases in the consumption of fossil fuels, resulting from the rapid growth of the global economy, are producing two major associated issues. The first is the accelerating depletion/exhaustion of existing fossil fuel reserves and the second is the affiliated environmental problems e.g., increasing greenhouse gas emissions and general air and water pollution. Taken together these items constitute the basis of an urgent, worldwide concern about the future, sustainable development and health of our society’s ecosystem. The need to develop and scale up sustainable, * Corresponding authors. E-mails: [email protected] (W.B. Hu); [email protected] (J.J. Zhang). 1 Chemical Society Reviews Page 2 of 114 clean energy sources and their associated technologies is recognized worldwide as an urgent priority. Most renewable clean energy sources are highly dependent on the time of day and regional weather conditions. Development of related energy conversion and energy storage devices is therefore required in order to effectively harvest these intermittent energy sources. In this regard, batteries, electrochemical supercapacitors (ESs) and fuel cells are all recognized as three kinds of the most important electrochemical energy storage/conversion devices. The ES, also known as a supercapacitor or ultracapacitor, has attracted considerable interest in both academia and industry because it has some distinct advantages such as higher power density induced by a fast charging/discharging rate (in seconds) and a long cycle life (>100,000 cycles) when compared to batteries and fuel cells. 1 Depending on the charge storage mechanism, ESs can be briefly classified as electrochemical double-layer capacitors (EDLCs), pseudocapacitors and hybrid-capacitors. Compared to both pseudocapacitors and hybrid-capacitors, EDLCs constitute the majority of currently available commercial ESs, mainly due to their technical maturity. In an evaluation of electrochemical energy devices, besides cycle-life (or lifetime), both their energy density and power density are the two most important properties. The relationship between these two important properties can be expressed by the Ragone plot. Fig. 1 shows the Ragone plots for several typical electrochemical energy storage devices.1 It can be clearly seen from these Ragone plots that supercapacitors are currently located between the conventional dielectric capacitors and batteries/fuel cells. Despite the much lower energy density compared to the batteries/fuel cells, ESs can have much higher power densities. This makes them promising for applications in current stabilization when accessing intermittent renewable energy sources. In addition, ESs have also attracted considerable interest in a wide variety of applications requiring high power density, such as portable electronics, electric or hybrid electric vehicles, aircraft and smart grids. For example, in the case of both hybrid and electric vehicles including fuel cell ones, ESs could provide high power density for short-term acceleration and recovery of energy during braking, thus saving energy and protecting the batteries from the high-frequency rapid discharging and charging process (dynamic operation). 2 Page 3 of 114 Chemical Society Reviews Fig. 1 Ragone plots for representative energy storage devices of batteries, fuel cells, and supercapacitors. Reprinted with permission from Ref. 2 (open access). However, the major challenge for ESs, when compared to both batteries and fuel cells, is their insufficient energy density (for example, EDLCs commonly have an energy density of <10 Wh kg –1; even for both pseudocapacitors and hybrid-capacitors, their energy densities are generally less than 50 Wh kg –1), 11 which cannot fully meet the growing demand of the applications where high energy density is required. To overcome this challenge, extensive work has been devoted to increase the energy density of ESs, 3, 4 in order to widen their application scope. Since the energy density (E) of ESs is proportional to the capacitance (C) and the square of the voltage (V), that 1 is: E = CV 2 , increasing either or both of the capacitance and the cell voltage are 2 effective ways to increase the energy density. This can be achieved through the development of electrode materials with high capacitance, electrolytes (electrolyte salt + solvent) with wide potential windows, and and integrated systems with new and optimized structure. To date, these developments can be briefly summarized as follows: (1) increasing the specific capacitance of carbon-based electrodes through the development of a novel carbon structure with a highly effective specific surface area and a high packing density;5 (2) developing pseudocapacitors based on pseudocapacitive materials (e.g., some electroactive transition-metal oxides and conducting polymers) with high specific capacitance contributed from the pseudo-capacitance;6 (3) enlarging the cell voltage via the development of a new electrolyte; and (4) exploring ESs with novel structures or new concepts, such as the hybrid (or asymmetric) capacitors, especially the lithium ion capacitors (LICs). 3 Note, these developments are all closely related with each other. Although it is relatively simple to fabricate the individual components (e.g., electrode material, electrolyte and structure) of the ESs, their interaction must be considered to promote the synergetic effect. For example, design and preparation of the porous carbon electrode materials should consider the matching between the pore structure and the size of the electrolyte ions in order to make a high capacitive electrode.7 The development of some electrolytes should consider their possible interaction with the electrode materials 3 Chemical Society Reviews Page 4 of 114 such as in the case of hybrid capacitors. Regarding the development of ES electrolytes, widening the potential window of an electrolyte solution, that is, enlarging the cell voltage ( V), can effectively increase 1 the energy density as seen from the equation of E = CV 2 . It is worth noticing that 2 increasing the cell voltage would be more efficient than increasing the electrode capacitance in terms of energy density improvement. This is because the energy density is proportional to the square of the cell voltage. Therefore, developing new electrolytes/solutions with wide potential windows should be given even higher priority efforts than the development of new electrode materials. As discussed above, the operating cell voltage of the ESs is largely dependent on the electrochemical stable potential window (ESPW) of the electrolytes if the electrode materials are stable within the working voltage range. For example, aqueous electrolyte-based ESs usually have an operating potential window of about 1.0–1.3 V because the aqueous electrolyte’s potential window is about 1.23 V (the potential window of H 2/O 2 evolution reactions at 1.0 atm

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