Date Published: February 21, 2017
Publisher: John Wiley and Sons Inc.
Author(s): Wenhua Zuo, Ruizhi Li, Cheng Zhou, Yuanyuan Li, Jianlong Xia, Jinping Liu.
Design and fabrication of electrochemical energy storage systems with both high energy and power densities as well as long cycling life is of great importance. As one of these systems, Battery‐supercapacitor hybrid device (BSH) is typically constructed with a high‐capacity battery‐type electrode and a high‐rate capacitive electrode, which has attracted enormous attention due to its potential applications in future electric vehicles, smart electric grids, and even miniaturized electronic/optoelectronic devices, etc. With proper design, BSH will provide unique advantages such as high performance, cheapness, safety, and environmental friendliness. This review first addresses the fundamental scientific principle, structure, and possible classification of BSHs, and then reviews the recent advances on various existing and emerging BSHs such as Li‐/Na‐ion BSHs, acidic/alkaline BSHs, BSH with redox electrolytes, and BSH with pseudocapacitive electrode, with the focus on materials and electrochemical performances. Furthermore, recent progresses in BSH devices with specific functionalities of flexibility and transparency, etc. will be highlighted. Finally, the future developing trends and directions as well as the challenges will also be discussed; especially, two conceptual BSHs with aqueous high voltage window and integrated 3D electrode/electrolyte architecture will be proposed.
With the increasing concerns of environmental issues and the depletion of fossil fuels, the emergence of electric vehicles and the generation of renewable wind, wave, and solar power are of great importance to the sustainable development of human society.1 Therefore, reliable energy storage systems such as batteries and supercapacitors (SCs) are key elements to enable these energy structure evolutions. In addition to traditional lead–acid, Ni–Cd, Ni–MH, lithium ion batteries (LIBs), and SCs, various advanced batteries such as lithium–air/–sulfur,2 sodium/aluminum ion batteries3, 4 and aqueous metal ion batteries5 have been emerging, and great efforts have been devoted to optimize their overall performance for future practical applications.6 Building better energy storage devices not only depends on the micro‐/nanostructure design of electrode materials but more crucially relies on the device’s configuration engineering.7 An energy storage system based on a battery electrode and a supercapacitor electrode called battery‐supercapacitor hybrid (BSH)8 offers a promising way to construct device with merits of both secondary batteries and SCs, as shown in Figure1. This hybridization is indispensable to meet with the demands of both higher energy and power densities for powering future multifunctional electronics, hybrid electric vehicles, and industrial equipment.
To illustrate the structure details and classification of BSHs and further understand the energy storage mechanism, SCs’ electrode is the key and must be distinguished from a battery electrode. Basically, electrode materials of SCs can be classified into electric double‐layer capacitive (EDLC) materials and pseudocapacitive materials. EDLC electrodes store charges via ion accumulation to form electric double layers at the interface between electrode and electrolyte; and the most widely used EDLC materials are various nanocarbons with high specific surface area and relatively low cost.29, 30 Pseudocapacitive electrodes store energy electrochemically through surface/near‐surface reversible faradaic reactions.31 Transition metal compounds (Mn‐/Fe‐/V‐/Mo‐based oxides/hydroxides/sulfides, etc.), conducting polymers (such as polyaniline, polypyrrole, and their derivatives),32 and heteroatom (N, O, B, P, etc.) doping carbon‐based electrodes[[qv: 6b]] have been extensively investigated as pseudocapacitive electrode materials. It should be emphasized that pseudocapacitors must possess the basic EDLC‐type electrochemical features.33, 34 Simon et al. showed their worries about the confusion between battery materials and pseudocapacitive materials and underlined their fundamental electrochemical differences.19 In general, qualified pseudocapacitive materials should exhibit a combination of properties that include the followings: (i) strong faradaic reactions at/near surfaces enabling high capacitance; (ii) EDLC‐like electrochemical behaviors over relatively long ranges of potential, that is, near rectangular CV plots and linear potential–time response of charge–discharge curves; (iii) fast charge storage kinetics that offers high power density (>103 W kg−1). Nevertheless, there is also “intercalation pseudocapacitance” that occurs via bulk intercalation like batteries but on the same timescale as redox pseudocapacitance, as well elaborated by Dunn and co‐workers6, 35, 36 Conventional RuO237, 38 and MnO231, 39 and newly emerged materials such as Nb2O5,36 metallic 1T MoS2,40 LaMnO3,21 and MXenes41 are eligible pseudocapacitive materials exhibiting good capacitive features, as shown in Figure2. By contrast, many Ni‐ and Co‐based compounds that follow diffusion‐controlled electrochemical process in aqueous alkaline electrolytes and display apparent plateau in charge‐discharge curves cannot be considered as pseudocapacitive materials.
With the rapid development of portable electronic devices, smart products, artificial intelligence, and micro‐/nanosystems, multifunctional electrochemical energy storage devices with flexible/stretchable/foldable/wearable, transparent, and intelligent functionalities are highly demanded.26 The integration of active materials on unusual current collectors/substrates enables electrodes and devices with new physical or chemical functionalities that cannot be achieved with common current collectors/substrates. Among various functional energy storage devices, flexible, and transparent ones are particularly attracting in the near future due to their potential use in smart wearable electronics and optoelectronic systems.
In emerging fields such as electrification of transportation and smart miniaturized electronics, technologies require much larger amounts of energy to be stored within much shorter time but at low cost. Therefore, substantial increase of both the energy and power densities of energy storage systems is highly necessary. Replacing one capacitive electrode of a symmetric SC with a battery electrode allows the generation of an attractive BSH device that is with wider cell voltage and larger capacity (thus higher energy density). With appropriate battery‐electrode architecture design, BSHs would possibly have high power density approaching conventional SCs. In this review, we have addressed several existing and emerging kinds of BSH devices. Typical examples of these devices (cathode, anode, electrolyte, cycling performance, energy/power densities, voltage, etc.) have been summarized in Table1. With over twenty years of development, some technologies such as Li‐ion BSH have been successfully commercialized. Nevertheless, the overall performance of BSH devices (particularly the energy and power densities) is still not so competitive with advanced batteries and SCs. In addition, with the consideration of sustainable development of modern society, non‐Li‐ion BSHs with naturally abundant resources and multifunctionalities are indispensable; most kinds of BSHs are still at their early stage and need substantial advancement. Based on the above, several future trends and challenges are discussed here.