Date Published: May 29, 2017
Publisher: John Wiley and Sons Inc.
Author(s): Bin Yao, Jing Zhang, Tianyi Kou, Yu Song, Tianyu Liu, Yat Li.
Paper‐based materials are emerging as a new category of advanced electrodes for flexible energy storage devices, including supercapacitors, Li‐ion batteries, Li‐S batteries, Li‐oxygen batteries. This review summarizes recent advances in the synthesis of paper‐based electrodes, including paper‐supported electrodes and paper‐like electrodes. Their structural features, electrochemical performances and implementation as electrodes for flexible energy storage devices including supercapacitors and batteries are highlighted and compared. Finally, we also discuss the challenges and opportunity of paper‐based electrodes and energy storage devices.
Flexible electronics have attracted extensive attention due to their potential for future hand‐held, potable consumer and wearable electronics. Electronic or optoelectronic components on flexible substrates enable novel applications, such as flexible display, electronic textile, artificial electronic skin, distributed sensors, etc.1, 2, 3, 4, 5, 6, 7, 8, 9 Whilst various flexible electronics are commercially available nowadays, the development of flexible energy and power sources for these devices is slow.10, 11 Such a “rate‐limiting step” has greatly impeded the commercialization of these electronics. In order to overcome this limiting factor, extensive efforts have been devoted to make flexible and high performance energy storage devices.12, 13, 14, 15, 16, 17, 18, 19
Paper, which is made of randomly interconnected cellulose fibers, is a product after a three‐step treatment of wood cellulose pulp suspension: dewatering, pressing and heating. Mineral fillers, such as calcium carbonate, chalk and clay, are usually mixed with the cellulose pulps to increase smoothness. To enhance the brightness of paper, fluorescent whitening agents (e.g. stilbenes) are added in the process of fabrication. The mechanical properties of paper can be readily tuned by adjusting the length, diameter and physical and chemical nature of cellulose fibers used for production. In general, papers made of longer and wider fibers are more robust than counterparts composed of shorter and thinner fibers.34 Various types of papers with different purposes (daily‐use office photocopying paper, weighting paper, filter paper, Kimwipes paper etc.) are widely used in daily lives. Besides mechanical properties, paper’s optical properties can also be changed. It has been reported that by filling inter‐fiber space using transparent materials (e.g., wax) with reflective constant that is closed to the cellulose (≈1.5), it is able to fabricate transparent papers.34 Reducing the diameter of cellulose fibers from micro‐meters (≈20 µm) to nanometers (≈20 nm) will increase paper transparency.52, 53, 54
Most of the electrodes for energy storage devices are generally made by mixing particulate active materials with polymeric binders e.g., polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE) and conducting agents (e.g. carbon black) with the help of appropriate solvent. The binders are used to glue the active materials and conducting agents as well as the current collector, whereas the conductive agent network helps to transfer electrons from active materials to current collector. Although this process is widely adopted in industry, the introduction of insulated binders increases both the contact resistance between particles as well as the resistance of the electrodes. Besides, the binders and additive conductive agents make up about 20–40% of the total electrode mass, which are seen as ‘dead mass’ because they do not contribute to charge storage, but instead decrease the energy density of both the electrodes and devices.102
The heavy demand of flexible electronics push the development of various flexible energy storage devices. Different types of energy storage devices have different scopes of application, depending on their electrochemical properties and working mechanism. For example, supercapacitors are suitable for electronics that required fast charging and discharging and long cycling stability, while lithium‐oxygen batteries fit the applications that require continuously and high energy supply. In this section, four different kinds of flexible paper‐based energy storages devices, including supercapacitors, Li‐ion batteries, Li‐S batteries and Li‐O2 batteries, will be introduced and discussed.
The rapid development of flexible electrodes has open up new opportunities for flexible energy storage systems and wearable electronics. Among them, paper‐based electrodes as a new class of electrode configuration have attracted a lot attention. In this review, we have summarized the recent advances in this area, with a particular focus on the methodology of fabricating these novel functional electrodes, including paper‐supported electrodes and paper‐like electrodes. Finally, in this section, we will discuss some key challenges and opportunities in this fast growing field. First, paper‐based electrodes have been widely used in flexible energy storage devices such as supercapacitors and Li‐ion batteries. However, their application in Li‐S and Li‐O2 batteries, as well as some new types of energy storage system like Na‐ion batteries, Mg‐ion batteries has been rarely investigated. In exploring the potential use of paper‐based electrodes in these areas, some issues should be considered. For example, to use paper‐based electrodes for Li‐S batteries, the electrode is required to have not only good electrical conductivity and flexibility, but also strong affinity with sulfur. When they are used in Na‐ion batteries and Mg‐ion batteries, a critical problem would be whether these paper‐based electrodes is mechanically/structurally robust that can accommodate large volume change during Na‐ion and Mg‐ion insertion and desertion, since these ions are much larger than Li‐ions.
The authors declare no conflict of interest.