Chemical Routes to Graphene-Based Flexible Electrodes for Electrochemical Energy Storage - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 417

The fabrication of such devices requires the development of new type of batteries
or supercapacitors that are flexible as their power sources. Typically, batteries and
supercapacitors consist of several major parts, which are electrodes (including a
positive electrode and a negative electrode), separator, and electrolyte, in which
two electrodes are spaced by a separator, and all these parts are soaked in an
electrolyte solution or gel [ 5 ]. A conventional electrode for battery or superca-
pacitor is fabricated on a metal substrate that is coated with a thin layer mixture of
active material, electrical conductor, and binder, as illustrated in Fig. 1 . This kind
of electrode configuration is not suitable for flexible or bendable application
conditions, because the heterogeneous interface between the active material layer
and the metal substrate is not strong enough to endure the shape changing process
[ 4 ]. The active material layer will be damaged or peeled off from the substrate
thereby causing the rapid performance fading.
Recent reports show that thin film or paper-like materials could be adopted as
key elements for application in flexible energy storage devices; several routes
toward the development of flexible batteries are being explored [ 6 - 9 ]. A range of
studies focus on batteries developed for disposable-card applications. However,
primary batteries can only produce current by an irreversible chemical reaction
and cannot be recharged, which therefore hindered their application in portable
electronic equipment [ 1 ]. Secondary Li-ion batteries (LIBs) and supercapacitors
are generally used to power portable electronic equipment [ 10 - 15 ]. Fabrication of
LIBs and supercapacitors into thin films while maintaining their high energy and
power density is crucial for their application in flexible devices. Making a flexible
LIB requires the development of soft electrode containing electrochemical-active
materials, such as metal oxide nanoparticles for cathodes and nanocarbons for
anodes, while flexible supercapacitor requires electrodes with high surface area for
electro double layer capacitance (EDLC) or high electrochemical activity for
pseudocapacitance [ 16 - 19 ]. There has been a great number of research works on
the development of electrode materials designed for LIBs and supercapacitors in
regular use conditions, in which conventional electrodes fabricated on metal
substrates are applied, and tremendous achievements have been accomplished [ 13 ,
20 - 27 ]. But the researches on the flexible electrode for the application in the soft
or bendable LIB and supercapacitors have just been carried out in the very recent
years.
For flexible devices, polymers can be used as substrate [ 4 ]. However, most
polymers cannot offer stability for electrode fabrication, since they degrade easily
at a wide range of environments. Also, poor adhesion between polymer and oxide
materials would be problematic upon long-term battery cycling. The free-standing
paper-like carbon-based materials, featured by light-weight, flexible, and high
conductivity, appear to be promising electrodes for wearable or rolling-up devices.
Flexible electrode materials made of carbon nanotubes (CNT) and their compos-
ites have been extensively studied [ 6 , 28 , 29 ]. Although these binder-free elec-
trodes afford wonderful mechanical properties and desirable electrochemical
properties, the still high production cost of CNTs and the difficulty in making