Flexible Energy Storage Devices Using Nanomaterials

Flexible electronics is the most recent upcoming technology in the electronics world. These flexible products include mobile phones, displays, surgical tools, measuring sensors, implantable sensors, automobile sensors, agricultural devices, environmental monitoring sensors and strain gauges [23]. In real-time operation, a flexible device has to satisfy major demands such as performance, low cost, flexible electrodes, more reliability, safety, high stability and longer life time. However, the structural design can also hamper the device stability, performance and its durability. The recent advancements in battery technology have been provoked by new technologies such as portable electronic devices with a compact size and low cost [24]. Reducing the battery weight and size, increasing the capacity and performance and obtaining more flexibility are possible when nanotechnology can be incorporated with battery technology [25]. By combining these two methods, the battery efficiency is improved drastically by exploiting various materials at the nanoscale that include nanoparticles, nanotubes and nanowires [26]. Nanomaterial research has changed the way things operate in the energy sector, particularly in the harvested energy storage, flexible electronics and wearable electronics.

Materials for Flexible Electrodes

Li-ion batteries are viewed as the new pitch of experimentation for flexible ESDs [27]. Li-ion batteries do not change their crystalline structure while charging and discharging. If a change occurs in the crystalline structure of the material, the life time and number of cycles will reduce to a great extent. So, in maintaining the electrode plate properties in a stable manner, the nanomaterials play a crucial role [28]. Nanomaterials such as nanotubes are easily implanted with Li-ions to achieve improved charge capacity of batteries, lack of reactivity with materials, long lifecycle and higher electron mobility. Also, they provide better flexibility under pressure.

Carbon Nanotubes

Carbon nanotubes (CNTs) are used in batteries due to their high elasticity, less weight and low density [29]. CNTs naturally exhibit good structural stability under loading conditions without structural deformation [30]. The nature of CNTs is

FIGURE 8.10 Inconel CNTs implantation on electrode.

super-compressibility under 15% strain, and once the strain is removed, the structure will retain its original position. Hence, CNTs can turn out to be the ideal materials for flexible battery applications that would benefit from their characteristics of light weight, structural stability and a combination of electrical conductivity, chemical and thermal stability [31]. The deposition of CNTs into a current collector is very difficult because the development of CNTs is a high temperature process and it requires less catalytic interaction. The substrate needs to be grown under well aligned conditions at high temperature. There are many successful CNT implementations reported with different materials such as Inconel and stainless-steel substrates [32]. A well- grown CNT anchored into a substrate is expected to provide excellent electrical conductivity. Figure 8.10 shows the Inconel implementation on an electrode. A large temperature range between 500°C and 820°C should be available for the growth of an Inconel substrate. The maximum growth rate is attained at 770°C at a rate of 2.8 mm/min [33].

Nanocomposites

CNTs are integrated into nano-polymer composites that are bound to have extreme mechanical properties for flexible storage devices. These composites are employed as various smart flexible devices in a wide range of electrical and electronic products [34]. In recent years, CNT-polymer (CNT-P) composites have been tested in supercapacitors and batteries to improve their mechanical stability [35]. A CNT-P consists of an ionic liquid at room temperature, cellulose and CNT, and it fits the characteristics of a spacer, an electrode and an electrolyte, and this provides better flexibility to the batteries [36,37]. The flexible battery devices are assembled as shown in Figure 8.11. A thin film of Li deposited by thermal evaporation is used as an electrode. A Li-based flexible battery is manufactured using a basic building block method. This battery consists of a Room Temperature Ionic Liquid (RTIL), nano-composite film and Li-metal layer. The cellulose layer acts as a spacer between the electrodes. So far, we have discussed the non-conducting polymers mixed with

FIGURE 8.11 Flow diagram for the fabrication of flexible battery devices.

CNTs. An improvisation in this technology has been proposed where conducting polymers are integrated with CNTs to improve conductivity [38].

Hybrid Nanostructures

Hybrid nanostructures can be effectively used in flexible battery manufacturing. In this process, nanotubes developed using multi-segmented nanowires are implanted in electrodes, and this helps in reducing the internal resistance, increasing the conductivity power density. In the manufacture of nanotubes, different methods using nanowires and nanoparticles are developed. The most commonly used method for developing a single dimension crystalline is the template method. This method is supposed to have more flexibility in terms of different varieties of nanomaterial arrangements. But growing of CNTs in a metal layer is very challenging, but this unique method of fabricating multi-segmented nanowire/nanotubes gives a pathway to develop CNTs on metal sheets, which can be further used for manufacturing flexible batteries [39]. Two and three segmented hybrid structures of CNTs are also made using the template method. This is achieved with the help of chemical vapor deposition and electro chemical deposition. A metal-CNT hybrid gives good conductivity as the metal sheet gets connected with all CNTs. This enables reduction of resistance and internal heat; hence the devices are able to store high energy [40]. As showm in Figure 8.12, porous anodized alumina (AAO) is prepared using a modified two- step anodizing process. Then, Au nanowires are deposited by the electrodeposition method on the AAO sheet. The remaining portion of the AAO template is deposited with CNTs by pyrolysis of acetylene at 650°C at a rate of 35 ml/min [41].

The intermetallic alloys are also explored for manufacturing flexible batteries, as they are expected to have large charge capacity but their number of cycles is lower.

FIGURE 8.12 CNTs deposited in the pores of an AAO template.

There are different suitable elements available that can form an alloy with Li, B, As, Sb, Sn, Pb and Mg. Likewise, different inactive materials such as Fe, Ni and Co are used in battery manufacturing. Therefore, still lots of elements are required to be explored to make more flexible batteries along with large charge capacity.