*1.4.1.1 Tungsten trioxide (WO3)*

Tungsten trioxide (WO3) is a noticeable anodic material for the intention that of its low-cost and rich oxidation states (W4+, W5+, W6+); WO3 has in modern times become visible as an apparent anodic electrode material in the development of pseudo-capacitive nature due to its exceptional electrochemical performance and global profusion [55, 56]. However, even though WO3 has well-known its potential as a proficient candidate for a widespread mixture of applications, it's an ideal applicant for thin film EES applications; the active material ought to contain high conducting nature and be capable to providing extraordinary electrochemical performance. Very few of reports on its presentation as an anodic active material in the assembly of a SC in addition to battery necessitate to further investigation in this pathway [57]. Recently author effectively achieved WO3 Nano structure decorated (**Figure 11a**) on the surface of thin films, grown in an in-situ annealed condition by using well established PLD coating unit. Furthermore High resolution transmission electron microscopy (HRTEM) investigation using the WO3 Nano particles with the morphology shown in **Figure 11b** in addition that the elemental distribution analyzes of W and O the color mapping images are shown in **Figure 11c** and **d**. From this contest author revealed that WO3 is the one of the opted anodic material for TFSC device fabrications.

#### *1.4.2 Cathodic materials*

Usually, TMOs such as Co3O4, MnO2, NiO, ZnO, V2O5, etc. are redox-active behavior and have been used as positive electrode materials for thin flexible energy storage. Most of the TMOs having good electronic conductivity, chemically stable, high theoretical specific capacities, low prices, abundance, and eco-friendly.

#### **Figure 11.**

*(a) FESEM morphology of WO3 Nano structures; (b) HRTEM Nano particles image; (c, d) HRTEM- EDAX color mapping images of W and O (Reprinted with permission from Ref. [28]. Copyright 2020 Royal Society of Chemistry).*

#### *1.4.2.1 Vanadium pentoxide (V2O5)*

Vanadium pentoxide (V2O5) is a well-known electrode active material for EES applications in the middle of vanadium family as stated by Whittingham et., Vanadium pentoxide has variable oxidation states (V5+, V4+, V3+, and V2+), permitting it to attain high capacity than the other TMOs and layered structure of V2O5 creates it highly striking for EES applications [58, 59]. V2O5 has also paying attention as an active material for improved green EES systems. V2O5 with diverse morphologies in an adequate particles and thin film Nano structures have been fabricated by a variety of methods. In particularly physical vapor deposition (PVD) techniques are promising tool for thin film Nano structure fabrication. Recently author fabricated V2O5 thin films by using thermal evaporation technique with different thicknesses such as 210 nm, 380 nm, and 540 nm respectively [46]. As fabricated films further gone to symmetric SC device assembly, further all devices subjected to investigate electrochemical studies. The thin film thickness of 540 nm (cross section **Figure 12b**) symmetric device showed better electrochemical performance as clearly indicated from CV curve shown in **Figure 12a**. Meanwhile, thin film electrodes annealed at 500°C showed redox active behavior than as-prepared film (**Figure 12c**). The post annealing condition is also important for SC device performance because the annealed film morphology (**Figure 12d**) clearly shows the larger grain size.

*Physicochemical Approaches for Thin Film Energy Storage Devices through PVD Techniques DOI: http://dx.doi.org/10.5772/intechopen.99473*

#### **Figure 12.**

*(a) CV curve comparison of V2O5 symmetric capacitors in different thicknesses; (b) FESEM cross sectional image of V2O5 thin film fabricated by thermal evaporation coating unit; (c) CV curve comparison of bare substrate and V2O5 thin film annealed at 500 °C in a three electrode configuration; (d) FESEM morphology of V2O5 thin film annealed at 500 °C (Reprinted with permission from Ref. [46]. Copyright 2019 American Chemical Society).*

In thermal evaporation technique, have some draw backs such as large molten materials are required for film fabrication. To overcome this issue author reported V2O5 thin film electrode fabrication by using PLD. PLD has some unique features such as layer by layer coatings, in-situ annealing condition, fine thickness control and inlet gases atmosphere while film fabrication. The author lastly reported work V2O5 Nano rods (**Figure 13a**) grown on flexible thin substrate with the help of PLD in an in-situ annealed 500°C at partial pressure atmospheric condition [28]. Further, the Nano structure investigation by using HRTEM is well agreed with Field Emission Scanning Electron Microscope (FESEM) morphology as displayed in **Figure 13b**, also the elemental distribution of vanadium and oxygen was uniformly distributed as presented in **Figure 13c** and **d**.

#### **1.5 Flexible electrodes for thin film energy storage**

As yet, it is still foremost contest to fabricate flexible thin electrodes with robustness mechanical belongings and outstanding electrochemical performance. The TFSC device fabrication current collector must be an essential tool to supply power to the active materials. Normally, conducting metal foils are used as substrates or electrodes for EEs devices [7, 60]. In particularly, TFSC device manufacture flexible current collectors can be needed; at the present time EES device fabrication usually used flexible electrodes such as 2 dimensional metal foils (Ti foil, Ni foil,

#### **Figure 13.**

*(a) FESEM morphological image of V2O5 thin film Nano rods grown by in-situ annealed at 500°C in a partial pressure atmosphere; (b) HRTEM Nano particle morphological image of V2O5 Nano rods; (c and d) HRTEM-EDAX color mapping images of V and O (Reprinted with permission from Ref. [28]. Copyright 2020 Royal Society of Chemistry).*

and stainless steel foil), conducting carbon clothes, and 3 dimensional arrays (Ni foam, cu foam, and graphite foam) have been widely used for the deposition of a combination of capacitive materials, conducting additives and binder. Nevertheless, metal foils are definitely corroded in aqueous electrolytes, which limits the lifetime of the devices [7, 30]. As a result, foregoing efforts have been attentive on the device design and fabrication of TFSC electrodes by way of non-metal materials. Even though, author used carbon paper substrates in aqueous electrolyte while fabrication of TFSC device used flexible Ni foam array is shown in **Figure 14a** and as prepared TFSC device shown in **Figure 14b**. In set **Figure 14b** clearly indicates Ni foam is one of suitable conducting flexible electrode for TFSC device manufacturing.

#### **1.6 Electrolyte for thin film energy storage**

The solid-state electrolyte is one of significant key components for fabrication of flexible TFSCs. In assessment to aqueous electrolytes, solid-state electrolytes are at ease to handle, and have superior reliability and an extensive range of working temperature. In addition, with a solid-state electrolyte can avoid a leakage issue, and consequently, which is reducing the device packaging cost [61]. The most extensively used solid-state electrolytes in TFSCs are gel polymeric mixture. A good solid state electrolyte is a non-toxic material, fabrication cost is low and with high ionic conducting nature, excellent stability, functioning at ambient temperature,

*Physicochemical Approaches for Thin Film Energy Storage Devices through PVD Techniques DOI: http://dx.doi.org/10.5772/intechopen.99473*

**Figure 14.**

*Photographical image representation at CSIR-CECRI, India (a) V2O5 thin film deposited on Ni foam substrate; (b) author group fabricated thin film device.*

better mechanical strength and an extensive potential window. In comparison gel polymer electrolytes exhibit superior ionic conductivity than dry solid-polymer electrolytes further down ambient conditions. Gel polymer electrolytes classically contains in a polymeric mixture as the host of an aqueous / organic solvent used as the plasticizer, and a secondary electrolytic salt. Poly ethylene oxide (PEO), poly vinyl alcohol (PVA), polyacrylonitrile (PAN) and poly (methyl methacrylate) (PMMA) are the maximum frequently used for preparing polymeric gel electrolyte mixtures. Author group reported fabrication of TFSC and Supercapbattery devices solid state PVA-KOH gel polymeric mixture was used [28, 46].

### **1.7 Significant parameters for estimating the device performance of flexible energy storage device**

There are two significant parameters for estimating performance of Flexible energy storage devices such as volumetric energy density and volumetric power density of a TFSC device can be evaluated by using Eqs. (1) and (2)

$$E = \frac{1}{2} \frac{C\_{cell} V^2}{3600} \left(\frac{m \,\mathrm{W} h}{cm^3}\right) \tag{1}$$

$$P = \frac{E}{\Delta t\_d} \times 3600 \left(\frac{m\,\mathrm{W}}{cm^3}\right) \tag{2}$$

Where Ccell is the specific capacitance of the TFSC device, V is the device working voltage and Δtd is the discharge time. Based on Eq. (1), to achieve high volumetric specific energy density and volumetric specific power density, there is a necessity to rise C and V even though reducing Rs. Make best use of the TFSC device specific capacitance and voltage window are straight approaches to magnify the volumetric energy density of TFSCs. The working voltage window is determined by the electrode active materials and electrolytes.

The dynamics of thin film solid state battery as well as Supercapbattery devices for estimating specific volumetric capacity from discharge rate performance can be evaluated by using Eq. (3)

*Management and Applications of Energy Storage Devices*

$$C\_v = \frac{i\Delta t}{3600\,v} \left(\frac{mAh}{cm^3}\right) \tag{3}$$

Where Cv is specific volumetric capacity of the supercapbattery device. υ is the volume of the thin film supercapbattery device and Δt is the discharge time. Author group reported the thin film supercapbattery device showed excellent rate performance and the device delivered maximum volumetric discharge capacity ~32 mAh cm−3 at a current density of 1.3 A cm−3 [28]. This is unique instance for thin film supercapbattery energy storage was stated via PLD system.

To investigate essentially meaningful volumetric energy and volumetric power densities of a TFSC device, it must be fabricated and examined as a widespread sized and enveloped device. The essential calculation of volumetric energy and volumetric power densities ought to be based on the total area as well as volume of the whole device together with the thin film electrodes, solid-state gel electrolyte, the separator, current collectors and wrapping materials. Author reported supercapbattery device delivered maximum volumetric energy density about 12.5mWh cm−3 is displayed in **Figure 15**. Furthermore, the thin film Supercapbattery device delivered the steady performance of cycle stability even if an assorted bending position is shown in **Figure 16a**. Finally, the flexible TFSC tested the practical viability by illuminating Blue Light Emitting Diode (LED) glow (**Figure 16b**) with the series combination thin film devices, TFSCs well thought-out to be probable candidates for use in biomedical and wearable Microelectronic applications.

#### **1.8 Reaction kinetic mechanism**

The supercapbattery device showed fast kinetics with good storage behavior. The investigated results are extremely specific and exciting in terms of stability, volumetric energy and power density. This development in the supercapbattery device characteristics are essentially attributed to the electrode fabrication where the PLD

**Figure 15.** *Ragone plot for thin film supercapbattery device.*

*Physicochemical Approaches for Thin Film Energy Storage Devices through PVD Techniques DOI: http://dx.doi.org/10.5772/intechopen.99473*

#### **Figure 16.**

*(a) Stability analysis of supercapbattery for different bent position; (b) photographic image representation for blue LED glow at CSIR-CECRI India (Reprinted with permission from Ref. [46]. Copyright 2019 American Chemical Society).*

#### **Figure 17.**

*(a) FESEM cross sectional image for V2O5 thin film fabricated by PLD; (b) EIS spectra for as fabricated supercapbattery device (Reprinted with permission from Ref. [28]. Copyright 2020 Royal Society of Chemistry).*

deposition process plays an important role in such a Micro/ Nano scale devices. In order to make such supercapbattery device, the charge and mass balancing is very much important to construct, however, it is challenging to balance the charge 100% in practical devices. Instantaneously, in thin film energy storage, balancing of the charge storage can be attained easily by controlling the film fabrication process with the help of advanced coating system. Author's present study, the mass of the thin film electrodes was optimized using the characteristics observed from the three electrode system. On other hand optimized thickness of thin film electrodes are playing very important role for device fabrication, here in author group fabricated thin film electrodes separately with the help of PLD and the thicknesses of WO3 and V2O5 thin film electrodes such as 1473 nm and 1075 nm is displayed in **Figure 17a**. Further this work reported total thickness of thin film supercapbattery device was 2.5 microns, even if the device presenting good conducting nature, Electrochemical Impedance Spectroscopy (EIS) is the best way to determining Resistance of any electrode or device. The thin film supercapbattery device showed very low charge transfer resistances Rct value 11.9 ohms it's clearly indicating EIS spectra is displayed in **Figure 17b**. Thus the supercapbattery device delivered better electrochemical performances.

## **1.9 Future scope**

Nano scale level thin film active materials brought significant improvement for the development of flexible thin film energy storage, Nano complex materials in the form of thin film facilitate accessible of electrolytic ions and an enhance the device rate capability. Nevertheless, an additional side reaction affected by increasing pore area must be taken into consideration for practical wearable and portable electronics. The flexible storage approach to combine in the form of thin film energy storage advantages of different active materials is a hopeful approach for forthcoming development.

Gradually thin film based composite energy storages demands have led to necessities for more specific functions in an electrochemical energy storage devices. Furthermore, outdated Supercapbatteries are undertaking modernizations in different directions to encounter the special necessities of modern society. Here, promising development ways for Supercapbatteries for future as follows

