Transparent Electrolytes: A Promising Pathway for Transparent Energy Storage Devices in Next Generation Optoelectronics

Anjumole P. Thomas, Akhila Das, Neethu T.M. Balakrishnan, Sajan Chinnan, Jou-Hyeon Ahn, Jabeen Fatima M. )., Prasanth Raghavan


To address the energy crisis, the scientific world is exploring renewable energy resources such as wind energy, hydro energy, solar energy, tidal energy etc., as well as materials with high performance at a low cost. As green energy, the advantages of lithium-ion batteries (LIBs) [1, 2] and supercapacitors [3] include large energy density, power density, long service life, and no environmental pollution. But most of the energy storage devices are rigid and do not meet today’s stringent requirements; therefore, flexible and transparent energy storage systems have been increasingly used for portable wearable devices, light emitting diodes [4], transistors [5], energy storage smart windows [3], gas sensors [6], and so on.

Transparent electronics are an emerging and promising technology that recently have gained more attention in various applications, such as in touch screens, TV displays, and solar cells [7]. All these electronic devices are inseparable from the support of energy storage devices, such as batteries and supercapacitors [8]. Typically, an energy storage device consists of electrodes, electrolytes, current collectors, separators, and packaging [9]. In order to make the whole device transparent, the first step is to make or use a transparent electrolyte, which is the major component of any energy storage device [7]. None of the other components are transparent, so a fully integrated energy storage device cannot be visualized without reducing the size of the active materials. Small devices possess a low voltage window that may not be practical for the above-mentioned applications [8]. For the successful use of transparent electrolytes, it is possible to use a series of energy storage devices to scale up the charge density and that can be used in modern applications [9].

Transparent electronic devices appear in a substantial number of applications including portable electronic devices and solar cells [10]. The concept of a fully transparent cell phone has attracted great attention recently. Transparent screens also have experienced rapid development in modern technological gadgets; however, the key to developing a transparent cell phone is to overcome the problem of a fully transparent battery. However, the research and development of transparent LIBs are still at an infant stage. The typical method to make a transparent device is to decrease the thickness of the battery. A commonly used method for making transparent devices is to reduce the thickness of active materials as demonstrated in carbon nanotubes [11, 12], graphene, and organic semiconductors. Low cost, safety, and high energy density are important factors in the development of energy storage devices. Solid- state lithium batteries are expected to be safe for rigorous use applications because a nonflammable solid-state electrolyte (SSE) is used instead of the flammable organic liquid electrolytes used in conventional LIBs [13]. This solid-state electrolytes can be either ceramic-based (solid ceramic electrolytes, SCEs) or polymer-based (solid polymer electrolyte, SPE) electrolytes

Suzuki et al. [13] reported that a Garnet-type Li7La,Zr2012 (LLZ) is a better transparent electrolyte candidate material for the LIBs because of its high stability and high lithium-ion (Li+-ion) conductivity at room temperature. LLZ is stable in contact with lithium metal and has an excellent electrical conductivity at room temperature [14]. Transparent 1.0 wt% Al20,-doped LLZ (A-LLZ) was prepared using hot isostatic pressing (HIP) treatment. Through HIP treatment, the relative density of the LLZ pellet was increased from 91.5% to 99.1%. The HIP-treated samples have both white and transparent areas, showing electrical conductivity of 9.9 x 104 S cm4 at 25° C. LLZ is an attractive material for future LIBs because of its high stability and Li+-ion conductivity at room temperature [15]. Puthirath et al. [16] reported a novel transparent flexible Li+-ion conducting solid polymer electrolyte (SPE) based on poly-dimethyl siloxane (PDMS) and polyethylene oxide (PEO) polymer blend. Advantages of atomically smooth PDMS and flexible high solvation matrix PEO for Li+-ion is considered for material selection in developing SSEs [17, 18]. PDMS holds the SSE as a laminated surface and provides high hydrophobicity to the Li+-ion conducting medium [16]. A synergistic effect, a down shift in percentage crystallinity and crystalline melting point of matrix polymer of SSE upon the addition of lithium salt such as LiPF6, LiC104, or LiBOB, is observed, which is due to the salting-in phenomenon. This improved the ionic conductivity and electrochemical performance of the SSE without affecting lithium transference number. Because of the 3D network structure formed by the cross-linking reaction, PDMS holds the SSE as a laminated surface by providing high water contact angle (WCA), ensuring the durability of SSEs for ambient conditions. A completely transparent and flexible supercapacitor can be realized from this SSE.

Pan et al. [19] reported a gel type LIB with a long cycle enabled by a dense transparent polymeric single-ion conductor. Polymer electrolytes were synthesized using the side-chain grafting method with 4-amino-4’trifuorolmethyl bisfbenzene sulpho- nyl)imide grafted on side chains of poly(ethylene-alt-maleic anhydride) with a grafting ratio of 50%. Blending lithiated side-grafted poly(ethylene-alt-maleic anhydride) with poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-co-HFP) via the solution cast method results in dense transparent film. The fabricated polymer-blend- electrolyte film has the ionic conductivity of 0.104 mS cm-' at room temperature, a tensile strength of 15.5 MPa, and percentage elongation at break of 5%. A gel type single-ion conductive polymeric LIB was assembled using the blended film as the separator as well as the electrolyte, LiFeP04/C mixed with ionomers as the cathode, and a lithium foil as the anode. The battery delivers a reversible discharge capacity of 100 mAhg-' at 1 C under room temperature for 1,000 cycles. The stable cyclic imide and comb-like structure are responsible for the excellent battery performance. Side-chain grafted single-ion conducting polymer electrolytes are well suited for large-scale production.

Fuentes et al. [20] reported that transparent conducting polymer electrolyte films were prepared by adding LiC104 to a mixture of chitosane and poly(amino propyl siloxane) in a molar ratio of 0.6:1 by sol-gel method. Molecular and morphological studies carried out by Fourier-transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) depend on the lithium concentration. About 0.8 mol lithium salt per chitosan can be added before the product lost transparence and molecular compatibility of the pristine chitosan/poly(amino propyl siloxane) polymer complex. When lithium salt exceeds the concentration limit, anisotropically oriented patterns were observed in the film. Self-supporting films obtained have transmittance of 80% in the visible spectral range. Transparency and ionic conductivity of the product is due to the layered nature of the nanocomposite.

Supercapacitors or electrochemical capacitors gained increasing attention as compared to traditional capacitors because of their large capacity, specific energy, wider working range, and longer service life [21]. With the development of modern electronic device demands, supercapacitors must be transparent, flexible, and fold- able [22, 23]. In order to make fully transparent supercapacitors, each component (electrode, separator, current collector, and electrolyte) must be transparent. Many researchers are working to make completely transparent and flexible supercapacitors for optoelectronics. Rodriguez et al. [24] developed a transparent PVP-LiC104 SSE using the dip-coating method, and its temperature dependence, aging time, lithium- salt concentrations, etc. are studied. Of all the types of conjugated polymers, PVP has attracted special attention because of its good environmental stability, easy processing, and excellent transparency. This electrolyte was used in the fabrication of electrochemical supercapacitors (PEDOT/PVP-LiC104/PEDOT). Ionic conductivity increases with temperature reaching a value of 3.34 x 10-3 S cnv1 at 60° C for a

LiC104/PVP weight ratio equal to 1.2. At higher LiC104/PVP ratios, conductivity decreases.

Wei et al. [25] reported a transparent, flexible, and solid-state supercapacitor based on room temperature ionic liquid (RTIL) gel and indium tin oxide (ITO) electrodes coated in a transparent polymer substrate without a separator, which enables the roll-to-roll technique for fabrication of supercapacitors used as printable devices. This was the first type of transparent electrochemical double layer capacitor (EDLC) based on ionic liquid gel, which introduced an environmentally friendly, safe, transparent electrolyte based on RTIL gel that is made of l-butyl-3-methylimidazolium chloride [BMIM][C1] and cellulose. RTILs are molten salts with a melting point close to or below room temperature. They are composed of ions of opposite charges that only loosely fit together. The supercapacitor reported in this paper using [BMIM][C1] and cellulose gel composite is a completely transparent and flexible electrochemical device that can be bent and twisted.

Jung et al. [26] reported creating transparent, flexible supercapacitors by assembling nanoengineered carbon electrodes, prepared in porous templates, with morphology of interconnected arrays of complex shape and porosity. Highly textured graphitic films act as electrodes and integrate with polymer electrolytes that act as a thin film supercapacitor. Solid polymer electrolytes with conformal electrolyte packing provide excellent mechanical stability and optical transparency to the supercapacitor. The design of the devices allows for mechanically flexible energy storage devices that could be integrated into unique applications that require high form factor and optical transparency, such as roll-up displays, wearable devices, and organic solar platforms.

High-performance SSEs are a major challenge for some applications in batteries, solar cells, fuel cells, etc. [27]. In the case of fuel cells, the electrolytic membrane should possess a medium-range operating temperature for many applications [28]. Vioux et al. [29] reported a versatile heat-resistant polymer electrolyte in which room temperature ionic liquids (RTILs) are confined in a porous silica matrix prepared using one-step nonhydrolytic sol-gel route. Ionogels are prepared from a mixture of tetramethoxysilane (TMOS), formic acid (FA) and RTIL. Ionogels are a versatile family of transparent, temperature-resistant SSEs and can operate in a wide range of temperatures.[30], A protonic conductive membrane is a very important element for polymer electrolyte fuel cells (PEFCs), while it is also applicable for proton sensor, separation, acidic catalyst, and so on. Honma et al. [31] and Advani et al. [32] reported transparent proton conducting hybrid SSE for proton exchange membrane (PEM) fuel cell (PEMFC). This chapter focuses on the preparation and properties of transparent electrolytes for different energy storage devices such as LIBs, super capacitors (SCs) and fuel cells (FCs).

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