Organic Electronics for Electrochromic Materials and Devices. Hong Meng
by dissolving metal salts in polar polymer hosts, which could be used to replace the liquid ionic solution. Liquid electrolytes possessing high ionic conductivity have several inevitable drawbacks such as the possibility of electrolyte leakage, low chemical stability, hydrostatic pressure considerations, and difficulty in assured sealing and are unsafe for practical applications especially in scaling‐up processes [8]. Comparing with liquid electrolytes, PE has several prominent advantages, such as high ionic conductivity, safety, flexibility, wide electrochemical windows, and so on [9, 10].
2.2 Requirements of Polymer Electrolytes in Electrochromic Applications
Factors that lead to the use of PE in an EC devices are as follows (Figure 2.1).
High optical transparencyHigh optical transparent PEs are highly desirable for EC applications. High transmittance enhances the transparency of an EC device in the bleached state. Electrolytes cannot affect the optical shifts during colored and bleached states.
Flexible and mechanically stretchablePE owing to flexible and mechanically stretchable advantages can be better applied in ECDs, especially in flexible and stretchable devices. The polymeric matrix is of vital importance. The PE requires to keep mechanical stability during bending and stretching states.
Multifunctional separatorThere is a direct contact between the two electrodes without any separator, which will lead to a short circuit. PE can act as a separator layer in an EC device, making these devices light and compact without the need for additional separators. Moreover, PEs demand intimate binding effect, making favorable electrical contact with the electrodes and good adherence to EC layers.
Wide range of working temperaturePEs inherently exhibit the ability to operate over a wide range of temperature as compared to the other counterparts.
Cation coordination abilityPolymers in electrolytes with benign cation coordination ability can better make for coordination bonding interactions with the metal cations.
Wide operation potential windowStable electrochemical window is the range of working potential in an EC device without breakdown of electrolyte itself. The maximum potential is determined by the potential of the oxidation reaction. Meanwhile, the minimum potential is determined by the potential of the reduction reaction. To ensure the long‐time and cycling stability of the ECD, the working potential window of a device should be considered in the electrochemical operating range of the electrolyte.
Thermal, photo, chemical, and electrochemical stabilityAn ideal PE should possess good thermal and photo stability. During electro chromic processes, the device may release heat, which may result in degradation of the EC devices. Similarly, an ideal electrolyte with photo stability prevents device from being destroyed via prolonged exposure to air. It is necessary to keep good electrochemical stability in the voltage range for ECDs. Undesired interaction effects between the EC layer and electrolyte should not exist in the ECD during EC processes so that devices can keep reversible and long lifetime performance for practical application. This chemical stability must be ensured both during the deposition and during the cycling processes. Currently, small molecule EC materials‐based ECDs have appeared in lots of researches. However, this kind of ECD mostly can be operated under higher potential. Therefore, PEs with wide potential window are demanded.
High ionic conductivity (>10−4 S/cm) with low electronic conductivityPEs should be a good ionic conductor and electronically insulating (σe < 10−12 S/cm) so that ion transport could be facilitated to minimize self‐discharge. In EC process, the essence of color change is the transfer of ions into and out of an EC film. The electrolyte should also maintain its high ionic conductivity even after thousands of cycles. The flexibility of polymer matrix chains in the amorphous phase allows ions to be transported frequently. This easy transport of ions is hindered in the crystalline phase, where the material is densely packed and there is not enough space to allow rapid transport of ions [11]. The ion transference number is important in the characterization of PEs. A large transference number can reduce concentration polarization of electrolytes during charge–discharge steps [12].
Easy to synthesize with low costThe price of PE cannot be the factor of resulting in hindering practical application. Inexpensive, easily available PEs are highly necessary for the successful commercialization and implementation of EC devices. It is worth mentioning that the thickness of PE film should be controlled and easy to operate.
Safe and environmentally friendlySafe and environmentally friendly are highly desirable for practical application of the ECDs. So far, ECDs have been widely used in people's daily lives, such as EC glasses, car mirrors, and cockpit glasses. A nontoxic, environmental, and recyclable electrolyte is a top priority in an EC device, which cannot jeopardize our health.
Figure 2.1 Main concerns in electrolyte.
2.3 Types of Polymer Electrolytes
PEs are formed by dispersing a salt into a neutral polymer matrix. The composition of the matrix plays a great role in mechanical strength, electrolyte/active material contact, flexibility, and processability in ECDs. PEs applied for ECDs can be mainly classified into gel polymer electrolytes (GPEs), self‐healing PEs, cross‐linking polymer electrolytes (CPEs), ceramic PEs, ionic liquid (IL) PEs, and gelatin‐based PEs.
2.3.1 Gel Polymer Electrolytes (GPEs)
GPE also known as plasticized PE was first introduced by Feuillade and Perche in 1975 [13]. GPEs are usually synthesized by incorporating a larger quantity of liquid plasticizer and/or solvents into a polymer matrix to form a stable gel with the polymer host structure, having relatively higher ambient‐temperature ionic conductivity [14]. Generally, GPE consists of an ionically conducting medium polymer such as poly(ethylene oxide) (PEO) and a metal salt (lithium) swollen with a suitable solvent. GPE could exhibit fast ion diffusive nature and cohesive properties comparable to solids [15]. The first one is known for its high conductivity with the H+ donors originating from, e.g. sulfuric (H2SO4) or phosphoric (H3PO4) acid [16]. In the second group mobile Li+ species are provided by dissolution of lithium perchlorates (LiClO4) [17], triflates (LiCF3SO3) [18], fluorophosphates (LiPF6) [19], or fluoroborates (LiBF4) [20] in protic solvents (propylene, acetonitrile [ACN], ethylene carbonates, etc.). As was reported, binary or ternary solvents, such as EC + PC and DMC + PC + EC, were also employed [21, 22]. These types of electrolytes are characterized by a higher ambient ionic conductivity but poor mechanical properties. Although most PGEs have high ionic conductivity of 10−3 S/cm at room temperature, poor mechanical properties and considerable viscosity inevitably result in internal short circuit and cell leakage. UV and heat irradiation are other potential factors for degradation of electrolytes. Plasticizers such as PC, EC, diethyl carbonate (DEC), and dimethyl carbonate (DMC) can be applied in developing the physical characteristics of the overall blend. Plasticizers improve the ionic conductivity by increasing the amorphous phase content, dissociating ion aggregates, increasing ionic mobility within the gel electrolytes, or lowering the glass transition temperature (Tg) of the system [23]. The balance between increasing ionic conductivity and decreasing mechanical strength needs to be maintained while plasticization occurs. Gel electrolytes have been used in many applications but most of recent published papers are about EC applications. In the GPE, the immobilized solvent in the polymer matrix has strong effects on the ionic conductivity. So far, PEO, poly methyl methacrylate (PMMA), poly(vinylidene fluoride) (PVDF), poly(vinylidene fluoride‐co‐hexafluoropropylene) (PVDF‐HFP), and poly acrylonitrile (PAN)‐based gel electrolytes have aroused researchers' interests [24, 25]. Recently, some of gel electrolytes have aroused researchers' interests, such as poly ethylene