Organic Electronics for Electrochromic Materials and Devices. Hong Meng

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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 (T_g) 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 oxide (PEO), polyethylene glycol (PEG) [26], boronate esters [27], sodium polystyrene sulfonate [28], waterborne polyurethane, [29] polyester and PEO [30], polyvinyl alcohol (PVA), PAN, PMMA, poly(ethyl methacrylate) (PEMA) and poly(2‐ethoxyethyl methacrylate) (PEO EMA), [31] poly vinyl chloride (PVC), poly(vinyl sulfones) (PVS), and PVDF [32].

      2.3.1.1 PEO‐/PEG‐Based Electrolytes

      PEO (with a molecular mass above 20 000 g/mol) or PEG (with a molecular mass below 20 000 g/mol) simply refers to an oligomer or polymer of ethylene oxide. PEO‐based electrolyte is the earliest and most widely studied matrix for the formation of PE. Since Armand proposed a PE based on polyethylene oxide (PEO) for lithium batteries in 1978, research in this field has been extensively carried out [33]. Polyethylene oxide (PEO) has a polar ether group and significant segment mobility. As a wide range of salt polymers, especially high lithium ion stable polymers, it has excellent compatibility [34]. But because of high crystallinity, low melting point, limited operating temperature range, low hydroxyl ion migration number, poor interface characteristics, and other shortcomings, it fails to achieve the desired effect. In PEO‐based electrolytes, the most common method of increasing ion dissociation is the use of low lattice energy salts and to add fillers, which may prevent formation of polymer crystals, resulting in fast ion transportation via an interaction between the fillers and electrolyte [35]. PEO–H₃PO₄ and (PEO)₈–LiClO₄ were studied for electrochromism in devices [36]. However, the low Li⁺ conductivity inhibits its applicability. Later, PEO–Li_SO₃CF₃ and PEO–LiN (SO₂CF₃)₂ were reported to reach higher ion conductivities [37, 38]. PEO used as electrolyte in the gel form with an ionic conductivity of 2 mS/cm was reported in WO₃ film ECD, which exhibited superior EC performance and memory characteristics [39]. In addition, PEO‐based GPE plasticized with ethylene carbonate/propylene carbonate or N‐butyl‐3methylpyridinium trifluoromethane sulfonylimide (PTFSI) has been studied by Desai et al. for the ECD [40]. Ionic conductivity can be significantly enhanced and the phase separation of the PEO and plasticizer was inhibited. Yang et al. studied the performance of the EC device prepared using poly(2,5‐dimethoxyaniline) (PDMA) and tungsten oxide (WO₃) as electrode materials and PEO‐LiClO₄ as GPE plasticized with polycarbonate [41]. Similarly various other hybrid electrolytes based upon PEO were successfully applied in EC applications [42].

      2.3.1.2 PMMA‐Based Polymer Electrolytes

Due to high degree of crystallization, low ionic conductivity at ambient temperature exists in PEO‐based PE [43]. PMMA along with PVDF PEs have recently caused great attention in EC application. PMMA including amorphous phase and flexible backbone could increase ionic conductivity. As was reported, PMMA can provide a high transparency, excellent environmental stability, good gelatinizing properties, high solvent retention ability, and excellent compatibility with the liquid electrolytes [44, 45]. Moreover, PMMA shows good interfacial stability toward electrodes and high solvation ability to form complexation between polymer and salt. Highly ionic conductive PMMA exists as a gel and is mainly used for electrochromism [45]. Anderson et al. majored on ECDs with films of W oxide and vanadium pentoxide and an intervening layer of a PPG–PMMA–LiClO₄ [46, 47]. Reynold and coworkers first reported a polymeric ECD using poly(3,4‐proplenedioxythiophene) (PProDOTMe₂) and poly[3,6‐bis(2‐(3,4‐ethylenedioxy)‐thienyl)‐N‐methylcarbazole] (PBEDOT‐N‐MeCz) СКАЧАТЬ