Ionics (2011) 17:479–483 DOI 10.1007/s11581-011-0563-1
REVIEW
Natural polymer-based electrolytes for electrochemical devices: a review Pradeep K. Varshney & Shikha Gupta
Received: 8 February 2011 / Revised: 30 March 2011 / Accepted: 15 April 2011 / Published online: 6 May 2011 # Springer-Verlag 2011
Abstract Polymer electrolytes are an important component of many electrochemical devices. Researchers have carried out a significant work for the development of polymer electrolytes. This paper reviews the recent developments in the area of polymer electrolytes using aqueous and nonaqueous-based natural polymers for developing a cheaper, ecofriendly, biodegradable, and widely used electrolytes as a substitute for existing synthetic polymer electrolytes. This paper also encompasses the merits and demerits of the different natural polymers used by the researcher. There is a scope to develop a nonaqueous-based natural polymer electrolyte as an alternate for synthetic polymer electrolyte for batteries and other electronic devices. Keywords Composite polymer electrolytes . Polymer electrolytes . Natural polymer electrolytes . Electrochemical devices
Introduction The production, storage, and distribution of energy are imperious necessities for the industry and modern society in general. Moreover, fields as aerospace, electronic circuitry, or the need for new architectures for computers require more and more devices as solid-state batteries, sensors, and P. K. Varshney : S. Gupta (*) Department of Chemistry, Faculty of Engineering and Technology, Manav Rachna International University (formerly CITM, Faridabad), Faridabad, Haryana, India e-mail:
[email protected] P. K. Varshney e-mail:
[email protected]
portable electrochemical devices. In such a context, the development of solid-state electrolytes is a central challenge to be faced by scientific research [1, 2]. In this regard, the best candidates for such applications can be materials as ceramics, polymers, hybrids, or gels [3]. In recent years, surveys have emphasized the development of solid polymer electrolytes (SPEs) as they offer some advantages under liquid electrolytes, such as higher temperatures of operation, no flowing and corrosion after damage, and ease of application to electrochemical devices. As already described in many papers and books, the SPEs are solid or gel ion-conducting membranes consisting of a salt dispersed in a polymer matrix forming the ionically conducting solid solution. During the last two decades, different systems have been extensively studied, and the most commonly studied polymer electrolytes are the complexes of Li salts with high molecular weight polyethylene oxide (PEO) [3]. PEO qualifies as a host polymer for electrolytes because of its high solvating power for lithium salts and compatibility with lithium electrode [4]. However, one of the major drawbacks of PEO-based solid polymer electrolytes is their low ionic conductivity (10−7–10−8 S/ cm) at ambient temperature, which limits their practical applications [5–7]. The commonly used amorphous polymers include Poly Acrylonitrile (PAN) and Poly Methyl Methacrylate (PMMA), and the high conductivity exhibited by them is due to “gel formation,” the polymer network encaging the liquid electrolyte. PMMA gel electrolytes exhibit high conductivity and good electrochemical stability; where their electrochemical stability window is reported up to 4.5 V (vs Li/Li+). PAN is reported to interact more with the liquid electrolyte taking active part in the conduction mechanism, while PMMA is regarded as more passive in nature [8, 9]. However, reasonable conductivity achieved by such plasticized film is offset by poor
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mechanical properties at high plasticized content [10]. Moreover, with long-time uses, leakage of organic liquid solvents from the polymer electrolyte may take place, which decreases the ionic conductivity with damage to the lithium electrode and other components. Fortunately, the problems can be effectively circumvented by proper dispersion of nano- or submicron-sized inorganic fillers to the polymer matrix, and it has been shown that fillers may greatly influence the properties of polymer electrolytes [11]. On the other hand, Bellcore group [12] proposed a different technique to synthesize GPEs wherein polymer membrane is soaked in electrolyte solution. This system can be more specifically described as a heterogeneous phase separated polymer electrolyte membrane. Advantages of this process lie in the fact that the critical moisture control in this process is required only at the time of assembling the cell and mechanical strength is retained as compared to that of solvent cast membranes. But unfortunately, the polymer membranes prepared by this technique undergo poor rate-capability [13]. The most important property of these electrolytes is their ionic conductivity that is due to the segmental motion of the polymer chains that continuously creates free volume into which the ions migrate, and this process allows them to propagate (Fig. 1). In such a system, the population of ions and electrons must be large in order to avoid problems with migration and Ohmic resistance. Most of the investigated SPEs are based on ion-conducting species, but also protonconductive polymer electrolyte systems based on polyacrylates or polymethacrylates have been reported [14, 15]. In most cases, these gel electrolytes are composed of a polymer matrix swollen with a solution of proton donor in a polar solvent and containing redox sites. Also in comparison to electrolytes with alkali metal salts, proton conductors are characterized by higher dynamics of ionic transport [16]. The other property of these electrolytes is ion transport which depends on many factors like degree of salt dissociation and its concentration, dielectric constant of host polymer, degree of ion aggregation, and mobility of polymer chains [17, 18]. An effective means for increasing the ionic conductivity of polymer electrolytes consists in plasticizing the polymer electrolyte with organic molecules such, as ethylene carbonate (EC), poly (ethylene glycol) (PEG), and glycerol, which have high dielectric constant and low vapor pressure [17, 19]. These plasticizers help in improving the electrical conductivity of polymer electrolytes by (1) increasing the amorphous phase content; (2) dissociating ion aggregates; and (3) lowering the glass transition temperature (Tg) [17]. Recent studies reveal that the composite polymer electrolytes (CPE) alone can offer improved electrolyte/ electrode compatibilities and safety hazards [20–22]. One
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Fig. 1 Block diagram of a typical electrochemical cell
of the most promising ways to improve the morphological and electrochemical properties of polymer electrolytes is addition of ceramic fillers [20–23]. The highly conducting ceramic fillers, zeolites [24] and ionites [25] as well as electrically neutral ceramic fillers [26] have been investigated. It has been well established that the addition of ceramic fillers improves the conductivity of polymer hosts and their interfacial properties in contact with the lithium electrode. This increase in ionic conductivity is explained by enhanced degree of amorphocity of the polymer chain or hindered recrystallization [27]. The composite polymer electrolytes as protonic conductors also find applications in fuel cell applications [28–31]. The ceramic fillers also play a vital role in enhancing the electrochemical properties of the electrolytes. In general, the ceramic fillers for the polymer matrix are broadly classified into two categories: active and passive. The active component materials are participated in conduction process, e.g., Li2N and LiAl2O3, while in inactive, the materials such as Al2O3, SiO2, and MgO [32–39] do not involve in the lithium transport process. The selection of fillers between active and passive components is quite arbitrary. In recent years, researchers have focused on the development of light and safe devices whose production, storage, and distribution of energy is preferentially at low cost. Therefore, the developments of new electrolytes to replace the existing traditional Synthetic Polymer Electrolyte (SPEs) are obtained from finite sources. One of the possibilities is to invent new electrolytes from renewable sources, and in this context, natural polymers can become a promising substitute for synthetic polymers. Among natural polymers, polysaccharides and proteins are best candidate due to their abundance in environment.
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These natural polymers are particularly interesting due to their chemical structure differences, richness in nature, economical, and principally biodegradable. These polymers are usually used in the cosmetic [40–42], pharmaceutical [43–46], and food industries [47–49] and can also be used to prepare SPEs. Most of SPEs are based on hydroxethyl cellulose [50], starch [51], chitosan [52–54], agar-agar [55], pectin [56], and gelatin [57], wherein the ionic conductivity has been reported in the order of 10−4 S/cm at room temperature. The electrochromic devices (ECDs) with good electrochromic properties have already been realized with such SPEs [58, 59], and they can be used in different electrochemical devices like smart windows [55, 57], electronic and optical devices [51], capacitors [60], etc. The initial findings of natural polymer-based electrolytes are summarized below: Cellulose-based electrolytes Solid polymeric electrolytes were obtained by the plasticization process of hydroxyethylcellulose (HEC) with different quantities of glycerol and addition of lithium trifluoromethane sulfonate (LiCF3SO3) salt. The samples were prepared in the form of transparent films with very good adhesion properties. The ionic conductivity measurements were obtained by impedance complex spectroscopy as a function of both salt contents and temperature. The best conductivity values of 1.07×10−5 S/cm at 30 °C and 1.06×10−4 S/cm at 83 °C were obtained for the samples of HEC plasticized with 58% of glycerol and containing [O]/[Li]=6. These results show that plasticized HEC is a very good material to be used for the preparation of new solid polymeric electrolytes. Besides these, it has attractive features such as intrinsic safety, ecocompatibility, and low production cost and industrialization potentials [50]. Starch-based electrolytes The amylopectin-rich starch plasticized with glycerol. The samples were characterized through ionic conductivity (σ) measurements, scanning electron microscopy, thermal analysis, and spectroscopy in the UV–Vis–NIR region. The results showed that the highest σ (1.1×10−4 S/cm at 30 °C) was obtained for the sample with n=[O]/[Li]=6.5 ratio. In addition, the samples plasticized with 30–35 wt.% of glycerol presented high ionic conductivity, transparency, and conduction stability. The ionic conductivity measurements as a function of lithium salt contents showed a maximum for n=6.5. The ionic conductivity as a function of time for amylopectin-rich starch plasticized with 30 wt.% of glycerol and containing [O]/[Li]=10 showed conduction stability over 6 months ðs 3:01 105 S=cmÞ. This results show that this system meets all the conditions
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required by optical and electronic devices that uses an electrolyte layer. [51]. Nuclear magnetic resonance (NMR) spectroscopy and complex impedance spectroscopy have been used to study the polymer electrolytes formed by amylopectin-rich starch, plasticized with glycerol and containing lithium perchlorate. The 7Li and 1H NMR linewidth narrowing occurs close to the glass transition temperature (Tg) of the plasticized electrolytes. The heteronuclear decoupling NMR experiments suggest a weaker Li–polymer interaction in the plasticized electrolyte when compared with the unplasticized ones. The effects of the plasticizer on the ionic mobility in these starch-based electrolytes were measured by NMR spin–lattice relaxation and conductivity. The 7Li NMR relaxation results indicate that the ionic mobility in these plasticized electrolytes seems to be controlled by the plasticizer molecules. The dynamical parameters obtained from the conductivity and NMR data demonstrate that the Li+ mobility is comparable to those found in others plasticized polymer electrolytes [61]. Chitosan-based polymer electrolytes Chitosan has been under extensive research on account of its specific properties, such as biocompatibility and bioactivity and also because of its promising potential in biomedical, pharmaceutical, and industrial applications [43, 44, 52]. Chitosan also constitutes a polymer host for electrolyte as it is able to dissolve ionic salts [62–64], and because when a chitosan membrane is swollen in water, its amino groups may be protonated, hence leading to protonic conductivity [65, 66]. The electrical properties of polymer electrolytes based on chitosan complexed with lithium and ammonium salts have been reported [53, 67, 68]. Conductivities of the order of 10−6 S/cm at room temperature were reported for chitosan/ poly (ethylene oxide) PEO blends with LiTFSI salt [67] and for the complex formed by chitosan, poly (aminopropylsiloxane) (pAPS) and LiClO4 [65]. Conductivities between 10−5 and 10−4 S/cm were reported for proton-conducting polymer electrolytes, based on chitosan and ammonium salts (NH4NO3 and NH4CF3SO3) [53, 58] and conductivity in the range of 10−6–10−4 S/cm for chitosan and κ-carrageenancontaining ammonium nitrate-based film which is used in electrical double-layer capacitor. [60] Agar-based polymer electrolytes In new type of polymer, electrolytes based on agar have been prepared to obtain proton-conducting polymer electrolytes. Agar similar to starch is a heterogeneous mixture of two polysaccharides: agaropectin and agarose. Although both polymers share the same galactose-based backbone,
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agaropectin is heavily modified with acidic side-groups, such as sulgate and pyruvate; agarose has a neutral charge. As a function of the temperature, the ionic conductivity exhibits an Arrhenius behavior increasing from 1.1×10−4 S/cm at room temperature to 9.6×10−4 S/cm at 80 °C temperature. The samples were made by using different quantities of acetic acid (0.1–3.0 g, 6.3–66.7 wt.%). All the samples showed more than 70% of transparency in the visible region of the electromagnetic spectrum, a very homogeneous surface, and a predominantly amorphous structure. All these characteristics imply that these polymer electrolytes are very attractive for electrochemical device applications, as electrochromic smart windows [55].
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chain segments and the dissociation of salts would be improved, which increases ionic conductivity. The ionic conductivity increases with the increase in the acetic acid concentration. The ionic conductivity decreases at higher acetic acid contents. The increase in the ionic conductivity with increasing acetic acid concentration can be related to the increase in the number of mobile charge carriers. The results suggest that this electrolyte is very attractive for electrochemical device applications, as electrochromic smart windows [57]. These above findings show that aqueous natural polymer electrolytes have relatively poor ionic conductivity due to the presence of water in the films and warrant their application in batteries and in other electrochemical devices.
Pectin-based electrolytes Conclusion Pectin is a natural polymer present in plants and as all natural polymers has biodegradation properties. Chemically, pectin is a polysaccharide composed of a linear chain of 1−>4 linked galacturonic acids, which is esterfied with methacrylatecrylatenol. The most important quality of an electrolyte is transparency, which is easily obtained with liquid electrolytes, but not so easily with solid electrolytes. However, some of polymer electrolytes, principally those based on natural polymers, can be obtained in the thin membrane forms with very good transparency in the visible range of electromagnetic spectrum. Pectin-based ionic conducting membranes show the optical transmittance in the 200–1,100-nm range, which increases in function of the wavelength from zero in the UV region at 289 nm to 85% depending on the sample. The plasticized pectin and LiClO4-based gel electrolyte were prepared and analyzed by spectroscopic, thermal, structural, and microscopic analyses. The best ionic conductivity values of 4.7×10−4 S/cm at 30 °C and 6.3× 10−3 S/cm at 80 °C were obtained. Thermal analysis shows that these electrolytes are stable up to 160 °C and 10% of weight loss is due to the absorbed water. These electrolytes were predominantly absorbed, and SEM surface visualization evidenced a uniform surface morphology of the samples. Pectin-based electrolytes shows good adhesion to the glass and steel, and are very promising materials to be used as solid electrolytes in electrochromic devices [56]. Gelatin-based electrolytes The gelatin-based proton-conducting polymer gel electrolytes containing acetic acid cross-linked with formaldehyde and plasticized with glycerol, and the ionic conductivity values are found to increase from 4.5×10−5 to 3.6×10−4 S/cm. When the temperature increases from room temperature to 80 °C, respectively, the films are found to be mechanically stable. At higher temperatures, the thermal movement of polymer
Based on the literature available so far, more efforts have been made for the development of synthetic polymer-based CPEs. These electrolytes have many advantages except their intrinsic safety, eco-compatibility, and high production cost and industrialization potentials. Now, in recent years, only a few literature reports are available on such natural polymer-based electrolytes. These natural polymer-based electrolytes are water-based systems so it exhibit poor ionic conductivity (10−3 to 10−5 S/cm) and poor shelf life, which warrants their application in electrochemical devices. To overcome these problems, there is a need to explore and develop nonwater-based natural polymer electrolytes or hybrid type of electrolyte by chemical or physical modifications so they could be used as an alternate electrolyte for all electrochemical devices. Acknowledgments The authors would like to thank Dean, Faculty of Engineering and Technology, Manav Rachna International University (formerly CITM Faridabad), Faridabad, for his kind support. The authors are grateful to All India Council of Technical Education (AICTE) for providing research grant under Research Promotion Scheme.
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