Solid polymer electrolytes as a prospective class of electrolytes

Electrolytes have been identified as one of the most influential components for the performance of electrochemical devices. In literature, there are several generations in the field of electrolytes and among them Solid Polymer Electrolytes (SPEs) have received a great interest. The first ever SPE has been introduced to the world around 1973. Thereafter, so many modifications have been done in order to uplift the characteristics of SPEs. In general, SPE is a blend of a polymer, a salt and solvent/s. With respect to SPEs, there are some special features of interest encompassing ionic conductivity, transference number etc. Various characterization techniques are being practiced to evaluate those properties and some of them are Electrochemical Impedance Spectroscopy (EIS), Cyclic voltammetry (CV) test, X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM). Looking back at the past as well as the present day of SPEs, it can envisage that they will appear in future with a huge number of reforms.


INTRODUCTION
Electrolytes play a pivotal role in electrochemical devices mainly serving as the medium for facilitating ion conduction between the anode and the cathode. Besides, they take care of avoiding short circuiting between those two electrodes guaranteeing safer operation. Upon physical state of electrolytes, there are two major categories of electrolytes as solid and liquid 1 . They both owe various merits and demerits when employing in applications. To overcome crucial issues such as low conductivity, leakage and evaporation, a substantial amount of methodologies have been practiced by experts in multidisciplinary areas to explore novel electrolytes with significant higher performance. As a result, SPEs have been In this review article, the evolution, the approaches launched to improve of SPEs, the current state and features of interest of SPEs will be discussed in brief followed by common characterization methods. Lastly, future predictions and conclusions are presented.

EVOLUTION OF SOLID POLYMER ELECTROLYTES
In 1973, Wright et al came up with the first ever SPE comprising PEO and NaSCN and in addition, they reported the semi crystalline structure of complexes between PEO and alkali metals 2 . As time passes, it was realized that conductivity of a SPE should be higher than 10 -4 Scm -1 to be used for device applications. Hence, the electrochemical properties of these systems were studied later and correlation between the ionic conductivity and the amorphous phase was highlighted 4 .
Next generation came up with modified SPEs with the main target of breaking crystalline nature of the structure. This has been motivated mainly by the realization of the fact that crystallinity hinders the ionic conductivity 5,6 . Some of the modification methodologies include cross-linked polymer matrices, blended polymers and comb branched copolymers 7-9 . But, many of those approaches were also not able to reach the accepted conductivity levels.
Introduction of composite SPEs gave a new insight for conventional SPEs and unlocked another important era. A remarkable attention was focused on using plasticizers for SPEs 10,11 . Those plasticizers such as ethylene carbonate (EC), propylene carbonate (PC) and dimethyl carbonate (DMC) were capable of introducing amorphous nature to the SPEs while lowering glass transition temperature and paving path for high conductivity. Simultaneously, the mechanical properties were affected negatively which pose some limitations for practical applications. With the untiring efforts of researchers to raise the conductivity while maintaining mechanical properties, addition of fillers was practiced. As fillers, nanoscale additives like silicon dioxide (SiO2), titanium dioxide (TiO2) have been considered 12,13 . They slightly reduce conductivity but the assistance for sustaining good mechanical properties is well compensated.
Another key milestone in the history of SPEs is the introduction of gel polymer electrolytes (GPEs) for the first time by Feuillade and Perche in 1975 as an alternative and a parallel approach to improve SPEs 14,15 . A GPE is consisted of a polymer matrix wherein a liquid electrolyte with a salt/s and solvent/s are encapsulated in it. Upon continuous discoveries and progress of SPEs and GPEs, it has been found that presence of plasticizers and solvents create some negative issues while enhancing properties.
Ionic liquids (ILs) have opened a new chapter in the history of SPEs as they have proved to be suitable substitutes for plasticizers and solvents 16,17 . In some occasions, they have been able to replace the salts serving a dual role.
In the new millennium, hybrid inorganic-organic SPEs have been able to draw the world wide attention as a class of very attractive SPEs. They exhibited excellent mechanical, thermal, chemical and electrochemical stability and high conductivity at room temperature which was of pressing importance to increase the suitability for applications [18][19][20] .

CURRENT STATE OF SPES WITH NOVEL APPROACHES
The major legacy of the present day world is indeed the concept of green technology and low cost. This has addressed the necessity for shifting of SPEs towards natural polymer materials which satisfy at once those two contradicting properties of safety and cheap price.
At present different types of natural polymers have received a notable attention in preparation of SPEs.

Starch based SPEs
Starch, a form of energy storage in plants, is the largest amount of carbohydrate in human diets and is found in substantial quantities in staple foods such as potatoes, wheat, corn, and rice 21,22 . It is a polymeric carbohydrate composed of repeating glucose monomers joined by glycosidic linkages forming a stable helix structure 23,24 . SPEs have been made when starch is plasticized with glycerol. This has several beneficial properties such as biodegradability, non-toxicity and high ionic conductivity. Mattos

Cellulose based SPEs
Cellulose is a biodegradable natural polymer which can be obtained from all plants.
According to the literature reviews, samples with Hydroxyl Ethyl Cellulose (HEC) with different quantities of glycerol and addition of lithium trifluoro methanesulfonate (LiCF3SO3) have been prepared in the form of transparent films with very good adhesive properties 26,27 . The best σ values of 1.07×10 −5 Scm -1 at 30 0 C and 1.06×10 −4 Scm -1 at 83 0 C were reported for the samples of HEC plasticized with 58% of glycerol. These results showed that plasticized HEC is a very good material to be used for the preparation of SPEs.

Chitosan based SPEs
Chitosan is a natural biopolymer with the chemical name of poly[(1,4)-N-acetylD-glucose-2-amine)] which has received an upsurge of interest from academia and industry over the past decades. Structure of chitosan is shown in fig. 1. It also has been used as a polymer host in solid/composite polymer electrolytes for batteries and capacitors by several groups 28,29 . A lithiumtrifluoromethanesulfonate / EC / chitosanbased electrolyte was studied by Osman and Arof in 2003 30 . When ethylene carbonate (EC) was used as a plasticizer in this composite SPE, it has been reported that the ionic conductivity improved due to the interaction between the lithium salt and the plasticizer.

Gelatin based SPEs
Gelatin is a synthesized biopolymer which is also a polysaccharide. The molecular structure of gelatin is shown in fig. 2.

Natural rubber (NR) based SPEs
The growing interest in the exploitation of new raw materials for the development of high quality SPEs makes Natural Rubber (NR) a material of choice among polymers. This is due to its low cost and wide availability. NR having the monomer isoprene ((C5H8)n) shown in fig.3 is obtained from the latex of several rubber yielding plants such as Hevea brasiliensis and Parthenia argentatum. But, it is to be noted that much preference is on the latex obtained from Hevea brasiliensis 34 . The rubber produced from latex contains relatively small quantities of protein (~2.2%), carbohydrates (~0.4%), natural lipids (~2.4%), glycolipids and phospholipids (~1.0%), inorganic materials (~0.2%), other compounds (~0.1%) 35 . The molar masses of rubber molecules range from 50,000 to 3,000,000 gmol -1 36 . There are several ongoing research activities in progress in the field of NR based SPEs during recent years.  To improve the stability, NR is sometimes chemically modified by introducing hydrophilic groups along the isoprene backbone or by epoxidization which yields Epoxidized Natuarl Rubber (ENR). The presence of polar epoxide groups in ENR provides rubber a compatibility with high polar fillers. ENR is commercially available in 25 mole % epoxidation (ENR 25) and 50 mole % epoxidation (ENR 50). The epoxy groups of ENR, which are randomly distributed, provide sites for chemical modifications or crosslinking with suitable compounds 42,43 . ENR appears as a masticated sheet after it is passed into the two-roll mill machine about six times. Properties are improved depending on the degree of epoxidation. Epoxidation is found to be increasing the polarity of natural rubber 44 . ENR can be prepared by using peracid formed by the reaction of acetic or formic acid with hydrogen peroxide. It produces rubber with randomly dispersed epoxide groups along the polymer backbone 45 . However, ENR suffers a drawback due to sticky nature which disables easy peeling off from substrate 46,47 .

Ionic conductivity()
From fundamentals, conductivity is defined as the degree to which a specified material conducts electricity and is calculated as the ratio of the current density in the material to the electric field which causes the flow of current 48  Ionic conductivity is normally symbolised with  and it correlated to as, Here, Rb is the bulk resistance of polymer electrolyte, t is the film thickness and A is surface area of electrolyte 49 .
 of SPEs is truly depending on temperature and its variation with temperature basically follows two behaviours. They are Arrhenius and Vogel Tamman Fulcher behaviours 50-52 .

Transference number
Transference number is defined as the ratio of the current flowing when a steady state current is passing through a cell following establishment of a stable concentration gradient, to the current flowing in the absence of a concentration gradient. There are two types of transference numbers. They are ionic/electronic transference number and cationic/anionic transference number. They all are symbolizing the contribution of ions, electrons, anions and cations to the conductivity 53 .

Thermal, chemical and electrochemical stability
To operate in an appropriate temperature range, SPEs must possess good thermal stability.
It is mainly because the devices which use SPEs have to be operated in diverse temeprature ranges. The solid state electrochemical devices are fabricated by sandwiching the SPE membranes between appropriate cathode and anode materials. In order to avoid undesired chemical reactions proceeding at the electrode/electrolyte interfaces, SPEs should possess a high chemical stability. In addition, they have to be stable through a wide potential window.
In other words, there should not be any current change within a certain potential window.
Preferably, a electrochemical stability domain is expected to extend from 0 V to as high as 4-5 V 54 .

Durability
Durability is a crucial factor for the continuous and constant operation of devices. It is exceptionally high in SPEs when compared to liquid and solid electrolytes. This is due to the retention of all components of a SPE without undergoing deterioration and also due to the good compatibility of SPEs with many electrodes. Unstable charge/discharge cycles are present if the durability is poor which affects the efficiency of devices very much.

Electrochemical impedance spectroscopy (EIS)
EIS is a technique that is used to study the electrical properties of materials and their interfaces over a wide range of frequencies and temperatures. The bulk and interface contribution can be separated using this method. It can also be used to study the ion conduction mechanism and dielectric relaxation in SPEs. Frequency response analyzer is the main device that is being used to get required data for calculating conductivities at different frequencies.
 is found via EIS using impedance measurements with an electrochemical cell consisting of the SPE film sandwiched between two electrodes. EIS could be used to measure the impedence. The impedance (Z) of a circuit is its opposition to the current, so for example in a circuit composed purely of resistors, the impedance is exactly equivalent to the circuit resistance. It can be represented by where V is the voltage and I is the current.
After gathering impedacne data, a graph is plotted with real part of impedacne (Z′) and imagianry part of impedance (Z′′) for the whole frequency range which is known as a Nyquist plot 55 .

DC polarization test
In electrochemistry, polarization is a collective term for certain mechanical side-effects (of can be calculated using DC polarization test according to the equation 57,58 .

tion=(Ii-It) / It (3)
Ii is the initial current and It is the steady current. If the ionic transference number (tion) of a SPE is close to unity, it highlights the dominant ionic contribution on conductivity. This means that the particular SPE is suitable for electrochemical applications. Interestingly, many of SPEs possess tion value very much close to unity 57,58 .
Cationic transference number (tc) can be calculated by using the following equation, where, Ic is the current carried by cations and Ia is the current carried by anions. Allowed values of transport numbers are believed to lay exclusively in the range from 0 to 1 59,60 .

Linear sweep voltammetry (LSV) and cyclic voltammetry (CV)
A typical linear sweep voltammogram is shown in fig. 5.

Fig. 5 Linear sweep voltammogram
In LSV, a fixed potential range is employed much like potential step measurements.
However in LSV, the voltage is scanned from a lower limit to an upper limit as shown above 61 . CV is an electrochemical technique which measures the current that develops in an electrochemical cell during charge and discharge processes 62,63 . It could be performed by varying the potential between a working electrode and a reference electrode while measuring the resulting current between the working and counter electrodes. The potential of the working electrode is controlled with respect to the reference electrode such as a saturated calomel electrode (SCE) or a silver/silver chloride electrode (Ag/AgCl). The current can be considered as the response to the potential excitation signal. A graph can be drawn between potential and current. It is known as a cyclic voltammogram which is a useful tool for identifying several details of a particular system 64 . A typical cyclic voltammogram is shown in fig. 6.

Fig. 6 A typical cyclic voltammogram
Cycling is done at a specific rate called as the scan rate. It serves as a determinant of the performance of the system under investigation. Modern instrumentation enables switching potentials and scan rates to be easily varied.

Scanning electron microscopy (SEM)
SEM provides surface morphological information of SPE films 65

X-Ray diffraction (XRD)
XRD is a powerful nondestructive technique for characterizing materials. X-ray diffractometer is the device used for this technology which produces the so-called diffractogram. It provides information on structures, phases, preferred crystal orientations and other structural parameters such as average grain size, crystallinity, strain, and crystal defects. X-ray diffraction peaks are produced by constructive interference of a monochromatic beam of X-rays scattered at specific angles from each set of lattice planes in a sample 69 . XRD analysis is used to determine the structure and crystallization of polymersalts complex by observing the appearance and disappearance of peaks. A sketch of a XRD pattern is given in fig. 8. The DSC set-up is composed of a measurement chamber and a computer. Two pans are heated in the measurement chamber. The sample pan contains the material being investigated. A second pan, which is typically empty, is used as a reference. The computer is used to monitor the temperature and regulate the rate at which the temperature of the pans changes. A typical heating rate is around 10 0 Cmin -1 . A differential scanning calorimetry graph is shown in fig. 9. Moreover, some companies have made efforts toward commercialization to promote the practical applications. Although substantial progress has been obtained, it is necessary to bear in mind that some fundamental issues concerning SPEs must be addressed before they meet the requirements for practical applications; the optimization of the ionic conductivity and the interfacial compatibility, for instance. Furthermore, fundamental knowledge as well as new design technologies is urgently needed to accelerate the industrialization process.
Some new device architectures and technological applications with SPEs are still at an early stage and require better evaluation procedures. In the field of polymer nano composite materials, the influence of several parameters remains unexplored. Specifically, the role of the size, shape of the nanoparticles and their alignment remains to be clarified. Molecularscale details regarding conductivity across conductive ceramics and polymer electrolyte interfaces has not been discussed largely. Moreover, there is still an open question of whether the polymer-nano particle interactions can indeed be tuned to achieve a simultaneous enhancement in conductivity and mechanical properties. To address such questions, more realistic force fields and models are essenial.

CONCLUSIONS
The storage of electrical energy is far more important in this century than it was in the previous decades. The necessity will be doubled during the next few years to come. This is mainly due to the high rising usage of multi-purpose appliances. In par with that, the use of SPEs in various devices has advanced much in recent years.
Since the introduction, SPEs have gone through a large number of developing stages in terms of improving their characteristics. Present day world has a high concern over green technology. Hence, it is very much needed to develop materials for SPEs which enable to raise the level of safety. On the other hand, due to economical constraints exist in many countries; money value is an important factor. Novel approaches are really important to address the low cost while maintain good performance. If SPEs are revitalized with more and more attractive features using suitable materials, demand on them will be doubled in the future. It is therefore the central focal point of attention of researchers in the fields like electrochemistry and material science to exploringe new SPEs with improved properties.

ACKNOWLEDGEMENT
Authors would like to acknowledge the financial assistance provided by National Research Council, Sri Lanka (NRC/17/006) and Wayamba University of Sri Lanka.