The diploma thesis entitled "INVESTIGATION OF THE EFFECTS OF IONIC LIQUIDS AND IONIC MIXTURES IN BIODEGRADABLE POLYMER ELECTROLYTES" was prepared by R. ON THE EFFECTS OF IONIC LIQUID AND IONIC MIXTURE AND BIODEGRADABLE POLYMER ELECTROLYTES" under the supervision of Dr MORRIS A G EZRA from the Department of Electrical Engineering and Electronics, Faculty of Mechanical Engineering and Natural Sciences.
MATERIALS AND METHODS 43
The dependence of log ionic conductivity (σ) values of CS:LiTFSI:DES polymer electrolytes at room temperature. Variation of log ionic conductivity (σ) values of CS:LiTFSI:[Amim]Cl polymer electrolytes as a function of [Amim]Cl concentration.
Separator in batteries
There are a few types of electrolytes present and each of the types is distinguished based on its physical conditions. The dissociation of the individual constituents in an organic solvent forming free ions has been attributed to the thermodynamic interaction between the solvent and solutes, in a process called solvation.
Polymer
The selection of a natural polymer in the development of a conductive medium requires a lot of attention, as it is highly crystalline in nature. Since mechanical stability and impact resistance carry equal weight with conductivity properties in the development of polymer electrolytes, an acceptable degree of crystallinity in the polymer is therefore required to achieve appreciable ionic conductivity with a combination of other desired properties.
Overall view
Problem statement
Research objectives
Scope of research
Preparation method
The nature of the samples in system I is present in both forms, a thin-layer membrane at lower DES concentration and consequently forming gel-like samples at higher plasticization. The CS system plasticized with [Amim]Cl produced gel-like patterns throughout the composition.
Characterisations
Introduction to polymer electrolytes
Gel polymer electrolytes
The properties of polymer gel depend on the structure of the polymer network that makes up the gel, as well as the interaction of the network and the solvent. Since the polymer networks are dissolved by a large amount of trapped solvent, gels generally possess high mobility.
Thin film polymer electrolytes
Advantages of polymer electrolytes
Thus, since most of the inventions are elaborated with a view to commercialization, the development of SPEs will be more attractive due to the ease of fabrication in shapes not found in the conventional liquid-containing systems. The biodegradable nature of the formed conductive matrix makes it more attractive as it is greener for the environment.
Limitations of polymer electrolytes
In this thesis we are involved in the development of polymer electrolytes using natural polymers. The cost of production is reduced as it is a renewable resource that is naturally available in the environment.
Methods of enhancing ionic conductivity
Polymer blending
Thus, it appears to be a promising material for use as a solid electrolyte in electrochromic devices. Blending with a cornstarch component not only looks environmentally friendly, but has achieved the desired properties. 2008) studied the performance of a perfluorosulfonic acid polymer (Nafion) blended with two different types of host polymers, namely cellulose acetate and microporous polytetrafluoroethylene (PTFE) separately.
Mixed salts system
Growth in the amorphous elastomeric phase improves ionic conductivity through reduction of microviscosity, which increases the mobility of lithium ions. The anion in the ionic salt causes structural disorder while the cation involved increases ionic conductivity.
Addition of plasticisers
One of them is [Amim]Cl and another type is DES which is a type of ionic mixture that shares the similar functions of ionic liquid. Other advantages of the used ionic liquid and ionic mixture will be discussed in the last subsection.
Corn starch
An initial approach in the development of CS-based polymer electrolytes was made by incorporating an ionic salt, which provides both anion and cation ionic species upon dissolution in the CS matrix. Anions are responsible for causing structural disorder in the CS matrix in order to increase ionic conductivity.
Cellulose acetate
This is because the acetate group (-Ac) connected to the oxygen atom (-O) in CA is comparably larger in atomic size than the hydrogen atom (-H) found in the cellulose structure. This type of host polymer also attracted the attention of many researchers in developing porous structures (Fischer et al., 2006; Reverchon and Cardea, 2007).
Lithium bis(trifluoromethanesulfonyl)imide
The undeniable outstanding properties of CA have attracted the development of electrolytes with the combination of a perfluorosulfonic acid polymer (Nafion) used in solid-state EDLCs (Subramaniam et al., 2008). The presence of the anion (TFSI-) will contribute to the structural disorder in the polymer matrix by breaking the hydrogen bond in the polar functional group.
Deep eutectic solvent
In recent years, most of the applications of DES have been focused on the development of electrolyte to be widely used in electrochemical devices such as. As a result, the incorporation of [Amim]Cl increases the ionic conductivity of polymer electrolytes, while maintaining the biodegradable properties of the matrix formed by natural polymers.
Materials
Synthesis of deep eutectic solvent (DES)
Preparation of polymer electrolytes
CS based polymer electrolytes
After the heating process, the solution was cooled to room temperature under constant stirring for about 30 minutes. Continue with an appropriate addition of LiTFSI and DES/[Amim]Cl to the viscous solution and again the solution was allowed to stir for another hour to allow good mixing between the added chemical components.
CA based polymer electrolytes
The initial step in the development of biodegradable polymer electrolytes was to dissolve an appropriate amount of CS powder in 15 mL of distilled water producing a milky solution. Then, the homogeneous polymer electrolytes were cast by spreading the viscous solution on a clean Teflon plate and dried in an oven at 55 oC for 8 h.
Physical appearance of the developed polymer electrolytes
Instrumentations
- Alternating current (AC) impedance spectroscopy
- Ubbelohde capillary viscometer
- Horizontal attenuated total reflectance-Fourier transform infrared (HATR-FTIR)
- X-ray diffractometry (XRD)
- Scanning electron microscopy (SEM)
- Atomic force microscopy (AFM)
- Thermogravimetric analysis (TGA)
This is the main analysis performed to test the ionic conductivity of the synthesized polymer electrolytes. This characterization technique is performed for phase identification, which is important for understanding the ionic conductivity of polymer electrolytes.
Conductivity studies at room temperature
In Region 3, a significant increase in ionic conductivity was first observed for the DES-50 sample before a drastic increase was calculated for the 60 wt. Further increase in DES content contributes to an almost constant increase in ionic conductivity.
Scanning electron microscopy (SEM)
Horizontal attenuated total reflectance-Fourier transform infrared (HATR-FTIR)
The characteristic peaks of CF3 and CF stretching that coexist at 1193 and 1142 cm-1, respectively, in pure LiTFSI, were shifted to a lower frequency at 1186 and 1133 cm-1 in DES-0 after complexation with pure CS. Further evidence to demonstrate complex formation was based on the peaks falling in the range of 1100-1300 cm-1 in pure LiTFSI splitting into 3 to 4 individual peaks and assigned at a lower frequency in DES-0 when complexed with pure CS.
Frequency dependence of loss tangent studies
The increase in the relaxation frequency (logmax ω) as a function of DES content is summarized in table 4.2. This was attributed to the increase in the number of Li+ ions participating in the relaxation process to aid the ionic conductivity.
Temperature dependent conductivity studies
All the tested samples obey Arrhenius theory which reveals that there is no phase transition in the plasticized polymer electrolyte matrix in the temperature range studied. This has been attributed to the increase in the amorphous elastomeric fraction in the CS: LiTFSI: DES matrix which facilitates rapid Li+ ion movement in polymer network with increase in temperature (Armand, 1986; Ratner and Shriver, 1988).
Thermogravimetric Analysis
The displacement of the maximum decomposition temperature of sample DES-20 to lower temperature reveals the decrease in the sample's heat resistance. Further increase in DES content leads to the decrease in both the heat resistance and thermal stability.
Conductivity studies at room temperature
The increase in ionic transfer capacity improves the ionic conductivity of the polymer electrolytes. This complexation in turn facilitates the mobility of Li+ ions with the aim of improving ionic conductivity.
Relative viscosity studies
Based on the collected results, it was found that an increase in DES concentration from 0 wt. The increase in Li + ion mobility as the increase in DES concentration matched well with the fluidity results, observed through an increase in the fluidity value from 0.04 (CA-0) to 0.19 (CA-60).
Frequency dependence of loss tangent studies
These non-bridging ions (Cl-) obtained from the miscibility of DES in the polymer matrix serve as a passage site for the Li+ ions. This was attributed to the increase in the number of Li+ ions participating in the relaxation process.
Scanning electron microscopy (SEM)
This composition appears to be the sample with the highest conductivity due to the increase in free void, neglecting the effect of increasing bead diameter. An increase in free vacancy improves the charge transfer mechanism and thereby increases ionic conductivity.
Horizontal attenuated total reflectance-Fourier transform infrared (HATR-FTIR)
The occurrence of complexation between pure CA and pure LiTFSI was confirmed based on the alternations in the CA-0 spectrum as depicted in Figure 5.5. The occurrence of complexation can be further confirmed by relying on the characteristic C=O peak present at 1726 cm-1 in CA-0.
X-ray diffractometry (XRD)
Complete dissolution results in an increase in the amorphous region of the CA matrix based on the absence of a moderately intense peak (2θ = 8.5 o, 10.5 o and 13.5 o) and a significant broadening of the diffraction peak of CA-0. This finding indicates a decline in the degree of crystallinity in the CA matrix as more DES are incorporated into the CA:LiTFSI:DES system.
Temperature dependent conductivity studies
It was observed that there is no phase transition in DES plasticized polymer electrolytes in the studied temperature range as all tested samples obey a common rule. This is attributed to the increase in flexibility of the polymer backbone induced by the decrease in crystallinity of the CA matrix.
Conductivity retained studies
Thus, the ionic conductivity decreases over time, caused by the dissipation of liquid component from the matrix. In view of the results for CA-20, CA-40 and CA-60, the ionic conductivity was further maintained as the DES concentration increased.
Thermogravimetric analysis
The maximum decomposition temperature and the total weight loss in percentage of the same samples are given in Table 5.2. In view of the results in Table 5.2, it can be understood that the incorporation of LiTFSI into CA matrix successfully increased both the heat resistivity and the thermal stability of pure CA.
Conductivity studies at room temperature
Further addition of [Amim] Cl in the polymer electrolytes contributes to a sharp decrease in the ionic conductivity. At low concentration of [Amim]Cl, there will not be a large free volume available in the polymer electrolytes for movements of Li+ ions, so at this point the low dielectric constant of [Amim]Cl exerts a significant influence on the ion conductivity trend.
Relative viscosity analysis
To some extent, this property dominates the free volume effect by leading to the formation of neutral ion multiples that reduce the ionic conductivity. While the ion conductivity trend in step 2 was dominated by the free volume effect, and as a result, no fluctuations in the ion conductivity are observed.
Frequency dependence of loss tangent studies
The availability of free volume in the formed matrix facilitates the mobility of Li+ ions and therefore improves the ionic conductivity. As clearly depicted, the magnitude of tan δ increases as an increase in the concentration of [Amim]Cl.
Temperature dependent conductivity studies
This in turn promotes inter- or intra-chain ion hopping and accordingly the ionic conductivity of the polymer electrolyte will be improved upon temperature rise. The variation of Ea for polymer electrolytes with different [Amim]Cl concentrations is summarized in Table 6.2 and the result shows that the Ea effectively decreases with the increase of the ionic conductivity value.
Horizontal attenuated total reflectance-Fourier transform infrared (HATR-FTIR)
Incorporation of [Amim]Cl leads to the observation of a small shoulder at 2851 cm-1 in CS-40, and further addition of [Amim]Cl to polymer electrolytes results in the formation of a moderate peak at 2856 cm-1 in CS-80. The occurrence of complexation between CS-0 and pure [Amim]Cl can be further demonstrated by the formation of a shoulder at 1295 cm-1 in CS-40.
Thermogravimetric analysis
This observation reveals the improvement in heat resistance, but this property was found to decrease with the incorporation of [Amim]Cl. This explains the decreasing trend in the maximum decomposition temperature with an increase in [Amim] Cl concentration.
Conductivity studies at room temperature
The continuous improvement of the ionic conductivity with increase of [Amim]Cl concentration can also be validated in terms of the increase of the non-bridging ions in [Amim]Cl(Cl-) upon dissociation in the CA matrix. This, in turn, increases the ionic transfer capacity, which then increases the ionic conductivity at room temperature.
Relative viscosity analysis
This intense drop reveals the disruption in the crystalline region of the CA:LiTFSI matrix due to the presence of free mobile ions (of the ionized [Amim]Cl) that enhances the amorphous nature in the polymer electrolytes. Therefore, a greater disturbance in the crystalline phase will occur, increasing the free volume between the polymer chains.
Atomic force microscopy (AFM)
An increase in the amorphous fraction coexisting between the spherulite boundary (represented by the light yellowish region) was evident after the addition of 80 wt. The changes observed in the surface morphology upon plasticization with increasing concentration of [Amim] Cl indicate a decrease in the crystallinity of the CA matrix.
Temperature dependent conductivity studies
This fraction gradually increases with rise in temperature which is evident from the continuous improvement in the ionic conductivity. Thus facilitating rapid Li+ ion movement in the polymer network with the increase in temperature.
Conductivity retained studies
Incorporation of [Amim]Cl into the polymer electrolytes significantly preserves the ionic conductivity throughout the storage time, making it more resistant to aging. Therefore, more traces of the liquid component will be trapped between the CA matrix providing a dilute environment to facilitate the movement of Li+ ions and thus maintaining ionic conductivity during storage time (Ramesh and Arof, 2009).
Horizontal attenuated total reflectance-Fourier transform infrared (HATR-FTIR)
The first attempt to prove the complexity was based on the shift of the characteristic C=O peak originally present at 1726 cm-1 in Figure 7.8 (a) and 1732 cm-1 in Figure 7.8 (b). Further evidence to prove the complexation focused on the peak at 1573 cm-1 in pure [Amim]Cl.
X-ray diffractometry (XRD)
It has also been observed that the diffraction peak significantly increased in width and decreased in intensity with the increase in [Amim]Cl concentration in the polymer electrolytes. Therefore, the increase in the amorphous region will cause a decrease in the energy barrier for the segmental movement of polymer electrolytes (Baskaran et al., 2006), which undeniably improves the charge transfer mechanism and induces greater ionic conductivity.
Frequency dependence of loss tangent studies
The increase in the number of non-bridging ions in [Amim] Cl (Cl-) increases the capacity of the Li+ ion transfer which may contribute to such findings. The magnitude of the tan δ increases with increase in [Amim]Cl concentration indicating the increase in the number of Li+ ions participating in the relaxation process to aid the ionic conductivity as attributed by the area under the loss factor peak.
Thermogravimetric analysis
The change in the thermal properties of the CA: LiTFSI: [Amim] Cl system with increasing [Amim] Cl concentration is shown in Table 7.4. In addition, the incorporation of [Amim] Cl has a significant change in the thermal properties.
At higher [Amim] Cl plasticization, almost uniform increase in ionic conductivity is observed, while in DES-containing system almost constant rise is obtained. The almost constant trend in ionic conductivity was obtained after reaching the maximum collapse in CS crystallinity even at 60 wt.
In Region 2, continuous increase in ionic conductivity is observed in both systems which is attributed to the increase in the mobility of Li+ ions. This is due to the high viscosity of DES that can form small aggregates in the matrix when the amount is increased, which consequently limits the Li+ ions mobility.
Comparison between CS and CA based polymer electrolytes
The ionic conductivity of polymer electrolytes was found to be maintained over a storage time as plasticization increases. Effect of nanosized silica in poly(methyl methacrylate)-lithium bis(trifluoromethanesulfonyl)imide-based polymer electrolytes.
