Study on the effect of lithium nitrate in ionic conduction properties based alginate biopolymer electrolytes

In this work, alginate (Shaanxi Orient Co.) and LiNO₃ (Sigma Aldrich Co.) were used for preparation as biopolymer electrolytes (BEs) film. By employing solution-casting technique, 2 g of alginate was dissolved in distilled water. Then, different composition of LiNO₃ were added into the alginate solution and stirred until became homogenous solution. A homogenous solution was casted into Petri dish and left dry in the oven at 60 °C for about 24 h. The sample then was further up drying in desiccators filled with silica gel. The composition of LiNO₃ and designation of sample for BEs system are tabulated in table 1.

2.2.1. Fourier transform infrared spectroscopy (FTIR)

2.2.2. Electrical impedance spectroscopy (EIS)

The ionic conduction of BEs system was analyzed using Electrical Impedance Spectroscopy (EIS). The present sample were cut into a suitable size and sandwiched between two stainless steel (SS) blocking electrodes with surface area is π cm² [13]. The conductivity of prepared samples were performed using a HIOKI 3532–50 LCR Hi-Tester with frequency range from 50 Hz to 1 MHz. The ionic conductivity, σ of alginate-LiNO₃ BEs system were calculated using equation:-

$$\begin{eqnarray}&&\sigma =\displaystyle \frac{t}{{R}_{b}A}\end{eqnarray} \tag{ 1 }$$

where, t (cm) is the thickness, A (cm²) is the electrode-electrolyte contact area and R_b (Ω) is bulk resistance of BEs system which can be extracted from Cole-Cole plot. The dielectric constant, ε_r (real part of complex permittivity) and loss, ε_i (imaginary part of complex permittivity) can be calculated based on the following equation:

$$\begin{eqnarray}&&{\varepsilon }_{r}=\displaystyle \frac{{Z}_{i}}{\omega {C}_{o}\left({{Z}_{r}}^{2}+{{Z}_{i}}^{2}\right)}\end{eqnarray} \tag{ 2 }$$

$$\begin{eqnarray}&&{\varepsilon }_{i}=\,\displaystyle \frac{{Z}_{r}}{\omega {C}_{o}({{Z}_{r}}^{2}+{{Z}_{i}}^{2})}\end{eqnarray} \tag{ 3 }$$

Here C_o = ε_oA/t is vacuum capacitance, ε_o is permittivity of free space, A is the area of electrode-electrolyte contact, t is the thickness of BEs system. The angular frequency ω is equal to ω = 2πf, where f is frequency of applied ac field. Z_r is the real part of impedance and Z_i is the imaginary part of impedance. The real, M_r and imaginary part, M_i of electrical modulus were calculated using the equation below:-

$$\begin{eqnarray}&&{M}_{r}=\displaystyle \frac{{\varepsilon }_{r}}{({{\varepsilon }_{r}}^{2}+{{\varepsilon }_{i}}^{2})}\end{eqnarray} \tag{ 4 }$$

$$\begin{eqnarray}&&{M}_{i}=\displaystyle \frac{{\varepsilon }_{r}}{({{\varepsilon }_{r}}^{2}+{{\varepsilon }_{i}}^{2})}\end{eqnarray} \tag{ 5 }$$

2.2.3. Transport parameter study

The transport properties of alginate-LiNO₃ BEs system were analyzed via FTIR deconvolution. Deconvolution technique was performed using Gaussian-Lorentz function, which adapted from Origin Lab Pro 8.0 software. In this method, the FTIR peaks due to complexation of alginate and LiNO₃ were selected based on dominant ionic movement. The area under the peaks was determined, and the free ions percentage (%) were calculated using the equation [14, 15]:

$$\begin{eqnarray}&&Percentage\,of\,free\,ions\left( \% \right)=\displaystyle \frac{{A}_{f}}{{A}_{f}+{A}_{c}}\times 100 \% \end{eqnarray} \tag{ 6 }$$

where A_f is an area under the peak representing the free ions region, A_c is the total area under the peak representing the contact ions. The transports parameter such as number of mobile ions (η), ionic mobility (μ) and diffusion coefficient (D) were calculated following this equation [16, 17]:-

$$\begin{eqnarray}&&\eta =\displaystyle \frac{M{N}_{A}}{{V}_{Total}}\times free\,ions( \% )\end{eqnarray} \tag{ 7 }$$

where:

$$\begin{eqnarray}&&{V}_{Total}=\left[\displaystyle \frac{weight}{density}\left(alginate\right)\right]+\left[\displaystyle \frac{weight}{density}\left(LiN{O}_{3}\right)\right]\end{eqnarray} \tag{ 8 }$$

$$\begin{eqnarray}&&\mu =\,\displaystyle \frac{\sigma }{\eta e}\end{eqnarray} \tag{ 9 }$$

$$\begin{eqnarray}&&D=\displaystyle \frac{KT\mu }{e}\end{eqnarray} \tag{ 10 }$$

where M is number of moles of dopant (LiNO₃) used, N_A is the Avogadro's constant (6.02 × 10²³ mol⁻¹), V_total is total volume of BEs system, k is the Boltzmann constant (1.38 × 10^–23 J K⁻¹), T is absolute temperature in Kelvin and e is electric charge (1.602 × 10^–19 C).

Figure 1 shows the FTIR spectra for alginate incorporate with various composition of LiNO₃. Based on figure 1, it clearly can be seen there is crosslinking reaction between lone pair electrons from alginate and cation from LiNO₃ at the groups of C–O–C, COO⁻, and –OH in the polymer matrix. The highlighted wavenumber which was believed the interaction has occurred between alginate and LiNO₃ is presented in figure 2.

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In the present work, figure 2(a) denotes the ether group (C–O–C) of alginate-LiNO₃ complexes in the region 900 to 1200 cm⁻¹. The absorption peak of BEs system was slight shifting from 1027 cm⁻¹ of ALNO-0 to 1030 cm⁻¹ of ALNO-3 and remain unchanged until ALNO-5. This indicates the presence of high electronegativity atom (oxygen) from the ether group does not creates any opportunities for interaction occurred between alginate and lithium salt since the ether group form strong bonding in the polymer backbone.

Figure 2(b) depicts another significant peak of alginate-LiNO₃ complexes between 1200 to 1800 cm⁻¹ which attributed to symmetrical and asymmetrical of carboxylate anion, COO⁻ [18]. It can be observed that the peak represent to COO⁻ asymmetric stretching of pure alginate (ALNO-0) shifted to higher wavenumber, from 1596 to 1605 cm⁻¹. It can be explained that with the increment of LiNO₃ in BEs system, it will enhance the dissociation of Li⁺–NO₃⁻ where positively charged cations from ionic dopant will more attracted to negatively charged of electron pairs from alginate [19]. The changes in wavenumber revealed the complexes between alginate and LiNO₃ via weak van der Waals attraction of dipole-dipole forces [20] and this would lead to affect the ionic conduction in BEs system. Besides, the Li⁺ from LiNO₃ function as electrophile positive ion in order to interact with polar group of alginate (Li⁺...⁻O) [21]. On the other hand, the absorption peak for COO⁻ symmetric stretching shifted to lower wavenumber from 1409 to 1400 cm⁻¹ which confirms that the migration of Li⁺ to the coordinating site (oxygen) of COO⁻ via electrostatic interaction in order to form (Li⁺...⁻OOC) [22]. It can be observed that there is appearance of new peak at 1362 cm⁻¹ belongs to lithium and nitrate ion stretching which was due to addition of LiNO₃ in polymer matrix. This new peak remain unchanged and similar observation was founded in Mobarak (2015) [23] work, where new appearance peak confirmed the presence of ion pairs (Li⁺ and NO₃⁻) upon addition of lithium salt and would influences the ionic conductivity of polymer-salt complexes. However, this peak shifted to the lower wavenumber upon addition of salt until sample ALNO-5 and this might due to the aggregation of dopant, which lead to decreasing number of mobile ions; hence, might affect ionic conductivity of BEs system [24].

Meanwhile, figure 2(c) presents FTIR pattern for alginate-LiNO₃ complexes in the range from 2800 to 3800 cm⁻¹. The weak peak at 2928 cm⁻¹ and strong absorption band at range 3000 to 3600 cm⁻¹ were related to CH₂ stretching and –OH stretching, respectively [25]. From ATR-FTIR analysis, it can be seen the addition of LiNO₃ showed no effect on the intensity or shifting of peak at CH₂ stretching. However, the absorption peak of –OH stretching band start to shift from 3368 cm⁻¹ for ALNO-0 to higher wavenumber peak at 3390 cm⁻¹ for ALNO-3 and remain unchanged until ALNO-5. According to Kadir et al (2018) [26], the shifted of polymer complexes to higher wavenumber indicates the dissociation of LiNO₃ that lead to the interaction between cation and lone pair of electrons in oxygen atom of hydroxyl group; thus, it expected to increase the number of mobile ions and conductivity in BEs system. The summarization for all changes in functional group of BEs system are tabulated in table 2.

According to Rani (2018) [27], impedance spectroscopy is a non-destructive and powerful technique to analyze the conduction properties of polymer electrolytes and their interfaces with electronically conducting electrodes. Figure 3 depicts the Cole-Cole and calculating fitted plots for alginate-LiNO₃ BEs at ambient temperature. In the present work, the Cole-Cole plot for a blocking electrode consist of a semicircle and a spike, and only a tilted spike. Based on figure 3, the Cole-Cole plot for ALNO-1, ALNO-2, ALNO-4 and ALNO-5 showed the combination of a tilted semicircle with an inclined (tilted) spike. The semicircle in impedance plot can be represented by parallel combination of bulk resistance, R_b and CPE element with another CPE element in series due to tilted spike; therefore, it confirms the semiconducting behavior [28]. The values of Z_r and Z_i associated to electrical equivalent circuit (EEC) of tilted semicircle and spike can be expressed as:-

$$\begin{eqnarray}&&{Z}_{r}=\displaystyle \frac{R+\,{R}^{2}{{k}_{1}}^{-1}{\omega }^{{p}_{1}}\,\cos \left(\displaystyle \frac{\pi {p}_{1}}{2}\right)}{1+2R{{k}_{1}}^{-1}{\omega }^{{p}_{1}}\,\cos \left(\displaystyle \frac{\pi {p}_{1}}{2}\right)+{R}^{2}{{k}_{1}}^{-2}{\omega }^{2{p}_{2}}}+\displaystyle \frac{\cos \left(\displaystyle \frac{\pi {p}_{2}}{2}\right)}{{{k}_{2}}^{-1}{\omega }^{{p}_{2}}}\end{eqnarray} \tag{ 11 }$$

$$\begin{eqnarray}&&{Z}_{i}=\displaystyle \frac{R+\,{R}^{2}{{k}_{1}}^{-1}{\omega }^{{p}_{1}}\,\sin \left(\displaystyle \frac{\pi {p}_{1}}{2}\right)}{1+2R{{k}_{1}}^{-1}{\omega }^{{p}_{1}}\,\cos \left(\displaystyle \frac{\pi {p}_{1}}{2}\right)+{R}^{2}{{k}_{1}}^{-2}{\omega }^{2{p}_{2}}}+\displaystyle \frac{\sin \left(\displaystyle \frac{\pi {p}_{2}}{2}\right)}{{{k}_{2}}^{-1}{\omega }^{{p}_{2}}}\end{eqnarray} \tag{ 12 }$$

where R is bulk resistance, k₁^–1 is the bulk geometrical capacitance of BEs, k₂^–1 is the electrical double layer capacitance at the electrode/electrolyte interface during the impedance measurement. The p₁ and p₂ is the ratio of the angle between the diameter of the depressed semicircle from Z_i axis, and p₂ is the skew parameter that controls the degree of the tendency of the title spike from the real axis. From the Cole-Cole plots, the values of parameters k₁ and k₂ were obtained by trial and error method until the fitted points give a good fit to the experimental points. The R_b value is determined from the interception between semicircle arc and the tilted spike with the real axis [29]. The presence of high frequency arc and low frequency spike in the complex impedance plots may be attributed to the bulk conduction process and the accumulation of charges at the electrolyte–electrode (blocking electrode) interface [30]. Here, the depressed semicircle denotes the bulk material while the tilted spike represents the electrical double layer [31]. It can be observed that the incomplete semicircle curve begin to disappear at high frequency and left with only the tilted spike upon higher addition LiNO₃. This phenomenon suggests that only the resistive component of the electrolyte prevails [32]. On the other hand, sample ALNO-3 shows the fully inclined spike in the impedance study. So, the values of Z_r and Z_i associated to EEC of only tilted spike can be expressed as:-

$$\begin{eqnarray}&&{Z}_{r}={R}_{b}+\,\displaystyle \frac{{k}_{2}\,\cos \left(\displaystyle \frac{\pi {p}_{2}}{2}\right)}{{\omega }^{p}}\end{eqnarray} \tag{ 13 }$$

$$\begin{eqnarray}&&{Z}_{i}=\displaystyle \frac{{k}_{2}\,\sin \left(\displaystyle \frac{\pi {p}_{2}}{2}\right)}{{\omega }^{p}}\end{eqnarray} \tag{ 14 }$$

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Since the spike in this study did not begin at the origin, the equivalent circuit is consist of a resistor and constant phase element (CPE) in series connection. CPE for the spike is due to the double-layer capacity of an inhomogeneous electrode surface [33]. A resistor in series with a CPE can represent the plot that takes the shape of a spike. Based on figure 3(c), ALNO-3 showed fully inclined spike with disappearance of incomplete semicircle and shifted to lower value attributable to dissociation of LiNO₃, which enables the free ions to participate in conduction process [34]. The calculated values of theoretical fitting parameter are presented in table 3. It clearly can be seen that the R_b value for ALNO-3 exhibits the minimum value of 51 Ω indicates that the sample becomes more capacitive than resistive [35]. Therefore, the fully inclined spike of sample ALNO-3 were expected to have higher conductivity value in BEs system.

The ionic conductivity value for entire sample of alginate-LiNO₃ BEs system at ambient temperature is presented in figure 4. The remarkable effect with incorporation of LiNO₃ enhanced the ionic conductivity by three order of magnitude until achieved optimum value at 1.10 × 10^–4 S cm⁻¹ for sample containing 15 wt% of LiNO₃ (ALNO-3). Alves et al (2018) [36] proclaim that ionic conductivity mainly affected by three factor which are (i) types of cation and anion, (ii) salt composition, and (iii) temperature. Generally, increment of LiNO₃ in BEs system cause the ionic conductivity to improved which was due to the concentration of mobile charge carriers from Li⁺. This also can be proven as discussed earlier in FTIR analysis, increment addition of LiNO₃ provide more cation to attracted with potential coordinating of alginate thus, the ionic conductivity will affected. It shows that addition above 15 wt. % of LiNO₃ lead to decreasing in ionic conductivity of BEs and this might due to the aggregation of ion cluster appeared at high level and increase the formation of overcrowded ions in system.

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The ionic conductivity for entire sample at various are presented in figure 5. It can been observed that the ionic conductivity increased with increasing temperature for entire sample. This was due to the formation of free volume provided by alginate from their chain and created unoccupied spaces for migration of ions [37]. The enhancement of ionic conductivity is also attributed to overall mobility of ion and polymer which determined by the free volume; hence, it will leads to the increased in ionic and segmental mobility that will assist ion transport and practically compensate the retarding effect of ion clouds [38]. The segmental motion of the polymer chain and dissociation of the dopant increased due to vibrational energy occurred in the space around its own volume of which believed to push against the hydrostatic pressure imposed by its neighboring atoms [39]. Therefore, the particles move unreservedly in the free volume around the polymer chain and lead to the enhancement of the ionic conductivity value [40]. Figure 6 depicts the log conductivity versus 1000/T for various BEs sample at the temperature range from 303 K to 348 K. It can be seen that all regression values are close to unity suggesting that the points lie on fitting lines, so the variations of conductivity with temperature is obey with the Arrhenius rule [41]:-

$$\begin{eqnarray}&&\sigma ={\sigma }_{o}\exp \left(-\displaystyle \frac{{E}_{a}}{{k}_{b}T}\right)\end{eqnarray} \tag{ 15 }$$

where E_a is the activation energy, σ is the ionic conductivity of BEs system, σ₀ is the pre-exponential factor, k_b is the Boltzmann constant, and T is the temperature in Kelvin. Figure 4 shows the activation energy for various composition of LiNO₃ at ambient temperature. The E_a was found to be decreases as the ionic conductivity of BEs increased due to higher flexibility of the alginate backbone based on enhancement of segmental mobility of its chains [42]. Therefore, it requires low activation energy to facilitates the motion of ions in the biopolymer network [43].

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Dielectric properties provide an important insight regarding on conduction complexes behavior in BEs system. The dielectric properties may vary in different types of polymer electrolyte system attributed to several factors including frequency of applied electrical field, temperature, structural characteristics or other external factors [44]. The variation of ε_r and ε_i as function of frequency at ambient temperature are shown in figures 7(a) and (b) respectively.

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It is clear can be seen from both figures where the value of ε_r and ε_i are higher at low frequency region due to presence of polarization effect which confirms non-Debye dependence [45]. In this work, both values of ε_r and ε_i were found to be higher for sample ALNO-3 and this attributable to increment in charge carrier concentration which lead the ionic conductivity to achieved optimum value [11]. At high frequencies, it can be observed both dielectric profiles are decreased and reached plateau conditon for entire samples. This mechanism might be due to the higher periodic reversal of the applied field where there is no charge build up at the interface between the sample and the electrode (space-charge) and also no excess ion diffusion in the path of field [46, 47].

Figure 8 depicts the temperature dependence of ε_r and ε_i for most conducting sample in BEs system (ALNO-3). It can be observed that both ε_r and ε_i of BEs system increased with increasing of temperature and this is due to the polarization of the mobile ions. As the temperature increased, degree of LiNO₃ dissociation and re-dissociation of ion aggregates increases due to higher content of free ions or charge carrier density; thus, it will lead to enhancement of ionic conductivity with temperature [46]. Table 4 showed the dielectric part from the literature as comparison for the present highest conducting BEs system. Its revealed that the present BEs is comparable with other types of polymer electrolytes system which implies their capability to stored a charge when apply in energy devices. A further investigation of the dielectric behavior can be more successfully achieved using a modulus formalism analysis.

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The real and imaginary part of modulus spectrum for ALNO-3 exhibited as shown in figure 9. Based on figure 9, both M_r and M_i values are increased at higher frequency region without any relaxation peak was observed. Increasing value for both modulus part is due to the distribution of relaxation processes over a range of frequencies [51]. In addition, a long tail at low frequencies depicts that there might be a large capacitance associated with the electrodes polarization effects which further confirms non-Debye behavior in the samples [52, 53]. According to Bakar et al (2015) [54], modulus part increase as frequency increase due to bulk polarization which can be differentiate into two elements; (i) transferred by electron and (ii) avalanche transit-time. In the present work, it is believed the bulk effects appeared in the complexes due to transferred by electron between host polymer and ionic dopant.

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Figure 10 depicts the FTIR deconvolution for various composition LiNO₃. According to Ren et al (2016) [55], strong absorption peaks at ∼1400 cm⁻¹ denotes the anion vibration mode of NO₃⁻ which proved the IR active in FTIR study. Arya et al (2019) [56] also reported that NO₃⁻ mode confirms the D_3h point group and the fundamental modes proved that peak at 1380 cm⁻¹ were correspond to asymmetric stretching of NO₃⁻ for LiNO₃. Therefore, the band of NO₃⁻ showed great importance for the investigation of ions dissociation in BEs system. In this work, the region between 1200 cm⁻¹ to 1500 cm⁻¹ were selected due to obvious ions interaction occurred in alginate-LiNO₃. Thus, the peak located at ∼1400 cm⁻¹ of LiNO₃ is believed due to the free anion (resulting from dissociation of ions) while peak located at ∼1350 cm⁻¹ and ∼1420 cm⁻¹ were attributed to contact ions (due to association of ions) which can be ion pairs or ion aggregates [57]. Figure 11 presents the percentage of free ions and the contact ions of alginate-LiNO₃ BEs system.

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It can be observed that the percentage of free ions increases and the percentage of contact ions decreases when LiNO₃ was added. The highest percentage of free ions was obtained for the optimum ionic conductivity of BEs system. This implies that more ions are dissociated and would provide with balanced interactions between alginate and LiNO₃; thus, assisting more ion conduction in BEs system [58]. The calculated value of the ionic mobility (μ) and the diffusion coefficient number (D) and the number of mobile ions (η) of the BEs were depicted in figure 12.

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Based on figure 12, the value μ and D were found to be similar with the trend of ionic conductivity where it rise sharply at ALNO-3 with maximum value of 4.07 × 10^–9 cm² V⁻¹ s⁻¹ and 1.06 × 10^–10 cm² s⁻¹, respectively. In the present work, addition of LiNO₃ in alginate definite the conjecture in FTIR study where the diffusion of Li⁺ from LiNO₃ and migrate to lone pair of COO⁻ of alginate contain with huge amount of ions and also faster rate. The μ and D will lead to the enhancement of ionic conductivity where it need lesser activation energy to hopping in the backbone of alginate [59]. Referring to figure 12, it can be inferred that the number of η increased linearly upon addition higher composition of LiNO₃ and this was attributed to the dissociation of dopant which lead to the overcrowding of mobile ions that obstruct the pathway for migration process; thus, the value of μ and D decreased [60].