Effect of incorporation of different plasticizers on structural and ion transport properties of PVA-LiClO₄ based electrolytes

Abstract

To date high ionic conducting polymer electrolytes are of great interest because of their potential applications in various electrochemical devices such as batteries, fuel cells, solar cells and super capacitors etc., as electrolytes. Ion conduction through polymer electrolytes can occur mostly in amorphous environment exists above their glass transition temperature (T_g). In order to improve ionic conductivity, many approaches such as addition of plasticizer, blending of polymers, nano composite have been employed. This paper reviews the influence of different plasticizers/additives on the ion transport mechanism of Poly(vinyl alcohol) (PVA)-LiClO₄ polymer electrolytes since poly vinyl alcohol is a semi crystalline, synthetic biodegradable polymer and lithium perchlorate is one of the most moisture resistant lithium salts. This review also reveals the relation between dynamical disorder in polymer electrolyte with ionic conductivity.

1. Introduction

Rechargeable Li ion cells which are the key component as power supply for portable, computing and telecommunication services, consist of two electrodes, a reductant (anode) and oxidant (cathode) which are separated by an electrolyte. Electrolyte plays the major role in transferring ionic components, created during the chemical reaction that occurs inside the cell from cathode to anode and vice-versa [1].

Lithium ion batteries are more attracted than other batteries because of its high operating voltage and high capacity parameters. Most of the commercial Li-ion batteries use liquid electrolytes that have serious issues like leakage, corrosion at electrodes which are to be solved. Solid electrolytes overcome the issues of liquid electrolyte and conduct electricity by the movement of ions with negligible electronic transport. The present focus in lithium ion batteries is to exploit safe polymer electrolytes as solid electrolyte and to finally fabricate batteries with full plastic structure. Such plastic Li⁺ batteries with solid electrolyte are anticipated low cost with mass production than their liquid counterparts. In addition, the presence of solid electrolytes allows packaging in light-weight, less volume plastic containers.

Polymer electrolyte refers to any macromolecular polymer chain with added ionic salt characterized by a significant ionic conductivity approximately of the order of 10⁻⁶ S cm^-1 [2]. They are having advantages such as natural seal, resistance to shock and vibration, resistance to temperature and pressure variation. Main drawbacks of this type of electrolytes are (i) low ambient conductivity and (ii) high interfacial resistance with electrodes. The above drawbacks are to be rectified by different approaches such as blending of two polymers [3, 4], plasticizing [5, 6] the polymer matrix using liquid additive, incorporation of nano sized fillers [7, 8, 9, 10] into polymer matrix etc.

Among them, addition of some small amount liquid into polymer matrix can plasticize the polymer chain as well as enhances the salt dissociation rate. Non volatile organic solvents or low molecular weight polymers are mostly used as plasticizer. For example, polyethylene glycol (PEG-200, PEG-400, PEG-600), dimethylforamide (DMF), ethylene carbonate (EC), propylene carbonate (PC) etc are reported as good plasticizers.

This short review is devoted to deal with PVA based polymer electrolytes, LiClO₄ based polymer electrolytes and plasticized PVA-LiClO₄ polymer electrolytes with various plasticizers. Further, discussions are made with two new types of plasticizers called Sulfolane (Tetra Methyl Sulfone) and Triton (Poly ethylene glycol p-tert octyl phenyl ether) in to PVA-LiClO₄ electrolyte.

2. Main text

2.1. PVA based solid polymer electrolytes (SPEs)

PVA based electrolytes are extensively being studied till now for different applications. These electrolytes really refer solvent-free PVA/salt complexes. Many research investigations have been dedicated to develop proton conducting [11, 12, 13, 14, 15, 16], Mg²⁺ conducting [17], Cu²⁺ conducting [18] and Li⁺ ion conducting [10, 19, 20, 21] and Ag²⁺ [22] ion conducting electrolytes using PVA as host polymer. The electrolytes exhibit conductivity in the range of 10⁻⁷ to 10⁻³ S cm^-1 at ambient temperature and are listed in Table 1. But, the membrane with very low conductivity cannot be used in battery applications.

Table 1.

Electrolyte composition and dc conductivity values for various ion conducting PVA based solid polymer electrolytes.

S. No	Electrolyte composition	dc conductivity at RT (S cm−1)	References
1.	PVA – CH3COONH4	5.62 × 10−6	[11]
2.	PVA – Cu(NO3)2	1.60 × 10−5	[18]
3.	PVA – AgNO3	7.56 × 10−7	[22]
4.	PVA – NH4Br	5.70 × 10−4	[13]
5.	PVA – NH4I	2.50 × 10−3	[14]
6.	PVA –LiAsF6 –TiO2	5.10× 10−4	[20]
7.	PVA – Poly acrylonitrile (PAN) – LiClO4	8.64 × 10−6	[15]
8.	PVA – Mg(NO3)2	7.32 × 10−7	[17]
9.	PVA – Poly vinyl pyrrolidone (PVP) – CH3COONH4	8.12 × 10−5	[19]
10.	PVA – H3PO4	2.56 × 10−3	[16]
11.	PVA – Chitosan – LiClO4	3.00 × 10−6	[10]
12.	PVA – Starch – LiBr	5.00 × 10−3	[21]

Hema et al [14] prepared proton conducting solid polymer electrolyte using ammonium iodide (NH₄I) as ionic salt and Dimethyl Sulfoxide (DMSO) as solvent. They clearly explained the correlation between crystallinity and dc conductivity of electrolyte films. They calculated crystallinity from Differential Scanning Calorimetry (DSC) studies. According to them, the film which contains low crystallinity shows maximum conductivity. This may be due to the reason that increase of amorphous nature reduces the energy barrier for ion movement and there by facilitates the fast ion transport.

PVA is a semi crystalline polymer. In order to prepare high conducting polymer electrolyte using PVA as host polymer, it is essential to reduce its semi-crystalline phase into amorphous using appropriate salts or plasticizers or fillers or blending polymer host. Rathod et al [10] achieved higher ambient temperature conductivity of the order of 10⁻⁶ by incorporating different concentrations of LiClO₄ into PVA- Chitosan matrix. The complex nature between salt and polymer host was confirmed by FT Raman and UV-Vis studies. It was found that the dielectric properties of the composites followed non-Debye behavior.

Blending the polymer host is other way to improve electrical, thermal, mechanical and electrochemical properties of polymer electrolyte. One of such work was done by Chatterjee et al. [21]. They prepared PVA-Potato starch-LiBr (Lithium bromide) blend polymer electrolytes. They achieved highest ambient temperature conductivity of 5 × 10⁻³ S cm^-1 for 20% of LiBr into blend matrix. Enhancement of the conductivity upon addition of salt is correlated to the enhancement of amorphous nature of polymer electrolyte and was also confirmed through XRD studies.

In summary, PVA based solid/blend/nano composite polymer electrolytes were studied by various research groups. In all types of polymer electrolytes, enhancement of conductivity is somehow correlated to the increase of amorphous region of polymer matrix.

2.2. PVA based plasticized polymer electrolytes

This section focuses the plasticized PVA electrolytes which are available in literature. The incorporation of substantial amount of plasticizer in the polymer electrolytes enhances the ionic conductivity of polymer electrolytes. Plasticizer facilitates the reduction of crystalline nature of the polymer matrix and also increases the polymer segmental mobility. It further results in greater ion dissociation which allows greater number of charge carriers for ion transport in the electrolyte. Many research groups have been working in the field of plasticized polymer electrolytes [23, 24, 25, 26, 27, 28, 29, 30, 31]. They have obtained ambient temperature conductivity values in the range of 10⁻⁷ – 10⁻³ S cm^-1. Electrolyte composition and their corresponding dc conductivity values are listed in Table 2.

Table 2.

Electrolyte composition and dc conductivity values for various ion conducting PVA based plasticized polymer electrolytes.

S.No	Electrolyte composition	dc conductivity at RT (S cm−1)	References
1.	PVA – Chitosan – NH4NO3 – Ethylene Carbonate	1.60 × 10−3	[23]
2.	PVA – KI – N-Methyl Pyrolidone-γ-butyrolactone	8.41 × 10−3	[24]
3.	PVA – PMMA – LiBF4 – Ethylene Carbonate	1.28 × 10−6	[25]
4.	PVA – PVP – KClO3 – Dimethyl formamide	7.4 × 10−7	[26]
5.	PVA – PVP – KOH – Propylene Carbonate - Ethylene Carbonate	1.5 × 10−4	[27]
6.	PVA – KOH – Al2O3-Propylene Carbonate	∼10−4	[28]
7.	PVA – LiClO4-[EMIM][EtSO4]	1.9 × 10−6	[29]
8.	PVA – NH4SCN – Dimethyl sulfoxide	2.58 × 10−3	[30]
9.	PVA – PMMA –LiClO4 –Dimethyl Phthalate	6.0 × 10−4	[31]
10.	PVA – LiClO4 – Dimethyl Phthalate	1.49 × 10−3	[32]

The effect of plasticizers (sulfolane/triton) on the structural properties of PVA - LiClO₄ matrix through XRD (Fig. 1) was studied by our group. The characteristic diffraction peak around 2θ = 20° of PVA which shows its semi-crystalline nature was disrupted upon addition of 20 mol% LiClO₄ into PVA matrix. The relative degree of crystallinity is calculated using the formula (Eq. 1),

$${\chi }_C=\frac{I_C}{I_C+I_a}\times 100\text{\%}$$ (1)

where, $I_C$ is integrated intensity of crystalline region.

Fig. 1.

XRD diffractogram of (a) Pure PVA (b) 80mol% PVA:20mol% LiClO₄ (c) 75.3 mol% PVA:20 mol% LiClO₄:4.7 mol% sulfolane (d) 69.9 mol% PVA:20 mol% LiClO₄:10.1 mol% sulfolane (e) 59.3mol% PVA:20 mol% LiClO₄:20.7 mol% sulfolane (f) 78.9 mol% PVA:20 mol% LiClO₄:1.1 mol% triton (g) 78 mol% PVA:20 mol% LiClO₄:2 mol% triton (h) 76 mol% PVA:20 mol% LiClO₄:4 mol% triton electrolytes.

$I_a$ is integrated intensity of amorphous region.

I_c and I_a values were obtained by deconvoluting the XRD spectra. (deconvoluted spectra are not shown). The calculated relative degree of crystallinity values are listed in Table 3.

Table 3.

Relative degree of crystallinity of PVA-LiClO₄-sulfolane/triton electrolytes.

S.No	Sample designation (mol%)	Relative degree of crystallinity (%)
1.	100:0 (Pure PVA)	19.08 ± 0.33
2.	80PVA:20LiClO4	16.86 ± 0.45
3.	75.3 PVA:20 LiClO4:4.7sulfolane	13.38 ± 0.35
4.	69.9 PVA:20 LiClO4:10.1sulfolane	11.78 ± 0.37
5.	59.3 PVA:20 LiClO4:20.7sulfolane	12.99 ± 0.33
6.	78.9 PVA:20 LiClO4:1.1triton	14.60 ± 0.46
7.	78 PVA:20 LiClO4:2 triton	15.44 ± 0.41
8.	76 PVA:20 LiClO4:4 triton	16.23 ± 0.44

While adding various amounts of sulfolane/triton into 80mol%PVA: 20mol% LiClO₄ electrolyte (Fig. 1b), amorphous nature of the electrolyte is further increased. Fig. 1 shows the XRD pattern of PVA-LiClO₄-Sulfolane (Fig. 1 (c–e)) and PVA-LiClO₄-Triton (Fig. 1 (f–h)) electrolytes.

From the Table 3, both plasticizers reduced the crystalline phases available in PVA-LiClO₄ matrix and thus it created sophisticated conducting pathways for charge carriers. Another important observation is that sulfolane effectively plasticize the PVA-LiClO₄ matrix than triton which is reflected on degree of crystallinity values. This result is consistent with the results of conductivity study where sulfolane based electrolytes shows higher conductivity than triton based electrolytes.

Kadir et al. [23] plasticized PVA-Chitosan blend matrix using Ethylene carbonate (EC). The conductivity was found to the order of 10⁻³ S cm^-1 for the addition of 70 wt% of EC. They calculated that the carrier concentration values are 6.57 × 10¹⁹ cm⁻³ and 2.20 × 10²¹ cm⁻³ for bare and 70 wt% EC incorporated electrolytes respectively and mobility values are 1.97 × 10⁻⁶ and 4.54 × 10⁻⁶ cm²V⁻¹S⁻¹ for plasticizer free and plasticizer added electrolytes respectively. Thus the conductivity enhancement is primarily due to carrier concentration enhancement and secondarily due to carrier mobility enhancement. They also observed that EC free electrolyte matrix has some regular arrangement with crystalline phase using Scanning Electron Microscopy (SEM) and X-ray diffraction (XRD) characterization. After the addition of EC, the observed regular arrangement and crystalline phase completely changed into amorphous. Effect of EC on the structural and morphological properties of (PVA-Chitosan) – NH₄NO₃ electrolyte are given in Fig. 2.

Fig. 2.

(a) SEM image (b) XRD pattern of plasticizer free PVA-Chitosan- NH₄NO₃ complex (c) XRD and SEM image of 70 wt % EC (d) Arrhenius plot of plasticizer free and plasticizer added electrolytes [23] "Reprinted from Electrochimica Acta, vol.55(4), M.F.Z Kadir, S.R.Majid, A.K.Arof, Plasticized Chitosan-PVA blend polymer electrolyte based proton battery, 1475–1482 copyright (2018) with permission from Elsevier".

Fig. 2 (d) clearly explained that temperature dependent conductivity obeys Arrhenius rule irrespective of plasticizer addition. It all reveals that plasticizer addition affects the crystallinity and morphological properties and not the conduction mechanism involved.

In some investigations, plasticizer addition did not improve the conductivity in significant manner but it efficiently enhanced the electrochemical properties. One of such work is reported in [26], in which authors used DMF to plasticize PVA-PVP-KClO₃ electrolyte. The conductivity value of bare electrolyte is reported to be 4.6 × 10⁻⁷ S cm^-1 while DMF plasticized electrolyte shows the conductivity value of 7.4 × 10⁻⁷ S cm^-1. It was found that DMF incorporated PVA-PVP-KClO₃ blend polymer electrolyte system shows an increased discharge time and is presented in Fig. 3.

Fig. 3.

Discharge characteristics of DMF free and DMF plasticized PVA-PVP-KClO₃ matrix [26] "Reprinted from Journal of Power Sources, vol.111(2), Ch.V.Subba Reddy, A.K.Sharma, V.V.R.Narasimha Rao, Effect of plasticizer on electrical conductivity and cell parameters of PVP + PVA + KClO₃, 357–360 copyright (2018) with permission from Elsevier".

The electrochemical cells were fabricated using the electrolytes that contain plasticizers like EC, PC and the performance of the cells was analyzed through discharge characteristic studies [27]. It was found that the discharge capacity of the cell using PC plasticized electrolyte delivers better capacity of 145 mAh g⁻¹ than the cell contains EC plasticized electrolyte which delivers capacity of 125 mAh g⁻¹. They concluded that PC effectively plasticizes the polymer matrix than EC.

2.3. PVA based blend and composite polymer electrolytes

As discussed earlier, in addition to plasticized polymer electrolytes, PVA based blend and composite electrolytes have been reported with large number of applications. Some of them are listed in Table 4.

Table 4.

PVA based blend and composite electrolytes and their ambient temperature conductivity along with applications.

S.No	Electrolyte composition	conductivity (S cm−1)	Applications	Reference
1.	PVA – PVP – L-Asparagine doped NH4Br	2.34 × 10−4	Proton battery	[33]
2.	PVA – Mg(Tf)2-EMITf	2.1 × 10−4	Mg ion battery	[34]
3.	PVA – Poly vinylidene Fluoride -LiCF3SO3-SiO2	9.4 × 10−4	Characterization only	[35]
4.	PVA – Poly vinylidene Fluoride -LiCF3SO3	2.7× 10−3	Characterization only	[36]
5.	PVA – Sulfonated Poly(Arylene Ether Ketone Sulfone) copolymers containing carboxylic acid groups	9.4 × 10−2	DMFC	[37]
6.	PVA – PAN-Mg(ClO4)3	2.96 × 10−4	Mg ion battery	[38]
7.	PVA/sodium alginate-Glutaraldehyde	9.1 × 10−2	DMFC	[39]
8.	Cyanoethylated polyvinylalcohol – PAN – LiClO4-Propylene carbonate	1.46 × 10−2	Characterization only	[40]
9.	PVA – Phosphonic acid grafted bis(4-ν-amino propyldiethoxy silyl phenyl) sulfone	4.6 × 10−2	Poly electrolyte/DMFC	[41]
10.	PVA – PAAS-KOH.H2O	0.1	Super capacitor	[42]
11.	PVA – LiBr-H2SO4	1.5 × 10−3	Solid acid PEs/Mg battery	[43]
12.	PVA – Poly(2-Acrylamido2-Methyl-1-Propane Sulfonic acid -Poly(ethylene glycol)bis(carboxymethyl)ether	0.1	DMFC	[44]
13.	PVA – Poly vinylidene -NH4SCN	1.09 × 10−3	Characterization only	[45]
14.	PVA – Poly vinylidene -Montmorillonite-LiTFSI	4.31 × 10−4	Lithium ion battery	[46]
15.	PVA – CH3COONH4- BmImBr	9.29 × 10−3	Characterization only	[47]
16.	PVA – CH3COONH4- BmImCl	5.74 × 10−3	PEMFC	[48]
17.	PVA – P(MA-co-AHPS)-Al2O3	1.08 × 10−3	Lithium ion battery	[49]
18.	PVA – Poly vinylidene fluoride-LiCF3SO3-SiO2	3.7 × 10−3	Characterization only	[50]
19.	PVA – Poly(2-Acrylamido2-Methyl-1-Propane Sulfonic acid)- Zeolitic Imidazolate Framework	0.134 at 60 °C	Characterization only	[51]
20.	PVA – Poly(2-Acrylamido2-Methyl-1-Propane Sulfonic acid)-1,2,4 trizole	2 × 10−3	Characterization only	[52]
21.	PVA – LiCF3SO3-N-methyl pyrrolidone	2 × 10−3	Super capacitor	[53]
22.	PVA – SSA	1.7 × 10−4 at 40 °C	DMFC	[54]
23.	PVA – CH3COOK	4.57 × 10−6	Potassium battery	[55]

Qiao et al. [44] reported low-cost proton-conducting semi-interpenetrating polymer (S-IPN) network of PVA/PAMPS blends with the addition of poly(ethylene glycol)bis(carboxymethyl) ether (PEGBCME) as plasticizer. PAMPS, which is ion conducting site is relatively low content in the blend resulting that S-IPN exhibits proton conductivity of 0.1 S cm⁻¹ at 25 °C. This blend makes higher power density of 51 mW cm⁻² at 80 °C. Composite solid polymer electrolyte (CSPE) based on montmorillonite (MMT) nano-clay fillers was reported [46]. In this report, the effect of MMT on the ionic conductivity and electrochemical properties of the CSPE have been demonstrated. Lithium ions transference number of 0.40 was also reported. Electrochemical performance was evaluated using LiFePO₄ cathode material and achieved a good electrochemical performance with low capacity fading on charge–discharge cycling.

Thus, it is noticed that irrespective of the types of electrolyte, PVA based electrolytes are extensively studied for the fabrication of various electrochemical and energy storage devices [33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55].

2.4. LiClO₄ as ionic dopant

The ionic conductivity of polymer electrolytes is impeded by ion segregation of changed species as ion pairs, or higher ion aggregates called charged multiples of ions. The less ionic conductivity is also observed when the salt with more lattice energy or large inter coulombic interaction is added with polymer host as ionic dopant.

If the solvation energy is strong enough, contact ion pair formation is rather avoided but formation of solvent-separated ion pairs, leading to quasi free ions results significant ionic conduction. Ion dissolution energy by polymer host must be larger than the ion pairing stabilization as marked by the lattice energy of dopant salt. Hence, salt selection is an important parameter in the preparation of polymer electrolytes.

Among all Li salts, LiClO₄ is moisture resistant and thus it eliminates the use of glove box for maintaining inert atmosphere for the preparation of electrolyte. Many investigations on polymer electrolytes are reported based on LiClO₄ as ionic salt such as polymer-LiClO₄ complexes [32, 56, 57, 58] and polymer blend-LiClO₄ complexes [59, 60]. Table 5 presents the list of electrolytes that use LiClO₄ as ionic dopant with its dc conductivity at ambient temperature.

Table 5.

Electrolyte composition and obtained dc conductivity values for LiClO₄ incorporated solid polymer electrolytes.

S.No	Electrolyte composition	dc conductivity at RT (S cm−1)	References
1.	Thermoplastic Polyurethane-LiClO4	8.89 × 10−5	[61]
2.	Poly ethylene oxide-Poly methyl methacrylate- LiClO4	∼10–5	[62]
3.	Poly vinyl chloride- Poly methyl methacrylate - LiClO4	7.17 × 10−3	[63]
4.	Poly ethylene oxide –Poly vinyl pyrrolidone - LiClO4	2.3 × 10−6	[59]
5.	Gelatin- LiClO4	1.04 × 10−4	[64]
6.	Poly ethyl methacrylate – LiClO4	2.34 × 10−6	[56]
7.	Poly ethylene oxide – LiClO4	∼10–6	[57]
8.	Poly vinyl pyrrolidone – LiClO4	∼5 × 10−5	[58]
9.	Poly vinyl acetate/Poly methyl methacrylate – LiClO4	1.76 × 10−3	[60]
10.	Poly vinyl pyrrolidone - LiClO4	IR only	[65]
11.	Poly vinylidene fluoride- LiClO4	∼10–5	[66]
12.	Poly(bis(pentyl amino phosphazene) - LiClO4	8.1 × 10−3	[67]
13.	Poly ethylene oxide -PES- LiClO4	3 × 10−5	[68]
14.	Poly propylene oxide-LiClO4	∼10−5 at 70 °C	[69]
15.	Poly ethylene oxide -LiClO4-α alumina	5.2 × 10−4	[70]
16.	Poly ethylene oxide - PVDF-HFP-LiClO4-TiO2	2.27 × 10−4	[71]
17.	Poly ethylene oxide - PVDF-HFP-LiClO4-SrTiO3	4.82 × 10−5	[72]
18.	Poly ethylene oxide - LiClO4-SiO2	2.3 × 10−5	[73]
19.	Poly vinyl chloride - LiClO4 – ZnO	3.7× 10−7	[74]

LiClO₄ addition destructs the crystalline phase which present in Poly ethyl methacrylate (PEMA) [56] and it is confirmed by obtained scherrer length values. LiClO₄ also enhances amorphous phase of polymer matrix and hence decreases the glass transition temperature [60] of the host polymer which lowers the energy barrier for segmental motion. From the Table 5, it is noticed that the polymer-LiClO₄ complexes show the moderate conductivity values. Nano composite polymer electrolytes have also been developed and reported by many researchers [[8], [73], [74], [75], [76], [77]] to enhance the mechanical strength as well as the conductivity. Another way to increase the electrical and electrochemical properties of polymer-salt complexes is adding plasticizer within the polymer salt matrix. The works based on plasticized polymer electrolytes with LiClO₄ as ionic salt, are reported by many researchers [[63], [66], [78], [79], [80], [81], [82], [83], [84], [89], [90], [91], [92], [93], [95]]. The conductivity values of plasticized polymer electrolytes which contain LiClO₄ as ionic salt are listed in Table 6.

Table 6.

Electrolyte composition and dc conductivity values for plasticized polymer electrolytes which contain LiClO₄ as ionic salt.

S.No	Electrolyte composition	dc conductivity at RT (S cm−1)	References
1.	P(VdF-co-HFP):PVAc – LiClO4		[78]
2.	P(VDF-co-HFP)/PMMA- LiClO4		[79]
3.	poly(vinyl sulfone) – and poly(vinylidene fluoride) – LiClO4/LiN(CF3SO2)2/LiAsF6	10−4 to 10−3	[80]
4.	PEO – PVP – LiClO4 – EC	2.72× 10−4	[81]
5.	PAN – LiClO4 – EC	IR only	[82]
6.	PAN – PC – 1,4-butyrolactone – DMSO – LiClO4	4.7 × 10−3	[83]
7.	PEO – P(VDF-co-HFP) – LiClO4 – PC	2.39 × 10−3	[63]
8.	PVDF – LiClO4 – PC	∼10–5	[66]
9.	Chitosan – Starch – Glyscerol – LiClO4	3.7 × 10−4	[84]
10.	PEO – LiClO4 – DOP-νAl2O3	4.29 × 10−4	[85]
11.	PMMA – LiClO4 – DMP-CeO2	5.36 × 10−5	[86]
12.	PMMA – LiClO4 – PC-SiO2	∼10–3	[87]
13.	P(VDFcoHFP) – PVAc – LiClO4 – EC-PC	2.3 × 10−3	[78]
14.	PVDFcoHFP – PMMA – LiClO4 – PC	∼10−3	[79]
15.	PVP – LiClO4 – PC-BaTiO3	1.2× 10−3	[88]
16.	PVS – PVDF – LiClO4 – EC-PC-Sulfolane	∼10−3	[80]
17.	PAN – LiClO4-ED4CN-PC	1.05 × 10−2	[89]
18.	PPC – LiClO4 – BMIMBF4	1.5 × 10−3	[90]
19.	PEO – LiClO4 – d2000	1.5 × 10−5	[91]
20.	PVDF – PVC – LiClO4 – EC – PC	4.68 × 10−3	[92]
21.	PVC – PAN – LiClO4 – EC – TiO2	4.46 × 10−3	[93]
22.	P(VDF-co-HFP) – PVDF – LiClO4 – PC – DEC	7.5 × 10−3	[94]

⁷Li NMR studies confirm [78] that mobility of charge carriers decreases with increasing PVAc content and is presented in Fig. 4. The line width is found to be broader with PVAc increment in the blend that indicates diminution of li⁺ mobility with PVAc.

Fig. 4.

⁷Li NMR spectra of PVDF-HFP/PVAc – LiClO₄- EC-PC electrolytes [78] "Reprinted from Electrochimica Acta, vol.46(10–11), Nam-Soon Choi, Young-Gi Lee, Jung-Ki Park, Jang-Myoun Ko, Preparation and electrochemical characteristics of plasticized polymer electrolytes based upon a P(VdF-co-HFP)/PVAc blend, 1581–1586, copyright (2018) with permission from Elsevier".

In an another investigation, authors [80] prepared plasticized polymer electrolytes by mixing poly vinyl sulfone (PVS) and poly vinylidene fluoride (PVDF) with high conductive solutions of LiClO₄, LiN(CF₃SO₂)₂ and LiAsF₆ which were dissolved in PC, EC and sulfolane mixtures. It has been found that PVS based electrolytes have shown the conductivity of the order of 10⁻⁴ S cm^-1 and PVDF based electrolytes have shown the conductivity of the order of 10⁻³ S cm^-1 at 30 °C. But, in the case of electrochemical stability, PVS based electrolytes have shown better stability (4.5–4.8 V vs Li/Li⁺) than PVDF based electrolytes (3.9–4.3 V vs Li/Li⁺).

In summary, it is concluded from the literature reports and our experimental work that plasticizer incorporation into polymer-salt preferably poly (vinyl alcohol) as host polymer and LiClO₄ as ionic dopant can have great impact on electrical and electrochemical properties. The enhancement of amorphous nature and disordered morphologies improves the electrochemical performances of the electrolytes.

2.5. Ion transport studies of plasticized PVA – LiClO₄ based electrolytes

The effect of plasticizers, sulfolane/triton incorporation on the ion transport properties of PVA-LiClO₄ electrolytes is discussed here. A series of PVA-LiClO₄ electrolytes are prepared and among them 80mol%PVA-20mol%LiClO₄ shows the conductivity of 5.64 ± 0.44 × 10⁻⁵ S cm^-1 at room temperature [95] and its Nyquist plot is given in Fig. 5a. The plot consist two regions: 1) incomplete semicircle at high frequency region and 2) inclined line at low frequency region. Typically, the semicircle part relates ionic conduction due to bulk of the sample and the inclined line is due to the effect of electrode polarization.

Fig. 5.

Nyquist plot of (a) PVA-LiClO₄ (b) sulfolane/triton plasticized PVA-LiClO₄ electrolyte.

From Fig. 5b, it can be seen that (sulfolane/triton) plasticized polymer electrolytes show only slanted line region. That is semi circle region is disappeared.

The representation of equivalent circuit corresponds to Nyquist plot is an easy way to provide complete picture of the system. In the present work, PVA-LiClO₄ and plasticized PVA-LiClO₄ electrolytes are fitted using z-fit software.

The equivalent circuit corresponds to the unplasticized electrolyte (Fig. 5a insert) shows parallel combination of resistance (R₁) and constant phase element (Q₁) along with another constant phase element (Q₂) in series. Conductivity of 80mol%PVA-20mol%LiClO₄ electrolyte is enhanced to 1.14 ± 0.20 × 10⁻² S cm^-1 and 1.92 ± 0.23 × 10⁻³ for the addition of 10 mol% sulfolane and 1 mol% triton respectively [95,96]. Plasticized PVA-LiClO₄ electrolytes at room temperature are shown in Fig. 5b. From the Fig. 5b, inclined line alone detected in the frequency range studied. The existence of the inclined line in the region reveals occurrence of polarization of ions at the electrode-electrolyte interface. Hence, the addition of plasticizer (Sulfolane and Triton) causes the enhancement of ion concentration in the PVA- LiClO₄ matrix. The disappearance of semicircle suggests the resistive component of polymer prevails. The angle between the inclined line and X-axis is less than 90^ο that suggests non-homogenous or roughness nature of electrode-electrolyte interface [97]. The corresponding equivalent circuit is given as insert Fig. 5b which is a series combination of resistance (R₁) and constant phase element (Q₂). Similar results were found in literatures [3, 31] in which PMMA-PVA LiClO₄ electrolyte doped with Dimethyl Phthalate (DMP) and PVA-LiClO₄ electrolyte doped with same DMP plasticizer depict Nyquist plot as slanted region only. Enhancement of charge carrier concentration is correlated by incorporation of plasticizer. Thus it proves that sulfolane/triton addition enhance the charge carrier concentration.

Fig. 6 shows the dielectric loss spectra for bare and sulfolane/triton plasticized PVA-LiClO₄ electrolytes.

Fig. 6.

Dielectric loss spectra of PVA-LiClO₄ and Sulfolane/Triton plasticized PVA-LiClO₄ electrolytes.

From the Fig. 6, plasticizer free electrolyte does not show any relaxation but shows only polarization. It reveals that PVA chains are rigid and conduction is only due to available free charge carriers. Sulfolane plasticized electrolyte shows two types of relaxations such as fast and slow relaxations. Triton plasticized electrolytes show single relaxation process which reveals plasticizer enhances segmental mobility of polymer chains and thus enhances the free volume there by facilitate the ion transport. That is relatively fast segmental motion coupled with mobile ion enhances the ion transport in PVA matrix. It proves that plasticizer addition not only enhances amorphousity of host polymer and carrier concentrations but also improves the segmental mobility of polymer chains. Thus plasticizers create an effective and sophisticated pathway for the transportation of ions. Fig. 7 shows the variation of tangent loss with frequency of PVA-LiClO₄ electrolyte with different plasticizers.

Fig. 7.

Loss tangent spectra of PVA-LiClO₄ and Sulfolane/Triton plasticized PVA-LiClO₄ electrolytes.

The tangent loss spectra characterized by peak appearing at a characteristic frequency. Relaxation peak is appeared for plasticizer free sample whereas relaxation peak is not observed in the measured frequency range for the plasticized electrolytes. At the same time, the magnitude of loss tangent for plasticized electrolytes is very high than unplasticized one. This is because of the reason that plasticizer increases the amorphousness content in the materials (which was already proven by XRD reports) and mobile dilute molecules (sulfolane and triton) speed up the segmental motion by increasing the available free volume. It is evidenced by shifting of peak maxima towards higher frequency side there by reduces the relaxation time. Similar results have also been observed in literatures [7,98,99]. Thus it is substantiated that plasticizer addition not only affects the structural properties of polymer-salt matrix but also influences on ion transport properties. On the comparison of two plasticizers sulfolane shows better performance than Triton due to availability of strong polar groups.

2.6. Static and dynamical disorder, ionic conductivity correlation

The structure and morphology of the polymer chain cause static disorder. Polymer materials normally do not show any crystalline phase in its structure and hence X-ray diffraction or optical microscopy techniques cannot be useful. The dynamic disorder is totally a complicated process than static disorder since the local environment of a particular site of the polymer undergoes considerable change with time due to liquid like motions [100].

Polymer electrolytes are usually studied above glass transition temperature, T_g, the temperature below which the polymer electrolyte behave as glass like and no flexible chain movement. If the temperature of the polymer electrolyte exceeds glass transition temperature, the local structure is in solid like structure with highly viscous nature along with fast changes in local bond as well as bond angle. It is also important to note that the materials above T_g can flow easily for a short chain polymer electrolyte and behave as liquid like. In dynamically disordered system, nearest atoms about a specific site will always changes with time.

The decoupling index, R_τ, (the ratio between relaxation time corresponding structural relaxation and τ_σ relaxation time corresponding to conductivity) is normally observed as unity in the polymer electrolytes. Sometimes it occurs below unity for concentrated electrolytes where structural relaxations do not contribute any ion movement. For most of the polymer materials, the temperature dependence of the conductivity can be represented by Vogel-Tamman-Fulcher formula and is given in Eq. (2).

$$\text{σ(T)}={\text{σ}}_{\text{0}}\text{exp}\left[-\text{B}/\left(\text{T}-{\text{T}}_{\text{0}}\right)\right]$$ (2)

As the polymer materials are always studied above the glass transition temperature, both static and dynamical disorders occur.

From the above discussions, addition of plasticizer could enhance the amorphous nature polymer-salt complexes. Hence static disorderness is enhanced and thus it reflects in the ionic conductivity values. At the same time incorporation of salt or/and liquid additive or plasticizer reduces the glass transition temperature (T_g) of polymer and hence dynamical disorder also enhances. The glass transition temperature (T_g) of PVA-LiClO₄ is found to be 63 °C. On the addition of Ionic Liquid (IL), T_g is further reduced where as T_g of pure PVA is 92 °C [29]. Hence above 60 °C, dynamical disorder also arises in addition to static disorder. Hence, addition of different types of plasticizer into polymer-salt matrix improves the static and dynamical disorder of the system and thus enhances the ionic properties.

2.7. Drawbacks of plasticized polymer electrolytes

Main drawback of plasticized polymer electrolytes is lower thermal stability while comparing solid polymer electrolytes (SPEs) and Nanocomposite Polymer Electrolytes (NCPEs). According to literature report [29], incorporation of Ionic Liquid 1-ethyl-3-methyl imidazolium ethyl sulfate [EMIM] [EtSO₄] into PVA-LiClO₄ matrix, reduces the thermal stability of bare electrolyte. The TGA curves of IL added PVA-LiClO₄ electrolyte is given in Fig. 8 (a). From Fig. 8 (a), it is clear that addition of IL into PVA-LiClO₄ system reduces the thermal stability. It is possibly due to complexation of EMIM cation of Ionic liquid with the hydroxyl group of the polymer, which destabilizes the adjacent C–H bond soft the PVA backbone. Fig. 8 (b) shows the TGA curves for Pure PVA, PVA-LiClO₄ and PVA-LiClO₄-Sulfolane/Triton electrolytes. Pure PVA shows thermal stability at 353 °C. Thermal stability is reduced to 252 °C for the addition of LiClO₄ into PVA matrix. The thermal stability of PVA-LiClO₄ complex is further reduced for the addition of various plasticizers such as sulfolane/triton.

Fig. 8.

TGA curves (a) for (a) 0 (b) 5 (c) 10 and (d) 15 wt % of IL added PVA-LiClO₄ electrolytes [29] "Reprinted from Journal of Physics and Chemistry of Solids, 73(2), A.L.Saroj, R.K.Singh, Thermal, dielectric and conductivity studies of PVA/Ionic liquid[EMIM}{EtSO₄} based polymer electrolytes, 162–168, copyright (2018) with permission from Elsevier" and (b) for PVA-LiClO₄ Sulfolane/Triton electrolytes.

Hence, while plasticizing the polymer matrix, conductivity and electrochemical properties are improved but thermal properties of polymer electrolytes become reduced. In order to compensate these properties, nano filler incorporation within plasticized polymer electrolytes has been suggested by many researchers [[85], [86], [87], [88], [101]]. The combined effect of filler and plasticizer on polymer-salt matrix can lead to better electrical, electrochemical as well as thermal properties.

3. Conclusions

Effect of inclusion of plasticizers on structural properties of polymer electrolytes is discussed with aid of literature reports available. It is evidenced from literature reports and from our work that the incorporation of substantial amount of plasticizer has effectively enhanced the amorphous nature, ionic conductivity, electrochemical performance of polymer electrolytes. Ion transport properties of prepared sulfolane/triton plasticized PVA-LiClO₄ are analyzed. It is found that sulfolane performs better than triton as plasticizer on the enhancement of ion transport properties. Incorporation of liquid additive into polymer-salt matrix may affects/reduces the thermal and mechanical stability of electrolyte. In order to balance, incorporation of nano filler (metal oxide) into plasticized polymer-salt complex is suggested. Conductivity enhancement by the addition plasticizer is also correlated with enhancement of static and dynamical disorderness of polymer-salt complexes.

Declarations

Author contribution statement

All authors listed have significantly contributed to the development and the writing of this article.

Funding statement

This work was supported by Science and Engineering Research Board, Department of Science and Technology, Government of India through sanction number SERB SR/FTP/PS-126/2010.

Competing interest statement

The authors declare no conflict of interest.

Additional information

No additional information is available for this paper.