Journal of Ravishankar University–B, 32 (1), 76-83 (2019)

Ion Transport and Materials Characterization Studies on Hot-Press Cast Zn²⁺ Conducting Nano-Composite Polymer Electrolyte (NCPE) Films: [90 PEO: 10 Zn (CF₃SO₃)₂] + xAl₂o₃

Shrabani Karan*, R.C. Agrawal

School of Studies in Physics & Astrophysics, Pt. Ravishankar Shukla University, Raipur – 492010, CG, India

*Corresponding Author:  shrabo12karan@gmail.com

[Received: 16 February 2019; Revised version: 17 March 2019; Accepted: 27 March 2019]

 

Abstract. Investigations on ion-transport and materials properties of poly (ethylene oxide) (PEO) based Zn²⁺ conducting Nano-Composite Polymer Electrolyte (NCPE) membranes: [90 PEO: 10 Zn (CF₃SO₃)₂] + xAl₂O₃, have been reported. NCPE films have been prepared by a completely dry hot-press cast technique using Solid Polymer Electrolyte (SPE) composition: [90 PEO: 10 Zn (CF₃SO₃)₂] as I phase and Al₂O₃ nano-filler particles (< 50 nm) as II- Phase dispersoid. In an earlier study, SPE used here as I phase host has been identified as optimum room temperature conducting film exhibiting (σ_rt) ~1.01 x 10^-5 S/cm. As a consequence of fractional dispersal of nano-filler particles in SPE, additional σ_rt enhancement of an order of magnitude was obtained. This has been referred as NCPE OCC film. Ion transport behavior in NCPE OCC has been characterized in terms of ionic conductivity (s), total ionic (t_ion)/cation (t₊) transport numbers which have been measured using different ac/dc techniques. Temperature dependent conductivity study has also been carried out to understand the mechanism of ion transport and to compute activation energy (Ea) from ‘log σ-1/T’ plot. Materials and thermal properties have been characterized with the help of SEM, XRD, FTIR and DSC / TGA techniques.

 

Keywords: nano composite polymer electrolyte (ncpe), hot-press casting procedure, ionic/cationic transference number, all-solid-state polymer batteries.

Introduction

Dry flexible polymer electrolyte films show great technological promises to develop all Solid-State electrochemical devices viz. primary /secondary batteries in any desired shapes/size including mini/micro/ printable batteries [Kim et al., 2015; Ponronch et al.,2015; Zhon et al., 2014; Quartarone et al., 2011; Agrawal et al., and Tarascon et al., 2001]. Pure polymers often show high insulating property. However, they can be made conducting i.e. electron / ion / mix conducting by complexing / dissolving electron and / or ion conducting salts in polymeric host. Ion conduction in polymer was reported for the first time in 1973 by Fenton et al and the first practical all Solid-State battery, based on Solid Polymer Electrolyte (SPE) i.e. poly (ethylene oxide) PEO complexed with Li⁺ salt, was demonstrated in 1979 by Armand et al. These breakthrough discoveries stimulated scientists / researchers worldwide to explore more for such materials. As a result, variety of ion conducting polymers involving different mobile ion species viz. H⁺ , Ag⁺ , Li⁺ , Na⁺ , K ⁺ , Mg^2+, Cu⁺ , Cu^2+, Zn²⁺ etc. have been discovered and tested for electrochemical device applications in the last nearly 4 decades [Kim et al., 2015; Ponronch et al.,2015; Zhon et al., 2014; Quartarone et al., 2011; Agrawal et al., and Tarascon et al., 2001; Fenton et al., 1973; Armand et al., 1979; Armand et al., 1986; Ratner et al., 1988; MacCallum et al., 1987; Murata et al., 1995; Bruce et al,; Gray et al., 1997;  Gray et al., 1999; Croce 1998; Appetecchi 2000]. Most of SPEs, reported in the past, used high mol. wt. polar polymer: poly (ethylene oxide) PEO as common host to complex ionic salts containing larger size anions. PEO exists in mixed semicrystalline / amorphous phase at room temperature. The amorphous region in PEO increases gradually as temperature increases followed by a characteristic semicrystalline to complete amorphous phase transition at ~ 690C. The degree of amorphousity predominantly controls the ion conduction phenomenon in the polymer-salt complexes. Larger is the amorphous region, higher is σrt. PEO also has relatively higher dielectric constant. Hence, it has an inherent ability to dissolve variety of ionic salts in larger proportion as compare to other polymers. The polar and flexible main chains of PEO dissociate the salt completely providing free ions for transport through amorphous region of polymer via interchain / intrachain segmental motion. However, majority of PEO based SPEs exhibit relatively lower value of ionic conductivity (σrt ≤ 10-4 S cm^-1) at room temperature, hence, not much suitable for practical device applications. Nevertheless, σ_rt can be increased substantially by fractional dispersal of low dimension (µm or nm) particles of an insulating / inert filler material such as Al₂O₃, SiO₂, TiO₂ etc. as IInd-phase dispersoid into SPE which acts as Ist-phase host [Agrawal et al., and Tarascon et al., 2001; Fenton et al., 1973; Armand et al., 1979; Armand et al., 1986; Ratner et al., 1988; MacCallum et al., 1987; Murata et al., 1995; Bruce et al,; Gray et al., 1997;  Gray et al., 1999; Croce 1998; Appetecchi 2000]. Such systems are referred to as Composite Polymer Electrolytes (CPEs) in general and Nano-Composite Polymer Electrolytes (NCPEs), if filler particles of nano dimension are dispersed. σrt- enhancement in CPEs/NCPEs is primarily attributed to increase of amorphous region in PEO due to dispersal of filler particles. Akin to 2- phase inorganic composite electrolytes [Agrawal et al., 1999], CPEs/NCPEs are 2 –phase organic composite electrolytes. The dispersal of nano particles also improves several other physical properties such as mechanical / electrochemical stability of the film, intimate electrode / electrolyte contacts as well as enhanced interfacial reactivity during battery operation etc. [Tarascon et al., 2001]. Dry polymer electrolyte films are usually prepared by traditional solution cast method. However, an alternate procedure, popularly referred to as hot-press (extrusion) technique, is currently being employed widely for casting SPE/NCPE films. Modern portable batteries, available today at large commercial scale, are mostly Li⁺ batteries based on lithium chemicals. These batteries recently encountered some serious safety hazards viz. inflammability and the reason identified primarily was the use of lithium chemicals [MacCallum et al., 1987; Murata et al., 1995]. Hence, on account of high-priority safety of the battery while in use, it has been felt strongly in the recent years to look for battery components i.e. electrolyte and electrodes completely free from lithium chemicals. Attempts have already been initiated and numbers of non-lithium chemical based dry polymer electrolytes have been investigated in the recent past [; Murata et al., 1995]. In view of this, the present paper reports synthesis of Zn²⁺ conducting dry polymer electrolyte systems. Zn chemical-based battery components are fundamental from the point of view of safety of batteries. Although, zinc electrode potential is only 0.76 V against the Standard Hydrogen Electrode (SHE) and electrochemical equivalence is 0.82 Ah g^–1 as compared to lithium electrode (3.05V vs. SHE; 3.86 Ah g^-1). However, there are number of advantageous features associated with zinc chemicals. They are inexpensive, non-toxic, stable, non-reactive and batteries of high specific / volumetric energy density can be fabricated. Moreover, ionic radii of Zn²⁺ (74 pm) and that of Li⁺ (68 pm) are quite comparable, while Zn²⁺ displaces twice as much charge as Li⁺ (Cairns et al.). In the present investigation, Zn²⁺ conducting NCPE films: [(90 PEO: 10 Zn (CF₃SO₃)₂] + x Al₂O₃ of varying filler concentration (x) have been prepared using SPE composition: [90 PEO: 10Zn (CF₃SO₃)₂] as I^st-phase host and Al₂O₃ nano filler particles (<50 nm) as II nd-phase dispersoid. SPE composition, used here as Ist-phase host, was identified earlier exhibiting optimum value of room temperature conductivity (σ_rt ~1.09 ×10^-6 S/cm) which is approximately three orders of magnitude higher than that of pure PEO (σ_rt ~3.9 ×10^-9 S/cm). Further, NCPE film exhibiting optimum room temperature conductivity (σ_rt), referred to as Optimum Conducting Composition (OCC) NCPE film, has been identified again and the ion transport / materials / thermal properties have been characterized in order to evaluate its possible / potential applications in All-Solid-State battery.

Experimental

NCPE films: [90 PEO: 10Zn (CF₃SO₃)₂] + x Al₂O₃ where x = 1, 2, 3, …...,10 wt (%), have been prepared by hot –press cast technique, originally proposed by Gray et al [Polu et al., 2014] and adopted by several groups [Appetecchi, et al.,1999]. Hot-press casting has several procedural advantages over the 4 traditional solution cast method and has been recognized as relatively quicker, least expensive, completely dry/solution free procedure for casting dry polymer electrolyte films. Pre-dried precursor chemicals: PEO (purity > 99%, Mw ~ 6x 10⁵, Aldrich, USA), and Zn (CF₃SO₃)₂ (98%, Aldrich, USA), Al₂O₃ (< 50 nm, > 99 % Sigma Aldrich, USA) have been used for casting the films. Initially, SPE film: [90 PEO: 10 Zn (CF₃SO₃)₂], to be used as Ist–phase host, has been hot- press cast as before [Karan et al., 2017]. Dry powders of PEO: Zn (CF₃SO₃) _2: 90: 10 wt (%) ratio were mixed physically for ~30-40 min. in an Agate mortar/ pestle, then the homogeneous mixture was heated close to melting/softening point of PEO with mixing continued for another ~30-40 min. As a result, a soft slurry/lump was obtained which was pressed between two cold SS blocks to form SPE film of uniform thickness ~100-250µm. NCPE films have also been prepared in the same way by mixing dry powders of PEO: Zn (CF₃SO₃₎₂: Al₂O₃: 90: 10: x (where ‘x’ is filler particle concentration wt. (%) as mentioned). All the experimental measurements were done on pre-dried film sample (dimension: thickness ~ 270 µm, area of cross section ~ 1.23cm²). NCPE OCC film has been identified form filler particle concentration dependent conductivity study. Characterization of ion transport property in NCPE OCC film was done in terms of conductivity (σ), total ionic (t_ion) and cationic (t₊) transference numbers. σ-measurements were carried out by Electrochemical Impedance Spectroscopy (EIS) using a multi frequency (1 mHz – 200 KHz) LCR- meter (HIOKI IM 3533, Japan). The film sample was placed between two SS electrodes. Temperature dependent conductivity was also studied to understand the mechanism of ion transport operative in the system and to compute activation energy (Ea) by linear list square fitting of ‘log σ- 1/T’ Arrhenius plot. The total ionic transference number (t_ion) was determined by dc polarization Transient Ionic Current (TIC) technique [Chandra et al., 1988; watanbe et al., 1958]. In this measurement, film sample, placed between SS (blocking) electrodes, was subjected to a fixed dc polarizing potential (V) ~ 1V and the current were monitored as a function of time. tion was evaluated from (Iion / IT) ratio obtained from ‘Current - Time’ TIC plot. Cation (Zn²⁺⁾ transport number (t₊) was measured separately using a combined ac/ dc technique [Evans, 1987]. The film sample, placed between Zn (non-blocking) electrodes, was subjected to a fixed dc potential ∆V ~ 1V and t+ was evaluated with the help of following equation:

 

𝑡₊ = 𝐼_S (Δ𝑉 − 𝐼₀ 𝑅₀) / 𝐼₀ (Δ𝑉−𝐼_S 𝑅_S) --------- (1)

 

Where, I₀/I_S and R₀/R_S, the initial/final current/resistance values before/after polarization were obtained from ‘Current -Time’ and Z΄ - Z΄΄ impedance plots respectively. The materials and thermal properties have been studied using SEM (JSM-IT300, In TouchScope^TM  Scanning Electron Microscope), XRD (D2 phaser model: 08 discover, X-Ray Diffractometer, Bruker, CuKα λ = 1.5405 A^o), FTIR (IR Affinity – 1, Fourier Transform Infrared Spectroscope, Shimadzu Japan) and DSC (STARe, SW 13.00, Diffraction Scanning Calorimeter METTLER) / TGA (STARe SYSTEM TGA1 SF/1100, Thermo Gravimetric Analyzer, METTLER) techniques. Also, from DSC thermograms, degree of crystallinity (Xc) has been evaluated with the help of the following equation [Shin et al., 2003; Ibrahim et al., 2012]

X_c = (ΔH_m/ ΔH◦_m) X 100 %

where ΔH_m is the heat enthalpy of pure and salt complexed polymers which was obtained from the area of respective endothermic peaks, ΔH⁰m (~ 213.7 Jg^-1) is the theoretical value of heat enthalpy of pure polymer having 100% crystalline phase. All-Solid-State battery in the cell configuration:

Zn (Anode) || NCPE OCC || (I₂+C+electrolyte) (Cathode)

The cell potential discharge performance has been studied with load resistances viz. 100 kΩ & 60 kΩ and some important cell parameters have been evaluated from the plateau region of the cell potential discharge profiles.

Results and Discussion

Characterization of Ion Transport Property

 

Room temperature conductivity study on NCPE films: [90PEO:10Zn(CF₃SO₃)₂] + x Al₂O₃:

Fig.1 shows ‘log σ - x’ variation at room temperature for different NCPE films: [90PEO:10Zn(CF₃SO₃)₂] + x Al₂O₃. Two σ – maxima (indicated by arrow) can be sighted in the plot at x = 2 and 8 wt. (%). Inset in the figure shows Z΄ - Z΄΄ plots for NCPE films: [90PEO:10Zn(CF₃SO₃)₂] + 2 Al₂O₃ and [90PEO:10 Zn (CF₃SO₃)₂] + 8Al₂O₃. The existence of two σ- maxima has often observed in majority of the CPE/NCPE films reported in the past and the reason assigned for this has been two kinds of conductivity mechanisms operative in the system and explained on the basis of two separate percolation threshold phenomenon [Ibrahim et al., 2012]. However, σ – enhancement in NCPE film may be related to several other factors viz. increase in degree of amorphous region in PEO due to dispersal of IInd-phase dispersoid. Increase in mobile ion concentration at the space charge region created at Ist / IInd –phase interface boundaries and/ or increase in ionic mobility due to creation of high conducting paths interconnecting different space charge regions. NCPE film: [90PEO:10 Zn (CF₃SO₃)₂] + 8Al₂O₃ exhibited relatively higher σ_rt (~ 1.01 x 10-5 S/cm), hence, has been referred to as NCPE OCC film. σrt-enhancement of nearly an order of magnitude has been achieved further in NCPE OCC film simply by dispersing fractional amount of Al₂O₃ nano–particles into SPE host and an overall σ_rt-increase of nearly four orders of magnitude from that of pure polymeric host PEO has been obtained. Table 1 lists σrt value for SPE host, NCPE OCC film along with pure PEO as well as values of E_a, t_ion and t₊ values obtained below for SPE host/ NCPE OCC films.

 

Table 1 Values of σrt , Ea, tion, t+ for NCPE OCC film: [90PEO: 10 Zn(CF3SO3)2] + 8 Al2O3and SPE host film: [90 PEO: 10 Zn(CF3SO3)2] [30] along with σrt of pure PEO
Film Sample	σrt	Ea	tion	t+	Ref
Pure PEO	3.2x10-9	-	-	-	-
SPE host: [90 PEO: 10 Zn (CF3SO3)2]	1.09x 10-6	0.27	0.17	0.97	30
NCPE OCC: [90PEO: 10 Zn (CF3SO3)2] +8Al2O3	1.01x 10-5	0.19	0.24	0.98	Present study

 

 

Figure 1. ‘Log σ - x’ plot for hot press-cast NCPE films: [90PEO: 10Zn(CF3SO3)2] + x Al2O3 (Inset) Z΄ - Z΄΄ plots for NCPE films: [90PEO:10Zn(CF3SO3)2] + 2 Al2O3 & [90PEO:10Zn(CF3SO3)2] + 8Al2O3.	Figure 2. ‘Log σ –1/T’ plot for NCPE OCC film: [90 PEO: 10 Zn (CF3SO3)2] + 8 Al2O3; SPE host film: [90 PEO: 10 Zn (CF3SO3)2] [30]

Temperature dependent conductivity study:

Fig.2 shows ‘log σ –1/T’ plot for NCPE OCC film: [90PEO: Zn (CF₃SO₃)2] + 8Al₂O₃. Similar plot for SPE film: [90PEO: 10 Zn (CF₃SO₃)₂] has been reproduced [30]. It can be noticed that initially conductivity increased linearly as temperature increased, followed by a slight upward change in slope around ~60-700C. This temperature region belongs to characteristic semicrystalline-amorphous phase change of PEO.

‘Log σ – 1/T’ plot below this temperature region can be expressed by following Arrhenius straight line equation, which is indicative of ion transport via jump/ hop mechanism:

𝒍𝒐𝒈 𝝈 = 𝒍𝒐𝒈 𝝈_𝟎 − 𝑬_𝒂 / 𝒌𝑻 ---------- (3)

Where: σ0 is pre-exponential factor, k is Boltzmann constant and Ea is the activation energy. E_a has been computed by least square linear fitting of ‘log σ – 1/T’ below 600C and found to be ~ 0.27eV (NCPE OCC film), 0.19 eV (SPE host [30]).

Total ionic (tion) and cationic (t+) transference number studies:

Total ionic transference number (ti_on) was evaluated by TIC technique [Chandr et al; watanbe et al.], as mentioned in Section 2. Fig. 3 shows ‘Current – Time’ TIC plot for NCPE OCC film: [90PEO:10 Zn (CF₃SO₃)₂] + 8 Al₂O₃. TIC plot (Not shown here) for SPE film: [90PEO: 10 Zn (CF₃SO₃)₂] appeared identical [Karan et al.,2017]. t_ion value obtained from the ratio (Iion/ IT), was in the range: 0.97 – 0.98 when both the film samples were polarized under fixed dc potential (1V) for nearly 150 min. tion is very close to unity and hence, indicative of the fact that SPE/NCPE OCC film materials are predominantly ionic. Cation (Zn²⁺⁾ transport number (t+) was determined using a combined ac/ dc technique [Ibrahim et al., 2012] and evaluated with help of equation (1), as mentioned in Section 2. The values of I0 / Is and R0 / Rs for NCPE OCC film were obtained from ‘Current - Time’ and Z΄- Z΄΄ complex impedance plots, shown in Fig. 4 and its inset respectively. Identical plots (Not shown here) were obtained for SPE host film [Karan et al., 2017]. t₊ was found to be ~ 0.24 & ~ 0.17 for NCPE OCC and SPE films respectively (see Table 1). It can be clearly noted that t+ value increased substantially after dispersal of Al2O3 in SPE film.

Figure 3. ‘Current -Time’ TIC plot for NCPE OCC film: [90 PEO: 10 Zn (CF3SO3)2] +8 Al2O3.	Figure 4. ‘Current - Time’ and (Z΄- Z΄΄) (inset) plots for NCPE OCC film: [90 PEO:10 Zn (CF3SO3)2] +8 Al2O3

 

Characterization of Materials Property

SEM Surface Morphology Study

Fig. 5 shows SEM surface morphology of pure PEO (a), SPE host (b) and NCPE OCC (c) films. All the surface images looked smooth along with the changes in the appearance due to complexing of salt (pattern b) and/or dispersal of filler material particles (pattern c).

Figure 5. SEM morphology study for: (a) pure PEO, (b) SPE host: [90 PEO: 10 Zn(CF3SO3)2], (c) NCPE OCC film : [90 PEO: 10 Zn(CF3SO3)2]+ 8 Al2O3

 

 

 XRD and FTIR studies

Fig.6 illustrates XRD patterns for pure PEO, complexing salt Zn (CF₃SO₃)₂, SPE host film: [90PEO: 10 Zn (CF₃SO₃)₂] [Karan et al., 2017] and NCPE OCC film: [90PEO: 10 Zn (CF₃SO₃)₂] +8 Al₂O₃. On comparing these patterns, it can be clearly noticed that the intensity of two main peaks of PEO at 2θ ~ 19.3° and 23.3° have been suppressed substantially after complexation of salt in PEO and/ or dispersal of nano-filler particles in SPE host. These changes clearly indicated the complexation/dissolution of salt in PEO and / or dispersal of filler material particles in SPE host. Also, suppression of main peaks of PEO is indicative of decrease in degree of crystallinity and/or increase in degree of amorphosity in host polymer Complexation of salt in PEO and dispersal of IInd –phase filler particles in Ist −phase SPE host have also been confirmed by FTIR spectroscopy. FTIR spectra for pure PEO, complexing salt Zn (CF₃SO₃)₂, SPE host [30] and NCPE OCC film are shown in Fig.7. In the spectra of PEO the vibrational bands appeared at ~ 2238, ~ 2163 and ~1963 cm^-1; peaks at ~525–530/~1200 cm^-1 are related to C-O-C bending / stretching and bands in the range ~ 750–950, ~1820, ~2900–3000, ~1475, ~845 cm^-1 belong to symmetrical/ asymmetrical stretching/ vibration of CH₂ group, CH₂ bending, CH₂ rocking etc. Characteristic bands of Zn (CF₃SO₃)₂ viz. symmetric deformation, asymmetric stretching and SO₃ modes of triflate ion (CF₃SO₃)- appeared at ~ 769.60, ~1151.50 and 663.51cm^-1 respectively. On comparing, these spectra with those of SPE host and NCPE OCC films, substantial changes can be noticed, which are further confirmations of complexation of salt in PEO and / or dispersal of filler material in SPE host.

Figure 6. XRD for: (i) pure PEO, (ii) Zn (CF3SO3)2 salt, (iii) SPE host: [90 PEO: 10 Zn (CF3SO3)2], (iv) NCPE OCC: [90 PEO: 10 Zn (CF3SO3)2] + 8 Al2O3	Figure 7. FTIR for: (i) pure PEO, (ii) Zn (CF3SO3)2 salt, (iii) SPE host: [90 PEO: 10 Zn (CF3SO3)2], (iv) NCPE OCC: [90 PEO: 10 Zn (CF3SO3)2] + 8 Al2O3

Characterization of Thermal Property

DSC and TGA studies

Fig.8 shows DSC thermograms for pure PEO, SPE host: [90PEO:10 Zn (CF₃SO₃)₂] and NCPE OCC: [90PEO:10 Zn (CF₃SO₃)₂] + 8 Al₂O₃. The sharp endothermic peak at (T_m) ~71.09 0C (curve a) belongs to the characteristic transition of pure PEO from mixed semicrystalline-amorphous to complete amorphous phase. One can notice that as consequence of complexation of salt in PEO and/ or dispersal of filler material in SPE host (curves ‘b’ & ‘c’), the peak position sifted slightly towards lower temperature region i.e. 69.66 and 65.98⁰C respectively. The peak area also reduced significantly. Shift in peak position is usually considered as confirmation of complexation of salt and/ or dispersal of filler material, while reduction in peak area relates to decrease in degree of crystallinity in PEO. The relative percentage of crystallinity (Xc) of PEO as well as SPE host and NCPE OCC films have been evaluated with the help of Eq.2 (shown in Section 2) and listed in Table 2 along with T_m and ΔH_m values. One can clearly notice the substantial reduction in degree of crystallinity and /or increase in degree of amorphosity of PEO after complexation of salt in PEO and/or dispersal of filler material in SPE host. These results clearly supported our XRD results discussed in Subsection 3.2. Fig.9 shows TGA curves for pure PEO (curve a), SPE host (curve b) and NCPE OCC (curve c) and Table 3 lists the values of decomposition temperature, weight (%) loss. It can be clearly noted that as a consequence of complexation of salt in PEO and / or dispersal of filler material in SPE host, the thermal stability of the film material increased significantly along with substantial decrease in weight loss (%).

Figure 8. DSC curves for: (a) pure PEO, (b) SPE host: [90 PEO: 10Zn(CF3SO3)2], (c) NCPE OCC: [90 PEO: 10  Zn(CF3SO3)2]+ 8 Al2O3	Figure 9. TGA curves for: (a) pure PEO, (b) SPE host: [90 PEO: 10 Zn(CF3SO3)2], (c) NCPE OCC: [90 PEO:10 Zn(CF3SO3)2]+ 8 Al2O3

 

Table 2. Values of Tm, ΔHm and Xc for pure PEO, SPE host: [90PEO: 10 Zn(CF3SO3)2] and NCPE OCC : [90PEO: 10 Zn(CF3SO3)2] +8Al2O3
Sample	Tm (◦C)	ΔHm (J/G)	Xc %
Pure PEO	71.09	175.15	81.9
SPE host:[90 PEO: 10 Zn(CF3SO3)2]	69.66	123.18	57.6
NCPE OCC:[90PEO: 10 Zn(CF3SO3)2]+8Al2O3	65.98	75.81	35.4

 

Table 3. The values of decomposition temperature and weight loss (%) for pure PEO SPE host: [90PEO: 10 Zn(CF3SO3)2] and NCPE OCC: [90PEO: 10 Zn(CF3SO3)2] +8Al2O3
	Decomposition Temperature (◦C)		weight loss %
Sample	Onset	Endset
Pure PEO	226.56	282.47	85.33
SPE host:[90 PEO: 10 Zn(CF3SO3)2]	307.54	325.52	65.97
NCPE OCC:[90PEO: 10 Zn(CF3SO3)2]+8Al2O3	307.74	325.09	6.85

 

All-Solid-State Battery: Cell Performance Study

Fig. 10 shows potential discharge profiles of All-Solid-State battery in the following cell configuration:

Zn (Anode) || NCPE OCC || (I₂+C+electrolyte) (Cathode)

The cells were discharged through two load resistances viz. 60 kΩ & 100 kΩ. Open circuit voltage (OCV) was found to be 1.54 V which is quite close to the value reported in the literature [Polu et al., 2014]. Except for an initial voltage drop which is due to usual polarization build up, the cell potential remained almost constant at ~1.14V (100 kΩ) & 1.08V (60kΩ) for ~ 171hrs & 121hrs respectively. Some important cell parameters, calculated in the plateau region of the discharge profiles, are listed in Table 4. These studies clearly indicated that batteries can perform quite satisfactorily under low current drain states.

 

Table 4. Some important parameters of Zn|| NCPE OCC || (I2+C+electrolyte) cell for two different load conditions at room temperature (27°C)
Load (kΩ)		OCV (V)		Current density (µA/cm2)	Discharge Capacity µA.hr		Specific Power (µW/g)		Specific energy (mW-h/g)
60	1.54		13.84			2178	13.48	1.63
100			8.69			1932	8.72	1.49

 

 

 

 

 

 

 

 

 

Figure 10. Cell potential discharge profiles of All-Solid-State Battery: Zn (anode)|| NCPE OCC || (I₂+C+electrolyte) (cathode)

Conclusion

A non-lithium chemical-based Nano Composite Polymer Electrolyte (NCPE): [90 PEO: 10 Zn (CF₃SO₃)₂] +8 Al₂O₃ has been synthesized using hot press cast technique in place of traditional solution cast method. SPE compositions: [90 PEO: 10 Zn (CF₃SO₃)₂] has been used as Ist-phase host and Al₂O₃ nano particles as IInd –phase dispersoid. As consequence of dispersing IInd-phase into Ist-phase, significant improvement in σ_rt and t₊ values have been obtained. An overall σ_rt-enhancement of ~ 4 orders of magnitude from that of pure PEO host has been achieved in NCPE film. Materials / thermal properties have been characterized using different techniques which confirmed the complexation of salt in PEO and /or dispersal of filler material in SPE host. As a consequence of complexation of salt in PEO and / or dispersal of filler in SPE host, the amorphous region in PEO increased substantially which in turn supported the increase in σ_rt and t₊. However, both the values of σrt and t+ obtained for the newly synthesized Zn²⁺ conducting NCPE films need to be improved further for the purpose of possible applications in high energy density All-Solid-State batteries.

Acknowledgement

Author would like to acknowledge Dr. Dinesh Kumar Sahu for SEM characterization study from State Forensic Science Laboratory Raipur, India.

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