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CHAPTER 2

EXPERIMENTAL TECHNIQUES

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EXPERIMENTAL TECHNIQUES

2.0 Introduction

The work reported in the present Ph. D. thesis basically involved experimental measurements

on various aspects of polymer electrolyte materials. Hence, this chapter has been completely

devoted to describe techniques / procedure adopted to carry out these experimental studies.

The synthesis of alkali ion conducting Solid Polymer Electrolyte (SPE) and Nano-

Composite Polymer Electrolyte (NCPE) membranes, as already mentioned in Chapter 1, as

well as experimental techniques employed for characterization of different material properties

have been discussed. X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM),

Fourier Transform Infra-Red (FTIR) were used to study structural / morphological /

spectroscopic responses while Differential Scanning Calorimetry (DSC) was done for the

analysis of thermal properties. The ionic conductivity was measured by ac technique at fixed

ac frequency (i.e. 5 kHz), while d.c. polarization and combined a.c. /d.c. techniques were used

to evaluate ionic transference numbers i.e. the total (tion) and cationic (t+). All – Solid – State

batteries were fabricated using optimised SPE & NCPE films sandwiched between appropriate

anode / cathode active materials and electrochemical properties were characterized in terms of

cell performance studies under varying load conditions. The details of material synthesis and

characterization techniques are given below in the following sub-sections.

2.1 Material Synthesis and Film Casting Techniques

As already mentioned in Chapter 1 (Section 1.6), following SPE / NCPE films have been

casted using hot-press technique:

• SPE films: (PEO: KNO3); (PEO: KIO3); (PEO: NaNO3).

• NCPE films: (PEO: KNO3) + x SiO2; (PEO: KIO3) + x SiO2; (PEO: NaNO3) + x SiO2.

Precursor chemicals of AR grade: Poly (ethylene oxide), PEO (purity ˃ 99%, Aldrich, Mw~6,

00, 000); ionic salts: KNO3, KIO3, NaNO3 (˃ 98%, Reidel, India) and filler particles of SiO2

(˃ 99%, size ~ 8nm, Aldrich, USA) have been used as received. In order to compare the

quality of hot-press casted films, some of the optimized SPE/NCPE films were also casted by

traditional solution-cast method. However, the hot-press technique has several merits over the

solution-cast method. It is a rapid, least-expensive and a completely dry procedure to prepare

polymer electrolyte films. Nevertheless, the film casting procedures by both the methods are

discussed below in brief.

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• Hot-press (Extrusion) technique: Hot-press technique for casting polymer electrolyte

films has recently gained wide acceptability in several research groups [Gray et al. 1986;

Prosini et al. 1998; Appetecchi et al. 2000; Capiglia et al. 2000; Agrawal et al. 2008, 2009,

2011). For hot-press casting SPE films, dry powders of polymeric host and salt in appropriate

wt. (%) / mol. wt. (%) ratios are mixed physically for about ~30-60 min., the homogeneously

mixed powder is then heated close to the melting point of polymeric host, with mixing

continued, for about ~ 30-40 min. This results into a soft slurry/ lump of polymer salt complex

which is then pressed (~ 1-2 ton/cm2) between two cold S.S. (Stainless Steel) blocks which

gives rise to a uniform thin membrane of solid polymer electrolyte of thickness ~ 100-150 µm.

For hot-press casting NCPE films, IInd-phase dispersoid filler materials in appropriate (wt. % /

mol. wt. %) proportion is added to dry SPE composition, identified as optimum conducting

composition, followed by heating around the melting / softening point of polymer and then

pressed to form film, as before. The total time required for hot-press casting of SPE/NCPE

films is approximately 1-2 hours.

• Solution cast method: This is one of the traditional procedures for casting films of Solid

Polymer Electrolyte (SPE) as well as Composite Polymer Electrolyte (CPE). In this

technique, appropriate amount of polymer and complexing salt (in mol wt. % / wt. %) are

dissolved separately in a common solvent (viz. methanol, double distil water and acetonitrile

etc.). The two solutions are then mixed together and stirred magnetically at room or at a

slightly elevated temperature for sufficient time (few hours to few days) ensuing the salt

complexation in the polymer host. For casting of Composite Polymer Electrolyte (CPE)

films, micro/nano sized filler particles are added to mixed (polymer + salt) solution during

stirring. The obtained viscous solution is then poured into the petri-dishes for casting SPE /

NCPE films through slow evaporation of the solvent. The thin film of uniform thickness ~

100-200 µm can be casted by this method.

In the present investigation, firstly, the solid polymer electrolyte films with varying salt

concentrations were hot-press casted. To identify the highest conducting films, referred to as

Optimum Conducting Composition (OCC) to be used as Ist – phase SPE host matrix for

casting NCPE films by dispersing SiO2- nano particles as IInd – phase dispersoid, composition

dependent conductivity studies have been carried out. SPE films: (70PEO: 30 KNO3),

(70PEO: 30 KIO3) and (75PEO: 25 NaNO3) exhibited highest room temperature conductivity

and hence employed as Ist– phase host matrix for casting of NCPE films, as mentioned above.

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2.2 Characterization Techniques

2.2.1 Structural / morphological / spectroscopic / thermal properties

X-Ray Diffraction (XRD)

X-Ray Diffraction (XRD) is an important experimental tool widely used for materials

characterization viz. determination of structure, phase etc. Atomic planes of crystalline/

polycrystalline solids scatter the incident X-ray beam of characteristic wavelength, as

schematically shown in Fig. 2.1 (a). The outgoing beams interfere constructively with each

other by satisfying the Braggs condition:

2d sinθ = nλ........................................... (2.1)

where ‘d’ is the interplaner distance, ‘θ’ is the angle of incident of X-rays, n is an integer and

‘λ’ is the wavelength of the incident X-rays. A well-defined sharp peak pattern is usually

obtained in case of crystalline/ polycrystalline solids. However, for amorphous/ glassy solids

with disordered structures broad/ hallow peak pattern is obtained in place of well defined peaks.

The size of crystallites in the polycrystalline samples can also be determined from a given

characteristic peak, as shown in Fig. 2.1 (b), using the well-known Scherer’s formula:

L = [0.9 λ / B] sin θB....................................... (2.2)

where B is Full Width at Half Maxima (FWHM) and θB is in radian.

In the present Ph. D. work, XRD analysis on different SPE/NCPE OCC including pure PEO

films were done primarily to confirm the complexation of salt in polymer host, existence /

increase of degree of amorphous phase in the film materials etc. Expert-pro MRD, Panalytical

diffractometer at Cu-Kα radiation has been used in the present investigation.

Scanning Electron Microscopy (SEM)

A scanning electron microscope (SEM) images a sample by scanning it with a high-energy

beam of electrons in a raster scan pattern. The electrons interact with the atoms of sample

producing signals which contain informations regarding the surface topography, composition, as

well as other properties such as electrical conductivity.

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(a)

(b)

Fig. 2.1 (a) Schematic representation of X-rays diffraction from the set of atomic planes,

(b) A typical diffraction peaks with width B at FWHM.

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In a typical SEM, as shown in Fig. 2.2, the electron beam is thermionically emitted from an

electron gun fitted with a tungsten filament cathode. Tungsten, a low cost metal, is normally

used in thermionic electron guns because it has the high melting point and low vapour

pressure, thereby allowing it to be heated for electron emission. Other types of electron

emitters include lanthanum hexaboride (LaB6) cathodes, which can be used in a standard

tungsten filament SEM if the vacuum system is upgraded and field emission guns (FEG),

which may be of cold-cathode type using tungsten single crystal emitters or the thermally-

assisted Schottky type, using emitters of zirconium oxide. The electron beam, which typically

has an energy ranging from 0.5 keV to 40 keV, is focused by one or two condenser lenses to a

spot ~ 0.4 nm - 5 nm in diameter. The beam passes through pairs of scanning coils or pairs of

deflector plates in the electron column, fall in the final lens, which deflect the beam in the x

and y axes so that it scans in a raster fashion over a rectangular area of the sample surface.

When the primary electron beam interacts with the sample, the electrons lose energy by

repeated random scattering and absorption within a teardrop-shaped volume of the specimen,

which extends from less than 100 nm to around 5 µm into the surface. The size depends on the

electron's landing energy, the atomic number of the specimen and the specimen's density. The

energy exchange between the electron beam and the sample results in the reflection of high-

energy electrons by elastic scattering, emission of secondary electrons by inelastic scattering

and the emission of electromagnetic radiation, each of which can be detected by specialized

detectors. The beam current absorbed by the specimen can also be detected and used to create

images of the distribution of specimen current. Electronic amplifiers of various types are used

to amplify the signals which are displayed as variations in brightness on a cathode ray tube.

The raster scanning of the CRT display is synchronized with that of the beam on the specimen

in the microscope, and the resulting image is therefore a distribution map of the intensity of

the signal being emitted from the scanned area of the specimen. The image may be captured

by photography from a high resolution cathode ray tube. In the modern machines, the image is

captured digitally and displayed on a computer monitor as well as saved in the computer's hard

disk. In the present investigation, SEM [model: JEOL – JXA 8100] has been used to study the

surface morphology only on the newly synthesized SPE/ NCPE OCC membranes. The SEM

micrographs were taken at low vacuum after sputtering the samples with gold to prepare the

conductive surface.

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Fig. 2.2 Schematic diagram of a Scanning Electron Microscope (SEM) with a CRT display.

Fourier Transform Infra-red Spectroscopy (FTIR)

Infrared (IR) spectroscopy is basically an absorption phenomenon in the infrared region of

the electromagnetic (em) spectrum (i.e. λIR ˃ λvis or ʋIR < ʋvis). A common instrument used for

this analysis is a Fourier transform infrared (FTIR) spectrometer. A schematic ray and block

diagrams of FTIR spectrometry is shown in Fig. 2.3 (a, b). FTIR technique is used to identify

and analyse vibrational / rotational changes in the chemicals. The infrared spectrum is divided

into three regions: near-, mid- and far- infrared. Near-IR with wave number ranging from

14000–4000 cm−1 (0.8–2.5 µm wavelength) is used to excite overtone or harmonic vibrations.

The Mid-IR in the range 4000–400 cm−1 (2.5–25 µm) is used to study the fundamental

vibrations and associated rotational-vibrational structures. Far IR in the range 400–10 cm−1

(25–1000 µm), lying adjacent to the microwave region, has low energy and may be used for

rotational spectroscopy. The names and classifications of these sub-regions are the

conventions and are based loosely on the relative molecular or electromagnetic properties. In

the present study, FTIR analysis in the SPE/NCPE including pure PEO films were done to

investigate ion-polymer interaction and possible conformational changes due to the addition of

salt and filler dispersoid material in the host polymer.

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(a)

(b)

Fig. 2.3 (a) Schematic representation of FTIR ray diagram, (b) Block diagram of the FTIR

spectrophotometer

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The FTIR spectra were recorded with the help of Shimadzu-IR Affinity spectrophotometer, in

the wave-number range 4000-400 cm-1 at room temperature (~27 0C).

Differential Scanning Calorimetry (DSC)

Differential Scanning Calorimetry (DSC) is used to study the thermal responses of the

materials. When the materials are heated, thermal changes are bound to occur except for in

some inert materials viz. Al2O3. In polymers, thermal changes such as softening of polymers,

glass transition, melting etc. take place which are invariably accompanied with change in the

heat enthalpy of the materials. These changes can be detected in Differential Scanning

Calorimetry or DSC in short. A block diagram of DSC experimental set-up is shown in Fig.

2.4. It contains two pans one for sample and other for reference material which does not

exhibit any thermal changes when heated. Both the sample and reference materials are heated

simultaneously. At the point of thermal change in the sample heat energy is either absorbed

(endothermic) or given away (exothermic reaction) with respect to reference material and can

be detected and recorded. Fig. 2. 5 (a, b, c) shows DSC thermal responses for glass transition,

crystallization and melting temperatures. DSC thermograms give lot of informations about the

material such as entropy heat enthalpy, latent energy, Tg, Tc, Tm points etc. which are most

relevant scientifically. Particularly, in case of polymeric materials, which are mostly semi-

crystalline amorphous in nature, exhibit variety of DSC transitions, as illustrated in Fig. 2.5 (a,

b, c).

Fig. 2.4 Experimental setup for DSC studies.

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Fig. 2.5: DSC thermograms (a) Glass transition (Tg), (b) Crystallization (Tc), (c) Melting

(Tm).

2.2.2 Electrical and Electrochemical Properties

(A) Electrical property

Conductivity measurements

The electrical conductivity (σ) is, in general, expressed as:

σ = G.l/A = 1/R. (l/A) Scm-1................... (2.3)

where G (=1/R) is the conductance, ‘R’ is the resistance, ‘l’ is the thickness and ‘A’ is the

cross-sectional area of the sample. The measurement of electrical conductivity in ionic

materials with ions as charge carriers is not as simple and straightforward as that in the

electronic materials. In electronic conductor, conductivity can be measured by applying dc

potential directly onto the sample using either two-probe or four-probe method. However, the

application of dc voltage on to the ionic conductor is restricted due to the fact that dc potential

leads to a fast build-up of interfacial polarization of mobile ions at the respective counter

electrodes, resulting into a rapid increase in the resistance as time passes. Hence, it becomes

difficult to measure the true bulk resistance of the sample by dc method. To avoid this problem

of build of polarization, a.c. method has been adopted widely for the measurement of the ionic

conductivity in the ionic materials. However, For the sake of completeness, conductivity

measurements using both the dc and ac methods are discussed below in brief.

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(i) Direct current (d.c.) method:

In this method, a constant d.c. potential or current is applied across a cell comprising of

sample kept between two metallic electrodes and the resulting current or potential is measured.

Based on the Ohm’s law, the conductivity can be expressed as:

σ = I/V.l/A...................................... (2.4)

where V and I are the voltage and current; l and A are thickness and area of cross-section

respectively of the sample. D.C. conductivity measurement is carried out often using two

techniques: (i) two probe and /or, (ii) four-probe. In two-probe method, a known d.c. potential

is applied across the sample and the resulting current is measured, which gives the value of

resistance. In four-probe method, a known d.c. current is made to flow between two outer

electrodes and the potential drop is measured across two inner electrodes, hence resistance R is

determined.

(ii) Alternating current (a.c.) method

This technique is based on analysing the response of a cell, comprising of an ion conducting

electrolyte sample sandwiched between two blocking or non-blocking electrodes, to a

sinusoidal a.c. signal and subsequent calculation of the impedance as a function of frequency

of the applied signal. The technique, broadly referred to as ‘Impedance Spectroscopy (IS)’,

enables us to evaluate and separate out the different resistance contributions viz. bulk, grain-

boundary, electrode / electrolyte interfacial resistances etc. to the overall electrical resistance.

The responses of electrode reactions at the electrode/electrolyte interface, the migration of ions

through the bulk and across the grain boundaries within the electrolyte occur in different

frequency domains [Mac Donald 1987; Bruce 1987]. Bauerly [1969] for the first time to

propose a generalized complex impedance/admittance measurement to extract the value of true

bulk conductivity from a.c. method using IS technique. Since then, substantial developments

have been recorded in employing this technique and reviewed extensively [Epelboin &

Keddam 1970; Mellander & Lunden 1986; Raistrick 1986; Badwal 1988; MacDonald 1987,

2005; Barsoukov & MacDonald 2005; Orazem & Tribollet 2008]. In Impedance Spectroscopy

technique, as mentioned above, a sinusoidal voltage signal of low amplitude is applied across

a solid electrolyte cell and the resulting current through the cell is measured. Generally, this

current is related to the voltage in two ways viz. (i) the ratio of the current and voltage

maxima, Vmax / Imax (analogous to resistance in d.c. measurements) and, (ii) the phase

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difference ‘θ’ between voltage and current. The combination of (i) and (ii) gives the

impedance ‘Z’ of the cell. Both the magnitude of impedance (|Z| = Vmax / Imax) and the phase

angle ‘θ’ vary with the applied frequency. The results of the complex impedance measurement

of a cell as a function of applied signal frequency can be displayed conventionally in a

complex plane in one of the following form of equations:

Complex impedance: Z* (ω) = Z’ – jZ’’ = Rs-j / ωCs ................................. (2.5)

Complex admittance: Y (ω) = Y’ + jY’’ = 1 / Rp + jωCp = G(ω) + jB(ω)...(2.6)

Complex modulus: M (ω) = 1 / ɛ (ω) = M’ + jM’’ = jωC0Z (ω)................. (2.7)

Complex permittivity

(Dielectric constant): ɛ (ω) = ɛ’ - jɛ’’............................................................. (2.8)

Loss Tangent: Tan δ = ɛ’’ / ɛ’ = M’’/M’ = -Z’/Z’’ = Y’/Y’’................. (2.9)

Complex resistivity: ρ (ω) = ρ’ + jρ’’ = Zx (C0 / ɛ0)........................... (2.10)

Complex conductivity: σ (ω) = σ’ – jσ’’ = Yx (ɛ0C0).............................. (2.11)

where j = √-1, C0 is vacuum capacitance of the cell, G is the conductance and B is the

susceptance (subscripts s & p are used for series and parallel combinations of the circuit

elements respectively).

Out of these parameters, the impedance and admittance plane representations provide useful

informations regarding various processes within the cell having different relaxation times.

Randles [1947] for the first time successfully demonstrated that various electrical processes

occurring within the cell could be represented by an electrical equivalent circuit comprising of

resistors, capacitors, inductors and their series and parallel combinations. Fig. 2.6 shows some

typical complex impedance (Z’- Z’’) plots for elementary circuit elements viz. R, C and RC

(series & parallel) combinations. The equivalent circuit represents almost the similar

phenomenological processes occurring within the cell, as it is connected in the form of

electrical circuit. Practically, the equivalent circuit mainly consists of resistors and capacitors

in terms of which the charge migration and polarization occurring within the cell can be

represented. Fig. 2.7 shows represents Z’-Z’’ plots for two typical electrochemical cells

sandwiched between a non-blocking (reversible) and a blocking electrode along with their

equivalent circuits. The intercept of the semicircle on the real Z’-axis given the value of bulk

resistance (Rb). A general practice is to represent one charge polarization or migration process

by one parallel RC combination. Hence, a model of equivalent circuit for a solid electrolyte,

kept between two electrodes in the sample holder, schematically shown in Fig. 2.8 can be

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represented by three parallel RC circuits connected in series. Fig. 2.9 shows these RC circuits

in series along with the corresponding Z’-Z’’ plot. The three semicircles clearly indicate the

different resistance contributions viz. Rb, Rgb, Rcl in terms of the intersections of semi-circles

on the real Z’- axis responding in different frequency domains. However, in the actual IS

measurements, such ideal semi-circular patterns are not obtained. Contrary to ideal spectrum

obtained from the equivalent circuit model, it is usually observed that in the real experimental

Z’-Z’’ plots the semi-circles are depressed with significant flattening and tilted spike.

Flattening of semi-circle and tilting of spike can be attributed to the ‘leaky capacitor’ in the

circuit, multiple or coupled reaction sequences, roughening of the electrodes etc. In the present

investigation, σ-measurements by IS was attempted initially on the newly synthesized sample

materials using a multi-frequency LCR Bridge [model: HIOKI 3522-50, Japan] in the

frequency range 1 mHz – 100 kHz. However, Z' - Z" responses were very ambiguous and not

distinct enough to explore the value of true bulk resistance (Rb) of the sample materials.

Hence, in place of frequency dependent Impedance Spectroscopy, σ-measurements on all the

samples was done at a fixed frequency (5 kHz). The experimental arrangement used for the

conductivity measurements is already shown earlier in Fig 2.8. The temperature dependent

studies were carried out by placing the whole electrode holder with the film specimen sample

sandwiched between SS-electrode in an insulated laboratory-built electric furnace.

Ion transport number measurements

In polymer electrolytes both the ions (cations & anions) may be mobile. Besides this, if free

electrons are also present in the system, they will also take part in the conduction process.

Hence, it becomes mandatory to know what fraction of current is carried by which mobile

species. This is referred to as measurement of transport or transference number. This

measurement can be carried out using dc polarization and/or combined ac/dc techniques. The

transport or transference number for charged species, ion (cations and anions) or electron /

hole, is defined as the ratio of conductivity due to it and the overall conductivity [Chandra

1981; Linford & Hackwood 1981; Brinkmann 1988; Hagenmuller & van Gool 1978]. Hence,

the total ionic transport number (tion) is expressed as:

tion = σion/σT = Iion/IT ............................ (2.12)

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Fig. 2.6: Some typical complex impedance plots for elementary circuit elements: R, C and

RC series / parallel combinations.

Fig. 2.7: Typical electrochemical cells using: (i) non-blocking electrode and (ii) blocking

electrodes and equivalent circuits along with responses in Z′-Z″ complex

impedance plane.

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Fig. 2.8: Schematic experimental arrangement of LCR meter for electrical conductivity

measurements.

Fig. 2.9: Generalized circuit network for a electrochemical cell and its response in the

complex impedance plane.

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and the electron /hole transport number (te,h) is expressed as:

te,h = σe,h/σT = Ie, h/IT .......................... (2.13)

where symbol have their usual meanings i.e. σion (Iion), σe,h (Ie, h), σT (IT) are the ionic,

electronic/hole and total conductivity (current) respectively. Ideally, for pure ionic conductors tion

= 1 and te, h = 0. The transference number lies between 0 and 1 for mixed conductors with

partially ionic and electronic conduction. Various experimental techniques have been used to

measure the ionic / electronic transference number viz. Tubandt method [Tubandt 1932],

Wagner’s polarization method [Wagner 1975], electrochemical cell-potential measurement

technique [Bouridah et al 1986], steady state current (d.c.) measurement technique [Blonsky et al

1986]. In most of the liquid / aqueous electrolytes as well as ion conducting polymer electrolytes,

cations and anions both contribute significantly to the total ionic conductivity and the electron

/hole conduction is negligibly small. Hence, cationic/anionic transport numbers are important

parameters which can be mathematically calculated by following equations:

t+ = ∑ I+ / ∑ (I+ + I-) ...............................(2.14)

t- = ∑ I- / ∑ (I+ + I-)................................ (2.15)

where ∑ (I+ + I-) is the total current expressed as the sum of the partial currents due to mobile

cations and anions present in the electrolyte. Combined ac/dc technique as suggested by Evans et

al [1987] and Watanabe et al [1988] is widely used to evaluate cationic transference number (t+).

In the present study both d.c. polarization and combined ac/dc methods have been used these

techniques are discussed below in brief.

(i) d.c. polarization technique for the evaluation of total ionic transport number (tion)

D.C. polarization technique was used for the first time by Wagner [1975, 1976] to determine

transport number of ions and electrons separately. The experimental arrangement is shown as

inset in Fig. 2.10. A d.c. electric potential is applied across the sample sandwiched between two

blocking electrodes (stainless steel in the present study) and the current is monitored as a function

of time. Some typical ‘current vs time’ plots are shown in Fig. 2.10. The peak current obtained

initially decreases rapidly with time due to polarization of mobile ions at the electrode /

electrolyte interface, after words the current either approaches zero (for pure ion conductor) or

attains a residual constant value (for mixed ionic/electronic conductor). The initial total current

(IT) is either due to ions solely or as a result of combined ionic and

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Fig. 2.10: Schematic experimental arrangement inset used in dc polarization technique and typical ‘current vs time’ plots.

electronic conduction, while the constant residual current is only due to electron conduction.

From the ‘current vs time’ plot the ionic (tion) and electron (te,h) transport numbers can be

determined using equations (2.12 and 2.13). In present study, this method has been used to

evaluate the total ionic transference number (tion) in different SPE/NCPE OCC films.

D.C. polarization technique is also very useful to evaluate ionic mobility (µ) directly in a solid

electrolyte system with simple mobile ion species. The technique widely referred as ‘Transient

Ionic Current’ (TIC) method [Watanabe et al 1987; Chandra et al 1988]. In this method, the

sample of thickness ‘d’, sandwiched between two graphite (blocking) electrodes, is subjected to

an external fixed dc potential ‘V’ for sufficient long time. As a result, the mobile ions get

polarized at the electrode/electrolyte interface. After attaining a state of complete polarization, the

polarity of the external dc potential is reversed, as a result the polarized ion cloud instantly start

travelling within the bulk towards the opposite end. The movement of ions constitutes a current

which can be monitored as a function of time in the external circuit with the help of an x-y-t

recorder (Graphtec, WX - 2300, Japan) or a DSO. The moment ion cloud reaches the other end, a

sudden drop in the current value can be witnessed. The peak in the ‘current – time’ plot

corresponds to the time of flight ‘τ’ for the ion cloud to cross the thickness ‘d’ of the film sample,

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as indicated in Fig. 2.10. Substituting the data in the equation: µ = d2/v.τ [cm2/V.s], ionic mobility

can be evaluated. Subsequently, n- values can also calculated with the help of ‘σ’ and ‘µ’ data,

using equation n = σ/µ.q. Temperature variations of ‘µ’ & ‘n’ usually follow Arrhenius type

behaviour, hence, the energies involved in the thermally activated processes can be computed

from the respective Arrhenius plots: ‘log µ-1/T’ & ‘log n -1/T’. TIC technique was used in the

present investigation to evaluate the overall ionic mobility and mobile ion concentration in some

SPE/NCPE OCC film materials. However, these measurements were unable to provide the

quantitative information of these parameters for the mobile cations and anions separately.

Although, dc polarization method separates out the ionic and electronic part of the conduction.

However, it fails to give any idea about cation and/or anions taking part in the conduction

process. Moreover, in this technique it is difficult to choose a perfect blocking electrode for

variety of ions such as H+, Li+, Na+, Mg2+ etc. Nevertheless, platinum and SS foils almost fulfil

the requirement of electrodes having blocking nature for these ions. Hence, to evaluate the

transference number for cations only a combined ac/dc technique was used with SS as blocking

electrodes and discussed below in brief.

(ii) Combined a.c./d.c. technique for the evaluation of cation transport number (t+)

The combined a.c./d.c. technique for the determination of cation transport number has been

proposed by Vincent and co-workers [Evans et al 1987] and Watanabe and co-workers

[Watanabe et al 1988], as mentioned above.

Vincent & Bruce Technique

In this technique, the electrochemical cell in the configuration: M | MX (Electrolyte) | M type is

polarized potentiostatically applying a small d.c. voltage ∆V and initial (I0) / final (Is) currents are

recorded. The cell is also subjected to a.c. impedance measurements prior to and after the

polarization to evaluate the resistance before (R0) and after (Rs) the polarization. Eventually, the

cationic transport number (t+) can be estimated following equation:

t+ = Is (∆V – I0R0) / I0 (∆V- I0Rs)........................... (2.16)

• Watanabe Technique

Watanabe and co-workers [Watanabe et al 1988] also proposed ac/dc technique for the

determination of cationic transport number. In this method the electrochemical cell having

electrolyte sandwiched between two electrodes which behaves as a non-blocking to one of the

mobile species viz. cation. Firstly, a.c. complex impedance (Z’-Z’’) plot of the cell is recorded

up to low frequency region (i.e. up to few mHz) and the values of the bulk resistance (Rb) and the

electrode-electrolyte charge transfer resistance (Re) are noted. The cell is then subjected a small

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d.c. polarization potential V (~ mV) for sufficient time. Since, the electrodes are non-blocking, a

steady state current (Is) is eventually obtained with the passage of time due to flow of cations.

Hence, the transport number of the cationic species can be known from the following equation:

t+ = Rb/ (V/Is) – Rct ........................ (2.17)

In the present study both the techniques are employed for the evaluation of cationic transport

number in the newly synthesized SPE / NCPE OCC films.

(B) Electrochemical property: Fabrication of All-Solid-State Battery and cell potential

measurements

In the present studies, all-solid-state batteries have been fabricated using SPE/NCPE OCC films

in the following cell configuration, at as also mentioned earlier in Chapter 1 (section 1.4.1).

Na| SPE / NCPE films | C+I2 + Electrolyte or MnO2 + C + Electrolyte

K | SPE/NCPE films | C+ I2 + Electrolyte or MnO2 + C + Electrolyte

Na/K metal (in the thin slice form) was used as anode and (C+I2) mixed with MnO2 and polymer

electrolyte as binder as cathode. The details on the preparation of active electrodes have been

discussed later in Chapter 6. The Open Circuit Voltage (OCV) values were measured and

compared with the theoretical values. The cell performances of the batteries have been tested

under varying load conditions. All the cell potential measurements were done with a high

impedance multi display digital multimeter ESCORT – 97. The overall dimension of theses

batteries were: area: 1.32 cm2 and thickness: 1-2 mm. Some important cell parameters have been

calculated from the plateau region of the cell potential discharge profiles. Following basic cell

parameters have been evaluated:

● Electric Power : P = VI = I2R = V2/ R [W]

● Electric Energy : E = VI.t = qV [Wh]

● Current Density : J = I /A [Amp/cm2]

● Discharge Capacity : Current × Discharge Time [Ah]

● Energy density (or Volume Capacity): Electric Energy/ Battery Volume [Wh/ cc]

● Specific Energy (or Weight Capacity): Electric Energy/ Battery Weight [Wh/ kg]

● Specific Power : Electric Power/ Battery Weight [W/ kg]