CMZIPTER - I I

EXPERIMENTAL TECHNIQUES

2.1 Introduction

2.2 Wet Chemical Processes for the Synthesis of Nanocrystalline Oxide Powders

2.2.1 Sol-Gel Process

2.2.2 Hydrothermal Process

2.2.3 Co-Prec~pitation Process

2.2.4 Polyol Process

2.2.5 Combustion Process

2.2.6 Other Wet Chemical Processes

2.3 Combustion Processes (Polymeric Precursor and Pechini Processes)

2.4 Characterization Techniques

2.4.1 TGiDTA and DSC

2.4.2 Founer Transform Infrared Spectroscopy (FTIR)

2.4.3 X-Ray Powder Diffraction (XRD)

2.4.4 Scanning Electron Microscope (SEM)

2.4.5 Transmisston Electron Microscope (TEM)

2.4.6 Vibrating Sample Magnetometer (VSM)

2.5 Fabrication of Lithium Cell

2.5.1 Preparation of Cathode

2.5.2 Preparation of Anode

2.5.3 Preparation of Electrolyte

2.5.4 Assembling of Lithium Cell

2.6. Characterization of Lithium Cell

2.6.1 Cyclic Voltammetry

2.6.2 Charge - Discharge Studies

References

CHAPTER - I1

EXPERIMENTAL TECHNIQUES

2.1 Introduction

Among the available lithium intercalated transition metal oxides, LiCoOl (layer),

LiMnaO4 (spinel) and LiNio jC00 sV04 (inverse spinel) have been most studied cathode

materials because of their effective electrochemical performance [l-71. Charge - discharge

characteristics of lithium cells are mainly depend on the physical and chemlcal properties

of the cathode material such as particle size, purity of the phase. homogeneity, chemical

stability, etc. [7-101. Nowadays, there is a great interest in the synthesis of nanocrystalline

cathode materials, which show an improved electrochemical performance due to its

unusual physiochemical properties compared to their bulk [ 10- 161. Also, nano science has

been viewed as the big, which is considered as impute for the next industrial revolution.

Synthesis and stabilization of nanostructures with required properties are the challenging

task, since nano scale materials have large surface area and high reactivity, which tend to

react themselves to form necking and hence, agglomerate into large secondary particles

[17]. Hence, synthesis process plays an important tool for the design of nanostructures with

required properties such as crystallite size, size distribution, shape, homogeneity, etc., in

order to meet the requirement for desired specific applications [ I 8-21].

The preparation of LiCoOz, LiMnzO4 and LiNio sCoo sV04 cathode powders by

conventional solid state reaction methods involve heating of precursor materials (in the

form of oxides or carbonates or organic derivatives) at higher temperatures, which results

in inhomogeneity, bigger crystallite, poor stoichiometry, phase impurity, etc. [22, 271.

These problems could be overcome using low temperature wet chemistry methods. Various

wet chemical techniques such as, sol-gel, hydrothermal, co-precipitation, polyol,

combustion. etc., have been used for the sqnthesis of u i d e range of multicomponent

nanocrystalline oxide powders, including cathode materials [28-351.

2.2 Wet Chemical Processes for the Synthesis of Nanocrystalline Oxide Powders

2.2.1 Sol-Gel Process

Sol-gel is the multi step process. mvolving chemlcal and physical processes

associated with hydrolysis, polymerization, gelatlon, condensation, drying and

densification [36]. This process generally starts with the mixing of metal alkoxides or salts

in water or in a suitable solvent (usually an alcohol) at ambient or slightly elevated

temperatures [36: 371. Schematic diagram of various steps involved in the sol gel process

are shown in fig. 2.1. In sol gel process, controlling the pH of starting solution is very

much important to avoid the precipitation as well as to form the homogenous gel, which

can achieved by the addition of base or acidic solutions [38). Apart from the above,

organic compounds with hydrophilic functional groups (hydroxides or carboxylates) in

small molecules such as citric acid, succinic acid, oxalic acid, tartaric acid, acrylic acid,

etc. and polymers like polyacrylic acid (PAA) and polyvinyl pynolidone (PVP) can be

used with metal ion sources to form the sol as well as control the particle size and

uniformity of the products [38-421. Chelation of metal ions by carboxylic acid groups lead

to a homogeneous distribution of the constituent ions in the obtained gel [43]. The gel

intermediate is further heated between 150 "C and 300 "C to eliminate volatile organic

components, excess water, etc., which results the dried intermediate powders. Single phase

nanocrystalline metal oxides are obtained after calcining of dried gel powder at

400-800 "C depends on the precursor chemical nature [44,45].

Dense Cera~nics

Fig. 2.1 Schematic diagram of various steps involved in the sol-gel process.

2.2.1.1 Advantages of Sol-Gel Process

1. Low temperature processing and consolidation 1s possrble.

2. Smaller particle size and morphological control in poivder synthesis.

3. Sintering at low temperature also possible.

4. Better homogeneity and phase purity compared to traditional ceramic method.

2.2.1.2 Disadvantages of Sol-Gel Process

1. Raw materials for this process is expensive (in the case of metal alkoxides)

compared to mineral based metal ion sources.

2. Products would contain high carbon content when organic reagents are used in

preparative steps and this would inhiblt densification during sintering.

3. Since several steps are involved, close monitoring of the process is needed.

2.2.2 Hydrothermal Process

Water is an excellent solvent for many ionic compounds. It can dissolve even non-

ionic compounds under high pressure and high temperature. In hydrothermal synthesis, the

above property of water has been effectively exploited for the preparation of fine powders

of metal oxides [46-481. Under these hydrothermal conditions, water plays two roles as

pressure transmitting medium and solvent for the precursors. Such hydrothermal

conditions effectively brings down the activation energy for the formation of final phase,

which can also speed up the reaction between the precursors, otherwise it would occur only

at very high temperatures [49, SO]. An autoclave is invariably employed to achieve

hydrothermal conditions. The pressures attained are in the range of 10 to 150 kilo bar

which depends on the chosen temperature of water ( >373 K). Powders are either

crystalline or amorphous depending on chosen hydrothermal conditions [51-551.

~ ~ d r o t h e n n a l method has certa~n advantages as well as some disad~~antages. tvhlch are

listed below.

2.2.2.1 Advantages of Hydrothermal Process

1. Powders are formed directly from the solution.

2. It is possible to control particle size and shapes by using different starting

materials and hydrothermal conditions.

3. Resulting powders are highly reactive, which aid in low temperature sintering.

2.2.2.2 Disadvantages of Hydrothermal Process

1. Prior knowledge on solubility of starting materials is required.

2. Hydrothermal slurries are potentially corrosive.

3. Accidental explosion of the high pressure vessel cannot be ruled out.

2.2.3 Co-Precipitation Process

In this process, the required metal cations from a common medium are co-

precipitated usually as hydroxides, carbonates, oxalates, formates or citrates [56-581. These

precipitates are subsequently calcined at appropriate temperatures to yield the final

powder. For achieving high homogeneity, the solubility products of the precipitate of metal

cations must be closer [59]. Co-precipitation results in atomic scale mixing and hence, the

calcining temperature required for the formation of final product is low, which lead to

lower particle size [60]. However, each synthesis requires its own special conditions,

precursor reactions, etc. Also, co-precipitation process required to control the

concentration of the solution, pH, temperature and stirring speed of the mixture in order to

obtain the final product with required properties [6 1-62].

2.2.3.1 Advantages of Co-Precipitation Process

1 . Homogeneous mixing of reactant precipitates reduces the reaction temperature.

2. Simple direct process for the synthesis of fine metal oxide powders, which are

highly reactive in low temperature sintering.

2.2.3.2 Disadvantages of Co-Precipitation Process

1. This process is not suitable for the preparation of high pure, accurate

stoichiometric phase.

2. This method does not work well, if the reactants have very different solubility as

well as different precipitate rate.

3. It is not having universal experimental condition for the synthesis of various

types of metal oxides.

2.2.4 Polyol Process

Ethylene glycol has been widely used in the polyol process for the synthesis of

metal (both pure and alloyed) nanoparticles due to its strong reducing power and relatively

high boiling point (-1 97 "C) [63,64]. Recently, it has been widely used for the synthesis of

nanocrystalline ceramic powders that involved, complexation with ethylene glycol,

followed by polymerization [64-671. In addition, ethylene glycol has been used to fabricate

meso structures of titania, tin dioxide, zirconia, and niobium oxide by forming glycolate

precursors because of its coordination ability with transition metal ions. This process

involves hydrolysis and inorganic polymerization carried out on the salts dissolved in a

polyol medium [68]. The polyol acts as a solvent for the precursor salts because of its high

relative permittivity, and allows one to carry out hydrolysis reactions under atmospheric

pressure in a large temperature range up to the boiling point of the polyol [69,70].

2.2.4.1 Advantages of Polyoi Process

1. Low temperature process.

2. It yields high pure organic free powders.

3. Ability to control the particle properties such as size, shape and uniformity, etc.

2.2.4.2 Disadvantages of Polyol Process

1. Large amount of polyhydroxy alcohol requirement.

2. Phase separation while synthesizing the multicomponent oxides.

3. Choosing the suitable polyhydroxy alcohol for individual processes.

4. Collecting and purifying the intermediate particles are complicated.

2.2.5 Combustion Process

Combustion is a complex sequence of chemical reactions between a fuel and an

oxidant accompanied by the production of heat or both heat and light in the form of either

a glow or flames. The combustion concept that using the art of rapid thermal degradation

of precursor chemicals reaction with oxygen has been effectively used for the synthesis of

variety of metal oxides in nanoscale [71-751. Based on the ke ls and their combinations

with the metal ion sources (commonly metal nitrates, acetates, hydroxides), combustion

process has classified into following categories [76-771.

2.2.5.1 Combustion of Fuel-Oxidant

Fuel-oxidant combustion technique involves an exothermic decomposition of a

fuel-oxidant precursors such as urea- nitrate, glycine- nitrate, DHF- nitrate, etc, relatively

at lower temperatures [78-801. Also, it explores very fast and self sustaining exothermic

reaction between the metal salts and organic fuels. The heat required for the phase

formation is supplied by the reaction ltself and not by an external source. Dunng the

combustion process, large volume of gases will evolve which prevent the agglomeration

and lead to the formation of fine powders with nano structures. Release of heat dunng the

combustion reaction depends on the fuel-oxidant stoichiometry in the precursor

composition. The fuel-oxidant stoichiometry is used to calculate, based on the thermo

dynamical concepts used in the field of propellants and explosives, for the required nature

of combustion process [8 I].

2.2.5.2 Polymeric Precursor Process

The polymeric precursor process is known to be simple cost effective and versatile

low temperature combustion process for the synthesis of rnulticomponent metal oxides

relatively lower temperatures [82 ] . The general idea of this process is to distribute the

metal ions atomistically throughout the polymeric structure and to inhibit their segregation

and precipitation [83]. Further heating of these polymeric intermediates at appropriate

temperatures, yields ultra fine nanocrystalline metal oxides. Generally hydroxy carboxylic

acids such as citric acid, tartaric acid, etc., are used as a polymerizing as well as chelating

agents in this process [84]. The physiochemical properties of the synthesized powders are

critically depend on the properties of polymeric intermediates, which influence on the

combustion parameters such as ignition temperature, evolved heat, combustion duration etc

[85]. Hence, wide range of polymeric precursors have been investigated in order to control

the structural properties of final products.

2.2.5.3 Pechini Process

Pechini process, also one of the combustion process, which is based on the ability

of carboxylic acids (citric acid, tartaric acid, polyacrylic acid, etc.) to chelate the various

metal Ions [86] . These metal carbox.la?es can under go polyehtenfication ~vhen, heated

with polyhydroxy alcohol (ethylene glycol, glycerol. polplnyl alcohol. etc..) and lead to

the formation of polymeric resin, with three dimensional nehvork [87-891. The cations are

uniformly distnbuted throughout polymeric resln, which inhibits the precipitation. Further,

the calcinations of dried resin yields ultra fine oxide powders at very low temperature [90-

951. The basic chemical reactions involved in the Pechini process are shown in fig. 2.2.

Apart from that, fig. 2.3 shows some other combustion process for the synthesis of various

nanocrystalllne metal oxides.

2.2.5.4 Advantages of Combustion Processes

Gel combustion methods show advantages over the other processes mainly due to

the following important facts,

1. Low cost and low temperature process (compared to alkoxide based sol gel

process).

2. Better control of stoichiometry.

3. Crystalline size of the final oxide products, produced by these methods is

invariably in the nanometer range.

4. Exothermic reaction makes product almost instantaneously.

5. Possibility of multicomponent oxides with single phase and high surface area.

2.2.5.5 Disadvantages of Combustion Processes

1. Contamination due to carbonaceous residue, particle agglomeration, poor control

on particle morphology.

2. Understanding of combustion behavior is needed to perform the controlled

combustion in order to get final products with desired properties.

3. Possibility of violent combustion reaction, which needs special production.

(a) Esterification

OH I 0

O\ C - C H 2 - C - C H 2 - C 4 \ t HO-CH2- CHI - OH

OH' C I OH

d 'OH

(b) Polymerization

(c) Chelation

80°C, H'

Fig. 2.2 Chemical reactions involved in the Pechini process.

Polymerization

- Hz0 I

I . Soft Explosion Route

A - Polymeric intermediate B - Expansion of polynleric intermediate at 100 "C C - Soft explosion of polymeric intermediate at 150 "C D - Nanocrystalline LiCoO-, Powders

11. Flame Combustion Route

A - Slurry of precursors B - Ignition of the slurry C - Combustion with flame D - Nanocrystalline Powders

Fig. 2.3 Different combustion processes for the synthesis of various nanocrystalline metal oxides

2.2.6 Other Wet Chemical Processes

Except above discussed processes. there are many other wet chemical process

available for the synthesis oi'multicomponent oxide powders such as microemulsion [96-

981, sonochemical [99-1011, microwave heating [ I 02-1 041. spray pyrolysis [ 105- 1071, etc.

This section will discuss briefly about few of the above mentioned processes.

2.2.6.1 Microemulsion

Microemulsion can be defined as an isotropic, thermodqnamically stable system

constituting the smaller droplets dispersed in an immiscible solvent and an amphiphilic

surfactant species on the surface on the micelle. The microemulsion mediated synthesis has

been particularly used for the size selective preparation and self assembly of nano particles

because of the excellent control on the particle size, which can achieved through

modification of micelle [96-98, 1081.

2.2.6.2 Sonoche~nical

Application of ultrasound in chemical synthesis has initiated a novel synthesis

process for the nanocrystalline materials. In this process, sound wave act as the energy

source. The under lying mechanism of sonochemical process consists of the formation and

collapse of the bubbles of the sol upon exposure to acoustic waves [99-1011. The collapse

of the bubbles leads to very high temperature, pressure, high heating and cooling rates,

which leads to the formation of required nano particles. Inter particles collision during the

ultrasonic agitation leads to changes in particle morphology, composition and reactivity

[109].

2.2.6.3 iilicrotvave Heating

Heating the materials by means of microwaves (~vhen the lnarerial has good

absorptivity in the microwave region) results in rapid unifonn heating throughout the bulk

[102-1041. This is in contrast to the radial heating in the conventional heating methods and

the resulting temperature gradient in the materials being heated. In recent years wide range

of nano structures such as metals, metal oxides, sulphides, has synthesized with controlled

morphologies using microwave assisted processes [110].

2.3 Combustion Processes (Polymeric Precursor and Pechini Processes)

Evidence from the literature and also our earlier work, it is found that the

physiochemical properties of the synthesized oxide powders were mainly influenced by the

microstructure of its polymeric intermediates [76, 11 11. Fine, non agglomerated powders

can not obtained by calcining the dense ridged polymeric intermediates [112, 1131. Also,

these dense ridged polymeric intermediates exhibit poor combustion during the calcination

process, which leaves the undecomposed residual organic derivatives in the final products.

Soft, porous and voluminous polymeric intermediates are required for the synthesis of ultra

fine, non agglomerated oxide powders with nanostructure. Physiochemical properties of

the fuels or polymerizing or chelating agents plays a major role in the formation of porous

and voluminous polymeric intermediates with lower ignition temperatures. Apart from the

above, it is necessary to optimize the various parameters such as metal ion to fuel ratio,

amount of water, processing temperature, etc., to obtain the final product with required

properties.

Hence, in the present investigations, various types of combustion processes were

investigated for the synthesis of nanocrystalline cathode powders (LiCoOz. LiMnz04 and

LiNio.~Coo.~V0~) with desired properties. Also, the optimized synthesis processes are

extended for the synthesis of nanocrystalline NiFelOl and ZrO; powders for various

applications. From the present investigation, a new polyneric resin process is developed

for the surface modification (coating of metal oxides oT,er nano particles) of nano

structured LiCoO?, LiMn204, LiNiojCoo..sVO? and ZrOz powders to attain the enhanced

properties. All the processes are explained in detail in their respective chapters.

2.4 Characterization Techniques

In combustion process, the mechanism of combustion reaction plays major role in

the physiochemical properties of final products, which are mainly depends on the

microstructure, structural coordination, porosity, etc., of the polymeric intermediates. The

heat generation during combustion reaction lead to the formation of nanocrystalline

cathode powders relatively at lower temperatures. To identify the correlation between the

nature of polymeric intermediates and the final product, it is necessary to monitor the

various steps involved in the synthesis process. Hence, in the present investigations, the

characterization techniques like FTIR, XRD, TGIDTA, DSC, SEM and TEM are used to

identify the structural coordination, phase, thermal behavior and microstructures of the

polymeric intermediates as well as the synthesized LiCoO2, LiMnzO4, L~N~c.sCOC,SVO~,

NiFez04 and ZrOz powders. Principles and measurements of various techniques employed

in the present work are briefly described in this chapter.

2.4.1 TGIDTA and DSC

2.4.1.1 Principles of Thermal Analysis

In thermal analysis, during the heating process, the substance may undergo phase

transitions and I or chemical decompositions. Phase transitions involve heat effects such as

exothermic or endothermic. If gaseous products are formed during a chemical

decomposition, the process would be accompanied by mass loss in addition to thc heat

effects. Thennal analysis methods can be classified in:o three categories. ( i )

thermogravimetric analysis (TGA) (ii) differential thermal analysis (DTA) and ( i i i )

differential scanning calorimetry (DSC). In this work, simultaneous TGt'DTA and DSC

experiments were carried out to investigate the thermal behavior of polymeric

intermediates [114- 1 151.

2.4.1.2 Therrno Gravirnetrie / Differential Thermal Analysis

The schematic representation of the typical TG/DTA system is shown in fig. 2.4.

The important components of this apparatus are: (i) sample holder-measuring unit, (ii)

furnace, (iii) temperature programmer and recording system

( ' Sample holder - measuring unit: Sintered high density alumina crucibles are generally

used as sample and reference holders. Thermocouples are used in measuring unit to

measure sample temperature (below 850 OC chromel-alumel thermocouple gives better

result and above 850 OC platinum, platinum-rhodium thermocouple are employed). The

sample holder forms the part of one arm of thermobalance.

(ii) Furnace: Heat source with large uniform temperature zone.

(iii) Temperature programmer and recording system: A device, which performs the rate of

change of temperature on furnace.

This system permits the flow of gas or vapor through the samples during heating or

cooling cycles.

(a) Thermogravimetric Analysis (TGA)

In thermogravimetry process, mass of the sample is precisely measured while it is

subjected to a predetermined heating or cooling. Sample may also be maintained

Furnace

Controller n

Control system and %ta recorder

Fig. 2.4 Schematic representation of TGIDTA.

~sothemallq at a fixed temperature. Tkern?obalanccs ha! rng \ensitn i t ) ln the range of pg

are a\ allable In the present generation thrnnogra\ in~etnc sqstem

(b) Differential Thermal Analysis (DTA)

In DTA, one measures the difference in temperature between the sample cell (T,)

and the them~ally inert reference (T,) and record this difference (T,-T,) as the function of

temperature, which give the information about the sample behavior such as exothermic

(heat emission dunng the reaction) or endothermic (heat absorption during the reaction).

An exothermic process is plotted with upward deflection while an endothermic process is

plotted with a downward deflection in the DTA curve [114].

2.4.1.3 Differential Scanning Colorimetry (DSC)

Differential scanning calorimetry (DSC) is a technique, which measures the

required energy to maintain the zero temperature difference, between a substance and an

inert reference material. Based on the function, DSC has classified into two different types

as power compensation and heat flux and it is shown in fig. 2.5. In power compensation

DSC the temperatures of the sample and reference are controlled independently using

separate, identical hmaces. The temperature uniformity of the sample and reference are

made by controlling the power input to the two furnaces. In heat flux DSC, the sample and

reference are assembled in a singie furnace. Enthalpy or heat capacity changes in the

sample cause a difference in its temperature relative to the reference. The difference in

energy required to maintain them at a nearly identical temperature is provided by the heat

changes in the sample [114, 1151.

Single heat Source

(a)

Temperature sensors

Indi~iclual heaters

Fig. 2.5 (a) Heat flux DSC and (b) power compensation DSC.

2.4.1.4 Characterization of the Poljrnerie intermediates b). TGiQTA and DSC

Polymeric intermediates were characterized b). thermal analysis to study their

thermal behavior. This thermal analysis was used to identify the ignitlon temperature of the

precursors. For this purpose. both thermogravimetric (TG) and differentla1 thermal analysis

(DTA) techniques were employed to characterize thermal and mass loss events on heating

the precursors using a simultaneous TGIDTA system of Labsys M/s. Setaram., France.

Approximately 3 mg of polymeric intermediates were used as samples and a heating rate of

10°C/min was employed. TGiDTA studies were carried out from room temperature to

&0O0C in flowing oxygen ambient. Also, DSC measurement was carried out for the

polymeric intermediate between 30 and 500 'C at the heatlng rate of 10 "C min" under

static air ambiance using a Mettler Toledo star"' system module DSC

82 1 e/500/57514141831578, Switzerland.

2.4.2 Fourier Transform Infrared Spectroscopy (FTIR)

FTIR is a powerful technique for identifying the structural coordination (types of

chemical bonds) in the substances such as solids, liquids and gases. It is based on the

interaction of IR radiation with the substance and the nature of interaction, which reveal

the properties of the substance [116].

2.4.2.1 Principles of FTIR Spectroscopy

When infrared radiation passes through a sample (solid, liquid or gas), certain

frequencies of the radiation are absorbed by the molecules of the substance leading to the

molecular vibrations. The frequencies of absorbed radiation are unique for each molecule

which provide the characteristics of a substance. Since, the strength of the absorption is

proportional to the concentration. FTIR can also be used for some quant1tatlr.e analysis

[117]. Schematic ofFTIR spectrometer IS shoisn in fig. 2.6.

2.4.2.2 Sample Preparation

Samples preparation for FTIR analysis are gi\.en belo\$..

(a) For liquid samples, the easiest is to place one drop of sample between two plates of

single crystal sodium chloride. The drop forms a thin film between the plates.

(b) Solid samples can be mixed with potassium bromide (KBr) as diluter to form a very

fine powder. The powder is then compressed into a thin transparent pellet. which is used

for taking FTIR spectrum.

(c) Polymer samples can be dissolved in a solvent such as methylene chloride and the

solution placed onto plates of single crystal sodium chloride. Further evaporation lead to

the formation of thin film of the original material on the plate, which is kequently used for

polymer samples.

(d) Solid samples also can be examined as a mull, which is prepared by thorough gnnding

( to avoid the scattering of IR radiation) of few mg sample with mulling oil (Nujol- liquid

paraffin is commonly used as a mulling agent). The mull is examined as a thin film

between flat sodium chloride single crystal plates.

2.4.2.3 Characterization of as Prepared as well as Calcined Polymeric Intermediates

by FTIR

In the present investigation, the FTIR spectra were recorded for the as prepared as

well as calcined polymeric intermediates (synthesized LiCoO:, LiMn204 and

L ~ N ~ O . S C O ~ . ~ V O ~ , ZrOz, NiFe:04 powders) using Shimadzu FTIR 8300/8700

spectrophotometer, 4cm-' resolution, auto gain, between the frequency range of 4000 and

Beuin ; C'onlbiaed Ibezjir~ splitter

Sei11p13 cell

i

4 Detector

A4nulog to digital cun~ertor

Recorder or Printer

Fig. 2.6 Schematic of FTIR Spectrometer.

400 c ~ n - ' . ?illnimurn amount of sampie i i a s mlscd ui;h s?c.ctral gradc KBr (as d~luter) and

the mixed powder was made into transparent pellet. 77le ineasurements were carried out for

the obta~ned pellets at room temperature.

2.4.3 X-Ray Powder Diffraction (XRD)

X-ray powder diffraction is one of the most important techniques to characterize

the powder samples. It is effectively used for the structural determination, phase analysis,

detection of preferred orientation, deduction of an orderldisorder phenomenon and also

determination of crystallite size of the powder samples [118]. The positions and the

intensities of the peaks are used to identify the structure, phase, etc., of the material [119].

2.4.3.1 Principle of X-Ray Diffraction

Consider two parallel monochrolnatic X-ray beams with the wave length of 2.

falling on the successive planes of the crystal at an angle of 8. Constructive interference of

the reflected rays from two successive planes occurs, only the path difference between the

two rays fulfils the Bragg's condition, nh = 2d sine, where n is integer and d is an inter

planar distance [118- 1 191.

2.4.3.2 Crystallite Size Calculation

The crystallite size of particles can be calculated by suitable analysis of X-ray line

broadening. Generally in XRD pattern, diffracted lines are not always sharp and may be

broadened due to instrumental factors and / or nature of the specimen (mainly crystallite

size). If the particles are very small, the lines are broader than usual. The broadening

increases with decreasing crystallite size. The crystallite size of the sample could be

determined using Scherrer's formula [120, 1211,

Where, A- Wavelength of the radiation used in A, Og- Bragg's angle in degees. P I 2 - Full

width at half maxima (FWIHM) in radians. 01 2 is calculated using the following

expression,

P, =(R? - P : ) ! : Where, PM- measured FWHM of the sample and, ps- measured FWHM of the Si standard.

Peak corresponds to the (1 11) plane of the Si standard was used to derive the instrumental

broadening. NBS silicon standard was used as standard for rhe estimation of instrumental

broadening.

2.4.3.3 Characterization of as Prepared as well as Calcined Polymeric Intermediates

by X-Ray Diffraction

Approximately 2 g of as prepared as well as synthesized cathode powders were

finely grounded and filled in the sample holder and mounted on the X-ray diffiactometer.

The X-ray diffraction patterns were recorded between 10" to 80' with scanning rate of

0.05" per second using a X' Pert PRO MPD, PANalytical (Philips) X-ray powder

diffiactometer using graphite monochromator employing Cu K, radiation. The obtained

patterns were compared with JCPDS data. In order to calculate the crystallite size, the 100

% peak was recorded at the very slow scan rate of ?4 ' per min.

2.4.4 Scanning Electron Microscope (SEM)

The scanning electron microscope (SEM) is extensively used to investigate the

microstructure of the polymeric intermediates as well as synthesized final products [122].

2.4.4.1 Principles of Scanning Electron 3licroscope

The schematic diagram of SEM instrument is shown in fig. 2.7. .A beam of

electrons is generated in the electron gun (through themionic or field emission), located at

the top. The beam is attracted towards the anode, condensed by a condenser Pens, and

focused as a very fine point on the sample by the objective lens. As tlme priinary electrons

strike the surface. they are inelastically scattered by atoms in the sampIe, which are

collected by secondary or backscatter detector, converted to a voltage. and amplified. The

amplified voltage is applied to the grid of CRT and forms the image. X-rays also produced

due to the interaction of electrons with the sample, which can be detected by SEM

equipped with energy dispersive X-ray spectrometer (EDS) and wavelength dispersive X-

ray spectrometer (WDS) that extended the possibility of chemical analysis [122- 1231.

2.4.4.2 SEM analysis of Polymeric intermediates

Microstructure of the polymeric intermediates was identified using scanning

electron microscope, Hitachi, S-340074, Japan. Small piece of the polymeric intermediate

was mounted on the conducting carbon tape pasted over the SEM stub. Since the

polymeric intermediates are poor conductors, gold or carbon coating was performed using

sputter coater in order to get better images.

2.4.4.3 SEM analysis of Synthesized final Products

Small amount of powder sample was dispersed in acetone and sonicated for few

minutes. Further, a drop of the mixture was spread on the conducting carbon tape pasted

over the SEM stub. Further, thin layer of gold was coated using sputter coater for better

conduction. Microstructure of the synthesized metal oxide powders was recorded at

different magnifications.

Electron gun

Electron beazn

Condenser lens

BSE Detector

I I / Amplifier 1

Fig. 2.7 Schematic representation of a scanning electron microscope (SEM).

2.4.5 Transn~ission Electron i\licroscope (TE3I)

In the field of nano science, transmission electron rnicroscopc is one of the most

tool, which is used for the direct imaging of lattice structure of solids. With the

assistance of energy dispersive X-ray spectroscopy (EDS) and electron energy loss

spectroscopy (EELS), the transmission electron microscope is a versatile and

comprehensive tool for characterizing the chemical and electronic structure at nanoscale.

2.4.5.1 Principle of the Transmission Electron Microscope

The transmission electron microscope (TEM) operates on the same basic principles

as the light microscope but uses electrons instead of light. Much lower wavelength of the

electrons makes it possible to get a resolution, which is thousands times better than the

light microscope. Using TEM, one can see very small objects to the order of a few

angstrom (10-lo m). For example, one can study small details of the different materials

down near to atomic levels. The possibility for high magnifications has made the TEM a

valuable tool in nanomaterials as well as medical and biological research [124].

Schematic representation of the transmission electron microscope (TEM) is shown

in fig. 2.8. An electron gun at the top of the transmission electron microscope emits the

electrons that travel through column of the microscope under high vacuum condition. TEM

uses electromagnetic lenses to focus the electrons into a very thin beam. The electron beam

then travels through the specimen, which is dispersed in the carbon coated copper grid.

Depending on the density of the material present, some of the electrons are scattered and

disappear from the beam. At the bottom of the microscope, an unscattered electrons hit a

fluorescent screen, which give rises to a image of the specimen displayed in varied

darkness, according to their density. The image can be recorded as photograph or digital

image for further analysis [125].

Electron Gun C

Anode

C------------ Computer Control

4 Diffraction

Image / Information

information

Monitor Viewer

Parallel a - Detector

Fig. 2.8 Schematic representation of transmission electron microscope (TEM).

2.4.5.2 TERl Analysis of NanocrystaIfine Powders

Transmission electron micrographs (TEM) ivere taken in a JEOL 200 FX

microscope operating an accelerating voltage of 200 kV. The samples were dispersed in

acetone and drops of the dispersion were transferred to a carbon coated copper gnd. Dried

copper grid was further used for the TEM analysis.

2.4.6 Vibrating Sample Magnetometer (VSM)

Field dependent magnetization plot, which is known as M-H as well as hysteresis

curve is an important information: which gives the magnetic properties of any magnetic

materials. Vibrating sample magnetometer an essential tool, which has been used to obtain

the hysteresis curve.

2.4.6.1 VSM Principles

The schematic diagram of VSM instrument is shown in fig. 2.9. Vibrating Sample

Magnetometer measures the magnetic properties of materials, which operates on Faraday's

law of induction. When a material is placed within a uniform magnetic field and made to

vibrate mechanically. This induces a voltage in the pick-up coils, which is proportional to

the magnetic moment of the sample [126].

2.4.6.2 VSM Analysis of Nanocrystalline Powders

Magnetic property of the as synthesized as well as surface modified nickel ferrite

particle was investigated using a Lake Shore, vibrating sample magneto meter (VSM),

USA. -5 mg of the samples were tightly packed in quartz crucible and mount on the VSM

sample holder and the hysteresis curve of the synthesized powder was recorded between

the magnetic field of +I- 15000 G.

s Sample

Personal Carnpu ter

Fig. 2.9 Schematic representation of vibrating sample magnetometer (VSM).

2 . j Fabrication of Lithium Cell

Electrochemical performance of the synthesized cathode powders ivas investigated

by characterizing the fabricated lithium cells using the newly de~eloped cathode materials

with respect to the lithium anode.

2.5.1 Preparation of Cathode

In general, electrode material is mixed with a conductive additive named acetylene

black (25 wt. %) and polyvinylidene difluoride (PVdF) (10 wt. %). Further. it will be made

into slurry through the addition of a dimethyl phthalate (NMP) solution as a solvent, which

lead to the formation of homogeneous paste. In some cases. particularly with very fine

materials, the addition of an extra solution was required to obtain the desired viscosity of

the slurry for better material coating.

The obtained sluny was coated in the form of thin layer on the aluminium foil (A

foil of -1.2 c m h i t h a tag for electrical connection was polished with successive grades of

emery, washed with double distilled water and etched for about 2 min in dilute HC1:

washed again, rinsed with double distilled water and air dried.) substrate of required

dimension. The electrodes were then dried overnight in a furnace at 80 O C and pressed in

order to get smooth surface [127].

2.5.2 Preparation of Anode

In the present investigation, pure lithium foil, from Aldrich, was used as anode.

Surface cleaned lithium foil was carefully cut into the same geometric surface area as the

cathode 11281.

2.5.3 Preparation of Electrolyte

The solvents were distilled thrice in argot? atmosphere and repeatedl) treated with

3 A molecular sieves prior to the preparation of the electroIj.te. The electrolyte used in the

present study was 1 M LiPF6 in ethylene carbonate (EC) and dimethyl carbonate (DMC)

mixture ( 1 : I ) [128].

2.5.4 Assembling of Lithium Cell

Cells were assembled into polypropylene test cell using an argon atmosphere

glove-box (MBraun model UNILAB). Lithium was employed for both counter and

reference electrodes. A Celgard (2400) porous polypropylene film was used as the inter-

electrode separator. Atmospheric sealing of the cell is achieved through warping the

threads of the bolts with teflon tap and the m - seal around the bolt heads [127, 1281.

2.6. Characterization of Lithium Cell

In order to identify the electrochemical performance of cathode materials, electro

chemical characterizations such as cyclic voltammetry and constant cunent charge 1

discharge studies were canied out for the fabricated lithium cells.

2.6.1 Cyclic Voltammetry

Cyclic voltanunetry is an effective tool for studyng the preliminary mechanism of

electrochemical reactions. It enables the electrode potential to be rapidly scanned in search

of redox couples. Potential - time wave form used for cyclic votammetry of an electrode is

shown in fig. 2.10. The technique involves sweeping the electrode potential fiom the

starting value V1 to the final potential V2 at a known sweep rate v [mV/s]. On reaching the

potential V2 usually known as the switching potential, the sweep direction is reversed

usuaily at the same rate. The io!tage scan r2te (i 1 is caicuiilied fioni the slope of the line

E139-1301.

important parameters of cyclic voltammetry are the rnas~munl and minlmum

potentials, whlch define the potential ivindow. The choice of thls potentla] wlndow must

take the stability of the eiectrolyte into account, so as to avoid ~ t s decomposit~on. The cell

is cycled in a potential window, where the potentla! applied on the working electrode is

continuously changed with constant rate. When the potentla1 is close to the value required

to oxid~ze the reduced species, there IS a substantla1 arlodrc current until the ox~dation is

complete [130]. The current that flows i s measured and plotted versus the potential as

shown in fig. 2.1 1.

From fig. 2.10, an Increase in the current response is observed at the potential.

where an electrochemical reactlon takes place. For a positive electrode, the part of the

curve under the zero line is the discharge curve, the one above the charge curve. The

surface under the cunle is proportional to the total amount of charge flown. The negative

sweep will lead to the reduction of the working electrode and results the negative current.

The overall shape of the cyclic voltammogram gives details of the kinetics of the electrode

process. Also, in cyclic voltammogram, the potential o f both the fonvard and reverse peaks

are voltage sweep rate dependent [129- 13 01. Electrochemical informations of the electrode

reactions are characterized by several important parameters and they are the cathodic (E,,)

& anodic (E,,) peak potentials, the cathodic (i,,) & anodic (i,,) peak currents, the cathodic

half- peak potential (Ep;2) and half wave potential (E112).

2.6.2 Charge - Discharge Studies

Charging and dischargmg of the cells were carried out between appropriate

potential range (for LiCo02 cathode materials it is found to be 3-4 V). However, for

Fig. 2.10 Potential - time wave form used for cyclic voltamrnetl

-100 0 20 0 400 600 Potential (mV)

Fig. 2.1 1 Typical cyclic voltammogram.

unknonn system. cvclic ~oltamrnctrq has to be perfonnec! first, in order to determine the

~otential range. A number of properties can be deternllned fron~ constant current charge-

discharge experiments, wh~ch are briefly gir en in the follou.lng sections.

2.6.2.1 Capacity

Capacity can be defined as the amount of current (amps) that the battery will

produce over a rated time (hours) and it is measured in mAh. (rnilliamps x hours) or. 1000

times bigger Ah, (Amp x hours). The capacity is usually lower, if a battery is discharged at

a large current (i.e. if the battery has to work fast it will usually produce a little less useable

energy). The measure of energy stored by a batter- is Volts x Amps x Hours or Watt.hour.

The specific capacity is the capacity divided by the mass of the electrode material and the

volumetric capacity is the capacity divided by the volume of the electrode materials

occupies.

2.6.2.2 Cycle Life

Cycle is defined as one sequence of charge and discharge. Using the charge-

discharge data, cycle life, i.e., capacity retention and variation with respect to the number

of cycles are examined. The number of cycles, which caused the discharge capacity to fall

80 % of the initial discharge capacity is an important measure to account the cycle life of

the fabricated battery.

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