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CHAPTER - I1
CHAPTER-I1
EXPERIMENTAL TECHNIQUES
A number of techniques are a~ai lable for the synthesis and
characterisation of cathode and polymer electrolyte materials for lithium ion
batteries. Some of the synthesis and charaterisation techniques employed in this
work are briefly explained here.
2.1. SYNTHESIS TECHNIQUES
Synthesizing techniques that Mere used for the preparation of cathode
and f'erroclectric flller materials \ \err solid-state. ball milling. solution co-
precipitation and citrate gel technique. Solvent cast technique mas used for the
synthesis of composite polymer electrolyte films.
2.1.1. Solid-state and Ball-milling technique
Grinding by hand in an agate mortar remains a useful method in modern
synthesis. For many small-scale studies, hand mortaring is perfectly adequate
for reducing the sizes of powder particles. Grinding of materials b) hand ~ i t h a
mortar and pestle was used in this study for mixing and particle-size reduction
of small amounts of material (-50 g). Once the homogenous precursor powder
Chapter - I1
was obtained, it was placed into a furnace and heated to a suitable temperature.
Gases were generated and released followed by chemical interaction to form
final product(s1. Generally. this material may be ground finel) again and then
put into a furnace a second time (for a ..2"d sinter". or to allow cysta ls to form
and grow in size). Sometimes powder materials are hydraulically pressed into a
pellet shape to increase the interactivity between the powder grains and fbrm a
higher quality andlor h~gher density final product
Ball mills. also known as centrifugal or planetary mills, are devices used
to rapidly grind materials to colloidal fineness (approximately I micron and
below) by developing high grinding energy via centrifugal andlor planetary
action. To grind a sample in a ball mill. the particle size should alread) have
been reduced to less than 10 mm. uslng a mortar and pestle if necessar). The
samples are placed in one of the b o ~ l s and several balls arc added. Samples
can be run wet or dry
Fig. 2.1. Grinding mechanism within the bowl of a ball mill.
D. Shanrnukaraj Ph.D. Thesis (2007) 68
Each bowl sits on an independent rotatable platform. and the entire
assembly of four bowls is also rotated in a direction opposite to the direction of
the bowl platform rotation. In planetary action, centrifugal forces alternately
add and subtract. The grinding balls roll halfway around the bowls and then are
thrown across the bowls. impacting on the opposite walls at high speed as
shown in Fig. 2.1. Grinding is further intensiiied by interaction of the balls and
sample. Planetary action gives up to 20g acceleration and reduces the grinding
time to about 213 of a simple centrifugal mill (one that simply spins around).
Grinding media is available in agate, sintered comndum, tungsten
carbide. tempered chrome steel. stainless steel. zirconium oxide, and polyamide
plastic. The exact type of bowl and balls that are used depend on the type of
material being ground. For example. very hard samples might requlre tungsten
carbide balls in steel ho\+ls. For Qpical use. agate is a good choice. As with any
riiethod of grinding. contaminat~on of the sample with the grinding unit
material can be a complication.
2.1.2. Co-precipitation
Chemical co-precipitation can provide uniform nucleation growth and
aging of the particles in the solution. The size and morphology of the panicles
can be manipulated by controlling the different reaction parameters: a
technique generally inexpensive to perform that relies on simple coordination
D. Shanrnukaraj Ph.D. Thesis (2007) 69
Chapter - I1
chemistry allowing the synthesis of the required solid "compound" w ~ t h the
desired composition and at high uniformity. On the other hand, the process of
establishing and controlling the precipitation conditions are quite complex.
Materials for co-precipitation are usually dissolved in a suitable solvent
most often water. For example Lu and Dahn [ l ] synthesized
I.i[Cr,Li,l~i.,,3,Mn(2,3.~,3,]02 with lithium acetate. chromium nitrate and
tnangnnese nitrate. all of wh~ch are easil) water soluble. After adding the
correct ratio of materials into continuously stirred water. they added
ammonium hydroxide solution and metal hydroxide products which started
forming a cloud of precipitation in the solution. Heating was done to evaporate
the water and the mix had a slightly jelly like consistency. The mix was then
d r ~ e d thoroughly by purtlng In an oven at 130 "C overn~ght and then ground up
In d mechan~cal mill to make sure no segregat~on of ~ngred~ents had occurred
du r~ng the preblous process The resultant powder ivas then heated at 900 C in
a turnace to ) ~ e l d LI[C~,LI I ,.3,Mn,2 3 > , I 10:
2.1.3. Sol-gel
The sol-gel technique can be described as chemical technique of
simplicity and effectiveness to synthesize different type of inorganic and
organic-inorganic hybrid materials. which can be used in solid state d e ~ i c e s .
Wide ranges of new and known materials containing oxide components have
D. Shanmukaraj Ph.D. Thesis (2007) 70
Chapter - I1
been successfully prepared in recent times 12-41, This process allows des~gning
rhc morphologc of electrochemical materials by which the propenics of the
surface and interface can be modified. Fig. 2.2 shows the schematic d~agram of
the sol-gel process. Sol-gel technique follows the route line of hydrolysis and
pol\.merisation to form amorphous or crystalline material at low temperature
processing in solut~on state [3 . 51.
The sol-gel process takes up two different routes to prepare various
tqpes ot amorphous nlater~al. (a) collo~dal process and (b) alkox~de routes
Colloidal process in\~olves the dispersion of particle of colloidal size in
aqueous s o l ~ e n t medlum to form a sol and thus formed sol is destabilised in a
controlled manner. by allowing the particles to approach each other to
overcome the stabilising barrier. This could be achieved by heating, freezing,
adjusting the pH of the sol to obtain a gel. The solvent from the gel can be
removed by maintaining it at a particular temperature and then sintered to give
a cps~a l l i ne~dense amorphous so l~d niaterial
Sol-gel process through alkoxide roure involves the following steps;
mixing, casting. gelation. aging, drying. chemical stabilisation and
densilication. Metal alkoxide is the main precursor chemical, which requires
the use of an organic solvent. normally an alcohol, In order to act as mutual
solvent for the alkoxide and water for hydrolysis. The citrate gel process is a
modified sol-gel technique in which the precursor ingredients include metal
nitrates dissolved in c i t r~c acid and water.
D. Shanmukaraj Ph.D. Thesis (2007) 71
Fast fire k' I Controlled slow
heating rate _1
Fig. 2.2. Schematic diagram of the sol-gel process.
D. Shlmmukamj ph.D. Thesis (2007) 72
Chapter - I1
2.1.4. Solvent casting technologv for the production of films
Nouadays. solvent cast technolog\, is becoming increasingl? attractive
for the production of films with extremel) high quality requirements. The
advantages of this technology include uniform thickness distribution.
maximum optical purity and extremely low haze. The optical orientation is
virtually isotropic and the films have excellent flatness and dimensional
stability. Solvent casting is an important commercial technique utilized to
fabricate thin layered films for diverse applications. Most familiar is the solvent
casting o f cellulose acetate (CA) for photographic films having good
dimensional stabilit!. clarit>, flexibility and fiacturr resistance. Some of the
advantages of solvent casting method (as compared to the melt process) are as
t'ollo\hs.
1. Higher quality (uniformity) and thinner film
2. Freedom from pinholes and gel marks
3. Purity and clarity
4. Lack of residual stresses
5. Possible to produce patterns or dull finishes
Thc solvent-cast process consists of dissolution of the film ingredicnts in
a suitable carrier that conveys the solution through a drier &here the solcent is
ecaporated. The resulting film is removed from the substrate and wound into
rolls or cut into desired shape as required
D. Shanrnukaraj Ph.D. Thesis (2007) 73
Chapter - I1 2.2. TEST CELL CONSTRUCTION
2.2.1. Cathode paste mixing
Materials to be evaluated as cathode materials for storing and releasing
Li should be in the form o f ' a powder uith particle size < -10 pm. Cathode
substrates should be prepared first. For CK2032 coin cells, Al disks ofdiameter
15.5 mm and thickness between 0.50 and 1.10 rnm are suitable. These
substrates are cleaned and de-burred by abrading with glass paper or SIC
abrasive paper and then degreased in alcohol in an ultrasonic bath. Care should
to be taken not to let the discs become re-oxidised by exposure to atmosphere
once abraded and degreased Discs are then weighed carefully and individually
so that their masses are known exactly.
If the cathode material has a rebistance of <-lo0 MR then it is mixed
\s ith -1 5 wt04 of acetylene carbon-black as a conductive matrix and - I ? \vt O h
of poly vinylidene difluoride (PVdf) as an adhesive. Cathode material of
resistance >I00 MR (eg: 5GR) are mixed with -20wt% of acetylene carbon-
black (to improve conductivity).
Powder ingredients are dry-ground in an agate mortar for 10 minutes.
then a fefi drops of ethanol is added to make a thin paste and the mix is wet-
ground in the agate mortar for I0 more minutes. Drops of dimethyl phthalate
(UMP) or normal methyl pyrolidinone (NMP) are then added and ground in the
mortar for 5 minutes to make a smooth. thin paste. The paste is applied to the
D. Shanmukaraj Ph.D. Thesis (2007) 74
Chapter - I1
entire upper surface of Al cathode substrates via a nylon or camel-hair artist's
brush and then allowed to dry in a vacuum oven between 50 and YO "C' for at
least one hour.
After drqing. cathode disks are compacted using a hydraulic press at
- 40 MPa between smooth dies to aboid voids. to homogenise and optimize the
density and thickness of the cathode coating. Then the disks are re-weighed to
determine (by subtraction) how much "active mass" has been added by the
cathode-pasting process. The active mass is vital for determining the exact
charge and discharge capacity of the material. Disks are then baked at high
temperature (120 - 170 "C) to make the paste adhere strongly to the Al
subitrates.
2.2.2. Cell construction and assembly
1-he fabricated electrodes are placed into an argon filled glove box
(MBraun Unilab) ready for assembly into CR2032 test cells. A schematic
diagram of a true electrode CR2032 test cell is presented in Fig. 2.3. Standard
'.half cell" test cells are assembled by placing the electrode to be tested at the
bottom of the cell followed by a microporous polqpropylene separator
(Celgunrd 250 '~ ' ) . Pure 1.i foil is used as an anode on a clean aluminium
backing disc. Electrolyte is then added to the cell which is Merck LP30-IM
LiPF6 in a 1:l mixture by volume of ethqlene carbonate (EC) and dimethyl
D. Shanmukaraj Ph.D. Thesis (2007) 75
c h a w - n
carbonate (DMC). A "crinkle washer" flat profile stainless steel spring is used
to provide the constant compaction function.
I C n s i n g (Hermetic)
Anode Subsink
Anode Maierpl
Electrolyte
P o l ~ s mepantor mernbnn
I
Fig. 2.3. Basic design of test cells for charge-discharge and similar tests.
2.3. CHARACTERISATION
2.3.1. Structural characterisation
2.3.1.1. X - Ray Dtflraction (XRD)
X-Ray Diffraction (XRD) measurements were carried out using x-ray
diflixctometers (XPERT Philips analytical and Philips PW 1730) with CU-K,
radiation. Scans were typically acquired at an accelerating voltage of 40 kV
and current 20 mA with a scan rate of one degree per minute. XRD
measurement was carried out by smearing the powder samples on slides using
ethanol. The x-ray diffiactometer (Philips PW 1730) is shown in Fig. 2.4.
D. Shanmubraj P~.D. Thesis (2007) 76
cham-n
Based on the interaction of x-rays with periodic arrangements of atoms in
crystalline materials, details on the structure of the material can be gathered.
The interactions which make this analysis possible are found to satisfy Braggs
Law as shown in Eqn. 2.1 (61.
nA = 2dhk, Sin8 2.1
Where
n is the order of reflection and is any integer such that sin0 i 1
h is the wavelength of x-ray
dhkl is the distance between two adjacent parallel planes
0 is the incident angle of the x-ray beam to the planes of atoms
Fig. 2.4. X-ray diffractometer (Philips PW173O).
By subjecting materials to incident x-rays at a variety of angles an x-ray
diffraction pattern can be produced. An X-Ray Diffraction (XRD) pattern
typically consists of a number of peaks of various intensities over a range of
angles. Analysis of the angular positions, peak intensities and shapes can then
he used to give information on the crystal structure and physical state of the
material being investigated [7].
Information that can he gained from application of XRD includes
identification and quantitative analysis of crystalline compounds. crystal
structural determination and analysis of residual stress and crystalline size [7]
Accurate determination of the interplanar distance (dhll) and relative intensities
is necessary for phase identification [7]. Routine identification can however be
carried out by comparison to the data puhlished in the x-ray pouder diffraction
file b> the Joint Committee on Powder Diffraction Standards (JCPDS).
Kesidual stress can affect peak width as a result of microstress while their
position can be influenced by marcostresses [7]. The peak width is however
also influenced by crystallite size with finer crystalline size materials exhibiting
broader peaks. Crystallite size can he estimated from XRD patterns using the
Scherrer equation as given in Eqn. 2.2 [8].
Where K is a constant which is frequently taken as 0.9
D. Shanmukaraj Ph.D. Thesis (2007) 78
h is the wavelength of the source used
t is the particle dimension
p is the peak width in radians
2.3.1.2. Fourier Transform Infrared (FTIR) spectroscopy
Molecular vibrational information can be obtained from the absorption
of infrared radiations and also from the inelastic scattering of light. The study
of molecular structure by spectrometry depends primarily on the existence of
the vibrating motion of atoms within the molecule. These motions in turn
depend on the nature and arrangement of constituent atoms. Radiant energy,
particularly infrared, incident upon matter, is affected by the presence of such
motions. A study of this behaviour of infrared radiation is thus capable of
giving indirect but very valuable information on molecular structure.
Fig. 2.5. Fourier Transform Infrared (FTIR) spectrometer.
D. Shanmukaraj Ph.D. Thesis (2007) 79
Chapter - I1
'l'he Fourier Transform Infrared (FTIR) measurements for the
synthesized cathode materials were taken using a FTIR spectrometer (ABB
ROMEM MB104) shown in Fig. 2.5. The samples were pelletised with KBr.
FI IK spectrometer is based on a Michelson interferometer that proxides a
spectrum in the time domain, which is Fourier-transformed by a computer to a
spectrum in the frequency domain. The sample can be scanned repeatedly and
the accumulated spectra can be averaged. thu5 producing a representative
~nfrared spectrum of a r e v high signal-to-noise ratio. This enables the
measurement of samples containing a x e p low concentration of the active
materials
Its main appl~cations are: to study the intramolecular forces,
intermolecular forces or degree of association in condensed phases and in the
determination of molecular symmetries. Other applications include. the
identification of functional groups or compound identification. determination
of' the strength of chemical bond. structural elucidation and the calculation of
therniodynamical properties.
2.3.2. Microstructural characterisation
The microstructure of the electrode materials is also important to the
electrochemical properties of materials and a variety of techniques can be used
to investigate it.
D. Shanmukaraj Ph.D. Thesis (2007) 80
Chapter - I1
2.3.2.1. Scanning Electron Microscopy (SEM)
Scanning Electron Microscopy (SEM) investigation was carried out
using an electron microscope (JEOL JSM-6460A) shown in Fig. 2.6. SEM is a
powerful technique for investigating the microstructure of a variety of
materials. The SEM is similar in many ways to a reflected light microscope but
it makes use of a beam of electrons rather than light. The working distance in
the SEM is the distance between the sample and the final objective lens and is
an important parameter for the SEM. A short working distance provides higher
Fig. 2.6. Scanning Electron Microscope (JEOL J S M M A ) .
Chapter - I1
resolutions hut at the expense of depth of field while a long working distance
probides a high depth of field at the expense of resolution. 'The \rorking
distance is not the only parameter that influences the image but also depends on
the size of the electron spot which in turn depends on the magnetic elecrron-
optical system which produces the scanning beam. The resolution is also
lim~ted by the size of the interaction volume. or the extent to which the material
interacts with the electron beam.
The SEM has compensating advantages. though, including the ability to
image a comparatively large area of the specimen: the ability to image hulk
~iiaterials (not just thin films or foils); and the variety of' analytical modes
abailable for measuring the composition and nature of the specirncn.
Depending on the instrument. the resolution can fall somewhere between less
than 1 nm and 20 nm. In general. SEM images are much easier to interpret than
Iransmission Electron Microscope (TEM) images. Prior to examination
samples were mounted on aluminium stubs with carbon conductive tape.
2.3.2.2. Transmission Electron Microscopy fTE.M)
The panicle size of the carbon coated LiFel'O, particles prepared in this
work u a s studied using a Transmission Electron Microscope (HRTEM. 300 kV
JEOL JEM-3000F) as shown in Fig. 2.7. The material to be studied was
suspended in a droplet of ethanol and some of the material adsorbed onto a Cu
b. Shanmukaraj Ph.D. Thesis (2007) 82
Fig. 2.7. Transmission Electron Microscope (HRTEM, 300 kV JEOL JEM-3000F).
TEM specimen grid by carefully immersing the Cu specimen grid in the
ethanol suspension. The Transmission Electron Microscope (TEM) operates on
the same basic principles as the light microscope but uses electrons instead of
light. TEMs use electrons as "light source" and their much lower wavelength
make it possible to get a resolution a thousand times better than with a light
microscope. Particles to the order of a few angstroms (10.'' m) can be viewed
with TEM. The possibility for high magnifications has made the TEM a
valuable tool in materials research. A "light source" at the top of the
microscope emits the electrons that travel through vacuum in the column of the
Chapter - I1
microscope. Instead of glass lenses focusing the light in the light microscope,
the TEM uses electromagnetic lenses to focus the electrons into a v e q thin
beam. The electron beam then travels through the specimen under study
Ilcpending on the density of the material present. some of the electrons are
scattered and disappear from the beam. At the bottom of the microscope the
unscattered electrons hit a fluorescent screen, which gives rise to a "shadow
image" of the specimen with its different parts displayed in varied darkness
according to their density. The image can be studied directly by the operator or
photographed with a camera.
2.3.3. Thermal analysis technique
2.3.3.1. Diffrrenfial Thermal .Ana!r.sis (DTA)
IlitTerential Thermal Anal!.sis (DTA) for the prepared pol?mer
elccti.ol~tes b a s taken using a d~tt'erential thcr~i~al anal)ser (Linseis) as shown
in Fig. 7.8. It IS one of the simplest and most Lvidely used thermal analysis
technique. The difference in temperature. AT. betiveen the sample and the
reference material is recorded while both are subjected to heating programme.
The classical DTA instrument is shown in Fig. 2.9. The DTA curves should be
marked either endo or cxo directions. The negative peak is called as an
cndotherrn (AH is positive, e.g. melting) and for an exotherm (AH is negat i~e .
e.y. oxidation) the peak will he in the posit~ve d~rection.
D. Shanmukaraj Ph.D. Thesis (2007) 84
Fig. 2.8. Differential thermal analyser (Linseis).
Fig. 2.9. Classical apparatus of differential thermal analysis (S = samplc; R = reference).
Chapter - I1
The reference material should have the following characteristics:
8 It should undergo no thermal events over the operating temperature
range.
It should not react with the sample holder or thermocouple
*:* Both thermal conductivity and heat capacity of the reference should he
similar to those of the sample.
AIZO; and SiC have been extensi~el) used as reference substance for
inurganic samples. while for organic compounds. especiall!, for polbmers. octyl
phthalate and silicone oil were used as reference substance.
2.3.4. Electrochemical characterisation
Techniques such as constant current charge'discharge and cyclic
\oltammetr> are useful for observing a \ a r i e ~ of electrochemical properties.
2.3.4.1. Constant current (galvanostatic) charge - discharge
In this study constant current charge-dischargt. testing was carried out
on 5 V-5 mA B a n e 9 Test System (BTS) (Neware) utilising a current ranging
from 20. 40. 80 and I00 PA. The experimental setup for testing cells is shown
schematically in Fig. 2.10. Charging and discharging of cells *ere carried out
hetween the voltage end points of 2.75 V to 4.25 V for LiFePOJ/C
materials and 2.75 V to 4.95 V for LiFe,.,Co, PO, cathode materials. The
D. Shanmukaraj Ph.D. Thesis (2007) 86
N e W u e BMey T e e System
( 8 Ch-el. I I Can el CsU Y2 CeU U3
Fig. 2.10. Contemporary equipment for charge-discharge testing.
"Neware" rack-mounted batter) testers can manage tests for upto 8 cells
simultaneously. The controlling sotiware used was either Nesare Celltest 3 1.
3.2 or t i w a r e RTS.
\'hen carrying out constant current charging to determine the capacity.
two different criteria are frequently used to determine the appropriate current to
be used. These are hascd on either the 'electrode surface area' or its 'mass'.
The current denslt) criterion is based on the electrode surface area. where the
cument density is simply the current divided by the electrodes surface
area.
D. Shanmukaraj Ph.D. Thesis (2007) 87
Chapter - I1
Fig. 2.11. Variation of current density with electrode diameter for a fixed current
of 50 uA.
Fig.
1:; I .'- I,. "
2.12. Variation of C-rate with electrode mass for a fixed current of 50 PA.
D. Shanmukaraj Ph.D. Thesis (2007) 88
Chapter - I1
'The C-rate is the other method. for e.g. LiFePO, has a theoretical
capacity of 170 mAhig over a period of time. A discharge current of 34 mAig
ocer five hours is referred to as the C'J5 rate ( 5 x 34 = 170) and 17 mAtg over
ten hours is referred to as CI10 rate. When the charging current is tixed, the
variation of the current density to the surface area of the electrode and C-rate to
the mass of the electrode is as shown in Fig. 2 . I 1 and Fig. 2.12. Such variation
should be taken lnto account as the measilred capacity \.aries with the current
densit) and C-rate employed
3 3.4 1 2. C'apuciry
A given discharge capacity can be represented as a reversible and
irreversible capacity as shown in Fig. 2.13.
Fig. 2.13. Schematic diagram illustrating reversible and irreversible capacih.
D. Shanmukaraj Ph.D. Thesis (2007) 89
Chapter - I1
The rebersiblc capacitj represents the portion of the capacltg that was
re~ers ible on charge and is given by the charge capacity. The irreversible
capacitc on the other hand is the portion of the capacity that cannot be
recovered on charge and is given bc the difference between the discharge and
charge capacities. Capacity is often expressed in mAh or .4h on commercial
cells.
'l'he specific capacity (niAh1g) is the capacity (mAh) divided b! the
mass (g) of the electrode material whereas the \olurnetric capacit! ( m n h em')
is the capacit) dl\ ided b) the \olume occupied b) the electrode material.
[Inless and otherwise stated the capacit? being referred to in t h ~ s thzsis is the
specific capacit?. The theoretical capaclt! can he calcul~ted using a
relationship between the number of moles of lithium in the reaction product
and the molar mass of the lithium host as shown in Eqn. 2.3.
Fn Specific Capacity =
3.hhl
Where t is Farada>'s conslant 96187 (C mole)
n is the number of moles of lithium in the reaction product
M is the molar mass in grams of the lithium host
0. Shanmukaraj Ph.D. Thesis (2007) 90
Chapter - I1
2.3.4.1.3. Cvcle l ~ f e
Cycle life is typically considered as the number of cycles it takes for the
d~scharge capacity to tall to 80% of the lnitial discharge capaci). C'!cle life
and variation of capacity with cycle number can he obtained from
charge'discharye data.
2 3.1 1.l . Chulomhic eflicfenoy
The coulombic efEciency can he calculated from the discharge and
iuhsequent charge capacity as shown in Eqn. 2.4. This can be considered as a
measure o f a nuniber of propenies including the ease ui th \\hich lithium ions
cdn be extracted fiom the structure (occurs during charging). .4 h~gher
coulomhic e f f i c~mcy indicates that lithiuln ions are more difficult to evtract
from the struclure and hcnce the re\erslbilir\ c~t'the reaction Ir reducsd
,, - Discharge capacity CoulombicEFficiency ( , o ) - xl00 2.4
Charge capacit)
2.3 -1 1 5. L)(ffcrcn/iul c,~~pacir>'
The differenliat~on of charge and discharge protiles (cnpacir? ~ i t h
respect to voltage) can be used to further assess the reaction mechanism and the
changes that occur in the reaction mechanism as shown in Fig. 2.14. The lower
portion of the differential capacity plot corresponds to the discharge bhile the
D. Shanmukaraj Ph.D. Thesis (2007) 9 1
Chapter - I1
upper to the charge. I f the corresponding charge-discharge peaks can be clearly
identified then the reversible potential E, and overpotential q can be calculated
using Eqn. 2.5 and 2.6. respectivel) 191.
Fig 2.14. Example ofa differential capacih plot.
D. Shanmukaraj Ph.D. Theois (2007) 92
Sharp peaks in differential capacity plots are typical of large crystalline
materials while broad peaks are typical of nanocrystalline materials where
reaction plateaus are less defined.
2.3.4.2. Cyclic L'o1tamrnelrq. (CV)
Cyclic Voltammetry (CV) for the synthesized polymer electrolytes was
carried out on an electrochemical work station (CH660B) over the \.ullage
range 0.01 V to 3.0 V at a scan rate of 1 mV per second. In Cyclic
Voltammetry (CV), the potential of a small, stationary working electrode is
changed linearly with time starting from a potentiai where no electrode reaction
occurs and moving to potentials where reduction or oxidation of a solute (the
material being studied) occurs. After traversing the potentiai region In which
one or more electrode reactions takes place. the direction of the linear sneep is
reversed and the electrode reactions of intermediate and products. formed
during the forward scan can be detected. The time scale of the experiment.
controlled by the scan (or sweep) rate and the total potential traversed. can be
varied over the range of 10'-10.' s through quantitative experiments.
The position (potential) of peaks in CV can be used to detcrm~ne the
reaction occurring at the given potential while the current describes the
intensity of the reaction. The separation of respecti\e oxidation and reduction
peaks for a given reaction also provides an indication of the stability and
reversibility of the reaction.
D. Shanmukaraj Ph.D. Thesis (2007) 93
- -
2.3.5. Ac impedance spectroscopy
Ac impedance spectroscopic measurements were carried out for the
synthesized cathode materials using a 1.CR meter (1-IP 4284A) and the
experimental setup is shown in Fig. 2.15. The impedance measurements for the
polymer electrolytes were taken using an electrochemical workstation
(CH 660R).
Fig. 2.15. Experimental setup for ac impedance analysis.
Ac impedance spectroscopy involves the application of a small potential
perturbation (E) at various frequencies (f) at a given dc potential (E&) as shown
in Fig. 2.16. The current response is monitored and as a result the variation of
resistance with frequency for the material can be examined.
/
Time
Fig 2.16. Basics of ac impedance technique.
Impedance spectroscopy is a powerful technique for the stud! of fast
ionic conductor materials. as it enables the bulk ionic conductl\it\ to be
resol\ ed tiom other resistl\,e or capacl t i~e elements \\lthin these conductors.
it single crystal ionic sample would show a sen~icircle in an impedance
plot as shown i l l Fig. 2. I 7 [ lo] . rrsult~ng from the bulk resistance and the hulk
capacitance of the crystal. This is modelled by an equibalent circuit of a
resistor in parallel with a capacitor and is shown in Fig. 2.18 [lo].
Measurement of the electrical conductivity for polqcr~stalline materials [ I I ]
using impedance spectroscopy provides information relating to the electrical
behavior of both the grain interiors and to the grain boundar? regions.
D. Shanmukaraj Ph.0. Thesis (2007) 95
Chapter - I1
Fig. 2.17 A simulation o f the semicircle obtained for an impedance plot of a 1 MQ resistor i n parallel w i th a 10 pF capacitor (an ideallsed ionic conductor). The crosses represent simulated ionic conductivity measurements from 0.1 Hz to 10 MHz at regular intervals in log (frequency) (101.
Fig. 2.18 The equivalent circuit for an idealised ionic conductor, consisting of a single sample capacitance i n parallel ~ i t h the bulk crystal resistance
1101.
D. Shanmukaraj Ph.D. Thesis (2007) 96
Chapter - I1
In Fig. 2.19 (a), the equivalent circuit for the electrical response of a
polycrystalline material is shown. The circuit has a direct relationship to the
frequency
2''
2'
'ha R e R,
Fig. 2.19. a) Equibalent circuit for the electrical response of a polycrystalline sample showing contributions from the grain interiors (gi), grain boundaries (gb) and electrolytelelectrode interface (e); b) Complex impedance plot corresponding to the circuit in a).
complex impedance plot (Fig. 2.19 b), in which 2". the imaginary part of the
complex impedance, is ploned against Z'. the real part. for a ~ i d e range of
frequencies (typically 10-'-10~ Hz).
D. Shanmukaraj Ph.D. Thesis (2007) 97
Chapter - I1
l 'he frequency increases as shown in the arrou in Fig. 2.19h. The
highest frequency is located at the origin. Comparing with the other two. the R,
could normally be ignored due to the use of very conductive metal except for
the imperfect contact. which causes interfhcial contact resistance [ I 1 j.
The capacitance C,,. Cgbr and C, corresponding to the grain interior,
grain boundary and electrode, respectivelq, can be obtained from the dielectric
relaxation peaks. uhich have their specific capacitance reference value 1121.
The ac theop is concerned uith sinusoidnl varqing currents and toitages
of the Ibrm
E = E,, (cos wt * .i sin on) 1 7
and the real part of the expression E,,cos wt and locos wt represents the
obsenable quant~ties. These equations all have the form I = E/Z uhere Z = R.
1,' ,jtr,C and joL for resistance, capacitance and induction respectively.
Modulus Z is known as electrical impedance and is exprrssed in unlts of ohms.
-1'11~ impedance behaves mathematicall) in sim~lar manner to resistance.
Z = ( I / R L 1 , ' - ' JWC
2.9
1~ 1 - (1 IR' + 11 t o 2 ~ 2 ) ' . 2.10
This is the same expression as found for the impedance by graphical means.
The frame of the complex impedance may be generalized to
z* = Z' - iZ" 2 I 1
D. Shanmukaraj Ph.D. Thesis (2007) 98
Chapter - XI
'The other related complex quantities similarly defined
Complex admittance Y' = Y' - jY" =(z')-' 2.12
Complex permitivity E' = Y' i ' oCc = E'- je" 2.13
Complex modulus M' = (c').' 2.14
These relationships form the basis of complex plane analqtical technique.
The impedance spectra convey information about the microscopic
dynamics o f the mobile carriers, because o(o) is the Fourier transform of the
autocorrelation function of the current density i. Its autocorrelation function is
I here i ( t )=--Zqjvi( t )
I
real function of time t, V is the volume of the sample. and the summation is
over the charge carriers. The charge on the mobile species and their yelocities
has been denoted by q, and v, respectlvel>. o. ki, T are angular frequency.
Boltzmams constant and temperature respectively
Thus in recent )ears t h ~ s techn~que has become a \<ell accepted
fundanietital tool for characterizing ionic conductors dut. to its ad\antagcs like
rapid acquisition of data (otien ~ i t h i n minutes). high accurac!. rcpeatahility
and high adaptability to a wide variety of different application.
0. Shanmukaraj Ph.0. Thesis (2007) 99
-
2.3.6. Temperature dependant ionic conductivity studies
Temperature dependant ionic conductivity studies for the polymer
samples were taken by casting the polymer samples on teflon moulds.
Preparing the sample in a teflon mould ensure the intactness of the sample
during conductivity measurement. The teflon mould was then placed in a
specially designed vacuum chamber coupled with a heating setup. Silver
electrodes were used for the conductivity measurements. The temperature
dependant conductivity setup used for the study is as shown in Fig. 2.20.
Fig. 220. Temperature dependant conductivity setup.
Chapter - I1
2.4. VARIABLES AFFECTING ELECTROCHEMICAL PROPERTIES
Variables affecting the electrochemical properties of materials include
the makeup and fabrication of the electrode. the electrochemical testing
procedure and those associated with the material themselves. Properties of the
materials that can influence the electrochemical properties ~nclude its
composition. particle size and its distribution as well as its morphology. A
number of variables from the fabrication of the electrode influence the
properties as well. such as the use and amount of conductive additives. hinder
choice and proportion as well as the film thickness and denslty. The
electrochemical properties are also influenced by the electrochemical test
procedure such as the testing current and the potential \\indo% in \\hich the
c>cles are cycled.
When nanostructured materials (crystallite size less than 10 nm) are
utilised a number of important material propert). changes occur [ I 31 which will
ultimately affect the electrochemical performance of such materials.
Nanostructured electrodes [14-171 and those composed of nanocrystalline
panicles [I81 have demonstrated better rate capahilih than those with larger
crystal sizes.
It is not just the crystalline size that can ~ntlucnce the properties but also
the distribution o i the particles [19]. Mass transport often limits the
electrochemical performance of lithium ion batteries and as a result the
D. Shanrnukaraj Ph.D. Thesis (2007) 101
- -- ~ -- -p
fabrication of electrodes as thin films (60-90 pm) consisting of small particles
(5-30 pm) on substrates of 20 Fm thick is common [20]. Shim el. al.
investigated the effect of electrode thickness and density on the electrochemical
properties of natural graphite electrodes [21 j . Dlft'erent densities bere achieced
by pressing the electrodes with a variety of forces includ~ng an unpressed
sample Pressing resulted in a decrease in both the reversible capacit). and
~rreversible capacity loss during the formation of cycles. The cycl~ng
performance of pressed electrodes was also more stable than those of unpressed
samples.
Takamura el . 01. examined the influence of conductive additive loading
on rlectrochemical properlies and found that homogeneity of the electrode was
more important 122). The addition of conductive a d d ~ t i ~ e s did improve
electrochemical performance however homogenous s lurp was found to be
n e c e s s q tbr high performance regardless of the presence of conductive
additives.
Dominko e!. al. made a similar observation and concluded the
distribi~tion of carbon black around active particles is critical. birh eben an
electrode consisting of on]) 2 wi% carbon black uniforml) distributed offering
better kinetics than one with 10 W% carbon black distributed non-unifbrmly
1231. The homogeneit) of the electrodes \\as also recognised as a ke! factor h!
Nanjundaswamy er. 01, in their examination of the coating techniques [24].
D. Shanmukaraj Ph.D. Thais (2007) 102
Franson el. al. examined the effect of carbon black and binder on
electrochemical properties [ 2 5 ] . In the voltage range 0.01-1.5V no difference in
cycling performance was observed between the binders polyvinyledene
fluoride (PVdf) and ethylene propqlene diene terpolynier (EPDM) though
irrcvcrsihle capacit) increased with increasing amounts of carbon black Cyclic
voltammetry in the same voltage range however showed an additional peak
during the first discharge at 0.35 V that \\,as associated with PVdF binder. In
many of these cases however the differences on a percentage basis are very
small and the results observed might in fact be within the normal error limits of
the experiment.
D. Shanmukaraj Ph.D. Thesis (2007) 103
Chapter - I1
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D. Shanmukaraj Ph.D. Thesis (2007) 101
Chapter - I1
12. H. Ye, 'Dielectric behaviour and thermal stability of synthetic diamond'.
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D. Shanrnukaraj Ph.D. Thesis (2007) 105
Chapter - If
24. K. S. Nanjundaswamy, H. D. Friend. C. 0. Kelly. D. J. Standiee. R. L .
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