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TRANSCRIPT
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CHAPTER 2
EXPERIMENTAL TECHNIQUES
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2.1 Introduction
Cathode materials investigated in this thesis are synthesized by one step/two steps/
three steps conventional solid-state reaction (SSR) method. High purity commercial
chemicals of transition metal oxides and lithium carbonate are used for SSR synthesis. In
the case of fluorine substituted spinel samples, (NH4)HF2 and LiF are used as fluorine
sources. Also, the cathode fabrication procedures and the cell assembly processes for all
materials are the same. The detailed information on the chemicals, synthesis and cathode
fabrication procedures, cell assembly processes and experimental facilities used in this
dissertation are described in this chapter.
2.2 List of Materials and Chemicals
The chemicals and materials used in this study for synthesis, fabrication and
characterization of different types of spinel cathode materials are listed in Table 2.1.
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Table 2.1 List of materials and chemicals used.
Chemicals/Raw Materials
Chemical Formula Purity (%)
Suppliers
Lithium Carbonate Li2CO3 Merck
Manganese Dioxide MnO2 Himedia
Lanthanum Oxide La2O3 99.99 Sigma-Aldrich
Chromium (III) Oxide Cr2O3 99.9 Sigma-Aldrich
Lithium Fluoride LiF 99.5 Merck
Ammonium Hydrogen
Difluoride (NH4)HF2 99.999 Sigma-Aldrich
Methanol CH3OH Merck
Carbon Powder C Alfa Aesar
Poly(Vinylidene
Fluoride) (-CH2CF2-)n Sigma-Aldrich
n-Methyl-2-Pyrrolidone C5H9NO 99.5 Sigma-Aldrich
LP31 Electrolyte EC:DMC=1:1 w/w Merck
CR2032 Coin-cell
Hardwares Stainless steel Japan
Swagelok Cell
Lithium Foil 99.9 Sigma-Aldrich
Aluminum Foil Sigma-Aldrich
Polypropylene
Membrane (Separator) Japan
Potassium
Permanganate KMnO4 ≥ 99 Sigma-Aldrich
Sulfuric Acid H2SO4 37 Sigma-Aldrich
Sodium Oxalate Na2C2O4 ≥ 99 Sigma-Aldrich
Potassium Bromide KBr Merck
Silver Ag 99.99 Sigma-Aldrich
Isoamyl Acetate ≥ 95 Sigma-Aldrich
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2.3 Materials Synthesis
In this research work, the following spinel cathode materials with a typical formula
are synthesized:
1. LiMn2O4-xFx, (x = 0, 0.05 and 0.15) by one step SSR method using LiF as fluorine
source.
2. LiMn2O4-xFx, (x = 0.00, 0.05, and 0.15) by three steps SSR method using
(NH4)HF2 as a fluorine source, for comparison.
3. LiLaxMn2-xO3.85F0.15, (x = 0.01, 0.02, 0.03, and 0.05) by three steps SSR method
using (NH4)HF2 as fluorine source.
4. LiLaxMn2-xO4, (x = 0.01, 0.02, 0.03, and 0.05) by two steps SSR method for
comparison.
5. LiLaxCryMn2-x-yO3.85F0.15, (x = 0.01, 0.02, 0.03, and 0.05 and y = 0.15 – 2x) by
three steps SSR method using (NH4)HF2.
6. LiLaxCryMn2-x-yO4, (x = 0.01, 0.02, 0.03, and 0.05 and y = 0.15 – 2x) by two steps
SSR method, for comparison.
7. LiMn2-x-yLixCryO3.85F0.15, (x = 0.02, 0.05, 0.075 0.1 and y = 0.15 – x) by three
steps SSR method using (NH4)HF2 as fluorine source.
As noted above, 26 samples are synthesized in total using solid-state synthesis method
2.4 Experimental Procedures
This section describes the details of the powder synthesis and cathode fabrication
procedures as well as cell assembly processes to study the physicochemical and
electrochemical properties. The majority of cathode materials studied in this thesis are
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synthesized by three steps solid-state reaction method to obtain the desired stoichiometric
products, high crystalline powder materials, small particle sizes which can be mixed to
maximize the surface contact area and homogenized products. These aspects play an
important role in the physicochemical and electrochemical properties of synthesized
materials.
2.4.1 Powder Preparation
Synthesis of high quality materials with desired properties is very important in the
experimental research work. The physical, chemical and electrochemical properties of
the electrode materials depend to a great extent on the synthesis methods. Several
synthesis methods have been developed for the preparation of cathode materials for
Li-ion batteries. Some of them are co-precipitation, Sol-Gel Process, Combustion
Process, molten salt method, hydrothermal method, microwave synthesis and solid-state
reaction method. However, every method has its own advantages and disadvantages. For
example, the conventional SSR method has the following disadvantages: inhomogeneity,
large particle size, agglomeration of the particles, irregular morphology, broad particle
size distribution, high calcination temperature to decompose the raw materials
completely, long heating time to complete the reaction, and difficult control of
stoichiometry [1-6]. Wet chemistry methods, which mostly use expensive and
environment-sensitive chemicals, require expensive apparatus and complicated synthesis
steps, making the process difficult [1,2]. Also, coprecipitation processes involve repeated
washing in order to eliminate the anions coming from the precursor salts used, making
the process complicated and time consuming.
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The term solid-state reaction (SSR) is often used to describe a chemical reaction
between solids. In this synthesis method, solvents are not used. Since, solids do not react
with each other at room temperature, solid-state synthesis needs much higher
temperatures and longer heating time than other techniques. The main advantages of
solid-state synthesis method are its simplicity in synthesis procedure, using low-cost and
widely available oxides as the starting materials, suitability for mass-production of cost-
efficient powders and environmental friendly technique, which means no toxic or
unwanted waste is produced after the synthesis procedure is completed [6,7].
Taking its advantage into consideration, we chose the SSR method for synthesis of
all our samples in powder form. Moreover, to achieve the desired objectives, we took
great care in the synthesis procedures: like using very high-purity starting
materials/chemicals, grinding the raw materials thoroughly for several hours, performing
three steps heating procedures to produce highly crystalline powder products, applying
slow cooling and heating rates during the calcination processes, and using platinum
crucibles which offer high resistance to chemical attack and inert to the reactants.
In the three step SSR methods, pre-calcination is done first involving heat treatment
at 500 oC for 5 hours to remove the CO2 gas, and then heated at 800 oC for 24 hours to
obtain the well crystallized non-fluorine substituted LiMn2O4, LiLaxMn2-xO4,
LiLaxCryMn2-x-yO4 and LiMn2-x-yLixCryO4 compounds. Since fluorine is influenced by
firing temperature, in the third step calcination is conducted at lower temperature of
500 oC for 5 hours to produce fluorine substituted compounds from the already
synthesized non-fluorine compounds and (NH4)HF2 as a source of fluorine. However, in
the single step procedure, the calcination process is done at 800 oC for 24 hours to obtain
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fluorine substituted LiMn2O4-xFx compounds. In this case, we used LiF as a source of
fluorine.
The various steps involved in SSR method for synthesis of cathode materials in the
dissertation are shown in the flow chart of Figures 2.1 and 2.2.
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Figure 2.1 Flow diagram of sample preparation by three steps conventional SSR route.
Stoichiometrically mixed raw materials to produce non-fluoride substituted materials
Wet grinding
Addition of 20 ml of methanol
Grinding thoroughly
Fine powder
1st heating at 500 oC for 5 hours
Addition of 20 ml of methanol
Grinding thoroughly
2nd heating at 800 oC for 24 hours
Grinding thoroughly
Mixing stoichiometric amount of fine powder and (NH4)HF2
Fine powder
Addition of 20 ml of methanol Grinding thoroughly
3rd heating at 500 oC for 5 hrs
Final products –fine powder
Grinding thoroughly
Grinding thoroughly
Grinding thoroughly
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Stoichiometrically mixed raw materials
Figure 2.2 Flow diagram of sample preparation by one-step conventional SSR route.
2.4.2 Pellet Preparation
For electrical properties study, the flow chart for the preparation of a pellet is shown
in Figure 2.3. The pellet preparation procedures for all samples are also the same. Pellet
of each sample is prepared from the calcined powder as an active material and polyvinyl
alcohol (PVA) as a binder. The proportion of binder to calcined powder is optimized for
better results. The calcined powder is initially ground in agate mortar for about
30 minutes. Further, the obtained powder is mixed with PVA and then ground for about
40 minutes. The binder added powder is then pressed at a pressure of 6 tons for
5 minutes in hydraulic press using a die set pressure technique to form circular disk
shaped pellets. Before pressing the powder in pellet form, uniform tapping of the die set
Addition of 20 ml of methanol
Wet grinding
Grinding thoroughly
Fine powder
Heating at 800 oC for 24 hrs
Final product-fine powder
Grinding thoroughly
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(filled with powders) is carefully adapted. The pellets are then sintered at 850 oC for
8 hours in air at heating and cooling rates of 5 oC/min. The surface layers of the sintered
pellets are carefully polished by fine emery paper to make their faces smooth and
parallel. The size of the pellets is around 10mm in diameter and 1.1 to 1.3 mm in
thickness. After polishing, the pellets are coated with silver paste on the opposite faces
which act as electrodes.
Figure 2.3 Flow chart illustrating the procedure for pellet preparation.
Grinding for about 30 minutes
Fine powder
Pressing into disk at 6 tones pressure for 5 minutes
10 mm circular disk
Strong circular pellet
Coating with silver paste Drying the paste
10 mm pellet
About 2 grams of calcined powder
Heating at 850 oC for 8 hrs
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2.4.3 Cathode Materials Fabrication
This section describes the fabrication process of cathode materials to study the
electrochemical properties. The fabrication procedure for all samples in this dissertation
is the same. Each electrode material is prepared from active materials in powder form,
conductive additive carbon black powder (to enhance the overall electronic conductivity
of the electrodes) and PVDF binder. The role of PVDF is for binding the powder
particles together, enabling the electrodes to adhere to the current collector foils and
preventing particle detachment during cycling.
As mentioned earlier, the development of high capacity cathode materials is one of
the primary objectives of this work. In this regard, the present study focused on methods
for producing high density and high capacity electrodes. Electrode density of the positive
cathode materials can be increased by using little amount of binder and carbon black
during cathode preparation, and by compressing the electrodes after drying [8]. The
compression process reduces the space between particles as well as the space between
carbon chains and allowing for more connectivity. Otherwise, electrical conductivity of
low carbon content electrodes is apparently quite poor if uncompressed due to the lack of
connectivity between the carbon chains. Based on this, 90 wt.% of active materials is
mixed with 7 wt.% carbon black and 3 wt.% PVDF binder for the fabrication of all the
spinel cathodes. The details of the fabrication process are shown in Figure 2.4.
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Figure 2.4 Flow chart illustrating the procedure for cathode fabrication.
Mixing active material and carbon black
Addition of some amount of methanol
Wet grinding
Fine powder
Mixing the fine powder with PVDF
Addition of some amount of NMP
Grinding thoroughly
Well mixed slurry
Paste on stainless steel/aluminum foil
Transferred to argon-filled glove box
Adhering the steel/foil by sheet of flat glass paper
Cleaning the steel/foil by acetone and deionized water
Paste the slurry again on stainless steel/aluminum foil
Drying in vacuum oven at 110 0C/hr for 2 hrs
Compress the cathode disk in a hydraulic press
Grinding the mixture for about 30 minutes
Wet grinding
Drying overnight in a vacuum oven before cell assembly
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2.4.4 Cell Assembly
The electrochemical cells used for this study are coin-type and Swagelok-type cells,
with a half-cell configuration. Throughout the thesis, all potentials are given with respect
to the metallic lithium electrode (Li/Li+). Polyethylene membrane is used as separator
and solution of 1.0M LiPF6 in a mixture of ethylene carbonate (EC) and di-methyl
carbonate (DMC) is employed as an electrolyte. The electrolyte solvents are mixed in the
proportion of 1:1 by weight (EC:DMC).
Figure 2.5 Schematic diagram of (a) Button-type coin cell, and (b) Swagelok-type cell Assembly.
The coin and Swagelok cells are then assembled inside a glove box under a high
purity argon atmosphere to avoid any traces of moisture or other contaminants get in
contact with cell parts. The assembling procedure is as follows: the fabricated cathode is
placed on the top of bottom cap with the active material facing up, and soaked in 3–4
drops of electrolyte. The separator is then placed on the top of the cathode and soaked
with 4–5 drops of electrolyte. Further lithium foil is put on the top of separator. Finally,
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the closing cap is placed at the top and pushed down slightly to close the system (Figure
2.5 (a) and (b)). At the end, the cells are left inside the glove box for some hours in order
to make sure that electrodes are completely soaked with electrolyte.
2.5 Materials Characterization
2.5.1 Physicochemical Characterizations
Different characterization methods are employed to analyze the physical and the
chemical performance of the cathode materials with the aid of various techniques:
TG/DTG, XRD, SEM, EDS, FT-IR Spectroscopy, Redox Titration, and Specific Surface
Area and Porosity measurement.
2.5.1.1 Thermal Analysis
Thermal analysis techniques are often used for characterization of materials.
Thermogravimetric and Differential Thermogravimetric (TG/DTG) are performed on
samples to determine changes in weight in relation to change in temperature. All the
measurements are carried out in oxygen atmosphere by heating the powdered samples
from room temperature to 850 oC at a heating and cooling rate of 10 oC/min. The
instrument used for this technique is Mettler Toledo TG/DTG 857e.
2.5.1.2 X-ray Powder Diffraction
X-ray powder diffraction (XRD) is one of the most powerful non-destructive
techniques which reveals information about the crystallographic properties of powder
samples, such as structure, crystallite size, lattice parameter, phase identification, purity
and so on [9-11]. It is also commonly used to identify unknown substances or
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compounds, by comparing diffraction data against a database maintained by Joint
Committee on Powder Diffraction Standards (JCPDS).
All samples prepared in this study are characterized by powder X-ray diffraction
using a Phillips XPERT-PRO diffractometer fitted with Cu Kα radiation (λ = 1.54060 Å)
at a setting of 40 mA and 45 kV between 2θ = 10o and 90o in step size of 0.017o with a
constant counting time of 24.765s. The unit cell lattice parameters, a, are obtained by the
least square fitting method from the d-spacing and the Miller indices, hkl values of (440)
diffraction peaks using Eq. 2.1 [11, 12].
2.1
Moreover, indexing is carried in comparison with Joint Committee on Powder Diffraction
Studies (JCPDS).
The crystal size, D, of the synthesized samples is determined from the XRD
patterns using the Scherrer’s equation 2.2 [11].
θβλ
cos9.0
=D
2.2
where λ is the wavelength of X-ray, θ is the Bragg angle, and β is the full width at half
maximum (FWHM) of the diffraction peak.
2.5.1.3 Scanning Electron Microscopy
Scanning electron microscope (SEM) is a commonly applied technique for the
analysis of the morphology of samples at very high magnifications with a characteristic
three-dimensional appearance.
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In the present study, SEM technique is employed to study the morphology and
micro structure of the synthesized powdered samples using JSM-6610 and Sirion
instruments. Before taking measurements, all the samples are coated with platinum in
Magnetron Sputter Coater.
2.5.1.4 Energy Dispersive X-Ray Spectroscopy
The energy dispersive X-ray spectroscopy (EDS or EDX) is an integral part of SEM
that qualitatively and quantitatively identifies the elemental composition of a particular
area of the analyzed sample in SEM [13]. However, EDS cannot detect the lightest
elements, Li, H, etc. The EDS spectrum plots the intensity of the x-rays for each of the
energies emitted. Analyzing this spectrum gives the atomic percentage of the different
elements present in a sample.
2.5.1.5 Fourier Transform Infrared (FT-IR) Spectroscopy
Fourier Transform Infrared (FT-IR) Spectroscopy is a simple and non-destructive
powerful tool for the identification of types of chemical bonds in the compounds by
producing infrared absorption spectra [14,15]. It is also used for identification of the
structure of samples as well as identification of unknown materials using the frequencies
of the vibrational modes.
FT-IR spectroscopy measurements are accomplished using transmittance method
with Potassium Bromide (KBr) as IR window in the wave0number region of
400 - 4,000 cm-1. A small amount of powder sample is mixed with KBr and ground in a
mortar with a pestle. The mixture is then pressed in a standard hydraulic press to form a
transparent pellet through which the beam of the spectrometer can pass. Before each
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measurement, the instruments (IRPrestige-21 and ALFA-T) are arranged to run with a
pellet of pure KBr only (no sample is added) kept in pellet holder to establish the
background, which is then automatically subtracted from the sample spectrum. This
technique helped to eliminate the instrument influence during measurements.
2.5.1.6 Redox Titration
The average oxidation state of Mn is determined by redox titration using sodium
oxalate (Na2C2O4) and potassium permanganate (KMnO4) [16,17]. The titration is
accomplished by the oxalic acid-permanganate back-titration method. About 50 mg of
each spinel sample is dissolved in 20 ml of an acidified 0.05N Na2C2O4 solution in 20ml
of 4N H2SO4. Further, the mixture is heated at 65 oC and stirred with magnetic stirrer for
about 30 minutes until all the powder has been dissolved completely in the solution in
order to reduce Mn(2+x)+ to Mn2+ as per the reaction in (2.3).
22)2(2
42 222 xCOMnMnOxC x +→+ +++− (2.3)
The unreacted C2O42- in the warm solution is then titrated against a 0.05N KMnO4
solution. The equivalence point at which the titrant and titrate are in stoichiometric
proportions is detected by the change of the solution from colorless to pink indicating that
the Mn(2+x)+ ions are no longer being reduced as per the reaction in (2.4).
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42 1022)(5 COMnMnOC +→+ ++− (2.4)
The average oxidation state of Mn is then determined based on the volume of KMnO4
consumed during back titration according to the reaction in (2.5):
(2.5)
where:
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• N1 is the normality of initially used C2O42- solution,
• N2 is the normality of KMnO4 solution,
• V1 is the volume of the initially added C2O42- solution,
• V2 is the volume of KMnO4 solution consumed by the unreacted Na2C2O4,
• Fw is the formula weight of the sample, and
• W is the sample weight.
2.5.1.7 Surface Area and Porosity Measurements
Surface area and porosity are important characteristics, capable of affecting the
physical and chemical properties like the electrical, thermal, mechanical, etc., of
materials. The most widely used technique for estimating the total exposed surface area
present in powder samples is Brunauer, Emmett, and Teller (BET) method. For surface
area measurements, different types of adsorbents such as water, nitrogen, oxygen and
toluene can be used. However, in this study, we used liquid nitrogen having cross-
sectional area 0.162 nm2 and boiling temperature of 77 K for adsorption technique [18].
This technique measures gas uptake (adsorption) for increasing partial pressure over a dry
powder sample and the release of gas (desorption) at decreasing partial pressures. The
resulting measurements produce adsorption isotherms which relate amount adsorbed to
the relative pressure.
The gas adsorption experiment using nitrogen as adsorbent is conducted on
Micromeritics ASAP 2020 surface area analyzer. About one gram of powder sample is
degassed at 350 oC for 6 hours under a vacuum of 10 mHg, which ensured removal of
any bound and capillary water present. After degassing the sample, it is exposed to
nitrogen gas at temperature of 77 K at a series of precisely controlled pressures. Nitrogen
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adsorption volumes are measured over the relative equilibrium adsorption pressure (P/Po)
range of 0.05 - 0.25. Further, the surface area of the samples is measured by applying the
BET analysis using multiple points of adsorption isotherm. The pore size distribution
(PSD) is obtained from Barrett-Joyner-Halenda (BJH) analysis.
2.5.2 Electrical Characterization
Complex Impedance spectroscopy (CIS), sometimes called AC impedance
spectroscopy, is a useful characterization technique for investigating the electrical
properties of materials. This technique is useful to investigate the electrical conduction
across intra-grain, grain boundary and electrode specimen interface. Moreover, the CIS
measurement technique is also useful to investigate the temperature and frequency
dependent behavior of ac conductivity and dielectric constant. The obtained results from
these analyses can provide information about the electrical behavior of the samples. The
electrical properties of some selected samples are performed by Phase Sensitive
Multimeter (Model: PSM 1700, UK) over the frequency range of 1 Hz to 1 MHz from
303.15 to 373.15 K. The dc conductivity of each is evaluated from the impedance
spectrum using the relation [19],
(2.6)
where L is the thickness of the pellet, is the bulk (grain) resistance and A is the area
of the pellet. To investigate the influence of frequency f on the conductivity as well the
dielectric constant, the frequency dependence ac conductivity and the real part of
dielectric constant of samples are calculated using the following relations [20,21],
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(2.7)
(2.8)
where is the permittivity of the free space, is dielectric loss tangent and C is the
capacitance. The activation energy (Ea) of samples is calculated from Arrhenius relation
[19],
(2.9)
Where is the pre-exponential factor, is the Boltzmann constant and T is the
absolute temperature. The activation energy is calculated by plotting versus103/T
and setting the slope equal to –Ea/Kb.
2.5.3 Electrochemical Characterizations
This section merely focuses on some specific aspects which are of particular
importance for the understanding of the electrochemical impedance spectroscopy, cyclic
voltammetry, and charge/discharge analysis.
2.5.3.1 Cyclic Voltammetry
Cyclic Voltammetry (CV) is the most widely used standard technique for studying
the qualitative and quantitative behavior of electrochemical performance of cells upon
cycling [22]. Basically, CV can provide information on redox processes that occur
during charge/discharge of the electrochemical cells. In CV technique, the
electrochemical cell is cycled in a potential window, where the potential applied to the
working electrode is scanned at a constant rate. The choice of this potential window must
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take into account the stability range of the chosen electrolyte in order to avoid its
decomposition. CV consists of a working electrode, counter electrode and reference
electrode setup [23]. The potential is measured between the working electrode and the
reference electrode which maintains a constant potential, and the current is measured
between the working electrode and the counter electrode. Then, the current flowing at
the working electrode is plotted as a function of the applied potential to give a cyclic
voltammogram.
In the present study, cyclic voltammograms are obtained by measuring the I-V
response at a scan rate of 10 mv.s-1 in the range of 3.0 to 4.5 V using a Biologic
potentiostat/galvanostat model VMP3 instrument. All parameters are carefully used to
obtain an optimized response of the materials under investigation. From the oxidation
and reduction peaks manifested on the CV plots, the cutoff voltages of capacity retention
study are determined.
2.5.3.2 Charge/Discharge Testing
Capacity retention experimentation is a useful technique to assess the
electrochemical performance of electrode materials. It measures the amount of charge
stored within an electrode under various experimental conditions over increasing cycle
numbers. The cyclability of the material is usually presented as the total charge or
discharge capacity, C, as a function of cycle number.
The Charge/discharge as well as capacity retention studies are performed in
CR2320 coin cells and Swagelok-type cell at room temperature. The studies are carried
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out using Biologic potentiostat/galvanostat model VMP3 and BT-2000 instruments in the
range of 3.0 to 4.5 V at 0.1C rate.
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