53

CHAPTER 2

EXPERIMENTAL TECHNIQUES

54

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

56

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

59

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.

60

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

61

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.

64

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,

66

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

67

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.

68

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

69

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).

2272

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

71

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],

72

(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

73

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

74

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.

75

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