3-1 Materials and Chemicals
The materials and chemicals used in this study are listed in Table 3-1. The chemicals were all of laboratory reagent grade and were used without any further purification. The de-ionized water used in every experiment was purified by a reverse osmosis equipment (Purelab Maxima/ELGA), in which the resistivity is 18.2 MΩ-cm.
Table 3-1 List of Materials and Chemicals
Chemical Formula Assay Supplier
Poly(dially dimethyl ammonium
Lithium hydroxide LiOH >98% Sigma-Aldrich
Graphite (KS6) C 99.9% Timcal
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ethyl carbonate/dimethyl carbonate1:2 in vol. with 4 vol%
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3-2 Preparation of Poly (lithium 4-styrenesulfonate)
The Poly (lithium 4-styrenesulfonate) (PSSLi) was produced by the neutralization of lithium hydroxide and Poly (4-styrenesulfonic acid) (PSSH). The schematic installation is shown in the Fig. 3-1. Firstly, 0.01g LiOH was dissolved in the 500ml distilled water as the titrant, and 10g 18wt % PSSH solution was used as the titrated solution. During the titration, the LiOH solution was slowly titrated into the titrated solution until the PH value reaches to 7.7 that was equal to the value of Di-water we measured. Secondly, the neutralized solution was dried on the hot plate in the water bath at 70 oC. Finally, sheets of PSSLi film were obtained and would be further use in the following process.
Figure 3-1 Schematic illustration of neutralization
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3-3 Polymeric Modification of Cathode Materials
3-3-1 Plasma Enhanced Chemical Vapor Deposition on Lithium-rich Nickel-Manganese Oxide Cathode Electrode
The plasma enhanced chemical vapor deposition (PECVD) was carried out in the low- temperature plasma system, Fig. 3-2 illustrates the schematic installation of the PECVD system. The argon gas was used as the plasma gas, and the cyclo-molecular octafluorocyclobutane (C4F8) was used as the precursor gas. During the chemical deposition, the plasma chamber was first exhausted with the vacuum pump until the pressure was lower than 0.15 torr. Following, the chamber was filled with plasma and precursor gas, the inlet flow rate was 50 and 25 sccm, respectively. Until the pressure in the plasma chamber reaches to 1.8 torr, the continuous radio frequency wave with the power of 15 watts was applied to excite the plasma gas to form the plasma. Upon the plasma start to interact with the precursor gas, it can force the carbon-carbon bond on the C4F8 to break down, forming the monomer of Teflon. And then, the monomer will further diffuse onto the inserted electrode in the chamber and polymerization will take place on the surface of the electrode. Finally, the Teflon film was obtained on the surface of the electrode.
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Figure 3-2 Schematic installation of the low- temperature plasma system.
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3-3-2 Single-polymeric Coating on Lithium Nickel-Cobalt-Manganese Dioxide Powder
Preparation of the single polymer-coated Li(NiCoMn)1/3O2particles was described as follows, the typical schematic coating process is presented in Fig. 3-3 and the flowchart is shown in Fig. 3-4. Firstly, 1g of poly(4-sodium styrene sulfonate) (PSSNa, Solid content = 30wt. %; Aldrich) was dissolved in 20 ml distilled water in advance and stirred at 600 rpm for half an hour with the agitator. Then, 15 g of Li(NiCoMn)1/3O2 particle was dispersed into the PSSNa-containing solution and mixed for one hour at 600 rpm. The resulting slurry was then dried in a rotary evaporator set at 70 oC, and the obtained powder was further dried in a vacuum oven at 70 oC for 12 hr. Finally, the dried powder was sieved through the number 270 meshed screen. The Li(NiCoMn)1/3O2 with 2 wt. % PSSNa-coated powder, hereafter was denoted as NCM-2%-PSSNa powder.
Preparation of the Li(NiCoMn)1/3O2 with 2 wt. % PSSLi-coated powder, hereafter denoted as NCM-2%-PSSLi powder was obtained from the same process with the NCM-2%-PSSNa powder, and the flowchart is also showed in Fig. 3-4.
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Figure 3-3 Schematic illustration of single-polymeric coating process
Figure 3-4 Flowchart of preparation of NCM-2%-PSSNa particles.
2 wt % of PSSNa (or PSSLi) was dissolved in 20 ml of DI
15g of Li(NiCoMn)1/3O2 was added in
Stirring at 600 rpm for 1 hr
Dried in a rotary evaporator set at 70 oC
Dried in the vacuum oven set at 70 oC for 12 hr
The dried powder was sieved through the 270 meshed screen Stirring at 600 rpm for 0.5 hr
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3-3-3 Multi-polymeric Coating on Lithium Nickel-Cobalt-Manganese Dioxide Powder
The multi-polymers, poly(dially dimethyl ammonium chloride) (PDDA, Solid content = 35wt. %; Aldrich) and poly(4-sodium styrene sulfonate) (PSSNa, Solid content = 30wt. %; Aldrich) were coated on the Li(NiCoMn)1/3O2particles, and the flowchart is shown in Fig. 3-5. This process was conducted in two steps as follows.
Firstly, 0.8 wt. % PDDA was dissolved in 20 ml distilled water and stirred at 600 rpm for half an hour in advance with the agitator. Then 15 g of Li(NiCoMn)1/3O2particles were dispersed into the PDDA-containing solution and mixed for one hour at 600 rpm.
After appropriately mixing at 600 rpm for one hour, the resulting slurry was then dried in a rotary evaporator set at 70 oC, and the PDDA-coated Li(NiCoMn)1/3O2particles were prepared to next coating step. Secondly, 1.2 wt. % PSSNa was dissolved in 20 ml distilled water and stirred at 600 rpm for half an hour in advance with the agitator. Then the pre-coating PDDA-coated Li(NiCoMn)1/3O2 particles were dispersed into the PSSNa-containing solution. After appropriately mixing at 300 rpm for half an hour, this resulting slurry was then dried in a rotary evaporator set at 70 oC, and the obtained powder was further heated in a vacuum oven at 70 oC for 12 hr. Finally, the dried powder was sieved through the number 270 meshed screen. The Li(NiCoMn)1/3O2 with 0.8 wt. % PDDA and 1.2 wt. % PSSNa-coated powder, hereafter it was abbreviated as NCM-2%-PDDA-PSSNa powder.
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Figure 3-5 Flowchart of preparation of NCM-(0.8% PDDA + 1.2% PSS) particles.
0.343 g PDDA (35wt %) was dissolved in 20 ml of DI water
15 g of Li(NiCoMn)1/3O2 was added in
Stirring at 600 rpm for 1 hr
Dried in a rotary evaporator set at 70 oC
0.6g PSSNa (30 wt %) was dissolved in 15 ml of DI water
15g of PDDA-coated Li(NiCoMn)1/3O2 was added in
Stirring at 300 rpm for 0.5 hr
Dried in a rotary evaporator set at 70 oC Stirring at 600 rpm for 0.5 hr
Dried in an oven set at 70 oC for 12 hr
The dried powder was sieved through the 270 meshed screen
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3-3-4 Preparation of 1%-Super P-PSSNa @ NCM-PDDA Composites
In this preparation, the 1% super P was coated on the Li(NiCoMn)1/3O2particles through the electrostatic self-assembly by ionic polymers. The flowchart is shown in Fig. 3-6. Firstly, 1 wt % PDDA was dissolved in 20 ml distilled water. Then 15 g of Li(NiCoMn)1/3O2 particles were dispersed into the PDDA-containing solution and mixed for one hour at 600 rpm to get a positively charge surface (NCM-PDDA(+)) and dried in a rotary evaporator set at 70 oC. Secondly, in the meantime, 1 wt % PSSNa was dissolved in 20 ml distilled water. 0.15 g of super P was dispersed into the PSSNa-containing solution and mixed for one hour at 600 rpm to get a negative charge surface (Super P-PSSNa(-)) and dried in a rotary evaporator set at 70 oC. Finally, the NCM-PDDA(+) particles were then mixed with the Super P-PSSNa(-) powders in 20 ml distilled water and mixed for half an hour at 300 rpm. This resulting slurry was then dried in a rotary evaporator set at 70 oC, and the obtained powder was further heated in a vacuum oven at 70 oC for 12 hr. The dried powder was sieved through the number 270 meshed screen.
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Figure 3-6 Flowchart of preparation of 1%-Super P-PSSNa@NCM-PDDA composites.
0.015g of Super P particles was added in
Stirring at 600 rpm for 1 hr
Dried in a rotary evaporator set at 70 oC
0.343 g PDDA (35 wt%) was dissolved in 15 ml of DI water
15g of Li(NiCoMn)1/3O2 was added in
Stirring at 600 rpm for 1 hr and dried in a rotary evaporator set at 70 oC Stirring at 600 rpm for 0.5 hr
Dried in a rotary evaporator set at 70 oC
Dried in the vacuum oven set at 70 oC for
0.6 g PSSNa (30 wt%) was dissolved in 20 ml of DI water
Pre-coated PSSNa-coated Super P was mixed with NCM-PDDA in 20 ml DI
Stirring at 300 rpm for 0.5 hr
The dried powder was sieved through the 270 meshed
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3-3-5 Preparation of Carbon Nanotube Wrapped and Polymer Coated Lithium Nickel-Cobalt-Manganese Dioxide Powder
The 2 wt % PSSNa (or PSSLi) was coated on the Li(NiCoMn)1/3O2particles through the typical single polymer coating process, as mentioned in section 3-3-2. The flowchart is shown in Fig. 3-7. 1wt % carbon nanotube (CNT) was added in the polymer-containing solution in the post half an hour of the one hour stirring step, and then kept stirring the slurry for left half an hour. This resulting slurry was then dried in a rotary evaporator set at 70 oC, and the dried powder was further heated in a vacuum oven at 70 oC for 12 hr. Finally, the obtained powder was sieved through the number 270 meshed screen. The CNT wrapped and polymer-coated Li(NiCoMn)1/3O2particles, hereafter they were abbreviated as 2%-PSSNa and NCM-1%-CNT-2%-PSSLi powder.
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Figure 3-7 Flowchart of preparation of NCM-1%-CNT-2%-PSSLi particles.
0.3 g PSSLi (or 1g PSSNa (30wt%))was dissolved in 20 ml of DI water
15g of Li(NiCoMn)1/3O2 was added in
Stirring at 600 rpm for 0.5 hr
Dried in a rotary evaporator set at 70 oC
The obtained powder were dried up in the vacuum oven for 12 hours at 70oC
The dried powder was sieved through the 270 meshed screen Stirring at 600 rpm for 0.5 hr
3g CNT (5wt% in water) was added in
Stirring at 600 rpm for 0.5 hr
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3-4 Material Characterizations and Analyses 3-4-1 Microscopy
Morphology observation of the material is essential and basic to provide some important and approximate messages in the initial stage before undergoing a further investigation, such as approximate particle size distribution, the shape of particles, the surface morphology, and porosity, etc. In this study, material morphology was examined by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and scanning transmission electron microscopy (STEM).
Surface morphology of the specimen was revealed by field-emission scanning electron microscope (JSM-7600F, JEOL) operated at 10 kV of accelerated voltage. In a combination of a detector for photons (X-MaxN, Oxford Instrument), the elemental composition can be qualitatively and quantitatively identified through energy dispersive X-ray spectroscopy (EDS). Before the examination, the samples were first pasted on a carbon-tape glued holder and then coated with platinum by ion sputtering to increase the conductivity.
TEM was performed on an electron microscope (JEM-1200EX II, JEOL) operated at 80 kV with 300 mm of camera length. All the samples were prepared by placing a drop of dilute solution, in w particles were ultrasonically dispersed in ethanol, on carbon-supported copper grids.
STEM was performed on an electron microscope (JEM-2100F, JEOL) operated at 200 kV with 80 mm of camera length.Like TEM, STEM requires very thin samples and looks primarily at beam electrons transmitted by the sample. One of its principal advantages over TEM is in enabling the use of other of signals that cannot be spatially correlated in TEM, including secondary electrons, scattered beam electrons, characteristic X-rays, and electron energy loss.
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3-4-2 X-ray Diffraction
X-ray Diffraction (XRD) technique was used to identify the phase of synthesized powders. Typically, information like phase purity, crystallinity, and size and shape of the unit cell can be obtained from an XRD pattern. The X-ray beam generated from the X-ray tube encounters the sample, and the diffracted photons obey Bragg’s law:
2dsinn ( 3-1)
Where n is the order of diffraction, a positive integer; λ is the wavelength of the incident beam; d is the distance between corresponding crystal lattice plane; θ is the incident angle between X-ray beam and atomic layers in the crystal, also called Bragg’s angle.
Fig. 3-8 demonstrates the relationship between the incident beam, diffracted beam, d-spacing and Bragg’s angle in Bragg’s law.
In this study, XRD measurements were performed on a Rigaku Ultima IV X-ray diffractometer with Cu Kα radiation ( = 1.5418 Å) used as the source of X-ray. The applying voltage and current are 40 kV and 40 mA, respectively. The scan-angle is typically in the range of 10o to 80o, with scan rate at 10o/min. Fig. 3-9 depicts the typical features of XRD experiment. The standard powder diffraction data of materials involved was cited from Powder Diffraction File (PDF-2, release 2004, published by International Centre of Diffraction Data (ICDD)).
3-4-3 X-ray Absorption Spectroscopy
The valence state of each element in samples was conducted in a X-ray photoelectron spectroscopy (VG Scientific/ESCALAB 250) equipped with a Al X-ray source (Kα, energy is 1486.6 eV) operated at 15 kV and 200 W and use a beam size of 400 μm and a pass energy of 20 eV for spectrum acquisition. The binding energy of the specified elements was measured. Due to the surface-sensitive property of
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photoelectron, the signal presented only reveals information of surface layer within about 10Å. The bulk property of samples could be obtained after removing the outer surface layer by ion etching.
3-4-4 Zeta Potential
The zeta potential of the particles was carried out on the Zeta Potential Analyzer (Malvern, Zetasizer Nano). The zeta potential is the electric potential in the interfacial double layer (DL) at the location of slipping plane, the electrostatic phenomenon in a solution for a charged particle was shown in Fig. 3-10. Zeta potential is not equal to the stern potential or surface potential in the double layer since these are defined at different locations, while it can be seen as the net potential of this electronic double layer. In this study, the zeta potential is an indicator to illustrate the sign of surface charge which is carried on the surface of the particles.
3-4-5 Fourier Transform Infrared Spectroscopy
Fourier transform infrared spectroscopy (FTIR) was conducted on an MFT-2000, Jasco Corporation. The analysis was carried out under reflection mode with a spectrum resolution of 1 cm-1 and 16 scans for every spectrum. An infrared (IR) spectrum represents a fingerprint of a sample with absorption peaks which correspond to the frequencies of vibrations between the bonds of the atoms making up the material.
Because each different material is a unique combination of atoms, no two compounds produce the exact same infrared spectrum. Therefore, infrared spectroscopy can result in a positive identification (qualitative analysis) of every different kind of material. In addition, the size of the peaks in the spectrum is a direct indication of the amount of material present. With modern software algorithms, infrared is an excellent tool for quantitative analysis[117].
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3-4-6 Inductively Coupled Plasma Analysis
The inductively coupled plasma-mass spectrometer (ICP-MS, Agilent 7500ce ) is an analytical technique used for the detection of trace metals in the specimen. It is a type of emission spectroscopy that uses the inductively coupled plasma to produce excited atoms and ions that emit electromagnetic radiation with wavelengths characteristic of specific elements in homogeneous solution system. Therefore, in this study, for examination of the dissolved transition metals, the cycled counter electrode, lithium metals, were picked out from the half-cell, and the residual organic solvent on them was removed in the desiccator by applying the vacuum pump overnight, the remained metal were dissolved into the 10 ml Di-water, and the trace amount of dissolution metals from the working electrode was measured by the ICP-MS technique.
The intensity of this emission is indicative of the concentration of transition metals dissolved from the active material.
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Figure 3-8 Schematic representative for the Bragg’s law
Figure 3-9 Basic features of XRD experiment
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Figure 3-10 Distribution of charges in a colloidal suspension
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3-5 Electrochemical Characterization 3-5-1 Preparation of Electrodes
In this thesis, the electrochemical performance was investigated by using CR2032 coin-type cells. To assemble a cell for the electrochemical analysis, the electrode must be prepared first. The electrode was made up of the aluminum foil and a layer of dried slurry. The slurry was composed of 80 wt. % active material, 8 wt. % graphite flake (KS-6), 4 wt. % carbon black (Super P) and 8 wt. % binder (polyvinylidene difluoride ,PVdF), listed in Table 3-2. In the beginning, the 6 wt. % PVDF was dissolved in the solvent, N-methyl pyrrolidone (NMP). Then the KS6 and Super P were poured in the binder solution. An agitator applying a propeller with a diameter of around 15 mm is used to disperse the solid content uniformly in the solution, and the slurry is vigorously stirred for 30 minutes at 4000 rpm. Next, the active material was added into the slurry with a further stirring for 60 minutes at 4000 rpm. Afterward, the preparation of the casting slurry had been finished. The well-dispersed slurry was uniformly spread onto the aluminum foil by the coating machine (All Real Technology Co.) and pre-dried at 140 °C in an ambient with around 50 μm in thickness. The electrode was then pressed by calendared press machine and then cut into discs with 12 mm in diameter. Further drying was conducted at 150°C in a vacuum oven for 8 hours to remove the residual solvent.
3-5-2 Cell-Fabricating Process
The resulting electrodes were assembled into coin cells (CR2032), which consists of the cathode, a stainless bottom cap, an upper cap with polypropylene ring to make sure well sealing, separator (Celgard 2400), a piece of stainless steel on which lithium foil is put and serves as a current collector of counter electrode, and a metallic leaf
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spring, as shown Fig. 3-11. The electrolyte was composed of 1.2 M LiPF6 in ethylene carbonate (EC) / dimethyl carbonate (DMC) (1 / 2 vol.%) with 4 wt. % fluoroethylene carbonate (FEC) was added as an additive for its positive effects on formation of solid-electrolyte interface (SEI). The cells were assembled in a glove box filled with argon gas, of which humidity and oxygen levels are lower then 0.5 and 10 ppm, respectively.
Table 3-2 Recipe of electrode slurry
Materials Composition (%, total solid=100%)
Active Material 80
KS6 8
Super P 4
PVDF 8
NMP (solvent) 200
Figure 3-11 Illustration of parts for the coin cells CR2032.
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3-5-3 Charge/Discharge Test
The charge-discharge test was employed to assess the electrochemical performances and characterize the charge-discharge features of the material. The common unit for this test is specific capacity, mAh/g, illustrating how much capacity it could deliver with one gram of active material. In this thesis, for the spherical Li1.1(Mn0.6Ni0.4)0.9O2 basedcells (LrMNO cells) , the cells were formation between 2.0 and 4.9 V for the first cycle under the current density of 10 mA/g and, the following cycles were under the current density of 20 mA/g between 2.0 and 4.6 V. The cyclic test was performed at 55 oC under current density of 0.3 C (1C = 225 mAh/g). On the other hand, for the Li(NiCoMn)1/3O2 based cells (NCM cells), the cells were formation at room temperature with 0.1 C (1C = 160 mA/g) between the voltage window from 2.5 to 4.4 V in the first three cycles. The rate test and the cyclic test were also conducted in 0.5, 1, 2, 3, 5, 10 C (1C=1 mA/g) and 0.3 C, respectively. For the tests at elevated temperature, the cells were put in an oven of which the temperature was kept at 55±1
oC. For the elevated higher cut-off operating potential test, the cell test was conducted between 2.5－4.6 V.
3-5-4 Electrochemical Impedance Spectroscopy
The AC electrochemical impedance measurement was conducted on the charge cycle upon to 4.0 V. For LrMNO cells, the constant current was applied at 20mA/g; For the NCM cells, the constant current was applied at 16 mA/g. The amplitude was 10 mV, and the frequency ranged from 100 kHz to 10 mHz. All the tested electrodes have the same area of 1.13 cm2. This electrochemical impedance spectroscopy (EIS) measurement was conducted by using AUTOLAB, Eco Chenie PSGTAT30.
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