2.3.1. X-Ray Diffraction (XRD)
X-ray diffraction (XRD) patterns for all the cathode materials were recorded by a Bruker AXS D8 ADVANCE X-ray Diffractometer with Ni-filtered Cu Kα (λ = 1.5406Å). The X-ray tube operating parameters were 40 kV and 40 mA. All materials were in powder form and the measurements were taken with 2 Theta (2θ) angle ranging from 10 to 70°, with a scanning rate of 0.02/s and a dwell time of 5.0/s. Rietveld refinement was achieved by using Topas 5 software, the atomic site occupancy, in particular the Ni and Li ion positions allowed for the refinement of the amount of the specific element to sit on the specific atomic coordinate. These were allowed to refine close to the Li1.2 Mn0.6Ni0.2O2 chemical formula [177]. Li2MnO3 [178] chemical
formula was also used to refine the XRD patterns of the produced materials, because Li1.2 Mn0.6Ni0.2O2 materials has Li2MnO3 composite in their structure. The
lattice parameter was allowed to be refined and subsequently fixed for further analysis. The composition is determined by the semi quantitative Rietveld refinement based on the crystal structure.
38 2.3.2. Scanning Electron Microscopy
Field emission scanning electron microscopy (FESEM) is a morphology technique used to survey mainly the size and the shape of particles. FESEM uses high energy beam of electrons to image the surface of the particles. High energy electrons interact with the surface of the sample and can be absorbed, reflected or excite the molecules in the sample and subsequently release a secondary electron. By detecting all these three electrons, information such as the surface structure, composition, size and shape of the particles of the molecules in the sample can be determined. SEM therefore has three major detectors namely: backscattered electron detector (reflected electrons), secondary electron detector (for secondary electrons) and x-ray detector (absorbed electrons). There are two types of emitters namely: thermionic emitter (scanning electron microscope) and field emitter (field emission scanning electron microscope. In this study a ZEISS ULTRA SS (Germany) field emission scanning electron microscope (FESEM) equipped with a secondary detector was used for the morphological analysis of the obtained powders. The carbon tape was placed on a sample holder and the cathode materials powders were placed on the carbon tape and coated.
2.3.3. Electron Dispersion X-Ray Spectroscopy (EDS)
Energy dispersive X-ray spectroscopy (EDS) technique offers qualitative analysis of the elemental composition of the materials. The technique is attached to the SEM, uses the image generated from SEM to allow for elemental information of the specimen under investigation to be generated. However the technique only has sensitivity for elements heavier than carbon. EDS spectra was done on three different areas in the sample to obtain mapping and spectra. In this study a ZEISS ULTRA SS (Germany) field emission scanning electron microscope (FESEM) equipped with energy dispersive x-ray detector was used for the qualitative elemental analysis of the obtained powders.
2.3.4. Transmission Electron Microscopy (TEM)
Transmission electron microscopy (TEM) similar to SEM is a morphology technique used to quantify sample structure, composition, size and the shape. TEM uses high energy beam and high speed electrons to image the internal structure of particles. Unlike SEM, electrons used in TEM pass/ transmit through the sample, has high
39 resolution and therefore can achieve atomic visualization. The sample is suspended on a copper grid as an ultrathin section [179]. In this study to study the structure cathode materials particles bright field TEM (JEOL JEM-2010F) was used.
2.3.5. Brunauer–Emmett–Teller (BET) Surface Area
The name BET was derived from the names of the three developers of the surface area method, namely; S. Brunauer, P. H. Emmet and E. Teller. BET is an analytical technique that gives information on the surface area of a sample. BET works on the physical adsorption of gas molecules on a surface of the sample. The amount of gas adsorbed by the sample at a given pressure is used to determine the surface area. The instrument uses inactive/inert gases that do not react when in contact with the sample to measure specific surface area. Generally, gases such as argon, krypton, carbon dioxide or Nitrogen are used as gaseous adsorbate. Nitrogen (N2) is a
common gaseous adsorbate for BET techniques and due to that the analysis is normally conducted at low temperatures -196.15 °C [180]. BET analysis involves sample heating and degassing under vacuum to remove foreign adsorbed molecules. Inert gas is then purged into the sample which is adsorbed, withdrawn and desorbed. To generate adsorption isotherms, the sample is then put under vacuum at low temperatures and subjected to various pressures. Gas molecules are then adsorbed or desorbed based on pressure variations. The area occupied by the adsorbate is then used to calculate the surface area of the sample. Surface area studies of the produced materials were determined using nitrogen physisorption incorporating the Brunauer-Emmett-Teller (BET) method. The measurements were recorded using a micrometric ASAT 2020 instrument physisorption analyser. The samples were pre-treated where 0.5 g of material was dried at 150 ˚C under vacuum for 3 hours prior to BET analysis. Nitrogen gas was used as the probe gas for the analysis.
2.3.6. Inductive coupled plasma mass spectroscopy (ICP-MS)
Elemental analysis of the xLi1.2MnO3 •(1-x) LiMn0.5Ni0.5O2 cathode materials
produced in this work were obtained using ICP-MS analytical tool. The technique is used to detect elemental concentrations in the samples and the concentration is measured in parts per billion (ppb). The tool is spectroscopic and employs inductively coupled plasma to excite ions and atoms of the sample. This excitement
40 phenomenon emits radiation at wavelengths characteristic to the respective element [181]. Thermo scientific iCAPQ inductively coupled plasma mass spectrometry (ICP- MS) was used to analyse the Mn and Ni atomic percentage. The powders were digested in aqua-regia and diluted in distilled water and further diluted 1000X before analysis. The ICP multi element standard solution 1 was used for calibration purposes, the standard was prepared in 5 folds namely: 20, 40, 60, 80 and 100 part per billion (ppb) in 3% nitric acid.
2.3.7. Thermogravimetric analysis (TGA)
Thermogravimetric analysis (TGA) is an analytical technique used to measure materials properties such as purity, decompositions reactions, decomposition temperatures, composition of the material and adsorbed moisture. This technique measures the material’s change in mass as a function of temperature. The TGA is equipped with a furnace to heat the sample and a sensitive weighing balance and measures the mass loss of a sample as it heated. In order to do a full thermal analysis of the cathode materials, Li1.2Mn0.6Ni0.2O2 were synthesized as described
previously in section 2.2.1 and dried at 100˚C in a vacuum oven for 4 hours to prepare for TGA analysis prior to calcination. Measurements were performed in air from room temperature to 1000˚C at a 10˚C/min rate and results were represented as % mass loss vs increasing temperature. The thermal decomposition of the cathode materials synthesized in this study were tested using Metter Toledo TGA/SDTA851 TGA instrument.
2.3.8. X-ray photo-electron spectroscopy (XPS)
X-ray photoelectron spectroscopy (XPS) is a non-destructive, surface-sensitive and quantitative technique used. XPS is used to analyze the structure of the atoms and get electronic and ionization information. This technique is based on the photoelectric effect which was described and developed by Albert Einstein in 1965. The X-ray beam is sent to irradiate the sample and the number of electrons ejected together with the kinetic energy is quantified. The technique is a surface-sensitive tool and measures elemental concentrations in parts per thousands (ppt), element- ionic state and empirical formulae of the elements forming the sample [182]. The nature of chemical bonds which exists between elements can also be quantified. The technique can identify all elements with atomic number above 3. XPS was used to analyze the elemental composition of the as-prepared cathode materials as well as
41 determine the oxidation states of Mn and Ni as well as their population in the structure. The photoelectron spectra was collected with a Kratos Axis Supra DLD spectrometer using Al Kα radiation (1486.6 eV). Data analysis was performed using the XPS Peak 4.1 program and Shirley function was used to subtract the background.