Advanced Materials Research Vol. 545

Abstract: Mesoporous SnP₂O₇ was synthesized via a surfactant templating method where an anionic surfactant, sodium dodecyl sulfate was used. X-ray diffraction (XRD) analysis indicates presence of mesostructure when the precursors were calcined at 200, 300 and 400 °C. Cyclic voltammetry tests carried out within 0-2.0 V (vs. Li/Li⁺) indicated that irreversible reduction of tin phosphate to form lithium phosphate phases and metallic tin occurred around 1.10 V and 0.69 V whereas the reversible alloying and de-alloying reaction involving lithium with tin occurred at 0.19 V and 0.52 V, respectively. Galvanostatic charge-discharge cycling tests carried out within 0-1.2V (vs. Li/Li⁺) showed that the mesoporous tin phosphate calcined at 400 °C exhibited a reversible discharge capacity of 738 mAh/g in the second cycle and upon reaching the tenth cycle, it retained a discharge capacity of 461 mAh/g. The relatively high capacity obtained for this anode was attributed to the mesoporous framework which provided larger surface area for reaction with lithium and minimized effect of volume changes experienced by the anode during repeated charging and discharging cycling.

Abstract: Mixed lithium nickel cobalt oxides are advantageous over LiCoO₂ due to decrease in cobalt content in the material. It is also better than LiNiO₂ which is known to be difficult to synthesize and has poor electrochemical properties. Partial substitution of nickel in lithium nickel oxide with cobalt significantly improves its electrochemical properties. In this work, a combustion method was used for synthesis of LiNi_1-xCo_xO₂ (x= 0.1 and 0.2). The starting materials used are nitrates of the metals or transition metals. The precursors obtained are used for thermal studies. The precursors of LiNi_1-xCo_xO₂ were annealed at a temperature of 700 °C for 24h. X-ray diffraction (XRD) showed that the materials are pure. A composite cathode comprising of the cathode active material, binder and graphite was fabricated. Charge-discharge profiles of the materials with a voltage range of 0.5 V - 4.5 V vs Li/Li+ were obtained in order to study their electrochemical characteristics. The materials exhibited several voltage plateaus attributed to different electrochemical reactions.

Abstract: Sodium based batteries presented a new and promising battery system. This work attempted to develop NaCoO₂ material using a new sol-gel method. The characterization of this sodium based material was done by using Thermogravimetric Analysis (TGA), X-Ray Diffraction (XRD) and Field Emission Scanning Electron Microscopy (FESEM). Cyclic voltammetry showed that the material exhibit reversible oxidation and reduction peaks which meant that a sodium based battery was possible for this material. Electrochemical charge-discharge profile was performed to study the voltage plateaus exhibited by the material. It showed discharge plateaus of between 3.2 to 2.3 V and another plateau at 0.6 V.

Abstract: Lithium aluminium titanium phosphate (LATP) with different stoichiometric ratios according to the formula Li_1+xAl_xTi_2-x(PO₄)₃ with x = 0, 0.2, 0.4, 0.5 and 0.8 are prepared by mechanical milling method. The structural and electrical properties of the prepared samples are investigated by X-Ray Diffraction (XRD), Differential Scanning Calorimetry (DSC) and A.C Impedance Spectroscopy (IS). The XRD results showed that the sample milled for 60 hours has very low crystallinity with conductivity value of ˜10^-7 S cm^-1. The conductivity is enhanced by one order of magnitude upon sintering of the samples at 900 °C for 6 hours. This enhancement may be attributed to formation of improved grain homogeneity and contacts.

Abstract: Lithium manganese vanadates are synthesized using the sol-gel method. The sol-gel method is a better synthesis method compared to that of the solid state reaction method and usually yields purer and more uniform sized particles. It also uses lower sintering temperatures and shorter time. This will be a savings in terms of costs of the material as heating requires a lot of electrical energy. Precursor materials acquired are subjected to thermal studies and a suitable sintering temperature is chosen in order to achieve pure, single phase compounds. As is well known it is not easy to get pure single phase final products for materials containing vanadium and manganese. From X-Ray diffraction (XRD), the results showed multi phase spinel lithium manganese vanadates. Battery fabrication is done using a composite cathode and a half cell is assembled in an argon filled glove box. Cell testing is done using a constant current charge-discharge procedure. The results show reasonable charge-discharge behaviour but the capacity is a little less than that for LiMn2O4.

Abstract: Lithium manganese oxide, LiMn2O4, is said to be the most promising cathode material to replace the present day ones used in commercial Li-ion batteries. However, it suffers from capacity fading. One of the ways to improve this material is by substituting some of the Mn atoms with some other metal or transition metal element to stabilize the material. Spinel type lithium manganese zinc oxide, LiMn(2-x)Zn\xO4 (x= 0.1, 0.2, 0.3, 0.4, 0.5) were prepared by sol-gel method. The compounds have a potential to improve pure LiMn2O4 performance. This method involves the mixing of acetates of metals and dissolved in absolute ethanol followed by the addition of tartaric acid. Tartaric acid acts as a gelling agent of the metal complexes and the compound was dried at low temperature to obtain the precursor of LiMn(2-x)ZnxO4. The synthesized product was annealed at a temperature of 850 oC for about 24 hour and X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) were done. Battery fabrications using the cathode materials were done using a non-aqueous electrolyte and Li metal as the anode. From charge-discharge studies, it was found that LiMn1.9Zn0.1O4 has better discharge capacity.

Abstract: LiCoO₂ is the commercial cathode material for Li-ion battery application with many advantages such as ease of preparation and good electrochemical properties. However, it has some limitations especially Co being expensive and toxic. Therefore, the substitution of Co in the LiCoO₂ by non-toxic and inexpensive transition metal elements will be an improvement. Partial substitution of Co by Mn has been done in this work via the self-propagating combustion (SPC) method. The materials are then characterized. The materials obtained were phase pure but the electrochemical discharge capacity is about 24 – 27 % less than that of LiCoO₂. However, the cycling behaviour of LiCo_0.9Mn_0.1O₂ and LiCo_0.8Mn_0.2O₂ over 15 cycles improved over that of LiCoO₂.

Abstract: The purpose of this research was to investigate and compare the electrochemical characterization of new cathode materials LiNi0.7Co0.2Fe0.1O2 and Li1.1Ni0.6Co0.2Fe0.1O2. The materials were prepared using a combustion synthesis method. Composite cathodes were fabricated and assembled in a coin-type cell. The discharge profile showed plateaus at 3.7 V and 1.6 V. The overlithiated sample exhibited a 10.4% increase of discharge capacity in the first cycle of and an increase of 43.7% in the 5th cycle compared to the non-overlithiated sample.

Abstract: Carbon additives are very important components of cathodes in Li-ion batteries. This is because carbon is an electronic conductor whereas cathode materials are ionic conductors. Without the presence of carbon, the electrons will not be able to flow and there will be space charge built-up in the materials. Carbon therefore facilitates the conductivity of charged species in the cathode materials and help to disperse the negative charge accumulation which may otherwise impede Li-ion diffusion within the cathodes. In this work, two types of carbon, namely, activated carbon (micron sized) and Denka Black (nano sized) were used in conjunction with the cathode materials LiCoO₂ and LiMn₂O₄. The amounts of cathode materials were kept constant while the amounts of carbon additives were varied. Galvanostatic charge-discharge was done over a voltage range of 4.2 V to 3.2 V. Results showed that Denka Black gives improved performance for both cathode material. This is believed to be due to the effect of nano sized particles of Denka Black.

Abstract: Anodic oxidation is an electrochemical method for the production of an oxide film on a metallic substrate. It involves the application of an electrical bias at relatively low currents while the substrate is immersed in an acid bath. The films can be very dense and stable, with a variety of microstructural characteristics. In the present work, films of the anatase polymorph of TiO₂ were formed on high-purity Ti foil (50 μm thickness) using phosphoric acid (0.3 M H₃PO₄). The conditions of oxidation involved the application of potentials (5 to 350 V) and current densities (5 to 60 mA.cm^-2) for 10 min at room temperature. The films were characterised using a digital photography, laser Raman microspectroscopy, and field emission scanning electron microscopy. The thicknesses of the oxide films on Ti were measured using a thin film analyser based on optical spectroscopy principles. The colours, thicknesses, and microstructures of the films depended strongly on the applied voltage and current density. At bias more than 15 V, single-phase anatase was observed to form on Ti at low (5 mA.cm^-2) and higher (up to 60 mA.cm^-2) current density.