Browsing by Author "Amaraweera, T. H. N. G."

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Development of expanded graphite from vein graphite via electrochemical exfoliation with sodium sulfate as an electrolyte
(Faculty of Graduate Studies, University of Kelaniya Sri Lanka, 2022) Wimalasoma, S. M. T. D.; Naranpanawa, H. M. H. D. K.; Amaraweera, T. H. N. G.; Wijayasinghe, H. W. M. A. C.
Vein graphite is a promising anode material for rechargeable lithium-ion batteries. Since lithium ions are scarce and expensive, research and development are focused on sodium and potassium rechargeable batteries. However, graphite structure should be modified through expanding the interlayer spacing to facilitate intercalation/de-intercalation of the bigger ions related to these future rechargeable batteries. Among the various methods, the electrochemical exfoliation process has been identified as a promising method to structural modification of the graphite to produce expanded and exfoliated graphite. Electrochemical exfoliation can be performed at room temperature within a shorter period with better efficiency. Hence, it is a more cost-effective and environment-friendly method that consumes less energy compared to mechanical and thermal exfoliation methods. However, detailed information on the investigations of electrochemical exfoliation of vein graphite are limited. Therefore, this study aims to investigate the possibility of producing expanded graphite from Sri Lankan vein graphite using electrochemical exfoliation. Electrochemical exfoliation of graphite rod (1 cm x 10 cm), cut from the vein graphite was carried out using 1 mol dm-3 Na2SO4 as an electrolyte and a Pt rod as a reference electrode under 10V DC voltage, for 30 minutes. The developed materials were characterized by X-ray diffractometer (Rigaku Ultima IV, Cu Kα radiation), Raman spectroscopy (Renishaw Invia, 514 nm laser), particle size analyzer (Horiba Nanopartica SZ-100), and Fourier-transform infrared spectroscopy (Thermoscientific Nicolet is 50, KBr pellet method). Crystallographic characterization using X-ray diffractometry revealed that the interlayer spacing of graphite had increased from 0.33859 nm to 0.33986 nm after the electrochemical exfoliation process. The ratio of the intensity of the D peak and G peak (ID/IG) of Raman spectroscopy was used to estimate the average defect density on the graphite surface after the electrochemical exfoliation. ID/IG of the edge plane of the graphite increased from 0.48 to 1.27 after the exfoliation. Similarly, the ID/IG of the basal plane of the graphite increased from 0.17 to 0.96. This reveals that the average defect density on the graphite edge and basal surface increased after the electrochemical exfoliation. Particle size analysis of expanded graphite was calculated by using the laser diffraction method. A median particle size of 1139.4 nm and polydispersity index value of 0.941 were reported for the exfoliated sample. Fourier-transform infrared spectroscopy analysis confirmed the oxidation of the graphite due to electrochemical exfoliation. Therefore, this study reveals the potential of producing expanded graphite by electrochemical exfoliation of vein graphite using Na2SO4 as an electrolyte. Further, material characterization and optimization of parameters such as electrolyte concentration and DC voltage, are currently undergoing to obtain expanded graphite for the investigations in intended rechargeable battery applications.
Investigating temperature dependence of lithium-ion diffusion through the silicon (111) surface
(Faculty of Graduate Studies, University of Kelaniya Sri Lanka, 2022) Ranaweea, R. M. L. H.; Samarakoon, Y. M. I. B.; Amaraweera, T. H. N. G.; Wijayasinghe, H. W. M. A. C.
Demand for high energy density rechargeable lithium-ion batteries has drastically increased in the last few decades. Graphite is the most common anode material used for rechargeable lithium-ion batteries. But it possesses less specific energy density hence is difficult to apply for high energy applications. Therefore, many studies to find out high specific capacity anode materials. One of the important high specific capacity, novel anode materials is silicon and its oxidative derivatives. But less diffusivity of lithium ions is one of the major drawbacks of these materials. Since it is very difficult to get the atomic picture of the anode during the lithiation process using analytical methods, computational methods have also been employed. Therefore, this research aims to carry out molecular dynamics simulations on silicon to study the migration of the lithium ions through silicon structure. Silicon model with 111 plane was modeled. X, Y and Z axis lengths of the lattice were around 30 Å, 26 Å and 50 Å respectively. Inside this lattice 896 of silicon atoms and 16 lithium atoms were placed. Modified embedded atom method (MEAM) potential is used to simulate the system by using Large-Scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) source code. After model validation, optimum voltage of diffusion for lithium ions, their mean square displacement (MSD) and diffusion coefficient (DC) are calculated. The optimum diffusion voltage for lithium ions is 2.1 V A-1. The DC of the lithium ions in silicon (111) surface at 300 K is 6.13 × 10-13 cm2 s-1 which is very close to the experimental values obtained in previous studies. Then the model was subjected to different temperatures (250 K to 450 K) while lithium ions were diffusing through 111 surfaces. DC of lithium ions was calculated at 10 K temperature gaps. Although there is an increment of the DC for lithium atoms with respect to temperature increments, it is not a stereotypical increment. Additionally, it has shown 93.85 % increase in DC for lithium ions when it goes from 250 K to 450 K. This shows a drop in friction acts on lithium atoms from the silicon environment as temperature rises.
Structural analysis of LiNi1/3Mn1/3Co1/3O2, Li0.96 Na0.04Ni1/3Mn1/3Co1/3O2 and Li0.96K0.04Ni1/3Mn1/3Co1/3O2 materials synthesized by Pechini method
(Faculty of Graduate Studies, University of Kelaniya Sri Lanka, 2022) Fernando, W. T. R. S.; Amaraweera, T. H. N. G.; Wijayasinghe, A.
Layered tri-transition metal oxides, specially LiNi1/3Co1/3Mn1/3O2 (NMC 333), have become a promising alternative to LiCoO2 electrode material in the rechargeable Lithium-Ion Battery (LIB). The electrochemical performances of NMC 333 mainly depend on its crystallographic structural properties including lattice parameters, the unit-cell, c/a ratio, volume, crystallite size (D), dislocation density(δ), and lattice strain. This study aims to synthesize LiNi1/3Mn1/3Co1/3O2, Li0.96Na0.04Ni1/3Mn1/3Co1/3O2, and Li0.96K0.04Ni1/3Mn1/3Co1/3O2 materials and study their structural properties. The Pechini method was used for powder synthesis in this study. The synthesized materials were characterized using X-ray diffraction (XRD). X-ray characterization confirmed the formation of only the single-phase layered hexagonal lattice (α-NaFeO2-type) structure without any impurity phase for all these prepared materials. Interestingly, while confirming the formation of layered structures, a better splitting of the (006)/(102) and (108)/(110) peaks appeared for Li0.96K0.04Ni1/3Mn1/3Co1/3O2 than that of LiNi1/3Mn1/3Co1/3O2 and Li0.96Na0.04Ni1/3Mn1/3Co1/3O2 in the diffractograms. The lattice parameters, i.e. a, c, c/a, the unit-cell volume, the crystallite size (D), and dislocation density(δ) are 2.8641(Å)̇, 14.2143(Å)̇, 4.9629, 100.979(Å3)̇, 77.45 nm,1.666×1014 m−2, for LiNi1/3Mn1/3Co1/3O2. While they are 2.8675(Å)̇, 14.2317(Å), 4.9630, 101.347(Å3)̇, 85.06 nm, 1.382×1014 m−2 for Li0.96Na0.04Ni1/3Mn1/3Co1/3O2 and 2.869 (Å)̇, 14.2421(Å)̇, 4.9641, 101.528(Å3)̇, 128.38 nm, 0.606×1014 m−2 for Li0.96K0.04Ni1/3Mn1/3Co1/3O2, respectively. It is also observed that the lattice parameters, the unit-cell volume, c/a, and the crystallite size are increased with the substitution of Li+ by Na+ and K+. It may be due to the radii of Na+ and K+ are bigger than that of Li+ and that will pave the way for increasing the interlayer space of the substituted materials with the substitution of bigger ions. The c/a ratio constitutes a direct indication of the cation mixing. Li0.96Na0.04Ni1/3Mn1/3Co1/3O2 and Li0.96K0.04Ni1/3Mn1/3Co1/3O2 exhibit higher c/a values than LiNi1/3Mn1/3Co1/3O2, supporting the observation that the substituting bigger ions such as Na+ and K+ into LiNi1/3Mn1/3Co1/3O2 suppresses the cation mixing and forms a well-defined layered structure. The micro-strain calculated for the LiNi1/3Mn1/3Co1/3O2, Li0.96Na0.04Ni1/3Mn1/3Co1/3O2, and Li0.96K0.04Ni1/3Mn1/3Co1/3O2 are 1.38×10−3, 2.17×10−3 and 1.46×10−3, respectively. This implies a slight difference in the crystallinity of the materials, as the micro-strain was slightly affected by substituting Na+ and K+. Crystallite size (D) was 77.45 nm, 85.06 nm, and 128.38 nm for LiNi1/3Mn1/3Co1/3O2, Li0.96Na0.04Ni1/3Mn1/3Co1/3O2 and Li0.96K0.04Ni1/3Mn1/3Co1/3O2, respectively. It exhibits an increment of crystallite size, indicating a lowering of the dislocation density with the substitution of bigger ions. Altogether, this study reveals that substituting Li+ with bigger ions of Na+ and K+ is improving the structural stability of NMC 333.