Abstract
The combustion of fossil fuel is a major source of air pollution problems. One of the possible solutions is to replace combustion vehicles gradually by electric vehicles. The electricity can be obtained from renewable energy sources such as solar panels and windmills. However, this electrical energy must be stored up for later use when required. Batteries are good and prospective as an energy storage system for powering the electric motor in electric vehicles. The lithium-ion (Li-ion) battery is more prospective than lead-acid, nickel-cadmium (Ni-Cd) and nickel-metal hydride (Ni-MH) batteries, has a relatively higher working voltage, less memory effect, a lower self-discharging rate, and a reasonable high energy density. In commercially available Li-ion battery, the specific capacity of the cathode is lower than the anode material. Hence, the battery performance can be improved by enhancing the specific capacity of the cathode.
Spinel lithium manganese oxide (LiMn2O4) cathode material is more environmentally friendly and less expensive than the typical layered lithium cobalt oxide (LiCoO2). LiMn2O4 also has higher voltage potential, higher electronic conductivity and ionic diffusivity, and good structure for Li+ ion diffusion with respect to olivine lithium iron phosphate (LiFePO4). However, the Jahn-Teller distortion, manganese dissolution, and surface-film formation are three determining phenomena causing the capacity fading of LiMn2O4. Possible methods to improve the battery performance are by reducing the capacity fading, improving the rate of diffusion, and improving the voltage by substitution. Compared with the modification by substitution and coating, modification of the particle size and morphology are less dependent on foreign materials. This study focuses on the effect of the particle size of LiMn2O4 particles as cathode materials in the submicron range to the electrochemical performance of Li-ion battery.
Two materials were synthesised in this study: manganese carbonate (MnCO3) as the precursor and lithium manganese oxide (LiMn2O4) as the cathode. MnCO3 was synthesised by chemical precipitation using low-cost manganese sulphate (MnSO4) and sodium bicarbonate (NaHCO3) salt. Two synthesising parameters were studied in controlling the particle size. The first one was the molar ratio between MnSO4 and NaHCO3. The particle size increased from approximately 0.43 μm to 0.87 μm in 50 mL hydrous ethanol when molar ratios were changed from 1: 10 to 1: 2. Another parameter was the amount of hydrous ethanol in the MnSO4 solution. The particle sizes were found in the range of approximately 0.43 μm to 5 μm when the hydrous ethanol added varied from 50 mL to 0 mL. This effect was not only observed by varying the amount of hydrous ethanol; other solvents such as acetone and PEG show similar trends. The possible reason for this size decrease is attributed to the effect of changes in the dielectric constant of the solution. A polynomial increase is observed between 1/r (r is the particle size of resultant MnCO3) and 1/ɛ (ɛ is the dielectric constant of the solution mixture).
LiMn2O4 was fabricated by calcination at 800°C with MnCO3 as a precursor and an extra 0.03 moles of lithium hydroxide (LiOH). MnCO3 was synthesised with a molar ratio between MnSO4 and NaHCO3 at 1: 10 with hydrous ethanol added ranging from 0 mL to 50 mL. Six LiMn2O4 samples with size ranging from 0.4 μm to 4.3 μm were studied. The LiMn2O4 structure found is the agglomeration of primary particles clustering into a secondary bigger particle. The secondary particle has a similar size to its precursor MnCO3.
The specific discharging capacities at a different current rate (C-rate) were found to be the highest when the secondary particle size of LiMn2O4 is 0.4±0.04 μm. The specific discharge capacities are 129 mAh/g at 0.1 C and 103 mAh/g at 5 C. The morphology of LiMn2O4 is aggregated primary particles forming a larger (secondary) particle. The rate discharge performance is similar to a solid particle or a hollow sphere according to the (secondary) particle size. For solid particles, the rate discharge capacity drops as the particle size increases due to the lengthening of the Li+ ion diffusion path. For hollow spheres, the rate discharge capacity is enhanced as the particle size increases because of the increase in the surface area for providing more sites for Li+ ion diffusion. The performance of LiMn2O4 having particle sizes ranging from 0.4±0.04 μm to 0.6±0.08 μm was like that of solid particles, while particle sizes ranging from 0.9±0.14 μm to 4.29±0.66 μm were like hollow spheres at a low C-rate. The lowest specific discharge capacities were from LiMn2O4 with the particle size at around 0.9 μm (100 mAh/g at 0.1 C and 23 mAh/g at 5 C).
The relationship between the lattice parameter and the electrochemical performance were studied. The lattice parameters of the resultant LiMn2O4 are in the range of 8.19 Å to 8.24 Å. For lattice parameters within the range of 8.21 Å to 8.24 Å, a better electrochemical performance was shown. The reduction of the lattice parameter (reference 8.238 Å) might be due to the substitution of the 16 d manganese site by Li+ ion. As the substitution leads to the increase of the concentration of Mn4+ ions, it alleviates the Jahn-Teller distortion. However, further reduction of the lattice parameter (lower than 8.20 Å) may increase the energy for Li+ ion hopping. Thus, the concentration of Mn3+ ion at the 4 V vs. Li/Li+ electrochemical reaction is reduced, leading to a bad performance.
| Date of Award | 9 Aug 2016 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Chi Yuen CHUNG (Supervisor) |
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