Electrochemical and Thermodynamic Study of Electrode Materials on Li-ion Batteries and Aqueous Energy Storage and Conversion Applications
The energy storage and conversion is one of the key issues for human beings to live sustainably on earth since our living environment has been deteriorating with the development of industrialization. We can alleviate the waste of energy consumption and corresponding environmental pollutions by storing and converting energy efficiently. The electrochemical cells are drawing considerable attention recently as a promising solution. In this thesis, electrode materials for Li-ion batteries and aqueous electrochemical cells are studied, focusing on the electrochemical and thermodynamic aspects.
First, transition metal difluorides, MF2 (M = Fe,Ni, and Cu) are explored. It is found that the conversion-reaction voltage is associated with the size of the converted metal nanoparticles. The surface energy of metal nanoparticles reduces the reaction energy, which decreases the conversion-reaction voltage. In addition, CuF2 electrodes are rechargeable when it is coated with NiO. NiO alleviates Cu dissolution into an electrolyte and enhances the cyclability of CuF2.
Second, Zn/β-MnO2 alkaline battery is studied as a promising rechargeable energy storage of high capacity. The nano-sized β-MnO2 cathode in the alkaline electrolyte of LiOH and KOH exhibit the average discharge capacity of 280 mAh g-1 over the first 100 cycles. It is found that the β-MnO2 transforms through proton intercalation and conversion reactions. The capacity is improved further with an addition of 4% mole fraction Bi2O3 in the nanosized β-MnO2.
Third, density functional theory (DFT) calculations are conducted for Li4Ti5O12 (LTO), its Gadolinium (Gd)-doped, and lithiated phases. The density of states (DOS) of LTO exhibits the property of an electrical insulator, however Gd-doped LTO is an electrical conductor which enhances the electrochemical performance. In addition, the formation energy of lithiated LTO phases is calculated to understand the reaction mechanism of LTO upon lithiation. The calculated results show that the lithiation proceeds by the two-phase reaction and there is no intermediate phase between two end phases: Li4Ti5O12 and Li7Ti5O12.
Lastly, oxygen evolution reaction (OER) on YBaCo4O7 (110) is investigated by DFT calculations. The results indicate that OER can be easily activated by YBaCo4O7 (110) due to its low overpotential. The free energy diagram exhibits the oxidation from O* to OOH*, which is the rate-determining step.