Electrical energy generation from clean and renewable energy sources is a topic of growing importance, considering the concerns over the environmental impacts and the resource constraints of fossil fuels, combined with the increasing worldwide energy demand. Development of low-cost energy storage systems is necessary to realize economical harvest of energy from intermittent renewable sources, such as the wind and solar energy. Lithium-ion battery, the state-of-the-art energy storage technology, provides high energy density and long cycle life, leading to its extensive use in portable electronic devices and its rapidly increasing application in electric vehicles. However, the large-scale application of electrical energy storage systems, to integrate renewable energy sources into the grid or to supply energy stored from local solar plants in remote areas, calls for important requirements: low-cost, material sustainability, and environmental safety. Lithium-ion batteries are presumed to fail these requirements as their cost is estimated to increase by the growth of electric vehicle market, due to the resource limitations of lithium.
Owing to the large abundance of sodium, sodium-ion battery technology is emerging as a promising alternative to the lithium-ion battery for large-scale applications, where sustainability and cost-effectiveness are more important criteria than gravimetric energy. Over the past few years, many efforts have been devoted to the development of sodium-ion batteries, including exploring new materials and novel chemistries and understanding the science underlying those systems. Layered oxides are the most studied and promising materials for the positive electrode in sodium-ion batteries; among them, P2-Na0.67[Mn0.5Fe0.5]O2 has attracted much attention from the research community. P2-Na0.67[Mn0.5Fe0.5]O2 is made from earth-abundant elements and delivers high specific energy, higher than 500 Wh.kg-1, which is comparable to LiFePO4 positive electrode material in lithium-ion batteries. Despite the advantages that P2-Na0.67[Mn0.5Fe0.5]O2 offers, instability in the ambient atmosphere and capacity fading are important challenges that hinder the commercial application of this material. Understanding of those aging mechanisms and implementing tailored cation substitutions to mitigate them have been the objective of this thesis.
The instability of layered sodium transition metal oxides in ambient atmosphere is known. The study presented in Chapter 3 shows the important impact of exposure of P2-Na0.67[Mn0.5Fe0.5]O2 to air on its electrochemical performance; this issue was underestimated in the previously reported studies and is probably not limited to this particular material. An air exposed P2-Na0.67[Mn0.5Fe0.5]O2 electrode demonstrates lower capacity and higher voltage polarization compared to an air-protected one. The nature of the reactivity of this material with air is investigated by a combination of thermogravimetric analysis, mass spectrometry, diffraction techniques, electron microscopy, and electrochemical measurements. A mechanism is proposed to describe this reactivity; carbonate anions are formed upon the exposure of the material to CO2, H2O, and O2 in air at room temperature and are inserted into the lattice, balanced by oxidation of Mn3+ ions to Mn4+. The Ni-substituted materials, P2-Na0.67[Mn0.5+yFe0.5-2yNiy]O2 (y = 0.1, 0.15) exhibit lower reactivity, as evidenced by the electrochemical performance.
The structural evolutions of P2-Na0.67[Mn0.5Fe0.5]O2 and P2-Na0.67[Mn0.65Fe0.20Ni0.15]O2 induced by electrochemical extraction and insertion of sodium ions upon charge and discharge are investigated by Operando X-ray diffraction measurements (Chapter 4). The materials undergo similar phase transitions: one at high voltage and one at low voltage. The phase emerging at high voltage is investigated by pair distribution function analysis and Mössbauer spectroscopy; the migration of transition metals out of MO2 layers into the interlayer space is proposed to occur at high voltage, induced by the stabilization of Fe4+ ions. The phase transitions are shown to have a detrimental impact on the electrochemical performance of the materials. Similar operando X-ray diffraction characterization, pair distribution function analysis, and electrochemical measurements are performed on P2-Na0.67[Mn0.66Fe0.20Cu0.14]O2 (Chapter 5). P2-Na0.67[Mn0.65Fe0.20Ni0.15]O2 outperforms the parent P2-Na0.67[Mn0.5Fe0.5]O2 and the Cu-substituted composition, owing to its increased structural stability upon cycling and higher specific capacity achieved by Ni2+/Ni4+ redox couple.
The redox processes involved in the cycling of P2-Na0.67[Mn0.66Fe0.20Cu0.14]O2 and Na0.67[Mn0.65Fe0.20Ni0.15]O2 are investigated by operando X-ray absorption spectroscopy. The evolution of the local structure of each transition metal upon charge and discharge is probed by the change in the X-ray absorption near-edge structure spectra collected at the transition metal K-edge. The data suggests the reversible contribution of oxide ions to the redox processes at high voltage
