Continuous efforts have been practiced on improving the energy density of conventional energy storage devices. Li-ion batteries are strongly considered for electric transportation and for facilitating the use of renewable energy, provided that improvements can be achieved in terms of cost, sustainability, and energy density. Therefore, high capacity materials, especially on cathode side, are in pressing needs. Among them, conversion-type metal oxyfluorides are gaining considerable attention, since they can deliver more than one electron per transition metal. However, synthesis of metal oxyfluorides normally requires high-temperature and high-pressure apparatus, hence limiting their practical application.

In this thesis, an in situ fluorination process on transition metal oxides is prop...[ Read more ]

Continuous efforts have been practiced on improving the energy density of conventional energy storage devices. Li-ion batteries are strongly considered for electric transportation and for facilitating the use of renewable energy, provided that improvements can be achieved in terms of cost, sustainability, and energy density. Therefore, high capacity materials, especially on cathode side, are in pressing needs. Among them, conversion-type metal oxyfluorides are gaining considerable attention, since they can deliver more than one electron per transition metal. However, synthesis of metal oxyfluorides normally requires high-temperature and high-pressure apparatus, hence limiting their practical application.

In this thesis, an in situ fluorination process on transition metal oxides is proposed to prepare Mn-based mixed-anion cathode materials for rechargeable Li/Na batteries. F^-containing compounds, such as AF, APF₆ (A = Li, Na), and fluorinated carbonate, are found to facilitate the in situ activation process by donating F^- to MnO upon oxidation. A reversible capacity of 220 mAh g^-1 can be obtained by activating MnO/C electrode in LiPF₆-based electrolyte at room temperature at C/50 rate. The activated electrode shows decent rate capability, delivering 207, 192, and 173 mAh g^-1 specific capacities at C/20, C/10, and C/5 rates, respectively. Advanced characterization techniques reveal that the reversibility is enabled by dynamic change of oxidation states and local environment of Mn. Overall, 0.8 ?^‒ transfer per Mn is estimated in Li-ion cells based on surface- and bulk-sensitive techniques. While the practical aspect of the fluorinated electrodes prepared in an in situ way remains to be proven, this electrochemical synthesis process opens an interesting direction to design novel Li(Na)-based electrode materials showing transformational advances.