| The ever-growing demand of energy and pursuit of renewable energy sources, i.e., wind, solar, and geothermal energies require urgent development of energy storage devices. Li- ion batteries have been considered as one of the most promising candidates as they assume the store energy responsibilities to meet high energy and high power target. Comparing to conventional Ni-Cd and Ni-MH batteries, Li- ion batteries possess overwhelming advantages of high energy density, environmental friendliness, and long cycle life, etc. Most importantly, they weigh less and take less space but deliver more energy. These properties lead to practical applications of Li- ion batteries in various devices with both high power and high energy requirements, from hybrid electric vehicles to consumer electronics such as cell phones and laptops. However, to use Li- ion batteries for large-scale energy storage, they require further improvements on cost, durability, and power density. Among various factors limiting the performance of Li- ion batteries, the most crucial one relates to the low capacity of current cathode materials. To reach the highest possible cathode capacity, all the possible oxidation states of the active material should be utilized. To address this, our effort has been focused on developing cathode materials with high capacities, including Li2MnSiO4 and LiF/Fe. Unlike most of cathode materials in the commercial market and under scientific research, which can deliver only one electron per formula unit leading to the highest capacity of 167 mAh/g in LiFePO4, Li2MnSiO 4 contains two lithium ions per transitional metal, and the full reduction of transitional metal Mn(II) from Mn(IV) allows complete delithiation of Li 2MnSiO4, which potentially enables both lithium ions to be extracted and therefore de livers a high theoretical capacity of around 330 mAh/g. For LiF/Fe, with the reversible conversion reaction of Fe into FeF3, three-electron transfer reaction can be realized by utilizing three valent state of metallic Fe, and this leads to a high theoretical capacity of 732 mAh/g. However, both Li2MnSiO4 and LiF/Fe suffer from low intrinsic conductivities, resulting in slow kinetics, and only a small portion of theoretical capacities have been realized. Our research has devoted to preparing Li2MnSiO4/C and LiF/Fe/C nanofibers by electrospinning and heat treatment. These carbon nanofiber-based cathodes facilitate the easy access of Li ions between the active particles, which dramatically promote effective Li ion diffusions, thereby improving the electron/ionic transfer rates and overcome the low intrinsic conductivities of Li2MnSiO4 and LiF/Fe. For Li2MnSiO4, doping treatments including Fe and Cr doping have been conducted to improve other intrinsic properties of Li2MnSiO4, such as unstable crystal structures upon Li ion insertion/extraction. Results show that Li- ions batteries using Li 2MnSiO4/C and LiF/Fe/C nanofiber cathodes offer advantages of improved energy densities, long cycle life and high discharge capacities. With electrospinning and heat treatment, the process also provides advantages of low cost, easy fabrication, and low pollution. The technological impact of preparing carbon nanofibers with controlled structures and compositions is not only instructive to the fundamental research of overcoming the limits of cathode materials, but also opens up new opportunities to design new-generation Li- ion batteries with high performance and good safety, which provides the possibility of turning into renewable energies from fossil fuels and coals as a relief of energy crisis. |
