Structural and Electrochemical Characterization of Sodium-Ion Insertion Electrodes.

摘要

With the alarming rate of fossil fuel consumption in the world, electrochemical energy storage technologies that are low in cost and high in performance will need to be developed. Na-ion batteries are now being considered an ideal alternative to lithium-ion batteries given that their intercalation properties are similar, the cost of sodium is low, and there are infinite reserves of sodium. However, since sodium is heavier and less electropositive than lithium, there is a gravimetric energy density penalty. Nonetheless, the work presented in this dissertation offers the prospect of safer, greener, and low cost rechargeable batteries. The first system we studied was sodium titanate (Na2Ti 3O7), where we modified the nanomorphologies of this compound, first into nanosheets and nanotubes, then optimized its electrochemical properties in the form of nanoplatelets for use as a negative electrode. It is noteworthy that a combination of nanoplatelets and nanosheets offered the best combination of high energy and high power densities. The next class of materials we studied were phosphates, as these materials exhibit higher operating voltages due to the inductive effect of the phosphate anion. However, slow kinetics plague phosphate-based compounds so nanostructuring and conductive coatings are required. For the second system, NaTi2(PO4)3, size-reduction and the use of reduced graphene oxide was investigated to solve this problem of slow kinetics. Similarly, for the third system, Na2FePO 4F, we utilized a novel (polyol) synthesis to prepare nanoparticles and also used reduced graphene oxide to promote high rate-capability of this material. The last part of the dissertation involves a pseudocapacitive energy storage system. This energy storage mechanism leads to both high energy and high power densities by suitably modifying the physical properties of the material of interest. For this work, we supported pseudocapacitive charge-storage of MoS2 nanoparticles using in-situ X-ray diffraction.