Doctor of Philosophy
Institute for Superconducting and Electronic Materials
Driven by demand for electric vehicles and grid-scale energy storage, a boom in lithium-ion batteries is currently underway to provide energy storage at great scale. The resulting sustained increased demand for lithium-ion battery raw materials may eventually make alternative battery chemistries attractive. Sodium-ion batteries currently represent the most mature such technology, although they remain far from market-ready. Much fundamental research and materials development remains to be done to improve their performance and understand their limitations. This thesis focuses on the development of cathode and electrolyte materials, with an overall goal of gaining a better overview of the materials available for sodium-ion batteries and a deeper understanding of their electrochemical behavior. To start, a series of Prussian Blue analogues (metal ferrocyanides) was studied, primarily focusing on sodium manganese ferrocyanide (MnPB) and sodium nickel ferrocyanide (NiPB). These materials complement each other: MnPB has a high capacity but poor electrochemical stability, while the opposite is true for NiPB. A range of MnPB-NiPB composites with both homogeneous and heterogeneous morphologies were prepared and characterized. Finally, the suitability of a range of polyurethane-based polymers for sodium-conducting gels was studied. The best-performing Prussian Blue material from the first part of the thesis was used as a prototypical cathode material to assess the polyurethane’s aptitude as a gel-polymer electrolyte for sodium-metal batteries.
The process to synthesize epitaxial Prussian Blue heterostructures exploited the unequal binding strength of chelation agents towards nickel and manganese ions, which prevents Ni2+ from reacting until the Mn2+ is consumed. Chelating agents are widely used to control the nucleation rate of Prussian Blue, but they have not to date been used to engineer targeted inhomogeneities at the particle level. This represents an attractive one-step synthesis of core-shell materials, which usually require multiple steps. A range of synthesis conditions were tested to optimize the products. The best-performing material has an electrochemical capacity of 93 mA h g-1, of which it retains 96% after 500 charge-discharge cycles – a considerable improvement on MnPB (37% after 500 cycles). The material’s rate capability is also notable: at 4 A g-1 it can reversibly store 70 mA h g-1, which is also reflected in its diffusion coefficient of ~10-8 cm2 s-1. The epitaxial outer NiPB layer probably stabilizes the structurally debilitating Jahn-Teller distortions MnPB undergoes as Mn2+ is oxidized to Mn3+. It is theorized that it exerts an anisotropic strain on the MnPB lattice, altering the orbital dynamics of its MnN6 octahedra and preventing (or pre-empting) the Jahn-Teller distortions. To support this mechanism, peak shifts in the x-ray diffraction pattern are analysed, confirming the presence of a highly spatially asymmetric macrostrain.
Gebert, Florian, Research on Prussian Blue cathode materials and polyurethane gel polymer electrolytes for sodium ion batteries, Doctor of Philosophy thesis, Institute for Superconducting and Electronic Materials, University of Wollongong, 2021. https://ro.uow.edu.au/theses1/1244
FoR codes (2020)
3402 Inorganic chemistry, 3403 Macromolecular and materials chemistry, 3406 Physical chemistry
Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.