Prussian blue analogue materials for energy storage applications

Sodium- and potassium-ion batteries are promising complementary technologies to lithium-ion, providing a low-cost, sustainable alternative that do not contain scarce or expensive elements. Prussian blue analogues (PBAs), an open-framework structure family of materials, are some of the best performing sodium- and potassium-ion cathode materials, exhibiting excellent rate capability and cyclability. The wide composition space and structural complexity of PBAs enables fine control of structure and electrochemical properties. In this thesis, PBA compositions suitable for use as anode materials and solid electrolytes in sodium-ion batteries are investigated.
Manganese hexacyanochromate is first examined as a model composition to study the electrochemical behaviour of transition-metal hexacyanochromates at low potential. Operating in a high-voltage aqueous sodium-ion electrolyte, manganese hexacyanochromate exhibits electrochemical activity at -0.86 V vs. SHE, one of the lowest reported reduction potentials. Ex situ analysis demonstrates that charge storage is from reversible sodium insertion and chromium redox. However, the specific capacity offered is too low for practical applications. To improve this the vacancy content of the material must be minimised and both transition metals must be electrochemically active. Low-vacancy chromium(III) hexacyanochromate(III) is identified as a target composition.
Previously, low-vacancy chromium(III) hexacyanochromate(III) has been of great scientific interest in the field of molecular magnetism due to its high predicted magnetic ordering temperature. However, preparing it has proved to be a challenging synthetic chemistry problem. In this thesis a novel synthesis method for preparing zero-vacancy chromium(III) hexacyanochromate(III) is developed and presented. It entails first synthesising rubidium chromium(II) hexacyanochromate(II) in a nonaqueous solvent, which is subsequently oxidised in water. The electrochemical and magnetic properties of zero-vacancy chromium(III) hexacyanochromate(III) are characterised.
The success of the chromium hexacyanochromate synthetic procedure was on account of understanding and controlling the thermodynamics of the precipitation reaction; a reaction in which a range of possible composition is available due to two competing charge neutrality mechanisms. A holistic model for determining the charge neutrality mechanism employed during PBA synthesis, based on basic thermodynamic and kinetic principles, is detailed. The model successfully rationalises experimental results and provides a basis to develop new synthetic procedures.
Finally, sodium zinc hexacyanoferrate is investigated as a potential solid electrolyte material. The impact of densifying pressure and level of hydration on the crystal structure and ionic conductivity is examined. Additionally, its electronic conductivity and electrochemical stability window, two critical properties, are studied.

Keywords: Manganese hexacyanochromate
Chromium hexacyanochromate
Potassium-ion battery
Zinc hexacyanoferrate
Prussian blue analogs
Sodium-ion battery
Advisor: Pasta, M