Surface Engineering Prussian White for High-Performance Sodium-Ion Battery Cathodes

Sodium (Na)-ion batteries (NIBs) are a promising energy storage technology, particularly for large grid-scale energy storage systems, because they use abundant and inexpensive raw materials and have similar manufacturing processes to lithium-ion batteries. Prussian White (PW), a material in the Prussian Blue analogue class, is a popular cathode material for NIBs; it has high energy density (comparable to the lithium iron phosphate cathode used in lithium-ion batteries) and tunable redox potentials. PW is made from inexpensive starting materials and is relatively easy to synthesize. Despite these benefits, PW materials are hygroscopic, and their ability to absorb and retain water in moist atmosphere makes them difficult to employ in non-aqueous batteries. Additionally, interactions with the electrolyte, including side reactions and metal dissolution, can cause PW cathode degradation during cycling. We have developed an ammonium treatment for PW that has a profound effect on the electrochemical behavior of PW cathodes including improved capacity retention during cycling, improved rate performance, and better resistance to water-induced cell degradation. We have characterized the impact of this ammonium treatment on PW crystalline structure, shelf-life, and electrochemical performance using X-ray diffraction, thermogravimetric analysis, X-ray photoelectron spectroscopy, inductively coupled plasma, Fourier-transform infrared spectroscopy, and extensive electrochemical testing to study how our synthesis and post-synthesis modifications impact the performance of PW cathodes. We show that our ammonium treated PW cathodes have improved stability against moisture, improved cycling stability, and improved rate performance.