Japanese researchers have achieved capacity retention of 91.2% after 300 cycles in a sodium-ion full cell by coating its layered oxide cathode with scandium.
The team from Tokyo University of Science and the Institute of Science Tokyo compared scandium surface coating with incorporating the element into the bulk structure of an O3-type sodium nickel manganese oxide cathode.
O3-Na[Ni₁/₂Mn₁/₂]O₂ offers relatively high reversible capacity but undergoes substantial volume changes during charging and discharging, causing its capacity to decline rapidly.
The researchers prepared samples containing 6%, 8% and 11% scandium in the cathode’s crystal structure. They also coated cathode particles using a scandium isopropoxide solution followed by annealing at 800°C.
Testing in coin-type sodium cells found that the unmodified cathode retained 18.6% of its capacity after 100 cycles. Capacity retention increased to 67.8% with 8% scandium doping and 75.4% with the scandium surface coating.
The researchers subsequently tested full sodium-ion cells combining the modified cathodes with hard-carbon anodes. After 300 cycles, the cell containing the scandium-doped cathode retained 71.4% of its initial capacity, while the coated cathode achieved 91.2% retention.
Associate professor Shinichi Kumakura of Tokyo University of Science said: “Sc ions can be incorporated either into the bulk structure or through an external coating. Both can improve the cycling performance, but the underlying mechanisms have not been elucidated. In this study, we explored how scandium improves battery performance of SIBs through both doping and coating, clarifying their distinct mechanisms.”
Operando and ex-situ X-ray diffraction, X-ray absorption spectroscopy and density functional theory calculations indicated that the two treatments worked in different ways.
Scandium incorporated into the cathode’s crystal lattice immobilised some sodium ions, which acted as structural pillars. This suppressed sodium-ion and vacancy ordering, reduced volume changes and helped stabilise the bulk material.
The surface treatment instead produced an O3-NaScO₂-like protective layer. This reduced reactions between the cathode and electrolyte, although it did not prevent a gradual loss of crystallinity within the cathode particles.
Professor Shinichi Komaba of Tokyo University of Science said: “Our findings show that a synergistic combination of bulk doping and surface coating is a promising strategy to improve performance of SIBs. This will help extend the lifespan of SIBs and consequently widen their practical application.”
The researchers acknowledged that scandium’s cost and limited availability would make its use in commercial sodium-ion batteries difficult. They intend to use the identified mechanisms to investigate more abundant and less expensive elements capable of providing comparable structural and surface protection.
The study was published in the journal Small.
Image caption: Scandium doping stabilizes the bulk structure, while a coating of the rare earth metal forms a protective layer, preventing side reactions.
Image credit: Professor Shinichi Komaba and Associate Professor Shinichi Komaba from Tokyo University of Science, Japan
