Researchers from Tokyo University of Science and the Institute of Science Tokyo in Japan have investigated how scandium (Sc) doping and coating can improve the durability and performance of sodium-ion batteries (SIBs) within the cathode element of the battery structure.
More specifically, they have introduced the scandium ion Sc3+ into an O3-Na[Ni1/2Mn1/2]O2 either by bulk doping or by surface coating. O3-Na[Ni1/2Mn1/2]O2 is attractive for SIBs because it offers relatively high capacity. However, it suffers from severe capacity loss during cycling, which the researchers sought to reduce by introducing Sc³.
“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,” explained Shinichi Kumakura, who led the research group. “In this study, we explored how scandium improves battery performance of SIBs through both doping and coating, clarifying their distinct mechanisms.”
The researchers synthesized Sc-doped O3-Na[Ni1/2Mn1/2]O2 (abbreviated as NNMO) samples with Sc contents of 0%, 6%, 8%, and 11% using a bulk-doping method, in which Sc³⁺ substitutes for Ni and Mn within the crystal structure. They also prepared Sc-coated samples by applying a scandium-containing precursor to the NNMO particles and then heat-treating them, creating a Sc-rich layer mainly at the particle surface, which they called NNMO-SC800. They then evaluated the electrochemical properties of the synthesized samples using coin-type aprotic Na cells.
The results showed that both doping and coating significantly improved the charging/discharging durability of the cells, improving capacity retention after 100 cycles from 18.6% for undoped NNMO to 67.8% for NNMSO at 8% Sc and 75.4% for NNMO-SC800. The researchers further evaluated Na-ion full cells using NNMSO at 8% Sc or NNMO-SC800 as cathodes and hard carbon as the anode – and found that after 300 cycles, the full cell employing NNMSO8 retained 71.4% of its initial capacity, while the cell using NNMO-SC800 retained 91.2%.
Then, to understand the origin of these improvements, the team performed detailed structural and mechanistic analyses using operando and ex situ X-ray diffraction, X-ray absorption spectroscopy, and density functional theory (DFT) calculations. These tests revealed that doping and coating improved cycling performance through distinct mechanisms: bulk doping stabilized the layered crystal structure and reduced volume changes during cycling, while surface coating formed a protective layer that suppressed interfacial degradation and side reactions.
The technique had appeared in “Scandium Doping and Coating for Improving O3-NaNi1/2Mn1/2O2 Electrode in Sodium Battery,” published in the journal Small.
“Our findings show that a synergistic combination of bulk doping and surface coating is a promising strategy to improve the performance of SIBs,” concluded Komaba. “This will help extend the lifespan of SIBs and consequently widen their practical application.”
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