In the race to secure affordable, sustainable alternatives to lithium-ion batteries, sodium-ion technology has increasingly emerged as a viable contender.
But material instability—particularly in sodium manganese oxide cathodes—has hampered its progress. Now, a team of Japanese researchers has identified a key material-level solution that could alter the trajectory of sodium-ion development: copper doping to eliminate stacking faults in β-NaMnO₂.
β-phase NaMnO₂ has long been recognized as a promising cathode material due to its high theoretical capacity and favorable voltage profile. However, the structure is prone to stacking faults (SFs)—crystallographic disruptions that mimic the α-phase and deteriorate electrode performance over repeated cycling. These faults not only reduce capacity retention but also obscure the understanding of the β-phase’s complex structural dynamics.
The central challenge lies in controlling these SFs during synthesis. β-NaMnO₂ is typically produced at high temperatures, which can result in sodium-deficient phases and introduce non-equilibrium defects. Until now, methods to mitigate SFs remained largely ineffective or resulted in trade-offs in electrochemical performance.
Copper Doping as Structural Stabilizer
A new study led by Professor Shinichi Komaba at the Tokyo University of Science presents a systematic approach to address these structural instabilities. The researchers doped β-NaMnO₂ with varying levels of copper, producing a series of NaMn₁₋ₓCuₓO₂ samples. The objective was to evaluate how Cu incorporation affects SF formation and cycling performance in sodium-ion half cells.
X-ray diffraction analysis revealed a clear trend: higher Cu content correlated with fewer stacking faults. Notably, the NMCO-12 sample, with 12% Cu doping, exhibited just 0.3% SF concentration—a dramatic reduction compared to the 4.4% seen in lightly doped NMCO-05. Electrochemical tests showed that while undoped samples degraded within 30 cycles, the NMCO-12 maintained its capacity over 150 cycles without noticeable loss.
This near-elimination of SFs offered a rare opportunity to study the intrinsic behavior of β-NaMnO₂ during Na insertion and extraction. In situ and ex situ XRD measurements, supported by density functional theory (DFT) calculations, revealed a distinct mechanism: drastic gliding of MnO₂ layers unique to the β-phase. These shifts were previously masked by structural disorder, making this finding not only a performance milestone but also a breakthrough in materials science.
From a cost perspective, manganese and sodium are far more abundant and geographically secure than lithium and cobalt. The use of Cu as a stabilizing dopant—an element already widely available—further enhances the scalability of this solution. Applications could extend from consumer electronics to electric vehicles and grid-scale storage, where cost and longevity are paramount.
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