Sodium (Na)-ion batteries have recently emerged as cost-effective and sustainable alternatives to lithium (Li)-ion batteries. Na, the sixth most abundant element on Earth, offers lower material costs and greater availability compared to Li-ion batteries. The design of cathode materials plays a key role in determining battery life and stability. Layered sodium manganese oxide (NaMnO2) has received increased attention from researchers for its use as a cathode material in Na-ion batteries.
NaMnO2 exists in two crystal forms: α-NaMnO2 and β-NaMnO2. The α-phase features a monoclinic layered structure, where planar MnO2 layers, consisting of edge-sharing distorted MnO6 octahedra, are stacked alternatively with Na-ions in between. β-NaMnO2, on the other hand, features corrugated or zig-zag layers of edge-sharing distorted MnO6 octahedra, also with Na-ions in between. Synthesis of β-NaMnO2 typically requires higher temperatures, often leading to Na-deficient phases.
Attempts to prevent Na-deficient phases produce non-equilibrium β-phases that exhibit several defects. The most notable among these are the stacking faults (SFs), formed by slipping of the crystallographic b-c plane, generating stacking sequences resembling the α-phase. Electrodes made from SF-containing β-NaMnO2 suffer from severe capacity reduction during charge/discharge cycles, limiting their practical applications. Moreover, SFs complicate the understanding of the material's solid-state chemistry.
In a new study, a research team led by Professor Shinichi Komaba from the Department of Applied Chemistry at Tokyo University of Science (TUS), Japan, investigated how copper (Cu) doping can stabilize SFs in β-NaMnO2. "In a previous study, we found that among the metal dopants, Cu is the only dopant that can successfully stabilize β-NaMnO2," explains Prof. Komaba. "In this study, we systematically explored how Cu doping can suppress SF and improve the electrochemical performance of β-NaMnO2 electrodes in Na-ion batteries." The team also included Mr. Syuhei Sato, Mr. Yusuke Mira, and Dr. Shinichi Kumakura from the Research Institute for Science and Technology, TUS. Their findings were published online in the journal Advanced Materials on July 15, 2025.
The team synthesized a series of highly crystalline, Cu-doped β-NaMnO2 samples (NaMn1-xCuxO2) with varying amounts of Cu, denoted as NMCO-00, -05, -10, -12, and -15, corresponding to Cu doping levels from 0% to 15%. The NMCO-00 sample served as the undoped reference. Through X-ray diffraction (XRD) studies, the team found that among the Cu doped samples, NMCO-05 exhibited the highest SF concentration at 4.4%, while in NMCO-12, the SF concentration was only 0.3%, indicating a clear suppression of SFs with increased Cu doping.
Electrochemical evaluation of electrodes made from the NMCO samples in Na half cells revealed significantly enhanced capacity retention in Cu-doped samples. While the undoped sample showed rapid capacity loss within 30 cycles, the SF-free NMCO-12 and -15 samples demonstrated excellent cycle stability, with the NMCO-12 exhibiting no capacity loss for over 150 cycles. These results suggest that the β-phase of layered NaMnO2 is inherently stable when SFs are eliminated.
Importantly, the SF-free structure allowed the researchers to examine the complex phase transitions that occur during Na insertion and extraction in these materials. Using a combination of in situ and ex situ XRD measurements, and density functional theory calculations, the researchers proposed a new structural model involving drastic gliding of the corrugated MnO2 layers. This gliding appears to be unique to the β-phase and was previously obscured by the presence of SFs, marking a major advancement in understanding the characteristic structural changes of the β-phase of NaMnO₂ during electrode reactions.
"Our findings confirm that manganese-based oxides are a promising and sustainable solution for developing highly durable Na-ion batteries," notes Prof. Komaba. "Owing to the relatively low cost of manganese and Na, this research will lead to more affordable energy-storage solutions for a variety of applications, including smartphones and electric vehicles, ultimately leading to a more sustainable future."
This study also demonstrates that stabilization of SF using Cu doping could resolve the supply chain vulnerabilities that are commonly faced with metals like lithium. Moreover, the study has potential implications in grid storage, electric vehicles, and consumer electronics.
The study offers valuable insights for developing more stable and long-lasting Na-ion batteries, leading to wider renewable energy adoption, aligning with the United Nations Sustainable Development Goal 7: Affordable and Clean Energy.
