Scientists Crack the Code for Stable and Long-Life Sodium-Ion Batteries

Sodium-ion (Na-ion) batteries have recently gained attention as affordable and sustainable alternatives to lithium-ion (Li-ion) batteries. Sodium, the sixth most abundant element on Earth, offers advantages such as lower material costs and greater natural availability compared to lithium. A key factor in enhancing the performance and lifespan of Na-ion batteries is the design of their cathode materials. One promising candidate is layered sodium manganese oxide (NaMnO₂), which has attracted growing interest from researchers.

NaMnO₂ exists in two distinct crystal structures: α-NaMnO₂ and β-NaMnO₂. The α-phase has a monoclinic layered structure, with flat MnO₂ layers made of edge-sharing, distorted MnO₆ octahedra, and sodium ions positioned between these layers. In contrast, the β-phase features zig-zag or corrugated layers of similar octahedra, also interspersed with sodium ions. Producing the β-phase typically requires high temperatures, which often results in sodium-deficient materials.

Efforts to avoid these sodium deficiencies can lead to non-equilibrium β-phases that contain various structural defects—most notably, stacking faults (SFs). These faults occur when the crystal’s b–c plane slips, creating stacking patterns that resemble the α-phase. Electrodes made from β-NaMnO₂ containing such faults tend to suffer significant capacity loss during charging and discharging, limiting their real-world usability. Additionally, these defects make it more difficult to fully understand the material’s solid-state chemistry.

In a recent study, a research team led by Professor Shinichi Komaba from the Department of Applied Chemistry at Tokyo University of Science (TUS), Japan, explored how doping with copper (Cu) can help stabilize stacking faults (SFs) in β-NaMnO₂. “In an earlier study, we discovered that copper is the only metal dopant capable of effectively stabilizing β-NaMnO₂,” said Prof. Komaba. “Here, we took a closer look at how Cu doping can suppress stacking faults and enhance the electrochemical performance of β-NaMnO₂ electrodes in sodium-ion batteries.” The research team also included Mr. Syuhei Sato, Mr. Yusuke Mira, and Dr. Shinichi Kumakura from TUS’s Research Institute for Science and Technology. Their findings were published in Advanced Materials on July 15, 2025.

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The team synthesized a series of highly crystalline, Cu-doped β-NaMnO₂ compounds (NaMn₁₋ₓCuₓO₂), labeled NMCO-00, -05, -10, -12, and -15, corresponding to Cu doping levels ranging from 0% to 15%. NMCO-00 served as the undoped control. X-ray diffraction (XRD) analysis revealed that the NMCO-05 sample had the highest SF concentration at 4.4%, while NMCO-12 showed a dramatically lower SF concentration of just 0.3%, clearly indicating that higher Cu doping effectively suppresses SF formation.

Electrochemical testing of these NMCO-based electrodes in sodium half-cells showed significantly improved capacity retention in the Cu-doped samples. While the undoped NMCO-00 experienced a sharp drop in capacity within just 30 charge-discharge cycles, the nearly SF-free NMCO-12 and -15 samples delivered outstanding cycling stability. Notably, NMCO-12 maintained its capacity with no measurable loss over more than 150 cycles. These results suggest that eliminating stacking faults is key to unlocking the inherent stability of the β-phase layered NaMnO₂ cathode material.

Crucially, eliminating stacking faults (SFs) enabled the researchers to closely study the complex phase transitions that take place during sodium insertion and extraction in β-NaMnO₂. Through a combination of in situ and ex situ X-ray diffraction (XRD) techniques and density functional theory (DFT) calculations, they proposed a new structural model that involves significant gliding of the corrugated MnO₂ layers. This type of layer movement appears to be a unique feature of the β-phase, which had previously been hidden by the presence of SFs—marking a significant breakthrough in understanding how the β-phase structure evolves during electrochemical cycling.

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“Our research confirms that manganese-based oxides hold strong potential as a cost-effective and sustainable option for building long-lasting sodium-ion batteries,” said Prof. Komaba. “Given the abundance and affordability of both manganese and sodium, this work could pave the way for more economical energy storage technologies for everything from smartphones to electric vehicles—helping move us toward a more sustainable future.”

The study also shows that stabilizing SFs through copper doping can help overcome supply chain challenges often associated with critical materials like lithium. Beyond consumer electronics and electric mobility, the findings could also benefit large-scale energy storage systems like power grids.

Ultimately, this research provides important insights into developing more reliable and longer-lasting Na-ion batteries, contributing to broader adoption of renewable energy and supporting the UN’s Sustainable Development Goal 7: Affordable and Clean Energy.