HMN 2026: How Atomic swap can improve phosphate cathodes for high-energy sodium-ion batteries

Most smartphones, portable computers and other devices on the market today are powered by lithium-ion (Li-ion) batteries. While these rechargeable batteries perform remarkably well, they are based on lithium, which is not as abundant as other materials and is not evenly distributed across different countries worldwide.

Over the past decades, energy engineers have thus been exploring the potential of rechargeable batteries based on other materials that are more abundant and affordable. These include sodium-ion (Na-ion) batteries, which utilize sodium ions (Na⁺) as charge carriers.

Like in other batteries, charge carriers in Na-ion batteries move between a cathode (i.e., an electrode attracting positively charged ions) and an anode (i.e., an electrode that attracts negatively charged ions). To improve the performance, stability, safety and energy density of batteries entails, engineers can design new cathodes or try to improve existing ones.

Researchers at Zhejiang University, Sun Yat-sen University and other institutes recently introduced a new approach to enhance iron-based phosphate cathodes for Na-ion batteries. Their proposed strategy, presented in a paper published in Nature Energy, entails modifying the atomic structure of cathodes based on the material Na₄Fe₃(PO₄)₂P₂O₇, replacing some iron atoms with vanadium ions (V³⁺).

“Fe-based polyanionic cathodes are promising for large-scale Na-ion batteries owing to their stability, safety and elemental abundance. However, their capacity remains limited by electrochemically inactive Na sites and irreversible Na loss,” wrote Xinyu Li, Duoduo Zhang and their colleagues.

“We identify that the Na⁺ coordination environment critically influences the Na-site accessibility and redox activity in Na₄Fe₃(PO₄)₂P₂O₇-type cathodes.”

Improving the structure of phosphate cathodes

As part of their study, Li, Zhang and their colleagues first studied iron-based phosphate cathodes that were previously integrated in Na-ion batteries. Specifically, they tried to determine why some Na⁺ ions in these cathodes remained electrochemically inactive and do not move freely while batteries are charging and discharging.

Based on the results of their analyses, they then devised a strategy designed to activate these inactive ions and thus improve both the capacity and energy density of Na-ion batteries. Their proposed approach entails replacing iron atoms at specific sites in the cathode with V³⁺ ions.

The researchers assessed the potential of their cathode design strategy in a series of computer simulations and real-world experiments. Their findings were very promising, as the substitution of selected iron atoms with V³⁺ ions successfully activated Na ions that were previously inaccessible, yielding improved battery capacities and energy densities.

“Combined experimental and theoretical analyses reveal that precise V³⁺ substitution at the Fe2 site harmonizes Na⁺ coordination geometry and softens the polyanionic framework, thereby activating previously inert Na sites and stabilizing high-voltage redox reactions above 4?V,” wrote the authors.

“The optimized Na_3.4Fe_2.4V_0.6(PO₄)₂P₂O₇ achieves full Na⁺ utilization (3.4 Na⁺, 150.7?mAh?g^?1) and a 52% increase in energy density (487?Wh?kg^?1), approaching the practical limit of Li-ion phosphate cathodes. It also demonstrates exceptional durability, over 10,000 cycles in the 2.1–4.5?V range and stable pouch-cell performance.”

Facilitating the deployment of Na-ion batteries

The cathode enhancement strategy introduced by Li, Zhang and their collaborators so far proved to successfully boost the capacity and energy density of Na-ion batteries. In the future, it could be refined further, applied to other phosphate-based cathodes, and tested in more real-world experiments under realistic operating conditions.

“These findings provide a coordination-based strategy to overcome intrinsic capacity limitations in phosphate cathodes, enabling high-energy, durable Na-ion batteries,” wrote the authors.

Eventually, this research team’s efforts could contribute to the introduction of safe, stable and high-energy rechargeable batteries that are more affordable than current Li-ion batteries. These batteries could potentially be deployed in both portable and large electronics, including electric vehicles.

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