Sodium-ion batteries have long been viewed as a promising alternative to lithium-ion technology, particularly for applications where low cost, abundant raw materials and large-scale energy storage matter more than achieving the highest possible energy density. Yet one of the most attractive cathode chemistries for sodium-ion cells has been held back by a frustrating materials problem: when iron becomes too abundant in the structure, the electrode can deliver impressive capacity at first but deteriorates rapidly during repeated charging and discharging. A new study now identifies the nanoscale structural events behind that failure and demonstrates a practical strategy for suppressing them in ampere-hour-level sodium-ion pouch cells.
The work, published in Nature Nanotechnology, focuses on iron-rich sodium layered oxides, a family of positive-electrode materials in which sodium ions move in and out of stacked oxide layers during battery operation. These compounds can exploit the Fe(III)/Fe(IV) redox couple, allowing iron ions to participate in the reversible transfer of electrons that stores electrical energy. Iron is especially attractive because it is inexpensive, widely available and less vulnerable to supply constraints than metals such as nickel or cobalt. However, the researchers found that increasing the iron concentration beyond roughly 33 atomic percent of the transition-metal sites creates a severe trade-off: capacity rises, but structural damage accumulates quickly during cycling.
The central discovery is that the degradation is not simply a consequence of large-scale phase changes visible across an entire electrode particle. Instead, it begins with the instability of iron’s octahedral coordination environment at the nanoscale. In the layered oxide structure, each iron ion is normally surrounded by six oxygen ions arranged approximately at the corners of an octahedron. This local geometry helps maintain the framework that supports sodium-ion transport. When the coordination environment becomes unstable during electrochemical cycling, iron ions can migrate away from their preferred sites and, in some cases, dissolve from the active material into the battery electrolyte.
That migration and dissolution trigger a chain reaction inside individual cathode particles. Regions affected by iron movement no longer expand and contract in exactly the same way as their surroundings. The resulting mismatch in local strain produces uneven mechanical stress, while defects such as dislocations accumulate within the particles. Eventually, the stress becomes concentrated along vulnerable internal regions, generating intragranular microcracks. These cracks are particularly damaging because they divide an initially coherent active particle into mechanically disconnected domains and create new surfaces that can react with the electrolyte.
The study further shows that the damage does not stop when the first cracks form. Driven by the non-uniform strain field, microcracks spread through the particles and interact with crystallographic defects. The researchers observed planar gliding, a process in which sections of the crystal shift along defined planes. Repeated gliding can reshape the external surface of the material, producing a distinctive stepped morphology. Such steps and cracks can interfere with the movement of sodium ions, expose fresh reactive surfaces and intensify chemical degradation, creating a feedback loop in which structural damage accelerates electrochemical capacity loss.
This nanoscale-to-microscale connection helps explain why iron-rich materials can fail even when their overall crystal structure appears to remain largely intact. Conventional structural measurements may average over millions of atoms and miss local coordination changes or the earliest stages of defect formation. By examining the electrode at much smaller length scales, the researchers linked iron instability directly to mechanical failure. Their findings suggest that the crucial design target is not merely the average composition of the cathode, but the ability of the local iron–oxygen environment to remain stable as sodium ions are repeatedly extracted and reinserted.
To reinforce that environment, the team introduced three dopant elements at the nanoscale: 1 atomic percent aluminium, 1 atomic percent yttrium and 3 atomic percent cobalt. The combined addition is designed to alter the local chemical and structural landscape without replacing the iron-rich character of the cathode. Aluminium and yttrium are expected to help stabilize the oxide framework, while cobalt can influence transition-metal–oxygen interactions and the electronic structure of the material. More broadly, the multi-element approach distributes the stabilizing effect across several chemical components rather than relying on a single dopant.
The modified material showed reduced iron migration and dissolution, which in turn suppressed the formation of internal cracks and limited planar gliding. This is significant because it addresses the proposed failure mechanism at its origin rather than attempting to repair damage after it has formed. By preserving the local octahedral coordination around iron, the doped oxide can better accommodate the repeated strain associated with sodium-ion removal and reinsertion. The result is a more mechanically coherent positive electrode with improved resistance to the chain of events that normally causes rapid capacity decay in iron-rich compositions.
The researchers then moved beyond half-cell experiments and assembled full sodium-ion pouch cells using the multi-element-doped positive electrode and a hard-carbon negative electrode. The cells reached a capacity of 2.7 ampere-hours, demonstrating that the approach can operate at a scale relevant to practical battery development rather than only in small laboratory test cells. Based on the total mass of the cell, the initial specific energy reached 121 watt-hours per kilogram at a current of 26 milliamperes per gram. When cycled at 130 milliamperes per gram and 25 degrees Celsius, the cells retained 83.4 percent of their discharge capacity after 2,000 cycles.
Those figures do not make sodium-ion batteries a universal replacement for lithium-ion cells, and the reported energy density remains dependent on the complete cell design, electrode balancing and testing conditions. Nevertheless, the results offer a compelling answer to a major obstacle facing iron-rich sodium layered oxides. The study shows that a material composed largely of abundant elements can be made more durable by controlling local coordination chemistry and the mechanical stresses that emerge from it. For sodium-ion technology, the implication is powerful: preventing a few unstable iron environments at the nanoscale may be the key to preserving performance across thousands of charge–discharge cycles and bringing cost-effective, large-format batteries closer to widespread use.
Subject of Research: Nanoscale structural degradation and multi-element doping in iron-rich sodium layered oxide cathodes for sodium-ion batteries.
Article Title: Multi-element nanoscale doping of iron-rich sodium layered oxides enables ampere-hour-level Na-ion batteries
Article References: Jin, RX., Lei, X., Su, XC. et al. Multi-element nanoscale doping of iron-rich sodium layered oxides enables ampere-hour-level Na-ion batteries. Nat. Nanotechnol. (2026). https://doi.org/10.1038/s41565-026-02257-3
Image Credits: AI Generated
DOI: https://doi.org/10.1038/s41565-026-02257-3
Keywords: sodium-ion batteries, iron-rich layered oxides, cathode materials, Fe(III)/Fe(IV) redox, nanoscale doping, aluminium, yttrium, cobalt, iron migration, microcracking, planar gliding, hard carbon, pouch cells, battery durability
