A new study suggests that one of the most persistent problems facing sodium-ion batteries may be solved not by changing what their cathode is made of, but by changing the shape of its microscopic crystal grains. Researchers have shown that tailoring the thickness of layered oxide particles along a single crystallographic direction can prevent internal cracking, reduce mechanical stress, and substantially improve battery life. The approach enabled a sodium-ion cathode to retain 96.7% of its capacity after 300 charge-discharge cycles at a demanding rate of 5 C, offering a potential route toward more durable and affordable energy-storage systems.
Sodium-ion batteries are attracting increasing attention as alternatives to lithium-ion technology, particularly for stationary storage and grid applications. Sodium is abundant, widely distributed, and generally less expensive than lithium, while sodium-based chemistries can be compatible with supply chains that avoid some of the critical-material constraints associated with conventional batteries. Yet sodium-ion cells still face important challenges. Layered transition-metal oxides can store substantial amounts of sodium, but the repeated insertion and removal of sodium ions causes their crystal lattices to expand and contract. When that movement is uneven, mechanical stress accumulates inside the active material and can eventually fracture the grains that carry out the electrochemical reactions.
The new research, led by a team at Wuhan University of Technology in collaboration with scientists from Xi’an Jiaotong University, focused on a P2-type layered oxide with the composition Na0.75Ni0.25Mn0.75O2. In these materials, sodium ions move between slabs formed by transition-metal and oxygen atoms. The structure is called “P2” because sodium occupies prismatic sites between the oxide layers, while the number refers to the approximate stacking arrangement of the oxygen framework. This architecture can provide attractive sodium-storage performance, but it is also vulnerable to structural changes during cycling, especially along the c-axis, the direction perpendicular to the principal oxide layers.
Rather than altering the chemical formula, the researchers designed two materials with the same composition but different grain geometries. The morphology-tailored sample, called MT-NaNMO, consisted of thinner prism-like primary grains measuring approximately 200 nanometers along the c-axis. The comparison material, C-NaNMO, contained grains roughly 800 nanometers thick in the same direction. This controlled comparison allowed the researchers to isolate the mechanical role of crystal dimensions. X-ray diffraction, Rietveld refinement, scanning electron microscopy, focused ion beam imaging, and electron microscopy confirmed that both samples preserved the P2 layered structure, while revealing a clear difference in the size and shape of their internal grains.
The key finding was that the two materials experienced broadly similar changes in their unit-cell dimensions during sodium extraction and reinsertion, but they did not distribute the resulting strain in the same way. In situ X-ray diffraction showed that the crystal lattice itself underwent comparable electrochemical breathing in both samples. The difference appeared at the grain scale. In the thicker C-NaNMO particles, the repeated lattice changes generated concentrated stress that distorted the crystal and promoted the formation of internal cracks. In the thinner MT-NaNMO grains, the shorter c-axis provided a more effective path for strain relaxation, preventing mechanical energy from accumulating in a small region.
High-resolution transmission electron microscopy and geometric phase analysis offered a direct view of this contrast. The tailored material retained more uniform lattice fringes and a comparatively even strain field during operation. By comparison, the conventional material exhibited pronounced lattice distortion and localized strain. Such localized stress is particularly damaging because it can initiate cracks inside a grain before any visible fracture appears at the particle surface. Once these cracks develop, electrolyte can penetrate newly exposed surfaces, triggering parasitic reactions that consume active sodium, increase interfacial resistance, and gradually reduce the battery’s usable capacity.
The researchers also used finite element analysis to model how the grain geometry affected stress evolution. The simulations indicated that reducing the c-axis dimension lowered the concentration of mechanical stress and produced a more homogeneous distribution throughout the grain. In a thick crystal, sodium-induced deformation must be accommodated across a longer distance, allowing tensile stress to build up and making cracking more likely. Shortening the vulnerable direction changes the way that deformation propagates, enabling the grain to release strain earlier and more evenly. The concept resembles the use of carefully engineered dimensions in other structural materials, where controlling the geometry can prevent a local defect from becoming a catastrophic failure.
The mechanical improvement was accompanied by electrochemical benefits. Electrochemical impedance spectroscopy indicated that MT-NaNMO offered faster sodium-ion transport and lower resistance than the comparison material. The thinner, better-preserved grains can maintain more continuous pathways for ion movement, while their reduced tendency to crack limits the creation of unstable interfaces. In half-cell testing, the optimized cathode retained 96.7% of its capacity after 300 cycles at 5 C, a high-rate condition in which a nominal full charge or discharge corresponds to approximately one-fifth of an hour. The results indicate that structural stability was not achieved at the expense of rapid electrochemical operation.
The strategy also showed promise beyond laboratory half-cells. When paired with a hard-carbon negative electrode in a full sodium-ion cell, the tailored cathode delivered an energy density of approximately 218.3 watt-hours per kilogram and retained 92.6% of its capacity after 300 cycles at 2 C. These values are significant because full cells expose materials to practical limitations that may not appear when a cathode is tested against an excess of sodium. The results suggest that c-axis dimension tailoring can contribute to a functioning cell architecture rather than merely improving an isolated electrode under idealized conditions.
The study, published online on May 20, 2026, in eScience Energy, points to a broader shift in how battery researchers may approach degradation. Particle miniaturization is already known to help relieve stress, but reducing every dimension of a particle can increase surface area, intensify side reactions with the electrolyte, and lower the tap density of an electrode. Directional tailoring offers a more selective alternative: instead of making particles uniformly smaller, engineers can focus on the crystallographic dimension most closely linked to stress concentration. By shaping how a grain responds to sodium-ion “breathing,” the approach may help extend cycle life while preserving useful electrode packing and energy density. The researchers’ findings suggest that future sodium-ion cathodes could be designed through a combination of chemical composition and mechanical architecture, bringing more resilient, lower-cost batteries closer to practical use in renewable-energy storage and grid-scale applications.
Subject of Research:
C-axis dimension tailoring to reduce lattice stress and intragranular cracking in layered sodium-ion battery cathodes.
Article Title:
Releasing lattice stress during Na+(de)intercalation in layered oxides through c-axis dimension tailoring
News Publication Date:
May 20, 2026
Web References:
https://www.sciencedirect.com/science/article/pii/S3050995526000450
eScience Energy
References:
DOI: 10.1016/j.esen.2026.100070
Journal: eScience Energy
Original article: “Releasing lattice stress during Na+(de)intercalation in layered oxides through c-axis dimension tailoring”
Image Credits:
Xing Zhou, Yongyuan Zhou, et al.
Keywords
Sodium-ion batteries, layered oxide cathodes, P2-Na0.75Ni0.25Mn0.75O2, lattice stress, c-axis tailoring, intragranular cracking, battery materials, energy storage, hard carbon, electrochemistry
