Reshaped battery crystals retain 96.7% capacity after 300 cycles, limiting stress

Researchers have found a way to make sodium-ion batteries more durable by changing the shape of their cathode crystals rather than altering their chemical composition. By reducing the thickness of a key crystal dimension, the team limited internal stress and cracking that normally develops as sodium ions repeatedly enter and leave the material. The researchers worked with a layered P2-type cathode, Na0.75Ni0.25Mn0.75O2, a material being studied for sodium-ion batteries because sodium is abundant and potentially cheaper than the lithium used in conventional batteries. The problem is mechanical. As sodium ions move into and out of the cathode during charging and discharging, the crystal lattice repeatedly expands and contracts. The changes are not uniform, creating stress inside individual grains. Over time, that stress can produce cracks, expose new surfaces to the electrolyte, and accelerate reactions that reduce battery performance. A team led by Wuhan University of Technology found that controlling the crystal’s c-axis dimension could help prevent this damage. Instead of simply making the particles smaller, the researchers created thinner, prism-shaped grains that could release lattice strain more evenly. Thinner crystals ease battery stress The team produced morphology-tailored NaNMO, or MT-NaNMO, and compared it with a conventional sample made from the same chemical composition. The key difference was the grain geometry. The tailored material had primary grains measuring about 200 nanometers along the c-axis, compared with about 800 nanometers for the conventional material. X-ray diffraction showed that both materials experienced similar changes in their crystal structure during charging and discharging, suggesting that the improvement came from how the grains handled the resulting strain. Microscopy provided a closer look at the difference. The thinner grains maintained more stable lattice structures and more uniform strain fields, while the thicker grains developed localized distortion and stress. Finite element analysis supported the finding, showing that reducing the c-axis dimension distributed mechanical stress more evenly throughout the grains. The tailored material also showed faster sodium-ion transport and lower resistance during electrochemical testing. The result was a significant improvement in cycling stability. The optimized cathode retained 96.7% of its capacity after 300 cycles at a high 5 C rate. The researchers described sodium-ion storage as a repeated “breathing” motion in layered oxide materials. In thicker grains, the repeated expansion and contraction can allow strain to accumulate until cracks form. Shortening the vulnerable direction gives that strain more room to dissipate before it causes structural damage. Battery design gets directional The approach could offer an alternative to simply reducing overall particle size. Smaller particles can release mechanical stress more effectively, but increasing surface area can also promote unwanted reactions with the electrolyte and reduce packing density. By targeting the c-axis specifically, the researchers focused on the direction most closely associated with tensile stress and internal cracking. That could allow cathode designers to improve mechanical stability without relying solely on smaller particles or changes to chemical composition. The researchers also tested the material in a full cell using hard carbon as the negative electrode. It reached an energy density of about 218.3 Wh kg-1 and retained 92.6% of its capacity after 300 cycles at 2 C. Sodium-ion batteries are being explored for applications including renewable-energy and grid-scale storage, where the abundance of sodium could help reduce dependence on more constrained battery materials. The study suggests that controlling crystal geometry could become another tool for improving their long-term durability. The study was published in eScience Energy. Get the latest in engineering, tech, space & science - delivered daily to your inbox.With over a decade-long career in journalism, Neetika Walter has worked with The Economic Times, ANI, and Hindustan Times, covering politics, business, technology, and the clean energy sector. Passionate about contemporary culture, books, poetry, and storytelling, she brings depth and insight to her writing. When she isn’t chasing stories, she’s likely lost in a book or enjoying the company of her dogs.