Ten Nanometers That Keep the Battery in Check

Ten Nanometers That Keep the Battery in Check

Yang-Kook Sun, a professor of battery engineering at Hanyang University in Seoul, has been designing cathodes for thirty years. Hundreds of compounds have passed through his laboratory — lithium-nickel oxides, lithium-manganese oxides, lithium-cobalt oxides. He has over 800 publications and an h-index exceeding 130 — he belongs to a narrow group of scientists who literally defined modern battery chemistry. But when one of his PhD students, Xinghui Liang, showed him images from a transmission electron microscope of the latest sodium cathode in March 2025, Sun just stared for a long moment. The crystal surface was smooth. No microcracks. After three hundred charge cycles.

For a decade, this was the holy grail of sodium batteries. O3-type cathodes — the cheapest, easiest to synthesize, with the highest energy density — would disintegrate every time after a few dozen cycles. The sodium ion is 30 percent larger than the lithium ion. When sodium ions leave the layered cathode structure during battery charging, the crystal shrinks. When they return — it swells. After fifty cycles, the stresses are so great that the material cracks. After a hundred — it disintegrates into dust.

Sun's team stopped this process with a layer ten nanometers thick. Fifty atoms, arranged side by side, act as both armor and anchor simultaneously. This solution does not require exotic materials — calcium sodium phosphate compounds are cheap, stable, available in tons. The scale of the problem and the scale of the solution could not be more distant from each other: a global lithium crisis versus a coating thinner than a virus envelope.

Sodium Instead of Lithium: Economics of Scale

When lithium carbonate prices skyrocketed from $15,000 to $80,000 per ton in 2022 — driven by demand for electric vehicle batteries — battery manufacturers around the world began frantically searching for an alternative. Lithium is rare: global resources estimated at 89 million tons are concentrated in Chile, Australia, China, and Argentina. Extraction is slow, water-intensive, and politically sensitive. Every ton of lithium from Chilean salars requires evaporating two million liters of brine — in the Atacama Desert, where water is worth its weight in gold.

Meanwhile, sodium is the sixth most common element in the Earth's crust — 2.36 percent of the crust's mass is sodium. It's in every drop of ocean water, in every crystal of table salt. Sodium carbonate costs about $200 per ton — a hundred times less than lithium carbonate under normal market conditions. For stationary batteries — energy storage at solar and wind farms, where cost matters more than weight — this difference is decisive.

The International Energy Agency estimated in its 2025 report that by 2030, global demand for stationary batteries will increase fifteenfold — to 1.5 terawatt-hours per year. For comparison: that's equivalent to the daily electricity consumption of the entire United States. At current lithium extraction rates, at most one-third of this demand can be met. The rest must come from sodium batteries — or there will be no storage, and without storage, there's no point in building solar farms.

CATL, the world's largest battery manufacturer, announced mass production of first-generation sodium-ion cells in 2024. BYD, HiNa Battery, Faradion — all major players are investing billions in sodium technology. The problem is that first-generation batteries had 30-40 percent lower energy density than lithium-ion and degraded faster. The O3 cathode — the very one Sun just saved from microcracking — is the key to the second generation: cheaper, more durable, with energy density approaching that of lithium. The difference between the first and second generations is precisely ten nanometers.

Fifty Atoms: How It Works

The key experiment was strikingly simple in concept — and devilishly difficult in execution. Sun's team took an O3-type cathode with the composition Na[Ni₀.₄Fe₀.₁Mn₀.₄Mg₀.₀₅Ti₀.₀₅]O₂ — a material already optimized for stability through magnesium and titanium doping — and coated its particles with a calcium phosphate precursor. During annealing at 650 degrees Celsius, a solid-state reaction occurred: the calcium phosphate reacted with residual sodium compounds on the surface, forming a uniform NaCaPO₄ coating — a fast ion conductor — just 10 nanometers thick. For comparison: that's one-tenth the thickness of a typical influenza virus. Ten thousand times thinner than a human hair.

This is the first protective layer. The NaCaPO₄ coating is like a selective membrane: it lets sodium ions pass without resistance, but blocks electrolyte molecules that would normally attack the cathode surface, causing degradation. However, the coating alone would not be enough to stop microcracking — those form deeper, within the crystal volume, where uneven distribution of sodium ions generates stresses.

This is where the second layer comes in — literally and figuratively. During annealing, some calcium atoms from the coating migrate into the cathode crystal, replacing sodium atoms in interlayer positions. Calcium has an ionic radius of 1.12 Å — it's larger than sodium (1.02 Å). The difference is just one-tenth of an ångström, one-hundredth of a nanometer. But at the crystal lattice level, this microscopic difference acts like a wedge: it physically blocks the oxide layers from sliding against each other during charging and discharging. In finite element method simulations (COMSOL), the team showed that this "pinning effect" reduces internal stresses by over 40 percent. Simultaneously, calcium, as a divalent element, interacts more strongly electrostatically with the oxide layers, stabilizing the structure at the atomic level.

The laboratory results are impressive, but it's the tests in industrial formats that make the difference. The optimized cathode retained 82.9 percent capacity after 300 charge-discharge cycles at 0.5C current — equivalent to a two-hour charge, typical for stationary applications. More importantly, the team tested it not only in miniature coin cells (CR2032, typical for laboratory research) but also in full-size pouch cells — flexible, rectangular packages, exactly the kind used by battery manufacturers. The pouch cell with a hard carbon anode and NaCaPO₄-coated cathode operated stably in a temperature range from minus 10 to plus 50 degrees Celsius — from the freezing winters of Poland to desert heat.

This last point is crucial for stationary applications. Solar farms in the Middle East and wind farms in the North Sea need energy storage that operates reliably in extreme temperatures, without expensive cooling or heating systems. Ten nanometers of NaCaPO₄ coating — a single grain of sand sliced ten thousand times — means the O3 cathode meets this condition for the first time.

Poland: Research Is There, Pipeline Is Not

Poland has a strong scientific position in the sodium battery race — and zero industrial one. In 2024, M. Zając's team from Jagiellonian University published a paper in Advanced Materials (impact factor 27.8) analyzing competing oxygen redox mechanisms in doped nickel-manganese cathodes for sodium batteries. The paper, cited 78 times, examines exactly the same phenomenon — the structural instability of O3 cathodes — from the perspective of oxygen chemistry: why oxygen atoms in the crystal lattice begin participating in electrochemical reactions and how this uncontrolled activity leads to degradation. This is fundamental knowledge without which materials engineers cannot design better cathodes.

This is not the only Polish lead. The Institute of Physical Chemistry of the Polish Academy of Sciences, Warsaw University of Technology, and AGH University of Science and Technology run their own research programs on cathode materials for sodium batteries. Polish teams regularly publish in Advanced Energy Materials, Energy Storage Materials and Journal of Materials Chemistry A. The problem is that between the laboratory and the factory in Poland, there is a vacuum — and it's not just about money.

South Korea, where Sun's team works, has an ecosystem where research results reach industry within months. LG Energy Solution, Samsung SDI, and SK On — three battery giants — have their own R&D departments that immediately test promising cathode materials in large-format cells. Every gram of cathode from Sun's laboratory can land in a prototype pouch cell within a week. Sun has collaborated with LG Energy Solution for over a decade — his earlier lithium cathodes are already in commercial batteries.

In Poland, there is not a single cathode materials factory. There is no pilot line for producing cathode powders at the kilogram scale — the first step from laboratory to industry. NCBR funds basic research, but between a research grant and industrial implementation, there is no bridge. The FENG program has €7.9 billion for 2021-2027, of which a significant portion is for the green transition — but the money goes mainly to large consortia, not to academic spin-offs that could scale an invention from the laboratory to a prototype.

The sodium battery market in Europe is expected to reach €7 billion by 2028. Northvolt, the European battery manufacturer, announced work on sodium-ion cells in 2024. Poland, with its chemical base (Grupa Azoty, PCC Rokita, Synthos) and academic base, could supply cathode materials to these factories. The first country to build a pilot line for producing NaCaPO₄ cathode powders at the kilogram scale will capture supply contracts for all European gigafactories. For now, that country is not Poland.

Meanwhile, Sun's team is already working on the next iteration. In the final paragraph of their paper in Nature Communications they write that the dual modification strategy — coating plus gradient doping — is universal. It works not just for this specific cathode composition, but for the entire family of O3 layered oxides, regardless of whether they contain nickel, manganese, iron, or titanium. In other words: they solved the problem not for one material, but for an entire class of materials. And because NaCaPO₄ and calcium are cheap and available, the process can be scaled immediately — no exotic precursors, no complicated equipment. All you need is a furnace, a precursor, and temperature control. The rest is chemistry that nature performs on its own — at the scale of ten nanometers.

Sources

Liang X., Song X., Sun H.H., Kim H., Kim M.-C., Sun Y.-K., High-energy and long-life O3-type layered cathode material for sodium-ion batteries, Nature Communications 16, 3505 (2025), DOI: 10.1038/s41467-025-58637-1.

Zając M. et al., Competing Mechanisms Determine Oxygen Redox in Doped Ni–Mn Based Layered Oxides for Na-Ion Batteries, Advanced Materials 36, 2309842 (2024), DOI: 10.1002/adma.202309842.