Rechargeable batteries could outlive their vehicles thanks to an ultrathin protective layer that does double duty, shielding cathode materials from corrosion while stabilising their atomic structure.

Nickel-rich cathodes power the latest electric vehicles. They pack more energy into each charge than older battery technologies. But that power comes at a cost: these cathodes degrade when pushed to high voltages, limiting how long batteries can last.

One solution has been to coat cathode particles with aluminium oxide—a ceramic layer just nanometres thick. It works. Batteries cycle longer. But until now, no one really understood why.

A team from the University of Cambridge used nuclear magnetic resonance spectroscopy—a technique that reveals atomic-level details—combined with density functional theory calculations to watch the coating evolve in real time. What they found changes how we think about battery protection.

The coating transforms

The aluminium oxide arrives as a disordered structure. Not quite crystalline, not quite random. The aluminium atoms sit in various configurations: some surrounded by four oxygen neighbours, others by five or six. These different arrangements matter.

When the coating encounters battery electrolyte—even trace moisture—it reacts. Undercoordinated aluminium sites, those with fewer oxygen neighbours, grab water molecules from solution. The structure rearranges. Coordination numbers shift. New chemical bonds form.

Then the real transformation begins. Battery electrolytes contain lithium hexafluorophosphate salt, which breaks down in the presence of water to produce hydrofluoric acid. Corrosive. Destructive to battery materials.

The aluminium oxide scavenges this acid. Fluorine atoms replace some oxygen and hydroxyl groups in the coating. The process is irreversible. Once transformed, the coating doesn't revert when the battery discharges.

Two roles, one coating

The coating protects in two distinct ways.

First, it neutralises harmful species before they reach the cathode surface. Those undercoordinated aluminium sites—the same ones that grab water—also bind fluorine preferentially. The aluminium-fluorine bond is stronger than aluminium-oxygen bonds. Thermodynamics favours the swap. Harmful chemicals get locked into the coating rather than attacking the cathode beneath.

Second, and less obviously, the coating stabilises oxygen atoms at the cathode surface.

Nickel-rich cathodes suffer from a fundamental problem: at high voltages, nickel and oxygen form covalent bonds with shared electrons. When batteries charge to high states, these bonds become unstable. Oxygen can form reactive species—essentially free radicals—that oxidise the electrolyte, generating carbon dioxide and carbon monoxide. The cathode surface reconstructs into an inactive phase. Capacity fades.

The computational analysis revealed something unexpected. Oxygen atoms coordinated to both aluminium and nickel show less orbital overlap between nickel d electrons and oxygen p electrons. Less hybridisation. Less covalency. The wide band gap nature of aluminium-oxygen bonding, characteristic of insulators, locally stabilises the oxygen.

X-ray absorption spectroscopy confirmed this. After 1,042 charge-discharge cycles—a brutal test—coated cathodes retained electrochemical activity at their surfaces. The nickel L-edge spectra showed preserved redox states. Uncoated cathodes had degraded significantly.

Gas evolution measurements told a complementary story. During charging, uncoated cathodes released carbon monoxide and carbon dioxide in a 1:2.5 ratio, consistent with complete oxidation of carbonate solvents. Coated cathodes showed a 1:1.2 ratio and lower total gas volumes. Different reaction pathways. Less destructive chemistry.

What happens during long-term cycling

After more than a thousand cycles to 4.3 volts, coated cathodes retained 71 percent of initial capacity versus 51 percent for uncoated materials. An improvement, but not a revolution.

Why the limited gain? Polycrystalline cathode particles crack during cycling as they expand and contract with lithium insertion and removal. Volume changes approach 5 percent. Microcracks expose fresh, unprotected surfaces. The coating, confined to the original particle exterior, can't protect these new interfaces.

Researchers observed this through diagnostic cells. After extended cycling, they disassembled batteries and paired aged cathodes with fresh lithium metal and electrolyte. The coated material delivered 120 milliamp-hours per gram—71 percent of its original capacity. Some fade came from loss of active material, but the cathode itself had degraded less than its uncoated counterpart.

From understanding to optimisation

The dual mechanism—chemical scavenging and electronic stabilisation—suggests design principles for next-generation coatings.

Ideal coatings should contain Lewis acid sites that preferentially bind fluorine over oxygen. They should also create interfacial chemistry that reduces covalency in metal-oxygen bonds at the cathode surface. Aluminium oxide satisfies both requirements, but other materials might perform better.

The coating's amorphous structure turns out to be advantageous. Crystalline coatings tend to have fewer reactive sites. The disorder provides multiple coordination environments for aluminium, including those undercoordinated sites essential for chemical scavenging.

Temperature matters too. The atomic layer deposition process used here operated at 120 degrees Celsius. Higher temperatures might crystallise the alumina, reducing its reactivity. Lower temperatures might not provide sufficient energy for uniform coating.

Single-crystal cathode particles, now entering commercial production, could benefit more from coatings than polycrystalline materials. Without the cracking that plagues agglomerates, protective layers would remain intact throughout the battery's life.

The bigger picture

Electric vehicles already dominate new car sales in several markets. Battery degradation remains one barrier to wider adoption. Consumers expect vehicles to last a decade or more. Batteries need to keep pace.

This research provides a framework for understanding how nanoscale interventions can extend battery lifetimes. The aluminium oxide coating isn't perfect. It adds resistance, slightly reducing initial capacity. It can't prevent cracking. But it demonstrates that carefully engineered interfaces can significantly slow degradation.

The insights extend beyond aluminium oxide. Any coating material must be evaluated for both its chemical reactivity with electrolyte species and its influence on electronic structure at the cathode surface. Both matter. Ignoring either misses half the story.

Future batteries may use entirely different cathode chemistries—lithium-rich layered oxides, or materials with even higher nickel content. The principles discovered here should transfer. Protect surfaces chemically. Stabilise oxygen electronically. Do both, and batteries last longer.

For now, the next generation of electric vehicles will likely use nickel-rich cathodes with nanometre-scale protective coatings. They'll charge faster and last longer than their predecessors. The chemistry is complex, but the outcome is simple: batteries that don't wear out before their cars do.

Credit & Disclaimer: This article is a popular science summary written to make peer-reviewed research accessible to a broad audience. All scientific facts, findings, and conclusions presented here are drawn directly and accurately from the original research paper. Readers are strongly encouraged to consult the full research article for complete data, methodologies, and scientific detail. The article can be accessed through https://doi.org/10.1039/d4ee03444a