Charge the battery before it's too low!

Some battery problems are loud. You can see the swelling, smell the overheating, or watch the charge plummet in real time. Others are quiet saboteurs, gnawing away at performance in ways almost no one notices until it’s too late. You probably know that some batteries suffer when they are charged while they still have some juice. But new research provides insights that contradict the general rule that, for your battery to last, you have to wait until it's almost running on fumes before plugging it into a charger.

A new study published in Advanced Energy Materials by Seungyun Jeon and colleagues from POSTECH, the Korea Institute of Science and Technology, Korea University, and Sungkyunkwan University has just shed light on one of these silent killers. They’ve discovered that lithium-ion batteries can suffer severe damage even when you operate them within so-called “safe” voltage limits, the very boundaries engineers have trusted for decades to prevent long-term harm.

The problem is something the team calls a quasi-conversion reaction, a reduction-induced oxygen loss process that strikes during battery discharge at voltages between 2.0 and 3.0 volts. This is far from the high-voltage conditions that were long assumed to be the main danger zone for lithium-ion cathodes.

The damage isn’t happening deep inside the bulk of the cathode material, but right at its surface, where the crystal structure quietly rearranges itself from its original layered form into a denser, less conductive “rocksalt” phase. This transformation comes with a loss of oxygen atoms, tiny defects that block lithium ions from flowing smoothly. Over time, that surface degradation chokes off performance, making it harder for the battery to deliver power.

The researchers focused on NMC cathodes, lithium nickel manganese cobalt oxides, which are the workhorses of modern EV and consumer electronics batteries. Engineers already know that charging these materials above about 4.3 V can destabilize the structure and lead to oxygen release. But Jeon’s team found that even during discharge, under much gentler conditions, certain surface oxygen atoms are vulnerable. When those atoms are coordinated with fewer neighboring metal ions, especially when manganese is scarce, they’re more likely to be stripped away, forming lithium oxides on the surface.

That might sound harmless, but it sets off a cascade of side effects. Lithium oxide doesn’t play nicely with the carbonate electrolytes used in most commercial batteries. The two react, creating an organic-rich layer that insulates the surface and gases like methane, carbon monoxide, and hydrogen. This layer increases resistance, while the gas generation can literally swell pouch cells until they bulge like overinflated pillows. In the team’s tests, lowering the discharge cutoff voltage from 3.25 V to 2.0 V caused some full-size cells to balloon from 0.2 cm to 1.7 cm thick after repeated cycling.

It’s a particularly nasty problem for nickel-rich cathodes, which already have fewer manganese atoms to help stabilize oxygen. In those materials, the vulnerable surface oxygens are even easier to dislodge, meaning the quasi-conversion reaction starts earlier and progresses faster. The study found that by simply avoiding deep discharges—keeping the voltage above about 3.25 V—cells retained dramatically more capacity over hundreds of cycles.

This finding is a big deal because it challenges one of the “safe operation” assumptions that battery designers, manufacturers, and even users rely on. For years, the focus has been on avoiding high-voltage damage during charging. Now, it’s clear that discharging too far, into a zone that barely contributes extra capacity, can be just as destructive.

The takeaway is as practical as it is scientific: if you want your high-performance lithium-ion battery to last, don’t just watch how high you charge it: watch how low you let it go. For electric vehicles, power tools, and even your phone, that could mean new recommendations for software limits and battery management systems.

If you want to learn more, the original article titled "Reduction-Induced Oxygen Loss: the Hidden Surface Reconstruction Mechanism of Layered Oxide Cathodes in Lithium-Ion Batteries" is available on Advanced Energy Materials at https://doi.org/10.1002/aenm.202404193