At first, the battery in your phone or electric car works just fine. It charges, discharges, and recharges with smooth regularity. But after months or years, the battery weakens. The charge doesn’t last. The range shortens. Scientists have long blamed this aging on stress from high voltages, particularly during charging.
Now, a team of scientists in South Korea has identified a silent saboteur within these batteries — and they’ve found a surprisingly simple way to stop it.
Why You Shouldn’t Let Your Phone Discharge to 0%
For years, researchers believed that the surface of a battery’s cathode — a material that stores and releases lithium ions — only degraded at high voltages. During charging, the cathode loses lithium and becomes unstable, leading to a structural collapse and the release of oxygen. This process transforms the material from its original layered structure into a more disordered, rocksalt-like phase. It’s like a brick wall morphing into a pile of gravel.
This degradation hinders the flow of lithium ions, gradually choking off the battery’s performance.
But the new study, led by Jihyun Hong at POSTECH and Jongsoon Kim at Sungkyunkwan University, reveals that this structural collapse can also occur in reverse — during discharging, and even when voltages stay below 3.0 volts.
Using a combination of advanced microscopy, spectroscopy, and computer simulations, the researchers studied commercial layered cathode materials like NMC622 and NMC811 (NMC stands for nickel-manganese-cobalt), which are commonly used in electric vehicles. They cycled these materials in half- and full-cell configurations while varying the discharge cut-off voltage — the lowest voltage the battery is allowed to reach before charging again.
Surprisingly, the lower the discharge voltage, the faster the battery degraded. And the damage was most severe at the surface of the cathode particles.
“Lowering the discharge cut-off voltage resulted in more severe capacity loss although the capacity accessible . . . is negligibly low,” the researchers note.
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The Quasi-Conversion Reaction
So, what’s going on? The team proposes that a quasi-conversion reaction takes place at these lower voltages. It’s not quite the same as the known “conversion reaction” used in battery anodes, where the entire structure breaks down. Instead, it’s a partial and localized process, focused on the surface.
At voltages between 2.0 and 3.0 volts, lithium re-enters the cathode, and oxygen atoms near the surface break away. This forms lithium oxide (Li₂O) and creates oxygen vacancies — holes where oxygen atoms used to be. These defects trigger the formation of the dreaded rocksalt phase.
“The surface of commercial layered oxide cathodes undergoes phase transformation and oxygen loss . . . through a reduction (lithiation) process,” the team explains.
Simulations showed that this surface oxygen loss happens at higher voltages than previously expected — because surface oxygen atoms are less tightly bound than those in the bulk. Their bonds are weaker, making them more prone to breakage even under seemingly benign conditions.
These reactions degrade the cathode’s structure and cause the battery to swell — a sign of internal failure. When batteries are discharged deeply — drained almost to empty — that’s when oxygen begins to escape from the cathode’s surface.
“We confirmed that when batteries are used until most of their capacity is depleted, the effects of the degradation process become increasingly pronounced,” the researchers noted.
Gas analysis confirmed the presence of a chemical storm inside the battery: 29 times more gas was generated in the deeply discharged cells. The electrolyte had been transformed into a cocktail of methanol, ethylene carbonate fragments, and polyethylene oxide.
“The significant and continued formation of gaseous byproducts… likely supports our proposed mechanism,” the authors conclude.
The damage was especially severe in high-nickel batteries. In one experiment, batteries that underwent deep discharge lost virtually all their storage ability after 250 cycles — retaining just 3.8% of their capacity. In contrast, batteries that were prevented from discharging too far maintained over 73% capacity even after 300 cycles.
A Simple Fix, Big Payoff
The solution is almost disarmingly simple: raise the discharge cutoff voltage. That means tweaking the battery management software so the battery stops discharging before the critical point where the quasi-conversion kicks in—around 3.0 volts.
By doing this, researchers dramatically slowed the release of oxygen and the formation of damaging gases. High-nickel batteries that once gave out prematurely showed a sharp increase in longevity.
“Discharge — the actual process of using a battery — has been largely overlooked until now,” said Professor Hong. “This research presents an important direction for developing longer-lasting batteries.”
Of course, such a tweak would come at the cost of reduced runtime.
If manufacturers implement the change in discharge protocols, consumers could see longer battery life without new hardware.
Better battery performance doesn’t always require exotic new materials or billion-dollar breakthroughs. Sometimes, it just takes a better understanding of how batteries live — and die — in the real world.
The findings appeared in the journal Advanced Energy Materials.
🔋 What You Can Do
- Try not to let your battery reach 0%. Even if your device says 0%, most have a safety buffer, but repeatedly running it low still risks degradation.
- Recharge before your device gets critically low, ideally when it’s around 20–30%.
- If possible, adjust settings (in EVs or power tools, for example) to limit how far the battery is discharged.
🚗 For EV Owners:
Some electric vehicles let you set a minimum discharge limit or avoid using the full range. This can protect the battery and extend its lifespan — even if it slightly reduces your day-to-day range.
📱 For Smartphone or Laptop Users:
You might not have full control over discharge voltage, but frequent deep discharges are best avoided. Topping up when you’re in the 20–30% range is a smart move.