The most common lithium-ion batteries suffer a previously unknown form of degradation when they run very low on charge, making it a good idea to give them a little extra juice before they run out entirely.
People under 40 might not believe it, but there was a time before lithium-ion batteries dominated the market. Back then most small devices relied on batteries that were not rechargeable, while the lead-acid batteries used in internal combustion car engines were the most common technology when repeated cycling was required.
Among the many factors that made lead-acid batteries inferior was the long-term damage they suffered when “flattened” (all their charge they held used up), ensuring they would never be as good again. Lithium-ion batteries not only offered far more charge for the same weight, turning electric vehicles from toys to serious competitors, but also held the promise of not having to worry about discharging too far.
Unfortunately, a new study reveals these claims aren’t true, at least for lithium-ion batteries with nickel-manganese-cobalt (NMC) cathodes. These currently make up around half the electric vehicle market, and are also used in smartphones, laptops, and many of the other devices on which we increasingly rely.
Professor Jihyun Hong of Pohang University of Science and Technology led a team that studied what happens to NMC cathodes when batteries are run flat. They identified a previously overlooked phenomenon they call a “quasi-conversion reaction” on the surface of the cathode.
The process involves oxygen escaping from the cathode surface and combining with lithium to form lithium oxide (Li2O). The Li2O reacts with carbonate-based electrolytes to produce gasses including carbon monoxide, methane, and hydrogen that damage the battery.
The effect was observed with a range of NMC cathodes, but was more pronounced when nickel concentrations were highest. That’s bad news because battery manufacturers have been replacing cobalt with nickel because of the high cost and environmental and human rights concerns associated with the world’s largest source of cobalt.
The team reports the effect can be astonishingly large for high nickel batteries that are regularly drained of all charge, but quite modest when the same batteries were monitored to avoid discharging too far. A 90 percent nickel battery (well above the market norm) that was deeply discharged repeatedly kept just 3.8 percent of its capacity after 250 cycles, making it effectively useless. Meanwhile, a control that was not allowed to fully discharge had almost three-quarters of its original capacity after 300 cycles.
“The impact of discharge—the actual process of using a battery—has been largely overlooked until now,” Hong said in a statement. “This research presents an important direction for developing longer-lasting batteries.”
Everyone knows their phones don’t hold their charge for as long as they get older. Partly that may be because of all the extra apps people add soaking up more electricity, but battery degradation is certainly a factor. Some loss of function is inevitable; entropy is coming for us all. However, overcharging batteries can also damage them.
That’s why many phone manufacturers advise you not to leave your phone on charge overnight, instead just charging it to 70 percent or so.
The same advice suggests not letting your phone run down below 30 percent, as this can also harm battery longevity. However, at least when it comes to extending the range on batteries in electric vehicles, the focus has been on avoiding overcharging, rather than discharging too deeply.
Hong’s team’s work suggests that focus has been misplaced, at least for the highest nickel concentrations. Batteries with lower nickel concentrations in their cathodes are not as drastically affected, and the research did not cover lithium iron phosphate (LFP) batteries, which dominate grid storage systems and are taking over the car industry.
Some people prefer to live life on the edge, only charging their phones or laptops when absolutely necessary, while others get anxious when the charge drops below 25 percent. It seems for the health of their devices, and maybe even the planet, the second group has it right.
The study is open access in Advanced Energy Materials.
Source Link: Why It’s A Bad Idea To Let Your Phone Battery Run All The Way Down
