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Researchers Design Rechargeable Battery with Improved Charge Capacity, Rate

Imagine a cell phone battery that stayed charged for more than a week – and recharged in just 15 minutes.

That dream battery could be closer to reality. Researchers at Northwestern University have created an electrode for lithium-ion batteries – rechargeable batteries like those found in cell phones and iPods – that allows the batteries to charge more quickly and hold a charge up to 10 times longer than current technology.

Researchers say the technology could pave the way for more efficient, smaller batteries that could be useful in electric cars.

A paper describing the research, “In-Plane Vacancy-Enabled High-Power Si-Graphene Composite Electrode for Lithium-Ion Batteries,” was published in October in the journal Advanced Energy Materials.

“We have found a way to extend a lithium-ion battery’s charge life by 10 times at the beginning of the battery’s life,” said Harold H. Kung, professor of chemical and biological engineering and the lead author of the paper. “Even after 150 charges, which would be one year or more of operation, the battery is still five times more effective than lithium-ion batteries on the market today.”

Lithium-ion batteries charge through a chemical reaction in which lithium ions are sent between two ends of the battery, the anode and the cathode. As energy in the battery is used, the lithium ions travel from the anode, through the electrolyte, and to the cathode; as the battery is recharged, they travel in the reverse direction.

With current technology, the capacity of a lithium-ion battery is limited in two ways. Its energy capacity – that is, how long a battery can maintain its charge – is limited by the charge density, or how many lithium ions can be packed into the anode or cathode. In current rechargeable batteries, the anode – made of layer upon layer of carbon-based graphene sheets – can only accommodate one lithium ion for every six carbon atoms. To increase capacity, scientists have previously experimented with replacing the carbon with silicon, as silicon can accommodate much more lithium: four lithium for every one silicon. However, silicon expands and contracts dramatically in the charging process, causing fragmentation and losing its charge capacity rapidly.

Meanwhile, a battery’s charge rate – the speed at which it recharges – is limited by another factor: the speed at which the lithium ions can make their way from the electrolyte into the anode. Currently, that speed is hindered by the shape of the graphene sheets: They are extremely thin – just one carbon atom thick – but by comparison, very long. During the charging process, a lithium ion must travel all the way to the outer edges of the graphene sheet before entering and coming to rest between the sheets. And because it takes so long for the lithium to travel to the middle of the graphene sheet, a sort of ionic traffic jam occurs around the edges of the material.

Now, Kung’s research team has combined two techniques to combat both these problems. First, to stabilize the silicon in order to maintain maximize charge capacity, they sandwiched clusters of silicon between the graphene sheets. This allowed for a greater number of lithium ions in the electrode while utilizing the flexibility of graphene sheets to accommodate the volume changes of silicon during use.Image demonstrates silicon clusters and holes that allow lithium ions to enter graphene sheets.

“Now we almost have the best of both worlds,” Kung said. “We have much higher energy density because of the silicon, and the sandwiching reduces the capacity loss caused by the silicon expanding and contracting. Even if the silicon clusters break up, the silicon won’t be lost.”

Kung’s team also used a chemical oxidation process to create miniscule holes (10-20 nanometers) in the graphene sheets – termed “in-plane defects” – so the lithium ions would have a “shortcut” into the anode. This reduced the time it takes the battery to recharge by up to 10 times. 

Next the researchers will begin studying changes in the cathode that could further increase effectiveness of the batteries. They will also look into developing an electrolyte system that will allow the battery to automatically shut off at high temperatures – a safety mechanism that could prove vital in electric car applications.

Other authors of the paper are Xin Zhao, Cary M. Hayner, and Mayfair C. Kung.

Read Forbes' article about the research.

Read Popular Science's article about the research.

-- Sarah Ostman