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Nanotech Solution Boosts Lithium-Ion Battery Performance

Plug-in electric vehicle (Idaho National Lab)

(Idaho National Lab)

Engineers at Northwestern University in Illinois have created an electrode for lithium-ion batteries that can hold 10 times the charge and recharge 10 times faster than current batteries. A paper describing the research was published last month in the journal Advanced Energy Materials (paid subscription required), and funded by the Energy Frontier Research Centers program in the Department of Energy.

Lithium-ion batteries are the power source for common electronic devices, such as smartphones and laptop computers, as well as plug-in electric cars. The research led by chemical and biological engineering professor Harold Kung addresses two limitations of lithium-ion batteries: their energy capacity and charge rate.

As energy in a lithium-ion battery is used, lithium ions travel from the anode at one end of the battery, through the electrolyte, and to the cathode at the battery’s other end. When recharging, the ions move in the reverse direction.

The energy capacity, or length of time lithium-ion batteries maintain a charge, is limited by their charge density, the number of lithium ions packed into the anode or cathode. The charge rate, or speed at which a battery recharges is limited by the speed of lithium ions traveling from the electrolyte into the anode.

In current rechargeable batteries, the anode is made of layered carbon-based graphene sheets that can only accommodate one lithium atom for every six carbon atoms. To increase energy capacity, scientists have previously tried replacing the carbon with silicon, as silicon can accommodate much more lithium — four lithium atoms for every silicon atom. But silicon has its own limitations; it expands and contracts dramatically in the charging process, causing fragmentation and loss of its charge capacity.

Also, graphene sheets are very thin, just one atom thick, but in terms of scale very long. When a lithium-ion battery charges, 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. This relatively extended route and length of time for lithium to travel to the middle of the graphene sheet creates an accumulation of ions around the edges of the material.

The researchers used two techniques to address these problems. To maintain maximum charge capacity the team first had to stabilize the silicon. For this task, the engineers sandwiched clusters of silicon between the graphene sheets, which allows more lithium atoms in the electrode, while taking advantage of graphene’s flexibility to accommodate the additional volume of silicon. Kung notes that the sandwiching technique reduces the capacity loss caused by silicon’s expanding and contracting.

To speed the recharging process, the researchers used a chemical oxidation process to create miniscule holes of 10 to 20 nanometers — 1 nanometer equals 1 billionth of a meter — in the graphene sheets. These nanoscale holes, called in-plane defects, provide a shortcut to the anode as well as a place for lithium ions to be stored when they react to the silicon. This technique cuts the time it takes the battery to recharge to as little as one-tenth that of current rates.

While Kung’s team’s paper focused on the battery’s anode, the researchers will next study changes in the cathode to further increase effectiveness of the batteries. The team also will look into an electrolyte system that allows the battery to automatically and reversibly shut off at high temperatures, a safety mechanism for electric car batteries highlighted by recent reports of a battery fire occurring in a test crash.

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