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Nanomaterial to Boost Lithium-Ion Battery Performance

Nanoscoop (RPI)

(Rensselaer Polytechnic Institute)

A new type of nanomaterial developed at Rensselaer Polytechnic Institute in Troy, New York could sharply reduce the time needed to recharge lithium (Li)-ion batteries, the type of battery found in electric automobiles, laptop computers, mobile phones, and other portable devices. Nanomaterials are made from technologies that operate at nanometer, or one-billionth of a meter, scale.

The new material, says its developers, withstands extremely high rates of charge and discharge that would cause conventional electrodes used in most of today’s Li-ion batteries to deteriorate and fail. It has a shape resembling a cone with a scoop of ice cream on top, and is thus called a “nanoscoop” (pictured right). The nanoscoop’s success lies in its unique material composition, structure, and size.

The Rensselaer research team, led by engineering professor Nikhil Koratkar, showed that a nanoscoop electrode could be charged and discharged at a rate 40 to 60 times faster than conventional battery anodes, while maintaining a comparable energy density. This performance was achieved over 100 continuous charge/discharge cycles.

The anode structure of a Li-ion battery physically grows and shrinks as the battery charges or discharges. When charging, the addition of Li ions increases the volume of the anode, while discharging has the opposite effect. These volume changes result in a buildup of stress in the anode. Too great a stress that builds up too quickly, as in the case of a battery charging or discharging at high speeds, can cause the battery to fail prematurely. Thus, most batteries in portable electronic devices like cell phones and laptops charge very slowly; the slow charge rate is intentional and designed to protect the battery from stress-induced damage.

The Rensselaer team says its nanoscoop is engineered to withstand this buildup of stress. The nanoscoop is made from a carbon base topped with a thin layer of aluminum and a ball of silicon — the “scoop ” — all in nanoscale. This structure, says the team, can accept and discharge lithium ions at extremely fast rates without significant damage.

The segmented structure of the nanoscoop allows the strain to be gradually transferred from the carbon base to the aluminum layer, and finally to the silicon scoop. This natural strain gradation provides for a less abrupt transition in stress across the material interfaces, leading to a stronger electrode.

Koatkar says batteries for all-electric vehicles must deliver high power densities in addition to high energy densities. Today, these vehicles use supercapacitors to perform power-intensive functions, such as starting the vehicle and rapid acceleration, with conventional batteries that deliver high energy density for normal cruise driving and other operations. Nanoscoops may enable these two separate systems to be combined into a single, more efficient battery unit.

Results of the study were published in the paper “Functionally Strain-Graded Nanoscoops for High Power Li-Ion Battery Anodes,” in the journal Nano Letters (paid subscription required).

Related: Univ/Corp Team Extends Lithium-Ion Capacity with Silicon

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