August 6, 2015

“Yolks” and “shells” improve rechargeable batteries

A new "yolk-and-shell" nanoparticle could boost the capacity and power of lithium-ion batteries.
The gray sphere at center represents an aluminum nanoparticle, forming the "yolk."
The outer light-blue layer represents a solid shell of titanium dioxide, and the space
in between the yolk and shell allows the yolk to expand and contract without damaging the shell.
In the background is an actual scanning electron microscope image of a collection of
these yolk-shell nanoparticles. Image: Christine Daniloff/MIT

Aluminum could give a big boost to capacity and power of lithium-ion batteries.

(August 6, 2015)  One big problem faced by electrodes in rechargeable batteries, as they go through repeated cycles of charging and discharging, is that they must expand and shrink during each cycle — sometimes doubling in volume, and then shrinking back. This can lead to repeated shedding and reformation of its “skin” layer that consumes lithium irreversibly, degrading the battery’s performance over time.

Now a team of researchers at MIT and Tsinghua University in China has found a novel way around that problem: creating an electrode made of nanoparticles with a solid shell, and a “yolk” inside that can change size again and again without affecting the shell. The innovation could drastically improve cycle life, the team says, and provide a dramatic boost in the battery’s capacity and power.

The new findings, which use aluminum as the key material for the lithium-ion battery’s negative electrode, or anode, are reported in the journal Nature Communications, in a paper by MIT professor Ju Li and six others. The use of nanoparticles with an aluminum yolk and a titanium dioxide shell has proven to be “the high-rate champion among high-capacity anodes,” the team reports.

Most present lithium-ion batteries — the most widely used form of rechargeable batteries — use anodes made of graphite, a form of carbon. Graphite has a charge storage capacity of 0.35 ampere-hours per gram (Ah/g); for many years, researchers have explored other options that would provide greater energy storage for a given weight. Lithium metal, for example, can store about 10 times as much energy per gram, but is extremely dangerous, capable of short-circuiting or even catching fire. Silicon and tin have very high capacity, but the capacity drops at high charging and discharging rates.

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