One big problem with electrodes in rechargeable batteries, going through cycles of charging and discharging, is they must expand and shrink during each cycle, sometimes doubling in volume, and then shrinking back. This leads to repeated shedding and reformation of the electrode’s “skin” layer that irreversibly consumes lithium, degrading the battery’s performance over time.
But a unique way around the problem has been found by researchers at MIT and Tsinghua University. By 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 concept could drastically improve cycle life, the team says, and provide a dramatic boost in the battery’s capacity and power.
The new findings use aluminum as the key material for the lithium-ion battery’s negative electrode, or anode, according to 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 current lithium-ion batteries, which are the most widely used kind 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).
Researchers have explored for many years 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 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.
In the nanotechnology field, there is a large divergence between what are called “core-shell” and “yolk-shell” nanoparticles.
The former have a shell that is bonded directly to the core, but yolk-shell particles feature a void between the two—equivalent to where the white of an egg would be. As a result, the “yolk” material can expand and contract freely, with little effect on the dimensions and stability of the “shell.”
“We made a titanium oxide shell,” Li says, “that separates the aluminum from the liquid electrolyte” between the battery’s two electrodes.
Stability of Coating
The shell does not expand or shrink much, he says, so the SEI coating on the shell is very stable and does not fall off, and the aluminum inside is protected from direct contact with the electrolyte.
After being tested through 500 charging-discharging cycles, the titanium shell gets a bit thicker, Li says, but the inside of the electrode remains clean with no buildup of the SEIs, proving the shell fully encloses the aluminum while allowing lithium ions and electrons to get in and out.
The result is an electrode that gives more than three times the capacity of graphite (1.2 Ah/g) at a normal charging rate, Li says.
The materials are inexpensive, and the manufacturing method could be simple and easily scalable, Li says. For applications that require a high power- and energy-density battery, he says, “It’s probably the best anode material available.”
Sa Li, Junjie Niu, Yu Cheng Zhao, Kang Pyo So, Chao Wang, Chang An Wang & Ju Li High-rate aluminium yolk-shell nanoparticle anode for Li-ion battery with long cycle life and ultrahigh capacity Nature Communications 6, Article number: 7872 doi:10.1038/ncomms8872
Illustration: 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. Credit: Christine Daniloff/MIT