A new ‘yolk-and-shell’ nanoparticle could boost the capacity and power of lithium-ion batteries. The grey 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 a scanning electron microscope image of a collection of these yolk-shell nanoparticles. Image: Christine Daniloff/MIT.One big problem faced by electrodes in rechargeable lithium-ion 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 reforming of an electrode’s ‘skin’ layer that consumes lithium irreversibly, degrading the battery's performance over time.
Now a team of researchers at the Massachusetts Institute of Technology (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. This innovation could drastically improve cycle life, the team says, and provide a dramatic boost in the battery's capacity and power.
As reported in the journal Nature Communications, in a paper by MIT professor Ju Li and six others, the nanoparticles comprise an aluminum yolk and a titanium dioxide shell. They are used as the key material for the lithium-ion battery's negative electrode, or anode, which has already proven to be "the high-rate champion among high-capacity anodes”, the team asserts.
Most current lithium-ion 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 it is extremely dangerous, capable of short-circuiting or even catching fire. Silicon and tin have very high capacities, but the capacities drop at high charging and discharging rates.
Aluminum is a low-cost option with theoretical capacity of 2Ah/g. But aluminum and other high-capacity materials, Li says, "expand a lot when they get to high capacity, when they absorb lithium. And then they shrink, when releasing lithium."
This expansion and contraction of aluminum anodes generates great mechanical stress, which can cause electrical contacts to disconnect. In addition, the liquid electrolyte in contact with aluminum will always decompose at the required charge/discharge voltages, forming a skin called the solid-electrolyte interphase (SEI) layer. Another detrimental effect of the repeated expansion and shrinkage is that it causes this SEI layer to shed particles. As a result, previous attempts to develop an aluminum electrode for lithium-ion batteries had failed.
That's where the idea of using confined aluminum in the form of a yolk-shell nanoparticle came in. In nanotechnology, there is a big difference between what are called ‘core-shell’ and ‘yolk-shell’ nanoparticles. The former have a shell that is bonded directly to the core, whereas yolk-shell particles feature a void between the two, around 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.” 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.
The team didn't originally plan it that way, says Li. "We came up with the method serendipitously, it was a chance discovery," he says. The aluminum particles they used, which are about 50nm in diameter, naturally have an oxidized layer of alumina (Al2O3). "We needed to get rid of it, because it's not good for electrical conductivity," Li says.
They ended up converting the alumina layer to titania (TiO2), which is a better conductor of electrons and lithium ions when it is very thin. To do this, they placed aluminum powders in sulfuric acid saturated with titanium oxysulfate. When the alumina reacts with the sulfuric acid, excess water is released and reacts with titanium oxysulfate to form a solid shell of titanium hydroxide with a thickness of 3–4nm. While this solid shell forms nearly instantaneously, if the particles stay in the acid for a few more hours, the aluminum core shrinks to become a 30nm-across ‘yolk’, showing that small ions can get through the shell.
The particles are then treated to get the final aluminum-titania (ATO) yolk-shell particles. After being tested through 500 charging-discharging cycles, the titania shell gets a bit thicker, Li says, but the inside of the electrode remains clean with no build-up of the SEIs. This proves that the shell fully encloses the aluminum while allowing lithium ions and electrons to get in and out. The result is an electrode that possesses more than three times the capacity of graphite (1.2Ah/g) at a normal charging rate, Li says. At very fast charging rates (six minutes to full charge), the capacity is still 0.66Ah/g after 500 cycles.
The materials are inexpensive, and the manufacturing method could be simple and easily scalable, Li says. For applications that require a battery with high power and a high energy-density, he says, "it's probably the best anode material available." Full cell tests using lithium iron phosphate as a cathode have been successful, indicating ATO is quite close to being ready for real applications.
This story is adapted from material from MIT, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.