“Yolks” and “shells” improve rechargeable batteries
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.
• Aluminum could give a big boost to capacity and power of lithium-ion batteries.
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.
Aluminum is a low-cost option with theoretical capacity of 2 Ah/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 particles generates great mechanical stress, which can cause electrical contacts to disconnect. Also, the liquid electrolyte in contact with aluminum will always decompose at the required charge/discharge voltages, forming a skin called solid-electrolyte interphase (SEI) layer, which would be ok if not for the repeated large volume expansion and shrinkage that cause SEI particles to shed. As a result, previous attempts to develop an aluminum electrode for lithium-ion batteries had failed.
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https://newsoffice.mit.edu/2015/yolks-and-shells-improve-rechargeable-batteries-0805
Source: www.mit.edu