The nanoparticle electrode features a solid shell, and a 'yolk' inside that can change size again and again without affecting the shell. The innovation could improve cycle life of rechargeable lithium-ion batteries.
A major 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. An electrode can double in volume before shrinking back. The movement can lead to repeated shedding and reformation of the electrode's 'skin' layer that consumes lithium irreversibly, degrading the battery’s performance over time.
The discovery, which uses aluminum as the key material for the lithium-ion battery’s negative electrode, or anode, have been reported in the journal Nature Communications, in a paper by MIT professor Ju Li and six others.
Most existing lithium-ion batteries use anodes made of graphite, a form of carbon. Graphite has a charge storage capacity of 0.35 Ah/g; for many years, researchers have explored other options that would provide greater energy storage for a given weight. Lithium metal can store about 10 times as much energy per gram, but is dangerous and capable of short-circuiting or even catching fire. Silicon and tin have 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 said, “expand a lot when they get to high capacity, when they absorb lithium. And then they shrink, when releasing lithium.”