Today's lithium-ion batteries are quite efficient due to improvements made by researchers that replaced the battery's graphite anode with one made from silicon. Now, new work from Drexel University and Trinity College in Ireland is seeking to boost that efficiency further by fortifying silicon anodes with a material called MXene.
Silicon anodes replacing graphite ones
"Silicon anodes are projected to replace graphite anodes in Li-ion batteries with a huge impact on the amount of energy stored," said Yury Gogotsi, PhD, Distinguished University and Bach Professor in Drexel's College of Engineering and director of the A.J. Drexel Nanomaterials Institute in the Department of Materials Science and Engineering, who was a co-author of the research.
"We've discovered adding MXene materials to the silicon anodes can stabilize them enough to actually be used in batteries."
Batteries function by holding charges in electrodes, the cathode, and anode. These charges are delivered to our devices as ions travel from anode to cathode and when the ions return to the anode the battery is recharged.
Silicon anodes can accept up to four lithium ions, while in graphite anodes, six carbon atoms take in just one lithium. Replacing graphite with silicon as the primary material in the Li-ion anode would improve its efficiency, but there is one problem. Silicon expands as it charges to the point that it can break.
An MXene solution
To bypass this issue, the researchers developed a novel method of mixing silicon powder into an MXene solution. The outcome is a hybrid silicon-MXene anode.
"MXenes are the key to helping silicon reach its potential in batteries," Gogotsi said.
"Because MXenes are two-dimensional materials, there is more room for the ions in the anode, and they can move more quickly into it -- thus improving both capacity and conductivity of the electrode. They also have excellent mechanical strength, so silicon-MXene anodes are also quite durable up to 450 microns thickness."
MXenes consist of a chemically etched layered ceramic material called a MAX phase. Researchers have produced more than 30 types of MXene to date.
The team of researchers used two of them to make the silicon-MXene anodes and found that all anode samples showed higher lithium-ion capacity than conventional graphite or silicon-carbon anodes. They reported a higher conductivity of up to 100 to 1,000 times more efficient.
"The continuous network of MXene nanosheets not only provides sufficient electrical conductivity and free space for accommodating the volume change but also well resolves the mechanical instability of Si," they write.
"Therefore, the combination of viscous MXene ink and high-capacity Si demonstrated here offers a powerful technique to construct advanced nanostructures with exceptional performance."
The researchers also note that the engineering of the MXene anodes lends itself easily to mass production. The study is published in Nature Communications.