By Battery Power Online Staff
February 25, 2019 | Silicon anodes promise to extend the charge-to-charge time for lithium-ion batteries by as much as 40%. But research from Drexel University and Trinity College in Ireland suggests that a 5x improvement could be achieved if the silicon is fortified with a special type of material called MXene, which prevents the silicon anode from expanding to its breaking point during charging.
The research was published last week in Nature Communications (https://doi.org/10.1038/s41467-019-08383-y).
A silicon anode improves battery capacity because each silicon atom can accept up to four lithium ions, while in traditional graphite anodes, six carbon atoms take in just one lithium. But as it charges, silicon also expands in size—as much as 300%—which can cause it to break and the battery to malfunction. By contrast, the Drexel and Trinity group’s method mixes silicon powder into a MXene solution to create a hybrid silicon-MXene anode.
“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, director of the A.J. Drexel Nanomaterials Institute in the Department of Materials Science and Engineering and co-author of the research, in a press release. “We’ve discovered adding MXene materials to the silicon anodes can stabilize them enough to actually be used in batteries.”
Gogotsi first discovered MXenes in 2011 (DOI: 10.1002/adma.201102306). The two-dimensional titanium carbide or carbonitride nanosheets can be used as a conductive binder for silicon electrodes and are produced by a simple and scalable slurry-casting technique without the need of any other additives.
MXene nanosheets distribute randomly and form a continuous network while wrapping around the silicon particles, thus acting as conductive additive and binder at the same time. The MXene framework also imposes order on ions as they arrive and prevents the anode from expanding.
“MXenes are the key to helping silicon reach its potential in batteries,” Gogotsi said in a press release. “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.”
Researchers have produced more than 30 types of MXenes to date, each with a slightly different set of properties. The group selected two of them to make the silicon-MXene anodes tested here: titanium carbide and titanium carbonitride.
MXenes are made by chemically etching titanium-aluminum carbide, a member of a family of layered ternary carbides called a MAX phase, leaving a stack of two-dimensional flakes. When mixed with water, the MXene ink is made of clean flakes with a hexagonal atomic structure, predominantly single-layered, the authors report.
Researchers also tested battery anodes made from graphene-wrapped silicon nanoparticles.
All three anode samples showed higher lithium-ion capacity than current graphite or silicon-carbon anodes used in Li-ion batteries and superior conductivity—on the order of 100 to 1,000 times higher than conventional silicon anodes—when MXene is added.
“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 silicon,” the authors write. “Therefore, the combination of viscous MXene ink and high-capacity silicon demonstrated here offers a powerful technique to construct advanced nanostructures with exceptional performance.”
Chuanfang Zhang, PhD, a post-doctoral researcher at Trinity and lead author of the study, also notes that the production of the MXene anodes, by slurry-casting, is easily scalable for mass production of anodes of any size, which means they could make their way into batteries that power just about any of our devices.
“Considering that more than 30 MXenes are already reported, with more predicted to exist, there is certainly much room for further improving the electrochemical performance of battery electrodes by utilizing other materials from the large MXene family,” Zhang said in the same press release.