Researchers from Drexel University say that adding MXene to silicon anodes could extend the life of Li-ion batteries by as much as five times. It’s able to do that because of the two-dimensional material’s ability to prevent the silicon anode from expanding to its breaking point during charging — a problem that’s prevented the use of silicon for some time.
In batteries, charge is held in the electrodes and delivered to our devices as ions travel from anode to cathode. The ions return to the anode when the battery is recharged. Battery life has steadily been increased by finding ways to improve the electrodes’ ability to send and receive more ions. Substituting silicon for graphite as the primary material in the Li-ion anode would improve its capacity for taking in ions because each silicon atom can accept up to four lithium ions, while in graphite anodes, six carbon atoms take in just one lithium. But, as it charges, silicon also expands — as much as 300 percent — which can cause it to break and the battery to malfunction. Most solutions to this problem have involved adding carbon materials and polymer binders to create a framework to contain the silicon, which is a complex process.
By mixing silicon powder into a MXene solution to create a hybrid silicon-MXene anode, 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. It’s the MXene framework that also imposes order on ions as they arrive and prevents the anode from expanding. MXenes are a key to helping silicon reach its potential in batteries because, since they 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 the 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 are made by chemically etching a layered ceramic material called a MAX phase, to remove a set of chemically-related layers, leaving a stack of two-dimensional flakes. Researchers have produced more than 30 types of MXene to date, each with a slightly different set of properties. The group selected two of them to make the silicon-MXene anodes for testing: titanium carbide and titanium carbonitride. They 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 as well as superior conductivity. The silicon-MXene anodes had on the order of 100 to 1,000 times higher conductivity than conventional silicon anodes.
“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,” according to the researchers. “Therefore, the combination of viscous MXene ink and high-capacity Si offers a powerful technique to construct advanced nanostructures with exceptional performance.” The process of slurry-casting MXene-silicon anodes is 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.