Going small with silicon potentially has big implications for lithium-ion battery capacity
Researchers at the University of California, Riverside (UCR) have developed a silicon anode for lithium-ion batteries that outperforms current materials and gets around previous issues that would cause the battery to be inefficient and quickly degrade (or even fail catastrophically) with use. As the researchers focus on scaling up production, the advance could pave the way for higher-performance electric vehicles, electronics and all-around portable power.
Graphite has long been used to build the negative electrodes in lithium-ion batteries, but as batteries improve, it is slowly becoming a performance bottleneck because of the limited amount of energy that it can store.
Silicon could store up to 10 times more energy per unit weight than graphite, but it currently has two main drawbacks. The first is low Coulombic effiency, which is to say that charge transferred to and from a silicon electrode incurs substantial losses. The second and more critical reason is that silicon contracts and then expands by as much as 300 percent with each charge cycle, forming cracks that reduce battery performance, create short circuits, and eventually cause a catastrophic failure of the cell.
A research team led by professors Mihri and Cengiz Ozkan has now developed an electrode consisting of sponge-like silicon nanofibers which, according to their study, gets around both problems (and more).
The researchers produced the nanofibers by applying a high voltage between a rotating drum and a nozzle emitting a solution of tetraethyl orthosilicate (TEOS). The material was then exposed to magnesium vapors to obtain sponge-like silicon fiber structures.
"Our silicon nanofibers have several structural advancements at the nanometer scale that help with the minimization of undesired large volume expansion as observed in other standard Si materials," M. Ozkan tells us.
The researcher told Gizmag that the nanofibers contain nanoscopic pores around 10 nanometers in diameter on their surface which, along with additional gaps between the fiber network, provide enough room for the silicon to expand into without damaging the cell. But there are also three more factors that reduce the undesired expansion of silicon: a one-nanometer shell of silicon dioxide; a second carbon coating that creates a buffer layer; and lastly, the size of the fibers themselves, which are between 8 and 25 nanometers (which is below the size at which silicon tends to fracture).
Lithium-ion cells currently need binders to hold together the particles of the active material and keep them in contact with the current collectors. Because these are not active materials, they make the battery bigger and heavier without directly contributing to performance. Although there have been prototypes of binderless batteries, these can’t scale because the active materials these cells use can only be produced in very small quantities.
The researchers say the new electrode material does away with the need for current collectors, polymer binders and conductive powder additives altogether, with silicon making up over 80 percent of the entire electrode by weight. The team tested the capacity of the electrode at 802 mAh/g after more than 600 cycles, with a Coulombic efficiency (previously one of the weak points of silicon electrodes) of 99.9 percent.
Unlike with previous binderless battery designs, this active material is much more well-suited to mass production, so the focus is now shifting on scaling up both the size and the quantity of the cells.
"Currently, our lithium-ion battery is in the coin-cell format," M. Ozkan told us. "We need to partner with a battery maker to scale up our battery technology into a pouch prototype format and then they can be used to power commercial portable electronics. How fast this can happen depends on how quickly we can partner with a battery maker."
An open-access paper describing the advance was published in the journal Nature Scientific Reports.